Electrosynthesis: A New Frontier in Aerobic Oxidation?

The formation of alcohols and carbonyl derivatives via oxidation of the corresponding Csp–H bonds is crucial to the chemical community. Historically chemists have relied for these transformations on the use of organic or inorganic oxidants, which are often toxic and produce stoichiometric amounts of harmful waste. As we strive for a more sustainable fine chemicals synthesis, for which typically larger amounts of waste per kilogram product are produced than for commodity chemicals, the use of greener oxidants has gained increasing attention. On paper, the high abundance, O content, low price, and the production of water as the only byproduct make O2 an ideal reactant. In a generalized and simplified view, air or pure O2 are used in combination with a catalyst (metal complex, organocatalyst, or combinations thereof) to facilitate the formation of a carbon-centered radical, which will then react with O2. These procedures have been widely applied to a number of molecules, but remain largely limited to the functionalization of activated C–H bonds, such as benzylic, allylic, or those alpha to a heteroatom. Unactivated aliphatic C–H bonds, on the other hand, are more difficult to oxidize; moreover, selectivity is difficult to achieve due to their typical omnipresence in organic molecules. Oxidation of such substrates is therefore currently performed by using strong oxidants such as methyl(trifluoromethyl)dioxirane (TFDO) or transition metal catalysts featuring specifically designed ligands, in combination with hydrogenperoxide (Scheme 1A). Unfortunately, the problematic synthesis and storage of TFDO, and the cost of ligands in the catalysts considerably hamper the application of such strategies in larger scale processes. In a recent Journal of the American Chemical Society communication, the Baran group disclosed a selective aerobic oxidation of unactivated C−H bonds making use of electrochemistry. One important advantage of electrochemical over chemical oxidation is the possibility to apply at will a specific potential to the system. Nonetheless, the direct electrochemical generation of a radical from unactivated aliphatic C−H bonds is still not a practical option. Unactivated C−H bonds typically possess the highest oxidation potential in a molecule, and at the required potentials degradation of other functional groups and the solvent will occur. Baran and co-workers were able to solve this problem by using quinuclidine as an electrochemical mediator (Scheme 1B). Quinuclidine can be oxidized at much lower potentials than the C−H bond itself and easily abstract the hydrogen atom from the substrate, indirectly generating a C-centered radical which will in turn react with O2, ultimately leading to the desired product. One downside to the setup is that although theoretically only a catalytic amount of mediator should be required in the process, in this case a stoichiometric amount had to be used to achieve good yields. A fluorinated alcohol, HFIP, as additive was also found to be crucial in the process, although it is not clear yet what its actual role is in the C−H activation. Interestingly, a recent benzylic aerobic oxidation process also uses this alcohol, in this case to achieve chemoselective oxidation to aldehydes. With this electrochemical approach, Baran and co-workers were able to perform the selective oxidation of linear and cyclic hydrocarbon moieties without affecting functionalities like ketones, alcohols, esters, amides, heterocycles, and epimerizable centers (Scheme 1C). An important feature is