Cycloheptatrienes, from norcaradienes

We assert in this review that, at this point in time, there are several examples of neutral molecules which have been shown to display either bond or no-bond homoaromaticity. These include, in addition to the boranes mentioned above in Section III. B, cyclohepta-triene, norcaradiene, bridged cycloheptatrienes and norcaradienes, semibullvalenes, bar-baralanes, bridged annulenes, etc. Confirmation of the homoaromatic character of these systems comes from thermochemical and spectroscopic studies, and force field and ab initio calculations. In particular, the work of Roth and coworkers must be mentioned in this connection in that they were the first to provide reliable resonance energies of a large number of these neutral molecules225 226. These authors have also demonstrated that systems such as bicyclo[2.1.0]pentene are homoantiaromatic. 

The overall mechanistic picture of these reactions is poorly understood, and it is conceivable that more than one pathway may be involved. It is generally considered that cycloheptatrienes are generated from an initially formed norcaradiene, as shown in Scheme 30. Equilibration between the cycloheptatriene and norcaradiene is quite facile and under acidic conditions the cycloheptatriene may readily rearrange to give a substitution product, presumably via a norcaradiene intermediate (Schemes 32 and 34). When alkylated products are directly formed from the intermolecular reaction of carbenoids with benzenes (Scheme 33 and equation 36) a norcaradiene considered as an intermediate alternatively, a mechanism may be related to an electrophilic substitution may be involved leading to a zwitterionic intermediate. A similar intermediate has been proposed143 in the intramolecular reactions of carbenoids with benzenes, which result in substitution products (equations 37-40). It has been reported,144 however, that a considerable kinetic deuterium isotope effect was observed in some of these systems. Unless the electrophilic attack is reversible, this would indicate that a C—H insertion mechanism is involved in the rate-determining step. 

The energy of activation for isomerization of norbornadiene itself is considerably largeThe reaction is mechanistically complex, and yields toluene and decomposition products as well as cycloheptatriene. Under the conditions used, part of the toluene is formed by isomerization of the cyclo-heptatriene , possibly via norcaradiene. In the gas phase the energy of activation for formation of cycloheptatriene from norbornadiene is about 51 kcal.mole and log is about 14.8 (refs. 24, 190). Activation parameters for the formation of toluene directly from norbornadiene in the gas phase are Ecf = 53 kcal.mole and log A — 14.2. These reactions probably involve initial cleavage of the C-1, C-7 bond in norbornadiene to yield an allylic diradical which can cyclize to norcaradiene (a precursor for both cycloheptatriene and toluene) as well as undergo other reactions. 

Also of interest is the finding that the hydrogen shift to give toluene occurs only from the transition state that involves retention at all levels of theory. Further, calculations on the cycloheptatriene-to-norcaradiene equilibrium at the B3LYP(RHF) level come closest to experiment, namely 6.5 kcal/mol for... 

A related >/4-norcaradiene tricarbonyliron complex is obtained upon reaction of tricy-clo[4.3.1.0l6]deca-2,4-diene with Fe3(CO)12 in boiling benzene (equation 143). However, the [4.3.1]propellane ring system is not retained in the analogous tricarbonylchromium complex. Instead, as suggested from solution NMR and solid state X-ray analyses, the complex assumes a homoaromatic structure, which is intermediate between a norcaradi-ene and a cycloheptatriene system (equation 144)193,194. It is noteworthy that the Cr(CO)3 group prefers the same conformation as the Fe(CO)3 group in the analogous norcaradiene iron complex. 

Formation of 648 from 647 was experimentally discarded. Since the most basic site of 646 is the carbonyl group, both Bronsted and Lewis acids should first coordinate to this position. In the case of Bronsted acid, protonation could occur at the electron-rich 9-position, and this was followed by deprotonation at the 1 la-position, which is promoted by the carbonyl protonation. In contrast, the Lewis acid cannot add to the 9-position and thus the skeletal rearrangement to give 648 took place. This kind of rearrangement through norcaradiene tautomers, shown in Scheme 128, is called walk rearrangement in thermal reaction of cycloheptatrienes <2002CL260>. 

Norcaradiene formation from a cycloheptatriene corresponds to a 1,3-ring-closure. On the basis of this reaction, aminocyclopropane 317 was obtained as a solid in 94% yield from piperidine and the tropylium ion 315 (equation 74). A rapid equilibrium between 316 and 317 was postulated in solution. Electrocyclic 1,3-bond connections also were involved in the fluctual behaviour of 9-azabarbaralanes " , in the formation of homoazepines (from nitrenes and cycloheptatriene" " ) and in a 6-azabenz[10]annulene system . ... 

