Role of Trace Amine in the Metathesis of Imines by CpTa(NR)Cl 2

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 metathesis of 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 obs of 1.29 × 10 - 4 min - 1 (σ 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( 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.