Mechanism of Sulfite-Driven, MeReO3-Catalyzed Deoxydehydration

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Mechanism of Sulfite-Driven, MeReO3‑Catalyzed Deoxydehydration of Glycols Peng Liu† and Kenneth M. Nicholas*,‡ †

Departments of Chemistry and Biochemistry, University of California at Los Angeles, Los Angeles, California, United States Departments of Chemistry and Biochemistry, University of Oklahoma, Norman, Oklahoma 73019, United States

‡

S Supporting Information *

ABSTRACT: The mechanism of the MeReO3-catalyzed deoxydehydration of glycols to olefins by sulfite salts has been investigated with density functional theory (DFT) calculations. Potential intermediates and transition states were evaluated for the three stages of the reaction: (a) dehydration of the glycol by an oxo−rhenium complex to form a Re− (O,O-glycolate), (b) sulfite-induced O transfer (to sulfate and a reduced oxo−Re), and (c) fragmentation of the ReV−glycolate to give the olefin and to regenerate MeReO3. Various sulfite, sulfate, and Na−sulfite/ sulfate species have been evaluated as reactants/products and as ligands. Alternative pathways have been analyzed differing in the order of steps a and b and whether and which sulfite/sulfate species are coordinated; the mechanism of sulfite association/sulfate loss has also been evaluated. Transition states and activation energies have been calculated for several of the key transformations, including H-transfer glycol dehydration, the LMeReVO(glycolate) fragmentations (L = H2O, NaSO3−, NaSO4−), and NaSO3− attack on oxo−ReVII species. The lowest energy catalytic pathway identified involves NaSO3− attack on an oxo oxygen of MeReO3 to produce MeReVO2(OSO3Na)− (22), glycol coordination by 22, followed by a series of H-transfer steps to ReO and/or Re−OSO3Na− to give MeReVO(glycolate)(OSO3Na)(H2O)− (26), concerted fragmentation of the ReV−glycolate 26 to olefin and MeReO3(OSO3Na)− (21), and dissociation of NaSO4− from 21 to regenerate MeReO3 (1). Fragmentation of the Re−glycolate 26 is turnover-limiting. The computational results are compared with available experimental observations for the MeReO3/sulfite and other DODH reactions as well as related oxo−metal-mediated O-transfer reactions and sulfite and phosphine oxidation.

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INTRODUCTION The drive to discover and to develop new chemical processes for the conversion of renewable biomass resources into chemicals and fuels has drawn attention to reactions that achieve selective deoxygenation of abundant, polyoxygenated feedstocks such as carbohydrates and polyols.1 The major efforts in this regard have focused on acid-catalyzed dehydration processes, some of which been developed with high efficiency to provide furan derivatives.2 Reductive processes, typically catalyzed by transition-metal compounds, that replace C−O bonds with C−H bonds (hydrodeoxygenation)3 and C−C unsaturation (deoxydehydration, DODH, eq 1) are also being actively investigated.

rhenium catalysts for the deoxydehydration reaction. Abu Omar described the MeReO3-catalyzed DODH of glycols and epoxides with hydrogen as the reductant.6 Bergman and Ellman reported the DODH of glycols by secondary alcohols promoted by Re2(CO)10 under aerobic conditions, producing the corresponding ketones as coproducts.7 Recently, Toste demonstrated that MeReO3 catalyzes DODH by secondary alcohols to unsaturated alcohols, unsaturated ethers, and polyenes,8 while Abu Omar reported on the MeReO3-catalyzed redox disproportionation of glycerol.9 Nicholas discovered that MeReO3 and perrhenate salts, ZReO4 (Z = Na+, NH4+, Bu4N+) catalyze DODH with sulfite salts as reductants.10 These investigations have led to the suggestion of a basic catalytic pathway involving three stages (Scheme 1): (1) an Otransfer redox reaction between the reductant and the oxo− ReVII species, (2) glycol condensation with an oxo−Re species to form a Re−glycolate, and (3) fragmentation (retrocyclization) of the ReV−glycolate to produce the olefin and to regenerate the oxo−ReVII species. Either sequence of the redox and dehydration steps 1 and 2 (path 1 vs 2) has been suggested to operate in the DODH process. Experimentally, it has been

The first metal-catalyzed DODH reaction was reported by Andrews employing arylphosphine reductants (eq 1, Red = PR3) and (C5Me5)ReO3 as the catalyst.4 Gable subsequently developed (tris-pyrazolylborate)ReO3 complexes as more robust DODH and epoxide deoxygenation catalysts.5 More recently, other groups have discovered new reductants and © 2013 American Chemical Society

