Z-Hexatriene

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)-1,3,5-hexatriene nitriles on reaction with various nucleophiles305. For example, reaction of phenyl lithium results in the corresponding hexatriene carbonitrile (equation 183). 

The photocyclization of (Z)-1,3,5-hexatriene represents a well-studied example of the influence of ground-state conformational equilibrium on the product composition (Havinga, 1973). The unsubstituted triene exists mainly in the s-trans-s-trans conformation and therefore cannot undergo photocyclization only Z-E isomerization has been observed. With the 2,5-dimethyl derivative the s-cis-s-cis conformation predominates and photocyclization yields dime-thylcyclohexadiene and some dimethylvinylcyclobutene. Finally, with the 2-methyl derivative the s-cis-s-trans conformation is preferred, and a relatively good yield of methylbicyclo 3.l.0]hexene as well as methylvinylcyclobutene and methyIhexatriene-1,2,4 is found. These results are collected in Scheme I. 

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

Cyclohexadiene itself undergoes smooth photochemical ring opening to Z-1,3,5-hexatriene in both the gas phase (4> = 0.13) and in solution (4> = 0.41). 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 -isomer vide infra). ... 

One reason for the rather low quantum yields for product formation from Z-1,3,5-hexatriene is thought to be that the ground state conformer distribution is heavily weighted in favor of the relatively unreactive fZf-conformer . Substituents and structural constraints have quite substantial effects on the conformer distribution, and hence on the quantum yields and distribution of products obsCTved. These effects are well understood, having been worked out independently by Havinga and coworkers, who examined the photochemistry of various 2-alkyl- and 2,5-dialkyl-l,3,5-hexatrienes (120 and 121, respectively) , and by Dauben and coworkCTs with their studies of the photochemistry of substituted 6,6,9,9-tetramethyl-A -hexalins (122). Both bodies of work were predicated on the effects of substituents at the 2- and/or 5-positions of the... 

We reach a similar conclusion when comparing benzene with the open-chain conjugated triene (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 (Z)-1,3,5-hexatriene is 337 kJ/mol (80.5 kcal/mol), a value which is 129 kJ/mol (30.7 kcal/mol) greater than that of benzene. 

FIGURE 4. Absorption spectrum of 3-cm layer of 1 1 mixture of Z-1,3,5-hexatriene and methylene-i odi de. 

FIGURE 9. The three planar conformations of the Z-1,3,5-hexatriene system and the corresponding photoisomers formed (formation of the 3E-isomer is not represented). 

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