Interfacial method of preparing ester-substituted diaryl carbonates

Abstract

High yields of ester-substituted diary carbonates such as bis-methyl salicyl carbonate were obtained by the condensation of methyl salicylate with phosgene in the presence of a phase transfer catalyst (PTC) in an interfacial reaction system in which the pH of the aqueous phase was greater than 9.3. Using the method of the present invention conversions of greater than 99% were obtained whereas under standard conditions using triethylamine as the catalyst conversions were limited to 70-75% of the methyl salicylate starting material even with a 20 mole % excess of added phosgene. The optimized conditions of the of the present invention use only a slight excess of phosgene and represent an attractive route for the manufacture of bis methyl salicyl carbonate and ester-substituted diaryl carbonates generally.

1. A method of preparing ester-substituted diaryl carbonates, said method having a contact time, said method comprising contacting an ester-substituted phenol with phosgene and a phase transfer catalyst in the presence of an organic solvent and an aqueous phase wherein the aqueous phase is maintained at a pH of at least about 9.3 throughout the contact time, said phosgene being used in an amount corresponding to between about 0.95 and about 1.20 moles of phosgene per mole of product ester-substituted phenol  diarylcarbonate. 
     
     
       2. A method according to  claim 1  wherein said ester-substituted diaryl carbonate has structure I 
                 
 
       wherein R 1  is independently at each occurrence C 1 -C 20  alkyl radical, C 4 -C 20  cycloalkyl radical or C 4 -C 20  aromatic radical, R 2  is independently at each occurrence a hydrogen atom, halogen atom, cyano group, nitro group, C 1 -C 20  alkyl radical, C 4 -C 20  cycloalkyl radical, C 4 -C 20  aromatic radical, C 1 -C 20  alkoxy radical, C 4 -C 20  cycloalkoxy radical, C 4 -C 20  arloxy radical, C 1 -C 20  alkylthio radical, C 4 -C 20  cycloalkylthio radical, C 4 -C 20  arylthio radical, C 5 -C 20  alkylsulfinyl radical, C 4 -C 20  cycloalkylsulfinyl radical, C 4 -C 20  arylsulfinyl radical, C 1 -C 20  alkylsulfonyl radical, C 4 -C 20  cycloalkylsulfonyl radical, C 4 -C 20  arylsulfonyl radical, C 1 -C 20  alkoxycarbonyl radical, C 4 -C 20  cycloalkoxycarbonyl radical, C 4 -C 20  aryloxycarbonyl radical, C 2 -C 60  alkylamino radical, C 6 -C 60  cycloalkylamino radical, C 5 -C 60  arylamino radical, C 1 -C 40  alkylaminocarbonyl radical, C 4 -C 40  cycloalkylaminocarbonyl radical, C 4 -C 40  arylaminocarbonyl radical, and C 1 -C 20  acylamino radical; and b is independently at each occurrence an integer 0-4. 
     
     
       3. A method according to  claim 2  wherein the ester-substituted diaryl carbonate is bis-methyl salicyl carbonate. 
     
     
       4. A method according to  claim 1  wherein said ester-substituted phenol has structure II 
                 
 
       wherein R 1  is C 1 -C 20  alkyl radical, C 4 -C 20  cycloalkyl radical or C 4 -C 20  aromatic radical, R 2  is independently at each occurrence a hydrogen atom, halogen atom, cyano group, nitro group, C 1 -C 20  alkyl radical, C 4 -C 20  cycloalkyl radical, C 4 -C 20  aromatic radical, C 1 -C 20  alkoxy radical, C 4 -C 20  cycloalkoxy radical, C 4 -C 20  aryloxy radical, C 1 -C 20  alkylthio radical, C 4 -C 20  cycloalkylthio radical, C 4 -C 20  arylthio radical, C 1 -C 20  alkylsulfinyl radical, C 4 -C 20  cycloalkylsulfinyl radical, C 4 -C 20  arylsulfinyl radical, C 1 -C 20  alkylsulfonyl radical, C 4 -C 20  cycloalkylsulfonyl radical, C 4 -C 20  arylsulfonyl radical, C 1 -C 20  alkoxycarbonyl radical, C 4 -C 20  cycloalkoxycarbonyl radical, C 4 -C 20  aryloxycarbonyl radical, C 2 -C 60  alkylamino radical, C 6 -C 60  cycloalkylamino radical, C 5 -C 60  arylamino radical, C 1 -C 40  alkylaminocarbonyl radical, C 4 -C 40  cycloalkylaminocarbonyl radical, C 4 -C 40  arylaminocarbonyl radical, and C 1 -C 20  acylamino radical; and b is an integer 0-4. 
     
