[2+2]-Cycloaddition and Subsequent <i>m</i><i>eso</i>/<i>r</i><i>ac</i> <i>a</i><i>nsa</i>-Metallocene Interconversion by Photolysis of a Bis(1,3-dialkenylcyclopentadienyl)zirconium Complex

6,6-Dimethylfulvene (5) was treated with acetone under basic conditions to yield 3-(1-methylethenyl)-6,6-dimethylfulvene (7). Deprotonation with LDA followed by transmetalation with 0.5 molar equiv of zirconium tetrachloride gave bis[1,3-di(1-methylethenyl)C5H3]zirconium dichloride (3a). Photolysis of 3a with Pyrex-filtered UV/vis light at ambient temperature led to a rapid intramolecular [2+2]-cycloaddition reaction to yield a 1:1 mixture of the meso- and rac-isomers of the corresponding singly cyclobutylene-bridged ansa-metallocene system 4a. Photolysis of this mixture with quartz-filtered UV light at −80 °C very slowly converted meso-4a to rac-4a. Over 2 days a 12:1 ratio of rac/meso-4a was achieved under these conditions.