Heterogeneous Olefin Aziridination Reactions Catalyzed by Polymer-Bound Tris(triazolyl)methane Copper Complexes

Article abstract of DOI:10.1002/ejic.202100526

Efficient olefin aziridination has been achieved with a tris(triazolyl)methane copper catalyst supported onto polystyrene. Aryl, alkyl and methoxycarbonyl-substituted olefins are converted into N-tosylaziridines in good to high yields. The solid catalyst is readily separated by filtration and recycled, allowing its reuse with no significant loss of the catalytic activity.

Full text of DOI:10.1002/ejic.202100526

European Journal of Inorganic Chemistry
Accepted Article
Title: Heterogeneous olefin aziridination reactions catalyzed by
polymer-bound tris(triazolyl)methane copper complexes
Authors: M. M. Díaz-Requejo, Pedro J. Perez, Manuel R. Rodríguez,
Pablo Etayo, Miquel A. Pericas, and Francisco Molina
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To be cited as: Eur. J. Inorg. Chem. 10.1002/ejic.202100526
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1/2020

European Journal of Inorganic Chemistry
10.1002/ejic.202100526
FULL PAPER
Heterogeneous olefin aziridination reactions catalyzed by
polymer-bound tris(triazolyl)methane copper complexes
[
a]
[a]
[b]
[b,c]
[a]
Manuel R. Rodríguez, Francisco Molina, Pablo Etayo, Miquel A. Pericàs,* Pedro J. Pérez,* M.
Mar Díaz-Requejo*[a]
Efficient olefin aziridination has been achieved with a tris(triazolyl)methane copper catalyst supported onto polystyrene. Aryl,
alkyl and methoxycarbonyl-substituted olefins are converted into N-tosylaziridines in good to high yields. The solid catalyst
is readily separated by filtration and recycled, allowing its reuse with no significant loss of the catalytic activity.
Introduction
Catalytic olefin aziridination constitutes a powerful tool for the
generation of carbon-nitrogen bonds from an array of available
alkenes, the reported catalytic systems allowing the achievement
of quantitative yields as well as excellent diastereo- and/or
[
1]
enantioselectivities. First developed as soluble catalysts, the
need of a readily separation from reactants/products led to the
use of their heterogeneous counterparts, with a significant degree
of success. Several supports have been employed to anchor the
successful molecular catalysts (Scheme 1), such as zeolites,[2]
silica[ 3 ] or carbon, either amorphous[ 4 ] or as nanotubes.[ 5 ]
Interestingly, the fixation of such catalysts onto polymeric
materials is rare. To the best of our knowledge, there is one
unique report for the use of a polymeric support in a catalytic olefin
aziridination system, a ruthenium-based system reported by
Gallo.[6] Regarding copper as the catalysts, there is only one
example in which the metal complex is attached to a polymeric
material, and induces the olefin aziridination reaction.[7]
We have previously described[8] the use of polystyrene-linked
tris(triazolyl)methanecopper(I) cationic catalysts for the transfer of
carbene units from diazo compounds to several saturated and
unsaturated substrates in batch and flow. Herein we report that a
Cu(I) complex attached to polystyrene catalyzes the transfer of
the NTs (Ts = p-toluenesulfonyl) group from PhI=NTs to olefins
bearing aryl, alkyl or carboxyl substituents, in good to high yields
and with acceptable ratios of recovery and recycling.
Scheme 1. The heterogeneous olefin aziridination reaction.
Results and Discussion
The soluble catalyst
[8]
In our previous work on carbene transfer reactions we employed
polystyrene-supported copper complex bearing
a
a
[
9 ]
tris(triazolyl)methane (TTM) ligand. The metal complex was
cationic in nature, an acetonitrile ligand completing the
coordination sphere, and PF
). Since we have now targeted such material as catalyst for olefin
aziridination reactions, the catalytic capabilities of the molecular
catalyst, i. e., [(TTM)Cu(NCMe)]PF (1) are needed for further
6
acting as the counterion (Scheme
1
6
comparison with the heterogeneous counterpart. This yet
unreported compound has been prepared by reaction of
[
a]
M. R. Rodríguez, F. Molina, Prof. Dr. M. M. Díaz-Requejo, Prof. Dr.
P. J. Pérez
Laboratorio de Catálisis Homogénea, Unidad Asociada al CSIC,
CIQSO-Centro de Investigación en Química Sostenible and
Departamento de Química, Universidad de Huelva, Campus de El
Carmen 21007 Huelva, Spain.
