Ruthenium(II) and rhodium(III) catalyzed asymmetric transfer hydrogenation (ATH) of acetophenone in isopropanol and in aqueous sodium formate using new chiral substituted aromatic monosulfonamide ligands derived from (1R,2R)-diaminocyclohexane

Article abstract of DOI:10.1016/j.tetasy.2008.04.036

A series of aromatic monosulfonamide ligands derived from (1R,2R)-diaminocyclohexane were synthesized with electron withdrawing and donating groups. These were complexed with Rh(Cp^*) or Ru(arene) and their catalytic efficiencies were compared in the ATH of acetophenone using sodium formate/water or isopropanol/KOH as the hydrogen source. Results suggest that substituents on the benzene ring of the sulfonamide have very little electronic impact on the enantioselectivity and mechanism of the reaction.

Full text of DOI:10.1016/j.tetasy.2008.04.036

Tetrahedron: Asymmetry 19 (2008) 1304–1309
Contents lists available at ScienceDirect
Tetrahedron: Asymmetry
journal homepage: www.elsevier.com/locate/tetasy
Ruthenium(II) and rhodium(III) catalyzed asymmetric transfer
hydrogenation (ATH) of acetophenone in isopropanol and in aqueous
sodium formate using new chiral substituted aromatic monosulfonamide
ligands derived from (1R,2R)-diaminocyclohexane
*
Norma A. Cortez, Gerardo Aguirre, Miguel Parra-Hake, Ratnasamy Somanathan
Centro de Graduados e Investigación, Instituto Tecnológico de Tijuana, Apartado Postal 1166, 22000 Tijuana, B.C., Mexico
a r t i c l e i n f o
a b s t r a c t
Article history:
A series of aromatic monosulfonamide ligands derived from (1R,2R)-diaminocyclohexane were synthe-
sized with electron withdrawing and donating groups. These were complexed with Rh(Cp*) or Ru(arene)
and their catalytic efficiencies were compared in the ATH of acetophenone using sodium formate/water
or isopropanol/KOH as the hydrogen source. Results suggest that substituents on the benzene ring of the
sulfonamide have very little electronic impact on the enantioselectivity and mechanism of the reaction.
Ó 2008 Elsevier Ltd. All rights reserved.
Received 23 January 2008
Accepted 24 April 2008
Available online 10 June 2008
1. Introduction
from trans-1,2-cyclohexanediamine. When used with Ru(II) and
Rh(III) complexes as catalysts in the ATH of ketones,6a the resulting
Noyori et al. reported the use of monotosylated diamines and
1,2-aminoalcohols as ligands for the ruthenium(II) catalyzed asym-
metric transfer hydrogenation (ATH) of ketones (Eq. 1).1 Since this
discovery, a significant number of new ligand-ruthenium(II), rho-
dium(III), and iridium(I) complexes have been used as catalysts
in the ATH of ketones.2–4
secondary alcohol was formed in >90% enantioselectivity and yield.
In the hope of discovering a more active and enantioselective cat-
alyst, we investigated the role of substituents on the ligand ArSO2
group, the influence of different hydrogen sources and the catalytic
ability of different metals [ruthenium(II) or rhodium(III)]. Noyori
et al. have shown that the ligand ArSO2 group is important for
maintaining reactivity; analogues with CF3SO2, C6H5CO, and CH3CO
were much less reactive.1 We found that replacing the ArSO2 group
with heterocyclic substituents on the sulfonamide, complexed to
RhIII(Cp*) led to excellent water soluble catalysts for ATH, resulting
in secondary alcohols with >90% enantioselectivity and yield.6b
Noyori’s work further shows that the effect of electron donor alkyl
groups (CH3, i-C3H7) on the g6-arene can also increase the catalytic
activity.1a A similar study with electron withdrawing groups on the
arene ring was reported by Süss-Fink et al. They showed that the
rate of ATH is dramatically decreased by an electron withdrawing
methyl carboxylate group on the arene ring in 2 (Fig. 1), compared
KOH
OH
OH
H
R
R
O
R1
R1
ð1Þ
Ru
Ru
Ar
SO2
Cl
H2N
Cl
O
N
H2N
Ph
Ph
Ph
Me
Recently, a number of researchers have shown that catalytic
O
R
ATH of ketones can be carried out efficiently using water under
noninert atmospheric conditions, increasing the potential indus-
trial applications.5 Water is inexpensive, readily available, and
environmentally benign, which has inspired the synthesis of novel
water soluble ligands for the ATH of ketones. We recently reported
a C2-symmetric water soluble bis(sulfonamide) ligand derived
OCH3
Ru
Ru
H
O2S NH
NH2
H
Ts
N
NH
Ts
N
NH
* Corresponding author.
E-mail addresses: miguelhake@yahoo.com (M. Parra-Hake), somanatha@
sundown.sdsu.edu (R. Somanathan).
1
2
3
Figure 1. Monosulfonamide ligand and metal complex.
0957-4166/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetasy.2008.04.036

N. A. Cortez et al. / Tetrahedron: Asymmetry 19 (2008) 1304–1309
1305
to electron donating methyl group in 1 (Fig. 1). When aqueous so-
Electron donors favored
dium formate is used as the hydrogen source,10 the latter is ten
times more reactive. A systematic study involving electron attrac-
tors and donors on the ArSO2, however, has not been reported. In a
recent study Mioskowski and Mohar reported the ATH of ketones
with electron withdrawing groups on the RuII (monosulfon-
amide-1,2-diphenylethane) complex.7 In this work, the authors
claimed that electron withdrawing groups such as, p-trifluoro-
methylbenzene, pentafluorobenzene, and nonaflate sulfonyl sub-
stituents enhance the enantioselectivity of the catalyst by
increasing the acidity of the N–H and decreasing the basicity on
the adjacent –NH2. They also reported a number of sulfonamide
ligands with electron withdrawing (NO2, Cl) and donating groups
(OCH3), which were used in the ATH of ketones.8a,b All of these
reactions were performed using HCOOH/Et3N as the hydrogen
source, and not isopropanol/KOH or aqueous sodium formate. Xiao
et al. demonstrated that the pH of the medium plays a major role in
the rate, TOF, and enantioselectivity, when HCOOH–NEt3 in water
is used as the hydrogen source in the ATH of ketones.9 Similarly,
studies have shown that electron attractors on the ketone
substrate also favor rate enhancement and selectivity in the
ATH.1–6,8c
These observations prompted us to explore the electronic role
of the substituents on the aryl sulfonamide of ligand 3 (R = electron
withdrawing or donating group). RuII(arene), RhIII(Cp*) complexes,
and electron donor or electron withdrawing groups in particular,
can relay the electronic effect through the sulfonamide bond to
the metal center (Fig. 2). Herein, we report the effect of these sub-
stituents on the rate, enantioselectivity, and yield in the ATH of
ketones, in the presence of aqueous sodium formate or isopropanol
as the hydrogen source. The secondary alcohol was obtained in
good yield and enantioselectivity.
electron attractors
favored
R
electron attractors
favored
H
R
Ru
O
S
N
N
H
O2
H
Figure 2. Substituent effects in the asymmetric transfer hydrogenation of ketones.
