Catalytic activity assessment of [RuCl2 (p-cymene) (PTA)] in the synthesis of enol esters

Article abstract of DOI:10.1007/s13738-015-0732-7

The addition of carboxylic acids onto 1-hexyne was catalyzed by [RuCl₂ (p-cymene) (PTA)] complex to give enol esters. Accordingly, the Markovnikov addition-type product was selectively obtained. The microwave-assisted reactions at 160 °C confir

Full text of DOI:10.1007/s13738-015-0732-7

J IRAN CHEM SOC
DOI 10.1007/s13738-015-0732-7
ORIGINAL PAPER
Catalytic activity assessment of [RuCl2 (p‑cymene) (PTA)] in the
synthesis of enol esters
Eric Musengimana1 · Claver Fatakanwa1
Received: 25 May 2015 / Accepted: 1 September 2015
© Iranian Chemical Society 2015
Abstract The addition of carboxylic acids onto 1-hexyne
was catalyzed by [RuCl2 (p-cymene) (PTA)] complex to
give enol esters. Accordingly, the Markovnikov addition-
type product was selectively obtained. The microwave-
assisted reactions at 160 °C confirmed the influence of the
temperature, where quantitative to excellent yields were
obtained in 30 min. The catalyst loading test proved that
[RuCl2 (p-cymene) (PTA)] complex can be efficiently used
with a 1/1500 ratio to carboxylic acid providing the TON
value of 1500.
importance. Within a given series of transition metal com-
plexes, the properties can vary widely and some result
from the steric and electronic effects of the ligands [1, 2].
Since many years, researches have been performed on the
development of new catalytic systems based on ruthenium,
more active and more selective, through the use of appro-
priate ligands that are optimized by introducing substitu-
ents able to modify the electronic and steric effects in the
catalytic site. Phosphines are used as ligands in numerous
metal complexes and the most popular example is the tri-
phenylphosphine ligand which is relatively stable in air and
undergoes a very slow oxidation. Unlike most of the metal–
amine complexes, metal complexes containing one or more
phosphines tend to be lipophilic. Phosphines are also com-
patible with the metals at various oxidation states. In addi-
tion, they do not participate directly in organometallic reac-
tions. Phosphine ligands are generally “spectators” rather
than “players”. However, they have the ability to modulate
deeply the stereo-electronic properties of the metal center
and, even if they do not generally participate directly in the
reactions, they frequently participate in the creation of a
coordination site.
1,3,5-Triaza-7-phosphatricyclo phosphine [3.3.1.1]
decane, commonly called “PTA”, has the distinction of
being a water-soluble phosphine thanks to the amine func-
tions [3]. Its cone angle is 103° and its basicity is provided
by the nitrogen atoms [4]. For 10 years, the PTA and its
derivatives have been increasingly successful in organo-
metallic chemistry [4–6]. Moreover, particular attention
has been paid to the use of (arene) ruthenium complexes
as catalysts or catalyst precursors for a wide range of
transformations.
Keywords [RuCl2 (p-cymene) (PTA)] · 1-Hexyne ·
Carboxylic acids · Enol esters · Catalyst loading ·
Microwave chemistry
Introduction
Nowadays, homogeneous catalysis of transition metal com-
plexes plays an important role in organic synthesis. Indeed,
the contribution of catalysis in reactions of oxidation,
reduction, polymerization, isomerization, coupling, car-
bonylation, as well as in metathesis reactions constitutes an
irrefutable evidence of their importance. Currently, organic
chemists are still searching for new, easily accessible and
more efficient catalysts, which can lead to further transfor-
mations in important areas of organic chemistry.
A variety of transition metal complexes are used as
catalysts in numerous reactions of academic and industrial
* Eric Musengimana
e.musengimana@ur.ac.rw
In particular, complexes of the type [RuX2 (arene)
(PTA)] commonly called “RAPTA” (ruthenium arene
PTA) are known for their antitumor and antiangiogenic
1
Department of Chemistry, School of Sciences, University
of Rwanda, PO Box 117, Butare, Rwanda
1 3

J IRAN CHEM SOC
properties of interest [3, 5–9]. However, it is surprising that
these complexes were less used in homogeneous catalysis.
In general, the catalytic performance of these ruthenium
complexes was shown to be affected by the nature of the
ligand, although its role was not elucidated. Among the few
reactions studied, the catalytic reduction of allyl alcohols
and the selective hydration of nitriles to amides are also
included [10, 11].
nitrogen bulky ligand, bidentate, electron donor are capa-
ble of inducing dimerization of terminal alkynes in enynes
[20, 24]. Thus, there is a high need to develop more effi-
cient paths that would allow minimizing such drawbacks.
