Highly Enantioselective [2+2+2] Cycloaddition of Enediynes Enabled by Cobalt/Organophotoredox Cooperative Catalysis

Article abstract of DOI:10.1021/acscatal.1c02410

Herein, we report dual cobalt and photoredox catalysis enabled [2+2+2] cycloaddition of enediynes to produce tricyclic cyclohexadienes bearing a quaternary bridgehead carbon. A variety of enediynes were used, and the corresponding cyclohexadienes were obtained in good to high yields. The use of a chiral ligand, (S)-Segphos, enabled a highly enantioselective reaction allowing access to highly enantio-enriched cyclohexadienes.

Full text of DOI:10.1021/acscatal.1c02410

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Letter
Highly Enantioselective [2+2+2] Cycloaddition of Enediynes Enabled
by Cobalt/Organophotoredox Cooperative Catalysis
Takeshi Yasui,* Rine Tatsumi, and Yoshihiko Yamamoto*
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ABSTRACT: Herein, we report dual cobalt and photoredox
catalysis enabled [2+2+2] cycloaddition of enediynes to produce
tricyclic cyclohexadienes bearing a quaternary bridgehead carbon.
A variety of enediynes were used, and the corresponding
cyclohexadienes were obtained in good to high yields. The use
of a chiral ligand, (S)-Segphos, enabled a highly enantioselective
reaction allowing access to highly enantio-enriched cyclohex-
adienes.
KEYWORDS: cobalt catalysis, asymmetric catalysis, metallaphotoredox, cycloaddition, enediynes
ver the past several decades, group 9 metal (Co, Rh, and
OIr) catalysts have contributed to the development of
[2+2+2] cycloaddition of unsaturated compounds, such as
alkynes, alkenes, and nitriles, which is one of the most efficient
methods for accessing polycyclic compounds in a highly atom-
economical fashion.1 In particular, the [2+2+2] cycloaddition
of enediynes can provide tricyclic cyclohexadienes, which can
be further transformed via functionalization of the diene
moiety, allowing access to complex sp3-rich molecules.2
Surprisingly, although nonprecious metal catalysts have gained
attention because of their economic and ecological benefits,
examples of the cobalt-catalyzed [2+2+2] cycloaddition of
enediynes are still very limited.3 Cyclopentadienyl cobalt(I)
complexes can facilitate the [2+2+2] cycloaddition of
enediynes, although in most cases, a stoichiometric amount
of cobalt is required because of the tendency of the cobalt
complex to form a stable η4-(1,3-cyclohexadiene) complex with
the cyclohexadiene product.4 Alternatively, low-valent cobalt
species generated in situ from an air-stable CoII salt (CoX2)
with phosphine or NHC ligands, using reductants such as Zn
and Mn, can also be used for the [2+2+2] cycloaddition of
enediynes, although a stoichiometric amount of cobalt salt is
still required (Scheme 1A).5 In 2013, Amatore, Aubert, Petit,
and co-workers reported the [2+2+2] cycloaddition of
enediynes using a catalytic amount of CoH(PMe3)4;6 however,
such CoI complexes are difficult to handle owing to their
instability.
with an amine reductant in the presence of an Ir photoredox
catalyst under blue LED irradiation.8 However, the cyclo-
addition reaction using the Co/photoredox cooperative
catalyst system is limited to the cycloaddition of 1,6-diynes
and alkynes, which can also be facilitated by conventional
catalyst systems such as CoII/Zn, RhI, and IrI.