2. Results and discussion
To explore the electronic effects on ATH catalysts, we
synthesized a series of monosulfonamide ligands with electron
withdrawing and donating groups 3a–r derived from trans-
(1R,2R)-1,2-cyclohexanediamine using our previously reported
method (Scheme 1).11 The new ligands were fully characterized
as outlined in Section 4.
The ligands 3a–h were then reacted with [RuCl2(arene)]2 and
the resulting complexes were tested as catalysts in the ATH of
acetophenone with isopropanol/KOH as the hydrogen source. The
results are shown in Table 1.
Catalysts with both electron withdrawing and donating groups
exhibited high enantioselectivities (82–94%) (Table 1) in the ATH
of acetophenone, when RuII(benzene) was used as the catalyst
RSO2Cl
Et3N / NaOH
O2
S
H2N
NH2
NH2
NH
R
F
(1R,2R)-Tartrate salt
3
F
F
CF3
f)
e)
d)
j)
a)
g)
b)
c)
F
CF3
CF3
F
OCH3
R=
k)
Cl
CH3
l)
h)
i)
Cl
Cl
F
CN
I
Cl
OCH3
p)
q)
r)
n)
o)
m)
OCH3
H3CO
CN
OCH3
COCH3
OC4H9
Scheme 1. Chiral monosulfonamide ligands.

1306
N. A. Cortez et al. / Tetrahedron: Asymmetry 19 (2008) 1304–1309
Table 1
Table 2
Asymmetric transfer hydrogenation of acetophenone in isopropanol/KOH, catalyzed
Asymmetric transfer hydrogenation of acetophenone in isopropanol/KOH, catalyzed
by RuII(arene)L* complexes
by RhIII(Cp*)L* complexes
O
O
OH
OH
H
H
[RhCl2(Cp*)]2
3, Metal precursor
iPrOH, KOH
3
I = [RuCl2(η6-benzene)]2 II = [RuCl2(η6-p-cymene)]2
Entry
Ligand
t (h)
HCOONa, H2Oa
t (h)
i-PrOH, KOHb
Metal precursor:
Yield (%)
100
98
91
100
>99
100
86
>99
100
94
eec (%)
Yield (%)
96
99
—
94
95
92
79
52
67
96
98
eec (%)
Entry
Metal complex
Liganda
Yield (%)
eeb (%)
1
2
3
4
5
6
7
8
9
3a
3b
3d
3f
3g
3h
3j
3k
3l
3m
3p
0.45
0.45
0.45
0.45
0.45
0.45
0.45
0.45
0.45
0.45
0.45
99
5
5
5
5
5
5
5
5
5
5
5
92
96
—
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
I
II
I
I
II
I
I
I
I
II
I
II
I
II
I
I
I
II
I
I
I
I
3a
3a
3b
3c
3c
3d
3e
3f
3g
3g
3h
1h
3i
3i
3j
3k
3l
3l
3m
3n
3o
3p
3q
3r
89
67
66
73
46
76
56
78
90
57
73
32
49
31
57
76
77
46
43
86
53
64
68
49
88
86
86
84
88
82
88
85
85
93
84
81
82
86
84
88
85
65
85
84
84
89
91
94
94
90
91
94
92
93
95
92
94
96
93
97
95
96
96
77
100
97
10
13
>99
Absolute configurations are all (R), assigned by comparing the specific rotations
with the literature values.6a,b
a
40 °C using a mixture of water/sodium formate under air.
Reaction conditions: 25 °C using a mixture of isopropanol/KOH, S/C 33.
Measured by GC analysis of the acetylated alcohol with chiral capillary column
b
c
b-DEXTM 120.
In summary, comparing the results from Tables 1 and 2 sug-
gests that RhIII(Cp*)L* exhibits a better catalyst in terms of rate
and enantioselectivity than RuII(arene)L*-based catalysts.1k The
rate enhancement in aqueous sodium formate observed with a
variety of Rh(Cp*)L*-based catalysts,2e,f,5a–c highly favors the mech-
anism hypothesized by Ogo, Süss-Fink, and Xiao.5c,o,10,12 In their
proposed mechanism, the RhIII(Cp*) complex reacts with sodium
formate, to give a formato intermediate species, which dissolves
in the hydrophobic ketone, and the reaction then takes place in
the substrate, leading to the catalytically, active metal hydride
species. Since we did not see any dramatic difference in rate,
enantioselectivity, or yield with electron donor or attractor groups,
we presume these groups are too far removed from the metal
center to have any significant electronic impact in the transition
state.
I
I
Absolute configurations are all (R), assigned by comparing the specific rotations
with the literature values.6a,b
a
Reaction conditions: 25 °C using a mixture of isopropanol/KOH, 5 h, S/C 33.
Measured by GC analysis of the acetylated alcohol with chiral capillary column
b
b-DEXTM 120.
and isopropanol as the source of hydrogen. Overall, however,
ligands with electron withdrawing groups (entries 1, 3, 4, 6,7, 8,
9, 11, 16, 17, and 20) gave slightly better yields (66–90%), com-
pared to ligands with electron donating groups (49–68%) (entries
21–24). Replacing RuII(benzene) with RuII(cymene) under the same
reaction conditions resulted in high enantioselectivities, but in
lower yields, probably due to steric interference from the isopropyl
group of the cymene.
3. Conclusion
In conclusion, we have synthesized a series of ligands with elec-
tron withdrawing and electron donating SO2Ar groups, and tested
their Ru(II) and Rh(III) complexes in the ATH of acetophenone to
study the effects of substituents on both enantioselectivity and
yield. Results suggest that, when used with the RuII(arene) com-
plex and isopropanol as the hydrogen source, electron withdraw-
ing groups, such as –CF3, and –F on the ligand tend to give
slightly higher yields compared to other groups, with the enantio-
selectivity showing little or no change. Replacing the RuII(arene)
moiety with RhIII(Cp*) and using ligands 3a–r resulted in excellent
enantioselectivities and yields in both isopropanol and aqueous
sodium formate. In these cases, no electronic effects on the selec-
tivity were observed. In summary, our results suggest that substit-
uents on the benzene ring of the sulfonamide have minimal
electronic impact on the enantioselectivity on the ATH.
