Selective Conversion of Various Monosaccharaides into Sugar Acids by Additive-Free Dehydrogenation in Water

Article abstract of DOI:10.1002/cctc.202000544

Abundant sugars of five and six carbon atoms are promising candidates for the production of valuable platform chemicals. Here, we describe the catalytic dehydrogenation of several pentoses and hexoses into their corresponding sugar acids with the concomitant formation of molecular hydrogen. This biomass transformation is promoted by highly active and selective catalysts based on iridium(III) complexes containing a triazolylidene (trz) as ligand. Monosaccharides are converted into sugar acids in an easy and sustainable manner using only catalyst and water, and in contrast to previously reported procedures, in the absence of any additive. The reaction is therefore very clean, and highly selective, which avoids the tedious purification and product separation. Mechanistic investigations using ¹H NMR and UV-vis spectroscopies and ESI mass spectrometry (ESI-MS) indicate the formation of an unprecedented diiridium-hydride as dormant species that correspond to the catalyst resting state.

Full text of DOI:10.1002/cctc.202000544

The European Society Journal for Catalysis
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Accepted Article
Title: Selective conversion of various monosaccharaides into sugar
acids by additive-free dehydrogenation in water
Authors: Jose A Mata, Andres Mollar-Cuni, Joseph P. Byrne, Pilar
Borja, Cristian Vicent, and Martin Albrecht
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ChemCatChem
10.1002/cctc.202000544
FULL PAPER
Selective conversion of various monosaccharaides into sugar
acids by additive-free dehydrogenation in water
[a]
[b]
[a]
[c]
*[b]
Andres Mollar-Cuni, Joseph P. Byrne, Pilar Borja, Cristian Vicent, Martin Albrecht, and Jose A.
*[a]
Mata
[a]
[b]
[c]
A. Mollar-Cuni, Dr. P. Borja and Dr. J. A. Mata
Institute of Advanced Materials (INAM), Centro de Innovación en Química Avanzada (ORFEO-CINCA)
Universitat Jaume I
Avda. Sos Baynat s/n, 12006, Castellón, Spain
E-mail: jmata@uji.es
@inam_uji
Dr. J. P. Byrne and Prof. Dr. M. Albrecht
Department of Chemistry & Biochemistry.
University of Bern
Freiestrasse 3, 3012. Bern, Switzerland
E-mail: martin.albrecht@dcb.unibe.ch
@albrecht_lab
Dr. C. Vicent
Servei Central d’Instrumentació Científica (SCIC)
Universitat Jaume I
Avda. Sos Baynat s/n, 12006, Castellón, Spain
Supporting information for this article is given via a link at the end of the document.
Abstract: Abundant sugars of five and six carbon atoms are
promising candidates for the production of valuable platform
chemicals. Here, we describe the catalytic dehydrogenation of several
pentoses and hexoses into their corresponding sugar acids with the
concomitant formation of molecular hydrogen. This biomass
transformation is promoted by highly active and selective catalysts
based on iridium-(III) complexes containing a triazolylidene (trz) as
ligand. Monosaccharides are converted into sugar acids in an easy
and sustainable manner using only catalyst and water, and in contrast
to previously reported procedures, in the absence of any additive. The
reaction is therefore very clean, and moreover highly selective, which
avoids the tedious purification and product separation. Mechanistic
acids produces useful chemical intermediates with applications in
food, pharmaceutical and other specific industries. Traditionally,
oxidation of sugars is carried out by enzymatic or heterogeneous
catalysts.[21,22] The main limitation for these approaches is a
rigorous control of the reaction conditions to avoid pH variations,
oxygen pressure and stability of the natural catalysts. For the
conversion of sugars, there is a need to develop selective
transformations avoiding tedious separation or purification
techniques. Unsurprisingly, therefore, the majority of catalysts for
the conversion of carbohydrates is restricted to materials and
enzymes. The use of homogeneous catalysts for the conversion
of sugars is scarce even though these types of catalysts generally
lead to highly selective transformations.[23,24] One of the reasons
for this underdeveloped chemistry is the often poor stability and
solubility of well-defined organometallic species in aqueous
media. However, recent developments have indicated substantial
progress of organometallic chemistry in water,[25–28] thus providing
new opportunities for biomass transformation in aqueous media
with homogeneous catalysts.
