Catalytic asymmetric oxidation of alkyl aryl sulfides mediated by a series of chiral N-alkyl-1,2-diphenylaminoethanol/titanium/water complexes

Article abstract of DOI:10.1081/SCC-120022167

A series of chiral N-alkyl-1,2-diphenylaminoethanol/titanium/water complexes were investigated as potential efficient catalyst for the enantioselective oxidation of alkyl aryl sulfides. The structure of the aminoethanols strongly influenced on the reactivity and enantioselectivity. Using(1S,2S)-N-methyl-1,2-diphenylaminoethanol (6) as ligand in oxidation of methyl phenyl sulfide, gave (S)-sulfoxide with moderate yield and ee.

Full text of DOI:10.1081/SCC-120022167

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Õ
SYNTHETIC COMMUNICATIONS
Vol. 33, No. 16, pp. 2793–2801, 2003
Catalytic Asymmetric Oxidation of Alkyl Aryl
Sulfides Mediated by a Series of Chiral
N-Alkyl-1,2-diphenylaminoethanol/
Titanium/Water Complexes
1
2,
3
YunGui Peng, XiaoMing Feng, * Xin Cui,
3
,
4
YaoZhong Jiang, * Michael C. K. Choi,
4
and Albert S. C. Chan
1
The School ofChemistry and Chemical Engineering,
Southwest Normal University,
BeiBei, ChongQing, China
2
Sichuan Key Laboratory ofGreen Chemistry and
Technology, College ofChemistry,
Sichuan University, Chengdu, China
Chengdu Institute ofOrganic Chemistry, Chinese
3
Academy ofSciences, Chengdu, China
4
Open Laboratory ofChirotechnology and Department
ofApplied Biology and Chemical Technology, The
Hong Kong Polytechnic University, Hong Kong, China
*
Correspondence: XiaoMing Feng, Sichuan Key Laboratory ofGreen
Chemistry and Technology, College ofChemistry, Sichuan University,
Chengdu 610064, China. YaoZhong Jiang, Chengdu Institute ofOrganic
Chemistry, Chinese Academy ofSciences, Chengdu 610041, China; E-mail:
xmfeng@pridns.scu.edu.cn.
2
793
DOI: 10.1081/SCC-120022167
Copyright & 2003 by Marcel Dekker, Inc.
0039-7911 (Print); 1532-2432 (Online)
www.dekker.com

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2
794
Peng et al.
ABSTRACT
A series ofchiral N-alkyl-1,2-diphenylaminoethanol/titanium/water
complexes were investigated as potential efficient catalyst for the
enantioselective oxidation ofalkyl aryl sulfides. The structure of
the aminoethanols strongly influenced on the reactivity and enantio-
selectivity. Using (1S,2S )-N-methyl-1,2-diphenylaminoethanol (6) as
ligand in oxidation ofmethyl phenyl sulfide, gave ( S )-sulfoxide with
moderate yield and ee.
Key Words: Aminoethanol; Titanium(IV) complexes; Chiral sulf-
oxide; Asymmetric catalysis.
Optically active sulfoxides have been extensively used as building
blocks in the pharmaceutical industry and chiral auxiliaries in asym-
[1]
metric synthesis. In recent year, a lot ofresearch have been carried
out in enantioselective oxidation ofreadily available prochiral sulfides
[2–10]
to sulfoxides.
Amongst the chiral ligands employed for the oxida-
tion, (1S,2S )-1,2-diphenylethan-1,2-diol has been proved to be an effec-
tive one. But it was found to decompose both via pinacol-type
transposition and via degradative oxidation when the reaction time was
over 2 h, and so further extension of the reaction time could even give rise
[7]
to a decrease ofee. (1S,2S )-1,2-Diphenylaminoethanol, which has the
same chiral carbon backbone and one different function group with
(
1S,2S )-1,2-diphenylethan-1,2-diol, and their derivatives as chiral ligands
had been successfully employed in many asymmetric catalytic reac-
[11–13]
tions.
from (1R,2S )-, (1S,2S )-1,2-diphenylaminoethanol in the enantioselective
oxidation ofsulfides.
We also have successfully used oxazolines which derivate
[14]
These results further prompted us to investigate
the catalytic efficiency ofchiral 1,2-diphenylaminoethanols and their
derivatives in the asymmetric oxidation ofsulfides. Herein, we wish to
report the results obtained in the asymmetric oxidation ofalkyl aryl
sulfides 1 to their sulfoxide 2 and sulfone 3 in the presence ofcatalytic
i
amount ofcomplexes of rmed in situ by reacting Ti(O Pr) , N-alkyl-1,2-
4
diphenylaminoethanols 4–11 and water (Sch. 1).
The results obtained from the enantioselective oxidation of methyl
phenyl sulfide (used as test substrate) indicated that 6 was an efficient
i
ligand (Table 1, Entry 3). Use of5 mol% of 6/Ti(O Pr) /H O as catalyst
4
2
ꢀ
in CCl at 0 C and 2.0 equiv. ofTBHP (70% in water) as oxidant resulted
4
in sulfoxide in 50% yield with 50% ee (S ). When the alkyl on N-atom
was changed, the yields and enantioselectivities decreased sharply.

