Catalysis/catalysts Lewis acids

The stereochemistry of GTP of MMA polymerization was measured for Lewis acid as well as for bifluoride catalysis. Lewis acid catalysts gave a ratio of syndiotactic heterotactic triads of 2 1 while bifluoride catalysis gave ratios near 1 1 [9, 41]. The amount of isotactic triads was about 5%. The effect of temperature on triad and diad composition provided data to calculate the difference in activation enthalpy (AAH ) and activation entropy (AAS ) for... 

In summary, water is clearly an extremely bad solvent for coordination of a hard Lewis acid to a hard Lewis base. Hence, catalysis of Diels-Alder reactions in water is expected to be difficult due to the relative inefficiency of the interactions between the Diels-Alder reactants and the Lewis-acid catalyst in this medium. 

First, the use of water limits the choice of Lewis-acid catalysts. The most active Lewis acids such as BFj, TiQ4 and AlClj react violently with water and cannot be used However, bivalent transition metal ions and trivalent lanthanide ions have proven to be active catalysts in aqueous solution for other organic reactions and are anticipated to be good candidates for the catalysis of aqueous Diels-Alder reactions. 

In summary, ligands tend to diminish the affinity of the substrate for the Lewis-acid catalyst as well as the extent of activation by this catalyst, once the ternary complex is formed. Only a few examples of ligand-accelerated catalysis " have been described... 

The first example of enantioselective catalysis of a Diels-Alder reaction was reported in 1979 . Since then, an extensive set of successful chiral Lewis-acid catalysts has been prepared. Some selected examples will be presented here together with their mechanistic interpretation. For a more complete... 

As anticipated from the complexation experiments, reaction of 4.42 with cyclopentadiene in the presence of copper(II)nitrate or ytterbium triflate was extremely slow and comparable to the rate of the reaction in the absence of Lewis-acid catalyst. Apparently, Lewis-acid catalysis of Diels-Alder reactions of p-amino ketone dienophiles is not practicable. 

In summary, we have demonstrated that it is possible to extend the scope of Lewis-acid catalysis of Diels-Alder reactions in water, by employing a chelating auxiliary. We envisage that analogues of 4.39 capable of undergoing a Mamrich reaction with 4.50 can be treated with reactive dienes in the presence of a Lewis-acid catalyst in water. 

First of all, given the well recognised promoting effects of Lewis-acids and of aqueous solvents on Diels-Alder reactions, we wanted to know if these two effects could be combined. If this would be possible, dramatic improvements of rate and endo-exo selectivity were envisaged Studies on the Diels-Alder reaction of a dienophile, specifically designed for this purpose are described in Chapter 2. It is demonstrated that Lewis-acid catalysis in an aqueous medium is indeed feasible and, as anticipated, can result in impressive enhancements of both rate and endo-exo selectivity. However, the influences of the Lewis-acid catalyst and the aqueous medium are not fully additive. It seems as if water diminishes the catalytic potential of Lewis acids just as coordination of a Lewis acid diminishes the beneficial effects of water. Still, overall, the rate of the catalysed reaction... 

Chapter 5 also demonstrates that a combination of Lewis-acid catalysis and micellar catalysis can lead to accelerations of enzyme-like magnitudes. Most likely, these accelerations are a consequence of an efficient interaction between the Lewis-acid catalyst and the dienophile, both of which have a high affinity for the Stem region of the micelle. Hence, hydrophobic interactions and Lewis-acid catalysis act cooperatively. Unfortunately, the strength of the hydrophobic interaction, as offered by the Cu(DS)2 micellar system, was not sufficient for extension of Lewis-acid catalysis to monodentate dienophiles. 

Lewis acids are defined as molecules that act as electron-pair acceptors. The proton is an important special case, but many other species can play an important role in the catalysis of organic reactions. The most important in organic reactions are metal cations and covalent compounds of metals. Metal cations that play prominent roles as catalysts include the alkali-metal monocations Li+, Na+, K+, Cs+, and Rb+, divalent ions such as Mg +, Ca +, and Zn, marry of the transition-metal cations, and certain lanthanides. The most commonly employed of the covalent compounds include boron trifluoride, aluminum chloride, titanium tetrachloride, and tin tetrachloride. Various other derivatives of boron, aluminum, and titanium also are employed as Lewis acid catalysts. 

