Trans cycloalkenes

In the smaller substituted cycloalkenes, the double bond can exist only in the cis configuration. The trans arrangement is prohibitively strained (as building a model reveals). However, in larger cycloalkenes, trans isomers are stable. 

So far we have represented cycloalkenes by structural formulas m which the double bonds are of the cis configuration If the ring is large enough however a trans... 

Dehydrohalogenation of cycloalkyl halides lead exclusively to cis cycloaUcenes when the ring has fewer than ten carbons As the ring becomes larger it can accommo date either a cis or a trans double bond and large nng cycloalkyl halides give mixtures of CIS and trans cycloalkenes... 

Among disubstituted alkenes trans RCH=CHR is normally more stable than as RCH=CHR Exceptions are cycloalkenes cis cyclo alkenes being more stable than trans when the nng contains fewer than 11 carbons... 

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... 

The racemization of medium-ring trans-cycloalkenes depends upon ring size and substitution, as indicated by the data below. Discuss these relative reactivities in terms of the structures of the cycloalkenes and the mechanism of racemization. 

While it is relatively easy to introduce a cis double bond into a small ring, it is very difficult to introduce a trans double bond. In fact, the smallest trans cycloalkene which has actually been isolated to date is cyclooctene. 

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 ... 

When the halogenation reaction is carried out on a cycloalkene, such as cyclopentene, only the trews stereoisomer of the dihalide addition product is formed rather than the mixture of cis and trans isomers that might have been expected if a planar carbocation intermediate were involved. We say that the reaction occurs with anti stereochemistry, meaning that the two bromine atoms come from opposite faces of the double bond—one from the top face and one from the bottom face. 

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... 

Almost all evidence indicates that the transition state must be planar. Deviations from planarity as in 17-3 (see p. 1322) are not found here, and indeed this is why six-membered heterocyclic nitrogen compounds do not react. Because of the stereoselectivity of this reaction and the lack of rearrangement of the products, it is useful for the formation of trans cycloalkenes (eight-membered and higher). 

Bis(diamino)alanes (R2N)2A1H were used for the hydroalumination of terminal and internal alkenes [18, 19]. TiCb and CpjTiCb are suitable catalysts for these reactions, whereas CpjZrCb exhibits low catalytic activity. The hydroaluminations are carried out in benzene or THF soluhon at elevated temperatures (60°C). Internal linear cis- and trans-alkenes are converted into n-alkylalanes via an isomerization process. Cycloalkenes give only moderate yields tri- and tetrasubstituted double bonds are inert. Hydroaluminahon of conjugated dienes like butadiene and 1,3-hexa-diene proceeds with only poor selechvity. The structure of the hydroaluminahon product of 1,5-hexadiene depends on the solvent used. While in benzene cyclization is observed, the reaction carried out in THF yields linear products (Scheme 2-10). 

Another important feature of this reaction is the synthesis of highly strained E-cycloalkenes. Thus E,Z-cycloocta-1,3-diene has been obtained from the Z,Z-compound in sensitized irradiation 309) (3.4), while E-cyclo-octene was obtained from the Z-isomer by direct irradiation (3.5) 310). The synthesis of a trans doubly bridged olefin has also been reported (3.6)311>. 

Wulfman, D.S., van Thinh, N., McDaniel, R.S., Pierce, B.W., Heitsch, C.W., and Jones, M.T., Metal salt-catalyzed carbenoids. IX. Catalysts in trialkyl phosphite-copper(I) complex catalyzed decomposition of diazomalonic esters in cycloalkenes, ]. Chem. Soc., Dalton Trans., 522, 1975. 

The stereochemistry of hydrogenation of 1,2-dimethylcyclQhexene and 1,2-dimethylcyclopentene is instructive. Each of these substances would be expected to yield only the ci5-l,2-dimethylcycloalkane via cis addition. Both cis and trans isomers, however, are formed from either of these two cycloalkenes when hydrogenated in the liquid phase (acetic acid) over reduced platinum oxide—one of the more stereoselective catalysts [57, 58). The ratio of isomers which is produced is a function of the pressure of hydrogen, the proportion of cis increasing with increasing pressure (Fig. 5). This fact implies that the trans isomer is formed via a... 

Allyltrimethylsilane (la) reacts with cycloalkenes such as cyclohexene and cyclo-pentene at room temperature to give stereospecifically tra j -l-trimethylsilyl-2-al-lylcycloalkanes. These products are formed through trans addition and an allylic inversion. 

Under the conditions which cause migration of the double bonds, e.g. treatment with sodium and especially potassium, the isolated double bonds can become conjugated, and thus they undergo reduction by metals. Some macrocyclic cycloalkadienes were reduced (predominently to trans cycloalkenes) by treatment with potassium on alumina in a hydrogen atmosphere [350]. 

The polymerization of the alkyne triple bond (Secs. 5-7d and 8-6c) and ring-opening metathesis polymerization of a cycloalkene (Secs. 7-8 and 8-6a) yield polymers containing double bonds in the polymer chain. Cis-trans isomerism is possible analogous to the 1,4-polymer-ization of 1,3-dienes. 

Ni and Pd initiators polymerize cycloalkenes through the double bond with cis 1,3-place-ment (Sec. 8-6a). The only reported polymerization of an acyclic 2-alkene, specifically trans-2-butene, involves the use of an a-diimine initiator [Leatherman and Brookhart, 2001]. 

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