Use of heterogenized metal complexes in hydrogenation reactions: Comparison of hydrogenation and CTH reactions

Article abstract of DOI:10.1007/s11164-015-1995-4

In the fine chemicals industry the preference for heterogeneous over homogeneous catalysts is well known. However, the activity and selectivity of heterogeneous metal complexes are usually better than those of their homogeneous counterparts. There has, consequently, been an increasing demand for heterogenized metal complexes which combine the advantages of both types of catalyst. In the production of fine chemicals, selectivity is probably its most important property of a catalyst. Chemoselectivity, regioselectivity, and enantioselectivity are very important in the synthesis of fine chemicals. Several examples of hydrogenation with heterogenized metal complexes have been studied, with emphasis on selectivity. Catalytic transfer hydrogenation is also discussed, as an alternative to classical hydrogenation.

Full text of DOI:10.1007/s11164-015-1995-4

Res Chem Intermed
DOI 10.1007/s11164-015-1995-4
Use of heterogenized metal complexes in hydrogenation
reactions: comparison of hydrogenation and CTH
reactions
1
1
•
Peter Bata1 Agnes Zsigmond Mihaly Gyemant
´
•
•
´
´ ´
Ambrus Czegledi Peter Kluson2
1
•
´
Received: 20 October 2014 / Accepted: 2 March 2015
ꢀ Springer Science+Business Media Dordrecht 2015
Abstract In the fine chemicals industry the preference for heterogeneous over
homogeneous catalysts is well known. However, the activity and selectivity of
heterogeneous metal complexes are usually better than those of their homogeneous
counterparts. There has, consequently, been an increasing demand for heterogenized
metal complexes which combine the advantages of both types of catalyst. In the
production of fine chemicals, selectivity is probably its most important property of a
catalyst. Chemoselectivity, regioselectivity, and enantioselectivity are very impor-
tant in the synthesis of fine chemicals. Several examples of hydrogenation with
heterogenized metal complexes have been studied, with emphasis on selectivity.
Catalytic transfer hydrogenation is also discussed, as an alternative to classical
hydrogenation.
Keywords Catalytic transfer hydrogenation Á Iron–phthalocyanine catalyst Á
Chemoselectivity
Introduction
During the last decade there have been dramatic developments in the use of
selective catalysis for production of fine chemicals. Among these processes
selective hydrogenation with soluble catalysts has become increasingly important in
the pharmaceutical industry, and, consequently, in catalysis research also. Both the
activity and the selectivity of these soluble complexes are excellent, but the
difficulty of recovering and recycling limit their usefulness in real processes.
´
Agnes Zsigmond
azsig@chem.u-szeged.hu
&
1
2
´ ´
Department of Organic Chemistry, University of Szeged, Dom ter 8, Szeged 6720, Hungary
Institute of Chemical Process Fundamentals, Academy of Sciences of the Czech Republic,
Rozvojova 135, 165 02 Prague, Czech Republic
123

P. Bata et al.
Because of these limitations, there has been increasing interest in the application of
heterogeneous catalysts. Heterogeneous systems can be either modified heteroge-
neous catalysts [1–3] or heterogenized homogeneous complexes [4].
Reduction of the carbonyl functionality of aldehydes and ketones by metal-
catalyzed transfer hydrogenation (CTH) with a suitable hydrogen donor is a
valuable synthetic tool which has proved to be an elegant alternative of
hydrogenation with molecular hydrogen [5]. Such organic molecules as unsaturated
hydrocarbons, primary and secondary alcohols, and formic acid and its salts have
been used as hydrogen sources in CTH reactions [6, 7]. Transition metals, for
example Rh, Ir, Ni, and Pd, and, for carbonyl reduction, Ru complexes and propan-
2-ol in the presence of a base, can catalyze the reduction [8]. Most practically used
homogeneous and heterogeneous catalysts are based on such precious metals as
ruthenium, rhodium, iridium, and palladium [9, 10]. As a result of their restricted
availability, high price, and potential toxicity, the search for more economical and
environmentally benign catalysts based on nonprecious analogues (Fe, Cu, Zn, Mn)
is a real and challenging objective. 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. It should also be highly active with
excellent selectivity toward a broad range of functional groups.
