Cobalt-Catalyzed Enantioselective Hydroboration/Cyclization of 1,7-Enynes: Asymmetric Synthesis of Chiral Quinolinones Containing Quaternary Stereogenic Centers

Article abstract of DOI:10.1002/anie.201903377

An asymmetric cobalt-catalyzed hydroboration/cyclization of 1,7-enynes to synthesize chiral six-membered N-heterocyclic compounds was developed. A variety of aniline-tethered 1,7-enynes react with pinacolborane to afford the corresponding chiral boryl-fun

Full text of DOI:10.1002/anie.201903377

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Accepted Article
Title: Cobalt-Catalyzed Enantioselective Hydroboration/Cyclization
of 1,7-Enynes: Asymmetric Synthesis of Chiral Quinolinones
Containing Quaternary Stereogenic Centers
Authors: Caizhi Wu, Jiayu Liao, and Shaozhong Ge
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201903377
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10.1002/anie.201903377
Angewandte Chemie International Edition
COMMUNICATION
Cobalt-Catalyzed Enantioselective Hydroboration/Cyclization of
1,7-Enynes: Asymmetric Synthesis of Chiral Quinolinones
Containing Quaternary Stereogenic Centers
Caizhi Wu, Jiayu Liao and Shaozhong Ge*
Abstract: An asymmetric cobalt-catalyzed hydroboration/cyclization
of 1,7-enynes to synthesize chiral six-membered N-heterocyclic
compounds was developed. A variety of aniline-tethered 1,7-enynes
react with pinacolborane to afford the corresponding chiral boryl-
functionalized quinoline derivatives in high yields with high
enantioselectivity. This cobalt-catalyzed asymmetric cyclization of
1,7-enyens provides a general approach to access a series of chiral
quinoline derivatives containing quaternary stereocenters.
reactions convert 1,6-enynes to five-membered carbocyclic or
heterocyclic compounds.[7] In contrast, the examples of catalytic
cyclization of 1,7-enynes, in particular, the corresponding
asymmetric reactions to prepare six-membered chiral
heterocyclic compounds,[8] are very rare.[9] This is presumably
due to the difficulty in forming a seven-membered 1,7-enyne-
metal chelating ring as compared to forming a six-membered
1,6-enyne-metal chelating ring for metal-catalyzed enyne
cyclization reactions. Instead, radical cyclization/cascade
reactions of 1,7-enynes readily occur due to the formation of
more stable six-membered cyclized radical intermediates.[10]
However, it is difficult to control the stereochemical outcomes for
these radical cyclization reactions.
Quinolines are privileged heterocycles that are present in a
variety of biologically active alkaloids and pharmaceutically
relevant molecules.[1] In particular, quinolinones are one of the
most useful and versatile families of antibacterial agents
(Scheme 1).[2] Due to their great synthetic importance, numerous
methods have been developed to prepare chiral quinoline
derivatives over the past few decades.[3] For example,
asymmetric hydrogenation and transfer hydrogenation of
quinolines have been extensively studied for this synthetic
purpose.[4] However, the majority of these approaches are
limited to quinoline derivatives bearing tertiary stereogenic
carbon centers, and enantioselective protocols to access
quinoline derivatives containing quaternary carbon centers are
extremely rare.[5] Therefore, the asymmetric synthesis of chiral
quinoline derivatives with quaternary stereocenters still remains
as a synthetic challenge.
To develop enantioselective cyclization reactions of 1,7-
enynes, we envisioned that the use of chiral catalysts based on
small transition metals, such as cobalt catalysts,[11] would help to
release the ring strain of 1,7-enyne-metal chelation. In view of
the importance of six-membered N-heterocycles in organic
synthesis and medicinal chemistry,[1,2] we are interested in
developing a Co-catalyzed asymmetric hydroboration/cyclization
of aniline-tethered 1,7-enynes to access boryl-functionalized
chiral quinoline derivatives. Aniline-tethered 1,7-enynes have
been widely employed for radical cyclization/cascade reactions
in recent years,[12] but enantioselective protocols to make chiral
quinoline derivatives from these 1,7-enynes still remain unknown.
