Copper-Catalyzed Chemoselective Silylative Cyclization of 2,2′-Diethynylbiaryl Derivatives

Article abstract of DOI:10.1021/acs.orglett.1c00968

In this protocol, copper-catalyzed diverse silylative carbocyclization reactions of 2,2′-diethynylbiaryl derivatives with silaboronate were reported. Three new and novel types of domino reactions for the copper-catalyzed transformation of silaboronate were discovered. The corresponding cyclobuta[l]phenanthrene, bis((silyl)methyl)phenanthrene, and silyl-substituted exocyclic diene products were chemoselectively formed with high efficiency.

Full text of DOI:10.1021/acs.orglett.1c00968

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Letter
Copper-Catalyzed Chemoselective Silylative Cyclization of 2,2′-
Diethynylbiaryl Derivatives
Meng Zhao, Ying Wang, Zi-Lu Wang, Jian-Lin Xu, Kai-Yang Dai, and Yun-He Xu*
Cite This: Org. Lett. 2021, 23, 3859−3863
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ABSTRACT: In this protocol, copper-catalyzed diverse silylative
carbocyclization reactions of 2,2′-diethynylbiaryl derivatives with
silaboronate were reported. Three new and novel types of domino
reactions for the copper-catalyzed transformation of silaboronate
were discovered. The corresponding cyclobuta[l]phenanthrene,
bis((silyl)methyl)phenanthrene, and silyl-substituted exocyclic
diene products were chemoselectively formed with high efficiency.
Scheme 1. Metal-Catalyzed C−Si Bond Formation
rganosilicon reagents play a crucial role in organic
O
synthesis.1 During the past several decades, organo-
silicon chemistry has substantially matured, and many new
methodologies have been discovered for the construction of
different C−Si bonds.2 Among them, classic hydrosilylation is
one of the most direct and powerful methods for introducing
the silyl group into unsaturated molecules (Scheme 1, 1a).3 It
should be noticed that the hydrosilylation strategy is limited to
the addition reaction pathway for the synthesis of organo-
silicon compounds, which hampers the magnification of the
product scope to a considerable extent. To solve this problem,
other silyl sources were explored to enhance the conversion
ability.4 In new approaches, the copper-catalyzed C−Si bond
formation using silaboronate reagents via the activation of the
Si−B interelement bond has achieved rapid development due
to its flexible reaction styles,5 including the currently developed
protosilyaltion,6 nucleophilic substitution,7 silaboration,8 and
radical addition (Scheme 1, 1b).9 However, it is worth pointing
out that the construction of complex molecules via this strategy
still has much room for development. Recently, copper-
catalyzed silylation-induced domino reactions have gradually
appeared as powerful tools to prepare functionalized organic
compounds with linear structures (Scheme 1, 1b).10
Compared with other transition-metal-catalyzed carbocycliza-
tion reactions of diynes for constructing cyclic molecular
skeletons,11 so far, examples of copper-catalyzed silylative
cyclization have very rarely been realized. In 2015, Tian and
coworkers elegantly demonstrated a copper-catalyzed asym-
metric silylative cyclization of cyclohexadienone-tethered
allenes to access bicyclo[4.3.0]nonanes.12 Following this
work, the enantioselective case of 1,6-enynes was reported by
the same group.13
Received: March 23, 2021
Published: May 10, 2021
Recently, during our investigation of the copper-catalyzed
silaboration of alkynes, the unexpected silaborative carbocyc-
https://doi.org/10.1021/acs.orglett.1c00968
© 2021 American Chemical Society
Org. Lett. 2021, 23, 3859−3863
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lization of hepta-1,6-diyne and octa-1,7-diyne was observed.
