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Air-stable chiral primary phosphines part (i) synthesis, stability and applications
This article reviews recent findings from our research groups on the preparation and utility of air-stable chiral primary phosphines. Enantiopure primary phosphines (R)-2 and (S)-3 have been synthesized and are remarkably stable to air-oxidation in both the neat state and in solution, by virtue of the extended π-ring system of the binaphthyl backbone. The extent of conjugation was found to be critical to their stability rather than the presence of bulky groups or heteroatoms. Despite this oxidative resistance, the ability to functionalize these atropisomeric primary phosphines remains, and they have proven to be highly useful chiral ligand precursors. Thus the electrophilic dichlorides (R)-8 and (S)-9 have been prepared, and used to synthesize the phosphonites (Rb,Rax)-13a, (Sb,Rax)-13b, (Rb,Sax)-14a and (Sb,Sax)-14b. By contrast, treatment of (R)-2 and (S)-3 with base to generate the phosphides, followed by the addition of a chiral cyclic sulfate gave the hybrid MOP phospholanes (R,R,Rax)-15a, (S,S,Rax)-15b(via their boranes) and the H-MOP phospholanes (R,R,Sax)-17a and (S,S,Sax)-17b respectively. The primary phosphines are also shown to undergo the hydrophosphination reaction: bis(hydroxymethyl)phosphine (R)-20 and the dicyanoethyl (R)-21 derivatives were prepared by the hydrophosphination of (R)-2 with formaldehyde and acrylonitrile respectively. Early results in the asymmetric hydrosilylation of styrene, and the addition of phenylboronic acid to 1-naphthaldehyde, demonstrate the potential of these new ligand libraries.