Novel C-S bond formation through α',β-elimination of tert-butyl sulfoxonium ylides: A facile approach to chiral sulfoxides

Article abstract of DOI:10.1055/s-0029-1217572

Chiral sulfoxides were prepared in excellent enantio-selectivity through a′,β-elimination of the tert-butyl substituted sulfoxonium ylide intermediates in situ generated from diazoacetates and (R)-N-tert-butylsulfinyl aldimines in the presence of a copper

Full text of DOI:10.1055/s-0029-1217572

LETTER
2183
Novel C–S Bond Formation Through a’,b-Elimination of tert-Butyl
Sulfoxonium Ylides: A Facile Approach to Chiral Sulfoxides
A
pproachtoChira
i
l
Sulf
o
a
a
Chengdu Institute of Organic Chemistry, Chinese Academy of Sciences, Chengdu 610041, P. R. of China
E-mail: whu@chem.ecnu.edu.cn; E-mail: jliao10@cioc.ac.cn
b
c
Graduate School of Chinese Academy of Sciences, Beijing 100049, P. R. of China
Department of Chemistry, East China Normal University, Shanghai 200062, P. R. of China
Received 24 March 2009
In this communication, we describe a novel reaction of
copper(I)-catalyzed diazo decomposition of diazocarbon-
yl compounds in the presence of tert-butyl sulfoxides.
tert-Butyl sulfoxonium ylides from the reaction were
found to be unstable, and underwent subsequential a¢,b-
elimination to give a-sulfinyl acetate derivatives with
inversion of absolute configuration of the sulfoxides
(Scheme 1).
Abstract: Chiral sulfoxides were prepared in excellent enantio-
selectivity through a¢,b-elimination of the tert-butyl substituted sulf-
oxonium ylide intermediates in situ generated from diazoacetates
and (R)-N-tert-butylsulfinyl aldimines in the presence of a copper(I)
catalyst.
Key words: chiral sulfoxides, sulfoxonium ylide, a¢,b-elimination
Ylides as reactive intermediates have been known to un-
dergo synthetically useful transformations.1 The reaction
of carbenes or carbenoids with sulfur compounds repre-
sents a useful approach to generate sulfonium ylides. As
useful intermediates in synthetic chemistry, the sulfonium
ylides have been used in many reactions, such as 1,2-
Stevens,2 2,3-sigmatropic rearrangements,3 and Corey–
Chaykovsky reaction.4
N2
Cu(I)
R
R
+
S
COOEt
OEt
S
H
O
O
O
Scheme 1 Copper-catalyzed reaction of ethyl diazoacetate with
tert-butyl sulfoxides
As important building blocks in organic synthesis, a-sulfi-
nyl acetates have been used to synthesize b-amino acid
and b-hydroxy acid derivatives exhibiting powerful anti-
bacterial properties.12 The a-sulfinyl acetates have been
also reported as key constituents for many naturally oc-
curring peptides, terpenes, alkaloids, macrolide and b-lac-
tam antibiotics.13
a¢,b-Elimination reaction of a sulfonium ylide derived
from di-tert-butyl or diethyl sulfide and ethyl diazoacetate
was reported to give corresponding ethyl tert-butyl- or
ethylthioacetate.5 In contrast, the reaction of carbenes
with sulfoxides to generate sulfoxonium ylides and subse-
quential transformations6,7–11 was scarcely reported de-
spite the potential usefulness in constructing sulfur-
containing molecules. Due to a competing attack of the
carbene on oxygen of the sulfoxide resulting in deoxygen-
ation of the sulfoxide, the reaction in generating the
desired sulfoxonium ylides was complicated.7,10 Never-
theless, a number of stable alkyl sulfoxonium ylides were
isolated in high yields through copper(I)- or rhodium(II)-
catalyzed diazo decomposition of diazoacetates in the
presence of alkyl sulfoxides.8,9–11 For example, Ando has
reported that the CuSO4-catalyzed decomposition of ethyl
diazoacetate in the presence of dimethyl or diphenyl sul-
foxide gave the corresponding sulfoxonium ylides.9 Sim-
ilarly the copper cyanide catalyzed decomposition of
various ethyl aryl diazoacetates in the presence of dimeth-
yl sulfoxide was found to give good yields of sulfoxonium
ylides.10 Moody has reported that the cyclic sulfoxonium
ylides were formed in the intramolecular interception of
rhodium carbenoids by sulfoxides.11
Our initial study began with the copper(I)-catalyzed reac-
tion of ethyl diazoacetate with racemic alkyl phenyl sulf-
oxides. No rearrangement products 2 were observed with
the use of methyl, ethyl, or benzyl phenyl sulfoxide as
starting materials (Table 1, entries 1–3). For example, the
reaction with ethyl phenyl sulfoxide gave corresponding
stable sulfoxonium ylide in 40% yield (Table 1, entry 2).
