Lewis acids water-stable

In this chapter Lewis-acid-mediated reactions have been summarized. While more than stoichiometric amounts of the Lewis acids were employed in conventional reactions, many efforts have been made to reduce the amounts of Lewis acid needed, and many truly catalytic reactions have been developed. Chiral Lewis-acid catalysis has been of great interest in the 1990s and early 2000s, and various combinations of metals and ligands have been investigated. Importantly, the established understanding that Lewis acids are easily hydrolyzed in water has been exploded. Water-compatible Lewis acids are stable in air and moisture, and are easily recovered and reused in many cases. These Lewis acids may solve recent environmental issues, and will be used further in many reactions in future. 

S. Kobayashi, S. Nagayama, T. Busujima, Lewis Acid Catalysts Stable in Water. Correlation between Catalytic Activity in Water and Hydrolysis Constants and Exchange Rate Constants for Substitution of Inner-Sphere Water Ligands J. Am. Chern. Soc 1998, 120, 8287-8288. 

The pivaloylamidobenzyl group was stable to acetic acid-water-90°, MeOH-NaOMe, iridium-induced allyl isomerization, and to many of the Lewis acids used in glycosylation. ... 

These results indicate that, during thermolyses of fructose-containing saccharides, di-D-fructose dianhydrides are formed readily, but subsequent isomerization is extremely slow—even in the presence of added acid. However, under these conditions, the protonating power of any acid is moot. At the high temperatures used, residual water would be driven off rapidly, unless the reaction vessel is pressurized therefore, reaction occurs in the anhydrous melt. It is presumably protonation of one of the ring oxygen atoms in the dianhydrides that constitutes the first step in isomerization, followed by scission of a C-O bond to yield one of the oxocarbenium ion intermediates postulated in Refs. 31 and 80. Such ions have also been postulated as intermediates in the isomerization of spiroketals to a more-stable product. This latter isomerization can be extremely facile 104 dilute aqueous acid,120 or non-aqueous Lewis-acid conditions121 have been used to effect such transformations. 

Transition-metal-based Lewis acids such as molybdenum and tungsten nitro-syl complexes have been found to be active catalysts [49]. The ruthenium-based catalyst 50 (Figure 3.6) is very effective for cycloadditions with aldehyde- and ketone-bearing dienophiles but is ineffective for a,)S-unsaturated esters [50]. It can be handled without special precautions since it is stable in air, does not require dry solvents and does not cause polymerization of the substrates. Nitromethane was the most convenient organic solvent the reaction can also be carried out in water. 

As mentioned several times Lewis acids are highly valuable catalysts but the most commonly used ones such as aluminium chloride and boron trifluoride are highly water sensitive and are not usually recovered at the end of a reaction, leading to a significant source of waste. In recent years there has been much research interest in lanthanide triflates (trifluoro-methanesulfonates) as water stable, recyclable Lewis acid catalysts. This unusual water stability opens up the possibility for either carrying out reactions in water or using water to extract and recover the catalyst from the reaction medium. 

Si. rra(pentafluorophenyl)boron was found to be an efficient, air-stable, and water-tolerant Lewis-acid catalyst for the allylation reaction of allylsilanes with aldehydes.167 Sc(OTf)3-catalyzed allylations of hydrates of a-keto aldehydes, glyoxylates and activated aromatic aldehydes with allyltrimethylsilane in H2O-CH3CN were examined. a-Keto and a-ester homoallylic alcohols and aromatic homoallylic alcohols were obtained in good to excellent yields.168 Allylation reactions of carbonyl compounds such as aldehydes and reactive ketones using allyltrimethoxysilane in aqueous media proceeded smoothly in the presence of 5 mol% of a CdF2-terpyridine complex (Eq. 8.71).169... 

