Substrate and product role in the Shvo's catalyzed selective hydrogenation of the platform bio-based chemical 5-hydroxymethylfurfural

Article abstract of DOI:10.1039/c4dt00304g

The bio-based substrate and target product 2,5-bishydroxymethylfuran (BHMF) demonstrated to influence the reaction kinetics in the homogeneous reduction of 5-hydroxymethylfurfural (HMF) catalyzed by the Ru-based Shvo's catalyst. A combined experimental an

Full text of DOI:10.1039/c4dt00304g

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Substrate and product role in the Shvo’s catalyzed
selective hydrogenation of the platform bio-based
chemical 5-hydroxymethylfurfural†
Cite this: Dalton Trans., 2014, 43,
10224
a,b
a
a
a,b
Thomas Pasini, Gavino Solinas, Valerio Zanotti, Stefania Albonetti,
a,b
a,b
a
a
Fabrizio Cavani,* Angelo Vaccari, Andrea Mazzanti,* Silvia Ranieri and
a
Rita Mazzoni*
The bio-based substrate and target product 2,5-bishydroxymethylfuran (BHMF) demonstrated to
influence the reaction kinetics in the homogeneous reduction of 5-hydroxymethylfurfural (HMF) cata-
lyzed by the Ru-based Shvo’s catalyst. A combined experimental and computational study supports an
2
important role of the –CH OH moiety which may be involved in the catalytic cycle toward the formation
of different intermediates from HMF and BHMF. The reaction is selective and leads to quantitative for-
mation of BHMF working under mild conditions. Furthermore, an optimized recycling procedure which
avoids the use of water, allows recover and reuse of the catalyst without loss of activity. The mechanistic
insights from this work may be extended to provide a general description of the chemistry of the Shvo’s
catalyst feeding further bio-based molecules.
Received 28th January 2014,
Accepted 17th April 2014
DOI: 10.1039/c4dt00304g
www.rsc.org/dalton
monomers in polymer industry. HMF reduction may be con-
ducted under stoichiometric conditions in the presence of
Introduction
9
10
Biorefinery concept is an important approach to address the NaBH
4
or by using aqueous solutions of HCHO and NaOH
increasing demand of energy and chemical building blocks for whereas typical catalysts reported for this reaction are copper
1
1
12
a diverse range of applications, that gradually may replace chromite, Ni RANEY®, Pt-, Pd- or Ru-supported cata-
1
5a,13
current dependence on fossil fuel resources. Among other lysts.
Main drawbacks in these reactions are the toxicity of
primary renewable building blocks, 5-hydroxymethylfurfural some of the catalysts employed and the high temperature
1
e
(HMF) is considered an important intermediate due to its rich (140–200 °C) and pressure (70–75 bar) required.
chemistry and potential availability from carbohydrates, such
as fructose, glucose, sucrose, cellulose and inulin. The struc- hydrogenation of polar double bonds provide a “green”
Even though metal–ligand bifunctional catalysts for the
2
1
4
tural moieties present in HMF, allow synthetic transformations alternative to stoichiometric reducing agents, homogeneous
to other target molecules by means of selective reactions such catalysis, that in principle might improve selectivity and
3
as oxidation of the formyl group, reduction of the formyl require mild reaction conditions, has been scarcely investi-
4
group and/or the furan ring, deoxygenation of the hydroxyl gated in this field (hydrogenation of HMF has been accom-
5
groups to 2,5-dimethylfuran, etherification and carbonyla- plished by hydrogen-transfer catalysts using HCOOH as
6
7
15
tion, and hydrogenolysis transformations. In particular, H-donor). On the light of these considerations we decided to
selective reduction of 5-hydroxymethylfurfural (HMF) provides employ a ruthenium based homogeneous hydrogenation cata-
5
access to the diol 2,5-bishydroxymethylfuran (BHMF), an lyst, known as the Shvo’s catalyst [Ph
4
(η -C
4
CO)
2
H(CO)
4
Ru
2
]-
1
6
important building block for the production of polymers and (μ-H) (1) (Fig. 1), which recently demonstrated to be effective
8,3c
7a
polyurethane foams.
These molecules may be also con- in the upgrading of bio-oil from white poplar by means of
verted into a range of polyol derivatives, with application as
a
Dipartimento di Chimica Industriale “Toso Montanari”, viale Risorgimento 4,
40136 Bologna, Italy. E-mail: rita.mazzoni@unibo.it, fabrizio.cavani@unibo.it,
andrea.mazzanti@unibo.it; Tel: +39 0512093700, +39 0512093680, +39 0512093632
b
Consorzio INSTM, Research Unit of Bologna, Firenze, Italy
†
Electronic supplementary information (ESI) available. CCDC 953038. For ESI
and crystallographic data in CIF or other electronic format see DOI: 10.1039/
c4dt00304g
Fig. 1 Chemical structure of the Shvo’s catalyst 1.
10224 | Dalton Trans., 2014, 43, 10224–10234
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1
7
hydrogenation of polar double bonds,
reduction of HMF to BHMF.
in the selective Table 1 Effect of temperature and H
2
pressure on BHMF yield. Reaction
conditions: HMF/1 = 1000, t = 1 h, [HMF] = 0.05 M, catalyst 1 = 1.71 mg,
1.58 µmol, toluene: 30 mL
The dinuclear complex 1 has been successfully applied in a
broad scope of hydrogen-transfer processes such as hydrogen-
ation of carbonyls and imines, oxidation of alcohols and
amines, and dynamic kinetic resolution of secondary alcohols
and primary amines in combination with lipases.
reduction reaction takes place by transferring a hydride
bonded to the metal centre) and a proton (bonded to a ligand)
Entry
H
2
pressure (bar)
Temperature (°C)
BHMF yield (mol%)
1
2
3
4
5
10
10
10
3
30
60
90
90
90
0
25
99
68
75
1
8
The
7
(
from the active catalytic species to a double bond. The reaction
mechanism for the hydrogen-transfer process has been a matter
1
9
of controversy, nevertheless several experiments and theore-
it is totally inactive at 30 °C (entry 1) while, increasing the reac-
tion temperature up to 60 °C, it leads to a slight formation of
BHMF (25% yield, entry 2). A significant improvement of the
catalytic activity is observed performing the reaction at 90 °C.
