Cyclooctane, conformations table

The internal angles in all the conformations given in Table 2 are considerably greater than the 111.5° found in cyclohexane. In the crown, chair-chair, twist-chair-chair, boat-chair, and twist-boat-chair, the angles are in the range of 115° to 117°. The experimentally determined internal and dihedral angles for several boat-chair cyclooctane derivatives (Table 1) are very similar to those calculated by Hendrickson and by Bixon and Lifson. 

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

In contrast to the parent compound, several cyclooctane derivatives and related compounds have had their structures determined by X-ray diffraction (Table 1). Most of these compounds have boat-chair conformations, but fra s-syM- m s-l,2,5,6-tetrabromocydooctane is a twist chair-chair, and crown conformations are found in octasulfur, the all-cfs tetramer of acetaldehyde, and related compounds. 

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. 

The nmr spectra of various rather simple cyclooctane derivatives discussed below are only consistent with the boat-chair conformation. Furthermore the structures in the crystalline state are boat-chairs (Table 1), with one exception which can be rationaUzed (see below). The conclusion that cyclooctane exists in solution as the boat-chair therefore appears inescapable. 

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. 

Cycloheptane and cyclooctane data on the thermal properties are also given in Table 3.1 They show little change from the cyclopentane and cyclohexane properties. Again, there is no indication of increasing amounts of conformational entropy in the transition entropies. For cyclooctane in solution H and NMR could prove ring-inversions and pseudorotation among the boat-chair conformations through the twist-boat-chair intermediate to very low temperatures (100 K). Only about 6% of the cydooctane could be found at about 300 K in the other three crown-family... 

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