Cyclodehydrogenation

Cyclodehydrogenation is a chemical reaction in which adjacent aryl units are fused by removing hydrogen, forming new carbon–carbon bonds and enlarging the fused-ring system. The process converts linear or sparsely connected arene precursors into polycyclic aromatic hydrocarbons (PAHs) with extended conjugation. It is a key step in the dehydrogenative synthesis of nanographenes and other PAHs from preorganized polyaryl substrates.

Most widely used forms are oxidative cyclodehydrogenations, often called Scholl-type reactions, which promote intramolecular ring closure between adjacent aryl rings. These reactions typically employ strong Lewis acids such as aluminum trichloride or ferric chloride, sometimes in combination with oxidants, and are carried out at elevated temperatures. Alternative oxidative methods use reagents such as DDQ (2,3-dichloro-5,6-dicyano-p-benzoquinone) or cerium(IV) salts. In addition, catalytic dehydrogenative cyclizations using transition metals (for example, palladium or copper catalysts under oxidizing conditions) have been reported. The exact mechanism can involve generation of aryl cations or radicals, followed by cyclization and rearomatization.

Applications of cyclodehydrogenation include the synthesis of classic PAHs such as pyrene, phenacenes, and larger nanographenes like hexabenzocoronene, often from preorganized hexaaromatic or hexaarylbenzene precursors. The method enables rapid construction of planar, highly conjugated rings but can suffer from regioselectivity challenges, side reactions, and limited yields for large systems. Substituents and substrate design are important to control fusion patterns and prevent over-oxidation.

See also Scholl reaction and oxidative cyclodehydrogenation.