After 1 h at this temperature a BHMF yield of 99% is obtained
without formation of any by-products, being the reaction com-
pletely selective toward hydrogenation of the aldehyde (entry
2
0
tical studies, support an outer sphere concerted carbonyl
hydrogenation. Isolation of ruthenium-alcohol complexes, as
intermediates, is far to be trivial and, to the best of our knowl-
edge, an equivalent species has been observed by Casey et al. at
1
19f
−
60 °C in an H-NMR experiment.
Herein we report about a work aimed to the investigation of
the Shvo’s catalyst 1 in the hydrogenation of the bio-based
HMF to BHMF both in homogeneous (toluene) and biphasic
3
). BHMF yield improves significantly when increasing the H2
pressure (see Table 1) from 3 (yield = 68%, entry 4) up to
0 bar (yield 99%, entry 3); at higher pressures, in fact, H dis-
(water–toluene) reaction mixtures under mild conditions. The
1
2
reaction is selective and a successful recycling method, tuned
so to avoid the use of water, is also presented and discussed.
Furthermore reactivity studies on the target reaction showed a
surprising substrate and product effect on the reaction rate.
A combined experimental and computational study supports
an important role of the –CH OH moiety which may be
2
involved in the catalytic cycle toward the formation of different
intermediates involving Ru species and HMF–BHMF.
solution in the liquid phase increases, favouring regeneration
of the active complex.
Conditions employed in Table 1 entry 3, leading to com-
plete conversion within 1 h, have been chosen in order to
follow the BHMF yield in function of time. The reaction dis-
plays some interesting and unusual kinetic features. Fig. 2
shows a comparison of the hydrogenation rates of HMF (solid
line) and benzaldehyde (dashed line), both reactions being
performed with the Shvo’s catalyst 1 under the same con-
ditions. Hydrogenation of the benzaldehyde shows a conver-
sion rate profile corresponding to that reported by Casey and
co-workers in the reduction of benzaldehyde catalysed by Shvo
Results and discussion
Effect of the reaction conditions on catalyst performances
19e
complex at 60 °C and 35 bar of H2, whereas HMF hydrogen-
The hydrogenation of 5-hydroxymethylfurfural (HMF) to 2,5- ation exhibits a very steep increase of conversion rate within
dihydroxymethylfurfural (BHMF) (Scheme 1) was carried out
employing homogeneous Shvo’s catalyst (1). Screening exper-
iments were initially developed in order to evaluate the activity
and selectivity of this catalyst toward the target reaction, and
to study the effect of hydrogen pressure, temperature and reac-
tion time on catalytic performances; all reactions have been
performed in toluene solution and conversions obtained by
HPLC analyses. The reaction is totally selective toward the for-
mation of BHMF, thus BHMF yields actually correspond to
HMF conversions.
Table 1 shows the behavior of catalyst 1 at different reaction
temperatures and H pressures. Under the conditions reported
2
Fig. 2 HMF hydrogenation to BHMF (▲) or benzaldehyde hydrogen-
ation to benzyl alcohol (○) at different reaction times. Reaction con-
2
ditions: p(H ) = 10 bar, T = 90 °C, substrate/1 = 1000, [substrate] =
Scheme 1 Hydrogenation of HMF to BHMF catalyzed by Shvo’s
catalyst 1.
0.05 M (HMF or benzaldehyde), catalyst 1: 1.71 mg, 1.58 µmol, toluene:
30 mL.
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the first 20 min of reaction (corresponding to about 70%), fol- Effect of biphasic water–toluene conditions on catalyst
lowed by a pronounced downward curvature (corresponding to performances
the 70–90% conversion range). An unexpected rate increase is
Easy separation and reuse of expensive catalysts or products by
then observed within the last 10 minutes of reaction (between
means of aqueous/organic biphasic reactions that employ
9
0% and complete conversion). These observations seem to
single phase soluble catalysts or products is an attractive goal.
Thus, we extended our studies to hydrogenation of HMF in a
water–toluene mixture with the aim of partitioning the formed
BHMF in water, and keeping the Shvo’s catalyst 1 in the
organic phase. Furthermore this in situ separation, should, in
principle, avoid the above underlined competitive influence of
the product on the reaction rate.
indicate an inhibition effect due to the reagent HMF and the
product BHMF. A more detailed investigation on this peculiar
reaction rate profile is described below (see in situ NMR
experiments).
Hydrogenation of HMF in the presence of co-reactants
Biphasic water–toluene reactions were carried out using
tailed off quantity of water. Catalyst 1 resulted to be selective
even in the presence of water. BHMF yields under different
conditions are collected in Table 3. High water–toluene ratio
resulted in lower conversion to BHMF (water–toluene = 10,
yield = 11% after 30 min, entry 1) compared to reactions per-
formed in toluene (yield = 80%, entry 5). Reduction of the
water content improves the catalytic activity: a toluene–water
ratio of 29/1 (yield = 68% after 30 min, entry 3) allows in fact
to reach quantitative yield within 120 min (entry 4).
In order to understand the BHMF influence on the catalytic
behaviour, a comparison between the activity as a function of
time for the Shvo’s catalyst 1 in toluene (solid line) and in the
optimized biphasic system (toluene–water ratio 29/1, dashed
line) is reported in Fig. 3. In the case of biphasic reaction the
lower conversion rate is ascribable to the high affinity of HMF
and BHMF for water solution which makes the substrate less
available to enter the catalytic cycle. Since, as the best of our
knowledge partition ratios (organic solvents–water) for BHMF
are not available in the literature, a biphasic NMR experiment
HMF hydrogenation has been performed in the presence of co-
reactants such as the furan ring containing BHMF, furfuryl
alcohol or 2,5-bismethoxymethylfuran respectively containing
2
two, one or zero –CH OH moiety in order to establish if the
reduction rate is affected by the presence of the two hydroxyl
groups of BHMF. The reactions have been performed under
the conditions employed, while starting from the pure HMF, to
reach a BHMF yield (corresponding to HMF conversion)
around 70% (entry 1, Table 2) and resulted in all cases to be
selective toward the transformation of HMF to BHMF. Reaction
mixtures analysis showed unreacted furfuryl alcohol or 2,5-bis-
methoxymethylfuran confirming that they are inert under the
conditions employed. Turn over frequencies calculated in the
presence of furfuryl alcohol and 2,5-bismethoxymethylfuran
(entry 3 and 4) are comparable with the one obtained starting
from the pure HMF (entry 1). On the other hand, if BHMF is
added from the beginning of the reaction a significant
reduction of the reaction rate is recorded (entry 2).
