WO2016126207A1 - Selective carbon-carbon bond cleavage by earth abundant vanadium compounds under visible light photocatalysis - Google Patents

Abstract

Provided herein a vanadium(V) complex of formula I, where R₁ to R₈ are as defined herein. Also provided herein are reactions making use of the vanadium(V) complex of formula I, such as selective sp³-sp³ carbon-carbon bond cleavage under visible light photocatalysis and photodegradation of lignin.

Description

Selective Carbon-Carbon Bond Cleavage by Earth Abundant Vanadium Compounds under Visible Light Photocatalysis

Field of Invention

This invention relates to novel catalytic compounds that are capable of selective carbon- carbon bond cleavage (i.e. sp³-sp³ C-C bond cleavage).

The increasingly severe effects of global climate change and environmental pollution due to fossil fuel combustion has led to growing interest in the development of non-food biomass as feedstocks for fuels and chemicals. In particular, the conversion of biomass into fine chemicals and pharmaceutical precursors is an attractive approach because of the inherent chemical complexity of biopolymers. Selective degradation of biomass, polymers, or macromolecules leads to the production of a number of smaller organic compounds, among which some could be valuable molecules that can be utilized as building blocks in organic synthesis or as fine chemicals and in the production of pharmaceuticals production. It is critical to control the degradation pathways to obtain specific and enriched products. The selective bond cleavage is an important strategy in obtaining smaller molecules with high yield from sources such as biomass lignin.

Numerous studies have been published on the C-C bond cleavage of macromolecules, such as persistent organic pollutants (POPs), both by molecular catalysts as well as photocatalysts (e.g. see Shappell, N.; Vrabel, M.; Madsen, P.; Harrington, G.; Billey, L; Hakk, H.; Larsen, G.; Beach, E.; Horwitz, C; Ro, K.; Hunt, P.; Collins, T. Environmental Science & Technology 2008, 42, 1296; and Lin, Y.; Li, D.; Hu, J.; Xiao, G.; Wang, J.; Li, W.; Fu, X. Journal of Physical Chemistry C 2012, 116, 5764). In these processes, the parent compounds are mineralised, i.e. converted to C0₂ and water. For example, the iron TAML complexes reported by Collins and co-workers degrade persistent organic dyes to C0₂ and water (Chahbane, N.; Popescu, D.-L; Mitchell, D. A.; Chanda, A.; Lenoir, D.; Ryabov, A. D.; Schramm, K.-W.; Collins, T. J. Green Chemistry 2007, 9, 49). On the other hand, degradation processes of organic pollutants by photocatalysis have generally utilised metal oxides (e.g. Ti0₂, ZnO) as heterogeneous photocatalysts (Gaya, U. I.; Abdullah, A. H. Journal of Photochemistry and Photobiology C: Photochemistry Reviews 2008, 9, 1 ). To date, all these processes result in the production of C0₂ and water as the major products. However, it would be ideal to develop a technology that leads to the formation of more valuable products through partial and selective degradation rather than conversion to C0₂ and water only.

Selective bond cleavage is an important tool for organic synthesis, and several reports have been published on the production of valuable compounds through C-C and other carbon- hetero atom bond cleavage reactions. For instance, Zhang and co-workers have shown that a 1 ,2-diketone can be obtained from a 1 ,3-diketone through selective C-C bond cleavage (Huang, L; Cheng, K.; Yao, B.; Xie, Y.; Zhang, Y. Journal of Organic Chemistry 2011, 76, 5732). The oxidative C-C bond cleavage of vicinal diols has also been reported using Na₂l0₄ (Sklarz, B. Quarterly Reviews - Chemical Society 1967, 21, 3). Okamoto et al. reported that the oxidative C-C bond cleavage of a 1 ,2-diol can occur with the use of an iron-porphyrin as the catalyst in the presence of dihydropyridine and molecular oxygen (Okamoto, T.; Sasaki, K.; Oka, S. Journal of the American Chemical Society 1988, 110, 1 187). Recently, biomass lignin model compounds have attracted much attention in selective degradation studies. The polymeric lignin, which constitutes up to 30% by weight in woody biomass, is especially attractive as a source of aromatic feedstock molecules (Xu, C; Arancon, R. A.; Labidi, J.; Luque, R. Chemical Society Reviews 2014, 43, 7485).

One of the biggest renewable resources is plant biomass, the major constituents of which are biopolymers such as cellulose, hemicelluloses and lignin. Lignin (Figure 1 ), which constitutes up to 30% by weight of biomass, is especially appealing as a source of aromatic small molecules such as phenols and aryl ethers. However, unlike cellulose and hemicelluloses that have already been utilized in biorefineries for biofuel production, lignin is often regarded as waste since it is notoriously resistant to oxidative, hydrolytic, photochemical, and even biological degradation. Consequently, lignin is typically burned for energy production despite the presence of rich chemical functionalities that would otherwise require energy-intensive, multi-step chemical syntheses. The chemoselective photochemical depolymerization of lignin remains an underexplored field. Biomass lignin (Figure 1 ) is a complex polymer that can realistically be harnessed as the largest source of inexpensive, easily accessible precursors for the production of numerous aromatic building blocks through judicious depolymerization (see D. M. Schultz and T. P. Yoon, Science, 2014, 343, 985). Current technology for the valorization of lignin involves harsh solvothermal or pyrolytic processes in the presence of excess reagents like acids or hydrogen gas. This causes the non-selective decomposition of lignin into many products with low yields or even mineralization and charring. A cost-effective, environmentally benign protocol for lignin depolymerization through chemoselective bond cleavage is required for the sustainable production of chemical feedstocks from biomass lignin.

Recently, the selective bond cleavage of lignin model compounds and, even lignin in some cases, by molecular catalysts has been reported in a few seminal papers. This includes the use of molecular catalysts based on Ru (see: J. Ellman et al., J. Am. Chem. Soc. 2010, 132, 12554-12555; and T. H. Vom Stein et ai, Angew. Chem. Int. Ed., 2015, 54, 5859-5863), Ni (see A. G. Sergeev and J. F. Hartwig, Science, 2011, 332, 439-443), and V (see Hanson, S. K.; Baker, R. T.; Gordon, J. C; Scott, B. L; Thorn, D. L. Inorganic Chemistry 2010, 49, 5611 ; Hanson, S.; Wu, R.; Silks, L. A. P. Angewandte Chemie, International Edition 2012, 51, 3410; Son, S.; Toste, F. D. Angewandte Chemie 2010, 49, 379; Zhang (ibid); Sedai et a/., ACS Catal., 2013, 3, 311 1 -3122; and Sedai er a/., ACS Catai, 2011 , 1, 794-804) under thermal conditions. For instance, Ellman adopted a tandem dehydrogenative-reductive ether cleavage approach with Ru complexes, whereas Hartwig reported efficient Ni complexes for the reductive hydrogenation of aryl ether linkages commonly found in lignin. On the other hand, Toste et a/., Hanson and Silks independently demonstrated the promise of earth- abundant V complexes in catalyzing selective C-0 and C-C bond cleavage in lignin model compounds with high yields at moderately elevated temperatures. Stahl (Nature, 2014, 515, 249-252) and Westwood (Angew. Chem. Int. Ed., 2015, 54, 258-262) also separately described novel two-step thermal depolymerization of lignin itself, through the initial oxidation of benzylic alcohols, followed by C-0 bond cleavage via retro Diels-Alder type pathways. The first successful degradation of lignin model compounds under ambient conditions was reported by Stephenson and coworkers (see Nguyen, J. D.; Matsuura, B. S.; Stephenson, C. R. J. Journal of the American Chemical Society 2013, 136, 1218), involving a tandem TEMPO-oxidation and photocatalytic C-0 bond cleavage using visible light irradiation. Their protocol is notable for the high yields and preservation of valuable alcohol, aldehyde, and ether functional groups in the products. However, the second step includes the use of expensive Ir-based photosensitizers and sacrificial amine-formate reductants, which could be improved if earth-abundant (photo)catalysts and reagents like air are utilized instead. Thus, there remains a need for ways to degrade lignin into useful materials.

The invention will be described by reference to the following drawings.

Figure 1 depicts a representative fragment of the biomass lignin and a model compound 11 used herein, showing an overlay of compound 11 on the lignin biomass. Figure 2 depicts a general reaction scheme showing the selective C-C bond cleavage by the vanadium catalyst of the current invention.

Figure 3 depicts Oak Ridge Thermal Ellipsoid Plots (ORTEPs) from single crystal X-ray diffraction experiments of (a) complex 2, and (b) complex 5.

Figure 4 is an ORTEP from single crystal X-ray diffraction experiments of the dimeric polymorph of complex 5. Figure 5 is an UV-vis absorption spectra of 1, 2, 4, and 5 in CH₃CN (0.10 mM).

Figure 6 is an UV-vis absorption spectra of VO(acac)₂ (a) and VO(OPr)₃ (b) in CH₃CN (0.10 mM). Figure 7 NMR (300 MHz, CD³CN) spectra of lignin model compounds before and after photocatalytic reactions, (a) Compound 11; (b) 11 after photolysis; (c) 11-¹³C2 after photoreaction; (d) 11-D1 after photoreaction (inset: ¹H-decoupled ²H NMR spectrum of 6-D); and (e) 11-D2 after photoreaction (inset: ¹H-decoupled ²H NMR spectrum after partial conversion. The peak at 4.27 ppm is from unreacted 11-D2). The unlabeled products 6 (rectangle), 9 (equilateral triangle), and formic acid (pentagon pointing up) are indicated at their distinctive chemical shifts. The ¹³C labeled 9-¹³C1 (star) and ¹³C labeled formic acid (diamond) have been split due to the ¹H coupling with ¹³C. The deuterated 6-D (square), 9-D (right-angled triangle), and formic acid (pentagon pointing down) have been detected by ²H NMR spectroscopy.

Figure 8 depicts a homemade batch reactor with circulating flow capabilities.

Figure 9 depicts FT-IR spectra of (a) chloromethyl polystyrene (PSR-CI), and (b) propargyloxymethyl polystyrene (PSR-O-propargyl).

Figure 10 depicts FT-IR spectra of (a) complex 3a grafted onto polystyrene (PSR-VO) - thereby forming complex 3c, and (b) complex 3a.

Figure 11 depicts FT-IR spectra of (a) non-functionalised Ti0₂, (b) Ti0₂ functionalised with propargyl phosphate (compound 47) and (c) complex 3a grafted onto the functionalised Ti0₂, thereby forming complex 48. Figure 12A depicts the diffuse reflectance UV-visible spectrum of complex 3a, while Figure 12B depicts the diffuse reflectance UV-visible spectra of non-functionalised Ti0₂ and complex 48 (b). We have surprisingly discovered more affordable, earth abundant vanadium-based catalysts that can perform very selective C-C bond cleavage reactions using visible light as the source of energy, under mild ambient conditions.

Thus, according to a first aspect of the invention, there is provided a compound of formula I:

wherein:

Ri represents F, CI, Br, I, CF₃, CN, S0₃R₉, SO2NR₁₀Rii, C(0)R₁₂, C(0)OR_13l C(0)NR₁₄Ri₅, C(NR₁₆)NR₁₇R₁₈, N₃, N0₂ and aryl, which latter group is optionally substituted by one or more substituents selected from halo, nitro, CN, N₃, Ci_-4alkyl, C_2-4alkenyl, and C_2-4alkynyl, OR_19a, S(0)_mR_19b, S(0)₂N(R_19c)(R_19d), N(R_19e)S(0)₂R_19f, N(R_19g)(R_19h); each R₂ and R₄ to R₇ independently represent:

(a) H,

(b) halo,

(c) C_1-6 alkyl, C_2-6 alkenyl, C_2-6 alkynyl and aryl, which latter four groups are optionally substituted by one or more substituents selected from halo, nitro, CN, N₃, C_1-4 alkyl, C_2-4 alkenyl, and C_2-4 alkynyl, OR_20a, S(O)_mR_20b, S(0)₂N(R₂oc)(R₂od), N(R₂₀e)S(0)₂R₂of,

(d) S(O)₂R_20i,

(e) S(O)₂N(R_20i)(R_20k),

(9) N₃; or R₆ represents a solid support, optionally wherein the solid support is attached to the rest of the compound of formula I by way of a linking group;

R₃ represents a C_1-6 alkyl group;

Rs represents

(a) H,

(b) Ci_6 alkyl, C_2-6 alkenyl, and C_2-6 alkynyl, aryl, which latter four groups are optionally substituted by one or more substituents selected from halo, nitro, CN, N₃, C_1-4 alkyl, C₂.₄ alkenyl, and C_2-4 alkynyl, OR₂i_a, S(0)_mR_21b, S(0)₂N(R_21c)(R_2ld), N(R_21e)S(0)₂R_21f,

each of R₉ to R₁₈, Rig_a to R_19h, R_20a to R₂₀i and R_21a to R_21h independently represent H or C_1-4 alkyl, which latter group is optionally substituted by one or more substituents selected from halo;

each m independently represents 0 to 2;

or a solvate and/or a dimer thereof.

As mentioned above, also encompassed by formula I are any solvates of the compounds. Preferred solvates are solvates formed by the incorporation into the solid state structure (e.g. crystal structure) of the compounds of the invention of molecules of a non-toxic pharmaceutically acceptable solvent (referred to below as the solvating solvent). Examples of such solvents include water, alcohols (such as methanol, ethanol, isopropanol and butanol), nitriles (such as acetonitrile, propionitrile, and butyronitrile), esters (such as ethyl acetate), ketones (such as acetone and ethyl methyl ketone), and dimethylsulphoxide. Solvates can be prepared by recrystallising the compounds of the invention with a solvent or mixture of solvents containing the solvating solvent. Whether or not a solvate has been formed in any given instance can be determined by subjecting crystals of the compound to analysis using well-known and standard techniques such as thermogravimetric analysis (TGA), differential scanning calorimetry (DSC) and X-ray crystallography. The solvates can be stoichiometric or non-stoichiometric solvates. Particularly preferred solvates are hydrates, and examples of hydrates include hemihydrates, monohydrates and di hydrates.

