1.3- Cyclohexadiene ring

FIGURE 5. On a cyclohexadiene ring, one allylic axial bond has been put in evidence. On the left, it defines a positive chirality (right-handed). The reverse holds for the case on the right. This representation is discussed in Section II.D.2.b... 

Unlike other diene photopericyclic reactions, cyclohexadiene ring opening proceeds with reasonable efficiency in phenyl-substituted derivatives, although the quantum yields are generally significantly lower than those of non-arylated systems. Two examples of the conversion of 93 to 94 are shown in equation 36178,179... 

The evolution of the shapes of the SC orbitals with the progress of the cyclohexadiene ring-opening is illustrated by Fig. 5. The symmetry-unique SC orbitals tj/i-ti/s are shown as three-dimensional isovalue surfaces at the cyclohexadiene end of the IRC segment (leftmost column of orbitals), at the TS (central column of orbitals) and at the hexatriene end of the IRC segment (rightmost column of orbitals). The reflections of t /i, tj/2 and /3 in the symmetry plane which is retained throughout this IRC interval, result in /6, /5 and tj/4, respectively. 

The geometry of the 1,3-cyclohexadiene ring can be described by the torsion angle a which represents the conformation along the linkage (C -C, -Cj-... 

It is apparent from Table 1 that the cyclohexadiene ring in 1,2-dihydrocorannule-nes is significantly flatter than either 1,3-cyclohexadiene itself or 9,10-dihy-drophenanthrene due to the constriction imposed by the remainder of the bowl-shaped ring system. Nonetheless, the lack of planarity means that substituents at may adopt either pseudoaxial (pa) or pseudoequatorial (pe) positions. However, the surface of the corannulene system is curved, and, unlike 1,3-cyclohexadiene and 9,10-dihydrophenanthrene, neither the two pa positions nor the two pe positions are equivalent. Thus, substituents in 1 -/ -1,2-dihydrocorannulenes may be (1) endo-pseudoaxial (endo-pa), (2) e do-pseudoequatorial (e/ido-pe), (3) exo-pseudoaxial (exo-pa), and (4) exo-pseudoequatorial (exo-pe), where endo and exo are related to concave and convex orientations, respectively. 

On the positive side, there are photochemical reactions that are essential for human health. One of these is the formation of vitamin D (the antirachitic vitamin) by irradiation of ergosterol. This photochemical reaction is an electro-cyclic ring opening of the cyclohexadiene ring of ergosterol of the type described in Section 28-2D. The product, previtamin D2, subsequently rearranges thermally to vitamin D2 ... 

Nucleophile addition to a quinone methide is formally a Michael addition reaction.153 However, an important difference between 1,6-addition of nucleophiles to / -quinone methides and conventional Michael addition reactions is the aromatization of the cyclohexadiene ring that accompanies this addition. The effect of aromatic ring formation on the thermodynamic driving force for 1,6-addition of water to p-1 has been evaluated by comparing the thermodynamics... 

Marvell, E. N., G. Caple, and B. Schatz Thermal valence isomeriza-tions Stereochemistry of the 2.4.6-octatriene to 5.6-dimethyl-1.3-cyclohexadiene ring closure. Tetrahedron Letters 1965, 385. 

Cyclohexadiene TS — Cyclohexadiene ring begins to open ring almost open... 

Given the concerted, almost synchronous nature of this gas-phase reaction it might seem reasonable to suppose that the electronic mechanism would resemble those for the Diels-Alder and cyclohexadiene ring-opening reactions, described above. However, our spin-coupled calculations along the IRC reveal a somewhat different picture [3]. 

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