Non-heme manganese complexes of C1-symmetric N4 ligands: Synthesis, characterization and asymmetric epoxidations of α,β-enones

Article abstract of DOI:10.1016/j.jorganchem.2012.03.034

A series of proline-derived C₁-symmetric chiral tetradentate N ligands and their manganese complexes have been synthesized and characterized. Their applications in enantioselective epoxidations of α,β-enones were explored reaching good yields and up to 71% ee at room temperature.

Full text of DOI:10.1016/j.jorganchem.2012.03.034

Journal of Organometallic Chemistry 715 (2012) 9e12
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Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
Non-heme manganese complexes of C1-symmetric N4 ligands: Synthesis,
characterization and asymmetric epoxidations of a,b-enones
,
,
a
a,
**
Bin Wang a b, Cheng-Xia Miao a, Shou-Feng Wang a, Fritz E. Kühn c , Chun-Gu Xia , Wei Sun
*
a State Key Laboratory for Oxo Synthesis and Selective Oxidation, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou 730000, P. R. China
b Graduate School of the Chinese Academy of Sciences, Beijing 100039, P. R. China
c Chair of Inorganic Chemistry and Molecular Catalysis, Catalysis Research Center, Technische Universität München, Garching 85747, Germany
a r t i c l e i n f o
a b s t r a c t
Article history:
A series of proline-derived C1-symmetric chiral tetradentate N ligands and their manganese complexes
Received 28 February 2012
Received in revised form
24 March 2012
have been synthesized and characterized. Their applications in enantioselective epoxidations of
enones were explored reaching good yields and up to 71% ee at room temperature.
a,b-
Ó 2012 Elsevier B.V. All rights reserved.
Accepted 30 March 2012
Keywords:
Epoxidation
a,b-Enone
N ligand
Manganese
1. Introduction
alkenes have been developed and many of these transformations
rely on chiral metal complexes in the presence of different oxygen
donors, such as salenemetal complexes or their derivates [16e20].
Tetradentate nitrogen based ligands (N4) and their metal
complexes have been drawing increasing attention. These
compounds have been used in both epoxidation, oxidations of
unactivated sp3 CeH bonds and other related reactions [21e34]. A
methane monooxygenase (MMO) model system, including an iron
complex of the MEP (MEP ¼ N,N0-dimethyl-N,N0-bis(2-pyridy-
lmethyl)ethylene-1,2-diamine) ligand and aqueous H2O2 were
originallyused byJacobsenet al. forepoxidations [21]. Subsequently,
Stack et al. described manganese complexes of chiral N4 ligands (S,S-
Catalytic asymmetric epoxidations are of high interest for the
preparation of non-racemic chiral intermediates in both pharma-
ceutical and chemical industry to generate enantiomerically pure
products [1e12]. In chemical industry mainly three areas are in
particular need for chiral compounds: flavor and aroma chemicals,
agricultural chemicals and speciality materials. Pharmaceuticals
are increasingly subject to numerous government regulations that
demand enantiomerically pure incredients, due to the sometimes
very different biological activities of the enantiomers. In 1979,
Schurig et al. achieved enantiomeric epoxidation of prochiral alkyl-
substituted olefins with a Mo(VI) complex bearing a chiral ligand.
The enantioselectivity, however, was low [13]. In 1980, Katsuki and
Sharpless reported on the asymmetric epoxidation of allylic alco-
hols, mediated by a titanium (IV) complex, using (þ)-(R,R)- or
(ꢀ)-(S,S)-diethyl tartrate as chiral ligands. The enantioselectivity
was very high, however, the titanium complex had to be applied in
stoichiometric amounts [14]. Later, a reduction of the catalyst:
substrate ratio was achieved and an X-ray structure of the titatium
tartrate catalyst could be obtained [15]. In the last decades, several
procedures for the synthesis of asymmetric epoxides starting from
MCP, MCP
¼
N,N0-dimethyl-N,N0-bis(2-pyridylmethyl)-cyclo-
hexane-1,2-diamine) in combinationwith peracetic acid (AcOOH) as
very fast and efficient epoxidation agents [22]. However, the
observed enantiomeric excesses (ee) were low. Besides, a series of
chiral N4 ligands containing two oxazoline rings instead of pyridine
rings were prepared by Pfaltz et al., and their manganese complexes
also only exhibited low enantioselectivities in olefin epoxidation
[25]. Costas et al. demonstrated that manganese complexes with
chiral tetradentate N4 ligands (S,S,R-MCPP) derived from (R,R-mcp)
by fusing pinene rings can promote the epoxidation reaction with
modest enantiomeric excess [26]. An ee value of 37% was obtained
when chalcone was chosen as the substrate.
