Cobalt-catalyzed (Z)-selective semihydrogenation of alkynes with molecular hydrogen

Article abstract of DOI:10.1039/c7cc01228d

Cobalt-catalyzed highly (Z)-selective semihydrogenation of alkynes using molecular H₂ was developed using commercially available and cheap cobalt precursors. A variety of (Z)-alkenes were obtained in moderate to excellent selectivities [(Z)-alkene/(E)-alkene/alkane ratio up to >99 : 1 : 1] and it was found that the readily available ethylenediamine ligand is crucial in determining the selectivity.

Full text of DOI:10.1039/c7cc01228d

View Article Online
View Journal
ChemComm
Accepted Manuscript
This article can be cited before page numbers have been issued, to do this please use: C. Chen, Y.
Huang, Z. Zhang, X. Dong and X. Zhang, Chem. Commun., 2017, DOI: 10.1039/C7CC01228D.
This is an Accepted Manuscript, which has been through the
Royal Society of Chemistry peer review process and has been
accepted for publication.
Volume 52 Number
1 4 January 2016 Pages 1–216
ChemComm
Accepted Manuscripts are published online shortly after
Chemical Communications
www.rsc.org/chemcomm
acceptance, before technical editing, formatting and proof reading.
Using this free service, authors can make their results available
to the community, in citable form, before we publish the edited
article. We will replace this Accepted Manuscript with the edited
and formatted Advance Article as soon as it is available.
You can find more information about Accepted Manuscripts in the
author guidelines.
Please note that technical editing may introduce minor changes
to the text and/or graphics, which may alter content. The journal’s
standard Terms & Conditions and the ethical guidelines, outlined
in our author and reviewer resource centre, still apply. In no
event shall the Royal Society of Chemistry be held responsible
for any errors or omissions in this Accepted Manuscript or any
consequences arising from the use of any information it contains.
ISSN 1359-7345
COMMUNICATION
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
=
3
rsc.li/chemcomm

Please do not adjust margins
Page 1 of 4
ChemComm
View Article Online
DOI: 10.1039/C7CC01228D
Journal Name
COMMUNICATION
Cobalt-Catalyzed (Z)-Selective Semihydrogenation of Alkynes with
Molecular Hydrogen
Caiyou Chen,a Yi Huang,a Zongpeng Zhang,a Xiu-Qin Dong,*a and Xumu Zhang*b,a
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
www.rsc.org/
A
cobalt-catalyzed highly (Z)-selective semihydrogenation of
alkynes using molecular H2 was developed using commercially
available and cheap cobalt precursors. A variety of (Z)-alkenes
were obtained in moderate to excellent selectivities [(Z)-
alkene/(E)-alkene/alkane ratio up to > 99:1:1)] and it was found
that the readily available ethylenediamine ligand is crucial in
determining the selectivity.
(Z)-Olefinic structures are widely found in many biologically
and pharmaceutically active molecules and are manufactured
in several fine and bulk chemical processes.[1] Convenient
methods have been developed for the preparation of (Z)-
alkenes, which include Wittig[2], Peterson[3] and Julia
olefination[4], olefin metathesis[5], and cross-coupling
reactions[6]. However, selective semihydrogenation of readily
available alkynes is the simplest and most straightforward
approach for the preparation of (Z)-alkenes. To date, many
transition-metal based catalysts have been intensively studied
for the (Z)-selective semihydrogenation of alkynes, which
include Pd,[7] Rh, [8] Au, [9] Ru, [10] Ni, [11] Cu, [12] V, [13] Cr, [14] Nb,
Scheme 1. Previous developed catalysts and drawbacks of Lindlar’s
catalyst.
With regard to the development of new catalysts, it is of
great value to use the nonprecious base metals to replace the
noble metals. The base metal such as cobalt is elegant for its
economic and environmental advantages, and more
importantly, for its unusual reactivities and selectivities in
hydrogenation reactions.[18] However, few cobalt-based
catalysts have been developed for the (Z)-selective
semihydrogenation of alkynes. Very recently, Liu et al.
developed a promising cobalt catalyst appended with pincer
ligands and excellent selectivities were obtained in the
semihydrogenation of a variety of alkynes.[19] Importantly, the
(Z) or (E) selectivity can be controlled by the ligands (Scheme
2a). However, ammonia borane was utilized as the hydrogen
source, which is not ideal from the atom economic and
environmental point of view. Recently, Fout et al. developed
an unique cobalt catalyst using molecular H2 as the hydrogen
source.[20] However, only the (E)-alkenes can be obtained
(Scheme 2b). Regarding to the (Z)-selective cobalt-catalyzed
semihydrogenation with molecular hydrogen, very recently
Beller et al also reported a heterogeneous cobalt catalyst.
