Lewis water-stable

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. 

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. 

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... 

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). 

Bismuth(m) salts, such as BiCl3, BiBr3, Bi(OCOR)3, Bi(N03)3, Bi(OTf)3, and Bi(NTf2)3, have been used as Lewis acid catalysts to mediate a variety of carbon-carbon bond-forming reactions.85 In some cases, true catalysts differ from the bismuth salts initially added. The most effective and frequently used catalyst is Bi(OTf)3, which is obtained as a hydrated or dehydrated form depending on the preparation methods.86,86a,86b Like lanthanide triflates, Bi(OTf)3 is water stable and reusable. 

Because of the extensive amount of waste generated in traditional Friedel-Crafts reactions, it is not surprising that this reaction has been studied in RTIL. Early examples included the use of catalytic chloroaluminate ionic liquids. However, the moisture sensitivity of such systems was a drawback. Therefore, water-stable rare-earth Lewis acids, such as Sc(CF3S03)3, have come to be used for these reactions.The same Lewis acid has also been used to catalyse Diels-Alder reactions in RTILs.Interestingly, in this example, the RTIL not only provided a means for recycling the catalyst but also accelerated the rate and improved selectivity. It has also been demonstrated that a moisture stable, Lewis acidic, catalytic ionic liquid could be prepared from choline chloride and zinc dichloride, and that this was an excellent medium for the Diels-Alder reaction. Yields of 90% or more were achieved in reaction times of between 8 min and 5h for a range of dienes and dienophiles. 

Yb(OTf)3 and Sc(OTf)3 are water-stable Lewis acids [73], and when combined with a surfactant such as SDS, the three-component coupling also proceeded ef-... 

On the other hand, in the course of our investigations to develop new synthetic methods, we have found that rare earth metal triflates [Sc(OTf)3, Yb(OT03, ] [2] and some other metal salts can be used as water-stable Lewis acids for activation of C=0 and C=N groups in water-containing solvents. 

While aldol reactions stated above were smoothly catalyzed by the water-stable Lewis acids in aqueous media, a certain amount of organic solvent such as THF or EtOH had to be still combined with water to promote the reactions efficiently. To avoid the use of the organic solvents, we have developed a new reaction system in which Sc(OTf)3 catalyzes Mukaiyama aldol reactions in pure water without any organic solvents in the presence of a small amount of a surfactant such as sodium dodecyl sulfate (SDS). 

The original methods for directed aldol and aldol-type reactions of aldehydes and acetals with silyl enolates required a stoichiometric amount of a Lewis acid such as TiCh, Bl i-OI y, or SnCl.j [18]. Later studies have introduced many Lewis acids which accelerate these processes with a catalytic quantity (vide infra). In addition, it has been found that fluoride ion sources also work as effective catalysts of the aldol reaction [19]. In the last decade, much attention has been paid for the development of diastereo- and enantioselective aldol reactions [20, 21], aqueous aldol reactions using water-stable Lewis acids [22], and novel types of silyl enolate with unique reactivity. 

Aqueous Aldol Reaction with Water-stable Lewis Acids... 

Kobayashi et al. have demonstrated fhat some metal salts (e.g. Fe(II), Cu(II), Zn(II), Cd(II), and Pb(II) perchlorates) other fhan rare earth metal salts are also water-stable Lewis acids and work as catalysts of fhe aqueous aldol reaction of SEE [75]. Metal salts wifh good catalytic activity have pKh values (/hydrolysis constant) from 4.3 to 10.08 and WERC (water exchange rate constant) greater than 3.2 X10 m s . If p/metal cations are readily hydrolyzed to give oxo-nium ions, which promote hydrolysis of SEE. Metal cations with pKh> 10.08 do not have sufficient Lewis acidity to promote the aldol reaction. When fhe WERC... 

Sinou and co-workers [73] studied the influence of different surfactants on the palladium-catalyzed asymmetric alkylation of l,3-diphenyl-2-propenyl acetate with dimethyl malonate in presence of potassium carbonate as base and non-water-soluble chiral ligands. Best results in activity and enatioselectivity (> 90% ee) were observed with 2,2 -bis(diphenylphosphino)-l,l -binaphthyl (BINAP) as ligand and cetyltrimethylammonium hydrogen sulfate as surfactant in aqueous medium. Water-stable Lewis acids as catalysts for aldol reactions were developed by Kobayashi and co-workers [74]. An acceleration of the reaction was indicated in presence of SDS as anionic surfactants. An additional promotion could be observed by combination of Lewis acid and surfactant (LASCs = Lewis acid-surfactant-combined catalysts) as shown in Eq. (3). Surfactant the anion of dodecanesulfonic acid. 

A recent report9 described the use of scandium triflate in conjunction with tributyltin cyanide, a more water stable cyanide source compared to TMSCN. These reaction conditions could be carried out in both organic and aqueous solutions. It was observed that rare earth triflates are stable Lewis acids in water. Thus, a variety of aldehydes 1 and amine 12 were converted to a-aminonitriles 13 in excellent yield. The spent reagent could be completely recovered and, along with the scandium reagent, recycled for subsequent use. 

The disadvantages associated with the use of chloroaluminate(iii) ionic liquids have led various authors to investigate alternative Lewis acidic ionic liquid media such as those based on zinc(ii) chloride [25], tin(ii) chloride [26] and indium(iii) chloride [27]. These ionic liquids are considerably more water stable although much less reactive than the chloroaluminate(iii) systems. 

Water-stable Lewis acids as catalysts for aldol reactions were developed by Kobayashi and co-workers [22]. A high promotion could be observed by combination of Lewis acid and surfactant (LASCs = Lewis acid-surfactant combined catalysts as shown in Eq. (2)). The surfactant is here the anion of dodecanesulfonic add. 

Ionic liquids can also exhibit superacidity (Brpnsted superacids are acids that are more acidic than pure H2SO4). The dissolution of gaseous HCl in acidic [emim][Cl]/AlCl3 (55mol% of Lewis acid) leads to a superacidic system that has similar properties to those of liquid HF and can be used for the protonation of arenes. The Brpnsted acidity of products dissolved in water-stable ionic liquids can be increased HNTf2 and TfOH display higher chemical activity in [bmim][NTf2] and [bmim][BF4] than in water. ... 

S. Kobayashi, Water Stable rare earth Lewis-acid catalysis in aqueous and organic solvents in organic synthesis in water, Paul A. Grieco, Ed., Blackie Academic and Professional, 1998, pp. 262-302. 

Water-stable rare-earth Lewis-acid catalysis in aqueous and organic solvents... 

WATER-STABLE RARE-EARTH LEWIS-ACID CATALYSIS... 

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