Dirhodium(II)-Catalyzed Cross-Coupling Reactions of Aryl Aldehydes with Arylboronic Acids in Water

Article abstract of DOI:10.1002/ejoc.201301564

In this report, dirhodium(II) catalysts with axial phosphanes ligands were employed to catalyze cross-coupling reactions of aromatic aldehydes with arylboronic acids to generate ketones in neat water. The overall reaction is proposed to occur through a cascade process involving the dirhodium-catalyzed addition of boronic acids to aldehydes followed by the dehydrogenative oxidation of alcohols.

Full text of DOI:10.1002/ejoc.201301564

SHORT COMMUNICATION
DOI: 10.1002/ejoc.201301564
Dirhodium(II)-Catalyzed Cross-Coupling Reactions of Aryl Aldehydes with
Arylboronic Acids in Water
[
a]
[a]
Yi Kuang and Yuanhua Wang*
Keywords: Rhodium / Cross-coupling / Oxidation / Phosphane ligands / Axial ligands
In this report, dirhodium(II) catalysts with axial phosphanes
ligands were employed to catalyze cross-coupling reactions
of aromatic aldehydes with arylboronic acids to generate
ketones in neat water. The overall reaction is proposed to
occur through a cascade process involving the dirhodium-
catalyzed addition of boronic acids to aldehydes followed by
the dehydrogenative oxidation of alcohols.
Introduction
attracted considerable attention and has emerged as a
powerful method in organic synthesis. However, reports
[
5]
Rhodium-catalyzed addition reactions of arylboronic on the use of rhodium catalysts to generate diaryl ketones
[
6]
acids with carbonyl-containing compounds have been well are so far very limited. To the best of our knowledge,
[
1]
I [2]
documented. The rhodium metal oxidation states Rh ,
the pioneering example was the synthesis of diaryl ketones
Rh , and RhIII[4] are all able to catalyze 1,2-addition re- from potassium trifluoro(organo)borates or arylboronic
II [3]
actions of arylboronic acids to aldehydes by forming di- acids with aryl aldehydes catalyzed by rhodium(I) by
[
6a,6c,6d]
arylcarbinols as final products. Owing to the potential Genet and co-workers.
In our laboratory, we are
cooperativity between the two metal sites, binuclear dirho- interested in developing highly efficient catalysts by com-
dium(II) complexes are attractive systems of study for the bining dirhodium compounds with various axial li-
activation of organic substrates.[ Recently, Gois and co- gands
3e]
[3,7]
and exploring the possibility of using these com-
workers reported that dirhodium(II) complexes with plexes to catalyze chemical transformations. Herein, we
unique paddlewheel structures[ bind axially to N-hetero- describe the application of dirhodium(II)/phosphane com-
3d]
cyclic carbene (NHC) ligands and catalyze the arylation plexes in water to catalyze a cascade reaction of aryl-
reaction in high yields.[
3a–3c]
Direct cross-coupling of aryl- boronic acids with aldehydes to synthesize diaryl ketones
boronic acids and aldehydes to generate diaryl ketones has (Scheme 1).
Scheme 1. DME = dimethoxyethane.
[
a] College of Chemistry, Sichuan University,
Results and Discussion
Chengdu 610064, P. R. China
E-mail: yhwang@scu.edu.cn
Gois and co-workers reported that NHC ligands bind
http://chem.scu.edu.cn/test/search/Zhindex
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejoc.201301564.
axially to Rh (OAc) to catalyze addition reactions of or-
2
4
ganoboronic acids with aldehydes to yield diarylmeth-
Eur. J. Org. Chem. 2014, 1163–1166
© 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
1163

Y. Kuang, Y. Wang
SHORT COMMUNICATION
anols.[3a–3c] Both alkyl/arylphosphanes and NHC ligands the base, although the reaction was heterogeneous, we ob-
are generally considered poor π acceptors and strong δ tained 4-methoxybenzophenone (3a) in 16% yield
donors. However, the conical shape of phosphane ligands (Table 1, entry 3). Encouraged by these results, various
can be significantly different from the planar shape of phosphane ligands including selected Buchwald ligands
NHC ligands. To understand how the phosphane ligands (e.g., RuPhos, BreetPhos) were tested. Results showed that
would affect the overall reactivity of dirhodium(II) cata- P(nBu) was the best ligand, which led to 3a in 84% yield
3
lysts, the addition reaction of 4-methoxyaldehyde (1a) and after purification (Table 1, entries 4–7). The reaction was
phenylboronic acid (2a) catalyzed by dirhodium(II) acet- less efficient if the NHC IMes·HCl ligand was employed
ate with phosphane ligands was chosen as the initial model (Table 1, entry 8). Almost no ketone product was pro-
for investigation.
