A general palladium-catalyzed carbonylative sonogashira coupling of aryl triflates

Article abstract of DOI:10.1002/chem.201002653

Trifling with triflates: A new protocol for the carbonylative Sonogashira reactions of aryl triflates (see scheme) that provides a significant extension of this interesting methodology is described.

Full text of DOI:10.1002/chem.201002653

DOI: 10.1002/chem.201002653
A General Palladium-Catalyzed Carbonylative Sonogashira Coupling of Aryl
Triflates
Xiao-Feng Wu,[a] Basker Sundararaju,[b] Helfried Neumann,[a] Pierre H. Dixneuf,[b] and
Matthias Beller*[a]
Alkynones represent an interesting structural motif found
in various bioactive molecules.[1] More importantly, they
constitute key intermediates in the synthesis of natural prod-
ucts[2] and, especially, heterocycles.[3] Traditionally, alkynones
have been synthesized by cross-coupling reactions of carbox-
ylic acid chlorides and terminal alkynes.[4] However, the sta-
bility and functional group tolerance of the respective acid
chlorides has limited the applications of this methodology.
Since the first report by Tanaka in 1981,[5a] palladium-cata-
lyzed carbonylative Sonogashira reactions of aryl halides
have become an interesting alternative for the synthesis of
alkynones and notable improvements have been
achieved.[5,6] For example, Mori and co-workers reported
carbonylation reactions at room temperature and with an
ambient pressure of CO.[5c] Xia[5b] and Ryu[5j] developed re-
cyclable catalyst systems based on heterogenization with
Fe3O4 particles or in the presence of ionic liquids. Moreover,
the latter group demonstrated the synthesis of alkynones
from iodoalkenes by applying a combination of a palladium
catalyst and UV irradiation.[5m] Recently, in addition to pal-
ladium-based catalysts, copper has been described as an
active metal for carbonylative Sonogashira reactions by
Bhanage and co-workers.[5n]
vantages of (hetero)aryl bromides compared with those of
the corresponding iodides, we very recently developed a
general procedure for carbonylative Sonogashira reactions
of this class of substrates.[7] Key to its success was the appli-
cation of a palladium/BuPAd2 (Ad=adamantyl) catalyst
system[8] in the presence of potassium carbonate as a base.
Based on this work, we became interested in further ex-
panding the substrate scope of this methodology to aryl sul-
fonates. To the best of our knowledge, carbonylative Sono-
gashira reactions utilizing aryl sulfonates as the starting ma-
terial have not been described before.
In addition to aryl halides, hydroxylated arenes are easily
available and frequently found in pharmaceuticals, agro-
chemicals, and polymers, as well as biologically active natu-
ral compounds.[9] It is known that phenols can be trans-
formed into their corresponding aryl triflates, which contain
a highly reactive leaving group. This principle has often
been used for the preparation of versatile intermediates in
modern organic synthesis, particularly in palladium-cata-
lyzed coupling reactions.[10]
Recently, some of us developed reductive carbonylation
and carbonylative vinylation reactions of aryl/vinyl tri-
flates.[11] Herein, in a continuation of this and other palladi-
um-catalyzed carbonylation reactions,[12] we wish to report a
general palladium-catalyzed synthesis of alkynones utilizing
aryl and alkenyl triflates.
Unfortunately, most of these synthetic developments were
only demonstrated with aryl iodides or iodoalkenes as the
substrates. Clearly, this has significantly limited the substrate
scope for this interesting methodology. Considering the ad-
Initially, we investigated the carbonylative Sonogashira re-
action of phenyl triflate and phenyl acetylene in the pres-
ence of [{PdACHTUNGRTENNG(U cinnamyl)Cl}2] (1 mol%) and different ligands
[a] 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)
Fax : (+49)381-1281-5000
(2 mol%) at 10 bar carbon monoxide with NEt3 as a base.
Selected results are shown in Table 1. Whereas monodentate
ligands did not give an appreciable amount of the product,
several bidentate phosphines converted phenyl triflate into
1,3-diphenylpropynone in moderate to good yield. No prod-
uct was detected in the presence of DPPM (1,1-bis(diphe-
nylphosphino)methane), DPPE (1,2-bis(diphenylphosphi-
no)ethane), DPPP (1,3-bis(diphenylphosphino)propane), or
DPPPe (1,5-bis(diphenylphosphino)pentane) as the ligand
(Table 1, entries 1–3 and 5). Applying ligands with theoreti-
E-mail: matthias.beller@catalysis.de
[b] B. Sundararaju, Prof. Dr. P. H. Dixneuf
Catalyse et Organomꢂtalliques
Institut Sciences Chimiques de Rennes
UMR 6226-CNRS-Universitꢂ de Rennes
Av. Gꢂnꢂral Leclerc, 35042 Rennes (France)
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201002653.
106
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Chem. Eur. J. 2011, 17, 106 – 110

