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

Palladium-Catalyzed/Copper-Mediated Decarbonylative Cross-Coupling of S-Pyrimidyl Thioesters for Biaryl Synthesis

A palladium-catalyzed/copper-mediated cross-coupling of S-pyrimidinyl thioesters with arylboronic acids to yield biaryls is described. The reaction is likely to proceed via cleavage of the S-C(O) bond and subsequent release of CO, rather than via cleavage of the S-C(pyrimidine) bond and release of SCO, as supported by the results of both experimental and computational studies. The investigation of the reaction scope with various S-pyrimidinyl thioesters and arylboronic acids showed that the reaction is significantly affected by the substituent of the thioester and the presence of a chelatable ortho substituent was found to increase reaction efficiency.

[(NHC)Pd(OAc)2]: Highly Active Carboxylate Pd(II)-NHC (NHC = N-Heterocyclic Carbene) Precatalysts for Suzuki-Miyaura and Buchwald-Hartwig Cross-Coupling of Amides by N-C(O) Activation

In the past eight years, the selective cross-coupling of amides by N-C(O) bond activation has emerged as a highly attractive manifold for the manipulation of traditionally unreactive amide bonds. In this Special Issue on Next-Generation Cross-Coupling Chemistry, we report the Suzuki-Miyaura and Buchwald-Hartwig cross-coupling of amides by selective N-C(O) cleavage catalyzed by bench-stable, well-defined carboxylate Pd(II)-NHC (NHC = N-heterocyclic carbene) catalysts {[(NHC)Pd(O2CR)2]}. This class of Pd(II)-NHCs promotes cross-coupling under exceedingly mild room-temperature conditions owing to the facile dissociation of the carboxylate ligands to form the active complex. These readily accessible Pd(II)-NHC precatalysts show excellent functional group tolerance and are compatible with a broad range of amide activating groups. Considering the mild conditions for the cross-coupling and the facile access to carboxylate Pd(II)-NHC complexes, we anticipate that this class of bench-stable complexes will find wide application in the activation of amide N-C(O) and related acyl X-C(O) bonds.

Samarium-Oxo/Hydroxy Cluster: A Solar Photocatalyst for Chemoselective Aerobic Oxidation of Thiols for Disulfide Synthesis

Oxidation contributes as a secondary driver of the prevailing carbon emission in the chemical industries. To address this issue, photocatalytic aerobic oxidation has emerged as a promising alternative. However, the challenge of achieving satisfactory chemoselectivity and effective use of solar light has hindered progress in this area. In this context, the present study introduces a novel homogeneous photocatalyst, [Sm6O(OH)8(H2O)24]I8(H2O)8 cluster (Sm-OC), via a unique auxiliary ligand-free oxidative hydrolysis. Using Sm-OC as catalyst, a solar photocatalyzed aerobic oxidation of thiols has been developed for the synthesis of valuable disulfides. Remarkably, this catalyst manifested a significant turnover number ≥2000 under tested conditions. Sm-OC-catalyzed aerobic oxidation showcased remarkable chemoselectivity. In thiol oxidations, despite the vulnerability of disulfides toward overoxidation, overoxidized byproducts or oxidation of nontarget functional groups was not detected across all 28 tested substrates. This investigation presents the first application of a lanthanide-oxo/hydroxy cluster in photocatalysis.

Visible-Light-Mediated Synthesis of Thioesters Using Thiocarboxylic Acid as the Dual Reagent

We have developed a visible-light-driven method for thioester synthesis that relies on the unique dual role of thiobenzoic acids as one-electron reducing agents and reactants leading to the formation of sulfur radical species. This synthetic process offers a wide scope, accommodating various thioacid and thiol substrates without the need for a photocatalyst.

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.

