Dehydrogenation of glycerol to dihydroxyacetone catalyzed by iridium complexes with P-N ligands

Article abstract of DOI:10.1039/c003542d

The chemoselective dehydrogenation of glycerol was catalyzed by organoiridium derivatives of the type [HIr(cod)L] (cod = 1,5-cyclooctadiene; L = Prⁿ-N(CH₂CH₂PPh₂)₂, Et₂NCH₂CH₂N(CH₂CH ₂PPh₂)₂, o-Me₂NC₆H ₄PPh₂) using hydrogen acceptors such as acetophenone, cyclohexanone, styrene and benzaldehyde. The catalytic reactions were performed in the absence of a basic cocatalyst in order to avoid decomposition of the desired product, i.e. dihydroxyacetone. Acceptor-less dehydrogenation was also observed either in the absence of a hydrogen acceptor, or as a parallel route, when the reaction was performed in the presence of acetophenone. The Royal Society of Chemistry 2010.

Full text of DOI:10.1039/c003542d

View Article Online / Journal Homepage / Table of Contents for this issue
PAPER
www.rsc.org/greenchem | Green Chemistry
Dehydrogenation of glycerol to dihydroxyacetone catalyzed by iridium
complexes with P–N ligands
Corrado Crotti,a,b Jan Kasˇparb,c and Erica Farnetti*b,c
Received 23rd February 2010, Accepted 14th April 2010
First published as an Advance Article on the web 24th May 2010
DOI: 10.1039/c003542d
The chemoselective dehydrogenation of glycerol was catalyzed by organoiridium derivatives
of the type [HIr(cod)L] (cod = 1,5-cyclooctadiene; L = Prn-N(CH2CH2PPh2)2,
Et2NCH2CH2N(CH2CH2PPh2)2, o-Me2NC6H4PPh2) using hydrogen acceptors such as
acetophenone, cyclohexanone, styrene and benzaldehyde. The catalytic reactions were performed
in the absence of a basic cocatalyst in order to avoid decomposition of the desired product, i.e.
dihydroxyacetone. Acceptor-less dehydrogenation was also observed either in the absence of a
hydrogen acceptor, or as a parallel route, when the reaction was performed in the presence of
acetophenone.
reported examples of glycerol oxidation to DHA include mi-
Introduction
crobial fermentation with Gluconobacter oxydans,3 which is the
Glycerol is the main byproduct of the biodiesel production
currently used method for industrial production, and electrocat-
process, and its availability has greatly increased in the last
alytic oxidation in the presence of TEMPO;4 heterogeneously-
decade. Therefore, glycerol is nowadays pointed out as one
catalyzed reactions are generally poorly selective,5 with the
of the most important renewable raw materials.1 However, the
exception of some platinum catalysts developed by Kimura
very reason that makes this molecule an attractive candidate
and co-workers.6 An efficient, selective, catalytic procedure for
for chemical reactions, i.e. its high degree of functionalization,
glycerol oxidation to DHA is presently lacking.
represents a severe limit to its selective transformation. As
a matter of fact, the most challenging problem concerning
glycerol valorization is represented by selectivity. Moreover, if
on one hand glycerol is a very abundant, safe organic building
block, on the other its physicochemical properties, such as
strong hydrophilicity and high viscosity, greatly increase the
experimental difficulties.
Among the most promising glycerol valorization routes2
is selective oxidation, which can lead to a variety of target
molecules (see Scheme 1), which are either commercially valu-
able themselves or are interesting building blocks. In fact, so
far, the market for these chemicals has been limited because
Scheme 1
of their high cost. Therefore, efficient, selective processes are
We recently proposed a novel route for glycerol valorization7
needed in order to make them widely and cheaply available. The
using homogeneous iridium-based catalysts of the type
development of catalytic processes that afford a single oxidation
[Ir(diene)(N–N)X] (diene = 1,5-hexadiene, 1,5-cyclooctadiene;
product with a good selectivity is therefore highly desirable. The
N–N = 2,2¢-bipyridine, 1,10-phenanthroline and substituted
catalyst must allow the control of glycerol oxidation towards
derivatives; X = Cl, I) to promote the chemoselective dehy-
either the primary hydroxyl groups, to produce glyceric acid,
drogenation of glycerol to DHA under hydrogen transfer con-
or the secondary alcohol function, to give dihydroxyacetone
ditions. The idea was that glycerol would behave as a hydrogen
(DHA) and hydroxypyruvic acid. DHA is presently employed
donor, as it can be considered as a disubstituted 2-propanol.
