Cycloaddition of CO2 to challenging N-tosyl aziridines using a halogen-free niobium complex: Catalytic activity and mechanistic insights

Article abstract of DOI:10.1016/j.mcat.2017.10.023

An efficient and facile approach to the regioselective synthesis of N-tosyloxazolidinones from the corresponding N-tosylaziridines and CO₂ was developed using dual catalytic systems involving an early transition metal coordination compound as a Lewis acid and a nucleophilic cocatalyst. Among the screened Lewis acids, halogen-free niobium pentaethoxide (Nb(OEt)₅) displayed the best catalytic activity when used in the presence of tetrabutylammonium iodide (TBAI). Systematic DFT calculations, supported by catalytic experiments, demonstrate that CO₂ insertion is the rate determining step for this process and it is highly dependent on the steric hindrance at the niobium center.

Full text of DOI:10.1016/j.mcat.2017.10.023

Molecular Catalysis 443 (2017) 280–285
Contents lists available at ScienceDirect
Molecular Catalysis
journal homepage: www.elsevier.com/locate/mcat
Research Paper
Cycloaddition of CO to challenging N-tosyl aziridines using a
2
halogen-free niobium complex: Catalytic activity and mechanistic
insights
a
a
b
b
Sunatda Arayachukiat , Prapussorn Yingcharoen , Sai V.C. Vummaleti , Luigi Cavallo ,
c,∗
a,∗
Albert Poater , Valerio D’Elia
Department of Materials Science and Engineering, School of Molecular Science and Engineering, Vidyasirimedhi Institute of Science and Technology
VISTEC), 21210, Wang Chan, Thailand
King Abdullah University of Science & Technology, KAUST Catalysis Center (KCC), 23955-6900 Thuwal, Saudi Arabia
Institut de Química Computacional i Catàlisi and Departament de Química, Universitat de Girona, c/Maria Aurèlia Capmany 6, 17003 Girona, Catalonia,
a
(
b
c
Spain
a r t i c l e i n f o
a b s t r a c t
Article history:
An efficient and facile approach to the regioselective synthesis of N-tosyloxazolidinones from the cor-
responding N-tosylaziridines and CO2 was developed using dual catalytic systems involving an early
transition metal coordination compound as a Lewis acid and a nucleophilic cocatalyst. Among the
screened Lewis acids, halogen-free niobium pentaethoxide (Nb(OEt)5) displayed the best catalytic activity
when used in the presence of tetrabutylammonium iodide (TBAI). Systematic DFT calculations, supported
by catalytic experiments, demonstrate that CO2 insertion is the rate determining step for this process
and it is highly dependent on the steric hindrance at the niobium center.
Received 8 September 2017
Received in revised form 17 October 2017
Accepted 20 October 2017
Available online 6 November 2017
Keywords:
Aziridines
Niobium
©
2017 Elsevier B.V. All rights reserved.
CO2 fixation
Catalysis
Cycloaddition of CO2
Mechanistic study
1
. Introduction
reaction of epoxides, amines and CO2 [28–31]. The synthesis of
N-tosyloxazolidinones from CO , being a class of compounds that
2
Carbon dioxide is an important greenhouse gas and a renewable
can serve as key intermediates for the preparation of synthons and
biologically active compounds [32–36], requires the cycloaddition
reaction of CO2 to N-tosylaziridines. Because of the presence of
the electron-withdrawing sulfonyl group at the nitrogen atom, this
reaction generally requires harsher reaction conditions than the
synthesis of the cognate N-aryl and N-alkyl compounds and very
few systematic attempts to develop an efficient catalyst for this
reaction have been undertaken.
