Lewis acid catalyzed reactions, water

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. 

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. 

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... 

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. 

Lewis acid catalysis in micellar systems [33] was first found in the model reaction in Table 14-6 [34]. While the reaction proceeded sluggishly in the presence of 0.2 eq. Yb(OTf)3 in water, remarkable enhancement of the reactivity was observed when the reaction was carried out in the presence of 0.2 eq. Yb(OTf)3 in an aqueous solution of sodium dodecyl sulfate (SDS, 0.2 eq., 35 mM), and the corresponding aldol adduct was obtained in 50% yield. In the absence of the Lewis acid and the surfactant (water-promoted conditions), only 20% yield of the aldol adduct was isolated after 48 h, while 33% yield of the aldol adduct was obtained after 48 h in the absence of the Lewis acid in an aqueous solution of SDS. The amount of the surfactant also influenced the reactivity, and the yield was improved when Sc(OTf)3 was used as a Lewis acid catalyst. Judging from the critical micelle concentration, micelles would be formed in these reactions, and it is noteworthy that the Lewis acid-catalyzed reactions proceeded smoothly in micellar systems [35]. It was also found that the surfactants influenced the yield, and that Triton X-100 was effective in the aldol reaction (but required long reaction time), while only a trace amount of the adduct was detected when using cetyltri-methylammonium bromide (CTAB) as a surfactant. 

Various metal salts such as rare earth metal triflates and copper triflate can function as Lewis acids in aqueous media. They can effectively activate aldehydes and imines in the presence of water molecules, and the first successful examples of Lewis acid-catalyzed reactions in aqueous solution have been demonstrated. Water-soluble aldehydes such as foimaldehyde could be employed directly in these reactions. Moreover, the catalysts could be easily recovered after the reactions were completed and could be reused. There are many kinds of Lewis acid-promoted reactions in industrial chemistry, and treatment of large amounts of the acids left over after the reactions have induced some severe environmental problems. From the standpoints of their catalytic use and reusability, the Lewis acids described in this chapter are expected to be new types of catalysts providing some solutions for these problems. 

Lewis acid-catalyzed reactions have been of great interest in organic synthesis because of their unique reactivities and selectivities, and for the mild conditions used [1]. Although various kinds of Lewis acid-promoted reactions have been developed and many have been applied in industry, these reactions must be carried out under strictly anhydrous conditions despite the general recognition of the utility of aqueous reactions [2]. The presence of even a small amount of water stops the reaction, because most Lewis acids immediately react with water rather than with the substrates and decompose or deactivate, and this fact has restricted the use of Lewis acids in organic synthesis. 

While the Lewis acid-catalyzed reactions of imines with silyl enolates are one of the most efficient methods for the preparation of b-amino esters, many imines are hygroscopic, unstable at high temperatures, and difficult to purify by distillation or column chromatography. It is desirable from a synthetic point of view that imines, generated in situ from aldehydes and amines, immediately react with silyl enolates and provide b-amino esters in a one-pot reaction. However, most Lewis acids cannot be used in this reaction because they decompose or deactivate in the presence of the amines and water that exist during imine formation. Due to the unique properties of Ln(OTf)3, their use as catalysts for the above one-pot preparation of b-amino esters from aldehydes was planned. 

The nse of water as a solvent in organic synthesis will play key roles in green chemistry. Despite the importance of Lewis acid-catalyzed reactions in laboratories as well as in industry, however, such reactions have not been carried ont in aqueons media, becanse Lewis acids were believed to hydrolyze rapidly in the presence of water. Contrary to this belief, we fonnd that rare earth and... 

