Enantioselective hydrosilylation of ketimines catalyzed by Lewis basic C2-symmetric chiral tetraamide

Article abstract of DOI:10.1016/j.tetasy.2007.03.008

l-Proline derived C₂-symmetric chiral tetraamide 5b was found to behave as an effective Lewis basic catalyst in the enantioselective hydrosilylation of ketimines, affording high isolated yields (up to 95%) and moderate to high enantioselectivit

Full text of DOI:10.1016/j.tetasy.2007.03.008

Tetrahedron: Asymmetry 18 (2007) 705–709
Enantioselective hydrosilylation of ketimines catalyzed by Lewis basic
C2-symmetric chiral tetraamide
Zhouyu Wang,a,b,c Siyu Wei,a Chao Wanga,c and Jian Suna,*
aNatural Products Research Center, Chengdu Institute of Biology, Chinese Academy of Sciences, Chengdu 610041, China
bDepartment of Chemistry, Xihua University, Chengdu 610039, China
cGraduate School of Chinese Academy of Sciences, Beijing, China
Received 28 November 2006; revised 3 March 2007; accepted 7 March 2007
Abstract—L-Proline derived C2-symmetric chiral tetraamide 5b was found to behave as an effective Lewis basic catalyst in the enantio-
selective hydrosilylation of ketimines, affording high isolated yields (up to 95%) and moderate to high enantioselectivities (up to 86% ee)
for a broad range of ketimines. A clear synergistic effect of the two identical diamide units of 5b was observed for asymmetric induction.
Ó 2007 Elsevier Ltd. All rights reserved.
1. Introduction
of a wide range of ketimines with up to 96% ee values.4a
Very recently, we successfully developed another highly
enantioselective amide catalyst 4, of which the substrate
scope is complementary to that of 3.4b
Chiral amides have recently been developed as efficient
Lewis basic organocatalysts for the asymmetric hydrosilyl-
ation of imines with trichlorosilane (HSiCl3).1–5 Matsu-
mura first reported that L-proline derived amide 1 (Fig. 1)
could catalyze this transformation with high yield and
low to moderate enantioselectivity (<66% ee).2a Later,
Malkov and Kocovsky disclosed that when L-valine de-
rived amide catalyst 2 was used as the catalyst, significantly
improved enantioselectivities (<92% ee) were obtained.3a
In our recent report, we presented the first highly enantio-
selective Lewis basic amide catalyst 3 which displayed a
broad substrate spectrum, promoting the hydrosilylation
A common feature of the molecular structures of catalysts
1–4 is that they all contain two closely positioned amide
functionalities (a diamide unit), including an N-formamide.
The chelation of the oxygen atoms of these two amide
groups with the central silicon atom of HSiCl3 was believed
to be a prerequisite for the operation of the catalyst over
the course of the reaction.2a,3 We were recently interested
in designing new, structurally simple C2-symmetric cata-
lysts bearing two such identical diamide units and evaluat-
ing their efficacies for the asymmetric hydrosilylation of
imines. The two identical diamide units could either func-
tion cooperatively or separately and thus endow the result-
ing C2-symmetric tetraamide catalysts with different
reactivities and/or enantioselectivities compared with the
diamide catalysts. The results might shed some light on
the mechanism of the Lewis base catalyzed asymmetric
hydrosilylation, of which the transition state models in
the literature are still debatable.2,3a,5b,c Thus we
prepared a set of a-amino acid derived C2-symmetric chiral
tetraamide catalysts 5–8 (Fig. 2) and tested their effects in
the hydrosilylation of N-aryl ketimines. Herein, we report
the results, which reveal for the first time that the C2-sym-
metric chiral tetraamide 5b6 behaved as an efficient enan-
tioselective Lewis basic catalyst, affording up to 95%
yield and 86% ee value.
t
SO (p- BuPh)
2
N
H
N
H
H
H
N
Ph
Ph
N
N
1
N
R
N
R
2
N
N
Ph
O
O
O
O
H
O
O
H
O
H
O
H
O
Ac
1
2
3
4
1
2
a, R = Ph
a, R = Ph
1
2
b, R = 1-Naphthyl b, R = 3,5-Me C H
2
6
3
Figure 1.
