Journal of the American Chemical Society, Vol.134, No.39, 16163-16166, 2012
Rhodium-Catalyzed Cascade Oxidative Annulation Leading to Substituted Naphtho[1,8-bc]pyrans by Sequential Cleavage of C(sp(2))-H/C(sp(3))-H and C(sp(2))-H/O-H Bonds
The cascade oxidative annulation reactions of benzoylacetonitrile with internal alkynes proceed efficiently in the presence of a rhodium catalyst and a copper oxidant to give substituted naphtho[1,8-bc]pyrans by sequential cleavage of C(sp(2))-H/C(sp(3))-H and C(sp(2))-H/O-H bonds. These cascade reactions are highly regio-selective with unsymmetrical alkynes. Experiments reveal that the first-step reaction proceeds by sequential cleavage of C(sp(2))-H/C(sp(3))-H bonds and annulation with alkynes, leading to 1-naphthols as the intermediate products. Subsequently, 1-naphthols react with alkynes by cleavage of C(sp(2))-H/O-H bonds, affording the 1:2 coupling products. Moreover, some of the naphtho[I,8-bc]pyran products exhibit intense fluorescence in the solid state.