A selective palladium-catalyzed carbonylative arylation of aryl ketones to give vinylbenzoate compounds

Article abstract of DOI:10.1002/chem.201202895

Preparation of enols: When treated with [{Pd(cinnamyl)Cl} ₂]/cataCXium A (nBuPAd₂, Ad=adamantyl) under an atmosphere of CO, aryl ketones react with aryl halides in a carbonylative C-O coupling reaction to form (Z)-vinyl benzoates (see scheme). Copyright

Full text of DOI:10.1002/chem.201202895

DOI: 10.1002/chem.201202895
A Selective Palladium-Catalyzed Carbonylative Arylation of Aryl Ketones to
Give Vinylbenzoate Compounds
Johannes Schranck,[a] Anis Tlili,[a] Helfried Neumann,[a] Pamela G. Alsabeh,[b]
Mark Stradiotto,[b] and Matthias Beller*[a]
The palladium-catalyzed (hetero)arylation of esters, ke-
tones, and related CH-acidic substrates has become an inter-
esting topic in organic synthesis in the last two decades.[1]
Different catalysts are available for this type of transforma-
tion and several synthetic applications have been described
since the appearance in the literature of seminal investiga-
tions by the research groups of Miura, Buchwald, and Hart-
propanations,[7] cycloaddition procedures,[8] asymmetric hy-
drogenations,[9] the preparation of enamides,[10] hydrofor
myl-
ation reactions;[11] furthermore, they can be used as aryl
ACHTUNGTRENNUNG
AHCTUNGTRENNUNG
halide equivalents.[12] However, the use of vinyl benzoates
has rarely been investigated; in particular, they have only
been used in enantioselective cyclopropanation reactions.[13]
Typically, the synthesis of substituted vinyl benzoates in-
volves the reaction between preformed O-trimethylsilyl enol
ethers and highly active benzoyl derivatives such as benzoyl
chloride, benzoyl fluoride, and benzoic anhydride.[14] This
method has low synthetic value because it involves a two-
step sequence and uses benzoyl compounds, which have lim-
ited availability. A straightforward method for preparing
vinyl benzoates is either the palladium- or ruthenium-cata-
lyzed addition of carboxylic acids to alkynes.[15] Notably, this
method is highly E/Z regioselectivity when terminal alkynes
are used (Scheme 1). However, the use of disubstituted al-
kynes leads to mixtures of isomers.[16] In addition, the iso-
merization of allylic esters has been reported, a transforma-
tion that also leads to mixtures of E and Z enol esters.[17]
Based on our long-standing interest in palladium-cata-
lyzed carbonylation reactions,[18] we recently became inter-
ested in the carbonylative coupling of CH-acidic com-
pounds.[19] In initial studies, and to our surprise, the corre-
sponding O-benzoylation compounds were formed. Owing
to the broad availability of (hetero)aryl halides and ketones,
wig.[2] In all of these coupling reactions new Csp2 Csp3 bonds
À
are formed, thus generating allylic and benzylic carbonyl
compounds.[3] As a recent example, Skrydstrup and co-work-
ers described a palladium-catalyzed carbonylative a-aryla-
tion reaction that gave 1,3-diketones.[4] In contrast to this
À
transformation, which involves C C bond formation, there
are very few reports describing carbonylative arylation reac-
À
tions of ketones and aryl halides that involve C O bond for-
mation.[5] The feasibility of such a reaction has been shown
by Kobayashi and Tanaka. In a single example, they showed
that iodobenzene and deoxybenzoin could be converted into
the corresponding vinylbenzoate by using harsh reaction
conditions (1208C, 20 bar of CO, 60 hours, NEt3 as solvent);
however, the stereoselectivity of the process, that is, the rel-
ative amount of Z- and E-configured product, was not inves-
tigated.[5b] In addition, the research group of Negishi descri-
bed the trapping of acylpalladium intermediates in intramo-
lecular carbonylative cyclization reactions that generate 5-
and 6-membered heterocycles.[6] So far, only three examples
of an intermolecular variant of this reaction has been re-
ported; these examples were limited to more active 1,3-dike-
tones and one example involved the transformation of a
cyclic ketone. Notably, in the case of cyclic starting materi-
als, stereoselectivity plays no role.[6a]
Vinyl acetates are valuable substrates for polymerizations,
are versatile acylating agents, and can be applied in cyclo-
[a] J. Schranck, Dr. A. Tlili, 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
E-mail: Matthias.Beller@catalysis.de
[b] P. G. Alsabeh, Prof. Dr. M. Stradiotto
Department of Chemistry
Dalhousie University
6274 Coburg Road, P.O. Box 15000
Halifax NS B3H 4R2 (Canada)
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201202895.
