Hydrogenation of esters catalyzed by ruthenium PN3-Pincer complexes containing an aminophosphine arm

Article abstract of DOI:10.1021/om500549t

Hydrogenation of esters under mild conditions was achieved using air-stable ruthenium PN³-pincer complexes containing an aminophosphine arm. High efficiency was achieved even in the presence of water. DFT studies suggest a bimolecular proton shuttle mechanism which allows H₂ to be activated by the relatively stable catalyst with a reasonably low transition state barrier.

Full text of DOI:10.1021/om500549t

Note
pubs.acs.org/Organometallics
Hydrogenation of Esters Catalyzed by Ruthenium PN3‑Pincer
Complexes Containing an Aminophosphine Arm
Tao Chen,†,‡ Huaifeng Li,† Shuanglin Qu,§ Bin Zheng,† Lipeng He,† Zhiping Lai,† Zhi-Xiang Wang,*,§
and Kuo-Wei Huang*,†
†Division of Physical Sciences and Engineering, King Abdullah University of Science and Technology, Thuwal 23955-6900, Saudi
Arabia
‡MOE Key Laboratory of Advanced Textile Materials & Manufacturing Technology, National Base for International Cooperation in
Science & Technology of Textiles and Daily Chemistry, Zhejiang Sci-Tech University, Hangzhou 310018, People’s Republic of China
§College of Chemistry and Chemical Engineering, University of the Chinese Academy of Sciences, Beijing 100049, People’s Republic
of China
S
* Supporting Information
ABSTRACT: Hydrogenation of esters under mild conditions
was achieved using air-stable ruthenium PN3-pincer complexes
containing an aminophosphine arm. High efficiency was
achieved even in the presence of water. DFT studies suggest
a bimolecular proton shuttle mechanism which allows H2 to be
activated by the relatively stable catalyst with a reasonably low
transition state barrier.
eduction of esters to the corresponding alcohols
represents one of the most important transformations in
R
organic synthesis. These reactions are typically achieved with
the use of stoichiometric or excess amounts of metal hydride
reducing reagents, such as LiAlH4,1 requiring strictly anhydrous
conditions with the formation of a stoichiometric amount of
metal salt waste. The development of environmentally friendly
procedures for organic reactions is always an important goal of
chemists. In this regard, direct hydrogenation of esters offers an
attractive atom-economical approach. Furthermore, because of
the worldwide research efforts on hydrogen production from
renewables as a major secondary fuel and energy carrier,2 the
utilization of H2 in organic synthesis is likely to play a role with
increasing importance.
Homogeneous catalysis systems for the direct hydrogenation
of esters used to be limited to activated esters, and usually high
temperature and high pressure of hydrogen are required.3
Catalytic hydrogenation of nonactivated esters under mild
conditions remains a research area of interest. In 2006, Milstein
and co-workers have reported the hydrogenation of non-
activated esters to the corresponding alcohols under relatively
mild and neutral conditions, catalyzed by a pyridine-based
PNN-pincer-type Ru(II) complex under 5.3 atm of hydrogen.4
After their discovery, great progress has been made toward
ester hydrogenation through utilization of bifunctional catalysts
based on metal−ligand cooperation (Figure 1).5 These catalysts
can be roughly divided into two classes: one adopts a CH2
group as spacer for the dearomatization−rearomatization of the
central pyridine-based ring for metal−ligand cooperation, and
the other utilizes the N group in the first coordination sphere
for bifunctional activation of H2. Among them, Milstein-type
pyridine-based pincer complexes showed significant catalytic
Figure 1. Ruthenium complexes used in the hydrogenation of esters.
activities under lower hydrogen pressure (typically less than 6
atm).4,5j−l
It is well known that, in comparison to C−H bonds, N−H
bonds are in general more acidic,6 and we have demonstrated
that the replacement of the CH2 group with an NH spacer
indeed favors the deproronation/dearomatization of the PNN-
pincer ligand and offers some enhanced or distinct reactivities.7
For example, a dearomatized ruthenium complex based on the
PN3P ligand displays good catalytic activities in the transfer
Received: May 22, 2014
© XXXX American Chemical Society
A
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Note
hydrogenation of ketones at 82 °C,7a and the replacement of
one strong phosphine arm by a weaker and smaller oxazoline
donor significantly increases the catalytic activity in the same
reaction.7b We have also demonstrated the enhanced
reactivities of ruthenium pincer complexes for the direct
coupling of amines to imines by introducing an NH arm.7c
Herein, we report this class of water-stable PN3-type ruthenium
pincer complexes as effective catalysts for the ester hydro-
genation under mild conditions.
