Synthesis of new (-)-menthylgermanium derivatives and its use in heterogeneous bimetallic catalysis

Article abstract of DOI:10.1016/j.jorganchem.2011.07.011

The syntheses and physical properties of tri-(-)-menthylgermanium chloride (2), tri-(-)-menthylmethylgermanium (5), and tri-(-)-menthylgermanium hydride (6) are reported. The preparation of a bimetallic catalyst derived from 5 and Pt supported on SiO

Full text of DOI:10.1016/j.jorganchem.2011.07.011

Journal of Organometallic Chemistry 696 (2011) 3440e3444
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Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
Note
Synthesis of new (ꢀ)-menthylgermanium derivatives and its use
in heterogeneous bimetallic catalysis
,
,
,
,
, ,2
*
María B. Faraoni a 1, Virginia Vetere b 2, Mónica L. Casella b 2, Julio C. Podestá a
**
a Instituto de Química del Sur (INQUISUR), Departamento de Química, Universidad Nacional del Sur-CONICET, Av. Alem 1253, 8000 Bahía Blanca, Argentina
b Centro de Investigación y Desarrollo en Ciencias Aplicadas “Dr. Jorge J. Ronco” (CINDECA), Departamento de Química, Facultad de Ciencias Exactas,
Universidad Nacional de La Plata-CONICET, calle 47 Nꢁ 257, 1900 La Plata, Argentina
a r t i c l e i n f o
a b s t r a c t
Article history:
The syntheses and physical properties of tri-(ꢀ)-menthylgermanium chloride (2), tri-(ꢀ)-menthylme-
thylgermanium (5), and tri-(ꢀ)-menthylgermanium hydride (6) are reported. The preparation of
a bimetallic catalyst derived from 5 and Pt supported on SiO2 and its use in the enantioselective catalytic
hydrogenation of acetophenone is also reported.
Received 12 May 2011
Received in revised form
6 July 2011
Accepted 7 July 2011
Ó 2011 Elsevier B.V. All rights reserved.
Dedicated to Prof. Dr. Pelayo Camps
(University of Barcelona, Spain) on the
occasion of his 65th birthday.
Keywords:
Tri-(ꢀ)-menthylgermanium chloride
Tri-(ꢀ)-menthylmethylgermanium
Tri-(ꢀ)-menthylgermanium hydride
Bimetallic catalyst
1. Introduction
implies the selective reaction of organometallic compounds of
several metals, such as Sn, Ge, Pb, with the surface of transition
In previous studies we have shown that organotin compounds
with (ꢀ)-menthyl ligands react with a monometallic catalyst
generating an organometallic phase e retaining the organic groups
on the surface e with remarkable properties for the selective
hydrogenation of carbonyl compounds [1]. Alleged toxicity of the
organotin derivatives aroused the interest for developing new
organometallic reagents that could replace the organotins. For this
reason, and as part of our ongoing studies on the preparation of
bimetallic catalysts derived from chiral organometallic compounds
in order to use them in heterogeneous catalysis [1,2], we consider it
of interest to carry out the preparation of bimetallic systems
derived from chiral organogermanium compounds and Pt sup-
ported on SiO2. Bimetallic systems were prepared by controlled
surface reactions, using techniques derived from the Surface
Organometallic Chemistry on Metals (SOMC/M). This approach
metals, giving rise to different types of surface structures, such as
grafted organometallic fragments, adatoms or bimetallic alloys.
A key step in the preparation of asymmetric heterogeneous
catalysts based on silica-supported platinum chemically modified
with chiral organometallic compounds is the synthesis of optically
pure organometallic derivatives. In order to carry out our studies,
we decided to attach the (1R,2S,5R)-(ꢀ)-menthyl [(ꢀ)-Men] group
to the germanium atom as chiral ligand. From our own experience
in organotin chemistry we knew that the direct attachment of the
(ꢀ)-menthyl group to the metal atom in germanium tetrachloride
might take place enantioselectively [2].
