Dimethyl(alkyl) sulfates as Lewis acid type catalysts for acetalization and related reactions of carbonyl compounds

Abstract

The catalytic activity of dimethyl sulfate (DMS) and its higher homologs in converting carbonyl compounds to O,O-dimethylacetal/ketals is demonstrated. Our mechanistic investigations proved that previously reported Schmitz and Langvad claims stating DMS as an alkylating agent and a source of sulfuric acid in the acetalization of aldehydes were erroneous. Carbonyl compounds were converted to methyl, ethyl acetals/ketals, 1,3-dioxalanes and 1,3-dioxanes, and cyclohexane-1,3-diones into vinylogous esters. Selective aldehyde protection and ketone reduction were preferentially achieved in a single step. This study represents the first instance of using alkylating agents, such as dimethyl, diethyl, diisopropyl, and di-n-butyl sulfates, as catalysts in organic transformations. The catalysis proceeds even in the presence of base, demonstrating that dialkyl sulfates act like Lewis acid-type catalysts. Noncovalent interactions arising from the polar covalent bonds of the alkyl group in dialkyl sulfate are responsible for catalytic activation. Acetalization was found to be instantaneous in the presence of DMS in methanol; this was attributed to the contribution from traces of hidden protons in the presence of base.