Push-pull substituent effect

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. 

The efficiency of the E/Z photoreaction - and especially the speed of the thermal Z/E reverse reaction - is strongly dependent upon the electronic and steric influence of the substituents on the arene units. 7r-Donors in para position accelerate it 7r-acceplors inhibit it push-pull substituents are particularly effective in lowering the barrier. 

The low C=C barriers in push-pull ethylenes compared to the 6S.S kcal/ mol in ethylene show that die effects of delocalization on the tr-electron energy in the transition state must be much greater than the effects in the ground state— that is, the important substituent effects on the barriers must occur in the transition state. Besides, an effect that improves delocalization in the ground state would be barrier raising, if it were not accompanied by an at least equal stabilization of the transition state. 

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. 

Bis [(trifluoromethyl)thio] acetaldehyde (83a) has been prepared from an enam-ine precursor (84), although refluxing in aqueous ethanolic HCl is required to effect this reaction.The aldehyde is less stable than its enol tautomer (83b), and many reactions typical of aldehydes fail. For example, addition of aqueous silver nitrate immediately yields the silver salt of (83b), rather than giving precipitation of (elemental) silver. The (trifluoromethyl)thio substituent has pseudohalogenic character and, together with the hydroxy group, stabilizes the alkene tautomer in the manner of a push-pull alkene. The enol-aldehyde equilibrium mixture in acetonitrile shows an apparent of 2.6 when titrated with aqueous hydroxide. 

In the NMR spectra, a push-pull effect of octahydropyrido[2,3- pyrimidine 28 was observed in the olefinic C-4a and G-8a due to the electronic behavior of their substituents. Thus, C a appears at low 6 values (88-93 ppm) and C-8a gives peaks at lower field (146.0-148.5 ppm) <1996JHC45>. 

Important classes of enamines are those having electron-withdrawing substituents, R2 and/or R3, at C(2), particularly when the substituent(s) act(s) by a +R effect, as in aminoenones and nitroenamines. The stronger and more extensive electron delocalization and the concomitant changes in bond orders of the resulting push-pull systems is well reflected in the NMR spectra and have been extensively investigated in this way. 

Proton NMR studies of 2-thiopheneamines bearing only an electron-withdrawing substituent in the ortAo-position, that is, the ester 261, indicate the existence of the imino form <2001M279, 2006T11311, 2006MI139>. Due to the push-pull effect, the imino form 262 can be generated (Scheme 24). 

Push, pull refer, respectively, to electron-donating or electron-withdrawing effects of substituents on the arsenic or carbon atoms). 

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