Fine tuning of molecular and supramolecular properties of simple trianglimines-the role of the functional group

Article abstract of DOI:10.1039/c6ra06095a

Chiral, triangular poly-azamacrocycles (trianglimines) readily available from enantiomerically pure trans-1,2-diaminocyclohexane and various aromatic dialdehydes, differ in their nature and substitution pattern. The highly symmetrical macrocycle having two electron-donating groups attached to the aryl moieties is formed under thermodynamic control that fulfilled the so called entropy of symmetry rule. Conversely, from the 2-nitroterephthaldehyde a kinetic product of trivial C₁ symmetry is solely obtained, whereas from 2-methoxyterepthaldehyde a mixture of C₃- and C₁-symmetrical macrocycles are formed. The factors that contribute to the mechanism of the macrocycle formation were determined on the basis of an experimental/theoretical approach. The non-symmetrical structure of the macrocycle resulted from a symmetrical intermediate that appeared during cyclocondensation. The chiroptical properties of the trianglimines were studied by means of experimental ECD and VCD methods supported by quantum-chemical calculations. The nitro-substituted trianglimine appeared to be a simple, low molecular weight supergelator forming in polar media of stable chiral organogels. The structure of the gel is affected by the nature and chirality of the dopant. The hexaimine macrocycles after reduction of the CN imine bonds formed trianglamines-useful chiral ligands in stereoselective synthesis. The Zn-trianglamine complexes were employed as catalysts for asymmetric hydrosilylation of prochiral ketones, providing products of enantiomeric excess up to 98%. This remains the best result obtained for Zn-diamine catalysed asymmetric hydrosilylation of ketones so far.

Full text of DOI:10.1039/c6ra06095a

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Fine tuning of molecular and supramolecular
properties of simple trianglimines – the role of the
functional group†‡
Cite this: RSC Adv., 2016, 6, 53358
ab
ab
J. Szymkowiakab and M. Kwit
*
*
J. Gajewy,
Chiral, triangular poly-azamacrocycles (trianglimines) readily available from enantiomerically pure trans-
1,2-diaminocyclohexane and various aromatic dialdehydes, differ in their nature and substitution pattern.
The highly symmetrical macrocycle having two electron-donating groups attached to the aryl moieties is
formed under thermodynamic control that fulfilled the so called entropy of symmetry rule. Conversely,
from the 2-nitroterephthaldehyde a kinetic product of trivial C1 symmetry is solely obtained, whereas
from 2-methoxyterepthaldehyde a mixture of C3- and C1-symmetrical macrocycles are formed. The
factors that contribute to the mechanism of the macrocycle formation were determined on the basis of
an experimental/theoretical approach. The non-symmetrical structure of the macrocycle resulted from
a symmetrical intermediate that appeared during cyclocondensation. The chiroptical properties of the
trianglimines were studied by means of experimental ECD and VCD methods supported by quantum-
chemical calculations. The nitro-substituted trianglimine appeared to be a simple, low molecular weight
supergelator forming in polar media of stable chiral organogels. The structure of the gel is affected by
the nature and chirality of the dopant. The hexaimine macrocycles after reduction of the C]N imine
bonds formed trianglamines – useful chiral ligands in stereoselective synthesis. The Zn–trianglamine
complexes were employed as catalysts for asymmetric hydrosilylation of prochiral ketones, providing
products of enantiomeric excess up to 98%. This remains the best result obtained for Zn–diamine
catalysed asymmetric hydrosilylation of ketones so far.
Received 7th March 2016
Accepted 23rd May 2016
DOI: 10.1039/c6ra06095a
www.rsc.org/advances
successful, quantitative synthesis of triangular hexaimine (tri-
anglimine) and its reduction product (trianglamine) received
Introduction
a wide attention as a convenient method for construction of
chiral, symmetrical aza-macrocycles.4–7 There are a few factors,
which contribute to favoring macrocyclization over acyclic
polycondensation. First, the symmetrical cyclic structure of the
product is controlled in the thermodynamically driven revers-
ible imination step. Second, the substrates should have limited
exibility, restricting the number of conformers accessible at
the reaction temperature. Third, the macrocycle chirality origi-
nates from the use of enantiomerically pure diamine substrate
of C2 symmetry usually. From the library of diamines, the most
frequently used chiral scaffold for synthesis of shape-persistent
macrocycles is the enantiomerically pure trans-1,2-dia-
minecyclohexane (DACH, 1).8 Among aromatic dialdehydes, the
terephthalaldehyde and its derivatives having linear formyl
groups arrangement are ideally geometrically predisposed to
provide trianglimines.
Highly symmetrical chiral macrocycles are of broad interest due
to their applications as molecular building blocks, chiral cata-
lysts and chiral discriminating agents. The widely used meth-
odology for facile synthesis of macrocycles is based on
thermodynamically controlled imination reactions of dia-
ldehydes with diamines. The subsequent reduction of readily
formed polyimine macrocycles of regular structure provide
chemically more stable and functionally feasible polyamines.1–3
In the [3 + 3] cyclocondensation reaction between equimolar
amounts of both: a dialdehyde and a diamine, triangular
products are the most readily formed. The rst reported
aDepartment of Chemistry, Adam Mickiewicz University, Umultowska 89B, 61 614
Poznan, Poland. E-mail: gajewy@amu.edu.pl; marcin.kwit@amu.edu.pl
bWielkopolska Center for Advanced Technologies (WCAT), Umultowska 89C, 61 614
Poznan, Poland
Structural modication of the trianglimine could be done at
the pre-macrocyclisation stage only, with the use of modied
substrates, usually dialdehydes. Analysis of literature prece-
dents led to conclusion, that the use of structurally modied
1,4-dialdehydes in synthesis provided mainly symmetrical
products, even when the aldehyde was non-symmetrical.9 The
† Dedicated to Prof. Janusz Jurczak on the occasion of his 75 birthday.
