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.

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.

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.

Recent advances in C/N-alkylation with alcohols through hydride transfer strategies

This review highlights the most recent reports in three powerful and ever-growing fields of borrowing hydrogen, acceptorless dehydrogenative coupling, and base-mediated hydride transfer strategies; which pave the way for generating reactive intermediates via shuttling hydrogen (or hydride) between starting materials without any need for an external hydrogen source to easily construct more complex structures. There is a thorough focus on diversifying the utility of alcohols for C/N-alkylation leading to the synthesis of branched ketones, alcohols, amines, indols, and 6-membered nitrogen-containing heterocycles such as pyridines and pyrimidines, various transformations with the focus on C-C and C-N bond-forming reactions via metal-based catalysis or metal-free approaches in this context to give a global overview in this area.

Methanol as a hydrogen source: room-temperature highly-selective transfer hydrogenation of α,β-unsaturated ketones

The described system offers an ideal, user-friendly protocol for the chemoselective homogeneous hydrogenation of α,β-unsaturated ketones at room temperature using methanol as a liquid organic hydrogen carrier. Excellent yields were achieved with an in situ-prepared phosphine-free Cp*Ir(III)/bipyridonate complex. Chemoselective reduction with other reducible functionalities and late-stage functionalization were also explored.

Iron-Catalyzed α-Methylation of Ketones Using Methanol as the C1 Source under Photoirradiation

A mild, environmentally benign approach for α-methylation of ketones utilizing methanol as the C1 source under visible light has been developed. The reaction conditions were favorable for a wide range of ketones with both aromatic and aliphatic backbones, allowing for good-to-excellent yields of the respective products. The tentative mechanism is postulated after preliminary mechanistic and kinetic experiments.

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.

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.

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

A series of nickel(ii) complexes supported by the new tridentate P3 and tetradentate P4 ligands act efficiently as catalysts for the N-alkylation of primary amines with alcohols.

Nitrile hydrogenation to secondary amines under ambient conditions over palladium–platinum random alloy nanoparticles

Nitrile hydrogenation over PdPt random alloy nanoparticles.

Solvent/metal-free benzimidazolium-based carboxyl-functionalized porphyrin photocatalysts for the room-temperature alkylation of amines under the irradiation of visible light

The improvement of novel sustainable catalytic methods for green chemical production is an emergent area in chemical science.

Reusable Co-nanoparticles for general and selective N-alkylation of amines and ammonia with alcohols

A general cobalt-catalyzed N-alkylation of amines with alcohols by borrowing hydrogen methodology to prepare different kinds of amines is reported. The optimal catalyst for this transformation is prepared by pyrolysis of a specific templated material, which is generated in situ by mixing cobalt salts, nitrogen ligands and colloidal silica, and subsequent removal of silica. Applying this novel Co-nanoparticle-based material, >100 primary, secondary, and tertiary amines including N-methylamines and selected drug molecules were conveniently prepared starting from inexpensive and easily accessible alcohols and amines or ammonia.

First-Row Transition-Metal Catalyzed Acceptorless Dehydrogenation and Related Reactions: A Personal Account

The development of sustainable catalytic protocols that circumvent the use of expensive and precious metal catalysts and avoid toxic reagents plays a crucial role in organic synthesis. Indeed, the direct employment of simple and abundantly available feedstock chemicals as the starting materials broadens their synthetic application in contemporary research. In particular, the transition metal-catalyzed diversification of alcohols with various nucleophilic partners to construct a wide range of building blocks is a powerful and highly desirable methodology. Moreover, the replacement of precious metal catalysts by non-precious and less toxic metals for selective transformations is one of the main goals and has been paid significant attention to in modern chemistry. In view of this, the first-row transition metal catalysts find extensive applications in various synthetic transformations such as catalytic hydrogenation, dehydrogenation, and related reactions. Herein, we have disclosed our recent developments on the base-metal catalysis such as Mn, Fe, Co, and Ni for the acceptorless dehydrogenation reactions and its application in the C-C and C-N bond formation via hydrogen auto-transfer (HA) and acceptorless dehydrogenation coupling (ADC) reactions. These HA/ADC protocols employ alcohol as alkylating agents and eliminate water and/or hydrogen gas as by-products, representing highly atom-efficient and environmentally benign reactions. Furthermore, diverse simple to complex organic molecules synthesis by C-C and C-N bond formation using feedstock alcohols are also overviewed. Overall, this account deals with the contribution and development of efficient and novel homogeneous as well as heterogeneous base-metal catalysts for sustainable chemical synthesis.

Chan-Lam Amination of Secondary and Tertiary Benzylic Boronic Esters

We report a Chan-Lam coupling reaction of benzylic and allylic boronic esters with primary and secondary anilines to form valuable alkyl amine products. Both secondary and tertiary boronic esters can be used as coupling partners, with mono-alkylation of the aniline occurring selectively. This is a rare example of a transition-metal-mediated transformation of a tertiary alkylboron reagent. Initial investigation into the reaction mechanism suggests that transmetalation from B to Cu occurs through a single-electron, rather than a two-electron process.

