General and selective iron-catalyzed transfer hydrogenation of nitroarenes without base

Article abstract of DOI:10.1021/ja2061038

The first well-defined iron-based catalyst system for the reduction of nitroarenes to anilines has been developed applying formic acid as reducing agent. A broad range of substrates including other reducible functional groups were converted to the corresponding anilines in good to excellent yields at mild conditions. Notably, the process constitutes a rare example of base-free transfer hydrogenations.

Full text of DOI:10.1021/ja2061038

ARTICLE
pubs.acs.org/JACS
General and Selective Iron-Catalyzed Transfer Hydrogenation of
Nitroarenes without Base
Gerrit Wienh€ofer,† Ivꢀan Sorribes,‡ Albert Boddien,† Felix Westerhaus,† Kathrin Junge,† Henrik Junge,†
Rosa Llusar,‡ and Matthias Beller*,†
†Leibniz-Institut f€ur Katalyse e.V. an der Universit€at Rostock, Albert Einstein Strasse 29a, 18059 Rostock, Germany
‡Departament de Química Física i Analítica, Universitat Jaume I, Av. Sos Baynat s/n, 12071 Castellꢀo de la Plana, Spain
S Supporting Information
b
ABSTRACT: The first well-defined iron-based catalyst system for the
reduction of nitroarenes to anilines has been developed applying formic
acid as reducing agent. A broad range of substrates including other reducible
functional groups were converted to the corresponding anilines in good to
excellent yields at mild conditions. Notably, the process constitutes a rare
example of base-free transfer hydrogenations.
’ INTRODUCTION
Aromatic amines are widely used as intermediates in the
synthesis of dyes, pigments, agrochemicals, and pharmaceu-
ticals.1 While primary aliphatic amines are produced by reductive
amination,2 reduction of nitriles,3 and direct amination of
Figure 1. Demanding substrates for selective nitro reductions.
alcohols,4 the most important method to obtain primary anilines
Iron as an abundant, cheap, and less-toxic element is an ideal
is based on the selective reduction of nitroarenes. In past years,
candidate to replace precious metals if comparable activities and
for many nitroarenes stoichiometric reduction procedures such
selectivities are achieved.13,14 To the best of our knowledge, iron-
as the Bꢀechamp process were replaced by more benign catalytic
catalyzed transfer hydrogenations of nitroarenes are only known
protocols. Today, noble metals are frequently used as hetero-
using toxic and hazardous 1,1-dimethyl-hydrazine.15 On the basis
geneous catalysts for the direct hydrogenation of nitroarenes.
of our recent report on the iron-catalyzed hydrosilylation of
However, a drawback of commercially available Ni or Pt catalysts
nitroarenes,16 we became interested in related transfer hydro-
is their missing chemoselectivity, and it is generally accepted that
genations, which obviously do not require the expensive silane
these catalysts cannot be applied for hydrogenation of substi-
tuted nitrobenzenes.5 Because of this lack of selectivity, for more
and might work at milder conditions.
functionalized substrates the sulphide reductions prevail in
industry.6 Although remarkable advancements have been achieved
’ RESULTS AND DISCUSSION
in the selective catalytic hydrogenation of nitroarenes via tuning of
the corresponding heterogeneous catalysts,7 still the development
of novel catalysts with broad functional group tolerance and high
activity represents an important challenge. In this respect, the use of
homogeneous catalysts based on inexpensive biorelevant metals
offers interesting opportunities especially for more challenging
substrates such as the ones shown in Figure 1.
In addition to catalytic hydrogenations, also transfer hydro-
genation provides an attractive option for the reduction of nitro-
arenes.8 For example, by applying formates as reducing agents,
no autoclaves are required, and the handling of hydrogen is
avoided.9,10 So far, Rh,9 Pd,9 Ru,9À11 Cu,12 and Co12 complexes
are known as catalysts for such transformations. However, rela-
tively long reaction times, high temperatures, and/or the price of
the metal are drawbacks of these protocols.
At the start of this project, we tested different iron, ruthenium,
copper, and cobalt salts in the presence of various commercially
available mono- and bidentate phosphines for the reduction of
nitrobenzene with formic acid at low temperature (40 °C).
