Palladium-Catalyzed Chelation-Assisted Regioselective Oxidative Dehydrogenative Homocoupling/Ortho-Hydroxylation in N-Phenylpyrazoles

A palladium-catalyzed pyrazole-directed regioselective oxidative C(sp2)-H functionalization of the N-phenyl ring in N-phenylpyrazoles to afford either a biaryl bis-pyrazole (via dehydrogenative homocoupling) or N-(o-hydroxyphenyl)pyrazole (via C-H oxygenation) or their mixture is described. The substitutions on the N-phenyl ring and the pyrazole ring and the dilution of the reaction medium with respect to the TFA/TFAA mixture (substrate concentration) have a remarkable influence on the outcome of the reaction. It was discovered that if the reactions were performed under highly dilute conditions (ca. 10 times) then N-(o-hydroxyphenyl)pyrazoles were the major or the sole products.