Palladium-Catalyzed Carbonylation of Amines: Switchable Approaches to Carbamates and N,N′-Disubstituted Ureas

A thiolated copper-hydride nanocluster with chloride bridging as a catalyst for carbonylative C-N coupling of aryl amines under mild conditions: a combined experimental and theoretical study

Atomically precise copper nanoclusters (Cu NCs), an emerging class of nanomaterials, have garnered significant attention owing to their versatile core-shell architecture and their potential applications in catalytic reactions. In this study, we present a straightforward synthesis strategy for [Cu29(StBu)12(PPh3)4Cl6H10][BF4] (Cu29) NCs and explore their catalytic activity in the carbonylative C-N coupling reaction involving aromatic amines and N-heteroarenes with dialkyl azodicarboxylates. Through a combination of experimental investigations and density functional theory studies, we elucidate the radical mechanisms at play. The crucial step in the catalytic process is identified as the decomposition of diisopropyl azodicarboxylates on the surface of Cu29 NCs, leading to the generation of oxyacyl radicals and the liberation of nitrogen gas. Subsequently, an oxyacyl radical abstracts a hydrogen atom from aniline, initiating the formation of an aminyl radical. Finally, the aminyl radical reacts with another oxyacyl radical, culminating in the synthesis of the desired carbamate product. This detailed analysis provides insights into the intricate catalytic pathways of Cu29 NCs, shedding light on their potential for catalyzing carbonylative C-N coupling reactions.

Deoxygenative Hydroboration of Aromatic Nitro Compounds Catalyzed by Tetra(diisopropylamido) Rare-Earth Metal-Lithium Bimetallic Complexes

The first example of the reduction of a nitro compound with HBPin catalyzed by tetra(diisopropylamido) rare-earth metal-lithium bimetallic complexes LiRE(NiPr2)4(THF) (RE = La, Nd, Sm, Gd, and Y) was disclosed. A series of aromatic nitro compounds were reduced to N-borylamines in high yields (up to 99%). The derivatives of N-borylamines─amides and carbamates─were obtained in a sequential one-pot manner. Furthermore, kinetic studies of the deoxygenative hydroboration of nitro compounds were carried out.

Advances in Cu nanocluster catalyst design: recent progress and promising applications

The quest for cleaner pathways to the production of fuels and chemicals from non-fossil feedstock, efficient transformation of raw materials to value-added chemicals under mild conditions, and control over the activity and selectivity of chemical processes are driving the state-of-the-art approaches to the construction and precise chemical modification of sustainable nanocatalysts. As a burgeoning category of atomically precise noble metal nanoclusters, copper nanoclusters (Cu NCs) benefitting from their exclusive structural architecture, ingenious designability of active sites and high surface-to-volume ratio qualify as potential rationally-designed catalysts. In this Minireview, we present a detailed coverage of the optimal design strategies and controlled synthesis of Cu NC catalysts with a focus on tuning of active sites at the atomic level, the implications of cluster size, shape and structure, the ligands and heteroatom doping on catalytic activity, and reaction scope ranging from chemical catalysis to emerging photocatalysis and electrocatalysis.

Mechanosynthesis of Polyureas and Studies of Their Responses to Anions

Polyureas (PUs) have already found wide practical applications, and various methods of their synthesis have been reported. In this manuscript, we wished to report the very first mechanochemical approach towards aromatic PUs via reactions between isomeric 2,2'-, 3,3'-, and 4,4'-diaminobiphenyls and triphosgene under solvent-free conditions following ball-milling. By using this synthetic approach, both PUs and azomethine-capped Pus were obtained. The fluorescence response of the above-mentioned PUs towards various anions in solutions were studied and selective fluorescence responses towards the hydroxyl and fluoride anions were observed.