Received: December 24, 2012 Published: February 26, 2013 1821

dx.doi.org/10.1021/om301251z | Organometallics 2013, 32, 1821−1831

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and (3) showed that the olefin was extruded with regeneration of MeReO3 upon heating to 80 °C. It was also noted that the catalytic conversion rates were dependent on the glycol structure and that addition of the sodium ion complexant, 15-crown-5, accelerates the Na2SO3-driven reactions. However, the identity and involvement of other nondetected intermediates, their relative stabilities, the transition states interconverting them, and the turnover-limiting step(s) are all uncertain and difficult to address experimentally because of the low solubility of the sulfite reductant in the organic solvents employed for the catalytic DODH reaction. The O-atom transfer redox component of the DODH process, especially with phosphine and sulfite reductants, bears upon other important oxo−metal-promoted processes. Oxo− molybdenum-catalyzed oxidation of phosphines by sulfoxides,16 for example, involves phosphine attack at the electrophilic oxo group and dissociation of the resulting phosphine oxide to produce the two-electron-reduced metal species. Recent mechanistic and computational studies of this O-transfer reaction have provided insight into the energetic and electronic features of the O-transfer process, which operates by a two-step mechanism involving phosphine attack on the oxo−metal oxygen followed by dissociation of the phosphine oxide.17 The O-transfer oxidation of sulfite to sulfate is catalyzed naturally by the sulfite oxidase molybdoenzyme18 and has been demonstrated in a few cases by synthetic oxo−Mo complexes.19 The details of the O-transfer process here are less well understood relative to the phosphine oxidation. Hille proposed that the enzyme-catalyzed reaction proceeds via S attack of sulfite on the electrophilic oxygen of OMo.20 Although there is experimental evidence for adduct formation between sulfite/ bisulfite and model oxo−Mo complexes,19 whether these adducts are derived from sulfite or bisulfite or if they involve addition at the Mo−oxo oxygen or at the metal is uncertain and has been considered computationally.21 Many of these same issues apply to the sulfite-driven DODH reaction as well. It was our objective in the present study to gain deeper insights into the mechanistic pathway of the sulfite-driven deoxydehydration of glycols catalyzed by MeReO3. In particular, we sought through computational analysis to identify the energetically accessible catalytic intermediates and key transition states and the energetics of their interconversions, and thus to identify the primary catalytic pathway and turnoverlimiting step(s) operating under experimental conditions.

Scheme 1

shown that PPh3 does deoxygenate Cp*ReO3 (path 2)11 and MeReO3 condenses with glycols to produce the ReVII−glycolate (path 1).12 However, in-depth experimental and theoretical mechanistic studies of these various DODH systems have been limited. Gable probed the mechanism of the Cp*− and (TPB)−Re(glycolate) fragmentation (cycloreversion) step experimentally and computationally,13 providing evidence for either an asynchronous concerted [3 + 2] process or a stepwise pathway via a metallaoxetane intermediate, depending on the steric and electronic nature of the glycol R groups. Several other studies of the reverse reaction, cycloaddition of oxo− metals to olefins, favor the concerted [3 + 2] process.14 Very recent computational studies of the MeReO3-catalyzed, H2-15a and alcohol-driven15b glycol DODH reactions found that the path 1 process is energetically favored for the former while the path 2 process is operative for the latter. In both cases the redox step was found to be turnover-limiting. Little is presently known about the mechanistic details of the sulfite-driven DODH reactions. During our experimental studies of these reactions we demonstrated the viability of the path 1 process promoted by MeReO3 by investigating relevant stoichiometic reactions (Scheme 2).10 Thus, we (1) confirmed the equilibrium formation (K = ca. 0.2) of MeReO2(glycolate) from the reaction of MeReO3 with styrene glycol at room temperature, (2) detected the conversion of the latter to a reduced glycolate upon reaction with PPh3 or sulfite,

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Scheme 2

COMPUTATIONAL METHODS

The geometries of all the complexes were optimized without constraints using DFT calculations with the Becke3LYP (B3LYP) functional.22 The following basis sets were used: 6-31G(d) for H, C, and O, 6-311+G(d,p) for Na and S, and LANL2DZ for Re.23 Frequency calculations were carried out to identify all energy minima structures (no imaginary frequencies) and transition states (one imaginary frequency; optimized using the Berny algorithm) and also to provide temperature-corrected Gibbs free energies and enthalpies at 298 K. To obtain energies of solvated complexes and transition states, a single-point (energy) calculation on the in vacuo optimized structure was carried out using the CPCM solvent model24 in benzene. Some transition state searches were guided by the mod-redundant method and followed by a full geometry optimization without constraints. Calculations were performed using the Gaussian 09 software package.25 Computations were carried out primarily on the OU Supercomputers Sooner and Boomer. Display graphics for structures were produced using the CYLview software.36 1822


