Exciton-Coupled Circular Dichroism Characterization of Monotopically Binding Guests in Host?Guest Complexes with a Bis(zinc porphyrin) Tweezer

Article abstract of DOI:10.1002/cplu.201800564

A stiff-stilbene-linked bisporphyrin tweezer with inherent helicity was used for exciton-coupled circular dichroism (ECCD) characterization of a series of monotopically binding amine guest molecules. CD signals were observed for a variety of monoamines at relatively low tweezer/amine (host/guest) ratios between 1 : 10 to 1 : 70. For the amines producing the most intense CD signals, a binding stoichiometry of 1 : 2 was found. A likely explanation is the presence of guest-guest interactions in the complexes. This is supported by the correlation observed between CD signal intensity and magnitude of possible noncovalent binding between the guests, which can be divided into three groups showing no, moderate and strong response, respectively. Further support for this rationalization comes from molecular modelling.

Full text of DOI:10.1002/cplu.201800564

Accepted Article
Title: ECCD characterization of monotopically binding guests in host-
guest complexes with a bis-(zinc porphyrin) tweezer
Authors: Sandra Olsson, Clara Schäfer, Magnus Blom, and Adolf
Gogoll
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Exciton-Coupled Circular Dichroism Characterization of
Monotopically Binding Guests in Host-Guest Complexes with a
Bis-(zinc porphyrin) Tweezer
Sandra Olsson,[a] Clara Schäfer,[b] Magnus Blom[a] and Adolf Gogoll*[a]
The tweezer itself consists of two metalloporphyrins connected
by a linker of varying degree of flexibility.The by far most
commonly used metalloporphyrin is ZnTPP.[3] As the zinc
porphyrin readily binds to amine groups these tweezers can be
very useful to determine the absolute stereochemistry of a
variety of natural and synthesised diamines.[9] Although hydroxy
and carboxyl groups show considerably weaker binding, guests
with these functional groups can be added in huge excess. Thus,
bis-porphyrin tweezers have been successively used on diols,
aminoalcohols amino acids and carboxylic acids.[10][11] However,
these systems rarely work for molecules with only one available
binding site, i. e. ditopic binding of the guest is essential. A few
cases have been reported where the absolute stereochemistry
of monoamines has been determined by utilizing bisporphyrin
tweezers and ECCD but they rely on secondary interactions of
the monotopically bound guest with either the tweezer linker or
with substituents on the porphyrin rings.[11][12][13][14] In most cases
where molecules with only one binding site are studied
derivatization is used to facilitate ditopic binding by providing a
second binding site.[15] Our group has recently developed a
bisporphyrin tweezer 1 with a linker enforcing inherent helicity
(Figure 2).[16]
Abstract: We have investigated the possibility to use a stiff-stilbene
linked bisporphyrin tweezer with inherent helicity for Exciton-Coupled
Circular Dichroism (ECCD) characterization of
monotopically binding amine guest molecules. CD signals were
observed for variety of monoamines at relatively low
tweezer:amine (host:guest) ratios between 1:10 to 1:70. For the
amines producing the most intense CD signals binding
a
series of
a
a
stoichiometry of 1:2 was found. A likely explanation is the presence
of guest-guest interactions in the complexes. This is supported by
observing a correlation between CD signal intensity and magnitude
of possible non-covalent binding between the guests, which we can
divide into three groups showing no, moderate or strong response,
respectively. Further support for this rationalization comes from
molecular modelling.
Introduction
Bisporphyrin molecular tweezers are well studied systems for
ditopic binding of guest molecules.[1][2][3] These systems can be
used in combination with Exciton-Coupled Circular Dichroism
(ECCD) to determine the absolute stereochemistry of chiral
guest molecules (Figure 1).[4][5][6][7][8]
Figure 2. cis-Bis-zincporphyrin tweezer 1. M- and P- enantiomers are shown.
Figure 1. Bisporphyrin tweezers can be used to measure the absolute
stereochemistry of guest molecules. The empty tweezer (top) gives no CD
signal on its own. By introduction of a binding chiral guest molecule (lower)
chirality is induced and a CD signal can be recorded.
[a]
[b]
S. Olsson, Dr. M. Blom, Prof. Dr. A. Gogoll
Department of Chemistry-BMC
Uppsala University
Uppsala, S-75123, Sweden
E-mail: adolf.gogoll@kemi.uu.se
C. Schäfer
Department of Chemistry and Pharmacy
Friedrich-Alexander-University Erlangen-Nürnberg
Erlangen, DE-91058, Germany
Figure 3. A diamine, L-lysine methylester ditopically bound to the bis-
zincporphyrin tweezer 1.
Supporting information for this article is given via a link at the end of
the document.
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Figure 4. alpha-amino acid methylesters used as guests for the tweezer 1. Most of the guests were tested as both (R) and (S) (i.e., D and L) enantiomers as
stated in the result tables.
rigid side chains. As our initial investigations showed a strong
CD signal for tyrosine methylester G9 but not for Phenylalanine
methylester G8, the guests G13-G19 were added to investigate if
further substituents might affect the binding.[16] To propose a
rationale for the ability of monotopically bound guests to induce
a CD signal the host-guest systems have been studied with
molecular modelling.
