Cis-trans stereoisomerism

Not all double-bonded compounds show cis-trans stereoisomerism. Cis-trans stereoisomerism is found only in alkenes that have two different groups attached to each double-bonded carbon atom. In 2-butene, the two different groups are a methyl and a hydrogen for each double-bonded carbon ... 

Stereoisomerism at double bonds is indicated in SMILES by / and . The characters specify the relative direction of the connected atoms at a double bond and act as a frame. The characters frame the atoms of a double bond in a parallel or an opposite direction. It is therefore only reasonable to use them on both sides Figure 2-78). There are other valid representations of cis/trans isomers, because the characters can be written in different ways. Further details are listed in Section 2,3.3, in the Handbook or in Ref, [22]. 

Cis-trans stereoisomerism m alkenes is not possible when one of the doubly bonded carbons bears two identical substituents Thus neither 1 butene nor 2 methyl propene can have stereoisomers... 

In order to answer these questions, Stang and Dueber (186) investigated the solvolysis of the stereoisomeric cis and trans 1,2-dimethyl-2-phenylvinyl triflates 232a and 232b, Rj = R2 = CH3. In 60% aqueous ethanol buffered with... 

The combination of cis-trans isomerism with iso-syndio and erythro-threo dispositions gives complex stractures as exemplified by the 1,4 polymers of 1-or 4-monosubstituted butadienes, such as 1,3-pentadiene (72, 73), and 2,4-pentadienoic acid (74, 75) and of 1,4-disubstituted butadienes, for example, sorbic acid (76). This last example is described in 32-35 (Scheme 6, rotated Fischer projection). Due to the presence of three elements of stereoisomerism for each monomer unit (two tertiary carbons and the double bond) these polymers have been classed as tritactic. Ignoring optical antipodes, eight stereoregular 1,4 structures are possible, four cis-tactic and four trans-tactic. In each series (cis, trans) we have two diisotactic and two disyndiotactic polymers characterized by the terms erythro and threo in accordance with the preceding explanation. It should be noted that here the erythro-threo relationship refers to adjacent substituents that belong to two successive monomer units. 

The all-trans-all-isotactic and all-trans-all-syndiotactic structures for the 1,4-polymerization of 1,3-pentadiene are shown in Fig. 8-6. In naming polymers with both types of stereoisomerism, that due to cis-trans isomerism is named first unless it is indicated after the prefix poly. Thus, the all-trans-all-isotactic polymer is named as transisotactic l,4-poly(l,3-penta-diene) or isotactic poly( -3-methylbut-l-ene-l,4-diyl). 

Ring formation also confers rigidity on molecular structure such that rotation about the ring bonds is prevented. As a result, stereoisomerism of the cis-trans type is possible. For example, 1,2-dimethylcyclopropane exists in two forms that differ in the arrangement of the two methyl groups with re-... 

The cis-trans isomerism of cyclohexane derivatives (Section 5-1 A) is complicated by conformational isomerism. For example, 4-tert-butylcyclohexyl chloride theoretically could exist in four stereoisomeric chair forms, 1, 2, 3, and 4. 

Stereoisomerism Indicates whether double bonds are cis/trans or E/Z, and indicates stereocenters (R, S), which we wall cover in the chapter on configuration. 

Although the term prochirality is frequently used, especially by biochemists, it suffers from a limitation which arises from a corresponding limitation in the definition of chirality. Molecules may display purely stereochemical differences without being chiral cis-tram isomers of olefins and certain achiral cis-trans isomers of cyclanes are examples. Thus (Fig. 2) (Z)- and ( )-1,2-dichloroethylene (4, 5) are achiral diastereomers, as are cis- and /rtww-1,3-dibromocyclobutanes (6, 7) being devoid of chirality these compounds have no chiral centers (or other chiral elements). Thus it is inappropriate to associate stereoisomerism with the occurrence of chiral... 

In 2, all three substituents may lie on the same side of the reference plane alternatively, A, B, or C may be on the opposite side from the other two. Isomerism resulting from these four arrangements is analogous to the more familiar cis-trans isomerism. Each of these arrangements is chiral, even in the absence of helicity (e.g., even when all rings are perpendicular to the reference plane), and may exist in two enantiomeric forms which differ in configuration with respect to the reference plane. Thus this plane may be treated as a plane of chirality. This is illustrated in the top portion of Fig. 1 where the two enantiomeric structures each have all three substituents on the same side of the reference plane. There are consequently 4x2=8 stereoisomeric structures, i.e., four dl pairs. 

