Chemoselective Hydrogenation of Nitroarenes Catalyzed by Molybdenum Sulphide Clusters

Article abstract of DOI:10.1002/cctc.201601496

Herein, we describe an atom efficient and general protocol for the chemoselective hydrogenation of nitroarenes to anilines catalyzed by well-defined diimino and diamino cubane-type Mo₃S₄ clusters. The novel diimino [Mo₃S₄Cl₃(dnbpy)₃]⁺ ([5]⁺) (dnbpy=4,4′-dinonyl-2,2′-dipyridyl, L1) trinuclear complex was synthesized in high yields by simple ligand substitution reactions starting from the thiourea (tu) [Mo₃S₄(tu)₈(H₂O)]Cl₄?4 H₂O (3) precursor. This strategy has also been successfully adapted for the isolation of the diamino [Mo₃S₄Cl₃(dmen)₃](BF₄) ([6](BF₄)), (dmen=N,N′-dimethylethylenediamine) salt. Applying these catalysts, high selectivity in the hydrogenation of functionalized nitroarenes has been accomplished. Over thirty anilines bearing synthetically functional groups have been synthesized in 70 to 99 % yield. Notably, the integrity of the cluster core is preserved during catalysis. Based on kinetic studies on the hydrogenation of nitrobenzene and other potential reaction intermediates, the direct reduction to aniline is the preferential route.

Full text of DOI:10.1002/cctc.201601496

Accepted Article
Title: Chemoselective hydrogenation of nitroarenes catalyzed by
molybdenum sulphide clusters
Authors: Rosa Llusar, Elena Pedrajas, Iván Sorribes, Artem L.
Gushchin, Yuliya A. Laricheva, Kathrin Junge, and Matthias
Beller
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10.1002/cctc.201601496
ChemCatChem
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Chemoselective hydrogenation of nitroarenes catalyzed by
molybdenum sulphide clusters
Elena Pedrajas,[a] Iván Sorribes,†[a] Artem L. Gushchin,[b] Yuliya A. Laricheva,[b] Kathrin Junge,[c]
Matthias Beller*[c] and Rosa Llusar*[a]
Abstract: Herein, we describe an atom efficient and general
protocol for the chemoselective hydrogenation of nitroarenes to
anilines catalyzed by well-defined diimino and diamino cubane-type
Mo3S4 clusters. The novel diimino [Mo3S4Cl3(dnbpy)3]+ ([5]+) (dnbpy=
4,4’-dinonyl-2,2’-dipyridyl, L1) trinuclear complex was synthesized in
high yields by simple ligand substitution reactions starting from the
thiourea (tu) [Mo3S4(tu)8(H2O)]Cl4·4H2O (3) precursor. This strategy
has also been successfully adapted for the isolation of the diamino
approach. Noteworthy, by choosing the right combination of the
metal center and enclosing ligands their activity can be tuned to
control the selectivity towards the desired product.
One of the most fundamental hydrogenation reactions in
chemistry involves the use of nitroarenes as starting materials,
since the resulting anilines are key building blocks and central
intermediates
for
the
chemical,
pharmaceutical
and
agrochemical industries.[3–5] Despite of the significant progress
achieved during last decades in both homogeneous[6–14]and
heterogeneous catalysis,[15–27] the environmentally benign
chemoselective hydrogenation of nitroarenes conducted at mild
conditions and using non-noble metal based catalysts remains
[Mo3S4Cl3(dmen)3](BF4)
([6](BF4)),
(dmen=
N,N’-
dimethylethylenediamine) salt. Applying these catalysts, high
selectivity in the hydrogenation of functionalized nitroarenes has
been accomplished. Over thirty anilines bearing synthetically
functional groups have been synthesized in 70 to 99% yield. Notably, as an actual and important topic for both academia and industry.
the integrity of the cluster core is preserved during catalysis. Based
on kinetic studies on the hydrogenation of nitrobenzene and other
potential reaction intermediates, the direct reduction to aniline is the
preferential route.
