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Perrhenate–Catalyzed Deoxydehydration of a Vicinal Diol: A Comparative DFT Study Jamaladin Shakeri, Hassan Hadadzadeh, Hossein Farrokhpour, Mohammad Joshaghani, and Matthias Weil J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.7b08884 • Publication Date (Web): 25 Oct 2017 Downloaded from http://pubs.acs.org on November 1, 2017

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The Journal of Physical Chemistry A

Perrhenate–Catalyzed Deoxydehydration of a Vicinal Diol: A Comparative DFT Study Jamaladin

Shakeri,a,b

Hadadzadeh,*a

Hassan

Hossein

Farrokhpour,*a

Mohammad

Joshaghani,b Matthias Weil c a

Department of Chemistry, Isfahan University of Technology, Isfahan 84156-83111, Iran

b

Faculty of Chemistry, Razi University, Kermanshah 67149, Iran

c

Institute of Chemical Technologies and Analytics, Division of Structural Chemistry, TU Wien,

Getreidemarkt 9/164-SC, A-1060 Vienna, Austria

Number of Pages: 30 Number of Figures: 5 Number of Schemes: 5 Number of Tables: 2 *Corresponding

authors,

Hassan Hadadzadeh Professor of Inorganic Chemistry Department of Chemistry, Isfahan University of Technology, Isfahan 84156-83111, Iran E-mail address: [email protected] Hossein Farrokhpour Associate Professor of Physical Chemistry Department of Chemistry, Isfahan University of Technology, Isfahan 84156-83111, Iran E-mail address: [email protected]

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Abstract Oxo–rhenium compounds, such as perrhenate salts, have demonstrated preferable activity in catalyzing the deoxydehydration (DODH) reaction in the presence of reductants. Here, the first computational details of the reported DODH mechanisms are presented using the density functional theory (DFT) (M06/6–311+G(d,p)/LANL2DZ) to investigate the conversion of a vicinal diol into the corresponding alkene by ReO4– as a catalyst. The DFT studies were carried out to evaluate the DODH mechanisms, from the energy point of view, for the conversion of phenyl–1,2–ethanediol to styrene by perrhenate anion in the presence of PPh3 as a reductant through a detailed comparison of two potential pathways including pathway A and pathway B. Pathway A includes the sequence of condensation of oxo–Re(VII) with diol before the reduction of Re(VII) to Re(V), whereas pathway B involves the reduction of oxo–Re(VII) to oxo–Re(V) before the condensation process. In pathway B, two basic routes (B1 and B2) are possible, which can take place through different reaction steps, including the extrusion of alkene from Re(V)– diolate in route B1, and the second reduction of the Re(V)–diolate by reductant and then the extrusion of alkene from the Re(III)–diolate intermediate in route B2. The intermediates and the Gibbs free energy changes, including ΔGog and ΔGosol, have been calculated for alternative pathways (A and B) in the gas and solvent (chlorobenzene and methanol) phases and compared to each other. In addition, the transition states and the activation energy barriers for two pathways (A and B) in the gas phase and in chlorobenzene have been calculated. The key transition states include the nucleophilic attack of PPh3 on an Re=O bond, the dissociation of OPPh3 from the rhenium moiety, the transfer of an H atom of diol to the oxo ligand in an oxo–Re bond through the condensation step, and the extrusion of styrene from the Re–diolate complexes. The DFT results indicate that the DODH reaction is thermodynamically feasible through both pathways (A and B).



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However, the calculations reveal that the perrhenate–catalyzed DODH reaction through pathway A has the lowest overall activation barrier energy among the DODH mechanism routes.

