Octa-1.3.5,7-tetraene

The size of the K-system chosen has important implication on the structural and functional aspects of metal binding. To explore the size effect calculations were performed on the cation-ir complexes of Li+ and Mg + with the Jt-face of linear and cyclic unsaturated hydrocarbons [45]. In the case of the acyclic Jt-systems, we started with the simplest system, e.g. ethylene followed by buta-1,3-diene, hexa-l,3,5-triene, and octa-1,3,5, 7-tetraene with 2, 3 and 4, conjugated jt units, respectively. These linear systems with two and more number of jt units can have various conformations wherein the jt units can have cis, trans or a combination of both cis and trans orientations. Similarly for cyclic systems cyclobutadiene, benzene, cyclooctateraene, naphthalene, anthracene, phenanthrene and naphthacene have been included. Thus a wide range of sizes for aromatic systems have been covered. 

Calculated transition structure for the electrocyclic closure of (3Z,5Z)-octa-1,3,5,7-tetraene to cycloocta-1,3,5-triene. (Adapted from reference 174.)... 

Modern experimental measurements and the new computational techniques just discussed are now providing results that can rationalize issues such as the efficiency of 1C at a surface crossing, the competition with fluorescence when an excited state barrier is present, and the relationship between the molecular structure at the intersection and the structure of the photoproducts. Experiments on isolated molecules in cold-matrices or expanding-jets have revealed the presence of thermally activated fast radiationless decay channels. For example, Christensen et al. have proposed that (under isolated conditions in a cool-jet) trans — cis motion in all-tra 5-octa-1.3,5,7-tetraene (all-trow -OT) induces the opening of an efficient nonadiabatic radiationless deactivation channel on Si (2Ag). We now discuss this experiment and complementary theoretical results that illustrate the way in which theory and experiment can be used in concert. 

Dienes react with molybdenum hexacarbonyl to give complexes of the type [Mo(CO)4(diene)] and [Mo(CO)2(diene)2], which are generally yellow, soluble in organic solvents, and readily sublimed. Cyclo-octa-1,5-diene (12, 79, 151), bicyclo[2,2,l]hepta-2,5-diene (175), a dimer of cyclo-octa-tetraene (12) and dimethyldivinylsilane (158) give the former type of complex with the structure (VI M = Mo), while butadiene (81) and cyclo-hexa-1,3-diene form the latter type (80). Tetracyclone gives the complex [Mo(CO)2(tetracyclone)2] (215). 

Cyclo-octatetraene.—Synthesis. Full details have been published on the preparation of stable cyclo-octatetraene bond-shift isomers. 1,2,3,8-Tetramethylcyclo-octa-tetraene was prepared from sulphone (140) by dialkylation, reduction, bromination, and dehydrobromination and shown to give Diels-Alder adduct (141 = H,... 

Cyclo-octa-l,5-diyne, prepared in 2% yield by dimerization of buta-l,2,3-triene, has been examined by X-ray diffraction and shown to be almost planar with C—C C bond angles of 159.3°. Irradiation of cyclo-octa-l,5-diyne gave buta-l,2,3-triene, and treatment with ba gave cyclo-octa-tetraene. Cyclo-octa-l,5-diyne underwent Diels-Alder reactions with two molecules of buta-1,3-diene and 2,3-dimethylbuta-1,3-diene, but gave complex mixtures of products when treated with bromine, iodine, dimethyl acetylenedicarboxylate, or tcne. ... 

Alternatively, they can be prepared from cyclo-octatetraene and the finely divided metals [127]. X-ray powder diffraction data indicate that these complexes are isomorphous with uranocene. In a similar manner, bis(cyclo-octa-tetraene)protactinum has also been synthesized [134] ... 

The most direct comparision with experiment can be made with the energy barriers to nonadiabatic isomerization. The computed versus experimental data are summarized in Figure 10 for butadiene = 4, hexatriene n = 6 and octa-tetraene n = 8, It can be seen that theoretical and experimental barriers agree to ca. 2 kcal mol . ... 

Ihble 3 Transition-structure Geometries, Activation Energies, and Reaction Energies of the Electrocyclic Ring Closure of 1,3,5,7-Octa-tetraene... 

