Acylium ion intermediate

Unlike the carbocation intermediate on the Friedel-Crafts alkylation, this acylium ion intermediate will not undergo rearrangement. 

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... 

A kinetic study of the acylation of phenol with phenyl acetate was carried out in liquid phase at 160°C over HBEA zeolite samples, sulfolane or dodecane being used as solvents. The initial rates of hydroxyacetophenone (HAP) production were similar in both solvents. However the catalyst deactivation was faster in dodecane, most likely because of the faster formation of heavy reaction products such as bisphenol A derivatives. Moreover, sulfolane had a very positive effect on p-HAP formation and a negative one on o-HAP formation. To explain these observations as well as the influence of phenol and phenyl acetate concentrations on the rates of 0- and p-HAP formation it is proposed that sulfolane plays two independent roles in phenol acylation solvation of acylium ions intermediates and competition with phenyl acetate and phenol for adsorption on the acid sites. Donor substituents of phenyl acetate have a positive effect on the rate of anisole acylation, provided however there are no diffusion limitations in the zeolite pores. 

The expected reaction to give A is a simple Friedel-Crafts acylation with the usual acylium ion intermediate. 

There is also evidence for the role of acylium ions in some acyl substitutions, such as the reactions of acyl halides with nucleophilic reagents in acetonitrile, nitromethane, and ethanol and the hydrolysis of some acyl fluorides. 203,206,207 ylium ions are more likely candidates for reaction intermediates under acidic conditions. Bender reported that the hydrolysis of a series of substituted methyl 2,6-dimethylbenzoates in 9.7 M sulfuric acid proceeds through an acylium ion intermediate. Hydrolysis of a series of substituted 2,6-dimethylbenzoyl chlorides in 99% CH3CN-H2O was determined to involve an acylium intermediate under acidic or neutral conditions, and p values (measured by cr" ) were found to be -3.85 and -3.73, respectively. The value of p under basic conditions was -I-1.20, suggesting that the base-promoted reaction does involve a tetrahedral addition intermediate. ... 

This occurs with sterically hindered acids, e.g. 2,4,6-trimethylbenzoic acid, using sulphuric acid catalysis, where the acylium ion intermediate reacts with the alcohol. 

A mechanistic proposal for these transformations involving a novel carbonyl-group-facilitated heterolytic fragmentation on the loss of an acylium ion intermediate was presented. 

The reaction shown is believed to involve an acylium ion intermediate. 

The carbonylation of formaldehyde and sequential esterification with methanol in BMI PFg IL promoted by p-toluenesulfonic acid/CF3S03Ag produces methyl glycolate with a significant improvement in the catalytic activity when compared with solid acid catalysts in organic solvents [67]. This effect is rationalized in terms of the stabilization effect of the IL on the acylium ion intermediate, HOCH2CO. ... 

Oxidative Ring Closure Reactions 4.03.4.1.1 C—N bond formation N—N bond formation C—S bond formation N—S bond formation O—C bond formation O—N bond formation S—S, S—Se and Se—Se bond formation Electrophilic Ring Closures via Acylium Ions and Related Intermediates Ring Closures via Intramolecular Alkylations... 

Electrophilic Ring Closures via Acylium Ions and Related Intermediates... 

There are alternatives to the addition-elimination mechanism for nucleophilic substitution of acyl chlorides. Certain acyl chlorides are known to react with alcohols by a dissociative mechanism in which acylium ions are intermediates. This mechanism is observed with aroyl halides having electron-releasing substituents. Other acyl halides show reactivity indicative of mixed or borderline mechanisms. The existence of the SnI-like dissociative mechanism reflects the relative stability of acylium ions. 

These reactions are presumed to occur through aroyl triflate intermediates which dissociate to aiyl acylium ions. Lithium perchlorate and scandium triflate also promote acylation. ... 

In highly concentrated acids, however, with leaving groups that facilitate bond cleavage, the A1 route may prevail, the intermediate being the acylium ion ... 

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... 

In some systems the acylium ion is formed reverisbly and to a detectable extent while in others, to be discussed in the next chapter, it is a hypothetical intermediate. The first evidence for a stable acylium ion was the fourfold depression of the freezing point of sulfuric acid by mesitoic acid.176 The presence of the acylium ion in the sulfuric acid solutions of carboxylic acids with fourfold depressed freezing points is confirmed chemically by the high yield of ester obtained on pouring the... 

There is no reason to believe that any of these reactions involve formation of an intermediate acylium ion (Bentley et al., 1984) but the extent of C—Cl bond breaking probably increases markedly with increasing electron release from a substituent. Kinetic primary chloride isotope effects show that C—Cl bond breaking increases with increasing electron release in hydrolyses of 4-substituted benzoyl chlorides in aqueous acetone (Fry, 1970)... 

Thus, the best compromises for Boc and Fmoc chemistries seem to be cyclohexyl and 2,4-dimethylpent-3-yl (Dmpn), which is of intermediate stability, and the removal of which by trifluoromethanesulfonic acid with the aid of thioanisole (see Section 6.22) leads to minimal imide formation (see Section 6.13). Points to note are that acidolysis of esters by hydrogen fluoride can lead to fission at the oxy-car-bonyl bond instead of the alkyl-oxy bond, thus generating acylium ions that can react with nucleophiles (see Sections 6.16 and 6.22), and that benzyl esters may undergo transesterification if left in methanol. The side reactions of cyclization (see Section 6.16) and acylation of anisole (see Section 6.22) caused by acylium ion formation do not occur at the side chain of aspartic acid.47-51... 

Koch-Haaf reaction, acylium ions, 42 160 Koch reaction, 34 126 Kolbel-Engelhardt reaction, 31 59 Kolbe reaction, 40 160-161 chemical identity of adsorbed intermediates, 38 18-19... 

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