Reductive Cross-Coupling between Unactivated C(aryl)–N and C(aryl)–O Bonds by Chromium Catalysis Using a Bipyridyl Ligand

Cr-catalyzed borylation of C(aryl)-F bonds using a terpyridine ligand

The defluoroborylation of fluoroarenes by chromium-catalyzed cleavage of unactivated C-F bonds is described. The reaction uses HBpin as the boron source, low-cost and commercially available chromium salt as the precatalyst, and terpyridine as a crucial ligand, providing a protocol with atom-efficient benefits and a wide range of applicable substrates for the functionalization of aryl C-F bonds. Preliminary mechanistic studies indicate that an unprecedented Cr-catalyzed magnesiation of the unactivated C-F bond occurred. The generated arylmagnesium intermediates then participated in the subsequent borylation reaction. The application of the strategy in the preparation of valuable derivatives is demonstrated by the late-stage functionalization of boronate ester groups.

Pd-NHC (NHC = N-Heterocyclic Carbene)-Catalyzed B-Alkyl Suzuki Cross-Coupling of 2-Pyridyl Ammonium Salts by N-C Activation: Application to the Discovery of Agrochemical Molecular Hybrids

2-Alkylpyridines are a privileged scaffold throughout the realm of organic synthesis and play a key role in natural products, pharmaceuticals, and agrochemicals. Herein, we report the first B-alkyl Suzuki cross-coupling of 2-pyridyl ammonium salts to access functionalized 2-alkylpyridines. The use of well-defined, operationally simple Pd-NHCs permits for an exceptionally broad scope of the challenging B-alkyl C-N cross-coupling with organoboranes containing β-hydrogen, representing a novel method for the discovery of highly sought-after molecules for plant protection.

Suzuki-Miyaura Cross-Coupling of 2-Pyridyl Trimethylammonium Salts by N-C Activation Catalyzed by Air- and Moisture-Stable Pd-NHC Precatalysts: Application to the Discovery of Agrochemicals

We report the first Suzuki-Miyaura cross-coupling of 2-pyridyl ammonium salts by highly selective N-C activation catalyzed by air- and moisture-stable Pd(II)-NHC (NHC = N-heterocyclic carbene) precatalysts. The use of well-defined and highly reactive [Pd(IPr)(3-CF₃-An)Cl₂] (An = aniline) or [Pd(IPr)(cin)Cl] (cin = cinnamyl) Pd(II)-NHC catalysts permits an exceptionally broad scope of the cross-coupling to furnish valuable biaryl and heterobiarylpyridines that are ubiquitous in medicinal chemistry and agrochemistry research. The overall process leverages the Chichibabin C-H amination of pyridines with N-C activation to enable an attractive strategy to the 2-pyridyl problem. The utility of the method to the discovery of potent agrochemicals is presented. Considering the importance of 2-pyridines and the versatility of N-C activation methods, we envision that this new C-H/N-C activation strategy will find broad application.

Nickel-Catalyzed Desulfonylative Reductive Cross-Coupling of Aryl Sulfones with Aryl Bromides

Herein, a nickel catalysis system for desulfonylative C(sp²)-C(sp²) reductive cross-coupling reactions of aryl sulfone derivatives with a range of aryl bromides has been established to form diverse biaryl compounds. The complex Ar-Ni(II)-SO₂CF₃ bearing a phosphine ligand through oxidative addition of aryl sulfone to Ni(0) species was isolated and confirmed by an X-ray, which provides solid evidence for the understanding of the C(Ar)-SO₂ bond activation and reaction mechanism.

Palladium-catalysed deaminative/decarboxylative cross-coupling of organoammonium salts with carboxylic acids

A palladium-catalysed carbon-carbon bond-forming reaction via deaminative/decarboxylative cross-coupling of organoammonium salts with carboxylic acids was developed. Under the reaction conditions, polyfluoroaromatic carboxylic acids, propiolic acids and α-cyano benzyl carboxylic acid reacted smoothly with benzyl ammonium salts to produce the corresponding carbon-carbon coupling products in good-to-excellent yields.

Palladium-Catalyzed Cross-Electrophile Coupling between Aryl Diazonium Salt and Aryl Iodide/Diaryliodonium Salt in H2O-EtOH

We report herein a mild highly chemoselective palladium-catalyzed cross-electrophile coupling between readily accessible aromatic diazonium salt and aryl iodide or diaryliodonium salt in water-ethanol (2:1) medium. Mechanistic studies revealed that ethanol is crucial to generate an active Pd(0) catalyst, and the counterion of the diazonium salt renders a cationic Pd(II) species that facilitates subsequent oxidative addition to aryl iodide/diaryliodonium salt. Silver(I) salt was crucial to retain the catalytic activity of palladium, removing the iodide ion as a precipitate.

