One-pot preparation of imines from nitroarenes by a tandem process with an Ir-Pd heterodimetallic catalyst

Article abstract of DOI:10.1002/chem.201000801

A new tandem catalytic process has been studied for a heterodimetallic complex containing both iridium and palladium fragments connected by a 1,2,4-trimethyltriazolyldiylidene ligand. The process implies the unprecedented preparation of imines from the direct reaction of nitroarenes and primary alcohols. The global process comprises the following steps: 1) reduction of the nitroarene to an amine, 2) oxidation of the alcohol to aldehyde, and 3) condensation of the aldehyde and the amine to form the corresponding imine. The oxidation of the alcohol to aldehyde is promoted by the iridium fragment, while the reduction of the nitro group to amine is facilitated by palladium. A wide set of different catalytic systems has been studied, showing that the Ir/Pd complex 1 is a highly active and stable catalyst in the preparation of imines.

Full text of DOI:10.1002/chem.201000801

DOI: 10.1002/chem.201000801
One-Pot Preparation of Imines from Nitroarenes by a Tandem Process with
an Ir–Pd Heterodimetallic Catalyst
Alessandro Zanardi, Jose A. Mata,* and Eduardo Peris[a]
Abstract: A new tandem catalytic pro-
cess has been studied for a heterodi-
ACHTUNGTRENNUNG
duction of the nitroarene to an amine,
2) oxidation of the alcohol to aldehyde,
and 3) condensation of the aldehyde
and the amine to form the correspond-
ing imine. The oxidation of the alcohol
to aldehyde is promoted by the iridium
fragment, while the reduction of the
nitro group to amine is facilitated by
palladium. A wide set of different cata-
lytic systems has been studied, showing
that the Ir/Pd complex 1 is a highly
active and stable catalyst in the prepa-
ration of imines.
ACHTUNGTRENNUNG
a
ligand. The process implies the unpre-
cedented preparation of imines from
the direct reaction of nitroarenes and
primary alcohols. The global process
comprises the following steps: 1) re-
Keywords: alkylation · domino re-
actions · imines · iridium · nitrogen
heterocycles · palladium
Introduction
tion, the development of catalytic methodologies that afford
high chemo- and regioselectivity under mild reaction condi-
tions is an intriguing area of research.[14]
The formation of carbon–nitrogen bonds is one of the most
important transformations in organic chemistry.[1] Of partic-
ular importance is the N-alkylation of amines by alcohols by
the borrowing-hydrogen methodologies.[2,3] Imines are inter-
mediates that are often detected in this synthetic proce-
dures, although their selective preparation has been rarely
studied[4,5] Imines are an important class of carbon–nitrogen
compounds, because they have a diverse reactivity and mul-
tiple applications in laboratory and industrial synthetic pro-
cesses.[6] The traditional preparation of imines implies the
reaction of ketones or aldehydes with amines in the pres-
ence of Lewis acid catalysts. They can also be obtained by
the oxidative condensation of amines,[7,8] and by oxidation
of amines.[8,9] In the past few years, the oxidative coupling of
alcohols and amines in the presence of oxygen to provide
imines was reported,[5,10,11] but the catalytic outcomes were
rather low. Very recently, Milstein and co-workers reported
an efficient method for the oxidative coupling of amines and
alcohols that produced imines in very high yields.[4]
Over the last few years we have been interested in the
formation of carbon-nitrogen bonds through the catalytic
coupling of alcohols and amines„[15,16] via “borrowing-hydro-
gen” methodologies.[3] We have also been interested in the
preparation of stable catalysts that may be compatible with
widespread applications,[17] especially using well-defined het-
erodimetallic complexes to be applied in the design of
tandem catalytic reactions.[18,19] In particular, the Ir/Pd com-
plex 1, in which the two metal centers are linked by the
1,2,4-trimethyltriazolyldiylidene (ditz) ligand, proved to be
an efficient catalyst for a series of tandem reactions com-
prising different transformations of haloacetophenones, typi-
cally catalyzed by Ir and Pd.[18] We envisaged that the appli-
cations of 1 may be extended to other tandem reactions by
combining the large and distinct library of transformations
for which Ir and Pd precatalysts are potentially active. In
this regard, we now report a general and efficient method
for the preparation of imines by the reaction of alcohols and
nitroarenes. The reaction implies a tandem process in which
a nitroarene is reduced to an aromatic amine by an alcohol,
The selective reduction of nitroarenes is a useful method
for the preparation of amines.[12,13] In the study of this reac-
[a] A. Zanardi, Dr. J. A. Mata, Prof. E. Peris
Departamento de Quꢀmica Inorgꢁnica y Orgꢁnica
Universitat Jaume I, 12071-Castellꢂn (Spain)
Fax : (+34)964-728-214
E-mail: jmata@qio.uji.es
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Chem. Eur. J. 2010, 16, 10502 – 10506

FULL PAPER
which is oxidized to the aldehyde. The coupling of the alde-
hyde and the amine selectively affords the corresponding
imine. Complexes 2–4 were used for comparison purposes.