We were gratified to find that the a¢,b-elimination product
2a was isolated in 28% yield with tert-butyl phenyl sulf-
oxide catalyzed by CuPF6(MeCN)4 (Table 1, entry 4).
Other Cu(I) complexes including Cu(acac)2, CuI, CuOAc,
and CuOTf were also tested in order to improve the reac-
tion yield (Table 1, entries 5–8). Among the catalysts,
only CuOTf was found to be effective and gave 25% yield
of 2a (Table 1, entry 8). It was found that Rh2(OAc)4 was
also an effective catalyst to give the desired product in
43% yield (Table 1, entry 9).14 The reaction was extended
to other diazo compounds by using Rh2(OAc)4 catalyst.
When methyl phenyl diazoacetate was employed, the cor-
responding product 3 was obtained in 21% yield (Table 1,
entry 10).14 The reaction failed to give the desired product
with the use of dimethyl diazomalonate (Table 1 entry
11).
SYNLETT 2009, No. 13, pp 2183–2187
x
x
.x
x
.2
0
0
9
Advanced online publication: 16.07.2009
DOI: 10.1055/s-0029-1217572; Art ID: W04009ST
© Georg Thieme Verlag Stuttgart · New York

2184
X.-Y. Guan et al.
LETTER
would provide us useful information regarding the reac-
tion mechanism. As shown in Table 2, the reaction of (R)-
tert-butyl phenyl sulfoxide [(R)-1a] with ethyl diazoace-
tate catalyzed by Rh2(OAc)4 gave optically active a-sulfi-
nyl acetates 2a in moderate yield (Table 2, entry 1).14 The
absolute configuration of (S)-ethyl 2-(phenylsulfinyl)ace-
tate (2a)15 was confirmed by comparison of the optical
rotation with literature values.13g A similar result was ob-
tained with sulfoxide (R)-1b bearing an electron-donating
3-MeO substituent (Table 2, entry 2).16 Extension of diazo
compound to methyl phenyl diazoacetate afforded corre-
sponding product 3 in 77:23 dr with 97.5% ee for the ma-
jor diastereomer, but in lower yield (Table 2, entry 3).17
Again, no desired product was observed for dimethyl dia-
zomalonate catalyzed by Rh2(OAc)4 or CuPF6(MeCN)4
(Table 2, entries 4 and 5).
Table 1 Reaction of Diazocarbonyl Compounds with Alkyl Phenyl
Sulfoxides
R2
N2
Cat
R1
OR3
OR3
+
R2
CH2Cl2
S
S
O
O
O
O
1
2 or 3
2: R2 = H, R3 = Et
3: R2 = Ph, R3 = Me
Entry R1
R2
R3
Et
Et
Et
Et
Et
Et
Et
Et
Et
Me
Catalyst
Yield (%)a
1
2
Me
Et
H
H
H
H
H
H
H
H
H
Ph
CuPF6(MeCN)4
CuPF6(MeCN)4
CuPF6(MeCN)4
CuPF6(MeCN)4
CuI
–
–
3
Bn
–
4
t-Bu
t-Bu
t-Bu
t-Bu
t-Bu
t-Bu
t-Bu
t-Bu
28 ( 2a)
trace
trace
trace
25 ( 2a)
43 ( 2a)
21 ( 3)
–
We next extended the reaction to optically active (R)-N-
tert-butylsulfinyl aldimines 4, because these chiral sub-
strates are easier to obtain from the condensation of corre-
sponding aldehydes with commercially available (R)-tert-
butylsulfinyl amide. A number of (R)-N-tert-butylsulfinyl
aldimines 4 were examined.18 Unfortunately, Rh2(OAc)4
catalyst failed to give the desired product 5 or 6 in this
case. CuPF6(MeCN)4 was identified to be the best catalyst
of choice and gave optically active a-sulfinyl acetates in
moderate yield with inversion of the absolute configura-
tion (Table 3, entries 1–7). The reaction tolerated the sulf-
oxide substrate with a good range of substitutions on the
aromatic ring except the substrate bearing o-substitution
on the phenyl ring. This is probably due to steric effect of
the ortho-substitution resulting in difficult formation of
corresponding sulfoxonium ylide intermediate (Table 3,
entries 8, 9). When methyl phenyl diazoacetate was em-
5
6
Cu(acac)2
7
CuOAc
8
CuOTf
9
Rh2(OAc)4
Rh2(OAc)4
Rh2(OAc)4
10
11
COOMe Me
a Isolated yield after column chromatography.