Such biologically inspired epoxidation catalysts in principle hold
great promise for synthetic applications. However, more work has
to be devoted to biomimetic systems, especially for designing new
* Corresponding author. Fax: þ49 89 289 13473.
** Corresponding author. Tel.: þ86 931 4968 278; fax: þ86 931 8277 088.
E-mail addresses: fritz.kuehn@ch.tum.de (F.E. Kühn), wsun@licp.cas.cn (W. Sun).
0022-328X/$ e see front matter Ó 2012 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2012.03.034

10
B. Wang et al. / Journal of Organometallic Chemistry 715 (2012) 9e12
ligand frameworks. Our group has introduced aromatic groups into
both 2-pyridylmethyl positions of the ligands (R,R-BPMCP) (Fig. 1)
[28]. With 1 mol % of manganese catalyst loading, the enantiose-
lecive epoxidation of olefins proceeds to nearly complete conver-
sion and enantiometic excess values of up to 89%. The respective
iron(II) complexes also show good performance in the same reac-
tion with up to 87% ee [35]. Recently, Talsi and co-workers (S,S-
BPBP) [36] and Costas et al. (S,S,ReBPBPP or R,R,ReBPBPP) (Fig. 1)
[37] disclosed that manganese complexes of N4 ligands composed
of a bipyrrolidine backbone and different pyridine donors are very
effective for the asymmetric epoxidation of olefins, respectively.
As a part of our research on oxidation catalyzed by non-heme
metal complex of N4 ligands [28,35,38], we report on the
synthesis of a series of C1esymmetric N4 ligands derived from
proline and various pyridine donors. Further, catalytic efficiency of
the corresponding manganese complexes of N4 ligands has been
chloromethylpyridine
hydrochloride
or
2-chloromethyl-6-
methylpyridine hydrochloride (2 mmol) and tetrabutylammo-
nium bromide, TBABr (0.04 g) were added directly as solids and the
resulting mixture was heated at reflux under Ar for 24 h. After
cooling to room temperature, the red mixture was filtered and the
filter cake was washed with CH2Cl2. The combined filtrates were
evaporated under reduced pressure. To the resulting residue, 1 M
NaOH (17.5 mL) was added and the mixture was extracted with
CH2Cl2 (4 ꢁ 15 mL). The combined organic phases were washed
successively with saturated aqueous solutions of NaHCO3, NaCl, and
finally H2O. The organic phase was dried over anhydrous Na2SO4
and the solvent was removed under reduced pressure to yield the
ligands L1eL4.
L1 [39]. 1H NMR (DMSO-d6, 400 MHz):
d 8.47 (m, 2H), 7.76e7.69
(m, 2H), 7.40 (dd, J ¼ 7.6 Hz, 2H), 7.26e7.20 (m, 2H), 4.25 (d,
J ¼ 14 Hz, 1H), 3.60 (s, 2H), 3.45 (d, J ¼ 14 Hz, 1H), 2.86e2.70 (m,
2H), 2.54e2.47 (m, 1H), 2.31 (dd, J ¼ 12.4, 7.2 Hz, 1H), 2.20 (s, 3H),
2.18e2.13 (m, 1H), 1.94e1.85 (m, 1H), 1.70e1.48 (m, 3H). 13C NMR
investigated in the asymmetric epoxidation of
peracetic acid as oxidant.
a,b-enones with
(DMSO-d6, 100 MHz):
d 159.87, 159.27, 148.88, 148.56, 148.53,
136.29, 122.60, 122.42, 121.96, 121.80, 64.11, 62.22, 61.08, 60.47,
54.28, 43.05, 29.55, 22.40.
