Reductive Molybdenum-Catalyzed Direct Amination of Boronic Acids with Nitro Compounds

Article abstract of DOI:10.1002/anie.201812806

The synthesis of aromatic amines is of utmost importance in a wide range of chemical contexts. We report a direct amination of boronic acids with nitro compounds to yield (hetero)aryl amines. The novel combination of a dioxomolybdenum(VI) catalyst and triphenylphosphine as inexpensive reductant has revealed to be decisive to achieve this new C?N coupling. Our methodology has proven to be scalable, air and moisture tolerant, highly chemoselective and engages both aliphatic and aromatic nitro compounds. Moreover, this general and step-economical synthesis of aromatic secondary amines showcases orthogonality to other aromatic amine syntheses as it tolerates aryl halides and carbonyl compounds.

Full text of DOI:10.1002/anie.201812806

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Accepted Article
Title: Reductive Molybdenum-Catalyzed Direct Amination of Boronic
Acids with Nitro Compounds
Authors: Samuel Suárez-Pantiga, Raquel Hernández-Ruiz, Cintia
Virumbrales, María R. Pedrosa, and Roberto Sanz
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201812806
Angew. Chem. 10.1002/ange.201812806
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10.1002/anie.201812806
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Reductive Molybdenum-Catalyzed Direct Amination of Boronic
Acids with Nitro Compounds
Samuel Suárez-Pantiga,* Raquel Hernández-Ruiz, Cintia Virumbrales, María R. Pedrosa, and Roberto
Sanz*
Dedicated to Prof. Dr. Francisco J. Fañanás on the occasion of his retirement.
Abstract: The synthesis of aromatic amines is of utmost importance
in a wide range of chemical contexts. We report a direct amination of
boronic acids with nitro compounds to yield (hetero)aryl amines. The
intermediate, which reacts with a proper alkylating or arylating
reagent. Therefore, the key success factor lies in reducing the
nitro group to the corresponding intermediate avoiding its
strongly favored over-reduction or its evolution through side-
reactions. In this field, Baran’s and Hu’s groups have described
the Fe-catalyzed synthesis of N-alkylanilines through an in situ
generated nitroso intermediate, which is subsequently trapped
by highly reactive alkyl radicals derived from alkenes[12] and alkyl
halides,[13] respectively (Scheme 1a). Conceptually different
electrophilic aminations have also been reported.[14] Within this
framework, Knochel developed the construction of diarylamines
by initial addition of arylmagnesium compounds to nitroarenes,
involving the intermediacy of arylnitroso and diarylhydroxylamine
derivatives (Scheme 1b).[15] More recently, Niggemann has
described an elegant reductive coupling of zinc organyls with
both aromatic and aliphatic nitro compounds. This is based in
the partial reduction of the nitro group with B2Pin2 that yields a
nitrenoid as the electrophilic amination reagent (Scheme 1c).[16]
Despite the potential of this transformation, it is currently
underexploited since a fine tuning of the nitrenoid could allow
the amination of less nucleophilic reagents such as boronic
acids.
novel combination of
a
dioxomolybdenum(VI) catalyst and
triphenylphosphine as inexpensive reductant has revealed to be
decisive to achieve this new CN coupling. Our methodology has
proven to be scalable, air and moisture tolerant, highly
chemoselective and engages both aliphatic and aromatic nitro
compounds. Moreover, this general and step-economical synthesis
of aromatic secondary amines showcases orthogonality to other
aromatic amine syntheses as it tolerates aryl halides and carbonyl
compounds.
The prevalence of aromatic amines in the chemical, materials,
agrochemical and pharmaceutical industries is a constant driver
for the innovation of synthetic methods.[1] The most general
transformations to prepare (hetero)aryl amines include direct
alkylation of amines with alkyl halides,[2] Buchwald-Hartwig,[3]
Chan-Lam-Evans[4] or Ullman-type[4a,5] CN cross-coupling
reactions and reductive amination.[6] These amination
methodologies employ amines as the nitrogen source,
particularly anilines, which are usually obtained by reduction of
nitroarene derivatives. As an alternative, other nitrogen-
containing compounds have also been used as starting
materials in CN bond forming aminations like nitrosoarenes,[7]
nitrenoids,[8] or hydroxylamines,[9] which are not commercially
available and, again, must be obtained in a previous synthetic
step by partial reduction of the nitro function. Therefore, the
development of a general and efficient methodology for the
synthesis of aromatic amines directly from abundant and readily
available nitro(hetero)arenes is very attractive and remains an
active research field. The use of nitro compounds as starting
materials not only improves step-economy, saving time and cost,
but may also allow tolerance of functional groups such as
unprotected amine.[10]
Although nitroaromatics are efficiently engaged as starting
materials in CN bond forming reductive cyclizations to
synthesize indoles or carbazoles,[11] the intermolecular amination
to afford anilines in one single operational step has proved to be
more challenging. The scarcely reported methodologies have in
common that the nitro group evolves to a partially reduced
[*]
Dr. S. Suárez-Pantiga, R. Hernández-Ruiz, C. Virumbrales, Dr. M.
