Reversible and Irreversible [2+2] Cycloaddition Reactions of Heteroallenes to a Gallaphosphene

[2+2] Cycloaddition reactions of gallaphosphene L(Cl)GaPGaL 1 (L=HC[C(Me)N(2,6-i-Pr2 C6 H3 )]2 ) with carbodiimides [C(NR)2 ; R=i-Pr, Cy] and isocyanates [RNCO; R=Et, i-Pr, Cy] yielded four-membered metallaheterocycles LGa(Cl)P[μ-C(X)NR]GaL (X=NR, R=i-Pr 2, Cy 3; X=O, R=Et 4, i-Pr 5, Cy 6). Compounds 4-6 reversibly react with CO2 via [2+2] cycloaddition at ambient temperature to the six-membered metallaheterocycles LGa(Cl)P[μ-C(O)O]-μ-C(O)N(R)GaL (R=Et 7, i-Pr 8, Cy 9). Compounds 2-9 were characterized by IR and heteronuclear (1 H, 13 C{1 H}, 31 P{1 H}) NMR spectroscopy and elemental analysis, while quantum chemical calculations provided a deeper understanding on the energetics of the reactions.

Multi-Talented Gallaphosphene for Ga-P-Ga Heteroallyl Cation Generation, CO2 Storage, and C(sp3 )-H Bond Activation

Gallaphosphene L(Cl)GaPGaL (2; L=HC[C(Me)N(2,6-i-Pr2 C6 H3 )]2 ), which is synthesized by reaction of LGa(Cl)PCO (1) with LGa, reacts with [Na(OCP)(dioxane)2.5 ] to LGa(OCP)PGaL (3), whereas chloride abstraction with LiBArF 4 yields [LGaPGaL][BArF 4 ] (4; BArF 4 =B(C6 F5 )4 ). 4 represents a heteronuclear analog of the allyl cation according to quantum chemical calculations. Remarkably, 2 reversibly reacts with CO2 to yield L(Cl)Ga-P[μ-C(O)O]2 GaL (5), while reactions with acetophenone and acetone selectively give compounds 6 and 7 by C(sp3 )-H bond activation.

More Than Just a Reagent: The Rise of Renewable Organohydrides for Catalytic Reduction of Carbon Dioxide

Stoichiometric carbon dioxide reduction to highly reduced C1 molecules, such as formic acid (2e^- ), formaldehyde (4e^- ), methanol (6e^- ) or even most-reduced methane (8e^- ), has been successfully achieved by using organosilanes, organoboranes, and frustrated Lewis Pairs (FLPs) in the presence of suitable catalyst. The development of renewable organohydride compounds could be the best alternative in this regard as they have shown promise for the transfer of hydride directly to CO₂ . Reduction of CO₂ by two electrons and two protons to afford formic acid by using renewable organohydride molecules has recently been investigated by various groups. However, catalytic CO₂ reduction to ≥2e^- -reduced products by using renewable organohydride-based molecules has rarely been explored. This Minireview summarizes important findings in this regard, encompassing both stoichiometric and catalytic CO₂ reduction.

Pyridinylidenaminophosphines: Facile Access to Highly Electron-Rich Phosphines

Electron-rich tertiary phosphines are valuable species in chemical synthesis. However, their broad application as ligands in catalysis and reagents in stoichiometric reactions is often limited by their costly synthesis. Herein, we report the synthesis and properties of a series of phosphines with 1-alkylpyridin-4-ylidenamino and 1-alkylpyridin-2-ylidenamino substituents that are accessible in a very short and scalable route starting from commercially available aminopyridines and chlorophosphines. The determination of the Tolman electronic parameter (TEP) value reveals that the electron donor ability can be tuned by the substituent pattern at the aminopyridine backbone and it can exceed that of common alkylphosphines and N-heterocyclic carbenes. The potential of the new phosphines as strong nucleophiles in phosphine-mediated transformations is demonstrated by the formation of Lewis base adducts with CO2 and CS2 . In addition, the coordination chemistry of the new phosphines towards CuI , AuI , and PdII metal centers has been explored, and a convenient procedure to introduce the most basic phosphine into metal complexes starting from air-stable phosphonium salt is described.

