Addition of pyrroles onto terminal alkynes catalyzed by a dinuclear ruthenium (II) complex

Article abstract of DOI:10.1016/j.jorganchem.2012.02.017

The addition of pyrroles onto alkynes has been catalyzed by a dinuclear ruthenium complex, Ru2(CO)4(PPh3)2Br4, resulting in the formation of geminal 2-vinylpyrroles in high yields under mild conditions. Further functionalization with pyrroles or alkynes to afford dipyrrolmethanes or 2,5-bis(vinyl)pyrroles via the vinyl functional group can readily be achieved. A mechanism involving cationic ruthenium complexes was proposed based on the product regioselectivity, deuteration and infrared spectroscopic studies carried out on the catalytic process.

Full text of DOI:10.1016/j.jorganchem.2012.02.017

Journal of Organometallic Chemistry 708-709 (2012) 58e64
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Addition of pyrroles onto terminal alkynes catalyzed by a dinuclear ruthenium (II)
complex
*
Sze Tat Tan, Yew Chin Teo, Wai Yip Fan
Department of Chemistry, National University of Singapore, 3 Science Drive 3, 117543 Singapore, Singapore
a r t i c l e i n f o
a b s t r a c t
Article history:
The addition of pyrroles onto alkynes has been catalyzed by a dinuclear ruthenium complex, Ru2(CO)
4(PPh3)2Br4, resulting in the formation of geminal 2-vinylpyrroles in high yields under mild conditions.
Further functionalization with pyrroles or alkynes to afford dipyrrolmethanes or 2,5-bis(vinyl)pyrroles
via the vinyl functional group can readily be achieved. A mechanism involving cationic ruthenium
complexes was proposed based on the product regioselectivity, deuteration and infrared spectroscopic
studies carried out on the catalytic process.
Received 30 December 2011
Received in revised form
9 February 2012
Accepted 16 February 2012
Keywords:
Ruthenium complexes
Pyrrole
Ó 2012 Elsevier B.V. All rights reserved.
Alkynes
Hydroarylation
1. Introduction
al. [18], it is therefore desirable to develop a regioselective system
for the exclusive formation of geminal hydroarylation products.
The synthesis of functionalized pyrroles is highly sought after
due to its importance in the study of natural products, biologically
active compounds and material science [1]. Various synthetic
methods, including the Knorr [2], Paal-Knorr [3], Hantzsch [4],
Piloty-Robinson [5] and aza-Wittig [6] synthesis have been repor-
ted for the production of functionalized pyrroles. In addition to
these methods, direct functionalization of pyrrole has also been
achieved via pyrrolyl CeH bond activation route [7], generating
much interest due to the ability to selectively functionalize organic
molecules. Subsequent addition of the activated site across alkynes
provides an elegant one-step reaction towards the formation of
vinylpyrroles (Scheme 1), which otherwise required multiple steps
[8]. The resulting vinylpyrrole can further be modified via its
unsaturated bond to produce a variety of useful pyrrolic products
[9], and even dipyrromethanes [10].
We have recently found that the bromoruthenium carbonyl
dimer, Ru2(CO)4Br4L2 (L ¼ CO or PPh3), catalyzes the hydroarylation
of pyrroles even at room temperature. The reaction proceeds
regioselectively to give geminal products in high yields (99%). It was
noted that although the ruthenium dimer has been shown to
catalyze hydroamination reactions, the formation of N-vinyl-
pyrroles (hydroamination products) were not observed [19]. The
selective nature of the system led us to propose Ru2(CO)4Br4L2 as an
excellent alternative catalyst to existing pyrrole hydroarylation
systems in producing 2-vinylpyrroles. Interestingly, by reacting
vinylpyrroles with pyrroles or alkynes to give the unique dipyrro-
methanes or 2,5-bis(vinyl)pyrroles respectively, we have demon-
strated the usefulness of the system for the development of
functionalized pyrroles. The products obtained are believed to have
potential applications in wide variety of areas.
