Radical addition reactions relative reactivity

The monomer reactivity ratios for many of the most common monomers in radical copolymerization are shown in Table 6-2. These data are useful for a study of the relation between structure and reactivity in radical addition reactions. The reactivity of a monomer toward a radical depends on the reactivities of both the monomer and the radical. The relative reactivities of monomers and their corresponding radicals can be obtained from an analysis of the monomer reactivity ratios [Walling, 1957]. The reactivity of a monomer can be seen by considering the inverse of the monomer reactivity ratio (1 jf). The inverse of the monomer reactivity ratio gives the ratio of the rate of reaction of a radical with another monomer to its rate of reaction with its own monomer... 

Several methods have been employed to extract the rate constant of the addition of nucleophiles to the aryl radicals from the kinetics of Sr I reactions. Relative reactivities of two nucleophiles towards the same aryl radical have been obtained from the ratio of the two substitution products after preparative-scale reaction of the substrate with a mixture of the two nucleophiles under photochemical or solvated-electron induction (Galli and Bunnett, 1981). 

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... 

Such relative lack of kinetic impact of olefinic fluorine substituents on alkyl radical addition reactions is consistent with Tedder s early studies on methyl affinities, the results of which are shown in Table 16, where the range of reactivities observed for the addition of methyl radical to ethylenes with varying fluorine content is seen to be relatively small [93,163],... 

Although peroxide-decomposition data have to be used with great caution, it can be concluded that double bonds are consumed by radical addition reactions. One can debate whether the unsaturations are consumed by multiple radical addition reactions or via consecutive radical addition/radical transfer sequences. The latter seems most likely, considering the low tendency of alkyl radicals for addition to alkyl-substituted double bonds under these relatively mild conditions. In radical addition reactions of this kind, the stabilisation of radicals due to polar effects is negligible. Experimental studies show that the reactivity is mainly controlled by steric effects [96]. The order of reactivity MNB > DCPD ENB > HD towards radical addition reactions as found by Fujimoto and coworkers [73, 74] is in line with these considerations. 

Generally, as the potential energy level of SOMO increases (becomes a more reactive radical), free radicals have nucleophilic character, while as the potential energy level of SOMO decreases (becomes a stable radical), free radicals have electrophilic character. Thus, when effective radical reactions are required, small energy difference in SOMO-HOMO or SOMO-LUMO interactions is necessary. For example, the relative reactivities of radical addition reactions of a nucleophilic cyclohexyl radical to alkenes,... 

Formation by a radical addition reaction The presence of methyl and ethenyl groups in the products indicates that shock waves destroyed the structure of benzene and formed some lower-molecular-weight radicals (e.g., methyl and ethenyl radicals). If we assume Uiat the formation of a MeNap or PhNap molecule is a result of attack by a methyl or a phenyl radical against naphthalene molecule, respectively, the yield relations in isomers estimated from tire reactivity indices would be 1-MeNap >2-MeNap and 1-PhNap >2-PhNap. These relative amounts are inconsistent with Uiose of the shock products. Therefore, it is unreasonable to invoke a radical reaction only for the reaction of MeBip and PhNap. 

Seleno radicals are relatively less reactive toward olefins, and these kinetic data suggest that the reverse process from -seleno alkyl radical intermediates contributes to the inefficiency of the radical addition reactions of selenium compounds (Scheme 15.58). 

Monomers which have been successfully polymerized using ATRP include styrenes, acrylates, methacrylates, and several other relatively reactive monomers such as acrylamides, vinylpyridine, and acrylonitrile, which contain groups (e.g., phenyl, carbonyl, nitrile) adjacent to the carbon radicals that stabilize the propagating chains and produce a suf cientiy large atom transfer equilibrium constant. The range of monomers polymerizable by ATRP is thus greater than that accessible by nitroxide-mediated polymerization, since it includes the entire family of methacrylates. However, acidic monomers (e.g., methacrylic acid) have not been successfully polymerized by ATRP and so also halogenated alkenes, alkyl-substimted ole ns, and vinyl esters because of then-very low intrinsic reactivity in radical polymerization and radical addition reactions (and hence, presumably, a very low ATRP equilibrium constant). 

Absolute rate constants for addition reactions of cyanoalkyl radicals are significantly lower than for unsubstituted alkyl radicals falling in the range 103-104 M V1.341 The relative reactivity data demonstrate that they possess some electrophilic character. The more electron-rich VAc is very much less reactive than the electron-deficient AN or MA. The relative reactivity of styrene and acrylonitrile towards cyanoisopropyl radicals would seem to show a remarkable temperature dependence that must, from the data shown (Table 3.6), be attributed to a variation in the reactivity of acrylonitrile with temperature and/or other conditions. 

