Ci s-trans-Isomerization

Comparison of the conformational behavior of different C = X substituents seems necessary either to acquire a more general knowledge of s-cis/ s-trans isomerism in heterocyclic systems or for a better understanding of the behavior of acyl derivatives. 

If other elements of chirality are present in the molecule, the descriptors should be arranged in the order central > carbocationic > axial > planar > torsional (note that Sokolov [6] uses a different order for the descriptors). The effects caused by such other types of chirality are discussed elsewhere in some detail [41], e.g., S-cis/S-trans-isomerism in ferrocenyl ketones and alkenes, as well as the chemistry of biferrocenyl (containing a direct bond between two ferrocenes). 

Monomolecular photochemistry of butadiene is rather complex. Direct irradiation in dilute solution causes double-bond cis-trans isomerization and rearrangements to cyclobutene, bicyclo[l.l.01butane, and l-methylcyclo-propene (Srinivasan, 1968 Squillacote and Semple, 1990). Matrix-isolation studies have established another efficient pathway, s-cis-s-trans isomerization (Squillacote et al., 1979 Arnold et al., 1990, 1991). 

The reaction pathways both for single- and double-bond isomerization enter the funnel region at less than one-third of the way toward the products, suggesting that the majority of excited-state molecules should decay back to the ground-state reactant. In addition, the excited-state barrier for singlebond isomerization is smaller than that for double-bond isomerization, reducing the quantum yield for cis-trans isomerization (since the product of s-cis-s-trans isomerization is just a different conformer, not a new cis-trans isomer of the reactant). Thus the low experimental value of = 0.034 reported for the quantum yield of /ran.v-hexatriene from r/.v-hexatriene (Jacobs... 

Irradiation of s-c/s-2,3-dimethylbutadiene yields both electrocyclic ring closure and double-bond isomerization products (Scheme 28). While the ring closure in. s-c/.s-butadiene, isoprene, 2-isopropylbutadiene, and 1,3-penta-diene is considerably less efficient than s-cis-s-trans isomerization, in s-cis-2,3-dimethylbutadiene it is about 50 times faster (Squillacote and Semple, 1990). This effect is counterintuitive, since the two methyl groups appear to hinder the rotation about the central C—C bond in butadiene. 

Fig. 15b shows that in all-trans-hex -l,3,5-tnene there are two distinct kinked Si/So conical intersections, one with the -(CH)3- kink located at the C2-C3-C4 position mediating simultaneous Z/E and s-ds/s-trans iso-merizations, and the other with the kink located at the C1-C2-C3 position, mediating simultaneous s-cis/s-trans isomerization and methylene twisting. The conical intersection of benzene,displaying the typical -(CH)s-kink arrangement, has already been reported in Fig. 8(a) of Sec. 2.2. The -(CH)3- kink intersection has been recognized in other aromatic systems such annulene, styrene,stilbene and azabenzenes ° and pyrrole, and antiaromatic molecules such as cyclo-l,3,5,7-octatetraene (see below). 

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