Vanadium-controlled crystallization of stereoisomers of NBu 4[VO2(N-salicylidene-isoleucinato)] through epimerization

Article abstract of DOI:10.1002/chem.201403125

Reported herein is a simple synthetic and crystallization procedure for sequential isolation of two stereoisomers of isoleucine-derived vanadium(V) complexes from a racemic mixture with three stereogenic centers and therefore eight hypothetical species. The products of this crystallization were characterized by electronic and vibrational circular dichroism, NMR spectroscopy, and polarimetry to compare the chiroptic properties of the enantiomerically pure analogues prepared from L-isoleucine and D-allo-isoleucine. NMR studies pointed to the yet unobserved phenomenon of vanadium-catalyzed epimerization of isoleucine. Vanadium-controlled resolution of stereoisomers: NBu₄[VO₂(N-salicylidene-L-isoleucinato)] is the first product of crystallization from a mixture containing Schiff bases derived from racemic isoleucine and vanadate accessible as a pure compound. Vanadium-catalyzed epimerization of isoleucine was observed in the solutions of [VO₂(N-salicylidene-isoleucinato)]^- complexes (see figure).

Full text of DOI:10.1002/chem.201403125

DOI: 10.1002/chem.201403125
Communication
&
Chirality
Vanadium-Controlled Crystallization of Stereoisomers of
NBu4[VO2(N-Salicylidene-isoleucinato)] through Epimerization
[a]
[a]
[a]
[b]
ˇ
ˇ
´
Lukꢀs Krivosudsky, Peter Schwendt,* Jꢀn Simunek, and Rꢁbert Gyepes
Abstract: Reported herein is a simple synthetic and crys-
tallization procedure for sequential isolation of two stereo-
isomers of isoleucine-derived vanadium(V) complexes
from a racemic mixture with three stereogenic centers
and therefore eight hypothetical species. The products of
this crystallization were characterized by electronic and vi-
brational circular dichroism, NMR spectroscopy, and polar-
imetry to compare the chiroptic properties of the enantio-
merically pure analogues prepared from l-isoleucine and
d-allo-isoleucine. NMR studies pointed to the yet unob-
served phenomenon of vanadium-catalyzed epimerization
Scheme 1. Stereoisomers of isoleucine.
of isoleucine.
fractionated by crystallization: PI (yield 32%), PII (yield ca. 32–
50%), and PIII (yield ca. 50–54%). Consequently we prepared
NBu4[VO2(N-salicylidene-l-isoleucinato)] (1) and NBu4[VO2(N-sal-
icylidene-d-allo-isoleucinato)] (2) starting from enantiomerically
pure l-Ile (2S,3S) and d-allo-Ile (2R,3S), respectively (for synthe-
sis details and crystal structures of 1 and 2 see Sections S1 and
S2 in the Supporting Information). The mode of chirality on
the vanadium center (A=anticlockwise vs. C=clockwise) ap-
pears to be induced by the configuration on the a-carbon
atom giving C configuration in 1 and A configuration in 2. No
other stereoisomers were detected as crystals or in the solu-
tions of 1 and 2.
Some characteristic properties of isolated products are sum-
marized in Table 1. 1H and 51V NMR studies confirmed that
there is only one diastereomer in PI and the observed chemi-
cal shifts correspond to 1, however, the same chemical shifts
would be found for [VO2(N-salicylidene-d-isoleucinato)]ꢀ,
which is an enantiomer to 1.[5] This enantiomer would have
the same magnitude of optical rotation, but with opposite
sign. Comparison of the signs of optical rotation for PI and
1 assured us that these compounds are identical. The diaste-
reomers of NBu4[VO2(N-salicylidene-isoleucinato)] can be distin-
guished by their crystal morphology with a rodlike crystal
shape for the complex derived from the l-isoleucine isomer
and a blocklike shape for the d-allo-isoleucine isomer derived
complex (Figure 1).
Vanadium Schiff base complexes have been investigated
mainly for the ability to catalyze asymmetric oxidations of or-
ganic substrates[1] and due to the ready capability of the Schiff
bases to introduce chirality into complexes.[2] However, not
only the chirality introduced by the Schiff base with stereogen-
ic centers has to be considered. Upon complexation of the
nonsymmetric Schiff base to the metal center with tetragonal
pyramidal[3] or octahedral[4] coordination environments an ad-
ditional stereogenic center at the metal site is generated.
Herein we report the preparation of chiral vanadium com-
plexes with N-salicylidene-isoleucinato Schiff base ligands, syn-
thesized from a racemic mixture of all four stereoisomers of
isoleucine (Scheme 1). From this mixture of eight hypothetical
isomers with three stereogenic centers, including metal-cen-
tered chirality, the same two stereoisomers were reproducibly
isolated by fractionated crystallization. This surprising result
was confirmed not only by stereochemical characterization of
the crystallized products using X-ray single-crystal diffraction,
but also by chiroptical methods.
