Efficient hydrogenation of ketones catalyzed by an iron pincer complex

Article abstract of DOI:10.1002/anie.201007406

Reductions made easy: A highly efficient iron catalyst effects the homogeneous hydrogenation of ketones under very mild conditions (see scheme). A mechanism including the insertion of the ketone into the Fei-H bond, followed by addition of hydrogen to a dearomatized intermediate is proposed.

Full text of DOI:10.1002/anie.201007406

Communications
DOI: 10.1002/anie.201007406
Catalytic Hydrogenation
Efficient Hydrogenation of Ketones Catalyzed by an Iron Pincer
Complex**
Robert Langer, Gregory Leitus, Yehoshoa Ben-David, and David Milstein*
The catalytic reduction of ketones to the corresponding
alcohols is an important reaction in organic chemistry, which
is usually catalyzed by complexes of precious metals (e.g. Rh,
Ir, Ru) using either H2 or iPrOH as hydrogen source.[1] In
particular, the reduction of ketones with gaseous hydrogen
provides an atom-economical synthetic method. The develop-
ment of iron-based catalysts with similar activity is desirable
due to their low toxicity, lower price, and the benign
environmental impact of iron compounds.[2]
Although considerable progress has been made in iron-
catalyzed transfer hydrogenations and hydrosilylations,[3] only
a few studies have been published on iron-catalyzed hydro-
genations.[4–7] Chirik and co-workers recently described the
synthesis of the well-defined catalyst [(iPrPDI)Fe(N2)2]
of cooperation between the ligand and the metal center,
involving aromatization–dearomatization of the ligand, plays
a key role in these reactions. To develop corresponding iron
catalysts with similar reactivity would be desirable. The
preparation of the paramagnetic FeII complexes [(PNN)Fe-
Cl2][10a] (PNN = 2-((di-tert-butylphosphinomethyl)-6-diethyl-
aminomethyl)pyridine) and [(tBuPNP)FeCl2] (tBuPNP =
2,6-bis(di-tert-butylphosphinomethyl)pyridine),[10a,b] as well
as the formation of the cationic complex [(PNN)FeCl(thf)]-
(PF6)[10a] has already been reported. The dinitrogen hydride
complex [(iPrPNP)Fe(H)2(N2)], described by Trovitch et al.,
catalyzes the hydrogenation of 1-hexene, but showed only
poor activity in the hydrogenation of cyclohexene, while the
complex [(iPrPNP)FeH2(SiH2Ph)(N2)] was not effective in
the hydrosilylation of alkenes.[7] Based on the unusual
reactivity of pyridine pincer based hydrido carbonyl RuII
complexes, we started to investigate the synthesis of analo-
gous iron complexes.
Herein we report the synthesis and characterization of the
new iron pincer complexes [(iPrPNP)Fe(CO)Br2] (1) and
[(iPrPNP)FeH(CO)Br] (2) (iPrPNP = 2,6-bis(diisopropyl-
phosphinomethyl)pyridine). The iron hydride complex 2 is
the most efficient iron complex catalyst reported to date for
the hydrogenation of ketones. The reaction takes place under
very mild conditions, with turnover numbers of up to 1880
using 4.1 atm hydrogen pressure at ambient temperature
=
(iPrPDI = ((2,6-CHMe2)2C6H3N CMe)2C5H3N), which was
capable of hydrogenating alkenes and alkynes with 0.3
mol% catalyst loading and 4 atm hydrogen pressure at
ambient temperature. Turnover frequencies of up to
1814 hÀ1 were reached with this system for the hydrogenation
of 1-hexene.[4] We are aware of only two iron complex systems
that are capable of catalyzing the hydrogenation of ketones to
alcohols. Casey et al. described the bifunctional complex
[{2,5-(SiMe3)2-3,4-(CH2)4(h5-C4COH)}Fe(CO)2H], which cat-
alyzes hydrogenation of ketones under mild conditions with
high chemo- and diastereoselectivity, resulting in 50 turn-
overs.[5] The diiminodiphosphine and diaminodiphosphine
iron complexes described by Morris and co-workers were
studied in the catalytic hydrogenation of acetophenone,
leading to high conversion at high pressure and excess of
base (T= 508C, p(H2) = 25 atm, TON 225; catalyst:base =
15).[6]
(26–288C).
