Protonation of P-Stereogenic Phosphiranes: Phospholane Formation via Ring Opening and C-H Activation.

Protonation of cyclopropanes and aziridines is well-studied, but reactions of phosphiranes with acids are rare and have not been reported to result in ring opening. Treatment of -Mes*PCHCHR (Mes* = 2,4,6-(-Bu)CH, R = Me or Ph, --) or -Mes*PCHCHPh (-) with triflic acid resulted in regiospecific -Markovnikov C-protonation with ring opening and cyclophosphination of a Mes* --Bu group to yield the phospholanium cations [PH(CHCHR)(4,6-(-Bu)-2-CMeCHCH)][OTf] (R = Me or Ph, -), which were deprotonated with NEt to give phospholanes -. Enantioenriched or racemic - both gave racemic . The byproduct [Mes*PH(CHCHMe)(OH)][OTf] () was formed from - and HOTf in the presence of water. Density functional theory calculations suggested that P-protonation followed by ring opening and hydride migration to C yields the phosphenium ion, [Mes*P(CHCHMe)][OTf], which undergoes C-H oxidative addition of an --Bu methyl group. This work established a new reactivity pattern for phosphiranes.