Screening of a library of hemisalen ligands in asymmetric H-transfer: Reduction of aromatic ketones in water

Article abstract of DOI:10.1016/j.crci.2013.08.011

A library of chiral hemisalen ligands (30) was realized. The ligands were synthesized by the condensation of salicylaldehyde derivatives with amino-alcohols (amino-indanol or substituted amino-ethanol) and characterized. These ligands associated with ruthenium (II) precursors were tested on the asymmetric transfer hydrogenation (ATH) of aromatic ketones by sodium formate in water. The different substituent pattern on the ligand (electronic and hindrance effects on different positions) as well as the ruthenium precursor were investigated. The best compromise in terms of conversion and chiral induction led to the complex [RuCl₂(mesitylene)]₂ coordinated to (1S,2R)-1-((E)-(3-(dimethyl(phenyl)silyl)-2-hydroxy-5-methoxy benzylidene) amino)-2,3-dihydro-1H-inden-2-ol (L25). It reduces acetophenone in 95% yield and 91% ee in 18 h at 30°C.

Full text of DOI:10.1016/j.crci.2013.08.011

C. R. Chimie 17 (2014) 403–412
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Preliminary communication/Communication
Screening of a library of hemisalen ligands in asymmetric
H-transfer: Reduction of aromatic ketones in water
Mourad Boukachabia a, Nicolas Vriamont b, Dominique Lambin b,
a,
Olivier Riant b, , Louisa Aribi-Zouioueche
*
*
a Laboratoire Catalyse asyme´trique e´co-compatible (LCAE), universite´ Badji-Mokhtar, BP 12, 23000 Annaba, Algeria
b
ˆ
Institute of Condensed Matter and Nanosciences Molecules, Solids and Reactivity (MOST), universite´ catholique de Louvain, batiment
Lavoisier P1, Louis-Pasteur, 1, bte 3, 1348 Louvain-la-Neuve, Belgium
A R T I C L E I N F O
A B S T R A C T
Article history:
A library of chiral hemisalen ligands (30) was realized. The ligands were synthesized by the
condensation of salicylaldehyde derivatives with amino-alcohols (amino-indanol or
substituted amino-ethanol) and characterized. These ligands associated with ruthenium
(II) precursors were tested on the asymmetric transfer hydrogenation (ATH) of aromatic
ketones by sodium formate in water. The different substituent pattern on the ligand
(electronic and hindrance effects on different positions) as well as the ruthenium
precursor were investigated. The best compromise in terms of conversion and chiral
induction led to the complex [RuCl2(mesitylene)]2 coordinated to (1S,2R)-1-((E)-(3-
(dimethyl(phenyl)silyl)-2-hydroxy-5-methoxy benzylidene) amino)-2,3-dihydro-1H-
inden-2-ol (L25). It reduces acetophenone in 95% yield and 91% ee in 18 h at 30 8C.
Received 24 May 2013
Accepted after revision 28 August 2013
Available online 29 January 2014
Keywords:
Asymmetric catalysis
Hydride transfer
Hemisalen ligands
Ruthenium
Aromatic ketones
´
ß 2013 Academie des sciences. Published by Elsevier Masson SAS. All rights reserved.
´
´
R E S U M E
´
Mots cles :
´
´
Une librairie de 30 ligands chiraux de type hemisalen est synthetisee. Les ligands sont
Catalyse asyme´trique
Transfert d’hydrure
Ligands hemisalen
Ruthe´nium
pre´pare´s par condensation de de´rive´s de l’alde´hyde salicylique avec des amino alcools
´
´
´
`
(amino-indanol ou un amino ethanol substitue). Ces ligands sont associes des
a
´
´
´
´
´
precurseurs de ruthenium (II) et testes dans la reduction de cetones aromatiques par
transfert asyme´trique d’hydrure (TAH) en pre´sence de formiate de sodium dans l’eau.
L’influence des facteurs ste´reoe´lectroniques ainsi que les pre´curseurs de ruthe´nium sont
Ce´tones aromatiques
´
´
´
etudies. Le meilleur compromis en termes de conversion et d’induction asymetrique
´
´
´
est realise avec le complexe [RuCl2(mesitylene)]2 : coordine au ligand (1S,2R)-1-((E)-
(3-(dimethyl(phenyl)silyl)-2-hydroxy-5-methoxy benzylidene)amino)-2,3-dihydro-1H-
´
´
´
`
inden-2-ol (L25), il reduit l’acetophenone avec 95 % de rendement et 91 % ee en 18 h a
30 8C.
ß 2013 Acade´mie des sciences. Publie´ par Elsevier Masson SAS. Tous droits re´serve´s.
1. Introduction
The conception of enantiomerically enriched molecules
is one of the most important preoccupations of organic
chemists [1]; thus, several catalytic asymmetric methodol-
*
Corresponding author.
E-mail addresses: olivier.riant@uclouvain.be (O. Riant),
ogies have been developed [2]. Catalytic enantioselective
reductionofketonesby hydrogentransfer reactionsisoneof
louisa.zouioueche@univ-annaba.org, lzouioueche@yahoo.fr
(L. Aribi-Zouioueche).
