Selective Arylation, Alkenylation, and Cyclization of Dibromonaphthols, Using Visible Light, via Carbene Intermediates

The photoreactivity of several 3-substituted-1,6-dibromo-2-naphthols has been investigated in neat acetonitrile in the presence of diluted Et3N and in aqueous buffered acetonitrile (pH 8, phosphate buffered), using visible light (450 nm). Hydrobromic acid loss in the presence of the base, for the unsubstituted naphthol, or heterolytic C-Br cleavage directly from the naphtholates, for the more acid 3-substutited naphthols (R=COOCH3, CONH2, CONMe2), generates electrophilic carbene intermediates, which have been successfully trapped by molecular oxygen, pyrrole, acrylonitrile, ethyl vinyl ether, and allyltrimethylsilane. Product distribution analysis reveals three types of products arising from (i) arylation, (ii) alkenylation, and (iii) cyclization reactions. The generation and the reactivity of a ketocarbene intermediates, as electrophilc diradicals, has been supported by laser flash photolysis, with the detection of both the carbene (λmax 510 nm) and 1,2-naphthoquinone-O-oxide (R=CONMe2, λmax 600 nm) in the presence of oxygen.