Oxidative kinetic resolution of 1-phenylethanol catalyzed by sugar-based salen-Mn(III) complexes

Article abstract of DOI:10.1016/j.carres.2008.04.022

Three new chiral salen-Mn(III) complexes with sugars at the C-5(5′) positions of the salicylaldehyde moieties of the salen ligand were synthesized. Their structures were characterized by FTIR, MS, and elemental analysis. The complexes together with two previously reported ones were successfully used as chiral catalysts for the oxidative kinetic resolution (OKR) of 1-phenylethanol using PhI(OAc)₂ as an oxidant and KBr as an additive. Excellent enantiomeric excess (up to 89%) of the product was achieved in 0.5 h at 20 °C. The results showed that the sugars at C-5(5′) of salicylaldehyde moieties in the ligand had influences on the catalytic performances of the complexes. It was concluded that the sugars with the same rotation direction of polarized light as the diimine bridge within the complex could enhance the chiral induction of the complex in the OKR of 1-phenylethanol, but the sugars with the opposite one would reduce that of the corresponding complex.

Full text of DOI:10.1016/j.carres.2008.04.022

Available online at www.sciencedirect.com
Carbohydrate Research 343 (2008) 1407–1413
Oxidative kinetic resolution of 1-phenylethanol catalyzed
by sugar-based salen–Mn(III) complexes
Furong Han, Jiquan Zhao,* Yuecheng Zhang, Weiyu Wang, Yanyan Zuo and Junwei An
School of Chemical Engineering, Hebei University of Technology, Tianjin 300130, PR China
Received 12 March 2008; received in revised form 13 April 2008; accepted 16 April 2008
Available online 22 April 2008
Abstract—Three new chiral salen–Mn(III) complexes with sugars at the C-5(50) positions of the salicylaldehyde moieties of the salen
ligand were synthesized. Their structures were characterized by FTIR, MS, and elemental analysis. The complexes together with two
previously reported ones were successfully used as chiral catalysts for the oxidative kinetic resolution (OKR) of 1-phenylethanol
using PhI(OAc)2 as an oxidant and KBr as an additive. Excellent enantiomeric excess (up to 89%) of the product was achieved
in 0.5 h at 20 °C. The results showed that the sugars at C-5(50) of salicylaldehyde moieties in the ligand had influences on the cat-
alytic performances of the complexes. It was concluded that the sugars with the same rotation direction of polarized light as the
diimine bridge within the complex could enhance the chiral induction of the complex in the OKR of 1-phenylethanol, but the sugars
with the opposite one would reduce that of the corresponding complex.
Ó 2008 Elsevier Ltd. All rights reserved.
Keywords: Salen–Mn(III) complex; a-D-Glucofuranose; a-D-Mannofuranose; Oxidative kinetic resolution; 1-Phenylethanol
1. Introduction
chloride founded by Katasuki and co-workers, which
also displayed high enantioselectivities in the OKR of
racemic secondary alcohols.4 Besides, Nishibayashi
and co-workers employed [RuCl2(PPh3)(ferrocenyloxa-
zolinylphosphine)] to catalyze the oxidative kinetic
resolution of racemic 1-indanol, obtaining an optically
active 1-indanol in good yield (turnover frequency
exceeds 80,000 hꢀ1) with high enantioselectivity (up to
94% ee).5 Inspired by the outstanding performance of
the salen ruthenium(II) complex in the OKR of second-
ary alcohols and high enantioselectivities and activities
of chiral salen–Mn(III) complexes on the asymmetric
epoxidation of nonfunctionalized olefins,6–11 subse-
quently, some salen–Mn(III) complexes were also used
as catalysts in the OKR of racemic alcohols and gave
good results.2,12–16 For example, an excellent enantio-
selectivity (up to 98% ee) was received when 4-chloro-
a-phenylethanol was resolved using PhI(OAc)2 as an
oxidant catalyzed by the traditional chiral salen–Mn(III)
complex.16
Optically active alcohols are useful chiral auxiliaries and
key synthetic intermediates in pharmaceutical, agro-
chemical, and fine chemical industries.1 The oxidative
kinetic resolution (OKR) of racemic alcohols is a poten-
tially attractive method to obtain optically active alco-
hols together with corresponding carbonyl compounds.2
To the best of our knowledge, there are several repre-
sentative nonenzymatic catalytic systems that are active
in the OKR of racemic alcohols. One is the sparteine–
Pd(II) complex reported by Stoltz and Sigman in 2001,
which can catalyze the aerobic OKR of secondary alco-
hols with high enantioselectivities (ee’s).3 However, the
method required a long reaction time and high loading
of catalyst when this catalyst was employed in the
OKR of racemic alcohols. Furthermore, the sparteine
is not easily available and very expensive. The second
one is the optically active nitroso salen ruthenium(II)
As is known, the attainment of a high enantioselectiv-
ity in the asymmetric epoxidation catalyzed by chiral
salen–Mn(III) complex relies on the steric and electronic
*
Corresponding author. Tel.: +86 22 60204279; fax: +86 22
26564733; e-mail: zhaojq@hebut.edu.cn
0008-6215/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved.
