Frans-Cycloheptene

The energy differences seem to be somewhat unrealistic, specifically if one considers that 15, 16, and 17 can be regarded as analogues of trans-cyclo-pentene, /rani-cyclohexene, and respectively, as frans-cycloheptene.12 As has... 

Since reactivity of alkenes increases with increasing alkyl substitution, hydration is best applied in the synthesis of tertiary alcohols. Of the isomeric alkenes, cis compounds are usually more reactive than the corresponding trans isomers, but strained cyclic isomeric olefins may exhibit opposite behavior. Thus, for example, frans-cyclooctene is hydrated 2500 times faster than cw-cyclooctene.6 Similar large reactivity differences were observed in the addition of alcohols to strained trans cycloalkenes compared with the cis isomers. frans-Cycloheptene, an extremely unstable compound, for instance, reacts with methanol 109 faster at —78°C than does the cis compound.7... 

Compare geometries of the cis and trans cycloalkenes. Are the double bonds incorporated into the trans compounds significantly more distorted than those incorporated into the analogous cis cycloalkenes Consider carbon-carbon bond lengths and the twisting and/ or puckering of the double bond. Are any distortions greater in frans-cycloheptene than in trans-cyclooctene ... 

Based on MM2 results, predictions have been made for the structure of frans-cyclohexene (43a) and frans-cycloheptene (39a). rrans-Cyclohexene... 

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). 

The copper(I) trifluorosulfonate catalyzed photoreaction of cycloheptene (38a) leads to the dimer 95 (114c). Formation of the latter was taken as support for the generation of frans-cycloheptene (39a) as an intermediate. However, isomerization of free (ran,s-cycloheptene (39a) seems to be faster than its cyclodimerization. 7 ran.s-cycloheptene (39a), obtained by photoreaction with methyl benzoate as sensitizer, isomerizes to ds-cycloheptene (38a) at 0°C (114b) but reacts at low temperature with acidified methanol to form 96 or with diazomethane to form 97 (114a). The stereochemistry of the pyrazoline 97 is trans, in accord with the stereospecific addition to trans-cycloheptene (39a). 

Th is was the d iscovery of H. Jendralla, Angew. Chem. Int Ed. Engl., 1980,19,1032. If you were really on the ball, you ll have noticed that a frans-cycloheptene is chiral, so this compound must be a single diastereoisomer though we don t know which. 

The smallest stable ring that can contain a trans double bond is cyclooctene—frans-cycloheptene can exist but is very unstable. 

Dissolution of the frans-cycloheptene-CuOTf complex in cycloheptene and evaporation of the solvent delivers a tris alkene complex of CuOTf containing one frans-cycloheptene and two cis-cycloheptene ligands. Heating frans-cycloheptene-CuOTf in neat c/s-cycloheptene delivers the trans,anti,trans,anti,trans trimer (eq 16). Experiments with c/s-cycloheptene-<4 show that the cyclotrimerization involves only frans-cycloheptene molecules, although the reaction is accelerated by the presence of cis-cycloheptene. A likely explanation for these observations is concerted template cyclotrimerization of a tns-trans-cycloheptene-CuOTf complex formed by ligand redistribution (eq 16). ... 

As previously mentioned, double bonds in relatively small rings must be cis. A stable trans double bond326 first appears in an eight-membered ring (frans-cyclooctene, p. 104), though the transient existence of frans-cyclohexene and cycloheptene has been demonstrated.327 Above about 11 members, the trans isomer is more stable than the cis.160 It has proved possible to prepare compounds in which a trans double bond is shared by two cycloalkene rings (e.g., 86). Such compounds have been called [m. n]betweenanenes, and... 

Section 5.7 Cycloalkenes that have trans double bonds in rings smaller than 12 members are less stable than their cis stereoisomers. frans-Cyclooctene can be isolated and stored at room temperature, but tra/ ,s-cycloheptene is not stable above -30°C. 

Calculate energy differences between cis and trans-cycloheptene and cis and trans-cyclooctene. Which is the more stable isomer for each compound Is there a significant (>.008 au or 5 kcal/mol) increase in the energy difference between cis and trans isomers in going from the seven to eight-membered cycloalkene Are your results consistent with the fact that frans-cyclooctenc is an isolable, stable compound while trans-cycloheptene is not ... 

As you now know (p. 1134) aU esters and lartones prefer to be in the conformation shown for the first compound so that they can enjoy stabilization by the anomeric effect. The second ester would also prefer o be like this too (first diagram below) but in this conformation it cannot cyclize at all. Even in the less favourable conformation it uses for reaction, it prefers conjugate to dirert substitution as the latter would give a strained truns-cycloheptene. The first compound can enjoy both the anomeric effert and conjugate addition as a frans-alkene in a ten-membered ring is fine. It is better than fine in the heterocyclic product as the lactone also enjoys the anomeric effect... 

Likewise, the trans/cis-isomerization of the more strained trans-cycloheptene (39a) is faster than that of frans-cyclooctene (37a) (114a, 246). It should be mentioned that a negative entropy of activation has been found for the isomerization of 39a. 

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. 

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