Migration of a group

An a-diazo ketone 1 can decompose to give a ketocarbene, which further reacts by migration of a group R to yield a ketene 2. Reaction of ketene 2 with water results in formation of a carboxylic acid 3. The Woljf re arrangement is one step of the Arndt-Eistert reaction. Decomposition of diazo ketone 1 can be accomplished thermally, photochemically or catalytically as catalyst amorphous silver oxide is commonly used ... 

Sigmatropic reaction (Section 30.8) A pericyclic reaction that involves the migration of a group from one end of a tt electron system to the other. 

Another example of migration of a group, in the original case Me, to a cationic carbon atom occurs in the acid-catalysed rearrangement of 1,2-diols, e.g. pinacol (cf. p. 218) Me2C(OH)C(OH)Me2 (34) to ketones, e.g. pinacolone, MeCOCMe3 (35) ... 

From a study of his and van Alphen s results, Huttel identified three classes of rearrangement (Scheme 40).137 Type A involved migration of a group from C-3 to unsubstituted C-4 (e.g., 114 - 11564), Type B an acyl migration from C-4 or C-5 to N (e.g., 116 -> 11764or 118 - 117137), and Type C an aryl migration from C-3 to N (e.g., 118 - 119137) the latter two modes were observed when all carbon atoms were fully substituted. 

We shall distinguish two different sigmatropic changes, which we represent schematically in Fig. 20, namely the migration of a group via a sigma orbital or by a pi orbital. 

Sigmatropic rearrangement (Section 22.8) The intramolecular migration of a group along a conjugated pi system. 

When one of the numbers in square brackets is T, the old and new o bonds are to the same atom, so we are dealing with the migration of a group around a conjugated system. 

Scheme 4. Possible mechanisms for the 1,2-migration of a group X in a free radical.

Key questions regarding the reactivity of silylene complexes concern their potential role in metal-catalyzed transformations. For the participation of intermediate silylene complexes in a catalytic cycle, low-energy chemical pathways must exist for the conversion of simple silanes to silylene ligands via activation processes at the metal center. Most probably, a key step in such silylene-forming processes would be the a-migration of a group from silicon to the metal. In search of such a reaction, we prepared the methyl silyl complex shown in Eq. 3. This complex is quite... 

As a method of synthesis of amines, the Hofmann degradation of amides has the special feature of yielding a product containing one less carbon than the starting material. As we can see, reaction involves migration of a group from carbonyl... 

Although this latter pathway is unlikely on the basis of the time dependence of the formation of trans-79a, 82a and 84a from cis-78a and Na/K presented in Table 4, page 17, the migration of a group R has been checked additionally by means of the... 

Perhaps one of the most widespread ways to form phenols is by the rearrangements of alicyclic dienones. The simplest variant of these isomerizations can be represented by acid-catalyzed transformation of 2,5-cyclohexadien-l-ones 297 to phenols 298 which proceeds with migration of a group R and aromatization of the ring (equation 144). Numerous versions of the dienone-phenol rearrangement were described in detail in many reviews ° . A list of other surveys and original papers can be found elsewhere. ... 

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