Convenient and general palladium-catalyzed carbonylative sonogashira coupling of aryl amines

Article abstract of DOI:10.1002/anie.201104653

Bar (alky)none: A general and efficient method for carbonylative Sonogashira coupling reactions of anilines to generate alkynones has been developed (see scheme; TFP=tri(2-furyl)phosphine). The reaction proceeds under mild conditions and no base is needed

Full text of DOI:10.1002/anie.201104653

Communications
DOI: 10.1002/anie.201104653
Cross-Coupling
Convenient and General Palladium-Catalyzed Carbonylative
Sonogashira Coupling of Aryl Amines**
Xiao-Feng Wu, Helfried Neumann, and Matthias Beller*
Alkynones represent an interesting structural motif that is
found in various bioactive molecules.[1] More importantly,
they constitute key intermediates for the synthesis of
numerous natural products,[2] especially heterocycles.[3] In
general, alkynones have been prepared by the reaction of acid
chlorides and alkynes. Unfortunately, this methodology is
limited with respect to functional group tolerance and
substrate stability.[4]
The first set of reactions was carried out using aniline and
phenylacetylene as a model system at ambient conditions
(328C). In the presence of 2 mol% Pd(OAc)2 and 6 mol%
TFP, different solvents were tested (Table 1, entries 1–6).
DMSO and DMF gave 43% and 38%, respectively, of the
desired product 1,3-diphenylprop-2-yn-1-one (Table 1,
Table 1: Palladium-catalyzed carbonylative sonogashira coupling of
Since the first report by Kobayashi and Tanaka in 1981,[5a]
notable improvements of palladium-catalyzed carbonylative
Sonogashira reactions of aryl halides have been achieved.[5,6]
A drawback to all these elegant developments is the necessity
to use iodo compounds as substrates. Hence, our development
of a general procedure for carbonylative Sonogashira reac-
tions of less expensive aryl bromides is interesting.[6a] Key to
the success was the application of a palladium/BuPAd2
catalyst system[7] in the presence of potassium carbonate as
base. In addition to aryl halides, carbonyaltive Sonogashira
coupling reactions of aryl triflates are also achieved.[6b] The
latter procedure opened the gate for using hydroxylated
arenes, which are frequently found in pharmaceuticals, agro-
chemicals, and natural products, as starting materials.[8]
Aryl amines are abundantly available and relatively
inexpensive. Compared to aryl halides, they have been
scarcely employed as electrophiles in palladium-catalyzed
coupling methodologies. However, they can be easily acti-
vated by simple diazotization under mild reaction condi-
tions.[9] Notably, the first carbonylation of isolated diazonium
salts was reported by Matsuda and co-workers.[10] Since then,
all other examples of such carbonylation processes required
isolated ArN2BF4 salts as coupling partners.[11] Given their
commercial availability, the hazards they pose, and their
tedious preparation, it would be interesting to develop
processes that can employ anilines directly as electrophiles.
Based on our continuing interest in carbonylations of
anilines: Optimization of reaction conditions.[a]
Entry
Ligand
Solvent
Yield [%][b]
1
2
3
4
5
6
7
8
TFP (6 mol%)
TFP (6 mol%)
TFP (6 mol%)
TFP (6 mol%)
TFP (6 mol%)
TFP (6 mol%)
–
DMSO (2 mL)
DMF (2 mL)
DMPU (2 mL)
THF (2 mL)
1,4-dioxane (2 mL)
toluene (2 mL)
THF (2 mL)
43
38
10
48
31
10
12
PPh3 (6 mol%)
P(o-tolyl)3 (6 mol%)
Xantphos (3 mol%)
DPEphos (3 mol%)
DPPP (3 mol%)
TFP (6 mol%)
THF (2 mL)
THF (2 mL)
THF (2 mL)
THF (2 mL)
THF (2 mL)
THF (1 mL)
2
23
<1
<1
<1
60
9
10
11
12
13
DMSO (1 mL)
THF (1 mL)
14
TFP (6 mol%)
81[c]
DMSO (1 mL)
THF (1 mL)
15
TFP (6 mol%)
55[c,d]
DMSO (1 mL)
[a] Reaction conditions: aniline (1.3 mmol), tBuONO (1.3 mmol), AcOH
(1.3 mmol), phenylacetylene (1 mmol), Pd(OAc)2 (2 mol%), CO
(10 bar), 328C, 16 h. [b] Determined by GC analysis of the reaction
mixture using hexadecane as an internal standard. Based on phenyl-
acetylene. [c] Aniline (2 mmol), tBuONO (2 mmol), AcOH (2 mmol).
