S N 2’ versus S N 2 Reactivity: Control of Regioselectivity in Conversions of Baylis–Hillman Adducts

TiCl 4 ‐induced Baylis–Hillman reactions of α,β‐unsaturated carbonyl compounds with aldehydes yield the ( Z )‐2‐(chloromethyl)vinyl carbonyl compounds 5 , which react with 1,4‐diazabicyclo[2.2.2]octane (DABCO), quinuclidine, and pyridines to give the allylammonium ions 6 . Their combination with less than one equivalent of the potassium salts of stabilized carbanions (e.g. malonate) yields methylene derivatives 8 under kinetically controlled conditions (S N 2’ reactions). When more than one equivalent of the carbanions is used, a second S N 2’ reaction converts 8 into their thermodynamically more stable allyl isomers 9 . The second‐order rate constants for the reactions of 6 with carbanions have been determined photometrically in DMSO. With these rate constants and the previously reported nucleophile‐specific parameters N and s for the stabilized carbanions, the correlation log k (20 °C)= s ( N + E ) allowed us to calculate the electrophilicity parameters E for the allylammonium ions 6 (−19< E