New C2-symmetric bis(sulfonamide)-cyclohexane-1,2-diamine-RhCp* complex and its application in the asymmetric transfer hydrogenation (ATH) of ketones in water

Article abstract of DOI:10.1016/j.tetlet.2006.09.141

C₂-symmetric bis(sulfonamide) ligands derived from trans-(1R,2R)-cyclohexane-1,2-diamine were synthesized and complexed with [Cp^*RhCl₂]₂ in situ and used in the ATH of aromatic ketones with aqueous sodium format

Full text of DOI:10.1016/j.tetlet.2006.09.141

Tetrahedron Letters 47 (2006) 8515–8518
New C -symmetric bis(sulfonamide)-cyclohexane-
2
*
1
,2-diamine-RhCp complex and its application in the asymmetric
transfer hydrogenation (ATH) of ketones in water
a
a
a
a
Norma A. Cortez, Ram o´ n Rodr ´ı guez-Apodaca, Gerardo Aguirre, Miguel Parra-Hake,
b
a,
*
Thomas Cole and Ratnasamy Somanathan
a
Centro de Graduados e Investigaci o´ n, Instituto Tecnol o´ gico de Tijuana, Apartado Postal 1166, Tijuana, BC 22500, Mexico
b
Department of Chemistry, San Diego State University, San Diego, CA 92182, USA
Received 25 July 2006; revised 22 September 2006; accepted 26 September 2006
Available online 17 October 2006
2
Abstract—C -symmetric bis(sulfonamide) ligands derived from trans-(1R,2R)-cyclohexane-1,2-diamine were synthesized and com-
*
plexed with [Cp RhCl ] in situ and used in the ATH of aromatic ketones with aqueous sodium formate as the hydrogen source. The
2
2
chiral secondary alcohols were obtained with 90–99% enantioselectivity and in 50–100% yield. Reductions in water were faster than
those in isopropanol/KOH.
Ó 2006 Elsevier Ltd. All rights reserved.
Chiral secondary alcohols are valuable intermediates in
hols were obtained in excellent enantioselectivities and
yields.
1
5,6e–j
the synthesis of physiologically active pharmaceuticals,
2
3
agrochemicals and flavor ingredients. In response to
the increasing demand for optically active secondary
alcohols, a variety of powerful catalytic procedures have
been developed. One such method involves the use of
monotosylated 1,2-diamines or amino alcohols as
ligands for ruthenium(II) catalyzed asymmetric transfer
hydrogenation (ATH) of ketones, a method developed
R
isopropanol/KOH
or HCOOH/NEt3
OH
H
R
O
R1
R
1
4
by Noyori and co-workers.
Rh
H N
Rh
N
ð1Þ
Rh
H N
Cl
O
Cl
N
Cl
N
Ts
H
2
2
Since this discovery, a significant number of new ligands
Ts
Ph
5
–8
have been reported for the ATH with ruthenium(II).
Ph
*
Recently, iridium and rhodium-Cp based catalysts have
also been shown to be efficient catalysts in the ATH of
ketones. Xiao, Ikariya (1), Gavriilidis (2), and Wills
3
1
5
9
10
2
1
1
*
(
3)
have independently shown Cp –Rh-diamine/
aminoalcohol complexes (Eq. 1) to be excellent catalysts
Binuclear complexes are known to give higher activities
and selectivities as compared to the mononuclear com-
plexes. This effect has been attributed to cooperative
activation of the substrate and reactants, enhancing
in the ATH (isopropanol/KOH or HCOOH/NEt ) of
3
ketones, leading to high enantioselectivities and yields.
Further, Xiao and other researchers have reported the
ATH of ketones under environmentally friendlier condi-
tions. Using aqueous sodium formate, secondary alco-
12a
product formation through steric or electronic effects.
Base on this association, we prepared rhodium and
ruthenium bis(sulfonamide) complexes as part of our
1
2b,c
continuing interest in this subject.
The chiral C2-
Keywords: Bis(sulfonamide) ligands; RhCp* complex; Asymmetric
transfer hydrogenation; Ketones.
symmetric bis(sulfonamide) ligands were derived from
the readily available trans-cyclohexane-1,2-diamine and
*
*
Corresponding author. E-mail: somanatha@sundown.sdsu.edu
complexed to Cp Rh and [(arene)RuCl ] and their
2
2
0040-4039/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2006.09.141

