Glycerol as solvent and hydrogen donor in transfer hydrogenation-dehydrogenation reactions

Article abstract of DOI:10.1016/j.tetlet.2009.08.035

Glycerol is employed successfully as a green solvent and hydrogen donor in catalytic transfer hydrogenation-dehydrogenation reactions. The glycerol donates hydrogen to various unsaturated organic compounds under mild reaction conditions and as a solvent, allows easy separation of products and catalyst recycling.

Full text of DOI:10.1016/j.tetlet.2009.08.035

Tetrahedron Letters 50 (2009) 5951–5953
Contents lists available at ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Glycerol as solvent and hydrogen donor in transfer
hydrogenation–dehydrogenation reactions
*
Adi Wolfson , Christina Dlugy, Yoram Shotland, Dorith Tavor
Green Processes Center, Chemical Engineering Department, Sami Shamoon College of Engineering, Bialik/Basel Sts., Beer-Sheva 84100, Israel
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 26 June 2009
Revised 22 July 2009
Accepted 17 August 2009
Available online 20 August 2009
Glycerol is employed successfully as a green solvent and hydrogen donor in catalytic transfer hydrogena-
tion–dehydrogenation reactions. The glycerol donates hydrogen to various unsaturated organic com-
pounds under mild reaction conditions and as a solvent, allows easy separation of products and
catalyst recycling.
Ó 2009 Elsevier Ltd. All rights reserved.
Keywords:
Transfer hydrogenation–dehydrogenation
Catalysis
Glycerol
Green solvent
Redox reactions are a family of reactions that deal with the
transfer of electrons between species. Several routes, using differ-
addition of catalytic amounts of an organic or inorganic base is also
required to activate the metal complex and/or to assist in proton
dissociation from the hydroxy group of the alcohol. Transfer hydro-
genation–dehydrogenation coupled reactions with heterogeneous
bases are also common. These reactions are proposed to operate
through the Meerwein–Ponndorf–Verley–Oppenauer mechanism,
which entails direct hydrogen transfer from a C–H of the alcohol
to a carbonyl carbon via a cyclic mechanism. A common solid base
catalyst is MgO, which catalyzes transfer hydrogenation in either
1
ent reducing agents and oxidants, are available for reduction and
oxidation, respectively, of organic compounds. In general, three
techniques exist for the reduction of unsaturated organic com-
pounds, which include the use of metal hydrides, employing
molecular hydrogen, or via transfer hydrogenation. Likewise,
organics can be oxidized by three main routes: (i) oxidation with
toxic inorganic oxidants, which usually generate high levels of pol-
lution; (ii) catalytic dehydrogenation or catalytic oxidative dehy-
drogenation, in which oxygen or peroxides are used as oxidants;
and (iii) catalytic transfer dehydrogenation, in which an organic
oxidant adsorbs the hydride and proton.
5
,7
gas or liquid phase. Recently it was reported that insoluble solid
alkaline metal hydroxides by themselves can also be used as cata-
lysts for transfer hydrogenation–dehydrogenation reactions in or-
8
ganic solvents.
Transfer hydrogenation–dehydrogenation reaction, in which
hydrogen is transferred from an alcohol to an unsaturated bond,
is an environmentally benign method for reducing unsaturated
compounds or oxidizing alcohols.2 One of the advantages of this
pathway is that it is performed in the absence of gaseous hydrogen
or oxygen, which requires precautions and special high pressure
equipment. Although transfer hydrogenation–dehydrogenation is
not as highly developed as catalytic hydrogenation or oxidation,
it is recognized as an attractive technology with a high potential
for wide industrial applicability.
Several organic compounds, such as 2-propanol and tetralin,
have been used as solvents and as hydrogen donors in the transfer
hydrogenation of unsaturated organic compounds. We recently
5
–5
reported that glycerol can also be successfully employed as a versa-
tileand alternative greensolvent ina variety of organic reactionsand
synthetic methodologies, all of which achieve high product conver-
9
–12
sions and selectivities.
We therefore explored the possibility of
usingglycerol, as with other alcohols, as an environmentally friendly
‘donor solvent’ in transfer hydrogenation–dehydrogenation reac-
1
3
tions. A polar, non-toxic, biodegradable, and recyclable liquid
manufactured from renewable sources, glycerol has promising
physical and chemical properties that facilitate the dissolution of
inorganic salts, acids, and bases and of enzymes and transition metal
complexes. Furthermore, it also dissolves organic compounds that
are poorly miscible in water and it is considered non-hazardous.
Hydrophobic solvents such as ethers and hydrocarbons, which are
immiscible in glycerol, enable the products to be removed by simple
A variety of homogeneous and heterogeneous catalysts can be
used in transfer hydrogenation–dehydrogenation reactions, such
as salts and complexes of Pd, Pt, Ru, Ir, Rh, Fe, Ni, and Co in addition
to their supported heterogeneous analogs.6 In many systems, the
*
Corresponding author.
E-mail address: adiw@sce.ac.il (A. Wolfson).
0
040-4039/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2009.08.035

