L. Li et al. / Journal of Organometallic Chemistry 794 (2015) 231e236
233
Scheme 1 [18,20,22]. Accordingly, due to conservation of mass, PC
total yield in cycloaddition reaction and PC conversion to DMC in
transesterification reaction could be calculated as follows: PC total
yield in cycloaddition reaction ¼ (PG yield þ PC yield) ꢁ 100%, and
PC conversion to DMC in transesterification reaction ¼ DMC yield/
PC total yield in cycloaddition reaction ꢁ 100%.
2.4. 1H NMR spectra
1H NMR spectra of initial and recovered DBC were recorded on a
VARIAN 400 MHz spectrometer, and the chemical shifts are re-
ported relative to TMS. For initial DBC (denoted as “0”), 1H NMR
(400 MHz, CDCl3):
1st recovered DBC, 1H NMR (400 MHz, CDCl3)
16H), 6.66e7.24 (m, 8H); the 2nd recovered DBC, 1H NMR
(400 MHz, CDCl3)
¼ 3.84e4.33 (m, 16H), 6.67e7.24 (m, 8H).
d
¼ 3.84e4.33 (m, 16H), 6.66e7.24 (m, 8H); the
d
¼ 3.37e4.33 (m,
d
3. Results and discussion
3.1. Cocatalyst screening of KCl catalyzed one-step synthesis of DMC
Fig. 1. The side-view and top-view photographs of the liquid after reaction. (a, d) KCl
alone as catalyst; (b, e) KCl as a catalyst and DBC as a cocatalyst; (e, f) after adding
about 10 mL of deionized water based on the above Fig. 1b.
As shown in Table 1, the inexpensive KCl was used alone as
catalyst, the PO conversion and DMC yield were 98.7% and 7.8%
(entry 1, Table 1), respectively. However, the DMC yield is still very
low, which may be due to quite low solubility of KCl in these re-
actants. Actually, there is quite amount of undissolved KCl after
reaction (Fig. 1a, d), which may be leads to lower interface between
catalysts and reactants, as a consequence, lower catalytic perfor-
mance. It is well known that crown ethers and alkali halides can
form an organometallic complex due to the good complexing
ability between crown ethers and alkali metal ions [30,31], and this
complexation is beneficial to solution of alkali halides in the reac-
tant system. Accordingly [2,4],-dibenzo-18-crown-6 (DBC) was
added into the reactant system, and the results show that there is
little undissolved KCl after reaction (Fig 1b, e), and the DMC yield
increases noticeably and reaches 23% (entry 2, Table 1), an increase
of 195% compared with KCl alone as catalyst. Meanwhile, DMC yield
of DBC alone is quite low (i.e., only 0.8%, entry 3, Table 1), sug-
gesting that DBC only plays the role of cocatalyst. Actually, the
complexation between DBC and KCl is not only beneficial to solu-
tion of KCl, but it also produces more “bare anions” used as free
nucleophilic groups, which might increase the probability of the
nucleophilic reaction (i.e., the first step cycloaddition reaction of
one-step synthesis of DMC, as shown in Scheme 1) [16] and, as a
consequence, the yield of one-step synthesis of DMC.
“eCH2eCH2eOe” units was also used in the reaction. The results
indicate that the DMC yield also increases and reaches 14%, an in-
crease of 79% compared with the DMC yield of KCl alone as catalyst
(entry 1, 4, Table 1). Similarly, the DMC yield of PEG4000 alone is
quite low (i.e., only 1.7%, entry 5, Table 1), suggesting that PEG4000
also plays the role of cocatalyst. This is probably because these
longer chains of “eCH2eCH2eOe” units in PEG4000 may intertwine
and form different sizes of rings, which partly match the radius of
Kþ, resulting in weaker co-catalytic effect of PEG4000 compared
with DBC. Therefore, the crown ethers as cocatalysts are our major
research interests upon which to focus.
3.2. Co-catalytic effect of the other typical crown ethers for one-
step synthesis of DMC catalyzed by different alkali halides
Firstly, two crown ethers (i.e., DBC and 18-crown-6) were used
as cocatalysts because of their good complexing ability for Kþ, the
catalytic performance of different potassium halides catalysts were
investigated in detail (entry 1e9, Table 2). It can be seen that the
DMC yields of potassium halides alone as catalysts are relatively
low (i.e., only 5.8e7.8 %, entry 1e3, Table 2). When DBC was used as
a cocatalyst, the DMC yields significantly increase by 106e195 %
(entry 7e9, Table 2), which is higher than the increase of 18-crown-
6 as a cocatalyst (i.e., the increase is only 9e47 %, entry 13e15,
Table 2). Thus, it can be indicated that both DBC and 18-crown-6
show a certain extent co-catalytic effect in the one-step synthesis of
As stated above, the formation of organometallic complex of
DBC and KCl and the co-catalytic effect of DBC mainly because
DBC's ringlike structure is constituted of “eCH2eCH2eOe” and the
cavity size just matches the radius of Kþ ions. Then, do the other
compounds with “eCH2eCH2eOe” units also show similar co-
catalytic effect for alkali metal ions? Accordingly, polyethylene
glycol (MW ¼ 4000, abbreviated as PEG4000) constituted of
Table 1
Cocatalyst screening of KCl catalyzing one-step synthesis of DMC.a
Entry
Catalyst
Cocatalyst
PO conversion (%)
Yield (%)
DMC
PC total yield in
cycloaddition reaction (%)
PC conversion to DMC in
transesterification reaction (%)
PG
7.8
29.8
1.4
16.7
5.6
PC
By-products (1 þ 2)
1
2
3
4
5
KCl
KCl
/
KCl
/
/
98.7
99.1
53.6
98.0
41.8
7.8
23.0
0.8
14.0
1.7
35.9
41.7
1.3
44.8
13.7
55.0
27.6
50.9
36.5
22.5
43.7
71.5
2.7
61.5
19.3
17.8
32.2
29.6
22.8
8.8
DBC
DBC
PEG4000
PEG4000
a
Reaction conditions: 3 mL (42.44 mmol) of PO; 7 mL (171.73 mmol) of methanol; 2 mmol of catalyst; 1 mmol of cocatalyst; initial pressure of CO2: 2 MPa; reaction
temperature: 130 ꢀC; reaction time: 8 h.