A molecularly defined iron-catalyst for the selective hydrogenation of α,β-unsaturated aldehydes

Article abstract of DOI:10.1002/chem.201300660

A selective iron-based catalyst system for the hydrogenation of α,β-unsaturated aldehydes to allylic alcohols is presented. Applying the defined iron-tetraphos complex [FeF(L)][BF₄] (L=P(PhPPh ₂)₃) in the presence of trifluoroacetic acid a broad range of aldehydes are reduced in high yields using low catalyst loadings (0.05-1 mol %). Excellent chemoselectivity for the reduction of aldehydes in the presence of other reducible moieties, for example, ketones, olefins, esters, etc. is achieved. Based on the in situ detected hydride species [FeH(H ₂)(L)]⁺ a catalytic cycle is proposed that is supported by computational calculations. Copyright

Full text of DOI:10.1002/chem.201300660

COMMUNICATION
DOI: 10.1002/chem.201300660
A Molecularly Defined Iron-Catalyst for the Selective Hydrogenation of
a,b-Unsaturated Aldehydes
Gerrit Wienhçfer,[a] Felix A. Westerhaus,[a] Kathrin Junge,[a] Ralf Ludwig,[b] and
Matthias Beller*[a]
Abstract: A selective iron-based catalyst system for the hydrogenation of a,b-unsa-
turated aldehydes to allylic alcohols is presented. Applying the defined iron–tetra-
phos complex [FeF(L)]ACTHNUTRGENN[UG BF4] (L=PAHCTUNGTREN(NGUN PhPPh2)3) in the presence of trifluoroacetic
Keywords: alcohols · aldehydes ·
acid a broad range of aldehydes are reduced in high yields using low catalyst load-
ings (0.05–1 mol%). Excellent chemoselectivity for the reduction of aldehydes in
the presence of other reducible moieties, for example, ketones, olefins, esters, etc.
is achieved. Based on the in situ detected hydride species [FeH(H2)(L)]+ a catalyt-
ic cycle is proposed that is supported by computational calculations.
hydrogenation
phosphines
· iron catalysis ·
Introduction
The reduction of aldehydes, especially a,b-unsaturated sub-
strates, is of particular interest for the production of flavors,
Scheme 1. Reduction of cinnamaldehyde to cinnamyl alcohol and
possible side products.
fragrances, and pharmaceuticals.[1] Typically, metal hydrides
such as sodium borohydride and lithium aluminum hydride
as well as other stoichiometric reagents are used for this
transformation on a laboratory scale. Obviously, the stoi-
chiometric amount of waste produced in these reactions is
not acceptable today. Catalytic hydrogenation is a more
benign method as theoretically no byproducts are formed.[2]
In general, heterogeneous catalysts are preferred in industry
for hydrogenation because they are facile to separate and
often can be recycled. However, often they show lower func-
tional-group tolerance relative to molecular-defined cata-
lysts, especially for other reducible moieties. Moreover, the
required harsher reaction conditions might lead to low
Here, the challenge is the selective reaction of the carbon-
yl group in the presence of the polarized C–C double bond
as the hydrogenation of the latter is thermodynamically fa-
vored.[3] Additional problems that can occur for aldehydes
are CHa-bond activation[4] followed by decarbonylation[5] or
Tishchenko-type dimerization.[6] So far, mainly catalyst sys-
tems based on precious metals, for example, Ir,[7] Rh,[8] Ru,[9]
Os,[10] and Cu[11] were used for this transformation, usually
with only moderate success. An important breakthrough
was the Ru-catalyzed reduction developed by Noyori and
co-workers.[9c] Here, full conversion was achieved in a short
reaction time (0.5 h) and with low catalyst concentration
(0.2 mol%). Unfortunately, only a narrow substrate scope
for aldehydes was presented and the catalyst was shown to
reduce other sensitive moieties, such as ketones, too.
