Sulfide oxidation catalyzed vanadyl complexes of N-salicylidene α-amino acids at low catalyst loading

Article abstract of DOI:10.1016/j.ica.2012.03.024

Sulfoxides are extensively used in chemical and pharmaceutical industries. Various catalytically synthetic methods were reported, but in common 1-10 (mol)% catalyst loading were demanded. Sulfide oxidation catalyzed by vanadyl complexes of N-salicylidene α-amino acids with catalyst loading as low as 0.03 (mol)% gave high yields at room temperature with aqueous hydrogen peroxide as an oxidant, which is a green, solvent-free, energy-saving and easy-operated protocol. In contrast, vanadyl complexes of N-salicylidene amino alcohols could not, even at 0.1 (mol)%. The possible reason is that the stronger acidity of N-salicylidene α-amino acids makes their vanadyl complexes stable to aqueous hydrogen peroxide. This rule may be used in designing other high efficient catalysts, especially with aqueous hydrogen peroxide as an oxidant.

Full text of DOI:10.1016/j.ica.2012.03.024

Inorganica Chimica Acta 388 (2012) 11–15
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Inorganica Chimica Acta
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Sulfide oxidation catalyzed vanadyl complexes of N-salicylidene
a-amino acids
at low catalyst loading
a,b,
⇑
c
d
Qingle Zeng
, Wen Weng , Xinghua Xue
a
State Key Laboratory of Oil and Gas Reservoir Geology and Exploitation, Institute of Green Catalysis and Synthesis, Chengdu University of Technology, Chengdu 610059, PR China
Mineral Resources Chemistry Key Laboratory of Sichuan Higher Education Institutions, College of Materials and Chemistry Chemical Engineering, Chengdu University of Technology,
b
Chengdu 610059, PR China
c
Department of Chemistry and Environmental Science, Zhangzhou Normal University, Zhangzhou 363000, PR China
College of Materials and Chemical Engineering, Hainan University, Haikou 570228, PR China
d
a r t i c l e i n f o
a b s t r a c t
Article history:
Sulfoxides are extensively used in chemical and pharmaceutical industries. Various catalytically synthetic
methods were reported, but in common 1–10 (mol)% catalyst loading were demanded. Sulfide oxidation
Received 24 November 2011
Received in revised form 1 March 2012
Accepted 8 March 2012
catalyzed by vanadyl complexes of N-salicylidene
a-amino acids with catalyst loading as low as
0.03 (mol)% gave high yields at room temperature with aqueous hydrogen peroxide as an oxidant, which
Available online 16 March 2012
is a green, solvent-free, energy-saving and easy-operated protocol. In contrast, vanadyl complexes of N-
salicylidene amino alcohols could not, even at 0.1 (mol)%. The possible reason is that the stronger acidity
Keywords:
Sulfide oxidation
Vanadyl complexes
N-Salicylidene a-amino acids
Hydrogen peroxide
of N-salicylidene
a-amino acids makes their vanadyl complexes stable to aqueous hydrogen peroxide.
This rule may be used in designing other high efficient catalysts, especially with aqueous hydrogen per-
oxide as an oxidant.
Ó 2012 Elsevier B.V. All rights reserved.
Catalyst loading
1
. Introduction
Sulfoxides are important chemicals extensively used in chemi-
2. Experimental
2.1. General
cal and pharmaceutical industries [1–5]. They are synthesized
mainly by oxidation of sulfides [6–8], catalyzed by titanium [9],
vanadium [10–18], iron [19], aluminum [20], heteropolyacid [21–
Chemical reagents: Amino acids were purchased from GL Bio-
chem (Shanghai) Ltd. VO(acac) , sulfides, and other chemicals were
2
2
3], molybdenum [24], etc. [25–26].
Early in 1987 Colonna reported oxidation of alkyl aryl sulfides
purchased from Sigma–Aldrich or local suppliers and used as
received.
catalyzed by vanadyl complexes of N-salicylidene
a
-amino acids
Experimental instruments: HPLC instrument (Shimadzu Scientific
Instruments with a model LC-20AB binary pump); Chiralcel OD-H
with tert-butyl hydroperoxide (TBHP) as an oxidant [27]. Later, Fuj-
ita disclosed that 10 (mol)% these vanadyl complexes with TBHP as
an oxidant gave chiral sulfoxides up to 14%ee [28].
