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Reaction diagram for rDA reaction of 4-OH-DHHP in water (both concerted and through zwitterion intermediate).
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The retro-Diels-Alder (rDA) reaction of partially saturated 2-pyrone molecules to form the 1,3-butadiene backbone and CO2 was studied using density functional theory (DFT) calculations in vapor-phase, polar and non-polar solvents. The activation barriers for the ring-opening and decarboxylation of the molecules were correlated to the type of substi...
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... between the activation barrier of the rDA reaction in a solvent to its dielectric constant. The activation barrier was observed to decrease with the increase in the solvent polarity and the substituent effect was linearly correlated with s p . Detailed reaction diagram for ring-opening and decarboxyl- ation of 4-OH-DHHP in water is outlined in Fig. 5. In polar solvent like water, 4-OH-DHHP may undergo the rDA reaction though a one-step concerted mechanism or a two-step mecha- nism via the formation of a zwitterionic intermediate. The activation and reaction energies for the partially saturated 2- pyrones were calculated for the rDA reaction for the concerted one step mechanism in ...
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... to stabilize the zwitterion intermediate through solvation and H-bonding. In contrast, non-polar solvents were unable to stabilize the zwitterion intermediates. Interestingly it was observed that, the zwitterion intermediate was stable in polar solvents only for 2-pyrones with electron donating substituents (-OH, -OMe and -NH 2 ) as shown in Fig. 5 for 4-OH-DHHP. For non or moderate electron donating groups like -H and -Me and for electron withdrawing substituents (-CHO and -CO 2 Me) the zwitterion intermediate was observed to be unstable and inclined to revert back to the corresponding product state. The activation energies for rDA reaction via the two-step zwitterion ...
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... The structure obtained was taken as an intermediate for QST optimization, resulting in a more refined structure closer to the TS geometry. The authors have already successfully used a similar DFT setup to study various reactions [43][44][45][46][47][48][49][50]. ...
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... Even though the rDA reactions are useful synthetic tools for the production of unsaturated compounds from bicyclic or cyclic compounds and are widely employed in the chemistry of 2-pyrones and cyclopentadienones, 87,88 the rDA reactions are clearly less explored compared to the DA reactions. 89,90 Utilizing the theory of microscopic reversibility, 91,92 the rDA reactions are typically explained by the reverse route, that is, the reaction going backward from the products to the bicyclic intermediates. Thus, FMO theory can also be applied to understand the reactivity trends. ...
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... Ty pical correlations between rate/DG°and FMO energy gap are frequently reported for furan DA cycloadditions:two such examples are provided in Figure 4B,C.A lternatively, other related reactivity descriptors are also useful, for instance the average of the diene ionization potential and dienophile electron affinity ((IP diene + EA dienophile )/2) [54] or the value of the electrostatic potential (ESP) at the ring critical point of the furan cycle. [52] Mapping rate parameters vs. orbital energies is quite likely to be of limited use in understanding the thermodynamically controlled furan DA reactions,h owever.I nt his respect, as will be showcased in Section 6, Bell-Evans-Polanyi relationships or complete reaction energy profiles,w hich look at both kinetic and thermodynamic barriers,h ave much higher practical utility. ...
Biomass‐derived furanic platform molecules have emerged as promising building blocks for renewable chemicals and functional materials. To this aim, the Diels–Alder (DA) cycloaddition stands out as a versatile strategy to convert these renewable resources in highly atom‐efficient ways. Despite nearly a century worth of examples of furan DA chemistry, clear structure–reactivity–stability relationships are still to be established. Detailed understanding of the intricate interplay between kinetics and thermodynamics in these very particular [4+2] cycloadditions is essential to push further development and truly expand the scope beyond the ubiquitous addend combinations of electron‐rich furans and electron‐deficient olefins. Herein, we provide pertinent examples of DA chemistry, taken from various fields, to highlight trends, establish correlations and answer open questions in the field with the aim to support future efforts in the sustainable chemicals and materials production.
