Nitrene transfer reactions catalysed by copper(I) complexes in ionic liquid using chloramine-T

Article abstract of DOI:10.1039/b813469c

The complex [Tpm^*,BrCu(NCMe)]BF₄ (Tpm ^*,Br = HC(3,5-Me₂-4-Br-pyrazolyl)₃) catalyses the aziridination of alkenes and the amidation of cyclic ethers with chloramine-T as the nitrene source and the ioni

Full text of DOI:10.1039/b813469c

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www.rsc.org/dalton | Dalton Transactions
Nitrene transfer reactions catalysed by copper(I) complexes in ionic
liquid using chloramine-T†
Israel Cano, M. Carmen Nicasio* and Pedro J. Pe´rez*
Received 4th August 2008, Accepted 14th October 2008
First published as an Advance Article on the web 28th November 2008
DOI: 10.1039/b813469c
The complex [Tpm*,BrCu(NCMe)]BF4 (Tpm*,Br = HC(3,5-Me2-4-Br-pyrazolyl)3) catalyses the
aziridination of alkenes and the amidation of cyclic ethers with chloramine-T as the nitrene source and
the ionic liquid [bmim]PF6 as the reaction medium (bmim = 1-n-butyl-3-methylimidazolium). High
conversions have been obtained over several cycles of catalyst recovery and reuse.
few systems have been reported to promote the aziridination
Introduction
reaction under conditions that allow easy catalyst and product
separation and further catalyst recycling.5,6 In the last decade
ionic liquids have emerged as a convenient reaction medium to
favour catalyst and product separation,7 although their use in
these nitrene transfer catalytic transformations is yet scarce. We
are only aware of two examples reporting the use of ionic liquids
as reaction media in aziridination reactions by nitrene transfer to
olefins. Kantam et al.8 performed the aziridination of alkenes in
The catalytic formation of carbon-nitrogen bonds is an area of
continuous research.1 Aziridines represent valuable intermediates
not only for the synthesis of wide range of nitrogen-containing
products,2 but also due to its presence in a number of naturally
occurring compounds with cytotoxic activity.3 Transition metal-
catalyzed nitrene transfer to olefins is an efficient route to the direct
synthesis of aziridines (Scheme 1).4 A reaction which sometimes
competes with aziridination is the insertion of the nitrene unit into
C-H bond of substrates to yield amidation products (Scheme 1).
Most of the work carried out in this area corresponds to
homogeneous systems in which, at the end of the reaction, product
purification supposes catalyst loss.4 On the other hand, very
[bmim]X (bmim = 1-n-butyl-3-methylimidazolium; X = BF4, PF6)
9
=
using PhI NTs as the nitrene donor and Cu(acac)2 as catalyst.
Jain and Sain10 reported the use of [bmim]X as reaction media as
well as promoters for the metal-free aziridination of alkenes with
chloramine-T and N-bromosuccinimide as catalyst. Regarding the
amidation reaction, and to the best of our knowledge, there is no
report on the use of ionic liquids as solvents to perform catalytic
nitrene insertion into carbon-hydrogen bonds.
During the last decade, we have developed a family of copper(I)
complexes which contain hydrotris(pyrazolyl)borate11 (Tpx) lig-
ands for carbene12 and nitrene13 transfer reactions to a variety of
organic substrates in conventional organic solvents. In search of re-
cyclable catalytic systems, we have recently developed a methodol-
ogy involving the use of ionic liquid (IL) to immobilise the catalyst.
This approach has been successfully applied in carbene transfer
reactions14 mediated by cationic Cu(I) complexes of formula
[TpmxCu(NCMe)]BF4 containing tris(pyrazolyl)methane ligands
(Tpmx), the neutral analogue of anionic Tpx ligands. On the basis
of the aforementioned lack of reported catalytic systems for nitrene
transfer and catalyst recycling, we have studied the use of those
cationic copper complexes as catalysts for such process. Herein, we
describe the catalytic capabilities of [TpmxCu(NCMe)]BF4 com-
plexes immobilised in [bmim]PF615 for alkene aziridination and C–
H amidation reactions using chloramine-T as the nitrene source.
