PHYSICAL ORGANIC-CHEMISTRY OF TRANSITION-METAL CARBENE COMPLEXES .2. KINETICS AND MECHANISM OF REACTIONS OF [METHOXY(PHENYL)CARBENE]PENTACARBONYLCHROMIUM(0) WITH PRIMARY ALIPHATIC-AMINES IN AQUEOUS ACETONITRILE

A kinetic study of the aminolysis of [methoxy(phenyl)carbene]pentacarbonylchromium(0) with five primary aliphatic amines in 20% acetonitrile-80% water (v/v) at 25-degrees-C is reported. Second-order rate constants (k(A)) increase with amine and OH- concentration, which indicates general base catalysis. Plots of k(A) vs amine or OH- concentration are curved and level off at high concentration; this implies a change in rate-limiting step and demonstrates that the reaction is stepwise. The mechanism is similar to that for ester aminolysis, i.e., the first step is nucleophilic addition of the amine to the substrate to yield a zwitterionic tetrahedral intermediate (T(A)+/-), followed by deprotonation of T(A)+/- to form T(A)- which, in a third step, is converted to products by general acid catalyzed methoxide expulsion. In contrast to ester aminolysis, where general base catalysis is a consequence of rate-limiting deprotonation of T(A)+/-, rate-limiting conversion of T(A)- to products is responsible for the general base catalysis with the carbene complex. Possible reasons for this contrast include a stronger stabilization of T(A)- and T(A)+/- and higher intrinsic barriers for the first and third steps of the reaction mechanism in the carbene-complex reaction. Structure-reactivity coefficients suggest that the transition state for the first step is T(A)+/--like while the transition state for the third step is T(A)--like.