Diastereoselective Synthesis and Catalytic Activity of Two Chiral cis-Dioxidomolybdenum(VI) Complexes

Article abstract of DOI:10.1002/ejic.201800285

Two enantiomerically pure chiral dioxidomolybdenum(VI) complexes (1 and 2) of the type [MoO₂L] (L = dianionic, tetradentate ONNO-ligand) were synthesized and investigated in enantioselective oxidation reactions. The solid-state structures of complex 1 and 2, determined via single-crystal X-ray diffraction analysis revealed two fundamentally different coordination geometries: a C1-symmetric cis-β isomer (Λ-1), and a C2-symmetric cis-α isomer (Δ-2). In both cases, only one of the two possible helical enantiomers (Λ- or Δ-helix) was formed. The complexes were examined as precatalysts in the epoxidation of the challenging prochiral substrate trans-stilbene, using either tert-butyl hydroperoxide (TBHP) or cumyl hydroperoxide (CHP) as oxidants. The asymmetric cis-β complex 1 was found to be significantly more active in the epoxidation than its cis-α counterpart 2, asymmetric induction was, however, negligible for both complexes. The complexes were also tested in catalytic enantioselective sulfoxidation reactions where chiral induction could be achieved, albeit small. The observed putative molybdenum oxido-peroxido intermediate 1-O₂ could be identified as an important precomplex before formation of the active catalyst in sulfoxidation.

Full text of DOI:10.1002/ejic.201800285

A Journal of
Accepted Article
Title: Diastereoselective synthesis and catalytic activity of two chiral
cis-dioxido molybdenum(VI) complexes
Authors: Md. Mehdi Haghdoost, Niklas Zwettler, Golara Golbaghi,
Ferdinand Belaj, Mojtaba Bagherzadeh, Jörg Anton
Schachner, and Nadia C. Mösch-Zanetti
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To be cited as: Eur. J. Inorg. Chem. 10.1002/ejic.201800285
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10.1002/ejic.201800285
European Journal of Inorganic Chemistry
FULL PAPER
Diastereoselective synthesis and catalytic activity of two chiral
cis-dioxido molybdenum(VI) complexes
Md. Mehdi Haghdoost,[a],† Niklas Zwettler,[b] Golara Golbaghi,[a],† Ferdinand Belaj,[b] Mojtaba
Bagherzadeh,[a] Jörg A. Schachner*[b] and Nadia C. Mösch-Zanetti[b]
Abstract: Two enantiomerically pure chiral molybdenum(VI) dioxido
complexes (1 and 2) of the type [MoO2L] (L= dianionic, tetradentate
ONNO-ligand) were synthesized and investigated in enantioselective
oxidation reactions. The solid state structures of complex 1 and 2,
determined via single-crystal X-ray diffraction analysis revealed two
fundamentally different coordination geometries: a C1-symmetric cis-
β isomer (Λ-1), and a C2-symmetric cis-α isomer (Δ-2). In both
cases, only one of the two possible helical enantiomers (Λ- or Δ-
helix) was formed. The complexes were examined as precatalysts in
the epoxidation of the challenging prochiral substrate trans-stilbene,
using either tert-butylhydroperoxide (TBHP) or cumylhydroperoxide
(CHP) as oxidants. The asymmetric cis-β complex 1 was found to be
significantly more active in the epoxidation than its cis-α counterpart
metal catalysts are successfully applied in several important
enantioselective transformations.[2,3]
Among methods for the synthesis of chiral-at-metal coordination
compounds, induction of chiral information from ligand to the
metal center is the most efficient and rational way.[4] This chiral
induction, which is also known as predetermination of chirality at
the metal center, has been previously reported for various
transition metal complexes,[5] underlining the potential of such
complexes for the application in asymmetric catalysis. Kol and
co-workers showed that predetermination of chirality around a
metal center strongly depends on the design of the chiral
ligand.[6] Thus, there is a great interest in investigating new
combinations of chiral ligands and metal cores in order to
2, asymmetric induction was, however, negligible for both complexes. prepare enantiopure chiral-at-metal complexes. Herein, we
The complexes were also tested in catalytic enantioselective
sulfoxidation reactions where chiral induction could be achieved,
albeit small. The observed putative molybdenum oxido-peroxido
intermediate 1-O2 could be identified as an important pre-complex
before formation of the active catalyst in sulfoxidation.
