Silver Ion Catalyzed Cyclization of Allenic Phosphonic Acids to 1,2-Oxaphospholenes

Abstract

We have previously described a general synthetic route to 1,2-oxaphosphol-3-ene 2-oxides (3) from propargyl alcohols (1), 1–4 proceeding via isolable allenic phosphoryl derivatives (2).^1–4 We

found² that phosphonic acids (2, R[dbnd]H) where both R₂ and R₃ were alkyl groups underwent spontaneous (but slow) cyclization to give 3 (R[dbnd]H). This reaction could also be catalyzed by any strong Bronsted acid. By contrast, if either R₂ or R₃ (or both) were hydrogen, 2 (R[dbnd]H) was completely inert toward concentrated acids even after prolonged exposure high temperature _{1, 2} Still, all derivatives of 2, including esters, could be cyclized by such electrophilic reagents as Br₂, Hg(OAC)₂, ArSCl, and ArSeCl_{3, 4}