Double bonds geometric isomers

In this chapter we shall talk about reactions similar to the ones on the previous page and we shall be interested in how to control the geometry of double bonds. Geometrical isomers of alkenes are different compounds with different physical, chemical, and biological properties. They are often hard to separate by chromatography or distillation, so it is important that chemists have methods for making them as single isomers. 

Iodine will add reversibly not only to Michael acceptors but also to most other alkenes. It can therefore be a useful reagent for equilibrating double bond geometrical isomers. 

Simple allylic alcohols and vinyl ethers can be coupled to give 2-alkoxytetrahydrofurans employing a Pd(ii)/Cu(ii) catalyst system under an oxygen atomosphere (Equation 101) <2006SL3110>. The reaction is stereospecific in that the (Z)-cinnamyl alcohol leads to the double-bond geometrical isomer of the product depicted in Equation (101) in 82% yield. 

Because these substances have two heterocyclic nuclei joined by a double bond, geometrical isomers should exist owing to prohibited... 

In all the reported convergent syntheses of LTA4, including the racemic form, the natural and three possible unnatural chiral forms, and various double-bond geometrical isomers, the epoxy aldehydo ester 56 has been found to be a key intermediate. Consequently a number of ingenious syntheses of the various stereo... 

Less successful, however, is the use of monosubstituted methylenecyclopropanes as substrates for the nickel(0)-mediated addition to the furanone substrate, leading to considerable side reactions. The corresponding palladium(0)-catalyzed process is very much more successful, providing the type A product as a mixture of double-bond geometrical isomers. 

When there is a substituent on the allyl double bond, geometric isomers are possible and two new stereocenters are formed. The transition structures in Scheme 5.3 illustrate how the -crotyl boron compound affords racemic anti addition product and the Z-crotyl compound affords the syn product. For the E isomer, the... 

Because two methyl groups are attached to the second carbon atom of the double bond, geometric isomers are not possible, b. Geometric isomers are possible. 

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