Alkenes relative nucleophilicity

There are also reactions in which electrophilic radicals react with relatively nucleophilic alkenes. These reactions are exemplified by a group of procedures in which a radical intermediate is formed by oxidation of readily enolizable compounds. This reaction was initially developed for /3-ketoacids,311 and the method has been extended to jS-diketones, malonic acids, and cyanoacetic acid.312 The radicals formed by the addition step are rapidly oxidized to cations, which give rise to the final product by intramolecular capture of a carboxylate group. 

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... 

As expected for an electrophilic addition, the reaction rate increases as alkyl groups are substituted on the double bond. The electron-donating alkyl groups make the alkene more nucleophilic. Table 11.1 lists the relative rates of bromination of a series of alkenes. As can be seen from this table, replacing all four of the hydrogens of ethene with methyl groups results in an increase in the rate of the reaction by a factor of 2 million. 

Hence the preferred reactivity of these alkyl radicals they are relatively nucleophilic and therefore prefer to react with electrophilic alkenes. Reaction between a nucleophilic alkyl radical and an unfunctionalized (and therefore nucleophilic) alkene is much slower. Similarly, radicals adjacent to electron-withdrawing groups do not react well with electrophilic alkenes. We can represent all this, on an energy level diagram. 

A related selectivity problem arises in addition reactions of unsaturated alkyl derivatives. The example shown on the bottom of Figure 1 illustrates that the 1 1 product 6 can only be generated in reasonable yield, if the nucleophilicity of 5 is higher than that of 4 and 6. An estimate for the relative nucleophilicities of olefinic TT systems in such reactions can be derived from the recently determined reactivity ratios of alkenes and alkynes towards arylcarbenium ions. An additional selectivity problem encountered in the reaction of 4 with 5 - compound 6 does not only incorporate a nucleophilic but also an electrophilic center will be discussed in the next section. 

When an alkene undergoes an electrophilic addition reaction with HBr, the first step is a relatively slow addition of a proton (an electrophile) to the alkene (a nucleophile). A carbocation intermediate (an electrophile) is formed, which then reacts rapidly with a bromide ion (a nucleophile) to form an alkyl halide. Notice that each step involves the reaction of an electrophile with a nucleophile. The overall reaction is the addition of an electrophile to one of the sp carbons of the alkene and the addition of a nucleophile to the other sp carbon (Section 5.6). 

Xenon difluoride [4, 5, 7, 8,10] is a white crystalline material obtained through the combination of fluorine and xenon m the presence of light The reagent is commercially available and possesses a relatively long shelf-life when stored cold (freezer) Xenon difluoride is very effective for small-scale fluormation of alkenes and activated nucleophilic substrates. The reactions are usually conducted between 0 °C and room temperature in chloroform or methylene chloride solutions Hydrogen fluoride catalysis is sometimes helpful Xenon difluoride reacts in a manner that usually involves some complexation between the substrate and reagent followed by the formation of radical and radical cation intermediates... 

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