Full text of DOI:10.1021/acscatal.8b01547

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Article
Copper-Catalyzed Stereo- and Enantio-selective 1,4-Proto-silylation of
#, #-Unsaturated Ketimines to Synthesize Functionalized Allylsilanes
Bing-Chao Da, QIU-JU LIANG, Yun-Cheng Luo, Tanveer Ahmad, Yun-He Xu, and Teck-Peng Loh
ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.8b01547 • Publication Date (Web): 18 May 2018
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ACS Catalysis
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Copper-Catalyzed Stereo- and Enantio-selective 1, 4-Proto-silylation of α,
β-Unsaturated Ketimines to Synthesize Functionalized Allylsilanes
Bing-Chao Da,a Qiu-Ju Liang,a Yun-Cheng Luo,a Tanveer Ahmad,a Yun-He Xu*a and Teck-Peng
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Loha, b aDepartment of Chemistry, University of Science and Technology of China, Hefei, Anhui, P. R. China
230026
bDivision of Chemistry and Biological Chemistry, School of Physical and Mathematical Sciences, Nanyang
Technological University, Singapore 637371
Supporting Information Placeholder
ABSTRACT: Copper-catalyzed conjugate proto-silylation reaction of α, β-unsaturated sulfonyl ketimines has been developed.
The corresponding E- and Z-stereoselective functionalized allylsilane products were obtained in good yields respectively via
tuning the ligands used in the reactions. Furthermore, the highly enantioselective (E)-β-tosylamine-substituted allylsilanes were
also achieved in the presence of chiral Pybox ligand. And the corresponding products could be easily transformed into other
useful synthons.
KEYWORDS: conjugate proto-silylation, allylsilanes, copper-catalyst, regioselectivity, stereoselectivity
imines have been documented. Firstly, in 2009 Palacios et al.
reported a non-enantioselective conjugate addition of amines
INTRODUCTION
Tremendous efforts have been devoted to discovering new
methodologies for the synthesis of the allylic silanes because
of their broad applications in organic synthesis and materials
to active α, β-unsaturated imines derived from α-amino
phosphonates.13a After that, Leung and co-workers developed
a
Pd-catalyzed asymmetric addition of Ph2PH to α,
β-unsaturated sulfonyl ketimines and got the solo
(Z)-configuration Recently,
enaminophosphines.13b
science.1 Among them, transition-metals such as Ni,2 Pd,3 Pt4
catalyzed silaborations of 1,3-dienes or allenes, and
Cu-catalyzed allylic substitution5 with use of Suginome
silaboronate reagent6 have emerged as very powerful tools
for the preparation of these compounds. Nevertheless, to
access the multi-substituted functionalized allylsilanes, the
development of practical and efficient methods still remains
highly desirable. Therefore, we design a copper-catalyzed
conjugate proto-silylation of α, β-unsaturated ketimines to
synthesize the stereo- and enantio-selective functionalized
allylsilanes via trapping the double bond generated in situ
during silyl-addition process (Scheme 1). It is worth noting
that the transition-metal-catalyzed silyl-additions to
conjugate carbonyl systems have attracted much attention.
a
copper-catalyzed conjugate borylation of ketimines was also
addressed to prepare β-boronate-substituted imines.13c
Unfortunately, so far there no study disclosed the stereo-
and/or enantio-selective silyl-addition to α, β-unsaturated
ketimines. In connection with our continuous interest in C-Si
bond formation,14 herein we would like to communicate the
results of copper-catalyzed conjugate proto-silylation of α,
β-unsaturated ketimines to access highly stereo- and
enantio-selective β-tosylamine-substituted allylsilanes.
