Non-precious metal carbamates as catalysts for the aziridine/CO2coupling reaction under mild conditions

Article abstract of DOI:10.1039/d1dt00525a

The catalytic potential of a large series of easily available metal carbamates (based on thirteen different non-precious metal elements) was explored for the first time in the coupling reaction between 2-aryl-aziridines and carbon dioxide, working under solventless and ambient conditions and using tetraalkylammonium halides as co-catalysts. The straightforward synthesis of novel [NbCl₃(O₂CNEt₂)₂],Nb^Cl, and [NbBr₃(O₂CNEt₂)₂],Nb^Br, is reported. The niobium complexNb^Cl, in combination with NBu₄I, emerged as the best catalyst of the overall series to convert aziridines with smallN-alkyl substituents into the corresponding 5-aryl-oxazolidin-2-ones.

Full text of DOI:10.1039/d1dt00525a

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Non-precious metal carbamates as catalysts for
the aziridine/CO2 coupling reaction under mild
conditions†
Cite this: Dalton Trans., 2021, 50,
5351
Giulio Bresciani,a,b Stefano Zacchini, b,c Fabio Marchetti a,b and
a,b
Guido Pampaloni
*
The catalytic potential of a large series of easily available metal carbamates (based on thirteen different
non-precious metal elements) was explored for the first time in the coupling reaction between 2-aryl-
aziridines and carbon dioxide, working under solventless and ambient conditions and using tetraalkylam-
monium halides as co-catalysts. The straightforward synthesis of novel [NbCl3(O2CNEt2)2], NbCl, and
[NbBr3(O2CNEt2)2], NbBr, is reported. The niobium complex NbCl, in combination with NBu4I, emerged as
the best catalyst of the overall series to convert aziridines with small N-alkyl substituents into the corres-
ponding 5-aryl-oxazolidin-2-ones.
Received 16th February 2021,
Accepted 16th March 2021
DOI: 10.1039/d1dt00525a
rsc.li/dalton
liquids,28 metal complexes,22,29,30 and metal–organic
frameworks.20,31 Despite many efforts to overcome the high
Introduction
Carbon dioxide is an inexpensive, largely available, and non- energy barrier of the aziridine/CO2 coupling, it is still necessary
toxic C1 synthon for organic synthesis and, in this regard, it has to work under high pressure of carbon dioxide and/or high
received attention from both academia and industry.1–4 Despite temperature conditions.20,24,32–34 As a matter of fact, environ-
the high thermodynamic stability and relative kinetic inertness mentally benign catalytic systems working at ambient tempera-
of this molecule,5 CO2 fixation has been involved in a wide ture and pressure are relatively rare.19,22,29
number of processes to obtain valuable chemicals6–9 such as
Homoleptic metal N,N-dialkylcarbamates constitute a class
the synthesis of five-membered cyclic carbamates (2-oxazolidi- of molecular compounds of general formula [M(O2CNR2)n]m,
nones). 2-Oxazolidinones are important intermediates in which are typically accessible from a one-pot reaction between
organic chemistry and their core is incorporated in pharmaceu- an amine and
a metal halide precursor under a CO2
tically active substances, such as the antibiotics Linezolid10 and atmosphere35–37 (eqn (1)). This synthesis is rather simple and
Tedizolid.11 Several synthetic strategies to access 2-oxazolidi- possesses a general character, so that metal carbamates have
nones have been evaluated making use of CO2, to be combined been prepared for almost all the transition metals and for
with either unsaturated compounds,12–14 amino-alcohols,15–17 several main group elements.
amino-epoxides18,19 or aziridines.20–22 The cycloaddition of
carbon dioxide to aziridines is one of the most attractive
MXn þ n CO2 þ 2n NHR2 ! ½MðO2CNR2Þnꢀ þ n ðNH2R2ÞX ð1Þ
methods (Scheme 1), and it generally requires both a catalyst
The monoanionic carbamato ligand is relatively robust38 and,
and a co-catalyst, acting as Lewis acid and nucleophilic activa-
according to the cases, it may act as monodentate, bidentate or
tor, respectively.22,23 In the last decade, a considerable diversity
bridging, adapting its coordination fashion to the environment
of catalysts has been investigated to promote this reaction, such
and offering the possibility for the generation of vacant coordi-
as N-heterocyclic compounds,24,25 superbases,26,27 ionic
nation site(s). Moreover, it has been demonstrated that the car-
aDipartimento di Chimica e Chimica Industriale, University of Pisa, Via G. Moruzzi
13, I-56124 Pisa, Italy. E-mail: guido.pampaloni@unipi.it
bCIRCC, via Celso Ulpiani 27, I-70126 Bari, Italy
cDipartimento di Chimica Industriale “Toso Montanari”, University of Bologna, Viale
Risorgimento 4, I-40136 Bologna, Italy
†Electronic supplementary information (ESI) available: NMR and IR spectra of
new niobium complexes and of organic compounds. CCDC 2062321 (NbCl) and
2062322 (NbBrCl). For ESI and crystallographic data in CIF or other electronic
Scheme 1 General synthesis of 2-oxazolidinone from the aziridine/
carbon dioxide coupling (R, R’ = alkyl or aryl).
