Synthesis of 3,3′-, 6,6′- and 3,3′,6,6′-substituted binaphthols and their application in the asymmetric hydrophosphonylation of aldehydes—an obvious effect of substituents of BINOL on the enantioselectivity
By using a direct ortho-lithiation strategy and/or Ni-catalysed coupling reaction, we have conveniently synthesized three new chiral polysubstituted BINOLs of vast synthetic potential from (S)-BINOL 1 in reasonable yields. Moreover, we have examined closely their influence on the enantioselectivity of the asymmetric Pudovik reaction catalyzed by chiral binaphthol-modified lanthanum alkoxides. The results show that the steric bulk of 3,3′-substituents of BINOL is responsible for a lowering of the enantioselectivity of the reaction, while coordination between the oxygens of the ortho-substituents and the lanthanum ion is beneficial in improving the asymmetric induction. 6,6′-Diphenyl-BINOL has the advantage over simple BINOL in giving the best asymmetric results. Further studies on this aspect are in progress.