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 σ_p values with a slope ca.–1. For a series of ring-substituted complexes (1) lgk₂ correlates with the pK_a 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.