1
investigations using H NMR and UV-vis spectroscopies and ESI
mass spectrometry (ESI-MS) indicates the formation of an
unprecedented diiridium-hydride as dormant species that correspond
to the catalyst resting state.
Introduction
Biomass represents an abundant and natural renewable
feedstock for the preparation of initial platform chemicals.[1–4] The
growing interest in the production of bio-sourced chemicals lies in
the circular character of carbon which represents a neutral carbon
process. Replacing the use of fossil fuels with transformation of
biomass is a major challenge for reduction of global CO
2
Figure 1. Selective dehydrogenation of sugars into sugar acids without
additives.
emissions.[5–9] The initial platform chemicals obtained from
biomass are sustainable intermediates for the production of
[10–13]
specific chemicals.
Carbohydrates are abundant in nature
and constitute one of the major components of plant biomass.[
Among them, monosaccharides of five and six carbon rings are
the basic units of cellulose and hemicellulose. Conversion of
these sugars, particularly D-Glucose and D-Xylose, represents an
interesting biomass source for the preparation of initial platform
chemicals.[16–20] Oxidation of sugars into the corresponding sugar
14,15]
Prompted by this expansion of organometallic chemistry into
aqueous media, we decided to explore the potential of iridium
complexes as homogeneous catalysts for the conversion of
glucose. In preliminary work, we demonstrated the formation of
gluconic acid using homogeneous Cp*Ir complexes with
1
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imidazole-based N-heterocyclic carbene (NHC) ligands.[29] This
transformation is formally an oxidation of glucose but is better
described as a dehydrogenative coupling of glucose and water
with the simultaneous formation of molecular hydrogen. Starting
from glucose, the process is highly selective towards the
formation of gluconic acid but requires strong acidic conditions. In
this context, modulation of well-defined organometallic species
with suitable ligands offers an attractive methodology to increase
activity and selectivity in the activation of functional groups. Here,
we demonstrate that the introduction of triazolylidene (trz) ligands
induces a substantial improvement of the catalytic properties of
these iridium complexes in the conversion of sugars into sugar
acids (Figure 1). The enhanced catalytic competence allows to
omit the harsh acid additives and significantly broadens the
substrate scope to efficient dehydrogenative coupling of other
carbohydrates. Moreover, this approach provided relevant
mechanistic insights into this transformation.
monitored by NMR spectroscopy and HPLC chromatography. As
a
representative example, the reaction progress for the
conversion of glucose to gluconic acid was profiled using complex
2 as catalyst precursor (Figure 3a). The mass balance confirms
that all glucose is converted to gluconic acid. Product analysis by
NMR spectroscopy and MS does not show the formation of any
other by-products. Interestingly, and contrary to our previous
observations using complex 1, complex 2 performs better without
additives. A >90% conversion of glucose is observed without the
2 4
addition of H SO after 7.5h. In contrast, the addition of acid (0.25
equiv.) reduced the conversion to 65% (Figure 3b).