format see DOI: 10.1039/d1dt00525a
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bamato ligand is prone to exchange its {CO2} fragment with to bidentate carbamato ligands,35,36 suggesting that the two
external carbon dioxide, thus enabling CO2 pre-activation.39,40 compounds are isostructural. The NMR spectra (CDCl3 solu-
Recently, our group described the use of a range of metal carba- tion) contain a single set of signals, a diagnostic 13C resonance
mates as active catalysts in carbon dioxide activation reactions, accounting for the bidentate carbamato ligands has been
such as the carboxylation of terminal alkynes,41 the cyclo- detected at ca. 169 ppm. It should be noted that the 13C reso-
addition to propargyl alcohols,42 and the coupling with nance related to the carbamato function is very sensitive to the
epoxides.40,43 Note that mixed halido-carbamato complexes of mode of coordination, in which terminal coordination (mono-
high valent metals may provide enhanced catalytic activity com- dentate carbamate) usually determines a significant upfield
pared to the corresponding homoleptic compounds.43,44
shift with respect to the bridging coordination (bidentate
Here, we present a screening study on the use of various carbamate).35,45,46 For instance, [Nb(O2CNMe2)5] and [Ta
metal carbamates, including two unprecedented hybrid (O2CNMe2)5], comprising both monodentate and bidentate
niobium(V) halido-carbamates, as catalysts for the reaction ligands, show 13C NMR signals fall at ca. 157 ppm and
between aziridines and CO2. Tetrabutylammonium halides 166 ppm in both complexes.47 The 93Nb spectrum of NbCl exhi-
have been selected as co-catalysts, according to what is fre- bits a broad peak centred at −482 ppm,48 whereas a clear 93Nb
quently described in the literature. All the reactions have been spectrum could not be recorded for NbBr, presumably due to
conducted under fixed ambient conditions, i.e. room tempera- the ligand exchange phenomenon.49 The X-ray molecular
ture and atmospheric pressure of carbon dioxide, in the light structure of NbCl was elucidated by single-crystal X-ray diffrac-
of their favourable incidence in terms of sustainability.
tion studies, and the view of the molecule is given in Fig. 1.
The compound is mononuclear, and the structural motif is in
accordance with the spectroscopic features: a niobium(V)
centre is coordinated to three chlorides in a pseudo-mer
arrangement, and two chelating carbamato ligands. The
overall geometry can be described as a pentagonal bipyramid,
with the two carbamates and one halide occupying equatorial
positions, and the other two halides in apical positions. A
search in the Cambridge Crystallographic Data Centre showed
that ca. 12% of the deposited structures containing niobium
display a coordination number of seven, whereas the most
common coordination number is six (37% of the entries).
Attempts to crystallize [NbBr3(O2CNEt2)2] afforded red crys-
tals of the mixed-halide derivative [NbX3(O2CNEt2)2], NbBrCl
(Fig. 2), probably due to some chloride-containing impurity in
commercial NbBr5.
Results and discussion
Synthesis of new niobium halido/carbamato complexes
With a view to the possible higher catalytic performance furn-
ished by mixed halido-carbamato complexes (see Introduction),
we synthesized novel NbV complexes [NbCl3(O2CNEt2)2], NbCl,
and [NbBr3(O2CNEt2)2], NbBr. The synthesis of NbCl and NbBr is
non-classical (see eqn (1)) and consists of the treatment of com-
mercial metal pentahalides with sodium carbamate in dichloro-
methane solution (eqn (2)). The metal products were separated
from the side-product sodium chloride by filtration, isolated in
67–85% yields and then fully characterized by elemental ana-
lysis, and IR and multinuclear NMR spectroscopy.
The structures reported in this paper resemble that pre-
viously reported for [NbCl3(S2CNEt2)2],49–51 including the
mutual trans arrangement of the ethyl groups.
NbX5 þ 2 NaO2CNEt2 ! ½NbX3ðO2CNEt2Þ2ꢀ þ 2 NaX
ðX ¼ Cl; NbCl; X ¼ Br; NbBrÞ
ð2Þ
Besides the unprecedented NbCl and NbBr, a large selection
of known metal carbamates has been investigated in the
The IR spectra (solid state) of both NbCl and NbBr display a
characteristic intense absorption at ca. 1620 cm−1, ascribable
Fig. 1 Molecular structure of [NbCl3(O2CNEt2)2], NbCl, with labelling. Displacement ellipsoids are at the 50% probability level. Main bond distances
(Å) and angles (°): Nb(1)–Cl(1) 2.3754(5), Nb(1)–Cl(2) 2.3392(5), Nb(1)–Cl(3) 2.3420(5), Nb(1)–O(1) 2.1149(12), Nb(1)–O(2) 2.0802(12), Nb(1)–O(3)
2.0998(12), Nb(1)–O(4) 2.0764(11), C(1)–O(1) 1.300(2), C(1)–O(2) 1.3016(19), C(1)–N(1) 1.309(2), C(6)–O(3) 1.303(2), C(6)–O(4) 1.299(2), C(6)–N(2)
1.309(2), O(1)–Nb(1)–O(2) 62.46(4), O(3)–Nb(1)–O(4) 62.72(4), Cl(1)–Nb(1)–Cl(3) 175.146(16), Cl(1)–Nb(1)–Cl(2) 91.391(15), Cl(2)–Nb(1)–Cl(3) 93.461
(16), O(1)–C(1)–O(2) 113.47(14), O(3)–C(6)–O(4) 113.29(14), sum at C(1) 360.0(2), sum at C(6) 360.0(2), sum at N(1) 360.0(2), sum at N(2) 360.0(2).
