Synthesis of a Rigid C3v-Symmetric Tris-salicylaldehyde as a Precursor for a Highly Porous Molecular Cube

Article abstract of DOI:10.1002/chem.201404829

The development of a synthetic approach to a C_3v-symmetric tris-salicylaldehyde based on triptycene is presented. The tris-salicylaldehyde is a versatile precursor for porous molecular materials, as demonstrated in the [4+4] condensation reaction with a triptycene triamine to form a molecular shape-persistent porous cube. The amorphous material of the molecular porous cube shows a very high surface area of 1014 m² g^-1 (BET model) and a high uptake of CO₂ (18.2 wt % at 273 K and 1 bar). Furthermore, during the multistep synthesis of the tris-salicylaldehyde precursor, a relatively rare (twofold) addition of the aryne to the anthracene in the 1,4- and 1,4,5,8-positions have been found during a Diels-Alder reaction, as proven by X-ray structure analysis.

Full text of DOI:10.1002/chem.201404829

DOI: 10.1002/chem.201404829
Full Paper
&
Organic Cage Compounds
Synthesis of a Rigid C3v-Symmetric Tris-salicylaldehyde as
a Precursor for a Highly Porous Molecular Cube
Sven M. Elbert, Frank Rominger, and Michael Mastalerz*[a]
Abstract: The development of a synthetic approach to a C3v-
symmetric tris-salicylaldehyde based on triptycene is pre-
sented. The tris-salicylaldehyde is a versatile precursor for
porous molecular materials, as demonstrated in the [4+4]
condensation reaction with a triptycene triamine to form
a molecular shape-persistent porous cube. The amorphous
material of the molecular porous cube shows a very high
surface area of 1014 m2 gÀ1 (BET model) and a high uptake
of CO2 (18.2 wt% at 273 K and 1 bar). Furthermore, during
the multistep synthesis of the tris-salicylaldehyde precursor,
a relatively rare (twofold) addition of the aryne to the an-
thracene in the 1,4- and 1,4,5,8-positions have been found
during a Diels–Alder reaction, as proven by X-ray structure
analysis.
of porous materials.[10,11] Since then, we and others have
shown that shape-persistent organic cage compounds can be
used as molecular units to form porous materials with different
pore sizes.[12,13] Recently, we introduced the synthesis of a dis-
crete boronic ester cage with pores in the mesoporous regime,
exceeding 2 nm and very high surface areas of 3758 m2 gÀ1
(BET).[12f] With these values, this material based on discrete
molecules is one of the most porous crystalline materials
known and comparable to some of the best performing
metal–organic frameworks (MOFs).[14] What distinguishes
porous cages from other porous (three-dimensional network)
materials is their solubility, and this has been exploited to pro-
cess cage compounds for several purposes, for example, by
embedding the cages into mesoporous silica or membranes to
enhance gas sorption selectivity;[15,16] making porous organic
alloys;[17] bringing them onto the surface of quartz crystal mi-
crobalances to enhance the selective recognition of certain an-
alytes in the vapour phase,[18] or post-functionalising them to
change their chemical stability[18b,19] or gas sorption proper-
ties.[20]
Introduction
Donald Cram finished his Nobel lecture in 1987 with the fasci-
nating topic of carcerands, which he conceptually called syn-
thetic molecular cells,[1] and envisioned the manifold possibili-
ties of such porous molecular cells. Nowadays, these kinds of
molecular organic structures he termed carcerands are better
known as organic cage compounds.[2] Indeed, since the first
description of shape-persistent organic cage compounds by
Vçgtle and Kiggen, who synthesised them for the selective rec-
ognition of Fe3+ [3]
, interest in those structures gained a renais-
sance through the introduction of the concept of dynamic co-
valent chemistry (DCC).[4] By applying DCC, cage compounds
are accessible in high yields from simple molecular building
blocks.[5] It was again the group of Cram who applied this
method in 1991 to connect two pre-organised molecular resor-
cinarene tetraldehydes with four phenylene diamines to form
covalent organic capsules in a one-pot, eightfold imine con-
densation reaction in high yields.[6] Since then, the method has
often been adopted to obtain organic cages in various geome-
tries and sizes.[5,7]
In 2009, Cooper et al.[8] and Atwood et al.[9] showed that or-
ganic cage compounds could be used as molecular units to
create porous materials with a reported maximum measured
surface area of 624 m2 gÀ1 (BET model). This approach to create
porous materials simply by self-assembling molecules through
weak interactions without making new bonds was unique at
that time; the contribution from the groups of Cooper and
Atwood could be seen as the birth of a new area in the field
As mentioned above, several geometries have been realised
for organic cages by applying DCC.[7,21b] Although molecular
cubes have been made previously by imine condensation, as
well as boronic ester formation, they either have not been in-
vestigated in terms of porosity or collapse upon desolvation of
the “virtually” porous material, due to the intrinsic flexibility of
the molecular structures.[21]
We envisioned that rigid C3v-symmetric triptycene tris-alde-
hyde 1 could be a potential precursor for synthesising a cube
in the stoichiometric [4+4] condensation reaction with tria-
mine 2[22] (Scheme 1), and due to its geometry and inherent ri-
gidity should be stable enough to form a porous molecular
material when evacuated.[11] Furthermore, the molecular struc-
ture of 3 contains relatively large quadratic windows (diame-
ter=1.01–1.14 nm) and a molecular void with a diameter of
d=1.31 nm between two opposite triptycene bridgehead pro-
[a] S. M. Elbert, Dr. F. Rominger, Prof. Dr. M. Mastalerz
Organisch-Chemisches Institut
Ruprecht-Karls-Universitꢀt Heidelberg
Im Neuenheimer Feld 270, 69120 Heidelberg (Germany)
E-mail: michael.mastalerz@oci.uni-heidelberg.de
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201404829.
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tons. Molecule 3, with the geometrical shape of an open cube
without peripheral side chains, should not be able to pack
tightly according to the concept of frustrated molecular pack-
ing.[23] Therefore, we assumed that cube 3, and thus, tris-salicy-
laldehyde 1 would be an attractive synthetic target. Herein, we
present the development of a synthetic method to the rigid
precursor 1. Through condensation reactions with 3, a highly
porous molecular cube is formed in the solid state with a high
specific surface area of 1014 m2 gÀ1 and a narrow pore size dis-
tribution, absorbing up to 18.4 wt% of CO2 at 273 K and 1 bar.
chlorotriptycenes in a syn/anti ratio
of 1.0:1.9. They attributed this
non-statistical ratio to the induc-
tive effect of the chloro substitu-
ents.[28] In 1986, Rogers and Averill
used the idea from Mori et al. and
extended the functionalities from
chloro to cyano, acetic acid methyl
ester and methyl substituents, and
also synthesised trisubstituted trip-
tycenes with mixed functional
groups.[29]
Figure 1. Numbering of tripty-
cene consistent with the 9,10-
dihydro-9,10-o-benzenoan-
thracene nomenclature.
To synthetically achieve tris-sali-
cylaldehyde 1, the tris-hydroxy-
substituted scaffold has to be formed within such a Diels–
Alder reaction in a fluoride-mediated addition of triflate 4[30] to
1,8-dimethoxyanthracene (5).[31] Addition of the in situ formed
aryne to positions 9 and 10 of the anthracene should lead to
the triptycenes 6a and 6b (Scheme 2).
Scheme 1. Synthesis of a rigid [4+4] cube (3) in a twelvefold Schiff base
condensation reaction of four molecules of 1 and four molecules of 2. The
arrow shows the pore diameter of about 1.31 nm, as extracted from an
AM1-optimised model.
Scheme 2. Diels–Alder reaction of triflate 4 and anthracene 5 to the desired
trisubstituted triptycenes 6a/6b and the unexpected by-products rac-6c
and rac-6d. The dashed box marks an inseparable mixture.
After reacting triflate 4 and anthracene 5 in dry acetonitrile
Results and Discussion
with caesium fluoride as a base for 40 h, monitoring of the re-
1
action by TLC showed four products and the H NMR spectrum
Synthesis and characterisation of the triptycene tris-salicy-
laldehyde precursor
of the crude product revealed some characteristic signals in
the region around d=5 ppm, which could not be assigned to
triptycene bridgehead protons (Figure 2a).
Non-substituted triptycene[24] is known to preferably react in
electrophilic aromatic substitution reactions at the 2,6,14- and
2,7,14-positions[22,25,26] (for numbering of the positions on the
triptycene scaffold, see Figure 1) due to the so-called fused
ortho effect.[27] Therefore, functional groups in the 1-, 8-, 13- or
16-positions have to be introduced into the triptycene scaffold
during the Diels–Alder reaction by choosing the corresponding
substituted aryne and anthracene as starting materials. For in-
stance, in 1971, Mori et al.[28] presented the first examples of
such substituted triptycenes by using 1,8-dichloroanthracene
and 6-chloroanthranilic acid as reactants to obtain the so-
called syn (1,8,13-substituted) and anti (1,8,16-substituted) tri-
Due to their low solubility, triptycenes 6a and 6b could be
isolated in 51% yield as a white solid after washing the crude
1
product with water, methanol and CH2Cl2. By H NMR spectro-
scopic analysis of the remaining white solid, it was confirmed
that a 2:1 mixture of syn-6a and anti-6b has been formed (Fig-
ure 2b). The ratio of the triptycene isomers was estimated by
integration of the signals resonating at d=5.53 and 5.85 ppm,
which could be assigned to bridgehead protons He and He’ of
the C3v- and Cs-symmetric triptycenes 6a and 6b, respectively.
The ratio of 2:1 deviates from the statistically expected 1:1
ratio and is comparable to the first findings by Mori et al.[28]
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Figure 3. Left: mesomeric effects of the methoxy groups on 1,8-dimethox-
yanthracene (5). Right: inductive effects of the methoxy group on a me-
thoxy-substituted aryne.
doublets (J=6.1 and 1.4 Hz) with coupling to the olefinic pro-
tons Hi (d=7.07 ppm, J=6.0 Hz) and Hj (d=7.24 ppm, J=1.1–
1.4 Hz). It was difficult to determine by 2D NMR spectroscopy
methods whether the formation of the 1,4-adduct occurred in
a syn or anti fashion. Therefore, we grew single crystals of 6c
suitable for X-ray analyses[34] by slow diffusion of n-pentane
into a solution of 6c in CH2Cl2 (Figure 4a). X-ray analyses
proved the syn addition of the aryne to the 1,4-position of the
anthracene to give rac-6c (for detailed information on the
crystal structure, see Table S1 in the Supporting Information).