RESULTS AND DISCUSSION
Initially, the reaction of copper-catalyzed conjugate
proto-silylation of α, β-unsaturated ketimine was examined
with use of ketimine 1a and Suginome silaboronate 2, and the
results were shown in Table 1. We first conducted the
reaction with various Cu(I) and Cu(II) salts under basic
condition (Table 1, entries 1-4). It was found that both of
them could provide the desired product. Especially, in
presence of 10 mol % Cu(OTf)2 and 20 mol % Na3PO4 as the
additive, the major product with a
For instance, in 1988 Hayashi initially disclosed
1,4-disilylation reaction with palladium catalyst.3a,
a
7a
Alternatively, Oestreich,7b Hoveyda,7c Riant,7d Córdova,7e
Santos,7f and Procter7g et al. elegantly developed different
strategies using Rh(I)-binap, Cu(I)-NHC, Cu(I)-dppf,
Cu(I)-amine or Cu(II)-4-picoline etc8 system for the
conjugate silyl-addition, respectively. Unfortunately, these
methods are difficult to access the allylsilane product.
On the other hand, a few examples of conjugate addition
to α, β-unsaturated imines to access nitrogen-containing
moieties have also been studied. However, the works mainly
focused on the construction of C-C bond by using organozinc
reagents,9 organocuprates,10 organoboronic acids,11 and other
carbon-based nucleophiles in the presence of Rh, La,12 or Cu
catalyst. So far, only scarce examples of heteroatom-carbon
bond formation via conjugate addition to α, β-unsaturated
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of (E)-isomer greatly increased in the presence of organic
1
2
3
4
5
6
7
8
base N, N'-diisopropylethylamine (DIPEA). Thus, we
hypothesized that increasing the steric hindrance around the
copper center could regulate the product’s stereoselectivity.
Therefore,
the
2,2 -bipyridine
with
strong
coordinated-ability was introduced to this catalytic system
(Table 1, entry 13). It is worth noting that the desired product
can be obtained in a moderate yield but with an excellent
(Z)-stereoselectivity. After further carefully screening the
copper
salts,
pleasingly,
the
(E)-isomer
of
9
β-tosylamine-substituted allylsilane could be isolated in high
yield and with excellent stereoselectivity by using Cu(CHB)2
(copper bis (4-cyclohexylbutyrate)) as a catalyst. Notably, the
copper catalyst and base used in reaction were both crucial
to afford the desired product (Table 1, entries 6 and 7).
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Scheme
Enantio-selective
β-Unsaturated Ketimine.
1.
Copper-Catalyzed
Stereo-
and
α,
1, 4-Proto-silylation
of
With the optimal conditions determined, the substrate
scope of α, β-unsaturated ketimine was explored for (Z)- and
(E)-stereoselective synthesis of β-tosylamine-substituted
allylsilanes, respectively. The results are presented in Tables
2 and 3. In both cases, different N-arylsulfonyl ketimines
Table 1. Optimization of Reaction Conditions of
Non-enantioselective Product.a
reacted
with
dimethylphenylsilylpinacolborane
(Me2PhSi-Bpin) 2. The results showed that the tosylsulfonyl
group more favored the formation of specifically
stereoselective product in a good yield. Next, the substituent
effect of R1 and R2 was also investigated. As indicated in
Tables 2 and 3, varying the position of R1 and R2 substituents
on the phenyl ring, regardless of the electron-donating or
electron-withdrawing group, the corresponding products
could be obtained in good to high yields and with a high to
excellent level of stereoselective control. Moreover, both
these processes tolerated a wide range of functionalized
groups, such as ester, halogens (F, Cl, Br) and heterocycles
(thienyl, furyl). The relative stereo configuration of
(Z)-isomer product 3a was confirmed by X-ray single crystal
diffraction analysis (CCDC No: 1837165).
