Acylium cation

The activation energies were computed to 3.0 (toward 183), 0.3 (toward 182), and 21.8 kcal/mol (toward 184) at the B3-LYP/6-31G level, and thus the mechanism leading to 182 is the preferred one. The transition states of all three reactions belong to concerted but asynchronous reaction paths. The transacetalization of 177 with acylium cations results in the formation of the thermodynamically stabilized 187 (Scheme 121) [97JCS(P2)2105]. 186 is less stable than 187, and 185 is destabilized by 32.5 kcal/mol. Moreover, transacetalization of 177 with sulfinyl cations is not a general reaction. Further computational studies on dioxanes cover electrophilic additions to methylenedioxanes [98JCS(P2)1129] and the influence... 

Obviously, the reaction rate of nucleophilic substrates with acylium cations increases as the induced electronic density on C4 increases, thus the N basicity also increases. In onr experiments the reaction rate followed the sequence benzenesulfonamide > p-nitrobenzenesulfonamide > p-... 

In fluorosulfonic acid the anodic oxidation of cyclohexane in the presence of different acids (RCO2H) leads to a single product with a rearranged carbon skeleton, a 1-acyl-2-methyl-1-cyclopentene (1) in 50 to 60% yield (Eq. 2) [7, 8]. Also other alkanes have been converted at a smooth platinum anode into the corresponding a,-unsaturated ketones in 42 to 71% yield (Table 1) [8, 9]. Product formation is proposed to occur by oxidation of the hydrocarbon to a carbocation (Eq. 1 and Scheme 1) that rearranges and gets deprotonated to an alkene, which subsequently reacts with an acylium cation from the carboxylic acid to afford the a-unsaturated ketone (1) (Eq. 2) [8-10]. In the absence of acetic acid, for example, in fluorosulfonic acid/sodium... 

The formation of a double bond during anodic oxidations can result from eliminations of protons, carbon dioxide or acylium cations. The electrooxi dative aromatization of dihydropyridine derivatives and heterocycles containing nitrogen atom (di-hydroquinoxalines, tetrahydrocinnolines) involves an ECE mechanism as previously... 

The sequence requires first simple A-acetylation with acetic anhydride. This product is then the substrate for the Friedel-Crafts reaction. The acylium cation is generated from acetyl chloride and aluminium chloride. 

You may perhaps remember that the acylium cation is the acylating species or, alternatively, you can deduce that the acyl chloride interacts with the Lewis acid AICI3, and is most likely to give AlCU. The acylium cation then emerges as the other part. This is resonance stabilized, as shown. 

Now the electrophilic attack of the aromatic system onto the acylium cation. The product is para substituted, so go for that site of attack. We then need removal of a proton, and chloride is the obvious base, but since this is complexed as AlC, we need the latter to decompose to chloride and AICI3. 

Cleavage a to carbonyl groups. Cleavage tends to occur a to carbonyl groups to give stable acylium cations. R may be an alkyl, -OH or -OR group. 

Acyclic cationic systems, chalcogen-halogen compounds, 35 299-301 Acylarsonic acids, 44 217 Acyldiazenido complexes, mechanisms of formation, 27 223-225 Acylium cation, 9 231, 241-243 Acylphosphonic acids, 44 217 Acylsilane, hydrolysis, 42 173 Adamantane... 

C-Acyl- and C-alkoxycarbonyl-pyrroles show a strong tendency to fragment to the acylium cation (Figure 15), which is well-stabilized by resonance. a-Acylpyrroles also yield a relatively intense fragment at m/e 66 corresponding to complete loss of the acyl group ... 

Difficulty Esterifiable Acids. The sterically hindered acids, such as 2.6-disubstituted benzoic acids, cannot usually be cslerified by conventional means. Several esters of sterically hindered acids such as 2,4.6-triisopropylbcnzoic acid have been prepared by dissolving 2 g of the acid in 14-20 mL of 100% H SOj. After standing a few minutes at room temperature, when presumably the acylium cation is formed, the solution is poured into an excess of cold absolute methanol. Most of the alcohol is removed under reduced pressure, about 50 ml. of water is added, and the distillation is continued under reduced pressure to remove the remainder of the methanol. The organic mailer is extracted with ether and treated with sodium carbonate solution. The ester is then distilled. Yields of esters made in this manner are 57-81%. 

Because the strength of the C=0 bond correlates with its stretching frequency, the latter increases when the substituent X becomes more electronegative (Table 2.2). Complete abstraction of the group X (e.g. for X = BF4 ) leads to the formation of acylium cations with a short and strong C=0 bond, as revealed by the high vibrational IR-frequency of these compounds [24] (Table 2.2). 

In mixed trifluoroacetic anhydrides, the strong electron-withdrawing tri-fluoroacetyl group polarizes the bonds to the anhydride oxygen in such a way that it tends to split to acylium cation and trifluoroacetate anion [/0/. Generally, the products of the reaction of hydroxy compounds with acyl trifluoroacetates are esters of carboxylic acids and trifluoroacetic acid, because the acyl carbonyl is better fit for a nucleophilic attack than the carbonyl next to the trifluoromethyl group [707]. 

Rearrangements are also no longer a problem because the electrophile, the acylium cation, is already relatively stable... 

Recently, the surprising ability of 4-alkyl-substituted 3-azapyrylium salts to undergo acylation at the 4-alkyl group by acylium cations was discovered157. This reaction gives 4-acylmethyl-3-azapyrylium salts 385 (equation 110). By comparing the spectral characteristics of 385 with those of model compounds 388 and 391 (equation 111) it was shown157 that salts 385 exist preferably in the enol form 386. This conclusion is confirmed also by AM 1 quantum-chemical calculations. 

IV. ACYLIUM CATIONS IN THE SYNTHESIS OF POLY(ARYLETHER KETONE)S... 

Reaction of the olcfm with the acid catalyst to form the carbocation. followed by forniation of the acylium-cation ... 

The main product, C, comes from the addition of both these electrophiles, but which adds first The ketone in A is meta-directing but the t-butyl group in B is para-directing. Product C has a par. relationship and must come from Friedel-Crafts acylation of B with the acylium cation. 

This mechanism (A = acid catalyzed, AC = acyl transfer, 1 = unimolecular) is observed in the esterification of 2,4,6-trisubstituted benzoic acids with R groups of moderate -i-M effect e.g. methyl). The Aac2 mechanism is blocked by the steric interference of the ortho substituents. Therefore, the acylium cation (17) is generated with anhydrous sulfuric acid and then treated with the alcohol (equation 2). R groups with strong -i-M effects, like methoxy, are not tolerable, as the aromatic nucleus undergoes sulfo-nation under the conditions. A variation of the AacI mechanism for aliphatic acids is achieved by using... 

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