Tris(4-bromophenyl)aminium Hexachloroantimonate-Mediated Intermolecular C(sp²)–C(sp³) Free Radical Coupling of Vindoline with β‑Ketoesters and Related Compounds

A powerful tris­(4-bromophenyl)­aminium hexachloro­antimonate (BAHA) mediated regioselective intermolecular coupling reaction of vindoline with a wide range of substrates that include β-ketoesters, β-diketones, β-ketoaldehydes, β-ketonitriles, β-ketolactones, β-ketolactams, β-cyanoesters, and malononitriles is detailed. The BAHA-promoted intermolecular sp³/sp² coupling, representing a special class of selective C–H functionalization reactions with direct carbon–carbon bond formation, proceeds with generation of a quaternary center bound to the aryl C15 center of vindoline capable of accommodating of the vinblastine C16′ methyl ester and functionalized for subsequent divergent heterocycle introduction. A comprehensive examination of the reaction scope, optimization of subtle reaction parameters, and key insights into the reaction mechanism are described. Contrary to what might be prevailing expectations, studies suggest the plausible mechanism entails initial single-electron oxidation of the substrate enolate, not vindoline, and subsequent regiospecific addition of the resulting electrophilic radical to vindoline. As such and beyond the new arylation reaction with vindoline, the studies define a host of new, previously unrecognized, applications of BAHA and related triarylaminium radical cations that arises from their ability to generate stabilized electrophilic radicals from β-ketoesters and related substrates under nonreducing and metal-free conditions. Those exemplified herein include mediating stabilized enolate free radical arylation, dimerization, allylation, alkene addition, and α-oxidation reactions.