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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]isoihdoles via intra-molecular 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.
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Submitted on : Friday, October 14, 2016 - 3:36:04 PM
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Mariem Brahim, Hamed Ben Ammar, Jean-François Soulé, Henri Doucet. Reactivity of 1-(2-bromobenzyl)-4-halopyrazoles in intermolecular and intramolecular Pd-catalysed direct arylations. Tetrahedron, Elsevier, 2016, 72 (29), pp.4312--4320. ⟨10.1016/j.tet.2016.05.079⟩. ⟨hal-01357418⟩

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