Water-soluble chiral monosulfonamide-cyclohexane-1,2-diamine-RhCp* complex and its application in the asymmetric transfer hydrogenation (ATH) of ketones

Article abstract of DOI:10.1016/j.tetlet.2007.04.116

Monosulfonamide ligands with heteroatom/heterocyclic systems were derived from trans-(1R,2R)-cyclohexane-1,2-diamine and complexed with [Ru(benzene)Cl₂]₂, [Cp^*RhCl₂]₂ in situ and used in the ATH of ar

Full text of DOI:10.1016/j.tetlet.2007.04.116

Tetrahedron Letters 48 (2007) 4335–4338
Water-soluble chiral monosulfonamide-cyclohexane-1,2-diamine-
*
RhCp complex and its application in the asymmetric
transfer hydrogenation (ATH) of ketones
Norma A. Cortez, Gerardo Aguirre, Miguel Parra-Hake and Ratnasamy Somanathan*
Centro de Graduados e Investigaci o´ n del Instituto Tecnol o´ gico de Tijuana, Apartado Postal 1166, Tijuana, B.C. 22500, Mexico
Received 16 March 2007; revised 17 April 2007; accepted 24 April 2007
Available online 29 April 2007
Abstract—Monosulfonamide ligands with heteroatom/heterocyclic systems were derived from trans-(1R,2R)-cyclohexane-1,2-di-
*
amine and complexed with [Ru(benzene)Cl
2
]
2
, [Cp RhCl
2
]
2
in situ and used in the ATH of aromatic ketones with aqueous sodium
formate as the hydrogen source. The chiral secondary alcohols were obtained with >93% enantioselectivity and >89% yield. Reduc-
tion in water was faster than in isopropanol/KOH. Addition of surfactants showed little or no effects.
Ó 2007 Elsevier Ltd. All rights reserved.
Optically pure secondary alcohols are valuable interme-
diates in the synthesis of physiologically active mole-
cules. One direct approach to these molecules is by
Although the catalytic ATH of ketones is operationally
simple compared to hydrogenation using H gas under
2
1
pressure, the reaction is less appealing for the industrial
applications, due to the use of 2-propanol/KOH or
catalytic enantioselective reduction of the corresponding
ketones, which has been extensively studied during the
7
formic acid/NEt3. On an industrial scale the waste
2
past decade. A particularly useful method in the direct
solvents generated by this process become an environ-
mental issue. Because there is an increasing demand
for environmentally friendly methods, the ATH per-
formed in water is now of great interest, because water
is safe, economical, and environmentally benign and is
considered a ‘greener’ solvent than most organic sol-
reduction of ketones is by asymmetric transfer hydroge-
nation (ATH). ATH is catalyzed by metal complexes
associated with various chiral ligands using 2-propanol
or HCOOH/NEt as the hydrogen source. The metal
3
complexes used are often derived from chiral 1,2-dia-
mine, 1,2-amino alcohol and phosphorous-containing
ligands, coordinated to Ru(II), Rh(III), and Ir(I) metals
8
vents as a medium for conducting reactions. Recently
a number of reports have appeared on the use of water
as a medium for ATH, using heterogenized or water-
3
–6
(
Eq. 1).
9
soluble metal complexes. Addressing this we recently
reported a C -symmetric bis(sulfonamide)-cyclohexane-
1,2-diamine (1)-RhCp complex which gave high enantio-
2
*
R
isopropanol/KOH
OH
H
R
O
selectivities and conversion in the ATH of ketones in
HCOOH/NEt3
R
1
R1
9a
aqueous sodium formate.
ð1Þ
Ar
Ts
O S
SO2
Rh
Rh
Cl
H N
2
N
2
Rh
Cl
H N
O
Cl
H N
SO2
N
NH
HN
Ph
2
2
Ph
Me
NH2
H N
2
Ph
1
Keywords: Monosulfonamide ligands; RhCp* complex; Asymmetric
transfer hydrogenation; Ketones.
Xiao and co-workers have used Noyori’s TsDPEN-Ru
and Cp Rh complexes in the ATH of ketones and
obtained high enantioselectivity and conversion in
*
*
Corresponding author. E-mail: somanatha@sundown.sdsu.edu
0040-4039/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2007.04.116