Interestingly, when RuII(arene) was replaced with RhIII(Cp*) in
the chiral ligand-complex, excellent enantioselectivities (90–97%)
and yields (90–99%) were obtained using identical reaction condi-
tions, irrespective of the nature of the substituent on the benzene-
sulfonamide ring (Table 2). When the source of hydrogen was
changed to aqueous sodium formate, the rate of the reaction
was enhanced, and high enantioselectivities (90–95%) and yields
(86–100%) were maintained. In addition, using Rh(Cp*)L* with
aqueous sodium formate, resulted in complete reduction in
45 min, compared to 5 h using isopropanol/KOH, representing a
sixfold rate enhancement. Our data suggest that with Rh(Cp*)L*
as catalyst, the effect of substituents on the enantioselectivity
and yields is minimal. As in our previous work,6b we found that
the use of a surfactant, along with sodium formate, had little or
no effect on the rate of ATH of ketones. The low yields observed
with some ligands in Table 1 may be attributed to the low solubil-
ity of the RuII(arene)L* complex in isopropanol; this is inferred
since the reaction mixture showed catalyst adhering to the walls
of the vessel, while with aqueous sodium formate a clear solution
was observed.
4. Experimental
Monosulfonamide ligands 3a–r were prepared by the reported
methodology,11 from the corresponding commercially available

N. A. Cortez et al. / Tetrahedron: Asymmetry 19 (2008) 1304–1309
1307
substituted benzenesulfonyl chloride with chiral (1R,2R)-1,2-
690 cmꢁ1
.
1H NMR (500 MHz, CDCl3):
d
7.93 (dt, J = 2.0,
cyclohexanediamine.
8.0 Hz, 1H), 7.63–7.51 (m, 2H), 7.16–7.32 (m, 2H), 2.89 (br s,
3H), 2.78 (dt, J = 4.1, 10.4 Hz, 1H), 2.45 (dt, J = 4.0, 10.9 Hz,
1H), 2.02–1.7 (m, 1H), 1.84–1.72 (m, 1H), 1.68–1.54 (m,2H),
4.1. Characterization of 3a–r
1.28–1.0 (m, 4H). 13C NMR (125 MHz, CDCl3):
d
158
4.1.1. N-(1R,2R)-(2-Aminocyclohexyl)-2-(trifluoromethyl)-
(JCF = 225.0 Hz), 134.4 (JCF = 6.0 Hz), 130.2, 128.5 (JCF = 7.5 Hz),
124.2 (JCF = 1.0 Hz), 116.4 (JCF = 10.1 Hz), 61.8, 54.5, 34.4, 32.2,
24.4, 24.9. Anal. Calcd for C12H17FN2O2S: C, 52.92; H, 6.29.
Found: C, 52.99; H, 6.35.
benzenesulfonamide 3a
25
Yellow oil (0.4 g, 97%); ½aꢀD ¼ ꢁ25 (c 0.13, CH2Cl2); IR (film):
mmax = 3593, 3367, 3064, 2935, 2857, 1853, 1587, 1445, 1346,
1178, 1028, 972, 774 cmꢁ1 1H NMR (500 MHz, CDCl3): d 8.3 (d,
;
J = 7.6 Hz, 1H), 7.85 (d, J = 8.9 Hz, 1H), 7.7–7.65 (m, 2H), 3.11 (br
s, 3H), 2.80 (dt, J = 4.03, 10.4 Hz, 1H), 2.48 (dt, J = 4.3, 10.9 Hz,
1H), 1.96–1.93 (m, 1H), 1.62–1.54 (m, 3H), 1.22–1.10 (m, 4H);
13C NMR (125 MHz, CDCl3): d 139.9, 132.6, 131.4, 128.4 (q,
JCF = 7.4 Hz), 127.4 (q, JCF = 32.9 Hz), 123.0 (q, JCF = 273.8 Hz), 60.6,
54.7, 34.9, 32.5, 24.9, 24.6; Anal. Calcd for C13H17F3N2O2S: C,
48.44, H, 5.32. Found: C, 48.48, H, 5.34.
4.1.6. N-(1R,2R)-(2-Aminocyclohexyl)-3-fluorobenzene-
sulfonamide 3f
25
Yellow crystals (0.42 g, 55%); mp 118–120 °C; ½aꢀD ¼ ꢁ64 (c
0.52, CH2Cl2); IR (KBr): mmax = 3348, 3259, 3083, 2933, 2854,
1590, 1476, 1328, 1223, 1156, 1083, 780, 679 cmꢁ1 1H NMR
;
(500 MHz, CDCl3): d 7.70 (ddd JH = 1.0, 1.3, JH–F = 7.8 Hz, 1H),
7.61 (ddd JH = 3.4, 1.7, JH–F = 8.2 Hz, 1H), 7.51 (ddd JH = 8.1, 7.9,
JH–F = 5.3 Hz, 1H), 7.27 (dddd, JH = 1.0, 1.7, JH–F = 7.3, JH = 1.0 Hz,
1H), 2.67 (dt, J = 4.3, 10.9 Hz, 1H), 2.58 (br s, 1H), 2.37 (dt,
J = 4.2, 11.0 Hz, 1H), 1.95–1.91 (m, 1H), 1.87–1.84 (m, 1H),
1.67–1.62 (m, 2H), 1.27–1.1 (m, 4H); 13C NMR (125 MHz,
4.1.2. N-(1R,2R)-(2-Aminocyclohexyl)-3-(trifluoromethyl)-
benzenesulfonamide 3b
25
Yellow crystals (0.45 g, 95%); mp 90–92 °C; ½aꢀD ¼ ꢁ34 (c 0.52,
CH2Cl2); IR (KBr): mmax = 3359, 3272, 3064, 2936, 2862, 1607, 1450,
CDCl3):
d 162.49 (d, JCF = 251.3 Hz), 143.1 (d, JCF = 36.7 Hz),
1326, 1162, 1130, 1070, 915, 806, 720, 696 cmꢁ1
;
1H NMR
130.8 (d, JCF = 7.6 Hz), 122.8 (d, JCF = 3.2 Hz), 119.7 (d,
JCF = 21.2 Hz) 114.6 (d, JCF = 24.2 Hz), 60.5, 54.8, 35.84, 32.7,
24.9, 24.8; Anal. Calcd for C12H17FN2O2S: C, 52.92; H, 6.29.
Found: C, 52.95; H, 6.30.