Cu
(mol %)
Base
(equiv)
Solvent
(V/V=2/1)
T
(℃)
E/Z
(%)b
Yield
(%)c
Entry
1
CuCl
CuBr
Na3PO4 (0.2)
Na3PO4 (0.2)
Na3PO4 (0.2)
dioxane-tBuOH
dioxane-tBuOH
dioxane-tBuOH
50
50
50
26:74
37:63
20:80
46
23
63
2
3
Cu(OAc)2
Na3PO4
(0.2)
dioxane-tBuO
4
Cu(OTf)2
50
7:93
91(89)
H
5
6
Cu(OTf)2
Cu(OTf)2
-
K3PO4 (0.2)
-
dioxane-tBuOH
dioxane-tBuOH
dioxane-tBuOH
dioxane-tBuOH
dioxane-tBuOH
dioxane-tBuOH
50
50
50
25
25
25
14:86
-
61
trace
trace
66
7
Na3PO4 (0.2)
Na3PO4 (0.2)
DIPEA (0.2)
DIPEA (3.0)
-
8
Cu(OTf)2
Cu(OTf)2
Cu(OTf)2
15:85
72:28
75:25
9
40
10
60
m-Xylene-tBuO
11
Cu(OTf)2
Cu(OTf)2
Cu(OTf)2
CuTc
DIPEA (3.0)
DIPEA (3.0)
DIPEA (3.0)
DIPEA (3.0)
DIPEA (3.0)
DIPEA (3.0)
DIPEA (3.0)
25
-5
-5
-5
-5
-5
-5
82:18
88:12
99:1
70
20
Table
2.
Non-enantioselective
Conjugate
H
m-Xylene-tBuO
12d
Proto-silylation of α, β-Unsaturated Ketimines for
the Synthesis of (Z)-isomer Allylsilane.a, b
H
m-Xylene-tBuO
13d, e
46
H
m-Xylene-tBuO
14 d, e, f
15d, e
16d, e, g
92:8
98:2
98:2
99:1
80
H
m-Xylene-tBuO
Cu(OAc)2
Cu(CHB)2
Cu(CHB)2
86(83)
92(90)
91(90)
H
m-Xylene-tBuO
H
m-Xylene-tBuO
H
17d, e, h
aReaction conditions: 1a (0.2 mmol), 2 (0.4 mmol), copper
catalyst, and base in the solvent (1.50 mL in total volume)
was heated for the indicated time. bThe ratio between E and Z
isomers was determined by crude H-NMR analysis. cThe
1
yield given is for single E or Z isomer which was determined
by 1H-NMR (2, 4, 5-trichloropyrimidine as an internal
standard). And the isolated yield is in the parentheses. dThe
reaction time is 36 h. e10 mol % 2, 2'-bipyridine was added as
the ligand. fCuTc
= ((thiophene-2-carbonyl)oxy)copper.
gCu(CHB)2 = copper bis(4-cyclohexylbutyrate). h5 mol %
Cu(CHB)2 was used in the reaction.
(Z)-configuration (Z/E = 97:3) was obtained in 89% isolated
yield (entry 4). Nonetheless, the change of base or reaction
temperature noticeably decreased the reaction efficiency
(Table 1, entries 5 and 8). It was also observed that the ratio
2
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PG
R1
Cu(OTf)2 (10 mol %)
Na3PO4 (20 mol %)
1,4-dioxane-tBuOH (2:1)
50 °C, 3 h
Table
3.
Non-enantioselective
Conjugate
NH SiMe2Ph
R2
NPG
Me2PhSi-Bpin
[2.0 equiv]
1
2
3
4
5
6
Proto-silylation of α, β-Unsaturated Ketimines for
the Synthesis of (E)-Isomer Allylsilane.a
+
R1
R2
3
Z-isomer
1
2
Cu(CHB)2 (5.0 mol %)
R1 SiMe2Ph
PG
NPG
2,2'-bipyridine (10 mol %)
DIPEA (3.0 equiv)
m-xylene-tBuOH (2:1)
