Enantioselective sulfoxidation catalyzed by polymer-supported chiral Schiff base-VO(acac)2 complexes

Article abstract of DOI:10.1016/j.tetasy.2004.06.041

The preparation of a series of polymer supported chiral Schiff bases derived from salicylic aldehydes and optically active amino alcohols is reported. These heterogeneous ligands have been complexed with VO(acac) ₂ and employed to catalyze the

Full text of DOI:10.1016/j.tetasy.2004.06.041

Tetrahedron:
Asymmetry
Tetrahedron: Asymmetry 15 (2004) 2467–2473
Enantioselective sulfoxidation catalyzed by polymer-supported
chiral Schiff base–VO(acac)2 complexes
Alessando Barbarini, Raimondo Maggi, Michele Muratori, Giovanni Sartori* and
Raffaella Sartorio
`
Clean Synthetic Methodologies Group, Dipartimento di Chimica Organica e Industriale dell’Universita, Parco Area delle Scienze
17A, I-43100 Parma, Italy
Received 14 May 2004; accepted 28 June 2004
Abstract—The preparation of a series of polymer supported chiral Schiff bases derived from salicylic aldehydes and optically active
amino alcohols is reported. These heterogeneous ligands have been complexed with VO(acac)2 and employed to catalyze the enan-
tioselective oxidation of sulfides to sulfoxides with hydrogen peroxide as an environmentally acceptable oxidant. The procedure
affords the sulfoxides in good yield and selectivity and with ee values comparable to those obtained with the homogeneous coun-
terparts. The catalyst can be used for at least four cycles without any significant decrease in both efficiency or enantioselectivity.
ꢀ 2004 Elsevier Ltd. All rights reserved.
1. Introduction
which are worthy of note, are Ti-beta zeolite,5 silica sup-
ported titanium/tartaric acid,6 a composite metal oxide
7
Optically active sulfoxides are an important class of
compounds useful as synthetic intermediates, chiral aux-
iliaries in asymmetric synthesis1 as well as bioactive
ingredients in the pharmaceutical industry.2 The asym-
metric oxidation of sulfides represents a convenient
chemo and stereoselective route to optically active
sulfoxides.
catalyst LiNbMoO6 and tungstate-exchanged Mg–Al
layered double hydroxide.8 The photochemical oxida-
tion has also been reported to occur with oxygen in
the presence of a nafion membrane doped with a lead
ruthenate pyrochlore catalyst9 or a clay-bound methyl-
ene blue.10
Particular attention has been paid to the asymmetric
sulfoxidation with some efficient procedures being
reported.11 Kagan, in continuation of his early studies,
described the enantioselective oxidation of sulfides by
cumyl hydroperoxide in the presence of Ti(OPri)4/
(R,R)-diethyltartrate with ee values up to 96%.12 This
methodology has been further developed by using
hydroperoxyfurans and pyrans as oxygen donors.13
The reaction has been performed by using electrophilic
oxidants because the initial oxidation of the highly
nucleophilic sulfide to a sulfoxide is an easier process
than the subsequent oxidation of the resulting much less
nucleophilic sulfoxide to sulfone.3
In recent years the reaction has been carried out with a
large number of homogeneous and heterogeneous cata-
lysts with particular attention to the use of eco-compat-
ible oxidants. Thus, the aerobic highly selective
oxidation of sulfides to sulfoxides has been carried out
in the presence of Fe(NO3)3–FeBr3.4
Much effort has also been directed towards the develop-
ment of methods for the enantioselective synthesis of
mono-oxides of cyclic dithioacetals, since these com-
pounds serve as chiral masked acyl groups.14
Moreover, different solid catalysts were utilized to effi-
ciently and selectively perform the reaction with hydro-
gen peroxide or its adduct with urea. Among them,
Although different oxidants and metal catalysts have
been described and continue to receive attention, current
efforts are mainly devoted to the development of oxida-
tion reactions using hydrogen peroxide, as this is an
inexpensive, readily available, environmentally benign
and atom efficient reagent. In this respect, the catalytic
system of a vanadium/chiral Schiff base described by
*
Corresponding author. Tel.: +39-0521-905551; fax: +39-0521-
905472; e-mail: giovanni.sartori@unipr.it
0957-4166/$ - see front matter ꢀ 2004 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetasy.2004.06.041

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A. Barbarini et al. / Tetrahedron: Asymmetry 15 (2004) 2467–2473
Bolm and Bienewald in 1995 is particularly attractive15
and consequently it was further studied and applied to
different substrates.16
10 and 11. Vanadium complexes 9V–12V were simply
prepared by stirring the supported ligand (an amount
corresponding to 0.150mmol of imine) with VO(acac)2
(0.100mmol) in CH2Cl2 at room temperature for 4h.
