Reactions of [Aryloxy(phenyl)carbene]pentacarbonylchromium(0) Complexes with Thiolate Ions. Decreasing Reactivity with Increasing Basicity of the Nucleophile

Claude F. Bernasconi, Moisés Pérez-Lorenzo, and Sara J. Codding
J. Org. Chem., 2007, 72, 9456–9463


A kinetic study of the reactions of thiolate ions with three Fischer-type [aryloxy(phenyl)carbene]pentacarbonyl chromium(0) complexes in 50% MeCN−50% water (v/v) is reported. Brønsted plots of the second-order rate constants are biphasic with an initial steep rise for weakly basic thiolate ions (βnuc ≈ 1.0) followed by a slightly descending leg with a negative slope (βnuc ≈ −0.2) for strongly basic thiolate ions. This indicates a change from rate-limiting leaving group departure at low pKaRSH to rate-limiting nucleophilic attachment at high pKaRSH. The negative βnuc values result from a combination of minimal progress of C−S bond formation at the transition state and the requirement for partial desolvation of the nucleophile before it enters the transition state. Possible factors that may affect the degree of bond formation in reactions of Fischer carbene complexes as well as reactions of other unsaturated electrophiles with thiolate ions are discussed.