NH SiMe2Ph
+
Me2PhSi-Bpin
[2.0 equiv]
2
R1
R2
PGHN
R2
4
1
-5 °C, 36 h
E-isomer
7
R
Z-3a, PG = Ts, Z/E = 93:7,
8
9
89% yield
PG = PhSO2, Z/E = 98:2,
80% yield
PG = 4-ClPhSO2,
Z/E = 91:9, 72% yield
SiMe2Ph
Ph
F
SiMe2Ph
Ph
Z-3b,
Z-3c,
SiMe2Ph
PGHN
TsHN
3a
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TsHN
E-4f
Ph
E-4m, E/Z > 99:1,
, R = Me,
E/Z > 99:1, 71% yield
E-4g, R = F,
E/Z = 97:3, 78% yield
E-4h, R = Cl,
E/Z = 94:6, 60% yield
, R = Br,
E/Z = 97:3, 74% yield
E-4k, R = OMe,
94% yield
E-4a, PG = Ts,
E/Z = 99:1, 90% yield
E-4b, PG = PhSO2,
E/Z > 99:1, 70% yield
, PG = 4-ClPhSO2,
E/Z = 98:2, 85% yield
E-4d, PG = 2-NaphthylSO2,
Z-3d, PG = 2- NaphthylSO2,
Z/E = 90:10, 82% yield
Ts
Ts
Ts
S
NH SiMe2Ph
NH SiMe2Ph
Ph
NH SiMe2Ph
Ph
SiMe2Ph
Ph
E-4c
Ph
TsHN
E-4i
R
E/Z = 98:2, 87% yield
E-4n, E/Z > 99:1,
84% yield
Z-3f, R = Me, Z/E = 93:7,
89% yield
Z-3g, R = F, Z/E = 91:9,
56% yield
Me
E/Z = 94:6, 69% yield
Z-3l,
Z/E = 94:6,
74% yield
Z-3e, Z/E = 92:8,
85% yield
Me
SiMe2Ph
Z-3h, R = Cl, Z/E = 95:5,
SiMe2Ph
Ph
Ts
Ts
NH SiMe2Ph
Ph
NH SiMe2Ph
Ph
62% yield
, R = Br, Z/E = 91:9,
72% yield
, R = COOMe, Z/E = 93:7,
50% yield
TsHN
Ph
TsHN
Z-3i
Z-3j
S
E-4e, E/Z = 98:2, 67% yield
E-4l, E/Z > 99:1, 71% yield
F
Ph SiMe2Ph
OMe
Z-3m
Ph SiMe2Ph
, Z/E = 93:7,
Ph SiMe2Ph
Z-3n, Z/E = 93:7,
Z-3k, R = OMe, Z/E = 85:15,
89% yield
51% yield
O
TsHN
TsHN
60% yield
TsHN
Ts
Ph
R
Ts
NH SiMe2Ph
Ts
NH SiMe2Ph
E-4o
NH SiMe2Ph
, R = Me, E/Z > 99:1,
87% yield
E-4q, R = OMe, E/Z = 97:3,
88% yield
E-4r, R = Cl, E/Z > 99:1,
83% yield
E-4s, R = Br, E/Z = 98:2,
73% yield
E-4t, R = Ph, E/Z = 98:2,
OMe
OMe
E-4w, E/Z = 97:3,
4u, E/Z > 99:1, 88% yield
S
Ph
96% yield
Ph
Ph SiMe2Ph
Ph SiMe2Ph
R
OMe
S
Z-3o, R = Me, Z/E = 90:10,
65% yield
Z-3p, R = OBn, Z/E = 91:9,
68% yield
Z-3q, R = OMe, Z/E = 91:9,
45% yield
Z-3r, R = Cl, Z/E = 94:6,
88% yield
Z-3s, R = Br, Z/E = 91:9,
61% yield
Z-3t, R = Ph, Z/E = 94:6,
86% yield
Z-3u, Z/E = 92:8,
TsHN
TsHN
Z-3v, Z/E = 95:5,
80% yield
63% yield
4v, E/Z = 99:1, 92% yield
E
-4x, E/Z > 99:1,
73% yield
75% yield
Ts
Ts
Ph
aReaction conditions: The mixture of 1 (0.2 mmol), 2 (0.4
mmol), Cu(CHB)2 (0.01 mmol), 2, 2'-bipyridine (0.02 mmol),
NH SiMe2Ph
O
NH SiMe2Ph
Ph
t
and DIPEA (0.6 mmol) in m-xylene/ BuOH (2:1, 1.5 mL) was
Z-3w, Z/E = 92:8,
stirred for 36 h at -5 °C under argon atmosphere. bThe
Z-3x, Z/E = 93:7,
71% yield
61% yield
isolated yield is for E- isomer only.
