Synthesis of Chiral Vicinal Diamines by Silver(I)-Catalyzed Enantioselective Aminolysis of N-Tosylaziridines

Article abstract of DOI:10.1002/anie.201610693

The kinetic resolution of 2-aryl-N-tosylaziridines and the asymmetric desymmetrization of meso-N-tosylaziridines by ring openings with various primary and secondary anilines, and aliphatic amines as nucleophile have been realized by using a single silver(I)/chiral diphosphine complex as catalyst for the first time. The simple starting materials, broad scope, and easy scalability render this protocol a practical way to chiral vicinal diamine derivatives.

Full text of DOI:10.1002/anie.201610693

Angewandte
Communications
Chemie
International Edition: DOI: 10.1002/anie.201610693
German Edition: DOI: 10.1002/ange.201610693
Ring Opening
Synthesis of Chiral Vicinal Diamines by Silver(I)-Catalyzed
Enantioselective Aminolysis of N-Tosylaziridines
Zhuo Chai,* Pei-Jun Yang, Hu Zhang, Shaowu Wang, and Gaosheng Yang
Abstract: The kinetic resolution of 2-aryl-N-tosylaziridines
and the asymmetric desymmetrization of meso-N-tosylaziri-
dines by ring openings with various primary and secondary
anilines, and aliphatic amines as nucleophile have been
realized by using a single silver(I)/chiral diphosphine complex
as catalyst for the first time. The simple starting materials,
broad scope, and easy scalability render this protocol a prac-
tical way to chiral vicinal diamine derivatives.
strategy, however, has achieved limited success despite the
numerous non-asymmetric catalytic examples reported over
the past decades.[6,12d] To date, systematic studies on this
reaction have only been done by using metal-BINOLate
complexes as efficient chiral Lewis acid catalysts, with
moderate to excellent enantioselectivities achievable (Sche-
me 1a).[7,8] An isolated single example with excellent enan-
tioselectivity by using chiral N,N’-dioxide/MgII complexes has
also been reported recently by Feng and co-workers.[9a] These
systems, however, are limited to the desymmetrization of N-
aryl/benzhydryl/acyl meso-aziridines and the amine nucleo-
philes are restricted to primary anilines only. Their scalability
has also been unexplored.
Chiral vicinal diamines are excellent platforms for the
development of numerous chiral ligands, organocatalysts, and
auxiliaries with broad utility in asymmetric synthesis.[1] They
are also a very important structural motif present in many
biologically active compounds and drug molecules.[2]
Although classical resolution or chiral pool strategy could
be used to afford chiral vicinal diamines, the cost and
availability of the racemic starting materials and/or the
chiral reagents could often limit the scope and practicality
of these methods. Thus, great efforts have been devoted to the
development of efficient practical catalytic asymmetric meth-
ods for their synthesis.[3]
The catalytic asymmetric ring opening of aziridines with
a nitrogen nucleophile represents a very efficient approach to
chiral vicinal diamines, with two differentiated amino groups,
for further useful diversified elaborations.[4–9] In this field, the
desymmetric ring openings of meso-aziridines using TMSN3
have been well-established under both chiral Lewis and
Brønsted acid catalysis,[5] which was pioneered by Jacobsen
and co-workers.[5a] Notably, RajanBabu and Parquette have
also realized elegant regiodivergent kinetic resolutions of
racemic 2-alkyl aziridines by TMSN3 with excellent stereo-
selectivities.[5b–d]
Scheme 1. Catalytic asymmetric aminolysis of aziridines. PG=protect-
ing group.
Despite the ready availability of N-tosylaziridines[10,11] and
extensive studies on the aminolysis of them over the past
decades,[6] the realization of catalytic asymmetric ring open-
ings of N-tosylaziridines with either TMSN3 or anilines as
nucleophile has been a long-standing challenge. Their inert-
ness toward TMSN3 in previous systems has been noted.[5a,g]
Inertness or very low enantioselectivity (< 10% ee) has also
been observed in asymmetric aminolysis catalyzed by metal/
BINOLate complexes.[7a,b,9b] 2-Aryl-N-tosylaziridines have
been reported to undergo quite fast background aminolysis
under neat conditions at room temperature.[6o] Moreover, the
catalytic asymmetric aminolysis of racemic aziridines, in
which the control of selectivity could be more challenging
because of additional regioselectivity issues, has hitherto been
unexplored.
