One-pot photocalalytic reductive formylation of nitroarenes via multielectron transfer by carbon nitride in functional eutectic medium

Article abstract of DOI:10.1016/j.jcat.2019.10.010

In the past years organic semiconductor photocatalysis has made the remarkable advances in developing efficient synthetic chemical routes to well refined structures. Until now, most efforts were focused on optimizing of the semiconductor photocatalyst, while solvents were largely ignored. Herein, we design a series of functional deep eutectic solvents (DES) with adjustable physicochemical properties – density, viscosity, glass transition temperature, and chemical activity towards reductive formylation of nitroarenes. The DES are prepared from abundant, sustainable resources, which makes them an affordable alternative to the ionic liquids, even on larger industrial scales. By combining the ammonium formate-based DES with carbon nitride photocatalyst, we enable the simultaneous six-electron reduction of nitro-compounds to the corresponding N-substituted formamides and benzimidazoles in one pot.

Full text of DOI:10.1016/j.jcat.2019.10.010

Journal of Catalysis xxx (xxxx) xxx
Contents lists available at ScienceDirect
Journal of Catalysis
journal homepage: www.elsevier.com/locate/jcat
One-pot photocalalytic reductive formylation of nitroarenes via
multielectron transfer by carbon nitride in functional eutectic medium
⇑
Yevheniia Markushyna, Antje Völkel, Aleksandr Savateev, Markus Antonietti, Svitlana Filonenko
Max-Planck Institute of Colloids and Interfaces, Department of Colloid Chemistry, Research Campus Golm, 14424 Potsdam, Germany
a r t i c l e i n f o
a b s t r a c t
Article history:
In the past years organic semiconductor photocatalysis has made the remarkable advances in developing
efficient synthetic chemical routes to well refined structures. Until now, most efforts were focused on
optimizing of the semiconductor photocatalyst, while solvents were largely ignored. Herein, we design
a series of functional deep eutectic solvents (DES) with adjustable physicochemical properties – density,
viscosity, glass transition temperature, and chemical activity towards reductive formylation of nitroar-
enes. The DES are prepared from abundant, sustainable resources, which makes them an affordable alter-
native to the ionic liquids, even on larger industrial scales. By combining the ammonium formate-based
DES with carbon nitride photocatalyst, we enable the simultaneous six-electron reduction of nitro-
compounds to the corresponding N-substituted formamides and benzimidazoles in one pot.
Ó 2019 Elsevier Inc. All rights reserved.
Received 17 July 2019
Revised 3 October 2019
Accepted 5 October 2019
Available online xxxx
Keywords:
Organic photoredox catalysis
Deep eutectic solvents
Carbon nitride
Photoreduction
N-arylformamides
1. Introduction
reactions widely used in organic chemistry. N-arylformamides
occupy pivotal positions in synthesis of building blocks for poly-
The turn of modern chemistry towards the principles of green
chemistry and sustainability drives the development of new,
robust and economically efficient synthetic methods designed to
improve the performance of chemical industry and minimize the
influence on the environment [1]. Creation of such sustainable pro-
cesses in chemistry includes improvements on different levels: uti-
lization of catalysts for decreasing the activation energy and
improving the reaction rate, decrease of pollution by utilizing recy-
clable, safe, and non-toxic solvents, conducting complex transfor-
mations in one-pot synthesis, minimization of reaction volume
by decreasing the amount of solvent used, and some more [2–4].
