Stereoselective iron-catalyzed alkyne hydrogenation in ionic liquids

Article abstract of DOI:10.1039/c3cc49679a

Iron(0) nanoparticles in ionic liquids (ILs) have been shown to catalyse the semi-hydrogenation of alkynes. In the presence of a nitrile-functionalised IL or acetonitrile, stereoselective formation of (Z)-alkenes was observed. The biphasic solvent system allowed facile separation and re-use of the catalyst. The Royal Society of Chemistry 2014.

Full text of DOI:10.1039/c3cc49679a

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Stereoselective iron-catalyzed alkyne hydrogenation
in ionic liquids†
Cite this: Chem. Commun., 2014,
50, 2261
Tim N. Gieshoff,a Alice Welther,a Michael T. Kessler,b Martin H. G. Prechtl*b and
Axel Jacobi von Wangelin*a
Received 20th December 2013,
Accepted 2nd January 2014
DOI: 10.1039/c3cc49679a
www.rsc.org/chemcomm
Iron(0) nanoparticles in ionic liquids (ILs) have been shown to
catalyse the semi-hydrogenation of alkynes. In the presence of a
nitrile-functionalised IL or acetonitrile, stereoselective formation of
(Z)-alkenes was observed. The biphasic solvent system allowed facile
separation and re-use of the catalyst.
Iron-catalysed hydrogenations are among the largest technical
processes (Haber–Bosch, gas-to-liquid)1 but are under-utilised
on the smaller scales of fine chemical, agrochemical, and
Scheme 1 Modular ion pair/ligand/iron catalysts for hydrogenations.
pharmaceutical manufacture and within academic synthesis controls the catalyst selectivity through coordination and an
programs. However, the current economic and environmental ionic liquid to allow catalyst separation. The potential of such
constraints have prompted reconsiderations of iron-catalysed modular systems was evaluated in stereoselective semi-
procedures.2 Hydrogenations of alkenes and alkynes with well- hydrogenations of alkynes (Scheme 1).
defined ligand-stabilised iron catalysts or heterogeneous species
Ionic liquids (ILs) based on azolium salts seemed to be perfectly
have been recently reported.3 Hydrogenations of alkynes in general suited for this task due to their ability to dissolve, stabilise, and
bear the dual challenge of product- and stereo-selectivity. modulate metal nanoparticles.8 Their physicochemical properties
Lindlar-type catalysts exhibit especially high versatility and are are widely adjustable by variation of substituents, the heterocycle,
the benchmark for semi-hydrogenations of (Z)-alkenes.4 This and counterions. Most importantly, azolium-ILs are immiscible
combination of an expensive noble metal catalyst (Pd/CaCO3) with non-polar solvents and have a negligible vapour pressure.
and toxic additives (Pb(OAc)2, quinoline) is clearly derogatory to the IL-stabilised precious metal nanoparticle catalysts were successfully
development of sustainable chemical processes, so an inexpensive applied to hydrogenations whereas the generation and utilization
nontoxic iron-catalysed alternative is highly desirable.5 However, of IL-embedded iron catalysts are as yet under-utilised.9
the search for new catalysts for technical applications is incomplete
We set out to study the catalytic activity of heterogeneous iron
without the implementation of efficient catalyst separation and species in low oxidation states generated by reduction of FeCl3 with
recycling technologies.6 Despite the higher selectivity of homo- ethylmagnesium chloride (Table 1).3c,e Hydrogenation of diphenyl-
geneous catalysts, most technical processes use heterogeneous acetylene (1) in the absence of a suitable ligand showed low selectivity
catalysts because of their ease of separation from the products. toward stilbene (2). The use of imidazolium ILs generally required
Within the scope of our iron catalysis program, we thus aimed elevated pressure and temperature. Geared by literature precedents
at merging the benefits of a separable heterogeneous catalyst with Pd, Ru, and Au nanoparticles, we employed nitrile-functionalised
with that of a highly dispersive ligand-modified catalyst. ILs as catalyst modifiers.9,10 Notably, the presence of a nitrile function,
We envisioned to capitalise on the hybrid concept of nano- the suppression of N-heterocyclic carbene formation,11 and the
particular iron catalysts7 in the presence of a ligand which absence of THF were crucial to a selective cis-hydrogenation in
IL-3.5 The replacement of THF with heptane also resulted in high
phase-partitioning at room temperature and allowed facile separa-
tion of the catalyst. The higher reaction temperature is believed to
enhance dispersion of the viscous IL with the non-polar phase.
