Bisnaphthol ligands

The MonoPhos family of ligands for the reduction of C-C double bonds, including enamides, is based on the 2,2-bisnaphthol (BINOL) backbone. These phosphoramidite ligands are comparatively inexpensive to prepare compared to bisphosphine ligands. For enamide reductions with MonoPhos (20) as ligand, it was found that the reaction is strongly solvent dependent. Very good enantioselectivities were obtained in nonprotic solvents. - ... 

The ee is highest, 86%, when the diastereomer 10 is used and it is in the same range as for 8, which has rotational freedom around the biaryl axis. When the diphosphite ligands are bisnaphthol derivatives 9a and 10, the three diastereomers of 9a resulted in low asymmetric inductions in comparison with the single isomer 10. 

In 1992, Takaya et al. reported on the use of a chiral diphosphite derived from bisnaphthol in the asymmetric hydroformylation of vinyl acetate [11], but the enantioselectivity achieved was only 50%. They noted that diphosphites led to more stable hydroformylation catalysts than diphosphines. This observation prompted Takaya, Nozaki et al. to synthesize the chiral phosphine-phosphite ligands (R,S)- and (R,R)-BINAPHOS (24, figure 17), which were expected to combine the high enantioselectivity obtained with diphosphines such as BINAP in asymmetric hydrogenation, with the apparently efficient coordination of the phosphite moiety [38]. Indeed, the Rh(I) complex of Ci-symmetric (R,S)-BINAPHOS provided much higher enantioselectivities than either C2 symmetric diphosphine ligands or diphosphite ligands, viz. more than 90% ee for a wide variety of both functionalized and internal aUcenes [38,39,40,41]. 

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