Well-Defined, Air- and Moisture-Stable Palladium-Imidazo[1,5-a]pyridin-3-ylidene Complexes: A Versatile Catalyst Platform for Cross-Coupling Reactions by L-Shaped NHC Ligands

Transition-metal-free photochemical reductive denitration of nitroarenes

We have developed a simple and mild photochemical process for the reductive denitration of nitroarenes under transition-metal-free conditions. This method is compatible with a broad range of functional groups, providing a practical and efficient approach for converting nitroarenes into denitrated arenes. The utility of this protocol is demonstrated through the prompt synthesis of dibenzoxepane.

Gold-catalysed amine synthesis by reductive hydroamination of alkynes with nitroarenes

Amines are the most pivotal class of organic motifs in pharmaceutical compounds. Here we provide a blueprint for a general synthesis of amines by catalyst differentiation enabled by triple Au-H/Au+/Au-H relay catalysis. The parent catalyst is differentiated into a set of catalytically active species to enable triple cascade catalysis, where each catalytic species is specifically tuned for one catalytic cycle. This strategy enables the synthesis of biorelevant amine motifs by reductive hydroamination of alkynes with nitroarenes. Using this triple cascade approach, we have achieved exceptional functional group tolerance, enabling the use of bulk chemical feedstocks as coupling partners for the amination of both simple and complex alkynes (>100 examples), including those derived from pharmaceuticals, peptides and natural products (>30 examples). The isolation and full crystallographic characterization of gold hydride and hydride-bridged gold complexes has garnered insights into the catalyst differentiation process of fundamental organometallic gold hydride complexes.

Palladium-Catalyzed Denitrative α-Arylation of Heteroarenes with Nitroarenes via C-H and C-NO2 Bond Activations

A general approach for the α-arylation of heteroarenes with nitroarenes via denitrative coupling is reported for the first time. Various heteroarenes, including derivatives of furan, benzofuran, pyrrole, indole, thiophene, and benzothiophene, can be arylated at the α-position in moderate to good yields. Mechanistic studies demonstrate that the reaction proceeds via a CMD pathway, with C-H bond activation as the rate-determining step. Furthermore, the scalability and applicability in the synthesis of a drug molecule exemplify the utility of this protocol.

Pd/NHC catalyzed reduction and coupling of nitroaromatics for the synthesis of diarylamines

Herein, we report a one-pot approach to diarylamines through the reductive homocoupling of nitroaromatics, employing triethylsilane as the reducing agent and Pd/NHC as the catalyst. This method enables nitroaromatics to serve both as electrophilic reagents and as precursors of nucleophilic reagents, allowing for the direct preparation of diarylamines without the need to isolate aromatic primary amines.

Palladium-Catalyzed Denitrative Synthesis of Aryl Nitriles from Nitroarenes and Organocyanides

A denitrative cyanation of nitroarenes using organocyanides and a palladium catalyst was developed. The key for this reaction was the utilization of an aminoacetonitrile as a cyano source to avoid the generation of stoichiometric metal- and halogen-containing chemical waste. A wide range of nitroarenes, including heteroarenes and pharmaceutical molecules, can be converted into aryl nitriles.

Wingtip-Flexible N-Heterocyclic Carbenes: Unsymmetrical Connection between IMes and IPr

IMes (IMes=1,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene) and IPr (IPr=1,3- bis(2,6-diisopropylphenyl)imidazol-2-ylidene) represent by far the most frequently used N-heterocyclic carbene ligands in homogeneous catalysis, however, despite numerous advantages, these ligands are limited by the lack of steric flexibility of catalytic pockets. We report a new class of unique unsymmetrical N-heterocyclic carbene ligands that are characterized by freely-rotatable N-aromatic wingtips in the imidazol-2-ylidene architecture. The combination of rotatable N-CH2 Ar bond with conformationally-fixed N-Ar linkage results in a highly modular ligand topology, entering the range of geometries inaccessible to IMes and IPr. These ligands are highly reactive in Cu(I)-catalyzed β-hydroboration, an archetypal borylcupration process that has had a transformative impact on the synthesis of boron-containing compounds. The most reactive Cu(I)-NHC in this class has been commercialized in collaboration with MilliporeSigma to enable broad access of the synthetic chemistry community. The ligands gradually cover %Vbur geometries ranging from 37.3 % to 52.7 %, with the latter representing the largest %Vbur described for an IPr analogue, while retaining full flexibility of N-wingtip. Considering the modular access to novel geometrical space in N-heterocyclic carbene catalysis, we anticipate that this concept will enable new opportunities in organic synthesis, drug discovery and stabilization of reactive metal centers.

L-Shaped Heterobidentate Imidazo[1,5-a]pyridin-3-ylidene (N,C)-Ligands for Oxidant-Free AuI /AuIII Catalysis

In the last decade, major advances have been made in homogeneous gold catalysis. However, AuI /AuIII catalytic cycle remains much less explored due to the reluctance of AuI to undergo oxidative addition and the stability of the AuIII intermediate. Herein, we report activation of aryl halides at gold(I) enabled by NHC (NHC=N-heterocyclic carbene) ligands through the development of a new class of L-shaped heterobidentate ImPy (ImPy=imidazo[1,5-a]pyridin-3-ylidene) N,C ligands that feature hemilabile character of the amino group in combination with strong σ-donation of the carbene center in a rigid conformation, imposed by the ligand architecture. Detailed characterization and control studies reveal key ligand features for AuI /AuIII redox cycle, wherein the hemilabile nitrogen is placed at the coordinating position of a rigid framework. Given the tremendous significance of homogeneous gold catalysis, we anticipate that this ligand platform will find widespread application.

Divergent Acyl and Decarbonylative Liebeskind-Srogl Cross-Coupling of Thioesters by Cu-Cofactor and Pd-NHC (NHC = N-Heterocyclic Carbene) Catalysis

Transition-metal-catalyzed cross-coupling reactions of thioesters by selective acyl C(O)-S cleavage have emerged as a powerful platform for the preparation of complex molecules. Herein, we report divergent Liebeskind-Srogl cross-coupling of thioesters by Pd-NHC (NHC = N-heterocyclic carbene) catalysis. The reaction provides straightforward access to functionalized ketones by highly selective C(acyl)-S cleavage under mild conditions. Most crucially, the conditions enable direct functionalization of a range of complex pharmaceuticals decorated with a palette of sensitive functional groups, providing attractive products for medicinal chemistry programs. Furthermore, decarbonylative Liebeskind-Srogl cross-coupling by C(acyl)-S/C(aryl)-C(O) cleavage is reported. Cu metal cofactor directs the reaction pathway to acyl or decarbonylative pathway. This reactivity is applicable to complex pharmaceuticals. The reaction represents the mildest decarbonylative Suzuki cross-coupling discovered to date. The Cu-directed divergent acyl and decarbonylative cross-coupling of thioesters opens up chemical space in complex molecule synthesis.