From the reaction of rac-isoleucine with salicylaldehyde and
NBu4VO3 we obtained products of three types, which could be
ˇ
´
[a] L. Krivosudsky, Prof. P. Schwendt, Dr. J. Simunek
Department of Inorganic Chemistry
Comenius University, Faculty of Natural Sciences
Mlynskꢀ dolina, 842 15 Bratislava (Slovakia)
Fax: (+421)2-60296-273
Repeated isolation of the first fraction of crystals with yields
between 20–30% showed, surprisingly, that PI consists of
nearly pure complex 1 and that the result is reproducible
(average a from 10 measurements was ꢀ177.4, which corre-
sponds to ee>99%, see Table S1 1 in the Supporting Informa-
tion). We repeated the synthesis with 10% excess of each ste-
reoisomer in the racemate (l-Ile, d-Ile, l-allo-Ile, d-allo-Ile). All
E-mail: schwendt@fns.uniba.sk
[b] Dr. R. Gyepes
Faculty of Education, J. Selye University
Bratislavskꢀ cesta 3322, 945 01 Komꢀrno (Slovakia)
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201403125.
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tion of ligand-to-metal charge transfer (LMCT) transi-
tions. The ECD spectra of 1 and 2 are perfect mirror
images, confirming the assumption that the sign of
the Cotton effect reflects the configuration of the
five-membered chelate ring of a Schiff base com-
plex[6] (in this region). In the present case, this means
that despite of being diastereomers, compounds
1 and 2 have mirror imaged ECD spectra because of
the opposite configurations on the vanadium and a-
carbon atoms: C, S in 1 and A, R in 2.
Table 1. Characteristic properties of 1, 2, PI, PII, and PIII.
1
2
PI
rod
PII
mixture of rods and blocks
PIII
block
+191.0
4.25
Crystal shape
rod
block
20[a]
½aꢁD
ꢀ177.4 +192.8 ꢀ178.4
+63.9
1H NMR[b]
a-carbon
1H NMR[b]
RCH=NR’
51V NMR[c]
4.22
4.25
4.22
4.22 (38%)
4.25 (62%)
8.29 (63%)
8.34 (37%)
8.34
8.30
8.34
8.29
ꢀ538.7 ꢀ537.8 ꢀ538.8 ꢀ537.7 (69%), ꢀ538.7 (31%) ꢀ537.6
[a] c=1 g/100 mL in CH3CN, cuvette length 1 dm. [b] Chemical shifts in ppm of the
hydrogen of the a-carbon and RCH=NR’ in CDCl3 at 600 MHz. [c] Chemical shifts in
ppm in CH3CN at 600 MHz, c=5ꢃ10ꢀ3 moldmꢀ3. No other signals were detected in
CH3CN solutions even after 48 h.
The superposition of the ECD spectra (Figure 2)
shows a perfect match of the spectra corresponding
to PI and 1, and the spectra corresponding to PIII
and 2. Product PII, which is a mixture of 1 and 2 with
Figure 1. Schematic representation of crystallization of products PI, PII, and
PIII from a racemic mixture of isoleucine. The crystals isolated first (PI) adopt
a rodlike crystal shape, whereas the product so far isolated last (PIII) form
block-shaped octahedral crystals. PII is a mixture of both types of crystals. In
crystal structures of PI and PIII the stereodescriptors of the chiral atoms are
given.
crystallizations provide 1 as the first product (Table S1 2 in the
Supporting Information). Product PII is a mixture of 1 and 2,
whereas PIII is nearly pure 2. Finally, X-ray structure analysis of
PI and PIII confirmed the absolute structures of 1 and 2, re-
spectively (see Section S3 in the Supporting Information for
Figure 2. Comparison of experimental ECD spectra in CH3CN solution (con-
centrations in moldmꢀ3: 1 2.0585ꢃ10ꢀ5, 2 2.148ꢃ10ꢀ5, PI 2.0406ꢃ10ꢀ5, PII
2.1122ꢃ10ꢀ5, PIII 2.0585ꢃ10ꢀ5) and simulated Boltzman averaged ECD spec-
tra at the CAM-B3LYP level. The spectra have been drawn by assuming Gaus-
sian band shape with a 0.35 eV bandwidth.
comparison of the crystal structures). Importantly, we did not
obtain any sign of crystallization of complexes with d-Ile and
l-allo-Ile.
The electronic circular dichroism (ECD) spectra of 1 and 2 in
CH3CN solution show ligand bands in the region 200–420 nm
(Figure 2). The broad band with a maximum at 365 nm be-
longs to the HOMO!LUMO transition (predominantly p!p*
character, see Figure S4 3 in the Supporting Information) and
has practically no origin at the metal site; this is supported by
the identical shape of UV/Vis and ECD spectra of the free Schiff
base and the corresponding complex (see Section S3 in the
Supporting Information). The bands at lower wavelengths cor-
respond to n!p* and p!p* transitions with a small contribu-
an excess of 2, adopts the shape of the ECD spectrum of 2.
