Cyclohexadiene-triene requirements

For a 1,3,5-triene the symmetry of the HOMO is /3. Therefore a thermal cleavage of cyclohexadiene should be such that the positive lobes must lie on the same side of the plane which requires a disrotatory motion. 

The l,3,5-triene-l,3-cyclohexadiene interconversion is a six-electron electrocycli-zation that requires a cis central double bond to occur.245 An important application of this rearrangement is the photocyclization of cis-stilbene to dihydrophenanthrene [Eq. (4.45)], which is usually further oxidized to phenanthrene 249... 

An important controlling factor is the conformation of the triene about the two single bonds that link the three double bonds (2.21) an s-c/s-s-cis conformation is required for 6-electron ring-closure, but 4-electron closure can occur also in an s-cis-s-trans conformation. The different conformations interconvert rapidly in acyclic trienes, but because each has different absorption characteristics, the wavelength of irradiation can influence the course of reaction dramatically. 2,5-Dimethylhexa-1,3,5-triene undergoes efficient cis trans isomerization at 254 nm (where the major absorbing species is the s-trans-s-frans conformation), but it cyclizes to a cyclohexadiene (2.22) using 313 nm radiation. 

The formation of the cis compound required substantially higher temperatures. In a geometrically constrained case that could permit ring expansion, Paul found that both the -triene and the cyclohexadiene were formed in a 3 1 ratio, respectively, kinetically (Scheme 7.38). ... 

The type of motion the orbital of the terminal carbon undergoes in an electrocyclic reaction can be detected only if substituents are bonded to these atoms. Substituents move as orbitals move. The thermal cyclization of (2A,4Z,6T)-octatriene provides an example of this effect. We consider 713 because it is the HOMO. It contains the highest energy electrons, so it is the frontier molecular orbital. As outlined above, disrotatory motion is required for a O bond to form at the ends of a conjugated triene. Disrotatory motion of the terminal 2p orbitals causes simultaneous disrotatory motion of the C-1 and C-8 methyl groups and yields r-5,6-dimethyl-l,3-cyclohexadiene (Figure 25.6a). 

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