Scheme 1 The cyclization of citronellal to p-menthane-3,8-diols.
a
Table 1 Influence of CO
2
pressure on citronellal cyclization
Table 2 Results of citronellal cyclization at different temperatures
Selectivity (%)
Selectivity (%)
◦
Entry
CO
2
/MPa
Conversion (%)
2
3
4
Entry
T/ C
Conversion (%)
2
3
4
1
2
3
4
5
6
7
8
9
0
1
—
1
4
6.5
7
7.5
8
—
6
7.5
12
8
42
49
50
53
46
43
16
93
97
28
27
22
20
17
20
20
20
24
13
12
21
58
73
76
79
77
76
76
70
86
85
76
15
5
4
4
3
4
4
6
1
3
3
1
2
3
4
5
90
100
110
120
130
60
73
80
83
83
16
17
19
21
26
79
81
76
74
71
5
2
5
5
3
Reaction conditions: (±)-citronellal: 1 mmol, water: 10 ml, CO
t: 2 h.
2
: 1 MPa,
b
b
b
1
1
b
16
-1
measurement as described in the literature, it was 6.25 mmol L
at 8.5 MPa CO
(see Fig S1, ESI†).
-
1
a
◦
2
, and increased to 25 mmol L at 11.2 MPa CO
2
Reaction conditions: (±)-citronellal: 1 mmol, water: 10 ml, T: 120 C,
t: 0.5 h. (S)-(-)-citronellal: 1 mmol, water: 10 ml, T: 100 C, t: 2 h.
b
◦
The solubility of CO and solution acidity increases with CO
2
2
pressure up to 7 MPa, so that the following reactions were carried
out at a CO pressure of 1 MPa. The effects of temperature
and p-menthane-3,8-diols condensation to p-menthane-3,8-diol
2
15
citronellal acetals which was referred to the literature.
In this work, the cyclization of citronellal in pressurized CO
O medium in the absence of additives has been studied.
The influence of CO pressure was examined and the results
are shown in Table 2. The conversion increased from 60% to
◦
2
–
83% when the reaction temperature was enhanced from 90 C
◦
H
2
to 120 C (entries 1 and 4), then it almost did not change at a
◦
2
higher temperature of 130 C. The selectivity to p-menthane-3,8-
◦
are shown in Table 1. When (±)-citronellal was used, both
the conversion and the selectivity to p-menthane-3,8-diols were
diols presented a maximum of 81% at 100 C (entry 2), then it
decreased at higher temperatures. Both temperature and acidity
of the system are important factors influencing the cyclization
of citronellal. Higher temperature and higher acidity favor the
present reaction, but the acidity of the system becomes weaker at
improved in the presence of CO
with increasing CO pressure, it increased to a maximum value
of 53% at 7.0 MPa CO (entry 5), which is about 6 times higher
than that (8%) obtained in the absence of CO (entry 1). The
changes of conversion with CO pressure may depend on the
solubility of CO and the acidity in the reaction solution. As
reported in the literature, the solubility of CO in the reaction
solution increases with CO pressure resulting in the solution
acidity noticeably increasing,
.8 to 3.24 when CO pressure increases from 0 to 7.0 MPa,
then it changes very little with pressure further increasing, it
2
, and the conversion increased
2
2
2
higher temperatures due to the solubility of CO
2
in the aqueous
2
2
phase decreasing under constant pressure. So that, the reaction
conversion increases and then changes slightly with increased
temperature. The higher selectivity to p-menthane-3,8-diols was
obtained at lower temperature due to the forward reaction of
isopulegols hydration to p-menthane-3,8-diols is an exothermic
reaction as discussed later.
2
2
2
2,3,6,8
the pH value changes from
6
2
The cyclization of citronellal in the CO
2
–H O system was
2
changes from 3.24 to 3.20 when CO
2
pressure was increased from
compared with the results in the presence of conventional liquid
acids and solid acid catalyst, the results are presented in Table 3.
Under the same temperature, sulfuric acid is more effective for
3
7
.0 MPa to 8.0 MPa. The selectivity to the desired product of
p-menthane-3,8-diols was larger in the CO
that in the pure water, and it did not change much with CO
pressure. When (S)-(-)-citronellal was used the conversion also
2
–H O system than
2
2
the cyclization of citronellal than acetic acid and 7 MPa CO
2
–
H
2
O system, and the selectivity to p-menthane-3,8-diols is also
increased with increasing CO
2
pressure. It increased to 93% and
, respectively (entries 9 and 10),
the highest with sulfuric acid (entries 1–3), in which the pH value
changed slightly (from 3.20 to 3.43) during the reaction. While,
97% at 6.0 and 7.5 MPa CO
2
which are more than 6 times higher than that in the absence of
CO (entry 8). While, at 12.0 MPa CO , the conversion decreased
largely due to the extraction of citronellal into CO phase and the
dilution effect (entry 11), because that the solubility of citronellal
in CO phase increased with increasing CO pressure, especially
at pressures above 7 MPa (critical pressure) from the solubility
the conversion and selectivity in CO
2
–H O system were 73%
2
2
2
and 81%, respectively. Those are higher than the values of 48%
and 73% obtained over the ZSM-5 catalyst (entries 4 and 5).
Furthermore, the conversion over ZSM-5 catalyst was increased
to 78% (entry 6) in the presence of CO
reaction solution being increased.
2
2
2
2
due to the acidity of the
1
228 | Green Chem., 2009, 11, 1227–1231
This journal is © The Royal Society of Chemistry 2009