The lower molar-absorption coefficient compared with pure 2
suggests a diastereomeric ratio 2:1 of roughly 70:30. This was
estimated from the amplitudes of the pure spectra of 1 and 2,
which cancel each other out because of a nearly mirror-image
appearance.
The combination of vibrational circular dichroism (VCD) with
quantum chemical calculations is a widely used method for
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Figure 4. 1H NMR chemical shifts of the hydrogen atoms on the a-carbon of
[VO2(N-salicylidene-l-isoleucinato)]ꢀ in solutions. The spectra were measured
immediately upon mixing the solution (50 mL) with CDCl3 (1 mL). After three
months standing at room temperature the diastereomeric ratio had in-
creased to 50:50. Similar behavior was observed for a solution of the N-sali-
cylidene-d-allo-isoleucinato Schiff base (see Figure S6 4 in the Supporting
Information).
Figure 3. Experimental VCD spectra were collected in a 0.1 mm BaF2 liquid
cell in CD3CN solution (concentrations in moldmꢀ3: 1 6.981ꢃ10ꢀ2,
a significant rate (Figure 4). This is evidence for the so far un-
observed vanadium-catalyzed epimerization of isoleucine. The
epimerization may occur in free amino acids, free Schiff bases,
or in a complex, because all of these species are present in the
solutions (see Figures S6 5, S6 6 in the Supporting Informa-
tion).
2
7.25ꢃ10ꢀ2, PI 6.981ꢃ10ꢀ2). The simulated VCD spectra represent a popula-
tion-weighted average of the spectra of nine conformers in respect to rela-
tive energies calculated at the B3PW91 level by using a polarizable continu-
um model (PCM) for the solvent (CH3CN). The spectra have been construct-
ed assuming Lorentzian band shape with 4 cmꢀ1 bandwidth and with fre-
quencies scaled by 0.97.
the determination of absolute configurations;[7] this method
was recently also used for chiral-at-metal complexes.[8] Experi-
mental VCD spectra of 1 and 2 (Figure 3) in CD3CN solution
are well reproduced by the calculated spectra, where the con-
formers of the given diastereomers present in the crystal struc-
ture were taken into account. The bands present in the observ-
able region correspond mainly to coupled CꢀH bending vibra-
tions (CH groups on the a-carbon and in the benzene ring)
with a contribution of CꢀO stretching vibration.
The VCD spectra of 1 and 2 differ at a band at 1350 cmꢀ1
,
which corresponds to the CꢀH bending vibrations (CH group
on the b-carbon) coupled with wagging vibrations of the
ꢀCH2ꢀ group. See Section S4 in the Supporting Information
for further details and deeper analysis of ECD and VCD spectra.
The crystallization of complexes from the racemic mixture
might be controlled by solubility. Whereas the different solubil-
ity allows for the separation of diastereomers 1 and 2, it does
not explain why P1 crystallizes first without the enantiomer
NBu4[VO2(N-salicylidene-d-isoleucinato)], which is supposed to
have identical solubility. On the other hand, the observed
products of successive crystallization may refer to the preferen-
tial formation of some stereoisomers[5] and/or to epimerization
of the amino acid.[9,10] The epimerization of isoleucine is not
easy, but possible,[10] and the transformation of l-Ile into
d-allo-Ile (i.e., epimerization at the a-carbon position) can be
performed by multiple-step organic reactions[11]
.
1
Figure 5. 51V NMR spectra were collected upon pouring the solution (500 mL)
into CH3CN (5 mL) and therefore are shifted in comparison to NMR spectra
of pure 1 and 2. The diastereomeric ratio is 62:38 prior to crystallization (a)
and 30:70 upon isolation of PI (b) in respect to l/d-complexes. The diaste-
reomeric ratio was also observed in 1H NMR spectra (see the Supporting In-
formation, Figures S6 7, S6 8 and Figures S6 9, S6 10 for full 51V NMR
spectra).
The H NMR spectra of solutions of the Schiff bases before
addition of tetrabutylammonium vanadate clearly demonstrate
that no spontaneous, diastereomeric transformation of the
Schiff base takes place, even after several weeks (see Fig-
ure S6 1–3 in the Supporting Information), whereas upon in-
troducing a solution of NBu4VO3 the conversion proceeds at
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a racemic solution with eight hypothetical stereoisomers. Al-
though not yet clear, we believe that this unexpected course
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stereoisomers of amino acids from complicated racemic
solutions.
Acknowledgements
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This work was supported by the Grant Agency of the Ministry
of Education of the Slovak Republic and Slovak Academy of
Sciences VEGA project no. 1/0336/13 as well as by the Slovak
Research and Development Agency (APVV-0510-12). Calcula-
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Academy of Sciences using the supercomputing infrastructure
acquired in project ITMS 26230120002 and 26210120002 sup-
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Steffen Lꢄdeke (Albert-Ludwigs-Universitꢅt Freiburg, Institut fꢄr
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Keywords: chiral
resolution
·
circular
dichroism
·
epimerization · Schiff bases · vanadium
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Received: April 16, 2014
Published online on June 17, 2014
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