During the reaction, the resins, originally yellow, turned
to green. The supported vanadium catalysts were fil-
tered, washed with CH2Cl2 several times and dried in
vacuo for 2h. The loading values of the imine ligand
in polymers 9–12 and the vanadium in complexes 9V–
12V are reported in Table 1.
Due to the great interest in the present topic, some
groups faced the problem of performing the reaction
in the presence of solid supported vanadium17 and tita-
nium18 chiral Schiff base catalysts performing the oxida-
tion of aryl alkyl sulfides in good yields and promising
ee values. Therefore, it is desirable to develop further
studies with the goal of making new solid catalysts
and achieving information on their efficiency on the ste-
reoselective sulfoxidation reaction.
In all materials, the vanadium content was considerably
lower than that of the Schiff base. This is probably due
to the fact that, during the polymerization process, var-
iable amounts of the ligand molecules are incorporated
within the framework of the material in hard to reach
regions, despite the use of toluene as porogen molecule.
In continuation of our involvement in this area,19 we
report herein the preparation of a small library of solid
supported chiral salicylaldimines and its use in the vana-
dium-catalyzed enantioselective oxidation of sulfides to
sulfoxides with hydrogen peroxide.
Initially we investigated the performances of catalysts
9V–12V in comparison with those of the homogeneous
monomers 5V–8V in situ prepared by adding VO(acac)2
(0.010mmol) to the ligand (0.015mmol) in CH2Cl2 as
previously reported15 in the model oxidation of thioan-
isole (1.0mmol) with H2O2 (1.1mmol) in the presence of
the catalyst (1mol% with respect to the thioether) in
CH2Cl2 (6mL) at 25ꢁC for 16h (see Table 2). In all
cases, product (S)-14a was detected as the major isomer.
2. Results and discussion
Bolm and Bienewald showed the apparent requirement
for a hydroxy group and a tert-butyl group on the
imine framework as well as a sterically demanding sub-
stituent, such as a tert-butyl group, at the C-6 of the aryl
moiety.15 For our purpose we needed a further OH
group (at the C-4 position of the aromatic ring)17 to
be utilized in the supporting procedure. In agreement
with previous studies from the literature, we decided
to utilize hydroquinone derivatives as building blocks
for the preparation of our chiral ligands.20 Concerning
the support, we decided to utilize organic polymers
(i.e., polystyrene and polyesters) avoiding the use of sil-
ica since VO(acac)2, the vanadium supplying compound,
could react with the surface silanols and structural
water21 affording a nonchiral supported vanadium
species.
