Water as a Hydrogenating Agent: Stereodivergent Pd-Catalyzed Semihydrogenation of Alkynes

Article abstract of DOI:10.1021/acs.orglett.9b00148

Palladium-catalyzed transfer semihydrogenation of alkynes using H₂O as the hydrogen source and Mn as the reducing reagent is developed, affording cis- and trans-alkenes selectively under mild conditions. In addition, this method provides an efficient way to access various cis-1,2-dideuterioalkenes and trans-1,2-dideuterioalkenes by using D₂O instead of H₂O.

Full text of DOI:10.1021/acs.orglett.9b00148

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
Water as a Hydrogenating Agent: Stereodivergent Pd-Catalyzed
Semihydrogenation of Alkynes
Chuan-Qi Zhao, Yue-Gang Chen, Hui Qiu, Lei Wei, Ping Fang, and Tian-Sheng Mei*
State Key Laboratory of Organometallic Chemistry, Center for Excellence in Molecular Synthesis, Shanghai Institute of Organic
Chemistry, University of Chinese Academy of Sciences, Chinese Academy of Sciences, 345 Lingling Lu, Shanghai 200032, China
S
* Supporting Information
ABSTRACT: Palladium-catalyzed transfer semihydrogenation
of alkynes using H2O as the hydrogen source and Mn as the
reducing reagent is developed, affording cis- and trans-alkenes
selectively under mild conditions. In addition, this method
provides an efficient way to access various cis-1,2-dideuter-
ioalkenes and trans-1,2-dideuterioalkenes by using D2O instead
of H2O.
onstruction of carbon−carbon double bond is of great
Scheme 1. Palladium-Catalyzed Transfer Hydrogenation
from Water
C
importance in organic synthesis since alkenes are
essential structural units in many natural products, pharma-
ceuticals, and agrochemicals.1 Transition-metal-catalyzed semi-
hydrogenation of alkynes using hydrogen gas has been
developed as a reliable tool for the stereoselective construction
of alkenes in the laboratory and on an industrial scale.2,3
Alternatively, transfer hydrogenation has emerged as a
promising tool to construct alkenes since it avoids the use of
a flammable gas and elaborate experimental setups.4 In this
context, various H donors such as formic acid,5 silanes,6
alcohols,7 and others have been used in catalytic semi-
hydrogenation of alkynes.8 However, there are few examples
of semihydrogenation reactions that use water as a hydro-
genating agent despite the cost and safety advantages
associated with its use.9,10 Further, due to the importance of
deuterium-labeled molecules as tools for biological research
and the elucidation of reaction mechanisms, the transfer
deuteriation of alkynes using D2O to obtain alkenes
deuteriated at olefinic positions is highly important.11,12
In 2005, the Hayashi group developed catalytic semi-
hydrogenation of diarylalkynes to provide trans-alkenes using
H2O and hexamethyldisilane as the hydrogen source (Scheme
1a).9 In 2007, Oltra and co-workers reported catalytic
semihydrogenation of alkynes to achieve cis-alkenes using
H2O and a stoichiometric amount of titanocene(III) chloride
with the aid of Lindlar catalyst (Scheme 1b).10a
Transfer hydrogenation of alkenes to alkanes with D2O was
also demonstrated, and incomplete deuterium incorporation
was obtained. In 2016, Stokes and co-workers elegantly
developed a diboron-mediated Pd-catalyzed transfer hydro-
genation of alkenes and alkynes using H2O as a hydride source
at room temperature (Scheme 1c).10b Nevertheless, catalytic
stereodivergent semihydrogenation of alkynes using H2O to
produce cis- and trans-alkenes with great selectivity and
efficiency is still lacking.13,14 Herein, we report a Pd-catalyzed
semihydrogenation of alkynes to access cis- and trans-alkenes
using water as the hydrogenating agent (Scheme 1d). In
addition, we demonstrate that cis-1,2-dideuterioalkenes are
selectively obtained from alkynes using D2O, the most
inexpensive and convenient deuterium source. A preliminary
mechanistic study indicates that trans-alkenes are derived from
cis-alkenes through isomerization and palladium nanoparticles
(PdNPs) with an average diameter of 1.3 nm are the active
catalysts.
