Alkoxy-substituted derivatives of π-conjugated acyclic anion receptors: Effects of substituted positions

Article abstract of DOI:10.1246/cl.2009.208

The synthesis, solid-state assemblies, and anion binding affinities of π-conjugated acyclic anion receptors with alkoxyphenyl substituents are discussed. The position of the alkoxy moiety is essential to determining not only the electronic state but also the anion binding affinity. Copyright

Full text of DOI:10.1246/cl.2009.208

208
Chemistry Letters Vol.38, No.3 (2009)
Alkoxy-substituted Derivatives of ꢀ-Conjugated Acyclic
Anion Receptors: Effects of Substituted Positions
Hiromitsu Maedaꢀ1;2 and Nazuki Eifuku1
1College of Pharmaceutical Sciences, Institute of Science and Engineering, Ritsumeikan University, Kusatsu 525-8577
2PRESTO, Japan Science and Technology Agency (JST), Kawaguchi 332-0012
(Received December 16, 2008; CL-081173; E-mail: maedahir@ph.ritsumei.ac.jp)
The synthesis, solid-state assemblies, and anion binding
whereas amphiphilic receptors that possess hydrophilic chains
(e.g., 1c) form solvent-assisted assemblies such as vesicular
structures in aqueous solution.6d Further, a ꢂ-alkyl-substituted
receptor 2a5b can be converted into various ꢀ-extended receptors
2b and 2c via an ꢁ-iodinated intermediate.6b,6c In order to con-
trol the anion binding affinities of acyclic receptors, both the
kinds of substituents on the aryl moieties and their positions
are essential. Here, we report the synthesis, solid-state assem-
blies, and anion binding affinities of acyclic anion receptors with
alkoxy units at the ortho, meta, and para positions of the side aryl
moieties (3o1, 3o8, 3m1, 3m8, 3p1, and 3p8; Figure 1c).
ꢁ-Alkoxy-substituted anion receptors (3o1, 3o8, 3m1, 3m8,
3p1, and 3p8) were synthesized in good yields by previously
reported procedures6a using the corresponding arylpyrroles as
starting materials. Chemical identifications for 3o1, 3o8, 3m1,
affinities of ꢀ-conjugated acyclic anion receptors with alkoxy-
phenyl substituents are discussed. The position of the alkoxy
moiety is essential to determining not only the electronic state
but also the anion binding affinity.
It is essential to control the binding properties for guest spe-
cies due to for example, the facile regulation of the conforma-
tions of ꢀ-conjugated oligomers and their assembled forms.1
Among the various available targets, inorganic and biotic anions
such as halides, acetates, and phosphates, ubiquitous in biology,
are essential, as seen in the activity of enzymes, transport of hor-
mones, protein synthesis, and DNA regulation.2,3 In contrast to
cyclic anion receptors with preorganized structures, acyclic re-
ceptors must dynamically change their conformations for bind-
ing.3e We have previously reported the synthesis of ꢀ-conju-
gated acyclic anion receptors (‘‘molecular flippers’’), BF2 com-
plexes of 1,3-dipyrrolyl-1,3-propanediones (e.g., 1a–1c and
2a–2c; Figure 1a),4–6 which bind anions using two pyrrole NH
groups and a bridging CH group with the inversion of the pyrrole
rings (Figure 1b). Using cross coupling reactions, various aryl
substituents can be introduced at the pyrrole ꢁ-positions. In fact,
we have reported the synthesis of ꢁ-aryl-substituted receptors
with long aliphatic chains (e.g., 1b), which constitute anion-re-
sponsive supramolecular organogels from octane solutions and
transition to solutions on the addition of appropriate anions,4b,6a
1
3m8, 3p1, and 3p8 were carried out by H NMR and MALDI-
TOFMS. The UV–vis absorption maxima (ꢃmax) of 3o1, 3m1,
and 3p1 in CH2Cl2 were observed at 513, 501, and 518 nm,
respectively, suggesting that alkoxy-substitution at the ortho
and para positions affords a red-shift while that at the meta
position has almost no effect in comparison with the ꢃmax value
of ꢁ-unsubstituted 1a (500 nm).6a Lengths of the alkoxy chains
give almost no effects on the electronic states. The order of these
ꢃ
max values is correlated to the order of the HOMO–LUMO gaps
(3o1, 3.007 eV; 3m1, 3.040 eV; 3p1, 2.969 eV) estimated by
DFT calculations.7 Further, 3o1, 3m1, and 3p1 are highly emis-
sive in CH2Cl2; the fluorescence wavelengths (and quantum
yields, ꢄF) for excitation at the corresponding ꢃmax are 542
(0.97), 533 (0.98), and 558 (0.95) nm, respectively.