Lewis-Acid Catalyzed. Recently, various Lewis acids have been examined as catalyst for the aldol reaction. In the presence of complexes of zinc with aminoesters or aminoalcohols, the dehydration can be avoided and the aldol addition becomes essentially quantitative (Eq. 8.97).245 A microporous coordination polymer obtained by treating anthracene- is (resorcinol) with La(0/Pr)3 possesses catalytic activity for ketone enolization and aldol reactions in pure water at neutral pH.246 The La network is stable against hydrolysis and maintains microporosity and reversible substrate binding that mimicked an enzyme. Zn complexes of proline, lysine, and arginine were found to be efficient catalysts for the aldol addition of p-nitrobenzaldehyde and acetone in an aqueous medium to give quantitative yields and the enantiomeric excesses were up to 56% with 5 mol% of the catalysts at room temperature.247... 

Recently, water-tolerating Lewis acid has been used to catalyze various Diels-Alder reactions in aqueous media. An important aspect of the Diels-Alder reaction is the use of Lewis acids for the activation of the substrates. While most Lewis acids are decomposed or deactivated in water, Bosnich reported that [Ti(Cp )2(H20)2]2+ is an air-stable, water-tolerant Diels-Alder catalyst.35 A variety of different substrates were subjected to the conditions to give high yields and selectivity (Eq. 12.6). 

Kobayashi has found that scandium triflate, Sc(OTf)3,36 and lanthanide triflate, Ln(OTf)3, are stable and can be used as Lewis catalysts under aqueous conditions. Many other Lewis acids have also been reported to catalyze Diels-Alder reactions in aqueous media. For example, Engberts reported37 that the cyclization reaction in Eq. 12.7 in an aqueous solution containing 0.010 M Cu(N03)2 is 250,000 times faster than that in acetonitrile and about 1,000 times faster than that in water alone. Other salts, such as Co2+, Ni2+, and Zn2+, also catalyze the reaction, but not as effectively as Cu2+. However, water has no effect on the endo-exo selectivity for the Lewis-acid catalyzed reaction. 

Sc(OTf)3 is stable in water, and effectively activates carbonyl and related compounds as a Lewis acid in water. This is remarkable, because most Lewis acids react immediately with water rather than the substrates, and are decomposed or deactivated. It has already been found that lanthanide trifiates Ln(OTf)3 (Ln = La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu) and yttrium trifiate Y(OTf)3 are stable in water, and can act as Lewis-acid catalysts in aqueous media.46-48 They are used catalytically in many reactions and can often be recovered and reused, because they are stable under the usual water-quenching conditions. 

Indium trichloride349-351 is a mild Lewis acid that is effective for various kinds of Lewis-acid-catalyzed reactions such as Diels-Alder reactions (Scheme 85), aldol reactions, and Friedel Crafts reactions. Since indium trichloride is stable in water, several aqueous reactions have been investigated (Scheme 85) indium(III) triflate is also used as a Lewis acid. 

Concept Conventional Lewis acids are moisture-sensitive and easily deactr vated by water. On the other hand, water-stable Lewis acid catalysts have been developed recently. 

Lewis acids as water-stable catalysts have been developed. Metal salts, such as rare earth metal triflates, can be used in aldol reactions of aldehydes with silyl enolates in aqueous media. These salts can be recovered after the reactions and reused. Furthermore, surfactant-aided Lewis acid catalysis, which can be used for aldol reactions in water without using any organic solvents, has been also developed. These reaction systems have been applied successfully to catalytic asymmetric aldol reactions in aqueous media. In addition, the surfactant-aided Lewis acid catalysis for Mannich-type reactions in water has been disclosed. These investigations are expected to contribute to the decrease of the use of harmful organic solvents in chemical processes, leading to environmentally friendly green chemistry. 

With these results in hand, we have next introduced new types of Lewis acids, e.g scandium tris(-dodecyl sulfate) (4a) and scandium trisdodecanesul-fonate (5a) (Chart 1).[1S1 These Lewis acid-surfactant-combined catalysts (LASCs) were found to form stable colloidal dispersions with organic substrates in water and to catalyze efficiently aldol reactions of aldehydes with very water-labile silyl enol ethers. 

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. 

Since our first paper181 on Lewis add catalysis in aqueous media appeared, many investigations and results in this area have been reported. Water-stable Lewis adds are now becoming common and useful catalysts in organic synthesis. These catalysts have been applied to various types of Lewis acid-catalyzed reactions. 

