Deconstruction of capsules using chiral spacers

Article abstract of DOI:10.1002/anie.201103031

Interconvertible host: Extended cavitands and capsules that recognize n-alkanes were generated using N-methyl glycoluril as a chiral spacer. The two host assemblies were interconverted by factors such as temperature, concentration, and guest length (see s

Full text of DOI:10.1002/anie.201103031

Communications
DOI: 10.1002/anie.201103031
Self-Assembly
Deconstruction of Capsules Using Chiral Spacers**
Yoshihiro Yamauchi, Dariush Ajami, Ji-Yeon Lee, and Julius Rebek Jr.*
Since their introduction, the shallow bowl-like resorcinarenes
have been elaborated into deeper cavitands through covalent
attachment of various aromatic panels.[1,2] The deeper cav-
itands are useful in a number of applications including
recognition,[3] catalysis,[4] sensing,[5] and switching.[6] They
also provide realistic analogs of enzyme active sites: they fold
around their target guests, isolate them from bulk solvent,
provide a hydrophobic pocket in a framework maintained by
secondary amide bonds and present the guests with reactive
functional groups.[7] A depth of approximately 1 nm is easily
achieved for cavitands,[8] but attempts to further deepen the
binding sites have been thwarted by conformational changes
that lead to solvophobic collapse[9] and the formation of
alternative structures that have no cavities.[10] We have now
used self-assembly to deepen cavitands as an alternative to
covalent synthesis and describe here the new container
structures. These arise from the application of a chiral
glycoluril as a spacer module that results in the expected
deeper cavitands but also causes the unexpected formation of
expanded, reversibly formed capsules.
Earlier, we reported that the cylindrical capsule 1·1[11] can
be expanded with glycolurils 2 when suitable guests are
present (Figure 1).[12] The hydrogen bonding possibilities
offered by glycolurils lead to a twisted “belt” of four spacer
elements inserted between the two cavitands of 1·1. The
extended complex 1·24·1 is held together through a network of
hydrogen bonds and each hydrogen bond donor and acceptor
site on the glycoluril finds a complement in the complex.[13]
Longer guests such as normal alkanes (from n-C19H40 to
C26H54) drive the assembly of hyperextended capsules.[14]
Undoubtedly, the hydrogen bonding is one of the driving
forces for the self-assembly. We considered “short circuiting”
the network by applying a N-monosubstituted glycoluril such
as 3. This was expected to dismember 1·1, stabilize[15] the deep
cavitand and, as a dividend, provide an asymmetric micro-
environment on its rim.
Addition of N-methylated glycoluril 3 (see Experimental
Section) to a solution of the normal alkanes, from n-C10H22 to
C14H30, encapsulated in 1·1 results in the new cavitand host
1·34 (Figure 2) with alkanes partially inside. As n-C11H24 is one
of the optimal alkane guests[16] for 1·1, a small amount of the
Figure 1. a) Chemical structure of cavitand 1, glycoluril 2, and N-
methylated glycoluril 3. b) Modeled structures of the cylindrical cap-
sule 1·1, extended capsule[9] 1·24·1, and new cavitand 1·34.
original complex persists, but with longer alkanes only the
new, open ended assemblies were observed. In these assem-
blies, only the first four carbons of the alkanes experience the
aromatic envelope of the cavitand and their proton signals
shift upfield by as much as Dd = ꢀ5 ppm. The CH2 groups of
the guests experience a chiral magnetic environment and
show diastereotopic signals. Several lines of evidence were
followed to confirm the details of the structure. The
integration of signals showed four N-methylated glycolurils
insert into the bifurcated hydrogen bonds of 1·1. The singlet at
ꢀ
d ꢁ 13.5 ppm indicates that all the imide N Hꢀs (Ha) are
equivalent; the array of glycolurils is highly symmetric and
diastereoselective assembly occurs; 1·(R)-34 and 1·(S)-34 were
selectively formed whereas 1·(R)-3x·(S)-34ꢀx (x = 1–3) were
not formed. The four methyl groups on 34 are oriented in the
same direction, represented as an up–up–up–up arrangement.
