Angewandte
Communications
Chemie
Molecular Recognition
Hot Paper
Unidirectional Threading into a Bowl-Shaped Macrocyclic Trimer of
Boron–Dipyrrin Complexes through Multipoint Recognition
Abstract: Bowl-shaped macrocycles have the distinctive
feature that their two sides are differentiated, and thus can be
developed into elaborate hosts that fix a target molecule in
a controlled geometry through multipoint interactions. We now
report the synthesis of a bowl-shaped macrocyclic trimer of the
boron–dipyrrin (BODIPY) complex and its unidirectional
À
threading of guest molecules. Six polarized Bd+
F
d- bonds are
directed towards the center of the macrocycle, which enables
strong recognition of cationic guests. Specifically, the benzyl-
butylammonium ion is bound in a manner in which the benzyl
group is located at the convex face of the bowl and the butyl
group at its concave face. Furthermore, adrenaline was
strongly captured on the convex side of the bowl by hydrogen
À
bonding, Coulomb forces, and C H···p interactions.
U
nidirectional guest binding is an important and challeng-
ing topic, not only in terms of precise molecular recognition,
but also for the development of elaborate artificial nano-
machines that achieve regulated motions.[1] Macrocyclic
compounds have often been employed as components of
molecular machines, such as rotaxanes, and their shape is
a fundamental factor to create the functions.[2] In contrast to
planar molecules, nonplanar macrocyclic hosts can interact in
a different way with guests on the two sides. Classical
examples of nonplanar bowl-shaped macrocycles are cyclo-
dextrins[3] and calixarenes.[4] Their unsymmetrical structures
gave rise to unidirectional threading of designed axle
molecules of (pseudo)rotaxanes.[5,6] However, they often
suffer from inversion of the bowl because of their flexibility,
or from the two sides of the macrocycles being very similar.
These issues adversely affect the binding ability and precisely
regulated molecular motions.
Figure 1. a) Difference in shape of para-phenylene-linked cyclic dipyr-
rin/BODIPY trimers and meta-phenylene-linked ones. b) Meta-phenyl-
ene-linked L-shaped dipyrrin trimer 1 and bowl-shaped BODIPY trimer
2. TFA=trifluoroacetic acid, DDQ=2,3-dichloro-5,6-dicyano-1,4-benzo-
quinone.
(ca. 1208)[10] instead of para-phenylene ones (ca. 1808)[9,11]
would result in trimeric macrocyclic frameworks with non-
planar, distorted shapes, because the sum of the interior
angles of the macrocycle would not be suitable for the
formation of planar polygons (Figure 1a). Herein, we report
the synthesis of a meta-phenylene-linked dipyrrin trimer 1,
and a bowl-shaped BODIPY trimer 2 formed by the reaction
of 1 with BF3·Et2O (Figure 1b). We also investigated the
ability of 2 to form an oriented [2]pseudorotaxane by
unidirectionally threading an ammonium guest.
We focused on boron–difluoride complexes of dipyrrins
(BODIPY)[7] as a constituent unit of the macrocycles.[8] We
previously reported trimeric macrocycles of dipyrrin/
BODIPY linked through its 3,5-positions by para-phenylene
moieties.[9] The macrocycles had planar triangular shapes as
a corollary of the angle of each unit (Figure 1a). We
envisaged that the employment of meta-phenylene linkers
A dipyrrin trimer 1 was synthesized in 16% yield by the
TFA-catalyzed condensation of a 2,6-bis(2-pyrrolyl)anisole
(5) and mesitaldehyde. The 1H NMR spectrum of 1 recorded
at 238 K showed three signals for the methoxy protons, thus
suggesting that the cyclic trimer lost its apparent C3 symmetry
(Figure 2a,b). A single-crystal X-ray diffraction analysis
revealed the unique L-shaped structure of 1 (Figure 2c,d).
The two dipyrrin moieties (denoted as units 2 and 3 in
Figure 2a,b and depicted in blue and red, respectively) and
one methoxyphenyl ring (unit 3) deviated significantly from
the plane. Another characteristic is that one pyrrole ring of
[*] Dr. T. Nakamura, G. Yamaguchi, Prof. Dr. T. Nabeshima
Graduate School of Pure and Applied Sciences and Tsukuba Research
Center for Interdisciplinary Materials Science (TIMS)
University of Tsukuba
1-1-1 Tennodai, Tsukuba, Ibaraki 305-8571 (Japan)
E-mail: nabesima@chem.tsukuba.ac.jp
index.html
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a dipyrrin (unit 3) was inverted. All the H NMR signals of
1
1 were assigned on the basis of H-1H COSY and ROESY
NMR measurements (see Figures S4 and S5 in the Supporting
Information). Characteristic transannular ROE effects (e.g.
a3-b2’) indicated that the solution structure of 1 is basically the
same as that obtained by the X-ray analysis.
Supporting information and the ORCID identification number(s) for
Angew. Chem. Int. Ed. 2016, 55, 1 – 5
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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These are not the final page numbers!