Revised Mechanisms for Aldehyde ... - ACS Publications

Jun 12, 2018 - ABSTRACT: It is widely believed that the Shvo catalyst (1) dissociates to form ... inactive for ester hydrogenation and for the accepto...
10 downloads 0 Views 2MB Size
Download PDF
Research Article Cite This: ACS Catal. 2018, 8, 6851−6861

pubs.acs.org/acscatalysis

Revised Mechanisms for Aldehyde Disproportionation and the Related Reactions of the Shvo Catalyst Dmitry G. Gusev*,† and Denis M. Spasyuk‡ †

Department of Chemistry and Biochemistry, Wilfrid Laurier University, Waterloo, Ontario N2L 3C5, Canada Canadian Light Source, 44 Innovation Boulevard, Saskatoon, Saskatchewan S7N 2V3, Canada

‡

Downloaded via FORDHAM UNIV on June 30, 2018 at 06:31:18 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

S Supporting Information *

ABSTRACT: It is widely believed that the Shvo catalyst (1) dissociates to form two active species in solution: the 18electron hydride RuH(CO)2[η5-C5(OH)Ph4] (2) and the naked 16-electron complex Ru(CO)2[η4-C5(O)Ph4] (3). This combined experimental/computational study demonstrates that a sustained presence of 3 is not viable in the reactions of alcohols and organic carbonyls; thus, 3 is better treated as nonexistent under the typical catalytic conditions. We propose a modified view where the key catalytic species are the hydride 2 and the 18-electron metal alkoxide intermediate Ru(OR)(CO)2[η5-C5(OH)Ph4] existing in equilibrium with the corresponding alcohol complex. An X-ray crystallographic study of 2 revealed an interesting dihydrogen-bonded dimer structure in the solid state. The mechanistic ideas of this paper explain the highly efficient Tishchenko-like aldehyde disproportionation reaction with the Shvo catalyst. Additionally, our observations explain why 1 is inefficient for hydrogenation of ethyl acetate and for the acceptorless dehydrogenative coupling of ethanol. Our findings provide practical guidance for future catalyst design on the basis of the Shvo ruthenium dimer prototype. KEYWORDS: Shvo catalyst, catalytic mechanisms, catalytic hydrogenation, catalytic dehydrogenation, aldehyde disproportionation

■

INTRODUCTION

Scheme 2. Formation of 2 and 3 from the Shvo Catalyst

1−3

4−7

The discovery and extensive subsequent investigations of the Shvo catalyst led to the development of the original ideas8−13 of metal−ligand cooperation in outer-sphere catalytic reactions. These ideas envisaged a process where the catalyst cycles between an 18-electron metal hydride and a coordinatively unsaturated (16-electron) intermediate according to Scheme 1. Thus, the diruthenium complex 1 is commonly viewed as a precursor to complexes 2 (18-electron) and 3 (16-electron, Scheme 2) under the catalytic conditions, where the 18-electron species hydrogenates organic carbonyls (aldehydes and ketones), whereas the 16-electron species can act as a dehydrogenation catalyst.4−7

Complex 2 could not be crystallized; however, the available NMR data are consistent with the proposed structure.14 On the other hand, 3 has never been observed. A dimer of 3, complex 4 in Scheme 3, was isolated and structurally characterized.15 Scheme 3. Structure of Diruthenium Complex 4

Scheme 1. Original Interpretation of the Bifunctional Outer-Sphere Catalytic Hydrogenation

Received: March 22, 2018 Revised: May 23, 2018 Published: June 12, 2018 © XXXX American Chemical Society

6851

DOI: 10.1021/acscatal.8b01153 ACS Catal. 2018, 8, 6851−6861

Research Article

ACS Catalysis

(these and other key NMR spectra are provided with the Supporting Information). Solvent removal followed by trituration of the residue with hexane afforded a yellow solid (1.41 g, 77%) of 2 containing traces of THF (7.5 mol %) and hexane (4 mol %). This material was insoluble in alkanes (hexane, isooctane); in other solvents (CD2Cl2, toluene, THF, Et2O, EtOAc, EtOH) the solubility of 2 appeared unlimited. NMR spectra of the freshly prepared solutions of 2 under Ar displayed no decomposition. Slow diffusion of hexane into a solution of 50 mg of 2 in 0.3 mL of toluene produced a mixture of dark red crystals of 1 and yellow crystals of 2, analyzed by X-ray crystallography. The structure of 1 obtained in our work displays only marginal differences in comparison with the known structure19 of the Shvo catalyst (see the Supporting Information for details), whereas complex 2 forms an unusual Ci-symmetrical dimer where the ruthenium fragments are connected by two intermolecular RuH···HO dihydrogen bonds as shown in Figure 1. The intermolecular RuH···HO distances are 1.97 Å in the X-ray structure or 1.75 Å when the crystallographic Ru−H and O−H bond distances of 1.54 and 0.76 Å are corrected to the more realistic values of 1.61 and 0.97 Å predicted by DFT calculations for 2 (vide infra). Other structural parameters of 2 are regular;20 the principal bond distances and angles are given in the caption of Figure 1. It has been known that 2 is hydrogen-bonded in solution; however, the proposed structural interpretation invoked a regular intermolecular OH···OH hydrogen bond between the molecules.21 Reactions of 1 and 2. Complex 2 very slowly eliminates H2 to give 1 in solution, and vice versa.9,22 Reaching an equilibrium, 1 + H2 ⇌ 2 2, takes ca. 3 days at 60 °C under 1 atm of H2 in toluene-d8, regardless of whether the reaction starts from 2 or from 1. Integration of the 1H NMR resonances at 4.51 (H2), −9.42 (2), and −17.83 ppm (1) gave the equilibrium constant K = [2]2[1]−1[H2]−1 = 0.57 (from 2), 0.58 (from 1) and ΔG333 = 0.4 kcal/mol (total metal concentration, [Ru] = 0.01 M). An alternative calculation using [H2] = p(H2) = 1 atm gave K = 0.0024 (from 1), 0.0026 mol L−1 atm−1 (from 2) and ΔG333 = 4.0 kcal/mol. This latter way of deriving K is more prone to errors since it requires accurate [1], [2], and p(H2) values. However, the difference in ΔG333 (3.6 kcal/mol) seems to be mostly due to the entropy of gaseous H2 being greater than the entropy of the dissolved H2 by 12 cal/(mol K) in toluene23 (TΔS = 4 kcal/mol at 333 K). Further measurements of the equilibrium constants gave the data of Figure 2 and the estimates of the reaction enthalpy and entropy: ΔH = 5.9 ± 0.5 kcal/mol and ΔS = 16.5 ± 1 cal/(mol K) (per mole of 1). The equilibrium 1 + H2 ⇌ 2 2 is affected by ethanol. The equilibrium constant K = [2]2[1]−1[H2]−1 increased 3-fold, from 0.58 to 1.81, at 60 °C with [EtOH]/[Ru] = 22 (0.22 M EtOH in toluene-d8, under 1 atm of H2). Furthermore, two oCH resonances of 2 shifted in the proton spectrum (see Figure 3) as well as the RuH resonance (from −9.42 to −9.34 ppm), whereas dimer 1 was little affected. These observations indicate that 2 is stabilized by hydrogen bonding with ethanol, like the related hydride RuH(CO)2[η5-C5(OH)Ph2Tol2] that exists as a hydrogen-bonded dimer in toluene-d8, yet becomes monomeric in this solvent in the presence of ethanol.21 Most, if not all, reported studies with the Shvo catalyst have been conducted at ≥60 °C. To investigate whether 1 is inert at room temperature, dimer 1 (0.005 M) was reacted with PPh3 or C5D5N (0.3 M each) in toluene-d8. Slow reactions ensued,