Scheme 1. Synthesis of vinyl benzoates. TBAF=tetra-n-butylammonium
fluoride.
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Chem. Eur. J. 2012, 18, 15592 – 15597

COMMUNICATION
we believed that using these compounds as starting materi-
als in a direct synthetic route employing carbon monoxide
to give vinyl benzoates would represent a valuable extension
of known procedures; this transformation would be especial-
ly valuable if it could be carried out in a stereoselective
manner.
Next, we tested some other palladium precursors in the
model reaction. [Pd
[{Pd(cinnamyl)Cl}2], yields of 57 and 55%, respectively,
being obtained; however, the use of Pd(OAc)2 did not pro-
mote the reaction to a significant extent (Table 1, entries 6–
8). [Cl2Pd(PPh3)2] and [Ni(cod)2]/dppb, systems that were
2ACHTUGNRTEN(NUNG dba)3] and PdCl2 were as effective as
AHCTUNGTRENNUNG
AHCTUNGTRENNUNG
A
ACHTUNGTRENNUNG
At the start of our work, we studied the reaction of iodo-
benzene with deoxybenzoin under 10 bar of CO in the pres-
ence of [{PdACHTUNGTRENNUNG(cinnamyl)Cl}2] and cataCXium A (nBuPAd2) as
developed by the research group of Negishi for the acylation
of more reactive systems, were tested as catalysts using NEt3
as base. Unfortunately, these catalysts turned out to be only
slightly active in the model reaction (Table 1, entries 9–10).
Moreover, variation of the solvent did not lead to any im-
provement in yield; carrying out the reaction in either 1,4-
dioxane, THF, or DMF led to low yields between 22 and
37% (Table 1, entries 11–14). However, when the concentra-
tion of iodobenzene was increased to 2 mmol, the reaction
temperature could be lowered to 608C, leading to a slight
improvement in the yield (65%; Table 1, entry 14). Finally,
we investigated the effect of the nature of the phosphine
ligand on the reaction using the optimized reaction condi-
tions (Scheme 2).
ligand, which has been shown to provide good catalyst per-
formance in many carbonylation reactions.[20] Because the
role of base was expected to be crucial for activating the
ketone, several bases were tested at the very beginning.
Whereas the desired 1,2-diphenylvinyl benzoate was ob-
served only in 6% yield when K2CO3 was used, the use of
Cs2CO3 afforded 63% of the product (Table 1, entries 1 and
2). Analysis of the product using 2D NMR spectroscopy
(COSY, NOESY, HMBC, and HSQC) revealed the selective
formation of (Z)-1,2-diphenylvinyl benzoate (for details see
the Supporting Information). The use of other bases, such as
NEt3, NaOtBu, and NaHMDS, did not provide any desired
product and only low conversions were observed (Table 1,
entries 3–5).
