Cyclooctatetraene-Annulene

Cyclooctatetraene, i.e., [8]annulene 3, was the first [4n] i-electron hydrocarbon to be studied [57, 64]. It was found that 3 adopts a tub conformation, that minimizes the electronic interaction between the double bonds, and makes the system less conjugated and thus nonaromatic [65]. Compound 3 undergoes a dynamic process of ring inversion, which involves D4h transition states that have been intensively studied [66]. 

Compound 3 was reduced to its dianion by Katz in his pioneering work concerning the reduction of annulenes [67]. Compound 3 was reacted with two moles of either Li or K, and in both cases the NMR spectrum showed a single sharp peak at low-field, which is characteristic of a planar aromatic ring (3 ). It was therefore concluded that reduction formed an aromatic anion, which con- 

The kinetic studies of the electron exchange between the radical anion and the dianion of 3, compared with the reaction between the radical anion and the neutral molecule [67c], provide insight into the structure of the radical anion. The reaction rate was found to be dependent on the counter-cation, in the order Li Na K. The electron exchange reaction occurs rapidly between the radical anion and the dianion, and much more slowly between the radical anion and the neutral molecule. This behavior is different from that observed in most aromatic hydrocarbons. Naphthalene shows extremely fast exchange between the radical anion and the neutral molecule, and a slower one with the dianion [69]. The exchange rates of the radical anion of 3 suggest that its carbon skeleton is similar to that of the dianion, and not that of the neutral molecule [67c]. 

Many studies in this area focus on the reduction of substituted 3, for example, the reduction of the four isomers of dimethylcyclooctatetraene [70]. These were reduced by K, and all the formed dianions were characterized as planar or essentially planar. The steric interactions and planarity of the radical anions of tert-butoxy-3 were also studied [71]. Another interesting derivative of 3 is 1,5,22,26-tetraoxa-[5,5]-(2,8)-dibenzo[a,e]cyclooctatetraenophane 10 [72], which is the first macrocycle incorporating two 3 units. A pair of diasteriomeric tetraanions were observed in the NMR when 10 was reduced with alkali metals. The NMR spectra and cyclovoltam-metric studies suggest that the two 3 subunits in 10 are flat dianions. 

The [12]annulene (4) system is thermodynamically unstable due to bond angle strain, steric interactions between the inner protons and the number of re-electrons [56, 57] therefore, this molecule has not been studied intensively. A possible configuration for 4 is one in which the double bonds are alternatively arranged in a 

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The general term annulene has been coined to apply to completely conjugated mono cyclic hydrocarbons with more than six carbons Cyclobutadiene and benzene retain then-names but higher members of the group are named [jcjannulene where x is the number of carbons m the ring Thus cyclooctatetraene becomes [8]annulene cyclodecapentaene becomes [10] annulene and so on... 

Divide the heats of combustion by the number of carbons The two aromatic hydrocarbons (benzene and [18]annulene) have heats of combustion per carbon that are less than those of the nonaromatic hydrocarbons (cyclooctatetraene and [16]annulene) On a per carbon basis the aromatic hydrocarbons have lower potential energy (are more stable) than the nonaromatic hydrocarbons... 

If the cycloaddition and cycloreversion steps occurred under the same conditions, an equilibrium would establish and a mixture of reactant and product olefins be obtained, which is a severe limitation to its synthetic use. In many cases, however, the two steps can very well be separated, with the cycloreversion under totally different conditions often showing pronounced regioselectivity, e.g. for thermodynamic reasons (product vs. reactant stability), and this type of olefin metathesis has been successfully applied to organic synthesis. In fact, this aspect of the synthetic application of four-membered ring compounds has recently aroused considerable attention, as it leads the way to their transformation into other useful intermediates. For example aza[18]annulene (371) could be synthesized utilizing a sequence of [2 + 2] cycloaddition and cycloreversion. (369), one of the dimers obtained from cyclooctatetraene upon heating to 100 °C, was transformed by carbethoxycarbene addition to two tetracyclic carboxylates, which subsequently lead to the isomeric azides (368) and (370). Upon direct photolysis of these, (371) was obtained in 25 and 28% yield, respectively 127). Aza[14]annulene could be synthesized in a similar fashion I28). 