‡ Electronic supplementary information (ESI) available: Experimental procedures,
calculation details, copies of NMR, MS and IR spectra, Fig. SI_1–SI_55, tabulated
total and relative energies for calculated structures, Cartesian coordinates,
conditions and retention times for HPLC separation of enantiomers, literature
references for the products of asymmetric hydrosilylation of ketones. See DOI:
10.1039/c6ra06095a
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decisive factor responsible for the preference of symmetrical reactions. At the rst stage, 2,5-dimethylanisole was oxidized to
(thermodynamic) over non-symmetrical (kinetic) product of 2-methoxyterepthalic acid.25 Then, the acid was reduced with
reversible reactions with the same bond formation enthalpies is diborane to the corresponding diol which was further PCC-
ascribed to so called “entropy of symmetry”.10–12 According to oxidized to the aldehyde 4. Despite many attempts we were
this rule, among several possible products, the one with the unsuccessful to synthesize 2,5-dinitroterephthaldehyde.
highest symmetry is characterized by the lowest relative energy.
The trianglimine macrocycles 5–7 were obtained via cyclo-
While the parent trianglimines have only limited applica- condensation reactions between equimolar amounts of 1 and
tions due to instability of C]N double bond,13 their reduced corresponding aldehyde 2–4 in dichloromethane solution. The
congeners were applied as chiral discriminating agents, ligands mixtures were stirred for 24 hours and aer evaporation to
and organocatalysts.14–16 Recently Tanaka et al., reported gela- dryness, the crude products 5–7 were obtained quantitatively.
tion properties of trianglimine containing azobenzene chro- The recorded 1H NMR and FT-IR spectra showed completely
mophore. The macrocycle is characterized by rather complex disappearance of respective aldehyde signals while mass spec-
structural behavior determined by the presence of –N]N– trometry conrmed [3 + 3] reaction stoichiometry. Subsequent
group. The gel is formed upon cooling of the benzene solution reductions of the trianglimines 5 and 6 with NaBH4 in
of the macrocycle and aer photoirradiation by UV light (300– dichloromethane–methanol mixture provide trianglamines 8
400 nm) for several hours is transformed to sol. The original gel and 9 with almost quantitative yields (Scheme 1). The triangl-
was recovered upon heating.13
amines 8 and 9 were used as zinc ligands for asymmetric
The [Zn–diamine]-catalyzed enantioselective hydrosilylation hydrosilylation of selected prochiral ketones. Due to the
of prochiral ketones is a practical and convenient synthetic formation of non-separable mixture of [3 + 3] products differ in
method providing optically active alcohols via reduction of the symmetry (vide infra), the trianglimine 7 was not reduced.
carbon–oxygen double bond. A typical catalyst system is formed
in situ from equimolar amounts of dialkyl zinc and a secondary characterization for all compounds were deposited in ESI.‡
diamine and reduction proceeds smoothly without using The UV and ECD spectra of trianglimines were measured in
Detailed experimental procedures and full spectroscopic
hydrogen gas.17–19 Recently reported chiral tetra- and hexamine chloroform. The gelation properties of trianglimine 6 were
macrocycles derived from DACH, in complexes with diethylzinc studied using solvents of different polarity and at various
efficiently catalyze asymmetric hydrosilylation of ketones and concentrations of the gelator. ECD spectra of gels were
imines with enantiomeric excess of the product up to 89% and measured using demountable quartz cell with path length of
>99%, respectively for alcohols and amines.20,21
0.01 cm. VCD spectra of 6 were measured in deuteriochloroform
Continuing our interest in macrocyclic compounds capable solution of concentration 20 mg per 0.15 mL, using BaF2 cell
of serving certain functions, we decided to synthesize chiral with 25 mm path length.
triangular macrocycles containing functionalities of the con-
To rationalize the experimental observations we performed
trasting character. Our goal was not the multiplication of new quantum-chemical calculations at the DFT level with the use of
compounds but rather in depth study of the structure, proper- B3LYP hybrid functional. The possible structures that include
ties and applications of relatively simple exemplary tri- both constitutional and conformational isomers of the imine
anglimines. Structure of the polyimine compounds were macrocycles as well as some model compounds were pre-
studied by means of experimental/theoretical approach, with optimized at the molecular mechanic level (MM3 force eld
particular emphasis on circular dichroism spectroscopy. We as implemented in Scigress soware).26 Then, all structures
expected the decisive role of substituents, attached to aromatic found at this stage were optimized at the B3LYP/6-31(d) level
rings, for tuning of molecular/supramolecular properties of the and re-optimized with the use of the same hybrid functional
trianglimines and trianglamines. Despite recent progress,22 the and enhanced triple-z basis set, 6-311G(d,p). To estimate
Zn-catalyzed asymmetric reduction is still relatively unexplored, solvent inuence on the structure and energy of the species
thus we applied the reduced macrocycles, previously obtained under study, the IEFPCM solvent model of chloroform was
at cycloimination stage, in asymmetric hydrosilylation of employed.27 The structures thus obtained were the real
ketones.
minimum energy isomers. The total and free energy values were
used to obtain the Boltzmann population at 298.15 K.
