A thiacalix[3]pyridine copper(I) complex as a highly active catalyst for the olefin aziridination reaction

Article abstract of DOI:10.1246/cl.2008.452

A copper(I) complex with thiacalix[3]pyridine, [Cu-(Py₃S ₃)]2(PF₆)₂, is a competent catalyst for aziridination of olefins. The reaction of styrene (1 equiv) with PhI=NTs (1 equiv) at 20°C produced N-(p-toluenesulfonyl)-2-phenylaziridine (71%, turnover number = 55) in the presence of 1.2 mol% catalyst under aerobic conditions. The turnover number increased up to 350 with a 0.12 mol % catalyst and a 5:1 styrene/PhI=NTs ratio in 41% yield. Copyright

Full text of DOI:10.1246/cl.2008.452

452
Chemistry Letters Vol.37, No.4 (2008)
A Thiacalix[3]pyridine Copper(I) Complex as a Highly Active Catalyst
for the Olefin Aziridination Reaction
Yuhei Tsukahara, Masakazu Hirotsu,ꢀ Shin-ichi Hattori, Yoshinosuke Usuki,ꢀ and Isamu Kinoshita
Department of Material Science, Graduate School of Science, Osaka City University, Sugimoto, Sumiyoshi-ku, Osaka 558-8585
(Received January 11, 2008; CL-080034; E-mail: mhiro@sci.osaka-cu.ac.jp, usuki@sci.osaka-cu.ac.jp)
A
copper(I) complex with thiacalix[3]pyridine, [Cu-
catalytic reaction. Here, we report an efficient aziridination of
olefins catalyzed by the air-stable copper(I) complex 1.
Aziridination reactions of styrene with [N-(p-toluenesul-
fonyl)imino]phenyliodinane (PhI=NTs)7 were carried out
using 1, or halogen-coordinated complexes, [Cu(Py3S3)Cl] (2)
and [Cu(Py3S3)I] (3).6a,6b,8,9 We initially employed 5 equiv of
styrene and 1 equiv of PhI=NTs (Table 1). In the use of
1 mol % of catalyst in acetonitrile (0.5 mol % for dimeric com-
plex 1), all the complexes catalyzed the aziridination to give
N-(p-toluenesulfonyl)-2-phenylaziridine (4). The dimeric com-
plex 1 (yield of 4: 81%) was more efficient than the halogen-
coordinated complexes (26% for 2, 66% for 3). In acetonitrile
solutions, the dimeric and halogen-coordinated complexes
would be converted to [Cu(Py3S3)(CH3CN)]þ by Cu–S and
Cu–X (X = Cl and I) bond cleavage, respectively. The better
yield of 4 by 1 is partly attributable to the poorer coordinating
property of the sulfide S atoms in 1 than the halogeno ligand
in 2 or 3.
Under aerobic conditions, the reactions were almost identi-
cal to those under an argon atmosphere. This is an important
advantage to the practical application considering that recently
developed copper catalysts, which are highly active for aziridi-
nation, require an inert atmosphere and anhydrous conditions.5
The resistance to oxidative degradation is consistent with the
fact that 1 is stable to aerobic oxidation in solution. The turnover
number (TON) of the aziridination reactions was markedly
improved by changing 0.5 mol % catalyst loading (79%,
TON = 160) to 0.1 mol % (41%, TON = 350), even though
(Py3S3)]2(PF6)2, is a competent catalyst for aziridination of
olefins. The reaction of styrene (1 equiv) with PhI=NTs
(1 equiv) at 20 ꢁC produced N-(p-toluenesulfonyl)-2-phenyl-
aziridine (71%, turnover number = 55) in the presence of
1.2 mol % catalyst under aerobic conditions. The turnover
number increased up to 350 with a 0.12 mol % catalyst and a
5:1 styrene/PhI=NTs ratio in 41% yield.
Aziridines are useful synthetic intermediates in route to the
nitrogen-containing compounds via nucleophilic ring-opening
reactions under relatively mild conditions.1 Nitrene transfer to
olefins mediated by transition-metal complexes is one of the
most straightforward synthetic strategies for the synthesis of
aziridines.1c,2 A wide variety of copper catalysts have been de-
veloped for the aziridination of olefins, and theoretical studies
have been performed.2c,3,4 The major problem to be solved is that
most catalytic aziridinations require high catalyst loading and a
large excess of olefin. Recently, several efficient aziridination
reactions catalyzed by pyridine- or pyrazole-based copper com-
plexes have been reported.5 However, the pursuit of promising
aziridination catalysts is still needed because the efficiency
depends on the substrates and additional conditions.
We have recently reported copper and rhodium complexes
of a sulfur-bridged macrocyclic polypyridine ligand, thiacalix-
[3]pyridine (Py3S3, Figure 1a).6 The mononuclear rhodium(II)
complex [RhII(Py3S3)2]2þ showed that the Py3S3 ligand tends
to stabilize the lower oxidation states of a metal center.6d The
dicopper(I) complex [CuI(Py3S3)]2(PF6)2 (1) has a dimeric
structure of Cu(Py3S3) units by compensatory coordination
through the sulfur atoms as shown in Figure 1b.6a In solution,
Table 1. Aziridination of styrene with Py3S3–CuI catalysts
NTs
Cu-Catalyst
2þ
the dimeric [Cu(Py3S3)]2 readily dissociates in the presence
+
+ PhI
PhI=NTs
1 equiv
of coordinating solvents (L) or ions (Xꢂ), such as CH3CN,
Clꢂ, Brꢂ, and Iꢂ, accompanied by Cu–S bond cleavage and
ligation of L or Xꢂ to the Cu atom to form monomeric
[Cu(Py3S3)(L)]þ or [Cu(Py3S3)(X)], respectively.6a,6b This
Solvent
5 equiv
Catalyst
/mol %
T/ꢁC,
Time/h
Yield of
4/%c
Solvent
TON
2þ
implies that [Cu(Py3S3)]2 can act as a highly unsaturated
1 (0.50)a
2 (1.0)a
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH2Cl2
CH2Cl2
CH3CN
CH3CN
r.t., 1
r.t., 1
r.t., 1
r.t., 1
r.t., 1
r.t., 1
r.t., 1
r.t., 1
0, 3
81
26
66
93
79
41
94
19
19
44
160
28
59
38
160
350
195
195
200
390
copper(I) catalyst for a variety of organic reactions. Furthermore
the monomeric species, also, are stable under air, and so the
complex may be robust toward oxidative degradation in the
3 (1.0)b
1 (2.5)b
1 (0.50)b
1 (0.12)b
1 (0.50)b
1 (0.10)b
1 (0.10)b
1 (0.10)b
(a)
(b)
2+
S
N
S
N
S
N
N
S
S
Cu
Cu
N
S
N
N
S
N
N
40, 1
aReactions were carried out at room temperature under an
argon atmosphere. bReactions were carried out at room tem-
perature under air. cYields are given relative to PhI=NTs.
S
S
2þ
Figure 1. Structures of (a) Py3S3 and (b) [Cu(Py3S3)]2
.
Copyright Ó 2008 The Chemical Society of Japan