What is the symmetry of the highest energy molecular orbital containing electrons in 1,3,5,7-octa-tetraene ... 

A number of binuclear complexes of stoicheiometry olefin Fe2(CO)6 (where olefin = cycloheptatriene, cyclo-octa-l,3,5-triene or cyclo-octa-tetraene) have each been shown by Mdssbauer studies to have equivalent iron atoms [87]. On the basis of this evidence it is proposed that in all cases the olefins bond to each iron by a r-enyl s tem (see 2.26). For example. 

Diazabicyclo[3,3,0]octa-2,6-dienes of type 391 undergo a sequence of electrocyclic reactions upon thermolysis and photolysis (82JPC338) to yield 4,5-diazaocta-l,3,5,7-tetraenes (393), probably via 392 (79CB2620). The influence of the substitution pattern on the conditions necessary for initiation of the reaction and on the valence isomeric equilibria have been studied (86JPC5552). 

Chemical Abstracts would name this as 8-phenyl-7-azabicyclo[4.2.01-octa-l,3,5,7-tetraene, which is unnecessarily complicated. Use of a replacement modification of a fusion name presents difficulties although the corresponding carbocycle should, in accord with IUPAC Rules, be named cyclobutabenzene, it is almost exclusively referred to in the literature as benzocyclobutene. Thus the name 2-phenyl-1-azacyclobuta benzene would be strictly correct, but 2-phenyl-1-azabenzocyclobutene is more in line with current usage. The fusion name based on azete avoids this problem. 

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). 

Using a relative rate method, rate constants for the gas-phase reactions of O3 with 1- and 3-methylcyclopentene, 1-, 3- and 4-methylcyclohexene, 1-methylcycloheptene, cw-cyclooctene, 1- and 3-methylcyclooctene, cycloocta-1,3- and 1,5-diene, and cyclo-octa-l,3,5,7-tetraene have been measured at 296 2 K and atmospheric pressure. The rate constants obtained (in units of 10-18 cm3 molecule-1 s-1) are as follows 1-methylcyclopentene, 832 24 3-methylcyclopentene, 334 12 1-methylcyclohex-ene, 146 10 3-methylcyclohexene, 55.3 2.6 4-methylcyclohexene, 73.1 3.6 1-methylcycloheptene, 930 24 d.s-cyclooclcnc, 386 23 1-methylcyclooctene, 1420 100 3-methylcyclooctene, 139 9 d.v.d.v-cycloocta-1,3-diene, 20.0 1.4 cycloocta- 1,5-diene, 152 10 and cycloocta-l,3,5,7-tetraene, 2.60 0.19 the indicated errors are two least-squares standard deviations and do not include the uncertainties in the rate constants for the reference alkenes (propene, but-l-ene, d.s-but-2-ene, trans-but-2-ene, 2-methylbut-2-ene, and terpinolene). These rate data were compared with the few available literature data, and the effects of methyl substitution have been discussed.50... 

DFT investigation of the eight-electron electrocyclization reactions of 1,8-disubsti-tuted (3Z,5Z)-octa-l,3,5,7-tetraenes has found that these reactions proceed via a Mobius helical aromatic transition state. It was also found that outward substituents were preferred to inward ones, regardless of the electronic nature of the substituents, and that torquoelectronic effects are overridden by secondary orbital, electrostatic,... 

Bicyclo[3.3.0]octa-l,3,5,7-tetraene (2), trivially called pentalene [26, 27], is the second member in the series of fully unsaturated oligoquinanes. Huckel MO theory predicts that this planar hydrocarbon with its 8 r-electron system should be an antiaromatic species [25]. 2-Methylpentalene (37) has been generated by a retro-Diels-Alder reaction and deposited as a film at —196 °C on a NaCl or quartz plate for its spectroscopic characterization. It rapidly dimerized upon warming the cold plates to temperatures above —140 °C [26]. Only two stable derivatives of pentalene not complexed to a metal [29], the hexaphenyl- (38) [30] and 2,4,7-tri-tert-butylpentalene (39) [31], have hitherto been reported (Figure 4). 

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