New chiral biscarboline N,N′-dioxide derivatives as catalyst in enantioselective reduction of ketoimines with trichlorosilane

Article abstract of DOI:10.1016/j.tetlet.2014.03.100

New axial N,N′-dioxide secondary amide derived from l-tryptophan was synthesized and firstly employed in catalytic enantioselective reduction of ketoimines with trichlorosilane. It was found that 4f was an effective catalyst with excellent reactivity and

Full text of DOI:10.1016/j.tetlet.2014.03.100

Tetrahedron Letters 55 (2014) 2948–2952
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Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
New chiral biscarboline N,N0-dioxide derivatives as catalyst in
enantioselective reduction of ketoimines with trichlorosilane
Yu-Ning Pei a, Yu Deng a,b, Jing-Liang Li a, Li Liu a, Hua-Jie Zhu a,
⇑
a Chinese Centre for Chirality, The Key Laboratory of Medicinal Chemistry and Molecular Diagnostics of Ministry of Education, College of Pharmacy Sciences of Hebei University,
Baoding, Hebei 071002, PR China
b State Key Laboratory of Photochemistry and Plant Resources in West China, Kunming Institute of Botany, CAS, Kunming, Yunnan 650204, PR China
a r t i c l e i n f o
a b s t r a c t
New axial N,N0-dioxide secondary amide derived from
L-tryptophan was synthesized and firstly employed
Article history:
Received 4 January 2014
Revised 18 March 2014
Accepted 20 March 2014
Available online 27 March 2014
in catalytic enantioselective reduction of ketoimines with trichlorosilane. It was found that 4f was an
effective catalyst with excellent reactivity and good enantioselectivity. Possible mechanism for the
catalytic procedure was tentatively proposed.
Ó 2014 Elsevier Ltd. All rights reserved.
Keywords:
Biscarboline N–O amides
Enantioselective reduction
Ketoimines
Trichlorosilane
Introduction
asymmetric additions of trichlorosilanes to aldehydes affording
the corresponding chiral alcohols with up to 99% ee. We hope to
Chiral amines are key components in numerous bioactive mol-
ecules found in nature products, drugs, pharmaceutical, and agro-
chemical active compounds.1–7 Catalytic asymmetric reduction of
ketoimines is an efficient and straightforward method to afford dif-
ferent chiral amines, and it represents a powerful and widely used
transformation for the synthesis of new stereogenic center with a
new formed CAN bond. Most of the effective catalysis is conducted
in the presence of transition metals.8–20 Recently, much attention
has been paid to the reduction using metal-free catalysts which
could be readily synthesized and is environment-friendly. Chiral
phosphoric acid employed as a catalyst in conjunction with Han-
tzsch esters as reductive reagents could afford high yields and good
stereoselectivities in this reduction.21–23 Chiral Lewis base in con-
junction with trichlorosilane also performed well in this enantiose-
lective reduction.24
develop a series of biscarboline amides with N,N0-dioxide and to
investigate their application in enantioselective reduction of ketoi-
mines with trichlorosilanes. To the best of our knowledge, such
chiral biscarboline amides have yet been studied as catalysts in
enantioselective reduction of ketoimines with trichlorosilane.
Results and discussions
Ligands 4a–4f were synthesized from 1 which can be readily
obtained from tryptophan (Scheme 1).30a Resolution of (+)-4 from
(ꢀ)-4 was performed through a chiral column with eluents DCM/
MeOH (V/V = 85/15 to 95/5, 2 to 2.5 ml/min). Optically active com-
pounds 4a–f with positive optical rotation values should have the
(R)-configuration and the resolution conditions are similar to our
previous reports.30b As a Lewis base, the oxygen of NAO could
coordinate with trichlorosilane, which may be applied for the
enantioselective reduction of ketoimines. We carried out the initial
experiment with compound 4b (10 mol %) as a catalyst. It exhib-
ited high reactivity (yield of 99%) and moderate enantioselectivity
(35%) in the enantioselective reduction of 5a. Encouraged by the
result, 4a to 4f were investigated and the results are summarized
in Table 1.
