An atom-economic approach to carboxylic acids via Pd-catalyzed direct addition of formic acid to olefins with acetic anhydride as a co-catalyst

Article abstract of DOI:10.1039/c5ob01180a

An effective Pd-catalyzed hydrocarboxylation of olefins using formic acid with acetic anhydride as a co-catalyst is described. A variety of carboxylic acids are obtained in good yields with high regioselectivities under mild reaction conditions without the use of toxic CO gas.

Full text of DOI:10.1039/c5ob01180a

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An atom-economic approach to carboxylic acids
via Pd-catalyzed direct addition of formic acid to
olefins with acetic anhydride as a co-catalyst†
Cite this: DOI: 10.1039/c5ob01180a
Received 11th June 2015,
Accepted 6th July 2015
a
a
a,b,c
DOI: 10.1039/c5ob01180a
Yang Wang, Wenlong Ren and Yian Shi*
www.rsc.org/obc
8
An effective Pd-catalyzed hydrocarboxylation of olefins using as catalysts at 100–200 °C. HCOOH presumably decomposed
formic acid with acetic anhydride as a co-catalyst is described. A to CO and H
variety of carboxylic acids are obtained in good yields with high conditions.
2
O at elevated temperatures under acidic
8
regioselectivities under mild reaction conditions without the use
Recently, we reported that olefins can be effectively hydro-
of toxic CO gas.
carboxylated with HCOOH and HCOOPh in the presence of a
Pd catalyst, giving a variety of carboxylic acids in good
Carboxylic acids are a class of chemically and biologically
9,10
yields.
Our studies showed that the hydrocarboxylation with
important compounds. They are present in a variety of phar-
HCOOH can also be carried out in the presence of a catalytic
1
maceuticals. Hydrocarboxylation of olefins provides
a
9
amount (20 mol%) of HCOOPh. This process illustrates that
straightforward approach to this class of molecules. Such a
HCOOH can be used directly for the hydrocarboxylation with
HCOOPh being a co-catalyst. During our ongoing efforts to
search for more active phenol-free co-catalysts, we have found
that the hydrocarboxylation of olefins can be achieved using
HCOOH in the presence of a catalytic amount of acetic anhy-
dride under mild conditions (Scheme 2). Very recently, Zhou
and co-workers reported an elegant hydrocarboxylation
2
,3
2
process has been traditionally carried out with CO and H O.
High pressure and high temperature are frequently used for
the reaction (Scheme 1). In addition, CO gas is difficult to
transport, store, and handle due to its toxicity. All these draw-
backs make hydrocarboxylation operationally inconvenient for
studies in laboratories, which calls for the development of
4
–6
effective hydrocarboxylation without using external CO gas.
11
process of alkynes with related catalytic systems. Herein, we
7
Direct addition of readily available HCOOH to olefins would
provide an attractive and atom-economic hydrocarboxylation
process. Thus far, there are only limited reports on hydrocar-
boxylation of olefins using formic acid, mostly with Ir and Rh
wish to report our own studies on the subject.
α-Methylstyrene (1a) was used as the test substrate for
initial studies. 3-Phenylbutanoic acid (2a) was obtained only in
1
5
2
2% yield when 1a was treated with HCOOH (2.0 equiv.),
mol% Pd(OAc) , and 20 mol% PPh in toluene at 70 °C for
4 h (Table 1, entry 1). The yield of 2a increased to 49% when
2
3
the reaction was carried out in the presence of 1.0 equiv. of
Ac O (Table 1, entry 2). Studies showed that the reaction
2
efficiency was highly influenced by the ligand used (Table 1,
entries 3–7). Carboxylic acid was obtained in 94% yield with
2
5
.0 equiv. of HCOOH and 1.0 equiv. of Ac
2
O in the presence of
Scheme 1 Traditional hydrocarboxylation with CO.
mol% Pd(OAc) and 10 mol% Xantphos in toluene at 70 °C
2
for 24 h (Table 1, entry 7). Trifluoroacetic anhydride was found
to be ineffective for the hydrocarboxylation under the reaction
a
State Key Laboratory of Coordination Chemistry, Collaborative Innovation Center of
Chemistry for Life Sciences, Center for Multi-molecular Organic Chemistry, School of
Chemistry and Chemical Engineering, Nanjing University, Nanjing, 210093, China.
