Catalytic Chemoselective and Stereoselective Semihydrogenation of Alkynes to E-Alkenes Using the Combination of Pd Catalyst and ZnI2

Article abstract of DOI:10.1021/acs.orglett.8b03295

An efficient E-selective semihydrogenation of internal alkynes was developed under low dihydrogen pressure and low reaction temperature from commercially available reagents: Cl₂Pd(PPh₃)₂, Zn⁰, and ZnI₂. Kinetic studies and control experiments underline the significant role of ZnI₂ in this process under H₂ atmosphere, establishing that the transformation involves syn-hydrogenation followed by isomerization. This simple and easy-to-handle system provides a route to E-alkenes under mild conditions.

Full text of DOI:10.1021/acs.orglett.8b03295

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
Catalytic Chemoselective and Stereoselective Semihydrogenation of
Alkynes to E‑Alkenes Using the Combination of Pd Catalyst and ZnI2
Radhouan Maazaoui,‡,† Raoudha Abderrahim,‡ Fabrice Chemla,† Franck Ferreira,†
Alejandro Perez-Luna,† and Olivier Jackowski*,†
†
́
́
Sorbonne Universite, CNRS, Institut Parisien de Chimie Moleculaire (IPCM), F-75252 Paris Cedex 05, France
‡
́
́
̀
́
́
Universite de Carthage, Faculte des Sciences de Bizerte, Laboratoire de Synthese Heterocyclique,7021 Jarzouna, Bizerte, Tunisia
S
* Supporting Information
ABSTRACT: An efficient E-selective semihydrogenation of internal alkynes was developed under low dihydrogen pressure and
low reaction temperature from commercially available reagents: Cl2Pd(PPh3)2, Zn0, and ZnI2. Kinetic studies and control
experiments underline the significant role of ZnI2 in this process under H2 atmosphere, establishing that the transformation
involves syn-hydrogenation followed by isomerization. This simple and easy-to-handle system provides a route to E-alkenes
under mild conditions.
he semihydrogenation of alkynes to alkenes is a
remarkable transformation of paramount importance in
Scheme 1. Selective Semihydrogenation of Alkynes to E-
Alkenes
T
synthetic fine chemistry.1 While reduction with Z-selectivity is
readily achieved through catalytic hydrogenation under H2
atmosphere with heterogeneous catalysts such as Lindlar’s
catalyst,2 obtaining E-selectivity requires Birch-type condi-
tions3 which often lack functional-group compatibility (see
Scheme 1a). To overcome this constraint, hydrosilylation/
desilylation strategies and related trans-hydrometalations have
been developed (Scheme 1b), although stoichiometric waste is
produced.4 In the same vein, catalytic E-selective semi-
hydrogenations of alkynes using hydrogen-transfer chemistry
or molecular hydrogen have been also explored.5 Yet, this
transformation remains challenging because most transition
metal-catalyzed semihydrogenations of C−C triple bonds
occur through cis-selective processes and E-alkenes are only
obtained upon isomerization of the initially formed Z-alkenes.
In such a situation, over-reduction of the alkene is a major
limitation. Hydrogen transfer semireductions of alkynes have
been completed using chromium reagents6 or transition-metal
catalysts such as Ir,7 Ni,8 Co,9 Ru,10 and Pd11 (Scheme 1c).
The use of molecular H2 has been also reported employing
different catalytic systems (Scheme 1d). While the metal-free
HB(C5F5)2/pentafluorostyrene system required high temper-
atures and pressures of H2 (140 °C and 50 bar),12 Co-,13 Fe-,14
and Ru-catalysts15 were disclosed to achieve trans-selective
alkyne semihydrogenations under lower temperatures and
pressures of H2 (below 90 °C and below 10 bar).
