Electron and -withdrawing substituents

For diaroy I peroxides (36, R ar 11. m- and / -electron withdrawing substituents retard the rate of decomposition while m- and/ -clcctron donating and all o-substituents enhance decomposition rates. The o-substiluent effect has been attributed to the sensitivity of homolysis to steric factors. 

In another aspect of the mechanism, the effects of electron-donating and electron-withdrawing substituents (p. 1065) indicate that the diene is behaving as a nucleophile and the dienophile as an electrophile. However, this can be reversed. Perchlorocyclopentadiene reacts better with cyclopentene than with maleic anhydride and not at all with tetracyanoethylene, though the latter is normally the most reactive dienophile known. It is apparent, then, that this diene is the electrophile in its Diels-Alder reactions. Reactions of this type are said to proceed with inverse electron demand ... 

In the reaction with butanone, an equilibrium between the CH3-activated complex and the CH2-activated complex is observed and it is revealed that the former is a thermodynamic product and the latter is a kinetic product. These results indicate that the relative reactivity of the C-H bonds is in the order of 2>1>3, and the large and electron-withdrawing substituents retard the reaction. A plausible mechanism is shown in Scheme 63. When the oxy-... 

Many radical reactions do show the expected small and non-specific response to substituents. Reaction 14 of Table XIV is an example it has a value of p not significantly different from zero and shows almost a random response to the polar nature of the substituent.438 The dissociation of hexaphenylethanes is obscured by experimental uncertainties but seems to be increased by both electron-releasing and electron-withdrawing substituents. 

It is clear that these solvolytic reactions of ipso-adducts are markedly dependent on reaction conditions strongly acidic conditions and electron withdrawing substituents will favour a heterolytic process lower acidity, higher temperatures and electron releasing substituents probably favour homolytic processes. With so many factors involved, each substrate under each set of conditions gives rise to its own particular behaviour. 

The processes depicted in Equation 5 all involve bond formation, while the reaction in Equation 6 requires cleavage of a carbon-manganese bond and, consequently, it is this step that is most likely to be rate limiting. Since the Hammett plots show that the reaction is accelerated by both electron donating and electron withdrawing substituents, it appears that the reaction can take place via tv/o different transition states, one electron rich and the other electron poor. 

A -Protonated 2-azabuta-l,3-diene undergoes 4+- -2-cycloaddition with alkenes activated by electron-donating and electron-withdrawing substituents.The 2+ - - 4-cycloadditions of l,3-dithian-2-ylium ions with buta-1,3-dienes follow second-order kinetics which do not exclude a mechanism involving a concerted pathway. The... 

Several reports deal with studies of rearrangement rates related to various electron-donating and electron-withdrawing substituents on the ring carbons and/or ring nitrogen in l,2-dihydro-2-imino-l-methylpyrimidine. 

With the arenesulfonyltriazoles (26), - polar, protic solvents, and electron-withdrawing substituents in the aryl group tend to favor the... 

An understanding of electron-donating and electron-withdrawing substituent effects is crucial to designing the synthesis of aromatic derivatives. For... 

A series of symmetrical and unsymmetrical derivatives containing electron-donating and electron-withdrawing substituents at position 6 of... 

Amide substrate scope and limitations were investigated using allyl alcohol 6a and 2-5 mol% of Bi(OTf)3 and KPF6 (Table 9). Using sulfonamides with electron-donating and electron-withdrawing substituents, the reaction completed within... 

Their yields are (1) higher in polar and lower in nonpolar solvents (2) increased by the presence of electron donating substituents in the phenolic nucleus and electron withdrawing substituents on the acyl moiety and (3) independent of the solvent viscosity. Their formation is possible even if the orf/io-position of the original aryl ester is substituted with — 1, + M substituents (e.g., chlorine, methoxy). They are extraordinarily stable to further photochemical reactions. 

This may be the formation of the ion-pair (281) which in non-polar solvents forms the second cr-bond to give the cycloadduct (280), but in polar solvents forms (282) as shown in Scheme 76. However, the possibility cannot be ruled out that in non-polar solvents the cycloaddition is concerted although formally forbidden, the activation energy for this may be lowered because the reacting 77-bonds have electron-donating and electron-withdrawing substituents attached to them. When the reactant is dicyanoacetylene, the thienopyrrolizine is formed even in dichloromethane. 

A wide variety of chromones has been prepared by this method since its introduction (01CB2475), and the literature abounds with examples. A fully detailed preparation of 3-ethylchromone is available (5SOSC(3)387). The presence of substituents in the aromatic ring of the acetophenone has minimal effect on the course of the reaction both electronreleasing and electron-withdrawing substituents are compatible with the synthesis. 

Chemical shifts of aromatic compounds occur between 120 and 150 ppm. On inclusion of electron releasing and electron withdrawing substituents as well as multiple substitution, this shift range may expand considerably (90-185 ppm). 

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