Such 1,4-adducts were first reported by Klanderman[35] and
recently by McKeown and co-workers,[36] but are still a rare ex-
ception in the reaction of arynes and anthracenes.[37] Again,
this rare cycloaddition can only be explained by frontier orbital
controlled regioselectivity, similar to the situation explained
above. The second isolated fraction showed a less complicated
Figure 2. 1H NMR spectra of: a) the crude product of the Diels–Alder reac-
tion depicted in Scheme 2, b) mixture of 6a/6b isolated in a 2:1 ratio, c) rac-
6c, and d) rac-6d. All spectra were recorded in [D6]DMSO at 500 (a and b)
and 400 MHz (c and d). Spectra in a) and b) were measured at 375 K. The as-
terisks indicate residual CH2Cl2.
This non-statistical product distribution was already observed
by others[28,29,32] and could be explained by frontier orbital con-
trolled regioselectivity during cycloaddition, similar to the de-
scriptions of Rogers and Averill.[29] In the [4+2] cycloaddition,
the anthracene represents the diene component. If the p elec-
trons of the central ring are involved in the reaction, the meso-
meric effects of the methoxy substituents have to be taken
into account, which means that a positive partial charge is
generated at position 9, and there is a negative partial charge
at position 10 (Figure 3, left). The aryne acts as a dienophile in
the reaction. The formally triple-bond electrons involved are lo-
cated in sp2 orbitals that lie in the s-bond plane of the molec-
ular scaffold.[33] Therefore, here the inductive effect of the me-
thoxy group over the s-bond scaffold has to be taken into ac-
count, rather than the mesomeric effect; this results in a nega-
tive partial charge at position 2 and a positive partial charge at
position 3 of the aryne (Figure 3, right). In principle, position 9
of the anthracene preferably interacts with position 2 of the
aryne by attractive coulomb interactions, which is a rational ex-
planation for the difference in the observed 2:1 syn/anti ratio
from a statistical 1:1 mixture, similar to that described previ-
ously by Mori et al.[28]
1
signal pattern in the H NMR spectrum than that of 6c (Fig-
The extract in CH2Cl2 contained two more Diels–Alder prod-
ucts, which could be isolated by column chromatography. The
molecular mass of the first by-product, 6c, was m/z 344 (EI-
MS), which was the same as that for triptycenes 6a and 6b. By
1H NMR spectroscopy, it was found that 6c was the 1,4-adduct
of the Diels–Alder reaction (Scheme 2). Two characteristic sin-
glets at d=8.09 and 7.64 ppm can be assigned to protons Hf
and Hg of the naphthyl substructure (Figure 2c). At d=
5.22 ppm, the bridgehead proton Hh resonates as a doublet of
Figure 4. Single-crystal X-ray structures of: a) rac-6c, and b) rac-6d. Only
one isomer of each is depicted. Carbon: light grey, oxygen: dark grey, hydro-
gen: white. Thermal ellipsoids are shown with 50% probability. For further
details, see Tables S1 and S2 in the Supporting Information.
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ure 2c) and in the EI-MS a mass signal of m/z 450 was found,
which corresponded to the twofold addition of the aryne to
the anthracene. 1H NMR spectroscopy indicates the structure
of 6d because of the characteristic singlets that can be as-
signed to protons Hi (d=7.49 ppm) and Hj (d=7.19 ppm) of
the central aromatic ring.
pounds 7a and 7b are not significantly more soluble than 6a
and 6b; therefore, these compounds are also not possible to
separate by column chromatography on reasonable scales.
Several attempts to fractionally crystallise the mixtures were
not successful. To obtain soluble derivatives of 7a/7b, esterifi-
cation with capronyl chloride in the presence of methyl imida-
zole to the corresponding esters 8a/8b was performed
(Scheme 3).
Those singlets appear to be shifted to high field relative to
those of 6c. Even more characteristic is the signal of bridge-
head proton Hk at d=4.99 ppm. Through crystallisation, again
from CH2Cl2 and n-pentane, suitable crystals of rac-6d have
been obtained (for detailed information, see Table S2 in the
Supporting Information), which further proved the twofold syn
addition in the formation of the 1,4,5,8-bis adducts as a racemic
mixture. Although 6d has been isolated in only 3% yield, it
should be noted that 1,4,5,8-bisadducts have not been de-
scribed in the cycloaddition of arynes to anthracenes and, to
the best of our knowledge, a similar product has only been ob-
served in the twofold addition of dicyanoacetylene to anthra-
cenes with bulky phenyl substituents at positions 9 and 10.[38]
In this context, it is worth mentioning that, for the formation
of rac-6d, in principle, a [4+2] Diels–Alder reaction at the
naphthalene scaffold of rac-6c has to occur; which is generally
rarely observed.[39] Most interestingly, other regioisomeric by-
products of the 1,4-additon and the 1,4,5,8-twofold addition
have not been found,[40] which once more could be explained
by the directing effects of the
To our delight, the desired C3v-symmetric ester 8a crystal-
lised from the reaction mixture to give the pure compound in
1
32% yield. The structure was proven by H NMR spectroscopy
by the appearance of just one set of signals for the aromatic
protons (d=7.25, 7.00 and 6.79 ppm) and two distinct singlets
for the bridgehead protons at d=5.82 and 5.50 ppm. Further-
more, crystals suitable for X-ray crystal-structure analysis were
obtained (Figure 5 and Table S3 in the Supporting Information)
and additionally proved the C3v-symmetric structure of 8a.
The mother liquor of crystallisation contained both isomers
(syn and anti) in a ratio of approximately 1:1, as determined by
the integration of characteristic signals in the 1H NMR spec-
trum. However, this mixture was difficult to separate by
column chromatography, as originally intended. Nevertheless,
this promising result of selective fractional crystallisation
brought us to test other esterifications to separate the two iso-
mers. Both the formed acetyl esters and benzyl esters showed
methoxy substituents in the re-
action, as discussed above.
To continue with the synthetic
sequence to form tris-salicylalde-
hyde 1, the two regioisomers 6a
and 6b have to be separated
prior to further reactions at the
aromatic scaffold. Although for
a dilute sample of the mixture of
triptycene 6a/6b two fractions
were detectable by analytical
HPLC, this method could not be
used for larger quantities, simply
because of the very low solubili-
ties of triptycenes 6a and 6b. In-
terestingly, this is in accordance
with other triptycene derivatives
with three substituents in posi-
tions 1, 8, 13 or 16 and seems to
be less dependent on the nature
of the substituent itself.[28,29,32]
To separate the two isomers
6a and 6b, they had to be trans-
formed into soluble derivatives
to make them processable, for
example, by column chromatog-
raphy. Therefore, the methyl
ethers were first quantitatively
cleaved by BBr3 to give the cor-
Scheme 3. Synthetic separation of syn- and anti-trisubstituted triptycenes. The dashed boxes mark inseparable
mixtures. [a] R=C5H11, RT, 16 h; [b] R=CH3, CH2Cl2, 08C–RT, 16 h; [c] R=Ph, RT, 16 h. Note: 7a and 7b were syn-
responding triptycenes 7a/7b
(Scheme 3). The hydroxyl com- thesized from 8a and 8b exclusively.
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crystal, see the Supporting Information) clearly proved the in-
troduction of formyl moieties and could be attributed to vibra-
tion of the carbonyl groups.
The selective formylation in the ortho position to the hy-
droxyl group of 7a can be proven by 2D NMR spectroscopy
(see the Supporting Information) and also by single-crystal X-
ray analysis: crystals with sufficient quality have been obtained
by slow diffusion of water into a solution of 1 in [D6]DMSO
inside an NMR tube (for data, see Table S7 in the Supporting
Information).
The obtained crystal structure proves ortho-selective formy-
lation as well as the proposed unidirectional orientation of all
three salicylaldehyde functions (Figure 6a). The three aldehyde
functions are co-planar to the aromatic phenyl rings. The aver-
age bond length of the C=O bonds is 1.23 ꢁ slightly elongated
in comparison to non-substituted benzaldehyde (1.21 ꢁ)[42] and
the average phenolic CÀO bond is in comparison to those in
simple phenols, such as 4-tert-butyl phenol[43] shortened from
1.39 to 1.36 ꢁ); these both indicate hydrogen-bond-assisted
resonance effects.[44] The molecules self-assemble anisotropical-
ly within the crystal through p–p stacking (the distance be-
tween two adjacent aromatic rings is d=3.68 ꢁ; see the green
dotted line in Figure 6b) and form one-dimensional channels
along the crystallographic c axis (Figure 6c) with a pore diame-
ter of about 0.7 nm. According to calculations by CrystalExplor-
er,[45] for 1 a specific surface area of 374 m2 gÀ1 should be ac-
cessible. However, investigations of the porosity of this materi-
al are beyond the scope of this contribution and investigations
in this respect will be reported elsewhere.
Figure 5. Single-crystal X-ray structure of triptycene tricapronyl ester 8a.
Only one molecule of the asymmetric unit is depicted. Carbon: light grey,
oxygen: dark grey, hydrogen: white. Thermal ellipsoids are shown with 50%
probability.
the same fractional crystallisation behaviour as the capronyl
esters, giving the acetyl ester of 9a in 43% yield and benzyl
ester 10a in 42% yield as fractionated crystals. In contrast to
the capronyl esters, here, both isomers remaining in the
mother liquor could be separated by column chromatography
to give an additional 13% yield of syn isomers 9a and 10a, in
addition to 19% of 9b and 16% of 10b. All isolated esters
have been characterised by NMR spectroscopy, MS and IR
spectroscopy; additionally, the purity was proven by elemental
analysis (see the Experimental Section and the Supporting In-
formation). For 9a, 9b and 10a, crystals of suitable quality for
X-ray structure analysis have also been obtained (see Ta-
bles S4–S6 in the Supporting Information).
The desired pure trihydroxytriptycene isomer 7a was ob-
tained in nearly quantitative yield by ester cleavage of 9a in
a mixture of ethanol and hydrochloric acid (Scheme 3). Under
similar conditions, anti-trihydroxytriptycene 7b was obtained
in 96% yield from 9b.
Finally, through the reaction of 7a under Duff conditions,[41]
tris-salicylaldehyde 1 was obtained in 47% yield as the sole re-
gioisomer (Scheme 4).
The 1H NMR spectrum of 1 shows a sharp signal at d=
9.96 ppm for the three aldehyde protons and a broad signal at
d=11.00 ppm, which corresponds to the hydroxyl protons.
