| Abstract
| - Rates for both rapid ligation and follow-up demethylation of CH3CoIIIPc by thiophenoxides in dimethylacetamide are reported. Remote substituent effect on the rates is presented. Mechanisms alternative to the favored SN2 have been ruled out. Alkyl group effect (Me > Et) and axial base inhibition of methyl transfer have been demonstrated. The rate constant for homolytic decay of [CH3CoPc]- anion radical of 2.8 s-1 has been determined by cyclic voltammetry. Methyl transfer from CH3CoIIIPc to aliphatic thiolates is extremely rapid.
- A two-step mechanism of the reaction of CH3CoIIIPc (Pc = dianion of phthalocyanine) with thiophenoxides in DMAhas 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 stepproceeds probably by an Id mechanism. Kinetic data for the follow-up methyl-transfer step yield second-order rateconstants and stability constants for CH3CoIIIPc(S−C6H4−X)- consistent with those estimated from concentrationdependence of the amplitude of the ligand-exchange step. Cyclic voltammetry provides first reduction potential forCH3CoIIIPc(DMA) of −1.42 V vs Fc+/Fc, which makes an OSET mechanism unlikely. Homolytic decay ofCH3CoIIIPc(SAr)- has also been ruled out. All of the kinetic data, including Hammett's ρ = −2.3 ± 0.1, N-donorinhibition, 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.
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