Hydrophilic diamine ligands for catalytic asymmetric hydrogenation of C=O bonds

Article abstract of DOI:10.1016/S0957-4166(02)00332-4

Asymmetric hydrogenations of phenylglyoxylate methyl ester and of acetophenone with catalytic amounts of iridium complexes containing hydrophilic chiral C₂-symmetric diamine ligands is reported. E.e. values of up to 68% are observed for complet

Full text of DOI:10.1016/S0957-4166(02)00332-4

TETRAHEDRON:
ASYMMETRY
Pergamon
Tetrahedron: Asymmetry 13 (2002) 1379–1384
Hydrophilic diamine ligands for catalytic asymmetric
hydrogenation of CꢀO bonds
A. Ferrand, M. Bruno, M. L. Tommasino and M. Lemaire*
Laboratoire de Catalyse et Synthe`se Organique, CNRS: UMR 5622, CPE, Baˆt. 308, 43 bd. du 11 novembre 1918,
69622 Villeurbanne, France
Received 5 June 2002; accepted 21 June 2002
Abstract—Asymmetric hydrogenations of phenylglyoxylate methyl ester and of acetophenone with catalytic amounts of iridium
complexes containing hydrophilic chiral C2-symmetric diamine ligands is reported. E.e. values of up to 68% are observed for
complete acetophenone hydrogenation in hydrophilic media. The use of such ligands allowed catalyst recovery without loss of
activity and enantioselectivity in at least four hydrogenation cycles. © 2002 Elsevier Science Ltd. All rights reserved.
1. Introduction
2. Synthesis of hydrophilic enantiomerically pure
diamines
Most of the catalysts used in homogeneous asymmetric
hydrogenation reactions involve chiral bidentate phos-
phorus-containing ligands1 which are often expensive
and easily oxidized. However, various chiral nitrogen
derivatives, such as C2-symmetric diamines have been
successfully used as chiral ligands in asymmetric transi-
tion metal-catalyzed reactions.2 Our laboratory has
developed a series of enantiomerically pure C2-symmet-
ric diamine and dithiourea ligands which are efficient
catalysts for the hydrogenation of CꢀO and CꢀC
bonds.3,4 We have also shown that combining (1R,2R)-
(+)-N,N%-dimethyl-1,2-diphenyl-ethylenediamine 2 with
[Ir(COD)2]BF4 as catalyst precursor could induce high
enantioselectivities in the hydrogenation of phenylgly-
oxylate methyl ester. From this point two areas of
research are possible: on one hand new diamines could
be synthesized in order to reach enantiomeric excess of
practical interest (e.e.=95%). On the other hand, the
modification of known ligands, with a view to facilitat-
ing the separation and recycling of the catalysts, could
be examined. For this last purpose, we present herein a
means of introducing hydrophilic groups onto the
diamine ligands (Scheme 1)
(1R,2R)-(+)-1,2-Diphenyl-ethylenediamine 1 is com-
mercially available. (1R,2R)-(+)-N,N%-Dimethyl-1,2-
diphenyl-ethylenediamine 2 was prepared according to
recently published procedures.5 Ligand 3, the methoxyl-
ated analog of diamine 2, was prepared by coupling the
corresponding imine as described by Alexakis and
Mangeney,5 with an improved yield of 98% of the (1:1)
d,l-/meso-diamine mixture (Scheme 2). Resolution was
not carried out on this crude mixture as reported
because we noticed that several recrystallizations in
ether led to a mixture enriched in d,l compound (d,l/
meso=75/25). Resolution was then possible with -(+)-
L
tartaric acid and (1R,2R)-(+)-N,N%-dimethyl-1,2-
(4-methoxyphenyl)ethylenediamine 3 is obtained in
more than 99.8% enantiomeric excess as determined by
31P NMR. When BBr3 was slowly added to a
dichloromethane solution of the protected diamine 3
and further deprotection in acidic media was carried
out (1R,2R)-(+)-N,N%-dimethyl-1,2-(4-hydroxyphenyl)-
ethylenediamine 4 was obtained in 80% yield and 97%
The synthesis of chiral diamines 3–5 is described and
we also report the use of diamine ligands 1–5 in the
presence of Rh, Ir and Ru precursors as chiral catalysts
for the asymmetric hydrogenation of CꢀO bonds. In the
case of hydrophilic systems containing diamines 3–5,
we describe the catalyst recovery studies.
