Carbocations Vinyl cations

George Olah published a perspective in which he provided an overview of the research of his group over five decades.1 Christopher Reed surveyed the chemistry of carborane acids, the strongest pure Brpnsted acids known, with a conjugate base that engages in little chemistry.2 These acids are employed to prepare highly acidic carbocations, including protonated benzene, protonated C6o, tertiary carbocations, vinyl cations, and silylium ions. The synthesis and properties of unusual... 

The allylic carbocation resulting from protonation of the center carbon might seem the obvious choice, but, in fact, the kinetically favored protonation leads to the vinyl cation... 

Figure 8.2 The structure of a secondary vinylic carbocation. The cationic carbon atom is sp-hybridized and has a vacant p orbital perpendicular to the plane of the tt bond orbitals. Only one R group is attached to the positively charged carbon rather than two, as in a secondary alkyl carbocation. The electrostatic potential map shows that the most positive (blue) regions coincide with lobes of the vacant p orbital and are perpendicular to the most negative (red) regions associated with the ir bond.

The stabilities of most other stable carbocations can also be attributed to resonance. Among these are the tropylium, cyclopropenium, and other aromatic cations discussed in Chapter 2. Where resonance stability is completely lacking, as in the phenyl (CeH ) or vinyl cations, the ion, if formed at all, is usually very short lived. Neither the vinyl nor the phenyl cation has as yet been prepared as a stable species in solution. ... 

As in the case of the base-catalyzed reaction, the thermodynamically most stable alkene is the one predominantly formed. However, the acid-catalyzed reaction is much less synthetically useful because carbocations give rise to many side products. If the substrate has several possible locations for a double bond, mixtures of all possible isomers are usually obtained. Isomerization of 1-decene, for example, gives a mixture that contains not only 1-decene and cis- and franj-2-decene but also the cis and trans isomers of 3-, 4-, and 5-decene as well as branched alkenes resulting from rearrangement of carbocations. It is true that the most stable alkenes predominate, but many of them have stabilities that are close together. Acid-catalyzed migration of triple bonds (with allene intermediates) can be accomplished if very strong acids (e.g., HF—PF5) are used. If the mechanism is the same as that for double bonds, vinyl cations are intermediates. 

These substituent effects are due to the stabilization of the carbocations that result from protonation at the center carbon. Even if allylic conjugation is not important, the aryl and alkyl substituents make the terminal carbocation more stable than the alternative, a secondary vinyl cation. 

Despite the successful prediction of chemical shifts for a great structural variety of carbocations some difficulties have been encountered for vinyl cations.47 The effect of electron correlation, basis sets and geometry on calculated NMR spectra of vinyl cations has been studied in some detail also for the parent vinyl cation in its linear form.48 Comparative experimental and computational NMR studies, however, have... 

The generation of a-ferrocenyl-P-silyl substituted vinyl cations of type 28 does not require superacidic conditions, they can be generated by protonation of l-ferrocenyl-2-trialkylsilyl alkynes with trifluoroacetic acid at room temperature. The SiR3-groups with larger alkyl substituents increase the lifetime of this type of carbocations. 

Siehl, H.-U. Excursions into Long-Lived Vipyl Cations NMR Spectroscopic Characterization a-Aryl Vinyl Cations, In Stable Carbocation Chemistry Prakash, G.K.S. Schleyer, P. v. R., Eds. Wiley New York, 1997 Chapter 5, p. 165 - 196. 

For a recent monograph on vinyl cations Dicoordinated Carbocations Rappoport, Z., Stang, P. J., Eds. Wiley New York, NY, 1997. 

A recent review on vinyl cations Rappoport, Z. Stang, P. J. Eds. Dicoordinated Carbocations, John Wiley Sons Chichester, 1997. 

The authors succeeded in preparing (20-25) a series of long-lived carbocations bearing vinyl and related groups in the p-position to the carbocationic center (la-d, 3). Their structures were confirmed by and 13C NMR spectroscopy. These cations can be considered as a-complexes of C-centered electrophilic agents, namely vinyl and methyl-substituted vinyl cations derived from 9,10-dimethylphenanthrene and 1,2-dimethylacenaphthylene, respectively. Quite naturally, they had not been prepared by electrophilic vinyl-... 

