Tetrahedral main-group compounds are normally configurationally stable, but P-epimerization of the chiral phosphiranium cations syn- or anti-[Mes*P(Me)CH CHPh][OTf] (Mes*=2,4,6-(t-Bu) C H ) occurred under mild conditions at 60 °C in CD Cl , resulting in isomerization to give a syn-enriched equilibrium mixture. Ion exchange with excess [NBu ][Δ-TRISPHAT] (Δ-TRISPHAT=Δ-P(o-C Cl O ) ) followed by chromatography on silica removed [NBu ][OTf] and gave mixtures of syn- and anti-[Mes*P(Me)CH CHPh][Δ-TRISPHAT]⋅x[NBu ][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.
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