Rhodium-Catalyzed Asymmetric Conjugate Alkynylation of β,γ-Unsaturated α-Ketoesters

Article abstract of DOI:10.1021/acs.orglett.7b00909

The first example of catalytic asymmetric conjugate alkynylation of β,γ-unsaturated α-ketoesters is reported. By using Rh(I)/(R)-DM-binap complex as the catalyst and diphenyl[(triisopropylsilyl)ethynyl]methanol as the alkynylating reagent, the alkynylation reaction proceeded smoothly to afford α-ketoesters bearing a propargylic chiral center at γ position in good yields with high enantioselectivities.

Full text of DOI:10.1021/acs.orglett.7b00909

Letter
pubs.acs.org/OrgLett
Rhodium-Catalyzed Asymmetric Conjugate Alkynylation of
β,γ‑Unsaturated α‑Ketoesters
Yanle Zhi, Jianhang Huang, Na Liu, Tao Lu,* and Xiaowei Dou*,†
†
†
†
,†,‡
†
‡
Department of Organic Chemistry and State Key Laboratory of Natural Medicines, China Pharmaceutical University, 24
Tongjiaxiang, Nanjing 210009, China
*
S Supporting Information
ABSTRACT: The first example of catalytic asymmetric
conjugate alkynylation of β,γ-unsaturated α-ketoesters is
reported. By using Rh(I)/(R)-DM-binap complex as the catalyst
and diphenyl[(triisopropylsilyl)ethynyl]methanol as the alkyny-
lating reagent, the alkynylation reaction proceeded smoothly to
afford α-ketoesters bearing a propargylic chiral center at γ
position in good yields with high enantioselectivities.
1
0
11
he asymmetric conjugate addition reaction is one of the
cobalt, and others as the catalyst have been explored.
Among all these catalytic systems, rhodium catalysis has
attracted much more attention in recent years since it showed
high reactivity and excellent chemo- and stereoselectivity,
especially for those substrates with an electron-withdrawing
T
most efficient methods for generating new stereogenic
1
carbon centers, while β,γ-unsaturated α-keto carbonyl
compounds represent an important class of substrates in such
2
transformations. Asymmetric conjugate 1,4-addition to those
12
substrates produces α-keto carbonyl compounds bearing a γ
chiral center, which are versatile synthons and can be readily
converted to valuable building blocks for the synthesis of a
number of natural products, bioactive compounds, and
group activated olefin; thus, we envisioned that a suitable
rhodium catalytic system might work for the aforementioned
transformation. As a breakthrough in rhodium-catalyzed
asymmetric alkynylation reaction, asymmetric conjugate
addition of terminal alkyne to acyclic α,β-unsaturated ketones
was first realized by Hayashi and co-workers, and the key of
the reaction was rational selection of sterically bulky chiral
bisphosphine ligand (DTBM-segphos) and terminal alkyne
(triisopropylsilyl)acetylene) to suppress alkyne dimerization.
A similar strategy was also applied to rhodium-catalyzed
asymmetric alkynylation of α,β-unsaturated aldehydes, nitro-
alkenes, and 5-benzylidene Meldrum’s acids. Later on, a
2
,3
pharmaceuticals. As a direct strategy for the formation of
12b
new carbon−carbon bonds and creating γ chiral centers,
2
3
asymmetric 1,4-addition of sp - and sp -carbon nucleophiles to
β,γ-unsaturated α-keto carbonyls (e.g., β,γ-unsaturated α-
ketoesters) has been extensively studied and a number of
(
2
−4
efficient reaction systems have been established. In contrast,
asymmetric conjugate addition of sp-carbon nucleophile to β,γ-
unsaturated α-keto carbonyls remains a formidable challenge.
To the best of our knowledge, no asymmetric alkynylation
12f
1
2i
12g
broader scope of enones was achieved with alkynylsilanols as
the alkynylating reagent.
12h
5
Recently, an easily accessible
reaction of those substrates has yet been reported. Herein, we
propargylic alcohol was utilized by Hayashi group as the
alkynylating reagent in asymmetric alkynylation of cyclic α,β-
unsaturated carbonyl compounds, and the combination of the
propargylic alcohol with a chiral Rh/diene catalyst was found to
be crucial for high reactivity and enantioselectivity. In this
Letter, we report our finding that, while the reported methods
were not effective for asymmetric conjugate alkynylation of
challenging substrates like β,γ-unsaturated α-ketoesters, a
reaction system consisting of propargylic alcohol and Rh/
bisphosphine catalyst worked efficiently to enable the desired
transformation.
report the first catalytic asymmetric conjugate alkynylation of
β,γ-unsaturated α-ketoesters (Scheme 1).