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Fig. 2 Molecular structure of [NbX3(O2CNEt2)2], NbBrCl, with labelling. Displacement ellipsoids are at the 50% probability level. Displacement ellip-
soids are at the 50% probability level. The positions labelled X(1), X(2) and X(3) are disordered Br/Cl. Main bond distances (Å) and angles (°): Nb(1)–X
(1) 2.4637(16), Nb(1)–X(2) 2.4726(17), Nb(1)–X(3) 2.5017(15), Nb(1)–O(1) 2.111(7), Nb(1)–O(2) 2.077(70), Nb(1)–O(3) 2.104(7), Nb(1)–O(4) 2.085(7), C(1)–
O(1) 1.302(12), C(1)–O(2) 1.288(12), C(1)–N(1) 1.306(13), C(6)–O(3) 1.293(13), C(6)–O(4) 1.307(13), C(6)–N(2) 1.298(12), O(1)–Nb(1)–O(2) 62.0(3), O(3)–
Nb(1)–O(4) 62.7(3), X(1)–Nb(1)–X(3) 176.63(6), X(1)–Nb(1)–X(2) 90.54(5), X(2)–Nb(1)–X(3) 92.75(5), O(1)–C(1)–O(2) 112.8(8), O(3)–C(6)–O(4) 114.0(8),
sum at C(1) 360.0(16), sum at C(6) 360.0(16), sum at N(1) 359.7(15), sum at N(2) 360.0(15).
Fig. 3 Structures of metal carbamates investigated in this work (blue: homoleptic compounds; red: mixed-ligand compounds). [M(O2CNEt2)3]n (M =
Fe, Al) and [Cu(O2CNEt2)2]n are polymeric materials.
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present work, including homoleptic compounds of group 4 [Si(O2CNEt2)4] and [Sn(O2CNMe2)4]. Interestingly, no traces of
and 5 metals, iron, cobalt, copper, silver, and main group another possible regioisomer, 3-methyl-4-phenyloxazolidin-2-
elements (Al, Sn, and Si), and mixed species with halido and/ one, were detected in the final mixtures. In alignment with
or oxido co-ligands of titanium(IV), niobium(V) and tungsten what was previously observed in other catalytic processes (see
(VI). The structures of these compounds are presented in Fig. 3. Introduction), halido-carbamato complexes exhibit superior
activity to their homoleptic counterparts (compare run 5 with
Catalytic studies
7 and 8 and run 1 with 2). This feature might be explained
based on a less steric hindrance around the metal centre in
the non-homoleptic species, favouring the coordination of the
substrate and thus accelerating the reaction. It must be noted
that this is the first time that compounds based on titanium,
tantalum, tungsten, silicon and tin are investigated as possible
catalysts for the aziridine/CO2 coupling reaction.
Following the preliminary screening and adopting the same
reaction conditions in terms of time, CO2 pressure and temp-
erature, a selection of active catalytic precursors was further
investigated in combination with different tetrabutyl-
ammonium halides (Table 2). The selected catalysts are good
candidates in view of a sustainable process, being based on
earth-abundant elements (Si or Sn) or nontoxic transition
metals (Nb, Ta).54 It was confirmed that a 5% molar concen-
tration of tetrabutylammonium bromide (TBAB) is necessary
to achieve satisfying yields of product, using [Si(O2CNEt2)4]
and [Sn(O2CNMe2)4]. On the other hand, the latter catalysts
showed much less performance when combined with tetra-
First, we tested the catalytic activity of carbamato complexes
(see Fig. 3) in the coupling reaction of 1-methyl-2-phenylaziri-
dine, which was selected as a model substrate, with carbon
dioxide from a balloon, in the absence of a solvent. The
systems were maintained at room temperature (ca. 25 °C) for
24 hours, and tetrabutylammonium bromide (TBAB) was
employed as a co-catalyst. The results of this preliminary study
are reported in Table 1.
