Biphasic hydrogenation of α,β-unsaturated aldehydes with hydrosoluble rhodium and ruthenium complexes

Article abstract of DOI:10.1007/s10562-011-0560-z

The hydrosoluble complexes [Rh(CO)(Pz)(L)]_2, and [RuH(CO)(CH₃CN)(L)₃][BF₄] [L = TPPMS (triphenylphosphinemonosulfonated) and TPPTS (triphenylphosphine-trisulfonated)] were evaluated in the selective hydrogenation of α,β-unsaturated aldhydes in biphasic media The Rh complexes produced the saturated aldehydes while the Ru complexes generated the alcohols with high chemoselectivity. The catalytic phase can be recycled up to five times.

Full text of DOI:10.1007/s10562-011-0560-z

Catal Lett (2011) 141:709–716
DOI 10.1007/s10562-011-0560-z
Biphasic Hydrogenation of a,b-unsaturated Aldehydes
with Hydrosoluble Rhodium and Ruthenium Complexes
•
Luis G. Melean Mariandry Rodriguez
•
•
Angel Gonzalez Beatriz Gonzalez
•
´
´
Merlin Rosales Pablo J. Baricelli
•
Received: 27 October 2010 / Accepted: 11 December 2010 / Published online: 15 February 2011
Ó Springer Science+Business Media, LLC 2011
Abstract The hydrosoluble complexes [Rh(CO)(Pz)(L)]2,
and [RuH(CO)(CH3CN)(L)3][BF4] [L = TPPMS (triphe-
nylphosphinemonosulfonated) and TPPTS (triphenylpho-
sphine-trisulfonated)] were evaluated in the selective
hydrogenation of a,b-unsaturated aldhydes in biphasic
media The Rh complexes produced the saturated aldehydes
while the Ru complexes generated the alcohols with high
chemoselectivity. The catalytic phase can be recycled up to
five times.
reacts, and this has been a very prolific research field over
the years.
Traditional heterogeneous hydrogenation catalysts lack
the selectivity desired for the process while the most
selective homogeneous ones present the well-known
drawbacks of difficult product separation and catalyst
recovery. Thus, the use of a biphasic liquid/liquid system in
which the catalyst is confined to an aqueous phase while
the reactants and products remain in an organic one is of
particular interest. This relatively new technology has
received a lot of attention ever since the introduction of the
biphasic oxo process by the Ruhrchemie/Rhone-Poulenc
(R/RP) in 1984, and it is now applied in all major types of
organometallic-catalyzed reactions [3].
Keywords Biphasic hydrogenation ꢀ a, b-Unsaturated
aldehydes ꢀ Hydrosoluble Rh ꢀ Ru complexes
1 Introduction
Pioneer work in the biphasic hydrogenation of a,b-
unsaturated aldehydes was conducted almost 20 years ago
by Grosselin and Mercier [4]. They reported the use of
Rh/TPPTS and Ru/TPPMS systems in the biphasic
hydrogenation of cinnamaldehyde, prenal, citral and cro-
tonaldehyde; the selectivity of the reactions was high
([95%) and depended on the metal used; thus the
Rh/TPPTS system favored the formation of the saturated
aldehydes while the Ru/TPPTS one selectively produced
the unsaturated alcohols for all substrates.
The selective reduction of a,b-unsaturated aldehydes, such
as cinnamaldehyde and citral, is a very important reaction
because the possible products, saturated aldehydes or
unsaturated alcohols (see Fig. 1), have found many uses in
the fragrance, cosmetics, flavor, and pharmacological
industries; also, they constitute relevant intermediates in
the synthesis of various fine chemicals [1, 2]. The reaction
is complicated by the presence of two moieties with similar
reactivity: the carbonyl group and the olefin carbon–carbon
double bond; in practice, it’s desired that only one of them
A few years later, Joo et al. [5] determined that the
selectivity of the Ru/TPPMS system in the biphasic
hydrogenation of cinnamaldehyde depended on the pH of
the reaction media and the major species present in the
equilibrium. At pH \ 3.3, the major specie formed was
RuHCl(TPPMS)3 which promoted the selective hydroge-
nation of the C=C bond while at pH [ 7.0, the equilibrium
shifts to favor the formation of Ru(H)2(TPPMS)4 which
selectively reduced the C=O bond.
