Direct C-H bond arylation of 2-hydroxybenzaldehydes with arylboronic acids via ligand-free palladium catalysis

Article abstract of DOI:10.1016/j.tetlet.2010.02.166

A mild and efficient ligand-free palladium-catalyzed direct C-H bond arylation reaction was developed to afford 2-hydroxybenzophenones in good to excellent yields from easily available 2-hydroxybenzaldehydes and arylboronic acids. The given reaction provided one of the easiest pathways for accessing 2-hydroxybenzophenones, and a variety of functional groups could be tolerated in this process.

Full text of DOI:10.1016/j.tetlet.2010.02.166

Tetrahedron Letters 51 (2010) 2593–2596
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Tetrahedron Letters
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Direct C–H bond arylation of 2-hydroxybenzaldehydes with arylboronic
acids via ligand-free palladium catalysis
*
Fei Weng, Chengming Wang, Bin Xu
Department of Chemistry, Shanghai University, Shanghai 200444, China
a r t i c l e i n f o
a b s t r a c t
Article history:
A mild and efficient ligand-free palladium-catalyzed direct C–H bond arylation reaction was developed to
afford 2-hydroxybenzophenones in good to excellent yields from easily available 2-hydroxybenzaldehy-
des and arylboronic acids. The given reaction provided one of the easiest pathways for accessing 2-
hydroxybenzophenones, and a variety of functional groups could be tolerated in this process.
Ó 2010 Elsevier Ltd. All rights reserved.
Received 11 December 2009
Revised 12 February 2010
Accepted 26 February 2010
Available online 3 March 2010
Keywords:
Arylboronic acids
C–H Bond arylation
2-Hydroxybenzaldehydes
2-Hydroxybenzophenones
Palladium-catalyzed
Aryl ketone template is generally recognized as a privileged
structure in medicinal chemistry, as exemplified by the marketing
drugs such as Tricor, and important framework in organic transfor-
mations.1 The prevalence of ortho-hydroxyl-substituted diaryl ke-
OH
HO
OH
O
O
OH O
OH
O
HOOC
O
OH
O
tones in
a
large number of biologically active compounds,2
OH
CH2OH
Morintrifolins A
natural products,3 and cosmetics,4 has resulted in a continued de-
mand for the development of general and flexible synthetic meth-
ods of this structural moiety (Scheme 1). Therefore, many useful
methods for their preparation have been developed, including
transformation from ortho-functionalized diaryl ketones5 or chro-
mones,6 acylation of benzoquinone7 or related arenes,8 Fries-type
rearrangement,9 addition of lithium or magnesium reagents to car-
boxylic derivatives,10 and coupling reactions of nitriles with boro-
nic acids.11 Although these methods are effective for the synthesis
of 2-hydroxybenzophenones, some of the reactions suffer from
lacking atom economy,6 incompatible with substrate scopes,7 han-
dling hazardous substances8a and harsh reaction conditions,10, and
inconvenient operation.8a,10
H3C
OH
OH O
HO
Monodictyphenone
Tenellone A
Scheme 1. Bioactive products with 2-hydroxybenzophenone moiety.
ples have been documented in the literatures involving the me-
tal-catalyzed coupling reactions from 2-hydroxyarylaldehydes
with hypervalent iodonium salts,13a,b boronic acids,13c,e or aryl ha-
lides18 via the cleavage of the formyl C–H bond and gave 2-
hydroxybenzophenones in low to moderate yields. In a program
aiming at developing novel approaches for the efficient construc-
tion of heterocycles and the evaluation on their biological proper-
ties,19 we herein reported an efficient direct aldehyde C–H bond
arylation reaction via ligand-free palladium catalysis in good to
high yields.
Initial study was performed by examining salicylaldehyde and
phenylboronic acid in the presence of Pd(OAc)2, CuCl2, and Na2CO3
in DMF under oxygen atmosphere and the reaction afforded the
product 3a in 67% yield (Table 1, entry 1). Among various palla-
dium sources screened (entries 1–5), PdCl2(PhCN)2 was found to
give the best result (entry 5) and no product could be detected in
the absence of palladium catalyst (entry 6). When the reactions
were conducted with excess or without CuCl2, 3a was given in de-
creased yields (entries 7 and 8). A variety of organic and inorganic
Recently, direct C–H bond arylation of aldehydes and their
derivatives to give aryl ketones in a green way has attracted lot
of attentions.12 In this context, diaryl ketones have been achieved
from aldehydes and organoboronic reagents13 or aryl halides,14
as well as obtained by arylation of N-tert-butyl-hydrazones,15 N-
pyrazyl16 or pyridinyl17 aldimines followed by hydrolysis. How-
ever, the existence of an ortho-hydroxyl group in aryl ketones led
to main impediment in some transformations.13g Only few exam-
* Corresponding author. Tel./fax: +86 21 66132065.
E-mail address: xubin@shu.edu.cn (B. Xu).
0040-4039/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2010.02.166

