Asymmetric transfer hydrogenation of ketones in aqueous solution catalyzed by rhodium(III) complexes with C2-symmetric fluorene-ligands containing chiral (1R,2R)-cyclohexane-1,2-diamine

Article abstract of DOI:10.1590/S0103-50532010000300005

Two C₂-symmetric bis(sulfonamide) ligands containing fluorene-chiral (1R,2R)-cyclohexane- 1,2-diamine were complexed to Rh ^III(Cp*) and used as catalyst to reduce aromatic ketones. The corresponding chiral secondary alcohols were obtained in 87-100percent ee and 85-99percent yield, under asymmetric transfer hydrogenation (ATH) conditions using aqueous sodium formate as the hydride source. With acetophenone, 94percent ee and 86-97percent yield was achieved with substrate/catalyst (S/C) ratio of 10,000.

Full text of DOI:10.1590/S0103-50532010000300005

J. Braz. Chem. Soc., Vol. 21, No. 3, 431-435, 2010.
Printed in Brazil - ©2010 Sociedade Brasileira de Química
0103 - 5053 $6.00+0.00
Asymmetric Transfer Hydrogenation of Ketones in Aqueous Solution Catalyzed
by Rhodium(III) Complexes with C2-Symmetric Fluorene-Ligands Containing
Chiral (1R,2R)-Cyclohexane-1,2-diamine
Rubén Montalvo-González,a,b Daniel Chávez,a Gerardo Aguirre,a
Miguel Parra-Hakea and Ratnasamy Somanathan*,a
aCentro de Graduados e Investigación, Instituto Tecnológico de Tijuana, Apartado Postal 1166,
Tijuana, B.C. 22500, México
bUnidad Académica de Ciencias Químico Biológicas y Farmacéuticas, Universidad Autónoma de
Nayarit, Ciudad de la Cultura Amado Nervo, Tepic, Nay. 63155, México
Dois ligantes C2-simétricos bis(sulfonamide) contendo o fluoreno-quiral (1R,2R)-ciclohexano-
1,2-diamina foram complexados com RhIII(Cp*) e usados como catalizador para redução aromática
de cetonas. Os alcoois secundários correspondentes foram obtidos com ee 87-100% e rendimento
de 85-99%, sob condições de transferência de hidrogenio assimétrica (THA), usando formato de
sodio aquoso como fonte de hidretos. Usando acetofenona, obteve-se ee de 94% e rendimento de
86-97%, com uma razão substrato/catalizador de 10.000.
Two C2-symmetric bis(sulfonamide) ligands containing fluorene-chiral (1R,2R)-cyclohexane-
1,2-diamine were complexed to RhIII(Cp*) and used as catalyst to reduce aromatic ketones. The
corresponding chiral secondary alcohols were obtained in 87-100% ee and 85-99% yield, under
asymmetric transfer hydrogenation (ATH) conditions using aqueous sodium formate as the hydride
source. With acetophenone, 94% ee and 86-97% yield was achieved with substrate/catalyst (S/C)
ratio of 10,000.
Keywords: bis(sulfonamide) ligands, RhIII(Cp*) complexes, asymmetric transfer hydrogenation
Introduction
only a limited number of catalysts have found application
in the industry. This drawback is mainly due to the cost of
metals, metal contamination, environmentally unfriendly
solvents and disposal problems, turnover number (TON)
and turnover frequency (TOF).7 Most of the development
on the homogeneous asymmetric catalysts has been made
from chiral ligands, which have been synthesized based on
phosphorus, amines or amino alcohols derivatives, and then
coordinated to RhIII, RuII or IrIII metal centres.8
Due to the demand of environmentally friendly methods,
the search for water soluble catalysts has received much
attention in recent years considering its safety, economical
and environmentally benign nature compared to most
organic solvents.9 Recently, a number of asymmetric
transfer hydrogenation (ATH) reports have appeared, using
heterogenized or water soluble metal complexes, and sodium
formate as the hydride source to get enantiopure chiral
alcohols from aromatic prochiral ketones.10 Addressing
this, we have reported a C2– symmetric bis(sulfonamide)-
cyclohexane-1,2-diamine ligand [(1), Figure 1] complexed
The demand for enantiomerically pure compounds have
been continuously increasing over the last three decades.
