ABSTRACT
Chiral carbon-carbon (C-C) and carbon-heteroatom (C-X) bonds are pervasive and very essential in natural products, bioactive molecules, and functional materials, and their catalytic construction has emerged as one of the hottest research fields in synthetic organic chemistry. The last decade has witnessed vigorous progress in Rh(I)-catalyzed asymmetric C-H functionalization as a complement to Rh(II) and Rh(III) catalysis. This review aims to provide the most comprehensive and up-to-date summary covering the recent advances in Rh(I)-catalyzed C-H activation for asymmetric functionalization. In addition to the development of diverse reactions, chiral ligand design and mechanistic investigation (inner-sphere mechanism, outer-sphere mechanism, and 1,4-Rh migration) will also be highlighted.
ABSTRACT
A rhodium-catalyzed 3-component conjunctive diastereo- and regioselective arylamidation of (homo)allylic sulfides, organon boronic acids, and dioxazolones is reported. These reactions deliver the 1,2-insertion and 2,1-insertion arylamidation products, respectively, for allylic sulfides and homoallylic sulfides. The enantioselective arylamidation of terminal and internal allylic sulfides is achieved, furnishing various 1,3-N,S compounds featuring one or two contiguous stereocenters in high yields and with high diastereo- and enantioselectivities. Mechanistic studies suggest a change in the turnover-limiting and selectivity-determining steps induced by the native and easily removable sulfide group.
ABSTRACT
The asymmetric hydrogenation (AH) of N-unprotected indoles is a straightforward, yet challenging method to access biologically interesting NH chiral indolines. This method has for years been limited to 2/3-monosubstituted or 2,3-disubstituted indoles, which produce chiral indolines bearing endocyclic chiral centers. Herein, we have reported an innovative Pd-catalyzed AH of racemic α-alkyl or aryl-substituted indole-2-acetates using an acid-assisted dynamic kinetic resolution (DKR) process, affording a range of structurally fascinating chiral indolines that contain exocyclic stereocenters with excellent yields, diastereoselectivities, and enantioselectivities. Mechanistic studies support that the DKR process relies on a rapid interconversion of each enantiomer of racemic substrates, leveraged by an acid-promoted isomerization between the aromatic indole and nonaromatic exocyclic enamine intermediate. The reaction can be performed on a gram scale, and the products can be derivatized into non-natural ß-amino acids via facile debenzylation and amino alcohol upon reduction.
ABSTRACT
Chiral cyclopentadienyl-rhodium(III) Cpx Rh(III) catalysis has been demonstrated to be competent for catalyzing highly enantioselective aziridination of challenging unactivated terminal alkenes and nitrene sources. The chiral Cpx Rh(III) catalysis system exhibited outstanding catalytic performance and wide functional group tolerance, yielding synthetically important and highly valuable chiral aziridines with good to excellent yields and enantioselectivities (up to 99 % yield, 93 % ee). This protocol presents a novel and effective strategy for synthesizing enantioenriched aziridines from simple alkenes. Various transformations were performed on the aziridine products, illustrating the versatility and synthetic potential of this protocol for constructing highly functionalized compounds.
ABSTRACT
Catalytic enantioselective hydroxylation of prochiral dihydrosilanes with water is expected to be a highly efficient way to access Si-chiral silanols, yet has remained unknown up to date. Herein, we describe a strategy for realizing this reaction: using an alkyl bromide as a single-electron transfer (SET) oxidant for invoking CuII species and chiral multidentate anionic N,N,P-ligands for effective enantiocontrol. The reaction readily provides a broad range of Si-chiral silanols with high enantioselectivity and excellent functional group compatibility. In addition, we manifest the synthetic potential by establishing two synthetic schemes for transforming the obtained products into Si-chiral compounds with high structural diversity. Our preliminary mechanistic studies support a mechanism involving SET for recruiting chiral CuII species as the active catalyst and its subsequent σ-metathesis with dihydrosilanes.
