Metal-free deoxygenation and reductive disilylation of nitroarenes by organosilicon reducing reagents

Article abstract of DOI:10.1002/chem.201801972

A metal-free deoxygenation and reductive disilylation of nitroarenes was achieved using N,N’-bis(trime-thylsilyl)-4,4’-bipyridinylidene (1) under mild and neutral reaction conditions, and a broad functional group tolerance was possible in this reaction. Mono-deoxygenation, giving a synthetically valuable N,O-bis(trimethylsilyl)phe-nylhydroxylamine (7a) as a readily available and safe phenylnitrene source from nitrobenzene, and double-deoxy-genation, giving N,N-bis(trimethylsilyl)anilines 8, were easily controlled by varying the amounts of 1 and reaction temperature as well as adding dibenzothiophene (DBTP). Reaction of 2-arylnitrobenzenes with 1 resulted in the formation of the corresponding carbazoles 14 via in situ-gen-erated phenylnitrene species derived by thermolysis of N,O-bis(trimethylsilyl)phenylhydroxylamines 7, followed by their subsequent intramolecular C H insertion. In addition, the intramolecular N N coupling reaction proceeded in the reduction of 2,2’-dinitrobiphenyl derivatives by 1, giving the corresponding benzo[c]cinnolines.

Full text of DOI:10.1002/chem.201801972

A Journal of
Accepted Article
Title: Metal-free Deoxygenation and Reductive Disilylation of
Nitroarenes by Organosilicon Reducing Reagents
Authors: Argha Bhattacharjee, Hiromu Hosoya, Hideaki Ikeda, Kohei
Nishi, Hayato Tsurugi, and Kazushi Mashima
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To be cited as: Chem. Eur. J. 10.1002/chem.201801972
Link to VoR: http://dx.doi.org/10.1002/chem.201801972
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Metal-free Deoxygenation and Reductive Disilylation of
Nitroarenes by Organosilicon Reducing Reagents
Argha Bhattacharjee, Hiromu Hosoya, Hideaki Ikeda, Kohei Nishi, Hayato Tsurugi,* and
Kazushi Mashima*[a]
Abstract: A metal-free deoxygenation and reductive disilylation of
nitroarenes was achieved using N,N’-bis(trimethylsilyl)-4,4’-
bipyridinylidene (1) under mild and neutral reaction conditions, and a
broad functional group tolerance was compatible in this reaction.
disilylation of nitroarenes to N,O-bis(silylated)hydroxyanilines
under mild reaction condition are highly demanded, which is also
valuable for the late stage organic transformation.
Mono-deoxygenation, giving
a
synthetically valuable N,O-
bis(trimethylsilyl)phenylhydroxylamine (7a) as a readily available and
safety phenylnitrene source from nitrobenzene, and double-
deoxygenation, giving N,N-bis(trimerhylsilyl)anilines 8, were easily
controlled by varying the amounts of 1 and reaction temperature as
well as adding dibenzothiophene (DBTP).
arylnitrobenzenes with resulted in the formation of the
corresponding carbazoles 14 via in situ-generated phenylnitrene
species derived by thermolysis of N,O-
bis(trimethylsilyl)phenylhydroxylamines 7, followed by their
subsequent intramolecular C—H insertion. In addition, the
Reaction of 2-
1
Scheme 1. Reductive deoxygenation of nitroarenes to hydroxyamines and
anilines.
intramolecular N—N coupling reaction proceeded in the reduction of
2,2’-dinitrobiphenyl derivatives by 1, giving the corresponding
benzo[c]cinnolines.
In this context, fully organic-based active and selective
reductants under the mild reaction condition are highly demanded.
