Cycloheptatrienyl ions

For N-vinylcarbazole in methylene chloride solutions cycloheptatrienyl ion has been shown to be a very efficient initiator, reacting by a rapid and direct addition to the olefin (82). A mechanistic scheme involving virtually instantaneous and quantitative initiation, rapid propagation (and transfer) and no true termination appears to operate, enabling rate constants for propagation kp, to be determined very simply from initial slopes of conversion/time curves. Under the experimental conditions used the initiators were almost totally dissociated and there seems every reason to suppose that the propagating cations are similarly dissociated (Section II.C.2). The derived rate constants therefore refer to the reactivity of free poly-(N-vinylcarbazole) cation, kp, and relevant data are summarised in Table 7. 

The six-electron, conjugated cyclic compound is particularly stable, and this condition is found not only in hydrocarbons, but also in the hetero-aromatic compounds, such as pyrrole. As with benzene, the cyclopentadienate and cycloheptatrienyl ions, here again there are three bonding orbitals which can accommodate six electrons, as shown in Fig. 2. 

Huckel realized that his molecular orbital analysis of conjugated systems could be extended beyond neutral hydrocarbons He pointed out that cycloheptatrienyl cation also called tropyhum ion contained a completely conjugated closed shell six tt electron sys tern analogous to that of benzene... 

Figure 11.14 shows a molecular orbital diagrfflTt for cycloheptatrienyl cation. There are seven tt MOs, three of which are bonding and contain the six tt electrons of the cation. Cycloheptatrienyl cation is a Hiickel (4n + 2) system and is an aromatic ion. 

Section 11.21 Species with six tt electrons that possess special stability include certain ions, such as cyclopentadienide anion and cycloheptatrienyl cation. 

According to the Hiickel criteria for aromaticity, a molecule must be cyclic, conjugated (that is, be nearly planar and have ap orbital on each carbon) and have 4n + 2 tt electrons. Nothing in this definition says that the number of p orbitals and the number of nr elections in those orbitals must be the same. In fact, they can he different. The 4n + 2 rule is broadly applicable to many kinds of molecules and ions, not just to neutral hydrocarbons. For example, both the cydopentadienyl anion and the cycloheptatrienyl cation are aromatic. 

Other kinds of substances besides benzene-like compounds can also be aromatic. For example, the cyclopentadienyl anion and the cycloheptatrienyl cation are aromatic ions. Pyridine, a six-membered, nitrogen-containing heterocycle, is aromatic and resembles benzene electronically. Pyrrole, a hve-membered heterocycle, resembles the cyclopentadienyl anion. 

Quasi-aromatic structures are also known in which the stabilised cyclic species is an ion, e.g. the cycloheptatrienyl (tropylium) cation (15, cf. p. 106), the cyclopentadienyl anion (16, cf. p. 275), both of which have 67te (n = 1), and even more surprisingly the cyclopropenyl cation (17, cf. p. 106) which has 2ne (n = 0) ... 

A special case of carbocation stability arises where the cation complies with the Hiiekel (4n+2) rule governing aromatic structures. Of these, the best known and most useful is the cycloheptatrienyl cation, more frequently referred to as the tropylium ion. For an informative and wide ranging account of structures, stabilities, properties and reactions of almost every type of carbocation, reference should be made to the series of monographs edited by Olah and Schleyer (18). 

Table 3. Ion pair dissociation data of cycloheptatrienyl cation salts (C7Hf) ...

The phenylsilyl cation CgHsSi 4 (85) and its interconversion to other possible isomers such as the silacylcoheptatrienyl cation (86) have attracted considerable experimental as well as theoretical interest in view of the possible similarities or differences of the well-known benzyl and cycloheptatrienyl cations243 and their silicon analogues. Thus, Beauchamp and coworkers244,245 have found that electron ionization of phenylsilane gives rise to both isomers 85 and 86, which do not interconvert at room temperature and can be readily distinguished by their ion-molecule reactions. 

The mass spectra of some metal carbonyl halides with it-cyclopen-tadienyl, 7t-cycloheptatrienyl, 7i-allyl and Tt-indenyl ligands have been investigated 52,38) The usual losses of carbonyl groups and C2H2 fragments are observed. The mass spectra of the chlorides exhibited a strong iodine memory effect, since ions expected for the corresponding iodides were also observed in their mass spectra. Pyrolysis of the halides... 

Use of triphenylmethyl and cycloheptatrienyl cations as initiators for cationic polymerization provides a convenient method for estimating the absolute reactivity of free ions and ion pairs as propagating intermediates. Mechanisms for the polymerization of vinyl alkyl ethers, N-vinylcarbazole, and tetrahydrofuran, initiated by these reagents, are discussed in detail. Free ions are shown to be much more reactive than ion pairs in most cases, but for hydride abstraction from THF, triphenylmethyl cation is less reactive than its ion pair with hexachlorantimonate ion. Propagation rate coefficients (kP/) for free ion polymerization of isobutyl vinyl ether and N-vinylcarbazole have been determined in CH2Cl2, and for the latter monomer the value of kp is 10s times greater than that for the corresponding free radical polymerization. 

Initiation with Tropylium Ion. When cycloheptatrienyl hexachlor-antimonate is used as initiator for tetrahydrofuran polymerization, the reactions are somewhat cleaner, and strong colors do not develop as readily as when the corresponding trityl salts are used (17). Rates of initiation are much lower, and the reaction is hardly noticeable at room temperature. However, at 50 °C. and above initiation is significant, and the polymerizations proceed almost to the expected theoretical conversion of monomer to polymer even when hexachlorantimonate is the anion (Table III). Therefore, the apparent low equilibrium conversion obtained with the rapidly initiating trityl salts is minimized in this case by the comparatively low rate of consumption of initiator. Once again GLC demonstrates clearly that the initiation reaction involves primarily hydride abstraction from the ether. 

The cycloheptatrienyl cation is called the tropylium ion. This aromatic ion is much less reactive than most carbocations. Some tropylium salts can be isolated and stored for months without decomposing. Nevertheless, the tropylium ion is not necessarily as stable as benzene. Its aromaticity simply implies that the cyclic ion is more stable than the corresponding open-chain ion. 

Although the tropylium ion forms easily, the corresponding anion is difficult to form because it is antiaromatic. Cycloheptatriene (pKa = 39) is barely more acidic than propene (piTa = 43), and the anion is very reactive. This result agrees with the prediction of Htickel s rule that the cycloheptatrienyl anion is antiaromatic if it is planar. 

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