The signals of the bridgehead protons can be found at d=
6.84 ppm and at d=6.02 ppm. CI-MS shows the signal of the
molecular ion [M]+ at m/z 386, as well as the signal of [M+H]+
with m/z 387, which indicates the formation of 1. By IR spec-
troscopy, the appearance of defined bands at n˜ =1660 (KBr
pellet) or 1649 cmÀ1 (Ge attenuated total reflectance (ATR)
Figure 6. a) X-ray crystal structure of 1. Thermal ellipsoids are shown with
50% probability. b) Enlarged packing of 1 within the unit cell. The green
dotted line shows the p–p stacking with a distance of d=3.68 ꢁ. Voids in
the crystal are shown in blue and turquoise. c) Packing of 1 as a capped
stick model and one-dimensional pores (blue and turquoise) formed along
the crystallographic c axis. One molecule of 1 is shown in black as a ball and
stick model. Carbon: grey; oxygen: red; hydrogen: white.
Scheme 4. Selective formylation of 7a in the ortho position to give tris-sali-
cylaldehyde 1. HMTA=hexamethylenetetramine, TFA=trifluoroacetic acid.
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Synthesis and characterisation of a porous [4+4] cube
Triptycene tris-salicylaldehyde 1 is a potential building block
for condensation reactions. Before using 1 in the reaction with
triamine 2 to synthesise a molecular cube, salicylimine 12 was
synthesised in a threefold condensation with para-toluidine
(11) in 73% yield (Scheme 5) as a model compound for spec-
troscopic comparison with molecular cube 3.
1
The H NMR spectrum of 12 shows a characteristic singlet
at d=8.93 ppm for the three imine protons and the signal for
the aldehydic protons at d=9.96 ppm of the precursor tris-sali-
cylaldehyde 1 are no longer detectable. The 13C NMR spectrum
of 12 also reveals successful imine condensation through
a signal at d=162.7 ppm for the imine 13C nuclei; the signal
for the aldehyde 13C nuclei of 1 at d=196.2 ppm is no longer
detectable. Suitable crystals of 12 for single-crystal X-ray analy-
sis were obtained by vapour diffusion of n-pentane into a solu-
tion of 12 in CH2Cl2 (Figure 7 and Table S8 in the Supporting
Information). It is known that salicylimines can undergo reversi-
ble proton transfer to form their keto–enamine tautomer.[46]
Tris-salicylimine 12 has an average bond length of d=1.35 ꢁ
for the phenolic CÀO bonds, which is comparable to those of
tris-salicylaldehyde 1 and again shorter than that for simple
phenols (1.39 ꢁ);[43] thus, the imine–ol tautomer is more pro-
nounced than the keto–amine tautomer. Furthermore, the
length of the imine C=N bond (d=1.28 ꢁ) is slightly longer
than that for non-hydrogen-bonded aromatic imines
(1.25 ꢁ).[47] These bond lengths are in a typical range for the
proposed salicylimine structure at 200 K and again indicates
hydrogen-bond-assisted resonance.[44] The absence of charac-
teristic signals for the carbonyl groups in 13C NMR and IR spec-
troscopy indicate that 12 is also more likely to be present in
the salicylimine tautomeric form at room temperature than in
the ketoamine tautomer.
Figure 7. X-ray crystal structure of 12. Carbon: light grey; oxygen: black; ni-
trogen: dark grey ; hydrogen: white. Solvent molecules are omitted for clari-
ty. Thermal ellipsoids are shown with 50% probability.
2524.78, which correlated with the calculated monoisotopic
mass expected for cubic cage compound 3 (m/z 2524.75). It is
important to exchange DMF used as a medium in the reaction
by immersing the material in dry diethyl ether (3ꢂ24 h) to
allow cage compound 3 to remain stable towards filtration.
Water has to be strictly excluded.[48]
When carefully worked up, dried cage compound 3 showed
only one sharp signal with the expected isotopic pattern (m/z
2525.3) by MALDI-TOF (Figure 8) and HRMS (MALDI; m/z
2524.78; see the Supporting Information).
Similar to the [4+6] exo-functionalised cage we described
earlier, [12b] lower reaction temperatures mainly result in the for-
mation of intermediates, as determined by MALDI-TOF MS.
To exclude the formation of catenanes, a mass spectrum in
the range of m/z 750–8000 was recorded and no further sig-
nals were detected (see also the Supporting Information).[12g,49]
Additionally, no smaller (e.g., [1+2] or [2+1]) or larger (up to
Cubic cage compound 3 was obtained in 65% yield by
a twelvefold Schiff base reaction of tris-salicylaldehyde 1 and
triaminotriptycene 2 after stirring the reaction mixture for
3 days at 1208C in dry DMF (Schemes 1 and 5).[12b] It was
found by MALDI-MS that the bright-yellow, still-wet solid
showed a pronounced signal with a monoisotopic mass of m/z
Scheme 5. Condensation reactions of tris-salicylaldehyde 1 with 11, to give model compound 12, and with triaminotriptycene 2 to give [4+4] cage com-
pound 3.
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110 and 160 ppm correlates well with that of model com-
pound 12. The signals of the bridgehead 13C nuclei of 3
appear at d=53–55 and 32–33 ppm, which is in the same
region as the bridgehead 13C nuclei of 12 (d=55.6 and
32.2 ppm).
Figure 8. MALDI-TOF MS of cage 3. The insets show the isotopic pattern
measured by HRMS (MALDI; top) and the calculated isotopic pattern
(bottom).
[12+13] or [13+12]) condensation products of the reaction of
tris-aldehyde 1 and triamine 2 could be detected either, if it is
assumed that these products or intermediates were similarly
ionised through the methods typically applied in MALDI. Un-
fortunately, cage compound 3 is not soluble enough for char-
acterisation by NMR spectroscopy methods (see also discussion
Figure 9. Comparison of the 13C CP MAS NMR spectrum of 3 (top) with the
13C NMR spectrum of model compound 12 (bottom) in [D6]DMSO at
125 MHz.
Also, by IR spectra, structural analogy can be proven. The IR
below) in solution. For insoluble molecular cages, it has been spectrum of 3 shows the same significant bands at n˜ =1619
controversially discussed, whether the compound contains ki- and 1596 cmÀ1 to those of compound 12 (n˜ =1618 and
netically formed polymers in different ratios as insoluble by- 1596 cmÀ1) for the strong C=N stretching band (see the Sup-
products or not. Thermodynamically, almost complete conver- porting Information). However, a small shoulder can be detect-
sion of the precursors to cages 3 should be entropically fav- ed at n˜ ꢀ1650 cmÀ1, which might be assigned to partly cleaved
oured, in comparison to a polymeric mixture. If no polymer is imine bonds to the corresponding aldehyde and amine units
formed kinetically, it seems to be reasonable to assume that under the conditions for the preparation of the IR sample (the
the thermodynamic equilibrium is reached with the formation signal for the aldehyde precursor resonates at n˜ =1649 cmÀ1);
of cage compound 3. Therefore, we monitored the condensa- this provides a clue for the reason behind the instability of the
tion reaction by MALDI MS (see the Supporting Information). It isolated cage compound (see also discussion above).
can clearly be seen that, at the beginning of the reaction,
The porosity of cage compound 3 was studied by gas sorp-
smaller soluble intermediates, such as [1+1] and [1+2] con- tion experiments with various gases at different temperatures.
densation products, are formed and are consumed with in- Prior to gas sorption experiments, compound 3 was investigat-
creasing reaction time to form larger (still soluble) molecular ed by thermogravimetric analysis (TGA), which suggested ther-
intermediates, such as [2+2] and [2+3] condensation products. mal stability up to 4008C (see the Supporting Information).
When the first precipitation occurs, the [4+4] cage compound
After thermal activation for 3 h at 2008C in high vacuum
3 is detected. Further monitoring revealed that, with increasing (2.6ꢂ10À2 mbar), the nitrogen sorption of 3 was measured at
reaction time, smaller intermediates were consumed and finally 77 K (Figure 10). The isotherm has a typical type I shape, which
exclusively cage compound 3 could be detected. Based on is characteristic for microporous materials, and shows a small
these observations and thermodynamic assumptions, it can be but pronounced hysteresis between adsorption and desorp-
concluded that no or very little polymeric by-products are tion.[50] The derived specific surface area was found to be 1014
formed.
To further characterise cage compound 3, a 13C CP MAS volume of
(BET model) or 1122 m2 gÀ1 (Langmuir model) with a micropore
micro =0.31 cm3 gÀ1 calculated by the t-plot
V
solid-state NMR spectrum was recorded and the signals were method.[51] QSDFT calculations[52] showed a narrow pore size
compared with a 13C NMR spectrum of model compound 12 distribution with a sharp maximum at 1.22 nm (Figure 10,
from solution (Figure 9). The most downfield-shifted signal of inset) in the microporous regime (N2 adsorption branch at
cage 3 appears at d=161.1 ppm for the imine carbon nuclei; 77 K, cylindrical/spherical pores, carbon material, fitting error
this is comparable to the signal for the imine 13C nucleus of 0.431%) This value is in good agreement with the diameter of
model compound 12 (d=162.7 ppm). Furthermore, in the typi- a calculated sphere fitting to the void of 3 (d=1.31 nm, see
cal carbonyl region of d=180–220 ppm, no succinct signal is Scheme 1). In comparison, by non-localised density functional
detected. Also, the range of aromatic signals of 3 between d= theory (NLDFT) calculations (N2 adsorption branch at 77 K, cy-
Chem. Eur. J. 2014, 20, 16707 – 16720
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lindrical/spherical pores, silicon material) with d=1.11 nm,
a smaller pore diameter was found. However, although the fit-
ting error (0.310%) is smaller than that for QSDFTone has to
keep in mind that the QSDFT method was developed for
amorphous materials.[52] All gas sorption data are reproducible,
for example, second and third batches of cage compound 3
gave BET surface areas of 1006 and 979 m2 gÀ1, respectively,
and the derived pore dimensions are also similar to those dis-
cussed above.
In comparison to other amorphous porous materials based
on discrete molecular units, the high surface area of
1014 m2 gÀ1 is, to the best of our knowledge, one of the high-
est values, reported so far.[12b,d,13a,36,54] Therefore, we also inves-
tigated the adsorption of other gases such as hydrogen,
carbon dioxide and methane.
The material adsorbs 7.29 mmolgÀ1 of H2 at 77 K and 1 bar,
which correlates to 1.47 wt%, and is comparable to co-crystal-
lised cages CC1S and CC3R (7.6 mmolgÀ1, 1.52 wt%) reported
by Cooper et al.[13f] and slightly higher than the value for
[4+6]-tBu cage (5.6 mmolgÀ1, 1.13 wt%),[12d] but lower than,
for example, the values for CC5R[13f] (Table 1) or TTBI
(10.8 mmolgÀ1, 2.2 wt%).[55] At higher temperatures, the
uptake of hydrogen was almost negligible: 0.17 mmolgÀ1
(0.034 wt%) at 273 K and 0.14 mmolgÀ1 (0.028 wt%) at 263 K.
