Ir(triscarbene)-catalyzed sustainable transfer hydrogenation of levulinic acid to γ-valerolactone

Article abstract of DOI:10.1002/aoc.6105

Sustainable iridium-catalyzed transfer hydrogenation using glycerol as the hydride source was employed to convert levulinic acid to γ-valerolactone (GVL) with exceptionally high turnover numbers (TONs) (500,000) and turnover frequencies (TOFs) (170,000 h^?1). The highly efficient triscarbene-modified iridium catalysts demonstrated good catalytic activities with low catalyst loadings (0.7 ppm) and good recyclability with an accumulated TON of over two million in the fourth reaction. In addition to glycerol, propylene glycol (PG), ethylene glycol (EG), isopropanol (IPA), and ethanol (EtOH) successfully transferred hydrides to levulinic acid, producing GVL with TONs of 339,000 (PG), 242,000 (EG), 334,000 (IPA), and 208,000 (EtOH), respectively. Deuterium-labeling experiments were conducted to gain insight into the reaction mechanism.

Full text of DOI:10.1002/aoc.6105

Received: 27 August 2020
DOI: 10.1002/aoc.6105
Revised: 27 October 2020
Accepted: 3 November 2020
F U L L P A P E R
Ir(triscarbene)-catalyzed sustainable transfer
hydrogenation of levulinic acid to γ-valerolactone
Kihyuk Sung | Mi-hyun Lee | Yeon-Joo Cheong | Hye-Young Jang
Department of Energy Systems Research,
Sustainable iridium-catalyzed transfer hydrogenation using glycerol as the
hydride source was employed to convert levulinic acid to γ-valerolactone
(GVL) with exceptionally high turnover numbers (TONs) (500,000) and turn-
over frequencies (TOFs) (170,000 h−1). The highly efficient triscarbene-
modified iridium catalysts demonstrated good catalytic activities with low cata-
lyst loadings (0.7 ppm) and good recyclability with an accumulated TON of
over two million in the fourth reaction. In addition to glycerol, propylene gly-
col (PG), ethylene glycol (EG), isopropanol (IPA), and ethanol (EtOH) success-
fully transferred hydrides to levulinic acid, producing GVL with TONs of
339,000 (PG), 242,000 (EG), 334,000 (IPA), and 208,000 (EtOH), respectively.
Deuterium-labeling experiments were conducted to gain insight into the reac-
tion mechanism.
Ajou University, Suwon, South Korea
Correspondence
Young Jang, Department of Energy
Systems Research, Ajou University,
Suwon 16499, South Korea.
Email: hyjang2@ajou.ac.kr
Funding information
National Research Foundation of Korea,
Grant/Award Numbers:
2015M3D3A1A01065436,
2019R1A2C1084021
K E Y W O R D S
iridium complex, sustainable reaction, transfer hydrogenation, triscarbene ligand,
γ-valerolactone
1 | INTRODUCTION
heterogeneous core-shell and bimetallic catalysts were
also reported for the transfer hydrogenation of levulinic
Efficient catalytic systems to convert biomass to high-
value chemicals have been extensively investigated
because sustainable, carbon-rich biomass has the poten-
tial to replace fossil fuels, thereby producing industrially
important chemicals and fuels without further increasing
the carbon concentration in the air. The synthesis of
γ-valerolactone (GVL), a renewable biofuel and green sol-
vent, is one of these important and useful biomass
conversions.[1–5] GVL is produced from biomass-derived
levulinic acid[6,7] by transition-metal-catalyzed hydroge-
nation or transfer hydrogenation.[8–22] While hydrogena-
tion systems for the synthesis of GVL from levulinic acid
involving various ligand-modified Ru and Fe catalysts are
being actively reported, transfer hydrogenation condi-
tions are not intensively studied; a few phosphine and
amine-modified Ir, Ru, and Pd catalysts have been uti-
lized in the presence of formic acid as a hydride
source.[9,15,18,21] In addition to homogeneous catalysts,
acid using formic acid.[23,24] In contrast to the transfer
hydrogenation of levulinic acid using formic acid, only a
few alcohol-mediated transfer hydrogenation reactions
have been employed for the production of GVL.[25–27]
Transfer hydrogenation has the benefits of avoiding
the use of hydrogen at high pressures in favor of sustain-
able liquid hydrogen sources.[28] Thus, it is valuable to
study efficient and sustainable transfer hydrogenation
conditions for the synthesis of GVL from levulinic acid.
