Catalytic reduction of acetophenone with transition metal systems containing chiral bis(oxazolines)

Article abstract of DOI:10.1016/S0022-328X(02)01767-9

The catalytic behaviour of several Ru, Rh and Ir systems containing bis(oxazoline) ligands (1-6) has been tested in the asymmetric reduction of acetophenone (7) to give 1-phenylethanol (8) by hydrogenation (Ir systems), transfer hydrogenation (Ir and Ru s

Full text of DOI:10.1016/S0022-328X(02)01767-9

Journal of Organometallic Chemistry 659 (2002) 186ꢀ195
/
www.elsevier.com/locate/jorganchem
Catalytic reduction of acetophenone with transition metal systems
containing chiral bis(oxazolines)
a,
Montserrat G o´ mez *, Susanna Jansat , Guillermo Muller , Michel C. Bonnet *,
a
a
b,
b
J e´ r e´ my A.J. Breuzard , Marc Lemaire
b
a
Departament de Qu ´ı mica Inorg a` nica, Universitat de Barcelona, Mart ´ı i Franqu e` s 1-11, E-08028 Barcelona, Spain
b
Laboratoire de Catalyse et Synth e` se Organique, UMR 5622, Universit e´ Claude Bernard Lyon I, 43 Bd du 11 Novembre 1918, 69622 Villeurbane
Cedex, France
Received 24 May 2002; received in revised form 24 July 2002; accepted 26 July 2002
Abstract
The catalytic behaviour of several Ru, Rh and Ir systems containing bis(oxazoline) ligands (1ꢀ
asymmetric reduction of acetophenone (7) to give 1-phenylethanol (8) by hydrogenation (Ir systems), transfer hydrogenation (Ir and
/6) has been tested in the
Ru systems) and hydrosilylation (Ir and Rh systems). Ligands 1 and 3 gave good activities, obtaining the best asymmetric induction
with Irꢀ
/
1 system in the hydrosilylation (ee up to 50% (S) of 8). In order to identify the catalytic precursors, Ru (9ꢀ11) and Ir (12)
/
complexes were synthesised and characterised. NMR studies of ruthenium complexes showed the existence of two main isomers in a
ca. ratio 3/1, in agreement with the PM3(tm) calculations carried out for 10.
#
2002 Elsevier Science B.V. All rights reserved.
Keywords: Homogenous catalysis; Chiral bis(oxazolines); Transition metals; NMR spectroscopy; PM3(tm) calculations
1
. Introduction
successful examples of catalytic asymmetric transfer
hydrogenation have been reported using Rh, Ir and
Ru catalysts with nitrogen-based chiral ligands. Excel-
lent enantioface-discrimination ability has been ob-
tained with different sources of hydrogen (formic acid
[7] and 2-propanol [8]) using as molecular catalyst
Asymmetric synthesis of secondary alcohols from
carbonyl compounds plays a singular role in homo-
geneous catalysis [1]. The catalytic process could be
performed in three ways: hydrogenation, hydrogen
transfer and hydrosilylation of prochiral ketones.
[RuCl (arene)] with chiral N-tosylated diamines [9],
2 2
Although catalytic asymmetric hydrogenation of
functionalised olefins and ketones have successfully
been achieved, reduction of simple aryl alkyl prochiral
ketones is more difficult, and only a limited number of
chiral catalytic systems (Ru, Rh) have given excellent
enantioselectivities, in particular Ru systems containing
diamines [10], and b-aminoalcohols [11], and also silica
supported catalysts [12]. These results suggest that a NH
group in the ligand backbone is crucial to obtain good
enantioselectivities. The ligand may promote a cyclic
transition state through hydrogen-bonding to the ketone
and this fact should have a beneficial effect on the
enantioselectivity. One example is the bis(oxazolinyl-
methyl)amine ligand, which allows the formation of the
chiral 1-phenylethanol (8) with ee up to 97% [13] and b-
aminoalcohols reported by Noyori getting ee up to 92%
both chiral diphosphane and diamine [2], and Rhꢀ
/chiral
diphosphine systems [3].
Transfer hydrogenation [4,5] and hydrosilylation [6]
using organic hydrogen donors have also been used for
the synthesis of secondary alcohols. Recently some
[8]. Recently, Moberg et al. has achieved high activities
using microwave irradiation, but the enantioselectivities
remained moderates [14].
In the last years C -symmetric chiral bis(oxazoline)
2
*
Corresponding authors. Tel.: ꢁ
725
E-mail address: montserrat.gomez@qi.ub.es (M. G o´ mez).
/
34-93-402-1271; fax: ꢁ34-93-490-
/
ligands [15] and C -symmetric phosphinoꢀ
/oxazolines
7
0
1
[16] have received a great attention for their successful
022-328X/02/$ - see front matter # 2002 Elsevier Science B.V. All rights reserved.
PII: S 0 0 2 2 - 3 2 8 X ( 0 2 ) 0 1 7 6 7 - 9

M. G o´ mez et al. / Journal of Organometallic Chemistry 659 (2002) 186ꢀ
/195
187
use in various catalytic processes. In particular, Irꢀ
/
bis(oxazolines) derived from oxalic and malonic acids
have not revealed reactivity in either hydrogenation or
hydrosilylation processes [17]. But rhodium complexes
containing bis(oxazolines) derived from oxalic and
malonic [18] and tartaric acids [19], pyridineꢀ
/bis(oxazo-
line) [20], bipyridineꢀbis(oxazolines) [21], and bis(pho-
/
sphinooxazolines) [22] have given good selectivities in
hydrosilylation processes. Also mono(oxazoline) ligands
as phenantrolineꢀ
/
oxazoline [23] and pyridineꢀoxazoline
/
[
24] have been tested in this process, being better the last
catalytic systems. On the another hand, in the hydrogen
transfer reaction of carbonyl bonds, catalytic rhodium
systems containing ligands led to high to moderate
enantioselectivity (up to 97% ee in the case of
Fig. 1. Bis(oxazoline) ligands, (S,S)-1, (R,R)-2, (S,S)-3, (Rc,Xa,Rc)-
, (Sc,Xa,Sc)-5 and (Sc,Xa,Sc)-6 (Xa means the axial absolute
configuration of the biphenyl backbone).
