Octatetraenes 1,3,5,7-Octatetraene

Another example of the role played by a nonradiative relaxation pathway is found in the photochemistry of octatetraene. Here, the fluorescence lifetime is found to decrease dramatically with increasing temperature [175]. This can be assigned to the opening up of an efficient nonradiative pathway back to the ground state [6]. In recent years, nonradiative relaxation pathways have been frequently implicated in organic photochemistry, and a number of articles published on this subject [4-8]. 

Figure 8-6 Approximate x-y Coordinates for the Alternant Hydrocarbons Ethene Through 1,3,5.7-Octatetraene.

Polymerization of alkynes by Ni" complexes produces a variety of products which depend on conditions and especially on the particular nickel complex used. If, for instance, O-donor ligands such as acetylacetone or salicaldehyde are employed in a solvent such as tetrahydrofuran or dioxan, 4 coordination sites are available and cyclotetramerization occurs to give mainly cyclo-octatetraene (cot). If a less-labile ligand such as PPhj is incorporated, the coordination sites required for tetramerization are not available and cyclic trimerization to benzene predominates (Fig. A). These syntheses are amenable to extensive variation and adaptation. Substituted ring systems can be obtained from the appropriately substituted alkynes while linear polymers can also be produced. 

However, uranocene can be made more air-stable by use of sufficiently bulky substituents, and 1,3,5,7-tetraphenylcyclo-octatetraene yields the completely air-stable [U( -CgH4Ph4)2], in which the parallel ligands are virtually eclipsed but the phenyl substituents staggered and rotated on average 42° out of the Cg ring plane (Fig. 31.10). 

The structure of product 3 is tub shaped, in agreement with the corresponding dibenzocyclo-octatetraene. 

While (Z)-l,2-bis(phenylsulfonyl)ethylene (140) does not add to dienes such as furan, cyclopentadiene, cyclo-octatetraene, indene and /f-naphthol, ( )-l,2-bis(phenylsulfonyl)ethylene (141) is more reactive and the reaction with furan proceeds at room temperature for 2 h to give the adduct in 95% yield. The reactivity of dienophiles having sulfonyl group in the [4 + 2]cycloaddition is shown in equation 10393,101. 

The pole strength profiles obtained in the outer valence region of the 1,3-trans butadiene, 1,3,5-trans hexatriene and 1,3,5,7-trans octatetraene compounds relate also typically to that found (10) with low-gap hydrogen chains. They nicely reflect the competition for intensity between the main and the correlation i.e. satellite) bands in that region. In both cases, the less energetic (HOMO LUMO (10,12)... 

An excellent agreement with the X-ray photoionization spectra of ethylene, butadiene and hexatriene (7) is obtained (12) (Figure 3) when including in our calculations the Gelius (36) photoionization cross sections for an Alka photon beam, by means of Eqs. (4) and (5). Such a direct comparison is impossible for octatetraene, a compound for which there is no available XPS data. 

WUlstatter, R. Waser, E. (1911) Uber Cyclo-octatetraen. Berichte der Deutschen Chemischen Gesellschaft, 44, 3423-3445. 

The overall transformation is the conversion of the carbon-sulfur bonds bond to a carbon-carbon double bond. The original procedure involved halogenation of a sulfide, followed by oxidation to the sulfone. Recently, the preferred method has reversed the order of the steps. After the oxidation, which is normally done with a peroxy acid, halogenation is done under basic conditions by use CBr2F2 or related polyhalomethanes for the halogen transfer step.92 This method was used, for example, to synthesize 1,8-diphenyl-1,3,5,7-octatetraene. 

Benzyne reacts with benzene to give a mixture of products in low yield. The original experiments 38> showed that the 1,4-cyclo-adduct (benzo-barrelene) (19), the valence-bond isomerised 1,2-cyclo-adduct (benzo-cyclo-octatetraene) (20), and the product of insertion into a carbon-hydrogen bond (biphenyl) (21), were obtained in 2,8, and 6% yields respectively. 

The analogues of the other products formed in the reaction of benzyne with benzene, 2,3,4,5-tetrafhiorobiphenyl (23), octafluorobiphenylene, and tetrafluorobenzocydo-octatetraene (26), have not been detected in the reactions of tetrafluorobenzyne with benzene. 

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>. 

The nucleophilic attack with 2-700 and 2-701 can proceed in a syn or anti manner to provide either 2-702 or 2-703, or both [357]. If2-703 is formed, it follows a charge-driven conrotatory opening of the cyclobutene ring with generation of the coiled 1,3,5,7-octatetraene 2-704. This intermediate is capable of a rapid helical equilibration [357] and a regioselective 8jt electrocyclic ring closure to give 2-705 [358]. 

Bis-l,2,4-triazole-3,5-diones such as 110 have also been used in Diels-Alder reactions, and give bispyridazines.171 The pyridazine derivative 111 is formed in quantitative yield from PTAD and 2,7-dimethyl-2,3,5,6-octatetraene,172 and the azadiene, 4-aza-l,3,5-triphenylpenta-2,4-diene, also reacts readily with PTAD to give 112.173 There are many other examples of Diels-Alder additions of ADC compounds to simple acyclic dienes which proceed entirely as expected the above selection has been limited to reactions of synthetic potential and with novel features. 

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... 

We will now consider the studies devoted to the next two linear polyenes 1,3,5-hexatriene and 1,3,5,7-octatetraene. First, we will present the results corresponding to geometries and conformational energies computed for these compounds. We will then discuss the computed frequencies and force fields. 

The most stable conformation of both hexatriene and octatetraene is the al -s-trans one. Figure 2 represents these structures schematically. 

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