Regioselective deuteriation kinetics of 2‐, 4‐, and 6‐methyl groups in D2O solutions of pyrylium and N‐methylpyridinium perchlorates possessing also 3‐methyl or 3‐phenyl groups

Abstract

AbstractBy 1H‐NMR it was found that 2, 3, 4, 6‐tetramethylpyrylium perchlorate (I) in D2O undergoes isotopic exchange at the 2, 4, and 6‐methyl groups with relative rates 5 : 40 : 1, respectively (assignments of α‐methyl NMR signals were substantiated by unambiguous synthesis). If the 3‐methyl group is replaced by a 3‐phenyl group, the relative rates for the pyrylium salt III are 2.5 : 49 : 1, respectively (in D2O + CD3OD). On the other hand, the relative rates for the 2, 4, and 6‐methyls in 1,2,4,6‐tetramethyl‐3‐phenylpyridinium perchlorate are 12 : 1 : 4, respectively (in D2O + CD3OD + NaOD). These rate differences allow the preparation of selectively deuteriated pyrylium or pyridinium salts, and of other aromatic or heterocyclic compounds starting from pyrylium salts with selectively deuteriated side‐chains.