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

The addition reaction of dibutylphosphane oxide [Bu2P(O)H] with alkynes proceeds efficiently in the presence of palladium-chelating phosphane-Bronsted 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 Bronsted acid affords a much lower yield. A mechanistic study has revealed that Bronsted acids (XOH) interact with oxygen in M- P(O)R-2 species (M = Pd, Pt) through hydrogen bonding to transform them to ionic M+ <- PR2 (OH center dot center dot center dot O-X) species, which was confirmed by NMR spectroscopy and X-ray crystallography. The phosphane-like PR2(OH center dot center dot center dot 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 Bronsted acid.