Radicals mesomerically stabilized

As initiators predominantly -diketones (especially 2,4-pentanedione, acetylac-etone, Acac) have been reported (see Figure 6.7 below). Enzymatic H-atom abstraction results in a mesomerically stabilized radical which initiates the polymerization mechanism [8]. A detailed discussion on the influence of the type of mediator and concentration can be found under Section 6.3.2.3. 

They provide protection during manufacture, molding, and use against the oxidative effect of oxygen and other oxidants. These are for the most part molecules that stop the chain reaction of the auto-oxidation process and are capable of forming highly stable, mesomerically stabilized, radicals. Without further additions, these stabilizers are consumed irreversibly in the course of the reaction. 

Experiments which could prove this statement are apparently not yet known. Because the gain in energy with the formation of a Pb—C bond (31 kcal/mole, see Section I,B) is relatively small, only those C=C groups can be attacked whose w-bond cleavage generates mesomerically stabilized radicals (compare the general statements in Stirling, 260), i.e.. 

The reaction course has not been elucidated (cf. also sodium hydroxide reagent). Hydrolyzation reactions and aromatizations are probably primarily responsible for the formation of colored and fluorescent derivatives. Substituted nitrophenols - e.g. the thiophosphate insecticides — can probably be hydrolyzed to yellow-colored nitro-phenolate anions by sodium hydroxide or possibly react to yield yellow Meisenheimer complexes. Naphthol derivatives with a tendency to form radicals, e.g. 2-naphthyl benzoate, react with hydrolysis to yield violet-colored mesomerically stabilized 1,2-naph-thalenediol radicals. 

The mechanism of the toluene and xylene oxidation bears a close resemblance to the oxidation of propene. Abstraction of a H-atom from the reactive methyl group and formation of a complex between the resulting radical and the catalyst is the first and probably the rate-determining step for both. However, the effect of the mesomeric stabilization of this radical complex is different. While a symmetrical allyl structure is formed from propene, an asymmetrical situation occurs for toluene and xylene, which is illustrated below for the case of toluene, viz. 

The localized bond model fails for odd conjugated systems (ions and radicals such as allyl or benzyl) the mesomeric stabilization of these can be estimated by the perturbation treatment of Section VIII. However, the localized bond model can still4 be used for other types of odd systems, i.e. ones where the... 

This stabilization of the radical intermediates, arising from a better mesomeric stabilization of radicals in the phthalimide moiety, consequently increase the exoenergicity of reactions and, according to the Bell-Evans-Polanyi principle, lowers the activation barrier and thus enables processes that are unknown from ketones. The unique photochemical reactivity of phthalimides will be demonstrated with some examples. 

Chemically, the air-drying of a nonconjugated oil such as linseed is characterized by the adsorption of 12-16% by weight of oxygen. The reactivity of drying oils is based on the mesomeric stabilization of the radical intermediate the unpaired electron is delocalized over several carbon atoms, and less energy is required to eliminate the proton as illustrated below (24). 

Methylations with methyl iodide were observed to proceed with high yields and stereoselectivities. Longer-chain alkyl iodides failed in most attempts. Allyl bromide reacts smoothly - however, products of low enantioenrichment (see 146g) result. We explain the fact by a single electron transfer (SET) during the alkylation. The intermediate formation of a mesomerically stabilized allyl radical supports the SET pathway [89]. A solution to this problem was most recently published by Taylor and Papillon who converted a lithio carbamate into the corresponding zinc cuprate prior to allylation [90]. Studies on the stereochemistry in a few metal-exchange reactions have been published by Nakai et al. [91]. 

Attack of monomer at the methylene carbon atom is less sterically hindered and yields a free radical that is more stable because the substituent group X stabilizes the free-radical site by steric hindrance and, in many cases, also by mesomeric stabilization. (Inductive effects are not important because the free-radical site bears no charge.) Thus the reaction is regioselective with mode (I) addition predominating. 