Amines are cheap and readily available nitrogen nucle-
ophiles, and the aminolysis of aziridines could provide a more
direct and alternative way to vicinal diamines. The develop-
ment of efficient catalytic asymmetric systems with this
[*] Prof. Dr. Z. Chai, P.-J. Yang, H. Zhang, Prof. Dr. S. Wang,
Prof. Dr. G. Yang
Key Laboratory of Functional Molecular Solids, Ministry of Education,
Anhui Laboratory of Molecule-Based Materials, College of Chemistry
and Materials Science, Anhui Normal University
1 East Beijing Road, Wuhu, Anhui 241000 (China)
E-mail: chaizhuo@mail.ahnu.edu.cn
Prof. Dr. S. Wang
Encouraged by our recent success in the asymmetric [3+2]
annulations of indoles with racemic N-tosylaziridines,[13a] we
became particularly interested in utilizing chiral Lewis acid
catalysis to address the above challenges. We report herein
the finding of commercially available AgI/chiral diphosphine
complexes as efficient chiral Lewis acid catalyst for highly
State Key Laboratory of Organometallic Chemistry, Shanghai Insti-
tute of Organic Chemistry, Chinese Academy of Sciences
Shanghai 200032 (P.R. China)
Supporting information and the ORCID identification number(s) for
the author(s) of this article can be found under http://dx.doi.org/10.
1002/anie.201610693.
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enantioselective aminolysis of N-tosylaziridines with various
anilines and aliphatic amines, and it is applicable to both the
kinetic resolution of racemic 2-aryl N-tosylaziridines and the
desymmetrization of meso-N-tosylaziridines (Scheme 1b).[12]
After an extensive screen of reaction parameters such as
various combinations of metal complexes and chiral ligands,
solvents, and temperature, the combination of AgSbPF6 and
(S)-DTBM-SEGPHOS in m-xylene at 158C was established
as the optimum reaction conditions (see the Supporting
Information for details). Then the reaction scope was probed
with regard to the kinetic resolution of different 2-aryl-N-
tosylaziridines (2) with anisidine (1a; Table 1). In general,
The high s factors obtained with substrates 2 f and 2i indicate
efficient kinetic resolutions of the racemic aziridines.
This reaction also proved applicable to various other
anilines to provide generally excellent results (Table 1).
Anilines bearing electron-donating substituents underwent
the reaction slower than those bearing electron-withdrawing
ones. This reactivity might be ascribed to the stronger Lewis
basicity of the former, which could coordinate to the Lewis-
acidic AgI center and thus could impair the activation of the
aziridines. Sterics also showed a pronounced influence on the
reaction rate: 2-substituted or 2,4,6-trisubstituted anilines are
relatively more sluggish in the reaction (3ea, 3 fa and 3ja),
while the ee values remained excellent. Pleasingly, for the first
time, secondary aromatic amines also performed efficiently in
the asymmetric aminolysis of aziridines (3ka–ma). Notably,
the primary anilines 1 are much more reactive than the
products 3, which bear a secondary amine moiety, as
nucleophiles in the reaction, though a relatively close
monitoring of the reactions is necessary to avoid further
undesired ring openings of the excess aziridines with such
products.
Table 1: Kinetic resolution of 2-aryl-N-tosylaziridines with anilines.[a–c]
This system is also amenable to the desymmetrization of
meso-aziridines (Table 2). Notably, such meso-aziridines are
inert in previously described ring openings with indoles under
chiral CuI/diphosphine catalysis.[13] Six-membered cyclic azir-
Table 2: Desymmetrization of meso-N-tosylaziridines with anilines.[a]
[a] Unless otherwise noted, reactions were run with 1 (0.1 mmol), 2
(0.22 mmol). [1]0 =0.2 M. [b] Yield of isolated product. [c] The ee values
were determined by chiral-phase HPLC. [d] Run with 2.0 equiv of
aziridine 2 f, 49% recovery of 2 f (92% ee), s=53. [e] Run at 08C. [f] Run
with 2.0 equiv of aziridine 2i, 48% recovery of 2i (77% ee), s=14.
s=Ln[(1ÀC/100)(1Àee/100)]/Ln[(1ÀC/100) (1+ee/100)]. PMP=para-
methylphenyl, Ts=4-toluenesulfonyl.
