Kinetic resolution in vanadium-catalyzed sulfur oxidation as an efficient route to enantiopure aryl benzyl sulfoxides

Article abstract of DOI:10.1055/s-2006-944187

Enantiopure aryl benzyl sulfoxides can be prepared using vanadium-Schiff base catalyzed sulfide oxidation and kinetic resolution of the resulting sulfoxide. The kinetic resolution can be effected in combination with asymmetric sulfide oxidation or instead starting from the racemic sulfoxide. Georg Thieme Verlag Stuttgart.

Full text of DOI:10.1055/s-2006-944187

LETTER
1569
Kinetic Resolution in Vanadium-Catalyzed Sulfur Oxidation as an Efficient
Route to Enantiopure Aryl Benzyl Sulfoxides
K
inetic Resolu
á
tion in Va
n
d
adium-Cata
r
d
Su
a
O
xidat
i
Department of Chemistry, Analytical and Biological Chemistry Research Facility, University College Cork, Cork, Ireland
Fax +353(21)4274097; E-mail: a.maguire@ucc.ie
Received 28 December 2005
tion in the oxidation of aryl alkyl sulfoxides using this sys-
Abstract: Enantiopure aryl benzyl sulfoxides can be prepared
tem.8 It was found the kinetic resolution was optimum in
using vanadium–Schiff base catalyzed sulfide oxidation and kinetic
resolution of the resulting sulfoxide. The kinetic resolution can be
chloroform at 0 °C.8 There are numerous examples of the
oxidative kinetic resolution of sulfoxides to sulfones most
of which were discovered during investigation of asym-
metric sulfide oxidation.9
effected in combination with asymmetric sulfide oxidation or in-
stead starting from the racemic sulfoxide.
Key words: asymmetric catalysis, oxidation, kinetic resolution,
sulfoxides, Schiff bases
We have recently investigated the enantioselective syn-
thesis of aryl benzyl sulfoxides using the vanadium-cata-
lyzed sulfide oxidation and wish to report here our
observations on both asymmetric sulfide oxidation and
kinetic resolution in sulfoxide oxidation. Furthermore
these processes can be employed in combination to yield
sulfoxides with high enantiopurity.
Enantiopure sulfoxides are useful as both building blocks
and chiral auxiliaries in organic synthesis.1 There are two
main routes used to prepare enantiopure sulfoxides, name-
ly nucleophilic displacement and enantioselective sulfur
oxidation.2 The Andersen synthesis, which involves the
nucleophilic displacement of a leaving group from a dia-
stereopure sulfinate ester is a popular method for the
preparation of enantiopure sulfoxides.3 The use of a chiral
auxiliary that could undergo two consecutive nucleophilic
displacements to give the enantiopure sulfoxide has also
been investigated, most recently by Ruano and Senanay-
ake.4,5
Initial work focused on using the Bolm oxidation method
to enantioselectively oxidize the aryl benzyl sulfide 1. A
range of Schiff base ligands were examined and it was
found the best results were obtained using ligand 2.10
Good enantioselectivity was observed as shown below
when oxidizing 1a using ligand 2 to prepare the catalyst.
It was noticed, however, that a considerable amount of
sulfone formation was taking place during the oxidation
(Scheme 1).
Enantioselective sulfur oxidation is particularly attractive
given that the prochiral sulfides are usually easily pre-
pared.2,3 To date relatively few enantioselective sulfur
oxidation methods have yielded enantiopure sulfoxides,
though many have resulted in highly enantioenriched sul-
foxides. Bolm et al. reported a robust oxidation method
based on vanadium in 1995.6 This involved the in situ for-
mation of a catalyst from vanadyl acetylacetonate and a
Schiff base. The oxygen source for this method is hydro-
gen peroxide, the reaction is not moisture sensitive and
can be carried out in an open reaction vessel. This reaction
method has attracted a lot of attention principally due to
its ease of use.7
This coupled with the high enantiopurity observed indi-
cated that kinetic resolution was possibly taking place.
Accordingly, an investigation was carried out to establish
if there was a relationship between the extent of sulfone
formation and enantiopurity. This was done by carrying
out the reaction using varying amounts of the oxidant. The
results of these experiments are shown Figure 1. The rela-
tionship between sulfone formation and enantiopurity of
the remaining sulfoxide can be clearly seen.
