Kharasch-type reactions

The palladium-catalyzed reaction of organotin compounds was first published by Eaborn s group in 1976 [1]. It was initially taken as a variation of the Kharasch type reaction, namely, a radical reaction (Scheme 2). 

Bromides are less reactive than the corresponding iodides in atom transfer processes. However, activated bromides such as diethyl bromomalonate and bromomalonitrile13 react with alkenes under Et3B-02 initiation. Kharasch-type reactions of bromotrichloromethane with alkenes are also initiated by Et3B-02.14 On the other hand, a remarkable Lewis acid effect was reported by Porter.15 Atom transfer reactions of an a-bromooxazolidinone amide with... 

Without an activator the reaction is assumed to be initiated by a catalytic amount of magnesium halide MgX, resulting from a Wurtz-type reaction, which would, following Kharasch and Reinmuth, account for the disproportionation products as well ... 

The above approach of mechanochemically initiated addition of reactive antioxidants on different polymers, such as rubbers and unsaturated thermoplastics such as ABS is illustrated here for thiol-containing antioxidants. For example, using thiol compounds (37) and (38) as the reactive antioxidants, Kharasch-type addition of the thiol function to the polymer double bond takes place during melt processing to give bound antioxidant adduct (see reaction 7) the polymer becomes much more substantive under aggressive environments. 

Kharasch called this the peroxide effect and demonstrated that it could occur even if peroxides were not deliberately added to the reaction mixture Unless alkenes are pro tected from atmospheric oxygen they become contaminated with small amounts of alkyl hydroperoxides compounds of the type ROOH These alkyl hydroperoxides act m the same way as deliberately added peroxides promoting addition m the direction opposite to that predicted by Markovmkov s rule... 

Although detailed kinetic studies on Kharasch and related additions are sparse, there is agreement that two different (but related to some extent) types of mechanism might be operative in these reactions. 

There are numerous publications on the improvements to the Kharasch reaction. The primary method for improvement was the introduction of more sophisticated initiators (instead of peroxides [22-26, 37]). It was shown that various Red-Ox type initiators, like metal salts in different oxidation states (Cu", Cu ", Fe ", etc.), salts with organic acids (Co acetoacetate, Mn stearate), and a metal carbonyl - Fe(CO)5 give adducts with practical yields [38, 39] and allow to carry the reaction at near room temperatures (Fig. 30). 

Scheme 8.28. Formally group-selection insertion of oxygen into enantiotopic C-H bonds, (a) An asymmetric Kharasch reaction [124], The catalyst is similar to that shown in Scheme 8.12, except that each oxazoline bears two methyl substituents at C-5. (b) Kinetic resolution of dihydronaphthalenes [125]. The reaction uses a Jacobsen epoxidation catalyst (Scheme 8.6, type A).

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