duced if organic solvents were added to the reaction sys-
We first chose PPh as the ligand and the diarylmeth- tem (Table 1, entries 9 and 10). This indicated that neat
3
anol was successfully formed in DME/water with KOtBu water was essential for the generation of ketone 3a if
at 90 °C under an atmosphere of N after 24 h. Changing K CO was used as the base. Further studies revealed that
2
2
3
the base to K CO prevented the reaction. Interestingly, if the dirhodium catalyst, the base, and the ligands were all
2
3
the reaction was conducted in neat water with K CO as needed for this reaction (Table 1, entries 12–14). The
2
3
Table 1. Optimization of the reaction conditions.[a]
Entry
Ligand
Solvent
Base
Yield [%][b]
[
c]
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
PPh
PPh
PPh
P(nBu)
P(OPh)
RuPhos
BrettPhos
IMes·HCl
P(nBu)
P(nBu)
P(nBu)
–
P(nBu)
P(nBu)
P(nBu)
P(nBu)
P(nBu)
P(nBu)
P(nBu)
P(nBu)
3
3
3
DME/H
DME/H
2
O
O
tBuOK
–
–
16
84
20
53
19
22
trace
trace
61
trace
–
Ͻ5
53
66
86
38
77
31
[
d]
2
K
K
K
K
K
K
K
K
K
K
K
K
–
2
2
2
2
2
2
2
2
2
2
2
2
CO
CO
CO
CO
CO
CO
CO
CO
CO
CO
CO
CO
3
3
3
3
3
3
3
3
3
3
3
3
H
H
H
H
H
H
2
2
2
2
2
2
O
O
O
O
O
O
3
3
[
d]
3
3
3
DME/H
toluene/H O
2
2
O
[
[
e]
0
1
2
3
4
5
6
7
8
9
0
f]
H
H
H
H
H
H
H
H
H
H
2
2
2
2
2
2
2
2
2
2
O
O
O
O
O
O
O
O
O
O
[
g]
3
3
3
3
3
3
3
3
NaHCO
PO ·7H
KOH
CO
tBuOK
CO
3
K
3
4
2
O
K
2
3
(3 equiv.)
[
h]
K
2
3
[
a] Unless otherwise noted, all reactions were performed by using aldehyde 1a (1.00 mmol), arylboronic acid 2a (2.00 mmol),
Rh (OAc) (3 mmol-%), ligand (6 mol-%), and base (1.00 mmol) in solvent (1 mL) at 90 °C for 24 h under an atmosphere of N . Cy
cyclohexyl. [b] Yield of isolated product. [c] DME/water = 5:1. The reaction gave the diarylmethanol in 88% yield. [d] DME/water
2
4
2
=
=
5:1. [e] Toluene/water = 5:1. [f] Ligand (3 mol-%). [g] No rhodium catalyst was used. [h] Rh
2
(OAc)
4
(1 mol-%) and ligand (2 mol-%).
1164
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Dirhodium(II)-Catalyzed Cross-Coupling Reactions
amount of axial ligand was essential to the yield of the with 2a did not afford any coupling product, which indi-
reaction. If a lower amount of P(nBu) was used, the ad- cated that this reaction was not compatible with hydroxy
3
dition reaction gave a lower yield (Table 1, entry 11). A substituents on the aromatic ring of benzaldehyde 1. We
series of bases was also examined: KOH and K CO gave also tested several arylboronic acids to react with 4-meth-
2
3
nearly identical results, and an excess amount of the base oxybenzaldehyde (1a; Table 2, entries 15–20). Electron-
3 equiv.) did not improve the yield of ketone 3a (Table 1, rich 4-methoxyphenylboronic acid (2b) generated the cou-
entries 15–19). Upon lowering the amount of Rh (OAc)4 pled product in good yields, whereas 4-Cl- and 4-CF -sub-
(
2
3
to 1 mol-% and the amount of P(nBu) to 2 mol-%, the stituted boronic acids 2e and 2f gave moderate yields of
3
addition reaction was sluggish and afforded ketone 3a in the ketone, and 4-CN-substituted boronic acid 2g did not
3
1% yield (Table 1, entry 20).
yield any of the desired coupling product. Similarly, bulk-
Under similar reaction conditions, benzaldehydes with ier arylboronic acids such as 2-methoxyphenylboronic acid
various electron-withdrawing and electron-donating sub- (2c) and 2-naphthylboronic acid (2d) were less reactive,
stituents (i.e., 1b–j) all reacted with 2a and 2b smoothly to and they delivered low yields of the ketone products.