Until very recently, the development of most hydrogena-
tion catalysts was based on noble metals. Hence, a major
goal in current catalysis research is the replacement of such
metals by more easily available biorelevant elements, which
offer economic and ecological advantages. In this respect,
homogeneous catalysis with iron complexes is especially at-
tractive,[12] due to the availability, price, and low toxicity of
the metal. Despite significant research efforts on the devel-
opment of structurally diverse iron catalysts for reduc-
tions,[13,14] to the best of our knowledge only two systems are
reported to catalyze the hydrogenation of aldehydes.[15] Al-
ready in 1983, Markꢀ et al. applied iron pentacarbonyl as a
catalyst precursor. Later on, Casey and Guan used the so-
chemoACHTUNGTRENNUNGselectivity. Alternatively, homogeneous catalysts usu-
ally work under milder reaction conditions. They can be
easily tuned by the choice of the metal and the ligand to
control all kinds of selectivity aspects.
The benchmark reaction for the development of new cata-
lyst systems for the hydrogenation of a,b-unsaturated sub-
strates is the reduction of cinnamaldehyde to the corre-
sponding unsaturated alcohol (Scheme 1).
[a] G. Wienhçfer, F. A. Westerhaus, Dr. K. Junge, Prof. M. Beller
Leibniz-Institut fꢁr Katalyse e.V. an der Universitꢂt Rostock
Albert-Einstein-Str. 29a, 18059 Rostock (Germany)
Fax : (+49)381-1281-5000
E-mail: matthias.beller@catalysis.de
[b] Prof. R. Ludwig
Universitꢂt Rostock, Institut fꢁr Chemie,
Dr.-Lorenz-Weg 1, 18059 Rostock (Germany)
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201300660.
Chem. Eur. J. 2013, 19, 7701 – 7707
ꢃ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
7701

Table 1. Iron-catalyzed hydrogenation of cinnamaldehyde.[a]
called Knçlker complex,[16] for the hydrogenation of mainly
ketones under mild conditions. With respect to the synthesis
of allylic alcohols both iron complexes were not selective
for the hydrogenation of a,b-unsaturated aldehydes.
For some time our group was interested in developing
iron-catalyzed reduction methods.[17] For example, we re-
ported an in situ generated iron–tetraphos system for the
transfer hydrogenation of nitro arenes[18] and terminal al-
kynes.[19] Based on this work, here we demonstrate for the
first time the highly selective iron-catalyzed hydrogenation
of a,b-unsaturated aldehydes and other aldehydes bearing
additional reducible groups as well.
Entry
Catalyst
Conv.
[%][b]
Yield
[%][b]
1
2
3
Fe
Fe
L
U
11
–
–
13
12
23
63
–
11
–
–
2
1
–
58
–
4[c]
5[c]
6[c]
7[c]
8[c]
9[c]
acetic acid/Fe
benzoic acid/Fe
p-toluenesulfonic acid/Fe
TFA/Fe(BF4)2·6H2O/L
phosphoric acid/Fe(BF4)2·6H2O/L
tetrafluoroboric acid (aq., 40%)
G
E
G
R
AHCTUNGTRENNUNG
Results and Discussion
10
–
Fe
Fe
ACHUTNGERN[NUG FeF(L)]ACHTGNUTRENNUNG
ACHTUNGTRENNUNG
10[d]
11[d]
A
>99
>99
92
97
Recently, we presented the straightforward synthesis of the
novel iron(II) fluoro
ACHTUNGTRENNUNG
phino}tetrafluoroborate complex (1, [FeF(L)]
AHCTUNGTRENNUNG
[a] Reaction conditions: cinnamaldehyde (0.5 mmol), catalyst (0.3 mol%,
M/L 1:1), 1208C, 20 bar H2, 2 h, THF (1.5 mL). [b] Determined by GC
analysis using n-hexadecane as an internal standard. [c] cat.: Fe-
AHCTNUGTREN(GNNU BF4)2·6H2O (0.3 mol%)+L; acid (5 mol%); iPrOH (1.5 mL). [d] TFA
(5 mol%).
complex allows for the hydrogenation of CO2 and bicarbon-
ates in good yields with high catalyst productivity and activi-
ty under basic conditions (Scheme 2).[20]
solvents at a comparably low catalyst concentration of only
0.1 mol% of Fe (Table 2). Under these conditions, polar
protic solvents showed decreasing conversions as the length
Table 2. Solvent screening for the catalytic hydrogenation of cinnamalde-
hyde.[a]
Scheme 2. Iron-catalyzed hydrogenation of CO2.
