Synthesis of cyclic carbonates by ruthenium(VI) bis -imido porphyrin/TBACl-catalyzed reaction of epoxide with CO2

Article abstract of DOI:10.1142/S1088424619501888

The catalytic activity of the ruthenium(VI) bis-imido porphyrin complex/TBACl binary system in promoting the CO2 cycloaddition to epoxides forming cyclic carbonates is here reported. The system was very efficient in catalyzing the conversion of differently substituted epoxides under mild experimental conditions (100 degC and 0.6 MPa of CO2). Even if the sole TBACl resulted active under the optimized experimental conditions, the addition of ruthenium species was fundamental to maximizing the reaction productivity both in terms of epoxide conversions and cyclic carbonate selectivities. A preliminary mechanistic study indicated a positive role of ruthenium imido nitrogen atom in activating carbon dioxide.

Full text of DOI:10.1142/S1088424619501888

Journal of Porphyrins and Phthalocyanines
J. Porphyrins Phthalocyanines 2019; 23: 1–8
DOI: 10.1142/S1088424619501888
Published at https://www.worldscinet.com/jpp/
Synthesis of cyclic carbonates by ruthenium(VI) bis-imido
porphyrin/TBACl-catalyzed reaction of epoxide with CO2
Caterina Damianoa, Paolo Sonzinia, Daniela Intrieria and Emma Gallo*a◊
aDipartimento di Chimica, Università degli Studi di Milano, via C. Golgi 19, 20133 Milano, Italy
Dedicated to Professor Roberto Paolesse on the occasion of his 60th birthday.
Received 6 November 2019
Accepted 27 November 2019
ABSTRACT: The catalytic activity of the ruthenium(VI) bis-imido porphyrin complex/TBACl binary
system in promoting the CO2 cycloaddition to epoxides forming cyclic carbonates is here reported. The
system was very efficient in catalyzing the conversion of differently substituted epoxides under mild
experimental conditions (100°C and 0.6 MPa of CO2). Even if the sole TBACl resulted active under the
optimized experimental conditions, the addition of ruthenium species was fundamental to maximizing
the reaction productivity both in terms of epoxide conversions and cyclic carbonate selectivities.
A preliminary mechanistic study indicated a positive role of ruthenium imido nitrogen atom in activating
carbon dioxide.
KEYWORDS: homogeneous catalysis, ruthenium, cyclic carbonates, carbon dioxide, epoxides.
systems for promoting the CO2 conversion into fine
INTRODUCTION
chemicals [6].
The increasing emission of carbon dioxide in the
Among all the CO2-based organic transformations,
atmosphere, coming from anthropic activities, is
the 100% atom-efficient cycloaddition of CO2 to
responsible for many environmental issues, such as
epoxides (Scheme 1) represents a valuable strategy for
global warming [1] and ocean acidification [2]. Thus, the
obtaining cyclic carbonates, which are extensively used
scientific community is currently working towards the
as solvents in chemical processes [7–9] and batteries
chemical valorization of carbon dioxide by transforming
[10] and they are also employed as useful synthetic
this greenhouse gas into high added-value compounds.
intermediates [11, 12].
This can determine a positive, albeit relatively small,
Several homogeneous and heterogeneous catalytic
impact on global CO2 levels [3–5]. The limited use of
systems mediate the formation of cyclic carbonates
carbon dioxide as a renewable C1-building block in
starting from CO2 and epoxides, and the most productive
organic synthesis is due to its inherent thermodynamic
procedures require the contemporary presence of Lewis
stability; accordingly, a large energy input must generally
acid (M) and Lewis base (Nu-) promoters [13–16]. One
be supplied for achieving the CO2 activation. In order to
of the most validated mechanistic hypothesis proposes
use CO2 as a feedstock under sustainable experimental
that the first coordination of an epoxide to the Lewis
conditions (avoiding the use of high temperatures, high
acidic metal is necessary for allowing the nucleophilic
CO2 pressures and extremely reactive reagents), it is
attack of the Lewis base to the three-membered ring. The
imperative to take full advantage of efficient catalytic
so-obtained compound A is nucleophilic enough to attack
carbon dioxide, and the subsequent ring closure reaction
forms the desired cyclic carbonate. Both the metal
catalyst and the Lewis base are restored for repeating the
◊ SPP full member in good standing.
catalytic cycle (Scheme 2).