2. Experimental
a 20
L2. ½ ꢂD ¼ ꢀ54.0 (c ¼ 0.1, CHCl3); 1H NMR (CDCl3, 400 MHz):
2.1. General information
d
8.53 (m, 1H), 7.65e7.60 (m, 1H), 7.49 (t, J ¼ 7.6 Hz, 1H), 7.40 (d,
1H and 13C NMR spectra were recorded on a Bruker Avance III
400 MHz spectrometer. The chemical shifts ( ) are reported in ppm
and coupling constants (J) in Hz. GCeMS were measured on Agilent
6890/5973 N. High resolution mass spectra (HRMS) were obtained
on a Bruker Daltonics APEX II 47e FT-ICR mass spectrometer or
a Bruker Daltonics microTOF-QII mass spectrometer. HPLC analysis
J ¼ 7.6 Hz, 1H), 7.25 (d, J ¼ 7.6 Hz, 1H), 7.16e7.10 (m, 1H), 6.98 (d,
J ¼ 7.6 Hz,1H), 4.36 (d, J ¼ 13.6 Hz,1H), 3.65 (dd, J ¼ 29.6, 14 Hz, 2H),
3.52 (d, J ¼ 14 Hz, 1H), 3.05e2.75 (m, 2H), 2.61 (dd, J ¼ 12.4, 4.8 Hz,
1H), 2.52 (s, 3H), 2.44(dd, J ¼ 12.4, 7.6 Hz, 1H), 2.35e2.20 (m, 4H),
2.05e1.95 (m, 1H), 1.76e1.60 (m, 3H). 13C NMR (100 MHz, CDCl3):
d
d
159.98, 159.07, 157.43, 148.98, 136.55, 136.31, 123.07, 121.77, 121.31,
119.86, 64.85, 62.90, 62.12, 61.35, 54.89, 43.47, 30.12, 24.46, 22.73.
HRMS (ESI-MS) calcd. for C19H27N4 [M þ H]þ: 311.2230, found:
311.2225.
was performed on Waters-Breeze (2487 Dual
l Absorbance
Detector and 1525 Binary HPLC Pump). Chiralpak OD, AD, OJ, AS
were purchased from Daicel Chemical Industries, LTD. Column
chromatography was generally performed on silica gel (200e300
mesh) and TLC inspections were on silica gel GF254 plates. All
reactions were carried out under argon in dried glassware. All
chemicals and solvents were used as received unless otherwise
stated. Diethyl ether (Na, benzophenone), acetonitrile (CaH2) were
distilled under argon prior to use. 4a and 4b were synthesized from
L3. ½a 2D0
ꢂ
¼ ꢀ57.0 (c ¼ 0.1, CHCl3); 1H NMR (CDCl3, 400 MHz):
d
8.51 (m, 1H), 7.65e7.58 (m, 1H), 7.52 (t, J ¼ 7.6 Hz, 1H), 7.44 (d,
J ¼ 8.0 Hz, 1H), 7.25(d, J ¼ 7.6 Hz, 1H), 7.16e7.11 (m, 1H), 7.0 (d,
J ¼ 7.6 Hz, 1H), 4.32 (d, J ¼ 14 Hz, 1H), 3.74e3.50 (m, 3H), 3.10e2.80
(m, 2H), 2.64e2.58 (m, 1H), 2.53 (s, 3H), 2.50e2.41 (m, 1H),
2.40e2.31 (m, 1H), 2.27 (s, 3H, CH3N), 2.06e1.97 (m, 1H), 1.76e1.60
(m, 3H). 13C NMR (100 MHz, CDCl3):
d 159.66, 158.95, 157.47, 148.89,
(S)-2-aminomethyl-1-N-Cbz-pyrrolidine
and
pyridine-2-
136.59, 136.33, 123.07, 121.84, 121.38, 120.04, 64.72, 62.70, 62.09,
62.21, 54.82, 43.31, 30.05, 24.49, 22.82. HRMS (ESI-MS) calcd. for
C19H27N4 [M þ H]þ: 311.2230, found: 311.2236.
carboxaldehyde or 6-methylpyridine-2-carboxaldehyde (For
details see supporting information).
L4. ½a 2D0
ꢂ
¼ ꢀ57.0 (c ¼ 0.1, CHCl3); 1H NMR (CDCl3, 400 MHz):
2.2. Synthesis of ligands L1eL4
d
7.53e7.45 (m, 2H), 7.29e7.21 (m, 2H), 6.98 (dd, J ¼ 7.6, 3.6 Hz, 2H),
4.29 (d, J ¼ 14 Hz, 1H), 3.82e3.47 (m, 3H), 3.03e2.70 (m, 2H),
2.60e2.45 (m, 7H), 2.44e2.35 (m,1H), 2.33e2.16 (m, 4H), 2.03e1.94
4a or 4b (2 mmol) and 10 mL of anhydrous acetonitrile were
added to a 25 mL flask. Then Na2CO3 (0.87 g, 0.82 mmol), 2-
(m, 1H), 1.75e1.60 (m, 3H). 13C NMR (100 MHz, CDCl3):
d 159.64,
159.40, 159.22, 157.40, 136.52, 136.50, 121.26, 121.19, 119.82, 119.64,
64.97, 63.01, 61.99, 61.47, 54.93, 43.46, 30.14, 24.51, 24.47, 22.81.
