Torsional strain, conformational isomers

In Table 7 the six-membered monocyclic dienes are represented by the conjugated 1,3-cyclohexadiene and its isomer 1,4-cyclohexadiene. 1,3-Cyclohexadiene has a nonplanar equilibrium conformation that is primarily influenced by three factors -electron interaction (optimal for a planar conformation) angle strain and torsion strain (both optimal for a planar conformation). The reduced overlap between the two --orbital systems is, for the observed C=C—C=C angle of 18°, estimated at ca 10% and should therefore not influence the conjugation stabilization drastically, compared to a conformation with coplanar C=C bonds. 

Conformational isomers represent minima on an energy surface, and all structures and the corresponding strain energies can be obtained by a careful analysis. This can be performed manually (such as in Sections 17.3 and 17.4) or automatically. An automatic procedure may involve a systematic search (grid search methods), a stochastic search (e.g., torsional Monte Carlo or cartesian stochastic, i.e., the random kick method) or molecular dynamics (see Chapter 5 and Section 16.5). Implemented in MOMEC is a random kick stochastic search module, and this has been shown to lead to excellent results, not only for conformational equilibria, but also for distributions of configurational isomers[37]. 

The conformation is thus not only free of angle strain but free of torsional strain as well. It lies at an energy minimum, and is therefore a conformational isomer. The chair form is the most stable conformation of cyclohexane, and, indeed, of nearly every derivative of cyclohexane. 

The consideration of the development of torsional strain in the transition state for nucleophilic addition afforded an explanation for product distributions observed in the addition of nucleophiles to substituted cyclohexanones. Dauben and co-workers reported that reduction of 4-methylcy-clohexanone with LiAlH4 produced 81% of the trans alcohol and 19% of the cis isomer. With 4-f-butylcyclohexanone, a conformationally more rigid struc-... 

---