Syntheses of acetonitrile ligated copper complexes with perfluoroalkoxy aluminate as counter anion and their catalytic application for olefin aziridination

Article abstract of DOI:10.1016/j.tetlet.2008.07.162

Acetonitrile ligated copper complexes with perfluoroalkoxy aluminate Al {OC (CF₃)₃}₄^- as weakly coordinating counter anion are successfully synthesized. Aziridination of various olefins with PhINTs catalyzed by

Full text of DOI:10.1016/j.tetlet.2008.07.162

Tetrahedron Letters 49 (2008) 5954–5956
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Tetrahedron Letters
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Syntheses of acetonitrile ligated copper complexes with perfluoroalkoxy
aluminate as counter anion and their catalytic application for olefin
aziridination
*
Yang Li, Bernd Diebl, Alexander Raith, Fritz E. Kühn
Molecular Catalysis, Fakultät für Chemie der Technischen Universität München, Lichtenbergstrasse 4, D-85747 Garching, Germany
a r t i c l e i n f o
a b s t r a c t
Acetonitrile ligated copper complexes with perfluoroalkoxy aluminate AlfOCðCF3Þ gꢀ as weakly coordi-
Article history:
3
4
Received 25 June 2008
Revised 22 July 2008
Accepted 29 July 2008
Available online 31 July 2008
nating counter anion are successfully synthesized. Aziridination of various olefins with PhINTs catalyzed
by copper(II) complex [Cu(II)(NCCH3)6][Al{OC(CF3)3}4]2 affords good to excellent yields (up to 96%) and
very high turnover frequency (higher than 5000 hꢀ1) under mild conditions.
Ó 2008 Elsevier Ltd. All rights reserved.
Keywords:
Copper
Olefin aziridination
Perfluoaluminates
Weakly coordinating anions
Aziridination of olefins is a very important transformation, since
aziridines are valuable synthetic intermediates for a wide range of
nitrogen-containing compounds1–3 and are often present as sub-
structures in natural products, exhibiting various biological prop-
erties, such as antitumor and antibiotic activities.4 However, the
catalytic aziridination of olefins had drawn little attentions before
1991, when a copper catalyst system for this reaction was reported
by Evans.5 Since then, various transition metal complexes such as
those of rhodium,6–8 copper,9–16 ruthenium,17,18 cobalt,19 iron,20,21
manganese,22 nickel,23 silver,24 and gold25 have been shown to cat-
alyze the transformation. The efficient systems usually contain
2–10 mol % of a soluble catalyst and nitrene sources, for example
N-tosyliminophenyliodinane (PhINTs) and bromamine-T.15
Very recently, a simple and cost efficient procedure to
synthesize [Ag(NCCH3)4][Al{OC(CF3)3}4] was reported.31 Herein,
we describe the syntheses of copper complexes (Fig. 1) [Cu(NC-
CH3)4][Al{OC(CF3)3}4] (1) and [Cu(NCCH3)6][Al{OC(CF3)3}4]2(2), as
well as first results of their applications in catalytic olefin
aziridination.
Complexes 1 and 2 were prepared from CuCl and CuCl2, respec-
tively, using the silver salt [Ag(NCCH3)4][Al{OC(CF3)3}4] in
metathesis reaction with good yields, as shown in Scheme 1.
a
Complexes 1 and 2 were characterized by IR spectroscopy,
elemental analysis, and 1H NMR. The IR shifts of
m(CN) absorption
are observed at 2268 cmꢀ1 and 2305 cmꢀ1 (1), and 2293 cmꢀ1 and
2332 cmꢀ1 (2). For 1H NMR, the chemical shift of the acetonitrile
protons of complex 2 (d = 1.96 ppm) is more close to that of
free acetonitrile (d = 1.83 ppm) than complex 1 (d = 2.14 ppm).
The results of elemental analysis also match the expectations
adequately.
Weakly coordinating counter anions (WCAs) have been applied
extensively in catalysis during the last decade, due to the fact that
WCAs have an effect of stabilizing the cations and rendering the
intermediate species more accessible to substrate coordination.
The applications of WCAs include polymerization reactions26–28
as well as various organic transformations.29,30,19 Yet, so far there
is only one report15 which shows WCAs having a significant posi-
tive effect on catalytic olefin aziridination. However, the reported
catalysts, which apply BfC6H3ðm-CF3Þ gꢀ as counter anion, were
The complexes were examined for their catalytic activity for
olefin aziridination (Scheme 2), with PhINTs as nitrene source
and styrene as substrate. As indicated in Table 1, only a moderate
yield was obtained with compound 1 (entry 1), yet an excellent
yield was achieved when compound 2 was applied (entry 2).
The yield remained excellent (90%) when only 0.1 mol % of 2 was
applied (entry 3), however, at the expense of a prolonged reaction
time. Nevertheless, with the same catalyst loading, the obtained
yield is still the best known to date for a copper-catalyzed styrene
aziridination. It is assumed that the robust WCA AlfOCðCF3Þ3gꢀ4 ,32
which stabilizes the Cu(II) cation and provides higher accessibility
2
4
generated in situ and no characterization data were available.
Hence, a more detailed exploration of the effect of WCAs on cata-
lytic aziridination would be most helpful and necessary.
* Corresponding author. Tel.: +49 (0)89 289 13096; fax: +49 (0)89 289 13473.
E-mail address: fritz.kuehn@ch.tum.de (F. E. Kühn).
0040-4039/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2008.07.162

