Cations as Lewis Acids

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

Further investigations are under way concerning the properties of the cations as Lewis acids in stoichiometric and catalytic reactions. 

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. 

The electrophile 4 adds to the aromatic ring to give a cationic intermediate 5. Loss of a proton from 5 and concomitant rearomatization completes the substitution step. Subsequent hydrolysis of the iminium species 2 yields the formylated aromatic product 3. Instead of the highly toxic hydrogen cyanide, zinc cyanide can be used. The hydrogen cyanide is then generated in situ upon reaction with the hydrogen chloride. The zinc chloride, which is thereby formed, then acts as Lewis acid catalyst. 

When a complex ion is formed from a simple cation, the electron pairs required for bond formation come solely from the ligands. Reactions such as these, in which one species donates an electron pair to another, are referred to as Lewis acid-base reactions. In particular—... 

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). 

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 ... 

Hydroxy-L-prolin is converted into a 2-methoxypyrrolidine. This can be used as a valuable chiral building block to prepare optically active 2-substituted pyrrolidines (2-allyl, 2-cyano, 2-phosphono) with different nucleophiles and employing TiQ as Lewis acid (Eq. 21) [286]. Using these latent A -acylimmonium cations (Eq. 22) [287] (Table 9, No. 31), 2-(pyrimidin-l-yl)-2-amino acids [288], and 5-fluorouracil derivatives [289] have been prepared. For the synthesis of p-lactams a 4-acetoxyazetidinone, prepared by non-Kolbe electrolysis of the corresponding 4-carboxy derivative (Eq. 23) [290], proved to be a valuable intermediate. 0-Benzoylated a-hydroxyacetic acids are decarboxylated in methanol to mixed acylals [291]. By reaction of the intermediate cation, with the carboxylic acid used as precursor, esters are obtained in acetonitrile (Eq. 24) [292] and surprisingly also in methanol as solvent (Table 9, No. 32). Hydroxy compounds are formed by decarboxylation in water or in dimethyl sulfoxide (Table 9, Nos. 34, 35). 

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. 

This reaction sequence of conjugate reduction followed by aldol reaction is known as the reductive aldol reaction. In certain instances, reductive elimination from the M-TM-enolate species may occur to furnish M-enolate, which itself may participate in the aldol reaction (Scheme 3). This detour may be described as the background path or stepwise path in one-pot. Indeed, it has been reported that certain cationic Rh complexes such as [Rh(COD)(DPPB)] (COD = 1,5-cyclooctadiene, DPPB = diphenylphosphinobutane) catalyze the aldol reactions of silyl enol ethers and carbonyl compounds by serving as Lewis acids [5-8]. 

Reactions of P-halogen-NHPs with Lewis acids leading to halide abstraction, or metathetic replacement of the halide by a nucleofugic, noncoordinating anion form by far the most important routes to access phosphenium ions in general [51, 52] and also 1,3,2-diazaphospholenium cations in particular. As Lewis acid assisted P-X... 

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). 

I 8.5 Cationic Zirconocene Complexes 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. 

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