Halogen Bonding Helicates Encompassing Iodonium Cations

Article abstract of DOI:10.1002/anie.201904817

The first halonium-ion-based helices were designed and synthesized using oligo-aryl/pyridylene-ethynylene backbones that fold around reactive iodonium ions. Halogen bonding interactions stabilize the iodonium ions within the helices. Remarkably, the distance between two iodonium ions within a helix is shorter than the sum of their van der Waals radii. The helical conformations were characterized by X-ray crystallography in the solid state, by NMR spectroscopy in solution and corroborated by DFT calculations. The helical complexes possess potential synthetic utility, as demonstrated by their ability to induce iodocyclization of 4-penten-1-ol.

Full text of DOI:10.1002/anie.201904817

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Accepted Article
Title: Halogen Bonding Helicates Encompassing Iodonium Cations
Authors: Alan Vanderkooy, Arvind Kumar Gupta, Tamás Földes, Sofia
Lindblad, Andreas Orthaber, Imre Pápai, and Mate Erdelyi
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201904817
Angew. Chem. 10.1002/ange.201904817
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10.1002/anie.201904817
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Entry for the Table of Contents
COMMUNICATION
Reactive halogen bonding helices:
Iodonium cations are stabilized via
three-centre, four-electron [N-I-N]+
halogen bonds with oligomeric pyridyl
ligands. Complexation of the iodonium
ions produces new helical structures
as predicted by DFT calculation and
confirmed by X-ray crystallography
and NOE spectroscopy. The helices
were also shown to be reactive in a
halofunctionalization reaction.
Alan Vanderkooy, Arvind Kumar Gupta,
Tamás Földes, Sofia Lindblad, Andreas
Orthaber, Imre Pápai, and Máté Erdélyi*
Page No. – Page No.
Halogen Bonding Helicates
Encompassing Iodonium Cations
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Halogen Bonding Helicates Encompassing Iodonium Cations
Alan Vanderkooy,[a] Arvind Kumar Gupta,[b] Tamás Földes,[c,d] Sofia Lindblad,[a] Andreas Orthaber,[b]
Imre Pápai,[c] and Máté Erdélyi*[a]
Abstract: We have designed and synthesized the first halonium ion
based helices by using oligo-aryl/pyridylene-ethynylene backbones
that fold around reactive iodonium ions. Halogen bonding
interactions stabilize the iodonium ions within the helices.
Remarkably, the distance between two iodonium ions within a helix
is shorter than the sum of their van der Waals radii. The helical
conformations were characterized by X-ray crystallography in the
solid state, by NMR spectroscopy in solution and corroborated by
DFT calculations. The helical complexes possess potential synthetic
utility, as demonstrated by their ability to induce iodocyclization of 4-
penten-1-ol.
located on the exterior of the helices. One notable exception is
the report by Ousaka et al. of manganese catalyzed
a
epoxidation inside an oligo(phenylene ethynylene) helix.[4a]
Motivated by the scarcity of examples utilizing the interior of
helical structures for reactivity, we decided to explore the effects
of positioning reactive iodonium ions in the interior of a helix
through the use of stabilizing [N–I–N]+ halogen bonds. This
approach is unique as helices have not previously been induced
by halonium cations.
N
I
N
I
N
I
Ph
Ph
BF4
BF4
BF4
Helices are indispensable secondary structures that serve
many functions in biochemistry. Additionally, helices derived
N
N
N
from synthetic polymers and oligomers have been the focus of
2]
enantioselective catalysis,[1] molecular recognition,[1a-c,
and
3]
selective molecular channels.[1a,
Helical conformations are
1
2
3
typically induced by non-covalent interactions such as hydrogen
bonding,[1a-c, 4] π-stacking,[1a-c, 5] and host-guest chemistry.[1a-c, 6]
Herein, we present synthetically applicable helices stabilized by
halogen bonds with halonium ions that occur in both solution
and in the solid state.
