Theoretical Study of the Ring-Opening of Epoxides Catalyzed by

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Theoretical Study of the Ring-Opening of Epoxides Catalyzed by Boronic Acids and Pyridinic Bases Luís Pinto da Silva J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b04157 • Publication Date (Web): 14 Jul 2017 Downloaded from http://pubs.acs.org on July 15, 2017

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The Journal of Physical Chemistry

Theoretical Study of the Ring-Opening of Epoxides Catalyzed by Boronic Acids and Pyridinic Bases Luís Pinto da Silva†‡* †

Chemistry Research Unit (CIQUP), Department of Chemistry and Biochemistry, Faculty of Sciences of University of Porto, R. Campo Alegre 687, 4169-007 Porto, Portugal.

‡

LACOMEPHI, Department of Geosciences, Environment and Territorial Planning, Faculty of Sciences of University of Porto, R. Campo Alegre 687, 4169-007 Porto, Portugal. * Corresponding author: Luís Pinto da Silva. E-mail: [email protected]

Abstract The ring-opening reaction of epoxides is a key step in the conversion of carbon dioxide (CO2), a greenhouse waste product of fossil fuel combustion, into value-added heterocyclic carbonates. Being an essential step in CO2 conversion, controlling its activation energy is essential in the development of methods in which heterocyclic carbonates can be obtained under mild conditions. We have employed a theoretical approach to obtain mechanistic insight into the ring-opening of a model epoxide, when catalyzed by both 2,6-phenylboronic acids and pyridinic bases. The presence of both catalysts decreases the activation energy of this process from ~60 kcal mol-1 to ~20 kcal mol-1. The electronic character of the substituent at the C2,6-position of phenylboronic acid has little effect on the activation energy. Nevertheless, electron-withdrawing substituents decrease the activation energy by increasing the acidity of the boronic acid group. It was also found that non-halogenated pyridinic bases present similar results to those provided by halide nucleophiles. Thus, two-component systems composed by phenylboronic acids and pyridinic bases appear to be a good choice for catalyzing the ring-opening reaction of epoxides under mild, metal-free and non-halogenated conditions.

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Introduction Our society still depends on the combustion of finite fossil fuel resources (coal, oil and natural gas) for meeting our energy requirements, and for obtaining raw materials for the synthesis of different chemicals.1-5 Available predictions indicate that in the near future these resources will decrease substantially and/or their availability will no longer be able to fulfil the demand for these raw materials.6 Therefore, researchers must face the challenge of developing sustainable methodologies for the production of required chemicals. It should be noted that while significant advances have been made in the development of renewable energy sources, fossil fuels are still likely to remain as one of the main energy sources for the next decades. One of the waste products of fossil fuel combustion is carbon dioxide (CO2), which is subsequently released into the atmosphere. Due to our dependence on fossil fuels, the atmospheric levels of CO2 has increased by about a third since the start of the Industrial Revolution.7 This increase in atmospheric CO2 has led to an increase in the average surface temperature of the earth, which means that CO2 is one of the primary anthropogenic greenhouse gases and a main responsible for climate change.7 Thus, reducing the levels of atmospheric CO2 is one of the most important issues of our days.

Scheme 1 - Coupling of CO2 and epoxides to yield heterocyclic carbonates.

Despite these harmful effects, CO2 can be considered (from a synthetic point of view) as an abundant, renewable, cheap and non-toxic C1 source.8-13 In fact, the coupling of CO2 to epoxides, to yield heterocyclic carbonates, has been found to be a promising method for converting this gas into useful organic compounds (Scheme 1).14-20 Heterocyclic carbonates 2 ACS Paragon Plus Environment

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have found use in a wide range of applications: as intermediates in the synthesis of pharmaceuticals, polymers and other fine chemicals;21,22 as electrolytes in lithium-ion batteries;23 as polar aprotic solvents.24 These heterocycles can also be converted into their linear analogues via transesterification.25 Given this, the development of efficient synthetic processes to exploit CO2 as a renewable carbon source could diminish our dependence on fossil fuels for raw materials. Moreover, such processes would turn CO2 into a valuable commodity and provide an economically attractive alternative to disposing of it into the atmosphere, which would help to deal with climate change. However, the thermodynamic stability and relative inertia of CO2 difficult the development of such processes, by creating the need for high energy starting materials and high reaction temperatures.26,27 In order to overcome such problems, the scientific community has been studying intensively the development of appropriated catalytic systems. The field of catalyzed CO2 conversion has been dominated by transition metal-based catalysts, being either heterogeneous or homogeneous in nature.28-30 Such systems present good results, as they operate at room temperature with good yields. However, organocatalysts possess attractive characteristics that favor their use, instead of metal-based catalysts. Organocatalysts are generally cheap, nontoxic and stable molecules, which present inertness towards moisture and air.2,5 In some cases, they can even be obtained from renewable feed stocks, opening the way for developing “carbon-neutrals” processes.5,31 Nevertheless, organocatalysts are still no match for metal-based catalyst, as they often work at high temperature and CO2 pressures, and require higher catalyst loadings and longer reaction times.2,5



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Scheme 2 - Proposed reaction mechanism for the ring-opening of an epoxide, when catalyzed by a HBD (in this work, 2,6-phenylboronic acid) and a nucleophile (in this work, pyridinic bases).

