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

Abstract Tetrahedral main‐group compounds are normally configurationally stable, but P‐epimerization of the chiral phosphiranium cations syn‐ or anti‐[Mes*P(Me)CH 2 CHPh][OTf] (Mes*=2,4,6‐(t‐Bu) 3 C 6 H 2 ) occurred under mild conditions at 60 °C in CD 2 Cl 2 , resulting in isomerization to give a syn‐enriched equilibrium mixture. Ion exchange with excess [NBu 4 ][Δ‐TRISPHAT] (Δ‐TRISPHAT=Δ‐P(o‐C 6 Cl 4 O 2 ) 3 ) followed by chromatography on silica removed [NBu 4 ][OTf] and gave mixtures of syn‐ and anti‐[Mes*P(Me)CH 2 CHPh][Δ‐TRISPHAT]⋅x[NBu 4 ][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.