Electronegative radicals

The biochemical basis for the toxicity of mercury and mercury compounds results from its ability to form covalent bonds readily with sulfur. Prior to reaction with sulfur, however, the mercury must be metabolized to the divalent cation. When the sulfur is in the form of a sulfhydryl (— SH) group, divalent mercury replaces the hydrogen atom to form mercaptides, X—Hg— SR and Hg(SR)2, where X is an electronegative radical and R is protein (36). Sulfhydryl compounds are called mercaptans because of their ability to capture mercury. Even in low concentrations divalent mercury is capable of inactivating sulfhydryl enzymes and thus causes interference with cellular metaboHsm and function (31—34). Mercury also combines with other ligands of physiological importance such as phosphoryl, carboxyl, amide, and amine groups. It is unclear whether these latter interactions contribute to its toxicity (31,36). 

A common feature of all of the systems considered is the presence of an open shell, i.e., a singly occupied MO hence, they are all called open-shell systems. The radicals can be electroneutral (radicals in a narrow sense), electropositive (radical cations), or electronegative (radical anions). Radical di-ions and tri-ions are less frequent. 

Because the addition steps are generally fast and consequently exothermic chain steps, their transition states should occur early on the reaction coordinate and therefore resemble the starting alkene. This was recently confirmed by ab initio calculations for the attack at ethylene by methyl radicals and fluorene atoms. The relative stability of the adduct radicals therefore should have little influence on reacti-vity 2 ). The analysis of reactivity and regioselectivity for radical addition reactions, however, is even more complex, because polar effects seem to have an important influence. It has been known for some time that electronegative radicals X-prefer to react with ordinary alkenes while nucleophilic alkyl or acyl radicals rather attack electron deficient olefins e.g., cyano or carbonyl substituted olefins The best known example for this behavior is copolymerization This view was supported by different MO-calculation procedures and in particular by the successful FMO-treatment of the regioselectivity and relative reactivity of additions of radicals to a series of alkenes An excellent review of most of the more recent experimental data and their interpretation was published recently by Tedder and... 

Use a VB analysis to show why radical recombination reactions generally do not possess an energy barrier. For simplicity, use the gas phase recombination of an alkyl radical R with an electronegative radical, X. ... 

Selectivity There is a preference to abstract the hydrogen atom that would produce the most stable radical (breaking the weakest bond is energetically favored). Table 11.1 can usually be used to predict which C-H bond abstraction is preferred. Exceptions occur with abstractions by very electronegative radicals such as chlorine radical. With CH3CH2CH2-ewg, chlorine radical preferentially abstracts the CH2 away from the ewg since that CH2 bears less of a partial plus. 

The organic moiety most suitable to establish a stable sigma carbon-metal bond in difficult cases seems to be an electronegative radical such as the phenyl or acetylenic group. Examples of new and interesting phenyl organometallic compounds are triphenylchromium (64), pentaphenyl-antimony, tetraphenyltellurium, and triphenyliodine (134)- Nickel and other metals can be transformed into acetylides (89). 

Phenyl-p-tolylmercury allowed to react in a sealed tube with GCI4 in the presence of benzoyl peroxide for 8 hrs. in a boiling water bath ayo a-tri-chloro-p-xylene (Y 82%) and phenylmercury chloride (Y 88%).— The less electronegative radical remains bound to mercury. F. e. s. A. N. Nesmeyanov et al., Izvest. 1960, 148 Tetrahedron IS, 683 (1962). 

Polar effects also appear to exist in the addition reactions of certain radicals to substituted ethylenes. It is clear from Table II that for electronegative radicals like difluoroamino or hydroxy and to a lesser extent trifluoromethyl radical and hydrogen atom, substitution with donor groups like methyl increases the rate of addition. Substitution with electronegative atoms like fluorine or chlorine has the effect of slowing the rate of addition by hydroxy or trifluoromethyl radicals but the rate of addition of methyl radicals is increased. 

Although the effect is relatively small it is in the same direction as found for electronegative radicals adding to substituted ethylene. 

Thus in the addition to PBN of benzoyloxy or t-butoxy radicals, both electronegative radicals, one gets the picture that the nitronyl function is rather electrophilic itself or the site of radical attack is quite elctrophilic, for if it were otherwise... 

This reaction promises to be a useful one for studies on polar effects involving the relatively electropositive tin radicals as a complement to work which has already been done on the electronegative radicals such as t-butoxy and chlorine atoms. 

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