We next turned our attention towards the reaction by us-
ing optically active tert-butyl phenyl sulfoxide. We were
very interested to see whether we can maintain the optical
purity at the chiral sulfur center after the reaction. This
Table 2 The Reaction of Diazocarbonyl Compounds with Enantiopure tert-Butyl Phenyl Sulfoxidesa
N2
R2
catalyst
CH2Cl2
OR3
OR3
+
R1
S
R2
R1
S
O
O
O
O
1
7
2 or 3
7a R2 = H, R3 = Et
2: R2 = H,
R3 = Et
3: R2 = Ph, R3 = Me
7b R2 = Ph, R3 = Me
7c R2 = COOMe, R3 = Me
Entry
R1
7
Catalyst
Yield (%)b
43 (2a)e
40 (2b)
21 (3)
n.r.
ee (%)c
90
1
H (1a)
7a
7a
7b
7c
7c
Rh2(OAc)4
Rh2(OAc)4
Rh2(OAc)4
Rh2(OAc)4
CuPF6(MeCN)4
2
MeO (1b)
H (1a)
99
3d
4
97.5
H (1a)
5
H (1a)
n.r.
a Reactions were performed on a 0.2 mmol scale (1/diazo = 1:3) in the presence of a catalyst (3 mol%) at 40 °C in CH2Cl2 (3.0 mL) under an
inert atmosphere.
b Isolated yield after column chromatography purification.
c The ee values were determined by HPLC on a chiral stationary phase.
d Diastereomeric ratio (R,S)-317/(S,S)-3 = 77:23, as determined by 1H NMR analysis of the crude reaction mixture.
e The absolute configuration of (S)-ethyl 2-(phenylsulfinyl)acetate 2a was confirmed by comparison of its optical rotation with that reported in
literature.13g
Synlett 2009, No. 13, 2183–2187 © Thieme Stuttgart · New York

LETTER
Approach to Chiral Sulfoxides
2185
Table 3 Copper-Catalyzed Reaction of Diazocarbonyl Compounds with the Optically Active N-tert-Butylsulfinyl Aldiminesa
R2
N2
CuPF6(MeCN)4
R1
N
R1
N
OR3
OR3
+
R2
S
S
CH2Cl2
O
O
O
O
5 or 6
4
5: R2 = H,
R3 = Et
6: R2 = Ph, R3 = Me
Entry
R1
Ph (4a)
R2
H
H
H
H
H
H
R3
Et
Et
Et
Et
Et
Et
Yield (%)b
55 (5a)
51 (5b)
42 (5c)
47 (5d)
40 (5e)
45 (5f)
ee (%)c
98
1
2
4-MeOC6H4 (4b)
4-O2NC6H4 (4c)
4-ClC6H4 (4d)
97.7
100.0
99.8
98
3f
4
5
4-BrC6H4 (4e)
6f
4-Cl-3-O2NC6H3 (4g)
99.8
O
7
H
Et
43 (5g)
99.2
O
(4i)
8
9
2,4-O2NC6H3 (4h)
H
Et
n.r.
2,4-MeOC6H3 (4f)
4-O2NC6H4 (4c)
4-O2NC6H4 (4c)
H
Et
n.r.
