ISOMERIZATIONS AND REARRANGEMENTS

Rearrangements. It will have been obvious from the Cr(C0)5... X discussion above that the matrix technique is particularly well suited to the examination of photochemically induced rearrangements and isomerization since the matrix holds the participating species in fixed positions and hence allows spectroscopic study of each, yet allows ready photochemical interconversion. 

Radical ions - charged species with unpaired electrons - are easily generated by a number of methods that are discussed in more detail below. Their properties have been characterized by several spectroscopic techniques, and their structures and spin density contributions have been the subject of molecular orbital calculations at different levels of sophistication. The behaviour of radical ions in rearrangement and isomerization reactions as well as in bond-cleavage reactions has been extensively studied [for recent reviews see Refs. 11-13 and references cited therein]. Useful synthetic applications, such as the radical-cation-catalyzed cycloaddition [14-20] or the anfi-Markovnikov addition of nucleophiles to alkenyl radical cations [21-25], have been well documented. In... 

The most intense interest in the addition of oxygen nucleophiles to Tr-allylpalladium complexes has centered on the delivery of OAc. For example, in allylpalladium-OAc complexes, acetate can be induced to migrate to the allyl ligand by the addition of CO (equation 46).162-164 Rearrangements and isomerizations of allyl acetates can also be readily accomplished via Pd catalysis (equations 47 and... 

Rearrangements and Isomerizations in Carbohydrate Chemistry, R. U. Lemieux, In Molecular Rearrangements, P. de Mayo (Ed.) Interscience Publishers (1964) p. 709. 

A discussion of classical rearrangements and isomerization reactions is a fitting end to this chapter. These types of reactions are extremely useful when considering their ability to convert readily available sugars to both scarce sugars and useful sugar derivatives. In this section, base-mediated isomerization reactions will be addressed, as will the Amadori rearrangement. 

Lemieux, R. U. Rearrangements and isomerizations in carbohydrate chemistry. Mol. Rearrangements (Paul de Mayo, editor. Interscience) 1964, 2, 709-769. 

Carboanionic Intermediates.—Whereas characterization of the carbocations active in cationic polymerization has been difficult, identification and quantification of the carboanionic centres in anionic propagations have been somewhat easier, largely as a result of a considerably reduced tendency to rearrange and isomerize. Nevertheless, detailed investigations continue in particular areas. Thus Bywater and Worsfold ° have used n.m.r. spectroscopy to probe the structure of various model compounds which closely resemble those presumed to be responsible for anionic propagations. Whereas in tetrahydrofuran solvent 2,5-diphenyl-2,5-dipotassiohexane and 2-lithio-4,4-dimethyl-2-phenyl-pentane display coupling constants consistent with the carbon atoms... 

Claisen rearrangement is widely used in organic synthesis. Thus, to obtain ortho-methoxylated phenethylamino derivatives as potent serotonin agonist 170, the strategy employed was based on the Claisen rearrangement and isomerization of the allyl fragment. The bromine atom was attached at an ortho-position to the hydroxy group in order... 

Although the multiplicity of excited states of heteroaromatic N-oxides is disputed, it is established that for pyridazine N-oxides the triplet state is involved in oxygen abstraction reactions, whereas the singlet state is involved in rearrangements and isomerizations. ... 

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