Z nomenclature

Protonation of the anion [SN2] by acetic acid in diethyl ether produces the thermally unstable sulfur diimide S(NH)2. Like all sulfur diimides, the parent compound S(NH)2 can exist as three isomers (Scheme 5.5). Ab initio molecular orbital calculations indicate that the (cis,cis) configuration is somewhat more stable than the (cis,trans) isomer, while the (trans,trans) isomer is expected to possess considerably higher energy. The alternative syn,anti or E,Z nomenclatures may also be used to describe these isomers. The structures of organic derivatives S(NR)2 (R = alkyl, aryl) are discussed in Section 10.4.2. 

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. 

The three isomers are cis/trans isomers with respect to the substituents -phenyl and -0-0 at C-4, and -OC2H5, = 0 at the P atom. For convenience, we use the E/Z nomenclature recommended for ethylene diastereoisomer assignment. If we replace the plane in ethylenes through the sp and sp atoms by a plane vertical to the phosphacyclohexadiene (2,4) ring (thought of in first approximation as planar) through C—4 and the phosphorus atom, we have to denote the isomer with -0—0 at C—4 and = O at P on the same side of this plane as Z , and that with -0-0 at C—4 and -OC2H5 at P on different sides as E diasteroisomer. 

Deprotonation of a-alkylated acetic acid esters (e.g., the propionic acid ester of Figure 10.13) with LDA at —78°C selectively yields the -enolatcs.Thc quotation marks indicate that this application of the term is based on an extension of the E/Z-nomenclature here, the Cahn-Ingold-Prelog priority of the O I, i1 substituent is considered to be higher than the priority of the OR group. The deprotonation of the ester shown in Figure 10.13 occurs via the strain-free transition state A. The alternative transition state B is destabilized by a 1,3-diaxial interaction. 

The following example shows the use of the E-Z nomenclature with cyclic stereoisomers that are not clearly cis or trans. 

Determine which compounds show cis-trans isomerism. Draw and label the isomers, using both the cis-trans and E-Z nomenclatures where applicable. 

Beware The terms cis and frans do not always translste directly into Z and E, Consider the preparation of an enamine from cyclohexanone, which forms a double bond that you d probably call cis (it s in a ring), But applying the rigorous rules laid down for E/Z nomenclature (p. 487), It is E, As for the useful terms syn and anti (Chapter 16), there are no rigid rules for deciding whether a double bond is cis or trans,... 

Throughout this chapter, ( )- and (Z)-nomenclature refers to oxime geomeOy, and has been indicated whenever specified in the original work. 

Using E,Z nomenclature, a cis isomer has the Z configuration and a trans isomer has the E configuration. 

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. 

The E/Z nomenclature was introduced in Chapter 7, and now that you have read Chapter 16 we can be more precise with our definition. For disubstituted alkenes, Ecorrespondsto trans and Zcorresponds to cis. To assign Eor Zto tri- or tetrasubstituted alkenes, the groups at either end of the alkene are given an order of priority according to the same rules as those outlined for R and S in Chapter 16. Ifthe two higher priority groups are cis, the alkene is Z if they are trans the alkene is E. Of course, molecules don t know these rules, and sometimes (as in the second example here) the Ealkene is less stable than the Z. 

Throughout this chapter, the /Z nomenclature wUl be employed to denote geometric isomerism about the double bonds of the polyene chain, while the cisitrans ( ( It ) nomenclature will be used only to denote conformational isomerism about the formal single bonds. 

This problem is intended to show the difficulty of using cis-trans nomenclature with any but the simplest alkenes. Cis and trans are ambiguous the first alkene in part (a) is cis if the two similar substituents are considered, but trans if the chain is considered. The E-Z nomenclature is unambiguous and is preferred for all four of these isomers. 

One special application comes when silyl enol ethers of allylic esters provide the starting material this is known as the Claisen-Ireland reaction. Esters normally form E-enolates 181 with LDA at low temperature. Because the E/Z nomenclature depends on the hierarchy of the substituents it is particularly ridiculous when applied to enol derivatives of esters. A lithium enolate (Li < C) would have the opposite stereochemical label from a silyl enol ether (Si > C) so a uniform scheme is adopted whereby the metal - O bond always has priority over the other. The mechanism 180 may remind you of the reasons for this - they are discussed in more detail in chapter 4. 

The first proposed nomenclature suggested that isomers should be called cis when W and Y are on the same side of the double bond and trans when they are on the opposite side (Fig. 1.2), provided W X and Y Z. However, this nomenclature was limited to the particular case where W and Y are identical. The more recent nomenclature of Cahn-Ingold-Prelog, based on the German Zusammen (Z) and Entgegen (E) notation, was extended to systems where W and Y are different substituents (Fig. 1.2). There is no direct relation between the two nomenclatures since they depend on the nature of substituents and so the Z isomer is not necessarily cis. Moreover, the order of priority is determined by the atomic number of each atom connected to the C=C double bond [1]. Although the E/Z nomenclature may also be applied to compounds B/B and C/C, these are considered as conformational isomers, whereas compounds A/A are configurational isomers. 

Z/E isomerism is not limited to true double bonds and may be used when sp2 electrons of a heteroatom are conjugated with a 7r-system to form a planar pseudo double bond. In particular, in the case of amides, the cis isomer is called E. Although the general tendency now is to use the E/Z nomenclature in chemistry, despite their inaccuracy cis and trans are still utilized by biochemists because they give a more readily understandable description of molecular shape, in particular for amides in peptides and proteins. When the chains are connected through a motif containing more than three dihedral angles (i.e. carbamates), the syn-anti... 

There are no rigid rules for deciding whether a variety of substituted double bonds are cis- or tram-, and hence the more systematic E/Z nomenclature is generally preferred (see Section 3.3.1.2). 

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. 

---