Those approaches are summarized in the twelve principles of
green chemistry [5,6]. Synthetic paths able to combine a whole
set of the green chemistry principles in one approach is desirable
to achieve in order to have high levels of efficiency with the lowest
environmental impact. This is particularly important in the field of
synthetic organic chemistry, where reactions are often slow and
tedious and require utilization of catalysts. Often large amounts
of highly volatile, flammable and environmentally questionable
solvents are used in the field [7,8].
mers, dyes and the synthesis of biologically active compounds
[9,10]. They are used as scaffolds for pharmaceuticals, such as
antibacterials and cancer chemotherapeutic agents, as well as in
agrochemical industry for fungicides, herbicides etc. production
[11,12]. Conventional methods used in industry for reduction of
nitrocompounds involve utilization of the sodium hydrosulfite,
iron, tin or zinc in ammonium hydroxide, with downsides of poor
selectivity, reaction side products and ecological impact [13]. The
second step involves formylation of the amines by using chloral,
phosgene, formic acid, carbon oxides or other formylating agents
[14,15]. By means of the specific catalysis, this complex two stage
process can be transformed into one pot synthesis, eliminating in
this way isolation of amines and decreasing the wastes. Typically
the one-pot synthesis of N-arylformamides from nitroarenes
requires expensive catalysts containing noble metal (Pd, Pt, Rh,
Ru, or Au) or more abundant transition metal (Fe, Co, or Ni). The
reaction requires high temperature and it is performed under the
high hydrogen pressure [16–19] in volatile organic solvents
(Table S2). Hydrogen gas is one of the most popular and attractive
reducing agents for the one-pot reaction, however, it requires spe-
cial equipment with potential safety issues regarding H2 handling.
Additionally its use is constrained by limited functional group
tolerance.
Synthesis of amines and their subsequent transformation into
formamide derivatives is of high importance among the essential
In the present contribution, we want to run this complex
transformation using photocatalysis to turn abundant solar energy
into the driving force of the chemical reaction. Conventional
⇑
Corresponding author at: Max Planck Institute of Colloids and Interfaces Am
Mühlenberg 1 14476 Potsdam.
E-mail address: svitlana.filonenko@mpikg.mpg.de (S. Filonenko).
https://doi.org/10.1016/j.jcat.2019.10.010
0021-9517/Ó 2019 Elsevier Inc. All rights reserved.
Please cite this article as: Y. Markushyna, A. Völkel, A. Savateev et al., One-pot photocalalytic reductive formylation of nitroarenes via multielectron trans-
fer by carbon nitride in functional eutectic medium, Journal of Catalysis, https://doi.org/10.1016/j.jcat.2019.10.010

2
Y. Markushyna et al. / Journal of Catalysis xxx (xxxx) xxx
photocatalysts for the hydrogenation of nitroarenes are doped with
metal or bimetallic nanoparticles [20]. In the presence of formic
acid the photocatalytic reaction can result in the formation of
N-arylformamides [21,22].
2. Results and discussion
2.1. Reactive eutectic media design
Carbon nitrides have been proposed as a versatile alternative to
the noble metal-based photocatalysts [23–26], and were already
favorably employed for the highly selective reduction of nitroare-
nes to anilines in aqueous media [27]. For the photocatalytic reac-
tions aiming at substrates reduction a special carbon nitride named
potassium poly(heptazine imide) (K-PHI) is particularly suitable.
Due to its chemical structure, K-PHI in the presence of electron
donors forms a long-lived radical anion [28,29]. This meta-stable
species is a reducing agent and was identified as a key intermedi-
ate in the reductive cyclodimerization of chalcones [30], tandem
coupling of the chalcones and tetrahydroisoquinolines [31], as well
as the C-H thiolation of toluene [28]. Earlier it has been demon-
strated that K-PHI can accumulate up to 1000 mmol of electrons
per one gram of the material [32]. In other words, it serves as an
electron pool to decouple photochemistry from chemical reduction
kinetics and potentially can be also used to enable multielectron
transfer processes. The functionality of the long-lived radicals of
carbon nitrides has been also shown in spontaneous H2 generation
from water [33–35].
Despite tremendous advances in designing robust semiconduct-
ing carbon nitride photocatalysts [36,37], the development of func-
tional solvents as task-specific reaction media for photocatalysis is
significantly less covered. This is surprising, as the solvent does not
only restricts redox activity by its electrochemical window (say
especially water), but it is also able to stabilize intermediates, i.e.
to decrease activation barrier, and to promote sorption and desorp-
tion of the products from the catalyst surface. Considering redox
photochemistry, usually a highly polar solvent is demanded. From
a reaction engineering point of view, high polarity of the solvent
also allows for the fine dispersion of the photocatalyst. These
requirements to the solvent are readily fulfilled by ionic liquids,
and here by special deep eutectic solvents.
