Alkyne hydroarylation in ionic liquids catalyzed by palladium(II) complexes

Article abstract of DOI:10.1002/cssc.201000039

The efficiency of dicarbene palladium(II) complexes and of simple palladium(II) acetate as catalysts for alkyne hydroarylation under liquid-liquid biphasic conditions involving an ionic liquid as the catalyst-containing phase is investigated. The results

Full text of DOI:10.1002/cssc.201000039

DOI: 10.1002/cssc.201000039
Alkyne Hydroarylation in Ionic Liquids Catalyzed by
Palladium(II) Complexes
Andrea Biffis,* Luca Gazzola, Cristina Tubaro, and Marino Basato[a]
The efficiency of dicarbene palladium(II) complexes and of
simple palladium(II) acetate as catalysts for alkyne hydroaryla-
tion under liquid-liquid biphasic conditions involving an ionic
liquid as the catalyst-containing phase is investigated. The re-
sults obtained under these conditions, both in terms of activity
and selectivity, are compared with those previously obtained
under homogeneous conditions. The catalytic efficiency of the
systems is found to be markedly dependent on the nature of
the anion of the ionic liquid. Suitable anions are found to sig-
nificantly improve the catalytic performance in comparison to
homogeneous conditions. Preliminary investigations on the re-
cyclability of the system as well as the on advantages of using
an acidic ionic liquid are also carried out.
Introduction
Ionic liquids have undergone a tremendous develop-
ment during the past decade, and this development
is still ongoing. Their applications range from sol-
vents for reactions in chemical synthesis to media for
electrochemical processes, and from active phases
for extraction and separation techniques to additives
for novel composite materials.[1] Reactions catalyzed
by acids (Brønsted- or Lewis-type) occupy a special
Scheme 1. Alkyne hydroarylation catalyzed by chelating dicarbene palladium(II) com-
plexes.
place among the many reactions investigated in ionic
liquids, because such reactions generally proceed
through cationic intermediates that are expected to
be stabilized by an ionic liquid, thereby leading to
substantial rate enhancements. Indeed, these reactions were
among the earliest to be investigated in these solvents and
numerous examples are known to date.[2] For the same reason,
reactions that require a cationic metal complex as catalyst
have also been intensively studied in ionic liquid media, be-
cause these solvents favor the generation of the catalytically
active cationic species through the dissociation of anionic li-
gands from the catalyst precursor.[2,3]
reaction promoter, thus decreasing the cleanliness of the pro-
cess and its technological applicability. Hence, we reasoned
that the use of ionic liquids as solvents for this reaction could
have at least two advantageous aspects: (1) ionic liquids
should facilitate the dissociation of anionic ligands from the
metal complex that leads to the formation of the catalytically
active species; and (2) ionic liquids should preferentially dis-
solve the strong acid promoter, allowing its separation and re-
cycling. Incidentally, ionic liquids have been previously em-
ployed as media for alkyne hydroarylations with alternative
catalytic systems and were found to dramatically improve the
catalytic performance.[7] Based on the above, we set out to
evaluate the catalytic efficiency of dicarbene palladium(II) com-
plexes in alkyne hydroarylation under liquid–liquid biphasic
conditions involving an ionic liquid as catalyst-containing
phase.
