Palladium-catalyzed aerobic oxidative carbonylation of arylboronate esters under mild conditions

Article abstract of DOI:10.1002/anie.201000460

(Figure Presented) "CO"n Air: The title reaction was carried out using [PdCl₂(PPh₃)₂ ] as the catalyst precursor under very mild conditions (balloon pressure of CO and air, at 40- 50°C), and produced a wide range of aryl carboxyl esters 2 in good to excellent yields. Remarkable selectivity between oxidative carbonylation and homocoupling of arylboronate esters l was also achieved.

Full text of DOI:10.1002/anie.201000460

Angewandte
Chemie
DOI: 10.1002/anie.201000460
Oxidative Carbonylation
Palladium-Catalyzed Aerobic Oxidative Carbonylation of
Arylboronate Esters under Mild Conditions**
Qiang Liu, Gang Li, Jun He, Jing Liu, Peng Li, and Aiwen Lei*
The transition-metal-catalyzed carbonylation involving CO
gas is a fundamental chemical transformation, which not only
extends the carbon chain length, but also introduces a
synthetically versatile carbonyl group. Theoretically, as
shown in Scheme 1, the Ar group could originate from an
transition metals, whereas the current transformation usually
requires the use of directing groups.[7–10] Furthermore, the
oxidative carbonylation of arylindium derivatives requires the
use of stoichiometric amounts of the oxidant desyl chloride.[11]
Herein, we report the first example of the oxidative carbon-
ylation of arylboronic acid derivatives under balloon pressure
of CO with air as the oxidant at 40!508C.
Arylboronic acid derivatives are air and moisture stable,
and are compatible with a broad range of common functional
groups.[12–14] For these reasons, they have been widely applied
in chemical syntheses.[15–17] Recently, the research groups of
Hou[18] and Iwasawa[19] have reported a nucleophilic addition
of arylboronate esters toward CO2 in the presence of copper
or rhodium catalysts, respectively. In the presence of CO, the
Suzuki carbonylation reaction was also realized by Beller and
co-workers, and resulted in the formation of biaryl ketones.[20]
However, there is no literature precedent of their oxidative
carbonylation to form carboxylate esters. Furthermore, with
respect to cost and simplicity, air is the ideal oxidant for
oxidative carbonylation.[21–26] The oxidative carbonylation of
amines or alcohols was investigated by employing air or
oxygen as the oxidant.[22,27–32] However, to the best of our
knowledge, no example has been reported regarding the
oxidative carbonylation of arylmetal reagents by employing
oxygen or air as the sole oxidant.
To probe the feasibility of our proposed study, we chose
the model substrates phenylBneop (1a; neop = OCH2C-
(CH3)2CH2O) and n-butanol to afford the carbonylation
product 2a, oxidative protodeboronation product 3a and
homocoupled product 4a. Upon variation of the reaction
conditions, different product distributions were observed
(Table 1). Among the palladium catalysts used as precursors
(Table 1, entries 1–11), [PdCl2(PPh3)2] showed the best result
(56% yield for 2a and 44% for 3a; Table 1, entry 7), whereas
the bidentate ligands gave poor results (Table 1, entries 1, 2,
and 6). Interestingly, Pd(OAc)2/PPh3 (1:2) and [Pd(dba)2]/
PPh3 (1:2) were not effective catalysts for this transformation
(Table 1, entries 10 and 11). Using [PdCl2(MeCN)2]/PPh3
(1:2) was equivalent to [PdCl2(PPh3)2], whereas [PdCl2-
(MeCN)2]/PPh3 (1:4) was less effective at producing the
carbonylation product 2a, however, it was an efficient
catalytic system for the oxidative homocoupling of 1a
(Table 1, entries 8 and 9).
Scheme 1. Two theoretical aryl carbonylation pathways (ArM; M=H,
In).
electrophile (ArX; Path I) or a nucleophile (ArM; Path II).
