Configurational Lability at Tetrahedral Phosphorus: syn/anti-Isomerization of a P-Stereogenic Phosphiranium Cation by Intramolecular Epimerization at Phosphorus.

Tetrahedral main-group compounds are normally configurationally stable, but P-epimerization of the chiral phosphiranium cations syn- or anti-[Mes*P(Me)CH2 CHPh][OTf] (Mes*=2,4,6-(t-Bu)3 C6 H2 ) occurred under mild conditions at 60 °C in CD2 Cl2 , resulting in isomerization to give a syn-enriched equilibrium mixture. Ion exchange with excess [NBu4 ][Δ-TRISPHAT] (Δ-TRISPHAT=Δ-P(o-C6 Cl4 O2 )3 ) followed by chromatography on silica removed [NBu4 ][OTf] and gave mixtures of syn- and anti-[Mes*P(Me)CH2 CHPh][Δ-TRISPHAT]⋅x[NBu4 ][Δ-TRISPHAT]. NMR spectroscopy showed that isomerization proceeded with epimerization at P and retention at C. DFT calculations are consistent with a mechanism involving P-C cleavage to yield a hyperconjugation-stabilized carbocation, pyramidal inversion promoted by σ-interaction of the P lone pair with the neighboring β-carbocation, and ring closure with inversion of configuration at P.