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The SO32−-driven reaction is calculated to be strongly exergonic (−25.0 kcal/mol, benzene solvent), being favored both enthalpically (−17.0 kcal/mol) and entropically (two reactants/three products). A calculated heat of reaction based on experimental heats of formation for the sulfite/sulfate couple30 and standard state heats of formation of ethylene glycol, ethylene, and water31 gives a value of −11.9 kcal/mol, in reasonable agreement with the DFT-calculated exothermicity in benzene. The overall reaction free energy calculated with NaSO3− as the reductant is less favorable (−6.0 kcal/mol), its endothermicity (+3.1 kcal/mol) being counterbalanced by the positive overall entropy change. These results indicate NaSO3− to be a weaker reductant than SO32−, as is seen in the lower reduction potential of monoanionic HSO3− relative to dianionic SO32−.32

RESULTS AND DISCUSSION The hybrid B3LYP density functional approach was employed for addressing the structures and energetics involved in the sulfite-driven, MeReO3-catalyzed DODH reaction. A mixed basis set for first- and second-row atoms of 6-31G(d), 6311+G(d,p) for third row (Na, S), and the effective core potential basis set LANL2DZ for Re was used. The method and basis sets have been established in prior studies of related species to provide accurate structures and reasonable energies (±2 kcal/mol) for the type of closed-shell intermediates likely involved in the present system.13,15,26 We consider first the form of sulfite involved in the experimental reactions. The catalytic DODH reactions use Na2SO3 with aromatic solvents at 150 °C, conditions under which Na2SO3,4 have very low solubility. Even with water as a coproduct and employment of undried solvents, the water available for salt solvation is estimated to be only about 0.4% by volume. The crown ether, 15-crown-5, significantly accelerates the MTO-catalyzed DODH reactions,10 presumably by sodium ion complexation, increasing the concentration of sulfitederived ions. We have considered both SO32− (and SO42−) and the likely more soluble ion-paired monoanions NaSO3−/ NaSO4− as potential reactants/products in the calculations. The calculated structures of SO32−, C3v/trigonal pyramidal, and SO42−, Td/tetrahedral, agree well with prior computational results27 and experimental aqueous and solid state data (although often associated with water or M+).28 The high lattice energies of sulfite and sulfate salts make it very unlikely that the free dianions will exist in the gas phase or in lowpolarity organic solvents. Our calculated structures for the NaSO3,4− ion pairs are shown below. Each was found to have C3v symmetry with the Na ion associating in a tridentate fashion to the three oxygen atoms of the ion. The NaSO4− ion pair has been detected, and our calculated structure agrees quantitatively with the gas-phase experimental structure (by PES) and that calculated by Wang and Nicholas.29 The NaSO3− ion pair apparently has not been identified experimentally or computationally studied. The binding of the sodium cation in a tridentate fashion to the three electron-rich oxygen atoms would appear to be the most stable option on electrostatic grounds, and it allows for S-centered nucleophilic reactivity. We expect that these and other Na complex species will exist in wet organic reaction media with additional water molecules bound to the sodium ion, but we adopt this simpler model, expecting it will not have a significant impact on the relative energies of important intermediates.

We computationally evaluated various potential intermediates in the ethylene glycol/SO32− DODH reaction catalyzed by MeReO3, expanding in detail the steps involved in the basic reaction stages of dehydration (to form a Re−glycolate), redox O transfer, and Re−glycolate fragmentation to form an olefin (Scheme 3). Two basic pathways (cycles 1 and 2) were considered, differing in the order of the redox and the dehydration stages; both paths meet at the ReV−glycolates 7 and 9, which can fragment to regenerate 1 or its H2O complex 10. Geometries and Gibbs free energies at 298 K for species 1− 19 were calculated first in vacuo and then with single-point energy calculations with the CPCM model in benzene; all were energy minima with the approximate geometries shown. We will not discuss in detail the calculated geometries of all these structures, most of which have not been detected experimentally, but we point out generally that most are calculated to be five-coordinate species with distorted-square-pyramidal or trigonal-bipyramidal geometries. The strong Lewis acidity of MeReO3 (1) is illustrated by its formation of highly stable fiveand six-coordinated adducts with strong Lewis bases such as amines and phosphines33 and as calculated with sulfite as in 12. The calculated structure of ReVII−glycolate 5 is nearly congruent with the X-ray-characterized structure of MeReO2(OCMe2CMe2O) (bond lengths and angles within ±5%).34

Scheme 3 also shows the calculated energy differences between the various intermediates for each individual step, on the basis of their Gibbs free energies (kcal/mol) in benzene. In cycle 1 (dehydration/redox/fragmentation) the mildly endergonic dehydration phase (1−5) involves glycol coordination (1 → 2), followed by H transfer from the glycol −OH to ReO (2 → 3), favorable H transfer from 3 to aquo complex 4, and

Sulfite-Driven DODH. The DODH reaction thermodynamics with both SO32− and NaSO3− as reductants and ethylene glycol as substrate were estimated using B3LYPcalculated free energies of reactants and products (eqs 2 and 3). 1823


