Fe(salan)-catalyzed asymmetric oxidation of sulfides with hydrogen peroxide in water

Article abstract of DOI:10.1021/ja071916+

A new chiral Fe(salan) complex was synthesized, and it was found to serve as an efficient catalyst for asymmetric oxidation of sulfides using hydrogen peroxide in water without surfactant. Not only alkyl aryl sulfides but also various methyl alkyl sulfides were oxidized to the corresponding sulfoxides with high enantioselectivities up to 96% ee. It should be noted that the turnover number of complex 4 amounts to 8000. Copyright

Full text of DOI:10.1021/ja071916+

Published on Web 06/29/2007
Fe(salan)-Catalyzed Asymmetric Oxidation of Sulfides with Hydrogen
Peroxide in Water
Hiromichi Egami and Tsutomu Katsuki*
Department of Chemistry, Faculty of Science, Graduate School, Kyushu UniVersity, Hakozaki,
Higashi-ku, Fukuoka 812-8581, Japan
Received March 19, 2007; E-mail: katsuscc@mbox.nc.kyushu-u.ac.jp
Enhancement of ecological sustainability as well as selectivities
in chemical transformation is of primary importance for realizing
green chemistry. Ecological sustainability of an organic reaction
Table 1. Oxidation of Methyl Phenyl Sulfide Using Fe(salan)
Complexes as Catalysta
1
2
depends on the reagent(s), solvent, and workup procedure used
for transformation. Thus, intensive effort has been devoted to
development of asymmetric reactions using highly atom-efficient
and green reagent(s) that co-produce only innocuous waste. For
example, asymmetric oxidation using molecular oxygen or aqueous
hydrogen peroxide that produces only water as the co-product is a
topic of current interest, and quite a few atom-efficient methods
1
using them have been reported. On the other hand, significant effort
has also been devoted to development of asymmetric reactions in
water, and a few efficient methods that use a metal complex as the
catalyst have been reported: Kobayashi et al. reported highly
enantioselective reactions in water with chiral Lewis acid-surfactant
yield of
yield of
ee of
entry
cat.
7ab (%)
8ab (%)
7ac (%)
3
combined catalysts. Asymmetric oxidation in water has also been
1
2
3
4
5
6
7
8
none
4
30
25
89
91
47
0
2
2
5
9
1
8
<1
4
d
actively studied. However, few highly enantioselective oxidations
1
2
3
4
4
4
5
10 (R)
d
10 (S)
88 (S)
96 (S)
95 (S)
96 (S)
with molecular oxygen or aqueous hydrogen peroxide in water have
been reported.4b Still, realization of asymmetric reaction using a
chiral catalyst in water without a surfactant is a challenge.
Optically active sulfoxides are found in many bioactive com-
d
d
d
d
e
f
92 (90)g
21
d
5
6
<5 (R)
pounds as a subunit, and they also serve as chiral auxiliaries. Thus,
7
many methods for asymmetric sulfoxidation have been developed,
and some of them use aqueous hydrogen peroxide as the oxidant;7b,8
a
The reactions were carried out in water on a 0.2 mmol scale in the
presence of 30% H2O2 and Fe(salan) complexes (2 mol %) at 20 °C, unless
b
1
Jacobsen et al. reported a Mn(salen) complex catalyzed enantiose-
otherwise mentioned. Determined by H NMR (400 MHz) spectroscopic
analysis. c Determined by HPLC analysis as reported in Supporting
Information. d Assigned as reported in Supporting Information. The reac-
9
lective sulfoxidation using aqueous H
2
O
2
in acetonitrile. Bolm et
e
al. achieved highly enantioselective sulfoxidation using a V- or
tion run for 0.5 h. f The reaction run with 1 mol % of 4. g Isolated yield
Fe-Schiff base complex/aqueous H
In 2005, Strukul et al. reported the first enantio- and chemoselective
sulfoxidation using a Pt/BINAP/H /surfactant (SDS) system in
2
O
2
-dichloromethane system.7
b,8e,i,k
(on a 0.4 mmol scale).
2
O
2
respectively, though slight over-oxidation was observed (entries 4
and 5). We also examined the reaction with 4 for 0.5 h and found
that the enantioselectivity was 95% ee, suggesting that the over-
oxidation was a poorly enantiomer-differentiating process (vide
infra) (entry 6). Moreover, reduction of the loading of 4 scarcely
affected the yield and the enantioselectivity (entry 7). The reaction
with 5 showed poor enantioselectivity (entry 8).