Heterogeneously catalyzed decarbonylation of thioesters by supported Ni, Pd, or Rh nanoparticle catalysts

Decarbonylation of thioesters has been actively studied using homogeneous metal catalysts as an attractive approach for synthesizing thioethers, which are widely utilized in various fields, because decarbonylation ideally requires no additives and produces CO as the sole theoretical byproduct. However, heterogeneously catalyzed decarbonylation of thioesters has not been reported to date, despite its importance for the construction of environmentally-friendly and practical catalytic systems. This study demonstrated a heterogeneously catalyzed system for the decarbonylation of various aryl thioesters to produce thioethers and CO by utilizing CeO2- or hydroxyapatite-supported Ni, Pd, or Rh nanoparticle catalysts. The Ni catalysts showed high catalytic activity, while the Pd catalysts possessed excellent functional group tolerance. The Rh catalysts were suitable for the selective decarbonylation of unsymmetrically substituted thioesters.

IPr*Oxa - a new class of sterically-hindered, wingtip-flexible N,C-chelating oxazole-donor N-heterocyclic carbene ligands

N-heterocyclic carbenes (NHCs) have emerged as a major direction in ancillary ligand development for stabilization of reactive metal centers in inorganic and organometallic chemistry. In particular, wingtip-flexible NHCs have attracted significant attention due to their unique ability to provide a sterically-demanding environment for transition metals in various oxidation states. Herein, we report a new class of sterically-hindered, wingtip-flexible NHC ligands that feature N,C-chelating oxazole donors. These ligands are readily accessible through a modular arylation of oxazole derivatives. We report their synthesis and complete structural and electronic characterization. The evaluation of steric, electron-donating and π-accepting properties and coordination chemistry to Ag(I), Pd(II) and Rh(I) is described. Preliminary studies of catalytic activity in Ag, Pd and Rh-catalyzed coupling and hydrosilylation reactions are presented. This study establishes the fluxional behavior of a freely-rotatable oxazole unit, wherein the oxazolyl ring adjusts to the steric and electronic environment of the metal center. Considering the tremendous impact of sterically-hindered NHCs and their potential to stabilize reactive metals by N-chelation, we expect that this class of NHC ligands will be of broad interest in inorganic and organometallic chemistry.

Application of Iodine as a Catalyst in Aerobic Oxidations: A Sustainable Approach for Thiol Oxidations

Iodine is a well-known oxidant that is widely used in organic syntheses. Thiol oxidation by stoichiometric iodine is one of the most commonly employed strategies for the synthesis of valuable disulfides. While recent advancements in catalytic aerobic oxidation conditions have eliminated the need for stoichiometric oxidants, concerns persist regarding the use of toxic or expensive catalysts. In this study, we discovered that iodine can be used as a cheap, low-toxicity catalyst in the aerobic oxidation of thiols. In the catalytic cycle, iodine can be regenerated via HI oxidation by O2 at 70 °C in EtOAc. This protocol harnesses sustainable oxygen as the terminal oxidant, enabling the conversion of primary and secondary thiols with remarkable efficiency. Notably, all 26 tested thiols, encompassing various sensitive functional groups, were successfully converted into their corresponding disulfides with yields ranging from >66% to 98% at a catalyst loading of 5 mol%.

Application of Oxazaborolidine Catalysts (CBS) on Enantioselective 1,4-Addition of Diarylphosphine Oxides to α,β-Unsaturated Thioesters

Here, we report the first catalytic enantioselective 1,4-addition of diarylphosphine oxides to α,β-unsaturated thioesters. Importantly, the most common and commercial oxazaborolidine (CBS) was employed as a catalyst for its new application without being activated by strong protonic acids or Lewis acids and led to the chiral thioesters in excellent yields and enantioselectivities. Furthermore, this method features mild reaction conditions (room temperature and air-insensitive), good substrate tolerance, and easy scalability.