Results and Discussion
The catalytic conversion of sugars into sugar acids was tested
Figure 3. Reaction profile monitored by HPLC-MS/MS of glucose conversion
using
a
series of Cp*Ir(carbene)Cl
2
complexes containing
(
black squares) and gluconic acid formation (red diamonds) using catalytic
quantities of complex 2 (2 mol%) at 110 ⁰C (bath temperature). Glucose (0.44
mmol) in 20 mL H O: a) without additives and b) with 0.25 eq. of H SO
different carbene ligands (Figure 2). Complex 1, containing an
Arduengo-type N-heterocyclic carbene ligand (NHC), was used
for comparative purposes. Complexes 2-4 contain a triazole-
derived ligand with different substituents that modulate the
electronic and the steric properties. The triazolylidene ligands do
not possess a neutral resonance structure and are therefore more
polar. Moreover, iridium trz complexes were observed to be highly
water-soluble,[30,31] which may be due in parts to the polar
structure, and in other parts due the presence of a backbone
nitrogen for potential hydrogen bonding.[32] Complex 4 contains a
carbohydrate wingtip group which has been introduced to probe
whether hydrogen bonding to sugar substrates favours the
catalytic process.
2
2
4
.
These results prompted us to study the catalytic activity of 1 – 4
in the conversion of glucose to gluconic acid with and without
additives (Table 1). Control experiments revealed that the
presence of iridium complexes is required for the production of
gluconic acid and hydrogen. We have not observed uncatalysed
sugar degradation under neutral or acidic conditions at 110 ⁰C
(Table 1, entry 1). Monitoring substrate conversion over time
using catalyst 4 showed high initial rates that, however, rapidly
slowed down. This is an indication of catalyst deactivation by
limited stability (Figures S4 and S5). In order to study the process
1
of catalyst deactivation we performed thermal stability tests by H
2
NMR spectroscopy. Solutions of complex 2 and 3 in D O (10 mM)
are stable at 100 ⁰C for at least 2 h without noticeable
decomposition whereas complex 4 completely decomposes
under these conditions into undefined products. Reducing the
temperature to 80 ⁰C increases stability and we have not
observed degradation for up to 1 h. These results show that
catalyst 2 and 3 containing alkyl or aryl N-substituents are stable
at the optimal catalytic conditions but catalyst 4 decomposes
rapidly, which accounts for the drop in catalytic activity of this
complex. Conversion of glucose at lower temperatures (80 °C) is
feasible, though requiring longer reaction times to achieve full
conversion (Table 1, entry 6).
Figure 2. Iridium complexes used in the catalytic dehydrogenation of sugars
featuring an NHC ligand (1) and triazolylidene (trz) ligands (2–4), respectively.
In a first set of experiments, we evaluated the catalytic activity of
complexes 1–4 in the conversion of glucose to gluconic acid. The
experimental set-up consists of the addition of the iridium complex
to a solution of glucose in deionised water. The mixture is heated
at reflux and the progress of the reaction and selectivity is
2
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ChemCatChem
10.1002/cctc.202000544
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dehydrogenation with the formation of molecular hydrogen. The
formation of hydrogen was qualitatively analysed by gas
chromatography using a TCD detector. The set-up for the catalytic
experiments only requires the addition of the catalysts into an
aqueous solution of the carbohydrate. However, it is important to
note that the reaction mixture should be maintained under
constant reflux in an open system, since the continuous release
of hydrogen from the reaction mixture facilitates conversion to the
sugar. The reaction is not sensitive to the presence of oxygen
indicating the high stability of catalysts under the reaction
conditions. Catalysts 1–3 are highly active in the production of
sugar acids, the isolated yields are >90% in most cases. As noted
for glucose conversion, complex 1 requires the addition of
additives for better catalytic performance, which compromises
yields to some extent. In contrast, no additives are necessary
when employing complexes 2 and 3 containing a triazolylidene
ligand, and isolation of the product is therefore particularly simple.
Table 1. Glucose dehydrogenation using catalysts 1–4.
Additive
SO (Eq.)
