Dioxidomolybdenum(VI) complexes containing ligands with the bipyrrolidine backbone as efficient catalysts for olefin epoxidation

Article abstract of DOI:10.1002/ejic.201300258

Air-stable chiral dioxidomolybdenum(VI) complexes of the type [MoO ₂(L)] (L = L1^2-; 1, L = L2^2-, 2) were prepared by the reaction of [MoO₂Cl₂] with the bis(phenol) ligands 1,4-bis(2-hydroxy-benzyl)-(S,S)-2,2′-bipyrrolidine [(S,S)-H₂L1] and 1,4-bis(2-hydroxy-3,5-di-tert-butylbenzyl)-(S,S)-2,2′-bipyrrolidine [(S,S)-H₂L2). They were characterized by NMR spectroscopy, mass spectrometry, elemental analysis, and single-crystal X-ray diffraction analysis, which revealed a distorted octahedral coordination geometry around the molybdenum(VI) center with a cis-α configuration. The Mo-O_oxo bond lengths are almost equal and vary only between 1.701 and 1.7091 A. Complexes 1 and 2 were found to be efficient and selective catalysts for various olefin epoxidation reactions. Catalyst loadings of 0.5 mol-%, the use of 2 equiv. of oxidant (tert-butyl hydroperoxide), as well as unchanged catalyst performance over three consecutive catalytic runs demonstrate the excellent stability of the catalysts. Various challenging substrates including 1-octene or limonene were converted selectively to their corresponding epoxides. However, no asymmetric induction was observed. The cis-α isomers of dioxidomolybdenum complexes with a chiral backbone exhibit higher catalytic epoxidation activity than the related cis-β complexes. Copyright

Full text of DOI:10.1002/ejic.201300258

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Olefin Epoxidation
R. Mayilmurugan, P. Traar,
J. A. Schachner, M. Volpe,
N. C. Mösch-Zanetti* ....................... 1–8
The cis-α isomers of dioxidomolybdenum
complexes with a chiral backbone exhibit
higher catalytic epoxidation activity than
the related cis-β complexes.
Dioxidomolybdenum(VI) Complexes Con-
taining Ligands with the Bipyrrolidine
Backbone as Efficient Catalysts for Olefin
Epoxidation
Keywords: Homogeneous catalysis / Epox-
idation / Molybdenum / Chirality
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DOI:10.1002/ejic.201300258
Dioxidomolybdenum(VI) Complexes Containing Ligands
with the Bipyrrolidine Backbone as Efficient Catalysts for
Olefin Epoxidation
Ramasamy Mayilmurugan,[a,b] Pedro Traar,[a] Jörg A. Schachner,[a]
Manuel Volpe,[a] and Nadia C. Mösch-Zanetti*[a]
Keywords: Homogeneous catalysis / Epoxidation / Molybdenum / Chirality
Air-stable chiral dioxidomolybdenum(VI) complexes of the
type [MoO2(L)] (L = L12–; 1, L = L22–, 2) were prepared by
the reaction of [MoO2Cl2] with the bis(phenol) ligands 1,4-
bis(2-hydroxy-benzyl)-(S,S)-2,2Ј-bipyrrolidine [(S,S)-H2L1]
and 1,4-bis(2-hydroxy-3,5-di-tert-butylbenzyl)-(S,S)-2,2Ј-bi-
pyrrolidine [(S,S)-H2L2). They were characterized by NMR
spectroscopy, mass spectrometry, elemental analysis, and
single-crystal X-ray diffraction analysis, which revealed a
distorted octahedral coordination geometry around the
molybdenum(VI) center with a cis-α configuration. The Mo–
Ooxo bond lengths are almost equal and vary only between
1.701 and 1.7091 Å. Complexes 1 and 2 were found to be
efficient and selective catalysts for various olefin epoxidation
reactions. Catalyst loadings of 0.5 mol-%, the use of 2 equiv.
of oxidant (tert-butyl hydroperoxide), as well as unchanged
catalyst performance over three consecutive catalytic runs
demonstrate the excellent stability of the catalysts. Various
challenging substrates including 1-octene or limonene were
converted selectively to their corresponding epoxides. How-
ever, no asymmetric induction was observed.