The carbonylation of organic halides (ArX) with CO,
pioneered by Heck and co-workers,[1a] has attracted broad
interest in the past 40 years, and is widely applied to the
synthesis of fine chemicals in industry.[1–3] In the reaction
systems shown in Scheme 1, the p-acceptor property of CO
renders low-valent metal catalysts such as Pd0 species to be
relatively electron deficient, and makes the oxidative addition
of organic halides towards Pd0 species difficult.[4] Conse-
quently, the reaction in Path I usually requires relatively harsh
conditions; for example, high temperatures or high CO
pressures.[5,6]
Recently, oxidative carbonylation of ArM (M = H, In)
with CO has emerged as a conceptually new alternative to
“classic carbonylation processes”.[7–11] The use of CO avoids
the reluctant oxidative addition step, which is mentioned
above in traditional carbonylations, and has the potential to
accommodate milder reaction conditions. The oxidative
carbonylation of ArH shows promise when catalyzed by
[*] Q. Liu, G. Li, J. He, Dr. J. Liu, P. Li, Prof. A. Lei
The College of Chemistry and Molecular Sciences
Wuhan University, Wuhan, Hubei (China)
Fax: (+86)27-6875-4067
E-mail: aiwenlei@whu.edu.cn
Homepage: http://www.chem.whu.edu.cn/GCI_WebSite/
main.htm
Prof. A. Lei
State Key Laboratory for Oxo Synthesis and Selective Oxidation,
Lanzhou Institute of Chemical Physics
Chinese Academy of Sciences, 730000 Lanzhou (China)
Next, different bases were then studied (Table 1,
entries 12–19). The results of triethylamine (B1) were similar
to those of 2,2,6,6-tetramethylpiperidine (B2) and 2,6-lutidine
(B3), however, other bases tested were less effective in this
reaction. Furthermore, the ratio of air to CO was also
screened. Interestingly, the yield of n-butyl benzoate (2a)
improved with an increase in the ratio of air to CO (Table 1,
[**] This work was supported by the National Natural Science
Foundation of China (20772093, 20972118, and 20832003), and the
Doctoral Fund of the Ministry of Education of China (20060486005).
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201000460.
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Table 1: Oxidative carbonylation of phenylboronate ester 1a.[a]
Table 2: Oxidative carbonylation of different arylboronic acid deriva-
tives.[a]
Entry [Pd] Catalyst
Base
Air/CO
Yield [%][b]
2a 3a 4a
Entry
1
Yield [%][b]
2a
3a
1
Pd(OAc)2/L1
Et3N
Et3N
Et3N
Et3N
Et3N
Et3N
Et3N
1:3
1:3
1:3
1:3
1:3
1:3
1:3
1:3
1:3
1:3
1:3
1:3
1:3
1:3
1:3
1:3
1:3
1:3
1:3
1:10
1:5
1:1
3:1
5:1
21 46
0
0
0
0
0
0
0
1
2
3
(PhBO)3
PhB(OH)2
PhBF3K
63
55
15
36
45
15
2
3
4
5
[PdCl2(dppf)]
0
0
0
0
0
0
0
0
0
0
[PdCl2(MeCN)2]/L2 (1:2)
[PdCl2(MeCN)2]/L3 (1:2)
Pd(OAc)2/py (1:4)
Pd(OAc)2/bpy
[a] Reaction conditions: Using 1 (0.25 mmol), Et3N (0.5 mmol), [PdCl2-
(PPh3)2] (0.0125 mmol), CO/air (1:3) at 1 atm pressure. [b] Yield
determined by GC analysis.
6
7
[PdCl2(PPh3)2]
56 44
8
9
[PdCl2(MeCN)2]/PPh3 (1:4) Et3N
[PdCl2(MeCN)2]/PPh3 (1:2) Et3N
0
12 80
56 44
26 34
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
Pd(OAc)2/PPh3 (1:2)
[Pd(dba)2]/PPh3 (1:2)
[PdCl2(PPh3)2]
[PdCl2(PPh3)2]
[PdCl2(PPh3)2]
[PdCl2(PPh3)2]
[PdCl2(PPh3)2]
[PdCl2(PPh3)2]
[PdCl2(PPh3)2]
[PdCl2(PPh3)2]
[PdCl2(PPh3)2]
[PdCl2(PPh3)2]
[PdCl2(PPh3)2]
[PdCl2(PPh3)2]
[PdCl2(PPh3)2]
Et3N
Et3N
nBuONa
CsF
K3PO4
dbu
Table 3: Palladium-catalyzed aerobic oxidative carbonylation of ArBneop
in nBuOH.[a]
0
0
32 68
32 67
45 53
47 50
dabco
0
0
B1
51 47
54 42
54 38
43 30
47 36
58 40
75 24
74 24
Entry
1
1
Ar
Yield [%][b]
75
B2
B3
1a
Et3N
Et3N
Et3N
Et3N
Et3N
2
1b
81
3
4
5
1c
1d
1e
85
80
79
[a] Reaction conditions: Using 1a (0.25 mmol), base (0.5 mmol), Pd
catalyst (0.0125 mmol), and air/CO in 1 atm pressure. [b] Yield deter-
mined by GC analysis. bpy=2,2’-bipyridine , dabco=1,4-diazabicyclo-
[2.2.2]octane, dbu=1,8-diazabicyclo[5.4.0]undec-7-ene, dppf=diphenyl-
phosphinoferrocene, py=pyridine.