To prove the versatility of this active species, initially we
tested the in situ generated complex for the model hydroge-
nation of cinnamaldehyde (Table 1, entry 1). Indeed, a nota-
ble conversion of 11% and high chemoselectivity was ob-
tained. On the other hand, the metal precursor Fe-
Entry
Solvent
Conv. [%][b]
Yield [%][b]
Selec. [%]
1
2
MeOH
EtOH
iPrOH
tAmOH
THF
91
69
82
15
14
–
29
24
72
15
14
–
32
35
88
>99
>99
–
3[c]
4
5
6
ACHTUNGTRENNUNG
toluene
AHCTUNGTRENNUNG
[a] Reaction conditions: cinnamaldehyde (0.5 mmol), [FeF(L)]ACHTUNGTRENNUNG[BF4]
tivity (entries 2–3). To improve the formation of the
assumed active iron hydride species various acidic co-cata-
lysts were added to the reaction mixture (entries 4–9).
(0.1 mol%), TFA (5 mol%), 1208C, 20 bar H2, 2 h, solvent (1.5 mL).
[b] Determined by GC analysis using n-hexadecane as an internal stand-
ard. [c] TFA (2 mol%).
While acetic and benzoic acid as well as p-toluenesulfonic
acid showed very low or no desired product yield, the addi-
tion of catalytic amounts of trifluoroacetic acid (TFA) led to
a significant increase of cinnamyl alcohol (58%; Table 1,
entry 7). Switching from isopropyl alcohol (iPrOH) as the
solvent to tetrahydrofuran (THF) led to quantitative conver-
sion and excellent yield (92% yield; entry 10)! For the de-
of their aliphatic chain increased (Table 2, entries 1–4). Ace-
tals were the main product when methanol and ethanol
were applied as the solvents. In contrast, tert-amyl alcohol
(tAmOH) showed no formation of acetals, but the conver-
sion was significantly lower. Best yields were obtained for
iPrOH, for which a high reactivity was observed with little
formation of the acetal. In THF, a nonprotic polar solvent,
the catalytic system was also active, whereas the nonpolar
toluene does not exhibit any reactivity (entries 5–6). Hence,
best activity at low catalyst loading was observed with
iPrOH as the solvent.
fined complex [FeF(L)]ACHTNUTRGNENU[G BF4] comparable results to the
in situ generated system were obtained (entry 11). Remark-
ably, in the latter reactions only cinnamyl alcohol was formed
and the conjugated double bond remained unaffected.
Next, we started to investigate the influence of the reac-
tion parameters in more detail. Here, we tested different
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Selective Hydrogenation of a,b-Unsaturated Aldehydes
COMMUNICATION
At this point it should be noted that catalytic hydrogena-
tions of carbonyl compounds are typically performed under
neutral or slightly basic conditions. Our iron catalyst shows
an unusual significant increase of activity in the presence of
acid. Therefore, we examined the influence of the acid con-
centration more carefully (Figure 1). Without any addition
hol was obtained (Table 3, entry 1). Raising the temperature
to 1208C and further to 1408C gave a yield of 72 and 93%,
respectively (entries 2–3). This demonstrates also the high
thermal stability of the active iron species. Under the opti-
mized reaction parameters full conversion was achieved at a
catalyst loading of only 0.05 mol%. Thus, a catalyst turnover
number of up to 2000 was observed, which is even superior
to other precious metal-based systems (entry 4).[9c,11]
To examine the substrate scope, we hydrogenated eight
different a,b-unsaturated aldehydes (Scheme 3). To suppress
potential side reactions, the catalytic experiments were per-
formed at 1208C and 20 bar hydrogen pressure in the pres-
ence of 0.2–0.4 mol% iron catalyst. Under these conditions,
all substrates were quantitatively reduced within few hours
to their corresponding alcohols. In none of the reactions did
the reduction of the C–C double bond or any other side re-
action occur to a significant extent.
