Palladium(II) acetate in pyridine as an effective catalyst for highly regioselective hydroselenation of alkynes

Article abstract of DOI:10.1021/jo048727j

(Chemical Equation Presented) A highly regioselective hydroselenation of terminal alkynes with benzeneselenol can be achieved by the combination of palladium acetate and pyridine, providing the corresponding terminal alkenes, (i.e., 2-phenylseleno-1-alkenes) as a sole product. In this hydroselenation, pyridine may act as a suitable ligand for active palladium intermediates.

Full text of DOI:10.1021/jo048727j

reaction is accompanied by the formation of further
addition product (2) of PhSeH to vinylic selenide, double
bond isomerization product (3), and 1-phenylseleno-1-
alkene (4) as byproducts (eq 1, and entry 1 in Table 1).
Palladium(II) Acetate in Pyridine as an
Effective Catalyst for Highly Regioselective
Hydroselenation of Alkynes
Ikuyo Kamiya,† Etsuyo Nishinaka,† and Akiya Ogawa*,‡
Department of Chemistry, Faculty of Science, Nara Women’s
University, Kitauoyanishi-machi, Nara 630-8506, Japan,
and Department of Applied Chemistry, Faculty of
Engineering, Osaka Prefecture University, 1-1 Gakuen-cho,
Sakai, Osaka 599-8531, Japan
ogawa@chem.osakafu-u.ac.jp
Received July 24, 2004
The product selectivity is somewhat increased by using
benzene as the solvent; however, byproducts 2-4 are still
formed similarly (entry 2, in Table 1). Thus, much effort
has been invested to improve the selectivity and efficiency
for this hydroselenation and we have successfully found
that palladium acetate in pyridine attains an excellent
product selectivity in the hydroselenation of alkynes with
benzeneselenol: 2-phenylseleno-1-alkene (1) was ob-
tained as a sole product (entry 5 in Table 1). On the other
hand, the reaction in the absence of catalyst did not
proceed efficiently (entry 6 in Table 1).
A highly regioselective hydroselenation of terminal alkynes
with benzeneselenol can be achieved by the combination of
palladium acetate and pyridine, providing the corresponding
terminal alkenes, (i.e., 2-phenylseleno-1-alkenes) as a sole
product. In this hydroselenation, pyridine may act as a
suitable ligand for active palladium intermediates.
TABLE 1. Influence of Solvents on Hydroselenationa
yield, %b
Recently, organosulfur compounds have been employed
as substrates for transition-metal-catalyzed reactions and
several synthetically useful reactions of them have been
developed in the presence of transion matal catalysts.1
In contrast, the transition-metal-catalyzed reactions of
organoselenium compounds have been scarcely investi-
gated, probably because the higher affinity of selenium
to transition metal catalysts may extensively decrease
the catalytic activity in the reactions of selenium com-
pounds.2 In 1992, we reported the first example of
transition-metal-catalyzed addition reactions of ben-
zeneselenol to terminal alkynes, in which palladium
complexes such as Pd(OAc)2, PdCl2, PdCl2(PPh3)2, PdCl2-
(PhCN)2, and Pd(PPh3)4 are used as the catalyst.2m
However, the product selectivity of hydroselenation is not
adequate in comparison with the corresponding pal-
ladium-catalyzed hydrothiolation of terminal alkynes
with benzenethiol, which provides the terminal alkenes
(i.e., 2-phenylthio-1-alkenes), as a sole product (cat. Pd-
(OAc)2, THF, 67 °C).3
entry
solvent
temp, °C
1
2
3
4
1
2
3
4
5
6c
THF
67
80
100
80
100
100
46
62
23
47
77
3
15
13
29
0
10
7
34
0
0
0
5
benzene
toluene
pyridine
pyridine
pyridine
<3
0
0
0
0
0
0
a Reaction conditions: Pd(OAc)2 (2 mol %), 1-octyne (1.0 mmol),
solvent (0.5 mL), PhSeH (1.0 mmol), 15 h. b Determined by 1H
NMR. c In the absence of catalyst.
