Pt-based chiral organotin modified heterogeneous catalysts for the enantioselective hydrogenation of 3,4-hexanedione

Article abstract of DOI:10.1016/j.apcata.2012.08.031

In this paper we have studied the liquid-phase enantioselective hydrogenation of 3,4-hexanedione using Pt-based catalysts, modified with chiral organotin compounds derived from the (-)-menthyl group: (-)-Pt-MenSnBu ₃ and (-)-Men₃Sn-Sn-(-)-Men₃. The organotin chiral modifiers were carefully synthesized and characterized in order to obtain optically pure compounds. The catalysts were prepared through a controlled surface reaction between the supported transition metal and the organometallic compound, using techniques derived from Surface Organometallic Chemistry on Metals (SOMC/M). The organobimetallic catalytic systems were found to be active and enantioselective in the hydrogenation of 3,4-hexanedione, yielding an enantiomeric excess of 25-27% for 4-hydroxy-3-hexanone.

Full text of DOI:10.1016/j.apcata.2012.08.031

Applied Catalysis A: General 445–446 (2012) 209–214
Contents lists available at SciVerse ScienceDirect
Applied Catalysis A: General
journal homepage: www.elsevier.com/locate/apcata
Pt-based chiral organotin modified heterogeneous catalysts for the
enantioselective hydrogenation of 3,4-hexanedione
Virginia Vetere , María B. Faraonib,1, Julio C. Podestá , Mónica L. Casella
a,∗
b
a
a
◦
CINDECA (CCT-La Plata, CONICET, UNLP), 47 n 257, 1900, La Plata, Argentina
INQUISUR (CCT-Bahía Blanca, CONICET, UNS), Av. Alem N. 1253, 8000, Bahía Blanca, Argentina
b
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 6 July 2012
Received in revised form 15 August 2012
Accepted 17 August 2012
Available online 1 September 2012
In this paper we have studied the liquid-phase enantioselective hydrogenation of 3,4-hexanedione using
Pt-based catalysts, modified with chiral organotin compounds derived from the (−)-menthyl group: (−)-
Pt-MenSnBu3 and (−)-Men3Sn-Sn-(−)-Men3. The organotin chiral modifiers were carefully synthesized
and characterized in order to obtain optically pure compounds.
The catalysts were prepared through a controlled surface reaction between the supported transi-
tion metal and the organometallic compound, using techniques derived from Surface Organometallic
Chemistry on Metals (SOMC/M). The organobimetallic catalytic systems were found to be active and
enantioselective in the hydrogenation of 3,4-hexanedione, yielding an enantiomeric excess of 25–27%
for 4-hydroxy-3-hexanone.
Keywords:
Enantioselectivity
3
,4-Hexanedione
Hydrogenation
(
−)-Menthyl derivatives
© 2012 Elsevier B.V. All rights reserved.
PtSn
SOMC/M
1
. Introduction
Most of the work reported in the literature on the heterogeneous
ketones, optically active hydroxyketones and diols, are widely used
in the synthesis of pharmaceutical products. An interesting exam-
ple is the hydrogenation of 1-phenyl-1,2-propanodione, whose
hydrogenation product (R)-1-hydroxy-1-phenyl-2-propanone is
an intermediate product in obtaining ephedrine derivatives.[8]
Fig. 1 outlines the reaction products of a relatively simple
␣-diketone, 3,4-hexanedione. As can be seen, in a first stage two ␣-
hydroxyketones are formed, (S)- and (R)-4-hydroxy-3-hexanone,
and in a second step (S,S)- and (R,R)-3,4-hexanediol and meso-
hexanediol appear. According to bibliographic reports for the
hydrogenation of 3,4-hexanedione using Pt/cinchonidine catalysts,
4-hydroxy-3-hexanone is mainly formed, the highest enantiomeric
excesses reported being of ca. 20% [3,9].
enantioselective hydrogenation of carbonyl compounds employs
catalysts based on Pt modified with alkaloids from the cinchona
family [1]. As is well known, these catalysts are particularly effec-
tive in the hydrogenation of ␣-ketoesters, allowing one to obtain
the corresponding ␣-hydroxy esters with enantiomeric excesses of
up to 98% [2]. To a lesser extent, these systems have been employed
in the hydrogenation of ␣-diketones, such as 2,3-butanedione,
2
1
,3-hexanedione, 3,4-hexanedione, 1,2-cyclohexanedione and
-phenyl-1,2-propanodione, reaching enantiomeric excesses gen-
erally much lower than those obtained for ␣-ketoesters [3–7].
The hydrogenation of ␣-diketones presents an interesting chal-
lenge, both in terms of regio- and enantioselectivity due to the
fact that these compounds have two carbonyl groups that may
be hydrogenated. Due to the presence of two conjugated keto
groups, the reaction proceeds in two consecutive steps: in the first
place, one C O group is hydrogenated and in a later stage the
other one is [4]. The products of the hydrogenation of these vicinal
The addition of tin compounds in general, and organotin com-
pounds in particular, have proven to have a great influence on
the behavior of platinum catalysts. Thus, the addition of tin to a
Pt/Al O3 reforming catalyst results in a higher catalytic activity
2
toward the aromatization of hydrocarbons [10,11], and also the use
of tin in Pt-based systems leads to a large increase in the overall
rate in the hydrogenation of ␣,␤-unsaturated aldehydes together
with a high selectivity toward the formation of the corresponding
unsaturated alcohols [12,13].