The judicious choice of a homogeneous catalyst, includ-
ing consideration of the modifying ligands would allow
efficient transfer hydrogenation to be conducted in the
presence of various sustainable hydride sources.
Glycerol has been employed in transition-metal-catalyzed
transfer hydrogenation owing to the sustainability of
glycerol derived from biomass and the usefulness
of glycerol-derived products, such as dihydroxyacetone
(DHA), propanediol, and lactic acid.[29–31] Thereby, the
Appl Organomet Chem. 2020;e6105.
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SUNG ET AL.
Voutchkova–Kostal group reported the Ir-catalyzed
transfer hydrogenation of levulinic acid using glycerol
under microwave heating conditions with a good turn-
over number (TON) and turnover frequency (TOF).[26]
Here, we report efficient Ir(triscarbene) catalysts for the
transfer hydrogenation of levulinic acid to GVL in the
presence of glycerol, propylene glycol (PG), ethylene gly-
col (EG), isopropanol (IPA), and ethanol (EtOH) with
excellent TONs and TOFs. A TON of 500,000 (TOF of
170,000 h−1) was observed when using glycerol as a
hydride source under conventional heating conditions
(Scheme 1). The efficiency and versatility of our catalysts,
which can use various biomass-derived alcohols
as hydride sources, increase the environmental benefit
of this protocol for the transfer hydrogenation of
levulinic acid.
catalysts), bases (10 mmol based on moles of OH−), H2O
(0.5 ml), and alcohols (glycerol, EG, and PG; 10 mmol)
were added into an autoclave. The mixture was heated at
150ꢀC for 3 h under nitrogen atmosphere. The reaction
mixture was then cooled down to ambient temperature,
diluted with H2O, and acidified with hydrochloric acid
(1.0 ml). The TONs and yields of γ-hydroxyvaleric acid
1
(GHV) and GVL were calculated by H NMR spectros-
copy in D2O, using isonicotinic acid as an internal stan-
dard. For the transfer hydrogenation of ethyl levulinate,
the experiment was performed the same as mentioned
above with ethyl levulinate (5 mmol) instead.
2.3 | General catalytic procedure for
transfer hydrogenation of levulinic acid
using IPA and EtOH
2 | EXPERIMENTAL
2.1 | General
Levulinic acid (580.6 mg, 5 mmol), catalysts (0.0033 μmol
for bimetallic catalysts, 0.0067 μmol for monometallic
catalysts), bases (10 mmol based on moles of OH−), H2O
(0.5 ml), and alcohols (IPA and EtOH; 5 ml) were added
into an autoclave. The mixture was heated at 150ꢀC for
3 h under nitrogen atmosphere. The reaction mixture
was then cooled down to ambient temperature, diluted
with H2O, and acidified with hydrochloric acid (1.0 ml).
The TONs and yields of GHV and GVL were calculated
Levulinic acid (98%), glycerol (99%), ethylene glycol
(99.8%), and EtOH (anhydrous, 99.5%) were purchased
from Sigma-Aldrich. Ethyl levulinate (98%) was obtained
from TCI, isopropyl alcohol (99.9%) was supplied from
Samchun Chemicals, and PG (99%) was from Acros
Organics. Catalysts 1, 10, 2, and 20 were prepared as
described in the literature.[32] The synthesis of catalysts 1,
10, 2, and 20 and the catalytic reactions were conducted
with headspace N2 without Schlenk techniques.
1
by H NMR spectroscopy in D2O, using isonicotinic acid
as an internal standard.
2.4 | Experimental procedure for the
recycling experiment
2.2 | General catalytic procedure for
transfer hydrogenation of levulinic acid
and ethyl levulinate using glycerol, EG,
and PG
First cycle: Levulinic acid (580.6 mg, 5 mmol), catalyst
2 (0.0033 μmol), Ba(OH)2 (5 mmol), H2O (0.5 ml), and
glycerol (5 mmol) were added into an autoclave. The mix-
ture was heated at 150ꢀC for 3 h under nitrogen atmo-
sphere. Then, the reaction mixture was cooled down to
ambient temperature. Second cycle: Levulinic acid
(580.6 mg, 5 mmol), Ba(OH)2 (5 mmol), H2O (0.5 ml),
and glycerol (5 mmol) were added to the mixture of first
cycle. The mixture was heated at 150ꢀC for 3 h under
nitrogen atmosphere. Then, the reaction mixture was
cooled down to ambient temperature. Third cycle:
Levulinic acid (580.6 mg, 5 mmol), Ba(OH)2 (5 mmol),
H2O (0.5 ml), and glycerol (5 mmol) were added to the
mixture of second cycle. The mixture was heated at
150ꢀC for 3 h under nitrogen atmosphere. Then, the reac-
tion mixture was cooled down to ambient temperature.
