Stereoselective Alkyne Hydrogenation by using a Simple Iron Catalyst

Article abstract of DOI:10.1002/cssc.201900926

The stereoselective hydrogenation of alkynes constitutes one of the key approaches for the construction of stereodefined alkenes. The majority of conventional methods utilize noble and toxic metal catalysts. This study concerns a simple catalyst comprised of the commercial chemicals iron(II) acetylacetonate and diisobutylaluminum hydride, which enables the Z-selective semihydrogenation of alkynes under near ambient conditions (1–3 bar H₂, 30 °C, 5 mol % [Fe]). Neither an elaborate catalyst preparation nor addition of ligands is required. Mechanistic studies (kinetic poisoning, X-ray absorption spectroscopy, TEM) strongly indicate the operation of small iron clusters and particle catalysts.

Full text of DOI:10.1002/cssc.201900926

Accepted Article
Title: Stereoselective Alkyne Hydrogenation by a Simple Iron Catalyst
Authors: Axel Jacobi von Wangelin, Bernhard Gregori, Felix
Schwarzhuber, Simon Pöllath, Josef Zweck, Lorena Fritsch,
Roland Schoch, and Matthias Bauer
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10.1002/cssc.201900926
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Stereoselective Alkyne Hydrogenation by a Simple Iron Catalyst
Bernhard J. Gregori,[a] Felix Schwarzhuber,[b] Simon Pöllath,[b] Josef Zweck,[b] Lorena Fritsch,[c] Roland
Schoch,[c] Matthias Bauer,*[c] and Axel Jacobi von Wangelin*[a]
Abstract: The stereoselective hydrogenation of alkynes constitutes
one of the key approaches to the construction of stereo-defined
alkenes. The majority of conventional methods utilize noble and toxic
metal catalysts. Here, we report a simple catalyst comprised of the
commercial chemicals Fe(acac)2 and diisobutylaluminiumhydride
(DIBAL-H) that enables the Z-selective semi-hydrogenation of
alkynes under near ambient conditions (1-3 bar H2, 30 °C, 5 mol%
[Fe]). Neither elaborate catalyst preparation nor addition of ligands is
required. Mechanistic studies (kinetic poisoning, XAS, TEM) strongly
indicate the operation of small iron cluster and particle catalysts.
products.[4] The addition of ethylene-1,2-diamine (en) as ligand
led to improved stereocontrol.[4f] A related cobalt catalyst was
prepared from Co(OAc)2·4 H2O, en, and NaBH4 in an
EtOH/THF/H2O solvent mixture.[5] In the absence of en, full
hydrogenation to the alkane was observed. The development of
operationally facile, inexpensive, and non-toxic procedures based
on iron catalysts is highly desirable as such methods would make
a great contribution toward higher sustainability in chemical
synthesis. A few stereoselective Fe-catalyzed hydrosilylations
have been reported which allow the downstream conversion of
the resultant alkenylsilanes to alkenes in a separate operation
(10 mol% [Fe], 10 mol% PBu3, 110 mol% (EtO)3SiH, 60 °C, 48 h,
Z/E up to 99/1).[6] Stereoselective Fe-catalyzed semi-
hydrogenations of alkynes have been largely neglected. An early
protocol was reported by Taira with the use of the heterogeneous
Urushibara catalyst.[7] However, high catalyst loadings (30 mol%)
and harsh reaction conditions (100 °C, 50 bar H2) were required.
High Z-stereoselectivities were achieved in biphasic semi-
hydrogenations of alkynes by iron nanoparticles in ionic liquids.[8]
This protocol allowed multiple catalyst recyclings with minimal
catalyst loss but required elevated temperature and pressure for
full conversion, most likely due to the high viscosity, low H2
solubility, and strong NP surface coordination of the ionic liquid.[9]
To further develop the field of stereoselective Z-hydrogenation of
alkynes, we believed that an effective yet operationally simple
protocol would fulfill the following criteria: i) the active catalyst is
prepared in situ by the reduction of a simple Fe salt with a
commercial reductant; ii) no complex or expensive ligands or
additives are required; iii) the catalytic hydrogenation is highly
stereoselective under mild conditions in common organic solvents.
