Enantioselective Strecker and Allylation Reactions with Aldimines Catalyzed by Chiral Oxazaborolidinium Ions

Article abstract of DOI:10.1021/acs.orglett.9b02280

Chiral oxazaborolidinium ion (COBI)-catalyzed enantioselective nucleophilic addition reactions of aldimines using tributyltin cyanide and allyltributylstannane have been developed. Various α-aminonitriles and homoallylic amines were synthesized in high yi

Full text of DOI:10.1021/acs.orglett.9b02280

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
Enantioselective Strecker and Allylation Reactions with Aldimines
Catalyzed by Chiral Oxazaborolidinium Ions
Ki-Tae Kang, Sang Hyun Park, and Do Hyun Ryu*
Department of Chemistry, Sungkyunkwan University, Suwon 16419, Korea
S
* Supporting Information
ABSTRACT: Chiral oxazaborolidinium ion (COBI)-cata-
lyzed enantioselective nucleophilic addition reactions of
aldimines using tributyltin cyanide and allyltributylstannane
have been developed. Various α-aminonitriles and homoallylic
amines were synthesized in high yield (up to 98%) with high
to excellent enantioselectivity (up to 99% ee). A rational
mechanistic model for the complex of COBI and aldimine is
provided to account for these enantioselective reactions.
amine compounds, highly enantioselective borenium ion-
nantioselective carbon−carbon bond-forming reactions
Ecatalyzed by chiral boron Lewis acids are useful and catalyzed carbon−carbon bond formations with imines have
not been reported to date.9 Herein, we describe the first
versatile synthetic methods for stereoselective generation of
chiral centers.1 Among them, chiral borenium ions2 (Scheme
successful development of highly enantioselective nucleophilic
addition reactions such as Strecker and allylation reactions of
aldimines using a COBI catalyst to obtain highly optically
active α-aminonitriles 4 and homoallylic amines 5 (Scheme
1B). A possible pretransition state assembly 3 of COBI and o-
hydroxyphenyl aldimine10 2e is also suggested (Scheme 1B).
Since COBI catalyst 1 has proven to be an effective catalyst
for enantioselective cyanosilylation of aldehydes and ketones
with TMSCN,6d,e we first decided to investigate 1 for the
catalytic enantioselective Strecker reaction11 with aldimine 2.
Initially, the enantioselective Strecker reaction between
trimethylsilyl cyanide and benzaldimines with various protect-
ing groups such as Bn, Boc, and Ph was examined in the
presence of 20 mol % COBI catalyst 1a activated by triflic acid.
However, the obtained α-aminonitriles 4a−4c were racemic
(Table 1, entries 1−3). Gratifyingly, when the protecting
group of the aldimine was changed to an o-hydroxy p-methyl
phenyl group,12 α-aminonitrile 4e was obtained in 94% yield
and 81% ee (Table 1, entry 5). Since o-methoxy-substituted
phenyl aldimine 2d did not provide any enantioselectivity
(Table 1, entry 4), we assumed that bidentate binding of the
imine nitrogen and o-hydroxy group to COBI catalyst 1a is
necessary to achieve high enantioselectivity as shown in
complex 3 (Scheme 1B). In complex 3, hydrogen-bonding
between the aldimine and ammonium of COBI 1a is essential.
To support this hypothesis, we tested Strecker reactions with
oxazaborolidine 1b and methylammonium type13 COBI 1c,
which does not have a proton for hydrogen-bonding to give
chirally inverted or racemic product 4e (Table 1, entries 6 and
7). Intermolecular hydrogen-bonding14,15 between COBI and
aldimine was further supported by low-temperature (−40 °C)
1A), boron cations in the form of a three-coordinate boron
center with a net positive charge, are powerful Lewis acids that
have been widely utilized as oxophilic activating agents for
carbonyl compounds.3 As a representative example, chiral
oxazaborolidinium ion (COBI)4 catalyst 1, which is generated
from the corresponding oxazaborolidine by activation with
strong Brønsted or Lewis acid, has been demonstrated to be an
effective catalyst for various enantioselective carbon−carbon
bond-forming reactions of carbonyl compounds, such as
cycloaddition reactions,4,5 nucleophilic 1,2-6 or 1,4-addition,7
and corresponding tandem reactions,8 leading to a family of
useful chiral building blocks containing carbonyl compounds.
However, despite the potential for practical syntheses of chiral
