Methyl transfer from CH3Co^IIIPc to thiophenoxides revisited: Remote substituent effect on the rates

Authors:

• Włodzimierz Gałęzowski

Abstract

A two-step mechanism of the reaction of CH₃Co^IIIPc (Pc = dianion of phthalocyanine) with thiophenoxides in DMA has been confirmed, and the visible spectrum of the inactive transient, CH₃Co ^IIIPc(SAr)^-, has been determined. Rapid rates for ligation of CH₃Co^IIIPc, yielding CH₃Co ^IIIPc(S-C₆H₄-X)^-, are virtually independent of X; this step proceeds probably by an I_d mechanism. Kinetic data for the follow-up methyl-transfer step yield second-order rate constants and stability constants for CH₃Co^IIIPc(S-C ₆H₄-X)^- consistent with those estimated from concentration dependence of the amplitude of the ligand-exchange step. Cyclic voltammetry provides first reduction potential for CH₃Co ^IIIPc(DMA) of -1.42 V vs Fc⁺/Fc, which makes an OSET mechanism unlikely. Homolytic decay of CH₃Co^IIIPc(SAr) ^- has also been ruled out. All of the kinetic data, including Hammett's p = -2.3 ± 0.1, N-donor inhibition, and alkyl group effect, Me > Et, indicate that the reaction is a normal S_N² methyl transfer, only very fast. Methyl transfer to aliphatic thiolates is also rapid and follows the same S_N² mechanism. Exceptional methyl-transfer reactivity of the phthalocyanine model sharply contrasting with the inertness of methylcobaloxime is explained. © 2005 American Chemical Society.