Although a wide variety of metal catalysts, including gold [11],
nickel [12], indium [13], palladium [14], rhodium [15] and ruthe-
nium [16] has been shown to catalyze the hydroarylation of N
heterocyclic compounds, it is only recently that palladium and
ruthenium complexes have been shown to catalyze the addition of
pyrroles onto unactivated alkynes [17]. Due to a lack of gem-
selective pyrrole hydroarylation systems as highlighted by Gao et.
2. Experimental section
2.1. General procedures
Triruthenium Dodecacarbonyl, Ru3(CO)12 (Aldrich, 99%), was
recrystallized from cyclohexane before use. Phenylacetylene
(98%), 1-hexyne (98%), propargyl alcohol (99%), pyrrole (98%),
1-methylpyrrole (99%) and 1-phenylpyrrole (99%) were obtained
from Alfa Aesar, and used without further purification. IR spectra
were collected with liquid samples in a cell with CaF2 windows and
* Corresponding author. Tel.: þ6565166823; fax: þ6567791691.
E-mail address: chmfanwy@nus.edu.sg (W.Y. Fan).
0022-328X/$ e see front matter Ó 2012 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2012.02.017

S.T. Tan et al. / Journal of Organometallic Chemistry 708-709 (2012) 58e64
59
Scheme 1. Markovnikov addition of pyrrole across a terminal alkyne occurs in the presence of a ruthenium catalyst to produce vinylpyrrole.
0.1 mm pathlength, with a Shimadzu IR Prestige-21 spectrometer.
1H NMR spectra were recorded with a Bruker AMX 500 Fourier
Transform Spectrometer at room temperature, using CDCl3 as
solvent. The chemical shifts were referenced to tetramethylsilane.
Organic product yields were calculated from the 1H NMR spectra
using reagent grade toluene as internal standard. Mass spectra of
the organic products are recorded with a Finnigan Mat 95XL-T
spectrometer.
was stopped after 4 h (or longer hours, if necessary) before excess
hexane was added to precipitate the catalyst. The solvent from the
filtrate was removed under reduced pressure, and the remaining
contents were passed through
a silica gel column, using
CHCl3:Hexane (3:1 v/v) solvent pair as eluant.
3. Results and discussion
The ruthenium complex, Ru2(CO)4(PPh3)2Br4 and Ru2(CO)6Br4,
were prepared from Ru3(CO)9(PPh3)3 and Ru3(CO)12 respectively
according to literature methods and characterised by FTIR spec-
troscopy [20].
Synthesis of Ru2(CO)4(PPh3)2Br4 can easily be achieved and
the obtained product can be purified conveniently. The complex
was found to be stable under normal atmospheric conditions. It
was found that the dinuclear ruthenium complex [21],
Ru2(CO)4(PPh3)2Br4, was able to catalyze the addition of pyrroles
across terminal alkynes with high efficiency (turnover number of
50 over 4 h) (Table 1, entry 1). The reaction proceeds at relatively
mild conditions, hence does not require any sophisticated setup.
The analogous bromoruthenium carbonyl dimer [21], Ru2(CO)6Br4,
was also able to catalyze the same reaction, although its activity
was much lower when compared to the phosphine complex
(Table 1, entry 2). Our preliminary observations suggest that the
lower activity of the carbonyl was probably due to electronic rather
than steric effect, as otherwise the system that contains bulky
phosphines will produce a lower yield. As the ruthenium dimer is
known to generate active ruthenium intermediates which are
2.1.1. Synthesis of Ru3(CO)9(PPh3)3
Ru3(CO)12 (0.5 g, 0.78 mmol) was refluxed with excess PPh3 in
hexane until a dark violet precipitate was formed. The residue was
recrystallized using CHCl3-hexane to give needle-like crystals (1 g,
0.75 mmol).
y
(CO) (CHCl3): 2041 (vw), 1972 (sh), 1969 (vs). 31P NMR:
36.9 ppm.
