Highly Chemo- and Stereoselective Transfer Semihydrogenation of Alkynes Catalyzed by a Stable, Well-Defined Manganese(II) Complex

Article abstract of DOI:10.1021/acscatal.8b00983

Herein we report unprecedented manganese-catalyzed semihydrogenation of internal alkynes to (Z)-alkenes using ammonia borane as a hydrogen donor. The reaction is catalyzed by a pincer complex of the earth-abundant manganese(II) salt in the absence of any

Full text of DOI:10.1021/acscatal.8b00983

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Highly Chemo- and Stereoselective Transfer Semihydrogenation of
Alkynes Catalyzed by a Stable, Well-defined Manganese(II) Complex
Aleksandra Brzozowska, Luis Miguel Azofra, Viktoriia Zubar, Iuliana
Atodiresei, Luigi Cavallo, Magnus Rueping, and Osama El-Sepelgy
ACS Catal., Just Accepted Manuscript • Publication Date (Web): 30 Mar 2018
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Highly Chemo- and Stereoselective Transfer Semihydrogenation of Al-
kynes Catalyzed by a Stable, Well-defined Manganese(II) Complex
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‡
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†
Aleksandra Brzozowska, Luis Miguel Azofra*, Viktoriia Zubar, Iuliana Atodiresei, Luigi
‡
†, ‡
†
Cavallo, and Magnus Rueping Osama El-Sepelgy,*
‡
KAUST Catalysis Center (KCC), King Abdullah University of Science and Technology (KAUST), Thuwal 23955-
6
900, Saudi Arabia
†
Institute of Organic Chemistry, RWTH Aachen University, Landoltweg 1, 52074 Aachen, Germany
KEYWORD. Base Metals, Manganese, Pincer Complexes, Ammonia borane, Alkynes, Semihydrogenation.
ABSTRACT: Herein we report a manganese catalyzed semihydrogenation of internal alkynes to (Z)-alkenes using ammo-
nia borane as a hydrogen donor. The reaction is catalyzed by a pincer complex of the earth abundant manganese(II) salt
in the absence of any additives, base or super hydride. The ammonia borane smoothly reduces the manganese pre-
catalyst [Mn(II)-PNP][Cl] to the catalytically active species [Mn(I)-PNP]-hydride in the triplet spin state. This manganese
2
hydride is highly stabilized by complexation with the alkyne substrate. Computational DFT analysis studies of the reac-
tion mechanism rationalize the origin of stereoselectivity towards formation of (Z)-alkenes.
INTRODUCTION
During the past months, manganese catalyzed hydro-
genation reactions have emerged. However, so far these
≡
are limited to the hydrogenation of C=O and C N bonds.
To the best of our knowledge, a well-defined manganese
complex catalyzing the (transfer) hydrogenation of un-
saturated hydrocarbons has never been reported. There-
fore, it was not surprising that our initial attempts to
employ these complexes in semihydrogenation of alkynes
were also unsuccessful.
Stereo-defined alkenes are amongst the most im-
portant organic compounds, ubiquitous in chemical,
material and pharmaceutical industries. Semihydrogena-
1
3
1
tion of alkynes to alkenes is a very important reaction in
2
organic synthesis. Until today, triple bond semireduction
relies mainly on the use of precious metal catalysis.
Among the known heterogeneous catalysts, the palladium
based Lindlar catalyst is the most prominent one. How-
3
ever, the necessity to use toxic lead salts as additive and
the product isomerization under the reaction conditions
are the major drawbacks of this method. Besides, various
4
modern heterogeneous catalysts, homogenous counter-
parts, such as the Rh-based Wilkinson’s and Schrock-
5
6
7
8
Osborn catalyst, palladium, ruthenium, and iridium
Scheme 1. Manganese Catalyzed Transfer Semi-
Hydrogenation of Alkynes.
catalysts have been intensively investigated for this reac-
tion.