It can be observed that late transition metal carbamates
and the aluminium species are not suitable to promote the
examined reaction. Complicated mixtures of products were
obtained in low amounts from [Fe(O2CNEt2)2], [Fe(O2CNEt2)3]
and [Ag(O2CNEt2)], presumably including low molecular oligo-
mers of polyamine, poly(urethane-amine) and piperazines.52,53
On the other hand, 3-methyl-5-phenyloxazolidin-2-one was
selectively generated in moderate to good yields when some
specific high valent metal complexes were employed. The TaV
homoleptic carbamate and the hybrid chlorido-carbamato
complexes of TiIV and NbV were found to be effective (61–68%
yields). Moreover, a comparable performance was exhibited by
Table 2 Catalytic conversion of 1-methyl-2-phenylaziridine into
3-methyl-5-phenyloxazolidin-2-one: effect of the co-catalyst
Table 1 Conversion of 1-methyl-2-phenylaziridine into 3-methyl-5-
phenyloxazolidin-2-one: effect of the catalyst
Entry
Catalyst
Co-catalyst (mol%)
Yield%
1
2
3
4
5
6
7
8
[Si(O2CNEt2)4]
[Si(O2CNEt2)4]
[Si(O2CNEt2)4]
[Si(O2CNEt2)4]
[Si(O2CNEt2)4]
[Si(O2CNEt2)4]
[Sn(O2CNMe2)4]
[Sn(O2CNMe2)4]
[Sn(O2CNMe2)4]
[Sn(O2CNMe2)4]
[Sn(O2CNMe2)4]
[Sn(O2CNMe2)4]
[NbCl3(O2CNEt2)2] (NbCl
NbCl
[NBu4]Cl (5)
[NBu4]Cl (2)
[NBu4]Br (5)
[NBu4]Br (2)
[NBu4]I (5)
[NBu4]I (2)
[NBu4]Cl (5)
[NBu4]Cl (2)
[NBu4]Br (5)
[NBu4]Br (2)
[NBu4]I (5)
[NBu4]I (2)
[NBu4]Br (5)
[NBu4]Br (2)
[NBu4]Cl (5)
[NBu4]I (5)
[NBu4]Br (5)
[NBu4]I (5)
[NBu4]Br (5)
[NBu4]I (5)
Traces
4
61
41
5
Entry
Catalyst
Yield%
1
2
3
4
5
6
7
8
[Ti(O2CNEt2)4]
[TiCl2(O2CNEt2)2]
[Zr(O2CNEt2)4]
[Hf(O2CNEt2)4]
[Nb(O2CNEt2)5]
5
61
47
19
41
14
68
61
66
33
13
5
3
Traces
4
65
47
2
3
68
58
70
86
61
53
66
[NbO(O2CNEt2)3]
[NbCl3(O2CNEt2)2] (NbCl
)
)
9
[NbBr3(O2CNEt2)2] (NbBr
[Ta(O2CNEt2)5]
[WOCl2(O2CNEt2)2]
[Fe(O2CNEt2)2]
[Fe(O2CNEt2)3]
[Co(O2CNEt2)2]
[Cu(O2CNEt2)2]
[Ag(O2CNEt2)]
10
11
12
13
14
15
16
17
18
19
20
9
10
11
12
13
14
15
16
17
18
)
4
4
4
NbCl
NbCl
[NbBr3(O2CNEt2)2] (NbBr
)
[Al(O2CNEt2)3]
[Si(O2CNEt2)4]
[Sn(O2CNMe2)4]
Traces
61
65
NbBr
[Ta(O2CNEt2)5]
[Ta(O2CNEt2)5]
Traces
Reaction conditions: aziridine 1.0 mmol, catalyst 1 mol%, co-catalyst Reaction conditions: aziridine 1.0 mmol, catalyst 1 mol%, T = 25 °C,
[NBu4]Br 5 mol%, T = 25 °C, pCO2 = 1 atm, t = 24 h. Yield determined pCO2 = 1 atm, t = 24 h. Yields determined by 1H NMR using 1,1,2,2-tet-
by 1H NMR using 1,1,2,2-tetrachloroethane as a standard.
rachloroethane as a standard.
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butylammonium chloride (TBAC) or tetrabutylammonium
iodide (TBAI).
According to the outcomes reported in Table 2,
[NbCl3(O2CNEt2)2] (NbCl), [Si(O2CNEt2)4] and [Sn(O2CNMe2)4],
Among the transition metal compounds, the hybrid in combination with the respective optimal co-catalyst, were
complex NbCl worked as the best catalyst. It was tested in com- explored for their activity in the coupling of carbon dioxide
bination with the three tetraalkylammonium salts (Table 2, with two other 1-alkyl-2-phenyl-aziridines (Table 3, runs 4–10).
runs 14–16), with the idea in mind that the possible exchange The catalytic system NbCl/TBAI emerged as the most perform-
between the chloride ligand and the external halide source55 ing one to convert 1-ethyl-2-phenylaziridine into the corres-
might induce favourable reaction pathways. As a matter of fact, ponding oxazolidinone in a regiospecific manner. Otherwise,
TBAB and TBAC afforded comparable yields (ca. 70%) of oxazo- the presence of an isopropyl N-substituent prevented the coup-
lidinone, and TBAI provided a better result (86% yield).
ling process under the employed mild conditions; some
unfavourable impact of the size of the nitrogen substituent on
the formation of five-membered oxazolidinones via CO2 fix-
ation was previously documented.21,25,32 The NbCl/TBAI combi-
nation was successfully employed to extend the synthesis to
3-methyl(ethyl)-5-aryloxazolidin-2-ones bearing different para-
substituents on the aryl ring (Table 3, runs 11–15).
