Chemistry of the five-membered zirconacycloallenoids: reactions with unsaturated substrates

Zirconocene, in situ generated by treatment of zirconocene dichloride with two molar equiv. of n-butyl magnesium chloride, is trapped by the enynes R–CC–C(Me)CH2 (R = –SiMe3, 7c; R = tert-butyl, 7b) to give the zirconacycloallenoids 6b and 6c, respectively. Compound 6c adds an additional equivalent of the enyne 7c to form the seven-membered alkyne insertion product 9 which itself exhibits a typical zirconacycloallenoid structure. Acetonitrile insertion into the Zr–C bond of 6c also results in the formation of a seven-membered metallacycloallenoid, isolated as the respective enamido tautomer 10. Compound 6c reacts with two molar equiv. of benzaldehyde to give a nine-membered cyclic allene. Compound 6b adds one molar equiv. of the enyne 7c to yield the nine-membered metallacycle 14 that contains two metal bonded allenoid moieties inside the ring structure.