Steric Effects in Free Radical Addition Reactions

IV Steric Effects in Free Radical Addition Reactions... 

For most vinyl polymers, head-to-tail addition is the dominant mode of addition. Variations from this generalization become more common for polymerizations which are carried out at higher temperatures. Head-to-head addition is also somewhat more abundant in the case of halogenated monomers such as vinyl chloride. The preponderance of head-to-tail additions is understood to arise from a combination of resonance and steric effects. In many cases the ionic or free-radical reaction center occurs at the substituted carbon due to the possibility of resonance stabilization or electron delocalization through the substituent group. Head-to-tail attachment is also sterically favored, since the substituent groups on successive repeat units are separated by a methylene... 

Steric effects, although clearly recognized, introduce relatively small rate retardations or increases in selectivity in all these examples, probably because the transition states of all these addition reactions are rather loose ones, i.e., they occur early on the reaction coordinate when the distances between the radical and the substrates are still rather large92 93,97. An extreme example of a free radical reaction which does not response heavily to steric effects, is the SRN1 -substitution reaction of Kornblum 14 by which bonds between two quaternary carbons can be formed with great ease and in good yield, as is shown by one of many published examples114. The decisive step... 

A study of the overall rate of addition of radicals to olefins shows us that polar forces, familiar to chemists working with ionic species in solution, are apparent in free radical reactions, i.e. in reactions involving uncharged species in the gas phase. The effects are smaller than in solution but are none the less clearly apparent. The results also show that polarity is not the whole story and that some of the trends observed in Tables 1 to 7 may be partly due to steric... 

It is now established that product radical stability is a consideration in determining the outcome of radical addition reactions only where a substituent provides substantial delocalization of the free spin into a n-systeni. Even then, because these reactions are generally irreversible and exothermic (and consequently have early transition states), resonance stabilization of the incipient radical center may play only a minor role in determining reaction rate and specificity." " Thermodynamic factors will be the dominant influence only when polar and steric effects are more or less evenly balanced. " ... 

Metzger and coworkers have shown in a series of papers (269-276) that alkanes can be added to alkenes and alkynes in thermally initiated free-radical chain reactions in the neat reactants at SCF conditions. These reactions have been demonstrated with a wide variety of substrates investigating various effects, including the influence of steric and polar substituents as well as product regioselectivity. The radical chain is initiated by a bimolecular reaction of the alkane with the alkene or alkyne to give two radicals. Addition, rearrangement, and elimination reactions have also been observed. No effect on the reaction rate constant near the critical point was observed on varying the physical state of the reaction mixture from liquid to supercritical to gas-phase conditions (276). 

The first step of a free radical aromatic substitution, the formation of the a-com-plex, is also an addition step. The o,m,p-product ratio therefore also responds to steric effects. This is shown for the free radical phenylation and dimethylamination of toluene and r.-butylbenzene in Table 8. The larger the substituent on the aromatic system and the bulkier the attacking radical, the more p-substitution product is obtained at the expense of o-substitution. In the phenylation reaction the yield of m-product also increases in contrast to the dimethylamination reaction. The substitution pattern of this latter reaction is, in addition to the steric effect, governed heavily by polar effects because a radical cation is the attacking species113. ... 

Intennoleciilar Reactions. The intermolecular version of free radical reactions of sugar-derived radicals consists mainly of addition onto suitably activated olefins, such as acrylonitrile, generally used in excess. This approach has been explored by Giese [102]. The stereochemical course of the reaction is dictated by steric effects of the vicinal substituents, as seen from the reaction of radical 71 where equatorial attack is favored over the axial with acrylonitrile (Scheme 28). Only equatorial attack is observed using... 

Cyclopentyl radicals substituted in the /1-position relative to the radical center are formed during the solvomercuration/reductive alkylation reaction of cyclopentene34. The organomer-curial produced in the first solvomercuration step is reduced by sodium borohydride and yields free cyclopentyl radicals in a radical chain mechanism. Addition of alkenes can then occur tram or cis to the / -alkoxy substituent introduced during the solvomercuration step. The adduct radical is finally trapped by hydrogen transfer from mercury hydrides to yield the tram- and ris-addition products, The transicis ratio depends markedly on the alkene employed and it appears that the addition of less reactive alkenes occurs with higher trans selectivity. In reactions of highly substituted alkenes, this reactivity control is compensated for by steric effects. Therefore, only the fnms-addition product is observed in reactions of tetraethyl ethenetetracarboxylate. The choice of alcohol employed in the solvomercuration step has, however, only a small influence on the stereoselectivity. 

Attack of monomer at the methylene carbon atom is less sterically hindered and yields a free radical that is more stable because the substituent group X stabilizes the free-radical site by steric hindrance and, in many cases, also by mesomeric stabilization. (Inductive effects are not important because the free-radical site bears no charge.) Thus the reaction is regioselective with mode (I) addition predominating. 

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