Asymmetric addition of benzothiazole to N-tert-butanesulfinyl imine for the synthesis of chiral α-branched heteroaryl amines

Article abstract of DOI:10.1016/j.tetlet.2012.09.131

This work concerns the asymmetric additions of benzothiazole to a variety of N-tert-butanesulfinyl imines with excellent diastereoselectivities (d.r. up to >99:1). Amino alkoxyl lithium was the key additive to obtain the excellent diastereoselectivity. More functionalized 4-methyl-5-vinyl thiazole and alkyl imines which can isomerize to enamines are also compatible substrates to the present protocol.

Full text of DOI:10.1016/j.tetlet.2012.09.131

Tetrahedron Letters 53 (2012) 6893–6896
Contents lists available at SciVerse ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Asymmetric addition of benzothiazole to N-tert-butanesulfinyl imine for the
synthesis of chiral a-branched heteroaryl amines
Jinlong Zhang a, Yuhong Yang a, Mei Wang a, Li Lin a, , Rui Wang a,b,
⇑
⇑
a Key Laboratory of Preclinical Study for New Drugs of Gansu Province, State Key Laboratory of Applied Organic Chemistry and Institute of Biochemistry and Molecular Biology,
Lanzhou University, Lanzhou 730000, PR China
b State Key Laboratory for Oxo Synthesis and Selective Oxidation, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou 730000, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 18 June 2012
Revised 18 September 2012
Accepted 29 September 2012
Available online 12 October 2012
This work concerns the asymmetric additions of benzothiazole to a variety of N-tert-butanesulfinyl imi-
nes with excellent diastereoselectivities (d.r. up to >99:1). Amino alkoxyl lithium was the key additive to
obtain the excellent diastereoselectivity. More functionalized 4-methyl-5-vinyl thiazole and alkyl imines
which can isomerize to enamines are also compatible substrates to the present protocol.
Ó 2012 Elsevier Ltd. All rights reserved.
Keywords:
Asymmetric synthesis
Benzothiazole
Diarylmethylamine
Diastereoselectivity
N-tert-Butanesulfinyl imine
Heteroaromatic compounds always exhibit broad bioactivity
and pharmacodynamic value. These compounds are also important
synthetic intermediates in organic chemistry. The most attractive
and challenging way for their structural diversity is direct func-
tionalization of arene via new bond construction. In recent years,
expanding progress has been developed in the direct functionaliza-
tion of benzothiazole at C2 position.1 These studies mainly focused
on the direct cross coupling strategy for the benzothiazole aryla-
tion,2 alkylation,3 alkynylation,4 carbonylation,5 iodination,6 ami-
nation,7 and sulfuration.8 Besides, limited studies were reported
in the addition of benzothiazole to alkyne,9 aldehyde10 as well as
ketone.11 However, the addition of benzothiazole to aldimine for
chiral heteroaromatic substituted amine synthesis has rarely been
reported. Asymmetric synthesis of chiral diaryl-methylamines has
attracted growing attentions,12 since these frameworks always
perform as key building blocks as well as important synthetic
intermediates for a series of bioactive compounds. As the forma-
tion of one C–C bond and one chiral center can be achieved in
one step, direct aryl transfer approach is considered to be the most
facile strategy. The present efforts mainly focused on the rhodium
catalysis for the synthesis of diarylmethylamine via new C–C bond
formation.13 While the use of chiral sulfinyl as the chiral auxiliary
has been widely exploited in various synthetic approaches,14 the
additions of aryl-Grignard reagent15 and aryl-lithium reagent16 to
N-tert-butanesulfinyl imine were recognized to be reliable alterna-
tive methods. However, to obtain excellent diastereoselectivity
still remains the most challenging issue. Although very limited
cases have disclosed the addition of benzothiazole to ketimine
for achiral tertiary amine synthesis,17 to the best of our knowledge,
the direct asymmetric addition of benzothiazole to imine to afford
chiral
Herein, we report a highly diastereoselective synthesis of chiral
-branched heteroaryl amines. The direct asymmetric addition of
a-branched aryl-heteroaryl amine has not yet reported.
a
benzothiazole to a series of N-tert-butanesulfinyl imines was car-
ried out with excellent diastereoselectivities. Amplification of the
chiral auxiliary effect was also observed with the aid of the amino
alkoxyl lithium additive chelation with N-tert-butanesulfinyl
imine.
The preliminary study indicated that the benzothiazole-metal-
lic reagents exhibit distinctly different nucleophilicity (Scheme
N
N
S
2a (100mol%)
alkali (150mol%)
M
Ph
N
tBu
tBu
THF, -78oC
S
S
1 (150mol%)
M= MgCl, MgI, Li
O
alkali= MeMgI or t-BuMgCl: No Reaction
LiHMDS: d.r.= 4:1
HN
S
N
S
O
Ph
n-BuLi: d.r.= 4:1
3a
⇑
Corresponding authors. Tel./fax: +86 931 8912567.
E-mail addresses: linli@lzu.edu.cn (L. Lin), wangrui@lzu.edu.cn (R. Wang).
Scheme 1. Preliminary study for the addition of 1 to imine 2a.
0040-4039/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tetlet.2012.09.131

