Trans-cis isomerism

We have seen before that such diradicals can close up to give cyclopropanes (17-36). Therefore, pyrolysis of cyclopropanes can produce not only propenes but also isomerized (cis trans or optically active inactive) cyclopropanes. See, for example, Berson, J.A. Balquist, J.M. J. Am. Chem. Soc., 1968, 90, 7343 Bergman, R.G. Carter, W.L. J. Am. Chem. Soc., 1969, 91, 7411. 

The internal hydrogen bond (N—H—N) is responsible for suppression of cis-trans isomerization in the singlet excited state of cA-l-(2-indolyl)-2-(2-pyridyl)ethene (32) of reaction 13. Clearly, the reverse isomerization is possible125. On the contrary, irradiation of c -l-(2-pyrrolyl)-2-(2-quinolyl)ethene126 (35) induces the isomerization to 34. The isomerization (cis-trans) is possible because the excitation allows the tautomerization of 35 to 36 (equation 14). 

In comparison, photoremovable protecting groups such as 69 also release their protected molecule via intramolecular lactonization, but the photorelease is initiated by cis-trans isomerization. Cis-trans isomerization has been extensively used to turn molecular switches off and on, but there are a few examples where cis-trans isomerization has been used to activate photorelease. Initially, Stoermer showed that photoinduced cis-trans isomerization of trans-69 yields 71, presumably via intramolecular lactonization of cis-lQ (Scheme 37). In a similar way. 

The distribution of energy levels among the adsorbed molecules of the same dye species could cover a range of several tenths of an electron volt (45,46,235). Dye aggregation, differences in aggregation from one type to another, and the presence of isomeric (cis-trans) forms of the molecules on the grain also could be factors contributing to a distribution of levels. 

Even so, the distinction between the two is sometimes a more subtle matter. Thus, in a photoisomerization a common excited state intermediate may undergo a transformation to either of the two isomeric cis-trans species of a planar ground-state molecule. These two transformations are virtually identical in nature, yet one leads back to the original species and is therefore a photophysical primary step, e.g., internal conversion or intersystem crossing, while the other leads to the chemically distinct isomer and should be called a photochemical primary step. As another example, the distinction between the formation of an excimer and of a photodimer lies in the instability and stability, respectively, of the dimeric species in the ground state. Excimer formation is usually considered as photophysical and photodimer formation as photochemical. These examples show that the classification of steps as photochemical and photophysical is in some cases arbitrary. 

Treatment of [PdCl2(COD)] with ferrocenyl bis-NHC (97) yielded complex (98), in which an isomerization cis/trans was observed (equation 12). It is worth noting that the free carbene was generated in situ using the strong base KN(SiMe3)2. ... 

GAl was found to be a useful index for the discrimination of cis/trans isomerism (- cis/trans descriptors) and to model the chromatographic behaviour of phosphorus derivatives. 

Chemical compounds that have the same molecular formula but different structural formulas are said to be isomers of each other. These constitutional isomers (or structural isomers) differ in their bonding sequence, i.e. their atoms are connected to each other in different ways. Stereoisomers have the same bonding sequence, but they differ in the orientation of their atoms in space. Stereoisomerism can be further divided into optical isomerism (enantiomerism) and geometrical isomerism (cis—trans isomerism). The relationships between the different types of isomerism are shown in Figure 4.1. 

Pico second spectroscopic investigations of cis -trans -isomerization of stilbene and the derivative (25) have demonstrated the presence of an intermediate which is involved in the isomerization. cis -trans -Isomerization of several pyridinium analogues of stilbene have been studied. The salts (26) also undergo singlet state trans -cis -isomerization on irradiation. A detailed mechanistic examination of these systems has been carried out and a physical study of the influence of solvent and quencher on the decay of the excited singlet state of the stilbene derivatives (27) has been reported. The cis -trans isomerization of bis -heteroaryl ethenes has been reported in detail. ... 

In view of the C=C and =C H stretching frequencies of the Z- and E-isomers of 4-octene, the isomeric cis/trans-cyclooctenes 36 and 37a differ surprisingly little. Similarly, the C=C frequencies of the bridgehead olefins 177 and 260 differ little from trans- 1-methylcyclooctene (37c) (Table 10). 

A database search yields more than 20 000 references that contain Z-E isomerization, cis-trans isomerization, or geometric isomerization as keywords, and the general tendency is an increase in the number of papers devoted to the kinetic aspects of cis-trans isomerization (CTI) in all fields. The main isomerization pathways have probably been discovered, though many remain the object of intense theoretical (see Chapter 7) and experimental research (see Chapters 4—6, 8-10, 13, and 14). In the present chapter, general CTI mechanisms will be divided into homolytic and heterolytic cleavage of the 7r-bond which allows isomerization, though some molecular motifs such as amides are able to switch from cis to trans via both processes. An overview of CTI in metal complex (mainly thermal, photochemical, and oxidative isomerizations) will be the purpose of Chapter 14 and will not be detailed here. 

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