Highly Enantioselective [3 + 2] Annulation of 3-Butynoates with β-Trifluoromethyl Enones Promoted by an Amine-Phosphine Binary Catalytic System

Article abstract of DOI:10.1021/acs.orglett.0c00681

We report a highly enantioselective [3 + 2] annulation between 3-butynoates and β-trifluoromethyl enones, furnishing trifluoromethylated cyclopentenes with three contiguous stereogenic centers in good yields, high diastereoselectivities, and excellent enantioselectivities. A unique catalytic system consisting of a simple amine and a chiral phosphine was devised, and the synergistic play of Lewis basic amine and phosphine was crucial for alkyne isomerization and subsequent cyclization. The protocol disclosed herein allows facile activation of 3-butynoates in phosphine-mediated asymmetric transformations.

Full text of DOI:10.1021/acs.orglett.0c00681

pubs.acs.org/OrgLett
Letter
Highly Enantioselective [3 + 2] Annulation of 3‑Butynoates with
β‑Trifluoromethyl Enones Promoted by an Amine−Phosphine Binary
Catalytic System
Huanzhen Ni, Yee Lin Wong, Mingyue Wu, Zhaobin Han, Kuiling Ding,* and Yixin Lu*
Cite This: https://dx.doi.org/10.1021/acs.orglett.0c00681
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ABSTRACT: We report a highly enantioselective [3 + 2]
annulation between 3-butynoates and β-trifluoromethyl enones,
furnishing trifluoromethylated cyclopentenes with three contiguous
stereogenic centers in good yields, high diastereoselectivities, and
excellent enantioselectivities. A unique catalytic system consisting
of a simple amine and a chiral phosphine was devised, and the
synergistic play of Lewis basic amine and phosphine was crucial for
alkyne isomerization and subsequent cyclization. The protocol
disclosed herein allows facile activation of 3-butynoates in
phosphine-mediated asymmetric transformations.
molecular architectures would be highly desirable, we therefore
ver the past decades, enantioselective nucleophilic
Ophosphine catalysis has emerged as a powerful approach set our goal to tackle this challenge.
We reasoned the key to the utilization of 3-butynoates in
to access a wide range of optically enriched cyclic structural
motifs.1 Ever since Lu’s pioneering work on phosphine-
catalyzed [3 + 2] annulation between electron-deficient allenes
and activated alkenes,2a the field of phosphine catalysis thrived
and a broad range of cyclization methods have been developed
over the years, including [3 + 2],2 [4 + 1],3 [4 + 2],4 and [4 +
4]5 annulation reactions, among others.6 Electron-deficient
allenes are undoubtedly the most common substrates among
all the suitable reaction partners in phosphine catalysis. In stark
contrast, alkynes are much less employed, likely due to the
difficulty associated with their activations. From a practical
viewpoint, alkynes are readily available; therefore, broad
utilization of alkyne substrates in phosphine-mediated
annulation reactions would be highly desirable. Among
different alkynes, the best studied substrates are 2-butynoate-
type alkynes. Although 2-butynoates were more often used in
asymmetric γ-additions,7 their utilizations in asymmetric
annulations are rare.8 Ynones are unique alkynes possessing
an acidic α′-proton, which were utilized as a C3 synthon in a
handful of [3 + 2] annulations by the groups of Fu,9a Shi,9b
Huang,9c and Guo.9d In comparison with 2-butynoates and
ynones, 3-butynoates were even less utilized and this is
somewhat surprising given the fact that this class of alkynes are
synthetically readily accessible.10 To the best of our knowl-
edge, there was only one report to date by Loh and co-workers
in which they described the utilization of 3-butynoate in (R,R)-
DIPAMP-catalyzed enantioselective [3 + 2] annulation with
chalcones.11 Apparently, a practical and general enantioselec-
tive cyclization strategy that makes use of 3-butynoates as a
viable reaction partner for the creation of important chiral
phosphine catalysis is how to effectively activate this type of
alkynes. In Loh’s elegant example, highly nucleophilic
bisphosphine (R,R)-DIPAMP was utilized for isomerizing 3-
butynoates to the corresponding allenes and subsequently
promoted the annulation reaction (Figure 1). We noticed that,
in the literature reports, nitrogen Lewis base-catalyzed
isomerization of 3-butynoates to allenoates was well
documented.12 We therefore envisioned that a binary catalytic
system containing both an amine and a phosphine may offer a
simple and practical solution to the activation of 3-butynoates.
The achiral amine catalyst and chiral phosphine catalyst are
Figure 1. 3-Butynoates in asymmetric phosphine catalysis.
