Abstract :
A two-step mechanism of the reaction of CH3CoIIIPc (Pc = dianion of phthalocyanine) with thiophenoxides in DMA has been confirmed, and the visible spectrum of the inactive transient, CH3CoIIIPc(SAr)-, has been determined. Rapid rates for ligation of CH3CoIIIPc, yielding CH3CoIIIPc(S-C6H4-X)-, are virtually independent of X; this step proceeds probably by an Id mechanism. Kinetic data for the follow-up methyl-transfer step yield second-order rate constants and stability constants for CH3CoIIIPc(S-C6H4-X)- consistent with those estimated from concentration dependence of the amplitude of the ligand-exchange step. Cyclic voltammetry provides first reduction potential for CH3CoIIIPc(DMA) of -1.42 V vs Fc+/Fc, which makes an OSET mechanism unlikely. Homolytic decay of CH3CoIIIPc(SAr)has also been ruled out. All of the kinetic data, including Hammettʹs (rho)= -2.3 (plus-minus) 0.1, N-donor inhibition, and alkyl group effect, Me > Et, indicate that the reaction is a normal SN2 methyl transfer, only very fast. Methyl transfer to aliphatic thiolates is also rapid and follows the same SN2 mechanism. Exceptional methyl-transfer reactivity of the phthalocyanine model sharply contrasting with the inertness of methylcobaloxime is explained.
