Capto-dative effect

The capto-dative effect has also been demonstrated by studying the bond dissociation process in a series of 1,5-dienes substituted at C-1, C-3, C-4, and C-6. 

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... 

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]. 

Asymmetric syntheses of heterocycles 82AG490 81RTC393 78S329. Capto-dative effects in synthesis of heterocycles 79AG982. 

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 ... 

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. 

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... 

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. 

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