Palladium-catalyzed carbonylative C-H activation of heteroarenes

Article abstract of DOI:10.1002/anie.201003895

Not only carbonylation: The first carbonylative cross-coupling reactions towards ketones using C-H activation have been developed. Various heteroarenes, such as oxazoles, thiazoles, and imidazoles were used as coupling partners in this methodology. DBU =

Full text of DOI:10.1002/anie.201003895

Communications
DOI: 10.1002/anie.201003895
À
C H Activation
À
Palladium-Catalyzed Carbonylative C H Activation of
Heteroarenes**
Xiao-Feng Wu, Pazhamalai Anbarasan, Helfried Neumann, and Matthias Beller*
À
Transition metal catalyzed C H functionalization reactions of
arenes and heteroarenes are finding increasing application in
the preparation of organic building blocks and therapeutically
important scaffolds.[1] Notably, these methods can avoid the
use of stoichiometric amounts of organometallic reagents
along with any problems associated with their synthesis,
stability, and/or functional group compatibility. Recent prom-
inent examples include: transition metal (Rh, Pd, Ru, Ni, Cu)
catalyzed arylation,[2a–e,i] alkylation,[2f,j] alkynylation,[2g,h] alke-
nylation,[2i,j] and benzylation[2i–k] of (hetero)arenes. In this
context, the related carbonylative coupling reactions of
new and improved route to the desired di(hetero)aryl ketones
À
by C H functionalization (Scheme 1).
Owing to the synthetic importance of di-(hetero)aryl
ketones,[4] and based on our continuing interest in carbon-
ylative coupling reactions,[6d–i,11] herein we disclose the first
carbonylative cross-coupling of aryl iodides with non-preac-
tivated heteroarenes to give a variety of di-(hetero)aryl
ketones.
Initially, we investigated the carbonylative cross-coupling
of iodobenzene 1 and benzoxazole 2 as a benchmark reaction
to give 2-(benzoyl)benzoxazole 3 (Table 1). In a preliminary
À
(hetero)arenes using C H functionalization to afford carbox-
ylic acid derivatives have been scarcely studied, and previous
systems have been limited to chelation-assisted intramolec-
ular reactions.[3] In particular, the apparently simply synthesis
of (hetero)aryl ketones from nonchelating substrates through
an intermolecular carbonylative coupling reaction has not yet
been reported.
À
Table 1: Palladium-catalyzed carbonylative C H functionalization of
benzoxazole.[a]
Among the various ways for the synthesis of (hetero)aryl
ketones that have been developed,[4] the palladium-catalyzed
carbonylative coupling reactions of aryl halides with organ-
ometallic reagents has gained recent interest in modern
organic synthesis (Scheme 1).[5,6] Typical organometallic
Entry
[{Pd}2]
[mol%]
Ligand (mol%) Yield of 3 [%][b] Yield of 4 [%][b]
1
2
3
4
5
6
2.5
2.5
2.5
2.5
2.5
2.5
2.5
5
PPh3 (10)
BuPAd2 (10)
dppp (5)
12
22
27
12
21
23
31
51
58
73
54
43
33
39
20
5
dppe (5)
dppb (5)
dpppe (5)
dppp (5)
dppp (10)
dppp (10)
dppp (10)
dppp (10)
12
11
3
0
0
7[c]
8[c]
9[c,d]
10[d,e]
11[d,e,f]
Scheme 1. Strategies for palladium-catalyzed diarylketone syntheses.
5
5
5
[8]
M=B(OH)2[11], SnBu3[10], SiR3[9], AlR2
.