The selective adsorption of carbon dioxide from nitrogen or
methane is an important topic to remove CO2 pre- or post-
combustive.[56] Therefore, we measured the adsorption of
those gases and calculated selectivities for mixtures based on
Henry’s law.[57] At 263 K and 1 bar, 4.82 mmolgÀ1 (21.2 wt%) of
carbon dioxide is adsorbed, and at 273 K and 1 bar
4.14 mmolgÀ1 (18.2 wt%) of carbon dioxide is adsorbed. The
high amounts of CO2 adsorbed are, so far, unprecedented for
materials based on discrete molecules (here the highest report-
ed value is 3.4 mmolgÀ1 for the [4+6] exo-cage compound
published by our group).[12b] However, the value is still substan-
tially lower than that, for example, for Mg-MOF-74[58]
(8.61 mmolgÀ1; 37.8 wt% at 298 K and 1 bar) or the amor-
phous organic framework material BILP-4[59] (24 wt% at 273 K
and 1 bar). In contrast to the high amounts of carbon dioxide,
at 273 K, only 1.27 mmolgÀ1 (2.08 wt%) of methane was ad-
sorbed and, at 263 K, 1.64 mmolgÀ1 (2.6 wt%) of methane was
adsorbed, which was a higher uptake than that for most crys-
talline cage compounds, except a mixture of CC1S and CC3R
(Table 1).
*
*
Figure 10. Nitrogen sorption isotherm at 77 K. : adsorption, : desorption.
Inset: quenched solid density functional theory (QSDFT) pore size distribu-
tion.
Powder X-ray diffraction (PXRD) measurements of the mate-
rial after gas sorption revealed the amorphous character of the
sample (see the Supporting Information); this suggested that
the QSDFT method might be better applicable to describe the
pore structure of the material and indeed fit better to the pore
dimensions of the molecular model. It should be mentioned
that describing the pore structure of amorphous materials de-
rived from organic cages by theoretical models is challenging
and, although some progress has been made by the groups of
McKeown[23] and Cooper,[53] it should be mentioned that there
is still a significant difference between experimental and theo-
retical gas sorption properties.
By fitting the Langmuir–Freundlich equation to the obtained
isotherms at 263 and 273 K, parameters for the Clausius–Cla-
pyron equation[60] were obtained to calculate the isosteric heat
of adsorption for carbon dioxide and methane, depending on
the uptake (see the Supporting Information). The obtained dif-
ferences in heats of adsorption were similar. At 0.01, 0.1 or
1 mmolgÀ1 of uptake, the heats of adsorption were QSt =
Table 1. Comparison of gas sorption data for cage compound 3 with those of other cage compounds published previously.[a]
Compound
Phase
SABET [m2 gÀ1
N2, 77 K
]
SALangmuir [m2 gÀ1
N2, 77 K
]
VMirco
[cmÀ3 gÀ1
CO2 [mmolgÀ1
at 1 bar, 273 K
]
CH4 [mmolgÀ1
at 1 bar, 273 K
]
H2 [mmolgÀ1
at 1 bar, 77 K
]
Ref.
]
3
amorphous
crystalline
amorphous
crystalline
crystalline
crystalline
crystalline
crystalline
1014
2071
919
744
30
437
1333
350[h]
1122
2327
1037
835
204
n.d.
0.31
0.77
0.30
0.26
4.1 (18.2 wt%)
2.7 (11.9 wt%)
3.4 (15.0 wt%)
2.7 (11.9 wt%)[b]
3.3 (14.5 wt%)[b]
3.3 (14.5 wt%)[d]
3.1 (13.6 wt%)[g]
2.5 (11.0 wt%)[i]
1.3 (2.08 wt%)
0.7 (1.12 wt%)
0.7 (1.12 wt%)
0.7 (1.12 wt%)[b]
0.3 (0.48 wt%)[b]
1.7 (2.73 wt%)[e]
n.d.
7.3 (1.47 wt%)
5.6 (1.13 wt%)
4.7 (0.94 wt%)
n.d.
this work
[12d,20]
[12b]
[12c]
[12c]
[13f]
[4+6]-tBu
[4+6]-exo
[2+3]cage
[2+3]-extend
CC1S/CC3R (1:1)
CC5-R
[c]
–
n.d.
n.d.
n.d.
0.13
7.6 (1.52 wt%)[f]
8.5 (1.71 wt%)[g]
n.d.
n.d.
n.d.
[13f]
[9]
Noria
n.d.
[a] SA=surface area. [b] Measured at 298 K. [c] The t-plot analysis revealed that the observed surface was not derived from micropores. [d] Determined at
1.2 bar. [e] Determined at 0.85 bar. [f] Determined at 1.17 bar. [g] Determined at 1.1 bar. [h] Determined with CO2. [i] Determined at 298 K at 30 bar.
Chem. Eur. J. 2014, 20, 16707 – 16720
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spectrometer equipped with a 4 mm 1H/X CP-MAS probe. The
Bruker standard pulse program cp was used with a rotational fre-
quency of 15 kHz, a ramped contact time of 2 ms, a recycle delay
of 4 s and a spinal 64 proton broadband decoupling with a radio-
frequency (RF) field of 100 kHz. The spectrum was referenced to
external adamantane (d=38.48 ppm). CI mass spectrometry was
performed with a Finnigan MAT SSQ-7000 instrument. MALDI-TOF
mass spectra were recorded with a Bruker Daltonics Reflex III spec-
trometer and HRMS experiments were carried out on a Fourier-
transform ion cyclotron resonance (FT-ICR) mass spectrometer so-
lariX (Bruker Daltonik GmbH, Bremen, Germany) equipped with
a 7.0 T superconducting magnet and interfaced to an Apollo II Dual
ESI/MALDI source. Dithranol or trans-2-[3-(4-tert-butylphenyl)-2-
methyl-2-propenylidene]malononitrile (DCTB) were used as matri-
ces. IR spectra were recorded as KBr pellets with a PerkinElmer
FTIR Spectrum 2000 spectrometer or on a Bruker Lumos spectrom-
eter on a Ge ATR crystal. Melting points (uncorrected) were record-
ed on a Bꢄchi B-545 or B-540 apparatus. Elemental analyses were
recorded on an Elementar Vario EL instrument (Ulm University and
Microanalytical Laboratory of the University of Heidelberg). TGA re-
sults were measured on a Mettler Toledo TGA/SDTA 851 instrument
À34.47, À28.43 and À22.92 kJmolÀ1, respectively, for methane
or QSt =À32.64, À27.59 and À22.44 kJmolÀ1, respectively, for
carbon dioxide. It is known that a virial method gives more re-
liable results at lower coverage due to the incorrect fitting of
the Langmuir–Freundlich equation to the low-pressure regime.
By applying the virial method, the heats of adsorption for zero
coverage have been calculated to be Qst =À24.23 kJmolÀ1 for
carbon dioxide and Qst =À26.46 kJmolÀ1 for methane. Both
calculation methods show that neither gas has a higher physi-
cal affinity to cage compound 3. This is also represented with
the low Henry’s law selectivities of carbon dioxide over meth-
ane. The Henry’s law selectivity was calculated by dividing the
Henry’s law constants of the gases at a defined temperature.
To calculate the Henry’s law constants, a non-linear Tꢃth equa-
tion was fitted to the isotherms and the obtained parameters
were used in Henry’s law (see the Supporting Information).[57]
This method should be more reliable than giving the selectivi-
ties as a ratio of the maximum uptakes of individual gases (the
so-called ideal selectivity). The mentioned selectivity of carbon
dioxide over methane was found to be S=7.10 at 263 K and
S=7.67 at 273 K. A high Henry’s law selectivity was found for
carbon dioxide over hydrogen (S=134 at 263 K and S=105 at
273 K) and for a mixture of CO2 and N2 selectivities of S=40.1
at 263 K and S=33.7 at 273 K for carbon dioxide over nitrogen
were calculated. These are, for instance, higher than those re-
ported recently for ZIF-300 and ZIF-302 with S=17 or 22,[61] or
NOTT-202 with S=27.3[62]
with
a a nitrogen flow of
heating rate of 108CminÀ1 and
50 mLminÀ1, or were recorded on a Mettler-Toledo TGA/DSC1 in-
strument with a TGA/DSC-Sensor 1100 equipped with a MX1 bal-
ance (Mettler-Toledo) and a GC100 gas control box for nitrogen
supply. TGA samples were measured in 70 mL Al2O3 crucibles. All
measurements were carried out under
a flow of nitrogen
(20 mLminÀ1). The surface area and porosity of 3 were character-
ised by nitrogen adsorption and desorption analysis at 77.35 K
with an autosorb computer-controlled surface analyser (AUTO-
SORB-iQ, Quantachrome). The sample was degassed at 2008C (3 h)
before being analysed. The BET surface area was calculated by as-
suming a value of 0.162 nm2 for the cross-sectional area of the ni-
trogen molecules in the pressure range P/P0 =0.01–0.1. The QSDFT
model and isotherm data were used to calculate the pore size dis-
tribution (Kernel N2-carb.gai). Crystal structure analysis was accom-
plished on an Agilent SuperNova Atlas (Dual Source) diffractometer
with a copper source (l(CuKa) = 1.54178 ꢁ) as well as on Bruker
APEX-II Quazar and Bruker APEX I diffractometers with molybde-
num sources (l(MoKa) = 0.71073 ꢁ). The structures were solved
and refined with SHELXTL 2008/4 software[65] and for absorption
correction the SADABS 2012/1 software[66] was used. Powder dif-
fraction was performed with a STOE Stadi P Ge(111)-monochromat-
ed copper radiation (l(CuKa)=1.54060 ꢁ). The diffractograms were
obtained with a Stoe linear PSD detector. Compound 3 was mea-
sured in a glass capillary (Ø=0.5 mm) as a sample container. Plastic
sheets pre-coated with silica gel (Merck Si60 F254) were used for
TLC. Glass columns packed with Merck silica 60 (mesh 40–60 mm)
were used for flash-chromatography purification. All reagents and
solvents were obtained from Fisher Scientific, Alfa Aesar, Sigma–Al-
drich, ProLabo or VWR and were used without further purification
unless otherwise noted. Triaminotriptycene 2,[22] triflate 4 and 1,8-
dimethoxyanthracene (5) were synthesised according to known
procedures.[30,31] For the assignment of NMR spectroscopy signals,
triptycene was seen as 9,10-dihydro-9,10[1,2]benzenoanthracene.[67]
Conclusion
A synthetic route to triptycene tris-salicylaldehyde 1 was devel-
oped, and used as a precursor in the synthesis of a molecular
porous cube, which formed an amorphous porous material
with a very high BET surface area (1014 m2 gÀ1) and a good
Henry selectivity for the adsorption for CO2 over N2 of S=33.7.
