Cyclooctane strain energy

Cyclononane, strain energy of, 114 Cyclooctane, strain energy of, 114 Cyclooctatetraene, bond lengths in, 524... 

The dicyclopenta[a,characteristic element of some terpenoids was obtained in 100% yield by a very facile Cope rearrangement of the highly functionalized divinylcyclobutane derivative 533 on heating in benzene at 55 °C for 4 h. The mild conditions can be due to participation of the lone pair of the sulfur atom or to the strain energy of the divinylcyclobutane fragment (equation 206)259. 

Semi-empirical strain energy calculations for cyclooctane have been carried out by four groups 10-13,35-37) (Table 2). The perspective drawings 38) in Fig. 1 were drawn by the computer program Ortep 39) with the parameters calculated by Hendrickson, n) Table 2 gives dihedral angles, the sets of... 

Strain energy calculations for other conformations in the cyclooctane class are unfortunately not available. In particular, there are no calculations for heterocyclic eight-membered rings. Finally, there is a need for more accurate and rehable calculations, which can give not only the equilibrium geometry and the strain energy, but also the vibrational frequencies. Only a very hmited amount of work has been done along these lines. 

It appears clear from the strain energy calculations and the experimental evidence (Section VII) that, at least for cyclooctane itself, the conformations of low energies can be grouped into three families which are separated by relatively high barriers (8 to 11 kcal/mole). The members of a given family, however, are separated from one another by much smaller barriers (0 to perhaps 4 kcal/mole). Table 7 contains a summary of these conclusions. 

Since strain energy calculations and experimental data strongly indicate that the barrier for interconversion from the crown family to the boat-chair family is of the order of 11 kcal/mole, it is not possible for cyclooctane to exist as a mixture of these two families, unless one family is present in such a small amount that its spectrum is lost in the noise. Thus, one family must be present to more than 95% in cyclooctane at —130 °C. >... 

Roberts ) originally considered a twist-boat conformation for IV, but the mounting evidence for a boat-chair conformation for cyclooctane and various derivatives, led Roberts and coworkers to suggest boat-chair conformations for IV also. ) Futhermore, the original explanation requires that pseudorotation of the twist-boat via the boat be of lower energy than the pseudorotation via the boat-boat and this is not supported by recent strain energy calculations. 

The BC-3 conformation for cyclooctanone is supported by recent strain energy calculations, which have already been mentioned (Section V). Qualitatively, the BC-3 conformation is also very reasonable, since the non-bonded repulsions between the 3 and 7 methylene groups in the cyclooctane boat-chair conformation are largely removed in the BC-3 form. The 1 position in the boat-chair also has the same kind of advantage that the 3 position has. However, the 3 position is also favored because of the relief of eclipsing strain which occurs in that position, but not in the 1 position (see Table 2 for dihedral angles in the boat-chair). This point will be amplified in the following discussion on methylenecyclooctane. 

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. 

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. 

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