New insights into the old reaction between acryloyl chlorides and pyridine

Abstract

Direct acylation reactions of alcohols with acid chlorides in the presence of pyridine leads to the formation of unexpected pyridinium derivatives as major products. Although this phenomenon was briefly reported several decades ago, a detailed structure elucidation of the intermediates and ionic products was missing. In this study, the formed pyridinium products are structurally characterized and the underlying reaction mechanism is discussed. The addition of reactants in the order acryloyl chloride—R-OH—pyridine yields a structure P1, which was tentatively proposed before. However, if the order of reactant addition was changed, that is, R-OH was added to a mixture of acryloyl chloride and pyridine, two new types of pyridinium derivatives (P2 and P3) were observed. Their formation implies the unprecedented β-addition of pyridine to acryloyl chloride followed by a Michael addition of the nucleophilic α-carbon or by an alkylation of the activated carboxyl group. The proposed reaction mechanism is supported by a detailed structural analysis of intermediates and products.