Since Kobayashi and co-workers25 reported that DMF could
activate trichlorosilane toward the reduction of aldehydes, aldi-
mines, and ketones, great efforts have been paid in this area and
tremendous progress has been achieved.26–29 However, catalysts
with NAO structures have been rarely reported in the asymmetric
hydrosilylation of imines with trichlorosilane. In our recent study,
a
N,N0-dioxide biscarboline was synthesized and utilized in
It was found that catalyst bearing cyclic amides such as (R)-4b, f
gave much higher ee than acyclic amides (entries 1, 3–5). And, al-
most quantitative conversions were observed with all ligands
⇑
Corresponding author. Tel./fax: +86 312 5994812.
E-mail addresses: zhuhuajie@hotmail.com, jackzhu2002@sina.com (H.-J. Zhu).
http://dx.doi.org/10.1016/j.tetlet.2014.03.100
0040-4039/Ó 2014 Elsevier Ltd. All rights reserved.

Y.-N. Pei et al. / Tetrahedron Letters 55 (2014) 2948–2952
2949
Ο
Ο
Ο
Ο
Ο
R1
R2
ΟΗ
Ο
Ο
N
N
N
N
Ν
Ν
N
a
b
Ν
Ν
Ν
Ν
Ν
Ν
Ν
R2
R1
ΟΗ
Ο
1
2
3a-3f
Ο
Ο
a
b
R1
R2
: R1=R2= -Pr
N
N
N
N
: R1=R2= -(CH2)4-
c
N
O
O
Ν
Ν
N
Ν
Ν
c: R1=R2= -Bn
d: R1=R2= -n-Bu
e: R1=R2= -i-Pr
O
O
+
Ν
Ν
R2
R1
f: R1=R2= -(CH2)5-
Ο
Ο
4f
4a-4f
Scheme 1. Synthesis of chiral ligands 4a–4f. Reaction conditions: (a) NaOH, H2O, MeOH, 60 °C; (b) (i) isobutyl-chloroformate (2.4 equiv), Et3N (2.4 equiv), DCM, 0 °C, 20 min,
(ii) amine (2.2 equiv), rt, 12 h; (c) DCM, m-CPBA (6 equiv), rt, 12 h.
Table 2
Table 1
Influence of the amount of 4f on the ee% in the reductions using 5a
Enantioselective hydrosilylation of ketoimine 5a catalyzed by 4a–4f
10 mol% Cat.*
Entry
4f (%)
Time
Yieldb (%)
eec (%)
Ph
Ph
HN
N
1
2
3
4
5
20
10
5
1
0.5
16
16
16
16
16
>99
>99
98
97
37
75
77
83
83
35
HSiCl3,CH2Cl2,0 oC
16 h
Ph
*
Ph
5a
6a
Entry
Ligand
Yieldb (%)
Eec (%)
Config.d
a
All reactions were performed using 5 (0.1 mmol) and HSiCl3 (2 equiv) in the
presence of 4f at 0 °C for 16 h.
Isolated yield.
1
2
3
4
5
6
4a
4b
4c
4d
4e
4f
99
99
99
99
>99
>99
47
57
50
57
42
77
S
S
S
S
S
S
b
c
Determined by HPLC.
Table 3
a
All reactions were performed using 5a (0.1 mmol) and HSiCl3 (2 equiv) in the
presence of 4f at 0 °C for 16 h.
Isolated yield.
Determined by HPLC.
The configuration was determined by comparing the sign of specific rotation
value with the literature value.