The
two
dearomatized
complexes
[(iPrPNPÀH)FeH(CO)(NCPh)] (3) and [(iPrPNPÀH)FeH-
(CO)(PhCOPh)] (4) were characterized by multinuclear
NMR spectroscopy and their reactivity was investigated.
Based on these results a mechanism for this reaction is
proposed.
We recently reported reactions catalyzed by pyridine-
based pincer complexes of ruthenium,[8] including hydro-
genation of esters to alcohols[9a] and hydrogenation of amides
to amines and alcohols under mild conditions.[9b] A new mode
[*] Dr. R. Langer, Y. Ben-David, Prof. Dr. D. Milstein
Department of Organic Chemistry
Weizmann Institute of Science, 76100 Rehovot (Israel)
Fax: (+972)8-934-4142
E-mail: david.milstein@weizmann.ac.il
Dr. G. Leitus
Chemical Research Support
Weizmann Institute of Science, 76100 Rehovot (Israel)
[**] This research was supported by the European Research Council
under the FP7 framework (ERC No 246837) and by the MINERVA
Foundation. R.L. received a postdoctoral fellowship from the
Deutsche Forschungsgemeinschaft (DFG). D.M. holds the Israel
Matz Professorial Chair of Organic Chemistry.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201007406.
Scheme 1. Synthesis of the FeII complexes 1 and 2.
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The reaction of FeBr2 with iPrPNP in THF under an
atmosphere of CO resulted in the formation of the blue
complex 1 in good yield (Scheme 1). The IR spectrum of 1
exhibits a strong absorption at 1944 cmÀ1 assignable to
coordinated CO. The 31P{1H} NMR spectrum shows a singlet
at d = 72.8 ppm, indicating that the two phosphorus atoms of
the PNP ligand are equivalent. The 1H NMR spectrum
exhibits one doublet of doublets at d = 1.44 ppm for the
methyl protons of the iPr groups, one multiplet at d =
2.87 ppm for the CH protons of the iPr groups and one
doublet at d = 4.05 ppm for the methylene protons, indicating
a CS or C2 symmetry of the complex and thus a trans
orientation of the two bromide ligands. The crystal structure
of 1 (Figure 1) displays a distorted octahedral geometry
Figure 2. Molecular structure of complex 2 with the thermal ellipsoids
set at 50% probability. Selected bond lengths [ꢀ] and angles [8]: Fe1–
Br1 2.547(11), Fe1–P1 2.218(1), Fe1–N1 2.045(3), Fe1–C1 1.730(5),
Fe1–H1 1.42(4), C1–O1 1.145(5); Br1-Fe1-P1 100.7(4), Br1-Fe1-N1
93.0(1), Fe1-C1-O1 173.7(4), P1-Fe1-P2 163.75(5), N1-Fe1-C1
168.7(19), Br1-Fe1-H1 179.7(19).
Table 1: Iron-catalyzed hydrogenation of acetophenone.[a]
Solvent
t [h]
Yield [%]
TON
TOF/hÀ1
(conversion [%])[b]
MeOH
EtOH
EtOH[c]
nPrOH
iPrOH
THF
23
21.5
4
21
20
24
22
13 (23)
94 (94)
85 (86)
30 (35)
9 (21)
0
260
1880
1720
600
180
–
11
87
430
29
9
Figure 1. Molecular structure of complex 1 with the thermal ellipsoids
set at 50% probability. Selected bond lengths [ꢀ] and angles [8]:
Fe1–Br1 2.484(1), Fe1–N1 2.046(4), Fe1–C1 1.746(5), Fe1–P1 2.302(1),
C1–O1 1.160(6); C1-Fe1-N1 177.4(2), Fe1-C1-O1 177.6(5), P1-Fe1-P2
166.71(5), Br1-Fe1-Br2 178.09(3), Br1-Fe1-P1 94.35(4), Br1-Fe1-N1
91.75(11).