For CD calculations only the real minimum energy
conformers that differed from the most stable one by less than 2
kcal molꢀ1 were taken into consideration.28 Rotatory strengths
Results and discussion
Materials and methods
From virtually unlimited number of the terephthaldehyde were calculated employing CAM-B3LYP,29 M06-2X,30 B2LYP31
derivatives we chose representative ones having either methoxy and LC-wPBE32 hybrid functionals, all in conjunction with 6-
or nitro groups. 2,5-Dimethoxyterepthaldehyde (2) was obtained 311++G(d,p) basis set and IEFPCM solvent model.27 Such an
according to the procedure previously reported by Kuhnert approach was successfully employed for us to study large
et al., starting from 1,4-dimethoxybenzene.23 2-Nitro- systems of high structural diversity.33 Since all the TD-DFT
terephthalaldehyde (3) was obtained aer reduction of 2-nitro- methods gave very similar results only these obtained with the
terephthalic acid with diborane followed by PCC-oxidation of use of IEFPCM/TD-CAM-B3LYP/6-311++G(d,p) method were
corresponding diol intermediate.24 2-Methoxyterepthaldehyde discussed here. The rotatory strengths were calculated using
(4) was obtained from 2,5-dimethylanisole in sequence of both length and velocity representations. Due to the negligible
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Scheme 1 Synthesis of trianglimines 5–7 and trianglamines 8 and 9 from 1 and aldehydes 2–4.
differences between the length and velocity calculated values, due to fractional integration of signals, however, we can
only the velocity representations were used in the present study. distinguish four set of signals appeared at spectral ranges 8–59–
The ECD and VCD spectra were simulated by overlapping 8.35 (a), 8.21–8.17 (b), 7.99–7.82 (c) and 7.56–7.48 (d) ppm. A
Gaussian or Lorenzian functions for each transition.34,35 For the
remaining results and detailed calculation procedure see
Theoretical section in ESI.‡
Structure
As a result of syntheses, we expected highly symmetrical prod-
ucts 5–7, in accordance with the previously mentioned entropy
of symmetry rule. Experimentally, the preference for the
formation of symmetrical over non-symmetrical product is
1
revealed in the reduced number of signals in the H and 13C
NMR spectra, corresponding to a symmetry of given macrocycle.
For molecular structures generated in silico the more symmet-
rical macrocycle should have lower relative energy than less-
symmetrical one(s).
For 5 only one CHN and one CHAr signals are observed in 1H
NMR spectrum (Fig. 1a), in agreement with postulated D3
symmetry of the molecule. From the two possible D3-symmet-
rical isomers, the one characterized by anti orientation of
methoxy groups and the nearest neighboring nitrogen imine
atoms is lower in energy by over 23 kcal molꢀ1 (see Fig. 1a and
SI_1–SI_2 and Table SI_1 in ESI‡), than isomer of syn arrange-
ments of MeO and C]N fragments, in agreement with previ-
ously reported data.23 The methoxy groups in 5 are oriented
perpendicular to the mean macrocycle plane and situated in
between imine and aromatic hydrogen atoms, whereas for the
higher-energy isomer electrostatic repulsion between oxygen
and nitrogen atoms in close proximity caused signicant
increase of energy.
1
The case of 6 is more complex. The H as well as 13C NMR
Fig. 1 Diagnostic aromatic region of the 1H NMR spectrum of 5 (a), the
spectra of 6 showed more signals than it was expected for the
aromatic part of the HSQC spectrum of 6 (b) and the diagnostic
C3-symmetrical molecule. In aromatic region of 13C NMR
regions of the 1H NMR spectrum of 7 (c). Insert shows the lowest-
energy calculated structure of 5–7, the latest two characterized by C1
spectrum of 6 there are 24 signals of respective carbon atoms,
which suggested the trivial C1 symmetry of the whole macro-
cycle. The 1H NMR spectrum alone is difficult to interpretation lines indicate possible attractive interactions.
symmetry (some hydrogen atoms were omitted for clarity). Dashed
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HSQC spectrum of 6 (Fig. 1b) allowed correlation between
respective signals and indicates that the 1H NMR signals a,
b correlate with the six signals of the imine carbon atoms a0,
whereas the 1H NMR signals c and d correlate with nine signals
of tertiary aromatic carbon atoms b0. The number of signals did
ꢁ
not change during variable temperature (within ꢀ60 to +50 C
range) NMR experiments that exclude conformational isom-
erism (see ESI, Fig. SI_39‡).
Aer reduction of 6 to amine 9, the a, b proton signals dis-
appeared whereas c, d remains in almost the same spectral
region. Observed in 1H NMR spectrum of 9 signals that
appeared at d ¼ 7.93 and 7.89 as set of two singlets matched to
protons in ortho position to both nitro and imino groups.
Measured integration of these signals is 2 to 1, in excellent
agreement with C1 symmetry of hexaamine and its parent hex-
aimine macrocycle 6.
Generated “in silico” the lowest-energy structure of 6 is
characterized by the highest allowed C3 symmetry. At each side
of the triangle the nitro group and the nearest neighboring
imine nitrogen atom are in anti conformation. The theoretical
results prove the entropy of symmetry rule, however they are
inconsistent with the experimental data. The predicted for the
C3-symmetrical structure downeld region of 1H NMR spectrum
should consist of 3 signals of aromatic protons (singlet and two
doublets) and two singlets originated from CHN methine
protons. The NMR spectra conrmed formation of the non-
symmetrical product (vide supra). Disregarding C3-symmetrical
isomers, not-formed in the reaction, the most preferable (on the
basis on DFT results) the C1-symmetrical isomer of 6, shown in
Fig. 1b (see also Fig. SI_3 and Table SI_1 in ESI‡), is charac-
terized by anti, anti, syn conformation of nitro groups related to
respective neighboring imine nitrogen atoms. In this particular
conformation, there are possible attractive interactions
involved negatively charged oxygen atom of one nitro group (in
syn conformation) and both the imine hydrogen atom and the
positively charged nitrogen atom from the neighboring nitro
group in anti conformation.
The noted discrepancy between experiment and the entropy
of symmetry rule may originate in preference to form kinetic
over thermodynamic macrocyclic product 6. Thereby, the mac-
rocycle structure is determined by the structure of intermedi-
ates appeared during cycloimination reaction.