and ubiquitous C1 source for the synthesis of chemicals [1,2]. The
latter approach is regarded as a potential strategy to contribute
managing the excess anthropogenic emissions by sequestration
of CO2 into useful chemicals and fuels [3–6]. With the volume
of CO2 currently converted to chemicals representing just a tiny
fraction of global emissions [7,8], the development of a diversi-
fied and versatile portfolio of products able to provide a long-term
storage of CO2 is highly desirable [9–15]. In this context, the
products of cycloaddition of CO2 to three-membered heterocycles
such as cyclic organic carbonates[16–19] and oxazolidinones[20]
are being widely investigated. Focusing on the latter family of
compounds, N-aryl and N-alkyl oxazolidinones can be mainly pre-
pared by the cycloaddition of CO2 to aziridines [21–24], by the
cycloaddition of isocyanates to epoxides[25–27] or by the one pot
Beside the application of Pd (dba) /PPh /TBAT (dba: diben-
2
3
3
zylideneacetone, TBAT: tetrabutylammonium difluorotriphenyl-
silicate) under mild conditions for the special case of N-tosyl-
5-vinylaziridines [37], in situ-generated N-heterocyclic carbenes
(NHCs) were reported to afford 5-aryl-3-tosyloxazolidin-2-ones in
◦
good yields at moderate temperatures and CO2 pressure (80 C,
20 bar) [38]. However, a high catalytic loading was required
(20 mol% diisopropylphenylimidazolium chloride (NHC precursor)
and 22 mol% KOtBu). LiBr (20 mol%) is a readily available catalyst for
the regioselective synthesis of the 4-aryl-3-tosyloxazolidin-2-ones
∗ Corresponding authors.
E-mail address: valerio.delia@vistec.ac.th (A. Poater).
http://dx.doi.org/10.1016/j.mcat.2017.10.023
2
468-8231/© 2017 Elsevier B.V. All rights reserved.

S. Arayachukiat et al. / Molecular Catalysis 443 (2017) 280–285
281
ious by-products as observed in the crude 1H NMR of the reaction
mixture.
regioisomers in moderate yields, but cannot afford the 5-aryl-3-
tosyloxazolidin-2-ones analogues [39].
Coordination compounds of early transition metal halides such
as yttrium, niobium and zirconium are readily available inexpen-
sive Lewis acids that do not require the synthesis of sophisticated
ligand systems to harness the metal center that could have an
impact on the costs, molecular weight and sustainability of the cat-
alyst [16]. These compounds have shown the ability to catalyze the
Among the selected early transition metal compounds, Nb(OEt)5
(Table 1, Entry 8) showed complete 1a conversion and the highest
isolated yield of the target product 1b with high regioselectivity.
Niobium-based catalysts showed generally the best regioselec-
tivity for 1b versus its 1c isomer. NbCl5 (Table 1, Entry 7)
afforded exclusively 1b, albeit in moderate yields. Nb(OEt)5 was
selected for further investigation (Table 1, Entries 9–13) taking
into account the use of different nucleophilic cocatalysts such
as TBAC (tetrabutylammonium chloride), TBAB; (tetrabutylammo-
nium bromide), pyridine bases of different nucleophilicity, DMAP
(N,N-dimethylamino pyridine) and PPY (4-pyrrolidinopyridine)
[47], and amidine base DBU (1,8-Diazabicyclo[5.4.0]undec-7-ene).
In agreement with previously published mechanistic studies on the
cycloaddition of CO to epoxides under ambient conditions [40,41]
2
even in the case when diluted CO from flue gas was employed as an
2
impure feedstock [42,43]. The mechanistic aspects of the cycloaddi-
tion reactions promoted by such catalysts have been investigated
revealing intriguing details; a bimetallic cooperative mechanism
for the activation of CO2 between two niobium centers being iden-
tified in solution and on silica support [44,45]. Nevertheless, their
application to the cycloaddition reactions of CO2 has been, so far,
limited to the synthesis of cyclic carbonates.
Therefore, we explore here the catalytic activity of group III–V
transition-metal complexes in combination with nucleophilic co-
catalysts, for the cycloaddition of CO2 to N-tosylaziridines. Beside
readily available metal halides, early transition metals alkoxides
were employed in this study in order to identify greener and
less corrosive halogen-free Lewis acids [46]. We show here that
Nb(OEt)5 is a readily available and efficient, halogen-free catalyst
for the synthesis of 5-aryl-3-tosyloxazolidin-2-ones under mild
conditions when used in the presence of co-catalytic amounts of
TBAI (Tetrabutylammonium iodide). Furthermore, the mechanism
of this reaction is investigated by DFT calculations demonstrating
cycloaddition of CO to aziridines, the cocatalyst plays a crucial role
2
by serving as a nucleophile for the ring-opening of the niobium-
coordinated aziridine substrate and as a leaving group in the final
step of cyclization following CO2 activation (vide infra in the DFT
section for a more detailed mechanistic picture) [21,48]. Further-
more, the regiochemistry of the initial nucleophilic attack of the
cocatalyst on the aziridine determines the regioselectivity of the
reaction [21,39].
