Catalytic transformation of bio-derived furans to valuable ketoacids and diketones by water-soluble ruthenium catalysts

Article abstract of DOI:10.1039/c5gc01376c

Bio-derived furans such as 2-furfural (furfural), 5-hydroxymethyl-2-furfural (5-HMF) and 5-methyl-2-furfural (5-MF) were successfully transformed to a ketoacid, levulinic acid (LA), and diketones, 1-hydroxyhexane-2,5-dione (1-HHD), 3-hydroxyhexane-2,5-dione (3-HHD) and hexane-2,5-dione (HD), under moderate reaction conditions using water soluble and recyclable 8-aminoquinoline coordinated arene-ruthenium(ii) complexes. Under the optimized reaction conditions using 1 mol% catalyst in the presence of 12 equivalents of formic acid at 80-100 °C, complete conversion of furfural to LA with high selectivity was achieved. Several experiments along with ¹H NMR spectral studies are described which provide more insights into the mechanism underlying the transformation of furans to open ring components. Experiments performed using structural analogues of the active catalyst inferred a structure-activity relationship for the observed superior catalytic activity of the 8-aminoquinoline coordinated arene-ruthenium(ii) complex. Furthermore, due to the high aqueous solubility of the studied complexes, high recyclability, up to 4 catalytic runs, was achieved without any significant loss of activity. Molecular identities of the studied 8-aminoquinoline coordinated arene-ruthenium(ii) complex were also confirmed using single-crystal X-ray diffraction studies.

Full text of DOI:10.1039/c5gc01376c

View Article Online
View Journal
Green
Chemistry
Accepted Manuscript
This article can be cited before page numbers have been issued, to do this please use: K. Gupta, D. Tyagi,
A. D. Dwivedi, M. M. Shaikh and S. K. Singh, Green Chem., 2015, DOI: 10.1039/C5GC01376C.
This is an Accepted Manuscript, which has been through the
Royal Society of Chemistry peer review process and has been
accepted for publication.
Accepted Manuscripts are published online shortly after
acceptance, before technical editing, formatting and proof reading.
Using this free service, authors can make their results available
to the community, in citable form, before we publish the edited
article. We will replace this Accepted Manuscript with the edited
and formatted Advance Article as soon as it is available.
You can find more information about Accepted Manuscripts in the
Information for Authors.
Please note that technical editing may introduce minor changes
to the text and/or graphics, which may alter content. The journal’s
standard Terms & Conditions and the Ethical guidelines still
apply. In no event shall the Royal Society of Chemistry be held
responsible for any errors or omissions in this Accepted Manuscript
or any consequences arising from the use of any information it
contains.
www.rsc.org/greenchem

Page 1 of 30
Green Chemistry
`
1
2
Catalytic transformation of bio-derived furans to valuable ketDoOaI:c10i.d103Va9ie/nCw5AdGrtCicl0e1O3n7l6inCe
diketones by water-soluble ruthenium catalysts
3
4
5
6
7
8
9
Kavita Gupta,a Deepika Tyagi,a Ambikesh D. Dwivedi,a Shaikh M. Mobin a and Sanjay
K. Singh*ab
aDiscipline of Chemistry, School of Basic Sciences, Indian Institute of Technology (IIT)
Indore, Indore 452 017, Madhya Pradesh, India. E-mail: sksingh@iiti.ac.in
bCentre for Material Science and Engineering, Indian Institute of Technology (IIT)
Indore, Indore 452 017, Madhya Pradesh, India
Abstract
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
Bio-derived furans such as 2-furfural (furfural), 5-hydroxymethyl-2-furfual (5-HMF) and
5-methyl-2-furfural (5-MF) was successfully transformed to ketoacid, levulinic acid
(LA), and diketones, 1-hydroxyhexane-2,5-dione (1-HHD), 3-hydroxyhexane-2,5-dione
(3-HHD) and hexane-2,5-dione (HD), at moderate reaction conditions using water
soluble and recyclable 8-aminoquinoline coordinated arene-ruthenium(II) complexes. At
the optimized condition using 1 mol % catalyst in the presence of 12 equivalent formic
acid at 80 ~ 100 °C, complete conversion of furfural to LA with high selectivity was
1
achieved. Several experiments along with H NMR spectral studies are described which
provide more insights into the mechanism of the transformation of furans to open ring
components. Experiment performed using structural analogues of active catalyst inferred
a structural-activity relationship for the observed superior catalytic activity of the 8-
aminoquinoline coordinated arene-ruthenium(II) complex. Furthermore, due to high
aqueous solubility of the studied complexes, high recyclability, up to 4 catalytic runs,
was achieved without any significant loss of activity. Molecular identities of the studied
8-aminoquinoline coordinated arene-ruthenium(II) complex was also confirmed by
single-crystal X-ray diffraction studies.
1

Green Chemistry
Page 2 of 30
`
View Article Online
1
1. Introduction
DOI: 10.1039/C5GC01376C
2
3
Development of new methodologies and catalysts for the transformation of biomass and
biomass derived compounds into biofuel components are highly desirable to reduce the
global dependence on the fossil fuel resources. Moreover, these transformations of
biomass can also produce several value added chemicals for potential applications in
chemical industries. However, direct usage of biomass is unsuitable so far.1-7 In this
regard, furans, an important feedstock of biomass obtained by selective dehydration of
hexose and pentose sugar, are considered as an important resource for the production of
bio-based chemicals.8-19 Furan derivatives such as 5-HMF, furfural and 5-MF are being
used as valuable C5/C6 resources for the synthesis of value added chemicals and biofuel
components based on LA, succinic acid, ketones, 1,6-hexanediol, cyclopentanone, adipic
acid, caprolactam, caprolactone, furoic acid and so on.20-36 Moreover, C5-C9 alkane can
also be produced by the hydrolytic ring opening and complete hydrogenation of furans
and its aldol adducts.36-38
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
Ketoacids and diketones have been successfully used to produce several value
added fine chemicals such as esters, alcohols, lactones, amines and cycloketones.39-41
According to US-DOE, LA (ketoacid) has been identified as one of the 12 important
target chemicals in their biomass program.41 Due to the presence of carbonyl group,
carboxylic group and α-H in LA, various sorts of compounds can be synthesized by
different type of reactions like halogenation, esterification, hydrogenation, condensation
etc. that can be used in drugs, agriculture, food, cosmetics and spice industry (Scheme
1).41 Looking into the pharmaceutical applications of LA, derivatives of LA, δ-
aminolevulinic acid or 5-aminolevulinic acid and acyloxyalkyl ester prodrugs of 5-
aminolevulinic acid have been engaged for photodynamic therapy (PDT) of tumor like
human Meningioma (Scheme 1).42-44 Moreover, the calcium salt of levulinic acid,
2

Page 3 of 30
Green Chemistry
`
1
2
3
4
5
6
7
8
9
calcium levulinate, has been identified as a new supplement for calcium.4V1iewδAr-ticle Online
DOI: 10.1039/C5GC01376C
aminolevulinic acid has also been used as a harmless, highly environmental compatible,
selective and biodegradable photoactivation weedicide which has a vital agricultural
application.41 Levulinates, α-angelica lactone and γ-valerolactone can be employed as
perfume materials and food additives.41 LA can also be used for the production of liquid
hydrocarbon fuels by decreasing oxygen contents through dehydration/hydrogenation
and increasing molecular weight through ketonization (Scheme 1).45 Alternatively, LA
can be upgraded to high energy density compounds such as γ-valerolactone or
methyltetrahydrofuran for blending with gasoline (Scheme 1).
0
10
11
Scheme 1 Catalytic transformation of furan derivatives to ketoacid and diketones
for potential industrial applications.
3

