Annulenes cyclooctatetraene

Azocine (4), even before its detection by Hedaya,48 was of theoretical interest because it is a heterocyclic analog of the 871 nonaromatic annulene cyclooctatetraene (COT). Although azocine itself was found to be quite unstable, earlier work by Paquette and co-workers52,53 on the synthesis of azocine derivatives indicated that appropriately substituted compounds could be more stable. Paquette68 has recently reviewed the chemistry of these compounds. The picture of azocine vis a vis cyclooctatetraene682 is outlined below. 

The [nlannulenes may be defined as cyclic polyaUcenes possessing a closed circuit of n Ti-conjugated pz-orbitals. The first three small annulenes are shown in Fig. 1 and include [4]annulene (cyclobutadiene), [6]annulene (benzene), and [8]annulene (cyclooctatetraene). The bracketed number, n, can be classified as either a 4n+2 (Hiickel aromatic/Mobous antiasomatic) or 4n (Huckel antiaromatic/ Mobius aromatic) delocalized n-electron species. That said, it is not surprising that the concept of aromaticity is closely associated with the annulenes. Indeed, the first three annulenes listed cover the concepts of Hiickel aromatic, antiaromatic, and nonaromatic properties (see Fig. 1). 

The next higher annulene, cyclooctatetraene, is readily determined to be nonaromatic. The bond lengths around the ring alternate as expected for a polyene, and thermochemical data provide no evidence of special thermodynamic stability. Neither is the molecule antiaromatic. It is readily isolated and has the chemical behavior of a polyene. The molecule has a tub shape,and therefore is not a planar system of the type to which the Huckel rule applies. 

The next higher annulene, cyclooctatetraene, is readily determined to be nonaromatic. The bond lengths around the ring alternate as expected for a polyene,... 

The first such system was prepared in 1956 by Sondheimer it was 1,3,5,7,9,11,13,15,17-cyclooctadecanonaene, containing 18 tt electrons (4n + 2, n = 4). To avoid the use of such cumbersome names, Sondheimer introduced a simpler system for naming cyclic conjugated polyenes. He named completely conjugated monocyclic hydrocarbons (CH)jv as [(Vjannulenes, in which N denotes the ring size. Thus, cyclobutadiene would be called [4]amiulene benzene, [6]annulene cyclooctatetraene, [8]annulene. The first almost unstrained aromatic system in the series after benzene is [18]annulene. 

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... 

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