Para-Selective Halogenation of Nitrosoarenes with Copper(II) Halides

Article abstract of DOI:10.1021/acs.orglett.5b03198

The para-selective direct bromination and chlorination of nitrosoarenes with copper(II) bromide and chloride is reported. Under mild reaction conditions, a range of halogenated arylnitroso compounds are obtained in moderate to good yields with high regioselectivity. Additionally, the versatility of the method is demonstrated by the development of a one-pot procedure to obtain the corresponding para-halogenated aniline- and nitrobenzene derivatives.

Full text of DOI:10.1021/acs.orglett.5b03198

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Letter
pubs.acs.org/OrgLett
Para-Selective Halogenation of Nitrosoarenes with Copper(II) Halides
Angela van der Werf and Nicklas Selander*
Department of Organic Chemistry, Arrhenius Laboratory, Stockholm University, SE-106 91 Stockholm, Sweden
S
* Supporting Information
ABSTRACT: The para-selective direct bromination and chlorina-
tion of nitrosoarenes with copper(II) bromide and chloride is
reported. Under mild reaction conditions, a range of halogenated
arylnitroso compounds are obtained in moderate to good yields with
high regioselectivity. Additionally, the versatility of the method is
demonstrated by the development of a one-pot procedure to obtain
the corresponding para-halogenated aniline- and nitrobenzene
derivatives.
rylnitroso compounds are versatile building blocks that can
undergo a broad range of transformations.1 Moreover,
Cl2/Br2, we herein report on a general and regioselective
halogenation method (Scheme 1b) and the subsequent one-pot
oxidation/reduction protocol to obtain functionalized anilines,
nitrosoarenes, and nitroarenes.
A
nitroso compounds are readily synthesized by nitrosation2 or by
redox reactions3 of nitrogen-containing starting materials. Most
of the known transformations of arylnitroso compounds are
operating on the nitroso group itself, with addition,4 N- or O-
nitroso aldol,5 annulation, and cycloaddition,6 and nitroso−ene7
reactions being the most important examples. Additionally, the
nitroso group can easily be reduced or oxidized to obtain amines,
hydroxylamines, or nitro compounds.1d In contrast, there are far
fewer examples of aromatic functionalization reactions of
nitrosoarenes.8−10
During the initial screening of reaction conditions14 for the
chlorination of nitrosobenzene (1a) with CuCl2, a number of
interesting observations were encountered (Scheme 2).
Scheme 2. Relevant Findings in the Screening of Conditions
Given the wide applicability of halogenated aromatics, the
development of regioselective halogenation methods has
received considerable attention.11−13 Conveniently, the use of
Cu(II) salts12,13 has been devised as a mild and selective
alternative to classical methods, both by organometallic pathways
(C−H functionalization, ortho-directing) and by oxidative
methods (for electron-rich arenes, para-directing).13k Interest-
ingly, the electron-withdrawing nitroso group has been
recognized to be selectively para-directing in the classical
halogenation with Br2 and Cl2.8a For nitrosobenzene, the para-
halogenated product was obtained in yields ∼40% or less. This
procedure requires a careful control of the reaction temperature
(−5 °C), and a repeated removal of HX formed. Moreover,
azoxyarenes and nitro compounds were formed as byproducts,
along with unreacted starting material (Scheme 1a).
In the presence of 1.0 equiv of CuCl2 in MeCN at 40 °C, the
para-chlorinated nitrosoarene 2a was obtained as a single
regioisomer in 24% yield (by crude 1H NMR) after 18 h, along
with 19% of unreacted starting material 1a (Scheme 2, conditions
A). Upon increasing the loading of CuCl2 to 2.0 equiv, ∼90%
conversion of 1a was observed after 8 h, and the NMR yield of 2a
was 49% (Scheme 2, conditions B). Around 7% of azoxyarene
was observed, whereas the remainder of the starting material was
not accounted for (vide infra). By using CuCl2 in a catalytic
amount (10 mol %) with NCS as an oxidant, 2a was formed in
19% yield after 24 h at 50 °C (Scheme 2, conditions C).
However, a similar yield (17%) was obtained in the absence of
CuCl2, and the use of NCS led to a significant amount of
nitrobenzene (33% yield) as a byproduct, demonstrating the
need for a mild oxidant, such as Cu(II) halides.
