Synthesis and conformational properties of nonsymmetric pillar[5]arenes and their acetonitrile inclusion compounds

Article abstract of DOI:10.1002/ejoc.201000718

The catalytic cyclocondensation of 1-butoxy-4-methoxy-2,5- bis(methoxymethyl)benzene (1d) affords a statistical mixture of the regioisomeric pillar[5]arenes 3a-d in high yield. The alkoxy groups are arranged stereoselectively in a mode so that they avoid

Full text of DOI:10.1002/ejoc.201000718

FULL PAPER
DOI: 10.1002/ejoc.201000718
Synthesis and Conformational Properties of Nonsymmetric Pillar[5]arenes and
Their Acetonitrile Inclusion Compounds
Yuhui Kou,[a] Hongqi Tao,[a] Derong Cao,*[a] Zhiyong Fu,[a] Dieter Schollmeyer,[b] and
Herbert Meier*[b]
Keywords: Conformation analysis / Cyclocondensation / Cyclooligomerization / Host-guest chemistry / Regioselectivity
The catalytic cyclocondensation of 1-butoxy-4-methoxy-2,5-
bis(methoxymethyl)benzene (1d) affords a statistical mixture
of the regioisomeric pillar[5]arenes 3a–d in high yield. The
alkoxy groups are arranged stereoselectively in a mode so
that they avoid steric interactions. The rotation of the benz-
ene rings is, at room temperature, fast in terms of the NMR
timescale and leads to a de facto Cs symmetry for 3a–c and
a C5h symmetry for 3d. All four structural isomers can encap-
sulate two CH3CN guest molecules. The structure determi-
nations are based on four crystal structure analyses (consti-
tutions) and NMR spectroscopic measurements (conforma-
tions).
compounds 1 (R1 ϶ R2), which have the reacting alk-
oxymethyl groups in nonequivalent positions of the benzene
ring, behave. Can regio- and/or stereoselectivity be observed
or not?
Introduction
Macrocyclic hosts, such as crown ethers,[1] cyclodex-
trins,[2] cucurbiturils,[3] and calixarenes,[4] play an outstand-
ing role in host–guest chemistry. The recently obtained pil-
lararenes[5,6] represent a novel type of macrocyclic hosts.
They are para-bridged analogues of calixarenes, which have
meta-bridges. [1.1.1.1.1]Paracyclophane[7] is the parent
compound of pillar[5]arenes.
Results and Discussion
We published in a short paper a catalytic process that
permits the efficient synthesis of pillar[5]arenes and the first
access to pillar[6]arenes.[8] Symmetrical 1,4-dialkoxy-2,5-
bis(ethoxymethyl)benzenes 1a–c react smoothly in CH2Cl2
at room temperature in the presence of p-toluenesulfonic
acid to afford pillar[5]arenes 2a–c (86–95% yield). Scheme 1
shows this novel type of cyclocondensation reaction. The
crucial question was then how nonsymmetrical starting
Synthesis
We investigated in this context the reactivity of 1-butoxy-
4-methoxy-2,5-bis(methoxymethyl)benzene (1d). Scheme 2
reveals that 1d can perform three different types (A–C) of
condensation reactions for the formation of the CH2 brid-
ges between the benzene rings. In principle, four arrange-
ments of three types of links, A, B, and C, can exist in a
cyclopentamer: A5, A3BC, ABACA, and ABCBC. Obvious
restrictions are that for each type B, the ring must contain
a type C and vice versa, and two types B as well as two
types C can never be direct neighbors or neighbors sepa-
rated by An. Scheme 3 illustrates the four possible re-
gioisomeric cyclopentamers of 1d. The alkoxy groups are
drawn in such a way that no steric interaction between the
two alkoxy groups in the ortho position of the CH2 bridges
exists. The inner alkoxy groups can be oriented upwards
and the outer alkoxy groups downwards or opposite.
When we assume that there is no electronic or steric pref-
erence for the reaction of 1d in the 2- or 5-position, which
means no regioselectivity of the condensation reaction, then
we can expect a product distribution 3a/3b/3c/3d = 5:5:5:1.
