Synthesis of Highly Substituted Racemic and Enantioenriched Allenylsilanes via Copper-Catalyzed Hydrosilylation of (Z)-2-Alken-4-ynoates with Silylboronate

Article abstract of DOI:10.1021/jacs.5b08279

Copper-catalyzed highly efficient hydrosilylation reaction of enynoates was developed. Under simple reaction conditions, various di-, tri-, and tetrasubstituted racemic allene products could be obtained in high yields. The asymmetric 1,6-addition of silyl group to the (Z)-2-alken-4-ynoates could be achieved under mild reaction conditions to afford the silyl-substituted enantioenriched chiral allene products in good yields and with high enantioselectivities.

Full text of DOI:10.1021/jacs.5b08279

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Communication
Synthesis of Highly Substituted Racemic and Chiral Allenylsilanes via
Copper-Catalyzed Hydrosilylation of (Z)-2-Alken-4-ynoates with Silylboronate
Min Wang, Zheng-Li Liu, Xiang Zhang, Pan-Pan Tian, Yun-He Xu, and Teck-Peng Loh
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.5b08279 • Publication Date (Web): 11 Nov 2015
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Synthesis of Highly Substituted Racemic and Enantioen-
riched Allenylsilanes via Copper-Catalyzed Hydrosilylation of
(Z)-2-Alken-4-ynoates with Silylboronate
Min Wang,†a ZhengꢀLi Liu,†a Xiang Zhang,a PanꢀPan Tian,a YunꢀHe Xua* and TeckꢀPeng Loha,b*
a Hefei National Laboratory for Physical Sciences at the Microscale and Department of Chemistry, University of Science and
Technology of China, Hefei, 230026, China
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b Division of Chemistry and Biological Chemistry, School of Physical and Mathematical Sciences, Nanyang Technological
University, Singapore 637371
Supporting Information Placeholder
ABSTRACT: Copperꢀcatalyzed highly efficient hydrosilylation
Chirality Transformation
Kinetic Resolution
R1
R3
reaction of enynoates was developed. Under simple reaction
conditions, various diꢀ, triꢀ and tetrasubstituted racemic allene
products could be obtained in high yields. The asymmetric 1,6ꢀ
addition of silyl group to the (Z)ꢀ2ꢀalkenꢀ4ꢀynoates could be
achieved under mild reaction conditions to afford the silylꢀ
substituted enantioenriched chiral allene products in good yields
and with high enantioselectivities.
LG
R1
R4
R4
R2
R3
Traditional Methods
a
b
R1
R2
R3
R4
c
d
This work:
R
noble transition-metal
catalyzed asymmetric
synthesis of allenes
cat. = [Cu]
+
PhMe2Si–Bpin
Chiral allenes are featured in many natural products, drugs
and advanced materials.1 They can also serve as useful synthetic
building blocks in organic synthesis.2 Accordingly, we have witꢀ
nessed an increase in the development of new methods for the
synthesis of enantioenriched chiral allenic compounds including
the use of transition metals and chiral organocatalysts.3 Inspired
by Elsevier’s pioneering work on the transitionꢀmetalꢀcatalyzed
chiral allene synthesis in 1989,4 many groups have developed
efficient catalytic enantioselective systems. For example,
Hayashi,5 Ogasawara,6 Ma7 and Frantz8 groups have elegantly
developed an array of palladium or rhodiumꢀcatalyzed enantioseꢀ
lective reactions for the synthesis of axially chiral allenes in modꢀ
erate to excellent enantioselectivity. On the other hand, the reꢀ
search groups of Arai with Shioiri,9 Tan,10 Zhang with Sun,11
Gong12 and Maruoka13 etc14ꢀ16 have successfully applied chiral
organocatalysts in the catalytic synthesis of enantioenriched alꢀ
lenes (Scheme 1, pathway c). Despite all these successes in cataꢀ
lytic systems, there are still limitations with existing methods such
as the need to use expensive metals, limited substrate scopes such
as the need to use symmetrical allenes as starting materials, the
possible formation of homopropargylic alcohols as byproducts
and the moderate selectivities obtained still provide opportunity
for the development of more efficient methods (Scheme 1, pathꢀ
ways a and b). Furthermore, enantioꢀ and regioselective synthesis
of trisubstituted17 and tetrasubstituted allenes13,18 as well as hetꢀ
eroatom substituted allenes5c,5d,5g,19 are still not wellꢀestablished.
Herein, in connection with our interest in the development of
copperꢀcatalyzed hydrosilylation reaction of unsaturated carbonꢀ
R'O
cat. = [Pd], [Rh]... ...
