Lewis acid catalysis Subject

In order to be able to provide answers to these questions, a Diels-Alder reaction is required that is subject to Lewis-acid catalysis in aqueous media. Finding such a reaction was not an easy task. Fortunately the literature on other Lewis-acid catalysed organic reactions in water was helpful to some extent... 

The addition of an enolsilane to an aldehyde, commonly referred to as the Mukaiyama aldol reaction, is readily promoted by Lewis acids and has been the subject of intense interest in the field of chiral Lewis acid catalysis. Copper-based Lewis acids have been applied to this process in an attempt to generate polyacetate and polypropionate synthons for natural product synthesis. Although the considerable Lewis acidity of many of these complexes is more than sufficient to activate a broad range of aldehydes, high selectivities have been observed predominantly with substrates capable of two-point coordination to the metal. Of these, benzy-loxyacetaldehyde and pyruvate esters have been most successful. 

Silyl enol ethers need Lewis acid catalysis for efficient Michael reactions, such as the more substituted (and conjugated) isomer 110 forming a 1,5-diketone 111 from cyclohexenone in good yield.39 This product 111 is a mixture of diastereoisomers as have been many of the products in this chapter. We have also seen some reactions giving single diastereoisomers but without explanation. It is high time that we addressed the question of stereoselectivity and this is the subject of the next chapter. 

The catalyzed cycloaddition of alkoxy or amino aldehydes has been the subject of extensive studies. The influence of different Lewis acids and protective groups on the diastereoselectivity has been investigated for various types of aldehydes and dienes. Induced and simple diastereoselectivities (endoiexo) in the [4 + 2] cycloaddition of aldehydes 1 are highly dependent on the Lewis acid applied as catalyst. Several reactions ofa-alkoxyaldehydes 1 to dienes 2 under Lewis acid catalysis have been performed to give adducts 3 and 435. Among the catalysts tested (see table below) the best results were achieved for the magnesium bromide catalyzed cycloaddition of 1 to several dienes. Adduct 3 was obtained as a single compound. 

Both activated and unactivated systems have been observed to react, and the reaction is subject to Lewis acid catalysis. 

Ene reactions. Chloral has been found to undergo the ene reaction with mono-, 1,1-di-, and 1,2-disubstituted alkenes. The ene reaction in general requires temperatures of about 100° to proceed and is usually not subject to acid catalysis however, in the chloral reaction, several Lewis acids have been found to be effective catalysts FeCU, AICI3, SnCU. 

Finally, if there could be a way in which in water selective ri Jt-coordination to the carbonyl group of an a,P-imsatLirated ketone can be achieved, this would be a breakthrough, since it would subject monodentate reactants to catalysis by hard Lewis acids ". ... 

Among the unique features of Sc(03SCF3)3 is its ability to function as a catalyst in hydroxylic solvents. Other dienophiles, including (V-acryloyloxazolidinones, also are subject to catalysis by Sc(03SCF3)3. Indium trichloride is another Lewis acid that can act as a catalyst in aqueous solution.40... 

Like so many other reactions, the ene reaction has been given new life by metal catalysis. The use of metals ranges from common Lewis acids, which simply lower the barrier of activation of the hetero-ene reactions to transition metal catalysts which are directly involved in the bond-breaking and -forming events, rendering reactions formal ene processes. This review is meant to serve as a guide to the vast amount of data that have accumulated in this area over the past decade (1994-2004). If a particular subject has been reviewed recently, the citation is provided and only work done since the time of that review is included here. Finally, the examples included within are meant to capture the essence of the field, the scope, limitations, and synthetic utility therefore, this review is not exhaustive. 

Allylic carboxylation. Diethyl oxomalonate (1) undergoes a thermal ene reaction with mono-, di-, and trisubstituted alkenes at 145 180°. The reaction is also subject to catalysis with Lewis acids, which can lead to a different ene product. The products are a-hydroxymalonic esters. The corresponding malonic acids are converted to carboxylic acids by bisdecarboxylation with NaI04 and a trace of pyridine- or with ceric ammonium nitrate (CAN). Diethyl oxomalonate then functions as an cnophilic equivalent of C02. 

Metal ions are Lewis acids and as such catalyze many reactions which are also subject to specific acid catalysis by the proton. Reactions in which metal ions are involved are often best described as metal ion-promoted reactions as the products of the reaction often remain bound to the metal ion. Although scattered references to metal ion-promoted reactions are to be found in the early literature it was not until the late 1950s that such reactions began to be studied in detail. A strong driving force has been the realization that some 30% of enzymes are metalloenzy-mes or require metal ions for activity. Many of the reactions dealt with in this article have been studied in an attempt to delineate possible mechanisms for enzymic processes. 

The application of IR spectroscopy to catalysis and surface chemistry was later developed in the fifties by Eischens and coworkers at Texaco laboratories (Beacon, New York) in the USA [7] and, almost simultaneously, by Sheppard and Yates at Cambridge University in the UK [8]. Mapes and Eischens published the spectra of ammonia chemisorbed on a silica-alumina cracking catalyst in 1954 [6], showing the presence of Lewis acid sites and also the likely presence of Br0nsted acid sites. Eischens, Francis and Pliskin published the IR spectra of carbon monoxide adsorbed on nickel and its oxide in 1956 [9]. Later they presented the results of an IR study of the catalyzed oxidation of CO on nickel at the First International Congress on Catalysis, held in Philadelphia in 1956 [10]. Eischens and Pliskin also published a quite extensive review on the subject of Infrared spectra of adsorbed molecules in Advances in Catalysis in 1958, where data on hydrocarbons, CO, ammonia and water adsorbed on metals, oxides and minerals were reviewed [11]. These papers evidence clearly the two tendencies observed in subsequent spectroscopic research in the field of catalysis. They are the use of probes to test the surface chemistry of solids and the use of spectroscopy to reveal the mechanism of the surface reactions. They used an in situ cell where the catalyst sample was... 

In 1986, Reetz et al. reported that chiral Lewis acids (B, Al, and ll) promoted the aldol reaction of KSA with low to good enantioselectivity [115]. The following year they also introduced asymmetric aldol reaction under catalysis by a chiral rhodium complex [116]. Since these pioneering works asymmetric aldol reactions of silyl enolates using chiral Lewis acids and transition metal complexes have been recognized as one of the most important subjects in modern organic synthesis and intensively studied by many synthetic organic chemists. 

Previous works have shown that the diacetylation of acyclic olefins or paraffins leads to the formation of 6-diketones which cyclise to pyrylium salts. 31,32 YhQ formation of 2 is striking. We have noted that a solution of monoketone 1, acetyl chloride and aluminium chloride was stable at room temperature. Consequently, we assume that the formation of diketone 2 does not involve the acylation of monoketone but requires the acylation of the /3, T-unsaturated ketone (5-acetyl-1-methylcyclopentene) or related species. We also observed the rearrangement of the -acetylated dienolate 4 in presence of aluminium chloride or titanium tetrachloride at low temperature to diketone 2. There is an increasing evidence that unsaturated groups undergo the particularly easy 1,5-sigmatropic rearrangement. Moreover, the 1,5-acyl shift was subjected to catalysis by Lewis acids. But the possibility of an intramolecular process was conclusively ruled out by a cross-over experiment with propionyl chloride (overall yield 85-90 %). 

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