EP0000478B1 - Process for the preparation of acyloins - Google Patents

Description

The invention relates to a process for the preparation of acyloin by condensing aldehydes in the presence of thiazolium salts.

The production of acyloin by condensation of aldehydes in the presence of low molecular weight thiazolium salts as a catalyst and bases as a cocatalyst is known from the literature. Corresponding reactions are described, for example, in J. A. Chem. Soc. 80 (1958), 3719-3726, J. Chem. Soc. Chem. Commun. 1973, 891 and Synthesis Commun. 1976, 734.

The known processes have the disadvantage that the thiazolium salts used as catalysts can only be removed from the reaction mixture using very complex methods. If it is not possible to completely free the crude reaction product from the thiazolium salt, the subsequent distillation gives rise to unpleasant-smelling decomposition products, some of which pass over with the distillate and severely impair the quality of the acyloins obtained. Another disadvantage is that the low molecular weight thiazolium salts can only be recovered from the wash water with great difficulty in pure form after working up the reaction mixture.

From Macromolecules, 1968, 1, pp. 452-455 it is known that poly- (4-vinylthiazole) and poly- (4-methyl-5-vinylthiazole), in partially quaternized form, develop catalytic activity in the condensation of furfural. It must be concluded from the results given there that a sufficient catalytic effect on the furoin condensation only occurs if the thiazole nucleus is connected to the polymer chain in the 5-position. The use of polyvinylthiazoles as catalysts is currently out of the question for industrial processes since the monomeric vinylthiazoles necessary for their preparation are difficult to access.

The object of the invention was therefore to find a process for the preparation of acyloin in which the disadvantages described do not occur. In particular, a catalyst should be found which can be prepared from easily accessible components and can be removed in a simple manner from the reaction mixture of the acyloin condensation.

in der R₁ und R₂ gleich oder verschieden sind und geradkettige oder verzweigte Alkylreste mit 1 bis 18 Kohlenstoffatomen, Cycloalkylreste mit 3 bis 18 Kohlenstoffatomen, Arylalkylreste mit 7 bis 18 Kohlenstoffatomen oder R₁ und R₂ zusammen einen Alkylenrest mit 2 bis 4 Kohlenstoffatomen darstellen, durch Kondensation entsprechender Aldehyde in Gegenwart von Katalysatoren und Basen, das dadurch gekennzeichnet ist, daß man als Katalysatoren Reaktionsprodukte aus Thiazolen der Formel

in der R₃ Wasserstoff oder einen Alkylrest mit 1 bis 5 Kohlenstoffatomen und R₄ Wasserstoff, einen Alkylrest mit 1 bis 5 Kohlenstoffatomen oder eine Hydroxyalkalgruppe mit 2 bis 4 Kohlenstoffatomen darstellen, und einem vernetzten Chlormethylpolystyrol verwendet.This object is achieved by a process for the preparation of acyloin of the formula

in which R ₁ and R _{2 are the} same or different and represent straight-chain or branched alkyl radicals with 1 to 18 carbon atoms, cycloalkyl radicals with 3 to 18 carbon atoms, arylalkyl radicals with 7 to 18 carbon atoms or R ₁ and R ₂ together represent an alkylene radical with 2 to 4 carbon atoms , by condensation of corresponding aldehydes in the presence of catalysts and bases, which is characterized in that reaction products of thiazoles of the formula

in which R _{3 is} hydrogen or an alkyl radical with 1 to 5 carbon atoms and R _{4 is} hydrogen, an alkyl radical with 1 to 5 carbon atoms or a hydroxyalkali group with 2 to 4 carbon atoms, and a crosslinked chloromethyl polystyrene.

in Betracht. In dieser Formel steht R3 für einen geradkettigen oder verzweigten Alkylrest mit 1 bis 18 Kohlenstoffatomen, einen Cycloalkylrest mit 3 bis 18 Kohlenstoffatomen, einen Arylalkylrest mit 7 bis 18 Kohlenstoffatomen oder für die Gruppe

in der n eine ganze Zahl von 2 bis 4 bedeutet.Aldehydes of the formula come as starting material for the production of acyloins by the process according to the invention

into consideration. In this formula, R3 represents a straight-chain or branched alkyl radical having 1 to 18 carbon atoms, a cycloalkyl radical having 3 to 18 carbon atoms, an arylalkyl radical having 7 to 18 carbon atoms or the group

in which n is an integer from 2 to 4.

