1,3,5-Cycloheptatriene structure

By analogy to additions of divalent carbon to the Cio aromatic framework, the molecule Cgi was expected to have the norcaradi-ene (II) or the cycloheptatriene (III) structure. Although an X-ray structure was not available, the UV-visible spectrum, NMR spectrum, and cyclic voltammetry supported the cycloheptatriene (III) structure. The researchers then calculated the relative molecular mechanics energies of II and III and found the cycloheptatriene structure stabilized by 31 kcal/mol with respect to the norcaradi-ene structure. Although the calculations do not confirm the structures, they provide additional supporting evidence. 

The pareitropone project began quite by accident after an unexpected observation expanded our thinking about potentially accessible targets for alkynyliodonium salt/alkylidenecarbene chemistry (Scheme 18). Treatment of the tosylamide iodonium salt 125 with base under standard conditions was designed to provide no more than routine confirmation of the aryl C-H insertion capabilities, which were first exposed in indoleforming reactions using tosylanilide anion nucleophiles and propynyl(phenyl)iodonium triflate,5b of the intermediate carbene 126. However, this substrate did not perform as expected, since only trace amounts of the 1,5 C-H insertion product 127 was detected. One major product was formed, and analysis of its spectral data provided yet another surprising lesson in alkynyliodonium salt chemistry for us. The data was only consistent with the unusual cycloheptatriene structure 129. 

Recently, important progress on iridacycloheptatriene has been reported. M-dacycloheptatriene 62 was isolated by the reaction of Ir diene Tp [Tp = hydrotris(3,5-dimethylpyrazolyl)borate] complex with DMAD (Scheme 5.25) [39]. The cycloheptatriene structure was stabilized by the coordination of water to an iridium center. The water ligand was labile and easily replaced by CO, MeCN, or PMes. Iridacyclopentadiene Tp 63 reacted with an excess amount of 2-butyne to... 

Cyclic polyenes are expected to show similar behavior thus, cyclopentadiene has El I of 9.00 e.v. rather similar to the methyl-butadienes and lower than that of butadiene itself. Cycloheptatriene, however, has El / = 8.55 e.v., somewhat higher than that of hex-atriene even allowing for the difference between UV and El determinations. The value is intermediate between that which would normally be expected for a cycloheptatriene structure ( 7.89 e.v. (47)] and that of toluene, El / = 9.23 e.v. (46), and suggests that tropylidene or tropylidene cation actually has r-bonding between the 2- and 7-positions, VI. 

When we say cycloheptatriene is not aromatic but cycloheptatrienyl cation is we are not comparing the stability of the two to each other Cycloheptatriene is a stable hydrocarbon but does not possess the special stability required to be called aromatic Cycloheptatrienyl cation although aromatic is still a carbocation and reasonably reac tive toward nucleophiles Its special stability does not imply a rock like passivity but rather a much greater ease of formation than expected on the basis of the Lewis struc ture drawn for it A number of observations indicate that cycloheptatrienyl cation is far more stable than most other carbocations To emphasize its aromatic nature chemists often write the structure of cycloheptatrienyl cation m the Robinson circle m a ring style... 

Both the cycioheptatrienyl radical and the anion are reactive and difficult to prepare. The six-Tr-electron cation, however, is extraordinarily stable. In fact, the cycioheptatrienyl 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. 

It has been shown how alkenylcarbene complexes participate in nickel(0)-me-diated [3C+2S+2S] cycloaddition reactions to give cycloheptatriene derivatives (see Sect. 3.3). However, the analogous reaction performed with alkyl- or aryl-carbene complexes leads to similar cycloheptatriene derivatives, but in this case the process can be considered a [2S+2S+2S+1C] cycloaddition reaction as three molecules of the alkyne and one molecule of the carbene complex are incorporated into the structure of the final product [125] (Scheme 82). The mechanism of this transformation is similar to that described in Scheme 77 for the [3C+2S+2S] cycloaddition reactions. 

Reaction of iron atoms with cycloheptatriene to form [Fe( r) -C7H7)-(t7 -C7H9)] was confirmed by another group 15) these workers determined the crystal structure of the species, demonstrating a sandwich structure with the open faces of the two 7j -systems skewed to each other. The temperature-dependent NMR spectrum of this species (16) indicated two types of fiuxional behavior in solution. Evidence for a 1,-2-shift mechanism of the l-5-i7-cycloheptatrienyl moiety in the structure shown. 

The molecules taking part in a valence tautomerization need not be equivalent. Thus, NMR spectra indicate that a true valence tautomerization exists at room temperature between the cycloheptatriene 110 and the norcaradiene (111). In this case one isomer (111) has the cw-l,2-divinylcyclopropane structure, while the other does not. In an analogous interconversion, benzene oxide and oxepin exist in a tautomeric equilibrium at room temperature. 

This kind of compound was obtained in the reaction of cycloheptatriene with dichloroazine CF3CC1=NN=CC1CF3 when heated at 70°C. A 1 1 mixture of rearranged adducts 31 and 32 was isolated and this latter compound was obtained as a mixture of two diastereomers in the ratio 77 23 (NMR spectroscopy, yield not given). The formation of these two compounds requires considerable skeletal rearrangement of any initial [3+2] or [3+6] cycloadduct and a satisfactory mechanism cannot be proposed. It was not possible to differentiate between structures 31 and 32 on the basis of the spectral data obtained (Equation 3) <1995JFC203>. 

G. Merling had obtained tropylium bromide in 1891 by brominating cycloheptatriene but could not guess its structure tropylium was discovered when prepared again via the same route by W. E. Doering and L. H. Knox in 1957, i.e., 66 years later.23... 

The l-oxa-2,4,5-cycloheptatrienes 602 and 603 were postulated to be intermediates in the rearrangement of certain (ethynylfuryl)oxiranes to furo[3,4-b]furans [251]. The replacement of the methylene groups of 1,2-cycloheptadiene (465) by SiMe2 groups and the introduction of substituents at the allene moiety allowed the preparation of isolable seven-membered ring allenes. Thus, Barton et ah [177] obtained 604 and Ando et al. [178] 605. A few reactions of these systems have also been studied [177, 252]. Both groups [178, 253] synthesized the [4.4]betweenallene 606 and determined its structure by X-ray diffraction. 

For cycloheptatriene and a series of its derivatives various thermal unimolecular processes, namely conformational ring inversions, valence tautomerism, [1,5]-hydrogen and [l,5]-carbon shifts, are known. An example of such multiple transformations was described65 which can provide a facile approach to new polycyclic structures by a one-step effective synthesis (yields up to 83%) of the two unique ketones 156 and 157. The thermolysis of the neat ether 151 at 200 °C for 24 h gives initially the isomeric allyl vinyl... 

As to the cation-radical version of this isomerization, there are testimonies on the transition of the norcaradiene carcass into the cycloheptatriene skeleton. Calculations at the B3LYP level shows that cycloheptatriene cation-radical is more stable than norcaradiene cation-radical by ca. 29 kJ mol (Norberg et al. 2006). Hydrocarbon ion-radicals with strained ring structures have a tendency to undergo facile rearrangement to enforce the unpaired electron delocalization and release their strain energy. 

Azulene can be written as fused cyclopentadiene and cycloheptatriene rings, neither of which alone is aromatic. However, some of its resonance structures have a fused cyclopentadienyl anion and cycloheptatrienyl cation, which accounts for its aromaticity and its dipole moment of 1.0 D. 

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