Carbocations aryl cations

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-... 

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... 

Carbocations have also been obtained by protonation of photochemically generated carbenes (see Eq. 17), by the fragmentation of photochemically generated cation radicals (see Eq. 18), and by the addition of one photochemically generated cation to an arene (or aUcene) to generate a second cation. As illustrated in Eq. 19, the last method has been employed to convert invisible carbocations into visible ones. Short-hved aryl cations and secondary alkyl cations are quenched by electron-rich aromatics such as mesitylene and 1,3,5-trimethoxybenzene in HEIP to give benzenium ions that can be observed by LEP in this solvent. 

Aryl cations are less stable than 1°, 2°, or 3° carbocations, so aryl diazonium salts are more stable than alkyl diazonium salts. 

Vinyl and aryl halides generally do not undergo SN1 or Sn2 reactions. An SN1 reaction would require ionization to form a vinyl or aryl cation, either of which is less stable than most alkyl carbocations. An Sn2 reaction would require back-side attack by the nucleophile, which is made impossible by the repulsion of the electrons in the double bond or aromatic ring. 

A7 y/amines also react Miith nitrous acid, but the alkanedLazoniutn products of these reactions are so reactive they can t be isolated. Instead, they lose nitrt en instantly to yield carbocations. The analogous loss ef from an aieaediazoniuRt ion to yield an aryl cation is disfavored by the instability of the cation. 

Phenols are formed when a diazonium salt is heated in boiling water. Nitrogen is evolved and the aryl cation reacts rapidly with water (Scheme 8.15). Since this is a nucleophilic displacement, it is preferable to use acidic conditions to ensure that no phenoxide ions are present, since this could react with unchanged diazonium salt. Sulfuric acid rather than hydrochloric acid is preferred for the diazotization to avoid trapping the highly reactive carbocation with chloride ion. 

On the other hand, absorption of a photon promotes an electron from a bonding to an antibonding orbital. This activation may lead to aryl-halide bond fragmentation generating directly a carbocation (phenyl cation). 

Common error alert If your mechanism has an alkenyl, alkynyl, or aryl carbocation as an intermediate, it is almost certainly incorrect. (One major exception aryldiazonium ions ArNz lose N2 to give aryl cations Ar. )... 

A new approach to alkene arylation has been developed using diaryliodonium salts and copper catalysis. The products formed may differ significantly from those usually found from Heck arylations and carbocation intermediates are likely to be involved. The possibilities for combining diaryliodonium salts and catalytic amounts of chiral copper complexes to generate electrophilic chiral aryl cation equivalents for asymmetric arylation reactions have been highlighted. i... 

The triarylmethyl cations are particularly stable because of the conjugation with the aryl groups, which delocalizes the positive charge. Because of their stability and ease of generation, the triarylmethyl cations have been the subject of studies aimed at determining the effect of substituents on carbocation stability. Many of these studies used the characteristic UV absorption spectra of the cations to determine their concentration. In acidic solution, equilibrium is established between triarylearbinols and the corresponding carbocations. 

The diarylmethyl cations listed in Table 5.1 are 6-7pATr+ units less stable than the corresponding triarylmethyl cations. This indicates that the additional aryl group has a cumulative, although not necessarily additive, effect on stability of the carbocation. Primary benzylic cations (monparylmethyl cations) are generally not sufficiently stable for determination of pATr+ values. A particularly stable benzylic ion, the 2,4,6-trimethyl-phenylmefliyl cation, has a pATr+ of — 17.4. 

As the cation becomes progressively more reluctant to be reduced than [53 ], covalent bond formation is observed instead of electron transfer. Further stabilization of the cation causes formation of an ionic bond, i.e. salt formation. Thus, the course of the reaction is controlled by the electron affinity of the carbocation. However, the change from single-electron transfer to salt formation is not straightforward. As has been discussed in previous sections, steric effects are another important factor in controlling the formation of hydrocarbon salts. The significant difference in the reduction potential at which a covalent bond is switched to an ionic one -around -0.8 V for tropylium ion series and —1.6 V in the case of l-aryl-2,3-dicyclopropylcyclopropenylium ion series - may be attributed to steric factors. 

Replacing an a-alkyl snbstituent by an a-aryl group is expected to stabilize the cationic center by the p-Jt resonance that characterizes the benzyl carbocations. In order to analyze such interaction in detail, the cumyl cation was crystallized with hexafluoroantimonate by Laube et al. (Fig. 13) A simple analysis of cumyl cation suggests the potential contributions of aromatic delocalization (Scheme 7.3), which should be manifested in the X-ray structure in terms of a shortened cationic carbon—aromatic carbon bond distance (C Cat). Similarly, one should also consider the potential role of o-CH hyperconjugation, primarily observable in terms of shortened CH3 distances. Notably, it was found experimentally that the Cai distance is indeed shortened to a value of 1.41 A, which is between those of typical sp -sp single bonds (1.51 A) and sp -sp double bonds (1.32 A). In the meantime, a C -CH3 distance of 1.49 A is longer than that observed in the tert-butyl cation 1 (1.44 A), and very close to the normal value for an sp -sp single bond. 

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. 

Aryl(trimethylsiloxy)carbenes. Acylsilanes (153) undergo a photoinduced C —> O silyl shift leading to aryl(trimethylsiloxy)carbenes (154).73,74 The carbenes 154 can be captured by alcohols to form acetals (157) 73 or by pyridine to give transient ylides (Scheme 29).75 LFP of 153 in TFE produced transient absorptions of the carbocations 155 which were characterized by their reactions with nucleophiles.76 The cations 155 are more reactive than ArPhCH+, but only by factors < 10. Comparison of 154 and 155 with Ar(RO)C and Ar(RO)CH+, respectively, would be of interest. Although LFP was applied to generate methoxy(phenyl)carbene and to monitor its reaction with alcohols,77 no attempt was made to detect the analogous carbocation. 

Early attempts to generate ot-aryl-(3-silyl substituted carbocations by ionization of 1,1-diphenyl-2-(trimethylsilyl)ethanol 10 usipg FS03H in S02C1F even at very low temperature of -140 °C were unsuccessful (77). Only 1,1-diphenylethyl cation 11 and trimethylsilyl fluorosulfate, fhe products of P-silyl cleavage were observed. 

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. 

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