4
bis(phosphinoꢀ
Although phosphinoꢀ
phinoꢀpyrrolylꢀoxazolines ligands [26], gave good
asymmetric inductions in the Ir-catalysed enantioselec-
tive hydrogenation of olefins and imines, respectively, to
the best of our knowledge, no studies about hydrogena-
tion of carbonyl groups with bis(oxazolines) systems,
have been reported.
/
oxazolines) [22]).
/oxazolines [25] and phos-
Complexes were prepared by reaction of the dimeric
/
/
precursors
Cl)(coe) ] ) with two equivalents of the appropriated
([Ru(p-cymene)Cl(m-Cl)]2
and
[Ir(m-
2
2
ligand, following the methodology previously described
25,29]. In these compounds, the bis(oxazoline) acts as
[
N,N-bidentate donor ligand. Addition of tetrafluorobo-
rate salt decreases the solubility of the complexes in the
reaction media. The complexes were characterised by IR
and NMR spectroscopy, mass FAB spectrometry and
elemental analysis.
The lack of these data prompted us to study the
catalytic behaviour for the acetophenone reduction
(
of several C symmetric chiral bis(oxazoline) ligands,
hydrogenation, hydrogen transfer and hydrosilylation)
2
Their infrared spectra show the CÄ/N stretching
containing aliphatic (1 and 2) and aromatic (3ꢀ
backbones (Fig. 1). While for ligands 1 and 2, the two
oxazoline groups are connected by two or four Csp3
respectively (aliphatic bridge), for 3ꢀ6, two (phenyl) or
four (biphenyl) Csp2, separate both heterocycles. To
identify the catalytic precursors, ruthenium (9ꢀ11) and
iridium (12) complexes were prepared.
/
6)
vibration indicating the presence of the bis(oxazoline)
ligand. The signal appears within a narrow interval
,
ꢃ1
between 1619 and 1628 cm , shifted to lower frequen-
ꢃ1
/
cies compared with free ligands (1672 and 1654 cm
for 1 and 3, respectively). Another strong band at 1054
/
ꢃ1
cm
is observed for 11 and 12 complexes due to the
tetrafluoroborate anion. Mass spectrometry data agree
with monometallic compounds. FABMS spectra for Ru-
complexes (9ꢀ
corresponding to [M], [Mꢃ
where M means the cation fragment of the complex.
FAB spectrum for 12 exhibits only one intense peak
/
11) show the same pattern of fragments,
2
. Results and discussion
.1. Synthesis and characterisation of ruthenium and
iridium complexes
/
Cl] and [Mꢃp-cymene],
/
2
ꢁ
corresponding to [Mꢁ
/
DMSO] (CHCl3ꢀDMSO mix-
/
ture was used as solvent for recording the spectra).
1
H-NMR spectra of Ru complexes were recorded in
‘
Aliphatic bridge’ (1ꢀ
bis(oxazolines) were prepared as previously described
15b,27,28] (Fig. 1). Ligands containing the biphenyl
backbone (4ꢀ6) are obtained as a mixture of two
diastereomers, (Xc, Ra, Xc) and (Xc, Sa, Xc) (Xꢂ
/
2) and ‘aromatic bridge’ (3ꢀ6)
/
CDCl at several temperatures (240ꢀ330 K). No iso-
/
[
3
meric composition change was observed in any com-
pound in the temperature range studied. These spectra
basically show the existence of two main species in a ca.
/
/R
for 4 and S for 5 and 6) due to the axial chirality of the
skeleton. Upon N,N-bidentated coordination, the li-
gands can afford seven- (1 and 3) or nine-membered (2,
3/1 ratio, although other minor species (less than 10%)
could be detected, probably dimeric species or diaster-
1
eomers because of the metal chirality [30]. H-NMR
4
and 5) metallic cycles. Ligand 6 can coordinate to the
metal by sulphur atom, giving bimetallic species, as
observed for palladium compounds [15b]. Because the
best catalytic results have been obtained with systems
containing 1 and 3 (see below), we synthesised their
experiment (1D and 2D NOESY spectra) for 10 allowed
us the structural determination of the main isomers.
Complexes 9 and 11 show broader signals than 10.
The main isomers are probably due to the relative
position of the substituents on the p-cymene fragment
and the bis(oxazoline) ligand, as stated in the literature
monometallic Ru (9ꢀ/11) and Ir (12) complexes (Scheme
1
).

188
M. G o´ mez et al. / Journal of Organometallic Chemistry 659 (2002) 186ꢀ195
/
Scheme 1. Synthetic route for Ru (9ꢀ11) and Ir (12) complexes.
/
for analogous complexes [31]. Then two arrangements
are possible: (i) the arene isopropyl and the chloride
groups on the same side, Iso-10; and (ii) the arene
methyl and the chloride groups on the same side, Me-10
[32]. Iso-10 isomer showed lower formation enthalpy
ꢃ1
than the Me-10 one (ꢃ
/
62.082 vs. ꢃ61.509 kcal mol ).
/
This result is in good accordance with the ratio observed
1
in the H-NMR spectrum (2.6/1 (calculated) vs. 3/1
(experimental)).
(
Fig. 2).
Upon coordination, the two oxazoline fragments are
not equivalent and therefore, the aromatic protons of
the arene moiety appear as four signals. For the major
isomer, signals at 5.51 and 5.44 ppm (protons in ortho
position to the methyl group) and 5.24 and 5.11 ppm
2.2. Catalytic acetophenone reduction
2.2.1. Asymmetric hydrogenation
The Irꢀ
hydrogenation of the acetophenone, using molecular
hydrogen gas as hydride source (Eq. (1)). The catalystꢀ
/
bis(oxazoline) systems (1ꢀ6) were tested in the
/
(
protons in ortho position to the isopropyl group) are
observed, due to the different donor electronic proper-
ties of both alkyl substituents (Table 1). The methyl
group of the p-cymene unit for the major isomer of 10
resonates at higher-field (1.15 ppm) than the minor
species (2.12 ppm) and the precursor ([Ru(p-cyme-
/
substrate ratio was 1/1000. The catalytic results are
summarised in Table 2. Catalytic runs were performed
either under in situ conditions (1 mol ˜ [Ir(cod) ]BF
and 1.5 or 3 mol ˜ of L*) or using the precursor 12,
previously synthesised, but no differences were observed
in both cases (entries 5 and 7, Table 2).
