Torsional isomerization

The atropoisomerism of N -phenylazoles, i.e. an isomerism of the type shown by biphenyl derivatives, has been studied by several authors. Some torsional angles, measured by X-ray crystallography (Section 4.04.1.3.1) are shown in Table 26. 

In simple chemical systems, it is often possible to make a good first guess at the dominant reaction pathway [25-28]. An example of such a reaction is the chair-to-boat isomerization in cyclohexane. In that pathway, a clever combination of two torsion angles provides an excellent reaction coordinate for the isomerization reaction [29,30]. 

The result of such a series of steps is depicted in Figure 9. This method has proved effective in isolating reaction pathways for conformational transitions involving localized torsional transitions including those involving a subtle isomerization mechanism [56]. 

Fig. 13.9. Schematic reaction profile for the EZ isomerization of stilbene. The reaction coordinate 6 is the torsion angle about the double bond. [From FI. Meier, Angew. Chem. Int. Ed. Engl. 31 1399 (1992)]. Reproduced by permission of Wiley-VCH.

Intramolecular allylzinc aldehyde additions lead, in one synthetic step, to bicyelic a-methylene-y-lactones41. Interestingly, the (Z)-isomer gave only the r/5-fused bicyclic products, which arises from a. vyw-selectivc addition, while the ( >isomeric bromide furnished a cisjtrans mixture (yield 56%, cisjtrans 72 28)41 indicating that the double-bond torsion and the intramolecular carbonyl addition have similar rates. A similar sequence was the key step in a confertin synthesis28. 

Muller et al. focused on polybead molecules in the united atom approximation as a test system these are chains formed by spherical methylene beads connected by rigid bonds of length 1.53 A. The angle between successive bonds of a chain is also fixed at 112°. The torsion angles around the chain backbone are restricted to three rotational isomeric states, the trans (t) and gauche states (g+ and g ). The three-fold torsional potential energy function introduced [142] in a study of butane was used to calculate the RIS correlation matrix. Second order interactions , reflected in the so-called pentane effect, which almost excludes the consecutive combination of g+g- states (and vice-versa) are taken into account. In analogy to the polyethylene molecule, a standard RIS-model [143] was used to account for the pentane effect. 

Furthermore, it is often possible to extract from the structural analysis of solid solvates a significant information on solvation patterns and their relation to induced structural polymorphism. An interesting illustration has been provided by crystal structure determinations of solvated 2,4-dichloro-5-carboxy-benzsulfonimide (5)35). This compound contains a large number of polar functions and potential donors and acceptors of hydrogen bonds and appears to have only a few conformational degrees of freedom associated with soft modes of torsional isomerism. It co-crystallizes with a variety of solvents in different structural forms. The observed modes of crystallization and molecular conformation of the host compound were found to be primarily dependent on the nature of the solvent environment. Thus, from protic media such as water and wet acetic acid layered structures were formed which resemble intercalation type compounds. 

The average shape of the radical will therefore be a compromise between strain due to hydrogen-hydrogen repulsion in the completely planar state and torsional strain of the partial double bonds in the skew state. Such a compromise should result in a propeller form in which the blades are slightly feathered out of the plane. It is possible that the radical exists in two isomeric forms, one corresponding to a symmetrical propeller and the other to a propeller in which one of the blades has been tilted the wrong way. Although such isomerism has been ob-... 

In certain problems of torsional isomerism we shall employ the dissection of a molecule A-B into two closed shell fragments A and B. On the other hand, most problems of torsional isomerism which we shall be dealing with in the first part of this work can be treated by employing the dissection shown below. The appropriate definitions are specified in parenthesis. 

Our discussion in this and certain subsequent sections will involve torsional isomerism. In such cases, we can use Eqs. (1) to (6) where Hy is approximated by kSy. Accordingly, the aforementioned equations become ... 

An alternative approach for determining the relative pi electronic stabilization of two torsional isomers utilizes a molecular dissection into two open shell radical fragments. This approach is illustrated by examining torsional isomerism in butadiene and 1,3,5-hexatriene. The ir MO s of the conformational isomers of 1,3,5-hexatriene can be constructed from the union of the n MO s of two formal allyl radicals. The two regiochemical modes of union of interest will be designated cis and tram ... 

Another example of torsional isomerism in X—Y-Y—X tetraatomic molecules is the relative energetics of cis and trans N2F2. 

As a second example, we consider torsional isomerism in 1,2-dihydroxyethylene. The six conformations of dihydroxyethylene along with their labels are shown below ... 

Finally, a few words about previous explanations of torsional isomerism in 1-substituted propene. The most common rationale for the observed trends has been steric hindrance. However, steric effects alone cannot account for the greater stability of the cis isomer. Hence, the steric interpretation should be rejected. 

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