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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., (© 2021 Wiley-VCH GmbH.)

Nucleophilic substitution results in inversion of configuration at the electrophilic carbon center (S N 2) or racemization (S N 1). The stereochemistry of the nucleophile is rarely considered, but phosphines, which have a high barrier to pyramidal inversion, attack electrophiles with retention of configuration at P. Surprisingly, cyclization of bifunctional secondary phosphine alkyl tosylates proceeded under mild conditions with inversion of configuration at the nucleophile to yield P-stereogenic syn-phosphiranes. DFT studies suggested that the novel stereochemistry results from acid-promoted tosylate dissociation to yield an intermediate phosphenium-bridged cation, which undergoes syn-selective cyclization., (© 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.)