B(C6F5)3-Catalyzed Reduction of Aromatic and Aliphatic Nitro Groups with Hydrosilanes

Article abstract of DOI:10.1002/ejoc.201600556

A transition-metal-free method for the hydrosilane reduction of aromatic and aliphatic nitro compounds to the corresponding amines is reported. Et₃SiH is found to be the optimal reductant in this B(C₆F₅)₃-catalyzed deoxygenation. The functional-group tolerance is, however, moderate.

Full text of DOI:10.1002/ejoc.201600556

DOI: 10.1002/ejoc.201600556
Communication
Deoxygenation
B(C F ) -Catalyzed Reduction of Aromatic and Aliphatic Nitro
6
5 3
Groups with Hydrosilanes
Digvijay Porwal and Martin Oestreich*
[a]
[a]
Dedicated to Professor Dieter Hoppe on the occasion of his 75th birthday
Abstract: A transition-metal-free method for the hydrosilane optimal reductant in this B(C F ) -catalyzed deoxygenation. The
6
5 3
reduction of aromatic and aliphatic nitro compounds to the functional-group tolerance is, however, moderate.
corresponding amines is reported. Et SiH is found to be the
3
Introduction
sion, respectively, were obtained with tetramethyldisiloxane
TMDS; Entries 5 and 6). Conversely, polymethylhydrosiloxane
(PMHS) was moderately effective in the reduction of the nitro
(
1
The ability of B(C F ) to activate hydrosilanes through η coor-
6
5 3
[
1]
dination led to the development of numerous reductive proc-
esses, including the deoxygenation of alcohols and ethers as
group (Entries 7 and 8); the use of PMHS had been a complete
[
2]
[4]
failure with phosphine oxides. Et SiH turned out to be the
3
[
3]
well as carbonyl and carboxyl compounds. Our recent finding
that B(C F ) also catalyzes the reduction of phosphine oxides
optimal reductant for this transformation, reaching 73 % con-
version in toluene and full conversion without any solvent (En-
tries 9 and 10). Less catalyst (5.0 vs. 10 mol-%) or hydrosilane
6
5 3
[
4]
using PhSiH as the stoichiometric reductant prompted us to
3
investigate the related transformation of nitro compounds into
the corresponding amines. However, hydrosilane reduction of
aromatic and aliphatic nitro compounds had remained idle for
(4.0 vs. 10 equiv.) as well as lower reaction temperatures (80 vs.
1
1
00 °C) led to poorer conversion without exception (Entries 11–
3). At 80 °C, traces of azobenzene were detected, indicating
[
5]
decades until the recent disclosure of several transition-metal-
[
6]
catalyzed protocols. An InI -catalyzed procedure for the prep-
aration of anilines by Sakai and co-workers is noteworthy, and
a transition-metal-free method by Benaglia and co-workers us-
3
6 5 3
Table 1. Optimization of the B(C F ) -catalyzed hydrosilane reduction of
[
7]
[a]
nitrobenzene.
[
8]
ing Cl SiH/Hünig's base stands out. Both display excellent
3
functional-group tolerance. We report here the B(C F ) -cata-
6
5 3
lyzed hydrosilane reduction of simple aromatic and aliphatic
nitro groups.
Results and Discussion
Entry
Hydrosilane (equiv.)
PhSiH3 (4.0)
Solvent
T [°C]
Conv. [%][c]
We began with testing various B(C F ) /hydrosilane combina-
tions in the reduction of nitrobenzene to aniline (1 → 2;
1
2
3
4
5
6
7
8
9
toluene
neat
toluene
neat
toluene
neat
toluene
neat
toluene
neat
neat
neat
neat
100
100
100
100
100
100
100
100
100
100
100
100
80
10
75
–
6
5 3
PhSiH
(EtO) SiH (4.0)
(EtO) SiH (10)
3
(10)
3
Table 1). The use of B(C F ) /PhSiH in toluene at 100 °C that
6
5 3
3
3
–
had yielded full conversion in the aforementioned phosphine
TMDS (4.0)
TMDS (10)
PMHS (4.0)
PMHS (10)
decomp.
traces
53
70
73
[
4]
oxide reduction showed hardly any consumption of the
nitroarene, but conversion rose to 75 % without solvent (En-
tries 1 and 2). No reaction was seen with (EtO) SiH (Entries 3
3
Et
3
SiH (4.0)
and 4), another result that is not in line with those of the phos-
1
0
Et
Et
3
SiH (10)
SiH (10)
quant.