We decided to look deeper into mechanism of the macro-
cycle 6 formation using 1H NMR and ESI-TOF MS measure-
ments, supported by DFT calculations (Fig. 2 and ESI‡).36 The
test reaction between equimolar amounts of 1 and 3 was con-
ducted in NMR tube in CDCl3 at concentration of 0.1 M, the
liberated water was not removed and a content of the test tube
was protected against the inuence of external environment.
The same conditions were used for parallel reaction conducted
in DCM. For MS measurements, the aliquot of the reaction
mixture was diluted by methanol to a concentration of 0.1 mM
and directly injected.
Fig. 2 Parts of the 1H NMR spectra measured at the time intervals for
the reaction between 1 and 3 (a) and exemplary ESI-TOF mass spectra
recorded after 1 hour from the start of the reaction (b).
aldehyde CHO signals and formation of 6 as the sole product
was observed within 7 hours. The prolongation of the reaction
time to 60 hours or gentle heating of the reaction mixture did
1
not alter the shape nor the number of signals observed in H
NMR spectrum.
The ESI-TOF MS measurements run parallel to NMR exper-
iments allowed to identify main reaction intermediates and to
propose the mechanism of formation the trianglimine 6
(Scheme 2). In an exemplary MS spectrum recorded aer 1 hour
from the reaction start, the highest intensity peak of m/z ¼ 308
corresponds to protonated [1 + 1] intermediate 10 which
condenses with another DACH molecule to form [1 + 2] product
12. Surprisingly, the [2 + 1] product 11 of m/z ¼ 437 (M + H) was
not detected, probably due to rapid interconversion into [2 + 2]
intermediate 13. Similarly, the [3 + 2] intermediate 14 and open
chain pentaimine 16 were not detected, which suggested very
fast process of macrocycle ring closing. Alternatively, triimine
13 may be formed by subsequent condensation of 10 with 1 and
then with aldehyde 3.
DFT calculations performed for intermediates 10 and 15
shed a light on the stereochemical course of this particular
macrocyclisation reaction. From the two possible monoimines
10a and 10b, differ in the position of the nitro substituent with
respect to the imine group, the one having NO2 group in ortho
position is energetically preferred by ca. 1.3 kcal molꢀ1. The
tetraimine species 15a–15c are the most important intermedi-
ates since their structure determine the whole structure of the
macrocycle. Symmetrical intermediate always provide the non-
symmetrical macrocycle, whereas from non-symmetrical one
may form either symmetrical or non-symmetrical product 6.
From isomers 15a–15c, the C2-symmetrical ones 15a and 15c
The cycloimination reaction started immediately aer mix-
ing of substrates. The 1H NMR spectra showed gradual change
in the region of the aromatic proton absorption as well as
between 4 and 3 ppm. The completely disappearance of the
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Scheme 2 Stepwise [3 + 3] cycloimination reaction between 1 and 3 (due to the clarity of presentation only a few from all possible sub-paths of
the reaction were shown). The values in the upper right corner indicate calculated relative free energies (DDG, in kcal molꢀ1) between respective
isomers differ in the NO2 group location (within each set of constitutional isomers only the lowest-energy conformers were considered here, see
also ESI‡ for the remaining results).
covered almost 90% of energetically allowed structures intermediates over the symmetrical ones (Fig. SI_7–SI_10 and
responsible for further formation of non-symmetrical macro- Table SI_10, ESI‡). However, the domination of non-
cycle 6.
symmetrical intermediates is not reected in preferred forma-
Similar to the above-mentioned example, in 1H (shown in tion of symmetrical macrocycle (vide supra). Thus, formation of
Fig. 1c) as well as in 13C NMR spectrum of 7 more signals were non-symmetrical macrocycle is energetically privileged indeed.
observed than expected for C3 symmetry. The number and the However, it remains in contradiction with the entropy of
shape of the diagnostic OCH3 signals (observed in H and 13C symmetry rule.
1
NMR spectra) suggested the formation of equilibrium mixture
The main difference between respective structures of inter-
of non-symmetrical and symmetrical macrocycles. Estimated mediates, provide either 6 or 7, is due to the role of substituent.
integration of the OCH3 signals is 70 to 30, in favor of the C1- The nitro groups are involved in intramolecular hydrogen
symmetrical macrocycle.
bonding that strongly stabilize the structure. Contrary, the
1
The H NMR spectra measured at the time intervals for the methoxy groups control the structure by electrostatic and to less
reaction between 1 and 4 showed gradual disappearance of CHO extend by sterical interactions.
signals. The reaction is completed within 1.5 hour and an
established equilibrium between C3- and C1-symmetrical isomers
did not change even if the reaction was prolongated to one week.
Chiroptical properties
From all possible calculated isomers, there are two charac-
ECD spectra of 5 and 6. The measured in chloroform solu-
terized by all-anti orientation of methoxy groups and the nearest tion and calculated at the TD-DFT level UV and ECD spectra of
neighboring nitrogen imine atoms. The C1-symmetrical struc- the macrocycles 5–6 are shown in Fig. 3a.
ture is lower in energy by 1 kcal molꢀ1 than the higher
The presence of aromatic chromophores within macrocycle
symmetrical one. The calculated DDG energy difference corre- caused generation of strong exciton-type Cotton effects in ECD
sponds to 85% abundance of the C1-symmetrical structure in spectra that correspond to the UV absorption maxima allowing
the mixture, in very good agreement with the experimental data interpretation of the experimental results on the basis of
(see Fig. 1c and SI_4 and Table SI_1 in ESI‡). The preference for empirical exciton chirality rule.37 In the UV spectrum of 5 two
C1-symmetrical, all-anti-7 is due to the minimization of O/N absorption maxima at around 361 and 276 nm appeared. The
and O/O electrostatic interactions.
