Isomerization and Related Processes

Few reactions can have been studied in both extent and depth as the photochemical trans-cis isomerization of stilbene. An up to date review on the subject prepared by Waldeck is helpful and comprehensive. 

Silver ions cause perturbation of the (E)-(Z) photoisomerization pathway for both stilbene and azobenzene . The efficiency of silver ions in this respect is compared with the effect of Nal which can only induce a heavy atom effect. Ag+ clearly forms complexes with both compounds. Observation of cis-trans conversion in olefin radical cations shows that electron transfer can bring about isomerization of stilbene derivatives. The efficiency of such processes obviously depends on the presence and nature of any substituents. Another study deals with photochemical generation, isomerization, and effects of oxygenation on stilbene radicals. The intermediates examined were generated by electron transfer reactions. Related behaviour probably occurs through the effect of exciplex formation on photoisomerization of styrene derivatives of 5,6-benz-2,2 -diquinoyE.  

The connection between alternative modes of isomerization and the shape of potential surfaces underlies understanding of the highly selective nature of the cis-trans photoisomerization process of 1-pyrenylethylene. Another specific example of the unique behaviour of an excited cis singlet state is the one way isomerization of 2-(3,3-dimethyl-l-butenyl)pyrene . The shape of the PE surface involved has been examined from a study of the details of the fluorescence spectra. 

The photoisomerization dynamics of diphenylbutadiene in both liquid and solid alkane environments has been analyzed in paper which is one contribution to the complete journal issue on the role of solvents in liquid state reactions. Several related topics of photophysical interest are discussed in this collection of papers. 

Intramolecular heavy atom effects influence the photoisomerization derivatives of 5,5-diphenyl-1,3-cyclohexadiene The homogeneous acid catalysis of the photoisomerization of trans-3-(2-hydroxy-benzylidene)-4,5-dihydrofuran-2(3H)-one and model mechanisms for isomerization of carbocyanines have both been analyzed. The process of photoisomerization of the biologically important rhodopsin and bacteriorhodopsin has been examined by a theoretical ab initio study of retinal analogues.  

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Photocatalysis of alkene isomerizations and related processes has also been noted for a number of other common carbonyls including Fe(CO)s, Fe(CO)4PPh3, Mo(CO)6, W(C0)6, Ru(CO)4PPh3 and Ru3(CO)i2 [61,74,79-82], In some cases quantum yields substantially in excess of unity have been reported consistent with the photolytic formation of a catalytically active species capable of numerous turnovers. The photocatalysis of 1-pentene hydrogenation by Fe(CO)s is accompanied by isomerization of the substrate to 2-pentene [57], Such a pathway could occur via a -hydride elimination pathway comparable to the reverse of step 2 in Scheme I, but a more likely pathway would be via a C-H insertion reaction of the intermediate Fe(CO)3(alkene) to give an alkyl hydride intermediate as in eq. 20. 

Ldoir, Luis F., Enzymic Isomerization and Related Processes... XIV Lemer, Aaron Bunsen, Metabolism of Phenylalanine and Tyro-... 

Combination of Substrate Isomerization and their Derivatization with Chemo- and Biocatalysts Resulting in Dynamic Kinetic Resolutions and Related Processes... 

In a related process, 1,4-dichlorobutene was produced by direct vapor-phase chlorination of butadiene at 160—250°C. The 1,4-dichlorobutenes reacted with aqueous sodium cyanide in the presence of copper catalysts to produce the isomeric 1,4-dicyanobutenes yields were as high as 95% (58). The by-product NaCl could be recovered for reconversion to Na and CI2 via electrolysis. Adiponitrile was produced by the hydrogenation of the dicyanobutenes over a palladium catalyst in either the vapor phase or the Hquid phase (59,60). The yield in either case was 95% or better. This process is no longer practiced by DuPont in favor of the more economically attractive process described below. 

Besides solid transition metals, certain soluble transition-metal complexes are active hydrogenation catalysts.4. The most commonly used example is tris(triphenylphosphine)-chlororhodium, which is known as Wilkinson s catalyst.5 This and related homogeneous catalysts usually minimize exchange and isomerization processes. Hydrogenation by homogeneous catalysts is believed to take place by initial formation of a rc-complex, followed by transfer of hydrogen from rhodium to carbon. 

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. 

Sufficient evidence has now been obtained to provide an explanation for the photorearrangement of pyran-4-ones to the isomeric pyran-2-ones.217 An oxabicyclohexenyl zwitterion and a cyclopentadienone epoxide have been established as intermediates in this rearrangement, and the process is illustrated for 3,5-dimethylpyran-4-one (270) in Scheme 21. Related transformations have been observed in 3-hydroxypyran-4-ones218 and in the... 

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