Rhodium-catalyzed oxidative 1:1, 1:2, and 1:4 coupling reactions of phenylazoles with internal alkynes through the regioselective cleavages of multiple C-H bonds.
The direct oxidative coupling of phenylazoles with internal alkynes proceeds efficiently in the presence of a rhodium catalyst and a copper oxidant accompanied by double or quadruple C-H bond cleavages. Thus, as a representative example, 4,5-diphenylpyrazolo[1,5-a]quinoline, 1-(1,2,3,4-tetraphenylnaphthalen-5-yl)pyrazole, and 1-(1,2,3,4,5,6,7,8-octaphenylanthracen-9-yl)pyrazole can be obtained selectively through the coupling of 1-phenylpyrazole and diphenylacetylene in 1:1, 1:2, and 1:4 manners, respectively. The reactions preferentially take place at the electron-deficient sites on the aromatic substrates. A comparison of reactivities of variously substituted and deuterated substrates sheds light on the mechanism of C-H bond cleavage steps. The reaction pathway is highly dependent on reaction conditions employed, especially on the nature of solvent. The influence of solvation of a key rhodacycle intermediate has been investigated computationally. In addition, some of the condensed aromatic products have been found to exhibit intense fluorescence in the solid state.
