Enantioselective Synthesis of Ferrocene- or Cymantrene-Fused Planar-Chiral Phospholes

Planar-chiral ferrocene-fused phosphole (S)-1a was prepared in an enantiomerically pure form by enantioselective transformation. The synthesis was started with Kagan's chiral ferrocenyl acetal (-)-2a, and bromo and (Z)-2-bromovinyl substituents were introduced at the 1- and 2-positions of the ferrocene platform in (S)-6a with controlling its planar chirality. The double Li/Br exchange on (S)-6a followed by the reaction with PhPBr2 provided (S)-1a in excellent yield. The structure of (S)-1a was determined by X-ray crystallography. Planar-chiral cymantrene-fused phosphole (S)-1b was also prepared by an analogous asymmetric transformation starting with Jaouen's chiral cymantrenyl acetal (+)-2b.