In the absence of guest molecules the tweezer exists as a
racemate, giving no CD signal. Upon binding of a chiral guest
the tweezer adopts preferably either one of its enantiomeric
conformations, resulting in a CD signal (Figure 3). To our
surprise, we observed that also a monoamine was able to
generate a CD response.[16] We here present a more systematic
investigation of this aspect. To investigate which monotopically
binding guest molecules are capable of producing a CD signal
when bound to tweezer 1, we have tested a library of
monoamines with varying sizes and substituents (Figure 4).
Amongst these, G1-G5 have aliphatic side chains whereas G6-G9
have more rigid structures. In addition to the guests derived from
amino acids the benzylamines G10 and G11 and the
naphtylbenzylamine G12 was added to investigate the effect of
Results and Discussion
Synthesis of compounds and initial binding study
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The synthesis of tweezer 1 followed our previously reported
method.[16] The tweezer in solution exists as a racemate that is
unresolvable as the energy needed for switching between P-
and M-enantiomeres (1.2 kJ/mol) is well below the available
thermal energy at room temperature (see supporting
UV-vis spectroscopy was used to confirm the binding of guests
to the tweezer 1. Binding is indicated by a red-shift of the Soret
and Q-bands of the porphyrin chromophore. For small guests
this may be counteracted by the blue shift induced when the two
porphyrin units get close to each other.[19][20] In reported ECCD
information). The studied guest molecules are shown in Figure 4. studies involving ditopically binding diamines, the size of the red-
Amino acid methylester hydrochlorides were prepared from the
corresponding amino acid or amino acid derivative by treatment
with thionyl chloride in methanol.[16][17] The free amines where
obtained by treatment of the hydrochlorides with sodium
methoxide in methanol solution.[18]
shift has been related to the size of the guest, with larger guests
giving red shifts of about 10 to 12 nm and smaller guests give
around 3 to 4 nm.[14] All the monoamine guest molecules bind to
the tweezer and both enantiomers bind equally well, with similar
red-shifts for the Soret- and Q-bands of 7 to 11 nm (supporting
information Table 1).[21]
Figure 5. Possible binding motifs for the case of 1:2 host-guest complexes.[26] Depicting the M-enantiomers of the tweezer.
Binding stoichiometry
The guests used in this study (cf. Figure 4) are all expected to
bind monotopically since only one functional group with the
possibility of strong binding to Zn-porphyrin is present.[7] With
ditopic binding 1:1 host:guest complexes are usually formed due
to a cooperative effect.[22][23] In contrast, monotopical binding is
expected to result in a 1:2 stoichiometry, which allows for
several binding motifs (Figure 5).[24] It is possible to get CD
signals from complexes between guests and monoporphyrins[25]
and it should be noted that a CD signal can be detected for
some of our guests in the presence of only ZnTPP. However, as
this signal is extremely low it has not been further taken into
account.
To produce a CD signal by induced helicity of the bisporphyrin at
least one guest needs to bind inside of the porphyrin cleft. For
Figure 6. Job plot of D-tyrosine methylester D-G9 with tweezer 1. The plot
shows the typical 1:2 shape with the maximum at 0.7. The theoretical curve is
shown in red and the theoretical maximum is at 0.66.
1:1 binding stoichiometry, this leaves only one CD-active option,
i. e. the guest placed inside of the bisporphyrin cleft. With the
guest in large excess, as is commonly the case in ECCD
measurements, we might also form 1:2 host:guest complexes
which might be CD-active. Of their three possible binding motifs
(Figure 5), endo-endo and endo-exo might be CD-active. These
kind of binding motifs have been studied by Rath et. al. for the
case of aminoalcohols binding to their bis-magnesium porphyrin
tweezer, showing that the endo-endo motif gives a much
stronger CD signal.[26]
As our system is different we do not rule out that we might also
get a reasonable signal from the endo-exo binding motif. The
endo-endo binding motif is only likely if the guest molecules can
have some sort of interaction, otherwise steric repulsion should
disfavour this motif, especially for larger guests.[10][26]
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To elucidate the host:guest ratio in the complexes formed with
our tweezer we used the “method of continuous variation” (Job
plot) for tweezer 1 with D-tyrosine methylester D-G9 (expected to
bind 1:2 monotopically), and (2S,3R,4R,5R)-1,6-diamino-1,6-
dideoxy-2,3,4,5-tetra-O-methyl sorbitol[27] (expected to bind 1:1
Determination of absolute stereochemistry by ECCD
The UV-vis study confirms that the guest molecules bind to
tweezer 1. To test if there is chirality transfer between guest and
tweezer (i. e., chirogenesis) the complexes were studied with
ECCD. The CD measurements were done at 25 °C by titration of
tweezer solution (24 - 33 µM in CH2Cl2) with guest solutions. Out
of 19 guests 11 gave CD signals (Table 1). For all guest
molecules the R- and S-enantiomers give the opposite CD
signals. In some cases we see a difference in the amplitude of
the signals between the enantiomers. This is due to slight
variations in solution concentration. For G7 and G15 this is due to
difference in optical purity (ee) of the commercially available
compounds.
ditopically), Figure
6
and Figure S123 in Supporting
information.[28] A job plot for one of the guests with less
possibility of guest-guest interaction (S-(-)-G13) was attempted
but failed due to the CD-signal being too low at these guest
concentrations.
