Theoretical and experimental studies on the mechanism of norbonadiene Pauson-Khand cycloadducts photoarrangement. Is there a pathway on the excited singlet potential energy surface?

A myriad of tricyclo[5.2.1.0]deca-4,8-dien-3-ones is prepared by using norbornadiene as alkene. The interpretation of the experiments is that the photorearrangement has taken place through [super 3]([pi][pi]*) state as the energy barrier involved in the initial C-C [gamma]-bond cleavage of the enone is much lower in the [super 3]([pi][pi]*) potential energy surface (PES) than in the [super 1](n[pi]*) PES.