Role of Trace Amine in the Metathesis of Imines by CpTa(NR)Cl2

Tantalum imido compound CpTa(=NBu t )Cl 2 (1) reacts with imines to give NR exchange (metathesis) products. Addition of <5 mol % of compound 1 to a 1:1 mixture of two different imines resulted in catalytic cross metathesisof the two imines to give an equilibrium mixture of imines. Kinetic studies of the reaction of 1 with excess imine (Bu t )HC=N(p-tolyl) (2a) gave an average k o b s of 1.29 × 10 min 1 (σ 0.28). Pseudo-first-order behavior was observed for 1. The dependence of k o b s 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(=NBu t )(NHBu t )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.