Ruthenium Porphyrin Catalyzed Synthesis of Oxazolidin-2-ones by Cycloaddition of CO2 to Aziridines

Article abstract of DOI:10.1002/ejic.201801208

The reaction between N-substituted-2-arylaziridines and CO₂ is efficiently promoted by ruthenium(VI) imidoporphyrin complexes and yields a mixture of 5-aryl (A) and 4-aryl (B) substituted oxazolidin-2-ones with a regioisomeric A/B ratio up to 9

Full text of DOI:10.1002/ejic.201801208

A Journal of
Accepted Article
Title: Ruthenium Porphyrin-Catalyzed Synthesis of Oxazolidin-2-ones
by Cycloaddition of CO2 to Aziridines
Authors: Daniela Carminati, Emma Gallo, Caterina Damiano,
Alessandro Caselli, and Daniela Intrieri
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To be cited as: Eur. J. Inorg. Chem. 10.1002/ejic.201801208
Link to VoR: http://dx.doi.org/10.1002/ejic.201801208

10.1002/ejic.201801208
European Journal of Inorganic Chemistry
COMMUNICATION
Ruthenium Porphyrin-Catalyzed Synthesis of
Oxazolidin-2-ones by Cycloaddition of CO2 to Aziridines
Daniela Carminati,a Emma Gallo,b* Caterina Damiano,b Alessandro Casellib and Daniela Intrieri.b
Abstract: The reaction between N-substituted-2-arylaziridines and
CO2 is efficiently promoted by ruthenium(VI) imido porphyrin
complexes and yields a mixture of 5-aryl (A) and 4-aryl (B)
substituted oxazolidin-2-ones with a regioisomeric A/B ratio up to
99:1. Several oxazolidin-2-one molecules were synthesized at
100°C and 0.6 MPa of carbon dioxide by using the low catalytic
loading of 1% mol. The formation of a deactivated compound,
deriving from the ruthenium catalyst, suggested a possible catalytic
role of nitrogen imido atoms.
However, in these cases the required use of aziridine as the
reaction solvent[10] restricts the reaction scope when the aziridine
is not a liquid compound or its synthesis consists in time-
consuming and/or expensive procedures.
It is important to remember that, while the analogous reaction of
CO2 with an achiral epoxide yields the corresponding cyclic
carbonate as a single isomer, the CO2 cycloaddition to an
aziridine molecule can produce two different regioisomers.
When R1  R2, the ring opening reaction can take place on two
inequivalent carbon atoms of the aziridine therefore, two
regioisomers can be formed in a ratio which strongly depends on
the nature of the promoter (Scheme 1).
Introduction
Over the last decades, aziridines have been extensively studied
as starting materials due to the high energy of the strained ring
which permits the transformation of aziridines into a large variety
of fine chemicals by ring-opening reactions.[1] Among all the
organic transformations involving aziridines, the insertion of CO2
into the three-membered ring is responsible for the synthesis of
oxazolidin-2-ones, which are the pharmaceutically active
moieties of several antibacterial and antimicrobial compounds,[2]
such as Linezolid,[3] Tedizolid[4] and Radezolid.[5] In addition,
oxazolidin-2-ones are widely used as chiral auxiliaries in various
asymmetric synthetic reactions.[6]
The reaction of aziridines with CO2 displays interesting features
in terms of eco-sustainability because the catalytic valorization
of the greenhouse CO2 gas, as a renewable C1 building block,[7]
is coupled with the 100% atom-efficiency of the ring-insertion
process. The CO2 cycloaddition into aziridines can be efficiently
promoted by several homogeneous and heterogeneous
species,[8] which are fundamental in ensuring good reaction
performances by applying eco-friendly experimental conditions,
such as low catalytic loadings, low reaction temperatures and
low CO2 pressures.
Scheme 1. The coupling reaction between carbon dioxide and either epoxides
or aziridines, which yields cyclic carbonates or oxazolidin-2-ones, respectively.
