Copper(ii) complexes incorporating poly/perfluorinated alkoxyaluminate-type weakly coordinating anions: Syntheses, characterization and catalytic application in stereoselective olefin aziridination

Article abstract of DOI:10.1039/c1dt10280j

The synthesis and characterization of a series of cationic copper(ii) complexes of the type [Cu(NCR)₆][Al(OC(CF₃) ₂R′)₄]₂ (R = CH₃, Ph; R′ = CF₃, Ph, PhCH₃), incorpora

Full text of DOI:10.1039/c1dt10280j

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PAPER
Copper(II) complexes incorporating poly/perfluorinated
alkoxyaluminate-type weakly coordinating anions: Syntheses,
characterization and catalytic application in stereoselective olefin
aziridination†
Yang Li,a Jiayue He,a Vineeta Khankhoje,a Eberhardt Herdtweck,b Klaus Ko¨hler,c Oksana Storcheva,c
Mirza Cokojab and Fritz E. Ku¨hn*a,b
Received 19th February 2011, Accepted 21st March 2011
DOI: 10.1039/c1dt10280j
The synthesis and characterization of a series of cationic copper(II) complexes of the type
[Cu(NCR)6][Al(OC(CF3)2R¢)4]2 (R = CH3, Ph; R¢ = CF3, Ph, PhCH3), incorporating
poly/perfluoronated alkoxyaluminates as weakly coordinating anions (WCAs) is presented.
Aziridination of various olefins, such as the unreactive olefins e.g. ethylhex-2-enoate and 1-decene, with
N-tosyliminophenyliodinane catalyzed by [Cu(NCR)6][Al(OC(CF3)2R¢)4]2 affords very good yields (up
to 96%) and high TOFs (up to 5000 h-1) under mild conditions. Using disubstituted olefins as
substrates, high stereoselectivities are obtained at room temperature. The to date highest cis : trans ratio
(98 : 2) of the obtained aziridines is achieved for cis-stilbene in good yield (85%) as well as promising
TOF (> 2000 h-1). The investigation of the solvent effect on yield and selectivity reveals that for certain
oleophilic substrates (1-decene), less polar solvents, such as dichloromethane are a better choice than
acetonitrile, which is commonly considered as the best solvent for olefin aziridination. Accordingly, a
mechanism involving a paramagnetic copper nitrene intermediate with both concerted and stepwise
pathways is proposed.
complexes incorporating [Al(OC(CF3)3)4]- show at least the
same or superior catalytic activities than other types of
Introduction
The development of weakly coordinating anions (WCAs)1 has
WCAs.
drawn increasing attention since 2001,2 when Krossing reported a
Our group has been investigating the role of
facile synthesis of silver polyfluoroalkoxyaluminates AgAl(OR)4,
WCAs as counteranions of catalytically active cationic
metal complexes since 2003.13 We recently disclosed
a
(R = CH(CF3)2, C(CH3)(CF3)2, C(CF3)3). The robustness, non-
oxidizing and weak coordinating ability of the perfluoroalkoxya-
luminate [Al(OC(CF3)3)4]- make it a very good candidate for
stabilizing extremely sensitive and electrophilic cations, such as
straightforward and cost efficient synthetic procedure
for acetonitrile ligated silver perfluoroalkoxyaluminate
[Ag(NCCH3)4][Al(OC(CF3)3)4],14 which avoids using ultrasound
and expensive/toxic AgF. Using [Ag(NCCH3)4][Al(OC(CF3)3)4]
as metathesis reagent, we communicated the synthesis and
3
+
+
7
Ag(P4)+ and related Ag salts,4 P5X2 ,5 CI3 ,6 [Bi4OF2Cl6]2+
and [Zn2]2+.8 Meanwhile, as is the case with perfluoroborates
and carboranes, per/polyfluoroalkoxyaluminates are of great
interest for catalysis, e.g. C–C bond formation,1k enantiose-
lective hydrogenation,9 asymmetric cycloisomerization,10 olefin
polymerization11 and ethylene oligomerization.12 In all cases,
characterization
of
[Cu(NCCH3)4][Al(OC(CF3)3)4]
and
[Cu(NCCH3)6][Al(OC(CF3)3)4]2, as well as their catalytic
application for olefin aziridination.15 Aziridination of various
olefins with N-tosyliminophenyliodinane (PhINTs) catalyzed by
[Cu(NCCH3)6][Al(OC(CF3)3)4]2 affords good to excellent yields
(up to 96%) and high turnover frequencies (> 5000 h-1) with
low catalyst loadings (0.1–1 mol%) under mild conditions. These
promising results demonstrate that the perfluoroalkoxyaluminate
[Al(OC(CF3)3)4]- has significant advantages over other anions
in copper-catalyzed olefin aziridination. Thus, we now extend
our research to analogous polyfluoroalkoxyaluminates, such
as [Al(OC(CF3)2Ph)4]- and[Al(OC(CF3)2PhCH3)4]-, which
are expected to provide better solubility to the complexes in
commonly used non-polar organic solvents.
aMolecular Catalysis, Catalysis Research Center, Technische Univer-
sita¨t Mu¨nchen, Ernst-Otto-Fischer-Str. 1, D-85747, Garching, Germany.
E-mail: fritz.kuehn@ch.tum.de
bChair of Inorganic Chemistry, Catalysis Research Center, Technische Uni-
versita¨t Mu¨nchen, Ernst-Otto-Fischer-Str. 1, D-85747, Garching, Germany.
E-mail: fritz.kuehn@ch.tum.de
cDepartment of Inorganic Chemistry, Technische Universita¨t Mu¨nchen,
Ernst-Otto-Fischer-Str. 1, D-85747, Garching, Germany
† Electronic supplementary information (ESI) available. For ESI and
crystallographic data in CIF or other electronic format see DOI:
10.1039/c1dt10280j
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Table 1 Anion effect on yield and regioselectivity for olefin
aziridination20b
Catalyst
Yield (%)
cis : trans
Cu(ClO4)2
Cu(acac)2
CuBr2
76
64
40
95 : 5
26 : 74
30 : 70
Table 2 Anion effect on yield for the aziridination of cyclooctene and
cyclopentene17b
Fig. 1 Copper(II) complexes of the general formula [Cu(NCR¢)6]-
[Al(OC(CF3)2R)4]2.
complexes 1–6 (Fig. 1, Scheme 1). A slight excess of silver salt
allows the complete chloride elimination.22 However, this method
can only be applied to compound 1, due to the fact that 1 is the
only complex soluble in polar solvents such as acetonitrile and the
unreacted silver salt can be readily removed by product extraction
with dichloromethane.