8516
N. A. Cortez et al. / Tetrahedron Letters 47 (2006) 8515–8518
Table 1. Asymmetric transfer hydrogenation of ketones with chiral ligands 1, 4–7, and metal complexes
[
Cp*RhCl2]2, [(benzene)RuCl2]2
or [(cymene)RuCl2]2, L*
R
OH
H
R
R
configurationa
O
water/sodium formate,
or isopropanol/KOH
R1
R1
*
Ketone
Ligand
t (h)
[(Benzene)RuCl
2
]
2
[(Cymene)RuCl
2
]
2
[Cp RhCl
2
]
2
b
b
b
S/C
ee (%)
Yield (%)
S/C
ee (%)
Yield (%)
S/C
ee (%)
Yield (%)
c
4
0.25
5
16
16
16
12
24
100
100
100
25
25
100
—
92
99
89
67
50
94
—
100
99
84
25
15
92
—
O
d
4
33
33
25
25
—
—
91
88
76
73
—
—
70
95
92
87
—
—
33
33
—
—
—
33
94
94
—
—
—
89
89
74
—
—
—
97
e
4
5
6
f
7
g
7
O
c
4
0.25
5
100
100
92
66
76
71
d
4
33
81
47
33
85
54
O
c
4
0.25
5
100
100
90
89
98
87
d
4
33
33
78
66
87
98
33
33
89
91
94
99
Br
N
O
d
4
5
100
88
83
O
c
4
0.25
5
100
100
97
97
50
90
d
4
33
33
91
99
51
28
33
33
99
99
89
74
O
O
c
4
0.25
5
100
100
>99
99
86
60
d
4
c
4
0.25
5
100
100
>99
99
100
56
d
4
33
80
89
33
99
99
NO2
a
b
c
d
e
f
5,6l
Absolute configurations were assigned by comparing optical rotations with the literature values.
Measured by GC analysis of the acetylated alcohol with chiral capillary column b-DEX 120.
TM
4
2
2
0 °C using a mixture of water/sodium formate in air and without base.
5 °C using a mixture of isopropanol/KOH.
8 °C using a mixture of formic acid/triethylamine in DMF.
Ref. 10.
Ref. 15.
g
applications in the ATH of aromatic ketones in aqueous
and isopropanol solvents were studied. We coupled
commercially available 1,3-benzenesulfonyl chloride
and resolved trans-(1R,2R)-cyclohexane-1,2-diamine to
O S
SO2
1
3a,b
2
give C -symmetric bis(sulfonamide) 4 in 95% yield.
2
NH
HN
N
N
This ligand proved to be an excellent candidate in the
*
ATH with RhCp and Ru(arene)-based catalysts, giving
NH2
H N
NH
Ts
HN
2
high enantioselectivity and yields with a variety of aro-
matic ketones. Importantly, higher enantioselectivities,
shorter reaction times and similar yields were obtained
with aqueous sodium formate solution than with isopro-
panol (Table 1). With ligand 4, acetophenone ATH was
complete in 15 min with 92% enantioselectivity and
Ts
4
5
NH
HN
HN
Ts
N
NH2
H
1
00% yield in aqueous sodium formate, as compared
NH
Ts
7
to 5 h in isopropanol/KOH. When the acetophenone is
made bulkier: propiophenone, a-tetralone, and 2-aceto-
naphthone, the reaction in isopropanol/KOH was slug-
gish and reasonable yields were obtained after 5 h. On
Ts
6
Figure 1. Structurally similar ligands.

N. A. Cortez et al. / Tetrahedron Letters 47 (2006) 8515–8518
8517
O
O
Rh
H
S
O
H
N
N
H
H
N
H
N
S
O
Rh
H
O
O
*
Figure 2. anti-Cp Rh bis(sulfonamide)–acetophenone transition state in the pro-R conformation.
*
the other hand, using ligand 4-Cp Rh and aqueous
sodium formate, the reaction gave high enantioselectiv-
ity and >95% yield in 30 min. Interestingly, under the
same reaction conditions, 3-nitroacetophenone gave
In conclusion, our results show that bis(sulfonamide)
ligand 4-Cp Rh(III) complex is an excellent catalyst in
*
the ATH, delivering fast reactions, high enantioselectiv-
ities and high yields using sodium formate as the hydro-
gen source, providing an alternative method for
conducting ATH in a less costly, simpler and ‘greener’
manner.
>
99% enantioselectivity and 100% yield. This suggests
a probable role of the polar nitro group as a surfactant,
enhancing the reactivity. Aliphatic ketones gave much
lower enantioselectivities and yields. 4-Acetylpyridine
also gave low yield when used with ligand 4/aqueous
sodium formate, but (cymene)RuCl /isopropanol/
Acknowledgements
2
KOH gave 91% enantioselectivity and 99% yield. These
results show the versatility of the bis(sulfonamide)
ligand 4 with Rh(III) or Ru(II) as catalysts in the
ATH of ketones under aqueous sodium formate and
isopropanol/KOH conditions.
We gratefully acknowledge support for this project by
CONACYT (Grant 37827-E), COSNET (Grant
498.01P and 414.03P) and graduate scholarships from
CONACYT for N.A.C. and R.R.-A.
The importance of a primary amine functional group
and its location in the C -symmetric ligand 4 is evident,
Supplementary data
2
based on comparison to the results with the structurally
similar ligands 5 and 6 (Fig. 1). Ligand 4 gave 99% ee/
Experimental procedures and spectroscopic and analyti-
cal data of compounds are supplied in the supplementary
data. Supplementary data associated with this article
can be found, in the online version, at doi:10.1016/
j.tetlet.2006.09.141.
9
9% yield, while ligands 5 and 6 gave 67% ee/25% yield
and 50% ee/15% yield, respectively, in the ATH of
*
acetophenone using [Cp RhCl ] as the catalyst with a
2
2
significant depression of rates using isopropanol/KOH.
However, with [(benzene)RuCl ] , ligands 5 and 6 gave
2
2
7
6% ee/92% yield and 73% ee/87% yield, respectively.
Comparing ligand 4 to the monosulfonamide 7 with
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*
10
14
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2
2
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