Viewing 3 as a key catalytic intermediate has significant implications for catalyst design and for the development of catalytic processes. For example, Menashe and Shvo reported a highly efficient method for the synthesis of symmetrical esters from aldehydes in the presence of a catalytic amount of the corresponding alcohols, according to Scheme 4.16 In Scheme 4. Catalytic Ester Synthesis with 1

comparison with the related Tishchenko reaction,17 the Shvo process is about 2 orders of magnitude more efficient and afforded unprecedented turnover numbers (TON), up to 20000. It was postulated that the catalytic amount of alcohol in Scheme 4 was required to produce the hemiacetal intermediate, to be dehydrogenated by 3 according to Scheme 5.16 Scheme 5. Mechanism of Ester Formation Proposed by Menashe and Shvo

We undertook this present study to get a better understanding of the catalytic properties and behavior of complexes 1−3 and to gain more information about their speciation in solution in the presence of several representative species (H2, PPh3, pyridine, ethanol, acetaldehyde). Among the principal results presented and discussed below, we have demonstrated that 1 is surprisingly efficient for the catalytic ester synthesis of Scheme 4 without added alcohol. We explain this observation with the help of a modified view of the bifunctional reactions where the catalyst cycles between the 18-electron metal hydride 2 and an 18-electron metal alkoxide intermediate. We also, for the first time, crystallized and structurally characterized complex 2, which forms an unusual dimer supported by two dihydrogen bonds. Finally, a DFT theoretical study of complexes 2 and 3 was carried out that provided new mechanistic insights, particularly into why the Shvo catalyst is inactive for ester hydrogenation and for the acceptorless dehydrogenative coupling of primary alcohols.

■

EXPERIMENTAL OBSERVATIONS Isolation and Structure of 2. Complex 2 forms from 1 under H2.18 In THF (12 h at 100 °C, p(H2) = 50 bar), this reaction afforded a clear solution that displayed 1H and 13 C{1H} NMR spectra consistent with the reported data for 2 6852

DOI: 10.1021/acscatal.8b01153 ACS Catal. 2018, 8, 6851−6861

Research Article

ACS Catalysis

Figure 1. Structure of 2 with the thermal ellipsoids at 50% probability. Most hydrogen atoms are omitted for clarity. Selected bond distances (Å) and angles (deg): Ru1−C1 2.294(2), Ru1−C2 2.256(2), Ru1−C3 2.253(2), Ru1−C4 2.266(2), Ru1−C5 2.257(1), Ru1−C30 1.876(2), Ru1− C31 1.881(2), Ru1−H1Ru 1.54 (3), C30−O2 1.143(2), C31−O3 1.139(2), C1−O1 1.348(2), O1−H1O 0.76(3), C1−C2 1.430(2), C1−C5 1.420(2), C2−C3 1.457(2), C3−C4 1.426(2), C4−C5 1.452(2), Ru1−X 1.907, Ru1−Ru1 6.004(1), X−Ru1−C30 128.7, X−Ru1−C31 131.3, X− Ru1−H1Ru 92.5, C30−Ru1−C31 92.5(1), C30−Ru1−H1Ru 88(1), C31−Ru1−H1Ru 81(1), where X is the centroid of the C5 ring. The RuH··· HO dihydrogen bond distance of 1.75 Å was obtained after changing the crystallographic Ru−H and O−H distances to 1.611 and 0.974 Å, respectively, found in the DFT optimized structure of 2.

toward Ru(C5D5N)(CO)2[η4-C5(O)Ph4] (6);24 both products formed along with equal amounts of 2. The known complexes 515,25 and 626 were conveniently monitored by following their distinct o-CH 1H resonances at 7.80 and 8.10 ppm, respectively. The 31P resonance of 5 appeared at 38 ppm. These experiments reveal that the Shvo catalyst 1 is reactive with two-electron donors at room temperature. In the next part, we present further observations of room-temperature reactions of 1 with acetaldehyde and ethanol, presumably involving formation of the unstable intermediates Ru(CH3CHO)(CO)2[η4-C5(O)Ph4] (7) and Ru(EtOH)(CO)2[η4-C5(O)Ph4] (8) together with 2. Catalytic Reactions. We first tested reactions of Scheme 6a,b: hydrogenation of ethyl acetate and acceptorless

Figure 2. Plot of R ln K vs 1/T for the equilibrium 1 + H2 ⇌ 2 2. Data used: K = [2]2[1]−1[H2]−1 = 0.36 at 43.5 °C, 0.58 at 60 °C, 0.83 at 74.5 °C, and 1.07 at 85 °C.

Scheme 6. Catalytic Reactions of Ethanol and Ethyl Acetate

dehydrogenative coupling of ethanol. Only traces of the products were observed: ca. 2 turnovers of ethyl acetate to ethanol and 12 turnovers of ethanol to ethyl acetate. We also tested the dehydrogenative coupling of ethanol to ethyl acetate with added acetone, according to Scheme 6c. This reaction gave 11% ethanol conversion and produced a 1:2 mixture of ethyl acetate and 2-propanol as expected. A trace amount of acetaldehyde was also seen as the characteristic quartet at 9.73

Figure 3. 400 MHz 1H{13C} NMR spectra of a mixture of 1 and 2 (total [Ru] = 0.01 M) fully equilibrated at 60 °C under 1 atm of H2 in toluene-d8 with and without added ethanol. Three solvent resonances are seen at 6.98, 7.02, and 7.10 ppm.

e.g. affording (in 3 h) 11% conversion of 1 toward Ru(PPh3)(CO)2[η4-C5(O)Ph4] (5) and 15% conversion 6853

DOI: 10.1021/acscatal.8b01153 ACS Catal. 2018, 8, 6851−6861

Research Article

ACS Catalysis

1 (δ(RuH) −17.83), formation of a mixture of 1 and 2 (δ(RuH) −17.87, −9.34) was apparent in the sample with added ethanol. Resonances of acetaldehyde were also seen at 9.13 (q) and 1.42 ppm (d, 3J = 2.8 Hz). Figure 4 illustrates the 1 H NMR spectra.

ppm (J = 2.8 Hz) in the 1H NMR spectrum of the reaction solution. In related experiments, we established that 2 was stable in neat ethyl acetate and gave 1H NMR spectra that did not change even after heating the sample at 80 °C for 20 min, in agreement with the observed lack of catalytic hydrogenation activity. In addition, complex 2 dissolved without decomposition in neat ethanol at 24 °C. Upon heating to 80 °C, some hydrogen loss from 2 and precipitation of 1 (insoluble in ethanol) were apparent; however, even after 1 h at 80 °C, ca. 60% of the initial amount of 2 persisted in solution and no resonances other than those of 2 and ethanol were evident in the NMR spectra. Our observations indicate that extrusion of H2 gas from ethanol solutions of 2 is too slow for efficient acceptorless dehydrogenation. With these observations in hand, we proceeded to the principal part of this investigation: the Shvo ester synthesis of Scheme 4. The original publication stated that, in the presence of 1, the reaction “was found to be an efficient process, provided a catalytic amount of alcohol is present in the reaction solution”.16 Since no data are available on the ester formation without added alcohol, we decided to investigate a typical processthe catalytic synthesis of ethyl acetate with 1 and 2 in a 1:1 mixture of anhydrous acetaldehyde and toluene (total 0.2 mol), using the metal concentration [Ru] = 10−5 M and the substrate to ruthenium ratio [CH3CHO]/[Ru] = 104. In these experiments, the Shvo catalyst 1 afforded TON = 3995 in 24 h at 60 °C. Complex 1 also produced ethyl acetate at 24 °C, albeit at a slower rate, reaching TON = 2468, 4962, and 7033 in 7, 14, and 21 days, respectively. This roomtemperature reaction accelerated from TOF = 156 per day−1 after 24 h to 242 per day−1 in 2 days. The initial rate of ethyl acetate formation was faster with 2 as the catalyst, affording 294 turnovers after 24 h at 24 °C (TON = 5186 after 15 days). Addition of 5 mol % ethanol (0.3 M) further accelerated the production of ethyl acetate with 1 to give TON = 4379 in 24 h at room temperature. The recorded values are accurate to ±2%. A minor byproduct was noted to form along with ethyl acetate in all experiments: e.g., giving TON = ca. 200 after 2 weeks with both 1 and 2 at 24 °C. While attempting isolation of this compound, we distilled a fraction containing the byproduct in toluene that could be unambiguously characterized by NMR spectroscopy. The NMR data are summarized in Scheme 7 and match that of a known compound, 1ethoxyethyl acetate.27,28 To investigate reasons behind the acceleration of the synthesis of ethyl acetate by added ethanol, we prepared two 0.005 M toluene-d8 solutions of 1, one with ethanol (0.33 M, EtOH/1 = 66) and the other in neat toluene-d8, as a reference. While the latter produced the expected 1H NMR spectrum of

Figure 4. Partial 400 MHz 1H{13C} NMR spectra of 1 (top trace) in toluene-d8 and of a mixture of 1 and 2 formed in toluene-d8 with ethanol after 4 h at 24 °C.