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N-Alkyl-1,2-diphenylaminoethanol/Titanium/Water Complexes
2795
O
O
S
i
S
Ti(O Pr) /Ligand 4-11/H O
4
2
S
+
Ar
R'
Ar
R'
Ar * R'
O
3
CCl , 0˚C, TBHP
4
1
2
Ph
Ph
Ph
Ph
OH
RHN
MeHN
OH
4
5: R=H; 6: R=Me
7
9
1
: R=Et; 8: R=Pr
i
: R=Pr ; 10: R=-(C H )-
6
11
1: R=PhCH2-
Scheme 1.
a
Table 1. Effect ofligand on the reactivity and enantioselectivity.
b
Yield (%)
c
Ee (%) (config.)
d
Entry
Ligand
Time (h)
1
2
3
4
5
6
7
8
4
5
23
36
45
41
65
63
64
50
50
26
50
47
44
40
33
34
15.3 (R)
5 (S )
50 (S )
3 (S )
4 (S )
2 (S )
2 (S )
2 (S )
6
7
8
9
10
11
a
i
Reaction conditions: sulfide/ligand/Ti(O Pr)
4
/H
2
O ¼ 1.0:0.1:0.05:1.0, reaction
ꢀ
temperature ¼ 0 C, CCl was used as solvent, 2.0 equiv. ofTBHP (70% in
4
water) was used as oxidant.
b
Isolated yield.
Determined by HPLC on a Daicel Chiralcel OB column.
c
d
[2a]
The absolute configuration was assigned by comparison with literature values.
Even changed from methyl to ethyl, the enantioselectivity of the product
varied from 50% ee to almost racemic. When use (1S,2S )-1,2-diphenyl-
aminoethanol (5) as ligand, both the yield and enantiomeric excess of
sulfoxide were very low (Table 1, Entry 2). This indicated that it was
significant inferior to (1S,2S )-1,2-diphenylethan-1,2-diol in the same cat-
alytic reaction procedure owing to the different electronic property and
coordinate ability ofthe amino and hydroxy group. The configuration of
C (1) influenced strongly on the enantioselectivity ofsul of xide. When

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796
Peng et al.
aminoethanol 4 was used as chiral ligand, the sulfoxide was obtained in
0% yield with 15% ee (R). Analyzing the configuration ofenantiomeric
5
excess ofsul fo xides and the structure ofcorresponding chiral aminoetha-
nols 4 and 6, we can conclude that the absolute configuration ofenan-
tiomeric excess ofsul fo xides was determined by the stereogenic center of
C (1) in aminoethanols.
In order to achieve good reactivity and high asymmetric induction,
several reaction parameters had to be optimized. A series ofsolvents
including CH Cl , DCE, C H , CCl , THF, Et O were screened. The
2
2
7
8
4
2
best solvent was found to be CCl . The effect oftemperature was also
4
tested, the reactivity and enantioselectivity varied with the temperature
ꢀ
from 0 to 20 C. The optimum reaction temperature was found to be
ꢀ
i
0
C. The effect ofa molar ratio of6 to Ti(O Pr) was examined (Table 2).
4
When 2.0 equiv. of 6 were used per Ti, the best yields and enantio-
selectivities were obtained (Table 2, Entry 3). We also studied the influ-
ence ofadding water when prepared the catalytic complex on the
reaction. It was found that the enantioselectivity was not obviously
influenced by it, but it causes the reactivity greatly increased. This
may be the result ofcausing the catalyst to of rm the dimmer and to
be activated.
The amount ofcatalyst also influenced the reactivity and the enan-
tioselectivity (Table 3). Five mole percent catalyst was sufficient to
achieve good enantioselectivity (Table 3, Entry 2). When the amount of
i
to Ti(O Pr)
Table 2. Effect ofmolar ratio of6
a
enantioselectivity.
4
on the reactivity and
b
Yield (%)
Enantioselectivity
i
6/Ti(O Pr)
c
Ee (%) (config.)
d
Entry
Total yield
2
Chemo (2:3)
4
1
2
3
4
5
1:1
1.2:1
2:1
59
60
77
53
56
49
51
67
46
51
83:17
85:15
87:13
86:14
91:9
11 (S )
21 (S )
52 (S )
49 (S )
54 (S )
2.2:1
3:1
a
Reaction conditions: 5% catalyst amount, sulfide/H O ¼ 1:1, reaction
2
ꢀ
temperature ¼ 0 C, reaction time ¼ 72 h, CCl
4
as solvent was used, 4.0 equiv. of
TBHP (70% in water) was used as oxidant.
b
Isolated yield.
Determined by HPLC on a Daicel Chiralcel OB column.
c
d
[2a]
The absolute configuration was assigned by comparison with literature values.