Green Chemistry
Page 4 of 30
`
View Article Online
1
2
Various processes and catalysts have been developed for transformatiDoOnI: o10f.10fu39r/aCn5GsC01376C
into open ring products, but only few are active at low temperature with high selectivity
of the desired product in aqueous medium. For instance Heeres et al. reported a kinetic
study of acid, H2SO4, catalyzed decomposition of 5-HMF to levulinic acid at 98-181 °C
temperature range.46 The transformation of furfural, 5-HMF and furfuryl alcohol to LA
and its ester was studied by Li et al. in the presence of Amberlyst 70 catalyst in water and
methanol at 170 °C and they reported that in aqueous medium polymerization dominates
over ring opening even in the presence of Pd/Al2O3 with Amberlyst 70, a less amount of
furfural was converted to methyl levulinate and levulinic acid using 70 bar H2 at 165 °C
in methanol.47-49 Hydrogenation/hydrolytic ring opening of 5-HMF to the corresponding
diketones was reported by Au nanoparticles in the presence of H3PO4.31 Similar kind of
5-HMF transformation to diketones was also reported over Pd/C catalyst using CO2/H2O
at 120 °C.32 Ebitani et al. studied hydrogenolytic ring opening of 5-HMF to 1,6-
hexanediol over Pd/ZrP at 140 °C using formic acid as a hydrogen source.34 However,
Kegnæs et al. studied the conversion of 5-HMF using O2 with 2.5 bar pressure in the
presence of Ru(OH)x/Al2O3 catalyst, achieved 25% LA at 140 °C.50 Very recently, Zhang
et al. investigated the catalytic transformation of bio-derived 5-HMF and its derivatives
to diketones using Cp*-Ir(III) complexes based catalyst under 20 bar pressure of H2 gas
and 110 °C.51 Above results inferred that so far the hydrogenation/ring opening of furans
were acquired under strident reaction conditions like usage of H2 gas with high pressure
and high temperature. Therefore, new catalytic processes have to be optimized to achieve
high activity and selectivity for the desired product, under moderate reaction conditions.
Over a period of years, arene-ruthenium(II) complexes have been established as
an efficient and active class of catalyst for the hydrogenation of carbonyl groups in
ketones, aldehydes, esters and C-N double bond in imines.52-56 Among several H-donor
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
4

Page 5 of 30
Green Chemistry
`
View Article Online
1
2
3
4
5
6
7
8
9
sources use for the hydrogenation reaction, formic acid offers unique properties. Being a
DOI: 10.1039/C5GC01376C
non-toxic organic acid, formic acid not only served the demand of a H-donor for
hydrogenation, it can also tune the acidity by delivering H+ ion. Moreover, the usage of
water as a solvent facilitated the hydrolytic ring opening of furans as well as offered a
green and clean reaction condition.
Acquaintance with the available facts, in this study we investigated highly
efficient, recyclable and water-soluble homogeneous 8-aminoquinoline coordinated
arene-ruthenium(II) complexes for the catalytic transformation of furan derivatives such
as furfural, 5-HMF and 5-MF to ketoacid, LA, and diketones, 1-HHD, 3-HHD and HD in
the presence of subsidiary amount of formic acid under moderate reaction conditions.
Moreover, time-scaled UV-visible spectroscopic studies were performed to observe the
10
11
12
13
14
15
16
17
1
progress of the catalytic transformation reaction. Using H NMR spectroscopic studies
and several other experiment, attempts were made to identify the key intermediate of the
catalytic reaction. Herein, we also reported the structural characterization of the highly
active 8-aminoquinoline coordinated arene-ruthenium(II) complexes engaged with the
catalytic transformation of furan derivatives using single-crystal X-ray diffraction
studies.
18
19
Scheme 2 Catalytic transformation of furans to ketoacid and diketone based
open ring compounds by arene-ruthenium(II) catalysts.
5

Green Chemistry
Page 6 of 30
`
View Article Online
1
2
2. Results and Discussion
DOI: 10.1039/C5GC01376C
2.1. Synthesis of arene-ruthenium complexes based catalysts
3
4
To perform ruthenium catalyzed transformation of furans in water, we first synthesized
highly water soluble arene-ruthenium(II) complexes containing chelating 8-
aminoquinoline ligand. The 8-aminoquinoline coordinated arene-ruthenium(II)
complexes [(η6-arene)RuCl(8-aminoquinoline)]Cl, [Ru]-1 (η6-arene = η6-benzene) and
[Ru]-2 (η6-arene = η6-p-cymene) were obtained, using the reported process with some
modification,56 in quantitative yield from the reaction of arene-ruthenium dimer
precursors [{(η6-benzene)RuCl2}2] and [{(η6-p-cymene)RuCl2}2], respectively, with 8-
aminoquinoline in methanol for 4 h at room temperature. Analogues of [Ru]-2 were also
5
6
7
8
9
10
11
12
13
synthesized
[(η6-p-cymene)Ru(PPh3)(8-aminoquinoline)]2Cl
([Ru]-3),
[(η6-p-cymene)RuCl(8-
[(η6-p-
cymene)RuCl(2,2’-bipyridine)]Cl
hydroxyquinoline)] ([Ru]-5).56-58
([Ru]-4)
and
14
15
16
17
18
19
20
21
22
23
24
The water soluble red-brown and orange-brown coloured complexes [Ru]-1 and
1
[Ru]-2, respectively, were characterized by H and 13C NMR spectroscopy, elemental
1
analysis and ESI-MS techniques. The H NMR spectra of [Ru]-1 and [Ru]-2 depicted a
downfield shift of the protons of 8-aminoquinoline in comparison to the free
uncoordinated ligand, which is consistent with the coordination of 8-aminoquinoline with
the Ru centre.59,60 Structural characterization of the complexes [Ru]-1 and [Ru]-2 were
further confirmed by single-crystal X-ray diffraction of the most appropriate crystals of
both the complexes (Fig. 1, and Tables S1, S2 and S3 in ESI). The geometry around the
ruthenium centre in both [Ru]-1 and [Ru]-2 complexes is pseudo-octahedral where the
η6-coordinated arene ring is in the apical position and remaining three vertices are
occupied by two nitrogen atoms of bidentate ligand 8-aminoquinoline and chlorine
6

Page 7 of 30
Green Chemistry
`
atom.61-63 The Ru-nitrogen bond distances for the coordinated 8-aminoquinoline are
DOI: 10.1039/C5GC01376C
View Article Online
1
2
3
4
5
6
2.097 Å and 2.094 Å for Ru-N(quinoline) and 2.126 Å and 2.134 Å for Ru-NH2 in [Ru]-
1 and [Ru]-2, respectively.64 The Ru-Cl bond distances are 2.4085 Å and 2.3984 Å for
complexes [Ru]-1 and [Ru]-2 respectively.64 The Ru-C bond distances are also
comparable with other similar complexes.64 The N1-Ru-N2 bite angles in complexes
[Ru]-1 and [Ru]-2 are found to be 78.66° and 78.71°, respectively.65-67
[Ru]-1
[Ru]-2
7
8
9
Fig. 1 ORTEP view of the η6-arene ruthenium(II) complex [Ru]-1 and [Ru]-2 (50
% probability thermal ellipsoids). Chloride counteranion and hydrogen atoms
(except those of –NH2) are omitted for clarity.
10
11
12
13
14
15
2.2. Catalytic transformation of furans into ketoacid and diketones
One pot catalytic transformation of furans into ketoacid and diketones was attempted by
employing ruthenium catalyst, [Ru]-1 and [Ru]-2, in water under moderate reaction
conditions. Initially, furfural, an agricultural by-product and readily available furan
derivative, was chosen as a model substrate for its transformation into LA in the presence
7

