Ni-catalyzed asymmetric decarboxylative Mannich reaction for the synthesis of β-trifluoromethyl-β-amino ketones

Article abstract of DOI:10.1039/c5ra02653a

A new Ni-catalyzed asymmetric decarboxylative Mannich reaction between (S_s)-N-t-butylsulfinyl-3,3,3-trifluoro-acetaldimine and β-keto-acids was developed, which was carried out at room temperature affording β-trifluoromethyl-β-amino ketones with excellent yields and diastereoselectivities.

Full text of DOI:10.1039/c5ra02653a

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DOI: 10.1039/C5RA02653A
A
Niꢀcatalyzed
(Ss)ꢀNꢀtꢀbutylsulfinylꢀ3,3,3ꢀtrifluoroꢀacetaldimine
βꢀtrifluoromethylꢀβꢀamino ketones with excellent yields and diastereoselectivities.
asymmetric
decarboxylative
Mannich
reaction
between
and
βꢀketoꢀacids
was
reported,
affording

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DOI: 10.1039/C5RA02653A
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ARTICLE TYPE
Ni-catalyzed asymmetric decarboxylative Mannich reaction for the
synthesis of β-trifluoromethyl-β-amino ketones
Ping Qian,a Yanling Dai,a Haibo Mei,a Vadim A. Soloshonok,c,d Jianlin Han*a,b and Yi Pana
Received (in XXX, XXX) XthXXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX
5
DOI: 10.1039/b000000x
A new Ni-catalyzed asymmetric decarboxylative Mannich
reaction
between
(Ss)-N-t-butylsulfinyl-3,3,3-trifluoro-
acetaldimine and β-keto-acids was developed, which was
carried out at room temperature affording β-trifluoromethyl-
10 β-amino
ketones
with
excellent
yields
and
diastereoselectivities.
Fluorine-containing compounds play an increasingly important
role in various chemical industries ranging from life- to material-
related sciences.1 In particular, the pharmaceutical industry
15 critically depends on the methodological developments in
fluorine chemistry for the design of more selective and potent
new drugs.2 One of these areas is the synthesis of polyfunctional
fluorinated derivatives possessing keto/hydroxy/amino groups.
However, preparation of such structurally complex compounds
20 still remains a significant synthetic challenge.3 Consistent with
our continuous research on the preparation of fluorinated
amines,4-9 we have developed a method for the synthesis of β-
trifluoromethyl-β-amino ketones10 via asymmetric Mannich
50 Scheme 1 Asymmetric decarboxylative Mannich reaction of β-
keto acid.
Initially, we chose -keto acid 2a and imine 1 as model
substrates for the reaction condition optimization, and the results
were shown in Table 1. We were pleased to find that the reaction
55 could proceed smoothly with CF3COOLi as catalyst in THF at
room temperature, affording the desired product 3a with 65%
yield and 96:4 diastereoselectivity after 12 h (entry 1). Pd(OAc)2
also worked for this reaction with excellent diastereoselectivity
(98:2 dr), but the yield decreased dramatically (35%, entry 2). To
60 further improve the reaction efficiency, several metal triflate salts
were examined (entries 3-11). Among the divalent metals, such
as Mg (entry 3), Ni (entry 4), Zn (entry 5), Ni-triflate catalyzed
reaction gave the best result (96% yield, 98:2 dr, entry 4).
Interestingly, a series of trivalent metals also could catalyze the
65 addition reaction with almost complete stereocontrol, however
the target product (R)(Ss)-3a was isolated in only moderate yields
(24%-67%, entries 6-11). Then, several other Ni(II) salt were
tried, such as Ni(acac)2 (entry 12), Ni(OAc)2·4H2O (entry 13),
NiNO3·6H2O (entry 14) and NiCl2·6H2O (entry 15). However, no
70 improvement was found at all. Solvent also showed great effect
on this reaction. The use of 1,4-dioxane (entry 20), diethyl ether
(entry 21), chloroform (entry 22) afforded the product (R)(Ss)-3a
with good diastereoselectivity but in rather lower chemical yields
(51%-72%). Finally, we conducted additional experiments to
75 optimize the loading amount of the catalyst (entries 23-24). These
experiments showed that decreasing the amount of Ni(OTf)2
catalyst to 10 mol% also could provide the desired product 3a
with 92% chemical yield and 99:1 dr when the reaction was
prolonged to 24 h.
reaction
between
(Ss)-N-t-butylsulfinyl-3,3,3-trifluoro-
25 acetaldimine 111 and acetophenone derived enolates. This
reaction gave moderate yields and diastereoselectivities.
