Electrochemically Enabled C4-H and C3-H Functionalization of 2-Phenyl Quinazoline and Quinoxaline through Dehydrogenative C-H/C-H, C-H/P-H, and C-H/O-H Cross-Coupling.

Abstract

Quinazoline moieties and particularly C4-substituted quinazoline scaffolds are widely distributed in biologically active molecules, and thus, direct C4-functionalization of quinazolines is the most convenient way to materialize new, straightforward, and sustainable strategies for the synthesis of useful medicinal targets. Retrospecting that, effort has been directed toward electrocatalytic C4-H bond diversification of quinazoline and related electron-deficient N-heterocycles (quinoxaline) offering C4 and C3 benzoyl-, acetyl-, phenol-, ether-, phosphonate-, and nitroalkane-incorporated N-heterocycles via a radical addition pathway under sacrificial oxidant- and additive-free conditions. Various coupling partners and quinazolines, as well as other structurally similar heterocyclic motifs, respond well, providing moderate to high yields of coupled products along with the gram-scale upgradation. Additionally, the performed control experiments and cyclic voltammetry investigations also nicely justified the proposed mechanism of the coupling process. Further, late-stage functionalization leading to the synthesis of indolo quinolines and vinyl-sulfonated products using the ruthenium-catalyzed skeletal transformation of benzoylated quinazoline 3b nicely appropriated the developed methodology. Finally, this reaction can be summarized as (a) anodic activation of the functionalized Hantzsch ester to furnish key radical species; (b) radical addition to an activated N-heterocycle; and (c) oxidation leading to the target product without the assistance of any metal chelation.