A covalent organic cage compound acting as a supramolecular shadow mask for the regioselective functionalization of C60

Article abstract of DOI:10.1039/d0sc03131c

A trigonal-bipyramidal covalent organic cage compound serves as an efficient host to form stable 1?:?1-complexes with C60 and C70. Fullerene encapsulation has been comprehensively studied by NMR and UV/Vis spectroscopy, mass spectrometry as well as single-crystal X-ray diffraction. Exohedral functionalization of encapsulated C60via threefold Prato reaction revealed high selectivity for the symmetry-matched all-trans-3 addition pattern. This journal is

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DOI: 10.1039/D0SC03131C
ARTICLE
A Covalent Organic Cage Compound Acting as a Supramolecular
Shadow Mask for the Regioselective Functionalization of C60
Viktoria Leonhardt,a,b Stefanie Fimmel,b Ana-Maria Krause,a,b and Florian Beuerle*,a,b
Received 00th January 20xx,
Accepted 00th January 20xx
A trigonal-bipyramidal covalent organic cage compound serves as an efficient host to form stable 1:1-complexes with C60
and C70. Fullerene encapsulation has been comprehensively studied by NMR and UV/Vis spectroscopy, mass spectrometry
as well as single-crystal X-ray diffraction. Exohedral functionalization of encapsulated C60 via threefold Prato reaction
revealed high selectivity for the symmetry-matched all-trans-3 addition pattern.
DOI: 10.1039/x0xx00000x
available.18 Elaborate tether synthesis and limitations in
postysynthetic modification further hamper this approach. In
contrast, any supramolecular prealignment of the reactants or
an in-situ activation of specific double bonds is much more
tempting. Seminal work by Guldi, Torres and coworkers on
Prato reactions for a phthalocyanine aldehyde showed the
highly selective formation of cis-1 bisadducts mediated by π-π
interactions between the chromophores.19 More recently, von
Delius and coworkers reported the preferred formation of
Introduction
Fullerenes act as spherically arranged electron-deficient
polyolefines.1 Their unique electronic structures facilitate
potential applications in medicine,2 photovoltaics3 and organic
electronics.4 Synthetically, addition reactions, e.g., Diels-Alder,5
Bingel,6 and Prato7 reactions or other dipolar cycloadditions,8
exploit the inherent strain energy of the curved double bonds,
thus leading to a multitude of exohedrally functionalized
derivatives.9 Ih symmetrical C60 as the most abundant fullerene
gives only one monoadduct. However, the number of potential
regioisomers quickly raises to 89d and 469a,e for bis- and
trisaddition, respectively. The precise spatial organization of the
individual addends identifies such multiple adducts as highly
promising building blocks for 3D molecular architectures, e.g.,
dendritic systems10 or extended metal-organic11 and
supramolecular12 frameworks. However, regioselectivity is
generally governed by a combination of statistical and kinetic
factors.13 Thereby, the second addition step usually serves as a
bottle neck for the selective formation of higher adducts and
individual regioisomers have to be purified, if at all possible, by
tedious HPLC chromatography. In some cases, specific addition
patterns such as C60Cl6,14 C60Ph5H,15 or octahedral
hexakisaddition9c,16 have been obtained in surprisingly high
purity under thermodynamic control.
trans-bisadducts for Bingel reactions on C60⊂[10]cyclo-
paraphenylene.20 When applying more sophisticated cage
structures with a spatially precise orientation of multiple pore
windows as hosts, the regioselective formation of higher
adducts appears feasible (Fig. 1).
a)
nBu
C60
CHCl3
O
O
O
O
O
B
B
B
O
B
O
O
O
O
B
B
O
O
C70
nBu
CHCl3
C60⊂1
C70⊂1
1
b)
Following a tether-directed remote functionalization approach
that was introduced by the Diederich group,17 all seven
sterically possible bisadducts for methanofullerenes have been
obtained by means of a covalent fixation of the two reactive
sites in suitable tether systems.9a For higher adducts however,
only a few tethers for privileged addition patterns are so far
complexation
regioselective
functionalization
work-up
Fig. 1. a) Supramolecular complexation of fullerenes C60 and C70 within the cavity of
covalent organic boronate ester cage 1 (molecular structures of inclusion complexes are
derived from semiempirical PM6 modeling); b) schematical representation for the use of
C60-cage complexes as shadow masks for a regioselective exohedral C60 functionalization.
a. Universität Würzburg, Institut für Organische Chemie, Am Hubland, 97074
Würzburg, Germany.
