Study of concerted and sequential photochemical Wolff rearrangement by femtosecond UV-vis and IR spectroscopy

Photoinduced Wolff rearrangements were studied by femtosecond time-resolved UV-Vis and IR transient absorption spectroscopy. For BpCN(2)CCCH(3) in acetonitrile the IR data indicate the presence of at least two mechanisms of ketene formation. The first process is fast proceeding in either (1)BpCN(2)COCH(3)*, or in a hot carbene, or in both species, while the second is slow proceeding through the intermediacy of a relaxed carbene. The slow time constant of the ketene formation dynamics obtained by ultrafast IR (700 ps) spectroscopy agrees with the relaxed carbene decay of 800 +/- 100 ps obtained by UV-vis absorption spectroscopy.