Application of the optimised conditions in a biphasic heptane–
IL-3 mixture to other phenylacetylenes resulted in moderate to
a Institute of Organic Chemistry, University of Regensburg, Germany.
E-mail: axel.jacobi@ur.de; Fax: +49 (0)941 943 4617
b Dept. of Chemistry, University of Cologne, 50939 Cologne, Germany
† Electronic supplementary information (ESI) available. See DOI: 10.1039/
c3cc49679a
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Table 1 Optimisation of reaction conditionsa
Fig. 1 Particle sizes and TEM image of Fe@IL catalysts.
Entry
Conditionsb
Solvent(s)
1/2/3c [%]
Z/E (2)c
1
2
3
4
5
6
7
8
4 bar H2, 45 1C, 8 h
1 bar H2, 18 1C, 8 h
50 bar H2, 50 1C, 16 h
4 bar H2, 18 1C, 16 h
50 bar H2, 50 1C, 16 h
50 bar H2, 50 1C, 60 h
60 bar H2, 80 8C, 44 h
30 bar H2, 80 1C, 20 h
THF
THF
0/2/93
1/73/21
2/3/93
94/2/2
82/15/2
23/75/1
1/94/3
0/1/98
n.d.
3/1
n.d.
n.d.
6/1
7/1
19/1
n.d.
THF–IL-1
THF–IL-2
THF–IL-2
THF–IL-3
hept–IL-3d
hept–IL-1d
Scheme 2 Formation of a dimer of IL-3 by treatment with EtMgCl.
a
b
[Fe]: 5 mol% FeCl3, THF, 20 mol% EtMgCl, THF, r.t., 30 min. For
experimental details, see ESI. Determined by 1H-NMR and GC-FID.
c
d
Removal of THF prior to reaction.
Table 2 Biphasic semi-hydrogenation of alkynes in heptane–IL-3a
Scheme 3 Intramolecular vs. intermolecular mode of bifunctionality.
Entry
Alkyne
R
Yield [%]
Z/E
shown in Scheme 2.13 If a nucleophile-assisted manipulation of
the nitrile is relevant to the control of selectivity, simple alkyl-
nitriles should exert a similar effect. We therefore examined
whether the intramolecular bifunctional motif of IL-3 could also
be expressed by a much simpler intermolecular combination of
ionic liquid and nitrile functions. Furthermore, the potential
occurrence of acetyl or imine moieties as degradation products
of the nitrile prompted us to also investigate the activity of ternary
catalyst systems comprising a pre-formed reduced iron species
(5 mol%), the non-functionalised ionic liquid IL-1, and various
carbonyl derivatives under the optimised conditions (Scheme 3
and Table 3). Without additives, the hydrogenation of 1 in
heptane–IL-1 afforded dibenzyl (3, >95% yield, Table 3, entry 1).
1
2
3
4
5
H
94
86
92
75
52
95/5
95/5
93/7
100/0
96/4
4-tBu
4-OMe
4-F
3-OMe
6
68
80
89/11
88/12
7
a
For experimental details, see ESI.
good yields of the corresponding alkenes and generally high
stereoselectivities toward the (Z)-isomers (Table 2).
Our observation that ILs can effectively stabilise nano-
particles is in full accord with literature reports.9,10 Droplets
of Fe-NPs in IL-1 and IL-3 were measured by transmission
electron microscopy (TEM), respectively (Fig. 1). Both species
are approximately 4–5 nm in diameter and slowly grow during
the catalytic reactions (to B8–20 nm after 24 h under hydro-
genation reaction conditions).
From a mechanistic point of view, the presence of two
electrophilic functions (imidazolium cations and nitrile
groups) and a strong base/nucleophile (EtMgCl) poses the
question of the actual nature of the ligand moiety that is
present under the reaction conditions. We treated IL-1 with
1 equiv. EtMgCl at room temperature for 30 min followed by
quenching with deuterium oxide (D2O) which afforded a
mixture of ring-deuterated (35%) and 2-deuteromethyl products
(10%).12 IL-3 did not undergo deuteration under identical
conditions (2H-NMR, MS). Instead, a dimer was formed which
was detected by GC-MS and ESI-MS and tentatively assigned as
Table 3 Hydrogenation of 1 with ternary Fe/IL-1/additive catalystsa
Additive
—
Entry
Conversion [%] 2 [%] Z/E (2)
1
2
100
96
99
3
93
85
95
95
95
38
0
67/33
96/4
96/4
97/3
94/6
96/4
95/5
—
3b,c
4b
5b
6b
7b
8b
9b
10b
11
MeCN + PhCO2Me
MeCN + PhCl
MeCN + Ph2CQCH2
MeCN + PhI
99
100
100
43
MeCN + PhNO2
5
R, R0 = Me
25
100
92
22
84
86
91/9
94/6
95/5
R, R0 = Ph
R = Me, R0 = OEt
a
b
c
For experimental details, see ESI. 100 mol% additive. 20 bar H2.
