Palladium-catalyzed coupling reactions: Carbonylative heck reactions to give chalcones

Article abstract of DOI:10.1002/anie.201002155

Chalcones made easy: Carbonylative Heck reactions of aryl and alkenyl triflate derivatives with carbon monoxide and aromatic olefins proceed in the presence of palladium catalysts (see scheme; dppp=1,3-bis(diphenylphosphino) propane, Tf=triflate; R=aryl,

Full text of DOI:10.1002/anie.201002155

Communications
DOI: 10.1002/anie.201002155
Palladium Catalysis
Palladium-Catalyzed Coupling Reactions: Carbonylative Heck
Reactions To Give Chalcones**
Xiao-Feng Wu, Helfried Neumann, and Matthias Beller*
In the past decades palladium-catalyzed coupling reactions
have emerged as a power tool for advanced organic syn-
natural products that belong to the flavonoids.[10] These
compounds display manifold biological activities including
anticancer, antiinflammatory, antioxidant, antimicrobial,
analgesic, antipyretic, antihepatotoxic, antimalarial, and anti-
allergic properties (Scheme 2).[11]
thesis.[1,2] Nowadays, the formation of C C, C O, and C N
À
À
À
À
bonds of aryl, heteroaryl, and vinyl X (X = I, Br, Cl, OTf,
OMs, etc.) compounds in the presence of homogeneous
palladium catalysts is of significant value for the preparation
of pharmaceuticals, agrochemicals, and advanced materials.
Both academic as well as industrial laboratories continuously
investigate and develop new applications in this area.
Among the different available coupling methods, three-
component carbonylation reactions,[3,4] such as the carbon-
ylative Suzuki process[5] and particularly the carbonylative
Sonogashira reaction,[6] create interesting building blocks,
which allow for a significant increase in molecular complexity
(Scheme 1).
Scheme 2. Selected examples of bioactive chalcones.
Initially, the reaction of bromobenzene with an excess
amount of styrene in the presence of carbon monoxide was
investigated. However, the desired 1,3-diphenylpropen-1one
was not obtained, instead traces of benzoic acid derivatives
and stilbene were formed. Apparently, the reactivity of the
benzoylpalladium(II) bromide, which is generated in situ, is
too low for reaction with the olefin. We assumed that in a
similar manner to palladium-catalyzed olefin/copolymeriza-
tions, a cationic palladium intermediate should be better
suited for this reaction.[12] Indeed, the reaction of phenyl
triflate with styrene and carbon monoxide (10 bar) in the
presence of 1 mol% of [{(cinnamyl)PdCl}2] and 1,2-bis(dia-
damantyl)xylylphosphine[13] gave the desired chalcone, albeit
in low yield (8%).
À
Scheme 1. Carbonylative three-component coupling reactions of aryl X
compounds.
To the best of our knowledge, to date only the related
intramolecular reaction of aryl iodides, carbon monoxide,
dihydrofurane, and cyclopentene have been described.[7,8]
This situation is somewhat surprising because the resulting
structural motif is present in numerous biologically active
compounds and some currently employed pharmaceuticals.[9]
Among the different (hetero)aryl vinyl ketones, 1,3-diaryl-
propen-1-ones (chalcones) constitute a well-known class of
In Table 1 selected results from the variation of solvents,
bases, and ligands are shown. Although monodentate ligands
did not give any appreciable amount of product, several
bidentate phosphine derivatives (except dppf) converted
phenyl triflate into the chalcone in 4–12% yield (Table 1,
entries 1–6). Optimization of the base when the best ligand,
1,3-bis(diphenylphosphino)propane (dppp), was applied, led
to a significant increase of the desired product (50–65% yield;
Table 1, entries 7–10). Variation of the solvent resulted in
75% yield of the isolated chalcone (Table 1, entry 13).
[*] X.-F. Wu, Dr. H. Neumann, Prof. Dr. M. Beller
Leibniz-Institut fꢀr Katalyse e.V.
Albert-Einstein-Strasse 29a, 18059 Rostock (Germany)
Fax: (+49)381-1281-5000
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. We also thank S. Leiminger, K. Mevius, Dr. W. Baumann,
Dr. C. Fischer, and S. Buchholz (LIKAT) for analytical support and S.
Leiminger for technical assistance.