R. Pedrosa, Prof. Dr. R. Sanz
Department of Chemistry
Universidad de Burgos
Pza Misael s/n, 09001 Burgos, (Spain)
E-mail: svsuarez@ubu.es; rsd@ubu.es
Scheme 1. Direct amination with nitroarenes: state-of-the-art.
Supporting information for this article is given via a link at the end of
the document.
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10.1002/anie.201812806
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Boronic acids are a relevant class of intermediates as they
exhibit wide functional group compatibility and synthetic
versatility and are readily available from commercial sources or
easily synthesized.[17] Based on these features, achieving a
direct amination of boronic acids with nitro compounds is a
major milestone, which repurposes these two accessible and
simple building blocks as coupling partners delivering an
excellent platform to easily construct diverse libraries of valuable
aromatic amines. Only very recently, during the preparation of
this manuscript, a protocol for this reaction by using PhSiH3 as
reductant and phosphetane P(III)/O=P(V) catalysis was reported
by Radosevich and co-workers.[18] Ideally, the design of a
moisture and air tolerant procedure and the utilization of an
affordable reducing agent would be crucial to provide a general,
simple and scalable methodology. Based on our experience in
molybdenum catalyzed reactions,[19] we hypothesized that the
Mo-catalyzed formation of a partially-reduced intermediate from
a nitro compound may allow the direct electrophilic amination of
boronic acids (Scheme 1d).
9
Me
Me
MoO2Cl2(dmf)2/bpy
PPh3
PPh3
>95
88
84:7
10
MoO2Cl2(bpy)2
82:<5
[a] Reaction conditions: nitroarene (0.2 mmol), PhB(OH)2 (0.3 mmol),
reductant (0.48 mmol), catalyst (5 mol%) in toluene (0.6 mL) at 100 ºC for 20
h, under air. [b] Determined by 1H NMR analysis with CH2Br2 as internal
standard.
With an optimized set of conditions in hand, the scope of
both the nitro compound and the boronic acid partners was
extensively studied. The ability of the reaction to tolerate
sensitive functional groups in the nitroaromatic component is
exceptional (Scheme 2). Halides are well tolerated in different
positions with excellent yields (24, 13, 14, 1719), allowing for
downstream CC, CO, and CN cross-coupling chemistry.
Alkyl (5, 20) and methoxy (6) electron-donating groups (EDG)
were also tested and remain unscathed. However, starting from
4-nitroanisole MW irradiation was required to achieve full
conversion. Particularly remarkable is that amination takes place
selectively over the nitro group when free amine is also present
without the need for protecting group chemistry (25). This
example clearly shows that the reaction does not proceed
through the amino group, which would allow unconventional
disconnections in retrosynthetic analysis. Notable performance
is accomplished with nitroarenes bearing strong electron-
withdrawing groups (EWG) such as cyano (7, 21), ester (10, 22)
or amide (11). Despite being a reductive process, ketones (8, 9,
23) and even aldehydes (15) are not reduced, as opposed to the
classic reductive amination that would again require protection
of these functional groups. Moreover, α,β-unsaturated carbonyls
and alkenes emerged unaltered affording the corresponding
diarylamines in good yields (12, 16). The process has also
revealed to be compatible with nitroarenes bearing several
functional groups in the same ring (2427) and with highly
sterically demanding ortho-disubstituted nitroarenes (30),
although with more moderate yields. At the same time,
polycyclic nitroarenes are well tolerated (28, 29). Remarkably,
nitroheteroarenes can also be used to deliver medicinally
relevant building blocks containing indole (31), pyridine (32) and
quinoline (33) scaffolds. Interestingly, selective mono-amination
could be achieved in molecules bearing more than one nitro
group (34).
Initially, model reaction between phenylboronic acid and 1-
chloro-4-nitrobenzene was chosen (Table 1) and a screen of
several reducing agents was performed.[20] Pinacol proved to be
unsuitable for this transformation as only direct reduction to the
corresponding aniline B was observed (entry 1).[19a] However,
PPh3 and P(OEt)3 were identified as viable reductants leading to
high yields of diarylamine A and only minor amounts of the
aniline side-product B (entries 2 and 3). However, when
nitrobenzene was tested as substrate the conversion decayed
considerably (entries 4 and 5). This decrease was even clearer
with 4-nitrotoluene as starting material (entry 6). After some
reoptimization,[20] best results were achieved by simple addition
of 2,2’-bipyridine (bpy) as ligand and using PPh3 as reductant
(entries 79). When pre-formed MoO2Cl2(bpy)2 was used
instead, similar results were obtained although conversion was
slightly lower (entry 10).
