Aqueous-phase hydrogenation and hydroformylation reactions catalyzed by a new water-soluble [rhodium]-thioligand complex

Article abstract of DOI:10.1016/j.cattod.2014.05.038

Rh(DHTANa) is a new water-soluble catalyst easily obtained by mixing in water the catalytic precursor [Rh(COD)Cl]₂ and the dihydrothioctic acid sodium salt (DHTANa). This catalyst showed to be very active in the hydrogenation of unsaturated substrates as 2-cyclohexen-1-one, the biomass-derived furfural and acetophenone. In this last case the catalytic system obtained by using as water-soluble ligand (R)-(DHTANa) afforded (R)-1-phenylethanol with very modest enantioselectivity. Rh(DHTANa) was active also in the aqueous biphase hydroformylation of styrene producing exclusively the two corresponding aldehydes with 80-86% selectivity toward the branched aldehyde 2-phenylpropanal. This new catalytic system was easily recycled in both hydrogenation and hydroformylation processes and no leaching phenomenon was observed.

Full text of DOI:10.1016/j.cattod.2014.05.038

Catalysis Today 247 (2015) 64–69
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Catalysis Today
journal homepage: www.elsevier.com/locate/cattod
Aqueous-phase hydrogenation and hydroformylation reactions
catalyzed by a new water-soluble [rhodium]–thioligand complex
a,∗
b
a
a
a
S. Paganelli , O. Piccolo , P. Pontini , R. Tassini , V.D. Rathod
a
Dipartimento di Scienze Molecolari e Nanosistemi, Università Ca’ Foscari Venezia, Calle Larga S. Marta 2137, 30123 Venezia, Italy
SCSOP, Via Bornò 5, 23896 Sirtori, Italy
b
a r t i c l e i n f o
a b s t r a c t
Article history:
Rh(DHTANa) is a new water-soluble catalyst easily obtained by mixing in water the catalytic precursor
Received 27 March 2014
Received in revised form 9 May 2014
Accepted 27 May 2014
[Rh(COD)Cl]2 and the dihydrothioctic acid sodium salt (DHTANa). This catalyst showed to be very active
in the hydrogenation of unsaturated substrates as 2-cyclohexen-1-one, the biomass-derived furfural
and acetophenone. In this last case the catalytic system obtained by using as water-soluble ligand (R)-
Available online 3 July 2014
(DHTANa) afforded (R)-1-phenylethanol with very modest enantioselectivity. Rh(DHTANa) was active
also in the aqueous biphase hydroformylation of styrene producing exclusively the two corresponding
aldehydes with 80–86% selectivity toward the branched aldehyde 2-phenylpropanal. This new catalytic
system was easily recycled in both hydrogenation and hydroformylation processes and no leaching
phenomenon was observed.
Keywords:
Rhodium
Dihydrothioctic acid
Thioligand
Aqueous catalysis
Hydrogenation
Hydroformylation
© 2014 Elsevier B.V. All rights reserved.
1
. Introduction
Homogeneous catalysis is an efficient tool to carry out a chem-
atoms of the protein, therefore we hypothesized that the metal
atom mainly interacts with the sulphur atoms [13,14]: the great
affinity of “soft” metals, such as rhodium, for the thiolic group
is well known [15]. As a matter of fact many rhodium com-
plexes modified with thioligands have been successfully employed
in catalytic reactions in homogeneous phase in organic solvents
[16–23]. Our goal was to study catalytic processes in the pres-
ence of water-soluble sulphur containing ligands: in particular we
used some thio-oligopeptides as ligands in the rhodium catalyzed
hydroformylation of styrene [14]. More recently, we turned our
attention to simple and cheap water-soluble low molecular weight
thioligands such as the aminoacid (l)-Cysteine and [(S)-1-[(S)-3-
mercapto-2-methylpropanoyl]pyrrolidine-2-carboxylic acid] (sold
ical process in a very active and selective way but the major
drawback of the homogeneous process is the separation of the
expensive catalyst from the product mixture that requires an
energy intensive process such as distillation. In recent years the
use of metallic species with suitable ligands has allowed chemists
to carry out reactions in biphasic systems so permitting an easy
separation of the used catalyst, confined in a phase different from
that of reagents and products, and its re-use in recycling experi-
ments; in particular, the use of both water as co-solvent for biphasic
reactions and easily recyclable water-soluble catalysts are highly
desirable for the realization of green processes [1–4]. In the last
years natural compounds, such as aminoacids, peptides, proteins
and sugars have been used as ligands for metallic species active as
catalysts [5–9]. In this context, some years ago we reported highly
efficient and chemoselective hydroformylation [10–12] and hydro-
genation [8] reactions using water-soluble complexes derived from
the interaction between Rh(CO) (acac) or [Ir(COD)Cl] , respec-
®
as a pharmacologically active ACE inhibitor, named Captopril ).
