Dynamic Topochemical Reaction Control−A Principle and Its Application in <i>ansa</i>-Metallocene Synthesis

The Cp-alkenyl and Cp-dienyl ligand systems 5 and 8 were prepared by fulvene routes. Subsequent transmetalation gave the respective (Cp-propenyl)2-, (Cp-butenyl)2-, (Cp-pentadienyl)2-, and (Cp-hexadienyl)2ZrCl2 and -HfCl2 complexes 6, 9, and 10. In the crystal these complexes favor metallocene conformations that feature the alkenyl and dienyl substituents far separated from each other at the open front side of the bent metallocene wedge. Photolysis in the solid state consequently did not result in an intramolecular C−C coupling reaction but led to E/Z isomerization of the alkenyl units. In solution rapid conformational metallocene equilibration takes place. Photolysis of the (Cp-alkenyl)2ZrCl2 complexes 6 in solution very efficiently resulted in the formation of the cyclobutylene-bridged ansa-zirconocene isomers 11 by rapid intramolecular [2 + 2] cycloaddition reaction by means of a dynamic topochemical reaction control. Photolysis of the (Cp-dienyl)2ZrCl2 and -HfCl2 complexes (9, 10) in solution proceeded analogously to efficiently yield the cyclooctadienylene bridged ansa-metallocenes (12, 13) by formal dynamic topochemical [4 + 4] cycloaddition reaction control.