Direct arylation of arene C-H bonds by cooperative action of NHCarbene-Ruthenium(II) catalyst and carbonate via proton abstraction mechanism

Article abstract of DOI:10.1021/ja710276x

Direct functionalization of sp² C-H bonds via ortho diarylation of 2-pyridyl benzene with arylbromides was achieved using ruthenium(II) catalysts containing a RuCl₂(NHC) unit and generated from [RuCl₂(arene)]₂ and two types of NHC precursors, pyrimidinium and benzimidazolium salts, in the presence of Cs₂CO₃. DFT calculations from RuCl₂(NHC)(2-pyridylbenzene) show that a proton abstraction mechanism, on cooperative actions of both the coordinated base and the Ru(II) center, is favored via a 13.7 kcal·mol-1 exothermic process affording an orthometalated intermediate with a 2.009 A Ru-C bond. Copyright

Full text of DOI:10.1021/ja710276x

Published on Web 01/10/2008
Direct Arylation of Arene C-H Bonds by Cooperative Action of
NHCarbene-Ruthenium(II) Catalyst and Carbonate via Proton Abstraction
Mechanism
Ismail O¨ zdemir,† Serpil Demir,† Bekir C¸ etinkaya,† Christophe Gourlaouen,‡ Feliu Maseras,*,‡
Christian Bruneau,§ and Pierre H. Dixneuf*,§
Institute of Chemical Research of Catalonia (ICIQ), AV. Pa¨ısos Catalans, 16, 43007 Tarragona, Spain,
Departament de Quimica, UniVersitat Autonoma de Barcelona, 08193 Bellaterra, Catalonia, Spain, Faculty of
Sciences and Arts, Ino¨nu¨ UniVersity, Malatya, Turkey, Faculty of Science, UniVersity of Ege, Izmir, Turkey, and
Institut Sciences Chimiques, Campus Beaulieu, UniVersite´ de Rennes, 35042 Rennes, France
Received November 21, 2007; E-mail: fmaseras@iciq.es; pierre.dixneuf@univ-rennes1.fr
Table 1. Influence of 1,3-Dinitrogen Salts LH+X- 3-5 on
Metal-catalyzed cross-coupling reactions constitute the most
useful method for C-C bond formation and the access to diaryls
Diphenylation of 1a
entry
LH+Cl-
2 (yieldb)
entry
LH+Cl-
2 (yieldb)
and heterocycles, via the coupling of an organic halide with a variety
of organometallics.1 The search for C-H bond activation2 has
allowed discovery of alternative routes to C-C bond formation
via catalytic economical functionalization of sp2 C-H bonds.3-6
Direct arylation of functional arenes and heterocycles can now be
performed especially with Pd7 and Rh8 based catalysts. Ruthenium-
(0) catalysts are contributing to this field by initial insertion of Ru-
(0) species into C-H bonds.4,9 Alternatively, some ruthenium(II-
IV) catalysts, containing phosphines,10 phosphine oxides,11 or
N-methylpyrrolidone,12 recently promoted the direct arylation of
arenes and heterocycles; however, the mechanism has not been
elucidated and an electrophilic substitution by a Ru(IV) species10
is still questionable. In contrast, it is established that ruthenium(II)
complexes, under mild conditions, allow orthometalation of func-
tional arenes with [RuCl2(arene)]2 and a base.13 Consequently, a
ruthenium(II) C-H bond functionalization via an orthometalation
process10d should be also considered.
1
2
3
4
4
71
89
88
84
5
6
7
8
5a
5b
5c
5d
88
58
79
81
3a
3b
3c
a Conditions: 1 mmol of 1 and 2.2 mmol of PhBr were added to a
mixture of [RuCl2(p-cymene)]2 (0.025 mmol), 0.10 mmol of salt LH+Cl-
(3-5), 3 mmol of Cs2CO3 in 2 mL of NMP (N-methylpyrrolidone), 20 h
at 120 °C. b Isolated yield %.
We have addressed this problem by studying the arylation of
2-pyridylbenzene, in the presence of new NHC-ruthenium(II)
catalysts and a base. We now report that the in situ generation of
catalysts, from [RuCl2(p-cymene)]2 and a pyrimidinium or benz-
imidazolium salt in the presence of Cs2CO3, selectively promotes
the diarylation of 2-pyridylbenzene with arylbromides (eq 1), and
that DFT calculations performed on the system RuCl2(NHC)(2-
pyridylbenzene) support an ortho proton abstraction by the coopera-
tive actions of both the carbonate and the ruthenium(II) center.
Figure 1. 1,3-Dinitrogen salts LH+X- 3-5 as NHC precursors.
Table 2. Influence of the Bromoarenes p-Y-C6H4Br
p-Y
−C6H4Br
yieldb of 6
−9
3a
3b
3c
5c
p-NC-C6H4Br
p-MeCO-C6H4Br
p-MeO-C6H4Br
p-Me-C6H4Br
6
7
8
9
75
88
84
87
77
82
87
91
56
76
60
80
51
70
66
71
a Conditions: as in Table 1 during 10 h at 120 °C. b Isolated yields.
[RuCl2(p-cymene)]2 was used to reveal the influence of salts 3-5
as the NHC precursor. The results in Table 1 show that the
pyrimidinium salts 3 and the benzimidazolium salts 5 give complete
conversion of 1 and good isolated yields of 2.
To gather information on the mechanism and to explore the scope
of ortho arylation, the influence of arylbromide p-Y-C6H4Br has
been evaluated, but only during 10 h at 120 °C to allow better
comparison. The ortho-diarylated products 6-9 were obtained in
good yields (eq 2, Table 2).
The catalytic system was explored on the basis of (i) the
formation of NHC-ruthenium(II) complexes on reaction of [RuCl2-
14
(p-cymene)]2 with imidazolinium salts and Cs2CO3 and (ii) the
coordination of carbonate on its reaction with RuCl2(NHC)(arene)
complexes.15 The diphenylation of 2-pyridylbenzene 1 with PhBr
was evaluated with catalysts prepared from 0.05 mmol of mono-
nuclear metal source, 0.10 mmol of pyrimidinium salt LH+X- 3b,
and an excess of Cs2CO3 under Table 1 conditions. [RuCl2(nbd)]n
and [RuCl2(p-cymene)]2 led to complete conversion of 1 into
product 2, isolated in 77 and 88% yields, respectively (eq 1, Figure
1).
† Ino¨nu¨ University and University of Ege.
‡ ICIQ and Universitat Autonoma de Barcelona.
§ Universite´ de Rennes.
The results show that the arylbromides containing a para electron-
donating group lead to similar yields of 8 and 9 as those with the
9
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J. AM. CHEM. SOC. 2008, 130, 1156-1157
10.1021/ja710276x CCC: $40.75 © 2008 American Chemical Society