To date, the cycloaddition of CO2 to aziridines has been less
studied than the equivalent reaction to epoxides[11] and even if it
is generally recognized that the presence of both an electrophilic
(E) and nucleophilic (Nu) species is required for the productive
insertion of CO2 into an aziridine ring, the reaction mechanism is
still under debate.[12] Analogously to the mechanism of the CO2
cycloaddition to epoxides,[11c,11d] the coordination of an
electrophilic species to the aziridine nitrogen atom could
promote the ring opening reaction by the nucleophilic agent. The
so-formed intermediate can therefore react with CO2, yielding
the desired oxazolidin-2-one. As reported in Scheme 1,
oxazolidin-2-ones can be formed in two regioisomers and when
2-substituted aziridines are employed as starting materials,
5-substitued oxazolidin-2-ones[8h,8i] are usually formed as the
prevalent regioisomer, due to the preferred nucleophilic attack
on the most substituted aziridine carbon atom (Scheme 2).
The synthesis of oxazolidin-2-ones from aziridines and CO2 can
also be performed in the absence of any catalytic species.[9]
Dr. D. Carminati
Department of Chemistry
University of Rochester
416 Hutchison Hall, Rochester, NY
Dr. D. Intrieri, C. Damiano, Prof. Dr. A. Caselli, Prof. Dr. E. Gallo
Department Of Chemistry
Università degli Studi di Milano
Via Golgi, 19 20133 Milano (I)
E-mail: emma.gallo@unimi.it
Supporting information for this article is given via a link at the end of
the document.
Scheme 2. The proposed mechanism for the formation of oxazolidin-2-ones.
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10.1002/ejic.201801208
European Journal of Inorganic Chemistry
COMMUNICATION
The present communication deals with the catalytic activity of
metal porphyrins in mediating the reaction of N-substituted-2-
arylaziridines with CO2 forming 5-aryl- (A) and 4-aryl-oxazolidin-
2-ones (B) with the large excess of the less-hindered
5-substituted regioisomer A over the 4-substituted one B.
Despite the efficiency of metal porphyrin complexes in promoting
the CO2 cycloaddition to epoxide,[11b] the use of these catalysts
for the synthesis of oxazolidin-2-ones has been very
limited.[8b,8g,13]
The reaction performed in the presence of the sole TBACl
(Table 1, entry 1) formed oxazolidinones in 20% yield and 1A/1B
ratio of 88:12. As reported in Table 1, iron(III) porphyrin
derivatives were less catalytically active than ruthenium
catalysts and, even if the reaction occurred with good
regioselectivities, moderate yields were achieved in the
presence of Fe(F20TPP)X species (F20TPP = dianion of meso-
tetrakis-(pentafluorophenyl) porphyrin; X = Cl[16] or OMe[17]
(Table 1, entries 2 and 3).
)
Low catalytic efficiencies were also observed in the presence of
[19]
Ru(TPP)CO[18] and Ru(TPP)(py)2
(TPP = dianion of meso-
tetrakis-(phenyl) porphyrin) (Table 1, entries 4 and 5), while a
slight enhancement of reaction yields was achieved by using
[Ru(TPP)OCH3)]2O[20] (Table 1, entry 6).
Results and Discussion
In view of the reactivity of zinc porphyrin complexes to promote
the CO2 insertion into epoxides,[11b,13] the model reaction
between 1-butyl-2-phenyl aziridine and carbon dioxide was first
performed in the presence of Zn(TPP) but the formation of the
desired compounds was not observed. Thus, the model reaction
was run in the presence of less electrophilic metal species, such
as iron and ruthenium porphyrin complexes. Obtained results
are reported in Table 1. Considering the well-known catalytic
role of ammonium salts[14] and their general use as co-catalysts
in metal porphyrin-promoted reactions of CO2 with
epoxides,[11b,15] tetrabutyl ammonium chloride (TBACl) was
initially tested alone in the model reaction and then employed in
a catalyst/TBACl/aziridine ratio of 1:10:100. All the reactions
were performed in a steel autoclave for 6 h at 100°C and 3 MPa
of CO2 by using benzene as the reaction solvent.
