Azobenzene-bridged calix[8]arenes

Article abstract of DOI:10.1016/j.tetlet.2006.08.067

An azobenzene bridge was introduced into the lower (or smaller) rim of p-tert-butylcalix[8]arene (1) and 1,5-calix[8]crown-3 (2) to form 1,4-singly bridged (3) and 1,5:3,7-doubly bridged (4) calix[8]arene derivatives, respectively. Trans and cis isomers o

Full text of DOI:10.1016/j.tetlet.2006.08.067

Tetrahedron Letters 47 (2006) 7809–7813
Azobenzene-bridged calix[8]arenes
a
a
b,
*
Grazia M. L. Consoli, Corrada Geraci, Placido Neri,
c
c,
*
Giacomo Bergamini and Vincenzo Balzani
a
Istituto di Chimica Biomolecolare, CNR, Via del Santuario 110, I-95028 Valverde, CT, Italy
Dipartimento di Chimica, Universit a` di Salerno, Via Ponte don Melillo, I-84084 Fisciano, Salerno, Italy
Dipartimento di Chimica ‘G. Ciamician’, Universit a` di Bologna, Via Selmi 2, I-40126 Bologna, Italy
b
c
Received 6 July 2006; revised 2 August 2006; accepted 21 August 2006
Available online 15 September 2006
Abstract—An azobenzene bridge was introduced into the lower (or smaller) rim of p-tert-butylcalix[8]arene (1) and 1,5-calix[8]-
crown-3 (2) to form 1,4-singly bridged (3) and 1,5:3,7-doubly bridged (4) calix[8]arene derivatives, respectively. Trans and cis iso-
mers of conformationally rigid 4 were isolated. The quantum yields of the trans–cis photoisomerisation reactions have been
measured.
ꢀ
2006 Elsevier Ltd. All rights reserved.
Development of molecular machines and switches
whose motion can be induced by light is a focus of inter-
est in supramolecular chemistry. Azobenzene is one of
ability and the azobenzene photochromic properties,
novel light-driven molecular switches may be produced.
1
the typical photochromic units whose cis–trans isomeri-
sation has been the basis for many functional molecules
and materials. In order to produce light-driven struc-
Because the azo group is usually considered to be a
chromogenic centre but a modest metal-chelating site,
we decided to insert an azobenzene bridge also in the
singly bridged 1,5-calix[8]crown-3 2. The presence of
an additional crown ether spanner could result advanta-
geous in both conferring to the hybrid system a higher
rigidity and reinforcing its metal-chelating ability.
2
1
0
tures, azobenzene moieties have been chemically intro-
3
duced into molecular systems such as macrocycles.
4
5
Thus, calix[4,6]arenes and calix[4]resorcinarenes with
azobenzene functionalities have been reported and their
complexing and photoresponsive properties have been
demonstrated.
To introduce an azobenzene bridge into the calix[8]arene
lower rim, we adapted previously reported procedures
for the preparation of singly and doubly bridged calix[8]-
Light-induced conformational change of a photorespon-
sive unit is efficiently conveyed to other parts of the
molecule in some cyclic systems with consequent
9
b,c,10,11
arenes.
Thus, treatment of substrate 1 or 2 with
0
12
3,3 -bis(a-bromomethyl)azobenzene in the presence of
a base and under UV irradiation (Scheme 1) afforded
1,4-singly bridged calix[8]arene 3 (33% yield) and
6
changes of chemical and physical properties. To the
contrary, it is also known that the isomerisation of
responsive units in some macrocycles is strictly con-
trolled by the size and the conformation of the rest part
1
3
1,5:3,7-doubly bridged calix[8]arene 4 (30% yield).
Lower yields were usually obtained in the absence of
UV irradiation. The obtained orange coloured com-
pounds were characterised by FAB-MS, NMR and UV
analysis.
4
c,7
of the ring.