in the cosmetics industry as a component in artificial tanning
Unfortunately, in spite of the excellent chemoselectivity of the
preparations. If available at a lower market price, it would also
catalytic systems employed, the final yields of DHA were lower
find various applications as a versatile building block. Previously
than expected due to the poor stability of the target molecule in
the presence of the necessary basic cocatalyst. It can be recalled
aCNR—Istituto Struttura della Materia, Unita` staccata di Trieste,
S.S.14, Km 163.5, 34149, Basovizza, Trieste, Italy
that the great majority of transfer hydrogenation catalysts
require a basic cocatalyst in order to achieve an appreciable
catalytic activity.8
In the course of previous studies on hydrogen transfer
reactions, in our laboratories, we had developed iridium
catalysts with P–N-type ligands that showed the unusual
bINCA (Interuniversity Consortium “Chemistry for the Environment”),
Research Unit of Trieste, Italy
cDipartimento di Scienze Chimiche, Universita` di Trieste, Via Giorgieri 1,
34127, Trieste, Italy. E-mail: farnetti@units.it; Fax: +39 040 5583903;
Tel: +39 040 5583938
This journal is
The Royal Society of Chemistry 2010
Green Chem., 2010, 12, 1295–1300 | 1295
©

View Article Online
Table 1 Hydrogen transfer reactions between glycerol and acetophe-
ability to promote the reaction in the absence of a ba-
sic cocatalyst.9,10 Such complexes, namely [HIr(cod)(PNP)]
none catalyzed by iridium complexesa
(PNP = Prn-N(CH2CH2PPh2)2), [HIr(cod)(P2N2)] (P2N2
=
Yield (%)b
Et2NCH2CH2N(CH2CH2PPh2)2) and [HIr(cod)(P-NMe2)] (P-
NMe2 = o-Me2NC6H4PPh2), appeared to be suitable candidates
to study glycerol dehydrogenation in the absence of a basic
cocatalyst, with the aim of minimising the decomposition of
the DHA product. Here, we report the results obtained by the
use of such catalysts, together with those observed with other
organoiridium compounds in the base-free hydrogen transfer
dehydrogenation of glycerol in the presence of a ketone, olefin
or aldehyde as a hydrogen acceptor, as well as some results of
acceptor-less glycerol dehydrogenation reactions.
Entry
Catalyst
1-Phenylethanol
DHA
1
2
3
4
5
6
7
HIr(cod)(PNP)
HIr(cod)(P2N2)
HIr(cod)(PCy2Ph)2
HIr(cod)(dppp)
HIr(cod)(P-NMe2)
IrCp*(NHC)Cl2
[IrCp*Cl2]2
8
<1
<1
1
3
3
6
<1
<1
1
2
1
1
1
a Experiment◦al conditions: [Ir] = 3.0 ¥ 10-3 M, [acetophenone]/[Ir] =
100, T = 120 C, reaction time = 1 h. b Based on the initial acetophenone
amount.
Results
In the catalytic reaction in the presence of 1 at 120 ◦C, after
1 h, GC analysis of the reaction mixture showed a conversion of
8%, measured as the decrease in acetophenone concentration
and the quantitative formation of the reduction product, 1-
phenylethanol. The only glycerol oxidation product detected was
DHA, but the yield of the latter compound was not quantitative,
probably due to partial thermal degradation. When the same
reaction was performed using 2 as the catalyst, only traces
of 1-phenylethanol and DHA were detected, even after longer
reaction times.
The iridium complexes [HIr(cod)L] (L = PNP (1), P2N2 (2)),
which were prepared by the reaction of [Ir(cod)(OMe)]2 with
one equivalent of the phosphine ligand, were in fact isolated as
2
their isomers [Ir(s,h -C8H13)L] (see Fig. 1).
