The Phenyl Cation

The phenyl cation (134) firstpostulated by Waters335 is a highly reactive species oflow stability and plays a fundamental role in organic chemistry—for example, in the chemistry of diazonium ions. According to gas-phase studies and calculations, its stability is between that of the ethyl cation and the vinyl cation.336 Since it is an extremely electrophilic and short-lived species, it could not be isolated or observed directly in the condensed phase. For example, solvolytic and dediazoniation studies under superacidic conditions by Faali et al.337,338 failed to find evidence of the intermediacy of the phenyl cation. Hyperconjugative stabilization via orf/zo-Me3Si or 

The first stable, long-lived carbocation observed was the triphenylmethyl (trityl) cation 135.6 y 

Mono- and dialkylarylmethyl cations can be obtained readily from the corresponding alcohols, alkenes, or halides in superacid solutions, such as HS03F-SbF5,350 HSO3CI and SbF5,348,349 and oleum.88 Structures 137-139 are representative alky-larylmethyl cations. 

Because of the high stability of the tertiary ions, these are preferentially formed in the superacid systems from both tertiary and secondary, and even primary, precursors.353 If, however, the tertiary carbocation is not benzylic, rearrangement to a 

Although the unsubstituted benzyl cation is still elusive, many substituted derivatives have been observed (cations 143-146).355,356 

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The phenyl cation is an extremely unstable cation, as is reflected by the high hydride affinity shown in Table 5.2. In this case, the ring geometry opposes rehybridization so the vacant orbital retains sp character. Because the empty orbital is in the nodal plane of the ring, it receives no stabilization firom the n electrons. 

Scaiano and Kim-Thuan (1983) searched without success for the electronic spectrum of the phenyl cation using laser techniques. Ambroz et al. (1980) photolysed solutions of three arenediazonium salts in a glass matrix of 3 M LiCl in 1 1 (v/v) water/acetone at 77 K. With 2,4,5-trimethoxybenzenediazonium hexafluorophos-phate Ambroz et al. observed two relatively weak absorption bands at 415 and 442 nm (no e-values given) and a reduction in the intensity of the 370 nm band of the diazonium ion. The absence of any ESR signals indicates that these new bands are not due to aryl radicals, but to the aryl cation in its triplet ground state. 

As mentioned at the end of Section 8.3, the MO investigation by Apeloig and Arad (1985) of the influence of trimethylsilyl substituents on the phenyl cation led to the discovery of a further reagent, in addition to arenediazonium ions, that is able to form aryl cations, namely 2,6-bis-(trimethylsilyl)phenyltriflate. This was a significant success in the field of predictions on aryl cations by theoretical work. 

A series of semiempirical MO treatments of the benzenediazonium ion and the phenyl cation were made in the 1960s and early 1970s. These results were superseded by some papers published between 1976 and 1981 by Schleyer s group (Dill etal., 1976, 1977), by Castenmiller and Buck (1977), by Vincent and Radom (1978), by the groups of Simonetta and Zollinger (Gamba et al., 1980), by Alcock et al. (1980 a), and by Tasaka et al. (1981). 

Furthermore, we have to keep in mind that differences in thermodynamic stability of reagent(s) and product(s) do not include a kinetic parameter, the activation energy. The assumption made by Vincent and Radom, as well as by Brint et al., that the addition of N2 to the phenyl cation is a reaction with zero activation energy may be correct for the gas phase, but perhaps not for reaction in solution. One must therefore add an activation energy barrier to the calculated thermodynamic stability mentioned above for the reverse reaction (C6HJ + N2 — C6H5NJ). 

Fig. 8-5. Important orbital interactions in the end-on and side-on addition of N2 to the phenyl cation (left-hand side and right-hand side respectively), Zollinger (1990).

Before I proceed with the discussion of the dediazoniation mechanism, it is necessary to spend some paragraphs considering the definition of the term crisis as used by Kuhn. As already discussed in Section 8.3 the crisis was terminated by the experiments which demonstrated that the first step in Scheme 9-2 is reversible (mechanism B), or in other words that a simple organic compound, the phenyl cation, does react with N2 molecules. 

Coming back to the mechanism of dediazoniation, mechnism B in Scheme 9-2 is consistent with all experimental data known in 1973. Mechanism B was, indeed, mentioned in that paper (Zollinger, 1973 a) as an explanation, but not proposed as the explanation because it violated the common knowledge mentioned above. If that reverse reaction of the phenyl cation is faster than the forward reaction with water or metal halides, the rate is dependent on the concentrations of compounds involved only in the second step of the mechanism, even if that step is much faster than the first (forward) step. 

The 15N content was indeed lower when the experiment was performed This result justified the publication of a preliminary communication (Bergstrom et al., 1974). Later work (Hashida et al., 1978 Szele and Zollinger, 1978a Maurer et al., 1979) involving sophisticated statistical treatments suggested that, in a weakly nucleophilic solvent such as trifluoroethanol, the phenyl cation is formed in two steps and not in one, as in mechanism B (see Scheme 8-4 in Sec. 8.3), the first intermediate being a tight ion-molecule pair. 

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. ... 

The wide utility of aryl diazonium ions as synthetic intermediates results from the excellence of N2 as a leaving group. There are several general mechanisms by which substitution can occur. One involves unimolecular thermal decomposition of the diazonium ion, followed by capture of the resulting aryl cation by a nucleophile. The phenyl cation is very unstable (see Part A, Section 3.4.1.1) and therefore highly unselective.86 Either the solvent or an anion can act as the nucleophile. 