Although reduction catalysts based on iron are highly attractive candidates
meeting these criteria, there are few examples of iron-catalyzed hydrogenation of
carbonyl compounds [11–14]. The first efficient iron system used for hydrogenation
of carbonyl compounds was developed by Casey and Guan in 2007 [15, 16]. By use
of this highly air-sensitive complex a variety of simple aldehydes and ketones were
reduced under mild reaction conditions. Unfortunately, this system had poor
chemoselectivity for industrially important a,b-unsaturated carbonyl compounds.
Milstein et al. [17–19] also developed an iron complex based on their work on
similar ruthenium complexes. Although this catalyst is still the most efficient iron-
based system for reduction of ketones, benzaldehyde was reduced in only 36 %
yield, even with increased catalyst loading, and a,b-unsaturated substrates gave
mixtures of hydrogenated products with moderate yields.
Selective hydrogenation of a,b-unsaturated aldehydes has attracted much interest
in homogeneous catalysis, because of the synthetic value of the corresponding
allylic alcohols [20]. Several good examples of catalysts, for example {RuCl2(-
TPPMS)2}2, (TPPMS = (3-sulfonatophenyl)diphenylphosphane sodium salt) have
been developed and used for hydrogenation of trans-cinnamaldehyde. With this
system, depending on pH, a variety of Ru hydride complexes were established as
active catalysts [21]. At pH \ 6 HRuCl(TPPMS)3 was the dominant Ru(II)
complex, and it catalyzed slow but selective hydrogenation of the C=C bond.
Conversely, at pH [ 8 H2Ru(TPPMS)4 was found to be an active and selective
catalyst for C=O reduction. Consequently, changing the pH of the solution could
shift the equilibrium between the two Ru species and invert the selectivity of
hydrogenation of trans-cinnamaldehyde.
Considering all of these results, and environmental and economic aspects,
heterogenization of these catalysts was performed. NaY zeolite was chosen as
support, because our previous experience has shown that this three-dimensional
123

Use of heterogenized metal complexes in hydrogenation…
zeolite is suitable for heterogenization of soluble complexes [22], either by
synthesis of the complex inside the zeolite or by anchoring it to the surface [23]. The
complexes RuCl2(PPh3)3 and {RuCl2(TPPMS)2}2 were heterogenized, and the
immobilized catalysts were used for hydrogenation of a,b-unsaturated aldehydes.
As continuation of this work we studied the CTH reaction of a,b-unsaturated
aldehydes on heterogenized complexes. In this contribution we wish to emphasize
the importance of selectivity, i.e. the type of selectivity in CTH reactions, and
whether it is possible to change this selectivity by changing the conditions. We also
wished to develop an iron-based, state-of-the-art hydrogenation catalyst prepared
from inexpensive materials and easy to synthesize and convenient to handle.
Experimental
Materials: preparation of the catalysts
1
2
RuCl2(PPh3)3 was purchased from Aldrich and used as received. {RuCl2(-
TPPMS)2}2 was prepared from TPPMS and RuCl3Á3H2O as described in the
literature [24]. The purity of the TPPMS ligand was checked by NMR
spectroscopy, by use of a Bruker Avance DRX 500 MHz instrument. NMR
spectroscopy showed that the sulfonated triphenylphosphane has one SO3-group
in position 3.
Iron phthalocyanine, Fe(Pc), was prepared under solvent-free conditions, by use
of microwave irradiation [22]. Iron(II) chloride (2.5 g, 0.02 mol) and
phthalonitrile (10.25 g, 0.08 mol) were used for the preparation; details are
reported elsewhere [25]. The final catalyst was a deep blue fine power
Anchoring the soluble complexes
NaY zeolite (Aldrich; 1.5 g) was suspended in 30 mL 96 % ethanol. Phospho-
tungstic acid hydrate (PTA; 288.0 mg, 0.1 mmol) was dissolved in 25 mL ethanol
and this solution was added dropwise into the zeolite suspension with efficient
stirring. The stirring was continued for 2 days at room temperature, under an Ar
atmosphere. The mixture was filtered and the solid residue was suspended in 30 mL
ethanol. RuCl2(PPh3)3 (96 mg, 0.1 mmol) was dissolved in 40 mL deoxygenated
ethanol and this solution was added slowly, with stirring, to the suspension. Stirring
was continued for another 2 days. The mixture was filtered and the residues was
washed with ethanol until a colorless solution was obtained. The light brown solid
material, 1.6 g, was dried at 30 ꢁC for 2 h under vacuum and for 1 day under argon,
affording 1.45 g catalyst with an Ru content of 36.3 lmol/g.
