Cobalt catalyzed stereodivergent semi-hydrogenation of alkynes using H2O as the hydrogen source

Article abstract of DOI:10.1039/c9cc01970g

Cobalt-catalyzed stereodivergent semi-hydrogenation of internal alkynes to alkenes is developed. The reaction proceeded through transfer hydrogenation under mild conditions using a base metal CoI₂ as the catalyst, and H₂O/MeOH as the hydrogen source with Zn as the reductant. The E/Z-selectivity of the product could be switched by changing the solvent and by inclusion/exclusion of a bidentate phosphine ligand (dppe). This method provides a simple and cost effective pathway for the synthesis of 1,2-dideuterioalkenes.

Full text of DOI:10.1039/c9cc01970g

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Marilyn M. Olmstead, Alan L. Balch, Josep M. Poblet, Luis Echegoyenet al.
Reactivity differences of Sc3N@C2n (2n 68 and 80). Synthesis of the
first methanofullerene derivatives of Sc N@D5h-C80
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DOI: 10.1039/C9CC01970G
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DOI: 10.1039/x0xx00000x
Cobalt Catalyzed Stereodivergent Semi-hydrogenation of Alkynes
using H2O as the Hydrogen Source
Received 00th January 20xx,
Accepted 00th January 20xx
Kangkui Li,†,§ Ruhima Khan,†,§ Xuexin Zhang,† Yang Gao,† Yongyun Zhou,*† Heng Tan,† Jingchao
Chen,† and Baomin Fan*†,‡
Cobalt-catalyzed stereodivergent semi-hydrogenation of internal asymmetric TH reactions using alcohols,8a amines8b,c as
alkynes to alkenes is developed. The reaction proceeded through hydrogen sources. We have recently demonstrated Pd/Zn co-
transfer hydrogenation under mild conditions using base metal catalyzed asymmetric TH of heterobicyclic alkenes using H2O
CoI2 as the catalyst, H2O/MeOH as hydrogen source with Zn as as the H source.8d Very recently, Mei et al reported the Pd-
reductant. The E/Z-selectivity of the product could be switched by catalyzed transfer semi-hydrogenation of alkynes with H2O as
changing the solvent and by inclusion/exclusion of a bidentate the H source and Mn as the reductant.9 Herein, we report a
phosphine ligand (dppe). This method provides a simple and cost very simple catalytic system for the semi-hydrogenation of
effective pathway for the synthesis of 1,2-dideuterioalkenes.
alkynes to the corresponding Z-selective alkenes. The reaction
is catalysed by CoI2 with H2O/MeOH as the H source and Zn as
the reductant. Interestingly, the stereoselectivity can be
switched to the E-alkene just by adding the bidentate
phosphine ligand (dppe) with H2O as the sole H source in
CH3CN solvent. We believe that our newly developed reaction
condition is more mild and cost effective for the semi-
reduction of alkynes to alkenes and for the synthesis of 1,2-
dideuterioalkenes using H2O as the hydrogen source.
Among the hydrogenation reactions, semi-reduction of alkynes
to alkenes is one of the most important transformations in
organic chemistry because of its widespread application in the
synthesis process of pharmaceuticals, agrochemicals, natural
products, and fine fragrance components.1 The most classic
method for the semi-reduction of alkynes is by using Lindlar
catalyst.2 There has been development of various alternate
catalysts for the transformation of alkynes to alkenes.3
A
Initially, the hydrogenation reaction of diphenylacetylene (1a)
to obtain 1,2-diphenylethene was chosen as the model system
for the present study. The reaction was carried out under the
catalytic system of CoI2 (5 mol%), Zn (3 equiv) and H2O (10
equiv) in CH3CN at 60 oC without using any ligand. The partially
reduced product was obtained only in trace amount (Table 1,
entry 1). To our delight, when the solvent was changed to
MeOH with other conditions remaining same, the desired
product with Z-selectivity was observed with 98% conversion
and Z/E ratio of 90:10 in 84 h (Table 1, entry 2). Other solvents
such as 1,4-dioxane, DCE, toluene, DMF, DMSO and THF failed
to deliver the desired product in appreciable yield. Gratifying,
further improvement in the Z/E selectivity was observed when
10 mol% of CoI2 was used (Table 1, entry 3). On further
detailed optimization of the reaction conditions, it was found
that the most suitable quantity of H2O is 10 equivalent and
number of catalytic systems involving transition metals in
combination with H2 gas or transfer hydrogenating agents has
been reported for reduction of alkynes to alkenes.4 The
replacement of expensive precious metals by cheap, less toxic,
environmentally benign and abundant base metals for similar
or better results is an important focus of current chemical
research. For the first time, Liu and co-workers reported the
cobalt catalysed transfer hydrogenation (TH) of alkynes to
generate both Z- and E-alkenes with NH3-BH3 as the hydrogen
source.5 Subsequently, cobalt-catalyzed Z-selective semi-
hydrogenation of internal alkynes under H2 atmosphere,6a
NH3BH3 as a transfer hydrogenating source6b was developed.