1631-0748/$ – see front matter ß 2013 Acade´mie des sciences. Published by Elsevier Masson SAS. All rights reserved.
http://dx.doi.org/10.1016/j.crci.2013.08.011

404
M. Boukachabia et al. / C. R. Chimie 17 (2014) 403–412
the most straightforward methods for the efficient access to
a large variety of enantiomerically enriched secondary
alcohols [3], which are widely used as building blocks for
bioactive molecules [4]. Nowadays, economic and ecologic
imperatives impose switching the usual organic solvents
withsafeones, likeionicliquids[5], supercriticalfluids[6]or
water to follow some criteria of green chemistry [7].
Enantioselective reduction of ketones via transfer hydro-
genation in water was developed more recently [8]. The first
system, described by Chung et al. [9], involved ruthenium
catalysts coordinated with amides derived from (S)-proline
for the reduction of o-substituted acetophenones with
sodium formate as an hydrogen source. On the same lines,
several ligands havebeentested, suchas bidentatediamines
[10], amino-amides [11], and amino-alcohols [12].
We are interested in preparing water-soluble ligands,
which would be effective in the enantioselective transfer
hydrogenation of ketones. In the course of our previous
investigations, we have studied the asymmetric transfer
hydrogenation of prochiral ketones in water using ruthe-
nium (II) associated with hydrosoluble ligands, such as N-
phenylprolinamide [10j]. The ATH reaction of aromatic
ketones in the presence of sodium formate as an hydrogen
source allowed reaching enantioselectivities up to 99%, and
the catalysts could be re-used 15 times [13]. We have also
reported the use of a multi-substrate screening method for
evaluating ligands for ruthenium-catalyzed asymmetric
hydrogen transfer in water of several aliphatic [14] and
aromatic [15] ketones. We have investigated various proline
amides and amino-alcohols; the proline amide derivative
prepared from (1R,2S)-cis-amino-indanol revealed as the
best ligand for most of the aromatic ketones used in the
multi-substrate reductions. This led us to choose amino-
indanol as a precursor in the synthesis of a library of well-
known hemisalen-type ligands that had not been previously
used as water-soluble complex catalysts of asymmetric
transfer hydrogenation of aromatic ketones. We report also
the influence of the metallic precursor on the enantioselec-
tivity of the reaction.
of salicylaldehyde derivatives and chiral 1, 2-diamines
or amino-alcohols [16]. The use of these ligands for
ATH reactions has been barely described in the literature.
Riant et al. [17a] reported that the catalysts based on
ruthenium (II) coordinated with hemisalen ligands can be
used for the ATH of acetophenone in iso-propanol. In the
same context, we have decided to exploit this type of
ligands associated with ruthenium (II) for the asymmetric
H-transfer reduction of aromatic ketones in water.
2.1. Asymmetric reduction of acetophenone with ligand L13
Our reference reaction uses the previously reported (1R,
2S)-amino-indanol as a chiral ligand for the enantioselec-
tive H-transfer reduction of acetophenone in water with
[RuCl2(p-cymene)]2 as the metallic precursor and sodium
formate as the hydride source [15]. The corresponding
alcohol was obtained in 71% ee. Ligand L13 was then
prepared via simple condensation of 3,5-di-tert-butyl
salicylaldehyde and (1R, 2S)-1-amino-indanol and used
as a ligand with the same reaction conditions (Scheme 1).
We were pleased to observe a substantial increase in
the measured selectivity of the reaction, since the desired
alcohol was obtained with 81% ee. Yet, the reaction
proceeds more slowly with this ligand than with the
corresponding amino-alcohol (full conversion in 72 h
with L13 instead of 24 h with amino-indanol). It indicates
that the salicylaldehyde moiety has
a measurable
influence on both enantioselectivity and reactivity of this
catalytic reaction and optimization of the ligand was
carried out to design a new fast and enantioselective
catalytic system.
Due to the racemization of the phenylethanol observed
in previous investigations [17] during the course of ATH of
acetophenone in iso-propanol with hemisalen ligands
under inert atmosphere, and its successful use for the
dynamic kinetic resolution of secondary alcohols [18], we
first decided to check the outcome of the asymmetric
reaction of the acetophenone with L13 over time.
Enantiomeric excess of alcohol was monitored by sampling
aliquots for chiral GC analysis. Fig. 1 shows the absence of
racemization of the alcohol, since no variation of ee was
observed within the three days required to reach the
complete conversion.
2. Results and discussion
The hemisalen ligands are Schiff bases, which can be
synthesized in a straightforward manner by the condensation
Scheme 1. Asymmetric reduction of the acetophenone with L13.

M. Boukachabia et al. / C. R. Chimie 17 (2014) 403–412
405
with (1S,2R)-cis-amino-indanol 7 delivered ligand L25 in
98% isolated yield.
2.3. Screening of the synthesized library of ligands for the
reduction of acetophenone
The metallic precursor [RuCl2 (p-cymene)]2 associated
with the chiral selected ligand was dissolved in water
under argon atmosphere. After one hour of stirring, sodium
formate and the ketone were added to the aqueous
solution. The reaction was monitored by chiral GC analysis.
The results are reported in Table 1.
The results from Table 1 showed that the reactivity and
the enantioselectivity of the catalytic system are strongly
dependent on the structural variations of the ligands, such
as the bulkiness of the substituents, the presence of
electron-attracting or donating groups or the nature of the
starting chiral amino-alcohol. The analysis of the results
detailed in Table 1 leads to the following remarks.
Fig. 1. Variation of the enantiomeric excess in the course of the reaction.
2.2. Synthesis of a library of ligands
A small library of 30 chiral ligands was then prepared in
order to investigate both electronic and steric properties of
the chiral tridentate ligand on the outcome of the reaction.