The mechanism of the metal hydride catalyzed (Z)-
selective semireduction of alkynes typically takes place via
hydrometallation followed by reductive elimination steps,
which requires a vacant coordination site on the metal
center. However, the recently reported manganese hydro-
genation catalysts are hexa-coordinated pincer-Mn(I)
complexes. Thus, in order to design a proper catalyst for
this transformation, our attention was directed to the use
of the five-coordinate manganese(II) dichloride which
after activation might be converted to tetra-coordinate
manganese(I) hydride. In contrast to the well-established
iron(II) and cobalt(II) hydrogenation catalysts, manga-
nese(II) dichloride has been reported to be inactive for
the hydrogenation of carbonyl compounds. This might be
Economic and environmental aspects persuade chem-
ists to replace toxic noble metal catalysts by first row base
metal alternatives. In this regard, few homogenous cata-
lytic systems have been reported so far. In 2013, Milstein
has reported the use of acridine-based PNP iron catalyst
for the (Z)-selective semihydrogenation of alkynes fol-
lowed by rapid isomerization to (E)-alkenes under pres-
9
surized dihydrogen. More recently, processes relaying on
10
11
12
the use of nickel, cobalt and copper based catalysts
have been disclosed. Nonetheless, these protocols may
suffer from the use of high catalyst loading, limited sub-
strate scope, and low levels of chemoselectivity or over-
reduction. Despite these advances, new efficient catalytic
systems based on earth-abundant metal catalysts would
be highly desirable.
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attributed to the low stability of the formed Mn(I) hy- yield (69%) and a slightly higher Z/E ratio (88:12) was
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dride in the absence of carbonyl ligands.
Ammonia borane (AB) is a low molecular weight
observed by the application of triazine-based PNP-
manganese complex Mn-2 (Table 1, entry 2). Subsequent-
ly, we tested the catalytic performance of the readily
available terpyridine-Mn(II) complex Mn-3, however,
only moderate results were obtained (Table 1, entry 3). To
our delight, the newly synthesized pyridyl-based PNP
complex Mn-4 showed full conversion and very good
–
1
(
30.87 g mol ), bench-stable, inexpensive solid bearing
both protic N–H and hydric B–H bonds. The good hydro-
gen capacity (16.9 % wt.) and possibility of recycling its
dehydrogenated products has inspired intensive studies
towards the use of ammonia borane as H storage mole-
2
selectivity (97:3, Table 1, entry 4). The use of MnCl as a
catalyst afforded low conversion and selectivity (Table 1,
entry 5), highlighting the crucial role of the ligand.
cule. However, reports pertaining to the use of ammonia
borane as a chemical reagent and in particular as hydro-
gen donor are scarce. We report herein the first example
of manganese catalyzed transfer hydrogenation of unsatu-
rated carbon-carbon bonds using ammonia borane as an
environmentally benign hydrogen donor (Scheme 1).
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Next, different solvents were tested. Replacing MeOH
by EtOH resulted in the same conversion but lower Z/E
ratio (Table 1, entry 6). The promising results in methanol
and ethanol have encouraged us to investigate more alco-
hols such as iPrOH and more acidic fluorinated solvents:
trifluoroethanol (TFE) and hexafluoroisopropanol (HFIP).
However, these alcohols proved to be unsuitable for this
reaction (Table 1, entries 7-9). Interestingly, running the
reaction in THF or dioxane afforded moderate results
RESULTS AND DISCUSSION
We started our study with the synthesis of several PNP
and NNN manganese pincer complexes (Mn-1-Mn-4,
Figure 1). These five-coordinated Mn-complexes could be
easily prepared by the reaction of the anhydrous manga-
nese dichloride with 1.2 equivalent of the corresponding
ligand in THF at room temperature. The synthesis and
the characterization of the complexes Mn-1 and Mn-2
(
Table 1, entries 10-11). Furthermore, reactions performed
in DMF or NMP led to very low conversions, probably due
to the strong coordination of these solvents with the cata-
lyst (Table 1, entries 12-13).
