Cationic electron-withdrawing

Norbornadienes Possessing Cationic Electron Withdrawing Groups Biphotochromic Norbomadiene Derivatives ... 

Unsymmetrically substituted dipyrromethanes are obtained from n-unsubstitued pyrroles and fl(-(bromomethyl)pyiToIes in hot acetic acid within a few minutes. These reaction conditions are relatively mild and the o-unsubstituted pyrrole may even bear an electron withdrawing carboxylic ester function. It is still sufficiently nucleophilic to substitute bromine or acetoxy groups on an a-pyrrolic methyl group. Hetero atoms in this position are extremely reactive leaving groups since the a-pyrrolylmethenium( = azafulvenium ) cation formed as an intermediate is highly resonance-stabilized. 

Because the carbon atom attached to the ring is positively polarized a carbonyl group behaves m much the same way as a trifluoromethyl group and destabilizes all the cyclo hexadienyl cation intermediates m electrophilic aromatic substitution reactions Attack at any nng position m benzaldehyde is slower than attack m benzene The intermediates for ortho and para substitution are particularly unstable because each has a resonance structure m which there is a positive charge on the carbon that bears the electron withdrawing substituent The intermediate for meta substitution avoids this unfavorable juxtaposition of positive charges is not as unstable and gives rise to most of the product... 

Conversely an electron withdrawing substituent destabilizes the cyclo hexadienyl cations corresponding to ortho and para attack more than meta Thus meta substitution predominates... 

The kinds of vinyl monomers which undergo anionic polymerization are those with electron-withdrawing substituents such as the nitrile, carboxyl, and phenyl groups. We represent the catalysts as AB in this discussion these are substances which break into a cation (A ) and an anion (B ) under the conditions of the reaction. In anionic polymerization it is the basic anion which adds across the double bond of the monomer to form the active center for polymerization ... 

Table S.5. Destabilization of 2-Substituted 2-Propyi Cation by Electron-Withdrawing Substituents...

The effect of the bond dipole associated with electron-withdrawing groups can also be expressed in terms of its interaction with the cationic u-complex. The atoms with the highest coefficients in the LUMO 3 are the most positive. The unfavorable interaction of the bond dipole will therefore be greatest at these positions. This effect operates with substituents such as carbonyl, cyano, and nitro groups. With ether and amino substituents, the unfavorable dipole interaction is overwhelmed by the stabilizing effect of the lone-pair electrons stabilizing 3. 

Anionic polymerization is better for vinyl monomers with electron withdrawing groups that stabilize the intermediates. Typical monomers best polymerized by anionic initiators include acrylonitrile, styrene, and butadiene. As with cationic polymerization, a counter ion is present with the propagating chain. The propagation and the termination steps are similar to cationic polymerization. 

Synthetic polymers can be classified as either chain-growth polymen or step-growth polymers. Chain-growth polymers are prepared by chain-reaction polymerization of vinyl monomers in the presence of a radical, an anion, or a cation initiator. Radical polymerization is sometimes used, but alkenes such as 2-methylpropene that have electron-donating substituents on the double bond polymerize easily by a cationic route through carbocation intermediates. Similarly, monomers such as methyl -cyanoacrylate that have electron-withdrawing substituents on the double bond polymerize by an anionic, conjugate addition pathway. 

Hetero-substituted 2-alkenyllithium compounds, however, show useful levels of regio- and stereoselectivity in reactions with aldehydes, in particular if the cation is held at one terminus of the allylie system by electron-withdrawing or chelating groups. 

Introduction of an electron-withdrawing carboxy function at the x-carbon produces particularly reactive A-acyliminium ions, the so-called glycine cation equivalents, which are of great utility for the synthesis of x-amino acids. 

For the approach of N2 in the xy-plane of the aryl ring, i. e., side-on addition, o-electron withdrawal by substituents will increase the ai-a interaction. A positive constant pF is therefore expected for addition to 4-substituted aryl cations. 7i-Elec-tron donation by substituents will increase the b2-b2 interaction, for which a negative value of the constant pR is plausible. For the approach of N2 along the z-axis, i.e., end-on addition, the ai-ai interaction should be at least as strong as for the side-on reaction as substantial positive value of pF is therefore expected. The b2-b2 interactions should approximatively cancel. Interactions between the two occupied orbitals should lead to a slight destabilization, but interaction with n should reduce the importance of the b2-b2 interaction relative to the ai-ai interaction, i.e., Pr < Pf -... 

A number of approaches have been tried for modified halo-de-diazoniations using l-aryl-3,3-dialkyltriazenes, which form diazonium ions in an acid-catalyzed hydrolysis (see Sec. 13.4). Treatment of such triazenes with trimethylsilyl halides in acetonitrile at 60 °C resulted in the rapid evolution of nitrogen and in the formation of aryl halides (Ku and Barrio, 1981) without an electron transfer reagent or another catalyst. Yields with silyl bromide and with silyl iodide were 60-95%. The authors explain the reaction as shown in (Scheme 10-30). The formation of the intermediate is indicated by higher yields if electron-withdrawing substituents (X = CN, COCH3) are present. In the opinion of the present author, it is likely that the dissociation of this intermediate is not a concerted reaction, but that the dissociation of the A-aryl bond to form an aryl cation is followed by the addition of the halide. The reaction is therefore mechanistically not related to the homolytic halo-de-diazoniations. 

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