Especially, the reaction need strong base (LDA) and must be
performed at low temperature (-78 oC). So, development of
simple and efficient method for the synthesis of chiral β-
30 trifluoromethyl-β-amino ketones becomes highly urgent.12
Decarboxylative Mannich reaction of β-keto acids, which
could form in situ the enolates under mild condition13 and easily
construct β-amino ketones structures in cascade mode,14 has
attracted broad attentions in synthetic chemistry community.
35 Recently, Lu group reported an organocatalytic decarboxylative
Mannich reaction of β-keto acids for the synthesis of chiral β-
amino ketones at room temperature with moderate
enantioselectivities (Scheme 1a).15 Very recently, Ma group
developed a Cu-catalyzed decarboxylative Mannich reaction of β-
o
40 keto acids at -20 C with excellent enantioselectivities (Scheme
1b).16 However, the asymmetric decarboxylative Mannich
reaction of β-keto acids for the synthesis of chiral
trifluoromethylated β-amino ketones has never been reported.
Herein, we would like to report a Ni-catalyzed asymmetric
45 decarboxylative Mannich reaction of trifluoro-acetaldimine at
room temperature for the synthesis of chiral -trifluoromethyl--
amino ketones with excellent chemical yields and high
diastereoselectivities (Scheme 1c)
This journal is © The Royal Society of Chemistry [year]
[journal], [year], [vol], 00–00 |1

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Table 1. Optimization of decarboxylative Mannich addition reaction
conditions.a
35 substituted phenyl group 2z, almost no desired product was found
even the reaction time was increased to 48 h.
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CF3
O
S
O
O
O
O
Ni(SO3CF3)2 (10 mol %)
DOI: 10.1039/C5RA02653A
S
F3C
N
R
OH
+
R
N
H
THF, rt, 24 h
2
(E)(Ss)-1
(R)(Ss)-3
O
CF3
O
S
O
CF3
O
S
O
CF3
O
S
Entry
Catalyst (mol %)
Solvent Time
(h)
Yield
(%)b
65
35
75
96
53
31
24
40
35
60
67
82
76
97
85
Drc
N
N
N
H
H
H
Cl
Et
F
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
20
CF3COOLi (30)
Pd(OAc)2 (30)
THF
THF
THF
THF
THF
THF
THF
THF
THF
THF
THF
THF
THF
THF
THF
1,4-
12
12
12
12
12
12
12
12
12
12
12
12
12
12
12
12
96:4
98:2
99:1
98:2
99:1
3b, 90% yield, 98:2 dr
3a, 92% yield, >99:1 dr
3c, 83% yield, 97:3 dr
O
CF3
O
S
O
CF3
O
S
Mg(SO3CF3)2 (30)
Ni(SO3CF3)2 (30)
Zn(SO3CF3)2 (30)
Fe(SO3CF3)3 (30)
In(SO3CF3)3 (30)
Sc(SO3CF3)3 (30)
La(SO3CF3)3 (30)
Sm(SO3CF3)3 (30)
Yb(SO3CF3)3 (30)
Ni(acac)2 (30)
O
CF3
O
S
N
N
N
H
H
H
Me
Br
99:1
3f, 99% yield, >99:1 dr
3e, 97% yield, 98:2 dr
3d, 85% yield, 95:5 dr
96:4
>99:1
96:4
>99:1
>99:1
82:18
80:20
97:3
O
CF3
O
S
O
CF3
O
S
O
CF3
O
S
N
N
N
H
H
H
EtO
Br
MeO
Cl
iPr
3i, 97% yield, >99:1 dr
3g, 99% yield, >99:1 dr
3h, 94% yield, 98:2 dr
O
CF3
O
S
O
CF3
O
S
Ni(OAc)2·4H2O (30)
NiNO3·6H2O (30)
NiCl2·6H2O (30)
Ni(SO3CF3)2 (30)
O
CF3
O
S
N
N
N
H
H
H
93:7
>99:1
N
51
3k, 88% yield, 96:4 dr
3l, 70% yield, 97:3 dr
3j, 91% yield, 97:3 dr
dioxane
ether
CHCl3
THF
THF
O
CF3
O
S
O
CF3
O
S
O
CF3
O
S
21
22
23
24
Ni(SO3CF3)2 (30)
Ni(SO3CF3)2 (30)
Ni(SO3CF3)2 (20)
Ni(SO3CF3)2 (10)
12
12
24
24
72
28
94
92
95:5
97:3
>99:1
>99:1
MeO
N
N
N
H
H
H
3m, 91% yield, 94:6 dr
3n, 90% yield, >99:1 dr
3o, 98% yield, >99:1 dr
a Reaction conditions: 2a (0.16 mmol), imine 1 (0.1 mmol), catalyst,
solvent (2 mL), at room temperature. b Isolated yields. c Determined by
19F NMR analysis.
