Elemental Sulfur-Promoted Aerobic Dehydrogenative Aromatization of Cyclohexanones with Amines

Halogen bonding accelerated aerobic dehydrogenative aromatization for 4-aminoquinoline preparation

This study presents a highly efficient method for 4-aminoquinoline derivative preparation under transition metal-free conditions. The process involves an aerobic oxidative dehydrative coupling of 2,3-dihydroquinolin-4(1H)-ones with various amines, including ammonia, resulting in high yields of the desired products. The method is also applicable to substituted 4-aminoquinoline derivative construction through a cyclization/dehydrative coupling cascade process starting from 2'-amino chalcones. Mechanistic studies reveal that iodine (I2) is consumed to produce 3-iodoquinolin-4-ol, which acts as a true catalyst with high catalytic efficacy (as low as 0.5 mol%). The presence of halogen bonding is critical in the inter-molecular transfer hydrogenation process to generate inactive quinolin-4-ol. Subsequently, using air/oxygen as the terminal oxidant, the iodine anion was oxidized to I2 to regenerate the 3-iodoquinolin-4-ol from quinolin-4-ol in the catalytic cycle. Key benefits of this methodology include its simplicity, transition metal-free conditions, environmentally-benign oxidant, and high atom economy, making it a valuable approach for synthesizing medicinally significant 4-aminoquinoline derivatives.

Acid-Promoted Amination of Cyclohexenone for the Divergent Synthesis of p-Aminophenols and Tertiary Amines

A tunable method for the selective preparation of p-aminophenol and tertiary amines from a secondary amine and cyclohexenone has been described. Nonaromatic cyclohexenones were used as an aryl source. The desired tertiary amine products were generated when using I2 as the catalyst. This approach yields single-site-selective p-aminophenol without using I2, and the 18O labeling experiments demonstrated that hydroxyl oxygen originates from O2.

Aminoselenation and Dehydroaromatization of Cyclohexanones with Anilines and Diselenides

A three-component cascade reaction involving cyclohexanones, anilines, and diaryl diselenides under metal-free conditions is reported. The ortho-selenation of cyclohexanones with diaryl diselenides, followed by sequential dehydroaromatization with anilines, enables the preparation of a variety of o-selanyl anilines in moderate to excellent yields. This innovative transformation is notable for its excellent tolerance of functional groups and is suitable for the late-stage modification of complex pharmaceuticals.

Elemental Sulfur Promoted Cyclization of Aryl Hydrazones and Aryl Isothiocyanates Yielding 2-Imino-1,3,4-thiadiazoles

We report a method for using elemental sulfur to facilitate the cyclization of aryl hydrazones and aryl isothiocyanates, affording biorelated 2-imino-1,3,4-thiadiazoles. Reactions progressed in the presence of elemental sulfur, N-methylmorpholine base, and DMSO solvent, while were tolerant of a wide range of functionalities including halogen, nitro, cyano, methylsulfonyl, and heterocyclic groups. The method appears to offer a general pathway for using simple, cheap, and stable reagents to afford triaryl-substituted 2-imino-1,3,4-thiadiazoles under relatively mild conditions.

An I2-DMSO catalytic manifold enabled aromatization for C-ring editing of podophyllotoxone

The precise aromatization of the C-ring of podophyllotoxone to access value-added dehydropodophyllotoxin derivatives conventionally requires the use of equivalent amounts of unsustainable oxidants and suffers from inefficiencies. Taking advantage of the hydridic character of the C8 and C8' of podophyllotoxone, we have developed an I2-DMSO catalytic manifold that enables a green and selective dehydrogenative aromatization to overcome these synthetic challenges. An unprecedented dehydrogenative amination of podophyllotoxone derivatives was also realized using aniline as the reaction partner.

Site-Selective Synthesis of Aryl Sulfides via Oxidative Aromatization of Cyclohexanones with Thiophenols

We have introduced a metal-free facile access for the thiolation/aromatization of cyclohexanones with thiophenols to the corresponding aryl sulfides. The dehydroaromatic reaction of non-aromatic cyclohexanones proceeded smoothly using oxygen as a green oxidant.

Electrochemical Desulfurative Cyclization Accessing Oxazol-2-amine Derivatives via Intermolecular C-N/C-O Bond Formation

A practical protocol has been established to access diverse oxazol-2-amine derivatives in one step via the electrochemical desulfurative cyclization of isothiocyanates and α-amino ketones. On the basis of the cycle of in situ generation of iodine/desulfurative cyclization/iodide anion regeneration, the reaction is performed under metal-free and external-oxidant-free electrolytic conditions to achieve the formation of intermolecular C-O and C-N bonds, providing oxazol-2-amines in moderate to excellent yields.

Recent advances in the transition metal-free oxidative dehydrogenative aromatization of cyclohexanones

Cyclohexanone is a simple and widely available raw material that can be obtained from lignin biomass, highlighting its renewable and sustainable features. Cyclohexanone, as an important synthon in organic chemistry, has been demonstrated to be viable for constructing functionalized arenes and benzoheteroarenes, with recent extensive development on transition metal-free oxidative dehydrogenative aromatization. This review focuses on recent research progress on the transition metal-free derivation of cyclohexanones via oxidative dehydrogenative aromatization.