Typically
in
academia,
homogeneous
catalytic
hydrogenations of nitroarenes and related compounds make use
of noble metals,[28] although economic and environmental
reasons are nowadays boosting the development of earth
abundant metal based catalysts such as iron, cobalt and to a
lesser extent, molybdenum.[29–40] In fact, examples of
molybdenum-catalyzed homogeneous hydrogenations are
scarce. In the mid-eighties, the group of Dubois reported the
Introduction
hydrogenation of
a
few nitroarenes, nitrosobenzene,
Catalytic hydrogenation is
a fundamental methodology in
phenylhydroxylamine and azoxybenzene by using dinuclear
molybdenum(IV) complexes containing bridging sulphide ligands
catalysis widely used for the conversion of several raw materials
to produce highly valuable chemicals.[1] In general, success of
this transformation relies on the use of transition metals as
catalysts. In terms of industrial applications, heterogeneous
catalysts are preferred because of their stability, ease of
separation and recycling.[2] However, when the hydrogenation
reaction has to be performed in a selective manner, the use of
molecularly defined homogeneous catalysts is still an attractive
as
catalysts.[41]
These
MeCpMo(µ-S)2(µ-SH)2MoCpMe
complexes or closely related dimers, are also active for the
hydrogenation of N=N bonds in azides and azo compounds as
well as for the reduction of C=N bonds in isocyanates,
isothiocyanates and imines. Although such clusters were able to
hydrogenate different unsaturated nitrogen functional groups,
their selectivity was not evaluated when other reactive groups
were present.
Later on, Bullok and co-workers showed that mononuclear
hydrido molybdenum carbonyl complexes were active catalysts
for the ionic hydrogenation of ketones under mild conditions.[42]
In this respect, the work of Martins on related molybdenum
complexes is also noteworthy.[43] In addition, Royo and co-
workers applied simple MoO2Cl2 for the catalytic hydrogenation
of alkynes, sulfoxides and nitroarenes. However, harsh
conditions were required for the nitroarene hydrogenation (120
ºC; 50 atm of H2).[44,45] In 2009, Baricelli et al. presented the
hydrogenation of olefins catalyzed by a molybdenum carbonyl
mononuclear complex.[46] More recently, this transformation and
the hydrogenation of imines were also accomplished by using
molybdenum nitrosyl hydrides as catalysts.[47,48]
Generally, catalytic systems based on transition metal
clusters have been developed along the past decades to a much
lesser extent than their monometallic counterparts.[49] However,
in 2012 we described cubane-type Mo3(µ3-S)(µ-S)3 clusters
functionalized with diphosphane ligands for the selective transfer
hydrogenation of nitroarenes applying formates as reducing
[a]
[b]
E. Pedrajas, Dr. I. Sorribes and Prof. Dr. R. Llusar
Departament de Química Física i Analítica
Universitat Jaume I
Av. Sos Baynat s/n, 12071 Castelló, Spain
Fax: +34-964-728066
E-mail: rosa.llusar@uji.es
Dr. A. L. Gushchin and Y. A. Laricheva
Nikolaev Institute of Inorganic Chemistry
Siberian Branch of the Russian Academy of Sciences
Novosibirsk State University
630090 Novosibirsk, Russia
Dr. K. Junge and Pof. Dr. M. Beller
Leibniz-Institute für Katalyse e. V. an der Universität Rostock
Albert Einstein Str. 29a, 18059 Rostock (Germany)
E-mail: matthias.beller@catalysis.de
Present address: Instituto de Tecnología Química
Universitat Politècnica de València-Consejo Superior de
Investigaciones Científicas
[c]
†
Av. De los Naranjos s/n, 46022 Valencia, Spain
Supporting information for this article is given via a link at the end of
the document.
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10.1002/cctc.201601496
ChemCatChem
FULL PAPER
agents.[50] The same cluster unit decorated with diamine ligands
has been used for the selective catalytic hydrosilylation of nitro
and azo compounds under mild conditions.[51] Although both
systems are attractive for the synthesis of specific anilines at
laboratory scale, they suffer from low atom-efficiency. Therefore,
we became interested in developing an environmentally
Advantageously, compared to the existing homogeneous Fe-
based catalyst system reported to date for the hydrogenation of
nitroarenes, no stoichiometric amounts of strong acids such as
trifluoroacetic acid are required to get significant reactivity.[6] As
expected, no reactivity was observed for the ligand-catalyzed
reaction in the absence of complex 3 (Table 1, entry 15). Hence,
friendlier catalytic system that makes use of molecular hydrogen, we focused our attention on the catalytic activity of the well-
which is a cheap and “green” reducing agent. Motivated by the
early work of Dubois on Mo2(µ-S)2(µ-SH)2 and the electronic and
structural similarities between these dimers and the cubane-type
Mo3(µ3-S)(µ-S)3 clusters, we decided to investigate the catalytic
potential of these trimetallic complexes in the presence of
various bidentate ligands. In this context, herein, we report for
the first time the direct hydrogenation of aromatic nitro
compounds catalyzed by well-defined Mo3S4 clusters bearing
diimine and diamine ligands.
