1-Aminopyridinium Ylides as Monodentate Directing Groups for sp3 C-H Bond Functionalization

Discovery of Dehydroabietylamine Derivatives as Antibacterial and Antifungal Agents

A diverse array of biologically active derivatives was derived by modifying the chemically active sites of dehydroabietylamine. Herein, we describe the synthesis of a new series of C-19-arylated dehydroabietylamine derivatives using a palladium-catalyzed C(sp3)-H activation reaction. Five analogues (3b, 3d, 3h, 3n, and 4a) exhibited antibacterial activity against Escherichia coli. Compound 4a exhibited strong inhibitory activity against DNA Topo II and Topo IV. Molecular docking modeling indicated that it can bind effectively to the target through interactions with amino acid residues. The synthesized compounds were tested in vitro for their antifungal activity against six common phytopathogenic fungi. The mechanism of action of compound 4c against Rhizoctorzia solani was investigated, revealing that it disrupts the morphology of the mycelium and enhances cell membrane permeability.

N-Amino Pyridinium Salts in Organic Synthesis

C-N bond forming reactions hold immense significance to synthetic organic chemistry. In pursuit of efficient methods for the introduction of nitrogen in organic small molecules, myriad synthetic methods have been developed, and methods based on both nucleophilic and electrophilic aminating reagents have received sustained research effort. In response to continued challenges - the need for substrate prefunctionalization, the requirement for vestigial N-activating groups, and the need to incorporate nitrogen in ever more complex molecular settings - the development of novel aminating reagents remains a central challenge in method development. N-aminopyridinums and their derivatives have recently emerged as a class of bifunctional aminating reagents, which combine N-centered nucleophilicity with latent electrophilic or radical reactivity by virtue of the reducible N-N bond, with broad synthetic potential. Here, we summarize the synthesis and reactivity of N-aminopyridinium salts relevant to organic synthesis. The preparation and application of these reagents in photocatalyzed and metal-catalyzed transformations is discussed, showcasing the reactivity in the context of bifunctional platform and its potential for innovation in the field.

HFIP in Organic Synthesis

1,1,1,3,3,3-Hexafluoroisopropanol (HFIP) is a polar, strongly hydrogen bond-donating solvent that has found numerous uses in organic synthesis due to its ability to stabilize ionic species, transfer protons, and engage in a range of other intermolecular interactions. The use of this solvent has exponentially increased in the past decade and has become a solvent of choice in some areas, such as C-H functionalization chemistry. In this review, following a brief history of HFIP in organic synthesis and an overview of its physical properties, literature examples of organic reactions using HFIP as a solvent or an additive are presented, emphasizing the effect of solvent of each reaction.

Diastereoselective palladium-catalyzed functionalization of prochiral C(sp3)-H bonds of aliphatic and alicyclic compounds

We highlight the reported developments of the palladium-catalyzed C-H activation and functionalization of the inactive/unreactive prochiral C(sp³)-H bonds of aliphatic and alicyclic compounds. There exist numerous classical methods for generating contiguous stereogenic centers in a compound with a high degree of stereocontrol. Along similar lines, the Pd(II)-catalyzed, directing group-aided functionalization of inactive prochiral/diastereotopic C(sp³)-H bonds have been exploited to accomplish the stereoselective construction of stereo-arrays in organic compounds. We present a concise discussion on how specific strategies consisting of Pd(II)-catalyzed, directing group-aided C(sp³)-H functionalization have been utilized to generate two or more stereogenic centers in aliphatic and alicyclic compounds.

Rh(III)-Catalyzed C-H Diamidation and Diamidation/Intramolecular Cyclization of N-Iminopyridinium Ylides with Dioxazolones

A highly efficient Rh(III)-catalyzed C-H diamidation and diamidation/intramolecular cyclization of N-iminopyridinium ylides with dioxazolones has been developed, providing diamidated products and benzoxazinone products in good to excellent yields. Notably, the tunable selectivity of this reaction can be controlled by simply switching the solvent and the temperature. This reaction features operational simplicity, a broad substrate scope, and a good functional group tolerance.

