more than 100 years ago,6a they have attracted much attention
in the past decade due to their ability to bind anions,6b neutral
substrates,6c and metal ions.6d Very recently, it was dem-
onstrated that tetraurea calix[4]pyrroles form dimeric capsules
reminiscent of that of tetraurea calix[4]arenes.7 The Ballester
group recently reported the synthesis and the self-assembly
in the solid state of a new R,R,R,R-isomer of the calix[4]-
pyrrole-resorcinarene system 1a.8 It was found that this
sparingly soluble macrocycle self-assembles in the solid state,
in the presence of tetramethylammonium chloride guest, into
a hexameric structure driven by hydrogen bonding and
electrostatic interactions. This study and the resemblance of
1a to resorcin[4]arene 2a3a prompted us to synthesize
compound 1b, the lipophilic calix[4]pyrrole analogue of
resorcin[4]arene 2b9 (see Figure 1).
Figure 2.
1H NMR spectra (400 MHz, 298 K) of 10 mM solution
of 1b (a) in CD3OD, (b) in CDCl3, in the presence of (c) 5 mM 3,
(d) 5 mM 4, and (e) 5 mM 5 in CDCl3. * indicates residual
protonated solvents.
The difference in line shape can be attributed, inter alia,
to different conformations that 1b adopts in different solvents,
a different dynamic, or even the formation of not very well-
defined aggregates in the CDCl3 solution. It is well-known
that calix[4]pyrrole conformation, for example, depends on
the polarity of the solvents used. For example, it was found
that the 1,3-alternate conformation is the most populated
conformation of calix[4]pyrroles in most of the solvents, but
the population of the 1,3-alternate conformer was found to
decrease as the polarity of the solvent increases.11 In the
presence of small anions, the cone form is the major
conformation as a result of anion-π binding between the
anion and NHs of calix[4]pyrrole.
To verify which of the options is responsible for the
peculiar line shape of 1b in CDCl3 solution, we reverted to
diffusion NMR12 which has been used for characterization
of supramolecular assemblies in solution and was found to
be extremely useful in studying both dimeric2c,13 and
hexameric capsules.3c,4c-e,14 Interestingly, the diffusion
coefficient found for the peaks of 1b in CDCl3 (see Figure
2b) was 0.22 ( 0.01 × 10-5 cm2 s-1. This diffusion
coefficient is somewhat smaller than that found for the
Figure 1. Structures of calix[4]pyrroles 1, resorcin[4]arenes 2,
trioctylamine (3), tridodecylamine (4), and trioctadecylamine (5).
Compound 1b was synthesized by the acid-catalyzed
condensation of 1-(3,5-dimethoxyphenyl)dodecan-1-one with
pyrrole and was isolated as the R,R,R,R-isomer after puri-
fication.10 The 1H NMR spectrum of 10 mM of 1b in CD3OD
is shown in Figure 2a. The spectrum contains sharp and well-
defined peaks of the R,R,R,R-isomer of 1b. The presence of
1
only three types of protons in the downfield part of the H
NMR spectrum (δ ) 5.80, 6.15, 6.17 ppm) of 1b suggests
that 1b is locked in the cone conformation or is in fast
1
exchange between different conformations on the H NMR
time scale.
Compound 1b was also found to be soluble in CDCl3, and
1
the H NMR spectrum of 10 mM 1b in CDCl3 is shown in
Figure 2b. Clearly, a different line shape is observed in
CDCl3 as compared to CD3OD (compare Figure 2b to 2a).
(11) (a) Wu, Y.-D.; Wang, D.-F.; Sessler, J. L. J. Org. Chem. 2001, 66,
3739–3746. (b) Blas, J. R.; Lo´pez-Bes, J. M.; Ma´rquez, M.; Sessler, J. L.;
Luque, F. J.; Orozco, M. Chem.sEur. J. 2007, 13, 1108–1116.
(12) Cohen, Y.; Avram, L.; Frish, L. Angew. Chem., Int. Ed. 2005, 44,
520–554.
(6) (a) Baeyer, A. Ber. Dtsch. Chem. Ges. 1886, 19, 2184–2185. (b)
Gale, P. A.; Sessler, J. L.; Kral, V.; Lynch, V. J. Am. Chem. Soc. 1996,
118, 5140–5141. (c) Gale, P. A.; Sessler, J. L.; Kral, V.; Lynch, V. J. Am.
Chem. Soc. 1996, 118, 12471–12472. (d) Floriani, C. Chem. Commun. 1996,
1257–1263.
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Y. J. Chem. Soc., Perkin Trans. 2 2002, 88–93. (b) Frish, L.; Vysotsky,
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