This demand is due to strict government regulations that
require evaluation of all the possible stereoisomers of a
compound and commercialization of a chiral product only
as a single enantiomer.1 Among small chiral molecules, the
chiral alcohols occupy an important place in the synthesis
of pharmaceuticals,2 cosmetic and food,3 and agricultural
chemicals.4 Thus, the catalytic enantioselective reduction of
prochiral ketones is one of the most practical ways to obtain
pure chiral secondary alcohols.5 Noyori’s enantioselective
reduction of prochiral ketones using molecular hydrogen
and chiral Ru-BINAP-amine complex has already found a
useful niche in the fine chemicals industry.6
However, in spite of the great advances that have been
reached in the area of homogeneous asymmetric catalysis,
*e-mail: somanatha@sundown.sdsu.edu

432
Asymmetric Transfer Hydrogenation of Ketones in Aqueous Solution Catalyzed
J. Braz. Chem. Soc.
with [RhIIICl2(Cp*)]2 which gave high enantioselectivities
and yields in the ATH of ketones in aqueous sodium
formate.10 Searching for a better and “greener” catalyst,
herein we report the synthesis of two new C2-symmetric
ligands (2 and 3), in situ formation of the catalysts with
[RhIIICl2(Cp*)]2 and their application in the ATH of twelve
ketones with aqueous sodium formate. Ligands 2 and 3 are
derived from (1R,2R)-cyclohexane-1,2-diamine and also
from 9-oxo-9H-fluorene-2,7-disulfonyl dichloride (for 2)
and fluorene-2,7-disulfonyl dichloride (for 3) (Figure 1).
In comparison to ligand 1, it is expected that the fluorene
backbone of ligands 2 and 3 will provide more rigidity to the
complexes inhibiting any possible tetradentate-coordination,
thereby making a more active complex and also enhancing
enantioselectivity, especially with bulky ketones.
3 was prepared following reported methodology13 from
commercially available fluorene, chlorosulfonic acid,
phosphorus pentachloride and (1R,2R)-cyclohexane-1,2-
diamine. Spectra and analytical data of all compounds are
given below.
(1R,2R)-9-Oxo-9H-fluorene-2,7-disulfonic acid bis-[(2-
aminocyclohexyl)-amide] (2)
To a stirred solution of L-tartrate salt of (1R,2R)-
1,2-diaminocyclohexane (0.79 g, 3 mmol) in aqueous NaOH
(2molL-1, 10mL)wasaddedtriethylamine(1mL, 7.5mmol)
and dichloromethane (50 mL). The mixture was cooled to
o
0 C and a solution of 9-oxo-9H-fluorene-2,7-disulfonyl
chloride (0.38 g, 1 mmol) in dichloromethane (30 mL)
was added over 30 min. After the addition was complete,
the mixture was allowed to warm to room temperature and
stirred for 24 h. The resulting reaction mixture was washed
with aqueous HCl (2 mol L-1, 3 × 50 mL) and the organic
phase was removed. The HCl wash liquor was collected
and adjusted to pH ca. 9 by adding NaOH pellets until a
yellow precipitate appeared. The basic aqueous solution
was then extracted with dichloromethane (3 × 30 mL).