(500 MHz, CDCl3): d 8.20 (s, 1H), 8.14 (d, J = 7.9 Hz, 1H), 7.82 (d,
J = 7.8 Hz, 1H), 7.67 (t, J = 7.8 Hz, 1H), 3.40 (br s, 3H), 2.78 (td,
J = 4.0, 10.6 Hz, 1H), 2.54 (td, J = 4.0, 10.9 Hz, 1H), 1.95–1.65 (m,
4H), 1.59–1.10 (m, 4H); 13C NMR (125 MHz, CDCl3): d 142.6,
131.6 (q, JCF = 33.3 Hz), 129.9, 129.1 (q, JCF = 3.5 Hz), 124.4, 124.0
(q, JCF = 3.91 Hz), 123.3 (q, JCF = 273.0 Hz), 60.2, 54.64, 35.3, 32.5,
24.8, 24.7; Anal. Calcd for C13H17F3N2O2S: C, 48.44; H, 5.32. Found:
C, 48.49; H, 5.36.
4.1.7. N-(1R,2R)-(2-Aminocyclohexyl)-5-fluoro-2-methyl-
benzenesulfonamide 3g
25
White crystals (0.22 g, 98%); mp 123–125 °C; ½aꢀD ¼ ꢁ63 (c
0.41, CH2Cl2); IR (KBr): mmax = 3584, 3288, 3076, 2934, 2859,
1604, 1484, 1450, 1392, 1322, 1268, 1228, 1183, 1157, 1071,
4.1.3. N-(1R,2R)-(2-Aminocyclohexyl)-4-(trifluoromethyl)-
946, 885, 822, 747, 695 cmꢁ1 1H NMR (500 MHz, CDCl3): d 7.75
;
benzenesulfonamide 3c
(dd, J = 8.7 and J = 2.8 Hz, 1H), 7.29–7.26 (m, 1H), 7.15 (ddd,
J = 2.8, 5.3, 8.1 Hz, 1H), 2.64 (s, 3H), 2.61 (td, J = 4.0, 10.7 Hz, 1H),
2.37 (td, J = 4.2, 10.0 Hz, 1H), 1.94–1.87 (m, 2H), 1.66–1.60 (m,
2H), 1.25–1.10 (m, 4H); 13C NMR (125 MHz, CDCl3): d 160.2
(JCF = 248.0 Hz), 140.3 (JCF = 6.2 Hz), 133.1 (JCF = 7.2 Hz), 132.0
(JCF = 3.7 Hz), 119.5 (JCF = 20.7 Hz), 116.8 (JCF = 24.9 Hz), 60.38,
54.7, 36.1, 32.5, 24.9, 24.8, 19.6; Anal. Calcd for C13H19FN2O2S: C,
54.52; H, 6.69. Found: C, 54.56; H, 6.70.
25
White powder (0.55 g, 96%); mp 149–152 °C; ½aꢀD ¼ ꢁ57 (c
0.26, CH2Cl2); IR (KBr): mmax = 3352, 3287, 3057, 2933, 2860,
1607, 1449, 1403, 1324, 1164, 1131, 1062, 1017, 952, 915, 845,
711 cmꢁ1 1H NMR (500 MHz, CDCl3): d 8.04 (d, J = 8.18 Hz, 2H),
;
7.78, (d, J = 8.30 Hz, 2H), 2.76 (br s, 3H), 2.71 (dt, J = 4.0, 10.4 Hz,
1H), 2.43 (dt, J = 3.6, 10.6 Hz, 1H), 1.95–1.92 (m, 1H), 1.88–1.86
(m, 1H), 1.67–1.63 (m, 2H), 1.24–1.1 (m, 4H); 13C NMR
(125 MHz, CDCl3): d 144.7, 134.3 (q, JCF = 32.7 Hz), 127.6, 126.1
(q, JCF = 3.9 Hz), 123.1 (q, JCF = 273.0 Hz), 60.3, 54.8, 35.9, 32.7,
24.8; Anal. Calcd for C13H17F3N2O2S: C, 48.44; H, 5.32. Found: C,
48.45; H, 5.33.
4.1.8. N-((1R,2R)-2-Aminocyclohexyl)-4-iodobenzene-
sulfonamide 3h
25
Yellow crystals (0.61 g, 90%); mp 105–108 °C; ½aꢀD ¼ ꢁ39 (c
0.42, CH2Cl2); IR (KBr): mmax = 3749, 3358, 3264, 2931, 2854,
4.1.4. N-(1R,2R)-(2-Aminocyclohexyl)-2,3,4-(trifluoro)benzene-
1643, 1562, 1448, 1322, 1159, 1083, 815, 729 cmꢁ1 1H NMR
;
sulfonamide 3d
25
(500 MHz, CDCl3): d 7.86 (d, J = 8.6 Hz, 2H), 7.62 (d, J = 8.7 Hz,
2H), 2.65 (td, J = 4.3, 10.0 Hz, 1H), 2.39 (td, J = 4.0, 10.0 Hz, 1H),
1.95–1.89 (m, 1H), 1.83–1.79 (m, 1H), 1.66–1.60 (m, 2H), 1.30–
1.10 (m, 4H). 13C NMR (125 MHz, CDCl3): d 140.9, 138.3, 128.5,
99.7, 60.4, 54.7, 35.7, 32.6, 24.9, 24.8; Anal. Calcd for C12H17IN2O2S:
C, 37.90; H, 4.51. Found: C, 37.94; H, 4.55.
Yellow crystals (0.18 g, 85%); mp 122–124 °C; ½aꢀD ¼ ꢁ46 (c
0.56, CH2Cl2); IR (KBr): mmax = 3361, 3294, 3063, 2934, 2860,
1607, 1504, 1466, 1336, 1244, 1161, 1076, 1037, 949, 819,
736, 660, 610 cmꢁ1
;
1H NMR (500 MHz, CDCl3): d 7.78–7.64
(m, 2H), 7.18–7.04 (m, 2H), 3.35 (s, 3H), 2.88–2.7 (m, 1H),
2.58–2.42 (m, 1H), 2.15–1.92 (m, 1H), 1.88–1.72 (m, 1H),
1.69–1.60 (m, 2H), 1.32–1.1 (m, 4H); 13C NMR (125 MHz,
4.1.9. N-(1R,2R)-2-(Aminocyclohexyl)-4-chloro-benzene-
CDCl3):
d 154 (dd, JCF = 13.5, 260.1 Hz), 149 (dd, JCF = 10.0,
sulfonamide 3i
262.2 Hz), 140 (dt, JCF = 14.3, 256.6 Hz), 127.1 (d, JCF = 12.0 Hz),
124.2 (dd, JCF = 3.7, 18.0 Hz), 60.4, 54.6, 35.3, 32.5, 24.8, 24.7.
Anal. Calcd for C12H15F3N2O2S: C, 46.75; H, 4.87. Found: C,
46.80; H, 4.93.