As part of our continued interest in a new family of organic
reducing reagents, N,N’-bis(trimethylsilyl)-4,4’-bipyridinylidene
(1), 1,4-bis(trimethylsilyl)-1,4-dihydropyrazine (2) and its
methylated derivatives (3 and 4), and 1,4-bis(trimethylsilyl)-2-
methylcyclohexa-2,5-diene (5) (see Figure 1), we used these
reagents to reduce various transition metals, main group
elements, and organic halides.[9] Herein, we report a step-wise
reductive deoxygenation of nitroarenes using organosilicon
Chemoselective reductive deoxygenation of nitroarenes is an
important transformation used to access synthetically useful
nitrogen-containing compounds such as N-hydroxyanilines and
anilines, because nitroarenes are cheap, stable, and readily
functionlizable nitrogen-containing feedstock. A wide variety of
deoxygenation reagents has been developed using metal-based
reductants such as iron, zinc, and tin powders combined with
acids, and heterogeneous catalysts (Raney Ni, Pd/C, Rh/C) in the
presence of dihydrogen, among others, despite the fact that these
are highly reactive reductants capable to concurrently reduce any
functional groups, resulting in low functional group tolerance in
organic skeletons (Scheme 1).[1] Other deoxygenation reagents
include main group compounds such as trichlorosilane/NR3,[2]
B2(pin)2/base,[3] and a sulfur containing system,[4] as well as
organic compounds such as dihydroarenes,[5] (hetero)aryl
alcohols,[6] glucose,[7] and fullerene/H2;[8] however, the use of
these compounds requires rather harsh or basic reaction
conditions, mostly giving the final aniline derivatives. Because of
the high synthetic utility of the N,O-bis(silylated)hydroxyanilines
as safe and effective arylnitrene sources in the organic
reductant
1 under neutral and mild conditions: at room
temperature, reduction of nitroarenes by 1 afforded synthetically
useful N,O-bis(silylated)phenylhydroxylamine derivatives, whose
thermolysis in the presence of dibenzothiophene (DBTP)
provided aniline derivatives through trapping the corresponding
arylnitrene species by DBTP.[10] Furthermore, reduction of 2-
arylnitrobenzenes by 1 produced carbazole derivatives via
insertion of nascent nitrene into a C—H bond, while reduction of
2,2’-dinitrobiphenyl derivatives resulted in the formation of the
corresponding benzo[c]cinnolines. Explosive arylazides are
generally used for arylnitrene formation; thus, our protocol for the
selective formation of N,O-bis(silylated)phenylhydroxylamine
derivatives as precursors for arylnitrenes from nitroarenes
possesses a significant synthetic advantage due to their safety
and the availability of the nitro compounds.
transformation, leading to
a variety of nitrogen-containing
compounds, selective mono-deoxygenation and reductive
SiMe3
N
SiMe3
[a]
Mr. Argha Bhattacharjee, Mr. Hiromu Hosoya, Mr. Hideaki Ikeda,
Mr. Kohei Nishi, Prof. Dr. Hayato Tsurugi,*
Prof. Dr. Kazushi Mashima*
SiMe3
R2
R1
R1
N
Me
R2
SiMe3
Department of Chemistry,
N
N
Graduate School of Engineering Science, Osaka University,
1-3, Machikaneyama-cho, Toyonaka, Osaka 560-8531, Japan
E-mail: tsurugi@chem.es.osaka-u.ac.jp
Me3Si
SiMe3
2 (R1 = R2 = H)
3 (R1 = H, R2 = Me)
4 (R1 = R2 = Me)
1
5
mashima@chem.es.osaka-u.ac.jp
Supporting information for this article is given via a link at the end of
the document.
Fig 1. Organosilicon reducing reagents 1-5.
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by trapping the in situ generated nitrene by DBTP to form the
corresponding sulfimine 10a.[10,15] It was exemplified that 1
reduced a sulfimine 10b, derived from Martin sulfurane reagent
and aniline,[16] to give 8a in quantitative yield (eq 1), strongly
suggesting that the sulfimine formation played a key role for
preventing the formation of azobenzene.
We first tested organosilicon compounds 1—5 for their
ability to reduce nitorobenzene (6a), and the results are
summarized in Table 1. The reaction of 6a with 1 (2.5 equiv) in
CD3CN at room temperature proceeded smoothly within 15 min
to quantitatively afford N,O-bis(trimethylsilyl)phenylhydroxyamine
(7a) as a consequence of removal of one of two oxygen atoms as
well as reductive disilylation of the nitrogen and oxygen atoms on
the nitro group (entry 1), in which two equivalents of 1 were
Scheme 2. Effect of DBTP for the reduction of nitrobenzene (6a) to N,N-
bis(trimethylsilyl)aniline (8a) by 1.