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Scheme 3

exergonic dissociation of water from 4 to give the ReVII− glycolate complex 5. Sulfite addition to glycolate complex 5 can result in coordination at Re (19) or at an oxo oxygen to produce the sulfate-coordinated ReV−glycolate 6; both conversions are very exergonic, but the latter is significantly more favorable. Sulfate dissociation from 6 to give the reduced glycolate complex 7 is highly endergonic, with the net Otransfer reduction from 5 to 7 being nearly thermoneutral (−1.2 kcal/mol). Alternatively, glycolate 6 could associate with H2O (to give 8) and then dissociate sulfate, very endergonically to produce aquo-coordinated, reduced glycolate complex 9. Fragmentation of 7 to MeReO3 (1) is calculated to be highly exoergic, whereas ethylene extrusion from the aquo complex 9 is significantly less favorable. The alternative cycle 2 (redox/ dehydration/fragmentation) begins with sulfite addition to 1; once again (Re)O attack to give the ReV−sulfate complex 11 is more favorable than attack at Re to form the ReVII−sulfato complex 12 by a substantial amount (10 kcal/mol). Sulfate dissociation from 11 to produce MeReO2 (15) is so highly endergonic as to be judged unrealistic in the catalytic process. A different pathway from sulfate complex 11, involving water association (to 13), sulfate dissociation (to 14), and glycol coordination to 16, is less costly energetically, but the sulfate dissociation step again appears to be energetically prohibitive. Subsequent stepwise glycol dehydration via the ReV complexes 17, 18, and 9 is moderately exergonic and more favorable than the dehydration process in the ReVII manifold of path 1. The highly exergonic sulfite association and very endergonic sulfate dissociation steps of the intermediate complexes (e.g., 5 → 6 → 7 and 1 → 11 → 15) suggests that sulfite and sulfate ions

would remain coordinated in various intermediates and not undergo unassisted dissociation to the free dianions. Transition structures and activation free energies were sought for several key steps in cycle 1 that are not substantially exergonic, beginning with the dehydration/glycolate formation process. The first of these is the H transfer from coordinated glycol −OH to ReO (2−3). The transition structure TS2−3 was found featuring a four-centered H transfer with a moderate activation barrier of 17.6 kcal/mol relative to 2. A comparably accessible transition state with ΔG⧧ = 18.3 kcal/mol from water complex 2·H2O can be located if H2O is incorporated to serve as a H shuttle in a six-membered arrangement as in TS2(H2O)−3. Smaller activation enthalpies for the H transfer to give aquo complex 4 and water dissociation to 5 are anticipated because of the exoergicity of these conversions and are supported by computational modeling of these steps by Bi and Lin in the hydrogen-driven, MeReO3-catalyzed deoxygenation of epoxides and DODH of glycols. 15a The computed barrier for dehydration in cycle 1 is much lower than the subsequent sulfate dissociation step (from 6 to 7 or from 8 to 9). Thus, we conclude that the dehydration stage of cycle 1 (from 1 to 5) is not turnover-limiting in the catalytic DODH reactions. This conclusion is consistent with the experimental observations of reversible, though slightly endergonic, formation of 5 from 1 + glycols at room temperature.10,12 The transition-state structures and activation energies for fragmentation of the ReV−glycolates 7 and 9 to produce ethylene and MeReO3 (1) were also evaluated. The concerted, synchronous transition structure TS7−1 was located for the fragmentation from glycolate 7 with a moderate ΔG⧧ value of 15.6 kcal/mol. Substantial and equal C−O bond lengthening 1824


