Alkanes, addition from alkyl halides

This reactivity makes alkenes an important class of organic compounds because they can be used to synthesize a wide variety of other compounds. For example, alkyl halides, alcohols, ethers, and alkanes all can be synthesized from alkenes by electrophilic addition reactions. The particular product obtained depends only on the electrophile and the nucleophile used in the addition reaction. 

Oxidative addition of an alkyl halide to two metal centers to form two products from one-electron oxidations are also known (Scheme 7.8). This overall transformation generates the two products M" -R and M" -X from two metal complexes M . Thus, this reaction parallels the addition of dihydrogen and alkanes to two metal centers, but it is believed to proceed by successive one-electron processes instead of a mechanism through a single transition state containing the reactant and two metal centers. This dinuclear process occurs when the metal complex undergoes facile one-electron oxidation, but slow two-electron oxidation. 

Shilov reported some of the earliest evidence that transition metal complexes could selectively cleave the C-H bonds of alkanes in a catalytic fashion. Shilov showed that H/D exchange would occur between alkanes and deuterated acid in the presence of platinum complexes (Equation 18.5 and Table 18.1). In addition, Shilov showed that the oxidation of alkanes occurred in the presence of a platinum(II) catalyst, although a platinum(IV) complex was needed as the oxidant. These reactions led to a mixture of alkyl halides formed from the halide of the Pt(IV) oxidant (Equation 18.6) and trifluoroacetate from the trifluoroacetic acid solvent. The cost of platinum(IV) as an oxidant makes this reaction impractical. However, these results provided hope that selective alkane functionalization could be developed because H/D exchange occurred faster at primary C-H bonds than at secondary C-H bonds (Table 18.1), and some selectivity for oxidations of primary C-H bonds over secondary C-H bonds was observed. As noted in Chapter 6, these results motivated a large number of groups to seek transition metal complexes that would insert into, or by other means selectively cleave, the C-H bond of alkanes and create products from this bond cleavage that could be observed directly. 

Depending on the solvent, supporting electrolyte, electrode material, and potential, it is possible to electrogenerate either alkyl radicals or carbanions, which can lead to the formation of dimers (R2), alkanes (RH), and olefins [R(-H)]. In addition, the solvent (SH) can act as a hydrogen atom donor or a proton donor. Also olefins can arise from the base-promoted dehydrohalogenation of RX, and other products (RB) can be formed via displacement of halide from RX by a base (B"). When cathode materials such as... 

The titanium-catalyzed addition of magnesium hydrides to alkanes was first reported by Ashby [22], but this reaction can be more conveniently carried out using alkyl-magnesium halides ( Bu, "Pr), because (3-hydride elimination from organotransition metal complexes to produce transition metal hydrides is generally very rapid (RCH2CH2MgX + M—X RCH.CH -M H—M + RCH- CH,). Thus, dimeriza-... 

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