An alkene-promoted borane-catalyzed highly stereoselective hydrogenation of alkynes to give Z- And E-alkenes

Article abstract of DOI:10.1002/chem.201405388

The stereoselective hydrogenation of alkynes to alkenes is an extremely useful transformation in synthetic chemistry. Despite numerous reports for the synthesis of Zalkenes, the hydrogenation of alkynes to give E-alkenes is still not well resolved. In particular, selective preparation of both Z- and E-alkenes by the same catalytic hydrogenation system using molecular H₂ has rarely been reported. In this paper, a novel strategy of using simple alkenes as promoters for the HB(C₆F₅)₂-catalyzed metal-free hydrogenation of alkynes was adopted. Significantly, both Z - and E-alkenes can be furnished by hydrogenation with molecular H₂ in high yields with excellent stereoselectivities. Further experimental and theoretical mechanistic studies suggest that interactions between H and F atoms of the alkene promoter, borane intermediate, and H₂ play an essential role in promoting the hydrogenolysis reaction.

Full text of DOI:10.1002/chem.201405388

DOI: 10.1002/chem.201405388
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&
Hydrogenation
An Alkene-Promoted Borane-Catalyzed Highly Stereoselective
Hydrogenation of Alkynes to Give Z- and E-Alkenes
Yongbing Liu,[a] Lianrui Hu,[b] Hui Chen,*[b] and Haifeng Du*[a]
Abstract: The stereoselective hydrogenation of alkynes to al-
kenes is an extremely useful transformation in synthetic
chemistry. Despite numerous reports for the synthesis of Z-
alkenes, the hydrogenation of alkynes to give E-alkenes is
still not well resolved. In particular, selective preparation of
both Z- and E-alkenes by the same catalytic hydrogenation
system using molecular H2 has rarely been reported. In this
paper, a novel strategy of using simple alkenes as promoters
for the HB(C6F5)2-catalyzed metal-free hydrogenation of al-
kynes was adopted. Significantly, both Z- and E-alkenes can
be furnished by hydrogenation with molecular H2 in high
yields with excellent stereoselectivities. Further experimental
and theoretical mechanistic studies suggest that interactions
between H and F atoms of the alkene promoter, borane in-
termediate, and H2 play an essential role in promoting the
hydrogenolysis reaction.
Introduction
same catalytic hydrogenation system with molecular H2 is of
great interest but it is a significant challenge.[4]
The stereoselective hydrogenation of alkynes to alkenes is an
extremely useful tool in synthetic chemistry.[1] Various hetero-
geneous and homogeneous metal catalysts have been devel-
oped for this transformation, which provide efficient routes for
the synthesis of Z-alkenes.[2] However, the preparation of E-al-
kenes by the hydrogenation of alkynes has been less devel-
oped.[3] For example, McQuillin and Muetterties both inde-
pendently reported the E-selective hydrogenation of alkynes
by using homogeneous rhodium catalysts.[3a,b] Similarly, Bargon
and co-workers employed a ruthenium complex for this trans-
formation.[3c] Very recently, Fꢀrstner and co-workers described
a practical and functional-group-tolerant E-selective hydroge-
nation of alkynes by using ruthenium catalysts.[3d] Milstein and
co-workers reported a highly E-selective hydrogenation, cata-
lyzed by an iron pincer complex, in which E-alkenes were pro-
duced by an in situ isomerization of Z-alkenes; but pure Z-al-
kenes could not be obtained in this catalytic system.[3e] It
should be noted that accessing both Z- and E-alkenes by the
In contrast to transition-metal catalysis, the metal-free cata-
lytic hydrogenation of alkynes has seldom been reported.[5]
The chemistry of frustrated Lewis pairs (FLPs) provides a prom-
ising approach for metal-free hydrogenation;[6,7] recently, some
important progress has been made for the hydrogenation of
unactivated multiple CÀC bonds.[5b,8–10] For example, Paradies
and Grimme reported a low-temperature hydrogenation of al-
kenes by using phosphine and B(C6F5)3.[9a,e] Later, Stephan and
Grimme realized a similar transformation that combines ethers
and B(C6F5)3 as the hydrogenation catalysts.[9b] In addition,
Wang and Li described a single Lewis acid HB(C6F5)2-catalyzed
hydrogenation of alkenes.[9c,11,12] Very recently, Repo and Pꢁpai
accomplished the first metal-free hydrogenation of unactivated
alkynes 1 to give Z-alkenes 2 by using ansa-aminohydrobor-
ane, which involves a hydroboration process (Scheme 1).[5b]
However, their attempts to use HB(C6F5)2 in combination with
bases as the catalyst proved to be unsuccessful.
