One-pot procedures for the formation of secondary aryl amines from nitro aryls

Article abstract of DOI:10.1055/s-0033-1339860

Strategies for the one-pot formation of secondary aryl amines from the corresponding nitro aryls by utilizing reductive amination procedures are discussed. The extension of this chemistry where a Suzuki-Miyaura cross-coupling is conducted between a boronic acid and bromonitrobenzene prior to the reductive amination in one-pot is also presented. Georg Thieme Verlag Stuttgart, New York.

Full text of DOI:10.1055/s-0033-1339860

2340▌
SYNPACTS
sOynnpaects-Pot Procedures for the Formation of Secondary Aryl Amines from Nitro
Aryls
Formation of Secondary Aryl Amines
Sindre Å. Lunde, Magne O. Sydnes*
Department of Mathematics and Natural Science, Faculty of Science and Technology, University of Stavanger, 4036 Stavanger, Norway
Fax +4751831750; E-mail: magne.o.sydnes@uis.no
Received: 24.07.2013; Accepted after revision: 03.09.2013
Abstract: Strategies for the one-pot formation of secondary aryl
amines from the corresponding nitro aryls by utilizing reductive
amination procedures are discussed. The extension of this chemistry
where a Suzuki–Miyaura cross-coupling is conducted between a
boronic acid and bromonitrobenzene prior to the reductive amina-
tion in one-pot is also presented.
Key words: one pot, Suzuki–Miyaura cross-coupling, reductive
monoalkylation, biaryl, secondary amine
Selective synthesis of secondary amines in high yield can
Sindre Åse Lunde (left) was born in 1987. He received his bachelor
degree in chemistry and environmental engineering at the University of
Stavanger in 2013. He is currently conducting his master’s studies in
be a challenge with various degrees of overalkylation to
the tertiary amine taking place.1 This is often a problem
environmental technology at the same institution. His subjects of inter-
est are organic chemistry, separation technology, and mathematics.
Magne O. Sydnes (right) was born in Oslo, Norway in 1973. He re-
ceived his MSc degree in synthetic organic chemistry from the Univer-
sity of Oslo in 1998. After a short stint in industry he commenced his
PhD studies in 2001 at the Australian National University, Canberra,
under the guidance of Professor Banwell. Since earning his PhD in
2004 he has been working as a postdoctoral fellow both in Australia and
Japan, including two years as a JSPS postdoctoral fellow in Professor
Isobe’s group at Nagoya University, Japan. In 2009 he joined Interna-
tional Research Institute of Stavanger, Norway, as a researcher. Since
December 2011 he has been an associate professor at the University of
Stavanger, Norway. His research interests include catalysis, natural
product synthesis, and chemical biology.
when alkyl halides are utilized as the alkylation agent, and
to a lesser extent when aldehydes are used as the alkyl
source where first the corresponding imine is formed fol-
lowed by a reduction of the imine, thus forming the sec-
ondary amine. A range of secondary aromatic amines are
present in biological active compounds,1 pharmaceuti-
cals,1,2 intermediates for drug synthesis,1 and dyes3 (Fig-
ure 1). Due to that fact, there is an interest in generating
new efficient strategies for the selective preparation of
these types of amines.
O
NH2
O
Nitro aryls, which can easily be prepared from the corre-
sponding aryls,5 are excellent starting materials for the
formation of the corresponding secondary aryl amines.
Traditionally, nitro aryls have been transformed into the
corresponding secondary amine in a two-step process.
First reduction of the nitro group to the primary amine fol-
lowed by alkylation.1 Lately a range of strategies have
been reported where nitro aryls have been converted into
the corresponding secondary amines by using one-pot re-
ductive monoalkylation strategies (Scheme 1). These re-
actions have utilized alcohols,6,7 aldehydes,8–15
ketones,16,17 and nitriles18–21 as the alkyl source together
with a range of catalysts. With a few exceptions the cata-
lysts used in these reactions were palladium on solid sup-
ports and hydrogen gas as the hydrogen source.
