Cyclooctatetraenes, preparation

The first breakthrough came m 1911 when Richard Willstatter prepared cyclooc tatetraene by a lengthy degradation of pseudopelletienne a natural product obtained from the bark of the pomegranate tree Today cyclooctatetraene is prepared from acetylene m a reaction catalyzed by nickel cyanide... 

CyclooctatetraenylCompounds. Sandwich-type complexes of cyclooctatetraene (COT), CgH g, are well known. The chemistry of thorium—COT complexes is similar to that of its Cp analogues in steric number and electronic configurations. Thorocene [12702-09-9], COT2Th, (16), the simplest of the COT derivatives, has been prepared by the interaction of ThCl [10026-08-1] and two equivalents of K CgHg. Thorocene derivatives with alkyl-, sdyl-, and aryl-substituted COT ligands have also been described. These compounds are thermally stable, air-sensitive, and appear to have substantial ionic character. 

The Hiickel rule predicts aromaticity for the six-7c-electron cation derived from cycloheptatriene by hydride abstraction and antiaromaticity for the planar eight-rc-electron anion that would be formed by deprotonation. The cation is indeed very stable, with a P Cr+ of -1-4.7. ° Salts containing the cation can be isolated as a product of a variety of preparative procedures. On the other hand, the pK of cycloheptatriene has been estimated at 36. ° This value is similar to those of normal 1,4-dienes and does not indicate strong destabilization. Thus, the seven-membered eight-rc-electron anion is probably nonplanar. This would be similar to the situation in the nonplanar eight-rc-electron hydrocarbon, cyclooctatetraene. 

Chemists in the early 1900s believed that the only requirement for aromaticity was the presence of a cyclic conjugated system. It was therefore expected that cyclooctatetraene,. as a close analog of benzene, would also prove to be unusually stable. The facts, however, proved otherwise. When cyclooctatetraene was first prepared in 1911 by the German chemist Richard Willstatter, it was found not to be particularly stable but to resemble an open-chain polyene in its reactivity. 

Three decades ago the preparation of oxepin represented a considerable synthetic challenge. The theoretical impetus for these efforts was the consideration that oxepin can be regarded as an analog of cyclooctatetraene in the same sense that furan is an analog of benzene. The possibility of such an electronic relationship was supported by molecular orbital calculations suggesting that oxepin might possess a certain amount of aromatic character, despite the fact that it appears to violate the [4n + 2] requirement for aromaticity. By analogy with the closely related cycloheptatriene/norcaradiene system, it was also postulated that oxepin represents a valence tautomer of benzene oxide. Other isomers of oxepin are 7-oxanorbornadiene and 3-oxaquadricyclane.1 Both have been shown to isomerize to oxepin and benzene oxide, respectively (see Section 1.1.2.1.). 

The 1.4-dihydro-l,4-diazocines prepared from iyn-benzene diimines (Section 1.4.1.2.) can be transformed to other derivatives by exchange of the substituents at nitrogen. For this purpose, the dipotassium salt of 1,4-diazocine is generated and then reacted with appropriate electrophiles. For example, reduction of the bistosyl derivative 3 gives a relatively stable dianion, a lOrr-electron system analogous to cyclooctatetraene dianion, which on protonation clearly gives the parent l,4-dihydro-l,4-diazocine (4, E = H) as the only product. 

A general entry into the azonine system, c.g. 4, is by photoinduced electrocyclic ring opening of the bicyclic valence isomer 3,9 11-13 which is readily prepared by earboethoxynitrene addition to cyclooctatetraene. 

Few 1 -benzothiophene-S-oxides 218 were obtained in moderate yields by treatment of 1-arylacetylenes 219 with sulfur dioxide and benzene in the presence of antimony pentafluoride250 (equation 127). A series of cyclic sulphoxides have been prepared by hydrolysis of the corresponding alkoxy sulphonium salts 220251-254 (equation 128). Syn-sulphoxide 221 was obtained in a low yield (15-20%) in the reaction of the dianion of cyclooctatetraene 222 with thionyl chloride255 (equation 129). 

Synthetic applications of other decarbonylation reactions are found in the conversion of cyclooctatetraene to barrelene 250), with the photodecarbonyla-tion of a Diels-Alder adduct as key step (2.31) and the preparation of tetrathioesters from 1,3-dithioles (2.32) 251). The most remarcable application of such a reaction up to date is the synthesis of tetra t.butyltetrahedrane from a tricyclic ketone precursor (2.33) 252). 