HRMS (ESI-MS) calcd. for C20H29N4 [M þ H]þ: 325.2387, found:
325.2383.
2.3. Synthesis of complexes C1eC4
Under an Ar atmosphere, Mn(CF3SO3)2 (0.1 mmol) was added to
a stirred solution of chiral ligand L1eL4 (0.1 mmol) in acetonitrile
(3 mL) or acetonitrile and dichloromethane (3 mL, v/v ¼ 2 : 1). The
reaction mixture was stirred for 2 h and the organic solvent was
evaporated under reduced pressure. The resulting solid was
washed several times with diethyl ether and dried under vacuum to
give the desired complexes.
C1 [40]: [ Mn(OTf)2MPP]. ESI-HRMS calcd. for C19H24F3MnN4O3S
[M-OTf]þ: 500.0896, found: 500.0880. Anal. Calcd. for
C20H24F6MnN4O6S2, C, 36.99; H, 3.72; N, 8.63. found C, 37.04; H,
3.79; N, 8.54.
Fig. 1. Selected ligands for the Mn-catalyzed steroselective epoxidation of alkenes.

B. Wang et al. / Journal of Organometallic Chemistry 715 (2012) 9e12
11
C2: [Mn(OTf)2 H,MeMPP]. ESI-HRMS calcd. for C20H26F3MnN4O3S
[M-OTf]þ: 514.1053, found: 514.1048. Anal. Calcd. for C21H26F6
MnN4O6S2$1.2CH2Cl2, C, 34.83; H, 3.74; N, 7.32. found C, 34.63; H,
3.77; N, 7.51.
C3: [Mn(OTf)2 Me,HMPP]. ESI-HRMS calcd. for C20H26F3MnN4O3S
[M-OTf]þ: 514.1053, found: 514.1050. Anal. Calcd. for
C21H26F6MnN4O6S2, C, 38.01; H, 3.95; N, 8.44. found C, 38.04; H,
3.99; N, 8.49.
C4: [Mn(OTf)2 Me,MeMPP]. ESI-HRMS calcd. for C21H28F3
MnN4O3S [M-OTf]þ: 528.1191, found: 528.1192. Anal. Calcd. for
C22H28F6MnN4O6S2$CH3CN, C, 40.11; H, 4.35; N, 9.75. found C,
40.15; H, 4.39; N, 9.80.
2.4. Representative procedure for an asymmetric epoxidation of 1,3-
diphenyl-propenone
The chiral complex C1 (0.25 ꢁ 10ꢀ2 mmol) and 1,3-diphenyl-
propenone (0.25 mmol) were added to CH3CN (1.0 mL) under an Ar
atmosphere and the solution was stirred for 3 min. Then 1.2 equiv
of 10% AcOOH (228 mg, 0.3 mmol, diluted with 0.5 mL of CH3CN)
was added dropwise over 20 min and the mixture was stirred at
room temperature for 60 min. After the reaction, the crude product
was purified by chromatography on silica gel (PET/EtOAc ¼ 40:1) to
afford the epoxide product in 82% yield.
3. Results and discussion
Scheme 1. Preparation of ligands L1eL4.
In general, linear N4 ligands such as MEP or MCP contain two
pyridine donors as well as two sp3 N donors. These ligands display
C2-symmetry. Their iron or manganese complexes are capable of
mimicking the chemistry of non-heme iron enzymes. Que et al.
