N -Protecting group tuning of the enantioselectivity in Strecker reactions of trifluoromethyl ketimines to synthesize quaternary α-trifluoromethyl amino nitriles by ion pair catalysis

Article abstract of DOI:10.1039/c9cc09151c

An enantioselective Strecker reaction to construct trifluoromethylated quaternary stereocenters with N-PMP and unexplored N-Boc trifluoromethyl ketimines catalyzed using an organophosphine dual-reagent catalyst has been developed. The enantioselectivities

Full text of DOI:10.1039/c9cc09151c

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Conformational control of nonplanar free base porphyrins:
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DOI: 10.1039/C9CC09151C
COMMUNICATION
N-Protecting Group Tuning the Enantioselectivity in Strecker
Reactions of Trifluoromethyl Ketimines to Synthesize Quaternary
-Trifluoromethyl Amino Nitriles by Ion Pair Catalysis
Received 00th January 20xx,
Accepted 00th January 20xx
Mengyuan Du,a Longhui Yu,b Ting Du,b Zhaokun Li,b Yueyang Luo,b Xiangyu Meng,b Zhengtao Tian,b
Changwu Zheng,*c Weiguo Cao*a and Gang Zhao*b
DOI: 10.1039/x0xx00000x
O
An
enantioselective
Strecker
reaction
to
construct
HN
O
HN
Ph
F3C
NH2
F3C
trifluoromethylated quaternary stereocenters with N-PMP and
unexplored N-Boc trifluoromethyl ketimines catalyzed by
organophosphine dual-reagent catalyst has been developed. The
enantioselectivities of the corresponding products with the same
catalyst could be switched by using different N-protecting group (N-
PMP or N-Boc). The trifluoromethyl amino nitriles were obtained in
high yield and high enantioselectivity in a short time and could be
easily converted to a variety of useful trifluoromethyl-containing
compounds.
Me COOH
OH
(S)-trifluoromethylalanine
UPPS
inhibitor
F3C
F
OH
Cl
NH
HO
F3C
N
O
N
H
H
Efavirenz
anti-HIV drug
Glycosldase
inhibitor
Trifluoromethyl-containing amines, amino acids and related
compounds play an important role in chemical building blocks,
biological compounds and drugs (Scheme 1).1-3 The catalytic
enantioselective Strecker reactions is one of the most practical
and facile methods for the transformation of aldimines and
ketimines into the corresponding optically active -amino
nitriles, which are easily converted into amino acids and amine
derivatives.4 In 2010, Enders and coworkers reported the first
catalytic enantioselective Strecker reaction of N-PMP-protected
Scheme
1
Representative biologically active compounds bearing
a
trifluoromethylated quaternary amine motif.
Michael additions,9a as well as aldol reactions9b in high
enantioselectivity. The reactions feature a low catalyst loading
and high efficiency under homogeneous ion-pairing catalysis
which has largely expanded the application of organophosphine
catalysis. In continued efforts to explore the asymmetric
organophosphine catalysis, we now wish to report protecting
group (PMP or Boc)-determined enantioselective Strecker
reactions of α-trifluoromethylated ketimines under ion pair
catalysis (Scheme 2). To the best of our knowledge, few reports
trifluoromethyl ketimines for the synthesis of α-quaternary α-
5e,
trifluoromethyl amino acids5a, and later, Zhou5b,
5g, Ma5c,
Wang5d and Nenajdenko5f reported similar reactions. They have
used bifunctional Lewis base-thioureas as organocatalysts to
promote the Strecker reactions with various imines. However,
most of the reactions need longer time to complete.
a) Previous work
O
Bn
NBoc
O
(CH3)3SiCN
cat. (0.1 mol%)
methyl acrylate (0.1 mol%)
Ph
NC
NHBoc
O
N
H
R1
Recently, our group has developed a novel asymmetric dual-
reagent catalyst,6,7,8 which is generated in situ by the chiral
amino-acid derived organophosphine and methyl acrylate, to
effectively control the Mannich reactions,6,7 Strecker reactions,8
O
NH
PPh2
R1
N
N
R2
CH2Cl2, -40 o
C
R2
R1 = X, Me, OMe
R2 = Me, Bn, PMB, PNB, Allyl
cat.
up to 99% yield
up to 99% ee
NO2
b) This work
a. Department of Chemistry, Innovative Drug Research Center, Shanghai University,
Shanghai 200444, P. R. China. E-mail: wgcao@staff.shu.edu.cn
asymmetric
dual-reagent catalysis
PG
N
NC NHPMP
Ar CF3
or
NC NHBoc
Ar CF3
b. Key Laboratory of Synthetic Chemistry of Natural Substances, Shanghai Institute
of Organic Chemistry, Chinese Academy of Sciences, 345 Lingling Road, Shanghai
200032, P. R. China. E-mail: zhaog@sioc.ac.cn
Ar
CF3
protecting group-determined
enantioselectivity
PG = PMP
Boc (unexplored)
c. School of Pharmacy, Shanghai University of Traditional Chinese Medicine,
Shanghai 201203, China. E-mail: zhengcw@shutcm.edu.cn
Electronic Supplementary Information (ESI) available: [details of any supplementary
information available should be included here]. See DOI: 10.1039/x0xx00000x
Scheme 2 Asymmetric dual-reagent catalysis in Strecker reaction.