The 2/1 ratio increased as 0.011 (0.5 h), 0.027 (1 h), 0.052 (2 h), 0.059 (3 h), 0.063 (4 h), and further to 0.099 after 3 days at 24 °C. A trace of ethyl acetate formed in the sample after 3 days. Our observations indicate that an equilibrium was established in toluene-d8: 1 + EtOH ⇌ 2 2 + CH3CHO. Estimates of the equilibrium constant after 4 h and 3 days gave ΔG = 11.0 and 10.1 kcal/mol, respectively. The latter value might be underestimated because of the secondary changes in solutionformation of ethyl acetate (an exothermic process, from acetaldehyde). We note a relationship between the formation of 2 from 1 under H2 and formation of 2 from 1 and ethanol. Whereas molecular hydrogen is directly available in the former reaction, dimer 1 dehydrogenates ethanol in the latter. The Gibbs energy of ethanol dehydrogenation can be estimated as ΔG = 10.1 kcal/mol at 25 °C, on the basis of the known enthalpy of reduction of acetaldehyde (−19.4 kcal/mol, in triglyme)29 combined with the standard molar entropies of the species.30 Thus, formation of 2 from 1 and ethanol is more endergonic in comparison to the reaction of 1 and H2 by ca. 10 kcal/mol because of the extra energy needed for the extrusion of H2 from ethanol. In summary, efficient catalytic conversion of acetaldehyde to ethyl acetate with 1 does not require added ethanol. The acceleration of the catalytic synthesis of ethyl acetate with added ethanol appears to be linked with the increase of [2] formed from 1 and ethanol. The lack of observable 3 or 4 in Figure 4 (bottom trace) agrees with the previously documented15 “instantaneous, quantitative, and selective formation” of 1 upon addition of ethanol to a toluene solution of 4. In practical terms, 3 is not compatible with ethanol; therefore, assuming a sustained presence of 3 in catalytic solutions containing alcohols is unrealistic.

Scheme 7. Byproduct of Ethyl Acetate Synthesis from Acetaldehydea

a

NMR data in toluene. 6854

DOI: 10.1021/acscatal.8b01153 ACS Catal. 2018, 8, 6851−6861

Research Article

ACS Catalysis

■

Scheme 8. Ethyl Acetate Formation/Hydrogenolysis with 2a

DFT COMPUTATIONAL DATA Reactions of complexes 1 - 3. We next turn to DFT methods to learn more about the stability and reactions of complexes 2 and 3. We first probed the stability of 3 in the presence of acetaldehyde and ethanol. Considering that the Shvo catalyst is typically used at elevated temperatures, the reaction Gibbs energies of Chart 1 were calculated at 60 °C. Chart 1. Calculated MN15-L Gibbs Energies of Formation of 4, 7, and 8 from 3 at 60 °C in Toluene Continuuma

a

All values are per mole of 3.

The observations of Chart 1 are straightforward: there is a strong thermodynamic preference of 3 to dimerize. Formation of the acetaldehyde and ethanol complexes 7 and 8 is less favorable; however, even the least exoergic reaction, toward 7, has ΔG = −9.9 kcal/mol and the associated equilibrium constant K = [7][3]−1[acetaldehyde]−1 > 106. It is not surprising that complex 3 has never been observed. Scheme 8 depicts the mechanism of acetaldehyde conversion to ethyl acetate with complex 2: 2 CH3CHO → EtOAc, ΔH° = −22.7 kcal/mol, ΔG° = −18.2 kcal/mol.30 The reverse (counterclockwise) process of Scheme 8 is ester hydrogenolysis: EtOAc + H2(1 atm) → EtOH + CH3CHO, ΔH° = +2.3 kcal/mol, ΔG° = +7.1 kcal/mol.30 Correspondingly, two sets of Gibbs energies are given in the scheme: one for the forward reaction of ester synthesis and the other (in parentheses) for the reverse reaction. The interpretation of Scheme 8 is based on the mechanistic ideas proposed by Grützmacher for a rhodium catalyst,31 and it is guided by the results of our recent work on aldehyde disproportionation catalyzed by pincer complexes of osmium.32,33 Related mechanisms were also considered by Hasanayn for the Milstein catalyst.34,35 The ester formation of Scheme 8 starts by the familiar concerted H2 transfer from 2 to acetaldehyde via TS1 to give Int 1, leading to the ethanol complex 8 (ΔG = −3.7 kcal/mol). The related ethoxide Int 2 is less stable than 8 by 8.6 kcal/mol; this preference for Ru(0) in 8 vs Ru(II) in Int 2 seems unusual in ruthenium chemistry, where Ru(II) compounds abound. The ethoxide Int 2 further reacts with acetaldehyde in an outer-sphere fashion in steps III and IV to give the relatively stable hemiacetal intermediate Int 3. After internal rotation of the hemiacetal, the final product is formed by H2 transfer in steps V and VI to regenerate catalyst 2. The bottleneck of the catalytic ester synthesis without added ethanol must be the low concentration of 2 in solution, most likely produced by the substitution: 1 + acetaldehyde ⇌ 2 + 7. Formation of ethyl acetate is relatively fast only because of the low barriers ΔG⧧ = 12.1 kcal/mol (ΔH⧧ = 5.9 kcal/mol) and ΔG⧧ = 13.9 kcal/mol (ΔH⧧ = 5.5 kcal/mol) from 2 to Int 1 and from 8 to TS2, respectively. Another potential problem is the possibility of ethanol displacement from 8 by acetaldehyde to give 7. This reaction is endergonic (see the energies of

a

The Gibbs energies (kcal/mol) are given for the clockwise process, 2 + acetaldehyde, and (in parentheses) for the reverse reaction, 2 + ethyl acetate. The origin of the energies (in toluene continuum, at 298.15 K) is 2 with acetaldehyde or 2 with ethyl acetate, for the forward and the reverse reactions, respectively; mass balance is ensured throughout.