Green Chemistry
Page 8 of 30
`
View Article Online
1
2
of [Ru]-1 catalyst. The reaction of furfural with [Ru]-1 catalyst (1 mol %) in the
DOI: 10.1039/C5GC01376C
presence of 24 equivalents of formic acid gave an excellent conversion with >99 %
selectivity for LA in 24 h at 80 °C (Table 1, entry 1). However, decreasing the reaction
temperature to 60 °C or lower (40 °C) resulted in a drastic decrease in the catalytic
conversion of furfural (Table 1, entries 2, 3). Interestingly, complete conversion of
furfural to LA was also achieved in the reaction performed with lower formic acid
content (12 equiv.) at 80 °C in 24 h. However, with further decrease in the formic acid
content or the reaction time resulted in lesser catalytic conversion of furfural to LA
(Table 1, entries 5, 6).
3
4
5
6
7
8
9
10
11
12
13
14
Table 1 Optimization of reaction conditions for the catalytic transformation of
furfural by arene-ruthenium(II) complexes in aqueous condition. Reaction
condition: furfural (1.0 mmol), formic acid (as specified), ruthenium catalyst (1
mol %), water (10 mL).
Entry
Catalyst
[Ru]-1
[Ru]-1
[Ru]-1
[Ru]-1
[Ru]-1
[Ru]-1
[Ru]-1
[Ru]-2
HCOOH (equiv.)
T (°C)/ t (h)
80/24
Conv./ Sel. (%)
>99/>99
8/>99
1
2
3
4
5
6
7
8
24
24
24
12
6
60/24
40/24
6/>99
80/24
>99/>99
46/>99
80/24
12
12
12
80/16
68/>99
60/16
2.5/>99
>99/>99
80/16
8

Page 9 of 30
Green Chemistry
`
View Article Online
DOI: 10.1039/C5GC01376C
9
[Ru]-2
[Ru]-2
[Ru]-2
[Ru]-2
[Ru]-2
[Ru]-2
[Ru]-2
[Ru]-2
[Ru]-2
--
12
12
12
8
60/16
90/12
100/7
80/16
80/16
80/16
80/16
80/16
80/16
80/16
80/16
23/>99
10
11
12
13
14
15
16a
17b
18c
19
>99/>99
>99/>99
85/93
4
37/91
2
21/82
1
5/56
12
12
12
0
57/>99
28/>99
d
e
[Ru]-2
b
c
a 0.5 mol % catalyst. 0.25 mol %. Reaction performed without catalyst. d Unidentified
polymeric compounds. e No conversion.
1
2
3
4
5
6
7
8
9
In contrast to [Ru]-1 catalyst, [Ru]-2 catalyst appeared to be more efficient (Fig.
2), as complete catalytic transformation of furfural to LA was achieved with [Ru]-2
catalyst at 80 °C in 16 h (Table 1, entry 8). Even at 60 °C, the activity of [Ru]-2 was
found to be much higher than that of [Ru]-1 under analogous conditions (Table 1, entries
7, 9). Moreover, increasing the reaction temperature to 90 or 100 °C reaction completion
time can be reduced to 12 and 7 h, respectively (Table 1, entries 10-11). The lower
activity of the benzene complex [Ru]-1 than those of p-cymene complex [Ru]-2 is
consistent with the beneficial effect of the donor groups at the arene ligand.56 Further,
analogous catalytic turnover can be achieved while performing the catalytic reaction with
lower catalyst amount of [Ru]-2, TOFs 7.0 h-1 (0.25 mol %), 7.1 h-1 (0.5 mol %) and 6.2
h-1 (1 mol %) (Table 1, entries 16-17). It is worth mentioning here that the presence of
both ruthenium catalyst and formic acid was found to be essential for the efficient
10
11
12
9

Green Chemistry
Page 10 of 30
`
View Article Online
1
2
conversion of furfural to LA, as reaction cannot proceed or end with humin formation, if
DOI: 10.1039/C5GC01376C
any of these (ruthenium catalyst and formic acid) was absent (Table 1, entries 18-19).
3
4
Fig. 2 Influence of temperature on the catalytic transformation of furfural to LA in
the presence of [Ru]-1 and [Ru]-2 catalysts.
5
6
2.2.1. Effect of catalyst structure on the catalytic transformation of furfural to LA
To further investigate the influence of unique structural advantages with [Ru]-2
catalysts on its catalytic efficiency, its catalytic activity was compared by performing
reaction using structurally analogous ruthenium catalysts having 2,2’-bipyridine ([Ru]-4)
and 8-hydroxyquinoline ([Ru]-5) ligands, in place of 8-aminoquinoline (Scheme 2), for
the catalytic transformation of furfural under the optimized reaction condition.
Interestingly, replacing the 8-aminoquinoline with 2,2’-bipyridine and 8-
hydroxyquinoline in ruthenium complexes [Ru]-4 and [Ru]-5, respectively,
demonstrated a decrease in the catalytic activity in the order of [Ru]-2 > [Ru]-5 > [Ru]-4
for the catalytic transformation of furfural to LA (as shown in Fig. 3 and Table S4 in
ESI). These results clearly depicted the role of N-H entities in the observed superior
7
8
9
10
11
12
13
14
15
16
10

Page 11 of 30
Green Chemistry
`
56
View Article Online
1
catalytic activity of [Ru]-2 catalysts, where, presumably, N-H entities forDmOI:h10y.1d0r3o9/gCe5nGC01376C
bonding to bring the furfural in close proximity of the ruthenium centre as well as
transfer of H+ ion to formyl group to assist the formic acid driven transfer hydrogenation
of furfural to furfuryl alcohol.68 Moreover, replacing the labile chloro group by bulky
PPh3 in [Ru]-3 resulted in significant decrease in the catalytic activity (17 %) suggesting
that under the reaction condition chloro group in the ruthenium catalyst is detached to
provide vacant site for the coordination of water or the format anion (Fig. 3).
2
3
4
5
6
7
8
9
Fig. 3 Influence of catalyst on the catalytic transformation of furfural to LA in the
presence of different arene-ruthenium(II) complexes. Reaction Conditions:
furfural (1.0 mmol), catalyst (1 mol %), formic acid (12 equiv.) and water (10 mL),
T = 80 °C.
10
11
12
2.2.2. Reaction kinetics and identification of reaction intermediates
13
14
15
16
Time-scaled UV-visible spectroscopic experiment (0 ~ 16 h) was performed to
monitor the progress of the catalytic transformation reaction of furfural to LA with [Ru]-
2 catalyst (1 mol %) in the presence of formic acid (12 equiv.) at 80 °C in water (Fig. 4).
As the time progressed, the intensity of the absorption band at 274 nm corresponding to
11

