Application of silica-supported Shvo's catalysts for transfer hydrogenation of levulinic acid with formic acid

Article abstract of DOI:10.1016/j.jorganchem.2017.05.039

Two triethoxysilylpropoxy-functionalized Shvo's catalyst precursors were synthesized and characterized by IR, NMR and HRMS. Both covalent anchoring and sol-gel methods were used for their immobilization. The homogeneous and immobilized catalysts were used for the transfer hydrogenation of levulinic acid with formic acid to form hydroxyvaleric acid, which was readily dehydrated to yield gamma-valerolactone. The immobilized catalysts prepared by the sol-gel method showed higher activity than the covalently grafted catalysts. Hot filtration tests showed no leaching of the immobilized Shvo's catalysts, opening the door to facile catalyst recycling.

Full text of DOI:10.1016/j.jorganchem.2017.05.039

Accepted Manuscript
Application of silica-supported Shvo's catalysts for transfer hydrogenation of levulinic
acid with formic acid
Dongmei He, István T. Horváth
PII:
S0022-328X(17)30336-4
10.1016/j.jorganchem.2017.05.039
JOM 19965
DOI:
Reference:
To appear in: Journal of Organometallic Chemistry
Received Date: 7 March 2017
Revised Date: 17 May 2017
Accepted Date: 18 May 2017
Please cite this article as: D. He, Istvá.T. Horváth, Application of silica-supported Shvo's catalysts for
transfer hydrogenation of levulinic acid with formic acid, Journal of Organometallic Chemistry (2017),
doi: 10.1016/j.jorganchem.2017.05.039.
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ACCEPTED MANUSCRIPT
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Application of Silica-Supported Shvo’s Catalysts for
Transfer Hydrogenation of Levulinic Acid with Formic Acid
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Dongmei He and István T. Horváth*
Department of Biology and Chemistry
City University of Hong Kong, Kowloon, Hong Kong
Email address of the corresponding author: istvan.t.horvath@cityu.edu.hk
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Highlights
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• Triethoxysilylpropyl-functionalized Shvo’s catalyst precursors were prepared,
characterized, and immobilized on silica by covalent anchoring and sol-gel methods.
• The immobilized Shvo’s catalyst precursors were used for the transfer hydrogenation
of levulinic acid with formic acid.
• Hot filtration tests showed no leaching of the immobilized Shvo’s catalysts.
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Dedicated to Professor John A. Gladysz on the occasion of his 65th birthday.
16 Abstract
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Two triethoxysilylpropyl-functionalized Shvo’s catalyst precursors were synthesized and
characterized by IR, NMR and HRMS. Both covalent anchoring and sol-gel methods
were used for their immobilization. The homogeneous and immobilized catalysts were
used for the transfer hydrogenation of levulinic acid with formic acid to form
hydroxyvaleric acid, which was readily dehydrated to yield gamma-valerolactone. The
immobilized catalysts prepared by the sol-gel method showed higher activity than the
covalently grafted catalysts. Hot filtration tests showed no leaching of the immobilized
Shvo’s catalysts, opening the door to facile catalyst recycling.
26 Graphical abstract
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29 Keywords
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Transfer hydrogenation, Shvo’s catalyst, immobilization, levulinic acid, formic acid,
gamma-valerolactone.
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1.
Introduction
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The overarching goal of the immobilization of homogeneous catalytic systems is
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the combination of high product yield with facile catalyst separation [1]. This can be
achieved by “breaking” the uniformity among the molecules of the catalyst and the
product(s) by rendering them in separable phases to allow their separation [2]. The
most frequently used variations involve two liquid phases with no, limited, or
temperature-regulated miscibility of the two phases [3] or a solid phase and a liquid
phase with no, limited, or temperature regulated solubility of the constituent(s) of the
solid phase in the liquid phase [1]. The attachment of ligands of homogeneous catalysts
to the surface of inorganic or organic solid supports via covalent bonds is a typical
method to prepare immobilized catalysts [2, 4, 5], which could be used in continuous
and batch reactors. In the latter case, simple filtration of the reaction mixture is enough
for separation and catalyst recycling.
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Homogeneous transfer hydrogenations are catalyzed by transition metal
catalysts [6, 7]. The Ru-catalyst precursor, {[2,3,4,5-Ph4(η5-C4CO)]2H}Ru2(CO)4(µ-H) (1)
(Scheme 1), was discovered by Shvo in 1984 [8-13]. Each ruthenium atom is ligated by
R
O
H
R
R'
H
Ru
R'
OC
R
R
R
O
R
O
R'
R'
H
H
CO
(2)
O
R
- H2
H2
R'
R'
R
Ru
Ru
Ru
CO
R'
OC
R'
CO
CO
OC
CO
- CO
CO
(4)
CO
R
O
(1)
R
R'
Ru
R'
OC
CO
(3)
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Scheme 1. Shvo’s catalytic systems (1a, 2a, 3a, 4a: R=R’=Ph; 1b, 2b, 3b, 4b: R=Ph and R’=p-MeOPh).
