Catalytic enantioselective mannich-type reaction via a chiral silver enolate

Article abstract of DOI:10.1021/ol403075u

A catalytic asymmetric Mannich-type reaction of alkenyl trichloroacetates with aldimines was achieved using SEGPHOS·AgOTf as the chiral precatalyst and N,N-diisopropylethylamine as the base precatalyst in the presence of 2,2,2-trifluoroethanol. Optically

Full text of DOI:10.1021/ol403075u

Letter
pubs.acs.org/OrgLett
Catalytic Enantioselective Mannich-Type Reaction via a Chiral Silver
Enolate
Akira Yanagisawa,* Yuqin Lin, Ryoji Miyake, and Kazuhiro Yoshida
Department of Chemistry, Graduate School of Science, Chiba University, Chiba 263-8522, Japan
S
* Supporting Information
ABSTRACT: A catalytic asymmetric Mannich-type reaction
of alkenyl trichloroacetates with aldimines was achieved using
SEGPHOS·AgOTf as the chiral precatalyst and N,N-
diisopropylethylamine as the base precatalyst in the presence
of 2,2,2-trifluoroethanol. Optically active β-amino ketones with
up to >99% ee were syn-selectively obtained in moderate to
high yields via the in situ generated chiral silver enolates.
he asymmetric Mannich-type reaction of enolates with
imines is a convenient way to produce nonracemic β-
chiral cocatalyst because the BINAP·AgOTf complex has
turned out to be an efficient chiral catalyst for the asymmetric
aldol reaction of aldehydes or ketones with alkenyl trichlor-
oacetates in the presence of a catalytic amount of dibutyltin
dimethoxide.6 When a 2:1 mixture of 1-trichloroacetoxycyclo-
hexene (1a)7 and 4-(trifluoromethoxy)aniline-derived benzal-
dimine 2a was treated with (R)-SEGPHOS (10 mol %), AgOTf
(20 mol %), and Bu2Sn(OMe)2 (10 mol %) in the presence of
MeOH (5 equiv) and MS 3 Å in THF at −20 °C for 48 h, 2-
((4-(trifluoromethoxy)phenylamino)(phenyl)methyl)-
cyclohexanone (3aa) was obtained in 27% yield with a syn/anti
ratio of 87/13. The syn isomer had 84% ee (Table 1, entry 1).
Then, we examined the effectiveness of a tertiary amine as an
additive in an effort to avoid the aforementioned product
inhibition and increase product yield. As a result, we found that
the isolated yield of 3aa improved to 55% in the presence of 20
mol % of N,N-diisopropylethylamine (entry 2). Triethylamine
was also effective in raising the catalytic activity of the catalysts
(entry 4). What was even more significant is that these amines
afforded the syn product in higher diastereoselectivity (syn/anti
= 97/3−99/1) with remarkable enantioselectivity (98% ee). In
marked contrast, the use of pyridine resulted in a totally
unacceptable chemical yield, a very low syn/anti ratio, and low
enantioselectivity (entry 3). In order to obtain better results, we
further attempted to optimize the reaction conditions. An
increase in the amount of N,N-diisopropylethylamine to 40 mol
% realized perfect syn/anti selectivity (>99/1) and enantiomeric
ratio (>99% ee) as well as a practically meaningful isolated yield
that exceeded 80% (entry 5). Lowering the reaction temper-
ature to −30 °C improved the chemical yield to 90% (entry 6).
To our great surprise, however, it proved that Bu2Sn(OMe)2
was not necessary for the present asymmetric Mannich-type
reaction. In fact, the reaction in the absence of the tin catalyst at
−30 °C for 25 h provided the target Mannich adduct in 63%
yield without reducing its syn/anti ratio and ee (entry 7).
T
amino carbonyl compounds, which are versatile synthetic
intermediates of natural products or other useful organic
molecules.1 To acquire such chiral Mannich products in
satisfactory isolated yields and high optical purity, a variety of
methods employing asymmetric catalysts have been devel-
oped.2,3 Among the methods reported so far, the chiral Lewis
acid catalyzed Mannich-type reaction of silyl enolates is one of
the most well accepted; however, it has an intrinsic
disadvantage in that an imine is readily hydrolyzed into an
aldehyde and an amine when a strong chiral Lewis acid catalyst
is used. In addition, the catalyst is deactivated by the Mannich
product that is a better ligand for the Lewis acid (i.e., product
inhibition).1a,4 We report here a novel example of the
enantioselective Mannich-type reaction of alkenyl trichloroace-
tates with aldimines using the SEGPHOS·AgOTf complex as
the asymmetric precatalyst and N,N-diisopropylethylamine as
the base precatalyst in the presence of 2,2,2-trifluoroethanol
(Scheme 1).