Synthesis of benzimidazolones via CO2 fixation and N-phenyl formamides using formic acid in presence of zinc embedded polymer complex

A merrifield supported heterogeneous zinc catalyst [Zn(Meri-Ald-Py)] was synthesized and applied for benzimidazolone synthesis through the fixation of carbon dioxide (CO₂) and for different N-formylated products synthesis under mild reaction conditions.

Pivotal Role of the Basic Character of Organic and Salt Catalysts in C-N Bond Forming Reactions of Amines with CO2

Organocatalysts promote a range of C-N bond forming reactions of amines with CO₂ . Herein, we review these reactions and attempt to identify the unifying features of the catalysts that allows them to promote a multitude of seemingly unrelated reactions. Analysis of the literature shows that these reactions predominantly proceed by carbamate salt formation in the form [BaseH][RR'NCOO]. The anion of the carbamate salt acts as a nucleophile in hydrosilane reductions of CO₂ , internal cyclization reactions or after dehydration as an electrophile in the synthesis of urea derivatives. The reactions are enhanced by polar aprotic solvents and can be either promoted or hindered by H-bonding interactions. The predominant role of all types of organic and salt catalysts (including ionic liquids, ILs) is the stabilization of the carbamate salt, mostly by acting as a base. Catalytic enhancement depends on the combination of the amine, the base strength, the solvent, steric factors, ion pairing and H-bonding. A linear relationship between the base strength and the reaction yield has been demonstrated with IL catalysts in the synthesis of formamides and quinazoline-2,4-diones. The role of organocatalysts in the reactions indicates that all bases of sufficient strength should be able to catalyze the reactions. However, a physical limit to the extent of a purely base catalyzed reaction mechanism should exist, which needs to be identified, understood and overcome by synergistic or alternative methods.

N, N', N″-trisubstituted guanidines: Synthesis, characterization and evaluation of their leishmanicidal activity

Leishmaniasis is a group of diseases caused by protozoan parasites from the genus Leishmania. There are estimated 1.3 million new cases annually with a mortality of 20,000-30,000 per year, when patients are left untreated. Current chemotherapeutic drugs available present high toxicity and low efficacy, the latter mainly due to the emergence of drug-resistant parasites, which makes discovery of novel, safe, and efficacious antileishmanial drugs mandatory. The present work reports the synthesis, characterization by ESI-MS, ¹H and ¹³C NMR, and FTIR techniques as well as in vitro and in vivo evaluation of leishmanicidal activity of guanidines derivatives presenting lower toxicity. Among ten investigated compounds, all being guanidines containing a benzoyl, a benzyl, and a substituted phenyl moiety, LQOF-G2 (IC_50-ama 5.6 μM; SI = 131.8) and LQOF-G7 (IC_50-ama 7.1 μM; SI = 87.1) were the most active against L. amazonensis intracellular amastigote, showing low cytotoxicity to the host cells according to their selectivity index. The most promising compound, LQOF-G2, was further evaluated in an in vivo model and was able to decrease 60% of the parasite load in foot lesions at a dose of 0.25 mg/kg/day. Moreover, this guanidine derivative demonstrated reduced hepatotoxicity compared to other leishmanicidal compounds and did not show nephrotoxicity, as determined by the analyses of biomarkers of hepatic damage and renal function, which make this compound a potential new hit for therapy against leishmaniasis.

Transition Metal-Free CO2 Fixation into New Carbon-Carbon Bonds

CO₂ is the ultimate renewable carbon source on Earth and the essential C₁ building block for carbohydrate biosynthesis in photosynthetic organisms. Modern synthetic chemistry is facing the key challenge of developing fundamental transformations, such as C-C bond formation, in a sustainable and efficient manner from renewable sources. In this Minireview, the most significant methods recently reported for CO₂ fixation under transition metal-free conditions are summarized, organized into three different chapters according to the nature of the chemical transformation that forges the new C-C bond. The focus is on the mechanistic aspects of the different CO₂ activation modes, with specific attention to those systems that operate under catalytic conditions.

