The increasing demand to produce enantiomerically pure compounds has advanced the field of asymmetric catalysis. Despite tremendous progress in the area of stereoselective catalysis, the hunt for new and efficient asymmetric transformations to produce chiral compounds remains a major ongoing challenge. High enantioselectivity mostly depends on the bite angle, steric bulk, chirality and electronics of the ligand in homogeneous catalysis. As a result, rational design of chiral phosphorus ligands in order to improve the reactivity and selectivity is still appealing. For example, the application of DIPAMP in the production of L-DOPA can be considered as a milestone in asymmetric homogeneous catalysis. Since then, a few P-stereogenic phosphine ligands such as BisP*, QuinoxP*, Binapine, TCFP, DuanPhos and MaxPHOS have been successfully applied in various asymmetric transformations. Herein, we present the synthesis and application of newly developed mixed donor P-P ligands. The design approach involves the combination of P-chiral phosphines with excellent π-accepting phosphites/phosphoramidites. The P-chiral PNP ligands are relatively easy to prepare on multigram scale and can easily be adapted with numerous chiral diols or amino alcohols. After the success in asymmetric hydrogenation, the PNP ligands have been applied in the asymmetric allylic substitution of diphenyl allyl acetate as a model substrate. A match/mismatch effect has also been observed by the fine tuning of the axially chiral BINOL moiety (O-X in Scheme 1) furnishing enantiomeric excess values up to 74% (Scheme 2).

Soumyadeep Chakrabortty

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