Metal Cations as Lewis Acids

Molecules as Lewis Acids Metal Cations as Lewis Acids... 

To remove an ion, we can use the fact that many metal cations are Lewis acids (Section 10.2). When a Lewis acid and a Lewis base react, they form a coordinate covalent bond and the product is called a coordination complex. In this section, we consider complexes in which the Lewis acid is a metal cation, such as Ag+. An example is the formation of Ag(NI 1,)2+ when an aqueous solution of the Lewis base ammonia is added to a solution of silver ions ... 

Cation- and anion-exchange resins are widely applied as catalysts, when reactions can be carried out at temperatures lower than 423 K [161-171], Cation-exchange resins in acid forms have Bronsted acid sites and when exchanged with metallic cations contain Lewis acid sites, while anion exchange resins forms base species to carry out base-catalyzed reactions [169], The principles previously described in this chapter for these sites in other catalysts, can be applied for the catalytic action of these polymers. 

Metal cations and other Lewis acids can replace protons as reagents/catalysts for carbonyl addition reactions. Metal cations, for example, are involved in hydride and organometallic addition reactions. Metal cations and Lewis acids are also key reagents in the aldol-type reactions that are considered in Section 7.7. 

Carbocationic Initiation of Alkene Polymerization Use of Cationic Transition Metal Complexes as Lewis-Acid Initiators Use of Cationic Organotransition Metal Complexes as Lewis-Acid Initiators... 

III. Use of Cationic Transition Metal Complexes as Lewis-Acid Initiators... 

Is the interaction between an ammonia ligand and a metal cation a Lewis acid-base interaction If so, which species acts as the Lewis acid ... 

Claisen rearrangement of lithium ester enolates which display characteristics of intramolecular chelation often proceed with reduced yields and the production of side products. Investigators have examined alternative metal cations or Lewis acids which offer increased stability, such as 121, and improvements of efficiency and yield. ... 

Lewis acids are defined as molecules that act as electron-pair acceptors. The proton is an important special case, but many other species can play an important role in the catalysis of organic reactions. The most important in organic reactions are metal cations and covalent compounds of metals. Metal cations that play prominent roles as catalysts include the alkali-metal monocations Li+, Na+, K+, Cs+, and Rb+, divalent ions such as Mg +, Ca +, and Zn, marry of the transition-metal cations, and certain lanthanides. The most commonly employed of the covalent compounds include boron trifluoride, aluminum chloride, titanium tetrachloride, and tin tetrachloride. Various other derivatives of boron, aluminum, and titanium also are employed as Lewis acid catalysts. 

Small, highly charged metal cations that can act as Lewis acids in water, such as Al3+ and Fe3+, produce acidic solutions, even though the cations themselves have no hydrogen ions to donate (Fig. 10.18). 

Removing electrons from a metal atom always generates vacant valence orbitals. As described in Chapter 20, many transition metal cations form complexes with ligands in aqueous solution, hi these complexes, the ligands act as Lewis bases, donating pairs of electrons to form metal-ligand bonds. The metal cation accepts these electrons, so it acts as a Lewis acid. Metal cations from the p block also act as Lewis acids. For example, Pb ((2 g) forms a Lewis acid-base adduct with four CN anions, each of which donates a pair of electrons Pb ((2 ( ) + 4 CN ((2 q) -> [Pb (CN)4] (a g)... 

Metal atoms and cations are Lewis acids. As valence electrons are removed from a metal atom, the remaining electron cloud undergoes an ever-larger pull from the nuclear charge. This decreases the size of the ion as well as its polarizability. Thus, Fe is softer than Fe " ", which is softer than Fe. ... 

The Lewis definition covers all AB cements, including the metal oxide/metal oxysalt systems, because the theory recognizes bare cations as aprotic acids. It is also particularly appropriate to the chelate cements, where it is more natural to regard the product of the reaction as a coordination complex rather than a salt. Its disadvantages are that the definition is really too broad and that despite this it accommodates protonic acids only with difficulty. 

Metal cations can lend electrophilic assistance to weaken the Pd—X bonds in the intermediate R-Pd —X. Either full fission of this bond, leading to the realization of a polar mechanism, or partial polarization, might take place. Soft Lewis acids (the cations of Cu, Ag, Tl) are used most often (see Chapter 9.8 for a discussion of how metal ions act as Lewis-acid catalysts). 

Judging from these findings, the mechanism of Lewis acid catalysis in water (for example, aldol reactions of aldehydes with silyl enol ethers) can be assumed to be as follows. When metal compounds are added to water, the metals dissodate and hydration occurs immediatdy. At this stage, the intramolecular and intermolecular exchange reactions of water molecules frequently occur. If an aldehyde exists in the system, there is a chance that it will coordinate to the metal cations instead of the water molecules and the aldehyde is then activated. A silyl enol ether attacks this adivated aldehyde to produce the aldol adduct. According to this mechanism, it is expected that many Lewis acid-catalyzed reactions should be successful in aqueous solutions. Although the precise activity as Lewis acids in aqueous media cannot be predicted quantitatively... 

In the course of our investigations to develop new chiral catalysts and catalytic asymmetric reactions in water, we focused on several elements whose salts are stable and behave as Lewis acids in water. In addition to the findings of the stability and activity of Lewis adds in water related to hydration constants and exchange rate constants for substitution of inner-sphere water ligands of elements (cations) (see above), it was expected that undesired achiral side reactions would be suppressed in aqueous media and that desired enanti-oselective reactions would be accelerated in the presence of water. Moreover, besides metal chelations, other factors such as hydrogen bonds, specific solvation, and hydrophobic interactions are anticipated to increase enantioselectivities in such media. 

