Efficient asymmetric transfer hydrogenation of N-sulfonylimines on water

Article abstract of DOI:10.1016/j.tet.2013.05.064

An efficient and green approach for synthesis of optically active amines was developed via asymmetric transfer hydrogenation of N-sulfonyl ketimines catalyzed by the chiral and lipophilic rhodium-amido complex on water. Higher reactivity and enantioselect

Full text of DOI:10.1016/j.tet.2013.05.064

Tetrahedron 69 (2013) 6500e6506
Contents lists available at SciVerse ScienceDirect
Tetrahedron
journal homepage: www.elsevier.com/locate/tet
Efficient asymmetric transfer hydrogenation of N-sulfonylimines on
water
,
,
Lei Wang a c, Qi Zhou a c, Chuanhua Qu b, Qiwei Wang a, Linfeng Cun a, Jin Zhu a,
Jingen Deng a
,b,
*
a Key Laboratory of Asymmetric Synthesis & Chirotechnology of Sichuan Province, Chengdu Institute of Organic Chemistry, Chinese Academy of Sciences,
Chengdu, 610041, China
b Key Laboratory of Drug-Targeting of Education Ministry, Department of Medicinal Chemistry, West China School of Pharmacy, Sichuan University,
Chengdu, 610041, China
c The University of Chinese Academy of Sciences, Beijing, 100049, China
a r t i c l e i n f o
a b s t r a c t
Article history:
An efficient and green approach for synthesis of optically active amines was developed via asymmetric
transfer hydrogenation of N-sulfonyl ketimines catalyzed by the chiral and lipophilic rhodium-amido
complex on water. Higher reactivity and enantioselectivity were observed on the hydrogenation of the
solid substrate in an aqueous suspension compared to organic homogeneous phases. In the heteroge-
neous aqueous reaction, the reactivity depends on stirring speed and the recrystallized conditions of the
solid substrate.
Received 25 December 2012
Received in revised form 2 May 2013
Accepted 17 May 2013
Available online 28 May 2013
Keywords:
Ó 2013 Elsevier Ltd. All rights reserved.
Asymmetric transfer hydrogenation
Rhodium-amido complex
N-Sulfonyl ketimines
On water
1. Introduction
significantly higher reactivity,5 better chemoselectivity,6 and
regioselectivity,7 even higher enantioselectivity in the presence of
Presently, there is an intense interest in developing greener
chemistry for both laboratory and industrial applications. Re-
placement of organic solvents and especially chlorinated ones, with
greener ones is an important question for green chemistry.1 Water
is an attractive solvent for organic reactions because it is low cost,
safer to operate (non-flammable, non-toxic), and negligible impact
for the environment, and often shows interesting features like
enhanced reaction rate and selectivity (chemo-, regio-, stereo-,
even enantio-) if compared to organic media used in the same
process.2,3 As a result, great effort has been inspired to this field and
effective aqueous phase organic reactions have been documented.3
Among the aqueous catalytic approaches, asymmetric transfer hy-
drogenation (ATH) presents several advantages.4e8 The hydrogen
source, sodium formate (HCO2Na), is water-soluble and produces
the volatile co-product, carbon dioxide (CO2), which facilitates the
purification of the reduction products. The transition metal-
catalysts are insensitive to water as well as air oxidation, and sta-
ble in aqueous media.5b Particularly, in the comparison to organic
media, the ATH of ketones was performed in water to exhibit
surfactants.3f,6,8a
Optically active amines are important building blocks for bio-
active molecules and natural products, and have also extensively
been used as chiral auxiliaries and resolving agents in asymmetric
synthesis.9 Asymmetric reduction of ketimines is the most direct
and efficient method for preparation of chiral amines.9,10 Because
ATH possesses an operational simplicity and avoids the handling of
hazardous chemicals such as metallic hydrides or molecular hy-
drogen, recently, the ATH has widely been applied to reduction of
ketimines.11 However, the water-mediated ATH was rarely in-
vestigated probably because of the instability of ketimines, espe-
cially acyclic ketimines.4 Previous works indicated that ATH of
cyclic ketimines and even ketiminiums was well performed in
aqueous media, but the acyclic N-sulfonylimines did not work un-
der the same conditions.8 Since optically active
a-arylethylamines
are key constituents of chiral drugs and bioactive compounds, such
as Rivastigmine, Cinacalcet, and oxazolone derivative (Fig. 1),9a we
herein like to report the preliminary results on the successful ATH
of N-sulfonyl arylketimines catalyzed by lipophilic diamine-
transition metal-catalysts in the aqueous suspension, which also
shows that the recrystallized conditions greatly affect the reactivity
of the solid substrates in the heterogeneous aqueous reactions.
* Corresponding author. Tel./fax: þ86 28 85238597; e-mail address: jgdeng@
cioc.ac.cn (J. Deng).
0040-4020/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tet.2013.05.064

L. Wang et al. / Tetrahedron 69 (2013) 6500e6506
6501
Fig. 1. Structures of pharmaceuticals and bioactive compound with chiral amine moieties.