As previously observed in the liquid phase,15 the reac-
tion with supported catalysts shows some ligand acceler-
ation26 and similarly the polymeric support (i.e., PS)
plays an activating effect [compare entries 1 and 2 of
Table 2 showing the activity of VO(acac)2 alone and in
the presence of a 50mg of copolymer styrene/DVB per
millimole of sulfide]. Moreover sulfoxide 14a was pro-
duced in quite similar yields by using the homogeneous
catalyst and the heterogeneous counterpart. Interest-
ingly, satisfactory and similar ee values were observed
when the ligand was supported on a polystyrene matrix
(compare entries 5 and 6 or 7 and 8 of Table 2), whereas
the use of polyester supports resulted in a considerable
lowering in yield and ee values (compare entries 3 and
4 or 9 and 10 of Table 2). This negative effect could be
the consequence of the fact that some VO(acac)2 is held
by the polar ester functional groups on the matrix
affording the racemic sulfoxide 14a. Moreover the poly-
styrene utilized, containing 17% DVB, undergoes a good
level of swelling in CH2Cl2 and consequently, space is
available within the resin, allowing easy access of reac-
tants to the surface active sites, whereas a lower swelling
factor is expected for polyesters in the same solvent.24
In Scheme 1, the preparation of four polymer-supported
ligands is depicted. First we prepared monomers 5–8 by
applying synthetic methodologies reported in the litera-
ture with some adjustment. Compound 120,22 was first
converted into intermediates 2–4 in 45–94% yield by
selective alkylation or acylation of the less hindered
OH group with methacryloyl chloride, 4-iodomethylstyr-
ene23 and vinyl chloroformate, respectively, in the pres-
ence of pyridine or K2CO3. Salicylaldimines 5–8 were
then prepared in 70–100% yield by simply mixing the
selected aldehyde with (S)-tert-leucinol or (1R,2R)-2-
amino-1-(4-nitrophenyl)-1,3-propanediol. All the thus
obtained monomers were successively copolymerized
affording rigid but porous copolymers useful as hetero-
geneous chiral ligands. In particular, compounds 5 and
Among the heterogeneous catalysts, 10V showed the
best stereocontrol in the model reaction (50% ee); this
value could be progressively improved by carrying out
the reaction in the presence of traces of methanol (Table
2, entry 11: 53% ee) at 0 ꢁC (Table 2, entry 12: 56% ee).17
The lower ee values observed with catalysts 7V and 11V
could be attributed to a competitive process caused by
different complexes involving the additional OH group.
8
were copolymerized with methyl methacrylate
(MMA) and ethylene glycol dimethacrylate (EGDMA)
in toluene and in the presence of AIBN under a nitrogen
atmosphere24 giving the polymethacrylate-supported lig-
ands 9 and 12. Similarly compounds 6 and 7 were copo-
lymerized with styrene and divinylbenzene (DVB) in
toluene and in the presence of AIBN under a nitrogen
atmosphere25 affording polystyrene-supported ligands
The efficiency of the catalyst 10V in the present reaction
was shown by the possibility of recycling it for almost

A. Barbarini et al. / Tetrahedron: Asymmetry 15 (2004) 2467–2473
2469
OH
vinyl chloroformate
pyridine, CHCl3
methacryloyl chloride
pyridine, CHCl3
CHO
rt, 4 h
rt, 4 h
1
OH
4-iodomethylstyrene 80 ˚C
K2CO3, acetone
OH
6 h
OH
O
OH
O
CHO
O
CHO
CHO
2
3
4
O
O
O
(1R,2R)-2-amino
(S)-tert-leucinol
dry EtOH
-1-(nitrophenyl)-
1,3-propanediol
rt
dry EtOH
rt
6 h
rt (S)-tert-leucinol
6 h dry EtOH
(S)-tert-leucinol rt
dry EtOH 6 h
6 h
NO2
HO
HO
N
HO
N
HO
OH
O
N
OH
O
OH
O
OH
O
N
OH
H
H
H
H
5
6
8
7
O
O
O
styrene, DVB 100 ˚C
AIBN, toluene 21 h
styrene, DVB 100 ˚C
AIBN, toluene 21 h
MMA, EGDMA 60 ˚C
AIBN, toluene 8 h
MMA, EGDMA 60 ˚C
AIBN, toluene 8 h
NO2
HO
HO
HO
HO
OH
O
N
OH
O
N
OH
O
N
OH
O
N
OH
H
H
H
H
9
10
11
12
PA1
PS
PA2
PS
PA = polyacrylate; PS = polystyrene
Scheme 1. Preparation of four polymer-supported ligands.
under radical polymerization reactions and (ii) the sup-
ported catalysts can be reused several times without low-
ering their efficiency.
Table 1. Loading values of imine and vanadium complexes
Entry Ligand Imine loading Complex Vanadium loading
(mmol/g)
(mmol/g)
1
2
3
4
9
10
11
12
0.220
0.328
0.587
0.093
9V
0.032
0.175
0.115
0.023
The solid catalyst was then applied to the oxidation of
some other methyl aryl sulfides with good yields and sat-
isfactory ee values (see Table 3).
10V
11V
12V
3. Conclusion
three further cycles after filtration, washing with CH2Cl2
and immediately reusing (first cycle: 75% yield, 56% ee;
second cycle: 76% yield, 57% ee; third cycle: 72% yield,
55% ee; fourth cycle: 70% yield, 51% ee).
