Lewis acids metal cations

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)... 

All these compounds are thermally stable and therefore are good solvents for solvothermal reactions. Since the amino group also donates its lone-pair electrons to Lewis acid (metal cation), metal cations are stabilized and triethanolamine is a tetradentate hgand. Laine et al. reported that some metal oxides and hydroxides... 

The methanolysis, azidolysis, and aminolysis of epoxy benzyl ethers and epoxy alcohols have been reported <93JOCl22l>. All the epoxides studied showed a tendency toward C-3 selectivity when a Lewis-acidic metal cation (Li+, Mg2+, or Zn2+) was added to the reaction mixture, suggesting that the nucleophilic attack in these instances is chelation-controlled (Equation (11), and see Table 2). 

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

Lewis add-base interactions are very common in chemistry and are often rather subtle. You are about to meet, in the next chapter, an important way of making C-C bonds by adding organometallics to carbonyl compounds, and in many of these reactions there is an interaction at some point between a Lewis acidic metal cation and a Lewis basic carbonyl group. 

Oxide surfaces are generally thought to consist of Br nsted acid (hydroxyl and basic (oxide and hydroxyl) sites as well as Lewis acid (metal cation) and basic (oxide) centres [3]. Probably the four high-surface-area oxides which have been used most in surface organometallic... 

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). 

Because of the fractional R-Li bond, clusters and polymers can reversibly form an open cluster, which traps the unsaturated substrate through multiple-point bonding (cf Schemes 10.5 and 10.7). Lithium cations assist the electron flow from the cuprate to the electrophile and, to achieve such cooperative action, a cluster of a particular size may be necessary. Lewis acid metals other than lithium (Zn , for example) will also play similar roles. 

Having generated suitable (partially) cationic, Lewis acidic metal centers, several factors need to be considered to understand the progress of the alkene polymerisation reaction the coordination of the monomer, and the role (if any) of the counteranion on catalyst activity and, possibly, on the stereoselectivity of monomer enchainment. Since in d° metal systems there is no back-bonding, the formation of alkene complexes relies entirely on the rather weak donor properties of these ligands. In catalytic systems complexes of the type [L2M(R) (alkene)] cannot be detected and constitute structures more closely related to the transition state rather than intermediates or resting states. Information about metal-alkene interactions, bond distances and energetics comes from model studies and a combination of spectroscopic and kinetic techniques. 

Frequently substantially more than catalytic amounts of a Lewis acid metal halide are required to effect Friedel-Crafts alkylation. This is due partly to complex formation between the metal halide and the reagents or products, especially if they contain oxygen or other donor atoms. Another reason is the formation of red oils. Red oils consist of protonated (alkylated) aromatics (i.e., arenium ions) containing metal halides in the counterions or complexed with olefin oligomers. This considerable drawback, however, can be eliminated when using solid acids such as clays,97 98 zeolites (H-ZSM-5),99,100 acidic cation-exchange resins, and perfluoro-alkanesulfonic acid resins (Nafion-H).101-104... 

To study the catalytic performances of the Lewis acid metal compounds for the soybean methanolysis in homogeneous conditions, we prepared complexes using the ligand 3-hydroxy-2-methyl-4-pyronate (maltolate) and the cations Sn+2, Zn+2, Pb+2 and Hg+2, as depicted in Figure 4.1 (Abreu et al., 2003). 

Catalytic reactions of hydrocarbons over zeolites are reviewed. The historical development of various mechanistic proposals, particularly of the carbonium ion type, is traced. In spite of numerous catalytic, spectroscopic, and structural studies which have been reported concerning the possible roles of Bronsted acid, Lewis acid, and cationic sites, it still is not possible to formulate a comprehensive mechanistic picture. New activity and product data for cumene cracking and isotope redistribution in deuterated benzenes over Ca-and La-exchanged Y zeolites is presented. Cracking of the isomeric hexanes over alkali metal-exchanged Y and L zeolites has been studied. This cracking is clearly radical rather than carbonium-ion in nature but certain distinct differences from thermal cracking are described. 

In type a., the separating zeolite layer is equipped with catalytic sites (Bronsted add sites, Lewis acid sites (cations, special Al-sites), metal clusters, catalytic complexes). In type b., the non-supported side of the zeolite layer serves as a support for catalytic entities, e.g. metal crystallites. In type c., zeolite crystals with catalytic power are embedded in a matrix, e.g. a polymer membrane. 

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