doi:10.1016/j.carres.2008.04.022

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F. Han et al. / Carbohydrate Research 343 (2008) 1407–1413
properties of the substituents on the salicylaldehyde
moieties of the salen complexes, in addition to the pres-
ence of a chiral diimine bridge.17,18 The presence and
properties of substituents on the C-5(50) positions of
the salicylaldehyde moieties of the ligand also have a sig-
nificant, although generally less important, influence on
enantioselectivity.19 Recent work showed that both the
chiral Mn(III) complexes of the ligands from the intro-
duction of sugar moieties into the diimine bridge20 and
the one derived directly from the sugar-based ligand21
had moderate ee induction in the asymmetric epoxida-
tion of unfunctional olefins. Inspired by the above re-
sults, we much lately reported a facile synthesis of
several sugar-based chiral salen–Mn(III) complexes
and their application in the asymmetric epoxidation of
unfunctional olefins.22 It was found that the carbo-
hydrate groups at the C-5(50) positions had an enhance-
ment on the asymmetric induction of the epoxides.
Based on the previous results, it is expected that the
Mn(salen) complexes with sugar moieties at C-5(50)
positions will also have some influences on the OKR
of racemic alcohols. Therefore, we report here the syn-
thesis of several new salen–Mn(III) complexes bearing
sugar groups using the reported method.22 The new
complexes, together with the previously reported ones,
were employed in the OKR of 1-phenylethanol, and
some helpful results were obtained.
was prepared as reported by Katsuki.24 Solvents were
redistilled prior to use. Other chemicals were purchased
from commercial sources and used as received.
2.2. Synthesis of the chiral Schiff bases and salen–Mn(III)
complexes
2.2.1. Synthesis of 1,2:5,6-Di-O-isopropylidene-3-O-
methylene-[5-(3-tert-butyl-2-hydroxybenzaldehyde)]-a-D-
glucofuranose (2G). Compound 2G was prepared by
the procedure reported in the literature.25 Light-yellow
20
solid: yield 67%; mp 96–98 °C; ½aꢂD ꢀ34.1 (c 0.80,
EtOH); 1H NMR (CDCl3): d 1.38 (s, 9H), 1.42 (s,
12H), 4.01–4.68 (m, 8H), 5.90 (d, J = 3.9 Hz, 1H), 7.41
(d, J = 2.4 Hz, 1H), 7.48 (d, J = 1.8 Hz, 1H), 9.87 (s,
1H), 11.80 (s, 1H); 13C NMR (CDCl3): d 197.0, 161.0,
138.6, 133.9, 131.0, 128.3, 120.3, 111.9, 109.1, 105.3,
82.6, 81.6, 81.3, 77.7, 76.7, 72.4, 71.8, 67.5, 34.9, 29.1,
26.9, 26.8, 26.7, 26.2; FTIR (KBr) m: 3423, 2992, 2969,
2936, 2862, 1655, 1617, 1456, 1440, 1384, 1374, 1321,
1265, 1226, 1212, 1167, 1152, 1081, 1024, 847, 771,
759 cmꢀ1; FABMS: m/z 450 [M]+. Anal. Calcd for
C24H34O8: C, 63.98; H, 7.61. Found: C, 63.71; H, 7.83.