T
Conv
Yield
(%)a
Entry
Cat.
o
a
H
2
4
( C)
110
110
110
110
110
80
(%)
1
2b
3b
4
-
-
-
-
-
1
1
2
2
2
3
4
4
44
88
100
81
90
71
35
51
44
88
100
81
71
67
33
45
0.25
-
5
0.25
6
-
7
-
-
110
110
110
8
9
0.25
1
Analysis of the crude product by H NMR spectroscopy revealed
Reaction conditions: Glucose (80 mg, 0.44 mmol), catalyst loading (2 mol%),
O (20 mL) for 20h. [a] Conversions and yields determined by 1
highly selective oxidation of the C1 position and clean formation
of the corresponding sugar acids and therefore, no further
purification was required (See ESI for detailed procedures and
characterisation). Of note, product analysis in the case of D-
glucose or D-xylose by NMR spectroscopy under neutral or acidic
conditions is complicated by the presence of different species due
to the acid/base equilibria coupled with partial lactonisation.
However, in basic media, these equilibria are shifted and only one
set of signals is observed.
deionised H
2
H
NMR spectroscopy and HPLC-MS/MS. [b] Data from reference 29.
In view of the excellent conversion of glucose to gluconic acid by
dehydrogenation we decided to explore the catalytic activity of
complexes 1–3 towards other C5 and C6 sugars (Table 2). A
range of pentoses (D-ribose, D-xylose and L-arabinose), hexoses
(
D-mannose and D-galactose) as well as 2-deoxy-D-glucose are
converted in water to the corresponding sugar acids by
Table 2. Reaction scope. Conversion of pentoses and hexoses into sugar acids.
Entry
Reaction
Cat.
1
Additive[a]
Conv (%)[b]
Yield (%)[b]
1
H
H
H
H
H
2
2
2
2
2
SO
4
4
4
4
4
100
87
2
3
4
5
6
7
8
9
2
3
1
2
3
1
2
3
1
2
3
1
2
3
-
100
100
100
99
93
95
80
98
99
91
91
91
95
71
68
92
94
85
-
SO
-
-
99
SO
100
100
100
100
74
-
-
10
11
12
13
14
15
SO
-
-
80
SO
100
100
98
-
-
1
6
7
2
3
-
-
100
100
90
92
1
Reaction conditions: Sugar (0.44 mmol), catalyst loading (2 mol%), deionised H
determined by H NMR spectroscopy and HPLC-MS/MS for selected experiments.
2
O (5 mL) at 110 ⁰C for 20h. [a] Additive 0.25 eq. [b] conversions and yields
1
3
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Monitoring of the reaction progress was performed by 1H NMR
spectroscopy, using L-arabinose as a model substrate with
catalyst 2 (Figure 4). The results show that the conversion of L-
arabinose into L-arabinoic acid is very fast at the initial stages and
then slows down and proceeds slowly. For instance, yields are
higher than 50% after 4 h and after 8 h the yields are close to 80%.
This is the general trend observed in the conversion of all the
sugars tested either by 1H NMR spectroscopy (Figure 4) or
HPLC/MS (Figure 3a) suggesting a first order reaction in catalyst.
S54). In the lower m/z region, we also observed doubly-charged
species of formula [Cp*Ir(trz)]2+ (m/z 250.6, not shown) attributed
2 2
O)
]2+
to the generation of the diaqua complex [Cp*Ir(trz)(H
containing labile H O ligands that are removed upon ionisation
2
under ESI conditions.
Scheme 1. Equilibrium of complex 2 in water.
+
[
Cp*Ir(trz)(Cl)] and
+
[
Cp*Ir(trz)(OH)(OH2)]
518.2
536.2
100
500.2
98.2
534.2
4
546.2
501.2
519.2
560.2
0
m/z
4
80
80
490
500
510
520
530
540
550
560
+
[
Cp*Ir(trz)(OH)]
5
18.2
1
00
516.2
+
[
Cp*Ir(trz)(OH)(OH) + Na]
519.2
5
58.2
0
m/z
4
490
500
510
520
530
540
550
560
Figure 5. ESI mass spectra of complex 2 recorded in water (top) and water with
NaOH 10-3 M (bottom). The peak at m/z 500.2 is attributed to a product ion from
species at m/z 518.2 and 536.2.
