Carbamate Formation Research Articles

Azole-Anion-Based Aprotic Ionic Liquids: Functional Solvents for Atmospheric CO2 Transformation into Various Heterocyclic Compounds.

The chemical transformation of atmospheric CO2 is of great significance yet still poses a great challenge. Herein, azole-anion-based aprotic ionic liquids (ILs) were synthesized by the deprotonation of weak proton donors (e.g., 2-methylimidazole, 4-methylimidazole, and 2,4-dimethylimidazole) with tetrabutylphosphonium hydroxide, [Bu4 P][OH]. We found that these ILs, such as [Bu4 P][2-MIm], could activate atmospheric CO2 through the formation of carbamates. The resultant carbamate intermediates could further react with various types of substrate, including propargylic alcohols, 2-aminobenzonitriles, ortho-phenylenediamines, and 2-aminothiophenol, thereby producing α-alkylidene cyclic carbonates, quinazoline-2,4(1 H,3 H)-diones, benzimidazolones, and benzothiazoline, respectively, in moderate-to-good yields. Thus, we have achieved the transformation of CO2 at atmospheric pressure, and we expect this method to open up new routes for the synthesis of various oxygen-containing heterocyclic compounds under metal-free conditions.

Cover Picture: Regioselective Organocatalytic Formation of Carbamates from Substituted Cyclic Carbonates (Adv. Synth. Catal. 13/2016)

Cover Picture: Regioselective Organocatalytic Formation of Carbamates from Substituted Cyclic Carbonates (Adv. Synth. Catal. 13/2016)

CO2 adsorption study on primary, secondary and tertiary amine functionalized Si-MCM-41

The present study was conducted to investigate the CO2 adsorption efficiency of siliceous mesoporous material known Si-MCM-41 and impacts of amine loading, amine type, CO2 pressure and adsorption temperatures on CO2 adsorption. Si-MCM-41 was impregnated with monoethanolamine (MEA), diethanolamine (DEA) and triethanolamine (TEA). The adsorption study was investigated from very low pressure to 1bar at three temperatures (i.e. 25, 50 and 75°C) in pure CO2 atmosphere. Pristine Si-MCM-41 shows maximum CO2 adsorption capacity of 27.78mg/g at 25°C and 1bar pressure. Monoethanolamine functionalized Si-MCM-41 with 10–50wt.% samples were investigated in order to optimize MEA loading for maximum CO2 adsorption. The 50wt.% MEA-Si-MCM-41 sample exhibited the highest CO2 adsorption capacity of 39.26mg/g at 25°C and 1bar pressure as compared to other samples. 50wt.% DEA-Si-MCM-41 and 50wt.% TEA-Si-MCM-41 samples were also investigated for their affinity to capture CO2 at the same conditions. Results show that 50wt.% MEA-Si-MCM-41 still gives the highest CO2 adsorption capacity while adsorption capacity decreased in order of monoethanolamine, diethanolamine and triethanolamine-functionalized Si-MCM-41. CO2 adsorption capacity of all adsorbents decreases with increasing adsorption temperature above 25°C. Fourier transform infrared spectroscopy (FTIR) analysis of CO2-saturated 50wt.% MEA, DEA and TEA-Si-MCM-41 samples shows the presence of transmission peaks associated to formation of carbamates.

Regioselective Organocatalytic Formation of Carbamates from Substituted Cyclic Carbonates

AbstractA highly regioselective catalytic approach has been developed towards carbamates derived from cyclic organic carbonates by reaction of the latter with amine reagents under organocatalytic control. For various combinations of carbonate and amine substrates, an organocatalyst (TBD: 1,5,7‐triazabicyclo[4.4.0]dec‐5‐ene) was used to increase the reaction kinetics while exerting excellent regioselective control. The current method is the first general approach towards the control over the regioselectivity of this reaction using a wide variety of easily accessed substituted organic carbonates.magnified image

Formation of ethyl carbamate in Goji wines: Effect of Goji fruit composition.