Green Chemistry
Page 12 of 30
`
View Article Online
1
2
3
4
furfural was also decreasing which inferred the consumption of furfural. A linear
DOI: 10.1039/C5GC01376C
relationship between ln(Ct/C0) and the reaction time (on the basis of absorbance for
furfural at 274 nm) was obtained which indicated that the catalytic transformation of
furfural to LA followed pseudo-first-order kinetics (rate constant = 3.7 × 10-5 s-1).
5
6
Fig. 4 Time-scaled UV-visible spectra (inset: kinetics profile) for the catalytic
transformation reaction of furfural to LA in the presence of [Ru]-2 (1 mol %) and
formic acid (12 equiv.) at 80 °C in water.
7
8
Interestingly, during 1H NMR spectral studies of the catalytic reaction performed
with varying content of formic acid (1 ~ 12 equiv.) along with expected product LA
furfuryl alcohol, a key intermediate for the catalytic transformation of furfural to LA, was
also observed. As shown in Fig. 5, with an increase in the content of formic acid the
relative selectivities for furfuryl alcohol and LA from furfural also varied, where the
selectivity towards LA is increasing while that of furfuryl alcohol is decreasing. The
identification of furfuryl alcohol further strengthened the fact that the catalytic
transformation of furfural to LA was proceeding through the initial ruthenium catalyzed
hydrogenation of furfural to furfuryl alcohol using formic acid as a hydrogen source.
Moreover, decrease in the selectivity of furfuryl alcohol with increasing formic acid
9
10
11
12
13
14
15
16
17
12

Page 13 of 30
Green Chemistry
`
1
contents indicated that presumably the intermediate furfuryl alcohol underwent foVrimewiAcrticle Online
DOI: 10.1039/C5GC01376C
2
acid promoted hydrolytic ring opening/hydrogenation to LA.
3
4
Fig. 5 Influence of formic acid (equivalent to furfural) on the [Ru]-2 catalyzed
transformation of furfural to LA and identification of the intermediate, furfuryl
alcohol.
5
6
To further investigate that the formation route of LA is from furfuryl alcohol
hydrolytic ring opening, a new set of experiment was performed under the optimized
reaction conditions in the absence of ruthenium catalyst using furfuryl alcohol as starting
material. Interestingly, a complete conversion of furfuryl alcohol to LA was achieved
even in the absence of any ruthenium catalyst (Table 2, entry 2). These observations
further inferred that the hydrogenation of furfural to furfuryl alcohol is a crucial step to
initiate the catalytic transformation of furfural to LA (Scheme 3). Furthermore, to
investigate if the activation of formic acid by ruthenium catalyst has impact on the
observed high conversion of furfural to LA, we examined the decomposition of formic
acid using [Ru]-2 catalyst in water, without furfural, at 60, 80 and 100 °C. Results indeed
inferred that [Ru]-2 catalyst activates formic acid and complete conversion of formic
acid to H2 was achieved in 6 and 3 h at 80 and 100 °C, respectively, whereas poor
7
8
9
10
11
12
13
14
15
16
17
13

Green Chemistry
Page 14 of 30
`
View Article Online
DOI: 10.1039/C5GC01376C
1
2
conversion was observed at 60 C (Fig. S1 in ESI). The observed trend is well in
accordance with the activity of ruthenium catalyst towards furfural transformation to LA
(Table 1, entries 8-9, 11). However, to examine that whether the in situ generated
hydrogen is assisting the hydrogenation of furfural to furfuryl alcohol, we performed an
experiment for the transformation of furfural using [Ru]-2 catalyst in the presence of H2
gas at 80 °C under optimized reaction conditions. In contrast to the high activity achieved
in the presence of formic acid, replacing formic acid with H2 gas resulted in no
conversion of furfural (Table S5 in ESI). This result further supported that the formation
of metal-hydride bond in the presence of formic acid and -NH moiety of 8-
aminoquinoline ligand facilitated the transfer hydrogenation for the conversion of
furfural to furfuryl alcohol.
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
Moreover, using other short chain organic acids, acetic acid, instead of formic
acid, resulted in only traces of conversion even after 16 h at 80 °C. Moreover, using
organic acids with increasing carbon chain, propionic acid, or the bulky organic acid, iso-
butyric acid also found inactive for the conversion of furfural to LA under the optimised
reaction conditions (Table S5 in ESI). These observations inferred that these acids could
not contribute in metal catalysed transfer hydrogenation. However, the slight conversion
observed was presumably due to the acid catalyzed protonation/hydrolytic ring opening
process of furan.69 Moreover, a facile transformation of furfuryl alcohol to LA with
formic acid or even with acetic acid in the absence of ruthenium, clearly inferred that the
transfer hydrogenation of furfural to furfuryl alcohol is a crucial step which can be taken
place in the presence of formic acid but not with other organic acids or H2 gas under our
optimised reaction conditions. Furthermore, screening of formic acid amount (1 ~ 12
equiv.) depicted that the amount of formic acid is also playing an important role for the
complete transformation of furfural to LA (Table 1, entries 8, 12-15). Moreover, formic
14

Page 15 of 30
Green Chemistry
`
View Article Online
1
acid played a dual role by acting as a H-donor to facilitate the hydrogenation DreOaI:c10ti.1o0n39/aCn5dGC01376C
by populating H+ ion in the reaction mixture to promote the hydrolytic ring opening of
the intermediate furfuryl alcohol to LA (Scheme 3, Fig. S2 in ESI).70 Furthermore, water
provided an additional advantage as the ruthenium catalyst [Ru]-1 and [Ru]-2 displayed
high aqueous solubility.
2
3
4
5
6
7
Scheme 3 Plausible reaction pathway for the catalyzed transformation of furfural
to LA on the basis of 1H NMR studies.
8
2.2.3. Catalytic transformation of furan derivatives
9
The activity of both [Ru]-1 and [Ru]-2 catalysts were further evaluated for
catalytic transformation of other furans having different substituents at 2 and 5 positions
such as 5-HMF and 5-MF under the optimized reaction conditions. Results are shown in
Table 2.
10
11
12
13
14
15
Table 2 Conversion and products distribution for the catalytic transformation of
furan derivatives in aqueous medium. Reaction Conditions: furans (1.0 mmol),
catalyst (1 mol %), formic acid (12 equiv.) and water (10 mL), T = 80 °C.
Entry
1
Furans
Time
(h)
Products
Conv./ Sel. (%)
Conv./ Sel. (%)
[Ru]-1
[Ru]-2
16
68/>99
>99/>99
47a, [62]a, b, c
15

Green Chemistry
Page 16 of 30
`
View Article Online
DOI: 10.1039/C5GC01376C
2
3
12
6
>99/>99d
>99/>99d
4
16
48
7/27:48:33
(a:b:c)
31/27:55:18
(a:b:c)
14/31:41.5:27.5
(a:b:c)
>99/52:21:27
(a:b:c)
>99/54:23:23b,e
(a:b:c)
5
16
30/>99 (a)
89/92:8 (a:b)
67a, [75]a, b, c (a)
a Isolated yield. b T = 100 °C. c Time = 7 h. d Reaction performed without catalyst. e Time
= 30 h
1
2
Notably, poor conversion of 5-HMF (7 % and 31 %) to open ring compounds was
observed after 16 h with both [Ru]-1 and [Ru]-2 catalysts (Table 2, entry 4). However,
higher conversion (>99 %) of 5-HMF to the corresponding open-ring compounds, 1-
HHD, LA and 3-HHD was achieved using [Ru]-2 after 48 h with the relative selectivities
of 52 % (1-HHD), 21 % (LA) and 27 % (3-HHD). Although, the transformation of 5-
HMF to 1-HHD was also reported earlier using other catalyst like Pd/C32 and Cp*-Ir(III)
complexes51 with elevated temperature (110-120 C) and/or high H2 pressure (20 bar),
our reported ruthenium catalysts, [Ru]-1 and [Ru]-2 also facilitated the formation of
other important compounds such as 3-HHD and LA, along with 1-HHD, under mild
reaction conditions. Analogous to furfural, 5-MF also underwent facile catalytic
3
4
5
6
7
8
9
10
16