N
H
1
N
ONa
H2N
N
NH
O
Cl
H
N
Cl
2
3
Figure 1 Example of a biological active compound {4-[(4′-amino-
phenyl)amino]-2H-benzo[h]chromen-2-one (1)},4 pharmaceutical
[Voltaren (2)],2 and dye [Mauveine A (3)]3 containing a secondary
aryl amine moiety in its structure
Nitro aryls have also been converted into their corre-
sponding carbamates (Boc and CO2Et) in one pot utilizing
Boc2O or ClCO2Et, respectively, Sn and NH4Cl in metha-
nol under ultrasound conditions.22 The method represents
an excellent strategy for the formation of protected prima-
ry amines.
SYNLETT 2013, 24, 2340–2344
Advanced online publication: 30.09.2013
DOI: 10.1055/s-0033-1339860; Art ID: ST-2013-P0697-SP
© Georg Thieme Verlag Stuttgart · New York
0
9
3
6
-
5
2
1
4
1
4
3
7
-
2
0
9
6

SYNPACTS
Formation of Secondary Aryl Amines
2341
Only when one equivalent of formaldehyde (37 wt% in
water) was used as the alkylation agent during the reduc-
tive monoalkylation of ethyl (4-methoxy-3-nitrophe-
nyl)acetate was some (16%) of the corresponding tertiary
amine formed together with 61% of the desired product.
In addition 17% of the primary amine were also isolated.
However, by altering the reaction conditions slightly,
namely conducting the imine formation under an atmo-
sphere of air followed by reduction with hydrogen gas
over Pd/C (10%) the secondary amine 9 could be isolated
in 99% yield.23
alkylation agent
alcohol6,7
aldehyde8–15
ketone16,17
nitrile18–21
catalyst
R1
R1
R2
N
NO2
H
Scheme 1 Alkylation agents used for reductive monalkylation of ni-
tro aryls
Our entry into this type of chemistry was by coincidence
due to a byproduct (secondary amine 4) formed during re-
duction of the corresponding nitro aryl [ethyl (4-methoxy-
3-nitrophenyl)acetate] to the corresponding primary
amine under an atmosphere of hydrogen gas over Pd/C
(10%) at room temperature in ethanol.23 The primary
amine was needed for the formation of the corresponding
aryl azide which was used for photochemistry studies.24
The formation of secondary ethyl amine 4 was possible
due to small quantities of acetaldehyde formed via oxida-
tive addition of palladium to the alcohol followed by β-
hydride elimination.25,26 The equilibrium for this forma-
tion was far towards ethanol, however, due to the presence
of the primary amine functioning as a trapping agent for
the aldehyde a relatively large yield (41%) of the second-
ary amine could be isolated after 48 hours.23 No trace of
the corresponding diethyl tertiary amine could be found in
the product mixture.
The synthetic strategy utilized for the latter synthesis of
secondary amine 9 could also be used in order to prepare
some benzyl-protected amines.26 This was possible de-
spite the fact that deprotection of benzyl-protected amine
is conducted by hydrogenation over palladium on solid
support, most often palladium on activated charcoal.27
However, the final hydrogenation, namely reduction of
the imine, had to be fine-tuned for each substrate. It was
also absolutely necessary to keep the exposure time for
the substrate to hydrogen to a minimum in order to avoid
deprotection. By utilizing this strategy it was only possi-
ble to obtain a few benzyl-protected amines in good yield
(Figure 3). The outcome could be improved by utilizing
the strategy reported by Sajiki and Hirota where a nitro-
gen-containing base was added to the reaction mixture as
a catalyst poison in order to improve the chemoselectivity
for the hydrogenolysis reaction.28 In our case triethyl-
amine was added prior to the final reduction of the imine
with hydrogen gas over Pd/C (10%).26 This strategy facil-
itates the direct formation of some benzyl-protected pri-
mary amines and represents an addition to Chandrasekhar
and co-workers’ direct formation of Boc-protected
amines.22 Utilizing this strategy it was also possible to
generate secondary amines with substitution on the benzyl
group albeit in mixed yields.26
We thought we could take advantage of the fact that the
tertiary amine was not formed under these conditions and
see if we could develop a useful reaction by adding the al-
kylation agent, viz. the aldehyde to the reaction mixture.
