Dative substituent

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. 

More recently, reductive elimination of aryl ethers has been reported from complexes that lack the activating substituent on the palladium-bound aryl group (Equation (55)). These complexes contain sterically hindered phosphine ligands, and these results demonstrate how steric effects of the dative ligand can overcome the electronic constraints of the reaction.112,113 Reductive elimination of oxygen heterocycles upon oxidation of nickel oxametallacycles has also been reported, but yields of the organic product were lower than they were for oxidatively induced reductive eliminations of alkylamines from nickel(II) mentioned above 215-217... 

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... 

La Mar (150) and Walker (156) have found a thermodynamic cis effect in the formation of hemichrome salts [Fe(TRP)L2]Cl (->-[27]) according to equilibrium (77) which was studied by 1H-NMR and optical spectroscopy for L = l-Melm (Table 22). As the electron-donating power of the para-phenyl-substituent of the porphyrin increases, the total formation constant, /J2, increases. This is because the product of the reaction contains a positively charged center which is stabilized by electron-donating groups. As a Hammet relation exists, the mesomeric part of the electronic transmission is also operative, and hence dative porphyrin-to-metal tr-bonding seems to be involved. 

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. 

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... 

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... 

Raman spectra of [V(acac)3] suggest that the contributions of dative n bonding should be small.246 In IR spectra, the shifts of v(M—O) bands were related to the electronic effects of the substituents.247... 

Nuclear magnetic resonance (NMR) spectroscopic data have been reported by Takeda et al.32 for silylene-isocyanide complexes 28-30, which are stable at room temperature due to the bulky substituents. The Aic coupling constants of 38.6 Hz (28), 22.1 Hz (29), and 1.0 Hz (30) are considerably smaller than that of a Si-C single bond and indicate a weak Si-C interaction as expected for a dative bond. 

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... 

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