Microwave-assisted synthesis of vinyl esters through ruthenium-catalyzed addition of carboxylic acids to alkynes

Article abstract of DOI:10.1071/CH08480

A rapid and efficient method is described for the selective synthesis of enol esters via the microwave-accelerated addition of carboxylic acids to terminal alkynes. The method employs the readily available [RuCl ₂(p-cymene)(PPh₃)] complex as catalyst without the need of bases, and reactions are complete in 20 min.

Full text of DOI:10.1071/CH08480

RESEARCH FRONT
Rapid Communication
CSIRO PUBLISHING
www.publish.csiro.au/journals/ajc
Aust. J. Chem. 2009, 62, 227–231
Microwave-Assisted Synthesis of Vinyl Esters through
Ruthenium-Catalyzed Addition of Carboxylic Acids
to Alkynes
François Nicks,A Lionel Libert,A Lionel Delaude,A and Albert DemonceauA,B
ALaboratory of Macromolecular Chemistry and Organic Catalysis, University of Liège,
Sart-Tilman (B.6a), B-4000 Liège, Belgium.
BCorresponding author. Email: a.demonceau@ulg.ac.be
A rapid and efficient method is described for the selective synthesis of enol esters via the microwave-accelerated addition
of carboxylic acids to terminal alkynes. The method employs the readily available [RuCl2(p-cymene)(PPh3)] complex as
catalyst without the need of bases, and reactions are complete in 20 min.
Manuscript received: 3 November 2008.
Final version: 19 December 2008.
Ruthenium–arene complexes are versatile and efficient cata-
lyst precursors for various important organic transformations,
such as olefin metathesis,[1] atom transfer radical reactions,[2]
transfer hydrogenation of ketones,[3] and isomerization of allylic
alcohols.[4] Ruthenium–arene complexes also catalyze the addi-
tion of carboxylic acids to terminal alkynes, affording vinyl
esters in high yields and excellent selectivities.[5] However, the
past decade has also witnessed the high potential of microwave
irradiation technology to accelerate reactions and, in many cases,
improve yields over conventional heating methods.[6] As part
of our work on microwave-assisted transformations,[7] we have
developedasimpleandefficientmethodforthesynthesisofvinyl
esters from carboxylic acids and 1-alkynes that uses microwave
irradiation and we now report our findings.
In 2003, Goossen and his coworkers reported that the addi-
tion reaction of carboxylic acids to terminal alkynes could
be drastically improved when small quantities of base were
added to catalyst systems generated in situ from the ruthenium
dimer, [RuCl2(p-cymene)]2 (p-cymene = 4-isopropyltoluene),
and phosphines, such as tri(2-furyl)phosphine, triphenylphos-
phine, and tris(4-chlorophenyl)phosphine.[5j] Using Goossen’s
preformed [RuCl2(p-cymene)(PAr3)] complexes (Ar = Fur
(2-furyl), Ph, and p-ClC6H4), bearing the phosphine ligands that
were used for the in situ-generated catalyst systems.
In a first set of experiments, we employed the ruthenium-
P(Fur)3 system that emerged from Goossen’s work,[5j] and
found that the reaction rates increased significantly with the
temperature (Fig. 1).
Of particular note is the observation that under our experi-
mental conditions, p-acetoxybenzoic acid was only partly sol-
uble in the reaction medium, especially at 50◦C. As expected,
its solubility increased with the temperature. At 70◦C, however,
the acid was not yet completely soluble and, in addition, some
1-hexyne (bp 71–72◦C) condensed on the cold inner surface
of the reactor, particularly after extended reaction times. To
overcome these problems, we had recourse to a CEM Discover
microwavereactor.Thesealedreactionvesselsusedinthisdevice
allow the reactions to be heated to well above the boiling point
100
80
60
40
20
0
work as
a starting point, we chose the reaction of
p-acetoxybenzoic acid with 1-hexyne according to Scheme 1
(R = p-AcOC6H4) as our model system and screened three
O
[Ru]
C4H9
ϩ
∆ or MW
R
R
OH
O
O
R
O
0
20
40
60
ϩ
ϩ
C4H9
Time [h]
O
O
R
O
C4H9
C4H9
Fig. 1. Variation in enol ester yield with respect to time for reactions
between 1-hexyne and p-acetoxybenzoic acid catalyzed by [RuCl2(p-
cymene)(P(Fur)3)] at 50◦C ( ), 60◦C ( ), and 70◦C ( ) under conventional
heating (1-hexyne, 1.5 equiv.; p-acetoxybenzoic acid, 1 equiv.; [RuCl2(p-
cymene)(P(Fur)3)], 0.008 equiv.; Na2CO3, 0.016 equiv.).
