Allyl radicals configuration

In the case of the [4+ 2]-cycloadditions, the diradical analogous to 172 should contain an allyl radical subunit in the side-chain having the Z-configuration. There the closure of the six-membered ring occurs also employing the central carbon atom of the pentadienyl radical system. A quantum-chemical study reproduced the preference of the step 172 —y 163 over that from 172 to 173 [47]. This may have its origin in the higher spin density at C3 of the cyclohexadienyl radical as compared with Cl and C5 [108]. 

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 ... 

VB model, though successful for the interactions between monovalent atoms, breaks down when 71 bonds are considered. The aim of this chapter is to bring a quantitative answer to a question which can be so summarized What is the nature of the driving force which makes benzene more stable in a D6h geometry than in an alternated Dih geometry of Kekule type Exactly the same type of question applies to the allyl radical which will also be investigated and will allow the study of the effects of configuration interaction (Cl) and basis set extension. 

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. 

Electronic Configurations of the Allyl Radical, Cation, and Anion 681... 

The electronic configuration of the allyl cation (Figure 15-12) differs from that of the allyl radical it lacks the unpaired electron in tt2, which has half of its electron density on Cl and half on C3. In effect, we have removed half an electron from each of Cl and C3, while C2 remains unchanged. This MO picture is consistent with the resonance picture showing the positive charge shared by Cl and C3. 

Figure 15-12 also shows the electronic configuration of the allyl anion, which differs from the allyl radical in having an additional electron in tt2, the nonbonding orbital with its electron density divided between Cl and C3. 

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... 

Kolbe electrolysis conditions. The 6,)/-unsaturated carboxylic acids (VII) give the isomeric 1,5-dienes (IX-XI) via the allylic radicals (VIII) [Eq. (11)] [54]. Through this reaction, the configuration of the double bonds is almost totally (> 90%) retained. 

Figure 29.7. Allyl system. Configuration of tt electrons in cation, free radical, and anion.

Only a few examples exist for the intermolecular trapping of allyl radicals with alkenes . The reaction of ot-carbonyl allyl radical 28 with silyl enol ether 29 occurs exclusively at the less substimted allylic terminus to form, after oxidation with ceric ammonium nitrate (CAN) and desilylation of the adduct radical, product 30 (equation l4). Formation of terminal addition products with frows-configuration has been observed for reaction of 28 with other enol ethers as well. 

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