Fast and selective iron-catalyzed transfer hydrogenations of aldehydes

Article abstract of DOI:10.1016/j.jorganchem.2013.06.010

An efficient iron-based catalyst system consisting of Fe(BF)4$6H2O and P(CH₂CH₂PPh2)3 [tetraphos, (PP3)] is presented for the highly selective transfer hydrogenation of aromatic, aliphatic, and a,b-unsaturated aldehydes. A wide range of substrates including aldehydes with other reducible functional groups gave the corresponding alcohols in good yields. Formic acid is applied as a cheap, environmentally benign and easy to handle hydrogen source. Notable features of the presented methodology are the fast reactions under mild conditions. Advantageously compared to most transfer hydrogenations, no stoichiometric amounts of base additives are required.

Full text of DOI:10.1016/j.jorganchem.2013.06.010

Journal of Organometallic Chemistry xxx (2013) 1e4
Contents lists available at SciVerse ScienceDirect
Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
Fast and selective iron-catalyzed transfer hydrogenations of aldehydes
Gerrit Wienhöfer, Felix A. Westerhaus, Kathrin Junge, Matthias Beller*
Leibniz-Institut für Katalyse an der Universität Rostock e.V., Albert-Einstein-Str. 29A, 18059 Rostock, Germany
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 29 April 2013
Received in revised form
An efficient iron-based catalyst system consisting of Fe(BF) $6H O and P(CH CH PPh ) [tetraphos, (PP )]
4
2
2
2
2 3
3
is presented for the highly selective transfer hydrogenation of aromatic, aliphatic, and
a
,b
-unsaturated
aldehydes. A wide range of substrates including aldehydes with other reducible functional groups gave
the corresponding alcohols in good yields. Formic acid is applied as a cheap, environmentally benign and
easy to handle hydrogen source. Notable features of the presented methodology are the fast reactions
under mild conditions. Advantageously compared to most transfer hydrogenations, no stoichiometric
amounts of base additives are required.
11 June 2013
Accepted 11 June 2013
Dedicated to Professor Dr. Dr. h.c. Wolfgang
A. Herrmann on the occasion of his 65th
birthday.
Ó 2013 Elsevier B.V. All rights reserved.
Keywords:
Aldehydes
Formic acid
Iron
Reduction
Transfer hydrogenation
1. Introduction
overcome all these shortcomings highly selective catalytic systems
need to be developed. Although protocols have been reported,
Catalytic transfer hydrogenations have received much attention
in recent years as a benign and practical method for the reduction
of carbonyl compounds [1]. Various catalyst systems were estab-
lished especially for the reduction of ketones including chiral
complexes [2]. In contrast, only few protocols have been developed
for the transfer hydrogenation of aldehydes as it is more difficult to
control the chemoselectivity of this transformation [3]. A general
problem in these reactions is the use of base which is typically
required in transfer hydrogenations. In the presence of base,
based on iridium [3b,10], rhodium [11], ruthenium [3a,12], and
nickel [13] catalysts, many of them lack selectivity and further
improvements are highly desirable.
In the search for more efficient and environmentally benign
catalytic systems, in the last decade the scientific focus shifted from
the use of precious metals to their first row analogs. Especially iron
catalysts received much attention as the metal is cheap, ubiquitous
and relatively non-toxic [14]. Based on our experience in iron-
catalyzed reductions [15], we became interested to develop a
general iron-catalyzed transfer hydrogenation of aldehydes.