–
–
–
0
–
[a] Reaction conditions: 2 (0.0025 mmol), KOtBu (0.005 mmol), sub-
strate (5 mmol), m-xylene (1 mmol), ethanol (3 mL), H2 (4.1 atm).
[b] Determined by GC analysis with m-xylene as internal standard [c] T=
408C.
around the metal center, with the CO ligand trans to the
pyridine nitrogen and the two bromine atoms in the apical
positions.
The hydrido complex [(iPrPNP)FeH(CO)Br] (2) was
prepared by treating 1 in THF with one equivalent of
NaHBEt3 (Scheme 1) and was isolated as a brown solid in
86% yield. The carbonyl vibration in the IR spectrum is
shifted to 1892 cmÀ1. The equivalence of both phosphorus
atoms is reflected in the observation of a singlet peak at d =
90.43 ppm in the 31P{1H} NMR spectrum. The 1H NMR
spectrum shows a triplet at d = À20.47 ppm (2JPH = 53.5 Hz)
for the hydrido ligand, four doublets of doublets for the
methyl protons, and two multiplets for the CH protons of the
iPr groups, indicating lack of symmetry in comparison with 1.
According to the crystal structure of complex 2 the FeII
center has a distorted octahedral environment including the
iPrPNP, hydride, carbon monoxide, and bromide ligands
(Figure 2). The CO ligand is coordinated trans to the pyridine
nitrogen, while the hydride is trans to the bromide ligand
pressure at ambient temperature (26–288C). Selected experi-
ments for the catalytic hydrogenation of acetophenone are
outlined in Table 1. It became apparent that catalytic hydro-
genation could only be facilitated in alcoholic solvents, with
ethanol giving the highest activity by complex 2. No hydro-
genation was observed in THF or neat acetophenone. In
MeOH and iPrOH rac-1-phenylethanol was obtained only in
poor yields of 13% and 9%, respectively (Table 1). The
hydrogenation of acetophenone in nPrOH gives rac-1-phe-
nylethanol in 30% yield, while in EtOH the best yield is 94%.
Interestingly, slight heating of the reaction mixture to 408C
gives after only 4 h rac-2-phenylethanol in 85% yield with a
turnover number of 1720 and a turnover frequency of 430 hÀ1.
This is the highest reported catalytic activity for ketone
hydrogenation by iron complexes. It should be noted that
both base and H2 are necessary for the reaction to occur. The
strong dependency of the yield on the solvent suggest that an
equilibrium with the alcoholic solvent might be involved, as
observed previously for ruthenium complexes.[11]
À
(Fe1 H1 1.42(4) ꢀ).