Effect of solvents and temperatures on the enantioselective reduction of ketoimine
5aa
b
c
Entry
Solvent
T (°C)
Yieldb (%)
eec (%)
d
1
2
3
4
5
6
7
8
9
10
11
12
13
DCM
DCM
DCM
DCM
DCM
DCM
THF
CHCl3
CH3CN
CH2ClCH2Cl
Toluene
CH3CH2OH
DMF
24
10
5
>99
>99
>99
>99
95
95
97
99
90
99
95
—
70
51
49
60
83
71
25
82
73
13
76
69
—
0
ꢀ10
ꢀ20
0
0
0
0
0
0
0
(entries 1–6). Absolute configuration of N-(1-phenylethyl)aniline
(6a) was assigned as (S) via comparing with previous experimental
results and our recent study.30b,31
Influence of the catalyst loading for the reduction was then
examined (Table 2). Surprisingly, when the catalyst amount in-
creased to 20 mol %, the ee% was lower than that with 10 mol %
of catalyst (entries 1 and 2). When it decreased to 1 mol %, the ee
was improved to 83% without slowing down the reaction (Table 2,
entries 2–4). However, if the catalyst was further reduced to
0.5 mol % or lower, both reactivity and enantioselectivity were sig-
nificantly decreased (entry 5). Therefore, 1 mol % of 4f was opti-
mized in the standard conditions.
0
a
All reactions were performed using 5a (0.1 mmol) and HSiCl3 (2 equiv) in the
presence of 4f at 0 °C for 16 h.
Isolated yield.
Determined by HPLC.
b
c
Effects of solvents and temperatures on the enantioselectivity
were examined as well (Table 3). Good ee were observed (83%)
at 0 °C. Neither higher nor lower temperature could increase the
enantioselectivity (entries 1–6). Furthermore, dichloromethane
was selected among chloroform, dichloroethane, toluene, acetoni-
trile, and THF (entries 6–10). The enantioselectivity in acetonitrile
was only 13% (entry 9). Interestingly, no reactivity and
enantioselectivity were observed when ethanol was used as the
solvent (entry 12).
After the reaction conditions were optimized, a range of N-aryl
ketoimines were investigated in the standard conditions by tri-
chlorosiliane in the presence of 1 mol % of catalyst 4f in dichloro-
methane at 0 °C. All the reductions using N-aryl ketoimines
afforded excellent yield up to 99% (Table 4). Ketoimines 5b and

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Y.-N. Pei et al. / Tetrahedron Letters 55 (2014) 2948–2952
Ph groups orientated at equatorial position (pre-S configuration, TS
Table 4
structure 8). The hydride of the first HSiCl3 will attack the C@N
and, the complex 8 will dissociate to the active catalyst 7 and the
addition product 9. Finally, this reaction can be quenched with sat-
urated aqueous solution of NH4Cl, and afford (S)-6a as the expected
product.
Enantioselective reduction of ketoimines 5a–l with trichlorosilane catalyzed by (R)-4f
R2
R2
(R)-4f (1 mol%)
N
5
HN
HSiCl3, CH2Cl2, 0 oC
16 h
R1
R3
R1
R3
*
Conclusion
6
In summary, we have developed a new chiral axial amide with
N,N0-dioxide moiety and firstly utilized in enantioselective reduc-
tion of N-aryl ketoimines with trichlorosilane. This catalyst affor-
ded high yields and good enantioselectivities under mild reaction
conditions. A six-membered transition state responsible for the
stereoselectivity was proposed in this catalysis.
Entry
5, R1, R2, R3
Yielda (%)
eeb (%)
ORc
Config.d
1
2
3
4
5
6
7
8
9
5a: Ph, H, Me
97
98
97
96
96
95
97
97
99
97
98
95
83
68
71
80
67
85
75
82
84
71
71
81
ꢀ12.0
ꢀ12.8
ꢀ8.2
14.5
S
S
S
S
R
S
S
S
R
S
R
S
5b: 4-F-Ph, H, Me
5c: 4-Cl-Ph,H, Me
5d: 4-Br-Ph, H, Me
5e: 4-NO2-Ph,H,Me
5f: 4-MeO-Ph,H,Me
5g: Ph, MeO, Me
5h: Ph, EtO, Me
5i: Ph, Me, Me
31.9
ꢀ15.3
ꢀ4.3
ꢀ15.8
4.6
Experimental section
10
11
12
5j: Ph, Et, Me
5k: Ph, Br, Me
5l: Ph, H, Et
ꢀ1.7
All reactions were monitored by TCL. Flash chromatography
was performed using silica gel (200–300 mesh). 1H NMR was re-
corded with 400 or 600 MHz in CDCl3 or DMSO with tetramethyl-
silane (TMS) as a reference. HPLC analysis was performed with
chiral columns. Optical rotations were recorded using Na 589 nm.
