Cyclohexane planar

The most common stereoselective syntheses involve the formation and cleavage of cyclopentane and cyclohexane derivatives or their unsaturated analogues. The target molecule (aff-cts)-2-methyl-l,4-cyclohexanediol has all of its substituents on the same side of the ring. Such a compound can be obtained by catalytic hydrogenation of a planar cyclic precursor. Methyl-l,4-benzoquinone is an ideal choice (p-toluquinone M. Nakazaki, 1966). 

An X-ray structure of 5yn-triepoxide (38) (74AG(E)539) shows that the cyclohexane ring is planar, with the three oxygens and sbt hydrogens lying in parallel planes 1.19 and 0.42 A respectively from the carbon plane. With all oxygens on one face , (38) forms a crystalline 2 1 complex with potassium iodide. 

Incorporation of stereogenic centers into cyclic structures produces special stereochemical circumstances. Except in the case of cyclopropane, the lowest-eneigy conformation of the tings is not planar. Most cyclohexane derivatives adopt a chair conformation. For example, the two conformers of cis-l,2-dimethylcyclohexane are both chiral. However, the two conformers are enantiomeric so the conformational change leads to racemization. Because the barrier to this conformational change is low (lOkcal/mol), the two enantiomers arc rapidly interconverted. 

Identify the lowest-energy conformer from among those provided cyclopropane, planar and puckered cyclobutane, planar and puckered cyclopentane and chair, half-chair, boat and twist-boat cyclohexane. (If... 

Figure 11.19 The geometric requirement for E2 reaction in a substituted cyclohexane. The leaving group and the hydrogen must both be axial for anti peri-planar elimination to occur.

The annulation of 4//-thiopyran and cyclohexane rings in 50a results in the planarity of the heterocycle and a half-chair conformation of the carbocycle (81KGS1342). On the other hand, a boat conformation of the 2//-thiopyran ring was found in the crystal of224b [91JCS(P2)2061], Other geometrical parameters were within the limits of the expected values (Fig. 2). 

The molecular structure of benzene, C(,H(, is planar. Is the molecular structure of cyclohexane, C6HI2, planar as well ... 

Entry 3 involves a catalyst derived from (/ , Trans-cyclohexane- 1,2-diamine. The square planar Cu2+ complex exposes the re face of the dienophile. As with the BOX catalysts, this catalyst has c2 symmetry. 

The six-membered rings in these T.S.s are more flexible than the five-membered T.S.—(81) above—and need not be planar (cf cyclohexanes v. cyclopentanes). Elimination may thus proceed, in part at least, from conformations other than the syn-periplanar, with the result that the degree of SYN stereoselectivity in these eliminations may sometimes be lower than that observed in the Cope reaction. Both reactions require higher temperatures than for the Cope reaction, carboxylic esters particularly so. 

Generally six-membered rings are more stable than five-membered ones and on the basis of the tetrahedral theory Sachse in 1890 pointed out that cyclohexane could exist in two forms known as boat and chair conformations and neither of which is planar. They are actually puckered i.e., non-planar and are free of angle strain and so are called strainless rings. 

Among the fused ring systems, the most important compound is decalin. This compound exists as the cis and trans forms and the existence of these two forms on the basis of non-planar structures was predicted by Mohr. He thought that the two decalins were formed by the fusion of two chair forms or two boat forms of cyclohexane and so he assigned, the following structures. 

The planar Haworth projection formulae bear little resemblance to the shape of the six-membered pyranoses that actually adopt a non-planar ring conformation comparable to that of cyclohexane. The chair form is the most... 

Five- and six-membered rings formed by coordination of diamines with a metal ion have the stereochemical characteristics of cyclopentane and cyclohexane. The ethylenediamine complexes have puckered rings and the trimethylenediamine complexes have chair conformations. The methylene protons are nonequivalent in these nonplanar conformations, taking on the character of equatorial and axial substituents. They are made equivalent as the result of rapid conformational inversion at room temperature, just as in the alicyclic compounds (Fig. 7.1). This has been observed in nmr studies of planar and octahedral complexes of ethylenediamine-type ligands with a number of metals. 

Note that it is not necessary to consider both forms of cyciohexane, where the methyl is either wedged (up) or dotted (down). If the cyclohexane ring were planar, the two structures would be the same, since one merely has to turn the structure over to get the other. Although the cyclohexane ring is not planar, it turns out that the two structures are still identical, because of the ring flip process. This is shown below. One set of conformers is simply the upside-down version of the other. 

We saw early in Section 3.3.2 that, if we draw cyclohexane in typical two-dimensional form, the bonds to the ring could be described as up or down , according to whether they are wedged or dotted. This is how we would see the molecule if we viewed it from the top. When we look at the molecule from the side, we now see the chair conformation the ring is not planar as the two-dimensional form suggests. Bonds still maintain their up and down relationship, but this means bonds shown as up alternate axial-equatorial around the ring they are... 

Now, although the cyclohexane ring is not planar, the overall consequences for trans- and cis-dimethylcyclohexane can be predicted by looking at the two-dimensional representations. 

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