Highly enantioselective (OC)Ru(salen)-catalyzed sulfimidation using N-alkoxycarbonyl azide as nitrene precursor

Article abstract of DOI:10.1016/S0040-4039(03)00609-9

Enantioselective imidation of alkyl aryl sulfides with N-alkoxycarbonyl azide as a nitrene precursor was effected by using (OC)Ru(salen) complex 1 as catalyst. The steric and electronic nature of the N-alkoxycarbonyl group was found to strongly affect the enantioselectivity and the reaction rate, and high enantioselectivity (up to 99% ee) and good chemical yields were achieved by using 2,2,2-trichloro-1,1-dimethylethoxycarbonyl azide as the nitrene precursor at room temperature.

Full text of DOI:10.1016/S0040-4039(03)00609-9

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 44 (2003) 3301–3303
Highly enantioselective (OC)Ru(salen)-catalyzed sulfimidation
using N-alkoxycarbonyl azide as nitrene precursor
Yuusuke Tamura, Tatsuya Uchida and Tsutomu Katsuki*
Department of Chemistry, Faculty of Science, Graduate School, Kyushu University 33, CREST,
JST (Japan Science and Technology), Hakozaki, Higashi-ku, Fukuoka 812-8581, Japan
Received 5 February 2003; revised 27 February 2003; accepted 28 February 2003
Abstract—Enantioselective imidation of alkyl aryl sulfides with N-alkoxycarbonyl azide as a nitrene precursor was effected by
using (OC)Ru(salen) complex 1 as catalyst. The steric and electronic nature of the N-alkoxycarbonyl group was found to strongly
affect the enantioselectivity and the reaction rate, and high enantioselectivity (up to 99% ee) and good chemical yields were
achieved by using 2,2,2-trichloro-1,1-dimethylethoxycarbonyl azide as the nitrene precursor at room temperature. © 2003 Elsevier
Science Ltd. All rights reserved.
in a non-asymmetric reaction.7,8 We also examined
sulfimidation using N-tert-butoxycarbonyl azide as the
nitrene source in the presence of complex 1 (Scheme
1).5 Although the reaction proceeded with moderate
selectivity, the reaction was slow. In the course of this
study, however, we found that the nature of the alkoxy-
carbonyl group affected the enantioselectivity and reac-
tion rate of the sulfimidation to a considerable extent
(Table 1).
Nitrene transfer reaction is the most important method
for the synthesis of optically active compounds includ-
ing a nitrogen functional group. Therefore, many cata-
lysts have been developed for this reaction and high
enantioselectivity has been achieved in many reac-
tions.1–3 However, there is still big room for improve-
ment in the atom efficiency of the nitrene precursor,
because
N-arylsulfonyliminophenyliodinanes
(ArSO2N=IPh) that are low atom efficient reagents
have been mostly used as nitrene precursors in those
reactions. Recently, the use of azide compounds as the
nitrene precursor, which produce the corresponding
nitrene and the by-product nitrogen, has been exam-
ined. Jacobsen et al. reported that arylsulfonyl azide
served as a nitrene precursor for asymmetric aziridina-
tion in the presence of a copper ion under photo-irradi-
ation.4 We found that (OC)Ru(salen) complex 1
catalyzed sulfimidation of alkyl aryl sulfides using aryl-
sulfonyl azide as the nitrene precursor without photo-
irradiation, in
a
highly enantioselective manner
(Scheme 1).5 However, removal of the arylsulfonyl
group from the resulting nitrogen compounds usually
needs harsh reaction conditions.6
On the other hand, Bach and Ko¨rber used N-tert-
butoxycarbonyl azide as the nitrene source for FeCl2-
catalyzed imidation of alkyl and allyl sulfides, though
Keywords: (OC)Ru(salen) complex; asymmetric catalysis; sulfimida-
tion; N-alkoxycarbonyl azide.
* Corresponding author. Fax: +81-92-642-2607; e-mail: katsuscc@
mbox.nc.kyushu-u.ac.jp
Scheme 1.
0040-4039/03/$ - see front matter © 2003 Elsevier Science Ltd. All rights reserved.
doi:10.1016/S0040-4039(03)00609-9