... Over the past, E g is been assumed in many instances to act as one of the descriptors to correlate reactivity of a catalyst [153,168,171,90,198,199,200,201] Figure 4. 5. The electronic charge density at each atom is also shown, whose relevance is discussed later. ...
... Thus, fundamental gap E g can be considered as an important feature of the clusters that determines their redox properties, and in many cases can serve as a descriptor of the catalytic activity[169,170,168,171,172,173,174,175].We therefore analyze the dependence of E g of the clusters on composition, temperature, and O 2 pressure, and determine the explicit role of different atomic species to modulate the E g values. Strikingly, we find that the nature of TM plays a negligible role in tuning the E g in TM x Mg y O z clusters. ...
The ultimate goal of research in heterogeneous catalysis is to engineer the efficient and optimized catalyst for wide range of catalytic processes.
The meaningful strategy to find suitable catalysts is to think what really limits the utility of existing catalysts.
Basically, the development and rational design of catalytic materials largely depend on the ability to grasp the knowledge of targeted functionality at atomistic level. Usually, under realistic conditions catalytic materials come into contact with reactive molecules of surrounding phase.
This induces the changes in local structure, composition and morphology of the catalyst.
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The first principles methods such as density-functional theory (DFT) combined with concepts from thermodynamics have become stand tools for the accurate description of underlying factors that drive the activity of catalyst in operating conditions.
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... Thermal smearing of 0.005 au is applied to improve the SCF convergence. A similar method has been used previously by us to study the retro Diels−Alder reaction in the gas phase and solvents 66,67 and methane aromatization over an Mo/ZSM-5 catalyst. 68 The model of Ag 6 nanocluster embedded on COF support was chosen to simultaneously study the activity of the Ag nanoparticle and the influence of the COF support. ...
The present work introduces the favourable synthesis of porous functionalized nanomaterials with excellent surface area, porosity and high CO2 capture ability to serve cyclizative reactions by incorporating CO2 molecule in highly reactive organic moieties. We have attempted to fulfil the target by the decoration of Ag NPs over the exterior surfaces of COFs (covalent organic frameworks) TpPa-1 and TpTta to achieve [email protected] and [email protected] nanomaterials with absolutely ordered structure. Characterizations of the nanocatalysts ([email protected] and [email protected]) have been performed by FE-SEM, thermogravimetric tools, N2 adsorption/desorption, TEM, FT IR, UV-Vis and PXRD analysis. These Ag NPs architectured porous COFs described excellent performances of the catalytic systems for the benign synthesis of cyclic carbamates from unsaturated amines in the presence of N-Iodosuccinimide (NIS) and several 2-oxazolidinone derivatives from propargylamine derivatives via cyclizative atmospheric CO2 capture under solvent-free and alkali-free conditions (sustainable approach).The microporous material [email protected] exhibited most excellent catalytic performance than [email protected] during the production of cyclic carbamates and oxazolidinones which indicates that the selection of Ag NPs decorated COFs with the excellent surface area has a contributory effect on carboxylative cyclization reaction. DFT studies furnished important information to establish the detailed mechanism of silver (0) catalyzed CO2 incorporation into unsaturated amine. These COFs enriched with N centres can interact with the Ag NPs at their outer exterior surfaces very tightly. These two nanocatalysts exhibited magnificent recycling efficiencies for the generation of cyclic carbamates and 2-oxazolidinones with almost zero silver leaching from the exterior part of the catalyst.
... 52 In addition, the electron donating groups at the C 4 position were observed to introduce an electronic effect in reducing the activation barrier of the rDA reaction. 53 The reactivity of the 2-pyrone molecule was correlated back to the resultant normal electron demand frontier molecular orbital (FMO) gap of the product diene and dienophile. 53 Khan et al. 53 reported a linear scaling relationship between the activation energy and FMO gap for ring-opening and decarboxylation reaction of such partially saturated 2-pyrones in the vapor phase and also in different solvent environments. ...