TD-DFT calculations, performed for model “isolated” single
Note, even one syn arrangement of OCH3 and C]N groups chromophore, indicated that the low-energy electronic transi-
within macrocycle structure caused in signicant energy tion involves HOMO and LUMO orbitals, whereas the higher
increase.
energy transition involves HOMO-1 and LUMO orbitals. These
The “in silico” estimated mechanism of the macrocycle 7 absorption maxima associated with two sets of bisignate exciton
formation suggested dominant role of non-symmetrical Cotton effects that may be correlated with molecular chirality.
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the chromophore and involved HOMO-3, HOMO-5 and LUMO
orbitals. In the real molecule, rotation of the chromophore
around C–C bonds is fast and caused almost complete cancel-
ation of the generated Cotton effects.
It is worth noting, that the overall shape of experimental UV
and ECD spectra of 7 (shown on Fig. SI_12, ESI‡) are very close
to these measured for trianglimine 5. However, the presence of
only one methoxy group in each aromatic arm of the triangle
caused blue shi of respective UV and ECD maxima. The UV
absorption bands that correspond to two exciton couplets
observed in ECD spectrum appeared at 328 and 273 nm. The
positive long-wavelength couplet appeared at around 320 nm is
due to the interactions between electronic transition dipole
moments, polarized almost perpendicular to the long axis of the
chromophore. Similarly to trianglimines 5 and 6, the second
negative exciton couplet (at around 270 nm) originated from
interactions between electronic transitions dipole moments
polarized along the long axis of the chromophore.
Changes in ECD spectra of 5 upon protonation. Different
substitution patterns were responsible for contrasting behavior
of the 5 and 6 observed upon protonation. Acidication of the
chloroform solution of 6 by methanesulfonic acid caused the
signicant decreasing of the intensity of both UV and ECD
absorption maxima, due to the decomposition of the macrocycle.
The hexaimine 5 is an exception. Gradual titrations of 5 by
methanesulfonic acid solution revealed in strong bathochromic
shi of the long-wavelength absorption maximum (see Fig. SI_14
and SI_15‡). Just one equivalent of the acid caused appearance of
the new absorption band at around 427 nm. Addition of more
than three equivalents of the acid shied this band by further ca.
30 nm into low-energy region of the spectrum. Addition of more
than 6 equivalents of the acid did not alter shape of the spectrum.
The same situation is observed in ECD spectra (see Fig. 4). The
rst long-wavelength Cotton effect was red-shied by 140 nm
from 384 to 522 nm retaining its intensity regardless the amount
of the acid added. The second most intense UV absorption band
remains not altered by acidication. The negative Cotton effect
associated with this band did not shi, however, its intensity
increased over two-fold for fully protonated macrocycle in
comparison with the non-protonated one.
Fig. 3 UV (left panel) and ECD (right panel) spectra of 5 and 6,
measured (black lines) in chloroform solution and calculated (blue
lines) at the IEFPCM/CAM-B3LYP/6-311++G(d,p) level (a). The calcu-
lated spectra were not wavelength corrected. Inserts shows polari-
zation of the electronic transition moments of the highest oscillator
strength. The Newman projections along the C*–C* bond in cyclo-
hexane moiety explain the origin of the long-wavelength positive
exciton couplet for 5 (b) and negative short-wavelength exciton
couplets (c) for 5 and 6 (functional groups were omitted for clarity).
The positive sign of the long-wavelength couplet originated
from interacting electronic transition dipole moments polar-
ized parallel to the line connected oxygen atoms in the chro-
mophore (perpendicular to the long axis of the chromophore
and mean macrocycle plane). The sign of the low-energy
couplet remains in disagreement with the negative diamine
chirality (see Fig. 3b).38 The second, higher-energy negative
exciton couplet, appeared at around 270 nm, is in agreement
with negative sign of N–C*–C*–N torsion angle and originates
from interactions between electronic transition dipole
moments polarized along the long axis of the chromophore
(see Fig. 3c).
The UV spectrum of 6 is characterized by strong absorption
maximum appeared at 259 nm. The electronic transition dipole
moments of the highest oscillator strength, polarized along the
long axis of the chromophore, generated negative exciton
couplet centered at around 250 nm. Contrary to 5, the electronic
transitions polarized perpendicular to the long axis of the
chromophore did not affect the experimental ECD spectrum of 6.
The above empirical analyses remain in excellent agreement
with the results of TD-DFT calculations. For macrocycle 5 the
experimental/theoretical approach to the analysis of ECD
spectra allowed to distinguish between syn and anti isomers.
ECD spectrum calculated for the higher-energy syn isomer
exhibits negative long-wavelength exciton couplet, in disagree-
ment with experimental data.
The same behavior is observed for molecules “in silico”. TD-
DFT calculations performed for optimized structures of non-
protonated, mono-, di-, tri-, tetra-, penta- and hexaprotonated
5 perfectly reected the experiment.
Note, that UV and ECD spectra measured during titration of
7 by methanesulfonic acid showed similar changes to those
seen during the titration of 5 (see Fig. SI_26 and SI_27, ESI‡).
Experimental and calculated VCD spectra of 6. Our analysis
of the trianglimine 6 chiroptical properties was extended on
vibrational spectroscopy. The experimental IR and VCD spectra
were measured in deuterated chloroform. The DFT calculations
were carried out using the same level of theory as it was used for
the geometry optimization of all isomers of 6. The experimental
and calculated for the lowest-energy C1-symmetrical isomer of 6
spectra are shown in Fig. 5.
For 6 the overestimated intensity of the long-wavelength
positive Cotton effects in comparison to the experimental one
is the results of the “static”, frozen geometry of the macrocycle.
