BrOnsted acids Friedel-Crafts reaction

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. 

Friedel-Crafts reactions have been studied in detail by Olah [74, 75]. These reactions result in the formation of carbon-carbon bonds and are catalyzed by strong Bronsted or Lewis acids. 

The oldest method of alkylation with ethylene is the liquid phase reaction using anhydrous aluminum chloride as the catalyst. This reaction is a form of the classic Friedel-Crafts reaction and was discovered in 1879 by Balsohn. Most Lewis and Bronsted acids are known to be active for olefin alkylations. Alkylation by H2SO1, and H3PO1, was first shown by Ipatieff, et al, in 1936 who extended the reaction to isoparaffins. For the liquid phase alkylation of benzene with ethylene, however, aluminum chloride is preferred over the other acids, although a co-catalyst or promoter is usually needed to obtain efficient alkylation. AICI3 when dissolved in benzene containing some HCl forms a complex which can be simply described as ... 

Many mechanisms in organic chemistry start with an acid/base reaction. This may be just a simple Bronsted-Lowry protonation of a hydroxyl group, which results in the activation of a C-OH bond or it may be a Lewis acid/base reaction as, for example, when aluminium trichloride complexes with a halogenoalkane in the first step of the Friedel-Crafts reaction. In each case, the initial intermediate usually reacts further and leads to the desired product. In inorganic chemistry, the acid/base reaction may be all that is of interest, e.g. the treatment of a carbonate with an acid to liberate carbon dioxide. However, it is unusual in organic chemistry for the acid/base reaction to be an end in itself. It is for this reason that acid/base characteristics are normally considered as a property of the molecule, similar to the nucleophilic and electrophilic properties to which they are closely related, rather than as a fundamental reaction type as is the case in inorganic chemistry. 

The conventional method for preparation of these aromatic ketones involves reaction of the aromatic hydrocarbon with a carboxylic acid derivatives using a Lewis acid (AICI3, FeCb, BF3, ZnCb, TiCl4) or Bronsted acids (polyphosphoric acid, HF). The major drawback of the Friedel-Crafts reaction is the need to use a stoicheiometrical quantity of Lewis acid relative to the formed ketone. This quantity is required due to the fact that the... 

Friedel-Crafts reactions Replacement of stoichiometric quantities of Lewis acids (AICI3, FeCls, BF3, ZnCh, TiClJ or Bronsted acids (F13P04, HF) by heterogeneous catalysts (zeolites, ion exchangers, heteropolyacids, etc.). 

Strong, solid Bronsted acid catalyst. It has a wide range of applications including alkene alkylations, Beckmann rearrangements and esterification reactions its uses in Friedel-Crafts reactions are described in section 16.5.1. 

Shi and coworkers have identified BF3 OEt2 as the best Lewis acid for the formation of the ring-opened product (149) (Equation 89) [91]. Other Lewis acids and Bronsted acids failed to yield any appreciable amount of the desired product in this novel Friedel-Crafts reaction. 

Apart from Bronsted acid activation, the acetyl cation (and other acyl ions) can also be activated by Lewis acids. Although the 1 1 CH3COX-AIX3 Friedel-Crafts complex is inactive for the isomerization of alkanes, a system with two (or more) equivalents of AIX3 was fonnd by Volpin to be extremely reactive, also bringing abont other electrophilic reactions. 

Thus it appears that carbonium ion reactions of the Friedel-Crafts type can only proceed in ternary or three component systems, i.e., in the presence of a suitable third component commonly called the cocatalyst. Consequently, it must be assumed that either the Bronsted acid or the Lewis acid forms a primary complex with the substrate — otherwise carbonium ion reactions would become termolecular which is extremely unlikely (58). Apparently the active ion pair is formed by either a bimolec-ular reaction between "primary complexes and the third component (a, b and c) or by a unimolecular rearrangement from a ternary complex... 

