A Cp*Ir(III) complex (1) of a newly designed ligand L1 featuring a proton‐responsive pyridyl(benzamide) appended on N‐heterocyclic carbene (NHC) has been synthesized. The molecular structure of 1 ...reveals a dearomatized form of the ligand. The protonation of 1 with HBF4 in tetrahydrofuran gives the corresponding aromatized complex Cp*Ir(L1H)ClBF4 (2). Both compounds are characterized spectroscopically and by X‐ray crystallography. The protonation of 1 with acid is examined by 1H NMR and UV‐vis spectra. The proton‐responsive character of 1 is exploited for catalyzing α‐alkylation of ketones and β‐alkylation of secondary alcohols using primary alcohols as alkylating agents through hydrogen‐borrowing methodology. Compound 1 is an effective catalyst for these reactions and exhibits a superior activity in comparison to a structurally similar iridium complex Cp*Ir(L2)ClPF6 (3) lacking a proton‐responsive pendant amide moiety. The catalytic alkylation is characterized by a wide substrate scope, low catalyst and base loadings, and a short reaction time. The catalytic efficacy of 1 is also demonstrated for the syntheses of quinoline and lactone derivatives via acceptorless dehydrogenation, and selective alkylation of two steroids, pregnenolone and testosterone. Detailed mechanistic investigations and DFT calculations substantiate the role of the proton‐responsive ligand in the hydrogen‐borrowing process.
A Cp*Ir(III) complex bearing a pyridyl(benzamide)‐functionalized N‐heterocyclic carbene (NHC) ligand exhibits aromatization/dearomatization on protonation/deprotonation. The proton‐responsive unit of the title compound plays a key role in catalysing α‐alkylation of ketones and β‐alkylation of secondary alcohols using primary alcohols via a borrowing‐hydrogen pathway.
Asymmetric reductive amination (ARA) of a prochiral carbonyl compound with an amine using a H2/hydrogen surrogate is a concise and operationally simple method for the synthesis of chiral amines. ARA ...proceeds via condensation of a carbonyl group with an amine/ammonia followed by the enantioselective reduction of the generated intermediate. The activation of reductant and stereoselective transfer of hydrogen to intermediate imine/enamine is often mediated by a chiral transition metal catalyst. Considering the wide applications of enantiopure amines in pharmaceuticals, agrochemicals, and materials, the development of effective catalysts for ARA has been intensively pursued in the last two decades. Since the first report by Blaser in 1999, this key research area has grown significantly in recent years, as reflected by the advances in catalyst design, diversifying substrate scope and better mechanistic understanding. Several highly efficient and general ARA methodologies applicable to challenging carbonyl and amine partners have been demonstrated, providing ready access to a variety of enantiopure amines. In this Review, we present the recent progress in ARA featuring diverse carbonyl and amine partners employing transition metal-catalysts. This Review provides an organized and critical discussion on catalyst engineering and evolution, expanding susbstrate scope and mechanistic insights. To conclude, the remaining challenges and opportunities in ARA are also highlighted.
A Cp*Ir(III) complex (1) bearing a proton-responsive hydroxy unit on an annulated imidazo1,2-a1,8naphthyridine based mesoionic carbene scaffold was synthesized by two different synthetic routes. ...The molecular structure of 1 revealed an anionic lactam form of the ligand. The acid–base equilibrium between the lactam-lactim tautomers on the ligand scaffold was examined by 1H NMR and UV–vis spectra. The pK a of the appendage −OH group in the lactim form of 1 was estimated to assess the proton transfer property of the catalyst. The catalytic efficacy of 1 for reductive amination of aldehyde was evaluated by utilizing three different hydrogen sources: molecular H2, i PrOH/KO t Bu combination, and HCOOH/Et3N (5:2) azeotropic mixture. The HCOOH/Et3N (5:2) azeotropic mixture protocol was found to be the best among the three different hydrogenation methods. Catalyst 1 hydrogenates imines chemoselectively over carbonyls under the reaction conditions. A range of aldehydes was reductively aminated to the corresponding secondary amines using the HCOOH/Et3N (5:2) azeotropic mixture. Further, catalyst 1 showed high efficiency for the reduction of a wide variety of N-heterocyclic imine derivatives. The lactam-lactim tautomerization of the ligand system is proposed for direct hydrogenation, whereas only the lactam form operates in the strongly basic medium ( i PrOH/KO t Bu). Under HCOOH/Et3N (5:2) conditions, the lactam scaffold is not protonated; rather, an outer-sphere hydride transfer from formate to the Ir is proposed, which is supported by 1H NMR and DFT calculations. Finally, ligand-promoted hydride transfer from metal-hydride to the protonated imine affords the corresponding amine. A close agreement between the experimentally estimated and computed thermodynamic/kinetic parameters gives credence to the metal-ligand cooperative mechanism for the imine hydrogenation reaction using the HCOOH/Et3N (5:2) azeotropic mixture.
