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

Reaction of the (alkenylcyclopentadienyl)lithium reagents [C5H4C(R)=CH2]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 reagents [(C5H3R1)C(R2)=CH2]Li (2d-f, R1 = isopropyl, R2 = phenyl or cyclohexyl and R1 = methyl, R2 = cyclohexyl) gave the metallocene complexes [(C5H3R1)C(R2)=CH2]2ZrCl2(3d-f). In each case a near to equimolar mixture of the respective rac and meso diastereoisomers was obtained. For 3d,e these were separated by fractional crystallization. rac-Bis[eta-1-(l-phenylethenyl)-3-isopropylcyclopentadienyl]zi rconium dichloride (rac-3d) was characterized by X-ray diffraction. It crystallizes in space group Pccn with cell parameters a = 22.122(l) angstrom, b = 15.594(l) angstrom, c = 16.576(1) angstrom, R = 0.045, and R(w) = 0.049. With the exception of 3c, these (alkenyl-CP)2ZrCI2 complexes 3 undergo rapid intramolecular [2 + 2] cycloaddition upon photolysis. The photostationary equilibrium reached is dependent on the substituent pattern of the starting materials and the wavelength of the light employed. Near to quantitative conversion to the novel cyclobutene-bridged ansa-metallocene complexes 11 is achieved when alkyl substituents are used and the irradiation is carried out at 450 nm. The photochemically induced coupling of readily introduced alkenyl functionalities represents a useful synthetic pathway to novel C2-hydrocarbyl-bridged ansa-metallocene systems of the early transition metals.