Introduction
The precise control over the stereochemistry of alkenes is pivotal
to the synthesis of numerous natural products, pharmaceuticals,
fragrances, and materials.[1] By virtue of its perfect atom economy,
great operational simplicity, and versatility, the semi-
hydrogenation of alkynes is an especially attractive tool for the
stereo-defined construction of alkenes. Importantly, both stereo-
isomers can be accessed from the same starting material by the
choice of the hydrogenation catalyst. The hydrogenative
synthesis of Z-alkenes has attracted great interest due to their
special stereochemistry, higher reactivity, and a shortage of
alternative methods. The partially poisoned and solid-supported
Lindlar-type catalysts (Pd@CaCO3, poisoned with quinoline and
Pb(OAc)2) are the most prominent catalyst class for Z-selective
semi-hydrogenations of alkynes.[2] These reactions typically
operate under very mild conditions, provide very high stereo-
selectivities, and exhibit broad functional group tolerance.
Numerous applications to the synthesis of complex natural and
technical products document the ease of operation, scalability,
versatility, and generality of this method.[3] However, the use of
the noble metal Pd and the presence of toxic additives (e.g.
PbSO4, PbO) constitute severe drawbacks. Alternative methods
have been developed with the more abundant 3d transition metals.
The Nickel-P2 catalyst, prepared from nickel(II) acetate and
NaBH4 in ethanol, exhibits very good stereoselectivities under
mild conditions and was successfully applied to the synthesis of
diverse Z-alkene-containing building blocks and natural
[a]
M.Sc. 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
Scheme 1. Transition metal-catalyzed (Z)-selective hydrogenation of alkynes.
[b]
[c]
F. Schwarzhuber, S. Pöllath, Prof. Dr. J. Zweck
Dept. of Physics, University of Regensburg (Germany)
Results and Discussion
B.Sc. Lorena Fritsch, Dr. Roland Schoch, Prof. Dr. Matthias Bauer
Institute of Inorganic Chemistry and Center for Sustainable Systems
Design (CSSD), University of Paderborn (Germany)
E-mail: matthias.bauer@upb.de
Lead discovery. With these framework conditions, we
investigated various conditions involving ferrous salts, ligands,
reductants, and solvents (Table 1). Our recent investigations into
the development of low-valent Fe catalysts documented the
Supporting information for this article is given via a link at the end of
the document.
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10.1002/cssc.201900926
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especially high activity of FeX2/DIBAL-H (DIBAL-H = diisobutyl-
aluminium hydride) catalyst mixtures in alkene hydrogenations.[10]
Such catalysts proved also to be effective in the hydrogenation of
the model substrate phenylprop-1-yne. FeCl2(THF) and
Fe(hmds)2 (hmds=hexamethyldisilazide, N(SiMe3)2)) formed very
effective complete hydrogenation catalysts that exclusively gave
the alkane (entries 1-3). With lower DIBAL-H concentration,
Fe(hmds)2 was an active catalyst of the [2+2+2]-cyclotrimerization
to the benzene derivatives.[11] At higher DIBAL-H concentrations,
the Fe catalysts rapidly formed black suspensions which
efficiently catalyzed the complete hydrogenation to the alkane
(entries 1, 3, 5, 9, 10). The combination of commercial iron(II)
acetylacetonate (Fe(acac)2) with DIBAL-H (ratio 1/2) in THF
exhibited highest catalytic activity at only 2 bar H2 pressure and
30 °C toward semi-hydrogenation (98% yield of alkenes) with very
high stereoselectivity (Z/E = 18/1, Table 1). It is important to note
that the Z/E ratios remained constant over the course of the
reaction and only slightly eroded at long reaction times due to
Table 1. Selected optimization experiments.[20]
Entry
1
MX2
x mol%
10
Alkenes [%][a]
0[b]
Z/E[a]
Alkane [%][a]
>99
FeCl2(THF)1.5
-
2[b]
3
5
10
<1
<1
-
-
7
>99
Fe(hmds)2
Fe(OAc)2
4
5
5
10
94
24
14/1
1/20
6
76
6
7
8[c]
9
5
28
98
14
<1
<1
14/1
18/1
5/1
-
<1
<1
<1
>99
>99
10
10
15
20
Fe(acac)2
10
-
competing
hydrogenation (over E-alkene).
Z-E-isomerization
and
preferred
Z-alkene
11
12
Ni(acac)2
Co(acac)2
10
10
<1
<1
-
-
>99
>99
13[d]
Cu(acac)2
10
<1
-
0
Substrate scope. The optimized reaction conditions were then
successfully applied to the semi-hydrogenation of diaryl alkynes
even at a near-ambient pressure of only 1.3 bar H2 (Table 2). The
general protocol tolerated various substitution patterns and
exhibited good functional group compatibility, e.g. carbonyls,
halides, esters, amines. Highly electrophilic substituents such as
nitro, cyano, pyridyl, and acidic protons were not tolerated.[1e,5,12]
Mostly, full conversions were observed (entries 1-7); (4-bromo-
phenyl) ethynylbenzene gave high Z-selectivity and minor
formation of dehalogenation products (entry 4).[13] No hydro-
dehalogenation was observed with fluoro and chloro derivatives.