Of particular importance to this work is the fact that in the
original Bolm paper it was reported that the high enantio-
selectivity of the sulfoxide product was due exclusively to
asymmetric oxidation of the sulfide to the sulfoxide and
kinetic resolution did not play a role.6 This is backed up
by the considerable amount of literature that has since
been published on this oxidation method in which kinetic
resolution has not been reported.7 However, in contrast
Jackson et al. have very recently reported kinetic resolu-
SYNLETT 2006, No. 10, pp 1569–1573
2
1
.0
6
.2
0
0
6
Advanced online publication: 12.06.2006
DOI: 10.1055/s-2006-944187; Art ID: D38405ST
© Georg Thieme Verlag Stuttgart · New York
Figure 1 Relationship between sulfone formation and ee (%) of
sulfoxide during vanadium-catalyzed sulfide oxidation.

1570
P. Kelly et al.
LETTER
I
I
OH N
HO
O
S
O
O
Ligand 2 (1.5 mol%)
S
S
VO(acac)2 (1.0 mol%)
3a
4a
1a
H2O2, 1.3 equiv,
CH2Cl2,
sulfone
sulfoxide
> 99% ee (S)
41% yield
r.t.
ratio sulfoxide to sulfone 47:53
Scheme 1 Vanadium-catalyzed asymmetric oxidation of aryl benzyl sulfides
Oxidations using a range of aryl benzyl sulfides produced ides were found to be largely similarly to the standard
similar results, with significant sulfone formation and Bolm procedure except that only 0.8 equivalents of the
high enantiopurity in sulfoxides occurring together. The oxidizing agent were used and the oxidizing agent was
results of this work are shown in Table 1. Notably sam- added portion wise over an extended period (70 min). This
ples of each of the aryl benzyl sulfoxides 3a–h with good portion wise addition agrees with that reported by
enantiopurities (71% to 99% ee) were obtained. However, Karpyshev who found that enantioselectivity of the Bolm
the enantiopurities of the alkyl benzyl sulfoxides 3i and 3j oxidation was enhanced when the oxidant was added over
were much lower.
an extended period.11
Carrying out the oxidation using racemic sulfoxides as The results obtained carrying out the kinetic resolution ex-
substrates yielded enantioenriched sulfoxides indicating periments on various racemic sulfoxides are summarized
that kinetic resolution was indeed taking place. Optimum in Table 2.
conditions for the kinetic resolution of the racemic sulfox-
Table 1 Asymmetric Oxidation of Aryl Benzyl Sulfides
O
VO(acac)2 (1.0 mol%)
O
O
ligand 2 (1.5 mol%)
S
Ar
S
Ar
S
Ar
R
R
R
H2O2 (1.1 equiv)
sulfone
4
prochiral sulfide
enantioenriched
CH2Cl2
r.t.
sulfoxide
1
3
1
R
Ar
Ph
3:4a,b
3
Yield (%)c
ee (%)d
Confige
1a
1b
1c
1d
1e
1f
4-MeC6H4
4-MeC6H4
4-MeC6H4
4-MeC6H4
4-MeC6H4
4-MeC6H4
4-BrC6H4
Ph
72:28
48
54
47.6
45
50
52
41
51
42
76
94
S
S
S
S
R
S
S
S
S
S
4-FC6H4
4-ClC6H4
4-MeC6H4
2-MeOC6H4
4-MeOC6H4
Ph
71:29
57:43
66:33
69:31
62:38
45:55
70:30
95:5
71
98
>99
>99
>99
>99
91
1g
1h
1i
Ph
t-Bu
Ph
56.3
10
1j
Me
Ph
83:17
a Sulfoxide:sulfone ratio determined by 1H NMR of crude product.
b In some cases the extent of oxidation is beyond what would be expected on the basis of the amount of oxidant employed, possibly due to
variation in the concentration of the H2O2.
c Yield of isolated product. Reactions were carried out on a 1 mmol scale.
d Determined by chiral HPLC.
e Configuration assigned by comparing optical rotations and/or HPLC elution to literature values or reference sulfoxides prepared using the
Andersen synthesis.
