Z and E nomenclature

For di-, tri- and tetra-substituted alkenes (with two, three and four substituents, respectively, on the double bond), alkenes can be named using the E,Z nomenclature. The groups on the double bond are assigned priorities based on a series of sequence rules. 

Assume carbon is bonded to two oxygens Assume carbon is bonded to three nitrogens Assume oxygen is bonded to two carbons Assume nitrogen is bonded to three carbons 

Examples (numbers on each carbon are given in italic to indicate priorities) 

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In this context, the E- and Z-nomenclature of ester enolates and silyl ketene acetals refers to their geometries where the carbonyl oxygen given highest priority irrespective of priority by CIP Cahn-Ingold-Prelog) rules. 

Another notation for conformers has been developed to describe the geometry about the single bond of 1,3-butadiene (4). We will see in Chapter 4 that theory predicts some double bond character for the C2—C3 bond, so there are two conformational energy minima separated by an energy maximum associated with rotation about the C2—C3 bond. We describe 4a as s-frans-l,3-butadiene, since the two double bond units are trans to each other across the formally single bond. Similarly, 4b is the s-cis conformer. With the advent of the (E) and (Z) nomenclature system, the notations s-(Z) and s-(E) have begun to replace s-cis and s-trans. 

It is useful to combine the (E) and (Z) nomenclature system with the s-cis, s-trans system when designating the stereochemistry and conformation of polyenes. In such cases s-cfs may be abbreviated as c and s-trans as t. For example, the photochemical reaction of 1,2-dihydronaphtheilene (5) first forms the cZc conformer 6, which then rotates about the single bond to form the cZt conformer 7 (equation 3.1). ... 

The sfereochemisfry of the more highly substituted alkenes is difficult to define using the cis and trans designations. Therefore, a more systematic manner of indicating stereochemistry in these systems has been developed that uses the E and Z nomenclature system. Draw the structures of the E and Z stereoisomers of l,4-diphenyl-2-butene-l,4-dione used in this experiment. In this case, which is cis and which is trans ... 

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 following nomenclature rules apply to describing the transition state structures. The terms exo/endo refer to orientation of the chain starting from the enophile and proceeding to the ene. Endo means that the chain points in the direction of the ene, whereas in an exo form it points away from the ene. E and Z describe the geometry of the double bond of the ene. Syn and anti characterize the positions of the vinylic hydrogen atoms relative to each other. 

The stereochemistry of the cycloadducts in intramolecular Diels-Alder reactions depends upon the different geometry of the possible transition states 37—40 whose nomenclature can be explained as follows The orientation with the chain connecting the diene and dienophile lying under or above the diene is called endo. The opposite means exo. E and Z mark the geometry of the diene double bond which is connected with the chain. Syn and anti describe the arrangement of the hydrogen atoms (or substituents) at the prestereogenic centers which are involved in the C-C bond formation.12... 

The compound has two chirality centres and three pseudo chirality centres. There is however, only one (achiral) diastereomer of the compound shown in the question. The two isomers can be distinguished from one another solely on the relative position of the chlorine or bromine atoms which lie in a plane which also happens to be the plane of symmetry of the molecule (this is the only symmetry element present, therefore the symmetry point group is Cs). It is possible in this instance to specify the configuration unequivocally using the descriptors E and Z. However, in systematic nomenclature the complete configuration of all the stereogenic centres is specified. Thus the (so-called) Z isomer is (ls,3r,5 ,6r,7S)-l,6-dibromo-3,6-dichloroadamantane and the isomer is (ls,3r,5 ,6s,7S)-l,6-dibromo-3,6-dichloroadamantane, i.e. the two isomers can be distinguished simply by the descriptor used for position 6. 

In order to be able to define the absolute stereochemistry of a molecule, we therefore use another system of nomenclature which was proposed by Cahn, Ingold and Prelog in 1956. This system uses the prefixes R- and S-, derived from the Latin words rectus (right) and sinister (left) and the same system of priority setting that was discussed above in relation to E- and Z-isomers. We start by assigning the priorities of the four groups... 

The E or Z nomenclature can also replace the older S)m-anti terminology. Compound 51 has been called sj/n-propiophenone oxime because the OH and the phenyl are on the same side of the double bond, while 52 has been called the anti isomer because these two larger groups are on opposite sides. The distinction between syn and anti depends on the size or complexity of the substituents, and that may not always be unambiguous. Moreover, many authors reserve the terms syn and anti to describe the stereochemical pathway of a reaction, not the stereochemistry of molecules. With the E or Z system, 51 is unambiguously the Z isomer, while 52 is the E oxime. 

The reaction of the unsymmetrical alkyne 2-hexyne with HCl poses a regiochemical problem. Two carbocations are possible—99 and 100—and both are secondary vinyl carbocations and are expected to be of essentially equal stability. It is anticipated that both will be formed in roughly equal amounts, and subsequent reaction with bromide ion will give two products 101 (3-chlo-ro-2-hexene) and 102 (2-chloro-2-hexene). Experimentally, the reaction of HCl with 2-hexyne generates 101 and 102 in roughly a 50 50 ratio. However, the reaction of the alkyne unit with HCl will generate carbocations 99 and 100 as a mixture ofE- and Z-isomers in both cases. (Nomenclature for E- and Z-alkenes was described in Chapter 9, Section 9.4.3.)... 

The stereochemical prefixes E and Z should be used particularly where cis or trans may be ambiguous. For example, compound (29) may certainly be named as an all-trans compound on the basis of the steric arrangement of the polyene side chain. The E,Z nomenclature is of course less ambiguous in such cases. 

Aside this confusion, there is a principal argument, not to use Evans convention, because the hard descriptors E andZ must not be redefined. The soft descriptors cis and trans, however, can be used without violation of the strict definitions of the unequivocal E and Z. Therefore, in this book, the recommendation of Eliel etal. [18] is followed using the soft descriptors cis and raKS, if a series or a class of enolates are addressed [19]. Thereby, cis means that the OM substituent is on the same side as the higher-priority group at the a-carbon atom, and trans means that the OM substituent is on the opposite side. Only in those cases, where an individual enolate is concerned, EtZ nomenclature is used according to its strict definition. 

The E/Z nomenclature is here extended to apply not only to the C=C double bond but also to the bond linking the C=C and C=Q groups, which has some double bond character. 

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