To expand the utility of this copper-catalyzed silaborative
carbocyclization of diynes for the synthesis of useful
phenanthrene derivatives, 2,2′-diethynylbiphenyl was tried as
a substrate. Surprisingly, the designed reaction did not work,
whereas three previously undeveloped and novel copper-
catalyzed domino reactions including the silycupration/dual
carbocyclization/protonation to form cyclobuta[l]-
phenanthrene (Schemes 1 and 2A), the silycupration/
exocyclic diene 4a, and bis((silyl)methyl)phenanthrene 5a,
three interesting products, were formed. Our aim was to
improve the reaction chemoselectivities and the yields of the
desired products. Significantly, when 10 mol % CuTC as a
catalyst, 12 mol % PPh3 as a ligand, and 2.0 equiv of Et3N as an
additive were applied in this reaction, the ratio of product 3a of
the three products was significantly increased (Table 1, entry
3). Therefore, other copper salts were screened to catalyze this
reaction. When CuCl2 was used, bis((silyl)methyl)-
phenanthrene 5 as a major product was obtained in 38%
yield (Table 1, entry 4). In contrast, product 3a was observed
when Cu(OTf)2 (copper(II) trifluoromethanesulfonate) was
employed (60% yield, Table 1, entry 5). To further improve
the yield of the cyclobuta[l]phenanthrene product 3a, different
alcohols as the solvent were investigated (Table 1, entries 6
and 7). With an increase in the steric hindrance of alcohol, the
yield of product 3a was improved. Finally, 3a could be
obtained in 82% isolated yield by using 1.5 equiv of PhMe2Si-
Bpin (2) in tert-amylalcohol solution (Table 1, entry 8). We
further optimized the reaction conditions for the highly
selective synthesis of product 4a. We noticed that the CuCl2
catalyst could favor the formation of exocyclic diene product
4a (Table 1, entry 4). Additionally, to achieve a higher yield of
product 4a, the loading of PhMe2Si-Bpin 2 was reduced (38%
yield, Table 1, entry 9). Therefore, after carefully screening
different phosphine ligands, the exocyclic diene product 4a
could be detected in 67% yield by using (o-tolyl)Cy2P (Table
1, entries 10−12). Furthermore, by increasing the concen-
tration of the reaction and decreasing the amount of
silylboronate reagent 2 used in the reaction, a 73% isolated
yield of 4a was obtained (Table 1, entry 14). When bipyridine
instead of the phosphine ligand was applied in reaction,
bis((silyl)methyl)phenanthrene 5a as a single product was
obtained (Table 1, entries 15−17). To improve the yield of
product 5a, ethanol was used as a proton source to screen
other solvents (Table 1, entries 18 and 19). Finally, it was
found that the desired product 5a was obtained in 85% isolated
yield with diethyl ether as the solvent (Table 1, entry 19).
Under the optimized reaction conditions (Table 1, entry 8),
the substrate scope of compound 1 to synthesize the
cyclobuta[l]phenanthrene derivatives was examined. The
results are summarized in Scheme 2. A series of substituted
cyclobuta[l]phenanthrene derivatives 3a−3l were synthesized
in good to excellent yields. The reaction was mainly affected by
the substituents on the aromatic rings. Both electron-donating
(3b, 3c, 3g, 3h, 3i) and electron-withdrawing (3d, 3e, 3j)
groups on the rings were well tolerated in this reaction.
Notably, the cyclobuta[s]picene derivative 3f could be
obtained in high yield. On the contrary, unsymmetrical
diethynylbiphenyl derivatives were also subjected to this
reaction. It was found that compound 1g with a methyl
group at the five-position of the biphenyl underwent the
protosilylation and cyclization processes to give the corre-
sponding substituted cyclobuta[l]phenanthrene product in
81% yield (3g/3g′ 71:29).14 Similarly, biphenyls with different
substituents (1h−1k) could be well tolerated to furnish the
corresponding products in good yields with moderate
regioselectivity under the standard reaction conditions.
