Groups, electron-donating and

It is thus obvious that the 2-thienyl group is both a better electron donator and electron acceptor than the phenyl group, facilitating both nucleophilic and prototropic reactions at the a-methylene carbon. 

The equilibrium situation for simple substituted 2-ulkenyl alkali metal derivatives can be estimated by a rule of thumb electron-accepting and electropositive substituents ( ) prefer the exo position, but electron-donating and electronegative substituents ( ), including alkyl groups, tend to occupy the endo position. With increasing steric demand of the substituent, the exoisomer becomes more favored. 

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 ... 

All substituents increase reactivity at ortho and para positions over that of benzene. There is no great difference between electron-donating and electron-withdrawing groups. 

Reference has already been made to electron-donating and electron-withdrawing groups, their effect being to render a site in a molecule electron-rich or electron-deficient, respectively. This will clearly influence the type of reagent with which the compound will most readily react. An electron-rich species such as phenoxide anion (36)... 

The Alper group [163] reported on a highly efficient double carbohydroamination for the preparation of a-amino carboxylic acid amides 6/1-345, starting from aryl iodides and a primary amine 6/1-344, in usually high yield (Scheme 6/1.88) both, aryl iodides with electron-donating and electron-withdrawing groups can be used. 

In cases where both types of compounds are present in significant amount, the use of a mixture of solvents, nucleophilic and electrophilic, was suggested. In such cases, an amphiprotic solvent could also be suitable (an amphiprotic solvent contains simultaneously an electron donating and electron accepting groups in a single molecule), for instance ethanolamine. 

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