Hydrogen-bond donors (HBDs) have emerged as a potential solution, in two-component catalytic systems, to solve the poor performance of organocatalysts (when comparing with metal-based catalysts).14,15,32-34 HBDs can interact with the O-donor of the epoxide via hydrogen bonding, which facilitates its ring-opening by nucleophilic attack of a halide anion (Scheme 2). HBDs can also stabilize the resulting oxyanion by hydrogen bonding. The synergy between halide nucleophiles and HBDs makes the ring-opening reaction of the epoxide more efficient, thus allowing the synthesis of heterocyclic carbonates at low temperature and CO2 pressure.14,15,32-34


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One such report was made very recently by Wang and Zhang.14 They have demonstrated experimentally that, when acting together with tetrabutylammonium iodide (a halide nucleophile), boronic acids can efficiently catalyze (as HBDs) the conversion of CO2 and epoxides into heterocyclic carbonates under mild conditions. This was achieved by decreasing substantially the activation barrier for the ring-opening of the epoxide.14 In this type of catalytic scheme, the ring-opening of the epoxide through a nucleophilic attack is generally considered to be the rate-limiting step, with a corresponding high activation energy.2,5,14,17,30 However, the addition of appropriated boronic acids decreased significantly the activation energy to about ~19 kcal mol-1, a quite low value.14 These authors also demonstrated that among a wide range of boronic acids, better results were obtained with 2,6-arylboronic acids.14 As 2,6-arylboronic acids facilitate CO2 conversion by organocatalysts due to their ability to catalyze the ring-opening of epoxides in environmental benign conditions,14 the objective of this work is to provide detailed mechanistic information regarding this important step (ring-opening of epoxides) in the already experimentally validated catalytic system,14 by employing a theoretical approach. Such information is crucial for the development of more efficient twocomponent organocatalysts. In this work will also be analyzed the substituent effect on 2,6arylboronic acids and their catalytic properties. Such information will allow to predict which are the most suitable 2,6-arylboronic acids for the catalyzed ring-opening of epoxides, under mild and metal-free conditions. Moreover, the work of Wang and Zhang14, as well of other authors when studying HBDs in epoxides’ ring-opening reactions,15,32-34 focused on the use of halides as nucleophiles. However, the use of halides can present some drawbacks, such as reactor corrosion.35 To this end, the catalytic activity of boronic acids will be assessed in the presence of non-halogen nucleophiles.

Computational Details



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All calculations were made with the Gaussian 09 program package.36 Geometry optimizations, frequency and single point calculations were made with the hybrid meta-GGA M06-2X density functional57 and the 6-31+G(d,p) basis set. The M06-2X functional presents good results in applications involving main-group thermochemistry, kinetics and noncovalent interactions.37,38 All calculations were made in implicit water by using the SMD implicit solvation model.39 SMD is based in the integral equation formalism model (IEFPCM) but with radii and nonelectrostatic terms for Truhlar and co-workers’ SMD solvation model. SMD has the advantage of including non-electrostatic terms in the calculations.39 The transition states were located by using the STQN method,40 namely the QST3 variant. This variant requires three molecular specifications: the reactants, the products and an initial structure for the transition state. The analysis of the interactions between phenylboronic acid and propylene oxide was performed by applying the QTAIM theory.41-43 The QTAIM approach can be used to identify the coordinates of critical points (CP) in which the gradient of electron density vanishes. The characteristics of CP derived from QTAIM analysis can be used as descriptors for different types of interactions.41-43 Stable CPs can be divided into four categories: maxima in electron density almost always correspond to nuclei, and the minimum can be a cage CP, bond CP or ring CP.41-43 The QTAIM analysis was performed by using the Multiwfn code.44 The M062X was used in combination with the 6-31+G(d,p) basis set given previous good results in the study of reaction mechanisms and energetics for different chemical and catalyzed reactions.45-49


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Results and Discussion

Figure 1 - Reactant (a) and transition state (b) structures of the ring-opening of propylene oxide, obtained at the M06-2X/6-31+G(d,p) level of theory, when catalyzed by 2,6-dimethylphenylboronic acid and pyridine.

To obtain information regarding the ring-opening of epoxides in reactions catalyzed by boronic acids and non-halogen nucleophiles, a model of the catalytic system was constructed. This model was composed by three components: a model epoxide (propylene oxide), a 2,6arylboronic acid, and a pyridinic base (in this case, pyridine itself). This work focused on 2,67 ACS Paragon Plus Environment


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arylboronic acids, as the work of Wang and Zhang14 demonstrated that this is the most promising type of boronic acids. Pyridinic bases were chosen as the non-halogenated nucleophiles. This type of base has already been used as organocatalysts in CO2 conversion, and alone these bases have shown the ability to activate both the epoxide and CO2, but still require higher temperatures and pressures.50,51 Calculations were made solely in water by using the SMD implicit solvation model, a green solvent which use is recommended for performing a reaction in more environmental benign conditions. Table 1 - Activation ∆Eact, ∆Hact and ∆Gact (in kcal mol-1) for the ring-opening reaction of propylene oxide, in the presence of different catalysts. The activation energies were obtained with thermal corrections. The calculations were made at the M06-2X/6-31+G(d,p) level of theory, in implicit water (modelled with the SMD model). Iodide was calculated with the basis set LANL2DZ with polarization and diffuse functions. Reaction components

∆Eact

∆Hact

∆Gact

Propylene oxide

60.4

60.4

60.2

Propylene oxide + 2,6-dimethylphenyl boronic acid + 17.5

17.5

20.5

pyridine Propylene oxide + pyridine

24.4

24.4

27.4

Propylene oxide + 2,6-dimethylphenyl boronic acid

53.9

53.9

54.4

Propylene oxide + 2,6-dimethylphenyl boronic acid + iodide

22.1

22.1

24.4

First, we have assessed what was indeed the catalytic effect of a HBD and a pyridinic base on the ring-opening of a model epoxide. To this end, the activation electronic (∆Eact), enthalpy (∆Hact) and free Gibbs (∆Gact) energies (Table 1) were calculated for the ring-opening of propylene oxide. These activation parameters were obtained for the unimolecular ring-opening of the epoxide, for the reaction catalyzed by both the HBD (2,6-dimethyl-phenylboronic acid, Scheme 2) and nucleophile (pyridine, Scheme 2), and catalyzed only either by the HBD or the nucleophile. The reactants and transition state structures for the ring-opening of propylene 8 ACS Paragon Plus Environment

