Cyclohexanes substituted, conformation

Spherical polar coordinates are used for conformational representation of pyranose rings in the C-P system. Unlike the free pseudorotation of cyclopentane, the stable conformations of cyclohexane conformers are in deeper energy wells. Even simong the (less stable) equatorial (6 = 90 ) forms, pseudorotation is somewhat hindered. Substitutions of heteroatoms in the ring and additions of hydroxylic or other exocyclic substituents further stabilize or destabilize other conformers compared to cyclohexane. A conformational analysis of an iduronate ring has been reported based on variation of < ) and 0 (28), and a study of the glucopyranose ring... 

The table shows the preference of a number of substituted cyclohexanes for the equatorially substituted conformer over the axially substituted conformer. 

Disubstituted Cyclohexanes Conformational Energies of Substitutents Conformation and Chemical Reactivity Six Membered Heterocyclic Rings Cyclooctane and Cyclononane Cyclodecane... 

In a monosubstituted cyclohexane, the substituent can be either in an equatorial or axial position. Equatorial positions are more spacious and in substituted cyclohexanes they are preferred. Cyclohexane rings flip between chair forms and establish an equilibrium. In the process of flipping, all equatorial positions become axial and all axial positions become equatorial. The equilibrium favors the chair in which substituents are equatorial. In monosubstituted cyclohexanes, the conformation in which the substituent is equatorial is favored. In disubstituted cyclohexanes where one group is axial and one equatorial, the equilibrium favors the chair form where the larger group occupies the more spacious equatorial position. 

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. 

Figure 6 Incremental calculation scheme to predict destabilization energies ED in monoaxial substituted (a), 1,2-diequatorial substituted (b), and 1,3-diaxial substituted (c) cyclohexane chair conformations in the LHASA program.

The larger the substituent on a cyclohexane ring, the more the equatorial-substituted conformer will be favored. 

Which will have a higher percentage of the diequatorial-substituted conformer compared with the diaxial-substituted conformer, frawi-l,4-dimethylcyclohexane or cw-l-ferf-butyl-3-methyl-cyclohexane ... 

We can calculate the steric energy of the conformations of cyclohexanes substituted with two different substituents using the same techniques described above. For example, consider the two conformations of c -4-methylcyclohexanol. 

The physical, chemical cind biological properties of a molecule often depend critically upo the three-dimensional structures, or conformations, that it can adopt. Conformational analysi is the study of the conformations of a molecule and their influence on its properties. Th development of modem conformational analysis is often attributed to D H R Bcirton, wh showed in 1950 that the reactivity of substituted cyclohexanes wcis influenced by th equatoricil or axial nature of the substituents [Beirton 1950]. An equcilly important reaso for the development of conformatiorml analysis at that time Wcis the introduction c analytic il techniques such as infreired spectroscopy, NMR and X-ray crystaillograph] which actucilly enabled the conformation to be determined. 

The overall picture of the many results which have been obtained with hetero-substituted cyclohexane rings is a very consistent one. Cyclohexane itself in its lowest energy conformation adopts the so-called chair conformation, as depicted in Figure 3 by the two outer formulae (a, b). These are contained in energy wells ca. 42 kJ moP deep. Another conformation, of low abundance in cyclohexane at normal temperatures, but which is important in some substituted derivatives, is the twist form (c, d). This is ca. 22 kJ moP less stable than the chair forms, and it lies on the lowest-energy pathway between them. 

Substitution on a cyclohexane ring does not greatly affect the rate of conformational inversion but does change the equilibrium distribution between alternative chair forms. All substituents that are axial in one chair conformation become equatorial on ring inversion, and vice versa. For methylcyclohexane, AG for the equilibrium... 

Energy differences between conformations of substituted cyclohexanes can be measured by several physical methods, as can the kinetics of the ring inversion processes. NMR spectroscopy has been especially valuable for both thermodynamic and kinetic studies. In NMR terminology, the transformation of an equatorial substituent to axial and vice versa is called a site exchange process. Depending on the rate of the process, the difference between the chemical shifts of the nucleus at the two sites, and the field strength... 

For substituted cyclohexanes, the slow-exchange condition is met at temperatures below about —50 C. Table 3.5 presents data for the half-life for conformational equilibration of cyclohexyl chloride as a function of temperature. From these data, it can be seen that conformationally pure solutions of equatorial cyclohexyl chloride could be maintained at low temperature. This has been accomplished experimentally. Crystallization of cyclohexyl chloride at low temperature affords crystals containing only the... 

The free-energy difference between conformers is referred to as the conformational free energy. For substituted cyclohexanes, it is conventional to specify the value of — AC ° for the equilibrium... 

Consider the conformations possible for 3-substituted methylenecyclohexanes. Dc you expect typical substituents to exhibit larger or smaller preferences for the equatorial orientation, as conpared to the same substituent on a cyclohexane ring ... 

The easiest way to visualize chair cyclohexane is to build a molecular model. (In fact do it now.) Two-dimensional drawings like that in Figure 4.7 are useful, but there s no substitute for holding, twisting, and turning a three-dimensional model in your own hands. The chair conformation of cyclohexane can be drawn in three steps. 

The chair conformation of cyclohexane has many consequences. We ll see in Section 1.1.9, for instance, that the chemical behavior of many substituted cyclohexanes is influenced by their conformation. In addition, we ll see in Section 2S.5 that simple carbohydrates such as glucose adopt a conformation based on the cyclohexane chair and that their chemistry is directly affected as a result. 

The same kind of conformational analysis just carried out for cis- and fraus-l,2-dimethylcydohexane can be done for any substituted cyclohexane, such as as-l-tert-butyl-4-chlorocydohexane (see Worked Example 4.3). As you might imagine, though, the situation becomes more complex as the number of... 

Drawing the Most Stable Conformation of a Substituted Cyclohexane... 

Anti periplanar geometry for E2 reactions is particularly important in cyclohexane rings, where chair geometry forces a rigid relationship between the substituents on neighboring carbon atoms (Section 4.8). As pointed out by Derek Barton in a landmark 1950 paper, much of the chemical reactivity of substituted cyclohexanes is controlled by their conformation. Let s look at the E2 dehydro-halogenation of chlorocyclohexanes to see an example. 

The principles involved in the conformational analysis of six-membered rings containing one or two trigonal atoms, for example, cyclohexanone and cyclohexene are similar. The barrier to interconversion in cyclohexane has been calculated to be 8.4-12.1 kcal mol . Cyclohexanone derivatives also assume a chair conformation. Substituents at C2 can assume an axial or equatorial position depending on steric and electronic influences. The proportion of the conformation with an axial X group is shown in Table 4.4 for a variety of substituents (X) in 2-substituted cyclohexanones. 

Wehle, D. Fitjer, L. Tetrahedron Lett., 1986, 27, 5843, have succeeded in producing two conformers that are indefinitely stable in solution at room temperature. However, the other five positions of the cyclohexane ring in this case are all spiro substituted with cyclobutane rings, greatly increasing the barrier to chair-chair interconversion. 

EXERCISE 6.30 Below you will see one chair conformation of a substituted cyclohexane. Draw the other chair (i.e., do a ring flip) ... 

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