Table 2 represents the results of the Pd(OAc)2-
catalyzed hydroselenation of various terminal alkynes.
(2) (a) Okamura, H.; Miura, M.; Kosugi, K.; Takei, H. Tetrahedron
Lett. 1980, 21, 87. (b) Murahashi, S.; Yano, T. J. Am. Chem. Soc. 1980,
102, 2456. (c) Uemura, S.; Fukuzawa, S.; Patil, S. R. J. Organomet.
Chem. 1983, 243, 9. (d) Cristau, H. J.; Chabaud, B.; Labaudiniere, R.;
Chistol, H. J. Org. Chem. 1986, 51, 875. (e) Ohe, K.; Takahashi, H.;
Uemura, S.; Sugita, N. J. Organomet. Chem. 1987, 326, 35. (f)
Takahashi, H.; Ohe, K.; Uemura, S.; Sugita, N. J. Organomet. Chem.
1987, 334, C43. (g) Ohe, K.; Takahashi, H.; Uemura, S.; Sugita, N. J.
Org. Chem. 1987, 52, 4859. (h) Tsumuraya, T.; Ando, W. Organome-
tallics 1989, 8, 2286. (i) Fukuzawa, S.; Fujinami, T.; Sasai, S. Chem.
Lett. 1990, 927. (j) Kuniyasu, H.; Ogawa, A.; Miyazaki, S.; Ryu, I.;
Kambe, N.; Sonoda, N. J. Am. Chem. Soc. 1991, 113, 9796. (k) Uemura,
S.; Takahashi, H.; Ohe, K. J. Organomet. Chem 1992, 423, C9. (l)
Kuniyasu, H.; Ogawa, A.; Higaki, K.; Sonoda, N. Organometallics 1992,
11, 3937. (m) Kuniyasu, H.; Ogawa, A.; Sato, K.; Ryu, I.; Sonoda, N.
Tetrahedron Lett. 1992, 33, 5525. (n) Ogawa, A.; Sonoda, N. J. Synth.
Org. Chem. Jpn. 1993, 51, 815. (o) Han, L.-B.; Choi, N.; Tanaka, M. J.
Am. Chem. Soc. 1996, 118, 7000. (p) Ogawa, A.; Kuniyasu, H.; Sonoda,
N.; Hirao, T. J. Org. Chem. 1997, 62, 8361. (q) Kuniyasu, H.;
Maruyama, A.; Kurosawa, H. Organometallics 1998, 17, 908. (r)
Ogawa, A.; Kuniyasu, H.; Takeba, M.; Ikeda, T.; Sonoda, N.; Hirao, T.
J. Organomet. Chem. 1998, 564, 1. (s) Ogawa, A.; Kudo, A.; Hirao, T.
Tetrahedron Lett. 1998, 39, 5213. (t) Nishiyama, Y.; Tokunaga, K.;
Sonoda, N. Org. Lett. 1999, 1, 1725.
In THF, this Pd(OAc)2-catalyzed hydroselenation of
terminal alkynes with benzeneselenol provides 2-phen-
ylseleno-1-alkene (1) as the major product, but the
* Address correspondence to this author. Phone/fax: 81-72-254-9290.
† Nara Women’s University.
‡ Osaka Prefecture University.
(1) Reviews: (a) Ogawa, A. In Main Group Metals in Organic
Synthesis; Yamamoto, H., Oshima, K., Eds.; Wiley-VCH: Weinheim,
Germany, 2004; Chapter 15. (b) Alonso, F.; Beletskaya, I.; Yus, M.
Chem. Rev. 2004, 104, 3079. (c) Ogawa, A. In Handbook of Organo-
palladium Chemistry for Organic Synthesis; Negishi, E., Ed.; Wiley:
New York, 2002; Chapter VII.6. (d) Kuniyasu, H. In Catalytic Hetero-
functionalization; Togni, A., Gru¨tzmacher, H., Eds.; Wiley-VCH: Wein-
heim, Germany, 2001; Chapter 7. (e) Kondo, T.; Mitsudo, T. Chem.