More importantly than extraordinarily high selectivities, the
materials showed high selectivities in combination with high
amounts of adsorbed gas (18.2 wt% of CO2 at 273 K and
1 bar); this might be more feasible for applications in gas sepa-
ration.[56]
In addition to being a reactant for cubic cages, tris-salicylal-
dehyde 1 is a versatile precursor to synthesise discrete trinu-
clear salphene complexes,[54a,63] and metal-assisted salphene or-
ganic frameworks,[64] which are currently under investigation in
our laboratories.
Experimental Section
General
NMR spectra were recorded on Bruker DRX 500 (1H NMR: 500 MHz;
13C NMR: 125 MHz), DRX 400 (1H NMR: 400 MHz; 13C NMR:
100 MHz) or DRX 300 (1H NMR: 300 MHz) spectrometers at 298 K,
unless otherwise mentioned. Chemical shifts (d) were given in ppm
and were calibrated with residual non-deuterated solvent signals
(CDCl3: dH =7.26 ppm, dC =77.0 ppm; [D6]DMSO: dH =2.50 ppm,
dC =39.5 ppm) as internal standards. The 13C CP-MAS NMR spectra
were measured on a standard-bore Bruker Avance III 500.13 MHz
Synthetic Procedures
Compounds 6a and 6b: 2-Methoxy-6-trimethylsilylphenyltrifluor-
methane sulfonate (4; 4.00 g, 12.2 mmol) was added to a suspen-
sion of
5 (2.50 g, 10.5 mmol) and caesium fluoride (5.00 g,
32.9 mmol) in dry acetonitrile (160 mL). The mixture was stirred
under argon for 40 h at room temperature. After removal of sol-
vent by rotary evaporation, the residual yellow solid was washed
Chem. Eur. J. 2014, 20, 16707 – 16720
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with water (250 mL), methanol (200 mL) and CH2Cl2 (200 mL) to
give of 6a and 6b as an inseparable mixture (1.86 g, 5.40 mmol,
51%) in a 2:1 ratio (determined by 1H NMR spectroscopy) as
a white solid. M.p. 3658C (dec); 1H NMR (500 MHz, [D6]DMSO,
375 K): d=7.06–7.02 (m, 9H; Ar-4,4’,5,5’,13’,16-H), 6.95–6.92 (m,
9H; Ar-3,3’,6,6’,14’,15-H), 6.69–6.68 (m, 11H; Ar-2,2’,7,7’,14,15’,
bridgehead-9’-H), 6.24 (s, 1H; bridgehead-9’-H), 5.85 (s, 1H; bridge-
head-10’-H), 5.53 (s, 2H; bridgehead-10’-H), 3.84 ppm (s, 27H;
OCH3); 13C NMR (125 MHz, [D6]DMSO, 375 K): d=153.8 (Ar-C-OCH3),
147.9 (Ar-C-4a,10a,11), 147.2 (Ar-C-4a’,10a’), 146.7 (Ar-C-12a’), 132.7
(Ar-C-8a’,9a’,11’), 132.4 (Ar-C-8a,9a,12), 125.4 (Ar-C-14’), 125.3 (Ar-C-
3’,6’), 125.2 (Ar-C-3,6,15), 116.1 (Ar-C-4,5,16), 116.0 (Ar-C-4’,5’13’),
109.1 (Ar-C-2,7,14), 108.9 (Ar-C-2’,7’), 108.8 (Ar-C-15’), 55.7 (Ar-C-
1,8,13-OCH3), 55.6 (Ar-C-1’,8’-OCH3), 55.5 (Ar-C-16’-OCH3), 53.1
(bridgehead-C-10), 46.2 (bridgehead-C-10’), 32.6 ppm (bridgehead-
C-9,9’); IR (KBr pellet): n˜ =3435 (br, w), 3036 (w), 2999 (m), 2961
(m), 2938 (m), 2904 (m), 2835 (m), 2648 (w), 2590 (w), 2521 (w),
2332 (m), 2198 (w), 1993 (w), 1912 (w), 1822 (w), 1741 (w), 1655
(w), 1596 (s), 1583 (s), 1484 (s), 1439 (s), 1324 (m), 1275 (s), 1197
(m), 1160 (w), 1103 (s), 1092 (s), 1065 (s), 980 (m), 959 (w), 934 (w),
870 (w), 852 (w), 789 (s), 777 (m), 751 (s), 729 (s), 632 (w), 592 (m),
542 (w), 516 (w), 486 (w), 454 (w), 405 (w) cmÀ1; MS (CI); m/z (%)=
373 (6), 346 (26), 345 (100) [M+H]+, 344 (51) [M]+, 313 (11); ele-
mental analysis calcd (%) for C23H20O3: C 80.21, H 5.85; found: C
80.43, H 5.61.
(Ar-C-5a,6a), 141.0 (Ar-C-12a,13a), 137.7 (olefin-C-15,17/16,18), 137.4
(olefin-C-15,17/16,18), 132.9 (Ar-C-11a,14a), 125.1 (Ar-C-3,9/4,8),
117.0 (Ar-C-6), 115.8 (Ar-C-3,9/4,8), 112.4 (Ar-C-13), 110.8 (Ar-C-2,10),
87.7 (bridgehead-C-12,14), 56.2 (Ar-OCH3), 54.5 (bridgehead-OCH3),
49.1 ppm (bridgehead-C-5,7); IR (KBr pellet): n˜ =3436 (br, w), 3066
(w), 2961 (m), 2935 (m), 2833 (m), 1613 (w), 1583 (m), 1478 (s),
1439 (w), 1329 (m), 1267 (s), 1227 (w), 1212 (w), 1190 (w), 1141 (m),
1120 (w), 1105 (w), 1067 (m), 1030 (m), 942 (w), 906 (w), 793 (w),
778 (w), 755 (w), 735 (w), 698 (w), 668 (m), 636 (w), 588 (w),
564 cmÀ1 (w); MS (EI); m/z (%): 452 (7), 451 (32) [M+H]+, 450 (100)
[M]+, 439 (5), 438 (11), 437 (21), 436 (33), 435 (7), 422 (5), 421 (12),
420 (19), 419 (23), 418 (10), 417 (6), 407 (6), 406 (8), 405 (14), 404
(17), 403 (15), 402 (8), 401 (7), 400 (5), 390 (5), 389 (7), 388 (9), 387
(9), 386 (7), 385 (5), 384 (5), 375 (6), 374 (6), 373 (6), 372 (7), 371 (6),
370 (6), 361 (5), 358 (5), 356 (5); elemental analysis calcd (%) for
C30H26O4·1.5H2O: C 75.45, H 6.12; found: C 75.17, H 5.76.
Compounds 7a and 7b: Boron tribromide in CH2Cl2 (1m, 40 mL,
40 mmol) was added dropwise to a suspension of 6a/6b (2:1
ratio; 3.00 g, 8.71 mmol) in dry CH2Cl2 (30 mL) at 08C. After com-
plete addition, the vine-red solution was allowed to stir overnight
at room temperature. The mixture was cooled again to 08C and
ice (150 g) and water (150 mL) were added. The layers were sepa-
rated and the residual suspension was extracted with ethyl acetate
(3ꢂ300 mL). The combined organic layer was washed with water
(2ꢂ200 mL) and brine (200 mL), and dried over sodium sulfate.
After removal of the solvent in vacuum, compounds 7a and 7b
(2.56 g, 8.46 mmol, 97%) remained as a white solid in a 2:1 mixture
(determined by 1H NMR spectroscopy). M.p. >4108C (dec at
3508C); 1H NMR (400 MHz, [D6]DMSO): d=9.41–9.36 (m, 9H; OH),
6.86–6.83 (m, 9H; Ar-4,4’,5,5’,16,13’-H), 6.77–6.71 (m, 9H; Ar-
3,3’,6,6’,15,14’-H), 6.50–6.46 (m, 11H; Ar-2,2’,7,7’,14,15’, bridgehead
À9-H), 6.10 (s, 1H; bridgehead À9’-H), 5.70 (s, 1H; bridgehead
À10’-H), 5.35 ppm (2H; bridgehead À10-H); 13C NMR (100 MHz,
[D6]DMSO): d=151.9 (Ar-C-16’), 151.8 (Ar-C-1’,8’), 151.7 (Ar-C-
1,8,13), 148.4 (Ar-C-4a,10a,11), 147.9 (Ar-C-4a’,10a’), 147.7 (Ar-C-12’),
131.5 (Ar-C-11’), 131.3 (Ar-C-8a’,9a’), 131.2 (Ar-C-8a,9a,12), 125.3 (Ar-
C-14’), 125.2 (Ar-C-3’,6’), 125.0 (Ar-C-3,6,15), 114.8 (Ar-C-
4,4’,5,5’,16,13’), 112.5 (Ar-C-2,2’,7,7’,14,15’), 53.7 (bridgehead -C-10),
46.7 (bridgehead -C-10’), 33.1 ppm (bridgehead -C-9); IR (KBr
pellet): n˜ =3306 (s), 2975 (w), 2847 (w), 2716 (w), 1889 (w), 1819
(w), 1601 (s), 1474 (s), 1459 (s), 1389 (m), 1389 (w), 1323 (w), 1296
(w), 1255 (s), 1224 (w), 1189 (w), 1160 (w), 1079 (w), 1059 (w), 1037
(w), 975 (w), 862 (w), 786 (w), 772 (w), 758 (m), 753 (m), 726 (s),
625 (w), 569 (w), 545 (w), 471 (w), 400 (w) cmÀ1; MS (CI); m/z (%):
331 (20), 304 (5), 303 (100) [M+H]+, 302 (24) [M]+; elemental analy-
sis calcd (%) for C20H14O3: C 79.46, H 4.67; found: C 79.38, H 4.55.
By-products were isolated by removing the solvent from the
CH2Cl2 extract followed by column chromatography (SiO2, petrole-
um ether/ethyl acetate, 7:1) and recrystallisation of the isolated
fractions from ethyl acetate.
