Enantioselective allylic amination of trifluoromethyl group substituted racemic and unsymmetrical 1,3-disubstituted allylic esters by palladium catalysts

Article abstract of DOI:10.1021/ol5008229

The palladium-catalyzed regio- and enantioselective allylic amination of trifluoromethyl group substituted racemic and unsymmetrical 1,3-disubstituted allylic esters has been accomplished. The enantioselective formation of the α-type allylic amines was at

Full text of DOI:10.1021/ol5008229

Letter
pubs.acs.org/OrgLett
Enantioselective Allylic Amination of Trifluoromethyl Group
Substituted Racemic and Unsymmetrical 1,3-Disubstituted Allylic
Esters by Palladium Catalysts
Motoi Kawatsura,*,† Shou Terasaki,† Maki Minakawa,† Takuya Hirakawa,‡ Kazunori Ikeda,‡
and Toshiyuki Itoh*,‡
†Department of Chemistry, College of Humanities & Sciences, Nihon University, Sakurajosui, Setagaya-ku, Tokyo 156-8550, Japan
‡Department of Chemistry and Biotechnology, Graduate School of Engineering, Tottori University, Koyama, Tottori 680-8552, Japan
S
* Supporting Information
ABSTRACT: The palladium-catalyzed regio- and enantiose-
lective allylic amination of trifluoromethyl group substituted
racemic and unsymmetrical 1,3-disubstituted allylic esters has
been accomplished. The enantioselective formation of the α-
type allylic amines was attained by the dynamic kinetic
asymmetric transformation (DYKAT).
Based on our previous study,6i we first conducted the allylic
he palladium-catalyzed asymmetric allylic substitution
Treaction is a powerful synthetic method to construct amination of the trifluoromethyl group containing the racemic
allyl ester 1a with 1-phenylpiperazine (2a) by [Pd(C3H5)-
(cod)]BF4 with (S)-BINAP at 25 °C, and the mixture of the γ-
prodcut 3aa (6% ee) and α-product 4aa (94% ee) was obtained
with a low regioselectivity (Table 1, entry 1). Although the
yield of 4aa was low, the formation of the enantiomerically
enriched product from the racemic substrate is interesting and
suggests that the reaction proceeds through the deracemization
pathway; therefore, we attempted to obtain the enantiomeri-
cally enriched α-product in a high yield with a high
enantioselectivity. The reaction at elevated temperature
increased both the yield and α-selectivity, but the enantiomeric
excess of 4aa decreased (entries 2 and 3). We further examined
other palladium catalysts and found that the combination of
[Pd(C3H5)Cl]2 with a silver salt effectively increased the yield
of 4aa with a high enantiomeric excess (entries 4 and 5).
Although the role of silver salt is not clear, the reaction with the
addition of 5 mol % of AgPF6 exhibited better results and
provided 4aa in 88% isolated yield (91% α-selectivity) with
93% ee (entry 6). We also examined the reaction without silver
salt and confirmed that the reaction proceeds with a low α-
selectivity (8%) (entry 7).
With these optimized reaction conditions in hand, we
investigated the asymmetric allylic amination of racemic 1a with
several amines. As shown in Table 2, reactions with the
optimized catalyst with six-membered cyclic amines, such as
morpholine (2b), 1-methylpiperazine (2c), or 4-phenylpiper-
idine (2d), smoothly proceeded and produced the desired α-
products 4ab, 4ac, and 4ad with good enantioselectivities
(Table 2, entries 1−3). The reaction of 1a with pyrrolidine
(2e) also exhibited a high enantioselectivity, but the yield was
chiral carbon−carbon or chiral carbon−heteroatom bonds, and
several types of reactions have been reported.1 Although there
are many reports about the asymmetric allylic substitutions of
symmetrical 1,3-disubstituted allylic esters2 or monosubstituted
allylic esters,3 there are only limited examples of the asymmetric
reaction for the unsymmetrical 1,3-disubstituted allylic esters.4,5
In particular, the asymmetric reaction of racemic and acyclic
unsymmetrical 1,3-disubstituted allylic esters is still very rare
because the reaction generally proceeds via a net retention
(double inversion) mechanism4 or kinetic resolution process,5,6
and there are only limited examples of the palladium-catalyzed
dynamic kinetic asymmetric transformation (DYKAT).7−9 For
example, Hoberg9a and Gais9b reported the palladium-catalyzed
DYKAT of acyclic unsymmetrical 1,3-disubstituted allylic
substrates with oxygen nucleophiles, and the reactions with
carbon nucleophiles were demonstrated by Pucheault in 2011.9c
More recently, Liao realized the allylic indolylation of acyclic
unsymmetrical 1,3-disubstituted allylic substrates.9d However,
to the best our knowledge, there is no clear report about the
palladium-catalyzed DYKAT of acyclic unsymmetrical 1,3-
disubstituted allylic substrates with nitrogen nucleophiles in
high yield.10 On the other hand, we have studied the palladium-
catalyzed regioselective allylic substitution of fluorine-contain-
ing allylic esters11 and reported the regioselective allylic
amination of the trifluoromethyl group substituted unsym-
metrical 1,3-disubstituted allylic esters including the synthesis
of an enatiomerically enriched product from a chiral
substrate.4g,6i,11b During the course of our studies, we achieved
the palladium-catalyzed allylic amination of the racemic
trifluoromethyl group substituted unsymmetrical 1,3-disubsti-
tuted allylic esters that provide enatiomerically enriched allylic
amines with both a high yield and enantioselectivity by
DYKAT.
