Silver-catalyzed imination of sulfoxides and sulfides

Article abstract of DOI:10.1021/ol0519442

(Chemical Equation Presented) Silver salts in the presence of a chelating ligand efficiently catalyze the stereospecific imination of sulfoxides and sulfides with sulfonylamides and Phl(OAc)₂ to afford sulfoximines and sulfilimines, respectivel

Full text of DOI:10.1021/ol0519442

ORGANIC
LETTERS
2
005
Vol. 7, No. 22
983-4985
Silver-Catalyzed Imination of Sulfoxides
and Sulfides
4
Gae Young Cho and Carsten Bolm*
Institute of Organic Chemistry, RWTH Aachen UniVersity, Landoltweg 1,
D-52056 Aachen, Germany
carsten.bolm@oc.rwth-aachen.de
Received August 11, 2005
ABSTRACT
Silver salts in the presence of a chelating ligand efficiently catalyze the stereospecific imination of sulfoxides and sulfides with sulfonylamides
and PhI(OAc) to afford sulfoximines and sulfilimines, respectively, in good yields.
2
8
Sulfoximines and sulfilimines have been used as building
manganese and ruthenium complexes have been shown to
1
2
blocks for chiral ligands and pseudopeptides, and various
be capable of catalyzing the conversion of sulfoxides into
sulfoximines. However, common disadvantages of those
methods are that they either lead to products with protecting
groups at the sulfoximine nitrogen (such as tosyl), that are
difficult to cleave to give synthetically valuable NH-
sulfoximines, or that they still use potentially dangerous
reagents such as Boc-azide as nitrogen source. In 2004, we
3
approaches have been developed for their synthesis. Most
of them start from a sulfide, which is oxidized and iminated
sequentially. For the latter step, initiations by metal catalysts
have recently attracted much attention, since they allow one
to avoid the use of toxic and potentially explosive reagents
such as hydrazoic acid and O-mesitylenesulfonylhydroxyl-
4
-6
7
amine (MSH).
For example, copper and iron salts or
(
5) MSH: (a) Tamura, Y.; Minamikawa, J.; Sumoto, K.; Fujii, S.; Ikeda,
M. J. Org. Chem. 1973, 38, 1239. (b) Johnson, C. R.; Kirchhoff, R. A.;
Corkins, H. G. J. Org. Chem. 1974, 39, 2458. (c) Tamura, Y.; Matushima,
H.; Minamikawa, J.; Ikeda, M.; Sumoto, K. Tetrahedron 1975, 31, 3035.
(d) Fieser, M.; Fieser, L. F. Reagents for Organic Synthesis; John Wiley &
Sons: New York, 1975; Vol. 5; p 430.
(6) For other metal-free sulfoxide iminations, see: (a) Use of electro-
chemistry: Siu, T.; Picard, C. J.; Yudin, A. K. J. Org. Chem. 2005, 70,
932. (b) Krasnova, L. B.; Hili, R. M.; Chernoloz, O. V.; Yudin, A. K.
ArkiVoc 2005, iv, 26. (c) Application of a rather unstable 3-acetoxyami-
noquinazolinone (QNHOAc) as nitrogen source (which requires the use of
toxic lead tetraacetate for its preparation): Karabuga, S.; Kazaz, C.; Kilic,
H.; Ulukanli, S.; Celik, A. Tetrahedron Lett. 2005, 46, 5225.
(7) Cu salts: (a) M u¨ ller, J. F. K.; Vogt, P. Tetrahedron Lett. 1998, 39,
4805. (b) Lac oˆ te, E.; Amatore, M.; Fensterbank, L.; Malacria, M. Synlett
2002, 116. (c) Cren, S.; Kinahan, T. C.; Skinner, C. L.; Tye, H. Tetrahedron
Lett. 2002, 43, 2749. (d) Tomooka, C. S.; Carreira, E. M. HelV. Chim. Acta
2003, 85, 3773. (e) Takada, H.; Ohe, K.; Uemura, S. Angew. Chem., Int.
