Vinylic carbocation, electronic stability

On the contrary sp2-hybride phenyl and vinyl carbocations are less stable in comparison with their sp3-hybride analogues. The resonance notably increases ion stability in the case of participation of heteroatomic n electrons (Scheme 5.4). 

Shi and coworkers found that vinyl acetates 68 are viable acceptors in addition reactions of alkylarenes 67 catalyzed by 10 mol% FeCl2 in the presence of di-tert-butyl peroxide (Fig. 15) [124]. (S-Branched ketones 69 were isolated in 13-94% yield. The reaction proceeded with best yields when the vinyl acetate 68 was more electron deficient, but both donor- and acceptor-substituted 1-arylvinyl acetates underwent the addition reaction. These reactivity patterns and the observation of dibenzyls as side products support a radical mechanism, which starts with a Fenton process as described in Fig. 14. Hydrogen abstraction from 67 forms a benzylic radical, which stabilizes by addition to 68. SET oxidation of the resulting electron-rich a-acyloxy radical by the oxidized iron species leads to reduced iron catalyst and a carbocation, which stabilizes to 69 by acyl transfer to ferf-butanol. However, a second SET oxidation of the benzylic radical to a benzylic cation prior to addition followed by a polar addition to 68 cannot be excluded completely for the most electron-rich substrates. 

The success of the reaction depends on the formation of the zwitterionic 7t-allylpalladium complex, which is subsequently trapped with the electron-poor olefins to afford the desired vinylcyclopentane. This is achieved through a nucleophilic addition of Pd onto the vinyl group, which results in an opening of the cyclopropane ring, revealing the zwitterionic 7i-allylpalladium complex. The presence of the ester moieties stabilizes the carbanion, while the carbocation is stabilized by the 7t-allylpalladium complex. This is followed by a Michael addition of the electron poor olefins onto the carbanion to form the second intermediate, which rapidly... 

If we consider the protonation of 2-butyne by HBr, then we obtain an unstable vinyl cation, 11.15. This is linear because the carbon atom is sp-hybridized, with an empty p-orbital. Thus, it can be attacked by bromide ion from either side, resulting in the formation of both E- and Z-alkene products (Figure 11.32). The instability of the vinyl cation intermediate means that alkynes react more slowly with electrophiles than do alkenes. The addition of a second mole of HBr to the alkene generally gives the geminal dibromide the intermediate carbocation is stabilized by interaction with the lone pair of electrons from bromine (Figure 11.33). 

A reasonable mechanism for the reaction in sulfuric acid and water is presented below. One of the alkyne groups is protonated resulting in the formation of a new C-H bond, and a resonance-stabilized vinyl carbocation, analogous to the one described for the bromination reaction above. The n electrons from the other alkyne attack this carbocation, resulting in the formation of a new C-C bond, and a new vinyl carbocation. Nucleophilic attack of water, followed by deprotonation gives the enol, which tautomerizes to form the ketone, as shown (next page) ... 

The initiator can be a radical, an acid, or a base. Historically, as we saw in Section 7.10, radical polymerization was the most common method because it can be carried out with practically any vinyl monomer. Acid-catalyzed (cationic) polymerization, by contrast, is effective only with vinyl monomers that contain an electron-donating group (EDG) capable of stabilizing the chain-carrying carbocation intermediate. Thus, isobutylene (2-methyl-propene) polymerizes rapidly under cationic conditions, but ethylene, vinyl chloride, and acrylonitrile do not. Isobutylene polymerization is carried out commercially at -80 °C, using BF3 and a small amount of water to generate BF3OH- H+ catalyst. The product is used in the manufacture of truck and bicycle inner tubes. 

There is also substantial stabilization of [4+] by electron delocalization from the cyclic a-vinyl group. This is shown by a comparison of the thermodynamic driving force (p Tr lies between —7.8 and —8.5) and absolute rate constant (ks = 1 -6 x 107 s 1) for the reaction of [4+] in 25% acetonitrile in water with the corresponding parameters for reaction of the resonance-stabilized l-(4-methoxyphenyl)ethyl carbocation in water (p Tr = — 9.4and s= 1 x 108 s Table 5). 

Different rate-determining steps are observed for the acid-catalyzed hydration of vinyl ethers (alkene protonation, ks kp) and hydration of enamines (addition of solvent to an iminium ion intermediate, ks increasing stabilization of a-CH substituted carbocations by 71-electron donation from an adjacent electronegative atom results in a larger decrease in ks for nucleophile addition of solvent than in kp for deprotonation of the carbocation by solvent. 