Chemistry Letters Vol.37, No.4 (2008)
453
higher than that for methyl acrylate (R1 = H, R2 = H, and
R3 = Me).
Table 2. Aziridination of styrene catalyzed by 1 at a 1:1 reagent
mole ratio
In summary, we have demonstrated that the thiacalix[3]-
pyridine dicopper(I) complex 1 shows high catalytic activities
for aziridination of various olefins. The compensatory coordina-
tion dimer 1 readily provides the mononuclear species with a
largely open site in solution, retaining the copper(I) oxidation
state. Since the reactions are performed under aerobic conditions
without the removal of additional ligands, 1 is easy to handle and
would be useful in practical synthetic processes.
NTs
1
+
+ PhI
PhI=NTs
1 equiv
CH3CN
1 equiv
aerobic, r.t., 1 h
1/mol %
Yield of 4/%a
TON
0.12
0.61
1.2
1.8
6.1
13
42
71
75
82
95
73
55
41
14
This work was supported by a Grant-in-Aid for Scientific
Research (No. 18350033) from the Ministry of Education,
Culture, Sports, Science and Technology of Japan.
aYields are given relative to PhI=NTs.
References and Notes
1
the yield decreased. Higher catalyst loading (2.5 mol %) of 1
led to high yield aziridination (93%), as shown in other catalyst
systems.2a–2c,3b–3d,5b In dichloromethane, the reactions with 1
provided 4 in yields similar to those in acetonitrile. Reaction
temperature little affected the yield of 4 in the 0.1 mol % catalyst
loading.
Aliphatic alkenes, cyclooctene and 1-octene, were also
aziridinated with 2.5 mol % 1 to produce the corresponding azir-
idines, 5 and 6, in 85 and 40% yields, respectively. The highest
catalytic activity of aziridination toward cyclooctene has been
shown with the copper complexes of [2.1.1](2,6)-pyridinophane
(1 mol % catalyst, 98% yield for cyclooctene).5b In these
systems, the facial coordination of the ligands could make facile
access of reactants to the remaining site possible.
It has been pointed out that the molar ratio of olefins and
PhI=NTs appears to be crucial for improving the yield of aziri-
dine: the excess of styrene was required.2a,3b,5b In our system, the
reaction with an equimolar amount of styrene gave a 71% yield
of the corresponding aziridine by using 1 mol % catalyst. As
summarized in Table 2, the yield of aziridines increased as the
amount of 1 increased. On the other hand, TON decreased with
increasing the amount of 1. These results suggest that the opti-
mal conditions are largely affected by the amount of catalyst.
An electron-withdrawing COOR group attached to the C=C
bond deactivates the substrate and generally causes a significant
decrease in yield for aziridination reactions. We investigated
the aziridinations of methyl acrylate, methyl cinnamate, and
ethyl 2-methylacrylate using 1 as a catalyst (Table 3). The
reactions of unsaturated esters (5 equiv) with PhI=NTs afforded
the aziridination products 7–9 in moderate yields. An electron-
donating group at the ꢀ position improves the yield for ethyl
2-methylacrylate (R1 = H, R2 = Me, and R3 = Et), which is
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Table 3. Aziridination of unsaturated esters catalyzed by 1
Ts
N
COOR3
+ PhI
R2
H
COOR3
+
R2
1 (2.5 mol %)
PhI=NTs
1 equiv
7
8
Y. Yamada, T. Yamamoto, M. Okawara, Chem. Lett. 1975, 361.
General procedure of aziridination reaction: To a solution of a
copper complex were added olefins (1–5 equiv) and PhI=NTs
(1 equiv). The mixture was stirred for 1 h at room temperature.
The yield of the corresponding aziridine was determined by
1H NMR using triphenylmethane as an internal standard. The
1H NMR spectra of the aziridine products were identical to the
reported data in the literature.
CH3CN
R1
R1
aerobic, r.t., 1 h
5 equiv
R1, R2, R3
Product
Yield/%a
TON
H, H, Me
Ph, H, Me
H, Me, Et
7
8
9
33
54
67
13
21
26
9
Supporting Information is available electronically on the CSJ-Jour-
nal Web site, http://www.csj.jp/journals/chem-lett/index.html.
aYields are given relative to PhI=NTs.
Published on the web (Advance View) March 15, 2008; doi:10.1246/cl.2008.452