*
Corresponding author. Tel.: +86 28 85211220; fax: +86 28 85222753;
e-mail: sunjian@cib.ac.cn
0957-4166/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetasy.2007.03.008

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Z. Wang et al. / Tetrahedron: Asymmetry 18 (2007) 705–709
Table 1. Asymmetric hydrosilylation of imine 9a with different catalysts
H
N
H
N
H
N
H
N
X
N
N
Ph
N
N
n
Ph
HN
10 mol % Catalyst
N
O
O
O
O
5
6
H
O
O
H
H
O
O
H
HSiCl3, CH2Cl2, 0 oC
16 h
Ph *
Ph
a, n = 0; b, n = 2
c, n = 3; d, n = 4
9a
10a
X =
Entry
Catalyst
Yielda (%)
eeb (%)
Configurationc
H
N
H
N
1
2
5a
5b
5c
5d
6a
6b
6c
6d
6e
6f
7
51
93
87
64
84
70
77
82
77
82
64
97
68
92
<10
77
43
25
47
11
19
24
<10
<10
16
<10
38
(R)
(R)
(R)
(R)
(R)
(R)
(R)
(R)
(R)
(S)
(S)
(S)
(R)
(R)
b
a
N
N
O
O
7
H
H
O
O
H
3
4
5
6
7
8
9
10
11
12
13
14
c
e
d
f
H
H
N
N
N
N
Ph
Ph
Ph
Ph
O
O
O
O
H
8
Figure 2.
8
11
12
20
2. Results and discussion
a Isolated yield based on the imine.
b Determined using chiral HPLC.
c Identified by comparison of the HPLC data with the literature data.
Catalysts 5 and 6 were easily prepared by starting from the
readily available N-Boc-L-proline and the corresponding
diamines according to Scheme 1. Catalysts 7 and 8 were
prepared similarly starting from the corresponding N-Boc
protected amino acids and ethylenediamine.
To explore if the two identical diamide units of the most
effective C2-symmetric tetraamide catalyst 5b are depen-
dent upon each other for the asymmetric induction, the
truncated molecules 11 and 12 (Fig. 3) were synthesized
and tested as catalysts in the hydrosilylation of 9a. Interest-
ingly, both 11 and 12 displayed dramatically decreased
enantioselectivities compared with 5b under identical con-
ditions (entries 13 and 14 vs entry 2, Table 1), clearly indi-
cating a synergistic effect of the two L-prolyl diamide units
of 5b.
With these catalysts in hand, we next tested their catalytic
efficiencies in the hydrosilylation of 9a with HSiCl3 at 0 °C
in CH2Cl2. As illustrated in Table 1, the linkage of the two
proline diamide units proved to have significant impacts on
the enantioselectivity of catalysts 5 and 6. For catalysts 5,
the two-carbon linkage seems to be the best. Catalyst 5b
gave an ee of 77% (entry 2), whereas 5a, 5c, and 5d with
either a shorter or longer linkage afforded much lower
enantioselectivities (entries 1, 3, and 4). Aromatic linkages
in 6a–c were found to be unfavorable (entries 5–7), so were
the substituted two-carbon linkages in 6d–f (entries 8–10).
Catalyst 5b was also found to be effective for the asymmet-
ric hydrosilylation of a broad range of other ketimines 9a–
k (Table 2), including aromatic and aliphatic ones. In the
Interestingly, while both the L-valine backbone and the L-
pipecolinic acid backbone have proven to be superior to
the L-proline backbone in the diamide catalyst system for
asymmetric induction,3,4a in the C2-symmetric tetraamide
catalyst system, a completely different scenario was
observed. Catalysts 7 and 8 with the L-valine backbone
and the L-pipecolinic acid backbone, respectively, both
exhibited much lower enantioselectivity than the L-proline
derived catalyst 5b under identical conditions (entries 11
and 12 vs entry 2).
H
N
H
N
H
N
N
N
O
O
O
H
O
H
O
11
12
Figure 3.
