Hydrogen migration, reactions involving

The appearance of the complex (XXXIV) is interesting in that its formation must involve hydrogen transfer from the free diene or its complexed form. Although intramolecular hydrogen migration reactions are frequently encountered, such intermolecular hydrogen transfer reactions are most uncommon. 

It is the purpose of this review11 to show that a fairly substantial number of quite different unimolecular dissociations of gaseous cation radicals can be described conveniently by this concept. Among the reactions discussed are some unusual cleavage processes, the occurrence of which were difficult to explain without the involvement of hidden hydrogen migrations. 

The photocyclization of enamides has been the subject of detailed study and provides a valuable approach to the synthesis of alkaloids. A comprehensive review has been published.31 A variety of reaction types has been reported. The N-benzoylenamine 33, for example, is converted on irradiation to the trans-lactam 34 by a process involving conrotatory photocyclization followed by a thermally allowed [l,5]-suprafacial hydrogen migration.32 The influence of substituents on this transformation has been studied.33 The enacylamine 35 undergoes an analogous cyclization to give a mixture of cis- and trans-lactams 36, the ratio of which is solvent... 

Hydrogen migrations have been demonstrated or invoked in several instances, although the detailed course of these reactions has not been fully elucidated. The involvement of hydrogen shifts is postulated based on experimental data, such as EPR or CIDNP spectra, the structure of reaction products, or on theoretical calculations (Section Although some migrations clearly involve a hydride... 

It was further found that the trapped radicals in polyethylene decayed more rapidly in a hydrogen atmosphere. The migration of the free valencies was therefore explained on the basis of a process similar to Reactions 13 and 14, assuming an exchange reaction involving the hydrogen which results from the radiolysis of the polymer. However, Reaction 13 is endothermic and therefore unlikely to occur at room temperature. 

Products of the type (24) also result from enolizable ketones without the formation of silyl enol ethers if the reaction is carried out in the presence of tertiary phosphines. The proposed mechanism involves the betaine R3P—SiMe2 as the silylene transfer agent. In preventing a 1,3-hydrogen migration, the phosphine may well induce dimerization prior to oxasilacyclopropane formation. The dioxadisilacyclohexane (24) can be reduced with LiAIHU to give dimethylsilyl-substituted carbinols, so the reaction is of synthetic value (Scheme 34) (78JA7074). 

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