Lewis acids, and

This chapter introduces the experimental work described in the following chapters. Some mechanistic aspects of the Diels-Alder reaction and Lewis-acid catalysis thereof are discussed. This chapter presents a critical survey of the literature on solvent ejfects on Diels-Alder reactions, with particular emphasis on the intriguing properties of water in connection with their effect on rate and selectivity. Similarly, the ejfects of water on Lewis acid - Lewis base interactions are discussed. Finally the aims of this thesis are outlined. 

Appreciating the beneficial influences of water and Lewis acids on the Diels-Alder reaction and understanding their origin, one may ask what would be the result of a combination of these two effects. If they would be additive, huge accelerations can be envisaged. But may one really expect this How does water influence the Lewis-acid catalysed reaction, and what is the influence of the Lewis acid on the enforced hydrophobic interaction and the hydrogen bonding effect These are the questions that are addressed in this chapter. 

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. 

In Chapter 1 mechanistic aspects of Are Diels-Alder reaction are discussed. The literature on the effects of solvents and Lewis-acid catalysts on this reaction is surveyed. The special properties of water are reviewed and the effects of water on the Diels-Alder reaction is discussed. Finally, the effect of water on Lewis acid - Lewis base interactions is described. 

In a generalized sense, acids are electron pair acceptors. They include both protic (Bronsted) acids and Lewis acids such as AlCb and BF3 that have an electron-deficient central metal atom. Consequently, there is a priori no difference between Bronsted (protic) and Lewis acids. In extending the concept of superacidity to Lewis acid halides, those stronger than anhydrous aluminum chloride (the most commonly used Friedel-Crafts acid) are considered super Lewis acids. These superacidic Lewis acids include such higher-valence fluorides as antimony, arsenic, tantalum, niobium, and bismuth pentafluorides. Superacidity encompasses both very strong Bronsted and Lewis acids and their conjugate acid systems. 

Brmnsted-Lewis Superacids. Conjugate Friedel-Crafts acids prepared from ptotic and Lewis acids, such as HCl—AlCl and HCl—GaCl ate, indeed, supetacids with an estimated value of —15 to —16 and ate effective catalysts in hydrocarbon transformation (217). 

Mote stable catalysts ate obtained by using fluorinated graphite or fluorinated alumina as backbones, and Lewis acid halides, such as SbF, TaF, and NbF, which have a relatively low vapor pressure. These Lewis acids ate attached to the fluorinated soHd supports through fluorine bridging. They show high reactivity in Friedel-Crafts type reactions including the isomerization of straight-chain alkanes such as / -hexane. 

Lactams can also be polymerized under anhydrous conditions by a cationic mechanism initiated by strong protic acids, their salts, and Lewis acids, as weU as amines and ammonia (51—53). The complete reaction mechanism is complex and this approach has not as yet been used successfully in a commercial process. 

Salt formation with Brmnsted and Lewis acids and exhaustive alkylation to form quaternary ammonium cations are part of the rich derivati2ation chemistry of these amines. Carbamates and thiocarbamates are formed with CO2 and CS2, respectively the former precipitate from neat amine as carbamate salts but are highly water soluble. 

According to Figure 3, hydroperoxides are reduced to alcohols, and the sulfide group is oxidized to protonic and Lewis acids by a series of stoichiometric reactions. The sulfinic acid (21), sulfonic acid (23), sulfur trioxide, and sulfuric acid are capable of catalyzing the decomposition of hydroperoxides to nonradical species. 

Ethers are weakly basic and are converted to unstable oxonium salts by strong acids such as sulfudc acid, perchlodc acid, and hydrobromic acid relatively stable complexes ate formed between ethers and Lewis acids such as boron trifluodde, aluminum chlodde, and Gtignatd reagents (qv) (9) ... 

V0R8RUGGEN Nucleosidasynthesis Synthesis of nucleosides by condensation of sugars with siiyl heterocycles and Lewis acids such as SnCU or trimethylsityl Inflate 3. 