Photolysis of a-diazo esters in the presence of benzene or benzene derivatives often results in [2-1-1] cycloaddition of the intermediate acylcarbene to the aromatic ring, thus providing access to the norcaradiene (bicyclo[4.1.0]hepta-2,5-diene)/cyclohepta-l,3,5-triene valence equilibrium. The diverse effects that influence this equilibrium have been discussed (see Houben-Weyl, Vol. 4/3, p509). To summarize, the 7-monosubstituted systems obtained from a-diazoacetic esters exist completely in the cycloheptatriene form, whereas a number of 7,7-disubstituted compounds maintain a rapid valence equilibrium in solution. On the other hand, several stable 7-cyanonor-caradienes are known which have a second 7t-acceptor substituent at C7 (see Section 1.2.1.2.4.3). Subsequent photochemical isomerization reactions of the cycloheptatriene form may destroy the norcaradiene/cycloheptatriene valence equilibrium. Cyclopropanation of the aromatic ring often must compete with other reactions of the acylcarbene, such as insertion into an aromatic C H bond or in the benzylic C H bond of alkylbenzenes (Table 7). 

The quantum yield for toluene formation is very low in solution but approaches unity in the gas phase at low pressures . The toluene was suggested to be formed from vibrationally excited ground state molecules, following rapid internal conversion from the excited singlet state manifold, perhaps involving the intermediacy of norcaradiene (bicyclo[4.1.0]hepta-2,4-diene) . The hot ground state mechanism for toluene formation has received considerable support from time-resolved and steady-state experiments on cycloheptatriene and several of its derivatives - ". ... 

An interesting example is the pressure-induced reaction of buckminsterfullerene Cfio with 1,3,5-cycloheptatriene [60]. Generally C o reacts as an electron-deficient dienophile or dipolarophile in numerous Diels-Alder or 1,3-dipolar cycloadditions and 1,3,5-cycloheptatriene as a diene. The reaction with C(,o is a rare example where both adducts derived from the norcaradiene as well as the cycloheptatriene are observed. 

Oxygen functionalization of some norcaradienes or cycloheptatrienes has been communicated. Thus the endoperoxides (31) and (32) have been obtained from the corresponding hydrocarbons.An investigation has also been made of the photosensitized oxidation of 7-methoxycycloheptatriene. The product, a [4 + 2]cycloadduct (33), could be thermally isomerized to 4-methoxytropone. 

Among possible alternatives to DPMR we discarded the cx.cx-elimination of benzidine 12 from biradical-zwitterions 2 and 3 and formation of carbene 23 based on the absence of trapped carbene products in experiments with cyclohexene and ethanethiol (Xgxc = 300 nm). However, norcaradiene 6 produces carbene 23 when subjected to 350-nm irradiation. Another plausible alternative to DPMR involves initial C-CN bond homolysis, which is energetically favorable in both excited states, followed by a photochemical reaction of Crystal Violet radical. We discarded this mechanism because none of the expected products of the photochemical reactions of Crystal Violet radical has been detected during the course of our work. Nevertheless, isolation of MGCN in the trapping experiment with ethanethiol points to the possible involvement of the C-CN bond homolysis in S. Finally, although norcaradiene, cycloheptatriene, and norbomadiene are valence tautomers and are easily interconverted into each other [188], products of the latter two kinds have been neither detected nor isolated in our study [66, 30]. 

QI — Q2 and Cl—C3 bonds. The most direct demonstration of this phenomenon comes from crystal structures.A typical example is shown in 11.10. A related effect allows us to understand why the norcaradienc-cycloheptatriene equilibrium, shown in 11.11, is sliiftcd toward the side of norcaradiene when the substituent R... 

Bicyclo[2.2.1]heptadiene (norbornadiene) gives cycloheptatriene upon heating with log k = 14.68 — 50 610/2.3/ r. Also formed in the reaction is cyclopentadiene and acetylene, the retro Diels-Alder products with log k = 14.68 — 51900/2.3/ r and toluene with log k = 14.23 — 53 A0/23RT. Most likely, the initial reaction proceeds via cleavage of the C1-C7 bond to give a biradical which can form norcaradiene and then cycloheptatriene or undergo a hydrogen shift to toluene, but the retro 4 -h 2 reaction must result from C1-C2 (and C3-C4) bond fission (Scheme 8.18). 

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