Cyclometalated Ruthenium Catalyst Enables Selective Oxidation of N-Substituted Tetrahydroquinolines to Lactams

Herein, we report an unusual α-methylene oxidation of N-substituted tetrahydroquinoline to lactams using the cyclometalated Ru(II)-complex as a catalyst. Cyclic-α-methylene C-H bonds are selectively oxidized under the reaction conditions even in the presence of α-methyl or reactive α-methylene C-H bonds. This methodology is also useful in the late-stage functionalization of pharmaceuticals. Mechanistic study demonstrates that the high-valent Ru(VI)-cis-dioxo species plays an important role in controlling selectivity.

Visible Light Mediated Chemoselective Hydroxylation of Benzylic Methylenes

We have developed a metal-free photocatalytic selective hydroxylation of benzylic methylenes to secondary alcohols. This approach utilizes low-cost eosin Y as photocatalyst, O2 as green oxidant, and inexpensive triethylamine as inhibitor for overoxidation. The mild reaction conditions enable the production of secondary alcohols with 56-95% yields, making it a promising and environmental-friendly method for the synthesis of secondary alcohols from benzylic methylenes.

Straightforward synthesis of functionalized γ-Lactams using impure CO2 stream as the carbon source

Direct utilization of CO2 into organic synthesis finds enormous applications to synthesize pharmaceuticals and fine chemicals. However, pure CO2 gas is essential to achieve these transformations, and the purification of CO2 is highly cost and energy intensive. Considering this, we describe a straightforward synthetic route for the synthesis of γ-lactams, a pivotal core structure of bioactive molecules, by using commercially available starting materials (alkenes and amines) and impure CO2 stream (exhaust gas is collected from the car) as the carbon source. This blueprint features a broad scope, excellent functional group compatibility and application to the late-stage transformation of existing pharmaceuticals and natural products to synthesize functionalized γ-lactams. We believe that our strategy will provide direct access to γ-lactams in a very sustainable way and will also enhance the Carbon Capture and Utilization (CCU) strategy.

Water-Promoted Carbon-Carbon Bond Cleavage Employing a Reusable Fe Single-Atom Catalyst

The development of methods for selective cleavage reactions of thermodynamically stable C-C/C=C bonds in a green manner is a challenging research field which is largely unexplored. Herein, we present a heterogeneous Fe-N-C catalyst with highly dispersed iron centers that allows for the oxidative C-C/C=C bond cleavage of amines, secondary alcohols, ketones, and olefins in the presence of air (O2 ) and water (H2 O). Mechanistic studies reveal the presence of water to be essential for the performance of the Fe-N-C system, boosting the product yield from <1 % to >90 %. Combined spectroscopic characterizations and control experiments suggest the singlet 1 O2 and hydroxide species generated from O2 and H2 O, respectively, take selectively part in the C-C bond cleavage. The broad applicability (>40 examples) even for complex drugs as well as high activity, selectivity, and durability under comparably mild conditions highlight this unique catalytic system.

Organophotocatalysed synthesis of 2-piperidinones in one step via [1 + 2 + 3] strategy

Six-membered N-containing heterocycles, such as 2-piperidinone derivatives, with diverse substitution patterns are widespread in natural products, drug molecules and serve as key precursors for piperidines. Thus, the development of stereoselective synthesis of multi-substituted 2-piperidinones are attractive. However, existing methods heavily rely on modification of pre-synthesized backbones which require tedious multi-step procedure and suffer from limited substitution patterns. Herein, an organophotocatalysed [1 + 2 + 3] strategy was developed to enable the one-step access to diverse substituted 2-piperidinones from easily available inorganic ammonium salts, alkenes, and unsaturated carbonyl compounds. This mild protocol exhibits exclusive chemoselectivity over two alkenes, tolerating both terminal and internal alkenes with a wide range of functional groups.

Biomass-Derived Single Zn Atom Catalysts: The Multiple Roles of Single Zn Atoms in the Oxidative Cleavage of C-N Bonds

The C-N bond cleavage represents one kind of important organic and biochemical transformation, which has attracted great interest in recent years. The oxidative cleavage of C-N bonds in N,N-dialkylamines into N-alkylamines has been well documented, but it is challenging in the further oxidative cleavage of C-N bonds in N-alkylamines into primary amines due to the thermally unfavorable release of α-position H from N-C_α-H and the paralleling side reactions. Herein, a biomass-derived single Zn atom catalyst (ZnN₄-SAC) was discovered to be a robust heterogeneous non-noble catalyst for the oxidative cleavage of C-N bonds in N-alkylamines with O₂ molecules. Experimental results and DFT calculation revealed that ZnN₄-SAC not only activates O₂ to generate superoxide radicals (·O₂ ^-) for the oxidation of N-alkylamines to generate imine intermediates (C=N), but the single Zn atoms also served as the Lewis acid sites to promote the cleavage of C=N bonds in imine intermediates, including the first addition of H₂O to generate α-hydroxylamine intermediates and the following C-N bond cleavage via a H atom transfer process.