C(sp2)-C(sp2) Reductive Cross-Coupling of Triarylphosphines with Aryl Halides by Palladium/Nickel Co-catalysis

Herein, we report the first general C(sp²)-C(sp²) reductive cross-coupling reaction of diverse triarylphosphines with a wide range of aryl halides by palladium/nickel co-catalysis. This protocol offers a unique route for the synthesis of biaryl compounds via the activation of inert C(Ar)-P bonds. The mechanistic studies demonstrate that the formation of the phosphonium salts in situ plays a key role in the catalytic cycle.

Chromium-catalyzed couplings of C(aryl)-SMe bonds for accessing arylated and alkylated benzaldehyde derivatives

Described here is the chromium-catalyzed cleavage of C(aryl)-SMe bonds leading to coupling with organomagnesium to give functionalized benzaldehydes under mild conditions. This reaction was promoted specifically by a low-cost and simple CrCl₂ salt used as a precatalyst, enabling synchronous activations of ortho-C(aryl)-SMe and ortho'-C(aryl)-H bonds to achieve difunctionalization of benzaldimines. This work provided a strategy for accessing arylated, alkylated, and diarylated benzaldehyde derivatives as a result of the couplings of C(aryl)-SMe and C(aryl)-SMe/C(aryl)-H bonds promoted with cost-effective Cr catalysis.

Catalytic Cleavage of Unactivated C(aryl)-P Bonds by Chromium

We describe here the coupling to transform aryl phosphine derivatives by the cleavage of unactivated C(aryl)-P bonds with chromium catalysis, allowing us to achieve the reaction with alkyl bromides and arylmagnesium reagents under mild conditions. Mechanistic studies indicate that catalytic cleavage of unactivated C(aryl)-P bonds is due to the in situ formed reactive Cr, followed by transmetalation and coupling with alkyl bromides.

Unreactive C-N Bond Activation of Anilines via Photoinduced Aerobic Borylation

Unreactive C-N bond activation of anilines was achieved by photoinduced aerobic borylation. A diverse range of tertiary and secondary anilines were converted to aryl boronate esters in moderate to good yields with wide functional group tolerance under simple and ambient photochemical conditions. This transformation achieved the direct and facile C-N bond activation of unreactive anilines, providing a convenient and practical route transforming widely available anilines into useful aryl boronate esters.

From aniline to phenol: carbon-nitrogen bond activation via uranyl photoredox catalysis

Carbon-nitrogen bond activation, via uranyl photoredox catalysis with water, enabled the conversion of 40 protogenetic anilines, 8 N-substituted anilines and 9 aniline-containing natural products/pharmaceuticals to the corresponding phenols in an ambient environment. A single-electron transfer process between a protonated aniline and uranyl catalyst, which was disclosed by radical quenching experiments and Stern-Volmer analysis, facilitated the following oxygen atom transfer process between the radical cation of protonated anilines and uranyl peroxide originating from water-splitting. ¹⁸O labeling and ¹⁵N tracking unambiguously depicted that the oxygen came from water and amino group left as ammonium salt. The 100-fold efficiency of the flow operation demonstrated the great potential of the conversion process for industrial synthetic application.

Photoinduced Deaminative Borylation of Unreactive Aromatic Amines Enhanced by CO2

Herein, direct unreactive C-N borylation of aromatic amines by a photocatalyst was achieved. The C-N borylation of aromatic amines with bis(pinacolato)diboron (B₂pin₂) proceeded using a pyrene catalyst under light irradiation to afford desired borylated products and aminoborane as a byproduct. The yield of the borylated product improved under a CO₂ atmosphere which probably reduced the inhibitory effect of aminoborane. Mechanistic studies suggested that the C-N bond cleavage and C-B bond formation proceeded via a concerted pathway.

Chromium-Catalyzed Cross-Coupling Reactions by Selective Activation of Chemically Inert Aromatic C–O, C–N, and C–H Bonds

Transition-metal-catalyzed cross-coupling has emerged as one of the most powerful and useful tools for the formation of C–C and C–heteroatom bonds. Given the shortage of resources of precious metals on Earth, the use of Earth-abundant metals as catalysts in developing cost-effective strategies for cross-coupling is a current trend in synthetic chemistry. Compared with the achievements made using first-row nickel, iron, cobalt, and even manganese catalysts, the group 6 metal chromium has rarely been used to promote cross-coupling. This perspective covers recent advances in chromium-catalyzed cross-coupling reactions in transformations of chemically inert C(aryl)–O, C(aryl)–N, and C(aryl)–H bonds, offering selective strategies for molecule construction. The ability of low-valent Cr with a high-spin state to participate in two-electron oxidative addition is highlighted; this is different from the mechanism involving single-electron transfer that is usually assigned to chromium-mediated transformations.