report on the synthesis of two molybdenum(VI) dioxido
complexes with enantiopure tetradentate ONNO-ligands. In
addition, we re-investigated the capability of these two chiral
molybdenum(VI) complexes 1 and 2 as catalysts for epoxidation
and sulfoxidation reactions focusing on the influence of the
coordination motif.[7,8]
Introduction
Results and Discussion
The synthesis and characterization of chiral metal complexes is
undeniably one of the abiding interests in synthetic inorganic
chemistry. Much of this interest is inspired by the prospective
practical application of such chiral complexes in asymmetric
catalysis. A broad range of asymmetric reactions can be
catalyzed efficiently by a variety of chiral transition metal
complexes to give products in high enantioselectivity. The
chirality in these complexes can be located either on the ligand
or at the metal center, or on both. In “chiral-at-metal” complexes,
the metal ion itself inherits the chiral information which
subsequently is transferred to the products in stereoselective
metal-catalyzed reactions.[1] In comparison to traditional chiral
complexes, where the chiral information is intrinsic to the ligand
framework (chiral-at-ligand), chiral-at-metal complexes gained
much less attention until recently. However, nowadays chiral-at-
Synthesis of chiral ligands. The salen-type ligand H2L1 has
been synthesized via Schiff base condensation of (R,R)-1,2-
diammoniumcyclohexane mono-(+)-tartrate with 2-hydroxy-1-
naphthaldehyde in excellent yield (Figure 1).[9,10] The obtained
ligand exhibited >98% enantiomeric excess determined by
HPLC analysis using a chiral column. Synthesis and analytical
data for ligand H2L1 have been previously published.[11–13]
The subsequent reduction of the imine moiety of H2L1 with
NaBH4 in MeOH gave the salan-type ligand H4L2 with 98% ee in
fair yield after recrystallization. The 1H NMR spectrum of the free
salen ligand H2L1 shows a peak at 8.78 ppm (CDCl3) that is
assigned to the C-H protons of the imine groups. Reduction of
H2L1 led to the disappearance of this signal, but new signals
appeared at 4.22-4.50 and 3.70 ppm (CDCl3). These signals
were ascribed to be -CH2- and NH protons, respectively,
consistent with the reduction of both imine moieties on the ligand
to amine groups. Ligand H4L2 was further characterized by FT-
IR spectroscopy and elemental analysis.
[a]
[b]
Md. M. Haghdoost, G. Golbaghi, M. Bagherzadeh
Chemistry Department
Sharif University of Technology
P.O. Box 11155-3615, Tehran, Iran
N. Zwettler, F. Belaj, J. A Schachner, N. C. Mösch-Zanetti
Institute of Chemistry, University of Graz
Schubertstr. 1, 8010 Graz, Austria
Email: joerg.schachner@uni-graz.at
https://chemie.uni-graz.at/en/bioinorganic-and-coordination-chemistry/
†
current address: INRS Institute Armand-Frappier Research Centre,
531, blvd de s Prairies, Laval, H7V 1B7 Canada
Supporting information for this article is given via a link at the end of
the document.
1
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10.1002/ejic.201800285
European Journal of Inorganic Chemistry
FULL PAPER
NMR spectroscopy also found no evidence for the formation of
another isomer in the reaction solution.
Figure 2. Possible geometric isomers for the coordination of a symmetric
tetradentate ONNO-ligand at a molybdenum(VI) dioxido metal center.
Figure 1. Ligands H2L1 (salen-type) and H4L2 (salan-type) employed in this
study.
Complex 1 was further characterized by IR spectroscopy,
elemental analysis, and mass spectrometry. The solid state
structure of complex 1 was additionally confirmed by single-
crystal X-ray diffraction analysis (CCDC 1028614). The
molecular structure of complex 1 is depicted in Figure 3, details
on crystallographic data and structure refinement are listed in
Table S1, selected bond lengths and angles are provided in
Table 1. The solid state structure of complex 1 confirms the
formation of a -Mo complex with cis-β configuration. The
complex features a six coordinate Mo atom with distorted
octahedral geometry in which the two oxygen atoms of the
naphtholate rings are in a cis configuration with an O-Mo-O
angle of 84.04(6)°. The Mo-O and Mo-N bonds trans to the oxido
ligands (Mo1-O12 2.1342(12) Å, Mo1-N2 2.2963(14) Å) are
distinctly longer than trans to imine nitrogen (Mo1-O22
1.9366(11) Å, Mo1-N1 2.1088(14) Å), reflecting the stronger
trans influence of the oxido groups. The [MoO2]2+ fragment has
the expected cis configuration (O1-Mo1-O2 104.04(6)°) and
Mo=O bond lengths (Mo1-O2 1.7062(11) Å, Mo1-O1 1.7141(13)
Å) are in the expected range.[22] Furthermore, the (R,R)-chirality
of the ligand backbone and the left-handed Λ-chirality around
the Mo atom can be seen in the X-ray crystal structure of the
complex.