30
[
4]
phine oxide deoxygenation. Decomposition and little conver-
[d]
1
1
3
1
1
2
3
Et
Et
3
SiH (4.0)
SiH (10)
60
81[
e]
3
[
a] Institut für Chemie, Technische Universität Berlin
Strasse des 17. Juni 115, 10623 Berlin, Germany
[a] Reactions were performed on a 0.16 mmol scale. [b] The formation of the
N-silylated aniline was confirmed by NMR spectroscopy for Et SiH as reduc-
tant. [c] Determined by GLC analysis with reference to the starting material;
E-mail: martin.oestreich@tu-berlin.de
http://www.organometallics.tu-berlin.de
3
Supporting information and ORCID(s) from the author(s) for this article are
available on the WWW under http://dx.doi.org/10.1002/ejoc.201600556.
decomp. = decomposition, quant. = quantitative. [d] 5.0 mol-% of B(C
used. [e] Traces of azobenzene detected.
6 5 3
F )
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Communication
that higher temperatures are required to achieve full reduction. the methyl ether intact (19; Entry 10). With 19 being a better
It is worth mentioning that Et SiH did not allow for the deoxy- donor than the other nitrobenzenes, formation of a more stable
3
[
4]
genation of phosphine oxides.
Lewis pair with B(C F ) could be the reason. Again, room-tem-
6 5 3
We then subjected various nitroarenes to the optimized con- perature ether cleavages with B(C
F
)
/Et
SiH are known in the
3
6
5
3
[
3a,3b]
ditions [B(C F ) (10 mol-%), Et SiH (10 equiv.), neat, 100 °C; literature,
but we did not observe any demethylation. 2-
6
5 3
3
Table 2]. The model reaction and the nitrotoluene isomers Nitrofluorene cleanly converted into the corresponding amine
afforded aniline and the toluidines, respectively, in high isolated (21 → 22; Entry 11). Benzylic halides are not compatible with
yields (1 → 2 and 3/5/7 → 4/6/8; Entries 1–4). The para-halo- the setup, and the respective toluidine is formed as a result
genated nitrobenzenes displayed different reactivities: The iodo of hydrodehalogenation (23 → 8; Entry 12). Interestingly, α-
derivative was reduced with hydrodehalogenation to yield anil- nitronaphthalene underwent the desired deoxygenation con-
[
9]
ine (9 → 2; Entry 5), while chemoselective reduction was ob- comitant with reduction of the nitrogen-bound arene ring (23
tained for the bromo and chloro derivatives (11/13 → 12/14; → 24; Entry 13). Related hydrogenations were disclosed by the
Entries 6 and 7); the fluoro derivative was too unreactive (15 → groups of Stephan and Du, requiring either stoichiometric or
[
10]
[13]
1
6; Entry 8). In turn, trifluoromethylation in the para position catalytic amounts of B(C F ) .
6 5 3
was tolerated (17 → 18; Entry 9) despite the fact that B(C F ) /
The formed anilines are not sufficiently Lewis-basic to en-
6
5 3
Et SiH was reported to promote the hydrodefluorination of gage in irreversible Lewis pairing with B(C F ) at 100 °C. How-
3
6 5 3
[
11,12]
alkyl fluorides at room temperature.
However, the elec- ever, the arene reduction of the intermediate α-aminonaphth-
tron-richer nitroanisole did not react and was reisolated with alene releases a more Lewis-basic alkylamine (cf. Table 2, En-
try 13) that could potentially lead to a more stable Lewis adduct
with the catalyst. To investigate this further, we tested repre-
Table 2. B(C
6
F
5
)
3
-catalyzed hydrosilane reduction of various nitroarenes.[a]
sentative aliphatic nitro compounds and were pleased to find
that both converted into the corresponding amine (26 → 27
and 29 → 30); these were directly acylated to facilitate their
isolation (27 → 28 and 30 → 31; Scheme 1).