25
Yellow oil (0.22 g, 88%); ½aꢀD ¼ ꢁ22 (c 0.31, CH2Cl2); IR (film):
mmax = 3348, 3287, 3065, 2933, 2858, 1585, 1476, 1448, 1393,
1325, 1161, 1088, 1013, 942, 915, 829, 752, 705, 623 cmꢁ1 1H
;
NMR (200 MHz, CDCl3):
d 7.87, (d, J = 8.6 Hz, 2H), 7.47 (d,
J = 8.6 Hz, 2H), 3.6 (br d, 3H), 2.76 (dt, J = 4.0, 10.2 Hz, 1H), 2.55
(dt, J = 4.2, 10.2 Hz, 1H), 1.96–1.94 (m, 1H), 1.74–1.5 (m, 3H),
1.22–1.1 (m, 4H); 13C NMR (125 MHz, CDCl3): d 139.9, 138.7,
129.2, 128.4, 60, 54.4, 34.7, 32.3, 25.2, 24.8, 24.5. Anal. Calcd for
4.1.5. N-(1R,2R)-(2-Aminocyclohexyl)-2-fluorobenzene-
sulfonamide 3e
25
Yellow solid (0.15 g, 75%); mp 110–112 °C; ½aꢀD ¼ ꢁ30 (c
0.66, CH2Cl2); IR (KBr): mmax = 3348, 3289, 3057, 2931, 2859,
C12H17ClN2O2S: C, 49.91; H, 5.93. Found: C, 49.98; H, 6.01.
1597, 1474, 1448, 1329, 1264, 1163, 1078, 946, 827, 765,

1308
N. A. Cortez et al. / Tetrahedron: Asymmetry 19 (2008) 1304–1309
4.1.10. N-(1R,2R)-2-(Aminocyclohexyl)-2,5-dichlorobenzene-
1145, 1066, 1094, 832 cmꢁ1 1H NMR (500 MHz, CDCl3):
; d
sulfonamide 3j
7.80 (d, J = 8.9 Hz, 2H), 6.95 (d, J = 8.9 Hz, 2H), 4.01 (t,
J = 6.5 Hz, 2H), 2.63 (dt, J = 4.0, 10.6 Hz, 1H), 2.37 (dt, J = 4.1,
11.1 Hz, 1H), 1.94–1.90 (m, 1H), 1.82–1.76 (m, 3H), 1.65–1.60
(m, 2H), 1.53–1.46 (m, 2H), 1.24–1.02 (m, 4H), 0.98 (t,
J = 17.3 Hz, 3H); 13C NMR (125 MHz, CDCl3): d 162.4, 132.3,
129.1, 114.6, 68.2, 60.4, 54.9, 35.5, 32.7, 31.1, 25.0, 24.8, 19.2,
13.8.; Anal. Calcd for C16H26N2O3S: C, 58.87; H, 8.03. Found:
C, 58.82, H, 7.99.
25
Pale brown solid (0.48 g, 94%); mp 146–149 °C; ½aꢀD ¼ ꢁ29 (c
0.11, CH2Cl2); IR (KBr): mmax = 3350, 3290, 3085, 2932, 2854,
1643, 1450, 1330, 1240, 1163, 1100, 1039, 947, 896, 824 cmꢁ1
;
1H NMR (500 MHz, CDCl3): d 8.1 (d, J = 2.2 Hz, 1H), 7.48–7.44 (m,
2H), 2.71 (dt, J = 4.0, 10.4 Hz, 1H), 2.45 (dt, J = 4.0, 10.9 Hz, 1H),
1.96–1.94 (m, 1H), 1.81–1.79 (m, 1H), 1.67–1.61 (m, 2H), 1.27–
1.09 (m, 4H); 13C NMR (125 MHz, CDCl3): d 139.7, 133.5, 13.4,
132.7, 130.9, 129.8, 60.9, 54.9, 35.3, 32.4, 24.8, 24.6; Anal. Calcd
for C12H16Cl2N2O2S: C, 44.59, H, 4.99. Found: C, 44.64, H, 5.04.
4.1.16. N-(1R,2R)-(2-Aminocyclohexyl)-2,5-dimethoxy-
benzenesulfonamide 3p
25
4.1.11. N-(1R,2R)-(2-Aminocyclohexyl)-4-chloro-2-methoxy-
Pale brown crystals (0.32 g, 99%); mp 108–110 °C; ½aꢀD ¼ ꢁ15 (c
benzenesulfonamide 3k
0.81, CH2Cl2); IR (KBr): mmax = 3294, 3071, 2933, 2856, 1583, 1493,
25
Pale brown solid (0.40 g, 91%); mp 159–161 °C; ½aꢀD ¼ ꢁ33 (c
1440, 1322, 1274, 1222, 1157, 1041, 816, 736, 695 cmꢁ1 1H NMR
;
0.57, CH2Cl2); IR (KBr): mmax = 3361, 3286, 2930, 2855, 1588,
(500 MHz, CDCl3): d 7.45 (d, J = 2.4 Hz, H), 7.07 (dd, J = 4.0, 8.8 Hz,
1H), 6.96 (d, J = 8.0 Hz, H), 3.93 (s, 3H), 3.82 (s, 3H), 2.64 (dt, J = 4.0,
11.0 Hz, 1H), 2.38 (dt, J = 4.0, 10.9 Hz, 1H), 1.98–1.90 (m, 2H), 1.78–
1.60 (m, 2H), 1.24–1.04 (m, 4H); 13C NMR (125 MHz, CDCl3): d
153.1, 150.1, 129.2, 120.0, 114.4, 113.6, 61.0, 56.7, 55.0, 35.0,
32.1, 24.9, 24.7. Anal. Calcd. for C14H22N2O4S: C, 53.48; H, 7.05.
Found: C, 52.91; H, 7.00.
1477, 1389, 1322, 1269, 1158, 1066, 1017, 895, 812, 721 cmꢁ1
;
1H NMR (500 MHz, CDCl3): d 7.90 (d, J = 2.8 Hz, 1H), 7.48 (dd,
J = 2.6, 8.8 Hz, 1H), 6.97 (d, J = 8.8 Hz, 1H), 3.97 (s, 3H), 2.62 (dt,
J = 4.1, 10.0 Hz, 1H), 2.38 (dt, J = 4.0, 10.4 Hz, 1H), 1.96–1.90 (m,
1H), 1.82–1.72 (m, 1H), 1.67–1.58 (m, 2H), 1.25–1.02 (m, 4H);
13C NMR (125 MHz, CDCl3): d 154.7, 133.9, 129.7, 125.7, 113.4,
60.9, 56.6, 54.9, 35.3, 32.3, 24.9, 24.7; Anal. Calcd for C13H19
ClN2O3S: C, 48.97; H, 6.01. Found: C, 49.00; H, 6.04.
