Palladium-Catalyzed Direct Arylation of Heteroaromatics with Activated Aryl Chlorides Using a Sterically Relieved Ferrocenyl-Diphosphane

Abstract

The palladium-catalyzed direct arylations at C3 or C4 positions of heteroaromatics are known to be more challenging than at C2 or C5 positions. Aryl chlorides are also challenging substrates for direct arylation of heteroaromatics. We observed that in the presence of a palladium-catalyst combining only 0.5 mol % of Pd­(OAc)₂ with the sterically relieved new ferrocenyl diphosphane <b>Sylphos</b>, the direct arylation at C3 or C4 of oxazoles, a benzofuran, an indole, and a pyrazole was found to proceed in moderate to high yields using a variety of electron deficient aryl chlorides. Turnover numbers up to 176 have been obtained with this catalyst. Assessment of the electron-donating properties of <b>Sylphos</b> from electrochemical studies and ¹<i>J</i>_PSe measurement on its selenide derivative indirectly indicated that the influence of steric properties of <b>Sylphos</b>–and in particular a less sterically congested environment at phosphorus due to a methylene spacer–are certainly dominant in its catalytic performance