Table 1. Hydrogenation of Ester Catalyzed by the
Ruthenium Complexes
a
b
b
entry
catalyst
conversion (%)
products (yield (%))
1
2
3
4
5
6
7
8
1
2
3
4
5
<5
71
benzyl alcohol (68), ethanol (68)
benzyl alcohol (70), ethanol (69)
benzyl alcohol (62), ethanol (63)
benzyl alcohol (73), ethanol (70)
benzyl alcohol (98), ethanol (97)
benzyl alcohol (98), ethanol (97)
benzyl alcohol (98), ethanol (98)
With the ruthenium PN3-pincer complexes containing an
aminophosphine arm (complexes 1−5, Figure 2; the X-ray
73
65
75
c
5
>99
>99
>99
d
5
e
5
a
Condition: Ru catalyst (10 μmol), KOtBu (80 μmol), and ethyl
benzoate (1.0 mmol) in toluene (4.0 mL) were heated to 120 °C
b
under 81 psi of H2 for 12 h. Conversions and yields were determined
c
d
1
by GC and H NMR. The H2 pressure was increased to 120 psi. A
100 mg portion of water was added to the reaction mixture under 120
e
psi of H2. The H2 pressure was increased to 400 psi and the reaction
time was shortened to 2 h.
Under the optimal reaction conditions, catalyst 5 has been
successfully tested for the hydrogenation of nonactivated esters.
Several aromatic alkyl esters, such as ethyl benzoate, methyl
benzoate, and butyl benzoate, were converted into the
corresponding alkyl alcohol and benzyl alcohol in high yields
under our conditions (Table 2, entries 1−3). Hydrogenation of
ethyl acetate afforded ethanol in 98% yield (Table 2, entry 4).
This result is particularly interesting, since it provides a 4.34 wt
% gravimetric hydrogen storage capacity when combined with
the quantitative dehydrogenation of ethanol to ethyl acetate we
developed earlier.7d Pentyl acetate was hydrogenated to 1-
pentanol under similar conditions in 96% yield (Table 2, entry
Figure 2. Various NH-spacer-containing ruthenium pincer complexes.
Table 2. Hydrogenation of Esters Catalyzed by the
a
Ruthenium Complex 5
Figure 3. Molecular structure of 4 with thermal ellipsoids drawn at the
30% level. Hydrogen atoms (except hydride) are omitted for clarity.
Selected bond distances (Å): Ru1−N1 2.054(5), Ru1−N3 2.099(6),
Ru1−P1 2.2323(16), Ru1−C19 1.774(8). Selected angles (deg): N1−
Ru1−C19 175.7(3), N1−Ru1−Cl1 83.37(15), P1−Ru1−C19 99.4(3),
N3−Ru1−P1 155.09(18).
structure of 4 is given in Figure 3), we examined their catalytic
activities for the hydrogenation of esters (Table 1). Under 81
psi of hydrogen at 120 °C for 12 h with a catalytic amount of 1
(1.0 mol %), less than 5% conversion of the ethyl benzoate
ester was found (Table 1, entry 1). To our delight, when
unsymmetrical PN3 ruthenium complexes 2−5 were used as the
catalysts under the same conditions, hydrogenation of ethyl
benzoate resulted in formation of benzyl alcohol in good yields
(Table 1, entries 2−5). With an increase of the hydrogen
pressure up to 120 psi, reduction of ethyl benzoate with
hydrogen resulted in almost quantitative yields of benzyl
alcohol and ethanol (Table 1, entry 6). Remarkably, the ethyl
benzoate was still fully hydrogenated to benzyl alcohol and
ethanol when 100 mg of water was added to the reaction
mixture (Table 1, entry 7). This implies that the solvent
without strict purification can be used for hydrogenation with
our catalysts. Furthermore, using 400 psi of hydrogen could
enable the reaction to be completed in 2 h (Table 1, entry 8).
