Lewis acids acylation

Various classes of cationic initiators have been used to polymerize lactones protic acids, Lewis acids, acylating agents, and alkylating agents. These initiators are often difficult to handle experimentally... 

Ashforth, R., Desmurs, J.-R. Friedel-Crafts acylation Interactions between Lewis acids-acyl chlorides and Lewis acids-aryl ketones. Ind. Chem. Library 996, 8, 3-14. 

FRIEDEL-CRAFTS ACYLATION INTERACTIONS BETWEEN LEWIS ACIDS / ACYL CHLORIDES AND LEWIS ACIDS / ARYL KETONES... 

This chapter focuses on the advantages achieved with different procedures and makes comparative analyses of the synthetic results. Furthermore, despite utility at small scale, the use of Lewis acid/acyl chlorides reagents on an industrial scale is frequently discouraged due to the large amounts of waste produced. Practically, the use of homogeneous Lewis acids in combination with acyl chlorides or anhydrides poses a serious problem for the efficient recovery and disposal of metal oxides and protic inorganic acid by-products. 

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. 

Evidence supporting the formation of 1 1 addition compounds is substantiated by the actual isolation of stable acyl haUde—Lewis acid complexes. 

PoIysuIfonyIa.tlon, The polysulfonylation route to aromatic sulfone polymers was developed independendy by Minnesota Mining and Manufacturing (3M) and by Imperial Chemical Industries (ICI) at about the same time (81). In the polymerisation step, sulfone links are formed by reaction of an aromatic sulfonyl chloride with a second aromatic ring. The reaction is similar to the Friedel-Crafts acylation reaction. The key to development of sulfonylation as a polymerisation process was the discovery that, unlike the acylation reaction which requires equimolar amounts of aluminum chloride or other strong Lewis acids, sulfonylation can be accompHshed with only catalytic amounts of certain haUdes, eg, FeCl, SbCl, and InCl. The reaction is a typical electrophilic substitution by an arylsulfonium cation (eq. 13). 

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. 

FRIEDEL - CRAFTS Alkylation-Acylation Alkylation or acylation ol aromatic compounds by means of alryl halides, alcohols.alkenes, acyl halides in the presence of Lewis acids... 

Friedel-Crafts acylation usually involves the reaction of an acyl halide, a Lewis acid catalyst, and the aromatic substrate. Several species may function as the active electrophile, depending on the reactivity of the aromatic compound. For activated aromatics, the electrophile can be a discrete positively charged acylium ion or the complex formed... 

The formation of acyl halide-Lewis acid complexes have been observed by several methods. For example, both 1 1 and 1 2 complexes of acetyl chloride, with AICI3 can be observed by NMR spectroscopy. The existence of acylium ions has been demonstrated by X-ray diffraction studies on crystalline salts. For example, crystal structure determinations have been reported for /i-methylphenylacylium and acetylium ions as SbFg salts. There is also a good deal of evidence from NMR measurements which demonstrates that acylium ions can exist in nonnucleophilic solvents. " The positive charge on acylium ions is delocalized onto the oxygen atom. This delocalization is demonstrated in particular by the short O—C bond lengths in acylium ions, which imply a major contribution from the structure having a triple bond ... 

Although the Lewis acids used as co-reagents in Friedel-Crafts acylations are often referred to as catalysts, they are, in fact, consumed in the reaction, with the generation of strong acids. There has been considerable interest in finding materials which could function as true catalysts. Considerable success has been achieved using lanthanide triflates. ... 

The anhydride or acyl chloride and the catalyst (proton acid or Lewis acid) interact leading to the acylating agent [formulated here for brevity as an acyl cation (83)]. ... 

The Curtius rearrangement can be catalyzed by Lewis acids or protic acids, but good yields are often obtained also without a catalyst. From reaction in an inert solvent (e.g. benzene, chloroform) in the absence of water, the isocyanate can be isolated, while in aqueous solution the amine is formed. Highly reactive acyl azides may suffer loss of nitrogen and rearrange already during preparation in aqueous solution. The isocyanate then cannot be isolated because it immediately reacts with water to yield the corresponding amine. 