[Au(Np#)Cl]: Highly Reactive and Broadly Applicable Au(I)─NHC Catalysts for Alkyne π-Activation Reactions

Cationic Au(I)─NHC (NHC = N-heterocyclic carbene) complexes have become an important class of catalysts for alkyne π-activation reactions in organic synthesis. In particular, these complexes are characterized by high stability of catalytic species engendered by strong σ-donation and metal backbonding. Herein, we report the synthesis and characterization of well-defined [Au(NHC)Cl] complexes featuring recently discovered IPr# family of ligands that hinge upon modular peralkylation of aniline. These ligands have been commercialized in collaboration with MilliporeSigma (IPr#: 915653; Np#: 915912; BIAN-IPr#: 916420). Evaluation of the [Au(NHC)Cl] complexes in a series of Au(I)─NHC-catalyzed π-functionalizations of alkynes, such as hydrocarboxylation, hydroamination and hydration, resulted in the identification of wingtip-flexible [Au(Np#)Cl] as a highly reactive and broadly applicable catalyst with the re-activity outperforming the classical [Au(IPr)Cl] and [Au(IPr*)Cl] complexes. The utility of this catalyst has been demonstrated in the direct late-stage derivatization of complex pharmaceuticals. Structural and computational studies were conducted to determine steric effects, frontier molecular orbitals and bond orders of this class of catalysts. Considering the attractive features of well-defined Au(I)─NHC complexes, we anticipate that this class of bulky and wingtip-flexible Au(I)─NHCs based on the modular peralkylated naphthylamine scaffold will find broad application in π-functionalization of alkynes in various areas of organic synthesis and catalysis.

Direct C(sp3)-H Acylation by Mechanistically Controlled Ni/Ir Photoredox Catalysis

ConspectusSynthetic chemists have consistently aimed to develop efficient methods for synthesizing ketones, which are essential building blocks in organic chemistry and play significant roles in bioactive molecules. Recent efforts have focused on using photoredox catalysis, which enables previously inaccessible activation modes, to synthesize ketones through the cross-coupling of an acyl electrophile and simple C(sp3)-H bonds. Over the past few years, we have worked on developing effective and versatile approaches for directly acylating activated hydrocarbons to forge ketones.Initially, thioesters were explored as the acyl source to achieve the direct acylation of ethers, but an unexpected thioesterification reaction was observed instead. To gain insights into this reactivity, we conducted the optimization of reaction conditions, substrate scope evaluation, and mechanistic studies. Drawing from our understanding of Ni/Ir photocatalysis obtained in this study, we subsequently developed a method for the direct acylation of simple hydrocarbons. The use of less-reactive amides as the acyl electrophiles was found to be critical for suppressing undesired pathways. This seemingly counterintuitive reactivity was carefully studied, revealing a substrate-assisted reaction mechanism in which the suppressed oxidative addition leads to early-stage nickel oxidation and C-H activation.To address the drawbacks of this method, which primarily arose from decarbonylative and transmetallative side pathways, we employed N-acyllutidiniums as the acyl electrophile. This prevented undesired decomposition pathways, enabling the use of α-chiral acyl substrates with the retention of their stereochemistry, particularly those derived from α-amino acids. The developed versatile methodology allowed us to access a diverse range of α-amino ketones and their homologues.Despite the elegant utility of Ni/photoredox catalysis in developing new synthetic methodologies, the precise behavior of nickel catalysts under redox conditions is incompletely understood. To gain insight into this behavior and develop new chemical reactions, we used a combination of experimental and computational methods. Our investigations revealed that devised adjustments to the reaction conditions in nickel/photoredox catalysis can result in significant differences in the reaction outcomes, providing chemists with opportunities to tailor reactions through carefully designed mechanistic strategies. We believe that continued efforts to study and apply nickel redox modulation will lead to the discovery of additional organic transformations.

Construction of Chalcogenated Methylene Chroman-3-ones via Palladium-Catalyzed Carbocyclization

We report herein the synthesis of exo-chalcogenated methylene chroman-3-ones via palladium-catalyzed intramolecular acyl-chalcogenation of alkyne with thio- and selenoesters. Chalcogen containing tetrasubstituted alkenes are obtained stereoselectively. This protocol tolerates various functional groups and heterocycles, affording the chroman-3-one products in moderate-to-good yields.