Direct Conversion of Low-Concentration CO2 into N-Aryl and N-Alkyl Carbamic Acid Esters Using Tetramethyl Orthosilicate with Amidines as a CO2 Capture Agent and a Catalyst

Herein, we report the direct conversion of low-concentration CO₂ (15 vol %), equivalent to the CO₂ concentration in the exhaust gas from a thermal power station, into carbamic acid esters (CAEs), which are precursors for pharmaceuticals, agrochemicals, and isocyanates. The reaction was performed using Si(OMe)₄ as a nonmetallic regenerable reagent and 1,8-diazabicyclo[5.4.0]undec-7-ene as a CO₂ capture agent and catalyst. This reaction system does not require the addition of metal complex catalysts or metal salt additives and is therefore simpler than our previously reported reaction system involving Ti(OR)₄ and a Zn(II) catalyst. A variety of N-aryl, N-alkyl, and bis CAEs (precursors of polyurethane raw materials) were obtained in moderate to high yields (45-77% for 6 examples, 84-89% for 7 examples). In addition, bis CAEs were successfully synthesized from simulated exhaust gas containing impurities such as SO₂, NO₂, and CO or on a gram scale. We believe that this method can eliminate the use of toxic phosgene as the raw material for isocyanate production and mitigate CO₂ emissions.

Urea Synthesis from Isocyanides and O-Benzoyl Hydroxylamines Catalyzed by a Copper Salt

In the presence of CuOAc, a series of unsymmetric ureas can be generated in moderate to good yields under mild reaction conditions (10 mol% of CuOAc, 2 equiv t-BuONa or PhONa, 30 °C), using aryl isocyanides and O-benzoyl hydroxylamines as the readily accessible starting materials. The reactions might undergo a cascade process involving isocyanide insertion into the N-O bond and Mumm-type rearrangement. This work represents a rare example of isocyanide insertion into N-O bonds, which would extend isocyanide insertion chemistry.

Structure-Activity Relationship and Mechanistic Studies of Bisaryl Urea Anticancer Agents Indicate Mitochondrial Uncoupling by a Fatty Acid-Activated Mechanism

Targeting the cancer cell mitochondrion is a promising approach for developing novel anticancer agents. The experimental anticancer agent N,N'-bis(3,5-dichlorophenyl)urea (SR4) induces apoptotic cell death in several cancer cell lines by uncoupling mitochondrial oxidative phosphorylation (OxPhos) using a protein-free mechanism. However, the precise mechanism by which SR4 depolarizes mitochondria is unclear because SR4 lacks an acidic functional group typically found in protein-independent uncouplers. Recently, it was shown that structurally related thioureas can facilitate proton transport across lipid bilayers by a fatty acid-activated mechanism, in which the fatty acid acts as the site of protonation/deprotonation and the thiourea acts as an anion transporter that shuttles deprotonated fatty acids across the phospholipid bilayer to enable proton leak. In this paper, we show that SR4-mediated proton transport is enhanced by the presence of free fatty acids in the lipid bilayer, indicating that SR4 uncouples mitochondria through the fatty acid-activated mechanism. This mechanistic insight was used to develop a library of substituted bisaryl ureas for structure-activity relationship studies and subsequent cell testing. It was found that lipophilic electron-withdrawing groups on bisaryl ureas enhanced electrogenic proton transport via the fatty acid-activated mechanism and had the capacity to depolarize mitochondria and reduce the viability of MDA-MB-231 breast cancer cells. The most active compound in the series reduced cell viability with greater potency than SR4 and was more effective at inhibiting adenosine triphosphate production.

A palladium-catalyzed Barluenga cross-coupling - reductive cyclization sequence to substituted indoles

A short and flexible synthesis of substituted indoles employing two palladium-catalyzed reactions, a Barluenga cross-coupling of p-tosylhydrazones with 2-nitroarylhalides followed by a palladium-catalyzed, carbon monoxide-mediated reductive cyclization has been developed. A one-pot, two-step methodology was further developed, eliminating isolation and purification of the cross-coupling product. This was accomplished by utilizing the initially added 0.025 equivalents of bis(triphenylphosphine)palladium dichloride, thus serving a dual role in the cross-coupling and the reductive cyclization. It was found that addition of 1,3-bis(diphenylphosphino)propane and carbon monoxide after completion of the Barluenga reaction afforded, in most cases, significantly better overall yields.