-
4.1.17. N-(1R,2R)-(2-Aminocyclohexyl)-4-methoxy-benzene-
sulfonamide 3q
25
4.1.12. N-(1R,2R)-(2-Aminocyclohexyl)-4-cyano-benzene-
Yellow solid (0.2 g, 80%); mp 96–98 °C; ½aꢀD ¼ ꢁ57 (c 0.26,
sulfonamide 3l
CH2Cl2); IR(KBr): mmax = 3356, 3281, 3048, 2931, 2857, 1695,
25
1497, 1321, 1258, 1154, 1093, 1026, 912, 834, 734, 668 cmꢁ1 1H
;
Yellow crystals (0.22 g, 91%); mp 126–128 °C; ½aꢀD ¼ ꢁ47 (c
0.45, CH2Cl2); IR (KBr): mmax = 3356, 3268, 3057, 2934, 2859,
NMR (500 MHz, CDCl3): d 7.83 (d, J = 9.2 Hz, 2H), 6.97 (d, J = 9 Hz,
2H), 3.7 (s, 3H), 2.65 (dt, J = 4.5, 10.4 Hz, 1H), 2.40 (dt, J = 4.0,
10.7 Hz, 1H), 1.95–1.91 (m, 1H), 1.83–1.81 (m, 1H), 1.66–1.60 (m,
2H), 1.25–1.10 (m, 4H); 13C NMR (125 MHz, CDCl3): d 162.8,
132.6, 129.2, 114.2, 60.2, 55.6, 54.8, 35.5, 32.7, 25, 24.8; Anal. Calcd
for C13H20N2O3S: C, 54.91; H, 7.09. Found: C, 54.95, H, 7.12.
2232, 1589, 1449, 1330, 1161, 1091, 836, 735 cmꢁ1 1H NMR
;
(500 MHz, CDCl3): d 8.05 (d, J = 8.2 Hz, 2H), 7.76 (d, J = 8.2 Hz,
2H), 3.16 (br s, 3H), 2.74–2.70 (m, 1H), 2.52–2.4 (m, 1H), 1.97–
1.92 (m, 1H), 1.82–1.78 (m, 1H), 1.65–1.62 (m, 2H), 1.26–1.16
(m, 4H); 13C NMR (125 MHz, CDCl3): d 144.8, 127.5, 126.3, 126.2,
60.2, 54.7, 35.6, 32.6, 24.8, 24.7; Anal. Calcd for C13H17N3O2S: C,
55.89; H, 6.13. Found: C, 55.90; H, 6.15.
4.1.18. N-(1R,2R)-(2-Aminocyclohexyl)-3-methoxy-benzene-
sulfonamide 3r
4.1.13. N-(1R,2R)-(2-Aminocyclohexyl)-3-cyano-benzene-
25
Light yellow solid (0.18 g, 80%); ½aꢀD ¼ ꢁ85 (c 1.0, CH2Cl2); IR
sulfonamide 3m
(KBr); mmax = 3358, 3288, 3068, 2931, 2858, 1596, 1481, 1315,
25
Yellow crystals (0.25 g, 88%); mp 104–106 °C; ½aꢀD ¼ ꢁ63 (c
1246, 1157, 1075, 1039, 944, 788, 735, 690 cmꢁ1
;
1H NMR
0.42, CH2Cl2); IR (KBr): mmax = 3339, 3277, 3066, 2932, 2854,
(500 MHz, CDCl3): d 7.53–7.36 (m, 3H), 7.12–7.01 (m, 1H), 3.83
(s, 3H), 3.3 (br s, 3H), 2.72 (dt, J = 3.9, 10.0 Hz, 1H), 2.47 (dt,
J = 4.0, 10.2 Hz, 1H), 1.98–1.92 (m, 1H), 1.70–1.53 (m, 3H), 1.28–
1.06 (m, 4H); 13C NMR (125 MHz, CDCl3): d 159.8, 142.3, 130.0,
118.9, 118.7, 111.6, 60.3, 55.6, 54.5, 34.8, 32.3, 24.8, 24.5; Anal.
Calcd for C13H20N2O3S: C, 54.91; H, 7.09. Found: C, 54.88; H,
7.05.
2233, 1581, 1445, 1328, 1156, 1089, 685 cmꢁ1
;
1H NMR
(500 MHz, CDCl3): d 8.22 (sa, 1H), 8.15 (d, J = 8.0 Hz, 1H), 7.86 (d,
J = 8.0 Hz, 1H), 7.67 (t, J = 8.8, 8.0 Hz, 1H), 2.84 (br s, 3H), 2.72
(dt, J = 4.0, 10.1 Hz, 1H), 2.40 (dt, J = 3.6, 10.2 Hz, 1H), 2.0–1.82
(m, 2H), 1.56–1.74 (m, 2H), 1.38–1.04 (m, 4H); 13C NMR
(125 MHz, CDCl3): d 153.5, 150.1, 129.1, 120, 114.4, 113.6, 61,
56.7, 56, 55, 35, 32.1, 24.9, 24.7; Anal. Calcd for C13H17N3O2S: C,
55.89; H, 6.13. Found: C, 55.84; H, 6.09.
4.2. Procedure for catalytic reactions
4.1.14. 4-Acetyl-N-(1R,2R)-(2-aminocyclohexyl)benzene-
4.2.1. General procedure for the asymmetric transfer
hydrogenation of ketones in isopropanol
sulfonamide 3n
25
Pale brown solid (0.41 g, 78%); 200–203 °C dec; ½aꢀD ¼ ꢁ28 (c
A mixture of the metal precursor [RuCl2(arene)]2 or [RhCl2Cp*]2
(0.0039 mmol), and the chiral ligand (0.0075 mmol) in freshly dis-
tilled 2-propanol was stirred at 80 °C for 30 min under an argon
atmosphere. A solution of potassium hydroxide in 2-propanol
was stirred at 50 °C for 30 min. Then the prochiral ketone in 2-pro-
panol was added to the catalyst solution, followed by KOH solu-
tion, and stirred at room temperature for the time indicated in
Table 1 for each individual reaction. Water was added to the reac-
tion mixture and then extracted with dichloromethane
(3 ꢂ 10 mL). The dichloromethane layers were combined, dried
over anhydrous MgSO4, filtered, and concentrated under reduced
pressure, and passed through a short silica gel path. The residue
containing the alcohol was acetylated using acetic anhydride.
