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Additive-Free Cobalt-Catalyzed Hydrogenation of Esters to Alcohols Jing Yuwen, Sumit Chakraborty, William W. Brennessel, and William D. Jones ACS Catal., Just Accepted Manuscript • Publication Date (Web): 20 Apr 2017 Downloaded from http://pubs.acs.org on April 20, 2017
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ACS Catalysis
Additive-Free Cobalt-Catalyzed Hydrogenation of Esters to Alcohols Jing Yuwen, Sumit Chakraborty, William W. Brennessel and William D. Jones* Department of Chemistry, University of Rochester, Rochester, New York, 14627
ABSTRACT: Here, we report an additive-free catalytic system for hydrogenation of carboxylic acid esters to alcohols with a welldefined cobalt pincer catalyst precursor. Various substrates including methyl, ethyl and benzyl esters have been evaluated under hydrogenation conditions, however, methyl esters have low reactivity compared to the corresponding ethyl and benzyl esters. The biomass derived γ-valerolactone (GVL) successfully formed 1,4-pentanediol with a turnover number of 3890 with this system. Metal-ligand cooperativity is probed with the related [PN(Me)P] derivative of the cobalt catalyst and the results suggest a nonbifunctional hydrogenation mechanism. Keywords: cobalt, hydrogenation, esters, alcohols, catalysis
chemistry.43 However, first-row transition-metal complexes typically react by one-electron pathways, rather than by the prevailing two-electron transformations of precious metals, resulting in the difficulty of predicting and controlling catalytic reactivity. Recently, remarkable advances have been made in the application of inexpensive, earth-abundant and nontoxic iron-44-47 and cobalt-based48-49 catalysts for ester hydrogenation. Of particular interest is the first cobalt catalyst, supported by a PNN-pincer ligand, for ester hydrogenation by Milstein et al. In their system, the cobalt PNN-pincer pre-catalyst required activation by NaHBEt3 and the amount of the base added to the system has a significant effect on the yield of alcohols.48 Recently, Elsevier and de Bruin reported another cobalt system of Co(BF4)26H2O paired with a tridentate phosphine ligand to hydrogenate both esters and carboxylic acids.49
Introduction The reduction of esters to alcohols is an important process for both laboratory organic synthesis and the chemical industry.1 Traditionally, stoichiometric amounts of metal hydride reagents (e.g., LiAlH4 and NaBH4) are applied to hydrogenation of esters to alcohols, but stoichiometric amounts of waste are generated.2 In contrast, catalytic hydrogenation with molecular hydrogen is environmentally benign, easy to operate, and fully atom economic. Heterogeneous catalysts are mainly employed for reduction of esters to alcohols in industry, however, extreme pressures (2000-3000 psi) and temperatures (200-300 °C) are required which often leads to side product formation.3-4 Hence, there is a growing demand to develop homogeneous ester hydrogenation catalysts that could be operate more selectively under mild conditions.
