Pyrene-based mono- and Di-N-heterocyclic carbene ligand complexes of ruthenium for the preparation of mixed arylated/alkylated arylpyridines

Article abstract of DOI:10.1021/cs500735u

By using two pyrene-based mono- and di-N-heterocyclic carbene ligands, two ruthenium complexes (one monometallic and the other dimetallic) have been obtained and fully characterized. The molecular structure of the dimetallic complex has been determined by

Full text of DOI:10.1021/cs500735u

Research Article
pubs.acs.org/acscatalysis
Pyrene-Based Mono- and Di-N-Heterocyclic Carbene Ligand
Complexes of Ruthenium for the Preparation of Mixed Arylated/
Alkylated Arylpyridines
Sergio Gonell and Eduardo Peris*
́ ́ ́
Departamento de Química Inorganica y Organica, Universitat Jaume I. Avda, Sos Baynat s/n, 12071 Castellon, Spain
S
* Supporting Information
ABSTRACT: By using two pyrene-based mono- and di-N-heterocyclic carbene
ligands, two ruthenium complexes (one monometallic and the other dimetallic)
have been obtained and fully characterized. The molecular structure of the
dimetallic complex has been determined by means of X-ray diffraction studies. The
electrochemical studies reveal that the metal−metal communication in the
dimetallic complex is weak. The catalytic activity of both complexes has been
tested in the arylation of arylpyridines with aryl halides and in the hydroarylation of
alkenes, where they showed similar activity. The sequential combination of these
two catalytic processes (hydroarylation of alkenes followed by arylation of the
resulting alkyl-substituted arylpyridine) allowed the preparation of mixed arylated/
alkylated arylpyridines. In this tandem process, the dimetallic complex afforded
activity higher than that of the monometallic complex. The activity was compared to
that shown by the [RuCl2(p-cymene)]2 complex. This reaction constitutes an
efficient method for reaching unsymmetrically substituted arylpyridines.
KEYWORDS: ruthenium, N-heterocyclic carbene, Janus, tandem, arylation of arylpyridines, hydroarylation of alkenes
analogue complexes, although we have tentatively attributed
INTRODUCTION
■
this improvement to the presence of the rigid polyaromatic
ligand, rather than to the polymetallic nature of the catalyst.
One of the rigid bis-NHC ligands that we recently described is
a pyrene-based bis-NHC, which was coordinated to rhodium
and iridium and displayed interesting photophysical properties
as well as unusual geometrical features arising from the planar
configuration of the ligand (A, Scheme 1).5 On the basis of this
ligand, and the related monometallic analogue (based on B), we
now report the preparation of two complexes of ruthenium, and
the exploration of their catalytic activity in a tandem process
that combines the chelation-assisted arylation of arylpyridines
and the subsequent hydroarylation of unactivated alkenes. We
initially chose ruthenium for the coordination of these new
The replacement of salt-generating chemical synthesis with
efficient catalyzed reactions that strive for atom economy is
having a significant impact on the manufacturing of fine
chemicals and pharmaceuticals. In this regard, the need for
efficient synthetic procedures for sophisticated products may
benefit from the use of tandem catalytic methods, which allow a
sequence of reactions to be conducted in a single reaction
vessel, without the need for purification of the reaction
intermediates.1 When considering the possibility of sequential
combination of a series of catalytic reactions, it has to be taken
into account that the catalyst has to be compatible with the
reaction conditions needed for each catalytic process, and also
with the generation of any residual materials in the preceding
steps.1a This means that it is important to explore the
compatibility of the individual catalytic reactions before they
are combined in the full tandem process.
Scheme 1. NHC-Based Ligands Used in This Work
We have been recently interested in the preparation of
heterometallic complexes for the design of sophisticated
tandem processes.2 Our idea was to combine two different
metals linked to a single-frame ligand,3 the idea being that each
metal promotes a mechanistically independent catalytic cycle.
