Reactivity of 1-(2-bromobenzyl)-4-halopyrazoles in intermolecular and intramolecular Pd-catalysed direct arylations

Abstract

The reactivity of 1-(2-bromobenzyl)-4-halopyrazoles in inter- and intra-molecular Pd-catalysed direct arylation was investigated. Conditions allowing the intermolecular C5-arylations of both 1-(2-bromobenzyl)-4-chloropyrazoles and 1-(2-bromobenzyl)-4-bromopyrazoles, without cleavage of the pyrazolyl and benzyl C-halo bonds, are reported. Using KOAc as the base, DMA as the solvent and 2 mol% of an air stable palladium catalyst, the target C5-arylated pyrazoles were obtained in moderate to good yields with a wide variety of aryl bromides. The synthesis of 3-halopyrazolo[5,1-a]isoindoles via intramolecular Pd-catalysed direct arylation, without cleavage of the pyrazolyl C-halo bonds is also described. Moreover, sequential Pd-catalysed C5-arylations followed by intramolecular direct arylation allowed the access to dibenzo[c,e]pyrazolo[1,5-a]azepine derivatives. The reactivity of the 2-bromobenzyl moiety of C5-arylated 1-(2-bromobenzyl)-4-halopyrazoles in intermolecular direct arylation or in Suzuki coupling is also described.