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two carbonyl and a functionalized cyclopentadienyl ligands, which are bridged with a
hydride ligand between the two ruthenium atoms and with a proton shared by two
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oxygen atoms attached to the cyclopentadienyl ligands. The combination of these
peculiar structural features provides remarkable stability even under air at ambient
conditions and a mechanism for reversible dissociation at higher temperatures leading
to the Shvo’s catalyst [1-OH,2,3,4,5-Ph4(η5-C5)]2RuH(CO)2 (2) and the 16-electron
species [2,3,4,5-Ph4(η4-C4C=O)]Ru(CO)2 (3) [14-18]. Although the latter has not been
observed yet, it could form readily from [2,3,4,5-Ph4(η4-C4C=O)]Ru(CO)3 (4) by CO
dissociation and could react with hydrogen or a broad range of hydrogen donors such
as secondary-alcohols or formic acid to generate 2 during transfer hydrogenation
reactions.
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The Shvo’s catalyst 2a was one of the first examples of a bifunctional ligand-
metal catalyst. In the case of hydrogenation of ketones with 2, a concerted transfer of
the hydride from the metal center to the carbon atom and a proton transfer from the
hydroxycyclopentadienyl ligand to the oxygen atom was proposed [14]. We have used
Shvo’s catalysts 2a for the transfer hydrogenation of levulinic acid (LA) to 4-
hydroxyvaleric acid (4-HVA), which readily undergoes dehydration to form gamma-
valerolactone (GVL) (Scheme 2.) [19]. It should be noted, that the equimolar mixture of
LA and FA is the co-products of the acid catalyzed hydration of 5-(hydroxymethyl)-2-
furaldehyde, which can be prepared by acid catalyzed dehydration of carbohydrates
[19].
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Scheme 2. Transfer hydrogenation of LA with FA to 4-HVA in the presence of Shvo’s catalyst 2a followed
by the dehydration of 4-HVA to GVL.
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Although the Shvo’s catalyst 2a could be separated from GVL by vacuum
distillation [18], the required energy could be a limiting factor for large scale
applications. Since the immobilization of Shvo’s catalysts could enable its facile
separation or use in continuous processes [20-21], we report the synthesis and
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characterization of triethoxysilylpropoxy-functionalized Shvo’s catalysts, which were
immobilized to silica and used for the conversion of LA and FA to GVL.
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2.
Results and Discussions
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The multistep synthesis of the catalyst precursors of the Shvo’s catalyst was
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achieved by a combination of known and new methods (Scheme 3). The reaction of
4,4’-dimethoxybenzil with 1,3-diphenylacetone resulted in 3,4-dimethoxyphenyl-2,5-
diphenyl-2,4-cyclopentadien-1-one (5) in 90% yield. 5 was converted to 3,4-
dihydroxyphenyl-2,5-diphenyl-2,4-cyclopentadien-1-one (6) in 88% yield by the addition
of three equivalents of BBr3 followed by hydrolysis at 0 °C [22]. The preparation of 3,4-
triethoxysilylpropoxyphenyl-2,5-diphenyl-2,4-cyclopentdien-1-one (7) was achieved in
43% yield by the coupling reaction of 6 and (3-iodopropyl)triethoxysilane (IPTES) in
acetonitrile in the presence of Cs2CO3 catalyst at 80 °C and 6 hours [23]. IPTES was
obtained from commercial available (3-chloropropyl)triethoxysilane (CPTES) in 95%
yield [24]. Refluxing 7 and 1/3 equivalent of Ru3(CO)12 in toluene for 32 hours resulted
in the mono-nuclear ruthenium complex 8, which was converted to the dinuclear
ruthenium complex 9 by stirring the solution of 8 in refluxing iso propanol for 40 hours.
Compounds 5-9 were purified by column chromatography and structurally characterized
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by IR (Figures S1-S5), H- and C-NMR (Figures S6-S15), and HRMS (Figures S16-
S19).
MeO
HO
O
OMe
Ph
Ph
Ph
Ph
O
Ph
Ph
1. 3 eq BBr3, CH2Cl2,
0
oC, 1 h
O
O
2. H2O, 0 oC, 1 h
88%
NaOH, Ethanol, reflux for 6 h
90%
O
MeO
MeO
HO
2 eq
I
Si(OEt)3
5
6
Cs2CO3, CH3CN, 80 oC, 6 h
43%
(EtO)3Si
(EtO)3Si
Si(OEt)3
O
O
O
O
O
O
H
H
Ph
Ph
Ph
(EtO)3Si
O
O
Ph
O
Ph
Ru
OC CO
Ph
Ru
Isopropanol
1/3 eq Ru3(CO)12
Ph
reflux for 40 h
43%
Toluene, reflux for 32 h
55%
OC
CO
Ru
Ph
(EtO)3Si
O
O
O
OC
CO
CO
Si(OEt)3
(EtO)3Si
9
8
7
(EtO)3Si
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Scheme 3. Synthesis of triethoxysilylpropoxy-group functionalized Shvo’s catalysts.