In the past decades, considerable efforts have been devoted
to the development of catalytic asymmetric alkynylation
12k
6
7
8
9
reaction, and various systems using copper, iridium, nickel,
Scheme 1. Asymmetric Conjugate 1,4-Addition of Carbon
Nucleophiles to β,γ-Unsaturated α-Ketoesters
We started the investigation with rhodium-catalyzed
asymmetric alkynylation of γ-unsaturated α-ketoester 1a, and
the results obtained under various conditions are summarized
in Table 1. In the first set of the experiments, the previously
Received: March 27, 2017
©
XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.7b00909
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
14
Table 1. Rhodium-Catalyzed Asymmetric Conjugate
Alkynylation of β,γ-Unsaturated α-Ketoester 1a
elimination and effectively avoid dimerization due to the
steric bulkiness, was chosen as the substitute for the terminal
alkyne (Scheme 2). With propargylic alcohol 2b as the
a
Scheme 2. Alkynylating Reagent 2a versus 2b
b
c
d
entry
2 (equiv)
Rh catalyst
yield (%)
ee (%)
n.d.
1
2
3
4
5
6
7
8
9
2a (2.0)
2b (1.2)
2b (1.2)
2b (1.2)
2b (1.2)
2b (1.2)
2b (1.2)
2c (1.2)
2d (1.2)
Rh/(R)-DTBM-segphos
12
<3
23
54
32
36
78
28
<3
alkynylating reagent, different chiral bisphosphine ligands
were then examined. Introduction of a less bulky segphos
ligand indeed improved the yield, but only very slightly (23%
yield, entry 3). However, bisphosphine ligands with a binap
backbone were found to be superior. Rh/(R)-binap-catalyzed
reaction gave the desired product in 54% yield with a high
enantioselectivity (89% ee, entry 4). The chemical yield
e
[RhCl(rac-Fc-bod)2]
Rh/(R)-segphos
Rh/(R)-binap
n.d.
89
Rh/(R)-H -binap
88
8
Rh/(R)-T-binap
89
Rh/(R)-DM-binap
Rh/(R)-DM-binap
Rh/(R)-DM-binap
90
n.d.
decreased to 32% with (R)-H -binap, indicating the importance
8
of the rigid binap backbone (entry 5). Screening of binap-type
ligand led to the finding of the optimal catalyst in this study,
a
Reactions were performed with 1a (0.15 mmol), 2 (0.18 mmol), and
Rh catalyst (5 mol % Rh) in toluene (1.0 mL) at 100 °C for 16 h.
[Rh(OH)((R)-DM-binap)] , which catalyzed the alkynylation
2
b
The catalyst was generated in situ from [Rh(OH)(coe) ] (5 mol %
reaction of 1a efficiently to give the alkynylation product in
78% yield with 90% ee (entry 7). Other propargylic alcohols
bearing smaller substitutions instead of the triisopropylsilyl
group were also verified. Due to the dimerization of propargylic
alcohols, the yield dropped with the existence of a trimethylsilyl
protecting group as anticipated (entry 8), and no desired
product was obtained with the existence of a phenyl
2
2
c
Rh) and chiral bisphosphine ligand (6 mol %). Yield of the isolated
product 3. The ee value was determined by HPLC analysis on a chiral
stationary phase column. Cs CO (10 mol %) was added. n.d. = not
determined.
d
e
2
3
reported reaction conditions were examined. The combination
of bulky DTBM-segphos coordinated rhodium catalyst and
14a
substitution group (entry 9).
(
triisopropylsilyl)acetylene 2a, which was reported before to
With [Rh(OH)((R)-DM-binap)] as the catalyst, further
2
efficiently catalyze the alkynylation of linear α,β-unsaturated
carbonyl compounds,
but in a low yield (12%, entry 1). Rhodium catalyst with a Fc-
bod diene ligand, which was developed to catalyze the
alkynylation of less reactive cyclic α,β-unsaturated carbonyl
compounds with alkynylating reagent diphenyl-[(triisopropyl-
optimization of reaction conditions was conducted, and the
results are summarized in Table 2. First, in situ generation of
the active rhodium catalyst from a more stable precursor
[Rh(OH)(cod)] instead of air sensitive [Rh(OH)(coe) ] led
12b,f
gave the desired alkynylation product,
2
2 2
to essentially the same reaction outcome (entry 1). Thus,
[Rh(OH)(cod)] was used as the rhodium source in the
2
12k
silyl)ethynyl]methanol 2b,
reaction, and only a trace amount of product was observed
entry 2). Both of the best conditions for acyclic and cyclic α,β-
showed low reactivity in this
following studies. The effect of different esters on substrate 1
was studied, and we found methyl ester was still the best in
terms of yield and enantioselectivity. Ethyl ester 1b gave a
comparable result, but in a slightly lower yield (entry 2).
