Trans-cis isomerisation

Ion pairs (contd.) intimate, 91, 249, 291 solvation, 45, 57, 81, 390 solvent separated, 91 Ipso substitution, 161, 169 steric effects in, 162 Isocyanate intermediates, 49, 122 Isomer distribution, 158 Isomerisation, cis- trans, 315 Isoprene, 323... 

Another ternaiy crystal (2-Cl-HStb )(Hmal )(H2Fu)o.s was reported in the same article from the crystallisation of maleic acid (H2Mal) with 2-Cl-stilhazole in solution, which produces an unexpected ternaiy crystal as consequence of the formation of H2FU due to the isomerisation cis-trans in situ of H2Mal in the solution. 

Detailed studies of 1 1 complex formation between and maleic and fumaric acids, which precedes reduction to succinic acid, cis-trans isomerisation and exchange of the double bond hydrogens, are relevant to the complex kinetics (A = substrate)... 

The reaction types which includes 2 + 2 cycloaddition and carried out by photochemical means have also great synthetic potentiality. The photodimerisation of cinnamic acids were among the earliest photochemical reactions to be studied. These compounds give good yield of dimers when irradiated in the crystalline state. In solution, cis-trans isomerisation is the dominent reaction. 

The initial act in the process of vision involves the photochemical cis-trans isomerisation of the 11-cis C=C bond of the retinal chromophore... 

Consider the photosensitised cis-trans isomerisation of stilbene The two isomers have different triplet energy levels (247kJmoT1 for the cis and 205kjmol-1 for the trans). The proportion of the cis isomer in the photostationary state varies with the energy of the photosensitiser (ET), as shown in Figure 8.5. 

Scheme 8.2 Sensitised cis-trans isomerisation of vitamin A acetate...

In the simple four-electron systems, a route for cis-trans isomerisation of a diene is made available by the photochemical reaction usually being a disrotatory ring closure and the thermal reaction being a conrotatory ring opening ... 

If the products formed from the direct and sensitised reactions differ then the triplet state formed on sensitisation is not the reactive excited state formed by direct irradiation. This situation is illustrated in Sections 8.2 and 8.3, where direct and photosensitised cis-trans isomerisation of alkenes was considered. 

Non-geminal bis, tris, and tetrakis secondary amino derivatives are found to undergo reversible cis trans isomerisations in the presence of amine hydrochlorides [141-145]. It is reported that aluminium trichloride also acts as a catalyst for these interconversions (Eq. 26) [142]. 

The diphenylallyl carbanion is conveniently formed by the abstraction of an allylic proton from the corresponding diphenyl propene. Under certain conditions it was found that the initial product formed from trans 1,3 diphenyl-2-methyl propene was the trans,trans anion which isomerised completely into the cis,trans conformation within a few minutes [3]. Clearly, the abstraction reaction proceeds under kinetic and-not thermodynamic control. 

Recently we discovered [11] that photolysis of the diphenylallyl carbanion with white light causes isomerisation of the trans,trans conformer to the cis,trans (scheme 1). When the source of the illumination is removed, conformational relaxation proceeds at a rate which is markedly sensitive to the nature of the ion pairing, being much slower for loose than for tight ion pairs. The results of an extension of this work will be presented in this paper together with an outline of the possible relevance of this photochemical phenomenon to the stereochemistry of the polymerisation of dienes. 

Exposure of a solution of loose ion pairs of the diphenylallyl anion to white light results in partial isomerisation of the solute into the cis,trans conformation (scheme 1). Whereas the trans,trans ion is planar, the cis,trans ion is non-planar as a consequence of steric interaction between the hydrogen... 

The processes of olefin exchange, double bond migration and cis—trans isomerisation, observed to occur concomitant with hydrogenation, may be accounted for by considering that the formation of the half-hydrogenated state is reversible. For olefin exchange we can write... 

---