alcohols with Ru-catalyst15b and a ruthenium hydride catalyst
was reported to require only 10 equiv of H2.18 Finally, the
heterogeneous combination of Pd3Pb/SiO2 and RhSb/SiO2
Atmospheric pressure of H2 can be also used. Dinuclear Ir-
complexes16 and an Ag−Ru bimetallic complex17 were
described to be efficient under 1 atm of H2. Recently,
propargylic alcohol derivatives can be transformed to E-allylic
Received: October 15, 2018
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.8b03295
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
a
Table 1. E-Selective Semi-Hydrogenation of Alkynes: Optimization of Reaction Conditions
entry
catalyst
solvent
additive (equiv)
1 (%)
2 (%) (E/Z)
3 (%)
1
2
3
4
5
6
7
8
9
Pd(PPh3)4
Pd(PPh3)4
THF
THF
THF
THF
THF
THF
Et2O
CH3CN
DMF
THF
THF
THF
THF
THF
THF
THF
THF
THF
THF
THF
THF
−
0
54
9
1
4
63 (1.3/1)
46 (5/1)
81 (64/1)
0
83 (61/1)
86 (E)
12 (5/1)
25 (2/1)
75 (1.1/1)
55 (1/3)
85 (1/3.5)
62 (11.5/1)
98 (2/1)
18 (1/11)
33 (1.5/1)
5 (nd)
37
0
MeZnI (2)
MeZnI (2)
Cl2Pd(PPh3)2
Cl2Pd(PPh3)2
Cl2Pd(PPh3)2
Cl2Pd(PPh3)2
Cl2Pd(PPh3)2
Cl2Pd(PPh3)2
Cl2Pd(PPh3)2
Cl2Pd(PPh3)2
Cl2Pd(PPh3)2
Cl2Pd(PPh3)2
Cl2Pd(PPh3)2
Cl2Pd(PPh3)2
Cl2Pd(PPh3)2
Cl2Pd(PnBu3)2
Cl2Pd(POiPr3)2
Cl2Pd(dppp)
Cl2Pd(xantphos)
Pd-PEPPSI-iPr
Cl2Pd(PPh3)2
10
99
13
12
0
0
4
3
13
38
0
0
4
0
1
0
0
Me2Zn (0.2)
Me2Zn (0.2) + ZnI2 (2)
Zn0 (3) + ZnI2 (2)
Zn0 (3) + ZnI2 (2)
Zn0 (3) + ZnI2 (2)
Zn0 (3) + ZnI2 (2)
Zn0 (3) + ZnCl2 (2)
Zn0 (3) + ZnBr2 (2)
Zn0 (3) + Zn(OTf)2 (2)
Zn0 (3) + LiI (2)
Zn0 (3) + MgI2 (2)
Zn0 (3) + AlI3 (2)
Zn0 (3) + ZnI2 (2)
Zn0 (3) + ZnI2 (2)
Zn0 (3) + ZnI2 (2)
Zn0 (3) + ZnI2 (2)
Zn0 (3) + ZnI2 (2)
Zn0 (3) + ZnI2 (1)
2
88
75
21
42
2
10
11
12
13
14
15
16
17
18
19
20
0
2
82
63
95
55
79
95
94
0
44 (20/1)
21 (4/1)
5 (nd)
6 (nd)
96 (E)
0
4
b
21
a
b
1
Product distribution was determined by H NMR analysis. Reaction time was 8 h.
was related to achieve trans-semireductions under 1 atm of H2
at 25 °C.19
no better results were obtained (Table 1, entries 7−9). Then,
the influence of the counterion of the zinc salt was investigated.
The use of anions such as Cl− and Br− was not selective (Table
1, entries 10 and 11), while softer donor anions provided good
E-selectivities (Table 1, entries 6 and 12). However, alkane
proportion was dramatically increased with the use of
Zn(OTf)2 (Table 1, entry 12). Metal salts with other main-
group cations were also considered. Replacing ZnI2 by LiI or
MgI2 or AlI3 diminished E-selectivity and/or conversion
(Table 1, entries 13−15). Definitively, ZnI2 emerged as the
salt of choice to combine reactivity (full conversion) and
selectivity (excellent E-stereoselectivity and low amounts of
alkane).
Finally, we investigated the influence of the palladium ligand,
but the results obtained with the simplest PPh3 ligand were not
improved. Indeed, a moderate conversion (44%) with a good
E-selectivity was obtained with a less electron-donating ligand
(Table 1, entry 17), while the uses of more electron-donating
or bidentate ligands were detrimental (Table 1, entries 16, 18,
and 19). With Pd complex bearing a NHC ligand, only 6%
conversion of alkyne was observed (Table 1, entry 20).
Conditions were fully optimized by diminishing the amount of
ZnI2 to 1 equiv (lower amounts offered erratic results) and
setting the reaction time to 8 h (alkane formation was limited
to 4%) (Table 1, entry 21).