consumed
hexamethyldisiloxane (1 equiv) as easily removable
byproducts.[11,12]
Under the same reaction conditions,
to
give
4,4’-bipyridine
(2
equiv)
and
bis(trimethylsilyl)dihydropyrazine derivatives 2 and 3 (each 2.5
equiv) afforded 7a in only about 20% yields (entries 2 and 3), and
tetramethyl-substituted compound 4 was much less effective to
give 7a in only a trace amount (entry 4). The use of compound 5
resulted in no reaction (entry 5). Such results were consistent
with the highest negative redox potential of 1 among 1—5. We
additionally examined a reaction with hexamethyldisilane, which
was reported to deoxygenate 6a at 240 °C in ortho-C6H4Cl2;[13]
however, under the same reaction conditions at room
temperature, no reaction was observed (entry 6).
Table 1.
Selective transformation of nitrobenzene to the N,O-
bis(trimethylsilyl)phenylhydroxyamine by organosilicon compounds.a
A variety of nitroarenes was subjected to the salt-free
reduction under the reaction conditions using 1 (4.0 equiv) and
catalytic amounts of DBTP (10 mol%) at 100 °C for 6 h, giving the
corresponding anilines after acidic work-up with 1N HCl, and the
Organosilicon
reagent
7a
Oraganosilicon
reagent
7a
Entry
Entry
Yield (%)b
Yield (%)b
results are summarized in Scheme 3.
Electron-deficient
1
2
3
1
2
3
>99
18
4
5
6
4
5
trace
nitroarenes 6b,c with cyano and chloro groups, which are poorly
compatible groups for metal-mediated reduction, were selectively
reduced to give the corresponding amines 11b,c in excellent
yields. In contrast, the reduction of 1-methyl-4-nitrobenzene (6d)
proceeded in a moderate yield of 55%. Alkene moiety in 6e was
kept intact during the reduction to give 11e in 90%, while the yield
of 11f with a carbonyl group was decreased to 61%. N-Acyl and
O-TBS-protected nitroarenes 6g and 6h were converted to the
corresponding anilines 11g and 11h in good yields without loss of
the protecting groups. Ester and sulfone groups in 6i,j were
tolerated, resulting in the productions of 11i,j in 84% and 95%
yields, respectively. The present protocol was also applicable to
reduce 1-nitronaphtharene (6k) to afford the corresponding amino
derivative 11k in moderate to good yields. Di-substituted m-
nitropyridine 6l was converted to the corresponding m-
aminopyridines 11l in 92% yield without any reduction of the
pyridine skeleton. Reductions of ethyl 5-nitrobenzofuran-2-
carboxylate (6m) and 5-nitroindoles (6n) selectively afforded the
corresponding amino compounds 11m,n in 77% and 62% yields
without any decomposition of the five-membered heterocycles.
When 2-nitrobiphenyl (12a) was used as the substrate, an
equimolar amount of DBTP was necessary to obtain 2-
aminobiphenyl (13a) as the major product in 63% yield with
carbazole (14a) in 21% yield after acidic work up (eq 2). Selective
formation of 14a was achieved in the absence of DBTP (vide infra).
0
0
21
Si2Me6
[a] Reaction condition: Nitrobenzene (6a, 0.040 mmol) in CD3CN (0.30 mL)
was dropwisely added to the organosilicon compound (0.100 mmol) in CD3CN
(0.30 mL) at room temperature for 5 minutes, and then all the mixture was
transferred to J-Young NMR tube. [b] Determined by 1H NMR analysis using
mesitylene as internal standard.
Further deoxygenation of the oxygen atom of 7a progressed
at higher temperature (Scheme 2): in situ mixture of 6a and 1 (4
equiv) in CD3CN at room temperature, giving 7a smoothly, was
heated at 100 °C over 6 h to completely consume 7a and give
N,N-bis(trimethylsilyl)aniline (8a) in 73% yield together with 1,2-
diphenyl-1,2-bis(trimethylsilyl)hydrazine (9a) in 11% as the major
byproduct, the latter of which was formed through the N=N bond
reduction of azobenzene by 1,[12] indicating that thermolysis of 7a
produced a nascent phenylnitrene.[14] In fact, phenylnitrene
formation was confirmed by the following experiment: treatment
of the in situ generated 7a with HNEt2 at 100 °C for 16 h gave 2-
diethylamino-3H-azepine, in which generated phenylnitrene was
in equilibrium with benzazirine as a key species for the azepine
formation.[12,14]
Thus, our protocol for the reduction of
nitrobenzene by 1 was one of the easily applicable and safety
method for arylnitrene generation. We found that the addition of
DBTP (10 mol%) dramatically increased the yield of 8a up to 97%
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Scheme 3. Substarte scope for salt-free reduction of various nitroarenes 6 to
the corresponding anilines 11 using 1.