The belt of 3 apparently attracts normal alkanes, since these
flexible molecules are generally not good guests for open-
ended container hosts: indeed, this is our first observation of
n-alkanes in a cavitand competing with an organic solvent.[17]
A solution of 1 and 3 without the n-alkane present does not
lead to 1·34 even though 1 bears C11-alkyl feet as potential
[*] Dr. Y. Yamauchi, Prof. D. Ajami, J.-Y. Lee, Prof. J. Rebek Jr.
The Skaggs Institute for Chemical Biology and Department of
Chemistry, The Scripps Research Institute
10550 North Torrey Pines Road, La Jolla, CA 92037 (USA)
E-mail: jrebek@scripps.edu
[**] We are grateful for financial support from the NSF/CHE 1037590
and the Skaggs Institute. Y.Y. thanks the JSPS for a Research
Fellowship for Young Scientists.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201103031.
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Figure 2. a) 1H NMR spectra (600 MHz, 280 K, [D12]mesitylene) of the 1·34 complex with normal alkanes (n-C10H22 to n-C14H30) (2 mm).
b) Chemical structure of the 1·34 complex. c) Proposed model of the extended capsule (1·34)ꢂn-C13H28 (the peripheral groups are removed for
clarity). Asterisks represent the signals of encapsulated n-alkane within 1·1.
guests. Both DOSY spectroscopy and 2D NOESY are
consistent with the proposed model of the chiral cavitand
1·34 shown.
Supporting Information) of this assembly shows cross-peaks
ꢀ
between the N-methyl signal (Hh) and the imide N H signals
(Ha’). This is not reasonable for cavitand 1·34, but quite
appropriate if two of these are brought together as a capsule
that sheds four glycolurils. These aspects are all consistent
with a capsule with formulation of 1·32·32·1 as shown in
Figure 3.
To our surprise, the presence of higher alkanes (from n-
C15H32 to n-C19H40) resulted in the formation of a new,
extended but once again capsular assembly, where the methyl
groups of 3 adopt an up–down–up–down orientation. The
NMR spectra (Figure 3) indicate that the alkanes compress to
fit into the new capsule as both the number of upfield-shifted
CH2 signals and Dd value increase. Integration of the signals
indicates the presence of four glycolurils, two cavitands, and
An alternative structure 1·34·1 in which the N-methylated
glycolurils adopt a twisted orientation[18] in the middle of the
capsule (similar to 1·24·1, where each glycoluril is rotated
clockwise or anticlockwise some 308 from 1·32·32·1) was also
considered. Calculations (DFT; see Supporting Information)
of the host frameworks indicated that this arrangement is
considerably (13.6 kcalmolꢀ1) less stable than that of 1·32·32·1
proposed. Also, 2D NOESY spectroscopy is consistent only
with structure 1·32·32·1. A comparison of the pattern of the
upfield-shifted signals of encapsulated n-C17H36 in 1·32·32·1
versus 1·24·1 indicated that the 1·32·32·1 is slightly longer than
ꢀ
one guest in the assembly. The imide N H signals (Ha, Ha’) are
the furthest downfield resonances (13–14 ppm) and they are
separated as capsuleꢀs symmetry is reduced from fourfold to
ꢀ
twofold. Two of the ureido N Hꢀs (He, Hf) form weaker
hydrogen bonds with imide carbonyls of the cavitands; the
ꢀ
remaining ureido N Hꢀs (Hg) hydrogen-bond to the carbonyl
of the adjacent glycoluril. The NOESY spectrum (see
Figure 3. a) 1H NMR spectra (600 MHz, 300 K, [D12]mesitylene) of 1·32·32·1 complex with normal alkanes (n-C15H32 to n-C19H30). b) Chemical
structure of 1·32·32·1 complex. c) Proposed model of the encapsulation complex (1·32·32·1)ꢂn-C17H36 (the peripheral groups were removed for
clarity).
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Communications
1·24·1 (see Supporting Information). The DOSY spectrum of a
assembly components than the latter. This was also supported
by 1H NMR spectra at 278C, in which the extended cavitand
(1·34)ꢂn-C13H28, gave rise to slightly broadened signals of the
ureido N-H groups (He, Hf, Hg, see Supporting Information),
implying intermediate exchange of bound and free states. The
exchange was slowed by cooling (1·34)ꢂn-C13H28 in the
mesitylene medium, and the CD intensity became stronger.