Lewis acid-base mechanism, 233, 234 Lewis acid catalysts, 13, 546 Lewis-acid-catalyzed ammonolysis, of nylon, 571 Lewis acids, 224 catalysis by, 68-69 Lewis bases, 338... 

Cationic bis(oxazoline) and pyridil-bis(oxazoline) Cu(ll) and Zn(ll) Lewis-acid catalysts. A comparative study in catalysis of Diels-Alder and aldol reactions [101]... 

Cycloaddition of 2-cyanoalk-2-enones with several conjugated dienes proceeded under zinc chloride catalysis.636 Zinc halides have also shown reactivity with phenylacetylenes.637 Zinc chloride is an effective Lewis acid catalyst in the Diels Alder reactions of the keto esters and the effects on stereochemistry of catalysts used have been examined.638... 

Traditionally, the production of LABs has been practiced commercially using either Lewis acid catalysts, or liquid hydrofluoric acid (HF).2 The HF catalysis typically gives 2-phenylalkane selectivities of only 17-18%. More recently, UOP/CEPSA have announced the DetalR process for LAB production that is reported to employ a solid acid catalyst.3 Within the same time frame, a number of papers and patents have been published describing LAB synthesis using a range of solid acid (sterically constrained) catalysts, including acidic clays,4 sulfated oxides,5 plus a variety of acidic zeolite structures.6"9 Many of these solid acids provide improved 2-phenylalkane selectivities. 

In the next step of the sequence, the authors sought to introduce a hydroxy-methylene substituent at the unsubstituted 7-position of the enone. This bond construction can be carried out by conducting a Baylis-Hillman reaction with formaldehyde. In this instance, the authors used a modification of the Baylis-Hillman reaction which involves the use of a Lewis acid to activate the enone [26]. Under these conditions, the enone 42 is treated with excess paraformaldehyde in the presence of triethylphosphine (1 equiv), lanthanum triflate (5 mol%), and triethanolamine (50 mol%). It is proposed that the lanthanum triflate forms a complex with the triethanolamine. This complex is able to activate the enone toward 1,4-addition of the nucleophilic catalysts (here, triethylphosphine). In the absence of triethanolamine, the Lewis acid catalyst undergoes nonproductive complexation with the nucleophilic catalyst, leading to diminution of catalysis. Under these conditions, the hydroxymethylene derivative 37 was formed in 70 % yield. In the next step of the sequence, the authors sought to conduct a stereoselective epoxidation of the allylic... 

The surfactant-aided Lewis acid catalysis was first noted1151 in the model reaction shown in Table 2. While the reaction proceeded sluggishly in the presence of 20 mol% Yb(OTf)3 in water, remarkable enhancement of the reactivity was observed when the reaction was carried out in the presence of 20 mol% Yb(OTf)3 in an aqueous solution of SDS (20 mol%, 35 mM). The corresponding aldol adduct was obtained in 50% yield. The yield was improved when Sc(OTf)3 was used as the Lewis acid catalyst. It was found that different kinds of surfactants influenced the product yield, and that TritonX-100, a neutral surfactant, was effective in the aldol reaction (but required long reaction time), while only a... 

Lewis acid catalysis. Anhydrous lanthanide(III) chlorides, particularly SmCl3, can function as low-cost but efficient nonhomogeneous Lewis acid catalysts for aldol and other reactions. More rapid reactions are observed when the soluble but expensive Eu(fod)3 is used as a lanthanide catalyst.1... 

O-stannylene acetals,84 glycals and 1,2-anhydrosugars,85 and to selective de-O-benzylation of position-2 with TIBAL, DIBAL-H57 or Lewis acid catalysts,86 or to the one-pot access to 3-O-benzyl-4,6-0-benzylidene glucosides by tandem catalysis recently reported.87 The 1,2-lactones recently reported by Linker and co-workers35 are also synthons which provide... 

Uncatalysed Diels-Alder reactions usually have to be carried out at relatively high temperatures (normally around 100 °C)73, often leading to undesired side reactions and retro-Diels-Alder reactions which are entropically favoured. The Diels-Alder reaction became applicable to sensitive substrates only after it was realized that Lewis acids (e.g. A Clg) are catalytically active56. As a consequence, Diels-Alder reactions can now be carried out at temperatures down to — 100°C85. The use of Lewis acid catalysts made the [4 + 2]-cycloaddition applicable to the enantioselective synthesis of many natural compounds51,86. Nowadays, Lewis acid catalysis is the most effective way to accelerate and to stereochemically control Diels-Alder reactions. Rate accelerations of ten-thousand to a million-fold were observed (Table 7, entries A and B). 

