Alkene stereoisomers

Cyclodextrins (CDs) have recently found use as stationary phases in gas-solid chromatography (GSC) [1-8, 12-14] and in gas-liquid chromatography (GLC) [8-11], because of their selective separation capability. Their application to separations of stereoisomers (alkenes, pinenes) and positional isomers of aromatics (xylenes, trimethylbenzenes) has been found to be very advantageous. The inclusion process, which underlies selective separations, is, with cyclodextrins, also affected by the presence of water. It is well known that cyclodextrins form crystal hydrates and that the water of crystallization participates in the formation of inclusion complexes [15]. On the formation of an inclusion complex, the water molecules included in the CD cavity are liberated preferentially. This liberation is further enhanced under the dynamic conditions of gas chromatography. It can thus be assumed that water also plays an important role in the equilibrium processes between CD and a guest (sor-bate) in the gaseous state. 

The cyclization to form very congested quaternary carbon centers involving the intramolecular insertion of di-, tri-, and tetrasubstituted alkenes is particularly useful for natural products synthesis[l36-138], In the total synthesis of gelsemine, the cyclization of 166 has been carried out, in which very severe steric hindrance is e.xpected. Interestingly, one stereoisomer 167... 

The Pd-catalyzed hydrogenolysis of vinyloxiranes with formate affords homoallyl alcohols, rather than allylic alcohols regioselectively. The reaction is stereospecific and proceeds by inversion of the stereochemistry of the C—O bond[394,395]. The stereochemistry of the products is controlled by the geometry of the alkene group in vinyloxiranes. The stereoselective formation of stereoisomers of the syn hydroxy group in 630 and the ami in 632 from the ( )-epoxide 629 and the (Z)-epoxide 631 respectively is an example. 

Cis-trans stereoisomerism m alkenes is not possible when one of the doubly bonded carbons bears two identical substituents Thus neither 1 butene nor 2 methyl propene can have stereoisomers... 

Write structural formulas or build molecular models and give the lUPAC names for all the alkenes of molecular formula C6H12 that contain a trisub stituted double bond (Dont forget to include stereoisomers)... 

The difference m stability between stereoisomeric alkenes is even more pronounced with larger alkyl groups on the double bond A particularly striking example compares as and trans 22 5 5 tetramethyl 3 hexene m which the heat of combustion of the cis stereoisomer is 44 kJ/mol (10 5 kcal/mol) higher than that of the trans The cis isomer IS destabilized by the large van der Waals strain between the bulky tert butyl groups on the same side of the double bond... 

In addition to being regioselective alcohol dehydrations are stereoselective A stereo selective reaction is one m which a single starting material can yield two or more stereoisomeric products but gives one of them m greater amounts than any other Alcohol dehydrations tend to produce the more stable stereoisomer of an alkene Dehydration of 3 pentanol for example yields a mixture of trans 2 pentene and cis 2 pentene m which the more stable trans stereoisomer predominates... 

In addition to being regioselective dehydrohalogenation of alkyl halides is stereo selective and favors formation of the more stable stereoisomer Usually as m the case of 5 bromononane the trans (or E) alkene is formed m greater amounts than its cis (or Z) stereoisomer... 

Although both stereoisomers yield 4 tert butylcyclohexene as the only alkene they do so at quite different rates The cis isomer reacts over 500 times faster than the trans The difference in reaction rate results from different degrees of rr bond develop ment in the E2 transition state Since rr overlap of p orbitals requires their axes to be parallel rr bond formation is best achieved when the four atoms of the H—C—C—X unit he in the same plane at the transition state The two conformations that permit this are termed syn coplanar and anti coplanar... 

Isomeric alkenes may be either constitutional isomers or stereoisomers There is a sizable barrier to rotation about a carbon-carbon double bond which corresponds to the energy required to break the rr component of the double bond Stereoisomeric alkenes are configurationally stable under normal conditions The configurations of stereoisomeric alkenes are described according to two notational systems One system adds the prefix CIS to the name of the alkene when similar substituents are on the same side of the double bond and the prefix trans when they are on opposite sides The other ranks substituents according to a system of rules based on atomic number The prefix Z is used for alkenes that have higher ranked substituents on the same side of the double bond the prefix E is used when higher ranked substituents are on opposite sides... 