Heterogenized {RuCl2(TPPMS)2}2 catalyst was prepared analogously from
90.1 mg (0.1 mmol Ru) {RuCl2(TPPMS)2}2 complex by use of 1.5 g NaY zeolite
and 0.1 mmol PTA, affording 1.5 g heterogenized catalyst with an Ru content of
2.0 lmol/g.
123

P. Bata et al.
Because the same heterogenization process was used this is a relatively large
difference. Our explanation for this difference is that the neutral RuCl2(PPh3)3 is
more easily anchored to PTA layers containing large polytungstate anions than the
bulky {RuCl2(TPPMS)2}2 which carries negatively charged sulfonated ligands.
The immobilized Fe(Pc) complex was prepared similarly from 1.5 g Al2O3,
386.7 mg (0.15 mmol) PTA, and 56.8 mg (0.1 mmol) Fe(Pc) complex; 1.5 g light
brown catalyst was obtained. The anchored catalyst was found to contain 35.8 lmol
Fe/g catalyst.
Catalyst characterization
FTIR spectra of the support, the Ru complexes, and the heterogenized samples, as
KBr pellets, were recorded in the range 400–4000 cm-1 by use of a Bio-Rad FTS–
65 A spectrophotometer. XRD spectra were obtained by use of a Philips PW-1830
diffractometer.
Because, during preparation of immobilized catalyst, the uncomplexed ligand
was removed by extraction, the metal content of the sample gave us the complex
content. The metal content of the anchored catalysts was determined by use of a
Jobin–Yvon 24 type ICP–AES instrument; 250 mg catalyst samples were dissolved
in 4 mL conc. HNO3.
Hydrogenation experiments
trans-Cinnamaldehyde was hydrogenated in a 30-mL batch reactor at 65 ꢁC and
0.4 MPa hydrogen pressure [26]. For hydrogenation of the C=C bond, acidic
conditions were used. RuCl2(PPh3)3 (10 mg, 10.42 lmol) or RuCl2(PPh3)3–NaY
(300 mg, 10.89 lmol Ru) was added to 3 mL 96 % ethanol, followed by 3 lL
(20 lmol) Et3N and 7.8 mg (30 lmol) PPh3. The catalyst precursors were
prehydrogenated overnight, then 50 lL substrate (0.396 mmol trans-cinnamalde-
hyde) was injected and the reactor was pressurized with H2 and the reaction was
initiated by starting the stirrer. For C=O reduction we used basic conditions, with
the same amount of Ru precursor, 13.5 mg (0.05 mmol) PPh3, and 6 lL
(0.04 mmol) Et3N. Samples were taken every hour from the reaction mixture,
and the products were analyzed by capillary gas chromatography (Hewlett Packard
5890 Series II) with a DB-5 column at 145 ꢁC. Reactions with TPPMS-containing
catalysts were conducted in the same way with 12.3 mg (0.03 mmol) or 20.5 mg
(0.05 mmol) TPPMS for the two different conditions.
Catalytic transfer hydrogenation
In a 5-mL Schlenk tube 0.0038 mmol Fe(Pc) catalyst was added to 1 mL propan-2-
ol and the tube was purged with Ar. Sodium hydroxide (0.19 mmol) in 0.5 mL
propan-2-ol and 0.38 mmol trans-cinnamaldehyde in 0.5 mL propan-2-ol were
injected into the reaction vessel. The reaction mixture was heated for 6 or 12 h at
80 ꢁC with efficient stirring. The solution was cooled to room temperature and
isolated by filtration through a plug of silica. Conversion was measured by GC
123

Use of heterogenized metal complexes in hydrogenation…
without further manipulations and the products were identified by use of
chromatographic standards or by GC–MS.
