Design of a robust Ru(salen) complex: Aziridination with improved turnover number using N-arylsulfonyl azides as precursors

Article abstract of DOI:10.1039/b407693a

A new robust fluorinated (OC)Ru(salen) complex was designed on the basis of an X-ray structure of its parent complex to show improved turnover numbers (up to 878) and enantioselectivities (up to 99%) in aziridination reactions using p-toluenesulfonyl (Ts)

Full text of DOI:10.1039/b407693a

View Article Online / Journal Homepage / Table of Contents for this issue
Design of a robust Ru(salen) complex: aziridination with improved
turnover number using N-arylsulfonyl azides as precursors{
Kazufumi Omura, Tatsuya Uchida, Ryo Irie and Tsutomu Katsuki*
Department of Chemistry, Faculty of Science, Graduate School, Kyushu University 33, Hakozaki,
Higashi-ku, Fukuoka 812–8581, Japan and CREST, Japan Science and Technology Agency (JST).
E-mail: katsuscc@mbox.nc.kyushu-u.ac.jp; Fax: 181 92 642 2607
Received (in Cambridge, UK) 25th May 2004, Accepted 29th June 2004
First published as an Advance Article on the web 13th August 2004
A
new robust fluorinated (OC)Ru(salen) complex was
the course of this study, however, the active intermediate in the
sulfimidation, the complex 1–azide adduct,12 was found to easily
undergo intramolecular C–H amination, giving a catalytically
inactive complex 2. Since it was impossible to determine the exact
designed on the basis of an X-ray structure of its parent
complex to show improved turnover numbers (up to 878)
and enantioselectivities (up to 99%) in aziridination reactions
using p-toluenesulfonyl (Ts) or p-nitrobenzenesulfonyl (Ns)
azide as the nitrene precursor; the latter is synthetically
advantageous since the Ns group is N-protecting and can be
removed under mild conditions.
1
location of the aminated carbon due to the complexity of the H
NMR spectrum of 2,12 we performed an X-ray analysis of complex
1 to pinpoint the amination site. A single crystal of 1 was obtained
by recrystallization from acetonitrile–water. The X-ray structure
given in Fig. 1{ disclosed that one of the meta-carbons of the
phenyl substituent on the 3- or 3’-naphthyl group is very close
Catalytic asymmetric nitrene transfer reactions such as aziridina-
tion, C–H amination and sulfimidation are current topics in
organic synthesis and much effort has been directed toward these
chemistries.1–4 Although high enantioselectivity has been achieved
in some reactions, most of them need N-arylsulfonyliminophenyl-
iodinanes as the nitrene precursors. Use of such precursors incurs
two problems in the reactions: (i) generation of iodobenzene as
waste material and (ii) difficulty in removal of the arylsulfonyl
group from the nitrene transfer products. In order to solve the first
issue, use of arylsulfonyl azides has commanded much attention.5–7
Although several reactions utilizing arylsulfonyl azide have been
reported, most of them are non-stereoselective5a,6,7 and the
asymmetric version is only moderately enantioselective.5b Besides,
some of them need severe reaction conditions like UV-irradiation
or heating to promote the nitrene transfer.5b,7 We have recently
found that (OC)Ru(salen) complex 1 is an efficient catalyst for
asymmetric sulfimidation8 and aziridination of conjugated terminal
olefins using arylsulfonyl azides as the nitrene precursor.9 These
reactions proceeded under mild conditions, but use of arylsulfonyl
azides such as p-toluenesulfonyl azide has left the second issue
unresolved. On the other hand, Fukuyama et al. have reported that
a p-nitro- or o, p-dinitrobenzenesulfonyl group can be removed
under mild conditions.10 Unfortunately, p-nitrobenzenesulfonyl
(Ns) azide was a less efficient nitrene precursor for the aziridination
with 1 as the catalyst (vide infra). Furthermore, these reactions need
another improvement: complex 1 was not very robust and the
turnover number (TON), e.g., in the aziridination, was moderate.9
˚
(3.59 A) to the N-atom of the ruthenium-bound acetonitrile. The
acetonitrile should be replaced with an azide compound upon
nitrene transfer reaction,12 and it was considered that the meta-
carbon should be preferentially aminated by the ruthenium-bound
azide. Thus, we expected that the Ru(salen) complex would become
robust, if the hydrogen atoms at the meta-carbons are protected
somehow from the C–H amination. Along this line, we synthesized
complexes 3, 4 and 5. The bulky p-silyl groups introduced in 3 and
4 were expected to protect the vicinal meta-hydrogen atoms
sterically against the intramolecular C–H amination. In the case of
5, the meta-hydrogen atoms were replaced with fluorine atoms.