0.22, CH2Cl2); IR (KBr): 3350, 3287, 3056, 2933, 2856, 1635,
1587, 1450, 1393, 1325, 1159, 1091, 910, 835, 735 cmꢁ1 1H
;
NMR (500 MHz, CDCl3):
d 7.57 (d, J = 8.4 Hz, 2H), 7.33 (d,
J = 8.24 Hz, 2H), 3.57–3.34 (m, 4H), 2.31–2.09 (m, 1H), 2.05 (s,
3H), 1.80–1.65 (m, 2H), 1.56–1.35 (m, 2H), 1.18–1.0 (m, 4H); 13C
NMR (125 MHz, CDCl3): d 160.9, 142.9, 142.2,125.9, 124.8, 63.03,
58.52, 34.89, 32.08, 24.7, 23.3. Anal. Calcd for C14H20N2O3S: C,
56.73; H, 6.80. Found: C, 56.66; H, 6.76.
4.1.15. N-(1R,2R)-(2-Aminocyclohexyl)-4-butoxy-benzene-
sulfonamide 3o
25
Yellow oil (0.29 g, 94%); ½aꢀD ¼ ꢁ33 (c 0.44, CH2Cl2); IR
(film): mmax = 3352, 3258, 2920, 2854, 1589, 1495, 1299, 1253,

N. A. Cortez et al. / Tetrahedron: Asymmetry 19 (2008) 1304–1309
1309
M. Tetrahedron: Asymmetry 1998, 9, 2971–2974; (d) Schwink, L.; Ireland, T.;
Püntener, K.; Knochel, P. Tetrahedron: Asymmetry 1998, 9, 1143–1163; (e) Petra,
D. G. I.; Kamer, P. C. J.; van Leeuwen, P. W. N. M.; Goubitz, K.; van Loon, A. M.;
de Vries, J. G.; Schoemaker, H. E. Eur. J. Inorg. Chem. 1999, 2335–2341; (f) Frost,
C. G.; Mendonca, P. Tetrahedron: Asymmetry 2000, 11, 1845–1848; (g) Peach, P.;
Cross, D. J.; Kenny, J. A.; Mann, I.; Houson, I.; Campbell, L.; Walsgrove, T.; Wills,
M. Tetrahedron 2006, 62, 1864–1876; (h) Västilä, P.; Wettergreen, J.; Adolfsson,
H. Chem. Commun. 2005, 4039–4041; (i) Cheung, F. K.; Hayes, A. M.;
Hannedouche, J.; Yim, A. S. Y.; Wills, M. J. Org. Chem. 2005, 70, 3188–3197;
(j) Mao, J.; Wan, B.; Wu, F.; Lu, S. Tetrahedron Lett. 2005, 46, 7341–7344.
4. (a) Gao, J. X.; Zhang, H.; Yi, X. D.; Xu, P. P.; Tang, C. L.; Wan, H. L.; Tsai, K. R.;
Ikariya, T. Chirality 2000, 12, 383–388; (b) Ohta, T.; Nakahara, S.; Shigemura, Y.;
Hattori, K.; Furukawa, I. Appl. Organomet. Chem. 2001, 15, 699–709; (c) Rhyoo,
H. Y.; Yoon, Y.; Park, Y. A. H. J.; Chung, Y. K. Tetrahedron Lett. 2001, 45, 5045–
5048; (d) Faller, J. W.; Lavoie, A. R. Organometallics 2001, 20, 5245–5247; (e)
Brunner, H.; Henning, F.; Weber, M. Tetrahedron: Asymmetry 2002, 13, 37–42;
(f) Reetz, M. T.; Li, X. J. Am. Chem. Soc. 2006, 128, 1044–1045; (g) Guo, R.; Elpelt,
C.; Chen, X.; Song, D.; Morris, R. H. Chem. Commun. 2005, 3050–3052; (h) Yim,
A. S. Y.; Wills, M. Tetrahedron 2005, 61, 7994–8004; (i) Cabou, J.; Brocard, J.;
Pélinski, L. Tetrahedron Lett. 2005, 46, 1185–1188; (j) Tan, D.-M.; Chan, K. S.
Tetrahedron Lett. 2005, 46, 503–505; (k) Sortais, J.-B.; Ritleng, V.; Voelklin, A.;
Holuigue, A.; Smail, H.; Barloy, L.; Sirlin, C.; Verzijl, G. K. M.; Boogers, J. A. F.; de
Vries, A. H.; de Vries, J. G.; Pfeffer, M. Org. Lett. 2005, 7, 1247–1250.
5. (a) Wu, X.; Vinci, D.; Ikariya, T.; Xiao, J. Chem. Commun. 2005, 4447–4449; (b)
Li, X.; Wu, X.; Chen, W.; Hancock, F. E.; King, F.; Xiao, J. Org. Lett. 2004, 6, 3321–
3324; (c) Wu, X.; Li, X.; Hems, W.; King, F.; Xiao, J. Org. Biomol. Chem. 2004, 2,
1818–1821; (d) Ma, Y.; Liu, H.; Chen, L.; Cui, X.; Zhu, J.; Deng, J. Org. Lett. 2003,
5, 2103–2106; (e) Himeda, Y.; Onozawa-Komatsuzaki, N.; Sugihara, H.;
Arakawa, H.; Kasuga, K. J. Mol. Catal. A: Chem. 2003, 195, 95–100; (f) Rhyoo,
H. Y.; Park, H.-J.; Suh, W. H.; Chung, Y. K. Tetrahedron Lett. 2002, 43, 269–272;
(g) Thorp, T.; Blacker, J.; Brown, S. M.; Crosby, J.; Fitzjohn, S.; Muxworthy, J. P.;
Williams, J. M. Tetrahedron Lett. 2001, 42, 4041–4043; (h) Bubert, C.; Blacker, J.;
Brown, S. M.; Crosby, J.; Fitzjohn, S.; Muxworthy, J. P.; Thorpe, T.; Williams, J. M.
Tetrahedron Lett. 2001, 42, 4037–4039; (i) Dwars, T.; Oehme, G. Adv. Synth.
Catal. 2002, 344, 239–260; (j) Rhyoo, H. Y.; Park, H.-J.; Chung, Y. K. Chem.
Commun. 2001, 2064–2065; (k) Jiang, L.; Wu, T.-F.; Chen, Y.-C.; Zhu, J.; Deng, J.-
G. Org. Biomol. Chem. 2006, 4, 3319–3324; (l) Li, B.-Z.; Chen, J.-S.; Dong, Z.-R.; Li,
Y.-Y.; Li, Q.-B.; Gao, J.-X. J. Mol. Catal. A: Chem. 2006, 258, 113–117; (m)
Matharu, D. S.; Morris, D. J.; Clarkson, G. J.; Wills, M. Chem. Commun. 2006,
3232–3234; (n) Grassert, I.; Kovács, J.; Fuhrmann, H.; Oehme, G. Adv. Synth.