Even though this reactivity has been recognized as unique,8c
no other methods for the halogenation of nitrosoarenes have, to
the best of our knowledge, been reported. Exploiting the unique
reactivity of nitrosoarenes, and with the use of Cu(II) halides as
mild, convenient, and less hazardous alternatives to elemental
When a stoichiometric amount of CuCl2 was used (Scheme 2,
conditions A and B), a long induction period was observed,
which led us to suspect that radicals may be involved. Thus, we
investigated the effect of TEMPO (2,2,6,6-tetramethylpiperidin-
Scheme 1. Halogenation of Arylnitroso Compounds
Received: November 5, 2015
Published: November 25, 2015
© 2015 American Chemical Society
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Letter
a
1-oxyl), CuCl, and BHT (2,6-di-tert-butyl-4-methylphenol) as
additives. The addition of CuCl (0.1 equiv) or TEMPO (1 equiv)
resulted in a complete loss of reactivity, 1a was found to be intact
after 18 h.14 However, the addition of BHT (1 mol %) did not
only reduce the induction period significantly but also allowed
for the reaction to be performed at room temperature. The para-
chlorinated product 2a was obtained in 47% yield by 1H NMR
(Scheme 2, conditions D). No conversion was observed without
BHT after 24 h at rt.
Scheme 4. Bromination of Nitrosoarenes
In order to improve the yield of the halogenation reaction,
several solvents, dilution, slow addition, and a lower reaction
temperature were screened. In addition, a number of published
halogenation methods were investigated for comparison. None
of the efforts led to a more efficient reaction than the use of
Cu(II) halides in a stoichiometric amount; see the Supporting
Information (Table S-1) for a selection of reaction conditions
investigated. We settled for a general method using 2.2 equiv of
the Cu(II) halide and 1 mol % of BHT and investigated the scope
of this transformation. We were pleased to find that the general
reaction conditions were applicable for a wide range of
substituted nitrosoarenes, yielding the monochlorinated prod-
ucts with high regioselectivity (Scheme 3).
a
General conditions: Nitrosoarene 1 (0.30 mmol), CuBr2 (2.2 equiv),
b
and BHT (1 mol %) in MeCN (1.2 mL). With ≤5% of 1,3-dibromo-
c
4-nitrosobenzene. With CuBr2 (0.1 equiv) and NBS (1.5 equiv) for
d
e
f
24 h at 50 °C. 11.0 mmol scale, general cond. 1H NMR yield. Time:
g
1 h. Time: 5 min, 1H NMR yield, with 2-bromo-3,4-dimethyl-1-
nitrosobenzene (3:1).
dibrominated product were observed, and by the short reaction
times required for dimethyl-substituted nitrosoarenes 1i and 1j
to avoid decomposition. In some cases, we observed di- or
tribrominated anilines as byproducts (<10%). Moreover, the
nitro-substituted substrate 1g, which was unreactive in the
chlorination (Scheme 3, 2g), was brominated in an acceptable
yield (3g, 40% yield). This result is remarkable as similar
halogenation methods for arenes only work for electron-rich
substrates or require directing groups.12,13 The bromination was
also scalable, giving 3b in 57% yield (1.25 g). The attempted
halogenation of OMe-, Me2N-, CHO-, and acyl-substituted
nitrosoarenes led to an intractable mixture of products. When
CuBr2 (0.1 equiv) and NBS (1.5 equiv) were used, comparable
yields of 3a and 3b were obtained. However, for CF3-containing
substrate 1f, only 5% yield of 3f was obtained under the catalytic
conditions. No formation of 3b was observed using NBS without
CuBr2.14
Furthermore, we were interested in comparing the reaction
outcome for a series of dimethyl-substituted nitrosoarenes (1i,
1j, 1m, and 1n). As seen in Schemes 3 and 4, the reactions of 2,5-
dimethylated nitrosoarene 1i afforded the para-halogenated
products 2i and 3i in excellent selectivity. Quite expectedly,
substrate 1j gave an isomeric mixture of ortho-halogenated
products 2j and 3j with a preference for functionalization at the
least hindered position (Schemes 3 and 4, 2j and 3j). However,
the halogenation of substrates 1m and 1n led to rather
unexpected outcomes (eqs 1 and 2).
a
Scheme 3. Chlorination of Nitrosoarenes
a
General conditions: nitrosoarene 1 (0.30 mmol), CuCl2 (2.2 equiv),
and BHT (1 mol %) in MeCN (1.2 mL). 11.0 mmol scale. Time: 4
h. Time: 5 h. For 7 h at 50 °C. For 24 h at 50 °C. With 2-chloro-
3,4-dimethyl-1-nitrosobenzene (4:1). Time: 7 h.
