Rhodium or ruthenium-catalyzed oxidative C-H/C-H cross-coupling: Direct access to extended π-conjugated systems

Article abstract of DOI:10.1002/anie.201207196

Doubling up: A chemo- and regioselective oxidative cross-coupling between various N-heteroarene-containing arenes and heteroarenes has been carried out by rhodium- or ruthenium-catalyzed twofold C-H activation, to deliver an array of highly functionalized π-conjugated systems. Copyright

Full text of DOI:10.1002/anie.201207196

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Communications
DOI: 10.1002/anie.201207196
À
C H Activation
À
À
Rhodium or Ruthenium-Catalyzed Oxidative C H/C H Cross-
Coupling: Direct Access to Extended p-Conjugated Systems**
Jiaxing Dong, Zhen Long, Feijie Song, Ningjie Wu, Qiang Guo, Jingbo Lan, and Jingsong You*
Extended p-conjugated structures containing bi-, ter-, tetra-,
and poly(hetero)aryl motifs are found in many biologically
active molecules, ligands, agrochemicals, pharmaceuticals,
natural products, and especially, in optical and photochromic
materials.[1] Conventional wisdom states that the construction
of bi(hetero)arenes mostly relies on the transition-metal-
oarene in the presence of 5.0 equiv of Cu(OAc)2 at 1708C.[5]
However, the heteroaryl coupling partners were limited to
À
those with acidic C H bonds (such as, benzoxazoles,
5-aryloxazoles, and caffeine). In recent years, rhodium com-
À
plexes have been shown to be powerful catalysts for C H
bond functionalization, with the advantages of high catalytic
efficiency, good functional group tolerance, and mild reaction
conditions, all of which are complementary to palladium
À
À
catalyzed C X/C M cross-coupling of a (hetero)aryl halide
or pseudohalide with a (hetero)aryl organometallic reagent.
The further extension of p-conjugated systems from bi-
(hetero)aryls to ter-, tetra- and poly(hetero)aryls usually
requires the installation of activating groups (such as, Cl, Br, I,
OTs, SiR3, BR2, SnR3, ZnR) onto the coupling partners.
However, these preactivation methods can suffer from long
multistep reaction sequences, poor regio- and/or chemo-
selectivity, and even poor availability. In particular, the
resulting prefunctionalized coupling partners may also pos-
sess inadequately stability for participation in the subsequent
coupling reactions. Moreover, heteroaryl halides generally
undergo coupling reactions reluctantly.[2] Therefore, direct
catalysis.[6] However, the rhodium-catalyzed oxidative C H/
À
À
C H cross-coupling reaction is still in its infancy, and the
selectivity and reactivity issues remain to be addressed. To
date, only the cross-coupling reactions of furan, thiophene,
and indole derivatives (Scheme 1, route A),[7] and directing-
group-containing arenes with alkenes (route B)[8] or arenes
(route C)[9] have been disclosed.
À
À
oxidative C H/C H cross-coupling between two (hetero)aryl
À
scaffolds through a twofold C H activation would be one of
the most ideal approaches to expand p-conjugated systems.[3]
Herein, we wish to explore a concise and general method to
elongate p-conjugated systems through the formation of
heteroaryl-aryl linkages.
À
Most of the successful twofold C H oxidative cross-
couplings between an arene and a heteroarene have been
achieved through palladium-catalyzed methods. Neverthe-
less, the arene coupling parters usually suffer from regiose-
lectivity problems and are used in considerable excess as
a solvent or co-solvent. This is due to the generally low
À
À
Scheme 1. Rh-catalyzed oxidative C H/C H cross-coupling. DG=
directing group, HetAr=heteraryl.
