Electron-donating and -withdrawing

The Camps reaction has been used to prepare a variety of anti-ulcer agents of the type 23. As can be seen from the yields, the reaction works equally well with electron donating and withdrawing aromatic rings. ... 

Fig. 17a, b Substituent effects on the cyclic orbital interactions the (a) p-o -q and (b) p-o -q interactions are strengthened (shown by bold lines) by the electron-donating and -withdrawing substituents, respectively... 

Although the reaction mechanism of this type of reactions is not fully elucidated, it is easily anticipated that no intramolecular special stabilization effect for the carbanion generated from decarboxylation is expected, different from the case of malonic acid-type compounds. Moreover, cinnamic acid derivatives that have both the electron-donating and withdrawing substituents have been reported to undergo this reaction. This fact suggests that the enzyme itself stabilizes the transition state without the aid of mesomeric and inductive effects of the other part of the substrate molecule itself. If such unknown mechanism also works for other... 

This homoenolate methodology has been extended to the use of nitrones 170 as electrophiles [72]. Scheldt and co-workers have shown that enantiomerically enriched y-amino esters 172 can be prepared with excellent levels of stereocontrol from an enal 27 and a nitrone 170 using the NHC derived from triazolium salt 164 (Scheme 12.37). The oxazinone product 171, formally a result of a [3-1-3] cycloaddition, is cleaved to afford the y-amino ester product 172. The reaction shows broad substrate scope, as a range of substituted aryl nitrones containing electron donating and withdrawing substituents are tolerated, while the enal component is tolerant of both alkyl and aryl substituents. 

Electronic effects seem to have very little influence on the overall enantioselectivity with both electron-donating and- withdrawing groups giving very small changes to the enantioselectivity when all other variables were the same. Substrate geometry seems to play a much more influential role with this particular class of substrates. 

Various types of aromatic aldehydes with electron donating and withdrawing groups were cleanly and rapidly condensed with hydroxylamine hydrochloride at 90 °C, giving the corresponding -isomer of oximes (with OH anti to aryl) in excellent yields in the presence of CUSO4. However, aromatic ketones such as benzophenone did not afford the corresponding oximes under these conditions. 

Table 4.9 Effect of Electron-Donating and -Withdrawing Substituents on the Rate of Displacement Reactions on Carbon and Sulfur...

The discussion above has pointed out some of the relationships between structure and concertedness in the decompositions of peroxy compounds and azo alkanes. Other structural variations have been investigated also, such as the effect of electron-donating and -withdrawing substituents and of geometry about carbon atoms at which radical character is developing. 

Because radicals are uncharged, one would expect that electron donation and withdrawal would be less important than in ionic reactions. Although this expectation is certainly correct, there is nevertheless ample evidence that transition states of radical reactions do have some polar character. In resonance terminology, one might describe the transition state of a homolysis as a resonance hybrid (25) of covalent, radical, and ionic structures, with importance of the ionic forms subject to influence by substituents in A and B. It should therefore... 

Electron-donating and -withdrawing substituents can polarize the conjugated system in the Nazarov Reaction, which facilitates the cyclization and gives better regioselectivity ... 

Complex (2) is an effective catalyst for the asymmetric hydroxylation of aromatic hydrocarbons with 2,6-dichloropyridine IV-oxide as terminal oxidant. Up to 76% ee was achieved for the catalytic hydroxylation of 4-ethyltoluene, 1,1-diethylindane, and benzylcyclopropane. Both electron-donating and -withdrawing substituents were found to accelerate the catalytic oxidation reaction. A large primary kinetic isotope effect (kH/kD = 11 at 298 K) was observed for the catalytic ethylbenzene-dio oxidation. A... 

It is now evident why the formation of the alkylvanadates is insensitive to the pKa of the parent alcohol. The electron density about vanadate is close to optimal, so small changes arising from variation in the ability of the ligand to donate or withdraw electrons are relatively unimportant components of complexation of the alcohols. It would be expected, and is also observed, that both resonance and inductive effects are small for the reaction of vanadate with phenols [4], Considering that the pKa values of the various substituted phenols are considerably lower than 15, an approximately linear relationship (Figure 9.3) between the pKa of the aryl-vanadate and the pKa of the ligand would be expected and can be predicted from the observations of alkylvanadates. There are only minor differences that arise because of resonance electron donation and withdrawal. 

Therefore, the reactivity and the regioselectivity of Ar-SE reactions with substituted benzenes can be predicted reliably. According to what has been stated above, one only has to identify the electron donating and withdrawing substituents in the substrate. The electronic effects of the most important functional groups are listed in Table 5.3, where they are ordered semiquantitatively the best electron donors are on top and the best... 

It is known that nonlinear optical activity is often associated with the presence of electron-donating and - withdrawing groups linked by a conjugated bridge,82,84,88 e.g. 4.89... 

How electron donation and withdrawal change chemical shifts... 

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