Stereoselective Chromium-Catalyzed Semi-Hydrogenation of Alkynes

Article abstract of DOI:10.1002/cctc.202000994

Chromium complexes have found very little applications as hydrogenation catalysts. Here, we report a Cr-catalyzed semi-hydrogenation of internal alkynes to the corresponding Z-alkenes with good stereocontrol (up to 99/1 for dialkyl alkynes). The catalyst comprises the commercial reagents chromium(III) acetylacetonate, Cr(acac)₃, and diisobutylaluminium hydride, DIBAL?H, in THF. The semi-hydrogenation operates at mild conditions (1-5 bar H₂, 30 °C).

Full text of DOI:10.1002/cctc.202000994

The European Society Journal for Catalysis
Supported by
Accepted Article
Title: Stereoselective Chromium-Catalyzed Semi-Hydrogenation of
Alkynes
Authors: Axel Jacobi von Wangelin, Bernhard J. Gregori, Michal
Nowakowski, Anke Schoch, Matthias Bauer, Josef Zweck,
and Simon Pöllath
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To be cited as: ChemCatChem 10.1002/cctc.202000994
Link to VoR: https://doi.org/10.1002/cctc.202000994
01/2020

10.1002/cctc.202000994
ChemCatChem
FULL PAPER
Stereoselective Chromium-Catalyzed Semi-Hydrogenation of
Alkynes
Bernhard J. Gregori,[a] Michal Nowakowski,[b] Anke Schoch,[b] Simon Pöllath,[c] Josef Zweck,[c] Matthias
Bauer,[b] and Axel Jacobi von Wangelin[a]*
Abstract: Chromium complexes have found very little applications as
hydrogenation catalysts. Here, we report a Cr-catalyzed semi-
hydrogenation of internal alkynes to the corresponding Z-alkenes with
good stereocontrol (up to 99/1 for dialkyl alkynes). The catalyst
comprises the commercial reagents chromium(III) acetylacetonate,
Cr(acac)3, and diisobutylaluminium hydride, DIBAL-H, in THF. The
semi-hydrogenation operates at mild conditions (1-5 bar H2, 30 °C).
report an operationally facile protocol for the semi-hydrogenation
of alkynes based on a catalyst comprising of commercially
available chromium tris(acetylacetonate), Cr(acac)3, and
diisobutylaluminium hydride, DIBAL-H, at very mild conditions
(Scheme 1, bottom).
Introduction
Alkenes are ubiquitous motifs in numerous organic molecules
including natural products, fragrances, pharmaceuticals,
agrochemicals, fine chemicals, and polymers. The meticulous
control over the stereochemistry of the C=C bond enables distinct
structural and functional properties of the alkene unit.[1] An
especially attractive access to both stereoisomers of alkenes from
the same starting material is the semi-hydrogenation of alkynes.[2]
High reactivities, mild reaction conditions, and good levels of
stereocontrol of this atom-economical reaction have been
reported in the presence of various transition metal catalysts. The
first and foremost method is the Lindlar-type Z-selective
hydrogenation[3] with Pd@CaCO3 catalysts that are poisoned with
quinoline and lead acetate. Later, the Nickel-P2 catalyst[4]
(Ni(OAc)2, NaBH4, ethanol) was reported to give similar
selectivities. Recently, major progress has been achieved toward
the application of late 3d transition metal catalysts (Fe,[5] Co,[6]
Ni,[4,7] Cu[8]), whereas much less attention has been devoted to
earlier 3d metals (Sc,[9] V,[10] Mn,[11] Cr[12]). Only two examples of
chromium-catalyzed semi-hydrogenation of alkynes were
reported: With (arene)tricarbonylchromium complexes[12a] at
harsh conditions (120 °C, 70 bar H2); with metallated porous
organic polymers but narrow scope and low selectivity.[12b,c]
Unlike the many applications of chromium complexes to
Scheme 1. Chromium-catalyzed Z-selective semi-hydrogenation of alkynes.
