Styrene electrophilicity

Aryl and vinylic bromides and iodides react with the least substituted and most electrophilic carbon atoms of activated olefins, e.g., styrenes, allylic alcohols, a,p-unsaturated esters and nitriles. 

In addition to benzene and naphthalene derivatives, heteroaromatic compounds such as ferrocene[232, furan, thiophene, selenophene[233,234], and cyclobutadiene iron carbonyl complexpSS] react with alkenes to give vinyl heterocydes. The ease of the reaction of styrene with sub.stituted benzenes to give stilbene derivatives 260 increases in the order benzene < naphthalene < ferrocene < furan. The effect of substituents in this reaction is similar to that in the electrophilic aromatic substitution reactions[236]. 

When an unsymmetrically substituted vinyl monomer such as propylene or styrene is polymerized, the radical addition steps can take place at either end of the double bond to yield either a primary radical intermediate (RCH2-) or a secondary radical (R2CH-). Just as in electrophilic addition reactions, however, we find that only the more highly substituted, secondary radical is formed. 

Florio et al. demonstrated that the lithiation/electrophile trapping of enantio-pure styrene oxide, as well as the (3-substituted styrene oxides 180 and 182, is totally stereoselective (Scheme 5.42) [66]. These results demonstrate that the intermediate benzylic anions are configurationally stable within the timescale of depro-tonation/electrophile trapping. 

The traditional means of assessment of the sensitivity of radical reactions to polar factors and establishing the electrophilicity or nucleophilieity of radicals is by way of a Hammett op correlation. Thus, the reactions of radicals with substituted styrene derivatives have been examined to demonstrate that simple alkyl radicals have nucleophilic character38,39 while haloalkyl radicals40 and oxygcn-ccntcrcd radicals " have electrophilic character (Tabic 1.4). It is anticipated that electron-withdrawing substituents (e.g. Cl, F, C02R, CN) will enhance overall reactivity towards nucleophilic radicals and reduce reactivity towards electrophilic radicals. Electron-donating substituents (alkyl) will have the opposite effect. 

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. 

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. 

Carbanionic sites exhibiting high nucleophilicity (such as those of styrene or dienes) can react with a number of electrophiles such as ... 

The electrophilic functions most commonly used in grafting onto processes are ester 141 144), benzylic halide 145,146) and oxirane, 47). Other functions such as nitrile or anhydride could be used as well. The backbone is a homopolymer (such as PMMA) or a copolymer containing both functionalized and unfunctionalized units. Such species can be obtained either by free radical copolymerization (e.g. styrene-acrylonitrile copolymer) or by partial chemical modification of a homopolymer (e.g. 

It has been shown (ref. 21) that a solvent which is both protic and nucleophilic, assists the formation of the bromination intermediates of moderately reactive olefins as styrenes in two ways, (Scheme 7). Firstly, the solvent initiates bromide ion formation electrophilically and, secondly, favours... 

Apart from a few studies (ref. 7), the use of deuterium kinetic isotope effects (kie s) appears to have had limited use in mechanistic studies of electrophilic bromination of olefins. Secondary alpha D-kie s have been reported for two cases, trans-stilbene fi and p-substituted a-d-styrenes 2, these giving relatively small inverse kie s of... 

Random copolymerization occnrs between butadiene and styrene [15]. There are no appreciable differences in the nncleophilic and electrophilic abilities between the radical centers with the vinyl and phenyl groups at the end of the growing polymer chain or in the donor/acceptor properties between the monomers. 

Hydroboration is highly regioselective and stereospecific. The boron becomes bonded primarily to the less-substituted carbon atom of the alkene. A combination of steric and electronic effects works to favor this orientation. Borane is an electrophilic reagent. The reaction with substituted styrenes exhibits a weakly negative p value (-0.5).156 Compared with bromination (p+ = -4.3),157 this is a small substituent effect, but it does favor addition of the electrophilic boron at the less-substituted end of the double bond. In contrast to the case of addition of protic acids to alkenes, it is the boron, not the hydrogen, that is the more electrophilic atom. This electronic effect is reinforced by steric factors. Hydroboration is usually done under conditions in which the borane eventually reacts with three alkene molecules to give a trialkylborane. The... 

Specific examples include treatment of o-stannyl benzyl alcohols with TFA,55 reactions of ketones and aldehydes with Lewis acids,56 and electrophilic selenation of styrenes.57... 

In a more recent contribution, O Shea and coworkers described a related process leading to substituted indoles 2-544 and 2-545 by an intermolecular addition of alkyllithium to a styrene double bond and reaction of the formed intermediate 2-543 with an appropriate electrophile (Scheme 2.123) [285]. Using DMF, C-2 unsub-... 

Lithiated epoxides have been found to react with a number of different activated electrophiles. A new study examines the reactivity of lithiated epoxides with nitrones to prepare 3,y-epoxyhydroxylamines, 46, and oxazetidine, 47 <06OL3923>. Upon deprotonation of styrene oxide, the lithiated reactant was then added to nitrone 45 to form the P,y-epoxyhydroxylamine 46 in good yield as a single diastereomer. A number of additional nitrones were examined as well and all provided similar yields of the 3,y-epoxyhydroxylamines. Treatment of 46 with additional base provided the 1,2-oxazetidine ring system 47 in excellent yield. Interestingly, none of the five-membered isoxazolidines from the 5-endo-tet cyclization were formed in this cyclization. 

Numerous p-values for various electrophilic additions to styrene itself are available (Schmid and Garratt, 1977). Strictly speaking, the reaction constants measure only the sensitivity of the reaction to substituent effects they depend at the same time on the solvent, on the position of the transition state on the reaction coordinate (charge magnitude) and on the way in which substituent effects are transmitted (charge location). In particular, the observed trend of p-values for the chlorination ( — 3.22 Yates and Leung, 1980), bromination (—5.7 Ruasse et al, 1978) and sulfenylation ( — 2.41 ... 

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