Rationally-designed S-chiral bissulfinamides as highly enantioselective organocatalysts for reduction of ketimines

Article abstract of DOI:10.1002/adsc.200700504

We recently reported the first example of S-chiral organocatalysts, that are highly efficient and enantioselective in substoichometric amounts, and which use a chiral monosulfinamide group as Lewis base to activate trichlorosilane (HSiCl₃) to reduce N-arylketimines. Aplausible mechanism involving two molecules of the monosulfinamde catalyst for the activation of HSiCl ₃ prompted us to design S-chiral bissulfinamides as new catalysts. We herein describe our findings that an easily prepared S-chiral bissulfinamide bearing a five-methylene linkage not only inherited the excellent substrate generality from the monosulfinamide catalysts, but also exhibited further improved enantioselectivity.

Full text of DOI:10.1002/adsc.200700504

UPDATES
DOI: 10.1002/adsc.200700504
Rationally-Designed S-Chiral Bissulfinamides as Highly Enantio-
selective Organocatalysts for Reduction of Ketimines
Dong Pei,a,b Yu Zhang,a,b Siyu Wei,a Meng Wang,a,b and Jian Suna,*
a
Natural Products Center, Chengdu Institute of Biology, Chinese Academy of Sciences, Chengdu 610041, Peopleꢀs
Republic of China
Fax : (+86)-28-8522-2753; e-mail: sunjian@cib.ac.cn
Graduate School of Chinese Academy of Sciences, Beijing, Peopleꢀs Republic of China
b
Received: October 18, 2007; Published online: February 26, 2008
Supporting information for this article is available on the WWW under http://asc.wiley-vch.de/home/.
Abstract: We recently reported the first example of
S-chiral organocatalysts, that are highly efficient
and enantioselective in substoichometric amounts,
and which use a chiral monosulfinamide group as
Lewis base to activate trichlorosilane (HSiCl3) to
reduce N-arylketimines. Aplausible mechanism in-
volving two molecules of the monosulfinamde cata-
lyst for the activation of HSiCl3 prompted us to
design S-chiral bissulfinamides as new catalysts. We
herein describe our findings that an easily prepared
S-chiral bissulfinamide bearing a five-methylene
linkage not only inherited the excellent substrate
generality from the monosulfinamide catalysts, but
also exhibited further improved enantioselectivity.
phenolic hydroxyl group of 3 was found most likely
not to play a chelating role,[3] we speculate that two
molecules of catalyst are bound to the chlorosilane
through their Lewis basic S=O groups. The main
function of the indispensable phenolic hydroxy group
might be to facilitate the assembly of two sulfina-
mides through hydrogen bonding (Figure 1). We envi-
sioned that the incorporation of two sulfinamide units
into one molecule through chemical bonds could lead
to highly effective new catalysts.[4] Thus, we designed
dimeric bissulfinamides 4–7 (Figure 2) as our second-
generation catalysts. Herein, we report our discovery
of a structurally simple new S-chiral bissulfinamide
organocatalyst (5d) that promotes the asymmetric re-
Keywords: asymmetric reduction; S-chiral bissulfi-
namide; ketimines; organocatalysis
Chiral sulfur centers have been well established as ef-
ficient and versatile stereo-controllers in asymmetric
synthesis and have been extensively used as the chir-
ality source of chiral auxiliaries and ligands.[1] The de-
velopment of S-chiral organocatalysts, however, had
met with little success.[2] Recently, we reported the
first example of S-chiral organocatalysts that are
highly efficient and enantioselective in substoichomet-
Scheme 1.
ric amounts.[3] S-Chiral monosulfinamides
3 (20
mol%) catalyzed the asymmetric reduction of N-aryl-
ketimines with trichlorosilane (HSiCl3) in high yield
and enantioselectivity (Scheme 1).
Our previous observation of a clear positive non-
linear effect of the enantiomeric excess (ee) of cata-
lyst 3b on the productꢀs enantioselectivity in the
asymmetric reduction of ketimine 1a (R1 =Ar=Ph,
R2 =Me)3 seemed to suggest that more than one mol-
ecule of such a monosulfinamide catalyst is involved
in the stereochemistry-determining step. Since the Figure 1. Proposed binding pattern.
Adv. Synth. Catal. 2008, 350, 619 – 623
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Dong Pei et al.
Table 1. Asymmetric reduction of ketimine 1a.[a]
UPDATES
Entry
Catalyst
Yield [%][b]
ee [%][c,d]
1
2
3
4
5
6
7
8
3a
4a
4b
4c
4d
4e
5a
5b
5c
5d
5e
6
84
85
77
80
78
80
79
42
61
93
71
30
84
89
90
89
90
91
86
63
75
90
91
83
66
89
9
10
11
12
13
Figure 2. Structures of the newly designed bissulfinamide
catalysts.