When Ru(TPP)CO or Ru(TPP)(py)2 (Table 1, entries 4 and 5)
1
were used as the catalyst, the H NMR spectrum of the crude
revealed an aziridine conversion of 40% and 27%, respectively
and the presence of several unidentified side-products. We
suggest that even if Ru (II) porphyrin complexes were poorly
active in promoting the oxazolidin-2-one synthesis, they can
catalyze side-reactions which were responsible for the partial
aziridine conversion.
Higher reaction yields were achieved by performing the reaction
in the presence of bis-imido ruthenium porphyrin catalysts
(Table 1, entries 7-11) which always promoted the formation of
the 5-regioisomer 1A as the major product.
Bis-imido complexes Ru(F20TPP)(NAr)2 (Ar = 3,5(CF3)2C6H3),
Ru(TMP)(NAr)2 (TMP
= dianion of meso-tetrakis-(mesityl)
porphyrin) and Ru(OEP)(NAr)2 (OEP = dianion of octaethyl
porphyrin) were synthesized by using the methodology already
reported by us for preparing Ru(TPP)(NAr)2.[21] On the other
hand, Ru(TPP)(NAr’)2 (Ar’ = 4(tBu)C6H4) was prepared by using
our reported procedure in which Ar’N3 efficiently reacts with the
electron-poor ruthenium(IV) [Ru(TPP)OCH3)]2O complex.[20]
Collected data indicated that the catalytic activity of bis-imido
ruthenium(VI) porphyrins was independent from the electronic
nature of the N-imido ligand. Similar results, in terms of yields
and regioselectivities, were obtained by using both
Ru(TPP)(NAr)2 (Ar = 3,5(CF3)2C6H3) and Ru(TPP)(NAr’)2
(Ar’ = 4(tBu)C6H4) catalysts, which show an electron-deficient
and electron-rich axial imido ligand, respectively (Table 1,
entries 7 and 8). The catalytic effect of the electronic and steric
nature of the porphyrin ring was also investigated by comparing
the catalytic efficiency of Ru(F20TPP)(NAr)2, Ru(TMP)(NAr)2 and
Ru(OEP)(NAr)2 porphyrin catalysts. Catalytic results of the
reactions performed in the presence of Ru(F20TPP)(NAr)2 and
Ru(TMP)(NAr)2 (Table 1, entries 9 and 10) indicated that both
Table 1. Metal porphyrin-catalyzed cycloaddition of CO2 to 1-butyl-2-
phenylaziridine.[a]
entry
catalyst
Yield %[b]
1A/1B ratio[b]
1
-
20
40
30
13
12
35
79
62
34
41
65
88/12
95/5
2
Fe(F20TPP)Cl
Fe(F20TPP)OMe
Ru(TPP)CO
Ru(TPP)py2
[Ru(TPP)(OCH3)]2O
3
92/8
4
86/14
95/5
5
6
91/9
electron-deficient and sterically-hindered ligands have
a
negative effect on the activity of the ruthenium catalyst. On the
other hand, the low-hindered bis-imido Ru(OEP)(NAr)2 complex,
which shows hydrogen atoms on meso-positions of the
porphyrin ring, promoted the oxazolidin-2-one synthesis with
65% yield and A/B ratio of 90:10 (Table 1 entry 11).
Considering that the best combination of yield and
regioselectivity was obtained in the presence of Ru(TPP)(NAr)2
(Table 1, entry 6), this latter complex was employed for
optimizing experimental conditions of the model reaction. The
effect of the tetrabutyl ammonium salt’s anion, temperature,
solvent polarity and CO2 pressure on the catalytic efficiency was
investigated and the obtained results are displayed in Table 2.
The catalytic influence of the nature of the anion of the tetrabutyl
ammonium salt was examined by employing tetrabutyl
ammonium iodide (TBAI), tetrabutyl ammonium bromide (TBAB),
[c]
7
Ru(TPP)(NAr)2
88/12
90/10
85/15
88/12
90/10
[d]
8
Ru(TPP)(NAr’)2
[c]
9
Ru(F20TPP)(NAr)2
[c]
10
11
Ru(TMP)(NAr)2
[c]
Ru(OEP)(NAr)2
[a] Reaction conditions: catalyst/TBACl/aziridine = 1:10:100 in benzene.