compounds. Finally, the N=S bond of 10a is reduced by 1 to form
N,N-bis(trimethylsilyl)aniline (8a) to regenerate DBTP. Overall,
three equiv of 1 are required for the reduction of both the N=O
and N=S functionalities together with the capture of the resulting
nitrene species, giving 8a.
Scheme 4. Plausible reaction pathway for the reduction of nitrobenzene 6a to
N,N-bis(trimethylsilyl)aniline 8a.
Because nitrenes are highly active and unstable species as
typically exemplified by thermolysis of azide compounds,[17] the
ability to generate them under mild and safe conditions is in high
demand. Thus, our protocol is applicable for synthesizing
carbazole derivatives from 2-nitrobiphenyl derivatives via in situ
nitrene generation, followed by subsequent C—H insertion. As
shown in Scheme 5, 2-nitrobiphenyl (12a) and 1 (2.5 equiv) in
CH3CN was mixed at room temperature to give N-([1,1'-biphenyl]-
2-yl)-N,O-bis(trimethylsilyl)hydroxylamine
(7b),
whose
thermolysis in cyclopentyl methyl ether (CPME) at 120 °C for 72
h afforded carbazole 14a in 92% yield after acidic work-up, though
the same reaction in CH3CN resulted in lower yield of 14a (70%).
We analyzed the substrate scope for the intramolecular
cyclization of nitro compounds (Scheme 5). When an electron-
withdrawing group was introduced at the meta-position to the C—
H bond that reacted with the in situ-generated nitrene, carbazole
derivatives 14b,c were obtained in good yields of 92% and 85%.
The methoxycarbonyl moiety in 12b was preserved intact during
the carbazole formation. 2-(4’-Methylphenyl)nitrobenzene (12d)
was converted to 2-methylcarbazole (14d) in 90% yield, while the
corresponding methoxy-substituted one (14e) was isolated in
moderate yield (71%). When 2-(2’-methylphenyl)nitrobenzene
(12f) was used as the substrate, cyclization occurred at the 6’-
position to afford 4-methylcarbazole (14f) in 85% yield, in which
C—H insertion of the 2’-methyl group was not observed.
Preferential formation of the five-membered ring also proceeded
for 2-naphthylnitrobenzene (12g), from which benzo[c]carbazole
(14g) was isolated in 82% yield. Nitrene insertion into the
thiophene C—H bond for 12h afforded thieno[2,3-b]indole (14h)
in 88% yield. 3-Nitro-2-phenylpyridine (12i) was converted to 5H-
pyrido[3,2-b]indole (14i) in 83% yield, in which the nitro group
bound to pyridine was transformed into nitrene during the reaction.
2-Alkenylnitrobenzene (12j) was also selectively converted to
indole derivative 14j in 60% yield.
We conducted the reaction of nitrosobenzene (15a) with 1 (1.5
equiv) at room temperature for 15 min to form 7a in 40% yield
together with a mixture of azobenzene and azoxybenzene, the
latter of which was finally converted to azobenzene in a total yield
of 39% after 24 h at room temperature, although the amounts of
7a were kept constant during the prolonged reaction time (eq 3).
The observed stability of 7a at room temperature excluded a
reaction pathway through the nitrene coupling reaction to produce
azobenzene. Such a reaction profile suggests that 7a reacted
with 15a to form azoxybenzene as the initial byproduct, followed
by reduction by 1 to give azobenzene.[12]
On the basis of these observations, we propose a plausible
reaction pathway for the reduction of nitrobenzene by 1 (Scheme
4): deoxygenation of nitrobenzene (6a) by an excess of 1 gives
N,O-bis(trimethylsilyl)phenylhydroxyamine (7a) along with
a
release of (Me3Si)2O. At a higher temperature, thermal
decomposition of 7a produces an unstable nitrene
intermediate.[14] DBTP works as a catalytic stabilizer to trap such
the nitrene species through the formation of the corresponding
sulfimine 10a to suppress the formation of byproducts such as azo
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Scheme 5. Salt-free synthesis of carbazoles and indoles from nitro compounds
12.
species that inserted into the C—H bond of the other aryl ring.
