Friedel-Crafts acylation reactions stability

The electrophile in a Friedel-Crafts acylation reaction is an acyl cation (also referred to as an acylium ion) Acyl cations are stabilized by resonance The acyl cation derived from propanoyl chloride is represented by the two resonance forms... 

Figure 16.9 Mechanism of the Friedel-Crafts acylation reaction. The electrophile is a resonance-stabilized acyl cation, whose electrostatic potential map indicates that carbon is the most positive atom (blue).

For the purposes of this review, we include probe molecules that can be either directly adsorbed or formed in situ. Examples of the latter case are carbenium ions and related electrophilic species. We will also consider several important heteroatom-substituted carbenium ions and heteroatom analogs of carbenium ions. Acylium ions are the intermediates in Friedel-Crafts acylation reactions (96). The most simple, stable acylium ion is the acetylium ion, 1, and others are formally derived by replacing the methyl group with other R groups. Oxonium ions, formed by alkylation of an ether, resemble carbenium ions but are in fact onium ions in terms of their structures. Their stabilization requires strongly acidic media, and like carbenium ions, oxonium ions have been proposed as intermediates in a... 

The similarities between organic tin and silicon compounds, particularly their abilities to stabilize positive charge on a -carbon atom, suggest that vinylic and allylic tin compounds should be useful substrates for Friedel-Crafts acylation reactions. However, very few examples of the acylation of stannanes under the action of Lewis acids have been reported, and this remains a field of Friedel-Crafts reactions not yet fully exploited. The acylation of 1,2-bis(tri-n-butylstannyl)ethylene in the presence of aluminum chloride offers a moderately yielding route to tributylstannyl-substituted enones, useful as precursors to 4,5-dialky lcyclopent-2-enones. 

Hard Lewis acid chloroaluminate ionic liquids show intense catalytic activity in the Friedel-Crafts acylation reaction however, they suffer from the same issues as anhydrous aluminum chloride. i Of particular interest to these reactions, aluminum chloride may be replaced by indium trichloride to form chloroindate(III) ionic liquids. The advantage of using indium trichloride compared with aluminum chloride is represented by its hydrolytic stability and reduced oxophilicity. Chloroindate(III) ionic liquids are synthesized by mixing l-butyl-3-methylimidazolium chloride [C4mim]Cl with anhydrous indium trichloride at 80°C. In the benzoyla-tion of anisole with benzoic anhydride (BAN) at 80°C, the best yield of... 

As shown in Scheme 1, the synthesis of fert-butyl phenalenyl radical 10 started from 2,7-di-ferf-butylnaphthalene in ten steps. Bromination of 2,1-di-tert-butylnaphthalene gave 6 in high yield, which was converted into aldehyde by lithiation followed by reaction with DMF. Successive Reformatsky reaction afforded ester 7, which upon reduction, hydrolysis, and Friedel-Crafts acylation reaction gave the phenalanone 8. The key intermediate 9 was then obtained as pale yellow crystals by reduction of 8 and subsequent dehydration. Oxidation of 9 with p-chloranil in degassed toluene led to a blue neutral radical solution while similar treatment in hexane gave deep blue needles. This crystal showed high stability in the absence of air, while changing into phenalanone derivatives and other byproducts in 1 week in air. 

Acylium ion (Section 16.3) A resonance-stabilized carbocation in which the positive charge is located at a carbonyl-group carbon, R—C=0 R—C=0+. Acylium ions are intermediates in Friedel-Crafts acylation reactions. 

Although these reactions operate under solvent-free conditions (with an excess of the arene), many Friedel-Crafts acylations utilize volatile and hazardous halogenated solvents. Here, their replacement by ionic liquids can considerably lower the environmental risks and provide a greener chemistry . Ionic liquids with their unique miscibility properties, high thermal stability and miniscule vapor pressure are valuable alternatives for the wide range of traditional solvents available. 

Stannic chloride is also used widely as a catalyst in Friedel-Crafts acylation, alkylation and cyclization reactions, esterifications, halogenations, and curing and other polymerization reactions. Minor uses are as a stabilizer for colors in soap (19), as a mordant in the dyeing of silks, in the manufacture of blueprint and other sensitized paper, and as an antistatic agent for synthetic fibers (see Dyes, application AND EVALUATION Antistatic agents). 

Instead, these heterocycles and their derivatives most commonly undergo electrophilic substitution nitration, sulfonation, halogenation. Friedel-Crafts acylation, even the Reimer-Tiemann reaction and coupling with diazonium salts. Heats of combustion indicate resonance stabilization to the extent of 22-28 kcal/ mole somewhat less than the resonance energy of benzene (36 kcal/mde), but much greater than that of most conjugateci dienes (about Tlccal/mole). On the basis of these properties, pyrrole, furan, and thiophene must be considered aromatic. Clearly, formulas I, II, and III do not adequately represent the structures of these compounds. 

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