Method for producing a homogeneous catalyst for the Tishchenko reaction

Abstract

The invention relates to a process for preparing a carboxylic ester by reacting an aldehyde in the presence of an aluminum alkoxide, wherein the aluminum alkoxide is obtained either by reacting an aluminum hydride with an aldehyde or by reacting a different aluminum alkoxide with a carboxylic ester.

Claims

1. A process for preparing a carboxylic ester, comprising: a) preparing an aluminum alkoxide of general formula (I):
Al(OCH.sub.2R.sup.1).sub.3(I), by reacting an aldehyde of general formula (II):
R.sup.1CHO(II), with an aluminum hydride; and b) reacting an aldehyde of general formula (II) in the presence of the aluminum alkoxide of the general formula (I) to give a carboxylic ester of general formula (III):
R.sup.1COOCH.sub.2R.sup.1(III); wherein R.sup.1 is an alkyl, alkenyl or alkynyl group.

2. The process of claim 1, wherein the reaction of the aldehyde of general formula (II) with the aluminum hydride takes place at a temperature of 70 C. to 30 C.

3. The process of claim 1, wherein R.sup.1 is a (C.sub.1-C.sub.12)-alkyl group or a (C.sub.2-C.sub.12)-alkenyl group and may optionally be substituted by one or more substituents selected from the group consisting of: (C.sub.1-C.sub.12)-alkyl, (C.sub.3-C.sub.12)-cycloalkyl, (C.sub.4-C.sub.12)-heterocycloalkyl, (C.sub.6-C.sub.20-aryl, (C.sub.4-C.sub.20)-heteroaryl, O(C.sub.1-C.sub.12)-alkyl, O(C.sub.3-C.sub.12)-cycloalkyl, S(C.sub.1-C.sub.12)-alkyl, S(C.sub.3-C.sub.12)-cycloalkyl, COO(C.sub.1-C.sub.12)-alkyl, COO(C.sub.3-C.sub.12)-cycloalkyl, CONH(C.sub.1-C.sub.12)-alkyl, CONH(C.sub.3-C.sub.12)-cycloalkyl, N[(C.sub.1-C.sub.12)-alkyl].sub.2, OH, NH.sub.2, and halogen.

4. The process of claim 3, wherein R.sup.1 is ethenyl or 2-propenyl.

5. The process of claim 1, wherein the aluminum hydride is a compound of empirical formula AlH.sub.3 or a complex metal aluminum hydride.

6. The process of claim 5, wherein the complex metal aluminum hydride is selected from the group consisting of: LiAlH.sub.4; NaAlH.sub.4; KAlH.sub.4; Mg(AlH.sub.4).sub.2; Ca(AlH.sub.4).sub.2; Ba(AlH.sub.4).sub.2; Sn(II)(AlH.sub.4).sub.2; Ti(IV)(AlH.sub.4).sub.4; and mixtures thereof.

7. The process of claim 6, wherein an aluminum halide selected from the group consisting of AlF.sub.3, AlCl.sub.3, AlBr.sub.3 or AlI.sub.3 is added to the reaction mixture in step a).

8. The process of claim 3, wherein the aluminum hydride is a compound of empirical formula AlH.sub.3 or a complex metal aluminum hydride.

9. The process of claim 8, wherein the complex metal aluminum hydride is selected from the group consisting of: LiAlH.sub.4; NaAlH.sub.4; KAlH.sub.4; Mg(AlH.sub.4).sub.2; Ca(AlH.sub.4).sub.2; Ba(AlH.sub.4).sub.2; Sn(II)(AlH.sub.4).sub.2; Ti(IV)(AlH.sub.4).sub.4; and mixtures thereof.

10. The process of claim 2, wherein R.sup.1 is a (C.sub.1-C.sub.12)-alkyl group or a (C.sub.2-C.sub.12)-alkenyl group and R.sup.1 may optionally be substituted by one or more substituents selected from the group consisting of: (C.sub.1-C.sub.12)-alkyl, (C.sub.3-C.sub.12)-cycloalkyl, (C.sub.4-C.sub.12)-heterocycloalkyl, (C.sub.6-C.sub.20-aryl, (C.sub.4-C.sub.20)-heteroaryl, O(C.sub.1-C.sub.12)-alkyl, O(C.sub.3-C.sub.12)-cycloalkyl, S(C.sub.1-C.sub.12)-alkyl, S(C.sub.3-C.sub.12)-cycloalkyl, COO(C.sub.1-C.sub.12)-alkyl, COO(C.sub.3-C.sub.12)-cycloalkyl, CONH(C.sub.1-C.sub.12)-alkyl, CONH(C.sub.3-C.sub.12)-cycloalkyl, N[(C.sub.1-C.sub.12)-alkyl].sub.2, OH, NH.sub.2, and halogen.

11. The process of claim 10, wherein R.sup.1 is ethenyl or 2-propenyl.

12. The process of claim 11, wherein the aluminum hydride is a compound of empirical formula AlH.sub.3 or a complex metal aluminum hydride.