to aldehyde).[20,15] The overall reaction, which constitutes a
new borrowing-hydrogen process, is disclosed in Scheme 1.
Scheme 1. Tandem reaction studied in this work.
We designed this reaction with the initial idea that an iri-
dium–palladium heterobimetallic complex would be the per-
fect candidate for the overall process. The iridium fragment
would facilitate the alcohol oxidation with the release of hy-
drogen, while the palladium fragment would enable the ni-
trobenzene reduction using the hydrogen released during
the iridium oxidation process. Table 2 shows the results of
Results and Discussion
In order to determine the ability of a series of homogeneous
catalysts toward the reduction of nitroarenes, we first stud-
ied the reduction of nitrobenzene with molecular hydrogen.
All the reactions were carried out at 1008C, with 1 atm of
H2. As can be seen from the data shown in Table 1, all the
Table 2. Catalyst screening for the reaction of nitrobenzene and benzyl-
AHCTUNGTRENNUNG
alcohol.[a]
Table 1. Reduction of nitrobenzene to aniline.[a]
Entry Catalyst
Cat. [mol%][b] t [h] N-Benzylidene-
aniline [%][c]
1
2
3
4
5
6
7
8
9
N
2
0.5
2
2
2
0.5
2
0.5
2
0.5
0.5
0.5
3
20
18
3
3
20
3
20
3
20
20
20
81
60
12
72
87
76
16
7
85
35
73
52
Catalyst
[{IrCp*Cl2}2]
t [h]
Aniline [%][b]
G
20
10
10
10
20
10
93
90
97
5
PdCl2
1
2
3
10
11
12
[a] Reaction conditions: nitrobenzene (0.3 mmol), Cs2CO3 (0.3 mmol),
anisole as internal reference (0.3 mmol), catalyst (2 mol%) in toluene
(500 mL). Hydrogen was added with a balloon filled with one atmosphere
of gas. Three cycles of vacuum/H2 were applied before putting the reac-
tion vessel in an oil bath at 1008C. [b] Yields determined by GC chroma-
tography.
[a] Reaction conditions: nitrobenzene (0.3 mmol), benzylalcohol
(5 mmol) used as solvent and reagent, Cs2CO3 (0.3 mmol), anisole as in-
ternal reference (0.3 mmol). The solution was heated at 1108C under
aerobic conditions. [b] Based on metal amount. [c] Yields determined by
GC chromatography.
palladium-containing complexes used showed an excellent
outcome in this reaction. In contrast, the iridium complexes
the reaction between nitrobenzene and benzylalcohol used
as reagent and solvent at 1108C in the presence of Cs2CO3.
As can be seen from the results shown in Table 2, all iridi-
um-containing complexes provided good conversions to the
imine, while the catalysts that only contain palladium afford-
ed negligible activities (see entries 3, 7, and 8). This result is
not surprising, since the palladium catalysts are not expected
to oxidize the alcohol to aldehyde and release the required
amount of hydrogen to reduce the nitrobenzene to aniline.
