WO2014012908A1

WO2014012908A1 - Transition metal carbene complex catalysed method for producing carboxylic acid esters from alcohols under dehydration

Info

Publication number: WO2014012908A1
Application number: PCT/EP2013/064957
Authority: WO
Inventors: Thomas Schaub; Stefan Rüdenauer; Daniel Schneider; Rocco Paciello
Original assignee: Basf Se; Basf Schweiz Ag
Priority date: 2012-07-17
Filing date: 2013-07-16
Publication date: 2014-01-23
Also published as: JP2015526414A; CN104470887A; CN104470887B; MX2015000827A; EP2874991A1

Abstract

The present invention relates to a method for producing carboxylic acid esters, comprising the reaction of at least one primary mono alcohol or a mixture of a primary mono alcohol and at least one alcohol differing from this in the presence of a transition metal carbene complex catalyst (K), which has as the central atom (M) at least one transition metal atom of group 8, 9 or 10 of the periodic table (IUPAC) and at least one monodentate N-heterocyclic carbene ligand, in the presence of a base, characterized in that the catalyst (K) is produced by reaction of a transition metal compound (V), which has at least one transition metal atom of group 8, 9 or 10 of the periodic table (IUPAC), but no carbene ligand, with an imidazolium salt in the presence of the primary mono alcohol and the base, wherein the reaction is a bulk reaction.

Description

TRANSITION METAL COLOR COMPLEX CATALYZED METHOD FOR

PREPARATION OF | CARBOXYLIC ACIDES FROM ALCOHOLS UNDER

DEHYDRATION

Description: The present invention relates to a process for the preparation of carboxylic acid esters by reacting at least one primary monoalcohol or a mixture of a primary monoalcohol and at least one different alcohol in the presence of a transition metal carbene complex catalyst K. Carboxylic acid esters are important chemical compounds which are used, for example, as solvents, plasticizers and Fragrances and flavorings are used. There are various methods for their preparation, wherein the reaction of a carboxylic acid with an alcohol is the most common: R ^a COOH + HOR ^b R ^a COOR ^b + H ₂ 0

Since this is an equilibrium reaction, which is usually acid catalysed, the water or carboxylic ester must be removed from the reaction mixture in order to achieve high conversions. Especially in the case of low-boiling carboxylic acids and low-boiling alcohols, undesirable azeotropes often form, which make the separation more difficult.

Another way of producing carboxylic acid esters is the reaction of an acid anhydride with an alcohol:

R ^a COOCOR ^a + HOR ^b R ^a COOR ^b + R ^a COOH

In this case, one equivalent of the carboxylic acid is formed, which must also be separated. Again, azeotropes often form, and two equivalents of the carboxylic acid is needed for one equivalent of the carboxylic acid ester.

Another route for the preparation of carboxylic acid esters is the transition metal complex-catalyzed reaction of alcohols under dehydrogenation, hereinafter referred to as direct ester formation. Alcohols are used as educts, at least one of the two reactants being a primary OH group, ie. H. a CH20H group must have:

R ^a CH ₂ OH + R OH R ^a COOR + 2 H ₂ In this type of carboxylic acid ester synthesis no carboxylic acid must be used, which is advantageous, since especially the lower carboxylic acids have an unpleasant odor. In addition, no water is formed in the reaction, which simplifies the work-up by distillation of the reaction mixture. As a result of the fact that the reaction conditions can be kept fairly mild, only a few by-products are formed.

The conversion of primary alcohols into carboxylic acid esters under catalysis with transition metal complexes has been described several times in the literature.

Tetrahedron Lett. 1981, 22, 5327-5330 and J. Org. Chem. 1987, 52, 4319-4327 describe the use of ruthenium complexes with phosphine ligands as catalysts for the preparation of carboxylic acid esters and lactones from primary alcohols.

J. Organomet. Chem. 1985, 282, C7-C10 describes catalysts for the preparation of carboxylic acid esters from primary alcohols using a ruthenium complex with tetraphenylcyclopentadienone ligand. Chem. Rev. 2010, 1 10, 681-703 describes various transition metal complex catalysts for direct ester formation from primary alcohols.

Organometallics 201 1, 30, 2180-2188, Organometallics 201 1, 30, 5716-5724 and J. Am. Chem. Soc. 2005, 127, 10840-10841 describe ruthenium tweezer complexes catalyzing the direct esterification of primary alcohols.

M. Nielsen et al., Angew. Chem. 2012, 124 describes ruthenium and iridium complex catalysts with multidentate nitrogen and phosphorus-organic ligands for the synthesis of ethyl acetate from ethanol.

Organometallics 201 1, 30, 6044-6048 describes ruthenium complex compounds based on N-heterocyclic carbenes as catalysts in the direct ester formation from primary alcohols. The ruthenium complex is advantageously added a phosphine.

A disadvantage of the methods described for the preparation of carboxylic acid esters is that the transition metal complex catalysts used have to be prepared by complex syntheses and optionally isolated. In addition, often oxidation-sensitive and expensive phosphine ligands are used, which is a complicated process Requires reaction under inert conditions. Furthermore, it is predominantly worked in larger amounts of an added solvent, whereby the reaction mixture must be worked up consuming and reduces the space-time yield.

It is an object of the present invention to provide a process for the transition metal complex catalyzed direct ester formation from primary monoalcohols or from mixtures of a primary monoalcohol and at least one different alcohol.

It has now surprisingly been found that this object is achieved by a process in which a primary monoalcohol or a mixture of a primary monoalcohol and at least one different alcohol in the presence of a base and at least one defined in more detail below Übergangsmetall- talcarbenkomplexkatalysators K a Transition metal atom of group 8, 9 or 10 of the Periodic Table (IUPAC) in substance, wherein preparing the catalyst K by reacting a suitable non-carbenoid transition metal compound V with an imidazolium salt in the presence of the primary monoalcohol and the base.