2. Results and discussion
reactions (entry 8 vs entries 9 and 10), most likely by increasing the
area of surface contact between the solid particles and aqueous
phases. However, the reaction rate obviously decreased under
sonication conditions (entry 11).14a The results suggest that the
reaction proceeds at the interface between the solid substrates and
water in other words ‘on water’. Although solid 1a and Rh-2a cat-
alyst produced in situ are only sparingly soluble in water
(Fig. 2),5b,13 the hydrophobic interactions and trans-phase hydro-
gen-bonding might contribute to accelerate the reactivity.2c,3b,e
Various types of transition metal complexes were also in-
vestigated as catalyst precursors (entries 12e14). Excellent yields
The previous reports show that N-tosyl ketimines are more
stable and efficient substrates for highly selective asymmetric hy-
drogenation in halogenated solvents.12 However, the development
of efficient especially environmental friendly process or catalyzed
system for the asymmetric reduction with higher enantioselectivity
and more abroad scope of ketimine substrates is still an important
research field. Initially, the ATH of N-tosyl ketimine 1a was tried by
using 1 mol % of metal-complex prepared via treating (R,R)-Ts-
DPEN (2a) and [RhCl2(Cp
)]2 in situ.4,13 Table 1 shows that the re-
*
activities and enantioselectivities significantly depend on reaction
media (entries 1e7). Interestingly, although 1a was well dissolved
in the common organic solvents, the best yield (98.7%) and enan-
tioselectivity (96.5% ee) were obtained with solid suspensions of 1a
in water (Fig. 2) by using water-soluble HCO2Na as a hydrogen
source after 1 h at 40 ꢀC (entry 7).14 Vigorous stirring promotes the
were obtained with both [RhCl2(Cp
precursors after 5 h, but slightly poor enantioselectivity was ob-
served with [IrCl2(Cp )]2. However, poor yield and low ee value
*
)]2 and [IrCl2(Cp )]2 as the
*
*
were provided by using [RuCl2(p-cymene)]2 as a metal precursor,
which is obviously different to the results of the imines catalyzed by
the water-soluble ligands.8a,b Decreasing the reaction temperature
Table 1
Scanning of different solvent, metal precursor, and stirring speeda
Entry
Solventb
Metal complex
T (ꢀC)
t (h)
Yieldc (%)
ee (%)
1
2
3
4
5
6
7
8
CH2Cl2
CH3CN
MeOH
EtOH
THF
EtOAc
H2O
H2O
H2O
H2O
H2O
H2O
[RhCl2(Cp
[RhCl2(Cp
[RhCl2(Cp
[RhCl2(Cp
[RhCl2(Cp
[RhCl2(Cp
[RhCl2(Cp
[RhCl2(Cp
[RhCl2(Cp
[RhCl2(Cp
[RhCl2(Cp
[RhCl2(Cp
*
*
*
*
*
*
*
*
*
*
*
*
)]2
)]2
)]2
)]2
)]2
)]2
)]2
)]2
)]2
)]2
)]2
)]2
40
40
40
40
40
40
40
40
40
40
40
40
40
40
28
28
40
1
1
1
1
1
1
1
0.5
0.5
0.5
1
5
5
5
27
27
5
Trace
Trace
44.6
47.3
83.5
91.4
98.7
96.7
35.4
29.1
17.2
99.1
99.0
21.8
57.1
30.3
82.6
n.d.d
n.d.
82.2
83.6
90.1
94.2
96.5
96.4
96.6
96.6
n.d.
96.9
94.3
50.8
97.6
95.8
95.2
9e
10f
11g
12
13
14
15
16
17h
H2O
H2O
H2O
H2O
[IrCl2(Cp
[RuCl2(p-cymene)]2
[RhCl2(Cp )]2
[IrCl2(Cp )]2
[RhCl2(Cp )]2
*
)]2
*
*
H2O
*
a
The reaction in organic media was carried out in 1 mL of solvent under argon atmosphere using 0.4 mmol 1a. [Rh-(R,R)-2a]/[1a]¼1:100 and 0.2 mL of a mixture of formic
acid and triethyl amine (TEAF, v/v¼5:2) as hydrogen source; the reaction in water was carried out in 1 mL of H2O under argon atmosphere using 0.4 mmol 1a. [M-(R,R)-2a]/
[1a]/[HCO2Na]¼1:100:750; stirring speed is 1200 rpm.
b
Organic solvent was dried before use.
Isolated yield.
c
d
n.d.¼not detected.
e
Stirring speed is 700 rpm.
f
Stirring speed is 300 rpm.
Under sonication conditions.
g
h
The reaction was carried out in 1 mL of H2O under argon atmosphere using 0.8 mmol 1a. [M-(R,R)-2a]/[1a]/[HCO2Na]¼1:200:1500.

6502
L. Wang et al. / Tetrahedron 69 (2013) 6500e6506
Fig. 2. Reaction process of 1a on water by using Rh-2a complex as a catalyst. (a) Precatalyst prepared in situ; (b) after adding 1a; (c) after adding HCO2Na and 1a; (d) reaction
completed.
from 40 ꢀC to 28 ꢀC, a significant decline of yield was observed even
prolong the reaction time to 27 h but the enantioselectivity slightly
increased (entries 15 and 16). The different catalyst loading was
also tested. When 0.5 mol % catalyst was used, only 82.6% yield was
obtained with 95.2% enantioselectivity after 5 h (entry 17).
Using N-Ts-imine 1a as the standard substrate, we screened
a wide range of chiral diamine ligands. The results were summa-
rized in Table 2. Most of these chiral sulfonyl diamine-complexes of
afford better results (entries 12 and 15). It indicated that various
substituents on the benzene rings as well as sulfonyl group of 2a
(entries 4e11), even with another chiral auxiliary group (entries 13
and 14) showed slight effect to the enantioselectivity of the prod-
uct. Considering the similar enantioselectivities (entries 1 and 10)
and the inconvenient preparation of 2j, we chose easily available 2a
as the ligand for further research.
