Acridines refer to the most important class of nitrogen containing aromatic compounds structurally similar to anthracene, forming an important class of bioactive heterocycles that display a wide range of bioactivities. Acridines present in natural products and synthetic dye-stuffs with their application as optical materials. Photochemical transformations mainly employ transition-metal complexes as catalysts, and organic photocatalysts offer more potentially unique reactivity. Among them, acridine catalysts have gained popularity as one of the classes of organic photocatalysts, which are characterized by high excited-state reduction potentials. And acridine catalysts need to be fine-tuned for their catalytic performance by functionalization of their structural core.
Bernthsen reaction is one of the classical methods for the synthesis of acridines. A diarylamine is heated with a carboxylic acid (or anhydride) and zinc chloride at 200-210 °C for several hours to form a 9-substituted acridine.
Fig.1 Bernthsen acridine synthesis
Acridine catalysts are widely used in photocatalytic decarboxylation, photocatalytic Michael-Mannich radical cyclocondensation of amines and photocatalytic C–O bond cleavage.
The decarboxylation functionalization of organic carboxylic compounds can directly convert carboxyl groups into various C-C, C-X bonds (X = halogen, O, N, S, B), which is an important research topic in the field of organic synthesis. Compared with the traditional metal-catalyzed decarboxylation reaction, the visible-light-mediated photocatalytic reaction is mild and environmentally friendly, and the systems can mediate direct decarboxylation of various carboxylic acids, showing promising prospects. As a new class of visible-light photocatalysts, 9-arylacridine and its derivatives induce photodecarboxylation from the singlet excited state of acridine-carboxylic acid complexes via a proton-coupled electron transfer (PCET) process. The acridine photocatalyst can generate decarboxylation radicals directly in the acridine/copper dual process, which achieves the direct decarboxylation amination of carboxylic acids. And this decarboxylation conjugate addition reaction improves the overall efficiency of the photocatalytic system.
Fig.2 Direct acridine catalyzed decarboxylation
A four-component green technique for sans metal combination of polyfunctionalized dihydro-2-oxypyrroles was developed using the Michael–Mannich cyclocondensation of amines, dialkyl acetylenedicarboxylaes, and formaldehyde. Acridine yellow G (AYG) is a highly fluorescent dye that has been shown to have photosensitive and photodynamic effects for use in photocatalysis. AYG is used as a photo-induced electron transfer photocatalyst (PET) for the synthesis of a radical of polyfunctionalized dihydro-2-oxopyrroles. This reaction achieves excellent yields as well as efficient reactions by using a small amount of photocatalyst. Among them, AYG as a photocatalyst exhibits various advantages, including high yield, energy efficiency, high atom economy, etc.
Fig.3 Acridine yellow G structure
The cleavage of the relatively unstable C–O bonds in the lignin structure provides a sustainable green route for the preparation of abundant aromatic compounds. Using 2-(2-methoxyphenoxy)-1-phenylethan-1-ol as a model substrate and C9-aryl N-methyl/aryl acridinium salt as a photocatalyst, it can efficiently catalyze the selective CβO-Ar bond cleavage of diol monoarylethers, resulting in the preparation of 1,2-diols in good yields that constitutes a vital step toward converting lignin to value-added low-molecular-weight aromatics. Thus, the best organic acridine photocatalyst candidates in the photoinduced C-O bond cleavage of β-O-4 lignin models are determined.
Fig.4 Photocatalytic C−O bond fragmentation and N-substituted acridinium salts
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