Recently, H2O has emerged as a source of hydrogen paving a
way for environmentally benign and green pathway. The
employment of H2O as a source of H in the transfer semi-
hydrogenation of alkynes to Z- and E-alkenes is still rare.7
Recently, our group has been paying interest in the
o
reaction temperature is 60 C. Other metals such as Mn, Fe
and Mg were not suitable for the present reaction indicating
that the most ideal reductant is Zn (3 equivalents). (See Table
S1 in supporting information for detailed optimization studies).
Table 1: Optimization Table for the Z-selective transfer hydrogenation
of 1aa
†
YMU-HKBU Joint Laboratory of Traditional Natural Medicine, Yunnan Minzu
University, Kunming, 650500, China; zhouyongyundf@163.com (Yongyun Zhou),
FanBM@ynni.edu.cn (Baomin Fan)
Key Laboratory of Chemistry in Ethnic Medicinal Resources, Yunnan Minzu
H
Ph
H
H
‡
[Co], Metal, H2O
Solvent, 60 o
+
Ph
Ph
University, Kunming, 650500, China.
Ph
H
Ph
Ph
C
§ These authors contributed equally.
1a
[Co]
2a (
Z)
Time Yieldb
(h) (%)
3a (
)
E
Electronic Supplementary Information (ESI) available: [details of any
supplementary information available should be included here]. See
DOI: 10.1039/x0xx00000x
Entry
Solvent
Metal
Z/Ec
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Journal Name
1
2
CoI2
CoI2
CoI2
CoI2
CoI2
CoI2
CH3CN
CH3OH
CH3OH
EtOH
Zn
Zn
Zn
Zn
Zn
Mn
48
84
78
84
84
84
Trace
-
selective TH products in good to excellent yielVdieswwArittichle Ohnilginhe
stereoselectivity (2b-2g). Interestingly, alkynes with electron
98
98
75
37
14
90:10
91:9
90:10
85:15
-
DOI: 10.1039/C9CC01970G
3d
4
withdrawing groups were also compatible (2h-2k). Gratifyingly,
thiophene containing heterocyclic compound 1l also reacted
well to give the desired product 2l in 54% yield with excellent
stereoselectivity. Diphenyl acetylene 1a gave the Z-alkene 2a
with a turnover number (TON) of 28 with 88:12 Z/E selectivity
5
6
iPhOH
CH3OH
aReaction Conditions: 1a (0.2 mmol), [Co] (0.01 mmol), Zn powder
(0.6 mmol), H2O (2 mmol) in solvent (2 mL). bThe total yield of Z
and E-alkenes was determined by 1H NMR spectroscopy. The Z/E
c
ratio was determined by 1H NMR spectroscopy. dAdded 0.02 mmol
CoI2.
on loading 1 mol% CoI2.
Table 3: Substrate scope for Z-selective transfer hydrogenation of
internal alkynes.a,b,c
There has been reports on the switching of the selectivity of
the semi-hydrogenation products to furnish either E or Z-
isomer under similar reaction conditions.5,7c,7d Inspired from
these results, we envisioned whether we can change the
stereoselectivity by changing the reaction conditions and the
result of our investigation is summarized in Table 2. So, our
study commenced with a typical reaction of diphenylacetylene
(1a), CoI2(dppp) (5 mol%), H2O (10 equiv) and Zn (3 equiv) in
CoI2 (5 mol%)
H
Zn (3 equiv)
H2O (10 equiv)
H
R
R
R1
R1
2a-s
MeOH, 60 C
1a-s
NMR Yield
Z/E ratio
…………………………………………………………………………………………………………
H
H
H
H
H
H
H
H
H
H
o
CH3CN at 60 C for 6 h. Gratifyingly, the reaction afforded the
Ph
Ph
Ph
Ph
Ph
semi-hydrogenated product in 97% with E-selectivity in the Z/E
ratio of 12:88 (Table 2, entry 1). Changing the solvent to
methanol resulted into trace amount of product (Table 2,
entry 2). To our delight, excellent yield with Z/E-ratio of 3:97
was furnished when dppe was used (Table 1, entry 4). Other
ligands such as dppm, dppf, 1,10-phen, (±)-BINAP, Xantphos,
dppb and dppen could not perform better than dppe. Tested
solvents other that CH3CN failed to improve the results.
