Rapid Communication. Imidazolium-linked cyclophanes

Full text of DOI:10.1071/CH99104

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Volume 52, 1999
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Aust. J. Chem., 1999, 52, 823–825
Rapid Communication.
Imidazolium-Linked Cyclophanes
Murray V. Baker, Mark J. Bosnich, Charlotte C. Williams,
Brian W. Skelton and Allan H. White
Department of Chemistry, University of Western Australia, Nedlands, W.A. 6907.
Cyclophanes have been of interest for many years and a
huge variety of cyclophanes are known. A recent book by
Vögtle1 reviewed cyclophane chemistry in depth and
included discussion of work with cyclophanes in fields as
diverse as aromaticity, host–guest chemistry, molecular
recognition, and self-replicating systems, to name just a few.
In the syntheses of many cyclophanes, competition
between inter- and intra-molecular cyclization reactions
results in low yields, and high-dilution conditions are often
required. Recently some metacyclophanes, in which two
benzene rings were linked by imidazolium units, were
reported.2 Synthetic details were not provided, but the meta-
cyclophanes, which have structures reminiscent of calix-
arenes, were reported to interact with certain anions in
solution and in the solid state.
high yield of 87% (after recrystallization) by treatment of
1,3,5-tris(bromomethyl)-2,4,6-trimethylbenzene with 1 equiv.
of 1,3,5-tris(imidazol-1-ylmethyl)-2,4,6-trimethylbenzene
in acetone (Scheme 1). Paracyclophanes (2) and (3) were
synthesized by analogous routes and obtained in yields of 77
and 67% respectively.
The imidazolium-linked cyclophanes, as their bromide
salts, are insoluble in acetone, sparingly soluble in dimethyl
sulfoxide, and freely soluble in water. In (D6)dimethyl sul-
foxide, the cyclophanes show beautifully simple n.m.r.
spectra. For example, the 1H n.m.r. spectrum of (1)
((CD3)2SO solution) contains just four singlets due to the
four sets of protons present (6×CH3, 6×CH2, 6×imidazolium
In this communication we report some of our own work
on imidazolium-linked cyclophanes: the synthesis and struc-
tural characterization of the imidazolium-linked (1,3,5)-
cyclophane (1) and related paracyclophanes (2)–(4).
Cyclophanes (1)–(4) are easily synthesized in high yield by
the reaction of (bromomethyl)benzenes with appropriate
(imidazol-1-ylmethyl)benzenes. For example, without
recourse to use of high-dilution conditions,3 the triply
bridged cyclophane (1) was obtained in an astonishingly
Manuscript received 27 August 1999
© CSIRO 1999
0004-9425/99/090823
10.1071/CH99104

M. V. Baker et al.
824
H 4/H 5, 3×imidazolium H 2). The 1H n.m.r. signal due to the
imidazolium H 2 protons occurs at unusually high field (␦
5.60), indicating that these protons are directed towards the
cyclophane cavity and the region shielded by the magneti-
In the family of paracyclophanes exemplified by (2)–(4),
compound (2) was consistently obtained in the highest yield
and (3) was also obtained in good yield, but attempts to syn-
thesize (4) without the use of high dilution afforded only
insoluble, presumably polymeric products. The methyl
groups present in the precursors of (2) and (3) would restrict
the conformational freedom of the intermediate (5), and in
1
cally anisotropic benzene rings. For comparison, the H
n.m.r. signal due to H 2 in 1,3-dibenzylimidazolium chloride
occurs at ␦ 10.79 (CDCl3 solution).4
Fig. 1. Projections of the cations of (1)–(3a): (a) down; (b) normal to (both slightly oblique to) their putative principal axes. For (1) and (3a) 20%,
– –
and for (2) 50%, thermal ellipsoids are shown for the non-hydrogen atoms. Crystallographic symmetries are m, 1, 1 respectively.