with an aryl substituent. However, bulkier alkyl-substituted
Pybox ligands especially the one with the tert-butyl
substituent will significantly decrease the product’s
enantioselectivity. This was possibly due to the weak
interaction between the ligand and the copper atom center
caused by the large steric hindrance. Thus, the less bulky
iso-butyl-substituted Pybox was prepared and examined for
this reaction. Pleasingly, the desired product was isolated in
90% yield and with 91.5:8.5 enantiomeric ratio. Furthermore,
we recognized that if connecting a phenyl substituent on the
pyridyl ring20 to result a π-π interaction between the ligand
and dimethylphenylsilyl group should increase the product’s
enantioselectivity. Thus, we synthesized the chiral (S,
S)-4-Ph-Pybox-sBu as a ligand for this reaction and observed
a slightly increased enantioselectivity of the product. It is
worth noting that our hypothesis has also been proved by the
poor results by changing the (S, S)-4-Ph-Pybox-sBu ligand
aReaction conditions: the mixture of 1 (0.2 mmol), 2 (0.4
mmol), Cu(OTf)2 (0.02 mmol), Na3PO4 (0.04 mmol) in
1,4-dioxane/tBuOH (2:1, 1.5 mL) solution was stirred for 3 h at
50˚C under argon atmosphere. The isolated yield is for Z-
isomer only.
b
Base on the investigation
for the synthesis
non-enantioselective E-isomer allylsilane products, we
embarked on developing the methodology of asymmetric
conjugate proto-silylation to α, β-unsaturated ketimines.
According to the previous reports on Cu(II)-catalyzed
asymmetric addition reactions, we first tested various chiral
ligands such as chiral phosphoric acid,15 phosphine,16 NHC,17
phosphoramide18 and oxazoline19 etc. for current asymmetric
silyl addition reaction. Unfortunately, only very poor
enantioselectivity of the desired product was observed.
When the commercially available Pybox-iPr (L1) was used to
this catalytic system, the compound 4a was obtained in a
high yield and with a moderate enantiomeric ratio (89:11).
Therefore, to obtain a high-level enantioselectivity, various
substituted Pybox ligands were subjected to this addition
reaction (Table 4). Consequently, after a careful screening,
we realized that the Pybox ligand with an alkyl-substituent
would afford a better result than the one bearing
into
(S,
S)-4-tert-butyl-Ph-Pybox-sBu
or
(S,
S)-4-Mes-Pybox-sBu, which will diminish the π-π interaction
between the phenyl rings due to their increased space
resistance. Finally, we found that the desired allylsilane 4a
could be obtained in 90% yield and with 95:5 enantiomeric
ratio in presence of
5
mol
%
copper bis
(4-cyclohexylbutyrate), 20 mol % (S, S)-4-Ph-Pybox-sBu, and
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2,6-di-tert-butylpyridine (3.0 equiv) in the mixed solvents of
m-xylene and tert-butanol (V: V = 2:1) at -15 ℃ (entry 10).
were observed for the electron-rich substrates, which could
be explained by the lower electrophilicity and stronger
interaction between the ketimine and the
[Me2PhSi-Cu(II)]/Pybox species generated in situ. The
compound 4x was determined to be (R)-configuration by
single-crystal X-ray diffraction analysis (CCDC No: 1837166).
1
2
3
4
Table 4. Optimization of Reaction Conditions for the
Synthesis of Enantioenriched (E)-Isomer Allylsilane.a
5
Taking into account the previous reports on activation of
Si-B bond and the formation of [Me2PhSi-Cu(II)] reactive
intermediate,7e we proposed a stereochemical model for
generating the enantioenriched β-tosylamine-substituted
allylsilane (Figure 1). Comparing, in transition state A with B,
the minimum steric interaction between the isobutyl group
of the ligand and the tosyl group of the substrate as well as
the π-π stacking interaction of phenyl rings, both favor the
nucleophilic [Me2PhSi-Cu(II)] intermediate to attack the
double bond from the Re-face and afford the (R)-isomer
preferentially.