Considering this, we wished to investigate
the effect of the introduction of an azobenzene bridging
unit into the conformationally flexible calix[8]arene
8
macrocycle (1). Intrabridging of calix[8]arene is a valid
approach to effectively preorganise this macrocycle and
has provided highly selective ionophores. It is conceiv-
Considering that calix[8]arene 1 in solution likely as-
sumes a pleated loop conformation, molecular model-
9
14
able that combining the well-known calixarene host
ling suggested that the spanning azobenzene can
potentially cross-link the 1,4 or 1,5-phenoxy groups of
1
(Fig. 1). Identity of the obtained compound 3 was
Keywords: Calix[8]arenes; Azobenzene; Photoswitch.
1
5
*
Corresponding authors. E-mail addresses: neri@unisa.it; vincenzo.
balzani@unibo.it
established on the basis of its NMR data and mole-
cular weight, fitting for a singly bridged calix[8]arene
0040-4039/$ - see front matter ꢀ 2006 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2006.08.067

7810
G. M. L. Consoli et al. / Tetrahedron Letters 47 (2006) 7809–7813
OH
OH
O
O
O
O H
HO
3,3'-bis(α-bromomethyl)-
H
azobenzene
O
HO
OH
HO
Y
Cs CO , DMF, rt, 72h
H
H
2
3
H
H
O
O
O
OH
OH
N
N
Y =
3
1
CH2 H C
2
X = -CH CH -O-CH CH -
2
2
2
2
O
OH
OH
O
O
O
HO
H
H
3,3'-bis(α-bromomethyl)-
Y
X
azobenzene
X
O
O
O
O
NaH, THF/DMF, rt, 24h
H
O
H
H
H
O
O
O
OH
4
2
Scheme 1. Synthesis of azobenzene-bridged calix[8]arenes.
mented rule.17 The occurrence of a resonance at
.29 ppm in the protonic spectrum of 3 can be consid-
ered diagnostic of the relegated singly bonded hydroxyls
8
1
7
characteristic of 1,4-bridging. Interestingly, the methyl-
1
ene region of the H NMR spectrum of 3, at room tem-
perature in CDCl , displayed five AX/AB systems for
3
ArCH Ar groups indicating that conformational mo-
2
tion of the calixarene skeleton is hampered. This is
attributable to the presence of the bridge associated with
a stabilising hydrogen bonding network (Fig. 1).
The azobenzene group in compound 3 can exist in either
a trans or cis configuration. Proton NMR spectrum of
the isolated compound 3 appeared consistent with the
trans isomer on the basis of the chemical shift of the azo-
1
8
benzene aromatic protons. In addition, low intensity
broad resonances were also observed, which could be
ascribed to the cis isomer. This is in accordance with
MM2 calculations (GB/SA model solvent for CHCl3)
that favoured the trans over the cis isomer of 3
Figure 1. Computer model of idealised conformation of 3 in the trans
configuration.
(
Fig. 1). Attempts to isolate the pure cis isomer have
derivative. Among the four possible singly bridged
regioisomers (1,2, 1,3, 1,4 or 1,5), the presence of four
singlets for the t-Bu groups and eight signals for C–O
and C–Bu quaternary carbons (147–153 ppm) was indic-
not been successful probably because of a fast cis–trans
interconversion.
The known caesium cation conformational templation
1
6
19
ative of a CH –CH symmetry compatible with a 1,2-
in 1,5-bridged calix[8]arene derivatives, which folds
2
2
or 1,4-bridged derivative. Discrimination between these
two regioisomers was obtained from the chemical shift
of the phenolic hydroxyls on the basis of a well docu-
the calixarene skeleton in the tube-shaped conforma-
tion, addressed also the introduction of the azobenz-
ene bridge into substrate 2, affording 1,5:3,7-doubly
2
0

G. M. L. Consoli et al. / Tetrahedron Letters 47 (2006) 7809–7813
7811
bridged regioisomer 4. It was univocally identified on
2
1
the sole basis of the NMR signal patterns. The pres-
ence of three tert-butyl signals (1:2:1 ratio) and two
AX/AB systems for ArCH Ar groups in the proton
2
spectrum (CDCl , 297 K) of 4 was a clear evidence of
3
its molecular structure bisected by two orthogonal 2-
fold symmetry elements (C -symmetry). The introduc-
2
v
tion of the second bridge in compound 4 was also
confirmed by FAB-MS.