Umpolung Reactivity of Aldehydes toward Carbon Dioxide

Carbon dioxide is an intrinsically stable molecule. Therefore, its activation requires extra energy input in the form of reactive reagents and/or activated catalysts and, often, harsh reaction conditions. Reported here is a direct carboxylation reaction of aromatic aldehydes with carbon dioxide to afford α-keto acids as added-value products. In situ generation of a reactive cyanohydrin was the key to the successful carboxylation reaction under operationally mild reaction conditions (25-40 °C, 1 atm CO₂ ). The resulting α-keto acids served as a platform for α-amino acid synthesis by reductive amination reactions, illustrating the chemical synthesis of essential bioactive molecules from carbon dioxide.

Trapping CO2 by Adduct Formation with Nitrogen Heterocyclic Carbenes (NHCs): A Theoretical Study

Carbon dioxide can form compounds with nitrogen heterocyclic carbenes (NHCs) based on azoles through noncovalent interactions or by covalent bonding. A narrow dependence on the carbene structure has been observed for the preference for one or the other type of bonding, as revealed by a series of physicochemical descriptors. In our survey, a set of NHCs based on the azole family (three classical, three abnormal, and one remote) was shown to bind CO₂ at the accurate G4MP2 computational level. In most cases, exothermic reaction profiles towards the covalently bound form were found, which reached stabilization enthalpies of up to -77 kJ mol^-1 for the remote carbene case. Both noncovalent and covalent minima and the corresponding transition state that connects them have been identified as stationary points along the reaction coordinate.

Coupling Reactions of Alkynyl Indoles and CO2 by Bicyclic Guanidine: Origin of Catalytic Activity?

Density functional theory calculations were used to investigate the three possible modes of activation for the coupling of CO₂ with alkynyl indoles in the presence of a guanidine base. The first of these mechanisms, involving electrophilic activation, was originally proposed by Skrydstrup et al. (Angew. Chem. Int. Ed. 2015, 54, 6682). The second mechanism involves the nucleophilic activation of CO₂ . Both of these electrophilic and nucleophilic activation processes involve the formation of a guanidine-CO₂ zwitterion adduct. We have proposed a third mechanism involving the bifunctional activation of the bicyclic guanidine catalyst, allowing for the simultaneous activation of the indole and CO₂ by the catalyst. We demonstrated that a second molecule of catalyst is required to facilitate the final cyclization step. Based on the calculated turnover frequencies, our newly proposed bifunctional activation mechanism is the most plausible pathway for this reaction under these experimental conditions. Furthermore, we have shown that this bifunctional mode of activation is consistent with the experimental results. Thus, this guanidine-catalyzed reaction favors a specific-base catalyzed mechanism rather than the CO₂ activation mechanism. We therefore believe that this bifunctional mechanism for the activation of bicyclic guanidine is typical of most CO₂ coupling reactions.

In Situ Generated Zinc(II) Catalyst for Incorporation of CO2 into 2-Oxazolidinones with Propargylic Amines at Atmospheric Pressure

Incorporation of CO₂ into heterocyclic compounds (i.e., 2-oxazolidinones) under mild conditions, especially at atmospheric pressure still remains challenging. The mononuclear Zn^II complex ZnCl₂ (TBD)₂ , where TBD=1,5,7-triazabicyclo[4.4.0]dec-5-ene, in this study was demonstrated as a robust catalyst for the carboxylative cyclization of propargylic amines with CO₂ to exclusively afford various 2-oxazolidinones in excellent yields. Notably, the Zn^II catalytic species is readily generated in situ from ZnCl₂ and TBD without pre-preparation and further isolation. Such a CO₂ fixation protocol could proceed smoothly under atmospheric pressure at mild temperature in an atom economic and environmentally benign manner. ¹³ C NMR and control experiments were performed to explore the possible interaction between Zn^II and the carbon-carbon triple bond of propargylic amine. The dual catalytic role of the Zn catalyst to enhance O-nucleophilicity of the carbamate anion intermediate and activate the carbon-carbon triple bond is proposed based on mechanistic investigations.

Metal-Organic Frameworks as a Potential Platform for Selective Treatment of Gaseous Sulfur Compounds

The release of various anthropogenic pollutants such as gaseous sulfur compounds into the environment has been accelerated as globalization has promoted the production of high-quality products at lower prices. Because of strict enforcement of mitigation technologies, advanced materials have been developed to efficiently remove gaseous sulfur compounds released from various source processes. Metal-organic frameworks (MOFs) are promising materials to treat sulfur compounds via adsorption, catalysis, or separation. Nonetheless, the practical applicability of MOFs is limited by a number of factors including loss of structural integrity after use, limited reusability of spent MOFs, and low stability toward omnipresent molecules (e.g., H₂O). Here, we provide a comprehensive assessment of MOF technology for the effective control of gaseous sulfur compounds. This review will thus help expand the fields of real-world application for MOFs with a roadmap for this highly challenging area of research.