We have been interested for some time in the use of ionic
liquids as reaction media for cationic complex catalysts, for use
in silane alcoholysis.[4] Herein, we report on the application of
ionic liquids as media for another reaction that is the object of
intense studies by our group. We have recently shown that
chelating N-heterocyclic dicarbene[5] palladium(II) complexes
such as 1 (Scheme 1; where X is an anionic ligand such as
halide, carboxylate, tetrafluoroborate, or triflate) are efficient
catalysts for the hydroarylation of alkynes with simple arenes:
depending on the reaction conditions, a product of formal
trans-hydroarylation of the triple bond A or a double-insertion
product B can be obtained.[6] The optimized reaction condi-
tions require that the catalyst bears weakly coordinating anion-
ic ligands, because the catalytically active species is a cationic
complex. Furthermore, the reaction involves a large amount of
a strong acid (trifluoroacetic, triflic, or tetrafluoroboric acid) as
[a] Dr. A. Biffis, L. Gazzola, Dr. C. Tubaro, Prof. M. Basato
Dipartimento di Scienze Chimiche
Universitꢀ di Padova
via F. Marzolo, 1–35131 Padova (Italy)
Fax: (+39)049-8275223
E-mail: andrea.biffis@unipd.it
834
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Alkyne Hydroarylation in Ionic Liquids Catalyzed by Palladium(II) Complexes
Results and Discussion
We started our investigation by studying the reaction between
pentamethylbenzene and ethyl propiolate in various commer-
cial ionic liquids (Scheme 2), adopting the reaction conditions
Figure 1. Arene conversion (%) vs. time for the hydroarylation reaction of
pentamethylbenzene and ethyl propiolate catalyzed by complex 1a in dif-
ferent ionic liquids: effect of the cation of the ionic liquid on the efficiency
&
of the reaction. Without ionic liquid ( ); [BMMIM][BF4] (+); [BMIM][BF4] ( );
*
~
&
*
[MeOct3N][NTf2] ( ); [BuMe3N][NTf2] ( ); [BMMIM][NTf2] ( ).
Scheme 2. Ionic liquids employed in this study.
previously optimized by us, which imply the use of an equimo-
lar amount of arene and alkyne, 0.1 mol% catalyst, and room
temperature.[6] The solvent for the reaction was a three-compo-
nent system comprising trifluoroacetic acid (1 equiv with re-
spect to the reagents), ionic liquid (3 mL), and 1,2-dichloro-
ethane (necessary to dissolve the arene) in an amount required
to reach an arene concentration of 2.1molLÀ1, that is, the
value that we previously found to be optimal for this type of
reaction.[6b] We checked initially that a strong acid was indeed
needed for the reaction even in the presence of an ionic liquid
by performing several blank tests in [BuMe3N][NTf2] or
[BMMIM][NTf2], using complex 1a in the absence of trifluoro-
acetic acid. The reaction did not proceed at all, neither at
room temperature nor at 508C.
Analysis of the conversion curves obtained with trifluoroace-
tic acid in the different ionic liquids clearly highlighted the fact
that the efficiency of the catalytic system was strongly depen-
dent on the nature of the anion of the ionic liquid. In fact, in
ionic liquids with the same anion but different cations the
system presented virtually the same activity (Figure 1). On the
other hand, by changing the anion (Figure 2) the activity
varied markedly and was found to be either higher or lower
than that recorded without ionic liquid.
Figure 2. Arene conversion (%) vs. time for the hydroarylation reaction of
pentamethylbenzene and ethyl propiolate catalyzed by complex 1a in dif-
ferent ionic liquids: effect of the anion of the ionic liquid on the efficiency of
~
&
the reaction. Without ionic liquid ( ); [BMMIM][BF4] (+); [BMMIM][NTf2] ( );
~
*
[BMMIM][PF6] ( ); [BMMIM][OTf] ( ).
showed the best conversion, while the ionic liquid with triflate
anion (OTfÀ) showed the worst conversion. On the other hand,
the selectivity of the system towards the trans-hydroarylation
product after 48 h was not influenced by the employed ionic
liquid and remained almost constant in the range 78–84%; the
rest of the converted reagents going into the double-insertion
product B (Scheme 1). These results highlight that, by a proper
choice of the ionic liquid, it is possible to significantly enhance
the catalytic efficiency of the system.