Synthesis of chiral molybdenum(VI) complexes. The
synthesis of complex 1 has been previously published, albeit the
racemic version derived from rac-H2L1 and without crystal
structure.[14,15] The so obtained complex rac-1 was tested in the
epoxidation of cyclohexene and 1-octene under TBHP-limiting
conditions.[14] The chiral molybdenum(VI) complexes [MoO2(L1)]
(1) and [MoO2(H2L2)] (2) were obtained by adding [MoO2(acac)2]
to one equivalent of the corresponding ligand in refluxing
methanol (Scheme 1). Upon cooling both 1 and 2 precipitated
and were isolated as yellow to brown solids in analytically pure
form in 60 and 55% yield, respectively. They are highly soluble
in CH2Cl2 and CHCl3, moderately soluble in methanol, and
insoluble in diethyl ether and aliphatic solvents. Proton NMR
experiments revealed that these complexes were stable at
ambient conditions in solid as well as in solution for several
weeks.
Scheme 1. Synthetic procedure for the preparation of complexes 1 and 2.
For tetradentate ONNO-ligands coordinated at a [MoO2]2+ core,
three geometrical isomers may in principle be formed (Figure 2).
At present, the trans structure has not been reported in literature
because of the unfavorable trans arrangement of the two oxido
ligands and is thus unlikely. In the cis-α isomer, the ligand
phenolate oxygen atoms coordinate trans to each other, which
results in a higher overall symmetry (C2). In the case of the
lower symmetry cis-β isomer, there is no symmetry axis (C1) as
the phenolate oxygen atoms of the ligand coordinate cis to each
other.
Both C2-symmetric cis-α and C1-symmetric cis-β configurations
for dioxido molybdenum(VI) complexes have been previously
reported, dependent on the employed ligand.[8,16,17–21] The 1H
and 13C NMR spectra of complex 1 showed distinct signals for
both ligand half-units, in accordance with the asymmetric cis-β
configuration. This is exemplified by the distinct chemical shifts
of the imine protons (9.51 and 8.86 ppm), arising from the
asymmetric coordination of the salen ligand to the metal center.
Figure 3. Molecular view (50% probability level) of complexes Λ-1 (top) and Δ-
2 (bottom). H atoms and solvent molecules are omitted for clarity.
2
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10.1002/ejic.201800285
European Journal of Inorganic Chemistry
FULL PAPER
Table 1. Selected bond lengths (Å) and angles (°) of complexes 1 and 2.
1
2
Mo1-O2
Mo1-O1
Mo1-O12
Mo1-O22
Mo1-N1
Mo1-N2
O1-Mo1-O12
O1-Mo1-N1
O1-Mo1-N2
O1-Mo1-O22
O1-Mo1-O2
O2-Mo1-N2
O2-Mo1-N1
O2-Mo1-O12
O2-Mo1-O22
N1-Mo1-O22
N1-Mo1-N2
N1-Mo1-O12
O12-Mo1-O22
O12-Mo1-N2
1.7062(11)
1.7141(13)
2.1342(12)
1.9366(11)
2.1088(14)
2.2963(14)
164.33(5)
91.17(6)
1.697(2)
1.712(2)
1.9662(17)
1.9450(17)
2.3180(9)
2.3051(7)
95.50(9)
163.42(9)
91.15(9)
97.65(9)
108.76(12)
160.05(9)
87.03(8)
98.22(8)
97.34(8)
84.90(6)
73.27(3)
76.74(5)
155.18(8)
80.61(6)
84.59(6)
Scheme 2. High diastereoselectivity of the coordination of H4L2 to the [MoO2]
101.90(6)
104.04(6)
167.61(6)
97.92(6)
164.33(5)
104.94(5)
149.82(6)
72.71(5)
core.