afford the corresponding diaryl ketones (i.e., 3b–j, v–x) in
In investigating the reaction mechanism of these dirho-
good yields (Table 2, entries 1–9, 21–24). Sterically bulkier dium-catalyzed coupling reactions, we found that di-
aldehydes such as 1-naphthylbenzaldehyde (1k) and 2- phenylmethanols formed quickly at the early stage of the
methoxybenzaldehyde (1l) also reacted with 2a to afford reaction and were then oxidized to diaryl ketones slowly
diaryl ketones 3k and 3l, albeit in lower yields (Table 2, afterwards. This phenomena was described previously in
entries 10 and 11). Similarly, the heterocyclic aldehyde 1- Genet’s reaction catalyzed by RhI.[6a,6d] Addition reactions
formylfuran (1m) and the aliphatic aldehyde cyclohexane- of arylboronic acids with aldehydes to form diarylmeth-
carbaldehyde (1n) were less effective substrates; they re- anol catalyzed by rhodium(I) or dirhodium(II) have been
[
2,3a–3c,4]
acted with 2a by generating the corresponding aryl well documented.
Thus, we focused on the mecha-
ketones in 55 and 30% yield, respectively (Table 2, en- nism of diaryl ketone formation. Different from Genet’s
tries 12 and 13). Incubating 4-hydroxybenzaldehyde (1o) reaction, in which the cosolvent acetone as a hydride ac-
ceptor was crucial to generate the desired ketone prod-
[
6a,6d]
ucts,
only water was used as the solvent in our reac-
Table 2. Dirhodium-catalyzed formation of diaryl ketones from
aldehydes and arylboronic acids.[
a]
tion, and this prompted us to explore possible alternative
pathways and the involvement of water in these reactions.
After treating diphenylmethanol 4a under the reaction
conditions without adding an arylboronic acid, ketone 3a
was formed in 91% yield (Table 3, entry 4). In the absence
of a metal catalyst or ligand, oxidation to the ketone did
not proceed (Table 3, entries 1 and 2). The above results
indicated that diarylmethanol 4a was anaerobically oxid-
ized to diaryl ketone 3a under the reaction conditions. No-
tably, water is crucial to the oxidation (Table 3, entries 3
and 4) and hydrogen is produced in this oxidation process
(see the Supporting Information).
Entry R1
R2
Product Yield [%][b]
1
2
3
4
5
6
7
8
9
C
6
H
5
(1b)
4-MeC
3-OMeC
2-naphthyl (1e)
(3,4-OCH O)C
3,4,5-(OMe)
4-CF (1h)
4-ClC (1i)
4-FC (1j)
1-naphthyl (1k)
2-OMeC (1l)
H (2a)
H (2a)
H (2a)
3b
3c
3d
3e
3f
3g
3h
3i
70
54
54
76
62
72
80
87
64
37
39
55
30
–
81
37
38
55
40
–
6
H
4
(1c)
(1d)
6
H
4
H (2a)
2
6
H
3
(1f) H (2a)
(1g) H (2a)
H (2a)
3 6
C H
2
3
C
6
H
H
4
H
4
4
Table 3. Anaerobic oxidation of diarylmethanol to diaryl ketone
product.[
a]
6
H (2a)
H (2a)
H (2a)
H (2a)
H (2a)
H (2a)
H (2a)
4-OMe (2b)
6
3j
3k
3l
1
0
1
1
6
H
4
1
2
1-furyl (1m)
Cy (1n)
3m
3n
3o
3p
3q
3r
3s
3t
3u
3v
3s
3w
3x
1
3
1
4
4-OHC
4-OMeC
4-OMeC
4-OMeC
4-OMeC
4-OMeC
4-OMeC
1-naphthyl (1k)
4-ClC (1i)
3,4,5-(OMe)
4-FC (1j)
6
H
4
(1o)
1
5
6
H
4
(1a)
(1a)
(1a)
(1a)
(1a)
(1a)
Entry
Catalyst
–
Ligand
Solvent
Yield [%][b]
1
6
6 4
H
2-OMe (2c)
2-naphthyl (2d)
4-Cl(2e)
1
7
6 4
H
1
2
3
4
P(nBu)3
–
P(nBu)3
P(nBu)3
H O
trace
trace
Ͻ5
2
1
8
6 4
H
Rh (OAc)
H O
2
2
2
4
4
4
2
1
9
6 4
H
4-CF
3
(2f)
Rh (OAc)
DME
H O
2
2
0
6 4
H
4-CN (2g)
4-OMe (2b)
4-OMe (2b)
Rh (OAc)
91
2
1
62
51
68
45
[
a] Unless otherwise noted, all reactions were performed by using
alcohol 4a (1.00 mmol), Rh (OAc) (3 mol-%), PR (6 mol-%), and
CO (1.00 mmol) at 90 °C for 24 h under an atmosphere of N
b] Yield of isolated product.