Unfortunately, none of the in situ-generated complexes catalyzed
the formation of aniline to any extent (see Table 1). However,
using a combination of Fe(BF4)2 6H2O and the so-called
3
tetraphos ligand tris[(2-diphenylphosphino)-ethyl]phosphine
[P(CH2CH2PPh2)3; (PP3)] led to significant activity and gave
aniline in 71% yield (Table 1, entry 10).
Notably, it was demonstrated that related catalyst systems are
able to generate hydrogen directly from formic acid or can be
Received: July 7, 2011
Published: July 08, 2011
r
2011 American Chemical Society
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|

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Table 1. Catalytic Reduction of Nitrobenzenea
Table 2. Influence of the Solventa
entry
solvent
conv. (%)b
yield (%)b
1
2
DCM
10
<1
14
8
3
<1
14
2
acetonitrile
DMF
3
entry
catalyst
conv. (%)b
yield (%)b
4
Et2O
5
MeOH
EtOH
48
89
92
82
74
9
35
81
73
63
67
2
1
2
RuCl3 H2O/PP3
<1
<1
<1
<1
<1
<1
<1
<1
19
<1
<1
<1
<1
<1
<1
<1
<1
12
71
<1
<1
<1
<1
<1
<1
<1
95
93
93
3
6
Ru(acac)3/PP3
7
nBuOH
iPrOH
THF
3
RuCl2(cod)/PP3
8
4
FeCl3 6H2O/PP3
3
9
5
Fe(acac)3/PP3
10
toluene
6
FeSO4 7H2O/PP3
a Reaction conditions: 40 °C, 1 h, 0.5 mmol of nitrobenzene, 0.01 mmol
of catalyst, 4.5 equiv of formic acid, 3 mL of EtOH. b Determined by GC
using n-hexadecane as an internal standard.
3
7
Cu(BF4)2 xH2O/PP3
3
8
Co(BF4)2 6H2O/PP3
3
9
FeF2/PP3
10
11
12
13
14
15
16
17c
18d
19d,e
20d,f
Fe(BF4)2 6H2O/PP3
78
Table 3. Influence of the Reducing Agenta
3
Fe(BF4)2 6H2O
<1
<1
<1
<1
<1
<1
<1
>99
>99
>99
3
conv. (%)b
yield (%)b
Fe(BF4)2 6H2O/tdmeg
entry
reducing agent
3
Fe(BF4)2 6H2O/BINAP
3
1
2
3
4
5
6
7
<1
>99
16
<1
86
4
Fe(BF4)2 6H2O/dppe
3
formic acid
Fe(BF4)2 6H2O/dppp
3
sodium formate
h
Fe(BF4)2 6H2O/tdme/PPh3
3
ammonium formate
sodium formate/water (1:1)
formic acid/triethylamine
acetic acid
<1
8
<1
2
Fe(BF4)2 6H2O/PPh3
3
Fe(BF4)2 6H2O/PP3
3
<1
<1
<1
<1
Fe(BF4)2 6H2O/PP3
3
Fe(BF4)2 6H2O/PP3
a Reaction conditions: 40 °C, 1 h, 0.5 mmol of nitrobenzene, 0.03 mmol
of catalyst, 4.5 equiv of reducing agent, 3 mL of THF. b Determined by
GC using n-hexadecane as an internal standard.
3
a Reaction conditions: 40 °C, 1 h, 0.5 mmol of nitrobenzene, 0.01 mmol
of catalyst (M/L ratio 1:1), 4.5 equiv of formic acid, 3 mL of EtOH.
b Determined by GC using n-hexadecane as an internal standard. c M/L
ratio 1:4. d 0.02 mmol of catalyst. e Room temperature, 8 h. f 70 °C, 5 min.
g 1,1,1-Tris(diphenylphosphino-methyl)ethane. h 1.5 mol% of each
ligand.
at all is observed. After that, we run the same reaction in the
presence of 5 equiv of formic acid (with respect to the catalyst)
and an excess of nitrobenzene (100 equiv). In this experiment,
we examined if the active catalyst is formed by reaction with
formic acid and subsequently is able to use hydrogen as the “true”
reducing agent. However, under these conditions, only traces of
aniline (1%) were detected, showing that the substrate is not
reduced by an iron-catalyzed direct hydrogenation.