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Y. Tamura et al. / Tetrahedron Letters 44 (2003) 3301–3303
Table 1. Asymmetric imidation of methyl phenyl sulfide with various N-carbamoyl azide in the presence of complex 1
Entry
R
% eea
Yield (%)b
1
2
3
4
5
6
7
CH3
13
18
36
71
8
21
18
28
22
37
62
93
n-C4H9
C6H5CH2
t-C4H9
C6H5
Cl3CCH2
Cl3CC(CH3)2
77
95
a Determined by HPLC analysis using DAICEL CHIRALPAK AD-H (hexane:i-PrOH=9:1).
b Isolated yield.
Enantioselectivity increased as the bulkiness of the
alkygroup of the alkoxycarbonyl group became larger
(entries 1–4). On the other hand, use of phenoxycar-
bonyl azide improved the chemical yield, albeit with
poor enantioselectivity (entry 5). From these results, we
speculated that the N-alkoxy- (or aryloxy)carbonyl
group would affect the enantioselectivity through a
steric factor and the reaction rate through an electronic
factor, though some aromatic effect seemed to affect
enantioselectivity adversely. Thus, we examined the
reaction with trichloroethoxycarbonyl azide and found
that reaction proceeded with moderate enantioselectiv-
ity and acceptable chemical yield (entry 6). Encouraged
by this result, we further examined the reaction with
2,2,2-trichloro-1,1-dimethylethoxycarbonyl azide that
desired reaction occurred when these reagents were
used.
We next examined imidation of various alkyl aryl
sulfides using 2,2,2-trichloro-1,1-dimethylethoxycar-
bonyl azide in the presence of complex 1 (Table 2). The
effect of a p-substituent on the phenyl group was first
examined: introduction of an electron-donating
methoxy group or of an electron-withdrawing chloro
group had no influence on enantioselectivity: imidation
of methyl p-methoxy- and p-chlorophenylsulfides
showed enantioselectivity as high as that of methyl
phenyl sulfide (entries 1 and 2). Imidation of ethyl
phenyl sulfide also showed good but somewhat reduced
enantioselectivity of 92% ee (entry 3). On the other
hand, introduction of an o-substituent remarkably
improved enantioselectivity (entries 4 and 5).
possesses
a bulky and electron-withdrawing alkyl
group, and high enantioselectivity of 95% ee as well as
good chemical yield of 93% was finally achieved (entry
7).
Typical experimental procedure for imidation of
sulfides was exemplified by the reaction of methyl
phenyl sulfide with 1 as the catalyst: complex 1 (1.9 mg,
2.0 mmol) was dissolved in dry toluene (1 ml), concen-
trated azeotropically in vacuo, and re-dissolved in
We also examined the use of other nitrene sources such
as acyl azides, benzoyl and p-nitrobenzoyl azides, and
alkyl azides, benzyl and p-nitrobenzyl azides, but no
Table 2. Asymmetric imidation of alkyl aryl sulfides with 2,2,2-trichloro-1,1-dimethylethoxycarbonyl azide in the presence of
complex 1
b
Entry
X
Y
R
% eea
%
1
2
3
4
5
MeO
Cl
H
H
H
H
H
H
Br
NO2
CH3
CH3
CH3CH2
CH3
CH3
96
95
92
98
99
91
88
87
74
99
a Determined by HPLC analysis using DAICEL CHIRALPAK AD-H (hexane:i-PrOH=9:1).
b Isolated yield.