6
7
8
1 f
1g
1h
95
66
61
9
1i
1j
74
69
10
entries 20–24). Therefore, the optimized conditions for the
oxidative carbonylation were 5 mol% of [PdCl2(PPh3)2] as
the catalyst, triethylamine as the base, and a ratio of air/CO
3:1 at 408C.
In Table 2, different arylboronic acid derivatives were
compared under the optimized reaction conditions. The
phenylboronic anhydride (63% yield of 2a; Table 2,
entry 1) performed slightly worse than 1a (75% yield of 2a;
Table 1, entry 23). The oxidative carbonylation of phenyl-
boronic acid produced 2a in 55% yield (Table 2, entry 2).
When potassium phenyltrifluoroborate was employed, the
yields of both 2a and 3a were poor.
[a] Reaction Conditions: Using 1 (0.25 mmol), Et3N (0.5 mmol), [PdCl2-
(PPh3)2] (0.0125 mmol), CO/air (1:3) at 1 atm pressure. [b] Yield of
isolated product.
entries 1–6). The yields for p-MeO phenylBneop 1 f and p-
fluoro phenylBneop 1h are 95% and 61%, respectively
(Table 3, entries 6 and 8). Ortho-substituted substrates, such
as o-Me phenylBneop 1b and naphthylBneop 1i and 1j gave
satisfactory results (Table 3, entries 2, 9, and 10).
A further investigation of the substrate scope was carried
out under the optimized reaction conditions. In Table 3,
various ArBneop substrates were tested. Substrates contain-
ing electron-donating groups provided higher yields (Table 3,
Furthermore, a variety of alcohols were tested and the
results are listed in Table 4. In the presence of isopropanol,
the reactions gave good results at 508C for 24 hours in the
presence of 10 mol% of palladium catalyst (Table 4,
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Table 4: Palladium-catalyzed aerobic oxidative carbonylation of ArBneop
in different alcohols.[a]
Scheme 3. The Gibbs free energy difference between cis- and trans-
[ArPd(OH)(L)2].
Entry
1
Ar
Alcohol (ROH)
Yield [%][b]
1[c]
2[c]
3[c]
1d
1 f
1g
iPrOH
iPrOH
iPrOH
78
75
70
carbonylation products were identified in the reaction of 1a
with a stoichiometric amount of [PdCl2(PPh3)2] when NEt3
was employed as the base [Eq. (1)]. In fact, starting material
4[c]
1k
1l
iPrOH
iPrOH
85
71
5[c]
6
7
8
9
1e
1i
1a
1 f
EtOH
EtOH
EtOH
MeOH
77
70
73
90
[a] Reaction conditions: Using 1 (0.25 mmol), Et3N (0.5 mmol), [PdCl2-
(PPh3)2] (0.0125 mmol), and CO/air (1:3) at 1 atm pressure. [b] Yield of
isolated product. [c] Used [PdCl2(PPh3)2] (0.025 mmol) at 508C.
1a was quantitatively recovered. However, when [PdCl2-
(PPh3)2] was replaced by a stoichiometric amount of [(h-
O2)Pd(PPh3)2], n-butyl benzoate (2a) and phenol (3a) were
obtained in 50% yield, respectively [Eq. (2)]. Similarly, in the
entries 1–5).[33] Ethanol and methanol worked well under the
standard reaction conditions (Table 4, entries 6–9).