The dichloromethane layers were combined, dried over
anhydrous MgSO4, filtered and the solvent was removed
underreducedpressuretoobtain 2asapaleyellow crystalline
solid (0.36 g, 72% yield); mp: decomposes at 280 °C;
[a]D25 = +22° (c 0.34, MeOH). IR (film) nmax/cm-1: 3291,
3254, 3092, 2924, 2905, 2358, 2355, 1716, 1609, 1451,
1319, 1149, 1067, 970, 716.1H NMR (500 MHz, CDCl3) d
8.21 (d, J 1.5 Hz, 2H), 8.13 (dd, J 7.8, 1.5 Hz, 2H), 7.80 (7.8
Hz, 2H), 2.78 (td, J 10.6, 3.8 Hz, 2H), 2.39 (td, J 10.9, 4.0
Hz, 2H), 1.96-1.88 (m, 4H), 1.70-1.50 (m, 4H), 1.30-1.02
(m, 8H) 13C NMR (500 MHz, CDCl3) d 190.4, 145.8, 144.2,
134.8, 133.6, 122.9, 121.8, 61.0, 54.6, 34.6, 32.5, 25.0, 24.6.
Experimental
All reagents were commercially available and
used without further purification. Melting points were
determined on a Fisher-Johns melting point apparatus
and are uncorrected. Infrared (IR) spectra were measured
on a Perkin Elmer FT-IR 1600 spectrometer. NMR were
recorded withVarian Nova 500 MHz. The spectra were run
in CDCl3 with tetramethylsilane (TMS) as internal standard.
Mass spectra were obtained on a Hewlett Packard 5989 MS
Spectrometer at 70 eV by direct insertion. Rotation was
measured with Perkin-Elmer 343 Polarimeter.
General procedure for the preparation of ligands 2 and 3
Bis(sulfonamide) ligand 2 was prepared following
reported methodology12 from the corresponding
commercially available 9-oxo-9H-fluorene-2,7-disulfonyl
dichloride and (1R,2R)-cyclohexane-1,2-diamine. Ligand
Figure 1. Bis(sulfonamide) ligands.

Vol. 21, No. 3, 2010
Montalvo-González et al.
433
Elemental Anal. for C25H32N4O5S2; Calcd.: C, 56.37%; H,
6.06%. Found: C, 56.40%; H, 6.10%.
35.9, 32.8, 24.9 (2C). Elemental Anal. for C25H34N4O4S2;
Calcd.: C, 57.89%; H, 6.61%. Found: C, 57.93%; H, 6.65%.
(1R,2R)-9H-Fluorene-2,7-disulfonic acid bis-[(2-amino-
cyclohexyl)-amide] (3)13
General procedure for the asymmetric transfer
hydrogenation of ketones in water
To a stirred solution of fluorene (1.0 g, 6.25 mmol)
in acetic acid (10 mL), chlorosulfonic acid (1.9 g,
18.75 mmol) was added dropwise at 0 °C. The reaction
mixture was refluxed for 2 h, cooled and poured into a
saturated aqueous solution of NaCl (30 mL) containing
NaOH (0.62 g, 15.62 mmol) to obtain a yellow precipitate.
The precipitate was washed with a saturated solution of
NaCl (3 × 30 mL), filtered and dried overnight at 60 °C to
give sodium fluorene-2,7-disulfonate (1.62 g, 4.38 mmol,
70%). This salt was mixed with phosphorus pentachloride
(2.29 g, 11 mmol) and phosphorus oxichloride (8.22 g,
53.64 mmol). The reaction mixture was refluxed for 6 h
and the phosphorus oxychloride was distilled off. The
resulting dry residue was pulverized in a mortar and
then treated with water. The precipitate was filtered and
washed with water to give 9H-fluorene-2,7-disulfonil
chloride (1.2 g, 3.28 mmol, 75%) as a light-reddish-
brown powder. The procedure to add the trans-(1R,2R)-
cyclohexane-1,2-diamine to the sulfonyl groups of the
9H-fluorene-2,7-disulfonil chloride and get ligand 3, is
was identical to that described for 2. Yellow crystalline
solid, (0.38 g, 69% yield); mp decomposes at 285 °C;
[a]2D5 = -57° (c 0.148, CH2Cl2). 1H NMR (500 MHz, CDCl3)
d 8.12 (d, J 1.4 Hz, 2H), 7.98 (dd, J 8.0, 1.4 Hz, 2H), 7.95
(8.0 Hz, 2H), 4.06 (s, 2H), 2.79 (td, J 10.5, 4.0 Hz, 2H),
2.37 (td, J 11.0, 4.0 Hz, 2H), 1.95-1.88 (m, 4H), 1.70-1.6
(m, 4H), 1.25-1.04 (m, 8H) 13C NMR (125.72 MHz, CDCl3)
d 144.7, 144.0, 140.4, 126.5, 124.1, 121.2, 60.5, 54.9, 37.1,
The metal precursor [RhIIICl2(Cp*)]2 (0.0025 mmol)
and the chiral ligand 2 or 3 (0.0025 mmol) were mixed
with water (0.7 mL) and heated in a water bath at 60 °C
for 1 h. Sodium formate (5 equiv. relative to substrate)
and the substrate (ratio S/C = 100 as in Table 1; other
S/C ratio presented in Table 2) were subsequently added.