Electrochemically enabled decyanative C(sp3)-H oxygenation of N-cyanomethylamines to formamides

Selective oxygenation of C(sp³)-H bonds adjacent to nitrogen atoms is a highly attractive strategy for synthesizing various formamide derivatives while preserving the substrate skeletons. Herein, an environmentally benign electrochemically enabled decyanative C(sp³)-H oxygenation of N-cyanomethylamines using H₂O as a carbonyl oxygen atom source is described, leading to the synthesis of a large class of formamides in good to excellent yields with a broad substrate scope under metal- and oxidant-free conditions. This electrochemical technology highlights the facile incorporation of N-formyl into some important bioactive molecules.

The adsorption of single Au atom and nucleation on γ-Al2O3 surfaces

Single-atom catalysts (SACs) in heterogeneous catalysts have attracted increasing attention and the adsorption and nucleation of single atom on the surface are closely related to the performance of the catalyst. The present work employed density functional theory calculations to examine the adsorption of single Au atom and nucleation on γ-Al₂O₃ surfaces at the atomic level. The effect of surface hydroxyls group on the adsorption and nucleation of single Au atom on γ-Al₂O₃ surfaces is explored. It was found that the spillover reactions of surface hydroxyls H atoms with the deposited Au_- are not available on the hydroxylated surface. The interaction of Au to the clean surface is the stronger than to the hydroxylated surface. The even-odd alternations of Au_x and weak binding of single Au atoms to γ-Al₂O₃ leads to large even-numbered Au cluster on the surface. Density of states and electron density difference analysis show that the electronic structure of Au/γ-Al₂O₃ is quite different from the reported Cu and Pd on Al₂O₃.

Regiospecific α-methylene functionalisation of tertiary amines with alkynes via Au-catalysed concerted one-proton/two-electron transfer to O2

Regioselective transformations of tertiary amines, which are ubiquitously present in natural products and drugs, are important for the development of novel medicines. In particular, the oxidative α-C-H functionalisation of tertiary amines with nucleophiles via iminium cations is a promising approach because, theoretically, there is almost no limit to the type of amine and functionalisation. However, most of the reports on oxidative α-C-H functionalisations are limited to α-methyl-selective or non-selective reactions, despite the frequent appearance of α-methylene-substituted amines in pharmaceutical fields. Herein, we develop an unusual oxidative regiospecific α-methylene functionalisation of structurally diverse tertiary amines with alkynes to synthesise various propargylic amines using a catalyst comprising Zn salts and hydroxyapatite-supported Au nanoparticles. Thorough experimental investigations suggest that the unusual α-methylene regiospecificity is probably due to a concerted one-proton/two-electron transfer from amines to O₂ on the Au nanoparticle catalyst, which paves the way to other α-methylene-specific functionalisations.

Oxidative α-Functionalization of 1,2,3,4-Tetrahydroisoquinolines Catalyzed by a Magnetically Recoverable Copper Nanocatalyst. Application in the Aza-Henry Reaction and the Synthesis of 3,4-Dihydroisoquinolones

The oxidative α-functionalization of 2-aryl-1,2,3,4-tetrahydroisoquinolines (THIQs) promoted by a versatile heterogeneous nanocatalyst consisting of copper nanoparticles immobilized on silica-coated maghemite (CuNPs/MagSilica) has been accomplished. The methodology was successfully applied in the cross-dehydrogenative coupling (CDC) reaction of N-aryl THIQs and other tertiary amines with nitromethane as a pro-nucleophile (aza-Henry reaction) and the α-oxidation of THIQs with O₂ as a green oxidant. Phosphite, alkyne, or indole derivatives were also shown to be suitable candidates for their use as pro-nucleophiles in the CDC reaction with THIQs. The catalyst, with very low copper loading (0.4-1.0 mol % Cu), could be easily recovered by means of an external magnet and reused in four cycles without significant loss of activity.

