Valorization of CO2 into N-alkyl Oxazolidin-2-ones Promoted by Metal-Free Porphyrin/TBACl System: Experimental and Computational Studies

Article abstract of DOI:10.1002/ejoc.202100365

The cycloaddition of CO₂ to N-alkyl aziridines was efficiently promoted by the convenient TPPH₂/TBACl binary catalytic system. The metal-free procedure was effective for the synthesis of differently substituted N-alkyl oxazolidin-2-o

Full text of DOI:10.1002/ejoc.202100365

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Valorization of CO into N-alkyl Oxazolidin-2-ones
2
Promoted by Metal-Free Porphyrin/TBACl System:
Experimental and Computational Studies
Caterina Damiano, Paolo Sonzini, Gabriele Manca,* and Emma Gallo*
[a]
[a]
[b]
[a]
Dedicated to Professor Franco Cozzi on the occasion of his 70th birthday.
The cycloaddition of CO to N-alkyl aziridines was efficiently
to 100% and excellent regioselectivities (up to 99%). The
mechanism of the reaction was proposed based on a DFT study
2
promoted by the convenient TPPH /TBACl binary catalytic
2
system. The metal-free procedure was effective for the synthesis
which indicated the formation of an adduct between TPPH and
2
of differently substituted N-alkyl oxazolidin-2-ones in yields up
TBACl as the effective catalytic active species.
[
7]
[8]
[9]
Introduction
drugs, such as Linezolid and Tedizolid, antidepressants,
[10]
[11]
such as Toloxatone and also in anti-cancer medicaments.
The progressive increasing of greenhouse gas emissions is
causing a constant rising in average global temperatures which
is responsible for extreme events all over the world. In order to
reverse this no longer sustainable trend, it is imperative to
reduce damaging emissions and develop sustainable circular
chemical processes, in which gaseous wastes are valorized as
new resources for the synthesis of fine-chemicals.
The catalyzed CO cycloaddition to aziridines usually occurs
2
in the presence of an electrophile/nucleophile (E/Nu) binary
system, where the electrophile is in charge of coordinating the
nitrogen aziridine atom in order to promote the opening of the
aziridine ring by the nucleophilic species (Scheme 1).
As reported in Scheme 1, the reaction produces two
regioisomers A and B (5- and 4-substituted oxazolidin-2-one,
respectively) in accordance with the presence of two inequiva-
lent aziridine carbon atoms where the nucleophilic attack can
take place.
Since carbon dioxide is one of the most harmful emissions
for the environment, the use of CO as a renewable, cheap and
2
nontoxic C1 synthetic building block is receiving increasing
[1]
scientific attention. Among all synthetic routes employing CO2
as a starting material, ring-insertion processes in epoxides and
aziridines represent 100% atom-efficient procedures for the
It should be noted that while the electrophile is usually
considered the reaction catalyst, the nucleophilic species, such
as quaternary ammonium salts or other Lewis bases (LB), is
[2]
eco-compatible production of cyclic carbonates
oxazolidinones, respectively.
and
indicated as the co-catalyst of the CO cycloaddition.
2
[3]
Even if the insertion of CO into aziridines can proceed in
2
[12]
[13]
Oxazolidinones constitute a versatile class of compounds
which, depending on substituents present on the molecular
the presence of the sole co-catalyst and neither the solvent
[1b,14]
nor catalyst
are necessary, a great improvement of the
[4]
skeleton, can be employed as synthetic intermediates, chiral
reaction productivity (higher yields and regioselectivities) and
experimental conditions sustainability (lower CO2 pressures,
lower temperatures and reduced reaction times) is generally
observed when the reaction is mediated by either
[5]
[6]
auxiliaries in organic synthesis and pharmaceuticals. In
particular, the oxazolidinone motif is found in antimicrobial
[15]
[3a,16]
[a] Dr. C. Damiano, Dr. P. Sonzini, Prof. E. Gallo
Department of Chemistry
homogeneous or heterogeneous
catalysts.
