Efficient Addition Reaction of Dibutylphosphane Oxide with Alkynes: New Mechanistic Proposal Involving a Duo of Palladium and Brønsted Acid

The addition reaction of dibutylphosphane oxide [Bu₂P(O)H] with alkynes proceeds efficiently in the presence of palladium‐chelating phosphane–Brønsted acid catalyst systems. Terminal alkynes afford branched‐structured products selectively. On the other hand, the same reaction using monodentate phosphane ligands or the reaction run in the absence of a Brønsted acid affords a much lower yield. A mechanistic study has revealed that Brønsted acids (XOH) interact with oxygen in M P(O)R₂ species (M=Pd, Pt) through hydrogen bonding to transform them to ionic M⁺←PR₂(OH⋅⋅⋅O⁻X) species, which was confirmed by NMR spectroscopy and X‐ray crystallography. The phosphane‐like PR₂(OH⋅⋅⋅O⁻X) moiety is coordinatively labile, as substantiated by the ligand exchange reaction with tert‐butyl isocyanide. A new mechanism that accommodates these observations has been proposed to rationalize the enhancement of catalytic activity and the regioselectivity induced by the Brønsted acid.