In the chemical literature there are few examples of organo-
germanium compounds containing (ꢀ)-menthyl groups attached
to the germanium atom. Thus, in a fairly recent paper Schiesser
et al. reported
a
four steps synthesis of (ꢀ)-menthyldiphe-
nylgermane in an overall yield of 40% starting from the alkylation of
GeCl4 with (ꢀ)-menthylmagnesium chloride (1) in THF using
a molar ratio Grignard reagent/GeCl4 ¼ 1:1 [3]. Santini et al. [4]
obtained (ꢀ)-menthylgermanium trihydride via oxidative addi-
tion of germylene GeCl2 into the carbon-chloride bond of
(ꢀ)-menthyl chloride; this reaction led to the organogermanium
hydride in 60% yield but a partial racemization was observed.
* Corresponding author. Tel.: þ54 291 4595101; fax: þ54 291 4595187.
** Corresponding author.
E-mail addresses: casella@quimica.unlp.edu.ar (M.L. Casella), jpodesta@uns.edu.ar
(J.C. Podestá).
1
Member of CIC-PBA, Argentina.
Member of CONICET, Argentina.
2
0022-328X/$ e see front matter Ó 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2011.07.011

M.B. Faraoni et al. / Journal of Organometallic Chemistry 696 (2011) 3440e3444
3441
These organogermanium compounds with chiral ligands are
being used at present to modify supported Pt heterogeneous
asymmetric catalysts and to study their enantioselectivity in
hydrogenation reactions. In order to illustrate these studies, in this
paper we report the results obtained in the synthesis of some new
(ꢀ)-menthyl substituted organogermanium derivatives, and in the
hydrogenation of acetophenone using a catalytic system of Pt
modified with tri-(ꢀ)-menthylmethyl germanium (5).
compounds demonstrate that the alkylations take place without
epimerization this demonstrating that direct alkylation of GeCl4
takes place enantioselectively.
Starting from tri-(ꢀ)-menthylgermanium chloride (2) we were
able to obtain the new organogermanium derivatives tri-
(ꢀ)-menthylmethylgermanium (5), tri-(ꢀ)-menthylgermanium
hydride (6), and tri-(ꢀ)-menthylgermanium bromide (7), according
to Scheme 1. Thus, alkylation of compound 2 with methyl-
magnesium iodide afforded compound 5 in 86% yield. On the other
hand, reduction of 2 with lithium aluminum hydride leads in 98%
yield to the corresponding organogermaniun hydride 6.
2. Results and discussion
2.1. Synthesis of tri-(ꢀ)-menthylgermanium derivatives
To estimate its reactivity, hydride 6 was allowed to react with
carbon tetrachloride. It was found that the decomposition of
a 0.077 M solution of 6 in carbon tetrachloride at 25 ꢁC takes place
in about 10 h. The reaction was followed by IR spectroscopy by
Our target was the synthesis of tri-(ꢀ)-menthylgermanium
derivatives. Taking into account our previous experience in orga-
notin chemistry [2b], we considered it possible to obtain organo-
germanium compounds with three (ꢀ)-menthyl groups attached to
the Ge atom via the direct synthesis using higher Grignard reagent
1/GeCl4 molar ratios than those used by Schiesser et al. In order to
determine the best reaction conditions needed to obtain the
desired triorganogermanium derivative in higher proportion, we
carried out the reaction between GeCl4 and 1 in THF under different
reaction conditions, as shown in Equation (1)
observing the disappearance of the GeeH absorption at 2000 cmꢀ1
.
On the other hand, the reaction between hydride 6 and carbon
tetrabromide led after 72 h to a mixture of hydride 6 and tri-(ꢀ)
menthylgermanium bromide (7), the former in higher proportion.
We were not able to separate this mixture by column chromatog-
raphy on silica gel 60. However, 13C NMR spectrum of the mixture
showed the 10 resonances corresponding to each compound and
this enabled us to establish the signals belonging to bromide 7.
