Reduction of furfural by Mn/2,4,6-Coll.HCl/H2O: Mechanistic aspects of this reaction

Full text of DOI:10.1002/aoc.4948

Received: 24 January 2019
DOI: 10.1002/aoc.4948
Revised: 28 February 2019
Accepted: 20 March 2019
F U L L P A P E R
Reduction of furfural by Mn/2,4,6‐Coll.HCl/H2O:
Mechanistic aspects of this reaction
Antonio Rosales Martínez1
| María Castro Rodríguez1 | Martín Jaraíz2 | Lourdes Enríquez2 |
Emilio Díaz Ojeda1 | Roman Nicolay Rodríguez Maecker3 | Felipe Cordobés Carmona1 |
Laura Pozo Morales1
1 Department of Chemical Engineering,
A simple, mild and safe reduction of furfural to furfuryl alcohol by the
Mn/2,4,6‐Coll.HCl/H2O system is described. An experimental study on the role
played by each of the reagents involved in this reduction and DFT calculations
are reported. With the aim to provide an explanation for the results obtained, a
complete mechanism is proposed.
Escuela Politécnica Superior, University of
Sevilla, 41011 Sevilla, Spain
2 Department of Electricity and
Electronics, University of Valladolid,
Valladolid, Spain
3 Petrochemical Engineering, Universidad
de las Fuerzas Armadas‐ESPE, 050150
Latacunga, Ecuador
KEYWORDS
furfural, Furfuryl alcohol, green chemistry, manganese, reduction reaction
Correspondence
Antonio Rosales Martínez, Department of
Chemical Engineering, Escuela
Politécnica Superior, University of Sevilla,
41011 Sevilla, Spain.
Email: arosales@us.es
1 | INTRODUCTION
and Meerwein‐Ponndorf‐Verley reduction with alcohol as
H‐donor have been reported.2b, c However, many of these
Biomass, a renewable non‐fossil carbon energy has
gained worldwide attention as concerns grow over global
warming, diminishing fossil fuel reserves and environ-
mental problems caused by CO2 emissions.[1] Therefore,
biomass is one of the most encouraging options for a sus-
tainable and green production of chemicals and fuels.
In this context, furfural (1), obtained from hemicellu-
lose, is identified as one of most important building
blocks, as well as a rich platform for value‐added
chemicals and lignocellulosic biofuels (Figure 1).[2]
Among these different chemical products, the furfuryl
alcohol (FA) (2) is the most important. It has been esti-
mated that approximately 65% of the furfural produced
each year is turned into FA. FA is widely used in the pro-
duction of initial building blocks for the preparation of
procedures are not affordable and usually hydrogen gas
as reducing agent is used. These methods, based on the
use of hydrogen gas as reducing reagent, often present
problems because high‐pressure equipment is needed to
handle the gas. In this context, the development of a
new procedure for the reduction of furfural that avoids
the presence of hydrogen gas and uses an inexpensive,
nontoxic and environmentally friendly metal, is desirable.
Taking this into account, the Mn/2,4,6‐Coll.HCl/H2O sys-
tem is a safe, selective and mild procedure which pro-
motes the chemoselective reduction of aldehydes.[4]
A
detailed knowledge of the mechanism involved in the
reduction of aldehydes by the Mn/2,4,6‐Coll.HCl/H2O
system is necessary to provide a platform for the design
of more and novel efficient processes. In this paper, we
report on the experimental conditions and reagents nec-
essarily required for the Mn/2,4,6‐Coll.HCl/H2O‐pro-
moted reduction of furfural. Additionally, DFT
calculations were carried out with the aim to verify the
reduction mechanism.
3
other chemicals, fuels and resins.1a, For this reason,
the selective hydrogenation of furfural has been exten-
sively investigated using environmentally acceptable
metal catalysis (Cu, Pd, Pt, Rh, Ru, Ni, Co and Zn).2a
More recently, other methods such as the electrocatalytic
Appl Organometal Chem. 2019;33:e4948.
wileyonlinelibrary.com/journal/aoc
© 2019 John Wiley & Sons, Ltd.
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https://doi.org/10.1002/aoc.4948

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ROSALES MARTÍNEZ ET AL.
