N-formylation and N-methylation of amines using metal-free N-heterocyclic carbene catalysts and CO2 as carbon source

Integrated "all-in-one" strategy to construct highly efficient Pd catalyst for CO2 transformation

The synthesis of high-value chemicals featuring C-C and/or C-heteroatom bonds via CO2 is critically important, yet efficiently converting thermodynamically stable and kinetically inert linear CO2 and propargylic amine to the heterocyclic compound 2-oxazolidinone with an integrated catalytic system continues to pose a considerable challenge. Herein, we have designed an "all-in-one" (AIO) palladium (Pd) catalyst (Cat1), distinguished by its co-coordination with acetylglucose (AcGlu) and bis(benzimidazolium) units at the Pd center, which promotes the cyclization of CO2 and propargylic amine achieving a highest turnover frequency (TOF) of up to 3456 h-1. Moreover, Cat1 demonstrates excellent stability across various temperatures, with its catalytic activity remaining unchanged even after 10 cycles. The catalyst Cat1 simultaneously activates propargylic amine and CO2, facilitating the formation of N-heterocyclic carbene (NHC)-CO2 adducts and AcGlu-CO2 philes from CO2 in simulated flue gas, a key factor in reaching unprecedented TOF values. The catalytic mechanism was elucidated through quasi-in-situ NMR and 13C-isotope labeling experiments. Notably, this is the first instance of an AIO Pd catalyst that enables the simultaneous capture, activation, and catalytic conversion of in-situ activated CO2 along with propargylic amine. The design strategy of this AIO catalyst introduces a novel approach to overcoming the challenges in the efficient conversion of inert CO2.

DFT Study of Functional Reduction of CO2 with BH3NMe3: The Real Role of Organic Catalyst TBD

The detailed mechanism of transition metal-free-catalyzed monomethylation of 2-naphthyl acetonitrile (1a) with CO2 in the presence of triazabicyclodecene (TBD) and BH3NMe3 was investigated using density functional theory. The C-methylation process proved to generate formaldehyde followed by the formation of the product via an alcohol rather than a methoxyborane intermediate. During the reaction, CO2 is activated to form the TBD-CO2 adduct and BH3NMe3 is changed into TBD-BH2 (IM2) in the presence of TBD. IM2 plays a real reducing role within the system due to the unique coordination capability of the B atom. In addition to enhancing the nucleophilicity of 1a through deprotonation by tBuOK, our research also indicates that the generated tBuOH not only assists in proton transfer to generate an alcohol intermediate but also promotes the regeneration of TBD.

CO2-Driven N-Formylation/N-Methylation of Amines Using C-Scorpionate Metal Complexes

C-scorpionate metal complexes, specifically, [NiCl2(tpm)]·3H2O, [CoCl2(tpm)]·3H2O and [PdCl2(tpm)] [tpm = hydrotris(1H-pyrazol-1-yl)methane], were effective in the N-formylation and N-methylation of amines using carbon dioxide, as carbon source, in the presence of sodium borohydride. Various parameters were studied, including reaction time, temperature, solvent volume, presence of additives, and catalyst amount. These parameters were found to have a significant impact on the selectivity of the product. [NiCl2(tpm)]·3H2O exhibited good conversion at 80 °C, but its selectivity towards formamide decreased with prolonged reaction time. Increasing the amount of [NiCl2(tpm)]·3H2O, the selectivity changed. [PdCl2(tpm)] showed different selectivity compared to [NiCl2(tpm)]·3H2O, while [CoCl2(tpm)]·3H2O presented poor results. Monitoring the reaction course by 1H NMR revealed the presence of an intermediate species that influenced product formation. These results highlight the versatility and catalytic potential of C-scorpionate metal complexes in the N-formylation/N-methylation of amines in the catalytic system (NaBH4/MeCN/CO2).