E-mail: perez@dqcm.uhu.es mmdiaz@dqcm.uhu.es
URL: www.uhu.es/ciqso
Prof. Dr. M. A. Pericàs, Dr. Pablo Etayo
tris(triazolyl)methane ligand with [Cu(NCMe)
4
]PF
6
at room
temperature (Scheme 2) in acetonitrile. After workup, solid
material was obtained for which NMR spectra showed somewhat
broad resonances for three equivalent azolyl rings, as well as
coordinated acetonitrile, as expected for complex 1. Interestingly,
HRMS studies showed the presence of two species in acetonitrile
[
b]
c]
Institute of Chemical Research of Catalonia (ICIQ), The Barcelona
Institute of Science and Technology (BIST), Avda. Països Catalans
solutions, with masses corresponding to [(TTM)Cu(NCMe)]PF
621, M) and [(TTM)CuPF6, 580, M). A plausible association-
dissociation equilibrium of the acetonitrile ligand would explain
such finding. The generation of the [(TTM)CuPF ] units was
6
(
16, 43007, Tarragona, Spain. E-mail: mapericas@iciq.es
URL: www.iciq.es
Prof. Dr. M. A. Pericàs
6
[
Departament de Química Inorgànica i Orgánica, Universitat de
Barcelona, 08028 Barcelona, Spain.
assessed when attempting to crystallize the copper complex 1
from dichloromethane-hexane solutions at room temperature.
1
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European Journal of Inorganic Chemistry
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Colorless crystals of a new dinuclear complex 2 (Scheme 2)
formed, its structure being determined by X-ray studies (Figure 1).
Its formation is explained straightforwardly due to the lability of the
remaining of the initial nitrene precursor PhI=NTs not converted
into aziridines was consumed upon forming TsNH , a common
2
byproduct in these transformations. Furthermore, the use of 2
instead of 1 showed no difference in the catalytic outcome.
acetonitrile ligand in 1 and the association of two (TTM)CuPF
6
units.
The molecules of complex 2 consist of two (TTM)Cu units in which
each copper center is bonded to two azolyl groups of one TTM
ligand and a third azolyl group from the other TTM fragment. This
Table 1. Aziridination of styrene using 1 as catalyst.[a]
pattern is identical to that previously described for the [TpMe2Cu]
2
complex,[10] the main distances and angles being in the expected
Entry
% [Cu]
[Cu]:PhI=NTs:Styrene
1:200:200
1:100:200
1:50:200
% Aziridine
1
0.5
1
57
75
2
3
4
5
6
2
96
5
1:20:200
>98
90
5
1:20:20
20
1:5:50
>98
[
a] Conditions: 0.0075 mmol of 1, 0.15 mmol (20 equiv) of PhI=NTs, 4
1
mL dichloromethane. Yields determined by H NMR of the reaction crude
using 1,3,5-trimethoxybenzene as internal standard. TsNH
for 100% of initial PhI=NTs not converted into aziridine.
2
accounted
Scheme 2. Synthetic routes to complexes 1 and 2.
ranges. The intermetallic Cu-Cu distance of 2.7091(11) Å is very
close to the sum of two van der Waals radii for copper (2 x 1.40 Å
After the initial screening with styrene, we employed less
activated olefins as substrates (Scheme 3). Thus, 1-hexene, cis-
cyclooctene and methyl methacrylate were tested in their
reactions with PhI=NTs catalyzed by 1. The linear, non-activated
alkene was converted into the corresponding aziridine in 66%
yield, whereas for the cyclic olefin a 93% yield was found. The
olefin bearing the electron-withdrawing methoxycarbonyl group
was also aziridinated in 80% yield. It is worth mentioning that the
latter transformation is challenging, and that only few copper
catalysts for aziridination reaction are also active for deactivated
olefins.[
=
2.80 Å), and therefore the copper-copper interaction is, at best,
very weak.
The availability of complex 1 led us to explore its use as catalyst
in the olefin aziridination reaction. A dichloromethane solution of
1
and styrene was prepared, and PhI=NTs was then added to the
stirred solution in one portion. The nitrene precursor slowly
dissolved, the final reaction time being marked by the complete
disappearance of solid PhI=NTs in the mixture. Several
11]
[
Cu]:[PhI=NTs]:[styrene] ratios were screened, with NMR analysis
of the reaction crude providing the data shown in Table 1. With 2-
% catalyst loading, referred to the nitrene precursor, very high
5
yields of the aziridine were obtained (entries 3,4), and a
remarkable 90% yield was found employing equimolar amounts
of PhI=NTs and styrene (entry 5). On the other hand, lower
Scheme 3. Aziridination of alkyl-, aryl- and methoxycarbonyl-substituted
olefins. Reaction conditions as in Table 1 with 200 equiv of olefin.