First fraction (Rf =0.23): rac-6c (930 mg, 2.70 mmol, 25%) as colour-
less crystals. M.p. 212–2138C; 1H NMR (400 MHz, [D6]DMSO): d=
8.09 (s, 1H; Ar-11-H), 7.64 (s, 1H; Ar-6-H), 7.35–7.29 (m, 2H; Ar-8,9-
H), 7.24 (dd, J=7.8, 1.5 Hz, 1H; olefin-13-), 7.07 (dd, J=7.7, 6.1 Hz,
olefin-14-H), 7.01 (dd, J=7.2, 1.1 Hz, 1H; Ar-4-H), 6.98–6.95 (m, 1H;
Ar-3-H), 6.89 (dd, J=7.0, 1.4 Hz, 1H; Ar-9-H), 6.72 (dd, J=8.1,
1.0 Hz, Ar-2-H), 5.22 (dd, J=6.1, 1.4 Hz, 1H; bridgehead-5-H), 3.92
(s, 3H; OCH3), 3.84 (s, 3H; OCH3), 3.75 ppm (s, 3H; OCH3); 13C NMR
(100 MHz, [D6]DMSO): d=155.0 (Ar-C-10), 154.5 (Ar-C-1), 147.0 (Ar-
C-4a), 143.2 (Ar-C-5a/11a), 142.3 (Ar-C-5a/11a), 137.7 (olefin-C-13),
137.1 (olefin-C-14), 131.6 (Ar-C-6a/12a), 131.4 (Ar-C-6a/12a), 126.5
(Ar-C-3), 126.1 (Ar-C-8), 121.8 (Ar-C-10a), 120.0 (Ar-C-6), 119.5 (Ar-C-
7), 116.5 (Ar-C-4), 112.1 (Ar-C-11), 110.6 (Ar-C-2), 104.6 (Ar-C-9), 87.3
(Ar-C-12), 56.2 (Ar-C-1-OCH3), 55.5 (OCH3), 55.3 (OCH3), 48.9 ppm
(bridgehead-C-5); IR (KBr pellet): n˜ =3436 (br, w), 3055 (w), 3000
(w), 2936 (m), 2833 (m), 2361 (w), 2342 (2), 1904 (w), 1725 (w),
1607 (w), 1587 (m), 1502 (w), 1477 (m), 1466 (m), 1439 (m), 1423
(w), 1369 (m), 1336 (m), 1267 (s), 1231 (m), 1201 (w), 1187 (w), 1128
(w), 1109 (w), 1065 (m), 1037 (m), 966 (w), 966 (w), 939 (w), 894
(w), 876 (w), 853 (w), 809 (w), 789 (w), 777 (s), 753 (m), 739 (w), 722
(w), 715 (w), 685 (m), 635 (w), 601 (w), 573 (w), 498 (w), 431 cmÀ1
(w); MS (EI); m/z (%): 346 (5), 345 (24) [M+H]+, 344 (100) [M]+,
331 (10), 330 (56), 329 (15), 315 (14), 314 (36), 313 (21), 312 (11),
302 (7), 301 (6), 300 (6), 299 (8), 298 (10), 297 (7), 296 (7), 295 (5),
286 (5), 285 (5), 284 (5), 254 (5); elemental analysis calcd (%) for
C23H20O3: C 80.21, H 5.85; found: C 80.02, H 5.84.
Compound 8a: A 2:1 mixture of 7a and 7b (150 mg, 0.50 mmol)
was suspended in dry N-methylimidazole (2 mL) and stirred for
30 min at room temperature under argon in a screw-capped vial.
Freshly distilled capronyl chloride (2 mL) was added to the result-
ing bright-red solution and the mixture was stirred for 16 h at
room temperature. The accrued suspension was poured into cold
water (50 mL); extracted with CH2Cl2 (3ꢂ40 mL); and the com-
bined organic layer was washed with dilute hydrochloric acid (1m,
2ꢂ40 mL), a saturated solution of sodium hydrogen carbonate
(50 mL), water (2ꢂ50 mL) and brine (50 mL). After being dried over
sodium sulfate, CH2Cl2 was removed in vacuo and n-pentane
(10 mL) was added to the resulting brown oil. Colourless crystals
were obtained overnight and isolated by suction filtration to give
8a (98 mg, 0.16 mmol, 32%). M.p. 1398C; 1H NMR (400 MHz,
CDCl3): d=7.25 (d, 3H; Ar-4,5,16-H), 7.00 (dd, J=8.1, 7.4 Hz, 3H;
Ar-3,6,15-H), 6.79 (dd, J=8.2, 0.8 Hz, 3H; Ar-2,7,14-H), 5.82 (s, 1H;
bridgehead À9-H), 5.50 (s, 1H; bridgehead À10-H), 2.64 (t, 6H;
Second fraction (Rf =0.15): rac-6d (154 mg, 0.34 mmol, 3%) as
a white solid after drying in high vacuum. M.p. 347–3488C (dec);
1H NMR (400 MHz, [D6]DMSO): d=7.49 (s, 1H; Ar-13-H), 7.19 (s, 1H;
Ar-6-H), 7.15 (dd, J=7.6, 1.4 Hz, 2H; olefin-16,18-H), 6.97–6.93 (m,
2H; olefin-15,17-H), 6.92–6.86 (m, 4H; Ar-3,4,8,9-H), 6.65 (dd, J=
7.7, 1.4 Hz, 2H; Ar-2,10-H), 4.99 (dd, J=6.0, 1.2 Hz, 2H; bridgehead-
5,7-H), 3.76 (s, 6H; OCH3), 3.73 ppm (s, 6H; OCH3); 13C NMR
(100 MHz, [D6]DMSO): d=153.9 (Ar-C-1,11), 147.9 (Ar-C-4a,7a), 143.2
Chem. Eur. J. 2014, 20, 16707 – 16720
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Full Paper
COCH2C4H9), 1.91–1.84 (m, 6H; COCH2CH2C3H7), 1.48–1.40 (m, 12H;
COC2H4CH2CH2CH3), 0.97 ppm (t, J=7.1 Hz, 9H; COC4H8CH3);
13C NMR (100 MHz, CDCl3): d=171.3 (C=O), 147.7 (Ar-C-1,8,13)
145.9 (Ar-C-4a,10a,11), 134.9 (Ar-C-8a,9a,12), 126.9 (Ar-C-3,6,15),
121.3 (Ar-C-4,5,16), 119.4 (Ar-C-2,7,14), 54.0 (bridgehead -C-10), 36.3
(bridgehead -C-9), 34.5 (Alk-C-1,1’,1’’), 31.7 (Alk-C-3,3’,3’’), 25.0 (Alk-
C-2,2’,2’’), 22.6 (Alk-C-4,4’,4’’), 14.1 ppm (Alk-C-5,5’,5’’); IR (KBr
pellet): n˜ =3500 (br, w), 3409 (w), 3062 (w), 3019 (w), 2952 (m),
2939 (m), 2870 (m), 2732 (w), 2359 (w), 2342 (w), 1946 (w), 1912
(w), 1759 (s), 1609 (w), 1594 (w), 1548 (w), 1474 (m), 1438 (w), 1421
(w), 1376 (m), 1317 (m), 1293 (w), 1232 (s), 1164 (s), 1150 (s), 1113
(m), 1060 (w), 1029 (w), 974 (w), 924 (w), 887 (w), 855 (w), 801 (m),
752 (m), 726 (m), 623 (w), 598 (w), 569 (w), 540 (w), 437 (w),
410 cmÀ1 (w); MS (CI); m/z (%): 626 (10), 599 (5), 598 (33) [M+H]+,
596 (25), 528 (5), 527 (8), 501 (5), 500 (31), 499 (100), 498 (65), 402
(12), 401 (9), 400 (20), 303 (6), 302 (25), 301 (5), 99 (6); elemental
analysis calcd (%) for C38H44O6: C 76.48, H 7.43; found: C 76.53, H
7.35.
C-4,5), 119.4 (Ar-C-2,7,15), 46.7 (bridgehead-C-10), 41.4 (bridgehead-
C-9), 20.8 (Ar-C-16-COCH3), 20.6 ppm (Ar-C-1,8-COCH3); IR (KBr
pellet): n˜ =3445 (br, w), 3062 (w), 3018 (w), 3001 (w), 2935 (w),
2419 (w), 2051 (w), 1926 (w), 1763 (s), 1612 (w), 1610 (w), 1586 (w),
1472 (m), 1439 (w), 1372 (m), 1211 (s), 1164 (m), 1072 (w), 1033
(m), 979 (w), 909 (w), 901 (w), 894 (w), 878 (w), 851 (w), 793 (w),
769 (w), 746 (w), 732 (w), 722 (w), 679 (w), 668 (w), 595 (w), 566
(w), 548 (w), 530 (w), 480 (w), 445 (w), 411 cmÀ1 (w); MS (CI); m/z
(%): 430 (11), 429 (45) [M+H]+, 428 (31) [M]+, 415 (10), 388 (24),
387 (100), 386 (32), 373 (9), 345 (9), 344 (16), 302 (19); elemental
analysis calcd (%) for C26H20O6: C 72.89, H 4.71; found: C 73.24, H
4.47.
A second fraction (Rf =0.68, 0.75; chloroform/ethyl acetate, 1:1)
gave a mixture of 9b (201 mg, 0.46 mmol, 5%) and 9a (267 mg,
0.63 mmol, 6%); yields were determined by integration of the
1H NMR signals of the bridgehead protons.
From a third fraction (Rf =0.75; chloroform/ethyl acetate, 1:1),
more 9a (562 mg, 1.31 mmol, 13%) was isolated with same analyti-
cal properties as the crystallized fraction. This gave an overall yield
of 9a of 56%.
Compounds 9a and 9b: A 2:1 mixture of 7a and 7b (2.91 g,
9.63 mmol) and N-methylimidazole (40 mL) were stirred for 2 h at
room temperature in dry CH2Cl2 (150 mL). The resulting bright-red
suspension was cooled to 08C and freshly distilled acetyl chloride
(40 mL) was added very slowly. After addition, cooling was re-
moved and the mixture was allowed to stir overnight at room tem-
perature. Cold water (150 mL) was added, the layers were separat-
ed and the aqueous layer was extracted with CH2Cl2 (4ꢂ200 mL).
The combined organic layer was washed with dilute hydrochloric
acid (1m, 2ꢂ250 mL), a saturated solution of sodium hydrogen car-
bonate (250 mL), water (2ꢂ250 mL) and brine (250 mL). After
drying over sodium sulfate, CH2Cl2 was removed by rotary evapora-
tion. The residual solid was re-dissolved in CH2Cl2 (100 mL), a layer
of the same amount of n-pentane was added, and the mixture was
kept at room temperature overnight. The resulting crystals were
separated by suction filtration. This was performed twice to give
pure isomer 9a (1.79 g, 4.18 mmol, 43%) as colourless crystals
after drying in high vacuum (6.3ꢂ10À2 bar, 1258C). M.p. 3228C
Compounds 10a and 10b: In a screw-capped vial, a 2:1 mixture
of 7a and 7b (150 mg, 0.50 mmol) and N-methylimidazole (2 mL)
were stirred at room temperature for 30 min. A solution of benzoyl
chloride (2 mL, 17.4 mmol) was added to the resulting bright-red
solution and the mixture was stirred for 16 h at room temperature.
