Iron catalysed direct alkylation of amines with alcohols

NNO Pincer-Supported Pd(II)-Catalyzed Reductive N-Alkylation of Challenging Nitroarenes with Alcohols via Borrowing Hydrogen Strategy

A sustainable catalytic synthesis of selective monoalkylated amines from nitroarenes and alcohols by new palladium(II)-NNO pincer-type complexes has been described. Herein, a series of Pd(II) complexes [Pd(NNO)PPh3] (1-3) are synthesized and characterized by analytical and spectroscopic (IR, NMR, and HR-MS) methods. The solid-state molecular structures of two complexes are established by X-ray single-crystal diffraction. Furthermore, the catalytic N-alkylation of challenging nitroarenes with primary and secondary alcohols has been performed by the well-defined palladium(II) complexes via borrowing hydrogen strategy. The current protocol offers a wide range of monoalkylated amines (26 examples) with a maximum yield of 87% utilizing 1 mol % of catalyst loading. Gratifyingly, the catalytic system works well under mild reaction conditions and atom economy with water is the only byproduct. Furthermore, control experiments confirm the formation of probable intermediates (aniline, aldehyde, and imine), and deuterium labeling authenticates the borrowing hydrogen mechanism. A gram-scale synthesis of an alkylated product clearly demonstrates the synthetic efficacy of the present catalytic methodology.

Iron- and Organo-Catalyzed Borrowing Hydrogen for the Stereoselective Construction of Tetrahydropyrans

Stereocontrolled oxa-Michael additions are challenging, given the high reversibility of the process, which ultimately leads to racemization of the newly formed stereocenters. When iron-catalyzed borrowing hydrogen from allylic alcohols was combined with a stereocontrolled organocatalytic oxa-Michael addition, a wide array of chiral tetrahydropyrans were efficiently prepared. The reaction could be performed in a diastereoselective manner from pre-existing stereocenters or enantioselectively from achiral substrates. The key to success was the reactivity of the iron complex, which was selective for allylic alcohol dehydrogenation and irreversibly led the reaction to the final product.

A sustainable strategic approach for N-alkylation of amines with activation of alcohols triggered via a hydrogen auto-transfer reaction using a Pd(II) complex: evidence for metal-ligand cooperativity

This work describes a new well-defined, air-stable, phosphine free palladium(II) [Pd(L)Cl] (1) catalyst. This catalyst was utilized for N-alkylation of amines and indole synthesis where H2O was found to be the by-product. A broad range of aromatic amines were alkylated using this homogeneous catalyst with a catalyst loading of 0.1 mol%. Greener aromatic and aliphatic primary alcohols were utilized and a hydrogen auto-transfer strategy via a metal-ligand cooperative approach was investigated. The precursor of the antihistamine-containing drug molecule tripelennamine was synthesized on a gram scale for large-scale applicability of the current synthetic methodology. A number of control experiments were performed to investigate the possible reaction pathway and the outcomes of these experiments indicated the azo-chromophore as a hydrogen reservoir during the catalytic cycle.

Titanium-Catalyzed Selective N-Alkylation of Amines with Alcohols via Borrowing Hydrogen Methodology

The N-alkylation of amines with alcohols using earth-abundant and nonprecious metal catalysts has gained considerable attention in the pharmaceutical industry. We described titanium-catalyzed synthetic protocol for N-alkylation of amines with alcohols via borrowing hydrogen or hydrogen autotransfer reactions. The methodology enables the selective monoalkylation of various substituted (hetero)aromatic amines in good to excellent yields (up to 97% yield). The importance of the protocol was further demonstrated by the applicability of earth-abundant metal catalysis and the synthesis of 32 N-alkylated amines. The work allows the utilization of titanium-based catalysts for various reactions to expand the nature blueprint in catalysis.

N-Alkylation of aromatic amines with alcohols by using a commercially available Ru complex under mild conditions

An N-alkylation procedure has been developed under very mild conditions using a known commercially available Ru-based catalyst. As a result, a wide range of aromatic primary amines has been selectively alkylated with several primary alcohols, yielding the corresponding secondary amines in high yields. The methodology also enables the methylation of anilines in refluxing methanol and the preparation of a set of heterocycles in a straightforward way.