Figure 1. Conversion of cinnamaldehyde in dependence of the amount of
TFA added (reaction conditions: 0.5 mmol substrate, 0.1 mol% catalyst,
20 bar H2, 1208C, 2 h, iPrOH).
For the cinnamaldehyde derivatives, the position of the
substituent does not affect the reactivity. Also a,b-unsaturat-
of TFA, the conversion is low
and only traces of the alcohol
are detected. However, upon
adding 2 mol% of TFA with re-
spect to the substrate the reac-
tivity dramatically increased. A
further raise of the acid concen-
tration resulted in decreasing
reactivity. Thus, 2 mol% of
TFA was used for the following
catalytic tests.
Examination of the influence
of the hydrogen pressure[21] re-
vealed an increase of cinnamyl
alcohol with increasing pres-
sure. Hence, the product yield
rose from 18 to 39 to 72 to
Scheme 3. Iron–tetraphos-catalyzed hydrogenation of a,b-unsaturated aldehydes (0.5 mmol). GC yields are
shown, isolated yields are in brackets.
95% going from 5 bar hydrogen to 10, 20, and 40 bar, re-
spectively. Finally, the reaction temperature was varied
(Table 3). At 1008C a moderate yield of 31% cinnamyl alco-
ed aldehydes without any aryl substituents, such as citral,
perillaldehyde, or trans-2-decenal are quantitatively
reduced.
In addition to unsaturated aldehydes various benzalde-
hydes bearing different functional groups were tested
(Table 4). Simple benzaldehydes with alkyl and halogen sub-
stituents were converted to their alcohols in quantitative
yields (Table 4, entries 1–8). For 2-fluoro-benzaldehyde the
solvent had to be changed to THF because the formation of
acetales could not be suppressed in iPrOH even at higher
catalyst loadings (entry 6). Next, more challenging benzalde-
hydes with other reducible substituents were investigated
(entries 9–14). Substrates bearing ketone, vinyl, ether, thio-
ether, and ester groups are selectively converted to the re-
spective benzyl alcohols leaving the substituent unaffected.
We believe this high chemoselectivity is remarkable since
specific hydrogenation of aldehydes in the presence of ke-
tones is basically not known. In the case of isophthalalde-
hyde, full conversion to 1,3-benzene-dimethanol took place
Table 3. Variation of the reaction temperature for the hydrogenation of
cinnamaldehyde.[a]
Entry
T [8C]
Conv. [%][b]
Yield [%][b]
Selec. [%]
1
2
100
120
140
140
40
82
97
31
72
93
98
78
88
96
98
3
4[c]
>99
[a] Reaction conditions: cinnamaldehyde (0.5 mmol), [FeF(L)]ACTHNUTRGNE[UNG BF4]
(0.1 mol%), 20 bar H2, 2 h, TFA (2 mol%), iPrOH (1.5 mL). [b] Deter-
mined by GC analysis using n-hexadecane as an internal standard.
[c] [FeF(L)]ACHTUNGTRENNUNG[BF4] (0.05 mol%), 40 bar H2.
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M. Beller et al.
Table 4. Hydrogenation of benzaldehydes and alkyl and heteroaromatic
aldehydes with the iron–tetraphos catalyst.[a]
Table 4. (Continued)
Entry Substrate
Cat. [mol%] Conv. [%][b] Yield [%][b]
20[c]
0.4
>99
99 (94)
[a] Reaction conditions: substrate (0.5 mmol), [FeF(L)]ACHTUNGTERGN[NNU BF4], 1208C,
Entry Substrate
1
Cat. [mol%] Conv. [%][b] Yield [%][b]
20 bar H2, 2 h, TFA (2 mol%), iPrOH (1.5 mL). [b] Determined by GC
analysis by using n-hexadecane as an internal standard; isolated yields
given in brackets. [c] 5 h. [d] THF.
