Chiral Schiff base ligated dioxomolybdenum (VI) complexes and their asymmetric catalytic properties in the epoxidation of styrene

Article abstract of DOI:10.3184/174751912X13457309578443

Two series of Schiff base dioxomolybdenum (VI) complexes of the general formula [MoO2(L)(MeOH)] (L = D-glucosamine- derived Schiff base) have been synthesised, characterised and their asymmetric catalytic activities evaluated through the epoxidation of st

Full text of DOI:10.3184/174751912X13457309578443

584 RESEARCH PAPER
OCTOBER, 584–586
JOURNAL OF CHEMICAL RESEARCH 2012
Chiral Schiff base ligated dioxomolybdenum (VI) complexes and their
asymmetric catalytic properties in the epoxidation of styrene
Yan Sui*, Dongsheng Liu, Ronghua Hu and Xiaobo Que
School of Chemistry and Chemical Engineering, Jiangxi Province Key Laboratory of Coordination Chemistry, Jinggangshan University,
Ji’an Jiangxi 343009, P. R. China
Two series of Schiff base dioxomolybdenum (VI) complexes of the general formula [MoO2(L)(MeOH)] (L = D-glucos-
amine-derived Schiff base) have been synthesised, characterised and their asymmetric catalytic activities evaluated
through the epoxidation of styrene. The molybdenum complexes with acetyl-protected D-glucosamine Schiff base
ligands exhibited higher enanatioselectivity than those with unprotected sugar ligands. Protecting D-glucosamine by
acetyl groups is an effective way to improve asymmetric catalytic epoxidation activities.
Keywords: D-Glucosamine, Schiff bases, molybdenum (VI) complex, asymmetric catalysis, epoxidation
During the past few decades, increasing attention has been
paid to chiral catalysts for enantioselective epoxidation of ole-
fins for the many applications of chiral epoxides.1,2 Although a
variety of carbon–carbon double bonds such as allylic alcohols
and α- and β-unsaturated esters can be catalytically epoxidised
with several metal complex catalysts,3–6 the asmmmetric epox-
idation of non-functionalised olefins is still a challenge despite
the success of Mn–salen and metalloporphyrin complexes.7–9
As a cheap and naturally occurring chiral compound, D-
glucosamine has been used to synthesise stable Schiff bases
complexes with transition-metal ions such as Fe(III), Co(II),
Ni(II), Cu(II) and Mo(VI).10 Metal complexes derived from
carbohydrates are of interest in metal-assisted or metal-
catalysed enantioselective synthesis such as cyclopropana-
tion,11 hydroformylation,12 hydrogenation13 and allylic alkyla-
tion14 but the potential of this class of complexes as catalysts
for enantioselective epoxidation of olefins still remains
sparse.15,16 We now report four kinds of dioxomolybdenum(VI)
complexes with Schiff base ligands derived from D-glucos-
amine and their catalytic activities for asymmetric epoxidation
of styrene. The synthetic routine is shown in Scheme 1.
Results and discussion
The Mo-complexes 4a–d were obtained by the reaction of
MoO2(acac)2 and the corresponding chiral Schiff base ligands
3a–d in methanol. In the IR spectra of 4a–d, the imino (CH=N)
bands are shifted compared to the free ligands due to the
coordination of the metal to the imine nitrogen.17 The two
bands in the region ca 900–920 cm−1 and ca 920–940 cm−1 can
be assigned to the symmetric and asymmetric stretching modes
of Mo=O double bonds, respectively, indicating the presence
of oxo–molybdenum centres18–20 which is also validated by
their Raman spectra. In addition, the values of ν (Mo–Nimino
)
at ca 320–370 cm−1, ν (Mo-Osugar) at about 420–480 cm−1 and
ν (Mo–Ophenolic) around 286 cm−1 were observed in their Raman
spectra and gave hint of are consistent with Mo coordination.
In the case of the sugar–OH groups being protected by acetyl
groups, one of them is selectively deacetylated and coordi-
nated to the metal centre during the reaction process. The
possible structures of the complexes are shown in Scheme 1.
Analytical data including IR, Raman, NMR, and elemental
analysis are in accord with their descriptions as monometallic
compounds with one ligand L and a coordinated solvent
molecule MeOH.