In doubly bridged derivative 4, calixarene macrocycle
undergoes a strong reduction of its conformational
mobility as evidenced by the appearance of one AB
and one AX system for ArCH Ar groups in the protonic
2
spectrum in CDCl . NMR data suggested that analo-
3
9
,22
Figure 2. Computer model of idealised conformation of 4 (side view)
in the trans (left) and cis (right) configurations (H atoms omitted).
gously to other doubly bridged calix[8]arenes,
com-
pound 4 assumes a folded conformation in which the
triads of contiguous aromatic rings in positions 2–4
and 6–8 adopt a 3/4 cone geometry (AX system with
Dd = 0.97 ppm and carbon resonances at 30.4 and 30.9
compounds 3 and 4 contained 85% and 80% of the trans
isomer, respectively, before irradiation. The photoiso-
23
merisation reactions were monitored from the spectral
variations (see Fig. 3) at k = 320 nm that corresponds
to the largest difference in absorbance of the two iso-
mers. Because of the partial overlap of the spectra of
the two isomers, upon irradiation a stationary state
is reached in which the trans/cis concentration ratio
ppm for ArCH Ar groups), while the bridge-head aryls
2
bearing the crown bridge are in an out orientation (AB
system with Dd = 0.05 ppm for ArCH Ar groups).
2
Also for compound 4, the presence of the azobenzene
moiety allows the existence of trans and cis isomers that
were separated by preparative TLC (SiO , CH Cl ). The
(
Table 1) depends on the molar absorption coefficients
2
2
2
of the two isomers at the excitation wavelength and on
the quantum yields of the forward and reverse reactions.
The true quantum yields (Table 1) were obtained by
extrapolation to zero irradiation time (Fig. 3, inset) after
suitable correction. The rate of the thermal cis ! trans
back reaction at 35 ꢁC was also measured for azobenz-
ene and compound 3. In the case of 4, however, the back
thermal isomerisation reaction was accompanied by
decomposition. The results obtained, gathered in Table
two isomers were characterised on the basis of their
2
1
NMR and UV spectra. Because the aromatic protons
of a trans azobenzene unit generally resonate at a lower
1
8
magnetic field with respect to the cis form, we assigned
the cis configuration to the more polar compound,
whose azobenzene aromatic protons resonate at 6.41,
7
.42, 7.75 and 7.98 ppm. Consequently, the trans
geometry was assigned to the other isomer showing
azobenzene proton resonances at 7.57, 7.58, 7.94 and
1
, show that the photochemical behaviour of the azo-
8
.31 ppm. Trans–cis geometry influences also the reso-
benzene moiety of compounds 3 and 4 is substantially
the same as that of free azobenzene. Within the experi-
mental error (± 15%), the quantum yields for the
nances relative to the calixarene ArH and crown
OCH groups, both of which undergo upfield shift in
2
2
1
the cis isomer.
Isomerisation of both pure trans and cis forms of 4
1
1
0
0
0
0
0
.2
.0
.8
.6
.4
.2
.0
occurred at room temperature in CDCl solution to give
3
0
.10
.05
in both instances a final 36:64 trans–cis equilibrium
ratio as found by integration of their NMR signals.
0
Also in this instance, MM2 energy-minimisation (GB/SA
model solvent for CHCl ) favoured the trans with
3
0.00
respect to the cis isomer of 4 (Fig. 2) by 28 kJ/mol.