Interestingly, when the 2′,6-diethynyl-2,3,4,5-tetrahydro-1,1′-
biphenyl (1l) reacted with silylboronate 2 under the optimized
reaction conditions, the ring-fused naphthalene derivative 3l
was isolated in 50% yield. The structure of cyclobuta[l]-
Scheme 2. Substrate Scope for the Synthesis of Products
a b
,
3
a
Condition A: The mixture of 1 (0.1 mmol), 2 (0.15 mmol),
Cu(OTf)2 (10 mol %), PPh3 (12 mol %), and Et3N (50 mol %) in
extra dry EtOH (0.2 M) was stirred at 30 °C in an oil bath for 4 h
b
c
under an argon atmosphere. Isolated yield. Regioselectivities (3/3′)
1
were determined by H NMR of the crude mixture.
carbocyclization/protonation to give silyl-substituted exocyclic
diene (Schemes 1 and 2B), and the dual silycupration/
carbocyclization/dual protonation to generate bis((silyl)-
methyl)phenanthrene (Schemes 1 and 2C) products were
achieved, respectively, with high chemoselectivity and
efficiency.
Initially, the 2,2′-diethynylbiphenyl 1a and Suginome
silaboronate 2 were selected for the model reaction to
optimize the conditions, and the results are shown in Table
1. First, the reaction was tried in the presence of CuTC
(copper(I) thiophene-2-carboxylate) as a catalyst in ethanol
solution via changing different phosphine ligands (Table 1,
entries 1−3). It was found that cyclobuta[l]phenanthrene 3a,
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a
Table 1. Optimization of Reaction Conditions
b
yield (%)
entry
[Cu] (mol %)
ligand (mol %)
solvent (extra dry)
2 (x equiv)
3a
4a
5a
1
2
3
4
5
6
7
CuTC
CuTC
CuTC
CuCl2
Cu(OTf)2
Cu(OTf)2
Cu(OTf)2
Cu(OTf)2
CuCl2
CuCl2
CuCl2
CuCl2
CuCl2
CuCl2
CuCl2
CuCl2
CuCl2
JohnPhos
BINAP
PPh3
PPh3
PPh3
EtOH
EtOH
EtOH
EtOH
EtOH
iPrOH
tAmOH
tAmOH
2.0
2.0
2.0
2.0
2.0
2.0
2.0
1.5
1.5
1.5
1.5
1.5
1.5
1.4
2.5
2.5
2.5
2.5
2.5
6
7
10
8
1
1
6
38
19
17
10
6
24
37
2
60
72
83
5
PPh3
PPh3
c
d
8
PPh3
88 (82)
9
PPh3
PtBu3
PCy3
(o-tolyl)Cy2P
(o-tolyl)Cy2P
(o-tolyl)Cy2P
6,6′-Me-bpy
dtbbpy
bpy
EtOH
EtOH
EtOH
EtOH
EtOH (0.4 M)
EtOH (0.4 M)
EtOH
EtOH
EtOH
4
28
6
4
3
38
50
59
67
73
15
2
1
13
9
8
10
11
12
13
e
d
14
3
75 (73)
15
16
17
18
19
5
58
69
53
CuCl2
CuCl2
bpy
bpy
EtOH (3.0 equiv) in DMSO
EtOH (3.0 equiv) in Et2O
d
85 (85)
a
Mixture of 0.1 mmol 1 (1.0 equiv), 2 (x equiv), copper catalyst (10 mol %), ligand (12 mol %), and Et3N (2.0 equiv) in extra dry solvent (0.2 M)
b
1
was stirred at 30 °C in an oil bath for 12 h under an argon atmosphere. (See the SI.) Determined by H NMR with the use of (CHCl2)2 as an
c
d
e
internal standard. 0.05 mmol Et3N (0.5 equiv), stirred for 4 h. Isolated yield. 0.05 mmol Et3N (0.5 equiv), stirred for 2 h. 6,6′-Me-bpy, 6,6′-
dimethyl-2,2′-bipyridine; dtbbpy, 4,4′-di-tert-butyl-2,2′-bipyridine.
a b
,
phenanthrene derivatives 3a was confirmed by X-ray single-
crystal diffraction analysis (CCDC no. 2063184).
