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A New Class of 3′-Sulfonyl BINAPHOS Ligands: Modulation of Activity and Selectivity in Asymmetric Palladium-Catalysed Hydrophosphorylation of Styrene



Palladium-catalysed monophosphorylation of (R)-2,2′-bisperfluoroalkanesulfonates of BINOL (RF=CF3 or C4F9) by a diaryl phosphinate [Ar2P(O)H] followed by phosphine oxide reduction (Cl3SiH) then lithium diisopropylamide-mediated anionic thia-Fries rearrangement furnishes enantiomerically-pure (R)-2′-diarylphosphino-2′-hydroxy-3′-perfluoralkanesulfonyl-1,1′-binaphthalenes [(R)-8ab and (R)-8g–j], which can be further diversified by Grignard reagent (RMgX)-mediated CF3-displacement [→(R)-8c–f]. Coupling of (R)-8a–j with (S)-1,1′-binaphthalene-2,2′-dioxychlorophosphine (S)-9 generates 3′-sulfonyl BINAPHOS ligands (R,S)-10a–j in good yields (43–82%). These new ligands are of utlility in the asymmetric hydrophosphonylation of styrene (1) by 4,4,5,5-tetramethyl-1,3,2-dioxaphospholane 2-oxide (2), for which a combination of the chiral ligands with either [Pd(Cp)(allyl)] or [Pd(allyl)(MeCN)2]+/NaCH(CO2Me)2 proves to be a convenient and active pre-catalyst system. A combination of an electron-rich phosphine moiety and an electron-deficient 3′-sulfone moiety provides the best enantioselectivity to date for this process, affording the branched 2-phenethenephosphonate, (−)-iso-3, in up to 74% ee with ligand (R,S)-10i, where Ar=p-anisyl and the 3′-SO2R group is triflone.