COMMUNICATION
DOI: 10.1002/chem.200800535
0
Partial Hydrogenation of Alkynesto cis-Olefinsby Using a Novel Pd –
Polyethyleneimine Catalyst
Hironao Sajiki,*[a] Shigeki Mori,[a] Tomoyuki Ohkubo,[a] Takashi Ikawa,[a, b]
Akira Kume,[a] Tomohiro Maegawa,[a] and Yasunari Monguchi[a]
The selective partial hydrogenation of alkynes to cis-al-
kenes represents an important class of chemical transforma-
tions that have found extensive use in the construction of an
enormous variety of, for example, bioactive molecules, natu-
ral products, industrial materials.[1] The important transfor-
mation has mainly been accomplished under ordinary hy-
drogen pressure and at room temperature using heterogene-
ous Pd catalysts such as Lindlar catalyst (Pd/CaCO3–Pb-
(terminal) alkynes as substrates. Therefore, the Lindlarꢀs
method is only applicable to the selective chemical transfor-
mation of disubstituted alkynes to cis-alkenes.[10] Thus, there
is a need for the development of novel selective catalyst
that can provide high yields and selectivity without any nu-
cleophilic additives and the chemical pretreatment by an en-
vironmentally harmful materials. Recently, Alonso and Yus
et al. reported an efficient method for the partial hydrogena-
tion of both internal and terminal alkynes using Ni nanopar-
ticles although the adjustment of the amount of the in situ
generated molecular hydrogen source (Li powder and
EtOH or iPrOH) is essential.[11] We now disclose a develop-
ment of very practical catalyst and an efficient partial hydro-
genation controlled by the catalyst activity that overcomes
these serious and longstanding problems.
Recently, we have reported that a Pd/C catalyst formed
an isolable complex with ethylenediamine (en) employed as
the catalytic poison via one-on-one interaction between Pd
metal and en, and the complex catalyst [Pd/C(en)] chemose-
lectively hydrogenated a variety of reducible functionalities
in the presence of an O-benzyl, O-TES or N-Cbz protective
group, benzyl alcohol or epoxide.[12] Besides, we also devel-
oped silk fibroin (Fib)-supported Pd0 catalyst for the chemo-
selective hydrogenation of alkynes, alkenes and azides in
the presence of other reducible functionalities.[13] During our
effort to create a new Pd catalyst for hydrogenation possess-
ing different chemoselectivity, we found that polyethylene-
imine (PEI, branched polymer, average molecular weight
approximately 25000) supported Pd0 catalyst selectively cat-
alyzed hydrogenation of only alkynes including both mono-
and disubstituted alkynes, without the reduction of other re-
ducible functionalities.[14]
ACHTREUNG
(OAc)2 in conjunction with quinoline)[2], Pdc[3] and Ni cata-
lysts such as low-active Raney Ni[4], P-1 Ni[5], P-2 Ni[6], and
Nic[7], and Au nanoparticles[8] and homogeneous Rh and Cr
complexes[9] as a catalyst. In these cases except Lindlar cata-
lyst, the narrow substrate-applicability, concomitant use with
a basic additive, low cis/trans selectivity, pyrophoric property
and/or operational complexity of the catalysts are undesira-
ble as a general synthetic methodology and these are why
such catalysts have never been a common selective catalyst
for the partial hydrogenation of an alkyne to a cis-alkene.
On the other hand, the Lindlar catalyst is widely applicable
and used as such a selective catalyst although the pretreat-
ment using environmentally harmful PbACTHRE(UNG OAc)2 and the si-
multaneous use with quinoline is necessary to achieve the
chemoselective and geometrically selective partial hydroge-
nation. Furthermore, the smooth over-reduction to the cor-
responding alkanes efficiently proceeded under the Lindlarꢀs
hydrogenation conditions by the use of monosubstituted
[a] Prof. Dr. H. Sajiki, S. Mori, T. Ohkubo, Dr. T. Ikawa, A. Kume,
Dr. T. Maegawa, Dr. Y. Monguchi
Laboratory of Medicinal Chemistry, Gifu Pharmaceutical University
5-6-1 Mitahora-higashi, Gifu 502-8585 (Japan)
Fax : (+81)58-237-5979
This paper describes the creation and application of a
novel Pd0–polyethyleneimine complex catalyst (Pd0–PEI).
The 5 % Pd0–PEI catalyst was prepared by introduction of
[b] Dr. T. Ikawa
Current address: Laboratory of Synthetic Organic Chemistry
Department of Pharmaceutics, School of Pharmaceutical Sciences
University of Shizuoka, 52-1 Yada, Suruga-ku, Shizuoka, 422-8526
(Japan)
PdACHTRE(UNG OAc)2 directly in the MeOH solution of deaerated PEI
under Ar atmosphere (Scheme 1). The resulting rust-colored
solution was stirred under H2 atmosphere at room tempera-
ture for 24 h and the solution changed gradually to black, in-
Supporting information for this article is available on the WWW
Chem. Eur. J. 2008, 14, 5109 – 5111
ꢁ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
5109