defined [Mo3S4Cl3L3]+ cluster, L = dnbpy (L1), and its analogous
dmen (L4)] derivative.[51]
Results and Discussion
Synthesis and characterization of the catalyst
In an exploratory study aimed to identify active species in the
hydrogenation of nitrobenzene (1a), we tested several trinuclear
molybdenum sulphide clusters, represented in Fig. 1, as
catalysts. The Mo3S4 and Mo3S7 cluster cores differ in the nature
of the bridging ligand, sulphide or disulphide. The nine outer
positions in Mo3S4 and the six ones in Mo3S7 are occupied by
groups of different nature: chloride, disulphide, thiourea and
water. For comparative purposes, the one dimensional
{Mo3S7Cl4}n polymeric phase has also been tested.
Scheme 1. Ligands tested in combination with complex 3 on the catalytic
hydrogenation of nitrobenzene (1a) to aniline (2a).
For the synthesis of the new diimino [Mo3S4Cl3(dnbpy)3]+
([5]+) cluster, we followed the general procedure developed by
some of us for the preparation of diimino Mo3S4 complexes
functionalized with 2,2'-bipyridine and 1,10-phenanthroline (L3)
ligands, according to equation 1.[52]
[Mo3S4(tu)8(H2O)]Cl4·4H2O (3) + 3 dnbpy (excess) à
à [Mo3S4Cl3(dnbpy)3]+ ([5]+) + 8 tu + H2O (1)
Reaction (1) took place in acetonitrile under reflux
conditions. After removal of the solvent, the product was
extracted with CH2Cl2 to eliminate the free thiourea ligand, and
further purified by silica-gel chromatography column to afford the
[5](PF6) salt in 80% yield. Its positive ESI mass spectrum in
CH3CN shows one single charged peak at m/z = 1748, which is
associated to the pseudomolecular [Mo3S4Cl3(dnbpy)3]+ ([5]+) ion
on the basis of the m/z value and its characteristic isotopic
pattern. Unfortunately, the presence of long alkyl chains in the
[5]+ cation precluded crystallization of any of its salts. The
molecular structure of [5]+ is similar to that of other diimine and
diamine derivatives of the cuboidal Mo3S4 cluster core according
to NMR spectroscopy. The asymmetric coordination of the
ligands to the Mo3(µ3-S)(µ-S)3 core with one nitrogen atom
attached trans and the other one cis to the capping sulfur atom
(µ3-S) is confirmed by six different signals of the dnbpy ligand
(L1) in the aromatic region of the 1H NMR spectrum (Figure SI1).
The 13C NMR spectrum, with ten different signals corresponding
to the non-equivalent aromatic rings of L1, also confirms this
structural arrangement (Figure SI2).
Figure 1. Mo3S4 and Mo3S7 cluster units.
As shown in Table 1 (entries 1-4), the trinuclear complex
[Mo3S4(tu)8(H2O)]Cl4·4H2O (3) (tu
= thiourea) dissolved in
methanol proved to be the most active system in the
hydrogenation of this model substrate (1a) at 20 bar of H2 and
80 ºC affording aniline (2a) in 44% yield (Table 1, entry 3). To
improve the product yield, various commercially available
phosphine and nitrogen-based ligands, shown in Scheme 1,
were tested in combination with the thiourea Mo3S4 complex 3
(Table 1, entries 5-14).
In this particular case, the use of phosphorous-based
ligands led to a detrimental activity. However, in the presence of
the diimine, 4,4’-dinonyl-2,2’-dipyridyl (dnbpy; L1), 4,4’-di-tert-
butyl-2,2’-dipyridyl
(dbbpy;
L2)
or
diamine
N,N’-
dimethylethylenediamine (dmen; L4) ligands, the yield was
improved up to 98-99% (Table 1, entries 5, 6, 8).