It is worth mentioning that the most widely used nitrene source
Scheme 1 Nitrene transfer reactions and nitrene sources.
4
=
is N-tosyliminophenyl-iodinane, PhI NTs, in spite of the fact
that its use supposes the generation of stoichiometric amounts of
iodobenzene as by-product. In recent years chloramine-T (sodium
N-chloro-p-toluensulfonamide), an inexpensive and commercially
available oxidant, has become an alternative nitrogen source
for aziridination and amidation reactions in the homogenous
phase.16
Laboratorio de Cata´lisis Homoge´nea, Departamento de Qu´ımica y Ciencia
de los Materiales, Unidad Asociada al CSIC, Campus de El Carmen s/n,
Universidad de Huelva, 21007, Huelva, Spain. E-mail: perez@dqcm.uhu.es,
mcnica@dqcm.uhu.es; Fax: +34 959219942; Tel: +34 959219956
† Electronic supplementary information (ESI) available: Experimental and
characterisation data. See DOI: 10.1039/b813469c
730 | Dalton Trans., 2009, 730–734
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Table 2 Reuse of [TpmxCu(NCMe)]+/[bmim]PF6 in the catalytic azirid-
Results and discussion
ination of styrene with chloramine-Ta
Olefin aziridination
Catalyst
Initially, we screened a series of four [TmpxCu(NCMe)]BF4 (Fig. 1)
complexes14 dissolved in [bmim]PF6 as potential catalysts for
the aziridination of styrene using anhydrous chloramine-T17 and
different reaction conditions. The results are summarized in
Table 1. In all cases, the desired aziridine was obtained in good
yield (60–90%) along with small amounts of TsNH2 as the only
by-product. No aziridine formation was observed in the absence
of catalysts. The best results (84% isolated yield) were obtained
1
2
3
Run Yieldb (conversion)c Yieldb (conversion)c Yieldb (conversion)c
1
2
3
4
5
6
52 (60)
70 (81)
70 (80)
60 (70)
35 (40)
64 (70)
65 (71)
70 (76)
60 (64)
83 (90)
85 (90)
92 (95)
83 (90)
81 (85)
61 (65)
˚
when using 5 mol% of 3 in [bmim]PF6 (3 mL) over activated 4 A
a Reactions carried out for 24 h at room temperature in the presence of
molecular sieves using 0.025 mol of the catalyst (5 mol%), 0.5 mmol of
chloramine-T, 2.5 mmol of styrene in 3 mL of [bmim]PF6. b Isolated yield
(average of two runs). c Substrate conversion determined by NMR analysis
using a internal standard.
molecular sieves with a styrene to chloramine-T mole ratio of
5:1 (entry 3). Such degree of conversion found for 3 was better
than those previously described for metal mediated aziridination
of styrene with chloramine-T,16a–c,e,j or bromamine-T18 as nitrene
sources in organic solvents and it is comparable to that reported by
Xia and co-workers16g for aziridinations in aqueous media using
CuI and a phase-transfer catalysts. However, in the only reported10
example of chloramine-T employed under ionic liquid conditions,
styrene was converted into the corresponding aziridine in 75%
yield, slightly lower than in our case for 3 as the catalyst.
Having optimized the reaction conditions, we examined the
recyclability of the Cu(I)/[bmim]PF6 catalytic systems in the
aziridination of styrene. The Cu(I) complex (5 mol%) was dis-
solved in [bmim]PF6 (3 mL) in the presence of molecular sieves and
the substrate and chloramine-T (5:1 mol ratio respect to styrene)
were added. Reactions were stirred for 24 h at room temperature.
