Cyclomagnesation of dienes catalyzed by a chiral ansa-zirconocene

Abstract

The biphenyl-bridged, chiral zirconocene complex rac-2,2'-biphenyl-bis(3,4-dimethylcyclopentadienyl)zirconium dichloride catalyzes the reaction of 1,6- and 1,7-dienes with excess dibutyl magnesium to bis(butyl-magnesio-methyl)-substituted cycloalkane derivatives. Analogous reactions occur with butyl magnesium chloride and with heteroatom-containing dienes. The preference of 1,6-dienes for trans-fused cyclization products and that of 1,7-dienes for cis-fusion at ambient and for trans-fusion at elevated temperatures is similar to that observed before for unsubstituted zirconocene complexes. The R-enantiomer of the biphenyl-bridged zirconocene complex gives trans-fused cyclization products with an optical purity of only 15 ± 1% ee. The stereochemistry of these cyclomagnesation reactions is explained in terms of the relative rates of mutually competing cyclization and transmetalation steps.