     
       5. A method according to  claim 4  wherein said ester-substituted phenol is selected from the group consisting of methyl salicylate, ethyl salicylate, isopropyl salicylate and benzyl salicylate. 
     
     
       6. A method according to  claim 1  wherein said phase transfer catalyst comprises a quaternary ammonium compound having structure III 
                 
 
       wherein R 3 -R 6  are independently a C 1 -C 20  alkyl radical, C 4 -C 20  cycloalkyl radical or a C 4 -C 20  aryl radical and X −  is at least one organic or inorganic anion. Suitable anions X −  include hydroxide, halide, carboxylate, sulfonate, sulfate, carbonate and bicarbonate. 
     
     
       7. A method according to  claim 6  wherein said phase transfer catalyst is selected from the group consisting of methyl tributyl ammonium chloride, tetrabutyl ammonium chloride and decyl trimethyl ammonium chloride. 
     
     
       8. A method according to  claim 1  wherein said aqueous phase is maintained at a pH in a range between about 9.3 and about 12. 
     
     
       9. A method according to  claim 8  wherein said aqueous phase is maintained at a pH in a range between about 9.3 and about 12 by the addition of aqueous alkali metal hydroxide, aqueous alkaline earth metal hydroxide, or a mixture thereof. 
     
     
       10. A method according to  claim 9  wherein the alkali metal hydroxide is sodium hydroxide. 
     
     
       11. A method according to  claim 1  wherein said solvent is a halogenated solvent. 
     
     
       12. A method according to  claim 11  wherein said halogenated solvent is methylene chloride. 
     
     
       13. A method according to  claim 1  wherein said solvent is a non-halogenated solvent. 
     
     
       14. A method according to  claim 13  wherein said solvent is toluene. 
     
     
       15. A method according to  claim 1  wherein the phase transfer catalyst is present in a range between about 0.1 and about 2 mole percent based upon the number of moles of ester-substituted phenol. 
     
     
       16. A method of preparing ester-substituted diaryl carbonates, said method having a contact time, said method comprising contacting an ester-substituted phenol with phosgene, a phase transfer catalyst, and a tertiary amine, in the presence of an organic solvent and an aqueous phase wherein the aqueous phase is maintained at a pH of at least about 9.3 throughout the contact time, said phosgene being used in an amount corresponding to between about 0.95 and about 1.10 molar equivalents based on said product ester-substituted phenol  diarylcarbonate, whereby at least 90% of the ester-substituted phenol is converted into product ester-substituted diaryl carbonate. 
     
     
       17. A method according to  claim 16  wherein said phase transfer catalyst comprises a quarternary ammonium compound having structure III 
                 
 
       wherein R 3 -R 6  are independently a C 1 -C 20  alkyl radical, C 4 -C 20  cycloalkyl radical or a C 4 -C 20  aryl radical and X −  is at least one organic or inorganic anion. Suitable anions X −  include hydroxide, halide, carboxylate, sulfonate, sulfate, carbonate and bicarbonate. 
     
     
       18. A method according to  claim 1  wherein the tertiary amine is selected from the group consisting of triethylamine, diispropyl ethyl amine, tributyl amine, and 1,4-diazabicyclooctane. 
     
     
       19. A method according to  claim 18  wherein the amine is triethylamine. 
     
     
       20. A method of preparing bis-methyl salicyl carbonate said method comprising contacting a two phase mixture of a solution of methyl salicylates in methylene chloride and an aqueous phase, with from about 0.95 to about 1.10 molar equivalents of phosgene and from about 0.1 to about 2 molar equivalents of a quaternary ammonium compound, said molar equivalents of phosgene and quaternary ammonium compound being based on the number of moles of methyl salicylate employed, said aqueous phase being maintained at a pH of between about 9.3 and about 12 by the addition of aqueous sodium hydroxide solution, whereby at least 90% of the methyl salicylate is converted into product bis-methyl salicyl carbonate. 
     
     
       21. A method according to  claim 20  wherein said quarternary ammonium compound has structure III 
                 
 
       wherein R 3 -R 6  are independently a C 1 -C 20  cycloalkyl radical, C 4 -C 20  cycloalkyl radical or a C 4 -C 20  aryl radical and X −  is at least one organic or inorganic anion. Suitable anions X −  include hydroxide, halide, carboxylate, sullfonate, sulfate, carbonate and bicarbonate. 
     
     
       22. A method according to  claim 21  wherein structure III is methyl tributyl ammonium chloride.