Radicals captodative

A semiempirical procedure based on isodesmic reactions is used to determine unknown heats of formation. The choice of the isodesmic reaction is critical for the validity of the approach. By definition, all the bonds are conserved in number and nature in this type of process. Also, the bond lengths of equivalent bonds in reactants and products should be as similar as possible. The stabihzation energies are based on the type of isodesmic reaction which is given for the influence of amino- and cyano-groups in a captodative radical in equation (3). Only radicals appear as reactants and... 

A special property of captodative radicals can neither be recognized in Leroy s nor in Pasto s work. The substituents may show additivity, less than additivity or, even, more than additivity in the calculated stabilization. If we combine the results in both schemes it has to be concluded that captodative radicals are not a distinguishable class of radicals. They should not obey different rules from radicals stabilized by non-captodative substituents. Radicals can be stabilized by all kinds of substituents, although to a different... 

In order to find out whether captodative substitution of a methyl radical can lead to persistency, the rate of disappearance by bimolecular selfreaction was measured for typical sterically unhindered captodative radicals (Korth et al., 1983). The t-butoxy(cyano)methyl radical, t-butylthio(cyano)-methyl radical and methoxy(methoxycarbonyl)methyl radical have rate constants for bimolecular self-reactions between 1.0 x 10 and 1.5 X 10 1 mol s Mn the temperature range —60 to - -60°C. The dilTusion-controlled nature of these dimerizations is supported by the Arrhenius activation parameters. Thus, it has to be concluded that there is no kinetic stabilization for captodative-substituted methyl radicals. On the other hand, if captodative-substituted radicals are encountered which are kinetically stabilized (persistent) or which exist in equilibrium with their dimers, then other influences than the captodative substitution pattern alone must be added to account for this phenomenon. 

It is evident from the data in Table 6 that, with only one exception (entry 13), the combination of two captor or two donor substituents does not produce an additive effect, whereas, without exception, the captodative combinations display synergetic behaviour. Thus, the delocalization of the unpaired spin density in captodative radicals is markedly increased in comparison to pure additive superposition of capto and dative effects. This result is all the more significant since two identical substituents do not... 

Cyclic carbohydrates bearing a cyano group at the anomeric center have, at this position, captodative radical-stabilizing groups similar to those at C-5 of the hexopyranuronic add compounds (see Section 11,2). In consequence,... 

Because the aromatic rings of compounds of this class render the anomeric centers benzylic, and also conceivably subject to captodative radical stabilization, photobromination results might be expected to correlate with those observed for the glycosyl cyanides (see Section 11,6). 

This factor is shown to be very important in several of the examples given in Section II the compounds described in Section 11,2 showing enhanced activity at C-5, those in 11,6 enhanced activity at C-l, and those in 11,5, enhanced activity at C-l or C-5, according to whether the carbonyl or oximo groups are at C-2 or C-4 of pyranoid derivatives. Captodative radical stabili-... 

The radical polymerization behavior of captodative olefins such as acrylonitriles, acrylates, and acrylamides a-substituted by an electron-donating substituent is reviewed, including the initiated and spontaneous radical homo- and copolymerizations and the radical polymerizations in the presence of Lewis acids. The formation of low-molecular weight products under some experimental conditions is also reviewed. The reactivity of these olefins is analyzed in the context of the captodative theory. In spite of the unusual stabilization of the captodative radical, the reactivity pattern of these olefins in polymerization does not differ significantly from the pattern observed for other 1,1-disubstituted olefins. Classical explanations such as steric effects and aggregation of monomers are sufficient to rationalize the observations described in the literature. The spontaneous polymerization of acrylates a-substituted by an ether, a thioether, or an acylamido group can be rationalized by the Bond-Forming Initiation theory. 

All the above experiments were run using equimolar amounts of radicals and illustrate the extreme stabilization found in captodative radicals. This led Viehe and coworkers to conclude that the stabilized captodatively substituted radicals. .. do not undergo typical reactions such as polymerization or hydrogen abstraction but rather they trap another radical R or dimerize [2]. 