Even if M is generally considered the reaction
“catalyst” and the Lewis base is labelled as the
*Correspondence to: Emma Gallo, Department of Chemistry,
Università degli Studi di Milano, via Golgi 19, 20133 Milan
(Italy), email: emma.gallo@unimi.it.
Copyright © 2019 World Scientific Publishing Company

2
C. DAMIANO ET AL.
O
showing an ammonium salt on the ligand periphery
[18, 32–34].
O
[cat]
O
O
Albeit the acidic nature of the catalyst is usually
necessary to maximize the reaction performance, we
recently reported that the “non-acidic” bis-imido
porphyrin complex Ru(TPP)(NAr)2 (1) (TPP = dianion of
tetraphenyl porphyrin, Ar = 3,5-(CF3)2C6H3), in combi-
nation with a tetrabutyl ammonium salt (TBAX), pro-
moted the oxazolidinone synthesis by the cycloaddition
of CO2 to aziridines [35]. Although the
+ CO2
R1
R2
R1
R2
Scheme 1. General route of the cycloaddition of CO2 to
epoxides forming cyclic carbonates
[M]
R2
reaction mechanism has not been studied
in detail, preliminary results showed that
R1
O
Lewis
base
the nitrogen atom of the axial imido ligand
on the ruthenium center may activate
CO2 molecule. The metal center, being
coordinatively saturated, doesn’t interact
with aziridine.
The good results obtained prompted us
to test the catalytic activity of the Ru(TPP)
(NAr)2 (1)/TBAX binary system in the CO2
cycloaddition to epoxides forming cyclic
carbonates.
O
Nu-
R1
R2
R1
[M]
Lewis acid
O-
[M]
Nu
O
Nu-
A
R2
O
O
O
R1
R2
CO2
[M]
-O
O
cyclic
carbonate
R1
Nu
R2
Scheme 2. Mechanistic proposal of the cyclic carbonate formation
RESULTS AND DISCUSSION
The catalytic activity of Ru(TPP)(NAr)2
(1) was first compared to that of other
ruthenium complexes in the synthesis of
“co-catalyst”, it is important to remember that while the
Lewis acidic species is not usually active in the absence of
the nucleophilic “co-catalyst”, the CO2 cycloaddition can
be catalyzed by the sole Lewis base [17]. The DFT study
of the TBAX-catalyzed transformation of propylene
oxide (PO) into porpylene carbonate (PC) [18] suggested
that the halide anion X- is nucleophilic enough to attack
the epoxide ring, thanks to the interaction of the TBA+
cation with the epoxide oxygen atom. The so-formed
intermediate undergoes the CO2 insertion and the desired
cyclic carbonate is formed by the following ring-closing
reaction. However, the presence of the M catalyst is
usually required for applying milder experimental
conditions, improving the regioselectivity and speeding
up the reaction.
Both homogeneous and heterogeneous porphyrin-
based catalysts were seen to be very efficient in
promoting the CO2 cycloaddition to epoxide in the
presence of the opportune co-catalyst (generally an
ammonium salt). Regarding homogeneous systems [19],
many metal porphyrin catalysts were tested and good
results were achieved by using either main group (Al,
Mg, Sn, Bi) [20–25] or transition (Cr, Co, Mn, Ru and V)
[26–31] metal derivatives. In addition, bifunctional metal
catalysts were developed by inserting both Lewis acidic
and nucleophilic moieties within the same molecular
skeleton. Outstanding catalytic results were obtained in
the presence of bifunctional metal porphyrin complexes
cyclic carbonate 2. Obtained data are reported in Table 1.
As shown in Table 1, the reaction occurred in the
presence of TBACl alone. 74% of the starting epoxide
was converted and the desired cyclic carbonate 2 was
formed with 62% of selectivity after 8 h at 100°C
under 0.6 MPa of CO2 (Table 1, entry 1). The catalytic
performance improved upon adding a ruthenium species
to the catalytic reaction; when the Ru(TPP)CO/TBACl
system was used in the ratio reported in Table 1 (entry 2),
the conversion increased from 74% to 77% and the cyclic
carbonate selectivity from 62% to 99%. The epoxide
conversion was further improved to 100% by replacing
Ru(TPP)CO with Ru(TPP)(NAr)2 (1), which mediated
the formation of the desired product 2 with 99% of
reaction selectivity. The compared systems mediated the
synthesis of 2 by different mechanisms, which involve
the sole TBACl containing both nucleophilic (Cl-) and
electrophilic (TBA+) components (Table 1, entry 1) or
the Lewis acid/base system as in the case of Ru(TPP)CO/
TBACl combination (Table 1, entry 2). The beneficial
effect of catalyst 1 (Table 1, entry 3) on the reaction
productivity indicated the occurrence of a reaction
mechanism in which the presence of a coordinatively
unsaturated and “acidic” metal center is not required.