The reaction mixture was stirred at 40 °C in air for the
time indicated in Table 1 and Table 2 for each individual
reaction. The reaction mixture was extracted with CH2Cl2
(3 × 10 mL). The CH2Cl2 layers were combined, dried over
anhydrous MgSO4, filtered and concentrated in vacuo. The
residue containing the alcohol was acetylated using acetic
anhydride and 4-DMPA (4-N,N-dimethylamino pyridine).
Results and Discussion
Ligands 2 and 3 complexed with RhIII(Cp*) were used
as catalysts for the ATH to a series aromatic prochiral
ketones (Figure 2) in aqueous sodium formate. The results
are shown in Table 1.
Rhodium Cp* complexed ligands 2 and 3 were tested for
catalytic activity in theATH of aromatic ketones under the
same conditions used for ligand 1.10 In general, complexes
2-RhIII(Cp*) and 3-RhIII(Cp*) gave better yields and good
enantioselectivities in theATH compared with ligand 1 for
ketones b, d and e.
In order to test the catalytic properties of these
complexes on the ATH of bulky ketones, we reduced
O
O
O
O
( )3
a
d
c
b
O
O
O
O
Cl
Br
Cl
g
f
e
h
O
O
O
O
O
Cl
F
Br
j
i
l
k
Figure 2. Structures of ketones used in the ATH.

434
Asymmetric Transfer Hydrogenation of Ketones in Aqueous Solution Catalyzed
J. Braz. Chem. Soc.
Using complexes 2-RhIII(Cp*) and 3-RhIII(Cp*) in theATH
of 2-chloroacetophenone (f), 2-bromoacetophenone (g)
and 3-chloropropiophenone (h), the corresponding chiral
alcohols were obtained in 91% ee, > 99% yield, 88% ee,
91% yield and 95% ee, > 99% yield, respectively. Thus,
the reduction of halo ketones in aqueous sodium formate
provides an excellent and inexpensive synthetic route
to synthesize physiologically active 1,2- and 1,3-amino
alcohol derivatives.
Table 1. Enantioselective transfer hydrogenation of ketones with chiral
ligands 2 and 3 complexed with [RhIIICl2(Cp*)]2 using aqueous sodium
formate
[RhIIICl2(Cp*)]2, L2 or L3
HCOO- Na+/H2O
OH
H
R
R
R configuration
O
Ar
Ar
ketone
2b
3b
eec
95
Yieldd
> 99
98
eec
Yieldd
> 99
99
a
b
c
d
e
f
94
93
Substituted aromatic ketones i-l also gave excellent
yields and enantioselectivities on the reduction to the
corresponding alcohols (Table 1).