In order to study the reactivity of the GeeC bonds in tri-
(ꢀ)-menthylmethylgermanium (5), we carried out the addition of
bromine in carbon tetrachloride to a solution of 5 in the same
solvent. No reaction was detected after 48 h at r.t., tetralkygerma-
nium 5 being recovered pure.
GeCl4
+
GeCl
+
(1)
(THF)
MgCl
*
*
*
*
C6H6
GeCl2
GeCl3
3
2
13C NMR characteristics of the new organogermanium
compounds 2, 3, 5, and 6 are collected in Table 2.
2
3
4
1
These organogermanium compounds with chiral ligands are
being used at present to modify supported Pt heterogeneous
asymmetric catalysts and to study their enantioselectivity in
hydrogenation reactions. In order to illustrate these studies, in this
paper we report the results obtained in the hydrogenation of ace-
tophenone using a catalytic system of Pt modified with tri-
(ꢀ)-menthylmethyl germanium (5).
In all the experiments the Grignard reagent was added dropwise
to the solution of germanium tetrachloride in benzene. The results
obtained are summarized in Table 1.
Compounds 2 and 3 were separated by column chromatography
(silica gel 60). Table 1 shows that, according to the ratio 1/GeCl4, in
all cases were obtained varying mixtures of chlorides 2 and 3, and
that in one case (entry 1) was detected the formation of
(ꢀ)-menthylgermanium trichloride. (4). These reactions were
carried out by using titrated solutions of 1 in THF, and the figures
given in Table 1 are an average of 6 experiments. Using higher ratios
1/GeCl4 did not improve the yield of 2, and even using a ratio
1/GeCl4 ¼ 10 neither the formation of tetra (ꢀ)-menthylgermanium
nor of hexa(ꢀ)-menthyldigermane was detected. This might
be connected with the steric hindrance of the voluminous
(ꢀ)-menthyl group. On the other hand, 1H NMR of these
2.2. Preparation and characterization of the catalysts
The preparation of the monometallic Pt/SiO2 catalyst (1 wt% Pt,
measured by atomic absorption spectrometry) was achieved by
means of ionic exchange, a procedure that allowed obtaining
a catalytic phase with high dispersion (H/Pt ¼ 0.65 and 0.55,
measured by H2 and CO chemisorption, respectively) [5]. Results
obtained by TEM showed a rather uniform distribution of metallic
particle size, centered around 2 nm. These characteristics of surface
homogeneity and dispersion are essential to assure a correct
preparation of organometallic catalysts through SOMC/M
Table 1
Alkylation of GeCl4 with (ꢀ) menthylmagnesium chloride.
Entry 1/GeCl4 Reaction conditionsa
Yield Product Ratios (%)c
Ratio
(%)b
R3GeCl R2GeCl2 RGeCl3
(2)
(3)
(4)
1) 0 ꢁC; 2) refx 12 h; 82
3) rt 4 h
30
70d
2
i. MeMgI (diethyl ether, DE) + (C6H6)
1
2
3
4
4
5
6
6
ii. 2 (DE) + LiAlH4 (DE).
iii. CCl4.
*
1) rt; 2) refx 12 h;
3) rt 15 h
79
80
89
53
60
70
47
40
30
GeMe
iv. CBr4/nHexane/72 h/r.t.
v. Br2/CCl4
i
3
1) rt; 2) refx 3 h;
3) rt 15 h
v
5
*
1) rt; 2) refx 20 h;
3) rt 15 h
GeCl
3
ii
a
All the reactions were carried out by adding the Grignard reagent in THF onto
2
a solution of GeCl4 in benzene; rt ¼ room temperature; rfx ¼ reflux.
iv
iii
+
GeBr
b
*
*
After chromatography.
