Titanium dioxide photocatalyzed oxygenation of naphthalene and some of its derivatives

Abstract

The titanium dioxide photocatalyzed oxygenation of naphthalene in water gives (E,Z)-2-formylcinnamaldehydes and 1,4-naphthoquinone besides traces of naphthols. 1-Substituted naphthalenes (both with an electron-withdrawing and -donating group) are likewise oxidised at about the same rate with a similar product distribution. The evidence obtained supports the hypothesis that the reaction involves transfer of a hydroxy group to naphthalene and reduction of oxygen to superoxide followed by coupling of the two species. In organic solvents the reaction is slower, more selective (a cyano group inhibits the reaction) and leads to phthalic anhydride and 1,4-naphthoquinone. The relation with the solvent-dependent course of the ozonation of the same substrates is commented upon.