This mere statistical ratio was strongly supported by the
experimental distribution 3a/3b/3c/3d = 4.1:4.4:5.0:1.0. The
total yield of isolated regioisomers 3a–d amounted to
79.7%.
Scheme 1. Catalytic cyclocondensation reaction of symmetric start-
ing compounds 1 to pillar[5]arenes 2.
[a] School of Chemistry and Chemical Engineering,
South China University of Technology,
Guangzhou 510640, P. R. China
E-mail: drcao@scut.edu.cn
[b] Institute of Organic Chemistry, University of Mainz,
Duesbergweg 10-14, 55099 Mainz, Germany
E-mail: hmeier@uni-mainz.de
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Nonsymmetric Pillar[5]arenes and their Acetonitrile Inclusion Compounds
mined on the basis of the four crystal structures. In solu-
tion, pillar[5]arenes 3a–d display an average planar confor-
mation on the NMR timescale, because the pillar confor-
mations can exchange rapidly, when the 1,4-phenylene units
turn so that the OCH3 groups can preferentially move
through the cavity.
Isomer 3d gave the simplest NMR spectra. Its number of
1H and 13C NMR signals corresponds exactly to one re-
peating unit, which means that 3d must have a highly sym-
metrical structure with a C5 axis. Moreover, the geminal
CH2 bridge protons as well as the geminal OCH2 protons
are chemically equivalent. Figure 1 and Table 1 show part
of the aliphatic region of the 1H NMR spectra of 3a–d.
Highly symmetrical isomer 3d (C5h) displays a singlet for
the five OCH3 groups (δ = 3.659 ppm) and a singlet for the
five CH2 bridges (δ = 3.748 ppm). For the less-symmetrical
regioisomers 3a–c (Cs), both singlets are split into five sing-
lets each. These singlets are partially overlapping, but their
superposition is less than the superposition of the 10 signals
of the aromatic protons or the 5 triplet signals of the OCH2
groups (see the Experimental Section).
Scheme 2. Formation of different types (A, B, C) of CH2 bridges
in condensation reactions of 1d (the numbering corresponds to
starting compound 1d).
The region 3.6 Ͻ δ(1H) Ͻ 3.8 ppm can be used to distin-
guish the four regioisomers.[9] An analogous splitting is ob-
The regioisomers could be separated by simple column served in the 13C NMR spectra. An exact structure corre-
chromatography. The sequence a–d corresponds to the se- lation is only possible on the basis of the four crystal struc-
quence of the column elusion. The structures were deter- ture analyses shown below.
Scheme 3. Possible regioisomers 3a–d, which can be obtained by the fivefold cyclocondensation reaction of 1d.
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Y. Kou, H. Tao, D. Cao, Z. Fu, D. Schollmeyer, H. Meier
FULL PAPER
plained by the fact that this is the only conformer of 3d in
which no steric interaction exists between the alkoxy groups
of neighboring benzene rings. ROESY measurements reveal
the absence of such interactions. The NMR spectra of 3a–
c were interpreted in the same way.
X-ray Studies
Due to the alkoxy groups, the pillar[5]arenes have high
electron density in the pillar area. Therefore, cations or elec-
tron-deficient compounds represent suitable guest mole-
cules. We crystallized compounds 3a–d in CH3CN and ob-
tained crystals, which were suitable for X-ray studies. Fig-
ures 2, 3, 4, and 5 show ORTEP plots of the obtained struc-
tures, which confirm exactly the regio- and stereochemistry
that was supposed in Scheme 3. The arrangement of the
alkoxy groups could lead to 4 and 16 diastereomeric pairs
of enantiomers for 3d and 3a–c, respectively, out of which
just one was realized in each regioisomer. All compounds
contain two guest molecules of CH3CN.
1
Figure 1. H NMR subspectra of 3a–d in CDCl3.
Table 1. Singlet signals of the CH2 bridges and OCH3 groups in
1
the H NMR spectra of isomers 3a–d (δ values in CDCl3 relative
to TMS as internal standard).
