Ruthenium-arene complexes bearing imidazol(in)ium-2-dithiocarboxylate ligands: Evaluation of their catalytic activity in the synthesis of enol esters

Article abstract of DOI:10.1016/j.jorganchem.2009.08.028

The catalytic activity of four ruthenium imidazol(in)ium-2-dithiocarboxylates was evaluated for the synthesis of vinyl esters through addition of 4-acetoxybenzoic acid to 1-hexyne, and compared to those of the parent ruthenium-N-heterocyclic carbene compl

Full text of DOI:10.1016/j.jorganchem.2009.08.028

Journal of Organometallic Chemistry 694 (2009) 4049–4055
Contents lists available at ScienceDirect
Journal of Organometallic Chemistry
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Ruthenium–arene complexes bearing imidazol(in)ium-2-dithiocarboxylate ligands:
Evaluation of their catalytic activity in the synthesis of enol esters
*
Quentin Willem, François Nicks, Xavier Sauvage, Lionel Delaude, Albert Demonceau
Laboratory of Macromolecular Chemistry and Organic Catalysis, University of Liège, Sart-Tilman (B.6a), B-4000 Liège, Belgium
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 30 June 2009
Received in revised form 12 August 2009
Accepted 19 August 2009
Available online 23 August 2009
The catalytic activity of four ruthenium imidazol(in)ium-2-dithiocarboxylates was evaluated for the syn-
thesis of vinyl esters through addition of 4-acetoxybenzoic acid to 1-hexyne, and compared to those of
the parent ruthenium–N-heterocyclic carbene complexes and [RuCl2(p-cymene)(PPh3)] (a standard cata-
lyst). It turned out that ruthenium imidazol(in)ium-2-dithiocarboxylates were poorly active and selec-
tive. Quantitative yields, indeed, were obtained only after extended reaction times. However, the
catalytic activity could be improved significantly under microwave heating or conventional heating in
a sealed tube at 160 °C, driving the reaction to completion in less than 4 h of reaction.
Ó 2009 Elsevier B.V. All rights reserved.
Keywords:
Alkynes
Carboxylic acids
Enol esters
N-Heterocyclic carbene ligands
Microwave
Ruthenium
1. Introduction
metathesis and atom transfer radical polymerization (ATRP) [3].
Furthermore, related in situ-generated palladium-based catalyst
Metal complexes bearing imidazol(in)ium-2-dithiocarboxylate
ligands constitute a new class of compounds in coordination chem-
istry. Preliminary experiments in this field were conducted by
Borer et al. who showed that 1,3-dimethylimidazolium-2-dithio-
carboxylate (Scheme 1) formed stable complexes with a range of
transition metal halides or nitrates [1]. The products obtained were
characterized by IR and UV–Vis spectroscopies only. Measure-
ments of electrical conductivity, magnetic susceptibility, or cyclic
voltammetry complemented the analyses in some cases, but no
NMR or XRD analyses were reported. As part of our research pro-
gram directed toward N-heterocyclic carbenes (NHCs) and related
chemistry [2], we recently reported the synthesis of ruthenium–
arene complexes bearing imidazol(in)ium-2-dithiocarboxylate li-
gands (1–4), which are stable, 18-electron cationic species
(Scheme 2) [3]. These compounds were fully characterized by var-
ious analytical techniques and the molecular structure of complex
1 was determined by X-ray diffraction analysis.
systems were moderately active for the Suzuki–Miyaura cross-
coupling of aryl halides with trans-2-phenylvinylboronic acid [4].
With well-defined ruthenium imidazol(in)ium-2-dithiocarboxy-
lates 1–4 in hand (Scheme 2), we became interested in investigat-
ing their catalytic activity. To this aim, we selected the synthesis of
enol esters from carboxylic acids and alkynes as the first test-reac-
tion, as it is well established that this process can be catalyzed by a
variety of ruthenium complexes with good to excellent yields and
selectivities [5]. In this paper, we report the results of this
investigation.
