Bisallyl radical

Table 3 lists all polyenes whose radical cations have been investigated by one or other of the above-described techniques and some of the structures listed are shown below the table. Note that some nonconjugated dienes do not retain their structure upon ionization [e.g. semibullvalene 104 (equation 61) or the cyclopentadiene dimers 126 and 294 (equation 62)] but break a bond to form a bisallylic radical cation, a rather common tendency of radical cations that have this possibility. 

Benzocyclobutadienequinone (199) S78 fluorenone.benzil, 4,5-phenanthryl-eneketone (200), o-phenanthrenequinone and acenaphthenequinone 579 yield stable ketyls on cathodic reduction in DMF or acetonitrile, and these have been characterized by ESR. An interesting electrocyclic isomerization of the anion radical 201 (Eq. (252) ) to the 2,2 -bisallyl radical anion 202 was observed by ESR, when 1,2-dimethylenecyclobutane (203) was reduced in THF at-90°C 581 ... 

Figure 4.3 Formation of bisallylic radicals of arachidonic acid via inter- and intramolecular hydrogen atom abstraction mechanisms.

Clearly, this indicates that the 3,3-shift transition state has little cyclohexane-1,4-diyl character. This is not unreasonable considering that bond formation in this case must occur with generation of a strained cyclopropane bond so the transition state, no doubt, has much more bisallyl radical character (see Chapter 7, Section 4.1). Indeed, radical stabilizing substituents on the cyclopropane ring bond being broken dramatically increase the rate of the 3,3-shift to the point where the transition state is nearly equi-energetic with starting materials (Scheme 10.35). ... 

Details of the coding system developed to handle the interconversions of 1,2-dimethylenecyclobutanes and the related biradicals have now appeared. Gajewski has presented stereochemical evidence for the interconversion of planar and orthogonal bisallyl radicals in the thermal rearrangement of trans-3,4-dimethyl-l,2-dimethylenecyclobutane (442). Pyrolysis of (442) at 230°C for 80 minutes gave a 9 % conversion into an 18 1 1 mixture of the isomers (443), (444), and (445). The recovered (442) was 19 % racemized, and the isomer (443) also appeared to be racemic. The pyrolysis products show the expected preference for conrotatory motions about the 1,2- and 3,4-bonds. The conrotatory out motion leads to the anti-anti-bismethallyl radical (446) which can only close to anri,anri-diethylidene-cyclobutane (445) or anti-l-ethylidene-2-methylene-3-methylcyclobutane (443). The small percentage of the syn-isomer (444) found in the products could arise from the... 

The conversion of (298) into (300) upon gas-phase thermolysis occurs by a Cope rearrangement and not by a [1,3] carbon shift or a bisallyl radical mechanism. Thus, similar treatment of (299) affords only (301). Rearrangement of 3-oxobicyclo-[3,2,l]oct-6-ene to 3-oxobicyclo[3,3,0]oct-6-ene occurs only at much higher temperature and presumably by way of a non-concerted mechanism. The nickel-catalysed reaction of norbornene with butadiene affords the 1 2 cxo-adduct (302) which upon thermolysis is converted into the valence isomer (303). Concerted and non-concerted Claisen-type rearrangements have been observed in the A, -phosphorin series. Tricyclo[7,3,0,0 ]dodeca-2,5,7,10-tetraene (304), which is available in six steps starting from bicyclo[3,3,0]octa-3,7-dien-2,6-dione, appears to be an essentially static structure (as assessed by n.m.r. spectroscopy) up to 141 The slow... 

Initiation of MMA polymerization by complexes such as (192) was shown to proceed via a bimetallic bis(enolate) intermediate, arising from the dimerization of a radical anion.478" 80 Such a mechanism481,482 explains why efficiencies with such initiators (calculated from polymer molecular weights) are always <50%. Using a similar methodology, the bimetallic bisallyl complex (198) was shown to polymerize MMA in a living fashion (Mw/Mn 1.1) and triblock copolymers with methacrylate and acrylate segments have been prepared. 

This conclusion is partly true because superoxide is unable to abstract hydrogen atom even from the most active bisallylic positions of unsaturated compounds, while perhydroxyl radical abstracts H atom from linoleic, linolenic, and arachidonic fatty acids with the rate constants of 1-3 x 1031 mol-1 s-1 [24], However, the superoxide damaging activity does not originate from hydrogen atom abstraction reactions but from one-electron reduction processes, leading to the formation of hydroxyl radicals, peroxynitrite, etc, and in these reactions perhydroxyl cannot compete with superoxide. 

However, it has been suggested that in contrast to traditional view about the inactivity of tocopheroxyl radical, a-Toc is capable of participating in chain propagation. This mechanism was discussed in detail [121-124], It has been proposed that at low free radical fluxes and in the absence of ascorbate or ubihydroquinone (both antioxidants are supposedly able to regenerate a-tocopherol) tocopheroxyl radical abstracts a hydrogen atom from the bisallylic position of unsaturated compounds ... 

Optically transparent materials with different properties are synthesized from monomers containing the allyl group CH2=CH-CH2-. Allyl monomers and their mixtures with other monomers are polymerized in the presence of radical initiators, which break down into radicals during thermolysis or under the influence of ultraviolet or ionizing rays. Such allyl monomers as diethylene glycol bisallyl carbonate, diallylphthalate, triallylcyanurate, and others are widely used. 

More recently, several reactions of RS radicals generated chemically have been characterized and kinetically described, such as H-atom abstraction from the C-H bonds and from amino acid side chain C-H bonds in model peptides, and from bisallylic methylene groups in polyunsaturated fatty acids (PUFA). ... 

Figure 5 Hemozoin-mediated lipid peroxidation. Redox cycling of iron in complexes like HZ can initiate lipid peroxidation of fatty acids like arachidonic acid (LH). Abstraction of the bisallylic hydrogen leaves an unpaired electron on the methylene carbon that can rearrange to form a reactive alkyl radical (L ). Oxidation of this radical leads to a peroxyl radical (LOO ), and on reduction, forms a lipid peroxide (LOOM) that can undergo additional reactions to yield a variety of secondary oxidation products.

All these examples clearly identify the difference in bond dissociation energy between C-H and S-H as a major factor determining the extent to which RS -induced H-abstraction occurs. The most striking case is the reaction of thiyl radicals with polyunsaturated fatty acids (PUFA) (eq. 15).36,37,42 strength of the bisallylic C-H bonds in the latter is on the order of = 300 kJ/mol, i.e., considerably smaller than that of the S-H bond in thiols (340-365 kJ/mol). 

One further aspect besides H-atom abstraction must, however, be considered in the reactions of thiyl radicals with PUFAs, namely, the possibility of concurrent RS additions to the double bonds. By analyzing a particular system in which the thiyl radical from mercaptoethanol had reacted with linolenic acid, Schoneich at came to the conclusion that the abstraction of bisallylic hydrogen and thiyl addition occur with comparable rates. The C-centered adduct radical formed in the addition reaction is, of course, prone for repair in the presence of thiols, i.e., will subsequently regenerate thiyl radicals via the back reaction of equilibrium 14. These, in turn, will re-enter into the abstraction / addition competition cycle and eventually all of the thiyl radicals will appear to have reacted via the abstraction route. Experimentally, overall efficiencies of up to 85 % have been measured and the difference to the limiting 100% efficiency may be accounted for by side and termination processes. But looking into the details, the situation is, in fact, even more complex. As shown by Schwinn at al., there is still another process which readily occurs within the RS -adducts to the double bonds, namely, cis-trans isomerization. Quantification of this is of imminent interest for the biological community. 

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