1,3,5,7-Octatetraene formation

The formation of the benzocyclo-octatetraene (26) undoubtedly involves a singlet species which undergoes benzo-vinyl bridging rather than the vinyl-vinyl bridging of the triplet which leads, eventually to compound (27) 51>52>. In accord with this the photo-isomerisation of the adduct (28) formed by the reaction of tetrafluorobenzyne with cyclohexa-1,3-diene, results in the formation of tetrafluorobenzo-dihydrosemibull-valene (29) 53>. 

For the synthesis of the bis-cyclo-octatetraene compound [5] (Krummel et al, 1987 Auchter-Krummel and Mullen, 1991), cyclo-octatetraene dianion was quenched with tetrabromoneopentane to give the bis-adduct [23], which exists in an equilibrium between valence isomers [23a] and [23b]. Hexacycle [23a] was actually isolated in about 60% yield (Fig. 2) (Krummel et al, 1987). Accordingly, in the subsequent dehydrogenation, the formation of [23a] must be avoided by working at low temperatures in this case it was possible to deprotonate the originally formed isomer [23b], obtaining a... 

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. 

After treatment of zirconacyclopentadienes with 2 equivalents of CuCl at room temperature, the addition of a halogenating agent such as NBS at —78 °C leads to the formation of octatetraenes 57a [7m]. This reaction involves slow bromination of the diene-dicopper compounds and coupling with the alkenyl bromide moiety (Eq. 2.40). 

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. 

If a stepwise cyclization mechanism is assumed—for example, for /j-Cg—two octatriene intermediates may be formed, viz. 1,3,5-octatriene would lead to ethylbenzene, and 2,4,6-octatriene to o-xylene (Scheme II). The dehydrogenation of the latter would give octatetraene, which, in turn, gives styrene via vinylcyclohexadiene. Dehydrogenation and cyclization of octatriene were reported to compete over chromia and molybdena catalysts (67) with less hydrogen present (e.g., in a pulse system with in helium carrier gas), styrene formation is enhanced. 

The semiempirical quantum chemical consideration led to the conclusion that the discrepancy can be a consequence of ion-pair formation (Zuilhof Lodder 1995). In the case of cyclo-octatetraene, the ion-pairing phenomenon deserves more detailed explanation. While the dianion of cyclo-octatetraene is completely planar and meets all the requirements of aromaticity, the anion radical of cyclo-octatetraene is a nonaromatic species and is not completely planar. In the equilibria just considered, both the anion radical and the dianion had alkali cations as counterparts. The dialkali salt of the dianion has two cations symmetrically located over and beneath the octagonal plane. Distortions from ion pairing between the dianion plane and these two alkali cations are reciprocally compensated. With the... 

A single electron transfer from cyclo-octatetraene dipotassium (C8H8K2) to 2- and 4-nitro-stilbenes in THF leads to the formation of paramagnetic potassium salts of the anion radi-... 

White, E. H., and R. L. Stern The formation of acetylenic and benzenoid compounds in the photodecomposition of 1.2.4.7-tetraphenylcyclo-octatetraene. Tetrahedron Letters 1964, 193. 

The second class of volatile products observed were hydrocarbons, namely the ionene compounds. The formation of these hydrocarbons during heating is also reflective of deodorization and frying conditions. The formation of low molecular weight aromatic hydrocarbons results from fragmentation of the carotene molecule. The losses of toluene and ionene compounds from B-carotene yield do-decahexaene and octatetraene, respectively. These nonvolatile degradation products have been previously reported in our laboratory (13, 14). 

The ligand L, which can be a triphenylphosphine molecule, hinders the fourth molecule of the acetylene from coordinating and, by preventing the formation of metallic nickel, makes the process catalytic. In fact, the same Schrauzer (163) obtained the polymerization of acetylene to cyclo-octatetraene by a stoichiometric reaction with bisacrylonitrilenickel without any phosphine. He interpreted the reaction course as the formation of the intermediate, Ni(C2H2)4, which then gives metallic nickel and the tetramer ... 

Other, less acidic alcohols show no reaction, while phenol derivatives result in the formation of dineopentyl derivatives. Finally, binuclear molybdenum alkylidenes are obtained by reaction of a Schrock carbene with a.cu-dienes such as divi-nylbenzene or with octatetraene [91]. 

Singlet sensitization of tetramethyl-l,2-dioxetan by pyrene results in the formation of triplet acetone.296 In a theoretical study related to the one reported in ref. 293, it was concluded that the triplet surface intersects the ground-state surface between (15) or (16) and cyclo-octatetraene. Evidence for triplet product... 

A report of an unidentified single product from the irradiation of protonated cyclo-octatetraene (174) has been made recently.113 A re-examination of the problem has identified the ion as (175).114 This ion can be produced by the protonation of the dihydropentalene (176). The formation of the ion (175) photochemically probably arises by an allowed disrotatory closure between C-l and C-5. This is followed by a double-hydride migration. These reactions are pictured in Scheme 19. A review of such processes has been published.115 The... 

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