Our interest in semihydrogenation was instigated by the
observation that zinc salts tamed the catalyst activity in the
catalytic hydrogenation of alkenes.20 We thus speculated that
catalytic systems modified with zinc salt additives could allow
for the partial hydrogenation of alkynes to alkenes with a
subsequent Z-to-E isomerization. Herein we report the
development of such a catalytic system competent for the
semihydrogenation of internal alkynes to E-alkenes (Scheme
1).
We initiated our study by investigating the partial hydro-
genation of diphenylacetylene 1 as model substrate (Table 1).
First, a control experiment was performed with Pd(PPh3)4 in
the absence of zinc additive: full conversion was observed but a
mixture of alkane, Z- and E-alkenes was obtained (Table 1,
entry 1). By contrast, adding MeZnI to the system (Table 1,
entry 2) allowed for ∼50% conversion with an encouraging E/
Z ratio (5:1). In this transformation, the isomerization step
without alkane formation is favored thanks to organozinc
reagents. We then considered using Cl2Pd(PPh3)2 as
precatalyst and its in situ reduction by MeZnI. A noticeable
improvement was observed: the E/Z ratio reached 64:1 while
keeping alkane formation at 10% (Table 1, entry 3). On the
other hand, when the reaction was conducted with Me2Zn as a
reducing agent (0.2 equiv), 1,2-diphenylethane 3 was the only
product (Table 1, entry 4). However, the desired catalytic
semihydrogenation was obtained upon addition of 2 equiv of
ZnI2 to the previous system: an excellent E-selectivity and a
low proportion of 3 were obtained (Table 1, entry 5). Zn metal
was then used as a reducing agent. E-stilbene 2 was delivered
in 86% yield with limited alkane formation (Table 1, entry 6).
We also screened different solvents under these conditions, but
We then considered the semihydrogenation of a range of
substrates bearing a variety of functional groups to examine if
the alkene formation could be sensitive to the electron density
of the C−C triple bond. For some cases, the reactions had to
be stopped before full conversion to prevent significant alkane
formation (Scheme 2). Nonsymmetrical diphenylacetylenes
having one ring substituted with one electron-donating group
proceeded with excellent E/alkane ratios (2b−2e). The
B
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Letter
a
Scheme 2. E-Selective Semi-Hydrogenation of Alkynes: Scope and Limitations
a
1
Product distribution was determined by H NMR analysis.
transformation was compatible with chloride substitution as
alkene 2f was obtained in a good yield with an acceptable E/
alkane ratio (90:10). Nonsymmetrical diphenylacetylenes
having one ring substituted with one electron-withdrawing
group also proceeded in excellent yields and E/alkane ratios
(2g−2i). Formation of CF3-substituted stilbene 2j required 72
h of reaction time and 20 mol % of Pd catalyst to reach 89%
yield and an excellent E/alkane selectivity (96:4). Conversely,
introducing an electron-donating group (OMe) in para
position on the other phenyl part to balance the CF3
electron-withdrawing effect led to a suitable yield (69%) and
a decent E/alkane ratio (85:15) within only 7 h (2k).
Compound 2l was obtained in a moderate yield, because of its
low solubility in THF that needed the addition of dichloro-
methane, but the E/alkane selectivity was still excellent.
Finally, nonsymmetrical heteroaryl derivatives were also good
partners, as illustrated by the semihydrogenation delivering 2m
and 2n. A slight decrease in the E/Z selectivity (93.5:6.5) was
observed for compound 2m, and this was attributed to
competitive binding of nitrogen to the catalyst that could
thwart the isomerization. Diene 2o formation required 20
mol % of Pd catalyst to reach 90% conversion with a total E-
selectivity. Replacing one aryl moiety by an alkyl motif was also
tolerated. Overhydrogenation was almost nonexistent (1%)
and a good conversion with an excellent E-selectivity (97:3)
was obtained for alkene 2p. Slightly worst results were
observed for the formation of 2q: E-selectivity was lower (E/
Z ratio = 91:9) and alkane formation was slightly higher (9%).
However, from 7-tetradecyne, no alkane was formed but the
isomerization step was not efficient enough as a 34:66 E/Z
ratio was detected for alkene 2r. The semireduction of
trimethylsilyl phenylacetylene gave only 10% of the corre-
sponding E-vinyl silane. In the reaction medium, it was
produced but it was directly transformed to ethylbenzene by
Pd-catalyzed desilylation and subsequent hydrogenation of
styrene.
