Radical addition steric effects

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... 

Other limitations of the reaction are related to the regioselectivity of the aryl radical addition to double bond, which is mainly determined by steric and radical delocalization effects. Thus, methyl vinyl ketone gives the best results, and lower yields are observed when bulky substituents are present in the e-position of the alkene. However, the method represents complete positional selectivity because only the g-adduct radicals give reductive arylation products whereas the a-adduct radicals add to diazonium salts, because of the different nucleophilic character of the alkyl radical adduct. ... 

A clear demonstration of the relative importance of steric and resonance factors in radical additions to carbon-carbon double bonds can be found by considering the effect of (non-polar) substituents on the rate of attack of (nonpolar) radicals. Substituents on the double bond strongly retard addition at the substituted carbon while leaving the rate of addition to the other end essentially unaffected (for example, Table 1.3). This is in keeping with expectation if steric factors determine the regiospeeificity of addition, but contrary to expectation if resonance factors are dominant. 

The above argument is also at odds with the conventional wisdom that the well-known tendency for monomer alternation in copolymerization can primarily be attributed to polar factors. It was suggested9 that, in most cases, radical stabilization could provide an alternate explanation. A discussion on the relative importance of steric polar and radical stabilization effects on radical addition appears in Section 2.3. 

In free-radical addition, the main effect seems to be steric. All substrates CH2=CHX are preferentially attacked at the CH2, regardless of the identity of X or of the attacking radical. With a reagent such as HBr, this means that the addition is anti-Markovnikov ... 

Szeimies recently published an impressive example of a steric effect on a Sr2 reaction at carbon for the addition of thiols to the central bond of bicyclo[ 1.1.0]-systems87. From the radical chain addition of thiophenol to 32 the stereoisomeric cyclobutanes 33a and 33b are obtained exclusively in 56% yield. The thiylradical... 

IV Steric Effects in Free Radical Addition Reactions... 

The combined influences of polar and steric effects and of the strength of the newly formed bond93 was also recognized in the reaction of a,0-unsaturated carbonyl compounds and similar electron deficient alkenes95 with organomercurials and NaBH4. For the addition of alkyl radicals to substituted styrenes, p assumed a... 

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. ... 

The addition of trialkylsilyl radicals to 1,2-disubstituted ethylene derivatives is subject to a steric effect [49], This shows itself in the greater Ee0 value for Et3Si addition to RCH=CHR compared with that for the addition of the same radical to CH2=CHR. The contribution of... 

No effect of this type is manifested for the addition of alkyl radicals to the same alkenes. Evidently, the steric effect involved in the addition of trialkylsilyl radicals to 1,2-disubstituted ethylene derivatives is due to the repulsion between the carbon and silicon atom, caused by the large size of the silicon atom in the reaction center of the transition state. 

The regioselectivity in radical addition reactions to alkenes in general has successfully been interpreted by a combination of steric and electronic effects1815,47. In the absence of steric effects, regiochemical preferences can readily be explained with FMO theory. The most relevant polyene orbital for the addition of nucleophilic radicals to polyenes will be the LUMO for the addition of electrophilic orbitals it will be the HOMO. Table 10 lists the HOMO and LUMO coefficients (without the phase sign) for the first three members of the polyene family together with those for ethylene as calculated from Hiickel theory and with the AMI semiempirical method48. 

In conclusion, the regiochemistry of radical attack at dienes appears to be rather predictable by considering steric and electronic effects. Attack is almost always preferred at the terminal carbon atoms of the diene. a-Substituents retard the addition significantly in all known cases while the steric effects of -substituents depend on the nature of the attacking radical. 

The cumulated Jt-system of allenes has been described as consisting of two comparatively unperturbed double bonds with regard to its reactivity towards nucleophiles or electrophiles [10]. Early reports on radical additions to 1,2-dienes, however, already pointed to peculiarities of the allene system concerning its reactivity towards intermediates with unpaired electrons [11-14], It was soon realized that no such correlation between polar and steric substituent effects existed, similar to what had been uncovered for the reaction of radicals with olefins, in order to predict selectivities in radical additions to cumulated dienes [4, 15],... 

Rate constants and Arrhenius parameters for the reaction of Et3Si radicals with various carbonyl compounds are available. Some data are collected in Table 5.2 [49]. The ease of addition of EtsSi radicals was found to decrease in the order 1,4-benzoquinone > cyclic diaryl ketones, benzaldehyde, benzil, perfluoro propionic anhydride > benzophenone alkyl aryl ketone, alkyl aldehyde > oxalate > benzoate, trifluoroacetate, anhydride > cyclic dialkyl ketone > acyclic dialkyl ketone > formate > acetate [49,50]. This order of reactivity was rationalized in terms of bond energy differences, stabilization of the radical formed, polar effects, and steric factors. Thus, a phenyl or acyl group adjacent to the carbonyl will stabilize the radical adduct whereas a perfluoroalkyl or acyloxy group next to the carbonyl moiety will enhance the contribution given by the canonical structure with a charge separation to the transition state (Equation 5.24). 

Many examples of the influence of steric effects on reactivity and regioselectivity in free radical additions are known. The anti-Markownikoff regioselectivity apparently is smalier than originally assumed and frequently dependent on... 

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 large An extreme example of a free radical reaction which does not response heavily to steric effects, is the Srn 1 -substitution reaction of Kornblum 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 examples The decisive step... 

Rate constants for ring openings of substituted cyclopropylcarbinyl radicals show the same types of kinetic effects as seen in radical additions to substituted alkenes with an obvious relationship to the thermodynamics of the reaction (Fig. 4.16). Minor steric effects influence the kinetics, however, and... 

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