reactivity of their C H bonds,[4] which limits their application
À
scope to some extent, especially for high-melting-point
substrates and those which are not readily available. In
2011, Miura and co-workers disclosed that the copper-
mediated oxidative cross-coupling could proceed between
a directing-group-containing arene and just 2 equiv of heter-
As part of our continued efforts to extend p-conjugated
systems,[10] we envisioned that a Rh-catalyzed oxidative C H/
À
À
C H cross-coupling between an arene and a heteroarene
would provide new access to poly(hetero)aryls (route D). In
our attempts to acheive this type of transformation without
using a large excess of either of the two partners, we expected
to encounter some obstacles: 1) the low reactivity of arene
[*] J. Dong, Z. Long, Dr. F. Song, N. Wu, Q. Guo, Prof. Dr. J. Lan,
Prof. Dr. J. You
À
Key Laboratory of Green Chemistry and Technology of Ministry of
Education, College of Chemistry, and State Key Laboratory of
Biotherapy, West China Medical School, Sichuan University
29 Wangjiang Road, Chengdu 610064 (PR China)
E-mail: jsyou@scu.edu.cn
C H bonds, 2) the notorious tendency for homo-coupling of
reaction partners (heteroarenes in particular),[11] and 3) trou-
bles with regioselectivity. Herein, we disclose the discovery
and development of the solution to these substantial chal-
lenges. Furthermore, the potential of the use of more
economical ruthenium catalysts in the dehydrogenative
cross-coupling between an arene and a heteroarene is
displayed for the first time.
[**] This work was supported by grants from the National Basic
Research Program of China (973 Program, 2011CB808600), and the
National NSF of China (21025205, 20972102, 21021001, and
J1103315J0104).
We initiated our investigation by examining the feasibility
of the rhodium-catalyzed cross-coupling of 2-phenylpyridine
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201207196.
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(1a) and 2-methylthiophene (2a) [Eq. (1); see also the
Supporting Information, Table S1].[12,13] Gratifyingly, ter-
(hetero)arene 3a was obtained in 61% yield when
[(Cp*RhCl2)2] was used in combination with AgSbF6
(20 mol%) and Cu(OAc)2 (3.0 equiv) in dioxane at 1408C
for 48 h. Further optimization of solvent improved the yield of
on the arenes, biaryls could be converted into the desired
products. The reaction also performed well when the pyridine
ring had a substituent at the 3 position (3d). Moreover, when
the phenyl ring was replaced by a naphthyl group, the reaction
also afforded the target molecule 3h in 66% yield.
Subsequently, we applied our method to other heteroar-
enes to prepare an array of 1,2-diheteroaryl-substituted
arenes (4; Scheme 3). Thiophenes with either electron-with-
drawing groups or electron-donating groups successfully
engaged in this reaction (4b–f and 4h–I; Scheme 3). Func-
tional groups, such as esters, aldehydes, acyls, and nitriles,
which are useful in further synthetic transformations, were
tolerated under the current conditions (4d–f and 4h;
Scheme 3). When 3-methylthiophene was employed as the
substrate, the coupling reaction occurred at the less sterically
hindered C5-position (4I; Scheme 3). Other electron-rich
heterocycles such as benzofurans and furans could also be
applied in the coupling reaction to produce products in
satisfactory yields (4j–l; Scheme 3). Moreover, electron-rich
five-membered heteroarenes, thiazoles, and oxazoles could
3a to 70%. However, the addition of both acid and base to the
reaction system diminished the reaction effectiveness. Other
oxidants, such as Cu(OPiv)2, Cu(TFA)2, and CuCl2 were
significantly less effective, whereas Ag2CO3 gave a compara-
ble result. In view of the high cost of silver salts, Cu(OAc)2
was preferentially chosen as the oxidant. In the absence of
AgSbF6, the yield decreased to 54%. The reaction did not
proceed without rhodium catalyst or Cu(OAc)2 oxidant.
Finally, the cross-coupling proceeded well with a catalytic
system composed of [(Cp*RhCl2)2] (5 mol%), AgSbF6
(20 mol%), and Cu(OAc)2 (3.0 equiv) in 1,2-dichloroethane
(DCE) at 1408C for 48 h.