10f
11f
Ph
Me
53 (6a)d
50 (6b)e
99/99
98/99
4-BrC6H4
Me
a Reactions were performed on a 0.2 mmol scale (4/diazo = 1:3) in the presence of CuPF6(MeCN)4 (3 mol%) at 40 °C in CH2Cl2 (3.0 mL) under
an inert atmosphere.
b Isolated yield after column chromatography purification.
c The ee values were determined by HPLC on a chiral stationary phase.
d Diastereomeric ratio (R,R)-6a/(S,R)-6a = 44:56, as determined by 1H NMR analysis of the crude reaction mixture.
e Diastereomeric ratio (R,R)-6b/(S,R)-6b = 60:40, as determined by 1H NMR analysis of the crude reaction mixture.
f (S)-N-tert-Butylsulfinyl aldimines were used.
ployed, the reaction also afforded corresponding products a¢,b-elimination to afford a-sulfinyl acetate. The a¢,b-
with 98–99% ee and moderate dr (Table 3, entries 10, 11). elimination of the sulfoxonium ylide intermediate is prob-
The structure of (R,R)-6a was conformed by single-crystal ably through a five-membered cyclic transition state and
X-ray structural analysis (Figure 1).20
The CD spectra of compounds (S)-4c and (R)-5c19 were
resulting in inversion of the stereochemistry in the product
(Scheme 2).
recorded in order to get information regarding to the abso- In summary, we report here the first example of elimina-
lute configuration of the product. As shown in Figure 2, tion of tert-butyl sulfoxonium ylide in situ generated from
the CD curve of (R)-5c showed the opposite trend against diazocarbonyl compounds and tert-butyl sulfoxides. The
the starting material (S)-4c, indicating that the product has reaction affords a-sulfinyl acetate with inversion of abso-
the opposite absolute configuration at the sulfur atom. The lute configuration. Optically active a-sulfinyl acetate can
reaction mechanism is proposed as following: carbene be made from chiral tert-butyl sulfoxides in moderate
formed by the copper-catalyzed decomposition of diazo- yield. This reaction provides an easy entry for the prepa-
acetate can easily attack on a long-pair electron of the sul- ration of chiral sulfoxides and should be applicable in
fur atom to give sulfoxonium ylide. The ylide carrying a asymmetric synthesis through additional transformations.
large tert-butyl group is unstable, and prefers to undergo
Figure 1 X-ray crystal structure of (R,R)-6a
Synlett 2009, No. 13, 2183–2187 © Thieme Stuttgart · New York

2186
X.-Y. Guan et al.
LETTER
(4) (a) Aggarwal, V.; Richardson, J. In Science of Synthesis,
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Tetrahedron Lett. 1969, 1979.
Figure 2 CD spectrum structure of (S)-4c and (R)-5c
N2
R
Cu(I)
R
S
OEt
+
S
H
H
O
O
O
COOEt
(10) Dost, F.; Gosselck, J. Tetrahedron Lett. 1970, 5091.
(11) Moody, C. J.; Slawin, A. M.; Taylor, R. J.; Williams, D. J.
Tetrahedron Lett. 1988, 6009.
α',β-elimination
–
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R
S
COOEt
O
Scheme 2 Proposed reaction mechanism
Supporting Information for this article is available online at
http://www.thieme-connect.com/ejournals/toc/synlett.
Acknowledgment
We are grateful for financial support from the National Science
Foundation of China (2077203 and 20872139) and financial sup-
port from Shanghai Municipal Education Commission (09ZZ45).
References and Notes
(1) (a) Padwa, A.; Weingarten, M. D. Chem. Rev. 1996, 96,
223. (b) Doyle, M. P.; Mckervey, M. A.; Ye, T. Modern
Catalytic Methods for Organic Synthesis with Diazo
Compounds; John Wiley and Sons: New York, 1998.
(c) Padwa, A.; Hornbuckle, S. F. Chem. Rev. 1991, 91, 263.
(2) For examples of [1,2]-Stevens rearrangement of sulfonium
ylides derived from metal carbenoids, see: (a)Kametani,T.;
Yukawa, H.; Honda, T. J. Chem. Soc., Chem. Commun.