A supported pyridylimine-cobalt catalyst for N-formylation of amines using CO2

N-Formylation of amines with CO2 as a cheap and non-toxic C1-feedstock and hydrosilane reducing agent is a practical and environment friendly method to synthesize formamides. This study describes an efficient and chemoselective mono-N-formylation of amines using CO2 and phenylsilane under mild conditions using a porous metal-organic framework (MOF)-supported single-site cobalt catalyst (pyrim-UiO-Co). The pyrim-UiO-Co MOF has a UiO-topology, and its organic linkers bear a pyridylimine ligated Co catalytic moiety. A wide range of aliphatic and aromatic amines are transformed into desired N-formamides in moderate to excellent yields under 1-5 bar CO2. Pyrim-UiO-Co is tolerant to various functional groups and could be recycled and reused at least 10 times. Mechanistic investigation using kinetic, spectroscopic and density functional theory studies suggests that the formylation of benzylamine proceeds sequentially via oxidative addition of PhSiH3 and CO2 insertion, followed by a turn-over limiting reaction with an amine. Our work highlights the importance of MOF-based Earth-abundant metal catalysts for the practical and eco-friendly synthesis of fine chemicals using cheap feedstocks.

Phosphite-imidazole catalyzed N-formylation and N-acylation of amines

A novel and convenient method for the N-formylation reaction of amines with DMF as a formylating agent has been developed, utilizing a catalytic amount of diethyl phosphite/imidazole. Diethyl phosphite, as a nucleophilic catalyst, plays a significant role in this conversion. The presented method has a broad substrate scope, and various N-formyl products were obtained in good to excellent yields. Moreover, by using DMA instead of DMF, the N-acetylation reaction was also successful. The reaction of o-phenylenediamines with DMF afforded the corresponding benzimidazoles. Furthermore, N-sulfonyl amidines were obtained in good to excellent yields by the reaction of sulfonamides with DMF under similar conditions.

Recent Advances On Direct Formylation Reactions

Aldehydes serve as the key functional group in organic synthesis and are valuable intermediates. The various advanced methods of direct formylation reactions have been reviewed in this article. Overcoming the drawbacks of the traditional methods of formylation, newer methods involving homo and heterogenous catalysts, one pot reactions, solvent free techniques are elaborated, which can be performed under mild conditions and using inexpensive resources.

Recreating the natural evolutionary trend in key microdomains provides an effective strategy for engineering of a thermomicrobial N-demethylase

N-demethylases have been reported to remove the methyl groups on primary or secondary amines, which could further affect the properties and functions of biomacromolecules or chemical compounds; however, the substrate scope and the robustness of N-demethylases have not been systematically investigated. Here we report the recreation of natural evolution in key microdomains of the Thermomicrobium roseum sarcosine oxidase (TrSOX), an N-demethylase with marked stability (melting temperature over 100 °C) and enantioselectivity, for enhanced substrate scope and catalytic efficiency on -C-N- bonds. We obtained the structure of TrSOX by crystallization and X-ray diffraction (XRD) for the initial framework. The natural evolution in the nonconserved residues of key microdomains—including the catalytic loop, coenzyme pocket, substrate pocket, and entrance site—was then identified using ancestral sequence reconstruction (ASR), and the substitutions that accrued during natural evolution were recreated by site-directed mutagenesis. The single and double substitution variants catalyzed the N-demethylation of N-methyl-L-amino acids up to 1800- and 6000-fold faster than the wild type, respectively. Additionally, these single substitution variants catalyzed the terminal N-demethylation of non-amino-acid compounds and the oxidation of the main chain -C-N- bond to a -C=N- bond in the nitrogen-containing heterocycle. Notably, these variants retained the enantioselectivity and stability of the initial framework. We conclude that the variants of TrSOX are of great potential use in N-methyl enantiomer resolution, main-chain Schiff base synthesis, and alkaloid modification or degradation.

N-Formylation of amines utilizing CO2 by a heterogeneous metal–organic framework supported single-site cobalt catalyst

Single-site cobalt-hydride supported on oxo-nodes of a porous aluminium metal–organic framework is a chemoselective and reusable catalyst for N-formylation of amines using CO2.

Mesoionic N-heterocyclic olefin catalysed reductive functionalization of CO2 for consecutive N-methylation of amines

A mesoionic N-heterocyclic olefin (mNHO) was introduced as a metal-free catalyst for the reductive functionalization of CO2 leading to consecutive double N-methylation of primary amines in the presence of 9-borabicyclo[3.3.1]nonane (9-BBN). A wide range of secondary amines and primary amines were successfully methylated under mild conditions. The catalyst sustained over six successive cycles of N-methylation of secondary amines without compromising its activity, which encouraged us to check its efficacy towards double N-methylation of primary amines. Moreover, this method was utilized for the synthesis of two commercially available drug molecules. A detailed mechanistic cycle was proposed by performing a series of control reactions along with the successful characterisation of active catalytic intermediates either by single-crystal X-ray study or by NMR spectroscopic studies in association with DFT calculations.

