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.