Hydrogenation of olefins using Hantzsch ester catalyzed by palladium on carbon

Article abstract of DOI:10.1016/j.tetlet.2008.12.062

Hantzsch 1,4-dihydropyridine (HEH), a well-known model compound of coenzyme NAD(P)H was found as an efficient reducing agent in hydrogenation of unactivated olefins catalyzed by Pd/C. α,β-Unsaturated ketones also underwent hydrogenation, affording the corresponding saturated ketones selectively.

Full text of DOI:10.1016/j.tetlet.2008.12.062

Tetrahedron Letters 50 (2009) 1026–1028
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Hydrogenation of olefins using Hantzsch ester catalyzed by palladium on carbon
*
Qiang Liu , Jing Li, Xiao-Xia Shen, Rui-Guang Xing, Jie Yang, Zhengang Liu, Bo Zhou
State Key Laboratory of Applied Organic Chemistry, College of Chemistry and Chemical Engineering, Lanzhou University, Lanzhou 730000, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Hantzsch 1,4-dihydropyridine (HEH), a well-known model compound of coenzyme NAD(P)H was found
Received 1 November 2008
Revised 7 December 2008
Accepted 12 December 2008
Available online 24 December 2008
as an efficient reducing agent in hydrogenation of unactivated olefins catalyzed by Pd/C. a,b-Unsaturated
ketones also underwent hydrogenation, affording the corresponding saturated ketones selectively.
Ó 2008 Elsevier Ltd. All rights reserved.
Keywords:
Olefins
a,b-Unsaturated ketones
Hantzsch ester
Palladium on carbon
Hydrogenation
Reduction
It is well known that Pd/C is a cost-efficient heterogeneous
catalyst and is used extensively in organic chemistry.1 Some
dehydrogenation reactions using Pd/C catalyst have been reported
but the yields were not satisfactory. Recently, Hayashi and his co-
workers developed an efficient dehydrogenation reaction using
Pd/C under an ethylene atmosphere.2 In the Pd/C-ethylene sys-
tem, ethylene played a crucial role and it was used to remove
hydrogen from the active surface of catalyst. Pd/C has also been
employed to catalyze dehydrogenative aromatizations of hydroa-
romatic compounds. For instance, a Pd/C-acetic acid system was
successfully applied to the oxidation of Hantzsch 1,4-dihydropyri-
dines (HEHs) to pyridines.3 HEH is being investigated as a safe,
easy-to-handle, cheap, and environmentally benign reagent for
the reduction of organic functional groups.4 In continuing our ef-
forts to develop HEH-based synthetic methods,5 we began to
wonder whether hydrogen which is produced by dehydrogenative
aromatizations of HEH in the presence of Pd/C could be efficiently
accepted by olefins.
vated olefins with HEH. Herein, we reported the results of our
research.
It had been reported that ethylene was not hydrogenated to
ethane in oxidative aromatization process to convert 1,4-dihydro-
pyridines to the corresponding pyridines by using a catalytic
amount of Pd/C in acetic acid.3 Hence, various solvents such as
CH3CN, THF, PhCH3, ClCH2CH2Cl, and EtOH were screened for the
hydrogenation of olefins, and the best results were observed under
ethanol conditions. Typical procedure is as follows: an ethanolic
solution (20 mL) of allyloxybenzene (1.0 mmol), HEH (1.1 mmol),
and 10% Pd/C (10 wt % of allyloxybenzene) was refluxed under stir-
ring and argon atmosphere for 4 h. After completion of the reaction
as monitored by TLC, the solvent was filtered and the filtrate was
concentrated under reduced pressure. The corresponding saturated
propoxybenzene was isolated in 86% yield by a column chromatog-
raphy (silica gel). Besides propoxybenzene, diethyl 2,6-dimethyl-
3,5-pyridinedicarboxylate, the corresponding oxidative aromatiz-
ing product of HEH, was also isolated (Scheme 1). All products
were identified by comparing its 1H NMR, 13C NMR, and EI-MS
spectral data.
Many studies have been conducted concerning the mecha-
nism of HEH’s reduction of various electron-deficient functional
groups.4a,6 Recently, there has been much interest in finding an
application for HEH as a biomimetic reducing agent in organo-
With this initial success, the scope of this protocol was exam-
ined for other substrates. Typical results are summarized in Ta-
catalytic and enantioselective reduction of
a
,b-unsaturated alde-
ble 1. It is seen from Table 1 that the present procedure
hydes,7 ,b-unsaturated ketones,8 and nitroolefins.9 To our
a
affords a very clean and efficient approach for hydrogenation
of aliphatic and aromatic olefins. Both terminal olefins (entries
1, 3, 4, and 5) and internal olefins (entries 2, 6, and 7) afforded
the saturated products in an excellent yield. The hydrogenation
of indene also proceeded smoothly giving indan in 89% yield
(entry 8).
knowledge, there is still no report on hydrogenation of unacti-
* Corresponding author. Fax: +86 931 8625657.
E-mail address: liuqiang@lzu.edu.cn (Q. Liu).
0040-4039/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2008.12.062

