Ruthenium(III) chloride catalyzed acylation of alcohols, phenols, and thiols in room temperature Ionic liquids

Article abstract of DOI:10.3390/molecules14093528

Ruthenium(III) chloride-catalyzed acylation of a variety of alcohols, phenols, and thiols was achieved in high yields under mild conditions (room temperature) in the ionic liquid 1-butyl-3-methylimidazolium hexafluorophosphate ([bmim][PF₆]). The ionic liquid and ruthenium catalyst can be recycled at least 10 times. Our system not only solves the basic problem of ruthenium catalyst reuse, but also avoids the use of volatile acetonitrile as solvent.

Full text of DOI:10.3390/molecules14093528

Molecules 2009, 14, 3528-3537; doi:10.3390/molecules14093528
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molecules
ISSN 1420-3049
www.mdpi.com/journal/molecules
Article
Ruthenium(III) Chloride Catalyzed Acylation of Alcohols,
Phenols, and Thiols in Room Temperature Ionic Liquids
Zhiwen Xi 1, Wenyan Hao 1, Pingping Wang 2 and Mingzhong Cai 1,*
1
Department of Chemistry, Jiangxi Normal University, Nanchang 330022, China;
E-mails: xizhiwen1983@163.com (Z.X.); wenyanhao@tom.com (W.H.)
2
Department of Chemistry, Jiujiang University, Jiujiang 332000, China;
E-mail: wpp9090@yahoo.com.cn (P.W.)
* Author to whom correspondence should be addressed; E-mail: caimzhong@163.com;
Tel./Fax: +86 7918120388.
Received: 3 July 2009; in revised form: 10 September 2009 / Accepted: 10 September 2009 /
Published: 10 September 2009
Abstract: Ruthenium(III) chloride-catalyzed acylation of a variety of alcohols, phenols,
and thiols was achieved in high yields under mild conditions (room temperature) in the
ionic liquid 1-butyl-3-methylimidazolium hexafluorophosphate ([bmim][PF6]). The ionic
liquid and ruthenium catalyst can be recycled at least 10 times. Our system not only solves
the basic problem of ruthenium catalyst reuse, but also avoids the use of volatile
acetonitrile as solvent.
Keywords: acylation; ruthenium; ionic liquid; green chemistry
Introduction
The development of environmentally friendly synthetic procedures has become a major concern
throughout the chemical industry due to continuing depletion of natural resources and growing
environmental awareness [1-4]. One of the prime concerns of industry and academia is the search for
replacements for environmentally damaging organic solvents used on a large scale, especially those
which are volatile and difficult to contain. Room temperature ionic liquids, especially those based
upon the 1,3-dialkylimidazolium cation, have attracted growing interest in the last few years [5-9].
They offer a possible alternative and ecologically sound medium compared to conventional organic