The pH value is another important factor to the ATH of ketones
and imines in water.5a,b,8a,b,15 Thus, the pH value of the reaction was
carefully examined (Table 3). The conversion and enantioselectivity
of 1a increased with elevated pH value by adjusting the ratio of
HCO2Na and HCO2H. When the initial pH value achieved about 4,
[RhCl2(Cp )]2 could provide a good to excellent enantioselectivity.
*
Lipophilic ligands Ts-DPEN (2a) and 30-NO2 substituted Ts-DPEN
(2j) exhibited the highest yields with excellent ee values (entries
1 and 10). However, water-soluble ligands 2l8c and 2o8d did not
Table 2
Scanning of ligand scopea
Entry
Ligand
Yieldb (%)
eec (%)
1
2
3
4
5
6
7
8
2a
2b
2c
2d
2e
2f
2g
2h
2i
2j
2k
2l
2m
2n
2o
78.2
33.3
62.1
41.7
55.2
47.6
7.46
7.24
32.9
87.5
27.9
36.4
44.4
33.9
22.1
96.8
88.2
96.2
93.3
95.5
93.8
n.d.
n.d.
9
94.8
96.1
96.5
94.5
96.3
95.8d
88.6
10
11
12
13
14
15
a
All reactions were stirred at 1200 rpm.
b
c
Determined by 1H NMR of the crude mixture in CDCl3, using 0.06 N mesitylene solution in CDCl3 as internal standard.
(R)-3a was obtained.
d
Detected as (S)-form product by comparing the sign of rotation with the data from literatures.11d,12

L. Wang et al. / Tetrahedron 69 (2013) 6500e6506
6503
role of a phase-transfer catalyst (PTC).3f,15 The results also suggest
that the reduction occurs on the surface of the solid particles.
After the optimal conditions had been established, the generality
of the ATH processes of N-sulfonylimines (1aeq) was explored. As
shown in Table 5, the reactions proceeded with highyields and good
to excellent enantioselectivities. For N-tosyl arylimines (1aem),
electronic effects of substituents on the aryl group largely affected
the reactivities (entries 1e13). Ketimines 1def bearing strong
electron-donating methoxyl group on the benzene ring proceeded
in longer time (24 h) than 1aec (entries 4e6 vs 1e3). By adding
Triton X-100, the rate of para- and meta-methoxyl substituted
ketimines 1d and 1e were greatly increased. Amines 3d with 84.8%
yield and 95.5% ee in 7 h and amine 3e, which is a key intermediate
of chiral drug Rivastigmine (Fig. 1), with 92.2% yield and 96.3% ee in
3 h were obtained, respectively. However, only a little ortho-
methoxyl substituted amine 3f was produced, even prolonging the
reaction time to 24 h (entry 6). Interestingly, ortho-trifluoromethyl
ketimine 1g could be smoothly reduced with 98% yield and 96.1% ee
in 24 h.17 Although the reduction of 1g was performed with Triton
X-100 in 7 h, lower ee value (93.9%) was provided (entry 7). The
chiral amine obtained from deprotection of 3g is a structural motif
of biologically active oxazolone derivative,18 and to the best of our
knowledge, this is the first asymmetric synthesis for optically active
1-(2-trifluoromethylphenyl)ethylamine via catalyzed asymmetric
reduction. Moreover, chiral amine 3h, which is an important in-
termediate of chiral drug, Cinacalcet, could be obtained in 96.2%
yield with 98.8% ee in the presence of 10 mol % of Triton X-100 in 8 h
(entry 8). It is interesting that the additional Triton X-100 could
slow down the reaction rate of hetero cyclic ketimines 1k and 1l
(entries 11 and 12). On the contrast, the reaction rate of benzothien-
2-yl imine 1m could be accelerated in the presence of Triton X-100
(entry 13). To further examine the efficiency of this catalyst system,
more N-sulfonyl ketimines were also tested. We found that cyclic
sulfonylimine 1n was only reduced well with [RuCl2(p-cymene)]2 as
a metal precursor in 98.3% yield and 96.1% ee (entries 15 vs 14).8a,19b
N-tert-Butylsulfonyl ketimines 1o and 1p, as well as N-p-methox-
ylphenylsulfonyl ketimine 1q were hydrogenated in high yields and
enantioselectivities in the absence of Triton X-100 (entries 16e18).
It is notable that hindered N-sulfonyl ketimines, such as 1g and 1h,
can be smoothly reduced with good yields and excellent enantio-
selectivities in this heterogeneous system.17
We sometimes found that the result of ATH of the model sub-
strate 1a is not reproducible and speculated on a change in the
crystallized conditions after the purification of 1a by chromatog-
raphy on silica gel, such as the ratio of EtOAc and petrol ether (PE),
which are the eluents. Thus, we recrystallized 1a under different
conditions. The highest yield (96.7%) was obtained in 0.5 h with 1a
recrystallized from a mixture of EtOAc and PE in 1:20 ratio (v/v) at
28 ꢀC. Decreased yields (79e87%) were observed with changing the
ratio of EtOAc and PE for the recrystallization of 1a. Compound 1a
recrystallized from a mixture of EtOAc and PE (1:20, v/v) at 0 ꢀC
gave lower yield (58%). Only 16% yield of 3a was provided by using
PE alone as recrystallized solvent at 28 ꢀC. However, quantitative
yield could be obtained by prolonging the reaction time to 6 h for 1a
obtained under all recrystallized conditions. These results showed
that the recrystallized condition is an important factor for the re-
duction of solid substrates in the heterogeneous aqueous phases,
which would lead to different dispersity of substrates in water.
