Ascorbic acid: A versatile catalyst for the synthesis of organic compounds

Reviewed by Emily Henderson, B.Sc.Sep 26 2020

One of the important objectives of green chemistry is the use of eco-friendly solvents and catalysts to perform chemical reactions. Catalysts such as organocatalysts, enzymes, and ionic liquids have shown very assuring results in synthesis by reducing the number of hazardous effects of chemical reactions. Among those catalysts, the activities of ascorbic acid are promising because of its extraordinary abilities.

Ascorbic acid (Vitamin C or ascorbate) is the most well-known vitamin found in different types of foods, most commonly in fruits. For many years, ascorbic acid has been used as a constituent of various dermatologic drugs and cosmetics.

However, because of the lack of an enzyme called gulonolactone oxidase, humans and apes cannot synthesize ascorbic acid. Ascorbic acid was first isolated in 1928 by Hungarian biochemist Albert Szent-Györgyi and he named it as hexuronic acid. In 1932, Haworth and King independently established the molecular structure of hexuronic acid and renamed it as ascorbic acid.

Ascorbic acid is an antioxidant. Its oxidation-reduction potential and the stability of its oxidation products contribute values to the application of ascorbic acid as an antioxidant.

In addition, as a water-soluble reducing agent and donor antioxidant, ascorbic acid can undergo three consecutive or active types: ascorbate (reduced form), dehydroascorbic acid (oxidized form) and ascorbate radical (intermediate oxidized form).

The two-way reactions among these three forms occur very simply due to the low one-electron transfer potential. As a consequence, ascorbic acid has been accepted as a chemically unstable substnace. It is observed that ascorbic acid promptly undergoes pH-dependent autoxidation and makes hydrogen peroxide.

In the presence of catalytic metals, this oxidation is accelerated. The combination of metal and ascorbic acid is a very efficient oxidizing system. This system is mainly employed for the hydroxylation of alkenes, aromatics, and other oxidation reactions. Besides, in the presence of catalytic metal ions, ascorbic acid can exert pro-oxidant effects. Ascorbic acid serves as a reducing cofactor for many enzymes.

In organic synthesis, ascorbic acid can be used as a reactant in the preparation of chiral synthons and also as a catalyst to accelerate the reaction.

Several important reactions are performed with ascorbic acid: These include the oxidation of amines to carbonyl compounds, cross-coupling of disulfides with aryl iodides, ATRA of polyhalogenated compounds to alkenes, ATRP of polyhalogenated compounds to alkenes, ATRC of polychloroamides to cyclic amides, Amination of aryl halides to primary aromatic amines, oxidation of sulfides, arylation of arenes with anilines, cyclization of aryl radicals with arenes, metal-free synthesis of aryl sulfides, oxidative arylation of vinyl arenes to 2-aryl acetophenones, photoreductive removal of O-benzyl groups, photocatalytic reductive fluoroalkylation of nitrones, reduction of N-heterocyclic nitroaryls to anilines, synthesis of cyclic carbonates from CO2, dehalogenation of vicinal dibromo-, α-halo-, and α,α-dibromocarbonyl compounds, synthesis of quinolinones, aminocoumarins and anilines, solvent-free synthesis of polyhydroquinoline and acridine derivatives, benzylic C-H oxidation, synthesis of benzimidazoles, synthesis of triazoles, synthesis of α-acyloxy carboxamides, synthesis of 2-arylbutanoates, synthesis of benzodiazepines, synthesis of benzoxazepines, synthesis of dihydropyrimidinones, and synthesis of xanthenes.

These examples are described in a review published in the journal, Current Organocatalysis. The examples included in this review demonstrate that ascorbic acid is a versatile catalyst for the synthesis of diverse organic compounds. Reactions catalyzed by ascorbic acid are performed in organic or aqueous media.

Ascorbic acid-catalyzed reactions follow diverse mechanistic pathways. The mechanism of most of the processes is proposed by the authors. The multiple functions of ascorbic acid are realized from the suggested mechanisms.

In view of the current situation of conducting organic reactions under environmentally benign circumstances, ascorbic acid-catalyzed mechanistically unique transformations are highly valuable. Based on this study, review, the authors anticipate that additional reactions will be discovered and investigated using ascorbic acid as a catalyst.​