Chair conformation interconversion

Cyclopentane is nonplanar, and the two minimum-energy geometries are the envelope and half-chair. In the envelope conformation, one carbon atom is displaced from the plane of the other four. In the half-chair conformation, three carbons are coplanar, vdth one of the remaining two being above the plane and the other below. The energy differences between the conformers are very small, and interconversion is rapid. All of the carbon atoms r idly move through planar and nonplanar positions. The process is called pseudorotation. 

Ring inversion (Section 3.9) Process by which a chair conformation of cyclohexane is converted to a mirror-image chair. All of the equatorial substituents become axial, and vice versa. Also called ring flipping, or chair-chair interconversion. 

Because chair cyclohexane has two kinds of positions, axial and equatorial, we might expect to find two isomeric forms of a monosubstituted cyclohexane. In fact, we don t. There is only one methylcyclohexane, one bromocydohexane, one cycJohexanol (hydroxycyclohexane), and so on, because cyclohexane rings are confbnnationally mobile at room temperature. Different chair conformations readily interconvert, exchanging axial and equatorial positions. This interconversion, usually called a ring-flip, is shown in Figure 4.11. 

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. 

This boat-chair difference is not immediately obtainable from experiment, e.g. the high-temperature conformational isomerization—and then quenching—of cyclohexane does not result in a chair — boat interconversion, but rather chair — twist-boat M. Squillacote, R. S. Sheridan, O. L. Chapman and F. A. L. Anet, 7. Am. Chem. Soc., 97, 3244 (1975). 

When one looks at the hydrogens in the chair conformation of cyclohexane, one can see that they are of two types. Six of them are parallel to the central rotational axis of the molecule, so are termed axial. The other six are positioned around the outside of the molecule and are termed equatorial. One might imagine, therefore, that these two t)T>es of hydrogen would have some different characteristics, and be detectable by an appropriate spectral technique. Such a technique is NMR spectroscopy but, at room temperature, only one type of proton is detectable. At room temperature, all hydrogens of cyclohexane can be considered equivalent this is a consequence of conformational mobility, and the interconversion of two chair conformations. 

The chair conformation is the preferred conformer for the oxane ring and substituted derivatives. In the case of poly-substitution (e.g., 1,3-diaxial groups), twist conformers can also participate in the equilibrium. Substituents can adopt the axial and equatorial positions ring interconversion between the chair conformers is fast on the NMR timescale at ambient temperature but becomes slow at low temperature (AG = 10.3 kcal mol- ) (73JA4634). 

The geometries of both 3,4-dihydro- and 3,6-dihydro-l,2-dioxin have been calculated at the Hartree-Fock (HF) and MP2 ab initio levels of theory. In each case, half-chair conformers were found to be the most stable structures followed by boat conformers which represent the transition states for ring interconversion <2000JST(503)145, 1997JGC1392,... 

The microwave spectmm of 3,6-dihydro-1,2-dioxin 2 was measured in the frequency range of 10-26 GHz at dry ice temperature <1994JST(323)79> the structures on the basis of the rotational constants are half-chair conformers which readily interconvert at ambient temperature by ring puckering. This interconversion process could be frozen out spectroscopically and its free energy of activation determined (AG = 9.82 kcal moF ) by low-temperature NMR. 

The boat-boat (417) and the twist-boat-boat (418) have low torsional strains but severe non-bonded repulsions, which, as usual, are transferred to internal angle strains. However, heteroatoms can modify these repulsions and certain transannular interactions can drastically reduce them. Even so, the boat-boat family is relatively unimportant as its energy is calculated to be quite high (12 kj mol-1) in cyclooctane. It probably serves as an intermediate for certain conformational interconversions of the boat-chair, especially when the twist-boat-chair pseudorotation itinerary is of high energy. In cyclooctane the boat-boat and its twisted partner have nearly the same energies and are not separated by a significant barrier. 

The different conformations of indolo[2,3-a]quinolizidines play an important role in predicting the thermodynamically more stable epimer (see below). The indolo[2,3-a]quinolizidine system can exist in three main conformations one C/D trans ring juncture (conformation a) and two C/D cis ring junctures (conformations b and c). These are in equilibrium by nitrogen inversion and cw-decalin type ring interconversion. Ring C is considered to be in a half chair conformation and ring D in a chair conformation, Scheme (16) [40]. 

The activation energy for interconversion of these two forms is about 10 kcal mole-1. The boat-chair conformation 9 is quite flexible and movement of its CH2 groups between the various possible positions occurs with an activation energy of only about 5 kcal mole-1. 

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