Our library of ligands contains two categories of struc-
tures: type A ligands synthesized from amino-indanol and
2.3.1. Ligands A
Ligand L1 and its enantiomer L12 are considered as
references for the whole ligands family type A. The
asymmetric reduction of acetophenone with the two
above ligands afforded the phenylethanol in respectively
95% and 80% conversions and 63% and 67% enantiomeric
excesses (entries 1 and 12).
type
B ligands synthesized from substituted amino-
ethanol (Fig. 2). The ligands are easily prepared from the
corresponding chiral amino-alcohol and a salicylaldehyde
derivative using well-known procedures [19].
This structure of the ligand allows us to easily vary the
steric hindrance around the metal atom as well as to
introduce electron-active substituents on the aryl ring of
the ligand (Fig. 3). The key step for the preparation of
hemisalen ligands is the synthesis of the non-commercial
salicylaldehydes. Scheme 2 describes the synthetic path-
way of ligand L25, which was finally selected to complete
our investigations.
A regioselective bromination of p-methoxyphenol 1
was performed with benzyltrimethylammonium tribro-
mide 2 in dichloromethane/methanol, to obtain dibromide
3 in 95% isolated yield. Silylation of the phenol group was
then carried out with chlorodimethylphenylsilane 4 in the
presence of the triethylamine in THF. Double-bromide
lithium transfer, silane migration, and formylation were
then carried out using a reported literature procedure in a
one-pot process with tertio-butyllithium and DMF [20],
and the silylated salicylaldehyde 6 was isolated with a 30%
yield. Lastly, the condensation of the salicylaldehyde 6
The presence of electron-donating substituents, t-Bu,
OMe, Ph, on the aromatic ring of the ligand induces an
enhancement of both reactivity and enantioselectivity of
the catalyst. Ligand L2, possessing a methoxy substituent
on the para position, led to a 81% enantiomeric excess with
100% conversion (entry 2). However, a slight decrease in
the enantiomeric excess was obtained using L3 with
methoxy substitution in the ortho position; phenylethanol
is isolated with 71% ee (entry 3). A similar result was
recorded with L16, possessing a dioxolane substituent
(entry 16). The nature of the electron-donating substituent
plays an important role on the reactivity of the catalytic
system. Ligand L13, having two tertio-butyl substituents,
afforded similar enantioselectivity to that of L2, but with
an increased reaction time (entry 13 versus 2). This
observation was confirmed by the replacement of a tertio-
butyl by a phenyl group (ligand L18); the conversion is
reduced to the half C = 51%, with a drastic decrease of the
enantioselectivity, i.e. ee = 7% (entry 18). With ligand L22
(entry 22), obtained from (1S,2S)-trans-amino-indanol,
both reaction rate (C = 88%) and enantiomeric excess of the
recovered alcohol (ee = 43%) are decreased compared to
L13, in spite of the presence of an electron-donating
substituent.
R
2 R3
R1
The introduction of electron-withdrawing substituents,
such as NO2, Cl, Br, I, in the ligand structure affects
negatively the catalytic performance of the ruthenium
complex. Ligand L4 inhibits the reaction (entry 4).
However, a moderate reaction rate C = 26% is observed
using the analogous o-substituted ligand L5, 38% ee for (S)-
phenyl ethanol (entry 5). Low enantiomeric excesses
(52% < ee < 58%) and conversions were recorded using
the halogenated ligands; L9–L11 (35% < C < 66%) (entries
9,10 and 11). We have noted a significant enhancement of
*
*
*
*
N
N
OH
OH
OH
X
R
R
X = OH or NHR'
Ligands type A
Ligands type B
Fig. 2. Structure of the library of ligands. Color online.

406
M. Boukachabia et al. / C. R. Chimie 17 (2014) 403–412
Ligands type A:
N
OH
MeO
N
OH
N
OH
N
OH
NH
OH
OH
N
OH
N
OH
OH
OH
O2N
OH
OH
O2N
MeO
OH
OH
NO2
NO2
OMe
L1
L2
L3
L4
L5
L6
L7
N
OH
N
OH
N
OH
N
OH
N
OH
N
OH
t-Bu
OH
O2N
OH
I
OH
Cl
OH
OH
Br
OH
t-Bu
L13
OMe
L8
I
Cl
L9
L10
L11
L12
N
OH
N
OH
N
OH
N
OH
N
OH
OH
N
OH
O
OH
NH
O
OH
OH
NHTs
Si
Ph
Ph
L14
L15
L16
L17
L18
L19
N
OH
N
OH
N
OH
N
OH
N
OH
N
OH
Br
OH
t-Bu
OH
t-Bu
L22
OH
MeO
OH
OH
NH
Ph
N
N
Si t-Bu
Ph
Si
Ph
PPh2
L23
Ph
L20
L21
L24
L25
Ligands type B:
O
Ph
Ph
Ph
OH
Ph
Ph
OH
Ph
Ph
Ph
Ph
Ph
N
N
N
OH
N
OH
N
OH
O
t-Bu
OH
t-Bu
OH
t-Bu
OH
OH
t-Bu
OH
t-Bu
L29
t-Bu
L30
Ph
t-Bu
L27
t-Bu
L28
L26
Fig. 3. Structure of the studied ligands.
Scheme 2. Synthesis of L25.

