Basicity Metal-Hydrogen Exchange

Because organolithium reagents are powerful bases,28V the metal-hydrogen exchange reaction with functionalized substrates plays an important role in many syntheses.288 jhis acid-base reaction is believed to 

Chapter 8. Nucleophilic Species That Form Carbon-Carbon Bonds 

Conjugative effects enhance the acidity of certain hydrogens.292a when the proton on a C—H bond is on an atom bearing a substituent that can delocalize electron density toward a more electronegative atom, that proton is more acidic. The conjugate base (the carbanion) formed after its removal is more stable because the 

d-Orbital effects are important only when atoms are present that are not in the second row of the periodic chart, such as sulfur, phosphorus, and transition metals. Carbanions can be stabilized by overlap of the electron pair with 3d orbitals of third row elements such as phosphorus and sulfur.300 Dithioacetal 288 is more acidic than acetal 289 by 5-6 pA a units. The carbanionic p orbital aligns itself to minimize electrostatic repulsion, as in 290.20 Forcing these orbitals into close proximity by confining them to a polycyclic system significantly decreases the acidity. Tris-sulfone (291) is less acidic than 292, but in this case solvation effects contribute to the difference in acidity.200,301 

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There are three general types of metallation reactions (metal-hydrogen exchange) that are commonly used to synthesize organoalkali metal compoimds from organic molecules Direct reaction with an alkali metal, reaction with an alkali metal hydride, or reaction with an organo- or amido-alkali metal. Since these reactions involve acid/base equilibria, they are dependent on both the C-H acidity of the organic molecule and the basicity of the alkali metal source. 

In the case of oxide catalysts or alkali metal-doped oxide catalysts, basic surface sites can be generated by decarboxylation of a surface metal carbonate exchange of hydroxyl hydrogen ions by electropositive cations thermal dehydroxylation of the catalyst surface condensation of alkali metal particles on the surface and reaction of an alkali metal with an anion vacancy (AV) to give centers (e.g., Na + AV — Na + e ). 

For cation exchange, both organic polymers and very insoluble inorganic materials can be utilized as the counter phase. The basic reactions involve exchange of (most often) hydrogen ions or alkali cations for polyvalent metal ions from the aqueous (or mixed aqueous/organic) solution. Electroneutrality must be maintained in the resin phase, as it is in the less polar organic phase in solvent extraction separations, so three equivalents of monovalent cations must be released when the trivalent lanthan-ide/actinide is bound by the resin. 

Base catalysis is most effective with alkali metals dispersed on solid supports or, in the homogeneous form, as aldoxides, amides, and so on. Small amounts of promoters form organoalkali comnpounds that really contribute the catalytic power. Basic ion exchange resins also are usebil. Base-catalyzed processes include isomerization and oligomerization of olefins, reactions of olefins with aromatics, and hydrogenation of polynuclear aromatics. 

Ion exchange, in which cation and/or anion resins are used to replace undesirable anionic species in liquid solutions with nonhazardous ions. For example, cation-exchange resins may contain nonhazardous, mobile, positive ions (e g., sodium, hydrogen) which are attached to immobile acid groups (e.g., sulfonic or carboxylic). Similarly, anion-exchange resins may include nonhazardous, mobile, negative ions (e.g., hydroxyl or chloride) attached to immobile basic ions (e.g., amine). These resins can be used to eliminate various species from wastewater, such as dissolved metals, sulfides, cyanides, amines, phenols, and halides. 

As discussed in the previous section, metal oxides have both acidic and basic properties. The acid-base properties of metal oxides have led to many interesting catalytic reactions. Catalytic reactions such as H2-D2 exchange, hydrogenation, isomerization, dehydrogenation, dehydrohalo-genation, and benzylation can be considered as examples of acid-base catalysis reactions.31-36 These reactions will be briefly discussed in the following section. The remarkable properties of MgO as a catalyst have been well documented in the literature and we shall discuss some of these unique catalytic properties. 

The next homologues are 1- and 2-butyne, where similar isomerizations have been observed [20] a recent report describes the reaction on a basic, alkali metal-exchanged zeolite [21]. As an unexpected product, an allene was obtained in reactions with hydrogen and a samarium catalyst [16, 22]. 

Recent work (Brown and Pearsall, 15) has indicated that while hydrogen aluminum tetrachloride is nonexistent, interaction of aluminum chloride and hydrogen chloride does occur in the presence of substances (such as benzene and presumably, olefins) to which basic properties may be ascribed. It may be concluded that while hydrogen aluminum tetrachloride is an unstable acid, its esters are fairly stable. Further evidence in support of the hypothesis that metal halides cause the ionization of alkyl halides (the products of the addition of the hydrogen halide promoters to the olefins) is found in the fact that exchange of radioactive chlorine atoms for ordinary chlorine atoms occurs when ferf-butyl chloride is treated with aluminum chloride containing radioactive chlorine atoms the hydrogen chloride which is evolved is radioactive (Fair-brother, 16). 

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