2. Experimental
2.1. Materials
All reactions were carried out under an inert atmosphere, using
reagents (Aldrich) and solvents (Aldrich, Labotec) dried and puri-
fied by standard techniques [6]. Ruthenium imidazol(in)ium-2-
dithiocarboxylates 1–4 [3], [RuCl2(p-cymene)(PPh3)] 8 [7], and
[RuCl2(p-cymene)(NHC)] 12–14 [8] were synthesized according
to the literature, and their NMR data were in accord with literature
values. Microwave-assisted syntheses were performed using a sin-
gle-mode Discover reactor from CEM Corp. (Matthews, NC) where
the temperature was monitored with an IR sensor. Thermogravi-
metric analyses were performed under nitrogen using a TA Q500
instrument and a HI-Res dynamic heating rate.
The catalytic activity of these complexes has never been re-
ported. However, preliminary experiments have shown that cata-
lyst systems generated in situ from the ruthenium dimer
[RuCl2(p-cymene)]2 (p-cymene = 4-isopropyltoluene) and imida-
zol(in)ium-2-dithiocarboxylates were poorly active in olefin
* Corresponding author. Tel.: +32 4366 3495; fax: +32 4366 3497.
E-mail address: A.Demonceau@ulg.ac.be (A. Demonceau).
0022-328X/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2009.08.028

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Q. Willem et al. / Journal of Organometallic Chemistry 694 (2009) 4049–4055
2.2.3. Microwave-assisted reactions at 160 °C
Me
A 10-mL glass vial containing a Teflon-coated stir bar was
charged with the catalyst (0.004 mmol), sodium carbonate (25
w-% in Celite, 0.008 mmol, 0.85 mg), and 4-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 water-saturated
toluene was added. The vial was capped under nitrogen, heated
to 160 °C (monitored by IR sensor) using a dynamic heating rate,
and then held at that temperature in a CEM Discover instrument
with 150-W microwave power, programmed so as t = 0 when the
desired temperature is attained. No simultaneous cooling was ap-
plied. After rapid air-cooling by the unit, the reaction mixture was
analyzed by GC.
S
N
+
–
N
S
Me
Scheme 1. Structure of 1,3-dimethylimidazolium-2-dithiocarboxylate.
R
S
N
+
N
Ru
CS2, KPF6
Cl
Cl
Cl
+
–
0.5
Cl
Cl
EtOH, Δ, 3 h
Ru
2.3. Analysis of the reaction mixtures and characterization of the
products
S
R
GC analyses were performed on a Varian Star 3400 CX gas chro-
matograph equipped with
(25 m ꢀ 0.25 mm; film thickness, 0.25
a
RSLM-150 capillary column
m) and a flame ionization
-
-
PF6
PF6
l
detector. The enol esters were purified by distillation under high
vacuum and characterized by 1H and 13C NMR spectroscopy,
according to the literature [5]. 1H NMR and 13C NMR spectra were
recorded at 298 K with a Bruker DRX 400 spectrometer operating
at 400.13 and 100.62 MHz, respectively.
Ru
Ru
S
S
R
Cl
S
S
R
R
N
N
+
+
N
N
R
3. Results and discussion
1 (R = Mes)
2 (R = Dip)
3 (R = Cy)
4 (R = Mes)
To evaluate the catalytic activity of ruthenium complexes 1–4
in the synthesis of enol esters, we investigated the reaction be-
tween 4-acetoxybenzoic acid and 1-hexyne at 60 °C, using a proto-
col that has already proven to be successful [9]. Noteworthily,
water-saturated toluene was used as the solvent, and sodium car-
bonate as an activator [5j]. As illustrated in Scheme 3 (R1 = 4-
AcOC6H4, R2 = n-C4H9), three adducts were expected from the reac-
tion: the Markovnikov product (5) as well as the anti-Markovnikov
isomers 6 (Z) and 7 (E).
Mes = mesityl (2,4,6-trimethylphenyl); Dip = 2,6-diisopropylphenyl
Cy = cyclohexyl
Scheme 2. Synthesis and structure of the ruthenium imidazol(in)ium-2-dithiocar-
boxylate complexes 1–4 used in this work.