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Chemie
Table 1: Synthesis of chalcone: Variation of the reaction conditions.[a]
Table 2: Reaction of phenyl triflate with carbon monoxide and different
styrene derivatives.[a]
Entry
Ligand
(2 mol%)
Base
(2 equiv)
Solvent
(0.5 mL)
Yield
[%][b]
Entry
1
Styrene
Chalcone
Yield [%][b]
71, 68[c]
1
2
pyridine
pyridine
DMF
DMF
4
8
3
4
5
6
7
diop
dppp
dppf
dpppe
dppp
pyridine
pyridine
pyridine
pyridine
NEt3
DMF
DMF
DMF
DMF
DMF
5
12
<1
4
2
72
65
3
4
5
6
95
92
74
88
8
dppp
DMF
63
54
9
dppp
DMF
10
11
12
13
14
15
16
17
18
19
dppp
dppp
dppp
dppp
dppp
dppp
dppp
dppp
dppp
dppp
DABCO (1 equiv)
DMAP (1 equiv)
NEt3
NEt3
NEt3
NEt3
NEt3
NEt3
NEt3
DMF
DMF
NMP
toluene
dioxane
toluene
toluene
toluene
toluene
toluene
50
2
67
75
67
[a] Phenyl triflate (1 mmol), alkene (6 mmol), CO (10 bar), [{(cinna-
myl)PdCl}2] (1 mol%), dppp (2 mol%), NEt3 (2 mmol), toluene
(0.5 mL), 1008C, 20 h. [b] Yield of isolated product. [c] 10 mmol scale.
66[c]
64[d]
72[e]
59[f]
66[g]
NEt3
[a] Phenyl triflate (1 mmol), styrene (6 mmol), CO (10 bar), [{(cinna-
myl)PdCl}2] (1 mol%), ligand (2 mol%), base (2 mmol), solvent
(0.5 mL), 1008C, 20 h. [b] Yields were determined by GC analysis with
hexadecane as the internal standard. [c] [{(cinnamyl)PdCl}2] (0.5 mol%),
dppp (1 mol%). [d] 3 equivalents of styrene. [e] 1.5 equivalents of
styrene. [f] CO (5 bar). [g] CO (2 bar). DABCO=1,4-diazabicyclo-
styrene and carbon monoxide to give the corresponding
chalcone derivatives (Table 3).
Arene compounds substituted with electron-donating or
-withdrawing groups at the para, meta, or even ortho position
gave the desired products in high yields (Table 3, entries 1–9).
Notably, alkenyl triflates—both internal and teminal ones—
reacted and gave moderate to good yields of the respective
dienyl ketones (Table 3, entries 10–17). It is well known that
such products are interesting building blocks, for example, for
Nazarov cyclizations. The required alkenyl triflates are easily
prepared from the corresponding ketones or aldehydes.[15]
Hence, our method represents a straightforward carbonyl
extension strategy.
With respect to the reaction mechanism, this novel
carbonylative Heck reaction should proceed through oxida-
tive addition of the aryl triflate to the Pd0 center, migration of
CO to give the acyl palladium complex, insertion of styrene
and final b-hydride elimination. In most of the cases, only a
small amount (< 5%) of the Heck coupling product was
detected. However, in the case of sterically hindered sub-
strates the amount of this product increased (Table 3,
entry 2).
[2.2.2]octane,
diop=4,5-bis(diphenylphosphinomethyl)-2,2-dimethyl-
1,3-dioxolane, DMAP=4-dimethylaminopyridine, DMF=N,N-dimethyl-
formamide dppf=1,1’-bis(diphenylphosphanyl)ferrocene, dpppe=1,5-
bis(diphenylphosphino)pentane, NMP=N-methyl-2-pyrrolidone.
Notably, when we decreased the catalyst loading to 0.5 mol%
or lowered the amount of styrene to 1.5 equivalents, the yield
was only slightly decreased (Table 1, entries 15–17). Advan-
tageously, the reaction can be performed at a relatively low
pressure of CO (2–5 bar) without significant decrease of the
yield (Table 1, entries 18–19).