*
GeH
GeH
c
By CG-MS; R ¼ (ꢀ)-menthyl.
d
Compounds 3 and 4 could not be separated; however, NMR spectra of chro-
3
3
3
6
matographic fractions enriched in both 3 and known compound 4 enabled us to
establish the composition of these mixtures by comparison with the reported data
for compound 4 [2].
7
6
Scheme 1. Synthesis of derivatives of tri-(ꢀ)-menthylgermanium chloride (2).

3442
M.B. Faraoni et al. / Journal of Organometallic Chemistry 696 (2011) 3440e3444
Table 2
OH
O
13C{1H} NMR spectra of compounds 2, 3, 5, and 6.a
7
R
Z
Compound N°
(-)-Men
Cl
(-)-Men
2
Cl
Cl
5
4
3
6
ethylbenzene
12
1-phenylethanol
acetophenone
1
Z
3
5
9
2
*
8
Ge
Me
H
9
10
8
R
6
2
(-)-Men
O
OH
Cn
Compound 2
Compound 3
Compound 5
Compound 6b
C1
C2
C3
C4
C5
C6
C7
C8
C9
C10
Z
39.01
45.24
26.48
35.32
34.57
38.83
22.78
30.75
21.93
16.82
e
36.31
45.22
26.43
35.37
34.53
38.80
22.84
31.07
22.05
16.48
e
34.80
45.49
26.71
35.71
32.66
39.90
23.02
30.61
22.16
16.74
ꢀ1.67
34.91
46.42
26.61
35.97
30.56
41.00
23.10
30.46
22.23
16.23
e
ethylciclohexane
13
cyclohexylmethylketone
10
1-cyclohexylethanol
11
Scheme 2. Reaction scheme for acetophenone hydrogenation.
been proposed in the literature, by the initial C]O hydrogenation
followed by dehydration and addition of hydrogen to the newly
formed C]C bond [6]. In general, the required product is 1-
phenylethanol (9).
Fig. 1 shows the results of selectivity, enantiomeric excess, and
conversion to 1-phenylethanol (9) at 180 min of reaction for the
hydrogenation of acetophenone, Pt, PteGeBu4 and PteMen3GeMe
systems. As can be seen, in terms of activity, the catalysts behave
quite differently. The monometallic catalyst is more active than the
germanium modified system (reached conversion was 95, 66 and
64% for Pt, PteGeBu4 and PteMen3GeMe respectively). To explain
these facts it is necessary to take into account the variations in the
characteristics of the active sites for hydrogenation that could
appear by the addition of either a nonchiral (GeBu4) or a chiral 5
(Men3GeMe) organogermanium modifier. Pt-based catalytic
systems modified with SnBu4, similar to those used in this work,
have been characterized by our research group using EXAFS, XANES
and XPS techniques in order to clarify the nature of the active sites
[7]. Those studies revealed that in organobimetallic systems there
is a convergence of geometric (dilution of Pt sites) and electronic
(increased electron density of Pt) effects due to the presence of
second metal. It was also found by XPS that metal modifier is
present on the surface of Pt both in ionic form and in metallic. Both
monometallic and organobimetallic systems presented in the
region corresponding to Pt 4f7/2 binding energy (BE) a single peak,
which is characteristic of platinum in the metallic state. The orga-
nobimetallic sample presented a shift of this peak toward lower BE.
This fact is indicative of the existence of an electronic effect of tin on
platinum. The Sn 3d5/2 spectrum for PtSn presents two bands, one
that can be ascribed to reduced tin, and another one at higher BE,
a
In CDCl3 solution except when otherwise stated; chemical shifts, , in ppm with
respect to central peak of chloroform.
b
In C6D6.
techniques. Germanium-modified catalysts were obtained by
a controlled surface reaction between the monometallic catalyst,
previously reduced, and a solution of the organometallic germa-
nium compound in a paraffinic solvent, such as n-decane. The
reaction takes place between 363 and 423 K and results in a system
with organogermanium moieties anchored to the surface. Equation
(2) represents the above mentioned process.
