Cyclooctatrienes, formation

Relatedly, one would have expected 1,3,5-cyclooctatriene to have a more negative enthalpy of formation than tropilidene by the same —20.6 kJmol-1. By contrast, the difference for these enthalpies of formation of species 86 and 87 as derived from experimentally measured enthalpies of formation is ca +12 kJ mol-1. From this we may deduce that tropilidene enjoys considerable stabilization due to homoaromatic interactions. While this conclusion is not new64, nonetheless we find it encouraging to see it corroborated. 

We suspect fewer problems would have arisen had Oth and coworkers (see Reference 97) decided to perform enthalpy of hydrogenation measurements on [18]annulene. Nonetheless, we note that Oth s suggested value for the enthalpy of formation of benzo-l,3,5-cyclooctatriene is within 2 kJ mol of that estimated summing Roth s enthalpy of formation of 1,3,5-cyclooctatriene and Liebman s (cited in Reference 68) benzoannelation constant. 

The predicted conrotatory cyclization of octatetraenes was confirmed for the case of the methyl-substituted compounds, which above 16 °C readily formed the cyclooctatrienes shown in equations 13 and 14)14. We conclude this section with an electrocyclic reaction involving ten TT-electrons, that is, the formation of azulene (17) when the fulvene 16 is heated (equation 15)15,16. 

The direct irradiation of 1,3,5-cyclooctatriene (184) in ether or hydrocarbon solvents leads to the slow formation of two stable isomers corresponding to disrotatory 47T-electrocyclization (185) and bicyclo[3.1.0]pentene (186) formation along with small amounts of the reduced product 187 (equation 69)279-281. Conventional flash photolysis experiments later showed that, in fact, the main primary photochemical process is the formation of a short-lived stereoisomer (r = 91 ms)282, most likely identifiable as ,Z,Z-184. The transient decays to yield a second transient species (r = 23 s) identified as Z,Z-l,3,5,7-octatetraene (188), which in turn decays by electrocyclic ring closure to regenerate 184282 (equation 70). The photochemistry of 184 has been studied on the picosecond timescale using time-resolved resonance Raman spectroscopy49. 

Let us compare anion-radicals with dianions, which are definitely stronger bases. For example, the cyclooctatetraene dianion (CgHg ) accepts protons even from solvents such as dimethylsulfoxide (DMSO) and V,V-dimethylformamide. The latter is traditionally qualified as an aprotic solvent. In this solvent, the cyclooctatetraene dianion undergoes protonation resulting in the formation of cyclooctatrienes (Allendoerfer and Rieger 1965) + 2H+ CgHjo. It is seen that... 

With the benzocycloheptatriene (83) 1,5-addition occurs with concomitant formation of the cyclopropanecyclooctatriene (84) (Equation (29)) <87TL555, 88AX(C)660>. Addition to the bicyclo[4.2.0]octa-2,4-diene tautomer is the predominant mode with cyclooctatrienes (Equation (30)) <85CB3357>. 

In their first report [93], Wagner and Nahm describe the photochemical formation of yellow cyclooctatrienes from ortho- and para-alkenyloxyacetophe-nones and ortho- and para-valerophenones. In another publication [94], they reported further on the process and showed that the final product in most cases is a bicyclo[4.2.0]octa-2,7-diene derivative (Scheme 17). 

The formation of the cyclooctatrienes was quenched in the presence of low concentrations of 2,5-dimethylhexa-2,4-diene or 2,3-dimethylbuta-l,3-diene. The photocyclization of the trienes was, however, unaffected by the presence of the dienes. The authors conclude that the primary photoreaction of intramolecular ortho cycloaddition involves the 3 tt, tt state of the arene and the subsequent cyclization of the cycloocta-l,3,5-triene arises from the singlet state. 