To fully understand the reason for the benefit of adding tin to
platinum catalysts, it should also be noted how tin is added. Among
the techniques of controlled preparation of catalysts are those
derived from the Surface Organometallic Chemistry on Metals
(SOMC/M), which consists of the reaction of organometallic com-
∗ Corresponding author. +54 221 421 1353.
E-mail addresses: vetere@quimica.unlp.edu.ar (V. Vetere), bfaraoni@criba.edu.ar
(
(
M.B. Faraoni), jpodesta@uns.edu.ar (J.C. Podestá), casella@quimica.unlp.edu.ar
M.L. Casella).
1
Member of CIC.
0
926-860X/$ – see front matter © 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.apcata.2012.08.031

2
10
V. Vetere et al. / Applied Catalysis A: General 445–446 (2012) 209–214
OH
2.1.2. Hexa-(−)-menthylditin (−)-Men Sn-SnMen
3
3
The reaction was carried out in a three-necked round-bottomed
flask, fitted with a reflux condenser, magnetic stirrer and pressure
equalizing ampoule, under N2 atmosphere. To a solution of 10.3 g
OH
O
(
S,S)-3,4-hexanediol
(
0.040 mol) of SnCl in 48 mL of dry benzene, in an ice bath, 150 mL
4
of 1.58 M solution (0.240 mol) of (−)-menthylmagnesium chloride
in dry THF was added dropwise. Upon completion of the addition,
the reaction mixture was heated under reflux for 60 h and then
it was allowed to cool to room temperature under stirring. A 10%
HCl solution (25 mL) and distilled water (25 mL) were added. After
addition of ethyl ether (200 mL), the organic phase was separated
OH
OH
O
(
S)-4-hydroxy-3-hexanone
OH
O
(
(
S,R)-3,4-hexanediol
R,S)-3,4-hexanediol
O
meso
3
,4-hexanedione
OH
OH
and dried on anhydrous MgSO . The solvent was distilled under
4
(
R)-4-hydroxy-3-hexanone
reduced pressure and the product was recrystallized from ethanol,
yielding 12.9 g (0.012 mol, 59.8%) of hexa (−)-menthylditin, mp:
OH
◦
20
◦
2
30 C (dec.), [␣]D = −214 (0.99 dry benzene).
(
R,R)-3,4-hexanediol
The compounds obtained were characterized by 1H NMR, 13
C
and 1 Sn in a Bruker ARX 300 instrument.
19
Fig. 1. Reaction pathway for 3,4-hexanedione hydrogenation.
2.2. Preparation and characterization of the catalysts
pounds of elements from the representative groups, transition
metals, lanthanides and actinides with the surface of supported
metals [14]. By applying this technique, it is not only possible to
prepare well-defined catalysts that may be alloys of a given com-
position but also catalysts in which organometallic fragments are
likely adsorbed (coordinated) at some particular crystallographic
positions of the metallic particles. These materials exhibit interest-
ing and unusual selectivities in many catalytic reactions [15,16].
Special mention should be made of organobimetallic catalytic
phases where one or more of the organic fragments have chiral
characteristics. In this case, by using SOMC/M techniques, hetero-
geneous chiral catalysts of very good quality in terms of chemo- and
enantioselectivity, as well as good stability and reuse capability,
have been obtained [17,18].
The monometallic catalyst was prepared by ion exchange, using
2
−1
SiO2 (Evonik (previously Degussa) Aerosil 200, 180 m g ) as sup-
port, which had been previously treated with an ammonia solution.
The functionalized solid was put in contact with an aqueous solu-
tion of [Pt(NH3)4]Cl2 of the appropriate concentration to obtain a
catalyst with a Pt content of 1 wt.%. After 24 h exchange at room
◦
temperature, the solid was filtered off, washed and dried at 105 C
and subsequently reduced in H2 flow at 500 C.
◦
The chiral organotin catalysts were prepared using techniques
derived from the Surface Organometallic Chemistry on Metals
(SOMC/M). After the reduction step, 0.25 g of monometallic catalyst
was put in contact with either (−)-MenSnBu3 or (−)-Men3Sn-Sn-
(−)-Men3 dissolved in n-decane. The reaction was carried out for
◦
4 h in H2 atmosphere at 120 C. Upon completion of the reaction,
The present paper reports on the controlled route for the prepa-
ration of chiral organobimetallic catalysts via SOMC/M techniques,
as well as an interesting application in a reaction requiring high
activity as well as chemo- and enantioselectivity, as is the liq-
uid phase hydrogenation of 3,4-hexanedione. The selected chiral
organotin compounds contain (−)-menthyl groups in their compo-
sition, and must be carefully synthesized and characterized in order
to assume a high optical purity.
the catalyst was washed repeatedly with n-heptane in Ar atmo-
sphere. These catalysts, which have a molar ratio Sn/Pt of 0.8, are
designated Pt-Men6Sn2 and Pt-MenSnBu3, and correspond to solids
containing organic groups anchored to their surfaces.