Rev. 2000, 100, 3205. (f) Ogawa, A. J. Organomet. Chem. 2000, 611,
463. (g) Beletskaya, I.; Moberg, C. Chem. Rev. 1999, 99, 3435. (h) Han,
L.-B.; Tanaka, M. Chem. Commun. 1999, 395.
(3) Kuniyasu, H.; Ogawa, A.; Sato, K.; Ryu, I.; Kambe, N.; Sonoda,
N. J. Am. Chem. Soc. 1992, 114, 5902.
10.1021/jo048727j CCC: $30.25 © 2005 American Chemical Society
Published on Web 12/17/2004
696
J. Org. Chem. 2005, 70, 696-698

TABLE 2. Hydroselenation of Terminal Alkynesa
TABLE 3. Catalytic Hydroselenation with Pyridine or
2,2′-Bipyridyl
entry
solvent
time, h
additive, mol %
yield, %a
1
2
3
4
5
pyridine
toluene
toluene
toluene
toluene
15
15
15
10
10
none
77
38
63
60
63
pyridine (40)
2,2′-bipyridyl (20)
2,2′-bipyridyl (20)
2,2′-bipyridyl (40)
a Determined by 1H NMR.
SCHEME 1. A Possible Pathway for the
Pd(II)-Catalyzed Hydroselenation
a Reaction conditions: Pd(OAc)2 (2 mol %), alkyne (1.0 mmol),
PhSeH (1.0-1.1 mmol), pyridine (0.5 mL), 15 h, 100 °C. b NMR
(isolated) yield. c HOCH2CHdCHSePh was obtained in 12% yield
(E/Z ) 42/58) as a byproduct. d p-MeO-C6H4-CHdCHSePh was
obtained in 38% yield (Z>95%) as a byproduct.
Substituted aliphatic alkynes gave the corresponding
hydroselenation products in high to excellent yields
regioselectively (entries 1-6). Exceptionally, in the case
of propargylic alcohol, regioisomer (3-phenylseleno-2-
propene-1-ol) was also obtained as a byproduct (entry 6).4
In general, the radical addition of PhSeH to aliphatic
alkynes requires longer reaction time under neutral
conditions in the presence of a very small amount of air
contamination or upon irradiation with room light.
Contrary to this, similar radical addition to aryl-
substituted alkynes proceeds faster.5 For example, the
radical addition of PhSeH to 1-hexyne requires a long
reaction time (240 h) affording 1-(phenylseleno)-1-hexene
in 45% yield, whereas the additon to ethynylbenzene is
accomplished within 5 min in the presence of a trace
amount of oxygen contamination, which provides 2-(phen-
ylseleno)styrene exclusively. With these facts in hand,
the result that the Pd(OAc)2-catalyzed hydroselenation
of 1-ethynyl-4-methoxybenzene provides the correspond-
ing 2-phenylseleno-substituted terminal alkene as the
major product (56%) is noteworthy (entry 7).
1-octene was successfully obtained in moderate yield with
high product selectivity (entry 2). In this reaction, the
use of a bidentate ligand such as 2,2′-bipyridyl (20 mol
%) in place of pyridine led to the increase in the yield of
the desired vinyl selenide with excellent regioselectivity
(entries 3-5). These results strongly suggest that pyri-
dine acts as a suitable ligand for an active palladium
intermediate.6
Although the elucidation of the precise mechanism
requires further detailed investigations, Scheme 1 shows
a possible reaction pathway, which includes the follow-
ing: (1) ligand exchange of the acetate group with the
PhSe group to give an Pd(SePh)2Ln as an active catalyst
and AcOH; (2) coordination of alkyne to the palladium
selenide species; (3) syn-selenopalladation of alkyne to
form â-cis-(phenylseleno)vinylpalladium intermediate (A);
(4) the protonolysis of the vinylpalladium intermediate
with PhSeH to provide 2-phenylseleno-1-alkene with
regeneration of the catalyst.
