Direct exchange of a ketone methyl or aryl group to another aryl group through ciC bond activation assisted by rhodium chelation

Article abstract of DOI:10.1002/anie.201206693

Swapped: Commercially available quinolinone derivatives (1 or 2, see scheme) were reacted with arylboronic acids in the presence of a Rh^I complex to give aryl(quinolin-8-yl)methanone products 3 in medium to good yields. A mechanism that involves the in situ oxidation of Rh^I to Rh^III by O₂ in the presence of CuI was proposed. Copyright

Full text of DOI:10.1002/anie.201206693

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Angewandte
Communications
DOI: 10.1002/anie.201206693
À
C C Activation
Direct Exchange of a Ketone Methyl or Aryl Group to Another Aryl
À
Group through C C Bond Activation Assisted by Rhodium
Chelation**
Jingjing Wang, Weiqiang Chen, Sujing Zuo, Lu Liu, Xinrui Zhang, and Jianhui Wang*
À
À
The development of transition-metal-catalyzed C C bond
In contrast, transition-metal-catalyzed C C bond forma-
cleavage has attracted much attention in recent years,[1]
because these reactions help chemists to form complex
molecules through nontraditional retrosynthetic disconnec-
tion through a coupling reaction of aryl halides and aryl
boronic acids, called the Suzuki–Miyaura reaction,[8] is an
important synthetic tool for organic chemistry. Recently,
many research groups have developed direct coupling reac-
tions.[2] Traditionally, C C bond cleavage can be achieved by
À
oxidative cyclometalation of three- or four-membered-ring
compounds to form more stable metallacyclic complexes;
these reactions are accompanied by the release of ring
strain.[3] For unstrained molecules, special driving forces are
required to overcome the high kinetic barriers associated with
the activation step of generating stable intermediates, such as
the stability of the organometallic intermediates and the
tions through transition-metal-catalyzed activation of aro-
matic C H bonds directed by a functional group. For
example, the reaction of a chelating aldehyde with iodoarenes
or organostannanes in the presence of Pd and Ru as
a cooperative catalyst gave the direct coupling products in
[9]
À
À
high yields by means of C H activation (Scheme 1,
À
ketones that are produced during the catalytic selective C C
bond cleavage of tertiary alcohols.[4] Directing a metal com-
À
plex to a particular C C bond using functional groups is
another strategy that is commonly used for activation of
À
unstrained C C bond. This concept was first demonstrated in
the early 1980s when a rhodium complex was used to catalyze
À
the C C bond cleavage of quinolinone derivatives at the
8 position;[5] a five-membered ring (as favored in cyclometa-
lation) was formed, and metal coordination to the nitrogen
Scheme 1. Coupling reactions through C H or C C activation.
À
À
À
atoms in the ligands was directed to the a-ketone C C bond.
Since then, the idea of using assisted chelation for C C
path A).[10] Similar to the recent emergence of C H activation
À
À
activation has been extended to other systems.[6d–g] Many
important reaction models based on chelation-assisted C C
in synthetic chemistry, some articles have also reported C C
À
À
À
bond formation with arylboronic acid after cleavage of a C
activation have been developed,[2,6a–c,7] such as an intra-
molecular carboacylation reaction with acylquinolines,[2]
a catalytic decarbonylation reaction,[7a] and a pyridinyl-
directed alkenylation through bond cleavage of secondary
arylmethanols.[7c] The discovery of these reactions, which are
CN or a C COOR bond of a substrate.
However, for
[11]
À
À
reactions with olefins, there are only a few examples of C C
À
bond formations in ketones after chelation-assisted C C
activation.[2,6] Herein, we report C C bond formation in
À
À
ketones through a chelation-assisted C C activation. The
À
based on C C activation, has greatly enriched the methods
available for the construction of complex molecules. How-
ever, compared with the diversity of synthetic methods that
have been developed for C H activation, reactions using C C
activation are relatively few. Avariety of efficient methods for
ketone was reacted with arylboronic acid, which is the
commonly used partner for coupling reactions (Scheme 1,
path B). In a ketone that has an N-directing group, this
reaction results in the direct exchange of a methyl or an aryl
group to another aryl group.