O
CF3
O
S
O
CF3
O
S
O
CF3
O
S
N
N
O
N
H
H
H
3q, 91% yield, 99:1 dr
3p, 86% yield, 96:4 dr
3r, 90% yield, 96:4 dr
O
CF3
O
S
O
CF3
O
S
O
CF3
O
S
5
With the optimized reaction conditions in hand, we next
proceeded to study the substrate scope of this decarboxylative
Mannich addition reaction (Scheme 2). The reaction showed a
wide range of β-keto acids scope, and proceeded smoothly to give
the corresponding β-trifluoromethyl-β-amino ketones 3a-y in
N
N
N
H
H
H
3u, 98% yield, 99:1 dr
3t, 96% yield, 97:3 dr
3s, 81% yield, 96:4 dr
O
CF3
O
S
O
CF3
O
S
O
CF3
O
S
S
N
N
N
H
H
H
10 excellent yields (70%-99%) and high diastereoselectivities (94:6-
>99:1). For the substrates with para-substituted phenyl group 2b-
2j, either electronic properties or steric bulk of the substituent had
almost no noticeable effect on the diastereoselectivity of the
reactions. For example, product 3g, bearing bulky iso-propyl
15 group was also isolated as diastereomerically pure compound in
quantitative chemical yield. The reactions with the substrates
containing meta-substituted phenyl ring, also could proceed
smoothly resulting in a bit lower chemical yields (3k-3m). To
have more structurally interesting derivatives, a di-substituted
20 starting β-keto acid 2o was examined in the reaction, and >99:1
diastereoselectivity was obtained along with 98% chemical yield
(3o). Naphthyl containing keto-acids 2x,y cleanly reacted with
imine 1 affording the target products 3x,y in excellent yields and
diastereoselectivity, and the reaction of 2-naphthyl substituted
25 keto-acids 2y gave a little bit better yield and diastereoselectivity
(98% yield, >99:1 dr). The reaction could also well tolerate the
heterocyclic and ester substituted groups as disclosed by the
addition reaction of β-keto acid 2w and 2p, giving the
corresponding products 3w,p with excellent results. Finally we
30 examined a series of aliphatic group containing β-keto acids 2q-t.
It was noticed that these substrates with alkyl groups also worked
very well in the decarboxylative Mannich reactions, yielding the
products 3q-t as diastereomerically pure compounds with
excellent yields. However, for the substrate with para-NO2
O
3w, 99% yield, >99:1 dr
3x, 95% yield, 97:3 dr
3v, 99% yield, >99:1 dr
O
CF3
O
S
O
CF3
O
S
N
N
H
H
O2N
3z, trace
3y, 98% yield, >99:1 dr
Scheme 2 Substrate scope of the asymmetric decarboxylative Mannich
addition reactions. Reaction conditions: 2 (0.16 mmol), imine 1 (0.1
a
b
40 mmol), Ni(OTf)2 (10 mol %), in THF at room temperature for 24 h.
Isolated yields. Determined on crude reaction mixtures by 19F NMR
c
analysis.
The next study was to determine the absolute configuration of
the major products 3. Throughout the study, we noticed that the
45 major products 3a-y have a different 19F-NMR data as compared
with the previously reported (S)(Ss)-β-trifluoromethyl-β-amino
ketones obtained from the asymmetric Mannich reaction.10 So,
we synthesized the diastereomer (S)(Ss)-3 using the reported
method10 and conducted its deprotection to free amine (S)-4
50 (Scheme 3a). Diastereomeric compound (R)(Ss)-3, obtained in
this work, was also deprotected to produce free amino-ketone
(R)-4 (Scheme 3c). Finally, the racemic-4 was prepared with the
same method for (R)-4 (Scheme 3b). Then, we conducted their
detailed analysis by using chiral HPLC (see SI), which clearly
55 showed that the compound obtained in the current
decarboxylative Mannich system and that from previous Mannich
2
|Journal Name, [year], [vol], 00–00
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Page 4 of 5
reaction10 are enantiomers (see SI), and have opposite absolute
configuration. So, the absolute configuration of compound 4 30 (Scheme 5). (R)(Ss)-3a undergoes deprotection, aminoacylation
trifluoromethyl-β-amino ketone product 3a was carried out
obtained in the current system is assigned as (R), and the absolute
configuration of major products 3a-y, were assigned as (R)(Ss)
accordingly.
and cyclization with phosphorus pentasulfDidOeI:t1o0g.1i0v3Ve9ie/twChe5ArRt(iAcRl0e)2-O26n5,l6i3n-Ae
diphenyl-4-(trifluoromethyl)-4H-1,3-thiazine17
with good
6
5
chemical yields. Also, almost no racemation was found during
the three-step transformations.