No Transition Metals Required - Oxygen Promoted Synthesis of Imines from Primary Alcohols and Amines under Ambient Conditions

The synthesis of imines denotes a cornerstone in organic chemistry. The use of alcohols as renewable substituents for carbonyl-functionality represents an attractive opportunity. Consequently, carbonyl moieties can be in situ generated from alcohols upon transition-metal catalysis under inert atmosphere. Alternatively, bases can be utilized under aerobic conditions. In this context, we report the synthesis of imines from benzyl alcohols and anilines, promoted by KOt Bu under aerobic conditions at room temperature, in the absence of any transition-metal catalyst. A detailed investigation of the radical mechanism of the underlying reaction is presented. This reveals a complex reaction network fully supporting the experimental findings.

Ir-Catalyzed Synthesis of Functionalized Pyrrolidines and Piperidines Using the Borrowing Hydrogen Methodology

The Ir(III)-catalyzed synthesis of 3-pyrrolidinols and 4-piperidinols combining 1,2,4-butanetriol or 1,3,5-pentanetriol with primary amines was carried out. This borrowing hydrogen methodology was further extended to the sequential diamination of triols leading to amino-pyrrolidines and amino-piperidines.

N-Alkylation of Amines by C1-C10 Aliphatic Alcohols Using A Well-Defined Ru(II)-Catalyst. A Metal-Ligand Cooperative Approach

A Ru(II)-catalyzed efficient and selective N-alkylation of amines by C1-C10 aliphatic alcohols is reported. The catalyst [Ru(L^1a)(PPh₃)Cl₂] (1a) bearing a tridentate redox-active azo-aromatic pincer, 2-((4-chlorophenyl)diazenyl)-1,10-phenanthroline (L^1a) is air-stable, easy to prepare, and showed wide functional group tolerance requiring only 1.0 mol % (for N-methylation and N-ethylation) and 0.1 mol % of catalyst loading for N-alkylation with C3-C10 alcohols. A wide array of N-methylated, N-ethylated, and N-alkylated amines were prepared in moderate to good yields via direct coupling of amines and alcohols. 1a efficiently catalyzes the N-alkylation of diamines selectively. It is even suitable for synthesizing N-alkylated diamines using (aliphatic) diols producing the tumor-active drug molecule MSX-122 in moderate yield. 1a showed excellent chemo-selectivity during the N-alkylation using oleyl alcohol and monoterpenoid β-citronellol. Control experiments and mechanistic investigations revealed that the 1a-catalyzed N-alkylation reactions proceed via a borrowing hydrogen transfer pathway where the hydrogen removed from the alcohol during the dehydrogenation step is stored in the ligand backbone of 1a, which in the subsequent steps transferred to the in situ formed imine intermediate to produce the N-alkylated amines.

N-Alkylation and N-Methylation of Amines with Alcohols Catalyzed by Nitrile-Substituted NHC-Ir(III) and NHC-Ru(II) Complexes

A series of nitrile-modified N-heterocyclic carbene (NHC) complexes of Ir(III) (2a-e) and Ru(II) (3a-d) have been prepared by transmetallation of [IrCp*Cl2]2 and [RuCl2(p-cymene)]2 forming an in situ NHC-Ag complex. The structures of all complexes were characterized by 1H NMR, 13C NMR, and Fourier transform infrared (FT-IR) spectroscopies. And the structures were clearly elucidated by performing X-ray diffraction studies on 2b, 3a, and 3c single crystals. The complexes of NHC-Ir(III) (2a-e) and NHC-Ru(II) (3a-d) were investigated in the N-alkylation reaction of aniline derivatives with benzyl alcohols to form N-benzyl amines and in the N-methylation reaction of aniline derivatives with methanol. Both reactions were performed in solvent-free media. The Ir(III) complexes (2a-e) were found to perform essentially better than similar Ru(II) complexes (3a-d) in the N-alkylation and N-methylation reactions. Among the Ir(III) complexes (2a-e), the best results were obtained with 2b. The catalytic mechanisms of both reactions were revealed by 1H NMR study. Formation of Ir-hydride species was observed for both reactions. This new report provides useful information to evaluate the activity of complexes and the differences in sensitivity between the NHCs.