And indeed, when ethyl (4-methoxy-3-nitrophenyl)ace-
tate was stirred under an atmosphere of hydrogen gas over
Pd/C (10%) in the presence of 1.1 equivalents of acetalde-
hyde at room temperature the corresponding secondary
ethyl amine was formed in 98% isolated yield in the
course of three hours.23 A range of secondary amines
could be formed by utilizing this procedure (Figure 2).
OMe
OMe
EtO2C
NHBn
NHBn
OMe
OMe
10 (1 h, 25 h, 0.25 h, 81%)
11 (2 h, 18 h, 0.25 h, 86%)
EtO2C
EtO2C
NHEt
NHR
OMe
4 R = Et (3 h, 98%)
6 (6 h, 99%)
5 R = n-Pr (4 h, 95%)
NHBn
HO2C
NHBn
OMe
F
12 (2 h, 48 h, 0.75 h, 99%)
13 (2 h, 4.5 h, 0.25 h, 58%))
Figure 3 Examples of some benzyl-protected amines generated by
reductive monoalkylation of the corresponding nitro compounds (re-
action time and isolated yields in parentheses). Reagents and condi-
tions: nitro aryl in EtOH was stirred under H2 (1 atm) over Pd/C
(10%) at r.t. for 1–2 h; benzaldehyde (1.4 equiv) was added, and the
reaction mixture was stirred at r.t. under an atmosphere of air for 4.5–
48 h; H2 (1 atm) at r.t. for 0.25–0.75 h.
HO2C
NHEt
EtO2C
NHEt
7 (16 h, 90%)
8 (7 h, 84%)
OMe
EtO2C
NHMe
9 (4 h, 99%)a
Figure 2 Examples of some secondary aryl amines prepared by one-
pot reductive monoalkylation of the corresponding nitro aryl (reaction
time and isolated yields in parentheses). a Formaldehyde solution (37
wt% in H2O) was used. Reaction conditions for the formation of com-
pounds 4–8: nitro aryl and aldehyde (1.1–1.7 equiv) in EtOH was
stirred under H2 (1 atm) over Pd/C (10%) at r.t. for 3–16 h.
Inspired by the early work of Sajiki and co-workers using
ligand-free Pd/C in Suzuki–Miyaura cross-coupling
reactions29,30 we were wondering if it was possible to con-
duct a cross-coupling with a boronic acid on to a haloni-
© Georg Thieme Verlag Stuttgart · New York
Synlett 2013, 24, 2340–2344

2342
S. Å. Lunde, M. O. Sydnes
SYNPACTS
trobenzene prior to the previously developed reductive
monoalkylation in one pot (Scheme 2). And indeed, upon
optimization of the reaction conditions it was possible to
obtain secondary biaryl amine 16a in 89% isolated yield
after column chromatography (Figure 4).31 The optimized
conditions utilized ethanol–water (12:1) as the solvent
system and the Suzuki–Miyaura cross-coupling was con-
ducted at reflux followed by cooling the reaction mixture
to room temperature. Addition of 10 equivalents of alde-
hyde and stirring the reaction mixture at room temperature
under an atmosphere of hydrogen gas resulted in smooth
reductive monoalkylation of the nitrobiaryl in predomi-
nantly good yields.