However, the (Z)-selectivities achieved were only moderate.[21]
Herein, we report a cobalt-catalyzed highly (Z)-selective
semihydrogenation of alkynes using molecular H2 as the
hydrogen source. Importantly, compared with the previous
studies, in which the ligands and catalysts were synthesized in
multiple steps, the cobalt catalyst we report herein is very
simple and practical, which is generated in situ from the com-
[15]
[16]
and even Fe
(Scheme 1a). However, the Lindlar’s
catalyst is still the common choice for the (Z)-selective
semihydrogenation of alkynes since it is practical, general, and
commercially available.[1c,7a] Despite these superiorities, the
Lindlar’s catalyst has significant drawbacks, which suffers from
isomerization of the (Z)-alkene to the (E)-alkene, shift of the
double bond, over-reduction to the alkane, and poor
stability.[17] Moreover, the lead species and quinoline are often
required which lead to harmful wastes and heavy metal
residual in the products (Scheme 1b).[17] As a result, efforts
into developing new and more efficient catalysts are highly
desirable.
a.College of Chemistry and Molecular Sciences, Wuhan University, Wuhan, Hubei,
430072, P. R. China.
b.Department of Chemistry, South University of Science and Technology of China,
Shenzhen, Guangdong, 518055, P. R. China.
xumu@whu.edu.cn; xiuqindong@whu.edu.cn
Electronic Supplementary Information (ESI) available: [details of any
supplementary information available should be included here]. See
DOI: 10.1039/x0xx00000x
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 1
Please do not adjust margins

Please do not adjust margins
ChemComm
Page 2 of 4
COMMUNICATION
Journal Name
Table 1. Screening of the reaction conditions. a
View Article Online
DOI: 10.1039/C7CC01228D
Scheme 2. Examples of cobalt-catalyzed semihydrogenation of
alkynes.
mercially available and cheap Co(OAc)2·(H2O)4, NaBH4 and
ethylenediamine. Employing this readily available cobalt
catalyst, a variety of alkynes with wide functional group
tolerance were converted to the (Z)-alkenes in moderate to
excellent selectivities (Scheme 2c).
The investigation of the semihydrogenation was initiated by
using 1,2-diphenylacetylene 1a as the standard substrate. To
our delight, 1a was converted to the desired (Z)-1,2-
diphenylethylene 2a with good selectivity in the presence of
the cobalt catalyst generated in situ from Co(OAc)2·(H2O)4,
NaBH4 and ethylenediamine using molecular H2, although the
substrate was not fully converted (Table 1, entry 1). Screening
of the cobalt precursor revealed that Co(II) species gave
promising results while Co2(CO)8 didn’t show any activity
(Entries 2-4). Co(OAc)2·(H2O)4 was selected as the precursor for
the slightly higher activity. Screening of the ligands revealed
that almost all the substrate was converted to the alkane
without ethylenediamine or using 2,2’-bipyridine (Entries 5-6),
demonstrating the key role of the ethylenediamine ligand in
determining the selectivity. Further screening of the amine
ligands revealed that introducing substituents on
ethylenediamine gave slightly lower reactivity while the
selectivity was not significantly influenced (Entries 7-10).
Phosphine ligands were also tried, but relatively lower activity
and selectivity were observed (Entries 11-12). It was found
that solvent played an important role in the activity of the
catalytic system. Changing the solvent from alcohol to THF,
CH2Cl2 or toluene resulted in almost no activity of the catalyst
(Entries 13-15). It was also found that water is helpful to
increase the selectivity and much lower selectivity was
observed when the reaction was conducted in the absence of
water (Entry 16). It is likely that water may act as a weak
coordinating ligand to slowdown the isomerization of (Z)-
alkene to (E)-alkene and the over-reduction. Considering this
possibility, the weak coordinating solvent, THF, was added as a
co-solvent to further increase the selectivity. To our delight,
a The reaction was conducted in 0.5 mmol scale, solvent/H2O = 2.0
mL/0.1 mL, Co = 1 mol%, NaBH4 = 2 mol%, ligand = 8 mol%, H2 = 2
b
1
bar, time = 12 h; Conversion of the substrate, determined by H
c
d
e
NMR; Determined by 1H NMR; PPh3 = 4 mol%; DPPP = 1,3-
f
bis(diphenylphosphanyl)propane, 2 mol%; EtOH:THF:H2O = 1.0
mL:1.0 mL : 0.1 mL; g time = 24 h; h H2 = 3 bar; i EtOH:THF:H2O = 1.0
mL:1.0 mL : 0.1 mL, time = 24 h, H2 = 3 bar, conducted outside the
j
glovebox; EtOH:THF:H2O = 1.0 mL:1.0 mL : 0.1 mL, time = 24 h, H2
= 3 bar, using non-degassed solvent.