This particular Cotton effects is associated with the electronic
transition polarized almost perpendicularly to the long axis of
As can be seen, the calculated and experimental spectra are
in reasonable agreement. Surprisingly, the region of strong
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Fig. 5 IR (top) and VCD (bottom) spectra of 6 measured in CDCl3
(black lines) and calculated at the IEFPCM/B3LYP/6-311G(d,p) level of
theory (blue lines). Vertical bars represent calculated dipole and
rotatory strengths. Calculated frequencies were not scaled.
However, the mechanical mixing of the gel destructed its
structure. The minimum gelation concentration determined
through a series of dilution experiments is as low as 4.8 mg
Fig. 4 ECD spectra measured during titration of 5 by methanesulfonic
acid (a) and calculated at the IEFPCM/TD-CAM-B3LYP/6-311++G(d,p)
level for non-protonated, mono-, di-, tri-, tetra-, penta- and hex- mLꢀ1 (6.3 ꢂ 10ꢀ3 M, 0.5% w/v) for the mixture composed of 1
aprotonated 5, optimized at the IEFPCM/B3LYP/6-311G(d,p) level (b).
part (vol.) of CHCl3 and 30 parts (vol.) of ethanol (Fig. 6a). We
established that this gelation occurs also for the chloroform
Wavelengths were not corrected.
solution of 6 mixed with an excess of other solvent, such as
alcohols, whereas non-polar solvents: hexane, toluene and
N]C absorption at around 1640 cmꢀ1 (denoted here as A) is
diethyl ether do not form a gel (see Table 1). From the protic
less convenient for analysis of VCD intensities. The observed in
solvent tested, differing in the length and structure of an alkyl
this region Cotton effects are very weak due to the almost total
chain, the linear alcohols gelated better. However, the elonga-
cancelation of rotatory strengths of opposite signs but equal
tion of the chain above 3 carbon atoms has reverse effect. While
intensities. The peak in IR spectrum appeared at around 1530
n-propanol forms a gel, the use of n-butanol and its isomers at
cmꢀ1 (B) that corresponds to positive Cotton effect in VCD
best lead to a partial gelation. On the other hand, the meth-
spectrum is associated with N–O asymmetric stretch. The
anolic solution of 6, at concentration below 4.8 mg mLꢀ1, does
observed broadening of the VCD signal is apparently due to the
not gelate. Note, pre-dissolving of 6 in chloroform is compul-
C1 macrocycle symmetry and overlapping of rotatory strengths
sory due to poor or even insolubility of the macrocycle in
originated from non-equivalent nitro groups. The characteristic
common non-chlorinated organic solvents. The probable gela-
strong negative Cotton effect appeared at 1080 cmꢀ1 (C) is
tion mechanism in ethanol is the formation of the cascades of
generated by rotatory strengths originated mostly from C–H
hydrogen bonds involving macrocycle's nitro and solvent's
bending and in IR spectrum is associated with peak of small
hydroxy groups.
intensity. Note, the region of aliphatic C–H bending modes in
The gel made from 6 was dried and analyzed by scanning
VCD spectrum is not well reproduced by calculations and
electron microscopy (SEM) to obtain visual insights into the
thereby difficult to interpretation.
molecular aggregate. The SEM photographs was shown in
Fig. 6b and exhibited the presence of irregular, rounded struc-
tures having lateral dimensions of a few microns and the height
Gelation properties
Trying to grow a single crystal suitable for X-ray diffraction of the order of tens/hundreds of nm. The material form amor-
analysis unexpectedly, a gel was obtained when 6 (15 mg) was phous phase, as revealed by PXRD analysis. On an AFM image of
dissolved in chloroform (0.1 mL) and the resulted solution was 500 nm ꢂ 500 nm scale is visible porous structure of the
diluted with an absolute ethanol at room temperature. The gel material.
thus obtained was translucent, slightly yellow colored and
Similar tests performed with 5, 7 as well as for the triangl-
stable at room temperature in closed container for a few weeks. amines 8 and 9 did not lead to gelation in any case. Surprisingly,
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Table 1 Gelation studies of 6 in various solvent mixtures
Concentration
Run
Additivea
(mg mLꢀ1
)
Observationb
1
2
3
4
5
6
7
8
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
MeOH
MeOH
n-PrOH
i-PrOH
n-BuOH
sec-BuOH
tert-BuOH
DMF
DMSO
MeCN
Toluene
AcOEt
13.6
9.4
7.1
5.8
4.8
3.7
5.8
4.8
5.8
5.8
5.8
5.8
5.8
5.8
5.8
5.8
5.8
5.8
G
G
G
G
G
PG
G
PG
G
PG
PG
PG
S
9
10
11
12
13
14
15
16
17
18
S
S
PG
S
S
a
All tests were performed using a constant amount of 5 (15 mg) in 0.1
mL of chloroform that was diluted by the second solvent. b G ¼ gelation,
PG ¼ partial gelation, S ¼ solution.
perpendicularly to the long axis of the chromophore and
canceled upon free rotation of the aryl part. As it was supposed,
gelation may proceed through hydrogen bonding formation
between the gelator and the solvent. Thus, the formation of
ordered superstructure may cause hindered internal rotation of
the aryl fragments around C–C bonds, thereby increase inten-
sity of the long-wavelength Cotton effect. The ECD spectrum
measured for gel containing all-(R)-6 and all-(R)-5 is strongly
affected by Cotton effects originated from the dopand. This
revealed in similar shape of the ECD spectrum measured for gel
and for solution of all-(R)-5. Contrary, in the same spectral
region the ECD spectrum measured for gel made from hetero-
chiral 5 and 6 exhibited Cotton effect of the opposite sign,
Fig. 6 Macrocycle 6 gels made from samples of different concen-
trations (w/v) (a). SEM images of macrocycle 6 gel (b) and X-ray
diffraction pattern of the quickly dried gel sample (c). Gels made from
macrocycle 6 doped with all-(S)-5 and all-(R)-5 (d). SEM images of
macrocycle 6 gel doped with all-(R)-5 (e) and X-ray diffraction pattern
of the quickly dried gel sample (f).