Introduction The transformation of renewable biomass–derived compounds to valuable chemicals and fuels has attracted increased attention for the development of new chemical processes in matter of selective chemical conversions.1-7 Much efforts have been focused on the dehydration8,9 and hydrodeoxygenation10-12 processes, and in particular, the deoxydehydration (DODH) of vicinal hydroxyl groups into unsaturated carbon–carbon bonds (alkenes) in the presence of a reductant and a catalyst.13-20 However, one fundamental challenge in this field is that the hydroxyl group removal requires the development of selective oxygen–removal processes for the dehydration and/or reduction (deoxygenation). In fact, the DODH reaction can be regarded as an alternative effective approach in combining two discrete reactions (dehydration and deoxygenation) to remove two adjacent hydroxyl groups from vicinal diols to afford alkenes. The DODH reaction has been widely studied since the early report by Cook and Andrews21 who employed aryl phosphine (PAr3) as the reductant and (C5Me5)ReO3 as the catalyst to convert diols and polyols into the corresponding alkenes. A number of researchers have expanded the range of DODH reactions that use high–valent oxo–rhenium catalysts, such as Cp*ReO3 (Cp* = 1,2,3,4,5–


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pentamethylcyclopentadienyl),21 CptttReO3 (Cpttt = 1,2,4–tri(tert–butyl)cyclopentadienyl),22 Tp*ReO3 (Tp* = hydrido–tris–(3,5–dimethylpyrazolyl)borate),23 MeReO3 (MTO),24 and Z+ReO4− (Z = Na, NH4, and Bu4N)24 along with reductants including PPh3,13 H2,25,26 Na2SO3,27 dihydroaromatic compounds,28 alcohols,29-33 and even elemental reductants (Zn, Fe, Mn, or C) (eq 1).34

The high oxidation state of rhenium in the employed oxo–rhenium compounds has been established as an important and capable catalyst for deoxydehydration reactions. However, inexpensive catalysts such as oxo–molybdenum35 and oxo–vanadium36 compounds have been also used in DODH reaction. There are many mechanistic theoretical studies on the various oxygen atom transfer (OAT) reactions

with

oxo–rhenium

catalysts

including

dehydration,

deoxygenation,

and

deoxydehydration reactions in literature.25,33 The mechanistic aspects of the rhenium–catalyzed alcohol to alkenes dehydration reaction have been investigated by Gebbink and co–workers.33 The mechanism of MTO–catalyzed deoxygenation of epoxides to alkenes has been studied with the aid of density functional theory (DFT) calculations.25 The mechanism of deoxydehydration reaction of diols and polyols by oxo–rhenium complexes has been recently of considerable attention.29-31 The first mechanistic scheme of such DODH reactions was suggested by Cook and Andrews.21 Wang's group30 have performed a computational study on the mechanism of DODH reaction and compared it with other mechanisms proposed by Toste et al.37 and Abu−Omar et al..38 A detailed theoretical DFT study on the DODH mechanism of the conversion of glycols to alkenes by MTO and sulfite salts has been investigated by Nicholas's research group.39 It should be noted that the previously proposed mechanisms29,37–39 for dehydration and deoxydehydration reactions involve 4 ACS Paragon Plus Environment


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only organometallic oxo–rhenium catalysts. There is no a theoretical report about the dehydration and deoxydehydration mechanisms in the presence of perrhenate (ReO4−) anion as the catalyst. In this work, for the first time, the theoretical details of the reported DODH mechanisms are presented for the perrhenate–catalyzed DODH reaction of a diol. Taking into consideration all the above, a detailed DFT computational study was carried out to evaluate the reported DODH mechanisms using the perrhenate anion, so that two potential pathways (A and B) are proposed for the DODH reaction and compared to each other. In particular, we have used DFT methods to identify the details related to the catalytic processes including the catalytic intermediates, key transition states, and activation barrier energies.