Considering the fact that cationic metal species are invoked
as intermediates in the catalytic cycle for the Fujiwara reac-
tion,[8] it is not entirely surprising that only the anion of the
ionic liquid influences the catalytic activity of the system, pre-
sumably upon interaction with the metal center. Although the
anions of most commercially available ionic liquids are usually
defined as noncoordinating, it has been demonstrated that
they can interact with cationic metal centers through coordina-
tion or by formation of intimate ion pairs.[9] Indeed, the activity
By using simple palladium(II) acetate in the ionic liquid
[BMMIM][NTf2] the catalytic efficiency was higher than in the
absence of the ionic liquid (Figure 3); this is not surprising
when considering that in both cases cationic species are
postulated as catalytic intermediates. However, the conversion
enhancement was much more pronounced when using cata-
lyst 1a (Figure 1). This difference may be explained by consid-
of the system decreased as a function of the coordinating abili-
À
ty of the different anions in the order NTf2À >PF6À >BF4
>
OTfÀ. The ionic liquids with the bis(trifluoromethylsulfonyl)i-
mide (NTf2À) anion, which is the least-coordinating one,
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A. Biffis et al.
Figure 4. Alkyne conversion curves (%) vs. time for the hydroarylation reac-
tion between pentamethylbenzene and ethyl propiolate in ionic liquid
[BuMe3N][NTf2] using different acids/anionic ligands.
Figure 3. Enhancement of catalytic activity through the use of ionic liquid
[BMMIM][NTf2] with catalyst 1a or Pd(OAc)2; [HTFA]=2.1m.
ering that whereas in the case of simple Pd(OAc)2 monocation-
ic species are invoked as catalytically competent species,[8] in
the case of complex 1a and related complexes experimental
evidence favors dicationic complexes as the species responsi-
ble for the observed catalytic activity.[6b] On the other hand,
the reactivity trend exhibited by Pd(OAc)2 in the different ionic
liquids is the same observed with the dicarbene complex: per-
forming the reaction in [MeOct3N][NTf2] resulted in a conver-
sion comparable to that obtained with [BMMIM][NTf2] (42% vs.
48% after 23 h), whereas much lower conversions were ob-
tained in [BMMIM][BF4] (18% conversion after 23 h).
Subsequently, ionic liquid [BuMe3N][NTf2] was used to com-
pare the activity and selectivity of the system with different
Brønsted-acid co-catalysts, such as triflic or tetrafluoroboric
acid, together with palladium dicarbene complexes with
weakly coordinating anionic ligands derived from the same
acids. These catalytic systems were previously investigated by
us in conventional organic solvents and were found to exhibit
widely different catalytic activities as well as reaction selectivi-
ties, depending on the nature of the employed acid.[6c] In par-
ticular, the use of HBF4 caused preferred formation of product
B (Scheme 1).
The catalysts were prepared in situ by reacting complex 1b
with two equivalents of the silver salt bearing as anion the
conjugated base of the employed acid (AgBF4 in the case of
HBF4 and AgOTf in the case of HOTf). The resulting conversion
curves have been expressed both as a function of the alkyne
consumption (Figure 4) and of the arene consumption
(Figure 5), because whenever significant amounts of product B
are formed the alkyne becomes the limiting reagent. The activ-
ity of the catalytic system in the ionic liquid was found to be
influenced by the type of acid and to follow the order HTFAꢀ
HOTf<HBF4. Acids stronger than HTFA could be in principle
expected to perform better, because it is generally accepted
that the rate-determining step of the catalytic cycle is the pro-
tonolysis of the Pd-vinyl intermediate.[8] On the other hand, in
the absence of ionic liquid we previously observed that the rel-
ative reactivity recorded with the various acids is not directly
proportional to their acidic strength; for example HBF4 (pKa
Figure 5. Arene conversion curves (%) vs. time for the hydroarylation reac-
tion between pentamethylbenzene and ethyl propiolate in ionic liquid
[BuMe3N][NTf2] using different acids/anionic ligands.
À4.9) performed better than HOTf (pKa À14).[6c] We have inter-
preted this observation as a balance between the strength of
the acid and the coordinating ability of its conjugated base,
À
because BF4 is a less-coordinating anion than OTfÀ.