2.2.2. Synthesis of 2,3:5,6-di-O-isopropylidene-1-O-meth-
ylene-[3-(5-tert-butyl-2-hydroxybenzaldehyde)]-a-D-man-
nofuranose (2M). Compound 2M was prepared by the
procedure reported in the literature.26 Light-yellow
20
liquid: yield 79%; ½aꢂD +112.0 (c 1.10, EtOH); 1H
2. Experimental
NMR(CDCl3): d 1.39 (s, 9H), 1.42 (s, 12H), 3.61 (q,
J = 3.9, 3.6, 3.9 Hz, 1H), 4.10 (d, J = 5.4 Hz, 2H), 4.47–
4.89 (m, 6H), 7.43 (d, J = 2.1 Hz, 1H), 7.54 (d,
J = 2.1 Hz, 1H), 9.87 (s, 1H), 11.81 (s, 1H); 13C
NMR(CDCl3): d 192.5, 156.5, 134.0, 129.9, 127.0,
123.4, 115.7, 109.3, 104.8, 97.1, 75.2, 74.6, 72.1, 68.8,
66.6, 62.3, 30.4, 24.7, 22.5, 21.2, 20.7, 20.6, 20.0; FTIR
(Film) m: 3446, 2957, 2872, 1652, 1620, 1457, 1440,
1372, 1324, 1267, 1212, 1187, 1157, 1118, 1069, 886,
846, 773, 755 cmꢀ1; FABMS: m/z 450 [M]+. Anal. Calcd
for C24H34O8: C, 63.98; H, 7.61. Found: C, 63.78; H, 7.55.
2.1. Methods and materials
1H NMR spectra were obtained in CDCl3 using a Bru-
ker AC 400 spectrometer. IR spectra in KBr pellets were
recorded on a Bruker Vector-22 spectrophotometer.
Melting points were determined on a Perkin XT-4
microscopic analyzer. Optical rotations were measured
on a Shanghai WZZ-2S/2SS digital rotation analyzer,
at ambient temperature using a 100-mm sample tube.
Fast-atom-bombardment mass spectrometry was per-
formed on a VG ZAB-HS mass spectrometer. Electro-
spray-ionization mass spectrometry was performed on
a LCQ Advantage mass spectrometer. Reaction prod-
ucts were analyzed on a Shandong Lunan Ruihong
gas chromatograph, SP-6800A, equipped with a Cyclo-
dex-b capillary column (30 m ꢁ 250 lm i.d.) using an
FID detector. 1,2:5,6-Di-O-isopropylidene-a-D-gluco-
furanose, 2,3:5,6-di-O-isopropylidene-a-D-mannofura-
nose, and 1-phenylethanol were purchased from Acros.
(1S,2S)-1,2-Diphenylethylenediamine (99%) was pur-
chased from Zhejiang Dongyang Lingxing Biochemistry
Co., Ltd PhI(OAc)2 was prepared using the procedure
reported in the literature23 (mp 160.2–160.5 °C).
(1S,2S)-N,N0-bis(3,5-di-tert-butylsalicylaldehyde)-1,2-di-
phenylethylenediamine Mn(III) chloride (complex 4N)
2.2.3. Synthesis of Schiff-base ligand 3Ga. A solution
of compound 2G (1.00 g, 2.2 mmol) and ethylenedia-
mine (0.07 g, 1.1 mmol) in dry EtOH (25 mL) was
refluxed for 2 h under a nitrogen atmosphere. EtOH
was removed under reduced pressure. The residue was
purified by chromatography (1:2 EtOAc–petroleum
ether) to afford, after removal of the solvent under re-
duced pressure, the Schiff-base 3 Ga as yellow solid
20
(0.65 g, yield 60%): mp 81–83 °C; ½aꢂD ꢀ56.0 (c 0.20,
EtOH); 1H NMR(CDCl3): d 1.31 (s, 9H), 1.42 (s,
12H), 3.95 (s, 2H), 4.00-4.34 (m, 5H), 4.55–4.58 (m,
3H), 5.87 (d, J = 3.6 Hz, 1H), 7.10 (s, 1H), 7.26 (s,
1H), 8.38 (s, 1H), 13.87 (s, 1H); 13C NMR(CDCl3): d
202.8, 167.2, 160.5, 137.9, 129.8, 129.5, 126.9, 118.4,
112.0, 109.2, 105.5, 83.0, 81.7, 77.6, 76.8, 72.7, 67.6,

F. Han et al. / Carbohydrate Research 343 (2008) 1407–1413
1409
60.8, 59.8, 48.9, 35.1, 29.5, 27.1, 26.5, 25.7; FTIR (KBr)
m: 3442, 2987, 2932, 1633, 1444, 1373, 1341, 1268, 1215,
1164, 1077, 1026, 848, 798, 776 cmꢀ1; FABMS: m/z 925
[M+H]+. Anal. Calcd for C50H72N2O14: C, 64.92; H,
7.84; N, 3.03. Found: C, 64.89; H, 8.04; N, 3.10.