The enol-imine–keto-amine and the anomeric equilibria of
1
3a and 3b may occur in solution. The H NMR spectra of
ligands 3a and 3b showed two sets of signals (5.0–5.5 and
4.6–4.8 ppm) for H-1 representing α and β-configurations of
sugar rings, respectively. In a low field, the signals assigned to
the imino-proton (CH=N) appeared as two singlets due to the
sugar residue in the α–β equilibrium,21 and the vinyl proton of
the keto-amine tautomer (C=CHN) gave two signals due to the
anomeric equilibrium. But these equilibria were not detected
in the acetyl protected D-glucosamine ligands (3c and 3d) and
complexes 4a–d, which were found to be pure α-configurated
compounds.
Catalytic asymmetric epoxidation of styrene
Complexes 4a–d were examined as catalysts in the asymmet-
ric epoxidation of styrene. Blank runs were performed and, as
expected, without catalyst, no significant epoxide formation
was observed under the applied conditions. The results were
listed in Table 1.
As shown in Table 1, the enantiomeric excess (ee) value of
4b is a little higher than 4a, the reason may be related to the
larger steric hindrance of naphthalene ring, which could
increase the rigidity of ligand. Complexes 4c and 4d exhibited
higher enantioselectivity than those with unprotected sugar
ligands 4a and 4b, 4d gave the highest ee value (about 67%).
Molybdenum complexes with acetyl protected D-glucosamine
Schiff base ligands exhibited higher enantioselectivity than
those with unprotected sugar ligands. Although the reason was
not very clear, we think that it might be related to the partial
Scheme 1 Synthetic route to D-glucosamine Schiff base
dioxomolybdenum (VI) complexes.
* Correspondent. E-mail: ysui@163.com

JOURNAL OF CHEMICAL RESEARCH 2012 585
Table 1 The results of catalytic asymmetric epoxidation of
styrene by D-glucosamine Schiff base dioxomolybdenum (VI)
complexes
(d, 1H, J1,2=8.37 Hz, sugar H-1), 5.63 (t, 1H, J = 9.25 Hz, sugar H-3),
5.00 (t, 1H, J = 9.6 Hz, sugar H-4), 4.31–4.20 (m, 2H, sugar H-6),
4.05 (d, 1H, J1,2 = 10.28 Hz, sugar H-5), 3.60 (t, 1H, sugar H-2), 2.04
(s, 9H, acetyl-1,3,4), 1.85 (s, 3H, acetyl-6). IR (KBr matrix, in cm−1)
1749 (C=O), 1629 (CH=N).
Catalyst
Temperature/°C
Time/h
Yield/%
ee/%
4a
30
0
30
0
50
30
0
50
30
0
4
6
4
6
3
4
6
3
4
6
41.2
27.3
58.7
42.6
74.9
53.3
36.8
92.2
79.5
54.0
22.0
23.8
28.7
32.1
37.2
62.0
62.3
47.6
66.5
67.2
Synthesis of D-glucosamine Schiff base dioxomolybdenum (VI)
complexes
4b
4c
4a–d were prepared according to the following procedure. One of
the Schiff base ligands 3a–d (5 mmol) was dissolved in methanol
(80 mL). After complete dissolution, MoO2(acac)2 (1.64 g, 5 mmol)
was added to the yellow solution. The mixture was allowed to react
for 4 h at room temperature, and then the volume was reduced to
ca 10 mL and diethyl ether (20 mL) was added to precipitate the com-
pound as a yellow solid. The solid was washed twice with diethyl
ether and dried in vacuo.
4d
4a. Yield, 82.7%. Anal. Calcd for C14H18MoN2O11: C, 34.6; H, 3.7;
N, 5.8. Found: C, 34.5; H, 3.5; N, 6.3%. 1H NMR (DMSO-d6) δ (ppm):
8.77 (s, 1H, HC=N), 8.67–7.07 (m, 3H, ArH), 5.65 (d, 1H, J = 3.52
Hz, sugar H-1), 5.62–3.58 (m, 6H, sugar ring H), 1.11 (m, 3H, metha-
nol). IR (KBr matrix, in cm−1) 3373 (–OH), 1637 (CH=N), 946
(Mo=O), 918 (Mo=O).