We have also been interested in the preparation of improved
catalysts based on N-heterocyclic carbene ligands (NHCs)
decorated with rigid polyaromatic systems.4 In our studies, we
repeatedly observed that the use of polymetallic catalysts
produces clear improvements in the catalytic efficiencies of the
complexes compared to those provided by the monometallic
Received: May 29, 2014
Revised: July 14, 2014
Published: July 15, 2014
© 2014 American Chemical Society
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Scheme 2. Preparation of Mono- and Bis-azolium Salts E and F
Scheme 3. Preparation of Complexes 1 and 2
ligands, because ruthenium complexes have shown an
extraordinary catalytic versatility in a wide set of non-
metathetical reactions applied for the production of fine
chemicals.6 Specifically, ruthenium(II) complexes often com-
bine a good stability with a high activity toward sp2 C−H bond
conversion, and the rapid development of this area has been
achieved following the seminal works by Oi and Inoue,7 as
exemplified by the publication of a number of interesting review
articles by Ackermann6e,8 and Bruneau and Dixneuf.9 This type
of reaction has also allowed the design of interesting tandem
processes, like those resulting from the combination of
arylation and hydrosilylation, both facilitated by Ru(II)
catalysts.10 We also contributed to the field by describing the
activities of a series of [RuCl2(arene)(NHC)] complexes in the
arylation and selective deuteration of a series of arylpyridines.11
On the basis of these previous findings, and connecting with
the recently described Ru(II)-catalyzed hydroarylations of
unactivated alkenes described by Ackermann,12 we decided to
combine both processes for the preparation of arylpyridines
with asymmetrically 2,2′-substituted phenyl rings. The reaction
constitutes an interesting example of combination of two atom-
economy reactions for the production of highly sophisticated
products, by the sequential activation of two identical ortho C−
H bonds of functional arenes. It has to be noted that this type
of reaction has not yet been achieved, in spite of its potential to
reach new mixed bifunctional polyaromatic molecules of
interest for molecular materials and as polymer precursors.
RESULTS AND DISCUSSION
■
The pyrene imidazolium salt E was obtained starting from 2,7-
di-tert-butylpyrene-4,5-dione, C, as shown in Scheme 2. The
annulation of this diketone with ammonium acetate and
formaldehyde in refluxing acetic acid afforded the related
pyrene-imidazole, D, in almost quantitative yield (97%). D was
dimethylated with MeI in refluxing acetonitrile in the presence
of NaHCO3, yielding the final pyrene-imidazolium, E, in 67%
yield. Salt E was characterized by means of nuclear magnetic
resonance (NMR) spectroscopy, mass spectrometry, and
elemental analysis. Bis-azolium salt F was prepared according
to the method that we previously described (Scheme 2).5
Salt E and pyrene-bis-azolium salt F were used as NHC
precursors for the synthesis of the related ruthenium
complexes, as depicted in Scheme 3. The preparation of
monometallic complex 1 was better achieved by trans-
metallating the NHC ligand from the in situ prepared silver-
NHC complex to [RuCl2(p-cymene)]2. Dimetallic complex 2
was obtained by deprotonation of bis-azolium salt F with
potassium bis(trimethylsilyl)amide (KHMDS) and subsequent
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Figure 1. Two perspectives of the molecular structure of 2. Hydrogen atoms and solvent (dichloroethane) have been omitted for the sake of clarity.
Thermal ellipsoids are shown at the 50% level of probability. Selected bond distances (angstroms) and angles (degrees): Ru(1)−Cl(1), 2.4378(15);
Ru(1)−Cl(2), 2.4231(17); Ru(1)−C(1), 2.097(5); Ru−(p-cymene)centroid, 1.693(5); Cl(1)−Ru(1)−Cl(2), 84.65(6); C(1)−Ru(1)−Cl(1),
91.24(14); C(1)−Ru(1)−Cl(2), 88.14(15).
addition of [RuCl2(p-cymene)]2. Both complexes were
characterized by NMR spectroscopy, mass spectrometry, and
elemental analysis. The 1H NMR spectrum of 1 confirms the 2-
fold symmetry of the molecule caused by the presence of a
mirror plane, as exemplified by the equivalency of the protons
of the methyl groups of the tert-butyl substituents, and the
three resonances assigned to the six protons of the pyrene. The
13C NMR spectrum of 1 displays characteristic signals due to
the carbene carbon at 184.2 ppm. The 1H NMR spectrum of 2
is in agreement with the 2-fold symmetry of the complex, as
illustrated by the presence of one single resonance due to the
four protons of the pyrene. The 13C NMR spectrum of 2 shows
the signal due to the carbene carbons at 186.7 ppm.