Imidazolium-Linked Cyclophanes
825
doing so may favour cyclization of (5) over polymeriza-
tion.3,5 Similar arguments have been used to explain the ease
of synthesis of certain macrocycles from precursors bearing
bulky substituents.3,5,6 In the reaction of 1,4-bis(bromo-
methyl)-2,5-dimethylbenzene with 1,4-bis(imidazol-1-yl-
methyl)-2,5-dimethylbenzene, two cyclophane products are
possible, the pseudo-geminal isomer (3a) and the pseudo-
ortho isomer (3b), and both isomers are formed (ratio 1.7:1
respectively). The pseudo-geminal isomer (3a) was the less
soluble isomer and was isolated by selective crystallization
from methanol. In principle, (3a) and (3b) can be intercon-
verted by rotation of one of the benzene rings about its
C1–C4 axis, but such rotation does not occur even at tem-
peratures up to 80°C; a pure sample of (3a) was unchanged
after being heated in (CD3)2SO at 80°C for 30 min.
(1), (2) and (3a) crystallized as hydrates from aqueous
solutions and were characterized by single-crystal X-ray
structural studies (Fig. 1). In each of the three structures, the
cation is disposed about a crystallographic symmetry
element—a mirror plane or an inversion centre—so that only
one-half of the formula unit is crystallographically indepen-
dent in each case. The benzene rings are parallel and sep-
arated by 5.15, 4.84 and 5.15 Å in (1), (2) and (3a)
respectively. The imidazolium rings are oriented with their
C2–H bonds pointing into the cavities between the benzene
rings, but oblique to the centres of the cavities. In no case is
any species trapped in the cavity; the water molecules form
hydrogen-bonded arrays with the bromide ions. The C–C
bond lengths and bond angles are generally all close to their
usual values, consistent with little or no strain within the
cyclophane systems; this absence of strain may be a signifi-
cant factor contributing to the high yielding syntheses of
these cyclophanes.
H, 5.2; N, 11.1%). 1H n.m.r. ␦ (D2O) 2.21, s, CH3; 5.61, s, CH2; 5.79,
br t, J 1.8 Hz, H 2; 7.97, d, J 1.8 Hz, H 4/H 5. 13C n.m.r. ␦ (D2O) 17.7,
CH3; 51.0, CH2; 127.8, C4/C5; 131.9, C2; 132.8, C; 144.5, C.
Cyclophanes (2) and (3) were synthesized in the same way as (1);
satisfactory elemental analyses and n.m.r. data were obtained.
Cyclophane (4) was synthesized by a similar procedure, but in less con-
centrated solution (50 mg of 1,4-bis(bromomethyl)benzene and 45 mg
of 1,4-bis(imidazol-1-ylmethyl)benzene in 200 ml of acetone).
Crystallography
Crystals for X-ray diffraction studies were grown by vapour diffu-
sion of dimethylformamide into aqueous solutions of (1), (2) and (3).
Cyclophane (1). (C33H39N6)Br3·2H2O, Mr 795.5. Monoclinic,
P21/m (No. 11), a 9.861(5), b 17.30(1), c 10.603(5) Å, ␤ 114.34(4)°, V
–3
–1
1648 Å3. Dc(Z = 2) 1.602 g cm ; F(000) 808. ␮ 37 cm ; specimen:
Mo
0.26 by 0.65 by 0.10 mm; Tmin,max (Gaussian correction) 0.59, 0.71.
2␪max 60°. Temperature c. 295 K. 2933 (= No) ‘observed’(I > 3␴(I)) out
of N 4935 unique single counter diffractometer reflections refined by
full matrix least squares (anisotropic thermal parameter refinement
C,N,O,Br; (x, y, z, Uiso)H refined) to conventional R on |F| 0.040, Rw (sta-
tistical weights) 0.048.
