Pd-catalyzed cross-coupling of aryl carboxylic acids with propiophenones through a combination of decarboxylation and dehydrogenation

Article abstract of DOI:10.1002/chem.201200829

A palladium-catalyzed cross-coupling reaction of aryl carboxylic acids with saturated propiophenones through a combination of decarboxylation and dehydrogenation to form Heck-type products was reported. In a glove box, a 25 mL tube equipped with a stir bar was charged with Pd(OAc)₂, PCy₃, propiophenone, 2-nitrobenzoic acid, Ag₂CO₃ and nBu₄NOAc HOAc. Then, the mixture was heated under nitrogen at 90°C in DMF for 24 h. After cooling down, the crude reaction mixture was analyzed by GC with n-dodecane as an internal standard to obtain 3a in 75% GC yield. Relatively weak bases, such as carboxylate salts, facilitated this reaction and the effect of the bases was a function of their solubility, while strong bases, such as K₃PO₄ and K₂CO₃ shut down the reaction completely. The simultaneous use of carboxylate salts and equimolar carboxylic acids significantly improved the yield of 3a, although the use of acetic acid alone was ineffective for the reaction.

Full text of DOI:10.1002/chem.201200829

DOI: 10.1002/chem.201200829
Pd-Catalyzed Cross-Coupling of Aryl Carboxylic Acids with Propiophenones
through a Combination of Decarboxylation and Dehydrogenation
Jun Zhou, Ge Wu, Min Zhang, Xiaoming Jie, and Weiping Su*[a]
Carboxylic acids are valuable as starting materials in syn-
thetic chemistry, because of their low cost, wide diversity,
and ready availability.[1] Under transition-metal catalysis, or-
ganometallic intermediates, generated by extrusion of CO2
from carboxylic acids, undergo cross-coupling reactions with
other organic compounds, which represents a promising
[2–5]
À
À
strategy to form carbon carbon
and carbon heteroatom
bonds.[6] The development of new decarboxylative cross-cou-
pling reactions is vital for the realization of the potential of
carboxylic acids to serve as versatile starting materials. In
this regard, the combination of two newly emerging fields in
À
chemistry, decarboxylative cross-coupling and C H func-
tionalization, would offer new transformations and expand
the application of carboxylic acids in organic synthesis.
Figure 1. Examples illustrating the importance of chalcones.
Since the decarboxylative Heck-type cross-coupling of
benzoic acids with olefins was reported by Myers and co-
workers, progress has been made toward improving the effi-
ciency and practicability of this reaction and expanding its
substrate scope.[3] Consequently, this decarboxylative Heck-
type reaction has been applied to a variety of olefins, includ-
ing unactivated alkyl-substituted olefins and cyclic a,b-unsa-
turated ketones. However, no examples of using acyclic aryl
vinyl ketones in decarboxylative Heck-type reactions have
been reported, in part, because these olefins are not com-
mercially available.[9] Usually, the preparation of a,b-unsatu-
rated carbonyl compounds starts from the corresponding sa-
turated carbonyl compounds and requires two or more syn-
thetic steps, such as a PdII-mediated dehydrosilylation of
silyl enol ethers (Saegusa oxidation)[10] and a halogenation–
dehalogenation sequence.[11] The Pd-catalyzed aerobic oxi-
dative dehydrogenation of ketones represents a promising
method for the facile synthesis of a,b-unsaturated ketones,
but is still limited to cyclic enones or b-arylated enones.[12]
Herein, we report a palladium-catalyzed cross-coupling
reaction of aryl carboxylic acids with saturated propiophe-
nones through a combination of decarboxylation and dehy-
drogenation to form Heck-type products [Eq. (1)], and we
also present a one-pot procedure involving this reaction that
enables the facile synthesis of quinoline derivatives from 2-
nitrobenzoic acids. Our findings not only provide a comple-
mentary method for the synthesis of chalcones, which are an
important class of biologically active compounds[7] (Figure 1,
A) and synthetically useful intermediates in the formation
of complex compounds, including natural products[8]
(Figure 1, B), but also demonstrate that the in situ dehydro-
genation of saturated substrates to the corresponding olefins
is a potentially useful strategy to streamline organic synthe-
sis.
À
Our interest in decarboxylative cross-coupling and C H
[a] J. Zhou, G. Wu, M. Zhang, X. Jie, Prof. W. Su
State Key Laboratory of Structural Chemistry
Key Laboratory of Coal to Ethylene Glycol and
Its Related Technology
functionalization led us to consider whether the in situ dehy-
drogenation of saturated ketones to enones can be coupled
with the decarboxylative cross-coupling process to form the
Heck-type product. The realization of such a sequence in
a tandem fashion would offer a straightforward and highly
efficient synthetic method, because of the avoidance of the
troublesome synthesis of enones. Although extremely
rare,[13] the coupling of an in situ formed dehydrogenated re-
active intermediate with a second reaction process proved
Fujian Institute of Research on the Structure of Matter
Chinese Academy of Sciences
Yangqiao west road 155#, Fuzhou, Fujian 350002 (P. R. China)
Fax : (+86)591-83771575
E-mail: wpsu@fjirsm.ac.cn
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201200829.
8032
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Chem. Eur. J. 2012, 18, 8032 – 8036

