Allyl radicals configurational stability

Butadiene reacts with chlorine under radical and ionic conditions to give both 1,2- and 1,4-addition products.259 The predominant formation of the trans isomer in radical 1,4-addition [Eq. (6.37)] was explained to result from the predominant transoid form of the starting compound and the configurational stability of the resonance-stabilized allylic radical intermediate ... 

The most likely multistep mechanism of this type is shown in the lower part of Figure 15.17. It is a two-step mechanism where the diastereomeric diradicals F and G are the two intermediates that allow for rotation about the configuration-determining C—C bond. Each of the two radical centers is part of a well-stabilized allyl radical (cf. Section 1.2.1). Biradicals F and G cyclize without diastereocontrol to deliver the [4+2]-cycloadducts biradical F forms a mixture of l 2trans,cis-[D]2-C and 12trans,trans [D]2-C, since a rotation about the C2—C3 bond is possible but not necessary. For the same reason, biradical G forms a mixture of 1 2cis,cis-[D]2-C and 12cis,trans [D]2-C. 

In these two latter cases it appears with certainty from the heat of combustion and from spectra that there is nothing special about the bonds themselves. The low dissociation energy must, therefore, find its cause in the special stability of the products of dissociation, in these cases the allyl radical. The particular stability of this radical follows from the resonance which is possible here between two equivalent configurations. H2C=CH— GH2 H2C—GH=GH2... 

Lipoxygenase enzymes are known to oxidize arachidonic acid at six possible positions. The chemical requirement for this oxidation (Eq. 4.1) appears simply to be a skipped cis diene which can produce (via the enzyme) a stabilized allylic radical, which then traps molecular oxygen to form a conjugated cis-trans diene hydroperoxide. Chart 4.1 shows the six possible mono-oxidation products as members of the HETE family. All of these compounds have been shown to be natural products. Only in the cases of 8 and 9 HETEs is the absolute stereochemistry not definitively known. Nevertheless, it is speculated that 8-HETE has the alcohol in the (5)-configuration and 9-HETE is of the (R) absolute... 

Any electron in the MO of an allyl system has the same energy as an electron in a 2p atomic orbital. Therefore, it makes no contribution to the net stability of the molecule, and it does not destabilize the n system either. That is why it is called a nonbonding orbital. Figure 11.8 shows the electron configurations of the n orbitals of the carbocation, radical, and carbanion. Figure 11.7b shows the molecular orbitals of the allyl carbocation. In this case, the nonbonding orbital,, is not occupied. 

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