Accordingly, the present invention relates to a process for the preparation of carboxylic acid esters, comprising reacting at least one primary monoalcohol or a mixture of a primary monoalcohol and at least one different alcohol in the presence of a Übergangsmetallcarbenkomplexka- catalytic K, which as the central atom M at least one transition metal atom of

Group 8, 9 or 10 of the Periodic Table (IUPAC) and at least one monodentate N-heterocyclic carbene ligand, in the presence of a base, characterized in that the catalyst K by reacting a transition metal compound V, the at least one transition metal atom of group 8, 9 or 10 of the Periodic Table (IUPAC), but having no carbene ligand, with an imidazolium salt in the presence of the primary monoalcohol and the base prepares to carry out the reaction in bulk.

Advantages of the process according to the invention are above all the mild reaction conditions, the low formation of by-products with simultaneously high space-time yield. In addition, it can be avoided in this way that the catalyst system must be prepared separately. Another advantage is the simplified work-up of the reaction mixture, since no solvent must be separated due to the reaction in substance. In addition, the low sensitivity to oxidation of the Catalyst system a reaction even under non-inert conditions. In addition, the method avoids the use of lower carboxylic acids which have an unpleasant odor. In the process according to the invention, at least one catalyst K which contains at least one transition metal atom of group 8, 9 or 10 of the Periodic Table (IUPAC) is used. Iron, cobalt, nickel, ruthenium, rhodium, palladium, osmium, iridium and platinum are among the transition metals of Group 8, 9 and 10 of the Periodic Table (IUPAC). Preference is given to catalysts K which contain at least one transition metal atom selected from ruthenium and iridium. Particularly preferred transition metal is ruthenium.

The catalyst K has at least one, z. B. 1, 2, 3 or 4, N-heterocyclic carbene ligands. Particularly suitable N-heterocyclic carbene ligands are imidazole ligands wherein the carbon in the 2-position is the carbenoid donor atom. These include in particular those of the formula (I),

wherein

R ¹ and R ⁴ independently of one another are C 1 -C 10 -alkyl, C 3 -C 10 -cycloalkyl, 5 to 10-membered heteroaryl which has 1, 2 or 3 heteroatoms selected from O, N and S as ring members or aryl, the four last-mentioned substituents are unsubstituted or may be substituted by one or more substituents selected from halogen, C ₁ -C 10 -alkoxy, CN, C ₁ -C 10 -alkyl, C ₃ -C 10 -cycloalkyl, phenyl and naphthyl;

R ² and R ³ independently of one another are hydrogen or have one of the meanings given for R ¹ or R ⁴ .

Here and below, the prefix "C _p - C _q -" used to define substituents indicates the number of possible C atoms of the substituent. Unless otherwise stated, the following general definitions apply to the terms used in connection with the substituents in the context of the present invention: "C 1 -C 10 -alkyl" denotes a linear or branched alkyl radical having 1 to 10 carbon atoms. Examples of C 1 -C 10 -alkyl are methyl, ethyl, n-propyl, isopropyl, n-butyl, 2-butyl, isobutyl, tert-butyl (2-methylpropan-2-yl), n-pentyl ( Amyl), 1-methylbutyl, 2-methylbutyl, 3-methylbutyl, 1, 1-dimethylpropyl, 1, 2-dimethylpropyl, 2,2-dimethylpropyl, 1-ethylpropyl, n-hexyl, 1-methylpentyl, 2-methylpentyl, 3 Methylpentyl, 4-methylpentyl, 1, 1-dimethylbutyl, 1, 2-dimethylbutyl, 1, 3-dimethylbutyl, 2,2-dimethylbutyl, 2,3-dimethylbutyl, 3,3-dimethylbutyl, 1-ethylbutyl, 2-ethylbutyl , 1, 1, 2-trimethylpropyl, 1, 2,2-trimethylpropyl, 1-ethyl-1-methylpropyl, 1-ethyl-2-methylpropyl, heptyl, octyl, 2-ethylhexyl, nonyl, decyl and their constitutional isomers. "C3-Cio-Cycloalkyl" represents a mono-, di-, tri- or tetracyclic alkyl radical having 3 to 10 carbon atoms. Examples are cyclopropyl, cyclobutyl, cyclopentyl, cyclohexyl, cycloheptyl, cyclooctyl and adamantyl.

"Aryl" represents an aromatic hydrocarbon radical which may be substituted or unsubstituted. Examples of unsubstituted aryl are phenyl, 1-naphthyl, 2-naphthyl and 9-anthryl. Examples of aryl which may be substituted with one or more C 1 -C 10 alkyl radicals as defined above are 2,6-di (isopropyl) phenyl, o-tolyl, m-tolyl, p-tolyl and mesityl. "Ci-Cio-alkoxy" stands for an alkyl group having 1 to 10 carbon atoms bonded via an oxygen atom. Examples are methoxy, ethoxy, n-propoxy, 1-methylethoxy (isopropoxy), n-butoxy, 1-methylpropoxy (sec-butoxy), 2-methylpropoxy (isobutoxy), 1, 1-dimethylethoxy (tert-butoxy), n-pentoxy, 1-methylbutoxy, 2-methylbutoxy, 3-methylbutoxy, 1, 1-dimethylpropoxy, 1, 2-dimethylpropoxy, 2,2-dimethylpropoxy, 1-ethylpropoxy, 2-ethylpropoxy, n-hexoxy, 1-methylpentoxy, 2-methylpentoxy, 3

Methylpentoxy, 4-methylpentoxy, 1-ethylbutoxy, 2-ethylbutoxy, 3-ethylbutoxy, 1, 2-dimethylbutoxy, 1, 3-dimethylbutoxy, 2,3-dimethylbutoxy, 1-ethyl-2-methylpropoxy and 1-isopropylpropoxy. "5- to 10-membered heteroaryl" denotes a mono- or bicyclic 5- to 10-membered aromatic ring which has 1, 2 or 3 heteroatoms selected from O, N and S as ring members. Examples are thienyl, benzothienyl, 1-naphthothienyl, thianthrenyl, furyl, benzofuryl, pyrrolyl, imidazolyl, pyrazolyl, pyridyl, pyrazinyl, pyrimidinyl, pyridazinyl, indolyl, isoindolyl, indazolyl, purinyl, isoquinolinyl, Quinolinyl, acridinyl, naphthyridinyl, quinoxalinyl, quinazolinyl, cinnolinyl, piperidinyl, carbolinyl, thiazolyl, oxazolyl, isothiazolyl and isoxazolyl.

