Singly occupied molecular orbital single electron transfer oxidation

Radicals can be either reduced (to anions or organometallics) or oxidized to cations by formal single electron transfer (Scheme 11).50 Such redox reactions can be conducted either chemically or electro-chemically51 and the rates of electron transfer are usually analyzed by the Marcus theory and related treatments.50 These rates depend (in part) on the difference in reduction potential between the radical and the reductant (or oxidant). Thus a species such as an a-amino radical with high-lying singly occupied molecular orbital (SOMO) is more readily oxidized, while a species such as the malonyl radical with a low-lying SOMO is more readily reduced. The inherent difference in reduction potential of substituted radicals is an important control element in several kinds of reactions. 

From a viewpoint of molecular orbitals, the oxidation process is explained by electron transfer from the highest occupied molecular orbital (HOMO) of a substrate molecule to the anode. Therefore, an increase in the HOMO level is the most straightforward method for activating the substrate toward anodic oxidation. When the substrate that we wish to oxidize has a similar or lower HOMO level than that of the other species, it is, in principle, very difficult or impossible to accomplish selective oxidation of the substrate. In this case, selective increase of the HOMO level of the substrate that we wish to oxidize by the introduction of an electroauxiiiary group serves as a powerful method for accomplishing the desired selective oxidation. In the case where several sites are susceptible to oxidation in a single molecule, a similar argument can be applied, and the use of an electroauxiiiary to activate a particular site that we wish to oxidize is effective for selective oxidation. 

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