The solid aromatic dianion salt of l,2-di-[ C][8]annulene has not been found to scramble the Cs even on heating to over 600 °C for 2hs. ° This behaviour of the cyclooctatetraene dianion is in contrast with that of neutral aromatic systems, which readily automerize in the gas phase. Apparently, when sufficient energy is applied... 

CONTENTS List of Contributors. Introduction to the Series An Editor s Forward, Albert Padwa. Preface, Randolph P. Thummel. Cyclooctatetraenes Conformational and ii-Elec-tronic Dynamics Within Polyolefinic [8] Annulene Frameworks, Leo A. Paquette. A Compilation and Analysis of Structural Data of Distorted Bridgehead Olefins and Amides, Timothy G. Lease and Kenneth J. Shea. Nonplanarity and Aromaticity in Polycyclic Benzenoid Hydrocarbons, William C. Herndon and Paul C. Nowak. The Dewar Furan Story, Ronald N. Warrener. Author Index. Subject Index. 

Eight-membered rings with two O, S or N atoms or combinations of these heteroatoms in a 1,2- or 1,4-relationship and three double bonds possess conjugated tt-electron frameworks and can be designated as dihetera[8]annulenes (78AHC(23)55). These 1,2- or 1,4-diheterocins are isoelectronic with the cyclooctatetraene dianion, and if planar represent potentially aromatic IOtt-electron systems. Considerable interest has attached to the degree of aromaticity of these compounds from both theoretical and experimental standpoints. Most theoretical treatments have led to the conclusion that 1,2-diheterocins, because of... 

Since protonation of cyclooctatetraene is known to yield the homotropylium ion (see Section 3.5.3.1), Schroder and co-workers474 reasoned that the homo[15]annu-lenyl cation 229 can be formed by the protonation of the [16]annulene (Scheme 3.7). 

Twin Excited States of Cyclobutadiene, Cyclooctatetraene, and Antiaromatic Annulenes... 

Next we apply this simple model to the annulenes. These compounds are monocyclic conjugated polyenes with the general molecular formula (CH) with n even. Thus benzene may be considered as [6]-annulene. As n increases, essentially all [ ]-annulenes are non-planar. For instance, cyclooctatetraene, (CH)s, has the well-know tub structure. However, [18]-annulene is nearly planar, as shown below (bond lengths in picometers are displayed in bold italic font). 

The general name annulene is sometimes given to rings that contain alternating single and double bonds in a single Lewis structure. Thus, benzene can be called [6]annulene, and cyclooctatetraene can be called [8]annulene. A number of larger annulenes have been prepared to determine whether they follow Hiickel s rule and are aromatic when they have 4n + 2 electrons in the cycle. 

For the double bonds to be completely conjugated, the annulene must be planar so the p orbitals of the pi bonds can overlap. As long as an annulene is assumed to be planar, we can draw two Kekule-like structures that seem to show a benzene-like resonance. Figure 16-3 shows proposed benzene-like resonance forms for cyclobutadiene and cyclooctatetraene. Although these resonance structures suggest that the [4] and [8]annulenes should be unusually stable (like benzene), experiments have shown that cyclobutadiene and cyclooctatetraene are not unusually stable. These results imply that the simple resonance picture is incorrect. 

Cyclooctatetraene is [8]annulene, with eight pi electrons (four double bonds) in the classical structure. It is a (41V) system, with N = 2. If Htickel s rule were applied to cyclooctatetraene, it would predict antiaromaticity. However, cyclooctatetraene is a stable hydrocarbon with a boiling point of 153 °C. It does not show the high reactivity associated with antiaromaticity, yet it is not aromatic either. Its reactions are typical of alkenes. 

Large-Ring Annulenes Like cyclooctatetraene, larger annulenes with (4A0 systems do not show antiaromaticity because they have the flexibility to adopt nonplanar conformations. Even though [12]annulene, [16]annulene, and [20]annulene are (4A0 systems (with N = 3,4, and 5, respectively), they all react as partially conjugated polyenes. 

Annulenes (meaning ring alkenes) are compounds with alternating double and single bonds. The number in brackets tells us how many carbon atoms there are in the ring. Using this nomenclature, you could call benzene [6]annulene and cyclooctatetraene [8]annulene— but don t. 

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