The widespread presence of C–H bonds at various sites of synthetic substrates renders C–H activation the most powerful platform for developing catalytic reactions for synthesis. Both the measurement for success and approaches for addressing this synthetic problem are distinct from the long-pursued methane oxidation or alkane functionalizations, where one extremely efficient reaction of one particular C–H bond is needed. In contrast, to realize the full potential of C–H activation for synthesis, four fundamental challenges must be addressed: developing diverse catalytic systems for wide range of carbon-carbon and carbon-heteroatom bond forming reactions (DiversitY); enantioselective C–H activation reactions via asymmetric metalation of C–H bonds (EnantioselectivitY); site selective metalation and functionalization of remote C–H bonds (Site-selectivitY); achieving catalytic cycles using sustainable oxidants such as molecular oxygen, aqueous hydrogen peroxides as the terminal oxidants (SustainabilitY). Despite a century-long efforts on understanding and developing C–H activation reactions, seeking solutions to these problems has met with limited success due to: lack of reactivity of broadly useful substrates; lack of well-defined stereomodel for enantioselective metal insertion; lack of predictable approaches for controlling site selectivity as majority of C–H bonds are electronically and sterically similar, hence difficult to distinguish; and most importantly, lack of ligands that can accelerate C–H activation reactions. By combining the weak coordination (entropy) from substrates and ligand acceleration (enthalpy), we have made substantial progress towards addressing these four challenges. Most notably, six generations of bi-functional ligands (MPAA, APAQ, APAO, MPAAm, MPAThio, Pyridine-Pyridone) have been developed to enable a wide range of enantioselective1-6 and site-selective7-10 C–H activation reactions of diverse classes of native substrates. Most recently, we have realized C–H hydroxylation using molecular oxygen or aqueous hydrogen peroxide as the terminal oxidants, paving the way for large-scale industrialization.11,12

20-Year Dancing with Palladium and C–H bonds: From Laboratory to Industrialization

ACS Spring 2022

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