The recent literature reviews the important use of particular
ruthenium (II) catalysts in various reactions of site-selec-
tive C–H functionalization from methodology to synthetic
applications [25].
The enol esters are raw materials for a wide variety of
organic reactions [12]. They are frequently used as interme-
diates in organic synthesis and medicinal chemistry [13].
A wide variety of methods have been developed for their
synthesis such as the transvinylation of carboxylic acids
by vinyl or isopropenyl acetates catalyzed by transition
metal complexes, the acetoxylation of olefins-catalyzed
Pd(OAc)2 that is an important industrial process and treat-
ing of enol or enolate by anhydrides or acid chlorides [14].
At present, the most straightforward and atom-economic
method is the 1:1 addition of a carboxylic acid to a terminal
alkyne catalyzed by a transition metal complex (Scheme 1)
[12, 13, 15].
In this contribution, the complex [RuCl2 (p-cymene)
(PTA)] has been used as a catalyst for the synthesis of enol
esters. The purpose of the study was the assessment of its
catalytic activity, where the influence of the structure of
acids, the time and the temperature of reaction, as well as
the catalyst loading were investigated.
Results and discussion
Influence of carboxylic acid for reactions performed
under conventional heating
In this latter reaction, mercury salts were firstly used as
catalysts and this was a considerable disadvantage given the
toxicity of this metal [16]. Other catalytic systems based
on iridium [13], zinc, palladium and rhodium were subse-
quently tested [16–19]. Of particular use are ruthenium-
based catalysts which were the best [12, 20–23]. The selec-
tivity of this reaction should not be ignored, since it leads
to the formation of three isomers: the Markovnikov prod-
uct and anti-Markovnikov-type products, Z and E. How-
ever, as side reactions, ruthenium complexes containing a
Our investigations started with the evaluation of addi-
tion of easily accessible acetic acid and various other aryl
carboxylic acids (1 equiv.) to 1-hexyne (1.5 equiv.) under
conventional heating. Preliminary experiments were per-
formed by heating the reaction at 60 °C with dry toluene
as solvent using a ruthenium loading catalyst of 0.8 mol %
and sodium carbonate 1.6 mol % (relative to the carboxylic
acid) as additive. The results obtained at this end are pre-
sented in Table 1. As shown in Table 1, all carboxylic acids
tested in the above-mentioned conditions reacted smoothly,
Scheme 1 Ru-catalyzed vinylation of 1-alkynes
Table 1 Influence of
Entry
Carboxylic acid, R:
Yielda (%)
12 h
Selectivityb (%)
conventional heating (60 °C)
for the synthesis of enol esters
by the addition of carboxylic
acid on 1-hexyne catalyzed by
[RuCl2 (p-cymene) (PTA)]
25 h
50 h
100 h
M
AMZ
AME
1
2
3
4
5
6
p-CF3–C6H4
p-AcO–C6H4
C6H5
38
3
77
8
100
15
83
68
22
45
–
95
3
2
30
100
100
46
74
83
9.5
3.5
3
7.5
2
18
17
4
41
34
10
22
94.5
95
p-CH3–C6H4
p-CH3O–C6H4
CH3
2
92
4.5
5
3.5
4
11
91
a, b
Yields were determined by GC with the use of an internal standard (isooctane). To insure the reproduc-
ibility, reactions were performed in triplicate and the mean value of the three experiments is reported here
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J IRAN CHEM SOC
Scheme 2 Dimerization of terminal alkyne: 1-hexyne (R2 = n-C4H9); formation of a mixture of three isomers
Table 2 Influence of microwave irradiation for the synthesis of enol
esters by the addition of carboxylic acid on 1-hexyne catalyzed by
[RuCl2 (p-cymene) (PTA)]
leading to the formation of the desired enol esters in quan-
titative to good yields after 100 h of heating in a conven-
tional oil bath at 60 °C.
Entry
Carboxylic acid R:
Yielda (%)
Selectivityb (%)
In general, a high selectivity toward the Markovnikov
products was observed, among the mixture of three pos-
sible adducts produced by the reaction: the Markovnikov
product (M), as well as the anti-Markovnikov isomers Z
(AMZ) and E (AME) as illustrated in Scheme 1. We were
pleased to find that this employed ruthenium catalyst is
exceedingly sluggish; although the reaction rate is slow.
Consequently, 100 h was necessary to reach complete con-
version in most cases. Interestingly, the crude reaction mix-
ture showed only trace amounts of dimerization products of
1-hexyne, although it was used in excess (1.5 equiv. rela-
tive to carboxylic acid) (Scheme 2).