The composition of the catalysts was determined by atomic
absorption spectroscopy (AAS) using a Varian Spectra AA55
instrument, after digestion in an acid solution and dilution. Tem-
perature Programmed Reduction (TPR) tests were performed in
a Quantachrome apparatus equipped with a thermal conductiv-
◦
ity detector. Each sample, previously calcined at 500 C for 4 h,
◦
was heated from room temperature to 800 C at a heating rate
2
. Experimental
◦
−1
3
−1
of 10 C min in a reducing gas stream (25 cm min , 5% H2 in
N ). Hydrogen and CO chemisorption measurements were per-
2
2
.1. Synthesis and characterization of (−)-menthyl derived
formed in pulse dynamic equipment with catharometric detection,
considering a stoichiometry of adsorption H/Pts and CO/Pts. The
determination of metallic particle size distribution was carried out
by transmission electron microscopy (TEM) in a JEOL 100Cx micro-
scope, having a resolution of 6 A˚ and an accelerating voltage of ca.
organotin compounds
2.1.1. (−)-Menthyltributyltin (1R,2S,5R)-(−)-MenSnBu3
To a solution of (−)-menthylmagnesium chloride in THF
(
5.3 mmol, 3.3 mL of a 1.59 M solution), a solution of n-tributyltin
1
00 kV. The samples were ground and ultrasonically dispersed in
chloride (1.5 g, 4.6 mmol, 1.24 mL) and triphenylphosphine (1.26 g,
.8 mmol) in dry THF (6 mL) was added from a pressure-equalizing
distilled water. To estimate the mean particle size, the particles
were considered spherical and the second moment of the distribu-
tion was employed. The expression used for the calculation was:
4
funnel, with stirring and under a nitrogen atmosphere. The mixture
was kept under stirring at room temperature for 20 h. Then, it was
decomposed with the addition, dropwise, of a solution of HCl 10%.
The organic phase was extracted with dichloromethane (10 mL),
washed twice with distilled water and dried with MgSO4.
The solvent was distilled under reduced pressure. To the residue
obtained hexane (3 mL) and dichloromethane (2 mL) were added
to precipitate the triphenylphosphine, which was removed by
filtration. The product was purified by chromatography on a sil-
ica gel 60 column, eluting with hexane (1.72 g, 4.0 mmol, 87%),
ꢀ
nidi3
nidi2
d = ꢀ
where n is the number of particles with d size. Over 200 particles
were measured.
i
i
The Sn/Pt atomic ratio of the organobimetallic catalysts
was measured spectrophotometrically, at 530 nm, by forming a
complex between Sn and phenylfluorone in a GBC Cintra 20
20
◦
[␣]D = −30.5 (1.5, benzene).

V. Vetere et al. / Applied Catalysis A: General 445–446 (2012) 209–214
211
spectrophotometer, following the procedure of reference [19]. For
the preparation of standard solutions metallic Sn (0.4930 g) was
used, which was dissolved in concentrated HCl (50 mL) and allowed
to stand for 24 h, heating without boiling. From this solution, the
necessary dilutions were made using 1:9 HCl as a diluent. Tests per-
formed for different concentrations of tin allowed us to determine
that the optimal working range is 0.02–0.12 ppm Sn.
2.3. Hydrogenation of 3,4-hexanedione
Hydrogenation of 3,4-hexanedione was carried out in autoclave-
type reactor at 1 MPa H pressure, with temperature ranging from
2
◦
4
0 to 80 C, using 0.25 g of catalyst and 2-propanol as solvent. The
catalysts obtained were transferred to the autoclave reactor in Ar
atmosphere. In each test, 0.4 mL of 3,4-hexanedione and 60 mL of
2
-propanol (0.06 M) were used. It was verified that the stirring
speed and the catalyst particle size used ensured that diffusional
limitations were negligible.
The course of the reaction was followed by taking a series
of micro-samples, which were analyzed in a Varian CP-3800 gas
chromatograph equipped with a capillary column CP wax 52 CB
(
30 m, 0.53 mm i.d.) and a FID detector. The reaction products
were identified using a GC/MS Shimadzu QP5050 with a capillary
TM
column Supelco SPB -5 (30 m, 0.25 mm i.d.). The enantiomeric
excess (ee) was calculated according to the following equation:
ee% = 100(S − R)/(S + R), from the data obtained by gas chromatog-
TM
raphy on a capillary column BETA DEX 120 (30 m, 0.25 mm i.d.).
3
. Results and discussion
3
.1. Synthesis and characterization of (−)-menthyl derived
organotin compounds
A key step in the preparation of the enantioselective heteroge-
neous catalysts used in this work is the synthesis of the organotin
compounds employed as chiral precursors. Due to the configura-
tional instability of the most common intermediates employed in
the synthesis of chiral organotin compounds, triorganotin halides,
many techniques aiming to stabilize the configuration of these
compounds have been developed [20–25].