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F. Weng et al. / Tetrahedron Letters 51 (2010) 2593–2596
Table 1
Table 2
Optimization of reaction conditionsa
Direct C–H arylation of salicylaldehyde with arylboronic acidsa
O
O
O
O
B(OH)2
[Pd], Base, Additive
60°C, 8 h, Oxygen
B(OH)2 PdCl2(PhCN)2, CuCl2
H
+
H
+
R
R
NaHCO3, DMF
Oxygen, 60 °C
OH
1
OH
3a-q
OH
1
OH
3a
2a
2a-q
Entry
Pd source
Additive
Base
Yieldb (%)
Entry Arylboronic acids
Products
O
Time
(h)
Yieldb
(%)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
Pd(OAc)2
PdCl2
PdCl2(CH3CN)2
PdCl2(PPh3)2
PdCl2(PhCN)2
/
CuCl2
CuCl2
CuCl2
CuCl2
CuCl2
CuCl2
CuCl2
/
CuCl2
CuCl2
CuCl2
CuCl2
CuCl2
CuCl2
CuCl2
CuCl2
CuCl2
CuCl2
CuCl2
CuCl2
CuCl2
CuBr2
Cu(OAc)2
Cu(OTFA)2
BQ
Na2CO3
Na2CO3
Na2CO3
Na2CO3
Na2CO3
Na2CO3
Na2CO3
Na2CO3
Et3N
LiOAc
NaOAc
KOAc
CsOAc
K3PO4
67
63
65
54
72
0
B(OH)2
R
2
OH
R
3
1
2
3
4
5
6
7
8
9
2a: R = H
3a: R = H
8
11
11
17
8
7
7
7
14
85c
86c
75
74c
96
77
80
PdCl2(PhCN)2
PdCl2(PhCN)2
PdCl2(PhCN)2
PdCl2(PhCN)2
PdCl2(PhCN)2
PdCl2(PhCN)2
PdCl2(PhCN)2
PdCl2(PhCN)2
PdCl2(PhCN)2
PdCl2(PhCN)2
PdCl2(PhCN)2
PdCl2(PhCN)2
PdCl2(PhCN)2
PdCl2(PhCN)2
PdCl2(PhCN)2
PdCl2(PhCN)2
PdCl2(PhCN)2
PdCl2(PhCN)2
PdCl2(PhCN)2
40c
60
0
2b: R = CH3
2c: R = OCH3
2d: R = OBn
2e: R = F
2f: R = Cl
2g: R = CF3
3b: R = CH3
3c: R = OCH3
3d: R = OBn
3e: R = F
3f: R = Cl
3g: R = CF3
46
33
17
43
46
56d
58
67
57
76
62d
85e
39
47
47
< 5
2h: R = COOC2H5
2i: R = OTs
3h: R = COOC2H5
3i: R = OTs
74
92
K2CO3
Cs2CO3
LiOH
O
R
R
B(OH)2
NaOH
OH
NaHCO3
NaHCO3
NaHCO3
NaHCO3
NaHCO3
NaHCO3
NaHCO3
10
11
12
13
14
2j: R = CH3
2k: R = OCH3
2l: R = F
2m: R = NO2
2n: R = CHO
3j: R = CH3
3k: R = OCH3
3l: R = F
3m: R = NO2
3n: R = CHO
8
10
7
7
8
70
66
86c
76c
68
a
The reaction was performed with salicylaldehyde (1.0 equiv), phenylboronic
O
CH3
B(OH)2
CH3
acid (1.2 equiv), palladium catalyst (5 mol %), additive (10 mol %), and base
(2.0 equiv) in DMF (1 mL) for 8 h under 1 atm of O2 on a 0.3 mmol scale. Cu(OT-
FA)2 = cupric trifluoroacetate, BQ = p-benzoquinone.
15
16
10
11
63
66
2o
2p
2q
OH
3o
3p
3q
b
Isolated yields.
Reaction was run under N2 with 2.0 equiv of CuCl2.
Reaction was run under air.
1.7 equiv of 2a was used.
c
O
CH3
B(OH)2
CH3
d
e
H3CO
OH
O
OCH3
bases were tested (entries 9–19), and 76% yield of 3a could be
afforded using NaHCO3 as the base (entry 19), while diminished
yield was obtained under aerobic condition (entry 20). The best
condition could be achieved when the amount of phenylboronic
acid was increased to 1.7 equiv (entry 21). Further screening con-
cerning oxidants which are frequently used in the palladium-cata-
lyzed reactions gave less effective results (entries 22–25).
B(OH)2
17
9
65c
OH
a
The reactions were performed with salicylaldehyde (1.0 equiv), arylboronic acid