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oxide, when catalyzed by 2,6-dimethyl-phenylboronic acid and pyridine, are present in Figure 1. Our calculations demonstrate that the synergistic effect exerted by the HBD and the nucleophile is quite significant. While the unimolecular reaction presents very high activation energies (about ~60 kcal mol-1), the presence of both catalysts decrease these parameters by about ~40 kcal mol-1. Our calculations also show that this significant decrease is only obtained when both catalysts are present, as the activation parameters obtained in the presence of only one catalysts were higher in both cases. Nevertheless, it can be seen that pyridine, by itself, is a much better catalyst than dimethyl-phenylboronic acid. It should be noted that when we add the decrease in the activation parameters caused only by the HBD (between 6.0-6.5 kcal mol-1) to the decreased caused solely by the base (between 32.836.0 kcal mol-1), we obtain values (38.8-42.5 kcal mol-1) very similar to the decrease in the activation parameters caused by the presence of both catalysts (39.7-42.9 kcal mol-1). This indicates that the addition of both catalysts causes only a cumulative effect in the activation parameters, and not a true synergistic effect between the nucleophile and the HBD. The activation parameters here calculated for the ring-opening of propylene oxide, in the presence of dimethyl-phenylboronic acid and pyridine, were very similar to those obtained theoretically by Wang and Zhang14. These authors obtained a ∆Eact of 18.8 kcal mol-1 for a model composed of propylene oxide, 2,6-dimethyl-phenylboronic acid and bromide (as the nucleophile). This similarity indicates that the catalytic activity of pyridine is comparable to halide nucleophiles, supporting its use (and related derivatives) as non-halogen nucleophiles. It should also be noted that the products, in the reaction catalyzed both by the HBD and the base, are more stable than the reactants by ∆G = -14.9 kcal mol-1. This parameter, combined with the low activation parameters, indicates that the ring-opening of epoxides (the rate-limiting step of CO2 conversion into heterocyclic carbonates) 2,5,14,17,30 should occur under mild conditions, when catalyzed by 2,6-arylboronic acids and pyridinic bases. To further compare the catalytic activity of pyridine with halide nucleophiles, when used with 2,6-phenylboronic acid, it was modeled the ring-opening reaction of propylene oxide when 9 ACS Paragon Plus Environment


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catalyzed by 2,6-diphenylboronic acid and iodide (Table 1). It should be noted that iodide was calculated with the M062X functional and the basis set LANL2DZ with polarization and diffuse functions. All other atoms were treated at the M062X/6-31+G(d,p) level of theory. The activation parameters of this system varied between 22.1 and 24.4 kcal mol-1, which are 3.9-4.6 kcal mol-1 higher than the parameters calculated for the pyridine-phenylboronic acid catalytic system. The ∆Eact obtained here for the propylene oxide-2,6-dimethylphenylboronic acid-pyridine system (17.5 kcal mol-1) also compares well with activation energies reported in the literature for the ring-opening reaction of epoxides. Xia et al. reported an ∆Eact of 31.2 kcal mol-1 for the ring-opening of ethylene oxide when catalyzed by the binary ionic liquid 1-buty-3-menthylimidazolium

bromide.52

Other

authors

have

showed

that

1-(3-aminopropyl)-3-

methylimidazolium chloride and 1-(3-carbamic acid propyl)-3-methylimidazolium chloride can catalyze the ring-opening of propylene oxide with ∆Eact between 17.4 and 30.2 kcal mol-1.53 Sun and co-workers reported ∆Eact of 21.2 and 21.2 kcal mol-1 for the ring-opening reaction of propylene oxide, when catalyzed by the ionic liquid HEBimBr-.54 Finally, Tassaing et al. determined ∆Gact of 27.7 (for tetrabutylammonium bromide) and 33.4 kcal mol-1 (for 1,5,7triaza-bicyclo[4.4.0]dec-5-enium bromide) for the catalyzed ring-opening of propylene oxide.34 As for metal-based catalysts, Maeda and co-workers found that bifunctional metalloporphyrins with quaternary ammonium bromides allowed the ring-opening reaction of propylene oxide to occur with ∆Eact between 14.3 and 17.0 kcal mol-1.55 Finally, Capacchione and co-workers reported ∆Gact of 18.1 kcal mol-1 for the ring-opening reaction of propylene oxide when catalyzed by a dinuclear Fe(III) complex, bearing dithioether-triphenolate-based ligands, and bromide.56 Thus, the comparison of ∆Eact and ∆Gact showed us that the inclusion of pyridine leads to activation parameters generally lower than the one presented by organocatalysts composed by HBDs and halide nucleophiles. Moreover, the inclusion of pyridine led to ∆Eact in line with some metal-based catalysts. Thus, these data support the substitution of halide nucleophiles by pyridine and related derivatives. 10 ACS Paragon Plus Environment

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To assess if pyridine can interact with boronic acids and in this way affect the resting state of this system, the binding energies were calculated for three models. The first one consists on propylene oxide, 2,6-dimethylphenylboronic acid and pyridine. The second one consists only on propylene oxide and 2,6-dimethylphenylborinic acid, while the third consists on the same boronic acid and pyridine. The calculated binding energies are of -7.4, -6.0 and -5.6 kcal mol-1, respectively. These energies were obtained by calculating the electronic energies of these complexes and all molecules composing them, at the M06-2X/6-31+G(d,p) level of theory. Thus, while all complexes present favorable binding energies, it is the complex consisting on the two-component catalytic system to present a more favorable binding energy. Nevertheless, the close energy binding energy presented by the 2,6-dimethylphenylboronic acid and pyridine indicates some potential for affecting the resting state of this system, which means that in the future development of new two-component catalytic systems the potential interaction between the HBD and the nucleophile should be explored and taken into account. To assess the substituent effect on the catalytic properties of 2,6-arylboronic acids, different functional groups, other than -CH3 (a weakly electron-donating substituent), were added to the C2- and C6-positions (Scheme 2). The substituents were -H, -CF3 (a strong electron-withdrawing group) and -NH2 (a strong electron-donating group). The activation parameters calculated for the ring-opening of propylene oxide, when catalyzed by these 2,6-arylboronic acids and pyridine, are shown in Table 2. The first conclusion to come to mind is that the substituent effect is not very significant, as the highest energy difference between the activation parameters of different HBDs was only of 2.4 kcal mol-1. Moreover, while -CF3 (a strong electronwithdrawing group) presented the lower activation parameters and -NH2 (a strong electrondonating group) presented the highest activation energies, -CH3 (a weakly electron-donating group) presented the second best results while -H presented the third best.

