Catalysis by Lewis acids

By far the most effective method, however, is catalysis by Lewis-acids. In organic solvents. 

Many important reactions involve catalysis by Lewis acids or bases. One of the most important of these is the type of reaction carried out by Charles Friedel and James Crafts. These reactions, known as the Friedel-Crafts reactions, actually involve several types of important processes. One of these is alkylation, which is illustrated by the reaction of benzene with an alkyl halide in the presence of A1C13, a strong Lewis acid. 

Figure 7.9 Mechanistic interpretation for catalysis by Lewis acids containing aromatic a-amino acids as ligands.

A broad range of compounds can be O-alkylated with carbene complexes, including primary, secondary, and tertiary alcohols, phenols, enols, hemiaminals, hydroxylamines, carboxylic acids, dialkyl phosphates, etc. When either strongly acidic substrates [1214] and/or sensitive carbene precursors are used (e.g. aliphatic diazoalkanes [1215] or diazoketones) etherification can occur spontaneously without the need for any catalyst, or upon catalysis by Lewis acids [1216]. 

The reaction of a-diazocarbonyl compounds with nitriles produces 1,3-oxazoles under thermal (362,363) and photochemical (363) conditions. Catalysis by Lewis acids (364,365), or copper salts (366), and rhodium complexes (367) is usually much more effective. This latter transformation can be regarded as a formal [3 + 2] cycloaddition of the ketocarbene dipole across the C=N bond. More than likely, the reaction occurs in a stepwise manner. A nitrilium ylide (319) (Scheme 8.79) that undergoes 1,5-cyclization to form the 1,3-oxazole ring has been proposed as the key intermediate. 

Control of reaction selectivities with external reagents has been quite difficult. Unsolved problems remaining in the held of nitrile oxide cycloadditions are (a) Nitrile oxide cycloadditions to 1,2-disubstituted alkenes are sluggish, the dipoles undergoing facile dimerization to furoxans in most cases (b) the reactions of nitrile oxides with 1,2-disubstituted alkenes nonregioselective (c) stereo- and regiocontrol of this reaction by use of external reagents are not yet well developed and (d) there are few examples of catalysis by Lewis acids known, as is true for catalyzed enantioselective reactions. 

Electrophilic catalysis by Lewis acids is also observed here no ambiguity arises with general acid catalysis, as Lewis acids and proton acids are not the same. An interesting example is the strong catalysis of thiolester hydrolysis by mercuric and silver ions. These soft acids presumably coordinate with the sulfur and, by virtue of the consequent electron withdrawal, make the carbonyl group much more susceptible to attack in the addition mechanism, or, in favorable cases, promote unimolecular SN1 cleavage of the sulfur-carbon bond.122... 

Allylsilanes undergo reactions with a large range of electrophiles, although catalysis by Lewis acids is often necessary. Some recent examples of substitution reactions of allylsilanes are discussed below. 

The only paper concerning catalysis by Lewis acids of the Diels-Alder reactions of these simple sulfinyl ethylenes was due to Ronan and Kagan [20], who studied the influence of TMSOTf in the reaction of compound (S)-l with cyclo-pentadiene and furane. In the first case, the reaction occurs at 0°C in 3 h, giving an 89 11 mixture of endo and exo adducts (overall yield 60%) with very high n-facial selectivity (de> 92%). The high efficiency of the catalyst increasing the reactivity of 1 also made possible its reaction with furan, which evolved with low endo/exo selectivity (55 45) and lower 7r-facial selectivity (de 70%) than that observed with cyclopentadiene. These excellent results were nevertheless, eclipsed by those reported in the same paper [20] concerning the activation of... 

Electrophilic reagents are Lewis acids (- acid-base theories). Electrophilic catalysis is catalysis by Lewis acids. The term electrophilic is also used to designate the apparent polar character of certain -> radicals, as inferred from their higher relative reactivities with reaction sites of higher -> electron density. See also -> electrophilicity. Ref. [i] Muller P (1994) Pure Appl Chem 66 1077... 