University of Milan
Via Golgi, 19, 20133 Milan, Italy
E-mail: emma.gallo@unimi.it
https://sites.unimi.it/emmagallogroup/
b] Dr. G. Manca
[
Istituto di Chimica dei Composti OrganoMetallici,
ICCOM-CNR
Via Madonna del Piano 10, 50019 Sesto Fiorentino, Italy
E-mail: gabriele.manca@iccom.cnr.it
www.iccom.cnr.it/index.php/it/istituto/personale2?view=-
member&task=show&id=132
Supporting information for this article is available on the WWW under
https://doi.org/10.1002/ejoc.202100365
Part of the “Franco Cozzi’s 70th Birthday” Special Collection.
©
2021 The Authors. European Journal of Organic Chemistry published by
Wiley-VCH GmbH. This is an open access article under the terms of the
Creative Commons Attribution License, which permits use, distribution and
reproduction in any medium, provided the original work is properly cited.
2
Scheme 1. General scheme of the CO cycloaddition to aziridine mediated
by the E/Nu binary system.
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Among all catalytic systems, which promote the CO cyclo-
2
Table 1. Synthesis of oxazolidin-2-ones 1 and 2 mediated by porphyrin/
addition to three-membered rings (epoxides and aziridines),
TBACl catalytic system.
those employing porphyrin metal catalysts in the presence of
[15b,17]
LB co-catalysts
and temperatures.
performed well under low CO pressures
2
In the last few years we have studied the performance of
porphyrin-based protocols in catalyzing the CO cycloaddition
2
[
b]
[b]
Entry
Catalyst
Yield 1 (%)
A/B ratio
Yield 2 (%)
A/B ratio
to both epoxides and aziridines. Besides the capacity of
[
b]
[b]
[18]
ruthenium porphyrins in mediating both reactions, we have
[
c]
1
2
3
4
5
6
7
TPPH
2
95
95
more recently discovered that the CO cycloaddition to N-aryl
2
87:13
95:5
aziridines performed well in the presence of metal-free
t
[c]
4- BuTPPH
2
69
83:17
61
94
[19]
porphyrin catalysts. The TPPH /TBACl (TPPH =meso-tetrakis
91:9
2
2
[
c]
4-CF
3
TPPH
2
80
phenyl porphyrin; TBACl=tetrabutyl ammonium chloride) sys-
tem demonstrated to be a convenient combination for
synthesizing N-aryl oxazolidin-2-ones. The study of the reaction
scope, using the optimized experimental conditions of 125°C,
85:15
92:8
[c]
4-COOHTPPH
2
84
86:14
69
99
86:14
[
c]
F
5
TPPH
2
76
87:13
88:12
1
.2 CO MPa and TPPH /TBACl/aziridine=1:5:100, revealed a
[c]
2
2
F20TPPH
2
43
84:16
63
74
large applicability of the protocol and a good tolerance of
different functional groups. In addition, the DFT investigation of
the reaction mechanism clarified the nature of species involved
in the catalytic cycle.
89:11
[
c]
OEPH
2
85
93:7
84:16
[
3
a] Reaction conditions: 1.5 M aziridine THF solution in a steel autoclave for
hours with catalyst/TBACl/aziridine=1:5:100 at 125°C and 1.2 MPa of
1
CO
2
. [b] Determined by H NMR using 2,4-dinitrotoluene as the internal
standard. [c] After 1 hour.
Results and Discussion
Synthetic Study
tetrakis(4-carboxyphenyl) porphyrin (4-COOHTPPH ) (entries 1
2
In view of the aforementioned results and the importance of
developing metal-free sustainable catalytic procedures, we here
present the synthesis of N-alkyl oxazolidin-2-ones by applying
and 4, Table 1) respectively, while the best regioselectivity was
registered by performing the reaction in the presence of TPPH2
(entry 1, Table 1)
[19]
the protocol described above for obtaining N-aryl derivatives.