the mixture was further heated for 3 h while exposed
to air. The solvent was removed under reduced pressure,
and the residue was extracted with CH2Cl2. The extract
was washed with brine, dried over Na2SO4, and concen-
trated to obtain a dark-brown powder 4M: (0.65 g,
20
63%); mp 77–78 °C; ½aꢂD ꢀ495 (c 0.02, CH2Cl2); FTIR
2.2.4. Synthesis of Schiff-base ligand 3Gb. Ligand 3Gb
was prepared by the procedure reported in the litera-
ture.27 Light-yellow solid: yield 86.4%; mp 202–203 °C;
(KBr) m: 2925, 1614, 1543, 1435, 1382, 1344, 1309,
1258, 1209, 1169, 1071, 1025, 853, 825, 577, 556 cmꢀ1
;
ESIMS: m/z 1130 [MꢀCl]+. Anal. Calcd for
C62H78ClMnN2O14: C, 63.88; H, 6.74; N, 2.40. Found:
C, 63.90; H, 6.59; N, 2.48.
20
½aꢂD ꢀ66.3 (c 0.12, EtOH); 1H NMR (300 MHz,
CDCl3): d 13.83 (s, 1H), 8.34 (s, 1H), 7.18–7.23 (m,
6H), 7.00 (d, J = 2.4 Hz, 1H), 5.84 (d, J = 3.9 Hz, 1H),
4.74 (s, 1H), 3.95–4.53 (m, 8H), 1.44 (s, 12H), 1.28 (s,
9H); 13C NMR (75 MHz, CDCl3): d 166.9, 160.4,
139.7, 137.7, 129.8, 129.7, 128.6, 128.2, 127.9, 126.9,
118.4, 112.0, 109.2, 105.5, 82.9, 81.6, 81.5, 80.2, 76.0,
72.7, 72.5, 67.5, 67.3, 35.0, 29.5, 27.1, 26.5, 25.7; FTIR
(KBr) m: 3439, 2986, 2930, 1633, 1440, 1370, 1340,
1263, 1214, 1160, 1076, 1025, 848, 798, 776 cmꢀ1; FAB-
MS: m/z 925 [M+H]+. Anal. Calcd for C62H80N2O14: C,
69.12; H, 7.48; N, 2.60. Found: C, 69.36; H, 7.24; N,
2.39.
2.3. General procedure for OKR of ( )-1-phenylethanol
A mixture of ( )-1-phenylethanol (0.122 g, 1 mmol),
salen–Mn(III) complex (0.023 g, 2 mol %), KBr
(0.009 g, 8 mol %), CH2Cl2 (2.0 mL), and water
(4.0 mL) was stirred in a 25-mL round-bottomed flask
for a few min at 20 °C. The oxidant PhI(OAc)2
(0.224 g, 0.7 mmol) was then added and the reaction
mixture was magnetically stirred for needed time at
20 °C. The reaction was monitored by a GC equipped
with a suitable chiral column. After the desired oxida-
tion level was achieved, the CH2Cl2 layer was separated,
and the water layer was re-extracted again with CH2Cl2.
The CH2Cl2 was combined, dried over sodium sulfate,
and then concentrated under vacuum. The alcohol in
the reaction mixture was separated by column
chromatography.
