Cyclooctane conformations

There are several more or less reasonable looking cyclooctane conformations. After much research it now is clear that the favored conformation is the boat-chair, 9, which is in equilibrium with a few tenths percent of the crown conformation, 10 ... 

Hendrickson has investigated symmetrical interconversion pathways between cyclooctane conformations. In some cases at least, these paths are probably not as low-energy as unsymmetrical paths, which unfortunately are more difficult to calculate. In the crown family, which includes the chair-chair and twist-chair-chair forms, the situation is quite clear. The twist-chair-chair is of lowest energy and conversion to the chair-chair form involves only the energy difference between the two forms. Thus the chair-chair is a transition state for the degenerate interconversion of twist-chair-... 

Table 7. Families of rapidly interconverting low-energy cyclooctane conformations...

Another transannular reaction, which likewise proceeds via carbene insertion, is the base-catalyzed decomposition of the tosylhydrazone of cyclooctanone. In general these carbenes react with a- and -C—H bonds to give alkenes and cyclopropanes. However, when the carbene carbon can approach distant C —H bonds, such as in the cyclooctane conformation, then bicyclo[3.3.0]octane derivatives are also formed from transannular insertion.Thus, cyclooctanone- and 5-phenylcyclooctanone tosylhydrazones reacted with sodium methoxide to give a mixture of mono- and bicyclic products 4-6 and 7-10, respectively, in the stated proportions. 

In the case of cyclooctane, a total of 11 conformations have been suggested for consideration and their relative energies calculated. The boat-chair conformation was calculated to be the most stable conformation. This prediction was confirmed by analysis of temperature-dependent NMR spectra of 1,1-difluorocyclooctane and 1,1,4,4-tetrafluorocyclooctane. A few of the most stable cyclooctane conformations are shown below ... 

Fig. 10. Energy level diagram for cyclooctane conformers. The boat-chair, BC, conformer is taken as the ground state (Zq = 0) with energy barriers (in kJ/mol) indicated on the diagram for the conversion to the high energy conformers Ej to 69. These are ...

Cycloheptane, cyclooctane, and cyclononane and other higher cycloalkanes exist in nonplanar conformations. 

At one time it was believed that cyclooctane occurs in the extended crown form and the saddle conformation as shown below but on the basis of calculations of minimum energy strain, Hendrickson (1964) and Wiberg (1965) suggested that neither of the above two forms is the correct picture. R. Srinivasav and T. Srikrishnan (Tetrahedron 27, 5, 1009-1012, 1971) showed that the molecule exists as the boat-chair form in a number of crystalline derivatives. 

Since carbocations are structurally similar to ketones, they are discussed here. Schleyer s force field incorporates carbocation parameters, and Harris is exploring their application in conformational analysis (188). The calculated angles in a series of rigid polycyclic carbocations correlated well with ketone infrared frequencies (188a). The calculated relative stabilities among various conformers of tertiary cations of methylcyclohexane, methylcycloheptane, and methyl-cyclooctane do not contradict the limited MNR observations of these species at low temperature (188b). 

An illustrative example for the potency of catalyst C is Paquette s highly efficient total synthesis of the natural products teubrevin G 180 and teubrevin H 181, which feature a cyclooctane core fused and spiroannulated to smaller oxygen containing rings. In the retrosynthetic analysis, the viability of an RCM step for annulation of a cyclooct-enone ring to the furan played a central role. Despite the presence of a conformational constraint by the furan ring in... 

The conformational properties of such eight-membered ring molecules have been reviewed fairly recently (74MI51900). Cyclooctane is the archetypical molecule in this class, and the heterocyclic analogs, such as the azocanes, oxocanes and thiocanes, as well as carbocyclic derivatives, such as cyclooctanone, all have closely related conformational features and a brief overview of their conformations will now be given. 

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. 

Exercise 12-23 A conformation of cyclooctane called boat-boat can be formed by having two gauche C—C—C—C segments, as shown in Figure 12-19. As drawn, this conformation has all hydrogens staggered and normal C-C-C bond angles. Explain why it is not a favorable conformation. Use of models will be very helpful. 

Figure 12-19 Boat-boat conformation of cyclooctane, based on two gauche forms of butane (see Figure 12-16 and Exercise 12-23).

Transannular cation radicals with the intramolecular sulfur-sulfur bond of the 2ct-1ct type generated from medium-ring disulfides like 1,5-dithiacyclooctane are an exception in terms of their stability, although they are not resistant to water (Musker 1980). ESR and resonance Raman spectroscopy studies revealed the existence of the 1,5-dithia-cyclooctane cation radical, with substantial bonding between the sulfur atoms (T. Brown et al. 1981 Tamaoki et al. 1989). Computations confirmed this statement and pointed out that the chair-boat conformer has the lowest energy as compared to other possible conformers (Stowasser et al. 1999). 

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