Catalytic Enantioselective Reaction of 2H-Azirines with Thiols Using Cinchona Alkaloid Sulfonamide Catalysts

Article abstract of DOI:10.1021/acs.orglett.7b04022

The first catalytic enantioselective reaction of 2H-azirines with thiols has been developed. The obtained aziridines can be converted to optically active oxazolines, aziridylamides, or α-sulfonyl esters. Transformation of these optically active aziridines

Full text of DOI:10.1021/acs.orglett.7b04022

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
Catalytic Enantioselective Reaction of 2H‑Azirines with Thiols Using
Cinchona Alkaloid Sulfonamide Catalysts
Shuichi Nakamura,*,†,‡ Daiki Hayama,† Masataka Miura,† Tsubasa Hatanaka,§ and Yasuhiro Funahashi§
†Department of Life Science and Applied Chemistry, Graduate School of Engineering, Nagoya Institute of Technology, Gokiso,
Showa-ku, Nagoya 466-8555, Japan
‡Frontier Research Institute for Material Science, Nagoya Institute of Technology, Gokiso, Showa-ku, Nagoya 466-8555, Japan
§Department of Chemistry, Graduate School of Science, Osaka University 1-1 Machikaneyama, Toyonaka, Osaka 560-0043, Japan
S
* Supporting Information
ABSTRACT: The first catalytic enantioselective reaction of 2H-azirines with thiols has been developed. The obtained aziridines
can be converted to optically active oxazolines, aziridylamides, or α-sulfonyl esters. Transformation of these optically active
aziridines showed that 2H-azirines act as β,β-dicarbocationic amine synthons.
he reaction of ketimines with some nucleophiles is
recognized as one of the most powerful and atom-
namely, 2H-azirines act as β,β-dicarbocationic amine equiv-
alents (Scheme 1). Herein our ongoing interest was extended
to the enantioselective reaction of 2H-azirines with thiols using
our original chiral catalysts.
The reaction of 2H-azirines 2, which were prepared in situ by
heating α-azidoacrylates 1 in dichloromethane at 150 °C in a
sealed tube, with p-bromobenzenethiol 3a (1.0 equiv) was
carried out in the presence of 10 mol % loadings of various
chiral catalysts 4a−g. The results are shown in Table 1. The
reaction of 1a with 3a using cinchonine 4a afforded product
T
economical synthetic methods for chiral amines having a
tetrasubstituted chiral carbon center. Therefore, there are many
papers that describe enantioselective nucleophilic reactions for
ketimines, although these reactions are not easy because of their
low reactivity and difficulties associated with their enantiofacial
control of ketimines.1 Among these, the enantioselective
reaction of ketimines with S-nucleophiles has not been very
fruitful, although the reaction gives chiral N,S-acetals, which are
attractive as biologically active compounds.2 The first
enantioselective reaction of ketimines with thiols was reported
by us3 and by the Enders group4 independently. These reports
described the reaction of ketimines derived from isatins with
alkyl or aryl thiols using chiral cinchona alkaloid sulfonamide
catalysts or phosphoric acid catalysts, respectively, to give
Scheme 1. Enantioselective Reaction of 2H-Azirines as
Ketimines with Thiols and Synthetic Utility of the Obtained
Aziridines
products with high enantioselectivity. Gredica
̌
k5 and Singh6
also reported that the enantioselective reaction of N-acyl cyclic
ketimines with thiols using chiral phosphoric acid catalysts gave
products in good yields with high enantioselectivities.7 On the
other hand, the enantioselective reaction of 2H-azirines as
ketimines with nucleophiles is also an important reaction
because it provides chiral aziridines, which can act as chiral
building blocks8 or biologically active compounds.9 However,
the enantioselective reaction of 2H-azirines with nucleophiles is
rare. We recently reported the first highly enantioselective
reaction of 2H-azirines with phosphites as nucleophiles using
bis(imidazoline)zinc(II) catalysts.10 However, there have been
no reports on the enantioselective nucleophilic reaction of 2H-
azirines with thiols, although the aziridines that were obtained
also accept some other reactions with other nucleophiles,
Received: December 26, 2017
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.7b04022
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
Table 1. Enantioselective Reaction of Azirines 2 with Thiol
3a Using Various Chiral Catalysts 4a−g
Table 2. Enantioselective Reaction of Azirine 2c with
a
a
Various Thiols Using Catalysts 4
b
time
(min)
yield
(%)
% ee
entry
3
R
5
(config.)