Other selected iridium catalysts, all of which are
known to promote hydrogen transfer under baseless condi-
tions, were tested for glycerol dehydrogenation. [HIr(cod)-
(PCy2Ph)2],12 [HIr(cod)(dppp)] (dppp = 1,3-bis(diphenylphos-
phino)propane)13 and [HIr(cod)(P-NMe2)]9 only gave low con-
versions (see Table 1, entries 3–5). On the other hand,
the complexes [Cp*IrCl2]2 (Cp* = pentamethylcyclopentadi-
enyl) and [Cp*Ir(NHC)Cl2] (NHC = 1,3-di-n-butylimidazol-
ylidene)14,15 promoted the formation of small amounts of DHA,
the main reaction products being, in both cases, mixtures of
cyclic ketals formed by glycerol acetalization (see Table 1,
entries 6 and 7).
Based on these results, further investigations focused on
catalyst 1. We tested this compound at longer reaction times and
obtained only moderate increases of conversion, measured as 1-
phenylethanol yield, but lower selectivities of DHA formation.
This was because a longer permanence in solution at 120 ◦C
caused a more pronounced degradation of the hydroxyketone.
On the other hand, reactions performed at lower temperatures
resulted in a worse conversion but a higher selectivity. Inter-
estingly, in some cases, the DHA yield was higher than that
of 1-phenylethanol (e.g. at 100 ◦C, 5% and 3%, respectively)
(vide infra).
Fig. 1 Structures of compounds 1 and 2
As evidenced by NMR studies with both P–N ligands, in
solution, the hydrido cyclooctadiene compound rearranged to
the insertion product with a coordinated cyclooctenyl group;
in the resulting equilibrium, at r.t., the latter species was the
prevalent one, whereas the amount of hydrido isomer [HIr(h ,h -
C8H12)L] increased by raising the temperature.
In previous studies, compounds 1 and 2 have proved to be
active catalysts for the hydrogen transfer reduction of various
ketones, using a secondary alcohol, such as 2-propanol or
cyclopentanol, as a hydrogen donor in the absence of a basic
cocatalyst.9,11
2
2
Catalytic reactions with acetophenone
The hydrogen transfer reactions between glycerol and acetophe-
none (see Scheme 2) were initially performed using the former
as the solvent.
Scheme 2
The use of pure glycerol leads to low catalyst solubility.
Unfortunately, the alternative use of a glycerol/water mixture
caused a remarkable loss of DHA yield, thus ruling out the
greenest possible cosolvent. Some organic solvents were then
tested (Table 2); toluene and anisole were chosen as possible
catalyst protecting agents, since at 100 ◦C they gave rise to two-
phase mixtures, the catalyst being more soluble in the cosolvent.
On the other hand, dioxane and tert-butanol were selected as
they gave rise to homogeneous solutions where the catalyst
dissolved more easily than in glycerol alone. In fact, in all cases,
Compounds 1 and 2, as well as all the other iridium catalysts
tested, proved to be only partially soluble in glycerol, even at
temperatures above 100 ◦C; however, the catalytic reactions
were never performed at temperatures higher than 120 ◦C,
as our previous studies7 had evidenced that the expected
product, DHA, undergoes significant thermal degradation at
such temperatures (after 1 h in the absence of catalyst, DHA
◦
◦
decomposition was lower than 10% at 100 C, 20% at 120 C
and 45% at 140 ◦C).