Cobalt catalysts have been successfully applied in hydrogenation reactions of various substrates, such as alkynes, alkenes, ketones, aldehydes and imines.50-57 In 2012, Hanson reported the cationic cobalt(II) alkyl complex [(PNHPcy)Co(CH2SiMe3)]BArF4 (1) (where PNHPcy = HN(CH2CH2PCy2)2) as an efficient catalyst precursor for hydrogenation of alkenes, aldehydes, ketones, and imines.50, 52, 57 Due to its high activity for hydrogenation of carbonyl functionalities, we became interested to study if the same cobalt catalyst could also reduce carboxylic acid esters, which have
In fact, since the 1980s, a variety of homogeneous catalytic systems have been reported for ester hydrogenation.5-14 Significant progress was achieved around 2006 by Milstein et al.15 and Firmenich SA16 who developed Ru-based homogeneous catalysts for hydrogenation of nonactivated esters under relatively mild reaction conditions. Since then, many other catalysts have been prepared to improve the efficiency of ester hydrogenation.17-42 The replacement of precious-metal-based catalysts by earthabundant metal catalysts is a prime goal in current catalytic
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inherently less reactivity compared to the corresponding aldehydes and ketones. It is noteworthy that the previously reported pathway for hydrogenation of aldehydes and ketones involves a metal-ligand cooperative mechanism.57 Here, however, we find evidence for a non-bifunctional hydrogenation mechanism for hydrogenation of esters to alcohols using the same cobalt catalyst.58
Results and Discussion To determine the optimum conditions for catalysis, we studied catalytic hydrogenation of benzyl benzoate in the presence of catalyst precursor 1 (0.01 mmol) under various conditions (Table 1). Combining benzyl benzoate (0.2 mmol) with 5 mol % of cobalt compound 1 in THF, under 20 bar of H2 pressure and at 150 °C produced 48% benzyl alcohol after 24 h (Table 1, entry 1). Increasing the reaction time to 48 h resulted in a similar yield of benzyl alcohol (45.2%, entry 2), while decreasing the reaction time led to a significantly lower yield of benzyl alcohol (17.4%, entry 3). Methanol has been used as solvent in some of the previously reported catalytic systems.29, 49, 59-60 When MeOH was used in place of THF as reaction solvent, almost all of the benzyl benzoate was consumed (97% conversion). However, only 53% of benzyl alcohol was formed, the remainder of the product being methyl benzoate. This result suggested that transesterification had occurred and that the methyl benzoate was not able to be further hydrogenated (entry 4). The hydrogen pressure has a significant impact on the overall yield of the ester hydrogenation reaction. Increasing the hydrogen pressure to 35 bar did not have any impact on the yield of benzyl alcohol - (entry 6), but increasing further to 55 bar increased the yield of benzyl alcohol to 90% (entry 7). Decreasing the hydrogen pressure to 10 bar only slightly decreased the yield of benzyl alcohol to 40%
(entry 5). Cobalt compound 1 decomposed when the reaction temperature was raised above 150 °C. A higher yield of benzyl alcohol was detected when the reaction temperature was lowered to 120°C (97%, entry 8). Only a 52% yield of benzyl alcohol was observed when the reaction was run at 80 °C (entry 9). We assume that cobalt compound 1 partially decomposes at 150 °C. The hydrogenation proceeded smoothly at a shorter reaction time (20 h) and lower catalyst loading (2 mol%, entry 10), but reducing the catalyst loading to 1 mol% resulted in a significant drop in the yield of benzyl alcohol (56% entry 11). Almost no benzyl alcohol was detected by GC under the same reaction conditions without cobalt catalyst 1 (entry 12). Next, we investigated the scope of the hydrogenation reaction with a range of ester substrates (Table 2). In the presence of 1 (2 mol%), 0.5 mmol of benzyl benzoate under 55 bar H2 at 120 °C in THF gave a 97% yield of benzyl alcohol after 20 h (Table 2, entry 1). No other side product was observed by gas chromatography. Under the same reaction conditions, ethyl benzoate afforded a 90% yield of benzyl alcohol (entry 3). Methyl benzoate turned out to be a challenging substrate, and only gave a 15% yield of benzyl alcohol (entry 2). Aliphatic esters, which Table 2. Catalytic hydrogenation of esters using the cobalt catalyst 1.a
Table 1. Optimization of the reaction conditions for hydrogenation of benzyl benzoate.(a)
entry
rxn time (h)
press (bar)
temp (°C)
cat. loading (mol%)
yield (%)(b)
1
24
20
150
5
48
2
48
20
150
5
45
3
6
20
150
5
17
4(c)
24
20
150
5
53
5
24
10
150
5
40
6
24
35
150
5
54
7
24
55
150
5
90
8
24
55
120
5
97
9
24
55
80
5
52
10
20
55
120
2
97
11
20
55
120
1
56
12
20
55
120
0
1
(a) Reaction conditions: catalyst (0.01 mmol), THF (1mL). (b) Determined by GC with respect to benzyl alcohol. (c) MeOH as solvent.