H
N
H
ClCO2Et
+
N
H2N
NH2
Et3N
CO2H
N
N
N
Boc
Boc
Boc
O
O
H
H
1. TFA/CH2Cl2
2. HCOOH/(AcO)2O
N
N
N
N
O
O
O
O
H
H
Scheme 1.

Z. Wang et al. / Tetrahedron: Asymmetry 18 (2007) 705–709
707
Table 2. Asymmetric hydrosilylation of various imines catalyzed by 5b
ity. The two identical diamide units of the catalyst proved
to work cooperatively for the asymmetric induction.
R4
R4
R5
HN
N
10 mol % 5b
R3
R5
HSiCl3, CH2Cl2, 0 oC
16 h
R3
*
4. Experimental
9
10
4.1. General procedure for the synthesis of catalysts
Entry
Imine
R
Yielda
(%)
eeb
(%)
To a solution of Boc-L-amino acid (10.0 mmol) and TEA
(1.0 g, 10.0 mmol) in THF (40 mL) was added ethyl chloro-
formate (1.1 g, 10.0 mmol) dropwise at 0 °C. After stirring
for 0.5 h, diamine (5.0 mmol) was introduced. The resulting
solution was continued to stir at 0 °C for 1 h, at room tem-
perature for 16 h, and then at reflux for 3 h. The reaction
was then cooled down to room temperature and diluted
with ethyl acetate. After filtration and removal of solvents
under reduced pressure, the residue was purified through
column chromatography on silica gel to give the N,N0-di-
Boc protected diamide, which was then treated with
20 vol % TFA in CH2Cl2 (40 mL). After stirring for 1 h,
the solution was concentrated under reduced pressure.
The residue was dissolved in formic acid (1.5 mL) and
the solution was then cooled down to 0 °C. Acetic anhy-
dride (1 mL) was added dropwise at the same temperature.
The mixture was warmed to room temperature and allowed
to stir overnight. After removal of solvents under reduced
pressure, the residue was purified though column chroma-
tography on silica gel to give pure product.
1
2
3
4
5
6
7
9a
9b
9c
9d
9e
9f
C6H5
4-BrC6H4
4-NO2C6H4
4-MeOC6H4
2-Np
93
95
88
82
90
82
74
77
74
74
75
70
70
61
Ph
N
R
6-MeONp
c-C6H11
9g
R
N
N
8
9
9h
9i
4-ClC6H4
4-MeOC6H4
95
77
68
73
Ph
Ph
10
11
12
13
9j
Et
93
88
87
78
78
80
79
86
Ph
R
9k
9l
nPr
nBu
iBu
9m
a Isolated yield based on the imine.
b Determined using chiral HPLC.
presence of 10 mol % 5b, 74–95% yields and 61–86% ee val-
ues were obtained.
4.1.1. N,N0-Di-(N-formyl-L-prolyl)-hydrazine 5a. Viscous
20
liquid, yield: 63%, ½aꢀD ¼ ꢁ137:0 (c 0.108, EtOH);
1H NMR (300 MHz, CD3OD): d = 1.89–2.30 (m, 4H),
3.30–3.35 (m, 2H), 4.38–4.54 (m, 1H), 8.25 (s, 1H); 13C
NMR (150 MHz, CD3OD): d = 22.5, 23.6, 29.6, 30.2,
44.0, 46.7, 56.4, 58.8, 162.1, 162.9, 171.4, 171.7. ESI
HRMS exact mass calcd for (C12H18N4O4+Na)+ requires
m/z 305.1220; found, 305.1226.
On the basis of the above experimental data, we propose
that the 5b-catalyzed hydrosilylation of ketimines occurred
via either transition state I or II,7 both of which could rea-
sonably explain the synergistic effects of the two diamide
units and the absolute configuration of the product as well
(Fig. 4).
4.1.2. N,N0-Di-(N-formyl-L-prolyl)-ethane-1,2-diamine 5b.
20
Viscous liquid, yield: 75%; ½aꢀD ¼ ꢁ81:25 (c 0.16, EtOH).
