Insertion Reactions at Cyclobutylene-Bridged ansa-Metallocene Complexes: A Quest for the Influence of Covering Phenylene Units

Abstract

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 η 2 -benzoyl ansa-zirconocene complexes 18-syn/anti, which were characterized by X-ray diffraction. tert-Butyl isocyanide insertion into the Zr-CH 3 bond of 12 gave the N-inside η 2 -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 η 2 -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-(η 2 -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 η 2 -iminobenzoyl metallocene isomerization were observed for the tert-butyl isonitrile insertion products of the cyclobutylene-bis(2-indenyl)ZrPh2 (13) and cyclobutylene-bis(C 5 H 4 )ZrPh 2 (15) systems, which indicates a pronounced influence of the covering phenylene groups on the σ-ligand chemistry in these rigid ansa-metallocene systems.