Olefin
Anion
Yield (%)
-
Cyclooctene
Cyclopentene
BArF
BArF
Cl-
80
98
14
25
65
4
4
-
OTf-
BArF
-
4
L = tripyridine ligand [2.1.1]-(2,6)-pyridinophane
To date, literature reports on the catalytic olefin aziridination
are mainly focused on the effects of cations,16 ligands,17 substrates18
and nitrene sources.18b,19 Only a few mentioned the anion effect.17b,20
Nevertheless, it is commonly accepted that catalysts incorporating
WCAs are superior to those with normal anions. The earlier work
of Evans20b,21 showed that simple copper salts with classic WCAs
Scheme 1 Synthesis of copper(II) complexes 1–6.
such as ClO4 , OTf- and BF4 , are superior to those with halides
or acac- as counteranions, affording much higher yields and better
-
-
Complex 1 can be further purified by slow diffusion of an
acetonitrile solution of 1 into dichloromethane at -35 ◦C. For
the synthesis of 2–6, a slight excess of copper(II) chloride is added.
The unreacted salt can be easily removed by re-dissolving the
products in dichloromethane. Further recrystallization of 2–6 with
dichloromethane and pentane–hexane only resulted in green oily
compounds.
stereoselectivities (Table 1).
17b
Caulton et al.
demonstrated that high catalyst activity
could be achieved in dichloromethane when the halide ligands
were removed by NaB(C6F5)4. The same group also reported
that the use of the fluorinated [B(C6F5)4]- afforded higher
catalytic activity and better thermal stability compared to the
non-fluorinated congener [B(C6H5)4]-. Furthermore, Vedernikov
showed that the catalytic activity of Cu(II) complexes increased
significantly (Table 2) when, for instance, OTf- was replaced by
the bulky, fluorinated BPhF -(BArF = [B(C6F5)4]- or [B(C6H3(m-
Solid-state structures
Blue crystals of 1 suitable for X-ray diffraction can be obtained
by slow diffusion of toluene into a concentrated acetonitrile
4
4
◦
CF3)2)4]-). In a previous communication,15 we have shown
that [Cu(NCCH3)6][Al(OC(CF3)3)4]2 affords a higher yield than
solution of 1 at -35 C. The X-ray crystal structure of 1 (Fig. 2)
proves the octahedral symmetry of the Cu(II) cation and the
non-coordinating nature of the anion. However, due to severe
unresolvable disorder problems in the anionic part, a detailed
discussion of the molecular structure of complex 1 is not possible.
The same problem was also reported previously for acetonitrile
ligated lanthanide complexes by Helm et al.22 Nevertheless, it is
very clear that the anion [Al(OC(CF3)3)4]- has no interactions with
the copper(II) cation in solid state, proving the non-coordinating
character of [Al(OC(CF3)3)4]-.
[Cu(NCCH3)6][BArF ]2.
4
The work presented here is our continuing investigation of the
anion effect on the copper-catalyzed olefin aziridination. We now
report on the synthesis and characterization of copper(II) com-
plexes with the general formula [Cu(NCR¢)6][Al(OC(CF3)2R)4]2
(R¢ = CH3, Ph; R = CF3, Ph, PhCH3), as well as their catalytic
application in olefin aziridination.
Results and Discussion
FT-IR spectroscopic studies
Synthesis and characterization
Complexes 1–6 were characterized by FT-IR using Nujol oil as
matrix. It was observed that when using KBr, the color of the
pellets often changed from green–blue to brown shortly after
Reaction of anhydrous copper(II) chloride with the silver salts of
various WCAs in acetonitrile or benzonitrile readily affords copper
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be argued that the anions in complexes 2–6 are also truly non-
coordinating in solid state.
EPR spectroscopic studies
The EPR spectra of the copper(II) complexes 1–6 (Fig. 4) are
typical for copper(II) systems (3d9, S = 1/2) in the distorted
octahedral coordination (tetragonal elongation) due to the Jahn–
Teller effect. The copper(II) compounds can be described by
an axially symmetric spin Hamiltonian in accordance with the
symmetry of the molecule (see the crystal structure). The 63,65Cu
hyperfine splitting (quartet of lines) is resolved only for the parallel
part of the EPR spectra of frozen solutions due to the interaction
of the unpaired electron with the nuclear spins of 63Cu and 65Cu (I =
3/2, naturalabundance ª69 and31% respectively, isotopicsplitting
2
2
not resolved). The clear relationship gII> g^ > 2.0 indicates a d(x
–y
)
ground state for the unpaired electron.26 The small deviations of g
values (see Table 3) and hyperfine coupling constants AII (63,65Cu)
are probably due to different nitrile ligands of the complexes,13h
different frozen solutions as well as packing effects in solid state.
Fig. 2 Ball-and-stick style plot of the cationic part of complex 1 in the
solid state.23
the measurement. This is probably a consequence of complex
decomposition by forming M–Br bonds.
The IR spectra of acetonitrile coordinated complexes 1, 3, 5
exhibit two sharp n(CN) absorptions with medium intensity at
2332, 2304 (1), 2332, 2301 (3) and 2331, 2302 cm-1 (5), respectively
(Fig. 3, upper half), which can be assigned to the fundamental
n2(CN) stretching mode and a combination mode of (n3 + n4),24
respectively. On the other hand, only one broad n(CN) absorption
is observed for the benzonitrile congeners 2, 4, 6 at 2280, 2274 and
2275 cm-1, respectively (Fig. 3, lower half). The higher energies
of all n(CN) absorptions compared to those of free acetonitrile
(2254 and 2293 cm-1) and benzonitrile (2256 cm-1) are caused
by s-donation of electron density from the free electron pair of
nitrogen to the metal center.24b,25 It is noteworthy that the n(CN)
absorptions are almost identical for acetonitrile coordinated
complexes 1, 3, 5, as well as for the benzonitrile congeners 2,
4, 6. This observation indicates that changing counteranions from
[Al(OC(CF3)3)4]- to [Al(OC(CF3)2PhCH3)4]- has little influence
on the coordination environment of the copper(II) cation. Based
on the IR data and the solid-state structure of 1 (Fig. 2), it can
Fig. 4 EPR spectrum of a frozen solution of complex 1.