M. Boukachabia et al. / C. R. Chimie 17 (2014) 403–412
407
Table 1
Screening of a library of hemisalen ligands for the asymmetric reduction of acetophenone in water.
b
Entry
Ligandsa
Time (h)
Yield (%)
Conv. (%)c
ee (%)c
Conf.d
Amino-indanol
L1
24
72
20
48
72
48
48
72
72
72
72
72
72
72
72
72
24
20
72
72
72
72
72
72
48
20
24
72
72
24
72
92
74
92
96
–
100
95
71
63
81
71
–
(R)
(S)
(R)
(S)
–
1
2
L2
100
100
0
3
L3
4
L4
5
L5
21
28
53
18
34
30
57
73
89
10
69
94
91
46
75
80
95
68
89
93
96
80
15
70
87
59
26
38
43
47
55
58
52
55
67
81
62
64
71
61
7
(S)
(R)
(S)
(S)
(S)
(S)
(S)
(R)
(S)
(R)
(S)
(S)
(S)
(R)
(R)
(R)
(R)
(S)
(R)
(R)
(R)
(S)
(S)
(S)
(S)
(S)
6
L6
32
7
L7
61
8
L8
20
9
L9
37
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
L10
L11
L12
L13
L14
L15
L16
L17
L18
L19
L20
L21
L22
L23
L24
L25
L26
L27
L28
L29
L30
35
66
80
100
47
71
100
100
51
80
87
64
46
43
56
77
83
56
10
33
34
12
85
100
88
100
100
100
100
67
87
100
66
The lines in bold represent the best results.
a
1 mmol of ketone, 5% of catalyst (ligand/ruthenium:0.05/0.025 mmol).
Isolated yield.
b
c
Determined by chiral GC.
d
Absolute configuration determined by analogy and comparison with literature data.
both reactivity and selectivity with an N-methyl piperazyl,
the metal was increased by grafting a bulky silane group in
the ortho position. A significant increase of the enantio-
meric excess was observed: after 72 h, a full conversion
was reached with the highest enantiomeric excess in our
series, ee = 87% (entry 19). Further increase of the steric
bulkiness with ligand L24 incorporating a tertio-butyldi-
phenylsilane group surprisingly gave a reduction of the
reaction time to 48 h, albeit with a slight decrease in the
enantiomeric excess to 77% (entry 24). Lastly, we decided
to combine the effects of the phenyldimethylsilyl group at
the ortho position of the salicylaldehyde (enantioselec-
tivity) and of the methoxy group at the para position
(reactivity) of the salicylaldehyde and prepared ligand L25.
To our delight, with this ligand, the reactivity was
increased, with total reduction of acetophenone after only
20 h, while a high enantioselectivity of 83% was main-
tained (entry 25).
the methyl substituent being in the ortho position of
the phenolic hydroxyl for L20 (entry 9 versus 20). A
competitive effect is observed when the ligand is grafted
with both electron-withdrawing and/or donating substi-
tuents. We have also confirmed the influence of the
methoxy substituent in para on the reactivity of the
catalyst: 61% conversion with ligand L7, compared to 20%
with ligand L8 (entry 7 versus 8).
The results recorded with ligand L15 (C = 71%, ee = 64%,
entry 15), are similar to those obtained with L1 and L12. It
seems that 2-hydroxy-1-naphtaldehyde is to be consid-
ered as electronically neutral. For ligands L14, L17, and
L21, both the reactivity and selectivity of the ruthenium
(II) complex are related to the substitution of amine.
Ligand L17, with a mesitylene substituent, gives a more
reactive catalyst (3 days of reaction time with L14 and L21
and 20 h with L17). However, the tosyl-substituent in
ligand L14 does not affect the enantioselectivity of the
catalyst (entries 14 and 17), while the substitution by a
phenyl group decreases the enantioselectivity, ee = 46% at
100% of conversion (entry 21) compared to the tosyl
group.
2.3.2. Ligands B
The hemisalen ligands of type B, based on simple
amino-alcohols induce moderate enantiomeric excess
(10% < ee < 56%) (entries 26–30). We have markedly noted
a drastic inhibition of either the selectivity and the
reactivity of the catalyst due to the bulkiness of the ligand,
The use of ligand L23, incorporating a diphenylpho-
sphino group, did not allow us to improve the catalyst
(entry 23). With ligand L19, the steric hindrance around
expected with ligands L26 and L29, where
a total
consumption of acetophenone has been recorded (entries

408
M. Boukachabia et al. / C. R. Chimie 17 (2014) 403–412
ring and gave rise to highly efficient chiral catalysts [21].
An easy approach was tested here, as we have previously
shown that the use of the symmetric arene ligands could
influence the outcome of enantioselective ruthenium-
catalyzed ATH reactions [17].
A small collection of
[RuCl2(arene)]2 was prepared and compared to the
commercially available [RuCl2(mesitylene)]2 using the
optimized reaction conditions and the best ligand L25
(Scheme 3). All the experiments were carried out under
similar reaction conditions, with a catalyst loading of 2.5%
Ru(II) and 5% of L25. The results are summarized in Table 2.
Compared to p-cymene ligand as a reference, decreasing
the number of substituents (entry 5) or the bulkiness of
one of the substituent (entry 2) led to a dramatic decrease
in the reactivity as well as of the enantioselectivity in the
case of the benzene ligand (entry 5). The hexamethylben-
zene ligand (entry 4) gave similar results (reactivity and
enantioselectivity) as p-cymene. We were pleased to
notice an increase in the enantioselectivity (91% ee) with
the ruthenium precursor coordinated by mesitylene (entry
3). A similar reaction time was observed with the last
metallic precursor, which was thus selected for the
optimised catalytic system.