The heterogeneous system
Once demonstrated that complex 1 promotes the transfer of
nitrene units from PhI=NTs to olefins, we moved onto the
Figure 1. Molecular structure for the molecules of 2. Hydrogen atoms have
been omitted. Relevant distances (Å) and angles (º) (see Supporting
heterogeneous system. The preparation of the solid catalyst has
[8,9]
been previously reported,
and it is briefly commented of the
Information for full data): Cu(1)-Cu(1)#1
.926(4); Cu(1)-N(4)#1, 2.014(4); Cu(1)-N(7)#1, 1.986(4); N(1)-Cu(1)-
Cu(1)#1, 109.04(11); N(1)-Cu(1)-N(4)#1, 129.71(15).
2.7091(11);
Cu(1)-N(1),
sake of clarity. A polystyrene material bearing TTM ligand was
reacted with the copper source at room temperature in acetonitrile
1
(
Scheme 4). After workup, all the TTM sites are covered by Cu(I)
catalyst loadings were not that productive (entries 1,2). The
2
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European Journal of Inorganic Chemistry
10.1002/ejic.202100526
FULL PAPER
ions, (31.4 mg Cu/g polymer) and the catalyst 1het is ready to be
used.
catalyst loading (20%) to disfavor the effect of the workup, five
consecutive runs provided the exclusive formation of the aziridine
Table 2. Comparison of the catalytic activity of 1 and 1het in the olefin
aziridination reaction[
a]
%
Aziridines
Catalyst 1het
Entry
Olefin
Catalyst 1
1
styrene
>989
63
>98
61
2
3
4
1-hexene
cis-cyclooctene
Methyl methacrylate
93
96
Scheme 4. Preparation of the heterogeneous catalyst 1het
.
80
70
[
a] See experimental for details
We first investigated the use of 1het as catalyst for the benchmark
reaction, i. e., the aziridination of styrene. Scheme 5 shows the
in >98% yield, as inferred from NMR studies of the reaction crude
after each run. Therefore, the anchoring of the soluble catalysts
onto the polymeric material does not affect to its catalytic behavior,
while allowing easy recycling and reuse.
results
het]:[PhI=NTs]:[styrene] ratio of 1:20:200, and for three
consecutive recyclings. The first cycle originated aziridines in ca.
8% yield. After filtration, washing of the solid and the addition of
from
a
series
of
experiments
using
a
[
1
9
Given the activated nature of the olefinic bond in styrene, we
wondered if the observed behavior of 1het could be extended to
the less activated olefins already tested in solution. Toward that
end, we run experiments with the three olefins 1-hexene, cis-
cyclooctene and methyl methacrylate with the results shown in
Table 2, which also contains those achieved under homogeneous
conditions for a direct comparison. There is no doubt that the
copper center promotes the aziridination reaction with similar
efficiencies no matter the reaction occurs in solution or under
heterogeneous conditions. Only the less activated olefin methyl
methacrylate shows a slight decrease in aziridination yield,
whereas the others verify the conversion into aziridines without
any effect of the nature (homogeneous or heterogeneous) of the
catalyst employed.
fresh solvent and reactants, the next two cycles gave nearly
identical results (97 and 94%), values also indistinguishable from
that obtained with the soluble catalyst. Considering those two
Conclusion
We have developed a cationic tris(triazolyl)methanecopper(I)
catalyst anchored onto a polymeric material (polystyrene) which
induces the efficient conversion of aryl-, alkyl or alkoxycarbonyl-
substituted olefins into aziridines upon transfer of the NTs moiety
from PhI=NTs. This represents the first example of a polymer-
immobilized catalyst suitable for the aziridination of olefins
covering
a range from electron-rich to electron-deficient
substrates. Interestingly, the heterogeneous catalyst can be
separated by filtration and reused without any reactivation
treatment, showing essentially unmodified activity over three
reaction cycles. Also noteworthy, the catalytic activity of the
immobilized catalyst is identical to that observed in solution by the
soluble, molecular counterpart.
Scheme 5. Styrene aziridination using 1het as catalyst: Top: 5% catalyst
loading. Bottom: 20% catalyst loading.