Another noticeable characteristic of captodative olefins is the influence of the reaction medium. The stabilizing effect of solvent on the persistency of a captodatively radical has been reported experimentally for the bond homolysis of bis(3,5,5-trimethyl-2-oxomorpholin-3-yl) [111], but was not found for the 2,3-diphenyl-2,3-dimethoxysuccinonitrile homolysis [112]. Theoretically the solvent-assisted stabilization las been predicted for the captodative substituted nitriles in solvent with large dielectric constants [113-114], Table 16 illustrates the solvent effect on the spontaneous thermal polymerizations [115]. The polymer yields are... 

These copolymerization parameters are only slightly influenced by the solvent used (Table 18) [116], suggesting a small solvent effect on the propagation reaction. The reactivity of methyl a-methoxyacrylate towards a polystyryl radical (l/r2) however tends to increase with increasing Ex value or dielectric constant of the solvent. Here again it appears that increased solvent polarity leads to an increased persistency of the captodative radical. 

Structure of Lewis Acid-Coordinated Captodative Radicals... 

The stabilization of benzhydryl 31 and triphenylmethyl 2 is less than additive, as expected for the non-planar propeller-like structures of these radicals, which do not allow the development of full conjugation. The angle of twist is probably very similar in benzhydryl and trityl radicals 73) and one is tempted to attribute to each twisted phenyl an additive stabilization of 6 kcal mol"1. On the other hand two cyano groups in 32 likewise stabilize a radical less than additively. Phenyl and cyano (33) and phenyl and methoxy (34) show additive stabilization. For one cyclopropyl group in 35 a little more than 1 kcal mol-1 stabilization can be counted and additivity follows consequently for 36. The captodative radical 37 is stabilized according to additivity... 

Some mechanistic aspects of the above cascade reaction deserve comment. Thus, after the intermolecular addition of the nucleophilic acyl radical to the alkene, the electrophilic radical adduct A, instead of undergoing reduction, reacts intramolecularly at the indole 3-position (formally a 5-endo cyclization) to give a new stabilized captodative radical B, which is oxidized to the fully aromatic system. (For a discussion of this oxidative step, see Section 1.5.)... 

Both electron-donating and electron-withdrawing effects stabilize radicals, as you have just seen. Some radicals are stabilized by an electron-withdrawing group and an electron-donating group at the same time. These radicals are known as captodative radicals. 

Gaudiano, G., Koch, T. H. Redox chemistry of anthracychne antitumor drugs and use of captodative radicals as tools for its elucidation and control. Chem. Res. Toxicol. 1991, 4, 2-16. 

The ionic stability of phenyl selenides can also be advantageous in the choice of addition reagents. Reagents serving as precursors to heteroatom-stabilized radicals are more accessible because of the poorer leaving group ability of the phenylseleno substituent, as shown in Scheme 18. Phenylseleno precursors to captodative radicals have been shown to be ambiphilic in nature, with successful additions to electron-rich as well as electron-deficient olefins [54], The stable precursor to a highly nucleophilic radical, 2-phenylseleno-l,3-dithiane has been shown to add to electron-deficient olefins [55]. 

It is this fact which leads to the captodative effect, that is the stabilization of captodative radicals 1, bearing simultaneous captor and donor substituents. In turn, this means that compounds such as 2 and 3 tend to react by radical pathways [3], via radical abstraction (C-X homolysis of 2) or radical addition (to the n system of 3). 

Dehydrodimers 58 can be obtained essentially quantitatively from captodative methylene compounds 56 upon oxidation (Scheme 10). For example, methyl a-methoxyacetate (c = C02Me, d = OMe) leads to the dimer of the captodative radical 57 in 91% yield upon reaction with -butyl peroxide [38]. This reaction can be extended, under certain conditions, to yield polymers with interesting properties... 

Indigo (62) is made from indoxyl (60) upon treatment with O2 in base, via the captodative radical 61 [40]. 

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