The major conformer, 1H NMR (600 MHz, CD3OD):
d = 1.87–2.29 (m, 8H), 3.31–3.36 (m, 4H), 3.64–3.71 (m,
4H), 4.26–4.30 (m, 2H), 8.25 (s, 2H); 13C NMR
(150 MHz, MeOD): d = 22.5, 23.67, 29.63, 29.7, 38.6,
38.9, 44.0, 46.8, 58.2, 60.2, 162.3, 162.4, 172.9, 173.2; ESI
HRMS exact mass calcd for (C14H22N4O4+Na)+ requires
m/z 333.1533; found, 333.1522.
O
HN
O
HN
O
HN
O
HN
N
N
N
N
O
O
O
H
H
H
Si
Si
Cl
Cl
Cl
H
RL
O
Cl
H
H
RL
RS
RS
Cl
Cl
4.1.3. N,N0-Di-(N-formyl-L-prolyl)-propane-1,3-diamine 5c.
N
N
20
I
II
Viscous liquid, yield: 73%, ½aꢀD ¼ ꢁ89:0 (c 0.132, EtOH).
The major conformer, 1H NMR (300 MHz, CD3OD):
d = 1.69–2.30 (m, 5H), 3.17–3.23 (m, 2H), 3.53–3.69 (m,
2H), 4.30–4.46 (m, 1H), 8.26 (s, 1H); 13C NMR
(150 MHz, CD3OD): d = 22.5, 23.7, 28.6, 29.8, 30.2, 36.1,
36.3, 44.0, 46.8, 58.2, 60.2, 162.3, 162.9, 172.7, 172.9. ESI
HRMS exact mass calcd for (C15H24N4O4+Na)+ requires
m/z 347.1690; found, 347.1673.
Figure 4.
3. Conclusions
In conclusion, we have shown that structurally simple C2-
symmetric tetraamides could work as effective Lewis basic
catalysts for the asymmetric hydrosilylation of ketimines.
Compound 5b, easily prepared by starting from L-proline,
catalyzed the hydrosilylation of a broad range of N-aryl
ketimines in high isolated yield with good enantioselectiv-
4.1.4. N,N0-Di-(N-formyl-L-prolyl)-butane-1,4-diamine 5d.
20
White solid, yield: 70%, mp 146.0–148.0 °C, ½aꢀD ¼ ꢁ111:0
(c 0.136, EtOH); 1H NMR (600 MHz, DMSO-d6):
d = 1.36–1.38 (m, 2H), 1.73–1.90 (m, 3H), 2.07–2.14 (m,

708
Z. Wang et al. / Tetrahedron: Asymmetry 18 (2007) 705–709
1H), 3.02–3.07 (m, 2H), 3.48–3.6 (m, 1H), 3.57–3.61 (m,
1H), 4.14–4.16 (m, 1H), 8.19 (s, 1H); 13C NMR
(150 MHz, DMSO-d6): d = 23.0, 24.0, 26.8, 26.9, 30.3,
30.6, 38.6, 38.8, 44.2, 46.7, 57.8, 59.7, 161.5, 161.9, 171.4,
171.8. ESI HRMS exact mass calcd for (C16H24N4O4+
Na)+ requires m/z 361.1846; found, 361.1826.
4.1.10. N,N0-Di-(N-formyl-L-prolyl)-(1S,2S)-1,2-diphenyl-
ethane-1,2-diamine 6f. White solid, yield: 55%, mp
20
1
139.0–143.0 °C, ½aꢀD ¼ ꢁ61:5 (c 0.104, MeOH); H NMR
(600 MHz, DMSO-d6): d = 1.20–2.00 (m, 8H), 3.22–3.30
(m, 2H), 3.39–3.53 (m, 2H), 4.13–4.18 (m, 1H), 4.35 (m,
3.18 Hz, 1H), 5.32 (d, J = 7.8 Hz, 1H), 5.40 (d, J =
7.44 Hz, 1H), 7.13–7.32 (m, 10H), 7.90 and 7.91 (s, 1H),
8.14 and 8.15 (s, 1H), 8.28 (d, J = 7.86 Hz, 1H), 8.41 (d,
J = 8.82 Hz, 1H); 13C NMR (150 MHz, DMSO-d6):
d = 22.8, 23.9, 29.8, 30.5, 44.1, 46.6, 56.8, 57.2, 57.7,
59.6, 127.2, 127.2, 127.3, 127.4, 127.5, 127.5, 128.2, 128.3,
140.4, 140.6, 161.3, 161.9, 171.2, 171.5. ESI HRMS
exact mass calcd for (C26H30N4O4+Na)+ requires m/z
485.2159; found, 485.2137.