The resulting suspension was poured into cold water (50 mL) and
extracted with CH2Cl2 (3ꢂ40 mL). The combined organic extract
was washed with diluted hydrochloric acid (1m, 2ꢂ40 mL), a satu-
rated solution of sodium hydrogen carbonate (50 mL), water (2ꢂ
50 mL) and brine (50 mL). After being dried over sodium sulfate,
CH2Cl2 was removed on a rotary evaporator. The residual off-white
solid was dissolved in CH2Cl2 (5 mL) and a layer of n-pentane was
added. After crystallisation overnight, colourless crystals were iso-
lated by suction filtration to give 10a (130 mg, 0.21 mmol, 42%).
M.p. 2948C; 1H NMR (400 MHz, [D6]DMSO): d=7.74 (dd, J=8.1,
1,0 Hz, 6H; Ph-2,6-H), 7.54–7.51 (m, 6H; Ar-4,5,16-H, Ph-4-H), 7.19–
7.15 (m, 3H; Ar-3,6,15-H), 7.11 (t, J=7.8 Hz, 6H; Ph-3,5-H), 6.99 (dd,
J=8.1, 0.6 Hz, 3H; Ar-2,7,14-H), 6.03 (s, 1H; bridgehead-H-9),
5.94 ppm (s, 1H; bridgehead-H-10); 13C NMR (126 MHz, 360 K,
[D6]DMSO): d=163.5 (C=O), 147.4 (Ar-C-1,8,13), 145.2 (Ar-C-
4a,10a,11), 134.4 (Ar-C-8a,9a,12), 132.9 (Ph-C-4), 128.7 (Ph-C-2,6),
128.0 (Ph-C-3,5/Ph-C-1), 127.9 (Ph-C-3,5/Ph-C-1), 126.0 (Ar-C-3,6,15),
121.1 (Ar-C-4,5,16), 119.0 (Ar-C-2,7,14), 52.2 (bridgehead-C-10),
35.5 ppm (bridgehead-C-9); IR (KBr pellet): n˜ =3449 (br, w), 3061
(w), 3032 (w), 2963 (w), 2344 (w), 1920 (w), 1740 (s), 1594 (w), 1473
(m), 1451 (w), 1314 (w), 1267 (s), 1245 (m), 1228 (s), 1171 (m), 1093
(s), 1060 (m), 1028 (w), 1016 (w), 1003 (w), 962 (w), 935 (w), 901
(w), 873 (w), 849 (w), 790 (w), 761 (w), 740 (w), 705 (s), 617 (w), 587
(w), 507 (w), 409 cmÀ1 (m); MS (MALDI-TOF): m/z (%): 711.5 (12),
653.5 (44), 637.5 (100) [M+Na]+, 614.5 (42) [M]+; elemental analysis
calcd (%) for C26H20O6 (614.64): C 80.12, H 4.26; found: C 79.92, H
4.16.
1
(dec); H NMR (400 MHz, [D6]DMSO): d=7.40 (d, J=7.3 Hz, 3H; Ar-
4,5,16-H), 7.08 (t, J=7.8 Hz, 3H; Ar-3,6,15-H), 6.84 (d, J=8.1 Hz, 3H;
Ar-2,7,14-H), 5.89 (s, 1H; bridgehead-9-H), 5.60 (s, 1H; bridgehead-
10-H), 2.43 ppm (s, 9H; COCH3); 13C NMR (100 MHz, [D6]DMSO): d=
168.6 (C=O), 147.6 (Ar-C-1,8,13) 145.5 (Ar-C-4a,10a,11), 134.6 (Ar-C-
8a,9a,12), 126.4 (Ar-C-3,6,15), 121.6 (Ar-C-4,5,16), 119.4 (Ar-C-2,7,14),
52.0 (bridgehead-C-10), 35.7 (bridgehead-C-9), 20.4 ppm (COCH3);
IR (KBr pellet): n˜ =3435 (br, w), 3063 (w), 3015 (w), 2932 (w), 2342
(w), 1773 (s), 1611 (w), 1593 (w), 1471 (m), 1438 (w), 1373 (m), 1209
(s), 1169 (s), 1076 (m), 1037 (m), 1019 (w), 973 (w), 905 (w), 879 (w),
863 (w), 847 (w), 798 (w), 772 (w), 751 (w), 741 (w), 722 (w), 696
(w), 683 (w), 667 (w), 596 (w), 543 (w), 505 (w), 478 (w), 415 cmÀ1
(w); MS (EI); m/z (%): 429 (16) [M+H]+, 428 (13) [M]+, 388 (25), 387
(100), 386 (16), 373 (7), 345 (6), 344 (10), 302 (7); elemental analysis
calcd (%) for C26H20O6: C 72.89, H 4.71; found: C 72.70, H 4.79.
The mother liquor of the filtration was freed from solvent by rotary
evaporation and the residual solid was separated by column chro-
matography (SiO2, chloroform/ethyl acetate, 99:1) to give 9b
(783 mg, 1.83 mmol, 19%). Rf =0.75 (chloroform/ethyl acetate, 1:1)
The solvent was removed from the mother liquor and the residual
solid was separated by column chromatography (SiO2, chloroform)
to give 10b (47 mg, 0.08 mmol, 16%) as a colourless solid. Rf =
0.37; m.p. 2878C; 1H NMR (400 MHz, [D6]DMSO): d=8.31 (d, J=
7.3 Hz, 2H; Ph-2’,6’-H), 8.03 (d, J=7.4 Hz, 4H; Ph-2,6-H), 7.87–7.83
(m, 1H; Ph-4’-H), 7.74–7.66 (m, 4H; Ph-3’,4,5’-H), 7.43 (t, J=7.9 Hz,
6H; Ar-4,5-Ph-3,5-H), 7.34 (d, J=7.2 Hz, 1H; Ar-13-H), 7.16–7.10 (m,
3H; Ar-3,6,14-H), 7.01 (t, J=8.5 Hz, 3H; Ar-2,7,15-H), 5.90 (s, 1H;
bridgehead-9-H), 5.74 ppm (s, 1H; bridgehead-10-H); 13C NMR
(126 MHz, 360 K, [D6]DMSO): d=164.1 (C=O), 163.8 (C=O), 146.3
(Ar-C-1,8,16), 145.4 (Ar-C-4a/10a/12), 145.2 (Ar-C-4a/10a/12), 145.2
1
as the first fraction; m.p. 2738C; H NMR (400 MHz, [D6]DMSO): d=
7.35 (d, J=7.3 Hz, 3H; Ar-4,5,13-H), 7,06 (t, J=7.7 Hz, 3H; Ar-
3,6,14-H), 6.82 (d, J=8.1 Hz, 3H; Ar-2,7,15-H), 5.82 (s, 1H; bridge-
head-9-H), 5.65 (s, 1H; bridgehead-10-H), 2.48 (s, 3H; Ar-16-COCH3),
2.46 ppm (s, 6H; Ar-1,8-COCH3); 13C NMR (100 MHz, [D6]DMSO): d=
169.4 (Ar-C-16-COCH3), 169.0 (Ar-C-1,8-COCH3), 146.7 (Ar-C-1,8,16),
145.6 (Ar-C-12), 145.5 (Ar-C-4a,10a), 137.1 (Ar-C-8a,9a,11), 135.9 (Ar-
C-3,6,14), 126.2 (Ar-C-3,6), 126.2 (Ar-C-15), 122.2 (Ar-C-13), 121.8 (Ar-
Chem. Eur. J. 2014, 20, 16707 – 16720
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(Ar-C-4a/10a/12), 136.5 (Ar-C-8a,9a/11), 135.4 (Ar-C-8a,9a/11), 133.5
(Ph-C-4’), 133.2 (Ph-C-4), 129.5 (Ph-C-2’,6’), 129.0 (Ph-C-2,6), 128.5
(Ph-C-1/1’/3,5/3’,5’), 128.3 (Ph-C-1/1’/3,5/3’,5’), 128.2 (Ph-C-1/1’/3,5/
3’,5’), 126.0 (Ar-C-3,6), 125.9 (Ar-C-14), 121.5 (Ar-C-13), 121.4 (Ar-C-
4,5), 118.9 (Ar-C-2,7,15), 47.1 (bridgehead-C-10), 41.7 ppm (bridge-
head-C-9); IR (KBr pellet): n˜ =3444 (br, w), 3062 (w), 1924 (w), 1736
(s), 1600 (w), 1584 (w), 1471 (w), 1451 (w), 1265 (s), 1237 (s), 1234
(s), 1176 (m), 1091 (s), 1060 (m), 1027 (w), 874 (w), 858 (w), 782 (w),
764 (w), 738 (w), 705 (s), 685 (w), 589 cmÀ1 (w); MS (MALDI-TOF):
m/z (%): 653.5 (59), 637.5 (100) [M+Na]+, 614.5 (12) [M]+; elemen-
tal analysis calcd (%) for C41H26O6: C 77.84, H 4.46; found: C 78.11,
H 4.30.
3008C (dec); 1H NMR (500 MHz, [D6]DMSO): d=11.00 (s, 3H; OH),
9.96 (s, 3H; CHO), 7.53 (d, J=7.6 Hz, 3H; Ar-3,6,15-H), 7.29 (d, 6H;
J=7.6 Hz, Ar-4,5,16-H), 6.84 (s, 1H; bridgehead-9-H), 6.02 ppm (s,
1H; bridgehead-10-H); 13C NMR (125 MHz, [D6]DMSO): d=196.2
(CHO), 154.3 (Ar-C-1,8,13), 153.6 (Ar-C-4a,10a,11), 131.9 (Ar-C-
8a,9a,12), 131.4 (Ar-C-3,6,15), 120.0 (Ar-C-2,7,14), 116.8 (Ar-C-4,5,16),
54.4 (bridgehead-C-10), 31.7 ppm (bridgehead-C-9); IR (KBr pellet):
n˜ =3429 (br, w), 2849 (w), 2753 (w), 1660 (s), 1623 (s), 1583 (w),
1494 (m), 1438 (m), 1387 (w), 1355 (w), 1323 (m), 1274 (m), 1232
(m), 1145 (w), 1080 (w), 1036 (w), 1075 (w), 925 (w), 860 (w), 772
(m), 762 (m), 619 (w), 578 (w), 514 (w), 489 (w), 464 cmÀ1 (w); MS
(CI); m/z (%): 415 (13), 388 (23), 387 (100) [M+H]+, 386 (29) [M]+;
elemental analysis calcd (%) for C23H14O6: C 71.50, H 3.65; found: C
71.76, H 3.81.
