1,3,6-Cyclooctatriene

Is cyclooctatetraene aromatic To tell, compare the first and second hydrogenation energies, leading to 1,3,5-cyclooctatriene and then to 1,3-cyclooctadiene. (The energy for hydrogen is provided at left.)... 

Similarly, the cyclobutane-fused bicyclic peroxide 19 was prepared by diimide reduction of the corresponding bicyclic endoperoxide derived from 1,3,5-cyclooctatriene (Eq. 14)31a). 

Like the [3.2.2]-peroxide, the parent system 23 could be prepared in two ways, either from 1,3-cyclooctadiene 28 345 or from 1,3,5-cyclooctatriene 31 a> (Eq. 18). 

The diimide reduction again proceeds sluggishly and several recycles are essential to achieve complete conversion. The doubly unsaturated endoperoxide is the major product in the singlet oxygenation of 1,3,5-cyclooctatriene (Eq. 14). 

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 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. 

Cyclooctatetraene was reduced electrochemically to cyclooctatetraenyl dianion. In DMF the product is mostly (92%) 1,3,5-cyclooctatriene at —1.2 V. If the potential is lowered the main product is 1,3,6-cyclooctatriene. Previous experiments, in which the anion radical was found to be disproportionated, were explained on the basis of reactions of the cyclooctatetraene dianion with alkali metal ions to form tightly bound complexes, or with water to form cyclooctatrienes. The first electron transfer to cyclooctatetraene is slow and proceeds via a transition state which resembles planar cyclooctatetraene102. 

XXIII. Valence Tautomerism of 1.3.5-Cyclooctatriene and Bicyclo(4.2.0)-octa-2.4-diene. J. Amer. chem. Soc. 74, 4867 (1952). 

Bimetallic platinum-ruthenium nanoparticles were synthesized from the reaction of Pt(dba)2 (dba = dibenzyhdeneacetone) with Ru(COD)(COT) (COD = 1,5-cyclooctadiene COT = 1,3,5-cyclooctatriene) in various propor-... 

Irradiation of 1,3-cyclooctadiene (Formula 378) which could, in principle, give acyclic products gives the bicyclic valence tautomer, (Formula 379) (167-169). 1,3,5-Cyclooctatriene gives Formula 381 among other products (170). 

The next homolog in the cyclic series, 1,3,5-cyclooctatriene, also closes in a readily reversible transformation to bicyclo[4.2.0]octadiene (Equation 12.63). Cope and his collaborators reported this valence isomerization in 1952. They were able to separate the isomers, which revert to the equilibrium mixture of 85 percent 51 and 15 percent 52 on heating at 100°C for 1 hr.113 Huisgen has reported activation parameters of A Hi = 26.6 kcal mole-1, AS = — 1 cal... 

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 photocycloaddition of alkenes to benzene rings can be classified into three types (2 + 2) (ortho), (3 + 2) (meta), and (4 + 2) (para) photocycloaddition, depending on the substituents, the reaction media, and the additives, as shown in Scheme 3. In general, para addition is very limited, and ortho and para adducts are more labile thermally and photochemically than the meta adducts. The (2 + 2) (ortho) photocycloadducts, bicyclo[4.2.0]octa-2,4-dienes 1, give thermally the ring-opened 1,3,5-cyclooctatrienes 4 and their following photochemical ring clo-... 

In the last 15 years, Wagner and his co-workers have developed the intramolecular ortho photocycloaddition of alkenes to the benzene ring of the acetophenone chromophore. In these photoreactions, the ring opening products of ortho photocycloadducts, 1,3,5-cyclooctatriene derivatives (220), and their secondary photocycloadducts, tricyclic compounds (221), were often produced [271,272] (Scheme 63). The triplet exciplexes generated from ir, ir -triplet states of acetophenone derivatives were postulated as the reactive intermediates [273],... 

---