1296 | Green Chem., 2010, 12, 1295–1300
This journal is
The Royal Society of Chemistry 2010
©

View Article Online
Table 2 Hydrogen transfer reactions between glycerol and acetophe-
Table 3 Hydrogen transfer reactions between glycerol and various
acceptors catalyzed by HIr(cod)(PNP)a
none catalyzed by HIr(cod)(PNP) in the presence of a cosolventa
Yield (%)b
Yield (%)b
Entry
Cosolvent
1-Phenylethanol
DHA
Entry
Acceptor
Hydrogenated acceptor
DHA
1
2
3
4
5
—
3
1
1
1
2
5
4
6
9
5
1
2
3
4
5
Acetophenone
Cyclohexanone
Styrene
trans-Ethylcrotonate
Benzaldehyde
3
4
6
<1
37
5
5
8
2
23
1,4-Dioxane
Toluene
Anisole
tert-Butanol
a Experimental conditions: [Ir] = 3.0 ¥ 10-3 M, [acceptor]/[Ir] = 100,
a Experimental conditions: [Ir] = 3.0 ¥ 10-3 M, [acetophenone]/[Ir] =
100, T = 100 ◦C, reaction time = 1 h, glycerol/cosolvent = 3 : 1. b Based
on the initial acetophenone amount.
T = 100 ◦C, reaction time = 1 h. b Based on the initial acceptor amount.
the catalyst was completely dissolved by the reaction mixture.
For all the reactions with a cosolvent (Table 2, entries 2–5), the
conversions (measured as the amount of acetophenone reduced
to 1-phenylethanol) did not exceed 2%. Rather surprisingly,
the yield of DHA was, in all cases, higher than that of 1-
phenylethanol, which could be explained in terms of two parallel
reaction paths, i.e. a hydrogen transfer between glycerol and
acetophenone and a dehydrogenation of glycerol without the
participation of a hydrogen acceptor.
Catalytic reactions without a hydrogen acceptor
The acceptor-less dehydrogenation of alcohols is an uncommon
reaction, only a few examples having been reported recently.16 It
is needless to point out the importance of this process due to its
excellent atom economy and E factor.1a
Fig. 2 Comparison of reaction TONs with and without acetophenone
as a hydrogen acceptor in the presence of a cosolvent.
A series of dehydrogenation reactions in the absence of a
hydrogen acceptor (see Scheme 3), with or without cosolvent,
led to the formation of DHA, thus confirming that catalyst
1 promotes acceptor-less dehydrogenation. Even better yields
were achieved using other solvents that featured miscibility with
glycerol, low volatility and a negligible hydrogen donor ability:
1,3-dimethoxybenzene and 2-methyl-1-phenyl-2-propanol gave
dehydrogenation TONs up to 11 in 1 h at 100 ◦C (compared to
a dehydrogenation TON of about 2 obtained in the analogous
reaction without cosolvent).
Catalytic reactions with various hydrogen acceptors
In order to assess to what extent the choice of hydrogen acceptor
affects the glycerol dehydrogenation, other hydrogen acceptors
were investigated, i.e. cyclohexanone, an aldehyde (benzal-
dehyde) and two olefins (styrene and trans-ethylcrotonate)
(Table 3).
The catalytic reactions performed with the two ketones gave
similar results (see Table 3, entries 1 and 2), whereas styrene
was superior to the conjugated olefin (Table 3, entries 3 and
4). Notably, when benzaldehyde was employed as a hydrogen
acceptor (Table 3, entry 5), DHA yields higher than 20% were
obtained. In this case, significant amounts of acetals were
also formed. Such compounds have been identified on the
basis of GC-MS and NMR data as the cyclic acetals formed
from glycerol and benzaldehyde, i.e. cis- and trans-2-phenyl-
[1,3]dioxan-5-ol, and cis- and trans-(2-phenyl-[1,3]dioxolan-
4-yl)-methanol.
Scheme 3
Reaction temperatures either higher or lower than 100 ◦C gave
poorer results, as DHA yields were lower e.g. at 80 ◦C, whereas
at 120 ◦C the final amount of DHA had decreased, probably
due to thermal degradation; the latter effect was also observed
after longer reaction times. In all cases, a comparison of two
similar reactions where the only difference was the presence or
absence of acetophenone as a hydrogen acceptor was extremely
satisfactory, confirming that, when acetophenone was employed,
the final total yield of DHA was the sum of the amount formed
via hydrogen transfer and that formed via dehydrogenation. A
graphical comparison of the TONs of the catalytic reactions
with and without hydrogen acceptor is shown in Fig. 2.
