One-Pot C?H Functionalization of Arenes by Diaryliodonium Salts

Article abstract of DOI:10.1002/anie.201603175

A transition-metal-free, mild, and highly regioselective synthesis of nitroarenes from arenes has been developed. The products are obtained in a sequential one-pot reaction by nitration of iodine(III) reagents with two carbon ligands, which are formed in situ from iodine(I). This novel concept has been extended to formation of aryl azides, and constitutes an important step towards catalytic reactions with these hypervalent iodine reagents. An efficient nitration of isolated diaryliodonium salts has also been developed, and the mechanism is proposed to proceed by a [2,2] ligand coupling pathway.

Full text of DOI:10.1002/anie.201603175

Angewandte
Communications
Chemie
International Edition: DOI: 10.1002/anie.201603175
German Edition: DOI: 10.1002/ange.201603175
Hypervalent Compounds
ꢀ
One-Pot C H Functionalization of Arenes by Diaryliodonium Salts
Marcus Reitti, Piret Villo, and Berit Olofsson*
Abstract: A transition-metal-free, mild, and highly regioselec-
tive synthesis of nitroarenes from arenes has been developed.
The products are obtained in a sequential one-pot reaction by
nitration of iodine(III) reagents with two carbon ligands, which
are formed in situ from iodine(I). This novel concept has been
extended to formation of aryl azides, and constitutes an
important step towards catalytic reactions with these hyper-
valent iodine reagents. An efficient nitration of isolated diaryl-
iodonium salts has also been developed, and the mechanism is
proposed to proceed by a [2,2] ligand coupling pathway.
then many other systems have appeared, where a catalytic
amount of iodoarene is combined with a stoichiometric
amount of an oxidant.
Iodonium salts and their cyclic counterparts benziodoxol-
(on)es (e.g. EBX and Togniꢀs reagent) are important io-
dine(III) reagents for the transfer of carbon ligands (aryl,
[3]
alkynyl, vinyl, CF , or CN groups) to a wide range of
3
[4]
nucleophiles. These reagents are generally synthesized in
one or more steps, isolated, and subsequently reacted with
a nucleophile. Kita and co-workers have demonstrated the
efficient in situ formation of diaryliodonium salts from other
[5]
H
ypervalent iodine compounds have become important
iodine(III) reagents in metal-free arylations (Scheme 1b),
and the group of Suna has utilized the same strategy in metal-
reagents for a wide variety of transformations, as they have
a valuable combination of properties including high reactivity,
[
6]
catalyzed arylations. However, to the best of our knowledge,
catalytic reactions with iodonium salts or benziodoxol(on)es
[1]
high functional-group tolerance, and low toxicity. The main
drawback with hypervalent iodine reagents is the stoichio-
metric formation of either an iodine(I) or iodine(III)
compound as waste, depending on the oxidation state (III
or V) of the starting reagent. This aspect lowers the atom
efficiency and may lead to product purification problems. The
groups of Ochiai and Kita reported the first catalytic uses of
[4,7]
remain unknown.
In our long-term quest for catalytic arylation conditions,
we reasoned that in situ formation of diaryliodonium salts
from arenes and iodoarenes in the presence of an oxidant,
followed by arylation of a suitable nucleophile under metal-
free conditions, would constitute an important step towards
this difficult goal. Herein we present our results of an efficient
arylation of sodium nitrite with diaryliodonium salts, which
can be formed in situ from arenes and iodoarenes under
oxidative conditions (Scheme 1c). The methodology com-
pares well to previous routes to nitroarenes, concerning
regioselectivity, functional-group tolerance, and ease of han-
[
2]
hypervalent iodine reagents in 2005 (Scheme 1a). Since
[8]
dling.
Synthetic routes to aromatic nitro compounds are impor-
tant, as nitroarenes are key intermediates in the preparation
[
9]
of many dyes, plastics, and explosives. They can be
[
9a]
synthesized from arenes under harsh acidic conditions, by
[
10]
treatment with [bis(trifluoroacetoxy)iodo]benzene (PIFA),
[11]
and by rhodium-catalyzed oxidative CꢀH activation, all of
which give regioisomeric mixtures. Regiospecific transforma-
tions include palladium-catalyzed nitration of aryl chlor-
Scheme 1. In situ formation of iodine(III) reagents. EDG=electron-
donating group, HFIP=1,1,1,3,3,3-hexafluoro-2-propanol, mCPBA=
m-chloroperbenzoic acid, Ts=4-toluenesulfonyl.
