Photocyclization of 2-(2′-hydroxyphenyl)benzyl alcohol and derivatives via o-quinonemethide type intermediates

Article abstract of DOI:10.1021/ja00020a033

A new photochemical reaction, the photocyclization of 2-(2′-hydroxyphenyl)benzyl alcohol (1) and derivatives to 6H-dibenzo[b,d]pyrans, is reported. The quantum yield for cyclization of 1 to give pyran 7 is 0.,50 ± 0.04 in aqueous solutions of pH > 10. At lower pH, φ is significantly lower. For example at pH 7, φ = 0.25 ± 0.03. Results from investigations of structure-reactivity, pH effects, and fluorescence data suggest a mechanism in which the primary step involves ionization of the phenol moiety to phenolate in S₁, which is probably followed by (or is concerted with) twisting of the phenyl rings to give a more planar species in the excited state. This is subsequently followed by (or is concerted with) a dehydroxylation step of the benzyl alcohol moiety (all on the S₁ surface) to give an o-quinonemethide type intermediate 20 and deactivation back to the ground-state surface. Electrocyclic ring closure of this intermediate gives the observed pyran. Nucleophilic trapping of this electrophilic intermediate by solvent (e.g., MeOH) to give the methyl ether is also observed when photolysis is carried out in MeOH. The proposed reaction pathway is unprecedented: it takes advantage of the tendency of biphenyl systems to be planar in the excited state as well as the enhanced electron-donating effect of the phenolate anion, which is required for the dehydroxylation step.