Markovnikov addition cation rearrangements

Radical ions - charged species with unpaired electrons - are easily generated by a number of methods that are discussed in more detail below. Their properties have been characterized by several spectroscopic techniques, and their structures and spin density contributions have been the subject of molecular orbital calculations at different levels of sophistication. The behaviour of radical ions in rearrangement and isomerization reactions as well as in bond-cleavage reactions has been extensively studied [for recent reviews see Refs. 11-13 and references cited therein]. Useful synthetic applications, such as the radical-cation-catalyzed cycloaddition [14-20] or the anfi-Markovnikov addition of nucleophiles to alkenyl radical cations [21-25], have been well documented. In... 

Although terminal alkenes provide the best yields, the Ritter reaction is also successful when using tri-substituted alkenes and haloalkenes. Markovnikov addition is generally observed. Rearranged products arise occasionally, especially with alkenes that are prone to cationic rearrangements (equation 165).233... 

Direct hydration (p. 380) proceeds through an intermediate carbocation that is captured by water to give the product of Markovnikov addition. The reaction is limited in utility because rearrangements of the initially formed cation to more stable species can lead to undesired, rearranged products. 

In the absence of a good nucleophile, carbocation rearrangements may occur following addition of an electrophile to the alkene double bond (Section 9-3). Rearrangements are favored in electrophilic additions of acids whose conjugate bases are poor nucleophiles. An example is trifluoroacetic acid, CF3CO2H. Its trifluoroacetate counterion is much less nucleophilic than are halide ions. Thus, addition of trifluoroacetic acid to 3-methyl-1-butene gives only about 43% of the normal product of Markovnikov addition. The major product results from a hydride shift that converts the initial secondary cation into a more stable tertiary cation before the trifluoroacetate can attach. 

The expected product forms from the reaction of the nucleophihc chloride ion with the secondary cation that forms when a proton adds to the double bond by Markovnikov addition. The isomeric product forms when a nucleophihc chloride ion reacts with a tertiary carbocation. This carbocation forms when the hydrogen atom at C-3 moves, with its bonding pair of electrons, to the adjacent secondary carbocation center (Figure 6.4). This rearrangement is called a 1,2-hydride shift because a hydride ion (H ) moves between adjacent carbon atoms. 

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