Table 2 - Activation ∆Eact, ∆Hact and ∆Gact (in kcal mol-1) for the ring-opening reaction of



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propylene oxide, in the presence of pyridine and C2,6-substituted phenylboronic acid. The activation energies were obtained with thermal corrections. The calculations were made at the M06-2X/6-31+G(d,p) level of theory, in implicit water. C2,6-substituents

∆Eact

∆Hact

∆Gact

-NH2

19.7

19.7

22.4

-CH3

17.5

17.5

20.5

-H

17.6

17.6

21.5

-CF3

17.3

17.3

20.0

Having reached these conclusions, we have focused our attention in uncovering the factors responsible for the different activation parameters, obtained when 2,6-phenylboronic acid is substituted with different functional groups. In Table 3 are presented the bond lengths (at the transition state structure) between the propylene oxide’s oxygen (OPO) and the hydrogens of the boronic acid group (HBA1 and HBA2, see Scheme 2), and between the hydrogens and oxygen heteroatoms (OBA1 and OBA2, see Scheme 2). From our calculations, it appears that decreasing activation energies are associated with increasing OPO-HBA1 bond lengths and decreasing OPOHBA2 bond lengths. Thus, the two hydrogen bonds formed between the different 2,6phenylboronic acids and propylene oxide, at the transition state, have different behaviors. In order to obtain further information regarding the hydrogen bonding between substituted 2,6phenylboronic acid and propylene oxide (at the transition state), the different complexes were characterized with a Quantum Theory of Atoms in Molecules (QTAIM) analysis.41-43 QTAIM is a model of electronic systems in which the principal features of molecular structures (atoms and bonds) are expressions of the system-observable electron-density distribution function. According to QTAIM, the molecular structure can be revealed by the stationary points of the electron density together with the gradients paths that originate and terminate at these points. Visual analysis (Figure 2) revealed the existence (for all four complexes) of the expected two hydrogen bonds between the boronic acid group and the propylene oxide’s oxygen. 12 ACS Paragon Plus Environment

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Table 3 - Bond lengths (in Angstrom) found at the transition state structure of the ringopening of propylene oxide, in the presence of pyridine and C2,6-substituted phenylboronic acid . C2,6-substituents

OPO-HBA1

OPO-HBA2

OBA1-HBA1

OBA2-HBA2

-NH2

1.99

1.62

0.98

1.00

-CH3

2.07

1.59

0.97

1.01

-H

1.99

1.63

0.98

1.00

-CF3

2.13

1.53

0.97

1.02

Figure 2 - Transition state complex between propylene oxide, C2,6-substituted phenylboronic acid and pyridine, after QTAIM analysis.

These hydrogen bonds were also characterized on basis of three QTAIM-derived parameters (Tables 4): the electron density ρ(r), its Laplacian ∇2(r), and the energy density H(r).41-43,57-60



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The low values of ρ(r) indicate that these are closed-shell interactions, as expected.41-43,57-60 ∇2(r) and H(r) can be used as indicators of strength of hydrogen bonding.41-43,57-60 Positive ∇2(r) and H(r) indicate weak hydrogen bonds, whereas medium-strength bonding is characterized by positive ∇2(r) and negative H(r). Strong hydrogen-bonding is identified by negative ∇2(r) and H(r). All studied hydrogen bonds can be identified as medium-strength interactions. Moreover, our analysis indicates that the OPO-HBA1 hydrogen bond is significantly weaker than the OPOHBA2 interaction (for all four complexes), given their corresponding H(r) values. This supports the conclusion reached before when analyzing the hydrogen bond lengths (Table 3), which was that the two hydrogen-bonding interactions have a different behavior. Table 4 - QTAIM-derived parameters for the two hydrogen bonds (OPO-HBA1 and OPO-HBA2) formed between C2,6-substituted phenylboronic acid and propylene oxide at the transition state structure (Figure 2). OPO-HBA1

OPO-HBA2

C2,6-substituents

ρ(r)

∇2(r)

H(r)

ρ(r)

∇2(r)

H(r)

-NH2

0.023

0.070

-0.001

0.060

0.149

-0.006

-CH3

0.020

0.060

-0.001

0.061

0.152

-0.008

-H

0.023

0.070

-0.001

0.055

0.147

-0.005

-CF3

0.018

0.055

-0.001

0.071

0.152

-0.015

It should also be noted that while the H(r) for the OPO-HBA1 bond does not change significantly between the four complexes, the same parameter does change for the OPO-HBA2 interaction. In fact, H(r) is higher for the two complexes with lower activation energies (when 2,6phenylboronic acid is substituted with either -CF3 and -CH3), and lower for the complexes with the higher activation barriers (when 2,6-phenylboronic acid is substituted with either -CF3 and CH3). Thus, our calculations indicate that the activation energies obtained with different substituents are related not with the strength of the two hydrogen-bonding interactions between the boronic acid and propylene oxide, but are only related to the strength of the OPO-HBA2 bond. 14 ACS Paragon Plus Environment

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Table 5 - NBO atomic charges of relevant atoms and moieties of the four complexes, at the reactant complex, obtained at the M06-2X/6-31+G(d,p) level of theory. C2,6-substituents