Both ligands were capable of promoting efficiently the hydrogena-
tion process in the presence of Rh or Ir catalytic precursors and,
at the same time, allowing an easy recovery and recycling of the
water-soluble catalytic complexes [24]. In these last months we
addressed our research to the use of a very simple molecule, e.g.
dihydrothioctic acid (DHTA), the reduced form of thioctic acid
(THA), which presents two SH groups capable, in principle, to
work as a bidentate ligand for the rhodium atom (Fig. 1). The pres-
ence of another functional group, COOH, in the molecule, when it
is salified, might favor the solubility in water of the Rh complex but
also create a potential third bonding site for the metallic species.
Finally, as this molecule presents a chiral center and the corre-
sponding enantiomers are easily prepared from a commercially
2
2
tively, with human serum albumin (HSA). Previous studies, carried
out by us on the catalytic system Rh/HSA, showed an outstand-
ing correspondence between the surface distribution of Rh and S
∗ Corresponding author. Tel.: +39 0412348592; fax: +39 0412348517.
E-mail address: spag@unive.it (S. Paganelli).
http://dx.doi.org/10.1016/j.cattod.2014.05.038
0
920-5861/© 2014 Elsevier B.V. All rights reserved.

S. Paganelli et al. / Catalysis Today 247 (2015) 64–69
65
3
4.3 (CH SH), 26.9 (CH CH2 CH ), 24.7 (CH CH2 COOH), 22.7
2 2
(CH2 SH). The corresponding sodium salt DHTANa was obtained by
treatment of 507 mg (2.46 mmol) of DHTA with one equivalent of
1
Na CO3 dissolved in 10 mL of deaerated H O. HNMR (H O/D O):
2
2
2
2
ı 2.92 (m, 1H, S CH), 2.6 (m, 2H, S CH ), 2.08 (t, J = 7.2 Hz, 2H,
2
CH2 COONa), 1.9–1.78 (m, 2H, HSCH CH CHSH), 1.72–1.3 (m, 10H,
2
2
(
CH ) , 2 SH).
Fig. 1. Thioctic acid (THA) and dihydrothioctic acid (DHTA).
2 4
The aqueous solution obtained was then used for the prepara-
tion of the Rh(DHTANa) complex. In the same way (R)-(DHTANa)
was prepared starting from (R)-thioctic acid.
available enantiopure precursor, we could set out to evaluate the
enantioselectivity in the hydrogenation of suitable substrates cat-
alyzed by rhodium-(DHTA sodium salt) complex, from now on
named Rh(DHTANa).
2.3. Rh(DHTANa) catalyst preparation
The catalytic complex Rh(DHTANa) was prepared in situ
2
. Experimental
by reacting, at room temperature under
a nitrogen purge,
[
Rh(COD)Cl]2 (12.4 mg, 0.025 mmol) with a solution of dihydroth-
2
.1. Materials and methods
ioctic acid sodium salt (DHTANa) (10.3 mg, 0.5 mmol) in 5 ml of
deaerated water until complete dissolution of the complex. HNMR
(
J = 7.2 Hz, 2H, CH2 COONa), 1.9–1.78 (m, 2H, HSCH CH CHSH),
1
the free ligand DHTANa and the corresponding rhodium complex
Rh(DHTANa) we can observe a clear shift of the protons present on
the carbon atoms originally bound to the thiolic groups, so indicat-
ing a possible chelation of Rh to the S atoms.