2
4
ne)Cl(m-Cl)] (2.13 ppm), because of the electronic effect
2
of the oxazoline phenyl ring over this methyl group. In
addition, 2D NOESY spectrum of 10 showed cross-
peaks between the protons of one oxazoline heterocycle
and the two aromatic protons (in ortho position to the
methyl substituent) of the p-cymene moiety for the
major isomer, while the minor isomer shows cross-peaks
between one 4?b and both aromatic protons in ortho
position to the isopropyl group (Fig. 3). These facts
suggest that the major isomer should be Iso-10, which
shows the minor steric effects. Exchange signals between
both isomers are also observed in the 2D NOESY
spectrum, which demonstrates that these species are in
equilibrium.
ð1Þ
Activity is improved when the Irꢀ
when L*ꢂ1ꢀ3, suggesting a higher stability of the
catalytic active species when the L* amount increases
(entries 1 vs. 2, 3 vs. 4 and 5 vs. 6, Table 2). Irꢀ1 system
/
L* ratio decreases,
/
/
/
gave the best activity (up to 85% of conversion, entry 2,
Table 2) but no asymmetric induction was observed.
Under the same conditions, catalytic systems with
Semi-empirical calculations (PM3(tm) level) were
carried out for both isomers Me-10 and Iso-10 (Fig. 4)
ligands containing the biphenyl backbone (4ꢀ
low activities (entries 8ꢀ12, Table 2), which are not
dependent on the IrꢀL* ratio. However, Irꢀ6 system is
slightly selective (eeꢂ21% (S), entry 12), probably due
/
6) showed
/
/
/
/
to the existence of other catalytic species because of the
thioether substituent [15b].
In conclusion, we can state that the stability of the
catalytic species is only achieved with excess of ligand
probably due to the lability of the bis(oxazolines) in
these Ir complexes. For the N,N-donor ligands (1ꢀ5),
/
the lack of enantioselectivity could be associated to the
monodentate coordination of the ligands, and only
Fig. 2. Proposed isomers for Ru complex 10.

Table 1
1
a
H-NMR data (d in ppm, CDCl
3 2
, 298 K) for 10 (500 MHz), 3 and [Ru(p-cymene)Cl(m-Cl)] (250 MHz)
Compound
4?b
4?a
4?
3?
p-Cymene
Iso-10 major
(75%)
0.40 (d, 7.0, 3H), 0.75 (d, 7.0, 3H),
0.96 (d, 7.0, 3H), 1.04 (d, 6.5, 3H)
2.43 (m, 1H),
2.66 (m, 1H)
4.42 (m, 1H),
4.83 (m, 1H)
4.24 (m, 2H), 4.27 Me
(t, 10.0, 1H), 4.55 Aromatic protons: 5.11 (d, 5.0, 1H), 5.24 (d, 5.5, 1H), 5.44 (d, 5.5, 1H), 5.50
a b c d
: 1.15 (s, 3H), Me , Me : 1.22 (d, 7.0, 3H), 1.40 (d, 6.5, 3H), H : 3.01 (m, 1H),
b
(
nd
pt, 7.5, 1H)
(bs, 1H)
Me : 2.12 (s, 3H), Me
(bs, 1H), 5.31 (d, 5.5, 1H), 5.50 (bs, 2H)
b
Me-10 minor 1.20 (d, 7.0, 3H), 1.31 (d, 6.5, 3H),
c
c
c
2.85 (m, 2H)
1.79 (m, 1H)
nd
a
b c d
, Me : nd , H : 3.16 (m, 1H), Aromatic protons: 5.20
c
b
(25%)
1.34 (d, 7.0, 3H), nd
0.87 (d, 6.7, 3H), 0.96 (d, 6.7, 3H)
3
3.99 (m, 2H)
3.99 (m, 2H), 4.29
m, 1H)
(
d
[
Ru]
Me
(
a d
: 2.13 (s, 3H), Meb,c : 1.25 (d, 6.9, 6H), H : 2.13 (m, 1H), Aromatic protons: 5.31
d, 5.9, 2H), 5.45 (d, 5.9, 2H)
a
b
c
Multiplicity (d, doublet; m, multiplet; s, singlet; t, triplet; p, pseudo; bs, broad signal) and coupling constants (in Hz) in parentheses.
Signals overlapped.
Not distinguished.
d
[
2
Ru]ꢂ[Ru(p-cymene)Cl(m-Cl)] .

190
M. G o´ mez et al. / Journal of Organometallic Chemistry 659 (2002) 186ꢀ195
/
Rhꢀ3 system gave a similar catalytic behaviour than
/
the analogous iridium catalyst (39% conversion, without
asymmetric induction), under the same conditions [33].
2.2.2. Asymmetric hydrogen transfer
Ru- (L*ꢂ1ꢀ5) and Ir- (L*ꢂ1ꢀ6) bis(oxazoline)
/
/
/
/
systems were tested in the asymmetric hydrogen transfer
process of acetophenone, using isopropanol under basic
Fig. 3. Selected NOE contacts (indicated by arrows) for Me-10.
conditions as hydrogen source (Eq. (2)). The catalystꢀ
/
substrate ratio was 1/20. The catalytic results are
summarised in Table 3 (for Ru) and Table 4 (for Ir).
ð2Þ
Concerning the ruthenium catalytic systems, Ruꢀ2
/
was the most active, giving 91% of 1-phenylethanol (8)
in 10 h (entry 5, Table 3). As observed for the Ir-
catalysed hydrogenations (see above), an excess of
ligand induces higher activity (entries 1 vs. 2 and 6 vs.
7
, Table 3), although the selectivity was practically
unchanged. Comparing ‘aliphatic backbone’ (1 and 2)
and ‘aromatic backbone’ (3ꢀ5) ligands, we observe that
the former systems are most active than the latter ones
entries 2 and 5 vs. 7, 10 and 11, Table 3). Then, this
Fig. 4. Optimised structures (PM3(tm)) for Me-10 and Iso-10.