This screening confirmed quaternary ammonium salts, and in
particular TBAI, as the most suitable nucleophilic co-catalysts.
Strong nitrogen nucleophiles such as DBU and PPY failed to pro-
duce any appreciable amount of product (Table 1, Entries 11, 13).
In the case of PPY, the starting material was recovered unreacted.
A possible explanation for this observation could be a stable inter-
action between strong aminopyridine nucleophiles and niobium
complexes as suggested by previous studies. [41]. In the case of
DBU, we observed complete conversion of the starting material
but the target product was not formed. In the 1H NMR spectrum
of the reaction mixture a main by-product was observed display-
ing a downfield shift with respect to the aziridine protons of 1a,
but lower than expected for the formation of the oxazolidinone
product. These signals are likely to arise from the formation of
the intermediate product of aziridine-ring opening without the
that the process of CO insertion is the rate determining step of the
2
whole reaction and that this step is highly dependent on the steric
hindrance at the niobium center.
2
. Experimental
General information, details on the computational methods and
on the preparation of the aziridine substrates according to pub-
lished procedures are provided in Appendix A (see Supplementary
material).
insertion of CO . The pair Nb(OEt)5/DMAP, being a completely
2
halogen-free system, produced 1b in moderate yields but with low
regioselectivity (Table 1, Entry 12) reflecting the higher tendency
of the strong pyridine nucleophile to attack the less sterically hin-
dered carbon atom of the aziridine ring [21]. Consistently, when
exploring the effect of the counterion of the quaternary ammonium
salt (Table 1, Entries 8–10), only a limited effect on the yield of the
reaction was observed, however, the regioselectivity decreased in
the order TBAI ꢀ> TBAB ꢀ> TBAC thus following the inverse trend
2
.1. Catalysis experiments
For a typical cycloaddition reaction, 1a (273 mg, 1 mmol),
Nb(OEt)5 (21 mL, 0.08 mmol, 8 mol%) and TBAI (24 mg, 0.08 mmol,
mol%) were dissolved under protective atmosphere in
8
a
diethylether (3 mL) in a 50 mL stainless steel autoclave. CO (30 bar)
was added and the reaction vessel was heated to 80 C. After
2
◦
stirring for 48 h, the reactor was cooled by an ice bath and the
residual pressure was carefully vented. After withdrawal of an
aliquot of the reaction for 1H NMR analysis of the crude reaction,
the reaction solvent was evaporated under reduced pressure and
the product was purified by flash column chromatography (hex-
ane/dichloromethane 8:2) yielding 1b (288 mg, 0.91 mmol, 91%).
−
−
−
of nucleophilicity (Cl ꢀ> Br ꢀ> I ) of the halogen anion in the
aprotic reaction medium [49]. Whereas the regioselectivity of the
nucleophilic ring-opening of phenyl substituted three-membered
heterocycles is generally directed to the phenyl-bearing carbon by
the electronic effects of the aromatic ring [50], it is likely that the
tendency to attack the least sterically hindered carbon increases
when more nucleophilic species (TBAB, TBAC and DMAP) are used
as cocatalysts leading to the observed decrease of regioselectivity.
Further investigation was dedicated to the study of the influ-
ence of the reaction parameters (temperature and pressure) on the
catalytic efficiency of Nb(OEt)5/TBAI in the cycloaddition of CO2
to 1a. When the reaction temperature was varied under 30 bar CO2
3
. Results and discussion
3.1. Catalytic investigation
◦
The initial catalysis study was performed at 80 C under 30 bar
CO using 2-phenyl-N-tosyl aziridine (1a) as a benchmark substrate
◦
2
pressure (Fig. 1a) a low yield of 1b was observed at 50 C. Neverthe-
(
Table 1). Diethyl ether (DEE) resulted as the best solvent for this
less, moderate yields of the target product were obtained already
◦
◦
reaction after an initial screening. In general, at the end of the reac-
tion the formed 1b isomer could be nearly completely isolated by
column chromatography. Therefore, the large discrepancy between
at 60 C whereas for T ≥ 70 C high to quantitative yields of 1b were
afforded.