Catalytic reactions with benzaldehyde
The use of benzaldehyde as a hydrogen acceptor in glycerol
dehydrogenation was further investigated. It is noteworthy that
for a long time aldehydes have not been considered good can-
didates as hydrogen acceptors,17 mainly due to their disposition
to undergo decarbonylation, thus forming catalytically-inactive
metal carbonyl derivatives; moreover, their low stability under
This journal is
The Royal Society of Chemistry 2010
Green Chem., 2010, 12, 1295–1300 | 1297
©

View Article Online
Table 4 Hydrogen transfer reactions between glycerol and benzalde-
With regard to the catalytic reactions reported here, our results
confirm that iridium-catalyzed glycerol dehydrogenation selec-
tively occurs at the secondary hydroxyl group, producing DHA
as the only product. Unfortunately, the chemical properties of
DHA make it susceptible to thermal degradation, thus lowering
the effective yields obtained. The need to avoid catalytic systems
that require a basic cocatalyst, towards which DHA has been
shown to be reactive, severely restricts the choice of catalysts that
can be employed. Actually, of the organoiridium compounds
tested here, only complex 1 provided good results in glycerol
dehydrogenation, whereas the other derivatives tested proved to
be less active.
hyde catalyzed by iridium complexesa
Yield (%)b
Entry
Catalyst
Benzyl alcohol
DHA
Acetals
1
2c
3d
4
HIr(cod)(PNP)
HIr(cod)(PNP)
HIr(cod)(PNP)
HIr(cod)(P2N2)
HIr(cod)(PNMe2)
37
46
18
5
23
24
12
8
7
16
14
15
23
5
4
7
a Experimental conditions: [Ir] = 3.0 ¥ 10-3 M, [benzaldehyde]/[Ir] = 100,
T = 100 ◦C, reaction time = 1 h. b Based on the initial benzaldehyde
amount. c Reaction time = 3 h. d [Benzaldehyde]/[Ir] = 200.
Rather important is the observation that acceptor-less glycerol
dehydrogenation does actually work. Even if limited yields of
DHA were achieved, the feasibility of the greenest possible glyc-
erol selective oxidation has unambiguously been demonstrated.
On the other hand, by an appropriate choice of hydrogen
acceptor, significant DHA yields can be produced in the absence
of cocatalysts, as well as cosolvents. If the use of either a ketone
or an olefin as a hydrogen acceptor only led to limited reaction
conversions, when benzaldehyde was employed, the hydrogen
transfer reactions occurred with considerable yields. Moreover,
with such an acceptor, compounds 2 and [HIr(cod)(P-NMe2)]
also promoted the hydrogen transfer reaction, although with a
catalytic activity lower than that of 1. Therefore, the best results
in the present studies were obtained by using benzaldehyde,
which is not commonly considered as a first choice hydrogen
acceptor.
The DHA yields reported here are quite high as, to our knowl-
edge, only a bismuth-modified optimized platinum catalyst has
given better results (37% yield of DHA at 70% conversion).20,21
Notably, as we recently reported, in the basic conditions typically
required by catalysts [Ir(diene)(N–N)X], the highest DHA yield
achieved was 13% at 33% conversion.7
Some features of the catalytic reaction are currently the object
of more detailed investigations, i.e. the reasons for catalyst
deactivation, which becomes significant after reaction times
of 1–2 h, and the competition after longer reaction times of
the acetalization reactions—which are themselves potentially
interesting processes.
the basic conditions generally required by hydrogen transfer
catalysis has made them poorly appreciated substrates in this
reaction. Only recently have reports appeared in the literature
demonstrating the successful reduction of aldehydes under
hydrogen transfer conditions.14b,18
Benzaldehyde was tested as a hydrogen acceptor along with
catalyst 1 under a range of different experimental conditions,
in addition to other iridium-based catalysts. The data reported
in Table 4 show that: (a) once more, longer reaction times led
to an increase of the overall conversion without a significant
increase in the DHA yield; moreover, when using benzaldehyde
as an acceptor, the amount of acetals formed increased with time
(see Table 4, entries 1 and 2); (b) higher aldehyde concentrations
([benzaldehyde]/[Ir] = 200 vs. 100) appeared to favour acetal-
ization rather than hydrogen transfer (compare Table 4, entries
1 and 3); (c) other iridium catalysts were less active than 1 with
respect to the hydrogen transfer reaction, whereas they appeared
to be more efficient in catalyzing the formation of acetals (see
Table 4, entries 4 and 5). The excess of benzyl alcohol with
respect to DHA (see Table 4, entries 1–3) could be attributed
to an iridium-catalyzed benzaldehyde dismutation; consistently,
both benzyl alcohol and benzoic acid were detected, allowing
benzaldehyde to react in the presence of 1 at 100 ◦C in wet
toluene.