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Table 1 Optimization of the reaction conditions.a
reaction (Table 1, entries 2-3). To our delight, improved
View Article Online
DOI: 10.1039/C9CC09151C
enantioselectivity (60% ee) was observed with thiourea catalyst
MeO
4c which implied the possible interaction between fluorine and
H-bonding.5b,13 The skeleton of the organophosphine catalysts14
was herein investigated, which shown the catalyst 4e from L-
tert-Leucine was the best. Other catalysts, 4f and 4g, from L-tert-
Leucine with different aryl group on thiourea moiety gave
similar results (Table 1, entries 3-7). The following screening of
solvents, catalyst loading, and the additives indicated the
reactions were best to be performed with 5 mol% of 4e in DCM
using methyl acrylate as the additive (Table 1, entries 8-16).
Lowering the temperature to -20 oC further improved the
enantioselectivity to 91% ee (Table 1, entries 17-18). The
absolute stereochemistry of 3a was determined to be R by
comparing the optical rotation with the reported value.5a, 5b With
the optimized reaction conditions in hand, a series of N-PMP
trifluoromethyl ketimines were investigated. It seemed that
substituents on the aryl ring including the electron-withdrawing
and electron-donating groups were well tolerated, giving the
corresponding products in high yields with high
enantioselectivities (Table 2, 3a-3j). In addition, the
heteroaromatic α-amino nitriles could also be obtained in 86%
yield and 86% ee (Table 2, 3k). Substrates with other protecting
groups such as N-(4-fluorophenyl) imine, N-(4-bromophenyl)
imine and N-(4-methylphenyl) imine also showed good results
(Table 2, 3l-3n).
OMe
NC CF3
N
4
catalyst , additive
N
H
CF3
TMSCN
+
solvent, temp.
1a
2
3a
R1
Bn
PPh2
PPh2
PPh2
X = O, R1 = Bn
X = S, R1 = Bn
: X = S, R1 = 2-(S)-Bu
X = S, R1 = tBu
4b :
O
NH
X
NH
S
NH
NH
4c :
4d
HN
CF3
R2
4e :
4f
CF3
CF3
: R2 = Ph
: R2 = 4-NO2C6H4
4g
CF3
4a
Entry
1
Cat.
Additive
T/h
3
Yield/%b
90
ee/%c
0.
4a
4b
4c
4d
4e
4f
Methyl acrylate
Methyl acrylate
Methyl acrylate
Methyl acrylate
Methyl acrylate
Methyl acrylate
Methyl acrylate
Methyl acrylate
Methyl acrylate
Methyl acrylate
Methyl acrylate
Benzyl acrylate
2
3
88
41
60
68
85
78
82
84
67
80
84
80
3
3
91
4
3
85
5
3
93
6
3
91
7
4g
4e
4e
4e
4e
4e
3
90
8d
9e
10f
11g
12
3
89
3
87
3
88
Table 2 The Strecker reaction of N-PMP trifluoromethyl ketimines catalyzed by
the dual-reagent catalysis.a,b,c
7
82
NPG
CF3
4e
cat.
(5 mol%), methyl acrylate (5 mol%)
DCM, -20 oC, 24 h
NC
R
NHPG
CF3
3
85
+
TMSCN
R
1
2
3
13
4e
tert-Butyl acrylate
3
82
84
NC
NC
NC
NC
NHPMP
CF3
NHPMP
CF3
NHPMP
CF3
NHPMP
CF3
Methyl vinyl
ketone
Cl
14
15
4e
4e
3
4
83
82
62
56
F
Cl
3a
3b
87% yield
88% ee
3c
89% yield
90% ee
3d
90% yield
89% ee
Acrylamide
89% yield
91% ee
N,N-Dimethyl
acrylamide
16
4e
4
80
83
NC
NC
NC
NHPMP
CF3
NHPMP
CF3
NHPMP
CF3
NC
NHPMP
CF3
Me
Cl
3ed
Br
Me
17h
18i
4e
4e
Methyl acrylate
Methyl acrylate
10
24
90
89
88
91
3f
3g
87% yield
92% ee
3h
85% yield
91% ee
45% yield
90% ee
90% yield
90% ee
F
a
Reaction conditions: 0.10 mmol 1a, 0.20 mmol 2, methyl acrylates (5 mol%),
NC
NHPMP
CF3
chiral phosphine 4 (5 mol%), DCM (1.0 mL), rt. b Isolated yield. c The ee value was
determined by chiral HPLC analysis. d CHCl3 was used as the solvent. e Toluenewas
NC
NH
NC
NHPMP
CF3
NC
S
NHPMP
CF3
CF3
CF3
MeO
f
h
used as the solvent. Et2O was used as the solvent. g 4e (1 mol %) was used.
