E-1,3,5-Hexatriene

In the region between 260 and 215 nm the gas-phase spectra of Z- and E-hexatriene are very similar. For Z-hexatriene the 0-0 transition is located at 252.2 nm (113 kcal/mole) with progressions of -410, /1260, and -1630 cm l. The solution spectrum shows three maxima, the Franck-Condon transition occurring at 253.7 nm (113 kcal/mole) the oscillator strength f 0.7 correlates with a radiative lifetime of the order of 10 9 sec. 

Below 215 nm the gas-phase spectrum of Z-hexatriene is much more diffuse and less Intense than that of E-hexatrlene. Again, four regions of bands can be distinguished 215 to 200 nm, 200 to 175 nm, 175 to 160 nm, and 160 to 145 nm, the latter bands being comparable to those of E-hexatriene in intensity. In solution Z-hexatrlene shows the so-called els-band at 198 nm observed in long-chain conjugated polyenes having at least one els double bond. 

No fluorescence of either E-hexatriene or Z-hexatriene has been detected even at low temperatures (22b). The quantum yield of fluorescence, if existent. Is estimated (32a) to be smaller than 0.01. Interestingly, the substituted trans-trlene tachysterol has been shown to emit fluorescent radiation. In contrast to its cls-lsomer precalciferol (30). 

For 2-methyl-E-hexatriene (cmax 43,000 ref. 68) the results are summarized In Figure 13. It appears that while the Z-Isomer is the major primary photoproduct, the methylvinylcyclobutene also is formed directly from the photoexclted E-triene. Other products, methylbicyclo[3.1.0]hexene and methyl-1,2,4-hexatrlene, are formed only after a certain time lag during which the proportion of methyl-Z-hexatrlene has Increased to an appreciable level. 

Similarly, Figure 14 shows that irradiation of 2,5-dimethyl-E-hexatriene ( max 43,400 ref. 68) gives rise to the Z-isomer and the vinylcyclobutene derivative as primary photoproducts. 

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)... 

It is quite possible that more highly dehydrogented products (e.g. hexadiene or hexatriene) may also be involved in the reaction sequence. However, none of these species was observed in the GLC. This is not surprising since both these species are highly reactive and may not have accumulated to any measurable extent. One could have used labeled diolefins or triolefins in mixture with n-hexane to test this possibility. Although this experiment was not attempted, we would speculate that most of the radioactivity would have been quickly incorporated into the benzene with a small amount perhaps flowing temporarily upstream into the olefins and the paraffin. 

In the case of 22b-e, the butadienes 24b-d and the hexatriene 24 e, respectively, are also obtained on trapping with cinnamaldehyde, the 1,3-diene 24a is even the sole reaction product. It is quite obvious that the olefins 24 are secondary products of the trapping reaction of 9 arising by photofragmentation of 22. The other product is phenyldioxophosphorane (23) which also numbers among the short-lived compounds of quinquevalent phosphorus with coordination number 3 (see Sect. 3.1). 

The thermochemistry of totally cumulated trienes, i.e. species with the C=C=C=C substructure, is very limited. Indeed, the sole examples we know are those reported by Roth, namely (Z)- and ( )-2,3,4-hexatrienes MeCH=C=C=CHMe, species 17 and 18. Their enthalpies of formation are identical to within experimental error, 265 kJ mol-1. This equality is altogether reasonable given the small Me—Me interaction across the 4-carbon, linear, cumulene chain in contradistinction to the 4.3 kJ mol-1 difference that is found for the isomeric (Z)-and (E)-2-butenes with their significantly smaller Me...Me distance. Are cumulated trienes unstable relative to cumulated dienes much as cumulated dienes are unstable relative to simple olefins Briefly regressing to cumulated dienes, this assertion is corroborated by the finding that species 3, i.e. 1,3-dimethylallene, has an enthalpy of decarbonization 18 of 144.5 kJmol-1 (reaction 12)... 