Received: March 19, 2014
Published: April 23, 2014
© 2014 American Chemical Society
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Organic Letters
Letter
Table 1. Palladium-Catalyzed Allylic Amination of rac-1a
with 2a
Table 2. Palladium-Catalyzed Enantioselective Allylic
Amination of rac-1 with Several Amines
a
b
[Pd] (mol %),
[Ag] (mol %)
yield (%) of
ee (%)
a
entry
3:4
(3 + 4)
of 4aa
c
1
[Pd(C3H5)(cod)]BF4
(10)
77:23
38:62
9:91
69
94
d
2
[Pd(C3H5)(cod)]BF4
(10)
>98
>98
94
87
67
77
86
93
87
3
4
5
6
7
[Pd(C3H5)(cod)]BF4
(10)
a
,
b
c
[Pd(C3H5)Cl]2 (5),
AgBF4 (10)
33:67
8:92
time
(h)
yield (%) of
% ee
a
entry
1
2
3:4
(3 + 4)
of 4
[Pd(C3H5)Cl]2 (5),
AgBF4 (5)
>98
1
1a
1a
1a
1a
1b
1a
1a
1a
1a
1b
1b
1b
1b
2b
2c
2d
2e
2e
2f
12
24
24
24
24
24
24
36
72
72
96
36
48
12:88
9:91
98 (83)
98 (82)
94 (90)
49 (41)
98 (77)
75 (59)
90 (85)
90 (80)
60 (49)
65 (50)
77 (76)
80 (58)
91 (4ab)
89 (4ac)
86 (4ad)
91 (4ae)
87 (4ae)
85 (4af)
91 (4ag)
87 (4ah)
86 (4ai)
86 (4ai)
93 (4aj)
91 (4ak)
92 (4al)
e
2
[Pd(C3H5)Cl]2 (5),
AgPF6 (5)
9:91
95 (88)
3
5:95
4
26:74
10:90
4:96
[Pd(C3H5)Cl]2 (5)
92:8
>98
a
1
5
The yields and ratios were determined by H NMR of the crude
b
c
6
materials using an internal standard. Determined by HPLC. The
reaction was conducted at 25 °C. The reaction was conducted at 40
°C. Isolated yield in parentheses.
d
7
2g
2h
2i
7:93
e
8
7:93
9
8:92
10
11
12
13
2i
8:92
low due to deacylation of the allyl substrate 1a with amine
(entry 4). Fortunately, changing the leaving group of the allyl
substrate from acetate to tert-butyl carbonate prevented the side
reaction, and a good yield was then obtained (entry 5). The
reaction with acyclic secondary amines 2g−i also afforded the
intended enantiomerically enriched α-products in moderate to
good yields with a high ee value (entries 7−10). We further
established that high enantioselectivities were attained for the
reactions of 1b with primary amines 2j−l (entries 11−13).
We next examined the reaction of other trifluoromethyl
group substituted allyl acetates with 2a (Table 3). The
reactions of 1,1,1-trifluoro-4-arylbut-3-en-2-yl acetates 1c−f
with 2a smoothly proceeded and provided the desired allylic
amines in good yields with high ee values (entries 1−4).
Unfortunately, we confirmed that 1g, which has an alkyl group
at the C-3 position instead of an aryl group, exhibited a low
reactivity and produced the γ-product 3ga (entry 5). The
reaction of (Z)-1a gave a (S)-4aa in 90% yield with 91% ee, and
the regioisomeric allyl acetate rac-5 also provided (S)-4aa in
80% yield with 87% ee (entries 6 and 7). These results suggest
that the reaction proceeds through the same reaction pathway
including the π-allylpalladium intermediate.
To clarify the deracemization step in this enantioselective
reaction of the racemic substrate, we examined the isomer-
ization reaction of rac- or (R)-3aa (96% ee) (γ-product) to α-
product 4aa under several catalyst conditions (Table 4). We
first confirmed that the isomerization had not occurred by the
combinations of [Pd(C3H5)Cl]2/AgPF6 or AgPF6/BINAP and
recovered (R)-3aa without decreasing the ee value (entries 1
and 2). However, we observed that the racemization of (R)-3aa
had occurred using the [Pd(C3H5)Cl]2/BINAP catalyst without
isomerization to the α-product 4aa (entry 3). These results
suggest that the BINAP-ligated palladium catalyst caused the
epimerization between (R)-3aa and (S)-3aa. We further treated
rac-3aa and (R)-3aa with [Pd(C3H5)Cl]2/(S)-BINAP/AgPF6
2j
3:97
2k
2l
2:>98
2:>98
67 (56)
a
1
The yields and ratios were determined by H NMR of the crude
b
materials using an internal standard. Isolated yield in parentheses.
c
Determined by HPLC.