Similarly to the ionic liquids, DES are tailorable solvents with
ionic nature and offer the possibility to adjust their physicochem-
ical properties to various needs by changing their composition
[38,39]. However, DES are much easier to prepare by simply mix-
ing the components with subsequent moderate heating or grind-
ing. As nontoxic and biodegradable compounds are often used in
DES formulations, and it is possible to form DES exclusively from
natural compounds (NADES) [40], they are considered as green sol-
vents for the future [41]. Until now, DES were mostly used as non-
reactive solvents [42]. By using organic compounds, which on the
one hand are components of DES, while on the other are reagents
for particular chemical transformation, further makes the whole
process more atom efficient, more dense and thereby ‘‘greener”.
The room temperature liquid mixture obtained in this manner is
not a solvent only, but more accurately is described as a reactive
eutectic medium [43].
In this work we made a step towards such a new solvent system
which is polar and possesses a wide electrochemical window to
run green photocatalytic redox reactions mediated by the transi-
tion metal-free potassium poly(heptazine imide). The eutectic
medium is based on ammonium formate, an organic salt made
up of two of the cheapest mass chemicals. It has a melting point
of À60.3 °C, which can be further lowered by addition of polar
educts and products. In addition, it is a ‘‘volatile salt”, i.e. it has a
pronounced equilibrium with the acid/base form, thus providing
additional options for secondary catalysis and ease of removal.
The combination of the transition metal-free heterogeneous car-
bon nitride photocatalyst with this new solvent was explored in
the one-pot reductive formylation of nitroarenes.
The eutectic media for this reaction were designed in the way to
efficiently conduct photocatalytic reaction, namely to form color-
less transparent liquid as a main requisition for using visible light,
and at the same time to provide the reagents necessary for reduc-
tion and subsequent formylation of the substrates. We also
restricted the choice of components to low molecular weight com-
pounds, in order to overcome high viscosity intrinsic to most of the
traditional DES and hindering their practical use. The components
are easily reachable from renewable resources and have a much
lower price compared to ionic liquids.
Conventional DES are formed as a result of non-covalent inter-
actions between quaternary ammonium or phosphonium halide
salts acting as hydrogen bond acceptors (HBA) and molecules able
to donate hydrogen (HBD), such as urea, carboxylic acids, polyols
or saccharides. Hydrogen bonding occurring between HBA and
HBD interfere with the ability of the initial compounds to crystalize
and cause liquefaction of the mixture under much lower tempera-
tures [44]. The same approach we used to form reactive eutectic
media, namely we used ammonium formate as an already low
melting HBA component and complemented it with the two differ-
ent HBDs - neutral and acidic. The smallest
a-hydroxy acid and
glycerol were used as HBD, expecting to achieve lower viscosity
of the mixture and enlarge freezing point depletion. In the photo-
catalysis, ammonium formate functions as an electron donor and
triggers formation of the long-lived radical anion of K-PHI, while
the latter than reduces nitroarene as will be shown below.
Ammonium formate was used for catalytic reductive formyla-
tion of nitroaromatic compounds before [16,22,45]. It is an attrac-
tive source of carbon for attaching CO, CHO, or methyl groups, and
can be used similarly to formic acid as sustainable and affordable
reducing agent, easily available as a major product of biomass pro-
cessing [46,47]. Recent findings showed simple electrochemical
path for the formic acid preparation from carbon dioxide with
95% efficiency, increasing the value of this chemical for industrial
applications [48]. Based on that, we used ammonium formate as
the HBA for the reactive eutectic media, and combined it with
the several different HBD. The choice of HBD was based on the abil-
ity of the compounds to form liquid at room temperature when
mixed with ammonium formate. Namely glycolic acid and glycerol
were found to be suitable counterparts with ammonium formate
and corresponding DES were used to host the reaction (Table S1).