Ethyl carbamate (EC) is a multisite carcinogen widely occurring in alcoholic beverages. In this investigation, solid-phase extraction combined with gas chromatography mass spectrometry was employed to determine EC contents during the fermentation and storage processes, and the effects of Goji varieties on its formation were also examined. The results indicated that natural and potential EC contents were significantly affected by the varied composition of Goji fruits. The analysis of chemical properties showed differences in hundred-grain weight, water contents, amino acids, and nitrogen-to-carbon ratio for Goji berries. Citrulline was completely degraded although it is routinely considered as a non-preferred nitrogen for yeasts. Due to compositional differences, Goji wines accumulated distinct urea levels that positively correlated with the potential EC contents. Furthermore, the temperature in both the production processes highly influenced EC formation. These results contribute to a more comprehensive understanding of EC formation, and in turn, controlling EC in the Goji wine matrix.

Reactive flame retardancy of cyanate ester/epoxy resin blends and their carbon fibre reinforced composites

Cyanate ester/epoxy resin (CE/EP) blends consisting of diglycidyl ether of bisphenol A (DGEBA) and novolac type cyanate ester (CE) were prepared and reactively flame retarded using an epoxy functional adduct of DGEBA and 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOPO). Moisture uptake was measured by the gravimetric method; consequent carbamate formation was detected by infrared spectrometry. Effects of cyanate ester : flame retardant ratio were determined on glass transition temperature (Tg), thermal stability and flammability. The addition of CE compensated the Tg decreasing effect of DOPO. Reactively flame retarded CE/EP carbon fibre reinforced composites had LOI of 48 V/V%, V-0 UL-94 rate and peak of heat release rate of 84 kW/m2.

Reaction dynamics of $$\hbox {CO}_2$$ CO 2 in aqueous amines from ab initio molecular dynamics: 2-amino-2-methyl-1,3-propanediol (AMPD) compared to monoethanolamine (MEA)

We present Car–Parrinello molecular dynamics simulations—combined with metadynamics—of the reactions accompanying the capture of $$\hbox {CO}_2$$ in an aqueous amine solution. The selected amine is 2-amino-2-methyl-1,3-propanediol, which has been investigated experimentally in the search for optimum absorbents. In analogy with the empirical search, we use as reference aqueous monoethanolamine (MEA) at 30 wt%, namely the chemical absorbent most frequently used for the removal of $$\hbox {CO}_2$$ from combustion gases. In particular, we refer to our own results which have led to a detailed characterization of several reaction paths for the absorption of $$\hbox {CO}_2$$ and the amine regeneration (Ma et al. in J Chem Theory Comput 11:3189, 2015). Our simulations refer to the zwitterion mechanism leading to the formation of a carbamate or carbamic acid and to the release of the molecule in solution. The scenario established for the reactions in MEA is confirmed, and in particular the role of the zwitterion mechanism also for $$\hbox {CO}_2$$ release. We find that the difference in the structure of the two molecules does not influence the dynamics of either the formation or the dissociation of the zwitterion. However, it affects its interaction with water in a significant way, thus reducing the probability of carbamate formation and its stability in solution.

Impact of solvents and surfactants on the self-assembly of nanostructured amine functionalized silica spheres for CO2 capture

Macroscopic SiO2 spheres with a homogeneous amine distribution were synthesized by a one-step emulsion based synthesis approach in a flow column reactor. The CO2 adsorption capacity of the nano-structured amine-functionalized silica spheres was studied in absence and presence of H2O. The structural properties were adjusted by varying solvents and surfactants during the synthesis and, at constant amine loadings, were found to be the main factor for influencing the CO2 sorption capacities. Under water-free conditions CO2 is bound to the amino groups via the formation of carbamates, which require two neighboring amino groups to adsorb one CO2 molecule. At constant amine concentrations sorbents with lower surface area allow to establish a higher amine density on the surface, which enhances the CO2 uptake capacities under dry conditions. In presence of H2O the CO2 adsorption changes to 1:1 stoichiometry due to stabilization of carbamates by protonation of H2O and formation of further species such as bicarbonates, which should in principle double the adsorption capacities. Low concentrations of physisorbed H2O (0.3mmol/g) did not impair the adsorption capacity of the adsorbents for CO2, while at higher water uptakes (0.6 and 1.1mmol/g) the CO2 uptake is reduced, which could be attributed to capillary condensation of H2O or formation of bulky reaction products blocking inner pores and access to active sites.