2.1.2. Synthesis of Ru2(CO)4(PPh3)2Br4
Ru3(CO)9(PPh3)3 (0.5 g, 0.37 mmol) was dissolved in cold
benzene, before excess Br2 was added. The initial purple ruthenium
turned dark yellow immediately. The solvent and unreacted Br2
were then removed under reduced pressure, and a cold trap was
installed to prevent the corrosive vapours from entering the
vacuum pump. At this point, the residue contains a majority of
Ru(CO)3(PPh3)Br2. This was redissolved in enough CHCl3 to give
a saturated solution, which was heated at 50 ꢀC for 30 min. Hexane
was then added to precipitate the product. The product was
recrystallized from a CHCl3-Hexane solvent pair to give the yellow
product (0.35 g, 0.30 mmol).
electron-deficient, the presence of strong
s-donating phosphines
would promote their formation to a larger extent than carbonyl
ligands.
In contrast, the mononuclear analogue [22], Ru(CO)2(PPh3)2Br2,
was found to be inert towards the same reaction (Table 1, entry 3).
Such an observation is expected due to the strong coordinating
property of PPh3, which hinders the generation of a vacant site on
the metal centre, especially at low reaction temperatures. On the
(Found: C 41.3, H 2.7, Br 27.2C40H30O4Br4P2Ru2 requires C 41.6,
H 2.6, Br 27.4); 31P NMR 41.3 ppm;
y(CO): (CHCl3) 2070 (s), 2012S
Table 1
Ruthenium-catalyzed hydroarylation of N-methylpyrrole, 1a, with 2aa
(s) cmꢁ1
.
2.1.3. Synthesis of Ru2(CO)6Br4
Ru3(CO)12 (0.2 g, 0.31 mmol) was dissolved in sufficient
benzene, before excess Br2 was added. The solution turned pale
yellow immediately. The solvent and unreacted Br2 were then
removed under reduced pressure, and a cold trap was installed to
prevent the corrosive vapours from entering the vacuum pump.
Hexane was added to the residue and heated at 50 ꢀC until a yellow
precipitate forms. The yellow precipitate was collected and
recrystallized using CHCl3-Hexane.
.
Entry
Catalyst
% Yieldb
1
2
3
4
5
6
7
Ru2(CO)4(PPh3)2Br4
Ru2(CO)6Br4
Ru(CO)2(PPh3)2Br2
Ru3(CO)12
Ru3(CO)12/NH4PF6
Ru3(CO)9(PPh3)3
[Ru2(CO)4(CH3COO)2]n
99
15c
0
0
20
0
(Found: C 10.4, Br 46.4C6O6Br4Ru2 requires C 10.2, Br 46.3);
y
(CO): (CHCl3) 2138 (s), 2078 (s) cmꢁ1
.
2.1.4. Typical procedure for catalytic reaction
2
Pyrrole (10 mmol) and alkyne (1 mmol) was added to a CHCl3
a
Reactions were carried out based on 1a (10 mmol) and 2a (1 mmol) with 2%
solution (1 mL) containing the catalyst (20
mmol). The reaction
catalytic loading.
mixture was then reacted at either room temperature (25 ꢀC) or at
50 ꢀC. Sampling was carried out at 30 min intervals. The reaction
b
Total yield w.r.t. amount of 2a used.
The low yield was due to further reaction of the product to give 5 in 25% yield.
c

60
S.T. Tan et al. / Journal of Organometallic Chemistry 708-709 (2012) 58e64
Table 3
contrary, the Br-bridged dinuclear Ru2(CO)4(PPh3)2Br4 complex
can readily dissociate to give, for example, the16-electron
Ru(CO)2(PPh3)Br2 species, which will then allow for substrate
coordination and subsequent catalysis.
Ru2(CO)4(PPh3)2Br4-catalyzed hydroarylation of pyrroles, 1, with alkynes, 2a
When other ruthenium complexes, such as Ru3(CO)12
,
Ru3(CO)9(PPh3)3 [23] and [Ru2(CO)4(CH3COO)2]n [24] were used,
the catalysis did not proceed efficiently (Table 1, entry 4e7). In the
case of Ru3(CO)12, the addition of acidic salt NHþ4 PF6ꢁ was required
to obtain substantial yield [18]. This presumably suggests that Ru-
catalyzed hydroarylation involves the formation of ionic species,
which are responsible for any catalytic activity.