Micellar media are formed from tensioactive molecules in aqueous solution. Mi-cellization is a manifestation of the strong self-association of water and water-like solvents [95]. Micelles are known to increase the solubilization of weakly polar substances in water and, as a consequence, their presence determines the magnitude of hydrophobic interactions. Micelles aggregate spontaneously in aqueous solution beyond a critical concentration which is a function of pressure [96]. As a result, pressure may induce an extra kinetic effect on the rate of organic reactions carried out in aqueous micellar systems. Representative ionic micelles are sodium dodecyl sulfate (SDS) and tetradecyltrimethylammonium bromide (TTAB). Recent examples demonstrate the beneficial effect of the presence of surfactants in Lewis acid-catalyzed reactions, a kind of biactivation [97]. 

To date, the focus has been on Lewis acid catalyzed reactions in water [149, 150]. In particular, Cu -catalyzed Diels-Alder (D-A), Michael addition, and fluorination reactions have been investigated (Fig. 13). 

Finally, the new concept of DNA-based asymmetric catalysis in which DNA serves as a chiral scaffold for catalyst design was described. To date, DNA-based asymmetric catalysis has been applied successfully to Lewis acid catalyzed reactions in water. This concept shows much promise for application in organic synthesis as it is inexpensive, experimentally straightforward, can be performed at a synthetically relevant scale, and the catalyst can be recycled readily. 

Another question people may ask is what kind of reactions can be carried out in water To put it simply, we know that most major reaction types that have historically been carried out in anhydrous organic solvent in organic chemistry, have their counterparts in water. Among these reactions types, the more notable ones are pericyclic reactions, organometallic reactions, transition metal catalyzedreactions,and lewis acid catalyzed reactions. 

Lewis acid-catalyzed reactions are of great interest due to their increased reactivity and selectivity under mild reaction conditions. A wide variety of reactions using Lewis acids have been developed, and they have been applied to the synthesis of natural and unnatural compounds. Traditionally, Lewis acids such as AICI3, BF3, TiCU, and SnCLt, have been employed in these reactions however, more than stoichiometric amounts of the Lewis acids are needed in many cases. Moreover, these Lewis acids are moisture sensitive and are easily decomposed or deactivated in the presence of even a small amount of water. Furthermore, these Lewis acids cannot be recovered and reused after the reactions are completed. In 1991, the first water-compatible Lewis acids, lanthanide triflates [Ln(OTf)3], was reported. ... 

Sc(0Tf)3 as a water-compatible Lewis acid catalyst has been reported to exhibit particularly high catalytic performances for a series of Lewis acid-catalyzed reactions the Friedel-Crafts alkylation, allylation reactions, Mukaiyama aldol condensation, and Mannich-type reaction [3,49,50]. The same metal triflate was reported to show catalytic activity for other Lewis acid-catalyzed reactions with carhonyl compounds (Equation (8.23)) in water. Its activity was indicated to he far superior to the other metal triflates, which was suggested as an indication that the high stahiUty of metal triflate-carhonyl compound complexes causes high catalytic performance for these reactions [7]. 

Iron is also considered as a water-insensitive metal. However, reports on the catalytic behavior of iron triflate are contradictory. Bonnamour reported that iron triflate is an efficient catalyst for the typical Lewis acid-catalyzed reactions, but is inactive for the reactions carried in water, as solvent [62]. On the other side, Choi et al. indicated that iron triflate, in situ... 

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... 

Jenner investigated the kinetic pressure effect on some specific Michael and Henry reactions and found that the observed activation volumes of the Michael reaction between nitromethane and methyl vinyl ketone are largely dependent on the magnitude of the electrostriction effect, which is highest in the lanthanide-catalyzed reaction and lowest in the base-catalyzed version. In the latter case, the reverse reaction is insensitive to pressure.52 Recently, Kobayashi and co-workers reported a highly efficient Lewis-acid-catalyzed asymmetric Michael addition in water.53 A variety of unsaturated carbonyl derivatives gave selective Michael additions with a-nitrocycloalkanones in water, at room temperature without any added catalyst or in a very dilute aqueous solution of potassium carbonate (Eq. 10.24).54... 

---