The reaction yield appears to be strongly dependent on CO2
pressure (Fig. 1b). Indeed, despite the catalyst displaying some
1
a conversion and 1b isolated yield observed In some cases (i.e.
Table 1, Entries 4, 6, 10–12) is to attribute to the formation of var-

2
82
S. Arayachukiat et al. / Molecular Catalysis 443 (2017) 280–285
Table 1
Application of various early transition metal coordination compounds as catalysts for the cycloaddition of CO2 to aziridine 1a in the presence of nucleophilic cocatalysts.
a
1a Conversion (%) /1b Isolated yield (%)c
b
(1b:1c)b
TOF(h )d
−1
Entry
Lewis acid
Nucleophile
1
2
3
4
5
6
7
8
9
TBAI
TBAI
TBAI
TBAI
TBAI
TBAI
TBAI
TBAI
TBAB
TBAC
DBU
DMAP
PPY
77/43
83/69
93/63
53/27
89/87
64/33
50/43
100/91
99/79
92/58
100/0
100/45
0/0
87:13
88:12
89:11
93:3
88:12
88:12
100
92:8
87:13
77:23
–
0.11
0.18
0.16
0.07
0.23
0.09
0.11
0.24
0.20
0.15
–
YCl3
ScCl3
ZrCl4
Zr(OEt)4
Ti(OEt)4
NbCl5
Nb(OEt)5
Nb(OEt)5
Nb(OEt)5
Nb(OEt)5
Nb(OEt)5
Nb(OEt)5
1
1
1
1
0
1
2
3
57:43
–
0.12
–
a
b
c
◦
Using 1a (1 mmol), catalyst (0.08 mmol, 8 mol%), nucleophile (0.08 mmol, 8 mol%) in 3 mL DEE at 80 C, 30 bar CO2 for 48 h.
Regioselectivity and 1a conversion determined by H NMR analysis of the crude reaction mixture.
Isolated yield of 1b after column chromatography.
Values relative to the isolated yield of 1b.
1
d
Fig. 1. (a) Dependency of 1b isolated yield on the reaction temperature under 30 bar CO2 pressure using Nb(OEt)5 (8 mol%) and TBAI (8 mol%) as a catalyst (Reaction time
◦
4
8 h); (b) dependency of 1b isolated yield on the CO2 pressure at 80 C using Nb(OEt)5 (8 mol%) and TBAI (8 mol%) as a catalyst (Reaction time 48 h).
activity already under just 5 bar CO , only low to moderate yields of
substrate could not be converted to the corresponding oxazolidi-
none as expected on the basis of the observed regioselectivity of
the reaction that implies a nucleophilic attack of the iodide anion
at the aziridine ring carbon bearing the aryl substituent (vide infra
for the DFT calculations). Furthermore, the use of Nb(OEt)5 as a
Lewis acid could be extended also to N-aryl and N-alkyl aziridines
(6a–8a, Table 2, Entries 5–8). For this different family of sub-
strates a new optimization of the reaction conditions had to be
2
1
b could be obtained for CO2 pressures below 30 bar. Under 40 bar
CO2 a quantitative yield of 1b could be obtained. The observed
trend suggests that the CO2 concentration in the reaction mix-
ture is a crucial factor for high reaction yields and that the step of
insertion/activation of CO2 might constitute the rate determining
step of the reaction (vide infra for DFT calculations). The applica-
tion of Nb(EtO)5 in the presence of quaternary ammonium salts
could be extended to other N-tosylaziridines (1a–4a) to yield the
corresponding 5-aryl-3-tosyloxazolidin-2-ones with high regios-
electivity (Table 2). Aziridines 1a-3a bearing no substituents or
electron withdrawing groups on the phenyl substituent could be
quantitatively converted to the target products (Table 2, Entries
carried out resulting in a slightly modified reaction protocol under
◦
comparable reaction conditions (T = 60–100 C, 30 bar CO ) as for
2
the N-tosylaziridines. The highest reaction yields were generally
obtained using TBAB as a nucleophile and THF (tetrahydrofuran) as
a solvent. Moreover, the catalyst loading could be reduced. Under
the optimized reaction conditions the same regioselectivity as for
the N-tosylaziridines was observed.
