C-H Bond Arylation of Pyrazoles at the beta-Position: General Conditions and Computational Elucidation for a High Regioselectivity

Direct arylation of most five-membered ring heterocycles are generally easily accessible and strongly favored at the alpha-position using classical palladium-catalysis. Conversely, regioselective functionalization of such heterocycles at the concurrent beta-position remains currently very challenging. Herein, we report general conditions for regioselective direct arylation at the beta-position of pyrazoles, while C-H alpha-position is free. By using aryl bromides as the aryl source and a judicious choice of solvent, the arylation reaction of variously N-substituted pyrazoles simply proceeds via beta-C-H bond functionalization. The beta-regioselectivity is promoted by a ligand-free palladium catalyst and a simple base without oxidant or further additive, and tolerates a variety of substituents on the bromoarene. DFT calculations revealed that a protic solvent such as 2-ethoxyethan-1-ol significantly enhances the acidity of the proton at beta-position of the pyrazoles and thus favors this direct beta-C-H bond arylation. This selective pyrazoles beta-C-H bond arylation was successfully applied for the straightforward building of pi-extended poly(hetero)aromatic structures via further Pd-catalyzed combined alpha-C-H intermolecular and intramolecular C-H bond arylation in an overall highly atom-economical process.