Further support for the solvolysis equilibrium depicted in Scheme
1
is provided by recording the ESI-MS of 2 at higher pH. Under
these conditions, it is expected that iridium-bound water
molecules in complexes [Cp*Ir(trz)(H O)
]2+ are deprotonated to
Figure 4. Time-resolved conversion monitored by 1H NMR spectroscopy. L-
Arabinose (0.44 mmol) conversion into L-Arabinoic acid using catalyst 2 (2
2
2
hydroxo ligands which are not removed under positive-mode ESI
conditions. Figure 5 displays the ESI mass spectrum of aqueous
solutions in the presence of NaOH (10-3 M solution). Species
mol%) without additives in H
dried under reduced pressure and dissolved in D
2
O (5 mL). Samples were taken at appropriate time,
O for NMR analysis. Signals
2
highlighted in green are diagnostic for determining yields, and resonances
highlighted in blue for conversions.
[Cp*Ir(trz)(OH)]+ (m/z 518.2) and [Cp*Ir(trz)(OH)
58.2) were detected by their m/z values, isotopic composition
and CID spectra (Figures S55, S56). Analysis of this solvolysis
equilibrium by 1H NMR spectroscopy using complex 2 in D
O,
+ Na]+ (m/z
2
5
Mechanistic investigations
2
In order to gain mechanistic insight into the conversion of sugars
we first addressed the chemical speciation of iridium complexes
in water using pre-catalyst 2 as representative complex. Catalyst
revealed the presence of a single set of signals for the Cp* and
the triazolylidene ligand, which is consistent with fast equilibria on
the NMR time scale.
2
is soluble in water and forms an orange solution. The solubility
Furthermore, we monitored the progress of the catalytic
conversion of sugars by NMR and UV/Vis spectroscopies and by
ESI-MS. The combination of these orthogonal techniques
of 2 in water is promoted by the polar nature of the triazolylidene
ligand and a facile chloride-water ligand exchange at iridium
(
Scheme 1) as identify by the chemical speciation determined
provides insights into the identity of intermediate species relevant
to the mechanism.[34,35] Notably, the conversion of sugars is
from ESI-MS and NMR spectroscopy.[33] Aqueous solutions of 2
were stirred and aliquots were extracted at different time intervals,
diluted with water to a final concentration of 5 × 10-4 M (based on
the initial Ir concentration), and directly introduced to the mass
spectrometer. The ESI-MS of 2 in water (Figure 5) shows two
intense signals containing the hydroxo-ligated species
accompanied by a diagnostic change of colour of the reaction
solution from orange to deep-red when using complexes 1–3. The
deep-red colour is formed at initial stages and maintained during
the entire catalytic reaction. This change in colour was analysed
in more detail using L-arabinose as model substrate and complex
[
Cp*Ir(trz)(OH)]+ (m/z 518.2) and [Cp*Ir(trz)(OH)(OH
2
)]+ at m/z
2
as the pre-catalyst (Figures 6a and 6b). The initially orange
colour originates from two broad absorptions at 340 nm and about
00 nm, the latter observed only as a shoulder. The spectrum of
+
5
36.2, the latter coexisting with [Cp*Ir(trz)Cl)] . Even though
species [Cp*Ir(trz)(OH)(OH
2
)]+ and [Cp*Ir(trz)Cl]+ overlap in the
4
MS, they could be unambiguously identified on the basis of their
distinctive collision induced dissociation (CID) spectra (Figure
the deep-red coloured solution during conversion of L-arabinose
features two distinct absorption bands at 445 and 545 nm.