6
7
8
9
Ligand
(mol
%)
Yield
Entry
Base
E / Z (%)b
er (%)d
(%)c
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
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39
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41
42
43
44
45
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60
1
L1 (10)
L2 (10)
L3 (10)
L4 (10)
L5 (10)
L6 (10)
L7 (10)
DIPEA
DIPEA
DIPEA
DIPEA
DIPEA
DIPEA
DIPEA
97:3
80:20
94:6
94
63
92
98
63
90
88
89:11
56.5:43.5
80:20
2
3
4
5
6
7
> 99:1
80:20
98:2
60:40
55.5:44.5
91.5:8.5
93.5:6.5
Table 5. Asymmetric Conjugate Proto-silylation of α,
β-Unsaturated Ketimines.a, b
Cu(CHB)2 (5.0 mol %)
(S,S)-4-Ph-Pybox-sBu (20 mol %)
R2 SiMe2Ph
95:5
NPG
2,6-di-tert-butylpyridine (3.0 equiv)
Me2PhSi-Bpin
R1
[2.0 equiv]
+
PGHN
R1
m-xylene-tBuOH (2:1)
R2
2,6-di-tert-butylpyridi
8
9
L7 (10)
L7 (15)
97:3
95:5
95:5
98:2
96:4
96:4
93(90)
90
94:6
94:6
-15 °C, 36 h
ne
4
1
2
chiral E isomer
R
2,6-di-tert-butylpyridi
ne
SiMe2Ph
Ph
SiMe2Ph
F
SiMe2Ph
L7
(20)
2,6-di-tert-butylpyri
10
11e
12
13
91 (90)
85
95:5
dine
PGHN
TsHN
Ph
TsHN
Ph
4a, PG = Ts, E/Z = 95:5,
90% yield , 95:5 er
4f, R = Me, E/Z = 97:3,
81% yield, 96:4 er
4m, E/Z = 98:2,
2,6-di-tert-butylpyridi
93% yield, 75:25 er
L7 (20)
93.5:6.5
90:10
93:7
ne
4b, PG = PhSO2, E/Z = 98:2,
4g, R = F, E/Z = 94:6,
S
87% yield , 92:8 er
80% yield, 94:6 er
SiMe2Ph
4c, PG = 4-ClPhSO2, E/Z > 99:1, 4h, R = Cl, E/Z = 94:6,
L8
(20)
2,6-di-tert-butylpyridi
91
74% yield, 85:15 er
4d, PG = 2-NaphthylSO2,
E/Z > 99:1, 61% yield, 92:8 er
80% yield, 90:10 er
4i, R = Br, E/Z = 94:6,
80% yield, 91:9 er
4k, R = OMe, E/Z = 83:17,
54% yield, 95:5 er
ne
TsHN
Ph
4n, E/Z = 97:3,
71% yield, 92:8 er
L9
(20)
2,6-di-tert-butylpyridi
91
ne
Me
SiMe2Ph
SiMe2Ph
TsHN
Ph
TsHN
Ph
4e, E/Z = 96:4,
4l, E/Z = 98:2,
59% yield, 90:10 er
Ph SiMe2Ph
57% yield, 94:6 er
Ph SiMe2Ph
Ph SiMe2Ph
O
OMe
TsHN
aUnless noted otherwise, the reaction was conducted
according to following conditions: 1 (0.2 mmol), 2 (0.4 mmol),
Cu(CHB)2 (0.01 mmol), ligand (0.04 mmol), and base (0.6
mmol) in m-Xylene/tBuOH (2:1, 1.5 mL) were stirred for 36 h
at -15 °C under argon atmosphere. And the isolated yield is
TsHN
TsHN
R
OMe
4w, E/Z = 97:3,
95% yield, 89:11 er
4o, R = Me, E/Z = 97:3,
79% yield, 95:5 er
4u, E/Z = 98:2,
70% yield, 92:8 er
4q, R = OMe, E/Z = 94:6,
79% yield, 95:5 er
4r, R = Cl, E/Z = 98:2,
79% yield, 92:8 er
Ph SiMe2Ph
Ph SiMe2Ph
S
b
TsHN
for E- isomer. The ratio of the isomers was determined by
TsHN
crude 1H-NMR analysis. cThe yield was determined by
1H-NMR (2, 4, 5-trichloropyrimidine as an internal standard).