The aziridine was extracted from the reaction mixture with diethyl
ether, leaving the catalyst19 and NaCl in the ionic liquid phase,
which was loaded with more chloramine-T and styrene. The results
are displayed in Table 2 and plotted in Fig. 2. For catalysts 1 and
2 the yields were comparable for the first four runs, the low yield
for the former in the first cycle being probably due to the presence
of adventitious water. After the fourth run the yield decreased
considerably suggesting a deactivation process of the catalytic
system. However, complex 3 demonstrated to have better catalytic
performance during five cycles, in an unprecedented behaviour
compared to other recyclable systems for olefin aziridination
reactions.5,6,8,10 Only some deactivation was observed in the sixth
run (Table 2, entry 6). It is interesting to point out that we did not
attempt the removal of NaCl by-product from ionic liquid after
each run. The role of the chloride anion will be discussed below.
Fig. 1 Tris(pyrazolyl)methane ligands used in this work.
Table 1 Aziridination of styrene with chloramine-T (CT) catalysed by
a
[TpmxCu(NCMe)]BF4 in [bmim]PF6
Entry
Catalyst
Styrene/CTb
Yield (%)c
1
2
3
4
5
6
7
8
9
[TpmCu(NCMe)]BF4, 1
[Tpm*Cu(NCMe)]BF4, 2
[Tpm*,BrCu(NCMe)]BF4, 3
[Tpm*,BrCu(NCMe)]BF4, 3
[TpmMsCu(NCMe)]BF4, 4
[TpmCu(NCMe)]BF4, 1
[Tpm*Cu(NCMe)]BF4, 2
[Tpm*,BrCu(NCMe)]BF4, 3
[TpmCu(NCMe)]BF4, 1
[Tpm*Cu(NCMe)]BF4, 2
[Tpm*,BrCu(NCMe)]BF4, 3
5:1
5:1
5:1
5:1
5:1
10:1
10:1
10:1
2:1
2:1
2:1
70 (51)d
65 (50)d
84 (73)d
65e
70
62
57
70
61
50
10
11
76
a Reactions carried out for 24 h at room temperature in the presence of
molecular sieves using 0.05 mol of the catalyst, 5% respect to chloramine-
T (CT) in ionic liquid (3 mL) as the solvent. b Styrene/CT = styrene to
chloramine-T mole ratio. c Isolated yields based on chloramine-T (average
of two runs), TsNH2 accounted for the remaining nitrene source. d In the
absence of molecular sieves. e In biphasic medium ([bmim]PF6-hexane).
Fig. 2 Plot of the conversion vs. number of cycles for the aziridination of
styrene with chloramine-T using complexes 1–3 as catalysts.
We have also studied the aziridination with chloramine-T
of a series of cycloalkenes using 3 as catalyst (eqn (1)–(2)).
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Table 3 Reuse of [Tpm*,BrCu(NCMe)]BF4 (3)/[bmim]PF6 in the catalytic
The yields of aziridines were significantly reduced relative to
that obtained with styrene as substrate (the remaining of the
initial nitrene precursor was converted into TsNH2). Interestingly,
cyclohexene was converted to a mixture of the 3-cyclohexenyl
sulfonamide (20%), the allylic amidation product, together with
the corresponding N-tosyl aziridine as the minor product (10%).
It is worth mentioning that allylic insertion is rarely observed for
copper-based aziridination catalysts.16c,h,i
amidation of THF with chloramine-Ta
Run
1
2
3
4
5
6
7
8
Yieldb
83
(90)
80
(95)
84
(92)
83
(95)
77
(92)
78
(90)
74
(83)
76
(80)
(Conv)c
(1)
a Reactions carried out for 3 h at room temperature in the presence of
molecular sieves using 0.025 mol of the catalyst (5 mol%), 0.5 mmol of
chloramine-T, 2 mL of substrate in 3 mL of [bmim]PF6. b Isolated yield
(average of two runs). c Substrate conversion determined by NMR analysis
using a internal standard.