Recently, we demonstrated that defined iron-tetraphos complexes
allow for the selective reduction of nitroarenes to their corre-
sponding anilines [16]. The best catalyst activity was observed in
the absence of base, which makes it a rare example of a base-free
transfer hydrogenation. Obviously, this might be favorable for the
reduction of aldehydes as it avoids aldol condensations. To the best
deprotonation in the
followed by aldol condensation. Another side-reaction is the
possible CH -bond activation of the aldehyde [4]. The latter reac-
a-position of the carbonyl group takes place
a
tion is known to precede with rhodium [5] and iridium [6] com-
plexes followed by decarbonylation. Beside the formation of
alkanes, the carbonyl group may also block the catalyst. In addition,
ruthenium- [7] or osmium-based systems [8] can lead to
Tishchenko-type dimerization. Another problem in the hydroge-
a
of our knowledge, iron is not known to insert into the CH -bond of
nation of
a
,
b
-unsaturated aldehydes is the control of the chemo-
the carbonyl group and therefore does not catalyze the decarbon-
ylation of aldehydes [17]. Next to the nitro reduction, the selective
hydrogenation of terminal alkynes to the corresponding alkenes is
also possible with this type of iron catalyst [18]. Interestingly, it
does not catalyze the reduction of CC-double bonds. This obser-
vation made it a promising candidate for the selective reduction of
selectivity. From a thermodynamic point of view, the CC-double
bond is more easily reduced compared to the aldehyde [9]. To
*
Corresponding author. Fax: þ49 381 1281 0.
E-mail address: matthias.beller@catalysis.de (M. Beller).
a,b-unsaturated aldehydes.
0
022-328X/$ e see front matter Ó 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.jorganchem.2013.06.010
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2
G. Wienhöfer et al. / Journal of Organometallic Chemistry xxx (2013) 1e4
2
. Experimental
Table 2
Iron-catalyzed selective reduction of cinnamaldehyde: influence of solvents.a
Fe(BF
4 2 2 3
) 6H O/ PP
Fe(BF
4
)
2
$6H
2
O (0.7 mg; 0.002 mmol) and tris[2-(diphenyl-
CH PPh ; tetraphos] (1.4 mg;
.002 mmol) are placed in a Schlenk-tube under argon atmosphere.
mL dry tetrahydrofurane is added and the purple solution is
L; 0.5 mmol) and 100 L n-
O
OH
HCO
2
H, solvent
phosphino)-ethyl]phosphine [P(CH
2
2
2 3
)
0
1
Entry
Solvent
Conv.b
Yieldb
[%]
Selec.b
[%]
stirred for 2 min. Cinnamaldehyde (63
hexadecane as an internal GC-standard are injected and a sample is
taken for GC-analysis. The solution is heated to 60 C and the re-
action starts by addition of 1.1 equiv formic acid (22 mL; 0.55 mmol).
After 2 h, a second sample is taken for GC-analysis and conversion
and yield are determined by comparison with authentic samples.
For the isolation, the reaction is scaled up by a factor of 20. When
the reaction is completed, the reaction solution is diluted with a
mixture of n-hexane and ethyl acetate (3:1), filtered through a plug
of silica and the solvent removed in vacuum.
m
m
[%]
ꢀ
1
2
3
4
5
6
7
Toluene
MeOH
i-PrOH
THF
EtOH
2-Me-THF
t-AmOH
e
e
e
70
25
31
48
9
5
7
16
31
16
9
64
99
33
99
99
14
14
a
Reactions conditions: 0.5 mmol cinnamaldehyde, 0.75 mol% [FeF(PP
mL solvent, 2 equiv FA, 40 C, 2 h.
Determined by GC using n-hexadecane as an internal standard.
3
)][BF
4
],
ꢀ
1
b
3
. Results and discussion
Variation of the catalyst concentration (Table 3, entries 4e7)
demonstrated that 0.5 mol% of the catalyst is sufficient to maintain
complete conversion and quantitative yield.
At the start of this project, we used cinnamaldehyde as the
benchmark substrate for the reduction of unsaturated aldehydes.
The catalytic experiments were performed in the presence of a
Best results were obtained applying one equivalent of formic
acid (Table 3, entries 8e11). This finding is in contrast to our pre-
vious reductions of nitroarenes, where an excess of reducing agent
was required due to partly decomposition of formic acid into
hydrogen and carbon dioxide [16]. Apparently, under the present
conditions such decomposition of formic acid does not take place
and no increase of pressure is observed. Using lower amounts of
acid led to no complete conversion (Table 3, entry 8).
The highest yield was obtained using a catalyst loading of 0.4 mol
% and a slight excess of 1.1 equivalents of formic acid (Table 3, entry
12). Notably, the reaction time can be significantly reduced at higher
catalyst loading. Hence, applying 2 mol% of catalyst complete con-
version was obtained within only 10 min (Table 3, entry 13).