[a] Dr. Y. Liu, Prof. Dr. H. Du
Beijing National Laboratory for Molecular Sciences (BNLMS)
CAS Key Laboratory of Molecular Recognition and Function
Institute of Chemistry, Chinese Academy of Sciences
Beijing 100190 (P. R. China)
Fax: (+86)10-62554449
Scheme 1. Metal-free hydrogenation of alkynes.
E-mail: haifengdu@iccas.ac.cn
[b] L. Hu, Prof. Dr. H. Chen
Our initial study indicated that the HB(C6F5)2-catalyzed hy-
drogenation of alkyne 1a occurred with quite a low conversion
(Scheme 2).[13] In 2007, Stephan described an interesting three-
component reaction of PtBu3, B(C6F5)3, and an alkene, which at-
tracted intensive mechanistic studies to investigate the
alkene–borane interactions.[14] Subsequently, Li reported a theo-
BNLMS, CAS Key Laboratory of Photochemistry
Institute of Chemistry, Chinese Academy of Sciences
Beijing 100190 (P. R. China)
Fax: (+86)10-82617315
E-mail: chenh@iccas.ac.cn
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201405388.
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Table 1. Optimization of reaction conditions.[a]
Entry
HB(C6F5)2
[mol%]
3a
[mol%]
Time
[h]
Conv.
[%][b]
Z:E[b]
1
2
3
4c
5d
6
10
10
10
10
10
10
12.5
10
20
20
20
20
20
20
20
10
6
20
20
24
24
>99
>99
>99
61
82
>99
>99
83:17
76:24
95:5
98:2
97:3
Scheme 2. Initial study on HB(C6F5)2-catalyzed hydrogenation of alkyne 1a.
25:75
<1:99
7
[a] All reactions were carried out with alkyne 1a (0.25 mmol), H2 (50 bar),
toluene (2.0 mL), at 1408C. [b] Determined by 1H NMR analysis of the
crude reaction mixture. [c] 1208C. [d] H2 (20 bar).
Scheme 3. Strategy for the alkene-promoted hydrogenation of alkynes.
retical study that suggested the formation of a weakly bound
alkene–B(C6F5)3 complex A (Scheme 3).[14c] Stephan’s investiga-
tion into the interaction between the vinyl group and the
boron atom of an alkene-tethered borane compound support-
ed the formation of a van der Waals borane–alkene com-
plex B.[14d] The reported borane–alkene interaction is expected
to activate the alkene. Conversely, this interaction is also likely
to activate the borane. Inspired by these results, and based on
our previous work on the development of borane catalysts
from alkenes,[15–17] we propose to take advantage of the activa-
tion that arises from this alkene–borane interaction to promote
the HB(C6F5)2-catalyzed hydrogenation of alkynes (Scheme 3).
Herein we report our efforts on this subject.
tested, but none of them were effective for promoting this re-
action (see Supporting Information, Figure S1).
Next, the reaction conditions were optimized. It was found
that the reactivity was improved when the amount of alkene
3a was increased to 20 mol% (Table 1, entries 1 vs. 2). Restrict-
ing the reaction time to 6 h led to >99% conversion of the
alkyne 1a to Z-alkene 2a selectively (entry 3). However,
a lower reaction temperature or H2 pressure both resulted in
reduced levels of conversions (entries 4 and 5). Significantly, E-
alkene 2a can also be obtained as a single isomer by using
more HB(C6F5)2 over a longer reaction time (entry 7). Thus,
both Z- and E-alkenes can be furnished through a simple ad-
justment of the reaction conditions.
We next examined the substrate scope for the synthesis of
the Z-alkene. As shown in Table 2, a variety of alkynes 1a–l, in-
cluding diaryl, dialkyl, and aryl–alkyl alkynes, were efficiently
hydrogenated to furnish the desired Z-alkenes 2a–l in 86–99%
yields with high levels of stereoselectivity (Table 2, entries 1–
12). It should be noted that the reaction time for the hydroge-
nation of alkynes must be strictly controlled to ensure the ac-
quirement of high Z-selectivity. The synthesis of E-alkenes by
the metal-free hydrogenation was also studied. As shown in
Table 3, the hydrogenation of alkynes 1a–d,f–k,m–o by using
HB(C6F5)2 (12.5–20 mol%) and alkene 3a (20 mol%) went
smoothly to give the corresponding E-alkenes 2 in high yields
and excellent levels of stereoselectivity (Table 3, entries 1–13).