1
3a
13b
were recently reported by Beller and Kempe, respec-
tively. However, these complexes were inactive hydro-
genation catalysts. Mn-3 was prepared from literature
a
Table 1. Optimization of the Reaction Conditions.
15
report. Furthermore, we have synthetized a new pyri-
dine-based manganese complex Mn-4 in 80% yield. The
crystal structure analysis of Mn-4 showed that the coor-
dination geometry of the manganese center is distorted
square pyramidal (Figure 1).
[Mn]
mol %)
c
entry
solvent conv.(%) 2a:3a
(
1
Mn-1 (2)
Mn-2 (2)
Mn-3 (2)
Mn-4 (2)
MeOH
MeOH
MeOH
MeOH
MeOH
EtOH
iPrOH
HFIP
27
69
62
>99
29
99
41
84:16
2
3
4
5
6
7
8
9
88:12
79:21
97:03
78:22
85:15
95:05
ND
MnCl (2)
2
Mn-4 (2)
Mn-4 (2)
Mn-4 (2)
Mn-4 (2)
Mn-4 (2)
Mn-4 (2)
Mn-4 (2)
Mn-4 (2)
Mn-4 (2)
Mn-4 (2)
Mn-4 (2)
Mn-4 (2)
Mn-4 (1)
Mn-4 (0.5)
06
08
87
63
14
TFE
ND
1
0
THF
72:28
79:21
80:20
83:17
83:17
ND
1
1
Dioxane
DMF
12
13
NMP
21
1
4
toluene
hexane
MeOH
MeOH
MeOH
MeOH
14
15
<5
<5
<5
>99
76
Figure 1. Manganese complexes tested in this study and X-
ray crystal structure analysis of Mn-4 (hydrogen atoms omit-
ted for clarity).
b
16
ND
c
1
7
ND
1
8
99:01
95:05
Next, we investigated the catalytic activity of the man-
ganese complexes Mn-1-Mn-4 in the hydrogenation of
diphenylacetylene as a model substrate. Methanol was
used as a solvent and ammonia borane as both, the poten-
tial catalyst activator and hydrogen donor. The NH-based
PNP manganese complex Mn-1 provided only low conver-
sion (27%) with 84:16 E/Z ratio (Table 1, entry 1). Higher
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a
Reaction conditions: 1 (0.25 mmol), AB (0.25 mmol), Mn
catalyst in 1 mL of solvent at 60 °C for 16 h. Conversion was
1
determined by H NMR using mesitylene as the internal
b
standard. Z/E ratio was determined by GC analysis. AB (0.025
c
mmol), HCOOH (0.5 mmol). AB (0.025 mmol), H (10 bar).
2
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The solvent screening shows that the non-polar sol-
vents; hexane and toluene failed to give the desired prod-
uct (Table 1, entries 14-15). Moreover, in order to exclude
molecular hydrogen as a reactant in described process,
the reaction was examined under hydrogen atmosphere
(10 bar) in the presence of catalytic amounts of AB. Im-
portantly, only traces of the desired product were detect-
ed (Table 1, entry 16). This result suggests that the reac-
tion takes place via direct transfer hydrogenation and not
via AB solvolysis followed by hydrogenation. Also, formic
acid has demonstrated to be unsuitable hydrogen donor
in this process (Table 1, entry 17).
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Following these results, we turned our attention to
screening of the reaction conditions with the respect to
the catalyst loading (Mn-4). Pleasingly, the reaction still
proceeded well with a catalyst loading as low as 1 mol %
(Table 1, entry 18). Subsequent reduction of the catalyst to
0.5 mol % resulted in lower conversion of 76% after 16 h
(Table 1, entry 19).
