Insertion Reactions at Cyclobutylene-Bridged <i>a</i><i>nsa</i>-Metallocene Complexes: A Quest for the Influence of Covering Phenylene Units

The cyclobutylene−bis(2-indenyl)zirconium dichloride complex (4), derived from an intramolecular photochemical [2+2] cycloaddition reaction of bis[2-(methylethenyl)indenyl]zirconium dichloride (3), was reacted with methyllithium or phenyllithium to yield the corresponding cyclobutylene−bis(2-indenyl)zirconium dimethyl (12) or diphenyl (13) complex, respectively. Insertion of carbon monoxide into the Zr−phenyl linkage of 13 yielded the “O-inside” η2-benzoyl ansa-zirconocene complexes 18-syn/anti, which were characterized by X-ray diffraction. tert-Butyl isocyanide insertion into the Zr−CH3 bond of 12 gave the “N-inside” η2-iminoacyl metallocene isomers 17-syn and 17-anti; the latter was characterized by X-ray diffraction. Under kinetic control tert-butyl isonitrile insertion into 12 gave the “N-outside” η2-iminoacyl metallocene isomers 25-syn/25-anti, which rearranged to 17-syn/anti at 243 K with Gibbs activation energies of ΔG⧧rearr = 18.0/17.8 ± 0.3 kcal·mol-1, respectively. The analogous rearrangement of the “N-outside” to “N-inside” isomers of the corresponding cyclobutylene−bis(cyclopentadienyl)Zr(η2-iminoacyl) reference systems (23-syn/anti to 24-syn/anti) is much faster (203 K: ΔG⧧rearr = 14.7/14.9 ± 0.3 kcal·mol-1). Similar differences in the kinetic “N-outside” to thermodynamic “N-inside” η2-iminobenzoyl metallocene isomerization were observed for the tert-butyl isonitrile insertion products of the cyclobutylene−bis(2-indenyl)ZrPh2 (13) and cyclobutylene−bis(C5H4)ZrPh2 (15) systems, which indicates a pronounced influence of the covering phenylene groups on the σ-ligand chemistry in these rigid ansa-metallocene systems.