Reaction was conducted at 0 oC. i Reaction was conducted at -20 oC.
3i
3j
3k
3l
87% yield
90% ee
88% yield
86% ee
86% yield
86% ee
93% yield
90% ee
of trifluoromethyl ketimines bearing a more cleavable N-Boc
group10 have been reported in Strecker reactions and other
catalytic asymmetric reactions.
Br
Me
NC
NC
NC
HN
CF3
NH
NHBoc
CF3
CF3
Initially, the commonly used N-PMP trifluoromethyl
ketimines were tested under dual-reagent catalysis in DCM at
room temperature. When the amide catalyst 4a was employed,
the product 3a was obtained in high yield which demonstrated
the ability of ion-pair catalysis11,12 in this Strecker reactions
although it was racemic (Table 1, entry 1). Then the double
hydrogen-bonding catalyst 4b and 4c were chosen to catalyze the
3'ae
3m
91% yield
82% ee
3n
92% yield
89% ee
94% yield
60% ee
a
Reaction conditions: 1 (0.10 mmol), 2 (0.20 mmol), methyl acrylate (5
mol%), chiral phosphine 4 (5 mol%), DCM (1.0 mL), -20 oC. b Isolated yield.
c The ee value was determined by chiral HPLC analysis. d Reaction time: 3 d.
e Reaction time: 1 h.
2 | J. Name., 2012, 00, 1-3
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NC
Ph CF3
NH2
O
a
tentative evaluation of the N-Boc
View Article Online
Br
DOI: 10.1039/C9CC09151C
trifluoromethyl ketimine gave the product with 60% ee in one
hour (Table 2, 3’a). This is very exciting since the Boc-
protecting group could be removed more easily in view of the
practicality of this reaction10 and this is the first example to use
N-Boc trifluoromethyl ketimine as the substrate in the
asymmetric Strecker reaction. To improve the enantioselectivity,
NH
5a
5d
90% yield
85% ee
92% yield
85% ee
BocHN
Ph
CF3
HCl (g),
CH3OH, 0 o
4-bromobenzoic acid
HBTU, DIPEA, DCM
C
NaBH4
H
Ph
CF3
DBU
NH2
CF3
Ph
NHBoc NiCl2·6H2O
NC
N
O
a
few more conditions were screened (see Supporting
Ph CF3
Toulene, 110 o
C
CH3OH, 0 o
C
NHBoc
HN
Information for details). When the reaction was performed at -72
oC, the product could be obtained in 91% yield and 85% ee in
only 2 hours, which demonstrated the superior efficiency of the
dual-reagent catalysis in Strecker reactions. As shown in Table
3, this reaction was applicable to various N-Boc trifluoromethyl
ketimines bearing electron-withdrawing substituents or electron-
donating substituents on the aromatic ring, furnishing the
expected products in high yields with good to high
enantioselectivities. The absolute configuration of the N-Boc
products was then determined to be S based on X-ray crystal
structural analysis of 3’i after recrystallization.17 To our surprise,
a switch of the enantioselectivity was observed comparing to the
N-PMP products. In general, tunable enantioselectivity in
catalytic asymmetric reactions is still very challenging especially
by using a single chiral catalyst.15 So far, limited examples have
been discovered under organocatalysis to tune the
enantioselectivity by varying the solvents,16a additives16b or
reagents.16c,16d These results not only demonstrate the versatility
of the organophosphine dual-reagent catalysis in controlling the
enantioselectivity, but also largely expanded its application in
constructing quaternary stereocenters under mild conditions.
The reaction could be conducted on gram scale without
decreasing the ee. The achieved α-trifluoromethyl-α-amino
nitrile 3’a was conveniently de-protected under acidic
3'a
85% ee
5c
5f
92% yield
85% ee
90% yield
88% ee
H2O2, Na2CO3
acetone
4-nitrophenyl
isothiocyanate,
DCM
O
Ph
CF3
NO2
NH2
NHBoc
S
BocHN
Ph
N
N
5b
5e
88% yield
85% ee
94% yield
86% ee
H
H
CF3
Scheme 3 Transformations of the product 3’a.
conditions and was transformed into varieties of new
compounds, such as the amide 5b and diamine compound 5c, in
good yields (Scheme 3). The compound 5c could be further
transformed into amide 5d, the amino-thiourea 5e and
imidazolidinone 5f under ordinary conditions. The divergent
synthesis of these derivatives provides an efficient way towards
the motifs containing the trifluoromethyled amines or analogs.
