Physical organic chemistry of transition metal carbene complexes. 27. Substituent effects on the nucleophilic substitution of [aryl(thiomethyl)carbene]pentacarbonylchromium(0) complexes by amines in aqueous acetonitrile

A kinetic study of the reactions of (CO)(5)Cr=C(SMe)C(6)H(4)Z (Z = 4-CF3, 4-Cl, 4-F, 4-Me, 4-MeO, and 4-Me2N) with n-butylamine, 2-chloroethylamine, piperidine, and 1-(2-hydroxyethyl)piperazine in 50% MeCN-50% water (v/v) at 25 degreesC is reported. In all cases the nucleophilic attachment of the amine to the carbene complex is rate limiting, which contrasts with the previously reported aminolysis of the methoxy analogues ((CO)(5)Cr=C(OMe)C(6)H(4)Z), where leaving group departure is rate limiting in most cases. The substituent effect on the nucleophilic attachment step is much weaker (average rho = 0.55) than for the methoxy analogue (rho = 2.03). It is shown that the small rho value for (CO)(5)Cr=C(SMe)C(6)H(4)Z is "normal" and reflects the near cancellation of the substituent effects on the partial positive and partial negative charges at the transition state. These findings provide evidence that the high rho value for (CO)(5)Cr=C(OMe)C(6)H(4)Z is "abnormal" and the result of the strong pi-donor effect of the methoxy group on the reactant carbene complex.