Selective reduction of alkynes catalyzed by palladium acetate with sodium methoxide as the hydride source

Article abstract of DOI:10.1016/S0040-4039(03)00029-7

Treatment of internal alkynes with sodium methoxide in the presence of Pd(OAc)₂ and PPh₃ in methanol for 48 h gave the reduction products, alkenes or alkanes in good chemical yields. This reaction proceeds through a palladium methanolate complex, followed by β-hydride elimination and reductive elimination.

Full text of DOI:10.1016/S0040-4039(03)00029-7

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 44 (2003) 1979–1981
Selective reduction of alkynes catalyzed by palladium acetate
with sodium methoxide as the hydride source
a,b
a,b
a,b
a
a,
Li-Lan Wei, Li-Mei Wei, Wen-Bin Pan, Shiow-Piaw Leou and Ming-Jung Wu *
a
School of Chemistry, Kaohsiung Medical University, Kaohsiung, Taiwan
b
Fooyin University, Kaohsiung county, Taiwan
Received 14 October 2002; revised 12 December 2002; accepted 20 December 2002
Abstract—Treatment of internal alkynes with sodium methoxide in the presence of Pd(OAc) and PPh in methanol for 48 h gave
2
3
the reduction products, alkenes or alkanes in good chemical yields. This reaction proceeds through a palladium methanolate
complex, followed by b-hydride elimination and reductive elimination. © 2003 Elsevier Science Ltd. All rights reserved.
One of the most useful synthetic applications of alkynes
is the stereoselective reduction to (E)-alkenes by alkali
metals in liquid ammonia or (Z)-alkenes by catalytic
hydrogenation. The production of highly pure (Z)-alke-
nes is often a key-step for the synthesis of natural
(5 mol%) in methanol for 48 h whereby 1,2-
diphenylethane 2a was obtained in 92% yield (Table 1,
entry 1). Reaction with unsymmetrically disubstituted
alkynes 1b and 1c also gave the totally reduced prod-
ucts 2b and 2c in 80 and 57% yields, respectively
(entries 2 and 3). This reaction was monitored by TLC
until all starting material had disappeared. Similarly,
both cis-stilbene and trans-stilbene under the same
reaction conditions gave 1,2-diphenylethane 2a in 88%
yields, in both cases (entries 4 and 5).
1
2
products, for example pheromones. The best method
for the selective reduction of alkynes to the (Z)-alkenes
3
4
is catalytic hydrogenation using Lindlar’s catalyst or
5
P-2 nickel. In addition to these, new catalysts have
been developed such as the ZnꢀCu catalyst and the
(Ph P)CuH] catalyst. In our study on the palladium
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7
[
3
6
catalyzed reaction of aryl iodides with diphenyl-
acetylene, we noted that Pd(OAc)2 with sodium
methoxide can selectively reduce triple bonds to the
Surprisingly when the reaction of 1a was carried out
under the same reaction conditions except in THF, only
cis-stilbene 3a was obtained in 80% yield (entry 6).
Hydrogenation of 1c under the same conditions for 72
h gave the alkene 3c in 61% yield, nearly quantitative
based on the recovered starting alkyne (entry 7). The
ability to partially reduce in THF was further investi-
gated by carrying out the reaction with cis and trans
stilbene. No reduction of cis and trans stilbene was
observed using THF as solvent (entries 8 and 9).
corresponding double bonds and leave the aromatic
8
ring unaffected. We now report the Pd(OAc) –sodium
2
methoxide mediated reduction of alkynes selectively to
either alkenes or alkanes, depending on the choice of
reaction conditions.
The results of this initial investigation are given in
Table 1. The reactions were first studied using diphenyl-
acetylene 1a (0.5 mmol) with sodium methoxide (5
9
We have previously reported that 1d cyclized to 4d
without Pd(OAc)2 catalyst. On the other hand, the
equiv.) in the presence of Pd(OAc) (5 mol%) and PPh3
2
reaction of 1d with NaOCH in the presence of 5 mol%
3
of Pd(OAc) at reflux for 48 h gave 3d and the cycliza-
2
tion product 4d in 50% and 17% yields, respectively
(
entry 10) (Scheme 1). Reaction of 1e gave only the
partially reduced product 3e in 37% yield along with
2% of a dimer (entry 11). These results suggest that an
electron-withdrawing group on the aromatic ring
retards the reduction.
3
Keywords: palladium and compounds; b-hydride elimination; hydro-
genation.
*
Corresponding author.
0
040-4039/03/$ - see front matter © 2003 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(03)00029-7

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L.-L. Wei et al. / Tetrahedron Letters 44 (2003) 1979–1981
Table 1. Palladium-catalyzed reduction of diarylacetylenes
a
a
Entry
Substrate
Solvent
Time (h)/temp. (°C)
2 (Yield, %)
3 (Yield, %)
1
2
3
4
5
6
7
8
9
1a
1b
1c
MeOH
MeOH
MeOH
MeOH
MeOH
THF
THF
THF
THF
MeOH
MeOH
48 (25)
48 (25)
48 (25)
48 (25)
48 (25)
24 (25)
72 (25)
48 (25)
48 (25)
48 (65)
48 (65)
2a (92%)
2b (80%)
2c (57%)
2a (88%)
2a (88%)
cis-Stilbene
trans-Stilbene
1a
1c
cis-Stilbene
trans-Stilbene
1d
1e
3a (80%)
3c (61%)
No reaction
No reaction
b
10
1
3d (50%)
3e (37%)
1
a
Yields refer to isolated yields. All of the compounds gave satisfactory 1H and 13C NMR spectral data.
Another product 4d was obtained in 17% yield.
b
Scheme 1.
8
We previously proposed a mechanism for this reduc-
tion, as shown in Scheme 2. This mechanism involves
first a palladium methanolate 4 proceeding through
(Z)-alkenes or alkanes in good yields. When the solvent
was changed to THF, the reductions stop at the alkene
stage. We have also found that b-hydride elimination of
palladium methanolate provides an alternative route
for hydrogenolysis of organopalladiums. This paper
proposes a useful application where internal alkynes are
stereoselectively reduced to either (Z)-alkenes or
alkanes.
10
b-hydride elimination to give adduct 5. Next, addition
of 5 to the alkyne 1 gives s-vinylpalladium adduct 6
which exchanges one ligand with sodium methoxide to
give 7. Adduct 7 proceeds via b-hydride elimination to
give adduct 8. Finally, adduct 8 undergoes reductive
elimination to give product 3. The proposed mechanism
implies that the hydrogens come from the a-hydrogens
of methanol.
Acknowledgements
In conclusion, we have demonstrated that the reactions
of internal alkynes with sodium methoxide in the pres-
ence of Pd(OAc) and PPh in methanol give either
We are grateful to the National Science Council of the
Republic of China for financial support.
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3

L.-L. Wei et al. / Tetrahedron Letters 44 (2003) 1979–1981
1981
Scheme 2.
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