1
[
Rh(COD)Cl] , 2-cyclohexen-1-one, furfural, acetophenone and
2
H O/D O): ı 3.61 (m, 1H, S CH), 3.12 (m, 2H, S CH ), 2.08 (t,
2 2 2
styrene were Aldrich products. Thioctic acid and (R)-thioctic
acid were a generous gift of Prochifar s.r.l. (Milan) and Sintac-
tica s.r.l. Cassina De’ Pecchi (Milan), respectively. GC analyses
were carried out on an Agilent 6850 A gaschromatograph (HP1
column 30 m × 0.32 mm × 0.25 ␮m) and GC–MS analyses were
performed by using an Agilent MS Network 5937 (HP-5MS
column 30 m × 0.25 mm × 0.25 ␮m). Enantiomeric excess of 1-
phenylethanol (X) was determined by GC equipped with a cyclodex
2
2
1
.72–1.3 (m, 10H, (CH ) , 2 SH). By comparing the HNMR of
2 4
In the same way (R)-Rh(DHTANa) was prepared starting from
R)-(DHTANa). The complex Rh(DHTANa)2 was prepared at the
(
B (Agilent) capillary column (30 m × 0.32 mm I.D. × 0.25 ␮m thick-
◦
above reaction conditions but by using a double amount of the
water-soluble thioligand DHTANa.
ness): initial T
C; initial t 1.1 min; solvent delay 4.48 min; T
5
0
◦
◦
ramp 1.3 C/min; final T1
C. (R)-configuration was determined by
50
polarimetric measurements by using a Perkin Elmer 241 polarime-
2
0
ter ([␣] D = + 43, c = 5 in CH OH for pure (R)-X). ICP-MS analysis
3
2.4. Hydrogenation experiments
was performed by using an Agilent 7500a-Series instrument.
All reactions were carried out following the procedure below
described for the hydrogenation of 2-cyclohexen-1-one (I). Experi-
2.2. Dihydrothioctic acid sodium salt (DHTANa) preparation
mental details for all the hydrogenations are reported in Tables 1–4.
Dihydrothioctic acid (DHTA) was prepared as described [25].
Thioctic acid (510 mg, 2.5 mmol) was dissolved in a solution
2.4.1. Hydrogenation of 2-cyclohexen-1-one (I)
of 210 mg (2.5 mmol) of Na CO in 12 mL of H O and cooled to
2
3
2
◦
In a Schlenk tube, 1 mL of a 0.005 M solution of Rh(DHTANa) in
deaerated water and 480 mg (5.0 mmol) of 2-cyclohexen-1-one (I)
0
C in an ice bath. NaBH4 (190 mg, 5 mmol) was added slowly and
◦
the temperature kept below 4 C while stirring for an additional
in 2 mL of deaerated H O were stirred under nitrogen. The Schlenk
2
h. The reaction mixture was acidified with 2 M HCl to pH 1 and
2
tube was then transferred into a 150 mL stainless steel autoclave
under nitrogen, pressurized with H2 and stirred for the due time
extracted with CHCl3 (3 × 10 mL). The combined organic phases
were dried over MgSO4 and filtered. Evaporation of the solvent
yielded 507 mg (98%) of DHTA as a clear colorless oil. The oil was
◦
at 60 C (Table 2). The reactor was then cooled to room tempera-
◦
1
ture and the residual gases released. Diethyl ether was added to
the reaction mixture and the organic phase was separated, dried
maintained under nitrogen at low temperature (−20 C). HNMR
(
2
CDCl ): ı 10.1 (bs, 1H, OH), 2.89 (m, 1H, S CH), 2.7 (m, 2H, S CH ),
3 2
on Na SO4 and analyzed by GC and GC–MS. The catalytic aqueous
.4 (t, J = 7.1 Hz, 2H, CH2 COOH), 1.92–1.4 (m, 8H, (CH ) ), 1.36 (t,
2
2
4
13
phase was recycled for further experiments after addition of fresh
of 2-cyclohexen-1-one (I).
J = 7.9, 1H, SH), 1.31 (d, J = 7.6, 1H, SH). C-NMR: ı 180.4 (COOH),
3.1 (SH CH2 CH2 CH), 39.7 (CH2 COOH), 39.1 (CH2 CH SH),
4
Table 1
Aqueous biphasic hydrogenation of cycloehex-2-en-1-one (I) catalyzed by Rh(DHTANa).