chiral column (4.6 mm  250 mm  5
lm, Daicel Chemical Indus-
tries, Ltd.); Infrared absorption spectrometer (Nicolet Avatar 360);
Electrospray ionizing mass spectrometer (ESI-MS) (Bruker Dalton
Esquire 3000 Plus Ion trap liquid chromatography–mass spectrom-
etry; nuclear magnetic resonance spectrometer (Bruker Advance II
300 MHz). The sulfoxides were identified by comparison of their
Besides, vanadyl complexes of N-salicylidene
demonstrate good to excellent enantioselectivity in aerobic oxida-
tion of -hydroxy esters and amides [29] and coupling of 2-naph-
thols [30–32]. As our continuous study on sulfide oxidation
15,33–35], we will investigate vanadyl complexes of N-salicylid-
a-amino acids
a
1
[
H NMR signals and ESI-MS with the literature data. The enantio-
ene amino acids in sulfide oxidation with aqueous hydrogen perox-
ide as a green oxidant (Scheme 1).
meric excess values of the sulfoxides were determined with HPLC
on a Daicel Chiralcel OD-H column (250 mm  4.6 mm  5
lm)
with an eluent of n-hexane/isopropanol (90/10) and a detecting
wavelength at 254 nm. The retention times of methyl phenyl sulfox-
ide are t
retention times of methyl p-chlorophenyl sulfoxide are t
6.1 min for (R) form, and t = 20.1 min for (S) form at a flow rate
of 1.2 mL/min. The retention times of methyl p-methoxyphenyl
1 2
= 9.7 min for (R)-form, and t = 11.7 min for (S)-form. The
⇑
Corresponding author at: State Key Laboratory of Oil and Gas Reservoir Geology
1
=
and Exploitation, Institute of Green Catalysis and Synthesis, Chengdu University of
Technology, Chengdu 610059, PR China. Fax: +86 28 84079074.
E-mail address: qinglezeng@hotmail.com (Q. Zeng).
1
2
0
020-1693/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.ica.2012.03.024

1
2
Q. Zeng et al. / Inorganica Chimica Acta 388 (2012) 11–15
O-
S+
R2
V Schiff base complex
0% H O2
S
Ar
R
3
Ar
R
N
O
O
2
1
2
V
O
R1
OH
Scheme 1. Sulfoxidation catalyzed by vanadyl Schiff base complexes.
R1
7
a, R = I, R = t-Bu
1
2
7
b, R = H, R = Bn
1
2
sulfoxide are t
form at a flow rate of 1.2 mL/min.
1 2
= 18.7 min for (R) form, and t = 22.4 min for (S)
Scheme 3. Amino alcohols derived vanadyl Schiff base complexes.
2.2. Vanadyl complexes of N-salicylidene
a-amino acids
The structures of these complexes were further studied in de-
tails by ESI-MS [36]. Amino alcohols and salicylaldehydes derived
preformed vanadyl complexes (Scheme 3) are prepared according
to the literature [15].
L
-Amino acid, 3,5-bis(tert-butyl)salicylaldehyde (10 mmol), so-
dium acetate trihydrate (40 mmol) were added in a solution of eth-
anol, tetrahydrofuran (THF) and water. The solution was heated to
2
reflux for one hour. And then VO(acac) (10 mmol) was added into
the solution and the solution was heated to reflux for another hour.
Low boiling point solvents ethanol and THF in the resulting purple
solution were removed under vacuo. The residual was extracted
3
.2. Asymmetric sulfoxidation catalyzed by vanadyl complexes of
Schiff bases derived from amino acids
2 2 2 4
with CH Cl , and the organic layer was dried with Na SO , and con-
With vanadyl complexes of Schiff bases derived from amino
acids at hand, we first evaluated their asymmetric sulfoxidation
with 30% aqueous hydrogen peroxide and without solvent.
densed in vacuo to give the corresponding vanadyl complex. These
vanadyl complexes were measured with ESI-MS. Moreover, the
ESI-MS fragmentation of these vanadyl Schiff base complexes and
their (oxo)-peroxovanadium Schiff base complexes were also stud-
ied systematically by us [32].
2 2
When 1 (mol)% catalyst and 10.5 equivalent 30% H O were
used, all of these vanadyl complexes transformed thioanisole into
its corresponding sulfoxide without any over-oxidation product
sulfone detected, indicating high chemical selectivity (Table 1).
