Transition metal carbene chemistry 2: kinetic studies on the nucleophilic substitution reactions of (CO)5Mdouble bond; length as m-dashC(SCH3)CH3 (M = Cr and W) with morpholine in aqueous acetonitrile

Kinetic studies of the aminolysis of [methyl(thiomethyl)carbene]pentacarbonyl chromium(0),(CO)5Crdouble bond; length as m-dashC(CH3)(SCH3) (1-Cr(S)) and [methyl(thiomethyl)carbene]pentacarbonyltungsten(0), (CO)5Wdouble bond; length as m-dashC(CH3)(SCH3) (1-W(S)), with morpholine, a secondary amine, in 50% acetonitrile–50% H2O (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 View the MathML source(TA±) in the first step, followed by deprotonation to form View the MathML sourceTA- in the second step, which, in the third step, converted to product by H2O 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 View the MathML source(k3H2OKa±/k-1Kw), water catalyzed and View the MathML source(k3BHKa±/k-1KaBH), BH+ catalyzed, leaving group departure were found for both the carbenes complexes. All these observations have been explained successfully.