(2)
C–H bond amidation of tetrahydrofuran
The reaction medium and the catalytic species
In view of the results found for cyclohexene, in which the amidation
of a C–H bond was observed, we decided to use 3 in the
amidation of the C–H bonds of tetrahydrofuran. In a typical
experiment, chloramine-T (0.5 mmol) was added to a solution
of 3 in [bmim]PF6 (5 mol% with respect to the nitrene source)
containing THF in excess (2 mL) in the presence of molecular
sieves. The mixture was stirred at room temperature for 3 h and the
product and excess of THF were extracted with diethyl ether. The
conversion was examined by 1H NMR affording 93% yield of a-
tosylamino tetrahydrofuran (eqn (3)). The use of tetrahydropyran
was also successful albeit with a lower conversion (60%). As
mentioned above, there are no examples of catalytic amidation
of C–H bonds by nitrene insertion in ionic liquids. We have
previously reported the use of TpBr3Cu(NCMe) as the catalyst and
chloramine-T as the nitrene source for the quantitative conversion
of thf into the nitrene insertion product.13d Other systems have
also been reported with such degree of conversions but using
In the system reported herein the ionic liquid [bmim]PF6 acts as
the sole reaction medium, in contrast to other examples reported
in which the IL is part of a biphasic system. In our case, the
reactants, the catalyst and the products are contained in the
ionic liquid. It is worth mentioning that NMR studies carried
out with a sample of the reaction mixture after the chloramine-T
was consumed showed only the expected products and the ionic
liquid, the latter remaining unaffected by the catalytic reaction.
The catalytic reactions are performed in the presence of activated
˚
4 A molecular sieves to eliminate adventitious water from the
reaction medium which is responsible for the formation of TsNH2
as by-product. Blank experiments performed with molecular sieves
in the absence of the copper complexes were unproductive, the
molecular sieves being not active to induce these nitrene transfer
reactions.
An interesting feature is the presence of chloride ions in the
reaction mixture, formed upon decomposition of the nitrene
source, chloramine-T. Davies and co-workers22 have reported that
halide impurity in ionic liquids dramatically drops the yield and
enantioselectivity in the asymmetric cyclopropanation reaction
of styrene catalysed by bis(oxazoline) Cu(I) complexes in ionic
liquids. It seems that in our system, the presence of sodium chloride
in high concentration in the ionic liquid had no influence on the
yields at least during the first four catalytic runs. However, in a
separate experiment, when chloramine-T was reacted with styrene
in the presence of complex 3 as the catalysts and [bmim]Cl was
added (80 equiv with respect to the catalyst), the reaction was
completed inhibited. Since the cation is identical to that in the IL,
it seems obvious that such inhibition is due to the chloride anions.
The lack of observation of such effect due to accumulation of NaCl
could be explained in terms of the different degree of dissociation
PhI NTs as the nitrene source.13d,20 But the most interesting feature
=
of this system is its potential to be reused. As shown in Table 3,
up to eight cycles were run with an overall loss of efficiency of
only 10%, again proving the potential of this system to separate
catalyst and products and to be recycled for several times with
an acceptable decrease of activity. It seems reasonable to propose
that the differences in activity shown between cyclohexene and
tetrahydrofuran must be related to their solubilities into the ionic
liquid, since both C–H bonds display similar bond dissociation
energies.21
(3)
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of [bmim]Cl and NaCl in the IL, the latter being more associated
in this medium.
relative to partially deuterated solvent peaks but are reported
relatively to tetramethylsilane.
To account for the above experimental results, we could invoke
to the formation, in the reaction medium, of a neutral complex
of composition TpmxCuCl, that would be in equilibrium with the
cationic complex, the real catalytic species (Scheme 2). The relative
amount of chloride ion in solution would govern the amount of
such catalytic species. It is also worth mentioning that theoretical
studies have shown that chloride ions in ionic liquids can interact
with the medium, and therefore a competition with the above
equilibrium could also occur.23 When Tpm*,BrCuI was employed as
the catalyst precursor, in methylene chloride as the solvent, minor
conversions (below 15–20%) were observed. This is in good accord
with the cationic species [TpmxCu]+ being the real catalytic species:
the neutral precursor delivers small amounts of the cationic species
through the equilibrium shown in Scheme 2.
General catalytic procedure for aziridination reactions
The catalyst (0.025 mmol, 5 mol%) was dissolved in [bmim]PF6
˚
(3 mL) in an ampoule containing 4 A activated molecular sieves.
