The photochemical Wolff rearrangement of 3-diazo-1,1,1-trifluoro-2-oxopropane revisited

Abstract

Ethyl 2-diazo-4,4,4-trifluoroacetoacetate (1a) and 3-diazo-1,1,1-trifluoro-2-oxopropane (1b) exhibit a deviating behavior in solution photolysis (hydrogen abstraction for 1a; Wolff rearrangement for 1b) [(a) F. Weygand, W. Schwenke and H. J. Bestmann, Angew. Chem., 1958, 70, 506; (b) F. Weygand, H. Dworschak, K. Koch and S. Konstas, Angew. Chem., 1961, 73, 409]. As shown by ¹³C-labelling of 1b this difference is not caused by rearrangement of the primarily formed α-oxocarbene to an isomeric α-oxocarbene presenting a hydrogen atom as a migrating substituent for the Wolff rearrangement. It is discussed that the singlet α-oxocarbene generated from 1a rapidly undergoes spin equilibration followed by hydrogen abstraction of the triplet α-oxocarbene. In contrast, due to a larger singlet–triplet splitting in the singlet α-oxocarbene generated from 1b, the intramolecular Wolff rearrangement on the singlet surface can efficiently compete with the singlet–triplet interconversion.