0
5
10
t / min
To evaluate the influence of the calixarene macrocycle
on the photoisomerisation of the azobenzene moiety,
experiments were performed in dichloromethane solu-
tion on compounds 3 and 4 and on azobenzene as refer-
ence compound. Irradiation was carried out with 320
and 432 nm light obtained with a monochromator from
a xenon lamp. Since the calixarene moiety does not
absorb above 300 nm in compounds 3 and 4, there is
no overlap with the relevant bands of the azobenzene
unit (320 and 430 nm) that have indeed the same shape
and are assumed to have the same intensities as those of
the free azobenzene moiety. From the absorption spec-
tra, we have estimated that the investigated samples of
2
50
300
350
400
450
500
550
λ / nm
Figure 3. Absorption changes of a dichloromethane solution of 4 upon
irradiation at 320 nm: solid line represents the absorption spectrum of
the non-irradiated solution and dashed line corresponds to the
photostationary state. The dotted curve corresponds to the photosta-
tionary state upon irradiation at 432 nm. Inset shows the quantum
yield of trans ! cis photoreaction as a function of time.

7812
G. M. L. Consoli et al. / Tetrahedron Letters 47 (2006) 7809–7813
a
Table 1. Isomerisation reactions in dichloromethane solution
4. (a) Hamada, F.; Matsuda, T.; Kondo, Y. Supramol. Chem.
1995, 5, 129; (b) Shinkai, S. In Comprehensive Supramo-
lecular Chemistry; Lehn, J. M., Atwood, J. L., Davies, J.
E. D., MacNicol, D. D., V o¨ gtle, F., Eds.; Elsevier Science
Ltd, 1996; Vol. 1, p 671; (c) Saadioui, M.; Asfari, Z.;
Vicens, J. Tetrahedron Lett. 1997, 38, 1187; (d) Kim, N.
Y.; Chang, S.-K. J. Org. Chem. 1998, 63, 2362; (e)
Pipoosananakaton, B.; Sukwattanasinitt, M.; Jaiboon, N.;
Chaichit, N.; Tuntulani, T. Tetrahedron Lett. 2000, 41,
ꢀ
1 c
)
k
irr = 320 nm irr = 432 nm
k
k
c!t (s
b
b
U
t!c
% trans
U
c!t
% trans
ꢀ6
Azobenzene 0.12
16
19
18
0.33
0.49
0.44
80
83
65
3 · 10
1 · 10
—
ꢀ6
3
4
0.13
0.13
a
b
c
At rt, unless otherwise noted.
At photostationary state.
9
095; (f) Halouani, H.; Dumazet-Bonnamour, I.; Lamar-
Initial reaction rate at 308 K. Compound 4 undergoes decomposition.
tine, R. Tetrahedron Lett. 2002, 43, 3785; (g) Lee, S. H.;
Kim, J. Y.; Ko, J.; Lee, J. Y.; Kim, J. S. J. Org. Chem.
trans ! cis photoisomerisation at 320 nm are the same
as those of free azobenzene, whereas the quantum yield
for the cis ! trans photoisomerisation at 432 nm is
slightly higher for the azobenzene-bridged derivatives.
As far as the values of % trans at the stationary states
are concerned, they are difficult to rationalise since they
depend not only on the quantum yield values, but also
on values of the molar absorption coefficients that have
been taken to be those of free azobenzene for all the
investigated compounds.
2
2
004, 69, 2902; (h) Chen, C.-F.; Chen, Q.-Y. New J. Chem.
006, 30, 143.
5
. (a) Ichimura, K.; Oh, S.-K.; Nakagawa, M. Science 2000,
288, 1624; (b) Husaru, L.; Gruner, M.; Wolff, T.;
Habicher, W. D.; Salzer, R. Tetrahedron Lett. 2005, 46,
3377.
. Shinkai, S.; Nakaji, T.; Ogawa, T.; Manabe, O. J. Am.
