Bases Lewis acid-base reactivity

Often the requisite THF oxonium ion is generated m situ by using a combination of reagents based on the Meerwein syntheses of trialkyl oxonium salts (150). These combinations include epichlorohydrin or a reactive haUde with a Lewis acid, a reactive hahde with a metal salt, or sometimes just a Lewis acid alone. The epoxide portion is often referred to as a promoter. 

Borane is very reactive because the boron atom has only six electrons in its valence shell. In tetrahydrofuran solution, BH3 accepts an electron pair from a solvent molecule in a Lewis acid-base reaction to complete its octet and form a stable BH3-THF complex. 

Many years ago, geochemists recognized that whereas some metallic elements are found as sulfides in the Earth s crust, others are usually encountered as oxides, chlorides, or carbonates. Copper, lead, and mercury are most often found as sulfide ores Na and K are found as their chloride salts Mg and Ca exist as carbonates and Al, Ti, and Fe are all found as oxides. Today chemists understand the causes of this differentiation among metal compounds. The underlying principle is how tightly an atom binds its valence electrons. The strength with which an atom holds its valence electrons also determines the ability of that atom to act as a Lewis base, so we can use the Lewis acid-base model to describe many affinities that exist among elements. This notion not only explains the natural distribution of minerals, but also can be used to predict patterns of chemical reactivity. 

TiIV-based Lewis acids are effective in ring-forming reactions such as Diels-Alder reactions (Scheme 25).94 Besides the usual TiX4 compounds (X = halide or alkoxide), Cp2Ti(OTf)2 is also a reactive catalyst for the Diels-Alder reaction,95 and it has been reported that [( PrO)2Ti(bpy)(OTf) ( PrOLQXOTf) is even more effective than Cp2Ti(OTf)2.96 In asymmetric synthesis, reactions with chiral dienophiles have been widely investigated. 

This review has described the synthesis, structure and reactivity of important classes of group 13/15 compounds such as Lewis acid base adducts and heterocycles. In addition, their potential to serve as single source precursors for the deposition of the corresponding binary materials by MOCVD process has been demonstrated. Because of the large number of compounds containing the lighter elements of group 15, N, P and As, these... 

Some experimental evidences are in agreement with this proposed mechanism. For example, coordinating solvents like diethyl ether show a deactivating effect certainly due to competition with a Lewis base (149). For the same reason, poor reactivity has been observed for the substrates carrying heteroatoms when an aluminum-based Lewis acid is used. Less efficient hydrovinylation of electron-deficient vinylarenes can be explained by their weaker coordination to the nickel hydride 144, hence metal hydride addition to form key intermediate 146. Isomerization of the final product can be catalyzed by metal hydride through sequential addition/elimination, affording the more stable compound. Finally, chelating phosphines inhibit the hydrovinylation reaction. 

The reactivity of the closely related system TpMe2PtMeH2 toward electrophiles in arene solvents has also been reported recently (68). The boron-based Lewis acid B(C6F5)3 induced elimination of methane and formation of an aryl(dihydrido) platinum(IV) complex via arene C-H activation (Scheme 17, A -> C). The active acid may be either B(C6F5)3 or alternatively a proton generated from B(C6F5)3 and trace water. It was proposed that the acid coordinates to a pyrazole nitrogen (shown in Scheme 17, B) forming an intermediate five-coordinate platinum(IV) complex, which readily eliminates methane. 

The mineral surface may be considered as a solid source of Lewis and/or BrfSnsted acidity and the reactive sites S as localized acidic or basic functional groups. Reactions involving such sites may be understood in terms of Lewis acid/base or BrfSnsted acid/base interactions ( 1, 5, 6, 8, 38). As the acidity of the reactive sites increases, increasingly weak bases are neutralized and reactive surface complexes (A S) may be formed. The term "acidity" is often used in the broad sense of the word, including both BrjSnsted and... 

Because metal ions bind to and modify the reactivity and structure of enzymes and substrates, a wide spectrum of techniques has been developed to examine the nature of metal ions which serve as templates, redox-active cofactors, Lewis acids/bases, ion-complexing agents, etc. 

The binding of a metal ion to a ligand can be considered in terms of Lewis acid-base theory (Lewis, 1923 Allred and Rochow, 1958 Brown and Skowron, 1990) because, in accepting an electronic pair, the metal ion acts as a Lewis acid. When a metal ion coordinates a ligand, it can affect the electron distribution of the ligand and therefore its reactivity. 

Reactions other than Lewis acid-base associations/dissociations are frequently observed wit donor molecules, leading notably to solvolysis, oxygen or sulfur abstraction, insertion reaction and carbon-carbon coupling reactions. The tendency to form metal-element multiple bonds i remarkable in this respect the activation of dinitrogen by tantalum or niobium is unique. Th formation and chemistry of constrained reactive metallacycles open another promisin fast-developing area, on the frontier with organometallic chemistry. 

Its unique reactivity comes from the fact that borane first forms a Lewis acid-base complex with the acid and then a boron-carboxylate intermediate which increases the reactivity of the boron hydride and delivers the hydride by an intramolecular reaction. As such it provides a selective way to reduce acids and produce alcohols in the presence of most other functional groups. 

On the other hand, many research groups have focused their attention on the application of chiral iron-based Lewis acids for the generation of chiral products. The stereodifferentiation of these Lewis acid-based catalysts is in many cases moderate or negligible at ambient temperatures. To obtain good selectivity, only such reactants that still exhibit good reactivity at low temperatures can be employed. Typical... 

Chandrakumar, K.R.S. and Pal, S., A systematic study on the reactivity of Lewis acid-base complexes through the local hard-soft acid-base principle, J. Phys. Chem. A, 106, 11775-11781, 20002c. 

Lewis acids based on titanium tend to aggregate and form dimers which are usually more reactive than their monomeric precursors (cf., Chapter 2). The degree of aggregation depends on the solvent, temperature, and the ligands attached to titanium no dimerization was detected by cryoscopy at -95° C in CH2CI2 [174], However, kinetic measurements of isobutene and styrene polymerizations indicate that polymerization is second order in titanium chloride [175,176], perhaps due to formation of a low concentration of the more reactive dimer or more stable Ti2Cl9-anions. However, polymerizations performed at lower [TiCl4] were reported to be first order in titanium chloride [105]. 

In a Lewis acid-base reaction, a Lewis base donates an electron pair to a Lewis acid. Most of the reactions in organic chemistry involving movement of electron pairs can be classified as Lewis acid-base reactions. Lewis acid-base reactions illustrate a general pattern of reactivity in organic chemistry. 

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