2-Aminophenyl-1H-pyrazole as a Removable Directing Group for Copper-Mediated C–H Amidation and Sulfonamidation

Palladium-Catalyzed Meta C─H Alkoxylation and Amidation via Polarity Reversal of Nucleophilic Reagents

Norbornene-mediated remote metaselective C─H functionalization of arenes have been limited to relatively weakly electronegative and "soft" species, such as aryl, alkyl, and alkylamino moieties. Herein, we describe the first example of the use of a nucleophilic reagent, such as an alcohol or amide, to replace the electrophilic reagent during the palladium-catalyzed meta C─H alkoxylation or amidation reaction of an arene. The reaction conditions are mild and highly site-selective, thereby facilitating the direct introduction of natural products or drug molecules containing hydroxyl or amido groups at the meta positions of arenes. In addition, the directing group is rapidly convertible into the corresponding aldehyde, which further enhances the applicability of the reaction. Control experiments and density functional theory (DFT) calculations revealed that alcohol and amide polarity reversal induced by hypervalent iodine reagents and the subsequent formation of a Pd(IV) intermediate via the oxidative addition of the aryl-norbornyl-palladacycle intermediate are crucial for promoting the entire catalytic reaction cycle.

Diastereoselective Synthesis of Cyclobutanes via Rh-Catalyzed Unprecedented C-C Bond Cleavage of Alkylidenecyclopropanes

The stereoselective synthesis of highly substituted cyclobutanes is essential for the development of lead candidates in drug discovery. Herein, we present a novel Rh(III)-catalyzed reaction pathway for synthesizing substituted cyclobutanes, which involves a concerted N-C bond formation and C-C bond cleavage between 2-aryl quinazolinones and alkylidenecyclopropanes. Notably, the combination of Rh(III) catalyst and HFIP solvent plays a critical role in facilitating the formation of cyclobutane rings.

A Copper-Catalyzed Synthesis of Pyridoquinazolinones via C(sp2)-H Functionalization and Annulation of Benzoic Acids with 2-Aminopyridines

A facile synthesis of pyridoquinazolinones has been accomplished via C(sp2)-H functionalization and annulation of benzoic acids with 2-aminopyridines using Cu(OAc)2 as catalyst in DMSO. The reaction involves the formation of amides which on ortho amination followed by trans-amidation afford a wide array of products under ligand/base-free conditions.

2-Pyridone-Directed CuII-Catalyzed General Method of C(sp2)-H Activation for C-S, C-Se, and C-N Cross-Coupling: Easy Access to Aryl Thioethers, Selenide Ethers, and Sulfonamides and DFT Study

We have demonstrated N-substituted 2-pyridones as an N,O-directing group for selective C(sp2)-H-activated thiolation, selenylation, and sulfonamidation of ortho C-H bonds of benzamides. This method utilizes a cost-effective Cu(II)-salt catalyst instead of precious metal catalysts, achieving high yields, including gram-scale synthesis and excellent functional group tolerance. We applied this protocol to access 30 different compounds with high yields, demonstrating thiolation of fluorine-substituted benzamides as well. Density functional theory (DFT) calculations support the mechanism, including acetate-supported concerted metalation deprotonation (CMD) steps and the unique role of dimethyl sulfoxide (DMSO) solvent. The facile synthesis of pharmaceutically important sulfonamides and other compounds highlights the method's potential in chemistry and medicinal chemistry.

Copper(II)-Mediated, Site-Selective C(sp2)-H Sulfonamidation of 1-Naphthylamines

An operationally simple and efficient protocol for copper(II)-mediated, picolinamido-directed C8-H sulfonamidation of 1-naphthylamine derivatives with various sulfonamides has been developed. Remarkably, this cross-dehydrogenative C-H/H-N coupling reaction exhibits a broad substrate scope with excellent functional group tolerance, is scalable, and enables an expeditious route to a library of unsymmetrical N-arylated sulfonamides in good to excellent yields with exclusive site selectivity.

Catalyst-Controlled C-H Allylation and Annulation of 2-Aryl Quinazolinones with 2-Methylidene Cyclic Carbonate

The site-selective modification of quinazolinone as a privileged bicyclic N-heterocycle is an attractive topic in medicinal chemistry and material science. We herein report the ruthenium(II)-catalyzed C-H allylation of 2-aryl quinazolinones with 2-methylidene cyclic carbonate. In addition, tandem C-H allylation and annulation are achieved under rhodium(III) catalysis, resulting in the formation of tetracyclic quinazolinones including a tertiary carbon center. Post-transformations of the synthesized products demonstrate the potential of the developed methodology. A series of mechanistic investigations were also performed.

