Allyl radical structure

The results of this study again call attention to the chameleonic nature of the C2 wave function for the Cope rearrangement. At small values of R the CASSCF wave function is essentially that for cyclohexane-1,4-diyl (structure A in Fig. 30.1) whereas, at large values of R the CASSCF wave function approaches that for two allyl radicals (structure... 

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 allyl radical would be expected to be planar in order to maximize n delocalization. Molecular structure parameters have been obtained from EPR, IR, and electron diffraction measurements and confirm that the radical is planar. ... 

To see why allylic radicals are so stable, look at the orbital picture in Figure 10.3. The radical carbon atom with an unpaired electron can adopt sp2 hybridization, placing the unpaired electron in a p orbital and giving a structure that is electronically symmetrical. The p orbital on the central carbon can therefore overlap equally well with a p orbital on either of the two neighboring carbons. 

Sphingomyelin, 1066-1067 Sphingosine, structure of, 1067 Spin density surface, allylic radical, 342... 

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. 

Later, successful determination of the molecular structure of the free allyl radical was achieved by high-temperature electron diffraction, augmented by mass spectrometry studies (Vaida et al., 1986). The structural parameters obtained for the allyl radical were rcc 142.8 pm, rcH 106.9 pm, accc 124.6°, ccH 120.9°. This was the first electron diffraction study of an unstable organic molecule. 

As a result, the radical is stabilized to the extent of about 20 kcal. Two structures of nearly equal energies can be written for the allyl radical resulting from the addition of butadiene ... 

The spectroscopy, structure, photochemistry, and unimolecular reactions of allyl radical have been studied extensively and reviewed recently.145 Possible dissociation channels of allyl radical, their energetics, and the potential energy barriers of the C3H5 system are shown in Figs. 20 and 21.145,146... 

Since A and B are equivalent resonance structures, the allyl radical should be much more stable than either, that is, much more stable than a 10 radical => the allyl radical is even more stable than a 3° radical. 

This is not a proper resonance structure for the allyl radical because it does not contain the same number of unpaired electrons as CH2=CHCH2. ... 

Hydrocarbons containing one or more triple bonds in addition to double bonds have been excluded from the tile, as have been radicals (e.g. the allyl radical C3H5 ) and aromatic molecules, i.e. molecules for which more than one unexcited resonance structure (Kekule structure) can be written. Consequently, hydrocarbons such as phenyl-substituted polyenes, or annulenes — bridged or unbridged—have not been included. 

With ligand 170 (R = Bn), Fahmi reports the formation of an equal amount of byproduct, formulated as the allylic imide 171, Eq. 103. Indeed, Fahmi suggests that this is the correct structure of the same byproduct observed by Katsuki et al. (116) (cf. Section III.A.4, Structure 161). Fahmi suggests that this product may be formed by insertion of solvent in copper benzoate intermediate 172, as illustrated in Scheme 12. The generated copper imidate 174 then reacts with the allylic radical and combines to provide the allylic amination product 175 that rearranges to the observed imide 171. 

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. 

The dimer 352 of 351 was isolated from the product mixtures of two experiments conducted to trap 351 by alkenes, one with 350 and the other with 354 as substrate. Although no cycloadduct with the alkene was observed in one case, the yield of 352 amounted to only 0.8%. Nevertheless, the structure of 352 is interesting, since it suggests that the tetramethyleneethane diradical assumed to be the intermediate undergoes ring closure preferentially between two different allyl-radical termini. 

Concerning the structure, the cyclopropane derivatives 524—526 deviate from the generally observed cycloadducts of cyclic allenes with monoalkenes (see Scheme 6.97 and many examples in Section 6.3). The difference is caused by the different properties of the diradical intermediates that are most likely to result in the first reaction step. In most cases, the allene subunit is converted in that step into an allyl radical moiety that can cyclize only to give a methylenecyclobutane derivative. However, 5 is converted to a tropenyl-radical entity, which can collapse with the radical center of the side-chain to give a methylenecyclobutane or a cyclopropane derivative. Of these alternatives, the formation of the three-membered ring is kinetically favored and hence 524—526 are the products. The structural relationship between both possible product types is made clear in Scheme 6.107 by the example of the reaction between 5 and styrene. 

Stabilized allyl radical will be stabilized further if substituents are introduced. This stabilization occurs to different degrees in the ground state and the transition structure for rotation. In the ground state the substituent acts on a delocalized radical. Its influence on this state should be smaller than in the transition structure, where it acts on a localized radical. In the transition state the double bond and the atom with the unpaired electron are decoupled, i.e. in the simple Hiickel molecular orbital picture, the electron is localized in an orbital perpendicular to the jt(- c bond. 

For example, radical allylic bromination of pent-2-ene must produce a mixture of three products. There are two allylic positions in the substrate, and either can suffer hydrogen abstraction. If hydrogen is abstracted from the methylene, then the two contributing resonance structures for the allylic radical are equivalent, and one product results when this captures a bromine atom. Abstraction... 

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