We also tested the role of the glycolic acid in the reduction process
by combining it with choline chloride (DES4). Ammonium acetate
was combined with glycolic acid in order to test the possibility of
one pot reductive acetylation in eutectic medium. Choline chlo-
ride:formic acid DES5 eutectic mixture was prepared to reveal
the activity of HCOOH in the photocatalytic reductive formylation
with K-PHI. Conventional choline chloride:urea DES6 was used as a
medium for a blank experiment.
Applicability of the media to host the reaction largely depends
on its physical properties allowing its practical use for industrial
processes [49,50]. Namely those properties are density and viscos-
ity, polarity and polarizability of the medium, which determine the
borders of practical use and abilities of DES to solubilize the reac-
tants and activate the catalyst. The first and probably most impor-
tant physicochemical characteristic in this relation is viscosity. For
the eutectic mixtures, it is determined by dimensions of holes
formed in molten salt comparing to the size of the ions [51]. If
the size of holes is larger comparing to the ion radii, they can easily
move into the vacant sites providing low viscosity of the mixture.
For instance, this correlation can be observed in the choline
Please cite this article as: Y. Markushyna, A. Völkel, A. Savateev et al., One-pot photocalalytic reductive formylation of nitroarenes via multielectron trans-
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Y. Markushyna et al. / Journal of Catalysis xxx (xxxx) xxx
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Table 1
On the other side, the ability of the ionic melts to form holes
with larger average size increases with decrease of the surface ten-
sion, since the liquids with low surface tension are able to form
cavities of larger size. Ammonium formate:glycolic acid medium
has surface tension value larger than conventional choline chlo-
ride:urea DES, however much smaller sized of the components
obviously improve mobility of the ions, which explains the low
value of viscosity of this medium.
Distortion of the crystallization in the ammonium formate
based media might also originate from the high ability of the for-
mate anions to form hydrogen bonds with HBD components, which
results in the variety of microstates with the different geometry of
molecules towards each other, but with similar energetic charac-
teristics. This energetic undecided behavior of formate anions pre-
vents ordering the molecules into crystal lattice.
Physicochemical properties of eutectic media.
Eutectic
media
Density, g cmÀ3
(25 °C)
Viscosity, mPas
(25 °C)
Glass transition
temperature, °C
DES1
DES2
DES3
DES4
DES5
DES6
1.32
1.25
1.24
1.20
1.16
1.25 [51]
317.0
1486.7
476.2
–
27.3
750 [53]
À60.3
À54.1
*
9.6
*
12 [54]
*
No glass transition detected.
chloride:ethylene glycol, choline chloride:glycerol, and choline
chloride:urea DES, where choline cations have larger sizes and thus
HBDs have higher mobility, which respectively decreases in the
row. Contrary to this, malonic acid molecules are larger than cho-
line chloride and thus when combined in DES, choline cations are
diffusing faster than malonic acid [52]. In this regard, low molecu-
lar weight components of the proposed reactive media provide
small ions with high mobility.
The practical use of the media for reaction is largely depends on
its thermal behavior, described by the borders of the liquid and
solid states and thermal stability within it. Generally the melting
temperature of the mixture can be to some extend predicted from
the enthalpy of formation of the starting materials, taking into
account the lattice energy of the mixture and entropy changes
Fig. 1. TGA-MS analysis of the DES1 (A–C) and DES3 (D) reactive media using a heating rate of 2.5 C/min under helium.
