Torsional Isomerism in Ring Systems

In the previous sections we have examined diverse acylic molecular systems where sigma and/or pi nonbonded interactions obtain. The same considerations are applicable to cyclic and bicyclic systems. In the space below, we provide an overview of torsional isomerism in such systems. 

A similar discussion can be offered for other 1,2-disubstituted cycloalkanes. 

The boat form of molecules of the type shown below may involve sigma nonbonded attractive interactions between the X and Y groups. 

Obviously, steric interactions disfavor the boat relative to the chair form. Nonetheless, systems can be designed where the bow to bow nonbonded attraction may become dominant. It is interesting to discuss in detail one example in order to illustrate our approach. 

Let us consider the molecule shown below dissected in the manner indicated. 

---

Torsion about one of the formal double bonds is invariably the most efficient excited singlet state decay process of acyclic polyenes, and also often occurs efficiently in cyclic systems of moderate-to-large ring size- . E.Z-isomerization in the excited singlet state manifold takes place about only one of the double bonds per photon, as was initially demonstrated for 2,4-hexadiene (5) by Saltiel and coworkers and has since been shown to be quite general. Table 1 contains a summary of quantum yields for the direct E,Z-photoisomerization, in solution, of acyclic and cyclic polyenes 1, 42, 43, 5-18 bearing various substituents. For the most part, quantum yields for direct E,Z-photoisomerization of aliphatic dienes are not highly dependent on the structure of the system (i.e. acyclic, cyclic or exocyclic). 

The above mechanism also accommodates the unusual stereochemical results observed with azirine 185. As was pointed out earlier, the formation of the thermodynamically less favored endo isomer 186 from 185 corresponds to a complete inversion of stereochemistry about the n system in the cycloaddition process. The stereochemical results have been rationalized by assuming that collapse of the trimethylene derivative 189 to the thermodynamically more favored exo isomer 190 results in a severe torsional barrier on ring closure. Collapse of 189 to the thermodynamically less favored endo isomer 186 moves the phenyl and methyl groups increasingly further apart and accounts for the formation of the less stable product. Supporting evidence for this rationale was obtained from the irradiation of the isomeric Z-2-butenyl-2/f-azirine 188. Photolysis of this azirine resulted in the quantitative formation of the same e do-azabicyclohexene (186) and is perfectly consistent with the preferred kinetic closure of intermediate 189. The formation of 186 from 2i/-azirine 188 also provides convincing support for this interpretation. 

---