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-dihydrophenanthrene-9-carboxylic acid (1) in 55% yield, trans-9,10-dihydrophenanthrene-9,10-dicarboxylic acid (2) in 11% yield, a trace of phenanthrene-9-carboxylic acid (3), and a trace of 10-[p-(N,N-dimethylamino)phenyl]-9,10-dihydrophenanthrene-9-carboxylic acid (5). Addition of cumene, a hydrogen donor, or water, a proton donor, decreased the yield of 2, while addition of certain salts increased its yield. 9-Carboxy-9,10-dihydrophenanthr-10-yl, generated by irradiation of phenanthrene-9-carboxylic acid in the 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 concentrations in DMSO changed the reaction pathway, greatly reducing the yields of 1 and 2, and phenanthrene-9-carboxylic acid (3) and 10-[p-(N,N-dimethylamino)phenyl]-9,10-dihydrophenanthrene-9-carboxylic acid (5) were formed instead. On the 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-dicarboxylic acid formation. Transient spectroscopic evidence in support of this proposal is presented.