= 1.271 V) and to the oxidation of the pyrene fragment (E1/2 =
1.462 V),13 with the first being quasi-reversible and the second
irreversible (see the Supporting Information for details). The
differencial pulse voltammetry (DPV) analysis of both
complexes allowed us to observe that the metal-centered
wave for dimetallic complex 2 is slightly broader than the
related wave displayed by monometallic complex 1. Taking the
DPV signal of 1 as a reference, we found the deconvolution of
the DPV curve for 2 shows two bands separated by 40 mV
(Figure 2). This separation indicates an essentially decoupled
The molecular structure of 2 could be determined by single-
crystal X-ray diffraction studies. The molecule consists of a
pyrene-bis-imidazolylidene bridging two ruthenium centers,
which complete their coordination sphere with a p-cymene and
two chloride ligands. The relative configuration of the ligands
about the ruthenium atoms is anti, based on the orientation of
the p-cymene ligands, therefore affording pseudo-C2h symme-
try. The bridging ligand is planar, as can be confirmed by the
side-on perspective of the molecule given in Figure 1. The Ru−
Ccarbene distance is 2.097 Å, and the through-space distance
between the metals is 13.39 Å, very similar to the M−M
distance provided by the same ligand in our previously reported
complexes of rhodium and iridium.5
Cyclic voltammetry (CV) studies were conducted to
determine whether there is any significant metal−metal
electronic communication across the pyrene-di-imidazolylidene
ligand in dimetallic complex 2. The complex displays a single
quasi-reversible one-electron oxidation wave centered at an E1/2
of 1.336 V. Complex 1 displays two oxidation waves
corresponding to the oxidation of the ruthenium center (E1/2
Figure 2. Relevant DPV section of complex 2 (1 mM in dry CH2Cl2
with 0.1 mM [NBu4][PF6] as the supporting electrolyte, scan rate of
50 mV s−1, Fc+/Fc used as an internal standard with E1/2 (Fc/Fc+) =
0.44 V vs the saturated calomel electrode). The convoluted curves
(gray dotted lines) were obtained from adding two potential-shifted
and weighted signals of monometallic complex 1.
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a
Table 1. Arylation of Arylpyridines Using Catalysts 1 and 2
a
Reaction conditions: KOAc (0.05 mmol), 5% catalyst loading (based on metal), 2 mL of NMP, room temperature for 1 h, then substrate (0.5
b
mmol), Ar-X (1.25 mmol), K2CO3 (1.25 mmol), 120 °C. Yields determined by gas chromatography using anisole (0.5 mmol) as an internal
standard. Isolated yields in parentheses.
a
Table 2. Hydroarylation of Terminal Alkenes with 2-Phenylpyridine, Using Catalysts 1 and 2
b
b
entry
R
catalyst
additive
solvent
monoalkylated (%)
bisalkylated (%)
1
2
3
4
5
n-octyl
2
2
2
2
1
2
2
1
2
1
2
KOAc
NMP
6
−
−
−
−
−
−
−
−
n-octyl
KO2CMes
KOAc
NMP
14
n-octyl
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
72
n-octyl
KO2CMes
KO2CMes
KO2CMes
KO2CMes
KO2CMes
KO2CMes
KO2CMes
KO2CMes
93 (81)
95
n-octyl
c
6
n-octyl
93
7
n-hexyl
n-hexyl
n-butyl
n-butyl
trimethylsilyl
94 (79)
96
8
9
94 (80)
97
−
−
9 (7)
10
11
90 (80)
a
Reaction conditions: 0.5 mmol of 2-phenylpyridine, 1.5 mmol of alkene, 0.15 mmol of additive, 5% catalyst loading (based on metal), 2 mL of
b
solvent, 120 °C, 20 h. Mes = 2,4,6-trimethylphenyl. Yields determined by gas chromatography using anisol (0.5 mmol) as an internal standard.
Isolated yields in parentheses. Reaction conducted in the presence of a drop of mercury.
c
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a
Table 3. Sequential Hydroarylation of Alkenes and Arylation of Arylpyridines Catalyzed by 1, 2, and [RuCl2(p-cymene)]2
b
entry
R
catalyst
PhX
t (h)
yield (%)
1
2
3
4
n-octyl
n-octyl
n-octyl
n-octyl
n-octyl
n-octyl
n-octyl
n-octyl
n-octyl
n-octyl
n-hexyl
n-hexyl
n-hexyl
n-hexyl
n-hexyl
n-butyl
n-butyl
n-butyl
n-butyl
n-butyl
n-butyl
n-butyl
1
1
2
2
2
2
2
2
2
PhBr
PhBr
PhCl
PhBr
PhBr
PhBr
PhBr
PhBr
PhBr
PhBr
PhBr
PhBr
PhBr
PhBr
PhBr
PhBr
PhBr
PhBr
PhBr
PhBr
PhBr
PhBr
24
72
24
24
24
24
72
90
24
24
24
48
24
72
24
24
48
24
72
90
24
48
38
51
48
52
c
5
3
d
6
15
7
8
61
71 (64)
50
e
9
10
11
12
13
14
15
16
17
18
19
20
21
22
[RuCl2(p-cymene)]2
52
1
32
1
42
2
44
2
73 (62)
51
[RuCl2(p-cymene)]2
1
37
1
49
2
45
2
70
2
77 (65)
45
[RuCl2(p-cymene)]2
[RuCl2(p-cymene)]2
60
a
Reaction conditions: 0.5 mmol of 2-phenylpyridine, 1.5 mmol of alkene, 0.15 mmol of KO2CMes, 5% catalyst loading (based on metal), 2 mL of
b
toluene, 120 °C, 20 h. Then 1 mmol of PhX and 1 mmol of K2CO3, 120 °C for the appropriate time. Yields determined by GC using anisol (0.5
mmol) as an internal standard. Isolated yields in parentheses. Two milliliters of H2O was added in the second step of the reaction. Two milliliters
of NMP was added in the second step of the reaction. The reaction was conducted in the presence of a drop of mercury.
