Facile palladium-catalyzed hydrocarboxylation of olefins without external CO gas

Article abstract of DOI:10.1021/ol502987f

An effective Pd-catalyzed hydrocarboxylation of olefins with phenyl formate and formic acid is described. A variety of carboxylic acids are obtained in good yields with high regioselectivities under operationally simple conditions without the use of toxic CO gas.

Full text of DOI:10.1021/ol502987f

Letter
pubs.acs.org/OrgLett
Facile Palladium-Catalyzed Hydrocarboxylation of Olefins without
External CO Gas
Yang Wang,† Wenlong Ren,† Jingfu Li,† Haining Wang,† and Yian Shi*,†,‡,§
†State Key Laboratory of Coordination Chemistry, Center for Multimolecular Organic Chemistry, School of Chemistry and Chemical
Engineering, Nanjing University, Nanjing 210093, China
‡Beijing National Laboratory for Molecular Sciences, CAS Key Laboratory of Molecular Recognition and Function, Institute of
Chemistry, Chinese Academy of Sciences, Beijing 100190, China
§Department of Chemistry, Colorado State University, Fort Collins, Colorado 80523, United States
S
* Supporting Information
ABSTRACT: An effective Pd-catalyzed hydrocarboxylation of olefins
with phenyl formate and formic acid is described. A variety of
carboxylic acids are obtained in good yields with high regioselectiv-
ities under operationally simple conditions without the use of toxic
CO gas.
arboxylic acids are an important class of organic
molecules and are present in a wide variety of biologically
Scheme 2. Palladium-Catalyzed Hydrocarboxylation without
CO
C
significant compounds.1 While various synthetic methods have
been developed for this class of molecules, hydrocarboxylation
via direct addition of an H and a COOH group onto a C−C
double bond also presents an attractive approach.2,3 However,
such processes have been frequently carried out with CO gas
under high pressure and high temperatures (Scheme 1), which
1.2 equiv of HCOOH (Table 1, entry 2). HCOOCH3 (4b),
HCOOEt (4c), and HCOOBn (4d) were found to be
ineffective for hydrocarboxylation under the reaction conditions
(Table 1, entries 3−5), likely due to the fact that the OCH3,
OEt, and OBn groups are less electron-withdrawing than the
OPh group. Among various Pd catalysts examined (Table 1,
entries 6−11), the best result was obtained with 5 mol % of
Pd(OAc)2 and 10 mol % of dppf, giving the carboxylic acid in
96% yield with 1.2 equiv of HCOOPh and 1.2 equiv of
HCOOH in toluene at 90 °C for 24 h (Table 1, entry 11).
While lower yields were obtained, the reaction could proceed at
lower reaction temperatures (Table 1, entries 13 and 14).
When the reaction was carried out in the absence of HCOOPh
(4a), carboxylic acid 2a was obtained in 35% yield (Table 1,
entry 15). When AcOH was used, 2a was isolated in 29% yield
(Table 1, entry 16).
The hydrocarboxylation reaction can be extended to various
substituted α-methylstyrenes and related olefins, giving the
corresponding carboxylic acids in 60−99% yield (Table 2,
entries 1−8) (the X-ray structure of acid 2b is shown in Figure
1). Essentially, only linear carboxylic acids were obtained in
these cases. Nonconjugated 1,1-disubstituted terminal olefins
were also effective substrates for the hydrocarboxylation, giving
the acid products in good yields with high regioselectivities
(Table 2, entries 9−11). The reaction can also be applied to
electron-deficient α,β-unsaturated ester, giving the acid in 84%
Scheme 1. Metal-Catalyzed Hydrocarboxylation with CO
hinders their applications and studies in the laboratories
because of the difficulty in transporting, storing, and handling
toxic CO gas. The development of effective, mild, and
operationally simple hydrocarboxylation processes without the
use of external CO gas is highly desirable.4 Thus, far, there are
only limited reports on hydrocarboxylation of olefins using
formic acid or esters as source of CO, mostly with Ir and Rh as
catalysts at >100 °C.5,6 As part of our efforts in the
carbonylation of olefins without external CO gas,7−9 we have
found that a wide variety of olefins can be efficiently
hydrocarboxylated with HCOOPh and HCOOH in the
presence of Pd catalyst under mild reaction conditions to
give the corresponding carboxylic acids in good yields with high
regioselectivities (Scheme 2). Herein, we report our preliminary
results on this subject.
α-Methylstyrene (1a) was used as the test substrate. Treating
1a with HCOOPh (4a), 5 mol % of Pd(OAc)2, and 20 mol %
of PPh3 in toluene at 90 °C for 24 h only gave phenyl 3-
phenylbutanoate in 12% yield (Table 1, entry 1). However, the
corresponding acid (3-phenylbutanoic acid) was isolated in
