Radicals capto-dative

Radicals are particularly strongly stabilized when both an electron-attracting and an electron-donating substituent are present at the radical site. This has been called mero-stabilization" or " capto-dative stabilization. This type of stabilization results from mutual reinforcement of the two substituent effects. Scheme 12.3 gives some information on the stability of this type of radical. 

A comparison of the rotational barriers in allylic radicals A-D provides evidence for the stabilizing effect of the capto-dative combination ... 

Scheme 12.3. Radicals with Capto-dative Stabilization...

The reaction rates and product yields of [2+2] cycloadditions are expectedly enhanced by electronic factors that favor radical formation. Olefins with geminal capto-dative substituents are especially efficient partners (equations 33 and 34) because of the synergistic effect of the electron acceptor (capto) with the electron donor (dative) substituents on radical stability [95]... 

The general structure with a cyanine unit at one terminus is represented in Figure 16. Two-electron transfer of the hybrid system produces another cyanine substructure via neutral radical state. In this case, a two step redox reaction is expected, because the neutral radical state is stabilized by the capto-dative substituents effect (19). Therefore, three colored sates will be achieved by the hybrid system. We call this system a cyanine-cyanine hybrid. 

Di(l-azulenyl)(6-azulenyl)methyl cation (24+) represented in Figure 17 exemplifies the cyanine-cyanine hybrid (20). Di(l-azulenyl)methylium unit in 24+ acts as a cyanine terminal group. The tropylium substructure stabilizes the cationic state (24+). Reduction of 24+ should afford the neutral radical 24, which is stabilized by capto-dative substitution effect, because 24 is substituted with azulenes in the donor and acceptor positions. The anionic state (24") is also stabilized by contribution of the cyclopentadienide substructure, which should exhibit the third color change in this system. 

This regioselectivity results from the capto dative effect of intermediate radicals which are stabilized by the presence of one electron-donor and one electron-acceptor substituents [2]. The stereoselectivity of a-bromosubstitution is in agreement with the... 

Finally, it must be noted that in a few cases radical anions have also been observed to be generated from even electron anions One example concerns the CID loss of a methyl radical from the (M — H) ion of methoxyaceto-nitrile (Dawson and Nibbering, 1980) as shown in (80). The capto-dative character (Viehe et al., 1979 Crans et al., 1980) of the generated radical... 

Enthalpy and entropy effects and their interrelationships are discussed. Resonance stabilization of radicals by more than one substituent (including capto-dative substitution) is frequently additive and in no example higher than this. A linear correlation is found between the central C—C-bond lengths in 1 and the strain enthalpies Hs, quite independent of the substituents X and their resonance contribution Hr. 

The close agreement between AHdiss and AH 6 in Fig. 4 allows the conclusion that the dimerization of the radicals 37 is a non-activated process. It has been confirmed independently by direct kinetic experiments76) that AH of recombination for radicals 37 is similar to or smaller than their barrier for diffusion. Capto-dative substituted radicals accordingly have no kinetic stabilization. 

Figure 9. Resonance structures present in protein-bound glycyl radical. A-C represent the so-called capto-dative effect, whereas D-F are resonances due to the backbone of neighbouring amino acids.

In 1991, the syntheses of sialyl C-glycosides were introduced by Bednarski et ah, Paulsen et ah, and Vasella et al. Bednarski and Paulsen exploited a 2-halogeno derivative of Neu5Ac as a precursor of the anomeric radical, which was anticipated to be stabilized by the capto-dative effect of the C1 carboxylate group and ring oxygen. Bednarski reacted the 2-chloro derivative 195 with allyltributyltin and a catalytic amount of bis(tributyltin), and photolized for 18 h to obtain C-glycoside 196 as a 1/1 of anomeric mixture in 65% yield (O Scheme 53) [129]. 

Type III enzymes operate anaerobically, and rather than a2 32 quaternary structure of the Type I enzymes, have an a2 + 32 structure, in which the permanent radical site, presumably stabilised capto-datively, lies on a main chain glycine residue of the dimeric a unit. The site is generated by a separate small iron-sulfur protein ((32 subunit) which brings about a homo-olytic cleavage of 5 -adenosylmethionine, whose fragments generate the glycyl radical. 

When the combinations X,Y and X, Y are of the capto-dative type, as is the case for an alkoxy and ester group, the enthalpy of bond dissociation is 10-15 kcal lower than when all four groups are electron attracting. When the capto-dative combination CN/NR2 occupies both X,Y and X Y positions, the enthalpy for dissociation of the C(3)-C(4) bond is less than lOkcal/mol. Scheme 11.3 gives some information on the stability of other examples of this type of radical. 

One-electron reduction of a-dicarbonyl compounds gives radical anions known as semidiones. Closely related are the one-electron reduction products of aromatic quinones, the semiquinones. Both the semidiones and semiquinones can be protonated to give neutral radicals that are relatively stable. The semidiones and semiquinones belong to the capto-dative class of radicals, having both donor and acceptor substituents. 

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