Physical Organic Chemistry of Transition Metal Carbene Complexes. Part 20: Kinetics and Mechanism of Reactions of (CO)5M[dbnd]C(OMe)C6H4X (M=Cr and W) with Primary Aliphatic Amines in Aqueous Acetonitrile. Substituent Effects

Abstract

A kinetic study of the reactions of (CO)5CrC(OMe)C6H4X (X=4-Cl, 4-F, H, 4-Me, 4-OMe and 4-NMe2) and the respective tungsten carbene complexes with primary aliphatic amines (mainly n-butylamine) in 50% MeCN–50% water (v/v) at 25°C is reported. The rate law is complex and shows OH− and amine catalysis at low amine and OH− concentrations but not at high concentrations. The results are consistent with a three-step mechanism: the first step is nucleophilic addition of the amine to the carbene complex to form a zwitterionic steady state intermediate, TA±, which, in the second step, is followed by a rapid deprotonation of TA± to form the anionic steady state intermediate TA−, while the third step involves water and RNH3+ catalyzed methoxide ion expulsion, respectively, to form the substitution product, (CO)5MC(NHR)C6H4X (M=Cr or W). It is shown that the third step is rate limiting at low OH− and amine concentrations while the first step is rate limiting at high OH− and/or amine concentrations. The reasons why TA± and TA− cannot be observed directly are discussed and a detailed analysis of the substituent effects on various kinetic parameters is presented.