A small amount of the alkanes that were generated by over-
hydrogenation were observed in the reactions of alkyne sub-
strates 1c and 1k (entries 3 and 13). For alkyne substrates that
contained n-butyl substituents, substantial C=C bond migra-
tion isomers were also formed (entries 11 and 12).
Results and Discussion
Pentafluorostyrene (3a) was used in the hydrogenation of
alkyne 1a to test the feasibility of our proposal. To our pleas-
ure, we found that alkene 3a could indeed promote this hy-
drogenation reaction efficiently to give the desired alkene 2a
in a quantitative conversion and with good Z-selectivity
(Scheme 4). In contrast, styrene (3b) had very little beneficial
influence on this transformation. The charging sequence of the
alkene 3, HB(C6F5)2, and alkyne 1a was found to have a signifi-
cant impact on the reactivity. Treating alkyne 1a with HB(C6F5)2
for 5 min before the addition of alkene 3a or 3b led to
a higher conversion to the product (Scheme 4, Conditions a vs.
b). In addition, a variety of commercially available alkenes were
The interesting results that were obtained for the metal-free
hydrogenation of alkynes encouraged us to investigate the
mechanism of the reaction by both practical experiments and
theoretical calculations. Two initial experiments were carried
out, as shown in Scheme 5, in which the hydrogenation of
alkyne 1a was terminated after 2 h. Even though different
charging sequences were employed, the alkenyl borane 4a
was formed in both reactions, which suggests that this species
is an intermediate in the hydrogenation reaction and its hydro-
genolysis is likely to be the rate-determining step in the cata-
lytic process. Notably, higher yields of borane 4a and alkene
Scheme 4. Alkene-promoted hydrogenation of alkynes: a) treating alkyne 1a
with HB(C6F5)2 for 5 min before addition of alkene 3, b) treating alkene 3
with HB(C6F5)2 for 5 min before addition of alkyne 1a.
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Table 2. Hydrogenation of alkynes to Z-alkenes.[a]
Table 3. Hydrogenation of alkynes to E-alkenes.[a]
Entry
Product (2)
Time
[h]
Conv.[b]
(Yield)[c]
[%]
Z:E[b]
Entry
HB(C6F5)2
[mol%]
Product (2)
Yield
[%][b]
Z:E[c]
1
2
3
4
5
12.5
12.5
20.0
12.5
20.0
E-2a: R=H
E-2b: R=4-Me
E-2c: R=4-Cl
E-2d: R=3,5-Me2
E-2m: R=4-MeO
97
98
98[d]
99
<1:99
<1:99
<1:99
<1:99
<1:99
1
2
3
4
5[d]
Z-2a: R=H
6
5.2
10
5.5
24
>99 (96)
97 (93)
95 (87)
95 (94)
92 (86)
95:5
97:3
96:4
96:4
Z-2b: R=4-Me
Z-2c: R=4-Cl
Z-2d: R=3,5-Me2
Z-2e: R=2-Me
95
97.3
6
7
8
9
12.5
15.0
12.5
20.0
E-2f: R=4-Me
E-2g: R=4-Ph
E-2h: R=3,5-Me2
E-2n: R=4-MeO
99
99
99
91
1:99
<1:99
3:97
6
7
8
Z-2f: R=4-Me
Z-2g: R=4-Ph
Z-2h: R=3,5-Me2
5.5
6
5.5
93 (91)
>99 (98)
92 (88)
98:2
95:5
97:3
<1:99
9
Z-2i
7
97 (93)
94:6
10
15.0
E-2i
98
<1:99
11
12
12.5
15.0
E-2j: Ar=Ph
E-2o: Ar=2-Np
89[e]
96[e]
5:95
4:96
10
11
Z-2j
6.5
24
93 (99)
97:3
92:8
Z-2k
>99 (86)
13
17.5
E-2k
96[d]
<1:99
[a] Conditions: alkyne 1 (0.25 mmol), alkene 3a (0.05 mmol), H2 (50 bar),
toluene (2.0 mL), at 1408C for 24 h. [b] Yield of isolated product. [c] Deter-
mined by 1H NMR analysis of the crude reaction mixture. [d] Including 5–
8% alkane. [e] Including 24–30% C=C bond migration isomers.
