Cyclooctatetraene, rearrangements

Cyclooctatetraene can be obtained on an industrial scale by metal carbonyl catalyzed thermal tetramerization of acetylene. If cyclooctatetraene is UV-irradiated at low temperature in the presence of acetone, it is reversibly rearranged to form semibullvalene (H.E. Zimmerman, 1968, 1970). 

The existence of a metalated epoxide was first proposed by Cope and Tiffany, to explain the rearrangement of cyclooctatetraene oxide (8) to cydoocta-l,3,5-trien-7-one (11) on treatment with lithium diethylamide. They suggested that lithiated epoxide 9 rearranged to enolate 10, which gave ketone 11 on protic workup (Scheme 5.4) [4],... 

Benzobarrelene is similar to barrelene in that rearrangement to benzo-cyclooctatetraene takes place from the singlet state while benzosemibullvalene is formed from the triplet state<44> ... 

The influence of radical stabilization on the outcome of the rearrangement reactions of a variety of dibenzobarrelenes has been evaluated178. A detailed analysis of the acetophenone-sensitized conversion of the cyano-substituted barrelenes into the corresponding semibullvalenes has been presented179. The outcome of the irradiation of the dibenzobarrelene 331 is dependent upon the excited state involved. Thus direct irradiation affords a cyclooctatetraene and sensitized irradiation converts it into the two... 

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. 

The interaction of cyclooctatetraene as a dienophile with the diazadiene, 3,6-bis(trifluo-romethyl)-l,2,4,5-tetrazine 226, is accompanied by nitrogen elimination and gives rise to the 1,1-adduct 227. The latter displays interesting thermal rearrangements depending on the solvent polarity and temperature (equation 73)87. In toluene solution a [l,3]-carbon... 

Optical spectroscopy has merits in identifying radical cations, particularly when their spectra are known independently. For example, the radiolysis of quadricyclane led to the observation of the known spectrum of norbornadiene radical cation. In another study, irradiation of cyclooctatetraene radical cation caused the color of the sample to change from bright red to royal blue, suggesting the conversion to a different species, the previously identified semibullvalene radical cation. Further irradiation of the latter led to a characteristic banded (vibrationally resolved) spectrum the nature of this spectrum suggested that the rearranged species may be a linear conjugated radical cation and helped in its identification as 1,4-dihydropen-talene radical cation. ... 

According to the generalized Woodward-Hoffmann rule, the total number of (4q + 2)s and (4r)0 components must be odd for an orbitally allowed process. Thus, Eq. (14) is an allowed, and Eq. (13) a forbidden sigmatropic rearrangement. The different fluxional characteristics of tetrahapto cyclooctatetraene (52, 138) and substituted benzene (36, 43, 125) metal complexes may therefore be related to orbital symmetry effects. 

Tetrahaptocyclohexatriene- and cyclooctatetraene-metal-tricarbonyl complexes undergo some unusual cyclo-addition reactions with tetracy-anoethylene in which the dienophile undergoes a 1,3 addition across the coordinated ligand and causes a rearrangement of the metal-carbon bonds, for example (99),... 

Cyanogen azide reacts with cyclooctatetraene (112) at room temperature in ethyl acetate to give alkylidenecyanamide.326 At 78°, a 31% yield of a mixture of the alkylidenecyanamide (113) (68%) and the 1,4-adduct 114 (32%) is obtained. Since cyanogen azide decomposes above 40° and since the alkylidenecyanamide does not rearrange to the 1,4 adduct under the reaction conditions, one may believe that the 1,4 adduct forms directly from cyanonitrene and cyclooctatetraene. This is a case of 1,4 addition studied by Anastassiou.328... 

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). 

Interestingly, treating (>/4-cyclooctatetraene)Fe(CO)3 with acetyl chloride under Friedel-Crafts reaction conditions yielded unexpectedly222-223 the (>/2,>/3-8-e.x0-acetyl bicy-clo[3.2.1]octadienylium)Fe(CO)3 cation complex, presumably by rearrangement of the intermediate bicyclo[5.1.0]octadienylium isomer (Scheme 8). The structure of the rearranged cation was confirmed from the X-ray crystal structure and from the typical 1,3-cr.ji-allylic products obtained upon nucleophilic reaction with LiAlD4 and NaCN. The nucleophilic reaction of the more bulky iodide occurs, however, on the metal. 

Exercise 22-41 Write reasonable mechanisms for the different oxidation reactions of cyclooctatetraene with mercuric ethanoate in ethanoic acid, methanol, and water solutions. Notice that compounds of the type Hg(OR)2 appear to act in some cases as OR-donating agents and also that the oxide produced from cyclooctatetraene and peroxyacids (Section 15-11C) rearranges readily in the presence of acids to phenylethanal. 

The most intriguing hydrocarbon of this molecular formula is named bullvalene, which is found in the mixture of products of the reaction given above. G. Schroder (1963, 1964, 1967) synthesized it by a thermal dimerization presumably via diradicals of cyclooctatetraene and the photolytical cleavage of a benzene molecule from this dimer. The carbon-carbon bonds of bullvalene fluctuate extremely fast by thermal Cope rearrangements. 101/3 = 1,209,600 different combinations of the carbon atoms are possible. 

In addition to these rearrangements of more general importance, a number of isomerizations have been observed which apply only to specific cases. Some cycloolefines, like cyclooctatetraene and cycloheptatriene, are converted almost quantitatively to the alkylaromats styrene and toluene respectively 18). Of possible interest for preparative chemistry are ring-chain isomerizations which have been observed with certain nitrogen compounds 19). When pyrrole is subjected to a discharge it is largely converted to croton nitrile. 

Triamantane (13) is obtained from the rearrangement of the heptacyclic dimer of cyclooctatetraene (18), after it has been elaborated to the C1S level by Simmons-Smith cyclopropanation and subsequent hydrogenolysis (Eq.(9))33l. The structure of the product, obtained in 5% yield, is again confirmed by an X-ray analysis 34>. 

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