Neither furfuryl alcohol nor 2,5-bismethoxymethylfuran
affect the reaction rate, supporting the hypothesis of an active
role played by the two hydroxyl groups of BHMF. The non-
furan derived 1,3-benzenedimethanol (containing two
2 8
in D O and toluene-d has been performed in order to evaluate
the distribution of substrate and product in the two solvents.
The spectra (available in the ESI†) show that the large majority
2
–CH OH moiety as the BHMF) has been also tested, but the
2
of HMF and BHMF reside in the D O phase. Actually in the
complete insolubility under the reaction conditions prevents
the co-reactant from participating to the catalytic cycle, leaving
unaltered the turn over frequency. Anyway all the co-reactants
tested contained the furan ring, so the difference between
non-inhibiting co-reactants and BHMF regards the presence of
present case a very low concentration of HMF and BHMF are
available during all the reaction reducing the reactant and
product inhibition. Indeed, the in situ BHMF removal operated
by water in biphasic mixture lead to a rate profile (Fig. 3 –
dashed line) similar to the one obtained performing benz-
aldehyde reduction (Fig. 2 – dashed line).
2
two –CH OH moieties.
Catalyst stability and recyclability
Table 2 Catalytic hydrogenation of HMF in the presence of co-reac-
tants. Reaction conditions: p(H
2
) = 10 bar, T = 90 °C, HMF/1 = 1000, In order to test the recyclability of catalyst 1 we have investigated
[
HMF] = 0.05 M, catalyst 1: 1.71 mg, 1.58 µmol, t = 20 min, HMF/co-reac-
its recovery and reuse employing two different recycling methods.
tant = 1.5
HMF conversion
(mol %)
TOF
(min )
−
1
Table 3 HMF hydrogenation to BHMF in biphasic systems, with
different toluene–water ratio. Reaction conditions: catalyst 1: 1.71 mg,
Entry
Co-reactant
None
BHMF
Furfuryl alcohol
1.58 µmol, HMF/1 = 1000, p(H
2
) = 10 bar, T = 90 °C
1
2
3
4
73
50
71
72
37
25
36
36
a
Entry
2
Toluene–H O (v/v)
Time (min)
BHMF yield (mol%)
2,5-Bismethoxymethylfuran
1
2
3
4
5
1/10
9/1
29/1
29/1
30
30
30
120
30
11
34
68
99
80
a
The same reaction performed in the presence of 1,3-
benzenedimethanol as co-reactant did not affect the catalytic activity
because it is completely insoluble under the conditions employed
(toluene, 90 °C).
Toluene
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was reached (1 h), BHMF quantitatively precipitated from the
toluene reaction mixture. Recover of the catalyst-containing
solution could be then performed by simple filtration at the
end of every cycle. No loss of activity was observed utilizing
this procedure during five runs (black bars in Fig. 4), empha-
sizing that the Shvo’s catalyst 1 can be easily recovered and
reused. Indeed by avoiding the water extraction step no catalyst
decomposition was observed, as confirmed both by the
absence of insoluble Ru-species in the recycled toluene-catalyst
1
solution and by its IR analysis, that showed the typical absor-
bance of Shvo 1 species in the carbonyl region (ν(CO) = 2039,
008, 1979 cm− ).
1
2
The latter recycling method is reproducible and represents
a convenient way of synthesis of BHMF in lab scale. The practi-
cal use of this preparation has been also verified on a larger
Fig. 3 BHMF yield as a function of the reaction time in: toluene (▲) or
biphasic mixture (toluene–H
2
: 1.71 mg, 1.58 µmol, HMF/1 = 1000, p(H ) = 10 bar, T = 90 °C.
2
O = 29) (○). Reaction conditions: catalyst scale. Experiments performed on 4.90 mmol of HMF lead to
1
quantitative conversion to BHMF. The potential of the catalyst
recycling has been confirmed up to nine cycles.
Product extraction by water. At first, since the catalyst is
In situ NMR experiments
insoluble in water, HMF and BHMF were recovered at the end
of every cycle (performed under conditions reported in
Table 1, entry 3) by water extraction. The toluene solution, con-
taining the used Shvo’s catalyst 1, was recycled. The white bars
in Fig. 4 correspond to the selective conversion of HMF to
BHMF in the five reaction cycles and show that yield progress-
ively decreases from 99% (first cycle) to about 30% (fifth
cycle). XRF analysis of the extracted water-phase indicated the
absence of water-soluble Ru species; on the other hand the
analysis of an insoluble residue found at the end of the re-
cycling experiments showed the presence of Ru traces, indicat-
ing that a partial catalyst decomposition occurs using this
approach. The isolated residue was completely insoluble in the
most common organic solvents and from its IR spectrum (KBr)
no absorbance ascribable to the Shvo 1 complex was detectable
confirming that it arises from catalyst decomposition.
Minute-by-minute monitoring of the Shvo’s catalyzed hydro-
genation of HMF. The hydrogenation of HMF (1.98 mg,
0
=
.0157 mmol) with catalyst 1 (0.0676 mg, 0.0628 µmol, HMF/1
250) under 3 bar of hydrogen at 90 °C in 0.7 mL of toluene-
d
8
was monitored by in situ NMR spectroscopy by warming at
90 °C the sample in the pre-heated NMR probe. The conver-
sion of HMF, which corresponds to the yield of BHMF, was
1
evaluated by integration of the H-NMR signals of HMF and
BHMF and spectra acquired every 60 seconds. During the
whole reaction carried out with HMF/1 ratio of 250, no signals
ascribable to intermediates or to the free Shvo catalyst were
observed, due to the tiny concentration. The complete conver-
sion was reached within 530 min. The kinetic curve depicted
in Fig. 5 (see ESI† for further details) confirmed that the con-
version of HMF (i.e. the yield of BHMF) follows a complex rate
law, as for the behavior observed performing the reaction in
autoclave (Fig. 3). The experiment was proved to be reproduci-
Product removal by filtration. The most effective recycle
method was obtained avoiding the use of water. Surprisingly
by cooling the autoclave as soon as the complete conversion
ble. A plot of ln([HMF]/[HMF] ) vs. time for the first
0
50 minutes of reaction (up to 23% conversion) showed to fit a
trend (Fig. 6) demonstrating a pseudo-first order rate for HMF
conversion in the first part of the reaction. On increasing the
amount of BHMF a decrease in the reaction rate was con-
firmed, as well as the step increase after 90% yield (in Fig. 5 –
around 430 min – as in Fig. 3 – around 50 min) which allows
to reach quantitative yield in 530 min (60 min in the autoclave
conditions).