In summary, water appears as an extremely unsuitable solvent for coordination of hard Lewis acids to hard Lewis bases, as it strongly solvates both species and hinders their interaction. Hence, catalysis of Diels-Alder reactions in water is expected to be difficult due to the relative inefficiency of the interactions between the Diels-Alder reactants and the Lewis acid catalyst. On the other hand, the high stereoselectivities and yields observed in biosyntheses, with water as the solvent, indicate that these rationalizations cannot entirely be true. As a matter of fact, we will demonstrate in the following that Lewis acid catalysis in water is not only possible, but also allows for effective as well as environmentally friendly reaction conditions. 

Furthermore, the use of a Lewis acid promoter leads to increased stereoselectivities (Table 19, entry C)252,254. Compared to the aprotic reaction, where allyl silane was used instead of allyl bromide and indium chloride, an almost complete reversal of the diastereos-electivity was found. It was demonstrated recently that the Lewis acid catalysed allylation reaction can be carried out efficiently without any organic solvent in saturated ammonium chloride solution255. Finally, Lewis acid catalysed Mannich reactions can be carried out conveniently in mixtures of organic solvents and water. However, the exact role of the Lewis acid catalyst has not been clarified (Table 19, entry D)253. The same reaction can be carried out in pure water with catalysis by indium trichloride256. 

The reverse reaction is catalysed by copper sulphate in an ethanol/water (50 50) mixture297 298. Indium(III) chloride catalysis of Diels-Alder reactions was also reported, but the effects were poor and comparison to uncatalysed reactions was made only in a few cases299,300. A very versatile Lewis acid catalyst for such reactions is methylrhenium trioxide (MTO)300. This catalyst can be used without a solvent, in pure organic solvents like chloroform and even in pure water. While the catalyst is active in the latter two solvents (Table 22), it gives the best results in water (Table 23). 

In subsequent studies it has been found that a combination of Lewis-acid and micellar catalysis can lead to huge (in fact, enzyme like) rate acceleration in water. In the absence of Lewis-acid catalysts, micelles tend to inhibit Diels-Alder reactions, largely because of the particular nature of the substrate binding sites at the micelle. This problem can be solved by adding Lewis-acid catalysts that bind effectively at the micellar surface. 

Abstract The term Lewis acid catalysts generally refers to metal salts like aluminium chloride, titanium chloride and zinc chloride. Their application in asymmetric catalysis can be achieved by the addition of enantiopure ligands to these salts. However, not only metal centers can function as Lewis acids. Compounds containing carbenium, silyl or phosphonium cations display Lewis acid catalytic activity. In addition, hypervalent compounds based on phosphorus and silicon, inherit Lewis acidity. Furthermore, ionic liquids, organic salts with a melting point below 100 °C, have revealed the ability to catalyze a range of reactions either in substoichiometric amount or, if used as the reaction medium, in stoichiometric or even larger quantities. The ionic liquids can often be efficiently recovered. The catalytic activity of the ionic liquid is explained by the Lewis acidic nature of then-cations. This review covers the survey of known classes of metal-free Lewis acids and their application in catalysis. 

Nitrones are the most widely studied of the 1,3-dipoles in the field of catalyzed enantioselective 1,3-dipolar cycloaddition reactions. Effective catalysis using a variety of chiral Lewis acid catalysts has been reported for the nitrone cycloaddition... 

Engberts and co-workers (Otto et al., 1996) reported a detailed study of a Diels-Alder reaction that was catalyzed by Lewis acids in water. They presented the results of the effects of Co Ni, Cu and Zn ions as Lewis acid catalysts on the rate and endo-exo selectivity of the DA reaction between the bidentate dienophiles 3-phenyl-l-(2-pyridyl)-2-propen-l-ones and cyclopentadiene in water (see fig. 6.6). Relative to the uncatalyzed reaction in acetonitrile, catalysis by 0.010 M Cu(N03)2 in water accelerates the Diels-Alder reaction by a factor of 79,300. Water does not induce an enhanced endo-selectivity for this reaction. 

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