Stereoisomers have the same order of atom attachments but different arrangements of the atoms in space. Cis-trans isomerism is one kind of stereoisomerism. For example, two substituents on a cycloalkane can be on either the same (c/s) or opposite (trans) sides of the mean ring plane. Stereoisomers can be divided into two groups, conformational isomers (interconvertible by bond rotation) and configurational isomers (not interconvertible by bond rotation). Cis-trans isomers belong to the latter class. 

Like proton transfer, photoisomerization is a fundamentally important photochemical process. The two most important forms of photoisomerization are valence isomerization and stereoisomerization. The latter is probably the most common photoinduced isomerization in supramolecular chemistry. It may occur in systems in which the photoactive component has unsaturated bonds which can be excited, and this effect may be exploited for optical switching applications. A number of interfacial supramolecular complexes capable of undergoing cis-trans photoisomerization have been studied from this perspective - some examples are outlined in Chapter 5. 

The monomeric units formed via 1,4-polymerisation of monomers of the CH2=CH-CH=CHR type display stereoisomerism at the double bond (cis trans) and at the tertiary carbon atom which can assume two opposite configurations. Hence, the obtained 1,4-polymers can appear as cis- 1,4-isotactic, cis-... 

So-called cis-trans materials are also examples of stereoisomerism, the prefix cis- coming from the Latin meaning "on the same side" and... 

Pyrethroids have two kinds of stereoisomerism, i.e., geometric isomerism and enantiom-erism. The cyclopropene ring shown in the general structure acts like a double bond, resulting in geometric isomers, cis/trans or Z/E forms, depending on whether the two substitutes (C-l and C-3) are on the same side (cis or Z) or on opposite sides (trans or E), as shown in the following text ... 

Stereoisomerism is the arrangement of atoms in molecules whose connectivity remains the same but their arrangement in space is different in each isomer (atoms are connected in the same sequence). The two main types of stereoisomerism are cis-trans or Z-E isomerism and optical isomerism. 

The stereoselective synthesis of unsaturated oxetanes has recently been achieved by Feigenbaum and coworkers.Previous studies have indicated that photochemical cis-trans isomerization of enals is rapid and results in the formation of equivalent amounts of stereoisomeric alkene adducts. " For example, irradiation of rran.r-crotonaldehyde and 2,6-dimethylfuran produced a 1 1 mixture of alkenic isomers (174) and (175) in 64% yield. Irradiation of 4-trimethylsilylbutyn-2-one and furan provided with S 1 stereoselectivity the bicyclic oxetane (176) in which the methyl group occupies the exo position, presumably because of the small steric requirement of the triple bond. Desilyation of the protected al-kyne produced an alkynic oxetane which was hydrogenated under Lindlar conditions to bicyclic vinyl-oxetane (177) attempts to use the unprotected butyn-2-one gave low isolated yields of oxetane because of extensive polymerization. The stereochemical outcome thus broadens previous alkynyloxetane syn-theses and makes possible the preparation of new oxetane structures that may be synthetically useful. 

Sensitized irradiation of cyclohexenes and cycloheptenes in protic media results in protonation. This phenomenon, which is not shared by other acyclic or cyclic olefins, has been attributed to ground-state protonation of a highly strained tran -cycloalkene intermediate. In aprotic media, either direct or triplet-sensitized irradiation of cyclohexene produces a stereoisomeric mixture of (2 -I- 2] dimers 49-51 as the primary products, with 50 predominating. The reaction apparently involves an initial cis-trans photoisomerization of cyclohexene followed by a nonstereospecific nonconcerted ground-state cycloaddition, promoted by the high degree of strain involved. In contrast, cycloheptene undergoes only a slow addition to the p-xylene used as sensitizer. 

Utley and coworkers [31,32] reported that the stereoisomeric ratio cis/trans) of 1,4-disubstituted cyclohexanes formed by the hydrogenation of the corresponding activated cyclohexenes at a mercury electrode depended on solvents and proton sources, and discussed the stereochemical mechanism in detail. On the other hand, according to Lessard and coworkers [33,34], the hydrogenation at a Raney nickel electrode always provides the trans isomers in excess. 

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