[
12]
ides, and either metal-mediated or metal-free conversion of
[13]
arylboronic acids.
Nitroarenes have also been obtained
from diaryliodonium salts, thus making this transformation
suitable for our envisioned one-pot reaction.
[
14]
[
*] M. Reitti, Dr. P. Villo, Prof. B. Olofsson
Department of Organic Chemistry, Arrhenius Laboratory
Stockholm University, 106 91 Stockholm (Sweden)
E-mail: Berit.Olofsson@su.se
The nitration reaction was first investigated using isolated
diaryliodonium salts, as a stepwise and general methodology
to nitroarenes could be preferable with advanced diaryl-
iodonium salts carrying sensitive functional groups. Various
nitrite sources, solvents, and temperatures were screened, and
Homepage: http://www.organ.su.se/bo/
Supporting information and the ORCID identification number(s) for
the author(s) of this article can be found under http://dx.doi.org/10.
high yields of nitroarenes were obtained with NaNO in either
2
1002/anie.201603175.
[8]
DME or EtOAc at 708C, the latter solvent being preferable
ꢀ
2016 The Authors. Published by Wiley-VCH Verlag GmbH & Co.
from an environmental perspective. The nitration proved
insensitive to moisture and air, and can be conveniently
carried out without special precautions.
Interestingly, the diaryliodonium counterion had a major
impact on the reaction, as shown in Table 1. Diphenyliodo-
KGaA. This is an open access article under the terms of the Creative
Commons Attribution Non-Commercial NoDerivs License, which
permits use and distribution in any medium, provided the original
work is properly cited, the use is non-commercial, and no
modifications or adaptations are made.
Angew. Chem. Int. Ed. 2016, 55, 1 – 6
ꢀ 2016 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
1
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Communications
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[
a]
Table 1: Counterion effect under optimized reaction conditions.
[
b]
NaOTf
equiv)
Yield [%]
X=PF6
(
X=OTf
X=BF4
X=OTs
0
0
1
94
91
18
68
94
18
.5
.0
86
87
[
a] Reaction conditions: NaNO (0.16 mmol) was added to
2
1
(0.14 mmol) and NaOTf (0–1 equiv) in EtOAc (1 mL). The suspension
1
was stirred at 708C for 16 hours. [b] Yield determined by H NMR
analysis using 1,3,5-trimethoxybenzene as internal standard. Tf=tri-
fluoromethanesulfonyl.
nium triflate (X = OTf) and hexafluorophosphate (X = PF6)
gave nitrobenzene (2a) in excellent yields, whereas the
corresponding tetrafluoroborate (X = BF4) and tosylate
(
X = OTs) resulted in poor conversion into 2a. This result
was surprising as tetrafluoroborate salts are usually efficient
[
4e,f]
in both metal-free and metal-catalyzed reactions.
We
hypothesized that addition of sodium triflate might improve
the outcome, and discovered that 1 equivalent of NaOTf was
indeed sufficient to provide 2a in 94% yield. The tosylate salt
could also be efficiently utilized upon addition of NaOTf, and
Scheme 2. Scope for the nitration with isolated salts 1. Reaction
conditions as in Table 1 with 1 (0.5 mmol). Yields are those of isolated
[
8]
the trend was verified with other tetrafluoroborate salts.
products. [a] NaOTf (1 equiv) was added. [b] NaNO (3 equiv) was
This finding is important, as anion exchange of diaryliodo-
nium salts can be tedious and waste producing. While the
exact role of NaOTf remains unknown, the additive effect
could be useful in other arylations where the counterion
influences the reaction.
2
used. [c] In CH Cl /H O 1:1. [d] 7 mmol scale.
2
2
2
[
4e–h,17]
2m, 2p). A strong ortho-effect
was observed in
The scope of the arene nitration was subsequently
reactions with ortho-substituted iodonium salts, which pro-
ceeded with excellent chemoselectivities (2e, 2g, 2h) despite
competing electronic preferences (2g). p-Methoxyphenyl
[15]
investigated with diaryliodonium salts 1 (Scheme 2). Tri-
flate, tetrafluoroborate, and tosylate salts could all be
employed, using NaOTf as an additive with the two latter
anions. Nitrobenzene (2a) and the halide-substituted prod-
ucts 2b–e were all isolated in high to excellent yields. The
reaction showed great tolerance for steric hindrance in the
ortho-position, as exemplified with the halide products 2d and
e, and with the high-yielding synthesis of the di-ortho-alkyl-
substituted nitroarenes 2 f and 2g. While the electron-rich
products 2h and 2i were obtained in moderate yields despite
[
17a,18]
(anisyl) or 2,4,6-trimethoxyphenyl (TMP)
dummy
groups were efficiently utilized in reactions yielding 2h, 2o,
2r, and 2s.