(a) Reaction conditions: substate (0.5 mmol), catalyst 1 (2 mol%), THF (1.0 mL), H2 (55 bar), 120 °C, 20 hours. (b) Yields were determined by GC. (c) Product yields are ratios at 100%
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ACS Catalysis conversion. Yields of methanol and ethanol are not reported. (d) Toluene was used as solvent.
are generally considered to be more challenging substrates than aromatic esters, could be hydrogenated to afford the corresponding aliphatic alcohols. Although ethyl butyrate and ethyl cyclohexanecarboxylate were reduced to the corresponding alcohols in relatively high yields (entry 5, 7), hydrogenation of methyl butyrate and methyl cyclohexanecarboxylate proved low-yielding reactions (entry 4, 6). Cobalt compound 1 has been reported for the hydrogenation of olefins;52 therefore, we tested it for hydrogenation of methyl cinnamate and ethyl cinnamate. As we expected, reduction of methyl cinnamate gave methyl 3-phenylpropanoate as major product and 3-phenylpropanol as minor product (entry 8), while for hydrogenation of ethyl cinnamate, almost all of the ethyl 3-phenylpropanoate was converted to 3-phenylpropanol under the same reaction conditions (entry 9). Not surprisingly, a very low methanol yield (7.5%, entry 10) was detected for hydrogenation of methyl formate. No 6-hydroxyhexanoic acid was observed upon hydrogenation of adipic acid monoethyl ester (entry 11), indicating that the compounc 1 is not compatible with carboxylic acid functionalities. Similar to our findings, Chianese et al. observed that methyl esters showed low activity compared to the corresponding ethyl and benzyl esters during hydrogenation with a Ru-based catalyst system.41 They observed that methyl esters display an intrinsically lower reactivity with their catalyst, as demonstrated by observing substantial increases in the rate of hydrogenation of methyl decanoate upon addition of increasing amounts of benzyl alcohol to the reaction. However, this is not the case with cobalt compound 1. An experiment was conducted where methyl cyclohexanecarboxylate was hydrogenated in the presence of increasing amounts of benzyl alcohol. If methyl esters are intrinsically less reactive than ethyl esters, we should observe faster hydrogenation when more benzyl alcohol is added. However, the data in Table 3 indicates that the yields of cyclohexylmethanol were decreased when more benzyl alcohol was added to the system, and increased amounts of benzyl cyclohexanecarboxylate was left. This suggests that the catalyst decomposed during the reaction, which could be attributed to catalyst poisoning by CO generated from the decarbonylation of MeOH.
was carried out in the presence of one bar of CO (see SI for details). The cobalt catalyst precursor 1 decomposed under 1 bar of carbon monoxide at room temperature to form the cobalt(I) complex [(PNHPcy)Co(CO)2]BArF4 (2). The X-ray structure of complex 2 is shown in Figure 1. Also, no catalytic reaction was observed when complex 2 was used as a catalyst instead of complex 1. Neither complex 2 (by NMR or IR) nor CO (by GC) were observed, however, during hydrogenation of methyl benzoate. Instead, a new cobalt(III) complex [(PNHPcy)Co(κ1PhCOO)(κ2-PhCOO)]BArF4 (3) was isolated after the hydrogenation reaction. The X-ray structure of complex 3 is shown in Figure 2. The formation of this complex might explain why methyl esters have low reactivity compared to the corresponding ethyl and benzyl esters. The ester CH3–O bond is cleaved by cobalt and consequently the formation of cobalt benzoate complex 3 inhibits the ester hydrogenation catalytic cycle. Similar results were published by Chirik,61 when they used a bis(imino)pyridineiron compound with methyl acetate or methyl benzoate; exclusive ester CH3–O bond cleavage was observed, while phenyl acetate yielded products from selective acyl C–OPh bond cleavage. Although ethyl benzoate underwent ester Et–O bond cleavage rather than acyl C–OEt bond cleavage in their iron system, the cobalt system displayed the opposite selectivity with the ethyl ester. In addition to aliphatic and aromatic esters, biomass derived γvalerolactone was also examined as a substrate for hydrogenation. The decrease in hydrogen pressure on the reactor gauge could be used to monitor the reaction (See SI). When 0.1 mol% of catalyst 1 was employed in the reaction, the hydrogenation process was finished within 5 h and gave a 98% yield of 1,4-pentanediol. Upon decreasing the catalyst loading to 0.01 mol %, the hydrogenation needed 3 d to complete (1 g scale), and a TON of 3890 for 1,4-pentanediol was obtained (Table 4).