N
N
BF4
BF4
I
I
Halogen bonding is a noncovalent interaction between an
electron deficient halogen donor and a Lewis base.[7] Halogen
bonding has received growing attention in recent years for its
use in a wide range of applications including catalysis,[8]
molecular recognition,[8a, 8b, 9] medicinal chemistry,[8a, 8b, 10] and
self-assembly.[8a, 8b, 11] Halonium ions are strong halogen bond
donors that tend to form symmetric three-center, four-electron
(3c4e) bonds, for example an [N–I–N]+ halogen bond between
two nitrogen Lewis bases.[12] Such 3c4e halogen bonds have
been previously used to form supramolecular capsules,[13] and
for stabilization of halonium ions in synthetic reagents applied to
halonium transfer and oxidation reactions.[14]
N
N
Ph
Ph
4
Figure 1. Examples of [N–I–N]+ halogen bond forming systems. 1 and 2 have
been previously studied[12b,
12c, 15]
and are included here for comparison,
whereas 3 and 4 are first reported in this study.
Our work has been inspired by the helical metal complexes
reported by Bosch et al.,[16] and by our previous work on [N–I–
N]+ halogen bonds (Figure 1). Examples of halogen bonding
helices are sparse,[6b, 17] and most have only been confirmed in
the solid state. A noteworthy exception is the work of Massena
et al., who have produced solution stable, anion induced,
halogen bonding triple helices.[6b, 17a]
Polymer and oligomer helices have been widely used in
organic synthesis.[1] The active functionalities are typically
Multivalent complex 4 contains two [N–I–N]+ halogen bonds.
Complex 4 was predicted to fold into a helix by Density
Functional Theory calculations (see Supp. Info. for details). This
geometry is stabilized by the strong halogen bonds and by five
aryl-aryl interactions. Partially unfolded conformations of 4 were
also examined, but they were found to be at least 6 kcal/mol less
stable than the helix form.
Motivated by the computational prediction, we synthesized
ligands 5 and 6 following a strategy based on Sonogashira
couplings and deprotections (Supp. Info.). The silver(I)
complexes 7 and 8 were formed as precursors to 3 and 4,
following a previously described pathway (Scheme 1).[18] The
formation of the silver(I) and iodonium complexes in CD2Cl2
were associated with large 15N NMR coordination shifts, 60 ppm
and 90 ppm, respectively, consistent with previous reports.[12a, 15]
[a]
A. Vanderkooy, S. Lindblad, M. Erdélyi
Department of Chemistry - BMC
Uppsala Universitet
Husargatan 3, 752 37, Uppsala, Sweden
email: mate.erdelyi@kemi.uu.se
A. K. Gupta, A. Orthaber
[b]
[c]
Department of Chemistry – Ångström Laboratory
Uppsala Universitet
Lägerhyddsvägen 1, 751 20, Uppsala, Sweden
T. Földes, I. Pápai,
Institute of Organic Chemistry, Research Center for Natural
Sciences, Hungarian Academy of Sciences
Magyar tudósok körútja 2, H1117 Budapest
Budapest, Hungary
[d]
T. Földes
Present address: Department of Chemistry, King’s College London,
London SE1 1DB, U.K.
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10.1002/anie.201904817
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Single crystals of 3, 4, and 8 were grown by vapor diffusion of
hexane into solutions of either dichloromethane or 1,2-
dichloroethane at reduced temperature. Single crystal X-ray
analysis corroborated the predicted helical structures (Figure 2,
Table 1, and Supp. Info.). The crystals of the [N–I–N]+ helical
complex 3 and of the [N–Ag–N]+ complex 8 contained both
helical enantiomers, whereas the enantiomeric helices of 4
exhibited homochiral self-sorting. The crystal structure of 4 is
similar to the computationally predicted conformation (Figure 2).
Bond distances and angles were in agreement with those
reported for the related complex 1 (Table 1),[19] and notably the
observed [N–I–N]+ angles were consistent with the linear
geometry of halogen bonds. The range of distances between
aromatic centroids deviates only slightly from the generally
accepted range for π-stacking interactions.[20] We have also
observed an edge-to-face type interaction for 8, with one of the
terminal phenyl groups showing a 37° angle with the overlying
aromatic plane and a short, 2.827 Å, CH-π distance. Such edge-
to-face type interactions involving the terminal aryl groups might
also occur in solution for all of the helices.
dispersion forces acting between the adjacent aryl and
ethynylene groups of 4. Furthermore, the electron deficiency of
aromatic rings is known to strengthen π-stacking interactions,[24]
which also contribute to the stabilization of the helical
conformation.