Catalyst recycling
The heterogenized catalysts were used in several subsequent runs. After the
reactions the catalyst was recovered by filtration under Ar, washed with ethanol,
dried in Ar, and then reused.
Results and discussion
With the objective of developing active, chemoselective, heterogeneous catalysts,
we prepared anchored {RuCl2(TPPMS)2}2 and RuCl2(PPh3)3 catalysts and used
these for hydrogenation of trans-cinnamaldehyde [26]. Fe(Pc), a new, efficient,
heterogeneous catalyst for catalytic transfer hydrogenation of carbonyl function-
ality, was also developed [27]. The heterogenized version of this complex was
prepared and used for CTH of trans-cinnamaldehyde. In this contribution we
compare the performance of the two systems, with emphasis on selectivity.
Catalyst characterization
The heterogenized catalysts were characterized by use of conventional spectro-
scopic methods. The FTIR spectra of the support (PTA–NaY), the {RuCl2(-
TPPMS)2}2 and RuCl2(PPh3)3 complexes, and the heterogenized samples were all
recorded. Comparison of these spectra shows convincingly that RuCl2(PPh3)3 is
anchored to the NaY support. The spectrum of the heterogenized catalyst contains
Fig. 1 The FTIR spectra of Fe(Pc), the support Al2O3, and the heterogenized complex
123

P. Bata et al.
bands, at 1493, 1440, 1076 cm-1, which are characteristic of the RuCl2(PPh3)3
complex.
Similar spectra were obtained for {RuCl2(TPPMS)2}2 and its heterogenized
analog. Comparison of the spectra leads to the same conclusion as for RuCl2(PPh3)3.
Similar characterization was performed for the Fe(Pc) complex (Fig. 1). The
complex and the heterogenized catalyst were characterized by FTIR spectroscopy
and the spectra contained characteristic bands (1515, 1494, 1333, 1164, 1118, and
752 cm-1) for (Pc).
To determine the metal complex concentration of the heterogenized catalysts,
samples were dissolved in conc. HNO3 and the metal content of these solutions was
determined by ICP–AES. The anchored RuCl2(PPh3)3 and {RuCl2(TPPMS)2}2 solid
catalysts were found to contain 36.3 lmol/g and 2.0 lmol/g Ru, respectively. The
metal content of the anchored Fe(Pc) was 35.8 lmol Fe/g catalyst.
Hydrogenation of trans-cinnamaldehyde
The properties of the heterogenized catalysts, {RuCl2(TPPMS)2}2–PTA–NaY and
RuCl2(PPh3)3–PTA–NaY, were investigated in the hydrogenation of trans-cin-
namaldehyde. When the homogeneous complexes were used under H2 a pH-
dependent equilibrium was observed between the monohydrido HRuCl(TPPMS)3
and dihydrido H2Ru(TPPMS)4 derivatives. These two Ru-hydrides have different
selectivity in the hydrogenation of a,b-unsaturated aldehydes. The monohydrido
derivative hydrogenates the C=C double bond whereas the dihydrido species is a
selective catalyst for hydrogenation of the C=O double bond [15, 16]. Consequently,
simply adjusting the pH of the aqueous phase can change the selectivity of the
hydrogenation of trans-cinnamaldehyde.
Selective hydrogenation of C=O bonds
For the hydrogenation reaction we used two slightly different conditions, on the
basis of the results obtained under homogeneous conditions [21]. The pH of our
ethanolic solutions was adjusted by addition of Et3N. For the C=O reduction, basic
conditions were used (discussed in the ‘‘Experimental’’ section). The catalyst
mixture was prehydrogenated for sufficient time to ensure the formation of the
O
O
Ph
CH2
CH2
Sald
C
C
H
H
H
H
C
C
CH2
Ph
CH2
Salc
CH2OH
Ph
Uald
CH2OH
H
H
C
C
Ph
Ualc
Scheme 1 The hydrogenation reaction of trans-cinnamaldehyde
123

Use of heterogenized metal complexes in hydrogenation…
catalytically active Ru hydrides. The products of the reaction are shown in
Scheme 1 (no other products were obtained during the hydrogenation reaction); the
results are listed in Table 1.
