Acid-catalysed [2H6]ethanolyses of alkyl orthoacetates: evidence for keten dialkylacetals as intermediates

Abstract

N.m.r. analyses of 2,6-dichlorobenzoic acid-catalysed [²H₆]ethanolyses of 1,1,1-triethoxyethane, 1,1,1-trimethoxyethane, and 2-ethoxy-2-methyl-1,3-dioxolan indicate that in each case the orthoacetyl methyl protons are subject to deuterium exchange. This exchange, however, proceeds much more slowly than the accompanying alkoxy-exchange. For 1,1,1-triethoxyethane, deuterium incorporation (47 atoms % at equilibrium) was confirmed by hydrolysis and mass-spectral analysis of the resultant ethyl acetate. A reaction mechanism in which a keten dialkylacetal is formed by elimination of a proton from an intermediate carboxonium ion is suggested to account for the observations.