Acylation dicationic intermediates

Klumpp, D. A. Rendy, R. Zhang, Y. Gomez, A. McElrea, A. Dicationic Intermediates Involving Protonated Amides Dual Modes of Reactivity Including the Acylation of Arenes. Org. Lett. 2004, 6,1789-1792. 

Carbo-acyl dicationic species have been proposed as intermediates in several reports, but these types of distonic superelectrophiles have not yet been sufficiently studied. Work by the Olah group has shown that pro-tonated carboxylic acids cleave to the acyl ions in superacidic media at temperatures above — 10°C.32 In principle, ionization of a second group (such as hydroxyl or olefinic) can generate a carbocationic site adjacent to... 

Other diprotonated acyl-pyridines have likewise been studied.61 In studies of 5-, 6-, 7-, and 8-hydroxyquinolines and 5-hydroxyisoquinoline, dicationic intermediates like 185 (Table 4) were found to be involved in superacid catalyzed reactions with benzene and cyclohexane.59 For example, 8-hydroxyquinoline (187) reacts in CF SOsH-SbFs to generate dications (188 and 189) and undergoes ionic hydrogenation in the presence of cyclohexane (eq 64). Compound 187 also reacts with benzene in suspensions of aluminum halides (eq 65). 

A one-pot synthesis of 1-indanones and 1-tetralones in good to excellent yields have been developed by reacting a series of alkenyl carboxylic acid derivatives with arenes in TfOH medium. The reaction involves dicationic intermediates involving intermolecular alkylation followed by intramolecular acylation (eq 31). These reactions have been further investigated. ... 

Friedel-Crafts acylation using nittiles (other than HCN) and HCI is an extension of the Gattermann reaction, and is called the Houben-Hoesch reaction (120—122). These reactions give ketones and are usually appHcable to only activated aromatics, such as phenols and phenoHc ethers. The protonated nittile, ie, the nitrilium ion, acts as the electrophilic species in these reactions. Nonactivated ben2ene can also be acylated with the nittiles under superacidic conditions 95% trifluoromethanesulfonic acid containing 5% SbF (Hg > —18) (119). A dicationic diprotonated nittile intermediate was suggested for these reactions, based on the fact that the reactions do not proceed under less acidic conditions. The significance of dicationic superelectrophiles in Friedel-Crafts reactions has been discussed (123,124). 

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