In fact, the presence of the two imido axial ligands on
the ruthenium atom doesn’t allow the coordination, and
consequent epoxide activation, onto the central metal.
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SYNTHESIS OF CYCLIC CARBONATES BY RUTHENIUM(VI) bis-IMIDO PORPHYRIN/TBACL-CATALYZED REACTION
3
Table 1. Synthesis of 4-((allyloxy)methyl)-1,3-dioxolan-2-one (2)a
(TMP = dianion of tetramesityl porphyrin) (Table 1,
O
entry 8), respectively. This result indicated that the
steric hindrance of the porphyrin skeleton has more
influence than its electronic nature in determining the
catalytic efficiency of the methodology.
[cat]
O
O
O
+
CO2
O
O
2
In view of the good catalytic results achieved by
using Ru(TPP)(NAr)2 (1)/TBACl binary system,
this combination was employed for the synthesis of
cyclic carbonates 6–12, which was also performed in
the presence of TBACl alone to better appreciate the
influence of the ruthenium porphyrin catalyst (Table 2).
In order to improve the solubilization of CO2 in the
reaction medium, the reactions mediated by the sole
TBACl were executed in THF where a better catalytic
performance in the synthesis of 2 was observed. When
working in THF, the epoxide conversion increased
from 74% (Table 1, entry 1) to 91% (Table 2, entry 1),
while the cyclic carbonate selectivity remained the
same (62% vs. 63%) supporting the importance of
maximizing the CO2 solubility in the reaction solvent
to amplify the catalytic efficiency.
Unfortunately, we were forced to replace THF
with benzene, as the reaction solvent, when catalyst
1 was employed in combination with TBACl, due to
the poor chemical stability of the ruthenium species
in coordinating solvents, such as THF. As shown in
Table 2, even if the catalytic performances were always
maximized by adding Ru(TPP)(NAr)2 (1), the positive
effect was strongly related to the nature of the starting
epoxide. When epoxide was mono-substituted with a
-CH2(EWD) group (Table 2, entries 1–3), the reaction
worked well both in the presence of TBACl and Ru(TPP)
(NAr)2 (1)/TBACl binary system. Compounds 2 (Table 2,
entry 1), 6 (Table 2, entry 2) and 7 (Table 2, entry 3)
were obtained with the complete conversion of the
starting epoxide and with very high similar selectivities
(94–99%) in the presence of the Ru(TPP)(NAr)2 (1)/
TBACl combination.
The positive effect of the Ru(TPP)(NAr)2 (1)
addition was more clearly observed in the synthesis of
8 (Table 2, entry 4), which involved an epoxide showing
an electron-donating substituent on the three-membered
ring. In this case, albeit the complete epoxide conversion
was observed either in the presence or in the absence
of catalyst 1, the addition of the ruthenium species was
responsible for an increase of the reaction selectivity
from 27% to 65%. Next, the catalytic influence of the
steric hindrance on the epoxide ring was investigated. The
presence of a phenyl substituent on the starting epoxide
did not hamper the formation of the corresponding cyclic
carbonate 9 (Table 2, entry 5) in good selectivities. A
moderate improvement of the reaction productivity was
registered by running the reaction in the presence of
Ru(TPP)(NAr)2 (1)/TBAClinsteadofTBAClalone.When
more sterically encumbered substrates were employed,
the effect of the ruthenium catalyst was less evident. The
synthesis of compound 10 (Table 1, entry 6) occurred
Entry
Catalytic system
TBACl
Conv. (%)b Select. (%)b
1c
2d
3d
4d
5f
74
77
62
99
99
99e
99
99
99
99
Ru(TPP)CO/TBACl
Ru(TPP)(NAr)2 (1)/TBACl
Ru(TPP)(NAr)2 (1)/TBACl
Ru(TPP)(NAr)2 (1)/TBACl
100
100e
100
100
93
6d,g Ru(TPP)(NAr′)2 (3)/TBACl
7d
8d
Ru(F20TPP)(NAr)2 (4)/TBACl
Ru(TMP)(NAr)2 (5)/TBACl
95
a Reactions were performed in a steel autoclave for 8.0 h at 100°C
and 0.6 MPa of CO2. b Determined by 1H NMR spectroscopy using
2,4-dinitrotoluene as the internal standard (uncertainty calculation: 1%).