OBA2

Boronic acid group

-NH2

-0.441

0.291

-CH3

-0.433

0.313

-H

-0.437

0.331

-CF3

-0.423

0.335

Further information can be obtained by analyzing the NBO atomic charges of relevant moieties of the four complexes, at the reactant stage (Table 5). Boronic acids are weak Lewis acids in aqueous solutions, and their acidity have been correlated with the strength of their hydrogenbond donating (and subsequently, with their activity as catalyst in the ring-opening of epoxides).14,61,62 The acidity of a given molecule is related to the charge density of the ionizable group’s heteroatom, which in this case are OBA1 and OBA2.60,63-65 The reduction of negative charge on the target heteroatom makes its a worse base, thereby stabilizing the deprotonated species over the protonated one which increases the acidity of the molecule.60,63-65 As QTAIM analysis indicated that the most important hydroxyl group for the catalytic activity is that of OBA2-HBA2, we have now focused only on OBA2. The NBO charge of OBA2 decrease with increasing electron-withdrawing character of the C2,6-substituents (NH2 < CH3 < H < CF3), indicating that the acidity of this hydroxyl group increases with increasing electron-withdrawing character of the substituent. This correlates very well with the computed activation energies, and explains the NH2 > CH3 > CF3 trend. The exception is when the C2,6-substituents of phenylboronic acid are hydrogen atoms. Nevertheless, this can be explained by the charge density of the boronic acid group (composed by the boron heteroatom and the two hydroxyl groups, Table 5). The positive charge of this group correlates well with the variation of the activation energies. The increase of positive charge density in the boronic acid group is



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expected to increase the stability of the interaction with propylene oxide’s oxygen heteroatom via electrostatic interactions, given the electronegativity of this latter heteroatom. Table 6 - Activation ∆Eact, ∆Hact and ∆Gact (in kcal mol-1) for the ring-opening reaction of propylene oxide, in the presence of 2,6-dimethyl-phenyl boronic acid and different pyridinic bases. The activation energies were obtained with thermal corrections. The calculations were made at the M06-2X/6-31+G(d,p) level of theory, in implicit water. Pyridinic base

∆Eact

∆Hact

∆Gact

Pyridine

17.5

17.5

20.5

DMAP

19.1

19.1

22.1

While we have performed this study by using pyridine as the nucleophile, it is its derivative 4dimethylaminopyridine (DMAP) that has been frequently used as catalyst in CO2 conversion reactions.50,51 One of the reasons for this is that DMAP is more basic than pyridine, owning to the resonance stabilization from the dimethylamine group, which potentially increases its nucleophilicity. Given this, we have also assessed the differences originating from the use of either DMAP or pyridine in the catalysis of the ring-opening of propylene oxide (in the presence of CH3-substituted phenylboronic acid). Contrary to what could be expected, pyridine provides lower activation energies by about~1.7 kcal mol-1 than DMAP (as seen in Table 6), despite the latter molecule being more basic than the former. Nevertheless, the energetic difference between the two reactions is not very significant. The energetic difference can be explained by charge density reorganization induced by the dimethylamine group, at the reactants stage (Table 7). The addition of the electron-donating dimethylamine group increases the negative charge of the pyridinic nitrogen by 0.052e (in comparison with pyridine). This is expected to increase repulsive electrostatic interactions between the pyridinic nitrogen and the carbon atom that is the target of the nucleophilic attack (CPO), given the electronegative character of the latter atom. It should be noted that while CPO is electronegative, the propylene oxide by itself has positive charge in both reactions. 16 ACS Paragon Plus Environment

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Table 7 - NBO atomic charges of relevant atoms and moieties of the reactant complex, of the ring-opening reaction of propylene oxide in the presence of 2,6-dimethyl-phenyl boronic acid and different pyridinic bases, obtained at the M06-2X/6-31+G(d,p) level of theory. Pyridinic base

Pyridinic nitrogen

Propylene oxide

CPO

Pyridine

-0.510

0.024

-0.154

DMAP

-0.562

0.028

-0.155

Conclusions In this work was studied the ring-opening mechanism of a model epoxide (propylene oxide) when co-catalyzed by 2,6-phenylboronic acids and pyridinic bases, by employing theoretical methods based on density functional theory. This reaction is considered to be an important step in the conversion of CO2, a greenhouse gas that is a waste product of fossil fuel combustion, into value-added heterocyclic carbonates. Our calculations indicate that in the presence of these catalysts this reaction should occur under mild conditions, given the low activation barrier found (about ~20 kcal mol-1). Moreover, the activation energies of the catalyzed reactions are only about one third of the ones corresponding to the unimolecular ring-opening (about ~60 kcal mol-1). This decrease is the result of a cumulative effect, and not a truly synergistic one, induced by the catalytic properties of the pyridinic base and the boronic acid group. While both catalysts are needed for obtaining activation barriers of about ~20 kcal mol-1, pyridinic bases were found to be much better catalysts than boronic acids. The electronic character of substituents at C2,6-positions of phenylboronic acid has little effect on the catalytic properties of this molecule. Changing from a strong electron-donating substituent to a strong electron-withdrawing one only changed the activation barrier by 2.4 kcal mol-1. Nevertheless, our calculations indicate that smaller activation barriers can be obtained 17 ACS Paragon Plus Environment


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with an increasing electron-withdrawing character of the C2,6-substituents. They increase the acidity of the boronic acid group, via charge density reorganization, which increases the strength of the hydrogen-bonds that it forms with the epoxide molecule. Our calculations also indicate that non-halogenated pyridinic bases provide similar results to halide nucleophiles. Moreover, while contrary to what could be expected, more basic pyridine derivatives present worse results than unsubstituted pyridine. The addition of electron-donating substituents increase the negative charge of the pyridinic nitrogen heteroatom, thereby increasing repulsive electrostatic interactions with the target carbon atom of the epoxide.

Supporting Information Cartesian coordinates of relevant structures, obtained at the M06-2X/6-31+G(d,p) level of theory.