4
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ChemCatChem
10.1002/cctc.202000544
FULL PAPER
from single crystal X-ray diffraction of complex 5 (hydrogen atoms except
Monitoring the reaction by ESI-MS revealed a signal at m/z 501.2,
which is maintained during the course of the reaction. The
mass/charge value and the simulated isotopic pattern distribution
support the formation of a dicationic diiridium-hydride species
3 3
SO
-) omitted for clarity). (d) ESI-MS of the
hydrides and counterions (2 CF
deep-red solution showing a base peak at 501.2 m/z that fits with [Cp*(trz)Ir(µ-
Ir(trz)Cp*]2+ and a minor peak (m/z 666.2) due to the deprotonated arabinoic
H)
acid-bound complex formulated as [Cp*Ir(trz)(C )]
2
+
5
O
6
H
9
.
(
Figure 6d). Other signals confirmed the reaction progress as
indicated by the observation of signals corresponding to
deprotonated arabinoic acid-bound complex formulated as
+
[
Cp*Ir(trz)(C
5
O
6
H
9
)] at m/z 666.2 (Figure 6d) and in the negative
scan mode the signal attributed to deprotonated arabinoic acid at
m/z 165. The presence of a hydride species was further confirmed
by monitoring the catalytic reaction by 1H NMR spectroscopy
using L-arabinose and 10 mol% of pre-catalyst 2. The spectra
showed a characteristic signal in the hydride region at –16.8 ppm,
integrating for two hydrogen nuclei. Based on these data and the
ESI-MS analysis, we postulate
intermediate with molecular formula [Cp*(trz)Ir(µ-H)
a
dimetallic species
5 as
2
+
2
Ir(trz)Cp*] ,
which contains two [Cp*Ir(trz)] fragments bridged by two hydride
ligands. Similar diiridium-hydride species have been described
before using N-heterocyclic carbene ligands.[36,37] In order to
further characterise this species, we synthesised the diiridium-
hydride complex 5 as a triflate salt starting from 2 by chloride
abstraction and exposure to
H
2
(See ESI for details).
1
Characterisation by ESI-MS and H NMR spectroscopy showed
the same ion fragment (m/z 501.2) and isotopic distribution and a
hydride resonance at -16.8 ppm, in excellent agreement with the
data of the intermediate formed during the catalytic reaction. The
molecular structure of complex 5 was unambiguously elucidated
by single crystal X-ray diffraction (Figure 6c). Most strikingly, the
isolated complex 5, is catalytically competent in the
dehydrogenative transformation of L-arabinose and achieves
activity comparable to those observed with complex 2, suggesting
an identical or at least closely related catalytically active species.
In addition, the UV/Vis spectrum of 5 in water shows two
characteristic bands at 445 and 545 nm, commensurate with the
diagnostic features of the catalytic solution (Fig. 7, see also
discussion above). These data therefore suggest that the
diiridium-hydride complex 5 is a relevant species in the reaction
mechanism as an off-cycle catalyst resting state. Most probably,
cleavage of the diiridium-hydride species provides an iridium
monohydride intermediate, which is able to bind the carbohydrate
or water and subsequently release a carbohydrate/water proton
2
Figure 7. UV/vis spectra of the solutions (H O) from the catalytic conversion of
L-arabinose. The bands at 445 nm and 545 nm are identical for the catalytic
reaction (blue line) and for the independentlyprepared diiridium-hydride species
5
(green line).
A plausible mechanism for the conversion of sugars into sugar
acids is proposed based on previous results[29] and
complemented by the new experimental evidence from this work
(
=
Scheme 2). Iridium complexes of general formula [Cp*IrL(Cl)
NHC or triazolylidene undergo water/halide ligand exchange
forming the aqua complexes that are the active catalytic species
Scheme 1). The labile H O ligands are substituted by L-
2
] (L
(
2
arabinose most probably in a chelate coordination via the
carbonyl and a hydroxyl group (A). Such chelation may rationalise
the selectivity of the oxidation, which does not affect any of the
primary or secondary alcohol units. Intermediate A featuring
triazolylidene as ancillary ligand was not observed by ESI-MS;
however, for the NHC pre-catalyst 1 and glucose, intermediate A
was readily observed. The enhanced stability of the sugar-bound
intermediate A in the case of the NHC-based catalyst correlates
2
and the metal-bound hydride as H .