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among them were conceived and their energies calculated in benzene medium for four catalytic cycles A−D involving NaSO 3 − /NaSO 4 −, differing according to the order of dehydration/redox stages and whether Re-coordinated NaSO3− or NaSO4− ions are involved. In cycle A (Scheme 4) dehydration to form a ReVII−glycolate proceeds first from MeReO3 (1) (without NaSO3− or NaSO4− Scheme 4. Cycle A and Re−O and C−C bond shortening along with significant planarization of the incipient ethylene fragment is indicative of a concerted electrocyclic process. We and others have observed experimentally that the facility of the DODH reactions is substrate-dependent across various catalysts and reductants, with styrene glycol reacting appreciably faster than other monoalkyl-substituted glycols;6−10 ethylene glycol itself has not yet been reported to undergo DODH. This substituent effect may derive from a stabilization of the transition state according to the stability of the developing olefin. The recent computational analysis of the hydrogenative DODH of styrene oxide and its glycol by MeReO3 gave a ΔHact value of 11.1 kcal/mol for the fragmentation step,15a consistent with transition-state stabilization by the developing conjugated olefin and the weaker benzylic C−O bond. We also find that fragmentation of the H2O-coordinated glycolate 9 proceeds through the symmetrical transition state TS9−10, but with a significantly higher activation barrier of 27.1 kcal/mol relative to the coordinatively unsaturated 7. A comparable increase of 10 kcal/ mol in ΔH⧧ for the corresponding aquo−styrene−Re− glycolate was reported.15a being coordinated), followed by the NaSO3− O-transfer association; this pathway is identical with cycle 1 in Scheme 3, except for attack by NaSO3− on the ReVII−glycolate 5 to produce the anionic NaSO4−-coordinated ReV−glycolate complex 20. The structure calculated for 20 features tridentate Na ion coordination to two sulfate oxygens and one of the glycolate oxygens. The isomeric adduct 20a of similar energy with Na coordination to two sulfate oxygens and the Re−O was also found. The S-based NaSO3− attack at the O−(Re) of 5 is highly exergonic, though substantially less so than for the corresponding reaction of SO32− with 5. On the other hand dissociation of NaSO4− from 20 or 20a to produce the fourcoordinate ReV−glycolate complex 7 is highly endergonic and hence is unlikely to proceed significantly under catalytic reaction conditions. In light of the very high barriers associated with the sulfatedissociating steps outlined in Scheme 3, it appears that neither cycle 1 nor cycle 2 is energetically viable for the operational sulfite-driven DODH catalytic pathway. Although an associative process for sulfite/sulfate substitution can be envisioned, the resulting tetraanionic intermediates or transition states would also likely be prohibitively high in energy (especially in nonpolar reaction solvents) and hence were not evaluated further. NaSO3-Driven DODH. The monoanionic ion pairs NaSO3− and NaSO4− and derived complexes appear to be more realistic species in terms of both solubility and energetic accessibility in low-polarity organic solvents, and hence these were also evaluated for their potential role in the sulfite-driven catalytic DODH reactions. Intermediates and interconversions

Fragmentation of the glycolate complex 20 to ethylene and NaSO4−-coordinated 21 is nearly thermoneutral. A transition state for this process, TS20−21, was located with a moderate ΔG⧧ of 33.2 kcal/mol with respect to 20. The long and unequal C−O distances for TS20−21 are indicative of a late, somewhat asynchronous transition state. The transition state TS20a‑21 1825


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derived from a higher energy isomeric complex 20a was also found, featuring a rather symmetrical glycolate geometry indicative of a more synchronous fragmentation and a comparable activation barrier of 32.0 kcal/mol with respect to 20a. The activation free energies for fragmentation of the NaSO4−-coordinated glycolates 20 and 20a are somewhat higher than that of the H2O-bound glycolate complex 9 (→10), and all three should be considered as energetically viable conversions at the catalytic reaction temperatures.

Scheme 6

and 34 are both viable intermediates in the DODH process, the less stable Re−SO3Na species 34 should readily convert to the more stable 22 under the moderately high temperatures (ca. 150 °C) of the catalytic reactions. Dissociative loss of NaSO4− from 22 to produce MeReO2 (15), however, is calculated to be energetically prohibitive (as was the case for free sulfate dissociation), rendering the subsequent dehydration sequence outlined in Scheme 5 inoperative under catalytic conditions, even though the individual steps leading to glycolate complex 9 are modestly exergonic and likely would have low energy barriers separating them. It is noteworthy that Na2SO3 does react with MeReO3 at elevated temperatures to produce a black unidentified species, which when heated with glycols can be converted to olefin.10 This indicates that a reduction first pathway may still be viable in the catalytic DODH process, but not likely passing through MeReO2 (15).35 The highly exergonic NaSO3− association with MeReO3 and highly endergonic NaSO4− dissociation led us to evaluate catalytic pathways and intermediates in which these ions remain coordinated. In cycle C (Scheme 7) the energetically favorable reductive addition of NaSO3− to MeReO3 (to 22) is followed

In Cycle B (Scheme 5) Re−O attack by NaSO3− on MeReO3 (1) precedes the dehydration sequence. The conversion of 1 to the sodium sulfate complex 22 is moderately exergonic, though less so than for the corresponding sulfite dianion addition. The alternative attack at rhenium to produce the S-coordinated ReVII complex 34 is less energetically favorable (by 4.0 kcal/ mol), indicating that the ReV−sulfate complex 22 is more stable than the isomeric sulfite complex 34. A mod-redundant search involving incremental approach of NaSO3− to MeReO3 failed to provide evidence of a transition structure leading to 22; therefore, it appears that this conversion would have little or no significant energy barrier. However, the potential interconversion of complexes 34 and 22 via an intramolecular process was suggested by the location of the transition structure TS34−22, in which the sulfite S is substantially slipped toward the O−Re unit in a product-like transition state with a low ΔG⧧ value of 12.0 kcal/mol (Scheme 6). Thus, although NaSO3− adducts 22 Scheme 5. Cycle B