7
[a]
Reactions were carried out with 2.0 equiv. of HSiCl3 on a
0.2 mmol scale in 1.0 mL of solvent.
Isolated yield based on the imine.
The ee values were determined using chiral HPLC.
Product 2a was S configured in all cases, as revealed by
comparison of the optical rotation with the literature
data.
duction of N-arylketimines by HSiCl3 with high effi-
ciency and excellent enantioselectivity.[5,6]
[b]
[c]
[d]
Initially, we simply connected two molecules of sul-
finamide 3a with a polymethylene tether which was
easily installed through the phenolic hydroxy groups.
The resulting dimeric bissulfinamides 4a–e of varying
tether length were then examined as catalysts for the
model reaction of 1a with HSiCl3 in dichloromethane
at À208C. To our delight, all these bissulfinamides ex- It is likely that the five-methylene tether enables 5d
hibited the same high level of enantioselectivity as to chelate with silicon with a most favorable geome-
the parent monosulfinamide 3a regardless of the try.
tether length (entries 1–6, Table 1). While 20 mol% of
Considering that the straight chain of the polymeth-
the catalyst loading of the monosulfinamide drove the ylene tether is quite conformationally flexible, we also
reaction to completion in 24 h, half the amount of the tried to constrain the sulfinamide groups into a less
bissulfinamides displayed similar reactivity.
flexible arrangement by utilizing a phenyl-mounted
In principle, if a cooperative binding of the bissulfi- tether (6 and 7 in Figure 2), which, unfortunately,
namide with silicon is essential to the stereocontrol, caused adverse effects on both the catalytic efficiency
variation of the linkage that affects the spacing and and the enantioselectivity (entries 12 and 13, Table 1).
thus the conformational alignment of the two sulfina- Notably, 7 with a phenyl-1,3-dimethylene tether dis-
mide functional groups should have a substantial in- played significantly higher reactivity and enantioselec-
fluence on the catalytic outcome. Since the enantiose- tivity than 6 with a phenyl-1,2-dimethylene tether, fur-
lectivity of bissulfinamides 4 was only slightly affected ther supporting the cooperativity of binding with bis-
by the polymethylene tether length, it seems to imply sulfinamides, since if there were no chelation with sili-
that there exists excessive spacing flexibility with the con, the reactivity and enantioselectivity of these
phenyl-containing linkage. Thus, we next simplified compounds would be independent of the relative po-
the linkage and prepared bissulfinamides 5a–e with sitioning of the two sulfinamide functionalities.
only a polymethylene tether connecting the two chiral
sulfinamide functional groups.
As expected, for the 5d-catalyzed reduction of 1a
with HSiCl3, a linear effect of catalyst ee on product
As expected, the efficacy of bissulfinamides 5 is enantioselectivity was observed (Figure 3). Thus, it is
largely dependent on the tether length (entries 7–11, clear that a single molecule of bidentate 5d partici-
Table 1). Compound 5d bearing a five-methylene pates in the reaction.
tether proved to be the most effective catalyst, afford-
To further improve the performance of catalyst 5d,
ing 93% yield and 91% ee (entry 10). Either decreas- 2,6-lutidine was tested as an additive in the reduction
ing or increasing the methylene unit number (cata- of 1a (entries 1–7, Table 2). We were delighted to find
lysts 5a–c and 5e) had negative effects on both the re- that the use of 0.3 equivalents of 2,6-lutidine was ben-
activity and enantioselectivity. These results clearly eficial to the enantioselectivity (entry 4). When the
imply a cooperativity of binding with bissulfinamides. amount of 2,6-lutidine was increased, the beneficial
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Adv. Synth. Catal. 2008, 350, 619 – 623

Rationally-Designed S-Chiral Bissulfinamides as Highly Enantioselective Organocatalysts
UPDATES
Table 3. Asymmetric reduction of various ketimines 1 with
catalyst 5d.[a]
Entry Imine
No. X=
Yield
[%][b]
ee
[%][c]
96
(92)
95
(92)
93
(90)
95
1
2
1a
H
91
1b p-CF3 95
1c p-NO2 90
3
4
5
1d p-Br
92
83
(92)
95
1e p-
OMe
(93)
94
(90)
93
Figure 3. Linear effect in the 5d-catalyzed reduction of keti-
mine 1a with trichlorosilane.