Table 2. Mn-Catalyzed (Z)-selective Transfer Hydrogena-
a
tion of Alkynes.
a
Reaction conditions: 1 (0.5 mmol), AB (0.5 mmol), Mn-4 (1 mol%) in
mL of methanol at 60 °C for 20 h, yields after column chromatog-
raphy, E/Z ratio is determined by GC analysis of the crude reaction
2
b
c
d
e
mixture; Mn-4 (2 mol%); Mn-4 (3 mol%); Mn-4 (4 mol%); AB
(
f
g
h
i
0.75 mmol),; AB (1 mmol); AB (1.5 mmol); AB (2 mmol); 70 °C.
In order to demonstrate the applicability of our man-
ganese catalyst system, we tested the transfer hydrogena-
tion of various internal alkyne substrates bearing different
electronic and steric properties (Table 2). Generally, all
investigated aromatic substrates with electron-
withdrawing groups and electron-donating groups were
converted into the corresponding (Z)-alkenes with very
good yields and excellent (Z):(E) selectivity 1a-1n. Inter-
estingly, sensitive functional groups such as nitrile, ester
and heterocycles were well tolerated under the mild reac-
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tion conditions. The semi-transfer hydrogenation could
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Scheme 2. Showcase of Manganese Catalyzed Purifica-
tion of cis-Alkenes without Isomerization.
also be applied to the mono- and dialkylacetylene 1o and
1p after using more catalyst loading and ammonia borane.
The protected allyl alcohol 2q could be obtained in 81%
yield and good selectivity. Finally, the pentynol-derived
alkyne 1r could be selectively semihydrogenated in good
yield (66%).
With this background knowledge we studied the reac-
tion mechanism for diphenylacetylene reduction into
1
7
stilbene through DFT modeling (Figure 2). Gibbs free
reaction and activation energies were calculated at 50 °C
in methanol as a solvent. The pre-catalyst we considered
is Mn-4, which we suggest can be reduced into the active
catalytic [Mn(I)-PNP]-hydride species (A, at Figure 2) by
NH BH . This hypothesis is based on the consideration
Apart from the synthetic value of the conversion of al-
kynes to alkenes, we decided to investigate the applica-
tion of the newly developed catalytic system in the purifi-
cation of alkenes from alkyne impurities, which is a highly
important industrial process to valorize the steam crack-
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g
that thermogravimetric analysis and differential scanning
ing bulk feedstocks. To demonstrate the process feasi-
bility, we hydrogenated diphenylacetylene in the presence
of large excess of (Z)-stilbene. To our delight, 0.03 mmol
of diphenylacetylene could be hydrogenated in the pres-
ence 100 equivalent of (Z)-stilbene using only 0.0003
mmol of the Mn-4. Importantly, (Z)-alkenes were ob-
tained with 99% purity with only traces of the isomerized
calorimetric studies proved that CoCl to be reduced into
2
cobalt(I) by induction of NH BH decomposition. These
3
3
findings served as the basis for demonstrating the reduc-
ing power of NH BH towards other compounds such as
3
3
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FeCl and AlCl species. Starting from the proposed ac-
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3
tive species A, two catalytic cycles working simultaneous-
ly can be proposed to explain the product distribution.
One is the alkyne reduction cycle, converting diphenyla-
cetylene into (Z)-stilbene, the second is the alkene isom-
erization cycle, converting (Z)-stilbene into (E)-stilbene.
(E)-alkene 3a and no over-hydrogenation product was
detected (Scheme 2).
Figure 2. Proposed reaction mechanism for manganese catalyzed semi-transfer hydrogenation of diphenylacetylene (1a) into
Z)-stilbene (2a) (reduction cycle) and internal conversion of (Z)-stilbene (2a) into (E)-stilbene (3a), (isomerization cycle). Cal-
(
–
1
culated Gibbs free reaction and activation energies, at 50 °C, are shown in kcal mol at PBE0+D3/TZVP computational level in
methanol as solvent.