/
(
Table 2
Enantioselective hydrogenation of acetophenone using [Ir(cod)
a
2
]BF
4
ꢀ
/
feature seems to indicate that a more flexible metallic
cycle induces better activities. When 9 and 10 were used
as preformed precursors, the catalytic behaviour was
similar to the catalyst generated in situ (entries 1 and 4; 6
and 9, Table 3).
bis(oxazoline) systems
b
Entry
L*
Irꢀ/L*
Conversions (%)
1
2
3
4
5
6
7
8
9
1
1
2
2
3
3
1/1.5
1/3
65
85
1/1.5
1/3
7
14
Table 3
Enantioselective transfer hydrogenation of acetophenone using Ruꢀ
/
1/1.5
1/3
26
40
a
bis(oxazoline) catalytic systems
c
3
1/1
32
b
b
Entry L* RuꢀL* Time (h) Conversions (%)
/
ee (%)
4
4
5
6
6
1/1.5
1/3
17
18
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
2
3
3
3
3
4
5
1/1
1/2
1/2
1/1
1/2
1/1
1/2
1/2
1/1
1/2
1/2
20
20
24
23
10
24
20
24
24
96
96
40
88
38
39
91
10
42
21
6
24(S)
25(S)
38(S)
28(S)
11(R)
24(S)
29(S)
32(S)
22(S)
5(R)
10
11
12
1/1.5
1/1.5
1/3
11
9
10 (21%(S))
c
d
a
Results from duplicated experiments. Reaction conditions: 0.020
ꢃ5
ꢃ5
mol of acetophenone, 2ꢄ10 mol of [Ir(cod)
2
]BF
in 10 ml of THF at 50 8C for 15 h.
Conversions and enantiomeric excesses determined by GC on
chiral column.
4
, 3ꢄ10 mol (or
ꢃ5
6
ꢄb 10 ) of ligand, 40 bar of H
2
c
d
0
1
90
60
c
Preformed complex 12. Reaction conditions: 0.020 mol of
ꢃ5
3(S)
acetophenone, 2ꢄ10
8C for 15 h.
2
mol of 12, 40 bar of H in 10 ml of THF at
5
0
a
Results from duplicated experiments. Reaction conditions:
ꢃ5
ꢃ5
1
3
2ꢄ10
ꢄ10
mol of acetophenone, 2.4ꢄ10
mol of [Ru(p-cymene)Cl(m-Cl)] and 6ꢄ10
mol) of ligand in 4 ml of iPrOH.
mol of tBuOK,
ꢃ6
ꢃ6
2
mol (or
ꢃ6
enantiomeric excess is achieved when the N,S-donor
ligand 6 is used, suggesting the stabilisation of chelate
metallic rings in the catalytic species.
12ꢄ10
b
Conversions and enantiomeric excesses determined by GC on
chiral column.
c
Reaction at 0 8C.
Reaction conditions: 12ꢄ10 mol of acetophenone, 2.4ꢄ10
mol of tBuOK, 6ꢄ10
d
ꢃ5
ꢃ5
ꢃ6
mol of 9 or 10 in 4 ml of iPrOH.

M. G o´ mez et al. / Journal of Organometallic Chemistry 659 (2002) 186ꢀ
/195
191
Both the activity and selectivity are dependent on
temperature. At 273 K, the activity decreases but the
selectivity increases. Under these conditions the best
selectivities are showed by systems containing seven-
Table 4
Enantioselective transfer hydrogenation of acetophenone using Irꢀ
bis(oxazoline) catalytic systems
/
a
b
b
Entry L* IrꢀL* Time (h) Conversions (%)
/
ee (%)
membered cycles, Ruꢀ
most selective (ee up to 38%, entry 3, Table 3).
For Ir-catalysed transfer hydrogenation, the most
active and selective system was Irꢀ3 (85% conversion
with 33% of ee, entry 5, Table 4). Systems containing
biphenyl backbone show lower activities and worse
/
1 and Ruꢀ3. The former is the
/
1
2
3
4
5
6
7
8
9
1
1
2
3
3
3
3
3
4
5
6
1/3
1/3
1/1
1/2
1/3
1/1
1/1
1/3
1/3
1/3
2.5
5
91
91
95
96
85
10
100
77
67
90
9(S)
11 (R)
18 (S)
32 (S)
33 (S)
31 (S)
32 (S)
18 (R)
5 (S)
3
/
3
1.5
3
c,d
c
3
selectivities (entries 8ꢀ
system Irꢀ4 is slightly enantioselective (18% of ee, entry
, Table 4). With these systems the selectivity does not
/10, Table 4), but the catalytic
3
/
6
8
0
8
5 (S)
depend on the temperature (entries 6 and 7, Table 4).
Comparing analogous systems, Ir catalysts are more
active than the Ru ones (Tables 3 and 4).
An inherent problem of transfer hydrogenation is the
reversibility of the reaction, which prevents complete
conversion and also causes a decrease of the enantio-
meric purity of the chiral alcohol (8). To avoid this
a
Results from duplicated experiments. Reaction conditions:
ꢃ5
ꢃ5
1
2ꢄ10
mol of acetophenone, 2.4ꢄ10
mol of [Ir(m-Cl)(coe) in 4 ml of iPrOH at 80 8C.
Enantiomeric exces and conversions determined by GC with chiral
columns.
mol of tBuOK,
ꢃ6
3
ꢄb 10
2 2
]
c
ꢃ5
ꢃ5
Reaction conditions: 12ꢄ10 mols of acetophenone, 2.4ꢄ10
ꢃ6
mol of tBuOK, 6ꢄ10
iPrOH.
mol of preformed complex 12 in 4 ml of
problem, other hydrogen donor systems as HCOOHꢀ
/
d
Reaction carried out at room temperature.
NEt , are described in the literature, but oxazoline
3
ligands are not stable under these conditions [34]. In
order to know the existence of the unfavourable reverse
process with our systems, control experiments using rac-
1
showed the best selectivity (eeꢂ
).
The Ir systems gave better activities (Irꢀ
conversion, entry 4, Table 5) and better enantioselec-
tivities (Irꢀ1, 50% of ee, entry 2, Table 5) than the Rh
analogous systems. Rh and Ir containing 4 and 6 ligands
gave very low activities (up to 6%, entries 5ꢀ8, Table 5).
/
33%, entry 1, Table
5
8
and acetone in 2-propanol (under the same basic
conditions), revealed that the reaction is negligible for
ruthenium and iridium systems except, for Irꢀ5 system
/
3, 95% of
/
/
which afforded acetophenone (7), with 17% conversion
after 42 h of reaction (Eq. (3)). No kinetic resolution of
rac-8 was observed in any case.