On the other hand, the steric behavior of the catalyst
influenced the catalytic activity more, as suggested by the
lower performances observed by using 5-(pentafluorophenyl)-
10,15,20-triphenyl porphyrin (F TPPH ) (entry 5, Table 1). As
In order to compare the reactivity of N-aryl aziridines with
that of N-alkyl aziridines, two model reactions producing
oxazolidin-2-ones 1 and 2 were performed in the presence of
the co-catalyst TBACl and different free porphyrins for also
assessing the dependence of the reaction productivity from the
chemical characteristics of the catalyst. The catalytic efficiency
of the different porphyrins employed was tested by running the
synthesis of 1 and 2 for only 3 hours and 1 hour respectively, in
order to avoid the complete aziridine conversion (Table 1).
Results of Table 1 show a general more pronounced
reactivity of N-alkyl aziridines with respect to N-aryl aziridines
and in fact, better yields and A/B ratios were always registered.
Catalytic data did not suggest a real dependence of the reaction
performance on the electronic characteristic of the substituent
placed in the para position of the porphyrin meso aromatic
rings.
5
2
expected, the negative effect increased by replacing F TPPH2
5
with meso-tetrakis(pentafluorophenyl) porphyrin (F TPPH ) (en-
2
0
2
try 6, Table 1), which shows on the catalyst skeleton four
pentafluorophenyl moieties instead of only one. The effect was
more marked in the synthesis of 1 due to the presence of the
more sterically encumbered aromatic group on the aziridine
nitrogen atom. It should be noted that the use of octaethylpor-
phyrin (OEPH ), showing ethyl substituents on porphyrin
2
pyrrolic groups, was responsible for a minor negative outcome
on the reaction productivity (entry 7, Table 1). However, the
analysis of collected data revealed a limited influence of the
electronic and steric characteristics of the employed porphyrin
on catalytic performances.
In fact, only a slight improvement of yields was achieved by
It is worth mentioning that the presence of porphyrin had a
general positive effect on catalytic reactions, which, if con-
ducted by using TBACl alone under experimental conditions
reported in Table 1, produced 1 and 2 in 54% (A/B=85:15) and
80% (A/B=90:10) yields, respectively.
synthesizing 1 and 2 oxazolidin-2-ones in the presence of meso-
t
tetrakis(4-tert-butylphenyl) porphyrin (4- BuTPPH ) (entry 2, Ta-
2
ble 1) rather than meso-tetrakis(4-trifluoromethylphenyl) por-
phyrin (4-CF TPPH ) (entry 3, Table 1) catalyst. The poor elec-
3
2
tronic effect of the para-substituent on the catalytic
performance was also suggested on the basis of the similar
regioselectivities always achieved independently from the
catalyst employed. Among all the porphyrins tested, the best
Thus, in consideration of the good catalytic results (entry 1,
Table 1) observed by employing unsubstituted TPPH and due
2
to its commercial availability at a reasonable cost, TPPH was
2
used to test the activity of the other tetrabutyl ammonium salts
TBAB and TBAI. By using the same experimental conditions
yield of 1 and 2 was obtained by applying TPPH and meso-
2
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reported in Table 1, 2A/2B were formed after one hour of
reaction with yield of 27% (A/B=97:3) and 42% (A/B=98:2) in
the presence of TBAB and TBAI, respectively.
(4A/4B). This observation was also supported by the good yield
achieved in the synthesis of 5A/5B where, the linearity of the
chain, linked to the nitrogen, should allow the approach of the
aziridine to the catalytic active center and in turn its conversion
into corresponding oxazolidinones 5A/5B. The same trend was
observed in the synthesis of compounds 6A/6B and 7A/7B. The
enlargement of the ring linked to N-aziridine atom caused the
evident reduction of the reaction yield from 95% to 52%. Also,
in this case, the steric hindrance of the starting aziridine did not
affect the regioselectivity and the A/B ratio was 94:6 in both
cases.