Table 1. Palladium-catalyzed benzoylation of deoxybenzoin: variation of
reaction conditions.[a]
Entry
[Pd] source
[{Pd(cinnamyl)Cl}2]
[{Pd(cinnamyl)Cl}2]
[{Pd(cinnamyl)Cl}2]
[{Pd(cinnamyl)Cl}2]
[{Pd(cinnamyl)Cl}2]
[Pd2A(dba)3]
Pd(OAc)2
PdCl2
[Cl2Pd
[Ni
(cod)2][e]
[{Pd(cinnamyl)Cl}2]
[{Pd(cinnamyl)Cl}2]
[{Pd(cinnamyl)Cl}2]
[{Pd(cinnamyl)Cl}2]
[{Pd(cinnamyl)Cl}2]
[{Pd(cinnamyl)Cl}2]
[{Pd(cinnamyl)Cl}2]
Base
Solvent
Yield [%][b]
Scheme 2. Variation of ligands. General reaction conditions: iodobenzene
(1.5 mmol), deoxybenzoin (1 mmol), [{Pd
ligand (6 mol%), Cs2CO3 (2 mmol), MeCN (1 mL), CO (10 bar), 808C,
20 h. Ad=adamantyl, dppf=1,1’-bis(diphenylphosphino)ferrocene,
ACHTUNGTRENUN(GN cinnamyl)Cl}2] (1.5 mol%),
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
G
K2CO3
Cs2CO3
NEt3
NaOtBu
NaHMDS
Cs2CO3
Cs2CO3
Cs2CO3
NEt3
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
DMF
dioxane
DMF
THF
MeCN
MeCN
MeCN
MeCN
6
E
61, 30[d]
0
ACHTUNGTRENNUNG
dppp=diphenylphosphinopropane.
N
0
0
57
0
R
R
G
U
The use of simple commercially available mono- and bi-
dentate phosphine ligands including PPh3, PCy3, DPPP, and
DPPF gave around 50% yield (Scheme 2). Considering that
cataCXium A was the optimal ligand thus far, we decided to
try some other di(1-adamantyl) aryl(heteroaryl) phosphines
in the model reaction. The use of imidazole-based ligands
L1 and L2 led to slightly lower yields of 39 and 43%. The
use of Me-DalPhos (L3) led to a low yield of product (9%),
whereas the use of the related P—N ligand, DalPhos (L4),
led to 37% yield.[21] The use of 2,5-dimethylpyrrole-substi-
tuted DalPhos variant L5 resulted in a good yield of 55%.
Nevertheless, cataCXium A remained the best ligand in our
model system.
55
E
E
0, 5[d]
0, 0[d]
22
G
NEt3
N
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
G
35
37
N
T
65[f]
22[g]
96[h]
40[i]
ACHTUNGTRENNUNG
ACHTUNGTRENNUNG
ACHTUNGTRENNUNG
[a] General reaction conditions: iodobenzene (1.5 mmol), deoxybenzoin
(1 mmol), [{Pd(cinnamyl)Cl}2] (1.5 mol%), cataCXium A (6 mol%),
Cs2CO3 (2 mmol), solvent (1 mL), CO (10 bar), 808C, 20 h. [b] Conver-
sion and yield were determined by GC using hexadecane as the internal
standard. [c] No additional ligand. [d] 1008C, CO (40 bar). [e] [NiACTHNUTRGEN(UNG cod)2]
ACHTUNGTRENNUNG
(5 mol%), dppb (10 mol%). [f] Iodobenzene (2 mmol), 608C. [g] Iodo-
benzene (2 mmol), Cs2CO3 (2 mmol), MeCN (2 mL), 408C. [h] Iodo-
ACHTUNGTRENNUNG
When a lower pressure of CO (2 and 5 bar) was used, de-
creased conversions and yields were obtained. Further varia-
tion of the reaction conditions such as substrate concentra-
tion and reaction temperature led to full conversion of sub-
strate and a 96% yield of the desired product (Table 1,
ACHTUNGTRENNUNG
dba=trans,trans-dibenzylideneacetone, dppb=diphenylphosphinobutane,
NaHMDS=sodium hexamethyldisilazide.
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M. Beller et al.
Table 2. Pd-catalyzed carbonylative coupling of aryl iodides with deoxy-
benzoin.[a]
entry 16). Notably, in all optimization reactions no side
products derived from deoxybenzoin were detected. Howev-
er, varying amounts of benzoic acid and benzaldehyde were
observed, especially in cases where high conversions and
low yields were observed, thus showing that side reactions
involving iodobenzene are the primary competing pathways
that affect the formation of the desired product.