4.1.5. N,N0-Di-(N-formyl-L-prolyl)-benzene-1,2-diamine 6a.
20
White solid, yield: 67%, mp 63.0–69.0 °C, ½aꢀD ¼ ꢁ69:5 (c
0.154, EtOH); 1H NMR (600 MHz, CDCl3): d = 1.87–
2.40 (m, 8H), 3.56–3.66 (m, 4H), 4.65 (t, J = 6.12 Hz,
2H), 7.07–7.16 (m, 2H), 7.58 (m, 2H), 8.26 (s, 2H), 9.20
(s, 2H); 13C NMR (150 MHz, CDCl3): d = 23.1, 24.3,
28.7, 30.6, 44.4, 47.2, 58.7, 61.1, 124.9, 125.9, 130.0,
161.8, 170.1. ESI HRMS exact mass calcd for (C18H22-
N4O4+Na)+ requires m/z 381.1533; found, 381.1540.
4.1.11. N,N0-Di-(N-methylformyl-L-valinyl)-ethane-1,2-di-
20
amine 7. White solid, yield: 48%, ½aꢀD ¼ ꢁ71:1 (c 0.242,
4.1.6. N,N0-Di-(N-formyl-L-prolyl)-benzene-1,3-diamine 6b.
CH3OH); 1H NMR (300 MHz, CD3OD); d = 0.84 (d,
J = 6.57 Hz, 6H), 1.19–1.24 (m, 1H), 2.97 (s, 3H), 3.29–
3.47 (m, 2H), 4.07–4.28 (m, 1H), 8.08 (s, 1H); 13C NMR
(150 MHz, CD3OD): d = 13.5, 17.5, 18.2, 18.3, 26.4, 26.5,
30.5, 38.4, 38.9, 39.7, 61.0, 68.1, 164.1, 164.5, 170.3,
170.4. ESI HRMS exact mass calcd for (C16H30N4O4+
Na)+ requires m/z 365.2159; found, 365.2148.
20
White solid, yield: 65%, mp 113.0–117.0 °C, ½aꢀD ¼ ꢁ160:0
1
(c 0.14, EtOH); H NMR (600 MHz, CDCl3): d = 1.83–
2.24 (m, 8H), 3.47–3.68 (m, 4H), 4.56 (t, J = 6.48 Hz,
2H), 6.95 (t, J = 8.16 Hz, 1H), 7.21 (d, J = 7.68 Hz, 2H),
7.70 (s, 1H), 8.26 (s, 2H), 9.77 (s, 2H); 13C NMR
(150 MHz, CDCl3): d = 24.3, 29.2, 47.1, 47.2, 58.8, 110.7,
115.0, 129.0, 138.4, 161.7, 161.8, 169.9, 171.1. ESI HRMS
exact mass calcd for (C18H22N4O4+Na)+ requires m/z
381.1533; found, 381.1534.