Zn(II)-Catalyzed Selective N-Alkylation of Amines with Alcohols Using Redox Noninnocent Azo-Aromatic Ligand as Electron and Hydrogen Reservoir

We report a sustainable and eco-friendly approach for selective N-alkylation of various amines by alcohols, catalyzed by a well-defined Zn(II)-catalyst, Zn(L^a)Cl₂ (1a), bearing a tridentate arylazo scaffold. A total of 57 N-alkylated amines were prepared in good to excellent yields, out of which 17 examples are new. The Zn(II)-catalyst shows wide functional group tolerance, is compatible with the synthesis of dialkylated amines via double N-alkylation of diamines, and produces the precursors in high yields for the marketed drugs tripelennamine and thonzonium bromide in gram-scale reactions. Control reactions and DFT studies indicate that electron transfer events occur at the azo-chromophore throughout the catalytic process, which shuttles between neutral azo, one-electron reduced azo-anion radical, and two-electron reduced hydrazo forms acting both as electron and hydrogen reservoir, enabling the Zn(II)-catalyst for N-alkylation reaction.

Catalytic and Mechanistic Approach to the Metal-Free N-Alkylation of 2-Aminopyridines with Diketones

N-alkylation of amines is an important catalytic reaction in synthetic chemistry. Herein, we report a simple strategy for the N-alkylation of 2-aminopyridines with 1,2-diketones using BF₃·OEt₂ as a catalyst. The reaction proceeds under aerobic conditions, leading to the formation of a diverse range of substituted secondary amines in good to excellent yields. A close inspection of the mechanistic pathway using various spectroscopic techniques and the computational study revealed that the reaction proceeds through the formation of an iminium-keto intermediate with the liberation of CO₂.

Enantioselective Imine Reduction of Dihydro-β-carbolines by Fe-Thiosquaramide Catalyst

Enantioselective imine reduction of dihydro-β-carbolines (DHBCs) is a reliable and powerful tool to construct bioactive chiral tetrahydro-β-carbolines (THBCs). Here, we report an efficient enantioselective imine reduction employing in situ generated Fe-thiosquaramides (Fe-TSQs) 3a and 3b as asymmetric organometallic catalysts to produce chiral THBCs (2a-h). The catalyst 3a at 15 mol % was found to be suitable for the substrates with alkyl and aryl groups which afford corresponding chiral THBCs with excellent enantioselectivities (up to ee 99%).

Hydrogen Borrowing: towards Aliphatic Tertiary Amines from Lignin Model Compounds Using a Supported Copper Catalyst

Upcoming biorefineries, such as lignin-first provide renewable aromatics containing unique aliphatic alcohols. In this context, a Cu-ZrO₂ catalyzed hydrogen borrowing approach was established to yield tertiary amine from the lignin model monomer 3-(3,4-dimethoxyphenyl)-1-propanol and the actual lignin-derived monomers, (3-(4-hydroxyphenyl)-1-propanol and dihydroconiferyl alcohol), with dimethylamine. Various industrial metal catalysts were evaluated, resulting in nearly quantitative mass balances for most catalysts. Identified intermediates, side and reaction products were placed into a corresponding reaction network, supported by kinetic evolution experiments. Cu-ZrO₂ was selected as most suitable catalyst combining high alcohol conversion with respectable aliphatic tertiary amine selectivity. Low pressure H₂ was key for high catalyst activity and tertiary amine selectivity, mainly by hindering undesired reactant dimethylamine disproportionation and alcohol amidation. Besides dimethylamine model, diverse secondary amine reactants were tested with moderate to high tertiary amine yields. As most active catalytic site, highly dispersed Cu species in strong contact with ZrO₂ is suggested. ToF-SIMS, N₂ O chemisorption, TGA and XPS of spent Cu-ZrO₂ revealed that imperfect amine product desorption and declining surface Cu lowered the catalytic activity upon catalyst reuse, while thermal reduction readily restored the initial activity and selectivity demonstrating catalyst reuse.

Half-Sandwich Iridium Complexes with Hydrazone Ligands: Synthesis and Catalytic Activity in N-Alkylation of Anilines or Nitroarenes with Alcohols via Hydrogen Autotransfer

Here, we synthesize a series of hydrazone-based N,O-chelate half-sandwich iridium complexes through a facile route. All air-stable iridium complexes show high catalytic activity in N-alkylation of a broad scope of aniline derivatives and alcohols with liberating water as the sole byproduct. This reaction provides a smooth route to synthesize diverse monoalkylated amines in good to excellent yields at moderate temperature with a low catalyst loading. Moreover, the challenging N-alkylation process using nitroarene substrates as coupling partners is also carried out in this catalytic system. The mechanistic study shows that the present iridium catalysis process proceeds through a hydrogen borrowing mechanism. All iridium(III) complexes 1-4 are characterized by infrared (IR) spectroscopy, nuclear magnetic resonance (NMR) spectroscopy, and elemental analysis.