Please cite this article as: Y. Markushyna, A. Völkel, A. Savateev et al., One-pot photocalalytic reductive formylation of nitroarenes via multielectron trans-
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Y. Markushyna et al. / Journal of Catalysis xxx (xxxx) xxx
Fig. 2. NMR spectra of neat ammonium formate (a, e), glycolic acid (b), and glycerol (f), water solution of ammonium formate and glycolic acid (c), water solution of glycerol
and ammonium formate (g), and eutectic media ammonium formate/glycolic acid (d) and ammonium formate/glycerol (h).
when the liquid is formed [38]. For the reactive eutectic media we
used low melting temperature components, which after mixing
resulted in the depletion of the glass transition temperature to
À60.3 °C for DES1, while for DES3 it resulted in less melting tem-
perature depletion down to only 9.6 °C (Table 1).
z = 29). Considerable amounts of the ions with m/z = 45
(O@C@NH2) evidence the presence of formamide, obtained from
the decomposition of ammonium formate, obviously enhanced
by the presence of glycolic acid. Other detected fragment has
m/z = 46 and corresponds to HCOOH, largely formed in the eutectic
media, which presumably stipulates the low onset temperature of
the decomposition. However, regarding the involvement of the
media in reduction and formilation reactions, we consider this
in situ formed HCOOH as an origin of the high activity of the media
towards reduction and subsequent formylation of the amino group
in the photocatalytic reaction.
Thermal behavior of DES3 similarly shows formation of the one
whole media with properties distinctive from the mechanical mix-
ture of the crude components. Decomposition occurs in two steps
with temperatures 135 °C and 209 °C, that are differ both from
glycerol (500 °C) and ammonium formate [55]. In accordance with
the MS data, first decomposition step is correlated with decompo-
sition of both components of DES3 and results mostly in formation
of mass fragments with m/z = 16, 17 and 18. Similarly to DES1
mass fragments with m/z = 45 and 46 are detected, enclosing the
presence of formic acid and formamide in the reactive medium.
Considerable amounts of H+2 are formed, providing high reductive
activity. It can originate both from decomposition of glycerol and
ammonium formate. Notably that onset temperature of DES3 is
While having a low melting point (Table 1), stability of the reac-
tive eutectic medium at higher temperatures is restricted, and
thermal decomposition of DES1 starts at above 60 °C. Considering
that the boiling points of components are well above this temper-
ature (180 °C and 100 °C for ammonium formate and glycolic acid
respectively), such lowering of the decomposition temperature is
an intrinsic property of the reactive medium obviously originated
from the strong intermolecular interactions between the compo-
nents. The interactions that determine thermal behavior of the
medium are distinctive from simple overlapping of the TGA curves
of the corresponding components. Particularly decomposition of
neat glycolic acid occurs at 100 °C, while in the eutectic mixture
first peak is observed at 151 °C followed by the second peak at
178 °C that results overall in 88% mass lost (Fig. 1). The MS data
reveals that the mass loss is determined by the products of the gly-
colic acid cleavage, resulting in representative mass fragments OH+
(m/z = 17), H2O+Å (m/z = 18), CO+2Å (m/z = 44) and m/z = 45
(HOAC„O+), and ammonium formate fragments associated with
NH+3Å (m/z = 17), NH2+ (m/z = 16), O@C@NH2 (m/z = 44), CHO+(m/
Please cite this article as: Y. Markushyna, A. Völkel, A. Savateev et al., One-pot photocalalytic reductive formylation of nitroarenes via multielectron trans-
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Y. Markushyna et al. / Journal of Catalysis xxx (xxxx) xxx
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higher comparing to DES1 probably due to lower acidity of the
media with glycerol. Thus, less formic acid is in situ formed in
the DES3 medium.
2.2. Photocatalytic reaction of reductive formylation in eutectic media
Reduction of nitroarenes is a common procedure in the chemi-
cal industry as well as in the fine organic synthesis, and it is crucial
to perform it in a sustainable manner. In this work we used K-PHI
as a highly active catalyst for nitrotoluene reduction. As both
reducing agent and reaction media eutectic mixtures are
employed. As a result we observed quantitative conversion of
nitrotoluene (Table 2, entry 1). The only product of the reduction
is the N-(p-tolyl)formamide – derivative of aniline. The photocat-
alytic reaction with K-PHI obviously leads to full reduction of nitro
group to amino group and to further formylation.
Investigation of the reaction conditions with blind reactions
proved that the presence of photocatalyst and light are necessary
to carry out the reaction. (Table 2, entries 2–4).