The job plots for tweezer 1 with D-tyrosine methylester D-G9 and
diamino sorbitol methylester both shows the expected shapes
for 1:2 binding and 1:1 binding respectively, confirming the
suggested stoichiometry of the complexes.[28]
Table 1. CD signal amplitudes (L × mol-1 × cm-1) for the complexes of the cis tweezer 1 with the guests giving a CD signal. Measurements were performed at
25 °C for CH2Cl2 solutions at tweezer concentration of 24 - 33 µM.
Guest
Host CD signal (signal sign and amplitude)
λ/Δε (nm)
Host:Guest
Ratio
[a]
ACD
456 (+136)
448 (-65)
L-G6
D-G6
1:10
1:10
+237
-214
429 (-36)
456 (-121)
448 (+55)
430 (+38)
450 (-321)
441 (+250)
L-G7
D-G7
1:10
1:10
-571
450 (+456)
441 (-288)
450 (-750)
441 (+562)
428 (+57)
450 (+760)
441 (-551)
429 (-51)
456 (+23)
437 (-14)
454 (+10)
441 (-11)
454 (-13)
441 (+16)
454 (+10)
433 (-12)
454 (-11)
+744
L-G9
D-G9
1:10
1:10
-1369
+1362
(R)-(+)-G12
(R)-(+)-G13
(S)-(-)-G13
1:1000
1:70
+37
+21
-29
1:70
(R)-(+)-G14
(S)-(-)-G14
1:70
1:70
+22
-27
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435 (+16)
455 (+5)
(R)-(+)-G15
(S)-(-)-G15
(R)-(+)-G16
(S)-(-)-G16
(R)-(+)-G17
(S)-(-)-G17
(R)-(+)-G18
(S)-(-)-G18
(R)-(+)-G19
1:60
1:60
1:70
1:70
1:70
1:70
1:70
1:70
1:70
1:70
+17
-30
+25
-27
+31
-36
+32
-41
-66
+67
435 (-12)
454 (-11)
435 (+19)
455 (+11)
437 (-14)
456 (-14)
436 (+13)
457 (+10)
445 (-21)
454 (-11)
439 (+25)
454 (+17)
436 (-15)
454 (-21)
441 (+20)
454 (-40)
436 (+26)
453 (+42)
438 (-25)
(S)-(-)-G19
[a]
A
CD
= Δε1 - Δε2 represents the total amplitude of the CD couplets.
Based on our results (Table 1) involving guest molecules with
different structures we can divide them into three groups based
on their CD response (Table 2).
The required host:guest ratio for tweezer 1 to produce the
maximum signal amplitude is relatively low at 10-70 -fold guest
excess (Figure 7). This compares favourably with typical ECCD
studies, where far higher excess (over 1000 -fold) is not
unusual.[8][13]
Unlike Phenylalanine methylester G8 the corresponding
aminoalcohol G19 gives a CD signal, however the observed
positive Cotton effect for the S-enantiomer is the inverse of the
expected one, whereas a negative 1st Cotton effect has been
reported for complexes of the S-enantiomer with other
bisporphyrin tweezers.[10][29] The R-enantiomer gives the
opposite, a negative Cotton effect. This unexpected result might
be caused by the guest binding in a way differing from the
literature systems, inducing the opposite helicity of the tweezer.
A similar situation was described by Rath et. al. where G19 was
binding to a bis(magnesium porphyrin) tweezer via the alcohol
group, not the amino group.[10] That this should be the case in
the present study is unlikely as alcohols bind much weaker to
zinc as compared to amines.[30][31]
Figure 7. CD-spectra of D-tyrosine G9, showing the change in signal intensity
with the change in host/guest ratio.
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We can see that very flexible guests with aliphatic side chains
(G1-G5) give no CD signal. The same is true for guests G8-G11
which are variations of the benzylamine motif. However, all other
guest molecules in the study give moderate to strong responses.
These results indicate that a relatively rigid structure is needed
for chirogenesis.
It is interesting that G10 and G11 give no CD signal whereas G9
gives a strong response and G13-G18 give moderate responses.
Guest G19 with a similar structure also gives a moderate
response but with the Cotton effects for its S and R-enantiomers
reversed as to the expected ones.[10][29] This leaves the binding
of G19 to tweezer 1 in ambiguity as binding to the hydroxyl group
is unlikely in the presence of an amine.[30][31] The guests giving
strong CD signals all have stronger potential of guest-to-guest
interaction than the guests in the group with moderate response
(Figure 8).
Figure 8. Schematic representation of suggested interactions between two
guest molecules binding in an endo-endo motif. A) Suggested interaction for
guest molecules of group 3 (G9 as an example). B) Suggested interaction for
guest molecules of group 2 (para-substituted α-MBA G13 as an example).
