Rhenium-catalyzed oxidation of sulfides with phenyl sulfoxide

Full text of DOI:10.1021/jo960178x

2
260
J . Org. Chem. 1996, 61, 2260-2261
Rh en iu m -Ca ta lyzed Oxid a tion of Su lfid es
w ith P h en yl Su lfoxid e
Sch em e 1
J effrey B. Arterburn* and Sherry L. Nelson
Department of Chemistry and Biochemistry,
New Mexico State University, Box 30001/ 3C,
Las Cruces, New Mexico 88003
solution 1H NMR, UV/vis, and FT-IR spectra of the
catalytically active reaction mixture from both precursors
were identical. The rate of oxygen transfer between
Me SO-d (10 equiv) and Me S (1 equiv) in CDCl was
Received J anuary 30, 1996
Sulfoxides are valuable functional groups in organic
chemistry and are typically prepared by the oxidation of
sulfides. A variety of reagents are capable of this
oxidation; however, many of these are useful only within
a limited range of sulfide structures.1 Overoxidation of
sulfides, resulting in formation of sulfones, and undesired
reactions of other functional groups are common prob-
lems, particularly when preparing biologically relevant
sulfoxides. In principle, oxygen atom transfer from a
sulfoxide oxidant could provide a very mild method for
oxidizing sulfides. The oxidation of sulfides by dimethyl
sulfoxide has been reported, although this reaction is
limited to sulfides with unusual structural features and
conditions involving high temperatures or acid catalysts.3
Sulfoxides are capable of oxidizing transition metals, and
it has been observed that metal-catalyzed oxygen atom
transfer from sulfoxides to phosphines occurs with a
variety of complexes.4 Bryan and Mayer et al. observed
the rapid equilibrium exchange of an oxygen atom
between dimethyl sulfoxide and dimethyl sulfide that was
2
6
2
3
measured by 1H NMR and found to be the same for
reactions using 0.05 mol % of either precursor I or II.
,2
The synthetic and commercial availability of ReOCl -
3
8
(PPh ) (II) make it the preferred catalyst precursor;
3 2
therefore, II was used in all of the subsequent reactions.
Reactions were carried out in organic solvents without
precautions to exclude O or H O; however, the activity
2
2
of the catalytic system decreases slowly after several
hours at ambient conditions and eventually produces
-
7
inert Re(VII) perrhenate salts [ReO ] . Control experi-
4
ments showed no sulfide oxidation occurs in the absence
of catalyst.
A series of sulfides were then reacted with ReOCl3-
(PPh ) (II) (0.05 mol %) and an excess of Me SO-d (10
3
2
2
6
equiv) in C D , and the results are presented in Table 1
6
6
,5
(method A). Simple dialkyl sulfides such as methyl and
n-butyl sulfide were oxidized efficiently with Me SO
2
(entries 1 and 2, Table 1), while the reaction of the bulky
catalyzed by a rhenium(V) oxo complex Re(O)Cl
OPPh
) (I).6 Complex I was easily prepared from the
reaction of dimethyl sulfoxide and Re(O)Cl (PPh (II).
3
(Me
2
S)-
tert-butyl sulfide gave only partial conversion to sulfoxide
and the remainder as unreacted starting sulfide (entry
3, Table 1). Propylene sulfide (entry 4, Table 1) was
unreactive under these conditions, while the 5-membered
cyclic sulfide tetrahydrothiophene (entry 5, Table 1) was
oxidized efficiently. Aryl sulfides were essentially unre-
active using these conditions (entries 7 and 8, Table 1).
Dialkyl sulfides possessing alcohol and protonated amine
groups gave 77% and 44% conversion, respectively,
without affecting these functional groups (entries 8 and
9, Table 1). (Methylthio)methyl acetate was unreactive
under these conditions. No sulfide overoxidation prod-
ucts such as sulfones were detected in the crude reaction
mixtures.
(
3
3
3 2
)
The accessibility of Re(V) oxo complexes and the potential
for using sulfoxides as safe, inexpensive oxidants led us
to investigate this chemistry further. We report here an
exceedingly mild, catalytic oxidation of sulfides using
diphenyl sulfoxide as the co-oxidant (Scheme 1). This
procedure is effective for a wide range of sulfides, it is
compatible with a variety of functional groups, and it is
very practical for preparing hydrophilic sulfoxides.