Reactions were performed in a steel autoclave for 6 h at 100°C and 3 MPa
of CO2. [b] Determined by 1H NMR spectroscopy using 2,4-dinitrotoluene
as the internal standard. [c] Ar = 3,5(CF3)2C6H3. [d] Ar’ = 4(tBu)C6H4.
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10.1002/ejic.201801208
European Journal of Inorganic Chemistry
COMMUNICATION
TBACl and tetrabutyl ammonium fluoride (TBAF) as co-catalysts
in the model reaction at 100°C and using benzene as the
reaction solvent. When the reaction was performed in the
presence of either TBAI or TBAF the product was not formed. It
should be noted that TBAF was employed as a THF solution
(1.0 M) due to its instability in the solid state[22] thus, considering
the reactivity of Ru(TPP)(NAr)2 in coordinating solvents,[21] THF
was probably responsible for the catalyst deactivation.
revealed a general decrease of reaction yields when the steric
hindrance around the aziridine nitrogen atom was increased.
When isopropyl (Table 3, compounds 7A/7B) or cyclopentyl
(Table 3, compounds 8A/8B) groups were bonded to N-aziridine
atom, the reaction productivity was low. Better results were
achieved if less-hindered cyclohexylmethyl (Table 3, compounds
9A/9B) or benzyl (Table 3, compounds 10A/10B) substituents
were linked to the nitrogen atom of the aziridine ring, also when
a methoxy group was present on the benzylic moiety (Table 3,
compounds 11A/11B and 12A/12B). It should be noted that in
the latter reactions, the 5-regioisomer was obtained as the sole
reaction product (A/B = 99:1). Considering that the catalytic
performance decreased when a steric hindrance was present
either on the nitrogen aziridine atom or the tetrapyrrolic core, an
unfavorable interaction between these two moieties during the
catalytic reaction can be envisaged.
Considering that similar results were obtained by using either
TBAB or TBACl (Table 2, entries 1 and 2), the latter salt was
employed for optimizing the other reaction conditions.
In view of the lack of the catalytic effectiveness at 50°C (Table 2,
entry 3), the effect of the solvent polarity was studied by
performing reactions at 100°C. Even if the addition of 10% of
DMF to the benzene solution was responsible for the formation
of regioisomer A as the only reaction product (A/B ratio=99:1)
(Table 2, entry 4), the moderate reaction productivity did not
justify a general use of this reaction medium. In view of the low
productivity of the reaction performed in dichloroethane (Table 2,
entry 5), the optimization of the CO2 pressure was executed in
benzene at 100°C. Similar results were obtained by using either
3 or 0.6 MPa of carbon dioxide (Table 2, entries 5 and 6) whilst
the decrease of CO2 pressure to 0.1 MPa was responsible for a
large decrease of the catalytic performance; oxazolidin-2-ones
were obtained in 35% yield and A/B ratio of 94:6 (Table 2, entry
7). The model reaction was also performed in a pressure tube
where the good A/B ratio of 93:7 was obtained in the very low
yield of 13%.
Table 3. Synthesis of oxazolidin-2-ones from N-substituted-2-arylaziridine and
CO2.[a]
Table 2. Optimization of experimental conditions of the model reaction of
[a]
1-butyl-2-phenylaziridine with CO2
T
(°C)
PCO2
(MPa)
1A/1B
entry
solvent
X-
yield %[b]
ratio[b]
3
1
2
3
4
5
6
7
100
100
50
C6H6
C6H6
C6H6
Br
Cl
Cl
Cl
Cl
Cl
Cl
68%
79%
-
85/15
3
88/12
-
3
3
100
100
100
100
C6H6/DMF(10%)
C2H4Cl2
C6H6
45%
22%
70%
35%
99/1
90/10
90/10
94/6
3
0.6
0.1
C6H6
[a] Reaction conditions: Ru(TPP)(NAr)2/TBAX/aziridine
=
1:10:100
[a] Reaction conditions: Ru(TPP)(NAr)2/TBACl/aziridine
= 1:10:100 (Ar =
(Ar = 3,5(CF3)2C6H3). Reactions were performed in a steel autoclave for 6 h.