Next, we investigated the general applicability of our catalyst
combination of Fe(BF)
ethyl]phosphine [P(CH
4
$6H
2
O and tris[2-(diphenyl-phosphino)-
; (PP )] using formic acid (FA) as
2
CH PPh
2
2
)
3
3
reducing agent (Table 1). The single components of the catalyst
system did not show any reactivity at all while the combination of
ligand and cationic iron species exhibited good conversion (Table 1,
3
entries 2e4). Adding the ligand PP to the iron salt the complex
[
FeF(PP )][BF ] is formed in situ which is indicated by a change of
3
4
the colorless solution into deep purple. Next, the defined complex
was applied under the same reaction conditions. As expected,
reactivity comparable to the in situ system was obtained (Table 1,
entry 5) [19]. Remarkably, in all the reactions the conjugated CC-
double bond is not attacked. Decarbonylation, Tishchenko-type
dimerization or aldol condensation is also not observed and full
selectivity towards the cinnamyl alcohol is obtained.
system. First, we focused on the reactivity of different a,b-unsatu-
rated aldehydes (Table 4). Cinnamaldehydes bearing substituents in
Next, we investigated the influence of different solvents on the
benchmark reaction. While in toluene no reactivity is observed,
applying different alcohols and ether gave high to moderate con-
versions (Table 2). However, in alcohols the major products were
the undesired hemiacetal and acetal. Here, cinnamic alcohol was
obtained only in minor amount. Tetrahydrofurane led to the high-
est yields and therefore was chosen as the solvent for further re-
actions. As shown in Table 3, the reaction temperature has a major
influence on the reactivity of the system. While the increase from
Table 3
Optimization of selected reaction parameters for the selective reduction of cinna-
maldehyde.
a
Fe(BF
4
)
2
6H
2 3
O/ PP
O
OH
HCO
2
H, THF
Conv.b
[%]
Yieldb
[%]
Selec.b
[%]
Entry
Catalyst loading
[
mol%]
ꢀ
ꢀ
2
0
C to 40 C raised the conversion from 11% to 31%, a further
1c
0.75
0.75
0.75
0.2
0.3
0.4
0.5
0.3
0.3
0.3
0.3
0.4
2.0
11
31
100
20
31
40
100
75
97
83
60
11
31
>99
20
31
40
>99
75
97
83
60
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
ꢀ
increase to 60 C led to full conversion (Table 3, entries 1e3).
d
2
3
4
5
6
7
Table 1
Iron-catalyzed transfer hydrogenation of cinnamaldehyde: comparison of the in
a
situ-generated system and the defined iron-tetraphos complex.
PPh
2
Fe(BF
4
)
2
6H
2
O/ PP
3
e
O
OH
8
9
3
PP =
P
PPh2
f
HCO
2
H
g
PPh
2
10
h
1
1
1
2
i
j
100
100
>99
>99
Conv.b
Yieldb
Selec.b
13
Entry
Catalyst
a
ꢀ
[
%]
[%]
[%]
Reactions conditions: 0.5 mmol cinnamaldehyde, 1 mL THF, 2 equiv FA, 60 C, 2 h.
b
c
d
e
f
Determined by GC using n-hexadecane as an internal standard.
1
2
3
4
5
e
e
e
e
ꢀ
2
4
0
1
1
0
0
C.
C.
Fe(BF
PP
Fe(BF
4
)
2
2
$6H
$6H
2
O
e
e
e
ꢀ
3
e
e
e
.75 equiv FA.
equiv FA.
.25 equiv FA.
4
)
2
O/PP
3
31
30
31
30
>99
>99
3 4
[FeF(PP )][BF ]
g
h
i
a
Reactions conditions: 0.5 mmol cinnamaldehyde, 0.75 mol% catalyst, 1 mL THF,
1.5 equiv FA.
1.1 equiv FA.
10 min.
ꢀ
2
equiv FA, 40 C, 2 h.
b
j
Determined by GC using n-hexadecane as an internal standard.