"Aliphatic olefins" are linear or branched, monounsaturated or polyunsaturated hydrocarbons having generally 2 to 10 carbon atoms. Examples are ethene, propene, 1-butene, 2-butene, 1, 3-butadiene and 2-methyl-prop-1-ene.

"Cycloolefins" are cyclic, monounsaturated or polyunsaturated hydrocarbons having in general from 4 to 10 carbon atoms. Examples are cyclobutene, cyclopentene, cyclohexene and 1,5-cyclooctadiene.

"Carbocyclic aromatics" are unsubstituted or monosubstituted or polysubstituted with alkyl residues aromatic compounds having generally 6 to 10 carbon atoms. Examples are benzene, naphthalene and p-cymene.

"Heteroaromatics" are unsubstituted or substituted aromatic compounds having generally from 5 to 10 ring atoms and having at least one heteroatom selected from O, N and S. Examples are furan, pyrrole, thiophene, imidazole, pyrazole, oxazole, isoxazole and thiazole.

"Aldehydes" are linear or branched aldehydes, generally having 1 to 10 carbon atoms. Examples are formaldehyde, acetaldehyde and propionaldehyde. "Ketones" are linear or branched ketones, generally having from 3 to 10 carbon atoms. Examples are acetone, butanone, 2-pentanone and 3-pentanone.

"Ci-Cio-carboxylate" are anions of saturated or unsaturated carboxylic acids having 1 to 10, in particular 1 to 4 carbon atoms. Examples of C 1 -C 10 -carboxylate are formate, acetate, acrylate, methacrylate and propionate.

"C 1 -C 10 -alkoxide" are radicals of linear or branched alcohols having 1 to 10, in particular 1 to 4, carbon atoms. Examples are methoxide, ethoxide, propoxide, n-butylate, 2-butylate and tert-butylate.

In formula (I), the variables R ¹ , R ² , R ³ and R ^4, independently of one another and in particular in combination, have the following meanings: In formula (I), R ¹ and R ⁴ independently of one another preferably represent a radical selected from C ¹ -C 10 -alkyl, C 3 -C 10 -cycloalkyl, benzyl and phenyl, where the phenyl ring is unsubstituted in the last two radicals or one or more times , z. B. 1 -, 2- or 3-fold, is substituted with Ci-C3-alkyl. In particular, R ¹ and R ^{4 are} C ₁ -C 10 -alkyl, especially C ₁ -C ₆ -alkyl.

In formula (I), R ² and R ³ independently of one another preferably represent a radical selected from hydrogen, C ¹ -C 10 -alkyl, C ³ -C 10 -cycloalkyl, benzyl and phenyl, the phenyl ring being unsubstituted or being unsubstituted in the latter two groups. or several times, z. B. 1 -, 2- or 3-fold, is substituted with Ci-C3-alkyl. In particular, R ² and R ³ represents hydrogen or Ci-Cio-alkyl, especially hydrogen or Ci-C6 alkyl, and most especially hydrogen.

In addition to at least one monodentate N-heterocyclic carbene ligand, the catalyst K may have at least one further ligand L. Ligands L are preferably selected from CO, hydride, aliphatic olefins, cycloolefins, carbocyclic aromatics, in particular benzene, naphthalene and p-cymene, heteroaromatics, in particular furan, pyrrole, imidazole and pyrazole, aldehydes, in particular formaldehyde, acetaldehyde and propionaldehyde, ketones , in particular acetone, butanone, 2-pentanone and 3-pentanone, halides, C 1 -C 10 -carboxylate, methylsulfonate, methylsulfate, trifluoromethylsulfate, tosylate, mesylate, cyanide, isocyanate, cyanate, thiocyanate, hydroxide, C 1 -C 10 -alkoxide, cyclopentadienide, Pentamethylcyclopentadienide and pent-tabenzylcyclopentadienide. Particularly preferred ligands L are p-cymene, chloride, CO, hydride, C 1 -C 10 -alkoxide and C 1 -C 10 -carboxylate.

Imidazolium salts which can be used in the process according to the invention are, above all, imidazolium salts of the general formula (II)

in which R ¹ , R ² , R ³ and R ^{4 have} the abovementioned meanings, in particular the meanings given there as being preferred, in particular or specifically indicated,

A stands for H or COO ^" and x- for the equivalent of an anion, in particular for halide, C 1 -C 10 -carboxylate, benzoate, MeCeHUCOO ^" , tosylate, methanesulfonate, trifluoromethanesulfonate, mesylate, cyanide, isocyanate, thiocyanate, tetrachloroaluminate, tetrabromoaluminate, tetrafluoroborate, hexafluorophosphate , Hexafluoroantimonate, sulfate, hydroxide

Bis (trifluoromethanesulfonyl) imide or methylsulfate, provided that X ^"is absent when A is COO ^" .

In particularly preferred imidazolium salts of the formula (II), the variables X-, A, R ¹ , R ² , R ³ and R ^4, independently of one another and in particular in combination, have the following meanings:

X- is preferably chloride, tosylate, methanesulfonate, trifluoromethanesulfonate, mesylate, tetrafluoroborate, hexafluorophosphate, hexafluoroantimonate, C 1 -C 10 -carboxylate, sulfate or methylsulfate, in particular chloride, methanesulfonate or C 1 -C 10 -carboxylate.