Substrates with bulkier ester moieties were found to be less
reactive with the lower yield and ee (entries 3 and 4). The
reaction was also run in 1,4-dioxane, which is a commonly used
solvent in rhodium-catalyzed conjugate addition reaction, but
the yield and ee were lower than those obtained in toluene
(entries 5 and 6). The high reaction temperature was necessary:
when the reaction temperature decreased to 80 °C, the
chemical yield dropped dramatically from 78% to 36% (entry
7). The propargylic alcohol 2b was found to be fully consumed
during the reaction, while part of 1a remained. Thus, additional
alkynylating reagent was added to improve the conversion of
(
unsaturated carbonyl substrates did not work for β,γ-
unsaturated α-ketoester, indicating that it is a more challenging
substrate and that a new reaction condition should be
developed. The low reactivity of Rh/DTBM-segphos catalyst
can be attributed to the steric hindrance between the bulky
ligand and substrate; thus, we expect the fine-tuning of
bisphosphine ligand might improve the catalytic reactivity.
Since the alkyne dimerization is the dominant reaction when
less bulky bisphosphine is used, terminal alkyne 2a might not
13
be a suitable alkynylating reagent in this case. Propargylic
alcohol-type alkynylating reagent 2b, which could generate
active alkynyl rhodium intermediate in situ via β-alkynyl
B
DOI: 10.1021/acs.orglett.7b00909
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
a
Table 2. Further Optimization of Reaction Conditions
substrates with different types of γ-substitutions such as γ-alkyl
and γ-styryl β,γ-unsaturated α-ketoesters were also tested;
however, the alkynylation reaction did not proceed probably
due to the low reactivity of those substrates. It should be
pointed out that no 1,2-addition product was formed in all the
examples.
The alkynylation product could be readily converted to other
important building blocks in organic synthesis. As demon-
strated in Scheme 3A, β-alkynyl β-aryl aldehyde 4a, which could
b
c
entry
1
conditions
yield (%)
ee (%)
1
2
3
4
5
6
7
1a
1b
1c
1d
1a
1b
1a
1a
toluene, 100 °C, 16 h
toluene, 100 °C, 16 h
toluene, 100 °C, 16 h
toluene, 100 °C, 16 h
1,4-dioxane, 100 °C, 16 h
1,4-dioxane, 100 °C, 16 h
toluene, 80 °C, 16 h
toluene, 100 °C, 16 h
78
70
60
62
69
63
36
82
90
90
84
89
86
86
n.d.
90
Scheme 3. Derivatization of the Alkynylation Product 3a (A)
and Proposed Stereochemical Pathway (B)
d
8
a
Reactions were performed with 1 (0.15 mmol), 2b (0.18 mmol, 1.2
equiv), and Rh catalyst (5 mol % Rh) in anhydrous solvent (1.0 mL).
The catalyst was generated in situ from [Rh(OH)(cod)] (5 mol %
2
b
Rh) and (R)-DM-binap (6 mol %). Yield of the isolated product 3.
The ee value was determined by HPLC analysis on a chiral stationary
phase column. 2b (0.21 mmol, 1.4 equiv to 1a) was used. n.d. = not
c
d
determined.
1
a, and the yield of product 3 was ultimately improved to 82%,
maintaining 90% ee (entry 8).
With the optimal reaction conditions, the scope of the
reaction was then examined. As summarized in Table 3, diverse
a
Table 3. Substrate Scope
not be obtained by direct alkynylation of α,β-unsaturated
12f
aldehyde, was derived from 3a in high yield without erosion
of enantioselectivity. In the presence of acetic acid and TBAF,
an intramolecular 5-exo-cyclization/isomerization process took
place to afford furan product 6a. To determine the absolute
configuration, 4a was further transformed to alcohol 5a, which
was determined to be of R-configuration by comparison of its
specific rotation with that reported. The absolute config-
uration of the product 3a was accordingly assigned to be R,
which is in good agreement with the proposed stereochemical
pathway: the coordination of β,γ-unsaturated α-ketoester to the
alkynyl−rhodium intermediate with its re face is more favorable
than the si face due to the steric repulsions between the α-
ketoester moiety and one of the aryl rings on the chiral
bisphosphine ligand (Scheme 3B).
In summary, we have developed the first asymmetric
conjugate alkynylation reaction of β,γ-unsaturated α-ketoesters.