In view of collected results, the TPPH /TBACl binary system
2
was chosen for studying the reaction scope of the N-alkyl
oxazolidinone synthesis.
Compounds reported in Table 2 were obtained by running
the reaction at 125°C and 1.2 CO MPa in the presence of 1%
2
mol of TPPH and 5% mol of TBACl. The reaction time was
2
increased from 1 hour (Table 1) to 6 hours to maximize the
productivity and in fact, 2A/2B compounds were formed in a
1
00% yield (instead than 95%). However, an identical 2A/2B
The reaction of N-benzylic aziridines occurred in good yields
ratio was observed in the two cases (compare Table 1 and
Table 2), suggesting the independence of the regioselectivity
on the reaction time.
The study of the reaction scope disclosed again that one of
the most important parameters influencing the catalytic
performance is the steric hindrance on the aziridine nitrogen
atom.
probably for the presence of the CH spacer, which allowed the
2
access of the incoming reagent to the catalytic center.
Compounds 8A/8B and 9A/9B were obtained in 67% and 90%
yields, respectively.
It is important to underline that all the yields reported in
Table 1 and Table 2 correspond to the substrate conversion
because for all tested aziridines a 100% of reaction selectivity
was registered.
As showed in Table 2, a decrease of the reaction yield was
n
observed by replacing the Bu group on the nitrogen (2A/2B
In order to explore the asymmetric version of the method-
ology, the synthesis of 2A/2B was performed in the presence of
the chiral porphyrin P* (Figure 1), which already demonstrated
to be a good chiral ligand to promote iron-based olefin
i
1
00% yield) with Bu substituent (3A/3B 84% yield). Then, when
t
the more encumbered Bu group was present on the aziridine
nitrogen center, the reaction yield drastically dropped to 18%
[20]
cyclopropanations.
Due to the general difficulty to accomplish stereoselective
ring-opening processes, the reaction formed a racemic mixture
of 2A (99% yield, 2A/2B=90:10) in the presence of P*/TBACl/
2
Table 2. Synthesis of N-alkyl oxazolidin-2-ones 2–9 mediated by TPPH /
TBACl catalytic system.
[
a]
aziridine=1:10:100 at 50°C and 1.2 MPa of CO . Thus, future
2
efforts will be devoted to investigating the stereospecific
transformation of enantiopure aziridines into corresponding
chiral oxazolidinones by fine-tuning the reported protocol.
Considering the very good results achieved by employing
bifunctional porphyrin catalysts in the CO cycloaddition to
2
[21]
epoxide, we modified F TPPH by replacing one pentafluoro
2
0
2
meso-aryl group with an aromatic moiety, displaying the
[
a] Reaction conditions: 1.5 M aziridine THF solution in a steel autoclave for
6
hours with TPPH
CO
standard.
2
/TBACl/aziridine=1:5:100 at 125°C and 1.2 MPa of
1
2
. [b] Determined by H NMR using 2,4-dinitrotoluene as the internal
Figure 1. Structure of chiral porphyrin P*.
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ammonium salt N(Bu) Cl at the end of a carbon chain. It should
cycloaddition to aziridine, a DFT study was undertaken and
results are illustrated in the next paragraph.
3
be noted that the presence of three CH groups between the
2
tetrapyrrolic core and the co-catalyst should allow good arm
mobility for shaping the molecular skeleton to the incoming
reagent.