Chart 1); however, when [acetaldehyde]/[Ru] = 104, termination of the cycle by ethanol displacement from 8 is a certain concern. The observed 28-fold acceleration of the room-temperature synthesis of ethyl acetate with 5 mol % EtOH can be rationalized as a combined effect of the increased concentration of the catalyst 2 produced from 1 with ethanol, plus certain stabilization of 8 by the added ethanol. The ideas of Scheme 8 also explain formation of the 1ethoxyethyl acetate byproduct. Int 3 can rearrange to give a 1ethoxyethoxide Ru(II) intermediate (analogous to the ethoxide Int 2) followed by an outer-sphere addition of acetaldehyde via a transition state similar to TS2 to give a 1-(1-ethoxyethoxy)ethanol Ru(0) intermediate. Dehydrogenation of the latter should produce 1-ethoxyethyl acetate. Since the principal reaction (toward ethyl acetate) must be fast via the low-energy TS3, the lifetime of Int 3 is most likely short; thus, the 1-(1ethoxyethoxy)ethanol Ru(0) intermediate forms infrequently. Along this line, polymerization of aldehydes might be a feasible process with metal alkoxide catalysts not capable of efficient hydride abstraction. The reverse (counterclockwise) process of Scheme 8 is the hydrogenolysis of ethyl acetate to ethanol and acetaldehyde. This organic reaction is endergonic; therefore, an efficient catalytic reduction of ethyl acetate to give 2 equiv of ethanol requires rapid consumption of the intermediate acetaldehyde. Acetaldehyde hydrogenation should not be a problem with 2; thus, the reason 2 is inefficient for the reduction of ethyl acetate is most likely because of the slow formation and/or 6855

DOI: 10.1021/acscatal.8b01153 ACS Catal. 2018, 8, 6851−6861

Research Article

ACS Catalysis

above 2·EtOH; therefore, the concentration of H2 in the reaction solution must be extremely low. It therefore seems that the acceptorless dehydrogenation of ethanol with 2 might be severely hindered due to slow diffusion of the trace amounts of H2 from the liquid into the gas phase. In this regard, we may again recall the SNS catalyst RuCl 2 (PPh 3 )[HN(CH 2 CH 2 SEt) 2 ] that forms RuH(OEt)(PPh 3 )[HN(CH2CH2SEt)2] when it is treated with NaOEt in ethanol.36 This catalytic system gave TON = 8900 for the conversion of ethanol to ethyl acetate in 24 h, under reflux.36 In conclusion, the instability of the ruthenium ethoxide Ru(OEt)(CO)2[η5C5(OH)Ph4] (Int 2 of Scheme 8) seems to be a problem in the catalytic reactions with 1 or 2.

splitting of the intermediate hemiacetal. The hemiacetal intermediate Int 3 is not prohibitively high at 12.3 kcal/mol; however, the reaction encounters the high-energy TS2 next, at 28.7 kcal/mol. The alternative elimination of 1-ethoxyethanol from Int 3 to give 3 is 27.6 kcal/mol uphill from ethyl acetate with 2. When possible reasons for the high-energy TS2 in the hydrogenation of ethyl acetate are analyzed, it is worth having a look at the products of reaction step III (from TS2): Int 2 and acetaldehyde. At this point, the reaction path is still at 23.5 kcal/mol and this relatively high energy is due to the unusual feature of the Shvo system where the Ru(II) ethoxide is unstable. In contrast, the Noyori-type complexes (i.e., those possessing a primary or secondary amino group on ruthenium) form Ru(II) alkoxides; for example, we isolated and structurally characterized a highly efficient ester hydrogenation catalyst, RuH(OEt)(PPh3)[HN(CH2CH2SEt)2].36 One can speculate that if the ethoxide Int 2 had the stability of the ethanol complex 8, then TS2 could have been stabilized proportionally, down to near 20 kcal/mol, and the ester reduction might have been possible with the Shvo catalyst. It is further instructive to look at the acceptorless dehydrogenation of ethanol of Scheme 9 proceeding by

■

DISCUSSION Discoveries of the Shvo and Noyori catalysts at the end of the past century inspired the development of the original ideas of metal−ligand cooperation in the catalytic outer-sphere hydrogenation reactions.38 As was mentioned in the Introduction, these ideas envisaged a process where the catalyst cycles between an 18-electron metal hydride and a coordinatively unsaturated (16-electron) intermediate according to Scheme 1. The mechanism of Scheme 1 captured much attention, perhaps not in the least because of the unconventional nature of the reaction where the substrate does not have to bind to the metal for catalysis. The elegant simplicity of a concerted transfer of a hydride and a proton in Scheme 1 has been appealing to chemists. Intuitively, it “makes sense” for the product alcohol to form accompanied by a 16-electron catalytic intermediate. The flaw in this thinking is apparent in the case of the Shvo catalyst when we recall that 3 rapidly reacts with ethanol. Thus, e.g., acetaldehyde hydrogenation cannot produce ethanol alongside the naked 16-electron 3. Since 2005, the ideas of Scheme 1 have undergone gradual revisions for the Noyori-type catalysts possessing a protic N− H group on the metal. The experimental studies by Bergens,39−43 Baratta,44 and their co-workers demonstrated that the Noyori-type hydrogenation catalysts undergo insertion with ketones in stoichiometric reactions, leading to the corresponding alkoxides. DFT calculations incorporating solvent effects established that the Noyori-type catalysts act via discrete H−/H+ transfer steps, via a zwitterionic alkoxide intermediate.31,45−50 These computational studies also suggested that the heterolytic splitting of H2 occurs between the metal and the oxygen of the alkoxide ion.47−51 Throughout the catalytic reaction, the protic N−H group is engaged via N− H···O hydrogen bonding, providing important stabilization to the transition states encountered in the H−/H+ transfer steps. A modern view of the outer-sphere catalytic hydrogenation can be summarized with the help of Scheme 10 (top). Several features of this scheme are distinctly different from the original interpretation of Scheme 1. The cooperating protic ligand [L]H is a catalyst accessory that first facilitates the hydride to carbonyl transfer via hydrogen bonding to the carbonyl oxygen, [L]-H···O and then stabilizes the alkoxide during the heterolytic activation of H2. The metal alkoxide intermediate of Scheme 10 is in equilibrium with the corresponding alcohol complex. This equilibrium may be important for the intramolecular rearrangement of the alkoxide ligand, as discussed in our recent paper33 and by others.31 The reverse of the hydrogenation process is the acceptorless dehydrogenation of an alcohol in Scheme 10 (bottom) that starts by the familiar protonation of the metal hydride by the alcohol,

Scheme 9. Acceptorless Ethanol Dehydrogenation with 2a

a The Gibbs energies are per mole of 2·EtOH, calculated with the MN15-L functional at 298.15 K in ethanol continuum, with H2 dissolved in ethanol.

protonation of the hydride by ethanol, followed by H2 elimination and catalyst regeneration via ethanol dehydrogenation. The calculated barriers are relatively high (22.3−24.6 kcal/mol). We know that the reaction of Scheme 9 should be feasible from 8 to 2·EtOH and acetaldehyde, on the basis of the observation of a successful transfer dehydrogenation of ethanol in the presence of acetone. Johnson and Bäckvall reported the rate-limiting barrier ΔG⧧ = 22.2 kcal/mol for the dehydrogenation of 1-(4-fluorophenyl)ethanol with 1 at 70 °C.37 The overall organic transformation EtOH → H2 + CH3CHO is endergonic; therefore, the catalytic dehydrogenation requires removal of H2 from the reaction mixture. A salient feature of Scheme 9 is the absence of a favorable (i.e., accompanied by a negative Gibbs energy) intermediate. The most stable species on the reaction path (8) is 6.3 kcal/mol 6856

DOI: 10.1021/acscatal.8b01153 ACS Catal. 2018, 8, 6851−6861

Research Article

ACS Catalysis

not require the free 1-ethoxyethanol intermediate in the catalytic solution and the reaction naturally does not need the 16-electron dehydrogenation catalyst 3. Traditionally, formation of 3 has been viewed as inherent and necessary in the Shvo chemistry. In consequence, attempts to improve the Shvo catalyst were focused on making the ligands on ruthenium bulkier, to stabilize and protect the 16electron ruthenium through increased sterics against reverting to the precatalyst 1 with 2. The observations of Schemes 8 and 9 allow seeing the fundamental problem of the Shvo catalyst in a completely different light: the Achilles heel of the system is the instability of the alkoxide Int 2. If reactions of 2 with carbonyls (or alcohols) led to the corresponding Ru(II) alkoxides, the formation of 1 would have been avoided. A lesser vulnerability of the Shvo catalytic system is the thermal instability of 2. This is not a real problem under H2 or in the presence of an alcohol but can lead to 1 in the catalytic disproportionation of aldehydes without added alcohol.

