Redox-Neutral Radical-Cation Reactions: Multiple Carbon–Carbon Bond Formations Enabled by Single-Electron Transfer

In this comprehensive paper, three redox-neutral reactions, including [2 + 2] and [4 + 2] cycloadditions and vinylcyclopropane rearrangements, are outlined from the viewpoint of energy conversion. These reactions demonstrate the power of electrosynthesis in the field of synthetic organic chemistry not only from the viewpoint of energy conversion but also from that of redox economy because four-, five-, and six-membered-ring skeletons are constructed without a change in oxidation state of the growing molecules in synthetic routes. The key for all of the reactions is precise control of single-electron transfer (SET) in lithium perchlorate/nitromethane solution, where oxidative SET is facilitated and the thus-generated radical cations are highly stabilized. SET processes can be visualized by plotting the highest occupied molecular orbital and spin density distributions to obtain theoretical pictures for a mechanistic understanding of the reactions; the deduced mechanisms are in good accordance with the reactions’ formal expressions