Compound
CH2
3a
3b
3c
3d
3.730
3.730
3.751
3.777
3.777
3.640
3.646
3.652
3.652
3.652
3.721
3.746
3.746
3.746
3.778
3.639
3.646
3.656
3.656
3.656
3.736
3.762
3.762
3.762
3.789
3.681
3.670
3.670
3.664
3.656
3.748
OCH3
3.659
Enantiotopic CH2 protons require a symmetry plane
through their carbon atoms. Therefore, we have to postulate
pillararene structures with a rotation of the benzene rings,
which is fast in terms of the NMR timescale. Structure 3d
(Scheme 3) has all OCH3 groups on one side and all OC4H9
chains on the other side of the macrocyclic ring. This con-
former corresponds to the crystal structure. If this structure
will be “frozen” in solution, it would have a C5 axis as well,
but all four proton pairs of different CH2 groups would be
diastereotopic and yet isochronous, which is unlikely. The
Figure 2. Crystal structure of pillar[5]arene 3a·2CH3CN, which
contains two slightly different molecules: (a) view from above and
preference for the conformer with C5 symmetry can be ex- (b) view from the side.
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Nonsymmetric Pillar[5]arenes and their Acetonitrile Inclusion Compounds
Figure 3. Crystal structure of pillar[5]arene 3b·2CH3CN: (a) view
from above and (b) view from the side.
Figure 4. Crystal structure of pillar[5]arene 3c·2CH3CN: (a) view
from above and (b) view from the side. C60-63–N1-2 is one disor-
der CH3CN.
Related to the pillar structure, the electropositive methyl
carbon atoms are in the inner region of the cavity and the
electronegative nitrogen atoms point outside.
Conclusions
Figure 6 illustrates the elementary cell of 3c·2CH3CN. It
Nonsymmetric hydroquinone derivative 1d in the cyclo-
confirms the distinct assignment of the pair of acetonitrile condensation reaction with p-toluenesulfonic acid as the
guests to each host molecule in the stack. catalyst gave a mixture of regioisomeric pillar[5]arenes
The molecular structures of 3a–d are more or less regular (79.7%). Separation by column chromatography afforded
pentagons, which can be characterized by their equal side four possible isomers 3a–d in a ratio that corresponds al-
lengths ᐉ, their angles α, which are close to 108 °C, and most to the mere statistic ratio of 1:5:5:5. Structure determi-
their diameters d1 and d2, which determine the cavity. nation (constitution and conformation) was based on crys-
Table 2 depicts these values for 3b–d (regioisomer 3a has tal-structure analyses and spectroscopic methods (1H
similar parameters but it exists in the crystal in two slightly NMR, 13C NMR, ROESY, MS: MALDI-TOF).
different molecules). The planes of the benzene rings are
On the basis of the here-discussed methoxy–butoxy sys-
almost perpendicular (β ca. 85°) to the plane of the macro- tems and their NMR spectroscopic studies, it is evident that
cycle, which is defined by the carbon atoms of the CH2 the reaction of 1-ethoxy-4-methoxyhydroquinone with pa-
[10]
bridges. The diameters d1 and d2 are in the range of 800 raformaldehyde in the presence of BF3·O(C2H5)2 should
and 900 pm, respectively, which resembles the diameter of be reinvestigated to distinguish between structural isomers
narrow nanotubes.
and stereoisomers.
Eur. J. Org. Chem. 2010, 6464–6470
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Y. Kou, H. Tao, D. Cao, Z. Fu, D. Schollmeyer, H. Meier
Table 2. Selected data of the crystal structures.
FULL PAPER
Crystallization of the obtained nonsymmetric pillar[5]-
arenes in acetonitrile yielded host–guest complexes with
two CH3CN molecules in the cavities (four crystal structure
analyses). The arrangement of the alkoxy chains in the crys-
tals corresponds to the least steric interaction of these
chains. Thus, 1 of 4 possible diastereomers is realized for
3d and 1 of 16 diastereomers for 3a–c each.
At room temperature in solution, the benzene rings show
fast rotation in terms of the NMR timescale. Therefore, 3a–
d exhibit an average planar conformation.
