Methyl transfer from MeCo(III)Pc to thiophenoxide

Abstract

Transfer of the cobalt-bound methyl in MeCo(II1)Pc to thiophenoxide ion was studied (HzPc is the planar macrocyclic phthalocyanine; the cobalt is held in the center in this plane). In dimethylacetamide solution, the reaction is rapid, requiring stopped Pow for the kinetics, and yielding MeSPh and Co(1)Pc-in good yield. The kinetics are not simple second order, but instead approach a constant rate at high [PhS-1 , attributed to the reversible formation of an inert complex with PhS-occupying the vacant octahedral site in MeCo(lII)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 &2 rate constant, k, which at 25 O C have values of ca. 9.4 X 103 I mol-' and 1.8 X lo4 I mol-', respectively.Although these values are rough, the ratio k / K is 6rm at 1.91 f 0.02 s-'; this is the limit of the rate at high [PhS-1. 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 &2 reactions. Further arguments in favor of the &2 mechanism are presented. This transfer of the methyl group from Co to S is part of the possible analogy to the vitamin B1z-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 CI-from the pyridine. No more than 0.5% methyl transfer took place. This system does mimic well the complete natural enzymatic process.

Vitamin B12 as an allosteric cofactor; dual fluorescence, hysteresis, oscillations and the selection of corrin over porphyrinThe chemistry of vitamin B12. Part 32. For part 31, see ref. 1.

“…In addition to the linear correlation between the logarithm of the rate constants for methylation of a range of Co(I) complexes and their Co(II)/Co(I) redox potentials, 24 further more qualitative evidence 3b shows a positive correlation between an increase in electron density on Co(I), whether caused by changes in the cis-or trans-ligand, an increase in the rate of Me + transfer from donor to Co(I), a decrease in the rate of onward Me + transfer from Co to a nucleophilic acceptor and an increase in the rate of Me − transfer to the electrophilic Hg(II) ion. The observed pattern is self-consistent and supports Lewis's conclusion 55 that the larger part of the rate differences is due to the thermodynamic term in the Marcus-Hush equation. As examples of decreasing reactivity of Co(I) with increasing Co(II)/Co(I) potential (vs. SCE), the salen complex (−1.2 V) can be methylated by both MeI and NMe 4 + , the Cbi (−0.74 V) by MeI (log k 2 /M −1 s −1 = 4.5 in MeOH) but not by NMe 4 + (a maximum rate of log k 2 /M −1 s −1 ≤ −5.7 in aqueous solution can be estimated from the negative results in ref.…”

The First X-ray Structural Evidence Demonstrating Thiolate Coordination in an Organocobalt B12Model Complex: Implications for Methionine Synthase

“…The methyl group of MeB 12 was reported to transfer to dithiothreitol, reaction ii ( t 1/2 = 1.5 h at 43 °C) . In contrast, the MeB 12 model CH 3 CoPc transferred a CH 3 group to benzenethiolate at ambient temperature with t 1/2 = 0.4 s . Wishing to test for modes of reactivity of thiolates with organocobaloximes other than S-ligation, we selected a cobaloxime with a strongly coordinating axial ligand trans to the methyl group to block the ligand-exchange reaction with EtS - (reaction iii).…”

“…The reported result that thiol-promoted Co−C bond cleavage (reaction ii) of organocobaloximes (LCo(DH) 2 R, Chart ) occurred under neutral to acidic conditions , could not be reproduced. − Treatment of MeCbl with the thiolate form of β-mercaptoethanol resulted in methylation of the thiolate and Co−C bond cleavage (reaction ii) . Another reaction involving methylation of a thiolate by an organocobalt compound (reaction ii) was reported recently for the five-coordinate model CH 3 Co III Pc (Pc = phthalocyanine). The thiolate ligation reaction pathway (reaction iii) has been invoked in several instances for MeCbl and organocobalt B 12 models, although no characterized isolated complexes of this type have been reported.…”

Reactions of nucleophiles with 5-(alkoxy)thianthrenium ions

“…Preference for reaction at sulfonium sulfur is also seen in the reactions of PhS À with 1f,g (Table 4) and with methoxydiphenylsulfonium tetrafluoroborate (2, 35,36 In sulfolane solution, this ratio of reactivities toward MeI is 37 000. 37 The ratio for displacement of methyl from cobalt-bound methyl in methyl cobalt(III)phthalocyanine in dimethylacetamide solution is 41 000. 37 Intrinsically, therefore, especially in analogy with reaction with MeOTs, one would expect PhS À to react more easily than I À (and Br À ) at the alkyl group and displace ThO from 1 and Ph 2 SO from 2.…”

“…37 The ratio for displacement of methyl from cobalt-bound methyl in methyl cobalt(III)phthalocyanine in dimethylacetamide solution is 41 000. 37 Intrinsically, therefore, especially in analogy with reaction with MeOTs, one would expect PhS À to react more easily than I À (and Br À ) at the alkyl group and displace ThO from 1 and Ph 2 SO from 2. Reaction at both sulfonium sulfur (43%) and carbon (12%) did ocur in reaction of PhS À with methoxymethylp-tolylsulfonium tetrafluoroborate (8) in dichloromethane (Scheme 7).…”