REDUCTIVE PHOTOCARBOXYLATION OF PHENANTHRENE - A MECHANISTIC INVESTIGATION

Irradiation of a solution of phenanthrene (PHN) in DMSO saturated with CO2 in the presence of N,N-dimethylaniline (DMA) produced 9,10-dihydr ophenanthrene-9-carboxylic acid(1) in 55% yield, trans-9,10-dihydrophe nanthrene-9,10-dicarboxylic acid (2) in 11% yield, a trace of phenanth rene-9-carboxylic acid (3), and a trace of mino)phenyl]-9,10-dihydroph enanthrene-9-carboxylic acid (5), Addition of cumene, a hydrogen donor , or water, a proton donor, decreased the yield of 2, while addition o f certain salts increased its yield, 9-Carboxy-9,10-dihydrophenanthr-1 0-yl, generated by irradiation of phenanthrene-9-carboxylic acid in th e presence of DMA, is proposed to be an intermediate in the formation of the acids,,The quantum yield for the formation of 2 increased to a maximum of 0.13 with increasing light intensity. High CO2 concentratio ns in DMSO changed the reaction pathway, greatly reducing the yields o f 1 and 2, and phenanthrene-9-carboxylic acid (3) and mino)phenyl]-9,1 0-dihydrophenanthrene-9-carboxylic acid (5) were formed instead. On th e basis of these results reduction of 9-carboxy-9,10-dihydrophenanthr- 10-yl with the phenanthrene radical anion is proposed to be a step in the mechanism accounting for trans-9,10-dihydrophenanthrene-9,10-dicar boxylic acid formation, Transient spectroscopic evidence in support of this proposal is presented.