Our group has studied the construction of tricyclic
frameworks bearing quaternary bridgehead carbons that are
found in many biologically active natural products, such as 11-
O-debenzoyltashironin, perforanoid A, and jiadifenolide.9
Recently, we reported a Ru-catalyzed [2+2+2] cycloaddition
of ene-yne-yne enediynes to provide 5−6−5 tricyclic
lactones;2b however, the enantioselective version of this
reaction has not been reported to date. Herein, we
demonstrate the [2+2+2] cycloaddition of enediynes mediated
by Co/photoredox cooperative catalysis, wherein a broad
variety of enediynes can be used (Scheme 1B). An
enantioselective reaction was also achieved using a chiral
bisphosphine ligand. To the best of our knowledge, this is the
first example of a highly enantioselective cycloaddition using
Co/photoredox cooperative catalysis.10
During our study on the transition-metal-catalyzed cyclo-
addition of enediynes, we found that known catalyst systems,
such as CoII/Zn, RhI, and IrI, were not effective for the
cycloaddition of enediyne 1a, while the Co/photoredox
Received: May 29, 2021
Revised: July 13, 2021
Published: July 15, 2021
Recently, the merger of transition-metal catalysis and
photocatalysis, known as metallaphotocatalysis, has emerged
as a powerful tool that enables versatile chemical trans-
formations.7 Rovis and co-workers, in a pioneering study,
reported the [2+2+2] cyclotrimerization of alkynes catalyzed
by in situ generated low-valent Co species from CoBr2(PCy3)2
https://doi.org/10.1021/acscatal.1c02410
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Letter
cooperative catalysis gave the corresponding cyclohexadiene 2a
in good yield (Table S1). After extensively screening the
reaction parameters, 2a was obtained in 94% yield on using 1
mol % of 4CzIPN,11 5 mol % of CoCl2, 10 mol % of PPh3, and
25 mol % of Et3N in THF, under blue light irradiation (448
nm) (Table 1, entry 1). The reaction using CoBr2 as a
precatalyst gave 2a in 94% yield (entry 2). The amount of
catalyst could be reduced to 1 mol % when CoCl2(PPh3)2,
prepared from CoCl2 and PPh3, was used, affording 2a in 91%
yield (entry 3). The reaction with [Ir(dF(CF3)2ppy)-
(dtbbpy)]PF6 instead of 4CzIPN resulted in a slightly lower
yield of 2a (entry 4). Replacement of PPh3 with several other
phosphine ligands, including PCy3, did not improve the yield
Scheme 1. Cobalt-Mediated [2+2+2] Cycloadditions of
Enediynes
i
of 2a (entry 5; also see Table S4). When Pr2NEt was used as
the reductant, the yield of 2a was slightly reduced (entry 6).
Switching the reaction solvent from THF to MeCN provided
2a in a decreased yield (entry 7). The formation of 2a was not
detected in the absence of either CoCl2, Et3N, 4CzIPN, or
light, demonstrating the vital role for all these components
(entry 8).
Next, the [2+2+2] cycloaddition reaction using various
enediynes 1 was examined under the optimized reaction
conditions (Scheme 2). Substrates bearing aryl, heteroaryl, and
alkyl groups at the alkyne terminus gave the corresponding
products 2b−2j in high yields, while the reaction of enediyne
1k, bearing a terminal alkyne moiety, resulted in a moderate
yield of the product 2k. Enediyne 1l bearing a nonprotected
alcohol could be transformed into 2l in moderate yield,
although the isolated yield was lesser as the column
chromatography had to be repeated twice to completely
remove the impurities. As an alternative method to introduce a
hydroxy group, the TBS-protected substrate 1m was
successfully converted into cycloadduct 2m. The introduction
of a methyl group (1n) at the alkene moiety instead of the
ethoxycarbonyl group did not affect the reactivity, and 2n was
still obtained in high yield. The cycloaddition of enediyne 1o
(R2 = H) proceeded smoothly, and 2o was obtained in high
yield without the formation of any β-hydride elimination
products. Enediyne 1p, bearing a cyclopentene moiety, was
successfully converted into tetracyclic cyclohexadiene 2p.