Scheme 1. Enantioselective Mannich-Type Reaction of
Alkenyl Trichloroacetates with Aldimines Catalyzed by
SEGPHOS·AgOTf Complex
We have previously reported that dibutyltin dimethoxide
behaves as a catalyst in the Mannich-type reaction of alkenyl
trichloroacetates with imines.5 The reaction occurs through a
tin enolate, and the tin methoxide is regenerated with the
assistance of MeOH. We envisioned that if a suitable chiral
cocatalyst could selectively interact with an imine without
interrupting the tin catalysis, the asymmetric version of the
Mannich-type reaction would be possible. Thus, we tested the
possibility of using a chiral phosphine·silver(I) complex as the
Received: October 25, 2013
Published: December 4, 2013
© 2013 American Chemical Society
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dx.doi.org/10.1021/ol403075u | Org. Lett. 2014, 16, 86−89

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Letter
Table 1. Enantioselective Mannich-Type Reaction of Alkenyl
Table 2. Enantioselective Mannich-Type Reaction of Alkenyl
Trichloroacetate 1a with Imine 2a Catalyzed by Chiral
Phosphine·AgOTf without Bu2Sn(OMe)2
Trichloroacetate 1a with Imine 2a Catalyzed by SEGPHOS·
a
a
AgOTf and Bu2Sn(OMe)2
b
amine
(mol %)
temp
(°C)
time
(h)
yield
b
ee (%)
d
chiral
phosphine
time
(h)
yield
(%)
ee (%)
d
c
c
entry
(%)
syn/anti
(syn)
entry
additive
syn/anti
(syn)
e
1
2
−20
−20
48
43
27
55
87/13
97/3
84
98
1
(R)-SEGPHOS MS 3 Å
(R)-SEGPHOS MS 3 Å
(R)-SEGPHOS MS 4 Å
(R)-SEGPHOS MS 5 Å
(R)-SEGPHOS Drierite
(R)-SEGPHOS MgSO4
(R)-SEGPHOS MS 4 Å
25
8
63
63
81
57
83
65
19
47
<1
99/1
99/1
99
99
(i-Pr)2NEt
2
3
4
5
6
(20)
16
16
17
16
16
16
17
>99/1
>99/1
>99
>99
86
3
4
5
pyridine (20)
Et3N (20)
−20
−20
−20
46
23
20
18
41
82
69/31
99/1
40
98
75/25
72/28
83/17
34/66
(i-Pr)2NEt
(40)
>99/1
>99
84
f
7
81
6
(i-Pr)2NEt
(40)
−30
−30
20
25
90
63
>99/1
99/1
>99
99
g
8
9
(R)-BINAP
MS 4 Å
MS 4 Å
<1
e
(RR)-t-Bu-
QuinoxP*
7
(i-Pr)2NEt
(40)
10
(R)-Tol-
MS 4 Å
20
7
86/14
93
a
Unless otherwise specified, the reaction was carried out using (R)-
SEGPHOS
SEGPHOS (10 mol %), silver triflate (20 mol %), dibutyltin
dimethoxide (10 mol %), alkenyl trichloroacetate 1a (2 equiv),
imine 2a (1 equiv), amine (20−40 mol %), methanol (5 equiv), and
a
Unless otherwise specified, the reaction was carried out using chiral
phosphine (10 mol %), silver triflate (20 mol %), alkenyl
trichloroacetate 1a (2 equiv), imine 2a (1 equiv), diisopropylethyl-
amine (40 mol %), 2,2,2-trifluoroethanol (3 equiv), and additive in
THF at −30 °C. Isolated yield of 3aa. Determined by H NMR
analysis. The value corresponds to the syn isomer of 3aa. Determined
by HPLC analysis. MeOH (5 equiv) was used in place of CF3CH2OH
(3 equiv). AgOTf (10 mol %) was used. The value corresponds to
b
c
1
MS 3 Å in THF. Isolated yield of 3aa. Determined by H NMR
d
analysis. The value corresponds to the syn isomer of 3aa. Determined
by HPLC analysis. The reaction was performed without Bu2Sn-
(OMe)2.
b
c
1
e
d
e
f
g
the anti isomer of 3aa. Determined by HPLC analysis.