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G. Wienhöfer et al. / Journal of Organometallic Chemistry xxx (2013) 1e4
3
Table 4
Table 5
Iron-tetraphos catalyzed reduction of
a
,
b
-unsaturated aldehydes.a
Iron-tetraphos catalyzed reduction of benzaldehydes bearing different functional
a
groups.
0
4
.4 mol%
6H O/ PP
Fe(BF
)
2
2
3
0.4 mol%
R
O
R
OH
4 2 2 3
Fe(BF ) 6H O/ PP
R
THF, 60 °C, 2h
.1 equiv HCO
R
O
OH
R
R
1
2
H
THF, 60 °C, 2h
2
.1 equiv HCO H
1
Entry
Substrate
Conv.b [%]
Yieldb [%]
Selec.b [%]
Entry
Substrate
Conv.b [%]
Yieldb,c [%]
Selec.b,c [%]
1
>99
>99
99
99
1
2
>99
>99
96
97
96
97
2c
99 (99)
99 (99)
3
>99
>99
99
99
99
99
3
>99
99
99
4d
4
5
6
7
8
9
>99
>99
>99
>99
>99
>99
>99
98
97
99
98
99
99
99
98
97
99
98
99
99
99
5
6
>99
>99
99
99
99
99
7
>99
>99
99
99
8c
(96)
(96)
a
Reactions conditions: 0.5 mmol substrates, 0.4 mol% Fe(BF
4
)
2
$6H
2
O/PP
3
, 1 mL
ꢀ
THF, 1.1 equiv FA, 60 C, 2 h.
b
Determined by GC using n-hexadecane as an internal standard.
Scaled up by the factor 20, isolated yield given in brackets.
c
d
1
mol% catalyst loading.
1
0
the a- or b-position showed excellent reactivity and gave up to 99%
yield (Table 4, entries 1e3). Cinnamaldehyde with a methoxy-
substituent in the ortho-position of the aromatic ring was fully
converted to the corresponding cinnamyl alcohol but required a
higher catalyst loading of 1 mol% (Table 4, entry 4). Furthermore,
we tested different naturally occurring terpenes (Table 4, entries 5e
d
11
>99
99 (98)
99 (98)
1
1
2e
3f
>99
>99
99 (99)
99 (97)
99 (99)
99 (97)
7
). To our delight, in each case quantitative yield was achieved and
no side products were observed. Similarly, the heteroaromatic 2-
furanacrolein gave the corresponding unsaturated alcohol in
excellent yield (99%; Table 4, entry 8).
To explore the functional group tolerance in more detail, we
started to investigate reactions of different substituted benzalde-
hydes (Table 5). First, simple benzaldehyde was fully converted to
benzyl alcohol (Table 5, entry 1). Benzaldehydes bearing different
alkyl groups gave yields of 97% and 99%, respectively (Table 5, en-
tries 2 and 3). Halogen-substituted substrates also showed full
conversion (Table 5, entries 4e7), and the position of the halide
substituent had no influence on the reactivity (Table 5, entries 6
and 7). Ether- and thioether-substituted benzaldehydes yielded the
corresponding alcohols again in nearly quantitative yield (99%;
Table 5, entries 9 and 10). Remarkably, benzaldehydes with sensi-
tive functional groups such as vinyl, acetyl and ester were reduced
with excellent chemoselectivity and the different reducible sub-
stituents were not attacked under these conditions (Table 5, entries
1
4g
>99
99
99
a
4 2 2 3
Reactions conditions: 0.5 mmol substrates, 0.4 mol% Fe(BF ) $6H O/PP , 1 mL
ꢀ
THF, 1.1 equiv FA, 60 C, 2 h.
b
Determined by GC using n-hexadecane as an internal standard.
Isolated yield given in brackets.
Scaled up by the factor 6.
Scaled up by the factor 20.
Scaled up by the factor 5.
c
d
e
f
g
2.2 equiv FA, product is the 1,3-benzenedimethanol.
aldehyde and 2-thiophenecarboxaldehyde full conversion was
achieved, the 2-pyridinecarboxaldehyde yielded only traces of the
corresponding alcohol (Table 6, entries 1e3). A similar behavior
was observed in the reduction of 3-ethynylpyridine, which yielded
only traces of the corresponding vinyl compound [18]. Finally, we
tested the aliphatic aldehydes octanal and the sterically demanding
2,2-diphenylcarboxaldehyde (Table 6, entries 4 and 5). In both cases
11e13). In the case of the isophthalaldehyde both formyl groups
were reduced giving a TON of 500 (Table 5, entry 14).