Experimental Section
General: 1H NMR (400 MHz) and 13C NMR (100 MHz) spectra
were recorded with a Bruker DRX 400 spectrometer by using
CDCl3 as the solvent and TMS as an internal standard. Mass spec-
tra (GC–MS) were recorded with an Agilent GC–MS-t5975 mass
spectrometer. Matrix-assisted laser desorption ionization time-of-
flight (MALDI-TOF) mass measurements were performed with an
Autoflex III smartbeam spectrometer. TLC was performed by
using commercial Merck silica gel plates (GF254), and visualiza-
tion was effected at 254 nm.
Figure 5. Crystal structure of pillar[5]arene 3d·2CH3CN: (a) view
from above and (b) view from the side.
1-Butoxy-4-methoxy-2,5-bis(methoxymethyl)benzene (1d): A mix-
ture of 1-butoxy-4-methoxy-2,5-bis(chloromethyl)benzene[11,12]
(3.1 g, 11.2 mmol) and NaOCH3 (5.13 g, 95.0 mmol) was heated at
reflux in CH3OH (60 mL) for 3–4 h. The mixture was concentrated,
treated with H2O (60 mL), and extracted with CH2Cl2 (3ϫ15 mL).
The combined organic phases were dried (Na2SO4), concentrated,
and recrystallized from C2H5OH. Colorless solid, yield 2.4 g (80%),
1
m.p. 41–42 °C (ethanol). H NMR (400 MHz, CDCl3): δ = 0.95 (t,
3J = 7.4 Hz, 3 H, CH3), 1.47 (m, 2 H, CH2), 1.72 (m, 2 H, CH2),
3.40 (s, 3 H, OCH3), 3.41 (s, 3 H, OCH3), 3.80 (s, 3 H, 4-OCH3),
3
3.94 (t, J = 6.4 Hz, 2 H, OCH2), 4.46 (s, 2 H, Ar-CH2), 4.48 (s, 2
H, Ar-CH2), 6.89 (s, 1 H, Ar-H), 6.91 (s, 1 H, Ar-H) ppm. 13C
NMR (100 MHz, CDCl3): δ = 13.9 (CH3), 19.3 (CH2), 31.5 (CH2),
56.1 (Ar-OCH3), 58.3, 58.4 (OCH3), 68.6 (OCH2), 69.2, 69.3 (Ar-
CH2O), 111.1, 112.7 (C-3, C-6), 126.1, 126.7 (C-2, C-5), 150.4,
150.9 (C-1, C-4) ppm. GC–MS: m/z (%) = 268 (72) [M]+·, 180 (100),
150 (90), 121 (25).
Figure 6. Arrangement of 3c in the elementary cell.
General Procedure for the Preparation of Pillar[5]arenes 3a–d: A
mixture of 1d (3.22 g, 12.0 mmol) and p-toluenesulfonic acid hy-
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Nonsymmetric Pillar[5]arenes and their Acetonitrile Inclusion Compounds
Table 3. Details of the X-ray crystal structure analyses of pillar[5]arenes 3a–d with encapsulated acetonitrile.
3a
3b
3c
3d
Empirical formula
Formula weight
Crystal system
space group
a [Å]
b [Å]
c [Å]
C60H80O10·2CH3CN
1043.35
C60H80O10·2CH3CN
1043.35
C60H80O10·2CH3CN
1043.35
C60H80O10·2CH3CN
1043.35
triclinic
triclinic
triclinic
triclinic
¯
¯
¯
¯
P1
P1
P1
P1
14.600(4)
21.059(6)
21.442(6)
71.102(4)
80.949(5)
77.290(4)
6058(5)
4
11.9884(14)
12.2548(14)
21.975(2)
82.887(2)
76.674(2)
77.049(2)
3052.7(6)
2
12.371(3)
15.814(4)
16.184(4)
80.729(4)
80.629(4)
83.087(4)
3068.5(12)
2
12.498(3)
14.169(3)
18.250(4)
78.225(3)
78.502(3)
75.391(3)
3024.0(12)
2
α [°]
β [°]