a
Condition: ruthenium complex 5 (10 μmol), KOtBu (80 μmol), and
ester (1.0 mmol) in toluene (4.0 mL) were heated to 120 °C under
400 psi of hydrogen for 2 h. Conversions of esters and yield of
alcohols were determined by GC and H NMR.
b
8
1
B
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Note
5). Notably, the reaction of a bulky ester, 2-methylbutyl 2-
methylbutanoate, with hydrogen resulted in a good yield of 2-
methylbutan-1-ol under the standard conditions (Table 2, entry
6). It was demonstrated that the bulky ester was not effectively
hydrogenated using Milstein’s catalyst.4 The benzyl benzoate
was also smoothly hydrogenated to the corresponding alcohols
in almost quantitative yield (Table 2, entry 7).
To understand the reaction mechanism, we performed a
density functional theory (DFT) study by using Gaussian 09.9
The hydrogenation of ethyl acetate (7) to ethanol was chosen
to compute the mechanism. As illustrated in Figure 4, the
Furthermore, in agreement with our previous finding that a
proton transfer shuttle (e.g., water or alcohol) plays a crucial
role in mediating various hydrogen transfer steps,10 the
hydrogen activation step requires a two-water shuttle, while
for other hydrogen transfer steps (TS2−TS4), a one-water
shuttle is sufficient. The energetic results indicate that the
hydrogen activation step with a barrier of 24.2 kcal/mol is the
rate-determining step of the whole catalytic cycle. The whole
transformation is exergonic by 7.7 kcal/mol. Thus, the
hydrogenation of ethyl acetate (7) to ethanol is feasible in
terms of both kinetics and thermodynamics.
In conclusion, we have demonstrated a new class of PN3-type
ruthenium pincer complexes as effective catalysts for ester
hydrogenation under mild conditions. These catalysts can
tolerate the presence of water, allowing the use of benchtop
solvents without prior purification. DFT calculations revealed a
termolecular proton shuttle mechanism for the activation of H2
offering a reasonably low transition state barrier, consistent with
the experimental observations.
EXPERIMENTAL SECTION
■
General Information. All syntheses were carried out under an
argon atmosphere using standard Schlenk and glovebox techniques
unless otherwise stated. Literature methods were applied for the
preparation of ruthenium complexes 1−4.7 Solvents were dried
according to known procedures and freshly distilled prior to use.
Synthesis of [2,2′-Bipyridin]-6-amine. NH3 was fed to a mixture
of 6-bromo-2,2′-bipyridine (3.0 mmol, 705 mg) and Cu2O (0.02
mmol, 3 mg) in glycol (5.0 mL) in an autoclave. The reaction mixture
was maintained under 10 atm of NH3 at 110 °C for 24 h. After the
mixture was cooled to room temperature, water (5.0 mL) was added
and the aqueous phase was extracted with DCM (5.0 mL × 3). The
combined organic layers were washed with brine (10.0 mL × 2) and
dried over MgSO4. The crude product was purified by column
chromatography on silica gel (hexanes/ethyl acetate 1/1) to afford a
white solid (410 mg, 80% yield). 1H NMR (500 MHz, CDCl3): δ 8.66
(d, 1H, J = 4.0 Hz), 8.25 (d, 1H, J = 7.9 Hz), 7.77 (t, 1H, J = 7.7 Hz),
7.70 (d, 1H, J = 7.5 Hz), 7.57 (t, 1H, J = 7.8 Hz), 7.26 (t, 1H, J = 5.0
Hz), 6.54 (d, 1H, J = 8.1 Hz), 4.53 (br, 2H). 13C{1H} NMR (125
MHz, CDCl3): δ 157.97, 156.38, 154.57, 149.11, 138.58, 136.73,
123.31, 120.94, 111.59, 108.87.