The same procedure has been extended to 60 minutes long
minute-by-minute NMR monitored experiments performed by
varying HMF/1 ratio, keeping constant catalyst 1 (0.0676 mg,
0
.0628 µmol) and all the other conditions (selected data are
available in the ESI†). Turn over frequencies calculated at
0 minutes (low conversion 1.5–12.8%) plotted versus HMF/1
1
ratio (Fig. 7), demonstrate an effect of the substrate on the
catalyst behaviour which could be both ascribed to the for-
mation of a complex between HMF and the active catalyst A in
Scheme 2 by means of the aldehyde moiety or to the formation
Fig. 4 BHMF yields obtained reusing the catalyst 1 employing different
recycling procedures: □ water extraction; ■ BHMF precipitation by fast
cooling. Reaction conditions: catalyst 1: 1.71 mg, 1.58 µmol, HMF/1 =
1
2
000, p(H ) = 10 bar; T = 90 °C, t = 1 h, toluene = 30 mL.
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Fig. 5 In situ NMR experiment (BHMF yield vs. time). Reaction conditions: catalyst 1 (0.0676 mg, 0.0628 µmol), HMF (250 eq., 1.98 mg,
0
.0157 mmol), toluene-d
8
(0.7 mL), 90 °C, p(H
2
) (3 bar). Spectra acquired every 60 seconds.
reagent was found (see ESI†), in agreement with the fact that a
substrate to catalyst ratio of 125, falls within the linear part of
the TOF vs. HMF/1 ratio graphic (Fig. 7).
From the above described results, a role for HMF and
BHMF in the kinetic behavior of the reaction has to be taken
into account. Obviously, BHMF inhibition (product inhibition)
is expected to slow the reaction rate at high conversions and,
on the contrary, HMF inhibition should inhibit the rate at low
conversions. The overall result is more complex to predict and
depends on the dominating effect: if HMF inhibition prevails,
then initial inhibition should be gradually alleviated, and at
high conversions BHMF inhibition would take over, giving rise
to a sigmoidal conversion curve; if BHMF inhibition is stron-
ger than HMF inhibition, then the conversion should not go
to completion and rates would strongly decrease with time.
The kinetic profile observed both in the autoclave (see Fig. 3)
and in the in situ NMR experiments (see Fig. 5) is more
complex (i.e. the pronounced rate decrease between 70% and
Fig. 6 Plot of ln([HMF]/[HMF]
0
) vs. time for hydrogenation of HMF
(
[HMF] = 0.022 M) with catalyst 1 (0.088 mM).
0
90%, followed by an acceleration between 90% and 100% con-
version) and not fully consistent with a gradual replacement of
the HMF inhibition effect with the BHMF inhibition. This
suggests a more complex inhibiting effect, or an even more
intriguing combination of inhibiting and substrate accelera-
tion effects that we are unable to explain at the present stage.
In addition, a number of in situ NMR experiments with
8
HMF or BHMF and the catalyst 1 in toluene-d under nitrogen
or hydrogen atmosphere have been also investigated, and
details are reported in the ESI.† Experiments have been per-
formed with a substrate/1 ratio = 4, that is much lower com-
pared to the experiments described in previous paragraphs.
The aim was to detect possible intermediates or evidence
potential interactions between catalytic species that might
affect hydrogenation (or dehydrogenation) and explain the
observed inhibition effect. Experiments included hydrogen-
Fig. 7 In situ NMR experiment (TOF vs. HMF/1). Reaction conditions:
catalyst 1 (0.0676 mg, 0.0628 µmol), toluene-d (0.7 mL), 90 °C, p(H
3 bar), reaction time (10 min).
8
2
)
(
2
ation (H 3 bar, at 90 °C) of HMF in the presence of 1 and reac-
of a complex between HMF and the 16 electron specie B in tions of HMF and BHMF, respectively, with catalyst 1 under N2
Scheme 2, also involved in the catalytic cycle, by means of the atmosphere. Both hydrogenation and dehydrogenation were
CH OH function.
observed by monitoring HMF and BHMF selective interconver-
2
In order to gain a better understanding of the reaction, the sion. Moreover, reaction between BHMF and catalyst 1 under
kinetic profile has been also NMR-monitored up to the end of H2 atmosphere confirmed that H atmosphere suppresses
2
1
9
the reaction in the case of HMF/1 = 125 (corresponding to dehydrogenation.
HMF] = 0.011 M). When performing the reaction with half-
Results were consistent with what reported for the reactivity
[
0
19
concentration, a less pronounced inhibition effect of the of Shvo complex 1 with other substrates, confirming that 1,
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Scheme 2 Possible intermediates in Shvo’s catalyzed hydrogenation of HMF.
upon activation at 90 °C, works as a reducing agent in the benzyl alcohol (the latter follows the catalytic cycle proposed
1
9e
form of the 18 electron species, or as an oxidative agent in the by Casey et al.).
form of the 16 electron species (A and B, respectively in DFT calculations were performed to check if a transition
2
1
Scheme 2). Neither intermediates, nor species that should be state corresponding to that reported by Casey could be influ-
associated to interactions between ruthenium complexes and enced by the presence of the second hydroxyl group. At the
products/reagents, were detected. Finally, any attempt to same time, the geometries and energies of some feasible inter-
isolate possible stable intermediates from the almost stoichio- mediates were calculated, in order to check if they really corre-
metric solutions used in these NMR experiments, gave crystals sponded to energy minima and if they could influence the
of Shvo complex 1. Since the structure of 1 was not yet avail- reaction rate. Proposed intermediates involving BHMF and Ru
able we report its crystal data in the Cambridge database (see species are shown in Scheme 2. Calculations have been per-
2
2
ESI†).
formed on the complete catalyst using B3LYP and the
2
3
LANL2DZ basis set on all atoms. The solvent (toluene) was
included in the calculations using the PCM method. Given
Proposed reaction intermediates and DFT calculations
24
2
0,25
Although no intermediates were experimentally detectable, in the known X-ray data and calculations
available for these
view of the peculiar behaviour of the reaction rate profile, we compounds, the helical disposition of the four phenyl groups
speculated that the presence of the second hydroxyl group in was employed in all the starting geometries.