Molecules 2009, 14
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solvents, as they are non-volatile, recyclable, thermally robust and excellent solvents for a wide range
of organic and inorganic materials. Furthermore, their high compatibility with transition metal
catalysts and limited miscibility with common solvents, enables easy product and catalyst separation
with the retention of the stabilized catalyst in the ionic phase [10]. These and related ionic liquids have
been successfully applied to hydrogenations [11], alkene dimerizations [12], Friedel-Crafts reactions
[13-14], Diels-Alder reactions [15-17], Heck reactions [18-20], Bechmann condensations [21-22],
Suzuki reactions [23-26], Baylis-Hillman reactions [27-28], Stille reactions [29], and Sonogashira
reactions [30].
The acylation of alcohols, phenols, and thiols is a common practice in organic synthesis [31]. In
these reactions, acid chlorides or anhydrides are often used as the acyl source in the presence of amine
bases such as triethylamine, pyridine, or DMAP [31]. Some methods employing Bu3P [32], Sc(OTf)3
[33-34], Sc(NTf2)3 [35], TMSOTf [36-37], Cu(OTf)2 [38], InCl3 [39], In(OTf)3 [40], Bi(OTf)3 [41-42],
and clays [43] have also been reported. However, many of these methods have some drawbacks such
as low yields, long reaction times, harsh reaction conditions, use of hazardous materials (e.g., DMAP
is highly toxic, Bu3P is flammable and air sensitive), and the use of expensive [33-37,40-42] and/or not
readily available reagents [33,34,41,42]. Some of the reported methods work well on primary or
secondary alcohols only and fail to protect tertiary alcohols or less reactive phenols. Recently, Kanta
De described ruthenium chloride catalyzed acylation of alcohols, phenols, thiols, and amines [44]. The
reaction proceeds in the presence of 5 mol% RuCl3 in acetonitrile at room temperature. Although this
method has the major advantages of high yields, short reaction times, and compatibility with other
protecting groups, the expensive homogeneous ruthenium catalyst can’t be recovered and reused and
acetonitrile is volatile and toxic. These disadvantages have so far precluded its practical applications.
In this paper, we describe the ruthenium-catalyzed acylation of alcohols, phenols, and thiols in room
temperature ionic liquids.
Results and Discussion
As a suitable solvent for the acylation of alcohols, phenols, and thiols, the moisture stable and
readily available 1-butyl-3-methylimidiazolium hexafluorophosphate ([bmim][PF6]) was chosen. The
reaction of benzyl alcohol with acetic anhydride in the presence of 5 mol% RuCl3 in [bmim][PF6] at
40°C afforded the desired benzyl acetate in 92% yield in 10 min. The acylation of a variety of
alcohols, phenols, and thiols with acetic anhydride in the presence of 5 mol% RuCl3 was investigated
(Scheme 1), the experimental results are showed in Table 1. As shown in the Table, primary and
secondary alcohols underwent the acetylation reactions in excellent yields (entries 1-4). Interestingly,
tertiary alcohols can also be acetylated smoothly in high yields without any side products being
observed (entry 5). Phenols are less reactive than alcohols toward acylation reactions but nevertheless
they underwent acylation smoothly. Compared with the acylation of primary alcohols, the acylation of
secondary or tertiary alcohols and phenols needed longer reaction times. The ionic liquid,
[bmim][PF6], has the advantage of increase in reaction rate over acetonitrile as the solvent. For
example, the acetylation reaction of 2-naphthol in [bmim][PF6] within 2.5 h gave the corresponding
acetylated product in 93% yield (entry 14), the same reaction in acetonitrile required 10 h to go to
completion, affording the acetylated product in 91% yield [44].

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Table 1. RuCl3-catalyzed acylation of alcohols, phenols, and thiols in [bmim][PF6]. a
Substrate
PhCH2OH
Time
10 min
1.5 h
Entry
Product
3a
Yield (%)b
1
92
OH
2
91
3b
10 min
30 min
3 h
OH
3
4
3c
90
93
OH
OH
3d
5
87
3e
2 h
2 h
OH
6
7
89
98
3f
OH
3g
O2N
I
OH
1.8 h
2 h
8
9
3h
3i
92
90
95
OH
Br
OH
2 h
10
3j
2.5 h
OH
11
12
3k
3l
88
90
OH
3 h
3 h
HO
OH
OH
13
14
3m
3n
90
93
2.5 h
2 h
15
3o
98
N
OH
16
17
18
19
PhSH
n-BuSH
4-ClC6H4SH
1 h
95
91
90
92
3p
3q
3r
3s
35 min
1.5 h
2 h
4-CH3C6H4SH
a Reaction was carried out with 1 mmol of substrate, 1.2 equiv of Ac2O, and 5 mol% RuCl3 in
1.5 mL of [bmim][PF6] at 40 °C under Ar. b Yields refer to pure isolated products.