zymes.[11,12] In this endeavor, we recently reported diox-
idomolybdenum complexes coordinated by a bis(phenolate)
ligand containing a 1,4-diazepane backbone (Figure 1,
H2L3).[12] These complexes are catalysts for the OAT reac-
tion from dimethyl sulfoxide to trimethylphosphane and
were found to be more efficient than the related molybd-
enum complex [MoO2(L4)] (Figure 1). We provided ample
evidence that the cis-β geometry imposed by the H2L3 li-
gand compared to the cis-α geometry of H2L4 (Figure 3)
leads to a more activated oxo group and thereby accelerates
the OAT reaction.[13,14]
Introduction
Olefin epoxidation is an important industrial reaction be-
cause epoxides are a valuable feedstock for a number of
value-added chemical transformations.[1] However, selective
epoxidations constitute an ongoing challenge in synthetic
organic chemistry. Molybdenum oxo complexes are capable
of catalyzing epoxidations of unfunctionalized olefins.[2–4]
Molybdenum complexes are also suitable catalysts for the
asymmetric epoxidation of olefins; however, enantio-
selectivities remain moderate.[5,6] Recently, Mo catalysts
bearing chiral hydroxamic acids led to a significant chiral
induction in several olefins.[7,8] In addition to the prominent
role that high oxidation state O=Mo moieties play in indus-
trial oxidation applications, such as ammoxidation, meta-
thesis, and hydrocarbon dehydrogenation,[4] they are also
found in a class of enzymes that catalyze biological oxygen
atom transfer (OAT) reactions.[9] For these reasons, the co-
ordination chemistry of dioxido–MoVI has also provoked
considerable interest for biological models.[10]
Figure 1. Bis(phenolate) ligands H2L1 and H2L2 used in this study;
H2L2,[18] H2L3,[12] and H2L4[13] have previously been reported.
We have an ongoing interest in the development of new
bioinspired models for molybdenum-containing OAT en-
In addition to our bioinspired research, we also devel-
oped several new Mo and Re catalysts for the epoxidation
of various olefins.[15–17] This prompted us to investigate the
feasibility of [MoO2(L3)] as a catalyst for epoxidation reac-
tions. Furthermore, we modified the bis(phenol) ligand with
[a] Institut für Chemie, Bereich Anorganische Chemie,
Karl-Franzens-Universität Graz,
Stremayrgasse 16, 8010 Graz, Austria
E-mail: nadia.moesch@uni-graz.at
Homepage: http://www.uni-graz.at/ancwww/
[b] School of Chemistry/Department of Physical Chemistry,
Madurai Kamaraj University,
a
chiral bipyrrolidine backbone, namely, 1,4-bis(2-
hydroxybenzyl)-(S,S)-2,2Ј-bipyrrolidine (S,S)-H2L1 and
1,4-bis(2-hydroxy-3,5-di-tert-butylbenzyl)-(S,S)-2,2Ј-bi-
pyrrolidine (S,S)-H2L2 (Figure 1). Whereas H2L1 repre-
Madurai 625021, India
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejic.201300258
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sents a new molecule, the tert-butyl-substituted ligand H2L2 troscopy confirmed the formation of complexes 1 and 2.
has previously been employed in group 4 metal complexes The EI mass spectra of 1 and 2 show prominent molecular
by Kol and co-workers.[18] No other metal complexes with ion peaks at m/z = 480 ([M]+) for 1 and m/z = 704 ([M]+)
H2L2 have been described.
for 2. The IR spectra of 1 and 2 exhibit two strong Mo=O
Thus, in this paper we report the synthesis and crystallo- bands for asymmetric and symmetric stretches at ca. 841–
graphic characterization of new chiral dioxidomolybdenum 888 and 926–927 cm–1, respectively, similar to those of
complexes of (S,S)-2,2Ј-bipyrrolidine-based bis(phenolate) other compounds of this type.[20] The molecular structures
ligands. In addition, their catalytic performance in epoxid- of 1 and 2 were further confirmed by single-crystal X-ray
ation reactions with various olefins was investigated and diffraction analyses (Figure 2).
compared to the performances other published MoVI com-
plexes.
Results and Discussion
Synthesis and Characterization of Ligands and Complexes
The synthesis of the ligand (S,S)-H2L2 by substitution
reactions between the (S,S)-2,2Ј-bipyrrolidine and the
bromomethyl derivatives of the corresponding phenols was
described previously by Kol and co-workers.[18] We found
reductive amination of (S,S)-2,2Ј-bipyrrolidine with 2-
hydroxy-benzaldehyde [to form (S,S)-H2L1] or 2-hydroxy-
3,5-di-tert-butylbenzaldehyde [to form (S,S)-H2L2] with so-
dium cyanotrihydroborate as the reducing agent to be
slightly more convenient as it is a one-step procedure and
similar overall yields were obtained. This synthetic route is
very similar to a method that was adopted for our recently
reported 1,4-diazepane-based ligands (Scheme 1).[12,19]
Figure 2. Molecular structures of 1 (top) and 2 (bottom). Hydrogen
atoms have been omitted for clarity.