In conclusion, we have shown a simple method for the
preparation of vanadium-Schiff base chiral complexes
supported on polymeric materials. These heterogeneous
and reusable catalysts promoted the oxidation of some
thioanisoles to the corresponding sulfoxides in good
yields and satisfactory ee values. Even if in the literature
The above reported results show that (i) the Schiff base
ligands utilized are stable enough to be polymerized

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A. Barbarini et al. / Tetrahedron: Asymmetry 15 (2004) 2467–2473
Table 2. Efficiency of vanadium-based catalysts in the enantioselective
oxidation of thioanisole
vanadium content were obtained by using an Ultima 2
Jobin Yvon Horiba instrument (ICP–AES). Silica gel
chromatography was performed with Merck silica gel
60 PF254. All chemicals and solvents were purchased
from commercial sources and used without further
purification.
O
S
S
catalyst (1%)
+
H2O2
CH2Cl2, 16 h, rt
13a
14a
All 1H NMR spectra were recorded on a Bruker AC-300
instrument in CDCl3 with tetramethylsilane as the inter-
nal standard. Data are reported as follows: chemical
shift in parts per million (d), multiplicity (s = singlet,
d = doublet, t = triplet, q = quartet, m = multiplet,
br = broad signal), coupling constant (Hz) and integra-
tion. Infrared spectra (IR) were obtained on a Nicolet
PC 5 spectrophotometer and absorptions are reported
in reciprocal centimetres. Mass spectra were obtained
on an HP 5971 A spectrometer (CI).
Entry
Catalyst
Yield (%)
Eea (%)
1
VO(acac)2
VO(acac)2/PSb
5V
21
53
79
61
73
72
66
64
70
51
78
75
––
––
61
39
60
50
18
12
56
35
53
56
2
3
4
9V
6V
5
6
10V
7V
7
8
11V
8V
9
10
11
12
12V
10Vc
10Vd
4.2. Preparation of aldehydes 2–4
a Determined by polarimeter.
4.2.1. 3-tert-Butyl-4-hydroxy-5-oxomethylphenyl meth-
acrylate 2. Toz a stirred solution of 3-tert-butyl-2,5-di-
hydroxybenzaldehyde (5mmol, 970mg) in dry
chloroform (4mL), pyridine (6mmol, 484mg, 0.48mL)
and methacryloyl chloride (10mmol, 1045mg, 0.97mL)
were successively added. After the mixture was stirred
for 4h at room temperature, the solvent was removed
under reduced pressure and the crude residue chromato-
graphed on a silica gel column using hexane/ethyl ace-
tate 96/4 as eluant to afford product 2 (590mg, 45%)
as a white solid; mp 53–54ꢁC; IR (KBr) 2953,
b The amount of PS was 50mg/mmol of 13a.
c Reaction in CH2Cl2/MeOH (250/1).
d Reaction in CH2Cl2/MeOH (250/1) at 0ꢁC.
Table 3. Synthesis of chiral sulfoxides
O
S
S
10V (1% mol)
+ H2O2
1
1727cmꢀ1; H NMR (300MHz, CDCl3) d 1.42 (s, 9H),
CH2Cl2/MeOH (250/1)
R
R
16 h, 0 ˚C
2.07 (d, J = 1.4Hz, 3H), 5.77 (q, J = 1.4Hz, 1H), 6.35
(s, 1H), 7.21 (d, J = 2.8Hz, 1H), 7.25 (d, J = 2.8Hz,
1H), 9.82 (s, 1H), 11.71 (s, 1H); MS (CI, m/z) 263
(M+ + 1), 262 (M+), 89, 61; Anal. Calcd for C15H18O4:
C, 68.69; H, 6.92. Found: C, 68.75; H, 6.87.