No competing reduction of carbonyl/carboxyl groups was
operating (entries 8 and 9). The nitrophenyl derivative underwent
nitro reduction (equimolar to catalyst concentration) and led to
decomposition of the catalyst (entry 10). (2-Pyridyl)ethynyl-
benzene exclusively afforded the alkane product; the thiophene
substrate exhibited very high stereoselectivity but low conversion
(entries 12, 13).[14] Alkyl-substituted alkynes were also competent
substrates in the Z-selective semi-hydrogenation (Table 3).
Terminal alkynes underwent rapid [2+2+2]-cyclotrimerization to
the benzene derivatives rather than hydrogenation (entry 1, 99%).
The NH2 group of 1-(2-aminophenyl)-1-hexyne was fully tolerated
(entry 8). Generally, reactions with 1-alkyl alkynes displayed
significantly higher stereocontrol than 1-aryl alkynes. Highest
yields and stereoselectivities were achieved with internal alkyl-
substituted alkynes. The 1-trimethylsilyl group gave moderate
yields and Z/E ratios (Scheme 2, top) while the bulky triisopropyl-
silyl (TIPS) substituent inhibited conversion. The general
conditions were applied to the synthesis of two natural products.
Semi-hydrogenation of 5-decynyl-1-acetate and 9-tricosyne
resulted in complete conversions to the Z-alkenes in very high
yields and perfect stereoselectivities, respectively (Scheme 2,
bottom). The 1-acetate product is the natural insect pheromone of
the turnip moth Agrotis segetum;[15] 9-Z-tricosene is the insect
pheromone of the common housefly Musca domestica.[16]
Conditions: 0.2 mmol alkyne, 0.4 mL THF. [a] Determined by 1H-NMR and
GC-FID. [b] Cyclotrimerization as major reaction. [c] Sequential addition of
Fe(acac)2 and DIBAL-H to alkyne/THF solution. [d] 5 h.
Table 2. Semi-hydrogenation of diaryl alkynes.
Entry
Alkyne
R
Alkenes [%][a]
Z/E[a]
1[b]
2[b]
3
4[c]
5
6
7[d]
8
H
2-F
4-F
95
98
93
70
98
88
84
96
66
0
9/1
17/1
4.5/1
24/1
9/1
5/1
10/1
9/1
24/1
-
>99/1
4-Br
3-OMe
4-OMe
4-OCF3
4-CO2Me
4-COMe
4-NO2
4-NHCOMe
9[e]
10[b]
11[c]
10
12[f]
13[c]
-
-
0[d]
12
-
>99/1
Conditions: 0.2 mmol alkyne, 0.4 mL THF. [a] Determined by 1H-NMR and
GC-FID. [b] 3 bar H2, 16.5 h. [c] 5 bar H2, 24 h. [d] 3 bar H2, 6 h. [e] 5 bar H2,
48 h. [f] alkane formed.
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Kinetic studies. The reaction order in catalyst [Fe] was
determined to be ~1.4 at low catalyst concentrations (5-25 mM)
and decreased to ~0.7 at higher concentrations (25-50 mM). The
reducing conditions in the presence of the co-catalyst DIBAL-H
and the stoichiometric reductant dihydrogen H2 suggest the
involvement of low-valent catalyst species which are very likely to
aggregate to larger particles in the absence of stabilizing ligands.
The unambiguous distinction between homotopic and heterotopic
catalysis mechanisms is very intricate.[17] We performed kinetic
poisoning experiments that are consistent with a heterogeneous
mechanism.[16a] Hg poisoning (addition of 200 equiv. Hg per [Fe]
2 h prior to start of hydrogenation reaction) showed a slight loss
of catalyst activity (Figure 1).[18] The heterogeneity of the catalyst
was further indicated by experiments in the presence of trimethyl-
phosphite, P(OMe)3.[17] Complete catalyst inhibition of a standard
reaction with 5 mol% [Fe] loading was achieved with only 1 mol%
P(OMe)3 (addition after 30 min reaction), which corresponds to a
particle size of approximately 10 nm. At higher P(OMe)3
concentrations, the reestablishment of moderate catalyst activity
was observed, most likely as a consequence of the leaching of
catalyst species into the bulk solvent phase. A similar effect was
recorded when dibenzo[a,e]cyclooctatetraene (dct) was used, a
selective poison of low-valent homogeneous metal species.[16b,c]
Upon addition of 30 mol% dct to the reaction at 30 min, the
catalyst was deactivated to about 2/3 of its original efficiency. We
deduce from these observations that heterotopic or
nanoparticulate catalyst species are operative, possibly in
equilibrium with homotopic soluble catalyst species.