Author Information Corresponding author: [email protected]. Notes: The authors declare no competing financial interest.

Acknowledgement This work was made in the framework of the project Sustainable Advanced Materials (NORTE01-00145-FEDER-000028), funded by “Fundo Europeu de Desenvolvimento Regional (FEDER)”, through “Programa Operacional do Norte” (NORTE2020). Acknowledgment to project POCI-01-0145-FEDER-006980, funded by FEDER through COMPETE2020, is also made. The Laboratory for Computational Modeling of Environmental Pollutants-Human Interactions (LACOMEPHI) is acknowledged. Luís Pinto da Silva also acknowledges a Post-


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Doctoral grant funded by project Sustainable Advanced Materials (NORTE-01-00145-FEDER000028). Projects PTDC/QEQ-QFI/0289/2014 and PTDC/QEQ-QAN/5955/2014 are also acknowledged. These projects are co-funded by FCT/MEC (PIDDAC), and by FEDER through “COMPETE - Programa Operacional Fatores de Competitividade” (COMPETE-POFC).

References (1) Desens, W.; Werner, T. Convergent Activation Concept for CO2 Fixation in Carbonates. Adv. Synth. Catal. 2016, 358, 622-630. (2) Cokoja, M.; Wilhelm, M.E.; Anthofer, M.H.; Herrmann, W.A.; Kuhn, F.E. Synthesis of Cyclic Carbonates from Epoxides and Carbon Dioxide by Using Organocatalysts. ChemSusChem 2015, 8, 2436-2454. (3) North, M. Synthesis of cyclic carbonates from epoxides and carbon dioxide using bimetallic aluminium(salen) complexes. ARKIVOC 2012, (i), 610. (4) Buttner, H.; Steinbauer, J.; Wulf, C.; Dindaroglu, M.; Schmalz, H.G.; Werner, T. Organocatalyzed Synthesis of Oleochemical Carbonates from CO2 and Renewables. ChemSusChem 2016, 10, 1076-1079. (5) Fiorani, G.; Guo, W.; Kleij, A.W. Sustainable conversion of carbon dioxide: the advent of organocatalysis. Green Chem. 2015, 17, 1375-1389. (6) Olah, G.A.; Goeppert, A.; Prakash, G.K.S. Beyond Oil and Gas: The Methanol Economy, Wiley-VCH, Weinheim, 2009. (7) Orr, F.M. CO2 capture and storage: are we ready? Energy Environ. Sci. 2009, 2, 449458. (8) Alonso, D.M.; Bond, J.Q.; Dumesic, J.A. Catalytic conversion of biomass to biofuels. Green Chem. 2010, 12, 1493-1513. (9) Liu, Q.; Wu, L.; Jackstell, R.; Beller, M. Using carbon dioxide as a building block in organic synthesis. Nat. Commun. 2015, 6, 5933.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(10)

Page 20 of 26

Kondratenko, E.V.; Mul, G.; Baltrusaitis, J.; Larrazabal, G.O.; Perez-Ramirez,

J. Status and perspectives of CO2 conversion into fuels and chemicals by catalytic, photocatalytic and electrocatalytic processes. Energy Environ. Sci. 2013, 6, 3112-3135. (11)

He, L.N.; Wang, J.Q.; Wang, J.L. Carbon dioxide chemistry: Examples and

challenges in chemical utilization of carbon dioxide. Pure Appl. Chem. 2009, 81, 20692080. (12)

Aresta, M.; Dibenedetto, A.; Angelini, A. Catalysis for the Valorization of

Exhaust Carbon: from CO2 to Chemicals, Materials, and Fuels. Technological Use of CO2 Chem. Rev. 2014, 114, 1709-1742. (13)

Aresta, M. Carbon Dioxide as Chemical Feedstock, Wiley-VCH, Weinheim,

2010. (14)

Wang, J.; Zhang, Y. ACS Catal. 2016, 6, 4871.

(15)

Alves, M.; Grignard, B.; Gennen, S.; Mereau, R.; Detrembleur, C.; Jerome, C.;

Tassaing, T. Organocatalytic promoted coupling of carbon dioxide with epoxides: a rational investigation of the cocatalytic activity of various hydrogen bond donors. Catal. Sci. Technol. 2015, 5, 4636-4643. (16)

He, Q.; O’Brien, J.W.; Kiteselman, K.A.; Tompkins, L.E.; Kurtis, G.C.T.;

Kerton, F.M. Synthesis of cyclic carbonates from CO2 and epoxides using ionic liquids and related catalysts including choline chloride-metal halide mixtures. Catal. Sci. Technol. 2014, 4, 1513-1528. (17)

Xu, B.H.; Wang, J.Q.; Sun, J.; Huang, Y.; Zhang, J.P.; Zhang, X.P.; Zhang, S.J.

Fixation of CO2 into cyclic carbonates catalyzed by ionic liquids: a multi-scale approach. Green Chem. 2015, 17, 108-122. (18)

Sun, Q.; Jin, Y.; Aguila, B.; Meng, X.; Ma, S.; Xiao, F.S. Porous Ionic

Polymers as a Robust and Efficient Platform for Capture and Chemical Fixation of Atmospheric CO2 ChemSusChem 2017, 10, 1160-1165.