RhIII-Catalyzed heteroarylation of N-2,6-difluorophenyl arylamides with heteroaryl boronate esters

An efficient strategy to aryl-heteroaryl formation via RhIII-catalyzed C–H heteroarylation of arenes with N-heterocyclic boronates has been disclosed.

Transition-Metal-Catalyzed, Coordination-Assisted Functionalization of Nonactivated C(sp3)-H Bonds

Transition-metal-catalyzed, coordination-assisted C(sp³)-H functionalization has revolutionized synthetic planning over the past few decades as the use of these directing groups has allowed for increased access to many strategic positions in organic molecules. Nonetheless, several challenges remain preeminent, such as the requirement for high temperatures, the difficulty in removing or converting directing groups, and, although many metals provide some reactivity, the difficulty in employing metals outside of palladium. This review aims to give a comprehensive overview of coordination-assisted, transition-metal-catalyzed, direct functionalization of nonactivated C(sp³)-H bonds by covering the literature since 2004 in order to demonstrate the current state-of-the-art methods as well as the current limitations. For clarity, this review has been divided into nine sections by the transition metal catalyst with subdivisions by the type of bond formation. Synthetic applications and reaction mechanism are discussed where appropriate.

1,2,4-Trifunctionalized Cyclohexane Synthesis via a Diastereoselective Reductive Cope Rearrangement and Functional Group Interconversion Strategy

Polyfunctionalized cyclohexanes are privileged scaffolds in drug discovery. Reported herein is a method for synthesizing 1,2,4-trifunctionalized cyclohexanes via diastereoselective reductive Cope rearrangement. The scaffolds obtained can be derivatized by orthogonal functional group interconversion to cyclohexanes bearing a 1-amide, 2-branched arylallyl, and variable 4-functional group.

RhIII-Catalyzed Direct Heteroarylation of C(sp3)-H and C(sp2)-H Bonds in Heterocycles with N-Heteroaromatic Boronates

Herein, we disclose a Rh^III-catalyzed heteroarylation of C(sp³)-H and C(sp²)-H bonds in heterocycles with organoboron reagents. This protocol displays high efficiency and excellent functional group tolerance. A range of heterocyclic boronates with strong coordinating atoms, including pyridine, pyrimidine, pyrazole, thiophene, and furan derivatives, can be extensively served as the coupling reagents. The direct heteroarylation method could supply potential application in terms of the synthesis of drug molecules with multiple heterocycles.

Pd-catalysed β-selective C(sp3)-H arylation of simple amides

An efficient Pd-catalysed β-C(sp³)-H arylation of diverse native amides with aryl iodides was developed. This protocol overcomes the necessity of the Thorpe-Ingold effect and features broad substrate scope and good functional group tolerance. The potential application of this protocol is collectively demonstrated by gram-scale synthesis and the synthesis of several bioactive molecules.

Hexafluoroisopropanol: the magical solvent for Pd-catalyzed C-H activation

Among numerous solvents available for chemical transformations, 1,1,1,3,3,3-hexafluoro-2-propanol (popularly known as HFIP) has attracted enough attention of the scientific community in recent years. Several unique features of HFIP compared to its non-fluoro analogue isopropanol have helped this solvent to make a difference in various subdomains of organic chemistry. One such area is transition metal-catalyzed C-H bond functionalization reactions. While, on one side, HFIP is emerging as a green and sustainable deep eutectic solvent (DES), on the other side, a major proportion of Pd-catalyzed C-H functionalization is heavily relying on this solvent. In particular, for distal aromatic C-H functionalizations, the exceptional impact of HFIP to elevate the yield and selectivity has made this solvent irreplaceable. Recent research studies have also highlighted the H-bond-donating ability of HFIP to enhance the chiral induction in Pd-catalyzed atroposelective C-H activation. This perspective aims to portray different shades of HFIP as a magical solvent in Pd-catalyzed C-H functionalization reactions.