Discussion
The present data confirm the difficulty of catalytic transfor-
mations of glycerol when it is used as both a reactant and a
reaction medium. In particular, the use of glycerol as a solvent
implies a generally low solubility of catalysts and reagents, the
unavoidable presence of water, with predictable consequences
on organometallic catalysts, and further problems related to its
high viscosity. These shortcomings might be overcome by the use
of a suitable cosolvent. However, this choice is, in most cases, in
contrast with the green chemistry principles stated by Anastas
and Kirchhoff.19 For this reason, in spite of the beneficial effect
of a cosolvent, as proved in our studies, no optimization of such
reactions was carried out, as our aim was not to tune up at
all costs the catalytic reaction, but rather prove that effective,
selective reactions can be performed on glycerol by using no
other reaction medium than glycerol itself. Furthermore, it turns
out that glycerol does not behave as simply a disubstituted
secondary alcohol, as proved by the difficult application of
known catalytic systems otherwise working for 2-propanol.
Conclusions
Glycerol selective dehydrogenation to DHA was achieved under
iridium-catalyzed, base-free hydrogen transfer conditions by
the appropriate selection of catalyst, hydrogen donor and
reaction conditions. Among all of the organoiridium derivatives
examined, only [HIr(cod)(PNP)] catalyzed the hydrogen transfer
reaction from glycerol to all the acceptors tested. With the use
of ketones and olefins, only low conversions were observed,
whereas the choice of benzaldehyde as the hydrogen acceptor
significantly raised the reaction conversion. DHA once more
proved to be an evasive molecule due to its propensity to undergo
thermal degradation, thus decreasing the actual yield of the
reaction.
The use of organic solvents improved the catalyst perfor-
mance; however, we preferred to stick to green chemistry prin-
ciples, so no detailed optimisation of the reactions performed
with cosolvents was carried out.
1298 | Green Chem., 2010, 12, 1295–1300
This journal is
The Royal Society of Chemistry 2010
©

View Article Online
Interestingly, the feasibility of acceptor-less glycerol dehy-
drogenation was also demonstrated, albeit in limited yield.
The development of such reaction would produce the greenest
possible route for glycerol dehydrogenation.
1.0 mL of cosolvent. The mixture was de-gassed as described
above.
Procedure for dehydrogenation reactions
In a typical acceptor-less dehydrogenation reaction, either
4.0 mL of glycerol or 3.0 mL of glycerol and 1.0 mL of cosolvent
were introduced into a Schlenk tube equipped with an argon
inlet. The solvent or mixture of solvents was de-aerated by
bubbling argon through a needle for 15 min. After the addition
of the catalyst (0.012 mmol), the reaction vessel was closed using
a serum cap and heated under vigorous stirring to 100 ◦C (or any
other chosen temperature) in a thermostated oil bath. In analogy
to the procedure followed for hydrogen transfer reactions, the
time when the oil bath reached 100 ◦C (or any other reaction
temperature, respectively) was considered as the start of the
catalytic reaction.
Experimental
General
Reactions and manipulations were all performed under an argon
atmosphere using standard Schlenk tube techniques.
Toluene was distilled over sodium; dioxane was distilled over
sodium benzophenone ketyl immediately before use. The GC
standard naphthalene was recrystallized from ethanol. All the
other chemicals were of reagent grade and were used as received
from their commercial supplier.
Iridium chloride hydrate, a loan from Johnson Matthey PLC,
was used as received.