1
2 (Z )
3 (E)
Markovnikov product
Anti-Markovnikov products
Scheme 1. Synthesis of vinyl esters through addition of carboxylic acids
to 1-hexyne.
© CSIRO 2009
10.1071/CH08480
0004-9425/09/030227

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F. Nicks et al.
Table 1. Synthesis of vinyl esters using conductive (ꢀ) or microwave (MW) heating
1-Hexyne, 1.5 equiv.; p-acetoxybenzoic acid, 1 equiv.; catalyst, 0.008 equiv.; base, 0.016 equiv.
Catalyst system
Reaction conditions
YieldA [%]
SelectivityB [%]
1
2
3
[RuCl2(p-cymene)(P(Fur)3)] + Na2CO3
ꢀ, 60◦C, 8 h
16
96
92
98
16
70
91
90
12
49
93
87
13
29
91
97
91
91
89
88
89
88
88
85
93
93
89
88
86
89
84
86
6
5
8
9
7
8
9
11
6
6
9
9
9
7
13
11
3
4
3
3
4
4
3
4
1
1
2
3
5
4
3
3
ꢀ, 60◦C, 24 h
ꢀ, 160◦C, 20 min
MW, 160◦C, 20 min
ꢀ, 60◦C, 8 h
[RuCl2(p-cymene)(PPh3)] + Na2CO3
[RuCl2(p-cymene)(P(p-C6H4Cl)3)] + Na2CO3
[RuCl2(p-cymene)(PPh3)] (no base)
ꢀ, 60◦C, 24 h
ꢀ, 160◦C, 20 min
MW, 160◦C, 20 min
ꢀ, 60◦C, 8 h
ꢀ, 60◦C, 24 h
ꢀ, 160◦C, 20 min
MW, 160◦C, 20 min
ꢀ, 60◦C, 8 h
ꢀ, 60◦C, 24 h
ꢀ, 160◦C, 20 min
MW, 160◦C, 20 min
ADetermined by GC using dodecane as internal standard.
BDetermined by GC.
100
80
60
40
20
0
of the solvent, and we found that the optimum conditions for our
reaction were heating the vessel to 160◦C for 20 min using 150W
of power. Thus, when [RuCl2(p-cymene)(P(Fur)3)] was used as
catalyst, a 98% yield of enol ester was obtained after 20 min
at 160◦C under microwave, against 16% under conventional
heating at 60◦C for 8 h (Table 1).
In addition, from the comparison of the different phosphines
used, we confirmed the trend reported by Goossen[5j] for con-
ventionally heated reactions: P(Fur)3 > PPh3 > P(p-ClC6H4)3,
as evidenced by the yields (96, 70, and 49%, respectively)
obtained after 1 day of reaction at 60◦C (Table 1). On the con-
trary, these phosphines were almost equipotent ligands under
microwave irradiation as 98, 90, and 87% yields, respectively,
were obtained after 20 min at 160◦C (Table 1). This result is
particularly interesting because PPh3 is a cheap ligand com-
pared with P(Fur)3 and P(p-ClC6H4)3. However, we also found
that [RuCl2(p-cymene)(PPh3)] did not require the use of sodium
carbonate under microwave irradiation (Table 1), contrary to
the synthesis of vinyl esters under conventional heating, which
was found to be drastically enhanced by the addition of small
quantities of base.[5j]
80
100
120
140
160
Temperature [°C]
Fig. 2. Variation in enol ester yield ( ) and selectivity (1 ( ), 2 ( ),
and 3 ( )) with respect to temperature for reactions between 1-hexyne
and p-acetoxybenzoic acid catalyzed by [RuCl2(p-cymene)(PPh3)] under
microwave irradiation for 30 min (1-hexyne, 1.5 equiv.; p-acetoxybenzoic
acid, 1 equiv.; catalyst, 0.008 equiv.; no base).