For a more detailed discussion of solvates and the methods used to make and characterise them, see Bryn et al., Solid-State Chemistry of Drugs, Second Edition, published by SSCI, Inc of West Lafayette, IN, USA, 1999, ISBN 0-967-06710-3. Compounds of formula I, as well as solvates and dimers of such compounds are, for the sake of brevity, hereinafter referred to together as the "compounds of formula I".

Compounds of formula I (and formula II as described hereinbelow) may contain double bonds and may thus exist as E (entgegen) and Z (zusammen) geometric isomers about each individual double bond. All such isomers and mixtures thereof are included within the scope of the invention.

Compounds of formula I (and formula II) may exist as regioisomers and may also exhibit tautomerism. All tautomeric forms and mixtures thereof are included within the scope of the invention.

Unless otherwise stated, the term "alkyl" refers to an acyclic unbranched or branched, or cyclic, hydrocarbyl radical, which may be substituted or unsubstituted (with, for example, one or more halo atoms). Unless otherwise stated, where the term "alkyl" refers to an acyclic group, it is preferably C _-6 alkyl (such as ethyl, propyl (e.g. n-propyl or isopropyl), butyl (e.g. branched or unbranched butyl), pentyl or, more preferably, methyl). Unless otherwise stated, where the term "alkyl" is a cyclic group (which may be where the group "cycloalkyl" is specified), it is preferably C_3-6 cycloalkyl. Unless otherwise stated, the term "alkenyl" refers to an acyclic unbranched or branched, or cyclic, hydrocarbyl radical containing one or more carbon to carbon double bonds, and which radical may be substituted or unsubstituted (with, for example, one or more halo atoms). Unless otherwise stated, where the term "alkenyl" refers to an acyclic group, it is preferably C₂-6 alkenyl (such as ethylenyl, propylenyl (e.g. n-propylenyl or isopropylenyl), butylenyl (e.g. branched or unbranched butylenyl), or pentylenyl). Unless otherwise stated, where the term "alkenyl" is a cyclic group (which may be where the group "cycloalkenyl" is specified), it is preferably C_4-6 cycloalkenyl. Unless otherwise stated, the term "alkynyl" refers to an acyclic unbranched or branched hydrocarbyl radical containing one or more carbon to carbon triple bonds and may also contain one or more carbon to carbon double bonds, and which radical may be substituted or unsubstituted (with, for example, one or more halo atoms). Unless otherwise stated, where the term "alkynyl" is used herein, it is preferably C₂-6 alkynyl (such as ethylynyl, propylynyl, butylynyl, or pentylynyl).

The term "halogen", when used herein, includes fluorine, chlorine, bromine and iodine.

The term "aryl" when used herein includes C₆-i₄ (such as C_6-i3 (e.g. C_{6- 0})) aryl groups, which may be substituted or unsubstituted. Such groups may be monocyclic, bicyclic or tricyclic and have between 6 and 14 ring carbon atoms, in which at least one ring is aromatic. The point of attachment of aryl groups may be via any atom of the ring system. However, when aryl groups are bicyclic or tricyclic, they are linked to the rest of the molecule via an aromatic ring. C_6-14 aryl groups include phenyl, naphthyl and the like, such as 1 ,2,3,4- tetrahydronaphthyl, indanyi, indenyl and fluorenyl. Most preferred aryl groups include phenyl.

When used herein, "solid support" may refer to an organic resin or to an inorganic substrate. Suitable organic resins for use as a solid support may include, but are not limited to, polystyrene resin (e.g. polystyrene/divinylbenzene resin) and poly(ethylene glycol)- polystyrene/divinylbenzene resin. Other suitable organic resins for use as a solid support may be as mentioned in The Combinatorial Index by Barry A. Bunin (author); Academic Press: San Diego, U.S.A., 1998. Suitable inorganic substrates for use as a solid support may include, but are not limited to, titanium dioxide, alumina, silica gel and zeolites.

When used herein, "linking group" refers to a moiety that covalently bonds a compound of formula I to a solid support. Suitable linking groups that may be used herein may be derived by reaction of a reactive moiety at R₆ (e.g. N₃₎ NH₂, or OH, amongst others) on a compound of formula I where R₆ is not a solid support with a suitable functional group on the solid support (e.g. a functionalised organic resin or a functionalised inorganic substrate). Suitable functionalised organic resins include those discussed in Chapter 3 of The Combinatorial Index (ibid), which is incorporated herein by reference. As illustrated in Example 1 D hereinbelow, a particular functionalised organic resin that may be mentioned herein is propargyloxymethyl polystyrene, which may be reacted in a Click reaction with a compound of formula I wherein l¾ is CH₂N₃ to form a 1 ,2,3-triazole that then forms part of the linking group that covalently links the polystyrene resin and the rest of the compound of formula I. Example 1 H hereinbelow illustrates the use of a functionalised inorganic substrate (Ti0₂ attached to a phosphate species with a pendant propargylic moiety) in a similar Click reaction, thereby forming a 1 ,2,3-triazole as part of the linking group that then links the Ti0₂ to the rest of the compound of formula I.

For the avoidance of doubt, in cases in which the identity of two or more substituents in a compound of formula I may be the same, the actual identities of the respective substituents are not in any way interdependent. For example, given that the compound of formula I has more than four R₂ groups, those R₂ groups may be the same or different. Similarly, in the situation in which R₄ and R₅ are both C₂ alkyl groups substituted by one or more C _-4 alkyl groups, the alkyl groups in question may be the same or different.

All individual features (e.g. preferred or particular features) mentioned herein may be taken in isolation or in combination with any other feature (including preferred or particular features) mentioned herein (hence, preferred or particular features may be taken in conjunction with other preferred or particular features, or independently of them).

Embodiments of the invention that may be mentioned include those that relate to compounds of formula I in which:

(a) Ri represents F, CI, Br, CF₃ and N0₂;

(b) each of R₂, R₅ and R₇ independently represent H, F, CI, Br, C_1-4 alkyl (which latter group is optionally substituted by one or more substituents selected from F, CI, Br, and OR_20a) and OR₂₀i (e.g. each of R₂, R₅ and R₇ represent H);

(c) R₄ represents H or ^fBu;

(d) R₃ represents C_1-4 alkyl;

(e) R₆ represents H, CH₂OH, CH₂OCH₃, CH₂N₃, or CH₂-(L)_y-solid support, where L is a linking group and y is 0 or 1 (e.g. R₆ represents H, CH₂OH, CH₂OCH_3l CH₂N₃, or a polymeric fragment of formula la:

SP

where the zig-zag line represents the point of attachment of R₆ to the rest of the compound of formula I and SP represents the solid support (e.g. polystyrene));

(f) R₈ represents H.

Other compounds of formula I that may be mentioned include the compounds of the examples described hereinafter. Thus, embodiments of the invention that may be mentioned include those in which the compound of formula I is a compound selected from the list:

(a)

, or a solvate and/or a dimer thereof.

The compounds of formula I represent novel vanadium catalysts that can be applied as photocatalysts for the selective cleavage of carbon(sp³)-carbon(sp³) single bonds next to alcohol (OH) groups, as shown in Figure 2. We hereinbelow describe the efficient synthesis of a new class of oxo-vanadium catalysts and their reactivity in the degradation of lignin model compounds, as well as other potential substrates. Unlike previous reports that require high temperatures (>60 °C) and expensive metals (e.g. ruthenium or iridium), or lead to complete mineralisation to C0₂, the compounds of formula I described herein function on irradiation with visible light (>420 nm, AM1.5 solar intensity), under mild, ambient temperature and pressure. The compounds of formula I exhibit exceptional selectivity, as evidenced by the transformation of a lignin model compound into the corresponding substituted benzaldehyde and an aryl formate. This reaction can be generalised to convert ethylene glycol into formic acid selectively.

Thus, in a second aspect of the invention, there is provided a use of a compound of formula I as described hereinbefore, as a catalyst for a chemical reaction. The use of the compound of formula I as a catalyst may relate to reaction of a compound of formula II:

wherein:

Ra and Rb independently represent H, alkyl, alkenyl, alkynyl (which latter three groups are optionally substituted with halo, nitro, CN, N₃, OR_22a, N(R₂₂b)(R22c)). aryl or heterocyclic group (which latter two groups are optionally substituted with halo, nitro, CN, N₃, OR_23a>

Rc represents H, alkyl, alkenyl, alkynyl (which latter three groups are optionally substituted with halo, nitro, CN, OR₂4_a, N(R₂4_b)(R24c)), aryl, heteroaryl (which latter two groups are optionally substituted with halo, nitro, CN, N₃, OR₂₅a. N(R₂5_bXR₂5_C)), O-alkyl, O-alkenyl, O- alkynyl (which latter three groups are optionally substituted with halo, nitro, CN, N₃, OR_26a, N(R₂6b)(R₂6c)), O-aryl or O-heterocyclic (which latter two groups are optionally substituted with halo, nitro, CN, OR_27a, N(R_27b)(R_27c)); each of R_22a - R_22c, R_24a - R_24c, and R_26a - R₂6_C independently represents H, halo or C_1-4 alkyl, which latter group is optionally substituted by one or more substituents selected from halo or OH; each of R_23a - R_23c, R_25a - R₂5_C, and R_27a - R_27c independently represents halo or C_1-4 alkyl, which latter group is optionally substituted by one or more substituents selected from halo. The term "heteroaryl" when used herein refers to an aromatic group containing one or more heteroatom(s) (e.g. one to four heteroatoms) preferably selected from N, O and S (so forming, for example, a mono-, bi-, or tricyclic heteroaromatic group). Heteroaryl groups include those which have between 5 and 14 (e.g. 10) members and may be monocyclic, bicyclic or tricyclic, provided that at least one of the rings is aromatic. However, when heteroaryl groups are bicyclic or tricyclic, they are linked to the rest of the molecule via an aromatic ring. Heteroaryl groups that may be mentioned include benzothiadiazolyl (including 2,1 ,3-benzothiadiazolyl), isothiochromanyl and, more preferably, acridinyl, benzimidazolyl, benzodioxanyl, benzodioxepinyl, benzodioxolyl (including 1 ,3-benzodioxolyl), benzofuranyl, benzofurazanyl, benzothiazolyl, benzoxadiazolyl (including 2,1 ,3-benzoxadiazolyl), benzoxazinyl (including 3,4-dihydro-2 - -1 ,4-benzoxazinyl), benzoxazolyl, benzomorpholinyl, benzoselenadiazolyl (including 2,1 ,3-benzoselenadiazolyl), benzothienyl, carbazolyl, chromanyl, cinnolinyl, furanyl, imidazolyl, imidazo[1 ,2-a]pyridyl, indazolyl, indolinyl, indolyl, isobenzofuranyl, isochromanyl, isoindolinyl, isoindolyl, isoquinolinyl, isothiaziolyl, isoxazolyl, naphthyridinyl (including 1 ,6-naphthyridinyl or, preferably, 1 ,5-naphthyridinyl and 1 ,8- naphthyridinyl), oxadiazolyl (including 1 ,2,3-oxadiazolyl, 1 ,2,4-oxadiazolyl and 1 ,3,4- oxadiazolyl), oxazolyl, phenazinyl, phenothiazinyl, phthalazinyl, pteridinyl, purinyl, pyranyl, pyrazinyl, pyrazolyl, pyridazinyl, pyridyl, pyrimidinyl, pyrrolyl, quinazolinyl, quinolinyl, quinolizinyl, quinoxalinyl, tetrahydroisoquinolinyl (including 1 ,2,3,4-tetrahydroisoquinolinyl and 5,6,7,8-tetrahydroisoquinolinyl), tetrahydroquinolinyl (including 1 ,2,3,4- tetrahydroquinolinyl and 5,6,7,8-tetrahydroquinolinyl), tetrazolyl, thiadiazolyl (including 1 ,2,3- thiadiazolyl, 1 ,2,4-thiadiazolyl and 1 ,3,4-thiadiazolyl), thiazolyl, thiochromanyl, thiophenetyl, thienyl, triazolyl (including 1 ,2,3-triazolyl, 1 ,2,4-triazolyl and 1 ,3,4-triazolyl) and the like. Substituents on heteroaryl groups may, where appropriate, be located on any atom in the ring system including a heteroatom. The point of attachment of heteroaryl groups may be via any atom in the ring system including (where appropriate) a heteroatom (such as a nitrogen atom), or an atom on any fused carbocyclic ring that may be present as part of the ring system. Heteroaryl groups may also be in the N- or S- oxidised form. Particularly preferred heteroaryl groups include pyridyl, pyrrolyl, quinolinyl, furanyl, thienyl, oxadiazolyl, thiadiazolyl, thiazolyl, oxazolyl, pyrazolyl, triazolyl, tetrazolyl, isoxazolyl, isothiazolyl, imidazolyl, pyrimidinyl, indolyl, pyrazinyl, indazolyl, pyrimidinyl, thiophenetyl, thiophenyl, pyranyl, carbazolyl, acridinyl, quinolinyl, benzoimidazolyl, benzthiazolyl, purinyl, cinnolinyl and pterdinyl. Particularly preferred heteroaryl groups include monocylic heteroaryl groups.

The term "heterocyclic" when used herein refers to an aliphatic 4- to 14-membered heterocyclic group containing one or more heteroatoms selected from O, S and N, which heterocyclic groups may comprise one, two or three rings and may be substituted or unsubstituted. Heterocyclic groups that may be mentioned include isoxazolidinyl, maleimido, 1 ,2- or 1 ,3-oxazinanyl, oxazolyl, pyrrolidinonyl, pyrrolidinyl, pyrrolinyl, sulfolanyl, tetrahydrofuranyl, thiazolidinyl, dioxanyl, hexahydropyrimidinyl, morpholinyl, piperazinyl, piperidinyi, pyranyl, tetrahydropyranyl, 3,4,5, 6-tetrahydropyridinyl, 1 ,2,3,4- tetrahydropyrimidinyl, 3,4,5,6- tetrahydropyrimidinyl and the like.