We observed the same diastereoselectivity with an (S,S)-
configured bispyrolidine salan ligand, that resulted exclusively in
the Λ-configured Mo complex.[21] However, in some cases the
formation of both diastereomers of Zn and Ti complexes were
observed, even if enantiopure salan ligands were used.[6,23,24]
Furthermore, a molybdenum(VI) complex was reported, that
gave a 1:1 mixture of diastereomers when a chiral salalen ligand
(only one imine moiety reduced to the corresponding amine)
was used.[20]
77.20(5)
84.04(6)
81.96(5)
In the case of complex 2, chelation of the salan ligand H4L2 to
the [MoO2]2+ core generates two additional stereogenic centers
at the amine nitrogen atoms, resulting in the possible formation
of two diastereomers. If non-chiral salan ligands are employed,
in general a racemic mixture of complexes is obtained.[23]
However, when a chiral non-racemic ligand like H4L2 is used,
one diastereomer may be formed preferentially, a phenomenon
which has been termed predetermination of chirality-at-metal.[2]
According to the NMR spectra, complex [MoO2(H2L2)] (2) was
obtained by reaction of the H4L2 ligand with [MoO2(acac)2]
precursor as a single diastereomer of C2-symmetry. Complex 2
displayed a single set of signals for the ligand in the 1H NMR
spectrum, in which only 6 signals for 12 aromatic hydrogen
atoms were observed. In addition, the 13C NMR spectrum
revealed three resonances at 24.24, 29.96 and 45.61 ppm for
the six cyclohexyl carbon atoms, consistent with a C2-symmetric
cis-α structure. Most importantly, there was no indication for the
formation of the other possible diastereomers in the recorded
NMR spectra. Single-crystal X-ray diffraction analysis also
confirmed this conclusion (CCDC 1028615).
The solid state structure of complex 2 exhibits the symmetric cis-
α configuration, in contrast to complex 1. The molecular
structure of complex 2 is depicted in Figure 3, details on
crystallographic data and structure refinement are listed in Table
S1, selected bond lengths and angles are provided in Table 1.
The structure reveals a six-coordinate Mo atom in a distorted
octahedral geometry, in which the oxido ligands are trans to the
amine nitrogens. The Mo-N bonds in complex 2 (Mo1-N1
2.3180(1) Å, Mo1-N2 2.3051(1) Å) are significantly longer than
those found in the complex 1 (Mo1-N1 2.1088(1) Å, Mo1-N2
2.2963(1) Å). However, the Mo=O bonds lengths are essentially
similar in both complexes. The solid state structure clearly
shows the (R,R)-chirality of the cyclohexyl backbone and (S,S)-
configuration of the nitrogen atoms. This kind of chelation leads
to an overall Δ-wrapping of the salan ligand around the
molybdenum center, consistent with chiral induction from ligand
to metal (Scheme 2). Such high diastereoselectivities were also
observed for other (R,R)-enantiopure salan ligands like H4L2,
leading to the Δ-configuration at the molybdenum atom.[17–20] Kol
and co-workers reported similar behavior for titanium(IV)
complexes, and could also show that the (S,S)-configured
cyclohexyldiamine ligand gives the opposite Λ-configuration at
the metal atom.[6,24,25]
Figure 4. Perspective view of the single-stranded helix along the 65 screw axis
(top); the molecules of 2 are helically arranged around the 65 screw axes
forming tubes parallel to the c-axes. These tubes are filled with disordered
methanol solvent molecules.
Complex 2 was further characterized by elemental analysis,
mass spectrometry as well as by IR spectroscopy. This chiral
complex crystallized from methanol as the solvent adduct
2·CH3OH in the rare enantiomorphic hexagonal space group P65
3
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10.1002/ejic.201800285
European Journal of Inorganic Chemistry
FULL PAPER
(Figure 4). Single-crystal structural analysis indicates that
complex 2·CH3OH exhibits an infinite cylindrical single-stranded
helical structure with M (left-handed) helicity. The molecules are
helically arranged around 65 screw axes forming tubes parallel to
the c-axes. These tubes are filled with disordered methanol
solvent molecules turning their methyl groups towards the
naphthalene rings. Obviously, a sufficient homochiral helicating
element in the complex introduces only one type of helicity
within the crystal.[26] Thus, in total, three different types of
chirality have been identified in the crystal structure of complex
2: configurational chirality on the cyclohexyl backbone,
conformational chirality on the molybdenum center, and
supramolecular chirality in the crystalline state.
Complexes 1 and 2 are similar in many respects, such as the
first coordination sphere, the Mo=O bond lengths, steric bulk and
solubility, even though they have different octahedral
configurations and ligand backbones. Therefore, under the same
reaction conditions, differences in the catalytic activity of these
complexes may likely be related to their isomeric configuration.