To study the unproductive decomposition of the reductant,
we analyzed the gas phase of the reaction mixture. Here, the ratio
of carbon dioxide to hydrogen was 3.2:1, demonstrating that
about 31% of the consumed formic acid was decomposed to
hydrogen. This finding explains the necessity of using 4.5 equiv of
formic acid for complete reduction of the substrate. In addition,
we advise for larger scale experiments to use appropriate safety
measures similar to reactions with hydrogen.
The reducing agent was also the subject of further investiga-
tions (see Table 3). Surprisingly, formic acid is superior in
comparison to various formates (Table 3, entries 2À5). Hence,
the addition of base, for example, triethylamine, which is usually
required for transfer hydrogenation, is not necessary but causes
even lower reactivity (Table 3, entry 6). This is a rare example of
base-free transfer hydrogenations and to the best of our knowl-
edge the first example of a base-free catalytic transfer hydrogena-
tion of nitro compounds.19
After that, we investigated the catalytic performance of defined
iron complexes with the general formula [FeX(PP3)][Y] (X = H,
H(H2), F, Cl; Y = BF4, BPh4). These complexes were tested also
for the reduction of nitrobenzene as shown in Table 4. While the
applied for hydrogenation with molecular hydrogen.17,18 Appar-
ently, not only hydrogen can be generated in the presence of iron,
but it is also possible to transfer hydrogen to organic substrates
with this catalyst system. As compared to all other tested
precatalysts, the behavior of the catalyst is unique: As shown in
Table 1, only the combination with Fe(BF4)2 6H2O exhibited
3
promising conversion toward the reduction of the nitro group,
while FeF2 led to slight conversion (Table 1, entry 9). Remark-
ably, also ruthenium precursors in combination with tetraphos
did not exhibit any reactivity under these conditions.
Next, the reaction conditions for the active catalyst system
were further optimized to allow for full conversion and up to 95%
yield of aniline (Table 1, entries 18À20). In contrast to other
protocols, this catalytic reduction can be performed even at room
temperature (Table 1, entry 19), while at 70 °C only 5 min
reaction time is necessary to complete the reduction without any
loss of selectivity (Table 1, entry 20).
The influence of the solvent was also investigated and is shown
in Table 2. Beside tetrahydrofuran, protic solvents are most
suitable for this reaction (Table 2, entries 5À9).
To ensure that the nitro group is reduced via a transfer
hydrogenation process and not by hydrogen generated from
dehydrogenation of the formic acid, we performed the iron-
catalyzed reduction of nitrobenzene at 5 bar of hydrogen
pressure. Notably, when no formic acid was present, no reactivity
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Table 4. Reduction of Nitrobenzene Applying Well-Defined
Fe-Complexesa
entry
catalyst
conv. (%)b
yield (%)b
1
2
3
4
5
6
7c
[FeCl(PP3)][BPh4]
[FeF(PP3)][BPh4]
[FeH(PP3)][BPh4]
[FeH(H2)(PP3)][BPh4]
[FeF(PP3)][BF4]
<1
78
<1
14
72
78
53
<1
73
<1
11
63
71
46
Figure 2. Concentration/time diagram for the reduction of nitroben-
zene to aniline.
Fe(BF4)2 6H2O/PP3
3
[FeH(PP3)][BPh4]
a Reaction conditions: 40 °C, 1 h, 0.5 mmol of nitrobenzene, 0.01 mmol
of catalyst, 4.5 equiv of formic acid, 3 mL of EtOH. b Determined by GC
using n-hexadecane as an internal standard. c Addition of 2 mol % CsF.
