Chair-to-boat

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]. 

TABLE 12.7. Equilibrium Data for Chair to Boat Conversion... 

Such chair-to-boat transitions have been suggested to play a role in determining polysaccharide structure (end-to-end lengths) in aqueous solution (51). 

Structural change from "chair"—to "boat"—form X-ray analysis of A... 

Figure 1 Relationships of the conformational free energy barriers in 1,3-dioxocane and some of its methylated derivatives. The full lines link barriers for boat-chair to twist-boat-chair processes and the dotted lines link barriers for boat-chair to boat-boat processes. Energies are in kJ mol-1...

Raman spectroscopy confirms that the dominant form of 3,4-dihydro-2H- pyran at room temperature is the half-chair <74JCP(60)3098) and an angle of twist of 23° is indicated. A pulsed IR laser study of the half-chair to boat interconversion of this dihydropyran has shown that provided at least 20 kJ mol-1 is absorbed a transient UV absorption at 237 nm is observed which is assigned to the boat form (80JA6407). 

It is possible to measure the spectral properties of the twist-boat form by a very elegant technique employed by F. A. L. Anet. Because the equilibrium constant for conversion of chair to boat increases with temperature, a considerable proportion of the molecules exist as the twist-boat form in the vapor at 800°. If such vapor is allowed to impinge on a surface cooled to 20°K, the film condensate contains about 25% of the twist-boat form. At this low temperature, the twist-boat form is converted to the more stable chair form at a very slow rate. Infrared spectra can be taken of the boat-chair mixture at 10°K. If the mixture is allowed to warm to 75°K, the normal equilibrium favoring the chair form is established in a short time. 

Wallis and Thompson [61] developed a potential energy surface using these spectroscopic and theoretical data and used it in molecular dynamics simulations to study the chair-to-boat conformational inversion. They followed up this study with simulations of the RDX conformational changes in dense xenon gas as a function of concentration. Since then a great deal more has been learned about the details of the potential from quantum chemistry calculations and could be used to improve the Wallis-Thompson model. 

The entropy term due to mixing of conformers will be temperature independent if conformational energies of the two forms are equal, as in a dl pair. The entropy term will be temperature dependent if the energies of the conformers are unequal, as when a methyl group moves from axial to equatorial during a chair-chair dip, or when a ring dips from chair to boat. 

The most important role of 9-substituents is the inhibition of chair-to-boat interconversions. Forcing the two cyclohexane rings in bicyclo[3.3.1]nonanes to remain in chair conformations results in compounds in which the through space endo-3 and 7-substituent interaction can be maintained even under severe steric hindrance (54). When the gem-dimethyl groups are introduced at position 9, computation with the MM2 force field shows that the boat form is no longer a stable conformation. For compounds 66-69 a single chair-chair... 

Figure 9 (a) French door and (b) sliding door gating mechanisms in hemicarceplex formation and dissociation, (c) Chair-to-boat... 

It is well established that the stable configuration of cyclohexane is the chair of symmetry Dad. Other, higher energy, forms are present only in quite small amounts at room temperature. However, their energies are low enough to contribute to the thermodynamic properties the proportion of the boat form increases rapidly with temperature and since the conversion of chair to boat is strongly endothermic, this conversion contributes appreciably to the heat capacity. 

The energy needed for the chair-to-boat conformational excitation (- 0.5 eV) is comparable to the critical energy for the loss of water (Green et al. 1975). Note that the tertiary-H is not available for direct transfer via a six-membered transition state in the cis-isomer this isomer eliminates water to a much lesser extent via a different mechanism. 

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