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20a could fragment to ethylene and sulfate complex 21, as presented previously through a concerted process (TS20−21, TS20a‑21) with activation free energies of 30−32 kcal/mol. An alternative path to 21 was also considered in which both H2O and ethylene are lost simultaneously from the aquo−glycolate complex 26. The transition structure TS26−21 was located, which again indicated a concerted, relatively synchronous retrocyclization process with a moderate ΔG⧧ value of 33.0 kcal/mol, an activation barrier comparable to those for the 20/ 21 conversion and accessible under the experimental reaction conditions.

by glycol association and dehydration via stepwise H-transfer intermediates 23−25. Interestingly, the energy-minimized structure that results from glycol coordination to sulfate complex 22 is a bisulfate complex 23 (structure below): i.e., the glycol proton is transferred to a sulfate oxygen atom. The 22 to 23 conversion is moderately endergonic. An isomeric Re−hydroxo complex, 24, regains the typical sodium sulfate coordination mode and is almost isoenergetic with the bisulfate derivative 23 (and probably could form directly via glycol reaction with 22). Optimization of a structure for H transfer to form a Re−OH2 complex instead produced the hydroxo− glycolate complex 25 with the hydrogen relocated again to a bisulfite ligand as shown below; the 24 to 25 isomerization is substantially exergonic. The lowest energy species in cycle C was found to be the Na-coordinated, H-bonded aquo complex 26, likely the catalyst resting state in this pathway. Transition states for the H-transfer steps interconverting 22−26 were not located, but we anticipate that activation energies for these steps will be low, as found for the analogous, and less energetically favorable, glycol to O−Re H-transfer conversion of 2 to 3. Water dissociation from the aquo−glycolate complex 26 to the sulfato−glycolate complex 20 is moderately endergonic. The isomeric sulfato−glycolate complexes 20 and Scheme 7. Cycle C

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Scheme 8

Scheme 9. Cycle D

The product-forming NaSO4− dissociation from 21 to 1 to complete cycle C is substantially endergonic (+16.1 kcal/mol) but appears to have little additional activation barrier judging by the continuous scan of the energy profile as a function of the Re−OSO3Na distance. We also evaluated an associative pathway for the replacement of coordinated Na−sulfate by Na−sulfite, as outlined in Scheme 8. The attack of NaSO3− on an oxo−Re unit of 21 could produce the five-coordinate bissulfate complex 27 (below) in a moderately endergonic process. The optimized geometry of 27 is nearly symmetrical trigonal bipyramidal with the two NaSO4− units occupying axial positions on Re and each Na ion tetracoordinated to three sulfite oxygens and a Re−O. The transition structure TS21−27 was located for the Na−sulfite addition to 21 that shows partial S(sulfite)−O(Re) bond formation (2.1 Å) with an elongated Re−O(sulfate) bond (2.28 Å). The activation energy for the 21 to 27 conversion via TS21−27 was calculated to be a marginally viable 41.7 kcal/mol. The slightly more stable isomeric intermediate 27a and the corresponding transition state TS21−27a leading to it were also found, but the activation

barrier was only 2.6 kcal/mol less than for the 21−TS21−27 process. The loss of NaSO4− from intermediate 27 to produce four-coordinate sulfate complex 22 is quite exergonic and would likely have little or no barrier. The much lower energy required for the dissociative loss of NaSO3− from 21 (to 1) relative to the associative process to 22 via 27 (27a) indicates that the dissociative pathway from sulfate complex 21 to regenerate MeReO3 (1) is favored. Finally the catalytic pathway cycle D (Scheme 9) was considered, in which the sequence of redox−dehydration− fragmentation proceeds with NaSO3− being retained as a spectator ligand. In this pathway the reduced sulfate adduct 22 undergoes substitution by NaSO3− by an associative process via adduct 28 to give the sulfite complex 29. Overall, this substitution process is calculated to be moderately endergonic with the association step to form dianionic intermediate 28 being very unfavorable energetically. Although a transition state for this association step was not sought, it is anticipated to have a high barrier, perhaps 40−50 kcal/mol, considering the activation energy for the associative process involving the 1828