Page 17 of 30
Green Chemistry
`
1
transformation to corresponding open ring components in the presence of [Ru]-2 catVaielwyAsrtticle Online
DOI: 10.1039/C5GC01376C
2
3
4
5
6
7
8
(Table 2, entry 5). Higher conversion (89 %) of 5-MF to 3-HHD and HD with relative
selectivities of 92 % and 8 %, respectively, was achieved with [Ru]-2 catalyst in 16 h.
One of the major product of 5-MF conversion, 3-HHD, also known as Henze’s ketol, was
previous identified in plant extracts and also known for its role in the human
biochemistry.71 Moreover, the catalytic transformation of 5-HMF and 5-MF was also
performed at 100 °C in the presence of [Ru]-2 catalyst and observed that the reaction
time can be considerably reduced.
9
10
11
12
13
14
15
16
17
18
19
20
Studies for the conversion of furan derivatives to open ring compounds have been
done particularly in autoclave using H2 gas with high pressure and high temperature. Li
et al. investigated that in the presence of Amberlyst 70 furfural convert to polymer in
water as well as methanol.47 They also proposed mechanism for the synthesis of
insoluble polymer from furfural in the presence of acid in aqueous medium.48 Even in the
presence of Pd/Al2O3 with Amberlyst 70, only 23.1 % and 2.4 % yield of methyl-
levulinate and levulinic acid was achieved from furfural using 70 bar H2 at 165 °C in
methanol.49 Furthermore, Kegnæs et al. studied the conversion of 5-HMF in the presence
of Ru(OH)x/Al2O3 catalyst and achieved 25 % (TOF = 0.8 h-1) to LA at 140 °C.50
Discrete from these results, we achieved complete conversion of furfural to LA with
yield up to 62 % (TOF = 8.85 h-1) using 8-aminoquinoline coordinated arene-
ruthenium(II) complexes in water and open atmospheric conditions at 80 ~ 100 °C.
21
22
23
24
25
2.2.4. Thermal stability of ruthenium catalyst
1
We performed H NMR experiments and thermal gravimetric analysis (TGA)
measurements to study the thermal stability of the ruthenium catalysts. In the course of
catalyst recyclability test (discussed in later section), the remarkable stability towards air
and water with an extended time period was observed for the [Ru]-2 catalyst. Further to
17

Green Chemistry
Page 18 of 30
`
View Article Online
1
2
explore the robust activity of the catalyst, thermal stability of ruthenium catalDyOsIt:s1,0.1[0R39u/C]-5G1C01376C
and [Ru]-2, in D2O was tested in NMR tube. The decomposition of the catalyst was
monitored by 1H NMR spectra taken at certain time interval (0, 24, 48 and 72 h). Results
are reported in Fig. S3 and S4 in ESI. Ruthenium catalysts displayed high thermal
stability and only traces of decomposition were observed after 72 h of heating at 80 °C,
whereas at elevated temperature 100 C, decomposition of the ruthenium catalyst started
even after 24 h (Fig. S5 in ESI). Moreover, the respective TGA graphs of catalyst [Ru]-1
and [Ru]-2 inferred that both the catalyst are stable up to 200 °C (Fig. S6 in ESI). The
first loss of ~29 % from [Ru]-2 catalyst in the temperature range 211 ~ 304 °C,
corresponds to the loss of p-cymene ring. Analogously, [Ru]-1 showed the elimination of
benzene ring corresponding to the loss of ~20 % in the temperature range 196 ~ 299 °C.
In view of the high thermal stability and catalytic activity shown by [Ru]-2 catalyst at 80
°C, it is reasonable to examine the recyclability of the ruthenium catalyst at this
temperature for the catalytic transformation of furfural to LA.
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
2.2.5. Recovery and recyclability of ruthenium catalyst
Even if the homogeneous catalyst provides various advantages, catalyst recovery
from the reaction mixture has some limitations. However, synthesized 8-aminoquinoline
coordinated arene-ruthenium(II) complexes having high solubility in water, offered us an
opportunity to easily recover the catalyst from reaction mixture and to reuse it in the
further transformation of furfural without any significant loss of the catalytic activity.
After individual catalytic run, organic components were extracted from reaction mixture
using an appropriate organic solvent while catalyst remained in the aqueous phase.
Aqueous layer was further used for next catalytic run by adding appropriate amount of
distilled water, for making the reaction volume 10 mL, furfural and formic acid. Fig. 6
18

Page 19 of 30
Green Chemistry
`
View Article Online
1
depicted a high recyclability of [Ru]-2 for the catalytic conversion of furfuralDtOoI: L10A.103u9p/C5toGC01376C
2
4 catalytic runs without any significant loss in the catalytic activity.
3
4
Fig. 6 Recyclability of [Ru]-2 catalyst for the catalytic transformation of furfural to
LA.
5
3. Conclusions
6
7
To summarise, we successfully explored the high catalytic performance of 8-
aminoquinoline coordinated arene-ruthenium(II) complexes, [Ru]-1 and [Ru]-2, for the
transformation of bio-derived furans such as furfural, 5-HMF and 5-MF to ketoacid (LA)
and diketones (1-HHD, 3-HHD and HD) based value added chemicals and bio-fuel
components in the presence of formic acid, as a hydrogen source, under moderate
reaction conditions. In the studied ruthenium catalyst, [Ru]-2 catalyst outperformed and
facile catalytic transformation of furfural to LA was achieved with excellent selectivity
(>99 %) in the presence of formic acid at 80 °C in water. Exploring the catalytic activity
of several structural analogues of p-cymene ruthenium complex, [Ru]-2 catalyst, with
different ligands suggested a unique structure-activity relationship in [Ru]-2, due to the
presence of N-H entities and a labile chloro group, which contributes to the superior
catalytic activity of [Ru]-2 catalyst. A plausible catalytic reaction pathway for the
8
9
10
11
12
13
14
15
16
17
19

