Noncatalytic Esterification in the Presence of Ammonium Salts

Normant and coworkers have recently reported that the reaction of solid potassium acetate with benzyl chloride in acetonitrile is catalyzed by a wide variety of tertiary di- and polyamines (Eq. 6.3) [5]. The catalytic efficiency depends on the cation as- 

The catalytic activity could be due to the coordination and enhanced solubilization of potassium acetate by the amine or it could be due to in situ quaternary ion formation (or both). Although the authors rule out the latter as a mechanism of catalysis, it seems unlikely that there is no component of this mechanism operating. 

In light of the observation that Wittig-Horner-Emmons reactions require no additional catalyst [6] and that certain a-phosphoryl sulfoxides catalyze basic two-phase alkylations [7], it is clear that more study is required. The alkylation of several carboxylate anions are recorded in Table 6.1. 

Several non-catalytic systems have been reported to give good yields of esters. For example, if an equimolar mixture of a carboxylic acid, an alkyl halide and a tertiary amine (most commonly triethylamine) are heated for a few hours at 140 °C, excellent yields of esters are usually obtained. Under these conditions, methylene chloride was found to give methanediol-Z /5-esters [9]. Examples of esters synthesized by this approach are presented in Table 6.2. 

The homogeneous reaction of tetrabutylammonium carboxylates (ion pair extraction) with methyl iodide in a two-phase system consisting of aqueous base and dichloromethane has also been reported. In most cases where the carboxylic acid was monobasic, excellent to quantitative yields of ester were observed. It was also noted that the esterification was insensitive to steric hindrance in the acid component under these conditions (see Eq. 6.4) [10]. 

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