With these new reaction conditions in hand, we next investi-
gated the effect of both the temperature and the reaction time on
the microwave-assisted synthesis of vinyl esters using [RuCl2(p-
cymene)(PPh3)] as the catalyst, in the absence of base. Thus,
temperatures not less than 140◦C (Fig. 2) and reaction times
longer than 20 min (Fig. 3) are key parameters for obtaining
quantitative yields. However, a control experiment performed
for 0 min at 160◦C, as programmed in the microwave set-up (i.e.
the reaction mixture was heated to 160◦C and once it got there,
the reaction was stopped by rapid air cooling), afforded the tar-
geted vinyl ester in 22% yield (Fig. 3). This phenomenon is in
fact related to the time that the microwave oven requires to reach
the desired reaction temperature (time zero).[7b,c,8]
test reaction using a thermostatted oil bath under otherwise iden-
ticalconditions(concentration, temperature, etc.), exceptthatthe
reactions were performed in sealed tubes, which were immersed
in the oil bath at 160◦C. Examination of Fig. 3 appears to demon-
strate that microwave-irradiated reactions were faster than their
conventionally heated counterparts. Indeed, under microwave
conditions, yields were as follows: 78, 88, and 95% for runtimes
of 5, 10, and 15 min, respectively, against 3, 39, and 81% for
conventionally heated reactions. In the latter cases, the reactions
started with the immersion of the sealed tube in the oil bath and,
contrary to microwave heating, the yield was obviously zero at
time t = 0 and the induction period that was evidenced (Fig. 3)
could be explained by the time needed for the heat to be trans-
ferred inside the sealed ampoule. In light of these results and
taking into account the temperature problems associated with
both methods, it is likely that microwave irradiation and con-
ventional heating are equipotent methods, thereby ruling out the
In many cases, authors ascribed the increased reaction rates
and improved yields for microwave-assisted transformations
compared with those using conventional heating to the so-called
‘microwave effect’. To assess this hypothesis, we conducted the

RESEARCH FRONT
Microwave-Assisted Synthesis of Vinyl Esters
229
100
MW
Table 2. Influence of the carboxylic acid on the [RuCl2(p-cymene)
(PPh3)]-catalyzed, microwave-assisted synthesis of representative
vinyl esters
1-Hexyne, 1.5 equiv.; carboxylic acid, 1 equiv.; catalyst, 0.008 equiv.;
no base. T 160◦C for 30 min
80
60
40
20
0
Acid
YieldA [%]
SelectivityB [%]
1
2
3
Acetic acid
97
100
97 (95)C
100 (99)C
100 (98)C
100 (99)C
73
84
85
85
85
83
17
12
12
12
11
14
10
4
3
3
4
p-Methoxybenzoic acid
p-Methylbenzoic acid
Benzoic acid
p-Acetoxybenzoic acid
p-Chlorobenzoic acid
0
10
20
30
100
80
60
40
20
0
3
∆
ADetermined by GC using dodecane as internal standard.
BDetermined by GC.
CIsolated yield.
H9C4
H9C4
H9C4
C4H9
ϩ
ϩ
C4H9
H9C4
0
10
20
30
4
5
6
Time [min]
Scheme 2. Dimerization products of 1-hexyne.
Fig. 3. Variation in enol ester yield ( , ) and selectivity (1 ( ),
2 ( ), and 3 ( )) with respect to time for reactions between 1-hexyne
and p-acetoxybenzoic acid catalyzed by [RuCl2(p-cymene)(PPh3)] under
microwave irradiation (top) and under conventional heating (bottom) at
160◦C (1-hexyne, 1.5 equiv.; p-acetoxybenzoic acid, 1 equiv.; catalyst,
0.008 equiv.; no base).
respectively). In sharp contrast, trimethylsilylacetylene did not
react at all with p-acetoxybenzoic acid and dimers were formed
in small amounts (6%).
