Regioselective palladium-catalyzed C H arylation of 4-alkoxy and 4-thioalkyl pyrazoles

C-H Bond Arylation of Pyrazoles at the β-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 α-position using classical palladium-catalysis. Conversely, regioselective functionalization of such heterocycles at the concurrent β-position remains currently very challenging. Herein, we report general conditions for regioselective direct arylation at the β-position of pyrazoles, while C-H α-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 β-C-H bond functionalization. The β-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 β-position of the pyrazoles and thus favors this direct β-C-H bond arylation. This selective pyrazoles β-C-H bond arylation was successfully applied for the straightforward building of π-extended poly(hetero)aromatic structures via further Pd-catalyzed combined α-C-H intermolecular and intramolecular C-H bond arylation in an overall highly atom-economical process.

Transition-metal-catalyzed C–H functionalization of pyrazoles

This review describes recent advances in transition-metal-catalyzed C–H functionalization reactions of pyrazoles to form new C–C and C–heteroatom bonds on the pyrazole ring.

Preparation of 1,5-Dihydropyrazolo[3',4':5,6]pyrano[3,4- b]pyridines via a Microwave-Assisted, Palladium-Catalyzed Regioselective C-H Heteroarylation of Electron-Rich Pyrazoles

Here we report the first synthesis of a family of novel heterocyclic compounds based on a 5-dihydropyrazolo[3',4':5,6]pyrano[3,4- b]pyridine core. In the course of our drug discovery programs, we had need to access the previously unknown 5-dihydropyrazolo[3',4':5,6]pyrano[3,4- b]pyridine core. Initial attempts required long reaction times, which led to degradation and side products. Reaction optimization identified a Pd-catalyzed, microwave-assisted C-H heteroarylation protocol for the rapid, general, and high yielding synthesis of this tricyclic core (as well as related analogs) suitable to drive optimization efforts.