6894
J. Zhang et al. / Tetrahedron Letters 53 (2012) 6893–6896
1). While using either MeMgI or t-BuMgCl as the deprotonating al-
kali,18 no addition product of 3a was observed. It seemed that the
benzothiazol-2-yl-magnesium halides were not active species in
the addition to imine 2a due to the low nucleophilicity. In the
replacement of Grignard reagent with either LiHMDS or n-BuLi,
the model reaction favorably accomplished and afforded the de-
sired 3a immediately after inducing 2a into the priorly prepared
benzothiazole-lithium reagent. Although the reaction proceeded
very rapidly, only a low diastereoselectivity of 4:1 d.r. was ob-
served due to the difficulty in stereocontrol for the mono-lithium
species. Therefore, a series of reaction conditions were optimized
for the reaction diastereoselectivity improvement.
As a general strategy, various additives were explored to deac-
tivate the nucleophile (Table 1).19 Using TMEDA had no effect on
diastereoselectivity improvement (entry 1). Impressive lithium salt
effect on fine tuning the nucleophilicity and stereoselectivity of the
organometallic reagent was disclosed in recent years.20 However,
poor conversion of 1 and only a trace of 3a that were observed in
the model reaction (Method A), may be due to the nucleophilicity
of benzothiazol-2-yl sodium generated via 1 and NaHMDS was
well restrained by the lithiated amino alcohols (A–E, entry 2). Then
different types of alkali as well as a combination order of the reac-
tion components were attempted to improve the diastereoselectiv-
ity. While di-lithium additive A with 2a were firstly combined and
added to the benzothiazol-2-yl potassium solution prepared in ad-
vance, diarylmethylamine 3a was afforded with an increased d.r.
value of 10:1 in 38% yield (Method B) (entry 3). Chiral lithium
complex was probably formed with the combination of chiral
imine 2a and the lithium additive, and the chiral auxiliary effect
of the N-tert-butanesulfinyl might be amplified. Thus, method B
was employed in the subsequent studies as the general procedure.
Using di-lithium B (entry 4) and C (entry 5) as the additive, the
model reaction proceeded smoothly and gave 3a with moderate
yields and high d.r. values (17:1 and 30:1, respectively). Although
the diastereoselectivity was greatly improved to 52:1, only a low
yield (32%) of 3a was obtained (entry 6). Amino alkoxyl lithium E
was found to be an ideal additive. A single diastereomer (d.r.
>99:1) of the desired 3a was afforded in a high yield of 81% (entry
7). The reaction diastereoselectivity was also greatly affected by
different types of alkali used for the C2 de-protonation of benzothi-
azole. While using KHMDS and additive E in method A, the model
reaction proceeded smoothly to give 3a in moderate yield (51%)
with dr value of 14:1 (entry 8). The additive effect was not ob-
served while using either LiHMDS (entry 9) or n-BuLi (entry 10) in-
stead of KHMDS. The addition of benzothiazol-2-yl sodium to 2a
gave 3a with moderate d.r. value (13:1) and 50% yield (entry 11).
No reaction occurred while using t-BuOMe as the solvent due to
the poor solubility of the benzothiazol-2-yl potassium (entry 12).
Good yield (74%) of 3a was obtained but with poor diastereoselec-
tivity (d.r. 2:1) when the reaction was carried out in toluene (entry
13). Both the yield and the diastereoselectivity of 3a decreased
sharply at a higher reaction temperature (À40 °C) (entry 14).
Non-basic lithium salt such as LiCl was explored in the model reac-
tion but displayed no effect in dr enhancement (entry 15).
Under the optimal conditions, the substrate scope for the asym-
metric benzothiazole addition was studied, and results are summa-
rized in scheme 2. After adding the mixture of aldimine 2 and