During the oxidation process long-life radicals are formed, which are mesomerically stabilized. These radicals can induce the oxidation of bitumen binder in the road surface or roofing felt, even at low temperatures under the influence of UV radiation. Moreover temperatures up to 60 °C or 80 °C may be reached at the surfaces of roads or roofing felts as a result of the IR radiation in sunlight. However, the energetic UV rays can only penetrate the surface for a few micrometers due to the dark color of the bitumen. 

Cyclic peroxides are formed preferably in polyisoprene rather than in polybutadiene, because the mesomeric stabilization of allylic radicals is much less enhanced in polybutadienoid than in polyisoprenic sequences, on account of the lack of tertiary carbon in the former. 

Radical cation (polaron) mesomeric stabilization with oxygen contribution. 

The same authors also observed an interesting difference in the behavior of the a- and (3-tetralone silyl enol ethers 3 and 4, providing a further indication for the presence of radical ions as reactive intermediates in this reaction. The a-tetralone silyl enol ether radical cation 36 reacted with nitrogen dioxide to form cation 37, whereas the (3-tetralone based radical cation 38 reacted much more slowly and gave a mixture of products (Scheme 8). Due to the mesomeric stabilization of the radical cation 38, its lifetime increased dramatically as observed by time resolved spectroscopy. This favors a cage escape of the radical cation and opens the possibility for further reactions. 

When identical silyl enol ethers are used in the coupling reaction (Rj = R/, R = R ), homocoupling to symmetrical 1,4-diketones can be achieved (Scheme 11). For the synthesis of unsymmetrical 1,4-diketones, the two silyl enol ethers must differ significantly in terms of their oxidation potentials. This can be realized by selecting monosubstituted silyl enol ethers (R = H) and 1,2-disubstituted silyl enol ethers for the coupling reaction. Another possible way to reduce the oxidation potential is by the use of mesomeric stabilization vide supra).In the coupling reactions presented so far, the reactivity of silyl enol ethers is twofold. The component that is more easily oxidized forms the radical cation and consequently the a-carbonyl radical. In contrast, the second component acts as an electron-rich double bond in the radical addition reaction. 

The product is exclusively carbon monoxide, and good turnover numbers are found in preparative-scale electrolysis. Analysis of the reaction orders in CO2 and AH suggests the mechanism depicted in Scheme 4.6. After generation of the iron(O) complex, the first step in the catalytic reaction is the formation of an adduct with one molecule of CO2. Only one form of the resulting complex is shown in the scheme. Other forms may result from the attack of CO2 on the porphyrin, since all the electronic density is not necessarily concentrated on the iron atom [an iron(I) anion radical and an iron(II) di-anion mesomeric forms may mix to some extent with the form shown in the scheme, in which all the electronic density is located on iron]. Addition of a weak Bronsted acid stabilizes the iron(II) carbene-like structure of the adduct, which then produces the carbon monoxide complex after elimination of a water molecule. The formation of carbon monoxide, which is the only electrolysis product, also appears in the cyclic voltammogram. The anodic peak 2a, corresponding to the reoxidation of iron(II) into iron(III) is indeed shifted toward a more negative value, 2a, as it is when CO is added to the solution. 

The explanation of "mesomerism" he argued, lies in energy relationships, namely, that there are two or more extreme forms of similar or equal energy, and the ordinary form and energy of the molecule can best be interpreted quantum mechanically. Ingold treated this idea more thoroughly in his 1937 Faraday Society article on the relation between chemical and physical theories of the source of the stability of organic free radicals.64... 

In solution the colorless 2,4,6-triphenylphenoxyl dimer attains a rapid equilibrium with its red monomer radical (dissociation constant in benzene 4 X 10 at 20°). The radical is surprisingly stable toward oxygen and can be stored in solution for a long time when it is protected from light. The stability of the 2,4,6-triphenylphenoxyl radical is ascribed to steric and mesomeric effects. The e.s.r. spectrum and an ENDOR-spectrum of the radical are described. 