2
0
the product BHMF and the presence of the OH in HMF itself
As widely proposed, calculated transition state for the
might be the principal responsible for the reaction rate inhi- reduction of HMF corresponds to the simultaneous transfer of
bition experimentally observed. This consideration implies two hydrogens from the active catalyst A to HMF in an outer-
2
6
that this moiety has to play a role during the catalytic cycle, sphere mechanism (see Fig. 8, center). Because of the pres-
and also the presence of the oxygen on furan should be con- ence of the free OH of HMF, at a variance with the literature
sidered. These functional groups were not present in the data, the actual transition state exhibits an additional inter-
2
corresponding hydrogenation reaction of benzaldehyde to action of the CH OH of HMF with one carbonyl of the ruthe-
Fig. 8 Center: transition state for the reduction of HMF and the two intermediates found by IRC calculations. On the left is reported the catalyst A
coordinated with HMF. On the right is reported the intermediate between B and BHMF. The phenyl groups of the catalyst were removed for clarity.
Distances in Å.
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nium by an hydrogen bond with the oxygen of CO. The calcu- sidered (with solvent), C and C′ are isoenergetic and more
−
1
−1
lated free energy barrier for the hydrogenation (9.2 kcal mol
)
stable than D by 2.9 kcal mol (mainly because of the entro-
pic factor).
2
0
is in agreement with other reported values.
From this transition state, an IRC calculation [in the IRC
Given the presence of two hydroxyl groups in BHMF, an
the solvent was not included to reduce the computational cost] alternative framework speculates the interaction of a second
was performed to find the intermediates on the forward and molecule of the 16-electron species B with the free hydroxyl
backward pathway (see ESI† for the graphics). In the case of moiety of complex C′ to yield a bis-ruthenium complex (B -
2
the backward directions, the optimization localized a situation BHMF). The optimization of the 2 : 1 complex at the B3LYP/
where HMF is coordinated to the active catalyst by two hydro- LANL2DZ level of theory confirmed that it did correspond to
gen bonds (Fig. 8, left), whereas the forward pathway optimiz- an energy minimum with global C
2
symmetry (Fig. 9).
ation found an intermediate where the BHMF is still A reliable calculation of the stabilization energy of B2-
coordinated to the catalyst by two hydrogen bonds (Fig. 8, BHMF with respect to the C′ complex is not straightforward
right). Both structures are minima of energy, as confirmed by because of the known effect of basis set superimposition error
2
7
frequency calculations. The latter intermediate subsequently (BSSE). When the stabilization energy between the B -BHMF
2
evolves to the 16-electron species B and BHMF. The presence and the 1 : 1 complex C′ was calculated by the sum of the ener-
2
0d
of the CH
2
OH moiety of HMF seems therefore to play a rele- gies of the 1 : 1 complex C′ and one molecule of free B,
the
vant role in the pre-TS coordination. To confirm the impor- dimeric structure was favored by 22.1 kcal mol− as internal
tance of the hydroxyl group of HMF, the analogue transition energy without thermochemical corrections, by 5.0 kcal mol
1
−
1
state was calculated for furfural and a completely different dis- if Gibbs free energies were considered (solvent included with
position of the furanic ring was found, whereas the O–H and PCM method). When BSSE was taken into account by the
2
8
Ru–H distances are identical in both structures (see ESI†). The counterpoise method (without solvent), the stabilization was
1
9b
−1
−1
latter TS is very similar to that proposed by Casey.
When BHMF is formed and leaves the catalytic cycle, it is
still able to compete with H
still 17.4 kcal mol (3.4 kcal mol with Gibbs free energies).
Thus, regardless the calculation method, the B -BHMF
2
2
in the coordination to the de- complex appears to be largely stabilized with respect to C, C′
hydrogenated catalyst B. Several intermediates, sketched as C, and D complexes. Although it is statistically disfavored due to
C′, D, E and B -BHMF in Scheme 2, might be proposed. The the high dilution and to the entropic factor, the large stabiliz-
2
optimized geometries of C, C′, D, and E did correspond to ation energy could be sufficient to drive the formation of the
energy minima (no imaginary frequencies were observed in dimer that subtract the active intermediate from the catalytic
the frequency analysis), and the optimized Ru–C bond length cycle, thus reducing the reaction rates when increasing the
were very similar to that observed in the solid state for similar BHMF concentration.
1
9i,20d
compounds.
Whilst the relative energy of E is much
As a last consideration, also HMF itself could be involved in
higher, the calculated energies for C, C′ and D are similar, and the formation of a complex similar to C′ where the free hydroxyl
the lowest energy structure depends on the calculation para- can coordinate with B (F in Scheme 3). The formation of this
meters. Intermediate C was calculated to be the most stable if intermediate might deliver inhibition of the reaction in the early
−
1
the internal energy is considered (by 2.2 kcal mol vs. C′ and stages, where the concentration of HMF is very high. Scheme 3
−
1
0
.5 kcal mol vs. D). If the solvent is included (toluene, PCM summarizes the energy profile for hydrogenation of HMF.
2
4
−1
method ) C was more stable than C′ and D by 2.0 kcal mol
At a first sight, it could seem odd that the A-HMF complex
and 1.2 kcal mol− respectively. If Gibbs free energy is con- is calculated to have the same energy of the separate
1
Fig. 9 DFT optimized structure for the B
2
-BHMF complex (PCM-B3LYP/LANL2DZ level). All the hydrogen atoms, except for the two hydroxyls
(
cyan), were removed for clarity. Distances in Å.
10230 | Dalton Trans., 2014, 43, 10224–10234
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Scheme 3 Energy profile for hydrogenation of HMF. Energy values (B3LYP/LANL2DZ level) are free Gibbs energies in kcal mol−1, and including the
solvent with PCM method. The same profile with thermal corrected enthalpies is reported in the ESI.† Dotted red and green lines indicate the for-
mation of intermediates that could inhibit the main reaction cycle.
2
0a
species,
because the two hydrogen bonds should substan- bition is likely to be related to the presence of two CH OH
2
tially stabilize the complex. However, the entropic factor disfa- functions. In addition, also HMF was found to exert some inhi-
vors the complex. When the thermal corrected enthalpy is bition effect (demonstrated by increasing HMF initial concen-
considered to produce the analogue scheme, this complex is tration in NMR monitored experiments). DFT calculations
−
1
stabilized by 12.6 kcal mol (see ESI†).
suggest the availability of several intermediates that might play
a role in the observed inhibition. Also the key transition state
of reduction seems to be affected by an additional interaction
of the hydroxyl group of HMF that interacts with one carbonyl
group of the catalyst.
