Dual C–H/C–H asymmetric cross-coupling unveiled

Although chemists have turned modifying unreactive C–H bonds into a routine event in organic synthesis, now and again a new approach still turns up. In one of the latest examples, De-Wei Gao, Qing Gu, and Shu-Li You of Shanghai Institute of Organic Chemistry have reported an asymmetric twofold C–H oxidative cross-coupling reaction of ferrocenes with heteroarenes that takes place without having to first prefunctionalize one or the other aryl coupling partners, which is usually required in cross-coupling strategies (J. Am. Chem. Soc. 2016, DOI: 10.1021/jacs.6b00127). The new reaction is a greener, less expensive method for preparing planar chiral ferrocenes, which are useful as chiral ligands and as asymmetric catalysts, You says. The team’s strategy involves a palladium catalyst with a protected chiral amino acid ligand that proceeds in a twofold manner, first activating the ferrocene and then the heteroarene. The atom-economical reaction avoids the need for an aryl halide or aryl organometallic precursor, proceeds using oxygen in the air as an oxidant instead of a metal oxidant, and doesn’t need a significant excess of either coupling partner. The team shows that the ferrocene derivatives can be developed as N,O and N,S and N,P bidentate planar chiral ligands and as chiral catalysts.