In order to estimate the effect of over-oxidation on enantiose-
lectivity, we examined oxidation of racemic sulfoxide (rac-7a)
under the present conditions (Scheme 1), and the oxidation of
the (R)-enantiomer was found to be ca. 2.5 times as fast as that of
the (S)-enantiomer. This agrees with the results described in Table 1.
We next examined the oxidation of other alkyl aryl sulfides with
4 as the catalyst under the optimized conditions (Table 2). Similar
levels of yields and enantioselectivities were obtained in the
oxidation of 6b and 6c carrying an electron-donating group at the
p-position, respectively (entries 1 and 2). On the other hand,
substantial over-oxidation was observed in the oxidation of p-chloro-
substituted sulfide 6d, and the corresponding sulfoxide of 94% ee
was obtained in decent yield (76%) together with sulfone 8d (24%)
(entry 3), while oxidation of o-chloro-substituted sulfide 6e gave
4b
water, albeit with modest enantioselectivity. However, substrates
of these previous methods are largely limited to alkyl aryl
sulfides.7 We have recently demonstrated that cis-â-Ti(salen)
,10
8b
11
and cis-â-Al-(salalen) complexes are efficient catalysts for asym-
metric sulfoxidation using H as oxidant, while they are less
2
O
2
efficient in water probably due to high oxophilicity of titanium and
aluminum ions. We expected that a combination of a less oxophilic
metal ion and a more donor ONNO ligand, such as the salan
ligand8 that is the fully reduced salen (or reduced salalen) ligand,
would enable enantioselective sulfoxidation in water. It is known
that various iron complexes serve as catalysts for oxidation, and
the iron ion makes complexes with di- and trinitrogen ligands.1
Herein, we report widely applicable asymmetric oxidation of
sulfides using a Fe(salan) complex/aqueous hydrogen peroxide
system in water.
j,12
16
3,14
Oxidation of methyl phenyl sulfide was first examined with
15
complexes 1-5 at 20 °C with 1.5 equiv of H
O
2 2
(Table 1). The
reactions with 1 and 2 were slow, and the enantioselectivities were
low (entries 2 and 3). On the other hand, the reactions with 3 and
4
proceeded smoothly with good and high enantioselectivities,
8940
9
J. AM. CHEM. SOC. 2007, 129, 8940-8941
10.1021/ja071916+ CCC: $37.00 © 2007 American Chemical Society

C O M M U N I C A T I O N S
Scheme 1. Oxidation of Racemic Sulfoxide Using the 4/H2O2
System
and at 20 °C with remarkably high catalyst turnover number. Further
study on the mechanism of this sulfoxidation is now proceeding in
our laboratory.
Acknowledgment. This paper is dedicated to the memory of
the late Professor Emeritus Yoshihiko Ito. Financial support
(Specially Promoted Research 18002011) from a Grant-in-Aid for
Scientific Research from MEXT, Japan, is gratefully acknowledged.
H.E. is grateful for the JSPS Research Fellowships for Young
Scientists.
Table 2. Asymmetric Oxidation of Various Sulfides Using 4a
1
Supporting Information Available: Experimental procedures, H
NMR spectra data for sulfoxides, and HPLC condition. This material
is available free of charge via the Internet at http://pubs.acs.org.
yield of
7b (%)
yield of
8b (%)
ee of
7c (%)
entry
R1
R2
96 (S)d
95 (S)d
References
1
2
3
4
5
p-MePh
p-MeOPh
p-ClPh
Me
Me
Me
Me
Me
Me
Et
Me
Me
Me
Me
b
c
d
e
f
91 (88)
92 (88)
76 (72)
97 (86)
99 (90)
80 (77)
78 (73)
93 (85)
82 (73)
82 (79)
91 (73)
9
8
(1) Reviews for oxidation using hydrogen peroxide or molecular oxygen: (a)
ten Brink, G.-J.; Arends, I. W. C. E.; Sheldon, R. A. Chem. ReV. 2004,
104, 4105-4123. (b) Irie, R.; Katsuki, T. Chem. Rec. 2004, 4, 96-109.
