Unusual Solvent Effect on the Cycloadditions of Aromatic Nitrile N-Oxide with Alkyl Substituted p-Benzoquinones in Ethanol–Water Systems

The second-order rate constants of 1,3-dipolar cycloaddition reactions of 2,6-dichlorobenzonitrile N-oxide (1) with 2,5-dimethyl-p-benzoquinone (2a) or 2,6-diisopropyl-p-benzoquinone in various solvents at 20°C were determined by spectrophotometric method. The rate of 1 and 2a in ethanol : water=60 : 40 solution was 5.55×102 dm3 mol−1 s−1, which was fourteen folds of that in chloroform (3.9×10−3 dm3 mol−1 s−1). This solvent effect and the low solubility of the cycloadduct in aqueous ethanol were successfully applied to the cycloadduct synthesis.