A is preferably hydrogen; R ¹ and R ⁴ are each, independently of one another, preferably a radical selected from C ¹ -C 10 -alkyl, C 3 -C 10 -cycloalkyl, benzyl and phenyl, where the phenyl ring is unsubstituted in the last two radicals or is mono- or polysubstituted, for example. B. 1 -, 2- or 3-fold, is substituted with Ci-C3-alkyl. In particular, R ¹ and R ^{4 are} C ₁ -C 10 -alkyl, especially C ₁ -C ₆ -alkyl;

R ² and R ³ independently of one another preferably represent a radical selected from hydrogen, C ¹ -C 10 -alkyl, C ³ -C 10 -cycloalkyl, benzyl and phenyl, where the phenyl ring is unsubstituted in the last two radicals or is mono- or polysubstituted, for example. B. 1 -, 2- or 3-fold, is substituted with Ci-C3-alkyl. In particular, R ² and R ³ represents hydrogen or Ci-Cio-alkyl, especially hydrogen or Ci-C6-alkyl and very especially hydrogen.

Examples of suitable imidazolium salts of the formula (II) are 1,3-dimethylimidazolium salts, 1-methyl-3-iso-propylimidazolium salts, 1,3-diethylimidazolium salts, 1-methyl-3-n-propylimidazolium salts, 1-methyl- 4-n-butylimidazoliumsalze,

1, 3,4,5-tetramethylimidazolium salts, 1, 3-di-n-propylimidazolium salts,

1,3-di-iso-propylimidazolium salts, 1,3-di-n-butylimidazolium salts,

1,3-di-sec-butyl-imidazolium salts, 1,3-di-tert-butyl-imidazolium salts,

1, 3-di-cyclohexylimidazolium salts 1, 3-di-adamantylimidazolium salts, 1, 3-diphenylimidazolium salts, 1, 3-ditolylimidazolium salts,

1, 3-di-xylylimidazolium salts, 1, 3-di-mesitylimidazolium salts,

1, 3-bis [2,6-di (isopropyl) phenyl] imidazolium salts,

in particular their chlorides, tosylates, methanesulfonates, trifluoromethanesulfonates, mesylates, tetrafluoroborates, hexafluorophosphates, hexafluoroantimonates, sulfates, Ci-Cio-carboxylates and methyl sulfates, especially the chlorides, methanesulfonates and C1-C10 carboxylates.

N-heterocyclic carbenes have similar ligand properties as trialkyl or triaryl phosphanes, but N-heterocyclic carbenes have the advantage over phosphanes of not being sensitive to oxidation. N-heterocyclic carbenes based on 1,3-substituted imidazoles are very readily accessible synthetically, see, for example, W.A. Herrmann, Angew. Chem. 2002, 41, 1290-1309; E. Peris, Top. Organomet. Chem. 2007, 21, 83-1 16; T.N. Tekavec et al., Top. Organomet. Chem. 2007, 21, 159-192; F. Glorius, Top. Organomet. Chem. 2007, 21, 1 -20, to which reference is made in its entirety.

The imidazolium salts on which the N-heterocyclic carbenes are based are commercially available and are known, for example, as ionic liquids as well as solvents.

The monodentate N-heterocyclic carbene ligand is prepared from the imidazolium salt in the presence of at least one base. Suitable bases are, for example, selected from hydrides, hydroxides, carbonates, alcoholates and amides of the alkali metals, the hydrides, hydroxides, carbonates, alkoxides and amides of the alkaline earth metals, organic amines, aryllithium compounds and alkyllithium compounds. Preferably, the bases are selected below

B1 alkali metal hydroxides, in particular LiOH, NaOH or KOH

B2 alkaline earth metal hydroxides, in particular Ca (OH) 2

B3 alkali metal hydrides, in particular LiH, NaH, KH

B4 alkaline earth metal hydrides, especially CaH2

B5 alkali metal aluminum hydrides, especially UAIH4

B6 alkali metal borohydrides, in particular NaBH ₄ , LiBH ₄

B7 alkali metal carbonates, in particular Na 2 CO 3, U 2 CO 3, K 2 CO 3

B8 alkali metal phosphates, especially K3PO4, Na3P0 ₄ B9 alkyl lithium compounds, in particular n-butyllithium, methyllithium, tert-butyllithium

B10 Aryllithium compounds, in particular phenyllithium

B1 1 alkali metal alcoholates, in particular lithium methoxide, lithium ethoxide, lithium n-propylate, lithium iso-propylate, lithium n-butoxide, lithium iso-butoxide, lithium n-pentylate, lithium n-hexylate, lithium -n-heptylate, lithium n-octylate, lithium benzylate, lithium phenolate, potassium methoxide, potassium ethanolate, potassium n-propylate, potassium isopropylate, potassium n-butylate, potassium iso-butoxide, potassium n-pentylate , Potassium n-hexylate, potassium n-heptylate, potassium n-octylate, potassium benzylate, potassium phenolate, sodium methoxide, sodium ethoxide, sodium n-propylate, sodium iso-propylate, sodium n-butoxide, sodium iso butyl acrylate, sodium n-pentylate, sodium n-hexylate, sodium n-heptylate, sodium n-octylate, sodium benzylate, sodium phenolate and the constitutional isomers of the alkali metal alcoholates mentioned B12 alkali metal bis (trimethylsilyl) amides, in particular Potassium bis (trimethylsilyl) amide, lithium bis (trimethylsilyl) amide, sodium bis (trimethylsilyl) amide

