Mechanisms of Electron-Transfer Oxidation of NADH Analogues and Chemiluminescence. Detection of the Keto and Enol Radical Cations

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Abstract
The radical cation of an NADH analogue (BNAH: 1-benzyl-1,4-dihydronicotinamide) has been successfully detected as the transient absorption and ESR spectra in the thermal electron transfer from BNAH to Fe(bpy)33+ (bpy = 2,2‘-bipyridine) and Ru(bpy)33+. The ESR spectra of the radical cations of BNAH and the dideuterated compound (BNAH-4,4‘-d2) indicate that the observed radical cation is the keto form rather than the enol form in the tautomerization. The deprotonation rate and the kinetic isotope effects of the keto form of BNAH•+ were determined from the kinetic analysis of the electron-transfer reactions. In the case of electron transfer from BNAH to Ru(bpy)33+, the chemiluminescence due to Ru(bpy)32+* was observed in the second electron-transfer step from BNA•, produced by the deprotonation of the keto form of BNAH•+, to Ru(bpy)33+. The observation of chemiluminescence due to Ru(bpy)32+* provides compelling evidence that the Marcus inverted region is observed even for such an intermolecular electron-transfer reaction. When BNAH is replaced by 4-tert-butylated BNAH (4-t-BuBNAH), no chemiluminescence due to Ru(bpy)32+* has been observed in the electron transfer from 4-t-BuBNAH to Ru(bpy)33+. This is ascribed to the facile C−C bond cleavage in 4-t-BuBNAH•+. In the laser flash photolysis of a deaerated MeCN solution of BNAH and CHBr3, the transient absorption spectrum of the enol form of BNAH•+ was detected instead of the keto form of BNAH•+, and the enol form was tautomerized to the keto form. The rate of intramolecular proton transfer in the enol form to produce the keto form of BNAH•+ was determined from the decay of the absorption band due to the enol form and the rise in the absorption band due to the keto form. The kinetic isotope effects were observed for the intramolecular proton-transfer process in the keto form to produce the enol form.