Ed. 1999, 38, 1288. Fe salts: (f) Bach, T.; K o¨ rber, C. Tetrahedron Lett.
1998, 39, 5015. (g) Bach, T.; K o¨ rber, C. Eur. J. Org. Chem. 1999, 64,
1033.
(8) Mn complexes: (a) Nishikori, H.; Ohta, C.; Oberlin, E.; Irie, R.;
Katsuki, T. Tetrahedron 1999, 55, 13937. (b) Ohta, C.; Katsuki, T.
Tetrahedron Lett. 2001, 42, 3885. Ru complexes: (c) Murakami, M.;
Uchida, T.; Katsuki, T. Tetrahedron Lett. 2001, 42, 7071. (d) Tamura, Y.;
Uchida, T.; Katsuki, T. Tetrahedron Lett. 2003, 44, 3301. (e) Murakami,
M.; Uchida, T.; Saito, B.; Katsuki, T. Chirality 2003, 15, 116. (f) Uchida,
T.; Tamura, Y.; Ohba, M.; Katsuki, T. Tetrahedron Lett. 2003, 44, 7965.
(
1) For recent examples, see: (a) Bolm, C.; Simic, O. J. Am. Chem.
Soc. 2001, 123, 3830. (b) Harmata, M.; Ghosh, S. K. Org. Lett. 2001, 3,
321. (c) Bolm, C.; Martin, M.; Simic, O.; Verrucci, M. Org. Lett. 2003,
, 427. (d) Bolm, C.; Verrucci, M.; Simic, O.; Cozzi, P. G.; Raabe, G.;
Okamura, H. Chem. Commun. 2003, 2816. (e) Bolm, C.; Martin, M.;
Gescheidt, G.; Palivan, C.; Neshchadin, D.; Bertagnolli, H.; Feth, M. P.;
Schweiger, A.; Mitrikas, G.; Harmer, J. J. Am. Chem. Soc. 2003, 125, 6222.
g) Langner, M.; Bolm, C. Angew. Chem., Int. Ed. 2004, 43, 5984.
Reviews: (h) Harmata, M. Chemtracts 2003, 16, 660. (i) Reetz, M. T.;
Bondarev, O. G.; Gais, H.-J.; Bolm, C. Tetrahedron Lett. 2005, 46, 5643.
3
5
(
(
j) Okamura, H.; Bolm, C. Chem. Lett. 2004, 33, 482 and references therein.
2) (a) Bolm, C.; Kahmann, J. D.; Moll, G. Tetrahedron Lett. 1997, 38,
169. (b) Bolm, C.; Moll, G.; Kahmann, J. D. Chem. Eur. J. 2001, 7, 1118.
(
1
(
c) Tye, H.; Skinner, C. L. HelV. Chim. Acta 2002, 85, 3272. (d) Bolm, C.;
M u¨ ller, D.; Hackenberger, C. P. R. Org. Lett. 2002, 4, 893. (e) Bolm, C.;
M u¨ ller, D.; Dalhoff, C.; Hackenberger, C. P. R.; Weinhold, E. Bioorg. Med.
Chem. Lett. 2003, 13, 3207.
(3) Reviews on sulfoximines and sulfilimines: (a) Johnson, C. R. Acc.
Chem. Res. 1973, 6, 341. (b) Pyne, S. G. Sulfur Rep. 1999, 21, 281. (c)
Taylor, P. C. Sulfur Rep. 1999, 21, 241. (d) Reggelin, M.; Zur, C. Synthesis
2
000, 1.
4) NaN3/H : (a) Fusco, R.; Tericoni, F. Chim. Ind. (Milan) 1965, 47,
1. (b) Johnson, C. R.; Schroeck, C. W. J. Am. Chem. Soc. 1973, 95, 7418.
