Palladium(II)-Catalyzed Direct Ortho Arylation of 4-Methyl-N-phenylpyridin-2-amines via C–H Activation/C–C Coupling and Synthetic Applications

Synthesis and Mechanistic Investigation of Bipyrazolo[1,5-a]pyridines via Palladium-Catalyzed Cross-Dehydrogenative Coupling of Pyrazolo[1,5-a]pyridines

The synthesis of a range of 3,3'-bipyrazolo[1,5-a]pyridine derivatives via direct cross-dehydrogenative coupling of pyrazolo[1,5-a]pyridine precursors is herein presented. This simple and efficient methodology involving palladium(II)-catalyzed C-H bond activation showed good functional group tolerance and product yield (up to 94%). Through the mechanistic insights gained from both kinetic isotope effect experimental studies and density functional theory calculations, a plausible reaction mechanism was outlined. Furthermore, subsequent derivatizations of the resulting 7,7'-diaryl-3,3'-bipyrazolo[1,5-a]pyridines, executed by performing palladium-mediated ortho C-H bond activation followed by hypervalent iodine-induced chlorination, rendered this series of compounds more extended π-conjugation and twisted conformations. Our study on these bipyrazolo[1,5-a]pyridine-based luminogens provides new opportunities for tailor-made organic luminescent materials.

Synthesis of Dibenzosuberones Bearing an Isoxazole Group via Palladium-Catalyzed Intramolecular C-H/C-Br Bond Cross-Coupling of Ortho-Aroylated 3,5-Diarylisoxazoles

A facile and efficient synthetic methodology for preparing dibenzosuberones via a C-H bond activation strategy is presented. The ortho-aroylated 3,5-diarylisoxazole was employed as the starting substrate to undergo palladium-catalyzed intramolecular C-H/C-Br bond cross-coupling to produce a variety of dibenzosuberones bearing an isoxazole group in 24 to >99% ¹H NMR yields. The dibenzosuberone structure was further confirmed by X-ray crystallography. The developed methodology exhibits very good functional group tolerance. In addition, a rational mechanism was presented for describing the reaction process. For the prepared dibenzosuberone, the use of Mo(CO)₆ as the catalyst can easily transform the isoxazole ring into the β-aminoenone group. Finally, the structure of the anticipated ring-opening product, dibenzosuberenone, bearing a β-amino-α-ketone group was secured by X-ray crystallography.

Discovery of Small-Molecule Cyclic GMP-AMP Synthase Inhibitors

Cyclic guanosine monophosphate-adenosine monophosphate (GMP-AMP) (cGAS), a cytosolic DNA sensor, plays an important role in the type I interferon response. DNA from either invading microbes or self-origin triggers the enzymatic activity of cGAS. Aberrant activation of cGAS is associated with various autoimmune disorders. Only one selective probe exists for inhibiting cGAS in cells, while others are limited by their poor cellular activity or specificity, which underscores the urgency for discovering new cGAS inhibitors. Here, we describe the development of new small-molecule human cGAS (hcGAS) inhibitors (80 compounds synthesized) with high binding affinity in vitro and cellular activity. Our studies show CU-32 and CU-76 selectively inhibit the DNA pathway in human cells but have no effect on the RIG-I-MAVS or Toll-like receptor pathways. CU-32 and CU-76 represent a new class of hcGAS inhibitors with activity in cells and provide a new chemical scaffold for designing probes to study cGAS function and development of autoimmune therapeutics.

Benzimidazopurine nucleosides from N6-aryl adenosine derivatives by PhI(OAc)2-mediated C-N bond formation, no metal needed

The reaction of a variety of N⁶-aryl 2'-deoxyadenosine and adenosine derivatives with PhI(OAc)₂ in 1,1,1,3,3,3-hexafluoro-2-propanol provides a facile access to benzimidazopurine nucleoside analogues by metal-free C-N bond formation with a purinyl nitrogen atom. These reactions likely proceed via radical-cation/radical processes as indicated by radical inhibition experiments.

C-H Activation in Primary 3-Phenylpropylamines: Synthesis of Seven-Membered Palladacycles through Orthometalation. Stoichiometric Preparation of Benzazepinones and Catalytic Synthesis of Ureas

The dimeric cyclometalated complexes [Pd2{κ(2)C,N-C6H4CH2CH2C(R)(Me)NH2-2}2(μ-Cl)2] (R = Me (1a), H (1b)) are prepared by reacting 1,1-dimethyl-3-phenylpropylammonium or 1-methyl-3-phenylpropylammonium triflate with Pd(OAc)2 in a 1:1 molar ratio and subsequent treatment with excess NaCl. The mononuclear derivatives [Pd{κ(2)C,N-C6H4CH2CH2C(R)(Me)NH2-2}Cl(L)] (L = PPh3, R = Me (3a), H (3b); L = 4-picoline (4-pic), R = Me (4a), H (4b)) were prepared from 1a,b by splitting the chloro bridges with the neutral ligands L. A conformational analysis of the mononuclear palladacycles in solution has been carried out. Insertion of CO takes place into the Pd-C bond of complexes 1a,b, affording Pd(0) and the tetrahydro-benzazepinone 5a or 5b, which possesses potential pharmacological interest. Additionally, the triflate salt A or B undergoes catalytic carbonylation with CO to afford the corresponding N,N'-dialkylurea, using Pd(OAc)2/Cu(OAc)2, in boiling acetonitrile. The crystal structures of 3a·(1)/2H2O, 3b, 4b·CHCl3, and 5a were determined by X-ray diffraction studies.

A Domino Oxidation/Arylation/Protodecarboxylation Reaction of Salicylaldehydes: Expanded Access to meta-Arylphenols

A method that allows salicylaldehydes to be efficiently transformed into meta-arylated phenol derivatives through a cascade oxidation/arylation/protodecarboxylation sequence is presented. We demonstrate that the aldehyde functional group can be used as a convenient removable directing group to control site selectivity in C-H activation. Aldehydes are easily introduced into the starting materials and the group is readily cleaved after the C-H functionalization event.

Stoichiometric to catalytic reactivity of the aryl cycloaurated species with arylboronic acids: insight into the mechanism of gold-catalyzed oxidative C(sp2)-H arylation

Based on the well-defined five-membered aryl gold(iii) complexes, [Au(tpy)X2] (3a and 3b) and [AuBr(Ph)(tpy)] (7), as well as the aryl gold(iii) complex [AuCl2(Ph)(tpy)] (8) (tpy = 2-(o-tolyl)pyridine) as reliable models, we present a detailed study of the mechanism for gold(iii)-catalyzed oxidative cross-coupling reactions between cycloaurable arenes and arylboronic acids. Here we report the direct evidence for a mechanistic proposal including arene C-H activation, transmetallation and biaryl reductive elimination. The chelation-assisted C-H activation strategy has been used for the development of the gold(iii)-catalyzed C-H bond arylation of arenes with aryl reagents to forge extended π-conjugated systems.

Salicylic acids as readily available starting materials for the synthesis of meta-substituted biaryls

Salicylic acids are shown to be readily available and versatile starting materials that easily undergo a tandem arylation–protodecarboxylation process under Pd-catalysis. The corresponding meta-arylphenols can subsequently be easily transformed into a variety of meta-functionalized biaryls, highlighting the versatility of this approach to access this structural motif.