Particular embodiments of the second and third aspects of the invention that may be mentioned herein include those that relate to compounds of formula (II) in which:

(i) Ra and Rb independently represent H, Ci_₆ alkyl (which latter group is optionally substituted with halo or OR_22a), or aryl (which latter group is optionally substituted with nitro, OR_23a); and

Rc represents H, alkyl (which latter group is optionally substituted with halo or OR₂4_a), aryl (which latter group is optionally substituted with halo, nitro, OR₂5_a), O-alkyl (which latter group is optionally substituted with halo, OR_26a)), O-aryl (which latter group is optionally substituted with halo, nitro, OR_27a); and

each of R_22a, R_24a and R_27a independently represents H, halo or Ci_-4 alkyl, which latter group is optionally substituted by one or more substituents selected from halo or OH; or

(ii) the compound of formula (II) is lignin or a lignin-like polymeric material.

When used herein "lignin-like polymeric material" refers to a polymer (e.g. a synthetic or natural) that comprises the structural motif displayed in the compound of formula II.

Yet further embodiments of the third aspect of the invention that may be mentioned herein include those in which:

(a) the solvent comprises acetonitrile and/or ethyl acetate;

(b) the reaction is conducted at a temperature of from 0°C to 100°C, such as from 20°C to 35°C;

(c) the reaction is conducted in the presence of a gas that comprises oxygen (e.g. oxygen or air).

The merits of the developed process are the affordable and easy synthetic access to the compounds of formula I. The operation of the compounds of formula I as a catalyst in a process is convenient since ambient conditions are used under irradiation. Although we highlight the use of visible light, sunlight, including the ultraviolet components can be used, with retention of the high selectivity towards C-C bond cleavage. No sacrificial reagents are required except air/ molecular oxygen and visible light. Due to the high selectivity of the C-C cleavage reaction, separation and purification of the products is convenient since the product mixture is not as complex compared to thermally activated reactions.

In summary, there is disclosed herein a photocatalyst compound of formula I that contains an earth abundant V^v oxo complex supported by a redox non-innocent salicylaldimine- derived ligand. A catalytic amount of a compound of formula I can effect the chemoselective, aliphatic C-C bond cleavage of representative lignin model compounds under ambient conditions with visible light (>420 nm) irradiation at moderate to high yields. As discussed hereinbelow, the product analysis is supported by isotope labeling studies, which confirm that the lignin model compounds cleave regioselectively into two primary degradation products. The resulting aryl aldehyde and aryl formates are important building blocks in organic synthesis, since they possess highly reactive and versatile aldehyde groups. Thus, the current invention provides a unique and eco-friendly approach to harvest solar energy to perform C-C activation reactions, which can potentially be employed in late stage transformations of complex organic molecules or disassembly and valorization of (bio)macromolecules in the future. In addition, it is noted that a compound of formula I can work with a wide variety of substrate compounds by cleavage of a C-C bond, where one of the carbon atoms involved in the bond cleavage reaction has a pendant hydroxyl group. Non-limiting examples that embody certain aspects of the invention will now be described.

Examples

General

The vanadium complexes used in this study were synthesized under dry argon (Ar) using standard Schlenk techniques. Deuterated solvents were purchased from Cambridge Isotope Laboratories and were used as received. All other chemicals were obtained from Sigma- Aldrich and were used as received. The ¹H and ¹³C NMR spectra were recorded at room temperature on a Bruker AVANCE 300 MHz spectrometer, whereas the ⁵¹V NMR spectra were obtained at room temperature on a JEOL 400 MHz spectrometer. The ¹H and ¹³C chemical shifts (δ reported in ppm) are referenced to the residual solvent signal(s), and the ⁵¹V NMR chemical shift is referenced externally to V^v(0)(CI)₃ (0 ppm). EXAMPLE 1 : Synthesis of vanadium-oxo complexes

A: Vanadium oxo complex (2)

Compound 4-fluoro-/V-(2-hydroxybenzylidene)benzohydrazide (1). Compound 4- fluorobenzohydrazide (A. Husain, A. Ahmad, M. M. Alam, M. Ajmal, and P. Ahuja, Eur. J. Med. Chem., 2009, 44, 3798-3804) (1.54 g, 10 mmol) was dissolved in absolute ethanol (15 mL) and heated to reflux. Salicylaldehyde (1.22 g, 10 mmol) in absolute ethanol (5 mL) was slowly added dropwise into the reaction mixture. After complete addition, the reaction mixture was heated at reflux overnight. Subsequently, the reaction mixture was cooled to room temperature during which a white precipitate formed. The white solid was filtered off, washed with cold ethanol three times, and dried under vacuum to provide the corresponding hydrazone derivative 1 in high yield (2.32 g, 90%). H N R (DMSO-of₆, 300 MHz): δ = 6.89 - 6.95 (m, 2 H), 7.30 (t, J = 6.0 Hz, 1 H), 7.38 (t, J = 9.0 Hz, 2 H), 7.55 (d, J = 6 Hz, 1 H), 8.00 - 8.05 (m, 2 H), 8.64 (s, 1 H). ¹³C{¹H} NMR (DMSO-c/₆, 100 MHz): δ = 115.6 (d, J = 21.8 Hz), 116.4, 118.7, 119.4, 129.3 (d, J = 2.9 Hz), 129.5, 130.4 (d, J = 9.1 Hz), 131.4, 148.3, 157.5, 161.8, 164.3.

Vanadium oxo complex (2) The vanadium (V) precursor, VO(OPr)₃ (227 pL, 1.0 mmol), was added dropwise to a stirred solution of 1 (0.258 g, 1.0 mmol) in methanol (MeOH, 20 mL), during which the reaction mixture turned dark red. The reaction mixture was heated at reflux for 30 min, followed by hot filtration. The filtrate was then cooled to room temperature and kept aside for crystallization to obtain 2 by slow evaporation (0.248 g, 70%). ¹H NMR (MeOD-c/₄, 300 MHz): <5 = 6.94 - 7.04 (m, 2 H), 7.20 (t, J = 9.0 Hz, 2 H), 7.50 (t, J = 7.2 Hz, 1 H), 7.61 (d, J = 7.8 Hz, 1 H), 8.12 - 8.17 (m, 2 H), 8.75 (s, 1 H). ¹⁹F{¹H} NMR (MeOD-d₄, 300 MHz): δ = - 1 10.6. ⁵¹V NMR (MeOD-d₄, 400 MHz): δ = -548.7. HR-ESI-MS calcd. for C₁₅H₁₂FN0₄V [M]⁺ m/z = 354.0221 , found 354.0205. UV-vis [acetonitrile, MeCN, A_max (ε M^"1 cm^'1)]: 270 (9264), 315 (6935), 395 (3308) nm. Elemental analyses for C₁₅H₁₂FN₂0₄V calculated: C, 50.86; H, 3.41 ; N, 7.91 %; found: C, 50.44; H, 3.35; N, 7.90%. Melting point: 164 - 166 °C.

Complex 2 is a new compound synthesized from a hydrazone benzohydroxamate ligand 1 , which is redox non-innocent. The ligand 1 was synthesized in two steps from commercial reagents. In the presence of stoichiometric equivalents of V(V) oxytripropoxide and 1, complex 2 was prepared in high yields.

B: Vanadium oxo complex (3a)

The hydrazone ligand above was synthesized by adapting the procedure as described for the synthesis of hydrazone ligand 1 (A. Husain, A. Ahmad, M. M. Alam, M. Ajmal, and P. Ahuja, Eur. J. Med. Chem., 2009, 44, 3798-3804) (yield 90%). The quantities of reagents used were as follows: 4-fluorobenzohydrazide (1.54 g, 10 mmol), 5- azidomethylsalicylaldehyde (1.77 g, 10 mmol), and ethanol (20 mL) as solvent. ¹H NMR (DMSO-d₆, 300 MHz): δ = 4.39 (s, 2 H), 6.97 (d, J = 8.1 MHz, 2 H), 7.29-7.32 (m, 1 H), 7.38 (t, J = 8.7 Hz, 2 H), 7.62 (s, 1 H), 8.0-8.05 (m, 2 H), 8.66 (s, 1 H), 1 1.25 (s, 1 H), 12.12 (s, 1 H) ppm. ¹³C{¹H} NMR (DMSC-d₆, 100 MHz): δ = 53. 1 , 115.4, 115.6, 116.7, 118.9, 126.4, 129.1 , 129.3, 130.3, 130.4, 131.8, 147.3, 157.2, 161.8, 164.3 ppm. ¹⁹F{¹H} NMR (DMSO-d₆, 282.40 MHz): δ = -108.02 ppm.

Vanadium oxo complex 3a

Complex 3a was synthesized by following the procedure as described for the synthesis of complex 2 (yield 70%). The quantities of reagents used are as follows: VO(OPr)₃ (227 μΙ_, 1.0 mmol), corresponding hydrazone ligand as described above (0.313 g, 1.0 mmol), and solvent methanol (20ml_). ¹H NMR (MeOD-c^, 300 MHz): δ = 4.36 (s, 2 H), 6.95 (d, J = 6.6 Hz, 1 H), 7.18 (t, J = 6.6 Hz, 2 H), 7.49 (d, J = 5.5 Hz, 1 H), 7.59 (s, 1 H), 8.1 1 - 8.15 (m, 2 H), 8.74 (s, 1 H) ppm. ¹⁹F{¹H} NMR (MeOD-tf 282.40 MHz): δ = -80.08 ppm. ⁵¹V NMR (MeOD-cf₄, 105.15 MHz): δ = -548.0 ppm. HRMS (ESI+, m/z) calculated for C₁₆H₁₃FN₅0₄V [Mf = 409.0381 , found 409.0391.

C: Vanadium oxo complex (3b)

A/-[(E)-(3-tert-butyl-2-hydroxy-phenyl)methyleneamino]-4-fluoro-benzamide was synthesized by adapting the procedure as described for the synthesis of hydrazone ligand 1 (A. Husain, A. Ahmad, M. M. Alam, M. Ajmal, and P. Ahuja, Eur. J. Med. Chem., 2009, 44, 3798-3804.) (yield 90%). The quantities of reagents used were as follows: 4-fluorobenzohydrazide (1.54 g, 10 mmol), 3-tert-butylsalicylaldehyde (1.78 g, 10 mmol), and solvent ethanol (20 mL). ¹H NMR (DMSO-de, 300 MHz): δ = 1.41 (s, 9 H), 6.88 (t, J = 7.8 MHz, 1 H), 7.27-7.31 (m, 2 H), 7.40 (t, J = 7.8 MHz, 1 H), 8.01-8.05 (m 2 H), 8.57 (s, 1 H), 12.24 (s, 1 H), 12.44 (s, 1 H) ppm. ¹³C{¹H} NMR (DMSO-cf₆, 100 MHz): δ = 29.2, 34.5, 115.5, 115.7, 117.6, 118.7, 128.5, 129.0, 129.5, 130.3, 130.4, 136.4, 150.9, 156.9, 161.7, 164.3 ppm. ⁹F{ H} NMR (DMSO-d₆, 282.40 MHz): δ = -107.8 ppm.

Vanadium oxo complex (3b)

Complex 3b was synthesized by following the procedure as described for the synthesis of complex 2 (dark brown solid, yield 75%). The quantities of reagents used are as follows: VO(OPr)₃ (227 μΙ_, 1.0 mmol), corresponding hydrazone ligand (0.314 g, 1.0 mmol), and solvent methanol (20ml_). ¹H NMR (MeOD-c/₄, 300 MHz): δ = 1.49 (s, 9 H), 6.97 (t, J = 7.8 Hz, 1 H), 7.19 (t, J = 8.7 Hz, 2 H), 7.49 (d, J = 7.8 Hz, 1 H), 7.57 (s, J = 7.8 Hz, 1 H), 8.13 - 8.18 (m, 2 H), 8.75 (s, 1 H) ppm. ¹⁹F{¹H} NMR (MeOD-d₄, 376 MHz): δ = -110.8 ppm. ⁵¹V NMR (MeOD-d₄, 105.15 MHz): δ = -552.4 ppm. HRMS (ESI+, m/z) calculated for Ci₆Hi₃FN₅0₄V [M]⁺ = 410.0828, found 410.0847. D: Grafting of Complex 3a onto a polystyrene Resin to provide complex 3c (PSR-VO) Synthesis of Propargyloxymethyl polystyrene Resin

To a stirred solution of propargyl alcohol (0.672 g, 12.0 mmol) in dry THF (50.0 mL) was added NaH (60% dispersed in mineral oil) (0.600 g, 15.0 mmol) slowly at 0 °C. After complete addition, the reaction mixture was stirred for 1 h. To this reaction mixture, chloromethyl polystyrene (1.20 mmol/g chloro) (1.0 g) and Nal (0.225 g, 1.50 mmol) were added successively. The reaction mixture was then heated to reflux for 24 h. After cooling to room temperature, 20 mL of deionized water was added and the reaction mixture was stirred for 30 min. The modified resin was collected by filtration. It was then washed with 100 mL of water, followed by 25 mL of THF, methanol, and finally with acetone. The yellowish brown solid (propargyloxymethyl polystyrene, PSR-O-propargyl) thus obtained was dried under vacuum overnight, and was characterized by FT-IR spectroscopy.

Successful synthesis of propargyloxymethyl polystyrene (PSR-O-propargyl) was confirmed by the FT-IR spectrum (Figure 9b) which contains the peaks of the alkyne C-H stretch (3290 cm^"1) and carbon-carbon triple bond stretch (2114 cm^"1). The disappearance of the C-CI band (1263 cm^"1) in the IR spectrum (Figure 9b) of the product further confirms the replacement of the chloro by a propargyloxy group. Grafting of complex 3a via Click reaction

Oxovanadium(V) complex 3a (0.123 g, 0.30 mmol) was dissolved in 1 :1 mixture of anhydrous DMF:THF (20 mL). To this solution was added propargyloxymethyl polystyrene (0.20 g) followed by /V-diisopropylethylamine (0.30 mL, 1.80 mmol, 6 equiv.) and copper(l) iodide (0.028 g, 0.15 mmol, 0.5 equiv.). The resulting mixture was vigorously stirred at ambient temperature under atmospheric argon conditions for 3 days. The heterogeneous reaction mixture was then filtered and the dark brown solid was sequentially washed with DMF (50 mL), THF (50 mL), MeOH (50 mL), and finally with acetone (50 mL). The solid was then kept under vacuum for 5 h and was characterized by FT-IR spectroscopy.

