Cycloheptatrienyl derivatives

We must look for stability on a comparative basis—as was done above with the cydopentadienyl and cycloheptatrienyl derivatives—and may find evidence of aromaticity only in the fact that one molecular species is less unstable than its relatives. The net effect of a great deal of elegant work is strongly to support the 4/1 + 2 rule. The question now seems rather to be over how unfavorable a combination of angle strain and multiple charge can aromaticity manifest itself ... 

The reaction between vanadium hexacarbonyl and cycloheptatriene was first utilized by Werner and Manastyrskyj to prepare ir-cycloheptatrienyl-vanadium tricarbonyl (IV) (115), a dark green volatile solid soluble in nonpolar organic solvents. This compound is one of the relatively few tt-cycloheptatrienyl derivatives. Werner and Manastyrskyj report the concurrent formation in this reaction of a brown solid insoluble in nonpolar... 

Treatment of the sodium salt Na[CsH5Cr(CO)3] with tropylium bromide does not result in a cycloheptatrienyl derivative 134). Instead, a coupling reaction occurs producing ditropyl (dicycloheptatrienyl) and [CsHsCr (CO)3]2... 

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). 

Other li acetylides Li-C=C-R with R = hexyl [21] or benzylether dendrons [22, 23] (up to the fourth generation) have also been attached to (Figure 3.3), and various different electrophiles have been used to complete the reaction with the intermediate li-fuUeride (Scheme 3.2 and Figure 3.3). Besides the protonation, alkyl-, benzyl-, cycloheptatrienyl-, benzoyl- or vinylether-derivatives or formaldehyde and dichloro-acetylene were used as electrophiles [12,20]. Most of these electrophiles are attached to the anion in the expected C-2 position. The 1,4-adducts are available by quenching the anion with the tropylium cation or benzoyl chloride [12]. The fuUerene anion can be stabilized by introduction of benzylether dendrons. The lifetimes of the anions change with the size of the dendrons [22]. 

A growing body of work deals with CpV(cycloheptatrienyl) complexes and derivatives, which are formally V(0), if the cycloheptatrienyl (tropylium) ligand is considered to have a +1 charge [11-13]. The parent complex has a facile reversible oxidation (—0.17 V) in TlTF but it cannot... 

This review deals with metal-hydrocarbon complexes under the following headings (1) the nature of the metal-olefin and -acetylene bond (2) olefin complexes (3) acetylene complexes (4) rr-allylic complexes and (5) complexes in which the ligand is not the original olefin or acetylene, but a molecule produced from it during complex formation. ir-Cyclopentadienyl complexes, formed by reaction of cyclopentadiene or its derivatives with metal salts or carbonyls (78, 217), are not discussed in this review, neither are complexes derived from aromatic systems, e.g., benzene, the cyclo-pentadienyl anion, and the cycloheptatrienyl cation (74, 78, 217), and from acetylides (169, 170), which have been reviewed elsewhere. 

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. 

A bimetallic iron tricarbonyl cation C8H9Fe2(CO)6 + (IV) is derived from cyclooctatriene (157). The Mossbauer resonance of spectrum of (IV) and the related cycloheptatrienyl-Fe2(CO)6 cation (V) (73) suggest that these cations may be considered as containing a bis(7r-allyl) and an allyl-diene system, respectively. In the second case (V), rapid valence tautomerism is invoked to account for the unique proton NMR signal in solution. 

Stabilization of an organic cation occurs whenever the electron-deficient carbon atom is conjugated with aryl groups or with atoms containing unshared pairs of electrons such as oxygen, nitrogen, or sulfur (7). Examples of cations which are useful in polymerization include triphenylmethyl (I), cycloheptatrienyl (tropylium) (II), and various alkyl (III), aryl (IV), and benzopyrylium (V, VI) derivatives. 

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). 

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... 

Initial attempts to prepare 7r-cycloheptatrienyl-iron tricarbonyl complexes were not successful—the products often being 7r-cycloheptadienyl-iron tricarbonyl derivatives (60). In 1964 Mahler, Jones, and Pettit (162) reported the synthesis of a complex [(C7H7)Fe(CO)3] BF4 by the following route ... 

---