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Initially, we tried to attach ligand 7 to the surface of silica gel by stirring a
solution of 7 in toluene with a certain amount of silica gel at room temperature until the
red color of the ligand in toluene almost disappeared. After washing and filtration, we
isolated a red powder. Then, a solution of Ru3(CO)12 in toluene was added and the
reaction mixture was refluxed for 24 hours. After filtration and drying, the final solid was
immediately tested as a catalyst for the conversion of LA and FA to GVL. Since the
catalytic activity was too low, we then tried the covalent anchoring of preformed
complexes 8 and 9 as well as their immobilization by the sol-gel method [25-26].
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In the case of covalent anchoring, the silica gel was sintered at 500 °C for 8
hours under nitrogen. Complex 8 was dissolved in d8-toluene to give a yellow solution
and its concentration was measured by 1H NMR. A known amount of sintered silica gel
was added and the slurry was stirred at room temperature for 72 hours resulting in the
silica supported catalyst 10a, which was isolated by filtration (Scheme 4). The
successful immobilization was indicated by the disappearance of the yellow color of the
d8-toluene solution and the disappearance of the 1H NMR peaks for 8. Complex 9 was
immobilized similarly to yield 11a. FT-IR of 10a and 11a showed the expected
characteristic bands for the terminal carbonyl ligands on the ruthenium.
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Scheme 4. Immobilization of triethoxysilylpropoxy-group functionalized Shvo’s catalyst 8 to silica.
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The immobilization of 8 and 9 was also performed by the in situ sol-gel method
resulting in 10b and 11b, respectively. Complex 8 was dissolved in ethanol to give a
yellow solution and then the appropriate amounts of NH4OH and tetraethyl orthosilicate
(TEOS) were added successively (Scheme 5). The resulting mixture was stirred at
room temperature for 48 hours to form a yellow precipitate, which was isolated as a
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yellow powder of catalyst 10b. The same sol-gel procedure was used to incorporate
complex 9 into the silica framework resulting in catalyst 11b. FT-IR of 10b and 11b
confirmed the presence of the terminal carbonyl ligands on the ruthenium.
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Scheme 5. Immobilization of triethoxysilylpropoxy-group functionalized Shvo’s catalyst 8 by the sol-gel
method.
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8
The surface area and the porous nature of silica before and after sintering, and
catalysts 10a, 10b, 11a, and 11b were established by nitrogen adsorption-desorption
experiments (Table 1. and Figures S20-S22).
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Table 1. Physical properties of silica supports and silica supported catalysts 10a, 10b, 11a, and
11b with their catalyst loading.
SBET
(m2/g)
482.62
135.89
115.35
126.43
7.60
Pore volume
(cm3/g)
0.59
DBJH
(nm)
4.09
7.91
7.33
7.42
14.14
16.47
Catalyst loading
(mmol/g)
N/A
Sample
Silicaa
Silicab
10a
11a
10b
0.31
0.25
0.25
0.02
N/A
0.10
0.09
0.11
11b
8.08
0.02
0.11
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aSilica before sintering. bSilica after sintering.
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Both, the surface area and pore volume of silica were reduced after sintering at high
temperature, as expected [27], and further reduced after the immobilization of the
ruthenium complexes to form catalysts 10a and 11a. The surface area and the pore
volume of catalysts 10b and 11b, prepared by the sol-gel method, were significantly
lower. In contrary, their pore diameter was two times greater than catalysts 10a and
11a.
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Compounds 1b and 4b and the two triethoxysilylpropyl-functionalized ruthenium
complexes 8 and 9 were used as homogeneous catalysts for the conversion of LA to
GVL (Table 2). The observed selectivity of LA to GVL was 99.90%, as expected [19].
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Table 2. Transfer hydrogenation of levulinic acid with formic acid in the presence of
homogeneous and heterogenized Shvo’s catalysts at 90 °C under nitrogen.
Catalyst
t (h)
n(FA)/n(LA)
n(LA)/n(catalyst) Yield of GVL (%) TON
TOF (h-1)
12*
24**
24**
24**
24**
24*
2.2a
2.2a
2.2a
2.5b
2.5b
2.5b
2.5b
2.5b
371
371
371
573
573
573
573
573
90.93 ± 0.39
88.74 ± 0.28
11.16 ± 0.28
20.06 ± 0.45
23.03 ± 0.18
87.91 ± 0.30
31.04 ± 0.27
66.98 ± 0.44
334
326
41
115
132
504
178
384
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14
2
5
6
21
6
16
1b
4b
8
9
10a
10b
11a
11b
30**
24*
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8
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*The experiments were done in duplicate. **The experiments were done in triplicate.
a1.28 g of LA (11.02 mmol) and 1.14 g of FA (24.77 mmol) were added in 4 mL of 1,4-
dioxane. b2 g of LA (17.2 mmol) and 2 g of FA (43.4 mmol) were added without
solvent.