Theoretical Investigation
The poor catalytic activity of the F TPPH /TBACl system
2
0
2
(
entry 6, Table 1), observed in the synthesis of 2A/2B, should
The theoretical study of the of N-aryl oxazolidin-2-one synthesis,
[19]
permit a better evaluation of any positive effect deriving from
the employment of a bifunctional catalyst in place of a binary
porphyrin/ammonium salt combination. The reaction of por-
phyrin 10
(
recently reported by us, suggested the catalytic cycle shown
in Scheme 3. The adduct 12, originated by the interaction of
TPPH2 with TBACl, reacted with N-aryl aziridine (aryl=
3,5(CF ) C H ) forming 13, which evolves to 14 by a ring
[22]
with N(Bu)3 in the presence of KCl yielded 11
3 2
6
3
À
Scheme 2), which mediated the synthesis of 2A/2B in 49%
opening reaction due to the nucleophilic attack of Cl to the
aziridine carbon atom. Thus, the so-obtained electron-rich
nitrogen atom activates CO2 yielding 15, whose negatively
charged oxygen atom provokes the displacement of the
chloride atom and the final 1A formation as the major
regioisomer. The overall free energy gain for the reaction was
yield and 99:1 A/B ratio.
As 2A/2B were obtained with 74% yield and A/B ratio of
8
9:11 in the presence of F TPPH /TBACl system (entry 6,
20 2
Table 1), the use of the bifunctional catalyst 11 had only a
positive effect on the reaction regioselectivity with a contempo-
rary decrease of the reaction yield. The unsatisfactory result in
term on the reaction productivity can be due either to the
requirement of a molar excess of the ammonium salt during
the catalysis (porphyrin/TBACl are usually employed in the 1:5
ratio) or the limited mobility of the N(Bu) Cl group during the
catalytic process.
In order to shed some light on the reaction mechanism and
clarify the role of the free porphyrin in mediating the CO2
À 1
estimated to be exergonic by À 2.2 kcalmol .
In order to study the similarities and differences between
the CO cycloaddition to N-aryl and N-alkyl aziridines, the latter
2
was theoretically analyzed by considering the effect of the
substituents onto the porphyrin macrocycle in the formation
and stabilization of the porphyrin/TBACl adduct. In addition, the
lack of advantages of using the bifunctional catalyst 11 was
also investigated.
3
In order to rationalize data, which were obtained by using
either the most catalytically active 4-COOHTPPH2 (entry 4,
Table 1) or the encumbered and less performing F TPPH
20
2
porphyrin (entry 6, Table 1), the reaction of these two porphyr-
ins with TBACl, producing corresponding adducts 16 and 17,
was modelled by DFT and compared to the formation of adduct
1
2 (Scheme 3).
Computational analysis revealed no significant difference in
the assembly of the two adducts 16 and 17, which occurred
À 1
with energy variations À 5.8 and À 4.5 kcalmol , respectively. It
is worth noting that the formation of both adducts was less
convenient than the synthesis of 12, which took place with an
À 1
energy gain of À 7.5 kcalmol . However, in all the three cases
Scheme 2. Synthesis of porphyrin 11.
the generation of the catalyst/co-catalyst adduct is a favorite
process that explains the positive role of both promoters in
favoring the CO cycloaddition.
2
Considering the negligible energy difference related to the
establishment of the two adducts 16 and 17 and the use of
TPPH2 for studying the reaction scope (Table 2), all the
subsequent calculations were carried out by using adduct 12 as
the first step of the 2A (Table 2) formation.
The reaction of 12 with N-butyl aziridine yielded adduct 18
À 1
(
Figure 2) with a free energy cost of +4.9 kcalmol .
Relaxed scan, obtained by a stepwise shortening of the
ClÀ C distance, revealed the presence of the Transition State
1
1
8
(Figure 2) which was optimized and achieved with a free
TS
À 1
energy barrier of 35.4 kcalmol . The high value of energy,
needed for reaching the TS, can explain the high temperature
required for the accomplishment of the CO cycloaddition (see
2
experimental).
Scheme 3. Proposed mechanism of the N-aryl oxazolidin-2-one 1A forma-
tion.
