9
76
C. Yue et al.
Herein, we report an efficient and environmentally
[MMPPA][X]
rt
R-OH
+
Ac2O
R-OAc
friendly acetylation of alcohols and phenols with acetic
anhydride in the presence of 1 mol% economical ionic
liquids based on morpholinium salts at room temperature
under solvent-free conditions in short reaction time. To the
best of our knowledge, acetylations with these ionic liquids
as catalysts are unprecedented in the open literature.
O
X
[MMPPA][X]:
N
SO H
3
H3C
-
-
-
-
X = HSO4 , CF3COO , BF4
Scheme 1
Results and discussion
First, we investigated the miscibility of the morpholinium
ionic liquids with other chemicals. All of the ionic liquids
were miscible with acetic anhydride and water, but were
immiscible with all products. Excess of acetic anhydride
was necessary for the reaction to proceed at a reasonable
rate and accelerated the solubility of solid substrates in the
acetic anhydride. The reaction of benzyl alcohol with
acetic anhydride was chosen as a model reaction to test the
catalytic activity of these Brønsted acidic ionic liquids and
no other solvents were added. The results are shown in
Table 1. No appreciable amount of the corresponding
product was detected even after prolonging the reaction time
up to 30 min (entry 1, Table 1) in the absence of catalyst.
using [MMPPA][HSO ] as catalyst, and the results are
4
summarized in Table 2. Good yields were obtained for a
variety of primary, secondary, tertiary, cyclic, benzylic
alcohols, and substituted phenols. Primary alcohols were
acetylated faster than secondary and tertiary alcohols
(entries 1, 4, and 5, Table 2). Furthermore, with the
increasing of the length of alkyl chain of primary alcohols
and the reaction time, the yields of acetylated products
were decreased smoothly (entries 1, 2, and 3, Table 2). It
was very interesting to note that tertiary alcohols such as
tert-butyl alcohol can also be acetylated in 64% yield and
there was no elimination product in the mixture through
GC (entry 5, Table 2). Under similar conditions, the effi-
cacy of the catalyst can be clearly visualized in the
acetylation of diols and triols (entries 6 and 7, Table 2).
The structural properties of phenols were examined
carefully and the results showed that the reactions were
affected by electronic and steric factors. Table 2 shows that
phenols with electron-donating groups were converted to
the corresponding phenyl acetates in higher yields and
shorter reaction time than those with electron-withdrawing
groups. Furthermore the selectivity in the acetylation of
diols was also high, as examined through HPLC.
The ionic liquid [MMPPA][HSO ] (N-methylmorpholinium
4
propanesulfonic acid ammonium hydrogensulfate) was
demonstrated to be the best catalyst. We reasoned that
[
MMPPA][HSO ], which has two catalytic groups (–SO H
4 3
-
and HSO4 ), possessed higher catalytic capacity than
MMPPA][BF ] (N-methylmorpholinium propanesulfonic
[
4
acid tetrafluoroborate) or [MMPPA][TFA] (N-methylmorph-
olinium propanesulfonic acid trifluoroacetate) both of which
have a sole –SO H group. Th amount of [MMPPA][HSO ]
3
4
was then optimized. The experimental results showed that
by increasing the amount of [MMPPA][HSO ] catalyst,
4
The stability and reusability of the Brønsted acidic
the acetylation was greatly accelerated. When 1 mol% of
the catalyst was employed, the acetylation of benzyl alco-
hol with acetic anhydride was completed in 99% yield at
room temperature (25 °C) under solvent-free conditions
ionic liquid [MMPPA][HSO ] were investigated in the
4
acetylation of benzyl alcohol with acetic anhydride as a
model reaction. The results listed in Table 3 show that
[MMPPA][HSO ] could be reused for four times without a
4
(
Scheme 1).
noticeable decrease in the catalytic activity. Furthermore,
no changes in the selectivity were observed. Comparing the
IR spectra of the reused ionic liquid with that of fresh ionic
liquid, small differences were observed. These results
demonstrated that the structure of the catalyst did not
change.
On the basis of these results, we examined the acetyla-
tion of various alcohols and phenols with acetic anhydride
Table 1 Acetylation of benzyl alcohol with acetic anhydride cata-
lyzed by Brønsted acidic ionic liquids at room temperature (25 °C)
under solvent-free conditions
In conclusion, it was demonstrated that [MMPPA][HSO4]
was a highly efficient, chemoselective, and reusable cata-
lyst for acetylation of alcohols and phenols with acetic
anhydride under solvent-free conditions at room tempera-
ture. In terms of environmental and economical factors,
a
Yield (%)
Entry
Catalyst
Time (min)
1
2
3
4
a
None
30
2
0
78
75
99
[MMPPA][BF
[MMPPA][CF
4
]
3
COO]
2
[MMPPA][HSO ] is also a promising alternative to the
4
[MMPPA][HSO
4
]
2
traditional catalysts. Therefore, we believe that the new
synthesis method reported here would contribute greatly to
Isolated yield of the acetylated product
1
23