Complex 2 was investigated as a catalyst for the hydro-
genation of ketones. In preliminary experiments various
solvents were tested for the hydrogenation of acetophenone,
using 0.05 mol% 2, 0.1 mol% KOtBu and 4.1 atm hydrogen
Based on these results the scope and limitations of catalyst
2 were explored, using EtOH as solvent and 4.1 atm hydrogen
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Table 2: Hydrogenation of ketones catalyzed by 2.[a]
presence of a nitrile or a primary amine group no catalytic
activity of complex 2 was observed (Table 2, entries 5 and 6),
probably as a result of preferential coordination of these
groups. Simple ketones such as benzophenone and cyclo-
hexanone were reduced in good yield of 74% and 67%,
respectively (Table 2, entries 7 and 8). Although the diketone
benzil was not completely soluble under the reaction con-
ditions, it showed good reactivity as well, resulting in selective
hydrogenation to yield the monohydrogenation product
benzoin, which quantitatively precipitated during the reaction
(67% yield, Table 2, entry 9). The trifluoro-substituted ace-
tophenone, which mainly forms the hemiacetal under the
reaction conditions,[12] thus lowering the effective concentra-
tion of the ketone, was reduced to the corresponding alcohol
in moderate yield of 54% (Table 2, entry 10). The reduction
of a,b-unsaturated ketones such as trans-4-phenyl-3-buten-2-
one or 2-cyclohexenone resulted in mixtures, where reduction
of the double bond also took place. 2-Acetylpyridine, which
can potentially bind to the iron center, was hydrogenated in
very good yield (Table 2, entry 13). Benzaldehyde was hydro-
genated only at lower substrate concentrations and with
higher catalyst loading (Table 2, entry 14). The reason for the
low activity of 2 in the hydrogenation of benzaldehyde is not
clear.[13,14]
In order to develop a mechanistic understanding of the
catalytic hydrogenation of ketones the reactivity of complex 2
was investigated in stoichiometric reactions. Treatment of 2
with one or two equivalents of base (LiHMDS or KOtBu) in
an aprotic solvent (e.g. THF, C6H6, toluene) results in
immediate decomposition to the Fe0 complex [(iPrPNP)-
Fe(CO)2], free ligand (iPrPNP), and unidentified paramag-
netic products.[15] In contrast to that, no change in 31P NMR
spectrum was observed, when two equivalents of KOtBu were
added to an EtOH solution of complex 2.[16] Nevertheless,
deprotonation to a dearomatized complex takes place in the
presence of 2–3 equivalents of an ancillary ligand like
benzonitrile (3)[14] or benzophenone (4),[14] which probably
stabilize the FeII center by binding at the vacant coordination
site (Scheme 2). The 31P{1H} NMR spectra of complexes 3 and
4 show AB-systems with doublet signals at d = 83.17 and
90.70 ppm in case of 3 (2JPP = 112.6 Hz), and doublet signals at
Entry
Substrate
Product
t [h]
Yield [%][b] TON
1
2
3
4
5
6
X=H
X=Cl
X=Br
X=Me
X=NH2
X=CN
21.5
18
22
22
24
94 (94)
86 (89)
78 (78)
72 (73)
0 (0)
1880
1720
1560
1440
0
24
0 (0)
0
7
24
24
70 (74)
64 (67)
1400
1280
8
9
20
67
1340
10
15
17
54 (54)
20 (95)
44
1080
1220
11[c]
29
12[c]
21
2 (97)
10
42
13
15
24
87 (88)
36 (39)
1740
288
14[d]
[a] Reaction conditions: 2 (0.0025 mmol), KOtBu (0.005 mmol), sub-
strate (5 mmol), ethanol (3 mL), m-xylene (1 mmol), H2 (4.1 atm).
[b] Yield of alcohol, determined by GC analysis with m-xylene as internal
standard (conversion of alcohol is in parenthesis). [c] 0.005 mmol 2 and
0.01 mmol KOtBu were used. [d] 2 mmol of PhCHO in 4 mL EtOH were
used
pressure at room temperature. Halogen or methyl substitu-
ents on the aromatic system had a small impact on the yield
(72–86% yield, Table 2, entry 2–4). Interestingly, no hydro-
À
genolysis of the C Cl or CBr bonds took place. In the
Scheme 2. Reactivity of 2 towards base.
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Angew. Chem. Int. Ed. 2011, 50, 2120 –2124

d = 82.73 and 89.82 ppm in case of 4 (2JPP = 102.7 Hz),
corresponding to two inequivalent phosphorus centers,
respectively. The hydride ligand exhibits a virtual triplet at
d = À15.92 ppm for complex 3 and a doublet of doublets at
d = À14.95 ppm for complex 4, indicating that the ancillary
ligands are trans to the hydride, respectively. A broadened
1
singlet at d = 3.89 ppm in the H NMR spectrum of 3, which
integrates to one and a doublet resonance at d = 64.8 ppm
(1JPC = 51.1 Hz) in the 13C-DEPTQ spectrum suggest the
1
formation of an anionic PNP system (confirmed by H-13C-
HSQC NMR).