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1b showed the highest activity, which was about two times of that of 4b indicating that
the dissociation of carbon monoxide from 4b requires higher activation than the
disproportionation of 1b. In the latter case, one of the products is the active species 2b
and the other is ready to activate FA to form 2b.
The attachment of the
triethoxysilylpropyl groups to two phenyl rings in the para-position has significantly
lowered the catalytic activity of 8 and 9.
The covalently grafted triethoxysilylpropyl-functionalized ruthenium catalysts 10a and
11a displayed somewhat higher activities than 8 and 9. The sol-gel catalysts 10b and
11b were even more active, the conversion of LA and FA to GVL after 24 hours was
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88% and 67% yield, respectively (Figure 3.). A typical H-NMR of the reaction mixture
o
after heating 17.2 mmol LA and 43.4 mmol FA in the presence of 10b at 90 C for 36
hours is shown on Figure S23.
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80
60
11b
10b
11a
10a
40
20
0
0
20
40
60
80
100
Time (h)
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Figure 3 Conversion of LA and FA to GVL with catalysts 10a, 10b, 11a and 11b.
Finally, hot filtration tests were used to study possible catalyst leaching to the
solution. We examined the conversion of LA and FA to GVL at 90 °C and 120 °C in the
presence of silica supported catalysts 10a and 10b. After 12 hours half of each reaction
mixture was filtered at the reaction temperature, and the conversion of GVL was
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measured by quantitative H NMR. Both, the filtered portion and the other portion with
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the catalyst were heated at reaction temperature for an additional 12 hours. The GVL
yield in the reaction mixture with the catalyst increased, as expected, while the filtered
portion showed no change of the GVL yield (Table 3).
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Table 3. Hot filtration test of 10a and 10b.
Temperature
(°C)
Conversion to GVL after
additional 12 hours heating (%)
Conversion to GVL
Catalyst
10a
after hot filtration (%) a
Remaining portion
Filtered portion
Remaining portion
Filtered portion
Remaining portion
Filtered portion
Remaining portion
Filtered portion
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16
45
24
84
41
95
55
90
120
90
16
24
41
55
10b
120
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a1.28 g of LA (11.02 mmol) and1.14 g of FA (24.77 mmol) in 2 mL of d6-DMSO.
The molar ratio of catalyst to substrate was 1:367
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16 3. Conclusions
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Triethoxysilylpropyl-functionalized ruthenium complexes were synthesized and
immobilized on two types of silica support. The homogeneous and immobilized catalysts
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were tested for the conversion of LA and FA to GVL. The catalytic activity of the
triethoxysilylpropyl-functionalized ruthenium complexes was lower than the conventional
Shvo’s catalyst precursors. The immobilized catalysts prepared by the sol-gel method
showed higher TONs, TOFs and yield of GVL as compared with covalently grafted
catalysts. Hot filtration tests confirmed that the catalytic activity is due to the supported
Shvo’s catalyst and not from some active species leached from the solid support to the
solution under the reaction conditions, opening the opportunity to facile catalyst
recycling.
10 4. Experimental
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4.1 General Procedures.
Unless stated otherwise, all reactions were carried out under nitrogen using standard
Schlenk techniques. All solvents used for the synthesis were purified and dried by
standard methods.
4,4’-Dimethoxybenzil, (3-chloropropyl)triethoxysilain (CPTES),
boron tribromide (BBr3), 1,3-diphenylacetone, ruthenium (III) chloride hydrate, Cs2(CO)3,
NaI, NaOH, NH4OH, HNO3, Si(OCH2CH3)4 (tetraethyl orthosilicate or TEOS), levulinic
acid, HCOOH, (CH3)2SO2 (dimethyl sulfone), CD2Cl2, CDCl3, d8-toluene and d6-DMSO
were purchased from commercial sources (Sigma-Aldrich, Acros and Merck) and used
as received. Modified literature methods were used for the synthesis of 3,4-bis(4-
methoxyphenyl)-2,5-diphenylcyclopentadien-3-one [27], Shvo’s catalyst precursors
{[3,4-(p-MeOPh)2-2,5-Ph2(η5-C4CO)]2H}Ru2(CO)4(µ-H) (1b) [13] and [3,4-(p-MeOPh)2-
2,5-Ph2(η4-C4CO)]Ru(CO)3 (4b) [14], Ru3(CO)12 [29], and ICH2CH2CH2Si(OCH2CH3)3
(3-iodopropyl)triethoxysilane or IPTES) [24]. All reactions were monitored by thin-layer
chromatography (TLC) using Merck aluminum sheets silica gel 60 F254 and visualized
with UV light. E. Merck silica gel 60 (230−400 mesh) was used for column
chromatography. The silica gel used for covalent grafting was purchased from Sigma-
Aldrich with high-purity grade.
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1H and 13C NMR spectra were recorded on a 400 or a 600 MHz Bruker
instrument at room temperature. The concentrations of LA and GVL were measured by
using 1H-NMR of the reaction mixture in the presence of known amount of
methylsulfonylmethane or biphenyl as internal standard. All chemical shifts were
recorded in ppm relative to residual CH2Cl2, CHCl3, toluene or DMSO on the δ scale.