4b.Yield, 80.5%. Anal. Calcd for C18H21MoNO9: C, 44.0; H, 4.3; N,
2.9. Found: C, 44.6; H, 3.9; N, 3.8%. 1H NMR (DMSO-d6) δ (ppm):
9.46 (s, 1H, HC=N), 8.34–7.13 (m, 8H, ArH), 5.61 (d, 1H, J = 3.67
Hz, sugar H-1), 4.28–2.72 (m, 6H, sugar ring H), 1.11 (m, 3H, metha-
nol). IR (KBr matrix, in cm−1) 3385 (–OH), 1626 (CH=N), 930
(Mo=O), 905 (Mo=O).
dissociation of the complexes in solutions. After partial
dissociation, the enol-imine–keto-amine and the anomeric
equilibria of ligands 3a and 3b could decrease the asymmetric
induction, but there were no such side effects for 3c and 3d
because these equilibria did not exist in the acetyl protected
D-glucosamine ligands. In addition, in all the test reactions, ee
values gradually increased and yields gradually decreased with
decrease of reaction temperature.
Experimental
D-Glucosamine hydrochloride, 5-nitrosalicylaldehyde, and 2-hydroxy-
1-napthaldehyde were purchased from Alfa Aesar. Other reagents
were analytic grade and used as received. MoO2(acac)2 was synthe-
sised according to the literature.22
C, H and N elemental analysis were carried out on a Perkin-Elmer
2400 elemental instrument. IR spectra were recorded on Spectrum
GX using polystyrene as a standard (KBr pellets). FT-Raman spectra
were undertaken with a Bruker RFS100/S apparatus using a laser
source of Nd/YAG λ=1064 nm of 200 mW and a Ge detector at 77 K.
1H NMR spectra were recorded using a Bruker AV-300 spectrometer.
GC analysis was obtained with an HP 6890 gas chromatograph
equipped with a capillary column (50.0 m × 320 µm × 1.05 µm
nominal) and a FID detector.
4c. Yield, 76.6%. Anal. Calcd for C20H24MoN2O14: C, 39.2; H, 4.0;
N, 4.6. Found: C, 39.5; H, 3.5; N, 4.3%. 1H NMR (DMSO-d6) δ (ppm):
8.55 (s, 1H, CH=N), 8.34–7.05 (m, 3H, ArH), 6.18 (m, 1H, sugar H-
1), 5.77 (d, 1H, J = 8.44 Hz, sugar H-3), 4.93 (m, 1H, sugar H-4), 4.21
(m, 2H, sugar H-6), 4.00 (m, 1H, sugar H-5), 3.68 (m, 1H, sugar H-2),
2.21 (s, 6H, acetyl-3,4), 1.88 (s, 3H, acetyl-6), 1.08 (m, 3H, metha-
nol). IR (KBr matrix, in cm−1) 3468 (–OH), 1753 (C=O), 1630
(CH=N), 944 (Mo=O), 908 (Mo=O).
4d. Yield, 79.8%. Anal. Calcd for C24H27MoNO12: C, 46.7; H, 4.4;
N, 2.3. Found: C, 46.8; H, 4.6; N, 2.4%. 1H NMR (DMSO-d6) δ (ppm):
8.77 (s, 1H, CH=N), 8.26–6.98 (m, 8H, ArH), 6.16 (d, 1H, J1,2 = 3.61
Hz, sugar H-1), 5.59 (m, 1H, sugar H-3), 4.90 (t, 1H, sugar H-4),
4.30–4.19 (m, 2H, sugar H-6), 3.98 (d, 1H, sugar H-5), 3.65 (t, 1H,
sugar H-2), 2.12 (s, 6H, acetyl-3,4), 1.89 (s, 3H, acetyl-6), 1.11 (m,
3H, methanol). IR (KBr matrix, in cm−1) 3447 (–OH), 1748 (C=O),
1631 (CH=N), 942 (Mo=O), 909 (Mo=O).