À
undergo arylation at the acidic C H bond in the C2 position
(4n–o; Scheme 3). When the C2 position of the thiazole ring
was blocked with an isobutyl group, heteroarylation selec-
With the optimized conditions in hand, we next set out to
explore the scope of the method with respect to the arene
coupling partners. As summarized in Scheme 2, a range of
biaryls containing N-heteroarenes such as pyridine, quinoline,
pyrimidine, and pyrazole smoothly underwent oxidative
cross-coupling with 2-methylthiophene (2a), delivering
a series of p-conjugated poly(hetero)arenes in moderate to
excellent yields. The positions of the substituents on the
arenes had a negligible effect on the transformation. Whether
the substituents were installed in the para- or ortho-position
À
À
Scheme 2. Rh-catalyzed cross-coupling of N-heteroarene-containing
Scheme 3. Rh-catalyzed C H/C H cross-coupling of aryl-substituted
quinolines or pyridines with various heteroarenes. Reaction conditions:
biaryl (0.25 mmol), heteroarene (0.75 mmol), [(Cp*RhCl2)2] (5 mol%),
AgSbF6 (20 mol%), Cu(OAc)2 (3.0 equiv) and DCE (2 mL) at 1408C for
48 h. [a] Ag2CO3 (3.0 equiv) instead of Cu(OAc)2, PivOH (0.5 mmol),
and DMF (2 mL) instead of DCE. [b] 1.0 mmol scale. [c] The
À
arenes (1) with 2-methylthiophene (2a) by double C H activation.
Reaction conditions: biaryl (0.25 mmol), 2-methylthiophene
(0.75 mmol), [(Cp*RhCl2)2] (5 mol%), AgSbF6 (20 mol%), Cu(OAc)2
(3.0 equiv) and DCE (2 mL) at 1408C for 48 h. [a] Ag2CO3 (3.0 equiv)
instead of Cu(OAc)2, pivalic acid (PivOH; 0.5 mmol), and DMF (2 mL)
instead of DCE. [b] 1.0 mmol scale.
diheteroarylated product 4q was obtained in 23% yield.
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tively occurred at the thiazole C5 position in 73% yield (4p;
Scheme 3,). Caffeine, a biologically important active alkaloid
with an imidazole skeleton, reacted with 2-phenylquinoline to
provide 4m in a synthetically useful yield (Scheme 3).
encountered in palladium catalysis, were not observed.[16]
2) The transformation was completely regioselective.
3) Only the mono-heteroarylated product of arenes was
obtained, except in the case of 4q. In particular, the ratio of
the two partners could be decreased to 3:1, unlike with other
dehydrogenative coupling reactions, which often require the
use of arenes as solvent or co-solvent.[4,9a]
To get some mechanistic insight, H/D exchange experi-
ments were carried out [Eqs. (2)–(4)]. When 2-phenylpyri-
dine (1a) alone was treated with D2O under the standard
reaction conditions for 1 hour, 80% of deuterated [D]-1a was
obtained [Eq. (2)]. On the other hand, only 7% H/D
scrambling was observed with benzothiophene (2b)
[Eq. (3)]. The exposure of 1a and 2b to D2O gave a similar
ratio of deuterated products under the same conditions
[Eq. (4)], which suggested that a reaction starting from
2-phenylpyridine was reasonable. Importantly, the cyclome-
talated RhIII complex 7 could catalyze the reaction of 1a and
benzoxazole (2o) to give 4o in 73% yield [Eq. (5); Cp* =
1,2,3,4,5-pentamethylcyclopentadienyl], which implied that
complex 7 could be a plausible intermediate in the catalytic
cycle.
Benzodithiophene (2q, BDT) and 3,6-di(thiophen-2-
yl)pyrrolo[3,4-c]pyrrole-1,4(2H,5H)-dione (2r) are important
structural units that frequently appear in organic functional
materials such as OLEDs and solar cells.[14] These versatile
dithiophenes smoothly coupled with bi(hetero)arene to
elongate p-conjugated systems through the formation of
heteroaryl–aryl linkages. As shown in Figure S1 in the
Supporting Information, the coupled poly(hetero)aryl prod-
uct 5b exhibited bathochromic shifts in absorption and
emission maxima with a strong red emission (lex: 566–
601 nm). It was found that 2q could undergo cross-coupling
to give both mono- and di-arylated products (5a and 5c).