13a-d
14a-d
Entry
R
Yield (%)
Eea (%)
1
2
3
4
5
6
H
75
70
80
79
76
54
56 (S)
52 (S)
45 (S)
51 (S)
52 (S)
57 (S)
Me
Br
4.2.2. 3-tert-Butyl-2-hydroxy-5-(p-vinylbenzyloxy)benz-
aldehyde 3. To a stirred solution of 3-tert-butyl-2,5-di-
hydroxybenzaldehyde (5mmol, 970mg) in dry acetone
(30mL), potassium carbonate (5mmol, 691mg) and 4-
iodomethylstyrene (6.5mmol, 1590mg) were succes-
sively added. The mixture was stirred for 6h at 80ꢁC
and after cooling to room temperature, the inorganic
salts were filtered off, the solvent removed under reduced
pressure and the crude residue chromatographed on a
silica gel column using hexane as eluant to afford prod-
uct 3 (1133mg, 73%) as a pale red solid; mp 30–33ꢁC; IR
Cl
OMe
NO2
a Determined by polarimeter.
more stereoselective homogeneous catalysts are de-
scribed to promote the reaction, this note represents
the first step towards a safe and economical route for
performing the enantioselective sulfoxidation reaction.
1
(KBr) 2957, 1727cmꢀ1; H NMR (300MHz, CDCl3) d
Further investigations are currently underway to ration-
alize the role of the heterogeneous support and to
improve the stereochemical control of the process.
1.42 (s, 9H), 5.03 (s, 2H), 5.28 (dd, J = 10.9 and
0.6Hz, 1H), 5.78 (dd, J = 17.6 and 0.6Hz, 1H), 6.74
(dd, J = 17.6 and 10.9Hz, 1H), 6.87 (d, J = 3.0Hz,
1H), 7.26 (d, J = 3.0Hz, 1H), 7.40 (d, J = 8.2Hz, 2H),
7.45 (d, J = 8.2Hz, 2H), 9.80 (s, 1H), 11.54 (s, 1H);
MS (CI, m/z) 310 (M+), 117, 91; Anal. Calcd for
C20H22O3: C, 77.39; H, 7.14. Found: C, 77.46; H, 7.08.
4. Experimental section
4.1. General
4.2.3. 3-tert-Butyl-4-hydroxy-5-oxomethylphenyl vinyl-
carbonate 4. To a stirred solution of 3-tert-butyl-2,5-
dihydroxybenzaldehyde (5mmol, 970mg) in dry chloro-
form (4mL), pyridine (6mmol, 484mg, 0.48mL) and
vinyl chloroformate (10mmol, 1065mg, 0.91mL) were
Elemental analyses were performed by a Carlo Erba
CHNS-0 EA 1108 Elemental Analyzer. Optical Rota-
tory Power (ORP) determination was obtained by using
a Perkin Elmer 341 polarimeter. Analytical measures on

A. Barbarini et al. / Tetrahedron: Asymmetry 15 (2004) 2467–2473
2471
successively added. After the mixture was stirred for 4h
at room temperature, the solvent was removed under re-
duced pressure and the crude residue chromatographed
on a silica gel column using hexane/ethyl acetate 96/4 as
eluant to afford product 4 (1242mg, 94%) as a pale yel-
J = 8.7Hz, 2H), 8.27 (s, 1H); MS (CI, m/z) 506
(M+ + 1), 505 (M+), 449, 387, 152; Anal. Calcd for
C29H32N2O6: C, 69.03; H, 6.39; N, 5.55. Found: C, C,
69.12; H, 6.45; N, 5.47.
low solid; mp 59–60ꢁC; IR (KBr) 2958, 1780, 1761cmꢀ1
;
4.3.4. (S)-3-tert-Butyl-4-hydroxy-5-[(1-tert-butyl-2-hydr-
oxy-ethylimino)-methyl]-phenyl vinyl carbonate 8.
1H NMR (300MHz, CDCl3) d 1.41 (s, 9H), 4.71 (dd,
J = 6.9 and 2.3Hz, 1H), 5.06 (dd, J = 13.7 and 2.3Hz,
1H), 7.14 (dd, J = 13.7 and 6.9Hz, 1H), 7.30 (d,
J = 2.9Hz, 1H), 7.33 (d, J = 2.9Hz, 1H), 9.84 (s, 1H),
11.76 (s, 1H); MS (CI, m/z) 265 (M+ +1), 264 (M+),
249, 61; Anal. Calcd for C14H16O5: C, 63.63; H, 6.10.