We then further studied the reaction conditions without
Bu2Sn(OMe)2 (Table 2). First, we checked the utility of 2,2,2-
trifluoroethanol instead of methanol because the more acidic
alcohol was anticipated to promote the reaction and as a result,
we found that the former was better than the latter in terms of
reaction time (entry 2 vs entry 1). To confirm the
indispensability of MS 3 Å, some other drying agents were
examined as an additive and consequently, MS 4 Å was found
to be the most effective in producing a high yield while
maintaining the stereoselectivity (entries 2−6). Drierite was
also a promising additive from the viewpoint of chemical yield
but it disturbed the asymmetric induction by the SEGPHOS·
AgOTf catalyst to some extent (entry 5). A significant
deceleration of the reaction was observed when a catalyst
prepared from a 1:1 mixture of SEGPHOS and AgOTf was
used (entry 7). Next, we investigated the asymmetric induction
ability by chiral phosphines other than SEGPHOS and
ascertained that SEGPHOS was the chiral ligand of choice
(entry 3 vs entries 8−10). The use of BINAP and t-Bu-
QuinoxP* led to utterly unsatisfactory results (entries 8 and 9).
With the optimized reaction conditions (entry 3 in Table 2)
in hand, we carried out the chiral silver-catalyzed asymmetric
Mannich-type reaction using diverse combinations of alkenyl
trichloroacetates and imines (Table 3). The introduction of an
electron-withdrawing group at the para or meta position (R4
substituent) of N-phenylbenzaldimine derivative (R3 = Ph)
promoted the reaction, giving the syn-adduct with a high
enantioselectivity of >99−98% ee without significant loss of
reactivity (entries 2−6). Subsequently, we executed a screening
for the R3 substituent of imines and found that not only
electron-deficient aromatic imines but also an electron-rich
aromatic imine showed remarkable stereoselectivities although
the isolated yields of their products were moderate (entries 7−
11). Benzaldimines possessing a N-protective group other than
aromatic groups did not give satisfactory results.8 The
aforementioned results further encouraged us to use the
alkenyl trichloroacetates of other cyclic ketones in the
asymmetric Mannich-type reaction. Both cyclopentanone and
cycloheptanone derivatives 1b and 1c furnished the desired
products with high optical purities and predominant syn
selectivity similar to those of cyclohexanone-derived alkenyl
trichloroacetate 1a (entries 12 and 13 vs entry 5). The reaction
of acyclic alkenyl trichloroacetate 1d (E/Z = 1/4) with imine
2a also provided a Mannich product in a somewhat low yield
with syn selectivity, but the optical purity (96% ee) of the syn
isomer was high (entry 14). In addition, we attempted a
reaction between propiophenone-derived alkenyl trichloroace-
tate (E/Z = 1/4) with imine 2a under reaction conditions
similar to those in entry 14; however, the desired product was
not obtained at all.
A plausible catalytic mechanism is indicated in Figure 1.
Initially, (R)-SEGPHOS·AgOTf reacts with an alcohol in the
presence of N,N-diisopropylethylamine to yield the corre-
sponding (R)-SEGPHOS·AgOR, which is the true catalyst in
the present asymmetric Mannich-type reaction. Next, the thus
formed chiral silver alkoxide is allowed to attack alkenyl
trichloroacetate 1 to generate chiral silver enolate 2. The
subsequent addition reaction of chiral silver enolate 2 with
aldimine 3 affords chiral silver amide of Mannich adduct 4.
Finally, protonation of chiral silver amide 4 with ROH results in
the formation of optically active β-amino ketone 5 with
regeneration of the chiral silver alkoxide. The rapid alcoholysis
of silver amide 4 promotes the catalytic cycle efficiently.
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Letter
Table 3. Enantioselective Mannich-Type Reaction of Alkenyl
Trichloroacetates 1 with Imines 2 Catalyzed by (R)-
a
SEGPHOS·AgOTf
Figure 2. Proposed cyclic transition-state structures.
model B having a boat conformation connects the E-enolate to
the anti product.9 Between these two transition-state models, A
is evaluated to be more favorable than B because the latter
possesses a thermodynamically unfavorable boat form.