Scheme 1. Catalytic Enantioselective Strecker and Allylation
Reactions of Aldimines
Received: July 2, 2019
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.9b02280
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
Table 1. Optimization of the Enantioselective Strecker
Reaction
Table 2. Substrate Scope for Enantioselective Strecker
a
a
Reaction
b
c
entry
4
R2
time (h)
yield (%)
ee (%)
d
1
2
3
4
5
6
7
8
4e
4f
4g
4h
4i
4j
4k
4l
4m
4n
4o
4p
4q
4r
4s
4t
Ph
2
2
2
2
2
2
2
12
12
24
4
4
2
2
72
12
18
98
93
96
96
94
91
93
94
92
91
95
91
94
96
85
82
91
95
97
91
94
91
92
94
82
83
87
99
93
92
91
67
77
91
4-MeC6H4
2-MeC6H4
4-MeOC6H4
2-MeOC6H4
4-FC6H4
2-FC6H4
4-ClC6H4
4-BrC6H4
4-CF3C6H4
1-naphth
2-naphth
2-furyl
9
10
11
12
13
14
15
16
17
2-thienyl
Et
i-Pr
e
e
e
4u
t-Bu
a
The reaction of aldimines (0.2 mmol) with Bu3SnCN (0.3 mmol)
was performed in the presence of catalyst (20 mol %) in 1.4 mL of
b
c
toluene at −40 °C. Isolated yield of 4. The ee of 4 was determined
d
by chiral HPLC. 1 mmol scale reaction was also performed to give
98% yield and 94% ee value. The one-pot reaction was also performed
e
to give 97% yield and 95% ee.17a One-pot reaction of aldehyde and 2-
aminocresol.
Scheme 2. Transformation of a Chiral α-Aminonitrile
a
The reaction of aldimines (0.2 mmol) with TMSCN (0.3 mmol) was
performed in the presence of catalyst (20 mol %) in 1.4 mL of toluene
b
c
at −40 °C. Isolated yield of 4. The ee of 4 was determined by chiral
d
HPLC. The reaction was performed with Bu3SnCN.
Scheme 3. Formal Synthesis of (S)-Levamisole
1H NMR analysis.16 The aldimine proton (NC−H) peak
was downfield shifted from δ = 7.95 to 8.41 ppm and the
+
doublet splitting from N−H···N of 3 observed at δ = 11.36
ppm had the same coupling constant (d, J = 15 Hz) as N = C−
H (δ = 8.41 ppm, d, J = 15 Hz).14
Next, we screened the catalyst structure and found that the
catalyst system with a 3,5-dimethylphenyl Ar substituent and
2-methoxyphenyl R substituent, activated by triflic acid, gave
the best result of 91% yield and 95% ee (Table 1, entry 9).
Next, we applied Bu3SnCN17 as a safer cyanide source17b
instead of TMSCN to afford 4e in higher yield (Table 1, entry
10).
At optimized reaction conditions for the catalytic
enantioselective Strecker reaction, we evaluated this method-
ology with various aldimines (Table 2). Regardless of the
electronic or steric properties of substituents on the aldimines,
highly optically active α-aminonitriles 4 were obtained (Table
2, entries 2−10). This catalytic system was also successfully
applied to reactions of various aromatic aldimines such as
naphthyl, furyl, and thienyl (Table 2, entries 11−14). While
primary and secondary alkyl-substituted aldimines provided
products with moderate enantioselectivities, tertiary butyl
aldimine provided the desired α-aminonitrile 4 with high
yield and enantioselectivity (Table 2, entries 15−17).
Further chemical transformation of the resulting optically
active α-aminonitrile 4u to confirm the absolute structure is
illustrated in Scheme 2. Oxidative cleavage of 6 with cerium
ammonium nitrate (CAN)18 after methylation led to highly
optically enriched α-aminonitrile 7 with a free amine group.
B
DOI: 10.1021/acs.orglett.9b02280
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
a
diamine HCl salt 11 in 78% overall yield with 97% ee after one
recrystallization. Comparison of the optical rotation data of 11
Table 3. Substrate Scope for Enantioselective Allylation
confirmed the absolute (S) stereochemistry of ent-4e.20b
A
two-step transformation was reported to the known
anthelmintic levamisole.20
Encouraged by the good results exhibited in Table 2, we
applied our new catalytic method to allylation reactions with a
range of aldimines and allyltributylstannane to obtain chiral
homoallylic amines 5. Performing a screen of COBI catalysts
with the o-hydroxyphenyl aldimine 2 identified COBI catalyst
1e as the optimal catalyst (see the Supporting Information for
details). The synthesis of chiral homoallylic amines is
important because they have been used as versatile building
blocks, after transformation to other functional groups, in
syntheses of biologically active natural products.21 As
summarized in Table 3, the reactions produced the