Visible Light Driven Metal-Free Photoredox Catalyzed α-benzylation and α-oxygenation of N-substituted tetrahydroisoquinolines: Applications to Synthesis of Natural Products

Herein, visible light mediated organophoto redox catalysed simple and convenient method for the a-benzylation and a-oxygenation of tertiary amines is demonstrated. Synthesis of novel thiophenol based donor acceptor organophotoredox catalysts 4a - 4d were investigated along with commercial catalyst 4-CzIPN ( 4e ). A diverse biologically active a-benzylated tetrahydroisoquinolines and natural products such as (±)-Norlaudanosine, (±)-laudanosine and (±)-xylopinine have been synthesized under the optimized conditions in yields ranging from from 62-91%. Exploitation of synthesized a-benzylated compound using present phtoredox catalyzed conditions gave rise to dehydyrogenative benzylic oxidation product under oxygen atmosphere which is known to display biologically and structurally important properties. Also, various N-protected tertiary amines were found to be suitable for the a-oxygenation reactions using catalyst 4e and resulted in good yields (61-85%).

Iron(III)-catalyzed α-cyanation and carbonylation with 2-pyridylacetonitrile: divergent synthesis of α-amino nitriles and tetrahydroisoquinolinones

Iron-catalyzed oxidative synthesis of N-aryl-substituted tetrahydroisoquinolines (THIQs) toward tetrahydroisoquinoline-based derivatives is reported. A wide range of α-amino nitriles and tetrahydroisoquinolinones are synthesized in moderate to good yields. This approach involves a new organic nitrile source, a cheap iron catalyst under an oxygen atmosphere, and temperature-controlled divergent synthesis and features complete selectivity and operational simplicity.

TBAI-Catalyzed S-H and N-H Insertion Reactions of α-Diazoesters with Thiophenols and Amines under Metal-Free Conditions

Mild, convenient, and effective TBAI-catalyzed S-H and N-H insertion reactions of α-diazoesters with thiophenols and aromatic amines under metal-free conditions have been described, furnishing a straightforward and general platform for the synthesis of various thioethers and 2-amino-2-oxoacetates in moderate to excellent yields. Moreover, this strategy features simple operation, mild conditions, broad substrate scope, and easy scale-up.

Photoredox aerobic oxidation of unreactive amine derivatives through LMCT excitation of copper dichloride

Taking advantage of the chlorine radical as a HAT catalyst, a versatile oxidation system for unreactive amines has been well established.

Surface ligands enhance the catalytic activity of supported Au nanoparticles for the aerobic α-oxidation of amines to amides

The catalytic aerobic α-oxidation of amines in water is an atom economic and green alternative to current methods of amide synthesis. The reaction uses O2 as terminal oxidant, avoids hazardous...

Regioselective Synthesis of 2° Amides Using Visible-Light-Induced Photoredox-Catalyzed Nonaqueous Oxidative C-N Cleavage of N,N-Dibenzylanilines

A visible-light-driven photoredox-catalyzed nonaqueous oxidative C-N cleavage of N,N-dibenzylanilines to 2° amides is reported. Further, we have applied this protocol on 2-(dibenzylamino)benzamide to afford quinazolinones with (NH₄)₂S₂O₈ as an additive. Mechanistic studies imply that the reaction might undergo in situ generation of α-amino radical to imine by C-N bond cleavage followed by the addition of superoxide ion to form amides.

Copper catalyzed late-stage C(sp3)-H functionalization of nitrogen heterocycles

Nitrogen heterocycle represents a ubiquitous skeleton in natural products and drugs. Late-stage C(sp3)-H bond functionalization of N-heterocycles with broad substrate scope remains a challenge and of particular significance to modern chemical synthesis and pharmaceutical chemistry. Here, we demonstrate copper-catalysed late-stage C(sp3)-H functionalizaion of N-heterocycles using commercially available catalysts under mild reaction conditions. We have investigated 8 types of N-heterocycles which are usually found as medicinally important skeletons. The scope and utility of this approach are demonstrated by late-stage C(sp3)-H modification of these heterocycles including a number of pharmaceuticals with a broad range of nucleophiles, e.g. methylation, arylation, azidination, mono-deuteration and glycoconjugation etc. Preliminary mechanistic studies reveal that the reaction undergoes a C-H fluorination process which is followed by a nucleophilic substitution.

Production of Piperidine and δ-Lactam Chemicals from Biomass-Derived Triacetic Acid Lactone

Piperidine and δ-Lactam chemicals have wide application, which are currently produced from fossil resource in industry. Production of this kind of chemicals from lignocellulosic biomass is of great importance, but is challenging and the reported routes give low yield. Herein, we demonstrate the strategy to synthesize 2-methyl piperidine (MP) and 6-methylpiperidin-2-one (MPO) from biomass-derived triacetic acid lactone (TAL) that is produced microbially from glucose. In this route, TAL was firstly converted into 4-hydroxy-6-methylpyridin-2(1H)-one (HMPO) through facile aminolysis, subsequently HMPO was selectively transformed into MP or MPO over Ru catalysts supported on beta zeolite (Ru/BEA-X, X is the molar ratio of Si to Al) via the tandem reaction. It was found that the yield of MP could reach 76.5 % over Ru/BEA-60 in t-BuOH, and the yield of MPO could be 78.5 % in dioxane. Systematic studies reveal that the excellent catalytic performance of Ru/BEA-60 was closely correlated with the cooperative effects between active metal and acidic zeolite with large pore geometries. The related reaction pathway was studied on the basis of control experiments.