On the other hand, those catalysts containing only iridium,
provide high yields of N-benzylideneaniline (entries 1, 2,
and 9). This result is remarkable, because it suggests that,
for these catalysts, the reduction of the nitrobenzene may be
due to a hydrogenation-transfer mechanism, rather than the
direct reduction with released molecular hydrogen from the
[{IrCp*Cl2}2] (Cp*=pentamethylcyclopentadienyl) and
3
produced a negligible amount of aniline.
Having proved the ability of the palladium-based com-
plexes to homogeneously catalyze the reduction of nitroben-
zene to aniline, and also confirming that the iridium com-
plexes used showed negligible activity in this reaction, we
decided to study the direct reaction of nitroarenes and pri-
mary alcohols. The process implies the following steps: 1)
reduction of a nitroarene to an amine, 2) oxidation of an al-
cohol to an aldehyde, and 3) condensation of the aldehyde
and the amine to form the corresponding imine. This would
imply a new tandem reaction comprising two different steps,
one of which is catalyzed by palladium (reduction of the ni-
troarene) and the other by iridium (oxidation of the alcohol
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J. A. Mata et al.
Table 3. Catalytic condensation of nitroarenes and primary alcohols.[a]
alcohol. The fact that the
{IrCp*} fragment may reduce
the nitro group by means of a
hydrogen transfer process in
the presence of iPrOH, was
previously pointed out by Entry
Fujita and co-workers, although
Catalyst
[{IrCp*Cl2}2]
R1
R2
t [h]
Product
Yield [%][b]
16
1
G
4-Me
C6H5
3
the conversions to aniline were
very low.[21,22] In a similar ex-
periment, we performed the re-
duction of nitrobenzene to ani-
line with iPrOH using catalyst 1
(2 mol%). After 20 h, we ob-
2
N
4-MeO
4-Me
4-MeO
H
C6H5
3
3
32
3
C6H5
83
tained a quantitative yield of
4
5
6
C6H5
3
55
aniline. The Ir–Pd heterodime-
tallic complex 1 provides the
best catalytic outcomes in this
reaction (entries 5 and 6), al-
though a mixture of the homo-
dimetallic complexes 2 and 3
(entry 11), also affords good
yields. As a consequence of the
stoichiometry of the process, in
all the reactions under study,
benzaldehyde was always de-
tected in an amount over two
equivalents with respect to the
imine.
To test the general applicabil-
ity of the process, we decided
to study the catalysts previously
described in the condensation
of different nitroarenes with a
series of primary alcohols. The
results are shown in Table 3.
The reactions were carried out
by using a primary alcohol as
the reagent and solvent at
1108C, with a catalyst loading
of 2 mol% in the presence of
Cs2CO3. The catalytic activity
of [{IrCp*Cl2}2] was very low
after 3 h (entries 1 and 2) com-
pared to the other catalytic sys-
4-Me-C6H5
4-MeO-C6H5
5
90
H
20
50 (43)
7
8
H
H
C6H5CH=CHCH2
20
12
17
92
C6H5CH2
9
10
11
12
13
14
4-Me
C6H5
3
12
12
3
82
4-Me
4-Me-C6H5
4-MeO-C6H5
C6H5
89
4-Me
83
4-MeO
4-MeO
4-MeO
75
4-Me-C6H5
4-MeO-C6H5
12
12
84
70 (65)
[a] Reaction conditions: nitroarene (0.3 mmol), benzylalcohol (5 mmol) used as solvent and reagent, Cs2CO3
(0.3 mmol), anisole as internal reference (0.3 mmol), catalyst (2 mol%). The solution was heated at 1108C
under aerobic conditions. [b] Yields determined by GC chromatography (isolated yields in parenthesis).
tems tried in this series of reactions. Longer reaction times
afforded better yields to the final imines, although these
were never above 60% after 20 h (data not shown in
Table 3). As expected, the general activity of the iridium-
only-based system is less prone to induce nitro reduction.