Mimicking the Cu Active Site of Lytic Polysaccharide Monooxygenase Using Monoanionic Tridentate N-Donor Ligands

In an effort to prepare small molecule mimics of the active site of lytic polysaccharide monooxygenase (LPMO), three monoanionic tridentate N donor ligands comprising a central deprotonated amide group flanked by two neutral donors were prepared, and their coordination chemistry with Cu(I) and Cu(II) was evaluated. With Cu(I), a dimer formed, which was characterized by X-ray crystallography and NMR spectroscopy. A variety of mononuclear and dinuclear Cu(II) species with a range of auxiliary ligands (MeCN, Cl^-, OH^-, OAc^-, OBz^-, CO₃ ^2-) were prepared and characterized by X-ray diffraction and various spectroscopies (UV-vis, EPR). The complexes exhibit structural similarities to the LPMO active site.

Carboxylate-Assisted Iridium (III)-Catalyzed C(sp2)-H Amidation of 2-Aroylimidazoles With Dioxazolones

The Ir(III)-catalyzed ortho C-H amidation of 2-aroylimidazoles with 3-aryldioxazolones as an amidating reagent is reported. The method provides a broad substrate scope with wide functional group compatibility. Mechanistic studies indicate that C-H bond cleavage is reversible and appears not to be the rate-determining step. The presence of an electron-donating group in the 2-aroylimidazoles and an electron-withdrawing group in the 3-aryldioxazoles significantly accelerates the reaction, suggesting that nitrene insertion is the rate-determining step.

Selective Mono- and Diamination of Ketones in a Combined Copper-Organocatalyst System

Herein, we report a simple and mild protocol for the chemoselective mono- and diamination of ketone using pyrazole as the amine source in a combined copper-organocatalyst system. Various substrates are compatible, providing the corresponding products in moderate to good yields. This strategy gives an efficient and convenient solution for the synthesis of α-pyrazole and α,α-dipyrazole ketone derivatives. The control experiment demonstrates that in situ generated hydrazone is a key intermediate in the transformation.

Pd-catalyzed bidentate auxiliary assisted remote C(sp3)-H functionalization

Pd-catalyzed C-H functionalisation affords effective synthetic tools to construct C-C and C-X bonds. Despite the challenges, the distal functionalization of C(sp³)-H bonds has witnessed significant developments and the use of bidentate auxiliaries has garnished this area by providing an opportunity to control reactivity as well as selectivity beyond proximal sites. This article covers the recent developments on the Pd-catalyzed bidentate auxiliary-assisted distal C(sp³)-H functionalization and is categorized based on the nature of functionalizations.

Copper-mediated oxidative C-H/N-H activations with alkynes by removable hydrazides

The efficient copper-mediated oxidative C-H alkynylation of benzhydrazides was accomplished with terminal alkynes. Thus, a heteroaromatic removable N-2-pyridylhydrazide allowed for domino C-H/N-H functionalization. The approach featured remarkable functional group compatibility and ample substrate scope. Thereby, highly functionalized aromatic and heteroaromatic isoindolin-1-ones were accessed with high efficacy with rate-limiting C-H cleavage.

Iridium-Catalyzed Regio- and Enantioselective Borylation of Unbiased Methylene C(sp3 )-H Bonds at the Position β to a Nitrogen Center

Reported herein is the pyrazole-directed iridium-catalyzed enantioselective borylation of unbiased methylene C-H bonds at the position β to a nitrogen center. The combination of a chiral bidentate boryl ligand, iridium precursor, and pyrazole directing group was responsible for the high regio- and enantioselectivity observed. The method tolerated a vast array of functional groups to afford the corresponding C(sp³ )-H functionalization products with good to excellent enantioselectivity.

Copper-Catalyzed Oxalamide-Directed ortho-C-H Amination of Anilines with Alkylamines

A copper-catalyzed oxalamide-directed ortho-C-H amination of anilines has been developed by using 1 atm of air as the sole oxidant. The protocol shows excellent functional group tolerance, and some heterocyclic amines including indole, benzothiophene, benzothiazole, quinoline, isoquinoline, and quinoxaline could be compatible in the reaction. The late-stage diversification of medicinal drugs demonstrates the synthetic utility of this protocol.

Bidentate Directing Groups: An Efficient Tool in C-H Bond Functionalization Chemistry for the Expedient Construction of C-C Bonds

During the past decades, synthetic organic chemistry discovered that directing group assisted C-H activation is a key tool for the expedient and siteselective construction of C-C bonds. Among the various directing group strategies, bidentate directing groups are now recognized as one of the most efficient devices for the selective functionalization of certain positions due to fact that its metal center permits fine, tunable, and reversible coordination. The family of bidentate directing groups permit various types of assistance to be achieved, such as N,N-dentate, N,O-dentate, and N,S-dentate auxiliaries, which are categorized based on the coordination site. In this review, we broadly discuss various C-H bond functionalization reactions for the formation of C-C bonds with the aid of bidentate directing groups.