25.1
ꢀ34.3
Unless specified, all reactions were carried out with 2 equiv of HSiCl3, 1% (R)-4f in
1.5 ml CH2Cl2 at 0 °C for 16 h.
a
Isolated yield.
Determined by HPLC.
The OR is measured by OR machine.
The configuration was determined by comparing the sign of specific rotation
b
c
Experimental procedure for preparation of 3e–4f N,N,N0,N0-tet
raisopropyl-9,90-dimethyl-9H,90H-1,10-bipyrido[3,4-b]indole-3,30-
dicarboxamide (3e)
d
value with the literature value.
5e with electron-withdrawing functional groups gave relatively
moderate stereoselectivities with 68% and 67% ee (entries 2, 5).
However, ketoimines 5f and 5h with the electron-donating group
gave higher stereoselectivities (85%, 82% ee, entries 6, 8).
Because it was the first time that biscarboline N–O amides were
employed in the enantioselective reductions, it is worth to discuss
its plausible mechanism. It should be different from the one that
the chiral catalysts contained a free –OH.30b In this case, the proton
of –OH could chelate with the N atom of the C@N group and then
the hydride on Si could transfer to imine carbon.
Firstly, the NAO group is necessary in the enantioselective
hydrosilylation. If this group did not exist, such as 3a, the product
was only formed with a very low yield (less 20%) and less than 5%
ee under the same conditions.
In our previous study, it was found that the two NAO oxygens
were trans and therefore independent from each other in the pres-
ence of two five-membered rings in the structure of 9,90-dimethyl-
3,30-di(pyrrolidine-1-carbonyl)-9H,90H-[1,10-bipyrido[3,4-b] indole]
2,20-dioxide. This geometry can affect the following transition state
structures and lead the product to different configurations.30c Pre-
sumably, the designed catalyst with six-membered ring skeleton
could process a similar geometry to the five-membered-ring
skeleton.
To a solution of 2 (0.45 g, 1 mmol) in anhydrous CH2Cl2 were
added Et3N (0.33 ml, 2.4 mmol) and isobutyl-chloroformate drop-
wise (0.31 ml, 2.4 mmol) at 0 °C. Piperidine (0.22 ml, 2.2 mmol)
was added after 30 min. The reaction was slowly warmed to
room temperature and detected with TLC. After the reaction fin-
ished, HCl aqueous solution (1 mol/L) was added to quench the
reaction. Saturated NaHCO3 aqueous solution was used to adjust
pH to 7–8 and washed with brine, dried with over anhydrous
Na2SO4. The solution was concentrated under reduced pressure
and the residue was purified with silica gel to get a yellow solid.
Yield of 88%. IR (KBr, cmꢀ1), 3428, 2969, 1639, 1445, 1392, 1312,
1237, 1131, 1006, 748. MS-ESI, m/z 616 [M+H]+. HR-MS m/z calcd
for C42H52N6O2 616.3530, found 616.3526. 1H NMR (400 MHz,
CDCl3) d 8.37 (s, 1 Hꢁ2), 8.17 (d, J = 7.8 Hz, 1 Hꢁ2), 7.58 (t,
J = 7.6 Hz, 1 Hꢁ2), 7.35 (m, 2 Hꢁ2), 3.42 (s, 3 Hꢁ2), 3.25 (m, 4
Hꢁ2), 1.74–1.40 (m, 2 Hꢁ2), 1.39–1.10 (m, 2 Hꢁ2), 0.98 (t,
J = 7.3 Hz,
3
Hꢁ2), 0.85 (t, J = 7.3 Hz,
3
Hꢁ2). 13C NMR
(100 MHz, CDCl3) d 164.2, 145.7, 137.2, 128.6, 122.3, 121.0,
120.6, 116.0, 109.8, 47.0, 44.8, 30.8, 29.9, 28.2, 20.3, 19.8, 14.0.
