Carbocations destabilization

Carbocations Destabilized by Electron-Withdrawing Groups Applications in Orgamc Chemistry Charpentier Manze, M, Bonnet Delpon, D Ad Carbocation Chem 1, 219-253 54 ... 

The secondary benzylic alcohol l-phenylethan-l,2-diol requires 20 hours of treatment at room temperature to produce a 64% yield of 2-phenylethanol (Eq. 43).137 Under the same conditions, methyl mandelate fails to undergo reduction, presumably because of the greater carbocation-destabilizing effect of a neighboring carboalkoxy compared to a hydroxymethyl group (Eq. 43).137... 

Carbocations Destabilized by Electron-Withdrawing Groups Applications in Organic Chemistry ... 

Epoxides with an oxymethyl substituent (RO-CR2) usually react with nucleophiles at the non-oxymethyl-substituted carbon atom, because of the carbocation-destabilizing effect of this group. This is also observed for hydride as nucleophile, as illustrated by the examples in Scheme 4.63. 

NMR data for regioisomeric pyrenyl-a-CF3-substituted carbocations are gathered in Figs 31 and 32.29 Carbocation destabilization increases n-participation and pyrenium ion character. This is reflected in larger AS values and the significant double-bond character which shields the carbocation. Because charge delocalization from the /J-position is poor, CF3-substitution can exert maximal destabilization. Thus, the alcohol precursor is not ionized instead, protonation occurs at a remote a-position. 

Methyl group releases electrons stabilizes carbocation than than Tnfluoromethyl group withdraws electrons destabilizes carbocaUon... 

Deactivating and meta directing These substituents are strongly electron withdrawing and destabilize carbocations They include... 

Some destabilized carbocations, although not observed directiy, could be generated in situ under Friedel-Crafts reaction conditions, for example, the destabilized diethyl malonyl cation (27). 

Electronegative, nonconjugating groups (which interact with an incipient carbocation only by an inductive or field effect) discourage attack at C. This is due to destabilization of transition state (49) by the juxtaposition of positive charge. 

The pATr+ values allow for a comparison of the stability of carbocations. The carbocations that can be studied in this way are all relatively stable carbocations. The data in Table 5.1 reveal that electron-releasing substituents on the aryl rings stabilize the carbocation (more positive pA r+) whereas electron-withdrawing groups such as nitro are destabilizing. This is what would be expected from the electron-deficient nature of the carbocation. 

Measurement of (R /R ) can be accomplished by cyclic voltammetry for relatively Stable species and by other methods for less stable cations. The values obtained for AG -range from 83kcal/mol for the aromatic tropylium ion to 130kcal/mol for destabilized betizylic cations. For stable carbocations, the results obtained by this method correlate with cation stabiUty as measured by pKf.+. Some of these data are presented in Table 5.3. 

Substitution reactions by the ionization mechanism proceed very slowly on a-halo derivatives of ketones, aldehydes, acids, esters, nitriles, and related compounds. As discussed on p. 284, such substituents destabilize a carbocation intermediate. Substitution by the direct displacement mechanism, however, proceed especially readily in these systems. Table S.IS indicates some representative relative rate accelerations. Steric effects be responsible for part of the observed acceleration, since an sfp- caibon, such as in a carbonyl group, will provide less steric resistance to tiie incoming nucleophile than an alkyl group. The major effect is believed to be electronic. The adjacent n-LUMO of the carbonyl group can interact with the electnai density that is built up at the pentacoordinate carbon. This can be described in resonance terminology as a contribution flom an enolate-like stmeture to tiie transition state. In MO terminology,.the low-lying LUMO has a... 

Methyl group releases electrons, stabilizes carbocation than than Trifluoromethyl group withdraws electrons, destabilizes carbocation... 

Substituent effect on the stability of the transition state for solvent addition (C, Figure 6). The difference in the values of ks = 1.4 x 107s 1 for the addition of a solvent of 50/50 (v/v) trifluoroethanol/water to Me-[10+] and ks = 1.0 x 109 s-1 for addition of the same solvent to Me-[8+] (Table 1) reflects the balance between (1) the destabilization of X-[10+] resulting from rotation about the —Ca bond, which results in a less stable carbocation and hence an... 

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... 

The destabilizing effect of a silyl group compared with an alkyl group in trivalent carbocations was explained by the weaker hyperconjugation of the Si-R a-bond (R = alkyl) relative to a C-R cr-bond (R = H or alkyl) and by electrostatic repulsion between the adjacent positively charged cationic carbon and the electropositive silicon (10). 

---