Catal. 2002, 344, 312–318; (o) Koike, T.; Ikariya, T. Adv. Synth. Catal. 2004, 346,
37–41; Wu, X.; Xiao, J. Chem. Commun. 2007, 2449–2466.
4.2.2. General procedure for the asymmetric transfer
hydrogenation of ketones in water (S/C 100)
A mixture of the metal precursor [RhCl2Cp*]2 (0.0039 mmol)
and the chiral ligand (0.0075 mmol) was heated in water (2 mL)
at 40 °C for 1 h in air without a base. HCOONa (5.7 mmol) and
the substrate were subsequently added (1.14 mmol). The reaction
mixture was stirred at 40 °C in air for the time indicated in Table
1 for each individual reaction. The reaction mixture was extracted
with ether (3 ꢂ 10 mL). The ether layers were combined, dried over
anhydrous MgSO4, filtered, and concentrated under reduced pres-
sure. The residue containing the alcohol was acetylated using ace-
tic anhydride.
Acknowledgments
We gratefully acknowledge support for this project by CONA-
CYT (Grant 37827-E), COSNET (Grants 498.01P and 414.03P),
DGEST (grant 310.06-P), and graduate scholarship from CONACYT
for N.A.C.
References
1. (a) Noyori, R.; Hashiguchi, S. Acc. Chem. Res. 1997, 30, 97–102; (b) Hamada, T.;
Torri, T.; Onishi, T.; Izawa, K.; Ikariya, T. J. Org. Chem. 2004, 69, 7391–7394; (c)
Hamada, T.; Torii, T.; Onishi, T.; Izawa, K.; Ikariya, K. T. Tetrahedron 2004, 60,
7411–7417; (d) Hamada, T.; Torii, T.; Izawa, K.; Noyori, R.; Ikariya, T. Org. Lett.
2002, 4, 4373–4376; (e) Watanabe, M.; Murata, M.; Ikariya, T. J. Org. Chem.
2002, 67, 1712–1715; (f) Koike, T.; Murata, K.; Ikariya, T. Org. Lett. 2000, 2,
3833–3836; (g) Noyori, R. Adv. Synth. Catal. 2003, 345, 15–32; (h) Yamakawa,
M.; Ito, H.; Noyori, R. J. Am. Chem. Soc. 2000, 122, 1466–1478; (i) Noyori, R.;
Ohkuma, T. Angew. Chem., Int. Ed. 2001, 40, 40–73; (j) Noyori, R.; Yamakawa,
M.; Hashiguchi, S. J. Org. Chem. 2001, 66, 7931–7944; (k) Murata, K.; Ikariya, T. J.
Org. Chem. 1999, 64, 2186–2187; (l) Ikariya, T.; Murata, K.; Noyori, R. Org.
Biomol. Chem. 2006, 4, 393–406.
2. (a) Chen, Y. C.; Wu, T. F.; Deng, J. G.; Liu, H.; Jiang, Y. Z.; Choi, M. C. K.; Chan, A. C.
S. Chem. Commun. 2001, 1488–1489; (b) Ros, A.; Magriz, A.; Dietrich, H.;
Fernández, R.; Alvarez, E.; Lassaletta, J. M. Org. Lett. 2006, 8, 127–130; (c)
Fukuzawa, S.-I.; Suzuki, T. Eur. J. Org. Chem. 2006, 1012–1016; (d) Kawasaki, I.;
Tsunoda, K.; Tsuji, T.; Yamaguchi, T.; Shibata, H.; Uchida, N.; Yamashita, M.;
Ohta, S. Chem. Commun. 2005, 2134–2136; (e) Wang, F.; Liu Cun, L.; Zhu, J.;
Deng, J.; Jiang, Y. J. Org. Chem. 2005, 70, 9424–9429; (f) Liu, P. N.; Deng, J. G.; Tu,
Y. Q.; Wang, S. H. Chem. Commun. 2004, 2070–2071; (g) Schlatter, A.; Kundu, M.
K.; Woggon, W.-D. Angew. Chem., Int. Ed. 2004, 43, 6731–6734; (h) Xing, Y.;
Chen, J.-S.; Dong, Z.-R.; Li, Y.-Y.; Gao, J.-X. Tetrahedron Lett. 2006, 47, 4501–
4508; (i) Matsunga, H.; Ishizuka, T.; Kunieda, T. Tetrahedron Lett. 2005, 46,
3645–3648; (j) Matharu, D. S.; Morris, D. J.; Kawamoto, A. M.; Clarkson, G. J.;
Wills, M. Org. Lett. 2005, 7, 5489–5491; (k) Cheung, F. K.; Hayes, A. M.; Morris,
D. J.; Wills, M. Org. Biomol. Chem. 2007, 5, 1093–1103.
6. (a) Cortez, N. A.; Aguirre, G.; Parra-Hake, M.; Cole, T.; Somanathan, R.
Tetrahedron Lett. 2006, 47, 8515–8518; (b) Cortez, N. A.; Aguirre, G.; Parra-
Hake, M.; Somanathan, R. Tetrahedron Lett. 2007, 48, 4335–4338.
7. Mohar, B.; Valleix, A.; Desmurs, J.-R.; Felemez, M.; Wagner, A.; Mioskowski, C.
Chem. Commun. 2001, 2572–2573.
8. (a) Šterk, D.; Stephan, M. S.; Mohar, B. Tetrahedron Lett. 2004, 45, 535–537; (b)
Šterk, D.; Stephan, M. S.; Mohar, B. Tetrahedron: Asymmetry 2002, 13, 2605–
2608; (c) Šterk, D.; Stephan, M. S.; Mohar, B. Org. Lett. 2006, 8, 5935–
5938.
9. Wu, X.; Li, X.; King, F.; Xiao, J. Angew. Chem., Int. Ed. 2005, 44, 3407–3411.
10. Canivet, J.; Labat, G.; Stoeckli-Evans, H.; Süss-Fink, G. Eur. J. Inorg. Chem. 2005,
4493–4500.
11. Ng, K.; Somanathan, R.; Walsh, P. J. Tetrahedron: Asymmetry 2001, 12, 1719–
1722.
12. Ogo, S.; Abura, T.; Watanabe, Y. Organometallics 2002, 21, 2964–2969.
3. (a) Palmer, M. J.; Walsgrove, T.; Wills, M. J. Org. Chem. 1997, 62, 5226–5228; (b)
Alonso, D. A.; Guijarro, D.; Pinho, P.; Temme, O.; Andersson, P. G. J. Org. Chem.
1998, 63, 2749–2751; (c) Everaere, K.; Carpentier, J.-F.; Mortreux, A.; Bulliard,