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 (Ph2CHCO2 -) sensitized with 1-cyanonaphthalene or 1,4-dicyanonaphthalene in THF or benzene yielded Ph2CH2 as the protonated product of the corresponding carbanion (Ph2CH-). This reaction was not affected by the presence of oxygen, and nanosecond transient absorption spectroscopy allowed the observation of Ph2CH- 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 Ph2CH•, 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, Ph2CH• was observed by laser flash photolysis. The free energy change (−ΔG) on the electron transfer is the crucial factor determining the radical/anion selectivity.