It is apparent that the heterogenized catalysts—both sulfonated and nonsulfonat-
ed—were active in the hydrogenation of trans-cinnamaldehyde in alcoholic
solution. Comparison of the TOF of the heterogenized and homogeneous catalysts
revealed the former had approximately the same or higher activity than the latter.
The higher activity of 2a–NaY is in a good agreement with our previous findings
[28], i.e. that use of heterogenized catalysts resulted in approximately the same or
higher rate of reaction than their homogeneous counterparts, leading to higher
specific activity than that of the homogeneous analogs.
There are important requirements when heterogenizing soluble complexes. The
immobilized complex should be as active and selective as its homogeneous
counterpart, which is not always observed. Sometimes, however, the performance of
the immobilized complexes is much better than that of the homogeneous analogs.
Several explanations of these phenomena are available in the literature. One
example is site-isolation theory, which explains the greater activity on the basis of
separation of molecules of the complex, because some complexes form dimers
which are not catalytically active [29]. If the heterogenized complex is a ‘‘ship-in-a-
bottle’’ type catalyst, the usual explanation is to suppose the entactic state theory,
which asserts that distorted geometry causes the higher activity [30]. Thomas et al.
[31] established the confinement concept theory, which postulates more interaction
between the complex and the reactants if the complex is immobilized.
Because our immobilized complexes are anchored, either the ‘‘site isolation’’ or
‘‘confinement concept’’ could be a good explanation.
Table 1 The product distribution from hydrogenation of trans-cinnamaldehyde on homogeneous and
heterogenized Ru–phosphane catalysts under basic conditions
Catalyst
Conversion (%)
TOF (h-1
)
Products distribution (%)
Sald
Salc
Ualc
H2Ru(TPPMS)4
H2Ru(TPPMS)4–NaY
H2Ru(PPh3)3
68.4
15.0
93.9
92.3
8.5
38.3
11.7
11.9
9.1
12.0
5.5
0
90.9
88.0
82.9
85.1
0
11.6
11.9
H2Ru(PPh3)3–NaY
3.0
Reaction conditions: 0.4 MPa H2, 65 ꢁC, 0.05 mmol PPh3 or TPPMS, 0.04 mmol Et3N, 0.396 mmol
substrate, 10.28 lmol 2a or 0.6 lmol 2a–NaY, 10.42 lmol 2b, 10.89 lmol 2b–NaY. 2a = H2-
Ru(TPPMS)4, 2b = H2Ru(PPh3)3
Definition of product distribution: the products formed, not including the starting material
Yield = conversion. Selectivity: if the products were A ? B, selectivity for A = A/(A ? B)
Abbreviations: Sald = saturated aldehyde (3-phenylpropanal), Salc = saturated alcohol (3-phenyl-
propan-1-ol), Ualc = unsaturated alcohol (3-phenylprop-2-en-1-ol)
123

P. Bata et al.
Under the conditions described above, all the catalysts have fairly good
selectivity for C=O hydrogenation. In other words, the synthetically important
product, the unsaturated alcohol, is formed in high yield.
Selective hydrogenation of C=C bonds
Under acidic conditions, selective hydrogenation of C=C was expected. Table 2
shows the results obtained.
Table 2 shows that all the catalysts were active in the hydrogenation reaction
under these conditions also. Similar to C=O hydrogenation, the specific activity of
the heterogenized catalysts was approximately the same or higher than that of the
homogeneous catalysts. Both sulfonated catalysts and the heterogenized nonsul-
fonated catalyst had some selectivity in favor of hydrogenation of C=C bond, with
3-phenylpropanal (the saturated aldehyde) as the major product. With 1b, however,
C=O hydrogenation remained predominant, even under acidic conditions.
Catalytic transfer hydrogenation of aldehydes
With the final objective of developing a new, efficient, heterogeneous catalyst for
catalytic transfer hydrogenation of carbonyl groups we prepared the Fe(phthalo-
cyanine) complex, Fe(Pc), and the heterogenized version of it. Iron complexes under
homogeneous conditions usually used in oxidation reactions; their applicability in
reductions has been rather neglected in the past. We also wished to examine the
possibility of replacing the generally applied rare transition metal catalysts with the
Fe(Pc) complex.