(1.7 equiv), PdCl2(PhCN)2 (5 mol %), CuCl2 (10 mol %), NaHCO3 (2.0–3.0 equiv), and
DMF (2 mL) under 1 atm O2 on a 0.5 mmol scale.
With the optimized conditions in hand, we next investigated
the scope of the reaction using various arylboronic acids (Table
2). The reactions proceeded smoothly in the presence of a variety
of functional groups, including benzoxyl, formyl, nitro, trifluoro-
methyl, and ethoxycarbonyl groups (entries 2–16). The electron-
rich arylboronic acids which are usually active substrates in Suzuki
reaction showed no better results than those reagents with elec-
tron-withdrawing groups (entries 3 and 4, 10 and 11). Generally,
arylboronic acids with electron-deficient properties are prone to
undergo homocoupling and protodeboronation side reactions.
However in our case, the electron-deficient substrates could react
smoothly with salicylaldehyde to give corresponding products
3e–i (entries 5–9) and 3l–n (entries 12–14) in good to excellent
yields, respectively. It should be noted that, active formyl group at-
tached to the boronic acid remained intact under the reaction con-
dition (entry 14). Substrates with ortho-substitution gave
comparatively lower yields, which may be due to the steric hin-
drance (entries 15 and 16). This reaction was not limited to simple
aromatic boronic acids, the 2-naphthylboronic acid was also
proved to be a good partner in this procedure and gave 3q in
65% yield (entry 17).
b
Isolated yields.
c
2.0 equiv of NaHCO3 was used.
hydes with phenylboronic acid, and the results are summarized in
Table 3. The presence of an alkyl or phenyl substituent on the
hydroxybenzaldehyde appeared to be more beneficial for the reac-
tion (entries 1–3 and 8–9). No significant electronic effects were
observed concerning the substituents on hydroxybenzaldehyde
(entries 1, 4, and 7), and the substrates containing multiple-bond
functional groups, such as azoic and alkenyl moiety, could be toler-
ated in this procedure (entries 5 and 6). Substrate 1i with a mor-
pholino-methyl group on the ortho position of the hydroxyl
group could also proceed well (entry 9), although longer reaction
time was needed to complete the reaction. The naphthalene con-
taining aldehyde 1j could run smoothly in the reaction (entry 10)
and the substrate 1k with duel formyl groups gave corresponding
double arylated product 4k in 68% yield (entry 11). When 2-form-
ylphenyl acetate 1l was subjected to the reaction using K2CO3 as a
base, deacylated product 3a was produced instead in 55% yield (en-
try 12).
To further explore the generality of this novel procedure, we
examined the reaction using a wide range of 2-hydroxy-benzalde-
A tentative mechanism for the palladium-catalyzed direct C–H
bond arylation reaction is proposed in Scheme 2.13f,20 Complexa-