Molecules 2009, 14
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Scheme 1. RuCl3-catalyzed acylation reaction in [bmin][PF6].
5 mol% RuCl3
[bmim][PF6], 40 oC
CH3COX R
(CH3CO)2O
R XH
+
3
2
1
X = O, S
To explore generality and scope further, the RuCl3-catalyzed acylation in [bmim][PF6] was
examined using other functionally and sterically diverse alcohols or phenols, as listed in Table 1.
When 2,6-dimethylphenol was subjected to the present method, acylation took place smoothly in
excellent yield (Table 1, entry 12). Similarly, sterically hindered substrates, (entries 5 and 11) were
acetylated to give a high yield of product. Resorcinol underwent acetylation smoothly to give benzene-
1,3-diyl diacetate in 90% yield (entry 13, Table 1). This method can be extended to acetylation
reactions of thiols, and a variety of thiols underwent acylation smoothly in excellent yields (entries 16-
19). Although metal triflates (Otera [41,42] type and others [33,34,36-38]) have recently been reported
as Lewis acid catalysts for acetylation reactions, these catalysts are expensive and commercially
unavailable (in some cases). The strong Lewis acid character of metal triflates also makes them
unsuitable in use with acid-sensitive substrates such as 1-ethynyl-1-cyclohexanol, necessitating the use
of excess acetic anhydride and/or low temperatures (0 to –20 °C) to suppress competitive side
reactions. Thus, 1-ethynyl-1-cyclohexanol was acetylated by Sc(OTf)3 at –20 °C using three
equivalents of acetic anhydride, while Bi(OTf)3-catalyzed acetylation of the same compound required
10 equiv of acetic anhydride. The RuCl3-catalyzed acetylation of 1-ethynyl-1-cyclohexanol in
[bmim][PF6] at 40 °C requires only 1.2 equiv of acetic anhydride. All the acetylated products were
characterized by IR, 1H-NMR, 13C-NMR.
Isolation of the acetylated products from the [bmim][PF6] reaction mixtures can be conveniently
achieved by extraction with diethyl ether three times. To evaluate the possibility of recycling the ionic
liquid and ruthenium catalyst used in the reaction, benzyl alcohol and acetic anhydride were allowed to
react in [bmim][PF6] at 40 °C for 10 min and then the product was extracted with diethyl ether for
three times affording a cleaned, ionic liquid catalytic solution. After the recovered ionic liquid
containing ruthenium catalyst was concentrated in vacuo (5.0 torr/r.t. for 1 hour), a second amount of
reactants were added and the process was repeated up to 10 times. It seems that there was no effect on
the rate and yield of the reaction during 1-10 cycles (Table 2), a result that is important from a
practical point of view.
Table 2. Ionic liquid and catalyst recycling in the acetylation of benzyl alcohol. a
Cycle
Yield (%)b
Cycle
Yield (%)b
1
2
3
4
5
92
91
92
90
91
6
7
8
9
10
90
91
90
89
89
a Reaction was conducted under the conditions of 1.0 mmol of benzyl alcohol and 1.2 mmol of
acetic anhydride in the presence of RuCl3 (5 mol%) in 1.5 mL of [bmim][PF6] at 40 °C for 10 min
under Ar. b Isolated yield based on the benzyl alcohol used.

Molecules 2009, 14
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Experimental
General
1
All chemicals were of reagent grade and used as purchased. H-NMR (400 MHz) spectra and
13C-NMR (100 MHz) spectra were recorded on a Bruker AC-P400 spectrometer using CDCl3 as the
solvent; TMS was the internal standard. IR spectra were determined on an FTS-185 instrument as neat
films. All reactions were carried out in pre-dried glassware (150 °C, 4 h) cooled under a stream of
dry Ar.
Typical procedure for the acetylation of benzyl alcohol in ionic liquids
Into a two-necked flask equipped with a magnetic stirring bar were placed ruthenium chloride
(0.05 mmol), benzyl alcohol (1.0 mmol), acetic anhydride (1.2 mmol), and [bmim][PF6] (1.5 mL)
under an Ar atmosphere. The mixture was stirred at 40 °C for 10 min, then cooled to 25 °C and
extracted with diethyl ether (3 × 10 mL). The recovered ionic liquid containing ruthenium catalyst was
concentrated in vacuo (5.0 torr/r.t. for 1 h) and reused in the next run. The combined ether extracts
were washed with saturated NH4Cl solution (10 mL), 1 N NaHCO3 solution (10 mL), and brine
(10 mL), dried over MgSO4, and concentrated under reduced pressure to give an oil, which was
purified by SiO2 column chromatography (eluent(s): hexane/EtOAc, 19:1).
1
13
Benzyl acetate (3a) [44]: Oil. H-NMR: δ 7.43-7.34 (m, 5H), 5.14 (s, 2H), 2.13 (s, 3H); C-NMR: δ
170.94, 135.94, 128.62, 128.33, 128.31, 66.35, 21.07; IR: ν (cm-1) 3035, 2956, 1742, 1498, 1455,
1228, 1027, 736, 698.
1
Cyclohexyl acetate (3b) [44]: Oil. H-NMR: δ 4.76-4.71 (m, 1H), 2.04 (s, 3H), 1.87-1.34 (m, 10H);
13C-NMR: δ 170.64, 72.69, 31.63, 25.35, 23.80, 21.44; IR: ν (cm-1) 2937, 1737, 1486, 1452, 1240,
1045, 908, 734.
1
i-Pentyl acetate (3c) [44]: Oil. H-NMR: δ 4.09 (t, J = 6.8 Hz, 2H), 2.05 (s, 3H), 1.72-1.64 (m, 3H),
13
0.94 (d, J = 7.2 Hz, 6H); C-NMR: δ 171.25, 63.15, 37.30, 25.05, 22.44, 21.02; IR: ν (cm-1) 2961,
1743, 1466, 1367, 1234, 1123.
1
Methallyl acetate (3d): Oil. H-NMR: δ 4.98 (s, 1H), 4.93 (s, 1H), 4.50 (s, 2H), 2.10 (s, 3H), 1.76 (s,
13
3H); C-NMR: δ 170.74, 139.90, 112.88, 67.74, 20.32, 14.15; IR: ν (cm-1) 2959, 1741, 1454, 1375,
1232, 1027, 912, 734; Anal. Calc. for C6H10O2: C, 63.13; H, 8.83. Found: C, 62.89; H, 8.62%.
1
1-Ethynylcyclohexyl acetate (3e) [44]: Oil. H-NMR: δ 2.61 (s, 1H), 2.15-2.11 (m, 2H), 2.05 (s, 3H),
13
1.88-1.80 (m, 2H), 1.66-1.59 (m, 4H), 1.56-1.49 (m, 1H), 1.39-1.30(m, 1H); C-NMR: δ 169.30,
83.65, 75.10, 74.22, 36.90, 25.08, 22.44, 21.94; IR: ν (cm-1) 3287, 2938, 2862, 2112, 1745, 1447,
1367, 1229, 1025.
1
Phenyl acetate (3f) [44]: Oil. H-NMR: δ 7.40 (t, J = 8.0 Hz, 2H), 7.25 (t, J = 7.6 Hz, 1H), 7.11 (d,
J = 8.0 Hz, 2H), 2.31 (s, 3H); 13C-NMR: δ 169.48, 150.73, 129.44, 125.83, 121.59, 21.13; IR: ν (cm-1)
3064, 2927, 1765, 1594, 1493, 1370, 1193, 748, 691.