Herein, we report
hydroboration/cyclization of 1,7-enynes to prepare chiral
quinolines and quinolin-2-ones containing quaternary
a highly enantioselective Co-catalyzed
a
stereogenic center. Furthermore, we show that chiral quinolin-2-
one products can be readily converted to other chiral quinoline
derivatives bearing two vicinal stereocenters by standard
functional group interconversions.
We initiated our studies on this cobalt-catalyzed
enantioselective hydroboration/cyclization of 1,7-enyne by
identifying effective chiral cobalt catalysts for the reaction
between the 1,7-enyne 1a and pinacolborane (HBpin). Cobalt
catalysts generated in situ from Co(acac)2 and various chiral
bisphosphine ligands were tested, and the results were
summarized in Table 1. In general, these reactions were carried
out with 1a as the limiting reagent in the presence of 3 mol%
Co(acac)2 and 4 mol% chiral ligand in toluene at room
temperature for 24 h, and the alkylboronate 2a was identified as
the major product for these reactions.
Scheme 1. Examples of bioactive natural and synthetic bioactive quinolinones
Transition-metal-catalzyed cyclization of 1,n-enynes is
considered as one of the most straightforward approaches to
access cyclic compounds.[6] The majority of these cyclization
The reaction catalyzed by the combination of Co(acac)2 and
(R,R)-DIOP (L1) proceeded sluggishly to very low conversions
of 1a and product 2a was obtained in <5% yield. The reactions
conducted by Co(acac)2 and chiral ligands L2-L7 occurred
smoothly to high conversions of 1a and afforded the desired
product 2a with modest to high isolated yields (5787%) and
with modest enantioselectivities (5483% ee). To our delight, the
reaction catalyzed by Co(acac)2 and (S,S)-Ph-BPE (L8) afforded
2a in high isolated yield (86%) and excellent enantioselectivity
[*]
C. Wu, Dr. J. Liao, Prof. Dr. S. Ge
Department of Chemistry, National University of Singapore
3 Science Drive 3, Singapore 117543 (Singapore)
E-mail: chmgsh@nus.edu.sg
Homepage: www.geresearchgroup.com
Supporting information for this article is available on the WWW
under http://angewandte.org or from the author.
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10.1002/anie.201903377
Angewandte Chemie International Edition
COMMUNICATION
(98% ee). In addition, we also tested other solvents (such as
THF, 1,4-dioxane, and cyclohexane) for the reaction catalyzed
by Co(acac)2/(S,S)-Ph-BPE, and similarly high yields and
enantioselectivities were obtained for the desired product 2a
(see the SI for the details).
anilide functionality do not undergo this cobalt-catalyzed reaction.
However, the 1,7-enyne with a MOM-protected anilide reacted
with HBpin to give a MOM-containing quinolin-2-one (2s in Table
2) in 93% yield with 94% enantioselectivity. The MOM-protection
could be readily removed by the reaction with concentrated
hydrochloric acid, yielding the corresponding quinolin-2-one (2s')
in 74% yield [eq. (3)], see the SI for the details).
Table 1. Evaluation of conditions for the asymmetric reaction of 1a.[a]
Table 2. The scope of 1,7-enynes for this hydroboration/cyclization. [a]
[a] Reaction conditions: 1a (0.200 mmol), HBpin (0.300 mmol), Co(acac)2 (6.0
mol), ligand (7.2 mol), toluene (0.5 mL), room temperature, 24 h, isolated
yields; ee was determined by chiral HPLC analysis; [b] The configuration of 2a
was determined by single-crystal X-ray analysis on the vinyl compound 3
obtained by the reaction of 2a with vinylmagnesium bromide [eq. (1)].
The scope of 1,7-enynes that undergo this cobalt-catalyzed
asymmetric reaction is summarized in Table 2. In general, a
wide range of anilide-tethered 1,7-enynes containing various
aromatic or aliphatic substituents on the alkyne unit (1b1o), on
the nitrogen atom (1p), on the aryl linker (1q1t), or on the
alkene moiety (1u1y) smoothly reacted with HBpin in the
presence of 3 mol% Co(acac)2 and 3.6 mol % (S,S)-Ph-BPE at
room temperature, yielding the corresponding enantioenriched
quinolin-2-ones (2b2y) in high yields (6296%) with excellent
enantioselectivities (9499% ee). This cobalt-catalyzed reaction
shows good functional group tolerance, and a variety of reactive
groups, such as fluoro (2g and 2t), chloro (2h), alkynyl (2i),
monosubstituted alkenyl (2j), cyano (2k and 2u), carboxylic
ester (2l and 2ab), amide (2m), and acetal (2n) groups, can be
tolerated under the identified reaction conditions. In addition,
1,7-enynes containing heteroarenes, such as thienyl (2o) and
pyridyl (2p and 2q) groups, also reacted to produce the
corresponding chiral quinolin-2-ones with high enantioselectivity.