MS and HRMS were measured on a Thermo-Finnigan MAT 95 KL or Bruker 9.4 Tesla
Fourier Transform Ion Cyclotron Resonance Mass spectrometer. IR spectra of samples
in KBr pellets or Nujol films were recorded in the range of 400−4000 cm−1 using an
AVATAR 360 FT-IR spectrometer. BET surface area analysis and BJH pore size and
volume analysis were conducted by Micromeritics Tri Star 3000 Surface Area and
Porosity Analyzer. Ruthenium analysis was done by a Perkin Elmer Elan 6100 DRC
ICP-MS.
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4.2
Synthesis and Characterization
4.2.1. 3-Iodopropyl-triethoxysilane (IPTES)
To a stirred solution of NaI (6.00 g, 40 mmol) in anhydrous acetone (40 mL) in a
250 mL three-neck round bottom flask equipped with a condenser and a pressure-
equalizing dropping funnel, 3(chloropropyl)triethoxysilane (CPTES) (4.82 g, 20 mmol)
was added dropwise. The reaction mixture was stirred at 60 °C for 60 hours under N2
and then cooled to room temperature. The solvent was removed under reduced
pressure and the solid residue was washed with diethyl ether three times. The
combined filtrate was concentrated under reduced pressure. The crude product was
purified by column chromatography using hexane-ethyl acetate (10:1) as the eluent to
afford the pure 3-iodopropyl-triethoxysilane (IPTES) as a colorless oil (6.31 g, 95%). 1H
NMR (400 MHz, d6-DMSO): δ 0.64−0.68 (m, 2H, CH2Si), 1.15 (t, 9H, J = 6.0 Hz, CH3),
1.75−1.83 (m, 2H, CH2), 3.28 (t, 2H, J = 6.0 Hz, ICH2), 3.75 (q, 6H, J = 8.0 Hz, OCH2).
13C{1H} NMR (100 MHz, d6-DMSO): δ 11.6 (CH2Si), 12.4 (CH2), 18.2 (CH3), 27.3
(OCH2), 57.8 (SiOCH2).
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4.2.2. 3,4-(p-MeOPh)2-2,5-Ph2(C4CO) (5)
3
4,4’-Dimethoxybenzil (2.90 g, 10.36 mmol) and 1,3-diphenylacetone (2.26 g, 10.36
mmol) were dissolved in 30 mL ethanol in a 100 mL two-neck round bottom flask
equipped with a condenser. The reaction mixture was warmed up nearly to the boiling
point of ethanol, followed by the slow addition of a solution of 0.36 g NaOH in 3 mL
ethanol. When the addition was finished, the reaction mixture was refluxed for 6 hours
followed by cooling to 0 °C. The crude product was then filtered and washed several
times with deionized water and ethanol, and dried under vacuum overnight at room
temperature to afford 5 (4.15 g, 90%) as a dark purple solid. IR (KBr) in cm−1: 1707 (s,
νC=O). 1H NMR (400 MHz, CD2Cl2): δ 3.77 (s, 6H, OCH3), 6.72 (d, 4H, J = 8.0 Hz,
aromatic H), 6.86 (d, 4H, J = 8.0 Hz, aromatic H), 7.20−7.29 (m, 10H, aromatic H).
13C{1H} NMR (100 MHz, CD2Cl2): δ 55.6 (OCH3), 113.7−160.3 (8 resonances,
aromatic), 125.2 (C2,5 of cyclopentadienone or Cp-dienone), 154.7 (C3,4 of Cp-dienone),
200.6 (C1 of Cp-dienone). MS (ESI) m/z (relative intensity) [M+H]+: 445.
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4.2.3 3,4-(p-HOPh)2-2,5-Ph2(C4CO) (6)
To an ice-cold solution of 5 (0.44 g, 1 mmol) in dry CH2Cl2 (9 mL) in a 50 mL
three-neck round bottom flask equipped with a condenser and a pressure-equalizing
dropping funnel, BBr3 (3.5 mL, 1 M in DCM) was added dropwise at 0 °C under N2 in 1
hour. After the addition of all BBr3, the reaction mixture was stirred at 0 °C for an
additional hour, and then quenched with ice-water (10 mL) at 0 °C. The reaction
mixture was diluted with ethyl acetate (20 mL), washed with saturated NaHCO3 (3 x 10
mL) and saturated NaCl (3 x 10 mL). The organic layer was dried over anhydrous
MgSO4. The solvent was removed under reduced pressure and the residue was
purified by column chromatography using hexane-acetone (2:1) as the eluent to afford 6
(0.37 g, 88%) as a black powder. IR (KBr) in cm−1: 3416 (s, νO–H), 1687 (s, νC=O). 1H
NMR (400 MHz, d6-DMSO): δ 6.60 (d, 4H, J = 8.0 Hz, aromatic H), 6.72 (d, 4H, J = 8.0
Hz, aromatic H), 7.27−7.15 (m, 10H, aromatic H), 9.77 (s, 2H, OH). 13C{1H} NMR (100
MHz, d6-DMSO): δ 115.0−158.0 (8 resonances, aromatic), 123.1 (C2,5 of Cp-dienone),
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154.8 (C3,4 of Cp-dienone), 199.8 (C1 of Cp-dienone). MS (ESI) m/z (relative intensity)
[M+H]+ 417, HRMS (ESI) calcd for C29H20O3 [M+H]+: 417.1485, found: 417.1481.