γ [°]
V [Å3]
Z
Dcalcd [gcm–3]
µ [mm–1]
F(000)
1.144
0.08
2256
1.135
0.076
1128
1.129
0.075
1128
1.144
0.076
1124
θ Range [°]
Reflections collected
R(int)
R1,wR[IϾ2σ(I)]
R1, wR(all data)
1.20–25.00
44932
0.1288
0.1084, 0.2674
0.276, 0.3648
1.71–25.20
22175
0.0209
0.0758, 0.2127
0.1113, 0.2584
2.58–25.03
7293
0.0070
0.0765, 0.2136
0.0894, 0.2351
2.65–25.03
21020
0.0250
0.0868, 0.2286
0.1089, 0.2537
drate (288 mg, 1.5 mmol) was stirred in CH2Cl2 (360 mL) at room (m, 10 H, OCH2); 6.766 (s), 6.780 (s), 6.787 (s), 6.795 (s), 6.818 (s),
temperature for 6–8 h. Water (100 mL) was added, and the water
layer was extracted with CH2Cl2 (3ϫ 25 mL). The combined or-
ganic phases were dried (Na2SO4) and portionwise (up to five por-
tions) separated by column chromatography [3ϫ60 cm SiO2, petro-
leum ether (b.p. 60–90 °C)/ethyl acetate 400:1] to afford 3a, 3b, 3c,
and 3d in a total yield of 79.7% and a ratio 4.1:4.4:5.0:1.0. When
too large portions of the reaction mixture were given on the col-
umn, mixed fractions – in particular between 3a and 3b were ob-
tained and the chromatography had to be repeated.
6.824 (s), 6.830 (s) (10 H, Ar-H) ppm. 13C NMR (100 MHz,
CDCl3): δ = 14.0 (CH3), 19.5 (CH2), 29.3, 29.4, 29.5 (CH2-bridge),
32.0 (CH2), 55.5, 55.6 (OCH3), 68.0, 68.1 (OCH2), 113.9, 114.7,
114.9 (Ar-H), 127.9, 128.1, 128.1, 128.2, 128.3 (Ar-Cq), 150.0, 150.5
(Ar-CqO) ppm. HRMS (MALDI-TOF): calcd. for C60H80O10
[M]+ 960.5844; found 960.6167.
Pillar[5]arene 3d: Colorless crystals, yield 126 mg (5.5%), m.p. 161–
162 °C (ethyl acetate/ethanol). 1H NMR (400 MHz, CDCl3): δ =
0.95 (t, 3J = 7.4 Hz, 15 H), 1.51 (m, 10 H, CH2), 1.75 (m, 10 H,
Pillar[5]arene 3a: Colorless crystals, yield 516 mg (22.4%), m.p.
142–144 °C (ethyl acetate/ethanol). 1H NMR (400 MHz, CDCl3):
δ = 0.91–0.96 (m, 15 H, CH3); 1.47–1.54 (m, 10 H, CH2), 1.72–
1.75 (m, 10 H, CH2), 3.640 (s), 3.646 (s), 3.652 (s) (15 H, OCH3);
3.730 (s), 3.751 (s), 3.777 (s) (10 H, CH2-bridge); 3.80–3.85 (m, 10
H, OCH2); 6.762 (s), 6.766 (s), 6.774 (s), 6.784 (s), 6.794 (s) (10 H,
Ar-H) ppm. 13C NMR (100 MHz, CDCl3): δ = 14.0 (CH3), 19.5
(CH2), 29.4, 29.6 (CH2-bridge), 31.9 (CH2), 55.6, 55.7 (OCH3),
68.0, 68.1 (OCH2), 114.0, 114.1, 114.8, 115.0, 115.1 (Ar-H), 128.1,
128.2, 128.3 (Ar-Cq), 150.1, 150.6 (Ar-CqO) ppm. HRMS
(MALDI-TOF): calcd. for C60H80O10 [M]+ 960.5844; found
960.6058.
3
CH2), 3.66 (s, 15 H, OCH3), 3.75 (s, 10 H, CH2-bridge), 3.82 (t, J
= 6.4 Hz, 10 H, OCH2), 6.76 (s, 5 H, Ar-H), 6.82 (s, 5 H, Ar-H)
ppm. 13C NMR (100 MHz, CDCl3): δ = 14.0 (CH3), 19.5 (CH2),
29.5 (CH2-bridge), 32.0 (CH2), 55.7 (OCH3), 67.9 (OCH2), 114.1,
114.7 (Ar-H), 128.1, 128.1 (Ar-Cq), 150.0, 150.5 (Ar-CqO) ppm.