2
2
6
H
4
2
4
3
2
3
3 6
C H
2
(1g) 4-OMe (2b)
4-OMe (2b)
K
[
2
3
2
.
24
6 4
H
[a] Unless otherwise noted, all reactions were performed by using
aldehyde
Rh (OAc)
in H
b] Yield of isolated product.
1
(1.00 mmol), arylboronic acid
2
(2.00 mmol),
(1.00 mmol)
On the basis of the above results and literature re-
2
4
(3 mol-%), P(nBu) (6 mol-%), and K
3
2
CO
3
[
1,6]
ports,
we propose that the reaction occurs through a
2
2
O (1 mL) at 90 °C for 24 h under an atmosphere of N .
[
cascade process involving dirhodium-catalyzed addition of
Eur. J. Org. Chem. 2014, 1163–1166
© 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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1165

Y. Kuang, Y. Wang
SHORT COMMUNICATION
the boronic acid to the aldehyde[
3a–3c]
followed by dehy- 21272162). Prof. Dr. Michael P. Doyle is thanked for helpful dis-
drogenative oxidation of an alcohol.[ For the formation
8]
cussions.
of the diarylmethanol coupling product, according to the
mechanism proposed by Gois and co-workers,[
3b]
[1] For recent reviews on Rh-catalyzed addition reactions of aryl-
boronic acids with carbonyl-containing compounds, see: a) F.
Glorius, Angew. Chem. 2004, 116, 3444–3446; Angew. Chem.
Int. Ed. 2004, 43, 3364–3366; b) T. Hayashi, K. Yamasaki,
Chem. Rev. 2003, 103, 2829–2844; c) K. Fagnou, M. Lautens,
the di-
rhodium catalyst activates the boronic acid with the aid of
the phosphane ligand and the base; then, direct transfer
of the phenyl group from the boron to the aldehyde occurs.
Subsequently, the formed diarylmethanol reacts with the
dirhodium catalyst to give a rhodium alkoxide. After β-
hydride elimination of the rhodium alkoxide, the aryl
ketone is produced as the dehydrogenated product. Re-
garding the dehydrogenation and hydrogen-release mecha-
nism, Saito and co-workers previously reported the selec-
tive dehydrogenation of 2-propanol catalyzed by
Chem. Rev. 2003, 103, 169–196, and references cited therein.
I
[
2] For selected examples of Rh -catalyzed 1,2-additions of aryl-
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1
481; b) R. B. C. Jagt, P. Y. Toullec, J. G. de Vries, B. L. Fer-
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[
9]
Rh (OAc) by adding PPh in situ. The unique dirho-
2
4
3
[
3d]
dium complex structure
and the tuning effect of the ax-
may account for this transformation. De-
[
7,10]
ial ligands
tailed mechanistic studies, especially the water effect, are
currently underway in our laboratory.
1
686.
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3] a) P. M. P. Gois, A. F. Trindade, L. F. Veiros, V. Andre, M. T.
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demonstrated that dirhodium complexes with axial phos-
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also found that these coupling reactions are highly de-
pendent on the solvent, and neat water proved to be essen-
tial in these reactions if K CO was used as the base. Fu-
[
[
4
484–4486; b) A. Fürstner, H. Krause, Adv. Synth. Catal.
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3
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Experimental Section
4
2
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General Procedures for Products: Rh
), aldehyde (0.3 mmol, 1.0 equiv.), arylboronic (0.6 mmol,
.0 equiv.), and K CO (0.6 mmol, 1.0 equiv.) were added to a
2 4
(OAc) (0.009 mmol, 3 mol-
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%
2
2
3
tube. The septum-sealed tube was evacuated and refilled with ni-
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2
S. Darses, J.-P. Genet, J. Am. Chem. Soc. 2004, 126, 15356–
perature, it was extracted with ethyl acetate (3ϫ 5 mL) and
washed with water (2ϫ 10 mL). The ethyl acetate layer was sepa-
rated and dried with Na SO . After evaporation of the solvent,
2 4
the residue was purified by flash column chromatography (ethyl
acetate/hexane) to give the desired diaryl ketone.
1
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Received: October 16, 2013
The authors are grateful for financial support from the National
Natural Science Foundation of China (NSFC) (grant number
Published Online: January 15, 2014
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Eur. J. Org. Chem. 2014, 1163–1166