B13 amines of the formula R ⁵ NH ₂ , where R ^{5 is} substituted or unsubstituted C ₁ -C 10 -alkyl, C ₁ -C ₄ -alkyl-P (phenyl) ₂ , C ₃ -C 10 -cycloalkyl, C ₁ -C 10 -heterocyclyl, where C ₃ Cio-heterocyclyl at least one heteroatom selected from N, O and S, Cs-Cu-aryl or d-do-heteroaryl, wherein d-do-heteroaryl contains at least one heteroatom selected from N, O and S, is

B14 amines of the formula R ⁶ R ⁷ NH, where R ⁶ and R ⁷ independently of one another are substituted or unsubstituted C 1 -C 10 -alkyl, C 1 -C ₄ -alkyl-P (phenyl) 2, C 1 -C 10 -cycloalkyl, C ₃ -C 10 -alkyl Heterocyclyl, in which C 3 -C 10 -heterocyclyl contains at least one heteroatom, selected from among N, O and S, Cs-Cu- aryl or Cs -do-

Heteroaryl, wherein d-do-heteroaryl contains at least one heteroatom selected from N, O and S, are

B15 amines of the formula R ⁸ R ⁹ R ¹⁰ N, where R ⁸ , R ⁹ and R ¹⁰ independently of one another represent substituted or unsubstituted d-do-alkyl, C 1 -C ₄ -alkyl-P (phenyl) 2, d-do-cycloalkyl, d-doheterocyclyl, wherein d-Cio-heterocyclyl contains at least one heteroatom selected from N, O and S, d-Ci ₄ -aryl or d-to-heteroaryl, wherein d-do-heteroaryl selected at least one heteroatom under N, O and S, contains

Particularly preferred are alkali metal hydroxides, especially potassium hydroxide, and alkaline earth metal hydroxides and the alkali metal alcoholates of the alcohols used in the process according to the invention, especially potassium 3-methyl-n-butoxide and potassium 2-methyl-n-butoxide. Specifically, potassium hydroxide is used as the base.

Suitable transition metal compounds V are complex compounds and salts of the transition metals of groups 8, 9 and 10 of the Periodic Table (I UPAC), preferably complex compounds and salts of ruthenium and iridium, particularly preferably complex compounds and salts of ruthenium.

Compounds which have a transition metal atom chosen from ruthenium and iridium and at least 2 ligands which are selected from p-cymene, chloride, benzene, CO, 1,5-cyclooctadiene, allyl, acetylacetonate, dimethyl sulfoxide, cyclopentadienyl are particularly suitable as transition metal compound V. , Pentamethylcyclopentadienyl, indenyl, cyclooctene, hydride, ethene and H2O.

Suitable transition metal compounds V are, for example, [Ru (p-cymene) Cl 2] 2,

[Ru (benzene) Cl2] n, [Ru (CO) 2Cl2] n, where n is in each case in the range from 1 to 100,

[Ru (CO) ₃ Cl ₂ ] 2, [Ru (1, 5-cyclooctadiene) (allyl)], RuCl ₃ -H ₂ O, [Ru (acetylacetonate) ₃ ],

[Ru (cyclopentadienyl) (CO) 2 Cl], [Ru (cyclopentadienyl) (CO) ₂ H], [Ru (cyclopentadienyl) (CO) ₂ ] 2,

[Ru (pentamethylcyclopentadienyl) (CO) 2 Cl], [Ru (pentamethylcyclopentadienyl) (CO) 2 H], [Ru (pentamethylcyclopentadienyl) (CO) ₂ ] 2, [Ru (indenyl) (CO) ₂ Cl], [Ru (indenyl ) (CO) ₂ H], [Ru (indenyl) (CO) 2] 2, ruthenocene, [Ru (1, 5-cyclooctadiene) Cl 2] 2, [Ru (pentamethylcyclopentadienyl) (1, 5-cyclooctadiene) Cl ], [Ru ₃ (CO) i ₂ ], lrCl ₃ -H ₂ O, KlrCu, K ₃ lrCl ₆ , [Ir (1, 5-cyclooctadiene) Cl] 2, [Ir (cyclooctene) 2 Cl] 2, [lr (ethene) 2CI] 2, [Ir (cyclopentadienyl) Cl 2] 2,

[Ir (pentamethylcyclopentadienyl) Cl 2] 2 and [Ir (cyclopentadienyl) (CO) 2] and

[Ir (pentamethylcyclopentadienyl) (CO) 2].

In the process of the invention, a primary monoalcohol or a mixture of a primary monoalcohol and at least one different alcohol thereof is converted to the carboxylic acid ester. When using alcohol mixtures, the mixed carboxylic acid esters may form. The reaction can be illustrated by the following two reaction equations:

2 R ^a CH ₂ OH R ^a COOCH ₂ R ^a + 2 H ₂

R ^a R ^b CH ₂ OH + OH R ^a COOR ^b + 2 H ₂

If one reacts a mixture of a primary monoalcohol and at least one different alcohol, preferably the primary monoalcohol in an amount of at least 50 mol%, based on the total molar amount of alcohol used.

Virtually all primary monoalcohols are suitable as primary monoalcohols in the process according to the invention. The primary monoalcohols can be linear, branched or cyclic. The primary monoalcohols usually have 3 to 10 carbon atoms. The primary monoalcohols generally have no further functional groups besides the one hydroxyl group. In particular, the primary monoalcohols are primary alkanols having preferably 3 to 10 C atoms, primary hydroxy-C 1 -C 4 -alkyl benzenes, and primary hydroxy-C 1 -C 4 -alkyl-C 3 -C 8 -cycloalkanes. Specifically, the primary monoalcohols are primary alkanols preferably having 3 to 10 C atoms and more preferably 3 to 6 C atoms. Examples of suitable primary monoalcohols are 1-propanol, 1-butanol, 2-methyl-n-propanol, 1-pentanol, 3-methyl-n-butanol, 2-methyl-n-butanol, 1-hexanol, 1-heptanol, 1 Octanol, 2-ethyl-n-hexanol, 2-propyl-n-heptanol, 1-hydroxymethylcyclohexane, benzyl alcohol and 2-phenylethanol. The primary monoalcohols can also be used as mixtures.