Fourth cycle: Levulinic acid (580.6 mg, 5 mmol),
Ba(OH)2 (5 mmol), H2O (0.5 ml), and glycerol (5 mmol)
were added to the mixture of third cycle. The mixture
Levulinic acid (580.6 mg, 5 mmol), catalysts (0.0033 μmol
for bimetallic catalysts, 0.0067 μmol for monometallic
SCHEME 1 Ir-catalyzed synthesis of γ-valerolactone (GVL)
from levulinic acid

SUNG ET AL.
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was heated at 150ꢀC for 3 h under nitrogen atmosphere.
Then, the reaction mixture was cooled down to ambient
temperature.
derived from the glycerol dehydrogenation. The reac-
tions of DHA with Ba(OH)2, Ca(OH)2, and KOH were
conducted, showing higher yields of lactate with
Ba(OH)2 (see Table S2). During the conversion of DHA
to lactate, Ba2+ is reported to form a complex with
reaction intermediates, resulting in improved yields of
lactate.[35] Without the consumption of DHA, DHA can
function as an acceptor of the hydride, which can be a
competing process of reducing levulinic acid by Ir–H.
When a carbonate base (K2CO3) was used instead of a
hydroxide base, there was no product (Entry 6). In our
previous work with iridium-catalyzed transfer hydroge-
nation and dehydrogenation of glycerol, it was apparent
that carbene-coordinated iridium catalysts were highly
efficient for the reaction of glycerol even at low concen-
tration.[32,33] Accordingly, the reaction was attempted
with lower iridium catalyst concentration; catalyst load-
ings of down to 0.0025 μmol (0.00005 mol%, 0.5 ppm)
showed consistently high TONs for GVL synthesis
3 | RESULTS AND DISCUSSION
The Ir catalysts used in transfer hydrogenation are illus-
trated in Figure 1. The catalysts 1, 10, 2, and 20 have
been reported in our previous work in which the cata-
lysts were used to dehydrogenate glycerol.[32] During
dehydrogenation, we observed that bimetallic catalysts
1 and 2 produced hydrogen faster than monometallic
catalysts 10 and 20, implying that the Ir–H formation
from bimetallic iridium catalysts and glycerol is facile.
In our recent report on the iridium-catalyzed transfer
hydrogenation of K2CO3 using glycerol,[33] we noticed
that the wing-tip substituent of the carbene ligand
affected the reduction of CO2 by Ir–H. Accordingly,
methyl and ethyl substituents at the wing tip of the
triscarbene ligand and the monometallic and bimetallic
structure of Ir(triscarbene) catalysts might affect the cat-
alytic activity of this reaction because this process also
includes Ir–H generation from Ir(triscarbene) and glyc-
erol and the reduction of levulinic acid by Ir–H. As a
result, Ir(triscarbene)-catalyzed transfer hydrogenations
of levulinic acid in glycerol are conducted as listed in
(Entries 7–10). Although
a
catalyst loading of
0.0025 μmol produced the highest TON, the yield of
GVL began to decrease. Catalyst loadings of 0.0033 μmol
for bimetallic catalysts and 0.0067 μmol for
monometallic catalysts (0.00007 mol%, 0.7 ppm) showed
high
TONs
(460,000–500,000)
and
TOFs
(150,000–170,000 h−1) (Entries 11–13). We did not
observe the effect of wing-tip substituents; both cata-
lysts 1 and 2 formed GVL with TON of 500,000 and
GHV with TON of 33,000, and catalysts 10 and 20
formed products with similar TONs. Compared with
CO2 reduction, the ketone reduction of levulinic acid
might occur readily, showing no catalytic activity differ-
ence induced by the ligand substituents. The bimetallic
and monometallic structure of catalysts did not change
TONs of GVL and GHV, dramatically. To compare cata-
lytic activities of H2 generation via Ir–H and the reduc-
tion of levulinic acid at the same temperature, the
dehydrogenation reactions using catalysts 1 and 10 were
attempted at 150ꢀC, showing hydrogen generation with
only catalyst 1 (TON = 860). Although bimetallic and
monometallic catalysts showed different dehydrogena-
tion reactivity at 150ꢀC, TONs are low. We did not
observe vigorous hydrogen generation at 150ꢀC. Overall,
the bimetallic and monometallic structure of catalysts
does not result in different reactivities in the reduction
of levulinic acid.