Table 3
Effect of the pH valuea
Entry Amount (mmol) HCO2HþHCO2Na pHb
pHc
1.92 11.0
1.62 30.8
5.95 90.7
6.84 89.2
7.25 95.9
8.23 96.1
Yieldd (%) eee (%)
1
2
3
4
5
6
7
2þ0
1.60
1.29
3.94
4.46
5.03
7.40
n.d.
3þ0
87.7
93.2
94.6
96.3
96.3
97.0
0.75þ2.25
0.5þ2.5
0.143þ2.857
0þ3
0þ3f
11.18 10.76 42.3
a
Reaction conditions: 0.002 mmol [RhCl2(Cp
*
)]2, 0.004 mmol (R,R)-2a, 0.4 mmol
1a, 1 mL of degassed water, stirring speed at 1200 rpm for 1 h.
b
Initial pH value.
c
pH value after reaction.
d
Determined by 1H NMR analysis of the crude mixture.
e
Detected as R isomer by comparing the sign of rotation with the data from
literatures.11d,12
f
Extra 0.5 mmol Na2CO3 was added.
significant acceleration of the reaction rate was observed (entries
1e3). It is notable that the best result was obtained in the basic
media (entry 6),8a,b however, in an organic media, the ATH of im-
ines is performed on acidic activation.16 The conversion decreased
obviously as further increasing of pH value by adding Na2CO3 (entry
7). The results also indicate that the ATH of N-sulfonyl ketimine 1a
may be activated by water via trans-phase hydrogen-bonding in
this aqueous suspension.2c,3b
Our previous works indicated that both the reactivity and
enantioselectivity could be promoted by the addition of micelle-
forming agents.3f,6b,d,,8a In this study, either the cationic surfac-
tants (CTAB) or anionic surfactant (SDS) showed significant de-
crease in both the yield and enantioselectivity (Table 4, entries 2
and 3) because of difficult solubilization of solid substrate in the
micellar media but easy transport of the catalyst in micellar pha-
se,6b,17 while the addition of nonionic surfactant Triton X-100 ac-
celerated the reaction effectively (entries 4e8). The different
amount of Triton X-100 was also screened, and it was showed that
with the addition of 10% Triton X-100, the reaction gave the best
results (entries 6 and 8). Here, Triton X-100 may play possibly the
Table 4
Scanning of surfactantsa
Entry
Additive
Amount (mol %)
t (min)
Yieldb (%)
ee (%)
1
2
3
4
5
6
7
8
d
CTABc
d
50
50
50
30
10
4
10
10
10
10
10
10
10
30
76.9
13.4
10.5
84.7
85.7
91.6
40.7
99.8
96.8
89.9
n.d.
97.0
97.2
97.9
97.2
97.8
SDSd
Triton X-100e
Triton X-100
Triton X-100
Triton X-100
Triton X-100
3. Conclusion
10
a
Reaction conditions: 0.002 mmol [RhCl2(Cp )]2, 0.004 mmol (R,R)-2a, 0.4 mmol
*
Asymmetric transfer hydrogenation of a wide range of N-sul-
fonyl ketimines has successfully been performed in high yields
(82e>99%) and enantioselectivities (91e99%) on water by
employing the chiral and lipophilic rhodium-amido complex as
a catalyst and using HCO2Na as a hydrogen source. In the
1a, 1 mL degassed water, stirring speed at 1200 rpm.
b
Isolated yield.
c
CTAB¼hexadecyl trimethyl ammonium bromide.
d
SDS¼sodium dodecyl sulfate.
e
Triton X-100¼octyl phenoxy poly ethoxy ethanol.

6504
L. Wang et al. / Tetrahedron 69 (2013) 6500e6506
Table 5
Substrate scope of ATH of N-sulfonyl ketiminesa
Entry
Imine
R1
R2
t (h)
Yieldb (%)
eec (%)
Conf.d
1
2
3
4
5
6
7
8
1a
1b
1c
1d
1e
1f
1g
1h
1i
1j
1k
1l
1m
1n
1n
1o
1p
1q
C6H5
Me
Me
Me
Me
Me
Me
Me
Me
Me
Et
1 (0.5)
5 (1)
5(2)
24 (7)
24 (3)
24 (24)
24 (7)
21 (8)
23 (2)
8(6)
19 (19)
8 (7)
24 (4)
12 (24)
10
99.1 (99.8)
92.0 (99.2)
92.4(92.5)
95.1 (84.8)
55.7 (92.2)
10.2 (12.7)
98.0 (95.0)
69.2 (96.2)
95.5 (99.6)
92.9 (87.0)
82.1 (67.2)
97.3(94.0)
39.9 (99.0)
n.r. (89.2)
98.3
96.9 (98.0)
96.5 (97.7)
96.0(96.5)
96.4 (95.5)
95.4 (96.3)
n.d. (n.d.)e
96.1 (93.9)
91.4 (98.8)
95.4 (97.4)
96.0 (96.7)
91.0 (90.4)
95.5(95.9)
82.5 (96.4)
n.d. (70.8)
96.1
R
R
R
R
4-MeC6H4
4-FeC6H4
4-MeOC6H4
3-MeOC6H4
2-MeOC6H4
2-CF3C6H4
1-Naph
2-Naph
C6H5
2-Fury
2-Thienyl
2-Benzothienyl
R
n.d.
(þ)
(ꢁ)
R
R
R
R
R
R
R
9
10
11
12
13
14
15f
16
17
18
Me
Me
Me
14
5
24
93.8
60.0
99.1
96.7
86.0
95.7
R
(ꢁ)
(þ)
a
Reaction conditions: 0.002 mmol [RhCl2(Cp )]2, 0.004 mmol (R,R)-2a, 0.4 mmol N-sulfonyl imine, 3 mmol HCO2Na, 1 mL degassed water.
