Synthesis and characterization of a high-purity chiral 5,5'-disulfonato-BINAP ligand and its application in asymmetric hydrogenation of β-keto esters

2,2′-Bis(diphenylphosphino)-1,1′-binaphthyl (BINAP) is a widely used chiral phosphine ligand, and the sulfonated BINAP is one of the important water-soluble BINAP derivatives. In this article, we report an improved oleum sulfonation method for the synthesis of the high-purity 5,5′-disulfonato-(S)-BINAP ligand with an industrially acceptable high yield. By optimizing the sulfonation conditions and improving the workup process, the yield of 5,5′-disulfonated products was enhanced to 73 %. Furthermore, an acetonitrile extraction protocol was developed for the purification of sulfonated products by efficiently removing phosphine oxides, by which the content of phosphine oxide can be controlled below 3 %. The structure of 5,5′-disulfonato-(S)-BINAP was characterized with the elemental analysis, FT-IR, 1H NMR, 13C NMR, 31P NMR, 1H-1H COSY, 1H-13C HSQC, 1H-13C HMBC, and HR-MS, which unambiguously confirmed that the sulfo groups are introduced at the 5,5′-position of the binaphthyl ring skeleton in (S)-BINAP. The prepared high-purity 5,5′-disulfonato-(S)-BINAP was evaluated in the Ru-catalyzed asymmetric hydrogenation of β-keto esters, which exhibited high catalytic activity, enantioselectivity and excellent catalytic stability.