Chem. Soc. 1980, 102, 5860, and references cited therein.
. Norikane, Y.; Kitamoto, K.; Tamaoki, N. Org. Lett. 2002,
6
7
8
4, 3907.
. (a) Gutsche, C. D. Calixarenes Revisited; Royal Society of
Chemistry: Cambridge, 1998; (b) Calixarene 2001; Asfari,
Z., Bohmer, V., Harrowfield, J., Eds.; Kluwer: Dordrecht,
In conclusion, the first azobenzene-bridged calix[8]-
arenes have been synthesised and trans–cis isomers of
the conformationally rigid 1,5:3,7-doubly bridged deriv-
ative 4 have been isolated. The photoresponsive behav-
iour of these compounds has also been investigated.
More in depth studies to evaluate the photoswitching
and complexing properties of 3 and 4 and to design ana-
logues with improved photophysical characteristics are
in progress.
2
001; Chapter 5, pp 89–109.
9. (a) Ikeda, A.; Suzuki, Y.; Akao, K.; Shinkai, S. Chem.
Lett. 1996, 963; (b) Geraci, C.; Chessari, G.; Piattelli, M.;
Neri, P. Chem. Commun. 1997, 921; (c) Gregoli, L.; Russo,
L.; Stefio, I.; Gaeta, C.; Arnaud-Neu, F.; Hubscher-
Bruder, V.; Khazaeli-Parsa, P.; Geraci, C.; Neri, P.
Tetrahedron Lett. 2004, 45, 6277.
0. Geraci, C.; Piattelli, M.; Chessari, G.; Neri, P. J. Org.
Chem. 2000, 65, 5143.
1. Singly bridged calix[8]arenes: (a) Geraci, C.; Piattelli, M.;
Neri, P. Tetrahedron Lett. 1996, 37, 3899; (b) Gaeta, C.;
Gregoli, L.; Martino, M.; Neri, P. Tetrahedron Lett. 2002,
1
1
Acknowledgements
4
3, 8875. Doubly bridged calix[8]arenes: (c) Geraci, C.;
Financial support from the Italian MIUR (Supramole-
cular Devices Project) is gratefully acknowledged. We
thank Professor Salvatore Giuffrida, Dr. Paola Ceroni
and Mrs. Silvia Parmeggiani for experimental assistance
and discussion.
Consoli, G. M. L.; Piattelli, M.; Neri, P. Collect. Czech.
Chem. Commun. 2004, 69, 1345.
0
1
1
2. 3,3 -Bis(a-bromomethyl)azobenzene was synthesized by
the procedure reported in: Bartels, E.; Wassermann, H.;
Erlanger, B. F. Proc. Natl. Acad. Sci. U.S.A. 1971, 8, 1820.
3. Procedure for the preparation of compound 3: A solution of
p-tert-butylcalix[8]arene
1
(250 mg, 0.19 mmol) and
References and notes
Cs CO (502 mg, 1.54 mmol) in 40 mL THF/DMF (3:1)
2
3
was stirred at room temperature for 20 min. Then a
0
12
1
. (a) Spichiger-Keller, U. S. Chemical Sensors and Biosen-
sors for Medical and Biological Applications; Wiley-VCH:
Weinheim, Germany, 1998; (b) Czarnik, A. W. Fluorescent
Chemosensors for Ion and Molecular Recognition; Amer-
ican Chemical Society: Washington, DC, 1993; (c) Amen-
dola, V.; Feringa, B. L. Molecular Switches; Wiley-VCH
GmbH: Weinheim, Germany, 2001; (d) Balzani, V.; Credi,
A.; Venturi, M. Molecular Devices and Machines—A
Journey into the Nano World; Wiley-VCH: Weinheim,
Germany, 2003.