Rapid Construction of Tetralin, Chromane, and Indane Motifs via Cyclative C-H/C-H Coupling: Four-Step Total Synthesis of (±)-Russujaponol F

The development of practical C-H/C-H coupling reactions remains a challenging yet appealing synthetic venture because it circumvents the need to prefunctionalize both coupling partners for the generation of C-C bonds. Herein we report a cyclative C(sp³)-H/C(sp²)-H coupling reaction of free aliphatic acids enabled by a cyclopentane-based mono-N-protected β-amino acid ligand. This reaction uses inexpensive sodium percarbonate (Na₂CO₃·1.5H₂O₂) as the sole oxidant and generates water as the only byproduct. A range of biologically important scaffolds, including tetralins, chromanes, and indanes, can be easily prepared by this protocol. Finally, the synthetic application of this methodology is demonstrated by the concise total synthesis of (±)-russujaponol F in a four-step sequence starting from readily available phenylacetic acid and pivalic acid through sequential functionalizations of four C-H bonds.

Faster palladium-catalyzed arylation of simple arenes in the presence of a methylketone: beneficial effect of an a priori interfering solvent in C–H activation

A methyl ketone accelerates the Pd-catalyzed C–H activation of simple arenes. A noninnocent solvent, it reacts in a reversible way without affecting the outcome of the reaction.

Nickel-catalyzed reductive coupling of homoenolates and their higher homologues with unactivated alkyl bromides

The catalytic generation of homoenolates and their higher homologues has been a long-standing challenge. Like the generation of transition metal enolates, which have been used to great affect in synthesis and medicinal chemistries, homoenolates and their higher homologues have much potential, albeit largely unrealized. Herein, a nickel-catalyzed generation of homoenolates, and their higher homologues, via decarbonylation of readily available cyclic anhydrides has been developed. The utility of nickel-bound homoenolates and their higher homologues is demonstrated by cross-coupling with unactivated alkyl bromides, generating a diverse array of aliphatic acids. A broad range of functional groups is tolerated. Preliminary mechanistic studies demonstrate that: (1) oxidative addition of anhydrides by the catalyst is faster than oxidative addition of alkyl bromides; (2) nickel bound metallocycles are involved in this transformation and (3) the catalyst undergoes a single electron transfer (SET) process with the alkyl bromide.

N-Aminopyridinium Ylide-Directed, Copper-Promoted Chalcogenation of Arene C-H Bonds

N-Aminopyridinium ylide-directing group is employed for copper-promoted chalcogenation of sp² C-H bonds with aryl and alkyl disulfides as well as diphenyl diselenide. Reactions proceed in hexafluoroisopropanol (HFIP) solvent at elevated temperatures and are promoted by copper(II) acetate.

Palladium-Catalyzed Direct Mono- or Polyhalogenation of Benzothiadiazole Derivatives

An unprecedented method for the palladium (II)-catalyzed direct halogenation of benzothiadiazole derivatives is reported here, which would allow the rational tuning of their electronic properties by further transformations. The key strategy is the combination of the double functions of benzothiadiazole as both an important moiety of functional materials and modifiable directing group. Various mono/polychlorinated, brominated, or iodinated benzothiadiazole products could be prepared efficiently, which can be converted into a wide range of valuable polyaromatic compounds with potential applications in material chemistry.

N-Iminopyridinium ylide-directed, cobalt-catalysed coupling of sp2 C-H bonds with alkynes

N-Iminopyridinium ylides are competent monodentate directing groups for cobalt-catalysed annulation of sp2 C-H bonds with internal alkynes. The pyridine moiety in the ylide serves as an internal oxidant and is cleaved during the reaction. The annulation reactions possess excellent compatibility with heterocyclic substrates, tolerating furan, thiophene, pyridine, pyrrole, pyrazole, and indole functionalities.

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.