E-mail: Yian.Shi@colostate.edu
b
Beijing National Laboratory for Molecular Sciences, CAS Key Laboratory of
Molecular Recognition and Function, Institute of Chemistry, Chinese Academy of
Sciences, Beijing 100190, China
c
Department of Chemistry, Colorado State University, Fort Collins, Colorado 80523,
USA
†
Electronic supplementary information (ESI) available: Experimental pro-
cedures, characterization data, and NMR spectra. See DOI: 10.1039/c5ob01180a
Scheme 2 Palladium-catalyzed hydrocarboxylation with HCOOH.
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a
a
Table 1 Studies of the reaction conditions
Table 2 Pd-catalyzed hydrocarboxylation of olefins
b
b
Entry
[Pd]
Ligand
Additive (equiv.)
Yield (%) Entry
Substrate
Product
Yield (%)
1
2
3
4
5
6
7
8
9
Pd(OAc)
Pd(OAc)
Pd(OAc)
Pd(OAc)
Pd(OAc)
Pd(OAc)
Pd(OAc)
Pd(OAc)
Pd(OAc)
2
2
2
2
2
2
2
2
2
PPh
PPh
dppe
dppp
dppb
3
12
49
NR
Trace
53
80
94
—
79
1
93
3
Ac
Ac
Ac
Ac
Ac
Ac
2
2
2
2
2
2
O (1.0)
O (1.0)
O (1.0)
O (1.0)
O (1.0)
O (1.0)
dppf
Xantphos
Xantphos
Xantphos
(CF
(EtCO)
3 2
CO) O (1.0)
2
O (1.0)
2
3
4
5
6
7
2b
2c
2d
2e
2f
85
88
71
81
90
54
10
Pd(OAc)
2
Xantphos
87
1
1
1
1
1
1
1
1
1
2
2
1
2
3
4
5
6
7
8
9
0
1
Pd(PPh
PdCl
3
)
4
Xantphos
Xantphos
Xantphos
Xantphos
Xantphos
Xantphos
Xantphos
Xantphos
Xantphos
Xantphos
Xantphos
Ac
2
Ac
2
Ac
2
Ac
2
Ac
2
Ac
2
Ac
2
Ac
2
Ac
2
Ac
2
Ac
2
O (1.0)
O (1.0)
O (0.2)
O (0.1)
O (0.2)
O (0.2)
O (0.2)
O (0.2)
O (0.2)
O (0.2)
O (0.2)
41
49
91
59
81
39
83
93
83
89
93
2g
2
Pd(OAc)
Pd(OAc)
Pd(OAc)
Pd(OAc)
Pd(OAc)
Pd(OAc)
Pd(OAc)
Pd(OAc)
Pd(OAc)
2
2
2
2
2
2
2
2
2
8
78
c
d
e
f
9
84
80
g
h
i
a
10
The reactions were carried out with 1a (0.50 mmol), HCOOH
(
1.00 mmol), [Pd] (0.025 mmol), ligand (0.050 mmol or 0.10 mmol,
P/Pd = 4/1), and anhydride (0.05–0.50 mmol) in toluene (0.10 mL) at
7
(
(
(
(
b
c
0 °C for 24 h unless otherwise stated. Isolated yield. In DCM
d
e
f
0.10 mL). In THF (0.10 mL). In CH
3
CN (0.10 mL). With Pd(OAc)
0.015 mmol) and Xantphos (0.015 mmol).
0.005 mmol) and Xantphos (0.005 mmol).