The mixture of compounds 2 and 3 afforded a very good
outcome for the coupling of p-nitrotoluene with benzylalco-
hol (entry 3), but provided only a moderate yield when p-ni-
troanisole was used (entry 4). Catalyst 1 showed a wider ap-
plicability, providing very good yields in the reactions of sev-
eral nitroarenes and benzylalcohols. Some aliphatic primary
alcohols were also used (1-hexanol, 1-butanol), but we did
not observe the formation of the imines. On the contrast, 2-
phenylethanol provided an excellent outcome in the conden-
sation with nitrobenzene (entry 8).
The different catalytic performances of 1 and 3 can be il-
lustrated by the oxidative condensation of p-bromonitroben-
zene and benzylalcohol. The results are shown in Table 4.
Interestingly, each catalyst leads to the formation of a differ-
ent product. While 3 affords the expected N-benzylidene-p-
bromoaniline (A), catalyst 1 provides the dehalogenated
imine (B). Pd-catalyzed dehalogenations of aryl-halides can
take place in iPrOH in the presence of a base, such as
Cs2CO3, and we have also previously observed that this pro-
cess can be promoted by catalyst 1. Because the catalytic de-
halogenation and the Suzuki–Miyaura reaction are inter-
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Tandem Reactions
FULL PAPER
Table 4. Oxidative condensation of p-bromonitrobenzene and benzyl-
mediate that easily dehydrates to yield the imine. For the Ir/
Pd catalysts, an alternative pathway for the reduction of the
nitro group could be envisaged. In a first step, the oxidation
of the alcohol to aldehyde would be promoted by the iridi-
um fragment generating an iridium hydride. In a second
step, an intramolecular hydride migration from iridium to
palladium in catalyst 1 would generate a palladium hydride
responsible of the nitro to aniline reduction. (Pathway B,
Scheme 2).
ACHTUNGTRENNUNG
alcohol.[a]
Entry
Catalyst
Additive
t [h]
Product
Yield (%)[b]
1
2
3
3
1
1
none
none
PhB(OH)2
3
20
40
A
B
C
78 (68)
71
77[c] (70)
Conclusion
[a] Reaction conditions: nitrobenzene (0.3 mmol), benzylalcohol
(10 mmol) used as solvent and reagent, Cs2CO3 (0.3 mmol) and catalyst
(2 mol%). T=1108C. [b] Yields determined by GC chromatography
using anisole as internal standard (isolated yields in parenthesis). [c] Ni-
trobenzene (0.3 mmol), benzylalcohol (10 mmol), phenylboronic acid
(0.3 mmol) Cs2CO3 (0.6 mmol) and THF (1 mL).
With this work we have extended the general “borrowing-
hydrogen” procedure in the N-alkylation of amines using al-
cohols. The amine is generated in situ by reduction from a
nitroarene. We have evaluated the activity of a series of cat-
alysts in the oxidative coupling of nitroarenes with benzylal-
cohols to afford the corresponding imines. For this reaction,
four homo- and heterodimetallic catalysts of Ir and Pd have
twined, sharing the oxidative addition step, we decided to
combine this reaction with the oxidative condensation of p-
bromonitrobenzene and benzylalcohol, by adding phenyl-
boronic acid to the reaction mixture. This new reaction pro-
vided a very good yield of the corresponding bisarylated
imine C (Table 4, entry 3), in a reaction in which the two in-
dividual actions of the iridium and palladium fragments
imply clearly distinct fundamental catalytic processes.
Based on these results, we believe that the overall tandem
reaction may occur through two different mechanistic path-
ways (Scheme 2), depending on whether iridium–palladium
or “ iridium-only” catalysts are used. For the “ iridium-
been tested. The results provided by the bisACTHNURTGNEU(GN iridium) com-
plexes are remarkable, although limited to several sub-
strates. However, the presence of the palladium fragment in
the Ir/Pd complex 1 improves the catalyst activity widening
its applicability, most probably because the presence of the
Pd opens a new reaction pathway in the reduction of the
nitro group by using the hydrogen released in the oxidation
of the alcohol by the iridium moiety.