From a structural viewpoint, 18TS presents a ClÀ C distance
1
of 2.12 Å and an elongation of N À C distance up to 2.22 Å. The
2
1
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bond to 1.91 Å, was obtained (Figure 3). The modest free
À 1
energy gain of À 2.2 kcalmol revealed a limited stabilization of
the negative charge at the N atom which is responsible for an
2
efficient interaction of 19 with CO2.
In fact, the synthesis of species 20 (Figure 4), was favored by
À 1
À 20.3 kcal mol , as also certified by the modelled IR C=O
À 1
stretching at ca. 1575 cm . Compound 20 displayed a bent CO
2
moiety with the O C O angle of 127° and the strong N À C
1
3
2
1
3
bond of 1.44 Å. The final synthesis of the major regioisomer N-
butyl oxazolidin-2-one 2A occurred through the formation of
the Transition State 20 (Figure 4) with a free energy barrier of
TS
À 1
+
9.4 kcalmol . The modelling of compound 20 revealed a
TS
quasi-linearity of the O C Cl angle (177°) and a trigonal
Figure 2. Optimized structures of 18 and 18TS. Hydrogen atoms are omitted
for clarity.
1 1
bipyramidal molecular geometry around the C atom (Figure 4,
the hydrogen atom was omitted for clarity), which coordinated
1
the approaching O nucleophile and the leaving Cl anion as
1
Transition State nature of 18TS was confirmed by the detection
axial ligands. The Transition State nature of 20TS was confirmed
À 1
of a single imaginary frequency at À 230.1 cm associated with
by the detection of
a
single imaginary frequency at
À 1
the shortening of the ClÀ C distance and simultaneous
À 188.1 cm , which was associated with the approaching of O
1
1
elongation of the N À C bond.
to C and the consequent departure of Cl.
2
1
1
After the Transition State, compound 19, showing a length-
ening of the N À C distance to 2.33 Å and a shortening of C À Cl
After the accomplishment of the Transition State 20 , the
TS
final product 2A was achieved with an energy gain of
2
1
1
À 1
À 31.9 kcalmol . In conclusion, the entire free energetic profile
of the CO cycloaddition to N-butyl aziridine yielding 2A
2
À 1
occurred with an overall free energy gain of À 4.7 kcalmol , as
shown in Figure 5. This value was larger than the overall free
À 1
energy gain of À 2.2 kcalmol which was calculated for the CO2
cycloaddition to N-aryl aziridines. The energetic difference
between the two processes is in accordance to the general
higher reactivity of the N-alkyl with respect to N-aryl aziridines
towards CO2 in the presence of free porphyrin molecules
(Table 1).
As discussed in the precedent paragraph, the bifunctional
porphyrin 11 was synthesized and tested to assess a possible
positive catalytic improvement of placing both the catalyst and
co-catalyst onto the same molecular skeleton. Unfortunately,
experimental results did not suggest any advantage of using 11
instead of the binary porphyrin/TBACl combination.
Figure 3. Optimized structure of 19. Hydrogen atoms are omitted for clarity.
Figure 4. Optimized structures of 20 and 20TS. Hydrogen atoms were
omitted for clarity.
Figure 5. Free energy profile of synthesis of 2A.
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Thus, to better understand the lack of any beneficial effect
The mechanism was proposed on the basis of DFT
calculations which revealed that the reaction between the
porphyrin and TBACl yielded an adduct, which was exoergoni-
cally formed and can be considered the real catalytic active
species of the reaction.
when the bifunctional catalyst was employed to promote the
synthesis of 2A, the structure of 11 was first modelled by DFT
calculations and compared to the X-ray molecular structure of a
[22]
similar porphyrin, presenting a N(Et) I group instead of the
3
N(Bu) Cl moiety of 11. The DFT investigation was conducted in
Finally, the bifunctional catalyst 11, showing the ammonium
salt co-catalyst on the porphyrin skeleton, was synthesized but
was less efficient than the binary porphyrin/ammonium salt
combination.