Catalyst-free synthesis of substituted pyridin-2-yl, quinolin-2-yl, and isoquinolin-1-yl carbamates from the corresponding hetaryl ureas and alcohols

A novel catalyst-free synthesis of N-pyridin-2-yl, N-quinolin-2-yl, and N-isoquinolin-1-yl carbamates utilizes easily accessible N-hetaryl ureas and alcohols. The proposed environmentally friendly technique is suitable for the good-to-high yielding synthesis of a wide range of N-pyridin-2-yl or N-quinolin-2-yl substituted carbamates featuring electron-donating and electron-withdrawing groups in the azine rings and containing various primary, secondary, and even tertiary alkyl substituents at the oxygen atom (48-94%; 31 examples). The DFT calculation and experimental study showed that the reaction proceeds through the intermediate formation of hetaryl isocyanates. The method can be applied to obtain N-isoquinolin-1-yl carbamates, although in lower yields, and ethyl benzo[h]quinolin-2-yl carbamate has also been successfully synthesized (68%).

Pd Nanoparticles and Mixture of CO2/CO/O2 Applied in the Carbonylation of Aniline

CO2 is a compound of high stability which proves useful in some organic syntheses as a solvent or component decreasing explosivity of gases. It is also a good carbonylating agent for aliphatic amines although not for aromatic ones, the latter being carbonylated with phosgene or, as in our previous works, with CO/O2 in the presence of Pd(II) complexes. In this work we have used the mixture of CO/O2 and CO2 for carbonylation of aniline to N,N’-diphenylurea. After optimization of the reaction conditions (56% of CO2 in CO2/CO mixture) we studied the activity of three kinds of pre-catalysts: (a) Pd(II) complexes, (b) Pdblack, and (c) palladium nanoparticles (PdNPs) in the presence of derivatives of pyridine (XnPy). The highest conversion of aniline (with selectivity towards N,N-diphenylurea ca. 90%) was observed for PdNPs. The results show that catalytic cycle involves Pd(0) stabilized by pyridine ligand as active species. Basing on this observation, we put the hypothesis that application of PdNPs instead of Pd(II) complex can efficiently reduce the reaction time.

Development and biological investigations of hypoxia-sensitive prodrugs of the tyrosine kinase inhibitor crizotinib

Despite the huge success of tyrosine kinase inhibitors as anticancer agents, severe side effects are a major problem. In order to overcome this drawback, the first hypoxia-activatable 2-nitroimidazole-based prodrugs of the clinically approved ALK and c-MET inhibitor crizotinib were developed. The 2-aminopyridine functionality of crizotinib (essential for target kinase binding) was considered as ideal position for prodrug derivatization. Consequently, two different prodrugs were synthesized with the nitroimidazole unit attached to crizotinib either via carbamoylation (A) or alkylation (B) of the 2-aminopyridine moiety. The successful prodrug design could be proven by docking studies and a dramatically reduced ALK and c-MET kinase-inhibitory potential. Furthermore, the prodrugs showed high stability in serum and release of crizotinib in an enzymatic nitroreductase-based cleavage assay was observed for prodrug A. The in vitro activity of both prodrugs was investigated against ALK- and c-MET-dependent or -overexpressing cells, revealing a distinct hypoxia-dependent activation for prodrug A. Finally, inhibition of c-MET phosphorylation and cell proliferation could also be proven in vivo. In summary of the theoretical, chemical and biological studies, prodrug derivatization of the 2-aminopyridine position can be considered as a promising strategy to reduce the side effects and improve the anticancer activity of crizotinib.

Zn(ii)@TFP-DAQ COF: an efficient mesoporous catalyst for the synthesis of N-methylated amine and carbamate through chemical fixation of CO2

Selective N-methylation and carbamate formation reactions were demonstrated via the chemical incorporation of CO₂ using a Zn-loaded TFP-DAQ COF (covalent organic framework) as an active catalyst under mild reaction conditions.

