Stereoisomers of alkenes

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... 

The complexity of the mixture of regio- and stereoisomers of alkenes which may arise in these dehydrohalogenations may be illustrated by a consideration of the possible elimination routes in the case of 3-bromohexane. 

Name and Identify stereoisomers of alkenes by using the f, /system of nomenclature. 

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. 

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... 

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 ... 

This is the Zaitsev product that we expect. The stereoisomer of this alkene is not prodnced, because the E2 process is stereospecific ... 

Because olefin migration was very rapid under the conditions used, internal alkenes could be used. Thus, treatment of ( )-2-pentene or ( )-3-pentene with Tf2NH (1.1 eq) and (p-ClC6H4)3P in chlorobenzene at 65 °C for 24 h both gave the 1-pentylphosphonium salt in 91% yields (Scheme 21). The use of mixtures of alkene regioisomers and stereoisomers for the reaction may have a synthetic advantage. 

The polymerization of 1,3-dienes is more complicated than that of alkenes because of the greater number of possible stereoisomers (Secs. 8-ld and 8-le). Table 8-8 shows the... 

The more energetic cis double bond in acyclic alkenes is epoxidized faster than the trans double bond.215 In contrast, opposite reactivity is observed for the stereoisomers of cycloalkenes, where the trans isomers are more reactive as a result of higher ring strain. This was demonstrated in the selective monoepoxidation of cis, trans-1,5-cyclodecadiene 235... 

Most, but not all alkenes, also have stereoisomers that are not identical because of different spatial arrangements of the component atoms. Thus there are two stereoisomers of 2-butene that differ in the geometric arrangement of the groups attached to the double bond. In one isomer, both methyl groups are on the same side of the double bond (c/.v-2-butene) and in the other, the methyl groups are on opposite sides of the double bond (rrans-2-butene) ... 

Write structural formulas for two alkenes of molecular formula C7H14 that are stereoisomers of each other and have a trisubstituted double bond. Give systematic names for each. 

The first investigations by Bryce-Smith et al. [46,67,153] on ortho photocycloaddition of an alkene to hexafluorobenzene have revealed yet another secondary reaction of ortho photocycloadducts. Irradiation of a solution of hexafluorobenzene in r/.v-cyclooctene leads to the rapid formation of seven adducts of which six were identified (i) the exo-meta adduct, (ii) a product that can be formed from the meta adduct by a thermal 1,5 H-shift but which apparently is also a primary product, (iii) an ortho adduct of which the configuration could not be established, (iv) a cyclooctatriene derivative formed by thermal ring opening of the ortho adduct, and (v) and (vi) two stereoisomers of 2,3,4,5,6,7-hexaflu-orotetracyclo[6.6.0.02,7.03,6]tetradec-4-ene. The experiment was repeated 9 years later by Sket et al. [151] with the important difference that cyclohexane was used as a diluent. The meta adduct (i) and its formal rearrangement product (ii) were not found. One ortho adduct (iii), the cyclooctatriene (iv), and the two tetracyclic products (v) and (vi) could be identified and their stereochemistry determined. From their results, the authors concluded that a second ortho adduct with the alternative stereochemistry must also have been formed. They also performed experiments in which the influence of the solvent on the course of the reaction was studied and found that the difference between their results and those of Bryce-... 

Diastereoselectivity in the (3 + 2) photocycloaddition of di-( )-menthyl, di-( )-8-phenylmenthyl, and di-( )-bomyl derivatives (398b-d) is largely dependent on the concavity of the auxiliary, steric bulk of the substituents of the alkenes, and reaction temperature [365] (Scheme 107). The highest de value was obtained when 8-phenylmenthyl ester 398c was employed, although the yield of the adducts was relatively low. The bomyl ester 398d showed poor diastereoselectivity. The effect of the chiral auxiliary seems to be reasonably explained in terms of the concavity of the auxiliary. When a-methylstyrene was used as the alkene, two stereoisomers of (3 + 2) adducts were obtained in lower yields than... 

Monocrotalic acid (36) was previously shown to have the (2R, 3R, 4R) absolute configuration.14 All eight stereoisomers of monocrotalic acid have been synthesized by Matsumoto and co-workers.15 ds-Hydroxylation of the ( )-(Z)-alkene... 

The Peterson Reaction allows the preparation of alkenes from a-silylcarbanions. The intermediate p-hydroxy silane may be isolated, and the elimination step - the Peterson Elimination - can be performed later. As the outcome of acid or base-induced elimination is different, the Peterson Olefination offers the possibility of improving the yield of the desired alkene stereoisomer by careful separation of the two diastereomeric p-hydroxy silanes and subsequently performing two different eliminations. 

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