Most reactions were completed within four hours, and the yields
of the sulfoxide were good to excellent (Table 1, entries 1–8). How-
ever, their enantioselectivities were poor (30–51%ee). When the
oxidation of thioanisole was done at room temperature (20 °C),
there was a little increase in the yield, but ee values decreased
slightly (Table 1, entries 9 and 10). Some more sulfides (p-ClPhSMe
and p-MeOPhSMe) were evaluated, but ee values were still low
2.3. Typical procedure of vanadium-catalyzed asymmetric
sulfide oxidation
Vanadyl Schiff base complexes (0.01 mmol) and certain sulfide
(
10 mmol) were added into a 25 mL flask equipped with a mag-
netic stir bar, and then the flask was stirred slowly for a certain
time. After that the reaction was quenched with aqueous NaHSO
3
,
and diluted with ethyl acetate. The resulting solution was washed
with water, saturated sodium chloride, and then dried with anhy-
drous MgSO , and condensed in vacuo to give crude product, which
4
was purified with silica column chromatography using petroleum
ether and ethyl acetate as an eluent to give pure sulfoxide as thick
oil.
(
Table 1, entries 11–12). However, these ee values are much higher
than that obtained with TBHP as a terminal oxidant and at
0 (mol)% catalyst loading reported by Fujita and his coworkers
1
(
<14%ee) [4].
On further research of sulfide oxidation, we discovered that this
catalyst system was effective at a catalyst loading as low as
.03 (mol)%, which is much lower than vanadium, iron, titanium
0
3
. Results and discussion
or other catalyst systems (Table 2 and Table 3).
3.1. Vanadyl complexes of Schiff base derived from amino acids
and from amino alcohols
3.3. Effects of catalyst loading on sulfide oxidation catalyzed
by the vanadyl complexes
The general synthetic method of vanadyl Schiff base complexes
6
derived from 3,5-bis(tert-butyl)salicylaldehyde and amino acids
Since vanadyl complexes of Schiff base derived from chiral
amino acids efficiently transformed several sulfides into their
is shown in Scheme 2.
R
O
R
O
O
R
O
N
ONa
N
O
NaOAc
But
1. VO(acac)2, air, reflux
2. CH Cl /H O
OH
+
OH EtOH/THF But
But
V
O
H N
OH
O
OH
2
2
2
2
But
3
4
But
But
5
e, R = Me
6
a, R = Ph
b, R = Bn
c, R = i-Pr
d, R = i-Bu
f, R = TBS-OCH
2
H
N
g, R =
h, R = t-Bu
CH2
Scheme 2. Synthesis of vanadyl complexes 6 of Schiff bases derived from chiral amino acids 4.

Q. Zeng et al. / Inorganica Chimica Acta 388 (2012) 11–15
13
Table 1
When 1 (mol)% catalyst loading were used, both complexes
nearly completely transform thioanisole into its sulfoxide within
Asymmetric sulfoxidation catalyzed by chiral vanadyl catalysts of Schiff base derived
from amino acids.a
8
h, and high yield were obtained (Table 2, entries 1, 5, 9 and
Entry
Sulfide
Catalyst
Time (h)
Ee (%)b
Yield (%)
13). When 0.5 (mol)% catalyst loading was used, both complexes
could nearly transform completely thioanisole into its sulfoxide
within 20 h, and high yields were also obtained (Table 2, entries
1
2
3
4
5
6
7
8
PhSMe
PhSMe
PhSMe
PhSMe
PhSMe
PhSMe
PhSMe
PhSMe
PhSMe
PhSMe
p-ClPhSMe
p-MeOPhSMe
6a
6b
6c
6d
6e
6f
6g
6h
6c
6h
6c
6c
8
8
8
8
8
10
16
8
8
8
33
44
49
30
41
41
43
51
46
49
40
41
90
86
91
81
88
84
90
93
92
94
82
85
2
, 6, 10 and 14).
But when 0.1 (mol)% catalyst loading were used, thioanisole
(
56% for 7a and 74% for 7b) were recovered by silica column chro-
matography, and low yield (28% for 7a and 26% for 7b) were ob-
tained for vanadium complexes of N-salicylidene
a-amino
c
9
alcohols (Table 2, entries 3 and 7).
1
1
1
0c
1
2
Surprisingly, when vanadyl complexes of Schiff bases of amino
acids 6c and 6h were used, there was near complete conversion
and excellent yields up to 97% were obtained even in a catalyst
amount of 0.1 (mol)% (Table 2, entries 11 and 15).
