Oxidative coupling, hydrogen peroxide

Permanent Hair Colorants. Permanent colorants produce hair coloration that lasts until the hair grows out. Color is formed inside the hair by hydrogen peroxide-induced coupling reactions of colorless dye precursors. A hiU range of shades can be obtained with this system and the permanent or oxidative hair colorants are considered to be the most important class of hair dyes. 

Intramolecular hydrosilylations of functionalized alkenes followed by hydrogen peroxide oxidation provide powerful methods for organic syntheses86-88. The reactions of allylic O-dimethylsilyl ethers 59 promoted by platinum catalysts, e.g. Karstedt s catalyst and Pt(PPh3)2(CH2=CH2), or rhodium catalysts, e.g. Rh(acac)(COD) and [RhCl(CH2=CH2)2]2> proceed via 5-endo cyclization to give oxasilacyclopentanes 60 with a couple of exceptions in which siloxatanes 61 are formed (Scheme ll)87,89. 

Hydrogen peroxide used over a wide pH range has been employed to oxidize sulfur compounds. The scope of the oxidation using hydrogen peroxide includes coupling of thiols to disulfides355 and thiols coupled with amines,356 which are used to produce industrially important compounds. For example, sulfenamides are accelerators for rubber vulcanization (Figure 3.91).357... 

Nitroxyl radicals of diarylamines can also be obtained on oxidation with hydrogen peroxide in the presence of vanadium ions. Resonance helps stabili2e these radicals. Eor example, the nitroxide from 4,4 -dimethoxydiphenylainine [63619-50-1] is stable for years, whereas the radical from the unsubstituted diphenylamine caimot be isolated. Substitution in the ortho and para positions also increases the stabiUties of these nitroxides by inhibiting coupling reactions at these sites. However, they are not as stable as the stericaHy hindered tetramethylpiperidyl radical. 

Disulfiram may be made by the reaction of diethyl amine with carbon disulfide in the presence of sodium hydroxide. The (CjHjljNCSSNa intermediate is oxidatively coupled using hydrogen peroxide to give disulfiram. 

A rapid and clean oxidation of sulphides to sulphoxides can also be carried out using the titanium(III) trichloride/hydrogen peroxide reagent35. On a milimole scale, the oxidation takes place in a time shorter than 20 min upon addition of a solution of hydrogen peroxide to a solution of the sulphide and titanium(III) trichloride in methanol at room temperature. It was suggested that the formation of a sulphoxide in this reaction resulted from a direct coupling of the hydroxy radical with cation radical 20 formed at the sulphur atom of the sulphide (equation 6). 

Radical-mediated silyldesulfonylation of various vinyl and (a-fluoro)vinyl sulfones 21 with (TMSlsSiH (Reaction 25) provide access to vinyl and (a-fluoro)vinyl silanes 22. These reactions presumably occur via a radical addition of (TMSlsSi radical followed by /)-scission with the ejection of PhS02 radical. Hydrogen abstraction from (TMSlsSiH by PhS02 radical completes the cycle of these chain reactions. Such silyldesulfonylation provides a flexible alternative to the hydrosilylation of alkynes with (TMSlsSiH (see below). On oxidative treatment with hydrogen peroxide in basic aqueous solution, compound 22 undergoes Pd-catalyzed cross-couplings with aryl halides. 

Redox reactions with metal porphyrins (MPs) as photocatalysts. A spectacular example here is the reaction that couples upon illumination with the sunlight, methanol oxidation to formaldehyde with the formation of hydrogen peroxide in be nzene-methanol mixture (90 10)... 

---