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Incidentally, this synthetic strategy can be used for the
Therefore, experiments for further optimization of reaction
conditions were conducted using these well-defined complexes
as catalysts. Comparing [5]+ and [6]+, the diimino complex
isolation of the [Mo3S4Cl3(dmen)3](BF4) ([6](BF4)) salt, previously
prepared from "in situ” generated [Mo3S4Clx(PPh3)y(solvent)z](4-x)+
species.[51] In the case of the diamino [6]+ cation, acid must be
added to prevent partial substitution of the terminal chlorine
ligands by hydroxo groups generated from adventitious water
molecules.
With well-defined complexes [5](PF6) and [6](BF4) in hand,
we tested them as catalysts for the hydrogenation of
nitrobenzene (1a) under the same reaction conditions used vide
supra (Table 1, entries 16, 17). Both complexes showed similar
reactivity compared with their corresponding in situ systems,
thus revealing that identical active species are formed.
showed
a
slightly higher reactivity at lower reaction
temperatures (Table SI1), and a quantitative yield of aniline (2a)
was achieved at 70 ºC (Table 1, entries 17-18) for both catalysts.
Next, the influence of the H2 pressure, solvent and catalyst
loading was studied in the presence of catalyst [5](PF6).
Interestingly, only a slight decrease of conversion and yield
(89% of 2a) was observed by decreasing the H2 pressure up to
15 bar (Table SI2). The use of other solvents different from
methanol led to a detrimental conversion of 1a. Whereas good
reactivity was achieved with ethanol, other polar solvents, such
as CH3CN and isopropanol, afforded significantly lower yields of
2a. No reaction took place in non-polar solvents as
a
Table 1. Molybdenum sulphide-catalyzed hydrogenation of nitrobenzene.[a]
consequence of the low solubility of catalyst [5](PF6) (Table SI3).
It should be note that no deuterated aniline was formed by using
CD3OD as solvent, thus precluding a solvent-mediated transfer
hydrogenation mechanism (see, Figure SI4 in the Supporting
Information). Finally, an optimal catalyst loading was found to
be 4 mol%, affording 2a in quantitative yield at 20 bar of H2, 70
ºC and using methanol as solvent (Table 1, entry18; see also
Table SI4).
Next, a reaction profile of the hydrogenation of 1a was
performed under standard reaction conditions using different
batch experiments (Figure 2). To our delight, no accumulation of
the carcinogenic and potentially explosive hydroxylamine (< 1%)
was observed during catalysis. Notably, no obvious induction
period is observed from the reaction profile, thus suggesting that
no fragmentation of the trinuclear cluster entity to lower
nuclearity molybdenum complexes is required to form the active
species in the reaction mixture.
Entry
Catalyst
Conversion [%][b]
Yield [%][b]
1
(TBA)2[Mo3S7Cl6] (1)
12
0
12
0
2
(NH4)2(Mo3S13)2H2O (2)
3
[Mo3S4(tu)8(H2O)]Cl4·4H2O (3)
46
1
44
1
4
{Mo3S7Cl4}n (4)
3/L1
5[c]
>99
>99
16
>99
22
29
18
32
34
19
-
>99
>99
15
98
22
29
11
13
26
12
-
6[c]
3/L2
7[c]
3/L3
8[c]
3/L4
9[c]
3/L5
10[c, d]
11[c, d]
12[c, d]
13[c, d]
14[c, d]
15
3/L6
3/L7
3/L8
3/L9
3/L10
L1
Figure 2. Concentration time diagram for the hydrogenation of nitrobenzene
(1a) to aniline (2a) catalyzed by the diimino [Mo3S4Cl3(dnbpy)3](PF6) ([5](PF6))
complex.
16
[5](PF6)
[6](BF4)
[5](PF6)
[5](PF6)
>99
>99
>99
52
98
>99
>99
52
17
To get further evidences about the cluster integrity during
catalysis, reaction mixtures obtained from batch experiments at
different reaction times were analyzed by electrospray ionization
mass spectrometry (ESI-MS) after dilution with ethyl acetate. As
shown in Figure 3 (see also Figure SI5), the peak centered at
18[e]
19[f]
m/z
=
1748
associated
to
the
pseudomolecular
[a] Reaction conditions: Nitrobenzene (0.1 mmol), H2 (20 bar), catalyst (5
mol%), MeOH (2 mL), 18 h, 80 ºC. [b] Determined by GC analysis using n-
hexadecane as an internal standard. [c] 3/Ligand (1:3.5). [d] 100 ºC. [e]
Catalyst (4 mol%), 70 ºC. [f] Catalyst was reused for the second time.