1 Introduction

2 Chromium-Catalyzed Kumada Coupling of Nonactivated C(aryl)–O and C(aryl)–N Bonds

3 Chromium-Catalyzed Reductive Cross-Coupling of Two Nonactivated C(aryl)–Heteroatom Bonds

4 Chromium-Catalyzed Functionalization of Nonactivated C(aryl)–H Bonds

5 Conclusions and Outlook

Mechanistic Diversity of Low-Valent Chromium Catalysis: Cross-Coupling and Hydrofunctionalization

ConspectusTransition-metal catalysis has traditionally been dominated by precious metals because of their high reactivity toward chemical transformations. As a cost-effective alternative, catalysis by earth-abundant group 6 metal chromium is underdeveloped, and its reactivity remains largely unexplored, although the industrially important Phillips catalyst, which is composed of Cr as the active metal, is currently used to supply almost 40% of the total world demand for high-density polyethylene. Cr has traditionally served in organoreagents with high-valent states (≥2+), which are typified by reactions involving Nozaki-Hiyama-Kishi (NHK) and Takai-Utimoto one-electron transfer processes. Given that low-valent metals usually facilitate the process of oxidative addition (OA), studying the catalysis of Cr in the low-valent state provides the opportunity to develop new transformations. However, probably because of the low stability of reactive low-valent Cr or the lack of catalytic activity of structurally stable complexes, there has been limited success with respect to developing catalysis promoted by low-valent Cr. In recent years, our group has probed the reactivity of low-valent Cr in catalysis by adopting a strategy of forming reactive Cr in situ. In this Account, we detail our efforts to study the catalytic behavior and mechanism of low-valent Cr in challenging transformations, such as the cleavage of chemically inert bonds for the cross-coupling and hydrofunctionalization of arenes and nitro motifs, by developing strategies to address the prominent selectivity issues. We highlight the finding that low-valent Cr, being formed in situ, possesses the intriguing ability to promote the catalytic cleavage of unactivated C-O, C-N, and C-H bonds to achieve the Kumada couplings and even to enable challenging cross-coupling between two unactivated C(aryl)-O/C(aryl)-N bonds. During these catalytic processes, Cr usually adopts a high-spin state to interact with chemicals, allowing for insertion into unactivated σ-bonds. The OA catalytic model involving a two-electron process for the cleavage of unactivated bonds has rarely been considered for Cr. We highlight the finding that Cr allows for the breakage of two chemically inert bonds in one catalytic cycle. This ability is intriguing because most transition metals are suitable only for the cleavage of one unactivated bond in catalysis. Mechanisms involving two-electron OA for Cr are unusual, with processes involving one-electron transfer more often proposed, as exemplified in the NHK reactions. These reactions provide efficient strategies for forming functionalized benzaldehydes, amides, anilines, and amines, usually with high levels of selectivity. We hope that this account will extend the scope of cognition to Cr catalysis.

Nickel-Catalyzed Reductive 2-Pyridination of Aryl Iodides with Difluoromethyl 2-Pyridyl Sulfone

A novel nickel-catalyzed reductive cross-coupling between aryl iodides and difluoromethyl 2-pyridyl sulfone (2-PySO₂CF₂H) enables C(sp²)-C(sp²) bond formation through selective C(sp²)-S bond cleavage, which demonstrates the new reactivity of 2-PySO₂CF₂H reagent. This method employs readily available nickel catalyst and sulfones as cross-electrophile coupling partners, providing facile access to biaryls under mild reaction conditions without pregeneration of arylmetal reagents.

NHC-Nickel Catalyzed C-N Bond Cleavage of Mono-protected Anilines for C-C Cross-Coupling

A Ni-catalyzed aryl C-N bond cleavage of mono-protected anilines, N-arylsulfonamides, has been developed. A new N-heterocyclic carbene derived from benzoimidazole shows high reactivity for the C-N cleavage/C-C cross-coupling reaction. The ortho-directing group is not required to break the C-N bond of sulfonyl-protected anilines, which are not limited to π-extended anilines. The mechanistic studies have revealed that a sulfamidomagnesium salt is the key coupling intermediate.