Electrophilic Amination with Nitroarenes

Article abstract of DOI:10.1002/anie.201705356

An exceptionally general electrophilic amination, which directly transforms commercially available nitroarenes into alkylated aromatic aminoboranes with zinc organyl compounds was developed. The reaction starts with a two-step partial reduction of the nitro group to a nitrenoid, which is used in situ as the electrophilic amination reagent. To facilitate isolation, the resulting air- and moisture-sensitive aminoboranes were reacted with a range of electrophiles. The method not only represents a direct transformation of nitro compounds into electrophilic amination reagents but also provides an elegant alternative to dehydrocoupling methods for the generation of aminoboranes.

Full text of DOI:10.1002/anie.201705356

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Accepted Article
Title: Electrophilic Amination with Nitroarenes
Authors: Marian Rauser, Christoph Ascheberg, and Meike Niggemann
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Electrophilic Amination with Nitroarenes
Marian Rauser+, Christoph Ascheberg+, and Meike Niggemann*
Abstract: An exceptionally general electrophilic amination, which
directly transforms commercially available nitroarenes with zinc
organyls to alkylated aromatic aminoboranes was developed. The
reaction starts with a two-step partial reduction of the nitro group to a
nitrenoid, which is used in-situ as the electrophilic amination reagent.
To facilitate isolation, the resulting air and moisture sensitive
aminoboranes were reacted with a range of electrophiles. The
partially reduced nitro group is directly engaged for further
functionalization. Despite the obvious advantages with regard to
step economy, cost efficiency and orthogonality, the direct
transformation of nitro compounds without the detour to the fully
reduced aniline remains largely underdeveloped and only a few
methods were recently developed (see Scheme 1, b).[10,11] This
is attributed to the fact, that a partial reduction of the nitro group
is hampered by both, imminent overreduction and the erratic
behavior of the typically encountered intermediate, a nitroso
compound, which readily dimerizes and is easily entangled in
side reactions.[12] The recent reactions therefore rely on radicals
that react instantaneously with an in-situ generated nitroso
compound. Side reactions and overreduction are forestalled, as
the residual concentration of the nitroso species is kept minimal.
To further broaden the utility of partial reductions of nitroarenes,
we sought for an intermediate less erratic than a nitroso
compound, yet still providing ample reactivity for further
functionalization.
We originally thought of a metal organic compound for the first
reduction step.[13,14] O-alkylation of nitro compounds yields a
nitronate, followed by alkoxy cleavage and formation of an
erratic nitroso compound. Conversely, a direct reaction of the
nitronate A with B2pin2 as a mild and selective reductant[15]
yields nitrenoid B.[16] B is an ideal reagent for an electrophilic
amination,[17] proceeding via addition - 1,2-migration.[18] To our
surprise, organo zincs, usually well tolerated in reactions of
compounds containing nitro groups,[19] proved effective also for
the first reduction step, when combined with B2pin2. To facilitate
isolation of the air and moisture sensitive aminoboranes D,[20] we
concluded the reaction either by a simple hydrolytic work-up, or
with the addition of various electrophiles.
protocol not only represents
a direct transformation of nitro
compounds into an electrophilic amination reagent but also provides
an elegant alternative to dehydrocoupling methods for the
generation of aminoboranes.
The vast majority of compounds important to chemical industries
contain at least one nitrogen atom. Therefore, methods for the
functionalization of N-H bonds are well established.[1,2,3] Likewise,
reductions of nitroaromatics to anilines date back to the 19th
century.[4] Stoichiometric reduction processes[5] have since been
complemented with catalytic procedures[6,7] including metal free
approaches.[8] Hence, the typical synthesis of N-alkylated
anilines starts with the hydrogenation of the commercially
available nitro compound, followed by the introduction of carbon
substituents in a second synthetic operation (see Scheme 1, a).
Operational simplicity was improved in some cases, with one-pot
procedures based on borrowing hydrogen catalysis.[9]
Table 1. Optimization.
Entry
Additive
Equiv.
Solvent
Conv. [%]
Yield [%]
1[a]
2
3
-
-
THF
THF
THF
THF
THF
THF
THF
THF
THF
DMF
DCE
toluene
toluene
toluene
toluene
60
100
59
56
83
55
54
100
100
85
33
69
35
40
47
40
42
52
81
74
62
95
94
31
40
LiOMe
NaOMe
KOtBu
PPh3
DMAP
pyridine
DIPEA
Et3N
Et3N
Et3N
Et3N
Et3N
1.0
1.0
1.0
3.0
3.0
3.0
3.0
3.0
3.0
3.0
3.0
2.0
2.0
2.0
4
5[b]
6
7[b]
8
9
10
11
12
13
14[c]
15[d]
Scheme 1: Alkylation of Ar-NO2
80
100
100
41
Et3N
Et3N
Nevertheless, for an ideal, more redox economic reaction, the
53
[a] conditions: 1a (0.2 mmol), 2.0 equiv B2pin2 (0.4 mmol), 1.2 equiv
ZnEt2 (0.24 mmol), rt, 12 h. [b] heated to 60 °C, [c] 1.0 equiv of B2pin2
used, [d] 0.5 equiv of ZnEt2 used.