Tethered and untethered ruthenium half-sandwich complexes were synthesized and characterized spectroscopically. X-ray crystallographic analysis of three untethered and two tethered Ru N-heterocyclic ...carbene (NHC) complexes were also carried out. These RuNHC complexes catalyze transfer hydrogenation of aromatic ketones in 2-propanol under reflux, optimally in the presence of (25 mol %) KOH. Under these conditions, the formation of 2-3 nm-sized Ru
nanoparticles was detected by TEM measurements. A solid-state NMR investigation of the nanoparticles suggested that the NHC ligands were bound to the surface of the Ru nanoparticles (NPs). This base-promoted route to NHC-stabilized ruthenium nanoparticles directly from arene-tethered ruthenium-NHC complexes and from untethered ruthenium-NHC complexes is more convenient than previously known routes to NHC-stabilized Ru nanocatalysts. Similar catalytically active RuNPs were also generated from the reaction of a mixture of RuCl
(p-cymene)
and the NHC precursor with KOH in isopropanol under reflux. The transfer hydrogenation catalyzed by these NHC-stabilized RuNPs possess a high turnover number. The catalytic efficiency was significantly reduced if nanoparticles were exposed to air or allowed to aggregate and precipitate by cooling the reaction mixtures during the reaction.
Tethered and untethered ruthenium half‐sandwich complexes were synthesized and characterized spectroscopically. X‐ray crystallographic analysis of three untethered and two tethered Ru N‐heterocyclic ...carbene (NHC) complexes were also carried out. These RuNHC complexes catalyze transfer hydrogenation of aromatic ketones in 2‐propanol under reflux, optimally in the presence of (25 mol %) KOH. Under these conditions, the formation of 2–3 nm‐sized Ru0 nanoparticles was detected by TEM measurements. A solid‐state NMR investigation of the nanoparticles suggested that the NHC ligands were bound to the surface of the Ru nanoparticles (NPs). This base‐promoted route to NHC‐stabilized ruthenium nanoparticles directly from arene‐tethered ruthenium–NHC complexes and from untethered ruthenium–NHC complexes is more convenient than previously known routes to NHC‐stabilized Ru nanocatalysts. Similar catalytically active RuNPs were also generated from the reaction of a mixture of RuCl2(p‐cymene)2 and the NHC precursor with KOH in isopropanol under reflux. The transfer hydrogenation catalyzed by these NHC‐stabilized RuNPs possess a high turnover number. The catalytic efficiency was significantly reduced if nanoparticles were exposed to air or allowed to aggregate and precipitate by cooling the reaction mixtures during the reaction.
N‐Heterocyclic carbene (NHC)‐stabilized Ru0 nanoparticles (NPs) were generated in situ from tethered and untethered ruthenium half‐sandwich complexes. These catalytically active Ru0 NPs were explored for transfer hydrogenation of aromatic ketones in 2‐propanol. The transfer hydrogenation catalyzed by these NPs possess a high turnover number. The catalytic efficiency was significantly reduced if these NPs were exposed to air or allowed to aggregate and precipitate by cooling the reaction mixtures.
The unsymmetrical amino-imidazolin-2-imine ligand HAmIm, 1,2-(DippNH)–C6H4–N=C(NiPrCMe)2 is employed in the synthesis of the iron(I) arene complex (AmIm)Fe(η 6 -C6H6) and the iron(II) neosilyl ...complex (AmIm)Fe(CH2SiMe3). These compounds are highly efficient precatalysts in H/D exchange reactions with deuterium (D2) in hydrosilanes. The scope comprises primary to tertiary silanes at a catalyst loading of 1 mol % at ambient temperature. In-depth mechanistic studies including various control experiments and the syntheses of isolated iron-hydride and iron-silyl compounds are performed. These studies reveal that the activation of both Fe(I) and Fe(II) complexes generates Fe–H/D species as key catalytic intermediates. An alternative catalytic pathway involving an iron-silyl intermediate, although shown to be less feasible by DFT calculations, may also be operative.