Y. Li et al. / Tetrahedron Letters 49 (2008) 5954–5956
5955
-
-
CF3
CF3
F3C
CF3
+
2+
F3C
CF3
NCCH3
H3CCN Cu NCCH3
NCCH3
NCCH3
NCCH3
F3C
F3C
F3C
CF3
CF3
CF3
F3C
F3C
F3C
CF3
CF3
CF3
O
Al
O
O
Al
O
H3CCN Cu
NCCH3
O
O
O
O
H3CCN
NCCH3
F3C
CF3
F3C
CF3
CF3
CF3
2
2
1
Figure 1. Acetonitrile ligated copper complexes 1 and 2.
tained. This is described as being due to decomposition of PhINTs
in the presence of copper-based catalysts, when long reaction
times are necessary to complete the reaction.
CH3CN
rt
[Ag(NCCH3)4][Al{OC(CF3)3}4]
+
CuCl
[Cu(NCCH3)4][Al{OC(CF3)3}4]
AgCl
+
Copper(II) chloride (entry 6) and copper salts with traditional
WCAs (entries 7 and 8) were also applied as catalysts under the
same conditions, however, only low yields were obtained. When
CH3CN
rt
2[Ag(NCCH3)4][Al{OC(CF3)3}4]
CuCl2
+
the counter anions were changed to the widely used BðC6F5Þꢀ
4
2AgCl
and ½BfC6H3ðm-CF3Þ g ꢁꢀ (entries 9 and 10), the yields were higher,
[Cu(NCCH3)6][Al{OC(CF3)3}4]2
+
2
4
2
but still significantly lower than the yields obtained with 2.
The substrate scope was then further explored, as indicated in
Table 2. Under the standard conditions (2 ml MeCN, 1–3 mol % cat-
alyst, 1 equiv of PhINTs, 5 equiv of olefin, 25 °C), the reactions pro-
ceed in good to excellent yields with both aromatic and aliphatic
olefins. With styrene as substrate, it is noteworthy that the turn-
over frequency (TOF) is as high as 5000 hꢀ1, which is—to the best
of our knowledge—the highest for a copper-catalyzed aziridination
reported in the literature to date (entry 1). With methyl-substi-
tuted styrenes, complex 2 affords the corresponding aziridines in
excellent yields similar to that of the styrene reaction (entries
2–4). Remarkably, the stereochemistry of both trans- and cis-b-
methylstyrene remains in the respective products. The results are
consistent with Evans’ earlier investigation, in which a Cu(ClO4)2
system was applied,34 and a concerted mechanism35,34 was sug-
gested. On the other hand, contrasting results36,25,20 reported by
other groups suggested a stepwise mechanism for gold, iron, and
even for copper systems, for example, CuBr2 and Cu(acac)2,34 indi-
cating the reaction mechanism of catalytic aziridination is both
metal and counter anion dependent. Sterically hindered cis/trans-
stilbene was also tested (entries 5 and 6). With cis-stilbene (entry
5) excellent yield was obtained, but only moderate stereospecific-
ity was achieved. This observation is due to the higher stability of
the trans isomer of the product. On the other hand, aziridination of
trans-stilbene (entry 6) leads to pure trans product. The relatively
low yield and longer reaction time is probably due to the applied
solvent (mixture of acetonitrile and dichloromethane), since
trans-stilbene is a solid which is barely soluble in acetonitrile. Sim-
ple cyclic olefins were also shown to be suitable substrates (entries
7 and 8). As expected, cis-cyclooctene, possessing a weaker and
more reactive C@C bond, is more reactive than cyclohexene. How-
ever, higher catalyst loadings and longer reaction times are neces-
sary for the less active vinylcyclohexane and 1-octene to obtain
satisfactory yields (entries 9 and 11). Nevertheless, the results
are still comparable or superior to the best literature values. Good
to excellent yields were obtained for less sterically hindered ali-
phatic olefins (entries 10, 12, and 13), including the electron defi-
cient substrate methyl methacrylate (entry 12).
Scheme 1. Syntheses of 1 and 2.
ACN, RT
Olefin
+
PhINTs
Aziridines
Cat.
Scheme 2. Catalytic olefin aziridination.
Table 1
Aziridination of styrene by copper complexesa
No.
Catalyst
Loading (mol %)
Time
Yieldb (%)
1
2
3
1
2
2
2
0.5
0.5
0.1
0.5
1.0
0.5
0.5
0.5
0.5
0.5
20 min
3 min
6 h
63
91
90
54
68
31
47
33
61
59
4c
5c
6
24 h
2
20 min
10 min
5 min
5 min
5 min
5 min
CuCl2
Cu(BF4)2
7
8
Cu(ClO4)2
[Cu(NCCH3)6][1]2
[Cu(NCCH3)6][2]2
9d,f
10e,f
a
Styrene (2.5 mmol), PhINTs (0.5 mmol), mesitylene (20
were stirred in 2.0 ml acetonitrile at room temperature.
ll) and the catalyst
b
GC yield.
c
0.5 mmol styrene.
d
1 = B(C6F5)4.
e
2 = B{C6H3(m-CF3)2}4.
For the syntheses and application of the two complexes, see Ref. 33.
f
of the cation to substrate coordination, is responsible for the
extraordinary high catalytic activity of 2. In all reactions, trace
amounts of p-toluene sulfonamide are detected as by-product by
GC. To further explore the catalytic activity of 2, a 1:1 ratio of sty-
rene and PhINTs was applied (entry 4). As expected, the reaction
goes to completeness with prolonged time but produces a higher
amount of by-product. Yet when a higher catalyst loading (entry
5) is applied, the reaction time decreases strongly from 24 h to
20 min. In both reactions (entries 4 and 5), much more p-toluene
sulfonamide is detected. The result is in accord with Evans’ earlier
report,5 in which considerable p-toluene sulfonamide was ob-
The WCA AlfOCðCF3Þ gꢀ was successfully incorporated into
3
4
copper complexes 1 and 2 by straightforward metathesis reaction
of CuCl/CuCl2 and [Ag(NCCH3)4][Al{OC(CF3)3}4].
A preliminary
investigation of their catalytic potentials shows that complex 2 is

