Hydrogen atom abstraction from toluene

Draw resonance structures for the possible radicals resulting from hydrogen atom abstraction from toluene. Which would you anticipate to be the most stable Why Compare energies for the different radicals (radical A, radical B,. ..). Is the lowest-energy radical that which you anticipated Are any of the alternatives significantly better than any of the others Explain your reasoning. 

Figure 7.19. Hydrogen atom abstraction from toluene by a 1, 2-6/5-(diphenylphosphino)-A/-methyl maleimide complex.

Intermolecular radical reactions have also been reported, including selective radical-radical coupling reactions. These reactions involve the formation of a-amino acid radicals (stabilized by the captodative effect), which can couple to, for example, benzyl radicals to form phenylalanine derivatives (Scheme 21). The benzyl radicals are generated by hydrogen-atom abstraction from toluenes using alkoxyl radicals derived from peroxides and/or aromatic ketone sensitizers. 

Essentially no incorporation of deuterium label was observed when the same reaction was carried out in toluene-dg. Since the formation of more than one equivalent of CO(n) hydroxide requires the transfer of hydrogen atoms, we hypothesized that the mechanism of this transformation involved the formation of a dinuclear p-peroxo complex, followed by rapid cleavage of the 0-0 bond and hydrogen atom abstraction from the Tp-ligand. The inferred activation of the primary C-H bonds of the rm-butyl groups of the ligand requires a highly reactive species thus the "activation" of O2 seems to have been accomplished. 

For aromatic hydrocarbons with side chain alkyl group such as toluene (C6H5CH3), the addition reaction to the benzene ring and hydrogen atom abstraction reaction from the alkyl group can occur simultaneously. 

It should also be emphasized that benzyl radicals can be formed from toluene by hydrogen atom abstraction reactions. This may involve, for example, reaction with reactive oxygen-centered radicals derived from the photolysis of peroxides (see Amino Acids and Peptides later in this chapter). 

Alkyl radicals can be obtained by abstraction of a hydrogen atom from an alkyl group by another radical. This method was utilized for the generation of benzyl radicals from toluene with iert-butoxy radical obtained on heating di- er -butyl peroxide. BenzoyP and carboxymethyP radicals have also been obtained by this method. The reaction gives rise to a complex mixture of products and therefore is of rather limited use. 

It is evident from the nature of the products, especially those formed with toluene present, that the photoreaction in weakly acidic medium involves incursion of a radical species. The complete suppression of reactions leading to the above products, in the presence of oxygen, strongly suggests that it is an excited triplet trityl ion which undergoes reaction. It is postulated that the primary photochemical process is the abstraction of a hydrogen atom by the triplet trityl ion to form the radical cation 90, which was proposed as an intermediate in the dimerization reactions carried out in strong acid (Cole, 1970). 

Some radicals (e.g., tert-butyl, benzyl, and cyclopropyl), are nucleophilic (they tend to abstract electron-poor hydrogen atoms). The phenyl radical appears to have a very small degree of nucleophilic character. " For longer chains, the field effect continues, and the P position is also deactivated to attack by halogen, though much less so than the a position. We have already mentioned (p. 896) that abstraction of an a hydrogen atom from ring-substituted toluenes can be correlated by the Hammett equation. 

The initiation step in the high-temperature oxidation of toluene is the pyrolytic cleavage of a hydrogen atom from the methyl side chain, and at lower temperatures it is 02 abstraction of an H from the side chain, namely... 

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