Lewis 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 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. 

Removing dioxolane and oxathiolane-protecting groups using the CeCh H20/ Nal system as an efficient water-tolerant Lewis acid 03SL2101. 

SCHEME 12. A water-tolerant Lewis acid catalysed Diels-Alder reaction... 

Using sol-gel procedures, scandium triflate, a known water-tolerant Lewis acid, has been immobilised on a silica gel which had previously covalently functionalised with a sulphonic acid linker. The formation of the silica-gel supported species 41 and 42 is shown in Figure 43. 

BfCgFsls, an air-stable, water-tolerant Lewis acid catalyst, is applicable to conjugate additions of SEE and KSA to a,79-unsaturated ketones as well as aldol and Mannich-type reactions of silyl enolates [29]. 

Allylation using allylic organostannane can be performed in aqueous media catalyzed by water-tolerant Lewis acids such as scandium and lanthanide triflates... 

Water-tolerant Lewis acids, which can coordinate the reactants, catalyze reactions even in water, but their accelerative effect is less pronoimced than in the reactions in organic media, probably by preventing the second factor (hydrogenbonding enhancement in the transition state) to operate with the same eff icien-cy [27]. 

Taking into account the competitive hydrolysis of the silyl enol ether, this reaction is remarkable. The method was shown to be general and was extended to a variety of aldehydes and several a,j9-unsaturated carbonyl compounds giving uniformly 1,4-addition with aldehydes and a mixture of 1,4- and 1,2-adducts in the case of ketones [187]. Later, this aqueous version of the Mukaiya-ma reaction was shown to give near quantitative yields in the presence of a water-tolerant Lewis acid such as ytterbium triflate [188]. Keeping with the same concept,copper(II) triflate [189],indium(III) trichloride [190],tris(pentafluoro-phenyl)boron [191] and scandium(III) triflate in the presence of a surfactant [192] have proved to be active catalysts. 

The demand for environmentally friendly chemistry and its widespread applicability have made water an increasingly popnlar solvent for organic transformations. Mixtures of water and other solvents snch as tetrahydrofnran are now commonly anployed for a number of organic transformations. For instance, the Lewis acid catalysed aldol reaction of silyl enol ethers, commonly known as the Mnkaiyama aldol reaction, which was firstly reported in the early seventies, can be carried ont in snch media. With titanium tetrachloride as the catalyst this reaction proceeds regioselectively in high yields, but the reaction has to be carried ont strictly nnder non-aqneons conditions in order to prevent decomposition of the catalyst and hydrolysis of the sUyl enol ethCTS. In the absence of the catalyst it was observed that water had a beneficial influence on this process (Table 4, entry D) . Nevertheless, the yields in the nncatalysed version WCTe still unsatisfactory. Improved results were obtained with water-tolerant Lewis acids. The first reported example for Lewis acid catalysis in aqueous media is the hydroxymethylation of silyl enol ethers with commercial formaldehyde solution using lanthanide trillates. In the meantime, the influence of several lanthanide triflates in cross-aldol reactions of various aldehydes was examined " " ". The reactions were most effectively carried out in 1 9 mixtures of water and tetrahydrofnran with 5-10% Yb(OTf)3, which can be reused after completion of the reaction (Table 19, entry A). Although the realization of this reaction is quite simple, the choice of the solvent is crucial (Table 20). 

---