Cyclohexene half-chair

CjHioOs Ethylene glycol monomethyl CeHio Cyclohexene half-chair) 0.332 0.012... 

The conformational principles which apply to rings other than six-membered ones are rather more sketchy because only six-membered rings adopt well-defined chair (or, for cyclohexenes, half-chair) conformations. But we can still give you some general guidelines and principles, and illustrate them with some important examples. We will look in detail at four- and five-membered rings. 

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. ... 

The conformation A is a half-chair, in which all atoms, except for the C-6 atom, are virtually in a single plane, from which the C-6 atom deviates due to rotation about the N-O(l) bond. The C(3)-N(2)-0(l)-C(6) dihedral angle 01 characterizes the degree of rotation. The N(2)-C(3)-C(4)-C(5) dihedral angle 02 corresponds to rotation about the C(3)-C(4) bond, which can occur independently of the first process and leads to a deviation of the C-5 atom from the plane of the nitronate fragment in the direction opposite to the C-6 atom. For the conformation A, 01=24.3° and 02 = 6.5°,that is, A = /01 / - /02/, which characterizes the difference in the degree of rotation about two bonds, is 17.8° (for cyclohexene, A=0). It should be noted that the results of these calculations are... 

Hetero-substituted cyclohexenes, e.g. dihydropyrans, exist in half-chair conformations. 

Analysis of the far IR-spectra of 3,4-dihydro-2//- pyran (13) (72JCP(57)2572> and 5,6-dihydro-2/f- pyran (14) (81JST(71)97> indicates that for both molecules the most stable conformation is a half-chair form. The barrier to planarity is greater for the former compound. These preferred structures are in accord with the half-chair conformation established for cyclohexene and its derivatives. The conformational mobility of cyclohexene is greater than that of the 3,4-dihydropyran. The increased stabilization of the pyran has been attributed to delocalization of the v- electrons of the alkenic carbon atoms and the oxygen lone-pairs (69TL4713). 

The origin of the observed 1,4-asymmetric induction in Michael reactions of chiral imines (Scheme 28) has been rationalized by conformational transmission of chirality. Thus, the phenethylamine auxiliary forces the cyclohexene part of the intermediate enamine into a half-chair conformation (140) that is 0.8 kcal mol-1 lower in energy than (141). Axial attack as shown then leads to the major product the energy difference between (140) and (141) roughly correlates with the observed diastereoselectivity ( 9 l).105... 

The absolute stereochemistry of the Al(6)-alkene alkaloids cocculine (56) and coccutrine (52) has also been established by X-ray analysis. It was found that the cyclohexene ring A exists preferentially in an approximate half-chair conformation in the free base, but this was altered to an envelope conformation on protonation of the nitrogen atom (49). 

It is impossible for the CO bonds of the product epoxide ring to adopt perfectly axial and equatorial positions. If you make a model of cyclohexene oxide you will see that the ring is a slightly deformed chair—it is more of a half-chair conformation in which four of the carbon atoms are in the same plane (you met this on p. 461). 

Epoxide openings are not alone in always giving diaxial products. We can give the general guideline that, for any reaction on a six-membered ring that is not already in the chair conformation, axial attack is preferred. You will see in later chapters that this is true for cyclohexenes, which also have the half-chair conformation described in the next section. Cyclohexanones, on the other hand, already have a chair conformation, and so can be attacked axially or equatorially. 

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 ). 

Hetero-substituted cyclohexenes, e.g., 3,6-dihydro-2/7-pyran, 3,6-dihydro-2//-thiin, and 3,6-dihydro-l,2-dioxin, exist in half-chair conformations (see Section 2.2.3.2). 

The pyrrolidine ring in 79 has an envelope conformation and the morpholino groups in 77 and 83 as well as the piperidino groups in 78 possess a chair conformation. The cyclohexene moieties in 77 to 82 exist in half-chair conformations. 

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 skeletal geometry of cyclohexene and 1,2-epoxy cyclohexane systems corresponds to a partially flattened chair or half-chair (8). The alternative "half-boat conformation has... 

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