Ring inversion cyclohexene

Fig. 19.2 Cyclohexene ring inversion profile calculated by MP2/6-31G(d,p) method using intrinsic reaction coordinate...

The same conclusion was drawn from the results obtained from careful studies of the stereochemistry of the glycol products formed on oxidation of cyclohexene with thallium(III) acetate 3, 83). When dry acetic acid was employed as solvent the product was mainly the tranr-diacetate (XI) in moist acetic acid, however, the mixture of glycol mono- (XII) and diacetates (XIII) which was obtained was mainly cis. These results have been interpreted in terms of initial trans oxythallation, ring inversion. 

Six-membered rings with more than one sp2 C atom do lose their chair conformation—they become flattened to some degree when there are one or more double bonds included in the ring. Cyclohexene, with just one double bond in the ring, has a half-chair conformation similar to that of its related epoxide, cyclohexene oxide. The usual conformational diagram of cyclohexene is shown below. The barrier for ring inversion of cyclohexene is around 22 kj mol-1 (about half that for cyclohexane ). 

In cyclohexene itself, the barrier to ring inversion is certciinly low at 5.3 kcal/mol m.ns) and it appears that the preferred conformation is a half-chair 39 in equilibrium with its mirror image conformation 40. There have been calculations of the energies of cyclohexene conformations by Bucourt and Hainaut and by AUinger and his colleagues as well as suggestions on the basis of experimental evidence These... 

The effect of introducing /j -hybridized atoms into acyclic molecules was discussed in Section 2.2.1, and it was noted that torsional barriers in 1-alkenes and aldehydes are somewhat smaller than in alkanes. Similar effects are seen when sp centers are incorporated into six-membered rings. Whereas the energy barrier for ring inversion in cyclohexane is 10.3 kcal/mol, it is reduced to 7.7 kcal/mol in methylenecy-clohexane ° and to 4.9 kcal/mol in cyclohexanone. The conformation of cyclohexene is described as a half-chair. Structural parameters determined on the basis of electron diffraction and microwave spectroscopy reveal that the double bond can be accommodated into the ring without serious distortion. The C(l)—C(2) bond length is 1.335 A, and the C(l)-C(2)-C(3) bond angle is 123°. The substituents at C(3) and C(6) are tilted from the usual axial and equatorial directions and are referred to as pseudoaxial and pseudoequatorial. 

In this chapter we would like to demonstrate that careful analysis of conformational properties of such simple molecules as cyclohexene, its derivatives and heterocyclic analogues provides new and unexpected information about general background of modern stereochemistry. Especially this concerns character of ring inversion process. 

Thus, the ring inversion process in cyclohexene includes three stage. The first stage involves the transition from an equilibrium half-chair conformation to a... 

Thus, ring inversion profile in cyclohexene strongly differs from classical form due to its top-flattened shape. It is possible to suggest that this difference is caused by correlation between changes of geometrical parameters of ring during... 

It is quite clear that unusual character of ring inversion profile in cyclohexene is determined by subtle balance of intramolecular interactions. Therefore replacement of one methylene group by exocyclic double bond should lead to some changes of these interactions accompanied by change of equilibrium conformation as well as ring inversion pathway. 

Table 19.3 Values of puckering parameters (puckering degree S and polar angles and ) [53] for ring conformation, barrier or ring inversion (Ejnv, kcal/mol) of cyclohexene 1 and its derivatives 2-8 calculated by MP2/6-311G(d,p) method [49]...

Thus ring inversion process in sulphur- and selenium-containing molecules is a multi-stage one similar to derivatives of cyclohexene with non-conjugated exocyclic double bonds. However, thiopyranes 17 and 20 have additional feature (Fig. 19.10). Energies of two transition states between half-chair and boat or twist-boat conformers considerably differ (AE is up to 3 kcal/mol) as calculated by different methods [62]. 

The rates of ring-inversion of perfluoro- and 1,2-dichloro-octafluoro-cyclohexenes have been studied by n.m.r. spectroscopy. The measured rate parameters for the perfluoro-compound are (at —111-5 °C) 6-83 0-10, 7-2 0-2 kcal mol- . As +2-6 1-6 e.u. Other studies of... 

In cyclohexenes, two sp carbon atoms must be accommodated in the ring, and the conformation is thus rather different. Two half-chair conformations, 7.34 and 7.35, are in equilibrium, with an energy barrier of 22.2 kJ moH to ring inversion. 

---