One may therefore tentatively conclude from this, in the case of a [2 + 2] cycloaddition or reversion, that a diradical transition state is most likely, if it is stabilized by substituents in the 2,5-position by means of a mesomeric interaction. Otherwise the equilibrium is shifted to the tetraphosphahexadiene. This explains the observations with respect to the reaction proceeding and hence the influence of the substituents at the two carbon atoms. If, in the case of a hindered orbital overlap between the substituents and the PC double bonds, a quasiaromatic interaction is blocked, the equilibrium is shifted to the tetraphosphahexadiene. On the other hand, if substituents in the 2,5-position are able to interact with the PC double bond, a mesomeric charge transfer into the side chain takes place, obstructing the aromatic transition state and so favoring the 1,4-cyclohexadiyl radical, which recombines to the bicyclic compound (Fig. 8). The carbon atom and its substituents... 

The EPR spectra of the radical cations derived from pyrrole solutions can all be simulated by electronic structures in which the unpaired electron is located in an orbital with the nodal plane on the nitrogen and (2) showing large coupling constant values at the 2- and 5-positions and small values at the 3- and 4-positions <2000J(P2)905>. The EPR parameters of the radical cations from 2,5-dimethyl-l-phenylpyrroles and 3,4-bis(alkylthio)-2,5-dimethyl-l-phenylpyrroles (Table 30) denote a marked stabilization of the radical cations by the sulfanyl groups through mesomeric effects. 

A careful analysis based on these experimental results excluded a chain propagation process [33a]. On account of the 3-position of the methylthio substituent in the thiophene ring, three isomeric dimers may be formed. The main reaction path can be deduced from the mesomeric forms of the radical cation 1. The two most important mesomeric structures are those with the unpaired electron in an a-position (II, II). Structure I, with the positive charge next to the methylthio sulfur, is preferred, because of the stabilizing -l- M effect of the methylthio substituent. Therefore, the 3,3 -connected dimer 2a is the essential... 

The size as well as the electronic properties (i.e. inductive and mesomeric effects) of the surrounding groups affects the stability of carbocations, carbanions and radicals. When bulky substituents surround a cation, for example, this reduces the reactivity of the cation to nucleophilic attack by steric effects. This is because the bulky groups hinder the approach of a nucleophile. 

Mechanistic studies reveal transformations of N, A -disubstituted PD 11 in autoxidized and photo-oxidized polymers and model hydrocarbons [57,58], Wurster s ion radicals 49, associated with ROO in CTC 41 are formed in the first step after electron transfer from 11 [3,24]. 49 is formed regardless of the oxidizing agent and is stabilized via electron delocalization in mesomeric forms [59] ... 

The first study on the structure of imidoyl radicals dates back to 1973 [11], when Danen described the ESR spectra of the radicals obtained by irradiation of cyclopropane solutions of some aldimines and di-/er -butyl peroxide. On the basis of the low g-values (2.0016) and the jff-hydrogen hyperfine splittings, the authors claimed that imidoyls are cr-radicals with a non-linear arrangement about the N=C-C bond. The facile abstraction of the aldiminic hydrogen, and hence the remarkable stabilization of imidoyl radicals, was explained by the intervention of the mesomeric forms 3a and 3b (Scheme 5), in which the unpaired electron is stabilized by interaction with the lone pair on nitrogen. 

Phenoxyls are generated by the complex-bonded radicals RO"2 [Co(III)] also from bis(3,5-di-tert-butyl-4-hydroxybenzyl)-sulphide CXCVIIa22l The bridge CH2-S— CH2 completely interrupts the mesomerism between both aromatic nuclei in the phenoxyl formed. However, although the atom S is separated from aromatic nuclei by a methylene group, its stabilization effect on the phenoxyls formed still occurs. This followed from the comparison with ESR spectra of phenoxyls from 4,4 -methylenebisphenols. 

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