slightly higher selectivity was observed while the activity was not
significantly influenced (Entry 17). And importantly, the use of THF
as a co-solvent addressed the problem of the low solubility of many
substrates in alcohol. In order to ensure full conversion of the
substrate, reaction time and H2 pressure were increased. To our
delight, the substrate was fully converted while the selectivity
remained high (Entries 18-19). It should be noted that the catalytic
system is sensitive to air. Conducting the reaction outside the golve
box or using non-degassed solvent resulted in low conversions
(Entries 20-21).
With the optimized reaction conditions established, a series
of substrates was smoothly hydrogenated to the Z-alkenes
with moderate to excellent selectivities (11:1:1~>99:1:1 Z/E/A
2 | J. Name., 2012, 00, 1-3
This journal is © The Royal Society of Chemistry 20xx
Please do not adjust margins

Page 3 of 4
ChemComm
Please do not adjust margins
Journal Name
COMMUNICATION
Table 2. Substrate scope of the cobalt-catalyzed
View Article Online
DOI: 10.1039/C7CC01228D
semihydrogenation a
Scheme 3. Gram scale semihydrogenation of the functionalized
alkynes 1g and 1m; except for the catalyst loading, the reaction
conditions are the same as described in Table 2.
Scheme 4. Measurement of the reaction rates.
selectivity (2m
methoxy group was introduced, excellent selectivities were
observed (2n 2p). Moreover, the reaction also tolerated
-2v). Especially, when the weak coordinating
-
functional groups such as acetyl (2s) and cyano (2t). It is
noteworthy that, compared with the nonfunctionalized
alkynes, the selectivities observed in the semihydrogenation of
the functionalized alkynes are distinctly higher. This is probably
because that the amine and alcohol moieties in the
functionalized alkynes could act as weak coordinating groups
to slowdown the isomerization and over-reduction. However,
if the coordinating ability of the functional group (such as
dimethyl amine or thiophene) is too strong, the catalyst could
be poisoned, which afforded low conversions (2w-2x).
In order to further demonstrate the practical utility of the
current method, the semihydrogenation of the functionalized
substrates 1g and 1m was conducted in gram scale. To our
delight, the reaction proceeded smoothly to give the desired
(Z)-product in high yield and selectivity (Scheme 3).
a
The reaction was conducted in 0.5 mmol scale, EtOH/THF/H2O
= 1.0 mL/1.0 mL/0.1 mL, ethylenediamine/NaBH4/Co = 8:2:1, H2
= 3 bar, conversion of the substrate and the Z/E/A ratio were
1
determined by H NMR or GC analysis, Z/E/A is the ratio of (Z)-
In an effort to reveal the reactivity order of the
functionalized and nonfunctionalized alkynes, we measured
alkene/(E)-alkene/alkane, S/C is the substrate/catalyst ratio.
ratio, 77%~98% Z-alkene yield). Good selectivities were
obtained in the semihydrogenation of non-functionalized
diaryl-substituted alkynes (Table 2, 2a-2d), although a slightly
lower selectivity was observed when the electron-withdrawing
fluorine atom was introduced on the para position (2d).
However, in the semihydrogenation of the alkyne attached
with aryl and alkyl substituents, the selectivity was relatively
lower
(2e). Nevertheless, the selectivity of the
semihydrogenation of the dialkyl-substituted alkyne still
remained high (2f). Substrate scope was then investigated in
the semihydrogenation of a variety of functionalized alkynes. A
series of (Z)-3-substituted protected allylic amines were
obtained with very high selectivities, regardless of whether the
substituents on the aromatic ring is electron-donating or
electron-withdrawing (2g-2l). A number of (Z)-3-substituted
allylic alcohols was also smoothly obtained with high
Scheme 5. Control experiments using MeOD, D2O and D2.