6
7
1f
H
77
1g OMe 80
(91)
Table 2. Asymmetric reduction of ketimine 1a with catalyst
5d and additives.[a]
c-
C6H11
1i i-Pr
75
(74)
82
8
9
1h
84
87
Entry Additive
Amount (equiv.)[b] Yield [%] ee [%]
(79)
95
(92)
92
(91)
92
(91)
72
(88)
83
(90)
91
1
2
3
4
5
6
7
8
none
93
90
90
91
82
67
<5
35
83
66
72
68
46
91
90
91
96
93
94
nd
95
95
95
92
94
93
10
11
1j p-Cl
93
2,6-lutidine 0.1
2,6-lutidine 0.2
2,6-lutidine 0.3
2,6-lutidine 0.4
2,6-lutidine 0.5
2,6-lutidine 1.0
1k p-Me 88
12
13
1l p-
OMe
1m o-
OMe
92
75
NEt3
0.3
0.3
0.3
0.3
0.3
0.3
9
i-Pr2NEt
DMAP
NMM
DBU
pyridine
HMTA0.3
p-
CF3Ph
14
15
1n
90
86
10
11
12
13
14
1o 2-Np
(88)
54
93
92
(92)
16
1p
84
[a]
Reactions were carried out with 10 mol% catalyst 5d and
2.0 equiv. of HSiCl3 on a 0.2 mmol scale in 1.0 mL of sol-
vent for 24 h.
94
(93)
91
17
18
1q Et
86
87
[b]
Based on the imine.
1r n-Pr
(91)
93
91
(93)
93
(86)
19
20
1s c-Pr
1t n-Bu
70
89
effect on the enantioselectivity persisted (entries 5
and 6); however, the reactivity was significantly low-
ered. When the amount of 2,6-lutidine reached one
equivalent, the reaction became totally inactive
(entry 7).
Several other bases were also examined as additives
in the 5d-catalyzed reduction of 1a (entries 8–14,
Table 2). In the presence of 0.3 equiv. of all these
bases, similar beneficial effects on the enantioselectiv-
ity were observed. However, unlike 2,6-lutidine, these
bases all had substantially negative effects on the re-
activity.
21
1u i-Bu
82
[a]
Reactions were carried out with 10 mol% catalyst 5d and
with 2.0 equiv. of HSiCl3 on a 0.2 mmol scale in 1.0 mL
of solvent at À208C for 24 h.
[b]
[c]
Isolated yield based on the imine.
The ee values were determined using chiral HPLC; the
data in parentheses are for catalyst 3b.
Finally, the highly efficient new bissulfinamide cata-
lyst 5d was tested in the reduction of various keti- finamide catalyst 3b. Moreover, 5d exhibited higher
mines under the optimal conditions to establish the enantioselectivity than 3b in most cases (see ee values
generality. As shown in Table 3, 5d inherited the ex- in parentheses for 3b[3]). Thus, the bissulfinamide cat-
traordinarily broad substrate scope from the monosul- alyst 5d proved to be, in general, a better catalyst
Adv. Synth. Catal. 2008, 350, 619 – 623
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Dong Pei et al.
UPDATES
Compound 5c: Yellowish oil, yield: 84%; [a]2D5: À16.2 (c
0.15, MeOH); 1H NMR (600 MHz, CDCl3): d=1.21 (s,
18H), 1.64 (m, 4H), 3.11 (m, 2H), 3.23 (m, 4H); 13C NMR
(150 MHz, CDCl3): d=22.6, 28.3, 45.4, 55.6; ESI-HR-MS:
m/z 319.1493, calcd. for (C12H28N2O2S2 +Na)+: 319.1484.
Compound 5d: Yellowish oil, yield: 87%; [a]2D5: À33.0 (c
0.19, MeOH); 1H NMR (300 MHz, CDCl3): d=1.18 (s,
18H), 1.35 (m, 2H), 1.54 (m, 4H), 3.03 (m, 2H), 3.13 (m,
4H); 13C NMR (150 MHz, CDCl3): d=22.6, 23.7, 30.6, 45.5,
than the monosulfinamide catalyst 3b for the reduc-
tion of ketimines with HSiCl3.
In summary, we have developed S-chiral bissulfina-
mides as highly efficient and enantioselective organo-
catalysts for the reduction of N-arylketimines with tri-
chlorosilane.[8] The structurally simple bissulfinamide
5d with a five-methylene tether not only inherited the
excellent substrate generality from the monosulfina-
mide catalyst we have previously developed, but also
exhibited further improved enantioselectivity. The de-
velopment of other S-chiral new organocatalysts is
under active investigation in this laboratory.
55.6;
ESI-HR-MS:
m/z=333.1642,
calcd.
for
(C13H30N2O2S2 +Na)+: 333.1641.
Compound 5e: Yellowish oil, yield: 77%; [a]2D5: À41.0 (c
0.15, MeOH); 1H NMR (300 MHz, CDCl3): d=1.15 (s,
18H), 1.29 (m, 4H), 1.51 (m, 4H), 2.98 (m, 2H), 3.14 (m,
4H); 13C NMR (75 MHz, CDCl3): d=22.5, 26.2, 30.8, 45.5,
55.4;
ESI-HR-MS:
m/z=347.1806,
calcd.
for
Experimental Section
(C14H32N2O2S2 +Na)+: 347.1803.
