The Methanol-Trimethoxyborane Azeotrope as a Solvent for Acid-Catalyzed Reactions. Methyl Esterification

Carboxylic acids UIIldergo a rapid and complete acid-catalyzed methyl esterification in the methanol-trimethoxyborane azeotrope. The reaction is first order in cariboxylic acid and first order in strong acid catalyst. Rate constants are very similar to those in methanol alone. These observations suggest that the mechanism is the same as that in solvent, methanol-AAc 2. The reaction can be made the basis of a convenient preparation of methyl esters. It has been used to prepare methyl benzoate, methyl lactate, and the methyl ester of phenylalanine. Acid-catalyzed acetal and ketal formation in the azeotrope is fast but incomplete. Methyl etherification of tertiary alcohols seems too limited, structurally, to be generally useful. No acid-catalyzed addition of methanol to acetonitrile or acrylonitrile could be observed