Electrophiles electron-donating group

Substituent effects on this kind of concerted, stereospecific epoxidation indicate that the peroxy-acid functions as an electrophile electron-donating groups activate the olefin vhile electron-withdrawing groups strongly deactivate it. When the electron flow at the transition state is reversed by using electron-poor olefins and peroxy anions for epoxidation, the reaction is stepwise and stereo-equilibrating and both cis- and trans-olefins (e.g., benzalacetophenone) yield trans-epoxide. (13. 14)... 

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. 

The 3-, 4-, 5- and 6-positions in the pyridazine nucleus are electron deficient due to the negative mesomeric effect of the nitrogen atoms. Therefore, electrophilic substitution in pyridazines is difficult even in the presence of one or two electron-donating groups. The first reported example is nitration of 4-amino-3,6-dimethoxypyridazine to yield the corresponding 5-nitro derivative. Nitration of 3-methoxy-5-methylpyridazine gives the 6-nitro-,... 

Activating group (Section 16.4) An electron-donating group such as hydroxyl (-OH) or amino (— NH2) that increases the reactivity of an aromatic ring toward electrophilic aromatic substitution. 

Electron-donating groups, effect on thiazolium ring cleavage, 33 Electrophilic reaction, of thiazolium salts, in basic medium, 34 Energetic transfer yield, of thiazoiocyanines, 78... 

The frontier orbital theory was developed for electrophilic aromatic substitution (Chapter Elements of a Chemical Orbital Theory by Inagaki in this volume). Application is successful to the ortho-para orientation (Scheme 23a) for the benzenes substituted with electron donating groups. The ortho and para positions have larger HOMO amplitudes. The meta orientation (Scheme 23b) for the electron accepting groups is under control of both HOMO and the next HOMO [25]. 

Additions of an electrophile (E) to the n bond (If) substituted by an electron-donating group (EDG) and an electron-accepting group (EAG) occur at the P and a positions, respectively. Transition states are considered as non-cyclic E-Il-EDG(EAG) systems (Scheme 16). 

Scheme 18 Orbital phase properties for the electrophilic substitutions of benzenes with an electron-donating group...

Scheme 40 Facial selectivity predicted on the basis of matching of the nucleophilicity of the diene having electron-donating group X and the electrophilicity of the dienophile having electron-withdrawing group Y...

Successful syntheses by classical electrophilic aromatic decarboxylation (Section III,E) offer promise that a range of aryl organometallics containing strongly electron-donating groups could be prepared for electrophilic metals (e.g., Tlm, PbIV, Aum). 

Low-valent, 18-electron (Fischer-type) carbene complexes with strong n-acceptors usually are electrophilic at the carbene carbon atom (C ). These complexes can undergo reactions similar to those of free carbenes, e.g. cyclopropanation or C-H insertion reactions. The carbene-like character of these complexes becomes more pronounced when electron-accepting groups are directly bound to C (Chapter 4), whereas electron-donating groups strongly attenuate the reactivity (Chapter 2). 

In accordance with a typical electrophilic aromatic substitution, this reaction is successfully accomplished by simply treating aromatic compounds activated by electron-donating groups, such as alkoxy, phenoxy, hydroxyl and thiophenoxy moieties, with tellurium tetrachloride in reflnxing chloroform, carbon tetrachloride or tolnene. ° ... 

Aryltellurium trichlorides and tribromides condense with aromatic compounds bearing electron-donating groups, giving unsymmetrical diaryltellurium dihalides. This electrophilic substitution therefore parallels that described for tellurium tetrachloride. 

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