Synlett 2006, No. 10, 1569–1573 © Thieme Stuttgart · New York

LETTER
Kinetic Resolution in Vanadium-Catalyzed Sulfur Oxidation
1571
Table 2 Vanadium-Catalyzed Kinetic Resolution of Aryl Benzyl Sulfoxides
VO(acac)2 (1.0 mol%)
O
S
O
S
O
O
ligand 2 (1.5 mol%)
Ar
S
Ar
Ar
H2O2 (0.8 equiv)
R
R
R
CH2Cl2
r.t.
racemic sulfoxide
enantioenriched
sulfone
3
sulfoxide
4
3
3
R
Ar
3:4a
3
Yield (%)b
ee (%)c
93
Selectivity factord Confige
3a
3b
3c
3d
3e
3f
4-MeC6H4
4-MeC6H4
4-MeC6H4
4-MeC6H4
4-MeC6H4
4-MeC6H4
4-BrC6H4
Ph
Ph
34:66
30:70
20:80
16:84
33:67
26:74
37:63
28:72
86:14
54:46
24
21
14
15
25
19
31
21
60
51
8.6
9.9
S
S
S
S
R
S
S
S
S
S
4-FC6H4
4-ClC6H4
4-MeC6H4
2-MeOC6H4
4-MeOC6H4
Ph
98.3
97.3
>99
>99
74
6.7
5.6
13.1
3.4
3g
3h
3i
>99
86
17.6
4.6
Ph
t-Bu
Ph
5.4
1.1
3j
Me
Ph
4
1.1
a Sulfoxide:sulfone ratio determined by 1H NMR of crude product.
b Yield of isolated product. Reactions were carried out on a 1 mmol scale.
c Determined by chiral HPLC.
d Selectivity factor calculated using Kagan’s formula using 1H NMR conversions.12,
e Configuration assigned by comparing optical rotations and/or HPLC elution to literature values or reference sulfoxides prepared using the
Andersen synthesis.
From the data summarized in Table 2 it can be concluded When experiments were undertaken in the absence of any
that aryl benzyl sulfoxides are very susceptible to kinetic of the following: the hydrogen peroxide, the vanadium or
resolution using this system. As stated earlier, Bolm the ligand, no kinetic resolution was observed indicating
reported that kinetic resolution was not responsible for the that each of these components is necessary for the enan-
high enantiopurity obtained when oxidizing aryl methyl tioselective oxidation.
sulfides.6 This is confirmed by the work of other groups
The results shown in Table 2 compare very favorably in
who did not report kinetic resolution taking place when
terms of yield and enantioselectivity to the results of other
oxidizing aryl methyl sulfides.7 However, Jackson et al.
published kinetic resolution methods. Most of the
have very recently demonstrated that when conducted in
published studies focus on aryl methyl sulfoxides.8,9 How-
chloroform with careful temperature control rather than
ever, Sudalai (tungsten–cinchona alkaloid catalyst) and
dichloromethane, kinetic resolution is possible.8 Evident-
Zhu (vanadium–salen catalyst) have reported kinetic re-
ly subtle steric and electronic factors result in efficient ki-
solution experiments using racemic aryl benzyl sulfoxides
netic resolution in the oxidation of aryl benzyl sulfoxides
as substrates.9j,k
observed in dichloromethane.
Table 3 Comparison of Results of Different Kinetic Resolution Methods
Catalyst
3a
3h
ee (yield), selectivity factora
ee (yield), selectivity factora
Vanadium–salen (Zhu9k)
91% (27%), 5.6
90% (25%), 5.0
93% (24%), 5.3
93% (26%), 5.8
82% (25%), 3.9
86% (21%), 3.8
Tungsten–cinchona Alkaloid (Sudalai9j)
Vanadium–Schiff base (this work)
a Selectivity factors calculated using Kagan’s formula, using isolated yields.12
Synlett 2006, No. 10, 1569–1573 © Thieme Stuttgart · New York

1572
P. Kelly et al.
LETTER
C14H13ClOS (%): C, 63.51; H, 4.95; Cl, 13.39; S, 12.11. Found: C,
63.66; H, 4.99; Cl, 13.15; S, 12.18. White solid; mp 164.2–
166.0 °C; HPLC: tR (R) = 34.5 min, tR (S) = 40.9 min (Chiracel OD-
H; flow rate 0.5 mL min–1; hexane–2-PrOH = 93:7; 10 °C); 98% ee;
[a]D20 –140 (c 0.5, CHCl3).