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Table 4 Biphasic semi-hydrogenations in MeCN–heptane–IL-1a
Entry
Alkyne
R
Yieldb [%]
Z/E
1
2
3
4
5
6
7
H
97
94
98
76
84
89
83
96/4
97/3
100/0
100/0
100/0
100/0
99/1
4-tBu
4-OMe
4-NH2
4-Br
4-Cl
4-F
Fig. 2 Consecutive hydrogenations of 1 with the identical catalyst phase
after liquid–liquid decantation.
catalytic activity were realised with the ternary catalyst Fe/IL-1/
MeCN in hydrogenations of 1 under standard conditions (Fig. 2).
ICP-OES analysis of the product phase showed o0.03% leaching
of iron (= 0.0015 mol%). Replacement of IL-1 with tetraalkyl-
ammonium bromides (n-butyl, n-decyl) cleanly gave the alkanes,
and product extraction with heptane failed. The heterogeneity of the
catalyst species in IL-1/MeCN/heptane was further documented by
the absence of inhibition upon addition of dibenzo[a,e]cycloocta-
tetraene (dct).15
In summary, we have developed a simple ternary iron catalyst
that enables the (Z)-selective semi-hydrogenation of alkynes in a
biphasic solvent mixture and the separation/reuse of the catalyst.
The nanoparticle catalysts (B5 nm) formed upon reduction of FeCl3
with EtMgCl. Acetonitrile effects stereocontrol; 1-butyl-2,3-dimethyl-
1,3-imidazolium triflimide (IL-1) allows catalyst separation from the
products and prevents particle aggregation.
8
9
10
4-CO2Me
2-Cl
2-F
53 (74)
79
82
100/0
100/0
100/0
11
12
76
90
99/1
100/0
13
14
15
Et
TMS
CO2Me
79
13 (19)
19 (40)
95/5
96/4
92/8
16
90
100/0
17
38 (70)
100/0
a
b
See ESI. Conversion in parentheses if not >90%.
This work was financed by the Ministry of Innovation,
Science, Research (NRW-returnee fellowship to M.H.G.P.), the
Evonik Foundation (fellowships to M.T.K., T.N.G.), the Robert-
To our delight, identical productivity and stereoselectivity to
those with the bifunctional IL-3 were observed when adding
acetonitrile (entry 2), even at 20 bar H2 (entry 3). The loading of
acetonitrile could be varied from 50 to 200 mol% without any
change in selectivity. Further addition of 100 mol% methyl
benzoate, chlorobenzene14 or 1,1-diphenylethylene, respectively,
resulted in no change in activity (entries 4–6). Iodobenzene slowed
down conversion while nitrobenzene acted as an inhibitor. Benzo-
phenone and ethyl acetate showed only slightly lower activity and
selectivity as MeCN (entries 10 and 11). Nanoparticles prepared
from EtMgCl and EtMgBr afforded identical catalytic results.
The employment of a ternary Fe/IL-1/additive catalyst con-
stitutes a significant simplification of the procedure and allows
shorter reaction times than with the bifunctional IL-3 (16 h vs.
2 d). Table 4 shows selected examples of hydrogenations of
various alkynes in the presence of 5 mol% iron catalyst and
100 mol% acetonitrile in the biphasic solvent mixture IL-1–n-
heptane. Generally, higher yields and stereoselectivities were
obtained compared with the reactions in IL-3 (Table 4). Free
NH2 groups, esters, and alkenes were tolerated. Bulky (TMS)
groups and carboxylates led to lower conversions. 1-Alkynes gave
mixtures of alkenes and alkanes. Hydrogenations proceeded also
in mono-phasic THF–MeCN or toluene–MeCN (40 h) mixtures
with similar selectivity, but the catalyst phase could not be
separated. The catalyst species was found to rapidly age in the
absence of IL-1 and lose activity after 48 h. On the other hand,
effective catalyst separations and multiple re-uses without loss of
¨
Losch-Foundation, and the DFG. We thank Dr L. Greiner and
S. Mariappan (Dechema) for performing TEM analyses.
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