The envisioned one-pot sequence was next investigated.
Unfortunately, synthesis of the diaryliodonium salts in either
EtOAc or DME was low yielding, hence the one-pot
sequence was further studied in dichloromethane using our
2
[
15]
established methodology. Bromobenzene (3b) and 4-bro-
moiodobenzene (4b) were treated with mCPBA and TfOH to
yield the symmetric 4-bromophenyl iodonium salt 1b in situ
addition of 3 equivalents of NaNO , the synthesis of 2h was
2
improved to 79% yield by changing the solvent. Moderately
electron-donating substituents were well tolerated, and alkyl-
substituted salts were efficiently transformed into 2 f, 2g, 2j,
and 2k. The synthesis of 2k was easily scaled up to 7 mmol,
thus displaying the robustness of the reaction. Likewise,
products with electron-withdrawing substituents were
obtained in high yields (2l–o). More elaborate diaryliodo-
nium salts could also be utilized, as demonstrated in the
synthesis of the carbonyl-substituted products 2p–q, and in
the reactions with pyridyl iodonium salts to provide the
heterocycles 2r and 2s in 78 and 88% yield, respectively.
Unsymmetric diaryliodonium salts were utilized in several
[
19]
(Scheme 3a). Sodium nitrite and NaHCO3
were then
added, thus providing nitroarene 2b and unreacted diaryl-
[14d]
iodonium salt as the major components. Addition of water
and adjustment of the pH value improved the outcome, and
delivered 2b in 58% yield with 3 equivalents of NaNO
2
[
8]
within 3 hours. Only the p-substituted product 2b was
observed, and the complete regioselectivity can be attributed
to the known, highly p-selective formation of diaryliodonium
[
4e,f,h]
salts.
Considering the three steps involved in this sequential
one-pot reaction (oxidation, formation of 1b, and nitration to
2b), this metal-free CꢀH nitration of bromobenzene is quite
[
16]
reactions, and the phenyl group was an efficient “dummy”
group in the nitration of electron-deficient aryl moieties (2l,
efficient (Scheme 3a). The resulting iodoarene 4b was
2
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These are not the final page numbers!

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Chemie
as well as ortho-substituents are well tolerated. The p-azido
product 2o was obtained in an unsatisfactory yield, which is
[
8]
likely due to the moderate-yielding salt synthesis. Hetero-
cyclic products could also be obtained in a one-pot manner, as
exemplified by the synthesis of 3-nitropyridine (2r).
The application of this novel one-pot concept to other
nucleophiles is currently underway in our laboratory, and
[
23]
preliminary results with sodium azide
Scheme 4).
are promising
(
Scheme 4. Preliminary investigation with azide as a nucleophile
1
(
H NMR yield).
A radical mechanism is unlikely, as addition of the radical
scavengers, 1,1-diphenylethylene and TEMPO, to the nitra-
[8]
tion in EtOAc did not hamper the reaction significantly.
Hence, we suggest that the reaction proceeds by formation of
a T-shaped intermediate containing a NuꢀI bond (Scheme 5).
The nitrite nucleophile could either form an NꢀI or OꢀI
intermediate (A and B), followed by ligand coupling to
provide the N-arylated product 2a or O-arylated product 6
(not observed). While ligand coupling by the [1,2] pathway is
the only possibility with some nucleophiles, recent computa-
tional studies have found that four- or five-membered
transition states can be favored.
Scheme 3. One-pot nitrations using 4 (0.5 mmol), 3 (1.1 equiv),
mCPBA (1.1 equiv), TfOH (2–3 equiv). and NaNO (3 equiv). Yields
[17a,b,24]
2
are those of isolated products. [a] Nitration at 808C, with <5%
impurity. [b] TsOH·H O (1 equiv) used instead of TfOH. [c] 2d/
2
2
i>20:1. [d] Anisole (5 equiv) used.
isolated in 72%, thus showing the recyclability of this
component. In the same fashion, chlorobenzene was con-
verted into 2c in 55% with recovery of the iodoarene 4c in
similar yield, and benzene was transformed into nitrobenzene
Scheme 5. Mechanistic possibilities.