COMMUNICATION
Table 1. The carbonylation of phenyl triflate with phenyl acetylene: the
testing of different ligands.[a]
Table 2. The carbonylation of phenyl triflate with phenyl acetylene: the
variation of the reaction conditions.[a]
Entry
Ligand
Bite Angle
Yield [%][b]
1
2
DPPM
DPPE
72
78
0
0
Entry
Solvent
Base
Yield [%][b]
3
4
5
6
7
8
9
10
11
12
DPPP
DPPB
86
98
–
96
96
92
98
102
111
132
0
28
0
48
49
19
35
46
71
0
1
2
3
4
5
6
7
8
9
toluene
toluene
toluene
toluene
toluene
heptane
dioxane
DMF
DBACO
TMEDA
DiPEA
K2CO3
K3PO4
NEt3
NEt3
NEt3
NEt3
NEt3
0
20
37
13
0
DPPPe
DPPF
DtBPF
BINAP
DIOP
DPEphos
Xantphos
DBFphos
9
48
67
11
83[c]
0[d]
63[c,e]
NMP
10
11
12
toluene
toluene
toluene
NEt3
NEt3
[a] Phenyl triflate (1.0 mmol), phenyl acetylene (1.2 mmol), CO (10 bar),
[{Pd(cinnamyl)Cl}2] (1 mol%), ligand (2 mol%), toluene (2 mL), NEt3
ACHTUNGTRENNUNG
(2 mmol), 1008C, 20 h. [b] Yield was determined by GC with hexadecane
as the internal standard.
[a] Phenyl triflate (1.0 mmol), phenyl acetylene (1.2 mmol), CO (10 bar),
[{Pd(cinnamyl)Cl}2] (1 mol%), Xantphos (2 mol%), solvent (2 mL), base
(2 mmol), 1008C, 20 h. DBACO=1,4-diazabicyclo[2.2.2]octane;
AHCTUNGTRENNUNG
AHCTUNGTRENNUNG
TMEDA=N,N,N’,N’-tetramethylethane-1,2-diamine; DiPEA=N,N-diiso-
propylethylamine. [b] Yield was determined by GC with hexadecane as
the internal standard. [c] 1108C. [d] 1208C. [e] CO (5 bar).
cal bite angles[13] between 96 and 988, such as DPPB (1,4-
bis(diphenylphosphino)butane), DPPF (1,1’-bis(diphenyl-
phosphino)ferrocene), DtBPF (1,1’-bis(di-tert-butylphosphi-
no)ferrocene), DIOP ((+)-2,3-O-isopropylidene-2,3-dihy-
droxy-1,4-bis(diphenylphosphino)butane)
or
DPEphos
((oxydi-2,1-phenylene)bis(diphenylphosphino)) led to 1,3-di-
phenylpropynone in 28–49% yield (Table 1, entries 4, 6, 7,
9, and 10). Best results were obtained with Xantphos (4,5-
bis(diphenylphosphino)-9,9-dimethylxanthene) as the ligand
(Table 1, entry 11).[14] Notably, the product of the competing
Sonogashira coupling (1,2-diphenylethyne) is not formed
under these conditions. However, no reaction occurred with
DBFphos (4,6-bis(diphenylphosphino)dibenzofuran), which
has the largest bite angle among the ligands tested here
(Table 1, entry 12).
Next, we investigated the influence of base, pressure, sol-
vent, and temperature on the reaction (Table 2). The use of
other common organic and inorganic bases resulted in lower
yields (Table 2, entries 1–5). Variation of the solvent showed
that heptane and NMP (N-methylpyrrolidone) are not suit-
Table 3. The palladium-catalyzed carbonylative Sonogashira reaction of
aryl and alkenyl triflates.[a]
Entry
1
Product
Yield [%][b]
81
3a
3b
2
80
ACHTUNGTRENNUNGable solvents (Table 2, entries 6 and 9), whereas DMF and
3
4
3c
3d
78
74
dioxane gave product yields comparable to toluene (Table 1,
entry 11; Table 2, entries 7 and 8). Increasing the tempera-
ture to 1108C led to an 83% yield, but further increases in
the temperature resulted in the decomposition of phenyl tri-
flate (Table 2, entries 10 and 11). The desired coupling re-
ACHTUNGTRENNUNGaction also took place at 5 bar CO pressure, although the
yield of 1,3-diphenylpropynone was somewhat decreased to
63% (Table 2, entry 12).
5
6
3e
3 f
50
72
Once the optimized conditions were identified, we investi-
gated the general applicability of this methodology
(Table 3). To our delight, the new palladium-catalyzed car-
bonylative Sonogashira reaction took place in the presence
of both electron-donating and electron-withdrawing sub-
Chem. Eur. J. 2011, 17, 106 – 110
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107