Catalytic olefin aziridination with complexes 1–6
Acetonitrile coordinated complexes 1, 3, 5 were examined for
their catalytic activities for olefin aziridination, using styrene as
model substrate. It was found that complexes 3 and 5 are as
active as 1 under standard conditions (Table 4, entry 1, 3, 5),
affording very good yields (> 90%) in less than 5 min. However,
the yields of aziridine decreased significantly to ca. 65% when
only 0.1 mol% catalysts were applied (Table 4, entry 4, 6). The
significantly lower activity of the two complexes probably results
from complex decomposition, indicated by the color change of the
Table 3 EPR parameters of the copper(II) complexes 1–6
Complex
gII
g^
AII (mT)
1a
2a
3a
4b
5a
6b
2.353
2.355
2.346
2.317
2.328
2.323
2.098
2.091
2.095
2.078
2.081
2.078
14.72
13.72
14.72
15.48
14.65
15.07
a frozen acetonitrile solution; b frozen benzonitrile solution.
Fig. 3 IR spectra of the copper(II) complexes 1–6 of the range
2000–2500 cm-1.
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Table 5 Aziridination of various olefins catalyzed by 1a
Table 4 Aziridination of styrene with complexes 1–6a
Entry
Cat.
Load. (mol%)
Time (min)
Yieldb
Entry
1
Olefin
Time
Prod.
Yieldb (%)
90
1c
2c
3
4
5
1
1
3
3
5
5
6
6
6
0.5
0.1
0.5
0.1
0.5
0.1
0.5
0.1
0.5
3
360
3
360
3
360
3
360
120
91
90
93
65
90
67
89
68
75
15 min
2a
2
3
4
x = Br
x = OMe
1.5 h
5 min
1.5 h
2b
2c
2d
84
92
70
6
7
8
9d
5
5 min
2e
2f
90
a All reactions were performed in 2 mL dry acetonitrile with 1 equiv. of
◦
PhINTs and 5 equiv. of olefin at 25 C. b Isolated yield. c Data from Ref.
15. d dichloromethane as solvent.
6
x = OMe
1 min
10 min
93
N. A.
7c
reaction solution from yellow–green to orange. Previous studies
also showed that the anion [Al(OC(CF3)2PhCH3)4]-is less robust,
decomposing in the presence of zinc(II) cation and small amounts
of water.27
8
9
10 min
1 min
2g
2h
73
84
It is reasonable to assume that under the standard conditions,
benzonitrile complexes would behave as do their acetonitrile
congeners, due to the presence of a large excess of acetonitrile.
Indeed, the assumption is supported by the catalytic results
obtained from complexes 5 and 6 (Table 4, entries 5, 7 and entries
6, 8). Yet, when dichloromethane is used as solvent (Table 4, entry
9), the reaction time and the yield are much lower. Hence, the
catalytic activity of complexes 2 and 4 were not tested.
a All reactions were performed in 2 mL dry acetonitrile with 3 mol%
catalyst, 1 equiv. of PhINTs and 5 equiv. of olefin at 25 ◦C unless otherwise
noted. b Isolated yield. c TsNH2 was formed quantitatively.
is increasingly pronounced with long reaction times. Despite the
insolubility in acetonitrile, the aziridination of highly oleophilic
1-decene (entry 8) still affords a relatively high yield (73%) within
10 min. The non-conjugated diene 4-vinylcyclohex-1-ene (entry 9)
was also tested. Only the ring double bond yields the aziridine,
whereas the C C vinyl group does not exhibit any reactivity. This
is in good accordance with our earlier work,15 where 80% yield had
been obtained within 20 min for cyclohexene, versus 25% within
8 h for vinylcyclohexane, showing that the C C bound of the
vinyl substituent is not prone to aziridination.
Olefin aziridination catalyzed by complex 1
Following the preliminary investigation,15 the scope of olefin
substrates was further extended. Various olefins were tested for
catalytic aziridination using the most active and robust complex
1 as catalyst. As shown in Table 5, all substrates except cinnamyl
alcohol (entry 7) were successfully aziridinated with very good
isolated yields.
Stereoselectivity
Electron deficient cinnamate ester and its p-substituted deriva-
tives afford high yields of aziridines (entries 1–3) within very
short reaction times. The influence of the p-substitutes at the
aromatic ring is clearly reflected by the necessary reaction times:
the moderately electron-donating methoxy group (entry 3) is much
more favored than the electron-withdrawing bromide (entry 2).
The same effect is also observed for p-substituted styrenes (entry
5, 6).
A noteworthy example is ethylhex-2-enoate (entry 4), which was
first reported as substrate for olefin aziridination. The very low
reactivity of ethylhex-2-enoate is caused by the strong electron
withdrawing character of the ester and the inert aliphatic chain.
Nevertheless, a satisfactory yield (70%) of aziridine 2d could be
obtained within a relatively short reaction time (1.5 h). Not unex-
pectedly, the system was not compatible with allyl alcohols (entry
7). The only isolated product, TsNH2, suggests decomposition of
PhINTs in the presence of hydroxyl functionality, which has been
assumed to act as proton source, accounting for undesired hydrol-
ysis of PhINTs.20b A small amount of TsNH2 was also detected
in other cases, especially when the reaction time was prolonged
due to less active substrates (entry 2, 4). This is in accordance
with our previous results15 as well as the findings of Evans et al.,21
who demonstrated that the decomposition of PhINTs to TsNH2
Evans¢ copper system20b involving classic WCAs, such as ClO4
-
and OTf-, was found to be very efficient in the stereoselective
aziridination of 1,2-disubstituted olefins. The investigation of the
influence of per/polyfluoroalkoxyaluminates on the stereoselec-
tivity in olefin aziridination is of great interest (Table 6). As
expected, all trans olefins lead to the desired trans disubstituted
aziridines in high yields (entries 1, 5, 9, 13) in the presence
of 1. The aziridination ofcis-disubstituted aliphatic olefins also
exhibits excellent stereoselectivities (entry 2, 6, 10), affording
almost exclusively cis aziridines.