O
Chart 1. Screening the Optimized Reaction Conditions of
CopperꢀCatalyzed Hydrosilylationtion of Enynoate 1a with
PhMe2SiꢀBpin.a
entry
cat. (mol %)
additive ( mol %) yield (%)b
1
2
CuCl (10)
CuBr (10)
CuI (10)
Et3N (10)
Et3N (10)
Et3N (10)
Et3N (10)
Et3N (10)
pyridine (10)
DABCO (10)
NaOAc (100)
--
90
92
88
90
57
78
50
75
--
3
4
Cu(OAc)2 (10)
CuTc (10)
CuBr (10)
CuBr (10)
CuBr (10)
CuBr (10)
--
5
6
7
8
9
10
Et3N (10)
--
a Unless noted otherwise, the reaction was conducted with 1a (0.2 mmol), 2
(0.4 mmol, 2.0 equiv), copper catalyst (0.02 mmol, 10 mol%) and NEt3 (0.02
mmol, 10 mol%) in anhydrous MeOH (1 mL) for 24 h at 28 °C under argon
atmosphere. b Isolated yield.
20
carbon bonds, we report an efficient method to obtain silylꢀ
First, the copperꢀcatalyzed hydrosilylation of enynoates was
investigated, and the results are shown as in Chart 1. It was found
that both Cu(I) and Cu(II) catalysts could afford the desired allenꢀ
ylsilane product 3a in high yield in the presence of 10 mol% triꢀ
ethylamine as the additive23 (Chart 1, entries 1ꢀ5). However, the
use of pyridine would slightly lower the product yield to 78%
(Chart 1, entry 6). And the use of DABCO or inorganic base sodiꢀ
um acetate would significantly hinder the product’s formation
allenes in good yields and with high regioꢀ and enantioselectivity
based on the use of cheap copperꢀcatalyst and silylboronate21 for
the 1,6ꢀsilyl addition to enynoates (Scheme 1, pathway d).18,22
Scheme 1. Methods for Enantioselective Synthesis of Chiral
Allenes.
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(Chart 1, entries 7 and 8 respectively). It was also noted that no
found that regardless of the electronꢀdonating or electronꢀ
1
2
3
4
5
6
7
8
desired product could be obtained in the absence of either base as
additive or copper salt as catalyst (Chart 1, entries 9 and 10 reꢀ
spectively). Therefore, the more airꢀstable and nonꢀmoisture senꢀ
sitive CuBr was chosen as the catalyst for this silylation reaction
and 10 mol% triethylamine as additive in MeOH solution at room
temperature.24
withdrawing groups installed on the substrates all could afford the
desired products in high yields albeit the nitro group leading to a
dramatically decreased yield (3g). When the thiophene group was
used as substituent, only moderate yield of the product could be
observed along with the recovering of the unreacted starting mateꢀ
rial. The lower conversion was probably caused by the strongly
coordinating ability of the heteroatom with copper catalyst (3t).
There was no obvious negative effect on the product’s formation
when the bulky cyclopropyl moiety was applied as the substituent
(3u). Other aliphatic substituents with different functionalities
could also be tolerated in reaction and afford the corresponding
products in excellent yields (3vꢀ3y). Furthermore, the formation
of diꢀ and tetrasubstituted allenylsilanes was studied, and the
products 3z, 3aa and 3ab could be generated in high yields reꢀ
spectively under
Chart 2. Hydrosilylation of Various Enynoates with PhMe2Siꢀ
Bpin.a,d
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PhMe2Si
CO2Et PhMe2Si
CO2Et
PhMe2Si
CO2Et
H
H
H
Me
Chart 3. Optimization of Copper(I)ꢀCatalyzed Asymmetric
Hydrosilylation of 1a with PhMe2Si−Bpin.a
3b, 3-Me, 82%
3c
3a
, 92%
3d
, 86%
MeO
, 4-Me, 91%
PhMe2Si
CO2Et
PhMe2Si
CO2Et
PhMe2Si
CO2Et
H
H
H
H
nBu
NO2
tBu
b
b
3g
, 56%
3e
3f
, 80%
, 80%
PhMe2Si
CO2Et
PhMe2Si
CO2Et
PhMe2Si
CO2Et
H
H
Ac
CO2Me
EtO2C
3h
, 64%
3i, 78%
3j
, 92%
PhMe2Si
CO2Et
CO2Et
CO2Et
PhMe2Si
PhMe2Si
H
H
H
Cl
F
b
3m, 3-Cl, 81%b
3n, 4-Cl, 92%
b
Br
3k
, 3-F, 85%
3o
, 84%
b
3l
, 4-F, 85%
CO2Et
PhMe2Si
CO2Et
CO2Et PhMe2Si
PhMe2Si
H
H
H
HO
b
3p, 80%b
PhMe2Si
3r
, 84%
3q
, 94%
Ph
CO2Et
PhMe2Si
CO2Et
CO2Et
PhMe2Si
H
H
H
S
c
b
3t
, 61%
3u
, 88%
3s
, 76%
, 85%
PhMe2Si
BnO
CO2Et
PhMe2Si
CO2Et
PhMe2Si
Cl
CO2Et
H
H
H
3x
, 83%
3v
3w
, 95%
PhMe2Si
CO2Et
CO2Et
CO2Et
PhMe2Si
CO2Et
87%
H
a Reaction was run under the following reaction conditions: 0.2 mmol enynoate
1, 0.3 mmol 2 (1.5 equiv), 5 mol% CuTC and 6 mol% L5 in 1 mL anhydrous
tAmOH at 0 oC under argon atmosphere for 48 h. b The reaction time was 72 h.
