Abstract:
The invention relates to a method of producing alkoxycinnamic acid ester (alkoxy cinnamate) by condensing C 5 -C 8  acetic acid esters with C 1 -C 4  alkoxybenzaldehyde in the presence of an alkaline and/or alkaline earth C 1 -C 4  alcohalate and removing the C 1 -C 4  alcohol that is produced by this reaction from the reaction mixture.

Description:
FIELD OF THE INVENTION 
     The invention relates to the preparation of alkoxycinnamic esters (alkoxycinnamates). 
     BACKGROUND OF THE INVENTION 
     2-Ethylhexyl 4-methoxycinnamate and 3-methylbutyl 4-methoxycinnamate and isomers thereof are known and efficient light protection agents for the UV-B region and are prepared on an industrial scale. 
     To prepare these products, processes are sought which are low-cost and can be carried out with large yields. 
     U.S. Pat. No. 5,527,947 describes a process in which C 1-C   4 -alkoxybenzaldehydes, such as anisaldehyde, and C 1 -C 4 -alkyl acetates, such as methyl acetate, are dissolved in an inert hydrocarbon, such as heptane, toluene or petroleum ether. In the presence of a strongly alkaline metal base, such as sodium methoxide, the feed substances react to give a mixture of the corresponding C 1-C   4 -alkyl-C 1-C   4 -alkoxy cinnamates, the alkali metal salts of the corresponding C 1-C   4 -alkoxy cinnamic acid and the C 1-C   4 -alkanols. Following acidification of the reaction mixture with a strong polybasic acid, such as sulfuric acid, the liberated acetic acid is esterified with the C 1-C   4 -alkanoles and distilled off. The mixture of C 1-C   4 -alkyl-C 1-C   4 -alkoxycinnamates and C 1-C   4 -alkoxycinnamic acids which remains is reacted with a C 5 -C 14 -alkanol in the presence of a metal salt of a strong polybasic acid, such as sulfuric acid. Following transesterification and esterification, the corresponding C 5 -C 14 -alkyl-C 1 -C 4 -alkoxycinnamate, e.g. 2-ethylhexyl-4-methoxycinnamate, is obtained with 83 to 87% yield. 
     SUMMARY OF THE INVENTION 
     We have found a process for the preparation of cinnamic esters of the formula                           
     in which 
     R 1  is an alkyl group having 1 to 4 carbon atoms and 
     R 2  is 2-ethylhexyl or 3-methylbutyl, 
     which comprises condensing acetic C 5 -C 8 -esters of the formula                           
     in which R 2  has the meaning given above, 
     with an alkoxybenzaldehyde of the formula                           
     in which R 1  has the meaning given above, 
     in the presence of an alkali metal alkoxide, and removing alcohol which form during the reaction. 
     DETAILED DESCRIPTION OF THE INVENTION 
     The process according to the invention can be illustrated by the following equation:                           
     Alkoxybenzaldehydes for the process according to the invention may, for example, be anisaldehyde, 4-ethoxybenzaldehyde, 4-propoxybenzaldehyde, 4-iso-propoxy-benzaldehyde, 4-butoxybenzaldehyde and 4-iso-butoxybenzaldehyde. 
     The acetic C 5 -C 8 -esters for the process according to the invention are 2-ethylhexyl acetate or 3-methylbutyl acetate. 
     Alkali metal alkoxides for the process according to the invention are preferably the sodium and potassium alkoxides of lower aliphatic alcohols (C 1  to about C 4 ), such as methanol, ethanol, propanol, isopropanol, butanol and isobutanol. Particular preference is given to sodium methoxide. 