In comparison, up to 10 mol% catalyst loading and sometimes
low temperatures (- 20 ◦C) are required to achieve moderate
yields and relatively high stereoselectivities when the classic WCAs
are applied.20b When cis-b-methylstyrene is used as substrate, the
stereoselectivity is only slightly lower (entry 10). In the cases
of 3 (entry 3, 7, 11) and 5 (entry 4, 8, 12), similar yields and
indistinguishable stereoselectivities are obtained, only with slightly
longer reaction times. From Evans¢ results and those presented
in this work, it is very clear that counteranions have significant
influence on the stereoselectivity of olefin aziridination, with
the trend from high to low: [Al(OC(CF3)2R)4]-> ClO4 , OTf>
Br-, acac-. From this observation, it can be deduced that classic
-
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Table 6 Aziridination of cis and trans disubstituted olefins catalyzed by 1, 3, 5a
Entry
Olefin
Cat.
Time (min)
Prod.
3a
Yield (%)b
cis : trans ratioc
< 1 : 99
> 99 : 1
> 99 : 1
99 : 1
1
2
3
4
5
1
1
3
5
1
12
12
15
13
15
83
85
87
85
80
3b
3b
3b
3c
2 : 98e
6
1
3
5
1
1
3
5
20
25
22
5
3d
3d
3d
3e
3f
81
78
80
90
95
93
92
97 : 3e
98 : 2e
98 : 2e
< 1 : 99
3 : 97
7
8
9d
10d
11d
12d
3
5
3f
3 : 97
5
3f
4 : 96
a All reactions were performed in 2 mL dry acetonitrile with 1–3 mol% catalyst, 1 equiv. of PhINTs and 5 equiv. of olefin at 25 ◦C unless otherwise noted.
b Isolated yield. c The ratio of cis and trans isomers was detected by 400 MHz 1H NMR. d 1 mol% catalyst loading. e The lower stereoselectivity was due
to small amount of cis/trans impurities in the trans/cis 1,2-disubstituted olefins.
Table 7 Aziridination of cis-stilbene under various reaction conditionsa
WCAs have a stronger interaction ability than WCA of the type
[Al(OC(CF3)2R)4]-. Indeed, in certain cases, the classical WCAs
Entry Cat.
Time (min) Temp. (◦C) Yieldb
cis : trans ratioc
actually coordinate to the metal center, even in coordinating
solvents such as acetonitrile.24a
1
1
1
1
3
3
2
4
4
5
240
1
25
180
30
25
0
92%
93%
85%
86%
81%
91%
55%
70%
63 : 37
98 : 2
97 : 3
60 : 40
55 : 45
53 : 47
48 : 52
53 : 47
2
3d
4
25
25
25
25
25
25
5e
6e
7e
8d,e
Aziridination of cis-stilbene
300
5
To further investigate the influence of counteranions on the
stereoselectivity of olefin aziridination, cis-stilbene was aziridi-
nated under various conditions (Table 7). Only very moderate
stereoselectivity (cis/trans = 63 : 37) can be achieved in the presence
of 1 (entry 1) under standard conditions. Yet at 0 ◦C, cis-2,3-
diphenylaziridine is obtained almost exclusively (cis/trans = 98 : 2)
with prolonged reaction time (4 h) and in similar high yield
(93%) (entry 2). This is consistent with Evans¢ report,20b in which
the cis/trans ratio of the aziridines was increased from 83 : 17
to 90 : 10 when the temperature was decreased from 0 ◦C to -
20 ◦C. Moreover, examination of the reaction with higher catalyst
loading (3 mol%) at room temperature gives similar results with
respect to yield (85%), reaction time (1 min) and stereoselectivity
(97 : 3) (entry 3). With complex 3 as catalyst, slightly lower yield
a All reactions were performed in 2 ml dry acetonitrile with 1 mol% catalyst,
1 equiv. of PhINTs and 5 equiv. of olefin at 25 ◦C unless otherwise noted.
b isolated yield. c The ratio of cis and trans isomers was detected by 1H
NMR measurement and was the average value of three parallel reactions.
d with 3 mol% loading. e dichloromethane as solvent.
(86%) and stereoselectivity (cis/trans = 60 : 40) is obtained (entry
4).
Comparable results could be obtained using dichloromethane as
solvent, only when applying significantly prolonged reaction times
(entry 5). The replacement of acetonitrile ligands by benzonitrile
leads to lower yields and worse stereoselectivities (entry 5, 7),
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Table 8 Aziridination of 1-decene under various conditionsa
Entry
Cat.
Solvent
Time
Yieldb
1
2
3
4
1
3
3
4
MeCN
MeCN
DCM
DCM
10 min
8 min
2 min
2 min
73%
75%
76%
80%
a All reactions were performed in 2 mL dry acetonitrile with 1 mol%
catalyst 3, 1 equiv. of PhINTs and 5 equiv. of olefin at 25◦C unless otherwise
noted. b Isolated yield
which can be attributed to the lesser thermal stability of ben-
zonitrile coordinated complexes.13h The result can be significantly
improved with higher loading of 4 (entry 8), yet still far away
from the best records. Again, the complex with [Al(OC(CF3)3)4]-
as counteranion (2) shows a higher catalytic activity than the one
incorporating [Al(OC(CF3)2Ph)4]- (4) (entry 6, 7).
Solvent effects
It is well established that for olefin aziridination reactions polar
solvents afford better results than non-polar solvents, mainly
because the nitrene source PhINTs dissociates much faster in polar
solvents such as acetonitrile or nitromethane. However, this is not
always the case. From Table 8 it is apparent that dichloromethane
provides better results than acetonitrile for the aziridination of
1-decene (entry 2, 3).
Additionally, in contrast to the results obtained for cis-stilbene,
complex 3 shows a higher catalytic activity than 1 (entry 1, 2). The
benzonitrile coordinated complex 4 is superior to its acetonitrile
congener 3 (entry 3, 4). These observations can be attributed to the
very high lipophilic 1-decene having higher affinity to the catalysts
of lower polarity, e.g. 3 and 4.
Scheme 2 Proposed mechanism for copper-catalyzed olefin aziridination.