ABSTRACT
This density functional theory (DFT) study reveals a detailed plausible mechanism for the Sc-catalyzed C-H cycloaddition of imidazoles to 1,1-disubstituted alkenes to form all-carbon quaternary stereocenters. The Sc complex binds the imidazole substrate to enable deprotonative C2-H bond activation by the Sc-bound α-carbon to afford the active species. This complex undergoes intramolecular cyclization (CâC into Sc-imidazolyl insertion) with exo-selectivity, generating a ß-all-carbon-substituted quaternary center in the polycyclic imidazole derivative. The Sc-bound α-carbon deprotonates the imidazole C2-H bond to deliver the product and regenerate the active catalyst, which is the rate-determining step. Calculations illuminate the electronic effect of the ancillary Cp ligand on the catalyst activity and reveal the steric bias caused by using a chiral catalyst that induce the enantioselectivity. The insights can have implications for advancing rare-earth metal-catalyzed C-H functionalization of imidazoles.
Subject(s)
Alkenes , Scandium , Alkylation , Carbon , Catalysis , ImidazolesABSTRACT
A series of half-sandwich Ir(III) complexes 1-6 bearing an amidato bidentate ligand were conveniently synthesized and applied to the catalytic Leuckart-Wallach reaction to produce racemic α-chiral primary amines. With 0.1 mol% of complex 1, a broad range of ketones, including aryl ketones, dialkyl ketones, cyclic ketones, α-keto acids, α-keto esters and diketones, could be transformed to their corresponding primary amines with moderate to excellent yields (40%-95%). Asymmetric transformation was also attempted with chiral Ir complexes 3-6, and 16% ee of the desired primary amine was obtained. Despite the unsatisfactory enantio-control achieved so far, the current exploration might stimulate more efforts towards the discovery of better chiral catalysts for this challenging but important transformation.
ABSTRACT
The development of new methods for the direct functionalization of unactivated C-H bonds is ushering in a paradigm shift in the field of retrosynthetic analysis. In particular, the catalytic enantioselective functionalization of C-H bonds represents a highly atom- and step-economic approach toward the generation of structural complexity. However, as a result of their ubiquity and low reactivity, controlling both the chemo- and stereoselectivity of such processes constitutes a significant challenge. Herein we comprehensively review all asymmetric transition-metal-catalyzed methodologies that are believed to proceed via an inner-sphere-type mechanism, with an emphasis on the nature of stereochemistry generation. Our analysis serves to document the considerable and rapid progress within in the field, while also highlighting limitations of current methods.
ABSTRACT
A chiral Cpx RhIII catalyst system inâ situ generated from a Cpx RhI (cod) precatalyst and bis(o-toluoyl) peroxide as activating oxidant was developed for a C-H activation/ring-opening sequence between aryl ketoxime ethers and 2,3-diazabicyclo[2.2.1]hept-5-enes. This transformation provides access to densely functionalized chiral cyclopentenylamines in excellent yields and enantioselectivities of up to 97:3 er. The reported method is also well suitable for asymmetric alkenyl C-H functionalizations of α,ß-unsaturated oxime ethers, furnishing skipped dienes with high levels of enantiocontrol.
ABSTRACT
An efficient Cpx RhIII -catalyzed enantioselective alkenyl C-H functionalization/[4+1] annulation of acryl amides and allenes is reported. The described transformation provides straightforward access to enantioenriched α,ß-unsaturated-γ-lactams bearing a quaternary stereocenter. The reaction operates under mild conditions, displays a broad functional-group tolerance, and provides 2H-pyrrol-2-ones with excellent selectivity of up to 97:3 er. Such scaffolds are frequently found in natural products and synthetic bioactive compounds and are of significant synthetic value. It is noteworthy that the allene serves as a one-carbon unit in the [4+1]-annulation.
