An efficient bimetallic rhodium catalyst for the direct arylation of unactivated arenes

Article abstract of DOI:10.1002/anie.200604988

(Chemical Equation Presented) Together we stand: A novel N,P-ligand-stabilized bimetallic rhodium complex 1 accomplishes an efficient nondirected arylation of unactivated arenes. Aryl halides with several functional groups (FG) undergo these direct coupling reactions. The key to the catalytic efficiency is the bimetallic nature of the catalyst. copy; 2007 Wiley-VCH Verlag GmbH & Co. KGaA.

Full text of DOI:10.1002/anie.200604988

Angewandte
Chemie
DOI: 10.1002/anie.200604988
Cross-Coupling Reactions
An Efficient Bimetallic Rhodium Catalyst for the Direct Arylation of
Unactivated Arenes**
Sebastian Proch and Rhett Kempe*
The synthesis of polynuclear coordination compounds and
especially of systems with different metal atoms or the same
metal atoms in different chemical environments is intensively
investigated.[1] Motivation for this interest results from
possible applications in homogeneous catalysis because
many highly efficient biocatalysts[2] and heterogeneous cata-
lysts[3] are multimetal systems.[4] Despite considerable prog-
ress concerning the coordination chemistry of polynuclear
compounds there are not many molecular bimetallic catalysts
which show a different substrate activation to the single
(separated) metal components.[5] Biaryls are an important
Scheme 1. Synthesis of 2. After washing with hexane, the product was
isolatedin 98% yield.
class of compounds that are accessible through a large variety
of metal-catalyzed cross-coupling reactions. Usually arylated
organoelement reagents (boron (Suzuki),[6] magnesium
(Kumada),[7] silicon (Hiyama),[8] tin (Stille),[9] or zinc
(Negishi)[10]) and aryl halides are coupled, in palladium- or
nickel-catalyzed reactions. An attractive alternative is a direct
[11,12]
À
arylation which involves a C H bond functionalization.
A prerequisite for an efficient reaction is a directing group.
The arylation of unactivated arenes, for example benzene,
which do not have a directing group, remains challenging.[13,14]
Fujita, Yamaguchi, and Nonogawa, for instance, were able to
couple aryl iodides in reactions with relatively high catalyst
loadings (5 to 10 mol%) and turnover numbers (TONs) up to
15[13] and Fagnou and Lafrance describe a palladium-cata-
lyzed arylation of benzene with aryl bromides in the presence
of pivalic acid.[14] We introduced a P,N-ligand system[15] and
report herein on a novel bimetallic rhodium complex which is
stabilized by such ligands and efficiently catalyzes the non-
directed direct arylation.[13,14] In addition to aryl iodides, aryl
bromides and chlorides can be cross-coupled. The bimetallic
assembly is necessary for catalysis because the two separated
single components are catalytically inactive.
Figure 1. Molecular structure of 2 (H atoms omitted). Selected bond
lengths [Š] andangles [ 8]: P1–Rh1 2.1761(15), P2–Rh1 2.1729(14), N4–
Rh1 2.130(4), N2–Rh1 2.143(4), Cl3–Rh4 2.3758(15), Cl4–Rh4
2.3679(12); N4-Rh1-N2 101.12(16), N4-Rh1-P2 81.76(12), P2-Rh1-P1
97.65(6), N2-Rh1-P1 81.67(12).
bimetallic complex—a saltlike compound—which consists of
a [Rh(cod)Cl2]À ion[17] and a P,N-ligand-stabilized cation
[(PN)2Rh]+. One of the two pyridyl moieties of the P,N-
ligands does not coordinate to the metal center and in
solution an exchange of the coordinated and the noncoordi-
nated pyridyl moiety is detected, this exchange requires a
The reaction of [bis(2-pyridyl)amino]diphenylphosphane
(1, PN) with half an equivalent of [{Rh(cod)Cl}2] (cod = 1,5-
cyclooctadiene) leads to 2 (Scheme 1). The NMR spectro-
scopic data of 2 are in accordance with the results of an X-ray
crystal-structure analysis (Figure 1).[16] Compound 2 is a
À
rotation around the P N bond. About 24 rotations per second
take place at room temperature (NMR spectroscopic experi-
ments).
[*] Dipl.-Chem. S. Proch, Prof. Dr. R. Kempe
Lehrstuhl Anorganische Chemie II
Universität Bayreuth
For the nondirected direct arylation of benzene with aryl
iodides compound 2 is a more efficient catalyst than the
catalysts described to date. Benzene is arylated by iodoben-
zene with a TON of 780 in a shorter period of time and under
milder conditions than the current best catalyst
([{Cp*Ir(H)Cl}2], TON = 14; Cp* = C5Me5; Table 1).[13] An
increased efficiency is observed for other substrates as well.