Experimental Computational details All calculations were carried out using the GAUSSIAN 09 suite of programs.40 The molecular structures of the considered complexes involved in the mechanisms were fully optimized without any symmetry restrictions in both gas and solvent phases using DFT calculations with the M06 functional.41 The 6–31+G(d) basis–set for the light atoms (C, H, and O), the 6–311+G(d,p) basis– set for the P atom, and the effective core potential (ECP), LANL2DZ, for the Re atom were used in the frequency calculations. The frequency calculations at the same level of theory have been also performed to identify all stationary points as minima (zero imaginary frequencies) or transition states (one imaginary frequency) and to provide temperature–corrected Gibbs free energies at 298.15 K. For further confirmation of the transition state structures, the intrinsic reaction coordinate (IRC) calculations were performed at the same level of theory. The PCM model was used for modelling the solvent in the frequency calculation (chlorobenzene and


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methanol). The energies of the frontier molecular orbitals (FMOs) for some intermediates, and the atomic charges were also determined at the same level of theory.

Results and discussion Theoretical Calculations As mentioned before, the mechanism of the oxo–rhenium catalyzed DODH reaction has been investigated both experimentally42 and computationally29-33 by numerous researchers but, there are still some vague points in the mechanism of the DODH systems which are clarified in this part and explained in detail in the next sections. It has been generally accepted that the DODH reaction by oxo–rhenium catalyzed includes three steps: condensation, condensing diol and the rhenium atom; reduction, reducing Re(VII) to Re(V); and extrusion, the division of Re–diolate into alkene and oxo–rhenium accompanied with an oxidative addition to the rhenium center. Proposed Mechanisms On the basis of previous studies,29-31 two potential pathways have been considered in this work; pathway A: first, the condensation of oxo–rhenium (VII) with the diol and formation of Re(VII)– diolate, then reduction of the rhenium atom by the reductant (Re(VII) to Re(V)), and finally, the extrusion of an alkene from Re(V)−diolate. This pathway, in brief, means condensation → reduction → extrusion (Scheme 1).



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Scheme 1. Pathway A: Proposed reaction mechanism for the DODH reaction of phenyl–1,2– ethanediol to styrene by ReO4– and PPh3 as the catalyst and the reductant, respectively.

Pathway B: first, the reduction of rhenium(VII) to rhenium(V) and then, the condensation of the diol with the Re atom, in brief, means: reduction → condensation. In pathway B, two basic routes (B1 and B2) are possible (Scheme 2) that can proceed through different reactions, i.e., the division of Re(V)–diolate into alkene and an Re(VII) complex (step V in Scheme 2) (means: reduction → condensation → extrusion); followed by a second reduction of Re(V)–diolate by PPh3, and then the extrusion of alkene from Re(III)–diolate (step VI in Scheme 2) (means: reduction → condensation → reduction → extrusion). Route B1 is based on the oxidative extrusion of an alkene from the Re(V)–diolate intermediate under the formation of an initial oxo– 7 ACS Paragon Plus Environment

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rhenium (VII) catalyst, but route B2 coincides with the second reduction of Re(V)–diolate that results to form the Re(III)–diolate intermediate.

Scheme 2. Pathway B: Proposed reaction mechanism for the DODH reaction of phenyl–1,2– ethanediol to styrene by ReO4– and PPh3 as the catalyst and the reductant, respectively.

DFT Evaluation of the Proposed Mechanisms The DFT calculations were used to evaluate the DODH mechanism for the conversion of phenyl–1,2–ethanediol into styrene in the presence of perrhenate as the catalyst and PPh3 as the reductant through a detailed comparison of the two potential pathways (Pathway A and Pathway 8 ACS Paragon Plus Environment


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B). The structures of all species in their ground electronic states were separately optimized in both gas phase and solution (chlorobenzene and methanol). The pathways shown in Schemes 1 and 2 are separately discussed, before the explanations about the corresponding transition states. The gas (Gogas) and the solvation (ΔGosolv) Gibbs free energies for each species in both pathways are summarized in Tables S1 and S2. The method for the calculation of ΔG in the solvent has been previously described in details.43,44 Pathway A: The proposed structures of the optimized key intermediates of pathway A in the solvent phase are schematically depicted in Scheme 3. Scheme 3. Optimized structures of the species involved in the pathway A in the solvent phase.


