Physical organic chemistry of transition metal carbene complexes.: Part 20:: Kinetics and mechanism of reactions of (CO)5M=C(OMe)C6H4X (M = Cr and W) with primary aliphatic amines in aqueous acetonitrile.: Substituent effectsd

A kinetic study of the reactions of (CO)(5)Cr=C(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 degrees 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, T-A(+/-), which, in the second step, is followed by a rapid deprotonation of T-A(+/-) to form the anionic steady state intermediate T-A(-), while the third step involves water and RNH3+ catalyzed methoxide ion expulsion, respectively, to form the substitution product, (CO)(5)M=C(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 T-A(+/-) and T-A(-) cannot be observed directly are discussed and a detailed analysis of the substituent effects on various kinetic parameters is presented. (C) 2000 Published by Elsevier Science Ltd.