Abstract halides

The radical-like products readily abstract halide ligands from halo-carbon solvents ... 

In the latter reactions, at least one step corresponded to a carbometallation reaction. Such carbometallations also arose in Heck reactions, and indeed, several conditions set up for such Heck reactions relied on silver salts.49 In these reactions, silver salts only seemed to abstract halide from intermediate palladium species their role could, however, be more complex. 

Silver salts are also common activators in numerous Suzuki coupling reactions. The earliest example could probably be found in the Kishi s palytoxin synthesis. Silver oxide as well as thallium hydroxide provided dramatic rate enhancements in the couplings of vinylboronic acids (Scheme 10.41).69 Both thallium and silver ions are most probably abstracting halide in palladium intermediates, but silver is clearly the most efficient. Moreover, with the right counterion, the silver salt also acts as a mild base and activator. 

Fluoride abstraction requires a tertiary C-F bond which has been termed the Achilles heel of saturated fluorocarbons [68]. The reasons for this puzzling difference in reactivity are not yet clear but this observation is important to consider given the widespread use of these reagents to abstract halides in organic and organometallic synthesis. 

The action of Lewis acids in abstracting halide (and other leaving groups), the action of Lewis acids on cocatalysts to increase H+ activity, and the change in infrared spectra of oxygenated organic compounds in the presence of Lewis acids all embody the same overlap with sp filled orbitals. Measurements of relative acidities of Lewis acids based on these three categories of phenomena should and do show a parallelism (Table IV). 

Section 5 15 Dehydrohalogenation of alkyl halides by alkoxide bases is not compli cated by rearrangements because carbocations are not intermediates The mechanism is E2 It is a concerted process m which the base abstracts a proton from the p carbon while the bond between the halogen and the a carbon undergoes heterolytic cleavage... 

FIGURE 8 11 When a Lewis base reacts with an alkyl halide either substitution or elimination can occur Sub stitution (Sn2) occurs when the Lewis base acts as a nu cleophile and attacks carbon to displace bromide Elimi nation (E2) occurs when the Lewis base abstracts a pro ton from the p carbon The alkyl halide shown is iso propyl bromide and elimi nation (E2) predominates over substitution with alkox ide bases... 

Acetylenic Grignard reagents of the type RC CMgBr are prepared not from an acetylenic halide but by an acid-base reaction in which a Grignard reagent abstracts a proton from a terminal aUcyne... 

Dihalocarbenes are formed when trihalomethanes are treated with a strong base such as potassium tert butoxide The trihalomethyl anion produced on proton abstraction dissociates to a dihalocarbene and a halide anion... 

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. 

Base-catalyzed proton abstraction from trimethylsulfoxonium halides (1) with formation of dimethylsufoxonium methylide (2) was described by Corey in 1962. Solutions of (2) in dimethyl sulfoxide are conveniently prepared from the chloride or iodide (1) by stirring with one equivalent of sodium hydride at room temperature. 

The anion produced by proton abstraction from ethyl acetoacetate is nucleophilic. Adding an alkyl halide to a solution of the sodium salt of ethyl acetoacetate leads to alkylation of the a carbon. 

Monomeric thiazyl halides can be stabilized by coordination to transition metals and a large number of such complexes are known (Section 7.5). In addition, NSX monomers undergo several types of reactions that can be classified as follows (a) reactions involving the n-system of the N=S bond (b) reactions at the nitrogen centre (c) nucleophilic substitution reactions (d) halide abstraction, and (e) halide addition. Examples of each type of behaviour are illustrated below. 

The cation [NSO(NPCl2)2] (14.11) is the proposed intermediate in this ring-opening polymerization process. This cation is extremely reactive, as illustrated by the isolation of the solvent-derived product 14.12 when it is generated by halide abstraction from the cyclic precursor with AICI3 in l,2-dichloroethane. °... 

The converse of polymerization is heterolytic bond scission leading either to R3Sn+ or R3Sn species. Tricoordinate organotin(IV) cations can readily be synthesized at room temperature by hydride or halide abstraction reactions in benzene or other solvents.For example, with R = Me, Bu or Ph ... 

The inverse of these compounds are the phosphadiazenc cations, prepared by halide ion abstraction from diaminohalophosphoranes in CH2CI2 or SO2 solution, c.g. ... 

The strong halide-abstracting properties of acidic (X(A1C13) = 0.67 ... 

Osmium carbonyl (Os3(CO)i2) acts as a photoinitiator of vinyl polymerization [20], which can function without a halide additive. The mechanism of photoinitiation is by a hydrogen abstraction from monomer to pho-... 

The key step in the radical chain decomposition of alkanesulfonyl halides as well as in the adduct formation of sulfonyl halides with alkenes (vide infra) is equation 8 in which the R radical abstracts an X atom from the sulfonyl halide to regenerate a sulfonyl radical. 

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