ABSTRACT
Dexamethasone (Dex) induces direct cytotoxicity to cultured osteoblasts. The benzimidazole derivative compound 991 ("C991") is a novel and highly-efficient AMP-activated protein kinase (AMPK) activator. Here, in both MC3T3-E1 osteoblastic cells and primary murine osteoblasts, treatment with C991 activated AMPK signaling, and significantly attenuated Dex-induced apoptotic and non-apoptotic cell death. AMPKα1 knockdown (by shRNA), complete knockout (by CRISPR/Cas9 method) or dominant negative mutation (T172A) not only blocked C991-mediated AMPK activation, but also abolished its pro-survival effect against Dex in osteoblasts. Further studies showed that C991 boosted nicotinamide adenine dinucleotide phosphate (NADPH) activity and induced mRNA expression of NF-E2-related factor 2 (Nrf2)-regulated genes (heme oxygenase-1 and NADPH quinone oxidoreductase 1). Additionally, C991 alleviated Dex-induced reactive oxygen species (ROS) production in osteoblasts. Notably, genetic AMPK inhibition reversed the anti-oxidant actions by C991 in Dex-treated osteoblasts. Together, we conclude that C991 activates AMPK signaling to protect osteoblasts from Dex.
Subject(s)
AMP-Activated Protein Kinases/metabolism , Apoptosis/drug effects , Benzimidazoles/administration & dosage , Dexamethasone/administration & dosage , Osteoblasts/drug effects , Osteoblasts/physiology , AMP-Activated Protein Kinases/drug effects , Animals , BALB 3T3 Cells , Dose-Response Relationship, Drug , Drug Interactions , Enzyme Activation/drug effects , Mice , Osteoblasts/cytology , Reactive Oxygen Species/metabolismABSTRACT
Albumin-conjugated multilayered nanoemulsion (albumin-MNE) of methyl prednisolone (MP) was developed to ensure the specificity of the drug at the spinal cord injury (SCI) site. MNE was prepared by emulsification followed by ionic deposition of oppositely charged polymer followed by albumin conjugation using N-hydroxysuccinimide. Prepared nanoemulsion was characterized for particle size, polydispersity index (PDI), zeta potential (Zp), pH, viscosity, and entrapment efficiency. It was further evaluated for shape and morphological analysis, in vitro release, cell viability, and in vivo efficacy against post SCI-like conditions in terms of behavioral assessment, histopathological evaluation, and immunoflorescence assay of the histological sections showing Bax-driven apoptosis. Entrapment efficiency, particle size, PDI, and Zp of spherical-shaped, smooth-surfaced MNE droplets were found to be 68.9%, 83.2 ± 14.4 nm, 0.231, and + 62.7 mV, respectively. In vitro release of MP from MNE and albumin-MNE was observed to be 68.5 and 72.2% after 96th hour of the study. MNE showed higher viability of astrocytes than MP solution. Albumin-MNE improved behavior of SCI rat and histopathological conditions in a very effective manner when compared with MNE. Immunoflorescence assay reveals explicit decline in mitochondrial-mediated apoptosis by sub-cellular upregulation of Bax at spinal cord injury site. In conclusion, albumin-MNE delivered MP specifically at SCI site and avoided its instant availability inside astrocytes culture. On account of which the chitosan stabilized, lecithin-emulsified, multilayered nanoemulsion of MP depicts higher efficacy and safety than MNE and may offer safe and effective mean for the treatment of post SCI-like conditions in human.
Subject(s)
Albumins/administration & dosage , Anti-Inflammatory Agents/administration & dosage , Methylprednisolone/administration & dosage , Spinal Cord Injuries/drug therapy , Administration, Intravenous , Animals , Anti-Inflammatory Agents/therapeutic use , Apoptosis , Astrocytes/drug effects , Cell Survival , Chitosan/chemistry , Drug Liberation , Emulsions , Female , Lipids , Methylprednisolone/therapeutic use , Nanostructures , Particle Size , Rats , Spinal Cord Injuries/pathologyABSTRACT
A highly efficient synthesis of enantioenriched spiroindolines by catalytic asymmetric dearomatization of indolyl dihydropyridines through a chiral phosphoric acid catalyzed enamine isomerization/spirocyclization/transfer hydrogenation sequence has been developed. This reaction proceeds under mild reaction conditions, affording novel spiroindolines in good yields (up to 88 %) with excellent enantioselectivity (up to 97 % ee). DFT calculations provide insights into the reaction mechanism as well as the origin of stereochemistry.
