Photochemical intramolecular [2+2] cycloaddition of bridged bis-azulenyl zirconocenes

Photochemical reaction of a rac / meso mixture of bridged bis-azulenyl zirconocenes was investigated. Irradiation of the rac / meso mixture of dichlorodimethylsilylenebis(2-methyl-4-phenyl-4H-azulenyl)zirconium resulted in a rapid conversion of only the meso isomer to give a novel bridged metallocene with a bis-azulenyl ligand cross-linked by a cyclobutylene ring, which was produced by intramolecular [2+2] cycloaddition, and the rac isomer was stable toward photoirradiation. The structure of the cyclobutylene-bridged metallocene derived from meso-dichlorodimethylsilylenebis(2-methyl-4-(2-fluoro-4-biphenylyl)-4H-azulenyl)zirconium was determined by X-ray crystallographic analysis. The structure shows a bond formation between two azulenyl rings at the 7- and 8-position, respectively, to form a four-membered ring. The solubility of cyclobutylene-bridged metallocenes in solvent was found to be much higher than those of rac / meso isomers. After photoirradiation of a rac / meso mixture, rac isomers were easily isolated.