As for the enantioselectivity control and switch under the dual-
reagent catalysis, we proposed the transition states of the
reactions with N-PMP and N-Boc substrates as indicated in
Scheme 4.5,13 The substrate 1a with PMP group could combine
to the catalyst 4e via the N-H and F-H bonding, while the
substrate 1’a with N-Boc group may interact with the catalyst 4e
via O-H and N-H bonding. The face selectivity of the complex
was controlled by the steric hinderance and the C=N bond should
be close to the attacking cyanide anion. Herein, the reaction in
TS-1 happened via the Re-face and in TS-2 happened via the Si-
face affording two products 3a and 3’a in opposite configuration.
Table 3 The Strecker reaction of N-Boc trifluoromethyl ketimines catalyzed by the
dual-reagent catalysis.a,b,c
4e
cat.
(5 mol%)
NBoc
CF3
1'
NC
R
NHBoc
CF3
+
TMSCN
R
methyl acrylate (5 mol%)
DCM, -72 o
C
2
3'
TS-1
TS-2
NC
NHBoc
CF3
NC
NHBoc
CF3
NC
NC
NHBoc
CF3
NHBoc
CF3
F
H
H
F
F3C
F3C
S
N
F3C
F3C
S
N
F
N
Ph2P
OMe
N
Ph2P
OMe
O
F
H
H
3'a
91% yield
85% ee
3'b
96% yield
79% ee
3'c
94% yield
89% ee
3'd
, 2 h
92% yield
83% ee
, 2 h
, 2 h
, 2 h
H
O
H
F
F
F
Si
CN
Si
N
O
C
N
O
CF3
CN
NC
NC
NHBoc
CF3
NHBoc
CF3
NC
NC
NHBoc
NHBoc
CF3
Cl
Br
CF3
MeO
Cl
Cl
3'e
3'f
98% yield
72% ee
3'g
98% yield
83% ee
3'h
, 10 h
95% yield
68% ee
, 5 h
85% yield
85% ee
, 3 h
, 4 h
Si-face attack
Re-face attack
NC
NC
NHBoc
CF3
NHBoc
CF3
NC
NHBoc
CF3
NC
S
NHBoc
CF3
NHPMP
CF3
NC
NC
NHBoc
CF3
O2N
Br
Me
3a
3'a
3'i
3'j
3'k
3'l
, 5 h
, 12 h
, 1 h
, 10 h
90% yield
70% ee
90% yield
70% ee
90% yield
84% ee
87% yield
77% ee
Scheme 4 Proposed transition states leading to the opposite configuration.
a
Reaction conditions: 1' (0.10 mmol), 2 (0.20 mmol), methyl acrylate (5
mol%), chiral phosphine 4e (5 mol%), DCM (1.0 mL), -72 C. b Isolated
yield. c The ee value was determined by chiral HPLC analysis.
o
In conclusion, we have achieved the catalytic asymmetric
Strecker reaction of trifluoromethyl ketimines by
organophosphine dual-reagent catalysis under mild conditions.
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 3
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(a) B. Huang, C. Li, H.-M. Wang, C.-H. Wang, VLie.wLAiruticalenOdnlJin.e-
9
With the same catalyst 4e, an enantioselectivity switch was
observed when the protecting group of ketimines is changed
from PMP to Boc. The reactions complete in a short time
affording chiral α-trifluoromethyl-α-amino nitriles in high yields
and good to excellent enantioselectivities.
L. Zhang, Org. Lett., 2017, 19, 5102; (b) Q.-W. Yu, L.-P. Wu,
DOI: 10.1039/C9CC09151C
T.-C. Kang, J. Xie, F. Sha and X.-Y. Wu, Eur. J. Org. Chem.,
2018, 3992. For the racemic version of dual-reagent catalysis,
see: X.Wang, F. Fang, C. Zhao and S.-K. Tian, Tetrahedron
Lett., 2008, 49, 6442.
10 For selccted examples, see：(a) L.-H. Sun, Z.-Q. Liang, W.-
Q. Jia and S. Ye, Angew. Chem. Int. Ed., 2013, 52, 5803; (b) I.
V. Kutovaya, O. I. Shmatova, V. M. Tkachuk, N. V.
Melnichenko, M. V. Vovk and V. G. Nenajdenk, Eur. J. Org.
Chem. 2015, 6749; (c) B. M. Trost, C.-I. (Joey) Hung and M.
J. Scharf, Angew. Chem. Int. Ed., 2018, 57, 11408.
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We are grateful to the National Natural Science Foundation of
China (Nos. 21272247, 21572247, 21871282) and the Chinese
Academy of Sciences (XDB 20020100).
Conflicts of interest
There are no conflicts to declare.
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