Scheme 10. Modern Interpretation of the Outer-Sphere Catalytic (De)hydrogenationa

■

CONCLUSIONS Chemists generally cannot change the properties and reactivity of molecules at will. Also, some changes even when possible may be prohibitively expensive for practical catalytic applications. Therefore, it is an open question as to whether (and how) the Shvo catalyst can be improved in future work. The objective, however, can be tentatively formulated as follows: it may be desirable of the next-generation Shvo 18electron hydride catalyst, when it is reacted with an alcohol such as ethanol, to give the corresponding 18-electron Ru(II) alkoxide and H2 accompanied by a reaction Gibbs energy near zero, e.g., within ±2 kcal/mol. This idea is based on the expectation that an ideal “all-in-one” catalyst broadly applicable and efficient for hydrogenations of aldehydes, ketones, and esters, as well as for the dehydrogenative coupling of alcohols and the coupling reactions of aldehydes to esters and (with primary amines) to amides, should maintain sufficient operating concentrations of the Ru(II) hydride and alkoxide intermediates in the reaction solution. It will be appropriate for the search for the next generation of the Shvo catalyst to be guided by DFT calculations.

a R and R′ can be a hydrogen atom or a hydrocarbon group. Dotted lines to the protic (red) hydrogen atoms indicate hydrogen bonds.

leading to formation and elimination of H2. When they are applied to the Shvo catalyst, the ideas of Scheme 10 can be expressed with the help of Scheme 11. We note two features of the latter that are special: (a) the H2 transfer is concerted with the Shvo catalyst49 and (b) the Ru(II) intermediates are relatively unstable vs Ru(0). Scheme 11. (De)hydrogenation Reactions of the Shvo Catalyst

■

MATERIALS AND METHODS

Experimental Details. The Shvo catalyst 1 was obtained from Strem Chemicals. All other chemicals were purchased from Sigma-Aldrich. Anhydrous-grade acetaldehyde was redistilled under argon and stored at −28 °C (this material contained 0.13 mol % of trimethyltrioxane formed at low temperature during storage). Ethyl acetate and toluene were filtered through activated basic alumina under argon. Toluened8 and ethanol were stored with 3 Å molecular sieves. The NMR spectra were collected on a Agilent DD2 400 MHz spectrometer. For quantitative integration, the 1H{13C} NMR spectra were obtained using 15° pulses and relaxation delays of 30−60 s (depending on the nature of the samples). The choice of the relaxation delay was guided by the 1H T1 relaxation times measured under argon in toluene, at room temperature as follows: toluene (CH3 3.3 s, CH ca. 20 s), ethyl acetate (CH2 13.0 s, COCH3 5.2 s, CH3 9.8 s), acetaldehyde (CH ca. 50 s, CH3 ca. 19 s), complex 1 (RuH 2.2 s, CH 1.6−1.7 s), complex 2 (RuH 3.9 s, CH 2.8 s). Preparation of Complex 2. A 1.8 g portion of 1 (1.66 mmol) in 29 mL of anhydrous THF was pressurized to 50 bar

Choosing among the mechanisms of Schemes 1, 10, and 11 is not only a problem of theory; there are practical implications of making the right choice. The importance of viewing catalysts as cycling between the 18-electron hydride and the alkoxide structures of Scheme 10/11 becomes apparent when the scope of the bifunctional outer-sphere catalytic reactions is extended to include the mechanistically related ester hydrogenation and the catalytic coupling of aldehydes leading to esters and (with primary amines52) to amides. These reactions can be consistently rationalized only with the alkoxides as catalytic intermediates, such as in Scheme 8. As we have proved here, acetaldehyde disproportionation with the Shvo catalyst does 6857

DOI: 10.1021/acscatal.8b01153 ACS Catal. 2018, 8, 6851−6861

Research Article

ACS Catalysis

bath preheated to 100 °C on a hot plate stirrer. This temperature was maintained for 10 h with stirring at 500 rpm. Acceptorless Dehydrogenative Coupling. In an argon glovebox, 11.5 mg of 2 was placed in a 50 mL Schlenk tube (also containing a micro stirbar) on a calibrated analytical balance accurate to 0.1 mg. Next, 9.59 g of ethanol was added. After the tube was taken out of the box, it was connected to a vacuum/Ar manifold. Under argon, the stopper was replaced by a finger condenser connected to a circulating refrigerated bath. When the temperature in the bath reached −5 °C, the flask was placed in a bath, the argon tank was closed, and the gas released as the bath temperature increased to 90 °C was allowed to pass freely through a mineral oil bubbler. Throughout the reaction, the temperature in the cold finger was maintained between −10 and −15 °C and in the bath at 90 °C with stirring at 1100 rpm. Crystal Structure Determination. The single-crystal Xray diffraction (SC-XRD) data for compounds 1 and 2 were collected using the Canadian Macromolecular Crystallography Facility CMCF-BM beamline at the Canadian Light Source (CLS).53 CMCF-BM is a bending magnet beamline equipped with a Si(111) double-crystal monochromator, Rayonix MX300HE CCD detector, and MD2 microdiffractometer equipped with Mini Kappa Goniometer Head. The data for compounds 1 and 2 were collected at 18.000 keV (0.689 Å) and 19.000 keV (0.653 Å), respectively, at 100 K. Cell refinement and data reduction were performed using SAINT.54 An empirical absorption correction, based on the multiple measurements of equivalent reflections, and merging of data were performed using SADABS.55 The space groups were confirmed by the XPREP routine.56 The structures of 1 and 2 were solved with SHELXL-201657 using conjugate-gradient refinement (CGLS) in the initial stages and then by full-matrix least-squares and difference Fourier techniques in the final stages. All non-hydrogen atoms were refined with anisotropic displacement parameters. The hydrides were extracted from the Fourier difference maps and refined using a full-matrix least-squares method. All other hydrogen atoms were set in the calculated positions and refined as riding atoms with a common thermal parameter. All publication materials were prepared using LinXTL58 and Mercury59 programs. Routine checkCIF and structure factor analyses were performed with Platon.60 Computational Details. The DFT calculations were carried out with Gaussian 1661 (with G09Defaults) using the MN15-L functional.62 The basis sets used for the geometry optimizations included Def2QZVP (with def2 ECP) for Ru and Def2SVP for all other atoms (together with the W06 density fitting basis set).63,64 The polarizable continuum model (IEFPCM) was used in all without exception calculations (solvent = toluene or ethanol), with the radii and nonelectrostatic terms of Truhlar and co-workers’ SMD solvation model (scrf = smd).65 Every geometry optimization was accompanied by a frequency calculation, followed by a singlepoint energy calculation at the MN15-L/Def2QZVP level. The reaction Gibbs energies were calculated by combining the single-point energies with the thermal corrections to enthalpy from the frequency calculations and the computed molar entropies corrected by −22 cal/(mol K) following the approach developed in our recent work.52 The molar entropy of H2 in ethanol was assigned the value of S = 17.3 cal/(mol K) at 298.15 K: i.e., 13.9 cal/(mol K) smaller23 than the calculated gas-phase S = 31.2 cal/(mol K).