Binding constants
Comparing guests from the three groups with their binding
constants shows that the guests giving a strong CD response
have a higher binding constant whereas the guests giving no or
moderate response, respectively have constants of the same
order of magnitude (Table 3).
There is some precedence in the literature for hydrogen bonding
between guests in 1:2 host:guest complexes enhancing the CD
signal. Thus, it is probable that such a strong interaction is
present for the complexes of G6, G7 and G9 in group 3.[10][26]
Table 2. Guest-molecules arranged in groups, depending on the strength of
their CD-signal when bound to tweezer 1. Groups 1, 2 and 3 give no, moderate
and strong CD response respectively.
Table 3. Ka for guests G1, G6, G9, G13, G19 and 1,6 diaminohexane measured
by UV-vis titration of tweezer 1 with guests. Also including binding constant of
free ZnTPP with 1,6 diaminohexane and pyridine, measured by NMR titration.
CD
Response
Ka/M-1
CD Response
Guest molecule
Group
Host
Guest molecule
Tweezer 1
Tweezer 1
Tweezer 1
Tweezer 1
Tweezer 1
Tweezer 1
ZnTPP
L-G1
D-G6
D-G9
S-G13
4 x 103
8 x 104
2 x 104
5 x 103
7 x 103
5 x 105
5 x 104
7 x 103
None
Strong
Strong
Moderate
Moderate
None
S-G19
1,6 diaminohexane [16]
1,6 diaminohexane [16]
Pyridine [31]
None
None
ZnTPP
1
None
This is consistent with the fact that all guests can bind to the
tweezer but not all complexes thus formed are CD active. The
higher stabilization of the CD-active complexes formed by G6, G7
and G9 might be reflected in the higher binding constant.
Comparing the monotopically binding guests to the ditopically
binding 1,6 diaminohexane with tweezer 1 we see a binding
constant one order of magnitude higher for the ditopic binding.
Comparing with the binding constants to free ZnTPP the results
for the diamino guest is one order of magnitude lower compared
to binding to the tweezer 1. Binding to pyridine that has only one
binding site we see the same size for the binding constant as for
the tweezer binding to the guests G1, G13, G19 with no or
moderate CD signals. This lends further support to the
suggested monotopic binding motif.
2
Moderate
3
Strong
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Figure 9. The tweezer 1 binding 1:2 to D-G9 (D-tyrosine methyl ester) and to S-(-)-G13 ((S)-(-)-para-Fluoro-α-methylbenzylamine). The empty tweezer in the
middle as a reference. Hydrogen bonds are shown in orange and -stacking in blue. A: Tweezer:D-G9 complex top view B: Tweezer:D-G9 complex side view C:

Empty tweezer top view D: Empty tweezer side view. E: Tweezer: S-(-)-G13 complex top view F: Tweezer: S-(-)-G13 complex side view.
Molecular modelling of monotopic binding motif
To rationalize the possible intermolecular guest-guest
interactions in the complexes we have used molecular modelling
(OPLS3 force field). The calculations show numerous
configurations with the guests outside the tweezer cavity but as
these cannot give rise to the CD signal we observe in the
experiments we have focused on the configurations with the
guests inside the tweezer cavity. As shown in Figure 9 the 1:2
complex of tweezer 1 with D-G9 (D-tyrosine methyl ester) can be
stabilized by hydrogen bonding between the two guest
molecules and by -stacking between the two guest molecules.
In contrast, when investigating the complex between tweezer 1
and S-(-)-G13 ((S)-(-)-para-Fluoro-α-methylbenzylamine) only -
stacking between the two guests stabilizes the 1:2 complex.
Figure 10. The tweezer 1 binding 1:2 to S-(-)-G13 ((S)-(-)-para-Fluoro-α-
As it is counterintuitive that G8-G11 did not show any CD signal
methylbenzylamine) and to S-(-)-G10 ((S)-(-)-α-Methylbenzylamine). Hydrogen
as their complexes should also be able to be stabilized by -
bonds are shown in orange and
-stacking in blue. A: Tweezer: to S-(-)-G13
stacking we performed some calculations on the tweezer-G10
complex. Our results suggest that the smaller G10 should be able
to induce chirogenesis in the same manner as G13 if binding 1:2
complex top view B: Tweezer: to S-(-)-G13 complex side view C: Tweezer: S-(-
)-G10 complex top view D: Tweezer: S-(-)-G10 complex side view.
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and -stacking between the two guests, Figure 10. The major
difference between the complexes of G10 and G13 is that the G10
complex shows guest-guest hydrogen bonding between the
amino groups. This might suggest that the binding to the
tweezer zinc atoms is not favored in this configuration and might
be an indication the guest molecule will not bind in this fashion.
chirogenesis. As most of our guest molecules have only one
possible binding site the monotopic binding motif is clearly
established. Monotopic binding is also supported by both our
experimental and computational investigations into the complex
stoichiometry. These investigations indicates a 1:2 host:guest
binding motif with both guests on the inside of the tweezer. Our
results support the suggestion that guest-guest interactions
stabilize the complexes and enhance the CD signal from endo-
endo binding motifs.