6 5 3
Scheme 1. B(C F ) -catalyzed hydrosilane reduction of representative ali-
phatic nitro compounds followed by amide formation.
Conclusions
We have reported herein a rare example of a transition-metal-
free hydrosilane reduction of aromatic and aliphatic nitro
[
8]
groups. The Lewis-acidic catalyst B(C F ) together with Et SiH
6
5 3
3
as the stoichiometric reductant as well as the high reaction
temperature limit the functional-group tolerance, but the reac-
tion is reasonably efficient for simple substrates.
Experimental Section
[
(
[
a] All reactions were performed on a 0.32–0.40 mmol scale using B(C
10 mol-%) und Et SiH (10 equiv.) without any solvent at 100 °C for 16 h.
b] Determined by GLC analysis with reference to the starting material. [c]
6 5 3
F )
In a glovebox, an oven-dried 1 mL screw-capped sealed tube with
a magnetic stir bar was charged with tris(pentafluorophenyl)borane
3
[
B(C F ) ; 10 mol-%], triethylsilane (Et SiH; 10 equiv.), and the indi-
6 5 3 3
Determined after purification by acid/base extraction followed by flash chro-
matography on silica gel. [d] Hydrodeiodination occurred, exclusively afford-
ing aniline (2) rather than 4-iodoaniline (10). [e] 1,2-Bis(4-bromo-
phenyl)diazene formed as byproduct (< 10 % yield). [f] Unreacted starting
material recovered by flash chromatography on silica gel.
cated nitro compound (0.32–0.40 mmol). The tube was sealed prop-
erly and transferred to an oil bath preheated at 100 °C. After 16 h,
the reaction mixture was cooled to room temperature and passed
through a small plug of silica gel using diethyl ether. The crude
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Communication
material was collected in a glass vial and subjected to GLC analysis
to determine the conversion (with respect to starting material). The
ethereal solution was then transferred to a separating funnel; aque-
ous HCl (20 %; 20 mL) was added followed by extraction with water
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8
814–8818; Angew. Chem. 2015, 127, 8938–8942 (regioselective deoxy-
genation of terminal 1,2-diols using Ph SiH and then Et SiH); k) L. L.
Adduci, T. A. Bender, J. A. Dabrowski, M. R. Gagné, Nat. Chem. 2015, 7,
76–581 (selective deoxygenation of C -derived polyols using Me Et-
SiH).
2
2
3
5
6
O
6
2
(2 × 10 mL). The aqueous layer was neutralized to pH 8 with aque-
ous NaOH (50 %), and the aqueous layer was backextracted with
diethyl ether (2 × 20 mL). The ethereal solution was dried with so-
dium sulfate, filtered, and the solvent was removed under reduced
pressure. The residue was purified further by flash column chroma-
tography on silica gel using cyclohexane/diethyl ether (9:1) as elu-
ent to afford the desired amine.
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[
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Acknowledgments
3
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This research was supported by the German Academic Ex-
change Service (predoctoral fellowship to D. P., 2014-2018) and
the Cluster of Excellence “Unifying Concepts in Catalysis” of the
Deutsche Forschungsgemeinschaft (EXC 314/2). M. O. is in-
debted to the Einstein Foundation (Berlin) for an endowed pro-
fessorship.
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Keywords: Boron · Lewis acids · Reduction · Si-H bond
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3 4
S clusters bearing diamine ligands using
2
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–
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6
F
5
)
3
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)
3
] , see: a)
Et
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3
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3
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Received: May 4, 2016
Published Online: ■
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© 0000 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Communication
Deoxygenation
D. Porwal, M. Oestreich* .................. 1–4
B(C F ) -Catalyzed Reduction of Ar-
6
5 3
omatic and Aliphatic Nitro Groups
with Hydrosilanes
Nibble off the Os: Tris(pentafluoro-
phenyl)borane, B(C F ) , catalyzes the
6
5 3
hydrosilane reduction of nitro groups
to afford the corresponding amines.
Both aryl and alkyl nitro compounds
are deoxygenated with triethylsilane
as the stoichiometric reductant. The
functional-group tolerance is moder-
ate.
DOI: 10.1002/ejoc.201600556
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