The FT-IR spectrum of the complex 3c (i.e. complex 3a grafted on polystyrene) shows that the peaks of alkyne C-H stretch (3290 cm^"1) and carbon-carbon triple bond stretch (21 14 cm^"1) are no longer present. Instead, the spectrum of the composite shows new peaks at 1620 cm^"1 (C=N stretch), 960 cm^"1 (C-N stretch), as well as 1001 cm^"1 (V=0 stretch), which arise from the complex grafted via click reaction.

E: Vanadium oxo complex (5)

4 S

Compound 2-(feri-butyl)-6-(((3-hydroxypropyl)imino)methyl)phenol (4). Compound 3- (terf-butyl)-2-hydroxybenzaldehyde (0.891 g, 5.0 mmol) and Na₂S0₄ (3.40 g, 24 mmol) were added to a stirred solution of 3-amino-1 -propanol (382 μί, 5.0 mmol) in MeOH (20 mL) and the reaction mixture was heated to reflux overnight under nitrogen. The reaction mixture was cooled to room temperature, filtered, and concentrated to give 4 as a yellow oil in quantitative yield (1.17 g, 100 %). ¹H NMR (CDCI₃, 300 MHz): δ = 1.44 (s, 9 H), 1.94 - 2.03 (m, 2 H), 3.71 (t, J = 6 Hz, 2 H), 3.79 (t, J = 6 Hz, 2 H), 6.81 (t, J = 7.5 Hz, 1 H), 7.11 (dd, J = 7.5, 1.5 Hz, 1 H), 7.32 (dd, J = 7.5, 1.5 Hz, 1 H), 8.38 (s, 1 H), 13.99 (bs, 1 H) ppm. ¹³C{¹H} NMR (CDCI₃, 75 MHz): δ = 29.5, 33.7, 35.0, 55.9, 60.4, 1 17.9, 118.8, 129.5, 129.8, 137.6, 160.7, 166.2 ppm.

Vanadium oxo complex (5)

The vanadium oxo complex 5 was prepared for control experiments as a mimic of the vanadium catalyst reported by Toste et al. The complex was synthesized with 4 and VO(OPr)₃ as the vanadium (V) precursor by adopting the procedure described earlier by Toste. ¹H NMR (MeOD-c/₄, 300 MHz): δ = 1.46 (s, 9 H), 2.05 - 1.88 (m, 2 H), 2.29 (d, J = 13.8, 1 H), 3.35 (s, 3 H), 4.04 (d, J = 10.8 Hz, 1 H), 4.24 (t, J = 12.3, 1 H), 5.55 (t, J = 9.6 Hz, 1 H), 6.85 (t, J = 7.5 Hz, 1 H), 7.32 (dd, J = 7.5, 1.5 Hz, 1 H), 7.49 (dd, J = 7.5, 1.5 Hz, 1 H), 8.51 (bs, 1 H). ¹ NMR (MeOD-d₄, 100 MHz): δ = -551 .1. UV-vis [acetonitrile, MeCN, A_max (ε M^'1 cm^"1)]: 220 (23142), 258 (14480), 336 (5346) nm. F: Vanadium oxo complex (41)

Compound (E)-4-fluoro-/V'-(2-hydroxy-5-(hydroxymethyl)benzylidene) benzohydrazide (40). To a pre-heated solution of 4-fluorobenzohydrazide (A. Husain, A. Ahmad, M. M. Alam, M. Ajmal, and P. Ahuja, Eur. J. Med. Chem., 2009, 44, 3798-3804) (0.33 g, 2.1 mmol) in absolute ethanol (10 ml_), 2-hydroxy-5-(hydroxymethyl)benzaldehyde (0.32 g, 2.1 mmol) was added dropwise following its dissolution in approximately 2 mL of absolute ethanol. After complete addition, the reaction mixture was vigorously stirred under reflux conditions for 12 h. During that period, a pale-brown solid material started to precipitate out of the mixture. Upon the completion of the reaction, the excess solvent was removed under vacuum and the remaining solid was washed with n-hexane. The solid was subsequently re-dissolved in ethanol and purified by precipitation with n-hexane. Finally, the hydrazide product 40 was collected by filtration and dried in vacuum (0.57 g, 95%). H NMR (DMSO-d₆, 500 MHz): δ = 4.43 (d, J = 5.4 Hz, 2 H), 5.1 1 (t, J = 5.6 Hz, 1 H), 6.89 (d, J = 8.3 Hz, 1 H), 7.24 (d, J = 8.4 Hz, 1 H), 7.38 (t, J = 8.7 Hz, 2 H), 7. 51 (s, 1 H), 8.00-8.03 (m, 2 H), 1 1.06 (s, 1 H), 12.08 (s, 1 H) ppm. ¹³C{¹H} NMR (DMSO-d₆, 100 MHz): δ = 62.4, 115.4, 115.7, 1 16.1 , 118.3, 127.4, 129.4, 130.1 , 130.3, 133.4, 148.1 , 156.3, 161.8, 163.0, 165.5 ppm. ¹⁹F{¹H} NMR (DMSO-d₆, 376.50 MHz): δ = -107.6 ppm. HRMS (ESI+, m/z) calculated for C₁₅H₁₄N₂0₃F [M+Hf = 289.0988, found 289.0986.

Vanadium oxo complex (41). Under a dry nitrogen atmosphere, the vanadium(V) precursor, VO(OPr)₃ (114 μΐ_, 0.52 mmol) was added dropwise to a pre-heated methanol solution (20 mL) of 40 (0.15 g, 0.52 mmol), which resulted in an immediate colour change of the reaction mixture from pale yellow to dark red. The mixture was vigorously stirred at 40° C for 30 min. Finally, the mixture was filtered and the excess solvent was removed in vacuum yielding 41 as a dark brown solid (0.38 g, 95%). ¹H NMR (MeOD-d₄, 400 MHz): δ = 4.61 (s, 2 H), 6.94 (d, J = 8.4 Hz, 1 H), 7.20 (t, J = 8.8 Hz, 2 H), 7.51-7.59 (m, 2 H), 8.13-8.17 (m, 2 H), 8.75 (s, 1 H) ppm. ¹⁹F{¹H} NMR (MeOD-cT 376.50 MHz): 6 = -1 10.7 ppm. ⁵¹V NMR (MeOD-d₄, 105.25 MHz): δ = -546.6 ppm. HRMS (ESI+, m/z) calculated for C₁₇H₁₈N₂0₆FV [M+MeOH+H]⁺ = 416.0589, found 416.0573.

G: Vanadium oxo complex (43)

Compound (E)-4-fluoro-/V-(2-hydroxy-5-(methoxymethyl)benzylidene)benzohydrazide (42). To a pre-heated solution of 4-fluorobenzohydrazide (A. Husain, A. Ahmad, M. M. Alam, M. Ajmal, and P. Ahuja, Eur. J. Med. Chem., 2009, 44, 3798-3804) (0.77 g, 5.0 mmol) in absolute ethanol (10 mL), 2-hydroxy-5-(methoxymethyl)benzaldehyde (0.83 g, 5.0 mmol) was added dropwise. After complete addition, the reaction mixture was vigorously stirred under reflux conditions for 12 h. During this period, the initial suspension turned into a dark solution. Upon completion of the reaction, excess solvent was removed under vacuum and the remaining solid was washed with n-hexane. The solid product 42 was subsequently washed with cold ethanol and dried in vacuum for several hours. (0.75 g, 50%). ¹H NMR (DMSO-d₆, 500 MHz): δ = 3.27 (s, 3 H), 4.34 (s, 2 H), 6.91 (d, J = 8.3 Hz, 1 H), 7.24 (d, J = 6.6 Hz, 1 H), 7.38 (t, J = 8.8 Hz, 2 H), 7.54 (s, 1 H), 8.00-8.03 (m, 2 H), 8.64 (s, 1 H), 11.14 (s, 1 H), 12. 10 (s, 1 H) ppm. ¹³C{¹H} NMR (DMSO- ₆, 100 MHz): δ = 57.2, 73.1 , 115.4, 115.6, 116.3, 118.5, 128.5, 129.1 , 129.4, 130.4, 131.2, 147.7, 156.8, 161.8, 163.0, 165.5 ppm. ¹⁹F{¹H} NMR (DMSO-c _6l 376.50 MHz): δ = -108.4 ppm. HRMS (ESI+, m/z) calculated for C₁₆H₁₆N₂0₃F [M+H]⁺ = 303.1 145, found 303.1143. Vanadium oxo complex (43). Under aerobic conditions, to a pre-heated stirring suspension of 42 (0.30 g, 1 .0 mmol) in methanol (10 ml.) was added vanadyl acetylacetonate (VO(acac)₂) (0.27 g, 1.0 mmol) which led to an immediate change of colour of the suspension from dark yellow to dark red. The suspension quickly turned into a solution and it was stirred under reflux for 2 h. Excess solvent was subsequently removed in vacuum and the solid product was repeatedly washed with n-hexane finally yielding 43 as a brown solid (0.38 g, 93 %). ¹H NMR (MeOD- ,, 400 MHz): δ = 3.39 (s, 3 H), 4.45 (s, 2 H), 6.94 (d, J = 8.4 Hz, 1 H), 7.20 (t, J = 8.6 Hz, 2 H), 7.51 (d, J = 8.4 Hz, 1 H), 7.59 (s, 1 H), 8.13-8.17 (m, 2 H), 8.75 (s, 1 H) ppm. ¹⁹F{¹H} NMR (MeOD-d₄, 376.50 MHz): δ = -1 10.6 ppm. ⁵ V NMR (MeOD-d₄, 105.25 MHz): δ = -546.3 ppm. HRMS (ESI+, m/z) calculated for C₁₇H₁₇N₂0₅FV [M+Hf = 399.0561 , found 399.0554.

H: Grafting of Complex 3a onto TiO? (P25) to provide composite 48

Synthesis of trimethylsilyl prop-2-ynyl hydrogenphosphate (46) for alkyne functional ization of Ti0₂ surface:

Trimethylsilyl prop-2-ynyl hydrogenphosphate (46) was synthesized di(cyclohexylammonium) prop-2-ynyl phosphate (45), which was in turn prepared by adopting a previous published procedure (Hall, A. D.; Williams, A. Biochemistry, 1986, 25, 4784-4790). The synthetic procedure of compound 46 is as follows. Compound 45 (0.334 g, 1.0 mmol) was dispersed in dry acetonitrile (5.0 ml_) under a N₂ atmosphere. To this reaction mixture, trimethylsilyl trifluoromethanesulfonate (0.340 mL, 2.0 mmol) was injected and the reaction mixture stirred. The reaction mixture slowly turned clear, and the solution was stirred for 1 h. The solvent was removed and the residue was dried under vacuum. Toluene (20 mL) was added to the residue and the mixture was sonicated. The precipitate was filtered off and the filtrate was concentrated under reduced pressure to distill off the toluene. A viscous oily residue was obtained and it was further dried under high vacuum for 30 min and then characterized by ¹H NMR spectroscopy. The spectroscopic data confirmed the formation of compound 46 (0.190 g, yield 91 %). ^ΊΗ NMR (CDCI₃, 300 MHz): δ = 0.35 (s, 9 H), 2.62 (s, 1 H), 4.69 (d, J = 9.3 MHz, 2 H), 11.85 (bs, 1 H) ppm.

Alkyne functionalization of Ti0₂ surface

Alkyne functionalization of the surface of Ti0₂ was carried out in the presence of 46. In a typical procedure, P25 Ti0₂ (0.20g, 21 nm particle size) was added to a 50 mL Schlenk flask and dried under vacuum at 120 °C for 6 h. After the sample was cooled to room temperature, the flask was refilled with argon and anhydrous dichloromethane (DCM) (5 mL) was injected, followed by 46 (0.062 g, 0.30 mmol) dissolved in anhydrous DCM (5 mL). The reaction mixture was stirred for 24 h. The residue was recovered by centrifugation, washed five times with DCM, and dried under vacuum for 1 h. The solid thus obtained is the alkyne functionalized Ti0₂ (47). It was characterized by FT-IR spectroscopy (curve 'b', Figure 11). The presence of characteristic alkyne C-H stretch at 3298 cm^"1 in the IR spectrum of 47 confirms the alkyne functionalization of Ti0₂.

rafting of complex 3a via Click reaction

After successful synthesis of alkyne functionalized 47, the oxovanadium complex 3a was grafted on 47 by the Huisgen cycloaddition click reaction. In a typical procedure, 3a (0.041 g, 0.10 mmol) was dissolved in a 1 :1 mixture of anhydrous DMF:THF (10 ml_). To this solution was added alkyne functionalized Ti0₂ (0.10 g) followed by A/,A/-diisopropylethylamine (0.10 ml_, 0.60 mmol) and copper(l) iodide (0.009 g, 0.05 mmol). The resulting mixture was vigorously stirred at ambient temperature under Ar for 3 days. The heterogeneous reaction mixture was then centrifuged and the yellow solid was sequentially washed with DMF (10 ml_), THF (10 ml_), MeOH (10 mL), and finally with acetone (10 ml_). The solid was then dried under vacuum for 5 h and was characterized by FT-IR spectroscopy.