Scheme 4. Proposed reaction mechanism for the hydrogenation of
ketones catalyzed by 2.
generate the aromatic complex A’ (Scheme 4). Ketone
coordination to A followed by isomerization to intermediate
Scheme 3. Reactivity 3 and 4 towards H2.
À
B makes insertion of the ketone into the Fe H bond possible.
Complex 4 reacts rapidly with hydrogen. NMR studies
indicate the formation of a hydrido alkoxo complex 6[14]
together with a trans dihydride complex 5 (Scheme 3).[14]
The selectively decoupled 31P{1H} NMR spectrum of the
mixture shows a triplet signal at d = 118.3 ppm for complex 5,
suggesting that a symmetric dihydride complex was formed,
and a doublet signal at d = 90.01 ppm for complex 6. The
hydride ligands of complex 5 exhibit a triplet at d =
The pentacoordinated alkoxo carbonyl complex C can react
with hydrogen to give the aromatic hydrido alkoxy complex
D. Elimination of the product alcohol would regenerate the
dearomatized species A.
In conclusion, we have developed an efficient iron catalyst
for the hydrogenation of ketones under mild conditions. Our
NMR studies suggest that the reaction proceeds through a
dearomatized intermediate. Ketone coordination, insertion of
1
À7.36 ppm (2JPH = 39.4 Hz) in the H NMR spectrum, while
the ketone into the Fe H bond of the dearomatized
À
the hydride ligand in 6 shows a triplet resonance at d =
À19.94 ppm (2JPH = 52.5 Hz). The appearance of triplet sig-
nals for both complexes in the 1H NMR spectrum is in
agreement with re-aromatized complexes. The three equiv-
alents of benzophenone that were employed to stabilize the
dearomatized species were fully converted to the correspond-
ing alkoxide. In contrast to the reactivity of 4, complex 3
reacts slowly under one atmosphere of hydrogen to give the
trans dihydride complex 5 (Scheme 3).[14] This observation is
in agreement with the low catalytic activity of 2 towards para-
cyanoacetophenone in the hydrogenation reaction. Thus, it is
unlikely that the ketone can be reduced directly by trans-
[(iPrPNP)Fe(H)2(CO)] without pre-coordination. Opening of
the phosphorus arm at room temperature to generate a vacant
coordination site is not reasonable.
intermediate, followed by addition of hydrogen with re-
aromatization likely play key roles in this mechanism. We are
currently aiming at expanding the scope of this novel ketone
hydrogenation reaction, as well studying its mechanistic
implications.
Experimental Section
General procedure for catalytic hydrogenation: A 90 mL Fischer-
Porter tube was charged under nitrogen with the catalyst
2
(0.0025 mmol), KOtBu (0.005 mmol), ketone (5 mmol), m-xylene
(1 mmol), ethanol (3 mL), and 4.1 atm hydrogen. The solution was
stirred at ambient temperature (26–288C) for the specified time and
the product alcohols were determined by GC with m-xylene as
internal standard, using a Carboxen 1000 column on a HP 690 series
GC system.
Based on these results a possible reaction mechanism for
the hydrogenation of ketones catalyzed by 2 might involve
intermediate formation of a reactive dearomatized species,
which is stabilized by reversible addition of ethanol to
Received: November 24, 2010
Published online: January 5, 2011
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Communications
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catalytic activity of 2. Benzene was not observed, negating the
possibility of deactivation by CO, that might have been
generated by decarbonylation of benzaldehyde.
Keywords: homogeneous catalysis · hydrogenation ·
iron complexes · ketones · pincer ligands
.
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[14] For details see Supporting Information.
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