To get more information about the transformation,
monitoring studies were conducted on the semihydrogenation
of diphenylacetylene (Figure 1). At the beginning (Figure 1),
rapid formation of Z-stilbene was observed and reached a
maximum after 4 h of reaction. In the same time, E-stilbene
was also accumulated and a 1:1:1 mixture of alkyne/E/Z was
observed after 4 h. During the following 3 h of reaction, the
amount of E-stilbene increased steadily to reach 97% whereas
both Z-stilbene and diphenylacetylene were totally consumed.
Thus, the semihydrogenation of diphenylacetylene into E-
stilbene was achieved within 7 h. These studies lend clear
evidence that E-alkene formation with our new catalytic
conditions involving the combination of ZnI2 and Pd0 complex
is achieved through a pathway entailing semihydrogenation
C
DOI: 10.1021/acs.orglett.8b03295
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Letter
instead of Cl2Pd(PPh3)2 when only Zn0 was present, 45%
conversion was obtained within 24 h but low E/Z selectivity
was observed (Table 2, entry 1). By contrast only 5%
conversion was observed with Cl2Pd(PPh3)2 (Table 2, entry
2). Seemingly, I2Pd(PPh3)2 precatalyst is more easily reduced
and the presence of ZnI2 allows to regain a high transformation
(Table 2, entry 3). In fact, ZnI2 appears to be involved at two
levels: the Pd0 generation through I2Pd(PPh3)2 formation and
the E-selective semihydrogenation kinetics. As isomerization
and over reduction are involved in this catalytic process,
specific experiments with E- and Z-stilbenes were then
conducted to acquire more information about these steps. In
a first experiment, H2 atmosphere was removed and no
reaction was observed (Table 2, entry 4). Then two reactions
were conducted on Z-stilbene under H2 atmosphere using
Me2Zn as a reducing agent. In the presence of ZnI2, a mixture
of E-stilbene and 1,2-diphenylethane 3 was obtained in 84/16
ratio (Table 2, entry 5), whereas, in the absence of ZnI2, only
compound 3 was obtained (Table 2, entry 6).
Finally, suppressing the precatalyst resulted in a strong drop
in the Z-stilbene isomerization (Table 2, entry 7). At last,
alkane formation from E-stilbene was also investigated and
ZnI2 appears to significantly reduce alkene hydrogenation
(Table 2, entries 8 vs 9). Overall, the newly developed catalytic
E-selective semihydrogenation of alkynes can be explained
through a syn-hydrogenation/isomerization pathway. First ZnI2
interacts with the PdII precatalyst to deliver I2Pd(PPh3)2,
which is reduced to Pd0. Note that ZnI2 can also dramatically
modify the nature of Pd species and, a “cocktail system” of
mononuclear Pd and/or clusters and/or nanoparticles could be
envisioned in the absence of any further information
concerning its specific structure.22 Preliminary studies
conducted in the presence of Hg0 suggest the existence of
heterogeneous species in our “cocktail system”.23 Hence, this
Figure 1. Monotoring studies: from diphenylacetylene toward E-
stilbene. (Proportion was determined by GC analysis.)
and subsequent isomerization. Alkane formation was only
detected after 6 h of reaction when the amount of E-stilbene
reached 75%. Furthermore, monitoring studies on isomer-
ization and over-reduction conducted, respectively, from Z-
stilbene and E-stilbene (see the Supporting Information)
suggest that 1,2-diphenylethane 3 arises essentially from the
reduction of E-stilbene.
Then, to understand the role of ZnI2 better, several
experiments were conducted. 31P NMR (162 MHz in
CDCl3) analysis evidenced that the peak for Cl2Pd(PPh3)2
shifted from 23.73 ppm to 13.31 ppm upon the addition of
ZnI2 (see the Supporting Information). By comparison, freshly
21
prepared I2Pd(PPh3)2 presented a signal at 13.11 ppm. This
suggests that chlorine exchange by iodine occurs and delivers
I2Pd(PPh3)2. This precatalyst was then used for other control
experiments (Table 2). First, by introducing I2Pd(PPh3)2
a
Table 2. Control Experiments: Role of ZnI2
a
1
Product distribution was determined by H NMR analysis.
