Cycloheptane chair

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. ... 

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). 

Possible conformations for cycloheptane include the comfortable appearing chair form, 7. However, this form has eclipsed hydrogens at C4 and C5 as well as nonbonded interactions between the axial-like hydrogens on C3 and C6. The best compromise conformation is achieved by a 30°-40° rotation around the C4-C5 bond to relieve the eclipsing of the hydrogens. This spreads the interfering hydrogens at C3 and C6 and results in a somewhat less strained conformation called the twist chair. The twist chair, 8, is very flexible and... 

After a short qualitative introduction to the principles of dynamic nuclear magnetic resonance spectroscopy , the proposed interconversion processes for cycloheptanes and cycloheptenes are explained in detail. According to calculations, the most favourable conformation for cycloheptanes seems to be the twist chair cycloheptenes prefer the chair form. Possible conformational processes for chair and boat forms are discussed and illustrated. 

These are not frequently used monomers. There exists a clear connection between the strain of various members in the series of cyclic hydrocarbon molecules and their heats of combustion (see Table 1). The high heats of combustion of the first members are the consequence of the C—C bond angle deviation from 109°28. In cyclohexane, the most stable cycloalkane which can exist in the chair conformation, the C—C bond angle value deviates very little from that observed in unstrained compounds. Cyclopentane exhibits the smallest deviation of the C—C bond angle from the theoretical value. Its higher heat of combustion is due to steric interactions of pairs of neighbouring hydrogen atoms. A similar situation is observed with cycloheptane [12a]. 

Cycloheptane is found in two main groups of conformers the chair form (including the half-chair) and the boat form (including the twist boat), with the chair form lower in energy by about 7 kcal/mol (ap-... 

Benzene annelation significantly increases the barrier to ring inversion processes. Saturated seven-membered rings can invert by pseudorotation (pseudorotation barrier is 2 kcal/mole) which has not been measured by NMR techniques. The AG for the ring inversion process for the benzene-annellated derivatives of cycloheptane (which probably exists in a chair form) was determined to be 10.9 kcal/mole at Tj, = — 57°C (186, 298, 322). Equilibria in tricyclic organic derivatives have been studied more extensively. Several different inversion processes have been reported and depend on structural type. The following are examples of ring inversion processes. 

The acetone-sensitized irradiation of the bis-pyrimidine (248) yields the [2 -t- 2]-cyclized product (249). A chair-like conformation is adopted by the cycloheptane part of the molecule (249) in the solid and in solution. The dimerization of the bichromophores (250) to yield the cycloadducts (251) was slow by comparison with the dimerization in analogous less heavily substituted systems. The slowness of the process is, it is thought, due to steric factors. ... 

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... 

P2i2j2 Z = 4 D, = 1.20 R = 0.041 for 1,929 intensities. The septan-oside has a distorted, twist-chair conformation, TCe,o. with the symmetry axis for the corresponding cycloheptane conformation passing through C-4. The distortion is toward the Ci conformation, similar to that observed in 5-0-(chloroacetyI)-l,2 3,4-di-0-isopropylidene-a-D-glucoseptanose. The two dioxolane rings have envelope conformations. 

The two-dimensional representation of cycloheptane as a planar molecule was 49 in Section 8.5.1. A chair conformation is possible that relieves the Baeyer strain and torsion strain, and two conformations are possible (49A and 49B). The top hydrogen atoms are marked blue and the bottom hydrogen atoms are marked red. There is some strain for the axial-like hydrogen atoms, but the larger size of the ring allows those hydrogen atoms to be further apart. Closer examination revels that two of the carbons of cycloheptane are nearly coplanar (the flat part of 49A and 49B). 

This flattening is due to the presence of an odd niimber of carbons in the ring and it means that there will be some torsion strain due to eclipsing bonds and atoms in this form of cycloheptane. Some twisting of the ring can occur to relieve this strain, but such pseudorotation may increase strain elsewhere in the molecule. This increase in strain makes conformations 49A and 49B for cycloheptane higher in energy than the chair conformations of cyclohexane. 

There is also a boat-like conformation, 49C, but the flat part of the seven-membered ring has diminished transannular strain because the hydrogen atoms are a little further apart. The strain energies for chair cycloheptane and boat cycloheptane are close, and one does not greatly predominate over the other. There are several other conformations for cycloheptane, as there are for cyclohexane, because the size and flexibility of the ring has increased however, at this point, no other conformations for cycloheptene will be discussed. 

In cycloheptane, the energies of the chair and boat conformations are about equal. This is not true with 1,1,4,4,5,5-hexamethylcycloheptane, however. 

The e.s.r. spectra of a series of cycloheptane semidiones and analogues are consistent with the cycloheptane semidiones adopting chair conformations. The e.s.r. spectra... 

Computations were performed for the N-fluoroquinuclidinium ion 17 formation of such species is known to facilitate fluorine transfer significantly. It is noted that the conformations of the seven-membered ring transition states TS7 and TS8 (Figure 3.10) are different and comparable to the known conformations of cycloheptane in the TS7 the ring adopts a chair conformation, whereas in TS8 the boat conformation is taken to expose the opposite enantioface. 

Semi-quantitative potential-energy curves for pseudorotation within chair and boat forms of 1,1-dimethyl- and 1,1,2,2-, 1,1,3,3-, and 1,1,4,4-tetramethyl-cycloheptanes have been constructed interconversion of the preferred twist-chair forms of the latter is calculated to proceed via the twist-boat form rather than by pseudorotation within the chair family. 

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