Cyclo alkenes

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... 

RUBBERY CYCLO-OLEFIN (CYCLO-ALKENE) POLYMERS... 

Rubbery Cyclo-olefin (Cyclo-alkene) Polymers 305... 

Thus far, chemists have been able to influence the stereoselectivity of macro-cyclic RCM through steric and electronic substrate features or by the choice of a catalyst with appropriate activity, but there still exists a lack of prediction over the stereochemistry of macrocyclic RCM. One of the most important extensions of the original metathesis reaction for the synthesis of stereochemi-cally defined (cyclo)alkenes is alkyne metathesis, followed by selective partial hydrogenation. 

Cyclopentane has the low chemical reactivity which is typical of saturated hydrocarbons, while 2-pentene is much more reactive. Similarly, ring structures containing double bonds, called cyclo-alkenes, can be shown to be isomeric with alkynes. 

In a detailed study of the reduction of the xylenes in the liquid phase on a 5% rhodium on carbon catalyst, Siegel and Ku (105) showed that both 1,2- and 2,3-dimethylcyclohexenes are formed from orlAo-xylene (Fig. 19). The initial (extrapolated) ratio, l,2-/2,3-, lies between 0.5 and 1 but rises as the reaction proceeds. If the initial distribution of cyclo-alkenes was random as previously postulated (97) the ratio should be 0.5. 

Among a variety of compounds whose molecular characteristics fall into this category, our current interests have further limited our discussion to the doublebond systems which may conveniently be classified as follows 1. chiral ( )-cyclo-alkenes, 2. chiral anti-Bredt rule compounds, 3. trans-doubly bridged ethylenes ( betweenanenes ), 4. bridged allenes, and 5. overcrowded olefins. 

Cycloalkynes may be formed by applying ring-closing metathesis to diynes.214 Partial hydrogenation of the product provides a stereoselective route to cis cyclo-alkenes that cannot be directly prepared in pure form by ring-closing metathesis ... 

The simple carbocation intermediate of Equation 10-1 does not account for formation of the antarafacial-addition product. The results with SN1 reactions (Section 8-6) and the atomic-orbital representation (see Section 6-4E) predict that the bonds to the positively charged carbon atom of a carbocation should lie in a plane. Therefore, in the second step of addition of bromine to cyclo-alkenes, bromide ion could attack either side of the planar positive carbon to give a mixture of cis- and trans-1,2-dibromocyclohexanes. Nonetheless, antarafacial addition occurs exclusively ... 

We also discuss the conformations of cycloalkanes, especially cyclohexane, in detail because of their importance to the chemistry of many kinds of naturally occurring organic compounds. Some attention also will be paid to polycyclic compounds, substances with more than one ring, and to cyclo-alkenes and cycloalkynes. 

Exercise 31-9 Explain how an alkene-metathesis catalyst might convert a cyclo-alkene into (a) a long-chain unsaturated polymer, (b) a mixture of large-ring polymers, and (c) a catenane (interlocking carbon rings like two links in a chain). 

Since sulfenylhalides exhibit high anti-selectivity in addition reactions to alky-nes 202) and since a bulky substituent at the sulfur should stabilize the trans-cyclo-alkene relative to the cis-alkene, trichloromethylsulfenylchloride was used as an electrophile. 

In addition, also spiro compounds can be synthesized using alkylidene-cyclo-alkenes as a dienophile moiety [56]. Finally, as an immense enlargement of the scope of this protocol, the domino Knoevenagel hetero Diels-Alder reaction can be run as a three component transformation mixing an aldehyde such as 2-99, a... 

Alkyl and cycloalkyl halides react with benzenetellurolate to yield (cyclo)alkyl phenyl telluriums, which are converted to (cyclo)alkyl phenyl tellurium dibromides upon treatment with bromine. Stirring of a mixture of these tellurium dibroraides and 0.5 M aqueous sodium hydroxide at 20° resulted in the formation of (cyclo)alkenes . Alkyl phenyl tellurium oxides were postulated as intermediates that eliminated phenyl tellurium hydroxide. 

In another investigation, Alder and Stein found that cyclopentene forms aphenyl azide adduct but is considerably less reactive than the bridged compounds of type (6) cyclohexene formed no adduct. Ziegler later found the reaction with phenyl azide in ether very useful for the characterization of cis- and tranj-cycloalkenes of medium-size rings. Thus cis- and tranj-cyclooctene form crystalline adducts melting at 87 and 111, respectively. Approximate relative reaction rates of frans-cyclo-alkenes with phenyl azide are shown under the formulas ... 

Bicyclo[n. 1.0]alkanes 22 with an endo leaving group X give allylic derivatives 24 of cw-cyclo-alkenes via c ,cw-allyl cations 23, regardless of the size of n and the experimental method [thermolysis, solvolysis, silver(I) assistance, deamination] used. In a secondary reaction HY is eliminated and c ,cw-1,3-cycloalkadienes are obtained. 

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