Selenenylation occurs in a similar manner. However, this process is much faster and cleaner if performed in the presence of a catalytic amount of TiCU at — 78°C. The catalysis by Lewis acid is essential for the related sulphenylation . A mechanistic rationalization, which is in accordance with a Lewis acid-catalysed cis-trans isomerization at low temperature , is depicted in equation 98. 

These resonance structures suggest the probability of ionic reactions, with electron donors attacking the carbonyl carbon, and electron acceptors attacking the oxygen or nitrogen. Catalysis by Lewis acids and bases should be common. 

Dihaloethanes alkylate arenes under Friedel-Crafls catalysis by Lewis acids to afford 1,2-diaryl-ated ethanes. Dolgov and Larinsuggested that the reaction process via the intermediate monoaryI-ated haloethane (Scheme 11) which affords the final product via a phenonium-type intermediate. 

Early examples of catalysis by lewis acid adducts involving manganese... 

Silanes are widely recognized as efficient reagents for reduction of carbonyl and heterocarbonyl functionality. In the case of alkyl and arylsilanes, the reaction requires catalysis by Lewis acids or transition metal complexes 1, 3] however, with more Lewis acidic trichloro or trialkoxysilanes, an altemative metal free activation can be accomplished. Thus, it has been demonstrated that extracoordinate silicon hydrides, formed by the coordination of silanes to Lewis bases, such as tertiary amines 7a], DMF [7b] or MeCN, and so on [7], can serve as mild reagents for the reduction of imines to amines [8]. In the case of trichlorosilane, an inexpensive and relatively easy to handle reducing reagent, and DMF as a Lewis basic promoter, the intermediacy of hexacoordinate species has been confirmed by Si NMR spectro scopy [7b]. 

Many chemical reactions involve a catalyst. A very general definition of a catalyst is a substance that makes a reaction path available with a lower energy of activation. Strictly speaking, a catalyst is not consumed by the reaction, but organic chemists frequently speak of acid-catalyzed or base-catalyzed mechanisms that do lead to overall consumption of the acid or base. Better phrases under these circumstances would be acid promoted or base promoted. Catalysts can also be described as electrophilic or nucleophilic, depending on the catalyst s electronic nature. Catalysis by Lewis acids and Lewis bases can be classified as electrophilic and nucleophilic, respectively. In free-radical reactions, the initiator often plays a key role. An initiator is a substance that can easily generate radical intermediates. Radical reactions often occur by chain mechanisms, and the role of the initiator is to provide the free radicals that start the chain reaction. In this section we discuss some fundamental examples of catalysis with emphasis on proton transfer (Brpnsted acid/base) and Lewis acid catalysis. 

There are more structural variables to consider in catalysis by Lewis acids than in the case of catalysis by protons. In addition to the hard-soft relationship, steric, geometric, and stereoelectronic factors can come into play. This makes the development of an absolute scale of Lewis acid strength difficult, since the complexation strength depends on the specific characteristics of the base. There are also variations in the strength of the donor-acceptor bonds. Bond strengths calculated for complexes such as H3N+-BF3" (22.0 kcal/mol) and (CH3)3N+-BH3 (41.1 kcal/mol) are substantially... 

In many instances, Diels-Alder reactions give poor yields and/or require excessively harsh reaction conditions and long reaction times. There are several fairly straightforward techniques that increase the rate and, in some cases, the selectivity of Diels-Alder reactions. The three most important methods are catalysis by Lewis acids, rate enhancement in aqueous media and the use of high pressure (usually greater than 5 kbar). These three methods will be the focus of this section. 

Interesting cases of catalysis by Lewis acids are reported in Table 4 (see also Section 4.1.6). 

The use of zinc chloride as a means of liquefying coal is not surprising since there has been an interest in process catalysis by Lewis acids (e.g., zinc chloride) from the very early days of coal liquefaction technology (Nowacki, 1979). The chemistry involved in the hydrocracking (and other) reactions is not fully understood and therefore any mechanistic speculations are not warranted here. 

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