C10H23
N
H
N
H
16a (3 h, 46 h, 89%)
17 (3 h, 23 h, 80%)
MeO
MeO
C6H13
N
H
N
H
18 (3 h, 24 h, 85%)
19 (3 h, 16 h, 88%)
one-pot
1) Suzuki–Miyaura
cross-coupling
Pd/C (10%)
2) reductive monoalkylation
H2 (1 atm), Pd/C (10%)
aldehyde
Me
N
H
N
H
B(OH)2
14
20 (4 h, 24 h, 99%)a
21 (4 h, 24 h, 98%)
+
Figure 4 Examples of some secondary biaryl amines formed in one-
pot utilizing 1-bromo-4-nitrobenzene [15a; reaction time (Suzuki–
Miyaura cross-coupling, reductive monoalkylation) and isolated
NH
Br
NO2
16
a
yields in brackets]. Formaldehyde solution (37 wt% in H2O) was
15
used. Reagents and conditions: boronic acid/1-bromo-4-nitrobenzene
(1:1), Pd/C (10%), and Na2CO3·10H2O (1.6 equiv) at reflux for 3–4 h;
aldehyde (10 equiv), H2 (1 atm) at r.t. for 16–46 h.
Scheme 2 One-pot reaction sequence with 1-bromo-4-nitrobenzene
(15a) and 1-bromo-3-nitrobenzene (15b)
As an initial experiment in the optimization process the
Suzuki–Miyaura cross-coupling was conducted at reflux
as in the previous work.31 In addition, the reductive mono-
alkylation sequence was also conducted at reflux utilizing
1.5 equivalents of cyclohexanecarboxaldehyde as alkyla-
tion agent (Table 2, entry 1). After 22 hours at reflux un-
der an atmosphere of hydrogen an 85% yield of azo
compound 22 (Figure 5) could be isolated by flash chro-
The conditions also facilitated the formation of the sec-
ondary amine 16b when 1-bromo-3-nitrobenzene (15b)
was used as the cross-coupling partner (Table 1, entry 2).
However, when 1-bromo-2-nitrobenzene (15c) was uti-
lized in the reaction the desired cross-coupling reaction
did not take place in the course of 24 hours at reflux (Ta-
ble 1, entry 3).
Although the conditions recently reported gave predomi- matography in addition to small amounts (14%) of the
nantly good yields for saturated aldehydes (Figure 431 and corresponding hydrazine 23. No trace of the desired sec-
Table 1, entry 2). The fact that 10 equivalents of the alde- ondary amine 16a could be found. The formation of com-
hyde were required in order for the reaction to reach com- pounds 22 and 23 was puzzling; however, its origin had to
pletion within one to two days at room temperature was come from incomplete reduction of the nitrobiaryl. It is
limiting the practicality of the chemistry (Table 1, entries known that both azo compounds and hydrazines are inter-
1 and 2). In an effort to improve the reaction on this point mediates during the reduction of nitrobenzenes with hy-
we set out to further optimize the reaction conditions. The drogen over Pd/C.32 Occasionally, it is possible to isolate
optimization work was carried out utilizing phenylboron- a few percent of these compounds, but seldom in large
ic acid (14) and 1-bromo-4-nitrobenzene (15a) as cross- yields. Since this product had not been observed in our
coupling partner and cyclohexanecarboxaldehyde as the previous work31 it was thought that the fact that the reduc-
alkylation agent.
tion of the nitro group was conducted at reflux resulted in
Table 1 Expanding the Scope with Respect to the Bromonitrobenzene (15)a
Entry
Compd
Cyclohexanecarboxaldehyde Time (h)b
Product
Yield (%)
1
15a
15b
15c
10 equiv
10 equiv
–
3, 46
5, 43
16a
16b
16c
89
83
–
2
3c
24, –
a Solvent (EtOH–H2O = 12:1), Suzuki–Miyaura cross-coupling conducted at reflux, and the reductive monoalkylation sequence was conducted
at r.t.
b The times given refer to the following reactions: Suzuki–Miyaura cross-coupling, reductive monoalkylation.
c The Suzuki–Miyaura cross-coupling did not proceed in the course of 24 h at reflux.