The study of the effect of temperature on N-(p-tolyl)formamide
yield was done as well (Table 2, entries 5–8). We found that higher
temperatures give higher conversions but instead lead to lower
selectivity, which is rather typical for the photochemical reactions.
Therefore we chose 30 °C as an optimal temperature for this
reaction.
Heterogeneous K-PHI can be easily recycled from the reaction
mixture and used again. After three cycles of using the same cata-
lyst we did not observe any changes in the catalysts activity
(Table 2, entries 1, 9–10). In order to compare the performance
of K-PHI we ran the reaction with other forms of CN catalysts.
The results showed that mpg-CN results in only 9% conversion
(Table 2, entry 11) and H-PHI gives higher conversion of 38%
(Table 2, entry 12), however not reaching the conversion of100%
obtained with K-PHI at the same conditions. With g-CN catalyst
we detected only traces of the product (Table 2, entry 13). It should
be mentioned that acid treatment of carbon nitride, as in case of H-
PHI, typically increases surface zeta-potential. [59] Taking into
The DES are formed due to the specific interactions, which, in
most cases, are hydrogen bonds between the components, and
can be monitored by the shifts of the signals of protons in NMR.
To avoid contacting with deuterated solvent, DES samples were
sealed in a glass capillary prior the measurements. We compared
the 1H NMR specters of two DES (Fig. 2d and h), namely DES1
and DSE3, with the NMR of the separate components (Fig. 2a–b,
e–f) and with NMR of their dissolved mixture (Fig. 2a and g). When
ammonium formate and corresponding HDB (glycolic acid or glyc-
erol) are mixed together and dissolved in water, the signals from
proton of formate anions on the NMR spectra of the solutions are
shielded comparing to pure ammonium formate due to the change
of pH from slightly basic to slightly acidic in mixture. In 1H NMR
spectrum deprotonation of formate anions leads to change from
8.13 ppm to 8.45 ppm for the singlet 1H(C) atom signal [56]. The
same signals of formate in NMR spectra of both DES has a proton
chemical shift to lower field, which is known to be a result of
hydrogen bonding [57]. Similarly due to hydrogen bonds formation
protons of glycerol in DES are also deshielded comparing to water
solution. The other issue of NMR spectra from DES1 and DES3 is
additional signals occurred due to chemical interaction between
components, which can occur for most of the traditional DES pre-
pared by heating method. The amount of impurities in DES
reported in literature can reach from 5 to 30% [58].
The DES1 medium has a low value of refractive index (1.459)
comparing to traditional choline chloride based DES with organic
acids [38]. Refractive index is connected to the electronic polariz-
ability of the medium, hence the medium has lower polarizability
due to the fact that it is formed by small ionic molecules with low
ability of polarization.
Table 2
Reaction conditions optimization of p-nitrotoluene reduction.a
b
c
Entry
Photocatalyst
Light
Temperature, °C
Time, h
Conversion,
%
Yield, %
1d
2
3
4
5
6
7
8
K-PHI
–
K-PHI
–
K-PHI
K-PHI
K-PHI
K-PHI
K-PHI
K-PHI
mpg-CN
H-PHI
g-CN
465 nm
465 nm
–
30
30
30
80
30
45
60
80
30
30
30
30
30
48 h
48 h
48 h
24 h
24 h
24 h
24 h
24 h
48 h
48 h
48 h
48 h
48 h
100
0
0
85
0
0
–
0
0
465 nm
465 nm
465 nm
465 nm
465 nm
465 nm
465 nm
465 nm
465 nm
61
58
88
85
100
100
9
55
52
78
73
87
85
8
9e
10f
11
12
13
38
1
35
traces
a
b
c
d
e
f
Nitrotoluene 0.1 mmol; K-PHI 5 mg, DES1 (ammonium formate: glycolic acid) 0.2 mL; T = 25 °C; Ar, LED module 465 nm.
Determined by 1H HMR.
Total yield of cis-/trans- isomers determined by 1H HMR.
