Mapping out the key carbon–carbon bond-forming steps in Mn-catalysed C–H functionalization

Detailed understanding of the mechanistic processes that underpin transition metal-catalysed reactions allows for the rational and de novo development of complexes with enhanced activity, efficacy and wider substrate scope. Directly observing bond cleaving and forming events underpinning a catalytic reaction is non-trivial as the species that facilitate these steps are frequently short-lived and present at low concentrations. Here we describe how the photochemical activation of a manganese precatalyst, [Mn(ppy)(CO)4], results in selective loss of a carbonyl ligand simulating entry into the catalytic cycle for Mn-promoted C–H bond functionalisation. Time-resolved infra-red spectroscopy (ps-ms timescale) allows direct observation of the species responsible for the essential carbon-carbon bond formation step and an evaluation of the factors affecting its rate. The mechanistic information prompted discovery of a new photochemically initiated manganese-promoted coupling of phenylacetylene with 2-phenylpyrindine. This study provides unique insight into the mechanistic pathways which underpin catalysis by an Earth-abundant metal, manganese.