Single-crystal X-ray analyses of methoxy-substituted 3o1,
3m1, and 3p1 elucidated the molecular structures along with
the formation of hydrogen-bonding dimers and ꢀ–ꢀ stacking
assemblies in the solid state (Figure 2).8 Ortho substituted 3o1
has the methoxy units facing the pyrrole NH, thus forming intra-
molecular N–HꢁꢁꢁO hydrogen bonds as observed in solution,
whereas the other receptors exhibit N–HꢁꢁꢁF–B interactions with
N(–H)ꢁꢁꢁF distances of 3.04 and 3.38 (3m1), and 3.15 and 3.16
˚
(3p1) A. Further, these methoxy-substituted receptors have
slightly distorted side aryl rings: the dihedral angles (minimum
and maximum values) between the aryl rings and the dipyrrolyl
core plane comprising 16 atoms are 12.1 and 18.5ꢂ for 3o1, 21.3
and 28.4ꢂ for 3m1, and 18.9 and 25.9ꢂ for 3p1. These values are
slightly smaller (3o1) than and comparable (3m1 and 3p1) to
those of 1a (20.0 and 28.6ꢂ). The smaller values for 3o1 are
due to the intramolecular hydrogen bonding between NH and
the oxygen atom from the methoxy group. The deviations in
˚
the mean plane consisting of 16 atoms are 0.11 A (3o1),
˚
0.042 A (3m1), and 0.043 A (3p1); these values are larger than
˚
that of 1a (0.035 A). Compared with 3m1 and 3p1, 3o1 has
edge-to-edge-like stacking assemblies.
˚
Figure 1. a) BF2 complexes of dipyrrolyldiketones 1a–1c, 2a–
2c; b) anion binding mode of 1a; c) alkoxy-substituted deriva-
tives (3o1, 3o8, 3m1, 3m8, 3p1, and 3p8).
Copyright Ó 2009 The Chemical Society of Japan

Chemistry Letters Vol.38, No.3 (2009)
209
alkoxy-substituted aryl moiety to the subunits of supramolecular
assemblies is currently underway.
This work was supported by Grant-in-Aid for Scientific
Research in a Priority Area ‘‘Super-Hierarchical Structures’’
(No. 19022036) from the MEXT and Ritsumeikan Global Inno-
vation Research Organization (R-GIRO) project (2008–2013).
The authors thank Prof. Atsuhiro Osuka and Mr. Shohei Saito,
Kyoto University, for the X-ray analysis, and Prof. Hitoshi
Tamiaki, Ritsumeikan University, for helpful discussions.
References and Notes
1
a) T. M. Swager, in Acetylene Chemistry, ed. by F. Diederich, P. J. Stang,
R. R. Tykwinski, Wiley-VCH, New York, 2005, Chap. 6. b) D. T.
McQuade, A. E. Pullen, T. M. Swager, Chem. Rev. 2000, 100, 2537.
a) Supramolecular Chemistry of Anions, ed. by A. Bianchi, K. Bowman-
Figure 2. Single-crystal X-ray structures (left, monomer or
hydrogen-bonded dimers; right, stacking structures) of a) 3o1,
b) 3m1, and c) 3p1. Atom color code: brown, pink, yellow,
green, blue, and red refer to carbon, hydrogen, boron, fluorine,
nitrogen, and oxygen, respectively.