Our initial experiments focused on determining the
scope of rhenium catalyzed sulfide oxidations using
Me
Re(O)Cl
with excess Me
active reaction mixture, spectroscopically consistent with
2
SO-d
6
as the oxidant. Both Re(V) oxo complexes
(Me S)(OPPh ) (I) and ReOCl (PPh (II) react
SO-d to give a pale green, catalytically
3
2
3
3
3 2
)
In order to improve the efficiency and extend this
oxidation chemistry to aryl sulfides, we investigated
2
6
SO) as an oxidant.9 The oxida-
diphenyl sulfoxide (Ph
2
7
the known complex Re(O)
2 2 2
(Me SO) Cl. The presence of
tions of a variety of sulfides using this procedure are
reported in Table 1 (method B). Cyclic and acyclic alkyl
sulfides were converted to sulfoxides nearly quantita-
3
1
1
free OdPPh was indicated by P{ H} NMR, and the
3
(1) (a) Madesclaire, M. Tetrahedron 1986,42, 5459. (b) Drabowicz,
tively with Ph
for completion than when Me
2
SO and required shorter reaction times
SO was used (entries 1-3
J .; Kielbasinski, P.; Mikolajczyk, M. Synthesis of Sulphoxides; J ohn
Wiley & Sons, Ltd.: New York, 1994; p 109.
2
(
2) The oxidation of sulfides with CH
3
ReO
3
/H
2
O
2
has been re-
and 5, Table 1), however, propylene sulfide (entry 4,
Table 1) was unreactive using these conditions. The
rhenium-catalyzed oxidation of the monoaryl sulfide
ported: (a) Adam, W.; Mitchell, C. M.; Saha-Moller, C. R. Tetrahedron
994, 50, 13121. (b) Vassell, K. A.; Espenson, J . H. Inorg. Chem. 1994,
3, 5491.
1
3
(
3) (a) Hsu, F. L.; Szafraniec, L. L.; Beaudry, W. T.; Yang, Y. C. J .
Org. Chem. 1990, 55, 4153. (b) Miotti, U. J . Chem. Soc., Perkin Trans.
1991, 617.
2
thioanisole (entry 6, Table 1) with Ph SO was rapid and
proceeded to 96% conversion. The reaction of phenyl
vinyl sulfide (entry 7, Table 1) using these same condi-
tions resulted in only 60% conversion after 15 min at
ambient temperature. No further conversion occurred
2
(
(
(
4) Holm, R. H. Chem. Rev. 1987, 87, 1401.
5) Kukushkin, V. Y. Coord. Chem. Rev. 1995, 139, 375.
6) Bryan, J . C.; Stenkamp, R. E.; Tulip, T. H.; Mayer, J . M. Inorg.
Chem. 1987, 26, 2283.
7) Grove, D. E.; Wilkinson, G. J . Chem. Soc. A 1966, 1224. The
complex ReO (Me SO) Cl is known to hydrolyze to HReO , and our
attempted sulfide oxidations in neat Me SO or H O were unsuccessful.
The organic soluble perrhenate salt Bu N[ReO ] also was not catalyti-
(
2
2
2
4
(8) J ohnson, N. P.; Lock, C. J . L.; Wilkinson, G. Inorg. Synth. 1967,
9, 145. Trichlorooxobis(triphenylphosphine)rhenium (II) is also com-
mercially available from the Aldrich Chemical Co.
2
2
4
4
cally active. The addition of excess pyridine to the active catalyst
results in a yellow solution, consistent with the Re(V) dioxo complex
2 2 2 2
(9) For the gas-phase reaction: Ph SO + Me S f Ph S + Me SO;
∆H ) -2.6 kcal/mol. Holm has introduced a thermodynamic scale of
oxo transfer reactivity based on the correlation between ∆H values
for gas-phase reactions and ∆G values. Holm, R. H.; Donahue, J . P.
Polyhedron 1993, 12, 571.
[
2 5 5 4
ReO (NC H ) ]Cl that has been reported from the reaction of (I) with
pyridine. This complex was also not catalytically active for sulfide
oxidation.
0
022-3263/96/1961-2260$12.00/0 © 1996 American Chemical Society

Communications
J . Org. Chem., Vol. 61, No. 7, 1996 2261
Ta ble 1. Syn th esis of Su lfoxid es by Rh en iu m -Ca ta lyzed
Su lfid e Oxid a tion s
The sulfoxides of methionine and S-methylcysteine are
of interest because of their biological activity, for protec-
tion of the sulfide group during peptide synthesis, as
1
2
chiral ligands, and as synthetic intermediates.