1
2
3a
3b
3c
3d
3e
3f
4-BrC6H4
4-ClC6H4
4-FC6H4
5ca
5cb
5cc
5cd
5ce
5cf
30
60
96
88
86
80
93
89
93
97
90
97
95
93
93
97
88
91
91
93
87
94 (R)
94 (R)
92 (R)
93 (R)
96 (R)
90 (R)
92 (R)
93 (R)
90 (R)
93 (R)
93 (R)
93 (S)
91 (S)
91 (S)
94 (S)
93 (S)
91 (S)
91 (S)
72 (R)
3
30
4
3-FC6H4
30
5
2-FC6H4
30
6
4-CF3C6H4
C6H5
30
b
7
3g
3h
3i
5cg
5ch
5ci
30
entry
1
R1
4
X
yield (%)
% ee (config.)
8
3-CH3OC6H4
3-CH3C6H4
2-naphthyl
benzothiazole
4-ClC6H4
4-FC6H4
30
1
2
1a
1a
1a
1a
1a
1a
1b
1c
1d
1c
1c
1c
Bn
Bn
Bn
Bn
Bn
Bn
Me
Et
4a
4b
4c
4d
4e
4f
10
89
87
87
98
88
83
99
94
87
96
96
95
30 (R)
58 (S)
81 (S)
78 (S)
92 (S)
74 (S)
92 (S)
93 (S)
92 (S)
94 (R)
94 (R)
93 (R)
9
30
10
10
10
10
10
10
10
10
10
1
10
3j
5cj
75
3
c
11
3k
3b
3c
3d
3e
3j
5ck
5cb
5cc
5cd
5ce
5cj
60
4
d
12
90
5
d
13
30
6
d
14
3-FC6H4
30
7
4e
4e
4e
4g
4g
4g
d
15
d
2-FC6H4
30
8
16
2-naphthyl
benzothiazole
benzoxazole
benzyl
75
9
tBu
Et
cd
,
17
18
19
3k
3l
5ck
5cl
120
120
120
10
11
12
cd
,
Et
e
3m
5cm
Et
0.5
a
Reaction conditions: 1c (0.12 mmol), 3a−m (0.10 mmol) and
a
Reaction conditions: 1a−d (0.12 mmol), 3a (0.10 mmol), and
b
catalyst 4g (1 mol %) were used. Enantiomeric excess was
determined by HPLC analysis using a chiral column. Catalyst (10
mol %) was used. Catalyst 4e was used. Catalyst (20 mol %) was
b
catalyst 4a−g (1 mol %) were used. Enantiomeric excess was
determined by HPLC analysis using a chiral column.
c
d
e
used.
5aa in good yield but with low enantioselectivity (entry 1).11
To our delight, the reaction using picolinamide catalyst 4b gave
product 5aa with better enantioselectivity than that obtained
with catalyst 4a (entry 2). Encouraged by this result, we
examined the reaction using chiral catalysts 4c−e having
various arenesulfonyl groups (entries 3−5). The reaction using
8-quinolinesulfonylated 9-amino-9-deoxy-epi-cinchonine cata-
lyst 4e12 afforded product 5aa with high enantioselectivity
(entry 5). On the other hand, the reaction using 1-
naphthalenesulfonylated 9-amino-9-deoxy-epi-cinchonine cata-
lyst 4f gave 5aa in high yield but with lower enantioselectivity
than that obtained using catalyst 4e (entry 6). The reactions
using methyl, ethyl, and tert-butyl esters 1b−d showed almost
the same enantioselectivity (entries 7−9). On the other hand,
the reaction of 2c with 3a using 8-quinolinesulfonamide catalyst
4g derived from cinchonidine gave the opposite enantiomer of
5ca with good enantioselectivity (entry 10). Good enantiose-
lectivity was still observed even when the catalyst loading was
decreased to 1 or 0.5 mol % (entries 11 and 12).
