Keggin structure heteropolyacid-catalyzed phosphinylation of secondary propargyl alcohols with phosphine oxides to γ-ketophosphine oxides†

Phosphotungstic acid with a Keggin structure as an efficient, simple and green catalyst for the phosphinylation of secondary propargyl alcohols with phosphine oxides to afford γ-ketophosphine oxides with up to 88% isolated yield was developed using dimethyl carbonate as a green solvent. Diaryl- or alkylaryl-substituted propargyl alcohols, and diaryl or arylalkylphosphine oxides could tolerate the system, which reduced the catalyst dosage, and avoided the use of multi-components and toxic solvents. More interestingly, phosphotungstic acid exhibited the best activity when 0.58 moles of water were added per mole of HPWA, elevating the yield from 55% to 85%. An ¹⁸O labelled product was afforded using trace H₂¹⁸O instead of H₂O, indicating the participation of water in the reaction. Besides, our work underscores the importance and effect of a small amount of water, acting to promote the transformation of secondary propargyl alcohols into enones, which should be the real intermediates of the reaction. A mechanism involving a carbocation, Meyer–Schuster rearrangement and Michael addition of enones was proposed.