Molecular Structures of 1 and 2
Single crystals suitable for X-ray diffraction analyses
Scheme 1. Synthesis of chiral complexes (S,S)-1 and (S,S)-2.
were obtained from a benzene solution of 1 and from a
Compounds [MoO2(L1)] (1, yield 71%) and [MoO2(L2)] toluene solution of 2 at room temperature. Molecular views
(2, yield 62%) were obtained by reacting equimolar of 1 and 2 are shown in Figure 2 together with the number-
amounts of [MoO2Cl2] and the ligands H2L1 or H2L2, ing scheme for donor atoms. The crystallographic details
respectively, in acetonitrile in the presence of 2 equiv. of are given in Table 1, and selected bond lengths and angles
Et3N (Scheme 1). Complex 1 conveniently precipitated from are provided in Table 2.
the reaction solution and was isolated upon filtration as a
Two phenolate O atoms and two amine N atoms of the
yellow powder. In contrast, 2 did not precipitate under the chiral bis(phenolate) ligand as well as the two terminal O
same conditions as 1 and was purified by flash chromatog- atoms form a distorted octahedral coordination sphere. In
raphy. From the orange fraction, crystalline yellow com- octahedral complexes, tetradentate ligands can in principle
pound 2 was isolated upon evaporation of the solvent. coordinate in two different ways, the so-called cis-α and cis-
Compound 1 can also be prepared by reaction of H2L1 β configurations (Figure 3).[12]
with [MoO2(acac)2] in methanol at a slightly elevated tem-
In the higher symmetry cis-α isomer, the coordination
perature (50 °C). Again, 1 precipitates from the reaction of the O atoms of the ligand results in a crystallographic
solution and can conveniently be isolated in 80% yield. derivative with C2 symmetry about an axis that passes
Both dioxidomolybdenum complexes 1 and 2 are soluble in through the Mo center and bisects the OOxo–Mo–OOxo and
organic solvents such as CHCl3, CH2Cl2, toluene, tetra- N–Mo–N angles. Thus, both phenolate O atoms occupy co-
hydrofuran (THF), and benzene. Interestingly, the new ordination sites trans to each other. In the case of the cis-β
complexes 1 and 2 were found to be very stable under ordi- isomer, the two OPhenolate donor atoms bind to cis coordina-
nary atmospheric conditions, whereas the diazepane com- tion sites, which thereby results in a lower symmetry species
plexes slowly decompose in air.[12] 1H and 13C NMR spec- (Ci). Both complexes 1 and 2 adopt the cis-α configuration.
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Table 1. Crystal data and structure refinement for [MoO2(L1)] (1)
and [MoO2(L2)]·(C7H8)3 (2).
All bond lengths including those of the Mo–Ooxo bonds
[1.701(2)–1.7091(13) Å] are very similar to those of pre-
viously reported Mo complexes with bis(phenolate) li-
gands.[12–14,19,21]
[MoO2(L1)] (1) [MoO2(L2)]·(C7H8)3 (2)
Empirical formula
Mr
Crystal system
Space group
a [Å]
b [Å]
c [Å]
α [°]
β [°]
C22H26MoN2O4 C97H140Mo2N4O8
478.39
triclinic
P1
1682.01
orthorhombic
P212121
9.2207(6)
30.6659(18)
31.9966(19)
90
7.7290(4)
10.0020(5)
13.6100(7)
100.460(2)
106.484(2)
90.978(2)
989.49(9)
100(2)
Epoxidation Catalysis
90
90
Complexes 1 and 2 were tested in epoxidation reactions
of selected olefins with tert-butylhydrogenperoxide (TBHP)
as the oxidizing reagent. The optimal reaction solvent and
temperature were determined by using the substrates cis-
cyclooctene and styrene with 1 as the catalyst (2 mol-%)
and 3 equiv. of TBHP. The results of the solvent and tem-
perature screening are summarized in Table 3.
γ [°]
V [Å3]
9047.4(10)
120(2)
0.71073
1.235
T [K]
Mo-Kα λ [Å]
Density [Mgm–3]
Z
0.71073
1.606
2
4
μ [mm–1]
F(000)
0.695
0.333
492
3592
No. of reflections collected 16395
139499
1.112
Goodness-of-fit on F2
1.027
Final R indices [I Ͼ 2σ(I)]
Table 3. Effect of solvent in the epoxidation of cyclooctene (1h)
and styrene (4h) at 55 °C.[a]
R1[a]
0.0188
0.0462
0.0458
0.0951
wR2[b]
Conversion [%][b]
Cyclooctene
R indices (all data)
R1[a]
0.0196
0.0466
0.0713
0.1086
Styrene
wR2[b]
Benzene
Chloroform
Toluene
65 (96)[c]
94
63
10 (60)[c]
20
12
[a] R1 = Σ||Fo| – |Fc||/ Σ|Fo|. [b] wR2 = Σw[(Fo – Fc ) /Σw[(Fo2)2]}1/2
.
2
2 2
Dichloroethane 18
22
Ͻ5
Ͻ5
Ͻ5
Table 2. Selected bond lengths [Å] and angles [°] for [MoO2(L1)] (1)
and [MoO2(L2)]·(C7H8)3 (2).
