Cobalt(III)-Catalyzed Intermolecular Carboamination of Propiolates and Bicyclic Alkenes via Non-Annulative Redox-Neutral Coupling

ortho-C-H Amidation of Phenols through Intramolecular Rearrangement of N-Phenoxyarylamides

This study presents an efficient and concise methodology devised for effectuating the ortho-C-H amidation of phenols through the rearrangement of N-phenoxybenzamide derivatives. A variety of substrates, equipped with different electron-withdrawing and electron-donating functional groups, react smoothly under mild basic conditions or even without a base, eliminating the need for excessive use of strong acids, Lewis acids, or costly transition-metal catalysts (e.g., [Cp*RhCl2]2 or [Cp*Co(MeCN)3](SbF6)2) as previously delineated. Notably, the observed regioselectivity predominantly favors the ortho-position of the phenol rings. Mechanistic investigations suggest that C-H bond cleavage is likely not the rate-determining step and that an intramolecular rearrangement might be involved.

Hydroxyl-Assisted and Co(III)-Catalyzed Redox-Neutral C-H Activation/Directing Group Migration of 2-Pyridones with Propargyl Alcohols: Synthesis of Tetrasubstituted Alkenes

This study provides a practical route to synthesize tetrasubstituted alkenes that involves Co(III)-catalyzed C-H bond activation and regioselective insertion of the alkyne, followed by chelation of the substrate hydroxyl to Co and migration of the pyridine group. Density functional theory studies revealed the origin of regioselectivity and elucidated the crucial role of the hydroxyl group for the migration of pyridine. The method can be conducted on a gram scale, is compatible with a wide range of substrates, and has a high functional group tolerance. To demonstrate its significance, the method was used for the late-stage modification of Fasudil. Furthermore, the synthetic significance of the method was demonstrated by the various derivatizations of the products, many of which exhibit intriguing fluorescence characteristics.

Co(III)-Catalyzed C6-Selective C-H Activation/Pyridine Migration of 2-Pyridones with Propiolates

A versatile Co(III)-catalyzed C6-selective C-H activation/pyridine migration of 2-pyridones with available propiolates as coupling partners was demonstrated. This method features high atom economy, excellent regioselectivity, and good functional group tolerance by employing an inexpensive Co(III) catalyst under mild reaction conditions. Moreover, gram-scale synthesis and late-stage modifications of pharmaceuticals were performed to prove the effectiveness of these synthetic approaches.

Transition-metal-catalyzed domino reactions of strained bicyclic alkenes

This review presents a comprehensive overview of transition-metal-catalyzed domino reactions of strained bicyclic alkenes, including both homo- and heterobicyclic alkenes. These compounds are important synthons in organic synthesis, providing an important platform for the construction of biologically/medicinally significant compounds which bear multiple stereocenters. The review has been divided according to the metal used in the reaction. An overview of the substrate scope, reaction conditions, and their potential applications in organic synthesis is discussed. A comprehensive outlook on the reactivity paradigms of homo- and heterobicyclic alkenes is discussed and should shed light on future directions for further development in this field.

Rh(III)-Catalyzed chemo-, regio- and stereoselective carboamination of sulfonyl allenes with N-phenoxy amides or N-enoxy imides

The Rh(III)-catalyzed chemo-, regio- and stereoselective carboamination of sulfonyl allenes has been realized by virtue of either N-phenoxy amides or N-enoxy imides simultaneously acting as the C- and N-sources, via redox-neutral tandem C-H activation/allene insertion/oxidative addition/C-N bond formation for the direct construction of allylamine derivatives equipped with an α-quaternary carbon center. This protocol features high atom-economy with good substrate compatibility and exhibits profound synthetic potential for late-stage C-H modification of complex molecules.

Hydroamination, Aminoboration, and Carboamination with Electrophilic Amination Reagents: Umpolung-Enabled Regio- and Stereoselective Synthesis of N-Containing Molecules from Alkenes and Alkynes

Nitrogen (N) is ubiquitously found in bioactive molecules, pharmaceutical agents, and organic functional materials. Accordingly, development of new C-N bond-forming catalysis has been one of the long-standing research subjects in synthetic organic chemistry. In this Perspective, recent advances in highly selective amination reactions with electrophilic amination reagents are described: by taking advantage of the concept of nitrogen umpolung, otherwise challenging aminofunctionalizations, such as hydroamination, aminoboration, and carboamination, of readily available feedstock-like alkenes and alkynes are possible, giving densely functionalized complex and often chiral alkylamines with high selectivity. The scope, limitations, and reaction mechanism are briefly summarized.