(c) Kaczorowaka, K.; Kolarska, Z.; Mitka, K.; Kowalski, P. Tetrahedron
24
<1
<1
<1
22
7
18
18
9
94 (S)d
96 (S)d
o-ClPh
d
o-MeOPh
o-MeOPh
Ph
95 (S)
2
005, 61, 8315-8327.
e
93 (S)d
81 (S)d
6
f
(
2) (a) Kobayashi, S.; Manabe, K. Acc. Chem. Res. 2002, 35, 209-217. (b)
Lindstr o¨ m, U. M. Chem. ReV. 2002, 102, 2751-2772. (c) Li, C.-J.; Chen,
L. Chem. Soc. ReV. 2006, 35, 68-83.
7
8
9
0
1
g
h
i
j
k
d
PhCH2
87 (S)
n-C8H17
n-C12H25
c-C6H11
89 (S)d
(3) (a) Azoulay, S.; Manabe, K.; Kobayashi, S. Org. Lett. 2005, 7, 4593-
4595. (b) Gu, Y.; Ogawa, C.; Kobayashi, J.; Mori, Y.; Kobayashi, S.
Angew. Chem., Int. Ed. 2006, 45, 7217-7220. (c) Boudou, M.; Ogawa,
C.; Kobayashi, S. AdV. Synth. Catal. 2006, 348, 2585-2589.
d
1
1
94 (S)
88 (S)d
(
4) (a) Sun, W.; Wang, H.; Xia, C.; Li, J.; Zhao, P. Angew. Chem., Int. Ed.
2003, 42, 1042-1044. (b) Scarso, A.; Strukul, G. AdV. Synth. Catal. 2005,
347, 1227-1234. (c) Bourhani, Z.; Malkov, A. V. Chem. Commun. 2005,
4592-4594.
a
The reactions were carried out on a 0.2 mmol scale in water (0.5 mL)
in the presence of 30% H2O2 (1.5 equiv) and 4 (1 mol %) at 20 °C, unless
otherwise mentioned. b Determined by 1H NMR (400 MHz) spectroscopic
(
5) Legros, J.; Dehli, J. R.; Bolm, C. AdV. Synth. Catal. 2005, 347, 19-31.
analysis. The values in parenthesis are isolated yields that were obtained
on a 0.4 or 0.5 mmol scale. Determined by HPLC analysis as reported in
Supporting Information. Assigned as reported in Supporting Information.
The reaction run on a 10 mmol scale with 0.01 mol % of 4 for 6 h in the
c
(6) (a) Fern a´ ndez, I.; Khiar, N. Chem. ReV. 2003, 103, 3651-3706. (b)
Pellissier, H. Tetrahedron 2006, 62, 5559-5601.
d
(
7) (a) Kagan, H. B.; Luukas, T. O. Catalytic Asymmetric Sulfide Oxidations.
In Transition Metals for Organic Synthesis, 2nd ed.; Beller, M., Bolm,
C., Eds.; Wiley-VCH: Weinheim, Germany, 2004; Vol. 2, pp 479-495.
e
presence of 30% H2O2 (1.2 equiv).
(b) Bolm, C. Coord. Chem. ReV. 2003, 237, 245-256.
(
2 2
8) For recent examples of asymmetric sulfoxidation using H O as terminal
sulfoxide 7e exclusively with 96% ee (entry 4). It is noteworthy
that the oxidation of o-methoxy-substituted sulfide gave the
corresponding sulfoxide 7f exclusively with almost the same
enantioselectivity as 7e (entry 5), and the turnover number of 4
amounted to 8000 (entry 6). The results suggested that the presence
of an o-substituent significantly suppresses over-oxidation. The
oxidation of ethyl phenyl sulfide 6g proceeded with somewhat
reduced enantioselectivity of 81% ee (entry 7). To our delight, the
present oxidation could be successfully applied to oxidation of alkyl
methyl sulfides (6h-k), giving the corresponding sulfoxides of 87-
oxidant, see: (a) Page, P. B.; Heer, J. P.; Bethell, D.; Lund, B. L.
