Electron paramagnetic resonance, ENDOR and TRIPLE resonance studies on substitution reactions of 2-methyl-1,4-benzoquinone and 2,3-dimethoxy-5-methyl-1,4-benzoquinone in alkaline methanol

Abstract

Substitution of 2-methyl-1,4-benzoquinone with methanol takes place in alkaline methanol, giving rise to methoxy-substituted methyl-1,4-benzoquinone radical anions. The relative signs and absolute values of the hyperfine coupling constants (hfcs) of the various mono-, di- and tri-substituted methoxymethyl-1,4-benzoquinones have been measured by EPR, ENDOR and TRIPLE resonance spectroscopy. The hfcs of OCH₃ and CH₃ protons had the same positive sign, except in the 3,5-dimethoxy-2-methyl-1,4-benzoquinone radical anion. The ENDOR signals of the methoxy protons could be distinguished from the methyl proton signals by using perdeuterated methanol as solvent. Partial optimizations of the geometry of the radical were performed by using the unrestricted MNDO method. In the most stable conformers the methoxyl groups had twisted out of the aromatic plane. INDO calculations were performed by using optimized structures, and the hfcs were compared with those obtained using standard bond lengths and angles. The hfcs of the ring protons were assigned by McLachlan calculations and support of the assignment was provided by the additivity relationship method. The general TRIPLE technique allowed identification of the methyl, methoxy and ring protons of the different methoxy-substituted methyl-1,4-benzoquinone radical anions.