α-Oxygenation of N-aryl/alkyl heterocyclic compounds via ruthenium photocatalysis

We herein report ruthenium(iii) photocatalyzed oxidation of N-aryl tertiary amines to the corresponding amides.

Aerobic oxidation of primary amines to amides catalyzed by an annulated mesoionic carbene (MIC) stabilized Ru complex

A Ru complex, stabilized by an annulated mesoionic carbene ligand, catalyzes the aerobic oxidation of a host of primary amines to amides in high yields and excellent selectivity. Kinetics, Hammett and DFT studies provide mechanistic insight.

Oxidize Amines to Nitrile Oxides: One Type of Amine Oxidation and Its Application to Directly Construct Isoxazoles and Isoxazolines

A facile oxidative heterocyclization of commercially available amines and tert-butyl nitrite with alkynes or alkenes leading to isoxazoles or isoxazolines is described. The unprecedented strategy of the oxidation of an amine directly to a nitrile oxide was used in this cyclization process. This reaction is highly efficient, regiospecific, operationally simple, mild, and tolerant of a variety of functional groups. Control experiments support a nitrile oxide intermediate mechanism for this novel class of oxidative cyclization reactions. Moreover, synthetic applications toward bioactive molecular skeletons and the late-stage modification of drugs were realized.

Metal-free photocatalysts for the oxidation of non-activated alcohols and the oxygenation of tertiary amines performed in air or oxygen

This protocol describes the use of 9-fluorenone as a cheap and non-toxic photocatalyst for the oxidation of non-activated alcohols performed under the irradiation of a blue light-emitting diode. It also describes the use of the similarly cheap and non-toxic photocatalyst rose bengal for the selective α-oxygenation of tertiary amines to produce the corresponding amides in a selective way using the same light source. We have provided detailed instructions on how to assemble the light-emitting diode equipment and set up the photocatalytic reaction, where an oxygen atmosphere is created with an O₂-filled balloon. Further details are provided using four example reactions that illustrate how this system works: alcohol oxidation to prepare terephthlalaldehyde and androstanedione, and amine oxidation to make 2-phenyl-3,4-dihydroisoquinolin-1(2H)-one and (4-((4-chlorophenyl)(phenyl)methyl)piperazin-1-yl)m-tolyl)methanone. The times needed to perform these photocatalytic reactions are 18, 76, 22 and 54 h, respectively. We believe that this protocol represents a robust methodology for the late-stage modification of amines and the selective oxidation of steroids.

Synthesis of heteroatom-containing pyrrolidine derivatives based on Ti(O-iPr)4 and EtMgBr-catalyzed carbocyclization of allylpropargyl amines with Et2Zn

The Ti(O-iPr)₄ and EtMgBr-catalyzed regio and stereoselective carbocyclization of N-allyl-substituted 2-alkynylamines with Et₂Zn, followed by deuterolysis or hydrolysis, affords the corresponding methylenepyrrolidine derivatives in high yields. It was found that Ti–Mg-catalyzed carbocyclization of N-allyl-substituted 2-alkynylamines with Et₂Zn is equally selective in dichloromethane, hexane, toluene, and diethyl ether. The reaction was tolerant to the presence of aryl, alkyl, trimethylsilyl, methoxymethyl and aminomethyl substituents on the alkyne. A selective method was proposed for the preparation of bis-pyrrolidine derivatives using Ti–Mg-catalyzed carbocyclization of bis-allylpropargyl amines with Et₂Zn.

The Ti(O-iPr)₄ and EtMgBr-catalyzed regio and stereoselective carbocyclization of N-allyl-substituted 2-alkynylamines with Et₂Zn, followed by deuterolysis or hydrolysis, affords the corresponding methylenepyrrolidine derivatives in high yields.

Dehydrogenation and α-functionalization of secondary amines by visible-light-mediated catalysis

A visible-light-mediated process for dehydrogenation of amines has been described. The given protocol showed a broad substrate scope, mild reaction conditions and excellent results without the requirement of tedious purification.

