Formyl Substituent at C‐4 of Pyrazoles: A Temporary Protecting Group for Regioselective Palladium‐Catalyzed Direct Arylation at C‐5

Pyrazoles with an aldehyde function at C‐4 underwent a palladium‐catalyzed direct arylation reaction to provide a regioselective approach to 5‐aryl‐substituted pyrazoles. The reaction proceeds in moderate to high yields with a variety of aryl bromides in the presence of 2 mol‐% of Pd(OAc) 2 as the catalyst. The use of an aldehyde function at C‐4 of the pyrazoles presents several advantages: (1) 4‐formylpyrazoles are easily prepared from hydrazine derivatives, ketones, and N , N ‐dimethylformamide (DMF), (2) the control of the regioselectivity of the arylation at C‐5 of the pyrazole, (3) the aldehyde substituent can easily be transformed into a wide variety of other substituents, and (4) the formyl group can be considered a temporary protecting group, as it can be removed by a straightforward reaction.