N-Dimethylation and N-Functionalization of Amines Using Ru Nanoparticle Catalysts and Formaldehyde or Functional Aldehydes as the Carbon Source

N-methylated amines are essential bioactive compounds and have been widely used in the fine and bulk chemical industries, as well as in pharmaceuticals, agrochemicals, and dyes. Developing green, efficient, and low-cost catalysts for methylation of amines by using efficient and easily accessible methylating reagents is highly desired yet remains a significant challenge. Herein, we report the selective N-dimethylation of different functional amines with different functional aldehydes under easy-to-handle and industrially applicable conditions using carbon-supported Ru nanoparticles (Ru/C) as a heterogeneous catalyst. A broad spectrum of amines could be efficiently converted to their corresponding N,N-dimethyl amines with good compatibility of various functional groups. This method is widely applicable to N-dimethylation of primary amines including aromatic, aliphatic amines with formaldehyde, and synthesis of tertiary amines from primary, secondary amines with different functional aldehydes. The advantage of this newly described method includes operational simplicity, high turnover number, the ready availability of the catalyst, and good functional group compatibility. This Ru/C catalyzed N-dimethylation reaction possibly proceeds through a two-step N-methylation reaction process.

Ionic-Liquid-Catalyzed Approaches under Metal-Free Conditions

ConspectusMetal-free catalysis is a promising protocol to access chemicals without metal contamination. Ionic liquids (ILs) that are entirely composed of organic cations and inorganic/organic anions have emerged as promising alternatives to molecular solvents and metal catalysts due to their unique properties such as structural tunability, the coexistence of multiple interactions among ions (e.g., electrostatic interaction, hydrogen bonding, van de Waals forces, acid/base interactions, hydrophilic/hydrophobic interactions, etc.), unique affinity for a wide range of chemicals, good chemical and thermal stability, and quite low volatility. ILs have shown potential applications in various chemical processes.In this Account, we systematically described our most recent work on IL-catalyzed approaches under metal-free conditions. The first section presents the IL-catalyzed strategies toward the transformation of CO2 to value-added chemicals, focusing on the CO2-reactive IL-catalyzed CO2 transformation to various heterocycles and the IL-catalyzed reductive transformation of CO2 to chemicals. In these approaches, we designed task-specific ILs that are able to chemically capture and activate CO2 via forming anion-based carbonate/carbamate or cation-based carboxylate/carbamate intermediates, thus further accomplishing its transformation to a series of heterocycles including quinazoline-2,4(1H,3H)-diones, cyclic carbonates, 2-oxazolidinones, oxazolones, and benzimidazolones under metal-free conditions. For the IL-catalyzed approaches to reducing CO2 with hydrosilanes to chemicals, we employed ILs capable of activating the Si-H bond in hydrosilanes and the N-H bond in amine substrates via H-bonding, thus achieving the reductive transformation of CO2 to formamides, benzimidazoles, and benzothiazoles via cooperative catalysis. The second section describes our finding on the IL-catalyzed hydration of the C≡C bond in propargylic alcohols. Azolate anion-based ILs that can chemically capture CO2 via the formation of carbamates could serve as robust nucleophiles to attack the C≡C bond in propargylic alcohols and then efficiently catalyze the hydration of propargylic alcohols to produce α-hydroxy ketones with the assistance of atmospheric CO2 gas under metal-free conditions. The third section unveils the cooperative catalysis strategy of hydrogen bond donors and acceptors of ILs for chemical reactions. In the hydrogen-bonding catalysis protocols, cations of the ILs act as H-bond donors and anions, as acceptors, forming H-bonds with the reactant molecules, respectively, in opposite ways, which can cooperatively catalyze the ring-closing C-O/C-O bond metathesis reactions of aliphatic diethers to O-heterocycles, the dehydrative etherification of alcohols to ethers, and direct oxidative esterification of alcohols to esters. We believe that these IL-catalyzed metal-free processes and strategies display promising practical applications, and their commercialization would bring great benefits to the production of the as-afforded value-added chemicals.