The higher catalytic activity of 1 may be attributed to a
labilization of one Mo=O bond in the cis-β complex, where a
negative naphtholate oxygen atom is trans to one oxido ligand,
whereas in the cis-α case observed in 2, neutral amine nitrogen
atoms are trans to both oxido ligands. These results are in
agreement with our previous report on the effect of the
geometrical configuration on catalysis by molybdenum(VI)
dioxido complexes. A cis-β complex was found to be a more
efficient catalyst for oxygen atom transfer between DMSO and
PMe3 than its sterically similar cis-α counterpart.[8]
Finally, UV-Vis and Circular Dichroism (CD) spectra were
recorded for complexes 1 and 2 in CH2Cl2 (see Figure S5). The
UV-Vis spectrum for 1 had previously been reported.[14] Complex
2 shows two resolved transitions in the UV-Vis spectrum. A
lower energy ligand-to-metal charge transfer centered at 386 nm
and a higher energy intra-ligand π-π* transition at 337 nm.[27]
The same features were reported for 1 at 434 and 322 nm.[14]
Literature on CD spectra of chiral Mo(VI) complexes is
scarce.[27,28] The CD spectra of 1 and 2 differ quite a bit, with
especially complex 2 giving a more complex spectrum because
of having more chiral centers. Similar to the observed
absorptions in the visible region of the UV-Vis spectrum, the CD
spectra show the strongest absorptions between 330 and 405
nm. Both complexes show a distinct peak of opposite sign in
their respective CD spectra. For complex 1, a peak of negative
sign at 366 nm can be observed ( = -3.7 M-1 cm-1), for
complex 2 a positive peak at 350 nm (( = 6.5 M-1 cm-1) (see
Figure S5). Form these spectra we can conclude that no
racemization of complexes 1 or 2 in solution occurs, thereby not
explaining the lack of chiral induction observed in catalytic
reactions.
Complexes 1 and 2 did not show any enantioselectivity in the
epoxidation of trans-stilbene. In contrast to a previous study on
highly enantioselective epoxidations using Mo complexes by
Yamamoto and coworkers, changing the oxidant to the more
bulky
cumylhydroperoxide
(CHP)
did
not
improve
enantioselectivity in our system, but significantly decreased the
conversion.[29] In addition, no enantiomeric excess (ee) was
observed when the epoxidation of trans-stilbene was carried out
at 0 or 50 °C respectively. However, the catalytic activity of 1
increased gradually with increasing the reaction temperature.
Even when the catalyst loading was lowered to 0.05 mol% of 1,
trans-stilbene was converted almost quantitatively into the
corresponding epoxide at 50 ºC after 4 h (Table 2, entry 3),
which is a significant improvement compared to previously
studied salan Mo complexes.[19,21]
100
80
Catalytic oxidations. Complexes 1 and 2 have been tested as
catalysts (0.5 mol% catalyst loading) in the epoxidation of
prochiral trans-stilbene using tert-butylhydroperoxide (TBHP, 1.5
equiv.) as oxidant. Using complex 1, the corresponding epoxide
was obtained as the sole product with 65% yield after 24h (Table
2, entry 2). Under identical conditions, complex 2 only reached
19% yield of trans-stilbene oxide (Table 2, entry 6). Under all
tested reaction conditions complex 1, with the Schiff base imine
backbone and cis-β configuration, was consistently more active
than complex 2 with the reduced amine backbone and cis-α
configuration (Figure 5).
60
1
2
40
20
0
0
5
10
15
20
25
t [h]
Table 2. Epoxidation of trans-stilbene with complexes 1 and 2.
Figure 5. Comparison of conversion profiles for complexes 1 and 2 in the
epoxidation of trans-stilbene; Conditions: 0.5 mol% catalyst, 1.5 equiv. TBHP,
CH2Cl2, rt.
However, complex 1 shows some enantioselectivity in the
oxidation of methylphenyl sulfide with hydrogen peroxide, as
summarized in Table 3.
Cat.a
Solvent
CH2Cl2
CH2Cl2
CHCl3
CH2Cl2
CH2Cl2
CH2Cl2
T (°C)
rt
rt
50
0
rt
time (h)
Yield (%)b
Sel. (%)b
>99
1
2
3
4
5
6
1
1
4
24
4
24
4
35
65
96c
5
1
19
>99
1c
2
>99d
>99
2
2
>99
>99
rt
24
a
Reaction conditions: 0.5 mol% catalyst, 0.5 mmol trans-stilbene (1 equiv.),
b
0.75 mmol (1.5 equiv.) TBHP, solvent (1 mL); selectivities and yields were
determined by HPLC;c 0.05 mol% catalyst was used; selectivity and yield
d
was also determined by GC-MS measurement.