1
–3). In the case of aziridine 4a, bearing a methyl group on the
phenyl ring, only moderate conversion to the target oxazolidinone
was achieved (Table 2, Entry 4). This observation could be tenta-
tively attributed to the effect of the increased electronic density at
the phenyl ring on the step of aziridine ring-opening by the com-
bined action of the Lewis acid and of the iodide anion [39]. A control
experiment using aziridine 5a (Table 2, Entry 5), displaying a qua-
ternary carbon at the functionalized aziridine ring, shows that this
3.2. DFT studies
There are only very few examples of DFT investigations for the
cycloaddition of CO2 to aziridines in the literature and only for
the case of N-aryl or N-alkyl aziridines [21,48,51]. In order to gain

S. Arayachukiat et al. / Molecular Catalysis 443 (2017) 280–285
283
Table 2
phenyl-substituted methine carbon atom by the cooperative action
of Nb(OEt)5 and TBAI, CO2 insertion and ring-closure to afford the
desired product. This mechanism is analogous to that accepted for
the synthesis of cyclic carbonates from CO2 and epoxides [52,53]
and is supported experimentally by the lack of reactivity of aziridine
a
Cycloaddition of CO2 to several aziridines catalyzed by Nb(OEt)5/TBAI.
5a (Table 2, Entry 5). For the sake of completeness we also studied
the attack of the iodide anion at the least substituted carbon atom
of the aziridine ring to clarify the origin of the observed regiose-
lectivity. Moreover, prompted by the cooperative effect observed
between atoms of niobium for the activation of CO2 when inves-
tigating its cycloaddition to epoxides [44,45], we considered the
possibility of a bimetallic mechanism in the step of CO2 inser-
tion. Following the formation of intermediate A by coordination
of Nb(OEt)5 by the nitrogen atom of the aziridine, its ring-opening
proceeds via C N bond cleavage by the nucleophilic attack of the
iodide anion on the carbon atom bearing the phenyl ring and
requires an energy barrier of 10.0 kcal/mol (the total energy bar-
rier for the step of ring-opening is therefore 14.3 kcal/mol). The
insertion of CO2 in the Nb N bond of the ring-opened intermedi-
ate B was found to be highly energy demanding, with a barrier of
−1
TOF (h )
Entry
Substrate
Yield (%)b
Regioselectivity (b:c)c
1
2
3
4
5
91
94
92
40
92:8
100
92:8
94:6
0.24
0.24
0.24
0.10
–
0.44
1.67
0.41
0
d
e
6
7
85 (traces)
80 (73)e
78
94:6
f
g
8
88:12
a
Using aziridines 1a–8a (1 mmol), Nb(OEt)5 (0.08 mmol, 8 mol%), TBAI
◦
(
0.08 mmol, 8 mol%) in DEE (3 mL) at 80 C, 30 bar CO2 for 48 h.
b
Isolated yield of the most abundant regioisomer.
c
d
e
Determined by 1H NMR analysis of the crude reaction mixture.
◦
Using Nb(OEt)5 (4 mol%), TBAB (8 mol%) at 100 C.
Refers to the outcome of the reaction carried out under identical conditions as
3
0.1 kcal/mol and leading to intermediate C. Cyclization of the latter
per the N-tosylaziridines.
intermediate via C O bond formation proceeds through a transition
state of 12.0 kcal/mol. The calculated Gibbs free energy of cyclized
intermediate D, that still bears the Nb moiety, confirms that the
process starting from intermediate A is exergonic, by 1.7 kcal/mol.
The upper reaction barrier is located at 31.3 kcal/mol for the step
of CO2 insertion that, therefore, represents the rate determining
step of the process. This barrier is consistent with the experimen-
f
◦
Using Nb(OEt)5 (2 mol%), TBAB (4 mol%) in THF at 100 C for 24 h.