Fig. 2. Structure of (−)-menthyltin derivatives ( ) H; ( ) C; ( ) Sn.
The preparation of compounds derived from (−)-menthyl tin
involves several steps where, as already mentioned, the main prob-
lem is the retention of the configuration of the chiral inductor
group until the end of the synthesis. The C1 carbon atom of the
In order to control the configuration of the metal, Schumann
et al. anchored a (−)-menthyl ligand to a tin atom (see Fig. 2)
(
−)-menthyl group has a marked tendency to epimerization, i.e. to
change its configuration, which can lead to obtaining a mixture of
product of little use as chiral modifier. One of the strategies used to
prevent the epimerization of this carbon atom is the addition of a
Lewis base such as triphenylphosphine during the synthesis [29].
Generally speaking, the synthesis of (−)-menthyl derived organ-
otin compounds involves the reaction between a triorganotin
halide and a Grignard reagent containing the (−)-menthyl group.
According to what is reported in the literature, the epimerization
could be related to the Lewis acidity of the triorganotin precur-
sor. The first step in the above-mentioned general reaction would
involve the transfer of an electron from the Grignard reagent to the
Lewis acidic Sn atom of the triorganotin compound. As a result of
this electron transfer, a radical cation of planar geometry is formed
losing its original stereochemistry. The addition of a Lewis base
results in an interaction between it and the triorganotin halide,
with the formation of an adduct. The adduct has different electronic
properties from the triorganotin halide, inhibiting the electron
transfer from the Grignard reagent and the subsequent inversion
of configuration of the C1 atom of the (−)-menthyl group.
[
26,27]. In 1994 Podestá and Radivoy obtained the hexa-(−)-
menthylditin from which other derivatives of (−)-menthyl could
be synthesized [28]. The studies of these authors confirmed that,
in the binding of (−)-menthyl ligand to a tin atom by reacting
(
−)-menthylmagnesium chloride and tin tetrachloride, the config-
uration could be retained.
Chiral organotin compounds containing the chiral carbon atom
directly bonded to the Sn atom exert a greater asymmetric induc-
tion than those with the chiral center in a more remote position
[
28]. Taking this fact into account, it was proposed to synthesize
(
−)-menthyltin derivatives for use in the preparation of chiral het-
erogeneous catalysts.
The (−)-menthyl radical, which will be called in abbreviated
form (−)-Men or just Men, has three chiral centers of configura-
tion (1R,2S,5R) (Fig. 2). In this figure it can also be seen that for
the two organometallic compounds synthesized in this work, (−)-
menthyltributyltin and hexa-(−)-menthylditin, the carbon atom of
the (−)-menthyl group designated as 1 is the one that is bound to
the Sn atom. It is expected that the enantioselectivity of the result-
ing catalysts will not only be given by this chiral center directly
attached to Sn, but also by a steric effect caused by the (−)-menthyl
group as a whole. Fig. 2 shows the spatial structure of these com-
pounds.
The synthesis of (−)-MenSnBu is represented by the following
3
equation:
(
−)-MenMgCl/THF/Ph3P
Bu SnCl
−→
(−)-MenSnBu3
3

2
12
V. Vetere et al. / Applied Catalysis A: General 445–446 (2012) 209–214
Table 1
1
3
C
a
n
d
1
1
9
S
n
N
M
R
c
h
a
r
a
c
t
e
r
i
s
t
i
c
s
o
f
s
y
n
t
h
e
s
i
z
e
d
c
o
m
p
o
u
n
d
s
.
a
ıCn [ J(13C,119Sn)]
n
MenSnBu3 b
Men6Sn2
1
C1 ( J)
32.57 (409.1)
47.24 (14.6)
27.11 (56.0)
36.04 (NO)
35.76 (58.2)
41.79 (16.9)
23.02
39.62 (196.4)
46.30 (19.0)
27.63 (46.3)
35.70 (NO)
36.54 (63.7)
45.50 (NO)
22.42
2
C2 ( J)
3
C3 ( J)
4
C4 ( J)
3
C5 ( J)
2
C6 ( J)
C7
3
C8 ( J)
34.0 (17.6)
22.49
34.43 (11.8)
22.10
C9
C10
16.17
−23.8
17.32
18.0
119Sn
0
100
200
300
400
500
600
700
800
Temperature/ºC
a
13
In CDCl3; chemical shifts, ı, in ppm with respect to TMS ( C spectra) and
Me4Sn (1 Sn spectra); coupling constants, nJ(Sn, C), in Hz (in parenthesis); NO, not
observed.
19
Fig. 3. Temperature Programmed Reduction of Pt/SiO2.
b
Other signals: 9.05 (295.9), 14.08; 28.04 (54.1), 29.82 (19.2).
expression for the Pt/SiO2 catalyst is 0.58. It can be seen that the
dispersion value obtained from TEM measurements is quite simi-
lar from the one obtained by H2 and CO chemisorption. The high
dispersion of the metallic phase is a consequence of the strong
interaction between the metal precursor and the support gener-
ated during the impregnation step and that is maintained even after
the reduction in H2 for obtaining the supported metal particles.
