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] (ppy = 2-phenylpyridine), results in selective loss of a carbonyl ligand simulating entry into the catalytic cycle for manganese-promoted C–H bond functionalization. Time-resolved infrared spectroscopy (on the ps–ms timescale) allows direct observation of the species responsible for the essential C–C bond formation step and an evaluation of the factors affecting its rate. This mechanistic information prompted the discovery of a new photochemically initiated manganese-promoted coupling of phenylacetylene with 2-phenylpyridine. This study provides unique insight into the mechanistic pathways underpinning catalysis by an Earth-abundant metal, manganese. Although mechanistic understanding can drive new reactivity development, the key bond-forming and -breaking steps in catalytic cycles are often sufficiently fast to elude observation. Here, the authors photochemically produce a key intermediate in Mn-catalysed C–H functionalization, and follow the subsequent steps—spanning processes occurring over seven orders of magnitude in time—using time-resolved infrared spectroscopy.