Unusual intramolecular [2 + 2] cycloaddition of allyl and vinylidene C=C bonds under mild conditions: a theoretical analysis

A theoretical analysis allows for the rationalization of the recently reported unusual formation under mild conditions of a cyclobutylidene ring from a diastereoselective 2 + 2] intramolecular cycloaddition of two C=C systems. The reaction takes place by heating in dichloromethane the vinylidene complexes Ru((eta(5),eta(3)-C(9)H(7))=C=C(R)H]kappa(1)-(P)-PPh(2)(C(3)H(5))](PPh(3))]BF(4)] (R = Ph, p-Me-C(6)H(4)) (1) yielding the bicyclic alkylidene complexes Ru((eta(5),eta(3)-C(9)H(7))kappa(2)-(P,C)-(=CC(R)HCH(2)CHCH(2)-PPh(2)](PPh(3))]BF(4)] (2). The proposed mechanism represents an alternative to the classical Woodward-Hoffmann's supra-antara approach.