After 1 h (or any other reaction time), the Schlenk tube was
cooled to r.t. and air let in under stirring. The addition of 10 mL
of methanol and the GC standard naphthalene provided the
final solution, which was analyzed by GC.
Instrumental
1H, 13C and 31P NMR spectra were recorded on a Varian
500 spectrometer operating at 500, 125.68 and 202.28 MHz,
respectively.
Analysis of the reaction mixtures
Infrared spectra were recorded in Nujol mulls on a Perkin-
Elmer System 2000 FT-IR spectrophotometer.
The composition of the final reaction mixtures was determined
by GC and GC-MS. Qualitative analysis was accomplished
by GC-MS using, where possible, authentic samples for com-
parison. Quantitative evaluation of product distributions were
performed by GC with naphthalene as the internal standard,
using response factors previously determined by the analysis
of standard solutions; the quantitative analysis thus performed
allowed a reproducibility within 1%.
The qualitative and quantitative analysis of acetals and ketals
formed as by-products in the catalytic reactions was performed
by NMR and GC-MS; the data were compared with those of
authentic samples obtained by conventional routes (i.e. acid-
catalyzed acetalization).
The chemical yields of the catalytic reactions were determined
by GC using an Agilent 6850 instrument equipped with an
Rtx-5 Restek capillary column (30 m length). The analysis of
reaction products was periodically sided by a parallel analysis
on a Hewlett-Packard 5890 Series II GC instrument coupled
to a Hewlett-Packard 5971A Mass Selective Detector equipped
with an identical column.
Synthesis of the iridium catalysts
Procedures reported in the literature were followed for the
preparation of [HIr(cod)(PNP)],9 [HIr(cod)(P2N2)],9 [HIr-
(cod)(P-NMe2)],10 [HIr(cod)(PCy2Ph)2],22 [HIr(cod)(dppp)],13
14
[Cp*IrCl2]2 and [Cp*Ir(NHC)Cl2].15
Acknowledgements
Johnson Matthey PLC is gratefully acknowledged for the
generous loan of iridium chloride. Support from the Interuni-
versity Consortium “Chemistry for the Environment” (INCA)
is acknowledged.
Procedure for hydrogen transfer reactions without cosolvent
In a typical catalytic reaction, 4.0 mL of glycerol were introduced
into a Schlenk tube equipped with an argon inlet and de-aerated
by bubbling argon through a needle for 15 min. After the
addition of the catalyst (0.012 mmol), the reaction vessel was
closed using a serum cap and heated under vigorous stirring to
the chosen reaction temperature in a thermostated oil bath. The
addition of the hydrogen acceptor (1.2 mmol) by a microsyringe
at the reaction temperature started the catalytic reaction.
After the desired reaction time, the Schlenk tube was cooled to
r.t. and air let in under stirring. 10 mL of methanol containing
the GC standard naphthalene were then added. The resulting
solution was further diluted and analyzed by GC.
References
1 (a) R. A. Sheldon, Chem. Commun., 2008, 3352; (b) P. Gallezot, Green
Chem., 2007, 9, 295.
2 (a) A. Behr, J. Eilting, K. Irawadi, J. Leschinski and F. Lindner,
Green Chem., 2008, 10, 13; (b) C.-H. Zhou, J. N. Beltramini, Y.-X.
Fan and G. Q. Lu, Chem. Soc. Rev., 2008, 37, 527; (c) M. Pagliaro, R.
Ciriminna, H. Kimura, M. Rossi and C. Della Pina, Angew. Chem.,
Int. Ed., 2007, 46, 4434.
3 D. Hekmat, R. Bauer and V. Neff, Process Biochem., 2007, 42, 71.
4 R. Ciriminna, G. Palmisano, C. Della Pina, M. Rossi and M.
Pagliaro, Tetrahedron Lett., 2006, 47, 6993.