4
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10.1002/ejic.201800285
European Journal of Inorganic Chemistry
FULL PAPER
Table 3. Oxidation of methylphenyl sulfide with complexes 1 and 2.
Yield
(%)b
69
45
98
77
99
99
99
Sel.
(%)b
>99
>99
91
98
95
74
98
ee
(%)
0
Cat.a
1c
1c
1
1
1
1
2
Solvent
T (°C)
time (h)
Scheme 3. Oxidation of racemic sulfoxide by complex 1 showing no kinetic resolution.
1
2
3
4
5
6
7
8
CH2Cl2
CH2Cl2
CHCl3
rt
0
rt
0
0
0
0
0
0.5
24
4
24
24
24
24
24
0
Compared to complex 1, 2 was expected to show a better chiral
induction since in this complex two chiral centers at the nitrogen
atoms are located in close proximity to the Mo center. But in
contrast, with complex 2, a generally lower and opposite
enantioselectivity was achieved (Table 3, entries 7 and 8).
These results could be explained by the observation that in 2,
because of the longer Mo-N bonds compared to 1 (2: Mo1-N1
2.3180(1) Å, Mo1-N2 2.3051(1) Å; 1: Mo1-N1 2.1088(1) Å, Mo1-
N2 2.2963(1) Å), the interaction between the metal and the
chiral diamine centers is much weaker than in complex 1. The
weak interaction may result in partial dissociation of the chiral
ligand (hemi-lability) from the molybdenum center during the
catalytic process.[35,36] NMR studies of the catalytic reaction in
CDCl3 did not show any sign of hydrolysis of the ligand.
However, the exchange between the coordinated and partially
uncoordinated ligand can take place faster than the NMR
timescale. Evidently, implementing chirality directly at the Mo
center in complex 2 could not provide better enantioselectivity.
We believe that in this case, the potentially weakly coordinated
chiral diamine backbone is responsible for the loss of chiral
induction.[35,36]
In order to further probe the mechanism of oxidation reactions,
we attempted to synthesize the molybdenum(VI) oxido peroxido
complex of 1, 1-O2, which is supposedly formed under catalysis
conditions. Addition of TBHP or H2O2-urea adduct to complex 1
in various solvents (e.g. MeOH, CHCl3, CH2Cl2) lead to a
multitude of unidentifiable products, as observed by NMR and IR
spectroscopy. By careful addition of aqueous hydrogen peroxide
however, to a solution of 1 in MeOH and refluxing, a color
change of the solution to bright red was observed. All attempts
however to isolate this material in analytically pure form or to
grow high quality single crystals were unsuccessful due to ligand
hydrolysis or other decomposition reactions. A brief discussion
of analytical data obtained for this putative oxido peroxido
complex can be found in the Supporting Information.
1.4 (S)
2.5 (S)
9.4 (S)
0.7 (S)
1.3 (R)
1.7 (R)
CHCl3
CH3OH
CH3CN
CHCl3
2
CH3OH
99
98
a
Reaction conditions: 0.5 mol% catalyst, 0.5 mmol methylphenyl sulfide (1
equiv.), 0.75 mmol (1.5 equiv.) H2O2, solvent (1 mL); bselectivities to sulfoxide,
yields and enantiomeric excesses were determined by HPLC. c 0.5 mmol (1
equiv.) of TBHP was used instead of H2O2.
It was found that enantioselectivity depends on the choice of
solvent. In particular, catalytic sulfoxidation in methanol using
complex
1 afforded (S)-methylphenyl sulfoxide in nearly
quantitative yield, 95% selectivity with a slightly higher ee of
9.4% (Table 3, entry 5) compared to other solvents (in CH2Cl2
0% ee; in CHCl3 2.5% ee; in CH3CN 0.7% ee). Noticeably, when
TBHP was used as oxidant under the same reaction conditions,
poor sulfoxide yield and essentially no enantioselectivity were
observed (Table 3, entries 1 and 2).
Investigation of the enantioselectivity as a function of time (or
conversion) revealed a slow and steady increase of ee reaching
a maximum value after 24 h (Figure 6). This is in contrast to the
results published by the groups of Kühn [30] and Gonçalves[31], in
which enantioselectivities decrease as the reactions progress.