with the reduced catalytic activity of this complex towards the
sugar acid formation.[38] The coordinated L-arabinose undergoes
a)
b)
c)
nucleophilic attack by water at the aldehyde, producing a gem-
diol species (B). These species have been postulated as key
intermediates in the conversion of alcohols or aldehydes to
carboxylic acids or esters in aqueous media.[39–48] The gem-diol
species releases a solvated proton, forms an iridium-hydride by
β-hydride elimination and delivers the L-arabinoic acid product
(
C,D). The iridium-hydride species is in equilibrium with the
d)
diiridium-hydride (5) which corresponds to the catalyst resting
state. In the final stage, the iridium hydride is protonated by the
previously released proton to form (E). We speculate that this
protonation step is greatly accelerated by the triazolylidene ligand
due to the potentially basic N2 site of this ligand,[32,49] which
enables an intramolecular and hence fast proton shuttle
mechanism from N2 to the iridium-bound hydride. Such
intramolecular proton transfer is not feasible with NHC complexes
and may therefore rationalise the need for a strong acid as
5
01.2
x4
1
00
5
01.2 502.2
01.7
502.7
100
666.2
100
5
664.2
5
00.2
500.2 502.7
665.2
667.2
5
00.7
0
m/z
0
662
m/z
5
00 501 502 503 504
664
666
668
670
0
m/z
350
400
450
500
550
600
650
700
Figure 6. (a) Initial orange solution in H
characteristic deep-red solution after 30 min. at 110 ⁰C. (c) ORTEP structure
2
O of L-arabinose with catalyst 2 and (b)
5
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(13C{ H} NMR) respectively, and referenced to SiMe4 (δ in ppm and J in
1
additive with those ligands, but not with triazole-derived carbenes.
Formation of (E) induces the release of molecular hydrogen and
regeneration of the initial diaqua species [2-(H O) ] .
2 2
Hz). NMR spectra were recorded at room temperature with the appropriate
deuterated solvent. UV/vis spectra were recorded on a Jasco
spectropolarimeter at room temperature using MiliQ water as solvent.
Hydrogen was analysed using gas chromatography with a GS-MOL 15
meters column ID 0.55 mm TCD from J&W Scientific, using N2 as carrier
gas.
2
+
High Resolution Mass Spectrometry (HRMS) and Collision Induced
Dissociation (CID) experiments. HRMS studies were conducted on a
QTOF Premier instrument with an orthogonal Z-spray-electrospray
interface (Waters, Manchester, UK) operating in the V-mode at a resolution
of ca. 10000 (FWHM). The drying and cone gas was nitrogen set to flow
rates of 300 and 30 L/h, respectively. A capillary voltage of 3.5 kV was
used in the positive ESI(+) scan mode. The cone voltage was adjusted to
a low value (typically Uc = 5−15 V) to control the extent of fragmentation
in the source region. Chemical identification of the Ir-containing species
was facilitated by the characteristic isotopic pattern at natural abundance
of Ir and it was carried out by comparison of the isotope experimental and
theoretical patterns using the MassLynx 4.1 software. Sample preparation
was carried out as follows: i) for aqueous speciation studies, aqueous
solutions of pre-catalysts were stirred and aliquots were extracted at the
required time intervals and ii) for the study of the progress of the catalytic
reaction, aqueous solutions of pre-catalysts were stirred under catalytic
conditions and aliquots were extracted at the required time intervals. In
both cases, samples were diluted with water to a final concentration of 5 x
1 -
0
4
M (based on the initial Ir concentration) and directly introduced to the
mass spectrometer. For CID experiments, the cations of interest were
mass-selected using the first quadrupole (Q1) and interacted with argon in
the T-wave collision cell at variable collision energies (Elaboratory = 3 -15 eV).