L. G. Melean ꢀ M. Rodriguez ꢀ P. J. Baricelli (&)
´
´
Centro de Investigaciones Quımicas, Facultad de Ingenierıa,
Universidad de Carabobo, Valencia, Venezuela
e-mail: pbaricel@uc.edu.ve
´
´
A. Gonzalez ꢀ B. Gonzalez ꢀ M. Rosales
´
´
´
Laboratorio de Quımica Inorganica, Departamento de Quımica,
Facultad Experimental de Ciencias. Universidad del Zulia,
Apdo. 526, Maracaibo, Venezuela
Several other groups have studied the biphasic hydro-
genation of unsaturated aldehydes with various degrees of
123

710
L. G. Melean et al.
R1
OH
[Rh(CO)(Pz)(L)]2 and [RuH(CO)(MeCN)(L)3][BF4] (L =
TPPMS and TPPTS) taking particular attention to the
recycling of the catalysts. These complexes were chosen
due to their ease of preparation, stability, high water sol-
ubility and our experience in their use in biphasic systems
previously reported [11–15].
R2
OH
Unsaturated alcohol
R1
R1
O
R2
Saturated alcohol
R2
Unsaturated aldehyde
R1
O
R2
Saturated aldehyde
2 Experimental
Fig. 1 Hydrogenation of a,b-unsaturated aldehydes
2.1 General Procedure
success. Sanchez-Delgado et al. [6] studied several Ru and
Os catalysts for the hydrogenation of cinnamaldehyde
using TPPMS and TPPTS as ancillary ligands. [6] They
found that both the Ru and Os systems were selective
towards the production of cinnamic alcohol (54–100%) and
that the Ru systems were more active than their Os coun-
terpart. The group of Andriollo et al. [7] synthesized
complexes of the type RuHCl(CO)(L)3, L = TPPMS and
TPPTS, and evaluated them in the biphasic hydrogenation
of cinnamaldehyde, finding that both systems produced the
saturated alcohol with high conversion but medium selec-
tivity (&50%).
Lopez-Linares et al. [8] studied the biphasic hydroge-
nation of cinnamaldehyde with MCl2(CH3CN)4/TPPMS
systems, where M = Ru and Os. The Ru (II) system
showed better activity and selectivity than its Os(II) ana-
logue and the selectivity of the systems towards the
unsaturated alcohol was enhanced by increasing the sub-
strate to catalyst ratio, salt concentration and ligand
tensoactivity. Nuithitikul and Winterbottom [9] reported
the biphasic hydrogenation of cinnamaldehyde with
RhCl(TFFTS)3 to the saturated aldehyde with a 99.9%
selectivity. The same researchers reported a comprehensive
study of the crucial parameters for the biphasic hydroge-
nation of cinnamaldehyde using Ru/TPPTS systems,
achieving a selectivity to the saturated alcohol of 96% and
to the unsaturated aldehyde of 97% if the reaction condi-
tions are carefully selected [10].
All manipulations were carried out under nitrogen using
standard Schlenck techniques. Organic solvents were dried
and purified by distillation over standard agents under N2
prior to use. All other chemicals were commercial products
and were used without further purification. All gases were
of high purity ([99.9%) and were purchased from Aga
Gases-Venezuela. [Rh(CO)(Pz)(L)]2 and [RuH(CO)(-
MeCN)(L)3][BF4] (L = TPPMS and TPPTS; Pz = pyraz-
olate, C3H3N2) were synthesized according to published
methods [11–15]. GC analyses were performed on a
Hewlett Packard 5971 Plus Series II chromatograph with a
flame ionization detector in an Ultra-1, DB-1 (10% dime-
thyl polysiloxane), 25 m, 0.32 mm, 0.52 lm column to
separate the products. Quantification was achieved by
using heptane as the internal standard and all peaks were
identified by GC/MS on a Digital Technology 5890/5971
coupled system using a Quadrex PONA 10% dimethyl
polysiloxane, 25 m, 0.52 lm column.
