Electron donating groups substituent effects

Substituent effects observed for this reaction are entirely consistent with those described for electrophilic substitution and addition —only reversed. That is, the reactivity of an arene in metal reductions is increased by electron-withdrawing groups and decreased by electron-donating groups. Substituents that can stabilize the anion-radical intermediate facilitate the reduction (see Exercise 22-35). 

When the double bond is substituted with alkyl groups or other electron donating groups, the effect of the substituents on the regiochemistry leads to the trend known as Markovnikov addition. Markovnikov addition predicts the dominant product to arise from nucleophilic addition to the more substituted carbon. Note that nucleophilic addition to the more substituted carbon forms the more sterically crowded and thus less stable product (Eq. 10.7). Therefore, Markovnikov addition predicts the kinetic product, not the thermodynamic product. 

A tertiary carbonium ion is more stable than a secondary carbonium ion, which is in turn more stable than a primary carbonium ion. Therefore, the alkylation of ben2ene with isobutylene is much easier than is alkylation with ethylene. The reactivity of substituted aromatics for electrophilic substitution is affected by the inductive and resonance effects of a substituent. An electron-donating group, such as the hydroxyl and methyl groups, activates the alkylation and an electron-withdrawing group, such as chloride, deactivates it. 

Styrenes may act as 2n and 4n components of the Diels-Alder reaction depending on the substitution site and the electronic effects of the substituent. Electron-donating groups at the a-carbon of the olefinic double bond enhance the dienic reactivity of styrenes [30]. 

More recent B-NMR studies at room temperature have shown that (acyloxy)diethylboranes are monomeric also in non-polar solvents like chloroform [(5( B) - 60 ppm] [46]. Under the same conditions the corresponding 9-BBN derivatives are monomeric (both types I and II) or dimeric, depending on the electron-donating or -withdrawing effect of the substituent on the carboxyl group, the temperature, and the concentration of the... 

As might be expected, substituents in the aromatic nucleus have a marked effect on the stability of ArN2 , electron-donating groups having a marked stabilising effect ... 

An extended form of (39) has been used to analyse kinetic substituent effects on bromination of alkenes GRaC=CR()R) , where G is a conjugatively electron-donating group and R is alkyl (Bienvenue-Goetz and Dubois, 1981). 

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