[a] Reaction conditions: 1,7-enyne (0.200 mmol), HBpin (0.300 mmol),
Co(acac)2 (6.0 mol), (S,S)-Ph-BPE (7.2 mol), toluene (0.5 mL), room
temperature, 24 h, isolated yields; ee was determined by chiral HPLC
analysis; [b] 50 C.
As shown in Table 2, a variety of anilide-tethered 1,7-enynes
containing para- and meta-substituted aryl groups at the acetylic
position reacted to afford the desired products (2b2n) with high
enantioselectivity. However, ortho-substituted aryl groups on the
alkyne unit cannot be tolerated. Furthermore, anilide-tethered
1,7-enynes containing a terminal alkyne group do not undergo
this cobalt-catalyzed hydroboration/cyclization under the
standard conditions. For example, the reaction of the 1,7-enyne
1ac with HBpin afforded (E)-vinylboronate 2ac as the major
product [eq. (2)]. In addition, 1,7-enynes containing primary
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10.1002/anie.201903377
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We also tested oxygen- and nitrogen-tethered 1,7-enynes,
such as 1ad1af [eq. (4)], for this cobalt-catalyzed asymmetric
reaction. In the presence of Co(acac)2/(S,S)-Ph-BPE, 1ad did
not react with HBpin, 1ae reacted with HBpin to afford the
desired product 2ae in 62% yield with only 77% ee, and 1af
reacted with HBpin to form a complex mixture of hydroboration
products with only a trace amount of the desired product 2af.
However, when (R,Sp)-Josiphos (L7 in Table 1) was used
instead of (S,S)-Ph-BPE, the reactions of these enynes
proceeded smoothly and afforded the desired six-membered
cyclic compounds 2aa2ac in good yields with high
enantioselectivities [eq. (4)]. The absolute configuration of 2aa
was assigned as (S) by single-crystal X-ray diffraction analysis.
derivatives containing two vicinal stereogenic centers. For
example, the reduction of 8 with NaBH4 produced 4-hydroxyl-
3,4-dihydroquinolin-2(1H)-one 9 in 86% isolated yield with 98%
ee.[13] The reaction of 8 with phenylmagnesium bromide followed
by the oxidation with NaBO3 afforded 10, a 3,4-dihydroquinolin-
2(1H)-one containing a chiral 1,3-diol subunit, in 65% yield with
98% ee. In addition, we also showed that the reaction of 8 with
vinylmagnesium bromide followed by the reaction with I2 formed
11, a 3,4-dihydroquinolin-2(1H)-one with chiral allylic alcohol and
1,6-diene structure motifs, in 74% yield with 98% ee. Quinolin-2-
one derivatives 9-11 were isolated as a single diastereomer by
flash chromatography on silica.
With the attempt to construct tertiary stereogenic centers, we
tested the chiral cobalt catalyst generated from Co(acac)2 and
(S,S)-Ph-BPE for the reaction of one 1,7-enyne (1ag) containing
a mono-substituted alkene. However，this reaction yielded the
uncyclized hydroboration product 2ag in 83% yield [eq. (5)].
To highlight the synthetic utility of this cobalt-catalyzed
asymmetric hydroboration/cyclization, we conducted a gram-
scale reaction of the 1,7-enyne 1a with HBpin under the
standard conditions, and this reaction afforded 2a in 92%
isolated yield with 98% enantioselectivity (Scheme 2A). The
enantioenriched boryl-functionalized quinolin-2-one products can
undergo a series of stereospecific transformations to produce
the corresponding chiral quinoline derivatives without the loss of
enantiopurity. For example, 2a could be oxidized by NaBO3 to
form the chiral alcohol 4 in 95% yield with 97% ee (Scheme 2A).
