Transition metal carbene chemistry 2: kinetic studies on the nucleophilic substitution reactions of (CO) 5M [dbnd]C(SCH 3)CH 3 (M = Cr and W) with morpholine in aqueous acetonitrile

Abstract

Kinetic studies of the aminolysis of [methyl(thiomethyl)carbene]pentacarbonyl chromium(0),(CO) 5Cr C(CH 3)(SCH 3) ( 1-Cr(S)) and [methyl(thiomethyl)carbene]pentacarbonyltungsten(0), (CO) 5W C(CH 3)(SCH 3) ( 1-W(S)), with morpholine, a secondary amine, in 50% acetonitrile–50% H 2O (v/v at 25 °C) is reported. The second-order rate constant ( k A in m −1 s −1) increases with amine concentration, giving a linear dependence with an intercept on the rate axis and a tendency towards leveling off at higher amine concentration. The reaction was found to undergo general base catalysis. The mechanism proposed is very similar to those for ester reactions, involving a nucleophilic addition of amine to the substrate to yield a zwitterionic tetrahedral intermediate ( T A ± ) in the first step, followed by deprotonation to form T A - in the second step, which, in the third step, converted to product by H 2O and/or conjugate acid of the base (BH +), assisted MeS − expulsion. The reactivity ( k 1) of 1-W(S) was found to be higher than that of 1-Cr(S), whereas, comparable ( k 3 H 2 O K a ± / k - 1 K w ) , water catalyzed and ( k 3 BH K a ± / k - 1 K a BH ) , BH + catalyzed, leaving group departure were found for both the carbenes complexes. All these observations have been explained successfully.