Migration of substituents

The ease of migration of substituents depends on their ability to stabilize... 

Analysis of the data in Table XVIII suggests that silene formation is kinetically the most favorable process. However, according to experiment, metallated silenes are formed. This is related to the fact that in polar solvents proton transfer from the carbon atom to silicon is intermolecular, which leads to a considerable decrease in the reaction barrier. We believe that when the migration of substituents from the carbon atom to silicon is suppressed, for example, by the introduction of two alkyl radicals, the elimination of phosphines resulting in silene formation becomes the most probable process. 

Carbohydrate chemistry contains many examples of intramolecular migration of substituent groups. The largest number of these involve acyl groups, but migrations in phosphoric esters, and certain ethers have also been reported. Surprisingly, the topic has not been the subject of a comprehensive review, but there are several useful summaries thereof.539-542 In this Section, mainly those examples that have... 

The reaction mechanism was considered to be oxidative cyclization, and pal-ladacyclopentene 32 was formed. Reductive elimination then occurs to give cyclobutene 33, whose bond isomerization occurs to give diene 28. The insertion of alkyne (DMAD) into the carbon palladium bond of 32 followed by reductive elimination occurs to give [2+2+2] cocyclization product 27. Although the results of the reactions of E- and Z-isomers of 29 with palladium catalyst 26a were accommodated by this pathway, Trost considered the possibility of migration of substituents. Therefore, 13C-labeled substrate 25 13C was used for this reaction. 

Migration of substituents on a benzene ring caused by valence isomerization of the ring is discussed in Section 5.3.4.3. 

The fact that sometimes 1.3- or 1.4-disubstituted arenes exhibit a fragmentation behaviour which is characteristic for the ortho isomer may be caused by different effects. Among these the most familiar ones are the migration of substituents (e.g., via valence isomerization) prior to elimination reactions. Alternatively the fragment (M—HZ)+ might be generated via consecutive radical eliminations14. ... 

Eliminations with additional migration of two groups or bonds lead to extremely profound chemical changes. For the sake of an easy survey the types with exclusive migration of substituents are treated first. Then follow those types with migration of one substituent and one single- or double-bond (aside from allylic shifts) and finally those types with migration of two bonds from the main chain. 

The success of this method is strongly dependent on the substituents on the starting diazo compound. Copper-catalyzed decomposition of various (alkyl)(dimethoxyphosphoryl)di-azomethane derivatives in benzene did not lead to the formation of cyclopropanes. In the same way, the photolysis of (benzyl)(dimethoxyphosphoryl)diazomethane or (benzoyl)(dimeth-oxyphosphoryl)diazomethane derivatives in benzene did not afford cyclopropane adducts, but alkenes or ketenes arising from migration of substituents. ... 

Initially, the separation of sialic acids was mainly carried out by cellulose chromatography at low temperature [6,237]. However, nowadays HPLC fractionations using different column materials, elution protocols and detection techniques have replaced this approach [6,11,268]. The application of HPLC has also introduced a rapid method for tentative assignments of sialic acids in complex mixtures, based on elution times of known standards, being more reliable when more than one HPLC procedure is followed. Moreover, a rapid method for quantification of released sialic acids has become available. Due to the relatively short HPLC runs, also fast transitions between members of the sialic acid family due to migration of substituents, introduction of substituents, cleavage of substituents, or other (enzymatic) modifications can easily be monitored. 

There are two properties of cations that must be considered before considering their reaction with a nucleophilic species regioselectivity in formation of the cation and migration of substituents (rearrangement). The first is easily dealt with. In general, the more stable cation is formed. Since 3 is essentially a carbon with an empty p orbital, electrons from the adjacent bonds will be attracted to that positive center (inductive effect). 

Fenton (4), Udenfriend (3), Hamilton (16), or Viscontini ( ) which have been considered as possible models for enzymatic hydroxylation do not lead to significant migration and retention of tritium during the formation of 4-hydroxyacetanilide (Table II). Thus, only model hydrox-ylating systems proceeding via cationic intermediates produce the migration of substituents characteristic of enzymatic hydroxylation. It appears... 

Some substituted 2-hydroxybenzenium ions were observed to rearrange into 4-hydroxy isomers by the migration of substituents different from CH3 ... 

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