Scheme 1. Possible Pathways for the Reduction of
Nitrobenzene
corresponding chloride and hydride species exhibit little or no
reactivity (Table 4, entries 1, 3, 4), the cationic [FeF(PP3)]+
complexes gave similar results as compared to the in situ-gener-
ated system (Table 4, entries 2, 5, and 6). Interestingly, the
addition of CsF to the [FeH(PP3)][BPh4] complex enhances the
reactivity dramatically (Table 4, entry 7), probably due to the
formation of the highly active [FeF(PP3)]+ cation. To exclude
the possibility of active copper or cobalt traces often present in
iron salts, we tested Cu(BF4)2 xH2O and Co(BF4)2 6H2O as
Scheme 2. Proposed Catalytic Cycle
3
3
catalysts.20 However, again no reactivity is observed at all
(Table 1, entries 7, 8). Additionally, atomic absorption spectros-
copy was performed on the active iron complexes, showing no
traces of Ni, Cu, Co, Ru, Pt, and Pd.
An important issue of the reduction of nitroarenes at larger
scale is the formation of unwanted hydroxylamines. This inter-
mediate is often less reactive as compared to the starting
nitroarene, which is a critical factor especially at lower
temperature.21 These compounds are known carcinogens and
potentially explosive at higher concentration due to their thermal
instability. To our delight, in the presence of our iron-catalyzed
system, no hydroxylamines (<1%) are observed during catalysis
(Figure 2). Hence, taking samples from a catalytic experiment, no
intermediates (>1%) are detected.
Beside the direct reduction of the nitro group, a second
pathway via condensation of the initially formed nitrosoarene
and the hydroxylamine is possible (Scheme 1). In this case, the
resulting condensation product is further reduced via azoben-
zene and 1,2-diphenyl-hydrazine to give aniline.
However, in the reaction mixture of the model reaction, only
small quantities of azobenzene are observed (,1%) as side-
products. Furthermore, direct iron-catalyzed reduction of azo-
benzene resulted only in 48% conversion. Here, about 40% of
phenylhydrazine is formed, and aniline is detected in minor
amounts (5%). This led us to the conclusion that the direct
reduction pathway is favored in the presence of the active iron
catalyst system.
will lead to the neutral complex 2. β-Hydride elimination from 2,
which is well-known in other formate decomposition reactions,22
liberates carbon dioxide and results in complex 3.
Subsequent protonation from formic acid leads to the corre-
sponding iron dihydride complex 4, which will reduce nitroben-
zene to nitrosobenzene and water. Subsequent reduction of the
latter intermediate in a similar catalytic cycle leads to phenyl
hydroxylamine and finally to aniline. As mentioned above, these
intermediates are not observed in the reaction mixture and
should be therefore faster reduced as compared to the initial
nitrobenzene.
Finally, to demonstrate the general applicability of this catalyst
system, various nitroarenes were tested (Table 5). Easy alkyl-
substituted nitroarenes are reduced in excellent yields up to
99% (Table 5, entries 1, 2). More importantly, for halogenated
nitrobenzenes, full conversion is achieved, giving yields of
In Scheme 2, the proposed mechanism of our Fe-catalyzed
transfer hydrogenation is shown. On the basis of the experiments
with isolated iron complexes (see Table 4), we propose the
[FeF(PP3)]+ cation as active catalyst. Coordination of formate
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Table 5. Iron-Catalyzed Reduction of Nitroarenesa
and dihalogenated nitroarenes exhibit reactivity similar to that of
the monohalogenated derivatives. For all substrates, no dehalo-
genation processes are observed.23
From a synthetic point of view, it is useful that reducible
moieties such as olefins (meta-nitrostyrene) and ketones (para-
nitroacetophenone) remained unaffected by the iron catalyst
(Table 5, entries 10, 11). Vinylaniline was obtained in 95%,
giving a yield comparable to the results of supported gold
catalysts reported from the Corma group.7a Other functional
groups such as esters, ethers, thioethers, and amines on the
nitrobenzene are also well tolerated to give the corresponding
anilines in good to very good yield without further optimization
(Table 5, entries 12À15). For substrates bearing the functional
moiety not directly attached, also excellent yields are achieved
(Table 5, entries 16, 17).