xy
2
Pt=S þ yGeR4 þ H2/PtðGeR4ꢀxÞy=S þ xyRH
(2)
R represents the remaining organic fragment bonded to germa-
nium, that is, methyl or (ꢀ)-menthyl groups. The amount of
germanium fixed was measured to determine the stoichiometry of
the bimetallic phase, expressed as the atomic ratio between Ge and
Pt through the ‘‘y’’ value (y ¼ Ge/Pt ¼ 0.4). In order to study the
specificity of the interaction between the monometallic catalyst
and the organogermanium compounds, blank experiments were
conducted in which the germanium precursor was contacted with
SiO2. No detectable amounts of germanium on the support were
observed under the experimental conditions of this work. Unfor-
tunately, it has not been possible to characterize in detail the chiral
catalytic systems, in particular as regards the structure of the
organogermanium phase attached to the platinum surface.
However, the results presented below, and other published previ-
ously for organotin compounds [1a,b], show that these systems
induce enantioselectivity in the hydrogenation of certain prochiral
carbonyl compounds, concluding that at least some (ꢀ)-menthyl
groups are anchored on the catalyst.
Conversion %
Selectivity % to 1-phenylethanol (9)
ee%
100
90
80
70
60
50
40
30
20
10
0
2.3. Catalytic tests
The main products of acetophenone (8) hydrogenation are
shown in Scheme 2. If the C]O bond is hydrogenated, 1-
phenylethanol (9) is obtained, a product of interest in pharma-
ceutic and perfume industry. On the other hand, the hydrogenation
of the aromatic ring leads to the production of cyclo-
hexylmethylketone (10). Finally, the subsequent hydrogenation of 9
or 10 leads to 1-cyclohexylethanol (11), a product used in the
manufacture of certain polymers. The formation of ethylbenzene
(12) and ethylcyclohexane (13) could be explained by hydro-
genolysis of the CeO bond of the intermediate alcohols or, as it has
Pt-SiO2
Pt-GeBu4
Pt-Men3GeMe
Fig. 1. Conversion, selectivity, and enantiomeric excess for the hydrogenation of ace-
tophenone at 180 min of reaction.

M.B. Faraoni et al. / Journal of Organometallic Chemistry 696 (2011) 3440e3444
3443
which is attributed to oxidized tin (Sn(II) and Sn(IV)). Thus, the
presence of organotin fragments blocks some active sites for the
hydrogenation on the Pt surface and, as a consequence, the catalytic
activity is reduced.
tetrachloride was purchased and (1R,2S,5R)-(ꢀ)-menthylmagne-
sium chloride (1) was obtained following known procedures [9].
3.2. Synthesis of (1R,2S,5R)-(ꢀ)-trimenthylgermanium chloride (2)
and (1R,2S,5R)-(ꢀ)-dimenthylgermanium dichloride (3)
Concerning the selectivity, Fig. 1 shows that the addition of
organogermanium compounds favors the formation of 1-
phenylethanol (9). In the case of PteGeBu4 and PteMen3GeMe
catalysts, the aromatic ring hydrogenation is strongly inhibited
perhaps by a dilution effect of Pt atoms, besides the electronic
effects produced by the presence of ionic germanium, as shown by
EXAFS/XANES [8]. Both effects would favor the acetophenone
molecule interaction in this catalyst through the oxygen atom of
the C]O group, and not via the aromatic ring, facilitating in this
way the formation of 1-phenylethanol (9). The existence of an
additional steric effect due to the presence of GeRx fragments on
the surface should not be discarded. Thus, the selectivity of 1-
phenylethanol obtained was 50%, 70 and 62% for Pt, PteGeBu4
and PteMen3GeMe respectively.