When a metal oxide surface is exposed to water, adsorption of water molecules takes place as shown in Equation 2.1. Cation sites can be considered as Lewis acids and interact with donor molecules like water through a combination of ion-dipole attraction and orbital overlap. Subsequent protonation and deprotonation of the surface hydroxyls produce charged oxide surfaces as shown in Equation 2.2 and Equation 2.3, respectively ... 

In this chapter, we have discussed the application of metal oxides as catalysts. Metal oxides display a wide range of properties, from metallic to semiconductor to insulator. Because of the compositional variability and more localized electronic structures than metals, the presence of defects (such as comers, kinks, steps, and coordinatively unsaturated sites) play a very important role in oxide surface chemistry and hence in catalysis. As described, the catalytic reactions also depend on the surface crystallographic structure. The catalytic properties of the oxide surfaces can be explained in terms of Lewis acidity and basicity. The electronegative oxygen atoms accumulate electrons and act as Lewis bases while the metal cations act as Lewis acids. The important applications of metal oxides as catalysts are in processes such as selective oxidation, hydrogenation, oxidative dehydrogenation, and dehydrochlorination and destructive adsorption of chlorocarbons. 

We have recently shown that metal-exchanged zeolites give rise to carbocationic reactions, through the interactions with alkylhalides (metal cation acts as Lewis acid sites, coordinating with the alkylhalide to form a metal-halide species and an alkyl-aluminumsilyl oxonium ion bonded to the zeolite structure, which acts as an adsorbed carbocation (scheme 2). We were able to show that they can catalyze Friedel-Crafts reactions (9) and isobutane/2-butene alkylation (70), with a superior performance than a protic zeolite catalyst. 

Clay Minerals as Lewis Acids. Lewis acid sites in a clay mineral are exchangeable (2) or structural ( 0) transition metal cations in the higher valence state, such as Fe + and Cu +, and octahedrally coordinated aluminum exposed at the crystal edges (38). Reduction of both exchanged and structural (octahedral) transition metal cations in the upper oxidation state is a reversible process (12,... 

Abstract The term Lewis acid catalysts generally refers to metal salts like aluminium chloride, titanium chloride and zinc chloride. Their application in asymmetric catalysis can be achieved by the addition of enantiopure ligands to these salts. However, not only metal centers can function as Lewis acids. Compounds containing carbenium, silyl or phosphonium cations display Lewis acid catalytic activity. In addition, hypervalent compounds based on phosphorus and silicon, inherit Lewis acidity. Furthermore, ionic liquids, organic salts with a melting point below 100 °C, have revealed the ability to catalyze a range of reactions either in substoichiometric amount or, if used as the reaction medium, in stoichiometric or even larger quantities. The ionic liquids can often be efficiently recovered. The catalytic activity of the ionic liquid is explained by the Lewis acidic nature of then-cations. This review covers the survey of known classes of metal-free Lewis acids and their application in catalysis. 

This review will concentrate on metal-free Lewis acids, which incorporate a Lewis acidic cation or a hypervalent center. Lewis acids are considered to be species with a vacant orbital [6,7]. Nevertheless, there are two successful classes of organocatalysts, which may be referred to as Lewis acids and are presented in other chapter. The first type is the proton of a Brpnsted acid catalyst, which is the simplest Lewis acid. The enantioselectivities obtained are due to the formation of a chiral ion pair. The other type are hydrogen bond activating organocatalysts, which can be considered to be Lewis acids or pseudo-Lewis acids. 

Aldolases are part of a large group of enzymes called lyases and are present in all organisms. They usually catalyze the reversible stereo-specific aldol addition of a donor ketone to an acceptor aldehyde. Mechanistically, two classes of aldolases can be recognized [4] (i) type I aldolases form a Schiff-base intermediate between the donor substrate and a highly conserved lysine residue in the active site of the enzyme, and (ii) type II aldolases are dependent of a metal cation as cofactor, mainly Zn, which acts as a Lewis acid in the activation of the donor substrate (Scheme 4.1). 

Modem work on these and related bare post-transition element clusters began in the 1960s after Corbett and coworkers found ways to obtain crystalline derivatives of these post-transition element clusters by the use of suitable counterions. Thus, crystalline derivatives of the cluster anions had cryptate or polyamine complexed alkali metals as countercations [8]. Similarly, crystalline derivatives of the cluster cations had counteractions, such as AlCLj, derived from metal halide strong Lewis acids [9]. With crystalhne derivatives of these clusters available, their structures could be determined definitively using X-ray diffraction methods. 

The side reaction of hydrogenolysis of the methyl-ruthenium intermediate to methane also may become predominant when the carbonyl insertion-methyl migration step of the process (Scheme 1) proceeds at a low rate. To reduce this drawback some Lewis acid promoters (i.e. metal alkali cations, classical Lewis acids such as AII3, SbCl etc.)... 

The rational design of chelating agents as antidotes requires a careful consideration of acid-base chemistry. Metal ions are Lewis acids, while the chelating agents or ligands are Lewis bases. The concepts of hardness and softness may be used to describe systematically the interaction between them. A hard metal cation is one that retains its... 

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