have found that iron complexes of N4 ligands (6-Me2-MCP, and 6-
Me2-BPBP) can efficiently catalyze asymmetric cis-dihydrox-
ylation with good enantioselectivity. The 6-methyl substituents on
pyridine rings play an important role for the selectivity of cis-
dihydroxylation and enantioselectivity [29,33]. Meanwhile, 6-
methyl substituents on pyridine rings may have a significant
effect on the asymmetric epoxidation of olefins. The (S)-2-
aminomethylpyrrolidine derived from naturally occurring proline
is thought to be a suitable diamine backbone due to its C1-
symmetry. To achieve this goal, the ligands L1eL4 as shown in
Scheme 1 were designed and synthesized. The synthetic route is
depicted in Scheme 1. Reductive amination of (S)-2-aminomethyl-
1-N-Cbz-pyrrolidine with pyridine-2-carboxaldehyde or 6-
methylpyridine-2-carboxaldehyde gave intermediates 2a and 2b,
which was used in an Eschweiler-Clarke methylation to yield the
intermediates 3a and 3b without further purification. Subse-
quently, the Cbz protecting group in 3a or 3b was removed using
a Pd-catalyzed hydrogenolysis. Finally, the desired ligands L1eL4
were obtained in good yields, following standard procedures for
the alkylation of substituted pyrrolidine backbones. The corre-
sponding manganese complexes of general formula [Mn(OTf)2-
(R,R’MPP)] C1eC4 were obtained by reaction of each ligand with
Mn(CF3SO3)2 in CH3CN under an argon atmosphere (Scheme 1).
Generally, this type of N4 ligands coordinate the manganese center
having no substituent on the pyridine groups exhibits higher
asymmetric induction (69%) (Table 1, entry 1), which is even higher
than that of Stack’s catalyst [Mn(OTf)2-(R,R-MCP)] involving chiral
cyclohexane-1,2-diamine (43%) (Table 1, entry 1 vs 7). Additionally,
the oxidant and reaction temperature were examined using C1 as
the catalyst. When H2O2 was used as oxidant, 1,3-diphenyl-pro-
penone was epoxidized with slightly lower yield and similar
enantioselectivity (Table 1, entry
1 vs 2) [28,40]. However,
a comparable enantioselectivity was obtained after lowering the
reaction temperature by prolonging the reaction time to 34 h
(Table 2, entries 3 vs 1). These results clearly indicate the ability of
enantioselective induction being inhibited by the 6-methyl
substituents on the pyridine rings.
Encouraged by these results, we applied the optimized catalyst
system for the asymmetric epoxidation of a,b-enones. The results
are summarized in Table 2. In most of the examples,
Table 1
Screening of catalysts and reaction conditions.a
Entry
Catalyst
Yield (%)b
ee (%)c
1
C1
C1
C1
C2
C3
C4
82
73
80
52
60
61
76
69
68
75
12
0
2d
3e
4
in a cis-a topology [22,26,28,40].
The catalytic experiments were started by testing the activity of
compounds C1-C4 and screening the epoxidation conditions with
1,3-diphenyl-propenone as model substrate, using peracetic acid as
oxidant (Table 1). All examined catalysts show decent catalytic
activities under the conditions applied. However, a noteworthy
dependence of the catalytic activity and the asymmetric induction
from the structure of the ligands was observed. The newly designed
catalysts C2-C3 with substituted pyridine groups exhibit medium
activities and low ees (Table 1, entries 4e6), while catalyst C1
5
6
7
5
43
R,R-MCPMn(OTf)2
a
Unless otherwise stated, reactions were carried out in 1.5 mL MeCN for 60 min
at room temperature with 0.25 mmol of chalcone, 1 mol% catalyst, 1.2 equiv of
peracetic acid.
b
Isolated yields.
Determined by HPLC analysis (Daicel chiral OD-H column).
6 equiv of H2O2 and 5 equiv of AcOH.
ꢀ20 ꢃC, 34 h.
c
d
e

12
B. Wang et al. / Journal of Organometallic Chemistry 715 (2012) 9e12
Table 2
Appendix A. Supplementary material
Asymmetric epoxidation of a,b
-enones catalyzed by C1.a
Supplementary material associated with this article can be
found, in the online version, at doi:10.1016/j.jorganchem.2012.03.
034.
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Ph
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Ph
Ph
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85 (14c)
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63 (14e)
91 (14f)
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75 (14i)
70 (14j)
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64
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p-NO2eC6H4
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p-FeC6H4
p-CleC6H4
o-CleC6H4
o-BreC6H4
p-FeC6H4
9
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12
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ꢀ
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b
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c
Determined by HPLC analysis (see supporting information).
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62e93% yield and 60e71% ee. It was found that the substituted
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low ee (Table 2, entry 2). However, the substrate displaying a p-Cl
group gives the best result (Table 2, entry 4). Substitutions on the
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We are grateful for financial support from the “Hundred Talents
Program” of the Chinese Academy of Sciences and National Natural
Science Foundation of China (21073210, 21133011 and 20873166).