Absolute rate constants for the attack of aryl radicals on a variety of substrates have been reported by Scaiano and Stewart (Ph ) 7 and Citterio at al. (/j-CIPh-).379,384 The reactions are extremely facile in comparison with additions of other carbon-centered radicals [e.g. jfc(S) = 1.1x10s M"1 s"1 at 25 °C].3,7 Relative reactivities are available for a wider range of monomers and other substrates (Tabic 3.b). Phenyl radicals do not show clear cut electrophilic or... 

The rate constant for p-scisskm is dependent on ring substituents. Rate constants for radicals X-Q.H. CCh are reported to increase in the series where X is / -Fi There is qualitative evidence that the relative rales for p-scission and addition are insensitive to solvent changes. For benzoyloxy radicals, similar relative reactivities are obtained from direct competition experiments10 as from studies on individual monomers when p-scission is used as a clock reaction.399,401... 

Waters61 have measured relative rates of p-toluenesulfonyl radical addition to substituted styrenes, deducing from the value of p + = — 0.50 in the Hammett plot that the sulfonyl radical has an electrophilic character (equation 21). Further indications that sulfonyl radicals are strongly electrophilic have been obtained by Takahara and coworkers62, who measured relative reactivities for the addition reactions of benzenesulfonyl radicals to various vinyl monomers and plotted rate constants versus Hammett s Alfrey-Price s e values these relative rates are spread over a wide range, for example, acrylonitrile (0.006), methyl methacrylate (0.08), styrene (1.00) and a-methylstyrene (3.21). The relative rates for the addition reaction of p-methylstyrene to styrene towards methane- and p-substituted benzenesulfonyl radicals are almost the same in accord with their type structure discussed earlier in this chapter. 

The relative reactivity of cyclopentadiene and ds-dichloroethylene toward triplet cyclopentadiene was found to be greater than 20 1 while that for cyclopentadiene and trans-dichloroethylene is less than 5 1. Thus the trans isomer is about four times more reactive toward the triplet cyclopentadiene than the cis isomer. An interesting temperature dependence of the product distribution of this reaction has been reported (Table 10.8). The data in Table 10.8 indicate that the relative amount of 1,4 addition [products (39) and (40)] is much more sensitive to temperature than 1,2 addition [products (35)—(38)], especially for the trans-olefin. The data also indicate that some rotation about the CHC1-CHC1 bond occurs in intermediate radicals derived from both cis- and trans-dichloroethylene. However, rotational equilibrium is not established at ring closure since the ratios of ds-dichlorocyclobutanes... 

The autoxidation of aldehydes, and of other organic compounds, may be lessened considerably by very careful purification—removal of existing peroxides, trace metal ions, etc.—but much more readily and effectively by the addition of suitable radical inhibitors, referred to in this context as anti-oxidants. The best of these are phenols and aromatic amines which have a readily abstractable H atom, the resultant radical is of relatively low reactivity, being able to act as a good chain terminator (by reaction with another radical) but only as a poor initiator (by reaction with a new substrate molecule). 

There are few addition reactions to a,/J-disubstituted enoyl systems 151 that proceed in good yield and are able to control the absolute and relative stereochemistry of both new stereocenters. This is a consequence of problematic A1,3 interactions in either rotamer when traditional templates such as oxazolidinone are used to relieve A1,3 strain the C - C bond of the enoyl group twists, breaking conjugation which results in diminished reactivity and selectivity [111-124], Sibi et al. recently demonstrated that intermolecular radical addition to a,/J-disubstituted substrates followed by hydrogen atom transfer proceeds with high diastereo- and enantioselectivity (151 -> 152 or 153, Scheme 40). 

The absolute rate constants for a variety of cyclizafions have been measured. In particular, the rates of decarbonylafion of a variety of alkoxycarbonyl radicals have been obtained by LFP studies on PTOC oxalates." From these data, rate constants for the reduction of alkoxycarbonyl radicals with BusSnH and their 5-exo cyclizafions were determined. Whereas cyclizations were slightly faster than the analogous alkyl radical 5-exo cyclizations, their reactions with BusSnH were 10 times slower, indicating that cyclization processes should be synthetically useful. The rate constants for the cyclization of a number of variously substituted a-amide radicals have been determined together with their relative reactivities towards reduction using BusSnH (Scheme 16). Cyclizations of secondary-based radicals were found to be similar to the corresponding alkyl-substituted radicals. In addition, the rate constants were subject to minor electronic... 

Aryl radical additions to anions are generally very fast, with many reactions occurring at or near the diffusion limit. For example, competition studies involving mixtures of nucleophiles competing for the phenyl radical showed that the relative reactivities were within a factor of 10, suggesting encounter control,and absolute rate constants for additions of cyanophenyl and 1-naphthyl radicals to thiophenox-ide, diethyl phosphite anion, and the enolate of acetone are within an order of magnitude of the diffusional rate constant. ... 

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