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 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 2 CH 2 R)(4,6-( t -Bu) 2 -2-CMe 2 CH 2 C 6 H 2 )][OTf] (R = Me 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.