In recent years,
a
large variety of organic and
b. Universität Würzburg, Center for Nanosystems Chemistry (CNC) & Bavarian
Polymer Institute (BPI), Theodor-Boveri-Weg, 97074 Würzburg, Germany.
metallosupramolecular cage receptors for fullerenes have been
reported.21 Selective binding was utilized for the separation of
fullerene mixtures22 and electron transfer was studied for dye-
†
Electronic Supplementary Information (ESI) available: [details of any
supplementary information available should be included here]. See
DOI: 10.1039/x0xx00000x
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attached complexes.23 Nitschke and coworkers reported on the For a quantitative analysis, CHCl3 solutions of fulleVrieewneAsrticCle60Oanlninde
selective encapsulation of Diels-Alder bisadduct mixtures within C70 were titrated with a stock solutionDoOfI: 10.1039/D0SC03131C
and absorption
the cavity of an FeII L6 cage. However, the regioselectivity for changes were plotted against cage concentration (see ESI
for
the encapsulation has not been investigated.24 The Clever group details). Global fitting to a 1:1 binding model revealed
used a bowl-shaped [PdII L3(MeCN)2]4+ cage as a supramolecular complexation 105 M−1
constants of 6.3±0.4 and
105 M−1 for C60 and C70, respectively (Fig. 2c,d).
1
†
8
⨯
2
protecting group for the selective monofunctionalization of 5.3±0.4
⨯
encapsulated C60.25 The square-planar arrangement of the four Apparently, there is no significant preference for either C60 or
cage windows for a metallosupramolecular fullerene sponge C70, which is presumably attributed to opposing effects of better
was recently utilized for the exclusive formation of all-e Bingel size and shape matching for C60 and larger dispersion
tetrakisadducts of C60, even under catalytic conditions.26 This interactions for C70.
masking strategy provides a significant improvement over the
multistep orthogonal transposition approach27 as the so far only
a)
2323.570
b)
C70
2323.57
839.898
2444.653
2324.573
2321.546
practicable synthetic route for this addition pattern. Despite
these initial examples, no cage templates to control the
inherently less selective Prato reaction have been reported so
far.
C
70
2441.685
C70⊂1
2445.654
2325.576
2326.625
2320.520
2440.539
2442.673
[1:1] Komplex
2442.673
C60⊂1
C60
2328
2320
2440
2448
1000
3000 m / z
1000
3000
5000 m / z
Results and Discussion
c)
d)
1
1
C60
1+C60
C60⊂1
C70
1+C70
C70⊂1
10
5
10
Recently, we utilized the unique orthogonal arrangement of the
three aromatic wings in C3v symmetrical hexahydroxy
tribenzotriquinacenes (TBTQs)28 for the design and synthesis of
covalent organic cage compounds21f,29 via dynamic covalent
boronate ester formation. By varying the bite angle of the
diboronic acid counterpart, a series of cages30 with trigonal-
bipyramidal, tetrahedral or cubic shape and sizes ranging from
1.9 to 3.5 nm (solvodynamic diameters derived from DOSY
NMR) were obtained. As fullerene binding was already shown
0
1.5
3
0
1.5
c(1) / µmol L−1
3
5
c(1) / µmol L−1
300
400 λ / nm 500
300
400 λ / nm 500
Fig. 2. MALDI-TOF MS (TCNQ, CHCl3) for 1:1 mixtures of 1 and a) C60 and b) C70 (insets
show comparison of measured and simulated isotope patterns); UV/Vis titration
experiments for complex formation between 1 and c) C60 (black 1, purple C60, dotted red
1 + C60, red C60⊂1 derived from global fit to 1:1 model, inset shows fit to 1:1 model at
λ = 329 nm) and d) C70 (black 1, brown C70, dotted blue 1 + C70, blue C70⊂1 derived from
global fit to 1:1 model, inset shows fit to 1:1 model at λ = 379 nm).