This class of compounds is defined to have some of the three conjugated double bonds found in the ring and others not. This class includes the isomeric 3,3 -bis(cyclohexenylidenes), 100 and 101. Roth shows us that the two isomers have the same enthalpy of formation within ca 1 kJmol-1, a difference somewhat smaller than the 4 kJmol-1 found for the totally acyclic 1,3,5-hexatrienes, 79 and 80 respectively. Naively these two sets of trienes should have the same (E)/(Z) enthalpy difference. Given experimental uncertainties, we will not attempt to explain the difference69. We may compare 100 and 101 with phenylcyclohexane, 102, an isomeric species which also has the same carbon skeleton. There is nearly a 110 kJ mol-1 enthalpy of formation difference between the semicyclic and cyclic trienes. We are not surprised, for the word cyclic is customarily replaced by aromatic when in the context of the previous sentence. 

While we know of no experimental thermochemical data for 123, Roth informs us that the enthalpy of formation of 124 is 259 kJmol-1. There are no experimental thermochemical data for 125 either, but it is easy to estimate the desired enthalpy of formation. We may either use the standard olefin approach with ethylene, 1,3-butadiene and (E)-l,3,5-hexatriene (i.e. with CH2=CH2, 33 and 79) or linearly extrapolate these three unsaturated hydrocarbons. From either of these approaches, we find a value of ca 225 kJ mol-1. Cross-conjugation costs some 35 kJ mol-1 in the current case. Interestingly, the directly measured cross-conjugated 1,1-diphenylethylene (126) is only ca 10 kJmol-1 less stable than its directly measured conjugated (E)- 1,2-isomer (40) despite the expected strain effects that would additionally destabilize the former species. 

We recall that Fang and Rogers, op. cit., measured the enthalpy of hydrogenation of the acyclic trienes in a nonpolar solvent instead of acetic acid as earlier reported. However, they did not remeasure the Z- and E-isomers separately but instead assumed the earlier measured difference is correct. Said differently, they assumed that the effect on the enthalpy difference of the Z- and -hexatriene is essentially independent of solvent. This is plausible but remains untested. 

It should be emphasized that this analysis is based on the assumption of some appreciable 2—5 overlap in all-c/s and cis-gauche 1,3,5-hexatriene. Furthermore, it should be pointed out that our analysis of the conformational preference of 1,3,5-hexatriene is aiming at revealing electronic patterns. In reality, the all-c/s conformation of 1,3,5-hexatriene is unfavorable due to repulsive interactions between the two methylene groups, i.e. conformational preference varies in the order all-frans > cis-gauche. 

The structural variety increases if the second (and further) substituent(s) is (are) not bound to the allene nucleus. For vinylallene (2), the additional vinyl group can be introduced at C-5, leading to 1,2,4,6-heptatetraene (22 only. E-isomer shown) or at C-4, providing 4-methylene-l,2,5-hexatriene (23), the former being an important substrate for cyclization reactions, as will be discussed in Section 5.5 (Scheme 5.2). 

The linear isomer of 225, (E)-l,2,4,6,7-octapentaene (229), is formed in addition to other products on treatment of the bisdibromocarbene adduct to (E)-l,3,5-hexatriene with methyllithium in diethyl ether at -40 C like 226, it is a highly unstable hydrocarbon [90]. Several attempts to characterize the Z-isomer 230 [90, 91] also met with failure. Although very likely generated as an intermediate in these experiments, 230 immediately cyclized to o-xylylene (231), which can be trapped, e.g., as a Diels-Alder addition product. 

Photochemical d.s jraw.v-confonnational interconversion is also known to occur in larger polyenes. Brouwer and Jacobs have reported the results of irradiation of E- and Z-2,5-dimethyl-l,3,5-hexatriene (14) in argon matrices at 10 K108. Irradiation of the -isomer gives rise to various retainers, while irradiation of the Z-isomer results only in ,Z-isomerization. Photochemical translcis conformer interconversion has also been observed for , -1,3,5,7-octatricnc in matrices at temperatures below 10 K32. 