As examples of aldehydes that are used as starting material in the process according to the invention The following can be used: acetaldehyde, acetoxy-acetaldehyde, propionaldehyde, n-butyraldehyde, iso-butyraldehyde, pivalinaldehyde, valeraldehyde, iso-valeraldehyde, capronaldehyde, onanthaldehyde, 2-ethylhexanal, caprylaldehyde, pelargonaldehyde, lauraldehyde, caprinaldehyde Myristic aldehyde, palmitic aldehyde, stearic aldehyde, cyclopropane-carbaldehyde, cyclobutane-carbaldehyde, cyclopentane-carbaldehyde, cyclohexane-carbaldehyde, cycloheptane-carbaldehyde, cyclooctane-carbaldehyde, cycloheptadecane-carbaldehyde, 2-methyl-cyclohexane-carbaldehyde-iso- (aldehyde) carbaldehyde- (1), 1-ethyl-cyclopentane-carbaldehyde- (1), 1,3-dimethyl-cyclopentane-carbaldehyde, phenylacetaldehyde, p-tolyl-acetaldehyde, 2-phenyl-propionaldehyde, 3-phenyl-propionaldehyde, 3- p-tolylpropionaldehyde, 4-phenylbutyraldehyde, 2-phenyl-isobutyraldehyde, 2-methyl-3-phenyl-propionaldehyde, 2-methyl-3-p-tolyl-propionaldehyde, 2-phenylvaleraldehyde, 4-p-tolylvaleraldehyde, 2-methyl-6-phenyl-hexanal- (1), cyclohexylphenyl-acetaldehyde, 2-benzyl-heptanal- (1), (2,4-diisopropylphenyl) acetaldehyde, succinic dialdehyde, glutaric dialdehyde and adipic dialdehyde.

The hydrocarbon radicals of the aforementioned aldehydes can also contain substituents, for example hydroxyl groups, ester or ether groups, as long as it is ensured that these substituents do not interfere with the acyloin condensation.

• 1,3-Thiazol, 4-Methyl-1,3-thiazol, 5-Methyl-1,3-thiazol, 4-Äthyl-1,3-thiazol, 5-Äthyl-1,3-thiazol, 4-tert.-Butyl-1,3-thiazol, 4,5-Dimethyl-1,3-thiazol, 4,5-Diäthyl-1,3-thiazol und 5-(2'-Hydroxyäthyl)-1,3-thiazol. Als besonders geeignet hat sich 5-(2'-Hydroxyäthyl)-4-methyl-1,3-thiazol erwiesen.

To prepare the catalysts to be used according to the invention, substituted thiazoles of the formula II are reacted with crosslinked chloromethylpolystyrenes at elevated temperature, preferably at 60 to 100.degree. The chloromethyl polystyrenes are known, some commercially available substances that can be prepared by chloromethylation of polystyrenes using formaldehyde and hydrogen chloride. Examples of possible thiazoles of the formula 11 are:

• 1,3-thiazole, 4-methyl-1,3-thiazole, 5-methyl-1,3-thiazole, 4-ethyl-1,3-thiazole, 5-ethyl-1,3-thiazole, 4-tert. -Butyl-1,3-thiazole, 4,5-dimethyl-1,3-thiazole, 4,5-diethyl-1,3-thiazole and 5- (2'-hydroxyethyl) -1,3-thiazole. 5- (2'-Hydroxyethyl) -4-methyl-1,3-thiazole has proven to be particularly suitable.

The thiazoles are generally allowed to act on the crosslinked chloromethyl polystyrenes in an inert solvent or in water. Aprotic solvents such as e.g. Dimethylformamide, acetonitrile. Dioxane or tetrahydrofuran in question. Excess thiazole can also be used as a solvent. It has proven to be expedient in any case to work with a stoichiometric excess of the thiazole in order to achieve a virtually complete conversion of the chloromethyl groups of the polystyrene resin. After the reaction, the polymer catalyst containing thiazolium groups is filtered off, extracted with an inert solvent, for example ether or alcohol, and finally dried.

Inorganic and organic bases are suitable as co-catalysts, for example sodium hydroxide, potassium hydroxide, sodium carbonate, sodium bicarbonate, potassium carbonate, sodium acetate, potassium acetate, disodium hydrogen phosphate, triethylamine, triethanolamine, N-methylpyrrolidine or basic ion exchangers.

The amount of catalyst used is not critical. In general, the method according to the invention is used. those amounts of the catalyst resin carried out, which - based on the aldehyde to be reacted - contain 0.1 to 20 mol%, preferably 0.5 to 5 mol% of quaternary thiazolium groups. However, amounts which correspond to less than one mol% are sufficient to carry out the process.