Y. Tamura et al. / Tetrahedron Letters 44 (2003) 3301–3303
3303
dichloromethane (0.5 ml). To this solution were added
methyl phenyl sulfide (11.7 ml, 0.1 mmol) and MS 4 A
3339–3342; (b) Liang, J.-L.; Yu, X.-Q.; Che, C.-M. Chem.
Commun. 2002, 124–125.
,
(20 mg), and the suspension was stirred for 0.5 h at
room temperature. To this suspension was added 2,2,2-
trichloro-1,1-dimethylethoxycarbonyl azide (34.0 mg,
0.13 mmol), and the whole mixture was stirred for
another 24 h at the temperature. The mixture was
chromatographed on silica gel (ethyl acetate) to give the
product (31.9 mg, 93%). The enantiomeric excess of the
product was determined to be 95% by HPLC analysis
using DAICEL CHIRALPAK AD-H (hexane:i-
PrOH=9:1).
3. (a) Takada, H.; Nishibayashi, Y.; Ohe, K.; Uemura, S. J.
Chem. Soc., Chem. Commun. 1996, 931–932; (b) Takada, H.;
Nishibayashi, Y.; Ohe, K.; Uemura, S.; Baird, C. P.; Sparey,
T. J.; Taylor, P. C. J. Org. Chem. 1997, 62, 6512–6518; (c)
Takada, H.; Nishibayashi, Y.; Ohe, K.; Uemura, S. Phos-
phorus, Sulfur Silicone Relat. Elem. 1997, 120 and 121,
363–364; (d) Baird, C. P.; Clark, A. J.; Rooke, S. M.; Sparey,
T. J.; Taylor, P. C. Phosphorus, Sulfur Silicon Relat. Elem.
1997, 120 and 121, 365–366; (e) Miyake, Y.; Takada, H.;
Ohe, K.; Uemura, S. J. Chem. Soc., Perkin Trans. 1 1998,
2373–2376; (f) Nishikori, H.; Ohta, C.; Oberlin, E.; Irie, R.;
Katsuki, T. Tetrahedron 1999, 55, 13937–13946.
4. Li, Z.; Quan, R. W.; Jacobsen, E. N. J. Am. Chem. Soc.
1995, 117, 5889–5890.
In conclusion, we were able to demonstrate that N-
alkoxycarbonyl azide was successfully used as the
nitrene precursor for the sulfimidation using chiral
ruthenium(II)(salen) complex 1 as the catalyst, when
the electronic and steric nature of the alkyl group in the
azide was appropriately adjusted. Further study on the
sulfimidation using N-alkoxycarbonyl azide as the
nitrene precursor is in progress in our laboratory.
5. Murakami, M.; Uchida, T.; Katsuki, T. Tetrahedron Lett.
2001, 42, 7071–7074.
6. o-Nitro- or o,p-dinitrophenylsulfonyl group has been
reported to be eliminated under the milder reaction condi-
tions: (a) Fukuyama, T.; Cheung, M.; Jow, C.-K.; Hidai, Y.;
Kan, T. Tetrahedron Lett. 1997, 38, 5831–5834; (b) Hidai,
Y.; Kan, T.; Fukuyama, T. Tetrahedron Lett. 1999, 40,
4711–4714.
7. (a) Bach, T.; Ko¨rber, C. Eur. J. Org. Chem. 1999, 1033–
1039; (b) Bach, T.; Ko¨rber, C. J. Org. Chem. 2000, 65,
2358–2367.
8. Cenini et al. also reported nitrene transfer reaction using
p-nitrophenyl azide as the nitrene source in the presence of
metalloporphyrin, but high reaction temperature was
required for the reaction: Cenini, S.; Tollari, S.; Penoni, A.;
Cereda, C. J. Mol. Cat. A: Chem. 1999, 137, 135–146.
References
1. (a) Faul, M. M.; Evans, D. A. In Asymmetric Oxidation
Reaction, Practical Approach in Chemistry; Katsuki, T., Ed.;
Oxford University Press: Oxford, 2001; Chapter 2.7; (b)
Jacobsen, E. N. In Comprehensive Asymmetric Catalysis II;
Jacobsen, E. N.; Pfaltz, A.; Yamamoto, H., Eds.; Springer:
Berlin, 1999; Chapter 17.
2. (a) Kohmura, Y.; Katsuki, T. Tetrahedron Lett. 2001, 42,