2
´
James, E. Garcıa-Espan˜a, Wiley-VCH, New York, 1997. b) Fundamentals
and Applications of Anion Separations, ed. by R. P. Singh, B. A. Moyer,
Kluwer Academic/Plenum Publishers, New York, 2004. c) Anion Sensing
in Topics in Current Chemistry, ed. by I. Stibor, Springer-Verlag, Berlin,
2005, Vol. 255, p. 238. d) J. L. Sessler, P. A. Gale, W.-S. Cho, Anion
Receptor Chemistry, RSC, Cambridge, 2006.
Table 1. Binding constants (Ka, Mꢄ1) of 1a (as a reference),
3m1, 3m8, 3p1, and 3p8 with various anions in the form of
3
a) F. P. Schmidtchen, M. Berger, Chem. Rev. 1997, 97, 1609. b) P. D. Beer,
´
P. A. Gale, Angew. Chem., Int. Ed. 2001, 40, 486. c) R. Martınez-Ma˜nez, F.
a
TBA salts in CH2Cl2
´
Sancenon, Chem. Rev. 2003, 103, 4419. d) P. A. Gale, R. Quesada, Coord.
Ka (1a)b Ka (3m1) Ka (3m8) Ka (3p1) Ka (3p8)
Chem. Rev. 2006, 250, 3219. e) P. A. Gale, Acc. Chem. Res. 2006, 39, 465.
a) H. Maeda, Eur. J. Org. Chem. 2007, 5313. b) H. Maeda, Chem.—Eur. J.
2008, 14, 11274. c) H. Maeda, J. Inclusion Phenom. 2009, in press.
a) H. Maeda, Y. Kusunose, Chem.—Eur. J. 2005, 11, 5661. b) H. Maeda, Y.
Kusunose, Y. Mihashi, T. Mizoguchi, J. Org. Chem. 2007, 72, 2612. c) H.
Maeda, Y. Fujii, Y. Mihashi, Chem. Commun. 2008, 4285. d) H. Maeda, Y.
Haketa, Y. Bando, S. Sakamoto, Synth. Met. 2009, 159, in press.
a) H. Maeda, Y. Haketa, T. Nakanishi, J. Am. Chem. Soc. 2007, 129, 13661.
b) H. Maeda, Y. Haketa, Org. Biomol. Chem. 2008, 6, 3091. c) H. Maeda,
Y. Mihashi, Y. Haketa, Org. Lett. 2008, 10, 3179. d) H. Maeda, Y. Ito, Y.
Haketa, N. Eifuku, E. Lee, M. Lee, T. Hashishin, K. Kaneko, Chem.—Eur.
J. 2009, 15, in press.
Clꢄ
Brꢄ
30000
57000
(1.9)
5100
(1.8)
25000
(0.83)
2100
(0.75)
420000
(2.0)
38000
(0.53)
150
26000
(0.67)
2600
(0.82) (0.86)
140000 330000
(0.67)
27000
(0.38)
240
24000
(0.80)
2400
4
5
2800
ꢄ
CH3CO2 210000 490000
(2.3)
H2PO4
(1.6)
28000
(0.39)
180
(0.33)
6
ꢄ
72000
540
38000
(0.53)
270
ꢄ
HSO4
(0.50)
(0.28)
(0.44)
7
8
M. J. Frisch, et al. (see the Supporting Information), Gaussian 03 (Revision
C.01), Gaussian, Inc., Wallingford CT, 2004.
Crystal data for 3o1 (from CH2Cl2/hexane): C25H21BF2N2O4,
aThe values in the parentheses are the ratios of the Ka values to the
Ka value of 1a. Ref. 6a.