the chloroform solution of 6 that contains the same weighting
amount of guest trianglimine 5 shows the same gelation abili-
ties (Fig. 6d). The concentration of each component 5 and 6 was
0.45% w/v and gels were formed aer diluting by absolute
ethanol, irrespective to chirality of 5. The gels were opaque and
aer drying, were analyzed by SEM and AFM techniques. Both
gels, differ in absolute conguration of the guest, are charac-
terized by the same microcrystalline morphology. The height of
the crystal is approx. 650 nm and the microcrystals may be
characterized as “twins” where each part of the crystal is char-
acterized by ca. 300 nm heights. The crystalline phase of the
mixed gels was conrmed by PXRD measurements. Unfortu-
nately, despite many attempts we were not able to obtain
a single crystal from the mixture containing 5 and 6.
As the materials obtained were chiral we decided to measure
their ECD spectra and confronted them with the ECD spectra
measured for a given compound in chloroform solution (see
Fig. 7).
In ECD spectrum measured for gel made from 6 signicantly
enhanced long-wavelength positive Cotton effect is observed. As
we mentioned above this particular Cotton effect is generated
Fig. 7 ECD spectra of gels made from all-(R)-6 (black line), mixture of
all-(R)-6 and all-(R)-5 (blue line) and mixture of all-(R)-6 and all-(S)-5
by interacting electronic transition dipole moments polarized (red line).
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agent, we used diphenylsilane and all reactions were carried out
for 24 h at room temperature in dry and degassed toluene. As
the test reaction we choose AHS of 4-methyacetophenone.
The initial examination of the effect of the ligand structure
on the yield and enantioselectivity of the test reaction provided
very promising results in terms of enantioselectivity compared
to that previously published by others and us (Table 2).20,22,39
The best results were obtained when the methoxy-substituted
trianglamine 8 was employed as ligand (Table 2, entry 1).
With the use of 8 the asymmetric induction reached 98% with
completely conversion of the substrate. To our best knowledge,
this is the best results obtained so far for zinc–diamine-based
catalytic systems. The trianglamine 9 gave product with good
asymmetric induction and conversion, however, these results
were not better to that obtained with the use of 8 and compa-
rable to the results obtained when non-modied trianglamine
was employed as the zinc ligand (Table 2, entry 2).
Having established 8 and 9 as suitable ligands for asym-
metric hydrosilylation a selected alkyl–aryl and aryl–aryl pro-
chiral ketones were reduced in above mentioned conditions (3.5
mol% of catalyst, 1.2 equiv. of silane in toluene). The results are
summarized in Table 2. In general, the use of 8 and 9 as ligands
provided products of the higher asymmetric induction levels
than those obtained with other trianglamines.20 On the other
hand, the increased enantioselectivity is with expense of the
isolated yield of the product, and the higher chemical yield did
not exceed 65%. Good results in term of enantioselectivity were
obtained for the products signicantly differ in the size of
substituents (for example, phenyl vs. cyclohexyl, Table 2, entries
5 and 6). The prochiral substrates of benzophenone-type gave
moderate levels of asymmetric inductions (Table 2, entries 13–
18) due to low stereo-difference between substituents of the
carbonyl group. The most problematic aryl-triuoromethyl
ketones were reduced with the very low level of asymmetric
induction (Table 2, entries 19–21), regardless the size of the
aromatic part.
Scheme 3 Zinc–trianglamine-catalyzed asymmetric hydrosilylation
of prochiral ketones.
which suggested less inuence of the dopand on overall chi-
roptical properties of the sample. In the higher energy region
(below 350 nm), both spectra are similar in the sequence of the
Cotton effects (positive/negative/positive). However, the inten-
sities of the respective Cotton effects are quite different and for
heterochiral gel is almost two-fold higher. The observed Cotton
effects are superposition of the intra- or intermolecular inter-
actions, however, the denitely distinguish between the two
effects is not possible at the moment.
Zn–trianglamine catalyzed asymmetric hydrosilylation of
prochiral ketones
Finally, we attempted to use the obtained trianglamines 8 and 9
as zinc ligands in the asymmetric hydrosilylation (AHS) of
various prochiral ketones (Scheme 3). The catalytically active
species were generated in situ by mixing of equimolar amounts
of diethyl zinc with respective ligand. The catalyst loading was
3.5 mol% in all cases, since the lower amounts caused signi-
cant decrease of asymmetric induction level. As the reducing
Table
2 Enantiomeric excesses of the products of asymmetric
hydrosilylation of ketones catalyzed by zinc–trianglamine (L*)
complexes
Entry
Ar
R
L*
eea [%]
Based on the previously published assumptions we calcu-
lated structure of trianglamine 8–zinc complex.20a For the sake
1
2
3
4
5
6
7
8
4-MeC6H4
4-MeC6H4
Ph
Ph
Ph
Ph
4-NCC6H4
4-MeOC6H4
1-Indanone
1-Indanone
1-Tetralone
1-Tetralone
Ph
Ph
Ph
Ph
Ph
Me
Me
Me
Me
Cy
Cy
Me
Me
8
9
8
9
8
9
9
8
8
9
8
9
8
9
8
9
8
9
8
8
9
98
83
85
78
91
84
57
70
87
97
77
69
51
73
24
25
21
5
9
10
11
12
13
14
15
16
17
18
19
20
21
2-MeC6H4
2-MeC6H4
3-MeC6H4
3-MeC6H4
4-MeC6H4
4-MeC6H4
CF3
Ph
Ph
10
4
30
2,4,6-Me3C6H2
2,4,6-Me3C6H2
CF3
CF3
Fig. 8 Calculated at the B3LYP/6-311G(d,p) level folded structure of
8$Zn(II) shown in two different projections: from side (a) and from the
top (b). Colored squares represent occupied (red) and unoccupied
(blue) quadrants.
a
Enantiomeric excesses were determined by HPLC with a CHIRALPAK
IA column.