12
Z-2l
10
91 (93)
96:4
[a] Conditions: alkyne 1 (0.25 mmol), HB(C6F5)2 (0.025 mmol), alkene 3a
(0.05 mmol), H2 (50 bar), toluene (2.0 mL), at 1408C. [b] Determined by
1H NMR or GC-MS analysis of the crude reaction mixture. [c] Yield of iso-
lated product. [d] HB(C6F5)2 (0.05 mmol) was used.
Scheme 6. Experiments on hydroboration and retrohydroboration from
alkene 3a.
Scheme 5. Controlled experiments to investigate the reaction outcome with
different charging sequences.
the alkene 3a and HB(C6F5)2 were combined prior to the addi-
tion of the alkyne is attributed to an additional retrohydrobo-
ration process followed by decomposition.
3a were observed for the reaction with the more productive
charging sequence. However, boranes derived from alkene 3a
and HB(C6F5)2 were not formed in these reactions (Scheme 5).
In fact, boranes 5 and 5’ can be formed very quickly by the hy-
droboration of alkene 3a with HB(C6F5)2 at room temperature.
These boranes can be further converted into borane 4a and
alkene 3a by a retrohydroboration process at high tempera-
ture (Scheme 6).[17] However, significant amounts of unidenti-
fied side products were also observed by 1H NMR analysis,
which most likely resulted from the decomposition of boranes
5 and 5’. Therefore, the relatively low reactivity observed when
Because various unsaturated compounds, including the
alkyne substrate, the alkene product, and alkene 3a, were all
involved in the catalytic reaction, competition experiments to
compare their rates of hydroboration with HB(C6F5)2 were car-
ried out. As shown in Scheme 7, the hydroboration of alkyne
1a, to furnish alkenyl borane 4a, was much faster than that of
alkenes 3a and Z-2a. This demonstrates that the hydrobora-
tion of alkynes is the predominant process, unless alkynes are
exhausted, and it further supports the proposal that 4a is an
intermediate in the catalytic reaction.
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Figure 1. 1H NMR spectroscopy (500 MHz) study at À808C.
Scheme 7. Competition experiments to determine the relative rates of the
hydroboration of alkyne 1a and alkenes 3a and Z-2a.
1H{19F} HOESY did not provide any useful information with
which to verify the proposed H–F interactions.
A controlled experiment was next conducted to understand
the effect that alkene 3a has on the hydrogenolysis of borane
4a. Indeed, this process was shown to be promoted by alkene
3a, whereas, in the absence of 3a, only a trace amount of the
desired product Z-2a was observed (Scheme 8). Furthermore,
various additives that contained the pentafluorophenyl moiety
but which lacked the vinyl group were subjected to the hydro-
genation reaction (Scheme 9). However, none of them proved
to be effective in promoting this reaction. These results and
the different activation abilities of alkenes 3a and 3b suggest
that both pentafluorophenyl and vinyl groups are essential for
the high productivity of the reaction.
The reason for the production of E-alkenes was also investi-
gated. As shown in Scheme 10, Z-alkene 2a is stable at 1408C;
but isomerization of Z-alkene 2a to give E-alkene 2a was ob-
Scheme 10. Isomerization of Z-alkenes to E-alkenes.
served at this temperature in the presence of 10 mol% of
HB(C6F5)2. The isomerization is proposed to proceed through
the hydroboration of alkene Z-2 with HB(C6F5)2 to form borane
6, followed by a retrohydroboration to generate the more
stable alkene E-2.[17] Therefore, it can be deduced that Z-al-
kenes are generated directly by hydrogenation and E-alkenes
are formed by a subsequent isomerization of the Z-alkenes.
Owing to the high reactivity of alkynes with HB(C6F5)2, the iso-
merization process is restricted when unreacted alkyne is still
present in the reaction mixture, which ensures that the Z-
alkene is the predominant product. Only after exhaustion of
the alkyne substrate can E-alkenes be obtained through the
isomerization of Z-alkenes. Moreover, alkene 3a was found to
effectively inhibit the over-hydrogenation of alkene E-2a,
which would produce alkane 7 (Scheme 11). A possible explan-
Scheme 8. Hydrogenolysis of borane 4a.
Scheme 9. Hydrogenation of alkyne 1a with different additives.