[Mo3S4Cl3(dnbpy)3]+ ([5]+) ion is successively broadened with the
reaction time as a consequence of the formation of trinuclear
species of general formula [Mo3S4Cl3-x(OMe)x(dnbpy)3]+.
Apparently, all these mixed outer-ligands clusters remain active
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10.1002/cctc.201601496
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during the catalytic reaction. No other peaks were observed
even after longer reaction time (4 h). Therefore, we decided to
perform the catalyst recycling in the model reaction after
recovering the catalyst by evaporation of the reaction mixture.
Interestingly, a moderate yield of 2a (52%) could be still
obtained in a second run (Table 1, entry 19).
Since no intermediates could be detected by quenching
the reaction at shorter times (see Figure 2), the hydrogenation of
some potential intermediates, such as N-phenylhydroxylamine,
nitrosobenzene, azoxybenzene, and azobenzene were
conducted in the presence of [5](PF6). In all cases, full
conversions of the starting material and nearly quantitative yield
of aniline (2a) were obtained. Thus, both generally accepted
reaction pathways, the direct reduction to aniline or the
dimerization via azobenzene,[53] seem to be possible in the
present reaction system (Table SI5 and Scheme SI1). To
differentiate between both routes, kinetics studies starting from
nitrobenzene, azoxybenzene and azobenzene have been
undertaken (Figure SI6). Reaction monitoring for the
nitrobenzene and azobenzene hydrogenation shows aniline as
the only reaction product while both aniline and azobenzene are
detected in the azoxybenzene hydrogenation. Reaction rates for
the nitrobenzene hydrogenation range between 3,5-4,9 mmol L-1
h-1 which are two to three times higher than the rates for the
azoxybenzene and azobenzene hydrogenation. That is, the
direct reduction to aniline is the preferential route. Incidentally,
the same reaction pathway is found to be kinetically favored for
the heterogeneous nitrobenzene reduction with hydrazine
catalyzed by oxygen-implanted molybdenum disulphide.[54]
Figure 3. ESI mass spectra from nitrobenzene hydrogenation monitoring (a: t
= 0 min; b: t = 90 min and c: t = 240 min (x2 > x1)).
Table 2. Mo3S4-catalyzed hydrogenation of different nitroarenes.a
Conv.
Yield
Conv.
(%)b
Yield
(%)b
Entry
1
Substrate
Entry
18
Substrate
(%)b
(%)b
>99
>99
>99
>99
95
87
2
>99
>99
19
90
(80)
20
21
>99
>99
94
80
3
4
>99
>99
>99
97
(90)
>99
(96)
5c
>99
99
22
23
>99
>99
98
(91)
94
(87)
6c
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Conv.
(%)b
Yield
(%)b
Conv.
(%)b
Yield
(%)b
Entry
Substrate
Entry
24
Substrate
7c
>99
>99
95
>99
>99
88
80
92
(80)
8
25
99
(89)
9
>99
>99
26
27
>99
>99
94
96
(80)
10d
(99)
11
12
>99
>99
98
93
28
29
>99
>99
88
(95)
13
14
15
>99
>99
>99
>99
>99
>99
30
31c
32
>99
>99
>99
70
91
>99
97
(93)
16
17
>99
>99
>99
98
84
85
33
34
>99
aReaction conditions: nitroarene (0.1 mmol), 20 bar H2, catalyst (5 mol%), MeOH (2 mL), 18 h, 70 ºC. bDetermined by GC analysis using
n-hexadecane as an internal standard; yields of isolated products given in parentheses. c100 ºC. d35 bar H2.