The FT-IR spectrum (curve 'c', Figure 1 1 ) of the complex 3a grafted on Ti0₂ (compound 48) shows that the peak of the alkyne C-H stretch (3298 cm^"1) is no longer present when compared to the FT-IR spectrum of compound 47 (curve 'b', Figure 11 ). Instead, the spectrum of the composite shows a new peak at 1618 cm^"1 (C=N stretch), which arises from the complex grafted via the click reaction. The diffuse reflectance UV-visible spectrum of composite 48 (curve 'b', Figure 12B) shows the presence of the absorption band above 400 nm, which is due to the ligand-to-metal charge transfer (LMCT) band of the complex 3a (Figure 12A). It further confirms the successful covalent attachment of 3a on Ti0₂. Single crystal X-ray structure of compounds

Vanadium oxo complex 2 A red block of 2 (C₁₆H₁₆FN₂0₅V), with approximate dimensions 0.120 * 0.180 ^χ 0.360 mm, was used for the single crystal X-ray crystallographic analysis. The X-ray intensity data were measured. The total exposure time was 0.39 h. The frames were integrated with the Bruker SAINT software package using a narrow-frame algorithm. The integration of the data using a monoclinic unit cell yielded a total of 21857 reflections to a maximum Θ angle of 31.09° (0.69 A resolution), of which 5189 were independent (average redundancy = 4.212, completeness = 99.8%, Rin, = 3.70%, R_sig = 3.22%) and 4378 (84.37%) were greater than 2a(F²). The final cell constants of a = 7.9908(3) A, b = 17.1771 (6) A, c = 11.9692(5) A, β = 100.2079(18)°, volume = 1616.87(11 ) A³, are based upon the refinement of the XYZcentroids of 5914 reflections above 20 σ(Ι) with 5.697° < 2Θ < 62.13°. Data were corrected for absorption effects using the multi-scan method (SADABS). The ratio of minimum to maximum apparent transmission was 0.881. The calculated minimum and maximum transmission coefficients (based on crystal size) are 0.7980 and 0.9260. The final anisotropic full-matrix least-squares refinement on F² with 232 variables converged at R1 = 3.32%, for the observed data and wR2 = 8.72% for all data. The goodness-of-fit was 1.039. The largest peak in the final difference electron density synthesis was 0.476 eVA³ and the largest hole was -0.374e7A³ with an RMS deviation of 0.068 eVA³. On the basis of the final model, the calculated density was 1.587 g/cm³ and the F(000) was 792 e^".

Selected crystal data and structure refinement for complex 2 is provided in Table 1 , while selected bond lengths (A) for complex 2 are provided in Table 2.

Chemical formula C_l6H₁₆FN₂0₅V

Formula weight 386.25

Temperature 103(2) K

Wavelength 0.71073 A

Crystal size 0.120 x 0.180 x 0.360 mm

Crystal habit red block

Crystal system monoclinic

Space group P2_1/c

Unit cell dimensions a = 7.9908(3) A a = 90°

b = 17.1771(6) A β = 100.2079(18)° c = 1 1.9692(5) A Y = 90°

V1-01 1.5893(11 ) V1-05 1.7630(10)

V1-03 1.8552(10) V1-02 1.9584(9)

V1-N1 2.1307(11 ) V1 -04 2.3443(10)

C1-02 1.3027(15) C1-N2 1.3131(16)

C1-C2 1.4768(17) C10-O3 1.3330(16)

C8-N1 1.2939(16) C15-05 1.4164(16)

C16-04 1.4250(17) N1-N2 1.3967(14)

Table 2

Vanadium oxo complex 5

A red block of C₁₅H₂2N0₄V, with approximate dimensions 0.200 ^χ 0.220 * 0.420 mm, was used for the single crystal X-ray crystallographic analysis. The X-ray intensity data were measured. The total exposure time was 0.33 h. The frames were integrated with the Bruker SAINT software package using a narrow-frame algorithm. The integration of the data using an orthorhombic unit cell yielded a total of 12950 reflections to a maximum Θ angle of 31.07° (0.69 A resolution), of which 4783 were independent (average redundancy = 2.708, completeness = 99.8%, R_int = 2.65%, R_sig = 3.77%) and 4492 (93.92%) were greater than 2o(F²). The final cell constants of a = 14.7897(4) A, b = 13.1867(3) A, c = 7.8841(2) A, volume = 1537.62(7) A³, are based upon the refinement of the XYZ-centroids of 5799 reflections above 20 σ(Ι) with 5.508° < 2Θ < 62.11 °. Data were corrected for absorption effects using the multi-scan method (SADABS). The ratio of minimum to maximum apparent transmission was 0.819. The calculated minimum and maximum transmission coefficients (based on crystal size) are 0.7690 and 0.8790. The final anisotropic full-matrix least-squares refinement on F² with 194 variables converged at R1 = 2.89%, for the observed data and wR2 = 7.40% for all data. The goodness-of-fit was 1.072. The largest peak in the final difference electron density synthesis was 0.397 eVA³ and the largest hole was -0.268e7A³ with an RMS deviation of 0.061 eVA³. On the basis of the final model, the calculated density was 1.431 g/cm³ and the F(000) was 696 e^". Vanadium oxo complex 5 (dimer) A red plate of C30H44N2O8V2, with approximate dimensions 0.060 * 0.160 * 0.400 mm, was used for the single crystal X-ray crystallographic analysis. The X-ray intensity data were measured. The total exposure time was 0.46 h. The frames were integrated with the Bruker SAINT software package using a narrow-frame algorithm. The integration of the data using a monoclinic unit cell yielded a total of 18722 reflections to a maximum Θ angle of 31.04° (0.69 A resolution), of which 5001 were independent (average redundancy = 3.744, completeness = 99.4%, R_int = 4.40%, R_sig = 4.79%) and 3899 (77.96%) were greater than 2o(F²). The final cell constants of a = 12.1706(6) A, b = 11.0345(5) A, c = 23.8844(12) A, β = 100.8835(17)°, volume = 3149.9(3) A³, are based upon the refinement of the XYZ21 centroids of 4220 reflections above 20 σ(Ι) with 5.024° < 2Θ < 60.83°. Data were corrected for absorption effects using the multi-scan method (SADABS). The ratio of minimum to maximum apparent transmission was 0.901. The calculated minimum and maximum transmission coefficients (based on crystal size) are 0.7830 and 0.9620. The final anisotropic full-matrix least-squares refinement on F² with 222 variables converged at R1 = 4.09%, for the observed data and wR2 = 10.48% for all data. The goodness-of-fit was 1.052. The largest peak in the final difference electron density synthesis was 0.432 eVA³ and the largest hole was -0.434e^"/A³ with an RMS deviation of 0.076 eVA³. On the basis of the final model, the calculated density was 1.397 g/cm³ and the F(000) was 1392 e^".

X-ray crystallographic coordinates for structures

2, 5, and 5 (dimer) described herein have been deposited at the Cambridge Crystallographic Data Centre (CCDC), under deposition numbers CCDC1058817, 1058818, and 1058819. These data can be obtained free of charge from the Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.

Discussion

The single crystal X-ray diffraction structures of complexes 2 and 5 are depicted in Figure 3. Complex 2 displays an octahedral geometry in the solid state supported by ligand 1 , with a characteristic V=0 bond distance of 1.59 A. A methoxide and a methanol ligand are bound, with the V-0 bond for the methanol ligand (2.34 A) being dramatically elongated compared to the methoxide (1.76 A). Complex 5 crystallizes in two different polymorphs from the same methanol (MeOH) solution, depending on the temperature. At low temperatures (-20 °C), the monomeric polymorph (Figure 3b) was isolated, featuring a square pyramidal V center possessing an axial oxo ligand. On the other hand, the dimeric, centrosym metric polymorph (Figure 4) crystallized at room temperature, with an octahedral V geometry and the alcohol group of ligand 4 behaving as the bridging donor. In solution with acetonitrile (CH₃CN) as the solvent, both 2 and 5 are believed to be monomeric or in equilibrium, with no perceptible effects on the photocatalytic activity. PREPARATION 1

Synthesis of lignin model compounds:

Compound 1-(4-ethoxy-3-methoxyphenyl)-2-(2-methoxyphenoxy)propane-1 ,3-diol (11).

The lignin model compound, 11 , was synthesized according to the procedure reported previously (S. Son, and F. D. Toste, Angew. Chem., Int. Ed., 2010, 49, 3791 - 3794). The product identity has been confirmed by NMR spectroscopy and HR-MS. ^ΊΗ NMR (CD₃CN, 300 MHz): δ = 1.34 (t, J = 6.9 Hz, 3 H), 3.06 (t, J = 6.9 Hz, 1 H), 3.60 - 3.75 (m, 2 H), 3.77 (s, 3 H), 3.79 (s, 3 H), 4.01 (q, J = 6.9 Hz, 2 H), 4.25 - 4.30 (m, 1 H), 4.82 (t, J = 4.8 Hz, 1 H), 6.81 - 7.02 (m, 7 H), ppm.

Compound 1-(4-ethoxy-3-methoxyphenyl)-3-methoxy-2-(2-methoxyphenoxy)propan-1- ol (13). The Iignin model compound, 13, was synthesized from 11 by a previously reported procedure (S. Son, and F. D. Toste, Angew. Chem., Int. Ed., 2010, 49, 3791 - 3794). The product identity has been confirmed by NMR spectroscopy and HR-MS. ¹H NMR (CD₃CN, 300 MHz): δ = 1.34 (t, J = 6.9 Hz, 3 H), 3.27 (s, 3 H), 3.49 - 3.61 (m, 2 H), 3.78 (s, 3 H), 3.79 (s, 3 H), 4.01 (q, J = 6.9 Hz, 2 H,), 4.44 - 4.48 (m, 1 H), 4.80 (d, J = 4.8 Hz, 1 H), 6.83 - 7.03 (m, 7 H).

Compound 1-(4-ethoxy-3-methoxyphenyl)-2-(2-methoxyphenoxy)ethanone (14). The

Iignin model compound, 14, was synthesized according to a previously published procedure (S. Son, and F. D. Toste, Angew. Chem., Int. Ed., 2010, 49, 3791 - 3794). The product has been identified by NMR spectroscopy and HR-MS. ¹H NMR (CD₃CN, 300 MHz) δ = 1.40 (t, J = 6.9 Hz, 3 H), 3.83 (s, 3 H), 3.86 (s, 3 H), 4.14 (q, J = 6.9 Hz, 2 H), 5.35 (s, 2 H), 6.83 - 6.85 (m, 2 H), 6.91 - 7.02 (m, 3 H), 7.52 (d, J = 2.1 Hz, 1 H), 7.65 (dd, J = 8.4, 2.1 Hz, 1 H).

Compound 1-(4-ethoxy-3-methoxyphenyl)-2-(2-methoxyphenoxy)ethanol (15). The Iignin model compound, 15, was synthesized by modifying a previously published procedure (S. Son, and F. D. Toste, Angew. Chem., Int. Ed., 2010, 49, 3791 - 3794). The product has been identified by NMR spectroscopy and HR-MS. ¹H NMR (CD₃CN, 300 MHz): δ = 1.36 (t, J = 6.9 Hz, 3 H), 3.64 (bs, 2 H), 3.81 (s, 6 H), 3.97 - 4.09 (m, 4 H), 4.90 - 4.94 (m, 1 H), 6.87 - 7.05 (m, 7 H).

16 17 Compound 1-(4-ethoxy-3-methoxyphenyl)propane-1 ,3-diol (17). The lignin model compound, 17, was synthesized as reported previously (S. Son, and F. D. Toste, Angew. Chem., Int. Ed., 2010, 49, 3791 - 3794). The product identity has been confirmed by NMR spectroscopy and HR-MS. ¹H NMR (CD₃CN, 300 MHz): δ = 1.35 (t, J = 6.9 Hz, 3 H), 1.81 - 1.89 (m, 2 H), 3.04 (bs, 1 H), 3.64 (bs, 2 H), 3.79 (s, 3 H), 4.01 (q, J = 6.9 Hz, 2 H), 4.74 (bs, 1 H), 6.85 (s, 2 H), 6.95 (s, 1 H).

Compound 1-phenylpropane-1 ,3-diol (20). The lignin model compound, 20, was synthesized by modifying a previously published procedure (S. Son, and F. D. Toste, Angew. Chem., Int. Ed., 2010, 49, 3791 - 3794). The product has been identified by NMR spectroscopy and HR-MS. ¹H NMR (CD₃CN, 300 MHz): δ = 1.79 - 1.87 (m, 2 H), 2.87 (bs, 1 H), 3.57 (bs, 1 H), 3.61 - 3.65 (m, 2 H), 4.80 (t, J = 6.0 Hz, 1 H), 7.24 - 7.27 (m, 1 H), 7.31 - 7.37 (m, 4 H).

Compound 1-(4-nitrophenyl)propane-1,3-diol (23). The lignin model compound, 23, was synthesized by modifying a previously published procedure (S. Son, and F. D. Toste, Angew. Chem., Int. Ed., 2010, 49, 3791 - 3794). The product has been identified by NMR spectroscopy and HR-MS. ¹H NMR (CD₃CN, 300 MHz): 5 = 1.80 - 1.87 (m, 2 H), 2.92 (bs, 1 H), 3.58 - 3.71 (m, 2 H), 3.89 (bs, 1 H), 4.95 (t, J = 6.3 Hz, 1 H), 7.58 (d, J = 8.7 Hz, 2 H), 8.18 (d, J = 8.7 Hz, 2 H).

Compound 3-(4-ethoxy-3-methoxyphenyl)-3-methoxy-2-(2-methoxyphenoxy)propan-1- ol (25). The lignin model compound, 25, was synthesized from 11 by adopting the previously reported procedure (S. Son, and F. D. Toste, Angew. Chem., Int. Ed., 2010, 49, 3791 - 3794). The product identity has been confirmed by NMR spectroscopy and HRMS. ¹H NMR (CD₃CN, 300 MHz): δ = 1.35 (t, J = 6.9 Hz, 3 H), 2.99 (t, J = 6.3 Hz, 1 H), 3.20 (s, 3 H), 3.69 - 3.73 (m, 2 H), 3.75 (s, 3 H), 3.77 (s, 3 H), 4.01 (q, J = 6.9 Hz, 2 H), 4.29 - 4.34 (m, 1 H), 4.38 (d, J = 6.6 Hz, 1 H), 6.78 - 6.98 (m, 7 H).