c The values of ee were determined by HPLC.
PhMe2Si
H
Ph
3aa,
EtO2C
3y
, 92%
3z, 82%
PhMe2Si
CO2R3
PhMe2Si
3ac
3ad
3ae
,
,
,
R = Me, 96%
R = iPr, 78%b
H
Me
, 71%
R = nBu 94%
,
the standard reaction conditions. Finally, other esters substituted
enynoates were applied to reaction, and high yields of products
could be observed (3acꢀ3ae).
3ab
a Unless noted otherwise, the reaction condition was performed under the folꢀ
lowing reaction conditions: 0.2 mmol enynoate 1, 0.4 mmol 2 (2.0 equiv), 10
mol% CuBr and 10 mol% Et3N in 1 mL anhydrous MeOH at 28 °C under
c
argon atmosphere for 24 hours. b The reaction time was 48 h. The reaction
As mentioned above, so far the reported species of enantiꢀ
oenriched allenes especially the heteroatomꢀsubstituted examples
remain limited. Therefore, we embarked on the development of
the synthesis of silylꢀsubstituted allenes (Chart 3). After tedious
screening of the copper catalysts, solvents and different chiral
ligands; finally it was found that the desired enantioenriched silylꢀ
substituted allene 3a* could be obtained in 80% yield and with
92% enantiomeric excess in the presence of 5 mol% CuTC (copꢀ
time was 72 h. d Isolated yield.
With the optimized reaction conditions in hand, various
enynoates as substrates were tested, and the results are summaꢀ
rized in Chart 2. First, the electronic and steric effects of different
functional groups on the phenyl ring were examined, and it was
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per(I) thiopheneꢀ2ꢀcarboxylate) and 6 mol% chiral bisoxazoline
configuration was less reactive than the cisꢀone to generate the
silylallene product in low yield and with poor enantioselectivity
(eq. 1ꢀ4).
t
ligand L5 in AmOH (tAm=tertꢀamyl) solution at 0 °C with stirꢀ
1
2
ring for 48 hours (Chart 3, entry 10).
3
4
5
Chart 4. Copper(I)ꢀCatalyzed Asymmetric Hydrosilylation of
Various Enynaoates with PhMe2Si−Bpin.a,e
6
7
8
9
PhMe2Si
PhMe2Si
CO2Et
PhMe2Si
CO2Et
PhMe2Si
CO2Et
CO2Et
H
H
H
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Me
3b*, 87%b,c
3c*,83%
91% ee
3a*80%
92% ee
92% ee
Me
CO2Et
CO2Et
PhMe2Si
PhMe2Si
H
H
H
3e*, 73%c
93% ee
3d*, 72%
90% ee
3f*, 71%
91% ee
nBu
tBu
MeO
PhMe2Si
CO2Et
PhMe2Si
CO2Et
PhMe2Si
CO2Et
H
H
H
The absolute configuration of 3s* was determined to be (R)ꢀ
3h*, 64%b
F
3k*,76% b,c
3i*,74%b,c
93% ee
91% ee
90% ee
EtO2C
(+) by singleꢀcrystal Xꢀray diffraction analysis of compound 5,
Ac
which could be obtained after twoꢀstep derivatization (Scheme 2).
PhMe2Si
CO2Et
PhMe2Si
CO2Et
CO2Et
25
Thus, reduction of the ester 3s* with LiAlH4 afforded alcohol 4
PhMe2Si
H
which reacted with 3,5ꢀdinitrobenzoyl chloride26 to give 5 in high
H
H
3l*,90%c,d
3n' ,83%c,d
Cl
3m*,85%b,c
92% ee
yield and with no loss of enantiomeric purity during the two steps.