Table 3 Catalytic conversion of 1-alkyl-2-arylaziridines into 3-alkyl-5-
aryloxazolidin-2-ones
Note that the straightforward synthesis of 3-methyl-5-arylox-
azolidinones from aziridines under sustainable conditions is
not a trivial task, and actually more expensive and elaborated
catalytic systems, as well as more drastic experimental con-
ditions, have been often reported to access the same products
as those efficiently obtained here. Selected examples from the
literature since 2018 are presented in Table 4. The preparation
of 5-(4-fluorophenyl)-3-methyloxazolidin-2-one via aziridine/
CO2 coupling is unprecedented.
Entry
X
R
Catalyst
Co-catalyst Yield%
1
2
3
4
5
6
7
8
H
H
H
H
H
H
H
H
H
H
Me [Si(O2CNEt2)4]
[NBu4]Br
[NBu4]Br
[NBu4]I
[NBu4]Br
[NBu4]Br
[NBu4]I
[NBu4]Br
[NBu4]Br
[NBu4]Br
[NBu4]I
[NBu4]I
[NBu4]I
[NBu4]I
[NBu4]I
[NBu4]I
61
65
86
31
34
53
Traces
Traces
Traces
5
50
41
58
49
Me [Sn(O2CNMe2)4]
Me [NbCl3(O2CNEt2)2] (NbCl
)
Et
Et
[Si(O2CNEt2)4]
[Sn(O2CNMe2)4]
NbCl
Et
iPr
iPr
iPr
iPr
[Si(O2CNEt2)4]
[Sn(O2CNMe2)4]
NbCl
Conclusions
9
10
11
12
13
14
15
NbCl
Metal carbamates are easily available and cost-effective com-
pounds which possess the ability to dynamically activate
carbon dioxide, and for this reason they have been successfully
employed as homogeneous catalytic precursors in some CO2
activation reactions. Herein, we describe a study aimed to
assess the potential of carbamates of a vast range of metal
elements in the widely investigated coupling reaction of aziri-
dines with carbon dioxide affording five-membered cyclic car-
Me Me NbCl
Me Et
Cl
Cl
F
NbCl
Me NbCl
Et
NbCl
Me NbCl
56
Reaction conditions: aziridine 1.0 mmol, catalyst 1 mol%, co-catalyst
5 mol%, T = 25 °C, pCO2 = 1 atm, t = 24 h. Yields determined by 1H
NMR using 1,1,2,2-tetrachloroethane as a standard.
Table 4 Selection of data from the literature for the coupling between carbon dioxide and 2-arylaziridines with N-methyl/ethyl substituents,
affording 5-aryl-2-oxazolidinones
Aziridine
Catalyst/cocatalyst
T (°C)
pCO2 (atm)
6
Reaction time (h)
6
Yield (%)
65
Ref.
33
RuVI imidoporphyrin complex/NBu4Cl
100
Carbodicarbene (CDC)
80
50
20
1
12
48
75
83
24
22
Di-europium complex/DBU
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bamates. A newly synthesized mononuclear niobium(V) mixed Synthesis and characterization of [NbX3(O2CNEt2)2] (X = Cl,
chlorido-carbamato complex emerged as the most active cata- NbCl; X = Br, NbBr)
lyst in combination with tetrabutylammonium iodide, con-
firming the previously observed tendency that non-homoleptic
carbamates based on high valent metals possess a superior
The preparation of NbCl is described in detail; NbBr was pre-
pared using an analogous procedure.
[NbCl3(O2CNEt2)2].
A suspension of NbCl5 (370 mg,
catalytic potential compared to the corresponding homoleptic
species. The regiospecific synthesis of 3-alkyl-5-aryloxazolidin-
2-ones was achieved in good yields from 2-aryl-aziridines with
small N-groups (methyl or ethyl). This process is featured by
environmentally benign conditions, i.e. the use of simple cata-
lysts based on Earth-abundant and/or non-toxic elements,
atmospheric pressure of carbon dioxide and room tempera-
ture. Remarkably, the syntheses of some of the products
obtained here have been recently described according to the
same reaction (aziridine/CO2 coupling), but usually require
drastic experimental conditions and/or more elaborated cata-
lytic systems, and are sometimes non-regioselective.22,24,56,57
On the other hand, the steric hindrance of the aziridine is a
limiting factor with reference to the present catalytic systems,
since a bulkier group on the nitrogen atom prevents the for-
mation of the related oxazolidinone.
1.37 mmol) in CH2Cl2 (30 mL) was treated with NaO2CNEt2
(390 mg, 2.80 mmol). The mixture was stirred for 20 hours at
room temperature, and then the resulting precipitate (NaCl)
was filtered off. The solvent was evaporated under vacuum, and
the residue was added to THF (10 mL). The tetrahydrofuran
solution was treated with hexane (30 mL), and the mixture was
cooled to ca. −30 °C. The obtained suspension was filtered, and
the solid was dried under vacuum affording NbCl (0.50 g, 85%
yield) as a moisture-sensitive yellow microcrystalline solid. Anal.
Calc. for C10H20Cl3N2NbO4: C, 27.8; H, 4.7; N, 6.5; Cl, 24.6.