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4.2.4 3,4-[p-(EtO)3Si(CH2)3OPh]2-2,5-Ph2(C4CO) (7)
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Compound 6 (0.88 g, 2.12 mmol) and cesium carbonate (1.38 g, 4.24 mmol)
were dissolved in dry CH3CN (200 mL) in a 500 mL two-neck round bottom flask
equipped with a condenser. IPTES (1.41 g, 4.24 mmol) was added dropwise by using a
micro-syringe. The reaction mixture was stirred at 80 °C for 6 hours under N2 and then
cooled to room temperature. The solvent was removed under reduced pressure. The
crude residue was re-dissolved in 10 mL of hexane and filtered. The solid was washed
with an additional 10 mL of hexane and then the filtrates were concentrated under
reduced pressure. The crude product was purified by column chromatography using
hexane - ethyl acetate (8:1) as the eluent to afford 7 (0.75 g, 43%) as a red sticky solid.
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IR (thin film) in cm−1: 1705 (s, νC=O). H NMR (400 MHz, d6-DMSO): δ 0.63−0.67 (m, 4H,
CH2Si), 1.12 (t, 18H, J = 6.0Hz, CH3), 1.68−1.75 (m, 4H, CH2), 3.74 (q, 12H, J = 8.0 Hz,
SiOCH2), 3.88 (t, 4H, J = 6.0 Hz, OCH2), 6.76 (4H, d, J = 8.0 Hz, aromatic H), 6.83 (4H,
d, J = 8.0 Hz, aromatic H), 7.13−7.29 (10H, m, aromatic H). 13C{1H} NMR (100 MHz, d6-
DMSO): δ 6.0 (CH2Si), 18.2 (CH3), 22.3 (CH2), 57.8 (SiOCH2), 69.1 (OCH2),
114.0−158.9 (8 resonances, aromatic), 124.2 (C2,5 of Cp-dienone), 154.4 (C3,4 of Cp-
dienone), 199.6 (C1 of Cp-dienone). MS (ESI) m/z (relative intensity) [M+Na]+ 847,
HRMS (ESI) calcd for C47H60O9Si2 [M+Na]+: 847.3668, found: 847.3666.
4.2.5. [3,4-(p-MeOPh)2-2,5-Ph2(η4-C4CO)]Ru(CO)3 (4b)
Ru3(CO)12 (60 mg, 0.09 mmol) and 5 (125.2 mg, 0.28 mmol) were suspended in
anhydrous toluene (20 mL) in a 50 mL two-neck round bottom flask equipped with a
condenser. The reaction mixture was stirred at 110 °C under N2 for 48 hours until the
color of reaction mixture changed from dark red to light red, then cooled to room
temperature. The toluene was removed under reduced pressure. The crude residue
was purified by column chromatography using hexane:ethyl acetate (2:1) as the eluent
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to afford 4b (147.2 mg, 83%) as a yellow solid. IR (KBr) in cm−1: 2079 (vs, νC≡O), 2020
(vs, νC≡O), 2008 (vs, νC≡O), 1642 (s, νC=O). 1H NMR (600 MHz, CDCl3): δ 3.74 (s, 6H,
OCH3), 6.64 (d, 4H, J = 6.0 Hz, aromatic H), 6.96 (d, 4H, J = 6.0 Hz, aromatic H),
7.21−7.26 (m, 6H, aromatic H), 7.47 (d, 4H, J = 6.0 Hz, aromatic H). 13C{1H} NMR (151
MHz, CDCl3): δ 55.1 (OCH3), 82.4 (C2,5 of Cp-dienone), 107.3 (C3,4 of Cp-dienone),
113.5−159.6 (8 resonances, aromatic), 174.1 (C1 of Cp-dienone), 194.7 (terminal
carbonyls on Ru). MS (ESI) m/z (relative intensity) [M+H]+ 631, HRMS (ESI) calcd for
C34H24O6 Ru [M+H]+: 631.0695, found: 631.0693.