In this work we also wished to optimize the reaction conditions for the CTH
reaction, using benzaldehyde as starting material. When the optimum conditions had
been determine, differently substituted aldehydes were then studied; the results are
listed in Table 3.
Table 2 Product distribution in the hydrogenation of trans-cinnamaldehyde by use of homogeneous and
heterogenized Ru–phosphane catalysts under acidic conditions
Catalyst
Conversion (%)
TOF (h-1
)
Product distribution (%)
Sald
Salc
Ualc
HRuCl(TPPMS)3
HRuCl(TPPMS)3–NaY
HRuCl(PPh3)3
54.5
30.6
25.5
32.7
8.5
66.7
3.6
62.2
58.5
41.6
53.9
9.5
19.8
6.7
28.3
21.7
51.8
35.7
HRuCl(PPh3)3–NaY
3.9
10.4
Reaction conditions: 0.4 MPa H2, 65 ꢁC, 0.03 mmol PPh3 or TPPMS, 0.02 mmol Et3N, 0.396 mmol
substrate, 10.28 lmol 1a or 0.6 lmol 1a–NaY, 10.42 lmol 1b, 10.89 lmol 1b–NaY.
1a = HRuCl(TPPMS)3, 1b = HRuCl(PPh3)3
Abbreviations: Sald = saturated aldehyde (3-phenylpropanal), Salc = saturated alcohol (3-phenyl-
propan-1-ol), Ualc = unsaturated alcohol (3-phenylprop-2-en-1-ol)
123

Use of heterogenized metal complexes in hydrogenation…
Table 3 Fe(Pc)-catalyzed CTH reaction of different aldehydes
Entry
Starting material
Product
Conversion (%)
1.
2.
3.
4.
5.
6.
7.
Benzaldehyde
Benzyl alcohol
[99
[99
91
2-MeO-benzaldehyde
4-NO2-benzaldehyde
3-NO2-benzaldehyde
2-NO2-benzaldehyde
4-Cl-benzaldehyde
2,4-diMeO-benzaldehyde
2-MeO-benzyl alcohol
4-NO2-benzyl alcohol
3-NO2-benzyl alcohol
2-NO2-benzyl alcohol
4-Cl-benzyl alcohol
2,4-diMeO-benzyl alcohol
57
25
[99
98
Reaction conditions: 4 h, 80 ꢁC, 2 mL i-propanol, 0.0038 mmol Fe(Pc), 0.38 mmol aldehyde,
0.095 mmol NaOH
As Table 3 clearly shows, all the substituted benzaldehyde derivatives were
active in this reaction. Excellent conversion was achieved for benzaldehydes
substituted with electron-donating groups, for example MeO and diMeO (Table 3,
entries 2 and 7); almost total conversion was even obtained for 4-Cl-benzaldehyde.
Substitution with electron-withdrawing substituents, for example NO2, reduced the
conversion. The smallest decrease was observed for substitution at the p position
and the highest for substitution at the o position (Table 3, entries 3–5). We explain
this phenomenon on the basis of an ionic type of hydrogenation mechanism. The
positively charged carbon atom of the formyl group and the positively charged
nitrogen atom of the nitro group are repelled by each other, so the activation energy
is highest if the NO2 group is in the o position and the lowest if it is in the p position.
With the heterogenized catalyst we used the same conditions as for the CTH
reaction under homogeneous conditions. This study, also, started with optimization
of the reaction conditions; results for selected aldehydes under the optimized
conditions are listed in Table 4.
Table 4 Fe(Pc)–Al2O3 in CTH of differently substituted aldehydes
Entry
Starting material
Product
Conversion (%)
1.
2.
3.
4.
5.
6.
7.