Table 1. Optimization of the Mo-catalyzed reductive coupling.[a]
Entry
X
Catalyst
Reductant
Conversion
[%][b]
Yield[b]
A:B [%]
1
2
3
4
5
6
7
8
Cl
Cl
Cl
H
MoO2Cl2(dmf)2
MoO2Cl2(dmf)2
MoO2Cl2(dmf)2
MoO2Cl2(dmf)2
MoO2Cl2(dmf)2
MoO2Cl2(dmf)2
MoO2Cl2(dmf)2/bpy
MoO2Cl2(dmf)2/bpy
Pinacol
PPh3
70
:54
78:12
72:16
55:8
94
P(OEt)3
PPh3
95
70
H
P(OEt)3
PPh3
62
40:15
35:15
>95:<5
88:6
Me
Cl
H
66
PPh3
>95
>95
PPh3
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10.1002/anie.201812806
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COMMUNICATION
NO2
NHPh
NHPh
MoO2Cl2(dmf)2/bpy (5 mol%)
+
PhB(OH)2
(1.5 equiv)
PPh3 (2.4 equiv), toluene
100 oC, 12 24 h
FG
FG
NHPh
NHPh
NHPh
X
R
O
R
X = H, 1 (83%)
X = Cl, 2 (92%)
X = Br, 3 (88%)
X = F, 4 (71%)
R = Me, 5 (80%)
R = OMe, 6 (84%)[a]
R = CN, 7 (78%)
EtO2C
R = Me, 8 (91%)
R = Ph, 9 (99%)
R = OMe, 10 (90%)
R = NH2, 11 (86%)
12 (64%)
NHPh
NHPh
NHPh
O
H
X
X = Cl, 13 (84%)
X = F, 14 (85%)
15 (97%)
16 (88%)
NHPh
NHPh
R
NHPh
NHPh
X
Cl
H2N
X = Cl, 17 (89%) R = Me, 20 (83%)
X = Br, 18 (92%) R = CN, 21 (90%)
Cl
Cl
Cl
25 (72%)
24 (86%)
X = I, 19 (84%)
R = CO2Me, 22 (34%)
R = Ac, 23 (56%)
Scheme 3. (Hetero)aryl boronic acid scope (isolated yields). Conditions:
nitrobenzene (0.5 mmol), in toluene (1.5 mL), under air. [a] Performed under
MW irradiation (135 ºC, 25 min).
NHPh
NHPh
Me HN Ph
Me
NHPh
F3C
X
X = Br, 26 (88%)
X = F, 27 (68%)
Afterwards, diarylamines, such as 4549, possessing both
functionalized coupling partners were also synthesized in high
yields (Scheme 4). The usefulness of this protocol in the
synthesis of bioactive compounds is shown with the efficient
preparation of flufenamic acid (50), an anthranylic acid
derivative.[22]
28 (82%)
29 (76%)
NHPh
30 (51%)
NHPh
NHPh
NHPh
Cl
N
NH
31 (63%)[a]
N
33 (87%)
O2N
32 (73%)
34 (73%)
Scheme 2. Nitroarene scope (isolated yields). Conditions: nitroarene (0.5
mmol), in toluene (1.5 mL), under air. [a] Performed under MW irradiation (135
ºC, 2540 min).
Next, a screen of diverse boronic acids was evaluated
(Scheme 3). The transformation took efficiently place with
excellent yields in presence of EWG in the aromatic ring such as
halides (13, 18, 35, 36) and ester (37). Moreover, o-substituted
boronic acids also proved to effectively promote the
transformation (20, 38). At the same time, electron-donating
alkoxy substituents were evaluated in different positions (6,
3942) providing satisfactory results without the need of MW
irradiation, thus ideally complementing the nitroarene scope.[21]
In addition, naphthyl (43), benzothienyl (44) and pyridinyl (32)
boronic acids revealed also suitable.
Scheme 4. Synthesis of highly functionalized diarylamines (isolated yields).
Conditions: nitroarene (0.5 mmol), in toluene (1.5 mL), under air.