of H2 in a 300 mL reactor and heated at 100 °C for 12 h. The reactor was opened in an Ar glovebox to display a clear lightly colored solution. THF was evaporated, and the residue was dried under vacuum of an oil pump for 1 h. Addition of 20 mL of hexane with trituration afforded a yellow solid that was filtered and carefully washed with 3 × 10 mL of hexane with stirring and pressing the solid against the walls of the fritted funnel with a spatula. The solid was dried under vacuum for 20 min and stored at −28 °C. The product was characterized in CD2Cl2 (0.1 M) and displayed ca. 7.5 mol % of THF and 4.1 mol % of hexane (effective Mw = 552.52 g). Yield: 1.42 g (77%). 1H NMR (400 MHz, CD2Cl2): δ 7.49 (m, 4H, CH), 7.32 (m, 6H, CH), 7.06−7.13 (m, 10H, CH), 4.86 (s, 1H, OH), −9.79 (s, RuH). A low-intensity peak (ca. 1.7% of the hydride of 2) was present at −10.06 ppm. The nature of this minor species is unknown. 13C{1H} NMR (100.46 MHz, CD2Cl2): δ 201.8 (CO), 135.7 (C), 133.3 (CH), 132.9 (CH), 131.8 (C), 130.8 (C), 129.0 (CH), 128.5 (CH), 128.1 (CH), 128.0 (CH), 104.4 (C), 91.2 (C). Equilibrium between 1 and 2 was followed by 1H NMR using solutions containing the total 0.01 M [Ru] in 0.9 mL of toluene-d8 sealed under H2 under the normal atmospheric pressure (ca. 1 atm of H2) in J. Young NMR tubes. The tubes were heated with the solution part fully immersed in an oil bath until the observed 2/1 ratio stopped changing (i.e., stopped increasing from 1 or decreasing from 2). For the NMR measurements, the samples were taken from the bath for 15−20 min without disturbing the solutions. It is presumed that liquid- to gas-phase H2 diffusion must be slow at room temperature, and the recorded H2 concentrations reflect those at the bath temperatures. Following the approach of Casey and co-workers,22 the concentration of H2 in solution was corrected for underintegration of the H2 resonance due to para-hydrogen by multiplying the integral by 4/3 to give 3.8 (43.5 °C), 4.1 (60 °C), 4.3 (74.5 °C), and 4.6 mM (85 °C) concentration values. For comparison, an H2 concentration of 3.2 mM in toluene was reported at 35 °C,23 lower than the 3.6 mM expected on the basis of the trend in this work. Also, the solubility in C6H6 is 3.45 mM at 45 °C. Thus, it appears that the values determined in this work might be slightly overestimated; however, the origin of the difference is not clear. Acetaldehyde Disproportionation. All reactions were performed under argon, in 20 mL vials equipped with micro stir bars and tight polypropylene screw-on caps, according to the following procedure. Acetaldehyde was taken from the freezer (at −28 °C) immediately prior to the preparation of the reaction solutions. First, 5.4 mg of the catalyst was taken in a vial on a calibrated analytical balance accurate to 0.1 mg. Next, toluene (0.1 mol) was added (followed by 5 mol % ethanol, when ethanol was used) and the vial was placed on a magnetic stirrer. Acetaldehyde (0.1 mol) was added next, and the vial was securely sealed. Ca. 0.8 mL samples of the reaction solution were retrieved from the vial for regular NMR analyses. The TON values were determined by integration of the 1 H{13C} NMR spectra, using the toluene −CH3 resonance as an internal standard. Hydrogenation. The hydrogenation of ethyl acetate was performed under initial p(H2) = 50 bar using 0.1 mol of the neat substrate (ca. 9.8 mL) in a 300 mL stainless-steel Parr reactor. The reactor was loaded and assembled inside of an argon glovebox. The pressurized reactor was placed into an oil 6858

DOI: 10.1021/acscatal.8b01153 ACS Catal. 2018, 8, 6851−6861

Research Article

ACS Catalysis

■

Ketones Catalyzed by trans-Dihydrido(diamine)Ruthenium(II) Complexes. J. Am. Chem. Soc. 2002, 124, 15104−15118. (11) Casey, C. P.; Johnson, J. B. Kinetic Isotope Effect Evidence for a Concerted Hydrogen Transfer Mechanism in Transfer Hydrogenations Catalyzed by [p-(Me2CH)C6H4Me]Ru(NHCHPhCHPhNSO2C6H4-p-CH3). J. Org. Chem. 2003, 68, 1998−2001. (12) Sandoval, C. A.; Ohkuma, T.; Muniz, K.; Noyori, R. Mechanism of Asymmetric Hydrogenation of Ketones Catalyzed by BINAP/1,2-Diamine-Ruthenium(II) Complexes. J. Am. Chem. Soc. 2003, 125, 13490−13503. (13) Casey, C. P.; Johnson, J. B. Kinetic Isotope Effect Evidence for the Concerted Transfer of Hydride and Proton from Hydroxycyclopentadienyl Ruthenium Hydride in Solvents of Different Polarities and Hydrogen Bonding Ability. Can. J. Chem. 2005, 83, 1339−1346. (14) Washington, J.; McDonald, R.; Takats, J.; Menashe, N.; Reshef, D.; Shvo, Y. Os(CO)4(η2-C2Me2)-Promoted Coupling of Alkynes and CO: Formation of (η4-C4Me2R2CO)Os(CO)3 (R = Me, Et, nPr) and Catalytic Activity of (η4-C4R4CO)Os(CO)3 (R = Me, Ph). Organometallics 1995, 14, 3996−4003. (15) Mays, M. J.; Morris, M. J.; Raithby, P. R.; Shvo, Y.; Czarkie, D. X-ray Structure, Reactivity, and Catalytic Properties of a (Cyclopentadienone)ruthenium Dimer, [(C4Ph4CO)(CO)2Ru]2. Organometallics 1989, 8, 1162−1167. (16) Menashe, N.; Shvo, Y. Catalytic Disproportionation of Aldehydes with Ruthenium Complexes. Organometallics 1991, 10, 3885−3891. (17) Törmäkangas, O. P.; Koskinen, A. M. P. The Tishchenko Reaction and Its Modifications in Organic Synthesis. Recent Res. Dev. Org. Chem. 2001, 5, 225−255. (18) Blum, Y.; Shvo, Y. Catalytically Reactive (η4-tetracyclone)(CO)2(H)2Ru and Related Complexes in Dehydrogenation of Alcohols to Esters. J. Organomet. Chem. 1985, 282, C7−C10. (19) Pasini, T.; Solinas, G.; Zanotti, V.; Albonetti, S.; Cavani, F.; Vaccari, A.; Mazzanti, A.; Ranieri, S.; Mazzoni, R. Substrate and Product Role in the Shvo’s Catalyzed Selective Hydrogenation of the Platform Bio-based Chemical 5-Hydroxymethylfurfural. Dalton Trans. 2014, 43, 10224−10234. (20) The coordination geometry around ruthenium in 2 closely resembles the structure of the related chloride complex, RuCl(CO)2[η5-C5(OH)Ph4]; see: Jung, H. M.; Choi, J. H.; Lee, S. O.; Kim, Y. H.; Park, J. H.; Park, J. Facile Synthesis of (η5-Ph4C4COH)(CO)2RuCl and Catalytic Oxidation of Alcohols with Chloroform. Organometallics 2002, 21, 5674−5677. (21) Casey, C. P.; Johnson, J. B.; Singer, S. W.; Cui, Q. Hydrogen Elimination from a Hydroxycyclopentadienyl Ruthenium(II) Hydride: Study of Hydrogen Activation in a Ligand-Metal Bifunctional Hydrogenation Catalyst. J. Am. Chem. Soc. 2005, 127, 3100−3109. (22) Casey, C. P.; Beetner, S. E.; Johnson, J. B. Spectroscopic Determination of Hydrogenation Rates and Intermediates during Carbonyl Hydrogenation Catalyzed by Shvo’s Hydroxycyclopentadienyl Diruthenium Hydride Agrees with Kinetic Modeling Based on Independently Measured Rates of Elementary Reactions. J. Am. Chem. Soc. 2008, 130, 2285−2295. (23) Hydrogen and Deuterium; Young, C. L., Ed.; Pergamon Press: New York, 1981; Vol. 5/6, pp 169−175. (24) Although conversion of 1 was always observed, the reaction rates were inconsistent. It seemingly made a difference whether the samples were in the NMR probe (i.e., in the dark) or outside (exposed to room lights) between the measurements. For example, in a control experiment with PPh3 where only traces of 5 and 2 formed after 0.5 h, 59% conversion of 1 occurred in the next 1.5 h when the sample was exposed to light; however, no further significant changes were seen after an additional 1 h when this sample was kept in the probe. (25) Meijer, R. H.; Ligthart, G. B. W. L.; Meuldijk, J.; Vekemans, J. A. J. M.; Hulshof, L. A.; Mills, A. M.; Kooijman, H.; Spek, A. L. Triruthenium Dodecacarbonyl/Triphenylphosphine Catalyzed Dehy-

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acscatal.8b01153. Representative NMR spectra, computed energies, IRC plots, and a summary of the crystal data collection and refinement parameters for 1 and 2 (PDF) Crystallographic data for 1 and 2 (CIF) Cartesian coordinates of the ruthenium species computed in this study (XYZ)

■

AUTHOR INFORMATION

Corresponding Author

*E-mail for D.G.G.: [email protected]. ORCID

Dmitry G. Gusev: 0000-0003-3302-356X Author Contributions

The manuscript was written by D.G.G. Both authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS The authors are thankful to the Natural Sciences and Engineering Research Council of Canada-Discovery grant program, CFI LOF program, SHARCNET, and Wilfrid Laurier University for support. Insightful comments and suggestions from Prof. Faraj Hasanayn are gratefully appreciated.