The interaction between borane 4a and alkene 3a was fur-
1
ther investigated by H NMR analysis. When a 1:1 mixture of
borane 4a and alkene 3a in [D8]toluene was cooled to À808C,
the resonances for Ha and Hc in alkene 3a were shifted down-
field slightly and the resonance for Hd in borane 4a was shift-
ed upfield, but the signal of Hb was unchanged (Figure 1). In
comparison, a 1:1 mixture of borane 4a and styrene (3b),
under the same conditions, did not exhibit any changes in the
1H NMR spectra. These observations suggest that an encounter
complex of alkene 3a and borane 4a is most likely formed
through the interactions of H and F atoms,[18] which are dis-
tinct from the interactions of the C=C bond and the boron
atom proposed by Li and Stephan.[14c,d] A possible explanation
for this difference is that the alkene 3a is electron-deficient,
relative to ethylene and the borane tethered olefin, but it pos-
sesses more acidic protons. Unfortunately, analysis by
Scheme 11. Hydrogenation of E-alkene 2a to alkane 7.
ation is that the hydrogenation of alkenes to alkanes is a rela-
tively slow transformation, and the competition reaction of
alkene 3a with HB(C6F5)2 is likely to inhibit the reaction of
alkene 2a with HB(C6F5)2 so as to restrict the production of
alkane 7.
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culations. By considering the non-bonding nature of the inter-
action (p–p stacking) between 3a and 4a, DFT dispersion cor-
rection (DFT-D3) method was employed in the corresponding
geometry optimizations. In the absence of 3a, the transition
state (TS) for the hydrogenolysis of 4a to give Z-2a (Figure 3a,
TS4a) was calculated to have an activation energy of 35.1 kcal
molÀ1. Upon addition of 3a into the system, the lowest energy
TS for the hydrogenolysis of 4a was calculated at 33.8 kcal
molÀ1 (Figure 3b, TS4a+3a). Thus, alkene 3a exerts a TS stabiliza-
tion of 1.3 kcalmolÀ1 in the hydrogenolysis process of 4a.
These calculations support the experimental results that are re-
ported in Scheme 8, which indicate that alkene 3a can pro-
mote the hydrogenolysis process. It should be noted that, ac-
cording to transition state theory, a reduction of 1.3 kcalmolÀ1
in the energy barrier of the rate-limiting step is sufficient to
speed up the reaction by an order of magnitude.
Figure 2. DFT-calculated reaction profile for the hydrogenation of alkyne 1a
to give Z-alkene Z-2a without the assistance of 3a, and subsequent isomeri-
sation to E-alkene E-2a. Relative Gibbs free energy (DG) are reported in kcal
molÀ1, and all geometries were optimized without DFT dispersion correc-
tion.
DFT calculations were then
conducted to obtain a mechanis-
tic understanding of the catalytic
pathway at the atomic level.
Figure 2 depicts the whole reac-
tion profile for the catalytic hy-
drogenation of alkyne 1a based
on DFT calculations, without
consideration of the effect of
alkene 3a. The catalytic cycle is
initiated by the alkyne hydrobo-
ration process to give 4a, which
needs to overcome an activation
energy of 18.9 kcalmolÀ1. Fol-
lowing this, the hydrogenolysis
of 4a to give Z-2a occurs, which
is the rate-limiting step in the
catalytic cycle with a substantial
Figure 3. DFT-optimized transition state structures (bond lengths reported in ꢃ) for the hydrogenolysis of 4a:
a) without 3a and b) with 3a, both optimized with the DFT dispersion correction method.
reaction barrier of 34.4 kcal
molÀ1. The formation of Z-2a
from 1a encompasses the cata-
lytic cycle, and HB(C6F5)2 is regenerated for hydroboration of
another molecule of alkyne 1a in the next catalytic cycle. The
energy profile from Z-2a to E-2a in Figure 2 describes the re-
spective isomerization process, which is promoted by
HB(C6F5)2. It involves the following three steps: a) hydrobora-
tion of Z-alkene 2a to form alkyl borane 8a, b) rotation around
the (C6F5)2B(H)(Ph)CÀCH2Ph single bond to form another con-
former 8aiso, and c) retrohydroboration of 8aiso to give alkene
E-2a. The rate-limiting step in this isomerization process is the
final retrohydroboration, which has an energy barrier of
20.9 kcalmolÀ1. This barrier, which is higher than that for
alkyne hydroboration (18.9 kcalmolÀ1), further supports the ex-
perimental observation that when the alkyne substrate is pres-
ent in the reaction mixture, the E/Z isomerization process is
not a kinetically favored process.