It is known that the reaction of [Pd(PPh3)4] with oxygen
affords the [(h-O2)Pd(PPh3)2] complex.[34] Jutand and co-
workers investigated the transmetalation between the [(h-
O2)Pd(PPh3)2] complex and PhB(OH)2, and a fast reaction
generated the trans-[PhPd(OH)(PPh3)2] complex.[35,36]
A
proposed catalytic cycle of the aerobic oxidative carbon-
ylation was shown in Scheme 2. Carbonylation of trans-
absence of CO and air at 408C using NEt3 as the base and
nBuOH as the solvent, 1a did not react with a stoichiometric
amount of [PdCl2(PPh3)2] either [Eq. (3)]. However, in air,
the homocoupled product biphenyl (4a) and oxidative pro-
todeboronation product phenol (3a) were generated in 56%
and 44% yield, respectively [Eq. (4)]. These comparative
Scheme 2. Proposed mechanism.
[ArPd(OH)(PPh3)2] in the presence of CO, ROH, and base
would result in the formation of product ArCOOR, and a
Pd0Ln species. The [(h-O2)Pd(PPh3)2] complex would be
regenerated from the reaction of [Pd(PPh3)n] with air.
In Scheme 3, a simplified model was used to compare the
energy difference between cis- and trans-[ArPd(OH)(L)2].
The trans complex was 4.8 kcalmolꢀ1 more stable than the
cis complex. Thus, we speculated that in this oxidative
carbonylation, cis-[ArPd(OH)(PPh3)2], initially produced
from the transmetalation of [(h-O2)Pd(PPh3)2] with
PhB(OH)2, would quickly transform into trans-[ArPd(OH)-
(PPh3)2].
results indicated that the transmetalation reaction could only
occur between phenylBneop and [(h-O2)Pd(PPh3)2] under
these reaction conditions; [PdCl2(PPh3)2] as the catalyst
precursor needs to convert into [(h-O2)Pd(PPh3)2] before
initiating the catalytic cycle.
To study whether peroxide species were formed in this
reaction, aliquots were tested using potassium iodide starch.
Interesting, the test paper did not turn blue in the early stage
of the reaction (the solution of the reaction was yellow).
When the yellow reaction solution turned red, the potassium
iodide starch test paper turned blue, thus proving that the
peroxides were formed during the reaction.
In the presence of CO gas, two experiments revealed the
different transmetalation reactivities of [PdCl2(PPh3)2] and
[(h-O2)Pd(PPh3)2] complex with 1a. No biphenyl or any
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Communications
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biaryl product formed in this oxidative carbonylation. How-
ever, arylboronic acid derivatives could be homocoupled in
the presence of oxygen.[37–39] The difference between the
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mainly attributed to the presence or absence of CO. Thus, we
presumed that it was CO which led the high selectivities
between the oxidative carbonylation and oxidative homocou-
pling, but we do not have enough data to confirm this at
present.
In conclusion, we have developed a novel oxidative
carbonylation reaction. Under a balloon pressure of CO/air
and at temperatures of 40!508C, readily available arylbor-
onic acid derivatives could be converted into their corre-
sponding carbonylative products using [PdCl2(PPh3)2] as the
catalyst precursor. It is the first example whereby arylboronic
acid derivatives can be transformed into carbonylative
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Experimental Section
General procedure: [PdCl2(PPh3)2] (0.0125 mmol) and arylboronate
ester (0.25 mmol) was added to a Schlenk tube equipped with a stirrer
bar. A balloon filled with air and CO (the ratio is 3:1) was connected
to the Schlenk tube through the side arm. The mixture was purged
with the mixed gas for 1 min before triethylamine (0.5 mmol) and 1-
butanol (2 mL) were added. The Schlenk tube was placed in an oil
bath and heated to 408C for 24 h, and then cooled to room
temperature. After the crude mixture was concentrated under
vacuum, the reaction mixture was purified by flash chromatography
on silica gel (eluent: petroleum ether/ethyl acetate 60:1) to afford the
product.
Received: January 26, 2010
Published online: April 1, 2010
Keywords: air oxidation · arylboronate esters ·
.
homogeneous catalysis · oxidative carbonylation · palladium
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