Ground- and excited-state reactivity of some ketenylcarbenes

The reactivity of ketenylcarbenes has been widely discussed on experimental and theoretical grounds in connection with the photochemical decomposition in cryogenic matrices of some substituted cyclobutenediones and maleic anhydrides. It was found that, although the presence of these intermediates was postulated in the decomposition mechanisms, only monofluoroketenylcarbene was experimentally evidenced and characterized by its IR spectrum. In all other cases, the experimental data revealed the formation of the corresponding cyclopropenones and, under continuing irradiation, of the alkynes. Theoretical calculations also point to their high reactivity towards the ring closure reaction. In order to gain an insight into the main factors determining the ketenylcarbene reactivity, ab initio calculations on both the ground (HF/6-31G* and MP2/6-31G*) and the first excited state (HF/6-31G*) were performed. The results indicate that the ring closure reaction is mainly determined by the electrostatic interaction between the carbene center and the Calpha atom of the ketenyl group. The role of the in-plane interaction between the molecular orbital localized on the carbene center and the first two vacant orbitals is also discussed. The excited-state optimized geometries allow for the characterization of the excited-state reactivity. Copyright (C) 2004 John Wiley Sons, Ltd.