4-Arylation of 3-alkoxypyrazoles

Following the study of the alkoxypyrazoles nitrogen's reactivity toward arylation or alkylation reactions, we report here our results on the introduction of various aryl groups on carbon 4 position of 3-alkoxypyrazoles. This was achieved from the corresponding 4-halogeno derivatives via a Suzuki-Miyaura aryl-aryl coupling reaction. The unexpected difficulties (lack of reactivity or unwanted halogen reduction) encountered in the C-4 arylation of NH-free 4-halogenopyrazoles led us to design solutions to this recurrent problem. The cleavage of the 3-alkoxy group was also investigated using hydrogen bromide in acetic acid or boron tribromide in dichloromethane. This led, in one case, to the observation of a remarkable neighboring group-assisted electrophilic aryl boronylation. This second part of our work paves the way to the synthesis of many original chemical libraries featuring 3-alkoxy 1,4-diaryl pyrazoles as well as the corresponding 1,4-diaryl pyrazol-3-ones.