To investigate why G19 gave the opposite cotton effect with our
tweezer 1 compared to previously reported bisporphyrin:G19
complexes we preformed some calculations on this system.[10][29]
Our calculations show that when forming a complex with the M-
enantiomer (that would give the negative cotton effect) there is
less stabilization from intermolecular interactions then when
forming the complex with the P-enantiomer (Figure 11). In the P-
tweezer:S-G19 complex we observe -stacking between the
guest molecules as well as hydrogen bonding between the guest
and the carbonyl oxygen in the linker, whereas the M-tweezer:S-
G19 complex only shows hydrogen bonding between the guest
molecules. We suggest that the stabilization from intermolecular
bonds in the P-tweezer:S-G19 complex is large enough to make
this complex energetically favored when compared to the M-
tweezer:S-G19 complex.
Experimental
General: Commercially available compounds were used without
purification. meso-Tetraphenylporphyrin was purchased from Porphyrin
Systems GbR, Germany. Amino acids and amino acid derivatives where
obtained from Sigma-Aldrich. Guests G10-G19 were obtained from Sigma
Aldrich. Analytical TLC was performed using Merck pre-coated silica gel
60 F254 plates and for column chromatography Matrex silica gel (60 Å,
35-70 µm) was used.
Computational details
The DFT calculations on the tweezer stiff-stilbene linker were performed
with the B3LYP functional as implemented in the Gaussian 16 program
package.[32][33] The SCRF solvent model with the SMD variation was
used with DCM as solvent.[34][35] Geometries were optimized using the 6-
31G(d,p) basis set.[36] Frequency calculations were performed at the
same level to confirm that a minimum had been reached and to extract
free energy corrections which were evaluated at 298.15 K. A stability
analysis was performed to ensure that a stable wave-function was
attained for all species.
Molecular structures for the tweezer 1 and its complexes were calculated
in MacroModel 9.9 with the OPLS3 force field and dielectric constant
9.1.[37] Conformational analysis was performed with the lowest energy
conformation from these scans as the starting structure. For
bisporphyrins and host–guest complexes, Zn–N distances were initially
constrained to 2.0 Å, resulting in final Zn–N distances of 2.1–2.3 Å.[8][38]
Figure 11. The tweezer 1 binding 1:2 to S-G19. The disfavoured M enantiomer
is shown on the left and the favoured P enantiomer on the right.
Conclusions
1H and 13C NMR spectra were recorded on Varian Mercury Plus (1H at
300.03 MHz, 13C at 75.45 MHz), on Aglient 400-MR DD2 (1H at 399.98
MHz, 13C at 100.58 MHz) or on Varian Unity Inova (1H at 499.94 MHz,
13C at 125.7 MHz) spectrometers at 25 °C unless noted otherwise.
Chemical shifts are reported indirectly referenced to tetramethylsilane via
residual solvent peaks. Circular dichroism spectra were recorded for
solutions in CH2Cl2 on a JASCO J-810 spectropolarimeter from 300-500
nm (400-500 nm for the job plots) at 25 °C using a 0.1 cm path length
quartz cell with 5 scans for the spectra of the tweezer-guest complexes
and 3 scans for the spectra in the job plots. CD-spectra were measured
in millidegrees and normalized into Δεmax (L ꞏ mol-1)/(λ (nm) units. For
We have shown that we can get strong to moderate CD signals
for a series of monotopically binding monoamine guests bound
to our bisporphyrin tweezer 1. With this improvement the range
of chiral molecules that can be analyzed by ECCD without prior
modification is significantly extended.
From testing a library of guest molecules with different structures
we show that our system works well for guests with relatively
bulky substituents. The comparison between G10 to G13 seems
to indicate the required size of the guest for successful
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comparison with literature data, molar ellipticities ϴ were converted to
ACD values with ACD = ϴ/32.982. UV-Vis spectra were recorded on a
Shimadzu UV-1650PC spectrophotometer using a 0.1 cm path length
quartz cuvette. Specific optical rotation was measured using a Perkin-
Elmer 241 polarimeter.
atmosphere, at room temperature for 3 - 3.5 days. Dichloromethane (60
mL) and water (60 mL) were added to the reaction mixture and the
phases were separated. The organic phase was washed with water (2 ×
100 mL), 5 % NaHCO3-solution (2 × 100 mL) and again with water (2 ×
100 mL). The organic phase was ten dried over Na2SO4 and the solvent
was evaporated. The crude product was used as such without further
purification. TPP-NH2 was obtained as a dark purple solid (233 mg, 0.370
mmol, 96 %). 1H NMR (400 MHz, CDCl3, 25 °C) δ 8.81 – 8.74 (m, 3H),
8.73 (d, J = 4.7 Hz, 1H), 8.68 (d, J = 4.8 Hz, 1H), 8.50 (d, J = 4.7 Hz, 1H),
8.21 – 8.12 (m, 7H), 7.86 – 7.65 (m, 13H), 4.42 (s, 2H), -2.71 (s, 2H).
The binding constants (Ka) for the complexes were determined by UV
titration utilizing the iterative fitting program published by P.
[39]
Thordarsson
in Matlab R2012b. Ka is calculated by (Eq. 1). The
standard error (SEy) is estimated by (Eq. 2).