Run
1
Sub/cat. (molar ratio)
t (h)
Conv. (%)
II yield (%)
III yield (%)
IV yield (%)
TOFa
500
500
500
4
4
4
4
4
4
4
6
6
6
99
99
99
84
69
69
55
99
99
99
95
90
93
82
68
68
54
96
97
97
4
9
6
2
1
1
1
3
2
2
nd
nd
nd
nd
nd
nd
nd
nd
nd
nd
124
124
124
105
173
173
138
165
165
165
b
2
b
3
b
4
500
5
6
7
1000
1000
1000
1000
1000
1000
b
b
8
9
0
b
b
1
◦
Substrate = 5 mmol; T = 60 C; p(H2) = 2 MPa; H2O = 2 ml; toluene = 1 ml. nd = not detected in the reaction mixture.
a
−1
TOF = (mol I/mol cat.)h
.
b
Experiment carried out by using the catalytic phase recovered from the previous run.

6
6
S. Paganelli et al. / Catalysis Today 247 (2015) 64–69
Table 2
Hydrogenation of cycloehex-2-en-1-one (I) catalyzed by Rh(DHTANa) in pure water.
Run
1
pH2 (MPa)
t (h)
Conv. (%)
II yield (%)
III yield (%)
IV yield (%)
TOFa
2
2
2
1
1
1
1
1
1
1
2
4
4
4
4
4
4
2
2
2
99
99
99
99
99
99
97
82
80
60
3
95
94
94
90
93
95
94
80
78
58
2
4
5
4
9
6
4
3
2
2
2
1
nd
nd
nd
nd
nd
nd
nd
nd
nd
nd
nd
248
248
248
248
248
248
485
410
400
300
2
b
2
3
b
4
5
6
b
b
7
8
9
0
1
b
b
b
1
1
2
18
c
◦
Substrate = 5 mmol; substrate/Rh (molar ratio) = 1000/1; T = 60 C; H2O = 2 ml. nd = not detected in the reaction mixture.
a
b
c
−1
TOF = (mol I/mol cat.)h
.
Experiment carried out by using the catalytic phase recovered from the previous run.
Experiment carried out in the presence of Rh(DHTANa)2.
Table 3
deaerated H O and 2 mL of toluene were stirred under nitrogen.
2
Aqueous biphasic hydrogenation of furfural (V) catalyzed by Rh(DHTANa).
The Schlenk tube was transferred into a 150 mL stainless steel auto-
clave under nitrogen, pressurized to 8 MPa with syngas (CO/H2 = 1)
Run
1b
t (h)
Conv. (%)
VI yield (%)
VII yield (%)
VIII yield (%)
TOFa
◦
and heated at 80 C for 18 h (see Table 5). The reactor was then
6
6
6
38
69
67
40
99
98
99
71
38
69
67
40
99
98
99
71
nd
nd
nd
nd
nd
nd
nd
nd
nd
nd
nd
nd
nd
nd
nd
nd
32
58
56
33
31
31
31
22
2
3
4
cooled to room temperature and the residual gases released. The
organic phase was separated and dried on Na SO . The two iso-
c
2
4
c
6
meric aldehydes XII and XIII were characterized by GC and GC–MS.
The aqueous phase was then used as catalyst for a consecutive
recycling experiment after addition of fresh styrene and toluene.
5
6
7
8
16
16
16
16
c
c
c
◦
Substrate = 2.5 mmol; substrate/Rh (molar ratio) = 500/1; T = 60 C; p(H2) = 2 MPa;
3
. Results and discussion
Solvent: H2O/THF (2 ml/1 ml). nd = not detected in the reaction mixture.
a
b
c
−1
TOF = (mol V/mol cat.)h
Solvent: H2O (2 ml).
.
Thioctic acid is known as a very useful natural antioxidant which
Experiment carried out by using the catalytic phase recovered from the previous
is both fat- and water-soluble and contains a carboxyl group and
two sulfur atoms. In nature exists in two forms, such as cyclic
disulfide (oxidized form, TA) or as open chain with the name of
dihydrothioctic acid, which shows two sulfydryl groups in position
run.
Table 4
Aqueous biphasic hydrogenation of acetophenone (IX) catalyzed by Rh(DHTANa).
