Nickel-Catalyzed Direct Alkylation of Heterocycles with α-Bromo Carbonyl Compounds: C3-Selective Functionalization of 2-Pyridones

Modern drug discovery relies on the continual development of synthetic methodology to address the many challenges associated with the design of new pharmaceutical agents. One such challenge arises from the enzymatic metabolism of drugs in vivo by cytochrome P450 oxidases, which use single-electron oxidative mechanisms to rapidly modify small molecules to facilitate their excretion. A commonly used synthetic strategy to protect against in vivo metabolism involves the incorporation of electron-withdrawing functionality, such as the trifluoromethyl (CF(3)) group, into drug candidates. The CF(3) group enjoys a privileged role in the realm of medicinal chemistry because its incorporation into small molecules often enhances efficacy by promoting electrostatic interactions with targets, improving cellular membrane permeability, and increasing robustness towards oxidative metabolism of the drug. Although common pharmacophores often bear CF(3) motifs in an aromatic system, access to such analogues typically requires the incorporation of the CF(3) group, or a surrogate moiety, at the start of a multi-step synthetic sequence. Here we report a mild, operationally simple strategy for the direct trifluoromethylation of unactivated arenes and heteroarenes through a radical-mediated mechanism using commercial photocatalysts and a household light bulb. We demonstrate the broad utility of this transformation through addition of CF(3) to a number of heteroaromatic and aromatic systems. The benefit to medicinal chemistry and applicability to late-stage drug development is also shown through examples of the direct trifluoromethylation of widely prescribed pharmaceutical agents.

Nitrogen-rich heterocyclic compounds have had a profound impact on human health, as these chemical motifs are found in a large number of drugs used to combat a broad range of diseases and pathophysiological conditions. Advances in transition metal-mediated cross-coupling have simplified the synthesis of such molecules; however, the development of practical and selective C–H functionalization methods that do not rely upon prefunctionalized starting materials is an underdeveloped area. 1–9 Paradoxically, the innate properties of heterocycles that make them so desirable for biological applications render them challenging substrates for direct chemical functionalization, such as limited solubility, functional group incompatibilities, and reagent/catalyst deactivation. Herein we report that zinc sulfinate salts 9 can be used to transfer alkyl radicals to heterocycles, allowing for a mild, direct and operationally simple formation of medicinally relevant C–C bonds while reacting in an orthogonal fashion to other innate C–H functionalization methods (Minisci, borono-Minisci, electrophilic aromatic substitution, transition metal-mediated C–H insertion, C–H deprotonation). 2–7,9 A toolkit of these reagents was prepared and reacted across a wide range of heterocycles (natural products, drugs, building blocks) without recourse to protecting group chemistry, and can even be employed in a tandem fashion in a single pot in the presence of water and air.

Herein, the development of visible light-mediated atom transfer radical addition (ATRA) of haloalkanes onto alkenes and alkynes using the reductive and oxidative quenching of [Ir{dF(CF(3))ppy}(2)(dtbbpy)]PF(6) and [Ru(bpy)(3)]Cl(2) is presented. Initial investigations indicated that the oxidative quenching of photocatalysts could effectively be utilized for ATRA, and since that report, the protocol has been expanded by broadening the scope of the reaction in terms of the photocatalysts, substrates, and solvents. In addition, further modifications of the reaction conditions allowed for the efficient ATRA of perfluoroalkyl iodides onto alkenes and alkynes utilizing the reductive quenching cycle of [Ru(bpy)(3)]Cl(2) with sodium ascorbate as the sacrificial electron donor. These results signify the complementary nature of the oxidative and reductive quenching pathways of photocatalysts and the ability to predictably direct reaction outcome through modification of the reaction conditions.