■

REFERENCES

(1) Blum, Y.; Shvo, Y. Catalytically Reactive Ruthenium Intermediates in the Homogeneous Oxidation of Alcohols to Esters. Isr. J. Chem. 1984, 24, 144−148. (2) Blum, Y.; Czarkie, D.; Rahamim, Y.; Shvo, Y. (Cyclopentadienone)ruthenium Carbonyl Complexes-A New Class of Homogeneous Hydrogenation Catalysts. Organometallics 1985, 4, 1459−1461. (3) Shvo, Y.; Czarkie, D.; Rahamim, Y. New Group of Ruthenium Complexes: Structure and Catalysis. J. Am. Chem. Soc. 1986, 108, 7400−7402. (4) Karvembu, R.; Prabhakaran, R.; Natarajan, K. Shvo’s Diruthenium Complex: a Robust Catalyst. Coord. Chem. Rev. 2005, 249, 911−918. (5) Samec, J. S. M.; Bäckvall, J.-E.; Andersson, P. G.; Brandt, P. Mechanistic Aspects of Transition Metal-Catalyzed Hydrogen Transfer Reactions. Chem. Soc. Rev. 2006, 35, 237−248. (6) Conley, B. L.; Pennington-Boggio, M. K.; Boz, E.; Williams, T. J. Discovery, Applications, and Catalytic Mechanisms of Shvo’s Catalyst. Chem. Rev. 2010, 110, 2294−2312. (7) Warner, M. C.; Casey, C. P.; Bäckvall, J.-E. Shvo’s Catalyst in Hydrogen Transfer Reactions. Top. Organomet. Chem. 2011, 37, 85− 125. (8) Yamakawa, M.; Ito, H.; Noyori, R. The Metal-Ligand Bifunctional Catalysis: A Theoretical Study on the Ruthenium(II)Catalyzed Hydrogen Transfer between Alcohols and Carbonyl Compounds. J. Am. Chem. Soc. 2000, 122, 1466−1478. (9) Casey, C. P.; Singer, S. W.; Powell, D. R.; Hayashi, R. K.; Kavana, M. Hydrogen Transfer to Carbonyls and Imines from a Hydroxycyclopentadienyl Ruthenium Hydride: Evidence for Concerted Hydride and Proton Transfer. J. Am. Chem. Soc. 2001, 123, 1090−1100. (10) Abdur-Rashid, K.; Clapham, S. E.; Hadzovic, A.; Harvey, J. N.; Lough, A. J.; Morris, R. H. Mechanism of the Hydrogenation of 6859

DOI: 10.1021/acscatal.8b01153 ACS Catal. 2018, 8, 6851−6861

Research Article

ACS Catalysis drogenation of Primary and Secondary Alcohols. Tetrahedron 2004, 60, 1065−1072. (26) Zhang, X.; Lu, Z.; Foellmer, L. K.; Williams, T. J. NitrogenBased Ligands Accelerate Ammonia Borane Dehydrogenation with the Shvo Catalyst. Organometallics 2015, 34, 3732−3738. (27) Pihlaja, K.; Lampi, A. Carbon-13 NMR Chemical Shifts - a Conformational Probe for 1-Alkoxyalkyl Esters. Acta Chem. Scand. 1986, 40b, 196−199. (28) Boden, R. M. Flavoring With 1-Ethoxy-1-Ethanol Acetate. U.S. Patent 4,296,137, 1981. (29) Wiberg, K. B.; Crocker, L. S.; Morgan, K. M. Thermochemical Studies of Carbonyl Compounds. 5. Enthalpies of Reduction of Carbonyl Groups. J. Am. Chem. Soc. 1991, 113, 3447−3450. (30) Standard Thermodynamic Properties of Chemical Substances. In CRC Handbook of Chemistry and Physics, 96th ed.; Haynes, W. M., Ed.; CRC Press/Taylor and Francis: Boca Raton, FL, 2016. (31) Zweifel, T.; Naubron, J.-V.; Büttner, T.; Ott, T.; Grützmacher, H. Ethanol as Hydrogen Donor: Highly Efficient Transfer Hydrogenations with Rhodium(I) Amides. Angew. Chem., Int. Ed. 2008, 47, 3245−3249. (32) Gusev, D. G. Dehydrogenative Coupling of Ethanol and Ester Hydrogenation Catalyzed by Pincer-Type YNP Complexes. ACS Catal. 2016, 6, 6967−6981. (33) Morris, S. A.; Gusev, D. G. Rethinking the Claisen−Tishchenko Reaction. Angew. Chem., Int. Ed. 2017, 56, 6228−6231. (34) Hasanayn, F.; Baroudi, A. Direct H/OR and OR/OR’Metathesis Pathways in Ester Hydrogenation and Transesterification by Milstein’s Catalyst. Organometallics 2013, 32, 2493−2496. (35) Hasanayn, F.; Al-Assi, L. M.; Moussawi, R. N.; Omar, B. S. Mechanism of Alcohol - Water Dehydrogenative Coupling into Carboxylic Acid and H2 Using Milstein’s Catalyst: A Detailed Investigation of the PES of the Reaction Between an Aldehyde and an Octahedral Ruthenium Hydroxide. Inorg. Chem. 2016, 55, 7886− 7902. (36) Spasyuk, D.; Smith, S.; Gusev, D. G. Replacing Phosphorus with Sulfur for the Efficient Hydrogenation of Esters. Angew. Chem., Int. Ed. 2013, 52, 2538−2542. (37) Johnson, J. B.; Bäckvall, J.-E. Mechanism of RutheniumCatalyzed Hydrogen Transfer Reactions. Concerted Transfer of OH and CH Hydrogens from an Alcohol to a (Cyclopentadienone) ruthenium Complex. J. Org. Chem. 2003, 68, 7681−7684. (38) Selected reviews: (a) Noyori, R.; Ohkuma, T. Asymmetric Catalysis by Architectural and Functional Molecular Engineering: Practical Chemo- and Stereoselective Hydrogenation of Ketones. Angew. Chem., Int. Ed. 2001, 40, 40−73. (b) Clapham, S. E.; Hadzovic, A.; Morris, R. H. Mechanisms of the H2-Hydrogenation and Transfer Hydrogenation of Polar Bonds Catalyzed by Ruthenium Hydride Complexes. Coord. Chem. Rev. 2004, 248, 2201−2237. (c) Grützmacher, H. Cooperating Ligands in Catalysis. Angew. Chem., Int. Ed. 2008, 47, 1814−1818. (d) Eisenstein, O.; Crabtree, R. H. Outer Sphere Hydrogenation Catalysis. New J. Chem. 2013, 37, 21− 27. (e) Zhao, B.; Han, Z.; Ding, K. The N-H Functional Group in Organometallic Catalysis. Angew. Chem., Int. Ed. 2013, 52, 4744− 4788. (f) Khusnutdinova, J. R.; Milstein, D. Metal−Ligand Cooperation. Angew. Chem., Int. Ed. 2015, 54, 12236−12273. (39) Hamilton, R. J.; Leong, C. G.; Bigam, G.; Miskolzie, M.; Bergens, S. H. A Ruthenium-Dihydrogen Putative Intermediate in Ketone Hydrogenation. J. Am. Chem. Soc. 2005, 127, 4152−4153. (40) Hamilton, R. J.; Bergens, S. H. An Unexpected Possible Role of Base in Asymmetric Catalytic Hydrogenations of Ketones. Synthesis and Characterization of Several Key Catalytic Intermediates. J. Am. Chem. Soc. 2006, 128, 13700−13701. (41) Hamilton, R. J.; Bergens, S. H. Direct Observations of the Metal-Ligand Bifunctional Addition Step in an Enantioselective Ketone Hydrogenation. J. Am. Chem. Soc. 2008, 130, 11979−11987. (42) Takebayashi, S.; Bergens, S. H. Facile Bifunctional Addition of Lactones and Esters at Low Temperatures. The First Intermediates in Lactone/Ester Hydrogenations. Organometallics 2009, 28, 2349− 2351.

