Rearrangement by hydrogen

During the oxidation of 2-methylpentane, five radicals CeHiaOO will be formed the rearrangement by hydrogen transfer of these can, in theory, produce a total of 21 radicals C HioOOH (24). 

Isomerization the carbonium ion intermediates can undergo rearrangement by hydrogen or carbon atom shifts, leading to, e.g. a double-bond isomerization of an olefin. Other important isomerization reactions are methyl group shift and isomerization of saturated hydrocarbons. 

The dione (Scheme 22) reacts photochemically with aromatic aldehydes in processes similar to the reactions between quinones and aldehydes. Irradiation of the diketone (343) leads to the formation of the remarkable diol (344). The structure of this product was verified by T-ray analysis. The reaction, brought about by irradiation through Pyrex in various solvents, proceeds via the intermediate keto alcohol (345), a compound isolated at shorter reaction times. There is some doubt in the minds of the authors as to whether the reaction arises by hydrogen abstraction from the y- or the s-positioh. They propose that if the former occurs then the rearrangement by hydrogen migration to yield (I) is kinetically favoured. 

Curran, D.P. and Lung, H.K. (1995) Acceleration of a dipolar Claisen rearrangement by hydrogen bonding to a soluble diaryl urea. Tetrahedron Letters, 36, 6647-6650. 

With 2-methyl- and 2,4-dimethylthiazole, the methyl thiirenium ion (m/e 72) is obtained, which can easily lose a hydrogen radical to give the ml ell ion (confirmed by the metastable peak). This latter can rearrange by ring expansion to give the thietenyl cation whose structure was confirmed in certain spectra by the presence of a metastable peak corresponding to the decomposition of the m/e 71 ion to give the thioformyl cation m/e 45, probably by elimination of acetylene. 

An unusual by-product was obtained in small yield in palladium-catalyzed reduction of 2-amino-4,5-dimethoxyindanone hydrochloride, The reduction was done in two stages first, a rapid absorption of 1 mol of hydrogen at 38 C to give the amino alcohol, and then a much slower reduction in the presence of HCIO4 at 70 "C. The rearranged by-product was shown to arise from attack of acid on the amino alcohol (50), Resistance to hydrogenolysis is characteristic of / -amino aromatic alcohols (56), a fact that makes reduction of aromatic oximino ketones to amino benzyl alcohols a useful synthetic reaction. 

Phenylhydroxylamine rearranges in sulfuric acid to give mainly p-aminophenol. Industrial routes to this compound have been developed in which phenylhydroxylamine, formed by hydrogenation of nitrobenzene in sulfuric acid over platinum-on-carbon, is rearranged as it is formed. Conditions are adjusted so that the rate of rearran ment is high relative to the rate of hydrogenation of hydroxylamine to aniline (15,17,86). An easy way to obtain a favorable rate ratio is to carry out the reduction with about 1% DMSO present in the sulfuric acid (79,81). 

It is important for acid-catalysed reactions to determine whether the reaction is specifically catalysed by hydrogen ions or whether general acid catalysis takes place. Specific acid catalysis has been conclusively demonstrated for the benzidine rearrangement by three different sorts of kinetic experiments. In the first, it has been shown41 by the standard test for general acid catalysis (by measuring the rate of reaction in a buffered solution at constant pH over a range of concentration... 

The rearrangement of the intermediate alkyl cation by hydrogen or methyl shift and the cyclization to a cyclopropane by a CH-insertion has been studied by deuterium labelling [298]. The electrolysis of cyclopropylacetic acid, allylacetic acid or cyclo-butanecarboxylic acid leads to mixtures of cyclopropylcarbinyl-, cyclobutyl- and butenylacetamides [299]. The results are interpreted in terms of a rapid isomerization of the carbocation as long as it is adsorbed at the electrode, whilst isomerization is inhibited by desorption, which is followed by fast solvolysis. 

Two of the three general types of secondary reactions resulting from photochemical a-cleavage of carbonyls, namely molecular rearrangement and hydrogen transfer to yield aldehydes or ketenes, have been discussed. The third type of reaction observed, decarbonylation, will be discussed in this section. The discussion will begin with the decarbonylation of small ring carbonyls. By way of example of this type of reaction, diphenylcyclopropenone decarbonylates upon photolysis to yield diphenylacetylene(57) ... 

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