Acyl halides with aromatic rings

Because acylation of an aromatic ring can be accomplished without rearrangement it is frequently used as the first step m a procedure for the alkylation of aromatic compounds by acylation-reduction As we saw m Section 12 6 Friedel-Crafts alkylation of ben zene with primary alkyl halides normally yields products having rearranged alkyl groups as substituents When a compound of the type ArCH2R is desired a two step sequence IS used m which the first step is a Friedel-Crafts acylation... 

Scheme 9.128. A representation of a pathway to effect the snbstitntion of an aromatic ring for a chlorine atom on an acyl fnnction. In principle, the reaction could occur by attack of the aromatic ring on the carbon of the carbonyl of the acyl halide or by reaction of the acyl halide with aluminum trichloride (AICI3) to produce an acyhum ion, which is then captured by the aromatic ring. See Scheme 6.88.

Polysulfonylation. The polysulfonylation route to aromatic sulfone polymers was developed independently by Minnesota Mining and Manufacturing (3M) and by Imperial Chemical Industries (ICI) at about the same time (81). In the polymerization 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 polymerization process was the discovery that, unlike the acylation reaction which requires equimolar amounts of aluminum chloride or other strong Lewis acids, sulfonylation can be accomplished with only catalytic amounts of certain halides, eg, FeQ3, SbQ, and InCl3. The reaction is a typical electrophilic substitution by an arylsulfonium cation (eq. 13). 

The monoarylated enamine can then be acylated to form a mixture of 2-acyl-2-arylcyclohexanone and 2-aryl-6-acylcyclohexanone. This was held to indicate that arylation affords two enamines, 62 and 63, which differ only in the position of the double bond. The reaction involving acyl halides could be explained under the assumption that in the more probable enamine structure (with conjugation through the free electron pair on nitrogen, the double bond, and the aromatic ring) it is the hydrogen atom on C(6) which is more acidic. 

Once an amino group has been introduced to an aromatic ring, it can be alkylated with an alkyl halide, acylated with an acid chloride or converted to a higher amine by reductive animation as already described for an alkylamine. 

Many organic compounds react with carboxylic acids, acyl halides, or anhydrides in the presence of certain metallic halides, metallic oxides, iodine, or inorganic acids to form carbonyl compounds. The reaction is generally applicable to aromatic hydrocarbons. Benzene, alkylbenzenes, biphenyl, fluorene, naphthalene, anthracene, acenaphthene, phenanthrene, higher aromatic ring systems, and many derivatives undergo the reaction. 

Recently Blum reported that chlorotris(triphenylphosphine) rhodium (XI) is an active catalyst for the decarbonylation of aroyl halides and showed several examples (2). But in this case too, the real catalyst seems to be chlorocarbonylbis(triphenylphosphine)rhodium (XII), which is formed in situ from XI by the stoichiometric reaction with acyl halides. Formation of alkyl halides by decarbonylation of acyl halides can be carried out by the Hunsdiecker reaction, but the reaction is unsatisfactory when applied to aroyl halides. Therefore, the decarbonylation reaction of aroyl halides by the rhodium complex is a new and useful means of introducing halogen onto the aromatic ring. 

Friedel-Crafts reaction /free-del kraft/ A method for the substitution of an alkyl or acyl group onto a benzene ring. The aromatic hydrocarbon (e.g. benzene) is refluxed with a haloalkane or acyl halide using aluminum chloride catalyst. The product is an alkylarene, e.g. ... 

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