Allylic radicals resonance delocalization

Ffe. 13 Mesomerism scheme for the allyl radical the delocalized Hiickel wave luncUon represented as a linear combination of resonating Lewis stmctures Wg and Wc... 

Allyl radical is a conjugated system in which three electrons are delocalized over three carbons The resonance structures indicate that the unpaired electron has an equal probability of being found at C 1 or C 3 C 2 shares none of the unpaired electron... 

The chain propagation step consists of a reaction of allylic radical 3 with a bromine molecule to give the allylic bromide 2 and a bromine radical. The intermediate allylic radical 3 is stabilized by delocalization of the unpaired electron due to resonance (see below). A similar stabilizing effect due to resonance is also possible for benzylic radicals a benzylic bromination of appropriately substituted aromatic substrates is therefore possible, and proceeds in good yields. 

Radicals with adjacent Jt-bonds [e.g. allyl radicals (7), cyclohexadienyl radicals (8), acyl radicals (9) and cyanoalkyl radicals (10)] have a delocalized structure. They may be depicted as a hybrid of several resonance forms. In a chemical reaction they may, in principle, react through any of the sites on which the spin can be located. The preferred site of reaction is dictated by spin density, steric, polar and perhaps other factors. Maximum orbital overlap requires that the atoms contained in the delocalized system are coplanar. 

The observation that in the case of PCSO there is no formation of propanol while allyl alcohol is formed from ACSO agrees with the resonance stabilization of the allyl radical and hence weaker bond for S-allyl than for S-propyl. The yield of allyl alcohol from irradiation of ACSO is considerably greater than that from S-allyl-L-cysteine, probably due to energy delocalization by the four p electrons of the S atom. 

The most common and also most effective mechanism of radical stabilization involves the resonant delocalization of the unpaired spin into an adjacent 7r system, the allyl radical being the prototype case. A minimal orbital interaction diagram describing this type of stabilization mechanism involves the unpaired electron located in a 7r-type orbital at the formal radical center and the 7r- and tt -orbitals of the n system (Scheme 1). 

When considering the stability of spin-delocalized radicals the use of isodesmic reaction Eq. 1 presents one further problem, which can be illustrated using the 1-methyl allyl radical 24. The description of this radical through resonance structures 24a and 24b indicates that 24 may formally be considered to either be a methyl-substituted allyl radical or a methylvinyl-substituted methyl radical. While this discussion is rather pointless for a delocalized, resonance-stabilized radical such as 24, there are indeed two options for the localized closed shell reference compound. When selecting 1-butene (25) as the closed shell parent, C - H abstraction at the C3 position leads to 24 with a radical stabilization energy of - 91.3 kj/mol, while C - H abstraction from the Cl position of trans-2-butene (26) generates the same radical with a RSE value of - 79.5 kj/mol (Scheme 6). The difference between these two values (12 kj/mol) reflects nothing else but the stability difference of the two parents 25 and 26. 

Stabilized by resonance delocalization indeed, they are even more stable than tertiary radicals. In the presence of a suitable initiator, bromine dissociates to bromine atoms that will selectively abstract an allylic or a benzylic hydrogen from a suitable substrate, generating the corresponding allyl and benzyl radicals. 

Resonance effects, on the other hand, can significantly affect the regiochem-istry of the cyclizadon. Resonance delocalization of the unpaired electron of a free radical stabilizes that radical. This is why the allyl radical is much more stable than the //-propyl radical. Thus, if a double bond is substituted with a group capable of providing resonance stabilization to a free radical, it undergoes free-radical addition much more readily than a double bond which cannot provide such resonance stabilization. 

The allyl radical has an odd number of electrons. The odd electron is in a p orbital, so the species is conjugated. It has two equally important resonance structures. The octet rule is not satisfied, so this radical is an unstable, reactive species. However, because of its large resonance stabilization, it is not as unstable as would be predicted on the basis of examination of a single structure without delocalization. Single-headed arrows are used to show movement of one electron, rather than electron pairs. Radicals are discussed in more detail in Chapter 21. 

Although free-radical halogenation is a poor synthetic method in most cases, free-radical bromination of alkenes can be carried out in a highly selective manner. An allylic position is a carbon atom next to a carbon-carbon double bond. Allylic intermediates (cations, radicals, and anions) are stabilized by resonance with the double bond, allowing the charge or radical to be delocalized. The following bond dissociation enthalpies show that less energy is required to form a resonance-stabilized primary allylic radical than a typical secondary radical. 

Like allylic cations, allylic radicals are stabilized by resonance delocalization. For example, Mechanism 15-2 shows the mechanism of free-radical bromination of cyclohexene. Substitution occurs entirely at the allylic position, where abstraction of a hydrogen gives a resonance-stabilized allylic radical as the intermediate. 

The allylic cyclohex-2-enyl radical has its unpaired electron delocalized over two secondary carbon atoms, so it is even more stable than the unsubstituted allyl radical. The second propagation step may occur at either of the radical carbons, but in this symmetrical case, either position gives 3-bromocyclohexene as the product. Less symmetrical compounds often give mixtures of products resulting from an allylic shift In the product, the double bond can appear at either of the positions it occupies in the resonance forms of the allylic radical. An allylic shift in a radical reaction is similar to the 1,4-addition of an electrophilic reagent such as HBr to a diene (Section 15-5). 

The true structure of the allyl radical is a hybrid of the two resonance structures. In the hybrid, the ji bond and the unpaired electron are delocalized. 

As with the allyl radical, it is the overlap of the p orbitals in both directions, and the resulting participation of each electron in several bonds that corresponds to our description of the molecule as a resonance hybrid of two structures. Again it is the delocalization of the tt electrons—their participation in several bonds— that makes the molecule more stable. 

Use SpartanVicw to examine spin surfaces for the allyl radical ahd the benzyl radical (CfiHsCHj ). Draw resonance structures that describe how the unpaired electron is delocalized in each. 

An important factor which influences the rate of rearrangement is the extent of overlap between the SOMO and the bond to be cleaved. In conformationally mobile cyclopropylmethyl radicals rotation can occur to maximize this overlap. Substituents at the radical center which withdraw electron density, such as terf-butoxycarbonyl or nitro, dramatically decrease the rate of ring cleavage (Table 1), presumably because they reduce the overlap. Similarly, the rate of rearrangement is much slower in l-(cyclopropyl)allyl (3), l-(cyclopropyl)prop-2-ynyl (4), and a-cyanocyclopropylmethyl (5) radicals where resonance delocalization removes electron density from the a-carbon atom. 

Allylic carbocations, free radicals, and carbanions are resonance stabilized. In each case the stabilization is the result of delocalization of the positive or negative charge or the free radical. Resonance forms differ in the position of electrons and charge but not atoms. Every atom in an allylic carbocation, free radical, or carbanion possesses a p-orbital and the pi-electrons and charges or unpaired electrons are delocalized throughout these orbitals. 

Consider a cell membrane lipid derived from linoleic acid, which is an unsaturated fatty acid. Hydrogen atom abstraction of an allylic hydrogen by a hydroxyl radical leads to a resonance-stabilized free allylic radical in which the unpaired electron is delocalized over C-9, C-11, and C-13. 

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