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N-Alkyl-1,2-diphenylaminoethanol/Titanium/Water Complexes
2797
a
Table 3. Effect ofcatalyst amount on the reactivity and enantioselectivity.
b
Yield (%)
Enantioselectivity
Catalyst
amount (%)
c
Ee (%) (config.)
d
Entry
Total yield
2
Chemo (2:3)
1
2
3
4
2.5
5
43
77
57
70
43
67
47
60
100:0
87:13
82:18
87:13
23 (S )
52 (S )
50 (S )
51 (S )
10
20
a
i
Reaction conditions: ligand 6/Ti(O Pr)
4
¼ 2:1, sulfide/H
2
O ¼ 1:1, reaction
ꢀ
temperature ¼ 0 C, reaction time ¼ 72 h, CCl
4
as solvent was used, 4.0 equiv. of
TBHP (70% in water) was used oxidant.
b
Isolated yield.
Determined by HPLC on a Daicel Chiralcel OB column.
c
d
[2a]
The absolute configuration was assigned by comparison with literature values.
a
Table 4. Effect ofoxidant on the reactivity and enantioselectivity.
b
Yield (%)
Enantioselectivity
Time
(h) Total yield 2 Chemo (2:3) Ee (%) (config.)
c
d
Entry
Oxidant
1
2
3
4
5
6
TBHP (1.5 equiv.) 48
TBHP (2.0 equiv.) 72
26
51
33
72
77
55
26
43
27
59
67
49
100:0
84:16
82:18
82:18
87:13
89:11
45 (S )
51 (S )
36 (S )
56 (S )
52 (S )
26 (S )
e
TBHP (2.0 equiv.) 50
TBHP (3.0 equiv.) 72
TBHP (4.0 equiv.) 72
CHP (2.0 equiv.)
72
a
i
Reaction conditions: sulfide/ligand 6/Ti(O Pr)
4
/H
2
O ¼ 1.0:0.1:0.05:1.0, reaction
ꢀ
temperature ¼ 0 C, CCl as solvent was used, TBHP (70% in water) was used as
4
oxidant.
b
Isolated yield.
Determined by HPLC on a Daicel Chiralcel OB column.
c
d
The absolute configuration was assigned by comparison with literature
[
2a,e]
values.
e
TBHP (5-6 M in decane) was used as oxidant.
catalyst was decreased to 2.5 mol%, the enantiomeric excess was only
23% (Table 3, Entry 1).
We also investigated the influence ofoxidant on this reaction (Table 4).
The best results were obtained when 3.0 equiv. ofTBHP (70% in water)
was used as oxidants (Table 4, Entry 4). TBHP (5–6 M in decane) was

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2
798
Peng et al.
a
Table 5. The asymmetric oxidation ofsulfides (ArSR).
b
Yield (%)
Enantioselectivity
0
c
Chemo (2:3) Ee (%) (config.)
d
Entry
Ar
R
Total yield
2
1
2
3
4
Ph
p-MeC H
Me
Me
Me
Et
72
73
60
46
59
65
55
42
82:18
89:11
92:8
56 (S )
39 (S )
42 (S )
34 (S )
6
4
6 4
p-BrC H
Ph
91:9
a
i
6/Ti(O Pr)
Conditions:
sulfide/ligand
4
/H
2
O ¼ 1.0:0.1:0.05:1.0,
reaction
ꢀ
temperature ¼ 0 C, reaction time ¼ 72 h, CCl
4
as solvent was used, 4.0 equiv. of
TBHP (70% in water) was used as oxidant.
b
Isolated yield.
Determined by HPLC on a Daicel Chiralcel OB column.
c
d
[2a]
The absolute configuration was assigned by comparison with literature values.
obviously inferior to TBHP (70% in water) (Table 4, Entries 2 and 3).
When using cumene hydroperoxide (CHP) as oxidant, only 26% ee of
sulfoxide was obtained (Table 4, Entry 6).
To further explore the potential of this catalyst system, we examined
oxidation ofother sulfides under the optimized condition; these results
are summarized in Table 5. In all cases, the enantiomeric excess ofthe
products was moderate (34 to 56% ee) and (S )-configuration sulfoxides
were obtained. Depending on the structural features of the substrates, the
chemical yields varied from 42 to 65%.
In conclusion, we have probed the possibility ofusing chiral N-alkyl-
,2-diphenylaminoethanols/titanium/water complexes as catalyst in the
1
enantioselective oxidation ofsulfides. The relationship between the struc-
ture ofchiral ligands and the absolute configuration ofsul of xides were
demonstrated. Further efforts to improve the catalytic efficiency ofthe
reaction protocol are in progress.
EXPERIMENTAL
Melting points were determined on a Southend SS25PH appartus
1
and were uncorrected. H NMR spectra were recorded with a Bruker-
300 spectrometer. IR spectra were recorded with a Nicolet 560 spectro-
photometer. Optical rotations were measured using a Perkin–Elmer 341
digital polarimeter, in a cell ofpath length 10 cm and are given in units of
ꢁ ꢁ1
1
2
10
deg cm g . Chiral HPLC analyses were performed on a Beckman-