Benzocarbazole Synthesis via Visible-Light-Accelerated Rh(III)-Catalyzed C-H Annulation of Aromatic Amines with Bicyclic Alkenes

A visible-light-accelerated Rh(III)-catalyzed C-H annulation of aromatic amines with bicyclic alkenes for the synthesis of benzocarbazole derivatives was developed. In this approach, with the cooperation of rhodium catalysis and visible-light irradiation, various aromatic amines reacted with oxabicyclic alkenes and azabicyclic alkenes smoothly at room temperature, delivering the corresponding bridged oxa or aza tetrahydro benzocarbazoles in good to excellent yields. Moreover, a series of benzo[b]carbazoles were synthesized conveniently through further aromatization in one pot. The potential of this method was demonstrated via directing-group removal, derivatization, a scale-up reaction, and fluorescence investigations.

Rh(III)-Catalyzed and synergistic dual directing group-enabled redox-neutral [3+3] annulation of N-phenoxyacetamides with α-allenols

By virtue of α-allenols as innovative three-carbon annulation components, the Rh(III)-catalyzed redox-neutral C-H coupling of N-phenoxyacetamides with α-allenols has been realized for the assembly of 4-alkylidene chroman-2-ol frameworks via an unusual [3+3] annulation. This transformation features good functional group tolerance, specific regio-/chemoselectivity and potential synthetic utility. Mechanistic studies reveal that synergistic coordination modes between the dual directing groups (-ONHAc and -OH) and the rhodium metal center account for the observed exclusive selectivity.

Cobalt(III)-Catalyzed Regioselective C6 Olefination of 2-Pyridones Using Alkynes: Olefination/Directing Group Migration and Olefination

Co(III)-catalyzed highly regio- and stereoselective direct C6 olefination of 2-pyridones with alkynes has been developed with the assistance of chelation. Upon variation of the reaction conditions, 2-pyridones react well with diaryl alkynes via a C6 olefination/directing group migration pathway to give the tetrasubstituted 6-vinyl-2-pyridones, but the C6-H olefination with terminal alkynes works effectively to afford only the C6-olefinated 2-pyridones. A judicious choice of a solvent and an additive is crucial for catalysis. The protocols feature 100% atom economy, excellent site selectivity, high stereoselectivity, an ample substrate scope, and good compatibility of functional groups. Synthetic applications are demonstrated, and experimental studies and density functional theory calculations are conducted to gain mechanistic insight into the two transformations.

Recent Advances in Transition-Metal-Catalyzed C–H Functionalization Reactions Involving Aza/Oxabicyclic Alkenes

Bicyclic alkenes, including oxa- and azabicyclic alkenes, readily undergo activation with facial selectivity in the presence of transition-metal complexes. This is due to the intrinsic angle strain on the carbon–carbon double bonds in such unsymmetrical bicyclic systems. During the past decades considerable progress has been made in the area of ring opening of bicyclic strained rings by employing the concept of C–H activation. This short review comprehensively compiles the various C–H bond activation assisted reactions of oxa- and azabicyclic alkenes, viz., ring-opening reactions, hydroarylation, and annulation reactions.

1 Introduction

2 Reactions of Heterobicyclic Ring Systems

2.1 Ring-Opening Reactions of Oxa- and Azabenzonorbornadienes

2.1.1 Reactions Using 7-Oxabenzonorbornadienes

2.1.2 Reactions Using 7-Azabenzonorbornadienes

2.2 Hydroarylation Reactions

2.3 Annulation Reactions

2.4 Other Reactions

3 Conclusion

Chemodivergent assembly of ortho-functionalized phenols with tunable selectivity via rhodium(III)-catalyzed and solvent-controlled C-H activation

Ortho-functionalized phenols and their derivatives represent prominent structural motifs and building blocks in medicinal and synthetic chemistry. While numerous synthetic approaches exist, the development of atom-/step-economic and practical methods for the chemodivergent assembly of diverse ortho-functionalized phenols based on fixed catalyst/substrates remains challenging. Here, by selectively controlling the reactivities of different sites in methylenecyclopropane core, Rh(III)-catalyzed redox-neutral and tunable C-H functionalizations of N-phenoxyacetamides are realized, providing access to both ortho-functionalized phenols bearing linear dienyl, cyclopropyl or allyl ether groups, and cyclic 3-ethylidene 2,3-dihydrobenzofuran frameworks under mild cross-coupling conditions. These divergent transformations feature broad substrate compatibility, synthetic applications and excellent site-/regio-/chemoselectivity. Experimental and computational mechanistic studies reveal that distinct catalytic modes involving selective β-C/β-H elimination, π-allylation, inter-/intramolecular nucleophilic substitution cascade and β-H' elimination processes enabled by different solvent-mediated and coupling partner-controlled reaction conditions are crucial for achieving chemodivergence, among which a structurally distinct Rh(V) species derived from a five-membered rhodacycle is proposed as the corresponding active intermediates.