* Corresponding author. E-mail: marc.lemaire@univ-lyon1.fr
Scheme 1. Chiral diamines used in hydrogenation reactions.
0957-4166/02/$ - see front matter © 2002 Elsevier Science Ltd. All rights reserved.
PII: S0957-4166(02)00332-4

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A. Ferrand et al. / Tetrahedron: Asymmetry 13 (2002) 1379–1384
Scheme 2. Synthesis of diamines 3–5.
e.e. (1R,2R)-(+)-N,N%-Dimethyl-1,2-(4-methoxytriethyl-
ene glycol-phenyl)ethylenediamine 5 is prepared by O-
alkylation of the protected diamine 4 in DMF at 110°C
with tosylated triethylene glycol and further acid depro-
tection. The enantiomeric excess remains very high, with
e.e. of 96%, but a moderate yield (33%) is obtained.
ruthenium, we employed the neutral complex,
[RuCOD(h3-C4H7)2]. As for Ru and Rh catalysts, less
than 30% e.e. was attained despite good activities, the
results will not be described in detail. Chiral diamine
1–5 (2 equiv.) were added under an argon atmosphere
to the iridium precursor in the chosen solvent. The
solutions were used for the hydrogenation of phenyl-
glyoxylate methyl ester. Results are summarized in
Table 1.
3. Asymmetric hydrogenation of phenylglyoxylate
methyl ester
We notice that (1R,2R)-(+)-1,2-diphenyl-ethylenedi-
amine 1 gave the lowest e.e. values (entries 1 and 2).
This may be due to the presence of two hydrogen atoms
on the nitrogen binding site, thus avoiding the forma-
tion of stereogenic centres when the diamine is coordi-
nated to iridium.3 Diamines 1 and 2 led to better
In previous work, Rh or Ir complexes of C2-symmetric
diamine 1 were used in the hydrogenation of phenylgly-
oxylate methyl ester.3 As we then demonstrated that
cationic species are more active than neutral ones, we
used here [Rh(NBD)2]BF4 and Ir[(COD)2]BF4. For

A. Ferrand et al. / Tetrahedron: Asymmetry 13 (2002) 1379–1384
1381
Table 1. Diamines 1–5 as chiral inducers for phenylglyoxyl-
ate methyl ester hydrogenation
clearly better than the result obtained with diamine 2
(27% e.e.).
Table 2. Diamines 2–5 as chiral inducers for acetophenone
hydrogenation
Entry
L*
Solvent
% Yield
% E.e. (R)
1
2
3
4
5
6
7
8
9
1
1
2
2
3
3
4
4
5
5
THF
H2O
THF
H2O
THF
H2O
THF
H2O
THF
H2O
99
100
100
73
100
96
100
100
98
31
15
80
56
45
50
49
42
43
54
Entry
L*
Solvent
% Yield
% E.e. (R)
1
2
3
4
5
6
7
8
2
2
3
3
4
4
5
5
THF
H2O
THF
H2O
THF
H2O
THF
H2O
71
49
44
100
50
82
63
27
46
35
61
24
51
48
10
100
43
77
enantioselectivities when the corresponding iridium cat-
alysts are formed in THF instead of water (entries 1
and 3 versus 2 and 4). A noticeable decrease in the
catalytic activity is observed when using water as the
solvent for ligand 2. Conversely, the results in THF are
similar to those obtained in water when ligands 3, 4 and
5 are used: the presence of the hydroxy, methoxy and
methoxytriethylene glycol groups on the diamine
favours the solubility of the catalytic systems in water,
which is one of the goals of our study. In addition,
chiral induction of these hydrophilic ligands (entries 6,
8, and 10) is about the same as that observed for the
diamine 2 in water (56%).
From both Tables 1 and 2, we notice that the introduc-
tion of the hydrophilic groups on the (1R,2R)-(+)-N,N%-
dimethyl-1,2-diphenyl-ethylenediamine 2 allowed the
use of water as solvent and improved both the activity
and enantioselectivity of the corresponding iridium cat-
alysts. This trend is clearer for the methoxytriethylene
glycol diamine 5. Despite this, the enantioselectivity of
the aqueous catalysts (e.e.=54%) still needs to be
optimized.
