In-cage formation of carbanions in photoinduced electron-transfer reactionof carboxylate ions

To develop a novel method for the photochemical generation of carbanions, t he photoinduced electron-transfer reaction of carboxylate salts was investi gated. The photolysis of tetra-n-butylammonium or metal ion/crown ether sal ts of diphenylacetate (Ph2CKCO2-) sensitized with 1-cyanonaphthalene or 1 b eta-dicyanonaphthalene in THF or benzene yielded Ph2CH2 as the protonated p roduct of the corresponding carbanion (Ph2CH-). This reaction was not affec ted by the presence of oxygen, and nanosecond transient absorption spectros copy allowed the observation of Ph2CH- immediately after the laser pulse. T he suggested reaction mechanism involves (1) photoexcitation of the sensiti zer, (2) the one-electron oxidation of the carboxylate ion, (3) the decarbo xylation of the resulting carboxyl radical generating Ph2CH., and (4) the b ack electron-transfer from the sensitizer radical anion to the radical to y ield the carbanion, which occurs smoothly within cage of the geminate radic al ion pair. When the sensitizer employed was 9,10-dicyanoanthracene, which has a less negative reduction potential, Ph2CH. was observed by laser flas h photolysis. The free energy change (-Delta G) on the electron transfer is the crucial factor determining the radical/anion selectivity.