26 27

Compound 2-(2-methoxyphenoxy)ethanol (27). The lignin model compound, 27, was synthesized as reported previously (S. Son, and F. D. Toste, Angew. Chem., Int. Ed., 2010, 49, 3791 - 3794). The product identity has been confirmed by NMR spectroscopy and HRMS. ¹H NMR (CD₃CN, 300 MHz): δ = 3.01 (bs, 1 H), 3.77 - 3.82 (m, 2 H), 3.81 (s, 3 H), 4.00 - 4.03 (m, 2 H), 6.88 - 6.97 (m, 4 H).

Compound 4-(1,3-dimethoxy-2-(2-methoxyphenoxy)propyl)-1-ethoxy-2- methoxybenzene (28). The lignin model compound, 28, was synthesized from 11. To a stirred solution of 11 (0.348 g, 1.0 mmol) in dry THF (5.0 mL) was added NaH (0.100 g, 2.50 mmol) followed by a 2.0 M solution of iodomethane (Mel, 1.2 mL, 2.4 mmol) in dichloromethane over 10 min. The reaction mixture was stirred at room temperature overnight. After the substrate 11 was consumed, the reaction was quenched by the addition of a saturated aqueous solution of NH₄CI. The mixture was extracted three times with ethyl acetate (EtOAc, 3 * 30 ml_) and the combined organic layers were dried over anhydrous MgS0₄, filtered, and concentrated by rotary evaporation. The resulting oily residue was purified by flash column chromatography on silica with hexanes/EtOAc (5:1 ). The product was isolated as a colorless viscous liquid (0.309 g, 82%) ¹H NMR (CD₃CN, 300 MHz): <5 = 1.35 (t, J = 6.9 Hz, 3 H), 3.19 (s, 3 H), 3.28 (s, 3 H), 3.59 (d, J = 4.8 Hz, 2 H), 3.73 (s, 3 H), 3.77 (s, 3 H), 4.02 (q, J = 6.9 Hz, 2 H), 4.36 (d, J = 5.7 Hz, 2 H), 4.51 - 4.56 (m, 1 H), 6.76 - 6.97 (m, 7 H). ¹³C NMR (CDCI₃, 75 MHz) δ = 14.9, 55.9, 56.0, 57.3, 59.4, 64.4, 71.5, 82.5, 82.8, 111.2, 112.2, 112.5, 118.3, 120.5, 120.9, 122,4, 131.0, 148.0, 148.1 , 149.2, 150.9.

Compound 1-(4-hydroxy-3-methoxyphenyl)-2-(2-methoxyphenoxy)propane-1,3-diol (31). The lignin model compound, 31, was synthesized according to the procedure reported previously (E. Baciocchi.C. Fabbri, and O. Lanzalunga, J. Org. Chem., 2003, 68, 9061- 9069). The product identity has been confirmed by NMR spectroscopy and HR-MS. ¹H NMR (CD₃CN, 300 MHz): δ = 3.06 (t, J = 6.0 Hz, 1 H), 3.73 - 3.78 (m, 2 H), 3.79 (s, 3H), 3.82 (s, 3 H), 4.24 - 4.29 (m, 1 H), 4.80 (t, J = 4.5 Hz, 1 H), 6.46 (bs, 1 H), 7.03 - 6.74 (m, 7 H).

Isotopically ¹³C-labelled 1-(4-ethoxy-3-methoxyphenyl)-2-(2-methoxyphenoxy)prop- ane-1,3-diol (11-¹³C2). The ¹³C-labelled congener 11-¹³C2 was synthesized by modifying a previously published procedure (S. Son, and F. D. Toste, Angew. Chem., Int. Ed., 2010, 49, 3791 - 3794). The product identity has been confirmed by NMR spectroscopy and HR-MS. ¹H NMR (CDCI₃, 300 MHz): δ = 1 .45 (t, J = 6.9 Hz, 3 H), 2.40 9 (bs, 2 H), 3.40 - 3.71 (m, 1 H), 3.87 (s, 3 H), 3.89 (s, 3 H), 3.90 - 3.93 (m, 1 H), 4.09 (q, J = 6.9 Hz, 2 H), 4.27 (bd, J = 78 Hz, 1 H), 4.98 (d, J = 4.8 Hz, 1 H), 7.09 - 6.82 (m, 7 H).

Deuterated 1-(4-ethoxy-3-methoxyphenyl)-2-(2-methoxyphenoxy)propane-1,3-diol (11- D1). The deuterated congener 11-D1 was synthesized by modifying a previously published procedure (S. Son, and F. D. Toste, Angew. Chem., Int. Ed., 2010, 49, 3791 - 3794). The product identity has been confirmed by NMR spectroscopy and HR-MS. The ¹H NMR spectrum showed that 11-D1 consisted of two diastereoisomers in a 4:6 ratio. ¹H NMR (CD₃CN, 300 MHz): δ = 1.35 (t, J = 6.9 Hz, 3 H), 3.05 (bs, 1 H), 3.72 - 3.40 (m, 2 H), 3.47 (bs, 1 H), 3.77 (s, 1.82 H, major diastereoisomer), 3.77 (s, 1.17 H, minor diastereoisomer), 3.79 (s, 1.2 H, minor diastereoisomer), 3.84 (s, 1.7 H, major diastereoisomer ), 4.01 (q, J = 6.9 Hz, 2 H), 4.18 (dd, J = 5.4, 3.9 Hz, 0.6 H, major diastereoisomer), 4.27 (dd, J = 5.1 , 3.9 Hz, 0.4 H, minor diastereoisomer), 4.98 (d, J = 4.8 Hz, 1 H), 6.84 - 7.09 (m, 7 H).

i) MeONa ii) UAIH₄/THF

CD₃CN/D₂0

Deuterated 1-(4-ethoxy-3-methoxyphenyl)-2-(2-methoxyphenoxy)propane-1 ,3-diol (11- D2). The deuterated congener 11-D2 was synthesized from compound 10. Compound 10 (0.376 g, 1.0 mmol) in dry DCM (15 mL) was stirred at room temperature overnight with pyridinium chlorochromate (0.259 g, 1.2 mmol) to obtain compound 33. Purification by flash column chromatography on silica yielded compound 33 as a white solid (0.318 g, 85%). The deuteration was carried out under moisture- and air-free conditions. A 50 mL oven-dried round-bottom flask (RBF) was filled with Ar, 33 (0.075 g, 0.20 mmol), and a catalytic amount of NaOMe (0.0011g, 0.020 mmol), following which CD₃CN (1.0 mL) was added. After stirring for 10 min, D₂0 (1.0 mL) was injected into the mixture and the solution was stirred at room temperature under Ar for 12 h. The solvent was removed under reduced pressure at 60 °C, and the residue was dried under vacuum. Subsequently the RBF was refilled with Ar. CD₃CN (1.0 mL) followed by D₂0 (1.0 mL) were injected into the RBF again and stirred for another 6 h. This sequence of solvent removal and stirring with deuterated solvents was repeated thrice for complete deuteration. Finally, after solvent removal, the reaction mixture was dissolved in CDCI₃ (2.0 mL) and filtered into another dry 50 mL RBF. The solvent was removed in vacuo and the residue was dried and stored under Ar. Dry THF (10 mL) was injected and the reaction mixture was cooled in an ice bath under stirring. Excess LiAIH₄ (0.038 g, 1.0 mmol) was added slowly to the reaction mixture and stirred at room temperature. After 24 h, aqueous NaOH (10% w/w, 0.50 mL) was added and the reaction mixture was stirred for another 10 min. The solvent was then removed in vacuo. The residue was extracted with CHCI₃ (3 x 10 mL) and dried to give the crude 11-D2. The product was purified by preparative TLC as a colourless oil (0.042 g, 60 %). ¹H NMR (CD₃CN, 300 MHz): δ = 1.34 (t, J = 6.9 Hz, 3 H), 3.06 (s, 1 H), 3.63 - 3.75 (m, 2 H), 3.77 (s, 3 H), 3.79 (s, 3 4.00 (q, J = 6.9 Hz. 2 H), 4.82 (s, 1 H), 6.81 - 7.02 (m, 7 H).

EXAMPLE 2: Degradation of lignin model compound 11 under thermal conditions

The degradation experiments were carried out in an NMR tube. Compound 11 (0.014 g, 0.040 mmol) in CD₃CN (0.50 ml.) with the corresponding catalyst (4.0 pmol) were used in each reaction. An internal standard 1,1 ,2,2-tetrachloroethane (4.2 μΙ_, 0.040 mmol) was used for the calculation of the conversion as well as the yield of the products. The reactions were conducted under air with a balloon to maintain a constant pressure in the sealed tube. The NMR tube was wrapped with aluminium foil during the thermal reactions to minimize the photochemical reactions. The ¹H NMR spectra were recorded before and after the reaction. When 5 was used as the catalyst, the reactions were carried out at 25 °C, 45 °C, and 80 °C for 24 h. The conversion and yield were calculated based on the ¹H NMR spectra. The reaction at 80 °C gave complete conversion and the major products formed were isolated by preparative TLC and their identities were confirmed by ¹H NMR spectroscopy. When 2 was used as the catalyst, the reactions were carried out at 25 °C and 80 °C for 36 h. The ¹H NMR spectra showed that there were no observable changes at room temperature, whereas about 25% of 11 was oxidized to 32 at 80 °C.

The results obtained by these experiments are summarised in Table 3. As expected, 5 showed catalytic reactivity towards lignin degradation through selective CO bond cleavage similar to the prior report by the Toste group. At room temperature, 5 showed slow reactivity with 11 (18 % conversion, entry 1 ), but gave >95% conversion with good selectivity under reflux conditions (entry 2). In contrast, catalytic amounts of 2 resulted in only about 25% conversion of 11 to the ketone (32) after oxidation of the benzylic alcohol under similar reaction conditions over 36 h (entry 4).

Temperature Reaction time Conversion

"

" Thcvieldsare derived from the Ή NMR spectra with 1 ,1,2,2-tetrachloroethane as an internal standard. All the reactions were carried out in CH₃CN in the dark under ambient air.

Table 3

The product selectivity is patently different under thermal conditions compared to photocatalytic conditions. However, 2 has a brown while 5 has a red color, alluding to the ligand-to-metal charge transfer (LMCT) transitions occurring in these d° V(V) oxo complexes. Accordingly, the UV-visible (UV-vis) spectrum of 2 exhibits an absorption band in the near- UV region with A_max at 396 nm (Fig. 5) tailing to 500 nm, which can be attributed to LMCT based on DFT calculations (vide infra).

The UV region consists of more intense absorptions typical of salicylaldimine intra-ligand charge transfers. Similarly, 5 also displays a near-UV absorption band with a blue-shifted A_max at 330 nm that tails into the visible region (Fig. 5). The UV-vis spectra of ligands 1 and 4 are illustrated in Fig. 5 for comparison. Both ligands evidently do not exhibit absorption above 380 nm, lending credence to the assignment of the visible absorption bands as arising from LMCT transitions. The UV-vis spectra of VO(acac)₂ and VO(OPr)₃ are shown in Fig. 6 on the same scale, and both complexes absorb less light in the visible region relative to 2, consistent with the low photodegradation product yields (see entries 1 and 2 of Table 4 hereinbelow). Notably, 2 also has a photoluminescence A_max at 480 nm with lifetimes of 9.9 ns (59%), 2.0 ns (29%), and <110 ps (12%) as determined by time-correlated single photon counting.

Compound 1-(4-ethoxy-3-methoxyphenyl)prop-2-en-1-one (12) ¹H NMR (CD₃CN, 300 MHz): δ = 1.40 (t, J = 6.9 Hz, 3 H), 3.86 (s, 3 H), 4.13 (q, J - 6.9 Hz, 2 H), 5.86 (dd, J = 10.5, 2.1 Hz, 1 H), 6.34 (dd, J = 17.1, 1.8 Hz, 1 H), 6.99 (d, J = 8.4 Hz, 1 H), 7.31 (dd, J = 17.1 , 10.5 Hz, 1 H), 7.53 (d, J = 2.1 Hz, 1 H), 7.63 (dd, J = 8.4, 2.1 Hz, 1 H) ppm.

EXAMPLE 3: (a) Photodegradation of lignin model 11 in the presence of different vanadium oxo complexes

6a (24 %)

Photodegradation experiments were carried out in an NMR tube. In a typical procedure, 11 (0.014 g, 0.04 mmol) and the vanadium catalyst were dissolved in CD₃CN (0.50 ml.) and sealed. Compound 1 ,1 ,2,2-tetrachloroethane (4.2 μΙ_, 0.040 mmol) was used as an internal standard to calculate the conversion and yield of the products. A balloon was used to maintain an atmospheric condition via a needle. The reaction mixture was irradiated for 24 h under visible light (> 420 nm) with AM1.5 solar intensity at ambient temperature. A continuous water circulator was used to maintain the temperature below 30°C. The photodegradation experiments were carried out with the different vanadium oxo complexes VO(OPr)₃, VO(acac)₂, 2, and 5 with concentrations summarised in Table 4. Complex 2 was found to be most effective for cleaving 11 into 6 and 9 as the major products through selective C-C bond cleavage. Complete consumption of 11 was observed under optimized conditions. Compound 4-ethoxy-3-methoxybenzaldehyde (6). ¹H NMR (CD₃CN, 300 MHz): <5 = 1.41 (t, J = 6.9 Hz, 3 H), 3.87 (s, 3 H), 4.15 (q, J = 6.9 Hz, 2 H), 7.07 (d, J = 8.1 Hz, 1 H), 7.40 (d, J = 1.8 Hz, 1 H), 7.49 (dd, J = 8.1 , 1.8 Hz, 1 H), 9.83 (s, 1 H) ppm. 2-methoxyphenylformate (9) ¹H NMR (CD₃CN, 300 MHz): δ = 3.82 (s, 3 H), 6.99 (t, J = 7.8 Hz, 1 H), 7.12 (d, J = 7.8 Hz, 2 H), 7.28 (t, J = 7.8 Hz, 1 H), 8.26 (s, 1 H) ppm.