[a] Reactions were run with 1 (0.1 mmol), 4 (0.1 mmol); [1]0 =0.2 M.
Yields are those of isolated products. [b] Run with 5 mol% of AgSbF6 and
7.5 mol% of L at 408C.
aziridines with electron-donating substituents on the Ar
group are more reactive than those with electron-withdrawing
ones, while similarly high yields and enantioselectivities were
obtained in both cases (3ab–ag). This observation, together
with the excellent regioselectivity, is similar to our previous
findings,[13a] thus suggesting the development of a positive
charge, to a significant degree, at the reacting carbon center
during the reaction process. Notably, varied degrees of
regioselectivities have been observed in previous relevant
systems using nonchiral Lewis acid catalysts.[6] Substitution at
the meta or ortho positions of the benzene ring would slow
down the reaction significantly, but showed no significant
influence on the yield and enantioselectivity (3ac and 3ad).
Notably, 1,2-disubstituted aziridines were also well-tolerated
in the reaction, and represents the first examples of this type
of aziridine in catalytic asymmetric aminolysis (3ah and 3ai).
idines worked particularly well in the reaction to furnish
generally excellent yields and ee values (5aa–ma), and both
primary and secondary anilines were tolerated equally well.
This reaction system also tolerates variations in the substitu-
entsꢀ positions of anilines better than previous aminolysis
systems, in which 4-substituted anilines are usually required to
achieve excellent enantioselectivity. Another mentionable
point is that the reactions were run at nearly room temper-
ature, while a temperature as low as À408C is required in
previous systems.[8] While the five-membered cyclic aziridine
is sluggish in the reaction, the product 5ac was obtained in
93% yield and 85% ee. The reaction did not proceed to
completion with the seven-membered cyclic and acyclic
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aziridines, even after a prolonged reaction time at 408C.
the best of our knowledge, they represent the first examples
However, the ee value is excellent for the former product 5ad,
while an appreciably reduced ee value was observed for the
latter 5ae. Similar reactivity difference has also been noted in
previous catalytic asymmetric ring openings of aziridines with
TMSN3,[5] and might be attributed to steric reasons and/or
ring strain changes of the aziridines.
We next turned to the very challenging aliphatic amines to
further probe the reaction scope (Scheme 2). The inertness of
such amines in the previous TiIV/BINOL system has been
attributed to their higher Lewis basicity, thus leading to strong
of catalytic asymmetric aminolysis of aziridines using ali-
phatic amines.
Gram-scale experiments were performed with both the
kinetic resolution of the racemic aziridine 2a and the
desymmetrization of meso-aziridine 4a, and the results
demonstrate the good scalability of this reaction
(Scheme 3). The kinetic resolution of 2a proceeded with
Scheme 2. Reactions with aliphatic amines. [a] Run with 5 mol% of
AgSbF6, 7.5 mol% of L at 08C. [b] Run with 5 mol% of AgPF6,
7.5 mol% of L. [c] [1]0 =0.5 M. [d] [1]0 =1.0 M. The reaction run at
[1]0 =0.2m required 120 h (for 5sa) and 192 h (for 5va), respectively,
to provide the almost same results.
Scheme 3. Scale-up and product transformations. TCCA=trichloro-
isocyanuric acid.