Optimization of the system has been performed by studying the
effect of various solvents on the catalytic process. Use of solvent is
necessary especially when the ruthenium complex does not
dissolve readily in the substrate. A variety of solvents, including
THF, CH3CN and CHCl3 has been used with the yields of the organic
products recorded in Table 2. As expected, the incomplete disso-
lution of the ruthenium complex in neat substrates, which corre-
sponded to a lower catalytic loading, led to a lower product yield
(Table 2, entry 1). When coordinating solvents (THF and CH3CN)
were used instead, a drastic drop in efficiency was observed
(Table 2, entry 3 and 4). We reasoned that the coordinating nature
of THF and CH3CN induced increased competition between solvent
and substrates for binding sites on the ruthenium centre, which
resulted in the reaction being hindered. The degree of inhibition is
proportional to the coordinating strength of the solvent, with the
stronger CH3CN ligand inactivating the reaction completely.
Addition of pyrroles across alkyne was observed in the presence
of Ru2(CO)4(PPh3)2Br4 catalyst, although excess amount of the
pyrrole is required (Table 3). A mild reaction condition was suffi-
cient to initiate the reaction using N-methylpyrrole (1a) and phe-
nylacetylene (2a) to give the 2-vinylpyrrole product 3a (Table 3,
entry 1e2). The reaction can further be optimized by slightly
increasing the temperature (Table 3, entry 3e4). When the reaction
of 1a with other alkynes was examined, it was noted that the yield
of the products were lower when aliphatic alkynes were used
(Table 3, entry5e7). The reduced reactivity is likely to be caused by
electronic factors since even 1-hexyne (2b) and propargyl alcohol
(2c), which are relatively less bulky than 2a, suffers from low effi-
ciency. There was no reaction between 1a and 3-methyl-1-pentyn-
3-ol (2d) (Table 3, entry 7). The reaction does not proceed with
internal alkynes, such as 3-hexyne and diphenylacetylene, even at
elevated temperatures and longer reaction times.
.
Entry
1
2
Temp/ꢀC
Time/h
3
%Yield of 3b
1
2
3
4
5
6
7
8
1a
1a
1a
1a
1a
1a
1a
1b
1b
1c
1c
2a
2a
2a
2a
2b
2c
2d
2a
2a
2a
2a
25
25
50
50
50
50
50
50
50
50
50
0.5
4
0.5
4
4
4
4
0.5
4
0.5
4
3a
3a
3a
3a
3b
3c
3d
3e
3e
3f
48
89(86)
65
99(97)
27
14
0
5
9
10
11
21(15)
7c
3f
0c
a
Reactions were carried out based on pyrrole (10 mmol) and alkyne (1 mmol)
with 2% catalytic loading.
b
Total yield w.r.t. amount of alkyne used. Isolated yields are in parentheses.
Further reaction of 3f gave the double addition product 4 in 91% yield.
c
a lower yield, since the catalysis is most likely to proceed via an
electrophilic aromatic substitution pathway if the pyrrole acts as
the nucleophile [17,25]. The low yield can also be caused by steric
factors, with the relatively more bulky phenyl group on 1b
hindering the reaction.
1c reacted with 2a to produce the corresponding vinylpyrrole
product 3f (Table 3, entry 10 and 11). However the catalysis did not
stop upon formation of 3f. Instead the product underwent further
reaction immediately to give the double addition product (dipyr-
romethane), 4 (Scheme 2, part A). Due to the reactivity of 3f in the
system, it could not be isolated to give a pure sample for analysis,
with its existence being detected only at the initial stages of the
reaction. We believed the lack of steric bulk on the N-atom of 1c
was responsible for the rapid formation of 4. For the case of 1a, the
additional methyl group together with the bulky ruthenium cata-
lyst would restrict further reaction of 3a. On the same note, using
the less bulky Ru2(CO)6Br4 catalyst will result in the formation of
the double addition product 5 under the same reaction condition
(Scheme 2, part B). It was noted that the yield of 5 can be increased
by lengthening the reaction time. From the above examples, it is
possible to selectively form either vinylpyrrole or dipyrromethane
products by tuning the steric bulk of the system.
