Mechanistic Studies on the Wolff Rearrangement: The Chemistry and Spectroscopy of Some α-Ketocarbenes

Article abstract of DOI:10.1021/ja00311a063

Photochemical extrusion of dinitrogen from diazo ketones 1-4 matrix isolated in argon at 10-15 K produces α-ketocarbenes 5, 6, and 8.UV-vis, infrared, and electron spin resonance spectroscopy identify the α-ketocarbenes, which are further characterized by trapping with carbon monoxide and dioxygen.Excitation (T₀-T₁) of the α-ketocarbenes leads to rapid ring contraction of 5 -> 9 and slow ring contraction of 6 -> 10 and 8 -> 12.This trend parallels the increasing degree of strain in the product ketenes.Shorter wavelength irradiation causes rapid ring contraction of 6 -> 10 and 8 -> 12.The stepwise Wolff rearrangement of 1-4 proceeds through an α-ketocarbene.Ring contraction occurs in a singlet excited state (S''') formed upon photolysis of the primary product α-ketocarbene (T₀).