Results and Discussion
Lead discovery
We have recently reported the use of co-catalytic iron(II)
bis(acetylacetonate)
and
DIBAL-H
in
alkyne
semi-
hydrogenations.[5a] A brief survey of 3d transition metal salts
revealed distinct activities in the model reaction of 1-phenyl-1-
propyne under low H2 pressure (2 bar): While Ni(acac)2 and
Co(acac)2 underwent complete hydrogenations to the alkanes,
chemoselective hydrogenation to the alkenes with high Z-stereo-
polymerizations,[13]
oxidations,[14]
and
organometallic
chemistry,[15] there is very little knowledge of the activity of
chromium catalysts in hydrogenation reactions.[12,16] Here, we
[5a]
control was observed with Fe(acac)2 and Cr(acac)3 (Table 1,
entries 1-4). To the best of our knowledge, the latter protocol
constitutes the first application of a simple Cr salt/metal hydride
co-catalyst to efficient alkyne semi-hydrogenations. CrCl2 and
CrCl3 afforded slightly higher stereoselectivities (Z/E = 17/1) but
with significantly lower yields (Table 1, entries 5, 6). For further
studies, we used the readily soluble, bench-stable, and
commercially available Cr(acac)3 (see ESI for further optimization
experiments).
[a]
B. J. Gregori, Prof. Dr. A. Jacobi von Wangelin*
Dept. of Chemistry, University of Hamburg
Martin Luther King Pl 6, 20146 Hamburg (Germany)
E-mail: axel.jacobi@uni-hamburg.de
[b]
[c]
Dr. M. Nowakowski, A. Schoch, Prof. Dr. M. Bauer
Dept. of Chemistry, University of Paderborn
Warburger Str. 100, 33098 Paderborn (Germany)
S. Pöllath, Prof. Dr. J. Zweck
Dept. of Physics, University of Regensburg
Universitaetsstr. 31, 93053 Regensburg (Germany)
Supporting information for this article is given via a link at the end of
the document.
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10.1002/cctc.202000994
ChemCatChem
FULL PAPER
Highly electrophilic and reduction-sensitive functional groups
underwent competing reduction which led to catalyst deactivation.
(4-Bromophenyl)ethynylbenzene gave minor dehalogenation
(<5%) which could be suppressed by higher catalyst loadings
(Table 3, entry 4). Carboxylate groups were fully preserved while
ketones, nitriles, and nitro substituents were reactive under the
conditions (traces of corresponding alcohol and amine products
detected by GC-MS). Internal alkynes bearing 1,2-dialkyl
substitution gave high yields and near perfect stereoselectivities
in most cases (Table 4). Several functionalized alkynes with
chloro, acetate, trimethylsilyloxy, and phthalimido substituents
afforded the desired Z-alkenes with >99/1 stereocontrol.
Table 1. Selected optimization experiments.[a]
Entry
MXn
x
Alkenes [%]
Z/E
18/1
-
Alkane [%]
1
2
3
4
5
6
7
Fe(acac)2
Co(acac)2
Ni(acac)2
Cr(acac)3
CrCl2
10
10
10
15
10
15
15
98
<1
<1
51
19
28
<1
<1
>99
>99
2
-
12/1
17/1
16/1
-
<1
1
Table 3. Semi-hydrogenation of diaryl alkynes.[a]
CrCl3
-
-
[a] Conditions: 0.2 mmol alkyne, 0.4 mL THF. Yields and Z/E ratios were
determined by 1H-NMR and GC-FID.