2.2 Biphasic Aqueous Hydrogenation of Unsaturated
Aldehydes
In a typical experiment, the catalyst precursor (4.4 lmol)
and the phase transfer agent (dodecyl-trimethylammo-
nium bromide, DTAB, or Cetil-trimethylammonium
chloride, CTAC, 6.3 lmol) were dissolved in 5 mL of
deoxygenated water and added into a stainless steel
reactor (25 mL, Parr) fitted with internal mechanical
stirring, a temperature control unit, and a sampling valve.
The system was purged three times, then charged to the
desired pressure of H2 and warmed to the desired tem-
perature. Once the desired temperature was achieved, a
solution of the substrate (2.2 mmol) in 5 mL of toluene
was added via a high-pressure burette and this was taken
as the initial time of the reaction. Samples of the reac-
tion mixture were periodically extracted and the system
total pressure was adjusted via a high pressure reservoir.
Once the samples were extracted, they were cooled, the
phases separated and the organic phase analyzed by GC
and GC–MS techniques.
Despite the many reports on hydrogenation of unsatu-
rated aldehydes in biphasic media that can be found in the
literature, almost no attention has been given to the recy-
cling of the catalytic phase, arguably the most important
advantage of a liquid/liquid biphasic system. Our group has
shown a long lasting interest in this kind of systems; we
have reported the biphasic hydroformylation and hydro-
genation of olefins, olefin mixtures and light naphtha cuts
with hydrosoluble rhodium, ruthenium and tungsten com-
plexes [11–15]. In this work we present the biphasic
hydrogenation of a,b-unsaturated aldehydes (cinnamalde-
hyde, a-methyl-cinnamaldehyde, crotonaldehyde and cit-
ral) with rhodium and ruthenium hydrosoluble complexes,
123

Biphasic Hydrogenation of Unsaturated Aldehydes
711
2.3 Recycling of the Catalysts in the Biphasic
Hydrogenation of Unsaturated Aldehydes
reaction proceeds as shown in Eq. 1. Trans-cinnamalde-
hyde (1a) can be hydrogenated to form 3-phenyl-propanal
(2a), cinnamic alcohol (3a), and 3-phenil-propanol (4a),
and the reaction conditions (pressure of H2, temperature,
concentration of DTAB, and substrate/catalyst ratio) were
systematically varied aiming to maximize the total con-
version as well as the selectivity towards the unsaturated
alcohol; the result of this initial series of experiments are
summarized in Table 1.
In a typical experiment, after the reaction had run its
course, the reactor was cooled to room temperature and
slowly vented. The reactor was connected to an inert line
(typically Ar) and the reaction mixture was transferred into
a Schlenck-type addition funnel with the rigorous exclu-
sion of air. The phases were separated; the aqueous cata-
lytic phase was re-introduced into the reactor and the
procedure described in Sect. 2.2 was repeated with a fresh
organic phase containing the substrate.
O
OH
O Ru*, H2 , CTAC
+
+
3a
2a
H2O/Toluene
OH
1a
3 Results and Discussion
4a
3.1 Biphasic Hydrogenation of a,b-unsaturated
Aldehydes using Hydrosoluble Ionic Ruthenium
Complexes
ð1Þ
The effect of the total H2 pressure can be seen in entries
1–6 in Table 1. As the pressure of hydrogen increased, the
amount of H2 dissolved in the biphasic system increased as
well generating an improvement in the total conversion of
1a, but having little effect on the product distribution, until it
reached a maximum of 37% at 800 psi. Further increments
The study started with the determination of the optimal
reaction conditions for the biphasic hydrogenation of a,b-
unsaturated aldehydes using trans-cinnamaldehyde as a
model molecule and the ionic ruthenium complex [RuH(-
CO)(CH3CN)(TPPMS)3][BF4] as catalytic precursor; the
Table 1 Determination of the optimal reaction conditions for the biphasic hydrogenation of cinnamaldehyde with the catalyst precursor
[RuH(CO)(CH3CN)(TPPMS)3][BF4]
Run P H2 (psi) T (8C) S/C
[DTAB] (mM) Conversion
(%)
Selectivity
Saturated
aldehyde (2a) (%) alcohol (3a) (%) alcohol (4a) (%)
TOF (h-1
)
Unsaturated
Saturated
1
200
400
600
800
1000
1200
800
800
800
800
800
800
800
800
800
800
800
800
800
800
80
80
200:1
200:1
200:1
200:1
200:1
200:1
200:1
200:1
200:1
200:1
200:1
200:1
200:1
100:1
300:1
400:1
500:1
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
21
23
24
37
29
10
14
28
40
58
67
79
84
47
42
41
39
41
45
13
37
39
36
41
37
47
44
28
28
30
33
38
16
20
24
33
34
25
28
17
46
50
43
38
45
53
27
10
44
43
35
24
21
41
50
35
33
53
54
70
18
11
21
21
19
0
7
8
2
3
80
8
4
80
12
10
3
5
80
6
80
7
50
28
62
28
27
32
38
63
38
26
32
33
22
18
13
5
8
60
9
9
100
120
140
150
160
100
100
100
100
100
100
100
13
19
22
26
28
8
10
11
12
13
14
15
16
17
18
19
20
21
27
33
21
23
7
300:1 1.25
300:1 2.5
300:1 5.0
Conditions: Ru = 0.0044 mmol, H2O = Toluene = 5 mL, t = 6 h
123

