Alkenes geometrical isomerism

As is the case with acyclic alkenes, geometrical isomerization becomes the major photoreaction observed for medium-sized cycloalkenes upon direct excitation, although the carbene-derived products are still obtained as minor products. Thus, direct irradiation of cyclohexene 31Z in aprotic... 

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). 

The strict geometrical requirements for elimination can be put to further use, as illustrated by elegant procedures for the geometrical isomerization of alkenes. Trimethylsilyl potassium (10) and phenyldimethylsilyl lithium (11) both effect smooth conversion of oxiranes into alkenes, nucleophilic ring opening being followed by bond rotation and spontaneous syn fi-elimination ... 

The alkenes make up a homologous series of hydrocarbons with the general formula C H2 . Alkenes show two types of structural isomerism, position isomerism and chain isomerism. Geometrical isomerism also exists because of the lack of free rotation about the C=C double bond. 

When geometrical isomerism is due to the presence of one double bond where only two substituents are present it is easy to designate the isomers by the terms cis and trans. But if the alkene is a tri- or tetrasubstituted one, the terms cis and trans become ambiguous and do not apply at all as in the following examples. 

To illustrate what is meant by geometric isomerism, we will consider the alkene but-2-ene (H3C-HC=CH-CH3). It contains a carbon-to-carbon double bond and the diagrams on the left show the spatial arrangement of the bonds around each carbon atom. Furthermore, this arrangement of atoms and bonds is planar and all the bond angles around the C=C double bond are 120°. The bonds are fixed in relation to one another. This means that it is impossible to rotate one end of an alkene molecule around the C=C double bond while the other end is fixed. This is one of the reasons why some alkenes can exhibit geometric isomerism. 

Concerning their structure and reactions, organic radical cations have been the focus of much interest. Among bimolecular reactions, the addition to alkenes and their nucleophilic capture by alcohols, which lead to C—C and C—O bond formation, respectively have been investigated in detail. Unimolecular reactions like geometric isomerization and several other rearrangements have also attracted attention. 

Problem 6.5 Which of the following alkenes exhibit geometric isomerism Supply structural formulas and names for the isomers. <... 

Butenes or butylenes are hydrocarbon alkenes that exist as four different isomers. Each isomer is a flammable gas at normal room temperature and one atmosphere pressure, but their boiling points indicate that butenes can be condensed at low ambient temperatures and/or increase pressure similar to propane and butane. The 2 designation in the names indicates the position of the double bond. The cis and trans labels indicate geometric isomerism. Geometric isomers are molecules that have similar atoms and bonds but different spatial arrangement of atoms. The structures indicate that three of the butenes are normal butenes, n-butenes, but that methylpropene is branched. Methylpropene is also called isobutene or isobutylene. Isobutenes are more reactive than n-butenes, and reaction mechanisms involving isobutenes differ from those of normal butenes. 

The reactions of alkenes and related compounds are grouped here into nine sections. The first five deal essentially with photoisomeriza-lion processes—geometrical isomerization about a carbon-carbon double bond, concerted (electrocydic) cyclization, concerted shifts (usually of hydrogen) along the ir-system, the di-jr-methane reaction. 

Finally, intense ultraviolet irradiation of perfluoro(2.3-dimethylbut-2-ene) (14) yields terminal alkene 15 quantitatively.35 Unfortunately, photolysis of more complex perfluoroalkenes gives mixtures of products resulting from migration of both allylic fluorine and allylic per-fluoroalkyl groups. Perfluorobut-2-ene only undergoes geometric isomerization. Pyrolysis of 14 at temperatures up to 300 C in the dark does not lead to detectable rearrangement. 

Geometric Isomerizations. Alkene isomerization can also be induced by photochemically induced electron exchange. Lewis and coworkers have shown, eq. 29 (86),... 

Reductive alkene isomerizations can also be induced by photochemical excitation. Geometric isomerization and rearrangement can be observed upon electron transfer sensitization with molecules with inverse electron demand. Thus, a substituted cinnamyl alcohol in the presence of excited p-dimethoxybenzene gave geometric isomerization and rearrangement characteristic of a free allyl cation, eq. 32 (94) ... 

Electron Transfer Induced Geometric Isomerization of Alkenes. . 162... 

The topic of electron transfer induced geometric isomerization is treated in a separate section, because we wish to emphasize the existence of two fundamentally different mechanisms of isomerization. Although both mechanisms are initiated by an electron transfer step, the key intermediates involved in these isomerizations are fundamentally different. The interaction of acceptor sensitizers with donor olefins leads to the isomerization of alkene radical cations, whereas the reaction of donor sensitizers with acceptor olefins leads, eventually, to the population of alkene triplet states. 

Another extensively investigated system involves the interaction of two alkenes, each capable of geometric isomerization, viz., the system stilbene-dicyanoethylene, which also illustrates the involvement of ground-state charge-transfer complexes. Excitation of the ground-state complex results in efficient Z - E isomerization of the stilbene exclusively, because the stilbene triplet state lies below the radical ion pair, whereas the dicyanoethylene triplet state lies above it (Fig. 11) [163-166]. 

In the rather short history of organic photochemistry, the geometrical E-Z photoisomerization has been exceptionally intensively studied for half a century and a number of reviews have been published [11-18], Although the geometrical isomerization of alkenes can be effected thermally, catalytically, and photochemically, one of the unique features of photoisomerization is that the photostationary EfZ ratio is independent from the ground-state thermodynamics but is instead governed by the excited-state potential surfaces, which enables the thermodynamically less-stable isomers... 

In addition to the locations of the double bonds, another difference of alkenes is the molecule s inability to rotate at the double bond. With alkanes, when substituent groups attach to a carbon, the molecule can rotate around the C-C bonds in response to electron-electron repulsions. Because the double bond in the alkene is composed of both sigma and pi bonds, the molecule can t rotate around the double bond (see Chapter 6). What this means for alkenes is that the molecule can have different structural orientations around the double bond. These different orientations allow a new kind of isomerism, known as geometrical isomerism. When the non-hydrogen parts of the molecule are on the same side of the molecule, the term cis- is placed in front of the name. When the non-hydrogen parts are placed on opposite sides of the molecule, the term trans- is placed in front of the name. In the previous section, you saw that the alkane butane has only two isomers. Because of geometrical isomerism, butene has four isomers, shown in Figure 19.12. 

For alkenes, in addition to structural isomerism, geometrical isomerism can also occur. Geometrical isomerism differs only in the arrangement of the atoms in space. 

Scheme 3 Bonding of alkene to FeMo-cofactor as later refined. Geometrical isomerism (E, Z) of C2R2H2 is not specified...

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