Entry
Alkyne
R
Alkenes [%]
Z/E
1[b]
-
98
3/1
2
2-F
4-Cl
4-Br
87
96
22 (98)[c]
7/1
3/1
6/1 (4/1)[c]
Substrate Scope. The optimized conditions (10 mol% Cr(acac)3,
30 mol% DIBAL-H) were then applied to the semi-hydrogenation
of various aryl-substituted alkynes at very mild conditions (1.3-
5 bar H2, 30 °C) and reaction times up to 24 h to assure (near)
quantitative conversions. 1-Aryl-2-alkyl acetylenes (Table 2) and
1,2-diaryl acetylenes (Table 3) showed high conversions but only
moderate Z/E-stereoselectivities (up to 10/1). Terminal alkynes
were not tolerated (Table 2, entry 3) while the reaction was
compatible with F, Cl, Br, NH2, OCF3, ester, TMS, and cyclopropyl.
3[b]
4
5[d]
6[d]
7[d]
8
2-OMe
3-OMe
4-OMe
4-OCF3
97
92
87
85
1/1
3/1
2/1
4/1
9
10[f]
4-COMe
4-CO2Me
30[e]
97
3/1
10/1
11
12[g]
4-NH2
4-NO2
0
0
-
-
Table 2. Semi-hydrogenation of aryl-alkyl substituted alkynes.[a]
[a] Conditions: 0.2 mmol alkyne, 0.4 mL THF. Yields and Z/E ratios were
determined by 1H-NMR and GC-FID. [b] 3 bar H2. [c] 20 mol% [Cr], 60 mol%
DIBAL-H. [d] 3 bar H2, 16 h. [e] Incomplete conversion. [f] 2 bar H2, 12 h. [g]
3 bar H2, 20 h.
Entry
Alkyne
R
Alkenes [%]
Z/E
Catalyst characterization. In an effort to elucidate the nature of
the active catalyst, we performed several spectroscopic studies.
The strongly reducing conditions (DIBAL-H, H2) and the absence
of stabilizing ligands may facilitate catalyst reduction to a low-
valent or naked metal species which ultimately may nucleate and
grow to larger particles. However, the clear distinction between
homogeneous and heterogeneous metal catalysts is not trivial,[17]
yet reaction progress analyses and kinetic poisoning experiments
can provide useful insight.[18] The semi-hydrogenation of 1-
phenyl-1-propyne with 10 mol% [Cr] under standard conditions
displayed quite effective catalyst inhibitions when 5 mol% tri-
methylphosphite, P(OMe)3, or trimethylphosphine, PMe3,
respectively, were added after 30 min reaction. As the observed
catalyst inhibitions were not complete, these results may indicate
the presence of a major portion of active heterogeneous catalysts
which are in equilibrium with homogeneous catalyst species.
1[b]
2
3[c]
4[d]
5[b]
6
Me
Et
H
c-Propyl
c-Hexyl
TMS
95
94
0
96
73
58[e]
6/1
6/1
-
6/1
9/1
4/1
7
8[f]
9[g]
10[h]
11[h]
H
87
96
43[e]
96
6/1
7/1
2/1
3/1
6/1
Me
OMe
Cl
NH2
87
[a] Conditions: 0.2 mmol alkyne, 0.4 mL THF. Yields and Z/E ratios were
determined by 1H-NMR and GC-FID. [b] 5 h. [c] 5 bar H2, 3 h. [d] 3 bar H2,
16 h. [e] Incomplete conversion. [f] 14 h. [g] 12 h. [h] 5 bar H2, 24 h.
