Phenol oxidation with hypervalent iodine reagents leads to the formation of quinone-type products or iodonium ylides, depending on the structure of the phenol. Trapping of either product is possible with a suitable reagent, and this method is often employed in tandem with a second process. Introduction In the presence of hypervalent iodine(III) reagents such as iodobenzene diacetate (IBD) or iodobenzene di(trifluoroacetate) (IBTA), phenols undergo oxidation to either quinones. or iodonium ylides. Phenols with an electron- withdrawing group in the para position form the latter, while most other phenols give the former (or derivatives thereof). Direct transformation of quinone products may occur through intramolecular Diels-Alder or Michael-type reactions. Bis(phenol) substrates undergo oxidative coupling under these conditions. Iodonium ylides are relatively stable, versatile compounds that undergo substitution and cycloaddition reactions. They are represented using two resonance forms, one zwitterionic (the "betaine" form) and the other neutral (the "ylide" form). (1) Mechanism and Stereochemistry Prevailing Mechanism The mechanism of phenol oxidation with hypervalent iodine reagents begins with the formation of an aryloxyiodonium(III) intermediate. Inter- or intramolecular nucleophilic attack then takes place, either in one step or in two via an oxenium ion. If the substrate contains a diene, the quinone thus produced may undergo intramolecular [4+2] cycloaddition. Alternatively, the presence of a second nucleophilic group may lead to Michael-type adducts (see the lower pathway of equation (2) below). (2) When the phenol contains an electron-withdrawing group in the para position and at least one ortho hydrogen, stable iodonium ylides result. The initial intermediates are iodonium salts, which eliminate HZ to form the ylide. Iodonium ylides undergo cycloaddition reactions with unsaturated functional groups, and react with nucleophiles and electrophiles to give substitution products. (3) Oxidative coupling of bis(phenols) takes place in the presence of iodine(III) reagents. The mechanism of this process is analogous to the formation of para-substituted quinones via intramolecular nucleophilic attack. Mixtures of products may result from attack at inequivalent ortho or para positions. (4) Scope and Limitations Phenolic oxidations may afford different products depending on both the reaction conditions and the structure of the substrate. 2-Substituted phenols form ortho quinones upon oxidation. These products are unstable and undergo dimerization. (5) When external nucleophiles are added to phenolic oxidations, further reactions of the nucleophile with the resulting quinone may occur. Intramolecular Diels-Alder reactions have been observed in this context. (6) In substrates appropriately substituted with a nucleophile, Michael addition may occur. Michael