Found: C, 28.0; H, 4.6; N, 6.6%; Cl, 24.3. IR (solid state): ˜υ/cm−1
= 2977w, 2942w, 1616vs (CvO), 1443w, 1410m, 1382w-m,
1310m, 1261w-m, 1197w-m, 1089m, 1020w-m, 979w, 839s,
799m, 776m-s. 1H NMR (CDCl3): δ/ppm = 3.59 (q, 2H, 3JHH = 7.2
3
Hz, CH2); 1.30 (t, 3H, JHH = 7.0 Hz, CH3). 13C{1H} NMR
(CDCl3): δ/ppm = 168.7 (CvO); 40.3 (CH2); 13.8 (CH3). 93Nb{1H}
(CDCl3): δ/ppm = −482 (s, Δν1/2 = 6 × 103 Hz). Crystals suitable
for X-ray analysis were obtained from a THF solution of 5
layered with hexane and stored at −30 °C for 5 days.
Experimental
Materials and methods
[NbBr3(O2CNEt2)2], NbBr. A highly moisture sensitive dark-
red solid, 67% yield from NbBr5 (390 mg, 0.79 mmol) and
NaO2CNEt2 (220 mg, 1.58 mmol). Anal. Calc. for
C10H20Br3N2NbO4: C, 21.3; H, 3.6; N, 5.0; Br, 42.4. Found: C,
21.5; H, 3.5; N, 5.1; Br, 42.1. IR (solid state): ˜υ/cm−1 = 2975w,
2934w, 1706w-m, 1621m-s (CvO), 1569s, 1443m-s, 1380s,
1314s-sh, 1260m, 1200m, 1097w-m, 1074m-s, 976w-m, 944vw,
Operations were conducted under a N2 atmosphere using stan-
dard Schlenk techniques; the reaction vessels were oven-dried
at 140 °C prior to use, evacuated (10−2 mmHg) and then filled
with N2. Carbon dioxide (99.99%) was purchased from Rivoira,
while organic reactants (TCI Europe, Merck or Strem) were
commercial products of the highest purity available, stored
under a N2 atmosphere as received. Solvents (Merck) were dis-
tilled before use over appropriate drying agents. Compounds
1
838m, 793m-s. H NMR (CDCl3): δ/ppm = 3.61 (m, 2H, CH2);
1.33 (m, 3H, CH3). 13C{1H} NMR (CDCl3): δ/ppm = 168.7
(CvO); 40.5 (CH2); 14.0 (CH3) ppm.
NaO2CNEt2,58 [M(O2CNEt2)4] (M
=
Ti, Zr, Hf),59
X-ray crystallography
[TiCl2(O2CNEt2)2],45 [M(O2CNEt2)5] (M = Nb, Ta),60 [NbO
(O2CNEt2)3],61 [WOCl2(O2CNEt2)2],58 [Fe(O2CNEt2)2],62 [Fe Crystals suitable for X-ray analysis were collected from THF
(O2CNEt2)3],63
(O2CNEt2)],66
[Co(O2CNEt2)2],64
[Al(O2CNEt2)3],43
[Cu(O2CNEt2)2],65
[Si(O2CNEt2)4],67
[Ag solutions layered with hexane (THF/hexane ratio 1 : 3). Crystal
[Sn data and collection details are reported in Table 5. Data were
(O2CNEt2)4]68 and 2-arylaziridines28 were prepared according recorded on a Bruker APEX II diffractometer equipped with a
to literature procedures. IR spectra (650–4000 cm−1) were PHOTON100 detector using Mo-Kα radiation. Data were cor-
recorded on a PerkinElmer Spectrum One FT-IR spectrometer, rected for Lorentz polarization and absorption effects (empiri-
equipped with a UATR sampling accessory. NMR spectra were cal absorption correction SADABS).71 The structures were
recorded at 298 K with a Bruker Avance II DRX 400 instrument solved by direct methods and refined by full-matrix least-
equipped with a BBFO broadband probe. 1H chemical shifts squares based on all data using F2.72 Hydrogen atoms were
(expressed in ppm) were referenced to the residual solvent fixed at calculated positions and refined using a riding model.
peaks69 while chemical shifts for 93Nb were referenced to exter- All non-hydrogen atoms were refined with anisotropic displa-
nal [NEt4]NbCl6. Yields of oxazolidinones were evaluated on cement parameters. The crystals of [NbBr2.06Cl0.94(O2CNEt2)2]
1H NMR spectra using 1,1,2,2-tetrachloroethane as an internal (NbBrCl) are racemically twinned with a refined Flack para-
standard. CHN analyses were performed on a Vario MICRO meter of 0.35(2).73 Within the structure of NbBrCl, the positions
cube instrument (Elementar). The halide content was deter- of the halide ligands are actually disordered Br/Cl. These have
mined by the Volhard method,70 by dissolving the solid been refined by applying dummy atom constraints (EADP and
samples in aqueous KOH at boiling temperature, followed by EXYZ lines in SHELXL) resulting in the following refined occu-
cooling to room temperature and addition of HNO3 up to pancy factor: X(1) = 0.703(9) Br and 0.397(9) Cl; X(2) = 0.577
acidic pH.