N-phenylpyrrole (1b) adds across 2a to produce the corre-
sponding 2-vinylpyrrole 3e, albeit with a lower yield (Table 3, entry
8 and 9). It is reasonable for the more electron-deficient 1b to have
Table 2
Catalytic reaction of 1a and 2a carried out in different solventsa
Hence, following the above explanation, it can be expected that
1c will add to 3a under the same condition. Indeed, the unique
mixed pyrrole product, 6, was formed when 1c and 3a were reacted
in the presence of Ru2(CO)4(PPh3)2Br4 catalyst (Scheme 2, part C).
The formation of 6 joins a list of useful dipyrromethanes, and points
towards the advantage of having a controlled system for develop-
ments in a wide variety of applications.
Next, it was observed that under a different set of reaction
conditions, vinylpyrroles 3 can also add across an alkyne to give
2,5-bis(vinyl)pyrroles, 7 and 8 (Scheme 2, part D and E). The reac-
tion can be controlled by simply adjusting the pyrrole-alkyne ratio
to give the desired product. When the amount of pyrrole is suffi-
ciently in excess of the alkyne (at least 10 equivalent of pyrrole to 1
.
Entry
Solventb
% Yieldc
1
2
3
4
Neat
CHCl3
THF
74
95
15
0
CH3CN
a
Reactions were carried out based on 1a (10 mmol) and 2a (1 mmol) with 2%
catalytic loading.
b
1 mL of solvent was sufficient to homogenize the reaction mixture.
Total yield w.r.t. amount of 2a used.
c

S.T. Tan et al. / Journal of Organometallic Chemistry 708-709 (2012) 58e64
61
Scheme 2. (A) 3f reacts immediately upon formation with another molecule of pyrrole to give 4. (B) Double addition involving 1a can only take place using the less bulky
Ru2(CO)6Br4 catalyst. The yield can be improved to 70% if the reaction time is doubled to 8 h. (C) Addition of 1c to 3a can occur due to the lack of steric bulk on the incoming pyrrole.
(D) Addition of 2a to 3a can occur when the pyrrole-alkyne ratio is reduced. (E) Similarly, addition of 2a to 3e can also occur.
equivalent of alkyne), 3 is formed exclusively. On the contrary,
decreasing the ratio will give rise to the formation of 2,5-bis(vinyl)
pyrroles. Based on our knowledge, synthesis of 2,5-bis(vinyl)
pyrroles, such as 7 or 8, is relatively unknown. Thus the obtained
results will present an attractive platform for the synthesis of bis-
functionalized pyrroles.
(Scheme 3). Since there was no sign of deuteration on the other
positions of the product, it was clear that alkyne C-D bond cleavage
did not occur during the reaction. This result is in contrast to other
ruthenium systems studied by Yi et. al., where H/D exchange was
observed at various positions on the product [18]. The lower
chemical shift experienced by the deuterated product is a result of
In an attempt to understand the reaction mechanism, deuterium
studies were carried out using d1-phenylacetylene. From the
reaction of 1a with PhCC-D, we have managed to obtain partial
deuteration of the alkenyl proton alkenyl proton of 2-vinylpyrroles
isotope shift effect (Fig. 1, d 5.53 and 5.32 ppm, compared to 5.54
and 5.34 ppm of 3a) [26]. The small amount of 3a formed may be
a result of a side reaction involving an H/D exchange of PhCC-D
with atmospheric H2O.
Scheme 3. Reaction of 1a with d1-phenylacetylene.