*
b
c
d
e
f
Isolated yield and the data in the parenthesis were obtained by the addition of 0.04 mmol Triton X-100.
Determined by chiral HPLC analysis.
The absolute configuration was determined by comparing the sign of rotation with the data from literature.11d,12,19,20
n.d.¼not detected.
Reaction conditions: 0.002 mmol [RuCl2(p-cymene)]2, 0.004 mmol (R,R)-2a, 0.4 mmol 1n, 3 mmol HCO2Na, 1 mL degassed water, 0.04 mmol Triton X-100.
heterogeneous aqueous reaction, reactivity depends on stirring
speed and the recrystallized conditions of the solid substrates.
Moreover, a significant enhancement of rate and ee value in the
reduction was observed by adding nonionic surfactant Triton X-
100. It is worth to note that the ATH of hindered N-sulfonyl keti-
mines was well performed in the aqueous suspension compared to
organic homogeneous phases. Thereby, a green approach for the
synthesis of key intermediates of bioactive compounds including
chiral drugs has been developed.
40 ꢀC for 1 h. N-Sulfonylimine 1 (0.4 mmol), HCO2Na (204 mg,
3 mmol)/and Triton X-100 (26 mg, 0.04 mmol) were successively
added. The solid suspension was stirred vigorously at 40 ꢀC. After
completion of the reaction (monitored by TLC), the reaction mix-
ture was extracted with EtOAc (5 mL) for three times. The com-
bined organic phases were dried over anhydrous Na2SO4 and
evaporated under reduced pressure. The crude product was dis-
solved in 1 mL of CDCl3, and the yield was determined by 1H NMR
analysis using 1,3,5-trimethylbenzene or dimethyl malate as an
internal standard. Isolated yield was obtained by using flash
chromatography.
4. Experimental section
4.1. General information
4.2.1. (R)-4-Methyl-N-(1-phenylethyl)benzenesulfonamide
(3a).11d,12 White solid, mp: 78e80 ꢀC, 98.0% ee, determined by
HPLC, Chiral AD-H column, 254 nm, i-PrOH/hexane¼20:80,
All commercially available reagents were used as received
without further purification. 1H NMR and 13C NMR were acquired at
300 MHz and 75 MHz, respectively. Enantiomeric excess was de-
termined by HPLC analysis on Chiralcel AD, OD, or OJ column
(Daicel Chemical Industries, LTD). Electrospray ionization high-
resolution mass spectra (ESI-HRMS) were recorded on a Bruke P-
SIMS-Gly FT-ICR mass spectrometer.
23
1.0 mL/min, tmajor¼7.75 min, tminor¼8.59 min.
[a]
D
þ64.6 (c 0.41, CHCl3). 1H NMR (CDCl3, 300 MHz)
d 1.42 (d,
J¼6.9 Hz, 3H), 2.38 (s, 3H), 4.42e4.51 (m, 1H), 4.84 (br d,
J¼6.7 Hz, 1H), 7.09e7.11 (m, 2H), 7.17e7.19 (m, 5H), 7.62 (d,
J¼8.3 Hz, 2H).
4.2.2. (R)-4-Methyl-N-(1-p-tolylethyl)benzenesulfonamide
(3b).19a White solid, mp: 111e113 ꢀC, 97.7% ee, determined by
4.2. General procedure for the asymmetric transfer hydroge-
nation of imines 1 and characterizations
HPLC, Chiral OJ-H column, 254 nm, i-PrOH/hexane¼30:70,
23
1.0 mL/min, tminor¼9.56 min, tmajor¼10.99 min.
[a]
D
þ74.5 (c 0.40, CHCl3). 1H NMR (CDCl3, 300 MHz)
d 1.41 (d,
A mixture of [RhCl2(Cp
*
)]2 (1.3 mg, 0.002 mmol), ligand 2a
J¼6.8 Hz, 3H), 2.28 (s, 3H), 2.39 (s, 3H), 4.36e4.45 (m, 1H), 4.90
(1.5 mg, 0.004 mmol) in 1 mL of degassed water was heated up to

L. Wang et al. / Tetrahedron 69 (2013) 6500e6506
6505
(d, J¼6.6 Hz, 1H), 6.96e7.02 (m, 4H), 7.19 (d, J¼8.0 Hz, 2H), 7.62
(d, J¼8.2 Hz, 2H).
(m, 2H), 2.35 (s, 3H), 4.18 (q, J¼7.2 Hz, 1H), 4.84 (d, J¼7.0 Hz, 1H),
6.99e7.01 (m, 2H), 7.10e7.16 (m, 5H), 7.53 (d, J¼8.1 Hz, 2H).
