Carbocations electron-withdrawing substituents

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. 

Substituent effects on ks. The replacement of an a-methyl group at the 4-methoxycumyl carbocation CH3-[14+] by an a-ester or a-amide group destabilizes the parent carbocation by 7 kcalmol-1 relative to the neutral azide ion adduct (Scheme 11 and Table 3) and results in 5-fold and 80-fold decreases, respectively, in ks for nucleophilic addition of a solvent 50/50 (v/v) methanol/water.33 These results follow the trend that strongly electron-withdrawing substituents, which destabilize a-substituted 4-methoxybenzyl carbocations relative to neutral adducts to nucleophiles, do not lead to the expected large increases in the rate constants for addition of solvent.28,33,92-95... 

Exceptions to the generally facile ionic hydrogenation of trisubstituted alkenes include the resistance of both 2-methyl-1-nitropropene (R = NO2) and 3,3-dimeth-ylacrylic acid (R = CO2H) to the action of a mixture of triethylsilane and excess trifluoroacetic acid at 50° (Eq. 85).234 The failure to undergo reduction is clearly related to the unfavorable effects caused by the electron-withdrawing substituents on the energies of the required carbocation intermediates. 

Charge delocalization pathways were gauged and compared on the basis of the magnitude of A8 C values. Some representative examples are shown in Fig. 8. It was concluded that the carboxonium group is a robust electron-withdrawing substituent whose electronic response is sensitive to steric factors. In this way, it could be used to modulate charge delocalization into PAHs and their carbocations, as a function of substitution position. 

Unlike a methyl group, which is slightly electron-releasing, a trifluoromethyl group is a powerful electron-withdrawing substituent. Consequently, a CF3 group destabilizes a carbocation site to which it is attached. 

When R1 = Me (electron-releasing) the form 181 is more significant, and the stabilized carbocation at C-4 is attacked by H20 (Scheme 72). With electron-withdrawing substituents R1 = CChEt, form 180, being more conjugated, is more significant, and H20 attacks at the 2-position (Scheme... 

To avoid any loss of benzyl ester protection during acidolytic removal of the benzyloxy-carbonyl and ferf-butoxycarbonyl groups, electron-withdrawing substituents were used to destabilize the intermediate benzyl cation and thus to increase the acid stability. In addition to the very useful 4-nitrobenzyl esters (vide infra), the picolyl ester (see Section 2.2.1.2.2.3) as well as halo-P l or cyano-P°°l substituted benzyl esters have been reported, the latter being rarely used for a-carboxy protection. Conversely, an increase in sensitivity toward acids can be achieved by introduction of electron-releasing substituents, such as methoxy or methyl groups. Addition of scavengers to quench intermediate carbocations and to prevent electrophilic substitutions at sensitive amino acid side chains is beneficial in the deprotection of such esters. 

The substitution reactions at the anomeric carbon usually proceed easily and via a SnI mechanism in most cases. However, it is not the case in the nucleophilic substitutions at nonanomeric sites. Because of the presence of vicinal electron-withdrawing substituents (OR or NHR) which strongly destabilize the intermediate carbocations, Sn2 displacement reactions instead of Sn 1 reactions are favored (O Scheme 1). 

Note the differences between electrophilic and nucleophilic aromatic substitutions Electrophilic substitutions are favored by electron-donating substituents, which stabilize the carbocation intermediate, while nucleophilic substitutions are favored by electron-withdrawing substituents, which stabilize a carbanion intermediate. The electron-withdrawing group that deactivate rings for electrophilic substitution (nitro, carbonyl, cyano. and so on) activate them for nucleophilic substitution. What s more, these groups are meta directors in electrophilic substitution, but are ortho-para directors in nucleophilic substitution. 

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