Half-chair conformation of cyclohexene

Figure 1.2.6 The major half-chair conformers of cyclohexene.

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). 

In the half-chair conformation of cyclohexane, four ac//acent carbon atoms are in one plane with the fifth above this piane and the sixth beiow it. You will this conformation again later—it represents the energy minimum for cyclohexene, for example. 

How should we draw this epoxide fused to a six-membered ring 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 (as we can call this epoxide) you will see that the ring is a slightly deformed chair—in fact it is like the half-chair conformation of cydohexene, in which four of the carbon atoms are in the same plane (you met this on p. 829). 

For the general case, a complicated scheme of alternate pathways has been pro-posed to convert the two half-chair conformations of a 3-substituted cyclohexene into two pairs of conformationally isomeric bromonium ions [(262X ( 63) and (264X (265)] and thence to four pairs of adducts (Scheme 4). This scheme is considered below, with reference to the stereochemistry of epoxide formation and hypobromous-add addition to olefins. A small allylic equatorial substituent does not greatly influence the site of electrophilic attack, and when the substituent is bulky the preference is for formation of (262). An axial substituent exerte a mild preference for (263) over (264). The nucleophilic step can also be influenced by shielding of the substituent An electron-with-... 

Miehael addition of carbon- and hetero-nucleophiles to the in x/tn-generated nitroso-cyclohexenes has been found to be highly stereoselective, leading predominantly (or exclusively) to products resulting from axial attack on a half-chair conformation of the nitrosoalkene. 336 0... 

Geminal proton-proton coupling constants have been determined for the allylic methylene protons in a series of selectively deuteriated cyclohexene derivatives, e.g. (65). On the basis of — AG° for 4-methylcyclohexene and 4-methoxycarbonylcyclohexene the half-chair conformation of (65) with equatorial methyl should be favoured by 0.2—0.3 kcal mol this is in accord with... 

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. ... 

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). 

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 ). 

Cyclohexenes The conformational equilibria for monosubstituted cyclohexenes are known to be less biased against the axial conformer than is the case with the saturated analog. This may be attributed to the absence of one 1,3-diaxial hydrogen interaction for either an axial or a pseudoaxial substituent in the half-chair conformations ... 

Like all cyclohexene derivatives, L-shikimic acid (61) assumes a so-called half-chair conformation. Introduction of a double bond into a cyclohexane derivative in a chair conformation forces four of the carbon atoms (1, 2, 5, and 6 in shikimic acid) into a plane. This causes distortion to a halfchair form, as, for example, in conduritol B (63a), the depiction of which shows the positions of the substituents. The distortion has little effect on the dispositions of the substituents opposite the double bond, namely, those on C-3 and C-4 in shikimic acid and those on C-2 and C-3 in formula... 

X-ray study of the 4-(3-chlorophenyl) derivative showed that the N containing ring adopted a boat conformation and the cyclohexene has a half-chair conformation (05AX(E)01634). The corresponding 4-(2-chloro-5-nitrophenyl) derivative has the chloro substituent in a syn-periplanar orientation with respect to the pyridine ring plane with the nitro group over it (04AX(E)0711). 

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. 

Scheme 4 (a) Cyclohexene oxide adopts a half-chair conformation that exists in two different helicities. (b) The isopropyl substituent of limonene-1,2-epoxide makes one of the two half-chair conformations energetically more favorable. 

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