First Resolution of a Free Secondary Phosphine Chiral at Phosphorus and Stereospecific Formation and Structural Characterization of a Homochiral Secondary Phosphine-Borane Complex.

The R(P) diastereomer of (-)-menthylmesitylphosphine, (R(P))-1, has been isolated with high configurational purity at phosphorus by fractional crystallization of an (R(P))-1/(S(P))-1 = 43/57 mixture from acetonitrile containing a trace of sodium acetylacetonate as a proton scavenger or by deboranation of the corresponding borane complex (S(P))-2 with diethylamine, thereby effecting the first resolution of a secondary phosphine chiral at phosphorus. The crystal and molecular structure of (S(P))-2 has been determined. The ready isolation of (S(P))-2 of 97% diastereomeric purity in 66% yield from an equilibrium (R(P))-2/(S(P))-2 = 28/72 mixture in n-hexane by second-order asymmetric transformation and its quantitative and stereospecific conversion under mild conditions into (R(P))-1 of similar purity augurs well for the future of the resolved secondary phosphines in stereoselective syntheses.