Acylium ion as intermediates

Friedel-Crafts acylation was studied and kinetic and spectroscopic evidence was reported for acylium ions as the key reactive intermediate. The acetyl cation was detectable (absorption band at 2308 cm ) in low concentration by in situ IR studies, during the acetylation of toluene. The 4-tert-butylbenzoyl cation was also observed as a weak band at 2283 cm during the course of an acylation of mesitylene. 

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. 

On the basis of all these experiments various mechanisms have at some stage been advanced for the Fries rearrangement involving the free acylium ion or as a tightly bound ion pair, Ji-complexes and cyclic intermediates. It is clearly impossible to reconcile all the experimental data by one reaction mechanism. It is probable that many such mechanisms are possible, each one operative under a certain set of conditions. 

As in the alkylation reaction, the reactive intermediate in Friedel-Crafts acylation can be a dissociated acylium ion or a complex of the acid chloride and Lewis acyl.49 Recent mechanistic studies have indicated that with benzene and slightly deactivated derivatives, it is the protonated acylium ion that is the kinetically dominant electrophile.50... 

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 detailed mechanism, or mechanisms, of the solvolysis of acid chlorides is still a matter of dispute. There are at least four possible mechanisms, (a)-(d) below, all of which have been proposed either to act separately or in various combinations, and there is a unified mechanism, that of Minato93 which will be discussed later. The bimolecular mechanisms (a) and (b) differ in that (a) includes a tetrahedral intermediate whereas (b) does not. The former is commonly accepted as the most likely for the bimolecular mechanism and the arguments against (b) have been stated in the introduction. There is, however, good evidence for (A), at least in the case of the hydrolysis of chloracetyl chloride94. The acylium ion mechanism (c) and the hydrated carbonium ion mechanism (d) are both unimolecular mechanisms. Whereas the acylium ion XXVII has never been directly observed in hydrolysis or alcoholysis reactions, it is favoured as an intermediate by many workers, although it is kinetically indistinguishable from XXVIII. 

Acylation with the acylium ion in the gas phase. An unusual experiment was performed by Seldes et aL <20010MS1069>. The N2-tautomeric form of a 5-substituted tetrazole was reacted in the gas phase with an acyl ion generated as the secondary reactive chemical by ionization plasma in a mass spectrometer. It was suggested that the mechanism of this process involved the formation of an acylated tetrazole intermediate, which could not be isolated in a condensed phase, and by rearrangement with nitrogen loss afforded an oxadiazole <20010MS1069> (cf. Section 6.07.5.2.2, Equation 16). This experiment has no preparative value but provides important information on the interaction mechanism between the neutral N-unsubstituted tetrazoles and electrophilic agents in the gas phase. 

Advantage can be drawn from the positive effect of phenol on PA transformation into p-HAP to improve the yield and selectivity of p-HAP production.[82 84] Thus, with a HBEA zeolite the yield and selectivity for p-HAP passes from ca. 5 and 28 % respectively with cumene solvent to 24 and 60% with phenol as a solvent .[84] Again sulfolane was shown to have a very positive effect on the selectivity for p-HAP and limits the catalyst deactivation. To explain these observations as well as the effect of P and PA concentrations on the reaction rates, it was proposed that sulfolane plays two independent roles in phenol acylation solvation of acylium ion intermediates and competition with P and PA for adsorption on the acid sites.1831... 

Acylium ion pairs, as well as the related oxonium complexes with Lewis acids, are recognized as the effective intermediates of aromatic (Friedel-Crafts) acylations. Kinetic studies apparently exclude that the electrophilic attack at the aromatic nucleus is by free acyl cations (Brown and Jensen, 1958). 

Under strongly acidic anhydrous conditions, carboxylic acids dehydrate to give the acylium ions, which you met as intermediates in the Friedel-Crafts reaction (Chapter 22),... 

It should be noted that the alkyl aryl ketones 191 react with the mixture of perchloric acid and acetic anhydride under the same conditions (room temperature, 10-20 h) to give the unsymmetrical pyrylium salts 197114,115. As shown in Reference 112 this reaction is also an example of electrophilic catalysis by acylium ions, proceeding via the cationic intermediates 195 and 196 (equation 62). 

The electron-withdrawing effect of the halogen, coupled with that of the carbonyl oxygen, leads to a very electron-deficient carbon, and this is not effectively counteracted by the lone pairs on halogens such as chlorine. Consequently, the carbonyl carbon atom is very sensitive to nucleophilic addition to form a tetrahedral intermediate. The collapse of the tetrahedral intermediate with the expulsion of the halide ion, which is a good leaving group, enhances the reactivity of the acyl halides (Scheme 3.64a). The direct fission of the acyl halide C-X bond leads to the formation of an electrophilic acylium ion (Scheme 3.64b). 

The expected reaction to give A is a simple Friedel-Crafts acylation with the usual acylium ion (text p. 554) as the reactive intermediate. 

Product B must arise from a Friedel-Crafts alkylation with the f-butyl cation as intermediate This comes from the loss of carbon monoxide from the acylium ion. Such a reaction happens oniv when the simple carbocation is stable. 

The first report of an intramolecular addition of a vinylsilane came from the Burke laboratory The reaction was us for the preparation of spiro[4.S]decadienones. As shown in Scheme 2, the addition of the vinylsilane to the intermediate acylium ion generated using titanium tetrachloride resulted in the formation of the enone system (3) -> (4). 

Competition studies reported by Kuwajima, " which also complement the results of Nakai," illustrate the limitations of the 3-effect as a tool for predicting the outcome of vinylsilane-terminated cyclizations (Scheme 4). Acylium ion initiated cyclizations of (7a) and (7b) gave the expected cyclopentenones (8a) and (8b). However, compound (7c), upon treatment with titanium tetrachloride, gave exclusively the cyclopentenone proiduct (8c) arising fr the chemoselective addition on the 1,1-disubstituted alkene followed by protodesilylation of the vinylsilane. The reversal observed in the mode of addition may be a reflection of the relative stabilities of the carbocation intermediates. The internal competition experiments of Kuwajima indicate that secondary 3-silyl cations are generated in preference to secondary carbocations (compare Schemes 3 and 4), while tertiary carbocations appear to be more stable than secondary 3-silyl cari ations, as judged by the formation of compound (te). 

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