Cycloheptane conformational analysis

Beginning with cycloheptane, which has four confomnations of similar energy, conformational analysis of cycloalkanes becomes more complicated. The sane fundamental principles apply to medium and large rings as apply to smaller ones—but there ar e more atoms and more bonds to consider and more confor-mational possibilities. 

Conformational analysis of rings larger than cyclohexane is more complicated. These rings are also less common than cyclohexane, so we discuss their conformations only briefly. As can be seen from Table 6.1, the seven-membered ring compound cycloheptane has only a small amount of strain. Obviously, it is nonplanar to avoid angle strain. It does have some torsional strain, but the overall strain is comparable to that of cyclopentane. It is a fairly common ring system. 

Seven-membered Rings.—The conformational analysis of cycloheptene oxide has been studied by and n.m.r. spectroscopy. The singlet due to the C-2 proton of [l,3,3- H3]cycloheptane oxide (3) separates into two peaks at low temperatures, the free energy of activation for interconversion of the two conformers being 7.9 kcal mol . The n.m.r. spectrum of undeuteriated cycloheptene oxide showed four... 

An extensive study of the conformational behavior of perhydroazepine 66 was carried out by Espinosa and coworkers105. This pure force field based analysis, with no reference to experiment or other theoretical methods, was part of a series of studies on cycloheptane, cycloheptene and some of their oxygen, sulfur and nitrogen heterocyclic analogs. MM2... 

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). 

By this analysis, planar cyclopentane should be most stable, then cyclohexane, cyclobutane (which is about the same as cycloheptane), and finally cyclopropane. However, this is not the correct order for the inherent stability of cyclic alkanes. The energy inherent to each ring is shown in Table 8.1. The data in this table clearly show that cyclopropane is the highest in energy, but they also show that cyclohexane is lower in energy than cyclopentane. Indeed, cyclopentane and cyclohexane are the more stable (lowest energy) cyclic alkanes in the series 45-49 because cUc alkanes are not planar. The pseudorotation mentioned before leads to conformations that are lower in energy than... 

Cycloheptane-1,2-semidTone" is a rigid species (t>10 sec.) up to at least 90. From an analysis of the esr hyperfine interactions it can be concluded that the single populated conformation is the staggered conformation 4(5). 

---