Rh(III)-Catalyzed C-H Activation/[3 + 2] Annulation of N-Phenoxyacetamides via Carbooxygenation of 1,3-Dienes

A unique Rh(III)-catalyzed C-H activation/[3 + 2] annulation of N-phenoxyacetamides has been developed for the construction of dihydrobenzofurans via carbooxygenation of 1,3-dienes. This transformation features a redox-neutral process with specific chemoselectivity, good substrate/functional group compatibility, and profound synthetic potentials. A preliminary exploration to realize their asymmetric synthesis have been also successfully demonstrated, which further strengthens the practicality of this approach.

Three-Component 1,2-Carboamidation of Bridged Bicyclic Alkenes via RhIII-Catalyzed Addition of C-H Bonds and Amidating Reagents

A three-component method is described for the preparation of syn-1,2-disubstituted bridged bicyclic compounds. The reaction was demonstrated for readily available aromatic and heteroaromatic C-H bond substrates with tertiary and secondary amide, lactam, pyrazole, and triazole directing groups and a variety of bridged bicyclic alkenes, including norbornene, benzonorbornadiene, oxygen- and nitrogen-bridged analogs, and an unsaturated tropinone. A broad dioxazolone scope was also observed. The use of a chiral Cp-derived RhIII catalyst enables asymmetric synthesis of products.

Directing group migration strategy in transition-metal-catalysed direct C-H functionalization

Very recently, directing group (DG) migration has emerged as a practical strategy for transition-metal-catalysed direct C-H activation, resulting in a highly atom-economical process and enabling the reusage of DG. Therefore, great progress has been made in developing multitasking DGs. In this tutorial review, we present the rapid advances of this novel strategy by analyzing and comparing the different types of migratable DGs (including N-O, N-C, N-N or O-C bond cleavage to trigger DG migration). The related mechanisms, as well as synthetic applications, are also mentioned.

Rhodium(III)-Catalyzed Alkenyl C-H Functionalization to Dienes and Allenes

An oxyacetamide-directed Rh(III)-catalyzed Z-type alkenyl C-H functionalization through a rare exo-rhodacyle intermediate is described, forming multisubstituted dienes and allenes. A variety of alkenes and propargylic carbonate coupling partners are suitable for this transformation with high regio- and stereoselectivity. The synthetic utility is demonstrated by the selective late-stage modification of the Z-type natural products as well as the synthesis of the unnatural β-amino acid.

Cp*Co(III)-Catalyzed C-H Hydroarylation of Alkynes and Alkenes and Beyond: A Versatile Synthetic Tool

The use of earth-abundant first-row transition metals, such as cobalt, in C-H activation reactions for the construction and functionalization of a wide variety of structures has become a central topic in synthetic chemistry over the last few years. In this context, the emergence of cobalt catalysts bearing pentamethylcyclopentadienyl ligands (Cp*) has had a major impact on the development of synthetic methodologies. Cp*Co(III) complexes have been proven to possess unique reactivity compared, for example, to their Rh(III) counterparts, obtaining improved chemo- or regioselectivities, as well as yielding new reactivities. This perspective is focused on recent advances on the alkylation and alkenylation reactions of (hetero)arenes with alkenes and alkynes under Cp*Co(III) catalysis.

Synthesis of ortho-Phenolic Sulfilimines via an Intermolecular Sulfur Atom Transfer Cascade Reaction

To expand the toolbox for the synthesis of ortho-phenolic sulfilimines, sigmatropic rearrangements were introduced to the field of sulfilimine chemistry. Herein we report a N-H sulfenylation/[2,3]-sigmatropic rearrangement cascade reaction. This mild reaction enables commercially available thiols to serve as the sulfenylation reagent and generates water as the sole byproduct. Moreover, the reaction has a wide substrate scope and can be conducted on a gram scale with excellent reaction efficiency.

Transition metal-catalyzed coupling of heterocyclic alkenes via C–H functionalization: recent trends and applications

Heterocyclic alkenes and their derivatives are an important class of reactive feedstock and valuable synthons. This review highlights the transition-metal-catalyzed coupling of heterocyclic alkenes via a C–H functionalization strategy.

Recent advances in cobalt-catalysed C–H functionalizations

Ready availability, low cost and low toxicity of cobalt salts have redirected the attention of researchers away from noble metals, such as Pd, Rh, and Ir, towards Co in the field of C–H functionalisation.