Moreover, we demonstrated that 2,2’-dinitrobiphenyl derivatives
16 were transformed into the corresponding benzo[c]cinnoline
derivatives 17. Further salt-free reductive transformations of
various organic compounds using a series of organosilicon
reductants are ongoing in our laboratory.
Acknowledgements
A.B. acknowledged a financial support by JICA FRIENDSHIP
program. This work was supported by JSPS KAKENHI Grant Nos.
JP26708012 (Grant-in-Aid for Young Scientist (A)) to H. T. and
JP15H05808 (Precisely Designed Catalysis with Customized
Scaffolding) to K. M. We thank Dr. Taiga Yurino for his
contributions at the early stage of this work.
Keywords: Reduction • Nitroarene • Nitrene • Deoxygenation •
Salt-free reduction
Moreover, we used
1 (4.0 equiv) to reduce 2,2’-
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dinitrobiphenyl derivatives to give the corresponding
benzo[c]cinnoline derivatives, and results are summarized in
Scheme 6. Treatment of 2,2’-dinitrobiphenyl (16a) with 4.0 equiv
of 1 at room temperature gave the corresponding silylated
hydroxylamine derivatives, followed by heating at 100 °C for 16 h
to selectively form benzo[c]cinnoline (17a) in 88% yield without
any nitrene insertion products in the reaction mixture. Yields of
17b and 17c with methyl and methoxy substituents at the 3,8-
position were moderate (42% and 53%). 2,2’-Dinitrobiaryl with
methyl groups at the para-position of the nitro group reductively
[2]
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transformed
into
the
corresponding
2,9-
dimethylbenzo[c]cinnolines 17d in 72% yield, whereas the
reductive benzo[c]cinnoline formation of 17f with fluorine atoms at
the 2,9-position proceeded in a moderate yield of 44%. Such
intramolecular N=N bond formation from dinitro compounds to
benzo[c]cinnolines was typically mediated by LiAlH4,[18] Lewis
acid/HSiR3,[19] and strong base under harsh reaction
conditions;[20] however, contamination of byproducts sometimes
disturbed the purification. Thus, this reaction represents one
practical synthetic protocol for preparing fused azo compounds.
[4]
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Scheme 6. Salt-free synthesis of benzo[c]cinnolines 17 from dinitrobiaryls 16.
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In summary, we demonstrated a new synthetic protocol to
generate arylnitrene from nitroarenes in a mild and safe manner
using an organosilicon reducing reagent, N,N’-bis(trimethylsilyl)-
4,4’-bipyridinylidene (1), and clarified the importance of DBTP for
producing the corresponding fully deoxygenated aniline
derivatives 8. 2-Nitrobiphenyls 12 were successfully converted to
carbazoles 14 in CPME by in situ generation of the arylnitrene
[11] Compound 7a was also prepared by treating phenylhydroxylamine with
chlorotrimethylsilane in the presence of bases.
[12] See Supporting Information.
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[15] Direct trapping of nitrene by sulfides is only reported for nitrenes with
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Entry for the Table of Contents (Please choose one layout)
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Argha Bhattacharjee, Hiromu Hosoya,
Hideaki Ikeda, Kohei Nishi,
Hayato Tsurugi,* and Kazushi Mashima*
Page No. – Page No.
Metal-free Deoxygenation and
Reductive Disilylation of Nitroarenes
by Organosilicon Reducing Reagents
N,N’-Bis(trimethylsilyl)-4,4’-bipyridinylidene (1) serves as an efficient nitroarene
reductant under mild and neutral reaction conditions to give N,O-
bis(trimethylsilyl)arylhydroxylamines and N,N-bis(trimethylsilyl)anilines.
2-
Nitrobiphenyl derivatives are converted to carbazoles, while 2-(2’-
nitroaryl)nitrobenzenes are transformed into benzo[c]cinnoline derivatives.
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