90% ee
F
90% ee
Cl
PhMe2Si
CO2Et
PhMe2Si
CO2Et
PhMe2Si
CO2Et
H
H
H
3s*, 63%b,c
91% ee
3q*,,71%
93% ee
3r*, 75%b,c
90% ee
Ph
CO2Et
PhMe2Si
Cl
PhMe2Si
CO2Et
PhMe2Si
PhMe2Si
PhMe2Si
CO2Et
CO2Et
CO2nBu
H
H
H
S
3u*,75%b,c
73% ee
3t*, 52%b,c
3x*, 76%
68% ee
90% ee
CO2Me PhMe2Si
CO2iPr
PhMe2Si
Me
H
H
3ad*, 74%b,c
3ab*, 74%
36% ee
3ac*, 81%
94% ee
91% ee
Scheme 2. Determination of the Absolute Configuration of
Compound 3s*
H
3ae*, 75%
91% ee
Scheme 3. Proposed Plausible Mechanism for the Copperꢀ
Catalyzed Asymmetric Hydrosilylation of Enynaoate.
a
Reaction was carried out according to the following reaction conditions: 0.2
mmol enynaoate 1, 0.3 mmol 2 (1.5 equiv), 5 mol % CuTC, 6 mol% and L5 in
1 mL anhydrous AmOH at 0 °C under argon atmosphere for 48 h. b The reacꢀ
t
tion time was 72 h. c The temperature was ꢀ5 oC. d The reaction time was 96 h. e
The values of ee was determined by HPLC.
Subsequently, the enynoates with different functional groups
including acetal (Chart 4, 3h*) and ester (Chart 4, 3i*) groups on
the phenyl ring were examined, and all the products could be
obtained in good yields and with high ee values. When the 9Hꢀ
fluorenꢀ and thiopheneꢀsubstituted enynoates were examined,
good enantioselectivity of the products albeit with slightly lower
yields could be observed (Chart 4, 3s* and 3t* respectively). Howꢀ
ever, aliphatic substituents were unfavorable for the enantioselecꢀ
tive hydrosilylation of enynoate, the desired products were affordꢀ
ed with moderate enantioselectivity but in good yields (Chart 4,
3u* and 3x*). It is worth noting that an example of chiral tetraꢀ
substituted allene 3ab* could be obtained in 74% yield, albeit with
only 36% ee. The substituent effect of ester group was further
investigated, and the results showed that neither the product’s
yield nor its enantiopurity could be affected (Chart 4, 3ac*, 3ad*
and 3ae*). Moreover, it was found that the enynoate with transꢀ
A plausible mechanistic pathway of CuTC and chiral bisoxꢀ
azoline ligand system catalyzed asymmetric hydrosilylation reꢀ
action of enynoates was depicted as in Scheme 3. First, a CuꢀSi
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(5) Matsumoto, Y.; Naito, M.; Uozumi, Y.; Hayashi, T. J. Chem. Soc.,
species I could be generated from silylboronate 2 with the help
of CuTC in tertꢀamyl alcohol, followed by coordination with the
enynoate 1a to give the intermediate II. After silylation, the enoꢀ
late copper species III generated in situ would be protonated to
furnish the desired silylallene product 3a. The released copper
catalyst IV would be delivered to next catalytic cycle.
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1
2
3
4
5
6
7
8
In conclusion, a copperꢀcatalyzed asymmetric synthesis of
highly substituted chiral allenes was developed. In this work, the
(Z)ꢀenynoates could react smoothly with the silylboronate, the
corresponding racemic and enantioenriched silylꢀsubstituted alꢀ
lene products could be obtained in good yields and high enantiꢀ
oselectivities under very mild reaction conditions respectively.
Therefore, current work provides an efficient method to prepare
allenylsilanes, as well as expands the scope of heteroatomꢀ
substituted allene species.
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ASSOCIATED CONTENT
Supporting Information
Experimental procedures and spectral data for all new compounds
(1H NMR, 13C NMR, HRMS) , and crystallographic data (CIF) of
compounds 5 in CCDC numbers: 1435363. This material is availꢀ
able free of charge via the Internet at http://pubs.acs.org.
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135, 18020.
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AUTHOR INFORMATION
Corresponding Author
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xyh0709@ustc.edu.cn
teckpeng@ntu.edu.sg
Author Contributions
†M.W. and Z.ꢀL.L. contributed equally to this work.
Notes
The authors declare no competing financial interests.
ACKNOWLEDGMENT
We gratefully acknowledge the funding support of National Sciꢀ
ence Foundation for Young Scientists of China (21202156), the
National Natural Science Foundation of China (21372210) and
the State Key Program of National Natural Science Foundation of
China (21432009). The authors are grateful to Mr. Koh Peng Fei
Jackson for his careful proofreading of the final manuscript. We
thank Dr. S. M. Zhou (HFNL, USTC) for the determination of the
crystal structure.
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(24) Other silylboronates were also tested in this reaction; please see the
details in SI.
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