5956
Y. Li et al. / Tetrahedron Letters 49 (2008) 5954–5956
Table 2
Acknowledgment
Aziridination of olefins with PhINTs catalyzed by 2
No.
Substrate
Ph
L. (mol %)
t
Prod.
Yielda (%)
93b
Y.L. is grateful to the Universität Bayern e. V. for a Ph.D fellow-
ship. The International Graduate School of Science and Engineering
(IGSSE) is acknowledged for financial support.
1
1.0
<1 min
1a
95 cis:trans = 96:4c
90 cis:trans = 1:99c
Supplementary data
2
3
1.0
1.0
5 min
5 min
1b
1c
Ph
Experimental procedures and spectral data for all compounds.
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.tetlet.2008.07.162.
Ph
4
1.0
3 min
1d
1e
1f
91
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Ph
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a
b
c
Isolated yield.
Yield determined by GC.
The ratio of cis and trans isomers was detected by 1H NMR.
d
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reaction media.
e
cis Isomer is not detected by 1H NMR.
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a very efficient catalyst for olefin aziridination. Its main advantages
include low catalyst loading, high yields, very high turnover num-
bers, short reaction times, high stereospecificity, a wide substrates
range and mild reaction conditions. However, the low solubility of
2 limits its application to solvents such as dichloromethane and
toluene. Examinations such as modifying the counter anion and
the ligands are currently going on in our laboratories, to lead to a
better solubility and possibly of even higher catalytic activity.
36. Minakata, S.; Morino, Y.; Oderaotoshi, Y.; Komatsu, M. Chem. Commun. 2006,
3337.