4.2.3. (R)-N-(1-(4-Fluorophenyl)ethyl)-4-methyl-benzenesulfona-
mide (3c).12,19a White solid, mp: 127e129 ꢀC, 96.5% ee, determined
by HPLC, Chiral OJ-H column, 254 nm, i-PrOH/hexane¼30:70,
4.2.10. (R)-N-(1-(Furan-2-yl)ethyl)-4-methyl-benzenesulfonamide
(3k).11d White solid, mp: 72e74 ꢀC, 91.0% ee, determined by HPLC,
Chiral OJ-H column, 254 nm, i-PrOH/hexane¼20:80, 0.8 mL/min,
23
1.0 mL/min, tminor¼8.83 min, tmajor¼10.00 min. [
a]
23 þ52.9 (c 0.54,
tminor¼13.06 min, tmajor¼15.45 min. [
a
]
þ81.6 (c 0.39, CHCl3). 1H
D
D
CHCl3). 1H NMR (CDCl3, 300 MHz)
3H), 4.40e4.49 (m, 1H), 4.99 (d, J¼6.7 Hz, 1H), 6.83e6.89 (m, 2H),
7.04e7.09 (m, 2H), 7.18 (d, J¼8.3 Hz, 2H), 7.59 (d, J¼8.2 Hz, 2H).
d
1.38 (d, J¼6.9 Hz, 3H), 2.39 (s,
NMR (CDCl3, 300 MHz)
d
1.44 (d, J¼6.9 Hz, 3H), 2.39 (s, 3H),
4.49e4.58 (m, 1H), 4.97 (d, J¼5.1 Hz, 1H), 5.98 (d, J¼3.2 Hz, 1H), 6.15
(dd, J¼3.0 Hz, 1.8 Hz, 1H), 7.16 (d, J¼0.9 Hz, 1H), 7.22 (d, J¼8.1 Hz,
2H), 7.68 (d, J¼8.2 Hz, 2H). 13C NMR (CDCl3, 75 MHz)
d 20.8, 21.5,
4.2.4. (R)-N-(1-(4-Methoxyphenyl)ethyl)-4-methyl-benzenesulfona-
mide (3d).12 White solid, mp: 116e118 ꢀC. 96.4% ee, determined by
HPLC, Chiral OJ-H column, 254 nm, i-PrOH/hexane¼30:70, 1.0 mL/
47.3, 106.1, 110.0, 127.0, 129.4, 137.6, 141.9, 143.1, 154.0.
4.2.11. (R)-4-Methyl-N-(1-(thiophen-2-yl)ethyl)benzenesulfonamide
min, tminor¼13.40 min, tmajor¼15.11 min. [
a
]
23 þ75.4 (c 0.47, CHCl3).
(3l).11d White solid, mp: 92e94 ꢀC, 95.9% ee, determined by HPLC,
D
1H NMR (CDCl3, 300 MHz)
d
1.40 (d, J¼6.8 Hz, 3H), 2.39 (s, 3H), 3.76
Chiral OD-H column, 254 nm, i-PrOH/hexane¼10:90, 1.0 mL/min,
20
(s, 3H), 4.36e4.45 (m, 1H), 4.68 (d, J¼6.8 Hz, 1H), 6.73 (d, J¼8.6 Hz,
2H), 7.01 (d, J¼8.7 Hz, 2H), 7.20 (d, J¼8.2 Hz, 2H), 7.62 (d, J¼8.2 Hz,
2H).
tmajor¼9.55 min, tminor¼11.21 min. [
a
]
D
þ70.0 (c 0.51, CHCl3). 1H
NMR (CDCl3, 300 MHz)
d
1.52 (d, J¼6.8 Hz, 3H), 2.41 (s, 3H),
4.69e4.78 (m, 1H), 5.01 (d, J¼7.6 Hz, 1H), 6.76 (d, J¼3.3 Hz, 1H),
6.81e6.83 (m, 1H), 7.11 (d, J¼5.0 Hz, 1H), 7.24 (d, J¼8.0 Hz, 2H), 7.71
4.2.5. (R)-N-(1-(3-Methoxyphenyl)ethyl)-4-methyl-benzenesulfona-
mide (3e).11d,12,19a White solid, mp: 62e64 ꢀC, 96.3% ee, de-
termined by HPLC, Chiral OJ-H column, 254 nm, i-PrOH/
(d, J¼8.1 Hz, 2H). 13C NMR (CDCl3, 75 MHz)
d 21.5, 23.8, 49.2, 124.3,
124.7, 126.6, 127.1, 129.5, 137.6, 143.3, 146.0.
hexane¼30:70, 1.0 mL/min, tminor¼10.53 min, tmajor¼11.80 min.
4.2.12. (R)-N-(1-(Benzothiophen-2-yl)ethyl)-4-methyl-benzenesulfo-
namide (3m).11d White solid, mp: 139e141 ꢀC, 96.4% ee, de-
termined by HPLC, Chiral AD-H column, 254 nm, i-PrOH/
þ68.0 (c 0.12, CHCl3). 1H NMR (CDCl3, 300 MHz)
d 1.40 (d,
23
[
a
]
D
J¼6.8 Hz, 3H), 2.37 (s, 3H), 3.68 (s, 3H), 4.37e4.46 (m, 1H), 5.24 (d,
J¼6.6 Hz, 1H), 6.59 (s, 1H), 6.69 (d, J¼7.8 Hz, 2H), 7.07e7.18 (m,
3H), 7.61 (d, J¼8.0 Hz, 2H).
hexane¼20:80, 1.0 mL/min, tminor¼13.74 min, tmajor¼16.16 min.
20
[a
]
þ90.3 (c 1.0, CHCl3). 1H NMR (CDCl3, 300 MHz)
d 1.58 (d,
D
J¼6.6 Hz, 3H), 2.32 (s, 3H), 4.79e4.90 (m, 2H), 6.95 (s, 1H), 7.15 (d,
4.2.6. (þ)-4-Methyl-N-(1-(2-trifluoromethylphenyl)ethyl)benzene-
sulfonamide (3g). White solid, mp: 151e153 ꢀC, 96.1% ee, de-
termined by HPLC, Chiral OD-H column, 254 nm, i-PrOH/
J¼8.0 Hz, 2H), 7.28e7.33 (m, 2H), 7.59 (dd, J¼1.6, 8.6 Hz, 1H), 7.69
(dd, J¼3.5, 8.4 Hz, 3H). 13C NMR (CDCl3, 75 MHz)
d 21.4, 23.4, 49.8,
121.1, 122.2, 123.4, 124.21, 124.23, 127.0, 129.3, 137.3, 139.1, 143.3,
146.4.