Received: February 21, 2020
https://dx.doi.org/10.1021/acs.orglett.0c00681
© XXXX American Chemical Society
Org. Lett. XXXX, XXX, XXX−XXX
A

Organic Letters
pubs.acs.org/OrgLett
Letter
a
anticipated to work in a synergistic manner; the amine
facilitates the isomerization of alkyne substrate, and the
resulting allenes will then be promoted by chiral phosphine
for subsequent enantioselective transformations (Figure 2).
Table 1. Initial Screening
Figure 2. Our working hypothesis.
Moreover, the possibility of combining different amines and
various phosphines adds in another attractive feature to this
proposed strategy, a tunable system will enable different
transformations of 3-butynoates, thus making them broadly
applicable in phosphine-catalyzed asymmetric reactions.
Organofluorine compounds are molecules of great impor-
tance in pharmaceutical industry and agrochemistry, and their
asymmetric synthesis is one of the most actively pursued areas
in organic synthesis. Chiral cyclopentenes containing a
trifluoromethylated stereogenic center are structural scaffolds
that are widely present in many bioactive molecules,13 and an
efficient method for creating CF3-containing stereogenic
centers has always been well-sought.14 Herein, we demonstrate
that simple combination of achiral amine and chiral
phosphines constitutes a powerful catalytic system for the
activations of 3-butynoates, leading to the creation of optically
enriched cyclopentenes bearing a CF3-containing stereogenic
center.
b
c
d
entry
phosphine
dr
yield (%)
ee (%)
e
1
(R,R)-DIPAMP
(R,R)-DIPAMP
3:1
70
90
82
74
95
91
85
76
81
84
87
trace
−
70
84
12
12
18
24
77
70
−65
30
99
−
2
3
4
5
6
7
8
9
10
11
>20:1
7:1
3a
3b
3c
3d
3e
3f
3g
3h
6:1
>20:1
>20:1
1:2
6:1
8:1
9:1
NUSIOC-Phos
NUSIOC-Phos
12:1
f
12
13
14
−
−
g
−
96
h
NUSIOC-Phos
11:1:2
75
We started our investigations by examining the reaction
between 3-alkynoate 1a and enone 2a in the presence of
amine/phosphine binary catalytic systems (Table 1). We first
tested the annulation reaction with (R,R)-DIPAMP,11 and
poor dr, moderate yield, and enantioselectivity were observed
(entry 1). Subsequently, we treated 3-alkynoate 1a with
triethylamine for 12 h, after which enone 2a and (R,R)-
DIPAMP were introduced. With this protocol, the [3 + 2]
annulation proceeded smoothly and cyclopentene 4a with
three contiguous stereogenic centers was obtained in good
yield and ee (entry 2). We then turned to our amino acid-
derived bifunctional phosphines. Both L-valine-derived catalyst
3a and L-threonine-derived phosphine-amide 3b were
ineffective in asymmetric induction and so were phosphine-
thioureas 3c and 3d (entries 3−6). Dipeptide-based
phosphines 3e−3h led to much improved enantioselectivities
but still only modest at most (entries 7−10). When our
recently developed NUSIOC-Phos15 was evaluated, we were
delighted to discover that the [3 + 2] annulation product 4a
was formed in high chemical yield, with good diastereose-
lectivity, and with 99% enantiomeric excess (entry 11). Next,
we evaluated the necessity of having both amine and
phosphine in the catalytic system. When the reaction was
performed in the presence of only NUSIOC-Phos, a trace
amount of product was detected, suggesting the important role
that triethylamine had played in the reaction (entry 12). If the
reaction was performed without NUSIOC-Phos, only isomer-
ization to the corresponding γ-substituted allenoate was
a
Reactions were performed by treating 1a (0.12 mmol) in toluene (1
mL) with Et3N (0.13 mmol) and stirred for 12 h at room
temperature, followed by the addition of 2a (0.10 mmol) and the
b
catalyst (10 mol %). Determined by crude 1H NMR analysis.
c
d
Isolated yield of the major diastereomer. Determined by HPLC
e
analysis on a chiral stationary phase. The reaction was performed
with 1a, 2a, and (R,R)-DIPAMP (10 mol %) in toluene. The reaction
was carried out in the presence of NUSIOC-Phos. The reaction was
performed in the presence of only Et3N. One-pot reaction was
f
g
h
carried out.
observed (entry 13). Lastly, we carried out one-pot reaction
and the results were less satisfactory. When triethylamine and
phosphine were added in a one-step, one-pot fashion, although
4a was obtained in good yield and excellent ee value, another
diastereomer was formed during the reaction (entry 14).