c Method A was used, except benzene was the reaction solvent. d Method
B was used. eAfter three consecutive reactions.f Performed by using 1.0 g
of starting epoxide and 1/TBACl/epoxide = 1:10:100 in 25.0 mL of
benzene. gAr′ = 3,5-(NO2)2C6H3.
The chemical stability of the catalytic Ru(TPP)
(NAr)2 (1)/TBACl system, under the experimental
conditions employed, was tested to investigate the
recyclability of the catalyst. The synthesis of cyclic
carbonate 2 was performed three consecutive times
by adding the suitable amount of epoxide, after each
complete consumption, to the initial catalytic mixture.
As shown in entry 4 of Table 1, the catalytic performance
was completely maintained, indicating the lack of
decomposition processes of the employed catalyst. Next,
in order to investigate a possible scale-up of the Ru(TPP)
(NAr)2 (1)/TBACl-mediated procedure, the synthesis of
2 was executed by using 1.0 g of the starting epoxide
(Table 1, entry 5).We were delighted to observe a complete
retention of the reaction productivity, which paves the
way to some practical applications of the procedure.
While the reaction efficiency was completely
maintained when the catalyst Ru(TPP)(NAr)2 (1) was
replaced by Ru(TPP)(NAr′)2 (3) (Ar′ = 3,5-(NO2)2C6H3)
(Table 1, entry 6), a slight decrease of the epoxide
conversion was observed when more sterically
encumbered porphyrin ligands were employed. In fact,
the complete conversion of the epoxide was not achieved
regardlessoftheelectroniccharacteristicsoftheporphyrin
ring. However, the very high epoxide conversions of 93%
and 95% were obtained in the presence of Ru(F20TPP)
(NAr)2 (4) (F20TPP = dianion of tetrapentafluorophenyl
porphyrin) (Table 1, entry 7) and Ru(TMP)(NAr)2 (5)
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C. DAMIANO ET AL.
Table 2. Synthesis of cyclic carbonates 2, 6–12a
Entry
Cyclic carbonate
TBAClb
Conv. (%)d
Ru(TPP)(NAr)2 (1)/TBAClc
Conv. (%)d sel. (%)d
sel. (%)d
O
O
O
1
91
63
100
99
O
2
O
O
O
2
3
100
100
84
99
100
100
94
99
OPh
6
O
O
O
Ph
7
O
O
O
4
5
100
27
95
100
100
65
99
Ph
Ph
8
O
O
O
81
Me
9
O
O
O
e
e
6
—
14f
—
15f
10
O
O
O
7
8
10
99
15
99
17
Cl
11
O
O
O
100
9
100
Me
Me
12
a Reactions were performed in a steel autoclave for 8.0 h at 100°C and 0.6 MPa of CO2. b TBACl/
epoxide = 10:100 in THF. c 1/TBACl/epoxide = 1:10:100 in benzene. d Determined by 1H NMR
spectroscopy by using 2,4-dinitrotoluene as the internal standard (uncertainty calculation: 1%). e The
selectivity was not determined because the unreacted epoxide was eliminated during the work-up due
to its low boiling point. fYields.
