Substituents, electron withdrawing Inductive effects from

With substituents such as OH and OMe that have unshared electron pairs, an electron-donating, i.e. base-strengthening, mesomeric effect can be exerted from the o- and p-, but not from the m-position, with the result that the p-substituted aniline is a stronger base than the corresponding w-compound. The m-compound is a weaker base than aniline itself, due to the electron-withdrawing inductive effect exerted by the oxygen atom in each case. As so often, the effect of the o-substituent remains somewhat anomalous, due to direct interaction with the NH2 group by both steric and polar effects. The substituted anilines are found to have related pAa values as follows ... 

Italian workers have reported extensive studies on the evaluation of a values for thienyl and furyl groups. These are collected in Table XII. The am, Om, and am values reveal the electron-withdrawing inductive effect of the rings, while as expected from the previous discussion of this section, these substituents fall into that small category whose op, oj>, and Op values are all different, revealing an ability to both accept and donate electrons by resonance under the appropriate incentives. 

The calculated pA B values depend on the substituents as expected from the electronic theories. The 3-iodo substituent behaves similarly to other 3-halo substituents 3-F (1.51), 3-Br (1.40) and 3-Cl (1.38). The CF3 substituent decreases the basicity through the same electron-withdrawing inductive effect as the COOMe, COMe, CN and NO2 substituents. The pATbij values follow the order of the <7 f field-inductive substituent constant as shown by Equations 7.49 and 7.50 and as illustrated in Figure 7.15. 

Fig. 12.23. A Mukaiyama aldol addition (-> C) and its reaction mechanism (bottom row). As shown here, this method can be exploited to obtain the poly-unsaturated aldehyde D. Under the conditions of the first reaction step the primary product C—which, like the substrate A, is an acetal—does not compete with A for still unconsumed enol ether B. This is due to the fact that the methoxy substituent in the oxocarbenium ion G, which would have to be regenerated from Cin order to undergo further reaction with B, destabilizes G because of its electron-withdrawing inductive (-1) effect.

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