Found: C, 63.72; H, 6.07.
(727mg, 100%). Pale yellow foam; mp 51–52ꢁC;
20
D
½aꢁ ¼ ꢀ4:0 (c 1, EtOH); IR (KBr) 3415, 1775cmꢀ1
;
1H NMR (300MHz, CDCl3) d 0.99 (s, 9H), 1.43 (s,
9H), 2.92 (dd, J = 9.4 and 2.9Hz, 1H), 3.75 (dd,
J = 11.0 and 9.4Hz, 1H), 3.95 (br d, J = 11.0 and
2.9Hz, 1H), 4.68 (dd, J = 6.2 and 1.9Hz, 1H), 5.04
(dd, J = 13.8 and 1.9Hz, 1H), 7.02 (d, J = 3.0Hz, 1H),
7.14 (dd, J = 13.8 and 6.2Hz, 1H), 7.14 (d, J = 3.0Hz,
1H), 8.30 (s, 1H); MS (CI, m/z) 364 (M+ + 1), 363
(M+), 276; Anal. Calcd for C20H29NO5: C, 66.09; H,
8.04; N, 3.85. Found: C, 66.17; H, 8.13; N, 3.80.
4.3. General procedure for the preparation of ligands 5–8
To a stirred solution of the selected benzaldehyde 2–4
(2mmol) in dry ethanol (20mL), the selected amino
alcohol (2mmol) was added. The stirring was continued
for 6h at room temperature, the solvent removed under
reduced pressure and the crude residue chromato-
graphed on a silica gel column using hexane/ethyl ace-
tate mixtures to afford the products 5–8.
4.4. General procedure for the preparation of polymers 9
and 12
To a stirred solution of the selected ligand 5 or 8
(1mmol) in dry toluene (15mL), MMA (2.33mmol,
233mg, 0.25mL), EGDMA (13.80mmol, 2735mg,
2.60mL) and AIBN (0.19mmol, 31mg) were succes-
sively added at room temperature under nitrogen. The
mixture was heated at 60ꢁC for 8h, the resulting
solid crushed in a mortar and the powder washed with
dichloromethane (100mL) and dried under vacuo for 6h.
4.3.1. (S)-3-tert-Butyl-4-hydroxy-5-[(1-tert-butyl-2-hydr-
oxy-ethylimino)-methyl]-phenyl
methacrylate
5.
(723mg, 100%). Pale yellow foam; mp 52–52.5ꢁC;
20
1
½aꢁ ¼ 4:1 (c 1, EtOH); IR (KBr) 3425, 1735cmꢀ1; H
D
NMR (300MHz, CDCl3) d 0.98 (s, 9H), 1.43 (s, 9H),
2.06 (s, 3H), 2.93 (dd, J = 9.4 and 2.8Hz, 1H), 3.74
(dd, J = 11.1 and 9.4Hz, 1H), 3.94 (dd, J = 11.1 and
2.8Hz, 1H), 5.74 (m, 1H), 6.33 (s, 1H), 6.95 (d,
J = 2.9Hz, 1H), 7.05 (d, J = 2.9Hz, 1H), 8.29 (s, 1H),
11.4 (br s, 1H); MS (CI, m/z) 362 (M+ + 1), 361 (M+),
292; Anal. Calcd for C21H31NO4: C, 69.78; H, 8.64; N,
3.87. Found: C, 69.72; H, 8.57; N, 3.93.
4.5. General procedure for the preparation of polymers 10
and 11
To a stirred solution of the selected ligand 6 or 7
(1mmol) in dry toluene (15mL), styrene (6.97mmol,
726mg, 0.80mL), DVB 80% (1.50mmol, 196mg,
0.21mL) and AIBN (0.19mmol, 31mg) were succes-
sively added at room temperature under nitrogen. The
mixture was heated at 100ꢁC for 21h, the resulting solid
crushed in a mortar and the powder washed with dichlo-
romethane (100mL) and dried under vacuo for 6h.
4.3.2. (S)-2-[N-3-tert-Butyl-5-(p-vinylbenzyloxy)salicy-
den]amino-3,3-dimethyl-1-butanol 6. (573mg, 70%).