5. Catalyst recovery tests
When
(1R,2R)-(+)-N,N%-dimethyl-1,2-diphenyl-1,2-
4. Asymmetric hydrogenation of acetophenone
ethylenediamine 2 combined with Ir[(COD)2]BF4 is
used as a catalyst for the hydrogenation of acetophe-
none, in water or a H2O/MeOH mixture, low activity
and enantioselectivity are observed, while iridium spe-
cies containing hydrophilic diamines 3–5 induce 61–
70% e.e. with complete conversion in methanol. The
first attempts to recover the catalysts were thus carried
out in a H2O/MeOH (2/1) mixture. At the end of the
hydrogenation run, pentane is added to the reaction
mixture and two phases form: the organic layer is
analyzed, while the aqueous phase containing the cata-
lyst is replaced in the autoclave for reuse. A methanolic
solution of acetophenone is then added to the catalyst
solution in order to start the second hydrogenation
cycle. Table 3 lists the results obtained with the
hydrophilic diamines 3–5.
We then extended our study to a more difficult sub-
strate: acetophenone, which is a better model for practi-
cal synthetic applications. The hydrogenation reactions
are carried out following the same experimental proce-
dure as for phenylglyoxylate methyl ester using a sub-
strate to catalyst molar ratio of 200 and 0.5% of
Ir[(COD)2]BF4. The corresponding results are listed in
Table 2.
As observed for phenylglyoxylate methyl ester, the
reduction of acetophenone catalyzed by the iridium/
diamine 2 complex in water gives lower activity and
enantioselectivity than the reaction performed in THF
(entries 1 and 2). Moderate conversions and enan-
tiomeric excesses are obtained in THF with hydrophilic
ligands 3–5 (entries 3, 5, and 7). When the same hydro-
genations are carried out in water we notice a clear
improvement of the catalytic activity but, except with
diamine 5 the enantioselectivity diminishes (entries 7
and 8). Even though maximum asymmetric induction in
water is only 48% with the hydrophilic diamines, this is
In the first cycle, the three diamines 3–5 allowed com-
plete acetophenone hydrogenation with similar enan-
tioselectivities: 57–66% e.e. In the second cycle, the
enantioselectivity was unchanged but the activity
decreased so significantly that it dissuaded us to start
further hydrogenation runs using the recycled catalyst.

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A. Ferrand et al. / Tetrahedron: Asymmetry 13 (2002) 1379–1384
Table 3. Diamines 3–5 as chiral inducers for acetophenone hydrogenation: catalyst recovery
Entry
L*
Cycle
% Yield
% E.e. (R)
1
2
3
4
5
6
3
3
4
4
5
5
1
2
1
2
1
2
100
69
100
70
100
43
63
57
66
62
58
54
Catalyst recovery can also be achieved using other
hydrophilic solvents. Ethylene glycol and glycerol were
introduced by Davies when studying the supported
aqueous-phase catalysts (SAPCs).6 This concept is
based upon the immobilization of the catalytic species
by means of the interaction of their hydrophilic ligands
with a hydrophilic solid (glass, silica) in a thin film of a
hydrophilic solvent (water, ethylene glycol). The
organic substrates are commonly dissolved in a hydro-
phobic media and the reaction takes place at the inter-
face of the two liquid phases.
The presence of methoxytriethylene glycol in the reac-
tions using diamine 2 allowed us to recover the cata-
lysts, which are sufficiently hydrophilic, in only 43%
yield after the second run. In the case of diamines 3–5
we observe that the corresponding catalytic activities
and enantioselectivities are preserved during the first
four cycles when methoxytriethylene glycol is added.
However, when increasing the methoxytriethylene gly-
col levels (entries 7 and 8), the catalytic activity
decreased significantly in the fifth run. Thus, to get
SAPC conditions we should increase the hydrophilicity
of our ligands by adding longer polyethylene glycol
chains.
We studied the influence of methoxytriethylene glycol
as additive in the catalyst recycling tests: the reactions
are performed in the aqueous solvent (H2O/MeOH) as
previously described and methoxytriethylene glycol is
added in a molar ratio of 1 with respect to the ligand.
The reaction media are extracted with pentane after the
test, so the aqueous phases could be reused for another
hydrogenation cycle. The corresponding results are
listed in Table 4.
6. Conclusion
Chiral diamine ligands have only been adapted recently
for use in asymmetric hydrogenation reactions although
they are more accessible than their phosphine counter-
parts. In the presence of chiral diamines, iridium can
Table 4. Methoxytriethylene glycol as additive for catalyst recovery agent
Entry
L*
Cycle
% Yield
% E.e. (R)
1
2
3
4
5
6
7
8
3
3
4
4
1–6
7
1–4
5
1–3
4
1–4
5
99–100
83
100
68
100
91
90–100
75
65–68
62
60–65
60
61–63
59
58–63
48
5
5
5a
5a
a Methoxytriethylene glycol/H2O/MeOH=1/2/1.