In the formation of tetraenes from bicyclo[4.2.0]octa-2,4-dienes, two bonds are broken. This may occur in one concerted reaction which can be regarded as a retro [2 + 2] cycloaddition. It is also possible that the central bond, being part of a cyclohexadiene system, is the first one to break in a thermal, concerted disrota-tory process that leads to a 1,3,5-cyclooctatriene derivative. Ring opening of the cyclooctatriene then might take place photochemically, again disrotatory, to produce a tetraene. This two-step sequence was first observed by Mirbach et al. [114] in their study of the photocycloaddition of the two parent molecules benzene and ethene. The same explanation for the formation of a tetraene was given by Nuss et al. [160] in their report on the intramolecular ortho photocycloaddition of ( )-6-(2-methoxyphenyl)-5,5-dimethyl-2-hexenenitrile (see Scheme 40). 

The formation of a derivative of cyclooctatriene had earlier been observed by Bryce-Smith et al. [153] during their investigation of the photoaddition of cis-cyclooctene to hexafluorobenzene. One of the products of this reaction is 2,3,4,5,6,7-hexafluorobicyclo[6.6.0]tetradeca-2,4,6-triene, formed by ring opening of the primary ortho adduct. The ortho adducts derived from hexafluorobenzene and its derivatives seem especially prone to undergo this isomerization (see below), but for derivatives of benzene itself, the reaction was not often reported prior to 1987. The study by Mirbach et al. [114] has been referred to earlier and Atkins et al. [120] mention in a note that endo-ortho adducts might be converted... 

The first investigations by Bryce-Smith et al. [46,67,153] on ortho photocycloaddition of an alkene to hexafluorobenzene have revealed yet another secondary reaction of ortho photocycloadducts. Irradiation of a solution of hexafluorobenzene in r/.v-cyclooctene leads to the rapid formation of seven adducts of which six were identified (i) the exo-meta adduct, (ii) a product that can be formed from the meta adduct by a thermal 1,5 H-shift but which apparently is also a primary product, (iii) an ortho adduct of which the configuration could not be established, (iv) a cyclooctatriene derivative formed by thermal ring opening of the ortho adduct, and (v) and (vi) two stereoisomers of 2,3,4,5,6,7-hexaflu-orotetracyclo[6.6.0.02,7.03,6]tetradec-4-ene. The experiment was repeated 9 years later by Sket et al. [151] with the important difference that cyclohexane was used as a diluent. The meta adduct (i) and its formal rearrangement product (ii) were not found. One ortho adduct (iii), the cyclooctatriene (iv), and the two tetracyclic products (v) and (vi) could be identified and their stereochemistry determined. From their results, the authors concluded that a second ortho adduct with the alternative stereochemistry must also have been formed. They also performed experiments in which the influence of the solvent on the course of the reaction was studied and found that the difference between their results and those of Bryce-... 

Recently, a formal ruthenium-catalyzed [4+2+2] cycloaddition of 1,6-diynesto 1,3-dienes gave conjugated 1,3,5-cyclooctatrienes and vinylcyclohexadienes [94] (Eq. 73). Insertion of a double bond in the ruthenacyclopentadiene can lead to the formation of tetraenes or vinyltrienes which undergo a thermal elec-trocyclization. 

Two successive cycles of Hofmann elimination lead to formation of cyclooctatriene. 

Under nominally aprotic conditions, 1,2-protonation dominates in naphthalene. Reduction of naphthalene in anhydrous acetonitrile containing tetraethyl ammonium p-toluenesulfonate yields 1,2-dihydronaphthalene, which is subsequently reduced to tetralin [169]. Similarly, reduction of I in anhydrous DMF gives 1,3,5-cyclooctatriene almost exclusively [53]. The formation of the thermodynamically more stable products is most probably due to base-catalyzed isomerization. 

Irradiation of (164) in a matrix of frozen benzene and THE brings about different photochemical results from those obtained from irradiation in solution phase. The solution phase reaction was carried out in benzene with added trifluoroacetic acid. This treatment yielded only the cyclooctatriene derivative (165). In the frozen system meta addition to the benzene ring results in the formation of the three products (166), (167) and (168). The photochemical reaction of the same chlorouracil (164) in the presence of p-xylene results in the... 

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