Green Chemistry
Page 20 of 30
`
1
1
2
3
4
5
6
7
8
conversion of furfural to LA was reasonably proposed on the basis of H NMR reVsieuwlAtsrticle Online
DOI: 10.1039/C5GC01376C
and several other experiments which showed that the formation of furfuryl alcohol, a key
intermediate, is a crucial step for the transformation of furans to LA. Moreover, high
aqueous solubility and stability of the studied ruthenium catalysts offered high
recyclability of the catalyst. We believe that the present study could provide perceptions
towards the synthesis of simple but active catalyst for the transformation of bio-derived
compounds to precious chemicals and bio-fuel components, and can be applicable for
many other catalytic reactions.
9
4. Experimental
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
4.1. Materials
All chemicals used in this study were analytical grade and used without further
purification. Chemicals were purchased from Sigma Aldrich and Alfa Aesar. Dichloro
bridged arene-ruthenium dimers, [{(η6-benzene)RuCl2}2] and [{(η6-p-cymene)RuCl2}2],
used as a precursors for the synthesis of [Ru]-catalysts, and complex [(η6-p-
cymene)RuCl(2,2’-bipyridine)]Cl
([Ru]-4)
and
[(η6-p-cymene)RuCl(8-
hydroxyquinoline)] ([Ru]-5) were synthesized according to the literature procedures.57-60
4.2. Instrumentation
1H NMR (400 MHz), 13C NMR (100 MHz) and 31P NMR (162 MHz) spectra were
recorded using CDCl3 or D2O on a Bruker Avance 400 spectrometer at 298 K. For NMR
studies, tetramethylsilane (TMS) was used as an external standard. Chemical shifts are
reported in ppm relative to the centre of the singlet at 7.26 ppm for CDCl3 and 4.75 ppm
1
13
for D2O in H NMR and centre of triple at 77.0 ppm for CDCl3 in C NMR. Coupling
constants are reported in Hertz (Hz) and the splitting patterns of NMR peaks are deputed
as: s, singlet; d, doublet; t, triplet; m, multiplet; br, broad. Single-crystal X-ray diffraction
studies of arene-ruthenium(II) complexes were carried out using Agilent Technologies
20

Page 21 of 30
Green Chemistry
`
View Article Online
1
Supernova CCD system. High-resolution mass spectra (HRMS) and ESI-MS spectra
DOI: 10.1039/C5GC01376C
2
3
were recorded on a micrOTF-Q II mass spectrometer. GC-MS analysis was performed on
Perkin Elmers GC gas chromatograph Clarus 680 coupled with a perkin Elmers mass
spectrometer Clarus SQ8T in the EI (Electron Impact) mode using a capillary column
ELITE 5 (30 m × 0.25 mm). Thermal gravimetric analysis was performed on the Metler
Toledo Thermal Analysis system.
4
5
6
7
4.3. General procedure for the synthesis of complexes
8
4.3.1. Preparation of complex [(η6-benzene)RuCl(8-aminoquinoline)]Cl ([Ru]-1):
The studied complexes were synthesized using the reported procedure with some
modification.56 In a round bottom flask [{(η6-benzene)RuCl2}2] (0.250 g, 0.5 mmol) and
8-aminoquinoline (0.144 g, 1.0 mmol) were dissolved in methanol (50 mL). The reaction
mixture was stirred for 4 h at room temperature. Solution volume was reduced and the
red-brown crystals of complex [Ru]-1 were grown in solution of methanol and diethyl
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
1
ether (1:10 v/v). Yield = 85 %. H NMR (400 MHz, D2O): δ (ppm) = 9.57 (d, 1H, J =
8Hz), 8.45 (d, 1H, J = 12Hz), 7.84 (d, 1H, J = 8Hz), 7.75 (d, 1H, J = 4Hz), 7.66-7.57 (m,
2H), 5.96 (s, 6H), 13C NMR (100 MHz, D2O): δ (ppm) = 156.20, 145.30, 139.40, 137.88,
129.27, 128.24, 128.10, 127.12, 123.79, 84.74. MS (ESI) m/z calculated for [(η6-
benzene)RuCl(8-aminoquinoline)]+: 358.9 [M+], found 358.0 [M+], [(η6-benzene)Ru(8-
aminoquinoline)]2+: 324.0 [M+], found 323.0 [M+]. Anal. Calcd. for [Ru]-1: C,
45.70; H, 3.58; N, 7.11. Found: C, 45.30; H, 4.01; N, 6.90.
4.3.2. Preparation of complex [(η6-p-cymene)RuCl(8-aminoquinoline)]Cl ([Ru]-2):
Under the analogous conditions of [Ru]-1 preparation, [(η6-p-cymene)RuCl(8-
aminoquinoline)]Cl ([Ru]-2) complex was synthesized by the reaction of [{(η6-p-
cymene)RuCl2}2] (0.306 g, 0.5 mmol) and 8-aminoquinoline (0.144 g, 1.0 mmol) in 50
1
mL of methanol. Yield = 87 %. H NMR (400 MHz, D2O): δ (ppm) = 9.55 (d, 1H, J =
21

Green Chemistry
Page 22 of 30
`
1
2
4Hz), 8.51 (d, 1H, J = 8Hz), 7.89 (d, 1H, J = 8Hz), 7.81 (d, 1H, J = 4Hz), 7.72 (m, V1ieHw A)r,ticle Online
DOI: 10.1039/C5GC01376C
7.65 (t, 1H, J = 8Hz), 6.03 (d, 1H, J = 4Hz), 5.91 (d, 1H, J = 4Hz), 5.80 (d, 1H, J = 4Hz),
5.71 (d, 1H, J = 4Hz), 2.65-2.58 (m, 1H), 2.10 (s, 3H), 1.05 (d, 3H, J = 8Hz), 0.90 (d,
3
13
4
3H, J = 4Hz), C NMR (100 MHz, D2O): δ (ppm) = 156.12, 145.27, 139.46, 137.83,
5
129.27, 128.47, 128.29, 127.36, 124.11, 103.17, 100.83, 84.87, 83.30, 82.68, 82.08,
30.40, 21.19, 21.07, 17.77. MS (ESI) m/z calculated for [(η6-p-cymene)RuCl(8-
aminoquinoline)]+: 415.0 [M+], found 415.1 [M+], [(η6-p-cymene)Ru(8-
aminoquinoline)]2+: 380.0 [M+], found 379.1 [M+]. Anal. Calcd. for [Ru]-2: C, 50.67; H,
4.92; N, 6.22. Found: C, 50.21; H, 4.54; N, 5.96.
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
4.3.3. Preparation of complex [(η6-p-cymene)RuPPh3(8-aminoquinoline)]2Cl ([Ru]-
3): Complex [Ru]-3 was synthesized using [Ru]-2. For that 0.050
g
(0.081 mmol) of [Ru]-2 and 0.022 g (0.081 mmol) of PPh3 were dissolved in 20 mL
methanol and stirred for 4 h. Yield = 85 % 1H NMR (400 MHz, D2O): δ (ppm) = 8.39 (d,
1H, J = 4Hz), 8.32 (d, 1H, J = 8Hz), 7.72 (d, 1H, J = 8Hz), 7.68 (d, 1H, J = 8Hz), 7.58-
7.50 (m, 2H), 7.44-7.40 (m, 3H), 7.26-7.18 (m, 12H), 6.44 (d, 1H, J = 8Hz), 6.34 (d, 1H,
J = 8Hz), 6.27 (d, 1H, J = 8Hz), 5.21 (d, 1H, J = 8Hz), 2.28-2.22 (m, 1H), 1.74 (s, 3H),
1.01 (d, 3H, J = 8Hz), 0.42 (d, 3H, J = 8Hz). 31P NMR (162 MHz, D2O): δ (ppm) =
41.27. MS (ESI) m/z calculated for [(η6-p-cymene)RuPPh3(8-aminoquinoline)]2+: 642.1
[M+], found 641.2 [M+], [(η6-p-cymene)Ru(8-aminoquinoline)]2+: 380.0 [M+], found
379.1 [M+]
4.4. General procedure for the catalytic transformation of furans to ketoacid and
diketones: The catalytic transformation of furans was carried out in open atmospheric
conditions. For this, furan derivative (1 mmol) was added to an aqueous solution (10 mL)
of arene-ruthenium(II) complex (1 mol %) in a round bottom flask and then formic acid
(12 equiv.) was added. Reaction mixture was stirred for a desired time at 80 ºC in an oil
22

