Lewis acid catalysis Friedel-Crafts acylation

Addition compounds of anhydrides and acyl halides have been known for many years (277) and it is surely through their basic interactions with Lewis acids that Friedel-Craft catalysis occurs. 

In electrophilic catalysis, the metal ion acts as a Lewis acid. An example from organic chemistry is the formation of an acylium ion from aluminum chloride and an acid chloride in Friedel-Crafts acylation reactions (Figure 2). In this case substrate activation results in cleavage of the C—Cl bond. In most cases, however, substrate activation by Lewis acids involves electron redistribution without bond breaking (Figure 3). 

A more important variation is the Friedel-Crafts acylation with acid chlorides and AICI3. As you saw in Chapter 13, add chlorides can give the rather stable acylium ions even in hydrolytic reactions and they do so readily with Lewis acid catalysis. Attack on a benzene ring then gives an aromatic ketone. The benzene ring has been acylated. 

Electrophiles for aromatic substitution include the halonium ion, the nitronium ion and the carbonium ion. The latter may be generated from alkyl and acyl halides using Lewis acid catalysis in the Friedel-Crafts reactions. 

Bismuth(lll) salts such as BiCls, BiBrj, Bi(OCOR)3, and Bi (OTf), [166] have been widely used as Lewis acid catalysts to mediate C-C bond formation. Bi (OTf) 3, Bi2O3, and BiCl, catalyze Friedel-Crafts acylation with acyl chlorides or acid anhydrides [167]. Both electron-rich and electron-deficient arenes are acylated in high yields under catalysis by Bi(OTf)3 (Scheme 14.82). Under microwave irradiation the catalytic activity of BiX3 (X = C1, OTf) in the acylation of aromatic ethers is enhanced [168]. The N-acyl group of p-substituted anilides migrates to the ortho position of the aromatic nucleus under BiCls catalysis [169]. Treatment of 2,3-dichloroanisole with the ethyl glyoxylate polymer in the presence of a catalytic amount of Bi(OTf)3 affords an a,a-diarylacetic acid ester quantitatively (Scheme 14.83) [170]. 

Carboxylic acid anhydrides or halides normally require the presence of a Lewis acid (often boron trifluoride) for Friedel-Crafts acylation of furans, though trifluoroacetic anhydride will react alone. Aluminium-chloride-catalysed acetylation of furan proceeds 7 x lO times faster at the a-position than at the P-position. 3-Alkyl-furans substitute mainly at C-2 2,5-dialkyl-furans can be acylated at a P-position, but generally with more difficulty. 3-Bromofuran is efficiently acetylated at C-2 using aluminium chloride catalysis. ... 

There is one type of Lewis acid catalysis with which we should be very familiar—a range of Lewis acids such as AICI3, SnCl, and FeClj catalyze the Friedel-Crafts alkylation and acylation of aromatic compounds (Figure 23.6 see Section 12.2.2). 

Acylation of aromatic compounds (Friedel-Crafts (FC) acylation), of great industrial interest, suffers from an important catalysis problem [69]. Most of the Lewis acids used as catalysts (traditionally metal chlorides such as A1C13) complex preferentially... 

One of the most important modifications of the Friedel-Crafts reaction involves the use of acid chlorides rather than alkyl halides. An acyl group, RCO—, becomes attached to the aromatic ring, thus forming a ketone the process is called acylation. As usual for the Friedel-Crafts reaction (Sec. 12.8), the aromatic ring undergoing substitution must be at least as reactive as that of a halobenzene catalysis by aluminum chloride or another Lewis acid is required. 

Examples here are Friedel-Crafts alkylations and acylations, in which the zeolite catalyst replaces the homogeneous Lewis acid. Zeolite catalysts can also be used in place of mineral acids and organic acids. However, these catalyst substitutions also involve changing the process from homogeneous to heterogeneous catalysis. 

In 1877, Charles Friedel and James Mason Crafts [30a, b] corporately discovered that treatment of amyl chloride with aluminum strips in benzene led to the formation of amylben-zene. This type of transformation was found to be general for alkyl halides and aromatics under the catalysis of Lewis acid. Along with the discovery of the closely related acylation [30c, d], these two men are best remembered by Friedel-Crafts reaction that bears their names. With various modem modifications that appeared in the Uterature, including enan-tioselective variants [31], Friedel-Crafts alkylation and acylation have already become one of the most powerful C—C bond forming reactions in organic chemistry [32]. These methods are recognized to date as of fundamental importance not only in acadania but also in industry [33]. As shown in Scheme 10.18, some heteroaromatics, instead of the aryl component or alcohol, and alkenes instead of halides can be used as suitable substrates. Also, other common Lewis acids like BFj, TiCl, SnCl, ScfOTOj, etc., and Brpnsted acids snch as HF, H SO, and superacids (e.g., HF SbFj, HS03-SbFj) can also used as catalysts. 

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