Geometric Isomerism and Chirality

Review Chapter 5. The inability of atoms in rings to rotate completely about their r bonds leads to cis-trans (geometric) isomers in cycloalkanes. 

In such diagrams, either (i) the flat ring is perpendicular to the plane of the paper, with the bond(s) facing the viewer drawn heavy and with the substituents in the plane of the paper and projecting up and down, or (ii) the flat ring is in the plane of the paper, with wedges projecting toward the viewer and dots away from the viewer. 

Since the ring C s are sp -hybridized, they may be chiral centers. Therefore, substituted cycloalkanes may be geometric isomers as well as being enantiomers or meso compounds. 

Problem 9.3 Indicate the geometric isomers, if any, for (n) 1,1,2-trimethylcyclopropane, (ft) 1,3-dimethylcy-clobutane, (c) 1,2,3-trimethylcyclohexane, (d) 1,2,4-trimethylcyclopentane, (e) I-methyl-2-propenylcyclopen-tane, (/) 2-chlorobicyclo(2.2.1 (heptane (common name is 2-chloronorbornane). 

To designate cis.trans, start at C and go around the ring increasing the numbers. 

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Another interesting example that combines both geometric isomerism and chirality consists of complexes of the type 27... 

Property values can also be modified to take into account geometric isomerism and chirality. 

Compounds containing more than one chiral centre or which are subject to geometric isomerism and, therefore, have more than just two mirror image forms. 

Systematic studies of organocuprate conjugate additions to three pairs of y-epimeric and geometrically isomeric y-chiral acyclic enones and enoates (159a,b) and (160a,b) have allowed one to generalize diastereofacial selectivity of these reactions (Scheme 20). 

Adjacent polyenes are hydrocarbons with double bonds between each pair of atoms in the polyene system they are called cumulenes2. Two types of cumulenes exist those with an even number of adjacent double bonds and those with an odd number. The former can exhibit chirality, the latter geometric isomerism. Allenes (propadienes) are the simplest members of the even-numbered type of cumulenes. 

We should compare this system with a 1,4-disubstituted cyclohexane such as 4-methylcyclo-hexanecarboxylic acid (see Section 3.4.4). There is a plane of symmetry in this molecule, so there are no chiral centres but geometric isomers exist, allowing cis and trans stereoisomers. The restrictions imposed by bridging have now destroyed any possibility of geometric isomerism. 

To examine the viability of CIM a number of photoreactions (electrocyclic reactions, Zimmerman (di-n) reaction, oxa-di-7i-methane rearrangement, Yang cyclization, geometric isomerization of 1,2-diphenyl-cyclopropane derivatives, and Schenk-ene reaction) which yield racemic products even in presence of chiral inductors in solution have been explored (Sch. 40) [187,189-200]. Highly encouraging enantiomeric excesses (ee) on two photoreactions within NaY have been obtained photocyclization of tropolone ethylphenyl ether (Eq. (1), Sch. 40) and Yang cyclization of phenyl benzonorbornyl ketone (Eq. (3), Sch. 40). The ability of zeolites to drive a photoreaction that gives racemic products in solution to ee >60% provides... 

Hammond and Cole reported the first asymmetric photosensitized geometri-r cal isomerization with 1,2-diphenylcyclopropane (Scheme 2) [29]. The irradiation of racemic trans-1,2-diphenylcylcopropane 2 in the presence of the chiral sensitizer (R)-N-acetyl-1 -naphthylethylamine 4 led to the induction of optical activity in the irradiated solution, along with the simultaneous formation of the cis isomer 3. The enantiomeric excess of the trans-cyclopropane was about 1% in this reaction. Since then, several reports have appeared on this enantiodifferentiating photosensitization using several optically active aromatic ketones as shown in Scheme 2 [30-36]. The enantiomeric excesses obtained in all these reactions have been low. Another example of a photosensitized geometrical isomerization is the Z-E photoisomerization of cyclooctene 5, sensitized by optically active (poly)alkyl-benzene(poly)carboxylates (Scheme 3) [37-52]. Further examples and more detailed discussion are to be found in Chap. 4. 

The final photoreaction that we discuss in the context of chiral auxiliary effect in photorearrangements is the geometric isomerization of 2,3-diphenylcy-clopropane-1-carboxylic acid derivatives 40 and 2,3-diphenyl-1-benzoylcyclo-propane derivatives 41 [279,282,287,290,296-300]. As shown in Schemes 25 and 26, the isomerization in the former occurs through the cleavage of the C2-C3 bond, while that in latter occurs through the cleavage of the Ci-C2 and Ci C3 bonds. We have examined twenty-two 2,3-diphenylcyclopropane-l-carboxylic acid derivatives and fourteen 2,3-diphenyl-1-benzoylcyclopropane derivatives appended with a variety of chiral auxiliaries. The results in solution and zeolites are presented in Schemes 27 and 28. 

Geometrical isomers are not necessarily optical isomers. For instance, the trans isomer of [Co(en)2Cl2]+ shown in Fig. 20.17 is identical to its mirror image. Since this isomer is superimposable on its mirror image, it does not exhibit optical isomerism and is therefore not chiral. On the other hand, cis-[Co(en)2Cl2]+ is not superimposable on its mirror image thus a pair of enantiomers exists for the complex ion, making the cis isomer chiral. 

Geometric isomerism was first defined by Wislicenus in 1887 as isomerism occurring in compounds where rotation is restricted by double bonds or ring systems. Geometric isomers do not rotate the plane of polarized light (unless they also contain a chiral center), and hence are not optically active. 

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