Under these optimized conditions, we next examined the
nucleophilic reaction of azirine 2c with various thiols 3a−m
using catalyst 4g (Table 2). The reaction of azirine 2c with
thiols 3a−f having electron-withdrawing groups such as bromo,
chloro, fluoro, and trifluoromethyl groups at the ortho, meta, or
para position gave the corresponding products 5ca−cf in good
yield with high enantioselectivity (80−96% yield, 90−96% ee;
entries 1−6). The reaction of benzenethiol (3g) and electron-
rich thiols 3h and 3i having methyl or methoxy groups also
gave products 5cg−ci with good enantioselectivity (entries 7−
9). Various bulky thiols such as 2-naphthalenethiol and
benzothiazolethiol were also applicable nucleophiles in this
reaction (entries 10 and 11). In addition, the reaction of 2c
with various thiols 3b−e,j−l using catalyst 4e afforded the
opposite enantiomer of the products compared with the
reaction using catalyst 4g with high enantioselectivity (entries
12−18). The reaction of azirine 2c with benzyl mercaptan
(3m) as an alkyl thiol also gave product 5cm in high yield with
good enantioselectivity (entry 19).13 These results are the first
examples of a highly enantioselective reaction of azirines with
sulfur nucleophiles.
We next examined the transformation of various aziridines 5
obtained (Scheme 2). The reaction of 5aa with p-nitrobenzoyl
chloride gave N-benzoylated aziridine 6, which reacted with 2.0
equiv of Lawesson’s reagent in situ to give oxazoline compound
7 (Scheme 2, eq 1). Although the role of Lawesson’s reagent is
not clear, it could activate the carbonyl oxygen and act as a
weak Lewis acid.14 These reactions implied that azirines act as
β,β-dicarbocationic amine synthons. The reaction of 5ca with
NH3 in methanol gave aziridylamide 8 without loss of
enantiopurity (Scheme 2, eq 2). The absolute configuration
of 8 was assigned as R by X-ray crystallographic analysis, and
the configurations of other products were tentatively assumed
by analogy. Furthermore, chiral α-amino sulfones, β-amino
sulfones, and α-sulfonyl esters are important classes of synthetic
targets because they often exhibit a broad range of biological
activities.15 Therefore, we next examined the oxidation of
sulfides to sulfones. The reaction of 5aa with m-CPBA in
dichloromethane at rt gave α-sulfonyl ester 9 in high yield
(Scheme 2, eq 3).
Next, we examined the gram-scale synthesis of aziridine 5aa.
The preparation of azirine 2a was carried out in toluene at
B
DOI: 10.1021/acs.orglett.7b04022
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
Scheme 2. (1) Transformation of 5aa to Chiral Oxazoline
Compound 7 Using Lawesson’s Reagent; (2)
Scheme 4. Assumed Reaction Cycle for the Enantioselective
Reaction of Azirines 2 with Thiols 3 Using Catalyst 4e
Transformation of 5ca to Aziridylamide 8; (3)
Transformation of 5aa to α-Sulfonyl Ester 9 Using m-CPBA
chiral catalyst 4e, and therefore, the thiol approaches the Si face
of the azirine to avoid steric repulsion, giving the S isomer of
the product.
reflux because the large-scale reaction using azide compound 1g
in a sealed tube in CH2Cl2 was dangerous. The reaction was
accomplished using 1.19 g of 1a, 0.92 g of 3a, and 1 mol %
catalyst 4g to give 5aa in 74% yield with 94% ee (Scheme 3).