20
hexane¼20:80, 1.0 mL/min, tmajor¼6.22 min, tminor¼7.80 min. [
a]
D
þ93.1 (c 1.0, CHCl3). 1H NMR (CDCl3, 300 MHz)
d
1.38 (d, J¼6.7 Hz,
3H), 2.36 (s, 3H), 4.82e4.86 (m, 1H), 5.95 (d, J¼4.5 Hz, 1H), 7.15 (d,
J¼8.1 Hz, 2H), 7.20e7.26 (m, 1H), 7.31e7.36 (m, 1H), 7.45e7.51 (m,
4.2.13. (R)-3-tert-Butyl-2,3-dihydrobenzoisothiazoline 1,1-dioxide
(3n).8a,19b White solid, mp: 124e126 ꢀC, 96.1% ee, determined by
HPLC, Chiral OD column, 254 nm, i-PrOH/hexane¼20:80, 1.0 mL/
2H), 7.65 (d, J¼8.2 Hz, 2H). 13C NMR (CDCl3, 75 MHz)
d 21.4, 24.8,
49.3, 125.1 (q, JCeF¼272.6 Hz), 125.3 (q, JCeF¼5.8 Hz), 126.9 (q,
JCeF¼30.0 Hz), 126.9, 127.1, 127.6, 129.3, 132.2, 136.9, 142.0, 143.2.
HRMS (ESI) calcd for C16H16F3NNaO2S [MþNa]þ: 366.0746, found:
366.0741.
min, tminor¼6.73 min, tmajor¼17.51 min. [
a
]
20 þ53.4 (c 0.4, CHCl3). 1H
D
NMR (CDCl3, 300 MHz)
d
1.00 (s, 9H), 4.35 (d, J¼4.9 Hz, 1H), 5.55 (br
s, 1H), 7.43e7.55 (m, 3H), 7.72 (d, J¼7.3 Hz, 1H). 13C NMR (CDCl3,
75 MHz)
d 26.4, 36.5, 67.2, 121.2, 125.9, 129.0, 132.2, 135.8, 138.1.
4.2.7. (ꢁ)-4-Methyl-N-(1-(naphthalen-1-yl)ethyl)benzenesulfona-
mide (3h).12a White solid, mp: 92e94 ꢀC, 98.8% ee, determined by
HPLC, Chiral OJ-H column, 254 nm, i-PrOH/hexane¼10:90, 1.0 mL/
4.2.14. (R)-2-Methyl-N-(1-phenylethyl)propane-2-sulfonamide
(3o).11d,19b White solid, mp: 79e81 ꢀC, 96.7% ee, determined by
HPLC, Chiral AD-H column, 254 nm, i-PrOH/hexane¼10:90, 1.0 mL/
23
min, tminor¼7.83 min, tmajor¼9.93 min. [
a
]
23 ꢁ14.8 (c 0.12, CHCl3). 1H
min, tmajor¼7.08 min, tminor¼8.56 min. [
a]
þ32.6 (c 0.31, CHCl3).
D
D
NMR (CDCl3, 300 MHz)
d
1.54 (d, J¼6.5 Hz, 3H), 2.17 (s, 3H),
1H NMR (CDCl3, 300 MHz)
d
1.32 (s, 9H), 1.58 (d, J¼6.7 Hz, 3H), 4.25
5.14e5.23 (m, 1H), 5.60 (d, J¼7.1 Hz, 1H), 6.88 (d, J¼8.0 Hz, 2H), 7.14
(t, J¼7.7 Hz, 1H), 7.26e7.33 (m, 3H), 7.45 (d, J¼8.0 Hz, 2H), 7.53 (d,
J¼8.1 Hz, 1H), 7.64e7.66 (m, 1H), 7.80e7.83 (m, 1H).
(d, J¼8.6 Hz, 1H), 4.63e4.72 (m, 1H), 7.27e7.38 (m, 5H).
4.2.15. (ꢁ)-2-Methyl-N-(1-(naphthalen-1-yl)ethyl)propane-2-
sulfonamide (3p). White solid, mp: 99e101 ꢀC, 86.0% ee, de-
termined by HPLC, Chiral AD-H column, 254 nm, i-PrOH/
4.2.8. (R)-4-Methyl-N-(1-(naphthalen-2-yl)ethyl)benzenesulfona-
mide (3i).11d,12,19a White solid, mp: 111e113 ꢀC, 97.4% ee, de-
termined by HPLC, Chiral OJ-H column, 254 nm, i-PrOH/
hexane¼20:80, 1.0 mL/min, tmajor¼5.42 min, tminor¼6.97 min.
[
a
]
ꢁ5.7 (c 0.34, CHCl3). 1H NMR (CDCl3, 300 MHz)
d 1.28 (s,
23
D
hexane¼30:70, 1.0 mL/min, tminor¼14.67 min, tmajor¼16.68 min.