for some π
-extended TBTQ derivatives,31 we tested the host
properties of these cages. After saturation with either C60 or C70
by standing over pristine fullerenes for 24 hours at room
temperature, fullerene uptake was analysed by UV/Vis
absorption spectroscopy. No additional absorption in the range
of 350 to 600 nm, which would indicate fullerene complexation,
was observed for CHCl3 solutions of a methoxy-protected TBTQ
Ultimately, single crystals suitable for X-ray diffraction were
obtained during reactivity studies (see below) for the C60
complex. C60⊂1
crystallizes in the monoclinic space group C2/c.‡
Fig. 3b shows an ORTEP representation of the 1:1 inclusion
complex. Whereas the host could be nicely refined, much higher
disorder was observed for the guest indicating a very low barrier
for internal C60 rotation.
precursor and a large cubic cage (Fig. S5,S6
†). In case of trigonal-
bipyramidal [2+3] cage however, a linear correlation between
1
the host concentration and additional absorption features
characteristic for C60 and C70, respectively, clearly indicated
uptake of both fullerenes (Fig. S7†).
Specific shifting of cage protons in 1H NMR spectra of
1 in CDCl3
in the presence of 3 equivalents of solid C60 revealed
instantaneous complex formation (Fig. 3a). The rather small
shifts might be explained by the assumption that spherical C60 is
freely rotating within the cavities and the fact, that the para-
and diamagnetic ring currents within the five- and six-
membered rings of C60 are almost cancelling each other out.
Directly after mixing, residual signals that can be attributed to
free boronic acid and catechol moieties (Fig. 3a) suggest that
encapsulation is achieved via partially opened cages and not via
Further evidence for encapsulation was obtained by MALDI-TOF
MS. The isotope patterns for the major signals at m/z = 2323.57
and 2442.67 are in excellent agreement with the respective 1:1
host-guest complexes C60⊂1 and C70⊂1 (Fig. 1). Remarkably, no
signals for the empty cages besides small peaks for pristine C60
or C70 are detected for 1:1 mixtures (Fig. 2a,b). Semiempirical
molecular modeling on the PM6 level revealed that the two
TBTQ units in
encapsulation of one fullerene molecule. The distance of the
two TBTQ closo-atoms in , C60 and C70 only slightly
changes from 1.55 to 1.66 and 1.69 nm (Fig. S1 ). In the case of
the larger cubic assemblies, only empty cages and no complexes
were detected in MALDI measurements (Fig. S10 ). Here, the
1 are indeed perfectly preorganized for efficient
direct slipping. After one hour, cages 1 are again fully closed and
1
⊂
1
⊂1
quantitative complex formation is observed in case of
equimolar mixtures or excess C60. For substoichiometric
fullerene concentrations, the appearance of two separate sets
†
†
of signals for both
stability for the assemblies, at least on the NMR time scale. For
C70 , chemical shifts are more prominent, with the significant
1 and C60⊂1 suggests a rather high kinetic
increased distance between two TBTQs does not allow any
favourable interaction of one fullerene with two TBTQs, thus
preventing complexation.
⊂
1
low-field shift for the aromatic TBTQ protons being particularly
2 | J. Name., 2012, 00, 1-3
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apparent (Fig. S2
ARTICLE
†
). Therefore, we speculate that C70 is constraints, a limitation to trisaddition wasVieawnAtritcicilpeaOtnelinde.