A. Ferrand et al. / Tetrahedron: Asymmetry 13 (2002) 1379–1384
1383
lead to higher enantioselectivities than the analogous
rhodium or ruthenium species. We have also demon-
strated that the diamines can be modified in order to
obtain specific solubilities of the catalytic systems
allowing their recovery in biphasic liquid media. The
introduction of different hydrophilic groups onto
(1R,2R)-(+)-N,N%-dimethyl-1,2-diphenyl-ethylenedi-
amine 2 allowed the asymmetric hydrogenation of
phenylglyoxylate methyl ester and of acetophenone in
aqueous media. In the hydrogenation of acetophenone,
catalyst recovery was possible for at least four hydro-
genation cycles with complete conversions and
unmodified enantioselectivities (up to 68%). This is
encouraging because the chiral induction is about the
same as that observed in THF and the yield is enhanced
(71% yield and 63% e.e. in THF). We are now focusing
on the SAPC applications of the iridium/ligand 5 com-
plexes, notably by increasing the length of the
polyethylene glycol groups on the ligand.
was cooled to −5°C before hydrolysis with 30%
aqueous NH4OH (250 mL). Saturated aqueous NH4Cl
(400 mL) was then added and the mixture stirred
overnight at room temperature. After removal of the
zinc dust by filtration through Celite, the liquid was
extracted with CH2Cl2 (2×250 mL) and Et2O (2×250
mL). The combined organic layers were concentrated
and 5 M HCl (400 mL) were added. The obtained
aqueous layer was extracted with Et2O (2×250 mL) and
basified with NaOH pellets. The organic layer was
dried over K2CO3 and the solvent was removed to
afford the crude product (98% yield). Two recrystalliza-
tions of the raw product from ether afforded a yellow
solid which is a d,l/meso (75/25) mixture of the diamine
3 (73% yield). The resolution of 31.2 g of this mixture
(containing 85.9 mmol of d,l diamine) was completed
with -(+)-tartaric acid (13.5 g, 90 mmol) in 96% etha-
L
nol. The precipitated salt was stirred with 2.5 M NaOH
(300 mL) and the aqueous phase was extracted with
Et2O (3×300 mL). The organic layer was dried on
K2CO3 and the Et2O was removed to give a white solid.
The enantiomeric excess was determined by formation
of PCl3 derivative, according to the published proce-
dure.5 After recrystallization in ether, 99.8% enan-
tiomerically pure diamine 3 was isolated (5.86 g, 25%
7. Experimental
7.1. General
1
yield); mp 119.5°C; [h]D=+35 (c 1, CHCl3); H NMR
Anhydrous THF (99.9% in a Sure/Seal™ bottle,
Aldrich), EtOH (96%), MeOH, CH2Cl2, Et2O and pen-
tane (Normapur) were used as received. Commercial
methylamine, p-anisaldehyde, zinc dust, 1,2-dibromo-
(CDCl3): 2.26 (2H, s, NH), 2.50 (6H, s, CH3-NH), 3.50
(2H, s, CH-NH), 3.75 (6H, s, CH3-O), 6.71 (4H, d,
J=8.7 Hz, CaromH, meta), 6.95 (4H, d, J=8.7 Hz,
CaromH, ortho); 13C NMR (CDCl3): 34.4 (CH3-NH),
55.2 (CH3-O), 70.3 (CH-NH), 113.4 (CaromH, meta),
ethane, chlorotrimethylsilane, -(+)-tartaric acid, DMF,
L
tosylchloride, AcOEt, HCl, NaOH, NaCl, NH4Cl,
128.9 (CaromH, ortho), 132.8 (Carom-CH), 158.5 (Carom
-
1
NH4OH, AcOEt/NEt3 and PCl3 were used. H and 13C
OCH3, para). Anal. calcd for C18H24N2O2: C, 71.97; H,
8.05; N, 9.33; O, 10.65. Found: C, 71.83; H, 8.27; N,
9.34; O, 11.01%.
NMR spectra were recorded on a Bruker AC-200
Fourier-transform spectrometer and l values are given
1
in ppm (200.13 MHz for H and 50.32 MHz for 13C).
Optical rotations were measured with a Perkin–Elmer
241 polarimeter. Elemental analysis is carried out by
the central analysis service of CNRS at Solaize.