Synthesis of unsymmetrical urea derivatives via one-pot sequential three-component reactions of cyclic 2-diazo-1,3-diketones, carbodiimides, and 1,2-dihaloethanes

An effective and operationally simple one-pot strategy has been developed for the synthesis of unsymmetrical urea derivatives via sequential three-component reactions of cyclic 2-dizao-1,3-diketones with carbodiimides and 1,2-dihaloethane.

Selective formation of phthalimides from amines, aldehydes and CO by Pd-catalyzed oxidative C–H aminocarbonylation

The Pd/Cu-catalyzed intramolecular C–H bond aminocarbonylation utilizing imines as the amine source has been developed. This transformation provides an efficient and straightforward protocol for the synthesis of phthalimides which widely exist in natural products, pharmaceutical and functional materials. Various functional groups are tolerated and the yields are up to 99%.

Synthesis of 11C-Labelled Ureas by Palladium(II)-Mediated Oxidative Carbonylation

Positron emission tomography is an imaging technique with applications in clinical settings as well as in basic research for the study of biological processes. A PET tracer, a biologically active molecule where a positron-emitting radioisotope such as carbon-11 has been incorporated, is used for the studies. Development of robust methods for incorporation of the radioisotope is therefore of the utmost importance. The urea functional group is present in many biologically active compounds and is thus an attractive target for incorporation of carbon-11 in the form of [¹¹C]carbon monoxide. Starting with amines and [¹¹C]carbon monoxide, both symmetrical and unsymmetrical ¹¹C-labelled ureas were synthesised via a palladium(II)-mediated oxidative carbonylation and obtained in decay-corrected radiochemical yields up to 65%. The added advantage of using [¹¹C]carbon monoxide was shown by the molar activity obtained for an inhibitor of soluble epoxide hydrolase (247 GBq/μmol–319 GBq/μmol). DFT calculations were found to support a reaction mechanism proceeding through an ¹¹C-labelled isocyanate intermediate.

Ruthenium-Catalyzed Urea Synthesis by N-H Activation of Amines

Activation of the N-H bond of amines by a ruthenium pincer complex operating via "amine-amide" metal-ligand cooperation is demonstrated. Catalytic formyl C-H activation of N,N-dimethylformamide (DMF) is observed in situ, which resulted in the formation of CO and dimethylamine. The scope of this new mode of bond activation is extended to the synthesis of urea derivatives from amines using DMF as a carbon monoxide (CO) surrogate. This catalytic protocol allows the synthesis of simple and functionalized urea derivatives with liberation of hydrogen, devoid of any stoichiometric activating reagents, and avoids the direct use of fatal CO. The catalytic carbonylation occurred at low temperature to provide the formamide; a formamide intermediate was isolated. The consecutive addition of different amines provided unsymmetrical urea compounds. The reactions are proposed to proceed via N-H activation of amines followed by CO insertion from DMF and with liberation of dihydrogen.

Palladium-catalyzed oxidative carbonylation of N-aryl enamino esters with CO and alcohols: synthesis of N-aryl aminomethylenemalonates

A novel palladium-catalyzed oxidative carbonylation of tri-substituted alkenes with CO and alcohols to synthesize N-aryl aminomethylenemalonates has been developed.

The dearomative annulation between N-2-pyridylamidine and CO2 toward pyrido[1,2-a]-1,3,5-triazin-4-ones

A base-promoted dearomative annulation between N-2-pyridylamidine and an atmospheric pressure of CO₂ was developed, affording a series of pyrido[1,2-a]-1,3,5-triazin-4-ones in moderate to excellent yields.

