Studies on the C-H/C-D exchange in imidazolium (pilocarpinium) and in 2- and 4-hydroximinomethyl pyridinium compounds

Abstract The H–D exchange of methylene protons in a side‐chain when situated between a positively charged N‐atom (α–CH2) and a ketone function was studied in various pilocarpinium (N‐methyl imidazolium) and hydroximinomethyl pyridiniums using NMR measurements. Structural factors which determine the course of the exchange are reported, and the role of the solvent was examined. Mass‐spectra of nondeuteriated and deuteriated 4‐hydroximinomethyl‐1‐phenacyl‐pyridinium chloride confirmed the fast exchange of the α–CHa. An alternative H–D exchange of the C2–H in the imidazolium moiety of certain pilocarpinium compounds without a ketone in the N‐chain was detected. Mechanisms for the two types of exchanges involving an ylid as intermediate and attack of a deuteron were proposed.