7.2.2.
(1R,2R)-(+)-N,N%-Dimethyl-1,2-(4-hydroxy-
phenyl)ethylenediamine 4. Diamine 3 (4.3 g, 14.3 mmol)
,
was poured into Et2O (70 ml) containing 4 A molecular
sieves (5 g) under magnetic stirring and acetaldehyde
(1.5 mL, 26.6 mmol) was added. When the diamine was
totally dissolved, the molecular sieves were removed by
filtration. The ether and the excess acetaldehyde were
also removed under vacuum to give the acetaminal
quantitatively (98%). A solution of BBr3 in CH2Cl2 (1
M, 54 mL) were slowly added to the a solution of the
acetaminal (4.0 g, 12.2 mmol) in CH2Cl2 (50 mL) at
−5°C. After stirring at room temperature for 20 h, the
mixture was slowly hydrolyzed by treatment with 0.75
M NaOH (250 mL). After evaporating the CH2Cl2, the
solution was filtered through Celite and the pH was
adjusted to 8.5 with 0.1 M HCl: the precipitate was
quantitatively (95%) recovered by filtration. The
formed acetaminal is then cleaved with 2.5 M HCl (20
mL) and stirred for 30 min. Et2O (2 mL) were then
added and stirring was continued for 30 min. The
aqueous solution was then evaporated to a volume of
10 mL and the pH adjusted to 8.5 with 0.1 M NaOH to
afford the solid diamine 4 recovered by filtration (2.92
g, 80%); mp 182°C; [h]D=+102 (c 1, DMSO); 1H NMR
(DMSO): 2.07 (6H, s, CH3-NH), 3.27 (2H, s, CH-NH),
6.50 (4H, d, J=8.3 Hz, CaromH, ortho), 6.79 (4H, d,
J=8.3 Hz, CaromH, meta); 13C NMR (DMSO): 33.9
7.2. Synthesis of diamines
(1R,2R)-(+)-1,2-Diphenyl-ethylenediamine 1 is com-
mercially available from Fluka, [h]D=+105 (c 1,
MeOH) and (1R,2R)-(+)-N,N%-dimethyl-1,2-diphenyl-
ethylenediamine 2 was prepared according to recently
published procedures.5
7.2.1.
(1R,2R)-(+)-N,N%-Dimethyl-1,2-(4-methoxy-
phenyl)ethylenediamine 3. Zinc dust (<10 mm, 50 g, 0.76
mol) was suspended in anhydrous THF (200 mL),
1,2-dibromoethane (3 mL) was added as an activator
under magnetic stirring. The mixture was heated under
reflux for 1 min, cooled to room temperature before
further 1,2-dibromoethane (6 mL) was added. A solu-
tion of 4-methoxybenzyl-N-methylimine (obtained by
addition of aqueous methylamine to p-anisaldehyde,
112 g, 0.75 mol) in anhydrous THF, (400 mL) was
added to the suspension. Chlorotrimethylsilane (200
mL, 1.58 mol) was added carefully over 30 min with a
dropping funnel keeping the temperature below +40°C.
The mixture was slowly heated to boiling temperature
and heated under reflux for 4 h. The reaction mixture

1384
A. Ferrand et al. / Tetrahedron: Asymmetry 13 (2002) 1379–1384
(CH3-NH), 70.0 (CH-NH), 113.4 (CaromH, ortho), 128.9
(CaromH, meta), 131.7 (Carom-CH-), 155.5 (CaromH-OH,
para). Anal. calcd for C16H20N2O2: C, 70.56; H, 7.40;
N, 10.29; O, 11.75. Found: C, 70.62; H, 7.55; N, 10.08;
O, 11.76%.
degassed by argon bubbling for 30 min. Chiral gas
chromatography analysis was carried out in a Shi-
madzu GC-14A chromatograph using a flame-ioniza-
tion detector and a Shimadzu C-R6A integrator within
a Lipodex A 25 m column.
7.3.1. Typical hydrogenation procedure. The metal pre-
cursor and the corresponding amount of chiral ligand
were dissolved in the chosen solvent under an argon
atmosphere at room temperature. After stirring for 30
min, the substrate was added and the resulting solution
transferred to a purged stainless steel autoclave, con-
taining a stirrer in a glass vessel. H2 pressure is fixed to
50 bars and the autoclave heated to 50°C. After stirring
overnight the autoclave is cooled and degassed. Solu-
tions are filtered through Celite before CPV analysis.