62
S.T. Tan et al. / Journal of Organometallic Chemistry 708-709 (2012) 58e64
Fig. 1. Alkenyl region in the 1H NMR spectrum showing the formation of deuterated 2-vinylpyrroles. The slight shift away from the non-deuterated product is caused by isotope
shift effect.
Infrared spectra obtained after the reaction revealed 2 sets of
peaks at (a) 2070 and 2013 cmꢁ1, and (b) 2056 and 1997 cmꢁ1 cor-
responding to the starting ruthenium complex Ru2(CO)4(PPh3)2Br4
and the inactive species Ru(CO)2(PPh3)2Br2 respectively (Fig. 2). The
presence of Ru(CO)2(PPh3)2Br4 complex was probably due to the
reaction of the dinuclear complex with free PPh3 released from
decomposed ruthenium intermediates. We believed that the active
ruthenium species must have a short lifetime and hence eluded
detection by FTIR spectroscopy.
Hence, we first proposed the idea of ionic species being actively
involved in our system (Scheme 4). Since pyrrole hydroarylation
follows electrophilic aromatic substitution, formation of cationic
ruthenium species would only favour the reaction. In addition,
the electron-deficient intermediate can be stabilized by a strong
s-donating phosphine, which probably also accounts for a higher
yield that was obtained when Ru2(CO)4(PPh3)2Br4 was used.
Formation of an ionic intermediate can also account for the
exclusive formation of the Markovnikov product. In the mecha-
nism, the cationic ruthenium species may be generated upon
dissociation of the ruthenium dimer to the monomer species. This
As mentioned earlier, Ru-catalyzed pyrrole hydroarylation
reactions most likely involve the formation of ionic species [18].
Fig. 2. FTIR spectrum obtained upon completion of the catalysis.

S.T. Tan et al. / Journal of Organometallic Chemistry 708-709 (2012) 58e64
63
Scheme 4. Proposed mechanism for the reaction of pyrroles with alkynes.
is followed by the expulsion of a bromide ion upon alkyne coor-
dination to give a species resembling intermediate i. Depending on
the R group, either intermediate i or ii would be favoured for charge
stabilization. When 2a was used for example, the resultant benzyl
carbocation formed would be relatively more stable than primary
aliphatic carbocations, hence driving the reaction forward and
accounting for the higher product yield associated with it. The
effect of deuteration can also be accounted for by intermediate ii.
The R group is able to move to either the cis or the trans position
relative to CeD if PhC^CeD is used as the substrate. Scheme 3
shows that the trans-position is slightly favoured. Subsequent
step involving the attack of pyrrole onto ii would be driven by the
electronic attraction between the electron-rich aromatic ring and
the carbocation respectively. Ring substitution occurred exclusively
at C-2 instead of the C-3 position due to a more extensive delo-
calization of the resultant positive charge throughout the pyrrole
ring. Subsequent rearrangement within intermediate iii would
form the addition product and complete the reaction cycle.
the formation of the unprecedented 1-(1H-pyrrol-2-yl)-1-(1-
methylpyrrol-2-yl)-1-phenylethane. A mechanism involving ionic
species was proposed in an attempt to understand the process. The
high regioselectivity of the system can be accounted for using the
relatively stable carbocation formed from the resulting intermedi-
ates during the reaction.
Acknowledgement
The project was supported by a research grant provided by NUS
143-000-430-112. S.T.Tan acknowledges the support of an NUS
scholarship.
Appendix. Supplementary data
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.jorganchem.2012.02.017.
Apart from this pathway, we have also considered other similar
mechanisms involving cationic ruthenium species. As the alkyne
CeH bond was observed to have remained intact throughout the
reaction, mechanisms involving the formation of ruthenium ace-
tylide species cannot be applied to our system [27]. Also we ruled
out the possibility of pyrrole being the first substrate to react with
the ruthenium, as there was no observation reaction between
pyrrole and the ruthenium starting material [28]. Lastly, we wish to
emphasize that while the ionic mechanism was able to account for
our experimental observations, we could not rule out any possi-
bilities of the reaction occurring via neutral pathways.
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