The para-methyl group was introduced for the convenience of its
synthesis. With these new complexes as catalysts, we first examined
aziridination of styrene (Table 1). Aziridination using 2 mol% of 3
as the catalyst gave the desired product in a quantitative yield
without diminishing enantioselectivity (entry 1). The reaction using
0.1 mol% of 3 gave the product in 41% yield, indicating that TON
was remarkably improved to 410 (entry 2). It is noteworthy that,
despite the presence of the bulky TBDMS group, 3 showed higher
catalytic activity than 1 (entry 3): turnover frequency (TOF) of
3 in the initial minute was 27, though TOF of 1 was 8. The reason
for this enhanced catalytic activity of 3 is unclear at present.
Aziridination with 4 also showed the same enantioselectivity, but 4
was a little inferior to 3 in terms of TON (entry 4). However, TON
was further improved up to 867 albeit with slightly diminished
enantioselectivity (85% ee), when 5 was used as the catalyst (entry
5). TOF of 5 in the reaction amounted to 28. Aziridination of
p-bromostyrene was also examined with complexes 3 and 5: both
reactions showed the same enantioselectivity of 90% ee, and the
higher TON of 878 was again attained by using complex 5 as
catalyst (entries 6 and 7).
Based on these results, we next examined aziridination of various
conjugated terminal olefins using 5 as a catalyst. All the reactions
In order to settle the second issue, we examined the use of
alkoxycarbonyl azide as the precursor and found that 2,2,2-
trichloro-1,1-dimethylethoxycarbonyl azide served as an efficient
precursor for asymmetric sulfimidation using complex 1.11 During
{ Electronic supplementary information (ESI) available: typical experi-
mental procedures, determination of enantiomeric excess and elementary
analysis for complexes 3 and 5. See http://www.rsc.org/suppdata/cc/b4/
b407693a/
Fig. 1 An ORTEP diagram for the X-ray structure of 1. The hydrogen
atoms and solvent molecules are omitted for clarity.
2 0 6 0
C h e m . C o m m u n . , 2 0 0 4 , 2 0 6 0 – 2 0 6 1
T h i s j o u r n a l i s ß T h e R o y a l S o c i e t y o f C h e m i s t r y 2 0 0 4

View Article Online
a
as nitrene precursors should be replaced by easily available and
atom-efficient azide compounds.
Table 1 Catalytic asymmetric aziridination of various olefins
Notes and references
{ Crystallographic data: 1, C61H44N2O3Ru?5CH3CN, M ~ 1159.36, T ~
123 K, orthorhombic, space group P212121, a ~ 13.5262(3), b ~
Entry
Catalyst
R or substrate
Yield (%)
ee (%)
TON
3
˚
˚
19.7791(5), c ~ 21.9184(6) A, V ~ 5863.9(3) A , Z ~ 4, R ~ 0.059 (I w
2s(I)), wR ~ 0.160 (all data), GOF ~ 0.97. CCDC 236739. See http://
www.rsc.org/suppdata/cc/b4/b407693a/ for crystallographic data in .cif
format.