The remarkably short distance between the two charged
iodonium ions of 4 is worth noting. The distance between the
iodoniums is approximately 0.1 Å less than the sum of the van
der Waals radii (rvdW = 1.98 Å as proposed by Bondi),[21] and also
less than the sum of the cationic radii as calculated by Rahm et
al. (r= 2.21 Å).[22] Despite the short distance, the Mayer bond
order[23] between the iodine atoms was calculated to be small
(BO = 0.01), suggesting negligible orbital overlap between the
iodoniums. The proximity of the iodonium ions could be
facilitated by the redistribution of the positive charge throughout
the conjugated π-system. Indeed, Natural Population Analysis
revealed the atomic charge on the iodonium ions of 4 to be
reduced (QI = +0.45), however, the electrostatic interaction
between the two [N–I–N]+ units is still repulsive (Supp. Info.).
Computations reveal that the enhanced stability and the
compact nature of the helical structure arises from attractive
Figure 2. Top: X-ray crystal structure of complex 4. Bottom: Superimposition
of the DFT computed conformation (green) of complex 4 with the X-ray crystal
structure (red).
N
N
N
N
I2
AgBF4
DCM
BF4
BF4
Ag
I
DCM
N
N
5
7
3
N
N
N
N
BF4
BF4
BF4
BF4
Ag
I
I2
AgBF4
DCM
N
N
N
N
N
N
DCM
Ag
I
N
N
8
4
6
Scheme 1. Formation of the helical iodonium complexes.
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Table 1. X-ray crystallographic data: selected bond lengths and angles
Complex
r(N–X) (Å)
r(X–X) (Å)
σ(N–X–N) (°)
r(Ar–Ar) (Å) [a]
8[b] (X = Ag)
1[d] [19] (X = I)
3[e] (X = I)
4[f] (X = I)
2.146(5) to 2.165(5)
2.255(3) to 2.261(3)
2.256(5) to 2.316(5)
2.248(5) to 2.302(5)
3.5541(8) to 3.6351(8)
174.9(2) to 178.7(2)
177.66(12) to 180.0
174.1(2) to 176.17(18)
175.26(18) to 177.03(17)
3.628(4) to 4.067(7) [c]
-
-
-
3.678(4) to 3.972(5)
3.641(3) to 3.914(4)
3.8621(7) to 3.887(1)
[a] Distances between the centroids of overlying intramolecular aryl groups. [b] Two slightly different helices occur in the crystal structure of 8. [c] The range
does not include the distance for the one instance of the CH-π interaction (Supp. Info.) [d] The unit cell of 1 contains multiple complexes. Data from
reference [19]. [e] The unit cell of 3 contains multiple complexes. [f] The range of values is given for both the M and the P helices, which were measured at
temperatures of 150 and 100 K respectively.
The helicity of 4 and 8 in solution was confirmed by NOE
correlations indicating dipolar relaxation between hydrogens of
distinct pyridyl groups (Figure 3 and Supp. Info.). Such proximity
of the pyridines implies a helical conformation, considering the
in a similar fashion to Barluenga’s reagent (1). The helical
structure of the complexes was demonstrated by X-ray
crystallography in the solid state, NMR spectroscopy in solution
and corroborated by computation on the DFT level. The distance
between iodonium ions in 4 is remarkably small, less than the
sum of the van der Waals radii. Our results extend the
possibilities for both the modulation of halonium transfer
reagents and for the exploitation of the interior of helices as
reaction environments.
1
symmetry of the complexes indicated by H, 13C, and 15N NMR.
NOE correlations also show evidence for a helix conformation of
ligand 6. DFT calculations found the helix to be a plausible
conformation of 6 (Supp. Info.).
N
I
Acknowledgements
N
H2
H
The authors would like to acknowledge funding sources
including: the Natural Sciences and Engineering Research
Council of Canada’s Postdoctoral Fellowship award (516910-
2018), and FORMAS (2017-01173). This study made use of the
NMR Uppsala infrastructure, which is funded by the Department
of Chemistry - BMC and the Disciplinary Domain of Medicine
and Pharmacy.
N
I
N
H1
H
H1
H2
Keywords: halogen bond, helix, halocyclization, iodonium ion,
.5
8.3
8.1
7.9
7.7
7.5
f1 (ppm)
7.3
7.1
6.9
6.7
6.
N–X–N, 3c4e, three-center, four-electron
Figure 3. One-dimensional 1H NOESY NMR spectrum of iodonium complex 4.
The triplet at 7.86 ppm is evidence for the close proximity of the two pyridyl
groups. The spectrum was obtained with selective excitation centered at 8.14
ppm.
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