GB2100260A - Catalytic hydrogenation and carbonylation of unsaturated compounds, and metal/phosphine complex catalysts - Google Patents

Abstract

A process for saturating or partially saturating and/or extending the carbon chain length of an unsaturated organic compound comprises contacting said compound with hydrogen and/or carbon monoxide in the presence as catalyst of a complex or compound of a platinum group metal, and an unsymmetrical biphosphine of general formula A-(CH2)n-B in which A and B are different phosphine moieties, A having the formula R2P and B having the formula either R'2P or RR'P where R represents an aryl group and R' represents an aralkyl, alkaryl or alkyl group and n is an integer between 1 and 10 inclusive, or an unsymmetrical monophosphine of formula R2R'P where R and R' are as defined above.

Description

SPECIFICATION Catalytic process/complex This invention relates to catalytic processes for the hydrogenation of unsaturated organic compounds and in particular provides a novel catalytic complex compound for use therein.

In our co-pending British patent application No. 8116362 filed concurrently herewith and we disclose a process for the preparation of symmetrical and unsymmetrical bisphosphine, some of which are novel compositions of matter.

We have now found that unsymmetrical bisphosphines may be used in association with known catalytically-active metals or compounds in catalytic processes and that the reactivity and/or selectivity of the catalyst is thereby modified. Catalytic processes of particular interest are those which saturate or partially saturate and/or extend the carbon chain length of an unsaturated organic compound. Such processes include hydrogenation and hydroformylation reactions. We have also found that unsymmetrical monophosphines are useful in enhancing reactivity and/or selectivity in such processes.

According to the invention, therefore, we provide a catalytic process for saturating or partially saturating and/or extending the carbon chain length of an unsaturated organic compound comprising contacting said compound with hydrogen and/or carbon monoxide in the presence as catalyst of a complex or compound of a platinum group metal, and an unsymmetrical bisphosphine of general formula A(CH2)nB in which A and B are different phosphine moieties, A having the formula R2P and B having the formula either R'2P or RR'P where R represents an aryl group and R' represents an aralkyl, alkaryl or alkyl group and n is an integer from 1 to 10 inclusive, or an unsymmetrical monophosphine of formula R2R'P where R and R' have the above definitions.The R and R' groups may be substituted, for example by an alkyl or an alkoxy group such as methyl or methoxy, preferably, for an aryl group, at the para position.

We have found that the use of an unsymmetrical mono or bisphosphine in a process according to the invention imparts to the catalysts remarkable selectivity in that, where a mixture comprising two or more unsaturated organic compounds is subject to the process, the catalyst is capable of distinguishing between different compounds with the result that one or some of said compounds is or are preferentially reacted.

While we cannot be certain of the chemical nature of the active catalytic species, we have found that it is convenient to add the platinum group metal and unsymmetrical phosphine to the reaction mixture in the form of a complex compound or, alternatively, to form a complex compound in situ, by adding a compound of platinum group metal to the unsymmetrical phosphine in solution. Such complex compounds where the phosphine comprises an unsymmetrical bisphsphine are novel compositions of matter. The invention also includes, therefore, a complex compound of a platinum group metal containing as ligand an unsymmetrical bisphosphine having the general formula A(CH2)nB, where A, B and n have the meanings as defined above.

By "platinum group metal" we mean platinum, rhodium, palladium, ruthenium and iridium and of these rhodium is especially useful.

Complex compounds of platinum group metal according to the invention contain, in addition to an unsymmetrical bisphosphine, ligands comprising H, CO and/or X where X is a halide or pseudohalide, preferably a halide, for example chloride. One suitable complex compound has the general formula IRhCI(CO)(A-(CH,),-B] , where A, B and n are as defined above and m is 1 or 2. A preferred range for n is from 3 to 6 inclusive.

The process according to the invention may be carried out in the presence or absence of a solvent for the reactants and/or in the presence of an excess of free phosphine and/or bisphosphine ligand.

Indeed, excess free ligand may itself act as a solvent or reaction medium. Where free mono or bisphosphine is present in excess, it may or may not be an unsymmetrical mono or bisphosphine.