Page 21 of 26

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(19)

Toda, Y.; Komiyama, Y.; Kikuchi, A.; Suga, H. Tetraarylphosphonium Salt-

Catalyzed Carbon Dioxide Fixation at Atmospheric Pressure for the Synthesis of Cyclic Carbonates. ACS Catal. 2016, 6, 6906-6910. (20)

Zhang, Z.; Fan, F.; Xing, H.; Bao, Z.; Ren, Q. Efficient Synthesis of Cyclic

Carbonates from Atmospheric CO2 Using a Positive Charge Delocalized Ionic Liquid Catalyst. ACS Sustainable Chem. Eng. 2017, 5, 2841-2846. (21)

Sakakura, T.; Kohno, K. The synthesis of organic carbonates from carbon

dioxide. Chem. Commun. 2009, 1312-1330. (22)

Dai, W.L.; Luo, S.L.; Yin, S.F.; Au, C.T. The direct transformation of carbon

dioxide to organic carbonates over heterogeneous catalysts. Appl. Catal. A 2009, 366, 212. (23)

Crowther, O.; Keeny, D.; Moureu, D.M.; Meyer, B.; Salomon, M.;

Hendrickson, M. Electrolyte optimization for the primary lithium metal air battery using an oxygen selective membrane. J. Power Sources 2012, 202, 347-351. (24)

Schaeffner, B.; Schaeffner, F.; Verevkin, S.P.; Borner, A. Organic Carbonates

as Solvents in Synthesis and Catalysis. Chem. Rev. 2010, 110, 4554-4581. (25)

Cokoja, M.; Bruckmeier, C.; Rieger, B.; Hermann, W.A.; Kuhn, F.E.

Transformation of Carbon Dioxide with Homogeneous Transition-Metal Catalysts: A Molecular Solution to a Global Challenge? Angew. Chem. Int. Ed. 2011, 50, 8510-8537. (26)

Markewitz, P.; Kuckshinrichs, W.; Leitner, W.; Linssen, J.; Zapp, P.; Bongartz,

R.; Schreiber, A.; Muller, T.E. Worldwide innovations in the development of carbon capture technologies and the utilization of CO2. Energy Environ. Sci. 2012, 5, 72817305. (27)

Peters, M.; Kohler, B.; Kuckshinrichs, W.; Leitner, W.; Markewitz, P.; Muller,

T.E. Chemical Technologies for Exploiting and Recycling Carbon Dioxide into the Value Chain. ChemSusChem 2011, 4, 1216-1240.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(28)

Page 22 of 26

Cheng, W.; Fu, Z.; Wang, J.; Sun, J.; Zhang, S. Znbr2-based choline chloride

ionic liquid for efficient fixation of co2 to cyclic carbonate. Synth. Commun. 2012, 42, 2564-2573. (29)

Wilhelm, M.E.; Anthofer, M.H.; Reich, R.M.; D’Elia, V.; Basser, J.M.;

Hermann, W.A.; Cokoja, M.; Kuhn, F.E. Niobium(v) chloride and imidazolium bromides as efficient dual catalyst systems for the cycloaddition of carbon dioxide and propylene oxide. Catal. Sci. Technol. 2014, 4, 1638-1643. (30)

Martín, C.; Fiorani, G.; Kleij, A.W. Recent Advances in the Catalytic

Preparation of Cyclic Organic Carbonates. ACS Catal. 2015, 5, 1353-1370. (31)

Wu, Z.; Xie, H.; Yu, X.; Liu, E. Lignin-Based Green Catalyst for the Chemical

Fixation of Carbon Dioxide with Epoxides To Form Cyclic Carbonates under SolventFree Conditions. ChemCatChem 2013, 5, 1328-1333. (32)

Wang, L.; Zhang, G.; Kodama, K.; Hirose, T. An efficient metal- and solvent-

free organocatalytic system for chemical fixation of CO2 into cyclic carbonates under mild conditions. Green Chem. 2016, 18, 1229-1233. (33)

Wang, J.Q. Hydrogen Bond Donor-promoted Fixation of CO2 with Epoxides

into Cyclic Carbonates: Moving Forward. Curr. Green Chem. 2015, 2, 3-13. (34)

Foltran, S.; Mereau, R.; Tassaing, T. Theoretical study on the chemical fixation

of carbon dioxide with propylene oxide catalyzed by ammonium and guanidinium salts. Catal. Sci. Technol. 2014, 4, 1585-1597. (35)

Cho, W.; Shin, M.S.; Hwang, S.; Kim, H.; Kim, M.; Kim, J.G.; Kim, Y.

Tertiary amines: A new class of highly efficient organocatalysts for CO2 fixations. J. Ind. Eng. Chem. 2016, 44, 210-215. (36)

Gaussian 09, Revision D.01, M.J. Frisch et al., Gaussian, Inc., Wallingford CT,

2013. (37)

Zhao, Y.; Truhlar, D.G. The M06 suite of density functionals for main group

thermochemistry, thermochemical kinetics, noncovalent interactions, excited states, and


Page 23 of 26

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


transition elements: two new functionals and systematic testing of four M06-class functionals and 12 other functionals. Theor. Chem. Account 2008, 120, 215-241. (38)

Zhao, Y.; Truhlar, D.G. Density functionals with broad applicability in

chemistry. Acc. Chem. Res. 2008, 41, 157-167. (39)

Marenich, A.V.; Cramer, C.J.; Truhlar, D.G. Universal Solvation Model Based

on Solute Electron Density and on a Continuum Model of the Solvent Defined by the Bulk Dielectric Constant and Atomic Surface Tensions. J. Phys. Chem. B 2009, 113, 6378-6396. (40)

Peng, C.; Ayala, P.Y.; Schlegel, H.B.; Frisch, M.J. Using redundant internal

coordinates to optimize equilibrium geometries and transition states. J. Comput. Chem. 1996, 17, 49-56. (41)

Bader, R.F.W. Atoms in Molecules. Acc. Chem. Res. 1985, 18, 9-15.

(42)

Bader, R.F.W. A Quantum-Theory of Molecular-Structure and its Applications.

Chem. Rev. 1991, 91, 893-928. (43)

Bader, R.F.W. A bond path: A universal indicator of bonded interactions. J.

Phys. Chem. A, 1998, 102, 7314-7323. (44)

Lu, T.; Chen, F. Multiwfn: A multifunctional wavefunction analyzer J. Comput.