6
and 8 (reduced form, DHTA) and the two forms are, however,
TOFa
ee%b (Conf.)
easily interconvertible via redox reactions [26]. DHTA can be easily
obtained from its oxidized form by NaBH4 reduction [25]; as such,
it is not soluble in water but its sodium salt (DHTANa) is highly
water-soluble and, in principle, could represent a good and cheap
chelating agent for rhodium catalytic precursors. To our knowledge
this simple molecule has never been used as ligand to carry out
aqueous biphasic catalytic hydrogenation and hydroformylation
processes, therefore we started our research by studying the hydro-
genation of 2-cyclohexen-1-one (I); this ␣,␤-unsaturated molecule
might represent a useful model substrate to evaluate both activ-
ity and selectivity of the new water-soluble catalyst Rh(DHTANa)
Run
1
Sub/cat. (molar ratio)
Conv. (%)
X yield (%)
500
500
500
100
100
70
57
30
95
65
70
57
30
95
65
18
14
8
5
3
15 (R)
8 (R)
4 (R)
5 (R)
5 (R)
c
2
3
4
5
c
c
◦
Substrate = 4 mmol; T = 60 C; p(H2) = 2 MPa; t = 20 h; H2O = 2 ml.
a
−1
TOF = (mol IX/mol cat.) h
.
b
Determined by GC equipped with cyclodex B (Agilent) capillary column
20
(
30 m × m 0.32 nm I.D: ×0.25 ␮m thickness); ([˛]D = + 43, c = 5 in CH3OH).
c
Experiment carried out by using the catalytic phase recovered from the previous
run.
(Scheme 1, Table 1). The catalytic precursor was prepared in situ
by mixing in deaerated water the dihydrothioctic acid sodium salt
2.5. Hydroformylation of styrene (XI)
and [Rh(COD)Cl] , maintaining a one to one molar ratio between
2
Rh and the ligand. A first experiment was carried out in the biphase
system water/toluene at 60 C and 2 Mpa of H2 for 4 h, by using a
In a Schlenk tube, 1 mL of a 0.005 M solution of Rh(DHTANa)
in deaerated water, 520 mg (5.0 mmol) of styrene (XI), 1 mL of
◦
Table 5
Aqueous biphasic hydroformylation of styrene (XI) catalyzed by rhodium complexes modified with DHTANa.
Run
1
Catalyst
Conv. (%)
XII yield (%)
XIII yield (%)
XIV yield (%)
XII/XIII
TOFa
Rh(DHTANa)
99
97
99
99
4
99
99
96
83
82
84
80
4
86
83
81
16
15
15
18
nd
13
16
15
nd
nd
nd
nd
nd
nd
nd
nd
5.2
5.5
5.6
4.4
55
54
55
55
2
55
55
53
b
2
b
3
b
4
5
6
7
8
Rh(DHTANa)2
b
5.6
5.2
5.4
b
b
◦
Substrate = 5 mmol; substrate/Rh (molar ratio) = 1000/1; T = 80 C; p(CO) = p(H2) = 4 MPa; t = 18 h; H2O = 2 ml; toluene = 1 ml. nd = not detected in the reaction mixture.
a
−1
TOF = (mol XI/mol cat.) h
.
b
Experiment carried out by using the catalytic phase recovered from the previous run.

S. Paganelli et al. / Catalysis Today 247 (2015) 64–69
67
Scheme 1. Hydrogenation of 2-cyclohexen-1-one (I).
Scheme 3. Hydrogenation of acetophenone (IX).
substrate/Rh molar ratio 500/1: conversion was 99% and selectivity
to cyclohexanone (II) was about 96%, being also cyclohexanol (III)
formed, even if in a very small amount (run 1, Table 1). Same results
were obtained in two consecutive recycling experiments and only a
low decrease of activity, but not of selectivity, was observed when
the catalyst was reused for the third time (84% conversion, 98%
selectivity) (run 4, Table 1). Due to the high activity of this catalytic
system we carried out some hydrogenations at the above reaction
conditions but using a substrate/Rh molar ratio 1000/1: in this case
a conversion decrease, using the same reaction time, was observed
atom, RH(DHTANa) , conversion was very low (18%) after 18 h at
60 C and 2 Mpa of H2 (substrate/Rh molar ratio = 1000/1) (run 11,
Table 2): the presence of two moles of the chelating thioligand
surely makes more difficult the coordination of the unsaturated
substrate to rhodium and the following oxidative addition of hydro-
gen.