Table 3. Palladium-Catayzed Enantioselective Allylic
Amination of Several Allylic Acetates with 2a
a,
b
c
entry
1, 5
1c
time (h)
3:4
yield (%) of (3 + 4) % ee of 4
1
2
3
4
5
6
7
42
24
36
36
12
12
24
8:92
10:90
11:89
5:95
98 (81)
88 (83)
96 (88)
85 (80)
31
88 (4ca)
89 (4da)
90 (4ea)
84 (4fa)
nd
1d
1e
1f
1g
>98:2
13:87
23:77
(Z)-1a
5
98 (90)
91 (4aa)
87 (4aa)
82 (80)
1
a
The yields and ratios were determined by H NMR of the crude
b
materials using an internal standard. Isolated yield in parentheses.
c
Determined by HPLC.
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Organic Letters
Letter
Table 4. Isomerization of 3aa to 4aa
Scheme 1. Possible Reaction Pathway of Enantioselective
Allylic Amination by DYKAT
a
b
entry 3aa
cat. (mol %)
yield (%)/ee of 3aa
98/95 ee (R)
4aa
1
2
3
R
R
R
[Pd(C3H5)Cl]2 (5)
AgPF6 (5)
0
0
0
0
(S)-BINAP (10)
AgPF6 (5)
98/96 ee (R)
93/17 ee (R)
cleavage,15 and the interconversion between (R)-3 and (S)-3
through the π-allyl complex I and II proceeded. (5) A highly
selective dynamic kinetic resolution took place during the
isomerization from 3 to 4 with Pd/chiral-BINAP/AgPF6.
Overall, the allylic amination of the racemic trifluoromethyl
group containing allylic ester 1 proceeded through the dynamic
kinetic asymmetric transformation (DYKAT) and provided the
enantiomerically enriched α-product (S)-4 in high yield with a
high enantiomeric excess.
[Pd(C3H5)Cl]2 (5)
(R)-BINAP (10)
AgPF6 (5)
4
5
R
90/96 ee (R)
10/9 ee (S)
rac
[Pd(C3H5)Cl]2 (5)
(S)-BINAP (10)
AgPF6 (5)
96/93 ee (S)
89/92 ee (S)
89/89 ee (R)
6
7
R
R
[Pd(C3H5)Cl]2 (5)
(S)-BINAP (10)
AgPF6 (5)
7/10 ee (S)
8/10 ee (R)
In conclusion, we demonstrated the enantioselective allylic
amination of the racemic trifluoromethyl group containing allyl
esters with amines using the [Pd(C3H5)Cl]2/(S)-BINAP/
AgPF6 catalyst. The reaction proceeds through the dynamic
kinetic asymmetric transformation (DYKAT) and the enantio-
merically enriched α-type allylic amines in a high yield with a
high ee value. Further investigation of the mechanistic details
and reaction with other nucleophiles will be the subject of a
future study.
[Pd(C3H5)Cl]2 (5)
(R)-BINAP (10)
AgPF6 (5)
a
The yields were determined by 1H NMR of the crude materials using
b
an internal standard. Determined by HPLC.
and confirmed that both reactions afforded the enantiomerically
enriched (S)-4aa with 93% ee and 92% ee, respectively (entries
5 and 6). On the other hand, another isomerization reaction,
which was conducted by changing the (S)-BINAP to (R)-
BINAP, provided (R)-4aa with 89% ee (entry 7). Furthermore,
we examined the reaction of (S)-1a (99% ee) and observed that
the enantiomerc excess of γ-product (R)-3aa was lost
immediately under the optimized catalyst conditions; the
value of enantiomeric excess of (R)-3aa was 83% ee and 31%
ee after 10 min12 and 45 min, respectively (eq 1).
ASSOCIATED CONTENT
* Supporting Information
■
S
Experimental procedures and spectral data for the products.
This material is available free of charge via the Internet at
http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Authors
■
*E-mail: kawatsur@chs.nihon-u.ac.jp.
*E-mail: titoh@chem.tottori-u.ac.jp.
Notes
The authors declare no competing financial interest.
Based on these results and our previous work, we propose
the possible reaction pathway for the formation of the
enantiomerically enriched α-product (S)-4 from the racemic
allyl acetate 1 by (S)-BINAP as follows (Scheme 1): (1) The
CF3-group substituted π-allylpalladium intermediate I and II13
were formed at the first step. (2) The π-allyl complex I
provided both enantiomericallyl enriched (R)-3 and (S)-4 with
a certain ratio by a net retention mechanism. (3) A rapid
interconversion between π-allylpalladium complex I and II also
occurred before the attack of nitrogen nucleophiles due to the
Pd(0)/BINAP catalyst at 60 °C.8g,9b,14 (4) The γ-product 3
also reformed π-allylpalladium complex with the C−N bond
ACKNOWLEDGMENTS
■
This work was supported by a Grant-in-Aid for Scientific
Research (C) from the Japan Society for the Promotion of
Science.
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