(
2a). One-pot CꢀH nitration of alkyl benzenes proved more
difficult, and the three-step reaction provided 2k in poor
The suggested pathways were studied by DFT computa-
[
8]
yield, which gratefully was considerably improved at higher
tions (Figure 1). Ligand exchange by a dissociative pathway
leads to a large increase in energy, whereas the associative
pathway via four-coordinated complexes, to give the inter-
mediates B1 and B2, is energetically favorable by 13 and
[
20]
temperature.
The synthesis of unsymmetric diaryliodonium salts (Ar ¼
Ar ) is often high yielding, hence we investigated the
nitration of mesitylene by in situ formation of a mesityl-
phenyl)iodonium salt (Scheme 3a). To our delight, the
1
6
2
[15]
ꢀ
1
25 kJmol , respectively. Ligand coupling via the three-
membered TS1 would give N-arylation, whereas coupling
via the three-membered TS2 or five-membered TS3 would
result in O-arylation. These pathways are, however, all too
high in energy to be plausible. The reaction is suggested to
give the N-arylated product 2a via the four-membered TS5
(
sterically hindered product 2 f was obtained with excellent
chemoselectivity (2a not observed). In all cases the iodoar-
enes 4 could be recovered in at least equivalent amounts from
the one-pot reactions, thus allowing reuse in subsequent
[
21]
°
ꢀ1
reactions.
(DG = 73 kJmol ), which has a lower energy barrier than
°
ꢀ1
The scope investigation was continued with a range of
unsymmetric diaryliodonium salts, thus delivering function-
alized nitroarenes in good yields with complete regioselec-
TS4 (DG = 87 kJmol ). The energy difference corresponds
to a 135:1 ratio in favor of 2a at 708C, thus explaining why 6 is
not observed.
[
8]
[
22]
tivity (Scheme 3b). Nitroarenes with electron-withdrawing
substituents were formed in excellent yields over the three
steps (2l, 2m). 1-Nitronaphthalene (2h), and the ortho-
halogenated products 2d and 2e were also obtained with high
efficiency, thus illustrating that electron-donating substituents
To conclude, nitroarenes have been efficiently synthesized
by a transition-metal-free arylation of sodium nitrite with
diaryliodonium salts. An interesting counterion effect was
observed, and the unprecedented use of NaOTf as an additive
enabled efficient arylations with such salts. The mechanism is
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ꢀ 2016 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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ꢀ1
ꢀ
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1
Keywords: arylation · density functional calculations ·
hypervalent compounds · oxidation · reaction mechanisms
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[
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2
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iodoarene on solid support for large-scale reactions. See: a) R.
Edwards, W. de Vries, A. D. Westwell, S. Daniels, T. Wirth, Eur.
J. Org. Chem. 2015, 6909 – 6916; b) D.-J. Chen, Z.-C. Chen,
Synlett 2000, 1175 – 1177.
1
2
[
22] The synthesis of unsymmetric diaryliodonium salts (Ar Ar IX)
generally requires combination of the more electron-deficient
Received: March 31, 2016
Revised: April 29, 2016
Published online: && &&, &&&&
2
1
Ar I with the more electron-rich Ar H, while arylation under
Angew. Chem. Int. Ed. 2016, 55, 1 – 6
ꢀ 2016 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
5
These are not the final page numbers!

Angewandte
Communications
Chemie
Communications
Hypervalent Compounds
M. Reitti, P. Villo,
B. Olofsson*
&&&& — &&&&
One-Pot CꢀH Functionalization of Arenes
Without a trace: Nitroarenes are obtained
in a sequential one-pot reaction from
arenes via diaryliodonium intermediates.
Aryl azides can also be synthesized with
this novel approach, where iodine(III)
reagents with two carbon ligands are
formed in situ from iodine(I), and sub-
sequently reacted with nucleophiles. The
presented one-pot method is an impor-
tant step towards catalytic reactions with
these hypervalent iodine reagents.
by Diaryliodonium Salts
mCPBA=m-chloroperbenzoic acid.
6
www.angewandte.org
ꢀ 2016 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2016, 55, 1 – 6
These are not the final page numbers!