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In pathway A, the first step of the profile is the condensation of perrhenate with phenyl–1,2– ethanediol to form the Re(VII)–diolate intermediate (3), that takes place through two steps I and II by transfer of two H atoms of phenyl–1,2–ethanediol to the same oxo ligand in perrhenate, leading to the formation of H2O and 3. Considering the electrostatic effect of solvent, the first dehydration (step I) is predicted to be nonspontaneous by +31.63 kcal/mol, while the second dehydration reaction (step II) is spontaneous by –13.28 kcal/mol (Table 1). The structure of Re(VII)–diolate 3 is in good agreement with the crystal structure of ReO3(diolate) has reported by Sillanpää' group (bond lengths and bond angles within ±5%).45,46

Table 1. Calculated values of ΔGog (gas-phase Gibbs free energy changes, kcal/mol), and ΔGosol (solvent-phase (chlorobenzene and methanol) Gibbs free energy changes, kcal/mol) of each step. Pathway A Step

ΔGog

ΔGosol Chlorobenzene

Pathway B ΔGosol Methanol

Step

ΔGog

ΔGosol Chlorobenzene

ΔGosol Methanol

I

+26.31

+31.63

+30.55

I[b]

+52.94

+57.69

+33.43

II

–9.39

–13.28

–13.24

II[b]

–24.02

–42.14

–21.67

III

+21. 17

+31.53

+10.64

III[b]

–22.35

–16.48

–17.19

IV

–27.35

–44.02

–26.28

IV[b]

+2.10

+5.05

+6.32

V[a]

–30.53

–28.26

–24.89

VI

+25.54

+35.49

+37.48

V′[a]

+2.68

+7.03

+9.61

VII

–27.85

–31.34

–33.49

VI′[a]

–32.39

–35.29

–33.32

VIII

–1.18

–11.32

–13.23

[a] Steps V, V′ and VI′ in pathway A and route B1 are same. [b] Steps I, II, III, and IV in route B1 and route B2 are same.

The nucleophilic attack of PPh3 on the oxo–Re(VII) moiety of 3 gives the five–coordinate intermediate (4) in a nonspontaneous process (ΔGosol = +31.53 kcal/mol). The optimized geometry 10 ACS Paragon Plus Environment


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of intermediate 4 is nearly trigonal–bipyramidal with the OPPh3 unit occupying the axial position and the oxo–ligands occupying the equatorial positions (Scheme 3). The dissociation of the intermediate 4 yields phosphine oxide (OPPh3) as well as a four–coordinated Re(V)–diolate 5 with a tetrahedral configuration. This process is entropically favored, because one species converts to two species and the theoretical calculations show that this process is spontaneous in the solvent phase (ΔGosol = –44.02 kcal/mol). One of the most curious subjects to be clarified in the DODH process is the mechanism of the extrusion of the alkene from Re(V)–diolate, which regenerates the initial catalyst. There are various ways for the extrusion (cycloreversion) of styrene from the corresponding ReO2–diolate intermediate 5. According to the prevouse proposed mechanisms,4752

we arrive at the choice between a reverse [3+2] cycloaddition (concerted retro–[3+2] process)

and formation of a metallaoxetane with a reverse [2+2] cycloaddition (methylene migration). In the concerted process, the extrusion of the alkene from Re(V)–diolate (5) yields styrene as well as initial catalyst 1 with ΔGosol = –28.26 kcal/mol (step V), but in the methylene migration process, firstly, intermediate 5 converts to Re(VII)–metallaoxetane (6) and then the extrusion of alkene occurs. The DFT calculations, performed in our work, reveal that this net process is spontaneous by a significant driving force (ΔGosol = +7.03, and –35.29 kcal/mol in the steps V′ and VI′, respectively). In addition, previous studies have reported that the solvent has substantial effects on the rate of conversion,27 for example, the DODH reaction in polar and/or hydroxylic solvents are considerably slower. However, a very low (

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