Figure 4. Computed mechanism for the hydrogenation of ethyl
acetate to ethanol catalyzed by 5. The free energies (in red) are mass-
balanced and relative to 5 + 7 + 2H2.
whole transformation includes the hydrogenation of 7 to
acetaldehyde (10) and ethanol and the hydrogenation of 10 to
ethanol. Starting from the active catalyst 5, the hydrogenation
of 7 (the black cycle in Figure 4) proceeds via three steps,
including hydrogen activation (5 + H2 → TS1 → 6), hydrogen
transfer from 6 to the carbonyl group of ethyl acetate via TS2,
leading to a hemiacetal intermediate (8) and 5, and
decomposition of the intermediate 8 to give ethanol and
acetaldehyde (10). The decomposition step takes place by
adding the hydroxyl group of 8 to the Ru···N active site of 5 via
TS3, followed by proton transfer from N(H) to (Et)O via TS4.
The hydrogenation of acetaldehyde 10 to ethanol (the blue
path) occurs through the process 10 + 6 →TS5 → ethanol.
The proposed mechanism was further supported by exper-
imental observations: (1) both benzaldehyde and acetaldehyde
can be fully hydrogenated under the same conditions; (2) when
the hydrogenation of ethyl benzoate was stopped at 30%
conversion, a trace amount of benzaldehyde was detected; (3)
when a 10:1 mixture of ethyl benzoate and benzaldehyde was
hydrogenated to 30% conversion, only a trace amount of
benzaldehyde was detected. These results suggest that
aldehydes are plausible intermediates in the ester hydro-
genation and they are hydrogenated once generated under the
same conditions before the esters are fully consumed.
Synthesis of N-(Di-tert-butylphosphino)-[2,2′-bipyridin]-6-
amine. To a suspension of [2,2′-bipyridin]-6-amine (86 mg, 0.5
mmol) in toluene (10.0 mL) was added triethylamine (70 μL, 0.5
mmol). The mixture was then cooled to 0 °C, and (tBu)2PCl (96 μL,
0.5 mmol) was added dropwise. Upon further cooling to −78 °C,
nBuLi (0.5 mmol, 320 μL of a 1.6 M solution in hexanes) was slowly
added. The solution was warmed to room temperature and stirred
overnight at 80 °C. The solution was then filtered ,and the solvent was
removed from the filtrate in vacuo. The remaining yellow oil was
washed with hexanes (2.0 mL × 3). The resulting product was directly
used for the next step without further purification. 31P{1H} NMR (162
1
MHz, CDCl3): δ 59.61 (s). H NMR (400 MHz, CDCl3): δ 8.66 (d,
1H, J = 4.2 Hz), 8.26 (d, 1H, J = 8.0 Hz), 7.78 (dt, 1H, J = 7.8 Hz, 1.8
Hz), 7.66 (d, 1H, J = 7.3 Hz), 7.59 (t, 1H, J = 7.8 Hz), 7.22 (dd, 1H, J
= 7.8 Hz, 2.4 Hz), 5.11 (d, 1H, J = 10.4 Hz), 4.67 (br, 1H), 1.18 (d,
18H, J = 12.0 Hz). HRMS (ESI): calcd for C18H27N3P m/z 316.19426
(M + 1)+, found 316.19464.
Synthesis of PNNCN-RuHCl(CO) Complex 5. In a 25 mL
pressure vessel equipped with a magnetic stirring bar were placed the
ligand N-(di-tert-butylphosphino)-[2,2′-bipyridin]-6-amine (158 mg,
0.5 mmol), Ru(PPh3)3HCl(CO) (476 mg, 0.5 mmol), and 10 mL of
dry THF in an argon glovebox. The flask was sealed and heated to 65
°C overnight. Then the reaction mixture was cooled to room
temperature and red precipitates were formed. The solid was filtered,
washed with pentane (5.0 mL × 3), and dried in vacuo to afford pure
complex 5 (197 mg, 82%). 31P{1H} NMR (242 MHz, CD3OD):
C
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Organometallics
Note
1
155.80 (d, J = 24.0 Hz). H NMR (600 MHz, CD3OD): δ 9.09 (s,
1H), 8.31 (d, 1H, J = 7.4 Hz), 8.10 (t, 1H, J = 6.5 Hz), 7.85 (t, 1H, J =
7.4 Hz), 7.73 (d, 1H, J = 7.4 Hz), 7.61 (t, 1H, J = 5.8 Hz), 7.16 (d, 1H,
J = 9.0 Hz), 1.44 (d, 9H, J = 14.4 Hz), 1.31 (d, 9H, J = 14.4 Hz),
−18.59 (d, 1H, J = 19.2 Hz). 13C{1H} NMR (150 MHz, CD3OD): δ
206.03 (d, J = 8.1 Hz), 163.18, 158.43, 155.38, 154.61,140.85, 139.59,
127.91, 123.96, 113.56, 111.84 (d, J = 6.2 Hz), 40.48 (d, J = 17.4 Hz),
40.32 (d, J = 23.2 Hz), 29.42 (d, J = 5.6 Hz), 29.05 (d, J = 4.4 Hz).