The presence of the ionic liquid further complicates the pic-
ture. The ionic liquid increases on the one hand the reaction
rate in the case of catalyst 1a with trifluoroacetic acid, as out-
lined above. On the other hand, a marked reduction in the re-
action rate compared to homogeneous conditions is seen in
the case of the 1b/AgOTf/HOTf system, whose performance in
the ionic liquid is almost the same as 1a/HTFA despite the
widely different strength of the two acids (pKa À0.25 vs. pKa
À14). Finally, the activity of the system 1b/AgBF4/HBF4 is
nearly the same with or without ionic liquid.
Looking at the different reaction selectivities recorded in the
ionic liquid with the three acids (Table 1), in HTFA the system
is selective towards the trans-hydroarylation product A, while
in HBF4 and HOTf product B predominates. This type of selec-
tivity reflects the observations in the absence of ionic liquid,
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ChemSusChem 2010, 3, 834 – 839

Alkyne Hydroarylation in Ionic Liquids Catalyzed by Palladium(II) Complexes
using one equivalent of HTFA as the acid promoter. The results
Table 1. Selectivity at 23 h for the reaction of pentamethylbenzene and
ethyl propiolate catalyzed by complex 1a or 1b/2 AgX using different HX
acids, both in the presence or absence of ionic liquid (IL) [BuMe3N][NTf2].
are reported in Table 2.
Although the reported conversions may appear modest,
they are fully comparable with those previously obtained in
the homogeneous phase with four equivalents of acid promot-
er (instead of one equivalent as in the present case).[6c] Conse-
quently, it can be safely stated that the catalytic system in the
ionic liquid exhibits a superior performance in comparison to
the homogeneous system.
Yield [%]
Arene
Alkyne
A
B
conversion [%]
conversion [%]
HTFA with IL[a]
HTFA without IL[a]
HBF4 with IL
HBF4 without IL
HOTf with IL
HOTf without IL[b]
45
20
15
7
13
45
9
54
23
56
50
40
65
63
26
97
93
67
72
3
40
43
26
0
We have also evaluated the possibility to use the ionic liquid
phase for recovering and recycling the complete catalytic
system, that is, the palladium catalyst and the acid employed
as co-catalyst. For this purpose, a reaction was run using com-
plex 1a with HTFA and ionic liquid [BuMe3N][NTf2]. After 48 h
[a] Reaction performed with complex 1a. [b] Additional by-products
stemming from double-bond isomerization and ester hydrolysis are
formed in this case, see Ref. [6].
Table 2. Hydroarylation reactions catalyzed by complex 1a using HTFA and the ionic liquid [BuMe3N][NTf2].[a]
except for HOTf, whose chemo-
selectivity is inverted in the ionic
liquid. Furthermore, using HOTf
in the ionic liquid, side reactions
such as hydrolysis of the ester
function and double bond iso-
merization, which negatively af-
fected the selectivity in organic
solvents, were found to be mini-
mized.
Arene
Alkyne
Product
Yield [%]
45
Arene conversion[b]
54(63)[c]
10
30
8
13(16)[c]
37(44)[c]
8(8)
These experimental findings
can be interpreted in the follow-
ing terms. The enhanced activity
recorded in the ionic liquid with
the system 1a/HTFA, as well as
the inverted reaction selectivity
observed with 1b/AgOTf/HOTf
are related to the weakening
effect of the ionic liquid on the
interaction between the palladi-
um centre and the anionic li-
10
10(10)
[a] Yields and conversions determined at 23 h. [b] Alkyne conversion in parentheses. [c] An additional product
stemming from double insertion of alkyne is formed in this case.