Once optimized reaction conditions were identified, we
investigated the influence of different styrene derivatives in
this reaction. To our delight the novel palladium-catalyzed
carbonylative vinylation took place with both electron-
donating and electron-accepting substituents and gave good
to very good yields (71–95% yield of isolated product;
Table 2, entries 1–6). Notably, reactions can be performed on
a 10 mmol scale without a decrease of the yield. Next, the
scope and limitations with respect to the aryl triflate were
explored.[14] Different aryl triflate derivatives, including
heteroaromatic compounds were coupled smoothly with
Finally, we demonstrated that activation of phenol and
subsequent palladium-catalyzed coupling reaction can be
conveniently performed in a “one-pot” manner. Without any
optimization, phenyl nonaflate was prepared at room temper-
ature in the presence of triethylamine, followed by addition of
the palladium precatalyst, the ligand, and further triethyl-
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Table 3: Reaction of different aryl and alkenyl triflate derivatives with CO and styrene derivatives.[a]
Entry
1
Triflate
Chalcone
Yield [%][b]
80
Entry
10
Triflate
Chalcone
Yield [%][b]
86
2
3
68
82
11
12
80
71
4
79
13
46
5
6
7
8
9
80
70
75
73
86
14
15
16
17
56
63
54
40
[a] Aryl or alkenyl triflate (1 mmol), styrene (6 mmol), CO (10 bar), [{(cinnamyl)PdCl}2] (1 mol%), dppp (2 mol%), NEt3 (2 mmol), toluene (0.5 mL),
1008C, 20 h. [b] Yield of isolated product.
Experimental Section
Synthesis of chalcone: The reaction was carried out in a Parr
Instruments 4560 series 300 mL autoclave containing an alloy plate
with wells for six 4 mL glass vials. [{(cinnamyl)PdCl}2] (1 mol%,
5.2 mg), dppp (2 mol%, 8.3 mg), and a magnetic stir bar were placed
in each of the vials, which were then capped with a septum equipped
Scheme 3. “One-pot” Heck carbonylation of phenol.
with an inlet needle and flushed with argon. Then, phenyl triflate
(1 mmol, 0.162 mL), styrene (6 mmol, 0.69 mL), NEt3 (2 mmol,
0.28 mL), and toluene (0.5 mL) were added to each vial with a
syringe. The vials were placed in an alloy plate, which was then placed
in the autoclave. Once sealed, the autoclave was purged several times
with CO, then pressurized to 10 bar at RT and heated at 1008C for
20 h. The autoclave was then cooled to RTand vented to discharge the
excess CO. Water (2 mL) was added, and the product was extracted
with diethyl ether (3 ꢀ 3 mL). The organic layers were washed with
brine, dried over Na2SO4, and evaporated with adsorption onto silica
gel. The crude product was purified by column chromatography on
silica gel (eluent: heptane/EtOAc = 40:1) to give the title compound
(148 mg, 71%) as a yellow solid. 1H NMR (300 MHz, CDCl3): d =
7.29–7.37 (m, 3H), 7.38–7.43 (m, 1H), 7.46 (d, 1H, J = 15.89 Hz),
7.44–7.47 (m, 1H), 7.49–7.61 (m, 3H), 7.58 (d, 1H, J = 15.66 Hz),
7.91–7.98 ppm (m, 2H). 13C NMR (75 MHz, CDCl3): d = 122.03,
amine. After heating for 20 hours, chalcone was obtained in
50% overall yield (Scheme 3).
In conclusion, we have developed the first carbonylative
Heck reactions of aryl and alkenyl triflates and aromatic
olefins. This method represents a “missing link” between the
already established carbonylative Suzuki and carbonylative
Sonogashira reactions. Starting from easily available aryl and
alkenyl triflates the corresponding unsaturated ketones are
obtained in generally good yields. The products obtained
represent useful building blocks for the synthesis of numerous
biologically active compounds.
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128.42, 128.47, 128.60, 128.93, 130.52, 132.76, 134.84, 138.17, 144.82,
190.52 ppm. GC-MS (EI, 70 eV): m/z(%) 208(M+, 75), 207(100),
179(20), 131(25), 103(15), 77(35). HRMS (EI): m/z calcd for C15H11O
776; d) J.-J. Brunet, R. Chauvin, Chem. Soc. Rev. 1995, 24, 89 –
95.
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À
(M H): 207.0804; found: 207.07984.
Received: April 12, 2010
Published online: June 22, 2010
Keywords: aryl triflates · carbonylation · ketones · palladium ·
.
vinylation
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Angew. Chem. Int. Ed. 2010, 49, 5284 –5288