摘要:
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-dibenzo[b,d]pyrans 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-dibenzo[b,d]pyran. 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 (PHI = 0.0015 in 100% 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.