Electron donating groups mechanism

Condensation of an aniline with a dione with loss of water provides enamine 16. Ketone protonation and cyclization forms 18 followed by loss of water provides quinoline 4. Some have suggested the formation of dication 19 as a requirement to cyclization. Cyclization of 19 to 20 and subsequent conversion to quinoline 4 requires loss of water and acid. Another rendering of the mechanism takes into account participation of an electron-donating group (EDG), which stabilizes intermediate 21. 

Reactivity factors in additions to carbon-hetero multiple bonds are similar to those for the tetrahedral mechanism of nucleophilic substitution. If A and/or B are electron-donating groups, rates are decreased. Electron-attracting substituents increase rates. This means that aldehydes are more reactive than ketones. Aryl groups are somewhat deactivating compared to alkyl, because of resonance that stabilizes the substrate molecule but is lost on going to the intermediate ... 

It is assumed that the mechanism proceeds via activation of the imine by the ruthenium catalyst (structure 169), followed by reaction with ethyl diazoacetate to generate a metal-bound ylide intermediate. Intramolecular ruthenium- assisted attack of the carbanion 170 onto the iminium ion provides the corresponding aziridine with moderate to high // selectivity. Imines bearing electron-donating groups (R2) showed significant rate enhancement. 

The electron-donating groups, if present in the alkene enhance the rate of the reaction. This is why the reaction is particularly rapid with tetraalkyl olefines. The electron-withdrawing groups in the peracids also increases the reaction rate. The following suggested mechanisms has been accepted for the conversions. 

From the above mechanism, it becomes obvious that the presence of electron withdrawing groups on the ring would accelerate the reaction, while electron-donating groups would retard it. The former give 1, 4-dihydrobenzenes, while the latter give 2, 5-dihydrobenzenes. 

The neutral and acid-catalysed mechanisms of hydrolysis of formamide, HCONH2, have been revisited and a comparison made between ab initio, semiempirical and DFT results Ab initio MO calculations on the alkaline hydrolysis of para-substituted acetanilides (135) in the gas phase have shown that the activation energy depends on the nature of electron-withdrawing groups (e.g. X = NO2, CN, Cl) but is invariant for electron-donating groups (X = NH2, OMe). ... 

Apparently, Eq. (29) represents a polar nonradical addition. If a two-step mechanism is conceived, intermediates of the type [XB=NRH] will be reasonable, though such cations proved to be rather unstable as isolated species (unless X represents a x-electron donating group) (33). Intermediates of the type HY—B(X)=NR would explain the fast reaction with protic bases of vanishing Bronsted acidity. The results, however, mentioned in Sections V, A, and V, C, favor to some extent the picture of iminoboranes as preferring electrophilic to nucleophilic attack. The high activity of amines can also be rationalized in terms of a concerted process, with a transition state of type VI. 

The cw effect , observed also in trisubstituted enol ethers , may as well be rationalized by a similar mechanism (Scheme 12). In this case, the electron-donating group that stabilizes the partially charged double bond carbon of the perepoxide in the syn transition state is the alkoxy moiety. Therefore, the favourable interactions of the singlet oxygen with the phenyl or alkoxy substituents direct the orientation of the perepoxide intermediate. 

Substituted olefins that are capable of forming secondary or tertiary carbo-nium ion intermediates polymerize well by cationic initiation, but are polymerized with difficulty or not at all free radically. In general, vinyl or /-alkenes that contain electron donating groups (alkyl, ether, etc) polymerize well via a carbo-cationic mechanism. 

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