20
D
Pale yellow foam; mp 129–130ꢁC; ½aꢁ ¼ ꢀ6:5 (c 1,
EtOH); IR (KBr) 3440cmꢀ1
;
1H NMR (300MHz,
CDCl3) d 0.98 (s, 9H), 1.43 (s, 9H), 2.92 (dd, J = 10.9
and 2.9Hz, 1H), 3.76 (t, J = 10.9Hz, 1H), 3.94 (br d,
J = 10.9Hz, 1H), 5.00 (s, 2H), 5.25 (d, J = 10.9Hz,
1H), 5.76 (d, J = 17.6Hz, 1H), 6.70 (d, J = 3.0Hz,
1H), 6.73 (dd, J = 17.6 and 10.9Hz, 1H), 7.06 (d,
J = 3.0Hz, 1H), 7.39 (d, J = 8.4Hz, 2H), 7.43 (d,
J = 8.4Hz, 2H), 8.29 (s, 1H); MS (CI, m/z) 410
(M+ + 1), 409 (M+), 292; Anal. Calcd for C26H35NO3:
C, 76.25; H, 8.61; N, 3.42. Found: C, 76.29; H, 8.56;
N, 3.49.
4.6. General procedure for the preparation of complexes
5V–8V
To a stirred solution of the selected ligand 5–8
(0.015mmol) in dry dichloromethane (6mL), VO(acac)2
(0.010mmol, 2.65mg) was added at room temperature.
The mixture was stirred at room temperature for 4h
and the resulting complex in solution employed for the
reaction.
4.3.3. (10R,20R)-2-[N-3-tert-Butyl-5-(p-vinylbenzyloxy)-
salicyden]amino-10-(p-nitrophenyl)-10,30-propandiol
7.
4.7. General procedure for the preparation of complexes
9V–12V
(908mg, 90%). Pale yellow foam; mp 117–118ꢁC;
20
D
½aꢁ ¼ ꢀ107:5 (c 1, EtOH); IR (KBr) 3439cmꢀ1
;
1H
NMR (300MHz, CDCl3) d 1.43 (s, 9H), 2.7–2.8 (m,
1H), 3.49 (q, J = 5.5Hz, 1H), 3.78 (d, J = 5.4Hz, 2H),
4.98 (s, 2H), 5.1–5.2 (m, 1H), 5.26 (d, J = 10.9Hz,
1H), 5.76 (d, J = 17.6Hz, 1H), 6.63 (d, J = 3.0Hz,
1H), 6.73, (dd, J = 17.6 and 10.9Hz, 1H), 7.09 (d,
J = 3.0Hz, 1H), 7.37 (d, J = 8.3Hz, 2H), 7.42 (d,
J = 8.3Hz, 2H), 7.58 (d, J = 8.7Hz, 2H), 8.22 (d,
To a stirred mixture of the selected polymer 9–12
(0.015mmol of supported ligand) in dry dichlorometh-
ane (6mL), VO(acac)2 (0.010mmol, 2.65mg) was added
at room temperature. The mixture was stirred at room
temperature for 4h and then the resulting complex fil-
tered, washed with dichloromethane (2 · 10mL) and
dried under vacuo.

2472
A. Barbarini et al. / Tetrahedron: Asymmetry 15 (2004) 2467–2473
2. See for example: (a) RP 73163: Pitchen, P. Chem. Ind.
(London), 1994, 636–639; (b) Ustiloxine: Hutton, C. A.;
White, J. M. Tetrahedron Lett. 1997, 38, 1643–1646; (c)
Methionine sulfoxide: Holland, H. L.; Brown, F. M.
Tetrahedron: Asymmetry 1998, 9, 535–538; (d) Esomep-
razole: Cotton, H.; Elebring, T.; Larsson, M.; Li, L.;
So¨rensen, H.; von Unge, S. Tetrahedron: Asymmetry 2000,
11, 3819–3825.
3. (a) Bonchio, M.; Conte, V.; De Conciliis, M. A.; Di Furia,
F.; Ballistreri, F. P.; Tomaselli, G. A.; Toscano, R. M. J.
Org. Chem. 1995, 60, 4475–4480; (b) Adam, W.; Haas, W.;
Lohray, B. B. J. Am. Chem. Soc. 1991, 113, 6202–6208.