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10.1002/cctc.202000994
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isosbestic point so that a steady conversion of a homogeneous
solution is dominant in the XAS data. The magnitude of band
shifts induced by the addition of DIBAL-H is becoming lesser
when approaching the 5th and 6th equivalent. The continuous band
shifts - and the fact that the most active Cr catalyst involves 3
equiv. DIBAL-H - may be indicative of the presence of multiple
equilibrating species among which the fully reduced species is not
the active catalyst.[5a,19] The comparison with the XAS spectrum
of a bulk Cr(0) foil (Figure 2, gray curve) demonstrates that
reduction by the co-catalyst DIBAL-H does not exclusively take
place at Cr under the catalytic conditions. In agreement with
earlier observations in the closely related Fe(acac)2-catalyzed
semi-hydrogenation,[5a] we postulate the operation of ligand-
centered reduction events. The XANES curvature of the reduced
species (with 6 equiv. DIBAL-H) showed the characteristic
features of elemental chromium (mainly the intense and broad
pre-peak and edge signals in the range of 5988-5998 eV) but still
was significantly different from the XANES spectrum of Cr(0). This
supports the notion that potentially formed Cr(0) particles
remaining in solution are rather small, in low concentration, or
occurring in a different coordination environment. Further, the
doublet-like white line between 6005-6020 eV did not disappear
completely so that the presence of acac or related ligands at
chromium ions cannot be excluded. This behavior is also reflected
in the EXAFS (extended X-ray absorption fine structure) analysis
(please see the ESI for details) which clearly document constantly
decreasing Cr-O coordination numbers upon addition of DIBAL-
H. In the same direction, the Cr-C contributions from the acac-
backbone decrease, while the Cr-O and Cr-C distances slightly
increase. Importantly, no Cr-Cr pairs of potential clusters could be
observed under the catalytic reaction conditions. In combination
with the XANES results, we therefore postulate that the major
reduction by the co-catalyst DIBAL-H occurs at the acac ligands
in solution phase.[5a]
Table 4. Semi-hydrogenation of dialkyl-substituted alkynes.[a]
Entry
1[b]
Alkyne
R
Alkenes [%]
99
Z/E
n-C5H11
>99/1
>99/1
2
18 (42)[c] (58)[d]
3
4
5[e]
Me
Ac
TMS
99
73
35
>99/1
>99/1
>99/1
6[f]
7[g]
8[e]
0
-
0
-
91
>99/1
9[g]
0
-
10[g]
89
6/1
[a] Conditions: 0.2 mmol alkyne, 0.4 mL THF. Yields and Z/E ratios were
determined by 1H-NMR and GC-FID. [b] 1.3 bar H2, 14 h. [c] 20 bar H2, 24 h.
[d] 5 bar H2, 48 h. [e] 20 bar H2, 48 h. [f] 1.3 bar H2, 20 h. [g] 5 bar H2.
100
50
0
Control reaction
Addition of
Poison
P(OMe)3
PMe3
0
60
120
180
240
Time [min]
Figure 1. Kinetic catalyst poisoning experiments with PMe3 and P(OMe)3,
respectively (each 0.5 equiv. per [Cr]; addition after 30 min of hydrogenation).
The catalyst activation by addition of DIBAL-H was studied by in-
operando X-ray absorption spectroscopy (XAS) at the chromium
K-edge.[14d,19] Figure 2 shows the XANES (X-ray absorption near
edge structure) spectra for the successive additions of up to 6
equiv. DIBAL-H to a solution of Cr(acac)3 in THF (in the absence
of alkyne and H2). At higher DIBAL-H concentrations, minor
amounts of a precipitate formed which is in accordance with the
particle formation monitored by transmission electron microscopy
(TEM, vide infra). The recorded set of spectra exhibits an
Figure 2. XANES spectra of co-catalyst mixtures of Cr(acac)3 with DIBAL-H.
Spectrum of bulk Cr(0) foil shown for comparison.
The notion of significant ligand reduction is further supported by a
detailed quantification of the by-products of the binary pre-catalyst
mixture (Scheme 2). Stoichiometric reaction of tris(benzoyl-
acetonato)chromium(III), Cr(bzac)3, and DIBAL-H afforded mostly
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10.1002/cctc.202000994
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ligand reduction by hydride addition and deoxygenation (Scheme
2b). In comparison with our earlier report of an iron-catalyzed
semi-hydrogenation protocol,[5a] the extent of ligand-centered
reduction is higher for Cr, which appears to reflect the inherent
redox potentials and Lewis acid character. The reaction order of
the semi-hydrogenation was determined to be ~1 in [Cr] catalyst
under the catalytic reaction conditions and slightly higher
(approaching 2.5) at lower catalyst concentrations (see the ESI).
