it can be seen that, although the difference in conversion was
not so great, very small amounts of t-EV and i-EV were
obtained over Ru(PPh ) Cl , indicating that the coordination
3 3 2
molar ratio of p-Ts/Pd increased from 40 to 100, the conver-
sion of tert-butyl alcohol and corresponding selectivity to t-EV
and i-EV increased while the selectivity to i-BE and ETBE
decreased (entries 2–5). When the p-Ts/Pd ratio was further
increased (entries 6 and 7), the conversion of tert-butyl alcohol
and total selectivity to t-EV and i-EV decreased. However, it is
worth noting that the variation of the selectivities for t-EV and
i-EV exhibit different trends: the former continues to increase,
while the later decreases remarkably, indicating that a strong
acidic environment is more favorable for t-EV formation. At
the same time, selectivities to by-products i-BE and ETBE also
increased, and a small amount of i-BE dimer was produced
when the molar ratio of p-Ts/Pd was too high.
number of triphenylphosphine to Ru plays an important role
in the formation of valerate; however, the reason is still not
clear at this stage. Pd was found to be the most effective cata-
lyst for hydroesterification of tert-butyl alcohol among these
transition metal complex catalysts, which is consistent with the
1
7
previous work conducted in organic solvents. A tert-butyl
alcohol conversion of 90.5 and 78.7% total selectivity to
t-EV and i-EV were obtained (entry 7). The reasons for the
different catalytic performances of these metal complexes on
the reaction are not clear at this time and further investigations
are needed to explain such differences.
The effect of the tert-butyl alcohol/Pd(PPh
on the reaction was also examined. When Pd(PPh ) Cl was
3 2 2
) Cl molar ratio
In order to show the easy isolation of the desired products,
one example of an isolated yield is given in the case of entry 7.
Through only the upper layer was recovered, a total ester yield
3
2
2
not used, as expected, only i-BE and ETBE were formed,
which may result from the dehydration of tert-butyl alcohol
itself and a dehydration reaction between tert-butyl alcohol
and ethanol in the presence of p-Ts (entry 8). Upon increasing
(
t-EV þ i-EV) of 60.1 and 96% purity was achieved, which
indicates that the desired products are mostly contained in the
upper layer.
3 2 2
the molar ratio of tert-butyl alcohol/Pd(PPh ) Cl , the con-
Under the same reaction conditions, the effect of various
ionic liquids on the hydroesterification reaction was examined
using Pd(PPh ) Cl as catalyst. The difference in product dis-
version of tert-butyl alcohol decreased correspondingly and
the selectivities to desired products (i.e., t-EV and i-EV) and
by-products (i.e., i-BE and ETBE) decreased and increased,
respectively (entries 9–13).
3
2
2
tribution between different ionic liquids in Table 1 is not
remarkable except for MBIm[PF ]. The ionic liquids employed
6
in this work can be classified as two kinds: miscible or
À
Effect of pressure, reaction time and temperature on the
hydroesterification of tert-butyl alcohol
immiscible with water. Ionic liquids that contain [BF ] or
4
À
[
version and product selectivities (entries 8–11). The formation
NO
3
]
are water miscible, and showed roughly similar con-
The effects of pressure, reaction time and temperature on
the hydroesterification reaction are shown in Table 3. With
increasing CO pressure, the conversion of tert-butyl alcohol
and total selectivity for t-EV and i-EV increased mono-
tonically, entries 1–3 and 5. The maximum selectivity for t-EV
was achieved at 6 MPa, the highest pressure used.
The reaction temperature, as expected, has a strong impact
on the catalyst performance (entries 4–6). In the region
between 373 to 413 K, the conversion of tert-butyl alcohol
and selectivity to esters increases to their maximum values at
of iso-butylene dimer, however, was observed with the water
immiscible MBIm[PF ] (entry 12), which may have some
6
bearing on the difference.
For the purpose of comparison, the same hydroesterification
rection was conducted using Pd(PPh
(
3 2 2
) Cl in organic solvents
entries 13, 14); it can be seen that the conversion of tert-butyl
alcohol was relatively lower and no ethyl tert-valerate was
produced, suggesting that the reaction medium plays an
important role in the reaction.
3
93 K and then decrease when the temperature was further
increased. Formation of iso-butylene dimer was also observed
at the highest temperature, which as higher temperatures are
more favorable to the dehydration of tert-butyl alcohol to iso-
butylene and the dimerization of iso-butylene.
Effect of molar ratios of p-Ts/Pd(PPh
alcohol/Pd(PPh Cl on the hydroesterification
of tert-butyl alcohol
3 2 2
) Cl and tert-butyl
3
)
2
2
The effect of the molar ratios of p-Ts/Pd(PPh
3
)
2
Cl
2
and tert-
butyl alcohol/Pd(PPh ) Cl on the reaction was investigated
At 393 K and 6 MPa of CO, the results of entries 5, 7, and 8
show that the conversion of tert-butyl alcohol increases with
time before leveling off after 18 h. At 18 h, 95% of the tert-
butyl alcohol has been consumed and both the selectivities to
t-EV and i-EV have reached their maximum values.
3
2
2
and the results are listed in Table 2. No reaction occurred
when p-Ts was not used, indicating that p-Ts is indispensable
for the carbonylation of tert-butyl alcohol (entry 1). As the
a
Table 2 Effect of p-Ts/Pd(PPh
3
)
2
Cl
2
and tert-butyl alcohol/Pd(PPh
3
)
2
Cl
2
molar ratios on the hydroesterification reaction
Selectivity (%)
Entry
p-Ts/Pd(PPh
3
)
2
Cl
2
tert-Butyl alcohol/Pd(PPh
3
)
2
Cl
2
Conv. (%)
i-BE
ETBE
t-EV
i-EV
Dimer
1
2
3
0
40
60
200
200
200
ꢁ0
—
13.4
8.7
16.1
7.1
8.6
10.8
23.7
8.2
10.2
14.8
29.0
26.466.3
—
—
—
—
—
—
73.7
86.7
7.8
27.9
17.7
12.5
14.1
20.9
24.9
76.3
17.2
43.7
53.9
61.9
7.67
12.3
63.5
14.1
14.3
15.1
—
15.1
7.6
5.9
2.1
50.9
61.3
—
64.6
55.5
47.4
—
59.4—
38.2
26.7
9.4—
—
4
5
6
7
8
9
0
1
2
3
80
200
87.0
100
120
140
100
100
100
100
100
100
200
200
200
90.5
89.1
81.1
59.2
88.7
75.5
70.3
65.2
63.3
—
0.8
1.7
—
b
1
100
400
1
1
1
1
—
—
600
800
1000
1.2
5.9
a
b
], 6.0 MPa CO, 393 K, 12 h, tert-butyl alcohol: 0.017 mol. In the absence of Pd(PPh
Reaction conditions: MEIm[BF
4
3
)
2
Cl
2
.
New J. Chem., 2002, 26, 667–670
669