Finally, having in hand an improved method for the synthesis
of vinyl esters from carboxylic acids and 1-hexyne, we became
interested in the maximum turnover number (TON) attained by
[RuCl2(p-cymene)(PPh3)] for the test reaction. To this aim, the
ruthenium complex was finely dispersed in Celite (3 wt-%)[9] in
order to introduce a precise amount of the catalyst to the reaction
mixture, which was then microwaved at 160◦C for 30 min. Not
surprisingly, the yield decreased with the catalyst loading (Fig. 4)
and, concomitantly, the turnover number reached a maximum
value of 1000 for an initial molar ratio [acid]:[complex] close
to 1500. Extended reaction times with heating at 160◦C did not
improve theTON value, revealing thereby the deactivation of the
catalyst.
In conclusion, we have developed a new, simple, and efficient
procedure for the regioselective synthesis of the Markovnikov-
type vinyl ester from carboxylic acids and 1-hexyne. This
method, which takes advantage of the microwave irradiation
technology, uses the readily available [RuCl2(p-cymene)(PPh3)]
complex as catalyst and works very rapidly using a catalyst load-
ing as low as 0.1 mol-% without the need of bases.The ease of the
microwave methodology suggests potential extensions to other
ruthenium-catalyzed transformations.[10]
so-called ‘microwave effect’. However, compared with conven-
tional heating, microwave irradiation is much more convenient,
providing a fast and direct heating of the reactants at the desired
temperature.
With optimized conditions in hand, we next examined the
reactivity of a few electron-rich and electron-poor carboxylic
acids (Table 2) and were pleased to find that, in all cases, the
vinyl esters were formed in – almost – quantitative yields (as
determined by GC) and could be isolated without any prob-
lems (>95%). In addition, the Markovnikov product (1) was the
major regioisomer (80–90%, Tables 1 and 2), and the (Z)-anti-
Markovnikov isomer (2) was preferred over the (E)-isomer (3).
Furthermore, theselectivityfortheMarkovnikovproduct(1)was
often slightly lower at higher temperatures. However, 1-hexyne
also underwent coupling (Scheme 2), albeit in low yields (<5%),
affording predominantly enyne 4.
We then investigated the influence of various terminal
alkynes on the microwave-assisted synthesis of vinyl esters using
p-acetoxybenzoic acid as the model carboxylic acid (Scheme 3
and Table 3). Not surprisingly, 1-hexyne and 1-octyne dis-
played similar reactivities and furnished the Markovnikov iso-
mer (7) as the major product (85% selectivity). Likewise,
t-butylacetylene and phenylacetylene reacted exquisitely well
with p-acetoxybenzoic acid. The conversion was complete after
30 min of microwave irradiation, but the selectivity was less
satisfactory. Indeed, the Markovnikov isomers (7) were pro-
duced with a yield of 77 and 64%, respectively, together with
the (Z)-anti-Markovnikov isomers (8) (21 and 29% yield,
Experimental
All reactions were carried out under an inert atmosphere, using
reagents (Aldrich) and solvents (Aldrich, Labotec) dried and
purified by standard techniques.[11] Microwave-assisted synthe-
ses were performed using a single-mode Discover reactor from
CEM Corp. (Matthews, NC) where the temperature was moni-
tored with an IR sensor. GC analyses were performed on aVarian
Star 3400 CX gas chromatograph equipped with an RSLM-150

RESEARCH FRONT
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F. Nicks et al.
O
O
[RuCl2(p-cymene)(PPh3)]
MW, 160°C, 30 min
ϩ
R
OH
AcO
O
O
ϩ
ϩ
R
O
O
O
R
R
AcO
AcO
AcO
7
8 (Z )
9 (E )
Scheme 3. Synthesis of vinyl esters through addition of p-acetoxybenzoic acid to 1-alkynes.