gands, which facilitates formation of the catalytically compe-
tent species (in the case of TFAÀ) and relieves the coordination
sphere of the metal thereby favoring the coordination of a
second molecule of alkyne to give product B (in the case of
OTfÀ). The system 1b/AgBF4/HBF4, in which the anionic ligands
interact only very weakly with the metal center, remains in-
stead unaffected. On the other hand, the decreased activity of
the system 1b/AgOTf/HOTf together with the lower incidence
of unwanted acid-catalyzed side reactions with the same
system indicates that the acidic strength of triflic acid in the
ionic liquid is diminished as a consequence of the greater sol-
vation of the protons by the anions of the ionic liquid.[10] These
solvation effects do not affect the performance of less strong
acids such as HTFA or HBF4.
an arene conversion of 72% was reached; the ionic liquid
phase was separated by simple extraction of the products and
of the unreacted reagents with n-pentane. Unfortunately
though, with this procedure a large amount of acid (ca. 60%
of the initially employed amount) was removed from the ionic
liquid, whereas ICP-AAS analysis of the palladium content in
the combined organic phases demonstrated that 95% of palla-
dium was retained in the ionic liquid phase. We checked the
catalytic efficiency of the catalyst containing phase after addi-
tion of fresh reagents as well as of acid to restore the initial
acid concentration. Only a moderate decrease (ca. 10%) in the
arene conversion was registered, which in the light of the very
low level of palladium leaching into the organic phase could
be the consequence of some degree of catalyst decomposition
in the ionic liquid phase. The selectivity of the system re-
mained constant, around 80% towards product A in both
cycles. Thus, upon using liquid-liquid biphasic conditions in-
volving an ionic liquid as catalyst-containing phase, a catalytic
system is obtained which allows to efficiently recover and recy-
cle the palladium catalyst but not the acid co-catalyst, which is
On the basis of the above, it can be inferred that use of an
ionic liquid with a suitable noncoordinating anion as the cata-
lyst-containing phase allows to operate the hydroarylation re-
action using a lower amount of a weaker acid compared to ho-
mogeneous conditions. We have proved this inference by sub-
jecting different arene and alkyne substrates to hydroarylation
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A. Biffis et al.
lost to a large extent during extraction. Experiments performed
with other acids such as triflic acid pointed out that also in this
case substantial acid leaching from the ionic liquid phase into
the organic phase takes place.
rate when weaker acids having more strongly coordinating
conjugated bases are employed as reaction promoters, thereby
allowing to operate under milder conditions. Through the use
of ionic liquids as the catalyst-containing phase, the palladium
catalyst can be efficiently separated from the reaction mixture
and recycled, whereas substantial amounts of the acid promot-
er are lost into the reagents and products phase. Work current-
ly in progress aims at developing alternative intrinsically acidic
ionic liquids in order to obtain a catalytic system that is fully
recoverable and recyclable.
In order to overcome this problem we envisaged to avoid
the need for an external acid, and consequently we decided to
test an intrinsically acidic ionic liquid such as [BIM3SH][NTf2]
(Scheme 3), which was synthesized by reaction of the known
zwitterionic compound BIM3S[11] with HNTf2. We chose HNTf2
as the protonating acid in order to obtain an ionic liquid with
À
the NTf2 counteranion, which gave the best results in the pre-
viously performed catalytic tests.
Experimental Section
General remarks: All manipulations were carried out using standard
Schlenk techniques under an atmosphere of argon or dinitrogen.
The reagents were purchased from Aldrich as high-purity products
and generally used as-received. Compounds 1a[6c] and 1b[6a] and
BIM3S[11a] were prepared according to literature procedures. All sol-
vents were used as-received as technical grade solvents. NMR
spectra were recorded on a Bruker Avance 300 MHz (300.1 MHz
for 1H and 75.5 MHz for 13C); chemical shifts (d) are reported in
units of ppm relative to the residual solvent signals.
Scheme 3. Acid ionic liquid [BMI3SH][NTf2].