2.2. General procedures for the addition of 4-acetoxybenzoic acid to 1-
hexyne
The results shown in Table 1 and Fig. 1 demonstrate that ruthe-
nium imidazol(in)ium-2-dithiocarboxylates 1–4 are exceedingly
sluggish catalysts. Indeed, at 60 °C, the reactions proceeded to
10–30% overall yield in 50 h and, after 200 h of reaction, no more
than 66% yield was obtained. Consequently, several hundreds of
hours or so were required for the reactions to reach completion
(Fig. 1 and Fig. Sd1 (see Appendix A. Supplementary data)). In
marked contrast, when [RuCl2(p-cymene)(PPh3)] (8, Scheme 4) a
typical catalyst for the synthesis of enol esters [9a] was used,
100% yield was obtained after 16 h only! We hypothesized that
the exceedingly low catalytic activity of 1–4 was due to an en-
hanced stability of the complexes. In order to rationalize the large
discrepancy observed between complexes 1–4 and 8 in terms of
2.2.1. Conventional reactions at 60 °C
A 50-mL dry round-bottom Pyrex vessel containing a stirring
bar and fitted with a three-way stopcock was charged with the cat-
alyst (0.02 mmol), sodium carbonate (0.04 mmol, 4.2 mg), and 4-
acetoxybenzoic acid (2.5 mmol, 450 mg), and then purged of air
(three vacuum–nitrogen cycles). 13 mL of a stock solution contain-
ing 1-hexyne (3.75 mmol), dodecane (internal standard) in water-
saturated toluene were 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
gas chromatography (GC).
O
[Ru]
+
R2
2.2.2. Conventional heating in sealed tubes at 160 °C
R1
OH
A 10-mL glass tube equipped with a stirring bar was charged
with the catalyst (0.004 mmol), sodium carbonate (25 w-% in Cel-
ite, 0.008 mmol, 0.85 mg), and 4-acetoxybenzoic acid (0.5 mmol,
90 mg), and then purged of air (three vacuum–nitrogen cycles) be-
fore a stock solution (2.6 mL) containing 1-hexyne (0.75 mmol)
and dodecane (internal standard) in water-saturated toluene
was added. The tube was sealed under vacuum and immersed in
an oil bath heated at 160 °C. After a given reaction time, the am-
poule was drawn out of the bath and cooled down to room
temperature.
O
O
O
+
+
R2
R1
O
R2
R1
O
R1
O
R2
5
6 (
Z
)
E
7 ( )
Markovnikov product
anti-Markovnikov products
Scheme 3. Synthesis of enol esters from carboxylic acids and terminal alkynes.

Q. Willem et al. / Journal of Organometallic Chemistry 694 (2009) 4049–4055
4051
Table 1
100
Influence of the catalyst on the addition of 4-acetoxybenzoic acid to 1-hexyne
catalyzed by complexes 1–4, 8, and 12–14.a
80
60
40
20
0
Complex
Reaction time (h)
Yield (%)b
Selectivities (%)c
5
6
7
1
50
200
50
200
50
200
50
200
8
15
50
200
50
200
25
14
51
29
66
11
46
26
64
62
99
61
98
56
98
83
99
36
35
29
31
31
29
34
37
35
38
1
1
2.5
2
7.5
7.5
3
39
30
2
33
36
39
32
3
24
42
28
35
4
25
40
27
35
8
95.5
95.5
80.5
81
3.5
3.5
17
12
13
14
0
200
400
600
800
1000
17
71.5
72
82.5
83
21
100
80
60
40
20
0
20.5
14.5
14
50
3
a
Reaction conditions: 4-acetoxybenzoic acid, 1.15 mmol; 1-hexyne, 1.725 mmol;
catalyst, 9.2 ꢀ 10ꢁ3 mmol; Na2CO3, 18.4 ꢀ 10ꢁ3 mmol; water-saturated toluene,
6 mL; temperature, 60 °C under nitrogen.
b
Determined by GC using dodecane as internal standard.
Determined by GC.
c
100
80
60
40
20
0
0
200
400
Time, h
600
800
1000
Fig. 2. Influence of the catalyst on the enol ester selectivities for reactions between
4-acetoxybenzoic acid and 1-hexyne catalyzed by complexes 2 (s, h, 4) and 13 (d,
j,
N
). Enol esters: 5 (s, d), 6 (h, j), and 7 (4,
N
). Reaction conditions: 4-
acetoxybenzoic acid, 1.15 mmol; 1-hexyne, 1.725 mmol; catalyst, 9.2 ꢀ 10ꢁ3 mmol;
Na2CO3, 18.4 ꢀ 10ꢁ3 mmol; water-saturated toluene, 6 mL; temperature, 60 °C
under nitrogen.