1
Table 1. TONs of each reaction were determined by H
NMR analysis (see Figure S1). Optimization began with
the reaction of bimetallic iridium catalyst 1 (0.1 μmol,
0.002 mol%), levulinic acid (5 mmol), and NaOH
(10 mmol) in a mixture of glycerol (10 mmol) and H2O
(0.5 ml) to form GVL (TON, 6800) and GHV (TON,
520) after HCl treatment (Entry 1). Testing other bases
(KOH, CsOH, Ca(OH)2, and Ba(OH)2) revealed that
dicationic hydroxide bases (Ca(OH)2 and Ba(OH)2)
showed much higher TONs for GVL and GHV (Entries
1–5).[34] The significant effect of Ba(OH)2 is presumed
to be related with the consumption of DHA which is
Next, the transfer hydrogenation of ethyl levulinate
was examined under the optimized reaction conditions
for levulinic acid (Table 2). As with the reactions of
levulinic acid, ethyl levulinate was converted to GVL
with excellent TONs and TOFs (Entries 1–4).
In addition to glycerol, PG, EG, IPA, and EtOH were
FIGURE 1 Ir catalysts used in transfer hydrogenation
evaluated as hydride sources (see the Table S1). As

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SUNG ET AL.
TABLE 1 Optimization of transfer
hydrogenation of levulinic acid
Entry Catalyst (μmol) Base (mmol) GVL (TON, yielda) GHV (TON, yielda)
1
1 (0.1)
NaOH (10)
KOH (10)
6800, 27%
530, 2.0%
530, 2.0%
—
2
1 (0.1)
7500, 30%
3
1 (0.1)
CsOH (10)
Ca(OH)2 (5)
Ba(OH)2 (5)
K2CO3 (10)
Ba(OH)2 (5)
Ba(OH)2 (5)
Ba(OH)2 (5)
Ba(OH)2 (5)
Ba(OH)2 (5)
Ba(OH)2 (5)
Ba(OH)2 (5)
1800, 7%
4
1 (0.1)
11,000, 43%
17,000, 67%
—
720, 2.8%
1100, 4.6%
—
5
1 (0.1)
6
1 (0.1)
7
1 (0.01)
1 (0.005)
1 (0.0033)
1 (0.0025)
10 (0.0067)
2 (0.0033)
20 (0.0067)
150,000, 60%
330,000, 67%
510,000, 67%
580,000, 58%
460,000, 62%
510,000, 67%
480,000, 64%
8800, 3.6%
21,000, 4.2%
34,000, 4.4%
36,000, 3.6%
54,000, 7.2%
33,000, 4.4%
32,000, 4.2%
8
9
10
11
12
13
Note: The mixture of catalysts, base, and levulinic acid (5 mmol) in glycerol (10 mmol) and H2O (0.5 ml) was
heated at 150ꢀC for 3 h, and the resulting mixture was treated with HCl. The TONs of bimetallic Ir catalysts
were calculated based on the amounts of iridium ions. The bold emphasis are the best TONs of this reaction
using different catalysts.
Abbreviations: GHV, γ-hydroxyvaleric acid; GVL, γ-valerolactone; TOF, turnover frequency; TON, turnover
number.
aThe yields of GVL and GHV were quantitated with 1H NMR with an internal standard (isonicotinic acid).
TABLE 2 Transfer hydrogenation
of ethyl levulinate to GVL
Entry
Catalyst (μmol)
1 (0.0033)
GVL (TON, yielda)
502,000, 67%
GHV (TON, yielda)
36,000, 4.8%
1
2
3
4
10 (0.0067)
505,000, 68%
30,000, 4.0%
2 (0.0033)
470,000, 63%
30,000, 4.0%
20 (0.0067)
490,000, 66%
41,000, 5.6%
Note: The mixture of catalysts, base, and ethyl levulinate (5 mmol) in glycerol (10 mmol) and H2O (0.5 ml)
was heated at 150ꢀC for 3 h, and the resulting mixture was treated with HCl. The TONs of bimetallic Ir
catalysts were calculated based on the amounts of iridium ions.
Abbreviations: GHV, γ-hydroxyvaleric acid; GVL, γ-valerolactone; TOF, turnover frequency; TON, turnover
number.
aThe yields of GVL were quantitated with 1H NMR with an internal standard (isonicotinic acid).
illustrated in Figure 2, the catalysts were most reactive in
the presence of glycerol. The TONs were slightly lower
with the secondary alcohols (PG and IPA) at ꢁ300,000.
