Methyl transfer from CH(3)Co(III)Pc to thiophenoxides revisited: remote substituent effect on the rates

A two-step mechanism of the reaction of CH(3)Co(III)Pc (Pc = dianion of phthalocyanine) with thiophenoxides in DMA has been confirmed, and the visible spectrum of the inactive transient, CH(3)Co(III)Pc(SAr)(-), has been determined. Rapid rates for ligation of CH(3)Co(III)Pc, yielding CH(3)Co(III)Pc(S-C(6)H(4)-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 CH(3)Co(III)Pc(S-C(6)H(4)-X)(-) consistent with those estimated from concentration dependence of the amplitude of the ligand-exchange step. Cyclic voltammetry provides first reduction potential for CH(3)Co(III)Pc(DMA) of -1.42 V vs Fc(+)/Fc, which makes an OSET mechanism unlikely. Homolytic decay of CH(3)Co(III)Pc(SAr)(-) has also been ruled out. All of the kinetic data, including Hammett's rho = -2.3 +/- 0.1, N-donor inhibition, and alkyl group effect, Me > Et, indicate that the reaction is a normal S(N)2 methyl transfer, only very fast. Methyl transfer to aliphatic thiolates is also rapid and follows the same S(N)2 mechanism. Exceptional methyl-transfer reactivity of the phthalocyanine model sharply contrasting with the inertness of methylcobaloxime is explained.