M. Beller et al.
CH3CO substituents mainly resulted in the non-carbonyla-
tive Sonogashira reaction.
Table 3. (Continued)
Entry
Product
Yield [%][b]
68
Mꢀller and co-workers,[16] as well as other groups,[17] have
elegantly demonstrated the potential of alkynones in various
cascade sequences for organic synthesis. Also, under our
conditions further modification of the alkynones is possible
without purification. As shown in Scheme 1, three different
7
3g
8
9
3h
3i
64
72
60
80
10
11
3j
Scheme 1. The cascade sequence for the synthesis of enaminones.
enaminones have been synthesized in a one pot-sequence
starting from cyclohexenyl triflate and phenyl acetylene.
Following the carbonylation reaction, aliphatic amines, dis-
solved in ethanol, were added to the reaction mixture. After
heating at 808C, the corresponding enaminones were ob-
tained in 53–60% isolated yield.
3k
12
3l
76
In conclusion, the first general, chemoselective protocol
for the carbonylative Sonogashira coupling reaction of aryl
triflates has been developed. Both electron-rich and elec-
tron-poor aryl triflates are smoothly coupled with aryl acety-
lenes under low pressures of CO. The use of easily available
vinyl triflates allows for an interesting extension of the sub-
strate scope. The obtained products represent useful build-
ing blocks for the synthesis of biologically active com-
pounds. As an example, the straightforward one-pot prepa-
ration of 1-cyclohexenyl-3-phenylenaminones was demon-
strated.
13
14
3m
3n
79
69
15
3o
83
[a] Aryl triflate (1.0 mmol), alkyne (1.2 mmol), CO (10 bar), [{Pd-
ACHTUNGTRENNUNG(cinnamyl)Cl}2] (1 mol%), L=Xantphos (2 mol%), toluene (2 mL), NEt3
Experimental Section
(2 mmol), 1108C, 20 h. [b] Isolated yield.
General procedure for carbonylative Sonogashira reactions: [{Pd-
AHCTUNGTRENNUNG
stituents in moderate to good isolated yields (50–81%;
Table 3, entries 1–8). At this point, it should be noted that
carbonylative Sonogashira reactions of aryl bromides con-
taining electron-withdrawing substituents or sterically hin-
dered aryl substrates are still challenging,[7] However, im-
proved yields can be obtained with the current system (50–
72%).
a
(1.0 mmol), phenyl acetylene (1.2 mmol), toluene (2 mL), and NEt3
(2 mmol) were injected into the vial by syringe. Then, the vial was placed
in an alloy plate, which was transferred into a 300 mL autoclave of the
4560 series from Parr Instruments, under an argon atmosphere. After
flushing the autoclave three times with CO, the pressure was adjusted to
10 bar and the reaction was performed for 20 h at 1108C. After the re-
AHCTUNGERTGaNNUN ction, the autoclave was cooled to room temperature and the pressure
The straightforward preparation of vinyl triflates from al-
dehydes or ketones[15] offers an attractive possibility for the
synthesis of 1-vinyl-3-arylproynones. Indeed, three different
vinyl triflates reacted well under our conditions and gave
the corresponding products in 60–80% yield (Table 3, en-
tries 9–11).[15] Furthermore, different aryl acetylenes were
tested and the corresponding 1,3-diarylalkynones were iso-
lated in good yields (Table 3, entries 12–15). However, ap-
plying more reactive alkynes with p-CN, p-NO2, p-CHO, p-
was carefully released. Water (6 mL) was added to the reaction mixture
and the solution was extracted with ethyl acetate (3–5ꢄ2–3 mL). The ex-
tracts were evaporated in the presence of a small amount of silica gel
and the crude product was purified by column chromatography on silica
gel by using n-heptane and n-heptane/AcOEt (50:1) as the eluent. The
product was obtained as a yellow oil.
General procedure for enaminone synthesis: After the carbonylative So-
nogashira reaction, the autoclave was cooled to room temperature and
the pressure was carefully released. Ethanol (1 mL) and amine
(1.5 equiv) were added to the reaction mixture, the vial was closed again
108
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Chem. Eur. J. 2011, 17, 106 – 110

Sonogashira Coupling
COMMUNICATION
and heated at 808C for 8 h. After cooling it to room temperature, water
(6 mL) was added to the reaction mixture and the solution was extracted
with ethyl acetate (3–5ꢄ2–3 mL). The extracts were evaporated in the
presence of a small amount of silica gel and the crude product was puri-
fied by column chromatography on silica gel by using n-heptane and n-
heptane/AcOEt (10:1) as the eluent.
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Keywords: alkynones
· aryl triflates · carbonylation ·
palladium · Sonogashira reaction
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Received: September 15, 2010
Published online: December 3, 2010
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