4.1.12. N,N0-Di-(N-formyl-L-piperidyl)-ethane-1,2-diamine
20
8. Viscous liquid, yield: 51%, ½aꢀD ¼ ꢁ114:4 (c 0.104,
1
EtOH), H NMR (600 MHz, CD3OD): d = 1.38–1.67 (m,
4.1.7. N,N0-Di-(N-formyl-L-prolyl)-benzene-1,4-diamine 6c.
8H), 2.25–2.33 (m, 3H), 2.80–2.85 (m, 1H ), 3.27–3.39
(m, 4H), 3.62–3.64 (d, J = 13.08 Hz, 2H), 4.24–4.26 (m,
1H), 4.32–4.34 (m, 1H), 4.90–4.92 (m, 2H), 8.11 (s, 2H);
13C NMR (150 MHz, CD3OD): d = 20.7, 20.8, 24.18,
25.2, 25.9, 27.0, 39.1, 39.1, 44.4, 51.3, 57.6, 163.5, 171.1,
171.5; ESI HRMS exact mass calcd for (C16H26N4O4+
Na)+ requires m/z 361.1846; found, 361.1856.
20
White solid, yield: 68%, mp 206.0–208.0 °C, ½aꢀD ¼ ꢁ194:0
(c 0.1, EtOH); 1H NMR (600 MHz, CDCl3): d = 1.89–2.37
(m, 4H), 3.64–3.75 (m, 2H), 4.63 (t, J = 6.15 Hz, 1H), 7.28
(s, 2H), 8.35 (s, 1H), 9.51 (s, 1H); 13C NMR (150 MHz,
CDCl3): d = 24.3, 28.3, 47.3, 58.7, 119.7, 120.0, 134.3,
161.8, 168.7. ESI HRMS exact mass calcd for (C18H22-
N4O4+Na)+ requires m/z 381.1533; found, 381.1536.
4.1.8. N,N0-Di-(N-formyl-L-prolyl)-(1S,2S)-cyclohexane-1,2-
4.2. General procedure for the synthesis of imines
diamine 6d. White solid, yield: 62%, mp 240.0–241.0 °C,
20
½aꢀD ¼ ꢁ148:3 (c 0.160, EtOH); 1H NMR (600 MHz,
A mixture of NaHCO3 (50 mmol), amine (10 mmol), ke-
tone (10 mmol) and activated molecular 4 A sieves (8.0 g)
˚
CD3OD): d = 1.29–1.39 (m, 4H), 1.78–2.27 (m, 12H),
3.45–3.71 (m, 6H), 4.25–4.44 (m, 2H), 8.23 (s, 2H); 13C
NMR (150 MHz, CD3OD): d = 22.5, 23.7, 24.4, 29.7,
30.2, 31.7, 44.0, 46.7, 52.7, 52.8, 58.2, 60.6, 162.2, 163.3,
172.7, 172.4. ESI HRMS exact mass calcd for (C12H18-
N4O4+Na)+ requires m/z 305.1220; found, 305.1226. ESI
HRMS exact mass calcd for (C18H28N4O4+Na)+ requires
m/z 387.2003; found, 387.2001.
in anhydrous toluene (10 mL) was heated at 80 °C for
12 h under an argon atmosphere. The mixture was filtered
through Celite. The filtrate was then evaporated in vacuo
and the product crystallized from appropriate solvents or
purified by distillation to give pure imine.
4.3. General procedure for catalytic hydrosilylation of imines
4.1.9. N,N0-Di-(N-formyl-L-prolyl)-(1R,2R)-1,2-diphenyl-
Under an argon atmosphere, trichlorosilane (40 lL,
0.4 mmol) was added dropwise to a stirred solution of
imine 9 (0.20 mmol) and catalyst 5b (6.2 mg, 0.02 mmol)
in anhydrous CH2Cl2 at 0 °C. The mixture was allowed
to stir at the same temperature for 16 h. The reaction was
quenched with a saturated aqueous solution of NaHCO3
(5 mL) and was extracted with EtOAc. The combined ex-
tracts were washed with brine and dried over anhydrous
MgSO4. Solvents were evaporated. The crude product
was purified by column chromatography (silica gel, hex-
ane/EtOAc) to afford pure amine 10. The ee values were
determined by using established HPLC techniques with
chiral stationary phases.