A second fraction gave further 10a (40 mg, 0.065 mmol, 13%). The
combined yield of 10a was 55%.
Compound 12: Compound 1 (100 mg, 0.26 mmol), 11 (83 mg,
0.78 mmol) and 4 ꢁ molecular sieves (3 g) were suspended in dry
CH2Cl2 (12 mL) in a screw-capped vial and heated at reflux for 5 h.
The resulting yellow solution was filtered through a syringe filter
(0.45 mm) and the solvent was removed on a rotary evaporator.
After drying in high vacuum (5.4ꢂ10À1 mbar, 2008C, 3 h) 13 was
obtained as a yellow solid (124 mg, 0.19 mmol, 73%). M.p. 2268C;
1H NMR (300 MHz, [D6]DMSO): d=13.83 (s, 3H; OH), 8.93 (s, 3H;
imine-H), 7.35–7.33 (m, 9H; Ar-3,6,15-H, tolyl-3,5-H), 7.27–7.24 (d,
J=8.3 Hz, 6H; tolyl-2,6-H), 7.19–7.17 (d, J=7.6 Hz, 3H; Ar-4,5,16-H),
6.92 (s, 1H; bridgehead-9-H), 5.82 (s, 1H; bridgehead-10-H),
2.32 ppm (s, 9H; OCH3), 13C NMR (125 MHz, [D6]DMSO): d=162.7
(imine-C), 154.8 (Ar-C-1,8,13), 150.8 (Ar-C-4a,10,11), 144.9 (tolyl-C-1),
136.6 (tolyl-C-1), 131.3 (Ar-C-8a,9a,12), 130.5 (tolyl-C-3), 130.0 (tolyl-
C-2), 121.2 (Ar-C-3,6,15), 117.5 (Ar-C-2,7,14), 115.5 (Ar-C-4,5,16), 54.3
(bridgehead-C-10), 32.0 (bridgehead-C-9), 20.7 ppm (OCH3); IR
(ATR): n˜ =3740 (w), 3026 (w), 2920 (w), 2711 (w), 1738 (w) 1618 (s),
1596 (s), 1566 (m) 1508 (m), 1441 (m) 1396 (w), 1368 (w), 1327 (w),
1308 (w), 1278 (m) 1202 (m), 1188 (m), 1188 (m), 1172 (m), 1145
(w), 1110 (w), 1084 (w), 1040 (m), 1017 (w), 973 (w), 943 (w), 875
(m), 819 (s), 786 (m), 758 (m), 722 (w), 710 (w), 669 (w), 643 cmÀ1
(w); MS (ESI); m/z: 653; elemental analysis calcd (%) for C44H35N3O3:
C 80.83, H 5.40, N 6.43, found: C 80.52, H 5.69, N 6.33.
Compound 7a: Compound 9a (1.30 g, 3.03 mmol) was suspended
in an mixture of ethanol/hydrochloric acid (1:2, v/v; 60 mL) and
stirred for 16 h at 858C. The obtained white solid was filtered,
washed with water (2ꢂ40 mL) and dried under high vacuum (5.1ꢂ
10À1 mbar, 1258C) to give 7a (820 mg, 2.71 mmol, 89%) as a white
1
solid. M.p. >4108C; H NMR (400 MHz, [D6]DMSO): d=9.35 (s, 3H;
OH), 6.85 (d, J=7.1 Hz, 3H; Ar-4,5,16-H), 6.73 (t, J=7.6 Hz, 3H; Ar-
3,6,15-H), 6.49–6.47 (m, 4H; Ar-2,7,14-H, bridgehead-9-H), 5.35 ppm
(s, 1H; bridgehead-10-H); 13C NMR (100 MHz, [D6]DMSO): d=151.7
(Ar-C-1,8,13), 148.4 (Ar-C-4a,10a,11), 131.1 (Ar-C-8a,9a,12), 125.0 (Ar-
C-3,6,15), 114.8 (Ar-C-4,5,16), 112.5 (Ar-C-2,7,14), 53.7 (bridgehead-
C-10), 33.0 ppm (bridgehead-C-9); IR (KBr pellet): n˜ =3308 (br, w),
3255 (s), 2954 (w), 2850 (w), 2718 (w), 2360 (w), 2342 (w), 1604 (s),
1477 (s), 1459 (w), 1394 (m), 1324 (w), 1257 (m), 1188 (w), 1161 (w),
1081 (w), 1059 (w), 1041 (w), 975 (w), 863 (w), 786 (w), 759 (w), 752
(w), 726 (s), 625 (w), 570 (w), 545 (w), 472 (w), 400 cmÀ1 (w); MS
(CI); m/z (%): 332 (6), 331 (27), 304 (20), 303 (100) [M+H]+, 302 (6)
[M]+; elemental analysis calcd (%) for C20H14O3: C 79.46, H 4.67;
found: C 79.60, H 4.79.
Compound 7b: Analogous to the synthesis of 7a, after workup,
compound 9b (250 mg, 0.58 mmol) gave 7b (170 mg, 0.56 mmol,
96%) as a white solid. M.p. >4108C (dec at 3708C); 1H NMR
(400 MHz, [D6]DMSO): d=9.42 (s, 3H; OH), 6.86–6.84 (m, 3H; Ar-
4,5,13-H), 6.77–6.73 (m, 3H; Ar-3,6,14-H), 6.49–6.47 (m, 3H; Ar-
2,7,15-H), 6.11 (s, 1H; bridgehead-9-H), 5.71 ppm (s, 1H; bridge-
head-10-H); 13C NMR (100 MHz, [D6]DMSO): d=151.9 (Ar-C-16),
151.8 (Ar-C-1,8), 147.9 (Ar-C-4a,10a), 147.6 (Ar-C-12), 131.5 (Ar-C-11),
131.3 (Ar-C-8a,9a), 125.3 (Ar-C-14), 125.2 (Ar-C-3,6), 114.9 (Ar-C-
4,5,13), 112.6 (Ar-C-2,7,15) 46.7 ppm (bridgehead-C-10); IR (KBr
pellet): n˜ =3291 (s), 2990 (w), 2975 (w), 2840 (w), 2706 (w), 1890
(w), 1616 (w), 1591 (m), 1463 (s), 1374 (m), 1316 (m), 1254 (s), 1223
(m), 1182 (w), 1191 (w), 1159 (w), 1075 (w), 1034 (m), 975 (w), 946
(w), 852 (w), 786 (w), 771 (w), 726 (s), 714 (w), 674 (w), 599 (w), 583
(w), 561 (w), 537 (w), 476 (w), 392 cmÀ1 (w); MS (CI): m/z (%): 331
(16), 304 (20), 303 (100) [M+H]+, 302 (73) [M]+, 285 (9); elemental
analysis calcd (%) for C20H14O3: C 79.46, H 4.67; found: C 79.61, H
4.76.
Compound 3: Compounds 1 (60 mg, 116 mmol) and 2 (46.4 mg,
116 mmol) were dissolved in dry DMF (24 mL) and a solution of TFA
in DMF (0.1m, 32 mL) was added. The mixture was stirred under
argon in a screw-capped vial at 1208C for 3 days. The resulting
yellow suspension was allowed to settle at room temperature and
the supernatant solution was removed with a Pasteur pipette. The
residual suspension was immersed in dry diethyl ether (15 mL). Sol-
vent was removed after 24 h and the process was repeated three
times. Finally, the suspension was filtered and the obtained yellow
solid was washed three times with dry diethyl ether (10 mL) to
give, after drying in vacuo, compound 3 as a yellow solid (48 mg,
19 mmol, 65%). M.p. >3108C; 13C CP MAS NMR: d=156.6, 145.5,
132.7, 123.5, 117.8, 114.0, 112.6, 110.6, 55.3, 55.0, 32.6 ppm; IR
(ATR): n˜ =2957 (w), 1706 (w), 1615 (s), 1596 (s), 1466 (m), 1439 (m),
1361 (w), 1325 (w), 1276 (w), 1220 (m), 1143 (w), 1082 (w), 1039
(w), 950 (w), 847 (w), 808 (w), 773 (w), 762 (w), 626 cmÀ1 (w); MS
(MALDI-TOF): m/z: 2525.3; elemental analysis calcd (%) for
C172H100N12O12·17H2O: C 72.92, H 4.77, N 5.93; found: C 72.99, H
4.49, N 5.94.
Compound 1: A suspension of 7a (290 mg, 0.96 mmol) and HMTA
(525 mg, 3.74 mmol, 3.9 equiv) in TFA (6 mL) was stirred at 708C
under an argon atmosphere. After 16 h, the bright-yellow solution
was poured into a mixture of CH2Cl2/hydrochloric acid (2m, 1:1 v/
v; 200 mL) and stirred overnight at room temperature. After sepa-
ration of the layers, the aqueous layer was extracted with CH2Cl2
(4ꢂ200 mL) and the combined organic layer was washed with
brine (250 mL). After drying over sodium sulfate and evaporation
of the solvent, the residual off-white solid was separated by
column chromatography (SiO2, CH2Cl2/ethyl acetate, 10:1 (Rf =
0.76)) to give crude 1, which was washed with n-pentane (40 mL)
to give pure 1 (179 mg, 0.46 mmol, 47%) as a white solid. M.p.
Acknowledgements
We would like to thank the “Fonds der Chemischen Industrie”
(FCI) and the “Deutsche Forschungsgemeinschaft” (DFG) for fi-
nancial support (project MA4061/4-2) and ChemPur as well as
Chem. Eur. J. 2014, 20, 16707 – 16720
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20, 10588–10597; d) J. R. Holst, A. Trewin, A. I. Cooper, Nat. Chem. 2010,
2, 915–920.
Solvay Fluor for the donation of chemicals. S.M.E. would like to
thank the Studienstiftung des Deutschen Volkes for a scholar-
ship. We also would like to thank Petra Krꢅmer for IR, Bernd
Kohl for TGA, Dr. Gang Zhang for gas sorption and Hendrik
Herrmann (all University of Heidelberg) for PXRD measure-
ments.
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Keywords: cage compounds
· carbon dioxide capture ·
cycloaddition · microporous materials · Schiff bases
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Received: August 12, 2014
Published online on October 21, 2014
Chem. Eur. J. 2014, 20, 16707 – 16720
www.chemeurj.org
16720
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