712
L. G. Melean et al.
in the total H2 pressure resulted in an inhibition of the
reaction, probably due to the formation of polyhydride-Ru
species or the coordination of molecular hydrogen to the Ru
centre, forming complexes like those reported by Kubas,
et al. [16, 17], which saturates the coordination sites and
renders the species unreactive towards hydrogenation. The
effect of the reaction temperature was studied in the range
from 50 to 160 °C (entries 4, 7–13). A strong dependence of
the conversion with the temperature can be observed; the
conversion increased steadily from 14 to 84% in the tem-
perature range studied; unfortunately, as the temperature
increased, the saturated alcohol became the mayor product.
The highest selectivity towards the unsaturated alcohol (3a)
was achieved at 100 °C; thus this was selected as the optimal
temperature for the remaining reactions.
it was attributed to the accumulation of the phase transfer
agent on the interface, creating a barrier which prevents the
contact of the substrate with the metal center.
In order to rule out the participation of metallic ruthe-
nium the hydrogenation reactions, the well-known mercury
test was carried out performing the hydrogenation of cin-
namaldehyde under the best operational condition but
adding one drop of mercury to the reaction mixture [19,
20]. The reaction showed no interference of the Hg in the
course of the reaction, confirming the absence of metallic
particles or colloids during the catalysis.
Once the optimal reaction conditions of temperature,
pressure, S/C ratio and concentration of DTAB were deter-
mined, they were employed to evaluate the catalyst pre-
cursor [RuH(CO)(CH3CN)(TPPMS)3][BF4] in the biphasic
hydrogenation of different a,b-unsaturated aldehydes: cro-
tonaldehyde (1b), a-methyl-cinnamaldehyde (1c), and citral
(1d) according to Eq. 2; the results are shown in Table 2.
The substrate/catalyst (S/C) ratio had an interesting
effect (entries 9, 14–17); as the S/C ratio was increased by
varying the amount of cinnamaldehyde added into the
reaction mixture while keeping the amount of Ru constant,
the TOF also increased; but the total 1a conversion
remained practically unchanged around 40%. This evi-
denced a first order dependence of the reaction kinetics
with respect to the substrate concentration. When the
substrate concentration was too high (S/C C 400:1), the
selectivity of the reaction was lost; equal amounts of the
three possible products were generated. Thus, a S/C ratio of
300:1 was selected for the remaining reactions.