5
reagents employed in such reactions are organoaluminium,[7]
organosilane,[8] and organotin compounds,[9] as well as aryl
boronic acids.[10] Clearly, the direct carbonylative coupling of
aryl halides with easily available heteroarenes would offer a
[a] [{(cinnamyl)PdCl}2], ligand, DMF (2 mL), DBU (1 mmol), CuI
(1.2 equiv), iodobenzene (1 mmol), benzoxazole (1.2 equiv), CO
(10 bar), 1208C, 20 h. [b] Yields were determined by GC, using
hexadecane as internal standard. [c] CO (30 bar). [d] 1.5 equiv of
benzoxazole. [e] CO (40 bar), 30 h. [f] CO (50 bar). DMF=N,N-dime-
thylformamide, Ad=adamantyl, dppp=1,3-bis(diphenylphosphanyl)-
propane, dppe=1,2-bis(diphenylphophanyl)ethane, dppb=1,4-bis(di-
phenylphosphanyl)butane,
tane.
dpppe=1,5-bis(diphenylphosphanyl)pen-
[*] X.-F. Wu, Dr. P. Anbarasan, 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
variation of palladium precursors and ligands, a simple
catalytic system consisting of [{(cinnamyl)PdCl}2]/PPh3 in
the presence of CuI[12] under 10 bar of CO pressure at 1208C
yielded the desired ketone 3 in 12% yield. This result
demonstrated that the target reaction was indeed possible,
however 43% of the noncarbonylative couple product 4 was
formed as the major product (Table 1, entry 1).
[**] We thank the state of Mecklenburg-Vorpommern and the Bundes-
ministerium fꢀr Bildung und Forschung (BMBF) and Alexander-
von-Humboldt-Stiftung for financial support. We also thank
Dr. W. Baumann, Dr. C. Fisher, and Dr. S. Buchholz (LIKAT) for
analytical support and S. Leiminger for technical assistance.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201003895.
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Table 2: Palladium-catalyzed carbonylative coupling of benzoxazole with
different aryl iodides.[a]
By testing different ligands, it turned out that chelating
bidentate phosphines gave better results compared to mono-
dendate ligands (Table 1, entries 2–6). Changing the bite
angle of the bidentate ligand showed a significant influence
on the outcome of the reaction. In the presence of 1,3-
bisdiphenylphosphinopropane (dppp) ketone 3 was obtained
in 27% yield (Table 1, entry 3). When increasing the CO
pressure to 30 bar, the yield of 3 increased slightly (Table 1,
entry 7). Addition of an excess (1.5 equiv) of benzoxazole
further improved the yield of 3 to 58%. Notably, the
chemoselectivity also improved considerably (Table 1,
entry 9). Finally, the best result was obtained at 40 bar CO
pressure to give the target compound in 73% yield with
excellent selectivity (Table 1, entry 10). Notably, at higher
pressure, the yield and chemoselectivity decreased (Table 1,
entry 11).
With acceptable conditions in hand (Table 1, entry 10), we
tested the scope of this methodology for different aryl iodides
(Table 2) and heteroarenes (Table 3). As shown in Table 2,
ortho-, meta-, and para-substituted aryl iodides worked well in
the present system to give moderate to good yields of 2-
aroylbenzoxazoles (40–67%; Table 2, entries 2–4). Similarly,
ethyl- and tert-butyl-substituted iodobenzenes led to their
corresponding ketones in 63% and 54% yield, respectively
(Table 2, entries 5 and 6). In addition, both electron-donating
and electron-withdrawing substituents on the arene ring were
Entry
1
Aryl iodide
Ketone
Yield [%][b]
70
2
3
4
67[c]
40[d]
55[c]
5
6
63[c]
À
well-tolerated in the carbonylative C H functionalization
54[c]
reaction and their corresponding ketones were isolated in
good yield (Table 2, entries 7–10). Heterocyclic iodide 3-
iodothiophene also underwent a smooth reaction to furnish
the corresponding di-(heteroaryl) ketone in 62% yield
(Table 2, entry 11).
7
75[c]
72[c]
45
After demonstrating the general applicability of this
À
procedure for aryl iodides, we explored the scope of the C
H-activation substrates. We chose 4-iodoanisole as the
coupling partner for the carbonylative coupling with various
other heteroarenes (Table 3). First, oxazoles and benzoxa-
zoles with different substitution patterns were tested. Encour-
agingly, yields of 60–70% were obtained in these carbon-
8
9[e]
À
ylative C H functionalizations (Table 3, entries 1–3). Simi-
larly, commercially available thiazole and substituted thia-
zoles as well as benzothiazole were effectively converted into
their corresponding ketones in 65-71% yield (Table 3,
entries 4–6). Notably, even imidazole, an important family
of heteroarenes, was also activated under these conditions
(Table 3, entry 7). On the other hand, the reaction of
bromobenzene with benzoxazole under the optimized con-
ditions for aryl iodides gave only 5% of the desired product
together with starting material and iodobenzene.
