Acyliumioni

Acyliumioni, commonly known in English as acylium ions, are positively charged carbonyl-containing species that act as highly reactive electrophiles in organic synthesis. They are resonance-stabilized cations with a general formula of R–CO+, often depicted by two canonical forms: R–C≡O+ and R–CO+. The positive charge is delocalized between the carbonyl carbon and the oxygen, which stabilizes the ion and enables rapid acyl-transfer reactions.

Formation and sources typically involve the in situ generation of the acylium ion from an acyl precursor under the action of a strong Lewis acid. A classic example is the reaction of an acyl chloride (RCOCl) with a Lewis acid such as AlCl3, which produces the acylium ion [RCO]+, often as a complex or paired with a counterion (for instance, [RCO]+ AlCl4−). Other routes include activated derivatives of carboxylic acids or anhydrides under suitable catalytic conditions. In many cases, the active species exists as a coordinated complex with a Lewis acid (e.g., RCO–AlCl3).

Reactivity centers on their role as potent electrophiles. Acylium ions readily undergo acylation of electron-rich arenes, forming aryl ketones after workup (Friedel–Crafts acylation). They can also engage alkenes and arenes in diverse addition and cyclization pathways, enabling the construction of carbonyl-containing frameworks. The reactions generally proceed under Lewis acidic conditions and favor formation of ketone products with high regioselectivity.

Applications span classic Friedel–Crafts acylation, synthesis of substituted aryl ketones, and various routes to complex carbonyl compounds in both small-molecule synthesis and polymer chemistry. Acylium ions thus serve as versatile, transient electrophiles for introducing acyl groups in organic transformations.