Stereochemistry refers to chemistry in three dimensions Its foundations were laid by Jacobus van t Hoff and Joseph Achille Le Bel m 1874 Van t Hoff and Le Bel mde pendently proposed that the four bonds to carbon were directed toward the corners of a tetrahedron One consequence of a tetrahedral arrangement of bonds to carbon is that two compounds may be different because the arrangement of their atoms m space IS different Isomers that have the same constitution but differ m the spatial arrangement of their atoms are called stereoisomers We have already had considerable experience with certain types of stereoisomers—those involving cis and trans substitution patterns m alkenes and m cycloalkanes... 

Just as It makes sense to have a nomenclature system by which we can specify the con stitution of a molecule m words rather than pictures so too is it helpful to have one that lets us describe stereochemistry We have already had some experience with this idea when we distinguished between E and Z stereoisomers of alkenes... 

Epoxidation of alkenes is a stereospecific syn addition Which stereoisomer of 2 butene reacts with peroxyacetic acid to give meso 2 3 epoxybu tane Which one gives a racemic mixture of (2/ 3/ ) and (25 35) 2 3 epoxybutane ... 

Long-chain primary alcohols, eg, triacontanol, can be prepared by the hydroboration, isomerization, and oxidation of the corresponding internal alkenes (437). The less thermodynamically stable stereoisomer can be transformed into the more stable one by heating, eg, i j -into /ra/ j -myrtanjiborane (204). 

How many alkenes have the molecular formula C5H10 Write their structures and give their lUPAC names. Specify the configuration of stereoisomers as cis or trans as appropriate. 

Although both stereoisomers yield 4-terr-butylcyclohexene as the only alkene, they do so at quite different rates. The cis isomer reacts over 500 times faster than the trans. 

Hydrogenation of each of the following chiral alkenes gives only a single stereoisomer. 

Draw the other stereoisomer that might have been obtained from syn addition of hydrogen to each alkene. Is the observed product for each addition also the thermodynamic product Compare energies for alkene A+ H2 observed and not observed and alkene B+H2 observed and not observed. What structural factors seem to be responsible for the relative stabilities of the two products of each hydrogenation reaction ... 

The TiX2-TADD0Late-catalyzed 1,3-dipolar q cloaddition reactions were extended to include an acrylate derivative [66]. In the absence of a catalyst, the reaction between nitrones 1 and acryloyl oxazolidinone 19b proceeded to give a mixture all eight regio-and stereoisomers (Scheme 6.23). However, application of in this case only 10 mol% of Ti(OTs)2-TADDOLate 23d as catalyst for the reaction of various nitrones 1 with alkene 19b, led to complete regioselectivity and high endo selectivity in the reaction and the endo products 21 were obtained with 48-70% ee (Scheme 6.23) [66]. 

Redncdons of y-nitroketones yield cyclic nitrones, which undergo inter- and intramolecular cycloadthdon to various alkenes. The result of adthdon to acrylonitrile is shown in Eq. 8.43, in which a rruxnire of regio- and stereoisomers is formed. - ... 

If dichlorocarbene is generated in the presence of an alkene, addition to the double bond occurs and a dichlorocyclopropane is formed. As the reaction of dichlorocarbene with ds-2-pentene demonstrates, the addition is stereospecific, meaning that only a single stereoisomer is formed as product. Starting from a cis alkene, for instance, only cis-disubstituted cyclopropane is produced starting from a trans alkene, only trans-disubstituted cyclopropane is produced. 

R Stereoisomers (Section 4.2) are compounds whose atoms are connected in the same order but with a different geometry. Among the kinds of stereoisomers we ve seen are enantiomers, diastereomers, and cis-trans isomers (both in alkenes and in cycloalkanes). Actually, cis-trans isomers are just another kind of diastereomers because they are non-mirror-image stereoisomers. 

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