(9,90-Dimethyl-9H,90H-1,10-bipyrido[3,4-b]indole-3,30-diyl)bis
(piperidin-1-ylmethanone) (3f)
After conformational searches using MMFF94S force field, it was
found that this catalyst with a six-membered ring has very similar
geometry (Fig. 1, up) with that of five-membered ring although the
reported catalyst with five-membered ring can catalyze enantiose-
lective allylation of aldehydes with allyltrichlorosilanes. The tran-
sition state (TS) in the reduction of ketoimine with HSiCl3 was
thereby proposed as illustrated in Fig. 1 (down).
In the conformation with the lowest energy, the NAO oxygens
were independent from each other in the presence of the two six-
membered rings (Fig. 1, up). When the first SiHCl3 approached to
the catalyst, it would chelate with one of the two OAN groups. Then,
the oxygen of amide C@O will chelate to the same silicon center and
form the intermediate 7 (only half structure illustrated for clarity in
Fig. 1). After this chelation, the second SiHCl3 and PhC(Me)@NPh will
bind with the intermediate 7 forming a 6-membered ring with two
Following the general procedure above, 3f was obtained as yel-
low solid yield 67%. IR (KBr) n = 3425, 2931, 1615, 1541, 1439,
1408, 1266, 1131, 1044, 756 cmꢀ1. MS-ESI, m/z 584 [M+H]+. HR-
MS m/z calcd for C36H36N6O2 584.2900, found 584.2886. 1H NMR
(400 MHz, CDCl3) d 8.53 (s, 1 Hꢁ2), 8.22 (d, J = 13.4 Hz, 1 Hꢁ2),
7.64 (t, J = 7.3 Hz, 1 Hꢁ2), 7.38 (dd, J = 14.2, 7.7 Hz, 2 Hꢁ2), 4.15-
3.36 (m, 4 Hꢁ2), 3.23 (s, 3 Hꢁ2), 1.87–1.13 (m, 4 Hꢁ2), 0.91 (m,
2 Hꢁ2). 13C NMR (100 MHz, CDCl3) d 167.7, 143.1, 135.9, 129.6,
122.0, 120.8, 115.8, 109.9, 48.7, 43.7, 31.8, 26.6, 25.5, 24.5.
3,30-Bis(diisopropylcarbamoyl)-9,90-dimethyl-9H,90H-1,10-bipyri-
do[3,4-b]indole 2,20-dioxide (4e)
To a stirred solution of 3e (0.29 g, 0.5 mmol) in CH2Cl2 (25 ml)
was added m-chlorobenzoperoxoic acid (m-CPBA) (75%, 1.5 mmol)

Y.-N. Pei et al. / Tetrahedron Letters 55 (2014) 2948–2952
2951
Two oxygens are trans and
independent
This is the geometry with
the lowest energy
(R)-(+)-4f
N
N
N
O
N
O
N
O
O
N
4f
SiHCl3
PhC(Me)=NPh
SiHCl3
Cl-
N
N
O
N
O
Cl
Cl
N
H
N
N
O
Si
Cl
Ph
N
O
H
Si
Cl
Ph
Pre-S
Cl
Cl
7
Cl
Si
Cl
H
Me
8: 6-memebered ring TS
Only half structure
illustrated here
Cl
Cl
H
Si
Cl
H2O
Ph
Ph
N
Ph
Ph
N
H
H
S
S
Me
6a
Me
9
Figure 1. The structure of free ligand (up, color picture) and the plausible transition state structures for the formation of (S)-6a from 5a.