57% yield when the reaction was carried out in the presence of
Received: October 10, 2014
© XXXX American Chemical Society
A
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a
a
Table 1. Screening of Reaction Conditions
Table 2. Hydrocarboxylation of Olefins
b,c
entry
4
[Pd]
ligand additive (equiv) yield (%)
1
4a
4a
4b
4c
4d
4a
4a
4a
4a
4a
4a
4a
4a
4a
-
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(PPh3)4
PdCl2(CH3CN)2
PdCl2
PPh3
PPh3
PPh3
PPh3
PPh3
PPh3
PPh3
PPh3
dppe
dppb
dppf
dppf
dppf
dppf
dppf
dppf
d
57
2
HCOOH (1.2)
HCOOH (1.2)
HCOOH (1.2)
HCOOH (1.2)
HCOOH (1.2)
HCOOH (1.2)
HCOOH (1.2)
HCOOH (1.2)
HCOOH (1.2)
HCOOH (1.2)
HCOOH (2.0)
HCOOH (2.0)
HCOOH (2.0)
HCOOH (2.0)
AcOH (2.0)
3
NR
NR
NR
33
4
5
6
7
20
8
trace
NR
78
9
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
10
11
12
96
97
e
13
80
f
14
45
15
16
35
4a
29
a
The reactions were carried out with 1a (0.50 mmol), 4 (0.60 mmol),
[Pd] (0.025 mmol), ligand (0.050 or 0.10 mmol, P/Pd = 4/1), and
additive (0.60 mmol) in toluene (0.50 mL) at 90 °C for 24 h unless
b
c
otherwise stated. Isolated yield. The other regioisomer was barely
1
detectable by H NMR analysis of the crude reaction mixture if there
was any. The corresponding ester (phenyl 3-phenylbutanoate) was
obtained in 12% yield. The reaction was carried out at 70 °C for 24 h.
The reaction was carried out at 50 °C for 24 h.
d
e
f
yield (Table 2, entry 12). When a related α,β-unsaturated
amide was subjected to the reaction conditions, the pyrrolidine-
2,5-dione product was obtained in 84% yield (Table 2, entry
13). In the case of vinylcyclohexane, two regioisomers were
obtained in 69% yield with a ratio of 5:1, favoring the linear
carboxylic acid (Table 2, entry 14). Cycloalkenes were also
effectively hydrocarboxylated to give the corresponding acids in
72−87% yields (Table 2, entries 15−18).
A precise understanding of the reaction mechanism awaits
further study. A couple of reaction pathways can be perceived.
A plausible catalytic cycle is proposed in Scheme 3. The Pd(0)
oxidatively inserted into HCOOPh to form palladium hydride
complex 5, which rearranged to palladium carbonyl complex
6.10 The olefin substrate was subsequently hydropalladated to
give complex 7, which underwent a migratory insertion to form
acylpalladium complex 8. The replacement of the PhO group
on the palladium of 8 by formate led to the formation of
complex 9, which underwent reduction elimination to form
anhydride 10 and to regenerate the Pd(0) catalyst. Anhydride
10 was converted to carboxylic acid 2a via decomposition11
and/or by reacting with PhOH. HCOOH appeared to be very
important for the efficient generation of the carboxylic acid.
Without HCOOH, only small amount of the phenyl ester was
formed (Table 1, entry 1), resulting from the reductive
elimination of complex 8. When the reaction was carried out
with catalytic amount of phenyl formate (4a) (20 mol %),
carboxylic acid 2a was isolated in 80% yield (Scheme 4), which
appears to be consistent with the regeneration of phenyl
formate during the reaction as proposed in Scheme 3.
a
The reactions were carried out with 1 (0.50 mmol), 4a (0.60 mmol),
Pd(OAc)2 (0.025 mmol), dppf (0.050 mmol), and HCOOH (1.00
mmol) in toluene (0.50 mL) at 90 °C for 24 h unless otherwise stated.
b
For entries 2, 11, and 14, 0.50 mmol of HCOOH was used. Isolated
c
1
yield. The ratio of two regioisomers was determined by H NMR
analysis. For entries 1−13, the other regioisomer was barely detectable
if there was any.
In summary, we have developed an efficient Pd-catalyzed
hydrocarboxylation of olefins under mild reaction conditions. A
wide variety of carboxylic acids has been obtained in 60−99%
yields with generally high regioselectivities. The reaction
process requires no handling of toxic CO gas. The reaction is
B
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ASSOCIATED CONTENT
* Supporting Information
■
S
Experimental procedures, characterization data, X-ray struc-
tures, data for the determination of enantiomeric excess, and
NMR spectra. This material is available free of charge via the
Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
■
*E-mail: yian@lamar.colostate.edu.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We gratefully acknowledge the National Natural Science
Foundation of China (21202082) and Nanjing University for
financial support. We also thank Mr. Ben Dong, Mr. Xingchao
Guan, and Miss Jie Dai at Nanjing University for experimental
assistance.
Figure 1. X-ray structure of compound 2b.
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