0
200
400
Time, h
600
800
1000
Fig. 1. Influence of the catalyst on the enol ester yield for reactions between 4-
acetoxybenzoic acid and 1-hexyne catalyzed by complexes 2 (s) and 13 (d).
Reaction conditions: 4-acetoxybenzoic acid, 1.15 mmol; 1-hexyne, 1.725 mmol;
catalyst, 9.2 ꢀ 10ꢁ3 mmol; Na2CO3, 18.4 ꢀ 10ꢁ3 mmol; water-saturated toluene,
6 mL; temperature, 60 °C under nitrogen.
Ru
Cl
Cl
PPh3
8
Fig. 3. TGA curves for complex 1.
Scheme 4. Structure of [RuCl2(p-cymene)(PPh3)] (8).
R2
R2
R2
catalytic activity, we probed their thermal stabilities in the solid
phase by thermogravimetric analysis (TGA). The decomposition
profiles shown in Fig. 3 and Fig. Sd4, albeit quite different, reveal
that the first sign of degradation appeared around 180 °C, corre-
sponding to the loss of the p-cymene ligand.
R2
R2
R2
It should also be noted that, although 1-hexyne was used in
excess (1.5 equiv. relative to the carboxylic acid), the reaction
generated only a trace amount of dimers 9–11 (Scheme 5). Also
9
10
11
Scheme 5. Dimerization products of 1-hexyne (R2 = n-C4H9).

4052
Q. Willem et al. / Journal of Organometallic Chemistry 694 (2009) 4049–4055
noteworthy was the observation that the formation of enol esters
was not selective and that the selectivities changed over reaction
time when complexes 1–4 were employed as catalyst precursors
(Fig. 2 and Fig. Sd1). Thus, in all cases, the amount of the Markovni-
kov adduct (5) slightly increased over reaction time, concomitantly
with the decrease of the (Z)-anti-Markovnikov isomer (6). The
changes in the thermodynamically more stable (E)-anti-Markovni-
kov isomer (7) are, however, more difficult to rationalize. These
observations strongly contrast with those made when [RuCl2(p-
cymene)(PPh3)] (8) was used as the catalyst precursor. With the
latter system, indeed, the reaction showed a high and constant
selectivity for the Markovnikov product (up to 96%) throughout
the run (Fig. Sd1).
Over the past decade, microwave heating has proven to be a
useful tool for rapid compound synthesis [10] and has found appli-
cation in catalysis [11]. Pursuing our research program [9], we car-
ried out the synthesis of vinyl esters in a sealed vessel heated at
160 °C in a CEM Discover monomode microwave reactor. We found
that as expected microwave-assisted syntheses catalyzed by 1–4
were much faster than related experiments performed under con-
ventional heating at 60 °C. The reaction times, indeed, could be re-
duced from several hundreds of hours at 60 °C to around 4 h under
microwave heating at 160 °C (Fig. 4 and Fig. Sd3). Because of the
ongoing debate over non-thermal microwave effects [12], we
decided to perform similar reactions in sealed tubes immersed in
a thermostatically controlled silicon oil heating bath kept at
160 °C. Contrary to our previous observations [9a], we observed
that the reactions were twice as fast under conventional heating
than under microwave heating (Fig. 4 and Table 2). It is also note-
worthy that at 160 °C the reaction rates were found not to be sig-
nificantly influenced by the substituents of the imidazol(in)ium
fragment, contrary to the selectivities, which again changed over
reaction time (Fig. 5 and Table 2).
We have previously shown that although imidazol(in)ium-2-
dithiocarboxylates are stable compounds, they slowly decompose
in the solid state upon thermal activation, more likely releasing
carbene ligands [3]. As a consequence, we believe that the changes
in the selectivities might be due to the involvement of a new cat-
alytic species produced during the decomposition of the coordi-
nated imidazol(in)ium-2-dithiocarboxylate ligands in complexes
1–4 when heated to 60 °C for an extended time, or to 160 °C for
a few hours. To substantiate this hypothesis, we investigated the
catalytic activity of three [RuCl2(p-cymene)(NHC)] complexes,
12–14 (Scheme 6), for the synthesis of enol esters.