Molecules 2009, 14
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1
4-Nitrophenyl acetate (3g) [44]: Oil. H-NMR: δ 8.28 (d, J = 8.8 Hz, 2H), 7.29 (d, J = 8.8 Hz, 2H),
13
2.36 (s, 3H); C-NMR: δ 168.46, 155.36, 146.15, 125.24, 122.48, 21.16; IR: ν (cm-1) 3113, 1762,
1591, 1521, 1487, 1348, 1193, 917, 868.
1
4-Iodophenyl acetate (3h) [45]: Oil. H-NMR: δ 7.67 (d, J = 8.8 Hz, 2H), 6.87-6.83 (m, 2H), 2.27 (s,
13
3H); C-NMR: δ 169.02, 150.53, 138.48, 123.81, 89.88, 21.11; IR: ν (cm-1) 3066, 2925, 1761, 1579,
1481, 1368, 1195, 1009, 908, 840.
1
4-Bromophenyl acetate (3i) [44]: Oil. H-NMR: δ 7.50 (d, J = 8.4 Hz, 2H), 7.00 (d, J = 8.4 Hz, 2H),
13
2.31 (s, 3H); C-NMR: δ 169.19, 149.67, 132.49, 123.45, 118.93, 21.12; IR: ν (cm-1) 3070, 2936,
1762, 1585, 1485, 1369, 1197, 1012, 907, 842.
1
4-Methylphenyl acetate (3j) [46]: Oil. H-NMR: δ 7.09 (d, J = 8.4 Hz, 2H), 6.89 (d, J = 8.4 Hz, 2H),
13
2.26 (s, 3H), 2.21 (s, 3H); C-NMR: δ 169.85, 148.42, 135.53, 129.99, 121.28, 21.16, 20.91; IR: ν
(cm-1) 3035, 2926, 1763, 1508, 1369, 1195, 1018, 909, 845.
1
2,5-Dimethylphenyl acetate (3k) [47]: Oil. H-NMR: δ 7.13 (d, J = 7.6 Hz, 1H), 6.97 (d, J = 8.0 Hz,
1H), 6.84 (s, 1H), 2.33 (s, 3H), 2.32 (s, 3H), 2.16 (s, 3H); 13C-NMR: δ 169.44, 149.17, 136.93, 130.87,
126.90, 126.81, 122.40, 20.92, 20.85, 15.76; IR: ν (cm-1) 2926, 1768, 1578, 1510, 1369, 1214, 1110,
893, 810.
1
2,6-Dimethylphenyl acetate (3l) [44]: Oil. H-NMR: δ 7.09-7.04 (m, 3H), 2.32 (s, 3H), 2.15 (s, 6H);
13C-NMR: δ 168.85, 148.24, 130.11, 128.60, 125.89, 20.47, 16.31; IR: ν (cm-1) 2926, 1761, 1476,
1370, 1215, 1166, 910, 770.
1
Benzene-1,3-diyl diacetate (3m) [46]: Oil. H-NMR: δ 7.35 (t, J = 8.0 Hz, 1H), 6.98 (d, J = 8.0 Hz,
2H), 6.92 (t, J = 2.0 Hz, 1H), 2.27 (s, 6H); 13C-NMR: δ 168.97, 151.13, 129.68, 118.97, 115.46, 21.06;
IR: ν (cm-1) 3077, 2939, 1769, 1600, 1486, 1371, 1199, 1125, 1015, 915.
1
2-Naphthyl acetate (3n) [44]: Oil. H-NMR: δ 7.87-7.79 (m, 3H), 7.56 (s, 1H), 7.49-7.46 (m, 2H),
7.25-7.22 (m, 1H), 2.36 (s, 3H); 13C-NMR: δ 169.72, 148.27, 133.72, 131.44, 129.43, 127.76, 127.64,
126.56, 125.72, 121.12, 118.54, 21.23; IR: ν (cm-1) 3059, 2926, 1756, 1599, 1512, 1373, 1217, 1154,
761.
Quinolin-8-yl acetate (3o) [48]: Oil. 1H-NMR: δ 8.89 (d, J = 2.0 Hz, 1H), 8.13-8.10 (m, 1H), 7.69-7.65
(m, 1H), 7.52-7.35 (m, 3H), 2.50 (s, 3H); 13C-NMR: δ 169.86, 150.51, 147.37, 141.19, 136.09, 129.53,
126.23, 125.98, 121.77, 121.55, 21.09; IR: ν (cm-1) 3042, 2935, 1764, 1598, 1500, 1368, 1208, 790.
1
S-Phenyl thioacetate (3p) [49]: Oil. H-NMR: δ 7.41 (s, 5H), 2.41 (s, 3H); 13C-NMR: δ 194.19,
134.52, 129.52, 129.28, 127.91, 30.29; IR: ν (cm-1) 3061, 2923, 1709, 1583, 1478, 1441, 1353, 1114,
950, 747.
1
S-n-Butyl thioacetate (3q) [50]: Oil. H-NMR: δ 2.87 (t, J = 7.6 Hz, 2H), 2.32 (s, 3H), 1.59-1.52 (m,
13
2H), 1.43-1.34 (m, 2H), 0.92 (t, J = 7.2 Hz, 3H); C-NMR: δ 196.04, 31.56, 30.61, 28.82, 21.93,
13.58; IR: ν (cm-1) 2961, 2874, 1694, 1465, 1354, 1136, 913, 735.