Synthesis of Schiff bases
The compound 1b was obtained from 1a according to the literature
method.23
To an aqueous solution of 1a (or 1b) (0.1 mol) in NaOH (100 mL,
1.0 mol mL−1) or Na2CO3 (100 mL, 0.5 mol mL−1) was added slowly
the aromatic aldehyde 2a (or 2b) (0.1 mol) in methanol (50 mL) at
ice-water bath temperature, and the mixture was then stirred at room
temperature overnight. The bright yellow precipitate was filtered and
washed with methanol and diethyl ether respectively, recrystallised in
methanol and dried in vacuo.
3a. Yield, 82.3%. Anal. Calcd for C13H16N2O8: C, 47.6; H, 4.9; N,
8.5. Found: C, 47.7; H, 5.0; N, 8.7%. 1H NMR (DMSO-d6) δ (ppm):
8.71 (s, HC=N), 8.55 (d, HC=N), 8.15–7.10 (m, 3H, ArH), 6.82 (s,
C=CHN), 6.79 (s, C=CHN), 5.47 (d, H-1´α), 5.22 (d, H-1´α), 4.86 (d,
H-1´β), 4.67 (s, H-1´β), 3.75–2.74 (m, sugar ring). IR (KBr matrix, in
cm−1) 3521, 3352 (–OH), 1654 (CH=N).
3b. Yield, 80.7%. Anal. Calcd for C17H19NO6: C, 61.2; H, 5.8; N,
4.2. Found: C, 61.5; H, 5.6; N, 4.4%. 1H NMR (DMSO-d6) δ (ppm):
9.06 (s, HC=N), 8.96 (d, HC=N), 8.12–7.17 (m, 8H, ArH), 7.01 (s,
C=CHN), 6.83 (d, C=CHN), 5.37 (s, H-1´α), 5.19 (d, H-1´α), 4.77 (d,
H-1´β), 3.75–2.73 (m, sugar ring). IR (KBr matrix, in cm−1) 3477,
3231 (–OH), 1636 (CH=N).
Catalytic asymmetric epoxidations with Mo-complexes 4a–d
Epoxidations of styrene by D-glucosamine Schiff base dioxomolyb-
denum(VI) complexes with cumene hydroperoxide (CHP) as oxidant
were carried out according to the following general procedure: a
mixture of catalyst (0.003 mmol), styrene (0.08 mol) and 1,2-dichlo-
roethane (5 mL) was placed in a three-necked round-bottomed flask
equipped with a condenser and a magnetic stirrer. The mixture was
stirred for 5 min at room temperature and then with the addition of
CHP (2 mL, ca 0.02 mol), the reaction was started. The course of the
reactions was monitored by quantitative GC analysis at certain time
intervals.
This work was supported by Natural Science Foundation of
Jiangxi Province (20114BAB203027 and 20114BAB203028)
and Department of Education of Jiangxi Province (JXJG-11-
15-6).
Received 10 June 2012; accepted 18 July 2012
Paper 1201366 doi: 10.3184/174751912X13457309578443
Published online: 28 September 2012
3c. Yield, 88.6%. Anal. Calcd for C21H24N2O12: C, 50.8; H, 4.9; N,
5.6. Found: C, 50.9; H, 5.0; N, 5.8%. 1H NMR (DMSO-d6) δ (ppm):
8.78 (s, 1H, CH=N), 8.56–7.05 (m, 3H, ArH), 6.23 (d, 1H, J1,2
=
9.17 Hz, sugar H-1), 5.66 (t, 1H, J = 9.63 Hz, sugar H-3), 5.01 (t, 1H,
J = 9.6 Hz, sugar H-4), 4.30–4.20 (m, 2H, sugar H-6), 4.04 (d, 1H, J1,2
= 11.36 Hz, sugar H-5), 3.65 (t, 1H, sugar H-2), 2.03 (s, 9H, acetyl-1,
3, 4), 1.89 (s, 3H, acetyl-6). IR (KBr matrix, in cm−1) 1755 (C=O),
1635 (vs).
3d. Yield, 76.9%. Anal. Calcd for C25H27NO10: C, 59.9; H, 5.4; N,
2.8. Found: C, 59.7; H, 5.6; N, 2.9%. 1H NMR (DMSO-d6) δ (ppm):
12.40 (s, 1H, OH), 8.95 (s, 1H, CH=N), 8.12–7.06 (m, 8H, ArH), 6.20
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