2
◦
The results obtained in the hydrogenation of 2-cyclohexen-1-
one (I) spurred us to investigate the activity of this new catalytic
system in the hydrogenation of furfural (FAL) (V) as the design
of novel and ecofriendly catalysts, at low cost, for the efficient
transformation of natural resources or biomass-derived platform
chemicals is a very promising approach [27–29]. Lignocellulose is a
potential sustainable and renewable organic carbon source and in
particular FAL is a promising building block for fuels and chemicals.
It is usually obtained by acid-catalyzed dehydration of pentoses,
five carbon sugars, such as xylose and arabinose, or produced by
fast pyrolysis of biomass. However, the high oxygen content makes
FAL unstable against condensation and polymerization reactions
that provide undesirable storage properties. Hydrogenation of FAL
(V) to furfuryl alcohol (FOL) (VI) is a method used to avoid these
undesired reactions [30,31]. FOL is also a very important inter-
mediate in the chemical industry since it is widely used in the
production of synthetic fibers, thermostatic resins, fine chemicals,
etc. [32–36]. It is mainly produced by hydrogenation of FAL (V)
either in gas or liquid phase. Gas-phase hydrogenation of furfural
is a high energy demanding process due to the high tempera-
tures necessary for vaporizing FAL [37]; moreover, a big amount of
byproducts is formed. On the other hand the liquid-phase process
requires solvent and high pressures of hydrogen but it is char-
acterized by a lower energy consumption and, generally, higher
selectivities to furfuryl alcohol are reached [35,38]. Recently fur-
fural has been hydrogenated to furfuryl alcohol in water, under
very mild reaction conditions, by using polymer stabilized Ru(0)
nanoparticles in the presence of cyclodextrins [39]. To our knowl-
edge, only few examples of hydrogenation of FAL to FOL catalyzed
by homogeneous metal complexes are reported in the literature
[40–43] but none of them describes the use of rhodium compounds.
(
5
69%) but the selectivity to cyclohexanone (II) was almost 99% (run
, Table 1). The catalytic aqueous phase was used in a consecutive
recycling experiment and both conversion and selectivity remained
unchanged. Disappointingly, a second recycling reaction showed a
decrease of the catalytic activity: conversion was 55% but selectiv-
ity to cyclohexanone (II) remained very high (run 7, Table 1). In
order to increase the substrate conversion, the reaction time was
prolonged to 6 h. Conversion in both first experiment, carried out
in the presence of fresh catalyst, and in two consecutive recycling
reactions was practically quantitative and also selectivity toward
cyclohexanone (II) was always 98% (run 8–10, Table 1). It is impor-
tant to note that the catalytic system Rh(DHTANa) showed to be
very stable ad no leaching phenomenon was observed. As a matter
of fact, the organic phase recovered after the first hydrogenation
reaction (run 8, Table 1) was used as catalyst in the homogeneous
◦
hydrogenation of undec-1-ene in toluene: after 4 h at 60 C and
2
MPa of H , undecane was not formed at all. Moreover, analysis of
2
the same organic phase, carried out by ICP-MS, indicated that not
more than 1 ppm of rhodium was extracted from the aqueous to
the organic phase.
Due to the good solubility in water of 2-cyclohexen-1-one (I) we
decided to carry out the reaction in a homogeneous aqueous system
using water as the sole reaction solvent (Table 2). Very good results
◦
were obtained at 60 C and 2 Mpa of H for 4 h and by using a 1000/1
2
substrate/Rh molar ratio: conversion was always 99% in both first
reaction and in two recycling experiments and cyclohexanone (II)
was the prevailing product (95% selectivity) (runs 1–3, Table 2).
