Free radical deoxygenation

A relevant reductive process, which has found wide application in organic synthesis, is the deoxygenation of alcohols introduced in 1975 by Barton and McCombie [58]. Reaction (4.28) shows that the thiocarbonyl derivatives, easily obtained from the corresponding alcohol, can be reduced in the presence of BusSnH under free radical conditions. The reactivity of xanthates and thiocarbonyl imidazolides [58] was successfully extended to 0-arylthiocarbonates [59] and (9-thioxocarbamates [60]. Several reviews have appeared on this subject, thus providing an exhaustive view of this methodology and its application in natural product synthesis [61-64]. 

Free Radical Deoxygenation of Thiocarbonyl Derivatives of Alcohols... 

The free-radical chemistry of polyhydric alcohols in deoxygenated solutions has been extensively investigated by using radiolysis and ra-... 

Schuchmann H-P, von Sonntag C (1988) The oxidation of methanol and 2-propanol by potassium peroxodisulphate in aqueous solution free-radical chain mechanisms elucidated by radiation-chemical techniques. Radiat Phys Chem 32 149-156 Schuchmann H-P, Wagner R, von Sonntag C (1983) y-Radiolysis of 2 -deoxycytidine-5 -phosphate in deoxygenated aqueous solutions. OH radical-induced alternations at the sugar moiety. Z Naturforsch 38b 1213-1220... 

In fact, with a few exceptions, neither 02 nor nitro compounds react with free radicals by ET (as would be required for an electron affinity relationship in its proper sense) but rather undergo addition reactions. The ensuing radicals show different decay routes as is discussed in Chapter 6.3. When DNA is irradiated in deoxygenated aqueous solution in the presence of 14C-labeled mitronidazole, some of the sensitizer remains attached to DNA (Willson et al. 1974). Misonidazole and practically all other nitroimidazoles have a lower reduction potential than 02, and much higher concentrations have to administered to achieve the same effect (Whillans and Hunt 1982). 

Tin hydride 2 prepared from hydroxyl acid behaves like the conventional Bu3SnH in standard free-radical reactions, but the tin-containing by-products are easily removed by mild hydrolysis which converts them into base-soluble materials. The performance of these tin hydrides is evaluated for a range of radical reactions involving halides, selenides, Barton-McCombie deoxygenation, and enyne cyclizations.72... 

A method for the hydrocyanation of alkenes and alkynes is based upon hydrozirconation followed by isonitrile insertion (Scheme 13) overall yields range from 45-90%. Certain examples do not exhibit the clean regiochemistry represented here, and these were discussed in Section 3.9.3.3.2. The hydrozirco-nation/protonolysis of equation (53) results in high yield, stereoselective formation of C-methyldeoxy-sugars, an alternative to free radical deoxygenation. ... 

Chemically, methods for deoxygenation and reductive deamination have seen additions in the last two decades which depart from those traditional approaches using hydride reduction of activated alcohols and diazene generation from aliphatic amines. New methods for amine activation, new complex hydrides and, in particular, methods using electron transfer and free radical processes have greatly expanded the range of substrate molecules which can be subject to efficient functional group removal. 

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