(43) Takebayashi, S.; Dabral, N.; Miskolzie, M.; Bergens, S. H. Experimental Investigations of a Partial Ru-O Bond during the MetalLigand Bifunctional Addition in Noyori-Type Enantioselective Ketone Hydrogenation. J. Am. Chem. Soc. 2011, 133, 9666−9669. (44) Baratta, W.; Chelucci, G.; Gladiali, S.; Siega, K.; Toniutti, M.; Zanette, M.; Zangrando, E.; Rigo, P. Ruthenium(II) Terdentate CNN Complexes: Superlative Catalysts for the Hydrogen-Transfer Reduction of Ketones by Reversible Insertion of a Carbonyl Group into the Ru-H Bond. Angew. Chem., Int. Ed. 2005, 44, 6214−6219. (45) Bertoli, M.; Choualeb, A.; Lough, A. J.; Moore, B.; Spasyuk, D.; Gusev, D. G. Osmium and Ruthenium Catalysts for Dehydrogenation of Alcohols. Organometallics 2011, 30, 3479−3482. (46) Bertoli, M.; Choualeb, A.; Gusev, D. G.; Lough, A. J.; Major, Q.; Moore, B. PNP pincer osmium polyhydrides for catalytic dehydrogenation of primary alcohols. Dalton Trans. 2011, 40, 8941−8949. (47) Dub, P. A.; Henson, N. J.; Martin, R. L.; Gordon, J. C. Unravelling the Mechanism of the Asymmetric Hydrogenation of Acetophenone by [RuX2(diphosphine)(1,2-diamine)] Catalysts. J. Am. Chem. Soc. 2014, 136, 3505−3521. (48) Dub, P. A.; Gordon, J. C. The Mechanism of Enantioselective Ketone Reduction with Noyori and Noyori-Ikariya Bifunctional Catalysts. Dalton Trans. 2016, 45, 6756−6781. (49) Dub, P. A.; Gordon, J. C. Metal-Ligand Bifunctional Catalysis: The ″Accepted″ Mechanism, the Issue of Concertedness, and the Function of the Ligand in Catalytic Cycles Involving Hydrogen Atoms. ACS Catal. 2017, 7, 6635−6655. (50) Dub, P. A.; Scott, B. L.; Gordon, J. C. Why Does Alkylation of the N-H Functionality within M/NH Bifunctional Noyori-Type Catalysts Lead to Turnover? J. Am. Chem. Soc. 2017, 139, 1245−1260. (51) Hasanayn, F.; Morris, R. H. Symmetry Aspects of H2 Splitting by Five-Coordinate d6 Ruthenium Amides, and Calculations on Acetophenone Hydrogenation, Ruthenium Alkoxide Formation, and Subsequent Hydrogenolysis in a Model trans-Ru(H)2(diamine)(diphosphine) System. Inorg. Chem. 2012, 51, 10808−10818. (52) Gusev, D. G. Rethinking the Dehydrogenative Amide Synthesis. ACS Catal. 2017, 7, 6656−6662. (53) Grochulski, P.; Fodje, M. N.; Gorin, J.; Labiuk, S. L.; Berg, R. Beamline 08ID-1, the Prime Beamline of the Canadian Macromolecular Crystallography Facility. J. Synchrotron Radiat. 2011, 18, 681−684. (54) SAINT; Bruker AXS Inc., Madison, WI, USA, 2012. (55) Krause, L.; Herbst-Irmer, R.; Sheldrick, G. M.; Stalke, D. Comparison of Silver and Molybdenum Microfocus X-Ray Sources for Single-Crystal Structure Determination. J. Appl. Crystallogr. 2015, 48, 3−10. (56) XPREP; Bruker AXS Inc., Madison, WI, USA, 2012. (57) Sheldrick, G. M. Crystal Structure Refinement with SHELXL. Acta Crystallogr., Sect. C: Struct. Chem. 2015, 71, 3−8. (58) Spasyuk, D. M. LinXTL; University of Montreal, Montreal, Canada, 2009. (59) Macrae, C. F.; Bruno, I. J.; Chisholm, J. A.; Edgington, P. R.; McCabe, P.; Pidcock, E.; Rodriguez-Monge, L.; Taylor, R.; van de Streek, J.; Wood, P. A. Mercury CSD 2.0-New Features for the Visualization and Investigation of Crystal Structures. J. Appl. Crystallogr. 2008, 41, 466−470. (60) Spek, A. L. Structure Validation in Chemical Crystallography. Acta Crystallogr., Sect. D: Biol. Crystallogr. 2009, 65, 148−155. (61) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Petersson, G. A.; Nakatsuji, H.; Li, X.; Caricato, M.; Marenich, A. V.; Bloino, J.; Janesko, B. G.; Gomperts, R.; Mennucci, B.; Hratchian, H. P.; Ortiz, J. V.; Izmaylov, A. F.; Sonnenberg, J. L.; Williams-Young, D.; Ding, F.; Lipparini, F.; Egidi, F.; Goings, J.; Peng, B.; Petrone, A.; Henderson, T.; Ranasinghe, D.; Zakrzewski, V. G.; Gao, J.; Rega, N.; Zheng, G.; Liang, W.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Vreven, T.; Throssell, K.; Montgomery, J. A., Jr.; Peralta, J. E.; Ogliaro, F.; Bearpark, M. J.; Heyd, J. J.; Brothers, E. N.; Kudin, K. N.; Staroverov, 6860

DOI: 10.1021/acscatal.8b01153 ACS Catal. 2018, 8, 6851−6861

Research Article

ACS Catalysis V. N.; Keith, T. A.; Kobayashi, R.; Normand, J.; Raghavachari, K.; Rendell, A. P.; Burant, J. C.; Iyengar, S. S.; Tomasi, J.; Cossi, M.; Millam, J. M.; Klene, M.; Adamo, C.; Cammi, R.; Ochterski, J. W.; Martin, R. L.; Morokuma, K.; Farkas, O.; Foresman, J. B.; Fox, D. J. Gaussian 16, Revision A.03; Gaussian, Inc., Wallingford, CT, 2016. (62) Yu, H. S.; He, H.; Truhlar, D. G. MN15-L: A New Local Exchange-Correlation Functional for Kohn−Sham Density Functional Theory with Broad Accuracy for Atoms, Molecules, and Solids. J. Chem. Theory Comput. 2016, 12, 1280−1293. (63) Weigend, F.; Ahlrichs, R. Balanced Basis Sets of Split Valence, Triple Zeta Valence and Quadruple Zeta Valence Quality for H to Rn: Design and Assessment of Accuracy. Phys. Chem. Chem. Phys. 2005, 7, 3297−3305. (64) Weigend, F. Accurate Coulomb-Fitting Basis Sets for H to Rn. Phys. Chem. Chem. Phys. 2006, 8, 1057−1065. (65) 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.

6861

DOI: 10.1021/acscatal.8b01153 ACS Catal. 2018, 8, 6851−6861