8
8
a
2 2
Reaction conditions: 10 mmol sulfide, 10.5 mmol 30% aqueous H O , 0.1 mmol
vanadyl complex of Schiff base derived from amino acids, 0 °C, except other
mentioned.
When 0.03 (mol)% amino alcohols derived vanadyl Schiff base
complexes 7a and 7b were used, most thioanisole was recovered,
and less than 10% yields were obtained (Table 2, entries 4 and 8).
Vanadyl complexes 6c and 6h of Schiff bases derived from amino
acids still gave high yields at a catalyst loading as low as
b
Major isomer of all of the products are S form.
Reaction temperature: room temperature (20 °C).
c
Table 2
0
.03 (mol)%, when the reaction time was prolonged to 144 h (Table
The effects of several vanadium complexes and catalyst amount on oxidation of
thioanisole.a
2, entries 12 and 16).
Vanadyl complex (6c) of Schiff base derived from valine was ap-
plied in the oxidation of various sulfides (Table 3). Some more sub-
strates were examined at 0.03 (mol)% catalyst loading at room
temperature for 144 h. The results of oxidation of various sulfide
without optimization are listed in Table 3. Various alkyl phenyl
sulfides, including methyl, ethyl, benzyl and allyl, gave good yields
(Table 3, entries 1–4). For aryl methyl sulfides, para-methyl-, flu-
oro-, bromo-phenyl methyl sulfides all produced sulfoxides with
high yields (Table 3, entries 5–7). Ortho-chlorophenyl methyl sul-
fide was also smoothly transformed into its sulfoxide with good
yield (Table 3, entry 8). Dialkyl sulfides, such as dibenzyl sulfide,
di-n-butyl sulfide, also gave good yields (Table 3, entries 9 and 10).
Entry
Catalyst
Amount (mol%)
Time (h)
Yield (%)
1
2
3
4
5
6
7
8
9
7a
7a
7a
7a
7b
7b
7b
7b
6c
6c
6c
6c
6h
6h
6h
6h
1
8
86
83
0.5
0.1
0.03
1
0.5
0.1
0.03
1
0.5
0.1
0.03
1
0.5
0.1
0.03
20
48
144
8
20
48
144
8
20
48
144
8
20
48
144
b
28 (56)
9 (74)b
91
82
b
26 (57)
8 (77)b
92
1
1
1
1
1
1
1
0
1
2
3
4
5
6
86
98
97
94
90
97
96
3.4. Experimental phenomena in sulfoxidation with vanadyl
complexes and explanation
a
Reaction conditions: 10 mmol thioanisole, 10.5 mmol 30% aqueous H
2 2
O , a
When 1 and 0.5 (mol)% vanadyl complexes 7a and 7b of Schiff
bases derived from phenylalaninol and tert-leucinol was used in
thioanisole oxidation, the catalysts remained brown in color during
the whole reaction. However, when 0.1 (mol)% vanadyl complexes
were used, the brown color of the reaction mixtures slowly disap-
peared and the solution finally turned colorless. It was also found
that thioanisole could not be transformed into sulfoxide com-
pletely, even if the reaction time was as long as 144 h (Table 2, en-
tries 1–8). But for amino acids derived vanadyl Schiff base
complexes 6c and 6h, although the catalyst amount decreased
from 1 to 0.03 (mol)%, the reaction mixtures maintained the purple
color of the catalyst from start to finish (Table 2, entries 9–16). This
proved the effectiveness of the catalysts.
certain amount of vanadyl complex of Schiff base derived from amino acids
(
0.1 mmol for 1 (mol)%, 0.01 for 0.1 (mol)%), room temperature.
The values in parentheses is the percent recover of thioanisole.
b
Table 3
Oxidation of various sulfides with 0.03 (mol)% vanadyl complexes 6c of Schiff bases
derived from amino acids.a
Entry
Sulfide
Sulfoxide
Yield (%)
1
2
3
4
5
6
7
8
9
PhSMe
PhSEt
PhSOMe
PhSOEt
96
85
83
86
84
82
89
83
96
85
PhSCH
PhSCH
2
Ph
CH = CH
PhSOCH
PhSOCH
2
Ph
CH = CH
2
2
2
2
p-TolSMe
p-FPhSMe
p-BrPhSMe
o-ClPhSMe
BnSBn
p-TolSOMe
p-FPhSOMe
p-BrPhSOMe
o-ClPhSOMe
BnSOBn
Why did the color of vanadyl complexes of Schiff bases derived
from amino alcohols disappear at low catalyst loading? Why did
those derived from amino acids maintain their color?