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reaction of NaSO3− with 21 to give 27 (Scheme 8). The dehydration sequence begins with glycol coordination to 29, followed by a modestly endoergic proton transfer to give the Re−hydroxo complex 31 and a second, more uphill proton transfer to produce the aquo−glycolate complex 32. Water dissociation from 32 to give the sulfite−glycolate complex 33 is calculated to be very exergonic. Transition states for each of these conversions were not sought, but the identification of low-energy proton-transfer transition states for the dehydration intermediates in path A (op. cit.) suggest that this part of cycle D would be energetically accessible under the conditions of the catalytic reaction. Finally, fragmentation of the sulfitecoordinated glycolate complex 33 to ethylene and sulfite derivative 34 is thermodynamically favorable. A transition state for this process, TS33−34, could be located that again is indicative of a concerted, nearly synchronous electrocyclic process. The calculated activation energy via TS33−34 is slightly greater (2 kcal/mol) than those determined for the fragmentations of the ReV−glycolates having water (TS9−10) or NaSO3− (TS20−21) as auxiliary ligands. Completion of cycle D could proceed in a stepwise fashion, via moderately endoergic dissociation of NaSO3− from 34 to give MeReO3 (1) followed by comparably exergonic NaSO3− addition to 1 to produce 22, or by a one-step, concerted Re-to-O slip of the sulfito unit (34 to 22), which, as indicated earlier (Scheme 6), has a low activation barrier of 12 kcal/mol. The most difficult steps of cycle D are thus the sulfite association with 22 to give complex 28 followed by the fragmentation of the glycolate complex 33. This sequence appears to be a marginally viable contributor to the catalytic DODH reaction with somewhat higher energy barriers than cycle C.

reactant (NaSO3− or SO32−) in the organic solvent is low and thus the redox step, which involves the sulfite species, may contribute to the observed rate, even if its rate constant (related to ΔG⧧) is smaller than that for glycolate fragmentation. It is noteworthy that computational analysis of the H2-mediated glycol DODH and epoxide deoxygenation catalyzed by MTO found the H2 reductive addition to MeReO2(glycolate) (5) to be rate-limiting with a calculated ΔH⧧ value of 34 kcal/mol.15a The computational study on MTO-catalyzed DODH with 3octanol indicated the reduction of MTO with alcohol is ratelimiting.15b It is apparent, therefore, that the nature of the reductant, its coordinating ability, and its mechanism of action may be critical to determining the facility of the DODH reactions. It would thus appear that the most active DODH catalyst/reductant pairs could be those that employ a low barrier reductant/catalyst combination in which the oxidized byproduct is weakly coordinating. Regarding the issue of sulfite (Na) attack at the Re center vs at a rhenium−oxo oxygen, our study shows that both processes are viable thermodynamically and kinetically: i.e., have low activation barriers. In all of the cases we investigated, the attack at an oxo oxygen to produce a ReV−sulfate derivative is favored thermodynamically. A frontier orbital analysis of the nucleophilic NaSO3− and electrophilic MeReO3 reactants (Scheme 10) shows that the former’s HOMO has significant contributions at both sulfur and at oxygen and the latter’s LUMO has Re−O π-antibonding character. Sulfite S attack at an oxo oxygen enables S−O bonding while contributing to a decrease in the Re−O bond order. Comparisons between the features of the present sulfite/MeReO3 reactions and the LMoO2-type molybdoenzymes and model complexes should be made cautiously, given the structural and electronic differences between these two classes of d0 complexes. We suggest that attack at Mo and at O should both be evaluated and that the typically hydrophobic enzyme active site and physiological pH could favor the involvement of monoanionic sulfite species such as NaSO3− and HSO3− (pKa = 7.2). Another interesting contrast can be drawn with the more extensively investigated O-transfer reactions of oxo−metal complexes, mostly of Mo, with phosphines. Recent mechanistic experimental and computational studies of these reactions point to the operation of a two-stage associative/dissociative process to achieve net O transfer, as indicated here for the sulfite Otransfer process. With most phosphines associative P−O adduct formation is rate-limiting (has a higher ΔE⧧ value and negative ΔS⧧ value), largely dominated by steric effects; the second dissociative step has a positive ΔS⧧ value and generally lower ΔE⧧ value.17 In contrast, our computations of the sulfite to sulfate O-transfer reaction with MeReO3 point to a substantially exoergic and low barrier sulfite addition step, followed by a more difficult (endoergic) dissociation of sulfate. This difference may be the combined result of the greater Lewis acidity and accessibility (lower coordination number) of MeReO3 relative to typical L3,4MoO2 complexes and the less hindered nature and likely greater nucleophilicity of anionic sulfite relative to tertiary phosphines. Our ongoing and future studies are focused on the discovery of new, more active, and economical catalysts and reductants for the DODH conversion of polyols, while gaining deeper insights into their catalytic mechanisms.

On comparison of the energetics of the putative catalytic cycles A−D, it is cycle C, which proceeds through initial reduction of 1 by NaSO3−, glycol dehydration, and Re− glycolate fragmentation, that is judged to provide the lowest energy reaction pathway. An energy profile incorporating the intermediates and transition states for cycle C is provided in Figure 1. The catalyst resting state is found to be aquo− glycolate complex 26. According to our analysis the turnoverlimiting (TOL) step for cycle C is the glycolate fragmentation, preferably from the aquo−glycolate 26, with a ΔG⧧ value of 33.0 kcal/mol or somewhat less favorably from the dehydrated glycolate complex 20. These conclusions are supported by the qualitative dependence of the DODH reaction rates on the glycol structure and the experimental operating temperatures of the sulfite-driven reactions.6−10 It was also found that the presence and nature of auxiliary ligands (L) on the MeReVO(glycolate)L complexes significantly affect the activation energies for their cycloreversion to olefin: L= none (15.6 kcal/mol), L = H2O (27.1 kcal/mol), L = NaSO4− (30−33 kcal/mol), L = NaSO3− (34.8 kcal/mol). We also note that in the sulfite-driven reactions the concentration of the sulfite 1829