[b] Determined by 1H NMR spectroscopy using 2,4-dinitrotoluene as the
internal standard.
3,5(CF3)2C6H3). Reactions were performed in a steel autoclave for 6 h at
100°C and 0.6 MPa of CO2. [b] Isolated yields. [c] Determined by 1H NMR
spectroscopy using 2,4-dinitrotoluene as the internal standard.
Then, the scope of the reaction was studied in the presence of
Ru(TPP)(NAr)2 1% mol, TBACl 10% mol at 100°C, 0.6 MPa of
CO2 and using benzene as the reaction solvent. N-substituted-
oxazolidin-2-ones were obtained with moderate to good isolated
yields and A/B ratio up to 99:1 (Table 3). Collected data
The replacement of C6H5 with the more electron-rich (Me)C6H4
group on the C2 position of the starting aziridine had a beneficial
effect. Similar yields and regioselectivities of the synthesis of
13A/13B, 14A/14B and 15A/15B compounds (Table 3) were
observed independently from the position of the methyl group on
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10.1002/ejic.201801208
European Journal of Inorganic Chemistry
COMMUNICATION
the phenyl substituent, pointing out that the steric hindrance of
C2 aziridine carbon atom does not influence the catalytic
performance.
Experimental Section
Synthesis of porphyrin complexes and aziridines, general catalytic
procedures as well as NMR spectra, IR, UV/Vis, ESI-MS spectroscopic
data of all compounds reported in the manuscript can be found in the
Supporting Information.
In order to study the catalyst robustness, the CO2 cycloaddition
to 1-butyl-2-phenylaziridine, forming 1A/1B oxazolidin-2-one
mixture, was performed for three consecutive times in the
presence of 1% mol of Ru(TPP)(NAr)2. Each reaction was run
for 6 hours at 100°C and, after checking that the aziridine
Keywords: Carbon dioxide fixation • Cycloaddition •
Homogeneous catalysis • Porphyrinoids • Ruthenium.
conversion was >95%,
a
same amount of 1-butyl-2-
phenylaziridine was added before recharging the autoclave with
0.6 MPa of CO2. At the end of the three runs, the 1H NMR
analysis of the reaction mixture revealed the formation of
corresponding oxazolidin-2-ones in 70% yield (with respect to
the total amount of added aziridine) and 1A/1B ratio of 90:10,
indicating that Ru(TPP)(NAr)2 maintains its catalytic activity for
at least three consecutive catalytic reactions (see SI for the
experimental procedure).
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1
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Conclusion
In conclusion, we report the catalytic activity of ruthenium
bis-imido porphyrin complexes in promoting the regioselective
cycloaddition of CO2 to aziridines. The procedure was effective
for the synthesis of N-substituted oxazolidin-2-ones in yields up
to 96% and regioselectivities up to 99:1. The formation of a
deactivated ruthenium compound suggested the occurrence of a
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cycloaddition to the aziridine ring.
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European Journal of Inorganic Chemistry
COMMUNICATION
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10.1002/ejic.201801208
European Journal of Inorganic Chemistry
COMMUNICATION
Entry for the Table of Contents
COMMUNICATION
Daniela Carminati, Emma Gallo,*
Caterina Damiano, Alessandro Caselli,
Daniela Intrieri.
Page No. – Page No.
Ruthenium Porphyrin-Catalyzed
Synthesis of Oxazolidin-2-ones by
Cycloaddition of CO2 to Aziridines
The catalytic activity of ruthenium(IV) bis-imido porphyrins in promoting the
insertion of CO2 into aziridines, forming oxazolidin-2-ones, is here reported.
This article is protected by copyright. All rights reserved.