The aryl group controls the α/γ regioselectivity in deuterations of 2‐(p‐substituted benzyl)‐4,6‐dimethylpyrylium salts (1)

Abstract A kinetic study is reported for the deuteration (in sodium formate‐con‐taning HCOOD at 50–73°) of 2‐benzyl‐4,6‐dimethylpyrylium perchlorate and of its derivatives possessing para‐substituents at the phenyl ring (NO2, Cl, Me, OMe). Intramolecular comparison of deuteration rates indicates that the 4‐methyl is always deuterated 8–10 times faster than the 6‐methyl; however, the 2‐methylene group is deuterated with a rate dictated by the nature of the aryl group : faster than the 4‐methyl with p‐NO2 and p‐C1 substituents, more slowly than the γ‐methyl group with p‐Me and p‐OMe substituents. Hammett plots show good correlations for relative deuteration rates of the methylene group, and for 1H‐NMR chemical shifts for methyl and methylene protons.