By inspection of the structure TS4a+3a in Figure 3b, we could
identify three hydrogen bonding interactions. Two of them are
between protons Ha/Hc of 3a and fluorine atoms Fa/Fc of the
pentafluorophenyl group in 4a. Interestingly, these results are
consistent with the NMR spectroscopic data reported in
Figure 1, which showed the downfield shifts of the resonances
that corresponded to Ha and Hc in 3a and the resonance for
Hb was not affected. These two hydrogen bonding interactions
facilitate the hydrogenolysis of 4a by anchoring alkene 3a in
an appropriate orientation relative to 4a. The other hydrogen
bond in TS4a+3a is between a proton of H2, which is being
transferred to the vinyl group of 4a, and the fluorine atom Fb
of 3a. This interaction can assist the HÀH bond cleavage of H2
in the hydrogenolysis process, which helps to stabilize TS4a+3a
relative to TS4a and thus accelerates the reaction. These three
interactions indicate that both pentafluorophenyl and vinyl
groups are necessary in alkene 3a to make it effective in pro-
moting the hydrogenolysis reaction, which has been further
To clarify the effect that alkene 3a has on the rate-limiting
step of the catalytic cycle, which involves the formation of Z-
2a from 4a, at the atomic level, we performed further DFT cal-
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corroborated in the experiments above (Schemes 4 and 9. An-
other significant structural feature is that the pentafluorophen-
yl group of 3a lies closely above Hd. This close proximity may
lead to shielding of Hd by the pentafluorophenyl ring, which
could explain why the NMR resonance of proton Hd in alkene
4a was shifted upfield. To further explore the interaction of 3a
with 4a, we also calculated the Gibbs free binding energy of
3a to 4a without H2 in system. The slightly exergonic binding
energy of 0.5 kcalmolÀ1 was calculated at À808C. As a result,
the binding of 3a to 4a should be disturbed upon an increase
in temperature due to the entropy effect and, consequently,
this interaction would become undetectable, which is consis-
tent with the NMR experimental observation. In addition, the
p–p stacking between 4a and 3a is apparent from the geome-
try of TS4a+3a, as well as in the complex of 3a and 4a.
According to the mechanistic study, a plausible catalytic
cycle is shown in Scheme 12. The initial hydroboration of
alkyne 1 with HB(C6F5)2 generates the intermediate borane 4.
Then, an encounter complex 9 of borane 4 and alkene 3a is
formed through HÀF interactions. This is followed by hydroge-
nolysis via transition state 10, which produces alkene Z-2 and
regenerates both the catalyst and alkene 3a. Finally, after the
exhaustion of the alkyne 1, a HB(C6F5)2-promoted isomerization
of the alkene Z-2 gives E-2.
both practical experiments and theoretical calculations indicat-
ed that the alkenyl borane, which is formed by the hydrobora-
tion of the alkyne substrate with HB(C6F5)2, is a key intermedi-
ate, and its subsequent hydrogenolysis is likely to be the rate-
determining step in the catalytic process. Significantly, the
newly identified HÀF interactions between alkene 3a, the al-
kenyl borane intermediate, and H2, are suggested to play a key
role in accelerating this hydrogenation reaction, which might
have other interesting applications in FLP chemistry.
Acknowledgements
We are grateful for Prof. Junfeng Xiang’s kind help with the
low-temperature NMR analysis and the generous financial sup-
port from the National Natural Science Foundation of China
(21222207, 21290194, 21221002, 21473215), and 973 program
(2011CB808600).
Keywords: alkene
· alkyne · hydrogenation · metal-free
catalysis · stereoselectivity
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All experimental details can be found in the Supporting In-
formation.
Conclusion
An alkene-promoted borane-catalyzed hydrogenation of al-
kynes has been realized to give both Z- and E-alkenes in high
yields with excellent stereoselectivities. This reaction belongs
to the very rare class of catalytic hydrogenation systems that
use molecular H2 to access both alkene isomers selectively just
by a simple adjustment of the reaction conditions. Z-Alkenes
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Received: September 24, 2014
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Published online on && &&, 0000
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Full Paper
FULL PAPER
With a little help from an alkene! A
HB(C6F5)2-catalyzed metal-free hydroge-
nation of alkynes, promoted by an
alkene through HÀF interactions, was
successfully achieved. Z- and E-alkenes
can be accessed respectively in high
yields with excellent stereoselectivities
(see scheme).
&
Hydrogenation
Y. Liu, L. Hu, H. Chen,* H. Du*
&& – &&
An Alkene-Promoted Borane-
Catalyzed Highly Stereoselective
Hydrogenation of Alkynes to Give Z-
and E-Alkenes
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Chem. Eur. J. 2015, 21, 1 – 8
www.chemeurj.org
8
ꢂ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ÝÝ These are not the final page numbers!