2.2.5. Synthesis of salen–Mn(III) complex 4Ga. A mix-
ture of 3Ga (0.33 g, 0.34 mmol) and Mn(OAc)2ꢃ4H2O
(0.17 g, 0.68 mmol) in EtOH (30 mL) was stirred under
reflux in an atmosphere of nitrogen for 4 h. Solid LiCl
(0.04 g, 1.02 mmol) was added and the mixture was fur-
ther heated for 3 h while exposed to air. The solvent
was removed under reduced pressure, and the residue
was extracted with CH2Cl2. The extract was washed with
brine, dried over Na2SO4, and concentrated to obtain a
dark-brown powder 4Ga (0.22 g, 65%): mp 135–137 °C;
3. Results and discussion
20
½aꢂD ꢀ156.3 (c 0.02, CH2Cl2); FTIR (KBr) m: 2957,
3.1. Synthesis of chiral salen–Mn(III) complexes
1615, 1544, 1440, 1383, 1339, 1302, 1264, 1210, 1165,
1076, 1026, 848, 823, 584, 537 cmꢀ1. ESIMS: m/z 978
[MꢀCl]+. Anal. Calcd for C50H70ClMnN2O14: C,
59.25; H, 6.96; N, 2.76. Found: C, 59.46; H, 7.10; N, 2.47.
The synthetic route for the chiral complexes 4Ga, 4Gb,
and 4M is shown in Scheme 1. First, ligand 3Ga and
complexes 4Ga, 4Gb were prepared according to our
previously published procedure.22 Chiral complex 4M
is directly synthesized from condensation product of
2M and (1S,2S)-1,2-diphenylethylenediamine in situ
due to decomposition of the ligand in the purifica-
tion process. The complexes were characterized by opti-
cal rotation, IR spectroscopy, elementary analysis, and
MS.
2.2.6. Synthesis of salen–Mn(III) complex 4Gb. Com-
pound 4Gb was synthesized according to our previous
procedure.27 Dark-brown powder: yield 86.8%; mp 68–
20
69 °C; ½aꢂD ꢀ480.4 (c 0.08, CH2Cl2); FTIR (KBr) m:
2926, 1613, 1543, 1435, 1382, 1344, 1309, 1258, 1209,
1165, 1070, 1026, 853, 825, 566, 553 cmꢀ1; ESIMS:
m/z 1130 [MꢀCl]+. Anal. Calcd for C62H78ClMnN2O14:
C, 63.88; H, 6.74; N, 2.40. Found: C, 63.70; H, 6.84; N,
2.35.
3.2. Catalysis of salen chiral Mn(III) complexes
In order to get an insight of the effect of the sugar moie-
ties at C-5(50) of salen–Mn(III) complexes on the chiral
induction of OKR, complexes 4Ga, 4Gb, 4M, and 4N
were employed in the OKR of 1-phenylethanol with
PhI(OAc)2 as an oxidant at 20 °C. Potassium bromide
was used as an additive because bromide salts play a dis-
tinctive role for the activation of both PhIO17 and
2.2.7. Synthesis of salen–Mn(III) complex 4M. A solu-
tion of compound 2M (0.8 g, 1.78 mmol) and (1S,2S)-
1,2-diphenylethylenediamine (0.19 g, 0.89 mmol) in dry
EtOH (30 mL) was refluxed for 2 h under a nitrogen
atmosphere. Mn(OAc)2ꢃ4H2O (0.44 g, 1.78 mmol) was
added, and the mixture was refluxed further for 4 h.
Then solid LiCl (0.11 g, 2.65 mmol) was added, and
18
PhI(OAc)2 for the oxidation of various alcohols to

1410
F. Han et al. / Carbohydrate Research 343 (2008) 1407–1413
R2
R1
R2
R1
H
O
O
O
O
O
O
O
N
N
O
R2
R1
R2
O
O
O
O
O
O
O
R1
O
O
HO
OH
O
H2N NH2
EtOH
O
ClH2C
OH
HO
O
O
O
O
O
t-Bu
3Ga-b
t-Bu
NaH, TBAI, THF
O
t-Bu
O
t-Bu
1
2G
R2
R1
R1
R2
O
O
O
O
N
N
Mn(OAc)2·4H2O
a: R1 = R2 = H
b: (S,S) R1 = H, R2 = Ph
Mn
Cl
O
O
O
LiCl, air
O
O
O
O
O
O
O
t-Bu
t-Bu
4Ga-b
Ph
H
N
H
Ph
H
H2N
H
Ph
Ph
NH2
O
O
1
O
O
N
O
O
O
O
O
O
O
O
O
O
O
O
Mn
HO
O
O
O
O
O
Cl
2 Mn(OAc)2·4H2O
LiCl, air
t-Bu
t-Bu
t-Bu
3
4M
2M
Scheme 1. Synthesis route for the complexes.
give ketones.12 The results are shown in Table 1. For
comparison, some results from the literature2,15,16 are
also listed in Table 1. In Table 1, krel = kfast/kslow, which
represent the relative rates of enantiomers of racemic
substrates to form a product.28 It can be calculated by
using equation
where ee is the enantiomeric excess of the secondary
alcohol and c is the conversion of the secondary alcohol.