100
80
60
40
20
0
60
50
40
30
20
10
0
0
2
4
6
8
0
2
4
6
8
Time, h
Time, h
Fig. 4. Influence of the heating mode on the enol ester yield for reactions between
4-acetoxybenzoic acid and 1-hexyne catalyzed by complex 1 at 160 °C. Conven-
tional heating in an oil bath (s); microwave heating (d). Reaction conditions: 4-
acetoxybenzoic acid, 0.5 mmol; 1-hexyne, 0.75 mmol; catalyst, 4 ꢀ 10ꢁ3 mmol;
Na2CO3, 8 ꢀ 10ꢁ3 mmol; water-saturated toluene, 2.6 mL; temperature, 160 °C
under nitrogen.
Fig. 5. Influence of the heating mode on the enol ester selectivities for reactions
between 4-acetoxybenzoic acid and 1-hexyne catalyzed by complex 1 at 160 °C.
Conventional heating in an oil bath (s,h,4); microwave heating (d,j,
N
). Enol
esters: 5 (s,d), 6 (h,j), and 7 (4,
N
). Reaction conditions: 4-acetoxybenzoic acid,
0.5 mmol; 1-hexyne, 0.75 mmol; catalyst, 4 ꢀ 10ꢁ3 mmol; Na2CO3, 8 ꢀ 10ꢁ3 mmol;
water-saturated toluene, 2.6 mL; temperature, 160 °C under nitrogen.
Table 2
Microwave heating vs. conventional heating for reactions between 4-acetoxybenzoic acid and 1-hexyne catalyzed by complexes 1–4, 8, and 12–14 at 160 °Ca.
Complex
Microwave heating
Yield (%)e
Conventional heating
Yield (%)e
Selectivities (%)f
Selectivities (%)f
5
6
7
5
6
7
1b
38
43
46
43
100
74
38
23
16
18
87
74
66
69
42
46
56
56
10
21
29
23
20
31
28
26
3
5
5
8
60
70
80
64
–
58
61
62
46
31
28
25
–
73
64
67
38
43
46
42
–
23
31
25
16
26
26
33
–
4
5
8
2b
3b
4b
8c
12d
13d
14d
72
74
a
Reaction conditions: 4-acetoxybenzoic acid, 0.5 mmol; 1-hexyne, 0.75 mmol; catalyst, 4 ꢀ 10ꢁ3 mmol; Na2CO3, 8 ꢀ 10ꢁ3 mmol; water-saturated toluene, 2.6 mL; tem-
perature, 160 °C under nitrogen.
b
Reaction time, 30 min.
Reaction time, 5 min.
Reaction time, 10 min.
Determined by GC using dodecane as internal standard.
c
d
e
f
Determined by GC.

Q. Willem et al. / Journal of Organometallic Chemistry 694 (2009) 4049–4055
4053
100
R
80
Ru
R
N
Cl
Cl
N
60
12 (R = Mes)
13 (R = Dip)
14 (R = Cy)
40
20
0
Scheme 6. Structure of the [RuCl2(p-cymene)(NHC)] complexes 12–14 used in this
work.
0.0
0.1
0.2
0.3
0.4
0.5
Time, h
During the past decade or so, Ru–NHC complexes have emerged
as effective catalysts for a variety of reactions [13], including olefin
metathesis [14]. To the best of our knowledge, however, only a few
studies have been reported on the use of Ru–NHC catalysts for the
synthesis of enol esters [15]. Not surprisingly, in these reactions
both the yields and the selectivities strongly depended on the cat-
alyst’s structure (Scheme 7). In particular, ruthenium complexes
bearing the 1,3,4-triphenyl-4,5-dihydro-1H-1,2,4-triazol-5-ylidene
ligand (15–18) generally favored the anti-Markovnikov isomers
[15a,d,e], whereas ruthenium–vinylidene complexes (20 and 21)
produced selectively the Markovnikov adduct [15b,c]. Further-
more, with Ru–NHC complexes bearing Schiff base ligands (22),
enyne formation took precedence of enol ester synthesis [15f].
