Photochemistry of Phenoxybenzyl Alcohols in Aqueous Solution: Photosolvolysis vs Photorearrangement to 6H-Dibenzopyrans

Article abstract of DOI:10.1021/jo00016a009

The photochemistry of three phenoxybenzyl alcohols (1-3) has been studied in MeOH, CH3CN, and in aqueous solution.It was found that both of the ortho-substituted phenoxybenzyl alcohols 1 and 2 gave the corresponding 6H-dibenzopyrans 6 and 10, via a mechanism believed to involve initial aryl C-O bond homolysis followed by rearrangement to give a 2-(2'-hydroxyphenyl)benzyl alcohol (biphenyl) derivative, which subsequently undergoes a photocyclization reaction to the corresponding 6H-dibenzopyran.The quantum yield for formation of 6 (from 1) was 0.0073 in neutral 6:4 H2O-CH3CN.Lower quantum yields for formation of 6 were observed on photolysis in pure organic solvents (Φ = 0.0015 in 100percent CH3CN).The meta-substituted isomer 3 did not give any reaction via a similar photocyclization process: its photochemistry involves initial aryl C-O bond homolysis followed by simple radical recoupling to give isomeric hydroxybiphenyls, as well as products derived from radical escape.In aqueous sulfuric acid solution (pH < 2), a competing acid-catalyzed photosolvolysis reaction was observed for all of these compounds (i.e., C-OH bond heterolysis with assistance of hydronium ion); it was the only observed reaction in moderately concentrated sulfuric acid solution.