Catalyst 1 has been used for the preparation of different
products using the same substrates and changing the addi-
tives. In a new illustration of
the wide applicability of the Ir/
Pd heterodimetallic catalyst, we
have also combined the oxida-
tive coupling of p-bromonitro-
benzene with benzylalcohol,
À
with the Suzuki–Miyaura C C
coupling, by adding phenylbor-
onic acid in the reaction
medium. This reaction provides
the corresponding bisarylated
imine in high yield.
Our work provides a new ex-
ample of tandem catalysis using
dimetallic complexes bearing
the
1,2,4-trimethyltriazolyl-
Scheme 2. Two possible mechanistic pathways for the tandem reaction.
AHCTUNGTERGdNNUN iylidene ligand. The use of
only one catalyst with two dif-
ferent metal fragments is an interesting option for the
design of catalytic sequential reactions with fundamentally
distinct mechanisms. For these types of processes, a mixture
of two catalysts can also be used, but the utilization of a het-
erodimetallic catalyst provides a series of advantages. We re-
cently suggested that the linkage through the 1,2,4-trime-
thyltriazolyldiylidene ligand may provide some catalytic co-
operativity between the two metals, but other advantages
have to be taken into account, as the lower atomic efficiency
only” catalysts, the reduction of the nitro group to the
amino group only proceeds in good yield for nitrobenzene
through the general “borrowing-hydrogen” mechanism
(Pathway A, Scheme 2). Based on our experience,[15] the ini-
tial stage of the reaction would be the oxidation of the
benzyl alcohol to benzaldehyde and the generation of a
metal hydride. The metal hydride will reduce the nitroben-
zene to aniline. One benzaldehyde equivalent condensates
with the generated aniline to produce and hemiaminal inter-
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10505

J. A. Mata et al.
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are currently in progress in our laboratory.
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General procedures: Compounds 1–4 were prepared according to litera-
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out under aerobic conditions and without solvent pre-treatment. Gas
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Catalytic studies
Reduction of nitrobenzene to aniline using H2: Molecular hydrogen was
added with a balloon filled with 1 atm of gas to a mixture of nitroben-
zene (0.3 mmol), Cs2CO3 (0.3 mmol), anisole as internal reference
(0.3 mmol) and catalyst (2 mol%) in toluene (500 mL). Three cycles of
vacuum/H2 were applied before putting the reaction vessel in an oil bath
at 1008C for the appropriate time. Yields and conversions were deter-
mined by GC chromatography.
Typical catalytic condensation of nitroarenes and primary alcohols: A
capped vessel containing a stirrer bar was charged with nitrobenzene
(0.3 mmol), benzylalcohol (5.0 mmol), Cs2CO3 (0.3 mmol), anisole as in-
ternal reference (0.3 mmol), and catalyst (0.5 or 2 mol%). The solution
was heated to 1108C for the appropriate time. Yields and conversions
were determined by GC chromatography. Products and intermediates
were characterized by GC/MS. Isolated products were characterized by
1H NMR and 13C NMR spectroscopy after column chromatography pu-
rification using mixtures of n-hexanes/ethylacetate. The confirmation of
the nature of the products was performed by comparison with the litera-
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Acknowledgements
We thank the financial support from the MCINN of Spain (CTQ2008-
04460) and Bancaixa (P1.1B2007-04 and P1.1B2008-16). We would also
like to thank the Generalitat Valenciana for a fellowship (A. Zanardi).
The authors are grateful to the Serveis Centrals dꢅInstrumentaciꢂ Cientꢀf-
ica (SCIC) of the Universitat Jaume I for providing us with spectroscopic
facilities.
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Received: March 30, 2010
Published online: July 22, 2010
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