Aromatics acylium ions

In their reactions with suitable nucleophiles, such as tt-aromatics or heteroatom donor nucleophiles, the readily polarizable linear acylium ions shift a Tt-electron pair to oxygen, bending the ions and developing an empty p-orbital at the carbocationic center. This enables the reaction with aromatics. The acetylation of benzene can be depicted as... 

Acyl cations (acylium ions) generated by treating an acyl chloride or acid anhydride with aluminum chloride attack aromatic rings to yield ketones The arene must be at least as reactive as a halobenzene Acyl cations are relatively stable and do not re arrange... 

Friedel-Crafts acylation of aromatic compounds (Section 12 7) Acyl chlorides and carboxylic acid anhydrides acylate aromatic rings in the presence of alumi num chloride The reaction is electrophil ic aromatic substitution in which acylium ions are generated and attack the ring... 

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... 

Aryl acylium ions have substantial charge delocalization into the aromatic ring. 

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. 

Drawbacks as known from the Friedel-Crafts alkylation are not found for the Friedel-Crafts acylation. In some cases a decarbonylation may be observed as a side-reaction, e.g. if loss of CO from the acylium ion will lead to a stable carbenium species 8. The reaction product of the attempted acylation will then be rather an alkylated aromatic compound 9 ... 

The Lewis acid complex 4 can cleave into an ion-pair that is held together by the solvent cage, and that consists of an acylium ion and a Lewis acid-bound phenolate. A fr-complex 6 is then formed, which further reacts via electrophilic aromatic substitution in the ortho- or para-position ... 

Brown and Jensen395 suggested that the rate equation (194) for the reaction of benzene with excess benzoyl chloride could be interpreted according to the mechanisms given by the reactions (201) and (202), (203) and (204) and (205) and (206) which refer to nucleophilic attack of the aromatic upon the polarised acyl halide-catalyst complex, upon the free acylium ion, and upon an ion pair derived from the acyl halide-catalyst complex, viz. 

In exceptional circumstances the acylium ion (or the polarised complex) can decompose to give an alkyl cation so that alkylation accompanies acylation. This occurs in the aluminium chloride-catalysed reaction of pivaloyl chloride which gives acylation with reactive aromatics such as anisole, but with less reactive aromatics such as benzene, the acylium ion has time to decompose, viz. 

Acidic chloroaluminate ionic liquids are able to generate acylium ions and are therefore ideally suited to Friedel-Crafts reactions. Acylation of mono-substituted aromatic compounds in acidic chloroaluminate ionic liquids leads almost exclusively to substitution at the 4-position on the ring [9] (Scheme 10.8). 

It is generally admitted that over zeolites, acetylation of aromatic substrates with acetic anhydride (AA) is catalyzed by protonic acid sites. The direct participation of Lewis sites was excluded by using two BEA samples with similar protonic acidities, but with very different Lewis acidities indeed, these samples were shown to have quasi-similar activities. The currently accepted mechanism is shown in Figure 12.6 for the anisole acetylation example. The limiting step of the process is the attack of anisole molecules by acylium ions. 

In Friedel-Crafts acylations, an acyl halide, almost always the chloride, in the presence of a Lewis acid is employed to acylate an aromatic ring. The process is initiated by polarization of the carbon-chlorine bond of the acyl chloride, resulting in formation of a resonance-stabilized acylium ion. 

The acylium ion is now our electrophile, and aromatic substitution proceeds in the predicted manner. 

In the 40 years since Olah s original publications, an impressive body of work has appeared studying carbocations under what are frequently termed stable ion conditions. Problems such as local overheating and polymerization that were encountered in some of the initial studies were eliminated by improvements introduced by Ahlberg and Ek and Saunders et al. In addition to the solution-phase studies in superacids, Myhre and Yannoni have been able to obtain NMR spectra of carbocations at very low temperatures (down to 5 K) in solid-state matrices of antimony pentafluoride. Sunko et al. employed a similar matrix deposition technique to obtain low-temperature IR spectra. It is probably fair to say that nowadays most common carbocations that one could imagine have been studied. The structures shown below are a hmited set of examples. Included are aromatically stabilized cations, vinyl cations, acylium ions, halonium ions, and dications. There is even a recent report of the very unstable phenyl cation (CellJ)... 

Lifetimes are longer in the more weakly nucleophilic TFE and HFIP, and cations whose existence is on the borderline in water and simple alkanols can become quite long lived, especially in HFIP. Benzenium ions such as protonated mesitylene can also be observed in HFIP, and there is an estimate for a simple secondary cation 77 60 pjjgj.g Qjjg estimate of the lifetime of an acylium ion (72), based upon the clock approach. Even with the powerful electron-donor 4-Me2N on the aromatic ring, this cation appears to be very short lived in water. 

Under the reaction conditions the direct acylation of the aromatic hydrocarbons with the acylium ion does not take place. Instead, the intermediate vinyl carboca-tion reacts with the aromatics to form the 20 aryl-substituted vinyl ketones. 

The first unequivocal evidence for the effective electrophile in an acylation reaction has been presented.59 Reaction of aromatics with aroyl triflates in organic solvents needs no catalyst and allows kinetic investigation in homogeneous solution. The rate-limiting step can be either dissociation of the triflate to the acylium ion or reaction of the latter with the substrate. Kinetic measurements in the presence of base establish firmly the mtennediacy of acylium ions. 

Eberlin and Cooks578 discovered that acylium ions react with neutral isoprene and other 1,3-dienes in the gas phase to form covalently bound adducts by polar [4 + 2+] Diels-alder cycloadditions. The general reaction is given in Scheme 18, where R may range from H and alkyl to unsaturated, aromatic and polar substituents. The formation... 

Acylium ion salts, characterized by crystal structures, react with electron-rich aromatics to give Friedel-Crafts products by way of benzenium ions these Wheland... 

Acid chlorides can be treated with aromatic rings in the presence of a Lewis acid to give aromatic ketones (Following fig.). The reaction involves formation of an acylium ion from the acid chloride, followed by electrophilic substitution of the aromatic ring. 

Importantly, a Friedel-Crafts formylation has not yet been successful. Formyl chloride and formic anhydride are not stable reagents. The mixed anhydride H—C(=0)—O—C(=0)CH3 acts as a formylating reagent in reactions with many nucleophiles (cf. Section 6.3.3). However, in reactions with aromatic compounds under Friedel-Crafts conditions, it acts as an acetylating agent rather than as a formylating agent. Last but not least, formic acid and mineral acids proceed to react via the acylium ion H—C=0 to form carbon monoxide and water in an OC-elimination. We ll solve this problem later. 

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