Page 23 of 30
Green Chemistry
`
View Article Online
1
bath. The progress of reaction was monitored by thin layer chromatography (DTOLI:C10).1.03A9/fCt5eGrC01376C
the reaction, reaction mixture was extracted with ethylacetate (5 × 10 mL), organic layer
was dried over anhydrous Na2SO4 and then solvent was evaporated under reduced
2
3
4
5
6
7
8
1
pressure to get the desired product. The reaction products were identified by H NMR,
13C NMR, HRMS and GC-MS. The conversion and selectivities of the products were
1
obtained by H NMR. Isolated yield of the products was also calculated. The aqueous
portion of the reaction mixture, containing arene-ruthenium(II) catalyst, was further used
for recyclability experiments.
9
10 Acknowledgements
11
12
13
14
The authors thank IIT Indore, CSIR (01(2722)/13/EMR-II), New Delhi and SERB-DST
(SB/FT/CS-028/2014), New Delhi for financial support. SIC, IIT Indore is acknowledged
for instrumental facilities. K.G. and A.D.D. thank CSIR, New Delhi for their JRF grants.
D.T. thanks UGC, New Delhi for SRF grant.
Notes and references
(1) L. Petrus and M. A. Noordermeer, Green Chem., 2006, 8, 861-867.
(2) L. Hu, G. Zhao, W. Hao, X. Tang, Y. Sun, L. Lin and S. Liu, RSC Adv., 2012, 2,
11184-11206.
(3) A. Corma, S. Iborra and A. Velty, Chem. Rev., 2007, 107, 2411-2502.
(4) A. Yepez, A. Pineda, A. Garcia, A. A. Romero and R. Luque, Phys. Chem. Chem.
Phys., 2013, 15, 12165-12172.
(5) R. A. Sheldon, Green Chem., 2014, 16, 950-963.
(6) R. Xing, A. V. Subrahmanyam, H. Olcay, W. Qi, G. P. van Walsum, H. Pendse
and G. W. Huber, Green Chem., 2010, 12, 1933-1946.
23

Green Chemistry
Page 24 of 30
`
View Article Online
(7) A. J. Ragauskas, C. K. Williams, B. H. Davison, G. Britovsek, J. CaDirOnI:e1y0.,10C39./CA5G.C01376C
Eckert, W. J. Frederick Jr., J. P. Hallett, D. J. Leak, C. L. Liotta, J. R. Mielenz,
R. Murphy, R. Templer and T. Tschaplinski, Science, 2006, 311, 484-489.
(8) A. Osatiashtiani, A. F. Lee, D. R. Brown, J. A. Melero, G. Morales and K. Wilson,
Catal. Sci. Technol., 2014, 4, 333-342.
(9) R. Alamillo, A. J. Crisci, J. M. R. Gallo, S. L. Scott and J. A. Dumesic, Angew.
Chem., Int. Ed., 2013, 52, 10349-10351.
(10) A. Yepez, A. Garcia, M. S. Climent, A. A. Romero and R. Luque, Catal. Sci.
Technol., 2014, 4, 428-434.
(11) A. Corma, S. Iborra and A. Velty, Chem. Rev., 2007, 107, 2411-2502.
(12) M. J. Climent, A. Corma and S. Iborra, Green Chem., 2014, 16, 516-547.
(13) J. Zhang and E. Weitz, ACS Catal., 2012, 2, 1211-1218.
(14) H. Kimura, M. Nakahara and N. Matubayasi, J. Phys. Chem. A, 2013, 117, 2102-
2113.
(15) S. E. Davis, L. R. Houk, E. C. Tamargo, A. K. Datye and R. J. Davis, Catal. Today,
2011, 160, 55-60.
(16) J. M. R. Gallo, D. M. Alonso, M. A. Mellmer and J. A. Dumesic, Green Chem.,
2013, 15, 85-90.
(17) R.-J. van Putten, J. C. van der Waal, E. de Jong, C. B. Rasrendra, H. J. Heeres and
J. G. de Vries, Chem. Rev., 2013, 113, 1499-1597.
(18) J. C. Serrano-Ruiz, R. Luque and A. Sepulveda-Escribano, Chem. Soc. Rev., 2011,
40, 5266- 5281.
(19) S. Liu, Y. Amada, M. Tamura, Y. Nakagawa and K. Tomishige, Green Chem.,
2014, 16, 617-626.
24

Page 25 of 30
Green Chemistry
`
View Article Online
(20) Y. Yang, Z. Du, Y. Huang, F. Lu, F. Wang, J. Gao and J. Xu, Green Chem., 2013,
DOI: 10.1039/C5GC01376C
15, 1932-1940.
(21) Y. Nakagawa, M. Tamura and K. Tomishige, ACS Catal., 2013, 3, 2655-2668.
(22) Y. Nakagawa, H. Nakazawa, H. Watanabe and K. Tomishige, ChemCatChem,
2012, 4, 1791-1797.
(23) W. Xu, H. Wang, X. Liu, J. Ren, Y. Wang and G. Lu, Chem. Commun., 2011, 47,
3924-3926.
(24) F. L. Grasset, B. Katryniok, S. Paul, V. Nardello-Rataj, M. Pera-Titus, J.-M.
Clacens, F. D. Campo and F. Dumeignil, RSC Adv., 2013, 3, 9942-9948.
(25) J. Chen, F. Lu, J. Zhang, W. Yu, F. Wang, J. Gao and J. Xu, ChemCatChem, 2013,
5, 2822-2826.
(26) T. Pasini, M. Piccinini, M. Blosi, R. Bonelli, S. Albonetti, N. Dimitratos, J. A.
Lopez-Sanchez, M. Sankar, Q. He, C. J. Kiely, G. J. Hutchings and F. Cavani,
Green Chem., 2011, 13, 2091-2099.
(27) G. Li, N. Li, S. Li, A. Wang, Y. Cong, X. Wang and T. Zhang, Chem. Commun.,
2013, 49, 5727-5729.
(28) Z. Xinghua, W. Tiejun, M. Longlong and W. Chuangzhi, Fuel, 2010, 89, 2697-
2702.
(29) S. Sitthisa and D. E. Resasco, Catal. Lett., 2011, 141, 784-791.
(30) A. D. Sutton, F. D. Waldie, R. Wu, M. Schlaf, L. A. P. Silks III and J. C. Gordon,
Nature Chem., 2013, 5, 428-432.
(31) J. Ohyama, R. Kanao, A. Esaki and A. Satsuma, Chem. Commun., 2014, 50, 5633-
5636.
(32) F. Liu, M. Audemar, K. D. O. Vigier, J.-M. Clacens, F. D. Campo and F. Jerome,
ChemSusChem, 2014, 7, 2089-2095.
25