1b
2b
3b
4b
5b
6
7
8
9
10
11
12
13c
3 (2)
3 (0.1)
1 (2)
Ph
Ph
Ph
Ph
100
41
71
23
78
33
79
34
65
61
20
30
36
87
87
87
87
85
90
90
87
87
96
86
86
—
410
36
4 (0.1)
5 (0.09)
3 (0.1)
5 (0.09)
5 (0.09)
5 (0.09)
5 (0.09)
5 (2)
230
867
330
878
378
722
678
10
Ph
1 For reviews of asymmetric aziridination (hereafter, abbreviated as AA),
see: (a) E. N. Jacobsen, Comprehensive Asymmetric Catalysis II, ed.
E. N. Jacobsen, A. Pfaltz and H. Yamamoto, Springer, Berlin, 1999, ch.
17, p. 607; (b) T. Katsuki, Comprehensive Coordination Chemistry II, ed.
J. McCleverty, Elsevier Science Ltd., Oxford, 2003, vol. 9, ch. 9.4, p. 207;
(c) P. Mu¨ller and C. Fruit, Chem. Rev., 2003, 103, 2905.
2 For asymmetric C–H amination, see: (a) I. Na¨geli, C. Baud,
G. Bernardinelli, Y. Jacquier, M. Moran and P. Mu¨ller, Helv. Chim.
Acta, 1997, 80, 1087; (b) Y. Kohmura and T. Katsuki, Tetrahedron Lett.,
p-BrC6H4
p-BrC6H4
p-O2NC6H4
2-C10H7
PhCMC
n-C6H13
n-C6H13
Indene
3 (2)
5 (2)
15
18
w99
a See ESI for the typical experimental procedures and the determina-
tion of ee. b Absolute configuration of the product is S. c Absolute
configuration of the product is 1S,2R.
2001,
42,
3339;
(c)
J.-L.
C.-M. Che, Chem. Commun., 2002, 124.
Liang,
X.-Q.
Yu
and
3 For non-enantioselective C–H amination, see: (a) J. P. Mahy, G. Bedi,
P. Battioni and D. Mansuy, Tetrahedron Lett., 1988,
29, 1927; (b) D. A. Evans, M. M. Faul and M. T. Bilodeau,
J. Am. Chem. Soc., 1994, 116, 2742; (c) P. Mu¨ller, C. Baud and
Y. Jacquier, Tetrahedron, 1996, 52, 1543; (d) J. Yang, R. Weinberg and
R. Breslow, Chem. Commun., 2000, 531; (e) X.-Q. Yu,
J.-S. Huang, X.-G. Zhou and C.-M. Che, Org. Lett., 2000, 2, 2233;
(f) J. Yang, R. Weinberg and R. Breslow, Tetrahedron Lett., 2000, 41,
8063; (g) S.-M. Au, J.-S. Huang, C.-M. Che and W.-Y. Yu, J. Org.
Chem., 2000, 65, 7858.
4 For asymmetric sulfimidation, see: (a) H. Takada, Y. Nishibayashi,
K. Ohe and S. Uemura, J. Chem. Soc., Chem. Commun., 1996, 931;
(b) H. Takada, Y. Nishibayashi, K. Ohe, S. Uemura, C. P. Baird,
T. J. Sparey and P. C. Taylor, J. Org. Chem., 1997, 62, 6512;
(c) Y. Miyake, H. Takada, K. Ohe and S. Uemura, J. Chem. Soc., Perkin
Trans. 1, 1998, 2373; (d) H. Nishikori, C. Ohta, E. Oberlin, R. Irie and
T. Katsuki, Tetrahedron, 1999, 55, 13937.
5 For aziridination, see: (a) H. Kwart and A. A. Khan, J. Am. Chem. Soc.,
1967, 89, 1951; (b) Z. Li, R. W. Quan and E. N. Jacobsen, J. Am. Chem.
Soc., 1995, 117, 5889.
6 For sulfimidation, see: (a) T. Bach and C. Ko¨rber, Eur. J. Org. Chem.,
1999, 1033; (b) T. Bach and C. Ko¨rber, J. Org. Chem., 2000, 65, 2358;
(c) H. Okamura and C. Bolm, Org. Lett., 2004, 6, 1305.