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54.4, H 7.6, N 4.7. 1H NMR (500 MHz, 298 K, CD2Cl2): δ = 10.40 (d, J =
5.9 Hz, 3H); 9.60 (d, J = 5.9 Hz, 3H); 8.43 (s, 3H); 8.17 (s, 3H); 7.64 (d, J
= 5.8 Hz, 3H); 7.30 (d, J = 5.8 Hz, 3H), 3.03 (t, J = 7.8 Hz, 6H), 2.82 (t, J
= 7.8 Hz, 6H), 1.90 (p, J12 = 7.8, J23 = 7.6 Hz, 6H), 1.75 (p, J12 = 7.5, J23
= 7.7 Hz, 6H), 1.53 (p, J12 = 7.5, J23 = 7.3 Hz, 6H), 1.45 (p, J12 = 7.6, J23
= 7.2 Hz, 6H), 1.33 (m, 60H), 0.93 (m, 18H) ppm. 13С{1H} NMR (500 MHz,
298 K, CD2Cl2): δ= 14.43 (s, СH3), 23.23 (s, CH2), 29.85 (s, CH2), 30.54
(s, CH2), 30.75 (s, CH2), 32.44 (s, CH2), 36.19 (s, CH2), 123.09 (s, CH),
124.98 (s, CH), 126.87 (s, CH), 128.09 (s, CH), 153.62 (s, CH), 156.19 (s,
CH), 156.93 (s, CH), 158.35 (s, CH), 158.46 (s, CH), 158.60 (s, CH) ppm.
ESI-MS (CH3CN, 20 V): m/z: 1748 [M+].
Next, the applicability of the well-defined catalyst
[Mo3S4Cl3(dnbpy)3](PF6) ([5](PF6)) was demonstrated in the
hydrogenation of more than 30 structurally diverse nitroarenes.
As shown in Table 2, nitroarenes bearing inert and/or less
reactive functional groups, such as alkyl, hydroxy, alkoxy and
biphenyl, were hydrogenated giving the corresponding anilines
in up to quantitative yields (Table 2, entries 1-6). Industrially
relevant halogen-substituted nitroarenes were smoothly
hydrogenated without any dehalogenation process affording the
desired halogen-substituted anilines in excellent yields,
independently of the number of halogens and their ring position
(Table 2, entries 7-13). In addition, other substituents including
trifluoromethyl and amino as well as thiomethyl groups were well
tolerated, too (Table 2, entries 14-16).
More demanding is hydrogenation of nitroarenes
containing easily reducible functional groups. For instance, the
chemoselective reduction in the presence of olefins continues to
be challenging.[17] To our delight, olefinic double bonds were
retained affording the corresponding anilines in high yields
(Table 2, entries 17-19). Interestingly, other nitroarenes
containing sensitive groups, such as ketones or sulfonamides,
were converted into the corresponding anilines in good to
excellent yields (Table 2, entries 20, 21). Moreover, carboxylic
acids derivatives, such as cyanides, esters and amides were
also tolerated (Table 2, entries 22-30). Finally, several
heteroaromatic amines, which are valuable building blocks for
New procedure for the synthesis of [Mo3S4Cl3(dmen)3](BF4):
A
mixture of [Mo3S4(tu)8(H2O)]Cl4⋅4H2O (0.150 g, 0.119 mmol), N,N'-
dimethylethylenediamine (42 µL, 0.394 mmol) and CH3CN (20 ml) was
refluxed for 3 hours and allowed to cool to room temperature. Then, a
freshly prepared 0.5 M in CH3CN solution of HBF4·ether complex (0.95
mL, 0.477 mmol) was added and the reaction mixture was stirred at room
temperature for 30 min. After filtration, the green solution was taken to
dryness, redissolved in CH2Cl2 and the precipitate of thiourea was
eliminated by filtration. The solution was concentrated under reduced
pressure and the desired product was precipitated by adding diethyl
ether (75 mL). Finally, the green solid was separated by filtration and
washed with diethyl ether to yield 0.08 g of an air stable product
characterized as [Mo3S4Cl3(dmen)3](BF4) (77% yield).
Catalytic Activity Test
the preparation of
a
variety of agrochemicals and
pharmaceuticals,[55] were afforded in 91-99% yield without
further optimization (Table 2, entries 31-34).