b
ꢀ
Mr ¼ 462:25, triclinic, P1 (no. 2), a ¼ 7:961ð6Þ, b ¼ 8:167ð5Þ,
c ¼ 18:427ð10Þ A, ꢁ ¼ 89:90ð3Þ, ꢂ ¼ 81:04ð2Þ, ꢅ ¼ 63:71ð3Þꢂ, V ¼
˚
The anion binding properties of the alkoxy-substituted re-
1
3
1058:0ð12Þ A , T ¼ 123ð2Þ K, Z ¼ 2, Dc ¼ 1:451 g/cm3, ꢆ(Mo Kꢁ) =
˚
ceptors have been examined by H NMR and UV–vis spectros-
0.110 mmꢄ1, 10356 reflections measured, 4803 unique (Rint ¼ 0:0684),
copy upon the addition of anions in the form of tetrabutylammo-
nium (TBA) salts. The binding constants (Ka) of 3m1, 3m8, 3p1,
and 3p8 for anions, as determined by UV–vis absorption spectral
changes (Table 1), in CH2Cl2 are almost comparable to those of
1a. There are no significant differences between methoxy- and
octyloxy-substituted receptors. The binding mode of for exam-
ple, 3p8 for anions has been deduced from the 1H NMR spectral
changes in CD2Cl2 (1 ꢃ 10ꢄ3 M) at 20 ꢂC in the presence of
2.0 equiv of TBACl: 1H NMR revealed downfield shifts for
the pyrrole NH, bridging CH, and o-CH from 9.57, 6.54, and
7.57 ppm to 12.26, 8.94, and 8.14 ppm, respectively. Further,
both o-CH sites of 3m1 and 3m8 associate with anions.9 In con-
trast, the intramolecular hydrogen bonding in ortho substituted
3o1 and 3o8 affords complicated binding modes in 1H NMR
along with low affinities for anions. This suggests that ortho
substituents can interfere with anion binding.
R1 ¼ 0:0630, wR2 ¼ 0:1286, GOF ¼ 1:026 (I > 2ꢇðIÞ). Crystal data for
.
3m1 (from CH2Cl2/hexane): C25H21BF2N2O4 CH2Cl2, Mr ¼ 547:17,
monoclinic, P21=a (no. 14), a ¼ 12:524ð5Þ, b ¼ 12:858ð4Þ, c ¼
3
16:237ð8Þ A, ꢂ ¼ 106:788ð18Þꢂ, V ¼ 2503:1ð18Þ A , T ¼ 123ð2Þ K,
Z ¼ 4, Dc ¼ 1:452 g/cm3, ꢆ(Mo Kꢁ) = 0.311 mmꢄ1, 23253 reflections
measured, 5699 unique (Rint ¼ 0:0822), R1 ¼ 0:0585, wR2 ¼ 0:1218,
GOF ¼ 1:049 (I > 2ꢇðIÞ). Crystal data for 3p1 (from CH2ClCH2Cl/
˚
˚
hexane): C25H21BF2N2O4, Mr ¼ 462:25, orthorhombic, Pbca (no. 61),
3
˚
˚
a ¼ 12:808ð3Þ, b ¼ 25:217ð6Þ, c ¼ 13:176ð2Þ A, V ¼ 4255:6ð15Þ A ,
T ¼ 123ð2Þ K, Z ¼ 8, Dc ¼ 1:443 g/cm3, ꢆ(Mo Kꢁ) = 0.109 mmꢄ1
,
34899 reflections measured, 4868 unique (Rint ¼ 0:1020), R1 ¼ 0:0420,
wR2 ¼ 0:0939, GOF ¼ 0:960 (I > 2ꢇðIÞ). Crystallographic data for 3o1,
3m1, and 3p1 have been deposited with Cambridge Crystallographic Data
Centre as supplementary publication no. CCDC-712987–712989. Copy of
the data can be obtained free of charge via www.ccdc.cam.ac.uk/conts/
retrieving.html (or from the Cambridge Crystallographic Data Centre,
12, Union Road, Cambridge, CB2 1EZ, UK; fax: +44 1223 336033; or
deposit@ccdc.cam.uk).
In summary, the positions of the substituent influences the
structures and properties, such as the organized structures and
anion binding behavior, of ꢀ-conjugated acyclic anion receptors
with alkoxyphenyl substituents. The introduction of a single-
9
Optimized structures of the receptors and anion binding complexes are
presented in the Supporting Information Ref 10.
10 Supporting Information is available electronically on the CSJ-Journal Web
site, http://www.csj.jp/journals/chem-lett/index.html.
Published on the web (Advance View) January 31, 2009; doi:10.1246/cl.2009.208