To a vigorously stirred solution of GeCl4 (5.64 g, 26 mmol) in dry
benzene (27 mL) at room temperature and placed in a two-necked
round-bottomed flask with a pressure-equalizing funnel and
a reflux condenser with a nitrogen seal attached, was added drop-
wise a solution of (1R,2S,5R)-(ꢀ)-menthylmagnesium chloride (1)
in dry THF (98.7 mL of a 1.58 M solution, 156 mmol). The mixture
was heated under reflux for 20 h and then was left for 15 h with
stirring at room temperature. HCl was then added (ca. 20 mL of a 10%
solution), and the mixture was transferred to a separatory funnel.
After Et2O addition (ca 150 mL), the organic layer was decanted,
washed three times with water, and then dried with anhydrous
MgSO4. The solvent was removed under reduced pressure. The
chromatogram of the crude product (12.3 g) showed a mixture of
two compounds: (1R,2S,5R)-(ꢀ)-trimenthylgermanium chloride (2)
(Rt: 24.12 min) and (1R,2S,5R)-(ꢀ)-dimenthylgermanium dichloride
(3) (Rt: 22.88 min) in a ratio 2/3 ¼ 70:30. Column chromatography
(silica gel 60) of the mixture afforded 2 (8.4 g, 16 mmol, 62%) and 3
(2.3 g, 5.4 mmol, 21%) in the fractions eluted with 99:1 and 93:7
The PteMen3GeMesystemalsoprovedtobeenantioselective;the
enantiomeric excess reaching values of 17% for 1-phenylethanol (9).
2.4. Conclusions
The main findings of this study are:
hexaneediethyl ether, respectively. Compound 2: mp 104e105 ꢁC;
22
[a
]
¼ ꢀ79.5ꢁ (c 1, C6H6); 1H NMR (CDCl3, Me4Si)
d ppm: 0.80 [d, 9H,
D
- It has been possible to prepare organobimetallic catalysts by
the selective reaction of an organogermanium compound
(GeBu4 and Men3GeMe) and a Pt-monometallic catalyst. These
systems have been obtained through controlled surface reac-
tions, using techniques derived from SOMC/M.
- These solids proved to be active catalysts in the hydrogenation
of acetophenone. The monometallic catalyst is more active
than the germanium modified system, possibly due to a dilu-
tion effect of the active sites on the Pt surface, due to the
presence of Ge.
3J(H,H) 6.7]; 0.89 [d, 9H, 3J(H,H) 6.4]; 0.94 [d, 9H, 3J(H,H) 6.8];
0.97e2.10 (m, 30H). MS (m/z, relative intensity): 387 [34,
(M ꢀ C10H19)þ], 249 [37, (M ꢀ C20H38)þ], 137 (76), 55 (100). Anal.
Calcd. for C30H57GeCl: C 68.53; H 10.93. Found: C 68.66; H 10.97.
22
Compound 3: mp 83e84 ꢁC; [
a]
¼ ꢀ82.3ꢁ (c 1, C6H6); 1H NMR
D
(CDCl3, Me4Si)
d
ppm: 0.80 [d, 6H, 3J(H,H) 6.9]; 0.87 [d, 6H, 3J(H,H)
6.5]; 0.93 [d, 6H, 3J(H,H) 7.0]; 0.99e2.00 (m, 20H). MS (m/z, relative
intensity): 388 [79, (M ꢀ Cl)þ], 352 [17, (M ꢀ 2Cl)þ], 249 [15,
(M ꢀ Cl ꢀ C10H19)þ], 213 [9, (M ꢀ 2Cl ꢀ C10H19)þ], 137 (5), 55 (100).
Anal. Calcd. for C20H38GeCl2: C 56.91; H 9.08. Found: C 57.03; H 9.10.
- The addition of organogermanium compounds favors the
formation of 1-phenylethanol. In the PteGeBu4 and PteMen3
-
3.3. Synthesis of (1R,2S,5R)-(ꢀ)-trimenthylmethylgermanium (5)
GeMe catalysts, the aromatic ring hydrogenation is strongly
inhibited perhaps by a dilution effect of Pt atoms, besides the
electronic effects produced by the presence of ionic germa-
nium. The existence of an additional steric effect due to the
presence of GeRx fragments on the surface should not be
discarded.
e The PteMen3GeMe catalyst proved to be enantioselective in
the hydrogenation of acetophenone, reaching values of enan-
tiomeric excess of 17% for 1-phenylethanol.