additive E to the priorly prepared benzothiazol-2-yl potassium
solution, the asymmetric addition was favourably accomplished
within 10 min. This approach can tolerate either electron-donating
or electron withdrawing substituted aryl aldimines, and readily af-
ford the desired diarylmethylamines with high yields and excellent
diastereoselectivities. In general, just one single diastereomer (d.r.
>99:1) was obtained in several cases. The asymmetric additions of
benzothiazole to p-substituted phenyl imine 2h, 2l, and 2m pro-
ceeded favourably to give 3h, 3l, and 3m with excellent diastere-
oselectivities (d.r. of 49:1, 50:1, and 49:1, respectively). The
diastereo-selectivity slightly decreased to 16:1 in the addition to
aldimine 2c, which contained a 3, 4-dimethyl substituted phenyl
group. High d.r. value (32:1) of 3d was obtained in 75% yield. Dia-
rylmethylamine 3f bearing a 4-methyl phenyl was also obtained
with a high d.r. value (16:1). Much hindered 3i was afforded with
a moderate diastereoselectivity (d.r. 3:1) in good yield (76%), due
to the great steric hindrance of the 2, 6-dichloro substituted phe-
nyl. Alkyl-substituted imines, which can easily isomerize to enam-
ines under basic condition, were also compatible substrates.
However, the asymmetric additions to 2n and 2o were well re-
strained while using additive E under standard conditions due to
the enamine formation. Thus, lithium additive E was avoided in
the asymmetric synthesis of 3n–o. Accordingly, the diastereoselec-
tivities decreased to 4:1 and 6:1 for 3n and 3o.
Table 1
Model reaction optimizationa
alkali, additive
THF, -78oC
1
+
2a
3a
N
N
H
N
H
N
OLi
OLi OLi
OLi OLi
OLi
OLi
OLi
additive: A
B
C
D
E
b
Entry
Method
Alkali
Additivec
d.r.d
Yielde
1
Method A
Method A
Method B
Method B
Method B
Method B
Method B
Method A
Method B
Method B
Method B
Method B
Method B
Method B
Method B
NaHMDS
NaHMDS
KHMDS
KHMDS
KHMDS
KHMDS
KHMDS
KHMDS
LiHMDS
n-BuLi
NaHMDS
KHMDS
KHMDS
KHMDS
KHMDS
TMEDA
4:1
77%
Trace
38%
61%
59%
32%
81%
51%
53%
61%
50%
Trace
74%
42%
70%
g
2f
3
N. D.
10:1
17:1
30:1
52:1
>99:1
14:1
4:1
A
B
C
D
E
E
E
E
E
E
4
5
6
7
8
9
10
11
12f,h
13i
5:1
Under the standard conditions, the asymmetric addition of 6-
methyl benzothiazole and 4-methyl-5-vinyl thiazole to 2a pro-
ceeded favourably to give diarylmethylamine 5 and 6 with excel-
lent diastereoselectivities (d.r. of 34:1 and 61:1) in moderate
yields (57% and 55%) (Fig. 1). However, it was found that the addi-
tion activity was significantly affected by the substitutes of the thi-
azole ring in the following study.21 Thiazole as well as 4-methyl
thiazole was also examined in the addition to imine 2a under stan-
dard conditions, but with very poor conversion.
To illustrate the significant effect of the lithium additive in
enhancing the reaction diastereoselectivity, possible transition
states of the intermediate directed by amino alkoxyl lithium E
were proposed (Fig. 2). While using (Rs)-2a in the reaction, the
intermediate X might be favourably formed under the chelation
of additive E with 2a and benzothiazol-2-yl potassium. Thus, the
addition of benzothiazol-2-yl to the imine group preferred Si-face
13:1
N. D.
2:1
24:1
5:1
E
E
LiCl
j
14
15
a
Unless otherwise noted, reactions were carried out with 1 (0.45 mmol), 2a
(0.3 mmol), and alkali (0.45 mmol) in THF at À78 °C.
b
Method A: 2a was added to the mixture of 1, additive (0.45 mmol), and alkali.
Method B: 2a and additive (0.315 mmol) in 3 mL THF were added to the mixture of
1 and alkali.
c
Prepared and stored as solid.
d
Determined by 1H NMR analysis.
e
Isolated yield of the two diastereomers.
Poor conversion of 2a; d.r. was not determined.
Similar results were observed while using additives A–E, respectively.
The solvent was t-BuOMe.
The solvent was toluene.
Carried out at À40 °C.
f
g
h
i
j