Where it is desired to isolate complex compounds according to the invention before use in a hydrogenation or hydroformylation reaction, they may be prepared by reacting together a solution of the selected unsymmetrical bisphosphine in an organic solvent and a precursor complex of the platinum group metal prepared from a readily-available compound of the metal. For rhodium complex compounds, a suitable precursor complex is dio chlorobis [cycloocta-1 ,5-dienerhodium(l)l, [RhCl(cod)]2, prepared from rhodium trichloride and cycloocta-1 ,5-diene. Alternatively, where it is desired to prepare the complex compounds according to the invention in situ, the bisphosphine is added under hydrogen or argon to a solution of [RhCl(C2H4)2]2 prepared by stirring an ethanolic solution of rhodium trichloride under ethylene.The substrate compound is then either injected straight away or, if the bisphosphine was added under argon, is injected after substituting hydrogen for argon as the blanket gas. Alternatively, the complex compound may be formed in toluene and the substrate added dissolved in ethanol.

Hydrogenation reactions according to the invention may be carried out at ambient temperatures (less than 300C) under a slight positive pressure of hydrogen (for example 1.02 atmospheres).

We have found that complex compounds according to the invention exist as microcrystalline powders, generally having a pale yellow colour, and may be solvated. The solvent is generally removable under vacuum. They are air-stable, although exposure to air over periods of a few weeks causes oxidation of the phosphine ligands to phosphine oxides. Surprisingly, complex compounds according to the invention are generally readily soluble in common organic solvents compared with their symmetrical bisphosphine analogues. This is significant in a consideration of the compounds as homogeneous catalysts. A further advantage results from their giving two Pnmr signals instead of only one for the symmetrical analogues, in that this allows the two phosphine groups in a ligand to be separately monitored during, for example, investigations of the mechanism of a catalytic reaction.

The invention will now be more particularly described with reference to the following Tables, figures and examples, of which: Table 1 gives physical data of various carbonylchlorobisphosphine rhodium(l) complexes according to the invention; Table 2 gives elemental analysis data for certain of the complex compounds of Table 1; Table 3 shows results of catalytic hydrogenation experiments using a process and compounds according to the invention compared with various prior art compounds used as catalyst, and includes data for rates and selectivity ratios: : Fig 1 shows the rate of 1 -hexene hydrogenation using rhodium complexes including various phosphines as ligands, arranged in order of increasing rate; Fig 2 shows the rate of cyclohexene hydrogenation using various rhodium complexes, arranged in order of increasing rate; Fig 3 shows selectivity ratios for the hydrogenation of 1 -hexene compared with cyclohexenes, arranged in order of increasing rate; and Fig 4 shows selectivity ratios for the hydrogenation of 1 -hexene compared with a- acetamidocinnamic acid, arranged in order of increasing rate.

TABLE 1 Carbonylchlorobisphosphinerhodium (I) Complexes

Complex Colour M.p., ( C) v(C))a, (cm-1) #pb, (ppm) 1JRhP, (H2) C Trans-[RhCl(CO)(dph]2 Pale yellow 140 # 1961 +23.7 121 Trans-[RhCl(CO)(Etdph]2 Pale yellow 88 # 1965 +23.8(-PPh2) 121 # # Trans-[RhCl(C)(c-hexdph]2 Pale yellow 102 # 1958 +22.5(-PPhEt) 119 Trans-[RhCl(CO)(Etdpb]2 Pale yellow 130(dec) 1962 +23.8(-PPh2) 124 # # Trans-[RhCl(CO)(Etdpp]2 Pale yellow 142(dec) 1967 +22.6(-PPhEt) 121 Complex of Medph Orange 90 # 1960, 1800 Complex of Medph Orange 88 # 1962, 1814 RhCl(C))4(p-tol2P(CH2)6PPh2) Bright yellow 1968 a In nujol. b + ppm is downfield with respect to 85% H3PO4 (external standard) C prior art example.

Abbreviations: dph = Ph2P(CH2)6PPh2, dec = decomposed.

Meedph R = CH3, n = 6 Etdph R = C2H5, n = 6 c-Hexdph R = cyclo C6H11, n = 6 Medpb R = CH3, n = 4 # in RPhP*(CH2) PPh2 where indicates a chiral phosphorus atom Etdpb R = C2H5, n = 4 Etdpp R = C2H5, n = 5 TABLE 2 Yields and Analytical Data for Carbonylchlorobisphosphinerhodium (I) Complexes