9H), 1.70 (d, J¼6.8 Hz, 3H), 4.39 (d, J¼8.3 Hz, 1H), 5.46e5.56 (m,
20
[
a
]
þ74.1 (c 0.50, CHCl3). 1H NMR (CDCl3, 300 MHz)
d 1.50
1H), 7.47e7.58 (m, 4H), 7.78e7.82 (m, 1H), 7.87e7.90 (m, 1H),
D
(d, J¼6.8 Hz, 3H), 2.21 (s, 3H), 4.61e4.66 (m, 1H), 5.43 (d,
J¼7.2 Hz, 1H), 6.99 (d, J¼8.0 Hz, 2H), 7.21e7.26 (m, 1H), 7.42e7.47
(m, 3H), 7.58 (d, J¼8.1 Hz, 2H), 7.64 (d, J¼8.2 Hz, 2H), 7.73e7.76
(m, 1H).
8.08 (d, J¼8.3 Hz, 1H). 13C NMR (CDCl3, 75 MHz)
d 24.2, 26.0, 53.6,
59.8, 122.6, 122.7, 125.5, 125.8, 126.5, 128.0, 129.0, 129.8, 133.9,
139.6. HRMS (ESI) calcd for C16H25N2O2S [MþNH4]þ: 309.1631,
found: 309.1625.
4.2.9. (R)-4-Methyl-N-(1-phenylpropyl)benzenesulfonamide
(3j).11d,12 White solid, mp: 115e117 ꢀC, 96.7% ee, determined by
HPLC, Chiral OJ-H column, 254 nm, i-PrOH/hexane¼30:70, 1.0 mL/
4.2.16. (þ)-4-Methoxy-N-(1-phenylethyl)benzenesulfonamide
(3q).20 White solid, mp: 96e98 ꢀC, 95.7% ee, determined by HPLC,
Chiral AD-H column, 254 nm, i-PrOH/hexane¼20:80, 1.0 mL/min,
23
min, tminor¼7.15 min, tmajor¼10.94 min. [
a]
23 þ48.1 (c 0.52, CHCl3).
tmajor¼10.39 min, tminor¼11.97 min. [
a]
þ7.0 (c 0.22, CHCl3). 1H
D
D
1H NMR (CDCl3, 300 MHz)
d
0.78 (t, J¼7.4 Hz, 3H), 1.66e1.86
NMR (300 MHz, CDCl3)
d
1.43 (d, J¼6.9 Hz, 2H), 3.84 (s, 3H),

6506
L. Wang et al. / Tetrahedron 69 (2013) 6500e6506
8. (a) Wu, J.; Wang, F.; Ma, Y.; Cui, X.; Cun, L.; Zhu, J.; Deng, J.; Yu, B. Chem.
Commun. 2006, 1766e1768; (b) Canivet, J.; Suss-Fink, G. Green Chem. 2007, 9,
4.40e4.49 (m, 1H), 4.76 (d, J¼6.6 Hz, 1H), 6.85 (d, J¼8.9 Hz, 2H),
7.08e7.11 (m, 2H), 7.16e7.24 (m, 3H), 7.65 (dd, J¼2.0, 8.9 Hz, 2H).
€
391e397; (c) Li, L.; Wu, J.; Wang, F.; Liao, J.; Zhang, H.; Lian, C.; Zhu, J.; Deng, J.
Green Chem. 2007, 9, 23e25; (d) Tang, Y.; Li, X.; Lian, C.; Zhu, J.; Deng, J. Tet-
rahedron: Asymmetry 2011, 22, 1530e1535.
Acknowledgements
€
9. (a) Hohne, M.; Bornscheuer, U. T. ChemCatChem 2009, 1, 42e51; (b) Breuer, M.;
€
Ditrich, K.; Habicher, T.; Hauer, B.; Ke
b
eler, M.; Sturmer, R.; Zelinski, T. Angew.
We thank National Science Foundation of China (Grant Nos.
20972154 and 21172152), National Basic Research Program of China
(973 Program, No. 2010CB833300), and Sichuan University for fi-
nancial support. Chengdu Institute of Organic Chemistry, Chinese
Academy of Sciences, and West China School of Pharmacy, Sichuan
University, contributed equally to this work.
Chem., Int. Ed. 2004, 43, 788e824.
10. For reviews on the asymmetric reduction of imines, see: (a) Kobayashi, S.;
Ishitani, H. Chem. Rev. 1999, 99, 1069e1094; (b) Blaser, H. U.; Malan, C.; Pugin,
B.; Spindler, F.; Steiner, H.; Studer, M. Adv. Synth. Catal. 2003, 345, 103e151; (c)
Cho, B. T. Tetrahedron 2006, 62, 7621e7643; (d) Nugent, T. C.; El-Shazly, M. Adv.
Synth. Catal. 2010, 352, 753e819; (e) Rueping, M.; Sugiono, E.; Schoepke, F. R.
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Synlett 2010, 852e865; (f) Fleury-Bregeot, N.; de la Fuente, V.; Castillon, S.;
Claver, C. ChemCatChem 2010, 2, 1346e1371; (g) Xie, J.-H.; Zhu, S.-F.; Zhou, Q.-L.
Chem. Rev. 2011, 111, 1713e1760; (h) Wang, C.; Villa-Marcos, B.; Xiao, J. Chem.
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Supplementary data
11. For recent examples on the asymmetric transfer hydrogenation of imines, see:
(a) Martins, J. E. D.; Clarkson, G. J.; Wills, M. Org. Lett. 2009, 11, 847e850; (b)
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NMR, HPLC, and HRMS spectra of products. Supplementary data
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17. 1-(2-Trifluoromethylphenyl)-N-tosylethanimine (1g) is only liquid substrate in
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reduced under Lee’s conditions,11d however, no reaction was observed for 3g
and 3h only gave 31.8% yield (99.8% ee) after 8 h.
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