The reaction of [Rh(cod)2][B(ArF)4] with two equivalents of 1
leads to [(PN)2Rh][B(ArF)4] (3; ArF = C6H3(CF3)2). Replac-
95440 Bayreuth (Germany)
Fax: (+49)921-552-157
E-mail: Kempe@Uni-Bayreuth.de
[**] We thank the BMBF, Degussa AG, and the Fonds der Chemischen
Industrie for financial support and Heidi Maisel for her support with
the laboratory work. This work was supportedby NanoCat, an
International Graduate Program within the Elitenetzwerk Bayern.
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
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Table 1: Turnover numbers (TONs) for the nondirected direct arylation
of benzene by aryl iodides.
Group 9 metal complexes are rare. For comparison, a
drastically higher activation enthalpy (Ea = 64 kJmolÀ1) was
observed for the activation of CH3Cl at [Ir(CO)2Cl2]À ions.[20]
In agreement with the relatively efficient stoichiometric
activation of chlorobenzene, 2 is able to couple aryl bromides
and chlorides catalytically (Table 2). The intermolecular
Product
2[a,b]
3[a,b]
4[b]
[{Cp*Ir(H)Cl}2][c,d]
780
45
0
0
14
0
0
0
0
0
0
0
0
13
3
Table 2: Nondirected direct arylation of benzene by aryl bromides and
chlorides.[a]
30
40
32
Aryl halide
Product
Yield [%]
X=Br[b]
X=Cl[c]
11
4
65
46
96
87
73
61
13
0
0
2
[a] Generated in situ from the corresponding rhodium–COD complexes
and 1.[19] [b] 708C, 24 h. [c] Cp*=pentamethylcyclopentadienyl. [d] 808C,
30 h; values from ref. [13].
83
83
70
59
ing the anion in 2 (use of 3) or the cation (use of [NEt4]
[Rh(cod)Cl2] (4))[18] by a rhodium-free counterion results in
no catalytic activity (Table 1). Thus, the bimetallic nature of 2
plays an important role in terms of the observed catalytic
efficiency.
[a] 708C, 24 h. Compound 2 was generatedin situ from [{Rh(cod)Cl}
and 1.[19] [b] 5 mol% 2. [c] 10 mol% 2.
]
2
Furthermore, we were interested in the question: can aryl
bromides and chlorides be activated by 2 as well and do they
undergo catalytic direct arylation as observed for iodides?
Compound 2 activates chlorobenzene in a (pseudo) first-
order reaction (Figure 2). The reaction order in chloroben-
zene, determined by the isolation method, is 0.99 Æ 0.01. The
temperature dependence of the rate constants of the activa-
tion reaction gave rise to the following activation parameters:
directed direct arylation using aryl chlorides has hardly
been described. An efficient ruthenium-based catalyst
system was recently introduced by Ackermann and co-
workers.[12t,u] The direct arylation of aryl bromides and
chlorides catalyzed by 2 tolerates functional groups, such as
nitro groups (Table 2), whereas the catalyst system based on
[{Cp*Ir(H)Cl}2] does not. For example, no conversion was
observed for the coupling of 4-iodonitrobenzene with ben-
zene.[13] From the selectivity data for the arylation of toluene
with [{Cp*Ir(H)Cl}2] it was concluded that the reaction
proceeded via radical intermediates.[13] The same conclusions
can be dawn from the corresponding data for the nondirected
arylation using our catalyst system. Arylation of toluene
(iodobenzene) catalyzed by 2 leads to 71% ortho-, 19%
meta-, and 10% para-phenyltoluene. The results of a Ham-
mett correlation (Figure 3) are also in accordance with radical
pathways. For the catalyst system 2, a slope of 1 = 1.33 Æ 0.02
(Figure 3) is found indicating the participation of radical
intermediates.[22]
DH° = (41.5 Æ 0.7) kJmolÀ1,
DS° = (À171 Æ 2) JKÀ molÀ.
À
Activation parameters for the activation of C Cl bonds by
For future work we are interested in the development of
the catalyst system, for instance, by P,N-ligand variation, in
addressing the selectivity issue, and in sequential activation
À
and functionalization of unactivated aromatic C H bonds. As
the investigated substrate range indicates, a directing group
Figure 2. (Pseudo-)first-order kinetics for the reaction of 2 andchlor-
obenzene (excess) at different temperatures ([2]t: concentration of 2 at
time t).
À
can easily be introduced by nondirected C H activation and
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À
by a further selective, directed, C H activation reaction.
Received: December 9, 2006
Published online: March 20, 2007
Keywords: activation parameters · cross-coupling · kinetics ·
.
N,P-ligands · rhodium
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Angew. Chem. Int. Ed. 2007, 46, 3135 –3138