We
investigated the rhenium-catalyzed oxidation of S-meth-
yl-L-cysteine and L-methionine ethyl ester derivatives
time
(h)
yield
(%)
methoda
with Ph
2
SO to further test the functional group tolerance
entry
1
substrate
R, R′ ) -CH3
1
1
and preparative value of this reaction (eq 1).
The
A
B
A
B
A
B
A
B
A
B
A
B
A
B
A
B
A
B
A
B
0.5 86
0.1 96
2.0 86
0.5 98
2.5 50
1.0 96
n
2
3
4
5
6
7
8
9
R, R′ ) - Bu
(1)
t
R, R′ ) - Bu
R, R′ ) -CH2CH(CH3)-
24
24
0
0
R, R′ ) -CH2CH2CH2CH2-
1.0 80
1.0 98
5.0 10
0.3 96
oxidation of S-methyl-L-cysteine ethyl ester hydrochloride
11) gave the sulfoxide (13) in 98% yield, and similar
reaction with the methionine derivative (12) gave the
sulfoxide (14) in 94% yield. No diastereoselectivity was
evident in the formation of the sulfoxide stereocenter in
these products.
R ) -Ph
(
R′ ) -CH3
R ) -Ph
24
0
R′ ) -CHdCH2
R ) -CH2CH3
R′ ) -CH2CH2OH
R ) -CH2CH3
R′ ) -CH2CH2NH3Cl
R ) -CH3
0.3 60, 75b
4.0 77
1.5 98, 96c
2.5 44
98,97c
0
In conclusion, the rhenium-catalyzed oxidation of alkyl
and aryl sulfides with Ph SO is mild, efficient, and rapid.
2
0.3
24
1
0
R′ ) -CH2OAc
0.3 77, 61c
This oxidation reaction is compatible with a variety of
functional groups and devoid of sulfone byproducts
common with other oxidants. Catalytic oxygen atom
transfer from sulfoxides provides a desirable alternative
to traditional peroxide-based organic oxidations. Mecha-
nistic studies, further synthetic applications of this
reaction, and the potential for developing stereoselective
oxidations are currently being investigated.
a
Reaction conditions: method A, 0.05 mol % of ReOCl3(PPh3)2,
0 equiv of Me2SO-d6, 1 equiv of RSR′ in C6D6 at 25 °C; method
B, 0.05 mol % of ReOCl3(PPh3)2, 1.3 equiv of Ph2SO, 1 equiv of
RSR′ in CDCl3 at 25 °C. 2.6 equiv of Ph2SO. Preparative scale
1
b
c
in CHCl3.
after the addition of more catalyst precursor, and de-
composition of the sulfoxide product became evident with
longer reaction times.10 When the reaction was per-
formed using a larger excess of Ph SO (2.6 equiv), the
2
sulfoxide yield was improved to 75%. Dialkyl sulfides
possessing alcohol and protonated amine groups (entries
and 9, Table 1) were oxidized faster and more efficiently
using Ph SO and without affecting these functional
groups. (Methylthio)methyl acetate, which was unreac-
tive with Me SO, gave 77% conversion to sulfoxide using
Ph SO as the oxidant. The workup for the preparative
scale reactions of the functionalized sulfoxides (entries
-10, Table 1) was very simple, since the product
sulfoxides were obtained easily by aqueous extraction.
No sulfones were detected in the reaction mixtures.
Ack n ow led gm en t is made to the donors of the
Petroleum Research Fund, administered by the ACS,
for support of this research.
Su p p or tin g In for m a tion Ava ila ble: Experimental de-
tails and spectroscopic data for all compounds (5 pages).
8
2
J O960178X
2
(11) Gen er a l P r oced u r e. The catalyst precursor II (42 mg, 50
µmol) was added to a solution of the sulfide (1.0 mmol) and diphenyl
sulfoxide (286 mg, 1.3 mmol) in CHCl (4 mL) and the mixture shaken
3
2
8
briefly to solubilize and then allowed to stand at 22 °C for 1.5-2 h.
11
Aqueous extraction (3 × 4 mL) provided the sulfoxides free of unreacted
2 3
starting compounds and reaction byproducts (Ph S and Ph PO);
concentration and drying in vacuo gave the corresponding sulfoxides.
12) The oxidation of methionine to methionine sulfoxide using
aqueous Me SO/HCl has been reported: Shechter, Y. J . Biol. Chem,
1986, 261, 66.
(
(
10) The structures of these decomposition products have not yet
been identified; however, the fact that no sulfone forms is significant.
2