Z-l,3,5-Hexatriene

We reach a similar conclusion when comparing benzene with the open-chain conjugated tr-iene (Z)-l,3,5-hexatriene. Here we compare two real molecules, both conjugated trienes, but one is cyclic and the other is not. The heat of hydrogenation of... 

The precise value of the resonance energy of benzene depends, as comparisons with 1,3,5-cyclohexatriene and (Z)-l,3,5-hexatriene illustrate, on the compound chosen as the reference. What is important is that the resonance energy of benzene is quite large, six to ten times that of a conjugated triene. It is this very large increment of resonance energy that places benzene and related compounds in a separate category that we call aromatic. 

Consider now the 1,1,6,6-tetramethylated derivative of (Z)-l,3,5-hexatriene (83), a species more properly named (Z)-2,6-dimethyl-2,4,6-octatriene and occasionally and trivially called ds-allo-ocimene . To estimate its enthalpy of formation, let us use simple olefin additivity along with ... 

In the transition state of the electrocyclization of (Z)-l,3,5-hexatriene to 1,3-cyclohexadiene (Scheme 22 entry 1) a new six-membered ring develops analogously... 

J. Rimmelin and G. Jenner, Tetrahedron, 30, 3081 (1974). A recent measurement of the pressure and temperature dependence of die electrocyclic ring-closure of Z-l,3,5-hexatriene to 1,3-cyclohexadiene in the range of 200 to 2500 bar and 100 to 125 °C does not show a significant temperature dependence of die activation volume (M. K. Diedrich and F. -G. Klarner, unpublished results). 

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. 

The simplest member of the family, Z-l,3,5-hexatriene (Z-HT), has been studied in detail in both the gas phase176,211-213 and in solution177,188,214 and illustrates all five of the productive photoreactions which are characteristic of the Z-l,3,5-hexatrienyl moiety. 

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. 

From a thermodynamic standpmnt, one can expect that for the thermal conversion of (1) to (2) and related derivatives, unless the temperature is sufficiently high or strain factors become predominant, cycloreversion will not be observed. A more complete discussion of the factors affecting the thermal equilibration of strained hexatrienes and cyclohexadienes will be discussed in Section 6.2.2.2.I. A kinetic study of the pericyclic transformation of the parent triene (1) to 1,3-cyclohexadiene (2) provided an activation energy of 29.9 kcal mol. Taking into account the heats of formation of reactant and product (40.6 kcal mol for (Z)-l,3,5-hexatriene and 25.4 kcal mol" for 1,3-cyclohexadiene), the... 

Scheme 3-42 Example of a twofold Heck reaction synthesis and reactions of ,Z, )-l,3,5-hexatrienes 159 [141, 194b,c].

Machiguchi, Nozoe and coworkers have very recently observed that in contrast to chemical reactivity of tropones, the tosylate of tropone oxime undergoes a facile ring-opening to 6-substituted (Z,Z,Z)-l,3,5-hexatriene nitriles on reaction with various nucleophiles . For example, reaction of phenyl lithium results in the corresponding hexatriene carbonitrile (equation 183). 

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... 

The validity of the principle can be demonstrated by comparing the product distributions obtained from irradiation of Z-l,3,5-hexatriene (Z-HT) with those of Z-2-methyl- and Z-... 

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