Table 3. Effect of added benzyl alcohol on the hydrogenation of methyl cyclohexanecarboxylatea
equiv BnOH added
yield c-hexylmethanol
yield benzyl c-hexanecarboxylate
0
26%
0
1
25%
2%
10
15%
9%
a Reaction performed in THF, with a total solution volume of 1.0 mL and 0.5 mmol methyl cyclohexanecarboxylate. Yields were measured by GC.
Indeed, we observed that the catalytic reaction was inhibited in the presence of carbon monoxide. Less than 1% benzyl alcohol was produced when the hydrogenation of benzyl benzoate
Figure 1. X-ray structure of complex 2 (molecular cation shown, thermal ellipsoid at 50% probability, hydrogen atoms except for H1 omitted for clarity). Selected bond distances (Å) and angles (°): Co1-C1 = 1.726(3), Co1-C2 = 1.804(3), Co1-N1 = 2.071(2), Co1-P1 = 2.2098(7), Co1-P2 = 2.2106(7), C1-Co1-C2 = 115.47(12), C1-Co1-N1 = 143.67(11), C2-Co1-N1 = 100.86(10), C1-Co1-P1 = 88.20(8), C2-Co1-P1 = 101.89(8), N1-Co1-P1 = 85.02(6), C1-Co1-P2 = 90.99(8), C2-Co1-P2 = 94.38(8), N1-Co1P2 = 85.22(6), P1-Co1-P2 = 162.34(3).
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Table 4. Catalytic hydrogenation of γ-Valerolactone using the cobalt catalyst 1. (a)
entry Pinit H2 Pfinal H2 rxn conv. GC isol. cat. (bar) (bar)(b) time loading (%) yield yield (%) (%)(c) (mol%)
Figure 2. X-ray structure of complex 3 (molecular cation shown, thermal ellipsoid at 50% probability, hydrogen atoms except for H1 omitted for clarity). Selected bond distances (Å) and angles (°): Co1-O3 = 1.908(3), Co1-O1 = 1.909(2), Co1-N1 = 1.962(3), Co1-O2 = 1.998(2), Co1-P1 = 2.2942(11), Co1-P2 = 2.3031(11), Co1-C1 = 2.319(4), O3-Co1-O1 = 166.14(10), O3-Co1-N1 = 97.32(12), O1-Co1-N1 = 96.54(11), O3-Co1-O2 = 99.17(10), O1Co1-O2 = 66.98(9), N1-Co1-O2 = 163.51(11), O3-Co1-P1 = 89.42(8), O1-Co1-P1 = 91.22(7), N1-Co1-P1 = 86.78(9), O2Co1-P1 = 93.54(7), O3-Co1-P2 = 89.18(7), O1-Co1-P2 = 91.83(7), N1-Co1-P2 = 86.38(9), O2-Co1-P2 = 93.68(7), P1-Co1P2 = 172.78(4), O3-Co1-C1 = 132.40(11), O1-Co1-C1 = 33.74(9), N1-Co1-C1 = 130.28(12), O2-Co1-C1 = 33.24(9), P1Co1-C1 = 92.68(8), P2-Co1-C1 = 93.49(8).