+
(
6
(
c) For an improved protocol, see: Brandt, J.; Gais, H.-J. Tetrahedron:
Asymmetry 1997, 6, 909. (d) For the toxicity of NaN3: Merck Index, 12th
ed.; Budavari, S., Ed.; Merck & Co., Inc.: Rahway, NJ, 1996; p 451.
1
0.1021/ol0519442 CCC: $30.25
© 2005 American Chemical Society
Published on Web 10/05/2005

introduced an improved procedure for the imination of sulfur
compounds, which utilizes a rhodium catalyst and readily
available trifluoroacetamide or sulfonylamides as nitrogen
2 3
AgOAc (entry 5) or Ag CO (incomplete conversion; not
included in Table 1). Next, a broad range of ligands was
tested, and as previously observed in silver-catalyzed aziri-
9
10a,b
source. Following this method, sulfoximines and sulfilimines
dinations and intramolecular amidations,
the imination
can easily be prepared from the corresponding sulfoxides
and sulfides, respectively, under very mild reaction conditions
at room temperature. Furthermore, the imination is stereospe-
cific, making this approach particularly attractive for the
synthesis of optically active products. A drawback of this
protocol, however, is the high cost of the rhodium catalyst
reaction of 1a required a very carefully selected reagent
combination. All attempts to use phosphines, diamines, or
pyridines remained unsuccessful, and finally, terpyridine and
4,4′,4′′-tri-tert-butyl-2,2′:6′,2′′-terpyridine (4,4′,4′′-t-Bu
proved to be the most suitable ligands giving silver catalysts
(from AgNO as silver source), which afforded sulfoximine
2 in 52 and 83% yield, respectively (Table 1, entries 1 and
3
tpy)
3
{
[Rh
2 4
(OAc) ]}.
11
Silver reagents have long been believed to have low
2). As expected from those results, the presence of a ligand
was essential for substrate conversion, and AgNO alone did
catalytic efficiency. Most commonly, they are silver(I)
complexes, which rely on the high Lewis acidity of the
involved metal species. Only recently have synthetically
attractive silver catalysts (including chiral ones) for oxida-
tions as well as other group-transfer reactions been devel-
3
not catalyze the imination. Furthermore, at room temperature
no reaction occurred in the absence of the catalyst. The
presence of a base such as MgO did not affect the yield,
and acetonitirile was superior to other solvents such as
dichloromethane, DMF, and THF (which gave 2 in 66, 60,
and 52% yield, respectively). In less polar solvents such as
toluene and diethyl ether the imination did not proceed at
all. Generally, under reflux full conversion was achieved
within 1 h, and slightly improved yields were observed (entry
1
0
oped. Here, we report on a novel application of silver
catalysts, which allows efficient imination of sulfides and
sulfoxides.
For the initial screening and optimization we chose methyl
phenyl sulfoxide (1a) as substrate and a nitrogen source
1
2
generated in situ from NsNH
2
and PhI(OAc)
2
. The effect of
6). The catalyst loading could be as low as 2 mol %,
although under those conditions an extended reaction time
and a lower yield compared to iminations with more catalyst
had to be accepted (Table 2). Combinations of 4 or 8 mol
various combinations of silver salts and ligands (8 mol %
each) on the formation of sulfoximine 2 was first examined,
and the most relevant results are summarized in Table 1.
Table 1. Optimization of the Silver-Catalyzed Imination with
Table 2. Optimization of the Catalyst Loading
a
Ns-NH
2
entry catalyst loading (mol %) reaction time (h) yield (%)
entry
silver salt
ligand
yield (%)
1
2
3
2
4
8
48
16
16
62
83
83
1
2
3
4
5
6
AgNO3
AgNO3
AgNO3
AgOTf
AgOAc
AgNO3
terpyridine
52
83
66
85
74
86
4,4′,4′′-t-Bu3tpy
4,4′,4′′-t-Bu3tpy
4,4′,4′′-t-Bu3tpy
4,4′,4′′-t-Bu3tpy
4,4′,4′′-t-Bu3tpy
b
c
%
of both silver salt and ligand led to the best results in the
a
imination of 1a, giving sulfoximine 2 in 83% yield after 16
h (Table 2, entries 2 and 3).