The influence of the counteranions on stereoselectivity can be
explained by the assumption that the coordination/interaction of
an anion to the copper center would influence the equilibrium of
the two nitrene forms in favor of triplet b (Scheme 2). Accordingly,
the stepwise pathway would be preferred. Compared to normal
anions and classical WCAs, WCAs of the type [Al(OC(CF3)2R)4]-
(R = CF3, Ph, PhCH3) are less likely to have interactions with the
cation [Cu(NCR¢)6]2+ and the nitrene [(NCR¢)4Cu NTs]+ (R¢ =
CH3, Ph). Therefore, rendering the catalysts higher stereoselectiv-
ity. Table 5 and 6 also show that the stereoselectivity is only slightly
lower when [Al(OC(CF3)3)4]- is replaced by [Al(OC(CF3)2Ph)4]-
or [Al(OC(CF3)2PhCH3)4]-, demonstrating that the latter two
are almost as weakly coordinating as the former. On the other
hand, Norrby’s theoretical calculations30 showed that the smaller
the copper-nitrene is, the lower energy of the singlet a will be
compared to the triplet b, and vice versa. This straightforwardly
explains why acetonitrile ligated copper complexes lead to a better
stereoselectivity than their benzonitrile congeners (Table 7, entry
5, 7) and other larger copper systems.20b,33
Mechanistic considerations
The reaction mechanism of metal-catalyzed olefin aziridination
reactions has been subject of extensive debate. Previous investiga-
tions by Mansuy,28 Evans,20b Jacobsen29 and Che16a indicate that
the mechanism is system dependent both respect to the metals, the
ligands and on the nature of the counteranions. As for copper
systems, two mechanisms have been suggested. While Evans
assumed a non-radical concerted mechanism with copper(II) as the
catalytically active oxidation state in nitrene transfer;20b Jacobsen
argued for a discrete Cu(III)-nitrene as reactive intermediate.29b,30
Herein, a mechanism, which involves both concerted and stepwise
pathways is proposed (Scheme 2).
Pe´rez et al.31 suggested a paramagnetic copper nitrene I as
intermediate in copper-catalyzed olefin aziridination. The assump-
tion was supported by Norrby’s theoretical investigation,30 which
assigned the resonance species a and b to singlet and triplet states,
respectively. Results from Table 6 suggest that the aziridination
of 1,2-disubstituted aliphatic olefins and cis-b-methylstyrene pro-
ceeds via a concerted mechanism20b via transition state c. Yet,
the results obtained with cis-stilbene indicate that the stepwise32
pathway would be favored, if radical d is stable enough. It is
apparent that a hypothetic radical d originating from cis-stilbene
has lower energy than that of cis-b-methylstyrene; therefore
more trans product was obtained for cis-stilbene under the same
conditions.
Conclusion
Copper(II) complexes 1–6 of the general formula
[Cu(NCR¢)6][Al(OC(CF3)2R)4]2 (R¢ = CH3, Ph; R = CF3,
Ph, PhCH3) are very efficient catalysts for olefin aziridination
with PhINTs as nitrene source. Highly inert substrates such as
ethylhex-2-enoate and 1-decene can be successfully aziridinated
under mild conditions. Contrary to previous reports, aziridination
of cis-stilbene is also stereoselective under certain conditions,
providing cis : trans ratio of the resulting aziridines up to 98 : 2.
The very weak coordinating ability of WCAs [Al(OC(CF3)2R)4]-
(R = CF3, Ph, PhCH3) is considered to be responsible for
the superior results obtained with respect to reactivity and
stereoselectivity. Interestingly, for highly lipophilic olefins, such
as 1-decene, dichloromethane provides better results than the
commonly used acetonitrile. Further work to extend the substrate
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scope and mechanistic studies are currently under way in our
laboratories.
The reaction mixture was stirred for 15 min. Thereafter, 20 mL
of diethyl ether was added. The mixture was stirred for another
15 min and the volatiles were removed in vacuo, affording the
product as a white solid, which was re-dissolved in dry DCM
(10 mL). Filtration and removal of the volatiles in darkness
afforded the product as a white powder. Yield: 85–93%.
Experimental Section
All preparations and manipulations were carried out under argon
atmosphere using standard Schlenk techniques. All solvents used
were dried by standard procedures. 1H, 13C and 19F NMR
measurements were performed on a JEOL 400 MHz spectrometer.
27Al NMR measurements were performed on a Bruker AVANCE-
DPX-400 MHz spectrometer. IR spectra were recorded on a
Perkin Elmer FT-IR spectrometer using Nujol. EPR spectra were
recorded with a JEOL JES-RE2X at X-band frequency (n ª 9.05
GHz, microwave power 2 mW, modulation frequency 100 kHz).
Elemental analyses were carried out at the Mikroanalytisches
Labor of TU Mu¨nchen. Metal analyses were determined with
a Vario EL/1100 metal analyzer.
[Ag(NCCH3)4][Al(OC(CF3)3)4]. AgNO3 (0.408 g, 2.4 mmol),
K[Al(OC(CF3)3)4] (2.415 g, 2.4 mmol); Yield: 2.78 g (93%).
Elemental Anal. Calc. (%) for C24H12AgAlF36N4O4 (1239.17): C,
23.26; H, 0.98; N, 4.52; F, 55.19; Found: C, 23.48; H, 0.93; N,
4.47; F, 55.36;1H NMR (400 MHz, CD2Cl2, 25 ◦C, d (ppm)):
2.01 (CH3, s, 12H); 13C NMR(101 MHz, CD2Cl2, 25 ◦C, d
(ppm)): 126.43, 123.51, 120.60,◦119.68, 117.70, 79.97, 2.25; 27Al
NMR (104.3 MHz, CD2Cl2, 25 C,d (ppm)): 29.33 (s); 19F NMR
(376.5 MHz, CD2Cl2, 25 ◦C, d (ppm)): -79.91 (s).