A hypothesis for the enantiofacial discrimination between an
imine and a silver enolate in the present asymmetric Mannich-
type reaction catalyzed by (R)-SEGPHOS·AgOTf is exhibited
in Figure 3. An aldimine approaches the α-carbon atom of a
a
Unless otherwise specified, the reaction was carried out using (R)-
SEGPHOS (10 mol %), silver triflate (20 mol %), alkenyl
trichloroacetate 1 (2 equiv), imine 2 (1 equiv), diisopropylethylamine
(40 mol %), 2,2,2-trifluoroethanol (3 equiv), and MS 4 Å in THF at
b
c
d
−30 °C. Isolated yield of 3. Determined by 1H NMR analysis. The
value corresponds to the syn isomer of 3. Determined by HPLC
e
f
analysis. The absolute configuration is shown in parentheses. MS 3 Å
Figure 3. Hypothesis for enantioface discrimination between an imine
and a silver enolate.
g
was used. The reaction was performed at −20 °C.
chiral silver enolate while avoiding steric repulsion from a
phenyl group of the chiral phosphine ligand. Thus, carbon−
carbon bond formation takes place selectively between the Re
face of the silver enolate and the Si face of the aldimine to
provide the (2S,1′S)-β-amino ketone.
In conclusion, we have developed a novel catalytic
asymmetric Mannich-type reaction. The use of in situ generated
chiral silver alkoxide as the chiral catalyst allows for the
synthesis of various nonracemic syn-α-alkyl-β-amino ketones
with enantioselectivities of up to >99% ee. The catalytic cycle
via a chiral silver enolate affords an alternative asymmetric
Mannich process. To the best of our knowledge, this is the first
example of the generation of a chiral silver alkoxide and its
application as a chiral catalyst to the asymmetric reaction via a
chiral silver enolate.10,11 Further studies on the extension of the
present silver catalytic system to other asymmetric reactions are
underway.
Figure 1. Plausible catalytic cycle for the asymmetric Mannich-type
reaction catalyzed by chiral silver alkoxide.
ASSOCIATED CONTENT
* Supporting Information
Experimental procedures and spectral data for products in
Tables 1−3. This material is available free of charge via the
Internet at http://pubs.acs.org.
■
From the aforesaid catalytic mechanism and the outcome
that cyclic alkenyl trichloroacetates reacted with aldimines syn-
selectively, cyclic transition-state structures A and B described
in Figure 2 can be presumed as the plausible models for the
Mannich-type reaction of the cyclic substrates. In these
assemblies, the imine coordinates to a silver atom of the
enolate to furnish a six-membered cyclic structure. Accordingly,
from the cyclic enolate (E-enolate), the syn-Mannich adduct is
formed through chair-type transition-state structure A, while
S
AUTHOR INFORMATION
Corresponding Author
■
*E-mail: ayanagi@faculty.chiba-u.jp.
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Organic Letters
Letter
(5) Yanagisawa, A.; Saito, H.; Harada, M.; Arai, T. Adv. Synth. Catal.
2005, 347, 1517.
(6) (a) Yanagisawa, A.; Ichikawa, T.; Arai, T. J. Organomet. Chem.
2007, 692, 550. (b) Yanagisawa, A.; Terajima, Y.; Sugita, K.; Yoshida,
K. Adv. Synth. Catal. 2009, 351, 1757.
(7) Libman, J.; Sprecher, M.; Mazur, Y. Tetrahedron 1969, 25, 1679.
(8) The reaction of 1a with diversely N-protected benzaldimines was
performed under the optimized reaction conditions. The results are as
follows (imine, additive, time, yield, syn/anti, enantioselectivity of syn
isomer): N-tosylbenzaldimine, MS 3A, 8 h, 75% yield, 92/8, 1% ee; N-
benzylbenzaldimine, MS 4A, 16 h, <1% yield.
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(10) Two examples of the synthesis of achiral silver alkoxides have
been reported: (a) Edworthy, I. S.; Rodden, M.; Mungur, S. A.; Davis,
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by MEXT/JSPS KAKENHI Grant
No. 25410107, The Naito Foundation, and The COE Start-up
Program in Chiba University led by Professor Takayoshi Arai.
We gratefully acknowledge the generous gift of (R)-SEGPHOS
and (R)-Tol-SEGPHOS from Takasago International Corpo-
ration and (R,R)-t-Bu-QuinoxP* from Nippon Chemical
Industrial Co., Ltd.
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