The krel values are generally considered to be more
useful for the evaluation and especially comparison of
the efficacy of kinetic resolution catalysts.12
It was observed that when catalyst was absent in the
reaction mixture, no enantioselectivity was achieved
(entry 1). In the case of 4Ga with an absence of chirality
in the diimine bridge moiety, an ee of 27% and a krel
lnð1 ꢀ cÞð1 ꢀ eeÞ
krel
¼
lnð1 ꢀ cÞð1 þ eeÞ
Table 1. OKR of 1-phenylethanol using different salen–Mn(III)complexes as catalystsa
HO
O
OH
Salen−Mn(III), PhI(OAc)2
+
Solvent/addition, 20 ºC, 30 min
Entry
Catalyst
Conversion (%)
ee (%)
krel
1
2
3
4
5
—
53
55
60
65
65
69
61
63
0
27
89
77
79
98
55
88
—
4Ga
4Gb
4M
4N
Ab
Bc
Cd
1.98
11.2
5.14
5.63
10.0
3.50
8.90
a Reaction conditions: ( )1-phenylethanol (1 mmol), salen–Mn(III) complex (2 mol %), PhI(OAc)2 (0.7 mmol), KBr (8 mol %), CH2Cl2 (2 mL), H20
(4 mL), 30 min, and reaction temperature 20 °C.
b Results are from Table 2 in Ref. 15; for the structure of the catalyst and the reaction conditions see the reference.
c Results are from Table 3 in Ref. 2; for the structure of the catalyst and the reaction conditions see the reference.
d Results are from Table 2 in Ref. 16; for the structure of the catalyst and the reaction conditions see the reference.

F. Han et al. / Carbohydrate Research 343 (2008) 1407–1413
1411
value of 1.98 were obtained (entry 2). This enantio-
selectivity is undoubtedly induced by the chiral sugar
groups at C-5(50). Thus, it can be concluded that the chi-
ral centers attached on C-5 of the salicylaldehyde moiety
have an influence on the OKR of 1-phenylethanol in a
weak manner. The highest ee and krel were on 4Gb. An
ee of 89% and krel value of 11 were obtained (entry 3).
For 4N the ee and krel were 79% and 5.63, respectively.
The results indicated that a-D-glucofuranose moities at
C-5 of the salicylaldehyde in the complex enhanced the
OKR of 1-phenylethanol. However, not all sugars at
C-5 had the same effect as a-D-glucofuranose. When
the sugar moiety was a-D-mannofuranose as in 4M, a
slight decrease of enantioselectivity for the OKR of
1-phenylethanol was observed in comparison with that
for 4N. The ee and krel were 77% and 5.14, respectively
(entry 4). Perhaps the sugars having the same rotation
direction of polarized light as the (1S,2S)-diimine bridge
in the complex could enhance the chiral induction of the
complex in the OKR of 1-phenylethanol, but the sugars
with the opposite one would reduce that of the corre-
sponding complex. The results indicate that complex
4Gb has better performance than the metal complexes
in the literature listed in Table 1 from the point of view
of krel.
Generally, solvent plays a critical role in the OKR of
secondary alcohols.12 Therefore, the effects of solvents
were carried out using 4Gb as a catalyst for the OKR
of 1-phenylethanol, and the results are summarized in
Table 2. In the case of H2O alone as a solvent (Table
2, entry 6) and KBr as an additive, a conversion of
51% with 20% of ee for 1-phenylethanol was obtained,
possibly due to only partial solubility of the catalyst
4Gb in the mixture of alcoholic substrate and water.