Breast cancer is the most diagnosed cancer among women. Approximately 15-20% of all breast cancers are highly invasive triple-negative breast cancer (TNBC) and lack estrogen, progesterone, and ERBB2 ...receptors. TNBC is challenging to treat due to its aggressive nature with far fewer targeted therapies than other breast cancer subtypes. Current treatments for patients with TNBC consist of cytotoxic chemotherapies, surgery, radiation, and in some instances PARP inhibitors and immunotherapy. To advance current therapeutics, we repurposed mebendazole (MBZ), an orally available FDA-approved anthelmintic that has shown preclinical efficacy for cancers. MBZ has low toxicity in humans and efficacy in multiple cancer models including breast cancer, glioblastoma multiforme, medulloblastoma, colon cancer, pancreatic and thyroid cancer. MBZ was well-tolerated in a phase I clinical trial of adults recently diagnosed with glioma. We determined that the half-maximal inhibitory concentration (IC
) of MBZ in four breast cancer cell lines is well within the range reported for other types of cancer. MBZ reduced TNBC cell proliferation, induced apoptosis, and caused G2/M cell cycle arrest. MBZ reduced the size of primary tumors and prevented lung and liver metastases. In addition, we uncovered a novel mechanism of action for MBZ. We found that MBZ reduces integrin β4 (ITGβ4) expression and cancer stem cell properties. ITGβ4 has previously been implicated in promoting "cancer stemness," which may contribute to the efficacy of MBZ. Collectively, our results contribute to a growing body of evidence suggesting that MBZ should be considered as a therapeutic to slow tumor progression and prevent metastasis.
Reactions of a host of metal precursors with pyridyl(benzamide)-functionalized C2-methyl-protected imidazolium salts L1H2I and L2HI afforded the metal–methyleneimidazoline (MIz) compounds Ru(L1-κC ...1)(p-cymene)I (1), Mn(L1-κC 1)(CO)3 (2), Ru(L2-κC 1)(p-cymene)ClPF6 (3), and Ir(L2-κC 1)(Cp*)ClPF6 (4) in the presence of different external bases, such as LiHMDS, Na2CO3, t BuOK, and NaH. However, the use of NaOAc led to the selective formation of the metal–mesoionic carbene (MIC) compounds Ru(L2-κC 5)(p-cymene)ClPF6 (5), Ir(L2-κC 5)(Cp*)ClPF6 (6), Ir2(L1-κC 5 )(Cp*)2IPF6 (8), and the ortho-metalated compound Ir(L1)(Cp*)I (7). All compounds have been characterized by spectroscopic techniques and X-ray crystallography. Being more acidic, the C2-methyl is readily deprotonated by the external base to give the metal–MIz products. A metal-bound acetate, in contrast, interacts selectively with the imidazolium C5–H and drives the reaction toward the metal–MIC formation. DFT calculations support a concerted metalation–deprotonation pathway for selective C–H activation and metalation.
The two donor modules of an annelated pyridyl–mesoionic carbene ligand (aPmic) have different σ- and π-bonding characteristics leading to its electronic asymmetry. A Pd(II) complex 1 featuring aPmic ...catalyzes the oxidation of a wide range of terminal olefins to the corresponding methyl ketones in good to excellent yields in acetonitrile. The catalytic reaction is proposed to proceed via syn-peroxypalladation and a subsequent rate-limiting 1,2-hydride shift, which is supported by kinetic studies. The electronic asymmetry of aPmic renders a well-defined coordination sphere at Pd. The favored arrangement of reactants on the metal center features an olefin trans to the pyridyl module and a t butylperoxide trans to the carbene. This arrangement gains added stability by the π-delocalization paved by the compatible orbitals on Pd, the pyridyl module, and the olefin that is perpendicular to the Pd(aPmic) plane. The π-interactions are absent in an alternate arrangement wherein the olefin is trans to the carbene. Density functional theory studies reveal the matching orbital overlaps responsible for the preferred arrangement over the other. This work provides an orbital description for the electronic asymmetry of aPmic.
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•Simple and convenient methods of generating soluble ruthenium nano-catalysts.•High activity and unique selectivity in a variety of transformations.•Applications in (de)hydrogenation, ...transfer hydrogenation, and oxidation.•Challenges in nanocatalysis: identification, separation, and asymmetric induction.
Metal nanoparticles exhibit unusual properties different from metal complexes and heterogeneous metals and hence draw considerable attention for applications in catalysis, magnetism, medicine, optoelectronics, and sensors. Herein we present an overview of the recent progress in catalysis using soluble ruthenium nanocatalysts (colloids). These nanocatalysts have been widely used for catalyzing the hydrogenation of various substrates particularly arenes due to the milder conditions and the unique selectivities achieved compared to those exhibited by classical heterogeneous catalysts. Ru(0) colloids have been also examined for catalyzing many different reactions including transfer hydrogenation, dehydrogenation, coupling reactions and CH activation, etc. Although in many of these transformations Ru(0) nanocatalysts exhibit high activities, there remain several challenges such as recovery of the soluble catalyst, catalysis by leached molecular clusters, and asymmetric catalysis with high enantioselectivity.