Conclusion
The overall results show that Shvo’s catalyst provides a valu-
able tool in the hydrogenation of biomass derived building
blocks. At the same time, the work pointed out some unfore-
seen effects associated to the interaction of alcohols and
polyols, commonly present in biomass products, with Ru
active catalytic species, which need to be considered and
studied in more detail for the development of effective hydro-
genation up-grading technologies.
The Shvo’s catalyst 1 has been successfully employed in the
selective homogeneous hydrogenation of HMF to BHMF. The
reaction occurs under mild conditions and it is fully selective
toward the formation of the desired product. The catalyst may
be successfully recycled avoiding the use of water and remov-
ing the produced BHMF from the reaction mixture by precipi-
tation and filtration.
Reactivity studies and in situ NMR monitored experiments
on the target hydrogenation reaction showed an unusual and
intriguing kinetic profile, being of the pseudo-first order in
the first part of the reaction, and assuming a complex rate in
the last part, which is likely to be associated to the sum of
different inhibition effects.
Experimental section
Materials
In particular, lowering of the reaction rate at conversions
higher than 70% suggests an inhibition effect due to the Solvents: water has been freshly distilled; methanol (MeOH),
BHMF product. This has been evidenced by further experi- toluene, CDCl , toluene-d (Sigma Aldrich) and reagents: tetra-
ments in which the BHMF resulted to reduce the turn over fre- phenylcyclopentadienone, furfuryl alcohol (Sigma Aldrich),
quency of catalyzed HMF reduction. Interestingly, only BHMF triruthenium-dodecacarbonyl (Ru (CO)12) (Strem), 5-hydroxy-
3
8
3
exhibits this effect and not related species such as furfuryl methylfurfural (HMF), 2,5-bishydroxymethylfuran (BHMF)
alcohol, or 2,5-dimethoxymethylfuran, indicating that inhi- (SAFC) have been employed as purchased. 2,5-Bismethoxy-
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methylfuran has been prepared as described in the The in situ experiments have been performed in a Varian Inova
2
9
1
13
literature.
600 ( H, 599.7, C, 150.8 MHz) spectrometers at 90 °C. A spec-
trum was acquired and recorded every 60 seconds.
Analytical methods
HPLC analyses where performed on an Agilent Technologies Calculations
260 Infinity instrument, equipped with a C-18 core shell
column 50 × 4.6 mm employing a solution of 80% of 0.01 M
H PO and 20% acetonitrile as mobile phase. Identification of
1
All the calculations were performed using Gaussian 09 rev
A.02. All the geometries were fully optimized at the B3LYP/
LANL2DZ level including the solvent with the PCM method.
Vibrational analysis were done on all the optimized geometries
to confirm if they corresponded to energy minima (no imagin-
ary frequencies) or to transition states (one imaginary fre-
quency). In the case of transition state for hydrogenation, an
3
4
compounds was achieved by calibration using reference com-
mercial samples. The NMR spectra were recorded using Varian
Inova 300 ( H, 300.1; C, 75.5 MHz), Varian Mercury 400 ( H,
1
13
1
1
3
1
13
399.9; C, 100.6 MHz), Varian Inova 600 ( H, 599.7, C,
150.8 MHz) spectrometers. Infrared spectra were recorded at
298 K on a Perkin-Elmer Spectrum 2000 FT-IR spectro-
30
IRC calculation was performed using standard parameters.
photometer. XRF analysis were carried out using a Panalytical
Axios Advanced equipped with a Rh-target X-ray tube (4 kW
power) at 298 K. Powder analysis were recorded in vacuum
Acknowledgements
−
5
(10 bar) while liquid tests were carried out using Helium at The authors thank the Ministero dell’Universitá e della Ricerca
atmospheric pressure.
(MIUR) (projects: “Designing metal containing molecular frag-
ments for advanced chemical applications” and “Catalytic
upgrading of the C5 fraction in ligno-cellulosic biorefineries”).
Catalyst preparation
5
The catalyst [Ph
4
(η -C
4
CO)
2
H(CO)
4
Ru
2
](μ-H) (1) has been pre- INSTM is acknowledged for co-financing the PhD project of
1
9g
pared following a procedure reported in the literature
refluxing 3 equivalents of tetraphenylcyclopentadienone with support through the FARB Project “Catalytic transformation of
equivalent of Ru (CO)12 in MeOH for 40 h. The catalyst has biomass-derived materials into high added-value chemicals”,
been purified by precipitation and identified by comparison 2014–2015.
by T.P. The University of Bologna is acknowledged for financial
1
3
1
13
with literature data (IR, H-NMR, C-NMR).
Hydrogenation of HMF
References
HMF hydrogenation were carried out using an autoclave (Parr
Instrument series 4560) reactor (300 mL) equipped with a
mechanical stirrer (0–1200 rpm) and provision for measure-
ment of temperature and pressure. In a typical experiment the
reactor was charged with HMF (0.200 g, 1.58 mmol), catalyst 1
1 (a) P. N. R. Vennestrom, C. M. Osmundsen,
C. H. Christensen and E. Taarning, Angew. Chem., Int. Ed.,
2011, 50, 10502; (b) A. Corma, S. Iborra and A. Velty, Chem.
Rev., 2007, 107, 2411; (c) C. H. Christensen, J. Rass-Hansen,
C. C. Marsden, E. Taarning and K. Egeblad, ChemSusChem,
2008, 1, 283; (d) J. N. Chheda, G. W. Huber and
J. A. Dumesic, Angew. Chem., Int. Ed., 2007, 46, 7164;
(e) M. J. Climent, A. Corma and S. Iborra, Green Chem.,
2011, 13, 520; (f) P. Gallezot, Chem. Soc. Rev., 2012, 41,
1538; (g) R. A. Sheldon, Chem. Soc. Rev., 2012, 41, 1437;
(h) P. Gallezot, Green Chem., 2007, 9, 295.
2 For recent reviews on HMF as a building block platform:
(a) R. J. van Putten, J. C. van der Waal, E. de Jong,
C. B. Rasrendra, H. J. Heeres and J. G. de Vries, Chem. Rev.,
2013, 113, 1499; (b) A. A. Rosatella, S. P. Simeonov,
R. F. M. Frade and C. A. Afonso, Green Chem., 2011, 13,
754.
