Cyclohexane transannular strain

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. 

As was indicated in Table 3.6 (p. 95), cyclodecane is significantly more strained than cyclohexane. Examination of the boat-chair-boat conformation reveals that the source of most of this strain is the close van der Waals contacts between two sets of three hydrogens on either side of the molecule. The hydrogens involved are shown in the drawing. Distortion of the molecule to twist forms to relieve this interaction introduces torsional strain. It seems reasonable to assume that suitably substituted cyclodecanes may be found in which the transannular van der Waials repulsion is sufficiently large to allow a conformation other than the boat-chair-boat to be the most stable. 

Ten-membered Rings.—rm 5,/ra/w-Bicyclo[8,4,0]tetradeca-2,8-diene (227 R = H) cyclized readily on treatment with acetic acid-perchloric acid to give a mixture of epimeric acetates (228) and tricyclic olefins. Protonation of the double bond occurs from the equatorial direction. Analogous cyclizations of (227 R = Me) and the cyclopropyl derivative (229) were also studied. All three cyclizations are fairly rapid because participation of the transannular double bond is accompanied by the release of medium-ring strain and they are also facilitated by the favourable conformation enforced on the 10-membered ring by the cyclohexane ring. ... 

Modest stability of the diketone. Cycloheptane-1,3-dione is a colorless oil. We foimd that crude san5>les of this material tended to discolor upon storage above 0 °C. This color could be removed by distillation or filtration through silica gel, but with associated loss of material (distillation was particularly inefficient, 60-65% recoveries were typical). This is in marked contrast to cyclohexane-1,3-dione, which is a stable, crystalline, white solid. Interestingly, the NMR spectra of these two materials in CDCI3 are also quite different the 6-meiribered diketone is completely enolized, whereas the 7-membered diketone exists exclusively in the diketo form. Transannular interactions or some other form of ring strain are presumably responsible for this difference in enolization propensity, and this correlates with the greater reactivity/instability of the 7-... 

Figure 4-7 Conversion of the hypothetical planar cyclohexane into the boat form. In the boat form, the hydrogens on carbons 2, 3, 5, and 6 are eclipsed, thereby giving rise to torsional strain. The inside hydrogens on carbons 1 and 4 interfere with each other sterically in a transannular interaction. The space-filling size of these two hydrogens, reflecting the actual size of their respective electron clouds, is depicted in the ball-and-stick model on the right.

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