Singlet Vinylcarbenes:  Spectroscopy and Photochemistry

The first direct spectroscopic characterization of singlet vinylcarbenes is reported. Several vinyl amidines were converted to the corresponding vinylchlorodiazirines through straightforward Graham oxidation. Irradiation of the diazirines at 8 K in N2 matrices gave the corresponding singlet vinylchlorocarbenes, which could be characterized by IR, UV/vis, and calculational modeling. Hence, vinylchlorocarbene, 1-methylvinylchlorocarbene, and 1-cyclopentenylchlorocarbene were all generated and investigated. The spectra are consistent with localized carbene structures compared to the very delocalized triplet vinylcarbenes. In all cases, the carbenes readily cyclize to the corresponding cyclopropenes on visible irradiation, together with H-shift in the parent vinylchlorocarbene. In the cyclopentenyl system, cyclization leads to a highly strained bicyclo[3.1.0]hexene.