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 3
Please do not adjust margins

Please do not adjust margins
ChemComm
Page 4 of 4
COMMUNICATION
Journal Name
B. M. Trost, Z. T. Ball, T. Jꢀge, J. Am. Chem. Soc. 2002, 124
6
7
,
the reaction rate of each substrate separately. As shown in
scheme 4, when the reaction was stopped after 4 h with 1 mol%
catalyst, 3-phenyl-propargyl alcohol 1m exhibited the highest
rate. To the contrary, the reaction rate of 3-substituted
protected propargylamine 1g was the lowest. It is notable that
the reaction rate of 2-butylphenylacetylene 1e is distinctly
higher than that of 1,2-diphenylacetylene 1a, which is probab-
ly due to the electron-richer feature of the former.
View Article Online
7922-7923.
For representative examples, see: a) DHO. I:L1in0d.1l0a3r9,/CH7eClvC.0C12h2i8mD.
Acta 1952, 35
, 446-450; b) B. M. Trost, R. Braslau,
Tetrahedron Lett. 1989, 30, 4657-4660; c) L. L. Wei, L. M. Wei,
W. B. Pan, S. P. Leou, M. J. Wu, Tetrahedron Lett. 2003, 44
1979-1981; d) M. Crespo-Quesada, F. Cardenas-Lizana, A.
Dessimoz, L. KiwiMinsker, ACS Catal. 2012, , 1773-1786; e)
R. M. Drost, T. Bouwens, N. P. van Leest, B. Bruin, C. J.
Elsevier, ACS Catal. 2014, , 1349-1357; f) J. Zhong, Q. Liu, C.
,
2
4
Control experiments were also conducted using deuterated
methanol and water to determine whether the alcohol solvent
and water serve as hydrogen donors. As shown in Scheme 5,
no deuterated products were observed when deuterated
methanol or water was used as co-solvents (Scheme 5a-5b).
This result suggests that alcohol solvent and water do not
serve as hydrogen donors. Furthermore, when the reaction
was conducted using D2, for all of the three products (2a’, 3a’,
and 4a’), the deuterium incorporation was higher than 95%
(Scheme 5c). As a result, H2 was considered to be the sole
hydrogen source and the transfer hydrogenation mechanism
was ruled out.
Wu, Q. Meng, X. Gao, Z. Li, B. Chen, C. Tunga, L. Wu, Chem.
Commun., 2016, 52, 1800-1803; g) S. Yang, C. Cao, L. Peng, J.
Zhang, B. Han, W. Song, Chem. Commun., 2016, 52, 3627-
3630; h) T. Mitsudome, T. Urayama, K. Yamazaki, Y. Maehara,
J. Yamasaki, K. Gohara, Z. Maeno, T. Mizugaki, K. Jitsukawa, K.
Kaneda, ACS Catal. 2016, 6, 666-670.
8
9
For representative example, see: R. R. Schrock, J. A. Osborn, J.
Am. Chem. Soc. 1976, 98, 2143-2147.
For representative examples, see: a) L. Shao, X. Huang, D.
Teschner, W. Zhang, ACS Catal. 2014, 4, 2369-2373; b) G. Li,
R. Jin, J. Am. Chem. Soc. 2014, 136, 11347-11354. c) E.
Vasilikogiannaki, I. Titilas, G. Vassilikogiannakis, M. Stratakis,
Chem. Commun. 2015, 51, 2384-2387; d) T. Mitsudome, M.
Yamamoto, Z. Maeno, T. Mizugaki, K. Jitsukawa, K. Kaneda, J.
Am. Chem. Soc. 2015, 137, 13452-11354; e) S. Liang, G. B.
Hammond, B. Xu, Chem. Commun., 2016, 52, 6013-6016.
10 For representative example, see: M. Niu, Y. Wang, W. Li, J.
Conclusions
Jiang, Z. Jin, Catal. Commun. 2013, 38, 77-81.
In conclusion,
a
cobalt-catalyzed highly (Z)-selective 11 For representative examples, see: a) C. A. Brown, V. K. Ahuja,
J. Chem. Soc., Chem. Commun. 1973, 15, 553-554; b) J. A.
Schreifels, P. C. Maybury, W. E. Swartz, J. Org. Chem., 1981,
semihydrogenation of alkynes using molecular H2 was
developed. Notably, this reaction system is very practical using
commercially available and cheap cobalt precursor and
ethylenediamine ligand. A variety of (Z)-alkenes were obtained
in moderate to excellent reactivities and selectivities. It was
found that the ethylenediamine ligand is crucial in determining
the selectivity. The reactivity order of each kind of substrate
was determined by measuring the reaction rate separately.
And control experiments revealed that H2 was the sole
hydrogen source. Further studies are underway to reveal the
mechanism of this cobalt-catalyzed semihydrogenation.
46, 1263-1269; c) G. Zhu, X. Lu, J. Org. Chem., 1995, 60
,
1087-1089; d) F. Studt, F. Abild-Pedersen, T. Bligaard, R. Z.