10
62
11[e]
62[d]
À
Taking previous reports on noncarbonylative C H-func-
[a] [{(cinnamyl)PdCl}2] (5 mol%), dppp (10 mol%), DMF (2 mL), DBU
(1 mmol), CuI (1.5 equiv), aryl iodide (1 mmol), benzoxazole (1.5 equiv),
CO (40 bar), 1208C, 30 h. [b] Yield of isolated product. [c] 1308C.
[d] 1008C, 40 h. [e] 5–10% of the noncarbonylative coupling product was
obtained.
tionalization into account, we proposed the following reaction
mechanism for this novel reaction (Scheme 2). It is well
known that oxidative addition of iodobenzene onto a ligated
palladium(0) species generates the arylpalladium(II) complex
I, which produces the key arylpalladium(II) complex II
following CO insertion.[5] Next, complex III can be formed
by transmetalation with the preformed copper/arene species
resulting from the heteroarene.[13] Reductive elimination of
the ketone from complex III completes the catalytic cycle and
regenerates the active palladium species. In some cases the
noncarbonylative coupling product was observed as a side-
product, thereby demonstrating that even at 40 bar of CO,
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Communications
Table 3: Palladium-catalyzed carbonylative coupling of heteroarenes.[a]
Entry
1
Heteroarene
Ketone
Yield [%][b]
65
2[c]
70
3[c]
60
66
65
71
56
Scheme 2. Proposed reaction mechanism. DBU=1,8-diazabicyclo-
[5.4.0]undec-7-ene.
4
reactions. Further extension of this chemistry towards other
aryl halides and heteroarenes is currently underway.
5[c]
6[c]
7[c]
Experimental Section
3: The reaction was carried out in a Parr Instruments 4560 series
300 mL autoclave containing an alloy plate with wells for four 12 mL
Wheaton vials. [{(cinnamyl)PdCl}2] (5 mol%), dppp (10 mol%), CuI
(1.5 equiv), benzoxazole (1.5 equiv) and a magnetic stirrer bar were
placed in each of the vials, which were then capped with a septum
equipped with an inlet needle and flushed with argon. Then,
iodobenzene (1 mmol), DBU (1 mmol), and DMF (2 mL) were
added to a 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 40 bar at room
temperature and heated at 1208C for 30 h. It was then cooled to room
temperature and vented to discharge the excess CO. Water (2 mL)
was added, and the product was extracted with ethyl acetate (3 ꢀ
3 mL). The organic layers were washed with brine, dried over Na2SO4,
and evaporated to yield the crude product. Purification by column
chromatography on silica gel (eluent: heptane/EtOAc = 100:0–40:1)
gave the title compound (156 mg, 70%) as a white solid.
[a] [{(cinnamyl)PdCl}2] (5 mol%), dppp (10 mol%), DMF (2 mL), DBU
(1 mmol), CuI (1.5 equiv), 4-iodoanisole (1 mmol), heteroarene
(1.5 equiv), CO (40 bar), 1308C, 30 h. [b] Yield of isolated product.
[c] 5–10% of the noncarbonylative coupling product was obtained.
transmetalation of the heteroaryl copper intermediate is
competitive with the carbonylation of the aryl palladium
species.[14] Notably, the palladium-catalyzed reaction of ben-
zoxazole with benzoyl chloride in the absence of CO
exclusively gave 2-phenylbenzoxazole as the decarbonylative
coupling product. This result also shows the reversibility of
the carbonylation step.
Received: June 27, 2010
Published online: August 25, 2010
À
Keywords: carbonylation · C H activation · heterocycles ·
ketones · palladium
À
In conclusion, the first carbonylative C H activation
.
reactions of heteroarenes to form diaryl ketones have been
developed. Applying various aryl iodides and different
heteroarenes, such as oxazoles, thiazoles, and imidazole in
the presence of a palladium/copper system affords the
À
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Chemie
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this unwanted side-reaction.
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