Mechanism of Phosphine-Catalyzed Novel Rearrangement of Vinylcyclopropylketone to Cycloheptenone: A DFT Study

The title reaction is theoretically investigated in detail using density functional theory. Three possible routes starting from keto- or enol-type vinylcyclopropylketone are considered in this work. Results indicate that phosphine catalyst would first attack at the three-membered ring (C3 position) rather than the terminal of alkene (C1 position) in vinylcyclopropylketone. It is found that the two-stage mechanism would be responsible for the title reaction. The first stage is the S_N2-type ring-opening of the keto-type vinylcyclopropylketone with phosphine catalyst. After the proton-transfer tautomerisms in the zwitterionic intermediates, the second stage is associated with the 7-endo-trig S_N2′-type ring closure of keto- or enol-type zwitterions to furnish seven-membered cyclic products and recover the catalyst. Moreover, it turns out that 7-endo-trig S_N2′-type ring closure would be highly asynchronous and could be well described as an addition/elimination process where the ring closure already finishes before the cleavage of the C–P bond. Computational results provide a deep insight into experimental observations.