Reactivity Lewis acid-base

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. 

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... 

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... 

Aldol reactions using a carbocation as an organocatalyst An organocatalytic aldol reaction based on a different concept was developed by the Chen group. The chiral triarylcarbenium ion 34 was used as a novel non-metallic Lewis acid catalyst in a Mukaiyama-type aldol reaction which led to enantiomerically enriched aldol products (Scheme 6.17) [67]. Although non-chiral trityl salt-mediated catalytic aldol reactions had previously been reported by Mukaiyama and co-workers [68], the construction of a suitable chiral carbenium ion remained a challenge. Optically active salts of type 34 were synthesized as Lewis acids based on a reactive carbe-... 

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. 

When the surface of fully dehydrated MgO is contacted with ethylene oxide at RT (260), the first surface product formed is the a adduct between the ethylene oxide molecules and the most coordinatively unsaturated Mg2+ ions of the surface, presumably localized on edges, steps, and corners (hereafter Mg ) (Fig. 9 IR bands labeled A) (Scheme 6). This first step is not unexpected since ethylene oxide, with its medium base strength, acts as a surface probe for the most reactive Lewis acid sites (three- and fourfold coordinated Mg2+ ions). This precursor species transforms at RT into a truly chemisorbed species, leading to the formation of a structure plausibly suggested to be the cyclic structure represented below (bands B in Fig. 9) because of the nucleophilic attack of the basic coordinatively unsaturated oxygen ion (O ) in adjacent position on the CH2 group (Scheme 7). 

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]. 

Pyridines in their transition metal complexes coordinate as )] (N), q (C,C), (N,C), or q ligands (Figured). By far the most common coordination mode is (N), in which the lone electron pair on the nitrogen donates to a Lewis acidic metal center (see Lewis Acids Bases). The M-N bond lies in the plane of the pyridine ring (Figure 6). Few reactivity studies of (N) pyridine ligands have been reported. One of potential interest for HDN is the base-catalyzed exchange of the pyridine protons for deuterium... 

Cationic zirconocene complexes are well-known very reactive species with interesting applications owing to their Lewis acidic character see Lewis Acids Bases). In particular, they are often involved into various catalytic processes including Ziegler-Natta type polymerizations see Ziegler-Natta Catalysts). 

The reactivity of the cationic Zr complexes is a direct consequence of their Lewis acidity see Lewis Acids Bases) (i) various substitution reactions can occur into the Zr-solvent weak bond, (ii) unsatnrated substrates (CO, alkenes, alkynes, or ketones) insert into the Zr-C bond, potentially leading to polymerization reactions (see Section 8.2), (iii) new organic ligands obtained after reaction in the coordination sphere of the metal can be spontaneously released by /3-H elimination see -Hydride Elimination), or (iv) C-H bond activation of suitable ligands can occur. 

---