First run
Second run
Third run.
Please cite this article as: Y. Markushyna, A. Völkel, A. Savateev et al., One-pot photocalalytic reductive formylation of nitroarenes via multielectron trans-
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Y. Markushyna et al. / Journal of Catalysis xxx (xxxx) xxx
Scheme 2. Photocatalytic synthesis of benzimidazole and benzoxazole via reduc-
tive formylation of nitroarenes.
Table 3
a
Study of the different DES compositions in nitrobenzene reductive formylation.
Entry
DES
Yield, %
1
2
3
4
5
6
DES1
DES2
DES3
DES4
DES5
DES6
100
0
0b
0
3
0
a
Substrate 0.1 mmol, K-PHI 5 mg, corresponding DES 0.2 mL, T = 30 °C, Ar
atmosphere, LED module 465 nm. Yield is determined by 1H HMR.
b
The main product is aniline in 86% yield.
Scheme 1. Reduction of nitroarenes to N-arylformamides catalyzed by K-PHI[a]. [a]
Substrate 0.1 mmol; K-PHI 5 mg, DES1 (ammonium formate: glycolic acid) 0.2 mL;
T = 30 °C; Ar, LED module 465 nm. Yield and conversion (given in parentheses)
determined by 1H HMR. The yields are given for cis-/trans- isomers.
account this fact and results from the Table 2, considerably higher
performance of K-PHI compared to mpg-CN and H-PHI we attri-
bute to two factors: (1) electron buffering in the K-PHI and (2) neg-
atively charged poly(heptazine imide) framework (while K+
compensate the negative charge). Negatively charged surface of
K-PHI is apparently beneficial for improved interaction with the
reagent molecule.
A scope of aromatic nitrocompounds was studied within this
very simple approach that we developed. The proposed method
can indeed be used for the reduction of both electronically rich
and electronically poor compounds (Scheme 1). Generally, when
polar organic solvents are used, substrates with electron with-
drawing groups have lower yields comparing to those with elec-
tron donating substitutes [16]. Similar tendency is observed in
reactive eutectic media, which is probably due to its polar nature.
Overall it is possible to say that weak electron donating groups,
such as halogens and alkyl substituents, do not have a strong influ-
ence on the conversion and yield. However, in case of a substrate
with a strong electron donating group (4-methoxynitrobenzene),
considerably lower yield of the product was observed, while in
conventional organic solvents it doesn’t have a considerable effect.
This peculiar property probably resulted from specific interactions
between the 4-methoxynitrobenzene and components of DES.
Despite the various yields, selectivity towards formylated aniline
was preserved in all substrates.
To take the advantages of the reductive formylation, we used
this reaction medium to run in situ cyclization of the intermediary
formamides. Based on the results of our study, we propose a
method for the synthesis of benzimidazoles, providing an example
Scheme 3. Reduction of nitroarenes to anilines catalyzed by K-PHI[a]. [a] Substrate
0.1 mmol; K-PHI 5 mg, DES3 (ammonium formate: glycerol) 0.2 mL; T = 30 °C; Ar,
LED module 465 nm. Yield and conversion (given in parentheses) determined by 1
H
HMR. [b] The other product is N-(4-methoxyphenyl)formamide.
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Y. Markushyna et al. / Journal of Catalysis xxx (xxxx) xxx
7
Scheme 4. The proposed mechanism of the photocatalytic reductive formylation of nitroarenes. (A) Overall reaction equation; (B) In situ formation of formic acid; (C)
Multielectron photocatalytic reduction of the nitroarene by K-PHI; (D) Formylation of the arylamine.
of its simplest representative (Scheme 2). In this reaction three
steps are accomplished in one pot: (1) reduction of nitro-group,
(2) formylation of NH2-group and (3) heterocyclization. The pro-
duct is obtained in 85% yield as evidenced by GC–MS.
Indeed, in all cases higher yields of the rather pure amino com-
pound were obtained. Thus we can rather selectively control the
direction of the reduction of aromatic nitro compounds either to
N-arylformamidines or anilines by choice of the reactive medium.