Further, the yield and the stereoselectivity of the reaction was
analysed by using different cobalt catalysts. However, there
was no improvement in the results. After extensive
investigation, we concluded that the most optimized reaction
conditions for the E-stereoselective transfer semi-
hydrogenation of 1a is the catalytic system of CoI2 (5 mol%),
dppe (0.012 mmol) and Zn (3 equiv) with H2O (10 equiv) as the
only hydrogen source in CH3CN at 60 oC (See Table S2 in
supporting information for detailed optimization studies).
Table 2: Optimization Table for the E-selective transfer hydrogenation
of 1aa
2c, 97%
92:8
over-reduced = 2%
2b, 95%
90:10
over-reduced = 2%
2d, 70%
95:5
2e, 95%
90:10
2a, 98%, 90:10
28%, 88:12d
over-reduced = 1% over-reduced = 1%
over-reduced = 2%
H
H
H
H
H
H
H
H
H
H
Ph
Ph
Ph
Ph
Ph
2j, 98%
91:9
Cl
F
MeO
Br
2i, 95%
80:20
over-reduced = 5%
H
2f, 96%
2h, 92%
60:40
over-reduced = 8%
H
2g, 81%
80:20
over-reduced = 3%
H
91:9
over-reduced = 3%
over-reduced = 2%
H
H
H
H
H
H
S
H
O
Ph
Ph
Ph
Ph
Ph
NC
HO
F3C
2o, 64%
80:20
over-reduced = 1%
2k, 50%
2l, 54%
92:8
2m, 94%
91:9
2n, 21%
76:24
O
95:5
over-reduced = 1% over-reduced = nd
over-reduced = 5% over-reduced = nd
H
H
H
H
H
H
O
H
H
Ph
H3C(H2C)4 (CH2)4CH3
O
Ph
H2N
Ph
O2N
2p, 78%
90:10
over-reduced = 2%
2r, 93%
90:10
over-reduced = trace
2q, 55%
90:10
over-reduced = 6%
2s, nd
…………………………………………………………………………………………………………
aReaction Conditions: 1a-s (0.2 mmol), CoI2 (0.01 mmol), Zn powder (0.6
mmol), H2O (2 mmol) in CH3OH (2 mL). bThe total yield of Z and E-alkenes
was determined by 1H NMR spectroscopy. cThe Z/E ratio was determined by
1H NMR spectroscopy. . 1a (0.3 mmol), CoI2 (1 mol%) in MeOH (3 mL) was
d
H
H
used. nd = not determined.
H
Ph
[Co], L, Zn, H2O
Solvent, 60 o
Ph
Ph
+
Ph
Ph
C
Ph
3a (
H
)
To our delight, functional groups such as ester, cyano, hydroxyl
and amino groups were well tolerated and afforded the
corresponding partially reduced Z-selective products 2m-2p in
low to high yields with good to excellent stereoselectivity. The
propargylic ester 1q also participated in the semi-
hydrogenation reaction. Unactivated alkyne 1r also reacted
well to give the corresponding alkene 2r in 93% yield. The
expected product 2s was not detected in the case of nitro
substituted alkyne. Instead, reduction of NO2 to NH2 took
place.
The compatibility of the developed catalytic system for the E-
selective transfer hydrogenation of various alkynes was also
explored (Table 4). Substituents on diarylacetylenes were not
much effective in determining the yield and selectivity as both
electron donating and withdrawing substituents were
amenable to the catalytic system giving the corresponding
alkenes (3b-3k) in good to excellent yields with good to
excellent stereoselectivity. Notably, acetylene containing
1a
2a (
)
Z
E
Entry
[Co]
L
Solvent
Time
(h)
6
72
48
10
48
48
16
18
Yield
(%)b
97
trace
14
99
trace
89
Z/Ec
1
2
CoI2(dppp)
CoI2(dppp)
CoI2
CoI2
CoI2
CoI2
CoBr2
CoCl2
-
-
CH3CN
CH3OH
CH3CN
CH3CN
CH3OH
1,4-dioxane
CH3CN
CH3CN
12:88
-
-
3:97
-
20:80
3:97
4:96
3d
4
PPh3
dppe
dppe
dppe
dppe
dppe
5
6
7
8
96
95
aReaction conditions: 1a (0.2 mmol), [Co] (0.01 mmol), ligand (0.012
mmol), Zn powder (0.6 mmol), H2O (2 mmol) in solvent (2 mL). bThe total
yield of Z and E-alkenes was determined by 1H NMR spectroscopy. cThe
Z/E ratio was determined by 1H NMR spectroscopy. dAdded 0.024 mmol
PPh3. L- Ligand.
