Radical Substitution of Benzylic and Allylic Hydrogens

Draw the stereoisomers of the major monobromination products obtained from the following reaction. 

An allylic radical has an unpaired electron on an allylic carbon and, like an allylic cation, has two resonance contributors (Section 8.13). 

Because electron delocalization stabilizes a molecule (Section 8.6), allyl and benzyl radicals are both more stable than other primary radicals. They are even more stable than tertiary radicals. 

We know that the more stable the radical, the faster it can be formed. This means that a hydrogen bonded to either a benzylic carbon or an allylic carbon will be preferentially substituted in a halogenation reaction. Because bromination is more highly regioselective than chlorination, the percent of substitution at the benzylic or allylic carbon is greater for bromination. 

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Section 9.5 Radical Substitution of Benzylic and Allylic Hydrogens 347... 

By far the most generally useful synthetic application of allyltributyltin is in the complementary set of transition metal- and radical-mediated substitution reactions. When the halide substrates are benzylic, allylic, aromatic or acyl, transition metal catalysis is usually the method of choice for allyl transfer from tin to carbon. When the halide (or halide equivalent) substrate is aliphatic or alicyclic, radical chain conditions are appropriate, as g-hydrogen elimination is generally not a problem in these cases. 

The relative stabilities of radicals follow the same trend as for carhoca-tions. Like carbocations, radicals are electron deficient, and are stabilized by hyperconjugation. Therefore, the most substituted radical is most stable. For example, a 3° alkyl radical is more stable than a 2° alkyl radical, which in turn is more stable than a 1° alkyl radical. Allyl and benzyl radicals are more stable than alkyl radicals, because their unpaired electrons are delocalized. Electron delocalization increases the stability of a molecule. The more stable a radical, the faster it can be formed. Therefore, a hydrogen atom, bonded to either an allylic carbon or a benzylic carbon, is substituted more selectively in the halogenation reaction. The percentage substitution at allylic and benzyhc carbons is greater in the case of bromination than in the case of chlorination, because a bromine radical is more selective. 

When chlorination or bromination of alkenes is carried out in the gas phase at high temperature, addition to the double bond becomes less significant and substitution at the allylic position becomes the dominant reaction.153-155 In chlorination studied more thoroughly a small amount of oxygen and a liquid film enhance substitution, which is a radical process in the transformation of linear alkenes. Branched alkenes such as isobutylene behave exceptionally, since they yield allyl-substituted product even at low temperature. This reaction, however, is an ionic reaction.156 Despite the possibility of significant resonance stabilization of the allylic radical, the reactivity of different hydrogens in alkenes in allylic chlorination is very similar to that of alkanes. This is in accordance with the reactivity of benzylic hydrogens in chlorination. 

The rate-determining step of the radical substitution reaction is hydrogen atom abstraction to form a radical. The relative rates of radical formation are benzylic allyl > 3° > 2° > 1° > vinyl methyl. To determine the relative amounts of products obtained from the radical halo-genation of an alkane, both probability and the relative rate at which a particular hydrogen is abstracted must be taken into account. The reactivity-selectivity principle states that the more reactive a species is, the less selective it will be. A... 

At the same time, delocalization of unpaired spin in the free-radical product appears to be important for the course of the substitution reaction. For example hydrogen shift in sabinene radical cation 39a leads to a conjugated system (40 ) nucleophilic attack on l-aryl-2-alkylcyclopropane radical cations 43 or 47 produces benzylic radicals nucleophilic attack on 39a generates an allylic species and attack on the tricyclane radical cations 55 or 56 forms tertiary radicals. Apparently, formation of delocalized or otherwise stabilized free radicals is preferred. 

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