Communication
doi.org/10.1002/chem.202101961
Chemistry—A European Journal
N-Heterocyclic Iod(az)olium Salts – Potent Halogen-Bond
Donors in Organocatalysis
Abstract: This article describes the application of N-
heterocyclic iod(az)olium salts (NHISs) as highly reactive
organocatalysts. A variety of mono- and dicationic NHISs
are described and utilized as potent XB-donors in halogen-
bond catalysis. They were benchmarked in seven diverse
test reactions in which the activation of carbon- and metal-
chloride bonds as well as carbonyl and nitro groups was
achieved. N-methylated dicationic NHISs rendered the high-
est reactivity in all investigated catalytic applications with
reactivities even higher than all previously described
monodentate XB-donors based on iodine(I) and (III) and the
strong Lewis acid BF3.
Figure 1. Selected examples of XB-donors based on iodine(I) with cationic N-
heterocyclic backbones (a) and iodine(III) (b).
Halogen bonding (XB) is a non-covalent interaction between an
electrophilic halogen donor (XB-donor) and a Lewis basic
acceptor (XB-acceptor). Halogen bonds are important intermo-
lecular interactions which find widespread applications in
crystal engineering, functional materials and in molecular
recognition.[1–10] In recent years XB was found to be an
innovative concept in organic synthesis and in this regard XB-
donors have been established as versatile catalysts.[11–14] The
vast majority of XB-donors are based on monodentate iodine(I)
derivatives with either a polyfluorinated or a N-heterocyclic
backbone, for example, the triazolium derivative 1 or the
pyridinium derivative 2 (Figure 1a), with cationic species being
typically more reactive than neutral derivatives.[15–23] Bidentate
XB-donors such as imidazolium 3 have been described as well
and were found to have a significantly higher reactivity than
monodentate analogues.[24]
attention as XB-donors, due to the high Lewis acidity of the
hypervalent iodine atom.[25] This Lewis acidity was severely
investigated in a plethora of theoretical[26–28] and experimental
studies.[29] After an initial report in 2015 by Han and Liu about
the use of diaryliodonium salts as catalysts for a solvent-free
Mannich reaction,[30] Huber and co-workers investigated iodo-
lium salts
4 (Figure 1b) in the Ritter-type solvolysis of
benzhydryl chloride and [4+2] cycloadditions.[31] Further XB-
mediated halide abstractions for the initiation of a cationic
polymerization[32] or for the activation of a metal halogen bond
followed.[33]
Although these iodolium salts only act as monodentate XB-
donors, their performance in most reactions is comparable to
bidentate iodine(I) derivatives, underlining their high potential.
Thiophene-based bidentate iodolium salt 5 as well as perfluori-
nated iodolium salt 6 were recently described by Huber and co-
workers, which so far show the highest reactivity among all
literature-described iodine(I) and iodine(III) XB-donors.[34,35]
In our group, we are strongly interested in the influence of
N-heterocyclic substituents on the chemical properties of
hypervalent iodine compounds, in particular aryl-λ3-
iodanes.[36–41] We focus our investigations on the stability and
the reactivity of these so far underrepresented reagents, always
in direct comparison to well-established non-stabilized or O-
stabilized derivatives. In this regard, we recently introduced N-
heterocycle-stabilized iodanes (NHIs) and found a remarkable
reactivity of these reagents which outcompetes well known
iodanes in a plethora of oxidative couplings - even in catalytic
applications (Figure 2a).[42,43] We also developed chiral N-hetero-
cycle-substituted iodoarenes as chiral iodane precursors and
applied them in a plethora of highly enantioselective couplings
Besides iodine(I) species, hypervalent iodine(III) derivatives,
in the form of (cyclic) diaryliodonium salts, received growing
[a] A. Boelke, T. J. Kuczmera, Prof. Dr. B. J. Nachtsheim
Institut für Organische und Analytische Chemie
Universität Bremen
Leobener Straße NW2C, 28359 Bremen (Germany)
E-mail: nachtsheim@uni-bremen.de
[b] Dr. E. Lork
Institut für Anorganische Chemie und Kristallographie
Universität Bremen
Leobener Straße NW2C, 28359 Bremen (Germany)
Supporting information for this article is available on the WWW under
© 2021 The Authors. Published by Wiley-VCH GmbH. This is an open access
article under the terms of the Creative Commons Attribution License, which
permits use, distribution and reproduction in any medium, provided the
original work is properly cited.
Chem. Eur. J. 2021, 27, 1–8
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© 2021 The Authors. Published by Wiley-VCH GmbH
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