Recent Progress in Methyl-Radical-Mediated Methylation or Demethylation Reactions

Methylation is an important process in biology. To date, radical-type methylation protocols have been developed in the late-stage modifications of privileged scaffolds in pharmaceutical research and in medicinal and materials sciences. As a result, the chemistry of methyl-radical-involved conversions, which was a significant topic of interest over the past few years, has been greatly developed. This review focuses on the most recent developments in catalytic radical methylation or demethylation protocols that were mediated by methyl radicals. We classify the reactions according to the origination of the methyl radical species to illustrate the diversity of radical-based mechanisms; these radical generation pathways include (1) photo-/electro-catalysis, (2) transition-metal catalysis, (3) transition-metal-free methylations, and (4) demethylation-triggered radical cyclization/cascade reactions. In addition to the mechanisms, some of the representative methyl-containing molecules that were produced from the specific reactions are presented and analyzed.