In Situ Nanoparticle Embedding for Authentication of Epoxy Composites

In situ reduction of chloroauric acid inside an amine-cured epoxy matrix leads to formation of gold nanoparticles which are embedded inside the part. This phenomenon is leveraged to design an authentication system for composites wherein the particles are embedded spatially and are invisible to the naked eye. Under UV light, the particles diffract light and create an easily visible path. The particles penetrate inside the part and create a permanent, cost-effective, tamper-proof code. The advantage of this technique is that this authentication system can be built in composite parts after fabrication of the composite structure. As very small amount (nanograms) of particles are present in the part, negligible change in the thermal characteristics of the parent matrix is observed. The particles can be embedded easily in carbon fiber as well as glass fiber reinforced epoxy structures.

Sodium periodate/TEMPO as a selective and efficient system for amine oxidation

A new metal-free protocol for promoting oxidation of amines in aqueous-organic medium was developed. NaIO₄ and TEMPO as the catalyst emerged as the most efficient and selective system for oxidation of differently substituted benzyl amines to the corresponding benzaldehydes without overoxidation. Unsymmetrical secondary amines underwent selective oxidation only at the benzylic position thus realising an oxidative deprotection of a benzylic group with an easy amine recovery.

NaIO₄/TEMPO efficiently allowed a metal-free oxidation of benzylamines to benzaldehydes with complete selectivity for N-benzyl residues in secondary amines.

Metal-Free Thermal Activation of Molecular Oxygen Enabled Direct α-CH2-Oxygenation of Free Amines

Direct oxidation of α-CH₂ group of free amines is hard to achieve due to the higher reactivity of amine moiety. Therefore, oxidation of amines involves the use of sophisticated metallic reagents/catalyst in the presence or absence of hazardous oxidants under sensitive reaction conditions. A novel method for direct C-H oxygenation of aliphatic amines through a metal-free activation of molecular oxygen has been developed. Both activated and unactivated free amines were oxygenated efficiently to provide a wide variety of amides (primary, secondary) and lactams under operationally simple conditions without the aid of metallic reagents and toxic oxidants. The method has been applied to the synthesis of highly functionalized amide-containing medicinal drugs, such as O-Me-alibendol and -buclosamide.

Development of a Hypoxic Radiosensitizer-Prodrug Liposome Delivery DNA Repair Inhibitor Dbait Combination with Radiotherapy for Glioma Therapy

Gliomas are highly radioresistant tumors, mainly due to hypoxia in the core region of the gliomas and efficient DNA double-strand break repair. However, the design of a radiosensitizer incorporating the two above mechanisms is difficult and has rarely been reported. Thus, this study develops a hypoxic radiosensitizer-prodrug liposome (MLP) to deliver the DNA repair inhibitor Dbait (MLP/Dbait) to achieve the simultaneous entry of radiosensitizers with two different mechanisms into the glioma. MLP/Dbait effectively sensitizes glioma cells to X-ray radiotherapy (RT). Histological and microscopic examinations of dissected brain tissue confirm that MLP effectively delivers Dbait into the glioma. Furthermore, the combination of MLP/Dbait with RT significantly inhibits growth of the glioma, as assessed by in vivo bioluminescence imaging. These findings suggest that MLP is a promising candidate as a Dbait delivery system to enhance the effect of RT on glioma, owing to the synergistic effects of the two different radiosensitizers.

Hypoxia-responsive ionizable liposome delivery siRNA for glioma therapy

Here, we report the hypoxia-responsive ionizable liposomes to deliver small interference RNA (siRNA) anticancer drugs, which can selectively enhance cellular uptake of the siRNA under hypoxic and low-pH conditions to cure glioma. For this purpose, malate dehydrogenase lipid molecules were synthesized, which contain nitroimidazole groups that impart hypoxia sensitivity and specificity as hydrophobic tails, and tertiary amines as hydrophilic head groups. These malate dehydrogenase molecules, together with DSPE-PEG2000 and cholesterol, were self-assembled into O'1,O1-(3-(dimethylamino)propane-1,2-diyl) 16-bis(2-(2-methyl-5-nitro-1H-imidazol-1-yl)ethyl) di(hexadecanedioate) liposomes (MLP) to encapsulate siRNA through electrostatic interaction. Our study showed that the MLP could deliver polo-like kinase 1 siRNA (siPLK1) into glioma cells and effectively enhance the cellular uptake of MLP/siPLK1 because of increased positive charges induced by hypoxia and low pH. Moreover, MLP/siPLK1 was shown to be very effective in inhibiting the growth of glioma cells both in vitro and in vivo. Therefore, the MLP is a promising siRNA delivery system for tumor therapy.