The ionic products of fragmentation were analysed with the time-of-flight
analyser. The isolation width was 1Da and the most abundant isotopomer
was mass-selected in the first quadrupole analyser.
Scheme 2. Mechanistic proposal for the conversion of sugars to sugar acids
and hydrogen.
Conclusions
General procedure for catalytic experiments. In a typical reaction,
iridium catalyst (2 mol%) was added to a sugar solution (0.44 mmol) in
deionised H2O (5 mL). The reaction vessel was connected to a reflux
condenser and was introduced in a pre-heated oil bath at 110 oC for the
appropriate time (standard time 20h). Reaction monitoring was carried out
following two methodologies: for the quantification of glucose/gluconic acid,
a previously described method based on reverse-phase UHPLC coupled
to ESI-MS/MS was used. For the rest of sugars and sugar acids, different
aliquots (400 µL) were directly extracted from the Schlenk tube with a
syringe. The solvent was removed under reduced pressure and the
residue was redissolved in D2O for 1H NMR analysis. At the end of the
reaction, the solvent was evaporated and the crude product was analysed
by 1H-NMR spectroscopy without further purification. In the case of D-
gluconic and D-xylonic acids the pH was adjusted to 12 by adding NaOH
for NMR characterisation.
Iridium complexes bearing triazolylidene ligands are efficient
catalysts for the conversion of sugars into sugar acids without
additives and using water as solvent. This transformation
produces molecular hydrogen as a valuable by-product along with
the selective formation of sugar acids. The conversion of sugars
represents an interesting biomass transformation for the
production of initial platform chemicals in a sustainable manner.
The reaction is highly selective and does not require purification
of the sugar acid products. Moreover, the catalysts are highly
promiscuous and convert a variety of carbohydrate substrates,
which contrasts the typically highly substrate-specific mode of
operation of enzymes. Noticeably, high conversion and selectivity
proceeds without any additional acid using triazolylidene
complexes, which is
a common requirement for sugar
dehydrogenation in the presence of its NHC congeners.
Mechanistic studies support the formation of a diiridium-hydride
as catalyst resting state in the conversion of sugars to sugar acids.
These results provide useful guidelines for further optimisation of
this valuable biomass transformation.
Procedure for hydrogen identification. After 4h reaction of a general
catalytic experiment, a 15 mL sample of the generated gas was collected
with a gastight syringe and the hydrogen content was qualitative analysed
by gas chromatography using a TCD detector.
Acknowledgements
Experimental Section
The authors thank the financial support from MICIU/AEI/FEDER
(RTI2018-098237-B-C22) and Universitat Jaume I (UJI-B2018-
General details. Catalysts 1 - 4 were synthesised according to reported
procedures.[50–52] D-glucose (99 %), D-gluconic acid (50 % aq. sol), D-
ribose (98%), D-xylose (99%), L-arabinose (99%), D-mannose (99%), D-
Galactose (98%) and 2-deoxy-D-glucose (99%) were purchased from
commercial suppliers and used as received. HPLC-grade water was
obtained from distilled water passed through a Milli-Q water purification
system. Nuclear magnetic resonance (NMR) spectra were recorded on
23). P. Borja thanks the Universitat Jaume I for a postdoctoral
grant. We acknowledge generous support from the European
Commission for a Marie Skłodowska Curie Individual Fellowship
to J. P. Byrne (Grant 749549) and from the European Research
Council to M. Albrecht (CoG 615653). The authors are very
1
Bruker spectrometers operating at 400 MHz ( H NMR) and 100 MHz
6
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Entry for the Table of Contents
A sustainable catalytic process for the conversion of sugars into sugar acids using water as solvent without additives.
Institute Twitter username: @inam_uji
Researcher Twitter username: @jose_mata_uji
Researcher Twitter username: @albrecht_lab
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