Suitable alcohols, which are optionally used in admixture with the primary monoalcohol, may be aliphatic, cycloaliphatic or aromatic, linear or branched. It may be secondary or tertiary alcohols. The alcohols other than the primary monoalcohols generally have from 3 to 10 carbon atoms. The alcohols other than the primary monoalcohols are preferably monoalcohols and, as a rule, have no further functional groups in addition to the one hydroxyl group. In particular, the alcohols other than primary monoalcohols are secondary or tertiary alkanols having preferably 3 to 10 C atoms, cycloalkanols having preferably 5 to 10 C atoms, secondary or tertiary hydroxy-C 1 -C 4 -alkylbenzenes and secondary or tertiary hydroxy -C-C4-alkyl-C3-C8-cycloalkanes. Examples of such alcohols are isopropanol, 2-butanol, tert-butanol, cyclopentanol, cyclohexanol, 1-phenylethanol, phenol or 1-methyl-n-butanol.

It is also possible to use mixtures of different alcohols which are obtained from fermentative sources, for example mixtures of 3-methyl-n-butanol and 2-methyl-n-butanol, which are obtained as by-products in the industrial production of bioethanol (so-called "fusel oils").

The primary monoalcohols as well as the optionally used further alcohols may moreover bear substituents which undergo reaction under the reaction conditions. inert behavior of the process according to the invention, for example alkoxy, alkene oxy, dialkylamino and halogen (F, Cl, Br, I).

In a specific embodiment of the invention, a primary aliphatic C3-Cio-alcohol or a mixture of primary aliphatic C3-Cio-alcohols, in particular a primary aliphatic C4-C6-alcohol or a mixture of primary aliphatic C4-C6-alcohols, to ,

In a very specific embodiment of the invention, the primary monoalcohol is isoamyl alcohol or a mixture of isoamyl alcohol with 2-methyl-n-butanol.

The process according to the invention takes place in the presence of a transition metal carbene complex catalyst K, as defined above. It has proved to be advantageous if the catalytically active part of the catalyst K is at least partially dissolved in the liquid reaction medium. In a preferred embodiment, at least 90% of the catalyst used in the process K are dissolved in the liquid reaction medium, more preferably at least 95% and most preferably more than 99%, each based on the total amount in the liquid reaction medium.

According to the invention, the previously defined transition metal carbene complex catalyst K is prepared by reacting a transition metal compound V with an imidazolium salt in the presence of the primary monoalcohol and the base. The amount of the metal component of the catalyst, preferably ruthenium or iridium, is generally 0.1 to 5000 ppm (parts by weight), in particular in the range of 1 to 2000 ppm, especially 50 to 1000 ppm, each based on the total weight of the liquid reaction mixture. The base is preferably used in an amount of 1 to 20 equivalents of base, in particular in an amount of 1 to 5 equivalents of base, based on 1 equivalent of the imidazolium salt.

The imidazolium salt is used preferably in an amount of 1 to 20 mol, especially in an amount of 1 to 6 mol, based on 1 mol of the transition metal in the transition metal compound V.

According to the invention, the transition metal carbene complex catalyst K is prepared in the presence of the primary monoalcohol and the base. This is usually done like this proceed by bringing the primary monoalcohol or the mixture of a primary monoalcohol and at least one different alcohol, the imidazolium salt, the transition metal compound V and the base together into the reaction space or the primary monoalcohol or the mixture of a primary monoalcohol and at least one different alcohol and optionally the base in the reaction chamber and for this purpose the catalyst-forming constituents and optionally added base.

The reaction of the primary monoalcohol or the mixture of a primary monoalcohol and at least one different alcohol is generally carried out at a temperature in the range of 20 to 250 ° C. The process according to the invention is preferably carried out at temperatures in the range from 100.degree. C. to 200.degree. C., more preferably in the range from 110.degree. To 200.degree. The reaction of the primary monoalcohol or the mixture of a primary monoalcohol and at least one different alcohol is generally at a total pressure of 0.1 to 20 MPa absolute, which is both the autogenous pressure of the alcohols or of the alcohol at the reaction temperature and the pressure a gas such as nitrogen, argon or hydrogen can be carried out. The process according to the invention is preferably carried out absolutely at a total pressure of up to 10 MPa absolute, in particular at a total pressure of up to 1 MPa.

According to the invention, the reaction of the primary monoalcohol or of the mixture of a primary monoalcohol and at least one different alcohol is carried out in substance, ie, no additional solvent is added to the reaction mixture. Accordingly, the content of solvent in the reaction mixture is less than 10 wt .-%, preferably less than 5 wt .-%, more preferably less than 2 wt .-% and in particular less than 1 wt .-%. The reaction time usually depends on the reaction temperature, the reactivity of the alcohols used and the desired conversion. As a rule, the reaction of the primary monoalcohol or the reaction of the mixture of a primary monoalcohol and at least one different alcohol will lead to the extent that the conversion, based on the primary monoalcohol, at least 10%, in particular at least 20% and especially at least 25%. It has proven to be advantageous not to carry out the reaction up to full conversion (100%, based on the primary monoalcohol), but only up to a conversion of not more than 80%, in particular not more than 70%. The time required for this reaction can the expert determine by routine tests. It is generally in the range of 15 minutes to 100 hours, especially in the range of 1 hour to 50 hours.

The esterification of the primary alcohol or the mixture of a primary monoalcohol and at least one different alcohol is naturally carried out with elimination of hydrogen (see above reaction equations). It has proven to be advantageous to remove the hydrogen from the reaction system. In this way you can increase sales. This is achieved either by stripping with the boiling unreacted alcohol or by supplying a stripping gas such as nitrogen, carbon dioxide or argon.

