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

Abstract

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 syn-Mes*PCH 2 CHR (Mes* = 2,4,6-(t-Bu) 3 C 6 H 2 , R = Me or Ph, syn-1-2) or anti-Mes*PCH 2 CHPh (anti-2) with triflic acid resulted in regiospecific anti-Markovnikov C-protonation with ring opening and cyclophosphination of a Mes* ortho-t-Bu group to yield the phospholanium cations [PH(CH 2Me or Ph, 3−4), which were deprotonated with NEt 3 to give phospholanes 5−6. Enantioenriched or racemic syn-1 both gave racemic 3. The byproduct [Mes*PH(CH 2 CH 2 Me)(OH)][OTf] (7) was formed from syn-1 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(CH 2 CH 2 Me)][OTf], which undergoes C−H oxidative addition of an o-t-Bu methyl group. This work established a new reactivity pattern for phosphiranes.