Kinetic resolution of the formed sulfoxide by enantioselective
oxidation to the sulfone was envisioned as a possible reason
(Scheme 3).[32] There are numerous examples of titanium[33] and
vanadium[34] catalyzed oxidative kinetic resolution of sulfoxides.
However, here we ruled out this possibility by starting with
racemic methylphenyl sulfoxide, which under the same reaction
conditions led to the formation of methylphenyl sulfone in 26%
overall yield with no enantiomeric enrichment (Scheme 3). As a
result, we conclude that the observed enantioselectivity in the
sulfoxidation is a result of the asymmetric oxidation of sulfide in
the first step rather than an oxidative kinetic resolution of newly
formed sulfoxide.
100
9.4
Conclusions
80
Two chiral molybdenum(VI) complexes with enantiomerically
pure tetradentate ONNO-ligands were prepared, featuring
different ligand arrangements. With the salen-type ligand H2L1
cis-β complex 1 was obtained, whereas the cis-α isomer 2 was
obtained for the salan-type, diamine-based ligand H4L2. The
chiral induction from the ligands H2L1 and H4L2 to metal led to an
efficient predetermination of chirality around the [MoO2]2+ core.
In addition, a helical stacking of molecules for complex 2 caused
the formation of supramolecular chirality in the crystals. Both
complexes have been tested as catalysts in asymmetric
epoxidation and sulfoxidation. The comparative catalytic study
revealed the superiority of cis-β complex Λ-1 over cis-α
counterpart Δ-2 in the catalytic epoxidation of trans-stilbene.
However, no enantiomeric excess in the epoxide was obtained.
On the other hand, using the same two complexes, the
6.4
60
Sulfoxidation using 1, with %ee
4.5
40
2.9
2.5
1.7
20
0
0
5
10
15
20
25
t [h]
Figure 6. Time-conversion profile for the asymmetric oxidation of methylphenyl
sulfide with catalyst 1 and %ee given for each sampling time; Conditions: 0.5
mol% 1, 1.5 equiv. TBHP, MeOH, 0 °C.
5
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10.1002/ejic.201800285
European Journal of Inorganic Chemistry
FULL PAPER
128.78, 128.53, 126.65, 123.34, 121.48, 121.27, 111.24, 58.47, 45.61,
29.96 , 24.24. EI-MS (70 eV) m/z: 554 (7) M+. FT-IR (ATR) (cm-1): 3245
(w, N-H), 3061 (w), 2932 (w), 2858 (w), 1615 (m), 1596 (m), 1464 (m),
1428 (m), 1396 (m), 1271 (m), 1235 (s), 1029 (w), 916 and 894 (vs,
Mo=O), 810 (m), 774 (s), 745 (s), 568 (s). Anal. Calc. for C28H28MoN2O4
(MW 552.50) (%): C, 60.87; H, 5.11; N, 5.07. Found: C, 60.53; H, 5.07; N,
5.37.
sulfoxidation of prochiral methylphenyl sulfide provided with high
yields of sulfoxide with some enantioselectivity.
Experimental Section
General Remarks. (R,R)-1,2-Diammoniumcyclohexane mono-(+)-
tartrate was prepared according to literature procedures.[9] Synthesis of
ligand H2L1 has been previously described.[11,12] All other chemicals were
used without purification as purchased from commercial sources. All
NMR spectra were recorded on a Bruker Avance 300 MHz spectrometer
(25 ºC) and mass spectrometry on an Agilent Technologies 5795C inert
XL MSD. CDCl3 was used as NMR solvent and chemical shifts are
reported in ppm relative to the solvent residual peak. FT-IR spectra were
obtained on a Bruker Alpha P Diamond FTIR spectrometer. GC-MS
analyses were performed with an Agilent 7890A GC system with an
Agilent 19091J-433 column coupled to a 5975C inert XL EI/CI mass
selective detector (MSD). Helium was used as the mobile phase. HPLC
with diode array detection (HPLC/DAD) analyses were carried out with
Catalytic reaction. Typical catalytic reactions (epoxidation and
sulfoxidation) were performed by stirring a solution of catalyst (0.5 mol%)
and the corresponding substrate (0.5 mmol, 1 equiv) in 1 mL of solvent.