The amount of base used is also not critical. In general, an amount which is equimolar to the thiazolium salt groups contained in the catalyst is used. However, it is preferred to work with 2 to 5 times the molar amount. When working in aqueous or aqueous-alcoholic solutions, the amount of base must be measured so that a pH of about 7.3 to about 8.5 is maintained.

When carrying out the process according to the invention, the aldehydes to be reacted are optionally dissolved in a solvent, with the catalyst suspended therein, stirred at room temperature or at elevated temperature until the desired degree of conversion is reached. In view of the sensitivity of the catalyst, the reaction temperature should not significantly exceed 100 ° C.

If the process according to the invention is carried out in the presence of a solvent, polar solvents such as water, methanol, ethanol, isopropanol, dioxane, tetrahydrofuran, acetonitrile, dimethylformamide, dimethyl sulfoxide, hexamethylphosphoric acid triamide and pyridine and mixtures of these solvents, in particular water / ethanol mixtures, are suitable.

After the reaction has ended, the catalyst is separated off from the reaction mixture by filtration. After distilling off the solvent which may be present, the liquid portions of the reaction mixture are worked up by the methods customary in organic chemistry, such as fractional distillation and recrystallization.

The separated catalyst can be reused after washing with a solvent and drying.

In a special embodiment of the process according to the invention, the thiazolium group-containing catalyst resin is used in the base form. This procedure can be used as a cocatalyst without an additional base. The base or OH form of the catalyst can be obtained very easily by treating the salt form of the thiazolium group-containing resin with an aqueous solution of a base such as sodium hydrogen carbonate until chloride ions are no longer present in the resin.

The products accessible via the process according to the invention are valuable fragrance and aroma substances; they are also valuable intermediates for organic synthesis.

The process according to the invention has the advantage over the known procedures that the catalyst used can be removed quantitatively from the reaction mixture in an extremely simple manner, namely by filtering. Any reduction in the quality of the acyloins produced by decomposition products which arise when the catalyst is distilled off is thereby avoided.

The following examples are intended to explain the invention, but not to restrict it thereto.

Example 1 (catalyst preparation)

40 g of crosslinked chloromethyl polystyrene with a chlorine content of 5.78% by weight were suspended in 100 ml of dimethylformamide and with 21.6 g of 5- (2'-hydroxyethyl) -4-methyl-1,3-thiazole for 72 hours stirred at 80 ° C. The resin was then filtered off, extracted with ether and dried in vacuo at 40 ° C./0.01 mbar. The resin containing 5- (2'-hydroxyethyl) -4-methyl-1,3-thiazolium groups had a chlorine content of 3.71% by weight.

Example 2

64.1 g (0.5 mol) of distilled caprylaldehyde, 23.8 g (0.025 mol) of thiazolium salt catalyst from Example 1 and 15.2 g (0.15 mol) of triethylamine were stirred at 90 ° C. in a nitrogen stream for 7 hours. The mixture was then diluted with ether and the catalyst was filtered off. The catalyst was extracted several times with ether. The ether and then unreacted caprylaldehyde and excess triethylamine were distilled off from the combined ether filtrates in a stream of nitrogen under normal pressure and then in a water pump vacuum. The residue obtained in this way (63.3 g) contained 94.9% by weight of capryloin by gas chromatography. For further purification, the capryloin was recrystallized from gasoline (65-95 °). 55 g (86% of theory) of pure capryloin with a melting point of 38-39 ° C. were obtained.

Example 3

110.0 g (2.5 mol) of freshly distilled acetaldehyde, 23.8 g (0.025 mol) of thiazolium salt catalyst from Example 1 and 15.2 g (0.15 mol) of triethylamine were in an autoclave with a magnetic stirrer under a predetermined nitrogen pressure of 6.1 bar for 8 hours at 80 ° C at 80 ° C. The maximum pressure obtained at 80 ° C was 7.1 bar. After cooling and opening the autoclave, the reaction mixture was taken up in the ether for easier removal of the catalyst and the catalyst was filtered off. After the ether had been distilled off, the residue was fractionally distilled under carbon dioxide in a water jet pump vacuum over a Vigreux column. 68.1 g (61% of theory) of pure acetoin of bp 37-38 ° C / 16 mbar were obtained.