The transfer hydrogenations using primary alcohols
(EG and EtOH) had TONs of ꢁ200,000. Overall, the sec-
ondary alcohol and multihydroxy alcohol hydride sources

SUNG ET AL.
5 of 7
for cyclization of GHV to GVL was not performed,
resulting in the TONs of GHV being shown in Figure 3.
As illustrated in Figure 3, the catalytic activity of the
Ir(triscarbene) catalyst was maintained up to the third
reaction, and in the fourth reaction, slightly decreased
TON value was observed. Notably, the accumulated
TONs for GHV increased from 519,000 in the first cycle
to 2,040,000 in the fourth cycle.
A reaction mechanism is illustrated in Scheme 2.
Based on the mechanism of the previously reported
Ir-catalyzed transfer hydrogenation of glycerol,[33] the
glycerol-bound Ir(I) complex undergoes β-hydride
elimination to afford II, which transfers a hydride to
levulinic acid to form GHV. As we reported, iridium
catalysts modified with bidentate ligands showed the
higher catalytic activity in transfer hydrogenation using
glycerol than monodentate ligands by inhibiting catalyst
deactivation.[33] The second iridium ion of catalyst
1 and the imidazolium of catalyst 10 are speculated to
pull levulinate into the reaction sphere as illustrated in
FIGURE 2 Catalytic activities for γ-valerolactone (GVL)
synthesis in various alcohols
promoted GVL synthesis with excellent TONs. Recently,
Voutchkova–Kostal group reported the water-soluble
iridium-catalyzed transfer hydrogenation of levulinic acid
using glycerol and EtOH under microwave heating con-
ditions to afford GHV with TON of 101,000 (glycerol) and
7580 (EtOH), respectively.[26] Our catalytic conditions
using Ir(triscarbene) catalysts and Ba(OH)2 promoted
these transformations with excellent TONs using various
alcohols such as glycerol, PG, IPA, and EtOH.
A recycling test using the isolated iridium catalyst
from the reaction mixture would be challenging because
of the low catalyst concentration. Therefore, the
reutilization of catalyst 2 was attempted by repeatedly
adding levulinic acid, Ba(OH)2, and glycerol to the reac-
tion mixture (Figure 3 and Table S3). The acid treatment
SCHEME 2 A plausible reaction mechanism and a
FIGURE 3 Recycling results
deuterium-labeling study

6 of 7
SUNG ET AL.
IV and IV0. As a result, a facile reduction of levulinate
occurs to provide high TONs. When the reaction
was conducted in deuterio-glycerol, deuterium was
incorporated in ꢁ92% of the GHV product at the
α-position (see Figure S2). The 8% of nondeuterated
GHV was attributed to hydrogen exchange between
Ir-D and water.[36,37]
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We have presented an Ir(triscarbene)-catalyzed transfer
hydrogenation protocol for the sustainable conversion of
levulinic acid to GVL. These robust and efficient
Ir(triscarbene) catalysts promoted levulinic acid reduc-
tion with superb TONs and TOFs using sustainable
hydrogen sources (glycerol, PG, EG, IPA, and EtOH). In
addition to levulinic acid, ethyl levulinate was converted
to GVL with high TONs and TOFs, like levulinic acid
reactions. Recycling experiments showed that the catalyst
could be recycled without losing catalytic activities up to
the third reaction with high TONs. The structural differ-
ence of catalysts did not affect the catalytic activity, but
multidentate carbene ligands might play an important
role in promoting this reaction with good recyclability
in the presence of extremely low catalyst loadings
(0.7 ppm).
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ACKNOWLEDGMENT
This study was supported by the National Research
Foundation of Korea (No. 2015M3D3A1A01065436 and
No. 2019R1A2C1084021).
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AUTHOR CONTRIBUTIONS
Kihyuk Sung: Data curation; methodology. Mi-hyun
Lee: Data curation; methodology. Yeon-Joo Cheong:
Data curation; methodology; project administration.
DATA AVAILABILITY STATEMENT
The data that support the findings of this study are
openly available in the Supporting Information.
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ORCID
Hye-Young Jang https://orcid.org/0000-0003-4471-2328
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How to cite this article: Sung K, Lee M,
Cheong Y-J, Jang H-Y. Ir(triscarbene)-catalyzed
sustainable transfer hydrogenation of levulinic acid
to γ-valerolactone. Appl Organomet Chem. 2020;
e6105. https://doi.org/10.1002/aoc.6105
SUPPORTING INFORMATION
Additional supporting information may be found online
in the Supporting Information section at the end of this
article.