Experimental Section
steps of filtering and washing, it is reasonable assuming that the
activity loss is due to the workup. When employing a larger
General Information: All air- and moisture-sensitive manipulations were
carried out with standard Schlenk techniques under nitrogen atmosphere
3
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European Journal of Inorganic Chemistry
10.1002/ejic.202100526
FULL PAPER
or in a glovebox (MBRAUN UNILAB). Solvents were purchased from
commercial sources, dried by distillation under nitrogen atmosphere using
the suitable drying agent and deoxygenated immediately before their use.
Reagents were acquired from suppliers and used without any further
purification. The TTM ligand,[9] the supported complex 1het[9] and the
nitrene precursor PhI=NTs[12] were synthesized according to literature
procedures. NMR spectra were recorded on the Agilent 400MR
spectrometer as solutions at 298 K and referenced to residual solvent
peaks. High Resolution Mass Spectroscopy (HRMS) experiments were
carried out at CITIUS- Universidad de Sevilla.
by the addition of PhI=NTs (372 mg, 1 mmol) in one portion. After 12h, the
mixture was filtered off. The solid was washed three times with
deoxygenated DCM and dried under a N stream before it was charged
2
with solvent and reactants for a next catalytic cycle. The liquid fractions
were collected, and volatiles were removed before the crude was
1
investigated by H NMR as described above for the homogeneous case.
Conflict of Interest
Synthesis of [(TTM)Cu(NCMe)]PF
ligand (0.15 mmol, 78 mg), Cu(MeCN)
6
(1). Tris(triazolyl)methane TTM
PF (0.15 mmol, 56 mg) were
The authors declare no conflict of interest.
4
6
dissolved in acetonitrile (3 mL) under nitrogen. After 12 h of stirring,
volatiles were removed under reduced pressure and the resulting colorless
oil was re-dissolved in dichloromethane (4 mL). Solvent evaporation under
vacuum provided a white solid of complex 1 (98 mg, 90%). Crystallization
of this solid from a 5:1 mixture of dichloromethane:hexane at room
temperature gave colorless crystals of complex 2.
Supporting Information
NMR and HRMS data for complex 1 and crystallograpic data for
complex 2.
1H NMR (400 MHz, CD
CN): δ 7.79 (br s, 3H), 7.31 (m, 15H), 5.51 (br s,
3
1
6
1
H), 3.03 (br s, 3H). 13C{ H} NMR (100 MHz, CD
30.0, 129.6, 129.1, 126.0, 55.1, 53.3. HRMS (HESI) [M-PF
30CuN10O: 621.1895. Found: 621.1886.
A second peak is observed in the HRMS experiment matching with
O: 580.1629. Found: 580.1614. Crystallographic data for
3
CN): δ 148.4, 136.0,
]+ (performed
6
in acetonitrile) calculated for C31
H
Acknowledgements
29 9
C H27CuN
We thank financial support from the Ministerio de Ciencia e
Innovación (CTQ2017-82893-C2-1-R), Junta de Andalucía (P18-
RT-1536 and PY20-00348) and Universidad de Huelva
complex 2 is available at the Cambridge Crystallographic Data Centre
under code CCDC 2089200.
(
P.O.Feder UHU-1260216 and UHU-1254043)
Catalytic experiments. (a) Homogeneous phase. Complex 1 (5.5 mg,
0
.0075 mmol) was dissolved in deoxygenated DCM (4 mL). The olefin was
then added in the desired excess, followed by the addition of PhI=NTs
55.8 mg, 0.15 mmol) in one portion. After 4h of stirring at room
(
Keywords: Catalyst anchoring • Heterogeneous catalysis •
Nitrene transfer • Olefin aziridination • Polymers
temperature, the solvent was removed under reduced pressure and the
1
crude was analyzed by
H
NMR spectroscopy using 1,3,5-
trimethoxybenzene as internal standard. The aziridines were identified
upon comparison with literature reports.[11] (b) Heterogeneous phase.
Complex 1het (100 mg of polymer, 0.05 mmol of Cu) was dissolved in
deoxygenated DCM (6 mL). Addition of the olefin (10 mmol) was followed
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4
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European Journal of Inorganic Chemistry
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Entry for the Table of Contents
%
Aziridine
00
1
80
60
4
0
20
0
1
2
3
4
5
#
cycles
A polymer-bounded copper complex efficiently catalyzes the olefin aziridination reaction. Catalyst activity is maintained upon
separation and reuse.
Institute and/or researcher Twitter usernames: @pedrojperez65 @CIQSO_UHU
5
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