/
Table 5
Enantioselective hydrosilylation of acetophenone with diphenylsilane
ð3Þ
a
using Rh and Irꢀbis(oxazoline) catalytic systems
/
b
b
Entry Precursor
Time (h)
Conversions (%)
ee (%)
c
1
2
3
4
5
6
7
8
9
1
1
1
Rhꢀ
/
1
6
32
89
48
95
6
33(S)
50(S)
0
c
Irꢀ/1
6
c
c
c
Rhꢀ
/
3
6
2
.2.3. Asymmetric hydrosilylation
Catalytic behaviour of Rh and Irꢀ
c
Irꢀ/3
5
0
nd
/
bis(oxazoline)
Rhꢀ
/
4
60
25
60
60
24
24
24
24
systems (L*ꢂ
/
1, 3, 4 and 6) were studied in the
asymmetric hydrosilylation process of acetophenone
c
Irꢀ/4
5
nd
Rhꢀ
/
6
0
c
Irꢀ
Rhꢀ
Rhꢀ
Rhꢀ
Rhꢀ
/6
1
with diphenylsilane in the presence of [Rh(m-Cl)(cod)]2
d
d
d
d
/
1
3
4
6
60
65
8
20(S)
0
nd
and [Ir(m-Cl)(cod)] as metallic precursors (Eq. (4)). This
2
0
1
2
/
catalytic reaction was performed in toluene at 50 8C.
/
The catalystꢀ
/
substrate ratio was 1/100. The results are
/
8
nd
summarised in Table 5.
a
Results from duplicated experiments.
Enantiomeric excess and conversions determined by GC with
b
chiral column.
c
ð4Þ
Reaction conditions: 2 mmol of acetophenone, 0.01 mmol of
or [Ir(m-Cl)(cod)] , 0.08 mmol of ligand and 3.2
[Rh)(m-Cl)(cod)]
2
2
mmol of diphenylsilane at 50 8C in 1 ml of toluene.
Reaction conditions: 2 mmol of acetophenone, 0.01 mmol of
d
Concerning the Rh-catalysed reactions, the most
active system was Rhꢀ3 (entry 3, Table 5), while Rhꢀ
[
Rh(m-Cl)](cod)
2
, 0.08 mmol of ligand and 3.2 mmol of diphenylsilane
/
/
at r.t. in 0.1 ml of CCl .
4

192
M. G o´ mez et al. / Journal of Organometallic Chemistry 659 (2002) 186ꢀ195
/
For Rh catalytic systems, the reaction were also
tions were performed using the SPARTAN program,
version 5.0 (Wave function Inc., Irvine, CA, 1997).
carried out in CCl at room temperature (entries 9ꢀ
/
12,
oxazoline systems, the
4
Table 5), which is, for the Rhꢀ
/
6
4.2. Preparation of chloro-(h -p-cymene)-{1,2-
best solvent to get excellent enantioselectivities in the
hydrosilylation reaction as stated by Brunner [24]. At
bis[(4?S)-(4?-isopropyl-3?,4?-dihydrooxazol-2?-
yl)]ethane-N,N}ruthenium(II) chloride (9)
0
8C, these Rh systems were not active, in contrast to
other analogous catalysts described in the literature [23].
Comparing both solvents, we can conclude that Rhꢀ
/
[RuCl(p-cymene)(m-Cl)] (110 mg (0.18 mmol)), 100
2
bis(oxazoline) systems in toluene show better activities
and similar selectivities than in CCl (entries 1 vs. 9 and
mg (0.39 mmol) of (S,S)-1 were dissolved in 13 ml of
THF and stirred at 55 8C for 3 h. The solvent was
removed and the residue washed with pentane. The
product was recrystallised from CH Cl and hexane
4
3
vs. 10, Table 5).
2
2
giving an orange solid (129 mg, 88%). IR (KBr)ꢂ
/
1619
ꢃ1
1
3
. Conclusions
(CÄ
major isomer (75%), 0.83 (d, JHH
CHMe ), 0.86 (d, JHHꢂ7.0 Hz, 3H, CHMe ), 0.93 (d,
/N) cm . H-NMR (500 MHz, 298 K, CDCl ): d
3
ꢂ6.0 Hz, 3H,
/
In summary, the catalytic systems containing two
/
2
2
carbon spacers (ligands 1 and 3) show the best results in
activity and enantioselectivity for the reduction of
acetophenone (7) to its corresponding secondary alcohol
JHH
ꢂ
/
7.0 Hz, 3H, CHMe ), 1.01 (d, JHHꢂ
/
7.0 Hz, 3H,
6.0 Hz, 3H, CHMe ), 1.27 (d,
2
CHMe ), 1.22 (d, JHHꢂ
JHH
(m, 2H, CH ), 1.97 (m, 1H, CHMe ), 2.20 (m, 1H,
/
2
2
ꢂ6.0 Hz, 3H, CHMe ), 1.68 (m, 1H, CHMe ), 1.87
/
2 2
(
8). This fact suggests that N,N-bidentated coordination
2
2
of L* is necessary to stabilise the catalytic species. The
most of catalytic systems containing biphenyl backbone
CHMe ), 2.24 (s, 3H, CH ), 4.00 (m, 2H, CH ), 4.20 (m,
2
3
2
6H, CH iCH), 5.31 (d, JHHꢂ
/
5.5 Hz, 2H, C H ), 5.44
6
2
4
ligands (4ꢀ
/
6) are the less active. If we compare the three
(d, JHH
2.12 (s, 3H, CHMe ), 5.45 (sa, 2H, C H ) ppm. Anal.
ꢂ
/
6.0 Hz, 1H, C H ) ppm; minor isomer (25%),
6 4
described reduction processes, hydrosilylation seems to
be the process of choice in order to obtain asymmetric
induction for 8 (up to 50% ee).
2
6
4
Found: C, 51.61; H, 6.81; N, 5.02; Cl, 12.70. Calc. for
C H Cl N O Ru: C, 51.77; H, 7.00; N, 5.09; Cl,
2
4
38
2
2
2
ꢁ
1
(
2.10%. MS (FAB positive) m/z 523 ([MꢃCl] ), 487
[Mꢃ
/
ꢁ
ꢁ
/
Cl] ), 353 ([Mꢃ
/
p-cymene] ). Melting point:
4
. Experimental
90 8C.