TABLE 1 Relative ratio of compounds obtained after stirring
furfural (1) with the Mn/2,4,6‐Coll·HCl/H2O system at 27 °C
2,4,6‐
THF (ml/ Mn
coll·HCl H2O
1
2
3
mmol 1) (equiv) (equiv) (equiv) (%) (%) (%)
1
2
3
4
5
6
7
8
9
20
10
30
30
30
30
30
30
30
8
8
8
8
8
8
0
6
4
8
8
8
8
6
6
2
2
2
0
3
4
3
3
3
3
3
3
3
4
4
10
10
10
10
10
10
10
10
10
0
21a 56a 23a
40a 36a 24a
19a 65a 16a
100a
FIGURE 1 Value‐added chemicals obtained from furfural
2 | RESULTS AND DISCUSSIONS
81a 19a
Here, we describe a set of experiments to optimize the
reduction of furfural by the Mn/2,4,6‐Coll.HCl/H2O sys-
tem. Their results provide solid experimental evidence
of the mechanism of reduction. These results are summa-
rized in Table 1.
86a 14a
100a
80a 20a
53a 39a 8a
17a 65a 18a
81a 19a
Initially we studied the reduction of furfural (1), using
the experimental conditions reported by Justicia et al. in
the reduction of aldehydes[4] (entry 1). After 24 hr stirring
1 with Mn dust (8 equiv), 2,4,6‐Coll.HCl (2 equiv), and
water (10 equiv) in THF (20 ml), to our satisfaction, FA
(2) and pinacol coupling products 3 were observed. The
relative ratio of 2 and 3 was 56/23 (calculated by 1H
NMR). The ratio of dl/meso of the 1,2‐diols was 1/1 (cal-
10 30
11 30
12 30
13 30
14 30
15c 30
2.5
5
81a 19a
15
5
10a 72a 18a
87b 13b
5
86a 14a
1
aRelative ratio determined on the basis of the 1H NMR spectrum of the reac-
tion crude.
culated by H NMR). The most important observation to
be made about this result is that pinacol coupling prod-
ucts were detected for first time when the Mn/2,4,6‐
Coll.HCl/H2O system is used as a reducer. We subse-
quently investigated the role played by each reagent in a
series of thirteen experiments (entries 2–13, 15). We
decided to start studying the effect of concentration on
conversion reduction (entries 2, 3). With 30 ml of THF
per one mmol of fufural (1) a significant quantity of
reduction product 2 (65%) was obtained (entry 3).
Then, we investigated the role played by 2,4,6‐
collidinium hydrochloride (2,4,6‐Coll.HCl) in a series of
four experiments with increasing amounts of this reagent
(entries 3–6). We decided to start with a control experi-
ment in the absence of 2,4,6‐Coll.HCl. After 24 hr of stir-
ring, 1 was recovered unchanged (entry 4), showing that
2,4,6‐Coll.HCl is required for the reduction of aldehydes
1 to 2 to take place. These results indicate that 2,4,6‐Coll.
HCl reactivates the manganese surface previously
deactivated due to the dissociative adsorption between
water and the manganese surface.[5] Nevertheless, with
2 equiv of 2,4,6‐Coll.HCl a significant level of 2 (65%)
was obtained (entry 3). A similar amount of 2 was
obtained using 3 or 4 equiv of 2,4,6‐Coll.HCl (entries 5–6).
bRelative ratio determined on the basis of the 1H NMR spectrum after flash
chromatography.
cFurfural was slowly added during 8 hr.
Then, we researched the role played by manganese. As
above, we started with a control experiment in the
absence of manganese (entry 7). Under these conditions,
furfural (1) was recovered quantitatively. This confirms
that manganese is essential for the reduction process.
We determined the exact amount of manganese required
to complete the reduction reaction by means of three
experiments with increasing amounts of this metal
(entries 5, 8–9). It was found that at least 6 equiv of man-
ganese are required to complete the process and to obtain
a good (80%) yield of FA (entry 8).
