Phenyl cations

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. 

Phenyl cations are formed by thermal decomposition of aryl diazonium ions. The cation is so extremely reactive that under some circumstances it can recrqrture the nitrogen... 

Calculate energies for reaction of 2-methyl-2-propyl chloride (to 2-methyl-2-propyl cation), 2-phenyl-2-propyl chloride (to 2-phenyl-2-propyl cation) mdphenyl chloride (to phenyl cation). (The energy of chloride is given at right.) Assuming that reaction (1) is normal , what is the effect of a benzene ring Does it facilitate or hinder loss of chloride ... 

Electrostatic potential map for phenyl cation shows most positively-charged regions (in blue) and less positively-charged regions (in red). 

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 These results were superseded by some papers published between 1976 and 1981 by Schleyer s group (Dill etal., 1976, 1977), by Castenmiller and Buck (1977), by Vincent and Radom (1978), by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. 

Phenyl cation, see Aryl cations Phenyldiazenyl radical, see Aryldiazenyl radical... 

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. 

Phenols from Diazonium Ion Intermediates. Aryl diazonium ions can be converted to phenols by heating in water. Under these conditions, there is probably formation of a phenyl cation. 

Vinylic and phenyl cations are highly unstable and do not form readily. 

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... 

When 4-/-butylcyclohex-1 -enyl(phenyl)iodonium tetrafluoroborate (3) is heated at 60 °C in chloroform, 1-fluorocyclohexene 4, 1-chlorocyclohexene 5 and l-(o-iodophenyl)cyclohexene 6 are formed with accompanying iodobenzene leaving group (eq 2).3 These three substitution products are best accounted for by formation of an ion pair involving cyclohexenyl cation 7. The cyclohexenyl cation 7 formed picks up fluoride from tetrafluoroborate and chloride from chloroform solvent, and recombines with the iodobenzene generated (eq 3). This kind of reactions with a counteranion and solvent are characteristic of unstable carbocations and are known in the case of phenyl cation generated from the diazonium salt in the Schiemann-type reaction.4... 

These opposite signs can be explained by considering a twofold orbital interaction between the two parts of an arenediazonium ion, namely between the jr-HOMO of the diazonio group and the cr-LUMO of the aryl residue, and between the jr-HOMO of the aryl residue and the jr-LUMO of the diazonio group. These two overlaps stabilize the C—N bond and reduce the rate of dediazoniation into a phenyl cation and a nitrogen molecule. The two opposing HOMO-LUMO interactions are shown in Figure 1. Thus... 

---