Calculations indicate that the reaction occurs through
a triplet state spin route (Table S1). For example, in the
case of the catalytic species A the triplet spin state is more
stable than the singlet spin state by 27.5 and 32.4 kcal
–1
mol at the PBE/SVP and PBE0+D3(BJ)/TZVP//PVE/SVP
levels of theory in methanol. These values are consistent
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with the value of 33.5 kcal mol calculated at the S4). Proton transfer from NH BH to the substrate via
–
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OPBE/SVP level in gas phase.
transition state D’-E’, with a Gibbs free energy of activa-
–1
The alkyne reduction cycle, converting diphenylacety-
lene into (Z)-stilbene, starts with coordination of diphe-
nylacetylene to the [Mn(I)-PNP]-hydride catalyst A, spon-
taneously forming the stable complex B, with a binding
tion of 35.5 kcal mol is again of high energy, which de-
presses reactivity. These results are in agreement with
experimental solvent screening experiments in Table 1.
The formation of (E)-stilbene may also be explained
via the alkene isomerization cycle shown in Figure 2.
Within this cycle the catalytic species A interacts with
(Z)-stilbene leading to complex G. This is a slightly en-
dergonic step, with a binding Gibbs free energy of 1.2 kcal
–
1
Gibbs free energy at 50 °C of –16.4 kcal mol in methanol.
The overall structure of B (see Figure S2) has a strong
metallacycloolefin character, as the C–C–Ph angles are
severely bent from 180° in the isolated substrate, to as-
sume an average value of 143°. The C–C bond, 1.34 Å, is
close to the value of 1.36 Å as in the isolated (Z)-stilbene,
compared to 1.23 Å in the isolated diphenylacetylene.
This first hydrogenation of the substrate, leading to
intermediate C, exhibits a Gibbs free activation energy of
–1
mol , which contrasts with the large binding energy of
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–1
diphenylacetylene to A (–16.4 kcal mol ). Hydride transfer
from Mn(I) of G to the closest C atom of stilbene leads to
intermediate H. This hydrogenation transfer exhibits a
–1
low relative activation barrier of 5.0 kcal mol and leads
–1
3
1
7.6 kcal mol via transition state B-C. Intermediate C
to a sp hybridization of the C atom of stilbene, which
presents a cis disposition of the Ph substituents on the
formed C=C double bond. The overall skeleton of the
reacting carbonaceous moiety is perpendicular to the PNP
plane, see Figure 3. The formed Mn–C bond is nearly
collinear with the Mn–N bond, with a Mn–N–C angle of
indeed allows easy rotation around the C–C bond, step H
to I (the relative rotational activation barrier is calculated
–1
to be 5.0 kcal mol ). β-H transfer to the Mn atom from I
via transition state I-J with Gibbs free energy barrier of
–1
4.6 kcal mol leads to (E)-stilbene, intermediate J. Finally,
dissociation of (E)-stilbene from the metal regenerates
the catalytic species A.
1
61°, and the large distance between Mn and the trans-
ferred H atom, 3.17 Å, indicates no agostic interaction.
The next step corresponds to MeOH interaction with
the Mn center, leading to intermediate D, with a free
–1
energy increasing in 2.6 kcal mol . The long distance
between the Mn and the O atom of MeOH, 3.60 Å, sug-
gests this interaction being dominated by dispersion forc-
es. Indeed, the enthalpy of association (between MeOH
and C to reach D) at the PBE0+D3(BJ)/TZVP//PVE/SVP
–
1
level amounts to –6.6 kcal mol , while at the
PBE0+D3/TZVP level, which is after addition of empirical
B
G
–1
dispersion term, it amounts to –11.1 kcal mol . Proton
transfer from MeOH to the substrate occurs via transition
state D-E, indicated to be the rate-limiting step with a
–1
relative activation Gibbs free energy of 25.6 kcal mol
(energy span from C to D-E). Once (Z)-stilbene is released
in a spontaneous process, the formed [Mn]-OMe inter-
mediate F reacts with NH BH to regenerate the starting
3
3
[Mn(I)-PNP]-hydride catalytic species A.