The reaction can be carried out in the customary devices or reactors known to those skilled in the art for liquid-gas reactions in which the catalyst is homogeneously dissolved in the liquid phase. For the process according to the invention, it is possible in principle to use all reactors which are fundamentally suitable for gas-liquid reactions under the given temperature and the given pressure. Suitable standard reactors for gas-liquid and liquid-liquid reaction systems are described, for example, in KD Henkel, "Reactor Types and Their Industrial Applications", Ullmann's Encyclopedia of Industrial Chemistry, 2005, Wiley-VCH Verlag GmbH & Co. KGaA, Chapter 3.3: "Reactors for gas-liquid reactions", to which reference is hereby made. Examples which may be mentioned stirred tank reactors, tubular reactors or bubble column reactors. The supply of alcohol, transition metal compound V, imidazolium salt and the base can be carried out simultaneously or separately. The reaction can be carried out discontinuously in batch mode or continuously, semicontinuously with recycling or without recycling. The average residence time in the reaction space is generally 15 minutes to 100 hours, in particular 1 hour to 50 hours.

After the reaction, the carboxylic ester is usually separated from unreacted alcohol, preferably by distillation. The catalyst K remains with the high boilers in the bottom of the distillation and can be reused. Unreacted alcohol can be recycled back into the reaction. The thermal separation of the alcohol or of the alcohols and of the carboxylic acid ester takes place by the methods known to those skilled in the art, preferably in an evaporator or in a distillation unit, comprising evaporator and column (s), which usually has several trays, a packing or packing.

The invention is explained in more detail by the following examples: Example 1 :

In a 250 mL three-necked flask equipped with a water separator and a thermometer was added 0.21 g of [Ru (1, 5-cyclooctadiene) Cl 2] 2, 0.7 g of 1-methyl-3-η-butylimidazolium chloride, 5 g Potassium hydroxide and 100 g of isomer mixture of 3-methyl-n-butanol and 2-methyl-n-butanol (distilled product obtained from fusel oil, 80 wt .-% 3-methyl-n-butanol, 20 wt .-% 2 -Methyl-n-butanol). The mixture was heated for 20 h with stirring (magnetic stirrer) at normal pressure and 170 ° C oil bath temperature under reflux. After the end of the reaction, 72.6% unreacted starting material mixture and 27.1% of the carboxylic acid ester (mixture of isomers, formed from 3-methyl-n-butanol and 2-methyl-n-butanol, conversion determined by GC-FI%) were present in the reaction mixture. From the reaction, 46.3 g were distilled off on a rotary evaporator (80 ° C oil bath temperature, 20 mbar) (mixture of educt mixture and carboxylic acid ester). The catalyst and the base remained in the liquid residue and could be reused. From the distillate of the coarse separation, the alcohol mixture was separated from the carboxylic ester by precision distillation, and 6.8 g of the pure carboxylic ester (mixture of isomers) were obtained in the bottom of the fine distillation in a purity of 99.2% (determined by GC-FI-%). Example 2:

In a 250 mL three-necked flask equipped with a water separator and a thermometer was added 0.21 g of [Ru (1, 5-cyclooctadiene) Cl 2] 2, 0.7 g of 1-methyl-3-n-butylimidazolium chloride, 5 g Potassium hydroxide and 100 g of 1 -hexanol filled. The mixture was heated for 20 h with stirring (magnetic stirrer) at normal pressure and 180 ° C oil bath temperature under reflux. At the end of the reaction, 44.1% unreacted starting material and 39.5% of the carboxylic ester (hexylhexanoate, conversion determined by GC-FI%) were present in the reaction mixture. The reaction can be worked up as described in Example 1.

Example 3:

In a 250 mL three-necked flask equipped with a water separator and a thermometer was added 0.21 g of [Ru (1, 5-cyclooctadiene) Cl 2] 2, 0.7 g of 1-methyl-3-n-butylimidazolium chloride, 5 g Potassium hydroxide and 100 g of benzyl alcohol filled. The mixture was heated under stirring for 20 h (magnetic stirrer) at atmospheric pressure and 240 ° C oil bath temperature under reflux. After the end of the reaction were in the reaction mixture 10.4% unreacted starting material, 4.7% benzaldehyde (intermediate of the reaction) and 67.4% of the carboxylic ester (benzyl benzoate, conversion determined by GC-FI-%). The reaction can be worked up as described in Example 1.

Claims

claims:

A process for the preparation of carboxylic acid esters which comprises reacting at least one primary monoalcohol or a mixture of a primary monoalcohol and at least one different alcohol in the presence of a transition metal carbene complex catalyst K, which as central atom M at least one transition metal atom of group 8, 9 or 10 of the Periodic Table (IUPAC ) and at least one monodentate N-heterocyclic carbene ligand, in the presence of a base, characterized in that the catalyst K by reaction of a transition metal compound V, the at least one transition metal atom of group 8, 9 or 10 of the Periodic Table (IUPAC), but does not have a carbene ligand, with an imidazolium salt in the presence of the primary monoalcohol and the base prepares to carry out the reaction in bulk.

A process according to claim 1, wherein the catalyst K has, as the central atom M, ruthenium or iridium.

Method according to one of the preceding claims, wherein the catalyst K at least one, z. B. 1, 2, 3 or 4, N-heterocyclic carbene ligands of the formula (I),

wherein

R ¹ and R ⁴ independently of one another are C 1 -C 10 -alkyl, C 3 -C 10 -cycloalkyl, 5 to 10-membered heteroaryl which has 1, 2 or 3 heteroatoms selected from O, N and S as ring members, or aryl, wherein the four last-mentioned substituents are unsubstituted or may be substituted by one or more substituents selected from halogen, C 1 -C 10 -alkoxy, CN, C 1 -C 10 -alkyl, C 3 -C 10 -cycloalkyl, phenyl and naphthyl; R ² and R ³ independently of one another are hydrogen or have one of the meanings given for R ¹ or R ⁴ .