Acetonitrile
Ethanol
Methanol
THF
Ͻ5
Ͻ5
Ͻ5
Ͻ5
[MoO2(L1)] (1)
[MoO2(L2)]·(C7H8)3 (2)
Ͻ5
Mo1–O1
Mo1–O2
Mo1–O3
Mo1–O4
Mo1–N1
Mo1–N2
O1–Mo1–O2
O2–Mo1–N2
O3–Mo1–O4
O1–Mo1–N2
N1–Mo1–N2
1.7091(13)
1.7082(13)
1.9472(12)
1.9605(12)
2.3898(14)
2.3870(17)
107.96(7)
159.08(6)
159.08(6)
92.41(6)
1.703(2)
1.701(2)
1.9392(18)
1.9425(17)
2.395(3)
[a] Reaction conditions: 2 mol-% 1, 3 equiv. TBHP, 0.5 mL solvent.
[b] Conversions were determined by GC–MS measurements, n-dec-
ane was used as internal standard. [c] Reaction performed at 80 °C.
2.377(2)
108.50(11)
157.63(10)
160.21(9)
93.44(10)
72.47(9)
The catalytic activity was found to depend strongly on
the solvent used. The highest conversions were obtained
with chloroform. Lower yields were obtained in benzene
and toluene, whereas MeOH, EtOH, THF, and acetonitrile
gave no desired product. This is likely because these coordi-
nating solvents bind to the Mo center and block the site for
attack by TBHP.[22] In general, with the more challenging
olefin styrene, significantly lower yields were obtained at
50 °C (e.g., 10% in benzene; 20% in chloroform). At 80 °C,
styrene oxide was obtained selectively in 60% yield in benz-
ene. Hence, to explore the reactions of additional olefins,
further experiments were performed in benzene at 80 °C.
Furthermore, the effect of catalyst loading as well as the
amount of oxidant on the epoxidation of cyclooctene was
tested with catalyst 1. As demonstrated in Figure 4, the con-
version of cyclooctene is high (Ͼ80%) down to a catalyst
loading of 0.5 mol-%. At less than 0.5 mol-% catalyst load-
72.49(5)
Figure 3. Two possible isomers of octahedral ligands coordinated
by tetradentate ligands.
This configuration is interesting, as the bipyrrolidine back- ing, the conversion dropped significantly. The use of two
bones in 1 and 2 hold similar steric constraints to our 1,4- equivalents of TBHP resulted in higher yields of epoxide
diazepane ligands, which adopt the cis-β configuration.[12] that for the catalytic runs with equimolar amounts of
However, the cis-α configuration of 1 and 2 is similar to TBHP. Therefore, the optimal reaction conditions were
that of the previously published complex [MoO2(L4)] (Fig- found to be benzene at 80 °C, 0.5 mol-% catalyst, and
ure 1) and its derivatives with less steric constraints.[13,14]
two equiv. of TBHP.
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the substrates (Ͻ 10%). High conversions were obtained
with cyclooctene and cyclohexene within 1 h with both cat-
alysts 1 and 2, whereas lower conversions were obtained for
the more challenging olefins styrene and 4-phenyl-1-butene.
In the case of styrene, the formation of benzaldehyde (8–
10%) was observed with both catalysts. Notably, both cata-
lysts 1 and 2 exhibit comparable activities in the epoxid-
ation reactions (Table 4, Entries 1–4). For this reason, we
chose only catalyst 1 to explore epoxidation reactions
towards additional aliphatic and aromatic olefins (Table 4,
Entries 5–12).
Table 4. Epoxidation of aliphatic and aromatic alkenes with 1 and
2.[a]
Entry Substrate
Catalyst Conv. t [h]
Selectivity
[%]
[%]
Figure 4. Effect of catalyst loading on the epoxidation of cyclo-
octene with 1 (blue diamonds 1 equiv. TBHP, red rectangles
2 equiv. TBHP). Reaction conditions: 2 mol-% 1, 80 °C, benzene,
1h.
1
cyclooctene[b]
1
2
95
94
1
1
1
24
1
1
5
5
5
5
Ͼ99
Ͼ99
Ͼ99
Ͼ99
Ͼ99
Ͼ99
90
[MoO2(L3)] 51
[MoO2(L3)] 79
cyclohexene[b]
styrene[b]
1
2
1
2
1
2
1
1
95
92
67
63
67
64
59
79
To evaluate the stability of 1, three successive catalytic
cycles were performed with cyclooctene as the substrate
(Figure 5). To a solution of 1 (0.5 mol-%) in benzene
(0.5 mL) was added substrate (1 mmol) and TBHP
(2 mmol). After 1, 2, and 24 h, samples for GC–MS mea-
surements were taken and another aliquot of substrate
(1 mmol) and TBHP (2 mmol) were added to the reaction
mixture. As seen in Figure 5, the catalytic activity remained
unchanged over 24 h, and complete conversion to the epox-
ide was achieved after each cycle. Cyclooctene oxide was
the only product detected by GC–MS measurements.