b
c
d
e
f
g
h
For ortho-substituted nitrosoarenes, para-chlorinated products
2b−f were obtained in moderate to high yields depending on the
electronic character of the substituent. The highest isolated yield,
74%, was obtained for the o-methyl derivative 2b. This reaction
was also performed on an 11.0 mmol scale to obtain 1.18 g of the
chlorinated product (69% yield). The less electron-rich nitro-
soarenes were obtained in lower yields and required longer
reaction times and/or elevated temperatures (Scheme 3, 2d−f).
No reaction was observed in an attempted chlorination of o-
nitronitrosobenzene (1g). Meta-substituted nitrosoarenes af-
forded the corresponding products in moderate to good yields
(2h−j), whereas for the para-substituted substrates, a prolonged
reaction time was required, and the products were identified as
the ortho-chlorinated nitroso derivatives 2j−l.
Upon replacing CuCl2 with CuBr2, the analogous para-
brominated nitrosoarenes were obtained in comparable yields
(Scheme 4). The most important difference was an increased
reactivity observed with CuBr2 as exemplified in the bromination
of nitrosobenzene (1a), where traces (≤5%) of the 2,4-
For nitrosoarene 1m, being substituted in the para-position
and in one of the ortho-positions with respect to the nitroso
group, the benzylic products 2m and 3m were selectively
obtained instead of the expected ortho-halogenated products. We
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speculate that the ortho-position may be less reactive on the basis
of the geometry and direction of the nitroso group,15 favoring a
quinomethide-type of reactivity. The ortho-disubstituted sub-
strate 1n was in contrast completely unreactive under the applied
reaction conditions, possibly due to a low solubility of dimerized
1n.16
As seen in Schemes 3 and 4, moderate yields were generally
obtained. For certain substrates, e.g., nitrosobenzene, the
formation of azoxyarenes was observed, and in most cases
∼10% of the starting material remained unreacted. Formation of
polymeric material and sublimation, generally observed for
nitrosoarenes, are other possible reasons for the moderate
isolated yields. In order to probe for possible regioisomeric
products and to acquire more information on the mass balance,
the chlorination of 1b was monitored by 1H NMR (Figure 1).
present in the reaction mixture from the halogenation step.17
Furthermore, the oxidation of the nitroso group could efficiently
be achieved in a one-pot sequence using Oxone (40−68% yield,
6b, 6f, 7b, and 7f). No regioisomeric products were obtained in
the one-pot oxidation or reduction reactions, which further
support the high regioselectivity of our halogenation protocol.
Furthermore, we were interested in comparing our results with
the halogenation of a selection of electron-rich arenes (8a−c)
with CuX2 (Scheme 5).
a
Scheme 5. Halogenation of Electron-Rich Arenes
a
General conditions: arene 8 (0.30 mmol) and CuX2 (2.2 equiv) in
b
MeCN (1.2 mL), stirred at rt for 24 h. For 6 days at 70 °C.
c
d
Temperature: 50 °C. 2.0 equiv of CuBr2, time 45 min, isolated with
2,4-dibromo-1,3,5-trimethoxybenzene (ratio: 1.0:0.08).
1
◆
Figure 1. Chlorination of 1b, monitored by H NMR (blue , 1b; red
●
, 2b).
We found that the brominated derivatives of phenol (8a) and
methoxybenzene derivatives 8b,c could be obtained in high
yields. Although the corresponding chlorination using CuCl2
worked reasonably well for 8a and 8c upon heating, anisole (8b)
did not react at all. These results are very similar to the ones
reported in the literature for the halogenation of electron-rich
arenes with Cu(II) salts under oxidative conditions.13c,g When
comparing the halogenation of nitrosoarenes 1 with the results
presented in Scheme 5, one can point out a couple of important
differences. First, the electron-deficient nitrosoarenes 1 typically
required shorter reaction times compared to the electron-rich
arenes 8. Moreover, no induction period was observed for the
halogenation of electron-rich substrates 8 as was seen for the
nitrosoarenes 1, which may indicate different overall mechanistic
pathways.