Cyclohexadiene itself undergoes smooth photochemical ring opening to Z-l,3,5-hexatriene in both the gas phase (d> = 0.13)176 and in solution (d> = 0.41)71,177. As is almost always the case, extended irradiation in solution leads to the formation of a variety of isomeric products due to secondary irradiation of the Z-triene and its E-isomer (vide infra)11. 

In solution, an initial photoequilibrium is established between the Z- and -isomers, while the rearrangement products 117 and 118 are formed along with traces of cyclohexadiene (CHD) over much longer irradiation times (equation 46). In solution, the major products are 3-vinylcyclobutene (117) and bicyclo[3.1.0]hex-2-ene (118) Z-l,2,4-hexatriene (119), which is a major product in the gas phase176,211, is formed in relatively low yields. The quantum yields for ,Z-photoisomerization of Z- and -l,3,5-hexatriene in pentane solution (265 nm excitation) are /, r = 0.034 and E—Z = 0.016, respectively188. 

Theoretical and time-resolved spectroscopic studies of triene photochemistry The dynamics of relaxation of the excited singlet states of E- and Z-l,3,5-hexatriene (HT) have recently been studied in the gas phase and in solution. In the gas phase, population of the 21 / state of the Z-isomer by internal conversion from the spectroscopic 11B state has been estimated to occur with a lifetime Tig of about 20 fs, while the lifetime of the 21 / state has been determined to be T2A =730 fs47. The lifetime of the latter in ethanol solution has been determined by Fuss and coworkers to be T2a = 470 fs52. A similar 21A lifetime has been reported for -l,3,5-hexatriene in cyclohexane and acetonitrile solution by Ohta and coworkers48. 

It should be noted that products like 443 and 447 are the normal products of photochemical reactions of acyclic 1,3,5-hexatrienes, as well as of divinyl aromatics, but are quite unusual for thermal transformations of such substrates. Presumably, the electrostatic repulsion between CF2 groups prevents the formation of conformation 450 which is necessary for the electrocyclic ring closure (i.e. 438 — 439 and 445 -> 446). Instead, it leads to conformation 451 which is favorable to generate the diradical and then the fused vinyl-cyclopropane intermediates 452 (equation 170). Note that the rearrangement 452 —> 453... 

There is no unity of opinion in the literature concerning a classification, i.e, whether to call these transformations aza-Claisen or aza-Cope rearrangements. It is accepted that the term aza-Claisen should be reserved only for those processes in which a carbon atom in the allyl vinyl ether system has been replaced by nitrogen357. Three different types of aliphatic 3-aza-Cope reactions which were studied theoretically are the rearrangements of 3-aza-l,5-hexadienes (610, equation 262), 3-azonia-l,5-hexadienes (611, equation 263) and 3-aza-l,2,5-hexatrienes (612, equation 264) (the latter is a ketenimine rearrangement )357. 

Migration of the metal along the polyene chain in (l,l- 2-l,3,5-hexatriene)CoCp occurs with an activation energy of 25.6 kcalmoT 1 (equation 15)136b. This barrier is ca 5-8 kcal mol 1 lower than that for metal migration in (triene)- or (tetraene)Fe(CO)3 complexes (see Section IV.E.l.d). 

T. Parassassi, F. Conti, M. Glaser, and E. Gratton, Detection of phospholipid phase separation. A multifrequency phase fluorimetry study of l,6-diphenyl-l,3,5-hexatriene fluorescence, J. Biol. Chem. 259, 14011-14017 (1984). 

R. M. Fiorini, M. Valentino, E. Gratton, E. Bertoli, and G. Curatola, Erythrocyte membrane heterogeneity studies using l,6-diphenyl-l,3,5-hexatriene fluorescence lifetime distribution, Biochem. Biophys. Res. Commun. 147, 460-466 (1987). 

L. Davenport, R. E. Dale, R. H. Bisby, and R. B. Cundall, Transverse location of the fluorescent probe l,6-diphenyl-l,3,5-hexatriene in model lipid bilayer membrane systems by resonance excitation energy transfer, Biochemistry 24, 4097-4108 (1985). 

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