Fig. 4. The best studied ligands.
26 and 29). The best results of asymmetric inductions are
presented in Fig. 4.
2.4. Optimization of the structure of the metallic precursor
2.5. Asymmetric reduction of various aromatic ketones
Will et al. reported the strong influence of the
substituents of the aryl group of the metallic precursor
of the catalyst on the ATH of ketones with chiral ruthenium
complexes, and it was shown that locking the arene ligand
to the chiral structure prevented the rotation of the arene
Further to the optimization of the catalyst, we tested
this system, combining [RuCl2 (mesitylene)]2 and ligand
L25 for the ATH reduction in water of a representative
variety of aromatic ketones (see Scheme 4).
Scheme 3. Reduction of acetophenone with different metallic precursors.
Table 2
Effect of the metallic precursor.
Entry
Metallic precursora
Time (h)
Yield (%)b
Conv. (%)c
ee (%)c
Conf.d
1
2
3
4
5
[RuCl2 (p-cymene)]2
[RuCl2 (p-xylene)]2
20
72
18
18
72
96
74
95
97
68
100
84
83
73
91
85
49
(R)
(R)
(R)
(R)
(R)
[RuCl2 (mesitylene)]2
[RuCl2 (hexamethylbenzene)]2
[RuCl2 (benzene)]2
100
100
77
The lines in bold represent the best results.
a
1 mmol of ketone, 5% of catalyst (ligand/ruthenium:0.05/0.025 mmol).
b
c
Isolated yield.
Determined by chiral GC.
d
Absolute configuration determined by analogy and comparison with literature data.

M. Boukachabia et al. / C. R. Chimie 17 (2014) 403–412
409
Scheme 4. Reduction of aromatic ketones with [RuCl2 (mesitylene)]2/L25 system.
Table 3
Reduction of aromatic ketones with [RuCl2 (mesitylene)]2/L25 system.
c
Entry
Alcoholsa
Time (h)
Yield (%)b
Conv. (%)
ee (%)c
Conf.d
1
2
3
4
5
6
7
8
9
10
15a
15b
15c
15d
15e
15f
15g
15h
15i
72
18
18
48
18
72
18
72
18
72
61
94
97
92
93
76
94
80
95
10
52
100
100
100
100
81
87
85
82
85
83
79
81
91
17
65
(R)
(R)
(R)
(R)
(R)
(R)
(R)
(–)
(S)
(–)
100
81
100
2
15j
a
1 mmol of ketone, 5% of catalyst (ligand/ruthenium:0.05/0.025 mmol).
Isolated yield.
b
c
Determined by chiral GC.
d
Absolute configuration determined by analogy and comparison with literature data.
The results gathered in Table
3
show that the
15i (17%) (entry 9). An increase in the length of the alkyl
chain allowed a better asymmetric induction. The reduc-
tion of propiophenone 14g led to a 81% ee for the alcohol,
versus a 91% ee for the alcohol, resulting from heptano-
phenone 14h (entry 8). The bulkiness of the structure of
ketones inhibits catalysis: only a 2% conversion is observed
after 72 h for the reduction of cyclohexylphenyl ketone 14j
(entry 10).
performance of the chiral catalytic system [RuCl2 (mesi-
tylene)]2/L25 depends on the structure of the substrate.
The presence of a methyl substituent in the ortho or in the
para position on the aromatic ring induces a decrease in the
reactivity of the catalyst. The conversion of para-methy-
lacetophenone 14f into the corresponding alcohol after
72 h is 81% instead of 52% for ortho-methylacetophenone
14a (entries 1 and 6). For substrates with substitution in
the meta position on the aromatic ring, good results have
been obtained. Total conversions were observed after 18 h
(14b, 14c, 14e) or 48 h (14d) and enantiomeric excesses
above 80%. Furthermore, we did not notice any electronic
effect according to the nature, whether electron-with-
drawing or electron-donating, of the substituent (m-CH3,
m-OCH3, m-Br, and m-CF3); they are completely converted
into the corresponding alcohols, in the highest enantio-
meric excess, i.e. 83% (entries 2 to 5). The asymmetric
reduction of propiophenone 14g afforded the correspond-
ing alcohol 15g in 81% ee (entry 7). Nevertheless, the
replacement of the methyl group by a trifluoromethyl
group in 14i led to a lower enantiomeric excess of alcohol
3. Conclusion
We have developed a family of chiral robust catalysts
based on the ruthenium (II) complex associated with new
chiral ligands, efficient for the enantioselective reduction
of aromatic ketones via asymmetric H-transfer reduction.
We have investigated a simple metallic precursor Ru(II)
coordinated with a variety of chiral hemisalen ligands for
the asymmetric H-transfer reduction in water of acet-
ophenone into phenylethanol. Hemisalen ligands prepared
from amino-indanol revealed themselves to be better
ligands than those synthesized from linear amino-
alcohols. Significant effects of electron-attracting or

410
M. Boukachabia et al. / C. R. Chimie 17 (2014) 403–412
electron-donating substituents on the ligand on the
reactivity and the selectivity of the catalyst were observed.