Table 3. Semi-hydrogenation of alkynes.
Entry
Alkyne
R
Alkenes
[%][a]
Z/E[a]
1
H
Me
Et
c-hexyl
c-propyl
0[b]
98
87
95
95
-
2[c]
3
18/1
16/1
22/1
13/1
4[d]
5[e]
6[d]
7[d]
8
H
94
84
58
36
9/1
Me
NH2
Cl
45/1
19/1
99/1
9
10[f]
n-C5H11
98
>99/1
11
Cl
OH
OMe
OTMS
95
0 (>99)[g]
91
>99/1
- (>99/1)[g]
>99/1
-
12[f]
13
14[h]
13
15[h]
16[h]
17
CN
0
-
99
20
>99/1
>99/1
Conditions: 0.2 mmol alkyne, 0.4 mL THF. [a] Determined by 1H-NMR and
GC-FID/MS. [b] full conversion to [2+2+2]-cyclotrimers. [c] 2 bar H2, 3 h. [d]
1.3 bar H2, 5.5 h. [e] 3 bar H2, 5.5 h. [f] 1.3 bar H2, 12 h. [g] in parentheses
reaction of equimolar mixture of alkyne and n-BuLi at 3 bar H2, 16 h. [h] 3
bar H2, 24 h.
Figure 1. Catalyst poisoning experiments with 200 equiv. Hg per [Fe] (addition
during catalyst formation and 2 h prior to hydrogenation), 6 equiv. DCT per [Fe]
(addition after 30 min of hydrogenation), and 0.02-0.2 equiv. P(OMe)3 per [Fe]
(addition after 30 min of hydrogenation), respectively.
Scheme 2. Top: Semi-hydrogenation of functionalized alkynes. Bottom:
Synthesis of two insect pheromones. Yields and isomer ratios were determined
by 1H-NMR and GC-FID.
Catalyst characterizations. As an element-specific and short-
range sensitive method, X-ray absorption spectroscopy (XAS) is
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a powerful tool to provide information on the oxidation state and
coordination environment of the absorbing atoms in solution and
is therefore highly instructive toward the investigation of catalyst
species.[19] We performed XANES- (X-ray absorption near-edge
structure) and EXAFS- (extended X-ray absorption fine structure)
analysis of the catalyst combinations: Fe(hmds)2/DIBAL-H and
Fe(acac)2/DIBAL-H.[20] The former catalyst contains a bulky,
redox-stable amido ligand that is easily displaced by a formal
hydride delivered from DIBAL-H. The resultant catalyst exhibited
very high activity in the complete hydrogenation of alkynes to the
alkanes and the [2+2+2]-cyclotrimerization.[10,11] The latter
catalyst formed from Fe(acac)2 and DIBAL-H enabled highly
selective semi-hydrogenation to Z-alkenes. However, the
presence of reduction-sensitive carbonyl moieties within the
acetylacetonate ligand can alter the mode of action of the co-
catalyst DIBAL-H (vide infra). The XANES spectra unambiguously
show the reduction of both Fe(acac)2 and Fe(hmds)2 upon
addition of DIBAL-H (Figure 2).[20] Although comparably small
changes of the edge energies of max. 1.7 eV were observed for
Fe(acac)2, the reduction of the whiteline intensity accompanied by
an increased prepeak intensity and broadening at 7113 eV
indicate an oxidation state of FeI (or lower) in the final reduction
state (after addition of 4 equiv. DIBAL-H) and the formation of
agglomerates in agreement with previous studies.[19] For
Fe(acac)2, the pronounced whiteline is only significantly reduced
after addition of 3 equiv. DIBAL-H, which corresponds to a higher
concentration of the reductant than in the catalytically active
species formed from Fe(acac)2 and 2 equiv. DIBAL-H. In contrast,
the Fe(hmds)2 spectrum, which showed significant differences to
unreduced Fe(acac)2 possibly due to the presence of pronounced
-resonance in the Fe-N bonds, changed already with 0.5 equiv.