13. The process of claim 12, wherein the complex metal aluminum hydride is selected from the group consisting of: LiAlH.sub.4; NaAlH.sub.4; KAlH.sub.4; Mg(AlH.sub.4).sub.2; Ca(AlH.sub.4).sub.2; Ba(AlH.sub.4).sub.2; Sn(II)(AlH.sub.4).sub.2; Ti(IV)(AlH.sub.4).sub.4; and mixtures thereof.

14. The process of claim 13, wherein an aluminum halide selected from the group consisting of AlF.sub.3, AlCl.sub.3, AlBr.sub.3 or AlI.sub.3 is added to the reaction mixture in step a).

Description

EXAMPLES

(1) Example 1 (In Situ Synthesis of the Catalyst and Subsequent Methallyl Methacrylate Synthesis)

(2) Methacrolein (MAL) (1007 g) was cooled to 25 C. while stirring in a 2 l 3-neck round-bottomed flask under an N.sub.2 atmosphere. LiAlH.sub.4 (10.86 g, 2 mol % based on the amount of MAL) was added carefully in portions within 1 h 15 min so that the temperature in the flask did not exceed 20 C. After stirring for a further 15 min at 20 C., AlCl.sub.3 (12.78 g, 0.67 mol % based on the amount of MAL) was added in portions (10 min) so that the temperature did not exceed 15 C. The reaction mixture was heated to 20 C. (approx. 30 min) and stirred for a further 65 h at 20 C. The conversion of MAL was 85%. The product was distilled under reduced pressure (50 mbar, boiling point of product: 79 C.). 716 g of methallyl methacrylate (MAMA) was obtained (78% yield based on free MAL not bound in the formed catalyst, purity 99.8%).

Example 2 (Preparation of the Catalyst by Ligand Exchange)

(3) 93.0 g of freshly distilled Al(Oi-Pr).sub.3 were mixed with 70 mg of TEMPOL (4-hydroxy-2,2,6,6-tetramethylpiperidin-1-oxyl), 70 mg of HQME (hydroquinone monomethyl ether) and 300 g of MAMA and distilled within 3 h using a distillation column with introduction of air. The distillation started at a bottom temperature of 118 C. and a pressure of 400 mbar. The pressure was slowly reduced within three hours to 70 mbar, so that a steady but slow condensation was apparent at the condenser. The top temperature reached values of at most 108 C. here. Initially the isopropyl methacrylate produced and then the excess MAMA were distilled off via the distillation. The end of distillation was apparent from the rise of the bottom temperature from 115 C. to 133 C., reduced boiling and reduction of the top temperature. An orange, clear and viscous bottoms product (119 g) and a clear, colorless distillate (232 g) were obtained. The bottoms product was used without further purification as catalyst for methallyl methacrylate synthesis (with the assumption of 3.81 mmol of [Al] to 1 g of this bottoms product). GC analysis of the distillate gave 31.3% by weight of methallyl methacrylate and 68.7% by weight of isopropyl methacrylate, corresponding to the degree of ligand exchange of approx. 91%.

Example 3 (Comparison with Preformed Aluminum Alkoxide Catalysts)

(4) Methacrolein was reacted to give methallyl methacrylate with in the presence of an aluminum alkoxide prepared by ligand exchange as per example 2. In this case the reaction time and temperature in the second reaction step (conversion of MAL to MAMA) were set in accordance with the following table (test Nos 1-9).

(5) In comparative tests preformed, commercially available aluminum alkoxides (2 mol % Al based on MAL) were mixed with 20 g of MAL (contains <0.1% dimeric methacrolein DiMAL) and 0.05 g of TEMPOL and stirred at the temperature specified in the table for the time indicated there (test Nos 10-12).

(6) Product specimens were analyzed after hydrolysis by GC. The product parameters ascertained are given in the following table.

(7) TABLE-US-00001 MAL MAMA MAMA T Time conversion yield selectivity No. Catalyst [ C.] [h] [%] [%] [%] 1 see example 2 23 24 36.3 32.7 90.1 2 see example 2 23 48 60.1 53.5 89.0 3 see example 2 23 72 76.6 65.4 85.4 4 see example 2 30 24 38.7 29.5 76.2 5 see example 2 30 48 66.7 42.5 63.7 6 see example 2 50 24 77.1 53.2 69.0 7 see example 2 50 48 93.9 58 61.8 8 see example 2 80 4 53.5 35.3 66.0 9 see example 2 80 24 89.6 42.1 47.0 10 Al(OiPr).sub.3 23 24 23.9 16.5 69.0 11 Al(OiPr).sub.3 23 48 41.3 25.7 62.2 12 Al(OnBu).sub.3 23 72 93.4 72.7 77.8

(8) It is found that using the catalyst from Example 2, prepared by ligand exchange, allows the attainment of yields, with higher selectivity at all times, that are at least just as high as with the preformed commercially available catalysts. The yield can moreover be increased by increasing the temperature.