Scheme 2c illustrates potential homogeneous and heterogeneous
catalyst species. We speculate that DIBAL-H acts as activator for
the pre-catalyst by ligand and Cr reduction. The softer metal Cr,
even more so in low oxidation states, engages in coordination of
the soft Lewis base alkyne to effect -bond activation.
We further wished to study the formation of nanoparticles from
reactions of Cr(acac)3 with 3 equiv. DIBAL-H (upon ageing for >30
min and solvent evaporation). Transmission electron microscopy
(TEM) measurements were performed to evaluate the size,
elemental composition, and structure of the resultant hetero-
geneous species. Figures 3a and 3b show the TEM images of
isolated Cr-nanoparticles before the Cu/C grid. An energy
dispersive X-ray (EDX) spectrum on particles in a ~1.5 μm² area
is given in Figure 3c. Beside the metal background noise of the
TEM device (Fe, Co, Cu; see ESI), only two clear Cr peaks are
present, proving the particles’ Cr content. The background does
not contain any Cr and Al. Electron diffraction measurements
further indicated a crystalline structure of the Cr(0) particles (see
ESI). Analysis of 225 particles gave an average particle size of
5.2 ± 0.1 nm with a standard deviation σ = 1.9 nm (Figure 3d).
Scheme 2. a) Trends of metal vs. ligand reduction in pre-catalyst mixtures of
M(bzac)3 and DIBAL-H. b) Analysis of products derived from ligand-centered
reduction of M(bzac)3 with DIBAL-H (relative GC-MS peak areas of products are
given). c) Potential catalyst species and catalyst characterization studies.
Conclusions
Among the rich literature of chromium catalysis in organic
transformations, there are only very few reports of chromium-
catalyzed hydrogenations. This work has demonstrated the
catalytic potential of the simple pre-catalyst mixture
Cr(acac)3/DIBAL-H in the stereoselective semi-hydrogenation of
alkanes. The protocol involves mild conditions (<5 bar H2, 30 °C)
and can be applied to various arylalkynes and alkylalkynes
including functionalized substrates bearing halogen, ester, and
amine groups. Spectroscopic studies indicate a significant
operation of ligand-centered reduction events en route to the
active Cr catalyst species, which most likely are both
homogeneous and heterogeneous in nature, and possibly
equilibrate under the conditions.
Figure 3. a) High-resolution TEM of two Cr nanoparticles; b) Overview images
of several nanoparticles; c) EDX spectrum; d) Size distribution of 225 particles.
charged with alkyne (0.2 mmol) and n-pentadecane (internal GC reference,
0.2 mmol, 42.5 mg). The catalyst suspension (see above, 0.4 mL,
0.02 mmol [Cr], 10 mol%) was added, the vial sealed with a septum,
punctured by a short needle, and transferred to a pressure-resistant
reactor equipped with a gas inlet. The reactor was purged with H2 for 1 min
while stirring the reaction. After three short cycles of (de)pressurizing
(2.0 bar, 5 sec, 1.3 bar), H2 pressure and temperature were set (1.3 bar,
30 °C). After 3-48 h, the reactor was cooled and the pressure released.
The vials were retrieved, saturated aqueous NaHCO3 (1.5 mL) and ethyl
acetate (1.5 mL) were added. The resulting suspension was stirred for
20 min, the organic phase separated and filtered through a SiO2 plug.
Yields and selectivities were determined by flash chromatography (SiO2,
n-pentane, ethyl acetate), quantitative GC, and NMR vs. internal reference.
Experimental Section
Catalyst preparation: Under an atmosphere of argon (Schlenk line or
glove box), an oven-dried vial was charged with a stir bar, Cr(acac)3
(17.6 mg, 50 µmol), and dry THF (0.85 mL). The mixture was stirred for
10 min at r.t. and DIBAL-H was added dropwise (150 µL, 1.0 M in toluene,
150 µmol, 3.0 equiv.). The resultant mixture was stirred for 20 min prior to
hydrogenation reactions (color-change from purple to black after 5 min).
