Abstraction of halides

A chain mechanism is proposed for this reaction. The first step is oxidation of a carboxylate ion coordinated to Pb(IV), with formation of alkyl radical, carbon dioxide, and Pb(III). The alkyl radical then abstracts halogen from a Pb(IV) complex, generating a Pb(IIl) species that decomposes to Pb(II) and an alkyl radical. This alkyl radical can continue the chain process. The step involving abstraction of halide from a complex with a change in metal-ion oxidation state is a ligand-transfer type reaction. 

Generation of Non-Heteroatom-Sub. itituted Carbene Complexes 89 3.1.2.4 a-Abstraction of Halides... 

Fig. 3.17. Generation of carbon-metal double bond.s by a-abstraction of halides [459 490 -492].

Electrophilic activation and hydrolysis of a-CF bonds in transition metal complexes is well precedented with strong Lewis or protic acids (Sect. 2.2). Abstraction of halide using AgBF4 in moist dichloromethane from the perfluor-obenzyl and perfluoropropyl complexes 1 and 2 affords the cationic aqua complexes 3+ and 4+ which exist as hydrogen bonded dimers in the solid state with tetrafluoroborate anions bridging the coordinated aqua ligands [69] (Scheme 4). 

The most common reaction of metal halides is substitution by nucleophiles to generate metal compounds containing new metal X-type ligand bonds or displacement of halide by a strong L-t5 e ligand to form a cationic species (Chapter 6). Abstraction of halide by silver salts to generate insoluble silver halides is also a common transformation of the metal-halide complexes described in Chapter 12. In polar media, halides are also known to undergo heterolytic dissociation to form solvated species. 

Methods to generate the unsaturated intermediate that adds alkane include photolysis of metal-carbonyl or -dihydride complexes, thermolysis of alkyl hydride complexes, and abstraction of halide with a cationic reagent or protonation of an alkyl group with an acid containing a weakly coordinating anion. For example, the photolysis of Cp Ir(CO)j or Cp IrfPMejJHj generates Cp Ir(L) (L = CO or and photolysis of the... 

The preferential abstraction of halide ions from chromium complexes has been studied,24 Scheme 3.8 ... 

Mercury(II) salts have been employed as reagents for the abstraction of halides and pseudohalide ligands from complexes of transition metals,26 Eq. 3.4 ... 

Free radicals can also be formed in situ by the abstraction of a halogen atom from an organic halide (RX) as in the sequence of reactions ... 

The catalytic hydrogenation of alkyl halides (RX) probably also proceeds via the intermediate formation of free radicals, which are formed in this case by the abstraction of a halogen atom (see Section III,B). 

There is a discussion of some of the sources of radicals for mechanistic studies in Section 11.1.4 of Part A. Some of the reactions discussed there, particularly the use of azo compounds and peroxides as initiators, are also important in synthetic chemistry. One of the most useful sources of free radicals in preparative chemistry is the reaction of halides with stannyl radicals. Stannanes undergo hydrogen abstraction reactions and the stannyl radical can then abstract halogen from the alkyl group. For example, net addition of an alkyl group to a reactive double bond can follow halogen abstraction by a stannyl radical. 

A. Cyclizations of halides terminated by hydrogen atom abstraction or halogen atom transfer 1a... 

This is an extremely useful reaction for the synthesis of alkenes. It involves the addition of a phosphonium ylid, e.g. (136), also known as a phosphorane, to the carbonyl group of an aldehyde or ketone the ylid is indeed a carbanion having an adjacent hetero atom. Such species are generated by the reaction of an alkyl halide, RR CHX (137), on a trialkyl- or triaryl-phosphine (138)—very often Ph3P—to yield a phosphonium salt (139), followed by abstraction of a proton from it by a very strong base, e.g. PhLi ... 

Silver salts are also employed to create more effective chiral catalysts by exchange of counter anions. For example, in the Mizoroki-Heck reaction of alkenyl or aryl halides, silver salts are employed to form effective chiral Pd intermediates by abstracting a halide group from the Pd11 precursor species (Scheme 53).227,228... 

It is a fundamentally important point that the reaction (12) involving abstraction of a hydride ion from the monomer by a cation is essentially different from Kennedy s reaction (6) in which the neutral metal halide molecule acts as a hydride ion abstractor [12]. 

B2H6 from BHg. The simplest reaction observed involves the abstraction of hydride ion from BHi by a boron halide to generate BH3 units which combine to form B2H6. Reaction (16) represents the general reaction observed. 

Abstraction of one of the metal-bound hydrides from complex 5a provides the cationic iridium(lll) complex 28, which is an efficient precatalyst for alkyl halide reduction in the presence of EtsSiH (Equation 12.11) [31]. 

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