Organocatalyzed CO2 Trapping Using Alkynyl Indoles

The first organocatalyzed trapping of CO2 through C-C and C-O bond formation is reported. Alkynyl indoles together with catalytic amounts of an organic base and five equivalents of CO2 resulted in the formation new heterocyclic structures. These tricyclic indole-containing products were successfully prepared under mild reaction conditions from aromatic, heteroaromatic, and aliphatic alkynyl indoles with complete regioselectivity. Further investigations suggest that C-C bond formation is the initial intermolecular step, followed by lactone-forming C-O bond formation.

Upgrading carbon dioxide by incorporation into heterocycles

Carbon dioxide is commonly regarded as the primary greenhouse gas, but from a synthetic standpoint can be utilized as an alternative and sustainable C1 synthon in organic synthesis rather than a waste. This results in the production of organic carbonates, carboxylic acids, and derivatives. Recently, CO2 has emerged as an appealing tool for heterocycle synthesis under mild conditions without using stoichiometric amounts of organometallic reducing reagents. This Minireview summarizes recent advances on methodologies for CO2 incorporation into N-, O-, and C-nucleophiles to provide various heterocycles, including cyclic carbamates, benzoxazine-2-one, 4-hydroxyquinolin-2-one, quinazoline-2,4(1 H,3 H)-diones, benzimidazolones, α-alkylidene cyclic carbonate.

Metal-free reduction of CO2 with hydroboranes: two efficient pathways at play for the reduction of CO2 to methanol

Guanidines and amidines prove to be highly efficient metal-free catalysts for the reduction of CO2 to methanol with hydroboranes such as 9-borabicyclo[3.3.1]nonane (9-BBN) and catecholborane (catBH). Nitrogen bases, such as 1,5,7-triazabicyclo[4.4.0]dec-5-ene (TBD), 7-methyl-1,5,7-triazabicyclo[4.4.0]dec-5-ene (Me-TBD), and 1,8-diazabicycloundec-7-ene (DBU), are active catalysts for this transformation and Me-TBD can catalyze the reduction of CO2 to methoxyborane at room temperature with TONs and TOFs of up to 648 and 33 h(-1) (25 °C), respectively. Formate HCOOBR2 and acetal H2C(OBR2)2 derivatives have been identified as reaction intermediates in the reduction of CO2 with R2BH, and the first C-H-bond formation is rate determining. Experimental and computational investigations show that TBD and Me-TBD follow distinct mechanisms. The N-H bond of TBD is reactive toward dehydrocoupling with 9-BBN and affords a novel frustrated Lewis pair (FLP) that can activate a CO2 molecule and form the stable adduct 2, which is the catalytically active species and can facilitate the hydride transfer from the boron to the carbon atoms. In contrast, Me-TBD promotes the reduction of CO2 through the activation of the hydroborane reagent. Detailed DFT calculations have shown that the computed energy barriers for the two mechanisms are consistent with the experimental findings and account for the reactivity of the different boron reductants.

Electrocatalytically switchable CO2 capture: first principle computational exploration of carbon nanotubes with pyridinic nitrogen

Carbon nanotubes with specific nitrogen doping are proposed for controllable, highly selective, and reversible CO2 capture. Using density functional theory incorporating long-range dispersion corrections, we investigated the adsorption behavior of CO2 on (7,7) single-walled carbon nanotubes (CNTs) with several nitrogen doping configurations and varying charge states. Pyridinic-nitrogen incorporation in CNTs is found to induce an increasing CO2 adsorption strength with electron injecting, leading to a highly selective CO2 adsorption in comparison with N2 . This functionality could induce intrinsically reversible CO2 adsorption as capture/release can be controlled by switching the charge carrying state of the system on/off. This phenomenon is verified for a number of different models and theoretical methods, with clear ramifications for the possibility of implementation with a broader class of graphene-based materials. A scheme for the implementation of this remarkable reversible electrocatalytic CO2 -capture phenomenon is considered.