Cycloheptatrienyl radical

Cycloheptatrienyl radical (C7H7) contains a cyclic completely ... 

Cycloheptatrienyl radical has seven tt electrons Therefore it does not satisfy the Huckel 4n + 2 rule and is not aromatic... 

Hydrogen Abstra.ction. These important reactions have been carried out using a variety of substrates. In general, the reactions involve the removal of hydrogen either direcdy as a hydrogen atom or indirectly by electron transfer followed by proton transfer. The products are derived from ground-state reactions. For example, chlorarul probably reacts with cycloheptatrienyl radicals to produce ether (50) (39). This chemistry contrasts with the ground-state reaction in which DDQ produces tropyhum quinolate in 91% yield (40). 

Similar arguments can be used to predict the relative stabilities of the cyclo-heptatrienyl cation, radical, and anion. Removal of a hydrogen from cyclohepta-triene can generate the six-77-electron cation, the seven-77-electron radical, 01 the eight-77-elec iron anion (Figure 15.6). All three species again have numerous resonance forms, but HiickeTs rule predicts that only the six-7r-electron cyclohep-tatrienyl cation should be aromatic. The seven-77-electron cycloheptatrienyl radical and the eight-77-electron anion are antiaromatic. 

Problem 15.10 I Show the relative energy levels of the seven 77 molecular orbitals of the cvclohepta-trienyl system. Tel) which of the seven orbitals are filled in the cation, radical, and anion, and account for the aromaticity of the cycloheptatrienyl cation. 

We naturally exclude here the cyclopropenyl, cyclopentadienyl and cycloheptatrienyl radicals, all of which can also be recognized as cyclic C H species much as we did not include in our discussion the enthalpies of formation of allyl and pentadienyl radical as part of our analysis of polyenes such as butadiene and hexatriene. 

The photo-oxidation of the aryl-substituted cycloheptatrienes 7-(/ -methoxy-phenyl)cycloheptatriene and 7-, 1- and 3-(/ -dimethylaminophenyl)cycloheptatrienes to the corresponding radical cations in de-aerated acetonitrile solution was accomplished by electron transfer to the electronically excited acceptors 9,10-dicyanoanthracene, iV-methylquinolinium perchlorate, iV-methylacridinium perchlorate and l,T-dimethyl-4,4-bipyridinium dichloride. In the case of l- p-methoxyphenyl)cycloheptatriene (62), deprotonation of the radical cation occurs successfully, compared with back electron transfer, to give a cycloheptatrienyl radical (63) which undergoes a self-reaction forming a bitropyl. If the photooxidation is done in air-saturated acetonitrile solution containing HBF4 and one of the acceptors, the tropylium cation is formed. Back electron transfer dominates in the / -dimethylaminocycloheptatrienes and the formation of the cycloheptatrienyl radical is prevented. 

Figure 27-18 Electron-spin resonance spectrum of cycloheptatrienyl radical produced by x irradiation of 1,3,5-cycloheptatriene. See Exercise 27-13.

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. 

Carbazole, like most aromatic amines, oxidizes readily via electron transfer. We recognized early that electron transfer may be an important initiation process for polymerizing the N-vinyl derivative. Some years ago we showed (29) that cycloheptatrienyl cation could act as an efficient one-electron transfer reagent, producing the appropriate cation radicals from reactive amines such as phenothiazine and tetramethyl-p-phenylene-diamine. It was also suggested that the product of the reaction between cycloheptatrientyl cation and carbazole itself was the carbazole cation radical. However, our recent work (21) has demonstrated that one-electron oxidation of carbazole leads directly to the 3,3-dicarbazoyl cation radical (VII). 

Figure 15.6 Generation of the cycloheptatrienyl cation, radical, and anion. Only the six---electron cation is aromatic.

Both the cycloheptatrienyl radical and the anion arc reactive and difficult to prepare. The six-7r-electron cation, however, is extraordinarily stable. In fact, the cycloheptatrienyl cation was first prepared more than a century ago by reaction of Br2 with cycloheptatriene (Figure 15.7), although its structure was not recognized at the time. 

In a landmark paper, Breslow and coworkers described the determination of pA), values of weak hydrocarbon acids by use of thermochemical cycles involving electrochemical reduction data for triarylmethyl, cycloheptatrienyl, and triphenyl- and trialkylcyclopropenyl cations and radicals [9aj. Later, they derived pATa data from standard oxidation potentials and bond-dissociation energies [9b, c]. The methodology was further developed by Nicholas and Arnold [10a] for the determination of cation radical acidities, and later modified and extensively used by Bordwell and coworkers [10b, c] so that homolytic bond-dissociation energies and cation radical... 

---