Synthesis and reactions of optically active phosphine‐boranes

Synthesis and reactions of optically active phosphine‐boranes have been investigated. Optically active secondary phosphine‐boranes, (S_p)‐and (R_p)‐menthyloxyphenylphosphine‐boranes, and (S)‐methylphenylphosphine‐borane underwent palla‐dium(0)‐catalyzed electrophilic arylation with o‐, m‐, or p‐iodoanisole. The stereochemistry of this arylation was largely dependent on the solvent and the base used. The reaction in acetonitrile proceeded with almost complete retention of configuration at the chiral phosphorus, whereas inversion of configuration was observed in ethereal solvents or toluene. The phosphorus‐oxygen bond of (R_p)‐menthyloxy (methyl) phenylphosphine‐borane and (S_P)‐menthy‐loxy(o‐methoxyphenyl)phenylphosphine‐boranewas reductively cleaved at −78°C by lithium naphtha‐lenide or Li/NH₃ with virtually net retention of configuration at phosphorus, providing secondary or tertiary phosphine‐boranes in excellent yields. New synthetic routes to optically pure C₂‐symmetric bisphosphine‐boranes possessing chirality at phosphorus have been developed on the basis of these stereochemical studies.