Enantioselective Synthesis of Planar-Chiral Phosphaferrocenes by Molybdenum-Catalyzed Asymmetric Interannular Ring-Closing Metathesis

摘要

Planar-chiral ferrocenes are important chiral scaffolds and have been utilized in various asymmetric reactions as ligands or catalysts with fair success. Recent entries to this class of compounds are heterometallocenes such as azaferrocenes and phosphaferrocenes, and these Lewis basic organometallics have proven their usefulness in asymmetric synthesis. Standard methods of obtaining nonra-cemic planar-chiral ferrocenes are based either on resolution of racemates or on diastereoselective metalation using chiral directing groups or external chiral bases. Their catalytic enantioselective synthesis is extremely rare, and further breakthroughs have been clearly awaited. Recently, we and others reported the preparation of various bridged metallocenes by interannular ring-closing metathesis, which have enabled us to modulate metallocenes by transition metal catalysis. And thus, we are interested in controlling metallocene-based planar chirality using chiral metathesis catalysts.