7 For C–H amination, see: (a) S. Cenini, S. Tollari, A. Penoni and
C. Cereda, J. Mol. Catal. A: Chem., 1999, 137, 135; (b) F. Ragaini,
A. Penoni, E. Gallo, S. Tollari, C. L. Gotti, M. Lapadula, E. Mangioni
and S. Cenini, Chem.–Eur. J., 2003, 9, 249.
showed high enantioselectivity of 87% ee and above together with
much improved TONs greater than 377 (entries 8–10). Further-
more, aziridination of 1-octene, a non-conjugated olefin, was also
found to proceed with high enantioselectivity (86% ee), albeit with
moderate yield (entry 11). Notably, this reaction did not occur
when complex 1 was used as a catalyst.13 It should be also men-
tioned that, in this reaction only, complex 3 showed somewhat
better TON than complex 5, though the reason is unclear (entry 12).
Aziridination of indene with 5 showed excellent enantioselectivity
of w99% ee, though the TON was again moderate (entry 13).14
As described above, Ns azide was a poor nitrene precursor for
aziridination using 1 as catalyst (Scheme 1). The high catalytic
activity of 5, however, prompted us to examine the nitrene transfer
reaction using Ns azide as the precursor. The reaction of styrene
proceeded with high enantioselectivity (84% ee), albeit with
moderate yield, to give the corresponding aziridine. The reaction
was completed when catalyst-loading was increased to 4 mol%.{
Aziridination of p-bromostyrene and 1-phenyl-3-buten-1-yne also
proceeded with high enantioselectivity and good chemical yield.
However, 1-octene did not undergo aziridination under the present
conditions.
In conclusion, we were able to reasonably design new robust
(OC)Ru(salen) complex 5. This new complex shows much higher
endurance and catalytic activity than 1, remarkably expanding the
scope of aziridination and possibly of other nitrene-transfer reac-
tions using azide compounds as nitrene precursors. In particular,
introduction of the complex allows the use of p-nitrobenzene-
sulfonyl azide as a useful precursor. Finally, the present study
demonstrated that iminoiodinane compounds now generally used
8 M. Murakami, T. Uchida and T. Katsuki, Tetrahedron Lett., 2001, 42,
7071.
9 K. Omura, M. Murakami, T. Uchida, R. Irie and T. Katsuki, Chem.
Lett., 2003, 32, 354.
10 (a) T. Fukuyama, C.-K. Jow and M. Cheung, Tetrahedron Lett., 1995,
36, 6373; (b) T. Fukuyama, M. Cheung, C.-K. Jow, Y. Hidai and T. Kan,
Tetrahedron Lett., 1997, 38, 5831.
11 Y. Tamura, T. Uchida and T. Katsuki, Tetrahedron Lett., 2003, 44, 3301.
12 T. Uchida, Y. Tamura, M. Ohba and T. Katsuki, Tetrahedron Lett.,
2003, 44, 7965.
13 AA of 1-hexene has been reported to proceed with 70% ee and 68% yield,
but N-p-CH3(C6H4)SO2NLIPh was used as the nitrene precursor:
D. Cho, S. J. Jeon, H. S. Kim, C. S. Cho, S. C. Shim and T. J. Kim,
Tetrahedron: Asymmetry, 1999, 10, 3833.
14 AA of indene has been reported by using copper-diimine (58% ee, 50%)
or Ru(salen) complex (83% ee, 68%) as the catalyst in the presence of
N-p-CH3(C6H4)SO2NLIPh: Z. Li, K. R. Conser and E. N. Jacobsen,
J. Am. Chem. Soc., 1993, 115, 5326; J. L. Liang, X. Q. Yu and C. M. Che,
Chem. Commun., 2002, 124.
Scheme 1 Catalytic asymmetric aziridination with NsN3.
C h e m . C o m m u n . , 2 0 0 4 , 2 0 6 0 – 2 0 6 1
2 0 6 1