General procedure of catalytic semi-hydrogenations: Under argon
(Schlenk line or glove box), an oven-dried 4 mL vial with a stir bar was
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10.1002/cctc.202000994
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[8]
a) T. Wakamatsu, K. Nagao, H. Ohmiya, M. Sawamura, Organometallics,
2016, 35, 1354-1357; b) K. Semba, R. Kameyama, Y. Nakao, Synlett
2015, 26, 318-322; c) H. Bao, B. Zhou, H. Jin, Y. Liu, J. Org. Chem. 2019,
84, 3579-3589; d) X. Han, J. Hu, C. Chen, Y. Yuan, Z. Shi, Chem.
Commun. 2019, 6922-6925; e) N. Cox, H. Dang, A. M. Whittaker, G. Lalic,
Tetrahedron 2014, 70, 4219-4231; f) G.-H. Wang, H.-Y. Bin, M. Sun, S.-
W. Chen, J.-H. Liu, C.-M. Zhong, Tetrahedron 2014, 70, 2175-2179; g)
F. Pape, J. F. Teichert, Synthesis 2017, 2470-2482.
Acknowledgements
This work was supported by the Priority Program “Materials
Synthesis” of the Deutsche Forschunggsgemeinschaft (DFG,
SPP 1708). We thank Karen Schaefer and Alja Wölk (both UHH)
for technical assistance. We gratefully acknowledge beam time at
P64, PETRA III of the Deutsches Elektronen-Synchrotron (DESY).
[9]
D. S. Levine, T. D. Tilley, R. A. Andersen, Chem. Commun. 2017, 11881-
11884.
Conflict of Interest
[10] H. S. La Pierre, J. Arnold, D. Toste, Angew. Chem. Int. Ed. 2011, 50,
3900-3903.
The authors declare no conflict of interest.
[11] Selected examples of semi-transfer-hydrogenations: a) A. Brzozowska,
L. M. Azofra, V. Zubar, I. Atodiresei, L. Cavallo, M. Rueping, O. El-
Sepelgy, ACS Catal. 2018, 8, 4103-4109; b) Y.-P. Zhou, Z. Mo, M.-P.
Luecke, M. Driess, Chem. Eur. J. 2018, 24, 4780-4784.
Keywords: chromium • hydrogenation • alkynes •
stereoselectivity • nanoparticles
[12] a) M. Sodeoka, M. Shibasaki, J. Org. Chem. 1985, 50, 1147-1149; b) K.
K. Tanabe, M. S. Ferrandon, N. A. Siladke, S. J. Kraft, G. Zhang, J. Niklas,
O. G. Poluektov, S. J. Lopykinski, E. E. Bunel, T. R. Krause, J. T. Miller,
A. S. Hock, S. T. Nguyen, Angew. Chem. Int. Ed. 2014, 53, 12055-12058;
c) J. Camacho-Bunquin, N. A. Siladke, G. Zhang, J. Niklas, O. G.
Poluektov, S. T. Nguyen, J. T. Miller, A. S. Hock, Organometallics 2015,
34, 947-952; d) F. Kallmeier, R. Fertig, T. Irrgang, R. Kempe, Angew.
Chem. Int. Ed. 2020, 59, 11789-11793.
[1]
a) A. Fürstner, G. Seidel, J. Organomet. Chem. 2000, 606, 75-78; b) A.
Fürstner, F. Stelzer, A. Rumbo, H. Krause, Chem. Eur. J. 2002, 8, 1856-
1871; c) G. L. Parker, L. K. Smith, I. R. Baxendale, Tetrahedron 2016,
72, 1645-1652; d) D. Srimani, Y. Diskin-Posner, Y. Ben-David, D.