O
2b
+
Ru/TPPMS, H2, DTAB
O
OH
3b
Toluene, H2O
1b
+
4b
OH
O
2c
+
OH
3c
O
Ru/TPPMS, H2, DTAB
Toluene, H2O
Finally, the effect of the concentration of a phase
transfer agent was studied since they have been proven to
facilitate the biphasic reaction of substrates that are insol-
uble in water. Dos Santos et al. [18] successfully used cetil-
trimethyl-ammonium chloride (CTAC) in the biphasic
hydroformylation of terpenes with a Rh/TPPTS system. In
Table 1 (entries 15, 18–20) it can be seen that small
increases in the DTAB concentration have little effect on
the total conversion, while the selectivity towards the
unsaturated alcohol increases slightly; this is probably due
to the inherent tensoactive properties of the TPPMS pre-
viously reported [7]. Further increments in the DTAB
concentration resulted in the inhibition of the reaction; dos
Santos et al. [18] observed a similar trend in their work and
1c
+
OH
4c
2d
3d
O
+
Ru/TPPMS, H2, DTAB
Toluene, H2O
O
1d
OH
+
4d
OH
ð2Þ
Table 2 Biphasic hydrogenation of a,b-unsaturated aldehydes with the catalyst precursor [RuH(CO)(CH3CN)(TPPMS)3][BF4]
Substrate
Conversion (%)
Selectivity
Saturated
aldehyde (2a–d) (%)
Unsaturated
alcohol (3a–d) (%)
Saturated
alcohol (4a–d) (%)
Cinnamaldehyde (1a)
Crotonaldehyde (1b)
a-Methyl-cinnamaldehyde (1c)
Citral (1d)
75
100
76
11
0
55
100
75
34
0
13
0
12
38
96
62
Conditions: Ru = 0.0044 mmol, [DTAB] = 2.5 mM, H2O = Toluene = 5 mL, t = 24 h, T = 100 °C, S/C = 300:1, pH2 = 800 psi
123

Biphasic Hydrogenation of Unsaturated Aldehydes
713
Table 3 Biphasic hydrogenation of a,b-unsaturated aldehydes with the catalyst precursor [RuH(CO)(CH3CN)(TPPTS)3][BF4]
Substrate
Conversion (%)
Selectivity
Saturated
aldehyde (2a–d) (%)
Unsaturated
alcohol (3a–d) (%)
Saturated
alcohol (4a–d) (%)
Cinnamaldehyde (1a)
Crotonaldehyde (1b)
a-Methyl-cinnamaldehyde (1c)
Citral (1d)
64
87
86
92
16
0
57
48
84
92
27
52
16
7
0
1
Conditions: Ru = 0.0044 mmol, [DTAB] = 2.5 mM, H2O = Toluene = 5 mL, t = 24 h, T = 100 °C, S/C = 300:1, pH2 = 800 psi
This difference in behavior of the sulphonated phos-
The reaction time was increased to 24 h to allow the
reactions to proceed to completion; however, trans-cin-
namaldehyde reaches a maximum of 75% conversion in
that period. Interestingly, the longer reaction time favors
the production of the fully saturated product 4a; the
amount of saturated aldehyde decreases until it only
accounts for about 10% of the final product distribution
while keeping the amount of cinnamic alcohol nearly
constant. When the substrate is changed to a-methyl-
cinnamaldehyde (1c), with a more sterically demanding
internal olefin, the conversion remains constant, but the
selectivity towards the unsaturated alcohol (3c) increases
considerably, reaching 75% while only 13% of the satu-
rated aldehyde (2c) is found. When citral (1d) is hydro-
genated, which also presents a sterically hindered olefin,
the unsaturated alcohol (3d) accumulates, but then the
reduction of its internal olefin to the saturated alcohol
(4d) competes with the initial reduction of the aldehyde.
Thus, only the two alcohols are found in the product
distribution with a ratio of 2:1 by the time the conversion
is near quantitative. The reduction of crotonaldehyde (1b)
proves to be quantitative and totally selective towards the
unsaturated alcohol (3b) under the reaction conditions
studied.
phines is explained by the tensoactive properties present in
the TPPMS that are absent in TPPTS, as described by
Andriollo et al. [7]. TPPTS, with its high solubility in
water, acts more like an electrolyte in solution while the
TPPMS accumulates in the interphase and accelerates the
reaction, thus, resulting in higher conversions. TPPTS has a
bigger cone angle due to its three meta-substituted phenyl
rings, which prevents the sterically hindered olefins from
coordinating to the metal center, thus, the alkene reduction
is inhibited and the selectivity towards the unsaturated
alcohol is enhanced. The variation of the selectivity could
also be attributed to a variation of reaction medium: while
with the TPPMS ligand the reaction occurs mainly at the
interface, with the TPPTS ligand the reaction occurs in the
bulk aqueous phase.
These systems compare favorably against various
molecular Ruthenium precursors reported in the literature
[6–8], taking into account their higher conversion while
having comparable selectivity towards the unsaturated
alcohols; besides most of the reports focused only on the
hydrogenation of cinnamaldehyde while the scope of our
system was wider, being possible to hydrogenate four
different substrates.