3 (a) T. Pasini, M. Piccinini, M. Blosi, R. Bonelli, S. Albonetti,
N. Dimitratos, J. A. Lopez-Sanchez, M. Sankar, Q. He,
C. J. Kiely, G. J. Hutchings and F. Cavani, Green Chem.,
2011, 13, 2091; (b) S. Albonetti, T. Pasini, A. Lolli, M. Blosi,
M. Piccinini, N. Dimitratos, J. A. Lopez-Sanchez,
D. J. Morgan, A. F. Carley, G. J. Hutchings and F. Cavani,
Catal. Today, 2012, 195, 120; (c) S. A. Davis, L. R. Houk,
E. C. Tamargo, A. K. Datye and R. J. Davis, Catal. Today,
2011, 160, 55; (d) M. A. Lilga, R. T. Hallen and M. Gray,
(1.71 mg, 1.58 µmol; HMF/1 molar ratio = 1000) and 30 mL of
toluene ([HMF] = 0.05 M, in case of homogeneous reactions)
or toluene–water (in variable ratio in case of biphasic reac-
tions). The autoclave was purged 2 times with N (40 bar) and
2
then pressurized at 10 bar (H ). The temperature was increased
to 90 °C and the reaction mixture was stirred at ca. 1000 rpm
for the time requested. At the end of reaction, the reactor was
cooled to room temperature and the final product BHMF (or
the mixture HMF–BHMF) was separated from catalyst 1 by pre-
cipitation (or by water extraction). All samples were carefully
weighed and analysed using HPLC. Particular care was devoted
to determination of the C balance, which was found to always
fall between 95 and 105% (calculated as the comparison
between converted HMF and product yields).
2
General procedure for in situ NMR experiments
In a J. Young valve equipped NMR tube, catalyst 1 (e.g.
0
1
.0676 mg, 0.0628 µmol) and the substrate HMF (e.g. 250 eq.,
.98 mg, 0.0157 mmol) were added and dissolved in 0.7 mL of
8
toluene-d under nitrogen atmosphere. The samples solutions
have been frozen in liquid nitrogen, the nitrogen inside the
tube removed under vacuum and restored with 3 bar of H2.
10232 | Dalton Trans., 2014, 43, 10224–10234
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Paper
Top. Catal., 2010, 53, 1264; (e) Y. Y. Gorbanev,
S. K. Klitgaard, J. M. Woodley, C. Christensen and
A. Riisager, ChemSusChem, 2009, 2, 672; (f) O. Casanova,
S. Iborra and A. Corma, ChemSusChem, 2009, 2, 1138;
J. Org. Chem., 2001, 66, 7931; (e) M. Yamakawa, H. Ito and
R. Noyori, J. Am. Chem. Soc., 2000, 122, 1466;
(f) K. J. Haack, S. Hashiguchi, A. Fujii, T. Ikariya and
R. Noyori, Angew. Chem., Int. Ed. Engl., 1997, 36, 285.
(
g) E. Taarning, I. S. Nielsen, K. Egeblad, R. Madsen and 15 T. Thananatthanachon and T. H. Rauchfuss, Chem-
C. H. Christensen, ChemSusChem, 2008, 1, 75; SusChem, 2010, 3, 1139.
h) A. Gandini, D. Coelho, M. Gomes, B. Reis and 16 (a) Y. Shvo, D. Czarkie and Y. Rahamim, J. Am. Chem. Soc.,
(
A. Silvestre, J. Mater. Chem., 2009, 19, 8656.
(a) Y. Nakagawa and K. Tomishige, Catal. Commun., 2010,
1986, 108, 7400; (b) Y. Blum, D. Czarkie, Y. Rahamin and
Y. Shvo, Organometallics, 1985, 4, 1459.
4
5
1
2, 154; (b) R. Alamillo, M. Tucker, M. Chia, Y. Pagan- 17 L. Busetto, D. Fabbri, R. Mazzoni, M. Salmi, C. Torri and
Torres and J. Dumesic, Green Chem., 2012, 14, 1413.
(a) T. S. Hansen, K. Barta, P. T. Anastas, P. C. Ford and 18 For recent reviews of Shvo’s catalyst: (a) M. C. Warner,
V. Zanotti, Fuel, 2011, 90, 1197.
A.
b) M. Chidambaram and A. T. Bell, Green Chem., 2010, 12,
253; (c) T. Thananatthanachon and T. B. Rauchfuss,
Angew. Chem., Int. Ed., 2010, 49, 6616; (d) Y. Roman-
Leshkov, C. J. Barrett, Z. Y. Liu and J. A. Dumesic, Nature,
Riisager,
Green
Chem.,
2012,
14,
2457;
C. P. Casey and J.-E. Backvall, Top. Organomet. Chem., 2011,
37, 85; (b) B. L. Conley, M. K. Pennington-Boggio, E. Boz
and T. J. Williams, Chem. Rev., 2010, 110, 2294;
(c) R. Karvembu, R. Prabhakaran and N. Natarajan, Coord.
Chem. Rev., 2005, 249, 911.
(
1
2
007, 447, 982.
(a) M. Balakrishnan, E. R. Sacia and A. T. Bell, Green Chem.,
012, 14, 1626; (b) P. Che, F. Lu, J. Zhang, Y. Huang, X. Nie,
J. Gao and J. Xu, Bioresour. Technol., 2012, 119, 433;
c) P. Lanzafame, D. M. Temi, S. Perathoner, G. Centi,
A. Macario, A. Aloise and G. Giordano, Catal. Today, 2011,
19 (a) C. P. Casey, J. B. Johnson, S. W. Singer and Q. Cui,
J. Am. Chem. Soc., 2005, 127, 3100; (b) C. P. Casey,
G. A. Bikzhanova, Q. Cui and I. A. Guzei, J. Am. Chem. Soc.,
2005, 127, 14062; (c) J. S. M. Samec, A. H. Ell, J. B. Aberg,
T. Privalov, L. Eriksson and J. E. Backvall, J. Am. Chem. Soc.,
2006, 128, 14293; (d) C. P. Casey, T. B. Clark and
I. A. Guzei, J. Am. Chem. Soc., 2007, 129, 11821;
(e) C. P. Casey, S. E. Beetner and J. B. Johnson, J. Am. Chem.