Using Nb(OEt)5 (4 mol%), TBAB (8 mol%) in THF at 60 C for 48 h.
g
◦
◦
tally observed requirement of at least 60 C for a significant but slow
conversion of the substrate. Going into further detail on this mech-
anism, we observed the lack of cooperativity of a second Nb(OEt)5
moiety in the step of CO2 insertion with the barrier of this process
increasing by 8.7 kcal/mol when a bimetallic mechanism was taken
into account (see blue dashed lines in Fig. 2) [54,55], basically due
to sterical reasons that do not allow access to an optimal confor-
mation for bending the entering CO2 [56], whereas the increase of
entropy here is lower with just one niobium moiety. Second, we
investigated the formation of the minor regioisomer arising from
the ring-opening of the aziridine ring at the less sterically crowded
methylene carbon (See values in brackets in Fig. 2). The step of
ring-opening was found to be 2.6 kcal/mol higher in energy than for
the major regioisomer since, electronically, the NBO charge on the
methylene carbon is lower by 0.182 e, thus less electrophilic [57].
Importantly, also all the other steps of the calculated mechanism for
the minor regioisomer (1c) were found to be more energy demand-
ing than for the formation of 1b with the rate-determining step of
CO2 insertion being over 7 kcal/mol higher in energy. Switching
to the NbCl5/TBAI system (See Fig. S1a), no qualitative differences
were observed, but, quantitatively, the corresponding transition
state A → B appeared more facile with a barrier of 6.6 just kcal/mol.
This barrier was significantly higher for the aziridine ring opening
at the methylene carbon (9.9 kcal/mol) thus explaining the per-
fect regioselectivity observed for this catalyst (Table 1, Entry 7).
However, the CO2 insertion step became much more challenging
with a barrier of 37.7 kcal/mol (thus 7.6 kcal/mol higher in energy
with respect to the corresponding barrier for Nb(OEt)5). This obser-
vation, consistent with the difference in catalytic activity found
experimentally, suggests the importance of the steric hindrance
at niobium center to facilitate the insertion of CO2 by favoring the
Nb-N bond cleavage. Indeed, when Nb(OEt)5 is used as a catalyst,
this distance elongates by 0.128 Å with respect to the case of NbCl5
(Fig. 3). At the same time the distance between the nitrogen atom
and the carbon atom of CO2 is 0.197 Å shorter for Nb(OEt)5. Also
for the case of NbCl5, the cooperativity by a second niobium atom
in the step of CO2 insertion was found to be disfavored, but only
by 1.3 kcal/mol. Last, the reaction profile was recalculated using
Fig. 2. Reaction mechanism for the cycloaddition of CO2 to aziridine 1a catalyzed by
Nb(OEt)5/TBAI (The values in brackets refer to the steps leading to the minor regioi-
somer and starting with the nucleophilic attack at the least substituted carbon atom
of the aziridine; Gibbs free energies in solvent are given in kcal/mol, the reaction
barrier in blue (see web version of the paper) refers to a bimetallic pathway of CO2
insertion).
deeper insight on the mechanism of the reaction, that could be cru-
cial for the development of more active catalysts, the cycloaddition
of CO2 to 1a was investigated using the M06/TZVP ∼ sdd//BP86-
D3/SVP ∼ sdd computational scheme (See Appendix A for details
in Supplementary material). We calculated the whole reaction
pathway of the cycloaddition of CO2 to aziridine 1a catalyzed by
Nb(OEt)5/TBAI (Fig. 2) leading to 5-aryl-3-tosyloxazolidin-2-one
(
1b). For the sake of comparison we studied also the same reac-
tion catalyzed by NbCl5/TBAI and Nb(OMe)5/TBAI (See Appendix
A, Fig. S1 in Supplementary material). The mechanism consid-
ered is represented by the steps of aziridine ring-opening at the

2
84
S. Arayachukiat et al. / Molecular Catalysis 443 (2017) 280–285
Fig.3. Molecular structures of the computed transition state B → C for (a) Nb(OEt)5 and (b) NbCl5 (selected distances are given in Å).
Nb(OMe)5/TBAI as a catalyst to understand, and at the same time,
remove the potential noise due to the alkyl chains, which might
rotate freely at the experimental conditions (See Fig. S1b). Apart
from other minimal energy differences in the calculated pathway
with respect to Nb(OEt)5, the rate determining barrier for the step
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