In addition to electron-deficient alkenes, under the catalysis of TiCLt, 1,2-allenylsi-lanes can react with aldehydes or N-acyliminium ion to afford five-membered vinylic silanes 71 and 72. Here the carbocations generated by a Lewis acid regiospecifically attack the C3 of the 1,2-allenylsilanes to produce a vinyl cation stabilized by hyper-... 

The vinyl halide product is then able to react with a further mole of HX, and the halide atom already present influences the orientation of addition in this step. The second halide adds to the carbon that already carries a halide. In the case of the second addition of HX to RC CH, we can see that we are now considering the relative stabilities of tertiary and primary carbocations. The halide s inductive effect actually destabilizes the tertiary carbocation. Nevertheless, this is outweighed by a favourable stabilization from the halide by overlap of lone pair electrons, helping to disperse the positive charge. 

The cationic polymerization of vinyl isobutyl ether at —40°C produces stereoregular polymers (structure 5.21). The carbocations of vinyl alkyl ethers are stabilized by the delocalization of p valence electrons in the oxygen atom, and thus these monomers are readily polymerized by cationic initiators. Poly(vinyl isobutyl ether) has a low Tg because of the steric hindrance offered by the isobutyl group. It is used as an adhesive and an impregnating resin. 

If a carbocation or a dication at the same time is also a Hiickeloid An + 2)jt aromatic system, resonance can result in substantial stabilization. The simplest 2jt aromatic system is the Breslow s cyclopropenium ion 206.434 439 Recently, electronic and infrared spectra of the parent ion cyclo-C3H3+ (206, R = H) in neon matrices440 and the X-ray characterization of the tris(trimethylsilyl) derivative were reported.441 The destabilizing effect of the silyl groups was found to be significantly smaller than in vinyl cations. The ion was computed to be more stable than the parent cyclopropenium ion by 31.4 kcal mol1 [MP3(fc)/6-31 lG //6-31G +ZPVE level]. The alkynylcy-clopropenylium ions 207 have been reported recently.442... 

Summary 1-Ary 1-2-trialkyIsilyl-substituted vinyl cations are characterized in solution by NMR spectroscopy. The NMR chemical shift data reveal the stabilization of the positive charge by a (5-silyl substituent. The order of hyperconjugative stabilization of a positive charge by P-substituents is H < alkyl < silyl. The P-silyl effect is dependent on the electron demand of the carbocation and decreases with better electron donating a-substituents. NMR spectroscopy is a suitable tool to investigate the competition between 7i-resonance and CT-hyperconjugation in these type of carbocations. 

Comparing p-silyl-substituted vinyl cations however with different a-aryl substituents shows that the stabilizing effect of a P-silyl groiq> is not constant but is dependent on the electron demand of the carbocation. Fig. 4 shows the pora-carbon C NMR chemical shift difference A8 between the p-silyl and P-unsubstituted vinyl cations (For the a-ferrocenyl substituted cation A8 C3,4 is used to probe the silyl effect). 

The first step of the mechanism involves the initial complexation of titanium tetrachloride to the carbonyl group of the electron-deficient alkene (enone) to give an alkoxy-substituted allylic carbocation. The allylic carbocation attacks the (trimethylsilyl)allene regiospecifically at C3 to generate vinyl cation I, which is stabilized by the interaction of the adjacent C-Si bond. The allylic Ji-bond is only coplanar with the C-Si bond in (trimethylsilyl)allenes, so only a C3 substitution can lead to the formation of a stabilized cation. A[1,2]-shift of the silyl group follows to afford an isomeric vinyl cation (II), which is intercepted by the titanium enolate to produce the highly substituted five-membered ring. Side products (III - V) may be formed from vinyl cation I. 

The greater the number of alkyl substituents bonded to the positively charged carbon, the more stable the carbocation will be. The order of relative stability of carbocations is tertiary benzyhc > allylic secondary > primary vinyl> phenyl. The nature of electron release by alkyl groups is not very clear. It may be an inductive effect, a resonance effect (hyperconjugation), or a combination of the two. When we refer to the inductive effect of the alkyl groups, it should be clear that this might well include a contribution from hyperconjugation. 

The fast reaction of benzylic and allylic halides is a result of the resonance stabilization that is available to the intermediate carbocations formed. Tertiary halides are more reactive than secondary halides, which are in turn more reactive than primary or methyl halides because alkyl substituents are able to stabilize the intermediate carbocations by an electron-releasing effect. The methyl carbocations have no alkyl groups and are the least stable of all carbocations mentioned thus far. Vinyl and aryl carbocations are extremely unstable because the charge is localized on an sp -hybridized carbon (double-bond carbon) rather than one that is sp -hybridized. 

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