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The OH-functionalized ammonium bromide 7 has been in-
vestigated previously as a catalyst in batch and continuous-
flow reactions, reaching turnover frequencies of 3000–
14000 hÀ1 for the cycloaddition of CO2 and epoxides.[11a,17] For
this reason, 7 was chosen as a benchmark for the catalysts 1–
6. Nevertheless, separation of 7 from the product is difficult
because of the good solubility of this catalyst in propylene
oxide (PO) and propylene carbonate (PC). In addition, the high
pressures and temperatures (3.5 MPa CO2, 1808C) that are re-
quired are highly undesirable from an environmental point of
view.
In the absence of a catalyst, no conversion of PO occurred
under the applied reaction conditions. For a more detailed
evaluation of the activity, the molar amount of the bromide
anion was constant for all investigated catalysts. Additionally,
this allows a direct comparison to previous results.[16] By vary-
ing the substituent of the side chain of the catalyst, the con-
version of PO increased in the order methyl<benzyl<n-octyl.
This trend is mainly attributed to the better solubility of 3 in
PO and PC. Catalysts 1 and 2 are insoluble, even in PC, where-
as 3 remained in solution during the entire course of the reac-
tion. After 16 h a PO conversion of 95% was achieved. This is
the best result for the synthesis of PC under the applied condi-
tions, so far. Compared with our previous results, the reaction
time was decreased from 22 h to 16 h.[16] The high activity of 3
presumably results from three neighboring hydrogen-bond
donors (a hydroxyl group and two imidazolium cation units),
which activate the epoxide and stabilize the ring-opened inter-
mediate.[9d] For a better insight into the influence of the OH
groups on epoxide activation, imidazolium bromides with only
two hydrogen-bond donors were synthesized and applied as
catalysts for the cycloaddition of CO2 and PO. For catalysts 4
and 5, one hydroxyl group and one imidazolium C2 proton
can act as hydrogen-bond donors, respectively, and two imida-
zolium C2 protons for catalyst 6. As mentioned previously for
1–3, the conversion of PO also increases with longer chain
length in the order methyl<n-octyl for catalysts 4 and 5. Nev-
ertheless, using 5 as catalyst resulted in 79% conversion of PO,
which is significantly lower than that achieved by using 3
(95%). The solubility of 3 and 5 in PC is comparable; therefore,
the observed yields of PC indicate a positive influence of the
second imidazolium C2 proton on the catalytic performance.
This is in accordance with the results of Kleij and co-workers,
who demonstrated that systems containing three hydrogen-
bond donors exhibit the best catalytic activity.[9d] For a better
understanding of the role of the hydroxyl group of 3, an analo-
gous cation without a hydroxyl group was investigated (com-
pound 6). When using 6 as catalyst, the PO conversion de-
creased to 58%, hence it can be stated that the hydroxyl
group is crucial for an efficient activation of the epoxide. Final-
ly, 3 was compared to 7 to classify the performance of the cat-
alyst. In this regard, compound 3 performed slightly better
than compound 7 (95 versus 92% PO conversion). Additionally,
PC can be easily separated from 3 with diethyl ether. When
using 7 as catalyst, PC can be only separated by distillation,
owing to the good solubility of 7 in PC. This is a highly ener-
getic process because of the high boiling point of PC and,
therefore, has a negative effect on the carbon footprint of the
reaction. Consequently, compound 3 offers the possibility of
a sustainable system for the chemical fixation of CO2. For this
reason, further investigations, such as optimization of the reac-
tion parameters, recycling studies, and substrate screening,
were performed using 3 as the catalyst.
Therefore, compounds 1–7 were used as catalysts under the
previously reported mild reaction conditions (10 mol% catalyst
loading, 708C, 0.4 MPa CO2, neat). The cycloaddition reaction
was conducted with CO2 and PO to yield PC.[16] In a typical ex-
periment, a Fisher–Porter bottle was charged with the catalyst
and substrate, pressurized with 0.4 MPa CO2 for 1 min and
heated to the desired temperature for 16 h. The yield and se-
lectivity were determined by using gas chromatography (GC).
The results are summarized in Table 1. In all examined cases,
the selectivity for PC was higher than 99%. A reaction time of
16 h was needed to reach 95% conversion with catalyst 3
(Figure 2).
Table 1. Synthesis of PC from PO and CO2 by using catalysts 1–7.[a]
Catalyst
Catalyst amount
[mmol]
PO
[mmol]
Conversion
[%][b]
Selectivity
[%][b]
–
1
2
3
4
5
6
7
0
10.0
10.0
10.0
10.0
10.0
10.0
10.0
10.0
0
31
57
95
32
79
58
92
–
ꢀ99
ꢀ99
ꢀ99
ꢀ99
ꢀ99
ꢀ99
ꢀ99
0.5
0.5
0.5
1.0
1.0
0.5
1.0
[a] Reaction conditions: 708C, 0.4 MPa CO2, 16 h. [b] Conversion and se-
lectivity were based on GC analysis.
The time-dependent conversion of PO was investigated by
a series of 12 identical experiments. The reactions were
stopped after a defined period and samples were taken and
analyzed by means of GC. The conversion of PO was plotted
versus time (Figure 2), showing that after 16 h almost quantita-
Figure 2. Studies of time-dependent conversion of PO; reaction conditions:
10.0 mmol PO, 5 mol% 3, 0.4 MPa CO2, 708C; PO conversion based on GC
results; Selectivity towards PC ꢀ99%.
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