Trans-Cycloheptenes

Cycloalkenes that have trans double bonds m rings smaller than 12 mem bers are less stable than their cis stereoisomers trans Cyclooctene can be isolated and stored at room temperature but trans 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 trans-cyclooctene is an isolable, stable compound while trans-cycloheptene is not ... 

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 trans-cycloheptene than in trans-cyclooctene ... 

Another measure of distortion is the shape of the highest-occupied molecular orbital (HOMO). This corresponds to the 7t bond. Is the orbital relatively undistorted in the cis compounds (as in cis-2-butene)l Is it more distorted in trans-cycloheptene than in trans-cyclooctene Explain why distortion in the HOMO is likely to be energetically unfavorable. 

HOMO of trans-cycloheptene reveals distortion of the molecule s 7t system. 

H. Jendralla, Angew. Chem., Int. xhe twisted alkene of a trans-cycloheptene is unstable and rotates to the much more stable Ed. Eng ., 1980,19, 1032. alkene even at 20°. It can rotate because the overlap between the p orhitals is very weak as the ... 

The same is true for three-, four-, five-, and seven-membered rings, though trans-cycloheptene has been observed fieetingly. An eight-membered ring, on the other hand, is just about large enough... 

It is more efficient to obtain cycloaddition products via a stable silver(I) triflate complex of trans-cycloheptenes that is preferably not isolated, e.g. formation of 21. ... 

Perhaps the madness of needing to have another diastereomer of a starting material in order for a reaction to be stereospecific is more obvious in the next example. Heptene 106 may be dihydroxylated stereospecifically to give 107. But the reaction of cycloheptene 108 would not be a stereospecific reaction because there is no such thing as a trans cycloheptene. Clearly this is crazy - both reactions are stereospecific. But see what your friends think. 

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). 

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). 

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 an attempt was made to produce rrcm.v-cycloheptene from trans-1,2-cycio-heptenethionocarbonate (8) by the same procedure, only cfy-cycloheptene was obtained. However, the reaction of trans-1,2-cycloheptenethionocarbonate (8) with trimethyl phosphite in the presence of 2,5-diphenyl-3,4-isobenzofurane (9) as trapping agent gave an adduct (10), m.p. 150°, isomeric with the known adduct of the benzofurane with cii-cycloheptene, m.p. 185°. It is apparent, therefore, that trans-cycloheptene is the primary product from trans- 1,2-cyclohcptenethionocarbonate. 

Alkenes exhibit large strain effects when factors of molecular geometry do not permit all the bonds to the two -hybridized carbons to be coplanar. An example that illustrates the twisting of an olefinic 7r-system can be found in trans-cycloheptene ... 

An isolable CuOTf complex of a highly strained alkene, trans-cycloheptene, is produced by UV irradiation of a hexane solution of cis-cycloheptene in the presence of CuOTf (eq 14).i Photocycloaddition of cycloheptene is also catalyzed by CuOTf. Surprisingly, the major product is not a trans,anti,trans dimer analogous to that formed from cyclohexene (eq 12) but rather a trans,anti,trans,anti,trans trimer (eq 15). ... 

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). ... 

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