’ SUMMARY
We have developed a novel transfer reduction of industrially
important nitroarenes. In the presence of well-defined iron
complexes or the in situ combination of Fe(BF4)2 6H2O and
3
P(CH2CH2PPh2)3, selective transfer hydrogenation of various
functionalized substrates to the corresponding anilines occurred
in good to excellent yields. The catalytic system already works at
room temperature, and this catalytic process proceeds without
any additional base, which is a common requirement for other
transfer hydrogenations.
’ EXPERIMENTAL SECTION
The general procedure for the reduction of nitrobenzene is as follows.
In a Schlenk tube under argon atmosphere, Fe(BF4)2 6H2O (6.75 mg,
3
0.02 mmol) and P(CH2CH2PPh2)3 (13.41 mg, 0.02 mmol) were
dissolved in 3 mL of dry EtOH, leading to the formation of a deep
purple solution. Dry nitrobenzene (51.5 μL, 0.5 mmol) and dry
n-hexadecane (100 μL) as internal standard were added. The reaction
mixture was heated to 40 °C, and formic acid (85 μL, 4.5 equiv) was
injected. The solution immediately turned brown. After 1 h, the mixture
was cooled, and a sample was taken from the slight yellow solution. All
catalytic reactions were performed at least twice to ensure reproduci-
bility. To determine the isolated yield of the anilines, the general
procedure was scaled up by the factor of 4, and no internal standard
was added. After completion, the solution was washed with a saturated
NaHCO3 solution. The mixture was then extracted with ethyl acetate
(three times), the combined organic layers were dried over MgSO4, and
the solvent was removed in vacuo. The anilines were purified by column
chromatography (silica; n-hexane/ethyl acetate mixture = 10:1 f 5:1).
The molecular defined complexes [FeH(PP3)]BPh4, [FeH(H2)-
(PP3)]+XÀ (XÀ = BF4, BPh4), [FeCl(PP3)]BF4, and [FeF(PP3)]+XÀ
(XÀ = BF4, BPh4) were synthesized according to literature protocols.24
All complexes were characterized via elemental analysis and HRÀMS,
in the case of [FeH(H2)(PP3)]+XÀ (XÀ = BF4, BPh4), via NMR
spectroscopy.
a Reaction conditions: 40 °C, 0.5 mmol of substrate, 0.02 mmol of
catalyst (M/L ratio 1:1), 4.5 equiv of formic acid, 3 mL of EtOH.
’ ASSOCIATED CONTENT
c
bDetermined by GC using n-hexadecane as an internal standard. Up
scaling by factor 4 and isolated yields given in brackets. dAddition of 0.02
S
Supporting Information. Extended version of Table 1,
b
e
mol % CsF. Second addition of 0.02 mol % catalyst and 4.5 equiv of
preparation ofthe defined complexes, and references. This material
f
g
formic acid after 1 h. 0.03 mmol of catalyst. 1.5 mol% catalyst and
second addition of 1.5 mol% catalyst after 1 h.
is available free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION
88À92% (Table 5, entries 3À9). The position of the halide
substituent did not induce any significant differences in reactivity,
Corresponding Author
matthias.beller@catalysis.de
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’ ACKNOWLEDGMENT
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(16) Junge, K.; Wendt, B.; Shaikh, N.; Beller, M. Chem. Commun.
2010, 46, 1769.
(17) Bianchini, C.; Peruzzini, M.; Polo, A.; Vacca, A.; Zanobini, F.
Gazz. Chim. Ital. 1991, 121, 543.
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tetraphos, see: (a) Bianchini, C.; Mell, A.; Peruzzini, M.; Vizza, F.;
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see: Lagaditis, P. O.; Lough, A. J.; Morris, R. H. J. Am. Chem. Soc.
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This work was supported by the state of Mecklenburg-
Vorpommern, the BMBF, the Deutsche Forschungsgemein-
schaft (Leibniz price to M.B.), and the Spanish Ministerio de
Ciencia e Innovaciꢀon (MICINN) for a doctoral fellowship
(FPU).
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dx.doi.org/10.1021/ja2061038 |J. Am. Chem. Soc. 2011, 133, 12875–12879