0.4
0.4
>99
>99
98
2
3
99 (95)
(entry 9). Finally, different heteroaromatic and aliphatic al-
dehydes were tested (Table 4, entries 15–20). Again, all sub-
strates including the industrially important citronellal were
fully converted to the desired alcohol with very good selec-
tivity. Further tests showed that alcohols and carboxyl acids
(e.g. benzoic acid) were not reduced and also do not inter-
fere in the reaction. Additionally, the ketimine benzopheno-
neimine was applied, but only hydrolysis to benzophenone
occurred.
To understand the high selectivity of this novel catalyst
system high pressure NMR spectroscopic investigations and
DFT calculations were performed. The resulting mechanistic
proposal is shown in Scheme 4. Initially, the starting com-
0.4
>99
97 (97)
4
0.4
0.4
1.0
0.4
>99
>99
>99
>99
99 (99)
99 (98)
98
5
6[d]
7
99
8
0.4
0.4
0.2
>99
>99
>99
99 (98)
98 (97)
99 (99)
9[c]
10
11[d]
0.4
0.2
>99
>99
99
12[c]
99 (99)
Scheme 4. Proposed catalytic cycle for the iron-catalyzed hydrogenation
of cinnamaldehyde.
13
14
15
0.2
0.2
0.8
>99
>99
>99
99
96
99
plex [FeF(L)]ACTHNUTRGNEUNG[BF4] is hydrogenated in two consecutive
steps: First, complex [FeF(L)]+ (1) forms the hydrogenated
species [FeH(L)]+ (2) under release of hydrogen fluoride. In
a second step, the cation [FeH(H2)(L)]+ (3) is formed by an
additional uptake of hydrogen. In contrast to 1 and 2, spe-
cies 3 is diamagnetic and was detected by NMR spectrosco-
16
17
18
0.4
0.6
0.4
>99
>99
>99
99
99
1
py showing a H NMR spectroscopic signal at d=À9.18 ppm
and two 31P NMR signals at d=142.9 (quartet) and
87.6 ppm (doublet). For all species the counter-ion is weakly
coordinated and thus does not interact in the catalytic cycle
as we have shown before for the transfer hydrogenation of
nitroarenes.[18] Then, cinnamaldehyde coordinates to 3 and is
reduced to cinnamyl alcohol. The product is released and 2
is subsequently rehydrogenated to generate 3. In situ NMR
spectroscopic investigations at 808C and 20 bar of H2
showed that 3 is formed when no substrate is present. After
95 (95)
19
0.6
>99
99
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Selective Hydrogenation of a,b-Unsaturated Aldehydes
COMMUNICATION
the addition of cinnamaldehyde the signal disappears. Once
full conversion is achieved the signal of 3 is observed again.
Notably, the observed complex 3 has a sixfold coordina-
tion and therefore in principle no free coordination site for
the substrate. However, Bianchini et al. proposed in their
mechanistic investigation of the related iron-catalyzed hy-
drogenation of alkynes that one phosphorus arm of the
ligand dissociates to leave free space for the substrate.[22] We
assume that this essential step is catalyzed by the acid. To
have a closer look into the role of acid in this reaction, we
calculated the relative energies and the Gibbs energies of
the intermediates with and without the addition of acid
(Scheme 5). As expected, the dissociation of one phosphorus
atom raises the energy level of the corresponding intermedi-
ates. For the acid-free route, the relative energy of inter-
mediate 4 is significantly higher relative to the protonated
intermediate 7 and represents the highest energy level of
the acid-free reaction cycle. Similarly, for the acid-catalyzed
pathway the relative energy of species 7 presents the highest
energy level, but is significantly lower compared to 4. Thus,
we propose that the acid-catalyzed mechanism is energeti-
cally favored compared to the acid-free route. The results of
the calculated Gibbs energies agree well with these findings.
Here, the acid-free route is unlikely as the pathway from 3
over 4 to 5 shows a total increase in the free enthalpy of
93.4 kJmolÀ1. In contrast, the dissociation of one phosphorus
atom and addition of the substrate in the acid-catalyzed
route is only slightly exothermic. Hence, the addition of acid
to the reaction system should promote the reaction, which is
in agreement with our experimental observations.