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SYNTHESIS OF CYCLIC CARBONATES BY RUTHENIUM(VI) bis-IMIDO PORPHYRIN/TBACL-CATALYZED REACTION
5
O
O
Bu
N
Bu
Ru(TPP)(NAr)2 (1)/TBACl
O
+
+
O
N
O
O
CO2 (0.6 MPa)
Ph
Ph
Ph
9, 99% yield
Ph
13, 21% yield
Scheme 3. Competitive cycloaddition of CO2 to an equimolar mixture of epoxide
and aziridine
O
O
O
O
R1
R2
+ 1
O
TBACl
path a
NAr
−
R1
R2
ArN
Oδ
O
Ph
Ph
Ph
N
Ru
CO2
Ph
N
N
N
ArN
O-(TBA)+
Ph
+
N
δ Ru
N
N
Ph
Ph
N
Ph
N
Ru
N
Ph
Ph
N
N
TBACl
-[Cl]
1 NAr
A
NAr
Ph
Ph
path b
14
NAr
Scheme 4. Formation of complex 14
with low selectivities both in the presence or absence of
1, due to the presence of a tetra-substituted carbon atom
on the epoxide ring. The addition of Ru(TPP)(NAr)2 (1)
did not promote the formation of the desired compound in
an acceptable amount. It should be noted that the epoxide
conversion was not determined in this case because its
low boiling point favored the complete elimination of the
reagent during the work-up, which was executed in order
to perform the NMR analysis of the crude. Very modest
catalytic results were also observed in the synthesis of
cyclic carbonate 11 (Table 1, entry 7) where the epoxide
was converted in a very low extent in both cases. In spite
of the small amount of the converted epoxide, a 99% of
the cyclic carbonate selectivity was obtained by using
both the catalytic procedures. Finally, the low yield in
which cyclic carbonate 12 was obtained could be due to
the pronounced attitude of 7-oxabicyclo[4.1.0]heptane
reacting with CO2 forming poly(cyclohexene carbonate)
alongside the corresponding cyclic carbonate.
As reported in the Introduction, the Ru(TPP)
(NAr)2 (1)/TBACl binary system also mediated the
CO2 cycloaddition to aziridines; thus, a competitive
experiment was performed to assess which, between
epoxides and aziridines, can be better activated by this
catalytic system. The reaction was performed by reacting
equimolar amounts of styrene oxide and 1-butyl-2-phenyl
aziridine with CO2 under the experimental conditions
reported in Table 2. The NMR analysis of the crude after
8 h revealed the lower reactivity of aziridine with respect
to that of epoxide. In fact, while epoxide was completely
converted into corresponding cyclic carbonate 9, only
21% of the aziridine conversion occurred to form
corresponding N-butyl oxazolidinone 13 with 99% of
selectivity (Scheme 3).
In order to better clarify the role of Ru(TPP)(NAr)2
in the catalytic reaction, some experiments were
executed. In our previously published paper on the CO2
cycloaddition to aziridines catalyzed by the Ru(TPP)
(NAr)2 (1)/TBACl binary system, we reported the
formation of the deactivated Ru(TPP)(NAr)(ArNCOO-
TBA+) complex (14) at the end of the catalytic reaction
[35]. The formation of this latter complex indicated that
the imido axial ligand on the ruthenium atom was able to
interact with CO2 thanks to the high electron density on
the nitrogen atom, as already indicated by our previous
DFT study [36].
The formation of complex 14 was also detected by
ESI-MS at the end of the catalytic reaction forming
cyclic carbonate 2. Analogously to what was observed in
the case of the oxazolidinone synthesis [35], ruthenium
complex 14 was catalytically inactive when used in
combination with TBACl for promoting the synthesis
of 2, which was formed with the same conversion and
selectivity values as achieved in the presence of TBACl
alone (see Table 1, entry 1). In view of the formation of
14, a positive interaction between Ru(TPP)(NAr)2 (1) and
CO2 was again supposed, and it may be responsible for
the formation of the putative CO2/1 adduct A (Scheme 4).
While the formation of intermediate A should represent
the first step of the cyclic carbonate formation in the
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6
C. DAMIANO ET AL.
presence of the epoxide (path a, Scheme 4), it could be
responsible for the formation of the inactive compound
14 when the epoxide is not present in the reaction
medium (path b, Scheme 4). Thus, intermediate A can
either evolve during the productive catalytic cycle or be
transformed into the deactivated catalytic compound 14
by a dead-end reaction.
Unfortunately, any attempt to detect the adduct
A between CO2 and complex 1 as well as products
containing chlorine atoms, which derive from the TBACl
decomposition forming 14, have failed up to now. Even if
the mechanism showed in Scheme 4 is not fully supported
by experimental data, the isolation of compound 14 at
the end of the catalytic reaction strongly indicated the
reactivity of ruthenium imido nitrogen atoms towards
carbon dioxide.