This characteristic of high dispersion of the monometallic system is
essential to ensure an adequate preparation of the organobimetallic
catalysts through SOMC/M techniques.
In the absence of triphenylphosphine, a mixture of (1R,2S,5R)-
MenSnBu3 and (1S,2S,5R)-MenSnBu3 epimers in a 3:1 ratio is
obtained.
The hexa-(−)-menthylditin was obtained by direct alkylation of
tin tetrachloride, SnCl , with (−)-menthylmagnesium chloride (−)-
4
MenMgCl. The following equation outlines the reaction involved in
this synthesis:
THF/benzene
(
−)-MenMgCl + SnCl4
−→ (−)-Men Sn–Sn-(−)-Men3
3
Temperature Programmed Reduction (TPR) results obtained for
the Pt/SiO system showed the presence of two peaks: one at about
2
During the course of this reaction, besides hexa-(−)-
◦
◦
2
50 C and the other at 500 C (Fig. 3). According to the litera-
menthylditin, tri-(−)-menthyltin chloride (−)-Men SnCl and
3
ture, the low temperature peak can be assigned to the presence
of Pt(IV) species, generated during the calcination pretreatment.
di-(−)-menthyltin dichloride (−)-Men SnCl are also obtained.
2
2
Studies on the variation of experimental parameters in the yield
of this reaction allowed optimizing the conditions leading to a
greater proportion of the desired product [30]. Tables 1 and 2
The high temperature peak may be assigned to species of the type
n−y
Pt (O Si )y
(n = +2 or +4), formed through the interaction of the
list the results of 1H, C and
13
119
metallic precursor and the support [32]. The peaks obtained were
broad and presented shoulders, which could be due to the different
environment of the ions on the support surface. From this profile,
Sn NMR characterization of both
compounds synthesized in this work. The results there presented
indicate that optically pure compounds were obtained, without
epimerization of the C1 atom.
◦
a reduction temperature of 500 C was chosen, in order to obtain a
completely reduced Pt/SiO catalyst.
2
Tin-modified catalysts were obtained by a controlled surface
reaction between the previously reduced monometallic catalyst
and the organotin compound dissolved in n-decane. At the temper-
ature of 120 C, which was selected for the preparation, the reaction
that takes place may be schematized by the following equation:
3
.2. Catalysts preparation and characterization
The Pt/SiO2 monometallic catalyst (1 wt.% Pt measured by
◦
atomic absorption) was prepared by ionic exchange, a method that
allows catalysts having a high dispersion to be obtained (H/Pt = 0.65
and 0.55, measured by H and CO chemisorption, respectively).
The results of Transmission Electron Microscopy (TEM) mea-
surements indicate that the particle size distribution of the
monometallic Pt/SiO catalyst is very narrow, with a mean particle
o
xy
120
C
2
Pt/SiO + ySnR +
H2 −→ Pt(SnBu ) /SiO + xyRH
4−x
2
4
y
2
2
In this equation, R indicates the organic fragments bonded to
the tin atom in the organotin precursor. After the reaction, the
product is an organobimetallic system with some organic groups
anchored to the surface. These organic fragments represent either
butyl or (−)-menthyl groups bonded to the tin. The amount of
tin fixed, measured spectrophotometrically, is indicated by the “y”
value (y = Sn/Pt = 0.8). Organobimetallic catalyst are designated Pt-
Men Sn2 and Pt-MenSnBu . Unfortunately, so far it has not been
2
diameter around 2.5 nm. The metal dispersion is calculated from
the Pt particle size distribution obtained from TEM measurements
by the following equation, assuming spherical particles [31]:
ꢀ
nidi2
nidi3
D = 6
MPt
ꢁPt
ꢀ
ꢀ
6
3
possible to determine the number and characteristics of organic
groups anchored to the surface. However, the results presented
in this paper and others previously published show that these
systems induce enantioselectivity in the hydrogenation of certain
prochiral carbonyl compounds, strongly suggesting that at least
some (−)-menthyl groups remain anchored to the surface [17,18].
where MPt and ꢁPt are the molar mass and density of Pt, respec-
tively; n is the fraction of particles with a diameter d , and ꢀ is the
i
i
4
2
−1
area occupied by 1 mol of Pt at the surface (3.75 × 10 m mol ).
The value of the dispersion calculated using the above-mentioned
Table 2
Furthermore, in the case of modifier MenSnBu , theoretical cal-
1
H NMR characteristics of synthesized compounds.a
3
culations carried out in our research group work show that the
difference in binding energy Sn–C (where C is the carbon atom
belonging to each substituent, which is attached to Sn) for butyl
Compound
ı, ppm (multiplicity, n H, coupling constants nJ in Hz), CDCl3
◦
MenSnBu3
Men6Sn2
0.54–0.93 (m, 24H); 0.94–1.89 (m, 22H)
0.73 (d, 18H); 0.79 (d, 18H); 0.84 (d, 18H); 1.07–2.17 (m, 60H)
(−)-menthyl is very small, the retention of menthyl groups on the
a
catalyst surface being highly probable [33].