MS (MALDI-TOF): m/z = 960.7 [M]+, 983.7 [M + Na]+, 999.8
[M + K]+.
Crystal Structure Analyses of 3a–d: Suitable crystals of compounds
3a, 3b, 3c, and 3d were mounted on glass fibers. Measurements
were made with a Smart 1000 CCD diffractometer with graphite-
monochromated Mo-Kα radiation. Data were collected at 110 K by
using scans to a maximum θ value of 25.00, 25.20, 25.03, 25.03.
The data were refined by full-matrix least-squares techniques on F2
with SHELXL-97, and the structures were solved by direct meth-
ods SHELXS-97.[13] All non-hydrogen atoms were refined aniso-
tropically. The hydrogen atoms were included at geometrically
idealized positions. Crystallographic data for 3a, 3b, 3c, and 3d are
summarized in Table 3.
Pillar[5]arene 3b: Colorless crystals, yield 558 mg (24.2%), m.p.
113–114 °C (ethyl acetate/ethanol). 1H NMR (400 MHz, CDCl3):
δ = 0.92–0.95 (m, 15 H, CH3); 1.47–1.52 (m, 10 H, CH2); 1.72–
1.75 (m, 10 H, CH2); 3.639 (s), 3.646 (s), 3.656 (s) (15 H, OCH3);
3.721 (s), 3.746 (s), 3.778 (s) (10 H, CH2-bridge); 3.79–3.83 (m, 10
H, OCH2); 6.755 (s), 6.761 (s), 6.754 (s), 6.778 (s), 6.790 (s), 6.800
(s), 6.809 (s) (10 H, Ar-H) ppm. 13C NMR (100 MHz, CDCl3): δ
= 14.0 (CH3), 19.5, 19.5 (CH2), 29.4, 29.6 (CH2-bridge), 31.9, 31.9
(CH2), 55.6, 55.7, 55.7, 55.8 (OCH3), 68.1 (OCH2), 113.9, 114.0,
114.1, 114.9, 115.0, 115.1 (Ar-H), 127.9, 128.0, 128.1, 128.1, 128.2,
128.3, 128.3 (Ar-Cq), 150.1, 150.5 (Ar-CqO) ppm. HRMS
(MALDI-TOF): calcd. for C60H80O10 [M]+ 960.5844; found
960.5995.
CCDC-795257 (for 3a) 77751 (for 3b), -77752 (for 3c), and -77753
(for 3d) contain the supplementary data for this paper. These data
can be obtained free of charge from the Cambridge Crystallo-
graphic
Data
Centre
via
www.ccdc.cam.ac.uk/data_
request/cif.
Pillar[5]arene 3c: Colorless crystals, yield 636 mg (27.6%), m.p.
131–132 °C (ethyl acetate/ethanol). 1H NMR (400 MHz, CDCl3):
δ = 0.93–0.98 (m, 15 H, CH3); 1.45–1.55 (m, 10 H, CH2); 1.74–
1.77 (m, 10 H, CH2); 3.656 (s), 3.664 (s), 3.670 (s), 3.681 (s) (15 H,
OCH3); 3.736 (s), 3.762 (s), 3.789 (s) (10 H, CH2-bridge); 3.81–3.86
Acknowledgments
We are grateful to the National Science Foundation of China
(20872038) and the Specialized Chinese Research Fund for the
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Y. Kou, H. Tao, D. Cao, Z. Fu, D. Schollmeyer, H. Meier
FULL PAPER
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[9] Concerning the proton signals of the CH2 bridges, it is plaus-
ible that the central singlet in the patterns of 3a–c belongs to
the bridge type A and the outer singlets to the bridge types B
and C (Figure 1). An exact proof for this assumption is difficult
in the ROESY spectra, as the corresponding cross-peaks are
very close to the diagonal peaks.
Doctoral Program of Higher Education (20060561024) for finan-
cial support.
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Received: May 19, 2010
Published Online: October 7, 2010
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