5
952
A. Wolfson et al. / Tetrahedron Letters 50 (2009) 5951–5953
O
OH
OH
O
O
Ru-catalyst
a.
1
2
1
2
R
R
+
+
HO
OH
R
R
+
+
HO
HO
OH
OH
base
Pd/C
H
1
R =Ph, Alkyl
2
R =H, Alkyl
H
H
OH
H
H
H
b.
H
HO
OH
R1
R2
R1
R2
1
R =Ph, Alkyl
2
R =H, Alkyl
NO2
NH2
c.
OH
O
Raney nickel
base
+
3
HO
OH
+
3
HO
OH
+
2 H O
2
Figure 1. Transfer hydrogenations of unsaturated organic compounds in glycerol: (a) carbonyl compounds; (b) alkenes; and (c) nitrobenzene.
extraction. Additionally, the high boiling point of glycerol makes dis-
tillation of the products an attractive, efficient separation technique.
In addition to the favorable solubilities of both the reactants and the
catalysts and the easy separation of the product in reactions with
glycerol, transition metal complex recycling is also facilitated by
glycerol.
In addition, both these chemicals are used as starting materials for
the synthesis of many other important chemical compounds.
In this study, glycerol was used for the first time as both solvent
and hydrogen donor in the transfer hydrogenation of various rep-
resentative unsaturated organic compounds while itself being
dehydrogenated to dihydroxyacetone (Fig. 1). Different homoge-
neous and heterogeneous catalysts were tested.
Catalytic transfer hydrogenations of representative unsaturated
organic compounds were studied in reactions that typically uti-
lized homogeneous and heterogeneous metal catalysts and a base
as co-catalyst. The results are summarized in Table 1. In a typical
procedure, the reaction was performed as follows: in a 10 mL vial,
the appropriate amounts of metal catalyst together with base were
added to 4 g of glycerol. Then, 0.20–0.55 g of substrate was added
to the vial with stirring and the mixture was heated to 70 °C in an
The production of many important organic intermediates can be
14,15
accomplished using glycerol as the raw material.
Aerobic
oxidation of glycerol over supported metal and bimetallic catalysts
resulted in a variety of products such as dihydroxyacetone, glycer-
aldehyde, glyceric acid, glycolic acid, hydroxypyruvic acid, oxalic
acid, and tartronic acid. Dihydroxyacetone, glyceraldehyde, and
derivatives of the two are valuable precursors for target products
in an assortment of applications. For example, dihydroxyacetone
and its derivatives are used extensively in the cosmetics industry.
Table 1
Transfer hydrogenations using glycerol as solvent and hydrogen donora
Entry
1
Substrate (g)
Benzaldehyde
Catalyst (g)
Base (g)
Time (h)
5
Yield (%)
25
Ru(p-cumene)Cl
2
2
2
2
-dimer (0.16)
-dimer (0.16)
-dimer (0.16)
-dimer (0.16)
KOH
(0.02)
KOH
(0.02)
NaOH
(0.024)
(
0.22)
Benzaldehyde
0.22)
Benzaldehyde
0.22)
Benzaldehyde
0.22)
Benzaldehyde
0.22)
Benzaldehyde
0.22)
2-Octanone
0.22)
Cyclohexene
0.5)
Cyclohexene
0.)5
Styrene
0.5)
Nitrobenzene
0.25)
Nitrobenzene
0.25)
2
3
4
5
6
7
8
9
Ru(p-cumene)Cl
Ru(p-cumene)Cl
24
1
99
20
(
(
Ru(p-cumene)Cl
Et
3
N
1
17
(
(0.1)
KOH
(0.02)
KOH
(0.02)
KOH
(0.02)
—
b
RuCl
2
(TPPS)
3
5
18
(
(0.18)
5% Ru/C
(0.3)
5
12
(
Ru(p-cumene)Cl
2
-dimer
20
5
21
(
5 wt % Pd/C
(0.03)
5 wt % Pd/C
(0.03)
5 wt % Pd/C
(0.03)
Raney-nickel
(0.1)
81
(
—
—
9
100
100
11
(
10
11
12
5
(
NaOH
(0.2)
NaOH
(0.2)
5
(
Raney-nickel
(0.1)
24
43
(
a
Reaction conditions: glycerol (4 g), 70 °C.
TPPS: tris-(3-sulfophenyl)phosphine trisodium salt.
b