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N-Alkyl-1,2-diphenylaminoethanol/Titanium/Water Complexes
2799
110B chromatographic system with a Beckman 168 UV detector (254 nm).
OB and OD column was purchased from Daicel Chemical Industries,
Ltd. Elemental analyses were recorded on a Carlo Erba-1106 instrument.
(
reported method.
prepared according to Lit.
according to literature procedures.
1R,2S ) and (1S,2S )-1, 2-diphenylaminoethanols were prepared by
(1R,2S )-N-methyl-1,2-diphenylaminoethanol 4 was
[15]
[
16]
The aminoalcohols 6–11 were prepared
[
17]
General Procedure for the Asymmetric Oxidation of Sulfides
To
0.05 mmol) in CCl4 (2.5 mL) was added Ti(O Pr) (0.025 mmol) by
a
suspension ofchiral
N-alkyl-1,2-diphenylaminoethanol
i
(
means ofa syringe. This mixture was stirred at r.t. fo r 2 h and the solvent
4
was removed under reduced pressure, and then CCl (2.5 mL) and water
4
(
(
9 mL) were added. The mixture was stirred at r.t. for 1 h, the sulfide
0.50 mmol) was introduced by means ofa syringe and the mixture was
ꢀ
cooled to 0 C. After 0.5 h, TBHP (70% in water) was added by means of
ꢀ
a syringe, and the mixture was stirred at 0 C for 72 h. The reaction
mixture was directly purified by flash chromatograph (2:1 petroleum
ether–ethyl acetate).
Methyl phenyl sulfoxide. ꢀH (300 MHz; CDCl ; Me Si): 2.72 (s, 3H,
3
4
ꢁ1
CH ), 7.57 (m, 5H, Ar). ꢁ
3
(mull)/cm : 1477, 1443, 1415, 1089, 1036. (S )
max
25
5
6% ee. ½aŠ ꢁ34.4 (c 1.64, CH COCH ).
D
3
3
p-Methylphenyl methylsulfoxide. ꢀ (300 MHz; CDCl ; Me Si): 2.42
H
3
4
(
s, 3H, PhCH ), 2.71 (s, 3H, SOCH ), 7.33 (d, 2H, J ¼ 7.98 Hz, Ar), 7.54
3
3
ꢁ
(mull)/cm : 1493, 1421, 1088, 1048. (S )
max
1
(
3
d, 2H, J ¼ 7.89 Hz, Ar). ꢁ
2
D
7
9% ee. ½aŠ ꢁ10.4 (c 1.84, CH COCH ).
3
3
p-Bromophenyl methylsulfoxide. ꢀ (300 MHz; CDCl ; Me Si): 2.72
H
3
4
ꢁ
(mull)/cm : 1469, 1417, 1387, 1084,
max
1
(
s, 3H, CH ), 7.60 (m, 4H, Ar). ꢁ
3
2
7
1
043, 1008, 516. (S ) 42% ee. ½aŠ ꢁ19.5 (c 0.60, CH COCH ).
D
3
3
Ethyl phenyl sulfoxide. ꢀH (300 MHz; CDCl ; Me Si): 1.20 (t, 3H,
3 4
J ¼ 7.4 Hz, CH CH ), 2.80 (m, 2H, CH CH ), 7.60 (m, 5H, Ar). ꢁ
3
2
3
2
max
ꢁ
1
27
(
CH COCH ).
mull)/cm : 1443, 1086, 1044, 1021. (S ) 34% ee. ½aŠD ꢁ30.9 (c 1.0,
3
3
ACKNOWLEDGMENTS
We thank the National Natural Science Foundation ofChina
No. 29832020) and The Hong Kong Polytechnic University ASD
Fund for financial support of the study.
(

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Peng et al.
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Received in Japan October 26, 2002

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