Cyclohexane twist conformation

Figure 8-6. Comparison of the radial distribution function of the ctiair, boat, and twist conformations of cyclohexane (hydrogen atoms are not considered).

For cyclohexane, there are two extreme conformations in which all the angles are tetrahedral. These are called the boat and the chair conformations and in each the ring is said to be puckered. The chair conformation is a rigid structure, but the boat form is flexible and can easily pass over to a somewhat more stable form known as the twist conformation. The twist form is 1.5 kcal mol (6.3 kJ mol )... 

From the pseudorotating transition state the inversion process proceeds via an intermediate minimum of D2-symmetry (twist-conformation) and across a symmetry-equivalent second pseudorotational transition state to the inverted chair-conformation. The symmetric boat-form of cyclohexane (symmetry C2v) corresponds to a one dimensional partial maximum, i.e. a transition state (imaginary frequency 101.6 cm-1). It links sym-... 

Figure 4.16 (a) Carbon skeleton and (b) line drawing of the twist conformation of cyclohexane. 

The energy barrier between the chair, boat, and twist conformations of cyclohexane are low enough to make separation of the conformers impossible at room temperature. 

For (203), models indicated that the isomer containing cis-syn-cis hydrogen atoms on the cyclohexane ring should be able to form clam-type complexes, provided the cyclohexane ring is in the flexible or twist conformation. The models suggested that the cavity defined by the ten oxygen donors would be ideal for K+. However, for the potassium and barium thiocyanate complexes, configurations of type (204) do not occur in the solid state. Instead, two molecules of the bis-crown coordinate simultaneously to two alkali metal ions - both these 2 2 complexes have structures of type (205). 

On each carbon, one bond is directed up or down and the other more or less in the plane of the ring. The up or down bonds are called axial and the others equatorial. The axial bonds point alternately up and down. If a molecule were frozen into a chair form, there would be isomerism in monosubstituted cyclohexanes. For example, there would be an equatorial methylcyclohexane and an axial isomer. However, it has never been possible to isolate isomers of this type at room temperature.219 This proves the transient existence of the boat or twist form, since in order for the two types of methylcyclohexane to be non-separable, there must be rapid interconversion of one chair form to another (in which all axial bonds become equatorial and vice versa) and this is possible only through a boat or twist conformation. Conversion of one chair form to another requires an activation energy of about 10 kcal/mol (42 kJ/mol)220 and is very rapid at room temperature.221 However, by... 

A detailed follow-up of the reaction by 13C NMR spectroscopy has shown that the initially formed species is an isomer of 1 with a trans-configuration of the germyl groups with respect to the plane of the cyclohexane ring which exists in the twist-conformation. Over a period of two hours at 25 °C, 85% of the trans-isomer 1 isomerizes into the cis-isomer of 1 and there is no further change in the ratio of the isomers. The crystalline cis-isomer of 1 has been isolated in a pure form and fully characterized. 

These cyclohexane-like rings are characterized by the presence of various non-planar conformations chair, boat, and twist conformations. The starting point when dealing with the stereochemistry of these six-membered rings is therefore conformational analysis. The method of choice is NMR spectroscopy as a result the preferred conformer(s) and the axial/equatorial position of attached substituents at the preferred conformer(s) are obtained. 

The less stable puckered conformation of cyclohexane, with both parts puckered upward. The most stable boat is actually the twist boat (or simply twist) conformation. Twisting minimizes torsional strain and steric strain, (p. 113)... 

The boat and the chair forms of cyclohexane show only equatorial and axial exocyclic positions. In contrast, the twist conformers display pseudoequatorial e pseudoaxial (a ) and isoclinal (i) positions [102]. 

Until recently knowledge of twist conformation of simple cyclohexanes could be said to be limited to certain di-f-butylcyclohexanes which if they existed as chair conformations would have an axial f-butyl group, to cyclohexane-1,4-dione and to certain highly substituted cyclohexanes. Undoubtedly twist conformations are quite common, it is unfortunate that reasonably direct evidence is often not available. 

The symmetric puckered conformations of cyclopentane are the Cs symmetric envelope (E) (10) with four carbon atoms in a plane and the C2 s)mrmetric twist (T) (11) with three carbon atoms in a plane [88]. Unlike cyclohexane, these conformations are of almost equal energy and are separated by barriers of about RT or less [89]. There are ten envelope conformations, each with one of the five carbon atoms out of the plane in one of the two directions, and ten corresponding twist conformations. The individual conformations freely exchange which atom or atoms are out of the plane, a process termed pseudorotation, and the whole sequence of conformations is called the pseudorotational itinerary (O Fig. 9). 

For substituted cyclohexanes, two conformational properties are of fundamental importance. A force field should be able to predict both the correct conformation of the ring system and the position (axial or equatorial) of a substituent. Fig. 7 shows the ability of the different force fields to predict the energy difference between the twist-boat and chair conformation of cyclohexane [44]. As can be seen in the figure most of the force fields reproduce this well. However, the energy difference is overestimated by several of the force fields, in particular by CVFF and UFF1.1. 

If the hydrogen atoms in cyclohexane are replaced by large substituents, the conformer with most of them in equatorial positions is preferred. The chair conformer is about 5 kcal/mol more stable than corresponding twisted conform-ers. The activation energy needed to get these high-energy conformers is about 12 kcal/mol (Fig. 1.2.5). Non-chair conformers therefore never exceed a molar fraction of about 0.1% at room temperature, but they may be enforced in molecular complexes (see Figs. 9.6.6 and 9.6.7). 

Eight member ring diauracycles show structures which can be described as elongated cyclohexanes if all the bonds involved are treated as single. As with cyclohexane, the saturated ring may adopt a chair, boat or twist conformation, and... 

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