Synthesis of Stable Diarylpalladium(II) Complexes: Detailed Study of the Aryl-Aryl Bond-Forming Reductive Elimination

Trends in carbazole synthesis - an update (2013-2023)

The interest of scientists in the carbazole core has risen steadily over the last 30 years, particularly over the last decade given its presence in several active pharmaceutical ingredients, functional materials and a wide range of biologically active natural products. The continuous development of more efficient, more (regio-)selective and "greener" methodologies to access the carbazole core is thus imperative. This review compares and evaluates synthetic strategies towards the carbazole core that have been reported since 2013, with a focus on their applicability towards the total synthesis of carbazole-containing natural products.

A new selective inhibitor for IMP-1 metallo-β-lactamase, 3Z,5E-octa-3,5-diene-1,3,4-tricarboxylic acid-3,4-anhydride

3Z,5E-Octa-3,5-diene-1,3,4-tricarboxylic acid-3,4-anhydride (ODTAA, 1) was isolated from Paecilomyces sp. FKI-6801 for its selective IMP-1 MBL inhibitory activity. The first total synthesis of 1 from the commercially available compound was achieved in 9 steps with 28% overall yield. Introduction of catechol to the maleic anhydride of 1 improved the IC₅₀ toward IMP-1 MBL and the inhibitory activity against IMP-1 MBL-producing P. aeruginosa. Treatment of the maleic anhydride scaffold with amine showed that the β-carbonyl-α,β-unsaturated carboxylic acid moiety is required as a pharmacophore for IMP-1 MBL inhibition.

Dearomatization-Rearomatization Strategy for Synthesizing Carbazoles with 2,2'-Biphenols and Ammonia by Dual C(Ar)-OH Bond Cleavages

Carbazole is an essential building block in various pharmaceuticals, agrochemicals, natural products, and materials. For future sustainability, it is highly desirable to synthesize carbazole derivatives directly from renewable resources or cheap raw materials. Phenolic compounds are a class of degradation products of lignin. On the other hand, ammonia is a very cheap industrial inorganic chemical. Herein, an efficient dearomatization-rearomatization strategy has been developed to directly cross-couple 2,2'-biphenols with ammonia by dual C(Ar)-OH bond cleavages. This strategy provides a greener pathway to synthesize valuable carbazole derivatives from phenols.

Palladium-Catalyzed Ortho C-H Arylation of Aniline Carbamates with Diazonium Salts under Mild Conditions: Expedient Synthesis of Carbazole Alkaloids

Despite the significant progress, C-H arylation with aryldiazonium salts is a major challenge because of the faster rate of oxidative addition compared to the C-H insertion, leading to a deleterious homocoupling product. Recently, this limitation has been overcome by merging a photoredox catalyst with transition-metal catalysts which proceeds through a distinct single electron-transfer mechanism. However, we have observed that the photoredox catalyst is not necessary for the C-H arylation of aniline rather chemical reactivity can be controlled by tuning the electronic nature of the substrate. We report, herein, a palladium-catalyzed C-H arylation of aniline carbamates with aryldiazonium salts under external oxidant, acid, base free conditions at room temperature. Mechanistic studies suggest that the present reaction proceeds through a directed electrophilic metalation pathway which is the slowest step. However, the oxidative addition may take place through either ionic (2e^-) or radical (1e^-) pathway to generate hypervalent Pd(IV) or Pd(III) intermediate, respectively. A facile reductive elimination from the hypervalent palladium complex furnishes the C-H arylation product under mild conditions. The carbamate directing group is easily removed from the product to obtain the corresponding ortho-arylated aniline, which is a precursor for plethora of carbazole alkaloids and other biologically active molecules. The reaction is scaled-up to gram scale to furnish the desired product in comparable yields. Finally, we have applied this C-H arylation methodology for the synthesis of series of carbazole alkaloids such as clausine V, clauszoline K, O-methoxymahanine, and O-methylmurrayamine-D.

Synthesis of Indoles through Palladium-Catalyzed Three-Component Reaction of Aryl Iodides, Alkynes, and Diaziridinone

The three-component reaction of aryl iodides, alkynes, and diaziridinone is described. The reaction provides an innovative synthetic approach for indoles. The approach features high efficiency, broad substrate scope, and excellent regioselectivity. C, C-Palladacycles should act as the intermediates. The C, C-palladacycles are obtained from simple aryl halides and alkynes and then reacted with diaziridinone to afford indoles.

Synthesis of Tetranuclear Palladium(II) Complexes and Their Catalytic Activity for Cross-Coupling Reactions

We have developed a short and simple synthesis of tetranuclear palladium(II) complexes that have been structurally confirmed by X-ray analysis. These complexes were formed in about 30 % overall yield by spontaneous metalation of dimethylaminoarene derivatives and exhibit a high stability. We have studied the utility of the tetranuclear palladium(II) complexes as precatalysts for Mizoroki-Heck and Suzuki-Miyaura cross-coupling reactions. Our novel complexes show excellent catalytic activities with high turnover numbers (TON) and high turnover frequencies (TOF) (e.g., for the Suzuki-Miyaura reaction: TON up to 538000 and TOF up to 23400 h^-1 at room temperature).