In general, the reductive elimination process of vic-
bis(phenylseleno)alkene from the vinylpalladium inter-
mediate (A) complex takes place smoothly in the presence
of phosphine ligands.2j,7 In this hydroselenation, however,
there is no phosphine ligand so that the reductive
elimination may be difficult. Consequently, the hydrose-
lenation product is obtained by trapping with PhSeH.
To get insight into the role of pyridine in this catalytic
reaction, we examined the catalytic hydroselenation by
using pyridine (or its derivatives) as ligands in toluene
(Table 3). When the Pd(OAc)2-catalyzed reaction of 1-oc-
tyne with PhSeH in toluene was conducted by employing
40 mol % of pyridine, the corresponding 2-(phenylseleno)-
(6) In the absence of pyridine, palladium selenide (Pd(SePh)2)
molecules as key species for this hydroselenation of alkynes easily react
with each other by the coordination of the selenide ligand to the other
palladium center to form polymerized complex, which is insoluble in
usual organic solvents and loses the catalytic activation. Therefore,
pyridine might inhibit the polymerization and protect the catalyst from
the poisoning.
(4) Although the reason 2-phenyl-2-propenols were formed as
byproducts in the case of propargylic alcohols is unclear, the coordina-
tion of the propargylic oxygen to palladium may contribute to the
formation of the regioisomers (2-phenylseleno-2-propenol).
(5) Ogawa, A.; Obayashi, R.; Sekiguchi, M.; Masawaki, T.; Kambe,
N.; Sonoda, N. Tetrahedron Lett. 1992, 33, 1329.
J. Org. Chem, Vol. 70, No. 2, 2005 697

115 µL, 1.0-1.1 mmol) under N2 atmosphere. The mixture was
heated at 100 °C with stirring for 15 h. After the reaction was
complete, the resulting catalyst was removed by filtration
through Celite, and the filtrate was evaporated under reduced
pressure. The purification of the products was performed by
preparative TLC (PTLC) on Wakogel B-5F silica gel with a mixed
solvent of hexane/ethyl acetate as an eluent.
In summary, we have developed the highly regioselec-
tive hydroselenation of terminal alkenes by the combina-
tion of Pd(OAc)2 and pyridine. Pyridine (or 2,2′-bipyridyl)
acts as a useful ligand for this reaction, and may prevent
catalyst poisoning by inhibiting the polymerization of the
palladium selenide catalysts. This catalytic system pro-
vides a useful method for the transition-metal-catalyzed
reactions of organoselenium compounds.
Acknowledgment. This research was supported by
a Grant-in-Aid for Scientific Research on Priority Areas
(A) (No. 14044063, Exploitation of Multi-Element Cyclic
Molecules) from the Ministry of Education, Culture,
Sports, Science and Technology, Japan. We acknowledge
the analytical section of Osaka University for elemental
analyses.
Experimental Section
General Procedure for the Hydroselenation. Into a two-
necked flask equipped with a reflux condenser and a magnetic
stirring bar were placed palladium acetate (4.5 mg, 0.02 mmol),
pyridine (0.5 mL), alkyne (1.0 mmol), and benzeneselenol (106-
Supporting Information Available: Analytical and spec-
tral data and representative 1H NMR sprectra for the Pd-
(OAc)2-catalyzed hydroselenation of 1-octyne in THF and
pyridine. This material is available free of charge via the
Internet at http://pubs.acs.org.
(7) (a) Ananikov, V. P.; Beletskaya, I. P.; Aleksandrov, G. G.;
Eremenko, I. L. Organometallics 2003, 22, 1414. (b) Ananikov, V. P.;
Malyshev, D. A.; Beletskaya, I. P.; Aleksandrov G. G.; Eremenko, I.
L. J. Organomet. Chem. 2003, 679, 162. (c) Ananikov, V. P.; Malyshev,
D. A.; Beletskaya, I. P.; Aleksandrov G. G.; Eremenko, I. L. J.
Organomet. Chem. 2003, 687, 451.
JO048727J
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