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4.2.6. {3,4-[p-(EtO)3Si(CH2)3OPh]2-2,5-Ph2(η4-C4CO)}Ru(CO)3 (8)
Ru3(CO)12 (25.8 mg, 0.04 mmol) and 7 (100 mg, 0.12 mmol) were suspended in
anhydrous toluene (10 mL) in a 50 mL two-neck round bottom flask equipped with a
condenser. The reaction mixture was stirred at 95 °C under N2 for 32 hours and then
cooled to room temperature. The toluene was removed under reduced pressure. The
crude residue was purified by column chromatography using hexane:ethyl acetate (4:1)
as the eluent to afford 8 (66.7 mg, 55%) as a yellow sticky solid. IR (thin film) in cm−1:
2081 (vs, νC≡O), 2022 (vs, νC≡O), 1970 (w, νC≡O), 1649 (s, νC=O). 1H NMR (600 MHz,
CDCl3): δ 0.72−0.74 (m, 4H, CH2Si), 1.21 (t, 18H, J = 6.0Hz, CH3), 1.83−1.88 (m, 4H,
CH2), 3.82 (q, 12H, J = 8.0 Hz, OCH2), 3.86 (t, 4H, J = 8.0 Hz, OCH2), 6.62 (d, 4H, J =
8.0 Hz, aromatic H), 6.93 (d, 4H, J = 8.0 Hz, aromatic H), 7.21−7.26 (m, 6H, aromatic
H), 7.45−7.47 (m, 4H, aromatic H). 13C{1H} NMR (151 MHz, CDCl3): δ 6.7 (CH2Si), 18.4
(CH3), 22.9 (CH2), 58.5 (SiOCH2), 69.9 (OCH2), 82.4 (C2,5 of Cp-dienone), 107.5 (C3,4 of
Cp-dienone), 114.0−159.2 (8 resonances, aromatic), 174.2 (C1 of Cp-dienone), 194.7
(terminal CO on Ru). MS (ESI) m/z (relative intensity) [M+H]+: 1011, HRMS (ESI) calcd
for C50H60O12RuSi2 [M+H]+: 1011.2753, found: 1011.2755.
4.2.7. {[3,4-(p-MeOPh)2-2,5-Ph2(η5-C4CO)]2H}Ru2(CO)4(µ-H) (1b)
Compound 5 (400 mg, 0.90 mmol) and Ru3(CO)12 (192 mg, 0.30 mmol) were
added in methanol (50 mL) in a 250 mL two-neck round bottom flask equipped with a
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condenser and heated at reflux for 48 hours. A bright yellow precipitate was formed
and the reaction mixture was cooled to room temperature. The methanol was removed
by filtration and the crude residue was washed with methanol to afford 1b (374.2 mg,
69%) as a yellow powder. IR (CH2Cl2) in cm−1: 2037 (vs, νC≡O), 2006 (m, νC≡O), 1978 (s,
νC≡O). 1H NMR (400 MHz, CD2Cl2): δ -18.47 (s, 1H, RuHRu), 3.67 (s, 12H, OCH3),
6.54−6.58 (m, 8H, aromatic H), 6.91−6.95 (m, 8H, aromatic H), 7.01−7.08 (m, 16H,
aromatic H), 7.12−7.17 (m, 4H, aromatic H). 13C{1H} NMR (100 MHz, CD2Cl2): δ 55.4
(OCH3), 88.4 (C2,5 of Cp), 103.5 (C3,4 of Cp), 113.2−154.8 (8 resonances, aromatic),
159.5 (C1 of Cp), 201.6 (terminal CO on Ru). MS (ESI) m/z (relative intensity) [M+H]+:
1207, HRMS (ESI) calcd for C66H50O10Ru2 [M+H]+: 1207.1569, found: 1207.1567.
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4.2.8. {{3,4-[p-(EtO)3Si(CH2)3OPh]2-2,5-Ph2(η5-C4CO)}2H}Ru2(CO)4(µ-H) (9):
Compound 8 (200 mg, 0.20 mmol) was dissolved in isopropanol (20 mL) in a
100 mL two-neck round bottom flask equipped with a condenser, and heated at reflux
for 40 hours. The reaction mixture was cooled to room temperature and then the
isopropanol was removed under reduced pressure. The crude residue was purified by
column chromatography using hexane/diethyl ether (1:1) as the eluent to afford 9 (167.4
mg, 43%) as a yellow sticky solid. IR (thin film) in cm−1: 2031 (vs, νC≡O), 2001 (m, νC≡O),
1972 (str, νC≡O). 1H NMR (600 MHz, CDCl3): δ -18.50 (s, 1H, RuHRu), 0.67−0.72 (m,
8H, CH2Si), 1.19 (t, 36H, J = 6.0 Hz, CH3), 1.76−1.86 (m, 8H, CH2), 3.80 (q, 24H, J =
8.0 Hz, OCH2), 3.79 (t, 8H, J = 8.0 Hz, OCH2), 6.52 (d, 8H, J = 12.0 Hz, aromatic H),
6.88 (d, 8H, J = 12.0 Hz, aromatic H), 6.97−7.02 (m, 8H, aromatic H), 7.08−7.13 (m,
12H, aromatic H). 13C{1H} NMR (151 MHz, CDCl3): δ 6.6 (CH2Si), 18.4 (CH3), 22.9
(CH2), 58.5 (SiOCH2), 69.8 (OCH2), 88.0 (C2,5 of Cp), 103.0 (C3,4 of Cp), 113.5−154.1
(8 resonances, aromatic), 158.6 (C1 of Cp), 201.2 (terminal CO on Ru). MS (ESI) m/z
(relative intensity) [M+H]+ 1968, HRMS (ESI) calcd for C98H122O22Ru2Si4 [M+H]+
1967.5705, found 1967.5707.
