Elimination from Substituted Cyclohexanes

Elimination from substituted cyclohexanes follows the same stereochemical rules as elimination from open-chain compounds. 

We just saw that to achieve the anti-periplanar geometry favored in an E2 reaction, the two groups being eliminated must be parallel (Section 10.6). For two groups on a cyclohexane ring to be parallel, they both must be in axial positions. 

In an E2 reaction of a substituted cyciohexane, the groups being eiiminated must both be in axiai positions. 

The more stable conformer of chlorocyclohexane does not undergo an E2 reaction, because the chloro substituent is in an equatorial position. (Recall from Section 3.13 that the more stable conformer of a monosubstituted cyclohexane is the one in which the substituent is in an equatorial position because there is more room for a substituent in 

Because only one of the two conformers undergoes an E2 reaction, the rate constant for the elimination reaction is given by k K, where A is a rate constant and eq = is an equilibrium constant. Therefore, the reaction is faster if Teq is large. Most molecules are in the more stable conformer at any given time. Therefore, will be large if elimination takes place by way of the more stable conformer, and it will be small if elimination takes place by way of the less stable conformer. 

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The loss of sulfur from substituted 4-phenyl-1-benzothiepins 7 can be achieved by heating in cyclohexane or carbon tetrachloride.90 In a similar way, but under mild conditions,14 the elimination of sulfur monoxide occurs from the corresponding 1-benzothiepin 1-oxides, reflecting the lower thermal stability of the sulfoxides in apolar solvents.85... 

More evidence comes from the reactions of substituted cyclohexanes. You saw in Chapter 18 that substituents on cyclohexanes can be parallel with one another only if they are both axial. An equatorial C-X bond is anti-periplanar only to C-C bonds and cannot take part in an elimination. For unsubstituted cyclohexyl halides treated with base, this is not a problem because, although the axial conformer is less stable, there is still a significant amount present (see the table on p. 462), and elimination can take place from this conformer. 

When a substituted cyclohexane undergoes an El reaction, the two groups that are eliminated do not have to both be in axial positions, because the elimination reaction is not concerted. In the following reaction, a carbocation is formed in the first step. It then loses a proton from the adjacent carbon that is bonded to the fewest hydrogens— in other words, Zaitsev s rule is followed. 

Optically active substituted alkylidene cyclohexanes were prepared from sulfinyl esters (obtained by carboxylation of sulfinyl anions) by thermolytic elimination of the sulfinyl group65. 

The metabolism of chlorexolone to three monohydroxylated derivatives (all apparently substituted in the cyclohexane ring) in both dog and man has been reported [334]. No storage in body tissues could be detected and elimination via the kidney appeared to follow oxidation to the polar metabolites (no unchanged drug could be recovered from urine). These findings contrast with those relating to thiazides which often appear unchanged in urine. 

Cesium fluoride promoted oxirane preparation starting from bis(sulfonyl fluorides) has been reported. In general, epoxide formation predominates over furan synthesis. Investigations of cyclohexane derivatives 1 show trans elimination to form 2 this still holds true when the syn chlorine is substituted by a bromine.Epoxide 4 is a versatile, general, enan-liomerically pure building block that is best synthesized by substitution of the mesylate 3. ... 

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