The use of a temporary protection by a chloro group at C5 of pyrazoles allows the synthesis of the 4-arylated pyrazoles, which were previously inaccessible by palladium-catalyzed direct arylation, with complete regioselectivity and in high yields using in most cases as little as 0.5-0.1 mol % Pd(OAc)(2) as the catalyst with electron-deficient aryl bromides. Moreover, from 5-chloro-1,3-dimethylpyrazole, sequential catalytic C4 arylation, dechlorination, catalytic C5 arylation reactions allowed the synthesis of a 4,5-diarylated pyrazole derivative.

Palladium-catalyzed direct arylation of 5-chloropyrazoles allows for the synthesis of 4-arylated pyrazoles with complete regioselectivity and high yields, using minimal catalyst and electron-deficient aryl bromides.

Paper

Palladium-catalyzed direct arylation of 5-chloropyrazoles: a selective access to 4-aryl pyrazoles.

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