Geometric isomers Compounds with alkenes

Geometric isomers Compounds with different arrangements of groups on the opposite sides of a bond with restricted rotation, such as a double bond for example, cis-trans isomers of certain alkenes. 

As we saw earlier, there are three structural isomers of the alkene C4H8. You may be surprised to learn that there are actually/owr different alkenes with this molecular formula. The extra compound arises because of a phenomenon called geometric isomerism. There are two different geometric isomers of the structure shown on the left, on page 597, under (1). 

Cyclopropane formation occurs from reactions between diazo compounds and alkenes, catalyzed by a wide variety of transition-metal compounds [7-9], that involve the addition of a carbene entity to a C-C double bond. This transformation is stereospecific and generally occurs with electron-rich alkenes, including substituted olefins, dienes, and vinyl ethers, but not a,(J-unsaturated carbonyl compounds or nitriles [23,24], Relative reactivities portray a highly electrophilic intermediate and an early transition state for cyclopropanation reactions [15,25], accounting in part for the relative difficulty in controlling selectivity. For intermolecular reactions, the formation of geometrical isomers, regioisomers from reactions with dienes, and enantiomers must all be taken into account. 

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. 

With this type of shift reagent, the silver ion binds to soft Lewis base donors, such as olefins, aromatics, phosphenes, and halogenated compounds. At the same time, silver remains as an ion pair with Yb(fod)4 in a specific orientation such that a paramagnetic shift is induced in the resonances of the organic soft base. This method has proven to be useful for the separation of NMR resonance peaks of cis and trans geometric isomer mixtures of long-chain alkenes and complex mixtures of aromatic molecules [5]. 

Compounds 1 and 2 can be regarded as non-interconvertible and are called geometric isomers. they also fulfil the definition of diastereoiso-mers, though they are only occasionally so called. The cis and trans nomenclature is unambiguous for disubstituted alkenes however, the situation is not clear-cut for tri- and tetrasubstituted alkenes. With these latter compounds, problems arise in the definition of which substituent is cis or trans to which. 

Alkenation of carbonyl compounds can be used as a general method for the synthesis of enol ethers. The Homer-Wittig reaction of the phosphorus-stabilized carbanion (8) with aldehydes or ketones gives the adducts (9), which on heating eliminate to give the enol ethers (10) as a mixture of ( )- and (Z)-isomers. Since the two diastereomeric adducts (9) can be separated and the elimination reaction is stereospecific, this method can be used to prepare the individual geometrical isomers of (10) according to Scheme 2. 

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). ... 

As these two examples demonstrate, the names of alkanes end with -ene and nnmbers are nsed to indicate the position of the double bond. The parent chain is nnmbered so that the lowest nnmber possible is given to one of the carbon atoms in the donble bond, regardless of any other substitnents present in the compound (for example, alkyl gronps or halides). The numbers in the names of alkenes refer to the lowest numbered carbon atom in the chain that is part of the C=C bond of the alkene. The name butene means that there are four carbon atoms in the longest chain. Because of restricted rotation abont the carbon-carbon double bond, alkenes can form geometric isomers (Section 4.4). In this case, the name of an alkene must also specify whether the isomer is cis or trans ... 

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