DIBAL-H.[20] Interestingly, the XANES spectra of [Fe(hmds)2 + 2
equiv. DIBAL-H] and [Fe(acac)2 + 4 equiv. DIBAL-H] showed very
similar spectral shapes.[20] This is in full accord with their identical
catalytic performance as (complete) hydrogenation catalysts that
exclusively deliver the alkane product (see Table 1, entries 3 and
9). We therefore deduce that both catalyst species are closely
related in their structural and electronic properties and that the
two acac ligands “quench” 2 equiv. DIBAL-H to give alcoholates.
The EXAFS data showed that both pre-catalysts Fe(hmds)2 and
Fe(acac)2 form dimers upon addition of DIBAL-H and with
increasing equivalents of the reductant even larger aggregates.
The maximum Fe-Fe-coordination number was 3 at a Fe-Fe
distance of 2.5 Å, corresponding to at least tetra-nuclear
agglomerates,[20] which formed from Fe(acac)2 with 3 equiv.
DIBAL-H and from Fe(hmds)2 already with 1 equiv. DIBAL-H.
Comparison of the EXAFS data in Figure 3 documents the much
less effective Fe-centered reduction of Fe(acac)2 by DIBAL-H in
agreement with the XANES analyses. Further results of our
EXAFS study are: The absorption fine structure of the highly
selective semi-hydrogenation catalyst [Fe(acac)2 + 2 equiv.
DIBAL-H] is significantly different from the bulk Fe0 foil reference.
The presence of the Fe-O contribution resulting from the [acac]-
(or derivatives thereof) was only observed with up to 2 equiv.
DIBAL-H. With higher DIBAL-H concentration, an abrupt change
to shorter Fe-O distances and lower Fe-O coordination numbers
(from 4-6 to 1-2) was recorded, which is in agreement with the
XANES results. We believe that coordination of a chemically
different O-donor ligand is responsible for the new Fe-O
contribution (e.g. the solvent THF or alcoholate derivatives
formed from hydride addition to acac). In both catalyst series, the
addition of further DIBAL-H did not significantly alter the nature of
the catalyst species. The requirement of higher DIBAL-H
stoichiometry for the generation of an equally active (complete)
hydrogenation catalyst from Fe(acac)2 (>3 equiv. DIBAL-H)
supports the notion that there is concomitant ligand reduction
operative.[20]
Figure 3. EXAFS spectra in R-space of Fe(hmds)2 with addition of DIBAL-H.
Fe(acac)2
Fe-foil
+ 1.0 equiv. DIBAL-H
+ 2.0 equiv. DIBAL-H
+ 3.0 equiv. DIBAL-H
7110
7120
7130
7140
energy [eV]
Figure 2. XANES spectra of Fe(acac)2 and after addition of DIBAL-H.
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Figure 5. TEM measurements. a: Nanoparticles embedded in residual matter
from solvent, acac, DIBAL-H. b: Size distribution of 300 particles. c: Hi-res TEM
of separated particles showing atom fringes; inset: corresponding diffractogram.
d: Line scan along the atom fringes shown in c (along green arrow).
Reaction mechanism. We have collected several synthetic,
kinetic, and spectroscopic data that characterize the operating
catalysts species under reaction conditions. The optimal stoichio-
metries of the FeX2 pre-catalysts and the reductant DIBAL-H that
form an active and selective semi-hydrogenation catalyst appear
to exclude Fe(0) particle catalysts (Table 1). With 2 equiv. DIBAL-
H (which could effect FeX2→Fe(0) reduction), the ferrous pre-
catalysts Fe(hmds)2, FeCl2, Fe(OAc)2 catalyze the complete
hydrogenation to the alkane instead. Contrary to these
observations, an active semi-hydrogenation catalyst is formed
from Fe(acac)2/DIBAL-H in a 1/2 ratio, which is a direct
consequence of the hydride buffering effect of acac. Consistent
with this hypothesis is the detection of reduced side products
derived from hydride addition to acac. The addition of aceto-
phenone exerts a similar buffering effect (Scheme 3A). Related
reduction products from the reaction of tri(benzoylacetonato)
iron(III) (Fe(bzac)3) and DIBAL-H were isolated in ~100% yield
(Scheme 3B). XANES and EXAFS spectra are in full support of
the initial hydride transfer to the ligand rather than the Fe centers.