To a solution of 2 (3.0 g, 5.7 mmol) in dry benzene (12 mL) at 0 ꢁC
under nitrogen, was added slowly and dropwise with stirring
a solution of the methylmagnesium iodide in diethyl ether (3.7 mL
of a 1.53 M solution, 5.7 mmol). The mixture was heated under
reflux for 3 h. After cooling at 0 ꢁC, the reaction mixture was
decomposed with a solution of HCl (ca 5 mL of a 10% solution). The
organic layer was separated and the aqueous layer was extracted
three times with diethyl ether (ca 2 mL each). The combined
organic extracts were dried over anhydrous MgSO4, and the solvent
was removed under reduced pressure. Column chromatography
3. Experimental
(silica gel 60) gave 5 as a white solid (2.48 g, 4.9 mmol, 86%); mp
22
3.1. General methods
65e66 ꢁC; [
a]
¼ ꢀ43.7ꢁ (c 1, C6H6); 1H NMR (CDCl3, Me4Si)
d ppm:
D
0.23 (s, 3H); 0.77 [d, 9H, 3J(H,H) 6.9]; 0.88 [d, 9H, 3J(H,H) 6.6]; 0.93
[d, 9H, 3J(H,H) 6.9]; 0.98e1.86 (m, 30H). MS (m/z, relative inten-
sity): 367 [1, (M ꢀ C10H19)þ], 228 [41, (M ꢀ C20H38)þ], 137 (100).
Anal. Calcd. for C31H60Ge: C 73.67; H 11.96. Found: C 73.48; H 11.92.
NMR spectra were recorded on a Bruker ARX 300 instrument,
using CDCl3 as solvent; chemical shifts (d) are reported in ppm with
respect to TMS for 1H and 13C NMR spectra. Infrared spectra were
recorded with a Nicolet Nexus FT spectrometer. CG-Mass spectra
were obtained with
a
GC/MS instrument (HP5-MS capillary
m) equipped with 5972 mass
3.4. Synthesis of (1R,2S,5R)-(ꢀ)-trimenthylgermanium hydride (6)
column, 30 m ꢂ 0.25 mm ꢂ 0.25
m
selective detector operating at 70 eV (EI). Program: 50 ꢁC for 2 min
with increase 10 ꢁC/min to 280 ꢁC. Elemental analyses (C, H) were
performed at UMYMFOR (Universidad de Buenos Aires) and
University of Cologne (Germany). Melting points were determined
in a Kofler hot stage and are uncorrected. Specific rotations were
To a suspension of LiAlH4 (0.14 g, 3.6 mmol) in dry diethyl ether
(8 mL) under atmosphere of nitrogen at room temperature, was
added dropwise a solution of (1R,2S,5R)-(ꢀ)-trimenthylgermanium
chloride (2) (1.9 g, 3.6 mmol) in dry diethyl ether (20 mL). The
mixture was heated under reflux for 7 h and a saturated solution of
ammonium chloride was then added (ca 10 mL). The organic layer
was separated, and the aqueous was extracted three times with
determined with a Polar
reagents used were analytical reagent grade. Germanium
L-mP, IBZ messtechnik. All the solvents and

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M.B. Faraoni et al. / Journal of Organometallic Chemistry 696 (2011) 3440e3444
diethyl ether. The combined organic extracts were dried over
anhydrous MgSO4. Removal of the solvent under reduced pressure
3.7. Hydrogenation reactions
gave 6 as a white solid (1.73 g, 3.5 mmol, 98%). IR (NaCl, cmꢀ1):
Both the racemic and the enantioselective hydrogenations of
acetophenone were carried out in a stirred autoclave type reactor at
a H2 pressure of 1 MPa and a temperature of 353 K, using 0.25 g
catalyst, and 2-propanol as solvent. In each test, 0.5 mL of aceto-
phenone and 60 mL of 2-propanol were used (0.07 M). The
experimental conditions for the catalytic tests were chosen so that
the reaction rate was not influenced by mass transfer. The course of
the reaction was followed in a Varian CP-3800 gas chromatograph
equipped with a capillary column CP wax 52 CB (30 m, i.d.
