General and highly efficient iron-catalyzed hydrogenation of aldehydes, ketones, and α,β-unsaturated aldehydes

Article abstract of DOI:10.1002/anie.201301239

EnvIRONmentally friendly: The title hydrogenation of aldehydes, ketones, and α,β-unsaturated aldehydes is reported. In the presence of the catalyst 1, primary, secondary, and allylic alcohols were obtained in good to excellent yields under mild reaction conditions. The catalyst is easily and inexpensively prepared, and is also stable to air, water, and column chromatography. Copyright

Full text of DOI:10.1002/anie.201301239

Angewandte
Chemie
DOI: 10.1002/anie.201301239
Reduction
General and Highly Efficient Iron-Catalyzed Hydrogenation of
Aldehydes, Ketones, and a,b-Unsaturated Aldehydes**
Steffen Fleischer, Shaolin Zhou, Kathrin Junge, and Matthias Beller*
The reduction of carbonyl compounds constitutes one of the
most important catalytic methods in synthetic organic
chemistry both on laboratory and industry scale.[1] The
products of this transformation, namely primary, secondary,
and allylic alcohols, represent key building blocks in the
pharmaceutical, agrochemical, fine-chemicals, and fragrance
industry.[2]
underrepresented.[5–7] Notably, Casey and Guan developed
the first efficient iron system for hydrogenation of carbonyl
compounds in 2007.[8] By using the highly air-sensitive
complex 3 (Scheme 1), various simple aldehydes and ketones
were reduced under mild reaction conditions. Unfortunately,
industrially important a,b-unsaturated carbonyl compounds
To date metal hydride reagents, hydrosilylation reactions,
and transfer hydrogenations are frequently employed for such
reductions. However, the stoichiometric formation of unde-
sired by-products does not meet the requirements for
sustainable synthesis. On the contrary, catalytic hydrogena-
tion represents a 100% atom-efficient methodology.[3] Often,
heterogeneous catalysts are used in industry because of their
facile separation and potential recyclability. However, their
major drawback is the limited tolerance towards functional
groups, especially towards other reducible moieties such as
olefins, alkynes, and nitro-, ester-, and amide functional
groups. Therefore, easily tuneable homogeneous hydrogena-
tion catalysts have attracted great interest during the last
decades. The majority of the known homogeneous and
heterogeneous catalyst systems are based on precious
metals such as ruthenium, rhodium, iridium, and palladium.[4]
As a result of their restricted availability, high price, and
potential toxicity, the search for more economical and
environmentally friendly catalysts based on nonprecious
analogues (Fe, Cu, Zn, Mn) is a real and challenging goal.
Clearly, a state-of-the-art hydrogenation catalyst should
be based on inexpensive materials (metal, ligand, or support)
and should be easy to synthesize and convenient to handle.
Additionally, it should be highly active and show excellent
selectivity towards a broad range of functional groups.
Even though reduction catalysts based on iron are highly
attractive candidates to meet these criteria, the field of iron-
catalyzed hydrogenation of carbonyl compounds still remains
Scheme 1. Synthesis of the tricarbonyl(h4-cyclopentadienone)iron com-
plexes 1 and 2.
showed very low chemoselectivity with this system. In 2011,
Milstein and co-workers developed a defined iron pincer
complex[9] based on their elegant work on similar ruthenium
complexes.[10] To date, this catalyst represents the most
efficient iron-based system for the reduction of ketones
[turnover number (TON) = 1880]. However, benzaldehyde
was reduced in only 36% yield even with increased catalyst
loading, while a,b-unsaturated substrates gave mixtures of
hydrogenated products with moderate yields.
Based on our interest in iron-catalyzed reductions,[11] we
recently set out to develop improved iron complexes for the
reduction of different kinds of carbonyl compounds. First, we
synthesized eight different tricarbonyl(h4-cyclopentadie-
none)iron complexes (1a–f and 2a–b) in a straightforward
two-step procedure starting from either commercially avail-
able 1,6-heptadyines or 1,7-octadiynes, iron pentacarbonyl,
and chlorotrialkylsilanes in excellent yield (Scheme 1 and
Table 1).[12]
[*] S. Fleischer, Dr. K. Junge, Prof. Dr. M. Beller
Leibniz-Institut fꢀr Katalyse an der Universitꢁt Rostock
Albert-Einstein-Strasse 29a, 18059 Rostock (Germany)
E-mail: matthias.beller@catalysis.de
In contrast to the known complex 3,[13] and Milsteinꢀs
pincer complex,[9] all synthesized iron complexes can be
handled without special precautions. In fact, they are stable in
air and water, and can be purified by column chromatography
on silica gel. Next, we wondered if the hydrogenation of
carbonyl compounds would be possible by using an in situ
Hieber base reaction to activate the tricarbonyl(h4-cyclo-
pentadienone)iron complexes.[13,14]
Considering the industrial importance of 1-arylethanols,
our initial catalytic investigations were carried out using the
hydrogenation of acetophenone (4a) as a benchmark reaction
(Table 1). As expected the blank reaction without the iron
Homepage: http://www.catalysis.de
Dr. S. Zhou
College of Chemistry, Central China Normal University
Luoyu Road, No. 152, 430079 Wuhan (P.R. China)
[**] This work has been funded by the State of Mecklenburg-Western
Pomerania, the BMBF, and the DFG. We thank Dr. W. Baumann, Dr.