3
THF solution and confirmed that, analogously to what observed
in the solid state, the ammonium cation lies on the tetrapyrrolic
core probably thanks to dispersion forces between the macro-
cycle and the alkyl chains (Figure 6). The interaction of 11 with
the incoming N-butyl aziridine yielded adduct 21 (Figure 6)
Since the TPPH /TBACl adduct is able to activate aziridine
2
ring towards a nucleophilic attack, the present report can open
the door to its use in other reactions where the ring-opening
process of aziridine is the key-step of the catalytic cycle.
À 1
with a high free energy cost of +9.7 kcalmol . The disfavoring
energy contribution was also confirmed by the large separation
between the chloride anion and the C center, which is 1.4 Å ca.
1
longer than in adduct 18 and implies a less favorite attack of
the chloride nucleophile to the aziridine carbon atom.
In view of the obtained computational and experimental
results, the formation of N-butyl oxazolidin-2-one 2A promoted
by the bifunctional porphyrin 11 was not investigated further.
Experimental Section
General methods. Unless otherwise specified, all the reactions were
carried out under nitrogen atmosphere by employing standard
Schlenk techniques and magnetic stirring. THF and benzene were
distilled over sodium and benzophenone and kept under nitrogen.
Styrene was distilled over calcium hydride and kept under nitrogen.
Acetone was distilled over calcium sulfate and kept under nitrogen.
Conclusion
[
23]
meso-Tetrakis phenyl porphyrin (TPPH2), 5-(pentafluorophenyl)-
[24]
1
0,15,20-triphenyl
porphyrin
(F TPPH ),
meso-tetrakis
octaethylporphyrin
5
2
The present manuscript reported the catalytic activity of the
[25]
(pentafluorophenyl) porphyrin (F TPPH ),
2
0
2
[
26]
TPPH /TBACl combination for the synthesis of N-alkyl oxazoli-
(OEPH2),
CF
meso-tetrakis(4-trifluoromethylphenyl) porphyrin (4-
[27]
2
din-2-ones. Among all catalytic reactions involving either
harmful metal catalysts or promoters, which are often obtained
through time-consuming procedures, the present protocol
consists of an eco-compatible, commercially available and low-
cost methodology. In addition, the low catalytic loading as well
3
t
TPPH
2
),
meso-tetrakis(4-tert-butylphenyl)
meso-tetrakis(4-carboxyphenyl) porphyrin (4-
porphyrin
[28]
(
4- BuTPPH ),
2
[29]
COOHTPPH2) and 5-(2-(3-bromopropoxy)phenyl)-10,15,20-trispen-
[
22]
tafluorophenylporphyrin (10)
were all synthesized following
1-(3,5-bis-Trifluoromethylphenyl)-2-
reported procedures.
[30]
phenylaziridine and all the N-alkyl aziridines were synthesized
[18a]
as the moderate CO pressure and temperature required further
following reported procedure.
All the other starting materials
2
were commercial products and used as received. NMR spectra were
recorded at room temperature either on a Bruker Avance 300-DRX,
favor the general application of the catalytic procedure in every
laboratory. The study of the role of the porphyrin skeleton in
modulating the catalytic efficiency revealed only a slight
dependence of the activity on the steric hindrance of the
macrocycle. The electronic behavior of the catalyst seems to
not affect the reaction performance.
1
13
operating at 300 MHz for H, at 75 MHz for C and at 282 MHz for
19
F or on a Bruker Avance 400-DRX spectrometers, operating at
1
13
19
4
00 MHz for H and at 100 MHz for C and at 376 MHz for F.
1
Chemical shifts (ppm) are reported relative to TMS. The H NMR
signals of the compounds described in the following were
attributed by 2D NMR techniques. Assignments of the resonances
1
3
in C 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 re-
corded on an Agilent 8453E instrument. Elemental analyses and
mass spectra were recorded in the analytical laboratories of Milan
University.