À
À
À
the activation of C C bonds is highly desirable in order to use
To achieve this reaction, 1-(quinolin-8-yl)ethanone (1a)
and phenylboronic acid (2a) were initially used as cross-
coupling partners to optimize the reaction conditions (see the
Supporting Information for details). The results show that the
reaction gave the desired product 3a in 93% yield when
[Rh(PPh3)3Cl] (known as Wilkinsonꢀs catalyst) was used as
the catalyst in combination with CuI (1 equiv) and K2CO3
(2 equiv) at 1308C for 18 h. When Ru complexes were used as
the catalyst, the desired product was obtained in less than 5%
yield. These results clearly show the special reactivity of the
À
unstrained C C s bonds as versatile synthons for the
formation of complex molecules, and there has been an
intense focus in this area.
[*] J.-J. Wang, W. Chen, S. Zuo, L. Liu, X. Zhang, Prof. Dr. J.-H. Wang
Department of Chemistry, College of Science Tianjin University
Tianjin 300072 (China)
E-mail: wjh@tju.edu.cn
[**] This work was supported by grants from the NSFC (21072149,
20872108).
À
Rh center for C C activation.
Next, the reactions of 1a with various arylboronic acids
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201206693.
containing electron-donating or electron-withdrawing groups
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Table 1: Catalytic coupling of acyl ketones with various arylboronic acids.[a]
[a] Reaction conditions: 1-(quinolin-8-yl)ethanone (0.10 mmol), substituted phenylboronic acid (0.25 mmol), CuI (0.20 mmol), K2CO3 (0.20 mmol),
[Rh(PPh3)3Cl] (0.01 mmol, 10.0 mol%), xylene (0.5 mL), 1308C, under air, 48 h; yields of isolated products are given based on 1a. [b] Sodium
tetraphenylborate was used as the substrate. [c] 18 h. [d] 36 h. [e] 24 h.
were explored using the optimized reaction conditions
(Table 1). When 1a was reacted with sodium tetraphenylbo-
rate, 3a was obtained in 59% yield, however, the reaction of
1a with phenylboronic acid gave 3a in 93% yield. Longer
reaction times (36 or 48 h) were required for arylboronic acids
with electron-donating groups, but the reaction with 1a gave
the corresponding products 3b–3h in good yields (74–90%).
The reaction of 1a with 3,4,5-trimethoxyphenylboronic acid
gave the desired product 3i in moderate yield (51%), because
this boronic acid with its three strongly electron-donating
groups is prone to be oxidized under the reaction conditions.
b-Naphthaleneboronic acid also exhibited excellent reactivity
under similar reaction conditions and gave the product 3j in
94% yield. It should be noted that the arylboronic acids with
halide substituents were stable in these coupling reactions.
For example, arylboronic acids with a Cl, F, or Br substituent
at the 4 position reacted with 1a to give 3k, 3l, and 3m in
83%, 57%, and 47% yield, respectively. Arylboronic acids
with a strong electron-withdrawing group at the 4 position,
such as COOEt or NO2, gave products 3n and 3o in 57% and
35% yield, respectively, thus indicating that the electronic
effect plays an important role in this reaction. Thiophen-2-
ylboronic acid with its electron-rich heteroaromatic ring was
also tested for the coupling reaction. The desired product 3p
was not isolated, which may be due to the strong complexion
of the sulfur atom with the transition metal and its competi-
tion with the directing group of the substrate. Furthermore,
the arylboronic acid with a methyl group at the ortho position
did not give product 3q, thus indicating that these reactions
are very sensitive to the steric hindrance of the boronic acid
substrates. Substituents on the quinoline ring also affected the
yield of the catalytic reaction. For example, the reaction of 1-
(6-methylquinolin-8-yl)ethanone with phenylboronic acid
gave product 3r in 69% yield, which is significantly lower
than the yield of the reaction with 1-(quinolin-8-yl)ethanone
under similar conditions. When 1-(6-methoxyquinolin-8-yl)-
ethanone or 1-(5,6-dimethoxyquinolin-8-yl)ethanone were
used, the quinoline rings of which bear one or two methoxy
substituents, the substrates reacted with phenylboronic acid to
give the desired products 3s and 3t in 65% and 75% yield,
respectively. The reaction of 1-(5-chloroquinolin-8-yl)etha-
none, which bears a Cl substituent at the 5 position of the
quinoline ring, gave product 3u in 81% yield. In addition, 1-
(benzo[f]quinolin-5-yl)ethanone reacted with phenylboronic
acid to give the desired product 3v in 57% yield, and 1-
(quinoxalin-5-yl)ethanone, which (as quinolines) also has an
N atom at the appropriate position as a directing group,
reacted with phenylboronic acid to give the desired product
3w in 89% yield. These results clearly show that the direct
exchange of a methyl to an aryl group can be achieved in
ketones with an N-directing group, and that this reaction can
occur with many different arylboronic acids under the
optimized reaction conditions.
