Research Article
Photoinduced Iron-Catalyzed ipso-Nitration of Aryl Halides via
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ABSTRACT: A photoinduced iron-catalyzed ipso-nitration of aryl halides with
KNO has been developed, in which aryl iodides, bromides, and some of aryl
2
chlorides are feasible. The mechanism investigations show that the in situ
formed iron complex by FeSO , KNO , and 1,10-phenanthroline acts as the
4
2
light-harvesting photocatalyst with a longer lifetime of the excited state, and the
reaction undergoes a photoinduced single-electron transfer (SET) process. This
work represents an example for the photoinduced iron-catalyzed Ullmann-type
couplings.
KEYWORDS: photoinduced, iron catalyzed, ipso-nitration, aryl halides, SET mechanism
INTRODUCTION
In recent years, photopromoted Ullmann- and Buchwald-
type coupling reactions of aryl halides with various nucleophile
agents (such as heteroaromatic nitrogen nucleophiles, amines,
■
Aromatic nitro compounds and their derivatives are prevalent
synthons in pharmaceuticals, pesticides, functional materials,
and diverse intermediates in organic synthesis. Most studies
on the synthesis of aromatic nitro compounds have focused on
electrophilic nitration of arenes, leading to low region
selectivity and overnitration. To resolve this issue, ipso-
nitration of aryl bromides and iodides was performed to
produce these compounds via copper-catalyzed Ullmann-type
couplings (Scheme 1A). In recent years, the ipso-nitration of
aryl chlorides has been achieved under palladium-catalyzed
conditions by Buchwald and Fors (Scheme 1A). Compared
1
thiols, and phenols) have received significant attention
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(
Scheme 2). Furthermore, copper and nickel catalysts are
always employed as photoredox catalysts in the reactions.
However, until now, using iron complexes as photoredox
2
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catalysts for organic transformation has been rare. Very
recently, photopromoted iron-catalyzed Kumada coupling of
aryl halides with aliphatic Grignard reagents has been
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9a
developed by Noel
of the Grignard reagent as a strong electron donor (E = 0.4 V
̈
and Alcazar (Scheme 3) in the presence
́
4
ox
10
vs Mg RE.), and a two-electron transfer mechanism is
proposed. However, without strong electron donors as
substrates (for example, potassium nitrite as the substrate in
with expensive transition-metal catalysis, iron-catalyzed cou-
pling reactions are obviously cost-effective. However, iron-
catalyzed ipso-nitration of aryl halides has not been reported
thus far.
this work; E = 2.031 V vs saturated calomel electrode (SCE),
ox
Figure S1), it can be questioned whether the photopromoted
coupling of aryl halides with nitrites could be realized and
which mechanism (SET vs two-electron transfer) could be
preferred? Furthermore, the application of iron as an earth-
abundant metal in photocatalysis is highly desirable, but it is
In the aforementioned copper- or palladium-catalyzed ipso-
nitration of aryl halides, the mechanisms were proposed via an
oxidative addition/reductive elimination process (two-electron
3
,4
transfer). Indeed, iron complexes can engage in two-electron
5
transfer procedures. However, single-electron transfer (SET)
6
is competitive and preferred in some situations. Furthermore,
the formal valence states of iron range from −II to +VI, and
the limited stability of (organo)iron complexes with low or
Received: May 20, 2021
Revised: June 8, 2021
Published: July 16, 2021
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high oxidation states may lead to complex reactivity profiles.
Therefore, the iron-catalyzed ipso-nitration of aryl halides
could not be taken for granted, and mechanistic investigations
into iron catalysis are complicated.
©
2021 American Chemical Society
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ACS Catal. 2021, 11, 9561−9568