Chem. 2012, 33, 580-592. (45)

Yuen, A.; Bossion, A.; Gómez-Bengoa, E.; Ruipérez, F.; Isik, M.; Hedrick, J.L.;

Mecerreyes, D.; Yang, Y.Y.; Sardon, H. Room temperature synthesis of non-isocyanate polyurethanes (NIPUs) using highly reactive N-substituted 8-membered cyclic carbonates. Polym. Chem. 2016, 7, 2105-2111. (46)

Du, Q.; Wang, Z.; Schramm, V.L. Human DNMT1 transition state structure.

Proceed. Natl. Acad. Sci. U.S.A. 2016, 113, 2916-2921. (47)

Chiaccio, M.A.; Legnani, L.; Caramella, P.; Tejero, T.; Merino, P. Pivotal

Neighboring-Group Participation in Substitution versus Elimination Reactions Computational Evidence for Ion Pairs in the Thionation of Alcohols with Lawesson’s Reagent. Eur. J. Org. Chem. 2017, 1952-1960. 23 ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(48)

Page 24 of 26

Liu, C.; Besora, M.; Maseras, F. Computational Characterization of the Origin

of Selectivity in Cycloaddition Reactions Catalyzed by Phosphoric Acid Derivatives. Chem. Asian J. 2016, 11, 411-416. (49)

Cakir, K.; Erdem, S.S.; Atalay, V.E. ONIOM calculations on serotonin

degradation by monoamine oxidase B: insight into the oxidation mechanism and covalent reversible inhibition. Org. Biomol. Chem. 2016, 14, 9239-9259. (50)

Shen, Y.M.; Duan, W.L.; Shi, M. Phenol and organic bases co-catalyzed

chemical fixation of carbon dioxide with terminal epoxides to form cyclic carbonates. Adv. Synth. Catal. 2003, 345, 337-340. (51)

Sankar, M.; Tarte, N.H.; Manikandan, P. Effective catalytic system of zinc-

substituted polyoxometalate for cycloaddition of CO2 to epoxides. Appl. Catal. A 2004, 276, 217-222. (52)

Luo, Z.; Wang, B.; Liu, Y.; Gao, G.; Xia. F. Reaction mechanisms of carbon

dioxide, ethylene oxide and amines catalyzed by ionic liquids BmimBr and BmimOAc: a DFT study. Phys. Chem. Chem. Phys. 2016, 18, 27951-27957. (53)

Chen, C.; Ma, Y.; Zheng, D.; Wang, L.; Li, J.; Zhang, J.; He, H.; Zhang, S.

Insight into the role of weak interaction played in the fixation of CO2 catalyzed by the amino-functionalized imidazolium-based ionic liquids. Journal of CO2 Utilization 2017, 18, 156-163. (54)

Liu, M.; Guo, K.; Liang, L.; Wang, F.; Shi, L.; Sheng, L.; Sun, J. Insights into

hydrogen bond donor promoted fixation of carbon dioxide with epoxides catalyzed by ionic liquids. Phys. Chem. Chem. Phys. 2015, 17, 5959-5965. (55)

Maeda, C.; Shimonishi, J.; Miyazaki, R.; Hasegawa, J.; Ema, T. Highly Active

and Robust Metalloporphyrin Catalysts for the Synthesis of Cyclic Carbonates from a Broad Range of Epoxides and Carbon Dioxide. Chem. Eur. J. 2016, 22, 6556-6563. (56)

Monica, F.D.; Vummaleti, S.V.C.; Buonerba, A.; Nisi, A.D.; Monari, M.;

Milione, S.; Grassi, A.; Cavallo, L.; Capacchione, C. Coupling of Carbon Dioxide with Epoxides Efficiently Catalyzed by Thioether-Triphenolate Bimetallic Iron(III) 24 ACS Paragon Plus Environment

Page 25 of 26

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Complexes: Catalyst Structure–Reactivity Relationship and Mechanistic DFT Study. Adv. Synth. Catal. 2016, 358, 3231-3243. (57)

Shaik, S.; Danovich, D.; Wu, W.; Hiberty, P.C. Charge-shift bonding and its

manifestations in chemistry. Nat. Chem. 2009, 1, 443-449. (58)

Rozas, I.; Alkorta, I.; Elguero, J. Behavior of ylides containing N, O, and C

atoms as hydrogen bond accepters. J. Am. Chem. Soc. 2000, 122, 11154-11161. (59)

Grabowski, S.J.; Sokalski, W.A.S.; Leszczynski, J. How short can the H center

dot center dot center dot H intermolecular contact be? New findings that reveal the covalent nature of extremely strong interactions. J. Phys. Chem. A 2005, 109, 43314341. (60)

Pinto da Silva, L.; Esteves da Silva, J.C.G. Theoretical study of the

nontraditional enol-based photoacidity of firefly oxyluciferin. ChemPhysChem 2015, 16, 455-464. (61)

Tabélé, C.; Curti, C.; Kabri, Y.; Primas, N.; Vanelle, P. Cross-Coupling

Synthesis of Methylallyl Alkenes: Scope Extension and Mechanistic Study. Molecules 2015, 20, 22890-22899. (62)

Dimitrijevic, E.; Taylor, M.S. Organoboron Acids and Their Derivatives as

Catalysts for Organic Synthesis. ACS Catal. 2013, 3, 945-962. (63)

Pinto da Silva, L.; Simkovitch, R.; Huppert, D.; Esteves da Silva, J.C.G.

Combined experimental and theoretical study of the photochemistry of 4- and 3hydroxycoumarin. J. Photochem. Photobiol. A 2017, 338, 23-36. (64)

Agmon, N. Elementary steps in excited-state proton transfer. J. Phys. Chem. A

2005, 109, 13-35. (65)

Hynes, J.T.; Tran-Thi, T.H.; Granucci, G. Intermolecular photochemical proton

transfer in solution: new insights and perspectives. J. Photochem. Photobiol. A 2002, 154, 3-11.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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TOC Graphic


Theoretical Study of the Ring-Opening of Epoxides Catalyzed by

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