Kinetics and mechanism of vinylation of ortho-palladated NN-dialkylbenzylamines by para-substituted styrenes

Abstract

The kinetics of vinylation of ortho-palladated NN-dialkylbenzylamines (1a–g) by CH2CHC6H4R3-p(R3= H, Cl, Br, Me, and MeO) to from the respective 2-dialkylaminomethylstilbenes (2) has been studied mainly at 30°C in acetic acid solvent. The reaction follows second-order kinetics: d[(2)]/dt=k2[(1)][styrene]. The rate constants k2 increase markedly in the presence of both alkali-metal perchlorates (MClO4) and small concentrations of perchloric acid, k2 being directly proportional to [HClO4]. It is shown that the salt effect originates from the solvolytic reaction MClO4+ HOAc ⇌ MOAc + HClO4. Conversion of dimer (1a) into the respective monomer (3) stops the reaction, but the latter species readily forms π-complexes with styrenes. The stability constants of 1 : 1 π-complexes have been determined and their Hammett correlation with a slope of –1.87 is established. For a series of substituted styrenes lgk2 is the linear function of Hammett σp values with a slope ca.–1. For a series of ring-substituted complexes (1) lgk2 correlates with the pKa of the parent benzylamines with a slope of 0.98. The results are interpreted in terms of a mechanism involving intermediate protonation of (1), π-co-ordination of alkene, and subsequent rate-determining intramolecular insertion of styrene into a Pd–C bond through a four-centred transition state.