288 Catalytic Asymmetric Synthesis of Planar-Chiral Metallocenes

Thursday, November 5, 2009: 4:00 PM
Longhorn (Camino Real Hotel)
Masamichi Ogasawara , Catalysis Research Center, Hokkaido University, Sapporo, Japan
Planar-chiral metallocenes are important chiral scaffolds in organic and organometallic chemistry. Their preparation in optically active forms, however, is still a challenging problem. Most of non-racemic such compounds were obtained either by diastereoselective metalation utilizing chiral ortho-directing groups or by optical resolution of preformed racemic compounds. Examples of catalytic asymmetric reactions of preparing scalemic planar chiral metallocenes are extremely rare. We recently reported preparation of [4]metallocenophanes by Ru- or Mo-catalyzed ring-closing metathesis reaction (RCM) of 1,1'-diallylmetallocenes or of phosphaferrocene analogues. In this presentation, two novel asymmetric protocols of preparing planar-chiral metallocene derivatives based on the RCM reaction will be described. The first method is kinetic resolution of racemic planar-chiral 1,1'-diallylferrocene derivatives by asymmetric ring-closing metathesis (ARCM). The reaction proceeds very efficiently to afford planar-chiral ferrocenes in high yield with excellent enantiomeric discrimination. The krel value (relative reaction rate) between the two enantiomers is up to 186 and nearly perfect kinetic resolution was realized. The second method is asymmetric desymmetrization of the prochiral triallylphosphfarrocenes. The ARCM proceeds with excellent enantioselectivity and the planar-chiral phosphaferrocenes are obtained in up to 99% ee. Scope and limitation of the protocols as well as comparison between the Mo- and the Ru-catalysts will be given.