DOI: 10.1039/D0SC03131C
complexed in a more rigid fashion with the benzenoid hexagons Furthermore, trans-3 and neighbouring addition patterns,
of the equatorial belt being in close proximity to the axial TBTQ which approach the 120 ° angle between two individual cage
units and the fivefold rotational axes located along one of the windows, should be favoured (Fig. 6 and S16
cage windows. This model is also in accordance with The Bingel6 and Prato7 reactions are the two most common
semiempirical PM6 calculations (Fig. S1 ). However, the methods for exohedral functionalization of fullerenes. For the
†).
†
exchange of the encapsulated C70 between the three windows twofold Bingel reaction, there is an intrinsic preference for
should be fast, at least on the NMR time scale, as we did not trans-3 and e-addition,13 whereas cis-addition is significantly
observe any splitting of the signals for
NMR spectrum of C60 (Fig. S3d ), a slight upfield shift from at the cyclopropane rings. For higher adducts, any pre-existing
143.24 to 141.65 ppm was observed for C60 upon complexation addends in e-position increasingly favour further e-addition,
(Fig. S3e
). Due to the lower solubility of C70, no 13C-NMR ultimately leading to the highly preferred Th symmetrical
spectrum could be measured in CDCl3. For C70
however, five hexakisaddition pattern.33 Building upon this inherent
1
in 1H-NMR. In the 13C- disfavoured due to the steric demand of the ester substituents
⊂
1
†
†
⊂
1
signals at 129.01, 143.72, 146.18, 146.63 and 149.63 ppm selectivity, Ribas and coworkers reported in the exclusive
appeared, which are in very good agreement with values formation of the all-equatorial tetrakisadduct for Bingel
reported for C70 in C6D6 (130.28, 144.77, 146.82, 147.52, reactions on a complex of C60 within a tetragonal prismatic
150.07).32
metallosupramolecular cage.26 For the Prato reaction however,
a much broader distribution of all eight possible bisadducts is
usually observed (Fig. S12b†).34 Only trans-1 and cis-1 isomers
are formed in lower yields due to statistical (t1) and steric (c1)
factors. Substantial cis-additions for smaller Prato addends
generally result in very complex mixtures of higher adducts. In
contrast to Bingel reactions, the isolation of specific
regioisomers has only been reported in rare cases.34a
*
a)
a
b
0.5 eq C60
24 h
C
60 ⊂1
Initial investigations for Bingel reactions on C60⊂1 gave
c
d
promising hints for the selective formation of specific bis- and
trisadducts at the early stages. At higher conversion however,
an increasing proportion of the undesired background reaction
on free C60 was observed due to partial decomposition of the
cages in the basic medium. Under Prato conditions though, cage
R-B(OH)2
3 eq C60
5 min
b
open
TBTQs
a
e
f
c
d
f
e
1
1
is sufficiently stable to restrict functionalization to the
encapsulated C60. As a model reaction, we chose the synthesis
of N-methylfulleropyrrolidine multiple adducts
7.8
7.6
7.4
7.2
4.6 ppm 4.4
b)
c)
C60(CH2NCH3CH2)n from sarcosine (Sar, N-methylglycine) and
paraformaldehyde (Fig. 4a).
Me
Me
a)
N
O
N
O
O
O
O
O
OB
O
O
BO
O
O
OB
O
O
BO
O
O
O
O
O
O
MeOH
work-up
O
H
N
OH
HO(CH2O)nH
PhMe
120 °C
N
O
N
N
O
N
O Me
O Me
Me
Me
O
OB
O
O
O
OB
O
O
O
O
C
60 ⊂1
4-t3,t3,t3⊂1
4-t3,t3,t3
Fig. 3. a) 1H-NMR spectra (400 MHz, CDCl3, rt) for complexation of 1 with C60, b) ORTEP
representation of single-crystal X-ray structure of C60⊂1 (thermal ellipsoids were set at
50% probability, C=grey, O=red, B=purple, H atoms were omitted for clarity) and c)
packing of C60⊂1 in the solid state indicating the accessible C60 surface in the three cage
windows.