Scalable, One-Pot, Microwave-Accelerated Tandem Synthesis of Unsymmetrical Urea Derivatives

We report a facile, microwave-accelerated, one-pot tandem synthesis of unsymmetrical ureas via a Curtius rearrangement. In this method, one-pot microwave irradiation of commercially available (hetero)aromatic acids and amines in the presence of diphenylphosphoryl azide enabled extremely rapid (1-5 min) construction of an array of unsymmetrical ureas in good to excellent yields. We demonstrate the utility of our method in the efficient, gram-scale synthesis of key biologically active compounds targeting the cannabinoid 1 and α7 nicotinic acetylcholine receptors.

Efficient preparation of carbamates by Rh-catalysed oxidative carbonylation: unveiling the role of the oxidant

The synthesis of a wide variety of carbamates from amines, alcohols and carbon monoxide has been achieved by means of a Rh-catalysed oxidative carbonylation reaction that uses Oxone as a stoichiometric oxidant. In-depth studies on the reaction mechanism shed light on the intimate role of Oxone in the catalytic cycle.

Polynuclear Gold [Au(I) ]4 , [Au(I) ]8 , and Bimetallic [Au(I) 4 Ag(I) ] Complexes: C-H Functionalization of Carbonyl Compounds and Homogeneous Carbonylation of Amines

The synthesis of tetranuclear gold complexes, a structurally unprecedented octanuclear complex with a planar [Au(I) 8 ] core, and pentanuclear [Au(I) 4 M(I) ] (M=Cu, Ag) complexes is presented. The linear [Au(I) 4 ] complex undergoes C-H functionalization of carbonyl compounds under mild reaction conditions. In addition, [Au(I) 4 Ag(I) ] catalyzes the carbonylation of primary amines to form ureas under homogeneous conditions with efficiencies higher than those achieved by gold nanoparticles.

Palladium-Catalyzed Carbamate-Directed Regioselective Halogenation: A Route to Halogenated Anilines

This study describes an efficient method for ortho-selective halogenation of N-arylcarbamates under mild conditions for the first time. Although being weakly coordinating, N-arylcarbamates act very well as a removable directing group for activation of C-H bonds. The developed procedure results in extremely valuable halogenated N-arylcarbmates that can further be hydrolyzed to halogenated anilines. The obtained reaction conditions showed broad scope and wide functional group tolerance. All the products were formed in good yields with extremely high selectivity.

Copper-catalyzed carbonylation of anilines by diisopropyl azodicarboxylate for the synthesis of carbamates

Herein, a simple Cu-catalyzed system for the synthesis of carbamates has been developed using anilines with diisopropyl azodicarboxylate as a carbonyl source under solvent-free conditions.

Heterobimetallic dinuclear lanthanide alkoxide complexes as acid–base bifunctional catalysts for synthesis of carbamates under solvent-free conditions

Heterobimetallic dinuclear lanthanide alkoxide complexes Ln₂Na₈(OCH₂CH₂NMe₂)₁₂(OH)₂ [Ln: I (Nd), II (Sm), III (Yb) and IV (Y)] were used as efficient acid–base bifunctional catalysts for the synthesis of carbamates and the N-Boc protection of amines.

An efficient one-pot synthesis of N,N′-disubstituted ureas and carbamates from N-acylbenzotriazoles

A facile one-pot synthesis of carbamates and N,Nʹ-disubstituted symmetrical ureas from N-acylbenzotriazoles has been devised. It is believed that, the intermediate acyl-azide undergo Curtius rearrangement.

(E)-α,β-unsaturated amides from tertiary amines, olefins and CO via Pd/Cu-catalyzed aerobic oxidative N-dealkylation

A novel Pd/Cu-catalyzed chemoselective aerobic oxidative N-dealkylation/carbonylation reaction has been developed for the efficient and straightforward construction of synthetically useful and bioactive (E)-α,β-unsaturated amide derivatives.

Quinoline-2-carboimine copper complex immobilized on amine functionalized silica coated magnetite nanoparticles: a novel and magnetically retrievable catalyst for the synthesis of carbamates via C–H activation of formamides

A novel and magnetically recoverable nanocatalytic system consisting of a magnetic silica based copper complex has been fabricated and applied in the synthesis of carbamates via C–H activation of formamides.