CH2Cl2 alone as a solvent showed the poorest conver-
sion (36%), ee (6%) and krel (Table 2, entry 7), which
may be attributed to the fact that KBr is insoluble in
CH2Cl2. Solvents like toluene and CH2Cl2 when mixed
with water gave high enantioselectivity (80–89%) in the
case of OKR of 1-phenylethanol (entries 10 and 11),
while cyclohexane and CH3CN, gave poor results (en-
tries 8 and 9). Out of all the solvent systems studied,
Table 2. OKR of 1-phenylethanol using 4Gb as catalyst in various solvent systemsa
HO
O
OH
Salen Mn(III), PhI(OAc)2
+
20 ºC, 30 min
Entry
Solvents
Conversion (%)
ee (%)
krel
1.76
1.31
2.91
4.38
4.99
6
7
8
9
10
11
H2O
CH2Cl2
CH3CN + H2O
Cyclohexane + H2O
Toluene + H2O
CH2Cl2 + H2O
51
36
37
42
68
60
20
6
24
37
80
89
11.17
a Reaction conditions: ( )1-phenylethanol (1 mmol), salen–Mn(III) (2 mol %), PhI(OAc)2 (0.7 mmol), KBr (8 mol %), organic solvent (2 mL), H2O
(4 mL), 30 min, and 20 °C.
Table 3. OKR of 1-phenylethanol using 4Gb as catalyst in the presence of various additivesa
HO
O
OH
Salen Mn(III), PhI(OAc)2
20 ºC, 30 min
+
Entry
Additives
Conversion (%)
ee (%)
krel
1.15
12
13
14
15
16
17
18
—
KCl
KI
KBr
NaBr
N(C2H5)4Br
N(n-C4H9)4Br
56
55
25
60
57
58
60
6
6
6
89
78
72
77
1.16
1.52
11.17
4.17
6.58
6.90
a Reaction conditions: ( )1-phenylethanol (1 mmol), salen–Mn(III) (2 mol %), PhI(OAc)2 (0.7 mmol), additive (8 mol %), CH2Cl2 (2 mL), H2O
(4 mL), 30 min, and 20 °C.

1412
F. Han et al. / Carbohydrate Research 343 (2008) 1407–1413
Table 4. OKR of 1-phenylethanol using 4Gb as catalyst in different reaction timesa
HO
O
OH
Salen Mn(III), PhI(OAc)2
+
20 ºC
Entry
Time (min)
Conversion (%)
ee (%)
krel
4.74
14
15
16
17
18
19
20
5
10
15
20
25
30
35
53
54
56
57
59
60
63
54
62
66
71
74
89
73
5.93
6.05
6.77
6.63
11.17
5.16
a Reaction conditions: ( )1-phenylethanol (1 mmol), salen–Mn(III) (2 mol %), PhI(OAc)2 (0.7 mmol), KBr (8 mol %), CH2Cl2 (2 mL), H2O (4 mL),
and 20 °C.
the 1:2 CH2Cl2–water was found to be the best one. The
same result was also observed in the literature12 with
other complexes as catalysts.
of 89% of the chiral secondary alcohol was achieved in
0.5 h. The study revealed that the sugars at the C-5(50)
moiety of salicylaldehyde parts had some influence on
the enantioselectivity of OKR of 1-phenylethanol. The
sugars with the same rotation direction of polarized
light as the diimine bridge in the complex could enhance
the chiral induction, but the sugars with the opposite
one would reduce that of the corresponding complex.
We also studied the effect of various additives on the
OKR of 1-phenylethanol using complex 4Gb as a cata-
lyst. The results are shown in Table 3. Phase-transfer
catalysts such as tetraethylammonium bromide and tet-
rabutylammonium bromide as additives gave moderate
to high enantioselectivity (ee, 72–77%) with conversions
(58–60%) (Table 3, entries 17 and 18), which were not as
good as we expected. Meanwhile, KCl and KI as addi-
tives gave very poor results (Table 3, entries 13 and
14). However, NaBr and KBr exhibited good enantiose-
lectivity (Table 3, entries 15 and 16). In the case of the
absence of an additive, the enantioselectivity of the reac-
tion dropped considerably (entry 12), which indicated
that the presence of bromide ion is essential for the acti-
vation of PhI(OAc)2 to carry out the OKR of alcohols in
the biphasic system. The results are in accord with those
reported in the literature.2,12
Acknowledgments
This work was supported by the NSFC of China (Grant
No. 20376017) and the Innovation Foundation of 100
Talents of Hebei Province, China.
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