Phosphorus, Sulfur Silicon 1999, 153-154, 247-258. (b) Saito, B.;
Katsuki, T. Tetrahedron Lett. 2001, 42, 3873-3876. (c) Pelotier, B.; Ason,
M. S.; Campbell, I. B.; Macdonald, S. J. F.; Priem, G.; Jackson, R. F. W.
Synlett 2002, 1055-1060. (d) Ohta, C.; Shimizu, H.; Kondo, A.; Katsuki,
T. Synlett 2002, 161-163. (e) Legros, J.; Bolm, C. Angew. Chem., Int.
Ed. 2003, 42, 5487-5489. (f) Miyazaki, T.; Katsuki, T. Synlett 2003,
1046-1048. (g) Thakur, V. V.; Sudalai, A. Tetrahedron: Asymmetry 2003,
14, 407-410. (h) Weix, D. J.; Ellman, J. A. Org. Lett. 2003, 5, 1317-
1
320. (i) Legros, J.; Bolm, C. Angew. Chem., Int. Ed. 2004, 43, 4225-
4228. (j) Sun, J.; Zhu, C.; Dai, Z.; Yang, M.; Pan, Y.; Hu, H. J. Org.
Chem. 2004, 69, 8500-8503. (k) Legros, J.; Bolm, C. Chem.sEur. J.
2
005, 11, 1086-1092. (l) Drago, C.; Caggiano, L.; Jackson, R. F. W.
Angew. Chem., Int. Ed. 2005, 44, 7221-7223.
(
9) Palucki, M.; Hanson, P.; Jacobsen, E. N. Tetrahedron Lett. 1992, 33,
7
94% ee (entries 8-11). Although slight over-oxidation was
111-7114.
observed, the enantiomer differentiation (krel) measured in the
oxidation of racemic methyl octyl sulfoxide was as small as 4.5
(
10) (a) High enantioselectivity greater than 80% in the oxidation of methyl
n-alkyl sulfides has been achieved with a Ti/DET/H O/CHP system:
Brunel, J.-M.; Diter, P.; Duetsch, M.; Kagan, H. B. J. Org. Chem. 1995,
2
(Scheme 1).
6
0, 8086-8088. (b) Recently, we found that oxidation of methyl n-octyl
The mechanism of iron-catalyzed sulfoxidation is not well-
sulfide using Ti(salen) catalyst proceeded with 91% ee: Matsumoto, K.;
Saito, B.; Katsuki, T. Chem. Commun. 2007, advance articles.
11) Yamaguchi, T.; Matsumoto, K.; Saito, B.; Katsuki, T. Angew. Chem., Int.
Ed. 2007, 46, 4729-4731.
understood.8
k,14
In the present oxidation, mixing of solid 4 with
(
(
1
sulfide in water immediately gives a biphasic liquid system. H
NMR analysis of the system (1:1 4/sulfide/D O) showed the
2 2
12) For epoxidation with a Ti(salan)/aqueous H O system, see: Sawada, Y.;
2
Matsumoto, K.; Kondo, S.; Watanabe, H.; Ozawa, T.; Suzuki, K.; Saito,
B.; Katsuki, T. Angew. Chem., Int. Ed. 2006, 45, 3478-3480.
broadened signals of the sulfide, indicating the formation of a
sulfide-coordinated 4. The generation of the biphasic system should
increase substrate concentration around 4. Thus, regardless of a
heterogeneous system, the reaction in water proceeded as rapidly
as that in methanol. Higher enantioselectivity was also observed
in water than in methanol (see Supporting Information).
In summary, we have achieved highly enantioselective sulfoxi-
dation with wide scope of application in water in the absence of a
surfactant by using a novel Fe(salan) complex/aqueous hydrogen
peroxide system. The reaction can be carried out in ambient air
(
13) Bolm, C.; Legros, J.; Paih, J. L.; Zani, L. Chem. ReV. 2004, 104, 6217-
6254.
(
14) It has been reported that Fe(salen) complexes catalyze oxidation enanti-
oselectively, only when PhIO is used as the oxidant.: Bryliakov, K. P.;
Talsi, E. P. Angew. Chem., Int. Ed. 2004, 43, 5228-5230.
(
2 2
15) H O was slightly decomposed under the conditions (see Supporting
Information).
(
16) For recent examples of kinetic resolution of racemic sulfoxide, see: (a)
Thakur, V. V.; Sudalai, A. Tetrahedron: Asymmetry 2003, 14, 407-410.
(b) Mohammadpoor-Baltork, I.; Hill, M.; Caggiano, L.; Jackson, R. F.
W. Synlett 2006, 3540-3544.
JA071916+
J. AM. CHEM. SOC.
9
VOL. 129, NO. 29, 2007 8941