Entry[a]
ArI
Product
Yield [%][b]
With optimized reaction conditions established, we turned
our focus to the scope and limitations of the reaction
(Table 2). On a preparative scale the model reaction with io-
dobenzene led to the isolation of (Z)-1,2-diphenylvinyl ben-
zoate in 88% yield (Table 2, entry 1).
1
88
Similarly, 2- and 4-iodotoluene were converted into the
corresponding products in good yields of 78 and 73%, re-
spectively (Table 2, entries 2, 4).
Although the use of 3-iodotoluene led to relatively low
conversion, the separation of the corresponding product
from the remaining deoxybenzoin by using column chroma-
tography proved to be difficult owing to their similar Rf
values. Therefore, a lower yield (55%) of the pure product
was obtained (Table 2, entry 3).
2
3
73
55
The presence of both electron-donating and electron-with-
drawing substituents on the aryl iodide substrates were tol-
erated and there is no obvious relationship between the
nature of the substituent on the aryl ring and yield. For ex-
ample, the use of 2-iodoanisole gave a yield of 55%, where-
as the product of 4-iodoanisole was obtained in 70% yield
(Table 2, entries 5–6). On the other hand, 4-fluoroiodoben-
zene resulted in the vinyl ester being isolated in 74% yield
(Table 2, entry 7) and the more electron-deficient trifluoro-
methyl-substituted products were obtained in yields of 57
and 44%, respectively (Table 2, entries 8 and 9). 2- and 3-Io-
dothiophene were converted into the corresponding vinyl
esters in 50 and 77% yield, respectively (Table 2, entries 10–
11).
We then examined the scope of the ketone coupling part-
ners. Using 1-phenyl-2-p-tolylethanone, the corresponding
product was isolated in 75% yield (Table 3, entry 2). In ad-
dition, benzoylation of chloro- as well as bromo-substituted
deoxybenzoin substrates proceeded smoothly and gave the
corresponding products in 66 and 52% yield, respectively
(Table 3, entries 3 and 4). To our delight, the use of 1,3-di-
phenyl-2-propanone gave the corresponding product in 86%
yield (Table 3, entry 5). The above results show that ketones
that are substituted with an aryl ring at the 2-position are
good substrates. Unfortunately, simple aliphatic ketones did
not react under the optimized reaction conditions, perhaps
due to inefficient enolate formation. However, the more ac-
tivated 2-methoxy-1-phenylethanone was applied successful-
ly in our novel procedure leading to the respective vinyl
ester in 69% (Table 3, entry 6).
4
5
6
7
8
78
70
55
74
44
9
57
77
Finally, we were interested in using aryl bromides instead
of the more expensive aryl iodides for the transformation.
To observe significant reactivity, the reaction temperature
had to be increased to 1208C, which afforded 43% of (Z)-
1,2-diphenylvinyl benzoate. Although, the use of other li-
gands did not result in any improvements, changing the sol-
10
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Chem. Eur. J. 2012, 18, 15592 – 15597

Carbonylative Arylation of Aryl Ketones
Table 2. (Continued)
COMMUNICATION
Table 4. Pd-catalyzed carbonylative coupling of aryl bromides with de-
oxybenzoin.[a]
Entry[a]
ArI
Product
Yield [%][b]
11
50
Entry[a]
Aryl bromide
Product
Yield [%][b]
[a] General reaction conditions: aryliodide (2 mmol), deoxybenzoin
(1 mmol), [{Pd(cinnamyl)Cl}2] (1.5 mol%), cataCXium A (6 mol%),
ACHTUNGTRENNUNG
Cs2CO3 (2 mmol), MeCN (2 mL), CO (10 bar), 608C, 20 h. [b] Yield of
isolated product.