ꢆ
ꢅ
ꢅ
ꢏ
ꢀ
ꢃ
ꢀ
ꢃ
ꢀ
ꢃ
ꢀ
ꢃ
ꢀ
ꢃ
ꢁꢂ
ꢄ
ꢆ ꢇꢈ ꢂ ꢉꢊ ꢁ ꢉ ꢊ ꢋ ꢍ ꢎ ꢈ ꢂ ꢉꢊ ꢁ
ꢊ
ꢅ ꢍ ꢎ 4 ꢁ
ꢀ ꢃ ꢀ ꢃ
ꢂ ꢉꢐ
ꢉ
(1)
ꢉ
ꢋ
ꢌ
ꢌ
Ethyl-3-oxoindane-5-carboxlate 5.[43] To a solution of 3-oxoindane-5-
ꢜ
carboxylic acid (1.00 g, 5.68 mmol) in ethanol (100 mL) concentrated
(5)
ꢛ
ꢏ∑ꢔ
ꢓꢕꢖꢗꢖꢘꢓꢙꢖꢚꢙ
ꢑꢒꢓ
ꢄ
(2)
ꢝꢘꢞ
12M HCl (0.47 mL) was added. The reaction mixture was heated to reflux
and left to stir overnight. The solvent was evaporated and the solid
residue was dried under vacuum. The product was received as a pale
brown solid (1.11 g, 5.45 mmol, 96%). 1H NMR (400 MHz, CDCl3, 25 °C)
δ 8.43 (m, 1H), 8.28 (dd, J = 8.0, 1.7 Hz, 1H), 7.56 (dd, J = 8.0, 0.8 Hz,
1H), 4.40 (q, J = 7.1 Hz, 3H), 3.20 (s, 2H), 2.82 – 2.71 (m, 2H), 1.41 (t, J
= 7.1 Hz, 3H).
Syntheses
(2-Nitro-5,10,15,20-tetraphenylporphyrinato)copper(II), Cu(II)TPPNO2
2.[40] meso-Tetraphenylporphyrin (TPP) (500 mg, 0.814 mmol) was
dissolved in CH2Cl2 (150 mL). Cu(NO3) ∙ 3H2O (560 mg, 2.32 mmol) was
grinded into a fine powder, dissolved in acetic anhydride (45 mL) and
added to the mixture. The reaction mixture was heated to 40°C and left to
stir for 2h. The reaction was let cool down to room temperature and left to
stir overnight. After full conversion (TLC) the reaction was quenched with
1M NaOH (35 mL). The phases were separated and the organic phase
was washed with 60% Na2CO3 (100 mL), saturated Na2CO3 (2 × 100 mL)
and H2O (100 mL). The washed organic phase was dried over anhydrous
Na2SO4 and the solvent was removed under vacuum. The crude product
was purified by column chromatography using CH2Cl2:pentane 1:1 as
eluent. Cu(II)TPPNO2 was isolated as a dark purple solid (541 mg, 0.750
mmol, 92%). UV-Vis: (CH2Cl2) λmax: 423 nm, 548 nm, 588 nm.[41]
Ethyl 3-(6-ethoxycarbonylindan-1-ylidene)indane-5-carboxylate 6.[16]
Under Argon atmosphere Zn-powder (0.96 g, 14.7 mmol) was suspended
in dry THF (30.0 mL). The suspension was cooled to 5 °C – 0 °C and
TiCl4 (1.39 g, 7.4 mmol) was added slowly with a syringe. The resulting
dark greenish slurry was refluxed for 2h. Afterwards a solution of ethyl-3-
oxoindane-5-carboxlate (500 mg, 2.45 mmol) in dry THF (10 mL) was
added to the reaction mixture and the mixture was stirred at reflux for 12h.
After complete consumption of the starting material (TLC, eluent
pentane:CH2Cl2:EtOAc 6:2:2) the reaction was allowed to cool to r.t. and
then quenched with saturated NH4Cl. The mixture was extracted with
diethyl ether (4 × 25 mL). The combined organic phases were washed
with brine (100 mL), dried over MgSO4 and the solvent was evaporated.
The residue was dissolved in diethyl ether (15 mL) and the precipitate,
the E-product, was filtered of and washed with diethyl ether. The filtrate
was evaporated and the residue was purified by column chromatography
using CH2Cl2:pentane 7:3 as eluent. The stiff-stilbene diethyl estersE-6
(186 mg, 0.495 mmol) and Z-6 (35 mg, 0,093 mmol) were isolated as
pale yellow solids, combined yield (48%, E:Z = 5:1).