With the optimization reaction conditions for the Z-selective
reduction, we investigated the substrate scope with respect to
various substituted alkynes (Table 3). Diaryl alkynes with
electron donating substituents afforded the corresponding Z-
2 | J. Name., 2012, 00, 1-3
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isomerization is not the reason for switching of seVleiecwtiAvritticyle O(nSleinee
Table S3 in supporting information forDOdI:e1t0a.i1l0e3d9/Cm9eCcCh0a1n9i7s0tiGc
studies).
heterocyclic group was also smoothly reduced to the
corresponding alkene 3l. Diarylacetylenes bearing -COOMe, -
CN, -OH and –NH2 groups were also well tolerated and
furnished the E-selective partially hydrogenated products (3m- Based on our mechanistic studies and literature
3p) Unfortunately, 3-phenylprop-2-yn-1-yl acetate 1q resulted precedence,5,10 we have hypothesized a plausible reaction
into diminished yield (3q) with appreciable stereoselectivity. mechanism for the present transformations (Scheme 2). For
The inactivated alkyne 1r gave the desired product 3r only in the Z-selective hydrogenation, firstly, reduction of CoI2 by Zn
trace amount. Reduction of NO2 group to NH2 and over to Co(I) will take place which will combine with H2O to
reduction to alkane was detected when nitro substituted generate intermediate A (Scheme 2(a)). Then, A will bind with
alkyne 1s was used. Notably, all the alkyne substrates did not 1a to form the complex B followed by insertion of alkyne into
undergo over reduction to the unwanted alkanes under the E- the Co-H bond to furnish alkenyl cobalt complex C. Next,
selective reaction conditions. On loading of 1 mol% CoI2, protonation of C by MeOH will lead to the formation of Z-
diphenyl acetylene 1a furnished the E-alkene 3a in 6:94 Z/E alkene 2a and the cobalt complex D. Intermediate D will
selectivity with a TON of 87.
interact with H2O and Zn to give out MeOH, Zn(OH)2 along with
regeneration of Co(I). The E-selective hydrogenation will follow
different pathway as it involves the ligand dppe and H2O is the
only H-source. It would probably follow the reaction pathway
as proposed in Scheme 2(b). Initially, combination of CoI2,
dppe and Zn will take place to generate the reduced Co-
complex E which will combine with D2O to form the
intermediate F. Further, F will interact with 1a to give complex
G which undergoes migratory insertion to generate H. In this
step, we hypothesized that in order to avoid the steric
hindrance caused by the Co/dppe complex in the conversion of
Complex G to H, the two aryl groups are rearranged into the
thermodynamically more stable trans positions in complex H.
Then, H will interact with D2O and Zn to furnish the final
product 3a and regenerate Co(I) complex E.