The combination of Rh/bisphosphine catalyst and propargylic
alcohol alkynylating reagent shows high reactivity and
enantioselectivity in conjugate alkynylation reaction, and its
application in the asymmetric alkynylation of other challenging
substrates is ongoing in our laboratory.
b
c
entry
3 (R)
3a (C H )
yield of 3 (%)
ee of 3 (%)
1
2
3
4
5
6
7
8
82
84
63
84
84
78
77
81
75
88
72
75
84
90
91
91
90
87
88
92
93
91
91
92
89
84
6
5
3b (4-Me-C H )
6
4
15
3c (4-OMe-C H )
6
4
3d (3-Me-C H )
6
4
3e (3-OMe-C H )
6
4
3f (4-CF -C H )
3
6
4
3g (4−F-C H )
6
4
3h (3−F-C H )
6
4
d
9
3i (3−Cl-C H )
6 4
d
10
11
12
13
3j (2−F-C H )
6 4
3k (3,5-F -C H )
2
6
3
d
d
3l (3-thienyl)
3m (3-furyl)
a
b
c
See footnote a of Table 2. See footnote b of Table 2. See footnote c
of Table 2. [Rh(OH)(cod)] (10 mol % Rh) and (R)-DM-binap (12
d
2
mol %) were used.
γ-aryl β,γ-unsaturated α-ketoesters could be employed in this
reaction system, and generally good yields and high
enantioselectivities were obtained. Both electron-donating and
electron-withdrawing groups substituted γ-aryl β,γ-unsaturated
α-ketoesters worked well under optimal reaction conditions
ASSOCIATED CONTENT
Supporting Information
■
*
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.7b00909.
(
entries 2−6). Besides, substrates with different halogen
substitutions at different positions (entries 7−10) did not
influence the yield and ee, as well as those disubstituted
substrate and heteroaryl substrates (entries 11−13). Other
Experimental procedures, characterization data, and
1
13
copies of H and C NMR spectra and HPLC charts
(PDF)
C
DOI: 10.1021/acs.orglett.7b00909
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
(
d) Sawano, T.; Ou, K.; Nishimura, T.; Hayashi, T. J. Org. Chem. 2013,
AUTHOR INFORMATION
■
*
*
7
8, 8986.
11) (a) Wu, T. R.; Chong, J. M. J. Am. Chem. Soc. 2005, 127, 3244.
b) Yamashita, M.; Yamada, K.; Tomioka, K. Org. Lett. 2005, 7, 2369.
c) Cui, S.; Walker, S. D.; Woo, J. C. S.; Borths, C. J.; Mukherjee, H.;
Corresponding Authors
(
(
(
E-mail: lut163@163.com.
E-mail: dxw@cpu.edu.cn.
Chen, M. J.; Faul, M. M. J. Am. Chem. Soc. 2010, 132, 436. (d) Blay,
G.; Cardona, L.; Pedro, J. R.; Sanz-Marco, A. Chem. - Eur. J. 2012, 18,
ORCID
1
̃ ̃
́
2966. (e) Villarino, L.; García-Fandino, R.; Lopez, F.; Mascarenas, J.
Xiaowei Dou: 0000-0002-0748-1279
Notes
L. Org. Lett. 2012, 14, 2996. (f) Ito, J.; Fujii, K.; Nishiyama, H. Chem. -
Eur. J. 2013, 19, 601. (g) Sanz-Marco, A.; Blay, G.; Vila, C.; Pedro, J. R.
Org. Lett. 2016, 18, 3538.
The authors declare no competing financial interest.
(
12) (a) Nishimura, T.; Katoh, T.; Takatsu, K.; Shintani, R.; Hayashi,
T. J. Am. Chem. Soc. 2007, 129, 14158. (b) Nishimura, T.; Guo, X.-X.;
Uchiyama, N.; Katoh, T.; Hayashi, T. J. Am. Chem. Soc. 2008, 130,
ACKNOWLEDGMENTS
■
(
1
576. (c) Nishimura, T.; Guo, X.-X.; Hayashi, T. Chem. - Asian J. 2008,
We thank the National Natural Science Foundation of China
21602253, 81673301), the Natural Science Foundation of
Jiangsu Province (BK20160749), and China Pharmaceutical
University for financial support.
3
, 1505. (d) Nishimura, T.; Tsurumaki, E.; Kawamoto, T.; Guo, X.-X.;
Hayashi, T. Org. Lett. 2008, 10, 4057. (e) Crotti, S.; Bertolini, F.;
Macchia, F.; Pineschi, M. Chem. Commun. 2008, 3127. (f) Nishimura,
T.; Sawano, T.; Hayashi, T. Angew. Chem., Int. Ed. 2009, 48, 8057.
(
(
g) Fillion, E.; Zorzitto, A. K. J. Am. Chem. Soc. 2009, 131, 14608.
h) Nishimura, T.; Tokuji, S.; Sawano, T.; Hayashi, T. Org. Lett. 2009,
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DOI: 10.1021/acs.orglett.7b00909
Org. Lett. XXXX, XXX, XXX−XXX