F. Weng et al. / Tetrahedron Letters 51 (2010) 2593–2596
2595
Table 3
Direct arylation of o-hydroxybenzaldehydes with phenylboronic acida
O
CHO
(HO)2B
PdCl2(PhCN)2, CuCl2
+
R
R
NaHCO3, DMF
Oxygen, 60°C
OR'
OH
1a-l
2a
4a-k
Entry
Hydroxyaldehydes
Products
Time (h)
Yieldb (%)
R1
CHO
OH
O
R1
R2
OH
R2
1
2
3
4
5
6
7
8
1a: R1 = NO2, R2 = H
1b: R1 = sec-Bu, R2 = H
1c: R1 = Ph, R2 = H
4a: R1 = NO2, R2 = H
4b: R1 = sec-Bu, R2 = H
4a: R1 = Ph, R2 = H
9
8
8
12
25
13
11
9
87c
90
80
79
74
54
78
92
1d: R1 = OCH3, R2 = H
4d: R1 = OCH3, R2 = H
1e: R1 = N2-p-C6H4CH3, R2 = H
1f: R1 = CH@CHPh, R2 = H
1g: R1 = H, R2 = OCH3
4e: R1 = N2-p-C6H4CH3, R2 = H
4f: R1 = CH@CHPh, R2 = H
4g: R1 = H, R2 = OCH3
1h: R1 = Ph, R2 = Ph
4h: R1 = Ph, R2 = Ph
N
N
82c
1i: R1 = tert-Bu, R2
=
4i: R1 = tert-Bu, R2
=
9
16
O
O
CHO
OH
OH O
10
11
82
1j
4j
OH
O
OH O
CH3
OHC
CHO
Ph
Ph
11
12
16
9
68d
55e
CH3
CHO
4k
3a
1k
1l
O
OAc
OH
a
The reactions were performed with hydroxybenzaldehyde (1.0 equiv), phenylboronic acid (1.7 equiv), PdCl2(PhCN)2 (5 mol %), CuCl2 (10 mol %), and NaHCO3 (3.0 equiv)
in DMF (2 mL) under 1 atm O2 on a 0.5 mmol scale.
b
Isolated yields.
c
Reaction was run at 80 °C with 2.0 equiv of phenylboronic acid.
With phenylboronic acid (4.0 equiv), PdCl2(PhCN)2 (10 mol %), CuCl2 (20 mol %) and NaHCO3 (5.0 equiv).
K2CO3 was used as a base.
d
e
O
In summary, we have developed a mild and efficient ligand-free
palladium-catalyzed arylation of 2-hydroxy-benzaldehydes with
organoboronic acids in good to high yields. The given reaction pro-
vided one of the easiest pathways for accessing 2-hydroxybenz-
ophenones. This new reaction involves formal aldehyde C–H
bond arylation and a variety of functional groups could be toler-
ated in this process. The scope, mechanism, and synthetic applica-
tions of this catalytic reaction are under investigation in our
laboratory.
OH
Ar
OH
H
O-PdLn-H
A
ArB(OH)2
oxidant
Pd(II)
+ base
O
O
Ar
OH
H
H
OPdLn
Ar
OH
O
O-PdLn-Ar
B
OH
Acknowledgments
C
We thank the National Natural Science Foundation of China
(No. 20972093), Shanghai Municipal Education Commission
(No. J50102 and 10YZ06) and Graduate Student Creative Founda-
tion of Shanghai University (No. SHUCX092033) for financial
support. The authors thank Professor Hongmei Deng for spectral
support.
Scheme 2. Plausible mechanism for the direct arylation of 2-hydroxy-benzalde-
hydes with arylboronic acids.
tion of the palladium catalyst with salicylaldehyde affords palla-
dium(II) hydride A. The formed palladium complex subsequently
reacts with arylboronic acid to give intermediate B via a transmet-
allation process. Addition of the carbon–palladium bond to the car-
bonyl group gives diarylmethoxypalladium intermediate C, which
then yields 2-hydroxybenzophenone and also regenerates the pal-
ladium catalyst A in the presence of salicylaldehyde and oxidant.
Currently, we cannot rule out the mechanistic possibility involving
the five-membered palladacycle intermediate which was formed
through chelation-assisted aldehyde C–H bond activation proce-
dure.21–23
Supplementary data
Supplementary data (data and copies of the 1H NMR, 19F NMR
and 13C NMR spectra for all compounds, and representative exper-
imental procedures) associated with this article can be found, in
the online version, at doi:10.1016/j.tetlet.2010.02.166.

2596
F. Weng et al. / Tetrahedron Letters 51 (2010) 2593–2596
8. For selected examples of acylation of arenes to give o-hydroxyl diaryl ketones,
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