(10) Br and 0.423(9) Cl; X(3) = 0.769(9) Br and 0.231(9) Cl.
5356 | Dalton Trans., 2021, 50, 5351–5359
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3
Table 5 Crystal data and measurement details of [NbCl3(O2CNEt2)2]
(s, 3H, C6H4Me); 2.28 (m, 1H, CH); 1.92 (d, JHH = 3.2 Hz, 1H,
CH2); 1.63 (d, 3JHH = 6.7 Hz, 1H, CH2).
1-Ethyl-2-(p-tolyl)aziridine.
(NbCl) and [NbBr2.06Cl0.94(O2CNEt2)2] (NbBrCl
)
A
colourless liquid. 1H NMR
NbCl
NbBrCl
(CDCl3): δ/ppm = 7.18 (m, 4H, C6H4); 2.55–2.43 (m, 2H, NCH2);
2.38 (s, 3H, C6H4Me); 2.32 (m, 1H, CH); 1.92 (m, 1H, CH2); 1.67
(d, 3JHH = 6.7 Hz, 1H, CH2); 1.26 (t, 3JHH = 7.2 Hz, 3H, CH3).
1-Methyl-2-(4-chlorophenyl)aziridine. A colourless liquid. 1H
NMR (CDCl3): δ/ppm = 7.27–7.14 (m, 4H, C6H4); 2.48 (s, 3H,
Formula
FW
T, K
C10H20Cl3N2NbO4 C10H20Br2.06Cl0.94N2NbO4
431.54
522.90
100(2)
100(2)
λ, Å
0.71073
Monoclinic
P21/c
15.3038(15)
16.5965(16)
13.8932(14)
90.886(2)
3528.3(6)
8
0.71073
Monoclinic
P21
7.4915(14)
12.296(2)
9.6832(18)
95.837(8)
887.4(3)
2
Crystal system
Space group
a, Å
b, Å
c, Å
3
NMe); 2.24 (m, 1H, CH); 1.85 (d, 1H, CH2, JHH = 3.0); 1.63 (d,
1H, CH2, 3JHH = 6.7).
1-Ethyl-2-(4-chlorophenyl)aziridine. A colourless liquid. 1H
NMR (CDCl3): δ/ppm = 7.28–7.18 (m, 4H, C6H4); 2.45 (m, 2H,
NCH2); 2.29 (m, 1H, CH); 1.86 (d, 3JHH = 3.2, 1H, CH2); 1.68 (d,
3JHH = 6.7 Hz, 1H, CH2,); 1.21 (t, 3JHH = 7.2 Hz, 3H, CH3).
1-Methyl-2-(4-fluorophenyl)aziridine. A colourless liquid. 1H
NMR (CDCl3): δ/ppm = 7.17, 6.97 (m, 4H, C6H4); 2.47 (s, 3H,
β, °
Cell Volume, Å3
Z
Dc, g cm−3
μ, mm−1
F(000)
1.625
1.147
1744
1.957
5.458
510
Crystal size, mm
θ limits, °
Reflections collected
Independent
reflections
Data/restraints/
parameters
0.25 × 0.21 × 0.14
1.331–28.999
56 714
0.22 × 0.16 × 0.12
2.114–26.000
8480
3
NMe); 2.24 (m, 1H, CH); 1.84 (d, 1H, CH2, JHH = 3.4); 1.60 (d,
1H, CH2, 3JHH = 6.6).
9373 [Rint
=
3474 [Rint = 0.0625]
NMR characterization of 5-phenyl-oxazolidin-2-ones
3-Methyl-5-phenyloxazolidin-2-one.28,74 1H NMR (CDCl3): δ/
0.0376]
9373/0/369
3474/1/185
3
ppm = 7.41–7.26 (m, 5H, Ph); 5.48 (t, JHH = 8.6 Hz, 1H, CH);
Goodness on fit on F2 1.089
1.018
3.91, 3.44 (t, 3JHH = 8.6 Hz, 2H, CH2); 2.92 (s, 3H, CH3).
R1 (I > 2σ(I))
wR2 (all data)
0.0259
0.0553
0.0427
0.0940
3-Ethyl-5-phenyloxazolidin-2-one.28,74 1H NMR (CDCl3): δ/
Largest diff. peak and 0.380/−0.600
1.368/−0.998
3
ppm = 7.29–7.20 (m, 5H, Ph); 5.33 (t, JHH = 8.7 Hz, 1H, CH);
hole, e Å−3
3
3.80 (t, JHH = 8.7 Hz, 1H, CH2); 3.30–3.15 (m, 3H, CH2
NCH2); 1.04 (t, 3JHH = 7.2 Hz, 3H, CH3).
+
3-Isopropyl-5-phenyloxazolidin-2-one.28,74 1H NMR (CDCl3): δ/
Reactions between 2-aryl-aziridines and carbon dioxide
3
ppm = 7.38–7.28 (m, 5H, Ph); 5.43 (t, 1H, JHH = 8.4 Hz, CH);
4.12 (m, 1H, CHiPr); 3.84, 3.33 (t, 3JHH = 8.6 Hz, 2H, CH2); 1.18,
1.12 (d, 3JHH = 6.6 Hz, 6H, CH3iPr).
