Regioselective Palladium-Catalyzed Direct C3-H Arylation of Triazolopyridazines with Aryl Thianthrenium Salts.

A regioselective palladium-catalyzed C3-H arylation of 1,2,4-triazolo[4,3-b]pyridazines with readily accessible aryl thianthrenium salts has been developed. This method provides direct access to a diverse array of C3-arylated triazolopyridazines in moderate to excellent yields under mild and operationally simple conditions. A broad substrate scope and excellent functional-group tolerance were observed for both coupling partners. The synthetic utility of the protocol was further demonstrated by gram-scale synthesis. Mechanistic studies, including competition and radical-trapping experiments, support a Pd(0)/Pd(II) catalytic pathway and rule out the involvement of radical intermediates. This work offers an efficient approach to the direct C3 functionalization of triazolopyridazines and highlights the potential of aryl thianthrenium salts as valuable arylation reagents in heterocycle synthesis.