Comparison of the results for the kinetic resolution of 3a
and 3h are summarized in Table 3. Interestingly the oxi-
dant used in all these reactions is hydrogen peroxide. No-
tably Sudalai reported lower enantiopurities when starting
from the sulfide, while Zhu’s method is not amenable to
the combination of asymmetric sulfide oxidation and ki-
netic resolution, highlighting the advantage of the current
method in which a combination of asymmetric sulfide
oxidation and kinetic resolution can results in high enan-
tiopurities when starting from the sulfides (see
Table 1).9j,9k
(S)-(–)-4-Methylbenzyl-4¢-tolyl Sulfoxide (3d)
1H NMR (300 MHz): d = 7.32–7.17 (4 H, m, Ar-H), 7.04 (2 H, d,
J = 7.0 Hz, Ar-H), 6.85 (2 H, d, J = 7.2 Hz, Ar-H), 4.07–3.88 (2 H,
ABq, J = 12.5 Hz, SCH2), 2.40 (3 H, s, Ar-CH3) ppm. 13C NMR (75
MHz): d = 141.93, 140.13, 138.43, 130.66, 129.95, 129.58, 126.62,
124.89, 63.91, 21.86, 21.61 ppm. Anal. Calcd for C14H16OS (%): C,
73.73; H, 6.55; S, 13.12. Found: C, 73.62; H, 6.55; S, 13.39. White
solid; mp 140.2–140.9 °C; HPLC: tR (R) 75.6 min, tR (S) = 83.0 min
(Chiracel OD-H; flow rate 1.0 mL min–1; hexane–2-PrOH = 98:2;
0 °C); >99% ee; [a]D20 –43 (c 0.5, CHCl3).
From this research it can be concluded that enantio-
enriched aryl benzyl sulfoxides can be readily prepared
using vanadium-catalyzed oxidation through a combina-
tion of both kinetic resolution and asymmetric sulfur
oxidation.
(R)-(–)-2-Methoxybenzyl-4¢-tolyl Sulfoxide (3e)
1H NMR (300 MHz): d = 7.34–7.18 (5 H, m, Ar-H), 6.97–6.91 (1
H, m, Ar-H), 6.90–6.77 (2 H, m, Ar-H), 4.14–3.93 (2 H, ABq,
J = 12.7 Hz, SCH2), 3.73 (3 H, s, Ar-OCH3), 2.40 (3 H, s, Ar-CH3)
ppm. 13C NMR (75 MHz): d = 158.03, 141.67, 140.96, 132.55,
130.18, 129.71, 124.85, 120.79, 118.64, 110.65, 59.37, 55.68, 21.82
ppm. Anal. Calcd for C14H16O2S (%): C, 69.20; H, 6.19; S, 12.32.
Found: C, 68.90; H, 6.15; S, 12.35. Clear oil; HPLC: tR (R) = 79.0
min, tR (S) = 116.0 min (Chiracel OD-H; flow rate 0.5 mL min–1;
General Procedure
Kinetic Resolution of Sulfoxides
Vanadyl acetylacetonate (2.6 mg, 0.01 mmol) and the ligand 2 (7.1
mg, 0.015 mmol) were stirred together in a 25-mL round-bottomed
flask in CH2Cl2 (1 mL) for 5 min. Racemic sulfoxide (1.0 mmol) in
CH2Cl2 (1 mL) was then added and the reaction mixture was stirred
for a further 5 min. Then, H2O2 (30% w/w, 0.8 mmol) was added
portion wise over 70 min. The reaction mixture was then stirred for
a further 30 min. The reaction was quenched by the addition of H2O
(5 mL). The layers were separated and the aqueous layer was
washed with CH2Cl2 (3 mL). The combined organic layers were
washed with brine (5 mL), dried and concentrated under reduced
pressure. The crude product was purified by flash chromatography
on silica gel (EtOAc–hexane, 2:3). The ee (%) values were deter-
mined by HPLC on chiral stationary phases, 254 nm, 20 °C,
Chiracel OD-H. All reactions were carried out at room temperature.
20
hexane–2-PrOH = 90:10; 10 °C); >99% ee; [a]D +30 (c 1.1,
CHCl3).
(S)-(–)-4-Methoxybenzyl-4¢-tolyl Sulfoxide (3f)
1H NMR (300 MHz): d = 7.37–7.17 (5 H, m, Ar-H), 6.93–6.90 (1
H, m, Ar-H), 6.88–6.77 (2 H, m, Ar-H), 4.29–3.97 (2 H, ABq,
J = 57.4 Hz, SCH2), 3.69 (3 H, s, Ar-OCH3), 2.39 (3 H, s, Ar-CH3)
ppm. 13C NMR (75 MHz): d = 159.97, 141.93, 140.00, 131.97,
129.95, 124.91, 121.57, 114.27, 63.38, 55.66, 21.86 ppm. Anal.