Role of Amine Functionality for CO2 Chemisorption on Silica.

The mechanism of CO2 adsorption on primary, secondary, and bibasic aminosilanes synthetically functionalized in porous SiO2 was qualitatively and quantitatively investigated by a combination of IR spectroscopy, thermogravimetry, and quantum mechanical modeling. The mode of CO2 adsorption depends particularly on the nature of the amine group and the spacing between the aminosilanes. Primary amines bonded CO2 preferentially through the formation of intermolecular ammonium carbamates, whereas CO2 was predominantly stabilized as carbamic acid, when interacting with secondary amines. Ammonium carbamate formation requires the transfer of the carbamic acid proton to a second primary amine group to form the ammonium ion and hence two (primary) amine groups are required to bind one CO2 molecule. The higher base strength of secondary amines enables the stabilization of carbamic acid, which is thereby hindered to interact further with nearby amine functions, because their association with Si-OH groups (either protonation or hydrogen bonding) does not allow further stabilization of carbamic acid as carbamate. Steric hindrance of the formation of intermolecular ammonium carbamates leads to higher uptake capacities for secondary amines functionalized in porous SiO2 at higher amine densities. In aminosilanes possessing a primary and a secondary amine group, the secondary amine group tends to be protonated by Si-OH groups and therefore does not substantially interact with CO2.

Mechanisms of CO2 capture in ionic liquids: a computational perspective.

We present computational studies of CO2 sorption in two different classes of ionic liquid. The addition of carbon dioxide to four superbase ionic liquids, [P3333][Benzim], [P3333][124Triz], [P3333][123Triz] and [P3333][Bentriz], was studied using the DFT approach and considering anions alone and individual ion pairs. The addition of CO2 to the anion alone clearly resulted in the formation of a covalently-bound carbamate function with the strength of binding correlated to experimental capacity. In the ion pair however the cation significantly alters the nature of the bonding such that the overall cohesive energy is reduced. Formation of a strong carbamate function occurs at the expense of weakening the interaction between anion and cation. In [N1111][l-ALA], a representative amino acid ionic liquid, evidence was found for a low-energy monomolecular mechanism for carbamate formation, explaining the 1 : 1 molar uptake ratio observed in some amino acid ionic liquids. The mechanism involves proton transfer to the carboxylate group of the aminate anion.

Effect of ‘wheat Qu’ addition on the formation of ethyl carbamate in Chinese rice wine with enzymatic extrusion liquefaction pretreatment

Ethyl carbamate (EC), which has been confirmed as probably carcinogenic to humans, can be formed during the alcoholic fermentation of Chinese rice wine. In this study, EC and two possible precursors (urea and arginine) were analysed in wines fermented from rice processed by enzymatic extrusion liquefaction pretreatment, with different wheat Qu content gradients from 6 to 18% (dry basis, d.b.). The EC concentration decreased 22.5% with the addition of wheat Qu reduced to 6% for the enzymatic extrusion-processed rice wine (EE). Moreover, although the arginine was generated 6.33–8.39 times higher in the enzymatic extruded rice, instead of the traditional treated rice at the same wheat Qu level, the corresponding level of urea formed from arginine was not high in the EE. In addition, rice treated by extrusion combined with enzymatic hydrolysis was found to be more utilizable and fermentable, and the lost quality of the final traditional rice wine with the low wheat Qu level (6–14%) could be improved using enzymatically extruded rice as the fermenting material. Therefore, it may be feasible to inhibit EC accumulation in Chinese rice wine by wheat Qu control, combined with enzymatic extrusion pretreatment for rice.