ethane-1,2-diamine 6e. White solid, yield: 52%, mp
20
222.0–225.0 °C, ½aꢀD ¼ ꢁ127:9 (c 0.104, MeOH); 1H
NMR (600 MHz, DMSO-d6): d = 1.47–2.10 (m, 8H),
3.18–3.61 (m, 4H), 4.17–4.38 (m, 2H), 5.21–5.31 (m, 2H),
7.11–7.22 (m, 10H), 7.94 and 7.91 (s, 1H), 8.25 and 8.23
(s, 1H), 8.44 and 8.39 (d, J = 7.44 Hz, 1H), 8.71 and 8.59
(d, J = 6.78 Hz, 1H); 13C NMR (150 MHz, DMSO-d6):
d = 22.9, 23.9, 29.9, 30.3, 30.6, 44.2, 46.7, 57.2, 57.8,
59.6, 127.2, 127.3, 127.4, 127.5, 127.6, 128.2, 128.3, 140.4,
140.5, 161.8, 162.1, 171.3, 171.8. ESI HRMS exact mass
calcd for (C26H30N4O4+Na)+ requires m/z 485.2159;
found, 385.2137.

Z. Wang et al. / Tetrahedron: Asymmetry 18 (2007) 705–709
709
3. (a) Malkov, A. V.; Mariani, A.; MacDougall, K. N.; Kocov-
sky, P. Org. Lett. 2004, 6, 2253; (b) Malkov, A. V.; Stoncius,
S.; MacDougall, K. N.; Mariani, A.; McGeoch, G. D.;
Kocovsky, P. Tetrahedron 2006, 62, 264.
4. (a) Wang, Z.; Ye, X.; Wei, S.; Wu, P.; Zhang, A.; Sun, J. Org.
Lett. 2006, 8, 999; (b) Wang, Z.; Cheng, M.; Wu, P.; Wei, S.;
Sun, J. Org. Lett. 2006, 8, 3045.
Acknowledgement
We are grateful for the financial supports from the
National Natural Science Foundation of China (Projects
20402014 and 20672107).
5. For the other types of Lewis base organocatalysts for the
asymmetric hydrosilylation of imines, see: (a) Pei, D.; Wang,
Z.; Wei, S.; Sun, J. Org. Lett. 2006, 8, 5913; (b) Malkov, A. V.;
Liddon, A.; Ramirez-Lopez, P.; Bendova, L.; Haigh, D.;
Kocovsky, P. Angew. Chem., Int. Ed. 2006, 45, 1432; (c)
Onomura, O.; Kouchi, Y.; Iwasaki, F.; Matsumura, Y.
Tetrahedron Lett. 2006, 47, 3751.
6. During the preparation of this manuscript, an analogous C2-
symmetric chiral tetraamide was reported to behave as a
stoichiometric Lewis basic activator for the allylation of
imines, see: Jagtap, S. B.; Tsogoeva, S. B. Chem. Commun.
2006, 4747.
References
1. For recent reviews on asymmetric Lewis base catalysis, see: (a)
Denmark, S. E.; Heemstra, J. R.; Beutner, G. L. Angew.
Chem., Int. Ed. 2005, 44, 4682; (b) Rendler, S.; Oestreich, M.
Synthesis 2005, 1727; (c) Koboyashi, S.; Sugiura, M.; Ogawa,
C. Adv. Synth. Catal. 2004, 346, 1023; (d) Denmark, S. E.; Fu,
J. Chem. Rev. 2003, 103, 2763; (e) Denmark, S. E.; Fu, J.
Chem. Commun. 2003, 167; (f) Denmark, S. E.; Stavenger,
R. A. Acc. Chem. Res. 2000, 33, 432.
7. Heptacoordinate silicon structures have been previously
reported, see examples: (a) Kano, N.; Nakagawa, N.; Kawa-
shima, T. Angew. Chem., Int. Ed. 2001, 40, 3450; (b) Holmes,
R.; Oestreich, M. Chem. Rev. 1996, 96, 927.
2. (a) Iwasaki, F.; Onomura, O.; Mishima, K.; Kanematsu, T.;
Maki, T.; Matsumura, Y. Tetrahedron Lett. 2001, 42, 2525; (b)
Onomura, O.; Kouchi, Y.; Iwasaki, F.; Matsumura, Y.
Tetrahedron Lett. 2006, 47, 3751.