Another surprising feature of this system is the coex-
istence of the two new assemblies 1·34 and 1·32·32·1 in the
presence of certain alkane guests. Figure 5 shows the distri-
bution of the assemblies as a function of glycoluril concen-
tration and temperature, where n-C14H30, n-C17H36, and n-
C18H40 were used as a guest, respectively. The overall process
is represented by the equilibrium (1):
mixture of 1·32·32·1 and 1·1 shows a smaller diffusion
coefficient for the former (see Supporting Information).
Likewise, a pairwise DOSY experiment shows nearly the
same diffusion coefficient for 1·1 and 1·34 (see Supporting
Information).
Normal alkanes longer than C19H40 do not fit into the
inner space of extended capsule 1·24·1.[18] Instead, the capsular
assembly elongates by recruiting two, three, or four belts of
(unsubstituted) glycoluril spacers in response to the increas-
ing length of the guest, represented as 1·(24)n·1 (n = 2–4).[14]
This is not possible for the N-methyl glycouril 3 as there are
no optimal arrangements to extend the dimensions of a
capsule while maximizing the number of hydrogen bonds.
Consequently, n-C20H42 and longer alkanes are not guests for
extended capsule 1·32·32·1. But n-C20H42 appears to be a good
guest for extended cavitand 1·34, since 1H NMR spectra of the
solution (see Supporting Information) is similar to that shown
in Figure 2a.
As mentioned above, the new assemblies emerge with
complete diastereoselectivity. This was confirmed by resolv-
ing the racemic glycoluril 3 on a chiral HPLC column and
repeating the alkane binding experiments using the single
enantiomer. The NMR spectra of the optically active
complexes were identical to those of their racemic counter-
parts (see Supporting Information). Accordingly, each
extended cavitand (1·34) and capsule (1·32·32·1) in the
spectrum of Figure 2a and Figure 3b, contains only one
enantiomer of the glycoluril shown as Figure 2c and 3c,
respectively. The CD spectra of the optically active assembly
with n-C13H28 and n-C17H36 guest were characteristic of their
structures (Figure 4); the CD signals of chiral assembly were
distinct from that of chiral 3. At 258C, (1·34)ꢂn-C13H28
showed weaker Cotton effects than (1·32·32·1)ꢂn-C17H36,
indicating the former has a less stable interaction between
ð1 ꢃ 32 ꢃ 32 ꢃ 1Þ ꢂ G þ Gfree þ 4 3 Ð 2 ð1 ꢃ 34Þ ꢂ G
ð1Þ
While the trends are consistent with the expectations
based on mass action (the excess 3 leads to more cavitands)
and entropy (higher temperatures favor more particles), there
are apparently some limits to these simple interpretations. For
example, since the n-C17H36 is the best fit for the capsular
assembly, the resulting complex (1·32·32·1)ꢂn-C17H36 will
tolerate an excess of 3. The degree of attraction between
host and guest is also an important factor in governing the
equilibrium.
In conclusion, we described formation of extended chiral
cavitand 1·34 as well as extended chiral capsule 1·32·32·1
through diastereoselective assembly of N-methylated glyco-
luril spacer 3 with the tetraimide cavitand 1. These host
structures were selectively constructed depending on the
length of guest molecules, component ratio, and temperature;
moreover they interconvert. The dimensions and conforma-
tions of normal alkane guests can be manipulated in these
hosts with changes in external stimuli. The reversible
encapsulation reveals molecular
behavior in confined spaces and
there is much evidence that encapsu-
lated molecules behave quite differ-
ently than those in dilute solution.[19]
The cavitands and capsules provide a
convenient method of isolating mol-
ecules and observing them in very
small spaces. There is also a broader
perspective in biology since the
majority of medicines are synthetic
molecules that exert their effects
when they fit into the small cavities
of proteins and nucleic acids.