O
CF3
O
S
O
CF3
NH2
35
1) aq HCl, MeOH, rt, 12h
2) Et3N, DCM, rt, 1h
a)
Ph
(S)(Ss)-3
N
Ph
(S)-4
H
obtained from previous Mannich
O
CF3
O
O
CF3
1) aq HCl, MeOH, rt, 12h
2) Et3N, DCM, rt, 1h
S
b)
Ph
racemic-3
N
Ph
racemic-4
NH2
H
O
CF3 O
O
CF3
NH2
1) aq HCl, MeOH, rt, 12h
2) Et3N, DCM, rt, 1h
S
Ph
(R)(Ss)-3
N
Ph
(R)-4
obtained from decarboxylative Mannich
c)
H
Scheme 5 Cyclization derivatization of 3a.
In summary, we developed an asymmetric Ni-catalyzed
decarboxylative Mannich reaction of chiral imine for the
Scheme 3 Preparation of enantiomeric free amines (S)-4, (R)-4 and
racemic-4 by deprotection of 3.
40 synthesis of -trifluoromethyl--amino ketone for the first time.
The reaction has a broad scope of keto-acid substrates and could
be carried out under room temperature with excellent chemical
yields and diastereoselectivities. The reaction provides a new and
easy way for the preparation of chiral trifluoromethylated -
45 amino ketones, even for the large-scale preparation.
We gratefully acknowledge the financial support from the
National Natural Science Foundation of China (Nos. 21102071
and 21472082). The Jiangsu 333 program (for Pan) and
Changzhou Jin-Feng-Huang program (for Han) are also
50 acknowledged.
According to the above results and previous reports,13-16
a
10 plausible mechanistic pathway for this decarboxylative Mannich
reaction was proposed in Scheme 4. Initially, catalyst Ni(OTf)2
reacts with-keto acid 2a to generate intermediate A. Then,
intermediate A adds to chiral imine 1 to form intermediate C via
the transition state B. In transition state B, the enolate hydrogen is
15 supposed to coordinate with the oxygen of the S-O group.
Therefore the enolate O-H bond can be weakened to generate the
negative charge on the corresponding carbon, which accelerates
the addition step. The intermediate C reacts with starting material
2a affording the intermediate D, along with the formation of
20 intermediate A for the next catalytic cycle. Fortunately, the
intermediate D has been detected by HRMS from the reaction
mixture. The final step of this reaction is decarboxylation,
resulting in the final product 3a with R configuration.
Notes and references
a
School of Chemistry and Chemical Engineering, Nanjing University,
Nanjing, 210093, China. Fax: 86-25-83686133; Tel: 86-25-83686133;
E-mail: hanjl@nju.edu.cn.
55 b High-Tech Research Institute of Nanjing University, Changzhou, 213164,
China.
c Department of Organic Chemistry I, Faculty of Chemistry, University of
the Basque Country UPV/EHU, 20018 San Sebastian, Spain.
d IKERBASQUE, Basque Foundation for Science, 48011 Bilbao, Spain
60 † Electronic Supplementary Information (ESI) available: [Experimental
O
Ni(OTf)2
S
N
O
O
O
O
F3C
1
ONiOTf
OH
2a
1
A
procedures, full spectroscopic data for compounds 3-6 and copies of H
- TfOH
NMR and 13C NMR spectra]. See DOI: 10.1039/b000000x/
O
O
Ph
1
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OH
CF3
O
S
2a
H
O
S
O
CMe3
O
HN
65
70
75
N
S
O
H
CMe3
O
HN
H
CF3
Ni
O
CF3
CO2NiOTf
O
CO2H
O
O
D
B
C
s-trans-(R)(Ss)
ONiOTf
A
O
S
CMe3
O
HN
- CO2
R1
CF3
3a
25
2
Scheme 4 Possible mechanism for the asymmetric decarboxylative
Mannich reaction.
Finally, the cyclization derivatization of the obtained β-
This journal is © The Royal Society of Chemistry [year]
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RSC Advances
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