Enediynes bearing a 1,2-disubstituted or 1,1,2-trisubstituted
alkene moiety were successfully transformed to the corre-
sponding cycloadducts (2q−2s) as single diastereomers based
on the stereochemistry of the double bond. With respect to the
tether moieties, tosylamide, malonate, amide, and ester groups
could be incorporated, affording 2t−2x in good to excellent
yields. Methyl, phenyl, and benzyloxymethyl groups, as well as
a hydrogen atom, were successfully introduced into the alkene
moiety of ester-tethered enediynes, leading to tricyclic lactones
2y−2bb in good yields. The reaction of enediyne 1cc bearing
an unsubstituted propiolate terminus resulted in the formation
of 2cc in low yield, which could be ascribed to the competing
intermolecular oligomerization reaction due to the relatively
high reactivity of the propiolate moiety. Regarding the ring size
of the cycloadducts, while 6−6−5 tricyclic cyclohexadiene
(2dd) was successfully prepared in good yield, 5−6−6 tricyclic
cyclohexadiene (2ee) was not obtained. The use of optically
active enediyne 1ff afforded the corresponding cyclohexadiene
2ff in 63% yield with 2:1 dr. Notably, the intermolecular
cycloaddition afforded bicyclic cyclohexadiene 2gg in 60%
yield.
Table 1. Selected Optimization Study
a
entry
deviation from standard conditions
yield of 2a [%]
1
2
3
none
CoBr2
1 mol % CoCl2(PPh3)2
[Ir(dF(CF3)2ppy)(dtbbpy)]PF6 instead of 4CzIPN
PCy3 instead of PPh3
iPr2NEt instead of Et3N
94
94
91
88
73
90
59
0
b
4
5
6
7
8
MeCN as solvent
without CoCl2, Et3N, 4CzIPN, or light
a
Determined by 1H NMR analysis of the crude reaction mixture.
Using purple LEDs (385 nm) instead of blue LEDs.
b
There are two possible pathways for the cycloaddition of
enediynes, viz., via a metallacyclopentadiene intermediate or a
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Scheme 2. Scope of the [2+2+2] Cycloaddition of Enediynes 1
a
b
c
NMR yield. A substrate (enediyne or diyne) was slowly added over 1 h and the mixture was stirred for additional 10 min. The reaction was
performed for 2 h.
metallacyclopentene intermediate. To elucidate the reaction
mechanism, the reaction of 1i, under standard conditions, was
conducted in the presence of an excess amount of methyl
propargyl ether (MPE) to trap the cobaltacycle intermediate.
This reaction yielded phthalan 3 in 47% yield along with 2i in
18% yield; however, formation of cyclohexadiene 4 was not
observed (Figure 1A). Moreover, intermolecular cycloaddition
of enyne 5 and MPE did not proceed (Figure 1B). These
results suggest that the cycloaddition of 1 is initiated by the
yne-yne coupling leading to a cobaltacyclopentadiene inter-
mediate. Next, we investigated the valence of the Co species
involved in the photoinduced catalytic cycle. In the presence of
a catalytic amount of CoCl(PPh3)3, the cycloaddition of 1a
proceeded smoothly without irradiation with LED light
(Figure 1C). If CoI species, such as CoCl(PPh3)3, are the
active catalyst, continuous irradiation would not be required to
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Scheme 3. Enantioselective [2+2+2] Cycloaddition of
Selected Substrates
a
b
The reaction was performed for 2 h. Enediyne was slowly added
over 1 h and the mixture was stirred for additional 10 min.
in Figure 1D. The intensity of the absorptions derived from the
ester carbonyl groups of 1a and 2a was reduced or increased
during irradiation, while the transformation of 1a into 2a was
suppressed during darkness. This result implies that the CoI
species is not generated in this catalytic system. On the basis of
these results, we propose a plausible reaction mechanism, as
shown in Figure 1E. At the initial stage of the reaction, the
photoinduced reduction of a CoII precatalyst occurred to
produce Co0 species. The yne-yne coupling facilitated by the
Co0 catalyst generates the cobaltacyclopentadiene intermediate
I. Subsequent photoinduced oxidation and insertion of the
pendant alkene into the Co−Csp2 bond provides the
intermediate II, which on reductive elimination furnishes 2
and the CoI species. Finally, the Co0 species is restored by
photocatalyst-mediated reduction of the CoI species. This
mechanism is consistent with the fact that the reaction can be
regulated by turning ON and OFF the light, as originally
suggested by Rovis et al.8a
Figure 1. Mechanistic study.