Eq. 2.41. Preparation of a tricyclic cyclooctatetraene from a bicyclic zirconacyclopentadiene and a diiododiene. 

Our success in super-stabilization of cation 6 led us to the preparation of a higher homologue, that is, cyclooctatetraene (COT), fully annelated with BCO units 9 (9). As compared with a large number of studies on its radical anion or dianions, the studies on the cationic species of COT have been quite limited. There have been only one study by Olah and Paquette on the substituted COT dication (70), which is a typical 6n Hiickel aromatic system, and few sporadic studies on radical cations, which involve indirect spectral observations, such as electronic spectra in Freon matrix at low temperature (77,72) and constant-flow ESR study (13). 

Hisano and coworkers298 prepared tricycle 476 by reaction of cyclopentadienone 475 with cyclooctatetraene (474) in refluxing benzene (equation 141). Cyclized [4 + 2] cycloadduct 477 was isolated as a by-product. 

The numerous transformations of cyclooctatetraene 189 and its derivatives include three types of structural changes, viz. ring inversion, bond shift and valence isomerizations (for reviews, see References 83-85). One of the major transformations is the interconversion of the cyclooctatetraene and bicyclo[4.2.0]octa-2,4,7-triene. However, the rearrangement of cyclooctatetraene into the semibullvalene system is little known. For example, the thermolysis of l,2,3,4-tetra(trifluoromethyl)cyclooctatetraene 221 in pentane solution at 170-180 °C for 6 days gave three isomers which were separated by preparative GLC. They were identified as l,2,7,8-tetrakis(trifluoromethyl)bicyclo[4.2.0]octa-2,4,7-triene 222 and tetrakis(trifluoromethyl)semibullvalenes 223 and 224 (equation 71)86. It was shown that a thermal equilibrium exists between the precursor 221 and its bond-shift isomer 225 which undergoes a rapid cyclization to form the triene 222. The cyclooctatetraenes 221 and 225 are in equilibrium with diene 223, followed by irreversible rearrangement to the most stable isomer 224 (equation 72)86. 

Uranocene itself was prepared by allowing cyclooctatetraene (COT) to react with potassium in dry, oxygen-free tetrahydrofuran (THF) at —30° followed by the addition of a THF solution of anhydrous uranium tetrachloride ... 

The synthesis of bis(rj8-cyclooctatetraene)uranium(IV) (uranocene)J from uranium tetrachloride and (cyclooctatetraene)dipotassium was first published in 1968.1 The method reported here is a modification of that procedure and is suitable for a large variety of cyclooctatefraene complexes.2-4 BisO 8-cyclo-octatetraene)uranium(IV) has also been prepared by the reaction of uranium tetrafluoride with (cyclooctatetraene)magnesium in the absence of solvent.5 Direct reaction of finely divided uranium metal with cyclooctatetraene vapors at 150° also gives some uranocene.5 However, both methods give low yields. 

Eight-membered heterocyclic systems embrace a very broad variety of compounds ranging from cyclic ethers and imines to highly labile analogs of cyclooctatetraene, and the properties and chemical behavior of these compounds of course depend entirely on the extent and location of unsaturation. For this reason, preparative methods and reactions are treated together for each ring system and oxidation level. 

Fully unsaturated azocines are n- equivalent heterocyclic analogs of cyclooctatetraene. Paralleling the latter, the few azocines that have been prepared exist in the monocyclic form, but reactions of azabicyclo[4.2.0] isomers are observed. In the 1,2-dihydroazocine series the equilibrium between mono- and bi-cyclic valence isomers can be controlled by alkoxycarbonyl substituents. 

By use of a similar method other more stable 1-substituted phosphiranes are prepared, 1-phenylphosphirane (123) being the most stable. When the cyclooctatetraene dianion is treated with dichlorophenylphosphine the bicyclic 1-phenylphosphirane derivative (124) is formed. At 70 °C in CHC13 it rearranges into the bicyclic phosphabicyclo[4.2.1]heptane derivative (125). Oxidation and photochemical reaction give the tricyclic phospholene 1-oxide derivative (126 equation (76)) (66JA3832). 

Cyclooctatetraene can be prepared readily by polymerization of ethyne in the presence of nickel cyanide ... 

Efforts to prepare pentalene, a bridged analog of cyclooctatetraene, have not been very successful so far. A substance that appears to be a methylpenta-... 

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