8-Aminoquinoline-Assisted Synthesis and Crystal Structure Studies of Ferrocenyl Aryl Sulfones

A copper-catalyzed 8-aminoquinoline-directed oxidative cross-coupling of the C-H bond of ferrocene with sodium arylsulfinates has been achieved. The robust copper catalyst tolerates a range of methyl, tert-butyl, bromo, chloro, iodo and nitro functional groups in the phenyl ring, and set the stage for the synthesis of substituted ferrocene sulfones. Furthermore, X-ray crystal structure study on several ferrocenyl sulfones reveals the tetrahedral geometry around sulfur; interestingly, the O-S-O angle is larger than the electropositive substituent C-S-C angle which could be explained by Bent's rule. Further, unusual intramolecular O(S)⋅⋅⋅N(amide) short contacts (2.925-3) and O(S)⋅⋅⋅C=O were also noticed in ferrocenyl sulfones.

N-Aminopyridinium Ylide-Directed, Copper-Promoted Amination of sp2 C-H Bonds

N-Aminopyridinium ylides are used as monodentate directing groups for copper-promoted C-H/N-H coupling of sp² C-H bonds with pyrazoles, imidazoles, and sulfonamides. Reactions proceed in fluorinated alcohol solvents at elevated temperatures and require use of 1.3-3 equiv of copper(II) acetate. This appears to be the first method for copper-promoted C-H/N-H coupling directed by a removable monodentate auxiliary in absence of added ligands.

2-Amino-5,6-difluorophenyl-1H-pyrazole-Directed PdII Catalysis: Arylation of Unactivated β-C(sp3)-H Bonds

Palladium-catalyzed arylation of unactivated β-C(sp³)-H bonds in carboxylic acid derivatives with aryl iodides is described for the first time using 2-amino-5,6-difluorophenyl-1H-pyrazole as an efficient and readily removable directing group. Two fluoro groups are installed at the 5- and 6-position of the anilino moiety in 2-aminophenyl-1H-pyrazole, clearly enhancing the directing ability of the auxiliary. In addition, the protocol employs Cu(OAc)₂/Ag₃PO₄ (1.2/0.3) as additives, evidently reducing the stoichiometric amount of expensive silver salts. Furthermore, this process exhibits high β-site selectivity, compatibility with diverse substrates containing α-hydrogen atoms, and excellent functional group tolerance.

Iodine-Catalyzed Regioselective Oxidative Cyclization of Aldehyde Hydrazones with Electron-Deficient Olefins for the Synthesis of Mefenpyr-Diethyl

A regioselective synthesis of polysubstituted dihydropyrazoles and pyrazoles through an iodine-catalyzed oxidative cyclization strategy of aldehyde hydrazones with electron-deficient olefins is described. The protocol adopts very mild reaction conditions and provides desirable yields. The reaction is supposed to proceed via a cascade C-H functionalization, C-N bond formation, and oxidation sequential processes. The overall simplicity and regioselectivity of the catalytic system make this approach a valuable and step-economical tool to construct a C-C bond for the synthesis of Mefenpyr-Diethyl.

Copper-mediated direct thiolation of aryl C–H bonds with disulfides

A directing group-assisted copper-mediated thiolation of aromatic amides with disulfides via direct C(sp²)–H activation has been developed.

3d Transition Metals for C-H Activation

C-H activation has surfaced as an increasingly powerful tool for molecular sciences, with notable applications to material sciences, crop protection, drug discovery, and pharmaceutical industries, among others. Despite major advances, the vast majority of these C-H functionalizations required precious 4d or 5d transition metal catalysts. Given the cost-effective and sustainable nature of earth-abundant first row transition metals, the development of less toxic, inexpensive 3d metal catalysts for C-H activation has gained considerable recent momentum as a significantly more environmentally-benign and economically-attractive alternative. Herein, we provide a comprehensive overview on first row transition metal catalysts for C-H activation until summer 2018.

Discovery of 2-(pyridin-2-yl)aniline as a directing group for the sp2 C–H bond amination mediated by cupric acetate

2-(Pyridin-2-yl) aniline was designed as a new, removable directing group in promoting C–H amination mediated by cupric acetate.

Visible light-induced C3-sulfonamidation of imidazopyridines with sulfamides

A visible light-induced regioselective sulfonamidation of imidazo[1,2-a]pyridines was developed using sulfamides as the nitrogen sources and aqueous NaClO solution as the oxidant under mild conditions.

Metal-free C–H sulfonamidation of pyrroles by visible light photoredox catalysis

Blue light and catalytic amounts of an acridinium dye are sufficient for the C–H sulfamidation of pyrroles.

Copper-mediated etherification of arenes with alkoxysilanes directed by an (2-aminophenyl)pyrazole group

(2-Aminophenyl)pyrazole directed copper mediated C–H etherification of (hetero)arenes is described.