Twisted chair

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). 

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. ... 

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. 

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. 

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-... 

Conformational inversion barriers of 5,10-substituted decalins (17) were determined by NMR, and MMI calculations were carried out to see whether the twist-chair (ct) or the boat-boat (bb), correspond to the higher barrier. The ct was found to be higher in cw-decalin (17, R = H), but the introduction of substituents at the ring junction is expected to raise the energy of the bb. The observed AG agreed with the calculated value for ct, which is about 5 kcal/ mol higher than that of bb when R = CH3 or OH (124). 

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). 

Electron diffraction measurements of gaseous 1,2,4-trioxolane point to either a C2 symmetry, as depicted at the bottom of Appendix A, or a Cj symmetry, corresponding to an envelope conformation. However, consideration of steric interactions favors the twisted chair conformation (288c) for the molecule. Table 5 presents the structural parameters derived from electron diffraction, assuming that both C—O distances are equal and that the H atoms are symmetrically placed with respect of the neighboring atoms. Assumptions on the vibrational amplitudes of the ring bonds do not affect the calculated values of Table 5, except for the H—C—H angle . ... 

Distortion of the chair conformation by substitution occurs in 2,2,6,6-tetramethyl-piperidin-4-one hydrochloride (45) (71AX(B)932), in which steric interaction between axial methyl groups leads to flattening of the ring. The resultant C(2)—C(6) intramolecular distance is 3.2 A, to be contrasted with 2.4 A in the non-methylated compound. A similar effect is to be found in other 2,2,6,6-tetramethyl derivatives (81AX(B)1771). In the related free radical nitroxide (46) the steric interactions are reduced by adoption of a symmetric twist chair form (74AX(B)790). 4-Chloro-5-methylamino-2,3,6-pyridinetrione monohydrate... 

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). 

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). 

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

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