J. Zhang et al. / Tetrahedron Letters 53 (2012) 6893–6896
6895
KHMDS
2 (100 mol%)
E (105 mol%)
tBu
attack. In contrast, Re-face attack was unfavorable due to the high
steric hindrance. This hypothesis was reliably proved by the abso-
lute configuration of the product. The absolute configuration of
product 3b was confirmed to be (Rs, S) via X-ray crystal analysis
(Fig. 3).22
In summary, we developed a facile and reliable approach for the
asymmetric synthesis of chiral a-branched heteroaromatic amine.
N
HN
R
S
N
S
(150mol%)
O
o
S
THF, -78 C
within 10min
3
1 (150mol%)
20 min
O
O
S
O
S
S
NH
tBu
NH
tBu
N
tBu
NH
Me
N
N
Using N-tert-butanesulfinyl as the chiral auxiliary, the asymmetric
addition of benzothiazol-2-yl potassium reagent to N-tert-butane-
sulfinyl imines gave the chiral diarylmethylamines with excellent
diastereoselectivities in high yields. Amino alkoxyl lithium was ex-
plored as an essential additive to amplify the chiral auxiliary effect
of the N-tert-butanesulfinyl via chelating with chiral imine. It is
considered to be a meaningful protocol for the diastereoselectivity
improvement in N-tert-butanesulfinyl directed asymmetric trans-
formations. The more challenging alkyl substituted imines, which
can isomerize to enamines, were also compatible substrates to
the present protocol. The asymmetric addition of more functional-
ized 4-methyl-5-vinyl thiazole was successfully carried out under
standard conditions. The asymmetric addition of other thiazoles
as well as oxazoles is under investigation.
S
S
S
Me
3a: 81%, d.r.>99:1
3b: 79%, d.r.>99:1
3c: 84%, d.r. 16:1
O
O
O
S
S
S
tBu
NH
NH
NH
tBu
N
tBu
N
Me
N
S
S
S
Me
Me
3d: 75%, d.r. 32:1
3e: 69%, d.r.>99:1
3f: 81%, d.r. 16:1
O
O
O
S
S
S
Cl
tBu
NH
tBu
NH
tBu
NH
Cl
N
N
N
S
S
S
Cl
Cl
3g: 72%, d.r.>99:1
3h: 68%, d.r. 49:1
3i: 76%, d.r. 3:1
O
O
O
Typical Experimental Procedure
S
S
S
tBu
NH
tBu
NH
tBu
NH
Br
N
N
N
Method A: To a stirred solution of benzothiazole 1 (0.45 mmol)
and the additive (0.45 mmol) in 3 mL THF, NaHMDS (0.45 mmol,
1.0 M in THF) was added drop-wise at À78 °C. After the mixture
being stirred for 20 min, imine 2a (0.3 mmol) in 3 mL THF was
added in. Poor conversion of both benzothiazole 1 and imine 2a
as well as the trace product 3a was observed by TLC. No further
workup was treated.
S
S
S
F
F3C
3j: 62%, d.r.>99:1
3k: 82%, d.r.>99:1
3l: 63%, d.r. 50:1
O
O
O
S
S
S
tBu
N
NH
tBu
NH
tBu
NH
OMe
N
N
C7H15
Method B (General procedure): At room temperature, (Rs) imine
2 (0.3 mmol) and the additive (0.315 mmol) was dissolved in
3 mL THF with vigorous stirring for about half an hour. The re-
sulted clear solution was added to the priorly prepared ben-
zothiazol-2-yl metallic reagent (0.45 mmol in 3 mL THF) at
À78 °C. The reaction was accomplished rapidly within 10 min
(monitored by TLC). Then the reaction was quenched with aqueous
saturated NH4Cl, extracted with DCM (10 mL Â 3), washed by brine
(10 mL), dried over Na2SO4, and concentrated under vacuum. The
crude product was purified by silica gel column chromatography
(petroleum ether/ethyl acetate 3:1–1:1). The diastereoselectivity
was determined by 1H NMR analysis of the crude product.
S
S
S
3m: 78%, d.r. 49:1
3n: 66%, d.r. 4:1
3o: 57%, d.r. 6:1
(0.3 mmol) and
Scheme 2. Asymmetric benzothiazole addition. Imine
2
E
(0.315 mmol) in 3 mL THF were added to a pre-stirred THF solution (3 mL) of 1
and KHMDS at À78 °C. All reactions were accomplished within 10 min. The stated
yield was isolated yield of the two diastereomers. The d.r. was determined by 1H
NMR analysis of the crude product. No additive E was used in the synthesis of 3n or
3o.
O
S
O
S
tBu
NH
tBu
NH
N
S
Me
S
N
Me
5
6
57%, d.r. 34:1
55%, d.r. 61:1
Figure 1. Asymmetric addition of 4-methyl-5-vinyl thiazole 4 to imine 2a.
Me2N
Me2N
Li
O
O
O
O
Li
N
N
K
N
K
N
S
S
S
S
H
R
R
H
X (favorable)
X'
(unfavorable)
Figure 2. Proposed transition states of the intermediate directed by amino alkoxyl
lithium E.
Figure 3. X-Ray structure of 3b.

6896
J. Zhang et al. / Tetrahedron Letters 53 (2012) 6893–6896
2011, 13, 2860–2863; (c) Kim, J. Y.; Cho, S. H.; Joseph, J.; Chang, S. Angew. Chem.,
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We gratefully acknowledge financial support from NSFC (nos.
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