Cis- and trans-alkenes

Gas-phase thermolysis of (69) and (70) at 600 C/12 Torr occurs to give the corresponding cis and trans alkenes respectively, suggesting a concerted [<, +<,2a] cycloreversion process (70JA1763). 

The formation of alkenes from thiirane dioxides may not be stereospecifically controlled in the presence of a sufficiently strong base and sufficiently acidic protons in the three-membered ring. Under such conditions (essentially those typical for the Ramberg Backlund reaction), epimerization via a carbanion intermediate produces an equilibrium mixture of thiirane dioxides19,99 and consequently a mixture of cis- and trans-alkenes. 

Other examples of syn elimination have been found in medium-ring compounds, where both cis and trans alkenes are possible (p. 158). As an illustration, we can look at experiments performed by Zavada et al. These workers subjected 1,1,4,4-tetra-methyl-7-cyclodecyltrimethylammonium chloride (9) to elimination and... 

If the Criegee mechanism operated as shown above, the cis/trans ratio for each of the two cross ozonides would have to be identical for the cis and trans alkenes, since in this mechanism they are completely cleaved. 

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). 

The submitters report that they currently use two 500-g columns run consecutively. After the first column is used, it is washed with methanol, all traces of solvent flushed through with compressed air, reequilibrated with eluant, then those fractions (from the first column) that contain a mixture of cis- and trans-alkenes are rechromatographed. 

SCHEME 78. Solvent-free oxidation of cyclic and acyclic cis- and trans-alkenes using (NHqlsPMonOqo on fluorapatite... 

The reaction of singlet oxygen with cis and trans alkenes shows an unexpected regioselectivity for hydrogen abstraction on the large alkyl group of the double bond . 

Hydrazoic acid is a reagent in the well-known Schmidt reactions in solution, but imidogen is probably not involved in these transformations. Systematic mechanistic studies of the photochemistry of HN3 in solution and the chemistry of NH with hydrocarbons, particularly cis- and trans- alkenes as per the detailed studies of CH2, have not been performed. 

The reaction is not concerted, as evidenced by reactions of cis and trans alkenes giving the same product mixture. 

Monofluoroalkanes and vicinal difluoroalkanes are dehydrofluonnated if strong enough bases are applied [10 12] In 5-fluorononane and fluorocyclodo-decane, elimination by means of sodium methoxide in methanol gives cis- and trans alkenes in respective yields of 8 and 21% and in ratios of 1 2 2 2 4, however, the bulky lithium diisopropyl amide in tetrahydrofuran produces franr-isomers almost exclusively The strength of the base does not have much effect on the rate of elimination, but the lithium cation causes considerable acceleration [10] (equation 10)... 

In the presence of excess bipyridyl, ruthenium trichloride is an effective catalyst for the selective epoxidation of alkenes by sodium periodate (equation 297). The epoxidation is syn and stereospecific for cis and trans alkenes.637... 

The addition to cis- and trans-alkenes under these conditions is stereospecific with retention of the alkene geometry, implying that the intermediate alkylidenecarbene has a singlet electronic state [47]. 

Both cis and trans alkenes can be formed from this compound by anti elimination. Draw a Newman projection of the conformation required to form each of these products and. on the basis of these projections, predict which of these products would be formed in larger amounts. 

The m-dioxolane (94) on heating to 160 °C gives a trans-oxirane as shown in equation (13). The reaction proceeds in high yield for (94) while the corresponding trans derivative is stable under the same conditions. Both the cis and the trans compounds are converted stereospecifically into the cis and trans alkenes, respectively, on treatment with hot acetic anhydride (70TL5223). The reaction for the trans compound is shown in equation (14), and the mechanism for the transformation is discussed in Section 4.30.2.2.5. 

Give a mechanism for the elimination reaction in the formation of tamoxifen from p. 489 and comment on the fact that it gives a mixture of cis and trans alkenes. 

The asymmetric epoxidation of homoallylic alcohols has continued to be a problematic area. A potential solution has recently been published <07JA286 07T6075>. The use of bis-hydroxamic acid 1 as a chiral ligand for a vanadium catalyst has provided both excellent yields and enantioselectivity. This method works well with both cis- and trans-alkenes. 

The cycloaddition of cyclopentyne is stereospecifically cis to both cis- and trans-alkenes.12 Only [2 + 2] cycloaddition is observed in the reaction of cyclopentyne with 1,3-butadiene (equation III). 

Tetrahedrane (11) is the ruthenium analog of the much-studied tricobaltnonacarbonyl clusters Co3(CO)9CR see Cobalt Organometallic Chemistty). The substitution chemistry of (11) has been studied. A starting material is prepared from (11) by reaction with BX3 (equation 2), which gives the chloro and bromo compounds (12). In addition, (11) can also be treated directly with compounds such as diynes to yield interesting substitution products. For example, when (11) is refluxed in THF with diphenylbutadiyne, cis- and trans-alkene isomers of two alkyne insertion regioisomers are formed (equation 3). The product seems to arise from dehydrogenation of one end of the diyne to yield cis and trans enynes and an imsaturated monohydride cluster intermediate, which then reacts with the enynes to yield the allylic derivative products... 

Unfortunately, the Zweifel s hydroboration procedure suffers from the same dis-adventage as his synthesis of cis and trans alkenes. It requires R2BH as a reagent. Application of the new synthesis of RjBH via hydridation of R2BX made the procedure general for essentially all R groups available through hydroboration (Eq. 76)... 

Cis and trans alkenes often have somewhat different physical properties. For example, ci5-2-butene has a higher boiling point (4 °C) than trans-2-butene (1 °C). This difference arises because the C—C single bond between an alkyl group and one of the double-bond carbons of an alkene is slightly polar. The sp hybridized alkyl carbon donates electron density to the sp hybridized alkenyl carbon. 

Because halogenation occurs exclusively in an anti fashion, cis and trans alkenes yield different stereoisomers. Halogenation of alkenes is a stereospecific reaction. 

---