4336
N. A. Cortez et al. / Tetrahedron Letters 48 (2007) 4335–4338
water.9b,m This prompted us to explore new diamine
ligands replacing the Tosyl group with a heteroatom
containing molecule, which we expected to be more
hydrophilic in nature and thus enhance the rate of reac-
tion in water.
[Cp RhCl ] or [RuCl (benzene)] , while those in water
*
2
2
2
2
were made by stirring the two species in water at 40 °C
for 1 h. The results are summarized in Table 1.
The ATH of acetophenone in 2-propanol with
RuCl (benzene)] and [Cp RhCl ] gave moderate
*
[
2
2
2 2
In this Letter, we disclose the facile synthesis of new
chiral sulfonamide-based ligands 2a–g, derived from
readily available trans-cyclohexane-1,2-diamine and
enantioselectivity
and
yield.
However,
using
*
[Cp RhCl ] with 2a–g in aqueous sodium formate led
2
2
to excellent enantioselectivty and yields, indicating that
*
*
complexed to [Cp RhCl ] and [RuCl (benzene)]
2a–g-RhCp complexes are better catalysts in terms of
2
2
2
2
(
Fig. 1). These ligands contain pyridine, imidazole, isox-
the rate and enantioselectivity in comparison to the
Ru complex with the same sulfonamide ligands. For in-
stance, the reduction of acetophenone led to a 100%
conversion with 93% ee in 30 min in water at 40 °C
and higher S/C ratio of 100 (entry 6). In comparison,
the reaction performed in 2-propanol using the same
catalyst at 25 °C gave 82% ee and 75% conversion, but
in 5 h at an S/C ratio of 33. These results showed that
azole, benzoxadiazole and thiophene groups and were
studied in the ATH of aromatic ketones in isopropanol,
as well as in water with HCO Na as the hydrogen source.
In order to evaluate the performance of our ligands in the
ATH, we compared the catalysts in isopropanol and
water with acetophenone as a model substrate. The pre-
catalysts used for reduction in isopropanol were pre-
pared by heating at 80 °C for 1 h ligands 2a–1g with
2
O
O
O
b
c
O2
a
R S NH NH2
O
O
O
(R,R)-2a-g
Br
F3C
e
d
f
2a, R=
2b, R= H C
3
CH3
N
N
O
O
O
O
Cl
Cl
Br
F
CH3
CH3
h
NO2
i
2c, R=
N N
N
g
O
O
O
2d, R=
2e R=
CH3
N
N
MeO
H C
3
CH3
j
H C
k
3
CH3
l
N
H C
3
O
2f, R=
2g, R=
N
N
O N
S
m
Figure 1. Monosulfonamide ligands.
Figure 2. Ketones tested in the ATH.
a
2
Table 1. ATH of acetophenone catalyzed by ligands 2a–g in isopropanol versus HCOONa/H O
b
c
*
*
Entry
Ligand
[(Benzene)RuCl
2
]
2
/isopropanol/
[Cp RhCl
2
]
2
/isopropanol/KOH
[Cp RhCl
2
]
2
/HCOONa/water
b
KOH
d
d
d
t (h)
S/C
ee (%)
Conv (%)
t (h)
S/C
ee (%)
conv (%)
t (h)
S/C
ee (%)
conv (%)
1
2
3
4
5
6
7
2a
2b
2c
5
5
5
5
5
5
5
33
33
33
33
33
33
33
77
84
86
80
79
79
86
26
11
64
88
17
56
12
5
5
5
5
5
5
5
33
33
33
33
33
33
33
92
89
89
87
85
82
27
96
91
96
67
93
75
32
0.5
0.5
0.5
0.5
0.5
0.5
0.5
100
100
100
100
100
100
100
90
94
89
89
—
93
12
>99
100
99
100
—
9
s
2d
2e
2f
100
15
2g
a
b
c
Absolute configuration of the alcohol is R.
5 °C using a mixture of isopropanol/KOH.
0 °C using a mixture of water/sodium formate in air.
2
4
d
TM
Measured by GC analysis of the acetylated alcohol with chiral capillary column b-DEX 120.