Initially, we chose diphenylacetylene (1a) as a model
substrate and H2O as a hydrogenating agent and probed
Received: January 14, 2019
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.9b00148
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
a b
,
various reaction conditions for the envisioned semihydroge-
nation. After extensive optimization, we found that 88%
isolated yield of cis-stilbene (2a) could be obtained in the
presence of 1 mol % of Pd(OAc)2, 2 equiv of Mn, and 80 equiv
of water in CH3CN at room temperature after 24 h (Table 1,
Scheme 2. Cis-Selective Alkyne Semihydrogenation
a b
,
Table 1. Reaction Optimization with Substrate 1
2a
b
b
conv
yield
entry
variation from standard conditions
none
20 equiv of H2O in lieu of 80 equiv of
H2O
(%)
2a/3a
(%)
c
1
2
98
10
96/4
89/11
88
8
52
75
3
4
40 equiv of H2O in lieu of 80 equiv of
H2O
160 equiv of H2O in lieu of 80 equiv of
H2O
55
82
96/4
95/5
5
6
DMF in lieu of CH3CN
5 mol % of Pd/C in lieu of
1 mol % of Pd(OAc)2
32
14
95/5
99/1
31
14
7
1 mol % of Pd(PPh3)4 in lieu of
1 mol % of Pd(OAc)2
0
0
8
9
Zn in lieu of Mn
no Mn or no Pd(OAc)2 or no H2O
0
0
0
0
a
Reaction conditions: 1a (0.25 mmol), Pd(OAc)2 (1 mol %), Mn (2
b
equiv), H2O (80 equiv), CH3CN (2 mL), rt for 24 h. The yield and
cis/trans ratio was determined by HNMR using dibromomethane as
the internal standard. Isolated yield.
1
a
b
c
Isolated yields are reported unless otherwise noted. Cis/trans ratio
c
of alkenes in parentheses was determined by GC−FID. The yield was
determined by 1H NMR with dibromomethane as the internal
standard.
entry 1). Decreasing and increasing the amount of water
resulted in lower yields (entries 2−4). Evaluating different
solvents revealed that CH3CN was optimal (entry 5). The
yield decreased significantly when Pd(OAc)2 was replaced with
Pd/C or Pd(PPh3)4 (entries 6 and 7). Finally, control
experiments indicated that Mn, Pd(OAc)2, and H2O are all
essential for this reaction (entries 8 and 9).
With the optimized reaction conditions in hand, the scope of
alkynes was investigated to test the generality and limitations
of this Pd-catalyzed semihydrogenation reaction. As shown in
Scheme 2, both aromatic (2b−s) and aliphatic internal alkynes
(2t−y) are readily hydrogenated to the desired cis-alkenes with
good to excellent yield and cis-stereoselectivity. Alkynes
substituted with a variety of functional groups such as ether,
alkyl, chloro, acyl, ester, cyano, amide, formyl, aldehyde,
hydroxyl, and amino groups (2b−x) were well tolerated under
the standard conditions. It is worth noting that thiophene- and
pyridine-substituted alkynes proved compatible (2h and 2i).
To our delight, nonconjugated internal alkynes also gave
excellent yield with high cis-stereoselectivity (2t, 2u, and 2w−
y).
reaction was conducted in DMF, excellent trans-selectivity
could be achieved with moderate yield (entry 2). Fortunately,
lowering the amount water to 20 equiv could further improve
the yield of the desired product (3a) to 80%, along with 20%
over-reduction product (4a) (entry 3). Thus, controlling the
amount of water is crucial to overcome the over-reduction side
reaction. However, the trans-selectivity decreased with further
lowering of the amount of water (entries 4 and 5). To our
delight, the conversion increased rapidly when Mg(OAc)2 was
added (entries 6−8). trans-Stilbene (3a) could be obtained in
good yield (89%) with excellent trans-selectivity using 7 equiv
of H2O and 3 equiv of Mg(OAc)2 (entry 8). The role of
Mg(OAc)2 is not fully understood at the current stage,
although it was proposed that Lewis acid may help to activate
H2O.16 Other Lewis acids such as MgCl2, MgBr2, LiOAc,
NaOAc, and Ba(OAc)2 are not as effective as Mg(OAc)2
(entries 9−13).
Trans-selective semihydrogenation of alkynes is more
challenging. We envisioned that trans-alkenes could be formed
from cis-alkenes at elevated temperature through isomerization,
since trans-alkenes are typically the thermodynamically favored
products.15 To our delight, trans-stilbene (3a) could be
obtained in moderate yield (42%) with 8% cis-stilbene (2a)
in the presence of 5 mol % of Pd(OAc)2, 2 equiv of Mn, and
80 equiv of H2O in CH3CN at 80 °C (entry 1, Table S7; see
the Supporting Information for details). The low yield is due to
the formation of alkane 4a by over-reduction. When the
Next, we explored the scope of trans-stereoselective
semihydrogenation of alkynes. As shown in Scheme 3,
aromatic internal alkynes are hydrogenated to the desired
trans-alkenes with satisfactory yield and excellent stereo-
selectivity (3a−l). Importantly, substrates containing various
functional groups, including alkyl, ether, amino, fluoro, ester,
acyl, cyano, thiophene, amide, and hydroxyl, were tolerated.
Unfortunately, 3-phenyl-2-propynenitrile and 3-phenyl-2-pro-
pyn-1-ol give low yields due to over-reduction and isomer-
ization.