Considering that cyclic carbonates were also formed
in the presence of TBACl alone and that all the reactions
were carried out by using a 1/TBACl molar ratio of 1:10,
it was difficult to assess the real contribution of complex
1 in catalytic reactions occurring in the presence of a
TBACl excess. To shed some light on this aspect, the
model reaction forming 2 was performed by using a
TBACl defect (1/TBACl/epoxide = 2:1:100) in order to
favor the catalytic activity of 1 at the expense of the free
TBACl. The reaction carried out under these conditions
occurred in a higher yield (93%) than that performed
in the presence of TBACl alone (57%), suggesting the
active role of the ruthenium species in the catalytic cycle.
It should be noted that the model reaction forming 2
didn’t occur in presence of the sole ruthenium complex 1.
In the suggested mechanism, while the ruthenium
complex is responsible for the CO2 activation, TBACl
is necessary for the epoxide ring-opening reaction.
This proposal was supported by the lack of interaction
between 2-((allyloxy)methyl)oxirane and complex 1, as
pointed out by the absence of any shift of original IR
signals when the two compounds were reacted in the
absence of CO2. On the other hand, when 2-((allyloxy)
methyl)oxirane was reacted with Ru(TPP)CO, which
presents a free coordinating site, the IR spectrum of the
resulting solution showed the shift of the C=O IR signal
of Ru(TPP)CO from 1957 cm-1 to 1817 cm-1. In addition,
the IR signal attributed to epoxide shifted from 1814 cm-1
to 1715 cm-1 upon its plausible coordination to the penta-
coordinated ruthenium atom.
[37], 3,5-bis(nitro)phenyl azide [37], Ru(TPP)CO (TPP =
dianion of tetraphenyl porphyrin) [38], Ru(TPP)(NAr)2
(1) (Ar = 3,5-(CF3)2C6H3) [39], Ru(TPP)(NAr′)2 (3) (Ar′ =
3,5-(NO2)2C6H3 [40], Ru(F20TPP)(NAr)2 (4) (F20TPP =
dianion of tetrapentafluorophenyl porphyrin) [35], and
Ru(TMP)(NAr)2 (5) (TMP = dianion of tetramesityl
porphyrin) [35] were synthesized by methods reported in
the literature or by using minor modifications to them. All
the other starting materials were commercial products and
used as received. Analytical data of cyclic carbonates 2,
6–12 [41, 42] and oxazolidinone 13 [35] were in accordance
with those reported in the literature. NMR spectra were
recorded at room temperature either on Bruker Avance
1
300-DRX spectrometer operating at 300 MHz for H
and at 75 MHz for 13C, or on a Bruker Avance 400-DRX
spectrometer operating at 400 MHz for 1H and at 100 MHz
for 13C. Chemical shifts (ppm) are reported relative to
1
TMS. The H NMR signals of the compounds described
in the following were attributed by 2D NMR techniques.
Assignments of the resonance in 13C NMR were made
by using the APT pulse sequence, HSQC and HMBC
techniques. Infrared spectra were recorded on a Varian
Scimitar FTS 1000 spectrophotometer. UV-vis spectra were
recorded on an Agilent 8453E instrument. Mass spectra
were recorded in the analytical laboratories of Milan
University.
General catalytic procedures
Method A. In a 100 mL glass liner equipped with a
screw cap and glass wool, TBACl (9.71 × 10-5 mol) and
the epoxide (9.71 × 10-4 mol) in a molar ratio = 10/100
were dissolved in THF (3.3 mL). The reaction mixture
was cooled with liquid nitrogen and the flask was
transferred into a stainless-steel autoclave; three vacuum-
nitrogen cycles were performed, and 0.6 MPa of CO2 was
charged at room temperature. The autoclave was placed
in a preheated oil bath at 100°C and stirred for 8 h, then
it was cooled at room temperature and slowly vented.
The solvent was evaporated to dryness and the crude
was analyzed by 1H NMR with 2,4-dinitrotoluene as the
internal standard.
Method B. The procedure illustrated for method A
was employed by using the ruthenium catalyst (5.14 ×
10-6 mol),TBACl (5.14 × 10-5 mol) and the epoxide (5.14 ×
10-4 mol) in the molar ratio = 1/10/100 and benzene
(3.3 mL) as the reaction solvent.
Synthesis of 4-((allyloxy)methyl)-1,3-dioxolan-2-
one (2). The epoxide 2-((allyloxy)methyl)oxirane was
employed. Collected data were in accordance with those
EXPERIMENTAL
1
reported in the literature [41]. H NMR (300 MHz,
General
CDCl3): dH, ppm 6.00–5.86 (1H, m), 5.31–5.21 (2H, m),
4.86–4.75 (1H, m), 4.50 (1H, t, J = 8.3 Hz), 4.43–4.36
(1H, m), 4.07 (2H, d, J = 4.6 Hz), 3.71–3.59 (2H, m).
