Oxygenation studies. Part 8. Catalytic oxygenations of arylphosphines at platinum(0)

Abstract

Experimental support for the view that trace protic species may play a key role in the catalytic co-oxygenation of arylphosphines at [Pt(PPh₃)₃] under nominally aprotic conditions has been sought but not found. Marked reductions in the catalytic rates are observed when PPh₃ and PMePh₂ are oxidised in the presence of trace levels of moisture or alcohols. The inhibiting influence of Ph₂PO₂H and p-MeC₆H₄SO₃H is even greater. Oxygen uptake for the oxygenation of PMePh₂, in benzene at 25 °C, is shown to follow the rate law below and is considered in terms of a general Rate (O₂)=[Pt]_tot(15.41 [phosphine][O₂]^–1+ 1 290)^–1 scheme for the co-oxygenation of arylphosphines at Pt⁰ in aprotic solvents. It is suggested that the ‘oxygen insertion’ step is best interpreted as intramolecular nucleophilic attack on two co-ordinated phosphines followed by breakdown of the resulting metallacycle by reductive elimination; a mechanism embracing features which may well occur in a range of co-oxygenations at Group 8 transition-metal centres.