4s, R = Br, E/Z = 97:3,
71% yield, 92:8 er
4t, R = Ph, E/Z = 98:2,
75% yield, 93:7 er
4v, E/Z = 92:8,
73% yield, 92:8 er
4x
, E/Z > 99:1,
77% yield, 94:6 er
d
And the isolated yield was given in the parentheses. The
enantiomeric ratios were determined by HPLC with the
chiral column. eCu(OAc)2 was used in place of Cu(CHB)2.
On account of the optimal conditions, the practicability of
4x
the
Cu(II)/(S,
S)-4-Ph-Pybox-sBu
species
catalyzed
asymmetric conjugate proto-silylation of various α,
β-unsaturated ketimines was confirmed (Table 5). The
reaction could be conducted with different ketimines with a
tolerance of both electron-donating and-withdrawing
substituents. And the desired E-stereoisomers with high
enantiomeric ratios were obtained in good to high yields. In
most of cases, better enantioselectivity and lower reactivity
aReaction conditions: The mixture of 1 (0.2 mmol), 2 (0.4
mmol), Cu(CHB)2 (0.01 mmol), (S, S)-4-Ph-Pybox-sBu (0.04
mmol), and 2,6-di-tert-butylpyrdine (0.6 mmol) in
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m-xylene/tBuOH (2:1, 1.5 mL) at -15 °C was stirred for 36 h
under argon atmosphere. The isolated yield is for E-isomer
Notes
b
1
2
The authors declare no competing financial interests.
only.
3
ASSOCIATED CONTENT
4
5
6
7
8
9
Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI:.
Supporting spectra and characterization data (PDF)
Crystallographic data for compound 3a and 4x (CIF)
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ACKNOWLEDGMENT
We gratefully acknowledge the funding support of the
National Natural Science Foundation of China (21672198), the
Open Project of Key Laboratory of Organosilicon Chemistry
and Material Technology of Ministry of Education,
Hangzhou Normal University (2017110) and the Collaborative
Innovation Center of Chemistry for Energy Materials
(2011-iChEM) for financial support.
Figure 1. Proposed Model for Formation of (R)-4a
Compound.
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2.
Transformation
of
β-Tosylamine-Substituted Allylsilanes.
Finally, the utility of β-tosylamine-substituted allylsilane
products was demonstrated in Scheme 2. Notably, the
gram-scaled synthesis of compound 4a was performed in 89%
isolated yield
(2.21 g) and without loss of the
enantioselectivity (95:5 er) in the presence of 5 mol %
Cu(CHB)2 catalyst. The reduction of 4a with use of
Et3SiH/BF3•OEt2 was carried out to provide the
β-tosylamine-substituted benzylic silane in good yield (90%)
and moderate diastereoselectivity (3.4:1 dr).21 In addition, the
hydrolysis product 6 was obtained in high yield and with
complete retention of enantiopurity when the compound 4a
was treated with sulfuric acid in ethanol.11b The
Fleming-Tamao oxidation22 of the compound 4a successfully
afforded the chiral 3-hydroxy-1,3-diphenylpropan-1-one 7 in
91% yield and 93:7 er over two steps.
CONCLUSION
In conclusion, we have developed
a
protocol of
copper-catalyzed stereo- and enantio-selective conjugate
silyl-addition to sulfonyl ketimines. This work offers a simple
and straightforward method for the synthesis of not only the
(Z)-β-tosylamine-substituted allylsilane but also the E-isomer
in good yields and with high enantioselectivities via simply
tuning the reaction temperature and the ligand used.
Moreover, the corresponding products are effective synthons
for the preparation of useful compounds.
AUTHOR INFORMATION
Corresponding Author
xyh0709@ustc.edu.cn
ORCID Yun-He Xu: 0000-0001-8817-0626
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