Soc., 2008, 130, 2285; (f) C. P. Casey and H. Guan, Organo-
metallics, 2012, 31, 2631; (g) C. P. Casey, S. Singer,
D. R. Powell, R. K. Hayashi and M. Kavana, J. Am. Chem.
Soc., 2001, 123, 1090; (h) J. B. Johnson and J. E. Backvall,
J. Org. Chem., 2003, 68, 7681; (i) C. P. Casey,
G. A. Bikzhanova, J. E. Backvall, L. Johansson, J. Park and
Y. H. Kim, Organometallics, 2002, 21, 1955.
6
2
(
1
2
75, 435; (d) O. Casanova, S. Iborra and A. Corma, J. Catal.,
010, 275, 236; (e) O. Casanova, S. Iborra and A. Corma,
J. Catal., 2009, 265, 109; (f) G. Papadogianakis, L. Maat and
R. A. Sheldon, J. Mol. Catal. A: Chem., 1997, 116, 179.
(a) T. Buntara, S. Noel, P. H. Phua, I. Melian-Cabrera,
J. G. de Vries and H. J. Heeres, Angew. Chem., Int. Ed., 2011,
7
5
0, 7083; (b) M. Chia, Y. J. Pagan-Torres, D. Hibbitts,
Q. Tan, H. N. Pham, A. K. Datye, M. Neurock, R. J. Davis
and J. A. Dumesic, J. Am. Chem. Soc., 2011, 133, 12675.
8
9
(a) A. Gandini, Green Chem., 2011, 13, 1061; (b) C. Moreau, 20 (a) A. Comas-Vives, G. Ujaque and A. Lledos, Organometal-
M. N. Belgacem and A. Gandini, Top. Catal., 2004, 27, 11.
(a) J. M. Timko and D. J. Cram, J. Am. Chem. Soc., 1974, 96,
lics, 2007, 26, 4135; (b) A. Comas-Vives, G. Ujaque and
A. Lledos, Organometallics, 2008, 27, 4854; (c) X. Zhang,
X. Guo, Y. Chen, Y. Tang, M. Lei and W. Fang, Phys. Chem.
Chem. Phys., 2012, 14, 6003; (d) D. Hollmann, H. Jiao,
A. Spannenberg, S. Bahn, A. Tillack, R. Parton, R. Altink
and M. Beller, Organometallics, 2009, 28, 473.
7
159; (b) S. Goswami, S. Dey and S. Jana, Tetrahedron, 2008,
6
4, 6358; (c) F. W. Lichtenthaler, A. Brust and E. Cuny,
Green Chem., 2001, 3, 201.
1
1
1
0 J. H. Turner, P. A. Rebers, P. L. Barrick and R. H. Cotton,
Anal. Chem., 1954, 26, 898.
1 T. Utne, J. D. Garber and R. E. J. Jones, US Patent,
21 M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria,
M. A. Robb, J. R. Cheeseman, G. Scalmani, V. Barone,
B. Mennucci, G. A. Petersson, H. Nakatsuji, M. Caricato,
X. Li, H. P. Hratchian, A. F. Izmaylov, J. Bloino, G. Zheng,
J. L. Sonnenberg, M. Hada, M. Ehara, K. Toyota, R. Fukuda,
J. Hasegawa, M. Ishida, T. Nakajima, Y. Honda, O. Kitao,
H. Nakai, T. Vreven, J. A. Montgomery Jr., J. E. Peralta,
F. Ogliaro, M. Bearpark, J. J. Heyd, E. Brothers,
K. N. Kudin, V. N. Staroverov, R. Kobayashi, J. Normand,
K. Raghavachari, A. Rendell, J. C. Burant, S. S. Iyengar,
J. Tomasi, M. Cossi, N. Rega, J. M. Millam, M. Klene,
J. E. Knox, J. B. Cross, V. Bakken, C. Adamo, J. Jaramillo,
R. Gomperts, R. E. Stratmann, O. Yazyev, A. J. Austin,
R. Cammi, C. Pomelli, J. W. Ochterski, R. L. Martin,
K. Morokuma, V. G. Zakrzewski, G. A. Voth, P. Salvador,
J. J. Dannenberg, S. Dapprich, A. D. Daniels, Ö. Farkas,
J. B. Foresman, J. V. Ortiz, J. Cioslowski and D. J. Fox,
3
,083,236, 1963, assigned to Merck & Co.
2 T. J. Connolly, J. L. Considine, Z. Ding, B. Forsatz,
M. N. Jennings, M. F. MacEwan, K. M. McCoy, D. W. Place,
A. Sharma and K. Sutherland, Org. Process Res. Dev., 2010,
1
4, 459.
1
3 (a) J. Lewkowski, ARKIVOC, 2001, 2, 17; (b) V. Schiavo,
G. Descotes and J. Mentech, Bull. Soc. Chim. Fr., 1991, 704;
(c) W. N. Haworth, W. G. M. Jones and L. F. Wiggins,
J. Chem. Soc., 1945, 1.A. C. Cope and W. N. Baxter, J. Am.
Chem. Soc., 1955, 77, 393.
4 See for examples: (a) S. E. Clapham, A. Hadzovic and
R. H. Morris, Coord. Chem. Rev., 2004, 248, 2201;
1
(
(
b) R. M. Bullock, Chem. – Eur. J., 2004, 10, 2366;
c) R. Noyori and T. Ohkuma, Angew. Chem., Int. Ed., 2001,
4
0, 40; (d) R. Noyori, M. Yamakawa and S. Hashiguchi,
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Paper
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GAUSSIAN 09 (Rev. A.02), Gaussian, Inc., Wallingford, CT, 26 The transition state was checked by frequency analysis that
2
009.
showed only one imaginary frequency corresponding to the
transfer of both hydrogens.
2
2 (a) P. J. Stephens, F. J. Devlin, C. F. Chabloski and
M. J. J. Frisch, J. Phys. Chem., 1994, 98, 11623; 27 S. F. Boys and F. Bernardi, Mol. Phys., 1970, 19, 553.
(b) A. D. Becke, J. Chem. Phys., 1993, 98, 5648.
28 L. Turi and J. J. Dannenberg, J. Phys. Chem., 1993, 97,
2
2
3 P. J. Hay and W. R. Wadt, J. Chem. Phys., 1985, 82, 299.
4 J. Tomasi, B. Mennucci and R. Cammi, Chem. Rev., 2005, 29 C. R. Illig, J. Chen, S. K. Meegalla and M. J. Wall,
2488.
1
05, 2999.
5 S. Brydges and M. J. McGlinchey, J. Org. Chem., 2002, 67,
688.
PCT WO, 052237, 2009, Assigned to Janssen
Pharmaceutica.
30 K. Fukui, Acc. Chem. Res., 1981, 14, 363.
2
7
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