b)
80
C60
60
40
20
0
2 (monoadduct)
3 (bisadducts)
4 (trisadducts)
Based on these analytical data, we concluded that the 1:1
complex C60
stability. A Space filling model from the X-ray structure (Fig. 3c)
further reveals that the aromatic scaffold of shields a
⊂1 forms with very high thermodynamic and kinetic
C60
2.0 eq
48h
C60 ⊂1 C60 ⊂1 C60 ⊂1 C60 ⊂1
8.9 eq
20h
3.3 eq
42h
5.2 eq
20h
8.9 eq
48h
1
Fig. 4. a) Cage-templated synthesis of N-methylfulleropyrrolidine trisadducts; b) relative
yields (based on HPLC integration) for unreacted C60 and Prato adducts 2 (mono), 3 (bis)
and 4 (tris) for control experiment (black) and cage-templated reactions (blue and
green).
significant part of the C60 surface apart from the three cage
windows arranged in a trigonal planar fashion. Therefore, we
speculated on the potential of C60⊂1 acting as a supramolecular
template for exohedral C60 functionalization. Assuming that
each window can only accommodate one addend due to steric
This journal is © The Royal Society of Chemistry 20xx
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and cage-templated reactions with C60⊂1 and b) 5 eq. Sar (20h), c) 9 eq. Sar (20h) and d)
The use of formaldehyde as carbonyl component prevents the
occurrence of complex mixtures of stereoisomers at the
pyrrolidine rings. The implementation of Me as a very small N-
substituent also ensures that any selectivity effect is primarily
based on the inherent properties of the template and not on the
steric demand of the addends. Ultimately, comprehensive data
on HPLC eluation profiles and spectroscopic signatures are
View Article Online
9 eq. Sar (48h, preparative scale); yields for 3-t3 and 4-t3,t3,t3 are indicated in light red
DOI: 10.1039/D0SC03131C
and red, respectively.
Intriguingly, even at low conversion significant amounts of D3-
symmetrical 4-t3,t3,t3 (3%) are formed as the only detectable
trisadduct. For this addition pattern, all three addends can be
perfectly centred within the cage windows, thus minimizing any
steric interactions between the addends and the cage walls.
By contrast, a control reaction on C60 with five equivalents of Sar
for 20 hours resulted in a complex and inseparable mixture of
available34a,35 as reference for monoadduct
regioisomeric bis- ( , n=2) and relevant trisadducts (
To test the templating effect of cage , varying equivalents of
Sar and excess formaldehyde were added to solutions of C60
in 1:1 dry CHCl3/toluene (or C60 in dry toluene as control)
followed by heating under reflux (Fig. 4a, see ESI for details)
2
(n=1), all
4, n=3).
3
1
⊂
1
many isomers with up to five pyrrolidine addends (Fig. S14a,d
Reaction on C60 with 9 equivalents of Sar for 20 hours
resulted in higher conversion and the predominant formation of
trisadducts (64%). According to MALDI-TOF MS for the
reaction solution (Fig. S15a ), all fullerenes are still
encapsulated and almost no higher adducts are formed.
†).