[d] CO (1 bar). DPEphos=(oxydi-2,1-phenylene)bis(diphenylphos-
phine), DPPP=1,3-bis(diphenylphosphino)propane, P(o-tolyl)3 =tri(o-
tolyl)phosphine, TFP=tri(2-furyl)phosphine, Xantphos=4,5-bis(diphe-
nylphosphino)-9,9-dimethylxanthene.
[12,13]
À
aryl X derivatives,
and considering the importance of
alkynones,[1–3] herein we report the palladium-catalyzed
coupling of in situ formed arenediazonium salts with carbon
monoxide and alkynes under mild reaction conditions. To the
best of our knowledge such carbonylative Sonogashira
coupling reactions of diazonium salts are not known to
date.[14]
entries 1–2), whereas 1,3-dimethyl-3,4,5,6-tetrahydro-2(1H)-
pyrimidinone (DMPU) only resulted in 10% of the product
(Table 1, entry 3). Similarly, toluene gave 10% yield (Table 1,
entry 6). In contrast, 48% of the desired product is achieved
in THF (Table 1, entry 4). Thus, we choose THF to investigate
the effect of different ligands: In the absence of a ligand only a
low yield (12%) is observed (Table 1, entry 7). Surprisingly,
the standard ligand, PPh3, inhibited this carbonylative cou-
pling and gave only 2% of the corresponding product
(Table 1, entry 8). In the same way, bidentate phosphine
[*] X.-F. Wu, Dr. H. Neumann, Prof. Dr. M. Beller
Leibniz-Institut fꢀr Katalyse e.V. an der Universitꢁt Rostock
Albert-Einstein-Strasse 29a, 18059 Rostock (Germany)
E-mail: matthias.beller@catalysis.de
[**] We thank the state of Mecklenburg-Vorpommern and the Bundes-
ministerium fꢀr Bildung und Forschung (BMBF) for financial
support.
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Table 2: Palladium-catalyzed carbonylative Sonogashira coupling of
Table 2: (Continued)
anilines: Variation of alkynes.[a]
Entry
Alkyne
Product
Yield
[%][b]
14
68
Entry
Alkyne
Product
Yield
[%][b]
15
16
17
18
19
89
92
80
83
85
1
2
3
4
5
81
80
75
69
59
[a] Reaction conditions: aniline (2 mmol), tBuONO (2 mmol), AcOH
(2 mmol), alkynes (1 mmol), CO (10 bar), Pd(OAc)2 (2 mol%), TFP
(6 mol%), THF (1 mL), DMSO (1 mL), 328C, 16 h. [b] Determined by GC
analysis of the reaction mixture using hexadecane as an internal
standard. Based on alkyne; average of two runs.
6
86
ligands were not suitable and gave trace amounts of the
alkynone together with the homocoupling product of phenyl-
acetylene (Table 1, entries 10–12). To our delight, a 60%
product yield is obtained by using a mixture of THF and
DMSO (v/v = 1:1) as the solvent (Table 1, entry 13). The yield
is further improved to 81% by increasing the amount of
aniline (Table 1, entry 14). Although most reactions were
carried out at 10 bar of CO, notably, the reaction can be
performed without pressure to give 55% of the carbonylation
product (Table 1, entry 15). With the best reaction conditions
in hand, we examined different substrates for this novel
carbonylative Sonogashira reaction. The generality of this
methodology is demonstrated by the results in Tables 2 and 3.
Simple alkyl- and methoxy-substituted aromatic alkynes
are successfully coupled with CO and give good yields of the
corresponding products (Table 2, entries 2–6). Besides elec-
tron-donating groups, electron-withdrawing substituents are
tolerated and lead to good yields (Table 2, entries 7–12).
Notably, aldehydes might be potentially oxidized by
tBuONO; however, to our surprise 75% of the corresponding
alkynone is produced (Table 2, entry 12). Furthermore seven
different aliphatic alkynes were transformed into the desired
alkynones in excellent yields (Table 2, entries 13–19). Com-
pared with aromatic alkynes, aliphatic alkynes are more
selective, and only trace amounts of the noncarbonylation
products were formed.
7
8
65
90
9
88
10
11
12
13
93
81
75
91
As demonstrated in Table 3 eleven different anilines were
tested to prove the generality of this methodology. Not only
electron-donating (Table 3, entries 1–3), but also electron-
withdrawing groups attached to the aniline are well tolerated
(Table 3, entries 4–9). Heterocyclic amines can also be used as
