Conformation of cyclohexenes

The conformation of cyclohexene is described as a half-chair. Structural parameters determined on the basis of electron diffiaction and microwave spectroscopy reveal that the double bond can be accommodated into the ring without serious distortion. ... 

That the high degree of torsional and other types of strain inherent in the triplet states or trans conformers of cyclohexene and cycloheptene may be responsible for their photochemical behavior is suggested by the reactions of compound (50), a moderately twisted olefin according to molecular models. Compound (50) quantitatively yields bicyclo[3.3.1]non-l-yl acetate (51) within 15 sec after being dissolved in glacial acetic acid(83> ... 

Cyclohexene and derivatives. The stable conformation of cyclohexene is the half-chair326 (Fig. 20). 

We looked at the conformations of cyclohexenes and cyclohexene oxides in Chapter 18. and we will look again at the stereochemistry of reactions of six-membered rings contai ning double bonds in Chapter 33. 

Half-chair, half-boat Terms used most commonly to describe conformations of cyclohexenes in which four contiguous carbon atoms atoms lie in a plane. If the other two atoms lie on opposite sides of the plane, the conformation is a half-chair if they are on the same side, it is a half-boat, as shown below. Also used for 5-membered rings, where three adjacent atoms define the plane. 

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. 

Figure 1.2.6 The major half-chair conformers of cyclohexene.

The presence of the double bond in cycloalkenes affects the conformation of the ring. The conformation of cyclohexene is a half-chair, with carbons 1, 2, 3, and 6 in the same plane, and carbons 4 and 5 above and below the plane. Conversion to the alternative half-chair occurs readily, with an energy barrier of 22.2 kJ/mol (5.3 kcal/mol), which is about one half that required for chair-to-chair interconversion in cyclohexane. Substituents at carbons 3 and 6 are tilted from their usual axial and equatorial orientations in cyclohexane and are referred to as pseudoaxial and pseudoequatorial. 

Half-chair conformation of cyclohexene may be described in terms of the torsion angle formed by the saturated carbon atoms (the C3-C4-C5-C6 torsion angle, see Fig. 19.1). According to experimental and theoretical data value of this angle is within 60-64° (Table 19.1). 

Symmetry of ring conformation implies existence of two symmetrical half-chair conformations of cyclohexene with the same energy. Based on IR and Raman data it was suggested that conformational transition from one half-chair conformation (HCl) to another one (HC2) may proceed via either boat conformation with Cs symmetry or planar structure with C2v symmetry (Fig. 19.1). 

Problem was solved by application of intrinsic reaction coordinate (IRC) procedure in combination with Density Functional Theory and second order Moller-Plesset perturbation theory methods [35]. It was demonstrated that potential energy surface around boat conformation is extremely fiat (Fig. 19.2) and contains wide plateau where energy of molecule remains almost the same. Conformation of cyclohexene ring is changed from one twist-boat to symmetric another twist-boat via boat conformation representing a central point of plateau. Width of plateau in terms of value of the C3-C4-C5-C6 torsion angle slightly depends on method of calculations (Table 19.2) and is varied within 30 0°. 

According to theoretical data [49] decrease of polarity of exocyclic double bond leads to shift of equilibrium conformation of cyclohexene ring from sofa to halfchair (Table 19.3). Unfortunately experimental data for molecules 3-7 are absent. In the case of methylene derivative 8 IR spectroscopic study [45] suggested a sofa conformation of cyclohexene ring similar to cyclohexenone 2. However, quantum chemical calculations by MK/6-311G(d,p) method indicated slightly asymmetric half-chair equilibrium conformation of ring in this molecule. 

The conformation of cyclohexene oxide (60) resembles that of its hydrocarbon analogue, bicyclo[4,l,0]heptane (32), and that of cyclohexene (14b), so that both types of thiee-membered ring appear to have the same configuration-determining effect as a double bond. However, this hypothesis does not seem to apply to the pair of molecules a-pinene oxide (61) and a-pinene (39). The four-bond bridge in the latter is planar, whereas (61) shows that in the oxide this is not so however, there is a methyl-to-oxygen non-bonded interaction which may account for this difference. 

Figure 1.5 Lower-energy conformations of cyclohexene and cyclooctene.

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