3
4
92
4-phenyl-1-butene[b]
Ͼ99
Ͼ99
Ͼ99
Ͼ99
5
6
1-octene[b]
18
5
5-o-tolyl-2-pentene[b]
(mixture of cis/trans)
α-terpineol[b]
7
8
1
1
1
1
1
1
1
1
1
74
95[e]
33
5
18[e]
5
75
Ͼ99[e]
35
R-(+)-limonene[b]
60[e]
50[e]
26
5[e]
18[e]
18
5
Ͼ99[e]
50[e]
9
allyl phenyl ether[b]
cis-stilbene[c]
Ͼ99
Ͼ99
Ͼ99
Ͼ99
10
11
12
50
53
54
trans-stilbene[c]
allyl alcohol[d]
5
18
[a] Reaction conditions: 0.5 mol-% catalyst, 2 equiv. TBHP, benz-
ene, 80 °C. [b] Reaction yields of epoxides determined by GC–MS
measurements; decane was used as internal standard. [c] Reaction
yields of epoxides determined by HPLC/DAD measurements; me-
sitylene was used as internal standard. [d] Reaction yields of epox-
ides were determined by 1H NMR spectroscopy; dichloroethane
was used as internal standard. [e] Reaction was performed at 55 °C.
For comparison, the previously reported complex
[MoO2(L3)] containing the diazepane ligand[12] (Figure 1)
was tested in the catalytic epoxidation of cis-cyclooctene. It
showed a much lower activity than 1 and 2 under the same
conditions, as after 1 h only 51% conversion was observed
(Table 4, Entry 1). However, [MoO2(L3)] is poorly soluble
in various solvents including benzene and chloroform as we
Figure 5. Consecutive catalytic runs: 1 mmol cyclooctene, 2 mmol
TBHP, 0.5 mol-% catalyst 1, benzene, 80 °C. After 1 and 2 h,
1 mmol cyclooctene and 2 mmol TBHP were added. Samples for
GC–MS measurements were taken 60 minutes after the addition of
substrate/oxidant.
To evaluate the catalytic scope of 1 and 2, the substrates have previously observed. Thus, we also attribute the lower
cyclooctene, cyclohexene, styrene, and 4-phenyl-1-butene epoxidation activity to the poor solubility of [MoO2(L3)] in
were investigated, as summarized in Table 4. The sterically benzene even at 80 °C. Therefore, [MoO2(L3)] was not fur-
demanding tert-butyl substituents in the H2L2 ligand are ther tested as an epoxidation catalyst.
expected to result in a more stable complex and also to tune
Several aliphatic, cyclic, and aromatic olefins were inves-
the electronic properties and, hence, the catalytic behavior. tigated with catalyst 1 in the epoxidation reaction. The cata-
All reactions were performed under the optimized condi- lytic activity was found to be generally lower for the cyclic
tions in benzene (0.5 mL) at 80 °C with 0.5 mol-% catalyst, olefins, both when the olefin is attached to an aromatic sys-
1 mmol substrate, and 2 mmol TBHP as oxidant. Control tem (Table 4, Entries 3, 4, 6, 10, and 11) or to an aliphatic
experiments without catalyst 1 or 2 gave low conversion of olefin (Table 4, Entries 5 and 12), and lowest conversion
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was observed for allyl phenyl ether (Table 4, Entry 9), for with the current system. A more soluble compound with a
which only 26% of the epoxide was detected after 18 h. The cis-β configuration would be highly desirable.
catalyst showed excellent selectivity except for styrene
(selectivity 90%), terpineol (selectivity 75%), and limonene
Conclusions
(selectivity 35%). For the latter two, olefinic secondary
products (α-terpineol: 2-hydroxycineole; limonene: limo-
nene dioxide) were detected by GC–MS measurements
when the catalysis was performed at 80 °C. At 55 °C, lower
conversions were obtained, but the selectivity increased. α-
Terpineol was subsequently converted selectively to the cor-
responding epoxide in a 95% yield within 18 h. Limonene
gave a 60% conversion to the epoxide within 5 h (selectivity
Ͼ99%). However, after 18 h, a loss in selectivity (50%) was
observed as the formation of limonene dioxide (50%) took
place.