Molecules 2009, 14
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S-4-Chlorophenyl thioacetate (3r) [51]: Oil. 1H-NMR: δ 7.40 (d, J = 8.4 Hz, 2H), 7.35 (d, J = 8.4 Hz,
2H), 2.44 (s, 3H); 13C-NMR: δ 193.50, 135.85, 135.72, 129.50, 126.34, 30.27; IR (film): ν (cm-1) 3087,
2923, 1707, 1574, 1477, 1353, 1093, 952, 821.
S-4-Methylphenyl thioacetate (3s) [51]: Oil. 1H-NMR: δ 7.28 (d, J = 8.0 Hz, 2H), 7.20 (d, J = 8.0 Hz,
2H), 2.37 (s, 3H), 2.35 (s, 3H); 13C-NMR: δ 194.53, 139.72, 134.47, 130.08, 124.54, 30.11, 21.36; IR:
ν (cm-1) 3025, 2923, 1708, 1598, 1494, 1352, 1117, 1094, 950, 807.
Conclusions
In summary, we have described the ruthenium-catalyzed room temperature acylation of alcohols,
phenols, and thiols with acetic anhydride in an ionic liquid. Easy product isolation, facile recycling of
the ionic liquid and catalyst, and avoiding use of easily volatile acetonitrile as solvent are important
advantages of the developed methodology.
Acknowledgements
We thank the National Natural Science Foundation of China (Project No. 20862008), the Natural
Science Foundation of Jiangxi Province of China (2008GQH0034), and the Graduates Innovation
Foundation of Jiangxi Province of China (YC08A045) for financial support.
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Sample Availability: Samples of the compounds 3a-s are available from the authors.
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