Methyl transfer from MeCo(III)Pc to thiophenoxide

Abstract

AbstractTransfer 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− in good yield. The kinetics are not simple second order, but instead approach a constant rate at high [PhS−], 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 SN2 rate constant, k, which at 25°C have values of ca. 9·4 × 103 l mol−1 and 1·8 × 104 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 [PhS−]. 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 SN2 reactions. Further arguments in favor of the SN2 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 0.5% methyl transfer took place. This system does mimic well the complete natural enzymatic process.