Methyleneketenes and methylenecarbenes. XVII. Generation of methyleneketene intermediates by pyrolysis of N-alkenoylpyrazoles and synthesis of simple phenols

Abstract

Flash vacuum pyrolysis of 1-acyl-3,5-dimethylpyrazoles forms ketenes, but the conditions may lead to decarbonylation of the ketene. The 1-cinnamoyl derivative (4) forms phenylacetylene at 750°/0.02 mm, via benzylideneketene and benzylidenecarbene. Simple phenols (2-naphthol, phenol, 4-methylphenol, 2,4-dimethylphenol, and 3,5-dimethylphenol) are obtained in high yield on pyrolysis of the appropriate dienoylpyrazoles (10), (13), (16), (18) and (21); the reaction proceeds by way of cyclization of dienylketenes.