To investigate the substrate scope of the novel silver
catalysis, sulfoxides 1a-d were iminated with various
sulfonylamides (Table 3). Gratifyingly, most iminating agents
Reaction conditions: sulfoxide 1a (1 equiv), AgNO3 (8 mol %), ligand
(
8 mol %), NsNH2 (1.2 equiv), PhI(OAc)2 (1.5 equiv) in CH3CN (0.1 M)
at room temperature. Use of CH2Cl2 as solvent instead of CH3CN. c Use
of 4 mol % of the catalyst under reflux instead of 8 mol % at room
temperature.
b
2 2
such as p-nosylamide (NsNH ), p-tosylamide (TsNH ), and
p-methyl-2-pyridinylsulfonylamide proved applicable, and
even at room temperature the corresponding sulfoximines
The screening showed that AgNO
entries 2 and 4) were superior to other silver salts such as
3
and AgOTf (Table 1,
3
-8 were obtained in moderate to excellent yield (up to
98%). The imination of 1a with 2-trimethylsilylethylsulfo-
nylamide (SesNH ) to give 7 was more difficult, and the
(
9) Okamura, H.; Bolm, C. Org. Lett. 2004, 6, 1305.
(10) For selected examples, see: (a) Cui, Y.; He, C. J. Am. Chem. Soc.
2
4
2
1
003, 125, 16202. (b) Cui, Y.; He, C. Angew. Chem., Int. Ed. 2004, 43,
210. (c) Cirakovic, J.; Driver, T. G.; Woerpel, K. A. J. Am. Chem. Soc.
002, 124, 9370. (d) Driver, T. G.; Woerpel, K. A. J. Am. Chem. Soc. 2004,
26, 9993. (e) Josephsohn, N. S.; Snapper, M. L.; Hoveyda, A. H. J. Am.
Chem. Soc. 2003, 125, 4018. (f) Ji, J.-X.; Au-Yeung, T. T.-L.; Wu, J.;
Yip, C. W.; Chan, A. S. C. AdV. Synth. Catal. 2004, 346, 42. (g)
Yanagisawa, A.; Kageyama, H.; Nakatsuka, Y.; Asakawa, K.; Matsumoto,
Y.; Yamamoto, H. Angew. Chem., Int. Ed. 1999, 38, 3701. (h) Yanagisawa,
A.; Touge, T.; Arai, T. Angew. Chem., Int. Ed. 2005, 44, 1546.
2
(11) A combination of AgNO3 and 4,4′,4′′-t-Bu3tpy also proved useful
in silver-catalyzed aziridinations and nitrene insertions as reported by Cui
and He (compare ref 10b,c).
(12) The microwave-assisted (200 W) silver-catalyzed imination of
methyl phenyl sulfoxide (1a) led to sulfoximine 2 in 85% yield (unopti-
mized) after 20 min, and it occurred in a sterospecific manner.
4984
Org. Lett., Vol. 7, No. 22, 2005

identical reaction conditions afforded the corresponding
sulfilimines 10 and 11 in 83 and 77% yield, respectively.
Interestingly, in contrast to the rhodium-catalyzed imination,
almost identical reactivities were observed in conversions
of sulfides and sulfoxides. Unfortunately, all attempts to
Table 3. Silver-Catalyzed Imination of Sulfoxidesa
15
prepare sulfodiimine 12 by imination of sulfilimine 10 with
NsNH and PhI(OAc) in the presence of the silver catalyst
2
2
failed, and no formation of 12 was observed (Figure 1).
a
Reaction conditions: sulfoxide (1.0 equiv), X-NH2 (1.2 equiv),
PhI(OAc)2 (1.5 equiv), AgNO3 (8 mol %), 4,4′,4′′-t-Bu3tpy (8 mol %) in
Figure 1.
b
c
CH3CN (0.1 M) at rt, ca. 16 h. Reaction time of 7 days. Use of 4 mol
of the catalyst under reflux instead of 8 mol % at room temperature.
