Cyclomagnesation of dienes catalyzed by a chiral ansa-zirconocene
نویسنده
چکیده
The biphenyl-bridged. chiral zirconocene complex rac-2.2' -biphenyl-bis( 3.4-dimethylcyclopentadienyl I zirconium dichloride catalyzes the reaction of 1.6and 1,7 -dienes with excess dibutyl magnesium to bis( butyl-magnesio-methyl '-substituted cycloalkanederivatives. Analogous reactions occur with butyl magnesium chloride and with heteroatom-t.:Ontaining dienes. The preference of 1.6-dienes for trans-fusedcyclizatioo 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 Irans-fused cyclization products with an optical purity of only 15 ± 1% ee. The stereochemistry of ihese cyclomagnesalion reactions is explained in terms of the relative fates of mutually competing cyclization and transmetalation steps.
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