Synthesis of 4-(phenoxymethyl)-1,3-dioxolan-2-one
(6). The epoxide 2-(phenoxymethyl)oxirane was
employed. Collected data were in accordance with those
Unless otherwise specified, all the reactions were
carried out under nitrogen atmosphere employing standard
Schlenk techniques and vacuum-line manipulations. All the
solvents were dried by using standard procedures unless
otherwise specified. 3,5-Bis(trifluoromethyl)phenyl azide
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SYNTHESIS OF CYCLIC CARBONATES BY RUTHENIUM(VI) bis-IMIDO PORPHYRIN/TBACL-CATALYZED REACTION
7
1
reported in the literature [42]. H NMR (300 MHz,
CDCl3): dH, ppm 7.30 (2H, t, J = 8.3 Hz), 7.01 (1H, t,
J = 7.3 Hz), 6.92 (2H, d, J = 7.9 Hz), 5.06–4.99 (1H,
m), 4.64–4.58 (1H, m), 4.54–4.50 (1H, m), 4.28–4.09
(2H, m).
was followed by using Ru(TPP)(NAr)2 (1) (1.72 ×
10-6 mol), TBACl (1.72 × 10-5 mol) and an equimolar
amount of 1-butyl-phenyl aziridine (1.72 × 10-4 mol)
and 2-phenyloxirane (1.72 × 10-4 mol) as the reagents in
1.1 mL of benzene. The NMR analysis of the crude
revealed the formation of 9 (99% yield) and 13 (21%
yield) and collected analytical data of 9 [42] and 13 [35]
were in accordance with those reported in the literature.
Synthesis of 3-butyl-5-pheniloxazalidin-2-one (13).
Collected data are in accordance with those reported in
Synthesis of 4-(chloromethyl)-1,3-dioxolan-2-one
(7). The epoxide 2-(chloromethyl)oxirane was employed.
Collected data were in accordance with those reported in
the literature [42]. H NMR (400 MHz, CDCl3): d, ppm
5.09–4.89 (1H, m), 4.61 (1H, t, J = 8.5 Hz), 4.45–4.38
(1H, m), 3.75 (2H, d, J = 5.1).
1
1
the literature [35]. H NMR (300 MHz, CDCl3): d, ppm
Synthesis of 4-ethyl-1,3-dioxolan-2-one (8). The
epoxide 2-ethyloxirane was employed. Collected data
were in accordance with those reported in the literature
0.94 (t, J = 7.2 Hz, 3H), 1.31–140 (2H, m), 1.51–1.58
(2H, m), 3.23–3.38 (2H, m) 3.43 (1H, t, J = 8.0 Hz), 3.92
(1H, t, J = 8.8 Hz), 5.49 (1H, t, J = 8.0 Hz), 7.28–7.42
(5H, m).
1
[41]. H NMR (400 MHz, CDCl3): d, ppm 4.66–4.60
(1H, m), 4.46 (1H, t, J = 8.2 Hz), 3.98 (1H, t, J = 8.0 Hz),
1.60–1.66 (2H, m), 0.97 (3H, t, J = 7.4 Hz).
Study of the interaction between 2-((allyloxy)methyl)
oxirane and Ru(TPP)CO by IR spectroscopy. 24 mL of
2-((allyloxy)methyl)oxirane (2.03 × 10-4 mol) in benzene,
showing the characteristic IR signal at 1814 cm-1, was
added to a benzene (0.3 mL) solution of Ru(TPP)(CO)
(15.0 mg, 2.03 × 10-5 mol), showing the characteristic
IR signal at 1957 cm-1. The IR spectrum of the resulting
solution displayed two new signals at 1715 cm-1 and
1817 cm-1 as well as the absence of signals at 1814 cm-1
and 1957 cm-1.
Study of the interaction between 2-((allyloxy)methyl)
oxirane and Ru(TPP)(NAr)2 (1) by IR spectroscopy.
The analogous reaction was performed by using Ru(TPP)
(NAr)2 (1) (IR signal at 1485 cm-1). No shifts were
observed upon the epoxide addition.