Conclusion
In summary, we have developed the first highly selective
iron-catalyzed hydrogenation of a,b-unsaturated aldehydes
to give synthetically valuable al-
lylic alcohols. The defined iron
complex [FeF(L)]ACHTUNRGTNEUNG[BF4] is also
able to catalyze the reduction
of a broad range of aromatic
and aliphatic aldehydes in the
presence of other reducible
moieties. In general, the hydro-
genations proceed in quantita-
tive yields at low catalyst load-
ings (0.05–1 mol% Fe) and nei-
ther the C–C double bond is at-
tacked nor side reactions from
a CHa-bond activation are ob-
served. In contrast to the major-
ity of known hydrogenation cat-
alysts, our catalyst system is
only active under acid condi-
tions.
Experimental Section
General: All reactions were carried
out under an argon atmosphere.
Chemicals were purchased from Al-
drich, Fluka, Acros, Alfa Aesar and
used without further purification.
NMR spectroscopic data were record-
ed on Bruker ARX 400 and Bruker
ARX 300 spectrometers. 13C and
1H NMR spectra were referenced to
signals of the deutero solvents. Gas
chromatography analysis was per-
formed on an Agilent HP-6890 instru-
ment with a FID detector and HP-5
capillary column (polydimethylsilox-
ane with 5% phenyl groups, 30 m,
0.32 mm i.d., 0.25 mm film thickness)
by using argon as the carrier gas. Gas
chromatography/mass analysis was car-
ried out on an Agilent HP-5890 instru-
Scheme 5. Proposed mechanism and the role of the acid in the catalytic cycle. Relative energies of complexes
and the Gibbs energy (kJmolÀ1) are calculated at the B3PW91/6-311G** level of theory. Addition of polariza-
tion functions including hydrogen.
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7705

M. Beller et al.
ment with an Agilent HP-5973 Mass Selective Detector (EI) and HP-5
capillary column (polydimethylsiloxane with 5% phenyl groups, 30 m,
0.25 mm i.d., 0.25 mm film thickness) by using helium as the carrier gas.
HRMS was performed on MAT 95XP (EI) and Agilent 6210 Time-of-
Flight LCMS (ESI). All hydrogenation reactions were carried out in an
eightfold parallel reactor array with a reactor volume of 3.0 mL (Institute
of Automation (IAT), Richard Wagner Str. 31/H. 8, 18119 Rostock-War-
nemꢁnde, Germany). The in situ NMR spectroscopic experiments were
performed in a sapphire tube[23] on a BRUKER Avance 400 spectrometer
by using a broadband observe probe, which was properly tuned and
matched each time. The integrals of the different spectra were measured
and calibrated by using standard NMR software (i.e. Topspin version 1.3,
BRUKER, Germany).
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General procedure for the hydrogenation of cinnamaldehyde: A stem
solution (1.5 mL) consisting of dry iso-propanol, [FeF(L)]ACTHNUTRGNE[UNG BF4] (1 mg,
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0.2 mol%) and trifluoroacetic acid (1 mL, 2 mol%) were placed in a auto-
clave under an argon atmosphere. Cinnamaldehyde (0.5 mmol, 63 mL)
was added via a syringe. Next, the argon atmosphere was replaced by
20 bar of hydrogen and the autoclave was heated to 1208C for 2 h while
the solution was continuously stirred. After cooling to room temperature,
n-hexadecane (100 mL) was added as an internal standard for GC analy-
sis. A sample was taken and subjected to GC analysis. Conversion and
yields are determined by comparison with authentic samples. For isola-
tion, no internal standard was added. Instead, the solution was filtered
through a pad of silica flushed with ethyl acetate. The solvent was
removed in vacuo.
Acknowledgements
This work was supported by the state of Mecklenburg-Vorpommern, the
BMBF, and the Deutsche Forschungs gemeinschaft (Leibniz prize to
M.B.).
Keywords: alcohols · aldehydes · hydrogenation · iron
catalysis · phosphines
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Chem. Eur. J. 2013, 19, 7701 – 7707

Selective Hydrogenation of a,b-Unsaturated Aldehydes
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Received: February 20, 2013
Published online: May 6, 2013
Chem. Eur. J. 2013, 19, 7701 – 7707
ꢃ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
7707