Palladium-Catalyzed C4,C5-Diarylation of Isoxazole-3-carboxylate by Double C–H Bond Functionalization

The Pd-catalyzed C4-arylation of 3,5-disubstituted isoxazoles via C-H bond functionalization has been largely described. By contrast, the reactivity of isoxazoles in C-H bond functionalization with both unsubstituted C4 and C5 positions remains largely unexplored. Herein, we report on the reactivity in Pd-catalyzed double C-H bond arylation of an isoxazole with unsubstituted C4 and C5 positions. Conditions for the palladium-catalyzed direct C4,C5-diarylation of ethyl isoxazole-3-carboxylate using aryl bromides as the aryl source are reported. This procedure tolerates several useful substituents on the aryl bromide such as nitrile, acetyl, formyl, benzoyl, alkoxycarbonyl, chloro, fluoro, trifluoromethyl, trifluoromethoxy, cyanomethyl,tertbutyl and methoxy at para- and meta-positions. Conversely, with ortho-substituted aryl bromides, mixtures of mono- and di-arylated isoxazoles were generally obtained. This methodology provides a simple one pot access to a wide variety of C4,C5-diarylated isoxazoles from commercially available substrates allowing to modify easily their biological properties.