COMMUNICATION
to be a viable approach for the rapid construction of com-
plex molecules from simple starting materials.
vephos (Davephos=2-Dicyclohexylphosphino-2’-(N,N-dime-
thylamino)biphenyl) and Xphos (Xphos=2-Dicyclohexyl-
phosphino-2’,4’,6’-triisopropylbiphenyl), and sterically bulky
tBu3P, were used as supporting ligands, no reaction occurred
(entries 17–21). Less sterically hindered phosphines, such as
PCp3·HBF4 (Cp=cyclopentyl) and iPr3P·HBF4, were ob-
served to be as efficient as PCy3 (Cy=cyclohexyl; entries 22
and 23).
We initiated our investigation by studying the reaction of
propiophenone (1a) with 2-nitrobenzoic acid (2a) under
a variety of reaction parameters (Table 1). In the presence
of Ag2CO3, additional bases were observed to have an effect
on the reaction outcome depending on the nature of the
base (entries 4–10), presumably, because bases play a key
role in the formation of the Pd-enolate intermediate that is
supposed to be the precursor to enones. Relatively weak
bases, such as carboxylate salts, facilitated this reaction and
the effect of the bases was a function of their solubility (en-
tries 4–7), while strong bases, such as K3PO4 and K2CO3,
shut down the reaction completely. The simultaneous use of
carboxylate salts (2 equiv) and equimolar carboxylic acids
significantly improved the yield of 3a (entry 11 vs. 5 and
entry 13 vs. 7), although the use of acetic acid alone was in-
effective for the reaction. The complex of tetrabutylammo-
nium acetate and acetic acid gave a similar result (entry 16).
The beneficial effect of acetic acid may stem from the acid-
After identifying the factors influencing the reaction out-
come, we next evaluated the substrate scope of this reaction
with respect to aromatic carboxylic acids by using the reac-
tion conditions of entry 16 in Table 1 (PdACTHNURTGNENG(U OAc)2 (15 mol%),
PCy3 (30 mol%), Ag2CO3 (2.5 equiv), nBu4NOAc·HOAc
(1.7 equiv), DMF, 908C, 24 h). Considering that the elec-
tronic nature of substituents on the benzene ring have a re-
markable influence on the acidity and decarboxylation rate
of benzoic acids, we reasoned that the reaction conditions
needed to be adjusted for different benzoic acids to achieve
efficient transformation. We were pleased to find that slight
modifications of the standard reaction conditions enabled
the use of a variety of benzoic acids to produce chalcones