C. Fischer, S. Buchholz, S. Schareina, A. Koch, and S. Rossmeisl (all
at the LIKAT) for their excellent technical and analytical support. S.F.
thanks the Evonik Stiftung for providing a scholarship.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201301239.
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Table 1: Different iron complexes for the catalytic hydrogenation of
Table 2: Influence of different reaction parameters on the iron-catalyzed
acetophenone.[a]
hydrogenation of acetophenone.[a]
Entry Solvent
1a (mol%)/
K2CO3 (mol%)
T [8C] p (bar) Yield[b] [%]
Entry
Fe catalyst (mol%)
Yield[b] [%]
TON
1
–
<1
<1
37
34
<1
38
22
21
21
–
–
3700
3400
–
3800
2200
2100
2100
–
1
2
3
4
5
6
7
8
EtOH/H2O
1:2
1:2
1:2
1:2
1:2
80
80
80
80
80
80
80
60
80
100
130
100
100
100
100
10
10
10
10
10
10
10
10
10
10
10
30
50
30
30
>99
>99
>99
>99
51
3
<1
8
77
60
63
>99
92
2[c]
3
1a: R=SiMe3 (1)
1a: R=SiMe3 (0.01)
1b: R=SiEtMe2 (0.01)
1c: R=SiiPrMe2 (0.01)
1d: R=SitBuMe2 (0.01)
1e: R=SiCyMe2 (0.01)
1 f: R=SiEt3 (0.01)
2a: X=CH2 (0.01)
2b: X=O (0.01)
iPrOH/H2O
DME/H2O
THF/H2O
DMF/H2O
DMSO/H2O
tol/H2O
iPrOH/H2O
iPrOH/H2O
iPrOH/H2O
iPrOH/H2O
iPrOH/H2O
iPrOH/H2O
iPrOH/H2O
iPrOH/H2O
4
5
6
7
8
9
10
11
12
1:2
1:2
0.1:0.5
0.1:0.5
0.1:0.5
0.1:0.5
0.1:0.5
0.1:0.5
0.05:0.25
0.01:0.05
9
<1
<1
<1
10
11
12
13
14
15
[Fe(Cp*)2] (1)
[{CpFe(CO)2}2] (0.5)
–
–
[a] General reaction conditions: acetophenone (1.0 mmol), Fe complex
1/2 (0.01 mol%), K2CO3 (0.05 mol%), iPrOH/H2O (0.5 mL/0.2 mL), H2
(30 bar), at 1008C for 17 h. [b] Determined by GC analysis using
hexadecane as an internal standard. [c] Reaction without base.
Cp=C5H5, Cp*=C5Me5, Cy=cyclohexyl.
>99
37
[a] General reaction conditions: acetophenone (1.0 mmol), 1a (0.01–
1 mol%) K2CO3 (0.05–2 mol%), organic solvent/H2O (0.5 mL/0.2 mL),
H2 (10–30 bar) for 17 h. [b] Determined by GC analysis using hexadecane
as an internal standard. DME=dimethoxyethane, DMF=N,N-dime-
thylformamide, DMSO=dimethylsulfoxide, THF=tetrahydrofuran.
catalyst did not show any activity (Table 1, entry 1). When 1a
was used in a mixture of iPrOH and H2O without base no
product formation was observed (Table 1, entry 2). To our
delight adding a tiny amount of K2CO3 (0.05 mol%) to the
reaction mixture led to the smooth hydrogenation of aceto-
phenone even with a catalyst loading as low as 0.01 mol%
(Table 1, entry 3; TON = 3700). This result represents the
highest TON for the hydrogenation of carbonyl compounds
with any iron-based catalysts to date. Comparable high TONs
of up to 3800 were observed when 1b and 1d were used as
catalyst precursors (Table 1, entries 4 and 6). While 1e, 1 f,
and 2a gave lower TONs (Table 1, entries 7–9), 1c and 2b did
not yield any product at all (Table 1, entries 5 and 10). When
the cyclopentadienone moiety was displaced by Cp and Cp*
no reaction occurred, and thus indicates the importance of the
non-innocent character of the ligand (Table 1, entry 11 and
12).