at 0 °C. The reaction was quenched with saturated NaHCO3 and
washed with brine after completion. The organic layer was concen-
trated and the residue was purified through silica gel chromatogra-
phy (DCM/MeOH = 50:1) to afford 4e as yellow solid, yield 65%. IR
(KBr) n = 3428, 2969, 1639, 1445, 1392, 1312, 1237, 1131, 1006,
1006, 749 cmꢀ1. MS-ESI, m/z 617[M+H]+. HR-MS m/z calcd for
C36H36N6O4 616.2798, found 616.2778. 1H NMR (400 MHz, CDCl3)
d 8.12 (s, 1 Hꢁ2), 7.98 (d, J = 7.8 Hz, 1 Hꢁ2), 7.49 (t, J = 7.5 Hz, 1
Hꢁ2), 7.33–7.21 (m, 2 Hꢁ2), 3.36 (s, 3 Hꢁ2), 3.30 (t, 4 Hꢁ2), 1.63–
1.17 (m, 6 Hꢁ2).13C NMR (100 MHz, CDCl3) d 161.9, 143.5, 137.9,
128.4, 124.6, 121.5, 121.3, 120.9, 120.8, 116.1, 109.8, 47.7, 42.9,
748 cmꢀ1
.
MS-ESI, m/z 649 [M+H]+. HR-MS m/z calcd for
C
38H44N6O4 616.3424, found 616.3417. 1H NMR (400 MHz, CDCl3)
29.5, 26.2, 25.3, 24.4. [a]D +492 (c 0.98, CHCl3).
d 8.10 (s, 1 Hꢁ2), 8.06 (d, J = 16.4 Hz, 1 Hꢁ2), 7.59 (t, J = 7.4 Hz, 1
Hꢁ2), 7.42–7.31 (m, 1 Hꢁ2), 3.84 (dt, J = 12.6, 6.3 Hz, 1 Hꢁ2),
3.60 (dt, J = 13.5, 6.8 Hz, 1 Hꢁ2), 3.44 (s, 3 Hꢁ2), 1.63 (d,
J = 6.7 Hz, 3 Hꢁ2), 1.59 (d, J = 6.7 Hz, 3 Hꢁ2), 1.31 (d, J = 6.4 Hz, 3
Hꢁ2), 1.14 (d, J = 5.8 Hz, 3 Hꢁ2). 13C NMR (100 MHz, CDCl3) d
162.5, 143.6, 138.3, 128.4, 124.5, 121.8, 121.1, 120.9, 114.6,
General procedure for enantioselective reduction of ketimines
with trichlorosilane
To a stirred solution of 4f (1 mol %) in anhydrous CH2Cl2, the
imine was added under nitrogen at 0 °C. After 30 min, trichlorosi-
lane (2 equiv) was added to the mixture dropwise and the reaction
was allowed to stir at the same temperature for 16 h. The reaction
was quenched by saturated NaHCO3 (2 ml) and allowed to warm
up to room temperature. The mixture was extracted by CH2Cl2
three times. The combined organic layers were dried with anhy-
drous Na2SO4 and concentrated with reduced pressure. The crude
product was purified by column chromatography (silica gel, hex-
ane/EtOAc) to afford pure amine 6.
109.8, 51.5, 46.2, 29.5, 21.3, 21.1, 20.6, 19.8. [
CHCl3).
a]
+568 (c 0.28,
D
9,90-Dimethyl-3,30-di(piperidine-1-carbonyl)-9H,90H-1,10-bipyrido
[3,4-b]indole 2,20-dioxide (4f)
Following the general procedure, a yellow power was obtained,
yield 63%. IR (KBr) n = 3441, 1634, 1446, 1392, 1335, 1256, 1132,

2952
Y.-N. Pei et al. / Tetrahedron Letters 55 (2014) 2948–2952
13. Pannetier, N.; Sortais, J. B.; Issenhuth, J. T.; Barloy, L.; Sirlin, C.; Holuigue, A.;
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Supplementary data associated with this article can be found, in
the
online
version,
at
http://dx.doi.org/10.1016/j.tetlet.
2014.03.100.
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