C O M M U N I C A T I O N S
distance of 1.868 Å is similar to that obtained in previous studies
of C-H activation by σ-bond metathesis.20 The role of the metal
and the formation of the Ru-C are critical in favoring the otherwise
unexpected deprotonation of a phenyl group by a relatively weak
base.
The above results show that more aryl and heterocyclic C-H
bond functionalizations should be possible with catalysts offering,
via metal/base cooperative proton abstraction, an easy initial
orthometalation path.
Supporting Information Available: Computational details, Car-
tesian coordinates, and absolute energies for all computed structures.
This material is available free of charge via the Internet at http://
pubs.acs.org.
Figure 2. B3LYP-optimized structures for complexes 3t-a, 4t-a, and 5t-
a. Selected distances are indicated in Å.
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Scheme 1. Possible Orthometalation Intermediates
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electron-withdrawing ones leading to 6 and 7. This absence of
discrimination by various arylbromides does not support an arene
electrophilic substitution process.
In contrast, as 2-pyridylbenzene with [RuCl2(arene)]2 easily leads
to orthometalation,13 a plausible mechanism via an initial ortho-
metalation-like process is presented in Scheme 1 (L ) NHC). The
intermediate II is expected to undergo PhBr reversible oxidative
addition to give III, that is able to release the arylation product 2
by reductive elimination. The less intuitive aspect is the critical
cleavage of the C-H bond. In principle, two mechanisms can be
postulated, the C-H oxidative addition producing a Ru(IV)
intermediate with a hydride ligand, or a direct proton abstraction16,17
from the phenyl by the base. We studied the process computation-
ally through DFT calculations18 on the system [Ru(IMe)(Cl)2(2-
pyridylbenzene)] (1t) (IMe ) N,N′-dimethylimidazolylidene),
- 19
corresponding to I, plus HCO3
.
The system 1t has several
isomers, all of them producing similar energy profiles. Only the
most stable one, labeled 1t-a, will be discussed here, and the
optimized structures for the other isomers are collected in the
Supporting Information.
Complex 1t-a presents an important agostic distortion, with an
ortho C-H distance of 1.179 Å. However, oxidative addition from
this species is disfavored, and the associated octahedral complex
with a hydride ligand 2t-a has a relative energy of 28.2 kcal/mol,
quite high above 1t-a. Both complexes are shown in the Supporting
Information. In contrast, the proton abstraction mechanism is much
-
easier. The introduction of HCO3 in the system results in its
coordination to ruthenium and the formation of an adduct, 3t-a,
22.9 kcal/mol below the separated fragments. This adduct evolves
to product 5t-a, with a Ru-phenyl bond, through transition state
4t-a. The transformation from 3t-a to 5t-a is exothermic by 13.7
kcal/mol, and the barrier is 13.9 kcal/mol. This low barrier indicates
a strong preference for the proton abstraction mechanism, even
-
when considering the weak HCO3 base.
The structures for species 3t-a, 4t-a, and 5t-a are shown in Figure
2. It is worth noticing that the hydrogen atom being transferred is
practically halfway between carbon (C-H distance, 1.383 Å) and
oxygen (O-H distance, 1.316 Å) in the transition state. In contrast,
the Ru-C bond is almost formed in the transition state 4t-a: the
Ru-C distance is 2.182 Å, whereas it is 2.009 Å in 5t-a. The Ru-H
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