Next, the influence of critical reaction parameters such as
solvent, temperature, pressure, and catalyst loading were
investigated. The hydrogenation of 3a showed full conversion
and excellent yields of greater than 99% when aqueous
alcoholic solutions such as EtOH and iPrOH as well as
aqueous ethereal solutions as DME and THF were used as
solvents (Table 2, entries 1–4). In a DMF/water mixture
a moderate yield of 51% was obtained (Table 2, entry 5),
whereas DMSO gave only trace amounts of 5a (Table 2,
entry 6). When a nonpolar solvent such as toluene was used in
combination with H2O no product is formed (Table 2,
entry 7). The observations that reductions are possible in
ethereal solvent mixtures of DME and THF, as well as in
DMF, and that no reduction occurred in the absence of
hydrogen gas, clearly indicate a hydrogenation mechanism.
Next, the influence of temperature and pressure were
investigated. In the range of 80–1308C the hydrogenation of
4a with a low hydrogen pressure of 10 bar gave good yields of
5a in the range of 60–77% even with a low catalyst loading of
0.1 mol% 1a (Table 2, entries 9–11). When 608C was applied
as reaction temperature the yield drops down to 8% (Table 2,
entry 8). Increasing the pressure from 10 bar to 30 bar led to
full conversion and an excellent yield of greater than 99%
(Table 2, entry 12). Finally, the catalyst loading was decreased
to 0.05 mol% and to our delight, the yield remained as high as
greater than 99% (Table 2, entry 14). Further lowering to
0.01 mol% of 1a yielded the product in a 37% yield (Table 2,
entry 15).
With the optimized reaction conditions in hand, we
investigated the scope and limitations of the iron-catalyzed
hydrogenation protocol. First, we considered the hydrogena-
tion of different benzaldehydes and aromatic ketones.
Besides the hydrogenation of acetophenone, benzaldehyde
was also hydrogenated smoothly in high yield (94%; Table 3,
entry 1). We then focused our attention on the influence of
different functional groups. Remarkably, different aryl alde-
hydes and ketones bearing various electron-withdrawing and
electron-donating groups in the ortho-, meta-, and para-
positions showed high reactivity with excellent yields in the
range of 94–99% with catalyst loadings of 0.1–0.5 mol%
(Table 3, entries 2–5). Furthermore, we investigated sub-
strates with pharmaceutically relevant trifluoromethyl, tri-
fluoromethoxy, amino, ester, and amide substituents as well as
heterocycles. To the best of our knowledge, there is no iron-
catalyzed hydrogenation reaction of carbonyl compounds
reported which tolerates these functional groups. We were
pleased to find that all these compounds were hydrogenated
smoothly and 5g–5k were obtained with good to excellent
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Table 3: Iron-catalyzed hydrogenation of aromatic/heteroaromatic alde-
yields (60– > 99%). Similarly, different substituted N-, O- and
hydes and ketones to primary and secondary alcohols.[a]
S-heteroaromatic aldehydes and ketones gave high yields of
(Table 3, entries 11–13). Notably, a-trifluoroacetophenone
was hydrogenated with excellent yield (> 99%) to the
corresponding alcohol 5o (Table 3, entry 14). In the hydro-
genation of the a-diketone benzil both keto functionalities
were reduced and the industrially important a-diol hydro-
benzoin is produced in 95% yield (Table 3, entry 15).
The hydrogenation of aliphatic aldehydes and ketones
using iron catalysts is only scarcely investigated. Therefore,
seven different aliphatic aldehydes as well as cyclic and
aliphatic ketones (4q–w) were explored. Again all substrates
were hydrogenated with good to excellent yields (85– < 99%)
to the corresponding alcohols 5q–w (Scheme 2). Sterically
Entry
1
Product
1a (mol%)
Yield[c] [%]
94 (89)
5b
5c
0.1
2
3
0.5
0.1
99
98
5d
4
5
6
7
5e
5 f
0.1
0.1
1
99 (96)
99 (96)
98
5g
5h
1
>99
8
9
5i
5j
0.1
0.5
>99
Scheme 2. Iron-catalyzed hydrogenation of aliphatic aldehydes and
ketones into allylic and secondary alcohols.
76
demanding substrates, for example, 4s, did not lead to a drop
in catalyst activity. Moreover, citronellal (4t) is selectively
reduced to citronellol (5t), which is an important compound
for the fragrance industry, in excellent yield of 98%. Notably,
the catalytic system leaves the double bonds untouched.
