Conghui Tang

Recent advances in transition-metal catalyzed reactions using molecular oxygen as the oxidant

For green and sustainable chemistry, molecular oxygen is considered as an ideal oxidant due to its natural, inexpensive, and environmentally friendly characteristics, and therefore offers attractive academic and industrial prospects. This critical review introduces the recent advances over the past 5 years in transition-metal catalyzed reactions using molecular oxygen as the oxidant. This review highlights the scope and limitations, as well as the mechanisms of these oxidation reactions (184 references).

Copper-Catalyzed C–H Azidation of Anilines under Mild Conditions

A novel and efficient copper-catalyzed azidation reaction of anilines via C-H activation has been developed. This method, in which the primary amine acts as a directing group by coordinating to the metal center, provides ortho azidation products regioselectively under mild conditions. This effective route for the synthesis of aryl azides is of great significance in view of the versatile reactivity of the azide products.

Cationic Cobalt(III)-Catalyzed Aryl and Alkenyl CH Amidation: A Mild Protocol for the Modification of Purine Derivatives

A cationic cobalt(III)-catalyzed direct C-H amidation of unactivated (hetero)arenes and alkenes by using 1,4,2-dioxazol-5-ones as the amidating reagent has been developed. This transformation proceeds efficiently under external oxidant-free conditions with a broad substrate scope. Moreover, 6-arylpurine compounds, which often exhibit high potency in antimycobacterial, cytostatic, and anti-HCV activities, can be smoothly amidated, thus offering a mild protocol for their late stage functionalization.

From Ketones to Esters by a Cu-Catalyzed Highly Selective C(CO)–C(alkyl) Bond Cleavage: Aerobic Oxidation and Oxygenation with Air

The Cu-catalyzed aerobic oxidative esterification of simple ketones via C-C bond cleavage has been developed. Varieties of common ketones, even inactive aryl long-chain alkyl ketones, are selectively converted into esters. The reaction tolerates a wide range of alcohols, including primary and secondary alcohols, chiral alcohols with retention of the configuration, electron-deficient phenols, as well as various natural alcohols. The usage of inexpensive copper catalyst, broad substrate scope, and neutral and open air conditions make this protocol very practical. (18)O labeling experiments reveal that oxygenation occurs during this transformation. Preliminary mechanism studies indicate that two novel pathways are mainly involved in this process.

Copper‐Catalyzed Aerobic Oxidative CC Bond Cleavage for CN Bond Formation: From Ketones to Amides

A novel copper-catalyzed aerobic oxidative C(CO)-C(alkyl) bond cleavage reaction of aryl alkyl ketones for C-N bond formation is described. A series of acetophenone derivatives as well as more challenging aryl ketones with long-chain alkyl substituents could be selectively cleaved and converted into the corresponding amides, which are frequently found in biologically active compounds and pharmaceuticals.

FeCl2‐Promoted Cleavage of the Unactivated CC Bond of Alkylarenes and Polystyrene: Direct Synthesis of Arylamines

Ironing it out: an efficient and convenient nitrogenation strategy involving C-C bond cleavage for the straightforward synthesis of versatile arylamines is presented. Various alkyl azides and alkylarenes, including the common industrial by-product cumene, react using this protocol. Moreover, this method provides a potential strategy for the degradation of polystyrene.

Rh-Catalyzed N–O Bond Cleavage of Anthranil: A C–H Amination Reagent for Simultaneous Incorporation of Amine and a Functional Group

A novel Rh(III)-catalyzed C-H bond amination with the simultaneous release of a formyl group at distal positions is realized employing anthranil as a new type of C-H amination reagent. This chemistry provides an efficient protocol for the synthesis of 2-acyl diarylamines, which are important structural motifs in many bioactive compounds. This new type of C-H amination reagent possesses the advantages of high atom economy, avoids the use of external oxidants, and enables further transformation of the amination products.

Conversion of Simple Cyclohexanones into Catechols

A novel I2-catalyzed direct conversion of cyclohexanones to substituted catechols under mild and simple conditions has been described. This novel transformation is remarkable with the multiple oxygenation and dehydrogenative aromatization processes enabled just by using DMSO as the solvent, oxidant, and oxygen source. This metal-free and simple system demonstrates a versatile protocol for the synthesis of highly valuable substituted catechols and therefore streamlines the synthesis and modification of biologically important molecules for drug discovery.

Rh‐Catalyzed Direct Amination of Unactivated C(sp3)−H bond with Anthranils Under Mild Conditions

C-N Bond formation is of great significance due to the ubiquity of nitrogen-containing compounds. Here, a mild and efficient Rh(III) -catalyzed C(sp(3) )-H aryl amination reaction is reported. Anthranil is employed as the nitrogen source with 100 % atom efficiency. This C-H amination reaction exhibits broad substrate scope without using any external oxidants. Mechanistic studies including rhodacycle intermediates, H-D exchange, kinetic isotope effect (KIE) experiments, and in situ IR are presented.

Metal-Free Nitrogenation of 2-Acetylbiphenyls: Expeditious Synthesis of Phenanthridines

An intermolecular nitrogenation reaction toward the synthesis of phenanthridines has been developed. This metal-free protocol provides a novel nitrogen-incorporation transformation using azides as the nitrogen source. Phenanthridines, which are of great interest in pharmaceutical and medicinal chemistry, are synthesized efficiently in one step. Moreover, the byproducts derived from the Schmidt reaction are inhibited, which further demonstrated the high chemoselectivity of this transformation.