Table 4: Substrate scope for E-selective transfer hydrogenation of internal
alkynes.a,b,c
CoI2 (5 mol%)
dppe (6 mol%)
Zn (3 equiv)
H
Ph
P
R1
H2O (10 equiv)
Ph
R
P
Ph
R
R1
H
CH3CN, 60 C
3a-s
NMR Yield
Ph
dppe
1a-s
Z/E ratio
………………………………………………………………………………………………………………………
H
H
H
H
H
Ph
Ph
Ph
Ph
Ph
Ph
H
Ph
H
H
H
H
H
H
3a, 99%
3b
, 99%
5:95
3d, 94%
24:76
H
3e, 93%
15:85
H
3c, 93%
6:94
3:97
87%, 6:94d
H
Ph
Ph
Ph
Ph
H
H
H
H
H
F
MeO
Cl
Br
3f, 98%
3g, 90%
3j, 70%
3h, 95%
3i
, 97%
H
H
7:93
5:95
5:95
5:95
5:95
CoI2 (5 mol%), Zn (3 equiv)
Ph
Ph
(a)
H
H
H
H
Ph
Ph
H
D2O (10 equiv), MeOH, 60 oC, 96h
1a (0.2 mmol)
Ph
Ph
Ph
82% isolated yield
Ph
S
H
D
H
H
O
H
H
H
3l, 76%
28:72
CoI2 (5 mol%), Zn (3 equiv)
F3C
NC
Ph
HO
Ph
(b)
3n, 79%
10:90
3o
6:94
Ph
Ph
, 92%
3k, 95%
3m, 84%
H2O (10 equiv), CD3OD, 60 oC, 96h
O
1a (0.2 mmol)
6:94
5:95
88% isolated yield
D
D
H
H
CoI2 (5 mol%), Zn (3 equiv)
H
Ph
Ph
(c)
Ph
H3C(H2C)4
H
Ph
Ph
Ph
D2O (10 equiv), CD3OD, 60 oC, 120h
O
Ph
1a (0.2 mmol)
85% isolated yield
(75% D incorporated)
H
H
H
(CH2)4CH3
O2N
O
H
H2N
(20%)
D
D
3p, 73%
24:76
3q
, 24%
CoI2 (5 mol%), dppe (6 mol%),
Zn (3 equiv)
D2O (10 equiv), CH3CN, 60 oC, 12h
3r, trace
Ph
Ph
3s, nd
(d)
38:62
(45%)
1a (0.2 mmol)
(20%)
D
....................................................................................................................
D (20%)
D
D
aReaction Conditions: 1a-s (0.2 mmol), CoI2 (0.01 mmol), dppe (0.012 mmol),
(45%)
b
(20%)
Zn powder (0.6 mmol), H2O (2 mmol) in CH3CN (2 mL). The total yield of Z
and E-alkenes determined by 1H NMR spectroscopy. cThe Z/E ratio
determined by 1H NMR spectroscopy. d1a (0.3 mmol), CoI2 (1 mol%) in
CH3CN was used.
Isolated yield: 86%
CoI2 (5 mol%), dppe (6 mol%),
Zn (3 equiv)
H2O (10 equiv), CH3CN, 60 oC, 12h
Ph
Ph
(e)
Ph
2a (Z)
No Z to E isomerization
Ph
2a (Z)
In order to gain an insight into the mechanistic pathway of the
reaction and to verify the source of hydrogen, deuterium-labelling
and control experimental studies were performed under the
standard reaction conditions (Scheme 1). No deuterated product
was obtained when D2O was used instead of H2O (equation a). The
reason for this observation is perhaps all of the D-atom of D2O got
exchanged with acidic H-atom of MeOH since methanol is present
in excess as compared to D2O. From the mechanistic studies, we
concluded that both H2O and MeOH are the sources of hydrogen
contributing one H each in the Z-selective hydrogenation reaction.
For the E-selective partial hydrogenation reaction, deuterium
incorporation at both vinylic positions was observed in the product
when D2O was used (equation d). This proves that H2O is the only
hydrogen source in the E-selective semi-hydrogenation reactions.
The control experiments also revealed that no E/Z-isomerization
takes place in the reaction (equation e ) indicating that E/Z-
Scheme 1: Mechanistic studies
CoI2
H2O
Zn
Zn(OH)2
[Co]I
MeOH
+
[Co]III
OH
Ph
H
H2O + Zn
A
Ph
1a
MeO [Co]
OH
D
H
[Co]III
Ph
OH
H
H
Ph
B
Ph
Ph
OH
[Co]
Ph
H
migratory
insertion
2a
MeOH
Ph
C
(a)
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 3
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Journal Name
Ph
Ph
Conflicts of interest
There are no conflicts of interest to declare.
View Article Online
DOI: 10.1039/C9CC01970G
P
P
CoI2
+
Ph
Ph
D2O
Zn
D
CoILn
Zn(OD)2
+
H/D
H/D
3a(D)
E
D2O + Zn
Notes and references
1
D
CoIIILn OD
F
(a) A. Corma, S. Iborra and A. Velty, Chem. Rev., 2007, 107,
2411; (b) M. Fouche, L. Roone and A. G. M. Barrett, J. Org.