solution of 3,3 -bis(a-bromomethyl)azobenzene (trans–
cis mixture, 142 mg, 0.38 mmol) in 40 mL of DMF was
added dropwise over a period of 2–3 h. The reaction
mixture was kept under low-pressure UV lamp, to increase
the amount of electrophile in the cis form, and vigorously
stirred at room temperature for 7 days. The solvent was
removed under vacuum and the residue was suspended in
0.1 N HCl. The crude product was recovered by filtration
and purified by column chromatography (SiO
2
, gradient
from 45:55 to 70:30 CH Cl /hexane) to afford orange
2
2
2
. (a) Rau, H. In Photochromism, Molecules and Systems;
D u¨ rr, H., Bouas-Laurent, H., Eds.; Elsevier: Amsterdam,
compound 3 in 33% yield. Procedure for the preparation of
compound 4: A solution of 1,5-calix[8]crown-3 2 (150 mg,
0.11 mmol) and 60% oil dispersion NaH (55 mg,
0.88 mmol) in dry THF/DMF (40 mL, 3:1, v/v) was
stirred at room temperature for 20 min. Then a solution of
1990, Chapter 4; (b) Jousselme, B.; Blanchard, P.;
Gallego-Planas, N.; Delaunay, J.; Allain, M.; Richomme,
P.; Levillain, E.; Roncalli, J. J. Am. Chem. Soc. 2003, 125,
0
2888; (c) Norikane, Y.; Kitamoto, K.; Tamaoki, N. J.
3,3 -bis(a-bromomethyl)azobenzene (81 mg, 0.22 mmol)
Org. Chem. 2003, 68, 8291.
in 20 mL of dry THF was added dropwise over a period
of 2–3 h. The mixture was kept under low-pressure UV
lamp and vigorously stirred at room temperature until the
disappearance of the starting material (2–3 days). The
solvent was removed under vacuum and the residue was
suspended in 0.1 N HCl. The crude product was recovered
3
. (a) Shinkai, S.; Minami, T.; Kasano, Y.; Manabe, O. J.
Am. Chem. Soc. 1983, 105, 1851; (b) Yoshii, K.; Machida,
S.; Horie, K. J. Polym. Sci. Part B: Polym. Phys. 2000, 38,
3098; (c) Tamaoki, N.; Koseki, K.; Yamaoka, T.; Man-
abe, O. Angew. Chem., Int. Ed. Engl. 1990, 29, 105.

G. M. L. Consoli et al. / Tetrahedron Letters 47 (2006) 7809–7813
7813
by filtration and purified by column chromatography
SiO , gradient from 15:85 to 60:40 CH Cl /hexane) to
A. C. J. Photochem. Photobiol. A: Chem. 2003, 154,
179.
(
2
2
2
afford compound 4 in 30% yield (pure trans plus a trans–
cis mixture). Pure cis isomer of 4 was obtained by
19. Consoli, G. M. L.; Cunsolo, F.; Geraci, C.; Neri, P. Org.
Lett. 2001, 3, 1605.
purification on preparative TLC (SiO
2
, 100% CH
2
Cl
2
).
20. Consoli, G. M. L.; Cunsolo, F.; Geraci, C.; Gavuzzo, E.;
1
1
4. Gutsche, C. D.; Gutsche, A. E.; Karaulov, A. I. J. Incl.
Neri, P. Tetrahedron Lett. 2002, 43, 1209.
21. H NMR data for compound 4 (400 MHz, CDCl , 297 K):
1
Phenom. 1985, 3, 447.