c
d
e
system, as the ones that we found for our previously dimetallic
complexes of iridium(I), based on the same ligand (the
coupling was 30 mV).5 This weak coupling compares well to
those of other π-linked di-NHCs14 and is in agreement with our
previous suggestion that the electronic communication across
polyaromatic-linked di-NHCs is mostly due to a σ-inter-
action,15 unless a proper orientation of the carbene and metal
orbitals is achieved by the use of the appropriate chelating
ligands.16
Complexes 1 and 2 were tested in the arylation of
arylpyridines with a series of arylhalides. The results are listed
in Table 1. For the study of this reaction, we used previously
described reaction conditions (120 °C in NMP)7b,17 and a
catalyst loading of 5 mol %, based on the amount of metal. As
shown in the results listed in the table, both complexes afforded
almost quantitative yields to the bisarylated arylpyridines, in
relatively short reaction times (1−5 h). These results clearly
improve the activity shown by other related [RuCl2(p-
cymene)(NHC)] complexes in the same reaction, for which
longer reaction times were needed and poorer selectivities were
achieved.11 The comparison of the activities afforded by
monometallic catalyst 1 and dimetallic complex 2 reveals that
there are not any significant differences in the activities afforded
by these two complexes.
addition of sp2 C−H bonds to unactivated alkenes and has the
potential to provide access to a wide variety of alkyl-substituted
bicyclic compounds, which are important motifs in medicinal
chemistry.18 The reactions were conducted using 2-phenyl-
pyridine and a terminal alkene, in the presence of a carboxylate
at 120 °C, and using a 5 mol % of catalyst loading (based on
the amount of metal). We were interested in determining
whether the reaction could be efficient using NMP as a solvent,
to explore the compatibility of this reaction with the arylation
of arylpyridines (reactions shown in Table 1), but the process
was completely inefficient. As shown by the results listed in
Table 2, the optimal reaction conditions were obtained when
KO2CMes and toluene were used, and the activities of the two
catalysts compare well with the previously reported results.12 As
observed before, we did not find any significant differences in
the catalytic activities provided by monometallic and dimetallic
catalysts 1 and 2, respectively. Interestingly, although the
reactions were very selective in the formation of the
monoalkylated phenylpyridines, for the reaction with trime-
thylvinylsilane a small amount of the bisalkylated compound
was observed. Because the reaction is conducted at a high
temperature and needs a long time to reach completion, we
considered it convenient to perform a mercury drop experi-
ment to discard the possibility that the reaction could be
heterogeneously catalyzed.19 As shown by the comparison of
the results listed in the table (compare entries 4 and 6), the
yields obtained do not dependently vary upon addition of the
Encouraged by these results, we then decided to test both
catalysts in the hydroarylation of unactivated alkenes. This
carboxylate-assisted reaction, recently developed by Ackerman
and co-workers,12 constitutes a convenient atom-economical
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drop of mercury, and therefore, we believe that the
heterogeneity of the process may be ruled out.