When higher catalyst loadings 1 and 0.5 (mol)% were used, the
amount of hydrogen peroxide permeated from aqueous hydrogen
peroxide by solution equilibrium was consumed by the catalyst
to oxidize thioanisole. When low catalyst loading of 0.1 (mol)% or
lower (0.03 (mol)%) was adopted for the sulfide oxidation, the
amount of hydrogen peroxide permeated from aqueous phase
could not be depleted in time. The excess hydrogen peroxide
would then attack the (oxo)-peroxovanadium Schiff base complex,
as shown in Scheme 4. This resulted in the decomposition of the
(oxo)-peroxovanadium Schiff base complex and production of van-
adyl diperoxo complex and the Schiff base N-salicylidene amino
n
n
n
n
1
0
Bu SBu
Bu SOBu
a
Reaction conditions: 20 mmol sulfide, 0.006 mmol 6c, 21 mmol 30% H
2 2
O , rt,
1
44 h.
sulfoxides with high yield within a short time, we continued to
evaluate their catalyst loading.
In the examination of the catalyst loading of these vanadyl com-
plexes of Schiff bases derived from amino acids, amino alcohols de-
rived vanadyl complexes were taken for a comparison (Table 2).

1
4
Q. Zeng et al. / Inorganica Chimica Acta 388 (2012) 11–15
H
O
OH
2
OH
V
2
R
1
R
1
R1
O
H
N
O
H O
N
O
H O
2
2
N
O
O
O
2
2
V
O
V
R
2
O
OH
R2
O
R2
O
O
O
H
O
O
H
R
2
R
2
R2
O
H
O
R1
OH
R
1
OH
O
V
O
O
O
N
N
+
V
O
OH
R2
O
R2
OH
O
vanadyl diperoxo
O
H
complex
R
2
R
2
Scheme 4. Probable decomposition way of the amino alcohol derived vanadyl complexes.
decreases, vanadyl complexes derived from amino diols will
decompose in aqueous hydrogen peroxide and only can be used
in organic hydroperoxide. This rule can be useful for chemists to
design highly efficient catalysts used in aqueous H O . In addition,
2 2
a green, low catalyst loading (0.03 (mol)%), solvent-free and easy-
operated sulfide oxidation protocol with vanadyl complexes of N-
salicylidene a-amino acids as catalysts was disclosed.
O
O
R1
O
O
R1
N
N
H O
H O
2 2
2
2
V
O
V
R2
O
OH
R2
O
O
O
O
H
R2
R2
O
O
O
H
H
H
O
O
R1
R
1
O
Acknowledgements
N
O
N
O
OH
O
V
O
V
The authors thank the National Science Foundation of China
(Grant No. 20672088), the Ministry of Human Resources and Social
Security of China (Grant No. ZF0022), the Science and Technology
Bureau of Sichuan (Grant No. 2011HH0016), the Opening Fund of
State Key Laboratory of Geohazard Prevention and Geoenviron-
ment Protection (Grant No. SKLGP2012K005), and the Cultivating
Programme for Excellent Innovation Team of Chengdu University
of Technology (No. HY0084) for financial support. We sincerely
thank Mr. Allan Chelashaw, College of Material Science and Engi-
neering, Donghua University for his polishing this paper.
R2
O
R2
O
O
O
O
H
H
R2
R2
Scheme 5. Amino acid derived vanadyl complexes stable to hydrogen peroxide’s
attack.
alcohol, which was verified by ESI-MS [36] and by thin layer chro-
matography. The catalyst became colorless once the vanadyl com-
plexes decomposed.
Amino acid Schiff bases derived has stronger acidity than amino
alcohol derived ones, so their vanadyl complexes are more resis-
tant to decomposition when attacked by hydrogen peroxide than
those derived from amino alcohols. Thus hydrogen peroxide attack
could not break the V–OC(@O) bond or V–O–Ar bond, so this kind
of vanadyl complexes could exist for a long time in the presence of
excess hydrogen peroxide, as shown in Scheme 5. The peroxovan-
adyl complexes of Schiff bases derived from amino acids are fairly
stable and their fragmentation have been studied in details in our
laboratory by tandem ESI-MS [36]. Thus, vanadyl complexes re-
mained purple, and there was continuous catalytic oxidation of
thioanisole.
Appendix A. Supplementary material
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.ica.2012.03.024.
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