Moreover, the reactions conditions used so far seem to
be broadly applicable to a wide variety of aryl carboxylic
acids bearing different substituents in a p-position. It is
important to note that the reactivity is directly influenced
by the strength of the acid; in agreement with the litera-
ture [26, 27]. Accordingly, the p-(trifluoromethyl) benzoic
acid pKa = 3.67 (Entry 1, Table 1), for example, is more
reactive than the other tested carboxylic acids. Indeed, it
requires 50 h to reach 100 % of conversion, whereas ben-
zoic acid (Entry 3, Table 1) and p-toluic acid pKa = 4.36
(Entry 4, Table 1) required about 100 h to reach comple-
tion. As to acetic acid (Entry 6, Table 1), p-methoxybenzoic
acid (Entry 5, Table 1) and p-acetoxybenzoic acid (Entry
2, Table 1) gave, respectively, 74, 46 and 30 % of yield
after 100 h of reaction. It thus appears that the reaction
rate increases with the strength of the acid. It is important
to note that p-acetoxybenzoic acid, however, does not fit
in this line that is expected from its pKa = 4.36, presum-
ably because this acid is very slightly soluble in toluene
and used as the reaction solvent. For the same reason, the
p-methoxybenzoic acid (pKa = 4.46) presents a weak yield
for the reaction. Accordingly, too low yields were observed
for the two acids 30 and 46 %, respectively, for p-acetoxy-
benzoic acid and p-methoxybenzoic acid. Regarding the
selectivity, the Markovnikov adduct predominated in all
cases (Table 1). If we make a comparison of the results
obtained with RAPTA-C with those obtained previously in
M
AMZ
AME
1
2
3
4
5
6
7
p-CF3–C6H4
p-Cl–C6H4
p-AcO–C6H4
C6H5
100
100
97
85.5
87
10.5
9
4
4
80.5
87.5
83
13
8
6.5
4.5
6
100
99
p-CH3–C6H4
p-CH3O–C6H4
CH3
11
11
21
95
83.5
68
5.5
11
100
a, b
Yields were determined by GC with the use of an internal stand-
ard (isooctane). Experimental conditions: irradiation in the micro-
wave reactor at 160 °C and reaction time 30 min. To insure the repro-
ducibility, reactions were performed in triplicate and the mean value
of the three experiments is reported here
the literature, it appears that RAPTA-C is effective for the
synthesis of enol esters at 60° C [24].
Influence of carboxylic acid for microwave‑assisted
reactions
In the search for a more rapid reaction rate, reaction condi-
tions were changed by performing the reaction in a micro-
wave reactor. We expected that the reaction time might
be improved. This would also allow us to overcome the
problems related to the part solubility of p-acetoxybenzoic
and p-methoxybenzoic acids observed while performing
experiments under conventional heating at 60 °C. Addition-
ally, at 60 °C small amounts of 1-hexyne condensed on the
cold surface of the glass reaction with a negative impact
of reducing yields for prolonged reaction time. Hence, we
turned our attention to the use of a CEM Discover micro-
wave reactor with the possibility of heating the reaction at
the above boiling temperature of the reaction solvent (tolu-
ene). Therefore, we set the optimum conditions at 160 °C
for 30 min using 150 W of microwave power; similar con-
ditions were widely used in the literature for such reactions
[23]. Table 2 shows the results obtained.
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J IRAN CHEM SOC
Fig. 1 Influence of temperature on the addition of benzoic acid,
p-acetoxybenzoic acid and acetic acid to 1-hexyne catalyzed by
[RuCl2 (p-cymene) (PTA)]. To insure the reproducibility, reactions
were performed in triplicate and the mean value of the free experi-
ments is reported here
We found that microwave-assisted reactions were sig-
nificantly faster than their related experiments carried out
by conventional convective heating in an oil bath. Table 2
shows that these conditions allow achieving great per-
formance for all the tested carboxylic acids. Thus, we
obtained excellent yields for p-acetoxybenzoic acid (Entry
3, Table 2) and p-methoxy benzoic acid (Entry 6, Table 2),
97 % and 95 %, respectively, against 30 % and 46 % under
conventional heating at 60 °C for 100 h (Table 1). Note that
the acetic acid has poor selectivity with respect to other
tested acids.
