Hexatriene to 1,3-Cyclohexadiene

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125 °C does not show a significant temperature dependence of die activation volume (M. K. Diedrich and F. -G. Klarner, unpublished results). 

Scheme 80. A 1,3,5-hexatriene to 1,3-cyclohexadiene-type anion radical electrocyclic reaction.

Thermal isomerization of cw-l,3-5-hexatrienes to 1,3-cyclohexadienes was first recognized in the pyrolytic conversion of previtamin D2 and vitamin D2, which interconvert by 1,7-hydrogen shifts, to pyro- and isopyrocalciferol at higher temperatures (Scheme 7.39). ... 

Transition structure calculated for the disrotatory closure of 1,3,5-hexatriene to 1,3-cyclohexadiene. (Bond angles are in degrees distances are in A. Reproduced from reference 25.)... 

James and Troughton ( ) obtained ethylene and 1,3,5-hexatriene as the primary products in their study on the reaction of diallyl with the ethyl radical at 134- 175 C. Furthermore, they obtained 1,3-cyclohexadiene as a successive product. Recently Orchard and Thrush (19) reported the thermal isomerization of 1,3,5-hexatriene to 1,3-cyclohexadiene at ca. 400 C and the consecutive formation of benzene at ca. 550 C. In the present work, 1,3-cyclohexadiene (reaction 17) and benzene (reaction 18) were obtained as the secondary products. The hydrogen atom produced in reactions 12,... 

The experimental enthalpy of activation for disrotatory thermal isomerization of cis-l,3,5-hexatriene to 1,3-cyclohexadiene in the gas phase at 100 C is 29.2 kcal/mol [13]. The reaction is exothermic by 14.5 kcal/mol [14, p. 127], so of the reverse reaction is 43.7 kcal/mol, but - in spite of its high activation energy - it is characterized as allowed by all of the common orbital symmetry criteria. In norcaradiene ([4.1.0]hepta-2,4-diene), the cyclopropane ring bridging Cl and Ce of cyclohexadiene has built the disrotation into the molecule, desymmetrizing it - and its monocyclic isomer, cycloheptatriene - to C, in which the 61 and ai orbitals correlate directly (Fig. 5.3). The rate of isomerization is so much faster that it had to be measured at low temperature (ca. 100 K) in a hydrocarbon glass [15] is only 6.3 kcal/mol ... 

FIGURE 20.56 The electrocyclic ring closure of c j-l,3,S-hexatriene to 1,3-cyclohexadiene is similar to a Cope rearrangement. 

Let us consider first the conversion of 1,3-butadiene into cyclobutene. This process is endothermic because of ring strain. Indeed, the reverse reaction, ring opening of cyclobutene, occurs readily upon heating. However, ring closure of d5 -l,3,5-hexatriene to 1,3-cyclohexadiene is exothermic and takes place thermally. Is it possible to drive these transformations in the thermally disfavored directions ... 

Another example is provided by the cyclization of 1,3-cii-5-hexatriene to 1,3-cyclohexadiene. The reaction proceeds via a cyclic transition state (Figure 1) that may bind singly or doubly charged cations (e.g., Na" ", Li" ", Mg " ", Be " "). 

---