5 (a) P. Fordham, M. Besson and P. Gallezot, Appl. Catal., A, 1995, 133,
L179; (b) N. Dimitratos, J. A. Lopez-Sanchez, D. Lennon, F. Porta,
L. Prati and A. Villa, Catal. Lett., 2006, 108, 147; (c) S. Carrettin, P.
McMorn, P. Johnston, K. Griffin, C. J. Kiely and G. J. Hutchings,
Phys. Chem. Chem. Phys., 2003, 5, 1329; (d) S. Carrettin, P. McMorn,
P. Johnston, K. Griffin, C. J. Kiely, C. A. Attard and G. J. Hutchings,
Procedure for hydrogen transfer reactions with cosolvent
The procedure was identical to that described in the previous
paragraph, but for the initial addition of a cosolvent. Typically,
the solvent mixture composition was 3.0 mL of glycerol and
This journal is
The Royal Society of Chemistry 2010
Green Chem., 2010, 12, 1295–1300 | 1299
©

View Article Online
Top. Catal., 2004, 27, 131; (e) P. Fordham, M. Besson and P. Gallezot,
Catal. Lett., 1997, 46, 195.
6 (a) H. Kimura, K. Tsuto, T. Wakisaka, Y. Kazumi and Y. Inaya, Appl.
Catal., A, 1993, 96, 217; (b) H. Kimura, Appl. Catal., A, 1993, 105,
147.
7 E. Farnetti, J. Kaspar and C. Crotti, Green Chem., 2009, 11, 704.
8 (a) S. Gladiali and E. Alberico, Chem. Soc. Rev., 2006, 35, 226; (b) G.
Zassinovich, G. Mestroni and S. Gladiali, Chem. Rev., 1992, 92, 1051.
9 C. Bianchini, E. Farnetti, M. Graziani, G. Nardin, A. Vacca and F.
Zanobini, J. Am. Chem. Soc., 1990, 112, 9190.
15 R. Corbera´n and E. Peris, Organometallics, 2008, 27,
1954.
16 (a) K. Fujita, N. Tanino and R. Yamaguchi, Org. Lett., 2007, 9,
109; (b) J. van Buijtenen, J. Meuldijk, J. A. J. M. Vekemans, L. A.
Hulshof, H. Koijman and A. Spek, Organometallics, 2006, 25, 873;
(c) J. Zhang, M. Gandelman, L. J. W. Shimon, H. Rozenberg and D.
Milstein, Organometallics, 2004, 23, 4026.
17 S. Gladiali and E. Alberico, in Transition Metals for Organic
Synthesis, ed. M. Beller and C. Bolm, Wiley-VCH, Weinheim, 2nd
edn, 2004, ch. 1.3, pp. 145–166.
10 E. Farnetti, G. Nardin and M. Graziani, J. Chem. Soc., Chem.
Commun., 1989, 1264.
11 E. Farnetti, N. Verma Gulati and M. Graziani, Gazz. Chim. It., 1993,
123, 165.
12 E. Farnetti, unpublished results.
18 (a) J. R. Miecznikowski and R. H. Crabtree, Organometallics, 2004,
23, 629; (b) A. N. Ajjou and J.-L. Pinet, J. Mol. Catal. A: Chem.,
2004, 214, 203.
19 P. T. Anastas and M. M. Kirchhoff, Acc. Chem. Res., 2002, 35,
686.
13 M. J. Fernandez, M. A. Esteruelas, M. Covarrubias and L. A. Oro,
J. Organomet. Chem., 1986, 316, 343.
14 (a) K. Fujita, S. Furukawa and R. Yamaguchi, J. Organomet. Chem.,
2002, 649, 289; (b) X. Wu, J. Liu, X. Li, A. Zanotti-Gerosa, F.
Hancock, D. Vinci, J. Ruan and J. Xiao, Angew. Chem., Int. Ed.,
2006, 45, 6718.
20 R. Garcia, M. Besson and P. Gallezot, Appl. Catal., A, 1995, 127,
165.
21 N. Wo¨rz, A. Brandner and P. Claus, J. Phys. Chem. C, 2010, 114,
1164.
22 M. Marigo, D. Millos, N. Marsich and E. Farnetti, J. Mol. Catal. A:
Chem., 2003, 206, 319.
1300 | Green Chem., 2010, 12, 1295–1300
This journal is
The Royal Society of Chemistry 2010
©