In CDCl3; chemical shifts, ı, in ppm with respect to TMS.

V. Vetere et al. / Applied Catalysis A: General 445–446 (2012) 209–214
100
213
Table 3
Selectivity at 90% of conversion for Pt-Sn2Men6 catalyst.
9
0
a
b
c
◦
ria
SI%b
SII%b
SIII%b
eeI%c
eeII%c
T ( C)
80
70
40
60
80
110
280
670
80
75
78
7
8
7
13
17
15
27
20
15
4
5
–
6
5
4
0
0
0
a
−1
−1
Initial reaction rate (ri) (␮mol s gPts ). Estimated between conversion 0 and
1
0%.
b
30
20
I = (S)- + (R)-4-hydroxy-3-hexanone;
II = (S,S)- + (R,R)-3,4-hexanediol;
III = meso-hexanediol.
c
ee% = 100(S − R)/(S + R).
1
0
0
0
0
0
100
100
100
200
Time / min
300
400
400
400
500
3.3. Hydrogenation of 3,4-hexanedione
The influence of temperature on catalytic activity was inves-
tigated between 40 and 80 C, 1 MPa H2 pressure and an initial
,4-hexanedione concentration of 0.06 M, employing the Pt-
1
00
◦
9
8
0
0
3
Sn Men catalyst. As can be observed in Table 3, the initial reaction
2
6
−
1
−1
70
60
50
40
30
rate increased from 110 to 670 ␮mol s gPts
when the tem-
◦
perature was raised from 40 to 80 C. After 180 min reaction,
over 90% conversion was achieved at the three temperatures.
Concerning selectivity, for all the studied temperatures, 4-hydroxy-
-hexanone was obtained as the major product with a selectivity
of ca. 80%.
Table 3 also lists the enantiomeric excess (ee%) achieved at the
3
2
1
0
0
0
three temperatures tested. The values shown in the table indicate
a decrease in the enantiomeric excess for the first hydrogenation
200
Time / min
300
500
◦
step from 27% to 15% when the reaction was carried out at 40 C and
◦
8
0 C, respectively. In an enantioselective hydrogenation reaction,
one of the enantiomers is to be produced predominantly over the
other. However, due to the similarity of the spatial arrangement of
the contributing intermediate activation complexes, the activation
energy that leads to the formation of either one enantiomer or the
other will not be very different and, therefore, it is expected that
a reduction in the reaction temperature would promote the differ-
entiation between them and, accordingly, a higher ee% would be
obtained. The achievement of higher ee% values with decreasing
reaction temperature is a fact that has been observed by several
authors in asymmetric heterogeneous catalysis [34,35].
100
9
8
7
6
5
4
3
2
1
0
0
0
0
0
0
0
0
0
0
Fig. 4 shows the variation in the molar composition of the reac-
tion mixture as a function of time obtained when studying the
◦
hydrogenation of 3,4-hexanedione, at 40 C, with the three cata-
200
300
500
lysts prepared: Pt (Fig. 4a), Pt-MenSnBu (Fig. 4b) and Pt-Sn Men
Time / min
3
2
6
(
Fig. 4c). As can be seen, for the three systems, after an induc-
Fig. 4. Concentration profiles (mol%) along the reaction on Pt (a); Pt-MenSnBu3
b) and Pt-Men6Sn2. ( ) 3,4-hexanedione, ( ) 4-hydroxy-3-hexanone, ( ) 3,4-
hexanediol, ( ) meso-3,4-hexanediol.
tion period, the hydrogenation takes place in two stages. In a first
step, up to about 150 min reaction, the hydrogenation of one of
the C O groups of the 3,4-hexanedione molecule occurs, yielding
solely 4-hydroxy-3-hexanone. At a later stage, by hydrogenation of
the hydroxyketone, the corresponding diols are obtained. As shown
in the figure, all the catalytic systems allow an almost complete
conversion of 3,4-hexanedione at about 250 min. However, for the
second reaction step there is some difference in the behavior of
the catalysts. Systems modified with organotin compounds (Fig. 4b
and c) seem to be somewhat more active than the monometallic
catalyst (Fig. 4a), in particular the Pt-Sn Men6 catalyst. The anal-
ysis of Fig. 4 shows that when using the systems Pt-Men Sn2 and
Pt-MenSnBu the hydrogenation of 4-hydroxy-3-hexanone occurs
faster. This is particularly clear in Fig. 3c that shows a sharp decline
in the composition of the hydroxyketone as a function of time, and
a significant increase in the amount of hexanediols obtained at the
end of the experiment (53% meso-hexanediol and 25% (S,S)- and
(
analogous to those presented here, have been characterized by our
research group using EXAFS, XANES and XPS techniques [36]. These
studies revealed that in the organobimetallic systems there is a con-
vergence of geometric and electronic effects due to the presence of
Sn. It has also been found that Sn is present on the surface of the
catalyst both in ionic state (Sn(II,IV)) and as a metal (Sn(0)). The
presence of ionic tin, having electrophilic properties, increases the
polarization of the C O group of the hydroxyketone thereby facil-
itating the attack of the chemisorbed hydrogen and favoring the
second stage of hydrogenation. For the catalytic systems studied
here, these facts allow us to explain why 3,4-hexanediol is obtained
from 4-hydroxy-3-hexanone. On the other hand, the presence of
(−)-menthyl groups attached to tin could contribute to the effect
observed by some kind of geometrical interaction.