Nickel-catalyzed hydrogenative coupling of nitriles and amines for general amine synthesis

Efficient and general methods for the synthesis of amines remain in high demand in the chemical industry. Among the many known processes, catalytic hydrogenation is a cost-effective and industrially proven reaction and currently used to produce a wide array of such compounds. We report a homogeneous nickel catalyst for hydrogenative cross coupling of a range of aromatic, heteroaromatic, and aliphatic nitriles with primary and secondary amines or ammonia. This general hydrogenation protocol is showcased by straightforward and highly selective synthesis of >230 functionalized and structurally diverse amines including pharmaceutically relevant and chiral products, as well as ¹⁵N-isotope labeling applications.

Chemical characteristics and sources of nitrogen-containing organic compounds at a regional site in the North China Plain during the transition period of autumn and winter

Organic nitrogen constitutes a significant fraction of the nitrogen budget in particulate matter (PM). However, the composition and sources of nitrogen-containing organic compounds (NOCs) in PM remain unclear currently in North China Plain (NCP), China. Rare local or regional studies on NOCs were conducted. In this study, ambient fine particles (PM_2.5) were collected in Xianghe, a regional background site in NCP, from 26 October to 26 December 2017. The insights from this study include NOC molecule identification, concentration level, and NOC sources and origins. Specifically, we have identified and quantified >90 NOC species, with urea being the most abundant, accounting for 39.7 ± 4.7% of the total NOC followed by free amino acids (FAAs; 21.9 ± 1.5%), cyclic NOCs (15.3 ± 4.5%), amines (14.8 ± 1.5%), alkyl amides (5.8 ± 0.5%), isocyanates (1.7 ± 0.2%), and nitriles (1.1 ± 0.2%). The time series of FAAs was well correlated (r = 0.51-0.68, p < 0.01) with the organic marker of levoglucosan and was moderately correlated with O_x (r = 0.29-0.41, p < 0.01), suggesting biomass burning and secondary formation were important FAAs sources. We also show that amines can be oxidized and/or reacted by aqueous-phase processing to form secondary aerosols, which are further enhanced by the involvement of iron in the catalytic process. Using the receptor model of positive matrix factorization (PMF), six factors were identified including coal combustion, crustal sources, biomass burning, industry-related sources, traffic emissions, and secondary aerosols. Source apportionment of NOC shows biomass burning was the dominant factor, accounting for 31.8% of the total NOCs. This study provides a unique dataset of NOCs at this regional background site in the NCP, with the insights of NOC chemical composition and sources gained in this study being important for future NOC modeling as well as NOC health effects studies.

Four- and five-coordinate nickel(ii) complexes bearing new diphosphine-phosphonite and triphosphine-phosphite ligands: catalysts for N-alkylation of amines

The reaction of Ph2PCH2OH with PhPCl2 and PCl3 in the presence of Et3N afforded new phosphonite compounds PhP(OCH2PPh2)21 and P(OCH2PPh2)32, respectively. The reaction between 1 and [NiCl2(DME)] in dichloromethane gave the five-coordinate complex [NiCl2(1-κ3 P,P,P)] 3. Conversely, 1 reacts with [NiCl2(DME)] in the presence of NH4PF6 in dichloromethane to yield the four coordinate ionic complex [NiCl(1-κ3 P,P,P)][PF6] 4. The reactions between 1, [NiCl2(DME)] and KPF6 in the presence of RNC (R = Xylyl, t Bu and iPr) in dichloromethane yielded the five coordinate monocationic [NiCl(1-κ3 P,P,P)(RNC)][PF6] (R = Xylyl) and dicationic [Ni(1-κ3 P,P,P)(RNC)2][PF6]2 (R = t Bu and iPr) complexes, respectively. The analogous reaction of 2 with [NiCl2(DME)] in the presence of KPF6 gave complex [NiCl(2-κ4 P,P,P,P)][PF6], 8. The structures of all complexes were determined by single crystal X-ray diffraction studies and supported by spectroscopic methods. To demonstrate their catalytic application, N-alkylation reactions between primary aryl amines, benzyl and 4-methoxy benzyl alcohols were found to proceed smoothly in the presence of 2.5 mol% of complexes bearing ligand 1 and <0.5 mmol of KOBu t in toluene at 140 °C. The C-N coupled products were formed in very good yields. Its substrate scope includes sterically encumbered, heterocyclic amines and aliphatic alcohol.