Due to the high activity of the catalytic system in simple water,
we studied the optimization of the reaction conditions lowering
H2 pressure to 1 MPA: again, conversion was almost quantita-
tive and it maintained practically unchanged also in two recycling
experiments (runs 4–6, Table 2). Then we decided to decrease the
reaction time to 2 h: the first experiment gave a 97% conversion and
a 97% selectivity to cyclohexanone (II); disappointingly, two con-
secutive recycling experiments showed a lower catalytic activity,
First, FAL (V) was hydrogenated in sole water as reaction solvent
◦
but after 6 h at 60 C and 2 MPa of H by using a substrate/Rh molar
2
ratio 500/1 conversion to furfuryl alcohol (VI) was only 38% (run 1,
Table 3) (Scheme 2). The reaction was then carried out in the system
water/tetrahydrofurane (THF) so allowing a better phase contact
between the water containing catalyst and the organic substrate.
By using the above reaction conditions, conversion was 69% and,
again, furfuryl alcohol (VI) was the only product formed (run 2,
Table 3). Activity and selectivity remained practically unchanged
in a recycling experiment while a further reused of the catalyst
afforded only 40% conversion but, again, furfuryl alcohol (VI) was
8
2 and 80% conversion, respectively, but the selectivity to cyclo-
hexanone (II) did not change (runs 8 and 9, Table 2). Noteworthy,
when the reaction was carried out by using the catalytic precursor
containing two moles of the water soluble thioligand per rhodium
Scheme 2. Hydrogenation of furfural (FAL) (V).

6
8
S. Paganelli et al. / Catalysis Today 247 (2015) 64–69
Scheme 4. Hydroformylation of styrene (XI).
obtained with complete selectivity (run 3 and 4, Table 3). In order
to enhance the substrate conversion, we decided to maintain the
mild reaction conditions adopted (T and P) but to prolong the
reaction time up to 16 h: conversion of V was now practically
complete affording the desired alcohol VI with total selectivity
process represents one of the most striking example of this cat-
alytic methodology [1–3,46–51]. Since 1984, C4 and C5 aldehydes
from propene or butene are produced by using the aqueous biphasic
OXEA process (former Ruhrchemie/Rhône-Poulenc) catalyzed by
the water soluble complex Rh/TPPTS (TPPTS = triphenylphosphine-
3,3 ,3 -trisulfonic acid trisodium salt) [48–51]. Besides phosphines
as TPPTS, many other new ligands and/or surfactants having dif-
ferent hydrophilic groups such as COOH, NR3, OH, etc. have
been designed to prepare new water soluble catalytic precursors
[1–3,47].
ꢀ
ꢀ
(
run 5, Table 3). Same results were obtained in two consecutive
recycling experiments but when the catalyst was reused for the
third time conversion was only 71%; despite the lower conversion,
again FOL was selectively obtained (run 8, Table 3). A comparison
of Rh(DHTANa) activity with other catalytic systems is rather dif-
ficult as most of the homogeneous catalysts used in this reaction
are based on ruthenium instead of rhodium and they all are active
in organic solvent. For example, Ru(TMHD)3 (TMHD = 2,2,6,6-
tetramethyl-3,5-heptanedione) selectivity reduces FAL to FOL in
The reaction of styrene with Rh(DHTANa) was carried out at
◦
80 C and 8 MPa of syngas (CO/H = 1/1) by using a substrate/Rh
2
molar ratio of 1000/1: after 18 h conversion was practically
quantitative and the branched aldehyde XII, as expected, was
the prevailing product [46], obtained with 85% selectivity (run
1, Table 5): both catalyst activity and selectivity maintained
unchanged in three recycling experiments (run 2–4, Table 5). Very
interestingly, some experiments were carried out also in the pres-
ence of the catalytic system containing two moles of DHTANa per
◦
THF at 80 C and 2.8 MPa of H2 but using a 100/1 substrate/Ru
molar ratio [40], while RuCl (PPh ) selectively produces FOL
2
3 3
◦
in 3 h at 70 C and 3 MPa of H in the presence of a relevant
2
amount of catalyst (substrate/catalyst molar ratio = 39/1) [41]. Also
ruthenium(II)-bis(diimine) complexes are very active and selec-
◦
tive in the hydrogenation of FAL to FOL in EtOH at 90–130 C and
rhodium atom [Rh(DHTANa) ]: the first reaction gave a conversion
2
1
–5 MPa of hydrogen pressure with the catalyst ranging from 0.4
practically negligible (4%), affording the only branched aldehyde
XII (run 5, Table 5), but in three consecutive recycling experi-
ments conversions were practically quantitative and selectivities
to 2-phenylpropanal (XII) very close to those previously observed
in presence of the catalytic system Rh(DHTANa) (Rh/DHTANa 1/1)
(run 6–8, Table 5): on the base of these results we can suppose that,
under the reaction conditions adopted, a molecule of DHTANa can
dissociate rendering available two possible coordination sites on
the rhodium atom so making easier the addition of both CO and H2
to smoothly complete the hydroformylation catalytic cycle. In all
the experiments ethylbenzene was never detected in the reaction
mixture. Despite the organic phase, after the first recycle experi-
ment (run 6, Table 5), appeared practically colorless, the increase
of activity, in principle, could be due to the rhodium leaching. With
the aim to evaluate a possible leaching phenomenon, the organic
phase was used as catalyst for the homogeneous hydrogenation
to 1 mol% [42]. Very interestingly, ruthenium–carbene complexes
selectively reduces FAL to FOL by transfer hydrogenation at 60 C in
◦
2
4 h but, in this case, the H-donor i-propanol inevitably produces
acetone as by-product [43]. At the light of these results we can
assert that not only Rh(DHTANa) is active in the green solvent water
but it is able to hydrogenate FAL to FOL at mild reaction conditions
and by using a rather low substrate/Rh molar ratio (500/1).