The best combination was found in a new silylated ligand
L25, hemisalen type. Optimization of the metallic pre-
cursor led us to prepare the catalytic system from [RuCl2
(mesitylene)]2 and L25. This system exhibits the highest
reactivity and selectivity (ee 91%) of our series and was
tested for aromatic ketones.
over celite and the residue was washed with hexane
(30 mL). The filtrate was concentrated in vacuo to afford
the silylether product as a colorless solid [20] (75% yield).
1H NMR (300 MHz, DMSO-d6)
d 7.63 (dd, J = 7.6, 1.7 Hz, 2H),
7.47–7.30 (m, 3H), 7.16 (s, 2H), 3.70 (s, 3H), 0.61 (s, 6H).
4.4. General procedure for the preparation of hemisalen
ligands
4. Experimental
The metallic precursors and the chiral ligands were
obtained according to the procedure described by Riant
et al. [17]. A round-bottomed flask was loaded with the
appropriate amino-alcohol (1 equiv.) in 5 mL of ethanol.
Next, 1 equiv of the aldehyde and a few drops of acetic acid
were added. The reaction was refluxed overnight. At the
end of the reaction, the mixture was concentrated in vacuo.
Purification by flash chromatography on silica gel or re-
crystallization was carried out if necessary.
4.1. General
NMR spectra were recorded on Bruker spectrometers
(300 MHz for 1H NMR, 75 MHz for 13C NMR). Chemical
shifts are reported in
the solvent resonance as the internal standard for 1H NMR
and chloroform-d (
77.0 ppm) for 13C NMR. Coupling
d ppm from tetramethylsilane with
d
constants (J) are given in hertz. Following abbreviations
classify the multiplicity: s = singulet, d = doublet, t = triplet,
q = quartet, m = multiplet, br = broad signal. The mass
spectra were obtained from the mass service at the
Catholic University of Louvain-la-Neuve, Belgium (FINNI-
GAN-MAT TSQ 7000 and FINNIGAN-MAT LQC spectro-
meters). IR spectra were recorded on a Shimadzu FTIR-
8400S spectrometer. The enantiomeric excesses were
measured by gas chromatography (ThermoFinnigan Trace
GC) equipped with an automatic autosampler and using a
4.4.1. (1S,2S)-1-((E)-(3,5-di-tert-butyl-2-hydroxy
benzylidene) amino)-2,3-dihydro-1H-inden-2-ol L22
20
(98% yield); (mp = 123 8C); [
NMR (300 MHz, DMSO-d6)
a]
D = +56.5 (c 1, EtOH). 1H
d
13.99 (s, 1H), 8.77 (s, 1H), 7.37
(d, J = 10.3 Hz, 2H), 7.22 (dd, J = 13.7, 5.2 Hz, 3H), 7.08 (d,
J = 7.0 Hz, 1H), 4.67 (d, J = 6.4 Hz, 1H), 4.44 (dd, J = 14.4,
7.4 Hz, 1H), 3.25 (dd, J = 15.4, 7.2 Hz, 1H), 2.88 (dd, J = 15.4,
8.1 Hz, 1H), 1.38 (s, 9H), 1.29 (s, 9H). 13C NMR (75 MHz,
DMSO-d6)
d 167.9 (s), 157.6 (s), 141.7 (s), 139.7 (d,
CHIRALSIL-DEX CB column (25 m; 0.25 mm; 0.25
m
m).
J = 7.0 Hz), 135.6 (s), 127.95 (s), 126.7 (d, J = 10.8 Hz), 126.2
(s), 124.8 (s), 123.9 (s), 117.9 (s), 79.3 (d, J = 19.0 Hz), 56.1
(s), 34.6 (s), 33.9 (s), 31.3 (s), 29.3 (s). IR
880 (w), 1100 (m), 1427 (m), 1630 (m, C5N), 2856 (br, OH).
MS (D-ESI; m/z): 366.22 [M + 1]+, (100%); 367.21 (25%).
HRMS: m/z = 366.2427 (calcd for C24H32O2N).
Retention times are reported in minutes. Optical rotations
were determined using a PerkinElmer 241 polarimeter at
room temperature, using a cell of a length of 1 dm and
n
(cmÀ1): 700 (s),
l
= 589 nm.
4.2. Preparation of 2,6-dibromo-4-methoxy-phenol 3
4.4.2. (1S,2R)-1-((E)-(diphenylphosphine)-2-hydroxy
To 4-methoxyphenol (11 mmol) in CH2Cl2 (10 mL)
was added a stirred solution of benzyltrimethylammo-
nium tribromide (24,8 mmol) in CH2Cl2 (70 mL) and
MeOH (30 mL). The reaction was stirred for 12 h and
the solvent was removed in vacuo. Methyl tert-butyl
ether (MTBE) (50 mL) was added and the precipitate
(benzyltrimethylammonium bromide) was collected
by filtration, and washed with MTBE (100 mL). The
organics were filtered through celite and concentrated to
provide the desired product as an orange solid that may
be used without purification [22] (95% yield). 1H NMR
benzylidene) amino)-2,3-dihydro-1H-inden-2-ol L23
20
(97% yield); (mp = 134 8C); [
NMR (300 MHz, DMSO-d6)
a]
D = –61 (c 1, EtOH). 1H
d
8.71 (d, J = 11.3 Hz, 1H), 8.07–
7.58 (m, 6H), 7.33 (dd, J = 47.7, 27.4 Hz, 13H), 6.85–6.53 (m,
1H), 4.98 (dd, J = 48.9, 2.8 Hz, 1H), 4.59 (s, 1H), 3.48 (d,
J = 6.6 Hz, 1H), 3.12 (d, J = 11.7 Hz, 1H), 2.91 (t, J = 13.8 Hz,
1H), 1.93 (s, 1H), 1.08 (t, J = 6.3 Hz, 2H). 13C NMR (75 MHz,
DMSO-d6)
d 172.7 (s), 166.6 (d, J = 32.4 Hz), 141.6 (d,
J = 30.0 Hz), 140.6 (s), 139.3 (s), 137.1 (s), 135.1 (s), 133.9
(d, J = 23.0 Hz), 131.8 (s), 128.9 (dd, J = 27.5, 9.7 Hz), 127.4
(s), 125.9 (s), 125.1 (d, J = 15.6 Hz), 117.4 (s), 114.7 (s), 74.0
(300 MHz, DMSO-d6)
3H).