N-Alkylation of organonitrogen compounds catalyzed by methylene-linked bis-NHC half-sandwich ruthenium complexes

An efficient ruthenium-catalyzed N-alkylation of amines, amides and sulfonamides has been developed employing novel pentamethylcyclopentadienylruthenium(II) complexes bearing the methylene linked bis(NHC) ligand bis(3-methylimidazol-2-ylidene)methane. The acetonitrile complex 2 has proven particularly effective with a broad range of substrates with low catalyst loading (0.1-2.5 mol%) and high functional group tolerance under mild conditions. A total of 52 N-alkylated organonitrogen compounds including biologically relevant scaffolds were synthesized from (hetero)aromatic and aliphatic amines, amides and sulfonamides using alcohols or diols as alkylating agents in up to 99% isolated yield, even on gram-scale reactions. In the case of sulfonamides, it is the first example of N-alkylation employing a transition-metal complex bearing NHC ligands.

The mechanism of direct reductive amination of aldehyde and amine with formic acid catalyzed by boron trifluoride complexes: insights from a DFT study

A volcano diagram of BF3 catalytic species and their activities was proposed for the DRA of aldehyde and amine with formic acid.

Metal-/catalyst-free one-pot three-component thioamination of 1,4-naphthoquinone in a sustainable solvent

A metal-/catalyst-free protocol for the direct thioamination of 1,4-naphthoquinone with thiophenols and amines using air as an oxidant in a green solvent has been developed. This environmentally friendly strategy was...

Disarming the alkoxide trap to access a practical FeCl3 system for borrowing-hydrogen N-alkylation

Disarming the alkoxide trap using an in situ reduction strategy to access a practical FeCl3 and N-heterocyclic carbene system for borrowing-hydrogen N-alkylation.

Electronic Ni–N interaction enhanced reductive amination on an N-doped porous carbon supported Ni catalyst

Reductive amination on Ni/N-doped porous carbon catalyst was enhanced by the formation of Ni–Nx sites and the electronic interaction of N and Ni species, which promoted the reductive amination of CO bonds and reduced the activation energy.

Copper–cobalt coordination polymers and catalytic applications on borrowing hydrogen reactions

A porous copper–cobalt polymer was synthesized and achieved applications for the N-alkylation of sulfonamides with alcohols, and carboxamides with alcohols.

Coordinatively Unsaturated Hf-MOF-808 Prepared via Hydrothermal Synthesis as a Bifunctional Catalyst for the Tandem N-Alkylation of Amines with Benzyl Alcohol

The modulated hydrothermal (MHT) synthesis of an active and selective Hf-MOF-808 material for the N-alkylation reaction of aniline with benzyl alcohol under base-free mild reaction conditions is reported. Through kinetic experiments and isotopically labeled NMR spectroscopy studies, we have demonstrated that the reaction mechanism occurs via borrowing hydrogen (BH) pathway, in which the alcohol dehydrogenation is the limiting step. The high concentration of defective -OH groups generated on the metallic nodes through MHT synthesis enhances the alcohol activation, while the unsaturated Hf⁴⁺, which acts as a Lewis acid site, is able to borrow the hydrogen from the methylene position of benzyl alcohol. This fact makes this material at least 14 times more active for the N-alkylation reaction than the material obtained via solvothermal synthesis. The methodology described in this work could be applied to a wide range of aniline and benzyl alcohol derivates, showing in all cases high selectivity toward the corresponding N-benzylaniline product. Finally, Hf-MOF-808, which acts as a true heterogeneous catalyst, can be reused in at least four consecutive runs without any activity loss.

Metal-Free Aminomethylation of Aromatic Sulfones Promoted by Eosin Y

A metal-free α-aminomethylation of heteroaryls promoted by eosin Y under green light irradiation is reported. A large variety of α-trimethylsilylamines as precursor of α-aminomethyl radical species were engaged to functionalize sulfonyl-heteroaryls following a Homolytic Aromatic Substitution (HAS) pathway. This method has provided a range of α-aminoheteroaryl compounds including a functionalized natural product. The mechanism of this late-stage functionalization of aryls was investigated and suggests the formation of a sulfonyl radical intermediate over a reductive quenching cycle.