Later, we investigated the role played by water in a
series of five experiments with increasing amounts of this
reagent (5, 10–13). As above, we started with a control
experiment in absence of water (dry TFH, entry 10). This
system (Coll.HCl/Mn) was previously introduced by
Gansäuer et al. in the radical chemistry catalyzed by
titanocenes.[6] In order to check the degree of conversion

ROSALES MARTÍNEZ ET AL.
3 of 6
of this reduction in comparison to that obtained in the
experiment collected in entry 5, the reaction was
quenched just after the fixed reaction time (24 hr). Under
these conditions, 2 (65%) was the main product. This
result reflects that the hydrogen atom needed in the
reduction process does not necessarily come from water,
as previously was reported.[4] The optimum yield of 2
(81%) was obtained with 2.5 or 5 equiv of water (entry
11 and 12). An increase in the amount of water (entry
13) results in a lower yield of 2 possibly due to a dissocia-
tive adsorption between water and the manganese surface
and the consequent inactivation of this metal.
After optimizing all the reagents involved in the reduc-
tion reaction, the treatment of furfural (1) with Mn dust (6
equiv), 2,4,6‐Coll.HCl (4 equiv), and water (5 equiv) in
THF (30 ml) afforded a mixture of FA (2) and 3 (92 mg,
95%). The relative ratio of 2 (87%) and 3 (13%) (entry 14)
was determined on the basis of their 1H NMR spectra.
Finally, in order to discard or confirm the presence of
free radicals, furfural (1) was added dropwise to the reac-
tion medium (entry 15). Under these conditions, the same
ratio of coupling products 3 was found in this reaction
(entry 15) and in that of entry 14. This result confirms that
free ketyl radicals are not involved in this reduction. This
conclusion was previously reported by Justicia et al.[4]
After all the optimization reactions were carried out,
the formation of pinacol coupling products 3 was not
avoided. However, different material transfer operations,
such as rectification, have been used for the purification
of furfuryl alcohol (2).[7]
In the original work it was proposed that the
metalloxirane was generated by η2–interaction between
the carbonyl group and the Mn surface.[4] In absence of
a water‐compatible regenerating agent (2,4,6‐Coll.HCl),
the manganese surface is inactivated and the key inter-
mediate 4 is not generated, and consequently, neither
reduction nor pinacol coupling products are obtained
(entry 4). In presence of 2,4,6‐Coll.HCl, the metal surface
is reactivated, and the intermediate 4 is formed. The pres-
ence of a donor source of hydrogen (water and/or 2,4,6‐
Coll.HCl) could promote the hydrolysis of 4 to 2. Also,
the subsequent coordination of 4 to a second molecule of
furfural (1) could lead to pinacol coupling products such
as 3 via five‐membered intermediate 5. The higher ratio
of 2 with respect to 3 in all experiments listed in Table 1
may be due to the fact that 2 is more favorable energeti-
cally.To corroborate Scheme 1, geometry optimizations
and energy calculations were performed with Gauss-
ian16[9] using DFT at the B3LYP‐D3/6‐31g(d) level of the-
ory in vacuo. We have verified that this level of
theory yields the lowest energy for multiplicity 6. The D3
dispersion‐correction
scheme,[10]
applying
the
Becke–Johnson damping,[11] was included for all calcula-
tions. Intermediates, products and transition states were
located by means of the GRRM[12] (Global Reaction Route
Mapping) program, linked to Gaussian16. Transition state
structures were optimized as saddle points at the B3LYP‐
D3/6‐31g(d) level of calculation. To verify that they corre-
spond to the expected reactant and product, intrinsic reac-
tion coordinate (IRC) calculations were performed at the
same level. To obtain the Gibbs Free Energy correction, a
vibrational analysis was performed at the same level of
theory, using the SMD continuum model[13] for the sol-
vent (tetrahydrofuran) and the experimental temperature.
The energies have been refined by single‐point calcula-
tions at the B3LYP‐D3/def2tzvpp level on the previously
optimized structures. The reported energies are given in
Hartrees. In the Scheme 2, these energies are in
kcal/mol relative to intermediate 4.