d
9.33 (s, 1H), 7.15 (s, 2H), 3.71 (s,
(s), 73.3 (s), 72.4 (s), 70.0 (s), 56.7 (s), 21.8 (s), 19.2 (s). IR n
(cmÀ1): 525 (s), 680 (s), 760 (s), 1435 (s, C–P), 1637 (m,
C5N), 2840 (br, OH). MS (D-ESI; m/z): 438.18([M + 1]+,
20%); 454.13 (100%); 455.13 (26%); 322.21(12%). HRMS: m/
z = 438.1617 (calcd for C28H25O2NP).
4.3. Preparation of (2,6-dibromo-4-methoxyphenoxy)
dimethyl (phenyl) silane 5
A
solution
of
2,6-dibromo-4-methoxy-phenol
4.4.3. (1S,2R)-1-((E)-(3-(tert-butyldiphenylsilyl)-2-
hydroxy-5-methylbenzylidene) amino)-2,3-dihydro-1H-
(10.1 mmol) in THF (10 mL) was cooled in an ice bath
and treated with triethylamine (12.1 mmol); chlorodi-
methylphenylsilane (11.1 mmol) was then added drop-
wise. The ice bath was removed, and the stirred mixture
was allowed to warm up to room temperature for over 2 h.
The solvent was removed in vacuo and the residue diluted
with hexane (80 mL). The mixture was then filtered out
inden-2-ol L24
(99% yield); (mp = 102 8C); [
NMR (300 MHz, DMSO-d6)
20
a]
D = –47.8 (c 1, EtOH). 1H
8.67 (s, 1H), 7.57–7.32 (m,
d
12H), 7.29–7.12 (m, 4H), 6.89 (d, J = 1.6 Hz, 1H), 5.26 (s,
1H), 4.83 (d, J = 5.3 Hz, 1H), 4.57 (t, J = 8.0 Hz, 1H), 3.09 (dd,
J = 15.7, 5.8 Hz, 1H), 2.89 (dd, J = 15.7, 5.0 Hz, 1H), 2.09

M. Boukachabia et al. / C. R. Chimie 17 (2014) 403–412
411
(s, 3H), 1.12 (s, 9H). 13C NMR (75 MHz, DMSO-d6)
d
166.4
4.6.3. R-(+)-1-(3-methoxyphenyl) ethanol 15c
(s), 164.9 (s), 142.0 (d, J = 22.5 Hz), 141.7–141.4 (m), 141.1
(s), 135.7 (d, J = 2.9 Hz), 135.2 (s), 134.3 (s), 129.0 (s), 128.0
(s), 127.6 (s), 126.6 (s), 125.9 (s), 125.2 (s), 124.5 (s), 121.7
(s), 117.5 (s), 73.7 (d, J = 19.7 Hz), 29.6 (s), 20.1 (s), 18.1 (s).
IR (cmÀ1): 610 (m), 760 (s), 1120 (m), 1425 (s, C–Si), 1633
(m, C5N), 2860 (br, OH). MS (D-ESI; m/z): 506.09 ([M + 1]+,
100%); 507.07 (36%); 508.06 (10%); 428.24 (42%). HRMS:
m/z = 506.2509 (calcd for C33H36O2NSi).
GC [CHIRALSIL-DEX CB (25 m; 0.25 mm; 0.25
isotherm 140 8C], tR = 7.79 min, tS = 8.0 min. [a] D = +28.7
m
m),
20
(c 1.47, MeOH) for 82% ee, {Lit [
for 99% ee (S)} [23].
a]D = –34.9 (c 0.84, MeOH)
4.6.4. R-(+)-1-(3-bromophenyl) ethanol 15d
GC [CHIRALSIL-DEX CB (25 m; 0.25 mm; 0.25 mm),
20
isotherm 140 8C], tR = 9.78 min, tS = 10.19 min.
[a
]
=
D
+24.5 (c 1.84, EtOH) for 85% ee, {Lit [a]D = –28.6 (c 1.78,
4.4.4. (1S,2R)-1-((E)-(3-(dimethyl (phenyl) silyl)-2-hydroxy-
5-methoxy benzylidene) amino)-2,3-dihydro-1H-inden-2-ol
L25
EtOH) for 99% ee (S)} [23].
4.6.5. R-(+)-1-[3-(trifluoromethyl) phenyl] ethanol 15e
GC [CHIRALSIL-DEX CB (25 m; 0.25 mm; 0.25 mm),
isotherm 140 8C], tR = 4.44 min, tS = 4.77 min.