Complex Yield, (%) Analysis (th.), (%) [RhCl(C)) (dph)]2 100 C 59.7(60.0), H 5.2(5.2), Cl 5.5(5.7) [RhCl(CO) (Etdph)]2 83 C 54.6(56.6), H 5.5(5.6), Cl 6.4(6.2), P 10.3(10.8) [RhCl(CO) (c-hexdph]2 73 C 58.4(59.3), H 5.8(6.1), Cl 6.0(5.7), P (9.8(9.9) [RhCl(CO) (Etdpb)]2 83 C 53.9(55.1), H 5.2(5.2), Cl 6.5(6.5), P 10.7(11.4) [RhCl(CO) (Etdpp)]2 78 C 53.4(54.2), H 5.1(4.9) P 11.8(11.7) RhCl2(C))4(p-tol2P(CH2)6PPh2) 78 C 49.6(50.0), H 4.9(4.2), Cl 8.2(8.2) Tables 1 and 2 show data for various complexes according to the invention compared with a symmetrical bisphosphine-containing complex, the symmetrical bisphosphine being Ph2(CH2)6PPh2, (dph), which were prepared by stirring a benzene solution of the phosphine and di-y-chlorobis [cycloocta-1 ,5-dinerhodium(l)], [RhCl(COD)]2, under an atmosphere of carbon monoxide. The data includes melting point, infrared spectra, 31P nmr spectra, and elemental analysis.

Structure determination.

It is known from studies of known compounds having symmetrical bisphosphine ligands (such as dph) that the ir stretching frequency of the carbonyl ligand is related to the ligand trans to it and that, where the trans ligand is chloride, the carbonyl stretching frequency is in the region 1950-1975 cm~'.

Since carbonyl stretching frequencies in this region were found for complexes according to the invention, it was concluded that the carbonyl and chloro ligands were mutually trans and that in consequence the phosphorus groups of the bisphosphine ligands were also mutually trans. X-ray crystallography studies or molecular weight measurements were required to determine whether the complexes were monomeric, trans-chelated, or dimeric, trans-bridged, as follows:-

trans-chelated

trans-bridged Such measurements for the Etdph complex indicated a dimeric structure but were not easily carried out on all compounds due to a tendency to low solubility. By comparison of various other data with that for the Etdph complex, a dimeric structure was assigned by analogy to most other complexes.

The complexes with Medph and Medpb, however, appeared to contain bridging carbonyl groups as well as trans carbonyl-chloride groups, making structure determination uncertain, and the unsymmetrical but achiral complex with p-tol2P(CH2)6PPh2 gave anomalous analytical and infra red data.

Complex compounds according to the invention are generally readily isolatable as pale yellow micro-crystaliine powders frequently solvated with one benzene molecule per dimeric complex molecule; the benzene is removable under a high vacuum.

TABLE 3 Experimental Results of Catalytic Hydrogenations [Rh] = 5 mmol.dm-1; P/Rh = 2; substrate/Rh = 100; PH2 = 1.02 atm.; T = 2BwC solvent = EtOH/toluene, (2:1 v/v)

Selectivity Ratio (see Phosphine Vmax, a(cm H2/min) with substrate:: discussion of Figs 3 and 4) α-Acetamido- | Rate 1-hex | Rate 1-hex 1-Hexene Cyclohexene cinnamic acid Rate c-hex Rate o-acet * Ph3P 6.9 4.4 0.8 1.6 8.6 MePh2P 5.5 1.4 0.8 3.9 6.9 EtPh2P 8.5 (7.0) 0.7 0.2 12.1 42.5 'PxPh2P 7.5 0.4 - 18.8 c-hex0h2 P 2.5 1.5 - 1.7 BzPh2P - 3.8 * Ph2PCH2PPh2 7.5 5.8 0.0 1.3 * Ph2P(CH2)2PPh2 7.4 10.3 0.1 0.7 74.0 * Ph2P(CH2)PPh2 7.3 3.4 - 2.1 EtPhP(CH2)3PPh2 | 5.5 | 1.3 | - | 4.2 * Ph2P(CH2)4PPh2 4.4 0.5 1.2 8.0 3.7 EtPhP(CH2)4PPh2 7.1 0.6 0t4 11.8 17.8 * Ph2P(CH2)6PPh2 10.3(4.2)c 7.5 0.8(0.8)0 1.4 12.9 EtPhP(CH2)6PPh2 4.7(1.0)C 0.5 0.0 9.4 c-hexPhP(CH2)6PPh2 4.0 2.8 1.5 1.4 2.7 a Maximum rate, usually taken over the first five minutes, but in some cases an induction period occurred; catalysts preformed under hydrogen. For turnover rate, multiply value in table by factor of 0.71.