However, we cannot exclude the secondary transfer of a formal
hydride anion through β-hydride elimination of acac-derived
alcoholates at Fe. XANES and EXAFS data further suggest that
the catalytic species is most likely not as far reduced as an Fe(0)
species but involves smaller metal nanoclusters bearing Fe-O
coordination motifs (acac or reduction products thereof). The
kinetic poisoning studies suggest an equilibrium between
homogeneous and heterogeneous catalysts. A similar result can
be deduced from the reaction order in catalyst which ranges from
~1.4 (1-5 mol% Fe) to ~ 0.7 (5-10 mol% Fe).[20] Under the ex-situ
conditions of TEM measurements, Fe particles have been
detected. The combined data from visual inspections of the
hydrogenation reactions, kinetic studies, poisoning experiments,
XANES, EXAFS, and TEM measurements led to a mechanistic
proposal that involves initial hydride addition to the acac-ligands
in Fe(acac)2 and subsequent formation of Fe cluster and particle
catalysts (Scheme 3C).
Figure 4. EXAFS spectra in R-space of Fe(acac)2 with addition of DIBAL-H.
We further consulted transmission electron microscopy (TEM)
measurements to prove the nature, size, and shape of the
potentially heterogeneous catalysts. In accordance with the
kinetic poisoning experiments, the TEM spectra documented the
presence of Fe particles from the reduction of Fe(acac)2 with
2 equiv. DIBAL-H and after vacuum treatment (Figure 5). A TEM
image with a 0.3 x 1.2 µm field of view showed many separate
particles embedded in organic remnants is displayed in Figure 4a.
The distribution of the particle size gave an average particle
diameter of 6.9 nm with a standard deviation of 2.6 nm (Figure
5b).[20] A high resolution TEM image proved the particles’
crystallinity (Figure 5c, see diffractogram in inset). A line scan
along the green arrow with an integration width of 3.6 nm is
displayed in Figure 5d. The lattice constant, averaged over 10
fringes, was 0.27 ± 0.02 nm which is in full agreement with the
presence of elemental Fe0.[21]
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Experimental Section
Catalyst preparation: Under an atmosphere of argon (Schlenk technique
or glove box), an oven-dried vial was charged with a stir bar and Fe(acac)2
(6.4 mg, 25 µmol), and dry THF (0.95 mL) was added. The mixture was
stirred for 10 min at r.t. and DIBAL-H was added dropwise (50 µL, 1.0 M in
toluene, 50 µmol, 2.0 equiv.). The resultant black mixture was stirred for
20 min before use.
Catalytic semi-hydrogenations: Under an atmosphere of argon (Schlenk
technique or glove box), an oven-dried 4 mL vial with a stir bar was
charged with the alkyne (0.2 mmol) and n-pentadecane as internal GC
reference (0.2 mmol, 42.5 mg). The catalyst suspension (see above, 0.4
mL, 0.01 mmol [Fe], 5 mol%) was added and the vial was closed with a
septum. The septum was punctured with a short needle and the vial
transferred to a pressure-resistant reactor equipped with a gas inlet. The
reactor was purged with H2 for 1 min while stirring the reaction vial. After
three short cycles of (de)pressurizing (2.0 bar, 5 sec, 1.3 bar), the H2
pressure and temperature were set (1.3 bar H2, 30 °C). After 5-16 h, the
reactor was cooled and the pressure released. The vials were retrieved,
and saturated aqueous NaHCO3 (1.5 mL) and ethyl acetate (1.5 mL) were
added. The resulting suspension was stirred for 20 min, after which the
organic phase was separated and filtered through a short SiO2 plug. Yields
and selectivities were determined from pure samples obtained by flash
column chromatography (SiO2, n-pentane, ethyl acetate) or quantitative
GC and NMR analyses vs. internal references.
Acknowledgements
This work was supported by the Priority Program “Materials
synthesis at room temperature” of the Deutsche Forschungs-
gemeinschaft (DFG, SPP 1708). We thank Michaela Lutz and
Veronica Scheidler (U Regensburg), Wolfgang Caliebe, Edmund
Welter, and the beam line staff of P64 and P65 at PETRA III
(DESY, Hamburg) for technical support.
Conflict of Interest
Scheme 3. A: Carbonyl additive effect; B: Hydride addition products of
Fe(bzac)3; C: Postulated mechanism involving acac-reduction by DIBAL-H.
The authors declare no conflict of interest.
Keywords: iron • hydrogenation • alkynes • stereoselectivity •
nanoparticles • TEM • X-ray absorption
Conclusions
An operationally facile and highly efficient iron-catalyzed semi-
hydrogenation of alkynes was developed. The simple,
inexpensive catalyst combination Fe(acac)2 / DIBAL-H enabled
the synthesis of various Z-alkenes in high yields and with high
stereocontrol under very mild conditions (1-3 bar H2, 30 °C).
