Twisted chair

Crown (411) Chair-chair (412) Twist-chair-chair 

Among the possible conformations of oxepane the boat, twist-boat and chair forms were calculated to be of higher energy than the predominant twist-chair conformations (which were found to exist in approximately equal proportions). 

Fully saturated seven-membered heterocycles with one or two heteroatoms are normally in mobile twist-chair conformations (Section 5.17.1.1, Chapter 5.18) (b-77SH(2)123). Annelation and the introduction of exocyclic double bonds can have profound effects oxepan-2-one, for example, is in a near chair conformation (67JA5646). 

The total spread in energies calculated for the four conformations is only 2.7 kcal/mol. The individual twist-chair conformations interconvert rapidly by pseudorotation. 

X-ray analysis of the 1,6-diol derivative (15) of the bicydo[4.4.1 ]-undecane ring system revealed the often calculated (9a,81) but rarely observed twist-chair cycloheptane ring. MM calculations confirmed that the observed conformation of 15 is the global minimum conformation (119). 

P2t Z = 2 Dx = 1.281 R = 0.06 for 1,338 intensities. The molecule contains a seven-membered, 1,3-dioxepane ring which has a conformation close to the twist-chair, with a two-fold axis through the mid- 

As ring size increases, there are progressively more conformations that need to be considered. For cycloheptane, four conformations have been calculated to be particularly stable. NMR investigations indicate that the twist-chair is the most stable. Various cycloheptane derivatives adopt mainly twist-chair conformations.  

Further experimental confirmation of the preferred geometry of oxepane (1) has been obtained from its IR and Raman spectra. These vibrational and rotational spectra have been rationalized in terms of the lowest energy twist-chair conformations pseudorotating via a chair form (771A2866,2876). 

Bourn (107) as 10.8 kcal mole-1 (AH4 temperature range 156—246 K solution of cyclohexane in carbon disulfide). By applying an interesting trapping technique Anet and coworkers (110) have recently measured a value of 5.3 kcal mole-1 for the barrier of the reverse twist/chair process (AH4 temperature range 72.5—74.0 K solid state). Our calculated (for the gas phase) activation enthalpies AH4 for the former process are at 73, 

Another example is the temperature-dependent study of the 31P-NMR spectra of the hydrogenation catalyst shown in Figure 11.11 [28]. At 240 K the 31P-NMR signals are equivalent, whereas at 123 K two sets of signals are observed which are attributed to a seven-membered ring in a twist-chair and a distorted boat conformation. 

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