Iron-catalyzed one-pot synthesis of quinoxalines: transfer hydrogenative condensation of 2-nitroanilines with vicinal diols

Here, we report iron-catalyzed one-pot synthesis of quinoxalines via transfer hydrogenative condensation of 2-nitroanilines with vicinal diols. The tricarbonyl (η4-cyclopentadienone) iron complex, which is well known as the Knölker complex, catalyzed the oxidation of alcohols and the reduction of nitroarenes, and the corresponding carbonyl and 1,2-diaminobenzene intermediates were generated in situ. Trimethylamine N-oxide was used to activate the iron complex. Various unsymmetrical and symmetrical vicinal diols were applied for transfer hydrogenation, resulting in quinoxaline derivatives in 49–98% yields. A plausible mechanism was proposed based on a series of control experiments. The major advantages of this protocol are that no external redox reagents or additional base is needed and that water is liberated as the sole byproduct.

A transfer hydrogenative condensation between 2-nitroanilines and vicinal diols for the synthesis of quinoxalines, with no additional oxidant, reductant and base.

Phosphine-Free Ruthenium Complex-Catalyzed Synthesis of Mono- or Dialkylated Acyl Hydrazides via the Borrowing Hydrogen Strategy

Herein, we report a diaminocyclopentadienone ruthenium tricarbonyl complex-catalyzed synthesis of mono- or dialkylated acyl hydrazide compounds using the borrowing hydrogen strategy in the presence of various substituted primary and secondary alcohols as alkylating reagents. Deuterium labeling experiments confirm that the alcohols were the hydride source in this cascade process. Density functional theory (DFT) calculations unveil the origin and the threshold between the mono- and dialkylation.

Chromium-Catalyzed Alkylation of Amines by Alcohols

The alkylation of amines by alcohols is a broadly applicable, sustainable, and selective method for the synthesis of alkyl amines, which are important bulk and fine chemicals, pharmaceuticals, and agrochemicals. We show that Cr complexes can catalyze this C-N bond formation reaction. We synthesized and isolated 35 examples of alkylated amines, including 13 previously undisclosed products, and the use of amino alcohols as alkylating agents was demonstrated. The catalyst tolerates numerous functional groups, including hydrogenation-sensitive examples. Compared to many other alcohol-based amine alkylation methods, where a stoichiometric amount of base is required, our Cr-based catalyst system gives yields higher than 90 % for various alkyl amines with a catalytic amount of base. Our study indicates that Cr complexes can catalyze borrowing hydrogen or hydrogen autotransfer reactions and could thus be an alternative to Fe, Co, and Mn, or noble metals in (de)hydrogenation catalysis.

NaCl as Catalyst and Water as Solvent: Highly E-Selective Olefination of Methyl Substituted N-Heteroarenes with Benzyl Amines and Alcohols

Oxidative coupling of benzylamines and alcohols with methyl substituted N-heteroarenes such as quinolines and quinoxalines has been achieved using chloride, a sea abundant anion as the catalyst for practical synthesis of a wide range of E-disubstituted olefins in aqueous medium. Detailed mechanistic studies and control experiments were carried out to deduce the reaction mechanism which indicated that in situ formed ClO₂^- is the active form of the catalyst. We have successfully carried out a 1 g scale reaction using this methodology, and five pharmaceutically relevant conjugated olefins were also synthesized by this method in moderate to good yields.

Brønsted acid/visible-light-promoted Markovnikov hydroamination of vinylarenes with arylamines

A Brønsted acid/visible-light-promoted Markovnikov hydroamination of vinylarenes with arylamines in the presence of TPT and CF₃CO₂H has been developed. This transformation provides a green approach to alpha-amino-substituted arylalkanes under metal-free conditions.

One-pot, chemoselective synthesis of secondary amines from aryl nitriles using a PdPt–Fe3O4 nanoparticle catalyst

We have developed a new catalytic method for the one-pot, cascade synthesis of unsymmetrical secondary amines via the reductive amination of aryl nitriles with nitroalkanes using a PdPt–Fe₃O₄ nanoparticle (NP) catalyst.

α-Alkylation of Ketones with Secondary Alcohols Catalyzed by Well-Defined Cp*CoIII -Complexes

Although α-alkylation of ketones with primary alcohols by transition-metal catalysis is well-known, the same process with secondary alcohols is arduous and complicated by self-condensation. Herein a well-defined, high-valence cobalt(III)-catalyst was applied for successful α-alkylation of ketones with secondary alcohols. A wide-variety of secondary alcohols, which include cyclic, acyclic, symmetrical, and unsymmetrical compounds, was employed as alkylating agents to produce β-alkyl aryl ketones.

C–H and N–H bond annulation of aryl amides with unactivated olefins by merging cobalt(iii) and photoredox catalysis

A mild, environment-friendly protocol has been developed to carry out the [4+2] annulation of aryl amides with unactivated olefins.