ABSTRACT
Chiral cyclopentadienyl (Cpx ) ligands have a large application potential in enantioselective transition-metal catalysis. However, the development of concise and practical routes to such ligands remains in its infancy. We present a convenient and efficient two-step synthesis of a novel class of chiral Cpx ligands with tunable steric properties that can be readily used for complexation, giving Cpx RhI , Cpx IrI , and Cpx RuII complexes. The potential of this ligand class is demonstrated with the latter in the enantioselective cyclization of azabenzonorbornadienes with alkynes, affording dihydrobenzoindoles in up to 98:2 e.r., significantly outperforming existing binaphthyl-derived Cpx ligands.
ABSTRACT
A highly efficient synthesis of the enantioenriched tetrahydro-ß-carbolines was developed by using a chiral phosphoric acid catalyzed Pictet-Spengler reaction of indolyl dihydropyridines. The reaction proceeds under mild reaction conditions to afford the desired chiral tetrahydro-ß-carbolines in good to excellent yields (up to 96 %) and high enantioselectivities (up to 99 % ee). With this method, a formal synthesis of tangutorine and a total synthesis of deplancheine were achieved in a highly efficient manner.
ABSTRACT
The first Pd0 -catalyzed intermolecular arylative dearomatization of ß-naphthols with aryl halides is described. It was found that Q-Phos could facilitate the palladium-catalyzed cross-coupling-type dearomatization of ß-naphthols, while avoiding O-arylation, to construct 2-naphthalenones in excellent yields and with high chemoselectivity.
ABSTRACT
A highly efficient catalytic asymmetric dearomatization of naphthols by means of an electrophilic amination reaction catalyzed by chiral phosphoric acid is presented. This protocol provides a facile access to functionalized ß-naphthalenone compounds with a chiral quaternary carbon center in excellent yields and enantioselectivity (up to 99% yield, up to 96%â ee).
ABSTRACT
An intermolecular asymmetric dearomatization reaction of ß-naphthols with nitroethylene through a chiral-thiourea-catalyzed Michael reaction is described. Enantioenriched functionalized ß-naphthalenones with an all-carbon quaternary stereogenic center could thus be easily constructed from simple naphthol derivatives in good yields and excellent enantioselectivity (up to 79% yield, 98% ee).
ABSTRACT
Here, a photocatalytic asymmetric multicomponent cascade Minisci reaction of ß-carbolines with enamides and diazo compounds is reported, enabling an effective enantioselective radical CâH functionalization of ß-carbolines with high yields and enantioselectivity (up to 83% yield and 95% ee). This enantioselective multicomponent Minisci protocol exhibits step economy, high chemo-/enantio-selective control, and good functional group tolerance, allowing access to a variety of valuable chiral ß-carbolines. Notably, diazo compounds are suitable radical precursors in enantioselective cascade radical reactions. Moreover, the efficiency and practicality of this approach are demonstrated by the asymmetric synthesis of bioactive compounds and natural products.
ABSTRACT
Anomalous Hall effect (AHE), one of the most important electronic transport phenomena, generally appears in ferromagnetic materials but is rare in materials without magnetic elements. Here, a study of La3MgBi5 is presented, whose band structure carries multitype Dirac fermions. Although magnetic elements are absent in La3MgBi5, the signals of AHE can be observed. In particular, the anomalous Hall conductivity is extremely large, reaching 42,356 Ω-1 cm-1 with an anomalous Hall angle of 8.8%, the largest one that has been observed in the current AHE systems. The AHE is suggested to originate from the combination of skew scattering and Berry curvature. Another unique property discovered in La3MgBi5 is the axial diamagnetism. The diamagnetism is significantly enhanced and dominates the magnetization in the axial directions, which is the result of the restricted motion of the Dirac fermion at the Fermi level. These findings not only establish La3MgBi5 as a suitable platform to study AHE and quantum transport but also indicate the great potential of 315-type Bi-based materials for exploring novel physical properties.