4. The method of claim 3, wherein the catalyst K in addition to at least one N-heterocyclic carbene ligand has at least one further ligand L, which is selected from CO, hydride, aliphatic olefins, cycloolefins, carbocyclic aromatics, heteroaromatics, aldehydes, ketones, halides, Ci -Cio-carboxylate, methylsulfonate, methylsulfate, trifluoromethylsulfate, tosylate, mesylate, cyanide, isocyanate, cyanate, thiocyanate, hydroxide, Ci-Cio-alkoxide, cyclopentadienide, Pentamethylcyclopentadienid and Pentabenzylcyclopentadienid.

5. The method according to any one of the preceding claims, wherein for the preparation of the catalyst K, an imidazolium salt of the general formula (II),

wherein R ¹ , R ² , R ³ and R ^{4 have} the meanings mentioned in claim 3,

A is H or COO ^" and the equivalent of an anion, in particular halide, C 1 -C 10 -carboxylate, benzoate, MeC ₆ H ₄ COO, tosylate, methanesulfonate, trifluoromethanesulfonate, mesylate, cyanide, isocyanate, thiocyanate, tetrachloroaluminate , Tetrafluoroaluminate, tetrafluoroborate, hexafluorophosphate, hexafluoroantimonate, sulfate, hydroxide, bis (trifluoromethanesulfonyl) imide or methylsulfate, provided that X ^"is absent when A is COO ^" .

Method according to one of claims 3 or 5, wherein in the formulas (I) and (II) the variables R ¹ and R ⁴ independently of one another are C ¹ -C 10 -alkyl, in particular branched C 3 -C 10 -alkyl, especially isopropyl, R ² and R ^{3 are} each hydrogen and A is hydrogen.

7. The method according to any one of the preceding claims, wherein the base is selected from hydrides, hydroxides, carbonates, alcoholates and amides of the alkali metals, the hydrides, hydroxides, carbonates, alcoholates and amides of alkaline earth metals, organic amines, aryllithium compounds and Alkylli- thium compounds, especially under alkali metal hydroxides and alkali metal alkoxides.

8. The method according to any one of the preceding claims, wherein the base in an amount of 1 to 20 equivalents of base, based on 1 equivalent of the imidazolium salt, is used.

9. The method according to any one of the preceding claims, wherein the imidazole zoliumsalz in an amount of 1 to 20 mol, based on 1 mol of the transition metal in the transition metal compound V, is used.

10. The method according to any one of the preceding claims, wherein the transition metal compound V is selected from complex compounds and salts of ruthenium or iridium, which have no carbene ligand, especially among complex compounds and salts of ruthenium.

1 1. The method of claim 10 wherein the transition metal compound V has at least one ruthenium and iridium-selected transition metal atom and at least 2 ligands selected from p-cymene, chloride, benzene, CO, 1,5-cyclooctadiene, allyl, acetylacetonate, dimethylsulfoxide, cyclopentene - tadienyl, pentamethylcyclopentadienyl, indenyl, cyclooctene, hydride, ethene and H ₂ 0.

12. The method of claim 10, wherein the transition metal compound V is selected from [Ru (p-cymene) Cl2] 2, [Ru (benzene) Cl2] n, [Ru (CO) 2Cl2] n, where n is in each case in the range of 1 up to 100, [Ru (CO) ₃ Cl ₂ ] 2 [Ru (1, 5-cyclooctadiene) (allyl)], RuC-HO, [Ru (acetylacetonate) 3], [Ru (dimethylsulfoxide) 4Cl 2],

[Ru (cyclopentadienyl) (CO) ₂ Cl], [Ru (cyclopentadienyl) (CO) ₂ H],

[Ru (cyclopentadienyl) (CO) 2] 2, [Ru (pentamethylcyclopentadienyl) (CO) 2 Cl]],

[Ru (pentamethylcyclopentadienyl) (CO) 2H],

[Ru (pentamethylcyclopentadienyl) (CO) 2] 2, [Ru (indenyl) (CO) 2 Cl]],

[Ru (indenyl) (CO) ₂ H], [Ru (indenyl) (CO) ₂ ] 2, ruthenocene, [Ru (1, 5-cyclooctadiene) Cl 2] 2, [Ru (pentamethylcyclopentadienyl) (1, 5-cyclooctadiene ) Cl], [Ru ₃ (CO) i ₂ ], IrCl ₃ -H ₂ O, KlrCu, K ₃ IrCl ₆ , [Ir (1, 5-cyclooctadiene) Cl] ₂ ,

[Ir (cyclooctene) 2CI] 2, [Ir (ethene) 2CI] 2, [Ir (cyclopentadienyl) Cl 2] 2,

[Ir (pentamethylcyclopentadienyl) (CO) 2].

13. The method according to any one of the preceding claims, wherein the transition metal compound V in an amount of 0.1 to 5000 ppm by weight, based on 1 part by weight of the primary monoalcohol or the mixture of a primary monoalcohol with at least one different alcohol used ,

Method according to one of the preceding claims, wherein reacting a primary aliphatic C3-Cio-alcohol or a mixture of primary aliphatic C3-Cio-alcohols, in particular a primary aliphatic C4-C6-alcohol or a mixture of primary aliphatic C4-C6-alcohols.

15. The process according to claim 14, wherein isoamyl alcohol or a mixture of isoamyl alcohol is reacted with 2-methylbutanol.

A process according to any one of the preceding claims, wherein the reaction of the at least one primary monoalcohol or the mixture of a primary monoalcohol with at least one different alcohol is carried out at a temperature in the range of 20 to 250 ° C.