promote the reaction because the CoI species, generated from
the CoII species during the initial irradiation, would facilitate
the reaction without any further irradiation. To test this
hypothesis, we performed a real-time analysis of the reaction of
1a using in situ FTIR spectroscopy. The reaction profiles
under alternating periods of irradiation and darkness are shown
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Notes
Finally, we investigated the enantioselective version of this
reaction using a chiral ligand. Several chiral biaryl bisphosphine
ligands were examined for the cycloaddition of 1a, leading to
the identification of (S)-Segphos as the most appropriate
ligand for enabling a highly enantioselective cycloaddition
reaction (Table S4). The results of the enantioselective
cycloaddition using the selected substrates are summarized in
Scheme 3. A variety of enediynes were applicable and the
enantio-enriched 5−6−5 tricyclic products were obtained in
moderate to high yields with excellent enantioselectivities.
However, several enediynes such as 1s, 1t, and 1dd failed to
undergo the cycloaddition. The intermolecular cycloaddition
also did not proceed with (S)-Segphos. Considering that the
reactivity seems to be affected by the structure around the
alkene moiety in the intramolecular reaction, it might be
assumed that the alkene insertion process is problematic due to
the relatively narrow cavity of the Co-Segphos complex which
enables the high enantioselectivity. The absolute configuration
of (S)-2a was determined by X-ray diffraction analysis of 7,
which was prepared from 2a (>99:1 er) in four steps (Scheme
S2).12 Moreover, the reaction of (S)-1ff using (S)-Segphos
furnished 2ff with excellent diastereoselectivity (>99:1 dr). In
contrast, the use of (R)-Segphos afforded 2cc with 1:10 dr,
albeit in low yield, indicating that the stereoselectivity of the
quaternary bridgehead carbon is controlled by the catalyst
rather than the substrate.
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This research was partially supported by the Platform Project
for Supporting Drug Discovery and Life Science Research
(Basis for Supporting Innovative Drug Discovery and Life
Science Research (BINDS) from AMED under Grant Number
JP21am0101099) and JSPS KAKENHI (Grant Number
JP21K05051).
REFERENCES
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In conclusion, we have developed a [2+2+2] cycloaddition
of enediynes mediated by cobalt and photoredox cooperative
catalysis. Several control experiments suggest that the reaction
proceeds via the cobaltacyclopentadiene intermediate formed
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addition, highly enantioselective cycloaddition was achieved
using (S)-Segphos as a chiral ligand.
ASSOCIATED CONTENT
■
sı
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acscatal.1c02410.
Additional tables of optimization data, experimental
1
procedures, characterization data, copies of H NMR
and 13C NMR spectra of all new compounds (PDF)
Crystallographic data for 7 (CIF)
AUTHOR INFORMATION
■
Corresponding Authors
Takeshi Yasui − Department of Basic Medicinal Sciences,
Graduate School of Pharmaceutical Sciences, Nagoya
University, Nagoya 464-8603, Japan; orcid.org/0000-
0002-7630-8736; Email: t-yasui@ps.nagoya-u.ac.jp
Yoshihiko Yamamoto − Department of Basic Medicinal
Sciences, Graduate School of Pharmaceutical Sciences,
Nagoya University, Nagoya 464-8603, Japan; orcid.org/
0000-0001-8544-6324; Email: yamamoto-yoshi@
ps.nagoya-u.ac.jp
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(c) Ravetz, B. D.; Wang, J. Y.; Ruhl, K. E.; Rovis, T. Photoinduced
Ligand-to-Metal Charge Transfer Enables Photocatalyst-Independent
Light-Gated Activation of Co(II). ACS Catal. 2019, 9, 200−204.
Author
Rine Tatsumi − Department of Basic Medicinal Sciences,
Graduate School of Pharmaceutical Sciences, Nagoya
University, Nagoya 464-8603, Japan
Complete contact information is available at:
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(12) X-ray crystallographic data for compound 7 have been
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details, see the Supporting Information.
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