3
1
5. H NMR data for compound 3: (400 MHz, CDCl
3
, 297 K)
trans isomer d 1.20 [s, C(CH
C(CH , 18H each], 3.52/4.53 (AX system, ArCH
J = 14.2 Hz, 8H), 3.79 (br t, OCH , 4H), 3.81/3.86 (AB
3
)
3
, 36H], 1.22, 1.25 [s,
d 1.20, 1.21, 1.25, 1.27 [s, C(CH
3
)
3
, 18H each], 3.40/4.08
3
)
3
2
Ar,
(
AX system, ArCH Ar, J = 14.5 Hz, 2H), 3.50/3.78 (AB
2
2
system, ArCH
2
Ar, J = 14.2 Hz, 4H), 3.62/3.76 (AB sys-
system, ArCH
2
Ar, J = 14.8 Hz, 8H), 4.09 (br t, OCH
2
,
tem, ArCH
ArCH Ar, J = 15.0 Hz, 4H), 4.03/4.17 (AB system,
ArCH Ar, J = 15.2 Hz, 4H), 4.70/4.82 (AB system,
2
Ar, J = 12.7 Hz, 2H), 3.96/4.32 (AB system,
4H), 5.31 (s, azoBn OCH
2
, 4H), 7.04 (br d, ArH, 4H), 7.13
2
(br d, ArH, 4H), 7.16 (s, ArH, 4H), 7.21 (s, ArH, 4H), 7.31
(s, OH, 4H) 7.55–7.59 (overlapped, azoBn ArH, 4H), 7.95
(d, azoBn ArH, J = 6.5 Hz, 2H), 8.32 (s, azoBn ArH, 2H);
2
OCH
.12, 7.16, 7.21 (br d, ArH, 2H each), 7.18 (d, azoBn ArH,
J = 7.1 Hz, 2H), 7.43 (t, azoBn ArH, J = 7.8 Hz, 2H), 7.93
d, azoBn ArH, J = 7.9 Hz, 2H), 8.29 (s, OH, 2H), 8.48 (s,
2
, J = 10.8 Hz, 4H), 7.01, 7.02, 7.05, 7.06, 7.09,
7
cis isomer d 1.21 [s, C(CH
18H each], 3.50, 3.52 (br t, OCH
(AX system, ArCH Ar, J = 14.1 Hz, 8H), 3.88/3.95 (AB
3
)
3
, 36H], 1.22, 1.24 [s, C(CH
3 3
) ,
2
, 4H each), 3.56/4.54
(
2
OH, 2H), 8.63 (s, OH, 2H), 8.89 (s, azoBn ArH, 2H).
6. The general principle of this approach has been detailed
in: (a) Neri, P.; Consoli, G. M. L.; Cunsolo, F.; Geraci, C.;
Piattelli, M. New J. Chem. 1996, 20, 443; (b) Neri, P.;
Geraci, C.; Piattelli, M. In Recent Research Developments
in Organic Chemistry; Pandalai, S. G., Ed.; Transworld
Research Network: Trivandrum, 1997; Vol. 1, pp 285–297.
7. For example of calixarene OH chemical shift as structural
probe see Ref. 10 and references cited therein.
system, ArCH
2
Ar, J = 15.2 Hz, 8H), 5.19 (s, azoBn
1
OCH , 4H), 6.41 (d, azoBn ArH, J = 7.9 Hz, 2H), 7.02
2
(br s, ArH, 4H), 7.13 (s, ArH, 8H), 7.34 (s, OH, 4H), 7.35
(br s, ArH, 4H), 7.42 (t, azoBn ArH, J = 7.6 Hz, 2H), 7.75
(s, azoBn ArH, 2H), 7.98 (d, azoBn ArH, J = 7.5 Hz, 2H).
22. (a) Ikeda, A.; Akao, K.; Harada, T.; Shinkai, S. Tetra-
hedron Lett. 1996, 37, 1621; (b) Geraci, C.; Bottino, A.;
Piattelli, M.; Gavuzzo, E.; Neri, P. J. Chem. Soc., Perkin
Trans. 2 2000, 185.
1
1
8. For cis–trans assignment of azobenzene units, see: Tait, K.
M.; Parkinson, J. A.; Bates, S. P.; Ebenezer, W. J.; Jones,
23. From this data it is evident that cis–trans isomerisation
occurred during the sample manipulations.