From the reaction profile above, it can be concluded the
reaction took off after a fairly long induction period (in MeCN-d3
at 20 °C (Figure 1), the reaction was found to require slightly
longer time). Moreover, 1H NMR analysis showed that only 80%
of the material could be identified as 1b or 2b in the crude
1
reaction mixture. The H NMR yield (64%) was slightly lower
than the isolated yield (74%), which also indicates that
complexation of 2b (and/or 1b) with copper could account for
some of the remaining material. Furthermore, by comparison
with a reference sample of the ortho-chlorinated product, neither
1H NMR nor HPLC analysis of the crude reaction mixture could
verify formation of the ortho-regioisomer in any traceable
amounts.14
Based on the observation that nitrosoarenes with EWGs
generally required more forcing reaction conditions compared to
nitrosoarenes with EDGs, we speculate that the mechanism
involves a rate-determining step in which a positive charge is built
up. Moreover, since the nitrosoarenes reacted unexpectedly rapid
(compared to arenes 8), it is not unlikely that the nitroso group is
temporarily converted into a more reactive functional group
during the course of the reaction. Indeed the electrophilic
bromination of nitrosoarenes with Br2 has been proposed to be
initiated by a HBr addition across the N−O bond.8c A similar
hydroxylamine type of intermediate could also be envisioned in
our case as we could observe azoxybenzene derivatives (formed
by the condensation of hydroxylamine with a nitrosoarene) as
byproducts in the reaction. The second equivalent of the Cu(II)
halide, which was necessary to obtain full conversion, may be
important for reoxidizing any hydroxylamine type of inter-
mediates or for the stoichiometric generation of X2 or HX, which
are likely to be involved (Scheme 6, A). Alternative mechanisms,
such as a nucleophilic attack of a halide anion on a Bamberger-
type18 of nitrenium intermediate (B), or pathways involving
radical species (C), cannot be ruled out. Whether BHT is
involved in any important redox reactions, or serves as a radical
scavenger to prevent unproductive radical pathways, remains
unclear.
In order to demonstrate the versatility of our method, we
envisioned that a series of halogenated arenes within the whole
landscape of nitrogen oxidation states could be accessed by a
one-pot halogenation−reduction/oxidation (Figure 2).
The in situ reduction of the halogenated nitrosoarenes was
achieved by the addition of NaBH4 to the halogenation reaction
mixture to obtain anilines 4b, 5b, and 5f in good yields. Aniline 4f
was found to be unstable under the reaction conditions (Figure
2). Interestingly, the reduction of the nitroso group was found to
be more efficient in the presence of the copper salts already
Figure 2. One-pot synthesis of halogenated anilines and nitroarenes:
step 1, halogenation; step 2, oxidation/reduction (see the Supporting
Information for details). (a) Product was not stable.
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Letter
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Scheme 6. Proposed Halogenation Intermediates
In short, we have developed an operationally simple para-
selective method for the bromination and chlorination of a wide
range of nitrosoarenes with Cu(II) halides. The resulting nitroso
derivatives were, in a one-pot procedure, conveniently trans-
formed into the corresponding anilines and nitroarenes. Thus,
with a limited set of starting compounds and with an easy-to-
execute method, a range of halogenated, nitrogen-functionalized
products can be obtained in high regioisomeric purity and decent
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ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.orglett.5b03198.
́ ́ ́ ̌ ́
ring, see: (a) Pilepic, V.; Lovrek, M.; Vikic-Topic, D.; Ursic, S.
Tetrahedron Lett. 2001, 42, 8519. (b) Seayad, J.; Patra, P. K.; Zhang, Y.;
Ying, J. Y. Org. Lett. 2008, 10, 953.
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Maugel, N.; Yu, J.-Q. Angew. Chem., Int. Ed. 2008, 47, 5215. (d) Zhao, X.;
■
S
Experimental procedures and characterization data (PDF)
́
Dimitrijevic, E.; Dong, V. M. J. Am. Chem. Soc. 2009, 131, 3466.
AUTHOR INFORMATION
Corresponding Author
■
(e) Dudnik, A. S.; Chernyak, N.; Huang, C.; Gevorgyan, V. Angew.
Chem., Int. Ed. 2010, 49, 8729. (f) Bedford, R. B.; Haddow, M. F.;
Mitchell, C. J.; Webster, R. L. Angew. Chem., Int. Ed. 2011, 50, 5524.
*E-mail: nicklas.selander@su.se.
Notes
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The authors declare no competing financial interest.
ACKNOWLEDGMENTS
This work was supported by the Swedish Research Council (VR)
and the Lars Hierta Memorial Foundation.
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