... 53 The reactivity of the 2-pyrone molecule was correlated back to the resultant normal electron demand frontier molecular orbital (FMO) gap of the product diene and dienophile. 53 Khan et al. 53 reported a linear scaling relationship between the activation energy and FMO gap for ring-opening and decarboxylation reaction of such partially saturated 2-pyrones in the vapor phase and also in different solvent environments. Polar solvents were observed to preferentially to reduce the activation barrier as compared to the nonpolar solvents due to the relative stabilization of the polarized transition state. ...
... 53 The reactivity of the 2-pyrone molecule was correlated back to the resultant normal electron demand frontier molecular orbital (FMO) gap of the product diene and dienophile. 53 Khan et al. 53 reported a linear scaling relationship between the activation energy and FMO gap for ring-opening and decarboxylation reaction of such partially saturated 2-pyrones in the vapor phase and also in different solvent environments. Polar solvents were observed to preferentially to reduce the activation barrier as compared to the nonpolar solvents due to the relative stabilization of the polarized transition state. ...
Technologies for the processing of lignocellulosic biomass into fuels and chemicals are generally focussed on selective chemical transformation of the three different types of constituents; cellulose, hemicellulose and lignin. In this regard, heterogeneous catalytic reactions are employed to defunctionalize and upgrade the platform molecules obtained selectively from these constituents. Herein, a selection of studies are discussed which are adapted to deoxygenate and valorise the biomass-derived platform molecules with a specific focus on understanding the reaction mechanisms and rational design of a heterogeneous catalyst. The selection of the deoxygenation process constituted a combination of two or three reactions. For example, ring-opening reactions of the cyclic compounds are studied with decarboxylation, dehydration, hydrogenation and/or Diels-Alder reaction carried out on metal, acid and/or oxide catalysts. The platform molecules studied here include an array of saturated lactones, 2-pyrones, cyclic ethers, furans and phenolic compounds. In the study of lactones, mechanistic insights are provided to understand the selectivity trend over the Brønsted and Lewis acid catalysts for ring-opening and decarboxylation reactions leading to the formation of α-olefins. For 2-pyrones, the integrated bio- and chemo-catalytic process is studied in which a 2-pyrone molecule obtained from fermentation media may undergo Diels-Alder, hydrogenation combined with ring-opening, decarboxylation and dehydration reactions to yield the target product. Ring-opening studies on cyclic ethers, including furanic compounds, are focused on mechanistic observations in the C-O hydrogenolysis reaction, leading to the design of bimetallic alloys to produce terminal diols and carboxylic acids. In extension to this, rational thoughts on the design of bimetallic catalysts are elucidated in the hydrodeoxygenation of the phenolic compounds.
... DFT calculations were performed in Material Studio 8 (Biovia, San Diego, USA) based DMol 3 (Delley, 2006) module using DNP (double numerical plus polarization) numerical basis set and Generalized gradient approximation (GGA) PW91 functional (Perdew and Wang, 1992). Previous studies with PW91 functional has shown to be in good correlation with experimental trends Chia et al., 2013;Gupta et al., 2016;Khan et al., 2016). Energy, force and atom displacement convergence criterion of; 0.0001 eV, 0.05 eV/Å and 0.005 Å, respectively, were used for the geometry optimization. ...
... 13 It was further demonstrated by us using ab initio DFT simulations, that in aqueous conditions, partially saturated 2-pyrone molecules may undergo direct ring-opening and decarboxylation via a retro Diels−Alder (rDA) reaction in which water helps in stabilizing the intermediate zwitterion. 13,16,17 Similar chemical transformations have been experimented on several other 2-pyrone molecules such as 2-pyrone-4,6-dicarboxylic acid (PDC), coumalic acid, and 5-hydroxy-4-methyl-5,6-dihydro-2H-pyran-2-one, which are obtained from polyketide biosynthetic routes. 8,18,19 Since these chemical transformations are generally performed in a solvent, a study on the effect of solvent forms a necessary component for successful commercial development. ...