(b) Photodegradation of 11 with complex 2 under an inert atmosphere

7 (8%) 0 %)

In a Pyrex glass vial (20 mL), 11 (0.035g, 0.10 mmol) and 2 (0.0035 g, 0.010 mmol) were dissolved in dry MeOH (5 mL) and the solvent was removed under reduced pressure by rotary evaporation, after which the mixture was dried in vacuo for 1 h. The vial was then refilled with Ar and MeCN (10 mL) was injected. The mixture was stirred in the dark for 10 min with continuous Ar bubbling. The vial was then sealed and irradiated under visible light (> 420 nm) with AM1.5 solar irradiation. Continuous water circulation was used to keep the temperature of the reaction mixture below 30°C. After 24 h of irradiation, the solvent was removed under reduced pressure by rotary evaporator, the residue was dried in vacuo and the ¹H NMR spectrum was recorded to identify the products and calculate the yields. (c) Photodegradation of 11 with complex 2 using unfiitered AM 1.5 solar irradiation

6a (49 %)

The photodegradation experiments were carried out in a NMR tube. In a typical procedure, 11 (0.014 g, 0.040 mmol) and 2 (0.0014 g, 4.0 μιηοΙ) were dissolved in CD₃CN (0.50 mL) and sealed. A balloon as used to maintain an atmosphere of air via a needle. The reaction mixture was irradiated with AM1.5 solar irradiation without the 420 nm cut-off filter, using a continous water circulator to maintain the temperatures below 30 °C. After 6 h of irradiation, the ¹H NMR spectrum showed that 11 was completely consumed. The products were identified and quantified with NMR spectroscopy.

Irradiation time Conversion

VO(OPr),

O O

II \

12 (30%) 20%

24 0

24 - 1 6 (39%), 6a (7%), 7 (9%),

9 (34%), HCOOH (10%)

24 6 (70%), 6a (24%). 7 (6%),

9 (61%), HCOOH (32%) 1 1 2 10 24 -3 6 (3%)

11 2 10 16 100 6 (50%), fta (49%), 7 (30%),

9 (54%), HCOOH (45%)

9 1 1 2 20 24 -35 6 (32%), fcl (3%), 7 (10%),

9 (25%), HOOOH (10%)

" The yields are calculated based on *H NMR spectra with 1,1 ,2,2-ietrachloroethane as an internal standard- Unless otherwise mentioned, the reactions were carried out in H ,CN under aerobic conditions with visible light irradiation ( . 420 nm, AM 1.5 solar irradiation). During irradiation, the temperature was maintained below 30 ^* (. by water circulation through a transparent glass jacket. Reaction performed in the dark. ' Reaction performed under an Ar atmosphere. ^d Reaction carried out with AMI. solar irradiation without visible light filter.

Table 4 (d) Photodegradation of 11 with complex 2 using continuous flow reactor

A continuous flow photoreaction was carried out in a homemade continuous flow batch photoreactor 100 with a white-light LED strip 170 used as the visible light source. The photoreactor was made with four similar glass condensers 105, each having very thin inner space to contain a maximum of 10 mL of reaction solution (see Figure 8). The condensers were connected with one another using silicon tubing 106. Each glass condenser was wrapped with a white light LED strip and covered with aluminium foil 180 to maximize the light absorption. The inlet 140 of the reactor was connected with the outlet 120 of a peristaltic pump 115 and the inlet 110 of the pump was connected with a long needle 107 which was kept in a glass reservoir 130. The outlet 160 of the reactor was connected with the reservoir 130 through another needle 108. When the pump was on, the reaction solution was circulated between the reservoir and the irradiated reactor. A balloon containing air 150 was also attached to the reservoir to maintain 1 atmosphere of air without solvent evaporation. The reactor can be used for up to 40 mL of the reaction mixture. The flow rate was measured to be 14 mlJmin.

A large scale photolysis of 11 (0.300 g, 0.86 mmol) was carried out with 10 mol% of 2. Due to the poor light transmission properties of the reaction solution, the photolysis experiment was conducted with a homemade flow reactor with a residence time of around 3 min (Figure 8). After 24 h of irradiation under visible light, the products 6 (67%) and 9 (56%) were isolated by column chromatography over silica.

Discussion Prompted by the absorption and emission properties of these complexes, we probed their photocatalytic behavior in the degradation of 11.

Remarkably, when a solution of 11 and 5 mol% of 2 was irradiated under ambient conditions by a solar simulator with a visible light filter (>420 nm) for 24 h, >40% of 11 was chemoselectively converted, in contrast to the thermal activity of 2 (Table 4, entry 5). The products have been identified as 4-ethoxy-3-methoxybenzaldehyde (6) and 2- methoxyphenylformate (9), arising from aliphatic C-C bond cleavage reactions under unusually mild conditions in the preserve of enzymatic processes. A catalyst concentration of 10 mol% 2 gave the best results with >95% consumption of 11 and high yields of 6 (70%) and 9 (61 %, entry 6). However, further increase in the concentrations of 2 led to lower conversion rates, possibly due to reduced light transmission through the sample (entry 9). Under the optimized conditions for visible light photodegradation of 11, formic acid, 4-ethoxy- 3-methoxybenzoic acid (6a), and guaiacol (7) were also observed or isolated by preparatory thin layer chromatography (TLC), as illustrated in Table 4. The clean and controlled transformation of 11 into chemical products that retain valuable functional groups occurs under exceptionally mild conditions, with the major energy inputs being light and the water circulator needed to maintain temperatures below 30°C.

A large scale photolysis of 11 (0.300 g, 0.86 mmol) was carried out with 10 mol% of 2. After 24 h of irradiation under visible light, the products 6 (67%) and 9 (56%) were isolated by column chromatography over silica.

To elucidate the nature of photocatalysis by 2, a series of control experiments were conducted to ascertain the role of reagents and reaction conditions. Lignin model compound 11 did not react when irradiated with visible light (A > 420 nm) for 24 h at ambient temperature, since it does not absorb visible light. Likewise, no reaction was observed when 11 was stirred in the dark with 2 for 24 h in air (Table 4, entry 4). Under an Ar atmosphere, 2 surprisingly remained photocatalytically active with a 3 % conversion, although the yields had clearly diminished (Table 4, entry 7). When the reaction was conducted with AM1.5 solar irradiation without the visible light filter, the rate accelerated resulting in complete consumption of 11 within 16 h (Table 4, entry 8). However, the yields of 6 and 2- methoxyphenylformate decreased vis-a-vis the optimized conditions (Table 4, entry 6), with increased amounts of 6a and guaiacol 7. Compound 6a is believed to have arisen by aerial oxidation of 6, whereas guaiacol 7 and formic acid could be derived from 2- methoxyphenylformate by hydrolysis, as supported by isotope labeling studies below.

Inspired by these findings, we found that 10 mol % of catalyst 5 under visible irradiation led to a 50% conversion of 11 under similar reaction conditions. Two products are identical with those under thermal conditions, namely 12 and guaiacol 7, while 6 and 9 are new and similar to the results with photocatalyst 2 (Table 4, entry 3). This experiment suggests that two simultaneous catalytic pathways may operate for 5, with the thermal reaction predominant over the photochemical reaction. When the colored complexes V'O(acac)₂ and V^vO(OPr)₃ were used under photocatalytic conditions, only trace amounts of 6 were identified by ¹H NMR spectroscopy (Table 4, entries 1 and 2). These experiments support the critical role of LMCT for visible light absorption and redox non-innocent ligands in mediating the photocatalysis, as verified by DFT calculations. Moreover, an unprecedented, general pathway for the selective photodriven aliphatic C-C bond cleavage adjacent to alcohol groups may be accessible for other V(V) oxo catalysts that absorb visible light.

EXAMPLE 4: Photodegradation with isotopically labelled lignin model compounds The isotopically labelled lignin model compounds 11-¹³C2, 11-D1, and 11-D2 were investigated under the optimized photocatalytic conditions with 2. In a typical procedure, the substrate (0.014 g, 0.040 mmol) and 2 (0.0014 g, 4.0 μιηοΙ) were dissolved in CD₃CN (0.50 mL) in an NMR tube and sealed. A balloon was used to maintain an aerobic environment via a needle. The sample was irradiated for 24 h under visible light (> 420 nm) with AM1.5 solar irradiation with a continuous water circulator used to maintain temperatures below 30 °C. After 24 h of irradiation, the products were identified and the yields were quantified by ¹H NMR spectroscopy.

Deuterated-4-ethoxy-3-methoxybenzaldehyde (6-D). ¹H NMR (CD₃CN, 300 MHz): δ = 1.41 (t, J = 6.9 Hz, 3 H), 3.87 (s, 3 H), 4.15 (q, J = 6.9 Hz, 2 H), 7.07 (d, J = 8.1 Hz, 1 H), 7.40 (d, J = 1.8 Hz, 1 H), 7.49 (dd, J = 8.1 , 1.8 Hz, 1 H).

Deuterated-2-methoxyphenyl formate (9-D) ¹H NMR (CD₃CN, 300 MHz): δ = 3.82 (s, 3 H), 6.99 (t, J = 7.8 Hz, 1 H), 7.09 - 7.14 (m, 2 H), 7.28 (t, J = 7.8 Hz, 1 H) ppm.

¹³C-2-methoxyphenyl formate (9-¹³C) ¹H NMR (CD₃CN, 300 MHz): δ = 3.82 (s, 3 H), 6.99 (t, J = 7.8 Hz, 1 H), 7.12 (d, J = 8.1 Hz, 2 H), 7.28 (t, J = 8.1 Hz, 1 H), 8.26 (d, J = 234 Hz, 1 H).

To ascertain the identity of the products and obtain additional mechanistic insights, we carried out isotope labeling studies.

Three different isotopically labeled lignin model compounds 11-¹³C2, 11-D1 , and 11-D2 were synthesized and used as substrates in NMR tube experiments with CD₃CN as the solvent under the optimal conditions from Table 4 entry 6. NMR experiments for ¹H, ²H, and ¹³C were conducted before and after irradiation. With the ¹³C labelled 11-¹³C2 as substrate, ^Relabeled formic acid (diamond-shapes) and 2-methoxyphenylformate (9-¹³C1, star shapes) were observed by ¹H NMR spectroscopy, with the formate protons split into doublets due to coupling with ¹³C (Fig. 7).

The ¹H (Fig. 7) and ¹³C NMR spectra of both products confirm the assignment of the ¹H NMR chemical shfts and the regioselective aliphatic C-C bond cleavage sites. When 11 is deuterated at the benzylic position (1 1 -D1 ), the ¹H NMR spectrum of the product mixture no longer displays the aldehyde proton signal at 9.8 ppm (Fig. 7d), while the ²H NMR spectrum (inset) clearly shows the corresponding deuterium label (square shape). The product 6-D was isolated by preparatory TLC and characterized by ¹H and ²H NMR spectroscopy as well as by HRMS. The spectroscopic data suggested that the benzylic hydrogen is not scrambled or exchanged during the photocatalytic C-C bond cleavage reactions. When another deuterated lignin model 11-D2 was subjected to photocatalytic degradation, we found that the reaction rate was slower and it generated deuterated 2-methoxyphenylformate (9-D, right-angled triangle, (Fig. 7e) and almost exclusively deuterated formic acid (pentagon pointing down, inset), further confirming the absence of isotope scrambling.

The absence of formic acid in the ¹H NMR spectrum with 11-D2 as the substrate also suggests that the formic acid produced during this photocatalytic reaction originated from the 2-methoxyphenylformate fragment, likely via hydrolysis (Fig. 7e and inset). This proposal is confirmed by the identification of guaiacol by NMR spectroscopy and liquid chromatography- mass spectrometry (LC-MS), although isolation by preparatory TLC proved challenging possibly due to decomposition of guaiacol. These isotope labelling studies confirm that 2 is a photocatalyst for the unique and selective C-C bond cleavage of 11, ruling out the benzylic hydrogen abstraction and oxidation pathways proposed in prior reports. The product distribution of the isotope labelling experiments is summarized in Table 5.

Table 5 EXAMPLE 5: Photodegradation of other lignin model compounds with complex 2

Other lignin model compounds 13, 15, 25, 28, and 31 were also investigated under similar photocatalytic reaction conditions in an NMR tube. Complex 2 was found to be inactive for 28 and 31 , whereas 13 and 15 gave C-C bond cleavage products 6 and 9 at slower rates. These experiments have been summarized in Table 6. Compound 25 gave a complex product mixture. Other substrates 17, 20, 23, and 27 were also investigated under similar photocatalytic reaction conditions. The ¹H NMR spectroscopic analysis revealed that 17, 20, and 23 degraded by C-C bond cleavage to the corresponding aldehydes along with secondary alcohol oxidation to their corresponding ketones. The products were isolated and their identities were confirmed by ¹H NMR spectroscopy and HR-MS. Compound 27 gave 9 and formic acid under photocatalytic conditions.

1-(4-ethoxy-3-methoxyphenyl)-3-hydroxypropan-1-one (18). ¹H NMR (CD₃CN, 300 MHz): δ = 1.40 (t, J = 6.9 Hz, 3 H), 3.13 (t, J = 6.0 Hz, 2 H), 3.86 (s, 3 H), 3.87 (t, J = 6.0 Hz, 3 H), 4.13 (q, J = 6.9 Hz, 2 H), 6.98 (d, J = 8.4 Hz, 1 H), 7.50 (d, J = 1.8 Hz, 1 H), 7.62 (dd, J = 8.4, 2.1 Hz, 1 H).

3-hydroxy-1-phenylpropan-1-one (21) ¹H NMR (CD₃CN, 300 MHz): δ = 3.18 (t, J = 6.0 Hz, 2 H), 3.89 (t, J = 6.0 Hz, 3 H), 7.49 - 7.54 (m, 2 H), 7.59 - 7.62 (m, 1 H), 7.96 - 7.99 (m, 1 H).

3-hydroxy-1-(4-nitrophenyl)propan-1-one (24) ¹H NMR (CD₃CN, 300 MHz): δ = 3.23 (t, J = 6.0 Hz, 2 H), 3.90 (t, J = 6.0 Hz, 3 H), 8.14 (d, J = 9.0 Hz, 2 H), 8.30 (d, J = 9.0 Hz, 2 H) ppm.

