Alkyl and hydrogen migrations

An agostic M—H—C interaction is a 3-centre 2-electron interaction between a metal centre, M, and a C—H bond in a ligand attached to M (e.g. structure 24.46). 

The insertion of CO into M—Cai yi bonds is weU exemplified in organometallic chemistry, and one industrial example (equation 24.41) is a step in the Monsanto process for the production of acetic acid (see Section 27.5). 

The reaction is also called CO insertion since the incoming CO molecule seems to have been inserted into the Mn—Cmc bond this name is misleading. If reaction 23.34 

Alkyl migrations are not confined to the formation of acyl groups, and, for example, alkene insertion involves the conversion of a coordinated alkene to a cr-bonded alkyl group. Equation 23.37 shows the migration of an H atom related alkyl migrations occur and result in carbon chain growth. 

The reactimi is also called CO insertion since the incoming CO molecule seems to have been inserted into the Mn—C e bond this name is misleading. If reaction 24.39 is carried out using CO, none of the incoming CO ends up in the acyl group or in the position trans to the acyl group the isolated product is 24.45. 

The migration of the methyl group is reversible and the decarbonylation reaction has been studied with the Relabelled compound the results are shown in Fig. 24.15. 

The distribution of the products is consistent with the migration of the Me group, and not with a mechanism that involves movement of the inserted CO. The 

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Moreover, because of the involvement of cationic intermediates, rearrangements can occur in systems in which a more stable cation can result by aryl, alkyl, or hydrogen migration. Oxymercuration-reduction, a much milder and more general procedure for alkene hydration, is discussed in the next section. 

In carbocation chemistry, 1,2-alkyl and -hydride migrations are very common. In contrast, the direct 1,2-migration of hydrogen atoms to radicals is virtually unknown. While useful migrations of carbon substituents to radicals are known, they frequently proceed by a sequence of addition to a rr-bond followed by elimination. The 1,2-migration of saturated alkyl groups to radicals is also virtually unknown. 

Isopentane and trimethylpentanes (Cg alkanes, often called abnormal products) are always formed when isobutane is alkylated even when it reacts with propene or pentenes. Formation of isopentane involves the reaction of a primary alkylation product (or its carbocation precursor) with isobutane followed by alkyl and methyl migrations and, eventually, j8-scission. Formation of Cg alkanes is explained by the ready formation of isobutylene either by olefin oligomerization-cleavage reaction or hydrogen transfer converting isobutane to isobutylene. Isobutylene thus formed may participate in alkylation jdelding Cg alkanes. 

Stoilov, I.L., J.E. Thompson, J-H. Cho, and C. Djerassi Biosynthetic studies of marine lipids. Stereochemical aspects and hydrogen migrations in the biosynthesis of the triply alkylated side chain of the sponge sterol strongylosterol. J. Am. Chem. Soc. 108, 8235 (1986). 

The fact that aryl groups migrate, but alkyl groups and hydrogen generally do not, leads to the proposition that 40, in which the odd electron is not found in the three-membered ring, may be an intermediate. There has been much controversy on this... 

An intermolecular 1,3-dipolar cycloaddition of diazocarbonyl compounds with alkynes was developed by using an InCl3-catalyzed cycloaddition in water. The reaction was found to proceed by a domino 1,3-dipolar cycloaddition-hydrogen (alkyl or aryl) migration (Eq. 12.68).146 The reaction is applicable to various a-diazocarbonyl compounds and alkynes with a carbonyl group at the neighboring position, and the success of the reaction was rationalized by decreasing the HOMO-LUMO of the reaction. 

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