100
80
60
40
20
0
Table 3. Influence of the alkyne on the [RuCl2(p-cymene)(PPh3)]-
catalyzed, microwave-assisted synthesis of representative vinyl esters
1-Alkyne, 1.5 equiv.; p-acetoxybenzoic acid, 1 equiv.; catalyst, 0.008 equiv.;
no base. T 160◦C for 30 min
Alkyne
YieldA [%]
SelectivityB [%]
7
8
9
1-Hexyne
1-Octyne
t-Butylacetylene
Trimethylsilylacetylene
Phenylacetylene
100
100
100
0
85
85
77
–
11
13
21
–
4
2
2
–
7
100
64
29
0.0
0.2
0.4
0.6
0.8
1.0
ADetermined by GC using dodecane as internal standard.
BDetermined by GC.
1200
1000
800
600
400
200
0
capillary column (25 m × 0.25 mm; film thickness, 0.25 µm)
and a flame ionization detector. The enol esters were purified
by distillation under high vacuum and characterized by 1H and
13C NMR spectroscopy, according to the literature.[5]
Conventional Heating
The reaction between p-acetoxybenzoic acid and 1-hexyne is
representative of the procedure utilized. A 50-mL dry round-
bottom Pyrex vessel containing a stirring bar and fitted with a
three-way stopcock was charged with the catalyst (0.02 mmol),
sodium carbonate (0.04 mmol, 4.2 mg), and p-acetoxybenzoic
acid (2.5 mmol, 450 mg), and then purged of air (three vacuum–
nitrogen cycles). A stock solution (13 mL) containing 1-hexyne
(3.75 mmol) and dodecane (internal standard) in toluene was
then added under nitrogen. The resulting mixture was placed
in an oil bath and allowed to react at 60◦C. The reaction was
monitored by withdrawing samples at regular time intervals from
the reaction mixture and analyzing them by GC.
0.0
0.2
0.4
0.6
0.8
1.0
Catalyst loading [mol-%]
Fig. 4. Variation in enol ester yield ( ) and turnover number (TON)
( ), with respect to catalyst loading for reactions between 1-hexyne
and p-acetoxybenzoic acid catalyzed by [RuCl2(p-cymene)(PPh3)] under
microwave irradiation at 160◦C (1-hexyne, 1.5 equiv.; p-acetoxybenzoic
acid, 1 equiv.; no base; reaction time, 30 min).
acid (0.5 mmol, 90 mg) and then purged of air (three vacuum–
nitrogen cycles) before a stock solution (2.6 mL) containing
1-hexyne (0.75 mmol) and dodecane (internal standard) in
toluene was added. The vial was capped under nitrogen, heated
to 160◦C (monitored by IR sensor), and then held at that tem-
perature in a CEM Discover instrument with 150-W microwave
power. No ramp and no simultaneous cooling were applied.After
rapid air-cooling by the unit, the reaction mixture was analyzed
by GC.
Conventional Heating in Sealed Tubes
A 10-mL glass tube equipped with a stirring bar was charged
with the catalyst (0.004 mmol) and p-acetoxybenzoic acid
(0.5 mmol, 90 mg) and then purged of air (three vacuum–
nitrogen cycles) before a stock solution (2.6 mL) containing
1-hexyne (0.75 mmol) and dodecane (internal standard) in
toluene was added. The tube was sealed under vacuum and
immersed in an oil bath heated at 160◦C. After a given reac-
tion time, the ampoule was drawn out of the bath and cooled
down to room temperature.
Acknowledgements
The authors thank the ‘Fonds pour la Formation à la Recherche dans
l’Industrie et dans l’Agriculture’(FRIA) for a fellowship to F. Nicks and the
‘Fonds National de la Recherche Scientifique’ (FNRS), Brussels, for finan-
cial support of the present work (grants FRFC 2.4565.07 and 2.4645.07) and
for the purchase of major instrumentation. We also thank BRS, Drogenbos,
Belgium, for helpful discussion and technical support.
Microwave Heating
A 10-mL glass vial containing a Teflon-coated stir bar was
charged with the catalyst (0.004 mmol) and p-acetoxybenzoic

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Microwave-Assisted Synthesis of Vinyl Esters
231
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