Fig. 2 Influence of reaction time on the addition of p-acetoxybenzoic
acid to 1-hexyne catalyzed by [RuCl2 (p-cymene) (PTA)]. Irradiation
with microwave at 160 °C under N2. To insure the reproducibility,
reactions were performed in triplicate and the mean value of the free
experiments is reported here
Influence of temperature for microwave‑assisted
reactions
Based on these results, we next investigated in detail the
influence of both parameters: the temperature and the reac-
tion time on the microwave-assisted synthesis of enol esters
using [RuCl2 (p-cymene) (PTA)]. Logically, a temperature
of less than 140 °C (Fig. 1) and reaction times longer than
30 min (Fig. 2) were set as the important parameters for
obtaining maximum yields and selectivities.
According to Fig. 2, it is clear that the yield increases
with the reaction time. This rapid increase was between 0
and 10 min and then slower beyond 20 min. The selectivity
is not affected by the reaction time.
Influence of reaction time for microwave‑assisted
reactions
Figure 1 shows that for the three acids studied, the yield
increases with the temperature. The selectivity varies slightly
for benzoic acid and p-acetoxybenzoic acid, whereas for ace-
tic acid the increase of the temperature has the effect of sig-
nificantly reducing the amount of the Markovnikov addition
product in favor of the anti-Markovnikov isomers.
Scope of the reaction
After establishing these optimized conditions on 1-hexyne
used as the model alkyne, we were interested to evaluate
the scope of the reaction by using other terminal alkynes.
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J IRAN CHEM SOC
Scheme 3 Addition of p-acetoxybenzoic acid to 1-alkynes catalyzed by [RuCl2 (p-cymene) (PTA)]
Table 3 Influence of alkyne on the synthesis of enol esters catalyzed
by [RuCl2 (p-cymene) (PTA)]
Entry Alkyne
Yielda (%) Selectivityb (%)
M
AMZ AME
1
2
3
4
5
1-Hexyne
100
100
100
0
85.5 10.5
85.5 11.5
4
3
3
–
6
Scheme 5 Exocyclic cyclization of 4-pentynoic acid catalyzed by
[RuCl2 (p-cymene) (PTA)]
1-Octyne
t-Butylacetylene
Trimethylsilylacetylene
Phenylacetylene
79
–
18
–
99
68
26
the most straightforward method described so far is the
addition of carboxylic acid to propargylic alcohol catalyzed
by ruthenium complexes [29]. By employing the optimized
conditions described so far in our work, the reaction of
benzoic acid and propargylic alcohol was selective to give
one product. It was isolated and characterized as 2-oxopro-
pyl benzoate (Scheme 4) with 99 % yield.
a, b
Yields were determined by GC with the use of an internal stand-
ard (isooctane). Experimental conditions: irradiation in the micro-
wave reactor at 160 °C and reaction time 30 min. To insure the repro-
ducibility, reactions were performed in triplicate and the mean value
of the three experiments is reported here
To do so, p-acetoxybenzoic acid was chosen as the model
carboxylic acid (Scheme 3 and Table 3). As expected,
1-hexyne and 1-octyne displayed almost the same reactiv-
ity (quantitative yield) and selectivity (85 %) toward the
Markovnikov isomer (Entries 1 and 2, Table 3). We were
also pleased to find that t-butylacetylene and phenylacety-
lene were reactive in those conditions. Although reactions
were complete in 30 min of irradiation on microwave, low
selectivities were observed, with 79 and 68 % of selectivity,
respectively (Entries 3 and 5, Table 3). Very surprisingly,
trimethylsilylacetylene was not reactive at all and small
amounts of dimers (9 %) were isolated (Entry 4, Table 3).
To further evaluate the scope of the reaction, we engaged
other functionalized terminal alkynes, propargylic alcohol
and 4-pentynoic acid in the optimized conditions. By add-
ing any carboxylic acid to propargylic alcohol, the reaction
should lead to hydroxyl enol ester, but this is not the case,
rather a beta-oxopropylic acid is formed [28] which is also
an interesting building block in organic synthesis. Various
methods for their synthesis are known in the literature, but
Starting from terminal alkynoic acid, cyclization prod-
ucts, endo-enolic lactones and exocyclic lactones are
formed. Most transition metal complexes which are more
selective for the cyclization of terminal alkynoic acids
lead to the formation of exocyclic enolic lactones. How-
ever, ruthenium complex was reported to be the best tran-
sition metal catalyst to furnish selectively the endocyclic
enolic ester [30]. To evaluate our methodology in this type
of reaction, 4-pentynoic acid was catalyzed by [RuCl2
(p-cymene) (PTA)] in optimized conditions. Only the exo-
cyclic enol lactone was isolated selectively with 85 % yield
(Scheme 5).