(Table 2). 2-Nitrobenzoic acids bearing a variety of addition-
promoted separation of the ion pair [nBu4N] CTHNUGTRNENUG
+A[OAc]À[14] and
the acid-promoted enolization of ketones. The choice of
phosphine ligands was also crucial for achieving this reac-
tion. When phenyl-substituted phosphines, such as PPh3, Da-
Table 2. Scope of aryl carboxylic acids.[a,f,g]
Table 1. Optimization studies for the decarboxylative olefination
reaction.[a,d]
Entry Ligand
Base ([equiv])
Acid ([equiv]) Yield of 3a
[b]
[%]
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
–
–
–
–
–
–
–
–
–
–
–
6
10
12
31
49
66
44
0
PCy3
–
PCy3
PCy3
PCy3
PCy3
PCy3
PCy3
PCy3
PCy3
PCy3
PCy3
PCy3
PCy3
PCy3
PPh3
KOAc (2)
KOAc (2)
PivOk (2)
Me4NOAc (2)
nBu4NOAc
K2CO3 (2)
K3PO4 (2)
Ph3N (2)
PivOk (2)
Me4NOAc (2)
nBu4NOAc (2)
–
nBu4NOAc (1.7) HOAc (1.7)
nBu4NOAc·HOAc (1.7)
nBu4NOAc·HOAc (1.7)
nBu4NOAc·HOAc (1.7)
nBu4NOAc·HOAc (1.7)
nBu4NOAc·HOAc (1.7)
nBu4NOAc·HOAc (1.7)
nBu4NOAc·HOAc (1.7)
nBu4NOAc·HOAc (1.7)
–
–
0
0
PivOH (2)
HOAc (2)
HOAc (2)
HOAc (2)
67
69
73
12
76
75[c]
0
0
0
0
0
[a] Reaction conditions: 1a (0.2 mmol, 1 equiv), 2 (3.0 equiv), Pd
(15 mol%), PCy3 (30 mol%), Ag2CO3 (2.5 equiv), nBu4OAc·HOAc
(1.7 equiv), DMF (1 mL), 908C, 24 h. [b] Cu(OAc)2 (2.5 equiv), DME
ACHTUNGTRENNUNG(OAc)2
AHCTUNGTRENNUNG
(DME=dimethyl ether; 1 mL), 90–1408C. [c] 5-Methyl-2-nitrobenzoic
acid (4 equiv) was used. [d] 808C. [e] CuF2 (2.5 equiv) was used. [f] Isolat-
ed yield. [g] When non-ortho-substituted benzoic acids were used, the
final chalcones were not obtained.
dppe
Davephos
Xphos
tBu3P
PCp3·HBF4
iPrP3·HBF4
al substituents afforded generally good yields. In some cases,
CuACTHNUTRGNE(NUG OAc)2 or CuF2 was found to be more efficient than
79
76
54
Ag2CO3 for the decarboxylative cross-coupling process, es-
pecially, when electron-rich and heterocyclic carboxylic
acids were used.
We further investigated the compatibility of functional
groups on the propiophenones (Table 3). Under the stan-
tBu2MeP·HBF4 nBu4NOAc·HOAc (1.7)
[a] Reaction conditions: 1a (0.2 mmol, 1 equiv), 2a (3.0 equiv), PdACTHNUTRGNE(NUG OAc)2
(15 mol%), ligand (30 mol%), Ag2CO3 (2.5 equiv), DMF (1 mL), 908C,
24 h. [b] GC yield. [c] 71% isolated yield. [d] The a-arylated product was
not detected. dppe=1,2-Bis(diphenylphosphino)ethane.
Chem. Eur. J. 2012, 18, 8032 – 8036
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www.chemeurj.org
8033

