Electron-transfer properties of active aldehydes of thiamin coenzyme models, and mechanism of formation of the reactive intermediates

The active aldehydes 2a-c^- derived from the reaction of 3-benzylthiazolium salts (1a⁺: 3-benzyl-4-methylthiazolium bromide, 1b⁺: 3-benzyl-4,5-dimethylthiazolium bromide, 1c⁺: 3-benzylthiazolium bromide) with o-tolualdehyde in the presence of DBU (1,8-diazabicyclo[5.4.0]undec-7-ene) are stable in deaerated MeCN at 298 K because of the steric bulkiness of the o-methyl group, which prohibits benzoin condensation with a second aldehyde molecule. The one-electron oxidation of fourteen different active aldehydes (2a-n^-) derived from various aldehydes occurs at -0.98 to -0.77 V vs. SCE in deaerated MeCN at 298 or 233 K and leads to formation of the corresponding radical intermediates (2a-n^•), which have been characterized by electron spin resonance (ESR) spectroscopy. The rapid rates of electron exchange between 2a-c^- and 2a-c^• were determined by the linewidth variations of the ESR spectra of 2a-c^• in the presence of different concentrations of 2a-c^-, demonstrating the efficient electron-transfer properties of the active aldehydes. The electron transfer from 2a^- to an outer-sphere one-electron oxidant, [Co^II(phen)₃]²⁺ (phen = 1,10-phenanthroline), whose one-electron reduction potential (E_red^o = -0.97 V) is about the same as the one-electron oxidation potential of 2a^- (E_ox^o = -0.96 V), occurs efficiently to yield the corresponding Co^I complex. The observed rate constants for formation of [Co^I(phen)₃]⁺ agree with those for formation of the active aldehyde examined independently. This agreement indicates that rate-determining formation of 2a^-, which is a very strong reductant, precedes the highly efficient electron transfer from 2a^- to [Co^II(phen)₃]²⁺.