Photocatalytic 1,2-Phosphorus-Migrative [3 + 2] Cycloaddition of Tri(<i>t</i>‑butyl)phosphine with Terminal Alkynes

Tri(t-butyl)phosphine and terminal alkynes undergo 1,2-phosphorus-migrative [3 + 2] cycloaddition in the presence of a proton source under photocatalytic conditions. The reaction exhibits broad functional group tolerance and affords substituted cyclic phosphonium salts, which are amenable to further derivatization by Wittig olefination. Theoretical studies suggest that the phosphorus 1,2-migration of a β-phosphonioalkyl radical proceeds through a phosphine radical cation–alkene complex as a pseudointermediate, and the two fragments in the intermediate are bound to each other through multiple noncovalent interactions.