A. Wolfson et al. / Tetrahedron Letters 50 (2009) 5951–5953
5953
oil bath. The reactions were run for 5–24 h and were then allowed
to cool. The products were separated by extraction with diethyl
ether (3 Â 5 mL). The combined organic phase was concentrated
under reduced pressure, and the resulting crude residue was ana-
lyzed by gas chromatography (GC) using a HP-1 column to deter-
mine the extent of conversion.
substrates over three cycles with diethyl ether (25 mL), after which
fresh substrate was added and the reaction was run under condi-
tions similar to the first trial. Benzaldehyde conversions after the
second and third reaction cycles were found to be 56%, and 48%,
respectively. The lower conversion may be attributed to partial
deactivation of the catalyst. However, since hydrogenation–dehy-
drogenation transfers are equilibrium reactions, the conversion of
benzaldehyde in the second cycle may has been affected by the
partial dehydrogenation of glycerol, the hydrogen donor, during
the first reaction cycle.
In conclusion, we have demonstrated that glycerol can be suc-
cessfully utilized as an environmentally friendly solvent and as
the hydrogen donor in the transfer hydrogenation of representa-
tive unsaturated organic compounds. Dehydrogenation of glycerol
resulted in the formation of dihydroxyacetone.
As illustrated in Table 1, glycerol can be used as a solvent and
hydrogen donor in the transfer hydrogenation of various unsatu-
rated compounds. When carbonyl and nitro-compounds were re-
duced (entries 1–7, 11 and 12, respectively), the addition of a
base as co-catalyst was essential, and performing the reaction with
a metal catalyst in the absence of a base did not yield any product.
However, performing the reaction with only KOH, under similar
reaction conditions, yielded the corresponding products in limited
amounts, and the conversions were lower than 15% even after 24 h.
It was found that both organic and inorganic bases could be used as
co-catalysts (entries 3 and 4). On the other hand, the transfer
hydrogenation of cyclohexene and styrene (entries 8–10) was
run only in the presence of Pd/C, while the addition of KOH deac-
tivated the catalyst, resulting in negligible amounts of cyclohexane
and ethylbenzene. In general, increasing the reaction temperature,
reaction time, or catalyst loading increased the conversions in all
the tested reactions. Finally, it was found that glycerol was
dehydrogenated to dihydroxyacetone as expected from the higher
oxidation potential of the secondary alcohols, however, mass
balance calculations showed that under the reaction conditions,
some of the dihydroxyacetone decomposed.
References and notes
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The use of glycerol as a solvent also allowed easy product sep-
aration and catalyst recovery. For the first reaction cycle, benzalde-
hyde (2 g), Ru(p-cumene)Cl
2
-dimer (0.12 g), and KOH (0.17 g) were
1
1
0. Wolfson, A.; Dlugy, C. Chem. Papers 2007, 61, 228–232.
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dissolved in 25 g of glycerol and the reaction mixture was heated
in an oil bath to 70 °C. At the end of the reaction, after 18 hours,
the conversion of benzaldehyde was 62%. A catalyst recycling test
was performed after extraction of the product and the remaining
12. Wolfson, A.; Dlugy, C.; Shotland, Y. Environ. Chem. Lett. 2006, 5, 67–71.
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1
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