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Further studies showed that phenyl(quinolin-8-yl)metha-
groups with phenyl ketone 3a afforded the corresponding
products 3d–3g in 45–71% yield (Table 2, entries 3–6).
Furthermore, naphthalen-2-yl boronic acid reacted smoothly
with 3a to give the desired product 3j in 55% yield (Table 2,
entry 7). However, no product was isolated when naphthalen-
1-ylboronic acid was reacted with 3a, again showing that
steric hindrance has a significant impact on the reaction
(Table 2, entry 8). Unfortunately, in sharp contrast to the
electron-donating groups, an electron-withdrawing group on
the aryl boronic acids inhibited the reaction of these
2
À
none (3a), which contains a C(sp ) C bond, underwent
similar conversions when it was reacted with substituted
arylboronic acids under conditions similar to the above-
mentioned studies. Rh-catalyzed coupling reactions of phe-
nyl(quinolin-8-yl)methanone (3a) with an arylboronic acid
that bears a methyl substituent at the meta or para position
proceeded smoothly to give 3b and 3c in 52% and 50% yield,
respectively (Table 2, entries 1 and 2). The reaction of
arylboronic acids that bear different electron-donating
2
2
[a]
À
Table 2: The C(sp ) C(sp ) bond coupling reaction of substituted ketones with various arylboronic acids.
[a] Reactions conditions: (quinolin-8-yl)methanone (0.05 mmol), substituted phenylboronic acid (0.125 mmol, 2.5 equiv), CuI (0.10 mmol), K2CO3
(0.10 mmol), [Rh(PPh3)3Cl] (0.005 mmol, 10.0 mol%), xylene (0.5 mL), 1308C, under air, 48 h. [b] Yields of isolated products are given. Biphenyl
products were also observed, their yields are given in Table S3 in the Supporting Information.
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substrates with 3a. For example, an F-substituted aryl boronic
acid displayed no reactivity under similar conditions (Table 2,
entry 9) and again, an aryl boronic acid with an NO2
substituent did not react with 3a to give the desired product
3n, thus indicating that the electronic effect has a strong
impact on the outcome of the reaction (Table 2, entry 10).
A comparison of the reactivity of a specific arylborane
with ketones 1 and 3 showed that the substituent of the ketone
may also have a significant effect on the reaction. Thus,
a series of substituted aryl ketones 3 f–3o were tested for the
À
catalytic C C bond grafting reaction with different arylbor-
onic acids (Table 2, entries 11–21). Ketone 3 f, which bears an
electron-donating methoxy group on the phenyl ring, was
found to undergo a coupling reaction with 3,5-dimethylphe-
nylboronic acid to form the desired product 3e in 54% yield
(Table 2, entry 11). It is worth noting that the yield of product
3c increased to 95% when naphthalen-2-yl(quinolin-8-yl)me-
thanone (3j) was reacted with 3-methylphenylboronic acid
under standard reaction conditions (Table 2, entry 12). To
gain further insight into the reaction, the reactions of some
ketones with electron-withdrawing groups on the aromatic
ring were also investigated. For instance, 4-chlorophenyl(qui-
nolin-8-yl)methanone (3k) reacted with arylboronic acids
that bear electron-donating groups to give the desired
products in 66–72% yield (Table 2, entries 13--15). As in
other Rh-catalyzed reactions,[7a,12] a Cl substituent on the
phenyl rings also worked in this catalytic system, which might
allow further modification of the product. The reaction of 4-
fluorophenyl(quinolin-8-yl)methanone (3l) with boronic
acids that bear electron-donating groups proceeded smoothly
to give the corresponding products in 61–70% yield (Table 2,
entries 16–18). However, when 4-nitrophenylboronic acid,
a substrate that bears a strong electron-withdrawing group,
was used to react with the ketone 3l, no product was obtained
(Table 2, entry 19). In contrast, the reaction of 4-nitrophe-
Figure 1. Proposed mechanism for the reaction of ketones with aryl-
boronic acids catalyzed by [Rh(PPh3)3Cl].