Reduced Reactivity of Amines against Nucleophilic Substitution via Reversible Reaction with Carbon Dioxide

The reversible reaction of carbon dioxide (CO2) with primary amines to form alkyl-ammonium carbamates is demonstrated in this work to reduce amine reactivity against nucleophilic substitution reactions with benzophenone and phenyl isocyanate. The reversible formation of carbamates has been recently exploited for a number of unique applications including the formation of reversible ionic liquids and surfactants. For these applications, reduced reactivity of the carbamate is imperative, particularly for applications in reactions and separations. In this work, carbamate formation resulted in a 67% reduction in yield for urea synthesis and 55% reduction for imine synthesis. Furthermore, the amine reactivity can be recovered upon reversal of the carbamate reaction, demonstrating reversibility. The strong nucleophilic properties of amines often require protection/de-protection schemes during bi-functional coupling reactions. This typically requires three separate reaction steps to achieve a single transformation, which is the motivation behind Green Chemistry Principle #8: Reduce Derivatives. Based upon the reduced reactivity, there is potential to employ the reversible carbamate reaction as an alternative method for amine protection in the presence of competing reactions. For the context of this work, CO2 is envisioned as a green protecting agent to suppress formation of n-phenyl benzophenoneimine and various n-phenyl–n-alky ureas.

Solvent‐Free Iridium‐Catalyzed Reactivity of CO2 with Secondary Amines and Hydrosilanes

AbstractThe complex [Ir(H)(CF3SO3)(NSiN)(coe)] (NSiN=bis(pyridine‐2‐yloxy)methylsilyl fac‐coordinated) (1) is an effective catalyst precursor for the solvent‐free synthesis of silyl carbamates from reaction of aliphatic secondary amines with CO2 and HSiMe(OSiMe3)2. The preferential formation of the silyl carbamate instead of the expected formamide or methylamine has proven to be consequence of an iridium‐catalyzed dehydrogenative Si−N coupling between the silane and the amine to afford the corresponding silyl amine, which under the reaction conditions reacts with CO2 to give the corresponding silyl carbamate.

Ethyl Carbamate Production Kinetics during Wine Storage

The kinetics of ethyl carbamate (EC) formation was investigated during wine storage to evaluate the potential risk of EC formation in wine. The study monitored the EC, urea and citrulline concentration at different storage temperatures and wine pH. We found that temperature and initial urea content had significant effects on EC formation. The decay of urea and citrulline fit a first-order reaction approaching equilibrium, but the pH has little effect on the content of ethyl carbamate. Based on these results, we constructed an equation to forecast the content of ethyl carbamate during wine storage.

The Transition States for CO2 Capture by Substituted Ethanolamines.

Quantum chemical studies are used to understand the electronic and steric effects on the mechanisms of the reaction of substituted ethanolamines with CO2. SCS-MP2/6-311+G(2d,2p) calculations are used to obtain the activation energy barriers and reaction energies for both the carbamate and bicarbonate formation. Implicit solvent effects are included with the universal solvation model SMD. Carbamate formation is more favorable than bicarbonate formation for monoethanolamine (MEA) both kinetically and thermodynamically. Increase of the steric hindrance on the C atoms around the N atom in substituted ethanolamines favors bicarbonate formation over carbamate formation with lower activation barriers and thereby higher reaction rates. In contrast, substitution by an N-methyl or N-ethyl group on MEA leads to a lower activation barrier for both carbamate formation and bicarbonate formation. As a result, higher reaction rates are expected as compared to MEA, and therefore these compounds have significant potential as industrial CO2 capturing solvents.

Mechanisms of CO2 Capture into Monoethanolamine Solution with Different CO2 Loading during the Absorption/Desorption Processes.