Experimental Section
1H and 13C NMR spectra were recorded on
a Bruker DRX-600 spectrometer with a
5 mm QNP probe, where chemical shifts
were determined with respect to non-
deuterated residue DMSO (d = 2.50 ppm)
and non-deuterated mesitylene (d =
6.63 ppm) for 1H NMR spectroscopy and
DMSO (d = 39.52 ppm) for 13C NMR
Figure 4. CD spectra in mesitylene of 3 (5.0ꢀ10ꢀ4 m, dotted lines) at 258C; the blue and red colors
correspond to the first and second fractions in the chiral HPLC separation, respectively. The solid
lines indicate the CD spectra of a) (1·34)ꢂn-C13H28 and b) (1·32·32·1)ꢂn-C17H36 under the same
conditions as 3, where the concentration of 3 is 5.0ꢀ10ꢀ4 m. Absorption spectra of the
corresponding solution for c) (1·34)ꢂn-C13H28, d) (1·32·32·1)ꢂn-C17H36, respectively.
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General procedure for formation of assemblies: To a solution of
cavitand 1 (2.0 mg, 1.2 mmol) in [D12]mesitylene (0.6 mL) in 5 mm
NMR tube was added 3 ([2.8 mg, 5.0 mmol] for 1·34 or [1.4 mg,
2.5 mmol] for 1·32·32·1). The neat guest (20 equiv) was added into the
solution and the tube was placed in an ultrasonic bath (230W) and
sonicated for 5–10 min or briefly heated with heat gun to give a clear
solution.
Received: May 3, 2011
Revised: June 15, 2011
Published online: August 24, 2011
Keywords: chirality · host-guest systems ·
.
reversible encapsulation · self-assembly
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spectroscopy as internal standards. Deuterated DMSO and mesity-
lene were obtained from Cambridge Isotope Laboratories, Inc. High-
resolution mass spectra (HRMS) was recorded on an agilent ESI-
TOF mass spectrometer. Preparative chiral HPLC was performed at
room temperature using ethanol/hexane (1:9) as an eluent on a
CHIRAL TECHNOLOGIES INC CHIRALCELL OD column
(2 cm-fꢁ 25 cm) using a SHIMADZU type liquid chromatograph
LC-6AD HPLC pump, equipped with SHIMADZU type SPD-10A
variable-wavelength UV/vis detector. CD and UV/vis spectra were
recorded on Aviv Model 202 Circular Dichroism Spectrometer and
Cary 50 UV-Visible Spectrophotometer, respectively. All the CD and
UV/vis spectra were measured in a 2 mm cell.
All reagents were obtained from commercial suppliers and used
without further purification. Deuterated NMR solvents were
obtained from Cambridge Isotope Laboratories, Inc., Andover,
MA, and used without further purification.
Synthesis of compound 3: To a dry THF solution (275 mL) of the
glycoluril derivative 2 (3.15 g, 5.74 mmol) was added NaH (275 mg,
11.6 mmol) and methyliodide (1.63 g, 11.5 mmol). The resultant
solution was stirred at room temperature for 12 h. The solvent was
evaporated in vacuo, and the residue was resolved in ethylacetate.
The ethylacetate layer was washed with water and brine, dried over
anhydrous Na2SO4, filtrated, and concentrated in vacuo. The crude
product was purified by column chromatography on silica gel
(gradient elution from hexane/ethyl acetate = 1:1 to ethyl acetate)
to give 3 as a white powder (1.18 g, 2.09 mmol) in 36% yield. 1H NMR
(600 MHz, 330 K, [D6]DMSO): d = 7.64 (s, 1H, NH), 7.30 (s, 1H,
NH), 7.18 (s, 1H, NH), 6.82 (d, 2H, J = 8.4 Hz, ArH), 6.68 (d, 2H, J =
8.4 Hz, ArH), 6.34 (d, 2H, J = 8.4 Hz, ArH), 6.31 (d, 2H, J = 8.4 Hz,
ArH), 3.13 (br, 8H, CH2), 2.53 (br, 3H, CH3), 1.39 (m, 8H, CH2), 1.26
(m, 8H, CH2), 0.88 ppm (m, 12H, CH3); 13C NMR (150 MHz, 300 K,
[D6]DMSO): d = 160.50, 159.62, 147.20, 147.13, 128.02, 127.91, 124.05,
121.39, 110.52, 110.27, 85.18, 79.99, 49.71, 49.68, 28.77, 25.86, 19.58,
13.82 ppm, HRMS (ESI +); m/z calcd for [M+H]+: 563.4068; found
563.4075.
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