Preliminary hydroarylation experiments performed in this
protic ionic liquid (1 equiv with respect to the reagents, as in
the previous tests) with catalyst 1a indicate that this system is
indeed catalytically active, even in the absence of an added ex-
ternal acid. However, the catalytic efficiency is moderate both
at room temperature (14% of conversion after 48 h) and at
508C (23% of conversion after 48 h; Figure 6). Moreover the
system is not selective; forming almost identical quantities of
both the trans-hydroarylation product A and the double inser-
tion product B.
Synthesis of the ionic liquid [BIM3SH][NTf2]: BIM3S (2.82 g,
6.1 mmol)
and
bis(trifluoromethanesulfonyl)imide
(1.71 g,
7.9 mmol) where stirred at 508C for 6 h, during which time both
solids melted, giving the 3-butyl-1-(butyl-4-sulfonyl)imidazolium
bis(trifluoromethylsulfonyl)imide [BIM3SH][NTf2]. The IL phase was
then washed several times with toluene and diethyl ether and fi-
nally dried in vacuum. The product purity was confirmed via NMR
as well as via titration with standard NaOH.
General procedure for the catalytic tests: Pentamethylbenzene
(13.2 mmol), the palladium(II) complex 1a or 1b (0.013 mmol) and
the silver salt (0.026 mmol, AgOTf when HOTf was used, AgBF4 for
HBF4; no silver salt was used with complex 1a) were placed in a
100 mL round-bottomed flask, previously evacuated and filled with
argon. The ionic liquid (3 mL), the acid (13.2 mmol), and 1,2-di-
chloroethane (1 mL when HTFA was used, 0.8 mL for HOTf, 0.6 mL
for HBF4) were then added and the resulting solution was stirred
at 258C for 5 min. Finally ethyl propiolate (13.2 mmol) was added
and the reaction mixture was further stirred at 258C for the times
indicated in the figures. Portions of the solution (0.2 mL) were
drawn off from the reaction mixture and analyzed by 1H NMR or
GC-MS.
In the recycling test with the ionic liquid [BuMe3N][NTf2] the prod-
ucts and the unreacted starting materials were extracted from the
ionic liquid with n-pentane (4ꢁ10 mL). The extent of palladium
leaching was determined by digestion in 6 mL hot aqua regia of
the residue obtained evaporating the volatiles from the organic
phase. The resulting solution was diluted to 100 mL with water
and its palladium content was determined by ICP-AAS.
Figure 6. Arene conversion curves (%) vs. time for the hydroarylation reac-
tion between pentamethylbenzene and ethyl propiolate, catalyzed by com-
plex 1a, in the acidic ionic liquid [BIM3SH][NTf2] compared to [BuMe3N]-
[NTf2].
General procedure for the catalytic tests reported in Figure 6: Pen-
tamethylbenzene (4.32 mmol) and the palladium(II) complex 1a
(4.3 mmol) were placed in a 100 mL round-bottomed flask, previ-
ously evacuated and filled with argon. The ionic liquid [BMI3SH]-
[NTf2] (4.32 mmol) and 1,2-dichloroethane (0.33 mL) were then
added and the resulting mixture was stirred at 258C for 5 min. Fi-
nally ethyl propiolate (4.32 mmol) was added and the reaction mix-
ture was further stirred at 258C or 508C for the times indicated in
the figure. Portions of the solution (0.2 mL) were drawn off from
Conclusions
Ionic liquids can be employed as reaction media for alkyne hy-
droarylations catalyzed by palladium(II) complexes. The nature
of the anion of the ionic liquid markedly influences the catalyt-
À
ic efficiency, and best results were obtained with the NTf2
1
anion. Such ionic liquids substantially increase the reaction
the reaction mixture and analyzed by H NMR.
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Alkyne Hydroarylation in Ionic Liquids Catalyzed by Palladium(II) Complexes
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Keywords: carbenes · CÀH activation · hydroarylation · ionic
liquids · palladium
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Received: February 12, 2010
Revised: April 12, 2010
ChemSusChem 2010, 3, 834 – 839
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