corresponding chiral homoallylic amines in good yields and
high enantioselectivities regardless of the electronic or steric
properties of substituents on the aldimines (Table 3, entries
1−14).
b
c
entry
5
R2
time (h)
yield (%)
ee (%)
d
e
1
5a
5b
5c
5d
5e
5f
5g
5h
5i
5j
5k
5l
5m
5n
5o
Ph
3
1
3
3
1
3
3
4
3
3
3
4
3
6
3
88
87
83
82
85
91
87
89
84
91
86
78
80
77
91
93
f
2
4-MeC6H4
3-MeC6H4
2-MeC6H4
4-MeOC6H4
4-FC6H4
2-FC6H4
4-ClC6H4
4-BrC6H4
4-CF3C6H4
1-naphth
2-furyl
94
92
96
90
95
93
92
92
93
90
94
93
94
90
f
3
f
4
f
5
6
7
8
9
10
11
12
13
14
15
Based on catalyst structures, aldimine protecting groups
1
(vide supra, Table 1) and H NMR analysis, the observed
2-thienyl
i-Pr
g
stereochemistry for the enantioselective nucleophilic addition
reactions using COBI catalyst 1e can be rationalized using the
transition-state model shown in Figure 1. In the pretransition
state assembly 3, shown in Figure 1, the aldimine group is
situated above the 3,5-dimethylphenyl group, which effectively
shields the re face (back) from attack by the nucleophiles.
Nucleophilic addition from the si face (front) of the aldimine
leads to the major enantiomers shown in Figure 1.
In summary, we report the first example of highly
enantioselective borenium ion-catalyzed carbon−carbon bond
formation of imines in Strecker and allylation reactions of
aldimines. In the presence of COBI catalyst, various α-
aminonitriles and homoallylic amines were obtained in good
yields and high to excellent enantioselectivities. The absolute
configuration of the products was the same as predicted by the
transition-state model in Figure 1. We believe that the
mechanistic model implied by complex 3 has useful predictive
power, and there are many potential uses for complex 3 in
catalytic enantioselective syntheses of various nitrogen-
containing compounds beyond those outlined here. Other
applications of 3 and further mechanistic studies are underway.
g
t-Bu
a
The reaction of aldimines (0.2 mmol) with allyltributylstannane
(0.22 mmol) was performed in the presence of catalyst (20 mol %) in
b
c
1.4 mL of toluene at −40 °C. Isolated yield of 5. The ee of 5 was
d
determined by chiral HPLC. 1 mmol scale reaction was also
performed to give 84% yield and 93% ee. The one-pot reaction was
also performed to give 87% yield and 93% ee.21c The absolute
e
configurations of 5a shown in Table 3 were assigned by measurement
of optical rotation and comparison with known substances.21c,22 For
f
details, see the Supporting Information. The reaction was performed
at 0 °C. One-pot reaction of aldehyde and 2-aminophenol.
g
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.9b02280.
Figure 1. Transition-state model for enantioselective nucleophilic
addition reactions with aldimine.
Experimental procedures and full analytical data (PDF)
Recrystallization of the hydrochloride salt gave optically
enriched (R)-aminonitrile hydrochloride 8. Comparison of
the optical rotation data of 8 confirmed the absolute (R)
stereochemistry of 4u.19
The synthetic utility of the present reaction and absolute
structure of ent-4e were further demonstrated by formal
synthesis of levamisole (Scheme 3). Enantioselective Strecker
reaction of benzaldimine 2e with the enantiomeric catalyst of
1f, ent-1f, provided ent-4e in 97% yield and 95% ee. Reduction
of ent-4e was followed by Boc protection to furnish protected
diamine 9 without loss of optical purity. Subsequent
deprotection of o-hydroxyphenyl and Boc groups produced
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: dhryu@skku.edu.
ORCID
Do Hyun Ryu: 0000-0001-7615-4661
Notes
The authors declare no competing financial interest.
C
DOI: 10.1021/acs.orglett.9b02280
Org. Lett. XXXX, XXX, XXX−XXX

(8) For selected examples of enantioselective tandem reactions with
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ACKNOWLEDGMENTS
■
This work was supported by National Research Foundation of
Korea (NRF) grants funded by the Korean government
(MSIP) (Nos. NRF-2016R1A2B3007119 and
2019R1A4A2001440).
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Organic Letters
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E
DOI: 10.1021/acs.orglett.9b02280
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