B
DOI: 10.1021/acs.orglett.9b00148
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
a b
,
a−c
Scheme 3. Trans-Selective Alkyne Semihydrogenation
Scheme 4. Cis-Selective Alkyne Semireduction with D2O
a
b
Isolated yields are reported unless otherwise noted. Cis/trans ratio
c
of alkenes in parentheses was determined by GC−FID. Method A: 1
(0.25 mmol), Pd(OAc)2 (5 mol %), Mn (2 equiv), Mg(OAc)2 (3
equiv), H2O (7−10 equiv), DMF (2 mL), 80 °C for 24 h. Method
B: 1 (0.25 mmol), Pd(OAc)2 (5 mol %), Mn (2 equiv), H2O (20
equiv), DMF (2 mL), 80 °C for 24 h. Cis/trans ratio of alkene
determined by H NMR.
d
e
a
b
Isolated yield unless otherwise noted. Cis/trans ratio of alkene in
1
c
the parentheses was determined by GC-FID. D/H ratio of alkene
d
was determined by 1H NMR. Reaction conditions A: 1 (0.25 mmol,
Encouraged by the feasibility of catalytic stereodivergent
semihydrogenation of alkynes using H2O as a hydride source,
we moved on to examine the deuteration of alkynes using D2O.
To our satisfaction, as shown in Scheme 4, cis-1,2-
dideuterioalkenes were obtained in good yields and excellent
stereoselectivity. Various functional groups, including ether,
halide, trifluoromethyl, ester, cyano, and amide, were tolerated
under the reaction conditions (5a−m). Importantly, high
deuterium incorporation was achieved under optimal reaction
conditions. Meanwhile, high deuterium incorporation of trans-
1,2-dideuterioalkenes can also be prepared in this protocol
(5n−p). To the best of our knowledge, this is the first example
of Mn-mediated stereodivergent deuteriation of alkynes to
achieve cis-alkenes and trans-alkenes using D2O.
1.0 equiv), Pd(OAc)2 (3−5 mol %), Mn (0.5 mmol, 2 equiv), D2O
(20 mmol, 80 equiv), CH3CN (2 mL) at room temperature for 24 h.
e
Reaction conditions B: 1 (0.25 mmol), Pd(OAc)2 (5 mol %), Mn (2
f
equiv), D2O (25 equiv), DMA (2 mL), 80 °C for 24 h. Cis/trans ratio
of alkene determined by H NMR.
1
Scheme 5. Synthetic Applications
The scalability of this transfer semihydrogenation was
evaluated using a reaction containing 10.7 g (60.0 mmol) of
substrate 1a (Scheme S1). Specifically, the reaction between 1a
and H2O furnished the desired product in 95% yield with
excellent cis-stereoselectivity (96:4), which showcases the
preparative utility of this semihydrogenation. The utility of
this catalytic stereodivergent semihydrogenation was further
demonstrated by synthesis of natural products of cis-
combretastatin A-4 (7) and analogues (8−10) (Scheme 5).17
To gain insight into the reaction mechanism, the kinetic
behavior of this reaction was investigated. The kinetic profile of
this reaction clearly showed that the cis-alkene intermediate 2a
was generated in the first 1.5 h and that 2a was further
transformed to the trans-alkene 3a through an isomerization
process (Figure S1; see the Supporting Information for
details).15 Furthermore, the kinetic isotope effect (KIE)
experiments were carried out (Scheme S2). Subjection of 1a
to the reaction conditions in the presence of an equimolar
mixture of H2O and D2O revealed a primary kinetic isotope
effect of 6.1. Parallel experiments show similar results(kH/kD =
5.0). These results indicate that the O−H cleavage of water is
the rate-determining step.10b Additionally, a mercury drop
experiment was performed and resulted in a significant
decrease in the conversion of 1a, which indicates that the
reaction may be involved in metal nanoparticles (Scheme
S3).18 As direct evidence in support of the nanoparticulate
nature of the catalyst, transmission electron microscopy
(TEM) revealed the in situ generated, narrowly dispersed Pd
C
DOI: 10.1021/acs.orglett.9b00148
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
nanoparticles with an average size of 1.3 nm (Figure S2). The
nanoparticles were also analyzed using energy-dispersive X-ray
spectroscopy (EDX) to confirm the composition of palladium
nanoparticles rather than the Pd−Mn bimetallic particles
(Figure S3). Thus, we establish for the first time that palladium
nanoparticles could be prepared via such a convenient method
using Mn as a reducing agent.19
Tian-Sheng Mei: 0000-0002-4985-1071
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was financially supported by the Strategic Priority
Research Program of the Chinese Academy of Sciences (Grant
No. XDB20000000), “1000-Youth Talents Plan”, NSF of
China (Grant Nos. 21772220 and 21821002), and S&TCSM
of Shanghai (Grant Nos. 17JC1401200 and 18JC1415600).
Based on our data, we propose the mechanism depicted in
Scheme 6. Initially, Pd(OAc)2 is reduced by Mn to generate
Scheme 6. Proposed Reaction Mechanism
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ASSOCIATED CONTENT
* Supporting Information
■
S
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Text, figures, and table giving experimental procedures
and compound characterization data (PDF)
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: mei7900@sioc.ac.cn.
ORCID
Yue-Gang Chen: 0000-0001-6012-1849
Ping Fang: 0000-0002-3421-2613
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DOI: 10.1021/acs.orglett.9b00148
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