Cyclophane (2). (C30H38N4)Br2·6H2O, Mr 722.6. Monoclinic, C2/c
(No. 15), a 25.829(4), b 9.151(1), c 18.509(3) Å, ␤ 127.379(2)°, V
–3
–1
3476 Å3. Dc(Z = 4) 1.382 g cm ; F(000) 1504. ␮ 23.8 cm ; speci-
Mo
men: 0.40 by 0.35 by 0.25 mm; ‘T’
0.69, 0.80 (‘empirical’ cor-
min,max
rection (‘SADABS’)). 2␪
58°. Temperature c. 153 K. No 3452
max
(F > 4␴(F)), N 4367merged (Rint = 0.017) from 19329 total BrukerAXS
CCD reflections, refined by full matrix least squares to R 0.025, Rw
0.033 ((x, y, z, Uiso)H refined for cation, constrained for H2O).
Cyclophane (3a). (C26H30N4)Br2·4H2O, Mr 630.4. Monoclinic,
P21/c (No. 14), a 9.976(3), b 11.799(5), c 13.319(3) Å, ␤ 110.92(2)°, V
–3
1464 Å3. Dc(Z = 2) 1.430 g cm ; F(000) 648. ␮ 28 cm–1; specimen:
Mo
0.36 by 0.58 by 0.60 mm; Tmin,max 0.27, 0.41 (Gaussian correction).
2␪max 60°. Temperature c. 295 K. Data collection and refinement as for
(1), N 4251, No 1793; R 0.040, Rw 0.051.
All structures. Monochromatic Mo K␣ radiation, ␭ 0.71073 Å.
Crystallographic data for (1)–(3a) have been deposited. Copies are
available (coordinates, thermal parameters, non-hydrogen geometries,
structure factors) from the Australian Journal of Chemistry, P.O. Box
1139, Collingwood, Vic. 3066, until 31 December 2004.
Experimental
Acknowledgment
Nuclear magnetic resonance spectra were recorded with
1
a Bruker ARX-500 spectrometer (500.1 MHz for H, 125.8 MHz for
We thank the Australian Research Council for financial
support.
13C) at ambient temperature and were referenced with respect to inter-
nal sodium 3-(trimethylsilyl)propane-1-sulfonate (␦ 0.00). Micro-
analyses were performed by the Microanalytical Laboratory of the
Australian National University, Canberra.
References
1
Vögtle, F., ‘Cyclophane Chemistry’ (John Wiley: Chichester 1993).
Alcalde, E., Alvarez-Rúa, C., García-Granda, S., García-
Synthesis of Cyclophanes
2
Cyclophane (1). A solution of 1,3,5-tris(imidazol-1-ylmethyl)-
2,4,6-trimethylbenzene (1.00 g, 2.77 mmol, synthesized by reaction of
1,3,5-tris(bromomethyl)-2,4,6-trimethylbenzene with imidazole in di-
methylformamide) in acetone (75 ml) was added dropwise with stirring
Rodriguez, E., Mesquida, N., and Pérez-García, L., Chem.
Commun., 1999, 295.
Dietrich, B., Viout, P., and Lehn, J.-M., ‘Macrocyclic Chemistry’
(VCH: Weinheim 1993).
Harlow, K. J., Hill, A. F., and Welton, T., Synthesis, 1996, 697.
Lindoy, L. F., ‘The Chemistry of Macrocyclic Ligand Complexes’
(Cambridge University Press: Cambridge 1989).
Richman, J. E., and Atkins, T. J., J. Am. Chem. Soc., 1974, 96, 2268.
van der Made, A. W., and van der Made, R. H., J. Org. Chem., 1993,
3
4
to
a
solution of 1,3,5-tris(bromomethyl)-2,4,6-trimethylbenzene7
5
(1.10 g, 2.77 mmol) in acetone (75 ml). The resulting solution was
allowed to stand for 1 week, during which time a white solid slowly pre-
cipitated. This solid was collected by filtration and recrystallized from
water/acetone, to give (1) as a white powder (1.90 g, 87%), m.p.
>300°C (Found: C, 52.0; H, 5.0; N, 11.1. C33H39Br3N6 requires C, 52.2;
6
7
58, 1262.