2
6
3
(
R,R)-3,4-hexanediol).
To explain these facts it is necessary to take into account the
Table 4 lists the selectivity to the reaction products at two lev-
els of conversion of 3,4-hexanedione. As can be seen, there is an
increase in the selectivity to diols, at the expense of the hydrox-
yketone, as the reaction evolves. It is worth mentioning that for
changes in the characteristic of the hydrogenation active sites that
may be caused by the addition of an organotin compound to the cat-
alytic system. Catalytic systems based on Pt modified with SnBu4,

2
14
V. Vetere et al. / Applied Catalysis A: General 445–446 (2012) 209–214
Table 4
Selectivity to products at 40 C, measured at two different levels of conversion.
approximately 25%. This value remained constant throughout the
reaction.
◦
Conversion %
Catalyst
Pt
Acknowledgements
Pt-MenSnBu3
Pt-Sn2Men6
X = 70
X = 98
94
71
96
84
>99
62
SI%a
This work has been sponsored by CONICET (Argentina) (PIP
0
185), Universidad Nacional de La Plata (Argentina) (Project X487),
ANPCyT (Argentina) (PICT 2644), CONICET (Argentina) (PIP 112-
00801-02272), and Universidad Nacional del Sur (Bahía Blanca,
X = 70
X = 98
2
12
1
6
<1
14
SII%a
2
X = 70
X = 98
4
17
3
10
<1
24
SIII%a
eeI%b
Argentina, PGI 24/Q041).
X = 70
X = 90
–
–
23
25
25
27
References
[
1] H.U. Blaser, H.P. Jalett, M. Studer, Catal. Today 37 (1997) 441–463.
[2] M. Sutyinszki, K. Szöri, K. Felflödi, M. Bartók, Catal. Commun.
25–127.
X = 70
X = 90
–
–
4
3
–
4
eeII%b
a
3
(2002)
1
I = (S)- + (R)-4-hydroxy-3-hexanone;
III = meso-hexanediol
II = (S,S)- + (R,R)-3,4-hexanediol;
[3] J. Slipszenko, S. Griffiths, P. Johnston, K. Simons, W. Vermeer, P. Wells, J. Catal.
179 (1998) 267–276.
[4] X. Zuo, H. Liu, J. Tian, J. Mol. Catal. A: Chem. 157 (2000) 217–224.
b
ee% = 100(S − R)/(S + R).
[
5] O.J. Sonderegger, T. Bürgi, A. Baiker, J. Catal. 215 (2003) 116–121.
6] E. Toukoniitty, V. Nieminen, A. Taskinen, J. Päivärinta, M. Hotokka, D.Yu. Murzin,
J. Catal. 224 (2004) 326–339.
[
none of the catalytic systems used in this work, we have detected
the formation of hemiketals between the solvent (2-propanol)
and 3,4-hexanedione or 4-hydroxy-3-hexanone. According to what
has been reported in the literature, these by-products have been
observed in the hydrogenation of 2,3-butanedione in the presence
of ethanol as solvent [4].
[
[
[
7] E. Toukoniitty, S. Franceschini, A. Vaccari, D.Yu. Murzin, Appl. Catal. A: Gen. 300
(
2006) 147–154.
8] V. Nieminen, A. Taskinen, M. Hotokka, D.Yu. Murzin, J. Catal. 245 (2007)
228–236.
9] D. Ruiz, C. Claver, P. Reyes, XVI Argentine Congress of Catalysis, 2009.
[
10] V.A. Mazzieri, J.M. Grau, C.R. Vera, J.C. Yori, J.M. Parera, C.L. Pieck, Catal. Today
07–108 (2005) 643–650.
1
Systems modified with organotin chiral inductors proved to
be enantioselective, especially in the first stage of hydrogenation
reaching, for both systems, enantiomeric excess values of approx-
imately 25% for 4-hydroxy-3-hexanone. However, these chiral
catalysts showed a low enantiodifferentiating capacity to the cor-
responding diols. In Table 4 are reported the enantiomeric excesses
obtained at two levels of conversion, which remain approxi-
mately constant. In addition, no significant variation in these values
throughout the experiment has been detected. From the results
presented in this work, it can be concluded that the structure of the
chiral modifier seems to have no influence on the enantiodifferenti-
ating ability of the systems. It is noteworthy that the enantiomeric
excess obtained for 4-hydroxy-3-hexanone is comparable to that
reported in the literature when classic Pt/cinchonidine systems are
employed as catalysts [4,9]. The organotin chiral catalysts proposed
in this paper have the additional advantage of being able to be
reused, maintaining their activity, and chemo- and enantioselec-
tivity in several successive cycles of hydrogenation, as has been
shown previously for the enantioselective hydrogenation of other
prochiral carbonyl compounds [17].