To clarify the potential participation of the N-H moiety of the PNHPcy pincer ligand in catalysis, reactivities of 1 and Nmethyl derivative 457 were compared. Hanson and co-workers have reported that complex 4 displayed no catalytic activity in hydrogenation of acetophenone (with 1 atm H2 pressure, 2 mol% catalyst loading), even upon increasing the temperature to 60 °C (c.f., 98% of 1-phenylethanol was detected by GCMS when acetophenone was hydrogenated using 2 mol% of 1 within 24 h at 25 °C and under 1 atm H2). Upon increasing the hydrogen pressure to 4 atm and using 10 mol% of 4, ca. 60% conversion of acetophenone to 1-phenylethanol was observed after 4 days at 60 °C.57 These results point toward a bifunctional hydrogenation mechanism of ketones at 25 °C with low hydrogen pressures. Since high hydrogen pressure (55 bar) and high reaction temperature (120 °C) were required for hydrogenation of esters with complex 1, the catalytic activity of complex 4 was tested under the same reaction conditions; similar hydrogenation performance was observed (Scheme 1). The results show that 4 performs almost as well as 1, suggesting that cobalt compound 1 hydrogenates carboxylic acid esters without the involvement of the N-H moiety. The mechanism of the ester reduction is complex, in that several intermediates must exist on the way to the alcohol product. In addition, the mechanism must take into account two important observations: (1) the N-Me complex 4 works almost as well as N-H complex 1, and (2) methyl esters are poor substrates, giving reduced yields of products. The isolation of biscarboxylate 3 strongly suggests that methyl esters are cleaved at the O–CH3 bond to produce the RCO2 ligands in this product, rendering the catalyst inactive. Scheme 2 shows a mechanism consistent with the above requirements, and is based on the classical Schrock−Osborn (inner-sphere) mechanism.62 Here, the ester carbonyl undergoes insertion into a cobalthydride bond to give a metal-bound hemiacetal. Coordination
1
55
32
5h
0.1
99.8
98
91.6
2
55
46
3d
0.01
40.2
38.9
35.7
(a) Reaction conditions: γ-valerolactone (10 mmol, 1 g), catalyst 1, no solvent, H2 (55 bar), 120 °C. (b) Inside volume of Parr reactor is 22mL, using ideal gas PV = nRT, we can calculate about 20 mmol hydrogen was consumed for entry 1, and 8 mmol hydrogen was consumed for entry 2. (c) See SI for details.
Scheme 1. Comparison of the reactivity of 1 and 4: hydrogenation of (a) benzyl benzoate and (b) γ-valerolactone. F
BAr4 H N Cy2P Co PCy2 CH2SiMe3
F
BAr 4 Me N Cy 2P Co PCy2 CH 2SiMe3
1
4
(a) O O
+ 2H2 (55 bar)
catalyst 2 mol% THF 120 oC
OH
2
with 1: 97% with 4: 91% (b) O
O
+ 2H2 (55 bar)
catalyst 0.1 mol% 120 oC
OH OH
with 1: 98% with 4: 93%
of dihydrogen generates an acidic hydrogen atom that can protonolyze the hemiacetal, generating one molecule of alcohol plus an O-bound aldehyde. Insertion of the aldehyde C=O bond into the Co-H bond give an alkoxide that can again coordinate dihydrogen leading to protonation and loss of a second molecule of alcohol. None of these reactions use the N–H moiety. With methyl esters, it is possible that the CoI can undergo an SN2 attack on the methyl group to eject a carboxylate, leaving an unstable methyl hydride that eliminates methane. The carboxylate could then trap the metal to produce ultimately bis-carboxylate 3. It is also worth pointing out that this system works without additives – i.e., no base is required. This stands in contrast to Milstein’s (PNN)Co catalyst that works only with added base. The base is responsible for formation of an ester enolate that undergoes subsequent cleavage and hydrogenation.48
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ACS Catalysis Scheme 2. Proposed mechanism for ester reduction.
Conclusions In summary, we have demonstrated that the inexpensive, earth-abundant molecular cobalt pincer complex 1 is an efficient catalyst precursor for the hydrogenation of carboxylic acid esters to the corresponding alcohols. In addition to the aliphatic and aromatic esters, biomass derived γ-valerolactone can be reduced to 1,4-pentanediol in gram scale and with a TON of 3890. Methyl esters have lower reactivity compared to the corresponding ethyl esters, most likely because CH3–O cleavage leads to formation of a bis-carboxylate derivative.
ASSOCIATED CONTENT Supporting Information. Crystallographic data for complexes 2 and 3. This material is available free of charge via the Internet at http://pubs.acs.org. (CCDC# 1533491-1533492)
AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected] Author Contributions
The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.
Notes The authors declare no competing financial interest.
ACKNOWLEDGMENT Acknowledgment is made to the U.S. Department of Energy Office of Science, Basic Energy Sciences Grant #DE-FG0286ER13569 for their support of this work.
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