Chem., 2012, 77, 3060; (c) K. Radkowski, B. Sundararaju and
A. Fürstner, Angew. Chem. Int. Ed., 2013, 52, 355; (d) D.
Srimani, Y. Diskin-Posner, Y. Ben-David and D. Milstein,
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I.C. Lennon, P. H. Moran and J. A. Ramsden, Acc. Chem. Res.,
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Busacca and A. A. Genazzani, J. Med. Chem. 2006, 49, 3033.
(a) H. Lindlar, Helv. Chim. Acta, 1952, 35, 446; (b) H. Lindlar
and R. Dubuis, Org. Synth., 1966, 46, 89.
(a) C. A. Brown and V. K. Ahuja, J. Org. Chem., 1973, 38,
2226; (b) B. M. Choudary, G. V. M. Sharma and P. Bharathi,
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Chem., 1980, 45, 1937; (d) J. C. Choi and N. M.Yoon,
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(a) A. M. Kluwer, T. S. Koblenz, T. onischkeit, K. Woelk and C.
J. Elsevier, J. Am. Chem. Soc. 2005, 127, 15470; (b) R. Iwasaki,
E. Tanaka, T. Ichihashi, Y. Idemoto and K. Endo, J. Org.
Chem., 2018, 83, 13574; (c) M. Leutzsch, L. M. Wolf, P.
Gupta, M. Fuchs, W. Thiel, C. Farès and A. Fürstner, Angew.
Chem. Int. Ed. 2015, 54, 12431; (d) K. Tani, A. Iseki and T.
Yamagata, Chem. Commun., 1999, 1821; (e) R. Shen, T. Chen,
Y. Zhao, R. Qiu, Y. Zhou, S. Yin, X. Wang, M. Goto and L.-B.
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Abderrahim, F.Chemla, F. Ferreira, A. Perez-Luna, and O.
Jackowski, Org. Lett. 2018, 20, 7544.
Ph
Ph
1a
D/H
[Co]
H/D
OD
H
[Co]
OD
H
D
G
(b)
2
3
Scheme 2: Plausible reaction mechanism
The effectiveness of the present stereoselective TH protocol
was further justified in the efficient synthesis of E-resveratrol
(Scheme 3). The semi-reduction of 1,3-dimethoxy-5-((4-
methoxyphenyl)ethynyl)benzene
(4)
furnished
the
corresponding E-alkene 5 in 84% isolated yield with 90:10 E/Z-
ratio. Finally, the synthesis of E-resveratrol was concluded
following the reported procedure.11
4
CoI2 (5 mol%)
dppe (6 mol%)
H3CO
H3CO
H3CO
H3CO
Zn (3 equiv)
OCH3
H2O (10 equiv)
MeCN, 60 oC, 12h
OCH3
4
5
84% isolated yield
1,3-dimethoxy-5-((4-
methoxyphenyl)ethy
nyl)benzene
BBr3
DCM, 0 oC-rt
HO
HO
OH
5
6
S.-M. Fu, N.-Y. Chen, X.-F. Liu, Z.-H. Shao, S.-P. Luo and Q. Liu,
J. Am. Chem. Soc., 2016, 138, 8588.
(a) C. Chen, Y. Huang, Z. Zhang, X,-Q. Dong and X. Zhang,
Chem. Commun., 2017, 53, 4612; (b) V. G. Landge, J.
Pitchaimani, S. P. Midya, M. Subaramanian, V. Madhu and B.
Ekambaram, Catal. Sci. Technol., 2018, 8, 428.
6
E-Resveratrol
82% yield
Scheme 3: Synthesis of E-resveratrol
Conclusions
In summary, we have developed a protocol for the semi-
hydrogenation of alkynes to the corresponding alkenes using
inexpensive Cobalt (II) iodide, Zinc and more cheap and green
water as the hydrogen source. We have also demonstrated the
tuning of Z to E-selectivity by changing the solvent with
addition of phosphine ligand dppe under the same catalytic
system. The applicability of the developed catalytic protocol to
a wide range of internal alkynes has also been investigated. In
most of the cases, the corresponding alkenes were observed in
good to excellent yields with excellent stereoselectivity. This
method is environmentally and economically more
advantageous over expensive transition metal catalysts for the
reduction of alkynes to alkenes and for the synthesis of 1,2-
dideuterioalkenes.
7
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We are grateful to the National Natural Science Foundation of
China (21572198) and the Applied Basic Research Project of
Yunnan Province (2017FA004, 2018FB021), and Yunnan
Provincial Key Laboratory Construction Plan Funding of
Universities for their financial support.
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