Visible light-induced photofixation of CO2 into benzophenone: roles of poly(p-phenylene) as photocatalyst and two-electron mediator in the presence of quaternary onium salts

Abstract

Visible light-induced photocatalytic fixation of CO₂ into benzophenone (1) has been carried out in CO₂-saturated DMF (N,N-dimethylformamide) using poly(p-phenylene) (PPP) as a heterogeneous photocatalyst and triethylamine as an electron donor. Diphenylglycolic acid (2) is produced as a CO₂-fixed product together with benzhydrol (3), benzopinacol (4) and 1,1-diphenylpropane-1,2-diol (5). The presence of quaternary onium salts such as tetraethylammonium chloride (Et₄NCl) increases the yield of 2 in the presence of CO₂ and those of 3 and 5 in the absence of CO₂. The soft onium cations from quaternary onium salts are proposed to stabilize diphenylcarbinol anion (9) as a common precursor of 2, 3 and 5 on the basis of the HSAB concept. Further, the enhanced formation of PPP dianions (7) through PPP radical anions (6) has been examined in the reduction of PPP with sodium metal in the presence of Et₄NCl. Photocatalysis of PPP in the CO₂ fixation is discussed in terms of onium salt effects and favorable photoformation of 6 and 7 as an electron pool in the presence of quaternary onium cations.