1
63
Table 3. Pd-catalyzed carbonylative coupling of iodobenzene with differ-
ent ketones.[a]
2
73
Entry[a] Ketone
Product
Yield [%][b]
3
4
71
51
1
88
2
3
75
66
5
6
49
73
4
5
6
52
86
69
[a] General reaction conditions: aryl bromide (2 mmol), deoxybenzoin
(1 mmol), [{Pd(cinnamyl)Cl}]2 (1.5 mol%), cataCXium A (6 mol%),
Cs2CO3 (2 mmol), THF (2 mL), CO (10 bar), 1208C, 20 h. [b] Yield of
AHCTUNGTRENNUNG
isolated product.
aryl bromides were transformed; 3-bromotoluene and 1-
bromo-4-tert-butylbenzene gave the corresponding vinylben-
zoates in yields of 73 and 71% (Table 4, entries 2 and 3). In
contrast, the use of aryl bromides bearing methoxy substitu-
ents led to slightly decreased yields (ca. 50%; Table 4, en-
tries 4 and 5). In addition, the reaction of 3-bromothiophene
showed that also heterocyclic compounds could be applied
successfully (Table 4, entry 6).
In conclusion, we have developed general reaction condi-
tions for the carbonylative coupling of aryl halides with ke-
tones to give vinyl benzoates. In contrast to previous proto-
cols involving palladium enolates generated from benzylic
ketones, the reaction conditions developed herein lead to se-
[a] General reaction conditions: iodobenzene (2 mmol), ketone (1 mmol),
[{Pd(cinnamyl)Cl}]2 (1.5 mol%), cataCXium A (6 mol%), Cs2CO3
(2 mmol), MeCN (2 mL), CO (10 bar), 608C, 20 h. [b] Yield of isolated
ACHTUNGTRENNUNG
product.
vent to THF resulted in good conversion of deoxybenzoin
and 63% yield of the corresponding vinyl benzoate
(Table 4, entry 1). Under these reaction conditions, several
À
lective C O bond formation. A total of 18 vinyl benzoates
were synthesized in 44–88% yield in a straightforward
Chem. Eur. J. 2012, 18, 15592 – 15597
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M. Beller et al.
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Experimental Section
General information: All reactions were performed using standard
Schlenk techniques (argon). Gas chromatography was performed on a
Hewlett Packard HP 6890N chromatograph with an HP5 column. Chemi-
cals were purchased from either Fluka, Aldrich, or Strem and were used
as received. All solvents were distilled (MeCN and DMF from P2O5, di-
oxane and THF fom sodium ketyl, and NEt3 from CaH2) and stored in
Aldrich Sure/store flasks under argon.
Synthesis of (Z)-1,2-diphenylvinyl benzoate: To each of six 4-mL glass
vials were added [{PdACHTUNGTRENNUNG(cinnamyl)Cl}2] (1.5 mol%, 7.8 mg), cataCXium A
(6 mol%, 21.5 mg), deoxybenzoin (1 mmol, 196.2 mg), Cs2CO3 (2 mmol,
651.6 mg), and a stirring bar. The vials were put into an alloy plate,
equipped with a septum and an inlet needle and then flushed with argon.
Acetonitrile (2 mL) and iodobenzene (2 mmol, 227 mL) were injected in
each vial. The alloy plate with the six vials was then placed in a 300 mL
autoclave (Parr Instruments 4560 series). At RT the autoclave was flush-
ed with CO and was then pressurized with CO to 10 bar. Afterwards, the
autoclave was heated to 608C for 20 h and cooled to RT; the remaining
CO and nitrogen gases were released slowly. After their removal from
the autoclave, 2 mL of water were added to each vial and the product
was extracted with ethyl acetate (3ꢃ5 mL). After drying the combined
organic phases over sodium sulphate and evaporating the solvent, the
crude product was purified by column chromatography using heptane/
MTBE (10:1) as eluent to give (Z)-1,2-diphenylvinyl benzoate as a white
solid.
Acknowledgements
We thank the state of Mecklenburg-Vorpommern and the Bundesminis-
terium fꢀr Bildung und Forschung (BMBF) for financial support. We also
thank Dr. W. Baumann, Dr. C. Fischer, and S. Buchholz (LIKAT) for an-
alytical support.
Keywords: carbonylation · enols · homogeneous catalysis ·
palladium · vinyl esters
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Carbonylative Arylation of Aryl Ketones
COMMUNICATION
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Received: August 13, 2012
Revised: September 18, 2012
Published online: November 9, 2012
Chem. Eur. J. 2012, 18, 15592 – 15597
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www.chemeurj.org
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