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(h5-C9H7){=C=C(R)H}(PPh3){k1-(P)-PPh2(C3H5)][BF4] affords cyclobutylidene complexes, which can be also obtained by reaction of terminal alkynes with [Ru(h5-C9H7)(PPh3){k3-(P,C,C)-PPh2(C3H5)}][PF6]. The reaction proceeds under mild conditions via vinylidene complexes, and the activation parameters of the coupling step were determined by kinetic studies