Physical Organic Chemistry of Transition Metal Carbene Complexes. 22.¹ Substituent Effects on the Reversible Cyclization of Fischer Carbene Complexes (CO)₅WC(SCH₂CH₂OH)C₆H₄Z in Aqueous Acetonitrile

A kinetic study of the reversible cyclization of (((2-hydroxyethyl)thio)arylcarbene)pentacarbonyltungsten(0) complexes, 2-SR(Z), in 50% MeCN−50% water (v/v) at 25 °C is reported. The results are consistent with Scheme 1, which shows a pathway in basic solution that involves rapid deprotonation of the OH group followed by rate-limiting cyclization and a pathway in acidic solution that involves direct nucleophilic attack by the OH group. Rate and equilibrium constants for the various elementary steps were determined as a function of the phenyl substituent. They were found to correlate well with the Hammett equation. The ρ values provide insights into (a) the reasons that the equilibrium constants are highly sensitive to substituent effects despite the fact that the negative charge of the adducts is strongly dispersed, (b) the mechanistic details of the cyclization pathway in acidic solution, and (c) the extra stabilizing effect of soft−soft interactions in previously studied adducts between a soft thiolate ion and the extra-soft carbene complex ( 2-SR(Z) ).