Synlett 2013, 24, 2340–2344
© Georg Thieme Verlag Stuttgart · New York

SYNPACTS
Formation of Secondary Aryl Amines
2343
incomplete reduction of the nitro functionality. However, ucts 26 and 27) in addition to starting material 24 (Scheme
how temperature influences the reduction reaction is at 3). Switching solvent to solvents with higher boiling point
this stage not known.
(toluene and xylene) did not improve the outcome of the
cross-coupling reaction. Further one-pot work with 1-
chloro-4-nitrobenzene was abandoned since the cross-
coupling reaction with this analogue did not perform to
expectation under our reaction conditions.
Cl
1a
+
N
NO2
N
N
N
H
H
24
22
23
Pd/C (10%),
Na2CO3⋅10H2O
reflux, 92 h
Figure 5 Structures of azo compound 22 and hydrazine 23
O2N
A revised procedure was then attempted where the reduc-
tion of the nitro group was conducted at room temperature
for three hours without the presence of aldehyde. Under
these conditions the nitro group was fully converted into
the primary amine, and the aldehyde (cyclohexanecarbox-
aldehyde) was added to the reaction mixture. The reaction
mixture was then stirred at reflux under an atmosphere of
hydrogen in order to facilitate the alkylation. This im-
proved the outcome, and the desired secondary amine 16a
was formed as the only product in 93% yield when 3.0
equivalents of aldehyde were used (Table 2, entry 2). At-
tempts to further reduce the number of equivalents of al-
dehyde added (Table 2, entries 3 and 4) and keeping the
same reaction time or shorter only resulted in lower isolat-
ed yields of the desired product.
NO2
NO2
25 16%
26 20%
27 3%
Scheme 3 Cross-coupling of 1-chloro-4-nitrobenzene with phenyl-
boronic acid
In conclusion, we have broadened the scope of the one-pot
reductive monoalkylation reaction by introducing a Suzu-
ki–Miyaura cross-coupling reaction prior to the alkylation
sequence. By such means a range of secondary biaryl
amines can be generated in one pot from the correspond-
ing bromonitroaryl. The cross-coupling chemistry should
also work well with other boronic acids then the once uti-
lized thus far by us. This chemistry should therefore facil-
itate the formation of a broad range of substituted
secondary aryl amines.
Table 2 Optimization of the Reaction Conditions Utilizing Phenyl-
boronic Acid and 1-Bromo-4-nitrobenzene in the Cross-Coupling Re-
action and Cyclohexanecarboxaldehyde as Alkylation Agent
Entry
Aldehyde (equiv) Time (h)
Yield of 16a (%)
Acknowledgment
b
1
2
3
4
1.5
3.0
1.2
1.5
3, 22a
–
The initial chemistry was discovered by serendipity during
M.O.S.’s postdoctoral work in Professor Minoru Isobe’s group at
the University of Nagoya. M.O.S. is grateful for the opportunity
Isobe gave him to study under his excellent supervision. He would
also like to thank Inoue Foundation for Science (IFS) (July 2006 to
June 2007) and Japan Society for the Promotion of Science (JSPS,
July 2007 to June 2009) for their generous support. Financial sup-
port from the University of Stavanger and the research program
Green Production Chemistry is acknowledged for providing fun-
ding for the work conducted at UiS. Last, but not least the resear-
chers whose names are found in the references are thanked for their
contributions to the development of one-pot chemistry.
3, 3, 7.5c
3, 3, 7.5c
3, 3, 3
93
65d
60d
a Suzuki–Miyaura cross-coupling 3 h at reflux, reductive monoalkyl-
ation at reflux for 22 h.
b Azo compound 22 was isolated in 85%, and hydrazine 23 was iso-
lated in 14% yield.
c Suzuki–Miyaura cross-coupling 3 h at reflux, reduction of the nitro
group to the amine 3 h at r.t., alkylation and reduction conducted at
reflux (see Table 2 for reaction time).
d Primary amine and imine were observed by TLC when the reaction
was worked up.
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S. Å. Lunde, M. O. Sydnes
SYNPACTS
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Synlett 2013, 24, 2340–2344
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