Green Chemistry
Page 26 of 30
`
(33) T. Buntara, S. Noel, P. H. Phua, I. Melian-Cabrera, J. G. de Vries and H. J. HeVeierweAsr,ticle Online
DOI: 10.1039/C5GC01376C
Angew. Chem., Int. Ed., 2011, 50, 7083-7087.
(34) J. Tuteja, H. Choudhary, S. Nishimura and K. Ebitani, ChemSusChem, 2014, 7, 96-
100.
(35) M. Sankar, N. Dimitratos, P. J. Miedziak, P. P. Wells, C. J. Kiely and G. J.
Hutchings, Chem. Soc. Rev., 2012, 41, 8099-8139.
(36) Q.-N. Xia, Q. Cuan, X.-H. Liu, X.-Q. Gong, G.-Z. Lu and Y.-Q. Wang, Angew.
Chem., Int. Ed., 2014, 53, 9755-9760.
(37) W. Xu, X. Liu, J. Ren, P. Zhang, Y. Wang, Y. Guo, Y. Guo and G. Lu, Catal.
Commun., 2010, 11, 721-726.
(38) M. Li, X. Xu, Y. Gong, Z. Wei, Z. Hou, H. Li and Y. Wang, Green Chem., 2014,
16, 4371-4377.
(39) I. Podolean, V. Kuncser, N. Gheorghe, D. Macovei, V. I. Parvulescu and S. M.
Coman, Green Chem., 2013, 15, 3077-3082.
(40) E. O’Reilly, C. Iglesias, D. Ghislieri, J. Hopwood, J. L. Galman, R. C. Lloyd and
N. J. Turner, Angew. Chem., Int. Ed., 2014, 53, 2447-2450.
(41) J. Zhang, S. B. Wu, B. Li and H. D. Zhang, ChemCatChem, 2012, 4, 1230-1237.
(42) G. Berkovitch, D. Doron, A. Nudelman, Z. Malik and A. Rephaeli, J. Med. Chem.,
2008, 51, 7356-7369.
(43) M. El-Khatib, C. Tepe, B. Senger, M. Dibué-Adjei, M. J. Riemenschneider, W.
Stummer, H. J. Steiger and J. F. Cornelius, Int. J. Mol. Sci., 2015, 16, 9936-9948.
(44) R. Vallinayagam, F. Schmitt, J. Barge, G. Wagnieres, V. Wenger, R. Neier and L.
Juillerat-Jeanneret, Bioconjugate Chem., 2008, 19, 821-839.
(45) J. C. Serrano-Ruiz and J. A. Dumesic, Catalysis for Alternative Energy Generation,
Springer, 2012, DOI: 10.1007/978-1-4614-0344-9_2.
26

Page 27 of 30
Green Chemistry
`
(46) B. Girisuta, L. P. B. M. Janssen and H. J. Heeres, Green Chem., 2006, 8, 701-7V0ie9w.Article Online
DOI: 10.1039/C5GC01376C
(47) X. Hu, R. J. M. Westerhof, L. Wu, D. Dong and C.-Z. Li, Green Chem., 2015, 17,
219-224.
(48) X. Hu, R. J. M. Westerhof, D. Dong, L. Wu and C.-Z. Li, ACS Sustainable Chem.
Eng., 2014, 2, 2562-2575.
(49) X. Hu, Y. Song, L. Wu, M. Gholizadeh and C.-Z. Li, ACS Sustainable Chem. Eng.,
2013, 1, 1593-1599.
(50) Y. Y. Gorbanev and S. Kegnæs, Top Catal., 2011, 54, 1318-1324.
(51) Z. Xu, P. Yan, W. Xu, X. Liu, Z. Xia, B. Chung, S. Jia and Z. C. Zhang, ACS
Catal., 2015, 5, 788-792.
(52) P. Kumar, R. K. Gupta and D. S. Pandey, Chem. Soc. Rev., 2014, 43, 707-733.
(53) B. L. Conley, M. K. Pennington-Boggio, E. Boz and T. J. Williams, Chem. Rev.,
2010, 110, 2294-2312.
(54) M. C. Warner, C. P. Casey and J.-E. Bäckvall, Top. Organomet. Chem., 2011, 37,
85-125.
(55) R. Karvembu, R. Prabhakaran and K. Natarajan, Coord. Chem. Rev., 2005, 249,
911-918.
(56) H. Türkmen, I. Kani and B. Cetinkaya, Eur. J. Inorg. Chem., 2012, 4494-4499.
(57) C. E. Welby, G. K. Armitage, H. Bartley, A. Wilkinson, A. Sinopoli, B. S. Uppal,
C. R. Rice, and P. I. P. Elliott, Chem. Eur. J., 2014, 20, 8467-8476.
(58) C. Gemel, R. John, C. Slugovc, K. Mereiter, R. Schmid and K. Kirchner, J. Chem.
Soc., Dalton Trans., 2000, 2607-2612.
(59) W.-H. Kwok, G.-L. Lu, C. E. F. Rickard, W. R. Roper and L. J. Wright,
Organomet. Chem., 2004, 689, 2511-2522.
27

Green Chemistry
Page 28 of 30
`
View Article Online
(60) R. Manikandan, G. Prakash, R. Kathirvel and P. Viswanathamurthi, Spectrochim.
DOI: 10.1039/C5GC01376C
Acta, Part A, 2013, 116, 501-508.
(61) D. Carmona, M. P. Lamata, F. Viguri, R. Rodríguez, F. J. Lahoz, I. T. Dobrinovitch
and L. A. Oro, Dalton Trans., 2008, 3328-3338.
(62) A. Singh, N. Singh and D. S. Pandey, J. Organomet. Chem., 2002, 642, 48-57.
(63) B. Therrien, G. Süss-Fink, P. Govindaswamy and C. Saïd-Mohamed, Polyhedron,
2007, 26, 4065-4072.
(64) S. Dayan, N. O. Kalaycioglu, J.-C. Daran, A. Labande and R. Poli, Eur. J. Inorg.
Chem., 2013, 3224-3232.
(65) R. E. Morris, R. E. Aird, P. d. S. Murdoch, H. Chen, J. Cummings, N. D. Hughes,
S. Parsons, A. Parkin, G. Boyd, D. I. Jodrell and P. J. Sadler, J. Med. Chem., 2001,
44, 3616-3621.
(66) D. Tyagi, R. K. Rai, A. D. Dwivedi, S. M. Mobin and S. K. Singh, Inorg. Chem.
Front., 2015, 2, 116-124.
(67) S. K. Singh, M. Trivedi, M. Chandra, A. N. Sahay and D. S. Pandey, Inorg. Chem.,
2004, 43, 8600-8608.
(68) A. Fujii, S. Hashiguchi, N. Uematsu, T. Ikariya, R. Noyori, J. Am. Chem. Soc.,
1996, 118, 2521-2522.
(69) C. R. Waidmann, A. W. Pierpont, E. R. Batista, J. C. Gordon, R. L. Martin, L. A.
P. Silks, R. M. West and R. Wu, Catal. Sci. Technol., 2013, 3, 106-115.
(70) G. M. G. Maldonado, R. S. Assary, J. Dumesic and L. A. Curtiss, Energy Environ.
Sci., 2012, 5, 6981-6989.
(71) D. G. Alberg, T. B. Poulsen, S. Bertelsen, K. L. Christensen, R. D. Birkler, M.
Johannsen and K. A. Jørgensen, Bioorg. Med. Chem. Lett., 2009, 19, 3888-3891.
28

Page 29 of 30
Green Chemistry
`
View Article Online
(72) M. A. Bennett, T.-N Huang, T. W. Matheson and A. K. Smith, Inorg. Synth., 1982,
DOI: 10.1039/C5GC01376C
21, 74-78.
(73) R. A. Zelonka and M. C. Baird, Can. J. Chem., 1972, 50, 3063-3072.
29