Captodative

There have heen many studies on the polymerizability of a-substituted acrylic monomers.3jU35 33S It is established that the ceiling temperature for a-alkoxyacrylates decreases with the size of the alkoxy group. 35 However, it is of interest that polymerizations of a-(alkoxymethyl)acrylates (67)3 15 and a-(acyloxymethyl)acrylates (68)and captodative substituted monomers (69, 70) 39 appear to have much higher ceiling temperatures than the corresponding a-alkylacrylates methyl ethacrylate, MEA). For example, methyl a-... 

Conversely, there are compounds in which nearly free rotation is possible around what are formally C=C double bonds. These compounds, called push-pull or captodative ethylenes, have two electron-withdrawing groups on one carbon and two electron-donating groups on the other (66). The contribution of di-ionic... 

It has been postulated that the stability of free radicals is enhanced by the presence at the radical center of both an electron-donating and an electron-with-drawing group.This is called the push-pull or captodative effect (see also pp. 159). The effect arises from increased resonance, for example ... 

There is some evidence in favor ° of the captodative effect, some of it is from ESR studies. However, there is also experimental and theoretical evidence against it. There is evidence that while FCH2 and p2CH are more stable than CH3, the radical Cp3- is less stable that is, the presence of the third F destabilizes the radical. " Certain radicals with the unpaired electron not on a carbon are akso very stable. Diphenylpicrylhydrazyl is a solid that can be kept for years. We have already mentioned nitroxide radicals. Compound 29 is a nitroxide radical so stable that reactions can be performed on it without affecting the unpaired electron (the same is true for some of the chlorinated triarylmethyl radicals mentioned above ). ot-Trichloromethylbenzyl(rer/-butyl)aminoxyl (30) is extremely stable. In... 

An interesting alcoholysis of epoxides has been reported by Masaki and coworkers <96BCSJ195>, who examined the behavior of epoxides in the presence of a catalytic amount of the Tt-acid tetracyanoethylene (TCNE, 85) in alcoholic media. Ring-opening is very facile under these conditions, typically proceeding via normal C-2 attack, as exemplified by styrene oxide (86). Certain epoxy ethers (e.g., 89) undergo C-1 attack due to anchimeric assistance. Analysis of the reaction mixtures revealed the presence of captodative ethylenes (e.g., 85) formed in situ, whieh were shown to be aetive in eatalyzing the reaction. The proposed mode of catalysis is represented by the intermediate 87. The affinity of these captodative olefins for... 

The electrophilic character of sulfur dioxide does not only enable addition to reactive nucleophiles, but also to electrons forming sulfur dioxide radical anions which possess the requirements of a captodative" stabilization (equation 83). This electron transfer occurs electrochemically or chemically under Leuckart-Wallach conditions (formic acid/tertiary amine - , by reduction of sulfur dioxide with l-benzyl-1,4-dihydronicotinamide or with Rongalite The radical anion behaves as an efficient nucleophile and affords the generation of sulfones with alkyl halides " and Michael-acceptor olefins (equations 84 and 85). 

One point of debate in defining the magnitude of the captodative effect has been the separation of substituent effects on the radical itself as compared to that on the closed shell reference system. This is, as stated before, a general problem for all definitions of radical stability based on isodesmic reactions such as Eq. 1 [7,74,76], but becomes particularly important in multiply substituted cases. This problem can be approached either through estimating the substituent effects for the closed shell parents separately [77,78], or through the use of isodesmic reactions such as Eq. 5, in which only open shell species are present ... 

The reaction energy for Eq. 5 amounts to -54.7 kj/mol at the ROMP2 level, indicating a substantial captodative stabilization for glycyl radical (9) even with this definition. 

With l,2-di(phenylthio)benzene (24), two successive two-electron steps [207] can be observed in aprotic DMF [E = —1.82 V and 2 =—2.11 V vs Ag/AgI 0.1 M / reference system). On the other hand, substitution by efficient electron-withdrawing groups might change dramatically the cathodic behavior of ArSR-type compounds. Apparently, the captodative character of substituents attached to the phenyl ring (see Structure 25) strongly... 

Effects of single, isolated substituents on physical properties, reactions, or equilibria can be treated successfully on the basis of dual substituent parameter (DSP) approaches. To deal with di- or poly-substituted systems is less straightforward. Here the question arises of how individual substituents combine their respective actions. It is in this context that the captodative substituent effect has to be seen. 

The concept of captodative substitution implies the simultaneous action of a captor (acceptor) and a donor substituent on a molecule. Furthermore, in the definition of Viehe et al. (1979), which was given for free radicals, both substituents are bonded to the same or to two vinylogous carbon atoms, i.e. 1,1- and 1,3-substitution, and so forth is considered. One might, however, also include 1,2-, 1,4-,. .. disubstitution, a situation which is more often referred to as push-pull substitution. Before discussing captodative substituent effects it might be helpful to analyse the terms capto and dative in more detail. 

The o-scales in Table 1 were derived for molecules with a closed shell of electrons and not for free radicals. However, the concept of captodative... 

Mutual conjugation of, as we describe it nowadays, + M and — M substituents is equivalent to an extra stabilization of the system. Thus, we can interpret this statement as the first formulation of the captodative effect even though the term was coined much later. The difficulty of organic chemists to comprehend the rather mathematically formulated theorem must have hindered its wider recognition and seems to be the reason that the phenomenon of interaction of -l-M and — M substituents has been reinvented several times since then. It is remarkable that these rediscoveries were always initiated by experimental studies in free radical chemistry. 

The familiarity with qualitative valence bond descriptions of substituent effects in combination with the known substituent effects in carbocations and carbanions led Viehe and his group to the postulate of a captodative effect for free radicals (Stella et ai, 1978 Viehe et al., 1979). They did not seem to be aware of the earlier work which was of a more physical organic character. The fact that carbocations [8] are stabilized by + M substituents, and carbanions [9] by - M substituents, raised the idea that free radicals, as... 

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