Conformation of cycloheptane

Fig. 2.17. The four most favorable conformations of cycloheptane. Reproduced from J. Org. Chem., 68, 9322 (2003), by permission of the American Chemical Society.

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. 

Beginning with cycloheptane which has four conformations of similar energy confer matronal analysis of cycloalkanes becomes more complicated The same fundamental principles apply to medium and large rings as apply to smaller ones—but there are more atoms and more bonds to consider and more conformational possibilities... 

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. 

Rings larger than six-membered are always puckered unless they contain a large number of sp atoms (see the section on strain in medium rings, p. 223). The energy and conformations of the alkane series cycloheptane to cyclodecane... 

Bocian DF, Strauss HL (1977) Vibrational spectra, conformations and potential functions of cycloheptane and related ox nes. J. Am. Chem. Soc. 99 2866... 

Cycloheptane, cyclooctane, and cyclononane and other higher cycloalkanes also exist in nonplanar conformations. The small instabilities of these higher cycloalkanes appear to be caused primarily by torsional strain and repulsive dispersion forces between hydrogen atoms across rings, called transannular strain. The nonplanar conformations of these rings, however, are essentially free of angle strain. 

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 contrast to the six-membered rings, the seven-membered cychc systems are much more flexible. The cycloheptane rings occur in complex pseudoro-tational equihbria that has numerous conformations of similar energy with the absence of significant pseudorotational barriers. Owing to these reasons the study of such compounds is difficult using experimental techniques and therefore condensation with a benzene ring or the introduction of a double bond freezes the pseudorotational equflibrium at a low temperature [107]. 

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... 

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