Calcd for C14H16O2S (%): C, 69.20; H, 6.19; S, 12.32. Found: C,
69.47; H, 6.09; S, 12.30. White solid; mp 124–125.1 °C; HPLC: tR
(R) = 33.4 min, tR (S) = 41.0 min (Chiracel OD-H; flow rate 0.5 mL
min–1; hexane–2-PrOH = 90:10; 10 °C); >99% ee; [a]D20 –87 (c 0.2,
CHCl3).
Sulfoxides 3b,d–f have been previously reported in racemic form
only.13 Sulfoxides 3a,g–j have been reported in enantioenriched
form.14–16 Sulfoxide 3c has not been previously reported.
(S)-(–)-Benzyl-4¢-bromophenyl Sulfoxide (3g)15
Analytical Data
HPLC: tR (R) = 44.4 min, tR (S) = 50.1 min (Chiracel OD-H; flow
(S)-(–)-Benzyl-4¢-tolyl Sulfoxide (3a)14
HPLC: tR (R) = 44.4 min, tR (S) = 51.9 min (Chiracel OD-H; flow
rate 1.0 mL min–1; hexane–2-PrOH = 98:2; 20 °C).
rate 0.5 mL min–1; hexane–2-PrOH = 94:6; 10 °C)
(S)-(–)-Benzyl-phenyl Sulfoxide (3h)16
HPLC: tR (R) = 28.0 min, tR (S) = 34.6 min (Chiracel OD-H; flow
rate 1.0 mL min–1; hexane/2-PrOH = 98:2; 20 °C).
(S)-(–)-4-Fluorobenzyl-4¢-tolyl Sulfoxide (3b)
1H NMR (300 MHz): d = 7.27–7.17 (4 H, m, Ar-H), 7.00–6.93 (4
H, m, J = 7.6 Hz, Ar-H), 3.99 (2 H, s, SCH2), 2.40 (3 H, s, Ar-CH3)
ppm. 13C NMR (75 MHz): d = 162.70 (d, JC,F = 245.6 Hz), 141.75,
139.16, 132.04 (d, JC,F = 8.3 Hz), 129.63, 124.94, 124.40, 115.38
(d, JC,F = 21.7 Hz), 62.34, 21.45 ppm. Anal. Calcd for C14H13FOS
(%): C, 67.75; H, 5.28; F, 7.65; S, 12.91. Found: C, 67.80; H, 5.30;
F, 7.46; S, 13.16. White solid; mp 174.6–175.3 °C; HPLC: tR (R) =
34.6 min, tR (S) = 40.3 min (Chiracel OD-H; flow rate 0.5 mL
min–1; hexane–2-PrOH = 93:7; 10 °C); 71% ee; [a]D20 –109 (c 0.4,
CHCl3).
(S)-(–)-Benzyl tert-butyl sulfoxide (3i)14
HPLC: tR (R) = 18.5 min, tR (S) = 23.4 min (Chiracel OD-H; flow
rate 1.0 mL min–1; hexane/2-PrOH = 96:4; 10 °C).
(S)-(–)-Benzyl methyl sulfoxide (3j)14
HPLC: tR (R) = 51.3 min, tR (S) = 58.3 min (Chiracel OD-H; flow
rate 1.0 mL min–1; hexane/2-PrOH = 96:4; 0 °C).
Acknowledgment
(S)-(–)-4-Chlorobenzyl-4¢-tolyl Sulfoxide (3c)
We are grateful to the IRCSET Embark initiative for funding. We
would also like to thank the following for their assistance: Eamonn
Moynihan, Niamh Lehane, Jay Chopra, Sarah Ormond, and Breda
Doyle.
1H NMR (300 MHz): d = 7.28–7.20 (6 H, m, Ar-H), 6.91 (2 H, d,
J = 7.6 Hz, Ar-H), 3.98 (2 H, s, SCH2), 2.40 (3 H, s, Ar-CH3) ppm.
13C NMR (75 MHz): d = 142.22, 139.52, 134.72, 132.06, 130.08,
128.97, 128.02, 124.79, 62.87, 21.87 ppm. Anal. Calcd for
Synlett 2006, No. 10, 1569–1573 © Thieme Stuttgart · New York

LETTER
Kinetic Resolution in Vanadium-Catalyzed Sulfur Oxidation
1573
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