2
2
2
g
11
94
With Pd(OAc)
With Pd(OAc)
h
i
0.015 mmol) and Xantphos (0.015 mmol) at 60 °C for 24 h. With
2
Pd(OAc) (0.015 mmol) and Xantphos (0.015 mmol) at 50 °C for 48 h.
c
12
96 (l : b = 2 : 1)
conditions (Table 1, entry 8). Good yields were obtained for
the carboxylic acid with propionic anhydride and glutaric
anhydride (Table 1, entries 9 and 10). Lower yields were 13
n = 1, 1m
n = 2, 1n
n = 3, 1o
n = 4, 1p
2m
2n
2o
70
89
88
72
1
1
1
4
5
6
obtained with Pd(PPh ) or PdCl as a catalyst (Table 1, entries
3
4
2
1
1 and 12). Further studies showed that the acid product can
2p
still be obtained in high yield (91%) when the amount of Ac
2
O
a
The reactions were carried out with
1.00 mmol), Pd(OAc)
Ac
1
(0.50 mmol), HCOOH
was reduced to 0.2 equiv. (Table 1, entry 13). Among the
solvents examined (Table 1, entries 13, 15–17), toluene gave
(
2
(0.015 mmol), Xantphos (0.015 mmol), and
b
2
O (0.10 mmol) in toluene (0.10 mL) at 70 °C for 24 h. Isolated
c
the highest yield for the product. Similar yield was obtained yield. Total yield of two regioisomers.
when the Pd(OAc) was reduced from 5 to 3 mol% (Table 1,
2
entries 13 and 18). High yields were also achieved when the
reaction temperature was reduced to 60 or 50 °C (Table 1,
entries 20 and 21).
boxylic acids in 80–94% yields. In all these cases,
As shown in Table 2, the hydrocarboxylation process can be hydrocarboxylation proceeded with high regioselectivity,
extended to various substituted α-methylstyrenes and 2-isopro- leading to the linear carboxylic acids. In the case of styrene,
penylnaphthalene, providing carboxylic acids in 54–93% yields two regioisomers were obtained in 96% yield with a l : b ratio
(
Table 2, entries 1–8). Non-conjugated 1,1-disubstituted term- of 2 : 1 (Table 2, entry 12). When 1-hexene was subjected to the
inal olefins (Table 2, entries 9 and 10), and electron-deficient reaction conditions, a mixture of three acid isomers was
α,β-unsaturated ester (Table 2, entry 11) were also effective sub- obtained in 92% yield with n-heptanoic acid being slightly
strates for hydrocarboxylation, giving the corresponding car- favored. Cycloalkenes were found to be effective substrates,
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Conclusions
In summary, we have developed an efficient Pd-catalyzed
hydrocarboxylation of olefins using readily available HCOOH
with Ac O as a co-catalyst under mild reaction conditions. A
2
Scheme 3 Gram scale hydrocarboxylation reaction.
variety of carboxylic acids can be obtained in good yields
without using external toxic CO gas. The hydrocarboxylation
reaction is atom-economic, operationally simple, and amen-
giving the corresponding carboxylic acids in 70–89% yields able to gram scale, which makes the current process a viable
Table 2, entries 13–16). However, when 1-methyl-1-cyclohex- method to prepare carboxylic acids from olefins. Further
(
ene was used as a substrate, a mixture of four acid isomers was efforts will be devoted to understanding the reaction mechan-
obtained in 60% yield. It appears that 1-methyl-1-cyclohexene ism, expanding the substrate scope, and developing an asym-
underwent double bond migration under the reaction con- metric process of the reaction.
ditions to give olefin isomers which led to the corresponding
We gratefully acknowledge the National Natural Science
acid isomers. (E)-α-Methylstilbene was not an effective sub- Foundation of China (0205131566) and Nanjing University for
strate under the current reaction conditions. Only a trace financial support.
amount of an unidentified acid was formed.
As illustrated in Scheme 3, the hydrocarboxylation reaction
can also be carried out on a gram scale without using a sealed
reaction vessel, and carboxylic acid 2a was isolated in 90%
Notes and references
yield (Scheme 3). This operationally simple process using
HCOOH under mild conditions provides a practical method to
produce carboxylic acids from olefins.
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