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-... 

Many valuable reviews of the chemistry of these species are given in the new book Dicoordinated Carbocations An introduction by Grob " is followed by reviews of various theoretical studies of vinyl cations, their gas-phase chemistry, their generation by nuclear decay, and their NMR spectroscopic characterization. Vinyl cation production by addition to acetylenes and allenes, by solvolysis, and photolytically are covered, together with the chemistry of the species generated in these various ways. The next chapter deals with the synthetic applications of vinyl cations,and alkynyl and aryl cations are covered in the last chapter. A review of the NMR spectroscopic and quantum-chemical investigation of vinyl cations in superacid media (also of dienyl and 1-cyclopropylvinyl cations) is published separately,as is a review of alkynylcar-... 

Protonation of the alkyne is actually less favourable than protonation of an alkene, because the resulting vinyl cation is sp hybridized, having a bonds to just two substituents, a it bond, and a vacant p orbital. A vinyl cation is thus less stable than a comparable trigonal -hybridized carbocation, since ip-hybridization brings bonding electrons closer to carbon it thus becomes less tolerant of positive charge. Protonation, when it occurs, will be on... 

To be really satisfactory, a Friedel-Crafts alkylation requires one relatively stable secondary or tertiary carbocation to be formed from the alkyl halide by interaction with the Lewis acid, i.e. cases where there is not going to be any chance of rearrangement. Note also that we are unable to generate carboca-tions from an aryl halide - aryl cations (also vinyl cations, see Section 8.1.3) are unfavourable - so that we cannot nse the Friedel-Crafts reaction to join aromatic gronps. There is also one further difficulty, as we shall see below. This is the fact that introduction of an alkyl substitnent on to an aromatic ring activates the ring towards fnrther electrophilic substitution. The result is that the initial product from Friedel-Crafts alkylations is more reactive than the... 

In the 40 years since Olah s original publications, an impressive body of work has appeared studying carbocations under what are frequently termed stable ion conditions. Problems such as local overheating and polymerization that were encountered in some of the initial studies were eliminated by improvements introduced by Ahlberg and Ek and Saunders et al. In addition to the solution-phase studies in superacids, Myhre and Yannoni have been able to obtain NMR spectra of carbocations at very low temperatures (down to 5 K) in solid-state matrices of antimony pentafluoride. Sunko et al. employed a similar matrix deposition technique to obtain low-temperature IR spectra. It is probably fair to say that nowadays most common carbocations that one could imagine have been studied. The structures shown below are a hmited set of examples. Included are aromatically stabilized cations, vinyl cations, acylium ions, halonium ions, and dications. There is even a recent report of the very unstable phenyl cation (CellJ)... 

There is direct evidence, from ir and nmr spectra, that the f-butyl cation is quantitatively formed when f-butyl chloride reacts with A1CI3 in anhydrous liquid HCI.246 In the case of olefins, Markovnikov s rule (p. 750) is followed. Carbocation formation is particularly easy from some reagents, because of the stability of the cations. Triphenylmethyl chloride247 and 1-chloroadamantane248 alkylate activated aromatic rings (e.g., phenols, amines) with no catalyst or solvent. Ions as stable as this are less reactive than other carbocations and often attack only active substrates. The tropylium ion, for example, alkylates anisole but not benzene.249 It was noted on p. 337 that relatively stable vinylic cations can be generated from certain vinylic compounds. These have been used to introduce vinylic groups into aryl substrates.250... 

Carbocations with trivalent carbon may have carbon with coordination number 2. Acyl ions have already been mentioned the vinyl cations, or car-bynium ions (26), have been detected as intermediates in addition of electrophiles to acetylenes and allenes and in solvolysis reactions with the highly reactive trifluoromethanesulfonate (triflate) leaving group.84 Vinyl cations are expected... 

They are both reactions of vinyl cations, either as such 2.170, or highly stabilized vinyl cations in the case of ketene 2.167, where the carbonyl group is a stabilized carbocation. 

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