     The acetic C 5 -C 8 -esters can be used in pure form or in a mixture with 2-ethylhexanol or 3-methylbutanol. In order to increase the economic feasibility of the process, it is advantageous to prepare 2-ethylhexyl acetate or 3-methylbutyl acetate before or during the condensation reaction in situ, and to use this reaction mixture without purification or washing for the condensation reaction. 
     This may be carried out, for example, by 
     a) transesterifying 2-ethylhexanol or 3-methylbutanol with an acetic C 1 -C 4 -ester, such as methyl acetate, ethyl acetate, propyl acetate, isopropyl acetate, butyl acetate or isobutyl acetate, in the presence of an alkali metal alkoxide and/or alkaline earth metal alkoxide to give the feed material 2-ethylhexyl acetate or 3-methylbutyl acetate, or 
     b) esterifying 2-ethylhexanol or 3-methylbutanol with acetic acid autocatalytically or in the presence of catalytic amounts of a strong acid, such as sulfuric acid or 4-toluenesulfonic acid, and subjecting the mixture to incipient distillation. 
     The mixture prepared under a) can immediately be taken with a C 1 -C 4 -alkoxybenzaldehyde, such as anisaldehyde, to the condensation reaction. 
     The mixture prepared under b) is prepared for the condensation reaction by adding alkali metal alkoxide. 
     The alkoxybenzaldehyde is preferably added in the temperature range from −10 to 120° C., particularly preferably from 10 to 30° C., over the course of from 0.5 to 5 hours, particularly preferably 1 to 2 hours. 
     After-stirring is carried out for 0.5 to 10 hours, preferably 1 to 3 hours, the reaction temperature being increased to 50 to 150° C., preferably 80 to 110° C. During the after-stirring, a vacuum of from 800 mbar to 2 mbar, preferably from 80 mbar to 40 mbar, is applied. 
     The alcohol which forms during the process according to the invention is preferably separated off during distillation in a vacuum. 
     In the after-stirring period under vacuum, residual alkoxybenzaldehyde is reacted to give the 4-alkoxycinnamic ester, and lower homologous 4-methoxycinnamates are transesterified to give the alkoxycinnamic ester according to the invention, 2-ethylhexyl 4-methoxycinnamate or 3-methylbutyl 4-methoxycinnamate. 
     The reaction mixture is then preferably admixed with a strong acid, such as sulfuric acid, sulfuric acid/NaHSO 4  or 4-toluenesulfonic acid. The mixture is then distilled up to a bottom temperature of 150° C., the 4-methoxycinnamic acid (about 20%) which forms as byproduct being esterified to give the alkoxycinnamic ester according to the invention. At the same time, acetic acid which has formed is esterified to give 2-ethylhexyl acetate or 3-methylbutyl acetate. After washing and distillation, 2-ethylhexyl 4-methoxycinnamate or 3-methylbutyl 4-methoxycinnamate is obtained with high yield (90 to 93%) and high purity (96 to 98%) in a simple distillation. 
     The intermediate fractions 2-ethylhexanol/2-ethylhexyl acetate or 3-methylbutanol/3-methylbutyl acetate which form during the distillation can be co-used again in the next condensation reaction. 
     The advantages of the process according to the invention are: 
     Use of low-cost raw materials 
     Minimizing of waste materials and reuse of secondary streams 
     Minimizing of organic contaminants in waste water 
     It is surprising that alkoxycinnamic esters can be prepared by the process according to the invention in high yields and in high purity by simple distillation. 
    