The effects of different nitrogen bases were next evaluated
(Table 2). Compared with triethylamine, N,N-diisopropyle-
thylamine (DIPEA) displayed virtually the same catalytic
activities, and high yield and an excellent ee value were
attainable (entry 2). Surprisingly, the employment of 1,4-
diazabicyclo[2.2.2]octane (DABCO) led to very poor yield of
the desired product. In addition, another unidentified
diastereomer was formed as the major product (entry 3).
Inorganic base, such as Cs2CO3, could also be used; while
diastereo- and enantio- selectivities remained excellent, the
yield was only modest (entry 4). Subsequent solvent screening
B
https://dx.doi.org/10.1021/acs.orglett.0c00681
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
pubs.acs.org/OrgLett
Letter
a
substituents were excellent substrates, and highly function-
alized cyclopentenes were obtained in good yields, with high
diastereoselectivities and nearly perfect enantioselectivities
(entries 1−12). Furthermore, different aryl-substituted 3-
butynoates were also found to be suitable, and good yields,
high diastereoselectivities, and extremely high enantioselectiv-
ities were attainable (entries 13−20). The absolute config-
urations of cyclopentenes 4 were assigned by analogy based on
the X-ray crystallographic analysis of product 4s.17 The
employment of alkyl-substituted 3-butynoate or β-trifluoro-
methyl enones in the reaction failed to yield the desired
product.16
Table 2. Examination of Base and Solvent
b
c
d
entry
base
Et3N
DIPEA
DABCO
Cs2CO3
Et3N
Et3N
Et3N
Et3N
solvent
dr
yield (%)
ee (%)
1
2
3
4
5
6
7
8
toluene
toluene
toluene
toluene
EtOAc
CH2Cl2
CHCl3
dioxane
12:1
12:1
5:1:17
10:1
9:1
87
85
14
71
80
67
72
84
99
98
−
99
98
76
98
97
3:1
4:1
The isomerization process of 3-butynoate 1a facilitated by
1
11:1
triethylamine was studied by H NMR analysis. The freshly
a
prepared 3-butynoate 1a contains 7.6% of allenoate 1a′. With
the addition of triethylamine, the percentage of allenoate in the
system increased rapidly within the first few hours and then
reached equilibrium (83%) at around 12 h (Supporting
Information). When NUSIOC-Phos and enone 2 were
introduced to the system, the [3 + 2] annulation took place,
which further shifted the equilibrium between 3-butynoate 1a
and allenoate 1a′ toward creation of more 1a′, until 1a was
fully consumed.
In conclusion, we introduced a novel concept of utilizing an
amine−phosphine binary catalytic system for effective
activation of 3-butynoate substrates in phosphine catalysis. In
the presence of triethylamine and NUSIOC-Phos, the [3 + 2]
annulation between 3-butynoates and trifluoromethylated
enones proceeded smoothly, affording trifluoromethylated
cyclopentenes with three contiguous stereogenic centers in
good yields, high diastereoselectivities, and nearly perfect
enantioselectivities. Notably, the method disclosed herein
allows for general and potentially broad application of 3-
butynoates in phosphine-mediated asymmetric annulation
reactions.18 Our investigations in this direction are ongoing.
Reactions were performed by treating 1a (0.12 mmol) in solvent (1
mL) with the base specified (0.13 mmol) and stirred for 12 h at room
temperature, followed by the addition of 2a (0.10 mmol) and
b
NUSIOC-Phos (10 mol %). Determined by 1H NMR analysis of the
c
d
crude mixture. Isolated yield of the major diastereomer. Determined
by HPLC analysis on a chiral stationary phase.
was performed, and toluene remained to be the most suitable
solvent for this reaction (entries 5−8). Notably, the reaction
also proceeded smoothly with 20 mol% Et3N employed in the
first step, leading to the formation of the annulation product
with virtually the same efficiency.
