Two Paths to Oxidative C−H Amination Under Basic Conditions: A Theoretical Case Study Reveals Hidden Opportunities Provided by Electron Upconversion

Traditionally, cross-dehydrogenative coupling (CDC) leads to C−N bond formation under basic and oxidative conditions and is proposed to proceed via a two-electron bond formation mediated by carbenium ions. However, the formation of such high-energy intermediates is only possible in the presence of strong oxidants, which may lead to undesired side reactions and poor functional group tolerance. Alternatively, oxidation under basic conditions allows the formation of three-electron bonds. In a three-electron bond, two electrons will occupy a bonding orbital and the third is forced into a high energy anti-bonding orbital (resulting in an “upconverted” highly-reducing species). The benefit of this “electron upconversion” process is in the ability to use milder oxidants (e.g., O2) and to avoid high-energy carbenium ion intermediates. To explore the scope of this approach, we directly compared two- and three-electron pathways using quantum mechanical calculations. We observed that the absence of a strong oxidant can shut down two-electron pathways in favor of a three-electron path. Additionally, we investigated key factors involved in the three-electron C−N bond formation by analyzing the cyclization of 42 radical anions. An interesting stereoelectronic feature for the reaction of highly stabilized diester anions is that only one of the ester groups is involved in the extra electron delocalization in the cyclized radical anion product. Hence, the effect of unproductive reactant stabilization can be removed in the cyclizations of monoester enolates rendering such reactions much more thermodynamically favorable.