W. Su et al.
Table 3. Scope of propiophenones.[a,e]
Table 4. One-pot synthesis of 2-substituted quinoline derivatives by de-
carboxylative cross-coupling and in situ reductive cyclization.[a]
[a] Isolated yield.
cies from the decarboxylative cross-coupling step, 2-substi-
tuted quinolines were obtained as selective hydrogenative
cyclization products in synthetically useful yields after 8 h at
708C. This result differs from the Pd/C, H2 catalyst system,
which affords mainly the 2-substituted 1,2,3,4-tetrahydroqui-
nolines.[8c]
In summary, a new Pd-catalyzed cross-coupling reaction
of carboxylic acids with saturated propiophenones that pro-
ceeds through the combination of decarboxylation and de-
hydrogenation has been developed. This protocol shows
a good functional group tolerance, which can be ascribed to
the identification of adequate bases or acids for different
substrates. Furthermore, a one-pot procedure for the synthe-
sis of 2-substituted quinolines has been established that in-
volves the cross-coupling reaction of carboxylic acids with
saturated propiophenones and a subsequent selective hydro-
genative cyclization process.
[a] Reaction conditions: 1 (0.2 mmol, 1 equiv), 2a (3.0 equiv), Pd
(15 mol%), PCy3 (30 mol%), Ag2CO3 (2.5 equiv), nBu4OAc·HOAc
(1.7 equiv), phenol (15 mol%), DMF (1 mL), 908C, 24 h. [b] KOAc
(1.5 equiv) was used. [c] KOAc (1.5 equiv) was used in the absence of
phenol. [d] KOAc (1.0 equiv) was used in the absence of phenol. [e] Iso-
lated yield.
ACHTUNGERTN(NUNG OAc)2
dard reaction conditions, methylpropiophenone or phenyl-
propiophenone generated the corresponding products in
moderate yields, while only traces of product were detected
when 3-nitropropiophenone was used. These observations
indicated that the substituents on the benzene ring of pro-
piophenones had a profound effect on the cross-coupling re-
action, presumably, because the a-H acidity varies as the
substituents change. In regard of this, adequate acids or
bases needed to be used to obtain satisfying yields. When
adding a catalytic amount of phenol[15] to the reaction
system, methylpropiophenone, phenylpropiophenone, ethyl
naphthyl ketone, 2’-fluoropropiophenone, and 4’-fluoropro-
piophenone underwent the dehydrogenative cross-coupling
reaction with 2-nitrobenzoic acid to afford the desired prod-
ucts in good yields (3n–s and 3u). Replacing
nBu4NOAc·HOAc with less soluble KOAc afforded good
yields for the reactions of propiophenones bearing electron-
withdrawing groups under otherwise identical conditions (3t
and 3v–3g). As such, this reaction is compatible with ester,
cyano, nitro, trifloromethyl, fluoro, chloro, alkenyl, and
phenyl substituents.
Experimental Section
General procedure: In a glove box, a 25 mL tube equipped with a stir
bar was charged with PdACTHNUTRGNEUNG(OAc)2 (0.0068 g, 0.03 mmol, 15 mol%), PCy3
(0.0168 g, 0.06 mmol, 30 mol%), propiophenone (0.0268 g, 0.2 mmol,
1 equiv), 2-nitrobenzoic acid (0.1002 g, 0.6 mmol, 3 equiv), Ag2CO3
(0.1379 g, 0.5 mmol, 2.5 equiv), and nBu4NOAc·HOAc (0.1226 g,
0.34 mmol, 1.7 equiv). Then, the mixture was heated under nitrogen at
908C in DMF (1.0 mL) for 24 h. After cooling down, the crude reaction
mixture was analyzed by GC with n-dodecane as an internal standard to
obtain 3a in 75% GC yield.
Acknowledgements
Financial
support
from
the
973
Program
(2011CB932404,
A one-pot procedure for the synthesis of 2-substituted
quinolines from 2-nitrobenzoic acid with propiophenones
was also developed (Table 4). At the end of the cross-cou-
pling reaction of carboxylic acids with propiophenones, hy-
drogen gas (1 atm) was introduced to the reactor vessel.
Under the coaction of hydrogen gas and the palladium spe-
(2011CBA00501), NSFC (20925102), the Key Project from CAS and NSF
of Fujian Province (2009HZ0005) is greatly appreciated.
Keywords: aryl carboxylic acids
dehydrogenation · palladium · propiophenones
·
decarboxylation
·
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Chem. Eur. J. 2012, 18, 8032 – 8036

Pd-Catalyzed Cross-Coupling of Aryl Carboxylic Acids
COMMUNICATION
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