Trans -Cyclohexene

Both cis and tran -cyclohexene have been synthesized, but only one of them can be isolated. Electrophilic addition of ROH to one isomer occurs spontaneously, while addition to the other isomer occurs only in the presence of a strong acid, such as sulfuric acid. Calculate the energy of protonation for each isomer cyclohexene protonated cyclohexene, trans-cyclohexene protonated trans-cyclohexene), and identify the more reactive isomer. Also examine electrostatic potential maps. Suggest an explanation to account for both the reactivity difference and the structural changes. (See also Chapter 7, Problem 5.)... 

A study [19] of the cycloaddition between substituted (E)-l-phenyl-1,3-butadienes 55 and substituted 1,1-dicyanoethylenes 56 leading to cis- and trans-cyclohexenes 57 and 58 (Equation 5.8) has shown that diastereoselectivity is markedly dependent on pressure. 

As previously mentioned, double bonds in relatively small rings must be cis. A stable trans double bond first appears in an eight-membered ring trans-cyclooctene, p. 134), though the transient existence of tran -cyclohexene and cycloheptene has been demonstrated. Above 11 members, the trans isomer is more stable than the cis. It has proved possible to prepare compounds in which a trans double bond is shared by two cycloalkene rings (e.g., 104). Such compounds have been called [m.n]betweenanenes, and several have been prepared with m and n values from 8 to 26. The double bonds of the smaller betweenanenes, as might be... 

Every discussion of the Diels-Alder reaction for 1,3-butadiene includes the observation that cycloaddition should occur only from the s-cis conformer to produce czs-cyclohexene. This conformational selectivity, however, further implies that cycloaddition of s-trans- 1,3-butadiene should lead to trans-cyclohexene, but the s-trans region of this potential surface has remained unexplored. A study of this problem shows that the concerted and stepwise reaction paths exist for both diene conformers, connecting them to the respective cyclohexene isomers.661 It is also demonstrated that the usual paradigm for the Diels-Alder reactions is incomplete a thorough understanding of this archetypal reaction requires consideration of the full range of processes shown in Scheme 6.10, not just those involving the s-cis conformer. 

A parallel electrooxidation of cis- or trans-cyclohexene-4,5-dicarboxylic acids would thus be expected to produce 1,4-cyclohexadiene, eqn 7. 

The Hydrolysis of cis and trans Cyclohexene Chlorohydrin. When trans cyclohexene chlorohydrin is treated with strong alkali, trans cyclohcxene glycol is obtained. Thus inversion does not appear to have occurred during the displacement. When the reaction is carried out with the cis isomer, cyclohexanone is formed.27 These facts can be explained if it is assumed that the first step in the reaction is the removal of a proton from the hydroxyl group (XXI to XXIII and XXII to XXIV), followed by the displacement of chloride ion by the alkoxide ion of the trans form (XXII1 to XXV) and by a hydride ion in the cis form (XXIV to XXVII). Displacement by alkoxide leads to cyclohexene oxide (XXV), and this intermediate can be further attacked by hydroxide ion to give the dl trans glycol (XXVI). Cyclohexanone is produced by the hydride displacement (XXIV to XXVII) ... 

Since the action of dilute alkali on trans cyclohexene clilorohydrin constitutes a synthetic preparation for cyclohcxene oxide,28 there seems to be little doubt that the epoxide is an intermediate in the reaction of the trans chlorohydrin. There appears to be no evidence for the hydride ion displacement shown for the cis isomer. 

Due to its thermal instability, the ring structure of trans-cycloheptene (39a) has only been characterized in trans-2-cycloheptenone (41) by its IR spectrum and by trapping experiments (47). To date, no direct structural evidence has been found for trans-cyclohexene (43a). It seems likely, however, that trans-l-phenylcyclohexene (43b) is the species detected by pulsed time-resolved photoacoustic calorimetry (cf. Section III. B) (48). 

Irradiation with UV light is the key step for the generation of transient trans-cyclohexenes (43) and frans-cycloheptenes (39). Laser photolysis of 1 -phenylcyclohexene (42b) in methanol at room temperature gave a species with a life time of 9 /is (48,109) (Scheme 2). 

Although s-trans is the more favorable conformer, reaction occurs with s-cis because this conformation has its double bonds on the same side of the single bond connecting them hence, the stable form of cyclohexene with a cis double bond is formed. Reaction of the s-trans conformer with ethene would give the impossibly strained trans-cyclohexene [Problem 9.10(a)], As the s-cis conformer reacts, the equilibrium between the two conformed shifts toward the s-cis side, and in this way, all the unreactive s-trans reverts to the reactive s-cis conformer. 

FIGURE 7.4 Hypothetical trans-cyclohexene. This molecule is apparently too strained to exist at room temperature. 

Especially interesting is the trans,anti,trans stereochemistry of the major cyclobutane product generated in the photodimerization of cyclohexene (eq 12). It was noted that the formation of this product may be the result of a preliminary CuOTf promoted cis-trans photoisomerization that generates a trans cyclohexene intermediate (eq 13). Since one face of the transC=C bond is shielded by a polymethylene chain, the trans-cyclohexene is restricted to suprafacial additions. Although a highly strained transcyclohexene intermediate could be stabilized by coordination with Cu, such a complex has not been isolated. 

When discussing such systems we automatically put the olefin in a cis geometry— that is, trans-cyclopropene seems like a pretty unreasonable structure. But what about trans-cyclohexene or frans-cyclopentene The smallest cycle into which a trans olefin can be embedded has been a subject of considerable study. It has long been known that trans-cyclooctene is relatively stable, with a strain energy of 16 kcal/mol. fra s-Cycloheptene has been prepared and experimentally characterized at low temp>eratures, and it has an estimated strain energy of 27 kcal / mol. The olefin in this structure is substantially distorted, as indicated in Figure 2.16. 

Use handheld molecular models to investigate the strained nature of hypothetical trans-cyclohexene. 

Bonneau, R., Some new examples of trans cyclohexenes properties characteristic of these species,... 

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