The observations mentioned above cannot be fully
explained by the mechanism previously reported.[4]
Instead, the mechanism tentatively proposed in Scheme 1
adequately accommodates the new experimental observa-
tions collected in Table 1. The coordination between 1
and manganese would provide the intermediate 4 in
solution. This type of metallaoxiranes (in free solution)
is well‐known. In fact, both titanaoxiranes and
zirconaoxiranes closely related to 4 have been reported.[8]
As we can see in Scheme 2, the energy barriers associ-
ated with the reduction of 4 or the coupling of 4 with 2
are similar. However, FA (2) is energetically more favor-
able than the coupling products 3, which is translated
into a greater proportion of 2 with respect to 3. In addi-
tion, we have verified that the initial barrier of path (c)
is practically the same for both coupling products, using
the SCW[14] algorithm, included in the GRRM package,
that provides quick, rough estimates of minimum energy
reaction pathways between two minima. These calcula-
tions corroborate the 1:1 ratio found experimentally for
coupling products 3. Cartesian coordinates and energies
of the computed structures are given in the supporting
information.
SCHEME
promoted reduction of furfural
1
Mechanism for the Mn/2,4,6‐Coll.HCl/H2O‐

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ROSALES MARTÍNEZ ET AL.
SCHEME 2 Free energy profile
Deoxygenated THF (10 mL, entry 2); (20 ml, entry 1)
or (30 ml, entry 3) was added to a mixture of Mn
(437.6 mg, 8 mmol) and 2,4,6‐Coll.HCl (307.2 mg, 2 mmol)
under an Ar atmosphere. Then, a solution of furfural (1)
(96.0 mg, 1 mmol) and water (180 mg, 10 mmol) in
1 ml of THF was added, and the solution was stirred for
24 h at 27 °C. THF was removed under vacuum and the
reaction was then quenched with 0.5 N HCl (15 ml) and
extracted with CH2Cl2. The organic layer was dried
(anhyd Na2SO4) and the solvent removed to afford the
compounds indicated in entries 1–3 in Table 1. The rela-
tive ratio (see entries 1–3 in Table 1) was determined on
3 | CONCLUSIONS
In summary, a safe, chemoselective, mild, affordable
method for the reduction of furfural (1) to furfuryl alco-
hol (2) based on the Mn/2,4,6‐Coll.HCl/H2O system, is
described. Moreover, a detailed experimental study of
the role played by effect of the concentration and reagent
involved in the reduction process is reported. Addition-
ally, a computational investigation confirmed low bar-
riers from metallaoxirane intermediate 4 to furfuryl
alcohol 2 and coupling products 3, and that 2 is the most
energetically favorable product. At the moment, we are
working in the semipreparative production of 2 from 1
using the described method.
1
the basis of their H NMR spectra.
4.3 | Role played by 2,4,6‐Coll.HCl
(entries 3–6)
4 | EXPERIMENTAL
4.1 | General methods
Deoxygenated THF (30 ml) was added to Mn (437.6 mg,
8 mmol) (entry 4) or a mixture of Mn (437.6 mg, 8 mmol)
and 2,4,6‐Coll.HCl (307.2 mg,
2 mmol, entry 3);
All the reactions were carried out under argon using stan-
dard Schlenk. THF was distilled from Na/benzophenone
under argon and was deoxygenated prior to use. THF
(460.9 mg, 3 mmol, entry 5) or (614.5 mg, 4 mmol entry
6) under an Ar atmosphere. Then, a solution of furfural
(1) (96.0 mg, 1 mmol) and water (180 mg, 10 mmol) in
1 ml of THF was added, and the solution was stirred for
24 h at 27 °C. THF was removed under vacuum and the
reaction was then quenched with 0.5 N HCl (15 ml;)
and extracted with CH2Cl2. The organic layer was dried
(anhyd Na2SO4) and the solvent removed to afford the
compounds indicated in entries 3–6 in Table 1. The rela-
tive ratio (see entries 3–6 in Table 1) was determined on
1
was deoxygenated by bubbling argon. H NMR spectra
were recorded at 300 MHz. All reactions were monitored
by thin‐layer chromatography (TLC). The TLC plates
were visualized with UV light and 7% phosphomolybdic
acid. Flash chromatography was performed on silica gel
60 (0.04–0.06 mm). Reagents and solvents were pur-
chased from commercial sources. Compounds 2 and 3
are commercial.