Scheme 4. Maximal Isolated Yields Obtained in the
Production of Unsymmetrically Substituted Arylpyridines
Using Catalyst 2
Given the activity of 1 and 2 in the arylation of arylpyridines
and in the hydroarylation of alkenes, we decided to combine
these two reactions in a tandem process to afford unsymmetri-
cally substituted phenylpyridines. Because of the deactivation of
the hydroarylation in NMP, which is the optimal solvent for the
arylation of arylpyridines, we decided to conducted the
sequence of reactions in toluene, a solvent that has already
proven to be adequate for this process.20 Considering that the
hydroarylation of alkenes selectively yields the monoalkylated
products, leaving an unfunctionalized C−H bond at position 2′
of the phenylpyridine, we designed the sequence by first
proceeding with the hydroarylation to form the monoalkylated
phenylpyridine and then adding the arylhalide to arylate the
remaining C−H bond and afford the final alkyl,aryl-substituted
phenylpyridine. The inversion of the sequence would facilitate
the formation of the bisarylated phenylpyridine, because
arylation is favored over hydroarylation. The reactions were
conducted by reacting 2-phenylpyridine with the terminal
alkene in toluene at 120 °C for 20 h, in the presence of
KO2CMes and 5 mol % of catalyst (based on metal), and then
adding the arylhalide and K2CO3. The results for the overall
process are listed in Table 3, where t (time) refers to the time
for the second step of the reaction (arylation). As shown by the
data listed in the table, the catalysts afford moderate yields to
the final unsymmetrically substuted phenylpyridines after 24 h
[gas chromatography (GC) yields 38−52%], but longer
reaction times (90 h) allow the achievement of the final
products in higher yields (GC, 71−77%; isolated, 62−65%),
which in fact indicates that the catalyst remains active for the
very long reaction process. Some previous works have
suggested that the arylation of arylpyridines may be favored
in the presence of water,21 and this is why we also performed a
reaction in which water was added to the second step of the
process. As shown in Table 3, the addition of water clearly
reduces the efficiency of the process, compared to that of the
reaction in which toluene is used (compare entries 4 and 5).
Interestingly, although dimetallic complex 2 showed activities
similar to that shown by monometallic complex 1 in the
individual steps of the reactions (Tables 1 and 2), now
dimetallic complex 2 shows an activity higher than that of 1, as
shown by the higher product yields provided by the dimetallic
complex. For comparative purposes, we also conducted the
reactions using [RuCl2(p-cymene)]2 as a catalyst, and we
observed that the activity of this complex is very similar to that
afforded by dimetallic complex 2. Because the overall process
requires a very long reaction time at a very high temperature,
we also considered it important to discard the possibility that
the process may be heterogeneously catalyzed, and therefore,
we performed the mercury drop test. As shown by the results
listed in the table (compare entries 4 and 9), when the reaction
was conducted in the presence of a drop of mercury, the final
yields were quasi-identical to the results obtained in the absence
of mercury. This result suggests that the process is
homogeneously catalyzed.
CONCLUSIONS
■
In summary, we have obtained a new diruthenium complex
based on a pyrene-bis-NHC Janus-type ligand and a related
monometallic complex of ruthenium with a pyrene-based-NHC
ligand. The catalytic activity of both complexes has been tested
in the arylation of arylpyridines with aryl halides and in the
hydroarylation of alkenes, where they showed very similar
catalytic activities. The sequential combination of these two
catalytic processes (hydroarylation of alkenes followed by
arylation of the resulting alkyl-substututed arylpyridine)
allowed the preparation of mixed arylated/alkylated arylpyr-
idines, by the successive C−H bond activation of two identical
C−H bonds. In this tandem process, dimetallic complex 2
afforded an activity higher than that of related monometallic
complex 1. The [RuCl2(p-cymene)]2 complex provided a
catalytic activity similar to that of 2. This reaction constitutes an
efficient method for reaching unsymmetrically substituted
arylpyridines and widens the accessibility to a wide range of
new materials that may have important applications in materials
chemistry and pharmaceuticals. To the best of our knowledge,
only one recent example describing the sequential chelating-
assisted monoarylation with two different arylhalides to reach
unsymmetrical ortho diarylated arenes has been reported, which
also included an interesting example of mixed ortho-arylated
and alkenylated arenes,21 but our work constitutes the first
example describing the access to mixed ortho-substituted alkyl/
aryl-phenylpyridines.
ASSOCIATED CONTENT
* Supporting Information
Experimental procedures and characterization data of all new
complexes and crystallographic details of complex 2 in the form
of a cif file. This material is available free of charge via the
Internet at http://pubs.acs.org.
■
S
AUTHOR INFORMATION
Corresponding Author
*E-mail: eperis@uji.es.
Notes
■
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
We gratefully acknowledge financial support from Ministerio de
Economia y Competitividad of Spain (CTQ2011-24055/
BQU). S.G. thanks the Ministerio de Ciencia e Innovacion
for a fellowship. We are grateful to the Serveis Centrals
́
d’Instrumentacio Cientıfica (SCIC) of the Universitat Jaume I
́
for providing us with all characterization techniques. We thank
Dr. Macarena Poyatos for conducting the cyclic voltammetric
studies and for revising the writing of the manuscript.
■
́
Scheme 4 shows the maximal yields obtained and the
reaction conditions used in the preparation of each unsym-
metrically substituted phenylpyridine using catalyst 2. Given
the complexity of the process, we consider these yields to be
remarkable.
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