Ligand-free Iron(II)-Catalyzed N-Alkylation of Hindered Secondary Arylamines with Non-activated Secondary and Primary Alcohols via a Carbocationic Pathway

N-H Insertion of Anilines on N-Tosylhydrazones Induced by Visible Light Irradiation

Diazo compounds and their precursors represent an interesting chemical category for organic synthesis. Particularly, N-tosylhydrazones have attracted attention for their easy accessibility and diverse reactivity, including carbene transfer reactions. We described a visible light-induced N-H insertion reaction of anilines on in situ-generated diazo compounds. Optimal conditions using DBU in toluene efficiently yielded the desired products. Mechanistic studies enabled us to trap a carbene intermediate that plays a key role in the transformation.

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.

Preparation of a novel cadmium-containing coordination polymer and catalytic application in the synthesis of N-alkylated aminoquinoline derivatives via the borrowing hydrogen approach

Herein, we report an efficient and straightforward approach for the synthesis of N-alkylated aminoquinoline derivatives by recyclable Cd-containing coordination polymer-catalyzed reactions of aminoquinolines with primary alcohols via the borrowing hydrogen strategy. In this work, a new type of coordination polymer [Cd(CIA)(phen)2(H2O)]n was successfully designed and fabricated. The molecular structure was corroborated by single-crystal X-ray diffraction and fully characterized by PXRD, FT-IR, TGA, and XPS. Importantly, this polymer revealed high catalytic activity for the N-alkylation reaction of 2-aminoquinoline and 8-aminoquinoline with inexpensive and low-toxicity alcohols as alkylating agents in excellent yields up to 95%. Interestingly, the present synthetic protocol was successfully applied for the gram-level synthesis of several biologically active compounds. In addition, several control reactions were carried out to investigate the possible mechanisms of this transformation. Finally, recycling experiments indicated that the cadmium coordination polymer showed good recovery performance for borrowing hydrogen reactions.

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.

Heterogeneous V2O5/TiO2-Mediated Photocatalytic Reduction of Nitro Compounds to the Corresponding Amines under Visible Light

The hydrogenation of nitro compounds to their corresponding amines is developed using a heterogeneous and recyclable catalyst (V₂O₅/TiO₂) under irradiation of blue LED (9 W) at ambient temperature. Hydrazine hydrate is used as a reductant and ethanol is used as a solvent, facilitating green, sustainable, low-cost production. The synthesis of 32 (hetero)arylamines and their pharmaceutically relevant molecules (five) are described. Significant features of the protocol include catalyst recyclability, green solvent, ambient temperature, and gram-scale reactions. Among the other aspects studied are ¹H-NMR-assisted reaction progress monitoring, control experiments for mechanistic studies, protocol applications, and recyclability studies. Furthermore, the developed protocol enabled wide functional group tolerance, chemo-selectivity, high yield, and low-cost, sustainable, and environmentally benign synthesis.

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.

A Theoretical Study on the Borane-Catalyzed Reductive Amination of Aniline and Benzaldehyde with Dihydrogen: The Origins of Chemoselectivity

Density functional theory calculations are used in this study to investigate the product selectivity and mechanism of borane-catalyzed reductive aldehyde amination by a H₂ reducing agent. Knowing that different boranes yield different products, two typical boranes, (B(2,6-Cl₂C₆H₃)(p-HC₆F₄)₂ and B(C₆F₅)₃), are studied. Of the seven possible pathways of B(2,6-Cl₂C₆H₃)(p-HC₆F₄)₂-catalyzed aldehyde amination analyzed herein, four are favorable. Three of the four favorable pathways involve imine intermediates, and the fourth is a Lewis acid-base synergistic pathway that involves amine-alcohol condensation. As for the B(C₆F₅)₃ catalyst, it forms a highly stable Lewis adduct with aniline, which impedes the hydrogenation of imine. Therefore, the product of B(C₆F₅)₃-catalyzed reductive amination of benzaldehyde and aniline is an imine. The linear relationship between the charge on the boron atom in the Lewis acid and the relative energies of the Lewis adduct and H₂ splitting transition state indicates that this parameter determines product selectivity. Indeed, when the natural charge on boron is larger than 1, an amine is produced, whereas when the charge is less than 1, an imine is produced. Hence, the selectivity of products can be controlled by adjusting the natural charge of the boron atom in the Lewis acid catalyst.

Hydrosilane-Mediated Electrochemical Reduction of Amides

Electrochemical reduction of amides was achieved by using a hydrosilane without any toxic or expensive metals. The key reactive ketyl radical intermediate was generated by cathodic reduction. Continuous reaction with anodically generated silyl radicals or zinc bromide resulted in chemoselective deoxygenation to give the corresponding amines.

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.

Organic synthesis with the most abundant transition metal–iron: from rust to multitasking catalysts

The promising aspects of iron in synthetic chemistry are being explored for three-four decades as a green and eco-friendly alternative to late transition metals. This present review unveils these rich iron-chemistry towards different transformations.

Ligand-Free Ru-Catalyzed Direct sp3 C-H Alkylation of Fluorene Using Alcohols

The sp³ C-H alkylation of 9H-fluorene using alcohol and a Ru catalyst via the borrowing hydrogen concept has been described. This reaction was catalyzed by the [Ru(p-cymene)Cl₂]₂ complex (3 mol %) and exhibited a broad reaction scope with different alcohols, allowing primary and secondary alcohols to be employed as nonhazardous and greener alkylating agents with the formation of environmentally benign water as a byproduct. A variety of 9H-fluorene underwent selective and exclusive mono-C9-alkylation with primary alcohols in good to excellent isolated yield (26 examples, 50-92% yield), whereas this reaction with secondary alcohols in the absence of any external oxidants furnished the tetrasubstituted alkene as the major product. Furthermore, a base-mediated C-H hydroxylation of the synthesized 9H-fluorene derivatives afforded 9H-hydroxy-functionalized quaternary fluorene derivatives in excellent yield.

Boron-Catalyzed N-Alkylation of Arylamines and Arylamides with Benzylic Alcohols

A sustainable boron-based catalytic approach for chemoselective N-alkylation of primary and secondary aromatic amines and amides with primary, secondary, and tertiary benzylic alcohols has been presented. The metal-free protocol operates at low catalyst loading, tolerates several functional groups, and generates H₂O as the sole byproduct. Preliminary mechanistic studies were performed to demonstrate the crucial role of boron catalyst for the activation of the intermediate dibenzyl ether and to identify the rate-determining step.

The N-alkylation of sulfonamides with alcohols in water catalyzed by a water-soluble metal–ligand bifunctional iridium complex [Cp*Ir(biimH2)(H2O)][OTf]2

A water-soluble dinuclear Cp*Ir complex bearing 4,4′,6,6′-tetrahydroxy-2,2′-bipyrimidine as a bridging ligand was found to be a highly effective catalyst for the N-alkylation of ketones with alcohols in water.

Cp*Co(iii)-catalyzed N-alkylation of amines with secondary alcohols

We report here the first high-valent cobalt(iii)-catalyzed direct alkylation of aromatic amines with secondary alcohols.