(98% yield); (mp = 111 8C); [a 20
]
D = –52 (c 1, EtOH). 1H
13.45 (s, 1H), 8.65 (s, 1H), 7.69–
20
NMR (300 MHz, DMSO-d6)
d
[a
]
=
D
6.59 (m, 12H), 5.22 (s, 1H), 4.76 (d, J = 5.2 Hz, 1H), 4.63–4.42
(m, 1H), 3.69 (s, 3H), 3.10 (dd, J = 15.5, 5.9 Hz, 1H), 2.92 (dd,
J = 15.5, 5.5 Hz, 1H), 0.52 (s, 6H). 13C NMR (75 MHz, DMSO-
+23.4 (c1.76, MeOH) for 83% ee, {Lit [
MeOH) for 99% ee (S)} [23].
a]D = –28.4 (c 1.26,
d6)
d
166.0 (s), 160.2 (s), 151.0 (s), 141.9 (s), 141.2 (s), 137.8
4.6.6. R-(+)-1-(4-methylphenyl) ethanol 15f
(s), 133.8 (s), 129.0 (s), 128.0 (s), 127.7 (s), 126.7 (s), 125.8
(s), 125.1(s), 124.7 (s), 117.6(s), 116.3 (s), 74.0 (d, J = 7.1 Hz),
55.5(s), 2.3(s).IR(cmÀ1):610(m), 760(s), 1120(m), 1425(s,
C–Si), 1633(m, C5N), 2860(br, OH). MS(D-ESI;m/z):418.08
([M + 1]+, 20%); 888.20 (100%); 889.17 (70%); 890.14
(35%); 208.19 (14%); 262.10 (13%); 340.15 (11%). HRMS:
m/z = 418.1833 (calcd for C25H28O3NSi).
GC [CHIRALSIL-DEX CB (25 m; 0.25 mm; 0.25 mm),
isotherm 140 8C], tR = 5.16 min, tS = 5.43 min. [
a]
D = +34.4
(c 1.19, MeOH) for 79% ee, {Lit [a]D = –43.5 (c 0.94, MeOH)
for 99% ee (S)} [23].
4.6.7. R-(+)-1-phenyl propan-1-ol 15g
GC [CHIRALSIL-DEX CB (25 m; 0.25 mm; 0.25 mm),
20
isotherm 140 8C], tR = 5, 59 min, tS = 5, 68 min. [
a
]
=
D
4.5. General procedure for the asymmetric reduction of
ketones
+23.2 (c1.27, CHCl3) for 81% ee, {Lit [
CHCl3) for 78% ee (R)} [15].
a]D = +22.8 (c 1.14,
In a Schlenk tube, the chiral ligand (0.05 mmol) and the
metallic precursor (0.025 mmol) were dissolved in water
(4 mL). After one hour of stirring at 30 8C, sodium formate
(10 mmol) and the ketone (1 mmol) were added to the
aqueous solution. The solution was maintained at 30 8C
until total reduction of the ketone. The formed alcohol was
separated from the catalyst by simple extraction with
pentane (2 Â 8 mL), and the organic layer was dried over
MgSO4, and concentrated in vacuo. The crude residue was
distilled in order to purify the alcohol.
4.6.8. (–)-1-phenyl heptan-1-ol 15h
GC [CHIRALSIL-DEX CB (25 m; 0.25 mm; 0.25 mm),
20
isotherm 160 8C], t+ = 9.83 min, t = 9.98 min. [
a
]
D = –
À
40.7 (c 1.65, CHCl3) for 91% ee, {Lit [
benzene) for 78% ee (S)} [24].
a]D = –25.0 (c 1.71,
4.6.9. S-(+)-3,3,3-trifluoro-1-phenylpropan-1-ol 15i
GC [CHIRALSIL-DEX CB (25 m; 0.25 mm; 0.25 mm);
isotherm 140 8C], tR = 5.80 min, tS = 5.90 min. [a] D = +4.33
20
(c 1.67, CCl4) for 17% ee, {Lit [a]D = +25.1 (c 0.81, CCl4) for
99% ee (S)} [25].
4.6. Chiral GC analysis
4.6.10. (–)-Cyclohexyl (phenyl) methanol 15j
The chemical analysis was performed by gas chroma-
tography (ThermoFinnigan Trace GC), equipped with an
automatic autosampler and using a CHIRALSIL-DEX CB
GC [CHIRALSIL-DEX CB (25 m; 0.25 mm; 0.25 mm);
20
isotherm 160 8C], t+ = 11.43 min, t = 11.52 min. [
a
]
=
D
À
–10.5 (c 0.19, CHCl3) for 65% ee.
column (25 m; 0.25 mm; 0.25 mm; carrier gas: hydrogen;
flow rate: 2.5 mL/min). Retention times are reported in
minutes.
Acknowledgments
MESRS (FNR 2000 and PNR) and FNRS are gratefully
acknowledged for financial support of this work. We thank
Dr M. Merabet-Khelassi for her contribution to this work.
4.6.1. R-(+)-1-(2-methylphenyl) ethanol 15a
GC [CHIRALSIL-DEX CB (25 m; 0.25 mm; 0.25 mm),
isotherm 140 8C], tR = 5.97 min, tS = 6.33 min.
20
[a
]
=
D
+54,2 (c 0.83, EtOH) for 87% ee, {Lit [
EtOH) for 99% ee (S)} [23].
a]D = –64.3 (c 1.04,
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