Next, transfer hydrogenations of different heterocyclic and
aliphatic aldehydes were performed (Table 6). While for furan-2-
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4
G. Wienhöfer et al. / Journal of Organometallic Chemistry xxx (2013) 1e4
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6H O/ PP
2
3
(1992) 2520e2524;
R
O
R
OH
THF, 60 °C, 2h
.1 equiv HCO
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Yieldb [%]
99
Selec.b [%]
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2
99
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4
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99
98
99
98
(
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[
[
(
5
>99
>99
96
97
96
97
(
14] For selected recent examples see: (a) N.S. Shaikh, S. Enthaler, K. Junge,
M. Beller, Angew. Chem. Int. Ed. 47 (2008) 2497e2501;
(
b) A. Boddien, F. Gärtner, R. Jackstell, H. Junge, A. Spannenberg, W. Baumann,
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120e5214;
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(
6
(
5
(
a
Ed. 50 (2011) 537e541;
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Angew. Chem. Int. Ed. 50 (2011) 1425e1429;
Reactions conditions: 0.5 mmol substrates, 0.4 mol% Fe(BF
4
)
2
$6H
2
O/PP
3
, 1 mL
ꢀ
THF, 1.1 equiv FA, 60 C, 2 h.
b
Determined by GC using n-hexadecane as an internal standard.
(
(
(
g) S. Fleischer, S. Werkmeister, S. Zhou, K. Junge, M. Beller, Chem. Eur. J. 18
2012) 9005e9010;
h) S. Das, B. Wendt, K. Möller, K. Junge, M. Beller, Angew. Chem. Int. Ed. 51
excellent yields of the corresponding alcohols were obtained. Also
the industrial important aliphatic aldehyde citronellal was sub-
jected to the reaction conditions and was fully converted to yield
citronellol in 97%.
(2012) 1662e1666;
(
(
(
i) S. Fleischer, S. Zhou, S. Werkmeister, K. Junge, M. Beller, Chem. Eur. J. 19
2013) 4997e5003;
j) V. Kelsen, B. Wendt, S. Werkmeister, K. Junge, M. Beller, B. Chaudret, Chem.
In conclusion, we have developed an efficient iron-tetraphos cata-
lyzed system for the highly selective transfer hydrogenation of aro-
matic, aliphatic and a,b-unsaturated aldehydes towards their alcohols.
Commun. 49 (2013) 3416e3418;
k) K. Junge, B. Wendt, S. Zhou, M. Beller, Eur. J. Org. Chem. (2013) 2061e
065;
l) S. Fleischer, S. Zhou, K. Junge, M. Beller, Angew. Chem. Int. Ed. 52 (2013)
(
2
(
In none of the reactions any significant amounts of side-products were
observed. Awide range of functional groups, especially other reducible
moieties, are tolerated. The reaction proceeds fast and under mild
conditions. Formic acid is applied as a cheap, environmentally benign
and easy to handle hydrogen source. Notably, no base is required
leaving it a rare example of a base-free transfer hydrogenation.
5120e5124.
[
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Acknowledgments
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(
(
(
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This research has been funded by the State of Mecklenburg-
Western Pomerania, the Federal Ministry for Education and Research
[16] G. Wienhöfer, I. Sorribes, A. Boddien, F. Westerhaus, K. Junge, H. Junge,
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[
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References
[
[
[
1] For reviews on transfer hydrogenation see: (a) S. Gladiali, G. Mestroni, in:
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Weinheim, 2004;
(
(
b) J.S.M. Samec, J.-E. Bäckvall, P.-G. Andersson, P. Brandt, Chem. Soc. Rev. 35
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Please cite this article in press as: G. Wienhöfer, et al., Journal of Organometallic Chemistry (2013), http://dx.doi.org/10.1016/
j.jorganchem.2013.06.010