4.2.9. Preparation and Characterization of 10a, 10b, 11a and 11b
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(a) In the glove box, a sample of about 0.1 mmol of 8 or 9 and 1.0 g silica gel
(sintered at 500 °C for 8 h under N2) were weighed into a 25 mL round bottom flask
containing a magnetic stir bar, then 6.0 mL of d8-toluene was added. At this point,
quantitative 1H NMR (t = 0 h) of a small sample was carried out. The resulting mixture
was stirred at ambient temperature for 72 hours. The formed yellow solid was
separated by filtration, washed with CH2Cl2 several times, and dried under vacuum at
room temperature overnight to yield catalysts 10a and 11a. The d8-toluene phase was
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analyzed by quantitative H NMR (t = 72 h) to determine the catalyst loading. The
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catalyst loading was confirmed by digestion of samples of 10a and 11a followed by ICP-
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MS analysis.
(b) A sample of about 0.1 mmol 8 or 9 and 3 mL of TEOS were dissolved in 4
mL ethanol in a 25 mL two-neck round bottom flask equipped with a condenser and a
magnetic stir bar under N2. 2 mL of ammonium hydroxide solution (25% NH3) was
added dropwise. The mixture was stirred at room temperature under N2 for 48 hours.
The produced light yellow powder was separated by filtration, washed with ethanol three
times, and dried under vacuum at room temperature overnight to yield catalysts 10b
1
and 11b. The liquid phase was concentrated and analyzed by quantitative H NMR to
determine the catalyst loading. The catalyst loading was confirmed by digestion of
samples of 10b and 11b followed by ICP-MS analysis.
4.2.10. Determination of ruthenium concentration immobilized Shvo’s catalysts
A sample of 35 mg of 10a, 10b, 11a or 11b was digested in 3 mL of
concentrated nitric acid at 80 °C for 24 hours. Corresponding blank samples were
prepared in the same way. Each solution was then diluted to 25 mL deionized water and
then ICP-MS analysis was conducted.
4.2.11. Transfer hydrogenation of levulinic acid with formic acid without solvent
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A mixture of levulinic acid (2 g, 17.2 mmol) and formic acid (2 g, 43.4 mmol)
was placed in a 25 mL two-neck round bottom flask equipped with a condenser. About
0.03 mmol of 9 or a known amount of silica-supported Shvo’s catalyst (10a, 10b, 11a or
11b) was added and the resulting solution was stirred at 90 °C for specified times. The
concentration of GVL at different times was determined by quantitative 1H NMR.
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4.2.12. Transfer hydrogenation of levulinic acid with formic acid in dioxane
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A mixture of levulinic acid (1.28 g, 11.02 mmol), formic acid (1.14 g, 24.77
mmol) and 1,4-dioxane (4 mL) was placed in a 25 mL two-neck round bottom flask
equipped with a condenser. About 0.03 mmol of 1b, 4b or 8 was added and the
reaction mixture was stirred at 90 °C for specified times. The concentration of GVL at
different times was determined by quantitative 1H NMR.
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4.2.13. Hot filtration test of heterogenized Shvo’s catalysts
To a solution of 1 mL levulinic acid (1.28 g, 11.02 mmol) and 1 mL formic acid
(1.14 g, 24.77 mmol) in 2 mL of d6-DMSO in a 25 mL two-neck round bottom flask
equipped with a condenser and a magnetical stirrer, a certain amount of 10a or 10b was
added. The resulting suspension was stirred at 90 °C or 120 °C for 12 hours. 2 mL of
the reaction solution was separated from the catalyst via a Pasteur pipette and put in a
syringe equipped with a 45 µm PTFE syringe filter. The filtered portion was then added
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to a septum capped NMR tube for H NMR test. At this point, the conversion to GVL
was recorded for t = 12 h. Both the remaining portion of the solution with catalyst and
the filtered portion without catalyst were heated to 90 °C for another 12 hours. Then 1H
NMR spectra of both portions were recorded to obtain the conversions to GVL at t = 24
h.
29 Acknowledgements
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This work was funded by the City University of Hong Kong and the Innovation
and Technology Support Programme of the Innovation and Technology Fund of the
Government of the Hong Kong SAR (ITS/079/13). Any opinions, findings, conclusions or
recommendations expressed in this material/event (or by members of the project team)
do not reflect the views of the Government of the Hong Kong Special Administrative
Region, the Innovation and Technology Commission or the Panel of Assessors for the
Innovation and Technology Support Programme of the Innovation and Technology
Fund. We also thank the Environment and Conservation Fund (ECF/31/2014) for partial
financial support.
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11 Appendix A. Supplementary data
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Supplementary data related to this article can be found at ….
13 References
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