Influence of the catalyst loading on microwave‑assisted
reactions
Given the appreciable activity observed at a temperature
above 140 °C RAPTA-C, we undertook to study the influ-
ence of catalyst loading. As a model reaction, we chose
Scheme 4 Addition of benzoic acid to propargylic alcohol catalyzed by [RuCl2 (p-cymene) (PTA)]
1 3

J IRAN CHEM SOC
Selectivityc (%)
Table 4 Influence of the
catalyst loading on the
Ratio [PhCO2H]/[Ru]
Reaction time (h)
Yielda (%)
Tonsb
addition of benzoic acid to
1-hexyne catalyzed by [RuCl2
(p-cymene) (PTA)], with the
reaction assisted by microwave
irradiation
M
AMZ
AME
Nd
Nd
Nd
125
250
500
1
1
100
100
72.5
100
72.5
100
75
125
250
88.5
87.5
87.5
87.5
87.5
87
7.5
7.5
7.5
7.5
7.5
8
4
1
363
5
2
500
5
1000
1500
1750
2
725
5
4
1000
1125
1500
665
5
4
5
7
100
38
87
8
5
7
77.5
78.5
79.5
73.5
73.5
13
9.5
10
9
12
18
12
18
63
1100
1520
820
11.5
11.5
14.5
14
87
2000
41
12
12.5
43
860
a, b, c
Yields were determined by GC with the use of an internal standard (isooctane), Nd not determined,
([Na2CO3]/[Ru] = 2) diluted in Celite (10 %). To insure the reproducibility, reactions were performed in
triplicate and the mean value of the three experiments is reported here
the addition of benzoic acid to 1-hexyne at 150 °C in the
microwave reactor. For a good precision of the catalyst
loading to the reaction, the ruthenium complex catalyst
and Na2CO3 were mixed in a ratio of [Na2CO3]/[Ru] = 2
and finely dispersed in Celite (10 %). In these last condi-
tions, we observed a quantitative response to a molar ratio
[PhCO2H]/[Ru] of 1500 where the selectivity was not
affected. Not surprisingly, above this ratio, the selectivity
and the overall yield decrease gradually with ruthenium
loading (Table 4). Finally, the turnover number (TON) does
not exceed 1500. To this end, we extended reaction times
to 18 h with microwave heating at 140 °C, but these con-
ditions did not improve the TON value, which presumably
indicates that the catalyst was deactivated.
Experimental section
All reactions were carried out under an inert nitrogen atmos-
phere, using reagents from Aldrich and Fluka. Microwave-
assisted syntheses were performed using a single-mode
Discover reactor from CEM Corp. (Matthews, NC), where
the temperature was monitored with an infrared sensor.
Analyses by gas chromatography (GC) were performed on
a Varian Star 3400 CX gas chromatograph equipped with an
RSLM-150 capillary column (25 m × 0.25 mm; film thick-
ness, 0.25 µm) and a flame ionization detector. The complex
[RuCl2 (p-cymene) (PTA)], commonly known as RAPTA-C,
was synthesized by applying a procedure described in the
literature [23]. The enol esters were purified by distillation
under high vacuum and characterized by H and 13C NMR
1
spectroscopy, according to the literature [28, 31–39].
Conclusion
Preparation of the stock solution (1‑hexyne)
In the present work, the catalytic activity of [RuCl2(p-
cymene)(PTA)] has been assessed through the synthe-
sis of enol esters by regioselective addition of carbox-
ylic acids to 1-hexyne. The results obtained show that
the enol ester was selectively obtained with the yield
depending on the pKa and the solubility of the used acid.
The microwave-assisted reactions have confirmed the
key role played by the temperature during the synthe-
sis, where at 160 °C only 30 min was enough to reach
quantitative to excellent yields. The test of catalyst load-
ing showed that [RuCl2(p-cymene)(PTA)] can be used
as catalyst in the synthesis of enol esters with maximum
TON value of 1500.
Toluene (115 mL), the internal standard isooctane (1 mL),
1-hexyne (4 mL) and three drops of water were added to a
250-mL flask. The obtained solution was cooled to −14 °C
and then degassed for 10 min.
Procedure for the synthesis of enol ester
Conventional heating reactions in sealed tubes
In a 20 mL Schlenk tube equipped with a stirring bar, car-
boxylic acid (1.15 mmol, 1 equiv.), catalyst (0.0092 mmol,
1/125 equiv.) and Na2CO3 (0.0184 mmol, 2/125 equiv.)
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J IRAN CHEM SOC
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and isooctane (internal standard) in toluene was added
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