a prolonged reaction time (48 h). On the other hand, the
reaction proceeded very fast under an atmosphere of pure O2
(1 atm); ketone 1a was consumed completely in less than 12 h
and the reaction gave 3a in 72% yield along with some
unidentified by-products. This evidence clearly points to the
important role of O2 in the catalytic cycle, and thus proves
that the in situ oxidation of RhI to RhIII by O2 in the presence
of CuI is the turnover-limiting step. Furthermore, the crude
product mixture of 3b and phenylboronic acid obtained under
1
air was analyzed by H NMR spectroscopy (Scheme 2). The
integration results indicated that the crude product mixture
contained 3a and 4-methyl-1,1’-biphenyl (3ba) in a molar
ratio of 1:0.9 (for the molar ratio of biphenyl and the
substituted quinolinone derivative from other reaction sys-
tems, see Table S3 in the Supporting Information). This result
proves that the reductive elimination of B to C, which is the
key step, occurs (Figure 1). Although the key intermediates
were not isolated, the above evidence confirm the proposed
mechanism.
nyl(quinolin-8-yl)methanone (3o),
a ketone that bears
a strong electron-withdrawing group, with phenylboronic
acid or 4-methoxyphenylboronic acid gave the desired
products 3a and 3 f in 93% and 91% yield, respectively
(Table 2, entries 20 and 21).
Based on these findings and previous reports, the follow-
ing reaction mechanism is proposed (Figure 1). First, a RhI
À
complex activates an acetyl C C bond of the chelating ketone
It should be noted that 8-benzoylquinolines have been
frequently used as a core structure for the design of modern
pharmaceuticals and related compounds,[15] such as tubulin
polymerization inhibitors,[15a,b] cannabinoid (CB1) receptor
ligands,[15c] drugs for the treatment of bone metabolic
disorders,[15d] and antiulcer agents.[15e] The current method
1a to afford a five-membered cycloacyl RhIII intermediate
A.[1,5,13] Transmetalation of A with arylboronic acid then gives
intermediate B, in which the Rh center bears a methyl and an
aryl group. Subsequent elimination of the methyl and phenyl
groups on B lead to RhI intermediate C after a phosphine
ligand insertion. The RhI complex C is then oxidized to RhIII
complex D by CuI in the presence of O2.[14] Another
transmetalation of RhIII complex D with arylboronic acid
gives aryl RhIII complex E. Finally, reductive elimination of E
gave product 3a and the RhI complex after the coordination
of two phosphine ligands.
In this proposed mechanism, O2 is the terminal oxidant.
To prove this hypothesis, a catalytic reaction was conducted in
an N2 atmosphere using phenylboronic acid and 1a as the
reagents. In contrast to the 93% yield obtained in air, this
reaction did not give 3a in isolatable quantities, even after
Scheme 2. Molar ratio of the product mixture.
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offers a possible synthetic pathway to efficiently form similar
compounds from readily available materials.
In conclusion, a new approach for ketone synthesis by
a rhodium-catalyzed direct exchange of a ketone methyl or
aryl group to another aryl group is described. A variety of
different substituted quinolinone derivatives were obtained in
moderate to good yields using commercially available quino-
linone derivatives. Further efforts to expand the scope of this
reaction are currently underway in our laboratories.
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Received: August 18, 2012
Published online: November 4, 2012
À
Keywords: C C activation · chelation · cross-coupling ·
quinolinones · rhodium
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