Photocyclization and Photoaddition Reactions of Arylphenols via Intermediate Quinone Methides

A series of five benzannelated derivatives of 2-phenylphenol were prepared, and their photochemistry was investigated. Two of these (3-phenyl-2-naphthol, 10, and 1-phenyl-2-naphthol, 11) were photoinert. For 2-(1-naphthyl)phenol (12) and 1-(1-naphthyl)-2-naphthol (13), ESPT took place to either the 2'-position or the 7'-position of the naphthalene ring to give quinone methides (QMs) that underwent either reverse proton transfer (RPT) or electrocyclic ring closure to give dihydrobenzoxanthenes. The intermediate QMs for 12 and 13 were detected and characterized by laser flash photolysis. For 2-(9-phenanthryl)phenol (14), ESPT took place either to the 5'-position to give a QM that underwent quantitative electrocyclic ring closure to give the corresponding benzoxanthene or to the 10'-position to give a QM that underwent RPT. If the solution contained methanol, the QM produced on ESPT to the 10'-position in 14 could be trapped as the photoaddition product. The compounds studied in this work demonstrate three possible reactions of QMs produced following ESPT to aromatic carbon atoms: (1) reverse proton transfer (RPT) to regenerate starting material; (2) addition of hydroxylic solvents to give the photoaddition product; and (3) electrocyclic ring closure to give benzoxanthene derivatives.