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Figure 1. Free energy profile for the deoxydehydration of ethylene glycol promoted by MeReO3 with NaSO3− as the reductant according to cycle C. The relative Gibbs free energies are with respect to 1 and are given in kcal/mol. Soc. Rev. 2011, 40, 5266−5281. (c) Zhang, X.; Tu, M.; Paice, M. G. Bioeng. Res. 2011, 4, 246−257. (d) Naik, S. N.; Goud, V. V.; Rout, P. R.; Dalai, A. K. Renewable Sustainable Energy Rev. 2010, 14, 578−597. (e) Alonso, D. M.; Bond, J. Q.; Dumesic, J. A. Green Chem. 2010, 12, 1493−1513. (f) Marshall, A. L.; Alaimo, P. J. Chem. Eur. J. 2010, 16, 4970−4980. (2) (a) Mascal, M.; Nikitin, E. B. Green Chem. 2010, 12, 370−373. (b) Mascal, M.; Nikitin, E. B. ChemSusChem 2009, 2, 859−861. (c) Mascal, M.; Nikitin, E. B. Angew. Chem., Int. Ed. 2008, 47, 7924− 7926. (d) Grochowski, M. R.; Yang, W.; Sen, A. Chem. Eur. J. 2012, 18, 12363−12371. (e) Yang, W.; Grochowski, M. R.; Sen, A. ChemSusChem 2012, 5, 1218−1222. (3) (a) Stanowski, S.; Nicholas, K. M.; Srivastava, R. S. Organometallics 2012, 31, 515−518. (b) Schlaf, M.; Ghosh, P.; Fagan, P. J.; Hauptman, E.; Bullock, R. M. Adv. Synth. Catal. 2009, 351, 789−800. (c) Deutsch, K. L.; Lahr, D. G.; Shanks, B. H. Green Chem. 2012, 14, 1635−1642. (d) Yu, W.; Zhao, J.; Ma, H.; Miao, H.; Song, Q.; Xu, J. Appl. Catal., A: General 2010, 383, 73−78. (4) Cook, G. K.; Andrews, M. A. J. Am. Chem. Soc. 1996, 118, 9448 − 9449. (5) Gable, K. P.; Ross, B. Feedstocks for the Future; American Chemical Society: Washington, DC, 2006; ACS Symp. Ser. 921, pp 143−155 (6) Ziegler, J. E.; Zdilla, M. J.; Evans, A. J.; Abu-Omar, M. M. Inorg. Chem. 2009, 48, 9998−10000. (7) Arceo, E.; Ellman, J. A.; Bergman, R. G. J. Am. Chem. Soc. 2010, 132, 11408−11409. (8) Shiramizu, M.; Toste, F. D. Angew. Chem., Int. Ed. 2012, 51, 8082−8086. (9) Yi, J.; Liu, S.; Abu-Omar, M. M. ChemSusChem 2012, 5, 1401− 1404. (10) (a) Vkuturi, S.; Chapman, G.; Ahmad, I.; Nicholas, K. M. Inorg. Chem. 2010, 49, 4744−4746. (b) Ahmad, I.; Chapman, G.; Nicholas, K. M. Organometallics 2011, 30, 2810−2818. (11) Herrmann, W. A.; Serrano, R.; Kusthardt, U.; Guggolz, E.; Nuber, B.; Ziegler, M. J. Organomet. Chem. 1985, 287, 329−334. (12) (a) Takacs, J.; Cook, M. R.; Kiprof, P.; Kuchler, J. G.; Herrmann, W. A. Organometallics 1991, 10, 316−20. (b) Zhu, Z.; AlAjlouni, A. M.; Espenson, J. H. Inorg. Chem. 1996, 35, 1408−1409.

Scheme 10

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ASSOCIATED CONTENT

S Supporting Information *

Text giving the complete ref 25 and tables giving xyz coordinates and energies for all numbered structures and interconversions. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENTS We thank Prof. K. N. Houk (UCLA) for hosting K.M.N. during a sabbatical leave and for his mentorship of P.L. We also acknowledge the OU Supercomputing Center for Education and Research (OSCER) for providing access to the Sooner and Boomer supercomputers. We are grateful for financial support provided by the U.S. Department of Energy (K.N., DE11ER16276) and by the National Science Foundation (P.L., CHE-1059084).

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REFERENCES

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Mechanism of Sulfite-Driven, MeReO3-Catalyzed Deoxydehydration

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