As also (R)-thioctic acid is readily commercial available, we used
the sodium salt of its reduced form, (R)-DHTANa, in the hydro-
genation of acetophenone (IX), in order to evaluate both activity
and enantioselectivity of the rhodium catalyst modified with this
ligand (Scheme 3).
The reaction was carried out using water as the sole reaction
solvent because of the fine miscibility of the substrate. Prelimi-
nary results were carried out at 60 C and 2 MPa of H2 for 20 h
with a substrate/Rh molar ratio of 500/1: in this case conversion
to 1-phenylethanol (X) was quite good (70%) but the enantiose-
lectivity to the (R)-isomer was really disappointing, being only
◦
◦
of undec-1-ene in toluene: after 18 h at 80 C and 8 MPa of hydro-
gen, undecane was formed in negligible amount (1%) so excluding
a relevant rhodium leaching. It is worth to note that the new cat-
alyst Rh(DHTANa) works efficiently without the requirement of
an excess of the water-soluble thioligand, so making this catalytic
system even more sustainable.
1
5% (run 1, Table 4); moreover, in this case, the catalytic precur-
sor did not demonstrate to be a robust system showing a strong
decrease of both conversion and enantioselectivity in two consec-
utive recycling experiments (run 2 and 3, Table 4). Also carrying
out the reaction at the above reaction conditions but with a 100/1
substrate/Rh molar ratio, the only positive result was a conversion
enhancement but the enantiomeric excess was very low (run 4,
Table 4).
4. Conclusions
Finally we evaluated the activity of Rh(DHTANa) also in the
hydroformylation of styrene (XI), chosen as model substrate
The preliminary results presented in this communication show
that Rh(DHTANa) seems to be an interesting new water-soluble
and recyclable catalyst for hydrogenation and hydroformylation
reactions, with fine-excellent chemo- and regio-selectivity, at least
in the studied application, so spurring us to extend the study
for knowing its applicability and limits in some relevant indus-
trial processes. Noteworthy, this new catalyst showed to be quite
robust and, on the contrary of the most common water-soluble
catalytic systems, no ligand excess is required for its activity and
recyclability. Furthermore, our approach to study simple, low-
molecular weight, commercial available and cheap thiocompounds
(
Scheme 4). Hydroformylation is one of the most important indus-
trial reactions catalyzed by soluble metal complexes [44–46]. The
most efficient catalysts in terms of both activity and selectivity
are rhodium complexes, capable to operate under mild condi-
tions [46]. In order to overcome the drawback of the separation
of the expensive catalyst from the product mixture, liquid–liquid
two-phase systems have been studied and developed [1,47]. In
particular, the use of environmentally more benign solvents such
as water has been developed [1–3,47] and the hydroformylation

S. Paganelli et al. / Catalysis Today 247 (2015) 64–69
[22] N. Khiar, R. Navas, E. Álvarez, I. Fernández, ARKIVOC (2008) 211–224.
69
for preparing new and promising water-soluble catalysts is, in our
opinion, fruitful for sustainable applications.
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