Capto-dative substituents

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. 

Change of configuration around the C(l)-C(2) or the C(l)-C(3) bond does not necessarily afford a ring-opened species as an intermediate. Conversion of 523 into 525 occurred via iminium ion (524), which is attacked by the nucleophile from the more hindered position forming the thermodynamically more stable product (equation 134) For endo-exo isomerization, see equation 91). Isomerization of 523 (R = Ph, R = H, X = CN) was discussed in terms of a diradical intermediate stabilized by the capto-dative substituent combination... 

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. 

Push-pull or capto-dative ethylenes can be represented by the general formula 1 where A and/or B represent electron-donating groups and X and/or Y represent electron-accepting groups. Other substituents may be aryl or alkyl groups. The conformational properties of such compounds have been the subject of much... 

The apparent suppression of deprotonation at the a-carbon may be due its capto-dative nature. This carbon atom is bonded to both an acceptor (C=0) and a donor (Hal), the corresponding carbanion is destabilized by the donor substituent. In connection with this tendency is the selective silylation [215] of l,3-oxathian-3,3-dioxide at C-4. 

The presence of capto-dative geminal substituents as in 24 facilitates the cyclodimerization process. A similar observation was made when a ji-conjugating group was substituted at the methylene carbon such as in 26.14... 

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... 

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. 

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. 

As radical initiator. with allyltributyltin appar capto-dative radical inten agent and radical initiator Aza-Claisen rearrma dimethoxyanilines to the substituent to direct the ii... 

A third method would be to to use a push-pull substituent effect [411 or the capto-dative or merostabilization method. An example of this is Kosower s radical [42] (l-ethyl-4-carbomethoxypyridinyl). This method has not yet been used to form organic metals. 

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