[Ag(NCCH3)2][Al(OC(CF3)2Ph)4]. AgNO3
(0.238
g,
1.4 mmol), K[Al(OC(CF3)2Ph)4] (1.45 g, 1.4 mmol); Yield:
1.50 g (90%). Elemental Anal. Calc. (%) for C40H26AgAlF24N2O4
(1189.46): C, 40.39; H, 2.20; N, 2.36;F, 38.33; Found: C 39.60,
General procedure for the synthesis of K[Al(OC(CF3)2Ph)4] and
K[Al(OC(CF3)2PhCH3)4]
◦
H 2.03, N 2.32; F, 37.98;1H NMR (400 MHz, CD2Cl2, 25 C, d
The synthesis and characterization of K[Al(OC(CF3)3)4] were
carried out according to the literature.14
(ppm)): 7.85 (d, J = 7.6, 8H), 7.27 (t, J = 7.2, 4H), 7.19 (t, J =
7.5, 8H), 2.00 (s, 6H); 13C NMR (101 MHz, CD2Cl2, 25 ◦C, d
(ppm)): 136.03, 128.18, 128.04, 127.92, 126.80, 125.28, 122.38,
119.48, 118.88, 79.86, 1.18;27Al NMR (104.3 MHz,CD2Cl2, 25 ◦C,
d (ppm)): 30.18 (s); 19F NMR (376.5 MHz, CD2Cl2, 25 ◦C, d
(ppm)): -74.63 (s).
Purified LiAlH42 (1.4 equiv.) was suspended in toluene (30 mL)
and HOC(CF3)2R (R = Ph, PhCH3) (4.0 equiv.) was added at
room temperature. The mixture was refluxed for 3 h, whereupon
a yellow–orange solution formed. The volatiles were removed in
vacuo, affording a yellow–orange oily solid, which was re-dissolved
in diethyl ether (20 mL) and treated with saturated aqueous KCl
solution (20 mL). The mixture was then stirred vigorously for
3 h at room temperature. After separating the aqueous layer and
precipitation, the organic phase was washed by saturated aqueous
KCl solution (10 mL ¥ 3), dried by MgSO4, filtered and brought
to dryness in vacuo to afford the crude product as a white to pale
yellow solid, which was re-dissolved in dry DCM (5 mL) and
layered with n-pentane (15 mL) at 0 ◦C. The desired product was
obtained as white to pale yellow crystals. Yield: 50–75%.
[Ag(NCCH3)2][Al(OC(CF3)2PhCH3)4]. AgNO3 (0.238 g,
1.4 mmol), K[Al(OC(CF3)2PhCH3)4] (1.519 g, 1.4 mmol); Yield:
1.48 g (85%). Elemental Anal. Calc. (%) for C44H34AgAlF24N2O4
(1245.56): C, 42.43; H, 2.75; N, 2.25;F, 36.61; Found: C, 42.39,
H, 3.02, N, 2.24; F, 36.14;1H NMR (400 MHz, CD2Cl2, 25 ◦C,
d (ppm)): 7.70 (d, J = 8.1, 8H), 6.99 (d, J = 8.2, ◦8H), 2.23 (s,
12H), 1.95 (s, 6H);13C NMR (101 MHz, CD2Cl2, 25 C, d (ppm)):
139.73, 133.49, 128.77, 128.32, 127.81, 125.88, 122.98, 120.09,
119.35, 80.41, 21.30, 2.15;27Al NMR (104.3 MHz, CD2Cl2, 25 ◦C,
d (ppm)): 30.27 (s); 19F NMR (376.5 MHz, CD2Cl2, 25 ◦C, d
(ppm)): -74.62 (s).
K[Al(OC(CF3)2Ph)4]. LiAlH4 (0.158 g, 4.2 mmol),
HOC(CF3)2Ph (2.0 mL, 11.9 mmol); Yield: 70%. Elemental
Anal. Calc.(%) for C36H20AlF24KO4 (1038.58): C, 41.63; H, 1.94;
1
F, 43.90; K 3.76;Found: C, 42.30; H, 2.28; F, 44.30; K, 4.00. H
General procedure for the synthesis of [Cu(NCCH3)6]-
[Al(OC(CF3)3)4]2(1), [Cu(NCCH3)6][Al(OC(CF3)2Ph)4]2 (3) and
[Cu(NCCH3)6][Al(OC(CF3)2PhCH3)4]2 (5)
NMR (400 MHz, CD2Cl2, 25 ◦C, d (ppm)): 7.70 (d, J = 7.6, 2H),
7.28 (t, J = 7.4, 1H), 7.15 (t, J = 7.8, 2H); 19F NMR (376 MHz,
CD2Cl2, 25 ◦C, d (ppm)): -74.50.
CuCl2 (0.95–1.1 equiv.) was added to an acetonitrile solution
(10 mL) of [Ag(CH3CN)x][Al(OC(CF3)2R)4] (2.0 equiv.) (x = 2,
4; R = CF3, Ph, PhCH3). The resulting mixture was stirred for
3 h under exclusion of light. After filtration, the volatiles were
removed in vacuo, affording the product as a green–blue solid. For
1, the crude product was re-dissolved in 1.5 mL dry acetonitrile,
filtered and layered by 10 mL dichloromethane to afford purified
product; for 3 and 5, the crude product was re-dissolved in 10 mL
dichloromethane, filtered and the volatiles were removed in vacuo
to afford purified products.
K[Al(OC(CF3)2PhCH3)4]. LiAlH4 (0.129 g, 3.4 mmol),
HOC(CF3)2PhCH3 (2.0 mL, 9.7 mmol); Yield: 55%. Elemental
Anal. Calc.(%) for C40H28AlF24KO4 (1094.69): C, 43.89; H, 2.58;
F, 41.65; K, 3.57; Found: C, 44.20; H, 2.89; F, 41.20; K, 3.32. 1H
NMR (400 MHz, CD2Cl2, 25 ◦C, d (ppm)): 7.56 (d, J = 8.3, 2H),
6.95 (d, J = 8.4, 2H), 2.25 (d, J = 5.8, 3H); 19F NMR (376 MHz,
CD2Cl2, 25 ◦C, d (ppm)): -74.52.
