Polyhalogeno-allenes and -acetylenes. Part 16. Further 1,3-dipolar cycloadditions to perfluoropropadiene

Abstract

The nitrones PhCHN⁺(R)O^–(R = Me, Et, PhCH₂, and Ph) react rapidly with perfluoropropadiene to give the corresponding 2-R-4-difluoromethylene-5,5-difluoro-3-phenylisoxazolidines in good yield, although these isoxazolidines are unstable and decompose during catalytic hydrogenation. Perfluoropropadiene also reacts with diazophenylmethane and diazodiphenylmethane, giving 4-difluoromethylene-3,3-difluoro-5-phenyl- and 5,5-diphenyl-Δ¹-pyrazolines. The diphenylpyrazoline decomposes on distillation giving 3-difluoromethylene-2,2-difluorodiphenylcyclopropane. The regiospecificity of these dipolar cycloadditions is discussed in relation to frontier orbital theory. Benzoyldiazophenylmethane decomposes during a slow reaction with perfluoropropadiene and the only adduct isolated is 3-difluoromethylene-2,2-difluoro-4,5-diphenyldihydrofuran. Diazodiphenylmethane reacts with perfluoropropyne to give a 1 : 1 adduct believed to be 5-fluoro-3,3-diphenyl-4-trifluoromethyl-3H-pyrazole.