* Prior art example.

Turning now to Table 3, results are shown for the hydrogenation according to the invention of unsaturated substrates exemplified by 1 -hexene, cyclohexene and a-acetamido-cinnamic acid, compared with various prior art processes. In these experiments catalysts were prepared in situ from [RhCl(C2H4)2]2 and phosphine in a stoichiometric ratio of 2:1 phosphine:rhodium. Ethanol was added to the toluene as reaction solvent to increase the polarity and hence the reaction rate.

The catalyst was generally prepared by stirring the phosphine and the complex in the solvent under a hydrogen atmosphere for half an hour in the catalytic cell. The unsaturated compound was then injected. Some catalysts were also prepared under an inert gas (argon) atmosphere which was then replaced with hydrogen.

In the hydrogenation reaction, hydrogen uptake generally began immediately on adding substrate and proceeded to a quantitative end-point. In some experiments, there was an induction period between adding the substrate and the beginning of hydrogen uptake. Repeated addition of fresh substrate once the reaction had ceased indicated little or no catalyst degradation, even after 1000 tumovers, although at this stage there was an apparent drop in reaction rate due to the dilution of reactant substrate with product and the concomitant decrease in polarity of the solvent. Reproducibility of results was generally good, exceptions being for the easily oxidised MePh2P and EtPh2P phosphines which were very mobile liquids difficult to handle in small quantities; rates for these varied by as much as 20%.

Fig 1 compares the rates of catalysis by complexes containing different phosphines for hydrogenation of 1 -hexene. There is no apparent correlation between rate and structure of phosphine, there being a factor of only five between the slowest and the fastest systems.

In Fig 2, which compares rates for hydrogenation of cyclohexane, more pronounced rate differences are apparent, there being a factor of almost 40 between the slowest and fastest systems. We believe that this may be due to this substrate being more sterically demanding than a terminal alkene. Again, there is no apparent correlation between rate and phosphine structure.

Hydrogenation of a-acetamido-cinnamic acid shows that rates were generally slower than with the other two substrates; this may be due to the amine group being coordinated to the metal and therefore not easily dissociated from the catalyst. No correlation was evident between rate and phosphine structure.

We have found that the apparent lack of correlation between rate and phosphine structure is due to the effect of both electronic and steric factors which, in the resultant, have a partial masking effect on each other. To achieve an accurate assessment of the effect of structure on rate, it is therefore necessary to reduce the electronic factor to insignificant levels and then to observe the effect of the steric factor. This was achieved on the assumption that 1-hexene, being a terminal normal alkene, is sterically undemanding, that is, the terminal unsaturated groups can approach the catalyst with equal ease from any direction, irrespective of the ligands present. The only effect influencing rate of hydrogenation is thus the electronic effect.

Cyclohexene, on the other hand, is likely to be more sterically demanding but the electronic effects may be assumed to be similar to those influencing the rate of 1 -hexene. Results for cyclohexene hydrogenation can therefore be corrected to show only the effect of steric influence. We have therefore calculated the selectivity ratio, that is, the ratio of the rate with 1 -hexene to that with a second substrate, for the various complexes, and a correlation between rate and structure is indeed observed, as shown in Fig 3 for the selectivity ratio of cyclohexene:1 -hexene and in Fig 4 for the selectivity ratio of a-acetamidocinnamic acid:1 -hexene.

The catalysts of those phosphines which are "sterically symmetrical" exhibit a lower selectivity ratio between 1 -hexene and cyclohexene, of 0.7 to 2.1. Here "sterically symmetrical" is taken to mean, in the case of monodentate ligands, all three groups on phosphorus being sterically similar, and in the case of bidentate ligands, the two phosphine groups attached to the backbone being sterically similar.

Phenyl and cyclohexyl groups are regarded as being sterically similar. The ligands Ph 3P, c hexPh2P(CH2)nPPh2 (n = 1,2,3,4,6), and c-hexPhP(CH2)6PPh2 all fall into this class and all show the low selectivity ratio.

In contrast, the complexes of stericaily unsymmetrical phosphines had higher selectivity ratios, of 3.9 to 1 8.8. These phosphines are of the types RPh2P or RPhP(CH2)nPPh2 (where R * Ph or c-hex, but is an alkyl group such as Me, Et or Per). All six of the ligands of this type showed a much greater ability to distinguish between 1-hexene and cyclohexene.