After stirring the mixture for 5 min, with the addition of oxidant (0.75 mmol,
1.5 equiv) the reaction was started. The reaction mixture was stirred at
the desired temperature and the reaction progress was monitored by GC-
MS and HPLC. For GC-MS analyses, samples were taken and the
reaction was quenched by addition of a small amount of MnO2 to
decompose excess oxidant. After centrifugation, sample aliquots were
diluted with ethyl acetate and injected into the GC column. For HPLC
analyses, samples were taken, dried, diluted with heptane/isopropanol
(90:10 v/v), and treated with a catalytic amount MnO2 to quench the
excess of peroxide. The resulting slurry was filtered through a pad of
Celite and the filtrate injected into a chiral HPLC column.
an Agilent 1200 series instrument equipped with
a chiral ASTEC
Cellulose DMP column (5 µm, 25 cm x 4.6 mm) with heptane/isopropanol
(90:10 v/v for epoxidation and 80:20 v/v for sulfoxidation) as the mobile
phase. CD measurements were performed on a Jasco J-1500 CD
Spectrometer. Diffraction measurements were performed on a SMART
APEX II diffractometer equipped with a CCD detector using graphite
monochromated Mo Kα (λ = 0.7107 Å) radiation. The structure was
solved by direct methods (SHELXS-97)27 and refined by full-matrix least-
squares techniques against F2 (SHELXL-97).28 Elemental analyses (C, H
and N) were performed using a Heraeus Vario Elementar analyser
(Department of Inorganic Chemistry, University of Technology Graz).
Supporting information (see footnote on the first page of this article): IR
spectrum of H2L1 (Figure S1), EI-MS spectra for 1 and 1-O2 (Figure S2)
are shown; full details of crystallographic data collection and refinement
as well as bond lengths and angles for 1 and 2 are given.
Acknowledgements
Synthesis of (R,R)-H4L2. The synthesis of H4L2 was performed
according to a literature procedure published for similar ligands.[15,37] 422
mg of (R,R)-H2L1 (1 mmol) were dissolved in methanol (15 mL), cooled to
0 °C, and 152 mg (4 mmol) of NaBH4 was added in one portion. The
resulting mixture was stirred at rt for 1 h and then refluxed for 2 h. After
The authors gratefully acknowledge support from NAWI Graz.
MMH would like to thank the Research Council of Sharif
University of Technology. G. Hofer, MSc and Prof. M. Oberer
from the Institute of Molecular Biosciences, Structural Biology
group, are acknowledged for recording Circular Dichroism
spectra (NAWI Graz Core Facility CD-Spectropolarimetry).
cooling to ambient temperature,
5 mL distilled water was added
cautiously. The resulting reddish solution was filtered and the precipitate
recrystallized from water and methanol to give H4L2 ligand. Yield: 170 mg
(0.4 mmol, 40%). 1H NMR (300 MHz, CDCl3, δ): 11.88 (s, 2H), 6.80-8.00
(m, 12H), 4.22-4.63 (m, 4H), 3.69 (s, 2H), 2.57 (m, 2H), 1.28-2.20 (m,
8H). 13C NMR (125 MHz, CDCl3,δ): 156.10, 132.18, 129.49, 128.88,
128.52, 126.64, 122.85, 121.06, 129.09, 111.27, 69.63, 51.71, 29.13,
24.18. FT-IR (ATR) (cm-1): 3294 (w, N-H), 2950 (w), 2862 (w), 1605 (m),
1485 (m), 1368 (m) , 1230 (m), 875 (m). Anal. Calc. for C28H30N2O2 (MW
426.56) (%): C, 78.84; H, 7.09; N, 6.57. Found: C, 78.94; H, 7.30; N, 6.50.
Keywords: molybdenum • coordination modes • oxido ligands •
chirality • epoxidation
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7
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10.1002/ejic.201800285
European Journal of Inorganic Chemistry
FULL PAPER
Entry for the Table of Contents
Layout 1:
FULL PAPER
The synthesis of two chiral cis-
dioxido Mo(VI) complexes bearing
enantiopure, tetradendate ligands is
described. For complex 2, exclusive
formation of the -helix due to
predetermination of chirality leads to
a highly diastereoselective synthesis
of this complex. Chiral induction,
however, in asymmetric oxidation
reactions was low.
Chiral complexes*
Md. Mehdi Haghdoost,[a],† Niklas
Zwettler,[b] Golara Golbaghi,[a],†
Ferdinand Belaj,[b] Mojtaba
Bagherzadeh,[a] Jörg A. Schachner*[b]
and Nadia C. Mösch-Zanetti[b]
Page No. – Page No.
Diastereoselective synthesis and
catalytic activity of two chiral cis-
dioxido molybdenum(VI) complexes
8
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