2-Nitro-5,10,15,20-tetraphenylporphyrin, TPPNO2 3.[41] Cu(II)TPPNO2
(1.13 g, 1.56 mmol) was dissolved in CH2Cl2 (225 mL) and concentrated
H2SO4 (6.35 mL) was added. The reaction mixture was let to stir for 2 h
until all starting material was consumed (TLC). The reaction was
quenched by adding ice-water (150 mL). The phases were separated
and the water phase was extracted with CH2Cl2 (4 × 75 mL). The
combined organic phases were washed with 10% NaHCO3 solution (2 ×
300 mL), water (2 × 300 mL) and brine (2 × 300 mL) and dried over
anhydrous Na2SO4. The residue was purified by column chromatography
using CH2Cl2 as eluent. TPPNO2 was obtained as a dark purple solid
General procedure for hydrolysis of Z stiff-stilbene diethyl ester 6 to
3-(6-carboxy-indan-1-1-ylidene)indane-5-carboxylic acid 7.[16] Z-or E-
stiff-stilbene diethyl ester (120 mg, 0.319 mmol), NaOH (150 mg, 3.75
mmol) and ethanol (20 mL) were added to a round-bottomed flask. The
reaction mixture was heated to reflux and let stir at reflux for 6h. The
solvent was removed under vacuum and the residue was dissolved in
H2O. The solution was acidified using HCl (6M). The formed precipitate
was collected via filtration, washed with water, dried overnight and used
without further purification.
1
(968 mg, 1.47 mmol, 94%). H NMR (400 MHz, CDCl3, 25 °C) δ 9.05 (s,
1H), 9.02 (d, J = 5.1 Hz, 1H), 8.95 (d, J = 5.1 Hz, 1H), 8.90 (m, 2H), 8.72
(m, 2H), 8.26 (m, 2H), 8.23 – 8.17 (m, 6H), 7.84 – 7.67 (m, 12H), -2.63
(br. s, 2H). UV-Vis: (CH2Cl2) λmax: 426 nm, 526 nm, 660 nm.
2-Amino-5,10,15,20-tetraphenylporpyrin, TPP-NH2 4.[42] TPPNO2 (250
mg, 0.387 mmol) was dissolved in dry, degassed dichloromethane (25
mL). Sn(Cl)2∙2H2O (873 mg, 3.87 mmol) was added followed by addition
of conc. HCl (2.60 mL). The solution was left to stir under inert
(3Z)-3-(6-Chlorocarbonylindan-1-ylidene)indane-5-carbonyl chloride
8.[16] Dicarboxylic acid 7 (59 mg, 0.184 mmol) and dry CH2Cl2 (5.5 mL)
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10.1002/cplu.201800564
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was added to a dried 25 mL round-bottomed flask. Oxalyl chloride (1.0
mL) and DMF (4 drops) were added to the reaction mixture. The mixture
was stirred at room temperature under nitrogen atmosphere for two
hours. Volatile components were removed in vacuo and the solid residue
was washed with dry diethyl ether in which the product was soluble. To
remove by-products the heterogeneous diethyl ether solution was
filtrated (using a grade 3 filter paper). The filtrate was evaporated to
obtain the acid chloride as a light brown solid (yield: 50 mg, 0.140 mmol,
76%). 1H NMR (400 MHz, CDCl3, 25 °C) δ 8.79 (d, J = 1.7 Hz, 2H), 7.96
(dd, J = 8.0, 1.7 Hz, 2H), 7.43 (d, J = 8.0 Hz, 2H), 3.10 (m, 4H), 2.90 (m,
4H).
recrystalized from diethyl ether. The crystalline product was filtered off,
washed with diethyl ether and left to dry overnight under high vacuum.
General procedure B. The amino acid (1eq) was put into a round
bottomed flask and methanol was added. The suspension was cooled to
0 °C using an icebath. Thionyl chloride (10 eq) was added dropwise
resulting in a homogeneous solution. Afterwards the reaction mixture was
heated to reflux and left to stir overnight. The reaction mixture was
allowed to cool to room temperature and the solvent was removed in
vacuo. The crystalline product was dried under high vacuum overnight.
Procedure for preparation of free amines.[16][18] Free amines were
obtained following published procedures. Products were confirmed by 1H
NMR. Spectral data were in agreement with the literature.
(3Z)-N-(5,10,15,20-Tetraphenylporphyrin-2-yl-ato)Zn(II)-3-[6-
(5,10,15,20-tetraphenylporphyrin-2-yl-ato)Zn(II)-carbamoyl)indan-1-
ylidene]indane-5-carboxamide 1.[16] TPP-NH2 4 (225 mg, 0.357 mmol)
was added to a dried round bottom flask. Z-stiff-stilbene dicarboxylic acid
chloride 8 (45 mg, 0.126 mmol) was dissolved in dry CH2Cl2 and was
added to the flask via syringe. Dry pyridine (0.2 mL) was added and the
reaction mixture was stirred at room temperature under argon
atmosphere and protected from light for 3-4 days. The solvent was
evaporated and the solide residue was redissolved in CH2Cl2 (50 mL).
The solution was washed with HCl ( 1 M, 50 mL) and brine (5 %, 50 mL).
The organic phase was dried over Na2SO4 and the solvent evaporated.
The crude product was added to a flask for metallation. Zn(OAc)2 ∙ 2H2O
(1.00 g), methanol (20 mL) and CH2Cl2 (20 mL) was added and the
reaction mixture was heated to 50 °C and stirred at this temperature for 1
hour. The solvents were evaporated and the crude metallated product
was purified by column chromatography on silica using CH2Cl2 as eluent.