%
In conclusion, we developed a novel silver-catalyzed sulfur
imination, which allows the preparation of various sulfox-
imines and sulfilimines from sulfoxides and sulfides, re-
spectively. The combination of AgNO and 4,4′,4′′-t-Bu tpy
reaction required a higher temperature to afford the product
in satisfying yield (Table 3, entry 5). Unfortunately, all
attempts to utilize trifluoroacetamide as the nitrogen source,
which affords a product with an N-protecting group that can
easily be cleaved by mild hydrolysis to provide synthetically
valuable NH-sulfoximines, remained unsuccessful under
silver catalysis.
With the objective to investigate the stereochemical path
of the silver-catalyzed imination and the hope that optically
active NH-sulfoximines could be accessed by subsequent
cleavage of the protecting group of the sulfoximine nitrogen,
sulfoxide (S)-1a with 83% ee13 was iminated in acetonitrile
3
3
forms a catalyst, which iminates the substrates under mild
reaction conditions at either ambient or elevated temperature
(reflux of acetonitrile). Since the nitrogen transfer proceeds
with retention of configuration at the stereogenic sulfur atom,
enantiopure sulfoximines are accessible from enantiomeri-
cally pure sulfoxides by this method. The use of readily
available sulfonylamides and iodobenzene diacetate results
in the formation of N-protected products, which can readily
be converted into their synthetically valuable NH-counter-
parts without epimerization. As a net result, the new catalytic
system circumvents the disadvantages of existing methodolo-
gies by introducing easy-to-cleave protective groups and the
use of both unproblematic iminating agents as well as less
under reflux using the catalyst prepared from AgNO
,4′,4′′-t-Bu tpy (8 mol % each), and the resulting nosylated
sulfoximine 2 (83% yield) was then deprotected by treatment
with a mixture of Cs CO and thiophenol in acteonitrile at
3
and
4
3
2
3
1
6
room temperature to give NH-sulfoximine (S)-9 (76% yield).
The absolute configuration of this product and its ee (83%)
revealed that the overall process proceeded with net retention
of configuration and without epimerization at the stereogenic
center (Scheme 1).14
cost-intensive metal catalysts. Further applications of the
silver-catalyzed imination process are currently under in-
vestigation.
Acknowledgment. The Deutsche Forschungsgemein-
schaft within the SFB 380 and the Fonds der Chemischen
Industrie are gratefully acknowledged for financial support.
Scheme 1
Supporting Information Available: Experimental pro-
1
13
cedures and full characterization ( H and C NMR data and
spectra, MS, IR, and CHN analyses) for all new compounds.
This material is available free of charge via the Internet at
http://pubs.acs.org.
OL0519442
Application of the silver-catalyzed imination reaction to
methyl phenyl sulfide and tert-butyl methyl sulfide under
(14) Cleavage of the SES-group by treatment with TBAF is also well-
established (compare ref 7c), and there is no indication that this protocol
leads to racemization.
(
15) For a recent contribution on sulfodiimines, see: Dehli, J. R.; Bolm,
(13) The enantiomer ratio was determined by HPLC using a chiral column
C. Synthesis 2005, 1058.
(
Chiralcel OJ, 20 °C, heptane/i-PrOH ) 85:15, 0.5 mL/min); tR(R): 36.8
(16) Noteworthy is the fact that [Rh2(OAc)4] is about 100 times more
expensive than AgNO3 (Aldrich prices per gram in 2005).
min, tR(S): 42.6 min.
Org. Lett., Vol. 7, No. 22, 2005
4985