Synthesis of 4-phenyl-1,3-dioxolan-2-one (9). The
epoxide 2-phenyloxirane was employed. Collected data
were in accordance with those reported in the literature
1
[42]. H NMR (300 MHz, CDCl3): d, ppm 7.47–7.34
(5H, m), 5.67 (1H, t, J = 8.0 Hz), 4.79 (1H, t, J = 8.4 Hz),
4.33 (1H, t, J = 8.0 Hz).
Synthesis of 4,4-dimethyl-1,3-dioxolan-2-one (10).
The epoxide 2,2-dimethyloxirane was employed. Collec-
ted data were in accordance with those reported in the
1
literature [41]. H NMR (400 MHz, CDCl3): d, ppm
4.13 (2H, s), 1.91 (6H, s).
Synthesis
of
4,5-diphenyl-1,3-dioxolan-2-one
(11). The epoxide 2,3-diphenyloxirane was employed.
Collected data were in accordance with those reported in
1
the literature [41]. H NMR (300 MHz, CDCl3): d, ppm
CONCLUSIONS
7.51–7.45 (6H, m), 7.35–7.36 (4H, m), 5.44 (2H, s).
Synthesis of hexahydrobenzo[d][1,3]dioxol-2-one
(12). The epoxide 7-oxabicyclo[4.1.0]heptane was
employed. Collected data were in accordance with those
In conclusion, we described the activity of the
ruthenium(VI) bis-imido porphyrin complex/TBACl
binary system in catalyzing the synthesis of cyclic
carbonates by the insertion of CO2 into the epoxide ring.
Under the optimized experimental conditions of 0.6 CO2
MPa and 100°C, the activity of TBACl was enhanced
by the addition of the ruthenium species, which was
fundamental for maximizing both the epoxide conversion
and the cyclic carbonate selectivity. The catalytic system
was effective for the CO2 cycloaddition to differently
substituted epoxides, which were completely transformed
into corresponding cyclic carbonates displaying a low
steric hindrance on the ring. The chemical stability of
the catalytic system allowed its efficient recyclability,
as proven by the reuse of the binary system for three
consecutive times. In addition, the reaction scale-up
was also carried out by transforming 1.00 g of starting
2-((allyloxy)methyl)oxirane into cyclic carbonate 2
(99% yield). Obtained data indicated that the presence
of a Lewis acidic transition metal catalyst is not always
required and that the coordinatively saturated and
“non-acid” ruthenium(VI) bis-imido porphyrin species
probably activates CO2 instead of epoxide, conversely to
what is usually proposed for transition metal-catalyzed
1
reported in the literature [41]. H NMR (400 MHz,
CDCl3): d, ppm 4.69–4.64 (2H, m), 1.91–1.79 (2H, m),
1.72–1.50 (4H, m), 1.48–1.33 (2H, m).
Recycle of Ru(TPP)(NAr)2 (1)/TBACl. Method B
was followed by using 2-((allyloxy)methyl)oxirane as
the reagent. After the consumption of epoxide, which
was monitored by TLC analysis (n-hexane/AcOEt =
8:2), 2-((allyloxy)methyl)oxirane was added again to the
catalytic mixture for two more consecutive times. The
1H NMR analysis of the crude revealed 99% of global
yield of compound 2.
Scale-up of the cyclic carbonate 2 synthesis. The
catalytic procedure was followed by using Ru(TPP)
(NAr)2 (1) (1.72 × 10-6 mol), TBACl (8.76 × 10-4 mol)
and 2-((allyloxy)methyl)oxirane as the epoxide reagent
(1.0 g, 8.76 × 10-3 mol) in the molar ratio 1/TBACl/
epoxide = 1/10/100 and benzene (25.0 mL) as the reaction
solvent. Compound 2 was obtained with 99% yield.
Comparison of the reactivity of 2-((allyloxy)methyl)
oxirane and 1-butyl-2-phenilaziridine towards CO2 in
the presence of 1/TBACl system. The catalytic procedure
Copyright © 2019 World Scientific Publishing Company
J. Porphyrins Phthalocyanines 2019; 23: 7–8

8
C. DAMIANO ET AL.
cyclic carbonate syntheses. This hypothesis can open
the door to several applications of Ru(porphyrin)(NAr)2-
based catalytic systems where CO2 can be activated in
other carbon dioxide valorization processes.
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