tetrahedral intermediate collapses, eliminating a phenol and forming an ester protonated at the carbonyl oxygen (22). Finally, deprotonation of the carbonyl oxygen yields the collected products and regenerates the acid catalyst (23). Boric acid-catalyzed Dakin oxidation Adding boric acid to the acid-catalyzed reaction mixture increases the yield of phenol product over phenyl carboxylic acid product, even when using phenyl aldehyde or ketone reactants with electron-donating groups meta to the carbonyl group or electron-withdrawing groups ortho or para to the carbonyl group. Boric acid and hydrogen peroxide form a complex in solution that, once added to the carbonyl carbon, favors aryl migration over hydrogen migration, maximizing the yield of phenol and reducing the yield of phenyl carboxylic acid. Methyltrioxorhenium-catalyzed Dakin oxidation Using an ionic liquid solvent with catalytic methyltrioxorhenium (MTO) dramatically accelerates Dakin oxidation. MTO forms a complex with hydrogen peroxide that increases the rate of addition of hydrogen peroxide to the carbonyl carbon. MTO does not, however, change the relative yields of phenol and phenyl carboxylic acid products. Urea-catalyzed Dakin oxidation Mixing urea and hydrogen peroxide yields urea-hydrogen peroxide complex (UHC). Adding dry UHC to solventless phenyl aldehyde or ketone also accelerates Dakin oxidation. Like MTO, UHP increases the rate of nucleophilic addition of hydrogen peroxide. But unlike the MTO-catalyzed variant, the urea-catalyzed variant does not produce potentially toxic heavy metal waste; it has also been applied to the synthesis of amine oxides such as pyridine-N-oxide. Synthetic applications The Dakin oxidation is most commonly used to synthesize benzenediols and alkoxyphenols. Catechol, for example, is synthesized from o-hydroxy and o-alkoxy phenyl aldehydes and ketones, and is used as the starting material for synthesis of several compounds, including the catecholamines, catecholamine derivatives, and 4-tert-butylcatechol, a common antioxidant and polymerization inhibitor. Other synthetically useful products of the Dakin oxidation include guaiacol, a precursor of several flavorants; hydroquinone, a common photograph-developing agent; and 2-tert-butyl-4-hydroxyanisole and 3-tert-butyl-4-hydroxyanisole, two antioxidants commonly used to preserve packaged food. In addition, the Dakin oxidation is useful in the synthesis of indolequinones, naturally occurring compounds that exhibit high anti-biotic, anti-fungal, and anti-tumor activities. See also Baeyer–Villiger oxidation Beckmann rearrangement Nucleophilic acyl substitution Reimer–Tiemann reaction References Organic oxidation reactions Name reactions