Remarkably, alkyl boronic acids were also compatible with
this methodology as it was demonstrated with the synthesis of
alkyl aryl amines 5154. Both primary and secondary alkyl
boronic acids could undergo amination with selected
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functionalized nitroarenes, although lower yields were obtained
compared with the formation of Csp2N bonds (Scheme 5).
compound would afford II. Next, reoxidation of the Mo center by
the coordinated nitro compound would deliver intermediate III
bearing a nitroso ligand. At this point two different pathways
NO2
NHAlk
HO
B
HO
could be considered. Path
A would involve a second
MoO2Cl2(dmf)2/bpy (5 mol%)
+
Alk
deoxygenation of III with PPh3 giving rise, after reduction of the
nitroso ligand and reoxidation of the metal center, to a new
molybdooxaziridine (or 2-nitroso) species IV.[26] The attack of
PPh3 (2.4 equiv), toluene
100 oC, 17 20 h
FG
FG
(1.5 equiv)
Me
Me
Me
HN
Me
Cl
Me
Me
the
N atom to the boronic acid would afford nitrenoid
HN
intermediate V, whose subsequent rearrangement would
generate aminoboronic acid VI. Alternatively, path B would
involve the release of free nitroso compound VII,[7c] which after
reaction with a second equiv of PPh3 would generate reduced
adduct VIII. Its interaction with the boronic acid would deliver
nitrenoid borate intermediate IX, which would eventually release
O=PPh3 and the same aminoboronic acid VI through 1,2-
migration of a nucleophilic R2 group.[27] Final hydrolysis would
provide the isolated secondary aromatic amine.
HN
NC HN
Br
51 (85%)
52 (55%)
53 (62%)
54 (56%)
Scheme 5. Amination of alkyl boronic acids (isolated yields). Conditions:
nitroarene (0.51 mmol), in toluene (1.53 mL), under air.
Finally, amination with the more challenging nitroalkanes
was evaluated,[23] in which only Niggemann’s very recent
approach was able to succeed.[16b] Preliminary results under
conventional heating at 100 ºC showcased low conversion and
so, MW irradiation was tested. To our delight, high conversion to
the desired alkylated anilines was achieved. Several primary
and more sterically demanding secondary nitroalkanes were
converted in presence of different aromatic boronic acids leading
to alkyl aryl amines 53 and 5560 in moderate to good yields
(Scheme 6). Interestingly, when nitromethane is employed as
substrate this procedure offers a facile access to selectively
monomethylated
N-methylanilines
alternatively
to
the
problematic mono N-methylation of anilines.
Scheme 7. Mechanistic proposal.
In summary, we have described a practical, efficient,
scalable, air and moisture tolerant procedure for the general
synthesis of secondary aromatic amines from nitro compounds
in a single synthetic operation. As these results demonstrate, the
covered chemical space of our methodology is much wider and
diverse than that achieved with classical reactions and includes
the synthesis of many highly substituted and functionalized
amines. Not only nitroarenes but also nitroalkanes proved to be
suitable substrates for this transformation. The wide availability
of nitro compounds and boronic acids, in conjunction with the
use of affordable PPh3 as reductant and an easily available
Mo(VI) catalyst, make this simple strategy advantageous to
construct libraries of (hetero)aryl amines.
Scheme 6. Nitroalkane scope (isolated yields). Conditions: nitroalkane (1
mmol), in toluene (1.5 mL), under air.
The described procedure is amenable to scale up due to the
ready availability of both nitro compounds and boronic acids, as
well as, the use of inexpensive PPh3 as reductant in combination
with a dioxomolybdenum(VI) catalyst easily prepared from
affordable Na2MoO4.[24] Accordingly, several nitroarenes were
submitted to the reaction conditions at 7 mmol scale to provide
gram amounts of aromatic amines 10 (1.42 g, 92%), 15 (1.19 g,
86%), 46 (1.33 g, 88%), and 51 (10 mmol scale, 1.10 g, 74%).
Acknowledgements
We are grateful to Junta de Castilla y León (BU022G18), Junta
de Castilla y León and FEDER (BU076U16) and Ministerio de
Economía y Competitividad (MINECO) (CTQ2016-75023-C2-1-
A plausible mechanism is depicted in Scheme 7. It is well
known that dioxomolybdenum(VI) catalyst I is easily reduced by
PPh3 to a Mo(IV) species,[25] which upon coordination to the nitro
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generated, because a primary or secondary nitroso alkane would
rapidly suffer tautomerization to oxime making inaccessible the
subsequent CN amination. However, nitrosoarene compounds
efficiently afforded the corresponding diarylamines when subjected to
the standard reaction. So, regarding the reaction with nitroaromatics,
the participation of free nitrosoarenes is feasible. See Supporting
Information for further details.
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10.1002/anie.201812806
Angewandte Chemie International Edition
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Samuel Suárez-Pantiga,* Raquel
Hernández-Ruiz, Cintia Virumbrales,
María R. Pedrosa and Roberto Sanz*
Page No. – Page No.
Reductive Molybdenum-Catalyzed
Direct Amination of Boronic Acids
with Nitro Compounds
CN Coupling: In the present work a new electrophilic amination of boronic acids
with simple and inexpensive nitro compounds has been developed allowing the
preparation of a variety of highly substituted and functionalized aromatic secondary
amines. The reported process employs affordable PPh3 as reducing agent, in the
presence of an easily available and highly stable dioxomolybdenum(VI) complex as
catalyst.
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