[*]
Marian Rauser+, Christoph Ascheberg+, Prof. Dr. Meike
Niggemann
Institute of Organic Chemistry, RWTH Aachen
Landoltweg 1, 52072 Aachen
E-mail: niggemann@oc.rwth-aachen.de
These authors contributed equally to this work.
Apart from being a direct transformation of nitro compounds into
an electrophilic amination reagent our approach is beneficial in
[+]
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10.1002/anie.201705356
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several ways. The actual product of the reaction is the
aminoborane D, the formation of which remains largely restricted
to dehydrocoupling methods,[21] which are particularly cumber-
some for aromatic and secondary amines[22] (selective formation
of the aminoborane prior to work up was proven by 1H-NMR-
analysis, see SI). The switch to an organometallic alkylating
reagent effectively prevents the over-alkylation typical for N-
functionalization with electrophiles. Finally, nucleophilic groups
such as NH2 or OH are tolerated without protection.
Table 3. Scope - Zinc Organyls.
Table 2. Scope - Nitroarenes.
[a] conditions: 1a (0.2 mmol), 2 equiv B2pin2 (0.4 mmol), 1.5 equiv ZnR2 (0.3
mmol), rt, 12 h, then E+ (1.0 mmol, 5.0 equiv.), 4 h. [b] 3.0 equiv of XZnR.
Directly converting the B-N bond with alternative electrophiles,
an acetylating work-up was attempted. Thereby, two different
substituents are incorporated directly at the nitrogen atom of the
former nitro group. Aminoborane D reacted readily to the
acetamide 3a (see Table 2). Following this procedure
nitroarenes 1a-u with EWG or EDG substituents in the o-, m-
and p-position react in good to excellent yields to acetamides 3
(EDG: 3b-g, 3q/r; EWG: 3h-m, 3s-u). Interestingly, even
halogen substituted substrates (3h-m) selectively gave the
desired products. Only with o-iodo-nitrobenzene 1m (3m), 10%
dehalogenation occurred, thus clearly demonstrating that the
reaction proceeds mainly via
a
non-radical pathway.[24]
Potentially sensitive functional groups, such as OTBDMS (3n),
alkynes (3q), alkenes (3r) and even keto (3t) and ester groups
(3s) were well tolerated. Also, the presence of a carboxylic acid
(3u) did not interfere, after a pretreatment with NaH. Finally, as
expected, the protection of OH and NH2 functionalities proved
unnecessary and solely the nitro group was alkylated in both
cases (3o and 3p).
[a] conditions: 1 (0.2 mmol), 2.0 equiv B2pin2 (0.4 mmol), 1.2 equiv ZnEt2
(0.24 mmol), rt, 12 h, then AcCl (1.0 mmol, 5.0 equiv.), 4 h, [b] 1.5 equiv of
NaH at rt, addition of ZnEt2 after 30 min.
Next, the transfer of different alkyl groups was investigated, with
After some variation of the stoichiometries (not shown), we
settled on 1.2 equiv ZnEt2, and 2 equiv of B2pin2 in THF as the
starting point for optimization (table 1, entry 1). These conditions
allowed for full conversion of nitrobenzene 1a and isolation of
aniline 2a in 33 % yield, after hydrolysis. Major side products
were derived from nitrosobenzene, thus, the influence of
different additives was tested.[23] Alkoxides gave mixed results.
While LiOMe had a positive impact on the reaction (entry 3),
NaOMe and KOtBu led to a drop in conversion (entries 4+5).
Among Lewis bases DIPEA and NEt3 gave full conversion
(entries 8+9), the latter significantly improved selectivity and
yield. The reaction was best carried out in toluene (entry 12),
and the amount of NEt3 reduced (entry 13). Lower amounts of
B2pin2 or ZnEt2 (entries 14+15) resulted in lower yields.
freshly prepared zinc organyls. ZnMe2 (3v), Zn(nBu)2 (3w)[25] as
[26]
well as Zn(allyl)2
(2d) and even the bulky secondary zinc
organyl[25, 27] efficiently yielded 2c. No limitation was encountered
upon switching to aryl based organometallics (2e).[25] Also,
organo zinc halides,[24,28] generally considered less reactive,
readily gave the desired products 2b, 3x, 3y and 2f. For aniline
derivatives with branched alkyl moieties and allyl groups the
acetylating workup proved inefficient.
Finally, different electrophiles were reacted with aminoborane D.