1986, 651. (b) Kim, G.; Kang, S.; Kim, S. N. Tetrahedron
Lett. 1993, 34, 7627. (c) Moody, C. J.; Taylor, R. J.
Tetrahedron Lett. 1988, 29, 6005.
(3) For examples of [2,3]-sigmatropic rearrangement of
ammonium ylides derived from metal carbenoids, see:
(a) Ma, M.; Peng, L. L.; Li, C. K.; Zhang, X.; Wang, J. B.
J. Am. Chem. Soc. 2005, 127, 15016. (b) Doyle, M. P.;
Tamblyn, W. H.; Bagbers, V. J. Org. Chem. 1981, 46, 5094.
(c) Vedejs, E.; Hagen, J. P. J. Am. Chem. Soc. 1975, 97,
6878. (d) Doyle, M. P.; Griffin, J. H.; Chinn, M. S.; Leusen,
D. J. Org. Chem. 1981, 46, 5094.
(14) Typical Procedure for the Synthesis of Compounds 2
and 3
To a refluxing CH2Cl2 (3 mL) solution of Rh2(OAc)4 (2.6
mg, 3 mol%), (R)-(tert-butylsulfinyl)benzene [(R)-1a, 36.4
mg, 0.2 mmol] was added ethyl diazoacetate (68.4 mg, 0.6
mmol) in CH2Cl2 (3 mL) over 1 h via a syringe pump. After
the addition was completed, the reaction mixture was cooled
to r.t. Solvent was removed, and the crude product was
purified by a flash column chromatography on silica gel
eluting with 20% EtOAc–light PE to give (S)-2a (18 mg) in
43% yield.
(15) Analytical Data of (S)-Ethyl 2-(Phenylsulfinyl)acetate
[(S)-2a]
TLC: Rf = 0.15 (PE–EtOAc, 5:1); [a]D20 –131 (c 1, EtOAc);
Synlett 2009, No. 13, 2183–2187 © Thieme Stuttgart · New York

LETTER
90% ee, determined by HPLC [Daicel Chiralpak AD-H,
Approach to Chiral Sulfoxides
2187
(2.2 mg, 3 mol%), (S)-N-(4-nitrobenzylidene)-2-methyl-
propane-2-sulfinamide (4c, 50.8 mg, 0.2 mmol) was added
ethyl diazoacetate (68.4 mg, 0.6 mmol) in CH2Cl2 (3 mL)
over 1 h via a syringe pump. After the addition was
completed, the reaction mixture was cooled to r.t. Solvent
was removed, and the crude product was purified by a flash
column chromatography on silica gel eluting with 20%
EtOAc–light PE to give (R)-5c (24 mg) in 42% yield.
(19) Analytical Data of (R)-N-(4-Nitrobenzylidene)-2-ethoxy-
2-oxomethanesulfinamide [(R)-5c]
flow rate 0.4 mL/min, hexane–2-PrOH = 90:10, 254 nm;
tR(major) = 37.2 min and tR(minor) = 40.0 min]. 1H NMR
(300 MHz, CDCl3): d = 1.20 (t, J = 7.1 Hz, 3 H), 3.68 (d,
J = 13.6 Hz, 1 H), 3.87 (d, J = 13.6 Hz, 1 H), 4.16 (q, J = 7.1
Hz, 2 H), 7.52–7.70 (m, 5 H) ppm. 13C NMR (75 MHz,
CDCl3): d = 13.98, 61.70, 62.01, 124.17, 129.36, 131.75,
143.07, 164.68 ppm. HRMS (EI): m/z calcd for
C10H12NaO3S [M + Na]+: 235.0405; found: 235.0399.