General procedure for the synthesis of [Ag(NCCH3)4]-
[Al(OC(CF3)3)4], [Ag(NCCH3)2] [Al(OC(CF3)2Ph)4] and
[Ag(NCCH3)2][Al(OC(CF3)2PhCH3)4]
[Cu(NCCH3)6][Al(OC(CF3)3)4]
0.1 mmol), [Ag(CH3CN)4][Al(OC(CF3)3)4] (0.248 g, 0.2 mmol);
Yield: 0.18 (80%); Elemental Anal. Calc. (%) for
C44H18Al2CuF72N6O8 (2244.04): C, 23.55; H, 0.81; N, 3.75;
(1). CuCl2
(0.0128
g,
An acetonitrile (10 mL) solution of AgNO3 (1.0 equiv.) was added
to an acetonitrile (10 mL) solution of K[Al(OC(CF3)2R)4] (1.0
equiv.) (R = CF3, Ph, PhCH3) in a pre-dried Schlenk in darkness.
g
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F, 60.96; Found: C 23.42, H 0.72, N 3.81; F, 61.10; Selected IR
2066; (e) A. Macchioni, Chem. Rev., 2005, 105, 2039; (f) S. Korbe, P. J.
Schreiber and J. Michl, Chem. Rev., 2006, 106, 5208; (g) W. H. Hersh,
J. Am. Chem. Soc., 1985, 107, 4599; (h) M. Bochmann, Angew. Chem.,
Int. Ed. Engl., 1992, 31, 1181; (i) L. Jia, X. Yang, A. Ishihara and T. J.
Marks, Organometallics, 1995, 14, 3135; (j) S. M. Ivanova, S. V. Ivanov,
S. M. Miller, O. P. Anderson, K. A. Solntsev and S. H. Strauss, Inorg.
Chem., 1999, 38, 3756; (k) T. J. Barbarich, S. T. Handy, S. M. Miller, O. P.
Anderson, P. A. Grieco and S. H. Strauss, Organometallics, 1996, 15,
3776; (l) L. Jia, X. Yang, C. L. Stern and T. J. Marks, Organometallics,
1997, 16, 842; (m) T. J. Barbarich, S. M. Miller, O. P. Anderson and
S. H. Strauss, J. Mol. Catal. A: Chem., 1998, 128, 289; (n) S. V. Ivanov,
J. J. Rockwell, O. G. Polyakov, C. M. Gaudinski, O. P. Anderson, K. A.
Solntsev and S. H. Strauss, J. Am. Chem. Soc., 1998, 120, 4224; (o) V. C.
Williams, W. E. Piers, W. Clegg, M. R. J. Elsegood, S. Collins and T. B.
Marder, J. Am. Chem. Soc., 1999, 121, 3244; (p) S. V. Ivanov, A. J.
Lupinetti, S. M. Miller, O. P. Anderson, K. A. Solntsev and S. H.
Strauss, Inorg. Chem., 1995, 34, 6419; (q) Y. Sun, M. V. Metz, C. L.
Stern and T. J. Marks, Organometallics, 2000, 19, 1625.
(Nujol, cm-1): n(CN), 2332, 2304, 1036, 972;
[Cu(NCCH3)6][Al(OC(CF3)2Ph)4]2 (3). CuCl2 (0.0148 g,
0.11 mmol), [Ag(NCCH3)2] [Al(OC(CF3)2Ph)4] (0.238 g,
0.2 mmol); Yield: 0.21 g (89%); Elemental Anal. Calc. (%) for
C84H58Al2CuF48N6O8 (2308.83): C, 43.70; H, 2.53; F, 39.50; N,
3.64; Found: C, 43.98; H, 2.64; N, 3.06; F, 38.80; Selected IR
(Nujol, cm-1): n(CN), 2332, 2301, 1082, 1036, 967;
[Cu(NCCH3)6][Al(OC(CF3)2PhCH3)4]2 (5). CuCl2 (0.0148 g,
0.11 mmol), [Ag(NCCH3)2][Al(OC(CF3)2PhCH3)4] (0.249 g,
0.2 mmol); Yield: 0.20 g (85%) Elemental Anal. Calc. (%) for
C92H74Al2CuF48N6O8 (2421.04): C, 45.64; H, 3.08; N, 3.47; F,
37.67; Found: C, 44.74; H, 3.08; N, 3.10; F, 36.90; Selected IR
(Nujol, cm-1): n(CN), 2331, 2302, 1036, 1023, 966;
2 I. Krossing, Chem.–Eur. J., 2001, 7, 490.
3 I. Krossing, J. Am. Chem. Soc., 2001, 123, 4603.
4 (a) A. Adolf, M. Gonsior and I. Krossing, J. Am. Chem. Soc., 2002, 124,
7111; (b) T. S. Cameron, A. Decken, I. Dionne, M. Fang, I. Krossing
and J. Passmore, Chem.–Eur. J., 2002, 8, 3386; (c) I. Krossing and A.
Reisinger, Angew. Chem., Int. Ed., 2003, 42, 5725.
General procedure for the synthesis of [Cu(NCC6H5)6]-
[Al(OC(CF3)3)4]2 (2), [Cu(NCC6H5)6][Al(OC(CF3)2Ph)4]2 (4) and
[Cu(NCC6H5)6][Al(OC(CF3)2PhCH3)4]2 (6)
5 I. Krossing and I. Raabe, Angew. Chem., Int. Ed., 2001, 40,
4406.
6 I. Krossing, A. Bihlmeier, I. Raabe and N. Trapp, Angew. Chem., Int.
Ed., 2003, 42, 1531.
7 A. Decken, G. B. Nikiforov and J. Passmore, Polyhedron, 2005, 24,
2994.
8 S. Schulz, D. Schuchmann, I. Krossing, D. Himmel, D. Blaser and R.
Boese, Angew. Chem., Int. Ed., 2009, 48, 5748.
9 S. P. Smidt, N. Zimmermann, M. Studer and A. Pfaltz, Chem.–Eur. J.,
2004, 10, 4685.
CuCl2 (1.1 equiv.) was added to a dry benzonitrile solution (4 mL)
of [Ag(CH3CN)x][Al(OC(CF3)2R)4] (2.0 equiv.) (x = 2, 4; R = CF3,
Ph, PhCH3). The resulting mixture was stirred for 4 h in darkness.
After filtration, the volatiles were removed in vacuo to afford the
product as a green solid, which was re-dissolved in 10 mL dry
DCM, filtered and the volatiles were removed in vacuo to afford
purified product.
10 C. Bo¨ing, G. Francio and W. Leitner, Adv. Synth. Catal., 2005, 347,
1537.