Diels-Alder reactions are one of the most famous examples which are accderated by a Lewis acid. Various water-stable Lewis adds such as Ln(OTf)3,1371 methylrhenium trioxide,1381 copper nitrate,1391 copper bis(dodecyl sulfate) (4b),1401 indium chloride,1411 and bismuth triflate1421 have been used for Diels-Alder and aza-Diels-Alder reactions in water. Furthermore, a catalytic asymmetric Dids-Alder reaction in water using a copper complex of an amino... 

Given the prevalence of bis(oxazoline)-copper catalysts as chiral Lewis acids, it seems appropriate to comment briefly on catalyst preparations, since differences arise in the nature of the catalyst complex. Triflate-derived catalysts are formed simply by combining the ligand and Cu(OTf)2 in a given solvent and stirring for an appropriate length of time (typically >2 h) to achieve complete dissolution and complexation, Scheme 14. The hydrated version is formed by addition of 2 equiv of water to this catalyst solution, followed by removal of solvent after 15 min of stirring. The hydrated triflate catalyst is bench stable for months. 

Abstract Several bismuth-catalyzed synthetic reactions, which proceed well in aqueous media, are discussed. Due to increasing demand of water as a solvent in organic synthesis, catalysts that can be used in aqueous media are becoming more and more important. Although bismuth Lewis acids are not very stable in water, it has been revealed that they can be stabilized by basic ligands. Chiral amine and related basic ligands combined with bismuth Lewis acids are particularly useful in asymmetric catalysis in aqueous media. On the other hand, bismuth hydroxide is stable and works as an efficient catalyst for carbon-carbon bond-forming reactions in water. 

Due to increasing demands for optically active compounds, many catalytic asymmetric reactions have been investigated in this decade. However, asymmetric catalysis in water or water/organic solvent systems is difficult because many chiral catalysts are not stable in the presence of water [19]. In particular, chiral Lewis acid catalysis in aqueous media is extremely difficult because most chiral Lewis acids decompose rapidly in the presence of water [20, 21]. To address this issue, catalytic asymmetric reactions using water-compatible Lewis acids with chiral ligands have been developed [22-29]. 

The aldolization, a major synthetic tool involving a nucleophilic addition to the carbonyl double bond, is usually conducted under either acidic or basic conditions (Denmark and Lee, 1992). The yields of the aqueous aldolization logically follow the electrophilicity of the aldehyde, rising up to 82% using p-nitrobenzaldehyde. It has been shown that these aqueous aldolizations can be catalyzed by water-stable Lewis acids such as ytterbium or other lanthanide inflates (Kobayashi and Hachiya, 1992). 

Compounds with a high HOMO and LUMO (Figure 5.5c) tend to be stable to selfreaction but are chemically reactive as Lewis bases and nucleophiles. The higher the HOMO, the more reactive. Carbanions, with HOMO near a, are the most powerful bases and nucleophiles, followed by amides and alkoxides. The neutral nitrogen (amines, heteroaromatics) and oxygen bases (water, alcohols, ethers, and carbonyls) will only react with relatively strong Lewis acids. Extensive tabulations of gas-phase basicities or proton affinities (i.e., —AG° of protonation) exist [109, 110]. These will be discussed in subsequent chapters. 

A carbocation is strongly stabilized by an X substituent (Figure 7.1a) through a -type interaction which also involves partial delocalization of the nonbonded electron pair of X to the formally electron-deficient center. At the same time, the LUMO is elevated, reducing the reactivity of the electron-deficient center toward attack by nucleophiles. The effects of substitution are cumulative. Thus, the more X -type substituents there are, the more thermodynamically stable is the cation and the less reactive it is as a Lewis acid. As an extreme example, guanidinium ion, which may be written as [C(NH2)3]+, is stable in water. Species of the type [— ( ) ]1 are common intermediates in acyl hydrolysis reactions. Even cations stabilized by fluorine have been reported and recently studied theoretically [127]. 

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