⊂
1
†
and reaction progress was monitored by MALDI-TOF MS. After
specific time intervals, methanolic workup resulted in
disassembly of the cages and the obtained fulleropyrrolidine
mixtures were analyzed by analytical HPLC. As expected,
multiple 1,3-dipolar cycloadditions on free C60 proceed rather
unselective. After reaction with two equivalents Sar for eight
4
†
Remarkably, only four out of the possible 46 regioisomers for
were detected (Fig. 5c). Alongside the symmetry-matched
t3,t3,t3 (25%) as the main product, only -t4,t4,t2 (7%),
t4,t3,t3 (8%) and -e,t3,t2 (24% together with
4
4-
4
4-
hours, all eight regioisomeric bisadducts
trisadducts were observed (Fig. 4b, black) in similar relative
yields as previously reported (Fig. 5a and S12
).34a By contrast,
reactions on C60 (Fig. 4b, blue) proceed more slowly but with
3 and small amounts of
4
3
-e) were
4
observed as side products. This selectivity can be nicely
explained by molecular models for the trans- and e-bisadduct
†
⊂
1
complexes
bonds centred in the third window leads exclusively to these
four isomers (Fig. 6). Cage acts as a supramolecular shadow
3⊂1 (see Fig. S17†), since any addition to double
significantly higher selectivity. Reaction with up to 5 equivalents
of Sar over 20 hours indicated low conversion with most of the
C60 being unreacted (39% based on HPLC integration) or only
1
mask that prevents over-functionalization and templates the
addition in trans-3 and related positions. Indeed, no cis-adducts
and only those four trisadducts, which best match the trigonal
planar arrangement of the cage windows, were observed
throughout the reaction time (Fig. 5).
converted to monoadduct
2 (34%). For the bisadducts 3 (24%),
cis-addition is completely prevented and the symmetry-
matched
3-t3 is formed as the main regioisomer (Fig. 5b).
a)
60
t2
C60
2 eq Sar
8h
t3
mono
40
20
0
t1
c1 t4
c3
t4
e
c2/c3
c1
c2
t2
t1
t3
b)
60
t2
t1
C
60 ⊂1
e
t3
5 eq Sar
20h
t3,t3,t3
40
20
0
t4
e
trans-4
e
e,t3,t2
t4,t4,t2
c)
C60 ⊂1
9 eq Sar
20h
60
t3,t3,t3
t3
t2
t1
+ e
t4,t4,t2
40
20
0
e + e,t3,t2
t4,t3,t3
t4
t4,t3,t3
t4,t4,t2
d)
60
t3,t3,t3
C60 ⊂1
trans-3
trans-2
9 eq Sar
48h
40
20
0
t3
t4,t3,t3
e,t3,t2
t4,t4,t2
t4,t3,t3
e,t3,t2
1
2
3
4
e
1
3
t
3
2
2
2
2
t
t
t
t
t
t
t
t
t
5
10
15
t / min
20
25
30
c
,
2/3
4, 3, 4, 3,
3,
c
t
t
t
,
e
t
,t3
,
,t
4
4,
e
2,
3
t
t
t
c
Fig. 5. Relative yields (left, calculated from HPLC profiles) for bisadducts 3 and trisadducts
4 and HPLC chromatograms (right) for a) control experiment with C60 (2.0 eq. Sar, 8h)
e,t3,t2
t3,t3,t3
4 | J. Name., 2012, 00, 1-3
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Fig. 6. Accessibility of specific addition patterns in the cage windows for a second
addition on 2⊂1 (top, first addend purple) and a third addition on 3-e⊂1 (middle left, e-
addends in red), 3-t4⊂1 (middle right, t4-addends in blue), 3-t3⊂1 (bottom left, t3-
addends in green) and 3-t2⊂1 (bottom right, t2-addends in yellow).
supramolecular control of the regioselectivVitieyw Arftoiclre Ontlhinee
intrinsically nonselective Prato reaction. They will pave the way
for novel applications of covalent organic cage compounds as
effective templates for spatially precise reactions on large
DOI: 10.1039/D0SC03131C
For the non-templated reaction,34a nine different trisadducts
have been isolated in a combined yield of 4.4% (Fig. S24 ) only
after tedious chromatographic separation that involved several
preparative TLC and HPLC steps. Thereby, -t3,t3,t3 was
4
spherical π-systems.