Catalyst free N-formylation of aromatic and aliphatic amines exploiting reductive formylation of CO2 using NaBH4

Herein, we report a sustainable approach for N-formylation of aromatic as well as aliphatic amines using sodium borohydride and carbon dioxide gas. The developed approach is catalyst free, and does not need pressure or a specialized reaction assembly. The reductive formylation of CO2 with sodium borohydride generates formoxy borohydride species in situ, as confirmed by 1H and 11B NMR spectroscopy. The in situ formation of formoxy borohydride species is prominent in formamide based solvents and is critical for the success of the N-formylation reactions. The formoxy borohydride is also found to promote transamidation reactions as a competitive pathway along with reductive functionalization of CO2 with amine leading to N-formylation of amines.

Heterogeneous Organocatalysts for the Reduction of Carbon Dioxide with Silanes

The utilization of carbon dioxide (CO₂ ) as feedstock for chemical industries is gaining interest as a sustainable alternative to nonrenewable fossil resources. However, CO₂ reduction is necessary to increase its energy content. Hydrosilane is a potential reducing agent that exhibits excellent reactivity under ambient conditions. CO₂ hydrosilylation yields versatile products such as silylformate and methoxysilane, whereas formamides and N-methylated products are obtained in the presence of amines. In these transformations, organocatalysts are considered as the more sustainable choice of catalyst. In particular, heterogeneous organocatalysts featuring precisely designed active sites offer higher efficiency due to their recyclability. Herein, an overview is presented of the current development of basic organocatalysts immobilized on various supports for application in the chemical reduction of CO₂ with hydrosilanes, and the potential active species parameters that might affect the catalytic activity are identified.

Fabrication and characterization of the ternary composite catalyst system of ZnGA/RET/DMC for the terpolymerization of CO2, propylene oxide and trimellitic anhydride

To achieve the poly(propylene carbonate trimellitic anhydride) (PPCTMA) with excellent performance, high molecular weight, enhanced yield and good thermal stability, the ternary composite catalyst system of zinc glutarate/rare earth ternary complex/double metal cyanide (ZnGA/RET/DMC) was proposed to perform the terpolymerization of CO₂, propylene oxide and trimellitic anhydride. Since the crystallinity and surface activity point of Zn–Co DMC could significantly influence the catalytic ability, mechanical ball milling was applied to increase the surface area of the Zn–Co DMC catalyst with better surface activity point. Moreover, the ZnGA/RET/DMC composite catalytic system and polycarbonate products were comparatively evaluated by XRD, SEM, FT-IR, TGA, NMR, XPS and TEM. Experimental results showed that the ZnGA/RET/DMC composite catalyst system displayed outstanding synergistic effect on the terpolymerization of CO₂, PO and TMA with better selectivity, activity, and higher molecular weight (M_w) tercopolymer than those of the individual catalyst. According to optimum reaction conditions, the M_w of PPCTMA could be up to 8.29 × 10⁴ g mol⁻¹, and the yield could be up to 66 g_polym/g_cat. The alternating tercopolymer, PPCTMA, showed wonderful thermal stability and high decomposition temperature (TGA_10% = 313 °C). A possible synergistic catalytic mechanism of the ZnGA/RET/DMC ternary composite catalyst system was also conjectured.

For poly(propylene carbonate trimellitic anhydride) with good yield, thermal stability and high molecular weight, a catalyst of zinc glutarate/rare earth ternary complex/double metal cyanide was used for terpolymerization of CO₂, propylene oxide and trimellitic anhydride.

Mechanistic insights into carbamate formation from CO2 and amines: the role of guanidine–CO2 adducts

This work explores the reactivity of a reversible superbase–CO2 zwitterion, which can be used as a stoichiometric source of CO2.

Transition Metal-Free Synthesis of Carbamates Using CO2 as the Carbon Source

Utilization of carbon dioxide as a C1 synthon is highly attractive for the synthesis of valuable chemicals. However, activation of CO₂ is highly challenging, owing to its thermodynamic stability and kinetic inertness. With this in mind, several strategies have been developed for the generation of carbon-heteroatom bonds. Among these, formation of C-N bonds is highly attractive, especially, when carbamates can be synthesized directly from CO₂ . This Minireview focuses on transition metal-free approaches for the fixation of CO₂ to generate carbamates for the production of fine chemicals and pharmaceuticals. Within the past decade, transition metal-free approaches have gained increasing attention, but traditional reviews have rarely focused on these approaches. Direct comparisons between such methods have been even more scarce. This Minireview seeks to address this discrepancy.