Having in hand [RuCl2(p-cymene)(NHC)] complexes 12–14 [8],
the test-reaction between 4-acetoxybenzoic acid and 1-hexyne
was first investigated at 60 °C, using the experimental conditions
described above. It turned out that Ru–NHC complexes 12–14 dis-
played a higher activity than ruthenium imidazol(in)ium-2-dithio-
carboxylates 1–4 (Fig. 1 and Table 1). Thus, with catalysts 12 and
13 bearing, respectively, a mesityl and a 2,6-diisopropylphenyl
group on the nitrogen atoms, the reactions proceeded to quantita-
tive yields in 200 h, against 50 h with the cyclohexyl-substituted
catalyst 14. The reactions were further accelerated at 160 °C, either
Fig. 6. Influence of the heating mode on the enol ester yield for reactions between
4-acetoxybenzoic acid and 1-hexyne catalyzed by complex 13 at 160 °C. Conven-
tional heating in an oil bath (s); microwave heating (d). Reaction conditions: 4-
acetoxybenzoic acid, 0.5 mmol; 1-hexyne, 0.75 mmol; catalyst, 4 ꢀ 10ꢁ3 mmol;
Na2CO3, 8 ꢀ 10ꢁ3 mmol; water-saturated toluene, 2.6 mL; temperature, 160 °C
under nitrogen.
under microwave heating or under conventional heating (Table 2
and Figs. Sd2 and Sd3), with a reaction time of 20 min for the
reactions to be complete. It is also notable that at 160 °C complexes
12–14 behaved similarly in terms of catalytic activity. Under
microwaves, for instance, the additions were 72–74% complete
over 10 min, regardless of the substituents on the carbene ligand.
We also investigated in detail microwave-assisted and conven-
tionally heated syntheses of enol esters at 160 °C under otherwise
identical conditions (stock solution, concentration, etc.). As illus-
trated in Fig. 6, both heating modes led to different reactivity pro-
files that converged to the same point, i.e. 100% yield after 20 min
of reaction. In Fig. 6, it is also observed that for the microwave-as-
sisted reactions the origin is not intercepted in the plot of yield
against time. Thus, the experiment performed during 0 min re-
vealed already 28% yield of enol esters! For the sake of understand-
PPh3
PCy3
Ph
Ph
Cl
Cl
Ru PPh3
Ru
Ru
Ru CHPh
N
N
Cl
Cl
Cl
Cl
Cl
Ph
Ph
Ph
Ph
Ph
Ph
N
N
N
N
N
N
N
N
Ph
N
N
Ph
Ph
15
16
17
18
R'
+
PCy3
Ru CHPh
PCy3
PCy3
Cl
Cl
Cl
Cl
Cl
N
O
-
Ru
C
CHR
Mes
Ru
C
CHR BF4
Mes
R
Ru CHPh
Cl
Mes
Mes
Mes
Mes
Mes
Mes
N
N
N
N
N
N
N
N
19
20
21
22
Scheme 7. Structure of Ru–NHC catalyst precursors for the synthesis of enol esters.

4054
Q. Willem et al. / Journal of Organometallic Chemistry 694 (2009) 4049–4055
100
80
60
40
20
0
4. Conclusions
In comparison with [RuCl2(p-cymene)(NHC)] and, in particular,
[RuCl2(p-cymene)(PPh3)] (a standard catalyst in this field), ruthe-
nium imidazol(in)ium-2-dithiocarboxylates 1–4 are poor catalyst
precursors for the synthesis of enol esters from carboxylic acids
and terminal alkynes. The results are in line with those reported
previously in ring-opening metathesis and atom transfer radical
polymerizations using catalyst systems generated in situ from
the ruthenium dimer [RuCl2(p-cymene)]2 and imidazol(in)ium-2-
dithiocarboxylates. However, the catalytic activity could be im-
proved significantly at 160 °C under microwave heating or under
conventional heating in sealed tubes, as indicated by the reduction
of the reaction times from hundreds of hours at 60 °C to 4 h or less
at 160 °C. Furthermore, the selectivities in enol esters are not sat-
isfactory and, in addition, they change over reaction time, indicat-
ing that the primary catalytic species are presumably transformed
into other ones during the reactions. Applications of these com-
plexes to other metal-catalyzed reactions are currently under
investigation in our group.