All starting materials were of the highest commercially
available grade and used without further purification. All
solvents used in the reactions were distilled from appropri-
General Procedure for the Synthesis of Imines
Amixture of NaHCO (50 mmol), amine (10 mmol), ketone
3
1
ate drying agents prior to use. H - and 13C NMR (300 or
(10 mmol), and activated 4 ꢂ molecular sieves (8.0 g) in an-
hydrous toluene (10 mL) was stirred at 808C for 12 h under
an argon atmosphere, and was then filtered through celite.
The filtrate was concentrated under vacuum. The crude
product was subjected to distillation or recrystallization to
give pure imine 1.
600 and 75 or 150 MHz, respectively) spectra were recorded
on Brucker AVANCE 600 and 300 spectrometers. Chemical
1
shifts of H and 13C NMR spectra are reported in ppm (d).
ESI-MS were recorded on a BioTOF Q. HPLC analyses
were performed with a Perkin-Elmer Series 200 UV/VIS de-
tector and Series 200 pump. Chiralpak OD-H, AD-H and
OJ-H columns were purchased from Daicel Chemical Indus-
tries, Ltd. All enantiomer ratios have been controlled by co-
injections of the pure sample with the racemic substrates.
General Procedure for the Catalytic Reduction of
Imines
Under an argon atmosphere, trichlorosilane (40 mL,
0.4 mmol) was added dropwise to a stirred solution of imine
1 (0.20 mmol), catalyst 5d (6.2 mg, 0.02 mmol) and 2,6-luti-
dine (7 mL, 0.06 mmol) in anhydrous CH2Cl2 at À208C. The
mixture was allowed to stir at the same temperature for
24 h. The reaction was quenched with a saturated aqueous
solution of NaHCO3 (5 mL) and extracted with EtOAc. The
combined extracts were washed with brine and dried over
anhydrous MgSO4 and the solvents were evaporated. Purifi-
cation by column chromatography (silica gel, hexane/
EtOAc) afforded pure amine 2. The ee values were deter-
mined using established HPLC techniques with chiral sta-
tionary phases.
General Procedure for the Synthesis of 5[7]
To a solution of tert-butanesulfinamide (242 mg, 2.0 mmol)
in CH2Cl2 was added anhydrous CuSO4 (640 mg, 4.0 mmol)
followed by OHC(CH2)nÀ2CHO (0.8 mmol). The mixture
G
was stirred at room temperature for 24 h and was then fil-
tered through a pad of Celite. The filter cake was washed
well with CH2Cl2 and the filtrate was concentrated under
vacuum. The residue was dissolved in MeOH (10 mL), and
NaBH4 (68 mg, 1.8 mmol) was introduced at 08C. The mix-
ture was stirred at the same temperature for 0.5 h. The reac-
tion was quenched with acetone (2 mL). After the volatiles
had been evaporated, EtOAc (50 mL) and saturated aque-
ous ammonia chloride (10 mL) were introduced. The organ-
ic layer was separated, washed with brine, dried over anhy-
drous Na2SO4, and concentrated under vacuum. The residue
was purified by column chromatography (silica gel, hexane :
EtOAc=1:1) to give the pure product.
Acknowledgements
We are grateful for financial support from the National Natu-
ral Science Foundation of China (20672107 and 20732006)
and from the Chinese Academy of Sciences (Hundreds of
Talents Program).
Compound 5a: White solid, yield: 87%; [a]2D5: À43.1 (c
0.14, MeOH); mp 120.0–122.08C; 1HNMR (600 MHz,
CDCl3): d=1.24 (s, 18H), 3.29 (m, 2H), 3.41 (m, 2H), 4.52
(brs, 2H); 13C NMR (150 MHz, CDCl3): d=22.7, 47.2, 5.9;
ESI-HR-MS: m/z=291.1162, calcd. for (C10H24N2O2S2
+
References
Na)+: 291.1171.
Compound 5b: Yellowish oil, total yield: 28%; [a]2D5:
À58.4 (c 0. 40, MeOH); 1HNMR (300 MHz, CDCl3): d=
1.22 (s, 18H), 1.88 (m, 2H), 3.28 (m, 4H), 3.89 (t, J=6.0 Hz,
2H); 13C NMR (150 MHz, CDCl3): d=22.7, 31.9, 43.9, 55.7;
ESI-HR-MS: m/z=305.1321, calcd. for (C11H26N2O2S2 +
Na)+: 305.1328.
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Adv. Synth. Catal. 2008, 350, 619 – 623

Rationally-Designed S-Chiral Bissulfinamides as Highly Enantioselective Organocatalysts
UPDATES
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[8] It should be noted that the present catalyst system is not
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Adv. Synth. Catal. 2008, 350, 619 – 623
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asc.wiley-vch.de
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