N. A. Cortez et al. / Tetrahedron Letters 48 (2007) 4335–4338
4337
a
2
Table 2. Aerobic ATH of ketones with Rh-2f in H O using S/C 100
O
OH
R
H
[
RhCl Cp*] , L*
2 2
R
L*=
O2
S
NH
NH2
HCOONa, H O
2
S
b
c
e
Entry
Ketone
t (h)
Without surfactant
CTAB
SDS
Conf. Abs.
d
d
d
ee (%)
Conv. (%)
ee (%)
Conv. (%)
ee (%)
Conv. (%)
1
2
3
4
5
6
7
8
9
0
1
2
3
a
b
c
d
e
f
g
h
i
0.5
6
2
4
2
2
6
2
6
6
6
6
6
93
92
99.8
>99
89
94
82
91
87
79
94
92
82
100
94
100
86
100
98
96
>99
93
74
98
94
94
90
100
100
92
73
80
91
93
95
95
93
70
100
80
98
86
99
100
13
>99
100
99
94
78
99.4
100
90.3
68
93
93
87
85
100
87
100
98
>99
100
100
>99
93
64
91
>99
16
R
R
R
R
R
R
R
R
R
R
R
R
R
1
1
1
1
j
k
l
99
94
95
95
92
80
m
96
a
b
c
4
0 °C using a mixture of water/sodium formate in air.
CTAB, cetyltrimethylammonium bromide.
SDS, sodium dodecyl sulfate.
Measured by GC analysis of the acetylated alcohol with chiral capillary column b-DEX 120.
d
e
TM
Absolute configurations were assigned by comparing optical rotations with the literature values.
the ketone reduction was drastically accelerated in
water. Ligands 2b and 2f (Table 1, entries 2 and 6) gave
the best results, consequently, we extended the use of the
(CTAB) or sodium dodecyl sulfate (SDS), showed a
marginal effect on the enantioselectivity and yields and
no rate enhancement was seen.
(
R,R)-2f-Rh-catalyst to a wide range of ketones (Fig. 2)
under the same conditions with an S/C ratio of 100.
Table 2 shows the results obtained.
In conclusion we have synthesized a number of water-
soluble monosulfonamide ligand–RhCp complexes
*
which gave good enantioselectivity and conversion in
the ATH using aqueous sodium formate as the hydro-
gen source. The reaction requires neither organic sol-
vents nor inert conditions or substrate solubility in
water. The aqueous phase catalysis thus provides an
attractive alternative for carrying out ATH in a safe,
economical, and ‘greener’ manner.
The results indicated that a ketone with an electron-
withdrawing substituent, such as –CF , –NO , –Br,
3
2
–
F, –Cl (Table 2, entries 5–9) gave a higher conversion
owing to rapid hydride transfer and selectivity, while
electron donating –OCH led to a lower enantioselectiv-
3
ity and yield (entry 6). 4-Acetylpyridine (Table 2, entry
1
3) also gave low conversion, probably due to substrate
interaction with the metal center. Increasing the alkyl
chain from methyl to ethyl and cyclic form led to lower
conversion with good enantioselectivity (Table 2, entries
Acknowledgments
1
2
, 2 and 4). Interestingly, the rate of ATH with catalyst
We gratefully acknowledge support for this project by
CONACYT (Grant 37827-E), COSNET (Grant
498.01P and 414.03P) and graduate scholarship from
CONACYT for N.A.C.
*
f-RhCp was much slower compared to C -symmetric
2
*
*
ligand 1-RhCp and RhCp –TsCYDN under identical
conditions.
9
a,b
It has been suggested that the rate
enhancement in aqueous sodium formate may be due
to the formation of formato species, which dissolves in
hydrophobic ketone, and the reaction takes place in
Supplementary data
9
d,e
*
the substrate.
In our case, with 2f-RhCp , the hetero-
atom probably competes with the formate anion intra-
or intermolecularly for the metal center, slowing the for-
mation of the catalytically active formato species. Thus,
Experimental procedures and spectroscopic and analy-
tical data of compounds are supplied in the supplemen-
tary data. Supplementary data associated with this
article can be found, in the online version, at
doi:10.1016/j.tetlet.2007.04.116.
*
our system is kinetically slower than 1-RhCp and
*
RhCp –TsCYDN. Recently, several reports have shown
that surfactants play an important role in the ATH of
9
h,o,r,s
*
ketones in water.
Although our ligand–RhCp or
Ligand–Ru complexes are water soluble, most ketone
substrates are generally hydrophobic. This prompted
us to investigate the effect of surfactants in our ATH
reaction. Addition of cetyltrimethylammonium bromide
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