6
4
.1. General data
4.3. Preparation of chloro-(h -p-cymene)-{1,2-
bis[(4?S)-(4?-isopropyl-3?,4?-dihydrooxazol-2?-
yl)]benzene-N,N}ruthenium(II) chloride (10)
All compounds were prepared under a purified
nitrogen atmosphere using standard Schlenk and va-
cuum-line techniques. Solvents were purified by stan-
dard procedures and distilled under nitrogen [35]. t-
[RuCl(p-cymene)(m-Cl)] (118 mg (0.19 mmol)), 115
2
mg (0.39 mmol) of (S,S)-3 were dissolved in 13 ml of
THF and stirred at 55 8C for 3 h. The solvent was
removed and the residue washed with pentane. The
product was recrystallised from CH Cl and hexane
6
BuOK, [Ru(h -p-cymene)Cl(m-Cl)]
[Ir(cod) ]BF ,
2 4
2
,
[
Rh(cod) ]BF , [Ir(m-Cl)(coe) ] , [Rh(m-Cl)(cod)] and
2
4
2 2
2
[
vious purification. NMR spectra were recorded on
Ir(m-Cl)(cod)] were purchased and used without pre-
2
2
2
giving an orange solid (163 mg, 70%). IR (KBr)ꢂ
/
1628
1
Varian XL-500 ( H), Varian Gemini ( H, 200 MHz),
1
ꢃ1 1
(CÄ
major isomer, 0.40 (d, JHH
(d, JHH 7.0 Hz, 3H, CHMe ), 0.96 (d, JHHꢂ
/N) cm . H-NMR (500 MHz, 298 K, CDCl ): d
3
1
3
and Bruker DRX 250 ( C, 63 MHz) spectrometers.
ꢂ7.0 Hz, 3H, CHMe ), 0.75
/
2
Chemical shifts were reported downfield from SiMe as
4
ꢂ
/
/
7.0 Hz,
2
standard. IR spectra were recorded on a Nicolet 520
FTIR spectrometer. FAB mass spectra were obtained on
a Fisons V6-Quattro instrument. The GC analyses, for
3H, CHMe ), 1.04 (d, JHHꢂ
/
6.5 Hz, 3H, CHMe ), 1.15
2
2
(m, 3H, CH ), 1.22 (d, JHHꢂ
/
7.0 Hz, 3H, CHMe ), 1.40
2
3
(d, JHH
2.66 (m, 1H, CHMe ), 3.01 (m, 1H, CHMe ), 4.24 (m,
ꢂ6.5 Hz, 3H, CHMe ), 2.43 (m, 1H, CHMe ),
/
2 2
chiral substances, was performed on a Hewlettꢀ
/
Packard
2
2
5
I/P
clodextrinꢀ
890 Series II gas chromatograph (25 m FS-cyclodex-b-
2H, CH ), 4.27 (t, JHHꢂ
/
10.0 Hz, 1H, CH ), 4.42 (m,
2
2
column:
polysiloxan; 25
Nagel and 30 m Cyclodex b from J&W
heptakis(2,3,6-tri-O-methyl)-b-cy-
1H, CH), 4.55 (pt, JHH
JHH 5.5 Hz, 2H, C H ), 5.24 (d, JHHꢂ
C H ), 5.44 (d, JHHꢂ
ꢂ7.5 Hz, 1H, CH ), 5.11 (d,
/
2
/
m
Lipodex from
a
ꢂ
/
/5.5 Hz, 1H,
6
4
Machereyꢀ
/
/
5.5 Hz, 2H, C H ), 5.51 (m, 1H,
6
4
6
4
Scientifique) with a FID detector. Elemental analyses
were carried out by the Serveis Cient ´ı fico-T e` cnics of the
University of Barcelona in an Eager 1108 microanalyser.
Melting and decomposition points were determined in a
C H ) ppm; minor isomer, 1.00 (m, 1H, CHMe ), 1.20
2
6
4
(d, JHH
ꢂ
/
7.0 Hz, 3H, CHMe ), 1.31 (d, JHHꢂ
/6.5 Hz,
2
3H, CHMe ), 1.34 (d, JHHꢂ
/7.0 Hz, 3H, CHMe ), 3.16
2
2
(m, 1H, CHMe ), 2.12 (s, 3H, CH ), 5.20 (bs, 1H,
3
2
5
10-B u¨ chi apparatus. The molecular mechanics calcula-
C H ), 5.31 (d, JHHꢂ
/
5.5 Hz, 1H, C H ), 5.51 (bs, 1H,
6
6
4
4

M. G o´ mez et al. / Journal of Organometallic Chemistry 659 (2002) 186ꢀ
/195
193
C H ) ppm. Anal. Found: C, 55.42; H, 6.27; N, 4.62; Cl,
6
and conversions were determined by GC on a chiral
column.
4
1
1.69. Calc. for C H BCl F N O Ru: C, 55.93; H,
2
28
38
2
4
2
6
.35; N, 4.52; Cl, 11.78%. MS (FAB positive) m/z 571
ꢁ
ꢁ
ꢁ
(
[Mꢃ
/
Cl] ). 533 ([Mꢃ
/
Cl] ), 397 ([Mꢃp-cymene] ).
/
4.7. General procedure for iridium and rhodium catalysed
in situ hydrogenation
M.p.: 115 8C.
6
.4. Preparation of chloro-(h -p-cymene)-{1,2-
4
The precursor (2ꢄ
/
10ꢃ5 mol, 10 mg of complex
Ir(cod) ]BF or 8.2 mg of [Rh(cod) ]BF ) and the
bis[(4?S)-(4?-isopropyl-3?,4?-dihydrooxazol-2?-
yl)]ethane-N,N}ruthenium(II) tetrafluoroborate (11)
[
2
4
2
4
substrate (2.40 g, 0.0200 mol of acetophenone) were
dissolved in 10 ml of THF in the autoclave. Then,
molecular hydrogen was introduced until 40 bar. The
reaction was stirred at 55 8C for15 h. Then the solution
was filtered over celite and purified by column chroma-
Compound 9 (94 mg (0.168 mmol)) was dissolved in
9 ml of CH Cl and washed with an aqueous solution
2 2
1
of NaBF (256 mg, 2.34 mmol). The organic phase was
4
dried over anhydrous Na SO . The solvent was removed
2
4
tography (SiO ; ethyl acetate). Enantiomeric excesses
2
under vacuum, affording an orange solid; (87.2 mg,
ꢃ1
and conversions were determined by GC on a chiral
column.