0.0
0.1
0.2
0.3
0.4
0.5
Time, h
Fig. 7. Influence of the heating mode on the enol ester selectivities for reactions
between 4-acetoxybenzoic acid and 1-hexyne catalyzed by complex 13 at 160 °C.
Conventional heating in an oil bath (s,h,4); microwave heating (d,j,
N). Enol
esters: 5 (s,d), 6 (h,j), and 7 (4,
N
). Reaction conditions: 4-acetoxybenzoic acid,
0.5 mmol; 1-hexyne, 0.75 mmol; catalyst, 4 ꢀ 10ꢁ3 mmol; Na2CO3, 8 ꢀ 10ꢁ3 mmol;
water-saturated toluene, 2.6 mL; temperature, 160 °C under nitrogen.
Acknowledgements
ing, it is worth mentioning that the CEM Discover microwave
instrument is programmed so as t = 0 when the desired tempera-
ture is attained and not as expected when irradiation starts. The
yield of 28% mentioned above is therefore related to the time that
the microwave reactor takes to reach 160 °C the desired tempera-
ture [16]. On the other hand, when the syntheses of enol esters
were performed under conventional heating, the reactions started
with the immersion of the sealed tube in the oil bath and the yield
was obviously zero at time t = 0. In light of these results and taking
into account the temperature problems inherent to both methods,
it is likely that conventionally heated reactions are slightly faster
than the microwave-heated protocols, as noted above for catalysts
1–4 (Fig. 4). It is also worth mentioning that for conventional heat-
ing experiments performed at 160 °C based on the temperature of
the oil bath, the actual temperature of the reaction mixture is
slightly lower, which might result in an extended lifetime of the
catalyst compared to reactions carried out in the microwave reac-
tor where the temperature of 160 °C is that of the solution, as
determined by the infrared sensor calibrated at regular time
intervals.
We also observed that with catalysts 12–14 the selectivities de-
pended, not only on the substituents on the carbene ligand, but
also on the temperature. Thus, the selectivities for the Markovni-
kov isomer (5) were always ꢂ10% lower at 160 °C than at 60 °C.
Furthermore, the selectivities in enol esters remained constant
throughout the run (Figs. 2 and 7, and Fig. Sd1–3), as with
[RuCl2(p-cymene)(PPh3)] (8) [9a].
Because we suspected that the changes in selectivities observed
with complexes 1–4 could be caused by a partial decomposition of
the imidazol(in)ium-2-dithiocarboxylate ligand, releasing the car-
bene and carbon disulfide, the efficacy of [RuCl2(p-cymene)(NHC)]
complexes 12–14 was tested in the presence of added CS2. Interest-
ingly, the reactions were not inhibited by CS2, and the selectivities
slightly decreased (Table Sd1). It was then investigated whether
the addition of silver salts with non-coordinating counter-ions to
ruthenium imidazol(in)ium-2-dithiocarboxylates 1–4 would gen-
erate more active cationic catalysts. To this end, silver tetrafluoro-
borate was elected as chloride scavenger. Addition of 1 to 3 equiv.
of AgBF4 (relative to complex 2) to the reaction mixture signifi-
cantly affects neither the yield, nor the selectivities (Table Sd1).
By contrast, a deleterious effect was evidenced when complex 2
and Na2CO3 were first treated with AgBF4 in acetonitrile for
30 min, before addition of the reaction mixture.
The authors thank the ‘Fonds pour la Formation à la Recherche
dans l’Industrie et dans l’Agriculture’ (F.R.I.A.) for a fellowship to F.
Nicks and the ‘Fonds National de la Recherche Scientifique’
(F.N.R.S.), Brussels, for financial support of this work (Grants
F.R.F.C. 2.4565.07 and 2.4645.07) and for the purchase of major
instrumentation.
Appendix A. Supplementary material
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.jorganchem.2009.08.028.
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