The mechanism of the photocatalytic nitroarenes reductive
formylation is sketched in Scheme 4. The overall equation describ-
ing the process is shown on Scheme 4a. In the first step, formic acid
is formed reversibly in situ from ammonium formate and glycolic
acid (Scheme 4b). The equilibrium is constantly shifted to the right
as formic acid is consumed in forthcoming transformations. In the
second step, K-PHI is converted to the excited state, K-PHI*, by
absorbing a photon. Due to relatively low oxidation potential of
formic acid (+0.4 V vs RHE),[62] electron transfer from formic acid
to the K-PHI* leads to the blue radical anion, K-PHIÅ– appearance.
Formation of long-lived tinted radicals of carbon nitrides, i.e.
charging of K-HI, readily occurs in the presence of electron donors
and was reported earlier [28,63,64]. Charging of K-PHI is
accompanied by decomposition of formic acid that is converted
into CO2 and 2H+. Evolution of CO2 was proved by precipitation
of BaCO3 upon bubbling reactor head space gasses through Ba
(OH)2 solution (Figure S1). During the step of K-PHIÅ– discharging,
protons from the formic acid decomposition and electrons stored
To get closer to the mechanism of the reaction as such, we stud-
ied the influence of DES components on the reaction. We tried
combinations of ammonium formate and glycolic acid with other
compounds (Table 3). Only DES5 (containing formic acid) gave N-
phenylformamide, however with extremly low 3% yield. Interest-
ing result was found using ammonium formate and glycerol med-
ium – no N-phenylformamide was detected, but instead aniline
was formed in 86% yield. DES screening data suggest that formate
(either anion or formic acid) are crucial for reduction of the nitro-
group, while to formylate the amino group effectively formic acid
is required to be generated in the eutectic media, as most formylat-
ing agents are reactive formyl derivatives generated in situ [60,61].
The results of a selective aniline synthesis are interesting as
well. A series of aromatic nitrocompounds were reduced to amin-
oderivatives using the ammonium formate: glycerol DES
(Scheme 3). Similarly to formylative amination, the lower conver-
sion of the substrate with a strong electron donating group (p-
methoxynitrobenzene) was observed.
Please cite this article as: Y. Markushyna, A. Völkel, A. Savateev et al., One-pot photocalalytic reductive formylation of nitroarenes via multielectron trans-
fer by carbon nitride in functional eutectic medium, Journal of Catalysis, https://doi.org/10.1016/j.jcat.2019.10.010

8
Y. Markushyna et al. / Journal of Catalysis xxx (xxxx) xxx
in the semiconductor are transferred to the nitro-compound. The
photocatalyst is recovered, and aniline is expelled. Finally, in the
last step free formic acid amidates the aniline into the correspond-
ing formamide.
Appendix A. Supplementary material
Supplementary data to this article can be found online at
https://doi.org/10.1016/j.jcat.2019.10.010.
As mentioned before, in the proposed reaction system sub-
strates with electron withdrawing groups, such as nitrile group,
lead to a higher conversion/yield of the reduced product. During
the photocatalytical process charging of K-PHI as a result of a
reduction by formed HCOOH to the radical-anion K-PHIꢀ– occurs
faster than the opposite discharging. This leads to accumulation
of negatively charged K-PHIÅ– species in the reaction mixture.
Therefore the interactions, such adsorption on the catalyst’s sur-
face and discharging of the catalyst, between negatively charged
K-PHIÅ– and lower density substrate occur easier and faster than
with the electron rich substrates.
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In the current work we made a step forward to improve the
photocatalytic organic reduction of aromatic nitro compounds
and shift it closer to the demands of the green chemistry. The
key step was the design of new eutectic media using affordable
and sustainable components able to efficiently host the photocat-
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Seven N-arylformamides were prepared in 17–89% yield within
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fer by carbon nitride in functional eutectic medium, Journal of Catalysis, https://doi.org/10.1016/j.jcat.2019.10.010