[Cu(NCC6H5)6][Al(OC(CF3)3)4]2 (2). CuCl2 (0.0148 g,
0.11
mmol),
[Ag(CH3CN)4][Al(OC(CF3)3)4]
(0.248
g,
11 (a) I. Krossing and A. Reisinger, Eur. J. Inorg. Chem., 2005, 1979;
(b) P. Hanefeld, M. Sigl, V. Bohm, M. Roper, H. Walter, I. Krossing,
U.S. Patent 2008/0293900, 2008; (c) B. Korthals, A. Berkefeld, M.
Ahlmann and S. Mecking, Macromolecules, 2008, 41, 8332.
12 (a) D. S. McGuinness, A. J. Rucklidge, R. P. Tooze and A. M. Z. Slawin,
Organometallics, 2007, 26, 2561; (b) A. J. Rucklidge, D. S. McGuinness,
R. P. Tooze, A. M. Z. Slawin, J. D. A. Pelletier, M. J. Hanton and P. B.
Webb, Organometallics, 2007, 26, 2782.
13 (a) M. Vierle, Y. Zhang, A. M. Santos, K. Ko¨hler, C. Haeßner, E.
Herdtweck, M. Bohnenpoll, O. Nuyken and F. E. Ku¨hn, Chem.–Eur. J.,
2004, 10, 6323; (b) M. Vierle, Y. Zhang, E. Herdtweck, M. Bohnenpoll,
O. Nuyken and F. E. Ku¨hn, Angew. Chem., Int. Ed., 2003, 42, 1307;
(c) S. Gago, Y. M. Zhang, A. M. Santos, K. Ko¨hler, F. E. Ku¨hn, J. A.
Fernandes, M. Pillinger, A. A. Valente, T. M. Santos, P. J. A. Ribeiro-
Claro and I. S. Goncalves, Microporous Mesoporous Mater., 2004, 76,
131; (d) A. Sakthivel, A. K. Hijazi, H. Y. Yeong, K. Kohler, O. Nuyken
and F. E. Ku¨hn, J. Mater. Chem., 2005, 15, 4441; (e) Y. Zhang, A. M.
Santos, E. Herdtweck, J. Mink and F. E. Ku¨hn, New J. Chem., 2005,
29, 366; (f) A. K. Hijazi, N. Radhakrishnan, K. R. Jain, E. Herdtweck,
O. Nuyken, H. M. Walter, P. Hanefeld, B. Voit and F. E. Ku¨hn, Angew.
Chem., Int. Ed., 2007, 46, 7290; (g) A. K. Hijazi, H. Y. Yeong, Y. M.
Zhang, E. Herdtweck, O. Nuyken and F. E. Ku¨hn, Macromol. Rapid
Commun., 2007, 28, 670; (h) Y. Li, L. T. Voon, H. Y. Yeong, A. K.
Hijazi, N. Radhakrishnan, K. Ko¨hler, B. Voitc, O. Nuyken and F. E.
Ku¨hn, Chem.–Eur. J., 2008, 14, 7997.
0.2 mmol); Yield: 0.22 g (85%); Elemental Anal. Calc. (%)
for C74H30Al2CuF72N6O8 (2616.46): C, 33.97; H, 1.16; N, 3.21; F,
52.28; Found: C, 34.97; H, 0.92; N, 3.51; F, 52.50; Selected IR
(Nujol, cm-1): n(CN), 2280, 1067, 1027, 974;
[Cu(NCC6H5)6][Al(OC(CF3)2Ph)4]2 (4). CuCl2 (0.0148 g,
0.11 mmol), [Ag(NCCH3)2] [Al(OC(CF3)2Ph)4] (0.238 g,
0.2 mmol); Yield: 0.22 g (83%); Elemental Anal. Calc. (%)for
C114H70Al2CuF48N6O8 (2681.24): C, 51.07; H, 2.63; N, 3.13; F,
34.01; Found: C, 49.48; H, 2.33; N, 3.05; F, 33.00; Selected IR
(Nujol, cm-1): n(CN), 2274, 1036, 1024, 965;
[Cu(NCC6H5)6][Al(OC(CF3)2PhCH3)4]2 (6). CuCl2 (0.0148 g,
0.11 mmol), [Ag(NCCH3)2][Al(OC(CF3)2PhCH3)4] (0.249 g,
0.2 mmol); Yield: 0.24 g (86%) Elemental Anal. Calc. (%) for
C122H86Al2CuF48N6O8 (2793.46): C, 52.45; H, 3.10; N, 3.01; F,
32.64; Found: C, 51.70; H, 2.99; N, 2.92; F, 32.70; Selected IR
(Nujol, cm-1): n(CN), 2275, 1040, 1024, 964;
Acknowledgements
14 Y. Li and F. E. Ku¨hn, J. Organomet. Chem., 2008, 693,
2465.
Y. L. thanks the Universita¨t Bayern e. V. for a PhD fellowship. J.-
Y. He is grateful to DAAD for a master stipend. The International
Graduate School of Science and Engineering (IGSSE) of Tech-
nische Universita¨t Mu¨nchen is also acknowledged for financial
support.
15 Y. Li, B. Diebl, A. Raith and F. E. Ku¨hn, Tetrahedron Lett., 2008, 49,
5954.
16 (a) S.-M. Au, J.-S. Huang, W.-Y. Yu, W.-H. Fung and C.-M. J. Che,
J. Am. Chem. Soc., 1999, 121, 9120; (b) Y. Cui and C. He, J. Am. Chem.
Soc., 2003, 125, 16202.
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(b) A. N. Vedernikov and K. G. Caulton, Org. Lett., 2003, 5, 2591;
(c) Z. Lu, Y. Zhang and W. D. Wulff, J. Am. Chem. Soc., 2007, 129,
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23 Crystal data of complex 1: C44H18Al2F72N6O8Cu,C7H8; Mr = 2336.29,
blue fragment; monoclinic, P21/c (No.: 14), a = 25.496(3), b = 14.855(2),
◦
3
˚
˚
c = 23.231(3) A, b = 112.430(5) , V = 8132.9(18) A , Z = 4, dcalc = 1.908
gcm-3, F000 = 4564, m = 0.508 mm-1, T = 123 K.
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5754 | Dalton Trans., 2011, 40, 5746–5754
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