Scheme 3. Large-Scale Synthesis of (R)-5aa Using 1 mol %
4g
Figure 1. Assumed transition state of the reaction of azirine 2 with
thiol 3 using 4e. H atoms have been omitted for clarity.
In conclusion, we have developed a highly enantioselective
reaction of azirines with various thiols. The reaction was
screened for a broad range of thiols. This approach is the first
example of a highly enantioselective reaction of azirines with
sulfur nucleophiles. The obtained aziridines can be converted to
various chiral compounds having a tetrasubstituted carbon
center without loss of enantioselectivity. Further studies of the
potential of these catalytic systems for other processes and the
enantioselective reaction of azirines are in progress.
The reaction of azirines 2 with thiols 3 using catalyst 4e,
which has heteroarenesulfonyl groups, gave products 5 with
higher enantioselectivity than that obtained using naphthalen-
sulfonylated catalyst 4f (Table 1, entries 5 vs 6). Therefore, the
heteroarenesulfonyl groups play an important role in enhancing
the enantioselectivity of the reaction. On the basis of these
results, a proposed catalytic cycle for the reaction of azirines 2
with thiols 3 is shown in Scheme 4. The acidic proton of 8-
quinolinesulfonamide in catalyst 4e can activate azirine 2 by
hydrogen bonding to give intermediate A, and the quinuclidine
moiety in catalyst 4e could also activate the thiol by hydrogen
bonding. The reaction of activated thiol 3 with azirine leads to
an adduct, which undergoes protonation and decomplexation
to give product 5 and regenerate catalyst 4e. In order to clarify
the assumed reaction cycle, we conducted a spectroscopic
analysis. ESI-MS analysis of the reaction mixture of 2c and 4e
showed intermediate A (cation mode; calcd for C33H36N5O4S+
as [A + H]+ 598.3, found 598.3). This signal supports our
proposed reaction mechanism.
ASSOCIATED CONTENT
■
S
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.7b04022.
1H and 13C NMR spectra and experimental procedures
for all new compounds (PDF)
Accession Codes
CCDC 1588168 contains the supplementary crystallographic
data for this paper. These data can be obtained free of charge
via www.ccdc.cam.ac.uk/data_request/cif, or by e-mailing
data_request@ccdc.cam.ac.uk, or by contacting The Cambridge
Crystallographic Data Centre, 12 Union Road, Cambridge CB2
1EZ, U.K.; fax: +44 1223 336033.
The assumed transition state for the reaction of an azirine 2
with a thiol 3 using catalyst 4e is shown in Figure 1. The
reaction of 2 with 3 proceeds in the coordination sphere of the
C
DOI: 10.1021/acs.orglett.7b04022
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
251. For azicemicins A and B, see: (c) Tsuchida, T.; Iinuma, H.;
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T.; Hamada, M.; Takeuchi, T. J. Antibiot. 1993, 46, 1772. (d) Tsuchida,
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AUTHOR INFORMATION
■
Corresponding Author
*E-mail: snakamur@nitech.ac.jp.
ORCID
Shuichi Nakamura: 0000-0001-5633-8367
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by a Grant-in-Aid for Scientific
Research on Innovative Areas “Advanced Molecular Trans-
formations by Organocatalysts” from MEXT (26105727) and
the Tatematsu Foundation.
(10) (a) Nakamura, S.; Hayama, D. Angew. Chem., Int. Ed. 2017, 56,
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resolution of 2H-azirines, see: (d) Hu, H.; Liu, Y.; Lin, L.; Zhang, Y.;
Liu, X.; Feng, X. Angew. Chem., Int. Ed. 2016, 55, 10098. For
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DOI: 10.1021/acs.orglett.7b04022
Org. Lett. XXXX, XXX, XXX−XXX