Sometimes acylium ions lose carbon monoxide to generate an ordinary carbonium ion. It will be recalled that free acyl radicals exhibit similar behavior at high temperatures. Whether or not the loss of carbon monoxide takes place seems to depend on the stability of the resulting carbonium ion and on the speed with which the acylium ion is removed by competing reactions. Thus no decarbonylation is observed in Friedel-Crafts reactions of benzoyl chloride, the phenyl cation being rather unstable. But attempts to make pivaloyl benzene by the Friedel-Crafts reaction produce tert-butyl benzene instead. With compound XLIV cyclization competes with decarbonylation, but this competition is not successful in the case of compound XLV in which the ring is deactivated.263... 

Instability of phenyl cation In case of haloarenes, the phenyl cation formed as a result of self-lonlsatlon will not be stabilised by resonance and therefore, S l mechanism Is ruled out. Because of the possible repulsion. It Is less likely for the electron rich nucleophile to approach electron rich arenes. Replacement by hydroxyl group... 

Theoretical calculations predict that the highest occupied Walsh orbital of the cyclopropene ring in 1 should stabilize a positive charge at C2, ° and this prediction was experimentally confirmed. Appearance energy measurements for the loss of Br from ionized 2-bromobenzocyclopropene (202 X = Br) suggest that the benzo-cyclopropen-2-ylium ion 291 is stabilized by more than 27.6 kcal/mol over the phenyl cation 292. Calculations (3-2IG ) give a stabilization energy of 23 and 22 kcal/mol of 291 over 292 and over the m-isomer 293, respectively. ... 

Because of the aromaticity effects, the electronic ground state of the phenyl cation C6H5+ (263) is singlet A, (67r-electronic structure) [88-... 

Dediazoniation itself is not dramatically dependent on the hydrogen fluoride/pyridine ratio, as shown from the dediazoniation of benzenediazonium tetrafluoroborate (previously isolated) in hydrogen fluoride/pyridine mixtures provided that the hydrogen fluoride/pyridine ratio is larger than 6, fluorobenzene is always obtained in quantitative yield (for hydrogen fluoride/ pyridine ratios below 6, byproducts result from the phenyl cation and free pyridine).10 These observations have resulted in new, very efficient syntheses of 4-fluorophenols 2 involving dediazoniation, in hydrogen fluoride/pyridine mixtures, of crystalline 4-hydroxy benzenediazonium chloride and tetrafluoroborate 3 (formed under anhydrous conditions)89,96 as well as quinone-diazide (formed from aminophenol in tetrafluoroboric acid).93,97 Such methods are more efficient than the Balz-Schiemann reaction under standard conditions. 

Insofar as electron impact mass spectrometry is concerned, the cycloproparenes almost always display a molecular ion. The primary source of fragmentation is by loss of a C(1) substituent radical to provide a cycloproparenyl cation. However, labelling studies have shown that loss of H from 1 and 11 (at least) occurs only after complete scrambling of the carbon atoms216217. The use of appearance energy measurements218 for the loss of Br from ionized 2-bromocyclopropabenzene (57a), when coupled with thermochemical data, have led to the prediction that cation 110 is more stable than the phenyl cation by at least 27.6 kcal mol"1 AHf of 110 is estimated at 311 kcalmol"1. 

The enchanced stability of the 2-cyclopropabenzenyl cation (110) over the phenyl cation has been discussed above (Section III). 

The phenyl cation has been the subject of high-level ab initio calculations.116 The ground state was shown to be a singlet, and the singlet-triplet gap was estimated.116 The gas-phase transfers of H+, H-, and H2O to Ph+ have been studied.117 Arenium... 

Protonation of fluorobenzene in the gas phase has been studied by infrared photodissociation (IRPD) spectroscopy by Solca and Dopfer.351 F-protonated fluorobenzene was formed in significant amount when protonation was carried out with CH5+. It was found to be the most stable isomer in the gas phase by quantum mechanical calculations [B3LYP/6-311G(2df,2pd) level] separated by a large energy barrier from the four Wheland intermediates. F-protonated fluorobenzene is best described as a weakly bound ion-dipole complex between the phenyl cation and HF. 

The mass spectrum of dimethylaminomethylferrocene exhibits as the base peak the molecular ion, as well as a significant peak for loss of one hydrogen, a feature commonly observed in the mass spectra of simple amines (186). The tertiary alcohol, diphenylhydroxymethylferrocene, shows as the base peak an ion at m/e 230 for which the structure (XLII) is proposed or a rearrangement product thereof. The molecular ion gives a peak of 11.2% relative abundance, and the other very strong peak in the spectrum is for the phenyl cation, C6H5+ (186). 

An electron transfer is proposed from the cluster toward the aromatic ring, leading to the formation of C6H5X- (NH3) + and followed by a rearrangement leading to the formation of NH2 + (NH3) H+. In a concerted mechanism, the NH2 attacks the phenyl cation leading to aniline+ + HX. 

Intersystem crossing (ISC) should be efficient for fragmentation to occur from the triplet in order to generate the phenyl cation with the same multiplicity if this condition is not fulfilled, sensitization with high-energy triplet donors can be used instead [13]. 

The excitation wavelengths used to produce Figure 8 were chosen so that the chlorobenzene ions formed at the two-photon level were constrained to <250 meV of excess energy (i.e. so that they lack sufficient energy to dissociate). Production of the phenyl cation requires three or more photons, as shown in Figure 6 that is, the initial ionization produces an excited state which lies above the threshold energy of the PhCl+ — Ph+ + Cl process. 

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