Example 4

25.6 g (0.2 mol) of freshly distilled caprylaldehyde was dissolved in a mixture of 50 ml of water and 50 ml of ethanol with stirring and cooling. 9.5 g (0.01 mol) of thiazolium salt catalyst from Example 1 were stirred into this solution at room temperature. After adding 1 g of disodium hydrogenphosphate in 5 ml of water, the pH of the solution was adjusted to 8.4 with 1N sodium hydroxide solution. The mixture was then heated to 60 ° C. in a stream of nitrogen and with vigorous stirring and kept at this temperature for 7 1/4 hours. After cooling, the reaction mixture was worked up as in Example 2. The resulting crude reaction product contained 73.2% by weight of capryloin, 2.3% by weight of aldol and 23.5% by weight of caprylaldehyde, as determined by gas chromatography.

Example 5 (catalyst preparation)

30 g of the 5- (2'-hydroxyethyl) -4-methyl-1, 3-thiazolium group-containing resin from Example 1 (chloride form) were stirred into 1 l of saturated sodium bicarbonate solution. The suspension was placed in an exchange column. The resin was then washed with saturated sodium hydrogen carbonate solution until the effluent filtrate was free of chloride ions. The catalyst, now in the OH form, was then washed with water and dried at 40.degree.

Example 6

16.0 g (0.13 mol) of distilled caprylaldehyde and 6 g (0.007 mol) of the thiazolium salt catalyst in the OH form from Example 5 were heated to 60 ° C. with stirring in a stream of nitrogen and kept at this temperature for 4 hours. After cooling, the reaction mixture was worked up as in Example 2. According to gas chromatographic analysis, the crude reaction product contained 89.0% by weight of capryloin and 11.0% by weight of capryl aldehyde.

Example 7

25.6 g (0.2 mol) of caprylaldehyde were dissolved in 100 ml of water / ethanol mixture as in Example 4. 9.5 g (0.01 mol) of thiazolium salt catalyst in the CI form according to Example 1 were stirred into the solution. After adding 1 g of disodium hydrogen phosphate in 5 ml of water, the pH of the solution was adjusted to 7.4. The mixture was heated to 60 ° C under nitrogen and held at this temperature for 7 1/4 hours. The reaction mixture was worked up as described in Example 2. According to gas chromatographic determination, the crude reaction product contained 75% by weight of capryloin, 19.2% by weight of caprylaldehyde, 2.6% by weight of acetal and 3.2% by weight of aldol.

Example 8

36.1 g (0.5 mol) of freshly distilled n-butyraldehyde and 9.2 g (0.01 mol) of 5- (2'-hydroxyethyl) -4-methyl-1, 3-thiazolium group-containing resin from Example 1 in of the OH form were heated to 70 ° C. for 5 hours while stirring in a stream of nitrogen. After cooling, the reaction mixture was worked up as in Example 2. The resulting crude reaction product contained 84.5% by weight of butyroin and 15.5% by weight of butyraldehyde, as determined by gas chromatography.

The crude product was fractionally distilled under carbon dioxide in a water jet pump vacuum over a Vigreux column. 24.7 g (69% of theory) of butyroin with a boiling point of 86 ° C./17 mbar were obtained. According to gas chromatographic analysis, this product consisted of 100% butyroin.

Example 9

An ion exchange column made of glass (inner diameter 1 cm; length 60 cm), which was provided with a glass jacket and a glass frit, was charged with 20 g of the thiazolium salt catalyst in the OH form according to Example 8. The column was filled with caprylic aldehyde under nitrogen. The column contents were heated to 80 ° C by passing hot glycerin through the liquid jacket. Fresh caprylic aldehyde was added at the top of the column under nitrogen from a dropping funnel to the same extent as the reaction product was drawn off via a glass tap at the bottom of the column. An average residence time of 2 hours was set for the reaction product.

According to gas chromatographic analysis, the reaction product drawn off at the foot of the exchange column consisted of 90% by weight of capryloin, the rest was unreacted caprylaldehyde.

Example 10 (catalyst preparation)

20 g (0.093 mol) of crosslinked polychloromethylstyrene with a chlorine content of 16.5% by weight were suspended in 40 ml of dimethylformamide and 30.8 g (0.215 mol) of 5- (2'-hydroxyethyl) -4-methyl- 1,3-thiazole stirred at 80 ° C for 24 hours. The resin was then filtered off, extracted with ether and dried in vacuo at 40 ° C./0.01 mbar.

The resin containing 5- (2'-hydroxyethyl) -4-methyl-1,3-thiazolium groups contained 8.61% by weight of ionic chlorine; the total chlorine content was 9.33% by weight.