95
100
100
97
85
100
100
97
87
98
98
Inordertostudythecatalyticactivityofthenewligandsand
the Rh(III) complexes (TON, TOF) in water/HCO2NaATH,
we reduced acetophenone (a) and 4’-fluoroacetophenone (j)
using different catalyst loading, reaction times and reaction
temperatures. Results are shown in Table 2, which indicate
that the catalyst activity and the enantioselectivities of both
complexes 2-RhIII(Cp*) and 3-RhIII(Cp*) are similar and
constant through 0.25 h to 5 h. Ligand 3 reached TOF of
3,100 mol mol-1 h-1 in theATH of acetophenone (a) [entry 4
tested for 0.25 h, at 40 oC] and 4,320 ligand mol mol-1 h-1
in the ATH of 4’-fluoroacetophenone (j) [entry 8 tested for
0.08 h and 55 oC].
92
95
90
> 99
89
92
> 99
93
g
h
i
88
87
95
> 99
> 99
> 99
> 99
> 99
96
> 99
> 99
> 99
> 99
> 99
91
91
j
91
90
k
l
89
89
91
90
aAbsolute configurations are all R, except for f and g;assigned by
b
comparing optical rotations with literature values;10 Ratio substrate/
catalyst (S/C) = 100; c Measured by GC analysis of the acetylated alcohol
with chiral capillary column β-DEXTM 120; d Reaction conditions: 0.25 h
at 40 °C.
Conclusions
propiophenone (b), valerophenone (c), tetralone (d),
2’-acetonaphthone (e) and 3-chloropropiophenone (h)
to the corresponding alcohols. Results shown on Table 1
indicate good to excellent enantioselctivities and yields.
a- and β-Haloalcohols are important intermediates
in the synthesis of physiologically active molecules.2,3,11
We have synthesized two new chiral C2-symmetric
ligands with a rigid fluorene ring skeleton with the two
cyclohexylamine groups well apart from each other. Both
complexes 2-RhIII(Cp*) and 3-RhIII(Cp*) turned out to
be very good catalysts in the ATH of aromatic ketones
Table 2. Enantioselective transfer hydrogenation of acetophenone (a) and 4’-fluoroacetophenone (j) with chiral ligands 2 and 3 complexed with
[RhIIICl2(Cp*)]2 using a mixture of sodium formate/water
H
O
[RhIIICl2(Cp*)]2, L2 or L3
HCOO- Na+/H2O
OH
R configurationa
R
R
entry
ketone
ratio (S/C)
time (h)
L-2
L-3
ee (%)b
yield (%)c
98
ee (%)b
94
yield (%)c
99
1
2
3
4
5
6
7
8
a
a
a
a
a
a
j
100
200
0.08
0.25
1
94
95
95
94
95
94
91
91
>99
>99
>99
>99
97
94
>99
>99
>99
98
1000
2000
5000
10 000
200
94
2
94
5
94
12
94
86
0.17
0.67
>99
>99
90
>99
>99
jd
1000
91
a Absolute configurations are R, assigned by comparing optical rotations with the literature values;10 b Measured by GC analysis of the acetylated alcohol with
chiral capillary column β-DEXTM 120; c at 40 °C; d at 55 °C.

Vol. 21, No. 3, 2010
Montalvo-González et al.
435
using aqueous sodium formate as the hydride source, with
excellent enantioselectivities, yields and TOF. With these
complexes, excellent enantioselectivities and yields on the
ATH of bulky ketones were also achieved, making them
good candidates for the ATH of other bulky ketones, or to
be anchored to a solid phase for the heterogenization of
homogeneous catalysts.
The prochiral a-chlorine, a-bromine and β-chlorine
ketones f, g and h were reduced to the corresponding
chiral alcohols in very high yields (> 99%) and good
enantioselectivities (89-91% ee), providing an alternative
synthetic route to obtain 1,2- and 1,3-halo alcohol
derivatives.
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We gratefully acknowledge support for this project by
ConsejoNacionaldeCienciayTecnología(CONACyTgrant
37827-E) and Dirección General de Educación Superior
Tecnológica (DGEST grant 310.06-P), and postdoctoral
fellowship from CONACyT for R. Montalvo-González.
9. Anastas, P. T.; Warner, J. C.; Green Chemistry: Theory and
Practice, Oxford University Press: New York, 2000.
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Received: June 15, 2009
Web Release Date: December 3, 2009