7.2.3.
(1R,2R)-(+)-N,N%-Dimethyl-1,2-(4-methoxytri-
ethylene glycol-phenyl)ethylenediamine 5. The protected
diamine 3 (1 g, 3.35 mmol, prepared as for the synthesis
t
of diamine 4) was slowly stirred with BuOK (0.850 g,
7.57 mmol) for 15 min under an argon atmosphere.
Anhydrous DMF (5 mL) was added and the mixture
heated to 110°C before the addition of triethylene
glycol tosylate derivative (3.2 g, 10.0 mmol). After 4 h
when all the protected diamine was reacted (TLC con-
trol), AcOEt (20 mL) was added and the mixture
cooled to room temperature. After filtration and evapo-
ration of the solvent, the brown oil obtained was
purified on silica gel column using AcOEt and AcOEt/
NEt3 (90/10) as eluant to separate the product of the
remaining protected diamine. After removal of the sol-
vent, a brown oil was obtained and stirred with HCl
(1.11 mL, 13.4 mmol) in water (20 mL) during 30 min.
This aqueous phase was saturated with NaCl and
extracted with AcOEt (6×30 mL). The organic layer
was dried over K2CO3 and the solvent was removed
under vacuum to give pure diamine 5 (0.402 g, 21%
7.3.2. Catalyst recovery procedure. After a first hydro-
genation cycle carried out as described above, pentane
(1 mL) is added to the reaction mixture. After shaking
and short centrifuging, the organic phase was separated
for CPV analysis while the aqueous phase was quickly
replaced in the cleaned and purged autoclave with the
corresponding amount of substrate solution. Hydrogen
pressure and temperature are then fixed for the second
hydrogenation run.
1
yield); [h]D=+67 (c 1, CDCl3); H NMR (CDCl3): 2.18
References
(3H, s, CH3-NH), 3.33 (3H, s, CH3-O), 3.40 (1H, s,
CH-NH), 3.50–4.00 (12H, m, O-CH2-CH2-O),), 6.73
(2H, d, J=8.5 Hz, CaromH, meta), 6.86 (2H, d, J=8.5
Hz, CaromH, ortho); 13C NMR (CDCl3): 34.4 (CH3-
NH), 59.0 (CH3-O), 69.9, 70.5, 70.6, 70.8, 70.9, 72.3
(O-CH2CH2-O), 77.4 (CH-NH), 114.0 (CaromH, meta),
1. Takaya, H.; Ohta, T.; Noyori, R. In Catalytic Asymmetric
Synthesis; Ojima, I., Ed. Asymmetric Hydrogenation;
Wiley-VCH: Weinheim, 1993; p. 20.
2. Fache, F.; Schulz, E.; Tommasino, M. L.; Lemaire, M.
Chem. Rev 2000, 100, 2159.
128.9 (CaromH, ortho), 133.2 (Carom-CH), 157.6 (Carom
-
3. Tommasino, M. L.; Thomazeau, C.; Touchard, F.;
Lemaire, M. Tetrahedron: Asymmetry 1999, 10, 1813.
4. Tommasino, M. L.; Casalta, M.; Breuzard, J. A. J.;
Lemaire, M. Tetrahedron: Asymmetry 2000, 11, 4835.
5. (a) Alexakis, A.; Aujard, I.; Mangeney, P. Synlett 1998,
873; (b) Alexakis, A.; Aujard, I.; Mangeney, P. Synlett
1998, 875; (c) Alexakis, A.; Frutos, J. F.; Mutti, S.;
Mangeney, P. J. Org. Chem. 1994, 59, 3326.
O, para). Anal. calcd for C30H48N2O8: C, 63.89; H,
8.57; N, 4.96; O, 22.67. Found: C, 63.98; H, 8.77; N,
4.49; O, 22.85%.
7.3. Catalytic hydrogenation of ketones
Acetophenone and phenylglyoxylate methyl ester
(Aldrich) were used as received. Commercial catalyst
6. Davies, M. E. In Aqueous-Phase Organometallic Catalysis;
Cornils, B.; Herrmann, W. A., Eds. Transitions to Hetero-
geneous Techniques (SAPC and Variations); Wiley-VCH:
Weinheim, 1998; pp. 241–251.
precursors
were
used:
Ir[(COD)2]BF4
and
[Rh(NBD)2]BF4 from Strem and [RuCOD(h3-C4H7)2]
from Acros. When an argon atmosphere was required,
Schlenk techniques were employed and solvents were