Benzaldehyde
Benzyl alcohol
[99
[99
85.5
67
2-MeO-benzaldehyde
4-NO2-benzaldehyde
3-NO2-benzaldehyde
2-NO2-benzaldehyde
4-Cl-benzaldehyde
2,4-diMeO-benzaldehyde
2-MeO-benzyl alcohol
4-NO2-benzyl alcohol
3-NO2-benzyl alcohol
2-NO2-benzyl alcohol
4-Cl-benzyl alcohol
2,4-diMeO-benzyl alcohol
33
70
97
Reaction conditions: 6 h, 80 ꢁC, 2 mL i-propanol, 0.04 g (0.0019 mmol) FePc–Al2O3, 0.19 mmol NaOH,
0.38 mmol starting material
123

P. Bata et al.
As is apparent from Table 4, under the optimized conditions with the
heterogenized complex all the aldehydes studied were reduced to the corresponding
alcohols with good to excellent yields, i.e., the immobilized Fe(Pc) complex was a
good catalyst for CTH reaction of different aldehydes. The most important
advantages of the immobilized complexes are easy separation and the possibility of
recycling.
Catalytic transfer hydrogenation of trans-cinnamaldehyde
With the developed catalytic system we examined the CTH reaction of trans-
cinnamaldehyde. This study was performed to answer the questions:
–
–
–
Does the CTH reaction work for a,b-unsaturated aldehydes?
Is there any selectivity in this reaction?
Is it possible to change the selectivity by changing the conditions?
The CTH reaction of a,b-unsaturated aldehydes was studied under the conditions
found to be optimum for simple aldehydes; the results obtained are listed in Table 5.
The data in Table 5 show that the CTH reaction works for a,b-unsaturated
aldehydes also. Some chemoselectivity was observed, because the major product
was the unsaturated alcohol when the original conditions were used for the catalytic
transfer hydrogenation. In this reaction the cinnamyl alcohol is the most valuable
product, so the CTH reaction is more than 40 % selective for the most valuable
product. The heterogenized catalyst was also active, but its activity was less than
that of the free complex. Selectivity, also, was different—no C=C hydrogenation
was observed and the proportion of the unsaturated alcohol was smaller.
Consequently on the heterogenized catalyst the saturated alcohol became the major
product. Because we were also interested in changing the selectivity, we performed
the CTH reaction of trans-cinnamaldehyde under more basic conditions, i.e. with
twice the amount of NaOH (Table 6).
Table 5 Fe(Pc)-catalyzed CTH reaction of trans-cinnamaldehyde
Catalyst
Conversion (%)
Product distribution (%)
Ualc
Sald
Salc
Fe(Pc)
67.2
58.3
65.3
56.5
44.5
38.1
44.1
37.5
12.9
0
42.5
61.0
41.2
62.0
Fe(Pc)–Al2O3
Fe(Pc)
14.6
0
Fe(Pc)–Al2O3
Reaction conditions: 4 h, 80 ꢁC, 2 mL i-propanol, 0.0038 mmol Fe(Pc), 0.38 mmol aldehyde,
0.095 mmol NaOH
Abbreviations: Ualc = unsaturated alcohol (3-phenylprop-2-en-1-ol), Sald = saturated aldehyde (3-
phenylpropanal), Salc = saturated alcohol (3-phenylpropan-1-ol)
123

Use of heterogenized metal complexes in hydrogenation…
Table 6 Fe(Pc)-catalyzed CTH reaction of trans-cinnamaldehyde under more basic conditions
Catalyst
Conversion (%)
Product distribution (%)
Ualc
Sald
Salc
Fe(Pc)
100
100
100
100
72.5
77.9
73.8
80.2
11.4
0
16.0
22.0
15.7
19.8
Fe(Pc)–Al2O3
Fe(Pc)
10.3
0
Fe(Pc)–Al2O3
Reaction conditions: 4 h, 80 ꢁC, 2 mL i-propanol, 0.0038 mmol Fe(Pc), 0.38 mmol aldehyde, 0.19 mmol
NaOH
Abbreviations: Ualc = unsaturated alcohol (3-phenylprop-2-en-1-ol), Sald = saturated aldehyde (3-
phenylpropanal), Salc = saturated alcohol (3-phenylpropan-1-ol)
Table 6 shows that when the conditions were changed the selectivity also
changed, in favor of the most valuable product, the unsaturated alcohol. Another
important observation is that we achieved full conversion on both types of catalyst.
With the heterogenized complex the trend was similar to that observed for the
homogeneous catalyst, i.e. the saturated aldehyde has not formed and, consequently,
selectivity for the most valuable product increased to approximately 80 %.
