METHYL TRANSFER FROM MeCo(III)Pc TO THIOPHENOXIDE

Article abstract of DOI:10.1002/poc.610070206

Transfer of the cobalt-bound methyl in MeCo(III)Pc to thiophenoxide ion was studied (H2Pc is the planar macrocyclic phthalocyanine; the cobalt is held in the center in this plane).In dimethylacetamide solution, the reaction is rapid, requiring stopped flow for the kinetics, and yielding MeSPh and Co(I)Pc^1- in good yield.The kinetics are not simple second order, but instead approach a constant rate at high ->, attributed to the reversible formation of an inert complex with PhS^- occupying the vacant octahedral site in MeCo(III)Pc, on the other side of the phthalocyanine plane from the methyl group.The kinetics allow the estimation of the equilibrium constant, K, and the S_N2 rate constant, k, which at 25 deg C have values of ca. 9.4 x 1E3 l mol^-1 and 1.8 x 1E4 l mol^-1, respectively.Although these values are rough, the ratio k/K is firm at 1.91 +/- 0.02 s^-1; this is the limit of the rate at high ->.An alternative mechanism, which is entirely consistent with the kinetics, involves a rate-determining homolysis of the Co-S bond of the same complex.The mechanism is not favored because the product yields are high for a radical combination process and alternative chain processes are kinetically unacceptable.Further, the rate constant is about what would be expected from the reactivity of other nucleophiles in S_N2 reactions.Further arguments in favor of the S_N2 mechanism are presented.This transfer of the methyl group from Co to S is part of the possible analogy to the vitamin B₁₂-promoted methionine synthesis in nature.The other step in the biological, enzymatic process is the transfer of methyl from the nitrogen of N-methyltetrahydrofolate to cobalt.An attempt to model this with the very reactive N-methyl-2,6-dichloropyridinium ion was unsuccessful; the reaction took an entirely different course, presumably initiated by electron transfer, but leading to substantial loss of Cl^- from the pyridine.No more than 0.5percent methyl transfer took place.This system does mimic well the complete natural enzymatic process.