(16) Analytical Data of (S)-Ethyl 2-(Phenylsulfinyl)acetate
[(S)-2b]
TLC: Rf = 0.12 (PE–EtOAc, 5:1); [a]D20 +84.5 (c 1, EtOAc);
100% ee, determined by HPLC [Daicel Chiralpak AS, flow
rate 0.6 mL/min, hexane–2-PrOH = 100:30, 254 nm;
tR(major) = 25.72 min and tR(minor) = 19.09 min]. 1H NMR
(300 MHz, CDCl3): d = 1.30 (t, J = 7.1 Hz, 3 H), 3.76 (d,
J = 13.6 Hz, 1 H), 3.90 (d, J = 13.6 Hz, 1 H), 4.31 (q, J = 7.1
Hz, 2 H), 8.05 (d, J = 8.8 Hz, 2 H), 8.36 (d, J = 8.8 Hz, 2 H),
8.76 (s, 1 H) ppm. 13C NMR (75 MHz, CDCl3): d = 14.15,
59.98, 62.24, 124.26, 130.39, 132.32, 138.32, 161.13,
164.06 ppm. HRMS (EI): m/z calcd C11H12N2NaO5S [M +
Na]+: 307.0365; found: 307.0359.
TLC: Rf = 0.13 (PE–EtOAc, 5:1); [a]D20 –120 (c 1, EtOAc);
99% ee, determined by HPLC [Daicel Chiralpak AD-H,
flow rate 0.4 mL/min, hexane–2-PrOH = 90:10, 254 nm,
tR(major) = 42.35 min and tR(minor) = 45.58 min]. 1H NMR
(300 MHz, CDCl3): d = 1.26 (t, J = 7.2 Hz, 3 H), 3.67 (d,
J = 13.5 Hz, 1 H), 3.85 (d, J = 10.8 Hz, 1 H), 3.86 (s, 3 H),
4.21 (q, J = 7.1 Hz, 2 H), 7.03–7.20 (m, 4 H) ppm. 13C NMR
(75 MHz, CDCl3): d = 14.02, 55.60, 61.90, 62.04, 108.41,
116.18, 118.23, 130.31, 144.61, 160.53, 164.73 ppm. HRMS
(EI): m/z calcd for C11H14NaO4S [M + Na]+: 265.0510;
found: 265.0505.
(20) Crystal Structure Data for (R,R)-N-(4-Nitrobenzylidene)-
2-ethoxy-2-oxomethane-1-benzylsulfinamide [(R,R)-6a]
C16H14N2O5S, Mw = 346.35, light yellow, orthorhombic, P2,
a = 5.70380 (10), b = 8.2382 (2), c = 34.4784 (8) Å,
a = 90.00, b = 90.00, g = 90.00, V = 1620.11 (6) Å3, Z = 4,
T = 296 (2) K, rcalcd = 1.420 Mg·m, F(000) = 720,
l = 0.71073 Å, m = 0.229 mm–1, R(F) = 0.0299 and
wR(F)2 = 0.0792 for 2722 observed reflections, I > 2s,
2.36° < q < 24.99°. CCDC 722774 contains the
(17) Analytical Data of (R,S)-Ethyl 2-(Phenylsulfinyl)acetate
[(R,S)-3]
TLC: Rf = 0.26 (PE–EtOAc, 5:1); [a]D20 –110 (c 1, EtOAc);
97.5% ee, determined by HPLC [Daicel Chiralpak AD-H,
flow rate 0.4 mL/min, hexane–2-PrOH = 90:10, 254 nm;
tR(major) = 44.7 min and tR(minor) = 50.9 min]. 1H NMR
(300 MHz, CDCl3): d = 3.84 (s, 3 H), 4.55 (s, 1 H), 3.87 (d,
J = 13.6 Hz, 1 H), 4.16 (q, J = 7.1 Hz, 2 H), 7.04–7.49 (m,
10 H). 13C NMR (75 MHz, CDCl3): d = 61.70, 78.02, 125.19,
128.50, 128.57, 129.22, 129.37, 129.94, 131.70, 141.18,
168.01 ppm. HRMS (EI): m/z calcd for C15H14NaO3S [M +
Na]+: 297.0561; found: 297.0556.
supplementary crystallographic data for this paper.
These data can be obtained free of charge via
www.ccdc.cam.ac.uk/conts/retrieving.html [or from the
Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB21EZ, UK; fax +44 (1223)336033; or
deposit@ ccdc.cam.ac.uk].
(18) Typical Procedure for the Synthesis of Compounds 5 and 6
To a refluxing CH2Cl2 (3 mL) solution of CuPF6 (MeCN)4
Synlett 2009, No. 13, 2183–2187 © Thieme Stuttgart · New York

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