Selective Oxidation of Primary C-H Bonds Using Late-Transition-Metal-Oxo Catalysts

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Project Summary/Abstract Advances in organic synthetic methodology can profoundly impact the development of new and useful medicines. For example, cross-coupling reactions have become indispensable tools in medicinal chemistry, provided entry to previously inaccessible chemical space, and enhanced drug discovery efforts. Recently, methods for the selective functionalization of C–H bonds have gained attention from the pharmaceutical industry due to their potential utility in the diversification of drug-like scaffolds. Toward this end, metal-catalyzed C–H functionalization reactions that take advantage of polar functional groups to direct site-selective C–H activation have been extensively explored. In comparison, methods that avoid the use of pre-installed directing groups, or “undirected” C–H functionalization reactions, are underdeveloped. Specifically, the selective and undirected metal-catalyzed activation of strong primary C(sp3)–H bonds in the presence of weaker C–H bonds represents an ongoing challenge in the field. Notably, such technologies would provide chemists with useful synthetic tools to install functionality at remote sites on bioactive molecules. Though methods for the undirected selective catalytic functionalization of methyl groups to forge C–C, C–B, and C–Cl bonds have recently emerged, a general catalytic oxidation (C–O bond formation) of unactivated primary C(sp3)–H bonds is unknown. Metal-stabilized carbenes, nitrenes, and oxenes are useful reactive intermediates that can insert carbon or heteroatom functionality into strong C(sp3)–H bonds with ligand-controlled selectivities. Although early- and mid- first-row transition-metal-oxo complexes have been intensively studied, first-row late-transition-metal-oxo species (LTM-oxo) are less explored despite their potential utility for C–H oxidation. Indeed, synthesizing LTM- oxo complexes represents a major challenge toward harnessing these highly reactive species as useful oxidants. The proposed research aims to develop a modular route toward a series of LTM-oxo complexes bearing a novel sterically-bulky triptycene-substituted dipyrrin ligand scaffold. This ligand architecture is expected to enforce kinetic stability of the complexes to facilitate isolation and characterization efforts. The ligand scaffold will also promote high-spin electronic configurations, which should weaken the M–O bond and render the complexes more reactive toward C(sp3)–H oxidation. Finally, the reactivity of transiently-formed and sterically encumbered LTM-oxo complexes will be harnessed to enable the selective and undirected catalytic oxidation of sterically unhindered methyl groups. This methodology will also be applied toward the selective late-stage functionalization of medicinally-relevant scaffolds. These efforts will result in the first general method for the catalytic undirected oxidation of primary C(sp3)–H bonds. Moreover, these studies will provide the first unambiguous characterization of high-spin LTM-oxo complexes and validate their synthetic utility for catalytic C–H oxidation.