Discussion

A systematic series of various lignin model compounds have been investigated under the same photocatalytic reaction conditions to explore the generality and selectivity of the C-C cleavage reactions (Table 6). Model compound 13 was degraded to yield 6 and 9, albeit with lower yields than for 11. Similarly, compound 15 was photocatalytically converted into 6 and 9 cleanly (Table 6), with a dramatically lower yield. These results suggest that the primary alcohol in 11 is important for facilitating the chemical reactivity, although our DFT calculations indicate that the lower yields from degradation of 13 and 15 over 24 h may have kinetic origins. The results for substrates 13 and 15, as well as the DFT studies also indicate that a reaction path involving the tandem oxidation of the primary alcohol to an aldehyde, followed by a retro-aldol reaction, is probably not operational.

A set of mono-aryl aliphatic alcohol substrates was also subjected to the same photocatalytic conditions, and the product distribution gave additional insights into the catalytic mechanism. After photolysis, 17 was oxidized to 6 and formic acid as well as 6a, but the ketone 18 was also observed as a minor product (Table 6), with no aldehyde derived from primary alcohol oxidation. Interestingly, the ratio of the oxidized ketones (21 and 24) to the corresponding aldehyde originating from C-C bond cleavage (benzaldehyde and 4-nitrobenzaldehyde, respectively) was found to rise when increasingly electron-deficient aryl groups were introduced at the benzylic alcohol (20 and 23 in Table 6). This phenomenon suggests a vital role of the aryl ring in dictating the selectivity of our unusual photocatalytic reactions. On the other hand, protection of the hydroxyl group of the benzylic alcohol as a methoxy ether provided 25, and the reaction yielded a more complex mixture including aldehydes and formic acid upon photodegradation.

When 27 (Table 6) was photolyzed with the same protocol, 9 and formic acid were observed in poor yields at low conversion. Furthermore, irradiation of the ether-protected model 28 and the phenolic substrate 31 , in the presence of 2, resulted in no degradation (Fig. 6). Alcohol functionalities appear to be essential to direct the selective C-C bond cleavage, but binding of the aryl ring to the vanadium center as a phenolate shuts down the catalytic activity. To probe whether unactivated alcohols can undergo this remarkable photocatalyzed C-C cleavage, both glycerol (a by-product of biodiesel production) 35 and 1-butanol were used as substrates. Gratifyingly, formic acid was observed as one of the products for both substrates as anticipated (Table 6), although the identification of the remaining products will require additional studies. The photocatalytic cleavage of C-C bonds in alcohols shows promise for further generalization.

O

- -OH Z ,

H OH

OH 37% conversion

35

36 3%

Conditions a: 10 mol% 2, CD₃CN, hv (λ > 420 nm), air, 24 h

Table 6 EXAMPLE 6: Photocatalytic reactions with complexes 3a, 3b and 3c

The procedure outlined in Example 3a was followed, except that complexes 3a, 3b, and 3c were used instead. The results are set out in Table 7.

6 (8%) 9 (5%) ^lal For all the reactions, 0.025 mmol of compound 11 was used. The yields are calculated based on the ¹H NMR spectra with 1 ,1 ,2,2-tetrachloroethane as an internal standard. The reactions were carried out in CD₃CN for complexes 3a and 3b, and CH₃CN for 3c under aerobic conditions with visible light irradiation (λ > 420 nm, AM1.5 solar irradiation).

Table 7: Photocatalytic C-C bond cleavage by complexes 3a, 3b, and heterogeneous catalyst 3c.

EXAMPLE 7: Photocatalytic reaction with complex 41

+ HCOOH

11 6 9 The following photocatalytic experiments were performed in an NMR tube. In a typical procedure, lignin model compound 11 (0.011 g, 0.032 mmol) and 41 (0.0013 g, 0.0031 mmol) were dissolved in CD₃CN (0.50 mL) and the mixture was placed in an NMR tube with oxygen access. The mixture was subsequently irradiated under visible light (λ > 420 nm) with AM 1.5 solar intensity at ambient temperature for 48 h. A continuous water circulator was used to maintain the temperature below 30° C. The irradiation would occasionally be interrupted so as to monitor the reaction progress by NMR spectroscopic analysis. The obtained data indicated that the dominant reaction products were aryl aldehyde (6) and aryl formate (9), while formic acid arises due to the hydrolysis of the formate. ¹H NMR (CD₃CN, 500 MHz): δ = 8.04 (s, 1 H), 8.27 (s, 1 H), 9.82(s, 1 H) ppm.

EXAMPLE 8: Photocatalytic reaction with complex 43

HCOOH

11 6 (15%) 9 (12%) (9%)

Similarly to the previous photocatalytic experiment, lignin model compound 11 (0.010 g, 0.030 mmol) and 43 (0.0012 g, 0.0030 mmol) were dissolved in CD₃CN (0.50 mL) and the mixture was placed in an NMR tube with oxygen access. The mixture was subsequently irradiated under visible light (λ > 420 nm) with AM 1.5 solar intensity at ambient temperature for 16 h. A continuous water circulator was used to maintain the temperature below 30° C. The irradiation would occasionally be interrupted so as to monitor the reaction progress by NMR spectroscopic analysis. After the reaction, 1 ,1 ,2,2-tetrachloroethane (3.1 μί, 0.030 mol) was added inside the NMR tube as an internal standard to calculate the conversion and the yield of the products. The NMR spectroscopic analysis showed that the main reaction products were aryl aldehyde (6, 15%), aryl formate (9, 12%), and formic acid (9%) as anticipated. ¹H NMR (CD₃CN, 500 MHz): δ = 8.03 (s, 1 H), 8.27 (s, 1 H), 9.82 (s, 1 H) ppm. The conversion of the starting material after 16 h was ca. 40%.

Claims

Claims

1. A compound of formula I:

wherein:

Ri represents F, CI, Br, I, CF₃, CN, S0₃R₉, SO₂NR₁₀Rn, C(0)R₁₂, C(0)OR₁₃, C(0)NR₁₄Ri₅, C(NR₁₆)NR₁₇R₁₈, N₃, N0₂ and aryl, which latter group is optionally substituted by one or more substituents selected from halo, nitro, CN, N₃, C_1-4alkyl, C₂. alkenyl, and C₂.₄ alkynyl, OR_19a, S(0)_mR_19b, S(0)₂N(R_19c)(R_19d), N(R_19e)S(0)₂R_19f, N(R_19g)(R_19h); each R₂ and R to R₇ independently represent:

(a) H,

(b) halo,

(c) alkyl, C_2-6 alkenyl, C_2-6 alkynyl and aryl, which latter four groups are optionally substituted by one or more substituents selected from halo, nitro, CN, N₃, C_1-4 alkyl, C_2-4 alkenyl, and C_2-4 alkynyl, OR_20a, S(O)_mR_20b, S(0)₂N(R_20c)(R₂od), N(R₂₀e)S(0)₂R₂o_f>

(d) S(O)₂R_20i,

(e) S(O)₂N(R_20l)(R_20k),

(g) N₃; or R₆ represents a solid support, optionally wherein the solid support is attached to the rest of the compound of formula I by way of a linking group;

R₃ represents a C_1-6alkyl group;

R₈ represents

(a) H,

(b) C_1-6 alkyl, C_2-6 alkenyl, and C₂-6 alkynyl, aryl, which latter four groups are optionally substituted by one or more substituents selected from halo, nitro, CN, N₃, C_1-4 alkyl, C_2- alkenyl, and C_2-4 alkynyl, OR_21a, S(0)_mR_21b, S(0)₂N(R_21c)(R_21d), N(R_21e)S(0)₂R_21f,

each of R₉to R₁₈, R_19a to R_19h, R₂o_a to R₂₀i and R_21a to R_21h independently represent H or C_1-4 alkyl, which latter group is optionally substituted by one or more substituents selected from halo;

each m independently represents 0 to 2;

or a solvate and/or a dimer thereof.

2. The compound of Claim 1 , wherein R_! represents F, CI, Br, CF₃ and N0₂.

3. The compound of Claim 1 or Claim 2, wherein each of R₂, R₅ and R₇ independently represent H, F, CI, Br, C_1- alkyl (which latter group is optionally substituted by one or more substituents selected from F, CI, Br, and OR_20a) and OR₂₀|.

4. The compound according to Claim 3, wherein each of R₂, R₅ and R₇ represent H.

5. The compound according to any one of the preceding claims, wherein R₄ represents H or 'Bu.

6. The compound according to any one of the preceding claims, wherein R₃ represents C_1-4 alkyl.

7. The compound according to any one of the preceding claims, wherein R₆ represents H, CH₂OH, CH₂OCH₃, CH₂N₃, or -(L)_y-solid support, where L is a linking group and y is 0 or 1.

8. The compound according to any one of the preceding claims, wherein R₈ represents H.

9. The compound according to any one of the preceding claims, wherein the compound of formula I is:

(a)

(g)

, or a solvate and/or a dimer thereof.

10. Use of a compound as described in any one of Claims 1 to 9 as a catalyst for a chemical reaction.

11. The use of Claim 10, wherein the chemical reaction is reaction of a compound of formula II:

wherein:

Ra and Rb independently represent H, alkyl, alkenyl, alkynyl (which latter three groups are optionally substituted with halo, nitro, CN, N₃, OR₂2a. N(R₂2b)(R22c)). aryl or heterocyclic group (which latter two groups are optionally substituted with halo, nitro, CN, N₃, OR₂3_a,

Rc represents H, alkyl, alkenyl, alkynyl (which latter three groups are optionally substituted with halo, nitro, CN, OR_24a, N(R₂4b)(R24c)), aryl, heteroaryl (which latter two groups are optionally substituted with halo, nitro, CN, N₃, OR_25a, N(R_25b)(R₂5c)), O-alkyl, O-alkenyl, O- alkynyl (which latter three groups are optionally substituted with halo, nitro, CN, N₃, OR₂6a, N(R26b)(R26c)), O-aryl or O-heterocyclic (which latter two groups are optionally substituted with halo, nitro, CN, OR_27a, N(R_27b)(R27c));

each of R₂2a - R22₀ 2_4a - R24_C and R₂6a - R26_C independently represents H, halo or C_1-4 alkyl, which latter group is optionally substituted by one or more substituents selected from halo or OH;

each of R₂3_a - R230 R25a - R250 and R_27a - R27C independently represents halo or C_1-4 a!kyl, which latter group is optionally substituted by one or more substituents selected from halo.

12. The use of Claim 11 , wherein in the compound of formula II:

Ra and Rb independently represent H, C_1-6 alkyl (which latter group is optionally substituted with halo or OR22a), or aryl (which latter group is optionally substituted with nitro, OR₂3_a); and Rc represents H, alkyl (which latter group is optionally substituted with halo or OR₂₄a), aryl (which latter group is optionally substituted with halo, nitro, OR_25a), O-alkyl (which latter group is optionally substituted with halo, OR₂₆a)), O-aryl (which latter group is optionally substituted with halo, nitro, OR_27a); and

each of R₂2_a, R2_4a and R_{2 a} independently represents H, halo or Ci_-4 alkyl, which latter group is optionally substituted by one or more substituents selected from halo or OH.

13. The use of Claim 11, wherein the compound of formula (II) is lignin or a lignin-like polymeric material.

14. A method of degrading a compound of formula II:

wherein:

Ra and Rb independently represent H, alkyl, alkenyl, alkynyl (which latter three groups are optionally substituted with halo, nitro, CN, N₃, OR_22a, N(R₂2b)(R22c)), aryl or heterocyclic group (which latter two groups are optionally substituted with halo, nitro, CN, N₃, OR₂3_a,

Rc represents H, alkyl, alkenyl, alkynyl (which latter three groups are optionally substituted with halo, nitro, CN, OR_24a, N(R₂₄b)(R24c))_> aryl, heteroaryl (which latter two groups are optionally substituted with halo, nitro, CN, N_3l OR₂5_a, N(R₂5b)(R25c))_> O-alkyl, O-alkenyl, O- alkynyl (which latter three groups are optionally substituted with halo, nitro, CN, N₃, OR₂6_a, N(R26b)(R26c))_> O-aryl or O-heterocyclic (which latter two groups are optionally substituted with halo, nitro, CN, OR_27a, N(R_27b)(R₂7c));

each of R_22a - R₂2c, R_24a - R₂₄c, and R_26a - R₂6_C independently represents H, halo or C_1-4 alkyl, which latter group is optionally substituted by one or more substituents selected from halo or OH;

each of R_23a - R_23c, R_25a - R_25c, and R_27a - R_27c independently represents halo or C_1-4 alkyl, which latter group is optionally substituted by one or more substituents selected from halo, by reacting the compound of formula II in the presence of a catalytic amount of a compound of formula I as defined in any one of Claims 1 to 9, a solvent and light.

15. The method of Claim 14, wherein in the compound of formula II:

Ra and Rb independently represent H, C_1-6 alkyl (which latter group is optionally substituted with halo or OR_22a), or aryl (which latter group is optionally substituted with nitro, OR_23a); and Rc represents H, alkyl (which latter group is optionally substituted with halo or OR_24a), aryl (which latter group is optionally substituted with halo, nitro, OR_25a), O-alkyl (which latter group is optionally substituted with halo, OR_26a)), O-aryl (which latter group is optionally substituted with halo, nitro, OR_27a); and

each of R_22a, R_24a and R_27a independently represents H, halo or C_1-4 alkyl, which latter group is optionally substituted by one or more substituents selected from halo or OH.

16. The method of Claim 14, wherein the compound of formula (II) is lignin or a lignin-like polymeric material.

17. The method of any one of Claims 14 to 16, wherein the solvent comprises acetonitrile and/or ethyl acetate.

18. The method of any one of Claims 14 to 17, wherein the reaction is conducted at a temperature of from 0°C to 100°C, such as from 20°C to 35°C.

19. The method of any one of Claims 14 to 18, wherein the reaction is conducted in the presence of a gas that comprises oxygen.