To demonstrate the importance of acetate anion in the
reaction system, a stoichiometric amount of the RhIII complex
Cp*ArRhCl 7 and 2-methylthiophene (2a) were heated at
1408C in DCE in the presence of 1.0 equiv of AgSbF6 for
48 h, but only a trace amount of the cross-coupled product 3a
was observed. Subsequently, NaOAc (3.0 equiv) was added to
the reaction system, and 3a was successfully obtained, albeit
in a 38% yield (Scheme 5). When Cu(OAc)2 was replaced by
CuCl2 in the catalytic system, only a trace amount of 3a was
obtained (Supporting Information, Table S1). Upon adding
an additional 3.0 equiv of NaOAc to the reaction system, 3a
was obtained in 35% yield. These observations indicated that
We finally turned our attention to exploring a more
economical alternative to rhodium complexes. Ruthenium
catalysts are usually inexpensive and have demonstrated
some catalytic properties similar to those of rhodium
catalysts.[15] We were pleased to find that a catalyst system
composed of [Ru(p-cymene)Cl2}2] and KPF6 was able to
promote the oxidative cross-coupling of 2-methylthiophene
with benzo[h]quinoline and 2-aryl quinoxaline in acceptable
yields (Scheme 4). These preliminary results indicated the
intrinsic potential of ruthenium to catalyze the oxidative
cross-coupling between two (hetero)arenes, and the detailed
results of these couplings will be reported in due course.
Compared to the previous examples, the Rh- or Ru-
À
À
catalyzed C H/C H cross-coupling reactions developed
herein exhibited the following advantages: 1) The homocou-
pling products of each coupling partner, which are frequently
Scheme 4. Ru-catalyzed direct heteroarylation of N-heteroarene-substi-
À
tuted arenes by double C H activation. Reaction conditions: biaryl
(0.25 mmol), 2-methylthiophene (0.75 mmol), [{Ru(p-cymene)Cl2}2]
(5 mol%), KPF6 (20 mol%), Cu(OAc)2·H2O (2.0 equiv), and DCE
(2 mL) at 1408C for 48 h.
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Received: September 5, 2012
Revised: September 22, 2012
Published online: November 26, 2012
Keywords: conjugated systems · cross-coupling reaction ·
.
Scheme 5. Reactions of the RhIII complex Cp*ArRhCl 7 and 2-methyl-
thiophene 2a.
poly(hetero)aryls · rhodium · ruthenium
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Scheme 6. Tentative mechanism for the oxidative cross-coupling of
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tion with the N-heteroaryl substituent of the biaryl and
À
subsequent C H bond activation of the arene. Next, IM1
reacts with heteroarene 2 to form rhodacycle IM2 through
À
a regioselective electrophilic C H substitution (SEAr; pre-
ferred for electron-rich substrates)[10b,11a] or a concerted
À
metalation deprotonation (CMD; preferred for acidic C H
bond-containing substrates).[5,9a] Reductive elimination of
IM2 would then afford the desired product. Finally, the
RhIII is regenerated by the oxidation of CuII or AgI salts to
fulfill the catalytic cycle.
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In summary, we have presented a Rh or Ru-catalyzed
oxidative cross-coupling between various N-heteroarene-
À
containing arenes and heteroarenes by twofold C H activa-
tion, which provides a direct means of expanding p-conju-
gated systems. The coupling reactions are highly chemo- and
regioselective and are applicable to various heteroarenes
(thiophenes, furans, oxazoles, thiazoles, and xanthines).
Further studies to elucidate the mechanism and to find new
applications for these reactions are currently underway.
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[16] 2-o-Tolylquinoline and benzothiophene gave homocoupled
products in trace and less than 5% yield, respectively (deter-
mined by 1H NMR spectroscopy), in the absence of another
partner under the reaction conditions. For details, see the
Supporting Information.
[11] For the inevitability of homo-coupling in the Pd or Rh-catalyzed
À
À
oxidative C H/C H cross-coupling of two heteroarenes, see
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