With the optimized reaction conditions in hand, we
proceeded to examine the general applicability of the method
(Table 3). Trifluoromethylated enones 2 bearing different aryl
a
Table 3. Reaction Scope
yield
c
ee
b
d
entry
R1 (1)/R2(2)
Ph (1a)/Ph (2a)
Ph (1a)/4-ClC6H4 (2b)
Ph (1a)/3-ClC6H4 (2c)
Ph (1a)/2-ClC6H4 (2d)
Ph (1a)/4-BrC6H4 (2e)
Ph (1a)/4-FC6H4 (2f)
Ph (1a)/4-CNC6H4 (2g)
Ph (1a)/4-MeC6H4 (2h)
Ph (1a)/4-MeOC6H4 (2i)
Ph (1a)/3,4-(MeO)2C6H3 (2j)
Ph (1a)/2-naphthyl (2k)
Ph (1a)/2-thienyl (2l)
4-ClC6H4 (1b) /Ph (2a)
3-ClC6H4 (1c) /Ph (2a)
4-FC6H4 (1d) /Ph (2a)
2-FC6H4 (1e) /Ph (2a)
4-BrC6H4 (1f) /Ph (2a)
4-EtC6H4 (1g) /Ph (2a)
4-MeC6H4 (1h) /Ph (2a)
4-MeOC6H4 (1i) /Ph (2a)
4
dr
(%)
(%)
ASSOCIATED CONTENT
■
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
4a
4b
4c
4d
4e
4f
4g
4h
4i
12:1
13:1
13:1
10:1
16:1
15:1
15:1
14:1
8:1
13:1
17:1
16:1
9:1
87
89
90
75
80
92
90
91
81
72
95
93
78
73
81
70
71
89
88
82
99
99
99
91
98
99
95
99
99
99
98
98
99
98
99
99
98
99
99
99
sı
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.orglett.0c00681.
Experimental details, analytical data and HPLC
chromatograms of products, and copies of NMR spectra
(PDF)
Accession Codes
4j
4k
4l
CCDC 1902409 contains the supplementary crystallographic
data for this paper. These data can be obtained free of charge
via www.ccdc.cam.ac.uk/data_request/cif, or by emailing
data_request@ccdc.cam.ac.uk, or by contacting The Cam-
bridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
4m
4n
4o
4p
4q
4r
4s
4t
8:1
10:1
8:1
8:1
AUTHOR INFORMATION
15:1
16:1
12:1
■
Corresponding Authors
Yixin Lu − Department of Chemistry, National University of
Singapore, 117543, Singapore; Joint School of National
University of Singapore and Tianjin University, International
Campus of Tianjin University, Fuzhou, Fujian 359297, PR
China; National University of Singapore (Suzhou) Research
Institute, Suzhou, Jiangsu, PR China 215123; orcid.org/
0000-0002-5730-166X; Email: chmlyx@nus.edu.sg
a
Reactions were performed by treating 1 (0.12 mmol) in toluene (1
mL) with Et3N (0.13 mmol) and stirred for 12 h at room
temperature, followed by the addition of 2 (0.10 mmol) and
b
NUSIOC-Phos (10 mol %). Determined by 1H NMR analysis of the
c
d
crude mixture. Isolated yield of the major diastereomer. Determined
by HPLC analysis on a chiral stationary phase.
C
https://dx.doi.org/10.1021/acs.orglett.0c00681
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
pubs.acs.org/OrgLett
Letter
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Kuiling Ding − State Key Laboratory of Organometallic
Chemistry, Center for Excellence in Molecular Synthesis,
Shanghai Institute of Organic Chemistry, Chinese Academy of
Sciences, Shanghai 200032, China; orcid.org/0000-0003-
4074-1981; Email: kding@mail.sioc.ac.cn
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Huanzhen Ni − Department of Chemistry, National University
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1713-3323
Yee Lin Wong − Department of Chemistry, National University
of Singapore, 117543, Singapore
Mingyue Wu − Department of Chemistry, National University of
Singapore, 117543, Singapore
Zhaobin Han − State Key Laboratory of Organometallic
Chemistry, Center for Excellence in Molecular Synthesis,
Shanghai Institute of Organic Chemistry, Chinese Academy of
Sciences, Shanghai 200032, China
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Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.orglett.0c00681
Notes
The authors declare no competing financial interest.
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Substituted pyroolopyridines and pyrrolopyrazines for treating cancer or
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(16) When n-Pr-substituted 3-butynoate was employed to react with
enone 2a, only a trace amount of product was observed. When
phenethyl-substituted β-trifluoromethyl enone was reacted with 3-
butynoate 1a, no desired annulation product was observed.
(17) CCDC 1902409 contains the supplementary crystallographic
data for this paper. These data can be obtained free of charge from
The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.
uk/data_request/cif.
ACKNOWLEDGMENTS
■
Y.L. acknowledges the Singapore National Research Founda-
tion, Prime Minister’s Office for the NRF Investigatorship
Award (Grant R-143-000-A15-281). Financial support from
the National University of Singapore (Grants R-143-000-695-
114 and C-141-000-092-001) and the National Natural
Science Foundation of China (Grant 21672158) are also
gratefully acknowledged. K.D. thanks NSFC (Grant
21790333) and CAS (Grants QYZDY-SSW-SLH012 and
XDB20000000) for generous financial support.
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Org. Lett. XXXX, XXX, XXX−XXX