Double C-H Functionalization of Indoles via Three-Component Reactions/CuCl2-Catalyzed Aerobic Dehydrogenative Coupling for the Synthesis of Polyfunctional Cyclopenta[b]indoles

A sequential Rh-2(OAc)4-catalyzed multicomponent reaction and CuCl2-catalyzed postcyclization process is developed to build polyfunctional cyclopenta[b]indoles in good yields with high diastereoselectivities in an atom- and step-economic fashion. The key discovery in this process is the CuCl2-catalyzed intramolecular aerobic dehydrogenative Csp(2)-Csp(2) cross-coupling of indole C2 with an enol functionality. Mechanistic studies including X-ray photoelectron spectroscopy analysis suggest that the catalytic cycle involves Cu(II) and Cu(I). This coupling reaction represents a unique example of aerobic Cu-catalyzed direct coupling of indoles with enols under mild conditions.