The olefin (2.5 mmol) and dried chloramine-T (0.5 mmol) were
added to the solution under a nitrogen atmosphere. The reaction
mixture is stirred at room temperature for 24h. The product was
extracted with diethyl ether (4 ¥ 4 mL) and the organic solvent
was evaporated to dryness to yield a crude solid residue which
1
was investigated by H NMR to determine the conversion using
trimethyl vinyl silane (0.5 mmol) as internal standard. The crude
residue was purified by column chromatography on silica gel
using petroleum ether/ethyl acetate (5:1) as eluent to afford the
aziridination product. Aziridines and allylic amidation products
were characterised by comparison of their spectroscopic data with
those previously reported.
General catalytic procedure for amidation reactions of cyclic ethers
The catalyst (0.025 mmol, 5 mol%) was dissolved in [bmim]PF6
(3 mL) in an ampoule containing molecular sieves. The substrate
(2 mL) and dried chloramine-T (0.5 mmol) were added to the solu-
tion under a nitrogen atmosphere. The reaction mixture was stirred
at room temperature for the specified time period (see Table 5).
Diethyl ether (4 mL) was added to separate the organic layer.
The ionic liquid phase was washed with diethyl ethyl (3 ¥ 4 mL).
The combined organic solution was evaporated to dryness and the
residue was investigated by 1H NMR to determine the conversion
(using trimethyl vinyl silane as internal standard). We observed
that the amidation products decomposed on chromatography. To
avoid decomposition, the crude residue was treated with hexane
and filtered to remove p-toluenesulfonamide. The filtrates were
evaporated and dried under vacuum to yield the pure amidation
products. Compounds were characterised by comparing their 1H
NMR with the previously reported data.
Scheme 2 Role of chloride ion.
Conclusions
In summary, we have shown that cationic tris(pyrazolyl)methane
copper(I) complexes can be employed as catalysts in nitrene
transfer reactions with readily available chloramine-T as nitrene
source and the ionic liquid [bmim]PF6 as the solvent. This protocol
has proved to be efficient for the preparation of aziridines and
C–H bond amidation products. The recovery and reuse of the
catalytic system have been successfully demonstrated in both type
of transformations. The use of [bmim]PF6 as reaction medium
in amidation reactions is unprecedented and opens up the scope
of application of ionic liquids as alternative solvents in catalytic
processes.
Study of reuse of catalytic systems
The aziridination or amidation reaction was performed as de-
scribed above. Once the product was extracted with diethyl ether
the ionic liquid phase was dried under vacuum and charged with
a second load of the substrate and chloramine-T (0.5 mmol)
under nitrogen. The mixture was stirred at room temperature
for a specific reaction period. After this time, the products were
extracted with diethyl ether and the remaining solution of the
catalyst in the IL was reused following the same procedure (see
Tables 2 and 3).
Experimental
General
Acknowledgements
All manipulations were carried out under a nitrogen atmosphere
using standard Schlenk techniques. Solvents and hydrocarbons
were dried and degassed before use. Chloramine-T trihydrate was
purchased from Aldrich and was dried by heating at 60 ◦C in
We thank the MEC (Proyecto CTQ2005-00324/BQU) and the
Junta de Andaluc´ıa (Proyecto P07-FQM-02745) for financial
support. IC thanks MEC for a research fellowship.
15
an oil bath under vacuum for 6 h. Ionic liquid [bmim]PF6 and
the complexes [TpmXCu(NCMe)]BF4 were prepared according
24
Notes and references
to literature procedures. NMR spectra were recorded on a Varian
Mercury 400 MHz spectrometer. 1H chemical shifts were measured
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24 (a) TpmX = Tpm* (see Fig. 1): D. L. Reger, J. E. Collins, A. L. Rheingold
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Tpm*,Br: M. Cvetkovic, S. R. Batten, B. Moubaraki, K. S. Murray and
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734 | Dalton Trans., 2009, 730–734
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