†
Conflicts of interest
4
There are no conflicts to declare.
obtained as the minor isomer in only 0.2% yield. When running
the cage-templated reaction (Fig. 4a) on a preparative scale, it
proved advantageous to add up to 9 equivalents of Sar in three
Acknowledgements
batches and to extend reaction times to 48 hours (see ESI
details). For these prolonged reactions, the emerging
decomposition of cage initiated some overreaction outside of
the cages (see Fig. S19a for MALDI-TOF MS). Apparently, all
mismatched isomers react faster, thus further simplifying the
remaining mixture and increasing the relative yield for the
† for
This work was financially supported by the Fonds der
Chemischen Industrie (Liebig fellowship for FB and doctoral
fellowship for SF), the DFG (BE4808/2-1) and the Collaborative
Research Network “Solar Technologies Go Hybrid” of the
Bavarian Ministry of Science, Research and the Arts.
1
†
matching 3-t3 and 4-t3,t3,t3 (Fig. 5d).
After methanolic work-up, all five remaining bis- and trisadducts
could be purified by one single automated flash
chromatography run without the need for tedious HPLC
separation. Finally, the four different trisadducts were isolated
as pure regioisomers in a one-pot-reaction starting from
pristine C60 with a combined yield of 16%. All products were
Notes and references
‡
Supplementary crystallographic data for complex C60⊂1 can be
obtained free of charge from The Cambridge Crystallographic
Data Centre via http://www.ccdc.cam.ac.uk/data_request/cif;
C60⊂1
(CCDC
1539894):
C60⊂C106H72B6O4⋅(C7H11BrO4)2,
C2/c, a=23.3258(10),
M=2801.23 g⋅mol−1,
monoclinic,
analyzed by MALDI-TOF MS (Fig. S22
†
) and comparison of
b=22.5169(10), c=31.3889(10) Å, α=90, β=91.177(2), γ=90°,
V=16482.7(12) Å3, Z=4, ρcalc=1.129 g cm−3, µ(CuKα)=1.129 mm−1,
T=100(2) K; 168505 independent measured reflections. F2
refinement, R1=0.1264, wR2=0.4437 (observed), 16220
independent observed reflections (Rint=0.0337) [|F0|>4σ(|F0|),
2Θ≤144.72°], 1100 parameters, 490 restraints.
UV/Vis (Fig. S23
†
) and 1H-NMR spectra (Fig. S20,S21
†
) to
literature data.34a When compared to the rather nonselective
control reaction, the selectivity for the cage-templated reaction
is completely reversed and the isolated yield for
main product is increased by a factor of 30 (Fig. S24†). The small
pore windows of enforce the formation of the unfavoured
4-t3,t3,t3 as the
1
1
2
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t3,t3,t3-addition pattern that would otherwise only be obtained
in trace amounts. On the other hand, the cage walls prevent
overreaction to higher adducts. In future work, we plan to
optimize reaction conditions for a further increase in isolated
amounts of pure trisadducts, modify the cage windows to
enhance the selectivity and develop more stable cages to
perform more sensitive reactions.
3
4
5
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encapsulation within the pores of trigonal bipyramidal covalent
6
7
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organic cage
moieties, strong 1:1 complexes C60
organic solvents and binding constants of 6.3±0.4
5.3±0.4
105 M−1, respectively, have been determined by
UV/Vis titrations. As evidenced by an X-ray structure for C60
1
. Due to suitable preorganization of its two TBTQ
and C70 are formed in
105 and
8
9
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⊂
1
⊂1
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⨯
⊂1,
only three distinct parts of the C60 surface are accessible for
exohedral functionalization. In prototypical Prato reactions on
C60
trisadduct
cage-free reactions, was observed. Cage
⊂
1
,
remarkable selectivity for the symmetry-matched
-t3,t3,t3, which is formed only in trace amounts in
acts as an efficient
4
1
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supramolecular shadow mask, whose small pore windows force
the formation of an otherwise unfavoured trisaddition pattern.
These exciting findings are the first example for
a
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J. Name., 2013, 00, 1-3 | 5
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Chemical Science
View Article Online
DOI: 10.1039/D0SC03131C
The taming of the Prato reaction: A covalent organic cage compound serves as a
supramolecular template for the regioselective functionalization of C60.