Hexa-l,3,5-triene

Unlike the well-known chemistry of the vinylcyclopropane-cyclopentene rearrangement, there is no general method for the rearrangement of alkynyl-cyclopropane to cyclopentene derivatives. One specific example is the pyrolysis of l-ethynyl-2-methylcyclopropane to methylenecyclopentene and other compounds [5]. At 530°C, l-ethynyl-2-methylcyclopropane (1) undergoes a [1,5]-hydrogen shift to give hexa-l,2,5-triene (2), which further isomerizes to methy-lenecyclopentenes 3 and 4 in 38 and 29% yield, respectively (Scheme 1). 

To demonstrate the quality of this simple approach we show in Figures 6 and 7 the PE spectra of 1,3-butadiene 2, (3 )-hexa-l,3,5-triene 161, (3E, 5 )-ocla-1,3,5,7-tetraene 234, [3]radialene 173, 3,4-dimethylidenecyclobutene 174 and fulvene 175. The observed positions I j1 of the tt-bands are collected in the third column of Table 2, and the eigenvalues xj obtained from standard HMO models in the fourth. A least-squares calculation yields the linear regression... 

Orbital mapping analysis of thermal isomerization of aza- and diaza-bicyclo[2.2.0]hexa-dienes has been carried out (80MI51205). Molecular orbital calculations (MNDO) of a series of bisdehydropyridines showed that the 2-azabicyclo[2.2.0]hexa-l,3,5-triene (125) is the least stable form having a calculated heat of formation of 728.4 kJ mol-1 (77CC539). 

The benzene cluster 114 can also be prepared from hexa-l,3,5-triene.83 Reaction of 2 with hexa-l,3,5-triene (a mixture of cis and trans isomers) in a sealed system in heptane at 155°C for 21 h results in the formation of the two isomers Ru6C(CO)i4 U-CH2(CH)4CH2 149 and 150 in 3 and 19% yield, respectively, together with the benzene derivative 114 in 22% yield (Scheme 25). 

FIGURE 5. Representative graphs Q for an independent electron model based on Hnear combinations of two-centre jr-orbitals Examples 1,3-bntadiene 2, (3 )-hexa-l,3,5-triene 161, heptafulvene and sesqnifnlvalene... 

Certain hexa-l,3,5-trienes 2, either employed as such or obtained in situ by photochemical conrotatory opening of respective cyclohexa-1,3-dienes 1, cyclize on ultraviolet irradiation to bicyclo[3.1.0]hex-2-enes 3. ... 

The hexa-l,3,5-triene system can be generated and reacted in situ from a 1,2-dihydronaph-thalene system,e.g. or from ort/to-divinylbenzenes. ... 

Bis(diisopropylamino)cycIopropen-2-ylidene amino hexa-l,3,5-triene-l,l-dicarbonitrile (1) Typical Procedure ... 

The scope of the Scheme can be extended to include (1,2-propadiene)-phosphonic dichlorides when an additional equilibrium between the dichloro-phosphonium chloride and a trichloro pentaco-ordinate compound can be considered. Thus, the chlorination of the 3-phosphylated-penta-l,2,4-triene compound (150 R == X = Cl) yields the linear (hexa-l,3,5-triene)-phosphonic dichloride (153 R = X = Cl) via the proposed intermediate equilibrium... 

Polyacetylene reacts with chlorine rapidly to give a white polymer that is equivalent to poly(l,2-dichloroethylene) and no stereospecificity has been reported on the chlorinated polymer. The addition of halogens is an important general reaction of carbon-carbon double bonds in a stereospecific and regiospecific sense. For instance, the electrophilic additions of chlorine to ethylene, buta-1,3-diene and hexa-l,3,5-triene have been shown to proceed by 1,2-trans (anti), 1,4-cis (syn) and 1,6-trans (anti) attack, respectively. This stereospecificity has been rationalized with a mixing rule of a-n orbital interaction. The reaction of a long conjugated polyene like polyacetylene with chlorine may produce an atactic chlorinated polyene, because random l,2n additions occur to result in a random addition product. ... 

FIGURE 8. 2-Methyl-Z-hexa-l,3,5-triene e = 22,400 (259 ntn) conformations and photoproducts. 

Calculate the wavelength of light that will be absorbed when a It electron in hexa-l,3,5-triene, CHj=CH-CH=CH-CH=CH2, is promoted from the highest occupied level to the lowest unoccupied level. The average C-C bond length in hexatriene can be taken to be 144 pm. Compare your answer with the experimentally observed wavelength of 258 nm. 

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. 

The pyrazoline (133) affords only bicyclo[3,l,0]hex-2-ene on thermolysis, but on photolysis this and hexa-l,3,5-triene are obtained. In the vapour phase at pressures below lOTorr the -triene (134) is obtained (8-10%), but at pressures higher than 50Torr the Z-isomer (135) ensues. The formation of triene is thought to proceed in a... 

Goldstein and co-workers have initiated a careful study of the mechanistic possibilities involved in the thermal decomposition of bicyclo[2,2,0]hexene. Analysis of the products, greater than 97 % cyclohexa-1,3-diene, and strict first order kinetics (AH = 32.15 0.09kcalmol" AS = 2.4 + 0.2e.u.) eliminated many pathways including those involving cis-hexa-l,3,5-triene. Use of the deuteriated substrate (269)... 

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