... 13 Theoretical attempts in modeling an aqueous phase reaction of biomass-derived 2-pyrones and lactones, using DFT, had shown some limitations with significant deviations from the experimental observations. 13,16,17 This was likely due to the static implicit and/or explicit water environment applied in the model, which was not able to capture the dynamic behavior of the solvent during the course of the reaction. DFT studies, in general, tend to completely ignore the dynamics of reaction components and solvents. ...
... The occurrence of these simultaneous reaction steps unequivocally illustrated a concerted mechanism which was also apparent from our DFT simulations. 16 Consequently, the ring-opening and decarboxylation reactions transformed a cyclic reactant molecule into butadiene and CO 2 as products ( Figure 2). Formation of butadiene was further elucidated by the variation of other bond lengths in the molecule as shown in Figure 2. The length of C 3 −C 4 and C 5 −C 6 bonds decreased from ∼1.53 to 1.34 Å illustrating a transition from the single bond order to double at the same 840th metadynamics step. ...
2-Pyrones have been identified as a potential platform molecules derived from biomass to produce high value chemicals. Theoretical studies utilizing Density Functional Theory (DFT) simulations have shown partially saturated form of these molecules undergoing retro Deils-Alder (rDA) reaction in both vapor and solvent conditions. Developing an understanding on the effect of solvent media is crucial to improve the processing of these biomass-derived compounds. In this context, DFT simulations had shown limited access due to either implicit solvent or static explicit solvent environment applied in the model, which did not account for the contribution from the dynamics. Herein, Car-Parrinello Molecular Dynamics (CPMD) simulations, in conjunction with metadynamics, were performed to understand the effect of dynamics of the solvent on the rDA reaction of partially saturated 2-pyrones. For this, compounds containing both electron donating and withdrawing groups as substituent were studied. It was observed that, in vapor phase, the results from CPMD simulations were in agreement with our previous DFT results. However, in water, a lowering in calculated activation barriers (by ≈10 kJ/mol) was observed, due to the dynamic behavior of solvent, differentially interacting with the reactant and the activated complex, along the minimum energy path. For hydroxyl substituted 2-pyrones, computed activation energy of 63 ± 7 kJ/mol was in agreement with the apparent activation energy measured experimentally (42 ± 18 kJ/mol). CPMD-metadynamics simulations were further applied to study the reaction energetics of complex 2-pyrone molecule undergoing similar rDA reaction.
... Interestingly, TAL was observed to ring open and decarboxylate at unprecedented low temperatures (<100 C) with high conversions (>99%) and selectivity (>99%) in water, without the requirement of a catalyst (Chia et al. 2013). It was demonstrated by ab initio DFT simulations, that in aqueous systems, partially saturated 2-pyrone molecules undergo direct ring opening and decarboxylation via a retro-Diels-Alder (rDA) reaction, in which water helps in stabilizing the zwitterionic intermediate Khan et al. 2016). TAL could therefore act as a potential precursor to produce important chemicals such as sorbic acid, bioactive molecules (e.g. ...
Development of processes to produce fuels and chemical from biomass offers an exciting opportunity to achieve a sustainable supply from renewable sources as compared to fossil fuel-based methods. Technologies relying only on chemical catalytic routes have shown limitations in achieving the desired yield of a product molecule. Therefore, recent developments in research have emphasized the importance of integrating a biocatalytic route to a chemo-catalytic route to produce commodity chemicals with high conversion and selectivity. Microorganisms including bacteria, fungus and algae are versatile in nature and have potential to yield platform molecules which can be upgraded to produce their petrochemical counterparts. Genetic engineering techniques combined with metabolic flux analysis are employed to further enhance the productivity. On subsequent purification, the platform molecule may be used as a reactant for chemo-catalytic processing to produce a range of high-value chemicals. In this way, a novel integrated fermentation and catalytic processing strategy is envisaged, which will open avenues for producing chemicals from renewable sources. The chapter covers the progress made in this direction by summarizing the routes for producing a platform molecule via biocatalytic transformations.