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Communications
Table 3: Palladium-catalyzed carbonylative Sonogashira coupling of
anilines: Variation of anilines.[a]
Entry
Aniline
Product
Yield
[%][b]
1
2
3
4
5
6
7
8
75
82
79
83
81
75
68
88
Scheme 1. Proposed reaction mechanism.
Initially, diazotization of the aniline takes place. Then,
oxidative addition of the resulting diazonium salt to Pd0 (1),
which is generated from Pd(OAc)2, with subsequent coordi-
nation to give 2 and then insertion of carbon monoxide forms
the acyl palladium complex 3. Under the assistance of acetate,
phenylacetylene is deprotonated and generates 4 which gives
the terminal product 5 after reductive elimination, and also
regenerates the active Pd0 species for next catalytic cycle.
In conclusion, the first general and efficient methodology
for carbonylative Sonogashira reactions of anilines has been
developed. The transformation proceeds under mild reaction
conditions, and no base is needed. Both aromatic and
aliphatic alkynes are suitable starting materials and 30
different kinds of alkynones were produced in moderate to
excellent yields.
Experimental Section
General comments: All reactions were prepared under argon
atmosphere. DMSO and THF were distilled from sodium ketyl or
CaH and stored in Aldrich Sure/Stor flasks under argon. Pd(OAc)2,
anilines, alkynes, AcOH, tert-BuONO and all the ligands were
purchased from Aldrich and used as received. Gas chromatography
analysis was performed on an Agilent HP-5890 instrument with a FID
detector and HP-5 capillary column (polydimethylsiloxane with 5%
phenyl groups, 30 m, 0.32 mm i.d., 0.25 mm film thickness) using argon
as carrier gas. Gas chromatography/mass analysis was carried out on
an Agilent HP-5890 instrument with an Agilent HP-5973 Mass
Selective Detector (EI) and HP-5 capillary column (polydimethylsil-
oxane with 5% phenyl groups, 30 m, 0.25 mm i.d., 0.25 mm film
thickness) using helium carrier gas.
9
62
10
11
60
55
Typical reaction procedure for carbonylative Sonogashira reac-
tions: Pd(OAc)2 (2 mol%) and TFP (6 mol%) were transferred into a
vial (4 mL reaction volume) equipped with a septum, a small cannula,
and a stirring bar. After the vial was purged with argon, DMSO
(1 mL), THF (1 mL), aniline (2.0 mmol), tert-butyl nitrite (2.0 mmol),
AcOH (2.0 mmol), and phenylacetylene (1 mmol) were injected into
the vial by syringe. Then, the vial was placed on an alloy plate, which
was transferred into a 300 mL autoclave of the 4560 series from Parr
Instruments under argon atmosphere. After flushing the autoclave
three times with CO, a pressure of 10 bar was set and the reaction was
performed for 16 h at 328C. After the reaction was complete, the
autoclave was cooled to room temperature and the pressure was
released carefully. Hexadecane (0.1 mmol) was added to the reaction
mixture and a portion of the mixture was taken for GC and GC/MS
analysis after proper mixing. For isolation the reaction mixture was
[a] Reaction conditions: aniline (2 mmol), tBuONO (2 mmol), AcOH
(2 mmol), phenylacetylene (1 mmol), CO (10 bar), Pd(OAc)2 (2 mol%),
TFP (6 mol%), THF (1 mL), DMSO (1 mL), 328C, 16 h. [b] Determined
by GC analysis of the reaction mixture using hexadecane as an internal
standard. Based on phenylacetylene; average of two runs.
starting materials and 6-aminobenzothiazole resulted in 55%
of the desired product (Table 3, entry 11).
In Scheme 1 the proposed reaction mechanism of this
diazotization/carbonylative coupling sequence is depicted.
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Angew. Chem. Int. Ed. 2011, 50, 11142 –11146

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Received: July 6, 2011
Published online: September 28, 2011
Keywords: cross-coupling · domino reactions · heterocycles ·
.
palladium · synthetic methods
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ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2011, 50, 11142 –11146