General details. The appropriate amounts of catalyst and co-
catalyst were introduced into a Schlenk tube. The tube was
evacuated with a vacuum pump and then filled with CO2. The
vacuum/CO2 sequence was repeated twice. Under a stream of
carbon dioxide, the selected aziridine (1 mmol) was added,
and the resulting mixture was stirred for 24 hours at room
temperature under atmospheric pressure of CO2 from a
balloon. An exact amount of 1,1,2,2-tetrachloroethane (0.2 mL)
was added, and an aliquot (ca. 0.1 mL) of the mixture was
mixed with CDCl3 (0.5 mL) in an NMR tube. Yields were deter-
mined by 1H NMR spectroscopy using 1,1,2,2-tetrachlor-
oethane as an internal standard.
3-Methyl-5-(p-tolyl)oxazolidin-2-one.30,74 1H NMR (CDCl3): δ/
ppm = 7.27–7.21 (m, 4H, C6H4); 5.46 (t, 3JHH = 8.4 Hz, 1H, CH);
3
3.90, 3.45 (t, JHH = 8.4 Hz, 2H, CH2), 2.94 (s, 3H, NCH3); 2.38
(s, 3H, C6H4CH3) ppm.
3-Ethyl-5-(p-tolyl)oxazolidin-2-one.28,74 1H NMR (CDCl3): δ/
ppm = 7.20–7.13 (m, 4H, C6H4); 5.39 (t, 3JHH = 8.3 Hz, 1H, CH),
3
3.83 (t, JHH = 8.6 Hz, 1H, CH2), 3.38–3.25 (m, 3H, CH2
+
3
NCH2), 2.30 (s, 3H, C6H4CH3), 1.12 (t, JHH = 7.3 Hz, 3H,
CH2CH3) ppm.
5-(4-Chlorophenyl)-3-methyloxazolidin-2-one.30,74 1H NMR
3
NMR characterization of organic products
(CDCl3): δ/ppm = 7.38–7.28 (m, 4H, C6H4); 5.45 (t, JHH = 8.3
1-Methyl-2-phenylaziridine. A colourless liquid. 1H NMR
(CDCl3): δ/ppm = 7.37–7.21 (m, 5H, Ph), 2.50 (s, 3H, NMe),
Hz, 1H, CH); 3.91, 3.40 (t, 3JHH = 8.3 Hz, 2H, CH2); 2.92 (s, 3H,
CH3) ppm.
3
5-(4-Chlorophenyl)-3-ethyloxazolidin-2-one.28,74
1H
NMR
2.28 (m, 1H, CH), 1.91 (d, JHH = 3.2 Hz, 1H, CH2), 1.63 (d,
3JHH = 6.7 Hz, 1H, CH2).
1-Ethyl-2-phenylaziridine.
3
(CDCl3): δ/ppm = 7.37, 7.29 (d, JHH = 7.9 Hz, 4H, C6H4); 5.45 (t,
3JHH = 8.3 Hz, 1H, CH); 3.92 (t, 3JHH = 8.3 Hz, 1H, CH2); 3.44–3.30
(m, 3H, CH2 + NCH2); 1.17 (t, 3JHH = 7.3 Hz, 3H, CH3) ppm.
A
colourless liquid. 1H NMR
(CDCl3): δ/ppm = 7.40–7.24 (m, 5H, Ph), 2.53–2.44 (m, 2H,
NCH2), 2.34 (m, 1H, CH), 1.94 (d, 3JHH = 3.2 Hz, 1H, CH2), 1.68
(d, 3JHH = 6.7 Hz, 1H, CH2), 1.26 (t, 3JHH = 7.2 Hz, 3H, CH3).
1-Isopropyl-2-phenylaziridine. A colourless liquid. 1H NMR
5-(4-Fluorophenyl)-3-methyloxazolidin-2-one.74
1H
NMR
(CDCl3): δ/ppm = 7.36–7.32, 7.11–7.07 (m, 4H, C6H4); 5.46 (t,
3JHH = 8.1 Hz, 1H, CH); 3.90, 3.42 (t, JHH = 8.4 Hz, 2H, CH2);
3
3
(CDCl3): δ/ppm = 7.35–7.24 (m, 5H, Ph), 2.40 (dd, JHH = 3.3
2.93 (s, 3H, CH3) ppm.
3
Hz, 3H, NMe), 1.95, 1.71 (d, JHH = 3.2 Hz, 1H, CH2), 1.67 (m,
3
3JHH = 6.2 Hz, 1H, CHiPr), 1.26, 1.24 (d, JHH = 6.3 Hz, 6H,
CH3iPr).
Conflicts of interest
1-Methyl-2-(p-tolyl)aziridine. A colourless liquid. 1H NMR
(CDCl3): δ/ppm = 7.16 (m, 4H, C6H4); 2.52 (s, 3H, NMe); 2.37
There are no conflicts to declare.
This journal is © The Royal Society of Chemistry 2021
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25 V. B. Saptal and B. M. Bhanage, ChemSusChem, 2016, 9,
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Acknowledgements
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