Homologation of 2a with LiCH2Cl produced the chiral
alkylboronate 5 in 78% yield with 97% ee (Scheme 2A). The
oxidative hydrolysis of quinolin-2-one 2y followed by the
concomitant intramolecular alcoholysis of the carboxylic ester
afforded the spirocyclic lactone 6 in 95% isolated yield with 99%
ee (Scheme 2B). Further oxidative cleavage of the C=C bond in
6 formed the chiral spirocyclic quinolin-2,4-done 7 in 85% yield
with 98% ee (Scheme 2B).
Scheme 2. Transformations of chiral quinolin-2-ones and one-pot synthesis of
quinolin-2,4-diones.
To verify the chelating effect of 1,7-enynes to the cobalt
catalyst, we conducted the control experiments using
biarylacetylene 12 and acrylamide 13, which have similar steric
hindrance around the double and triple bonds with the anilide-
tethered 1,7-enynes. However, the reactions of 12 or 13 with
HBpin did not occur under the standard conditions [eq. (6) and
eq. (7)]. The results of these two control experiments and an
intramolecular competition reaction between a 1,7-enyne and a
more reactive vinylarene [eq. (8)] suggest that the chelation of
1,7-enyne to the cobalt catalyst enables this catalytic
hydroboration/cyclization of 1,7-enynes to occur (see the SI for a
possible pathway for this catalytic hydroboration/cyclization of
Furthermore, we combined this asymmetric cobalt-catalyzed
hydroboration/cyclization and oxidative cleavage of the C=C
bond of the resulting quinolin-2-one to develop a one-pot
procedure to synthesize chiral quinolin-2,4-diones (Scheme 2C).
The enyne 1a smoothly underwent these one-pot sequential
reactions to yield the corresponding quinolin-2,4-dione 8 in 90%
isolated yield with 98% ee. Further functionalization of the
quinolin-2,4-dione 8 allows the access of several quinolin-2-one
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In summary, we have developed a highly enantioselective
cobalt-catalyzed hydroboration/cyclization of 1,7-enynes to
prepare chiral boryl-containing six-membered cyclic compounds.
A wide range of aniline-tethered 1,7-enynes reacted with
pinacolborane to produce the corresponding chiral quinolin-2-
ones in high isolated yields with excellent enantioselectivity in
the presence of a chiral cobalt catalyst generated in situ from
Co(acac)2 and (S,S)-Ph-BPE or (R,Sp)-Josiphos ligand. The
chiral quinolin-2-one products can be readily converted to a
variety of chiral six-membered compounds, such as N-
heterocyclic primary and tertiary alcohols, alkenes, spirocyclic
lactones, and quinolin-2,4-diones. Therefore, this Co-catalyzed
asymmetric hydroboration/cyclization of 1,7-enynes provides a
general foundation to access a series of chiral quinolinine
derivatives containing quaternary stereogenic centers.
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Acknowledgements
This work was supported the Ministry of Education (MOE) of
Singapore (No. R-143-000-A07-112).
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Keywords: hydroboration/cyclization • 1,7-enynes • cobalt •
asymmetric catalysis • N-heterocycles
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[13] The absolute configuration of compound 9 was assigned as (3S,4R) by
the single-crystal X-ray analysis on the conressponding diol 9-OH
obtained by the oxidation of 9 with NaBO3, see SI for the details.
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10.1002/anie.201903377
Angewandte Chemie International Edition
COMMUNICATION
COMMUNICATION
Caizhi Wu, Jiayu Liao and Shaozhong
Ge*
Page No. – Page No.
Cobalt-Catalyzed Enantioselective
Hydroboration/Cyclization of 1,7-
Enynes: Asymmetric Synthesis of
Chiral Quinolinones Containing
Quaternary Stereogenic Centers
Asymmetric Catalysis: A highly enantioselective cobalt-catalyzed
hydroboration/cyclization of 1,7-enynes with pinacolborane (HBpin) using a catalyst
generated from Co(acac)2 and (S,S)-Ph-BPE was developed (see the Picture). This
cobalt-catalyzed asymmetric cyclization of 1,7-enyens provides a general approach
to access a series of chiral quinoline derivatives containing quaternary
stereocenters.
This article is protected by copyright. All rights reserved.