Comparison of hydrogenation and the CTH reaction of trans-
cinnamaldehyde
By studying the hydrogenation of trans-cinnamaldehyde we were able to shift the
selectivity from C=O hydrogenation to C=C hydrogenation by changing the
basicity. This study was performed on homogeneous and heterogenized Ru(phos-
phane) catalysts, and in ethanolic solution the basicity was changed by varying the
amount of Et3N. Because of the industrial importance of the unsaturated alcohol,
this finding seems very interesting and useful, especially because heterogenized
complexes have been proved to be recyclable through several runs without any
significant change in selectivity and activity whereas traditional hydrogenation, as is
well known, always has the risk of high pressure and the complexes used are
expensive.
These aspects motivated investigation of the CTH reaction with the same
substrate and study of the possibility of changing the selectivity. We also wished to
use a more economical and readily accessible catalyst, Fe(phthalocyanine). Fe(Pc),
both homogeneous and heterogenized, has been proved to be an active catalyst of
the CTH reaction of trans-cinnamaldehyde. By using a specific amount of NaOH
the industrially more important product, the unsaturated alcohol, was formed as a
major product. At the same time, however, C=C bond breaking occurred, producing
substantial amounts of benzyl alcohol and benzaldehyde. We were unable to reduce
the formation of these side products by increasing the amount of NaOH and
reducing the reaction time. This finding meant that, although we could produce the
unsaturated alcohol with reasonable selectivity, and application of the CTH reaction
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was simpler than traditional hydrogenation, and our ‘‘state-of-the-art’’ catalyst was
working very well, more investigation was required if the CTH reaction was to
become a real alternative to traditional hydrogenation.
Recycling of immobilized catalysts
The most important advantage of immobilization is the possibility of recycling,
which is crucial if the complex is expensive or difficult to obtain. Because Ru–
phosphane complexes are expensive, we checked the recyclability of these
complexes (Fig. 2).
As Fig. 2 clearly shows, we could recycle the catalyst for three consecutive runs
without significant changes of conversion and selectivity. Because Fe(Pc) is a
readily available, inexpensive material we did not investigate recyclability.
However, the heterogenized version of this system should be recyclable according
to our previous experience.
Conclusion
With the final objective of developing a ‘‘state-of-the-art’’ hydrogenation catalyst
which was made from inexpensive materials, was easy to synthesize, and was
convenient to handle we prepared an immobilized version of Fe(Pc) and used it in
CTH reactions of aldehydes. In this work we compared use of heterogenized Ru–
phosphane complexes in the hydrogenation of trans-cinnamaldehyde and in
catalytic transfer hydrogenation of the same substrate. For anchored Ru(phosphane)
Fig. 2 Product distribution for three consecutive hydrogenation of trans-cinnamaldehyde on 2b–NaY
catalyst
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Use of heterogenized metal complexes in hydrogenation…
catalysts the chemoselectivity was changed by changing the basicity. Under basic
conditions the unsaturated alcohol was the major product whereas under acidic
conditions C=C hydrogenation occurred. The anchored Ru(phosphane) complex
was recyclable without substantial change in activity. Catalytic transfer hydrogena-
tion was performed over an inexpensive iron complex, Fe(Pc), which has been
proved to be active in CTH reactions of aldehydes. In this work we extended the
application of this catalyst to reduction of trans-cinnamaldehyde, with emphasis on
selectivity. Our results show that catalytic transfer hydrogenation is suitable for
reduction of a,b-unsaturated aldehydes also. Fe(Pc) was proved to be an
inexpensive, effective catalyst for reduction of trans-cinnamaldehyde, a reaction
which is easy to perform and convenient to handle. Even the selectivity is good;
under suitable conditions the most valuable product, the unsaturated alcohol, is
obtained in good yield.
By comparing the two hydrogenation techniques we established that CTH could
be a valid option for reduction: it is effective, convenient to handle, and is suitable
for a,b-unsaturated aldehydes also. Although we have not achieved chemoselec-
tivity by use of the Ru(phospane) system, other advantages of the CTH reaction
makes it applicable in practical synthesis.
Acknowledgments The authors wish to express their thanks and gratitude for research grant 103191
OTKA NN, awarded in support of this research.
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