General procedure for the reduction of nitroarenes: A 4 mL glass vial
containing a stirring bar was sequentially charged with the molybdenum
catalyst (9.5 mg, 0.0050 mmol of [Mo3S4Cl3(dnbpy)3](PF6)), nitroarene
(10 µL, 0.097 mmol), n-hexadecane (15 µL; added as an internal
standard) and 2 mL of methanol. Afterwards, the reaction vial was
capped with a septum equipped with a syringe and set in the alloy plate,
which was then placed into a 300 mL autoclave. Once sealed, the
autoclave was purged three times with 30 bar of hydrogen, then
pressurized to 20 bar and placed into an aluminum block, which was
preheaated at 70ºC. After 18 h, the autoclave was cooled to room
temperature and the hydrogen was released. Ethyl acetate (2 mL) was
then added, and a sample was taken to be analyzed by GC. To
determine the isolated yields of the anilines, the general procedure was
scaled up by the factor of five, and no internal standard was added. After
completion of the reaction, the mixture was purified by silica gel column
chromatography (n-heptane/ethyl acetate mixtures) to give the
corresponding anilines. For anilines obtained from the substrates 27 and
29 (Table 2), the isolation procedure was different because the products
were insoluble in ethyl acetate. After the catalytic reaction, the mixture
was taken to dryness and a small amount of CH2Cl2 was added to solve
the catalyst. Then, the non-soluble aniline was recovered by filtration.
Conclusions
In conclusion, well-defined diimino and diamino Mo3S4 cubane-
type clusters have been established as excellent catalysts for
the hydrogenation of functionalized nitroarenes. Notably, the
procedure is highly chemoselective in the presence of other
reducible functional groups. A variety of anilines containing
synthetically relevant functional groups are obtained in good to
excellent yields. Reaction monitoring and electrospray ionization
mass spectrometric measurements support the integrity of the
cluster unit during catalysis.
Experimental Section
Catalyst Preparation
Synthesis
of
[Mo3S4Cl3(dnbpy)3](PF6):
A
mixture
of
Acknowledgements
[Mo3S4(tu)8(H2O)]Cl4⋅4H2O (0.40 g, 0.32 mmol), 4,4'-dinonyl-2,2'-
bipyridine (0.42 g, 1.0 mmol) and CH3CN (40 ml) was refluxed for 4-5
hours. After cooling to room temperature, the reaction mixture was taken
to dryness, redissolved in CH2Cl2 and the non-soluble thiourea was
eliminated by filtration. The solution of [Mo3S4Cl3(dnbpy)3]Cl was loaded
onto a silica gel column. After the column was washed with CH2Cl2,
The financial support of the Spanish Ministerio de Economía y
Competitividad (Grant CTQ2015-65207-P), Universitat Jaume I
(Research project P1.1B2013-19), Generalitat Valenciana
(PrometeoII/2014/022) and Russian Foundation for Basic
Science (Grant 15-03-02775a) is gratefully acknowledged. The
authors also thank the Serveis Centrals d’Instrumentació
Cientifica (SCIC) of the Universitat Jaume I for providing us with
mass spectrometry and NMR techniques. E. Pedrajas thanks
the University Jaume I for a predoctoral fellowship.
elution with
a solution of KPF6 in acetone (10 mg/ml) afforded a
concentrated brown solution, which was evaporated to dryness,
redissolved in CH2Cl2 and filtered in order to eliminate the inorganic salts.
Finally, the resulting solution was allowed to evaporate slowly in air to
give 0.48
g (80 %) of the desired complex as a brown solid.
C84H132N6Cl3F6Mo3PS4(1896.53): calcd. C 53.3; H 7.0, N 4.4; found C
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10.1002/cctc.201601496
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10.1002/cctc.201601496
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FULL PAPER
Growing the molybdenum cluster
age: Well-defined diimino and
diamino Mo3S4 cubane-type clusters
are applied as chemoselective
catalysts for the hydrogenation of
functionalized nitroarenes. This atom
efficient procedure allows for the
preparation of anilines bearing
Elena Pedrajas, Iván Sorribes, Artem
L. Gushchin, Yuliya A. Laricheva,
Kathrin Junge, Matthias Beller* and
Rosa Llusar*
35 substrates
Cluster
Catalysis
High selectivity
Page No. – Page No.
Chemoselective hydrogenation of
nitroarenes catalyzed by
molybdenum sulphide clusters
H2 (20 bar)
synthetically
relevant
functional
70 ºC, MeOH
groups in high yields.
This article is protected by copyright. All rights reserved.