Recent advances in liquid hydrosilane-mediated catalytic N-formylation of amines with CO2

Carbon dioxide is an ideal raw material for the synthesis of complex organic compounds because of its rich, non-toxic, and good physical properties. It is of great significance to transform CO₂ into valuable fine chemicals and develop a green sustainable cycle of carbon surplus. Based on hydrosilane as a reducing agent, this work summarizes the recent applications of reductive amidation of CO₂ using different catalysts such as organocatalysts, ionic liquids (ILs), salts, transition metal complexes, and solvents. The main factors affecting the reductive amidation of CO₂ and the possible reaction mechanism are discussed. Moreover, the future orientation and catalytic systems of the formylation of amines with CO₂ and hydrosilane are prospected.

This review depicts different types of catalyst systems developed for upgrading of amines and carbon dioxide into N-formylated products in the presence of hydrosilane, with attention on reaction mechanism and process optimization.

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.

Metal-Free N-Formylation of Amines with CO2 and Hydrosilane by Nitrogen-Doped Graphene Nanosheets

N-Formylation of amines with carbon dioxide (CO₂) as a carbonyl source is emerging as an important way for CO₂ transformation into high-value-added chemicals; however, the developed catalytic systems mainly focused on transition-metal-based homogeneous catalysts. Herein, we reported rationally designed nitrogen-doped graphene nanosheets (NG) as metal-free catalysts for N-formylation of various amines with CO₂ and hydrosilane to formamide under mild conditions. The NG catalyst displayed a wide amine scope with the desired formamide yields up to >99%, demonstrating its comparable catalytic performance to the reported transition-metal-based catalysts. Our experimental research reveals that the N-formylation of aniline involves an initial NG-promoted CO₂ hydrosilylation with PhSiH₃ to silyl formate and a subsequent nucleophilic attack of the aniline to give N-formanilide. Moreover, the key step of CO₂ hydrosilylation can be simplified to a pseudo-first-order reaction under a high CO₂ concentration with an observed reaction rate constant (k_obs) of 226 h^-1 at 40 °C and an apparent activation energy (E_a) of 34 kJ mol^-1. In sharp contrast, a k_obs of 23 h^-1 and E_a of 47 kJ mol^-1 were observed under catalyst-free conditions. Our theoretical investigation indicates that NG-promoted CO₂ hydrosilylation corresponds to an exergonic reaction (ΔG = -0.53 eV), which is much lower in energy state than that of catalyst-free conditions (ΔG = -0.44 eV). Finally, the NG showed outstanding recyclability in the N-formylation reaction with almost unchanged catalytic performance during twelve-time recycling. This research thus represented a breakthrough in metal-free transformation of CO₂ into fine chemicals with low-cost, environment-friendly, and carbon-based catalysts to replace the scarce and expensive transition-metal-based catalysts.

Tailored Cobalt-Catalysts for Reductive Alkylation of Anilines with Carboxylic Acids under Mild Conditions

The first cobalt-catalyzed hydrogenative N-methylation and alkylation of amines with readily available carboxylic acid feedstocks as alkylating agents and H₂ as ideal reductant is described. Combination of tailor-made triphos ligands with cobalt(II) tetrafluoroborate significantly improved the efficiency, thus promoting the reaction under milder conditions. This novel protocol allows for a broad substrate scope with good functional group tolerance, even in the presence of reducible alkenes, esters, and amides.

B(C6F5)3: a robust catalyst for the activation of CO2 and dimethylamine borane for the N-formylation reactions

In this work, B(C₆F₅)₃ is utilized as an organocatalyst for the transition-metal-free N-formylation of amines using carbon dioxide (CO₂) as a C1 source and dimethylamine borane (Me₂NH·BH₃) as a green hydrogen transfer source at 80 °C.

Reductive Coupling of CO2 , Primary Amine, and Aldehyde at Room Temperature: A Versatile Approach to Unsymmetrically N,N-Disubstituted Formamides

We present a simple, metal-free, and versatile route to synthesize unsymmetrically N,N-disubstituted formamides (NNFAs) from CO₂ , primary amine, and aldehyde promoted by an ionic liquid (1-butyl-3-methylimidazolium chloride) at room temperature. This approach features wide scopes of amines and aldehydes, and various unsymmetrical NNFAs could be obtained in good to excellent yields. The ionic liquid can be reused for at least five runs without obvious activity loss.

Chemical reduction of CO2facilitated by C-nucleophiles

This feature article describes recent advances in chemical reduction of CO₂facilitated by carbon-based molecular nucleophiles.