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

Abstract

A kinetic study of the aminolysis of [methyl(thiomethyl)carbene]pentacarbonyl chromium(0), (CO) 5Cr C(CH 3)(SCH 3) (Cr–S) and [methyl(thiomethyl)carbene]-pentacarbonyltungsten(0), (CO) 5W C(CH 3)(SCH 3) (W–S), with primary aliphatic amines namely aminoacetonitrile (AA), glycylethyl ester (GEE) and glycinamide (GA) in 50% MeCN–50% H 2O (v/v) at 25 °C is reported. The second-order rate constant ( k A, M −1 s −1) increases with amine concentration, leveling off at higher amine concentration. The general base catalysis was confirmed from the dependence of k A on [AA] f, [NMM] f ( N-methylmorpholine) and also on [OH −] for the M–S–AA reaction. The mechanism proposed is very similar to those for ester reactions, proceeding in a step-wise fashion. β nuc( k 1) for the reactions of primary amines was found to be 0.31 ± 0.02 and 0.29 ± 0.02 for Cr–S and W–S, respectively, which are of the same order as that for the reaction of Cr–SR, but lower than Cr–OR reactions with β nuc( k 1) = 0.60. The reactivity of W–S was found to be higher than Cr–S. All these observations have been explained successfully.