1
N), 1054 (BF ) cm . H-
8
5%). IR (KBr)ꢂ
/
1620 (CÄ
/
4
NMR (250 MHz, 298 K, CDCl ): d 0.92 (d, JHHꢂ
/
7.0
7.0 Hz, 3H, CHMe ),
3
Hz, 6H, CHMe ), 0.94 (d, JHHꢂ
/
2
2
1
.01 (d, JHH
Hz, 6H, CHMe ), 1.68 (m, 1H, CHMe ), 1.89 (m, 2H,
ꢂ
/
7.0 Hz, 3H, CHMe ), 1.25 (d, JHHꢂ
/
6.0
4.8. General procedure for iridium and ruthenium
catalysed hydrogen transfer with preformed complex
2
2
2
CH ), 1.99 (m, 1H, CHMe ), 2.15 (s, 3H, CH ), 2.22 (m,
2
2
3
1
CH), 5.35 (d, JHH
H, CHMe ), 4.00 (m, 2H, CH ), 4.20 (m, 6H, CH and
2
The precursor (complexes 10ꢀ
/
12) (3ꢄ
10ꢃ3 mmol)
/
2
2
ꢂ
/
5.5 Hz, 2H, C H ), 5.46 (d, JHHꢂ
/
were dissolved in 2 ml of a solution 0.012 M of tBuOK
in isopropanol at r.t.. The resulted solution was stirred
during 30 min. Then 2 ml of a solution 0.06 M of
acetophenone in isopropanol was added. The reaction
was stirred at r.t. (for ruthenium-catalysed) or at 80 8C
(for iridium-catalysed) until the substrate was totally
consumed (unless stated otherwise), and monitored by
GC.
6
4
5
N, 4.59. Calc. for C H BClF N O Ru: C, 47.77; H,
.9 Hz, 1H, C H ) ppm. Anal. Found: C, 47.11; H, 6.23;
6
4
2
4
38
4
2
2
6
.10; N, 4.49%. MS (FAB positive) m/z 523 ([Mꢃ
/
ꢁ
ꢁ
ꢁ
BF ] ), 487 ([Mꢃ
/
Cl] ), 353 ([Mꢃp-cymene] ). Melt-
/
4
ing point: 102 8C.
4
.5. Preparation of bis(cis-cyclooctene)-{1,2-bis[(4?S)-
4?-isopropyl-3?,4?-dihydrooxazol-2?-yl)]benzene-
(
N,N}iridium(I) chloride (12)
4.9. General procedure for iridium and ruthenium
catalysed in situ hydrogen transfer
[
Ir(m-Cl)(coe) ] (228 mg (0.25 mmol)), 153 mg (0.51
2
2
mmol) of (S,S)-3 were dissolved in 10 ml of diclor-
omethane and stirred at room temperature (r.t.) for 1 h.
The solution was washed with an aqueous solution of
The precursor (3ꢄ
/
10ꢃ3 mmol) (1.8 mg of [Ru(p-
cymene)Cl(m-Cl)] , 2.7 mg of [Ir(coe) Cl] ) and 6, 12 or
2
2
2
ꢃ3
1
8ꢄ
/
10
mmol of ligand were dissolved in 2 ml of a
NaBF (128 mg, 0.17 mmol). The organic phase was
4
solution 0.012 M of tBuOK in isopropanol at r.t.. The
resulted solution was stirred during 30 min. Then 2 ml of
a solution 0.06 M of acetophenone in isopropanol was
added. The reaction was stirred at r.t. (for ruthenium-
catalysed) or at 80 8C (for iridium-catalysed) until the
substrate was totally consumed (unless stated other-
wise), and monitored by GC.
dried over anhydrous Na SO . The product was recrys-
2
4
tallised from CH Cl and hexane giving an orange solid
2
2
(
cm . Anal. Found: C, 51.06; H, 6.51; N, 3.50. Calc. for
183 mg, 45%). IR (KBr)ꢂ
/
1622 (CÄ/N), 1054 (BF4)
ꢃ1
C H BF N O Ir: C, 51.23; H, 6.65; N, 3.60%. MS
2
3
4
32
4
2
ꢁ
(
FAB positive) m/z 793.5 ([MꢁDMSO] ). M.p.:
/
1
85 8C.
4.6. General procedure for iridium-catalysed
hydrogenation with preformed complex
4.10. Ruthenium catalysed hydrogen transfer of acetone
and rac-8
The precursor (10 mg, 2ꢄ
/
10ꢃ5 mol of 12) and the
[Ru(p-cymene)Cl(m-Cl)] (3ꢄ
/
10ꢃ3 mmol) (1.8 mg)
10 mmol (4.5 mg) of 5 were dissolved in 2 ml
2
ꢃ3
substrate (2.40 g, 0.0200 mol of acetophenone) were
dissolved in 10 ml of THF in the autoclave. Then,
molecular hydrogen was introduced until 40 bar. The
reaction was stirred at 50 8C for 15 h. Then the solution
was filtered over celite and purified by column chroma-
and 12ꢄ
/
of a solution 0.012 M of tBuOK in isopropanol at r.t..
The resulted solution was stirred during 30 min. Then 2
ml of an equimolar solution 0.06 M in rac-8 and acetone
in isopropanol were added. The reaction was monitored
by GC.
tography (SiO ; ethyl acetate). Enantiomeric excesses
2

194
M. G o´ mez et al. / Journal of Organometallic Chemistry 659 (2002) 186ꢀ195
/
[
[
2] (a) R. Noyori, T. Ohkuma, Angew. Chem. Int. Ed. 40 (2001) 40;
b) K. Abdur-Rashid, A.J. Lough, R.H. Morris, Organometallics
0 (2001) 1047.
4.11. General procedure for rhodium catalysed in situ
hydrosilylation
(
2
3] (a) Q. Jiang, Y. Jiang, D. Xiao, P. Cao, X. Zhang, Angew. Chem.