Mechanistic studies were indicative of smaller metal nanoclusters
and nanoparticles as active catalyst species. For the first time, X-
ray absorption spectroscopy was used to study the reductive
formation of the catalyst. The superior activity and selectivity of
catalyst species formed via ligand-centered hydride reduction of
acac rather than the direct hydride coordination to the metal
center is a hitherto unknown effect in such hydrogenation
reactions and worthy of further investigations. The use of
commercial catalysts and reagents and the absence of any
expensive, toxic, or complex ligand makes this catalytic semi-
hydrogenation attractive for lab-scale and larger scale operations.
[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) E. Breitmeier, Terpene. Aromen, Düfte, Pharmaka,
Pheromone, Wiley-VCH, Weinheim, 2005; e) D. Srimani, Y. Diskin-
Posner, Y. Ben-David, D. Milstein, Angew. Chem. Int. Ed. 2013, 52,
14131-14134; f) W.-Y. Siau, Y. Zhang, Y. Zhao, Top. Curr. Chem. 2012,
327, 33-58; g) R. H. Grubbs, T. M. Trnka, In: Ruthenium in Organic
Synthesis (ed.: S.-I. Murahashi), Wiley-VCH, Weinheim, 2004; h) A.
Welther, A. Jacobi von Wangelin, Curr. Org. Chem. 2013, 17, 326-335.
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.
[2]
[3]
[4]
E. N. Marvell, T. Li, Synthesis, 1973, 8, 457-468.
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, 53, 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,
This article is protected by copyright. All rights reserved.

10.1002/cssc.201900926
ChemSusChem
FULL PAPER
J. Org. Chem. 1970, 35, 1900-1904; f) C. A. Brown, V. K. Ahuja, J. Chem.
[14] C. Chen, Y. Huang, Z. Zhang, X.-Q. Dong, X. Zhang, Chem. Commun.
2017, 53, 4612-4615.
Soc., Chem. Commun. 1973, 553-554.
[5]
[6]
C. Chen, Y. Huang, Z. Zhang, X.-Q. Dong, X. Zhang, Chem. Commun.
2017, 53, 4612-4615.
[15] Z. A. Chobanyan, Russ. J. Appl. Chem. 2004, 77, 2036-2037.
[16] A. M. Moiseenkov, B. Schaub, C. Margot, M. Schlosser, Tetrahedron Lett.
1985, 26, 305-306.
a) S. Enthaler, M. Haberberger, E. Irran, Chem. Asian J. 2011, 6, 1613-
1623; b) S. C. Bart, E. Lobkovsky, P. J. Chirik, J. Am. Chem. Soc. 2004,
126, 13794-13807; c) C. Belger, B. Plietker, Chem. Commun. 2012, 48,
5419-5421; d) M. D. Greenhalgh, A. S. Jones, S. P. Thomas,
ChemCatChem. 2015, 7, 190-222; e) J. Chen, J. Guo, Z. Lu, Chin. J.
Chem. 2018, 36, 1075-1109; M. D. Greenhalgh, D. J. Frank, S. P.
Thomas, Adv. Synth. Catal. 2014, 356, 584-590.
[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; c) D. Gärtner, A. L.
Stein, S. Grupe, J. Arp, A. Jacobi von Wangelin, Angew. Chem. Int. Ed.
2015, 54, 10545-10549.
[18] V. Artero, M. Fontecave, Chem. Soc. Rev. 2013, 42, 2338.
[19] a) A. Welther, M. Bauer, M. Mayer, A. Jacobi von Wangelin,
ChemCatChem. 2012, 4, 1088-1093; b) M. Bauer, T. Kauf, J. Christoffers,
H. Bertagnolli, Phys. Chem. Chem. Phys. 2005, 7, 2664–2670; c) R. B.
Bedford, Acc. Chem. Res. 2015, 48, 1485–1493; d) C. Cassani, G.
Bergonzini, C.-J. Wallentin, ACS Catal. 2016, 1640–1648; e) R. Schoch,
W. Desens, T. Werner, M. Bauer, Chem. Eur. J. 2013, 19, 15816–15821;
f) H. Takaya, S. Nakajima, N. Nakagawa, K. Isozaki, T. Iwamoto, R.
Imayoshi, N. J. Gower, L. Adak, T. Hatakeyama, T. Honma, M. Takagaki,
Y. Sunada, H. Nagashima, D. Hashizume, O. Takahashi, M. Nakamura,
Bull. Chem. Soc. Jpn. 2015, 88, 410-418; g) A. M. Messinis, S. L. J.