Q. Liu et al. / Tetrahedron Letters 50 (2009) 1026–1028
1027
EtO2C
Me
CO2Et
EtO2C
Me
CO2Et
Me
O
O
10 wt% Pd/C
+
+
EtOH, reflux
4 hr.
N
H
Me
N
HEH
Scheme 1.
Table 1
Pd/C-catalyzed hydrogenation of olefins with HEHa
Entry
Substrate
Product
Yieldb (%)
88
O
O
1
O
O
2
3
4
5
89
90
90
99
6
7
99
99
OMe
OMe
8
90
a
Reaction conditions see text.
b
Isolated yields. All products are known compounds and were characterized by EI-MS, 1H NMR, and 13C NMR.
a
,b-Unsaturated ketones also underwent the present hydroge-
well with unactivated olefins. But the advantage of HEH-based
hydrogenation was shown significantly in hydrogenation of ,b-
nation, although a relatively more amount of Pd/C (20 wt % of
substrates) and a longer reaction time (8 h) were needed to com-
plete the reaction. In these cases, 1,4-reduced products were ob-
tained selectively and no alcohol product was observed. Typical
results are summarized in Table 2. The procedure worked well
on aliphatic 2-en-1-one (entry 1), phenyl aliphatic 2-en-1-one
(entry 2), and chalcone analogs (entries 3–5).The method was
also successfully extended to 1,4-dien-3-one (entry 6), 2,4-dien-
1-one(entry 7), and 1,6-diene-3,5-dione (entry 8) systems. In
the present case, all double bonds were reduced by using
2.1 equiv of HEH and the corresponding saturated ketones were
isolated as the major products.
a
Unsaturated ketones. For instance, the Pd/C-catalyzed hydrogena-
tion of chalcone with molecular hydrogen resulted in a mixture
of 1,3-diphenylpropan-1-one (73%) and 1,3-diphenylpropan-1-ol
(21%) even in 15 min, while the present method gave 1,3-diphenyl-
propan-1-one in 96% yield.
In summary, the Pd/C-catalyzed hydrogenation of olefins
with HEH is described. Although the catalyzed reduction of
electron-deficient olefins with HEH has been well studied,6–9
there is no report on hydrogenation of unactivated alkenes with
HEH. The present reaction expends the application of HEH as a
biomimetic reducing agent in synthetically useful organic trans-
formations, and provides a simple and easy way for the hydro-
genation of olefins without expensive reagents and pressurized
conditions.
The comparison of the present method with H2 hydrogenation
under ordinary pressure in the presence of 10% Pd/C at room tem-
perature was also carried out. It seems that both methods worked

1028
Q. Liu et al. / Tetrahedron Letters 50 (2009) 1026–1028
Table 2
Selective reduction of a
,b-unsaturated ketones by HEH in the presence of Pd/Ca
Entry
1
Substrate
Product
Yieldb (%)
93c
O
O
O
O
O
2
3
4
5
6
7
89
O
96
O
O
96
MeO
Cl
OMe
MeO
Cl
OMe
O
O
81
O
O
93d
88d
90d
O
O
O
O
O
O
8
a
The reaction was carried out in refluxing ethanol (25 mL) for 8 h using
a,b-unsaturated ketones (1.0 mmol), HEH (1.1 mmol) in the presence of 10% Pd/C (20 wt % of
corresponding a,b-unsaturated ketones).
b
Isolated yields. All products are known compounds and were characterized by EI-MS, 1H NMR, and 13C NMR.
The yield was determined by GC–MS.
2.1 Equiv of HEH was needed to complete the reactions.
c
d
3. Nakamichi, Natsuki; Kawashita, Yuka; Hayashi, Masahiko Org. Lett. 2002, 4,
Acknowledgments
3955.
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5. (a) Zhang, J.; Jin, M.-Z.; Zhang, W.; Yang, L.; Liu, Z.-L. Tetrahedron Lett. 2002, 43,
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Synthesis 2006, 771.
We thank the National Natural Science Foundation of China
(Grant No. 20702023) and Special Fund for Doctoral Program from
the Ministry of Education of China (Grant No. 20070730040) for
financial support.
6. (a) Yasui, S.; Ohno, A. Bioorg. Chem. 1986, 14, 70; (b) Fukuzumi, S.; Tanaka, T.. In
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