coordination with the Lewis metal center,[8a] and we believe it
would be detrimental to both the catalytic efficiency and
asymmetric induction of a chiral Lewis acid catalyst. Indeed,
under our system, aliphatic amines in general are much more
sluggish than aromatic ones, as larger catalyst loading, higher
reaction temperature, and prolonged reaction times are
generally required. The kinetic resolution of the racemic
aziridine 2a with the benzyl amine 1n proceeded to give two
regioisomeric products (separated by column chromatogra-
phy) with a poor regioselectivity, and a moderate ee value for
the major product 3na (Scheme 2a).[14] The use of the
aziridine 2g, whose two carbon atoms are more electronically
biased, circumvented the regioselectivity issue to provide
3ng–vg as the major products with almost exclusive regiose-
lectivities in high yields and enantioselectivities. The desym-
metrization of the aziridine 4a could also be done with both
primary and secondary aliphatic amines to provide the
products 5na–va in moderate-to-high yields and ee values
(Scheme 2b). Notably, performing the reaction at a higher
concentration could greatly accelerate the reaction without
affecting the yield and enantioselectivity as demonstrated in
the cases of 5sa and 5va. Though such results are somewhat
less successful as compared to those obtained with anilines, to
still very high efficiency (s = 40), and the recovered aziridine
could be converted into not only the ent-3ba by aminolysis
with a liquid amine, following a literature method, but also
the ent-3aa by aminolysis with a solid amine under AgI/
phosphine catalysis, with almost complete retention of the
stereochemical integrity. Thus, with this strategy, the two
enantiomers of a chiral diamine could be obtained by using
a single chiral catalyst. It is worth mentioning that such
diamine products have been used not only as precursors to
various diamine organocatalysts/ligands for asymmetric syn-
thesis,[15] but also as starting materials to access chiral
tetrahydroquinoxalines[6k,16] and indolines[6o] of important
biological interests. Notably, the scale-up experiment of the
desymmetrization of 4a, run at a higher concentration,
proved to be more efficient than the small-scale reaction
and retained the excellent enantioselectivity. Removal of the
PMP protecting group[17] of 5aa and subsequent tosylation of
the amine provided the chiral disulfamide 6, which is a useful
chiral ligand and ligand precursor.[18] Moreover, the detosy-
lation of 5qa and subsequent reaction with phenyl isothio-
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cyanate provided the compound 7, which belongs to the
widely used family of Takemotoꢀs bifunctional thiourea
catalysts. The preparation of such organocatalysts usually
takes multistep syntheses.[19] The absolute configurations of
the products 5na, 5qa, 6, and 7 were assigned by comparison
of specific optical rotation values with literature data (see the
Supporting Information for details).
In conclusion, we have realized the first catalytic highly
enantioselective aminolysis of both racemic and meso-N-
tosylaziridines with various anilines and aliphatic amines by
using a single catalyst system consisting of a silver(I) salt and
a chiral diphosphine ligand. The broad reaction scope, simple
starting materials, and good scalability of this transformation
provide an efficient and practical way to a wide range of very
useful chiral diamine derivatives. Detailed mechanistic stud-
ies[20] as well as expansion of the chiral catalyst system to other
transformations of aziridines are ongoing in our laboratories.
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Acknowledgements
Financial support of this research from the National Natural
Science Foundation of China (NSFC Nos. 21202001,
21472001, and 21432001), the National Basic Research
Program of China (2012CB821600), and the Special and
Excellent Research Fund of Anhui Normal University are
gratefully acknowledged.
Conflict of interest
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The authors declare no conflict of interest.
Keywords: amines · aziridines · desymmetrization ·
kinetic resolution · silver
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coordinative acidic additive to alleviate the basicity of both 1n
and the resulting diamine products accelerated the reaction but
led to a significant decrease in the enantioselectivity. Replacing
the Ts group with other sulfonyl groups, such as 8-quinoline-
sulfonyl, 2-naphthalenesulfonyl, 4-(trifluoromethyl)benzenesul-
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29; b) L.-W. Xu, J. Luo, Y. Lu, Chem. Commun. 2009, 1807;
Manuscript received: November 1, 2016
Final Article published: && &&, &&&&
Angew. Chem. Int. Ed. 2016, 55, 1 – 6
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
5
These are not the final page numbers!

Angewandte
Communications
Chemie
Communications
Ring Opening
Z. Chai,* P.-J. Yang, H. Zhang, S. Wang,
G. Yang
&&&& — &&&&
Synthesis of Chiral Vicinal Diamines by
Silver(I)-Catalyzed Enantioselective
Aminolysis of N-Tosylaziridines
Two birds, one stone: The nucleophilic
ring opening of N-tosylaziridines by var-
ious primary and secondary aromatic and
aliphatic amines for both racemic azir-
idines (by kinetic resolution) and meso-
aziridines (by desymmetrization) has
been realized by using a single silver(I)/
chiral diphosphine complex. The method
provides a practical and efficient way to
a variety of useful chiral vicinal diamines.
6
www.angewandte.org
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2016, 55, 1 – 6
These are not the final page numbers!