Compared to previously published systems, complexes 1
and 2 represent highly active catalysts.[2,6,23–25] For example,
related dioxidomolybdenum complexes containing (R,R)-
1,2-diaminocyclohexane-derived Schiff based ligands re-
quired 22 h to convert 65% cyclooctene and 11.8% of cis-
methylstyrene, respectively.[26] Mitchell and Finney reported
Chiral dioxidomolybdenum complexes of bis(phenolate)
ligands with the bipyrrolidine backbone have been isolated
and studied as epoxidation catalysts. The complexes exhibit
a distorted octahedral coordination geometry around the
molybdenum(VI) center with a cis-α configuration. They
are found to be significantly more stable and more soluble
than related complexes with a diazepane backbone with a
cis-β configuration. Complexes 1 and 2 are efficient and
selective catalysts in epoxidation reactions of various ole-
fins; however, they did not show enantioselectivity. Future
work is dedicated to the modification of the catalytic system
to increase the efficiency and, thus, obtain chiral induction
under ambient conditions and to deduce the influence of
the geometry.
a
dioxidomolybdenum(VI) complex containing
a
Experimental Section
monoanionic tridentate ligand as catalysts for the epoxid-
ation of aliphatic olefins in 75–90% yields, but 28 h were
needed.[24] Gonçalves and co-workers have found a highly
active pyrazolate-based dioxidomolybdenum catalyst for
the epoxidation of cis-cyclooctene with TBHP, but more
challenging substrates were not investigated.[25] In the epox-
idation of styrene to styrene oxide catalyzed by 1 and 2,
the obtained yields are good,[16,27] and the selectivities are
superior to those of many other systems.[17,27,28]
Unfortunately, asymmetric induction was found to be
negligible with both 1 and 2 at room temperature and at 55
and 80 °C. To achieve high enantioselectivity in Mo-cata-
lyzed epoxidation is challenging. Several chiral catalytic
molybdenum complexes have been investigated,[6] but a sys-
tem that exhibits significant chiral induction was only re-
cently reported by Yamamoto and co-workers. They have
developed an asymmetric catalytic oxidation of olefins with
up to 92%ee by using an in situ generated catalyst from
[MoO2(acac)2] and chiral bishydroxamic acids (BHA).[7,8]
We have previously shown [MoO2(L3)] to be a better cat-
alyst for the oxygen atom transfer (OAT) reaction from di-
methyl sulfoxide to trimethylphosphane than the cis-α com-
pound [MoO2(L4)]. We attributed this to the cis-β configu-
ration, which leads to a structural situation in which one
oxo O atom is located trans to a phenolate O atom and the
General: (S,S)-2,2Ј-Bipyrrolidine d-tartrate trihydrate, 2-hydroxy-
benzaldehyde, 2-hydroxy-3,5-di-tert-butylbenzaldehyde, sodium
cyanotrihydroborate and [MoO2Cl2] (Aldrich) were used as re-
ceived, unless noted otherwise. All solvents were dried by a solvent
purification system from Innovative Technology inc. and flushed
with argon prior to use. All deuterated solvents were purchased
from Deutero GmbH and dried with molecular sieves. Celite and
aluminum oxide were purchased from commercial sources and
dried in vacuo at 300 °C. NMR spectra were recorded at 300 MHz
with a Bruker AMX 360 spectrometer. Chemical shift values are
given in parts per million (ppm) and are referenced to residual pro-
tons in the solvent. Electron impact mass spectra were measured
with an Agilent 5973 MSD-Direct Probe instrument. Elemental
analyses were performed with a Heraeus Vario Elementar auto-
matic analyzer at the Institute of Inorganic Chemistry, Graz Uni-
versity of Technology, Austria. IR spectra were recorded with a
Bruker alpha FTIR spectrometer with the use of attenuated total
reflectance (ATR) as the sampling technique. GC–MS analyses
were performed with an Agilent 7890A GC system coupled to a
5975C inert XL EI/CI mass selective detector (MSD) equipped
with an Agilent 19091J-433 column (30 mϫ0.32 mmϫ2.5 mm)
and He as the mobile phase. HPLC with diode array detection
(HPLC/DAD) analyses were performed with an Agilent 1200 series
instrument equipped with
(4.6ϫ150 mm, 5 μm) column with acetonitrile/water (65:35 v/v) as
the mobile phase.
a
Zorbax Eclipse XDB-C18
other to an amine N atom. Thus, the faster oxygen atom Synthesis of Ligands: To a solution of the 2-hydroxybenzaldehyde
derivative (4.0 mmol) in methanol (20 mL) were added (S,S)-2,2Ј-
bipyrrolidine tartrate (0.69 g, 2.0 mmol) and a small amount of
acetic acid. Sodium cyanotrihydroborate (0.38 g, 6.0 mmol) in
methanol (5 mL) was added dropwise to the stirred solution. After
the solution had been stirred for 3 days at 25 °C, it was acidified
by the addition of conc. HCl and then evaporated almost to dry-
ness under reduced pressure. The residue was dissolved in saturated
aqueous Na2CO3 solution (20 mL) and extracted with CHCl3 (3ϫ
20 mL). The combined extracts were dried with anhydrous Na2SO4
and filtered. This crude product was purified by flash chromatog-
transfer can be explained by an activation of the Mo=O
bond owing the different trans effect imposed by the phen-
olate group rather than the amine in the asymmetric cis-β
configuration. Theoretical calculations support the mech-
anistic idea that the trans O–Mo=O oxo atom is actually
transferred.[12] In [MoO2(L4)], as well as in 1 and 2, both
oxo groups are located trans to a N atom. The low solubil-
ity of [MoO2(L3)] prevented us from fully investigating its
catalytic activity. Thus, the contribution of the geometry to
the activity in epoxidation reactions cannot be deduced raphy with an ethyl acetate/cyclohexane mixture (8:2), and evapora-
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/KAP1
Date: 05-06-13 18:11:35
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FULL PAPER
tion of the fraction gave a white pasty material. Recrystallization
from CH2Cl2 at room temperature gave the ligands as white solids.
