Palladium-Catalyzed <i>ortho</i> C–H Activation/Arylation of 7-Arylpyrazolo[1,5- <i>a</i> ]pyridines: Synthesis of Biaryl-Substituted Mono- and Bipyrazolo[1,5- <i>a</i> ]pyridines with AIEE Properties

In this study, we present a facile synthetic methodology for the synthesis of 7-(2-biaryl)-substituted pyrazolo[1,5-a]pyridines directly from 7-arylpyrazolo[1,5-a]pyridines via palladium-mediated ortho-C–H activation/arylation. This transformation can be readily accomplished under both catalytic and stoichiometric conditions, facilitated by the intrinsic directing ability of the pyrazolo[1,5-a]pyridine scaffold. Through the stoichiometric reactions, key 7-phenylpyrazolo[1,5-a]pyridine-derived palladacycles bearing acetate or trifluoroacetate ligands were isolated and structurally characterized by X-ray crystallography. To gain mechanistic insight into the C–H bond cleavage step, both parallel and intermolecular kinetic isotope effect (KIE) studies were conducted, providing evidence in support of the proposed reaction pathway. Finally, the developed methodology enables the practical synthesis of 7,7′-bis(2-biaryl)-substituted 3,3′-bipyrazolo[1,5-a]pyridines via an intermolecular C–H/C–H cross-coupling reaction, affording a new class of aggregation-induced enhanced emission (AIEE)-active luminogens.