Lewis acids water tolerant

Interest in the aqueous medium spread quickly and many, sometimes surprising, discoveries were made [3]. Today pericyclic [4], condensation [5], oxidation [6] and reduction [7] reactions are routinely carried out in aqueous medium. The recent discovery of water-tolerant Lewis acids such as lanthanide triflates, Bi(OTf)j, Sc(OTf)j and Y(OTf)j has revolutionized organometallic chemistry [5a, 7]. 

In the case of Lewis acids, protic solvents such as water or alcohol can strongly influence their reactivity, cause it to react via an alternative path to the one desired, or even cause decomposition. Recently, rare earth metal triflates were used to develop water tolerant Lewis acids that can be used in many organic reactions. ... 

Rare Earth Metal Trifluoromethanesulfonates as Water-Tolerated Lewis Acid Catiaysts Organic Synthesis," Kobavashi. Synlett, 1994, 679... 

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... 

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). 

A variety of other transition-metal-based Lewis acids are used in organic transformations. Some of them are water tolerant and promising as sources of highly functionalized Lewis-acid catalysts. 

In the preceding paragraphs the advantages of water in uncatalysed Diels-Alder reactions were outlined. An important question is whether these advantages can be transferred to Lewis acid catalysed reactions as well. Since the majority of Diels-Alder reactants are likely to have a negligible tendency to interact with Lewis acid catalysts in water, this issue was addressed only recently. The first step was the development of water-tolerant catalytic systems for [4 + 2]-cycloadditions, and there are now a few examples of Lewis acid catalysed Diels-Alder reactions that not only tolerate the presence of small amounts of water281-284 but even benefit from it285. An example of a Lewis acid catalysed Diels-Alder reaction carried out in water THF mixture is presented in Scheme 12286. 

In this paragraph it was demonstrated that Lewis acid catalysis can be extended to aqueous media. Although water is likely to alter the complexation step, the use of Lewis acids is not restricted to organic solvents. Most importantly, the advantageous effects of Lewis acid catalysis and water are often additive. Since the development of catalytic systems which are water-tolerant or even benefit from the presence of water is still in its infancy, these results are highly promising and open new avenues for future research. 

In 1991, Cottier et al. classified the acid catalysts, known to be active for the production of HMF, in five groups organic acids, inorganic acids, organic and inorganic salts, Lewis acids, and others [60]. As reported in Table 1, heterogeneous catalysts are rarely used for the production of HMF mainly due to the difficulty to design a water-tolerant solid catalyst. 

As in conventional AlCl3-promoted acylations the ketone product forms a strong complex with the chloroaluminate IL. Lanthanide triflates, in particular Sc(OTf)3, have been widely studied as water-tolerant Lewis acids in a variety of transformations, including Friedel-Crafts alkylations and... 

The absorption of species from the atmosphere is common to all electrolyte solutions and clearly the absorption of water is the biggest issue. This is not solely confined to ionic liquids, however, as all electroplaters who deal with aqueous solutions of acids know, if the solution is not heated then the tank will overflow from absorption of atmospheric moisture after some time. In the aqueous acid the inclusion of water is not a major issue as it does not significantly affect the current efficiency or potential window of the solution. Water absorption is also not such a serious issue with eutectic-based ionic liquids and the strong Lewis acids and bases strongly coordinate the water molecules in solution. The presence of up to 1 wt.% water can be tolerated by most eutectic-based systems. Far from having a deleterious effect, water is often beneficial to eutectic-based liquids as it decreases the viscosity, increases the conductivity and can improve the rate of the anodic reaction allowing better surface finishes. Water can even be tolerated in the chloroaluminate liquids to a certain extent [139] and it was recently shown that the presence of trace HQ, that results from hydrolysis of the liquid, is beneficial for the removal of oxide from the aluminum anode [140]. 

The effectiveness of various substituted BINOL ligands 12-16 in the Zr(IV)-or Ti(IV)-catalyzed enantioselective addition of allyltributyltin to aldehydes was also investigated by Spada and Umani-Ronchi [21], The number of noteworthy examples of asymmetric allylation of carbonyl compounds utilizing optically active catalysts of late transition metal complexes has increased since 1999. Chiral bis(oxazolinyl)phenyl rhodium(III) complex 17, developed by Mo-toyama and Nishiyama, is an air-stable and water-tolerant asymmetric Lewis acid catalyst [23,24]. Condensation of allylic stannanes with aldehydes under the influence of this catalyst results in formation of nonracemic allylated adducts with up to 80% ee (Scheme 3). In the case of the 2-butenyl addition reac-... 

As mentioned earlier (Section 1.5) another example of novel catalysis in an aqueous medium is the use of lanthanide triflates as water-tolerant Lewis acid catalysts for a variety of organic transformations in water [39]. 

One approach is to incorporate Lewis acids into, for example, zeolites or me-soporous silicas [141]. For example, incorporation of Sn(IV) into the framework of zeolite beta afforded a heterogeneous water-tolerant Lewis acid [142]. It proved to be an effective catalyst for the intramolecular carbonyl-ene reaction of citronellal to isopulegol [143] (Fig. 2.43) in batch or fixed bed operation. Hydrogenation of the latter affords menthol (Fig. 2.43). 

Benzothiazolines 72 are prepared by means of gallium(III) triflate mediated condensation of 2-aminobenzenethiol with fluorinated ketones followed by cyclization <07OL179>. Gallium triflate is a stable, water tolerant, recoverable, reusable, and environmentally friendly Lewis acid. Even when used in catalytic amounts, gallium triflate provides sufficient acidity required for the synthesis of fluorinated benzothiazolines. 

Abstract This paper proposes new ways of preparation of hybrid silicones, i.e. an alternated multiblock seqnence of silicone and alkyl spacers, via a polycondensation process catalyzed by the tris(pentaflnorophenyl)borane, a water-tolerant Lewis acid, between methoxy and hydrogeno fnnctionalized silanes and siloxanes at room temperature and in the open air. The protocol was first developed with model molecules which led to polydimethylsiloxane (PDMS) chains, in order to seize the best experimental conditions. Several factors were studied such as the contents of each reactants, the nature of the solvent or the rate of addition. The best conditions were then adapted to the synthesis of hybrid silicones, condensing alkylated oligo-carbosiloxanes with methoxy or hydrogeno chain-ends and complementary small molecules. A systematic limitation in final molar masses of hybrid silicones was observed and explained by the formation of macrocycles, which cannot redistribnte or condense further while formed. 

Unlike main group Lewis acids, catalysts for the Diels-Alder reaction containing metals reviewed in this chapter are of recent origin. Major expectations in this area are linked to desirable properties such as catalyst stabiUty, selectivity and effectiveness, tolerance of impurities (e.g. water), and asymmetric modification. Over the past few years an increasing number of literature reports show this to be an area of high potential. 

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