1
the basis of their H NMR spectra.
4.2 | General procedure for Mn/2,4,6‐Coll.
HCl/H2O‐mediated reduction of furfural (1)
4.4 | Role played by Mn (entries 5, 7–9)
Deoxygenated THF (30 ml) was added to 2,4,6‐Coll.HCl
(460.9 mg, 3 mmol, entry 7) or a mixture of 2,4,6‐Coll.
Effect of concentration (entries 1–3).

ROSALES MARTÍNEZ ET AL.
5 of 6
HCl (460.9 mg, 3 mmol) and Mn (218.8 mg, 4 mmol, entry
9); (328.2 mg, 6 mmol, entry 8) or (437.6 mg, 8 mmol, entry
5) under an Ar atmosphere. Then, a solution of furfural (1)
(96.0 mg, 1 mmol) and water (180 mg, 10 mmol) in 1 ml of
THF was added, and the solution was stirred for 24 h at
27 °C. THF was removed under vacuum and the reaction
was then quenched with 0.5 N HCl (15 ml) and extracted
with CH2Cl2. The organic layer was dried (anhyd Na2SO4)
and the solvent removed to afford the compounds indi-
cated in entries 5, 7–9 in Table 1. The relative ratio
(see entries 5, 7–9 in Table 1) was determined on the basis
of their 1H NMR spectra.
4.7 | Slow addition of furfural (1) (entry 15)
Deoxygenated THF (30 ml) was added to a mixture of Mn
(328.2 mg, 6 mmol) and 2,4,6‐Coll.HCl (614.5 mg, 4 mmol)
under an Ar atmosphere. Then, a solution of furfural (1)
(96.0 mg, 1 mmol) and water (90 mg, 5 mmol) in 2 mL
of THF was slowly added during 8 hr and the solution
was stirred for 24 hr at 27 °C. THF was removed under
vacuum and the reaction was then quenched with 0.5 N
HCl (15 ml) and extracted with CH2Cl2. The organic layer
was dried (anhyd Na2SO4) and the solvent removed. The
organic layer was dried (anhyd Na2SO4) and the solvent
removed to afford the compounds indicated in entries
15 in Table 1. The relative ratio (see entry 15 in Table 1)
1
was determined on the basis of their H NMR spectra.
4.5 | Role played by water (entries 5, 10–13)
Deoxygenated THF (30 ml) was added to a mixture of Mn
(437.6 mg, 8 mmol) and 2,4,6‐Coll.HCl (460.9 mg, 3 mmol)
under an Ar atmosphere. Then, furfural (1) (96.0 mg,
1 mmol, entry 10) in 1 mL of THF or a solution of furfural
(1) (96.0 mg, 1 mmol) and water (45 mg, 2.5 mmol, entry
11); (90 mg, 5 mmol, entry 12); (180 mg, 10 mmol, entry
5) or (270 mg, 15 mmol, entry 13) in 1 ml of THF was
added, and the solution was stirred for 24 hr at 27 °C.
THF was removed under vacuum and the reaction was
then quenched with 0.5 N HCl (15 ml) and extracted with
CH2Cl2. The organic layer was dried (anhyd Na2SO4) and
the solvent removed to afford the compounds indicated in
entries 5,10–13 in Table 1. The relative ratio (see entries
5,10–13 in Table 1) was determined on the basis of their
1H NMR spectra.
ACKNOWLEDGEMENTS
Financial support from the Ministerio de Economía y
Competitividad (MINECO) (Project CTQ2015‐70724‐R)
is gratefully acknowledged. A. Rosales acknowledges the
University of Sevilla for his position as professor.
ORCID
Antonio Rosales Martínez
https://orcid.org/0000-0003-
0182-0548
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SUPPORTING INFORMATION
Additional supporting information may be found online
in the Supporting Information section at the end of the
article.
How to cite this article: Rosales Martínez A,
Castro Rodríguez M, Jaraíz M, et al. Reduction of
furfural by Mn/2,4,6‐Coll.HCl/H2O: Mechanistic
aspects of this reaction. Appl Organometal Chem.
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