C?H Bond Arylation of Pyrazoles at the β-Position: General Conditions and Computational Elucidation for a High Regioselectivity |
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Authors: | Dr Xinzhe Shi Dr E Daiann Sosa Carrizo Marie Cordier Dr Julien Roger Prof?Dr Nadine Pirio Prof?Dr Jean-Cyrille Hierso Prof?Dr Paul Fleurat-Lessard Dr Jean-François Soulé Dr Henri Doucet |
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Institution: | 1. Univ Rennes, CNRS ISCR-UMR 6226, 35000 Rennes, France;2. Université de Bourgogne, Institut de Chimie Moléculaire de, l'Université de Bourgogne, UMR CNRS 6302, Université, Bourgogne Franche-Comté (UBFC), 9 avenue Alain Savary, 21078 Dijon, France |
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Abstract: | 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. |
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Keywords: | beta-C?H arylation catalysis heterocycle palladium solvent effect |
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