(s, Mo=O), 927 (s, Mo=O), 1236 (s), 1440 (s), 1467 (s), 2950 (w).
C38H58MoN2O4 (704.34): calcd. C 64.94, H 8.32, N 3.99; found C
64.88, H 8.68, N 3.60. Single crystals suitable for X-ray diffraction
analysis were obtained from a toluene solution of 2 at room tem-
perature.
1,4-Bis(2-hydroxybenzyl)-(S,S)-2,2Ј-bipyrrolidine
[(S,S)-H2L1]:
Yield 0.84 g (49%). 1H NMR (300 MHz, CDCl3): δ = 8.84 (s, 2
H), 7.17 (m, 2 H), 6.98 (m, 2 H), 6.80 (m, 4 H), 4.24 (d, J = 13.7 Hz,
2 H), 3.40 (d, J = 13.7 Hz, 2 H), 3.09 (m, 2 H), 2.95 (m, 2 H), 2.26
(m, 2 H), 2.04 (m, 2 H), 1.81 (m, 6 H) ppm. 13C NMR (75 MHz,
CDCl3): δ = 158.0, 129.3, 128.5,122.8, 119.8, 116.5 (Ar-C), 65.4
(CH), 58.6 (CH2), 55.4 (CH2), 25.8 (CH2), 24.3 (CH2) ppm.
C22H28N2O2 (352.48): calcd. C 74.97, H 8.01, N 7.95; found C
75.11, H 8.07, N 7.67.
General Procedure for Epoxidation: The epoxidation of alkenes cat-
alyzed by molybdenum complex 1 or 2 with tert-butylhydrogenper-
oxide (TBHP) was performed according to the following general
procedure: a mixture of catalyst (0.5 mol-%), benzene (0.5 mL),
and alkene (1 mmol) was placed in a screw-capped glass vial. A
TBHP solution (2 mmol) in n-decane was then added, and the re-
sulting mixture was stirred at 80 °C. Aliquots were taken periodi-
cally from the reaction mixture, quenched with MnO2, and diluted
with ethyl acetate. The reaction progress was monitored by GC–
MS, HPLC, or 1H NMR spectroscopy, and the amount of substrate
converted as well as the products obtained were expressed relative
to an internal standard (n-decane, dichloroethane, mesitylene). The
products were identified by comparison of available standards. If
authentic samples were not readily available, the products were
identified by GC–MS analyses.
1,4-Bis(2-hydroxy-3,5-di-tert-butylbenzyl)-(S,S)-2,2Ј-bipyrrolidine
1
[(S,S)-H2L2]: Yield 0.76 g (33%). H NMR (300 MHz, C6D6): δ =
10.67 (s, 2 H), 7.51 (d, J = 2.4 Hz, 2 H), 6.93 (d, J = 2.4 Hz, 2 H),
3.79 (d, J = 13.4 Hz, 2 H), 3.07 (d, J = 13.4 Hz, 2 H), 2.79 (m, 2
H), 2.47 (m, 2 H), 1.80–1.38 (m, 5 H), (s, 18 H), 1.45–1.21 (m, 5
H), 1.38 (s, 18 H) ppm. 13C NMR (75 MHz, C6D6): δ = 154.9,
141.0, 136.0, 123.2, 123.0, 122.7 (Ar-C), 65.3 (CH), 59.5 (CH2),
54.9 (CH2), 35.4 (CH2), 34.4 (CH2), 32.0 [C(CH3)3], 30.0
[C(CH3)3], 25.7 (CH2), 24.0 (CH2) ppm. Resonances in CDCl3 are
consistent with literature data.[18]
Single-Crystal X-ray Data Collection and Structure Solution: Single
crystals of 1 and 2 of suitable size were selected from the mother
liquor, immersed in paraffin oil, and then mounted on the tip of a
glass fiber. The data were collected by using Mo-Kα (λ = 0.71073 Å)
radiation and a Bruker SMART APEX 2 diffractometer equipped
with a CCD area detector at 100 K. The crystallographic data are
collected in Table 1. The APEX II[29] program was used to collect
frames of data, index reflections, and determine the lattice param-
eters; the SAINT program[29] was used for integration of the inten-
sity of reflections and scaling; the SADABS program[30] was used
for absorption and Lorrentz polarization correction; and the
WINGX suite of programs[31] was used for space group and struc-
ture determination, and least-squares refinements on F2. The struc-
tures were solved by direct methods (SIR-92).[32] The non-hydrogen
atoms were located in successive difference Fourier syntheses. The
final refinement was performed by full-matrix least-squares analy-
sis. Hydrogen atoms attached to the ligand moiety were fixed in
calculated positions and allowed to refine isotropically. CCDC-
830131 and -830132 contain the supplementary crystallo-
graphic data for this paper. These data can be obtained free of
charge from the Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif.
