Formation of a Cyclobutylidene Ring: Intramolecular [2 + 2] Cycloaddition of Allyl and Vinylidene CC Bonds under Mild Conditions

Intramolecular [2 + 2] cycloaddition of two CC bonds in vinylidene complexes [Ru(η5-C9H7){CC(R)H}(PPh3){κ-(P)-PPh2(C3H5)][BF4] affords cyclobutylidene complexes which can be also obtained by reaction of terminal alkynes with [Ru(η5-C9H7)(PPh3){κ-(P,C,C)-PPh2(C3H5)}][PF6]. The reaction proceeds under mild conditions via vinylidene complexes, and the activation parameters were determined by kinetic studies.