Phosphoramidates are cleaved with HCl saturated THE (70-94% yield). Their Stability is dependent on the alkyl group, the methyl derivative being the least stable. They also have good stability to organic acids and Lewis acids. ... 

Trifluoroacetates of silver, mercury(II), thallium(lll), lead(IV), and lodme(III) are synthetically valuable reagents that combine the properties of strong electrophiles, oxidizers, and Lewis acids Furthermore, trifluoroacetate anions are stable to oxidation, are weak nucleophiles, and usually do not cause any contamination of the reaction mixture... 

The carboranyl alcohol can also be prepared from the stannyl carborane and an aldehyde using Pd2(dba)3-CHCl3/dppe. The carborane is stable to Brpnsted and Lewis acids and to LiAlH.. 

A large number of Brpnsted and Lewis acid catalysts have been employed in the Fischer indole synthesis. Only a few have been found to be sufficiently useful for general use. It is worth noting that some Fischer indolizations are unsuccessful simply due to the sensitivity of the reaction intermediates or products under acidic conditions. In many such cases the thermal indolization process may be of use if the reaction intermediates or products are thermally stable (vide infra). If the products (intermediates) are labile to either thermal or acidic conditions, the use of pyridine chloride in pyridine or biphasic conditions are employed. The general mechanism for the acid catalyzed reaction is believed to be facilitated by the equilibrium between the aryl-hydrazone 13 (R = FF or Lewis acid) and the ene-hydrazine tautomer 14, presumably stabilizing the latter intermediate 14 by either protonation or complex formation (i.e. Lewis acid) at the more basic nitrogen atom (i.e. the 2-nitrogen atom in the arylhydrazone) is important. 

This section deals with Bronsted acid and Lewis acid catalyzed reactions, excluding Friedel-Crafts reactions, but including reactions such as nitrations, halogenations, and Claisen rearrangements. Friedel-Crafts reactions are discussed in the subsequent Sections 5.1.2.2 and 5.1.2.3. 

Both the Bronsted and Lewis acid sites on the catalyst generate carbenium ions. The Bronsted site donates a proton to an olefin molecule and the Lewis site removes electrons from a paraffin molecule. In commercial units, olefins come in with the feed or are produced through thermal cracking reactions. 

Thermolysis rates are enhanced substantially by the presence of certain Lewis acids (e.g. boron and aluminum halides), and transition metal salts (e.g. Cu ", Ag1).46 There is also evidence that complexes formed between azo-compounds and Lewis acids (e.g. ethyl aluminum scsquichloridc) undergo thermolysis or photolysis to give complexed radicals which have different specificity to uncomplexed radicals.81 83... 

Further discussion on the effects of the reaction media and Lewis acids on lacticily appears in Section 7.2. Attempts to control laciicily by template polymerization and by enzyme mediated polymerization are described in Section 7.3. Devising effective means for achieving stereochemical control over propagation in radical polymerization remains an important challenge in the field. 

Oxido[10]annulene closely resembles l,6-methano[lOJannulene in many of its spectral properties, particularly in its proton magnetic resonance, ultraviolet, infrared, and electron spin resonance spectra,1 but is chemically less versatile than the hydrocarbon analog due to its relatively high sensitivity toward proton and Lewis acids. 

Identify Bronsted and Lewis acids and bases in a chemical reaction (Self-Test 10.2). 

Reactivity and diastereoselectivity in the thermal and Lewis-acid-catalyzed Diels-Alder reactions of A/-sulphinylphosphoramidates [114]... 

Zeolite and Lewis-acid catalysis in Diels-Alder reactions of isoprene [20b]... 

Posner G. H., Anjeh T. E. N., Carry J. C., French A. N. A New and Efficient Asymmetric Synthesis of an A-Ring Precursor to Physiologically Active 1-a-Hydroxyvitamin D3 Steroids Proc. - NOBCChE 1994 21 383-389 Keywords inverse electron-demand Diels-Alder cycloadditions, (S)-lactate and Lewis acids (-)-Pr(hfc)3 with benzyl vinyl ether... 

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