In-Cage Formation of Carbanions in Photoinduced Electron-Transfer Reaction of Carboxylate Ions

To develop a novel method for the photochemical generation of carbanions, the photoinduced electron-transfer reaction of carboxylate salts was investigated. The photolysis of tetra- n -butylammonium or metal ion/crown ether salts of diphenylacetate (Ph 2 CHCO 2 - ) sensitized with 1-cyanonaphthalene or 1,4-dicyanonaphthalene in THF or benzene yielded Ph 2 CH 2 as the protonated product of the corresponding carbanion (Ph 2 CH - ). This reaction was not affected by the presence of oxygen, and nanosecond transient absorption spectroscopy allowed the observation of Ph 2 CH - immediately after the laser pulse. The suggested reaction mechanism involves (1) photoexcitation of the sensitizer, (2) the one-electron oxidation of the carboxylate ion, (3) the decarboxylation of the resulting carboxyl radical generating Ph 2 CH •, and (4) the back electron-transfer from the sensitizer radical anion to the radical to yield the carbanion, which occurs smoothly within cage of the geminate radical ion pair. When the sensitizer employed was 9,10-dicyanoanthracene, which has a less negative reduction potential, Ph 2 CH • was observed by laser flash photolysis. The free energy change (−Δ G ) on the electron transfer is the crucial factor determining the radical/anion selectivity.