Milstein, Angew. Chem. Int. Ed. 2013, 52, 14131-14134; e) W.-Y. Siau,
Y. Zhang, Y. Zhao, Top. Curr. Chem. 2012, 327, 33-58; f) R. H. Grubbs,
T. M. Trnka, In: Ruthenium in Organic Synthesis (Ed.: S.-I. Murahashi),
Wiley-VCH, Weinheim, 2004.
[13] a) K. H. Theopold, Eur. J. Inorg. Chem. 1998, 15-24; b) C. Bariashir, C.
Huang, G. A. Solan, W.-H. Sun, Coord. Chem. Rev. 2019, 385, 208-229;
c) R. Grauke, R. Schepper, J. Rabeah, R. Schoch, U. Bentrup, M. Bauer,
A. Brückner, ChemCatChem 2020, 12, 1025-1035.
[2]
a) J. A. Delgado, O. Benkirane, C. Claver, D. Curulla-Ferré, C. Godard,
Dalton Trans. 2017, 46, 12381-12403; b) A. Fürstner, J. Am. Chem. Soc.
2019, 141, 11-24; c) J. Chen, J. Guo, Z. Lu, Chin. J. Chem. 2018, 36,
1075-1109; d) K. C. K. Swamy, A. S. Reddy, K. Sandeep, A. Kalyani,
Tetrahedron Lett. 2018, 59, 419-429.
[14] J. Muzart, Chem. Rev. 1992, 92, 113-140.
[15] a) L. A. Wessjohann, G. Scheid, Synthesis 1999, 1-36; b) J. Li, P.
Knochel, Synthesis 2019, 2100-2106; c) X. Zeng, X. Cong, Org. Chem.
Front. 2015, 2, 69-72; d) M. Sodeoka, M. Shibasaki, Synthesis 1993,
643-658.
[3]
[4]
a) H. Lindlar, R. Dubuis, Org. Synth. 1966, 46, 89-91; b) H. Lindlar, Helv.
Chim. Acta 1952, 35, 446-450; c) W.-Y. Siau, Y. Zhang, Y. Zhao, Top.
Curr. Chem. 2012, 327, 33-58.
[16] a) T. J. Thomas, A. Brenner, J. Mol. Catal. 1983, 18, 197-202; b) W.
Kramarz, S. S. Kurek, J. Mol. Catal. 1985, 33, 305-310.
a) F. Alonso, I. Osante, M. Yus, Tetrahedron 2007, 63, 93-102; b) X. Wen,
X. Shi, X. Qiao, Z. Wu, G. Bai, Chem. Commun. 2017, 5372-5375; c) J.
M. Khurana, A. Gogia, Org. Prep. Proced. Int. 1997, 29, 1-32; d) Z. A.
Chobanyan, Russ. J. Appl. Chem. 2004, 77, 2036-2037; e) C. A. Brown,
J. Org. Chem. 1970, 35, 1900-1904; f) C. A. Brown, V. K. Ahuja, J. Chem.
Soc., Chem. Commun. 1973, 553-554.
[17] a) J. A. Widegren, R. G. Finke, J. Mol. Catal. A 2003, 198, 317–341; b)
R. H. Crabtree, Chem. Rev. 2012, 112, 1536–1554.
[18] D. Gärtner, S. Sandl, A. Jacobi von Wangelin, Catal. Sci. Technol. 2020,
10, 3502-3514.
[19] a) A. Jabri, C. B. Mason, Y. Sim, S. Gambarotta, T. J. Burchell, R.
Duchateau, Angew. Chem. Int. Ed. 2008, 47, 9717−9721; b) L. H. Do, J.
A. Labinger, J. E. Bercaw, ACS Catal. 2013, 3, 2582−2585; c) J. Rabeah,
M. Bauer, W. Baumann, A. E. C. McConnell, W. F. Gabrielli, P. B. Webb,
D. Selent, A. Brückner, ACS Catal. 2013, 3, 95-102; d) B. Venderbosch,
J.-P. H. Oudsen, L. A. Wolzak, D. J. Martin, T. J. Korstanje, M. Tromp,
ACS Catal. 2019, 9, 1197−1210.