Synthesis of 5-(2-(3-(butyl) ammoniumpropoxy)phenyl)-10,15,20-
3
trispentafluorophenylporphyrin chloride (11). 5-(2-(3-Bromopro-
poxy)phenyl)-10,15,20-trispentafluorophenylporphyrin
(10)
100 mg, 0.133 mmol), tributylamine (246 mg, 1.33 mmol) and KCl
99 mg, 1.33 mmol) were dissolved in 10 mL of dry acetone and
(
(
refluxed for 72 hours. Then, the solvent was evaporated to dryness
and the reddish residue was purified by flash chromatography
(
SiO , gradient elution from DCM to DCM/MeOH 97:3) to get the
2
1
purple solid 11 (25% yield). H-NMR (400 MHz, CDCl ): δ 9.01–9.00
(
7
1
3
βpyrr
βpyrr
Ar
m, 2H, H ), 8.93–8.92 (m, 6H, H ), 8.05–8.03 (m, 1H, H ), 7.88–
.83 (m, 1H, H ), 7.56–7.41 (m, 2H, H ), 1.58–1.53 (m, 2H, H
.47–1.38 (m, 10H, H ) À 0.23 (t, J=6.9 Hz, 9H, H ), À 0.33–
Ar
Ar
CH2
)
CH2
CH3
CH2
NH
19
À 0.47(m, 12H, H ), À 2.88 ppm (s, 2H, H ). F NMR (376 MHz,
CDCl ) δ À 136.39 (m, 3F), À 137.54 (dd, J=24.6, 8.5 Hz, 1F), À 137.79
3
(m, 2F), À 150.71 (t, J=20.9 Hz, 2F), À 151.00 (t, J=20.9 Hz, 1F),
Figure 6. Optimized structure of 11 and 21.
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doi.org/10.1002/ejoc.202100365
À 160.62 (td, J=22.4, 8.4 Hz, 2F), À 160.77–À 161.08 (m, 3F),
Organomet. Chem. 2021, 932, 121629; c) M. M. Heravi, V. Zadsirjan, B.
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13
À 161.29 ppm (td, J=22.4, 8.4 Hz, 1F). C NMR (101 MHz, CDCl ) δ
3
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1
1
6
57.67, 147.82, 145.35, 143.65, 141.09, 138.91, 136.45, 134.87,
31.32, 129.76, 121.05, 119.52, 115.48, 113.47, 102.98, 102.32, 77.34,
4.80, 57.08, 54.39, 22.36, 21.85, 17.85, 12.26 ppm. LR-MS (ESI): m/z
+
À
calcd for (C H ClF N O): 1162.49, found 1126.6 [M ], 35.4 [X ].
59
47
15
5
Elemental Analysis calcd. for (C H ClF N O): C (60.96), H (4.08), N
59
47
15
5
(
6.02), found: C (61.78), H (4.95), N (6.65). UV-Vis λmax (DCM)/nm (log
ɛ): 414 (5.18), 508 (4.00), 539 (3.17) 584 (3.52) 637 (2.89). IR ν
max
À 1
(DCM)/cm : 3322, 3058, 2986, 2961, 2931, 2874, 2860, 1650, 1519,
[
1
499, 1482, 1266, 990. Pf >350°C.
General catalytic procedure. In a 2.0 mL glass liner equipped with
À 6
a screw cap and glass wool, the desired catalyst (3.75×10 mmol),
À 5
À 4
TBACl (5.2 mg, 1.87×10 mmol) and aziridine (3.75×10 mmol)
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.2 MPa of CO was charged at room temperature. The autoclave
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CD, PS and EG gratefully acknowledge the University of Milan-Italy
for PSR 2020 – financed project ‘‘Sustainable catalytic strategies
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Keywords: Aziridine
Oxazolidinone · Porphyrin
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Carbon dioxide
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Manuscript received: March 24, 2021
Revised manuscript received: April 27, 2021
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by Wiley-VCH GmbH