Ac2O (3a), Bz2O, BzCl (4f) and Boc2O (4g) proved suitable. Also,
a tosyl group was easily introduced (4e).Tertiary amines were
accessed via the addition of (pseudo)halides such as allyl
bromide (4a), propargyl bromide (4b), benzyl bromide (4d) or
butyl triflate (4c, see SI for a Table).
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10.1002/anie.201705356
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To gain further insight into the mechanism we ran a number of
control experiments (see SI for a detailed discussion). No
conversion of the nitroarene occurred with B2pin2 alone or
B2pin2/NEt3. Likewise, as expected, it did not react in the
absence of B2pin2. Single electron transfer does not seem to
play a significant role, as even iodo-nitrobenzenes 1k+m retain
their halogen substituents.[24] Furthermore, time resolved 11B-
NMR analysis of the reaction clearly shows that a borate species
containing an sp3-hybridized B-atom is formed under the
Scheme 2: Proposed Pathway for the Partial Reduction 2
We also evaluated the potential intermediacy of a nitrosoarene,
as described for radical mediated reactions,[10,11] following a
mechanism explored by Knochel.[33] It was found very unlikely
due to nitrosobenzene’s poor performance under the reaction
conditions (see Scheme 3) and the absence of hydroxylamines
in any of our control experiments. A similar result was obtained
in the absence of B2Pin2. Hence, nitronate A does not fragment
to a nitrosoarene,[11b,11d] but is transformed directly to the
nitrenoid B upon reaction with B2pin2 (see Scheme 2). The C-N
bond forming reaction step towards aminoborane D, occurring
via addition of ZnR2 to nitrenoid B, followed by 1,2-migration, is
well documented.[18,34] Upon work-up the B-N bond in
aminoborane D is cleaved with the provided electrophile.[19] The
role of the NEt3 additive remains currently unknown.[35]
reaction conditions (δ
=
5.0 ppm, see Figure 1).[29] Its
concentration first rises and is highest about 40 min into the
reaction from which point onwards it gradually decreases,
thereby assigning it a role in the early stages of the reaction.
Furthermore, this coincides with the cessation of the formation of
EtBpin, which also occurs at around 40 min into the reaction. As
EtBpin is formed upon the transfer of the nucleophilic Bpin
moiety from [B2pin2Et]ˉ, which, in turn, is formed via the addition
of one ethyl group from Et2Zn to B2pin2,[30] we presume that the
gain in nucleophilicity stemming from the sp3-hybridization of
one of the B-atoms in the borate[23] allows the first reduction step
of the nitroarene to the nitronate A to occur, which neither the
zinc organyl or the B2pin2 were capable of alone. The
nucleophilicity might even be further reinforced via the formation
of a zincate intermediate (see SI for details).[30a,31]
Scheme 3: Control Experiment - Reaction of Ph-NO
In summary we describe an electrophilic amination of zinc
organyls with nitroarenes. Directly transforming commercially
available nitro compounds, the reaction provides a broad variety
of aminoboranes, which are very difficult to access via
conventional methods. Preliminary mechanistic investigations
indicate for a two-step partial reduction of the nitro group to a
nitrenoid, which is used in-situ for the electrophilic amination of
zinc organyls, and prove the selective formation of
aminoboranes prior to work-up.
Figure 1: Superimposed spectra - 11B{1H}-NMR (128 MHz, C6D6) - of the
model reaction of PhNO2, 1.2 eq. ZnEt2, 2.0 eq. B2Pin2 and 2.0 eq. NEt3
recorded at intervals of 20 min, start: 20 min reaction time, end: 3h.
Acknowledgements
Furthermore, the change of the ratio between the B-O signals at
δ = 22.3/21.8 ppm,[32] which continues until the reaction is
complete after 3 h, might reflect the transformation of nitronate A
to the aminoborane D as shown in Scheme 2 for the following
reasons: a) Significant amounts of B2pin2 are consumed during
this period of the reaction, albeit the formation of [B2pin2Et]ˉ and
EtBpin have ceased. b) Product formation continues. c) One of
the B-O signals, tentatively assigned to pinBOˉ is gradually
increasing in intensity.
We are grateful to Dr. Christoph Räuber for his assistance with
the NMR-experiments.
Keywords: aminoboranes • nitro functionalization • electrophilic
amination • nitrenoid • nucleophilic boron
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10.1002/anie.201705356
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Marian Rauser, Christoph Ascheberg,
Meike Niggemann*
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Electrophilic Amination with
Nitroarenes
Nitro-Power not wasted! An exceptionally general electrophilic amination of zinc
organyls was realized with commercially available nitroarenes, yielding alkylated
aromatic aminoboranes. Therefore, the partially reduced nitro group is directly
engaged as an electrophilic nitrogen intermediate. To facilitate isolation,
aminoboranes were reacted with electrophiles, thus incorporating two different
substituents at the N-atom of the former nitro group in a one pot procedure.
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