[5]
a) B. J. Gregori, F. Schwarzhuber, S. Pöllath, J. Zweck, L. Fritsch, R.
Schoch, M. Bauer, A. Jacobi von Wangelin, ChemSusChem 2019, 12,
3864-3870; b) T. N. Gieshoff, A. Welther, M. T. Kessler, M. H. G. Prechtl,
A. Jacobi von Wangelin, Chem. Commun. 2014, 2261-2264; c) S.-i. Taira,
Bull. Chem Soc. Jpn. 1962, 35, 840-844; d) K. Hata, I. Motoyama, K.
Sakai, Org. Prep. Proced. Int. 1972, 4, 179-209; e) G. Wienhöfer, F. A.
Westerhaus, R. V. Jagadeesh, K. Junge, H. Junge, M. Beller, Chem.
Commun. 2012, 4827-4829. Selected examples of E-selective semi-
hydrogenations: f) D. Srimani, Y. Diskin-Posner, Y. Ben-David, D.
Milstein, Angew. Chem. Int. Ed. 2013, 52, 14131-14134; g) K. Radkowski,
B. Sundararaju, A. Fürstner, Angew. Chem. Int. Ed. 2013, 52, 355-340.
a) K. Tokmic, A. R. Fout, J. Am. Chem. Soc. 2016, 138, 13700-13705; b)
K. Li, R. Khan, X. Zhang, Y. Gao, Y. Zhou, H. Tan, J. Chen, B. Fan, Chem.
Commun. 2019, 5663-5666; c) C. Chen, Y. Huang, Z. Zhang, X.-Q. Dong,
X. Zhang, Chem. Commun. 2017, 4612-4615; d) S. Fu, N.-Y. Chen, X.
Liu, Z. Shao, S.-P. Luo, Q. Liu, J. Am. Chem. Soc. 2016, 138, 8588-8594;
e) E. Qi, X. Liu, L.-B. Qu, Q. Liu, Y. Lan, J. Catal. 2018, 362, 25-34.
a) X. Wen, X. Shi, X. Qiao, Z. Wu, G. Bai, Chem. Commun. 2017, 5372-
5375; b) K. Murugesan, C. B. Bheeter, P. R. Linnebank, A. Spannenberg,
J. N. H. Reek, R. V. Jagadeesh, M. Beller, ChemSusChem 2019, 12,
3363-3369; c) H. Konnerth, M. H. G. Prechtl, Chem. Commun. 2016,
9129-9132; d) K. Schütte, A. Doddi, C. Kroll, H. Meyer, C. Wiktor, C.
Gemel, G. van Tendeloo, R. A. Fischer, C. Janiak, Nanoscale 2014, 6,
5532-5544; e) B. Bridier, J. Pérez-Ramírez, A. Knop-Gericke, R. Schlögl,
D. Teschner, Chem. Sci. 2011, 2, 1379-1383; f) N. O. Thiel, B. Kaewmee,
T. T. Ngoc, J. F. Teichert, Chem. Eur. J. 2020, 26, 1597-1603.
[6]
[7]
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10.1002/cctc.202000994
ChemCatChem
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Entry for the Table of Contents
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Stereoselective semi-hydrogenations
of alkynes were developed with a
simple [Cr(acac)3/DiBAL-H] catalyst
mixture. The reactions operate under
mild conditions (30°C, 1-5 bar H2) and
display very good yields and Z/E
ratios (>99/1 for alkyl substituents).
B. J. Gregori, M. Nowakowski, A.
Schoch, S. Pöllath, J. Zweck, M. Bauer,
and A. Jacobi von Wangelin*
Page No. – Page No.
Stereoselective Chromium-Catalyzed
Semi-Hydrogenation of Alkynes
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