    
     EXAMPLES 
     
       
                 
         
             
             
         
      
     
     Example 1 
     H 2 SO 4 /NaHSO 4  Variant 
     A 6 l jacketed vessel is charged with 
     2 106 g of 2-ethylhexanol=16.20 mol 
     1 200 g of methyl acetate=16.20 mol 
     187 g of sodium methoxide=3.46 mol 
     At 20 to 30° C., 440.0 g of anisaldehyde=3.24 mol are metered in over about 80 minutes. The mixture is then heated to 100° C., a mixture of methanol and methyl acetate distilling off. As the distillation abates, a vacuum is applied at 60 mbar. At 100° C./60 mbar a total of approximately 920 g of low-boiling components are distilled off over 2 hours. For neutralization, 262 g of 70% strength sulfuric acid=1.87 mol are metered in at 100° C. over 15 minutes. Then, over 2 hours, approximately 160 g of water are removed azeotropically up to a bottom temperature of from 145 to 150° C. After cooling to &lt;100° C., the mixture is washed with 700 g of water. Following phase separation, the mixture is after-washed with 200 g of water and 2 g of technical-grade sodium hydroxide solution. The organic phase is then distilled over a 10 cm Vigreux column. 
     
       
         
               
               
               
               
             
           
               
                   
               
               
                   
                   
                 Head 
                   
               
               
                   
                 Bottom 
                 temperature/ 
                   
               
               
                 Fraction 
                 temperature 
                 mbar 
                 Weight 
               
               
                   
               
             
             
               
                 1 
                 67-200° C. 
                 52-125° C./2.5 
                 1910 g (in each case 40-50% 
               
               
                   
                   
                   
                 of 2-ethylhexanol/2- 
               
               
                   
                   
                   
                 ethylhexyl acetate 
               
               
                 2 (Main 
                 −250° C. 
                 −165° C./2.0 
                  865 g (97% of 2-ethylhexyl 
               
               
                 Fraction) 
                   
                   
                 4-methoxycinnamate) 
               
               
                   
               
             
          
         
       
     
     The yield is 90% of theory. 
     Example 2 
     4-Toluenesulfonic Acid Variant 
     A 6 l jacketed vessel is charged with 
     2 106 g of 2-ethylhexanol 16.20 mol 
     1 200 g of methyl acetate=16.20 mol 
     187 g of sodium methoxide=3.46 mol 
     At 20 to 30° C., 440 g of anisaldehyde=3.24 mol are metered in over about 80 minutes. The mixture is then heated to 100° C., a mixture of methanol and methyl acetate distilling off. As the distillation abates, a vacuum is applied at 60 mbar. At 100° C./60 mbar a total of approximately 920 g of low-boiling components are distilled off over 2 hours. For neutralization, 243 g of 70% strength sulfuric acid=1.73 mol (=equimolar amount) are metered in at 100° C. over 15 minutes. 20 g of 4-toluenesulfonic acid=3%, based on mol of Na methoxide, are added. 
     Further procedure as in Example 1. 
     
       
         
               
               
               
               
             
           
               
                   
               
               
                   
                   
                 Head 
                   
               
               
                   
                 Bottom 
                 temperature/ 
                   
               
               
                 Fraction 
                 temperature 
                 mbar 
                 Weight 
               
               
                   
               
             
             
               
                 1 
                 64-195° C. 
                 51-111° C./15 
                 1814 g (in each case 40-50% 
               
               
                   
                   
                   
                 of 2-ethylhexanol/2- 
               
               
                   
                   
                   
                 ethylhexyl acetate 
               
               
                 2 (Main 
                 −250° C. 
                 −165° C./1.0 
                 870 g (98% of 
               
               
                 Fraction) 
                   
                   
                 2-ethylhexyl 4- 
               
               
                   
                   
                   
                 methoxycinnamate) 
               
               
                   
               
             
          
         
       
     
     The yield is 91.5% of theory. 
     Example 3 
     H 2 SO 4 /NaHSO 4  Variant With 2-Ethylhexanol/2-ethylhexyl Acetate Recycled 
     
       
                 
         
             
             
         
      
     
     A 6 l jacketed vessel is charged with 
     1685 g of 2-ethylhexanol/2-ethylhexyl acetate recycled consisting of 675 g=5.2 mol of 2-ethylhexanol and =5.9 mol of 2-ethylhexyl acetate) 
     480 g of 2-ethylhexanol=3.7 mol 
     720 g of methyl acetate=9.7 mol 
     187 g of sodium methoxide=3.46 mol. 
     At 20 to 30° C., 440 g of anisaldehyde=3.24 mol are metered in over about 80 minutes. 
     Further procedure analogous to Example 1, H 2 SO 4  variant. 
     
       
         
               
               
               
               
             
           
               
                   
               
               
                   
                   
                 Head 
                   
               
               
                   
                 Bottom 
                 temperature/ 
                   
               
               
                 Fraction 
                 temperature 
                 mbar 
                 Weight 
               
               
                   
               
             
             
               
                 1 
                 65-198° C. 
                 48-78° C./1.4- 
                 1700 g (38% of 2- 
               
               
                   
                   
                 1.7 
                 ethylhexanol, 61% of 2- 
               
               
                   
                   
                   
                 ethylhexyl acetate) 
               
               
                 2 (Main 
                 −265° C. 
                 −170° C./1.6 
                 906 g (96.1% 
               
               
                 Fraction) 
                   
                   
                 of 2-ethylhexyl 4- 
               
               
                   
                   
                   
                 methoxycinnamate) 
               
               
                   
               
             
          
         
       
     
     The yield is 92.7% of theory.