Example 11 (catalyst preparation)

20 g (0.093 mol) of crosslinked poly-chloromethylstyrene with a chlorine content of 16.5% by weight were suspended in 50 ml of distilled water and 30.8 g (0.215 mol) of 5- (2'-hydroxyethyl) -4-methyl -1,3-thiazole stirred at 80 ° C for 24 hours. The resin was then filtered off, washed with water, extracted with ethanol to remove excess 5- (2'-hydroxyethyl) -4-methyl-1,3-thiazole. The cleaned resin was dried in vacuo at 40 ° C / 0.01 mbar.

The resin catalyst thus obtained contained 7.17% by weight of ionic chlorine; the total chlorine content was 7.68% by weight.

Example 12 (catalyst preparation)

30 g (0.06 mol) of 5- (2'-hydroxyethyl) -4-methyl-1, 3-thiazolium group-containing resin according to Example 11 were stirred into 1 l of saturated sodium bicarbonate solution. The suspension was transferred to an exchange column. The resin was then washed with saturated sodium bicarbonate solution until the effluent was free of chloride ions. The catalyst, now in the OH form, was then washed with water and dried at 40 ° C./0.01 mbar.

Example 13

110.0 g (2.5 mol) of freshly distilled acetaldehyde and 9.7 g (0.021 mol) of the thiazolium salt catalyst in the OH form according to Example 12 were in an autoclave with a magnetic stirrer under a predetermined nitrogen pressure of 3 bar for 1 hour heated to 80 ° C for a long time. The pressure rose to 8.1 bar. After cooling, the reaction mixture was diluted with ether to facilitate separation, and the catalyst was filtered off. The ether was distilled off from the filtrate. The residue was fractionally distilled under carbon dioxide as a protective gas in a water jet pump vacuum over a Vigreux column. 93.5 g (85% of theory) of acetoin from Sdp. Get 39-43 ° C / 17 mbar. According to gas chromatographic analysis, it was a 100% pure product.

Example 14

1000 g (22.7 mol) of acetaldehyde and 214 g (0.449 mol) of the thiazolium salt catalyst in the OH form according to Example 12 were in an autoclave, which was provided with a device for water cooling and water heating, under a predetermined nitrogen pressure of 2.3 bar heated to 90 ° C. until the reaction started. The reaction temperature was then kept at 105 ° C. with water cooling, the pressure rising to 7.6 bar. As soon as the pressure dropped, a further 2000 g (45.4 mol) of acetaldehyde were metered in from a stock autoclave within 40 minutes. The reaction temperature was kept at 90 to 95 ° C. by water cooling. After the addition was complete, the reaction was stirred for 105 minutes to complete. The catalyst was filtered off from the resulting reaction mixture. The unreacted acetaldehyde was distilled off in a water pump vacuum. When the residue was fractionally distilled, 1912 g of acetoin of bp 37 to 39 ° C./16 mbar (64% of theory) were obtained.

Claims (4)

1. A process for the production of acyloins corresponding to the following formula

in which

R₁ and R₂ are the same or different and represent straight-chain or branched-chain alkyl radicals containing from 1 to 18 carbon atoms, cycloalkyl radicals containing from 3 to 18 carbon atoms, arylalkyl radicals containing from 7 to 18 carbon atoms or R₁ and R₂ together represent an alkylene radical containing from 2 to 4 carbon atoms, by the condensation of corresponding aldehydes in the presence of catalysts and bases, characterised in that the catalysts used are reaction products of thiazoles corresponding to the following formula

in which

R₃ represents hydrogen or an alkyl radical containing from 1 to 5 carbon atoms and R₄ represents hydrogen, an alkyl radical containing from 1 to 5 carbon atoms or a hydroxyalkyl group containing from 2 to 4 carbon atoms, and a crosslinked chloromethyl polystyrene.

2. A process as claimed in Claim 1, characterised in that the catalyst resin is used in quantities containing from 0.1 to 20 mole percent and preferably from 0.5 to 5 mole percent of quaternary thiazolium groups, based on the aldehyde to be reacted.

3. A process as claimed in Claims 1 and 2, characterised in that the catalyst used is a reaction product of 5-(2'-hydroxyethyl)-4-methyl-1,3-thiazole and a crosslinked chloromethyl polystyrene.

4. A process as claimed in Claims 1 to 3, characterised in that the catalyst resin containing thiazolium groups is used in the base form.