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

Ikuo Nakanishi, Shinobu Itoh, Shunichi Fukuzumi

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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, [CoII(phen)3]2+ (phen = 1,10-phenanthroline), whose one-electron reduction potential (Eredo = -0.97 V) is about the same as the one-electron oxidation potential of 2a- (Eoxo = -0.96 V), occurs efficiently to yield the corresponding CoI complex. The observed rate constants for formation of [CoI(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 [CoII(phen)3]2+.

Original languageEnglish
Pages (from-to)2810-2818
Number of pages9
JournalChemistry - A European Journal
Issue number10
StatePublished - Oct 1999


  • Coenzymes
  • Cyclic voltammetry
  • ESR spectroscopy
  • Radicals
  • Thiamin


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