[MoO2(L1)] [(S,S)-1]: The complex was prepared by adding an ace-
tonitrile solution (3 mL) of (S,S)-H2L1 (0.176 g, 0.5 mmol) in the
presence of two equiv. of Et3N (0.10 g, 140 μL, 1.0 mmol) to a solu-
tion of [MoO2Cl2] (0.099 g, 0.5 mmol) in acetonitrile (2 mL). The
mixture was stirred for 6 h, and a yellow precipitate formed. The
precipitate was collected by filtration, washed with small amounts
of acetonitrile, and dried in vacuo to yield 1 (0.170 g, 71%). Alter-
natively, complex 1 can also be prepared by the addition of a meth-
anol solution (5 mL) of (S,S)-H2L1 (0.352 g, 1 mmol) to a solution
of [MoO2(acac)2] (0.326 g, 1 mmol) in MeOH (5 mL). The yellow
solution was stirred at 50 °C, and a yellow precipitate formed
within 10 min. The solid was collected by filtration and washed
1
with hot methanol to yield 1 (0.383 g, 80%). H NMR (300 MHz,
CDCl3): δ = 7.22 (m, 2 H), 6.96 (m, 2 H), 6.85 (m, 4 H), 5.02 (d,
J = 14.3 Hz, 2 H), 3.62 (d, J = 14.3 Hz, 2 H), 3.53 (m, 2 H), 3.17
(m, 2 H), 2.82 (m, 2 H), 2.11 (m, 2 H), 1.90 (m, 2 H), 1.74 (m, 2
H), 1.43 (m, 2 H) ppm. 13C NMR (75 MHz, CDCl3): δ = 159.3,
129.6, 129.5, 122.4, 120.5, 118.8 (Ar-C), 63.6 (CH), 60.4 (CH2),
54.9 (CH2), 24.0 (CH2), 20.8 (CH2) ppm. MS (EI): m/z (%) = 480
(100) [M]+. IR: ν = 497 (s), 564 (s), 754 (s), 888 (s, Mo=O), 926 (s,
˜
Mo=O), 1262 (s), 1449 (s), 1482 (s), 2959 (w) cm–1. C22H26MoN2O4
(478.39): calcd. C 55.23, H 5.48, N 5.86; found C 55.11, H 5.43,
N 5.86. Single crystals suitable for X-ray diffraction analysis were
obtained from a benzene solution at room temperature.
Supporting Information (see footnote on the first page of this arti-
1
cle): H NMR spectra of (S,S)-1 and (S,S)-2.
Acknowledgments
[MoO2(L2)] [(S,S)-2]: The complex was prepared by adding an ace-
tonitrile solution (3 mL) of (S,S)-H2L2 (0.288 g, 0.5 mmol) in the
presence of two equiv. of Et3N (0.10 g, 140 μL, 1.0 mmol) to a solu-
tion of [MoO2Cl2] (0.099 g, 0.5 mmol) in acetonitrile (2 mL). The
mixture was stirred for 6 h, and an orange solution was obtained.
The pure compound was isolated by flash chromatography with an
ethyl acetate/cyclohexane mixture (8:2). The orange fractions were
collected, and the solvents were evaporated in vacuo to yield 2 as
a yellow crystalline material (0.218 g, 62% yield). 1H NMR
(300 MHz, C6D6): δ = 7.56 (d, J = 2.39 Hz, 2 H), 6.82 (d, J =
2.37 Hz, 2 H), 4.89 (d, J = 14.4 Hz, 2 H), 3.42 (m, 2 H), 3.21 (d, J
= 14.4 Hz, 2 H), 2.86 (m, 2 H), 2.69 (m, 2 H), 1.67 (s, 18 H), 1.55
(m, 2 H), 1.38 (m, 6 H), 1.33 (s, 18 H) ppm. 13C NMR (75 MHz,
C6D6): δ = 158.0, 142.1, 138.5, 124.4, 124.3, 122.5 (Ar-C), 64.1
(CH), 61.3 (CH2), 55.9 (CH2), 35.7 (CH2), 34.5 (CH2), 31.9
[C(CH3)3], 30.8 [C(CH3)3], 23.9 (CH2), 20.5 (CH2) ppm. MS (EI):
Financial support by the Austrian Science Foundation (FWF)
(P19309-N19) is gratefully acknowledged.
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Published Online: ᭿
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