The most important method for the synthesis of aromatic ketones 3 is the Friedel-Crafts acylation. An aromatic substrate 1 is treated with an acyl chloride 2 in the presence of a Lewis-acid catalyst, to yield an acylated aromatic compound. Closely related reactions are methods for the formylation, as well as an alkylation procedure for aromatic compounds, which is also named after Friedel and Crafts. 

The reaction is initiated by formation of a donor-acceptor complex 4 from acyl chloride 2, which is thereby activated, and the Lewis acid, e.g. aluminum trichloride. Complex 4 can dissociate into the acylium ion 5 and the aluminum tetrachloride anion 4 as well as 5 can act as an electrophile in a reaction with the aromatic substrate ... 

Depending on the specific reaction conditions, complex 4 as well as acylium ion 5 have been identified as intermediates with a sterically demanding substituent R, and in polar solvents the acylium ion species 5 is formed preferentially. The electrophilic agent 5 reacts with the aromatic substrate, e.g. benzene 1, to give an intermediate cr-complex—the cyclohexadienyl cation 6. By loss of a proton from intermediate 6 the aromatic system is restored, and an arylketone is formed that is coordinated with the carbonyl oxygen to the Lewis acid. Since a Lewis-acid molecule that is coordinated to a product molecule is no longer available to catalyze the acylation reaction, the catalyst has to be employed in equimolar quantity. The product-Lewis acid complex 7 has to be cleaved by a hydrolytic workup in order to isolate the pure aryl ketone 3. 

As acylating agent, a carboxylic anhydride may be used instead of the acyl halide. The reaction then yields the arylketone together with a carboxylic acid, each of which forms a complex with the Lewis acid used. The catalyst therefore has to be employed in at least twofold excess ... 

The synthesis of an alkylated aromatic compound 3 by reaction of an aromatic substrate 1 with an alkyl halide 2, catalyzed by a Lewis acid, is called the Friedel-Crafts alkylation This method is closely related to the Friedel-Crafts acylation. Instead of the alkyl halide, an alcohol or alkene can be used as reactant for the aromatic substrate under Friedel-Crafts conditions. The general principle is the intermediate formation of a carbenium ion species, which is capable of reacting as the electrophile in an electrophilic aromatic substitution reaction. 

The mechanism for that step is closely related to that of the Friedel-Crafts acylation. Upon subsequent hydrolysis the o-substituted Lewis acid-coordinated phenolate 7 is converted to the free o-acylphenol 2. By an analogous route, involving an electrophilic aromatic substitution para to the phenolate oxygen, the corresponding para-acylphenol is formed. 

While the Friedel-Crafts acylation is a general method for the preparation of aryl ketones, and of wide scope, there is no equivalently versatile reaction for the preparation of aryl aldehydes. There are various formylation procedures known, each of limited scope. In addition to the reactions outlined above, there is the Vdsmeier reaction, the Reimer-Tiemann reaction, and the Rieche formylation reaction The latter is the reaction of aromatic compounds with 1,1-dichloromethyl ether as formylating agent in the presence of a Lewis acid catalyst. This procedure has recently gained much importance. 

Acidic chloroaluminate ionic liquids have already been described as both solvents and catalysts for reactions conventionally catalyzed by AICI3, such as catalytic Friedel-Crafts alkylation [35] or stoichiometric Friedel-Crafts acylation [36], in Section 5.1. In a very similar manner, Lewis-acidic transition metal complexes can form complex anions by reaction with organic halide salts. Seddon and co-workers, for example, patented a Friedel-Crafts acylation process based on an acidic chloro-ferrate ionic liquid catalyst [37]. 

Table 4 Effect of Lewis Acid on the Intrinsic Viscosity and Degree of Acylation for the Various Fractions of Modified Block PS...

Because hydride ion is a base as well as a nucleophile, the actual nucleophilic acyl substitution step takes place on the carboxylate ion rather than on the free carboxylic acid and gives a high-energy dianion intermediate. In this intermediate, the two oxygens are undoubtedly complexed to a Lewis acidic aluminum species. Thus, the reaction is relatively difficult, and acid reductions require higher temperatures and extended reaction times. 

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