[11] L.S. Carvalho, C.L. Pieck, M.C. Rangel, N.S. Fígoli, C.R. Vera, J.M. Parera, Appl.
Catal. A 269 (2004) 105–116.
[
12] G.F. Santori, M.L. Casella, G.J. Siri, H.R. Adúriz, O.A. Ferretti, Appl. Catal. A: Gen.
9 (2000) 141–149.
[13] G.F. Santori, M.L. Casella, O.A. Ferretti, J. Mol. Catal. A: Chem. 186 (2002)
23–239.
1
2
[
14] O.A. Ferretti, M.L. Casella, in: J.M. Basset, R. Psaro, D. Roberto, R. Ugo (Eds.),
Modern Surface Organometallic Chemistry, Wiley-VCH, Weinheim, 2009, pp.
239–291.
[
[
15] J.P. Candy, B. Didillon, E.L. Smith, T.B. Shay, J.M. Basset, J. Mol. Catal. 86 (1994)
179–204.
16] G.F. Santori, A.G. Moglioni, V. Vetere, G.Y. Moltrasio Iglesias, M.L. Casella, O.A.
Ferretti, Appl. Catal. A: Gen. 269 (2004) 215–223.
[17] V. Vetere, M.B. Faraoni, G.F. Santori, J.C. Podestá, M.L. Casella, O.A. Ferretti, J.
Catal. 226 (2004) 457–461.
[
18] V. Vetere, M.B. Faraoni, G.F. Santori, J.C. Podestá, M.L. Casella, O.A. Ferretti, Catal.
Today 107–108 (2005) 266–272.
[19] V. Vetere, PhD Thesis, UNLP, Argentina, 2007.
[
[
[
20] M. Gielen, Pure Appl. Chem. 52 (1980) 657–668.
21] M. Gielen, Y. Tondeur, Nouv. J. Chim. 2 (1978) 117–118.
22] G. Van Koten, J.G. Noltes, A.L. Spek, J. Organometal. Chem. 118 (1976)
183–189.
[
[
[
[
[
[
23] J.T.B.H. Jastrzebski, E. Wehman, J. Boersma, G. van Koten, K. Goubitz, D.J. Heij-
denrijk, J. Organometal. Chem. 409 (1991) 157–162.
24] J.T.B.H. Jastrzebski, E. Wehman, J. Boersma, G. van Koten, J. Organometal. Chem.
413 (1991) 43–53.
25] G. van Koten, J.T.B.H. Jastrzebski, J.G. Noltes, W.M.G.F. Pontenagel, J. Kroon, A.L.
Spek, J. Am. Chem. Soc. 100 (1978) 5021–5028.
26] H. Schumann, B.C. Wassermann, F.E. Hahn, Organometallics 11 (1992)
4
. Conclusions
2
803–2811.
27] H. Schumann, B.C. Wassermann, J. Pickardt, Organometallics 12 (1993)
051–3063.
28] J.C. Podestá, G.E. Radivoy, Organometallics 13 (1994) 3364–3365.
The main conclusions of this study are summarized below:
3
-
-
It has been possible to obtain (−)-menthyl derivatives of organ-
otin compounds with retention of the configuration of the
chirality-inducing group.
It has been possible to prepare organobimetallic heterogeneous
catalysts by a surface controlled reaction, between the chiral
[29] D. Dakternieks, K. Dunn, D.J. Henry, C.H. Schiesser, E.R.T. Tiekink,
Organometallics 18 (1999) 3342–3347.
[
[
30] G.E. Radivoy, PhD Thesis, UNS, Argentina, 1997.
31] J. Yang, V. Tschamber, D. Habermacher, F. Garin, P. Gilot, Appl. Catal. B: Environ.
83 (2008) 229–239.
[32] L.W. Ho, C.P. Hwang, J.F. Lee, I. Wang, C.T. Yeh, J. Mol. Catal. A: Chem. 136 (1998)
93–299.
33] J.F. Ruggera, A.B. Merlo, V. Vetere, M.L. Casella, J. Mol. Struct. Theochem. 953
2010) 91–97.
[34] D.Yu. Murzin, P. Mäki-Arvela, E. Toukoniitty, T. Salmi, Catal. Rev. 47 (2005)
75–256.
35] J. Otera, K. Sakamoto, T. Tsukamoto, A. Orita, Tetrahedron Lett. 39 (1998)
201–3204.
2
organotin compound and the monometallic Pt/SiO catalyst.
2
[
-
-
-
The chiral heterogeneous systems obtained have been active in
the liquid phase hydrogenation of 3,4-hexanedione.
An increase in the ee% obtained with a decrease in the reaction
temperature has been observed.
(
1
[
3
The systems Pt-Men Sn2 and Pt-MenSnBu3 proved to be enan-
[36] J.M. Ramallo López, G.F. Santori, L. Giovanetti, M.L. Casella, O.A. Ferretti, F.G.
Requejo, J. Phys. Chem. B 107 (2003) 11441–11451.
6
tioselective reaching an ee% for 4-hydroxy-3-hexanone of