When the phosphine is changed from TPPMS to its tri-
sulphonated analog TPPTS, maintaining all reaction con-
ditions constant, interesting results are obtained as sum-
marized in Table 3. In general, when TPPTS is used, the
total conversion after 24 h are about 10% less compared to
the original system. The selectivity of the reactions shows
mixed results: when the substrates are sterically hindered in
the olefin (1c and 1d) the alcohols are generated almost
exclusively, with the unsaturated ones (3c and 3d)
accounting for over 84% of product distribution. However,
when the substrates show little sterical impediment in the
olefin, the C=C double bond shows a high tendency to be
reduced, thus in the case of crotonaldehyde (1b) a 1:1
mixture of both alcohols are found while in the case of
cinnamaldehyde (1a) some of the saturated aldehyde is
present as well as both alcohols.
3.2 Biphasic Hydrogenation of a,b-unsaturated
Aldehydes using Hydrosoluble Binuclear Rhodium
Complexes
Our group has studied the hydrosoluble catalyst precursors
[Rh(CO)(Pz)(L)]2, where L is TPPMS and TPPTS, in the
biphasic hydrogenation and hydroformylation of olefins,
olefin mixtures and naphthas [11, 12, 21]. Now we extend
the scope of our catalytic systems into the selective
hydrogenation of a,b-unsaturated aldehydes. The work
began with the determination of the optimal reaction con-
ditions with the system Rh/TPPMS and cinnamaldehyde as
model molecule, as described in Eq. 1, in the same way as
previously described for the Ru/TPPMS system (vide
infra). The concentration of the phase-transfer agent was
varied from 0 to 7.5 mM, the temperature was studied in
123

714
L. G. Melean et al.
Table 4 Biphasic hydrogenation of a,b-unsaturated aldehydes with the catalyst precursor [Rh(CO)(Pz)TPPMS]2
Substrate
Conversion (%)
Selectivity
Saturated
aldehyde (2a–d) (%)
Unsaturated
alcohol (3a–d) (%)
Saturated
alcohol (4a–d) (%)
Cinnamaldehyde (1a)
Crotonaldehyde (1b)
a-Methyl-cinnamaldehyde (1c)
Citral (1d)
65
100
45
80
100
84
13
0
7
0
5
3
11
14
28
83
Conditions: Rh = 0.0044 mmol, [CTAC] = 5.0 mM, t = 24 h, T = 70 °C, pH2 = 300 psi, S/C = 500:1
Table 5 Biphasic hydrogenation of a,b-unsaturated aldehydes with the catalyst precursor [Rh(CO)(Pz)TPPTS]2
Substrate
Conversion (%)
Selectivity
Saturated
aldehyde (2a–d) (%)
Unsaturated
alcohol (3a–d) (%)
Saturated
alcohol (4a–d) (%)
Cinnamaldehyde (1a)
Crotonaldehyde (1b)
a-Methyl-cinnamaldehyde (1c)
Citral (1d)
87
97
75
55
84
100
90
8
0
7
0
3
0
7
87
12
Conditions: Rh = 0.0044 mmol, [CTAC] = 5.0 mM, t = 24 h, T = 70 °C, pH2 = 300 psi, S/C = 500:1
the range of 60–100 °C, the H2 pressure was varied from
100 to 800 psi, and the S/C ratio was adjusted from 250:1
to 1500:1. The best reaction conditions, with higher
conversion and selectivity towards the saturated alde-
hyde, were determined to be pH2 = 300 psi, T = 70 °C,
[CTAC] = 5.0 mM, S/C = 500:1. The change of phase
transfer agent from DTAB to CTAC has no observed effect
on the system since they have a similar cationic nature of
the quaternary ammine and aliphatic chains of similar
length; however, the CTAC comes as an aqueous solution
which makes it easier to measure the small amounts used in
this study.
reaching only 28% conversion in 24 h. In all cases, the
selectivity towards the saturated aldehydes was high, in the
range of 80–84%.
When the phosphine in the system was changed to
TPPTS, maintaining all other conditions fixed, a positive
effect on the reaction was observed as shown in Table 5.
