SYNTHESIS OF ANSA-METALLOCENES BY INTRAMOLECULAR PHOTOCHEMICAL [2+2] CYCLOADDITION OF BIS(ALKENYLCYCLOPENTADIENYL)ZIRCONIUM COMPLEXES

Reaction of the (alkenylcyclopentadienyl)lithium reagents [C5H4C(R)=CH 2]Li (2a-c, R = cyclohexyl, phenyl, or o-tolyl) with ZrCl4(THF)2 gave the metallocene complexes [C5H4C(R)=CH2]2ZrCl2 (3a-c). Complex 3c was characterized by X-ray diffraction. It crystallizes in space group Cm with cell parameters a = 6.802(1) angstrom, b = 28.105(2) angstrom, c = 6.860(l) angstrom, beta = 114.5(1)-degrees, R = 0.023, and R(w) = 0. 030. The reaction of the (1-alkenyl-3-alkylcyclopentadienyl)lithium re agents [(C5H3R1)C(R2)=CH2]Li (2d-f, R1 = isopropyl, R2 = phenyl or cyc lohexyl and R1 = methyl, R2 = cyclohexyl) gave the metallocene complex es [(C5H3R1)C(R2)=CH2]2ZrCl2(3d-f). In each case a near to equimolar m ixture of the respective rac and meso diastereoisomers was obtained. F or 3d,e these were separated by fractional crystallization. 1-(l-pheny lethenyl)-3-isopropylcyclopentadienyl]zi rconium dichloride (rac-3d) w as characterized by X-ray diffraction. It crystallizes in space group Pccn with cell parameters a = 22.122(l) angstrom, b = 15.594(l) angstr om, c = 16.576(1) angstrom, R = 0.045, and R(w) = 0.049. With the exce ption of 3c, these (alkenyl-CP)2ZrCI2 complexes 3 undergo rapid intram olecular [2 + 2] cycloaddition upon photolysis. The photostationary eq uilibrium reached is dependent on the substituent pattern of the start ing materials and the wavelength of the light employed. Near to quanti tative conversion to the novel cyclobutene-bridged ansa-metallocene co mplexes 11 is achieved when alkyl substituents are used and the irradi ation is carried out at 450 nm. The photochemically induced coupling o f readily introduced alkenyl functionalities represents a useful synth etic pathway to novel C2-hydrocarbyl-bridged ansa-metallocene systems of the early transition metals.