Luckham, P. P. Wells, D. Gianolio, E. K. Gibson, H. M. O´Brien, H. A.
Sparkes, S. A. Davis, J. Callison, D. Elorriaga, O. Hernandez-Fajardo, R.
B. Bedford, Nat. Catal. 2019, 2, 123-133.
[7]
[8]
[9]
a) S.-i. Taira, Bull. Chem Soc. Jpn. 1962, 35, 840-844; b) K. Hata, I.
Motoyama, K. Sakai, Org. Prep. Proced. Int. 1972, 4, 179-209.
T. N. Gieshoff, A. Welther, M. T. Kessler, M. H. G. Prechtl, A. Jacobi von
Wangelin, Chem. Commun. 2014, 50, 2261-2264.
a) C. Vollmer, C. Janiak, Coord. Chem. Rev. 2011, 255, 2039–205; b) L.
S. Ott, R. G. Finke, Coord. Chem. Rev. 2007, 251, 1075-1100; c) M.
Zahmakiran, S. Özkar, Nanoscale 2011, 3, 3462-3481.
[10] T. N. Gieshoff, U. Chakraborty, M. Villa, A. Jacobi von Wangelin, Angew.
Chem. Int. Ed. 2017, 56, 3585-3589.
[11] D. Brenna, M. Villa, T. N. Gieshoff, F. Fischer, M. Hapke, A. Jacobi von
Wangelin, Angew. Chem. Int. Ed. 2017, 56, 3585-3589.
[12] For selected recent reports of stereoselective Z-hydrogenations of
alkynes that exhibited good functional group tolerance, see for example:
a) A. K. Ghosh, Y. Wang, J. T. Kim, J. Org. Chem. 2001, 66, 8973-8982;
b) A. Gollner, K.-H. Altmann, J. Gertsch, J. Mulzer, Chem. Eur. J. 2009,
15, 5979-5997; c) V. Hickmann, M. Alcarazo, A. Fürstner, J. Am. Chem.
Soc. 2010, 132, 11042-11044; d) K. Semba, R. Kameyama, Y. Nakao,
Synlett 2015, 26, 318-322; e) F. Pape, N. O. Thiel, J. F. Teichert, Chem.
Eur. J. 2015, 21, 15934-15938; g) C. F. Heinrich, E. Widemann, J. Sanz,
R. Lugan, T. Heitz, F. Pinot, M. Miesch, L. Miesch, Eur. J. Org. Chem.
2015, 1130-1136; h) K. Tokmic, A. R. Fout, J. Am. Chem. Soc. 2016, 138,
13700-13705.!
[20] For details, see Electronic Supporting Information (ESI).
[21] R. W. G. Wyckoff, Crystal Structures 1963, 1, 7. b) Lattice parameter of
Fe-Al-alloys: 0.286–0.293 nm, A. N. Sinha, Trans. J. I. M. 1984, 25, 900-
901; c) Interplanar spacing of Fe3O4: 0.251 nm, H. E. Ghandoor, H. M.
Zidan, M. M. H. Khalil, M. I. M. Ismail, Int. J. Electrochem. Sci. 2012, 7,
5734-5745; d) Interplanar spacing of α-Fe2O3: 0.171 nm (116 plane),
Interplanar spacing of γ-Fe2O3: 0.211 nm (311 plane), 0.253 nm (220
plane), R. A. Bepari, P. Bharali, B. K. Das, J. Saudi. Chem. Soc. 2017,
21, 170-178.
[13] W. M. Czaplik, S. Grupe, M. Meyer, A. Jacobi von Wangelin, Chem.
Commun. 2010, 46, 6350-6352.
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Entry for the Table of Contents (Please choose one layout)
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Here, we report a simple
catalyst comprised of
Bernhard J. Gregori, Felix
Schwarzhuber, Simon Pöllath,
Josef Zweck, Lorena Fritsch,
Roland Schoch, Matthias
Bauer, and Axel Jacobi von
Wangelin*
commercial Fe(acac)2 and
diisobutylaluminium hydride
that enables the Z-selective
semi-hydrogenation of alkynes
in high yields and with high
stereo-control. Neither
Page No. – Page No.
elaborate catalyst preparation
nor addition of ligands is
required. Mechanistic studies
indicate the operation of small
Fe cluster catalysts.
Stereoselective Alkyne
Hydrogenation by A Simple
Iron Catalyst
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