Sørensen, C. H. Christensen, J. K. Nørskov, Science 2008, 320
1320-1322; e) E. Richmond, J. Moran, J. Org. Chem. 2015, 80
6922-6929;
,
,
12 For representative examples, see: a) K. Semba, T. Fujihara, T.
Xu, J. Terao, Y. Tsuji, Adv. Synth. Catal. 2012, 354, 1542-1550;
b) N. O. Thiel, J. F. Teichert, Org. Biomol. Chem., 2016, 14
,
10660–10666; c) F. Pape, N. O. Thiel, J. F. Teichert, Chem. Eur.
J. 2015, 21, 15934 – 15938; d) A. M Whittaker, G. Lalic, Org.
Lett., 2013, 15, 1112–1115.
13 For representative example, see: H. S. La Pierre, J. Arnold, F.
D. Toste, Angew. Chem., Int. Ed., 2011, 50, 3900-3903.
14 For representative example, see: M. Sodeoka, M. Shibasaki, J.
Org. Chem. 1985, 50, 1147-1149.
This work was supported by the grant from Wuhan University
(203273463, 203600400006), the support of the Important Sci-Tech
Innovative Project of Hubei Province (2015ACA058), the National
Natural Science Foundation of China (Grant No. 21372179, 15 For representative examples, see: a) T. L. Gianetti, N. C.
Tomson, J. Arnold, R. G. Bergman, J. Am. Chem. Soc. 2011,
133, 14904-14907; b) Y. Satoh, Y. Obora, J. Org. Chem. 2011,
76, 8569–8573.
21432007, 21502145) and Natural Science Foundation of Hubei
Province (Grant No. 2016CFB449).
16 For representative examples, see: a) L. Ilies, T. Yoshida, E.
Nakamura, J. Am. Chem. Soc., 2012, 134, 16951-16954; b) D.
Srimani, Y. Diskin-Posner, Y. Ben-David and D. Milstein,
Angew. Chem., Int. Ed., 2013, 52, 14131-14134.
17 R. Chinchilla, C. Najera, Chem. Rev., 2014, 114, 1783-1826.
18 a) P. J. Chirik, Acc. Chem. Res. 2015, 48, 1687-1695; b) R. H.
Morris, Acc. Chem. Res. 2015, 48, 1494-1502. (c) T. Zell, D.
Milstein, Acc. Chem. Res. 2015, 48, 1979-1994.
Notes and references
1
2
a) J. Burwell, L. Robert, Chem. Rev. 1957, 57, 895-934; b) E. N.
Marvell, T. Li, Synthesis 1973, 8, 487; c) C. Oger, L. Balas, T.
Durand, J. Galano, Chem. Rev. 2013, 113, 1313-1350; d) A. M.
Kluwer, T. S. Koblenz, T. Jonischkeit, K. Woelk, C. J. Elsevier, J.
Am. Chem. Soc. 2005, 127, 15470-15480.
a) G. Wittig, U. Schꢀllkopf, Chem. Ber. 1954, 87, 1318-1330; b)
B. E. Maryanoff, A. B. Reitz, Chem. Rev. 1989, 89, 863-927; c)
19 a) S. Fu, N.-Y. Chen, X. Liu, Z. Shao, S.-P. Luo, Q. Liu, J. Am.
Chem. Soc. 2016, 138, 8588-8594.
20 K. Tokmic, A. R. Fout, J. Am. Chem. Soc. 2016, 138, 13700-
13705.
W. S. Wadsworth, W. D. Emmons, J. Am. Chem. Soc. 1961, 83
1733-1738.
,
21 F. Chen, C. Kreyenschulte, J. Radnik, H. Lund, A.-E. Surkus, K.
3
4
5
D. J. Peterson, J. Org. Chem. 1968, 33, 780-784.
P. R. Blakemore, J. Chem. Soc. 2002, 2563-2585.
a) B. K. Keitz, K. Endo, P. R. Patel, M. B. Herbert, R. H. Grubbs,
J. Am. Chem. Soc. 2012, 134, 693-699; b) F. Z. Dꢀrwald, In
Handbook of Metathesis. Vol. 1; R. H. Grubbs, Ed.; Wiley-
VCH Verlag: Hoboken, NJ, 2004; Vol. 43, pp 395-396.
Junge, M. Beller, ACS Catal. 2017, 7, 1526-1532.
4 | J. Name., 2012, 00, 1-3
This journal is © The Royal Society of Chemistry 20xx
Please do not adjust margins