Int. Ed. 37 (1998) 1100;
4
.11.1. In CCl4
A solution of ligand (0.08 mmol), [Rh(m-Cl)(cod)] (5
(b) K. Abdur-Rashid, A.J. Lough, R.H. Morris, J. Am. Chem.
Soc. 123 (2001) 7473.
4] (a) M.J. Palmer, M. Wills, Tetrahedron Asymmetry 10 (1999)
2
mg, 0.01 mmol), and acetophenone (240 mg, 2 mmol) in
tetrachloromethane (0.1 ml) was stirred for 1 h at r.t.
under argon atmosphere. After diphenylsilane (590 mg,
[
2
045;
b) F. Fache, E. Schulz, M.L. Tommasino, M. Lemaire, Chem.
Rev. 100 (2000) 2159.
(
3
.2 mmol) was added to the solution at ꢃ5 8C, the
/
reaction mixture was stirred at r.t. and monitored by
GC. The reaction solution was quenched with methanol
and a small amount of CaCO3 during 30 min. The
solution was washed with water and extracted with
diethyl ether. Enantiomeric excesses and conversions
were determined by GC on a chiral column.
[5] G. Zassinovich, G. Mestroni, Chem. Rev. 92 (1992) 1051.
[6] H. Nishiyama, Hydrosilylation of carbonyl and imino groups, in:
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[
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.11.2. In toluene
[
[
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11] M. Yamakawa, I. Yamada, R. Noyori, Angew. Chem. Int. Ed. 40
A solution of ligand (0.08 mmol) and [Rh(m-Cl)(cod)]2
5 mg, 0.01 mmol), in toluene (1 ml) was stirred for 1 h
(
(
2001) 2818.
at 50 8C under argon atmosphere. After acetophenone
240 mg, 2 mmol) and diphenylsilane (590 mg, 3.2
[12] A.J. Sandee, D.J.I. Petra, J.N.H. Reek, P.C.J. Kamer, P.W.N.M.
Leeuwen, Chem. Eur. J. 6 (2001) 7.
(
[
[
[
13] Y. Yiang, Q. Jiang, X. Zhang, J. Am. Chem. Soc. 120 (1998) 3817.
14] S. Lutsenko, C. Moberg, Tetrahedron Asymmetry 12 (2001) 2529.
15] (a) A.K. Ghosh, P. Mathivanan, J. Cappiello, Tetrahedron
Asymmetry 9 (1998) 1;
mmol) were added to the solution at r.t., the reaction
mixture was stirred at 50 8C and monitored by GC. The
reaction solution was quenched with methanol and a
small amount of CaCO during 30 min. The solution
3
(b) M. G o´ mez, S. Jansat, G. Muller, M.A. Maestro, J. Mah ´ı a,
Organometallics 21 (2002) 1077;
was washed with water and extracted with diethyl ether.
Enantiomeric excesses and conversions were determined
by GC on a chiral column.
(c) M.A. Peric a` s, C. Puigjaner, A. Riera, A. Vidal-Ferran, M.
G o´ mez, F. Jim e´ nez, G. Muller, M. Rocamora, Chem. Eur. J.
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18] G. Helmchem, A. Krotz, K.-T. Ganz, D. Hansen, Synlett (1991)
57.
[
4.12. General procedure for iridium catalysed in situ
hydrosilylation
(
(
[
[
[
[
[
A solution of ligand (0.08 mmol) and [Ir(m-Cl)(cod)]2
2.6 mg, 0.01 mmol), in toluene (1 ml) was stirred for 1 h
7
(
at 50 8C under argon atmosphere. After acetophenone
240 mg, 2 mmol) and diphenylsilane (590 mg, 3.2
2
(
19] Y. Imai, W. Zhang, T. Kida, Y. Nakatsuji, I. Ikeda, Tetrahedron
Asymmetry 7 (1996) 2453.
mmol) were added to the solution at r.t., the reaction
mixture was stirred at 50 8C and monitored by GC. The
reaction solution was quenched and analysed as de-
scribed above (see Section 4.11).
20] N. Nishiyama, M. Kondo, T. Nakamura, K. Itoh, Organome-
tallics 10 (1991) 500.
21] (a) H. Nishiyama, S. Yamaguchi, S.-B. Park, K. Itoh, Tetrahe-
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(
b) H. Brunner, R. St o¨ riko, Eur. J. Inorg. Chem. (1998) 783.
[
[
22] S. Lee, C.W. Lim, C. Eui Song, I.O. Kim, Tetrahedron
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Acknowledgements
23] S. Gladiali, L. Pinna, G. Delogu, E. Graf, H. Brunner, Tetra-
hedron Asymmetry 1 (1990) 931.
We would like to thank Spain’s Ministerio de Ciencia
y Tecnolog ´ı a (BQU2001-3358) for its financial support.
S.J. thanks Fundaci o´ n Domingo Mart ´ı nez for its
financial support and M.L. Tommasino for her help in
catalytic reactions and GC analyses.
[
[
24] H. Brunner, U. Obermann, Chem. Ber. 122 (1989) 449.
25] D.G. Blakmond, A. Lightfoot, A. Pfaltz, T. Rosner, P. Schinder,
N. Zimmermann, Chirality 12 (2000) 442.
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Pfaltz, Adv. Synth. Catal. 5 (2001) 343;
(
b) By hydrosilylation: I. Takei, Y. Nishibayashi, Y. Arikawa, S.
Uemura, M. Hiday, Organometallics 18 (1999) 2271;
c) Y. Nishibayashi, I. Takei, S. Uemura, M. Hiday, Organome-
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Karman Avenue, suite 370. Irvine, CA 92612, USA.
1
[33] Reaction conditions: 0.020 mol of acetophenone, 2ꢄ
/
10ꢃ5 mol of
30] (a) D. Carmona, C. Cativiela, S. Elipe, F.J. Lahoz, M.P. Lamata,
M.P. L o´ pez-Ram de V ´ı u, L.A. Oro, C. Vega, F. Viguri, Chem.
Commun. (1997) 2351;
ꢃ5
ꢃ5
[Rh(cod) ]BF , 3ꢄ
2
4
/
10
mol (or 6ꢄ
/
10 ) of ligand, 40 bar of
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