Synthesis of an Amino-Functionalized <i>ansa</i>-Zirconocene by Intramolecular Photochemical Enamine [2+2]Cycloaddition

Deprotonation of 6-dimethylamino-6-methylpentafulvene (3a) with methyl lithium yielded the (1-dimethylaminoethenyl)Cp lithium reagent 4a. Its treatment with zirconium tetrachloride under carefully controlled reaction conditions gave the corresponding bis(enamino-Cp)ZrCl2 complex 5a. The product 5a is very sensitive to acid-catalyzed Mannich condensation. Photolysis of 5a (HPK 125, Pyrex filter) resulted in a rapid intramolecular [2+2]cycloaddition reaction to yield the cis-bis(dimethylamino)cyclobutylene-bridged ansa-zirconocene complex 7a in a photostationary equilibrium (5a:7a ≈ 30:70). After acid-catalyzed Mannich conversion of the minor component (5a to 6a) the new photoproduct 7a was isolated pure by fractional crystallization. The complexes 5a and 7a were both characterized by X-ray diffraction.