Catalyst lewis-acid

The most widely used reactions are those of electrophilic substitution, and under controlled conditions a maximum of three substituting groups, e.g. -NO2 (in the 1,3,5 positions) can be introduced by a nitric acid/sul-phuric acid mixture. Hot cone, sulphuric acid gives sulphonalion whilst halogens and a Lewis acid catalyst allow, e.g., chlorination or brom-ination. Other methods are required for introducing fluorine and iodine atoms. Benzene undergoes the Friedel-Crafts reaction. ... 

The most effective Lewis-acid catalysts for the Diels-Alder reaction are hard cations. Not surprisingly, they coordinate to hard nuclei on the reacting system, typically oxygen atoms. Consequently, hard solvents are likely to affect these interactions significantly. Table 1.4 shows a selection of some solvents ranked according to their softness. Note that water is one of the hardest... 

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. 

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. 

The rate of the Lewis-acid catalysed Diels-Alder reaction in water has been compared to that in other solvents. The results demonstrate that the expected beneficial effect of water on the Lewis-acid catalysed reaction is indeed present. However, the water-induced acceleration of the Lewis-add catalysed reaction is not as pronounced as the corresponding effect on the uncatalysed reaction. The two effects that underlie the beneficial influence of water on the uncatalysed Diels-Alder reaction, enforced hydrophobic interactions and enhanced hydrogen bonding of water to the carbonyl moiety of 1 in the activated complex, are likely to be diminished in the Lewis-acid catalysed process. Upon coordination of the Lewis-acid catalyst to the carbonyl group of the dienophile, the catalyst takes over from the hydrogen bonds an important part of the activating influence. Also the influence of enforced hydrophobic interactions is expected to be significantly reduced in the Lewis-acid catalysed Diels-Alder reaction. Obviously, the presence of the hydrophilic Lewis-acid diminished the nonpolar character of 1 in the initial state. 

Alkyl halides by themselves are insufficiently electrophilic to react with benzene Aluminum chloride serves as a Lewis acid catalyst to enhance the electrophihcity of the alkylating agent With tertiary and secondary alkyl halides the addition of aluminum chlonde leads to the formation of carbocations which then attack the aromatic ring... 

Halogenation (Section 12 5) Chlorination and bromination of arenes are carried out by treatment with the appropriate halogen in the presence of a Lewis acid catalyst Very reactive arenes undergo halogenation in the absence of a catalyst... 

As improvements over P-methylumbeUiferone (55—57), 4-methyl-7-amino-coumarin [26093-31-2] (12a) and 7-dimethylamino-4-methylcoumarin [87-014] (12b) (58—61) were proposed. These compounds are used for brightening wool and nylon either in soap powders or detergents, or as salts under acid dyeing conditions. They are obtained by the Pechmaim synthesis from appropriately substituted phenols and P-ketocarboxyflc acid esters or nitriles in the presence of Lewis acid catalysts (see Coumarin). 

Uses. Boron triduoride is an excellent Lewis acid catalyst for numerous types of organic reactions. Its advantages are ease of handling as a gas and the absence of undesirable tarry by-products. As an electrophilic molecule, it is an excellent catalyst for Friedel-Crafts and many other types of reactions (63-65) (see Friedel-craftsreactions). 

Sulfur tetrafluoride [7783-60-0] SF, replaces halogen in haloalkanes, haloalkenes, and aryl chlorides, but is only effective (even at elevated temperatures) in the presence of a Lewis acid catalyst. The reagent is most often used in the replacement of carbonyl oxygen with fluorine (15,16). Aldehydes and ketones react readily, particularly if no alpha-hydrogen atoms are present (eg, benzal fluoride [455-31-2] from benzaldehyde), but acids, esters, acid chlorides, and anhydrides are very sluggish. However, these reactions can be catalyzed by Lewis acids (HP, BF, etc). 

Lewis acid catalysts, such as AlCl or BF, coordinate strongly with non-bonded electron pairs but they iateract only weakly with bonded electron pairs. Therefore, n-donon reagents, such as alkyl haUdes, can react with Lewis acid catalysts even under complete exclusion of moisture or any other proton source ... 

Ultimately, as the stabilization reactions continue, the metallic salts or soaps are depleted and the by-product metal chlorides result. These metal chlorides are potential Lewis acid catalysts and can greatiy accelerate the undesired dehydrochlorination of PVC. Both zinc chloride and cadmium chloride are particularly strong Lewis acids compared to the weakly acidic organotin chlorides and lead chlorides. This significant complication is effectively dealt with in commercial practice by the co-addition of alkaline-earth soaps or salts, such as calcium stearate or barium stearate, ie, by the use of mixed metal stabilizers. 

Displacement of activated chlorine atoms also proceeds with certain types of organic compounds, but only in the presence of Lewis acid catalysts. Particular examples include epoxides, polyhydric alcohols, trialkylphosphites (12), and P-aminocrotonates (13). These additives are commonly used in conjunction with metallic stabilizers to provide complete, high performance, commercial stabilizer packages. 

Cationic polymerization of coal-tar fractions has been commercially achieved through the use of strong protic acids, as well as various Lewis acids. Sulfuric acid was the first polymerization catalyst (11). More recent technology has focused on the Friedel-Crafts polymerization of coal fractions to yield resins with higher softening points and better color. Typical Lewis acid catalysts used in these processes are aluminum chloride, boron trifluoride, and various boron trifluoride complexes (12). Cmde feedstocks typically contain 25—75% reactive components and may be refined prior to polymerization (eg, acid or alkali treatment) to remove sulfur and other undesired components. Table 1 illustrates the typical components found in coal-tar fractions and their corresponding properties. 

Several methods are available to supplement the phenol alkylations described above. Primary alkylphenols can be produced using the more traditional Friedel-Crafts reaction. Thus an -butylphenol can be synthesized direcdy from a butyl haUde, phenol, and mild Lewis acid catalyst. Alternatively, butyryl chloride can be used to acylate phenol producing a butyrophenone. Reduction with hydrazine (a Wolff-Kishner reduction) generates butylphenol. 

The ammonolysis of phenol (61—65) is a commercial process in Japan. Aristech Chemical Corporation (formerly USS Chemical Division of USX Corporation) currently operates a plant at Ha verb ill, Ohio to convert phenol to aniline. The plant s design is based on Halcon s process (66). In this process, phenol is vapori2ed, mixed with fresh and recycled ammonia, and fed to a reactor that contains a proprietary Lewis acid catalyst. The gas leaving the reactor is fed to a distillation column to recover ammonia overhead for recycle. Aniline, water, phenol, and a small quantity of by-product dipbenylamines are recovered from the bottom of the column and sent to the drying column, where water is removed. 

The problems associated with predicting regioselectivity in quinone Diels-Alder chemistry have been studied, and a mechanistic model based on frontier molecular orbital theory proposed (85). In certain cases of poor regioselectivity, eg, 2-methoxy-5-methyl-l,4-ben2oquinone with alkyl-substituted dienes, the use of Lewis acid catalysts is effective (86). 

Acetalation. As polyhydroxy compounds, carbohydrates react with aldehydes and ketones to form cycHc acetals (1,13). Examples are the reaction of D-glucose with acetone and a protic or Lewis acid catalyst to form l,2 5,6-di-0-isoprop5lidene-a-D-glucofuranose [582-52-5] and its reaction with benzaldehyde to form 4,6-0-benzyhdene-D-glucopyranose [25152-90-3]. The 4,6-0-(l-carboxyethyhdine) group (related to pymvic acid) occurs naturally in some polysaccharides. 

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