Though the mechanism of MEA-CO2 system has been widely studied, there is few literature on the detailed mechanism of CO2 capture into MEA solution with different CO2 loading during absorption/desorption processes. To get a clear picture of the process mechanism, (13)C nuclear magnetic resonance (NMR) was used to analyze the reaction intermediates under different CO2 loadings and detailed mechanism on CO2 absorption and desorption in MEA was evaluated in this work. The results demonstrated that the CO2 absorption in MEA started with the formation of carbamate according to the zwitterion mechanism, followed by the hydration of CO2 to form HCO3(-)/CO3(2-), and accompanied by the hydrolysis of carbamate. It is interesting to find that the existence of carbamate will be influenced by CO2 loading and that it is rather unstable at high CO2 loading. At low CO2 loading, carbamate is formed fast by the reaction between CO2 and MEA. At high CO2 loading, it is formed by the reaction of CO3(-)/CO3(2-) with MEA, and the formed carbamate can be easily hydrolyzed by H(+). Moreover, CO2 desorption from the CO2-saturated MEA solution was proved to be a reverse process of absorption. Initially, some HCO3(-) were heated to release CO2 and other HCO3(-) were reacted with carbamic acid (MEAH(+)) to form carbamate, and the carbamate was then decomposed to MEA and CO2.

Cyanate as an Active Precursor of Ethyl Carbamate Formation in Sugar Cane Spirit.

The thermodynamic and kinetic aspects of ethyl carbamate (EC) formation through the reaction between cyanate and ethanol were investigated. The rate constant values for cyanate ion decay and EC formation are (8.0 ± 0.4) × 10(-5) and (8.9 ± 0.4) × 10(-5) s(-1), respectively, at 25 °C in 48% aqueous ethanolic solution at pH 4.5. Under the investigated experimental conditions, the rate constants are independent of the ethanol and cyanate concentrations but increase as the temperature increases (ΔH1(⧧) = 19.4 ± 1 kcal/mol, ΔS1(⧧) = −12.1 ± 1 cal/K, and ΔG1(⧧) = 23.0 ± 1 kcal/mol) and decrease as the solution pH increases. According to molecular modeling (DFT) that was performed to analyze the reaction mechanism, the isocyanic acid (HNCO) is the active EC precursor. The calculated ΔG1(⧧), ΔH1(⧧), and ΔS1(⧧) values are in very good agreement with the experimental ones.

Discovery of a Single Monooxygenase that Catalyzes Carbamate Formation and Ring Contraction in the Biosynthesis of the Legonmycins.

Pyrrolizidine alkaloids (PAs) are a group of natural products with important biological activities. The discovery and characterization of the multifunctional FAD-dependent enzyme LgnC is now described. The enzyme is shown to convert indolizidine intermediates into pyrrolizidines through an unusual ring expansion/contraction mechanism, and catalyze the biosynthesis of new bacterial PAs, the so-called legonmycins. By genome-driven analysis, heterologous expression, and gene inactivation, the legonmycins were also shown to originate from non-ribosomal peptide synthetases (NRPSs). The biosynthetic origin of bacterial PAs has thus been disclosed for the first time.

Discovery of a Single Monooxygenase that Catalyzes Carbamate Formation and Ring Contraction in the Biosynthesis of the Legonmycins

AbstractPyrrolizidine alkaloids (PAs) are a group of natural products with important biological activities. The discovery and characterization of the multifunctional FAD‐dependent enzyme LgnC is now described. The enzyme is shown to convert indolizidine intermediates into pyrrolizidines through an unusual ring expansion/contraction mechanism, and catalyze the biosynthesis of new bacterial PAs, the so‐called legonmycins. By genome‐driven analysis, heterologous expression, and gene inactivation, the legonmycins were also shown to originate from non‐ribosomal peptide synthetases (NRPSs). The biosynthetic origin of bacterial PAs has thus been disclosed for the first time.

Back Cover: Discovery of a Single Monooxygenase that Catalyzes Carbamate Formation and Ring Contraction in the Biosynthesis of the Legonmycins (Angew. Chem. Int. Ed. 43/2015)

Back Cover: Discovery of a Single Monooxygenase that Catalyzes Carbamate Formation and Ring Contraction in the Biosynthesis of the Legonmycins (Angew. Chem. Int. Ed. 43/2015)