G-H
B-C
A key feature of the above reaction pathway is that the
initial hydrogen transfer to diphenylacetylene, step B to
C, occurs with formation of a Mn-vinyl bond presenting a
cis disposition of the Ph substituents. Since no free rota-
tion around the C=C double bond of the substrate is pos-
sible, stereoselectivity towards the (Z)-alkene is imposed.
Our attempts to locate an alternative transition state in
which proton transfer from an external MeOH to diphe-
nylacetylene occurs first followed by the hydride transfer
from the Mn–H moiety failed.
H
C
Figure 3. Minima and transition states for the hydride trans-
fer steps in the reduction (B to C) and isomerization (G to H)
Furthermore, the potential role of NH BH as proton
3
’
3
’
donor instead of MeOH during the D to E step has been
cycles. (For clarity, iPr groups are in soft color).
2
also investigated. The large activation Gibbs free energy
–1
of 37.1 kcal mol (relative to C) clearly rules out this route
in presence of MeOH (Figure S3). Finally, we also ana-
lyzed both the reduction and isomerization cycles in THF,
where NH BH is the sole source of proton (see Figure
Comparison between the reduction and the isomeriza-
tion cycles shows that coordination of diphenylacetylene
–1
to the catalytic species A is favored by 17.6 kcal mol over
3
3
–
1
(
Z)-stilbene coordination (–16.4 vs. 1.2 kcal mol ). Steric
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effects play a minor role in determining this large differ-
ence in the binding energies, as a similarly large differ-
*Email: luis.azoframesa@kaust.edu.sa
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ORCID
ence, 14.0 kcal mol , is calculated for coordination of
diphenylacetylene and (Z)-stilbene to a model system
where the iPr substituents on the P atoms of PNP ligands
are replaced by smaller methyl groups, –17.3 and –3.3 kcal
Magnus Rueping: 0000-0003-4580-5227
Osama El-Sepelgy: 0000-0003-3131-4988
Luigi Cavallo: 0000-0002-1398-338X
Luis Miguel Azofra: 0000-0003-4974-1670
–1
mol , respectively. Further, coordination of the simplest
alkyne (acetylene) and alkene (ethylene) to this model
system with methyl substituents on the P atoms, displays
again a large difference in the binding Gibbs free energies,
Notes
The authors declare no competing financial interest
–
1
ASSOCIATED CONTENT
–
18.3 and –7.8 kcal mol , confirming that preferential
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The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acscatal.XXXXXXX.
alkyne coordination over alkene coordination is dominat-
ed by electronic effects.
Considering that the substrate coordinated intermedi-
ates B and G are connected by intermediate A, the Cur-
tin-Hammett principle can be used to relate the faster
kinetics of the alkyne reduction cycle to the energy differ-
ence between the highest laying transition states along
the two cycles, which is transition states D-E and G-H.
Consistently with the experimental evidence, the former,
along the alkyne reduction cycle, is favored by 2.4 kcal
Experimental and DFT details (PDF)
Crystallographic data (CIF)
Optimized geometries (XYZ)
ACKNOWLEDGMENT
L.M.A. and L.C., acknowledge King Abdullah University of
Science and Technology (KAUST) for support. Gratitude is
also due to the KAUST Supercomputing Laboratory using the
supercomputer Shaheen II for providing the computational
resources.
–1
mol over the latter, taking place along the alkene isom-
erization cycle.
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19
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AUTHOR INFORMATION
Corresponding Authors
*Email: Osama.Elsepelgy@rwth-aachen.de
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ing non-electrostatic terms. (Full computational details can be
found in the Supporting Information).
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the OLYP Functional. J. Chem. Theory Comput. 2007, 3, 689-702.
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(19) During publication process, a related manganese cata-
lyzed semihydrogenation of alkynes to E-Olefins has been re-
ported, see: Zhou, Y.-P.; Mo, Z.; Luecke, M.-P.; Driess, M., Stere-
oselective Transfer Semi-Hydrogenation of Alkynes to E-Olefins
with N-Heterocyclic Silylene–Manganese Catalysts. Chem. Eur. J.
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