General procedure C. Amino acid methyl ester hydrochloric salt (1 eq)
was dissolved in methanol (2 mL). NaOMe (1 eq) was added to the
solution and the mixture was swirled for 45 seconds. Diethyl ether (20
mL) was added to the reaction mixture and the precipitate was removed
by filtration through a celite plug. The solvent was removed and the
residue was dissolved in methanol (1 mL) and diethyl ether (10 mL) were
added to crystalize remaining impurities. The precipitate was again
removed by filtration through a celite plug and the solvent was removed
and the product was dried under high vacuum. The free amine was dried
and stored under inert gas at 8 °C.
The product was further purified by
a
subsequent column
chromatography on silica using CH2Cl2: methanol:acetic acid 96:4:1 as
eluent. The solid product was washed with methanol several times which
yielded the product as an air and light sensitive purple solid (70 mg,
0.042 mmol, 32 %). 1H NMR (400 MHz, CDCl3) δ 9.25 (s, 1H), 8.81 (s,
2H), 8.78 (dd, J = 4.7, 2.6 Hz, 6H), 8.72 (d, J = 4.7 Hz, 2H), 8.43 (d, J =
4.7 Hz, 2H), 8.37 (d, J = 4.7 Hz, 2H), 8.09 (t, J = 6.0 Hz, 10H), 8.03 –
7.99 (m, 5H), 7.69 (td, J = 13.8, 7.7 Hz, 11H), 7.62 (d, J = 7.7 Hz, 2H),
7.47 – 7.41 (m, 9H), 7.39 – 7.35 (m, 2H), 7.08 (t, J = 6.2 Hz, 4H), 6.90 (t,
J = 7.7 Hz, 4H), 3.25 – 3.18 (m, 2H), 3.08 – 3.02 (m, 6H). UV-Vis:
(CH2Cl2) λmax: 424 nm, 551 nm.
(2S,3R,4R,5R)-1,6-Diamino-1,6-dideoxy-2,3,4,5-tetra-O-methyl
sorbitol. This guest was prepared as previously described.[27] 1H NMR
(500 MHz, CDCl3) δ d=3.53 (s, 3H; OCH3- 4), 3.50 (s, 3H; OCH3-2), 3.48
(s, 3H; OCH3-3), 3.47 (dd, 3J(H,H )=5.6, 3.4 Hz, 1H; H-3), 3.42 (s, 3H;
OCH3-5), 3.42 (dm, 3J(H,H)=3.4 Hz, 1H; H-4), 3.37 (ddd, 3J(H,H)=6.6,
6.6, 4.0 Hz, 1H; H-5), 3.33 (ddd,3J(H,H)=5.6, 5.4, 3.6 Hz, 1H; H-2), 2.99
3
3
(dd, J(H,H)=13.5, 3.6 Hz, 1H;H-1a), 2.96 (dd, J(H,H)=13.2, 4.0 Hz, 1H;
H-6a), 2.85 (dd, 3J(H,H)=13.5, 5.4 Hz, 1H; H-1b), 2.78 ppm (dd,
3J(H,H)=13.2, 6.6 Hz, 1H; H-6b);
Acknowledgements
Preparation of hydrochloric salts.[16],[44]-[50] The compounds were
obtained by a slight modification of the literature precedures. Products
were confirmed by 1H NMR. Spectral data were in agreement with the
literature.
This study made use of the NMR Uppsala infrastructure, which
is funded by the Department of Chemistry - BMC and the
Disciplinary Domain of Medicine and Pharmacy. Financial
support by the Swedish Research Council (grant nr. 621-2012-
3379) and by the Carl Tryggers Foundation (CTS 16:156) is
gratefully acknowledged. The computations were performed on
resources provided by the Swedish National Infrastructure for
Computing (SNIC) at National Supercomputer Centre (NSC),
Linköping University.
General procedure A. The amino acid (1eq) was put into a round
bottomed flask and methanol was added. The suspension was cooled to
0 °C using an icebath. Thionyl chloride (10 eq) was added dropwise
resulting in a homogeneous solution. Afterwards the reaction mixture was
heated to reflux and left to stir overnight. The reaction mixture was
allowed to cool to room temperature and the solvent was removed in
vacuo. The formed hydrochloric salt of the amino acid methyl ester was
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Entry for the Table of Contents
FULL PAPER
A bis(zincporphyrin) tweezer is used
to produce strong to moderate ECCD
signals with monotopically binding
mono-amine guest molecules. The
stiff- stilbene linker induces inherent
helicity of the tweezer which in free
form exists as a racemate. For host-
guest complexes, the CD signals
reach their maximum amplitude at
relatively low host:guest ratios ranging
from 1:10 – 1:70. Molecular modelling
is used to investigate the possibility of
stabilizing guest:guest interactions.
Sandra Olsson, Clara Schäfer, Magnus
Blom and Adolf Gogoll*
Page No. – Page No.
Exciton-Coupled Circular Dichroism
Characterization of Monotopically
Binding Guests in Host-Guest
Complexes with a Bis-(zinc
porphyrin) Tweezer
((Insert TOC Graphic here: max.
width: 5.5 cm; max. height: 5.0 cm))
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