Geometrical isomers alkenes, geometry

The loss of stereochemical memory in the non-vertical excited state implies that stereospecific, concerted reactions of an alkene singlet state may take place from the vertical state. Of particular importance is that the change from cis or trans geometry to something in between opens up a route for converting one geometrical isomer of an alkene to another, and this is a photoisomerization reaction that will be described in the next section. 

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. 

It is often more convenient, as in the Heck reaction, to use a stable and soluble Pd(II) derivative such as bis(triphenylphosphine)palladium(II) chloride instead of Pd(0). This is rapidly reduced in situ to give a coordinatively unsaturated, catalytically active, palladium(O) species. The geometry of the alkene is generally preserved so that cis (Z) and trans (E) dichloroethylene give the two different geometrical isomers of the enyne below in >99% stereochemical purity as well as excellent yield. 

We can use sulfenyl-lactonisation to illustrate the stereospecificity of the process when the alkene differs in geometry. Rokach28 has shown that the geometrical isomers of the unsaturated acid E-and Z-182 give the correct diastereoisomers of the lactones from anti addition of Cl and R S. 

A pair of geometric isomers about the alkene can result from the condensation process. Eschenmoser demonstrated that sulfide contraction on the secondary thiolactam produced exclusively the (Z)-isomer (equation 3). Other studies have indicated sole formation of the ( )-isomer using tertiary thiolactams and bromoacetates or a mixture of isomers when a-substituted electrophiles were employed (vide infra). However, most synthetic applications of the Eschenmoser reaction have not stringently identified the geometry about the resulting alkene since the double bond is later reduced or equilibrated in the final product. A systematic investigation to identify factors that may influence the stereochemical outcome has not been reported. 

There are many examples of this type of reaction with both aromatic and heteroaromatic substrates. For successful electrocyclization, the central alkene must have Z geometry, however, as the action of light on stilbenes promotes E-Z isomerization, it is possible to start with either geometrical isomer of the substrate, or indeed a mixture of isomers. In a synthesis of cervinomycin A, photochemical electrocyclization of the mixture of E- and Z-diaryl alkenes 347 gave the polycyclic aromatic compound 348 after oxidation with iodine (3.219). ... 

SECTION 24.3 The names of alkenes and alkynes are based on the longest continuous chain of carbon atoms that contains the multiple bond, and the location of the multiple bond is specified by a numerical prefix. Alkenes exhibit not only structural isomerism but geometric (cis-tmiis) isomerism as well. In geometric isomers the bonds are the same, but the molecules have different geometries. Geometric isomerism is possible in alkenes because rotation about the C=C double bond is restricted. 

In addition to geometry, alkenes also differ from open-chain alkanes in that the double bonds prevent the relatively free rotation that is characteristic of carbon atoms bonded by single bonds. As a result, alkenes can exhibit geometric isomerism, the same type of stereoisomerism seen earlier for the cycloalkanes (Section 1.9). There are two geometric isomers of 2-butene ... 

As stated earlier, geometric isomers of alkenes are broadly classified as diastereomeric, since they are stereoisomers that are not enantiomers. The usual specification of geometry as cis or trans is deeply entrenched, but suffers from the same problems of ambiguity as does the Fischer convention, in that it is based on... 

There are two types of geometries with a coordination mrmber of 4. The [Zn(NH3)4] and [CoCU] ions have tetrahedral geometry, whereas the [Pt(NH3)4] ion has the square planar geometry. In Chapter 11 we discussed geometric isomers of alkenes (see p. 365). Square planar complex ions with two different monodentate ligands can also... 

Geometric isomers of alkenes are diastereomeric, since they are stereoisomers but not enantiomeric. The specification of the geometry of double bonds as cis and trans suffers from the same ambiguity as specifying configuration by the Fischer convention that is, it requires a subjective judgment about the similarity of groups. The sequence rule is the... 

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