HRMS (ESI): calcd for C19H27N3OPRu m/z 446.09352 (M − Cl)+,
found 446.09439. Anal. Calcd for C19H27ClN3OPRu: C, 47.45; H,
5.66; N, 8.74. Found: C, 47.19; H, 5.81; N, 8.42.
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Typical Procedure for the Catalytic Hydrogenation. To a
mixture of 5 (4.8 mg, 0.01 mmol), KOtBu (9 mg, 0.08 mmol), and
toluene (4.0 mL) in a Parr high-pressure reactor was added the ester
(1.0 mmol). The dark red solution was purged with H2 and stirred
under 400 psi of H2 at 120 °C. The products were analyzed by gas
1
chromatography (GC) or H NMR using mesitylene as an internal
standard.
ASSOCIATED CONTENT
* Supporting Information
Text, figures, tables, and a CIF file giving experimental details
and NMR spectra, computational details, and crystallographic
data for 4. This material is available free of charge via the
Internet at http://pubs.acs.org.
■
S
AUTHOR INFORMATION
Corresponding Authors
*E-mail for Z.-X.W.: zxwang@ucas.ac.cn.
*E-mail for K.-W.H.: hkw@kaust.edu.sa.
■
(9) (a) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.;
Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Mennucci,
B.; Petersson, G. A.; Nakatsuji, H.; Caricato, M.; Li, X.; Hratchian, H.
P.; Izmaylov, A. F.; Bloino, J.; Zheng, G.; Sonnenberg, J. L.; Hada, M.;
Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima,
T.; Honda, Y.; Kitao, O.; Nakai, H.; Vreven, T.; Montgomery, J. A., Jr.;
Peralta, J. E.; Ogliaro, F.; Bearpark, M. J.; Heyd, J.; Brothers, E. N.;
Kudin, K. N.; Staroverov, V. N.; Kobayashi, R.; Normand, J.;
Raghavachari, K.; Rendell, A. P.; Burant, J. C.; Iyengar, S. S.;
Tomasi, J.; Cossi, M.; Rega, N.; Millam, N. J.; Klene, M.; Knox, J. E.;
Cross, J. B.; Bakken, V.; Adamo, C.; Jaramillo, J.; Gomperts, R.;
Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.;
Ochterski, J. W.; Martin, R. L.; Morokuma, K.; Zakrzewski, V. G.;
Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Dapprich, S.; Daniels, A.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We are grateful for generous financial support from the King
Abdullah University of Science and Technology. T.C. thanks
the Natural Science Foundation of China (21302170), the
Young Researchers Foundation of Key Laboratory of Advanced
Textile Materials and Manufacturing Technology, Ministry of
Education (2012QN13), and the Science Foundation of
Zhejiang Sci-Tech University (1201839-Y).
̈
D.; Farkas, O.; Foresman, J. B.; Ortiz, J. V.; Cioslowski, J.; Fox, D. J.
Gaussian 09, Gaussian, Inc., Wallingford, CT, 2009. (b) All structures
were optimized in the gas phase at the M06/BSI level where BSI
represents a basis set with 6-31G(d,p) for non-metal atoms and SDD
for Ru. The energies were then improved by M06/BSII//M06/BSI
single-point calculations with solvent effects accounted for by the
SMD solvent model, using the experimental solvent (toluene). BSII
denotes a basis set with 6-311++G(d,p) for non-metal atoms and SDD
for Ru. All calculations were carried out using the Gaussian 09
program; more details are given in the Supporting Information..
(10) Qu, S.; Dang, Y.; Song, C.; Wen, M.; Huang, K.-W.; Wang, Z.-X.
J. Am. Chem. Soc. 2014, 136, 4974−4991.
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dx.doi.org/10.1021/om500549t | Organometallics XXXX, XXX, XXX−XXX