The above hypothesis indicates that the groups attached to phosphorus are important, by virtue of their steric properties, for steric recognition of a substrate by a catalyst. This suggests that altering the stereo-chemistry at phosphorus enables a catalyst to be tailored to a specific substrate, hence improving the stereospecificity of the catalyst.

Cyclohexyl fits in this scheme as sterica!ly similar to phenyl, despite the fact that electronically it would be more similar to alkyl than phenyl. This supports the assumption that electronic effects can be eliminated to allow the steric effects to be investigated.

We have observed similar effects in the use of a-acetamidocinnamic acid as the second substrate, although the relationship between phosphine structure and selectivity ratio is not as clear with cyclohexene. We believe that this is due to the electronic situation with (x-acetamidocinnamic acid being more different from 1 -hexene that is cyclohexene, since the -NHCOCH3 and -COOH functional groups in this substrate exert extensive electronic effects.Nevertheless, a certain general trend is observed, in that for a given backbone, exchanging phenyl for ethyl at phosphorus always greatly increases the selectivity ratio, for example: Phosphine Selectivity Ratio (rate 1-hex/rate a-acet.) Ph3P 8.6 EtPh2P 42.5 Ph2 P(CH2)4 PPh2 3.7 EtPhP(CH2)4 PPh2 17.8 Ph2P(CH2)6PPh2 12.9 EtPhP(CHz)6 PPh, Thus, other things being equal, lowering the symmetry of the phosphine ligand increases the degree of recognition between catalyst and substrate.

This "lock and key" effect is a mutual phenomenon -- both catalyst and substrate should have as many steric factors as possible for them to readily recognise each other and assemble together in a stereo-chemically precisely-defined structure. The fact that the sterically less symmetrical phosphines generally give high selectivity ratios is consistent with their provision of more steric features to control or influence the approach, orientation, interaction and hydrogenation of the substrate.

This argument, considered from the opposite viewpoint, predicts that sterically less symmetrical substrates should generally give higher selectivity ratios than more symmetrical analogues, other things being equal. a-Acetamidocinnamic acid is much less symmetrical than cyclohexene, that is, it has more features for the catalyst to recognise, and indeed the selectivity ratios are generally much higher for ez- acetamidocinnamic acid (values often and higher being common) than for cyclohexene (values mostly less than two). Thus with the symmetrical phosphines Ph3P, P h2P(CH2)2PPh2 and Ph2P(CH2)6PPh2, selectivity ratios are 8.6, 7.4 and 12.9 respectively for a-acetamidocinnamic acid, and 1.6, 0.7 and 1.4 respectively for cyclohexene. Comparing the Fig 4 (for a-acetamidocinnamic acid) with Fig 3 (for cyclohexene), it is striking that selectivity ratios vary widely in the former case, but in the latter, the broad plateau covering most of the sterically symmetrical phosphines suggests that cyclohexene is a relatively "featureless" substrate, and no matter what (symmetrical) phosphine is present, is hydrogenated at about two-thirds of the corresponding rate for 1 -hexene. Most surprisingly, we have found that the unsymmetrical phosphines are indeed able to distinguish from one another such featureless and almost isomeric molecules as cyclohexene and 1 -hexene, the chemical properties of which are so similar as to make them quite difficult to distinguish by prior art processes.

Claims (3)

1. A catalytic process for saturating or partially saturating and/or extending the carbon chain length of an unsaturated organic compound comprising contacting said compound with hydrogen and/or carbon monoxide in the presence as catalyst of a complex or compound of a platinum group metal, and an unsymmetrical bisphosphine of general formula A(CH2)nB in which A and B are different phosphine moieties, A having the formula R2P and B having the formula either R'2P or RR'P where R represents an aryl group and R' represents an aralkyl, alkaryl or alkyl group and n is an integer between 1 and 10 inclusive, or an unsymmetrical monophosphine of formula R2R'P where R and R' are as defined above.

2 A complex compound of a platinum group metal containing as ligand an unsymmetrical bisphosphine having the general formula A(CH2)nB where A has the formula R2P and B has the formula either P'2P or RR'P where R is an aryl group, R' is aralkyl, alkaryl or alkyl group and n is an integer from 1 to 10 inclusive.

3. A process or compound as claimed in Claim 1 or Claim 2 respectively in which R and/or R' are/is substituted.