Aromatic rings radical attack

Another polycyclization possibility has been described by Julia.In this case, an aromatic ring is attacked in the last step (Scheme 146). The radical 396 generated from the corresponding cyanoester by BP initiation in cyclohexane at 80°C gives the cyclized trans compound 398 (42% yield) in addition to the monocyclization product. Compound 399, when treated with BP in refluxing benzene, gives the trans compound 402 in 20% yield. ... 

All laromatics. The aromatic ring is fairly inert toward attack by oxygen-centered radicals. Aromatic acids consisting of carboxyl groups substituted on aromatic rings are good candidates for production by LPO of alkylaromatics since thek k /k ratios are low. TerephthaUc acid [100-21 -0]... 

The halogenation reaction conditions can be chosen to direct attack to the methyl group (high temperature or light to form free-radicals) or the aromatic ring (dark, cold conditions with FeX present to form electrophilic conditions). 

Alkyl Side Chains of Aromatic Rings. The preferential position of attack on a side chain is usually the one a to the ring. Both for active radicals such as chlorine and phenyl and for more selective ones such as bromine such attack is faster than that at a primary carbon, but for the active radicals benzylic attack is slower than for tertiary positions, while for the selective ones it is faster. Two or three aryl groups on a carbon activate its hydrogens even more, as would be expected from the resonance involved. These statements can be illustrated by the following abstraction ratios ... 

In these reactions a new carbon-carbon bond is formed, and they may be given the collective title coupling reactions. In each case, an aUcyl or aryl radical is generated and then combines with another radical (a termination process) or attacks an aromatic ring or alkene to give the coupling product. ... 

Evidently, the activation energy for a thermally neutral reaction with participation of a hydrogen atom or a radical (alkyl, alkoxyl, etc.) is higher in these cases where there is a iT-bond or an aromatic ring adjacent to the attacked C—H bond. This effect is a property of the structures themselves, and the n-bond exerts a dual effect on the reaction center. On the one hand, by weakening the C—H bond the ir-bond in the a-position lowers the enthalpy... 

For example, the fact that ions of m/z [90 + R]+ and [104 + R]+ arise directly from the molecular ions of sulfones (Scheme 5.20) confirms a transformation with a new C-C bond formation between carbon atoms of the small ring and of the second benzene ring prior to the fragmentation of the M+. In this case the cyclopropyl moiety (maybe iso-merized) retains the charge and unpaired electron and attacks the second aromatic ring by a nucleophilic or radical mechanism. 

This novel anodic methoxyiation may proceed via the fluorosulfonium ion B in a Pummerer-type mechanism as shown in Scheme 6.11. In this mechanism, the cation radical A of the sulfide is trapped by a fluoride ion, and this step should suppress side reactions from the cation radical A (such as dimerization and nucleophilic attack on an aromatic ring) even when deprotonation of A is slow due to the weak electron-withdrawing Rf groups or electron-donating substituents on the benzene ring. Since fluoride ions are much weaker nucleophiles compared to methoxide, it is reasonable that the methoxyiation predominates in methanol. 

Oxidative Polymerization Reactions. Clays can initiate polymerization of unsaturated compounds through free radical mechanisms. A free radical R", which may be formed by loss of a proton and electron transfer from the organic compound to the Lewis acid site of the clay or, alternatively, a free radical cation, R+, which may be formed by electron transfer of an electron from the organic compound to the Lewis acid site of the clay, can attack a double bond or an aromatic ring in the same manner as an electrophile. The intermediate formed is relatively stable because of resonance, but can react with another aromatic ring to form a larger, but chemically very similar, species. Repetition of the process can produce oligomers (dimers, trimers) and, eventually, polymers. 

The first step of other high-order alkylated aromatics proceeds through pyrolytic cleavage of a CC bond. The radicals formed soon decay to give H atoms that initiate the H2—02 radical pool. The decay of the initial fuel is dominated by radical attack by OH and H, or possibly O and H02, which abstract an H from the side chain. The benzylic H atoms (those attached to the carbon next to the ring) are somewhat easier to remove because of their lower... 

Scheme 6.12 Intramolecular addition of silyl radicals to aromatic rings ortho- vs Ipw-attack...

The intrinsic stability of the aromatic n system has two major consequences for the course of reactions involving it directly. First, the aromatic ring is less susceptible to electrophilic, nucleophilic, and free-radical attack compared to molecules containing acyclic conjugated n systems. Thus, reaction conditions are usually more severe than would normally be required for parallel reactions of simple olefins. Second, there is a propensity to eject a substituent from the tetrahedral center of the intermediate in such a way as to reestablish the neutral (An + 2)-electron system. Thus, the reaction is two step, an endothermic first step resulting in a four-coordinate carbon atom and an exothermic second step, mechanistically the reverse of the first, in which a group is ejected. The dominant course is therefore a substitution reaction rather than an addition. 

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