Palladium-catalysed, directed C-H coupling with organometallics

Transition metal (TM)-catalysed C-H coupling using organometallic reagents is developing into a distinct transformation area for the construction of carbon-carbon (C-C) bonds. Among different TMs (Ru, Rh, Pd, Co, Fe), complexes of palladium(II) are emerging as the most effective and versatile catalysts and have already asserted a proven track-record for the catalytic activation and coupling of unactivated sp² and sp³ C-H bonds with sp ² and sp³ organometallic reagents. A majority of these TM-catalysed C-H coupling reactions utilise a heteroatom contained in substrates as a directing group. In the last decade, both traditional as well as non-traditional but synthetically useful functional groups have been extensively exploited as directing groups. Novel directing groups based on mechanistic understanding have also been designed to enhance the reactivity of Pd(II) catalysts towards C-H bonds. In addition, the unprecedented application of exogenous ligands to Pd(II)-catalysed C-H coupling is also on the rise, and this approach has already demonstrated an extraordinary level of success not only in breaking unactivated C-H bonds enantioselectively, but also in improving reactivity of catalysts towards these C-H bonds.