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of simplicity we replaced diethylzinc by dimethylzinc. From the transition dipole moments polarised along or perpendicular to
considered here possible geometries of Zn–8 complexes, differ the long axis of the chromophore.
in the ligand conformation, the folded one is lower in energy
The study on chiroptical properties of trianglimine 6 were
than the fully extended, as revealed by computation at the expanded on vibrational spectroscopy. The most convenient
B3LYP/6-311G(d,p) level. The estimated DDG energy difference regions for interpreting IR and VCD spectra correspond to N–O
between extended form, characterized by all-trans arrangement asymmetric stretch and C–H bending. Cotton effects associated
of rotatable torsion angles and the folded one, where some of with C]N absorption are very small due to the mutual
the torsion angle are in gauche conformation is over 6 kcal cancelation of rotatory strengths of opposite signs.
molꢀ1
.
The calculated folded structure of mononuclear
Suitable substitution pattern allowed tuning of molecular and
14,20a
[8$Zn(II)] complex (shown in Fig. 8) is close to preferred by free supramolecular properties of the macrocycle. The presence of
trianglamine conformation.14 The complex consists of two electron donating groups in 5 affected its chiroptical properties
occupied and two unoccupied quadrants, thus, resembles the especially when proton donor is present in solution. Addition of
structure of ruthenium and rhodium species used for asym- small amounts of acid caused strong bathochromic shi of UV
metric hydrogenations. The unoccupied quadrants are acces- maxima. Color of the solution of 5 changed to deep red during
sible to approaching substrates and the role of sterical titration. Since 5 is optically active this property may be useful in
hindrance in the occupied quadrants can be ascribed to two supramolecular chemistry to construct a new macrocyclic recep-
from the three apexes of the triangle. The shielding effect is tors for chiral carboxylic acids. On the other side, the presence of
additionally tuned by the presence of methoxy groups, which an electron withdrawing group, like nitro, is responsible for
explains thereby the relatively low yield of the product.
unprecedented gelation properties. Due to its efficiency, the nitro-
substituted trianglimine may be treated as low molecular weight
supergelator,40 which forms, in polar media, stable chiral orga-
nogels at concentration as low as 0.4% m/v. The gelation can be
useful for purposes, such as the enantiomeric separation of
Conclusions
We presented here synthesis and structural studies on chiral racemates or chiral recognition. The morphology of the gel is
macrocyclic triangular hexaimines and hexaamines. These strongly affected by addition of other chiral macrocycle. Whereas
compounds are readily available through cyclocondensation for gels made from “pure” 5 the material obtained aer drying is
reaction between optically active vicinal diamines and aromatic amorphous, addition of both enantiomers of 5 to chloroform
aldehydes. The structure and contrasting properties of the solution of 6 let to obtain microcrystals aer drying.
product is affected by the nature of substituent in aromatic
Despite the symmetry, the reduced trianglamines 8 and 9
moiety. Although triangular shape of cycloimination product is originated from 5 and 6, respectively, are suitable zinc ligands
not altered, symmetry of the given macrocycle is strongly- that may be used in asymmetric catalysis. The presence of
substituent-dependent. The 2-nitroterephthaldehyde provide methoxy group, for instance, increased level of the asymmetric
non-symmetrical product, which is apparently formed under induction in zinc-catalyzed asymmetric hydrosilylation of
kinetic control. The structure of the macrocycle is determined ketones. The enantiomeric excesses determined for products of
by the structure of reaction intermediates. Energetically pref- AHS reactions, catalyzed by mononuclear 8$Zn(II) complex
erable symmetrical intermediates provided always non- reached 98%, which remains the best result obtained for Zn–
symmetrical macrocycle.
amine AHS of ketones so far.
Contrary, both the symmetrical 5 and non-symmetrical 7 are
The application of trianglamines reported here and similar
formed under thermodynamic control. In the last case, the compounds to other elds of catalytic asymmetric synthesis and
higher-energy C3-symmetrical isomer is formed as well. The to construction of supramolecular assemblies capable of
equilibrium between isomers of 7 differ in symmetry did not serving certain functions is currently under progress in our
change upon time. The 1H NMR spectra measured for solution laboratory.
of 7 at the regular time intervals do not show any change in
proportion of respective isomers.
Acknowledgements
For 5 and 7 the structure of the macrocycle is mainly
controlled by the tendency to minimization of electrostatic This work was supported by research grant from National Science
interactions between oxygen and imine nitrogen atoms. Due to Center (NCN) Poland no UMO-2012/06/A/ST5/00230. All calcula-
the presence of only one methoxy group in each aromatic part of tions were performed in Poznan Supercomputing and Networking
the triangle, the repulsive O/O interactions are responsible for Center (grant no. 217). The authors express their gratitude to Dr P.
´ ´
Połrolniczak (Institute of Non-Ferrous Metals, Division in Poznan)
the preference of C1- over C3-symmetrical structure of 7.
The measured ECD spectra of trianglimines 5 and 6 are and Dr Ł. Majchrzycki (WCAT) for their attempts to take powder
conveniently interpret using exciton-chirality formalism. The X-ray diffraction, SEM and AFM measurements.
negative exciton couplet is in agreement with negative chirality
of N–C*–C*N angle and reected absolute conguration of the
stereogenic centers. The ECD spectrum of macrocycle 5 is
Notes and references
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