Role of trace amine in the metathesis of imines by CpTa(=NR)Cl2

Tantalum imido compound CpTa(=NBut)Cl-2 (1) reacts with imines to give NR exchange (metathesis) products. Addition of < 5 mol % of compound I to a 1: 1 mixture of two different imines resulted in catalytic cross metathesis of the two imines to give an equilibrium mixture of imines. Kinetic studies of the reaction of I with excess imine (Bu-t)HC=N(p-tolyl) (2a) gave an average k(obs) of 1.29 x 10(-1) min(-1) (sigma 0.28). Pseudo-first-order behavior was observed for 1. The dependence of k(obs) on the concentration of 2a was found to be nonlinear and dependent on catalytic amounts of contaminant amine p-tolylamine (9). Reaction of 1 with tert-butylamine 6) results in the formation of the imide/amide CpTa(=NBut)(NHBut)Cl 7) and HCl. Compound 7 did not react with imines to give exchange products, so it is proposed that the reaction of 1 with imines is significantly accelerated by the presence of HCl.