D
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Letter
trans-Hydrometalation of Internal Alkynes. Applications to Natural
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and ZnI2 is essential to promote isomerization without alkane
formation.
In conclusion, we have developed a catalytic process for the
semihydrogenation of internal alkynes with excellent E-
selectivity. Monitoring studies and control experiments
underline the significant role of ZnI2 in this process,
establishing that the transformation involves syn-hydrogena-
tion, followed by isomerization under a H2 atmosphere.
Finally, this simple and easy-to-handle system constitutes a
route to E-alkenes under mild conditions. Further studies are
currently under investigation to determine the role of ZnI2
more precisely and to elucidate the structure of the active
catalytic species.
ASSOCIATED CONTENT
■
S
* Supporting Information
(9) Fu, S.; Chen, N.; Liu, X.; Shao, Z.; Luo, S.; Liu, Q. Ligand-
Controlled Cobalt-Catalyzed Transfer Hydrogenation of Alkynes:
Stereodivergent Synthesis of Z- and E-Alkenes. J. Am. Chem. Soc.
2016, 138, 8588−8594.
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.8b03295.
Experimental data, product characterizations, and
spectral data for new compounds (PDF)
(10) (a) Li, J.; Hua, R. Stereodivergent Ruthenium-Catalyzed
Transfer Semihydrogenation of Diaryl Alkynes. Chem. - Eur. J. 2011,
17, 8462−8465. (b) Schabel, T.; Belger, C.; Plietker, B. A Mild
Chemoselective Ru-Catalyzed Reduction of Alkynes, Ketones, and
Nitro Compounds. Org. Lett. 2013, 15, 2858−2861. (c) Musa, S.;
Ghosh, A.; Vaccaro, L.; Ackermann, L.; Gelman, D. Efficient E-
Selective Transfer Semihydrogenation of Alkynes by Means of
Ligand-Metal Cooperating Ruthenium Catalyst. Adv. Synth. Catal.
2015, 357, 2351−2357. (d) Kusy, R.; Grela, K. E- and Z-Selective
Transfer Semihydrogenation of Alkynes Catalyzed by Standard
Ruthenium Olefin Metathesis Catalysts. Org. Lett. 2016, 18, 6196−
6199.
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: olivier.jackowski@sorbonne-universite.fr.
ORCID
Alejandro Perez-Luna: 0000-0002-5578-9024
Olivier Jackowski: 0000-0001-9162-3294
Notes
(11) (a) Shirakawa, E.; Otsuka, H.; Hayashi, T. Reduction of
Alkynes into 1,2-dideuterioalkenes with Hexamethyldisilane and
Deuterium Oxide in the Presence of a Palladium Catalyst. Chem.
Commun. 2005, 5885−5886. (b) Luo, F.; Pan, C.; Wang, W.; Ye, Z.;
Cheng, J. Palladium-catalyzed Reduction of Alkynes Employing
HSiEt3: Stereoselective Synthesis of trans- and cis-Alkenes.
Tetrahedron 2010, 66, 1399−1403. (c) Shen, R.; Chen, T.; Zhao,
Y.; Qiu, R.; Zhou, Y.; Yin, S.; Wang, X.; Goto, M.; Han, L.-B. Facile
Regio- and Stereoselective Hydrometalation of Alkynes with a
Combination of Carboxylic Acids and Group 10 Transition Metal
Complexes: Selective Hydrogenation of Alkynes with Formic Acid. J.
Am. Chem. Soc. 2011, 133, 17037−17044. (d) Zhong, J.-J.; Liu, Q.;
Wu, C.-J.; Meng, Q.-Y.; Gao, X.-W.; Li, Z.-J.; Chen, B.; Tung, C.-H.;
Wu, L.-Z. Combining Visible Light Catalysis and Transfer Hydro-
genation for in situ Efficient and Selective Semihydrogenation of
Alkynes under Ambient Conditions. Chem. Commun. 2016, 52,
1800−1803.
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
́
We thank to Universite de Carthage for financial support (to
R. Maazaoui).
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mmol). When mercury was directly added at the beginning to the
reaction mixture no conversion was observed after 6 h. In a second
experiment, reaction was carried out normally within 2 h (conversion
≈ 40%) and then Hg0 was added. No further evolution was observed
during the following 20 h.
F
DOI: 10.1021/acs.orglett.8b03295
Org. Lett. XXXX, XXX, XXX−XXX