4.8. General procedure for the preparation of chiral
sulfoxides 14
To a mixture of the supported catalyst 10V (46mg) in
dichloromethane/methanol 250/1 (6mL) at 0ꢁC in a
Schlenk tube equipped with a magnetic stirrer, the se-
lected sulfide (1.0mmol) and H2O2 30% (1.1mmol,
0.11mL) were successively added. The mixture was stir-
red at 0ꢁC for 16h and then the catalyst removed by fil-
tration on
a Buchner funnel and washed with
¨
dichloromethane (3 · 5mL). The organic phase was
washed with distilled water (20 mL), dried over anhy-
drous Na2SO4 and the solvent removed under reduced
pressure. The crude residue was chromatographed on
silica gel plates (25 · 25cm) using hexane/ethyl acetate
20/80 to afford the products.
´
4. Martın, S. E.; Rossi, L. I. Tetrahedron Lett. 2001, 42,
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Schuchardt, U. Angew. Chem., Int. Ed. 1997, 36,
1144–1163.
4.8.1. (S)-Methyl phenyl sulfoxide 14a.27 Colourless
oil, yield: 75%, ee 56%, [a]D = ꢀ80.1 (c 0.69, abs EtOH)
{Lit. [a]D = 143 (c 0.69, abs EtOH)}.
`
6. Fraile, J. M.; Garcia, J. I.; Lazaro, B.; Mayoral, J. A.
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325–326; (b) Komatsu, N.; Hashizume, M.; Sugita, T.;
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4.8.2. (S)-Methyl p-methylphenyl sulfoxide 14b.28
White solid, yield: 70%, ee 52%, [a]D = ꢀ77.8 (c 2.00,
acetone) {Lit. [a]D = +145, ee 99.5% (c 2, acetone)}.
4.8.3. (S)-Methyl p-bromophenyl sulfoxide 14c.29
White solid, yield: 80%, ee 45%, [a]D = ꢀ58.7 (c 1.50,
CHCl3) {Lit. [a]D = ꢀ75.7, ee 58% (c 1.50, CHCl3)}.
4.8.4. (S)-Methyl p-chlorophenyl sulfoxide 14d.30
White solid, yield: 79%, ee 51%, [a]D = ꢀ80.6 (c 0.95,
CHCl3) {Lit. [a]D:ꢀ158 (c 0.95, CHCl3)}.
4.8.5. (S)-Methyl p-methoxyphenyl sulfoxide 14e.31
White solid, yield: 76%, ee 52%, [a]D = ꢀ88.0 (c 1.20,
CHCl3) {Lit. [a]D = ꢀ162.4, ee 96% (c 1.2, CHCl3)}.
4.8.6. (S)-Methyl p-nitrophenyl sulfoxide 14f.28 White
solid, yield: 54%, ee 57%, [a]D = ꢀ90.1 (c 0.75, CHCl3)
{Lit. [a]D = +156.9, ee 99.3% (c 0.75, CHCl3)}.
Acknowledgements
15. Bolm, C.; Bienewald, F. Angew. Chem., Int. Ed. Engl.
1995, 34, 2640–2642.
This research was supported by the Ministero dellÕIst-
16. See for example: (a) Vetter, A. M.; Berkessel, A. Tetra-
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Ostrycharz, E.; Siedlecka, R. Tetrahedron: Asymmetry
1999, 10, 3457–3461; (c) Ohta, C.; Shimizu, H.; Kondo,
A.; Katsuki, T. Synlett 2002, 161–163.
17. Pelotier, B.; Anson, M. S.; Campbell, I. B.; Macdonald, S.
J. F.; Priem, G.; Jackson, R. F. W. Synlett 2002,
1055–1060.
`
ruzione, dellÕUniversitae della Ricerca (MIUR), Italy
(National Project ÔStereoselezione in Chimica Organica.
Metodologie e ApplicazioniÕ). AB thanks INCA Con-
sortium for support. The authors are also grateful to
Mr. Pier Antonio Bonaldi (Lillo) for technical assistance
and to the Centro Interdipartimentale Misure (CIM) for
the use of NMR and mass instruments.
18. Green, S. D.; Monti, C.; Jackson, R. F. W.; Anson, M. S.
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