The study of cocatalysis in cationic polymerizations is extremely complicated by the fact that Lewis acids can participate in a variety of ill defined reactions (91). Satchell (91, 92) regards the hydrogen exchange reaction between Bronsted acids and aromatics catalyzed by Lewis acids as a prototype for Friedel-Crafts catalysis. He postulates that cocatalytic efficiency is determined by the stability of the complex anion B -f HX + SnCl4 - BH SnCl4Xe. A simple enhancement of conventional acidity by B + HX -> BH Xe as proposed by Plesch (93) and Russel (94) is considered to be unimportant. The stability of the complex and its catalytic activity are determined by the electron density... 

The first example of a Friedel-Crafts type reaction in an ionic liquid medium dates back to 1976 when the electrochemical oxidation of hexamethylbenzene in [C2py]Br-AlCl3 afforded a mixture of alkylated polyphenyl compounds.[69] Other early examples include the alkylation of benzene in C2Ciim C1-A1C137 and the acylation of ferrocene in [C2Ciim]I-AlClJ71 There are now numerous examples of Lewis- or Bronsted acid-catalysed Friedel-Crafts type reactions in ionic liquids. These include alkylation,[72 76] acylation,[71,77"83] arylation,[77 841 sulfonylation,[851 sulfoamylation[86] and O-acetylation of alcohols.[87,881... 

Ionic liquids containing chloroaluminate anions are strong Lewis acids. This characteristic, coupled with the fact that they are relatively easy to handle, makes them attractive alternatives to standard Lewis and Bronsted acids such as HF. The IL can function both as a catalyst and a solvent for acid-catalyzed processes. Some of the first reactions studied in ILs were Friedel-Crafts alkylations and arylations. The [emim][Al2Cl7] affords complete conversion of benzene and acetyl chloride to acetophenone in less than 5 minutes at room temperature. 

In 2004 Terada and his group revealed that 2 methoxy furan (45) that can be considered as a vinylketene 0,0 acetal was sufficiently nucleophilic to engage Boc imines 44 in Bronsted acid catalyzed Friedel Crafts type reaction to furnish y aminoalkyl substituted furans 46 in excellent yields and up to 97% ee (Table 5.9) [ 16]. As already explained, phosphoric add 47 protonates the imine, thus generating a chiral contact ion pair the anionic counterion of which controls the enantioselectivity of the C C bond forming event. Although typically 2 mol% ofthe Bronsted acid were... 

The conventional method of preparation of these aromatic ketones is the homogeneous Friedel-Crafts acylation of aromatic hydrocarbons with carboxylic acid derivatives using Lewis acids (A1C13, FeCl3, BF3, ZnCl2, TiCl4) or Bronsted acids (polyphosphoric acid, HF). For this purpose, stoichiometric to excess amounts of the catalyst are required for the reaction to proceed (ref. 1). 

In this chapter, the use of solid acids as heterogeneous catalysts for the Friedel-Crafts acylahon reaction is described. Our review is split up into seven sechons, describing the application of zeolites, clays, metal oxides, sulfated zirconia, heteropoly acids. Nation, and other less-utilized solid catalysts (i.e., graphite). When possible, the relationship between the acid properhes of the solids (namely, Bronsted and Lewis types) and the catalytic efficiency is shown, as well as the role of the active site location on the catalyst surface. ... 

From the preceding experimental data and comments, it is clear that both Lewis and Bronsted acid sites exist on sulfated oxide catalysts, but the detailed role of fhe two types of acidity in various reactions, including Friedel-Crafts acylation, is still controversial. 

This work incorporates all acid-catalyzed Friedel-Crafts-like acylation reactions. Thus, classic Lewis and Bronsted acid types are considered together with more innovative and advanced multicomponent superacid catalysts, ranging from rare earth triflates or triflimides and their... 

Acylation of aromatic compounds is a widely used reaction for the production of fine chemicals. The classic methodology for acylation is Friedel-Crafts, this uses stoichiometric amounts of Lewis acids, such as metal halides, but some Bronsted acids, such as polyphosphoric and sulphuric acid, can also be used as catalysts. Moreover, the reaction is carried out homogeneously and the catalyst must be destroyed to obtain the final products, generating a great amount of corrosive and toxic waste. 

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