0.53 mm) and a FID detector. The reaction products were identified
using a GC/MS Shimadzu QP5050 with a capillary column SUPELCO
SPBTM-5 (30 m, i.d. 0.25 mm). The enantiomeric excess (ee) was
calculated according to the following expression: ee% ¼ 100(S ꢀ R)/
(S þ R), taking into account data obtained with a BETA DEXÔ 120
capillary column (30 m length and i.d. 0.25 mm) in a Varian CP-
3800 gas chromatograph.
22
nGeeH 2000 [4]; mp 90e91 ꢁC; [
(C6D6, Me4Si)
a
]
D
¼ ꢀ105.3ꢁ (c 1, C6H6); 1H NMR
d
ppm: 0.89 [d, 9H, 3J(H,H) 6.9]; 0.92 [d, 9H, 3J(H,H)
6.2]; 1.00 [d, 9H, 3J(H,H) 6.9]; 1.21e2.28 (m, 30H); 4.35 (s, 1H,
GeeH). MS (m/z, relative intensity): 353 [16, (M ꢀ C10H19)þ], 215
[50, (M ꢀ C20H38)þ], 137 (100). Anal. Calcd. for C30H58Ge: C 73.33; H
11.90. Found: C 73.06; H 11.86.
3.5. Catalysts preparation
The monometallic catalyst was prepared by ion exchange, using
SiO2 as support, previously treated with ammonia solution. At pH
between 9 and 11, SiO2 behaves as a cationic exchanger, and under
this condition, was contacted with an aqueous solution of
[Pt(NH3)4]Cl2 in an appropriate concentration so as to obtain 1 wt%
Pt in the resulting catalyst. After 24 h of exchange at room
temperature, the solid was separated by filtration, washed and
dried at 378 K, and subsequently reduced in H2 flow at 773 K for 2 h.
The modification of monometallic catalysts by germanium
addition was carried out by using surface organometallic chemistry
on metals (SOMC/M) techniques [10]. Based on the deep analysis of
catalytic systems analogous to the ones here presented, but con-
taining tin, the experimental conditions were selected so as to
achieved the desired Ge/Pt atomic ratio [7a,11]. After the reduction
stage, a definite quantity of the monometallic catalyst (0.25 g) was
put to react in H2 atmosphere with GeBu4 dissolved in n-decane at
393 K. Once the reaction was complete, the catalyst was washed
with several portions of n-heptane in Ar atmosphere. This catalyst,
which has a molar ratio Ge/Pt ¼ 0.4, is denominated PteGeBu4 and
corresponds to a solid containing butyl groups anchored on the
metallic surface. The preparation of the chiral organometallic
catalyst was performed by following an analogous procedure, but
using a solution of the chiral organotin compound tri(ꢀ)-men-
thylmethylgermanium in n-decane. The Ge/Pt ratio used was also
0.4. This catalyst is denominated PteMen3GeMe.
Acknowledgments
JCP acknowledge grants from ANPCyT (Capital Federal,
Argentina/BID 1728/OC-AR PICT Nꢁ 2467), CONICET (Buenos Aires,
Argentina, PIP 112-200801-02272), and Universidad Nacional del
Sur (Bahía Blanca, Argentina, PGI 24/Q024). MLC and VV gratefully
acknowledge CONICET (PIP 0185) and Universidad Nacional de La
Plata (Project X487) for the financial support. A grant for a short
visit to Germany of one of us (JCP) from the Alexander von Hum-
boldt Foundation is also gratefully acknowledged.
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