Finally, we were interested in the hydrogenation of a,b-
unsaturated aldehydes. In principle three different products
are possible resulting from either the hydrogenation of the
double bond, the carbonyl group, or the hydrogenation of
both functionalities. Therefore, product mixtures are often
observed. Gratifyingly, our catalyst system selectively reduces
10
11
12
5k
5l
0.1
0.5
0.1
99 (92)
95
5m
>99 (97)
=
the C O bond, thus yielding allylic alcohol 7 as the only
product (Scheme 3). The further transformation of allylic
alcohols into key building blocks of complex molecular
architectures is well established in organic synthesis.[15]
Cinnamyl alcohol (7a), as well as the a- and b-substituted
cinnamyl alcohols 7b–e were also obtained in good to
excellent yields (90– > 99%). The industrially important
perillyl alcohol (7 f) and geraniol (7g) were delivered in 98–
99% yields, and again no hydrogenation of the double bonds
was observed. Simple aliphatic trans-2-decenal (6h) was also
reduced nicely and the corresponding allylic alcohols 7h was
obtained with excellent yield. We also wanted to investigate
the scale-up of the reaction procedure to a synthetically
applicable scale of 5grams of starting material. Gratifyingly,
13
14
5n
5o
0.5
0.5
82
>99
15
5p
0.5
>99[c]
[a] General reaction conditions: aldehyde or ketone 4 (1.0 mmol), 1a
(0.01–1 mol%), 5-fold excess K2CO3 (0.05–5 mol%), iPrOH/H2O
(0.5 mL/0.2 mL), H2 (10–30 bar) for 17 h. [b] Determined by GC analysis
using hexadecane as an internal. Yield of isolated product given within
parentheses. [c] cis/trans ratio 4:1.
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CO2 by a Hieber base reaction. After protonation, the 18-
electron iron(II) species 3 is formed, and 3 can transfer one
hydride and one proton to reduce 6a and the 16-electron
iron(0) species 9 is formed. Oxidative addition of hydrogen
closes the catalytic cycle by formation of the iron(II) hydride
complex 3.
In conclusion, we have demonstrated that easy-to-synthe-
size, air- and water-stable iron complexes such as 1a can be
used for the efficient hydrogenation of carbonyl compounds.
The compound 1a is conveniently activated in alcohol/water
solutions using inexpensive K2CO3 as a base. Notably, the
catalyst system shows a high tolerance towards a variety of
functional groups such as halides, methoxy, nitro, trifluoro-
methyl, trifluoromethoxy, amines, esters, amides, and N-, O-,
and S-heterocycles. High yields, up to greater than 99%, were
observed in the hydrogenation of aromatic and aliphatic
aldehydes and ketones as well as a,b-unsaturated aldehydes
to primary, secondary, and allylic alcohols. TONs of up to
3800 are obtained and the catalytic procedure can be easily
scaled up. All this makes the new protocol highly attractive
for the selective reduction of various carbonyl compounds.
Scheme 3. Iron-catalyzed hydrogenation of a,b-unsaturated aldehydes
into allylic alcohols.
Received: February 12, 2013
Published online: && &&, &&&&
the hydrogenation of 5grams of 6c with a catalyst loading of
0.5 mol% 1a yielded 7c in 96% after distillation (for
1H NMR spectrum of the crude reaction mixture, see the
Supporting Information).
Keywords: alcohols · hydrogenation · iron · reduction ·
synthetic methods
.
On the basis of previous work of the groups of Casey[8] and
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following mechanism (Scheme 4). As no hydrogenation
occurs in the absence of K2CO3, first the base has to activate
the 18-electron iron(0) precatalyst 1a. Hydroxide adds to one
carbon monoxide, which is coordinated to the iron center of
1a and the hydride complex 8 is formed with the release of
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4
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Angew. Chem. Int. Ed. 2013, 52, 1 – 6
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
5
These are not the final page numbers!

.
Angewandte
Communications
Communications
Reduction
EnvIRONmentally friendly: The title
hydrogenation of aldehydes, ketones, and
a,b-unsaturated aldehydes is reported. In
the presence of the catalyst 1, primary,
secondary, and allylic alcohols were
obtained in good to excellent yields under
mild reaction conditions. The catalyst is
easily and inexpensively prepared, and is
also stable to air, water, and column
chromatography.
S. Fleischer, S. Zhou, K. Junge,
M. Beller*
&&&& — &&&&
General and Highly Efficient Iron-
Catalyzed Hydrogenation of Aldehydes,
Ketones, and a,b-Unsaturated Aldehydes
6
www.angewandte.org
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2013, 52, 1 – 6
These are not the final page numbers!