The reactivity order remained constant, but in all cases
the conversion increased by about 20–30% while the
selectivity towards the saturated aldehyde increased
slightly (84–100%). Once again this enhance in activity
can be attributed to the different tensoactive properties
between the TPPMS and TPPTS. Since it has been
reported that an excess of phase transfer agent inhibits the
reaction [18], and in the Rh systems a high concentration
of 5.0 mM of CTAC was used (compared to 2.5 mM in
the Ru systems), the use of the TPPTS favored the
reaction in the Rh case for its lack of additional tenso-
active properties while the Ru one was enhanced by the
use of TPPMS.
Once the optimal reaction conditions were selected, they
were employed in the biphasic hydrogenation of several
a,b-unsaturated aldehydes as described in Eq. 2. The
results are presented in Table 4. It is evident than the Rh/
TPPMS system requires milder reaction conditions than its
Ru analog; lower temperature and pressure, higher S/C
ratios and shorter reaction times result in high conversions
and very high selectivity towards the saturated aldehyde in
accordance with the results of Grosselin and Mercier [4].
As it was the case with the Ru systems, crotonaldehyde
presents the highest reactivity, reaching quantitative con-
version with a complete selectivity towards the saturated
aldehyde in about 12 h. Now that the reaction is focused on
the C=C double bond, the steric impediment around it
determines the reactivity order; thus, cinnamaldehyde
reacted more rapidly than a-methyl-cinnamaldehyde and
citral; the reaction of this latest substrate being rather slow,
Although, to the best of our knowledge, there are no
reports of biphasic molecular, water-soluble rhodium pre-
catalysts to compare our system in the hydrogenation of
unsaturated aldehydes, in the pioneering work using in situ
formed catalysts the activity and selectivity obtained was
high in all substrates except for citral, where the reported
conversion was only 19% [4]. Our Rh/TPPMS and Rh/
TPPTS systems, with the addition of the phase-transfer
agent, reached conversion of citral of up to 55%; a
remarkable improvement over the previous work.
123

Biphasic Hydrogenation of Unsaturated Aldehydes
3.3 Reuses of the Catalyst Precursors
715
4 Conclusions
Arguably, the greatest benefit of a biphasic system is the
ability of easily separate the aqueous phase containing the
catalyst from the organic phase where the products are
present [3], and reusing the catalyst by loading fresh sub-
strate. In this order of ideas, we evaluated the recycling
ability of our catalyst precursor [Rh(CO)(Pz)(TPPMS)]2 in
the biphasic hydrogenation of cinnamaldehyde. The metal
content in the organic phase was determined by atomic
absorption analysis in each run and the results show that
less than 5 ppm were present in the organic phase, indi-
cating that practically all of the rhodium remains in the
aqueous phase. The results are shown in Figs. 2 and 3. It is
evident that for the first three cycles the activity remains
constant, dropping slightly for cycles 4 and 5, probably due
to the oxidation of the phosphine. The selectivity towards
the unsaturated aldehyde remains high, and unaltered
throughout the recycling of the catalytic phase; demon-
strating the stability of our catalyst precursor and evi-
dencing its potential for industrial applications.
The hydrosoluble precatalysts [Rh(CO)(Pz)(L)]2 and [RuH
(CO)(CH3CN)(L)3][BF4] (L = TPPMS and TPPTS)
proved to be effective in the biphasic hydrogenation of a,b-
unsaturated aldehydes: trans-cinnamaldehyde, a-methyl-
cinnamaldehyde, crotonaldehyde and citral. The Rh com-
plexes were more active, required milder reaction condi-
tions and preferentially reduced the C=C double bonds to
form the corresponding saturated aldehydes; while the Ru
ones, under harsher conditions, attacked the carbonyls to
preferentially generate the unsaturated alcohols. The Ru/
TPPMS system was slightly more active than its TPPTS
analog; however, the reactivity was reversed for the Rh
systems in which the TPPTS-complex was more active.
In both cases the TPPTS-bearing systems were more
selective towards their corresponding major products. The
Rh/TPPMS system could be recycled five times without
any significant loose of activity or selectitity, demon-
strating its stability and feasibility of use in industrial
applications.
Acknowledgments We thank FONACIT (Caracas) for the financial
support Project F-97-003766, CONIPET Project 97-003777 and
CDCH-UC Project UAC-397-08. We are grateful to the CYTED
Project V.9, Laboratory of synthesis and characterization of new
materials—IVIC and the University of Carabobo for permitting the
publication of this work.
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