Scheme 3. Substrate Scope for the Synthesis of Product 4
Previously, copper-catalyzed protosilylation and carbocycli-
zation domino reactions of diynes were still undiscovered.
Herein the silyl-substituted exocyclic diene product 4a was
successfully obtained in 73% isolated yield (Table 1, entry 14),
but it was found that this kind of exocyclic diene compound
could not be exposed to the air for a long time due to its partial
decomposition at ambient temperature. The reason might be
O2-induced oligomerization. This high reactivity indicated that
the silyl-substituted exocyclic dienes 4 could be a candidate for
Diels−Alder cyclization. Therefore, the substrates with both
electron-donating and electron-withdrawing groups were
representatively chosen to react with silaboronate 2 under
the optimized reaction conditions. The corresponding
products 4c and 4h were obtained in 55 and 65% isolated
yields, respectively (Scheme 3).
As shown in Scheme 4, a series of bis((silyl)methyl)-
phenanthrene compounds 5 were prepared in good yields
under the standard reaction conditions. Various functional
groups including electron-donating and electron-withdrawing
groups were also well tolerated (Scheme 4). It was believed
that the silyl-substituted exocyclic dienes were the inter-
mediates during these carbocyclization/dual protosilylation
domino reactions. To confirm the relationships of the
previously described domino reactions, the products 3a, 4a,
and 5a were isolated and subjected to the mutual trans-
formations under the corresponding standard reaction
conditions, respectively.15 It was observed that no reaction
happened between the products 3a and 4a at all. On the
a
Condition B: The mixture of 1 (0.1 mmol), 2 (0.14 mmol), CuCl2
(10 mol %), (o-tolyl)Cy2Ph (12 mol %), and Et3N (50 mol %) in
extra dry EtOH (0.4 M) was stirred at 30 °C in an oil bath for 2 h
b
c
under an argon atmosphere. Isolated yield. 60 °C in an oil bath,
stirred for 4 h. Yield was determined according to the mixed weight
and the H NMR ratio between 4c and 5c.
d
1
contrary, when compound 4a was used as the substrate,
product 5a was obtained in 88% yield.
On the basis of the previously described results and previous
work, a plausible mechanism for the synthesis of products 3a,
4a, and 5a from 2,2′-diethynylbiphenyl is depicted in Scheme
5. First, the LCu-SiMe2Ph species I could be generated from
silylboronate 2, CuX, and Et3N in alcohol. After the
silylcupration of one triple bond of 2,2′-diethylbiphenyl, a
vinyl cuprate species III would be formed. Subsequently, the
dienylcuprate intermediate IV generated in situ via carbocyc-
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experiments on exploring the diverse utilities of these
compounds are in progress in our laboratory.
Scheme 4. Substrate Scope for the Synthesis of Products
4
a b
,
ASSOCIATED CONTENT
■
sı
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.orglett.1c00968.
1
Experimental procedures, characterization data, and H,
19F, and 13C NMR spectra (PDF)
Accession Codes
CCDC 2063184 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.
AUTHOR INFORMATION
■
Corresponding Author
Yun-He Xu − Hefei National Laboratory for Physical Sciences
at the Microscale, and Department of Chemistry, University
of Science and Technology of China, Hefei, Anhui 230026, P.
R. China; State Key Laboratory and Institute of Elemento-
Organic Chemistry, Nankai University, Tianjin 300071, P. R.
China; orcid.org/0000-0001-8817-0626;
a
Condition C: The mixture of 1 (0.1 mmol), 2 (0.25 mmol), CuCl2
(10 mol %), bpy (12 mol %), Et3N (2.0 equiv), and EtOH (3.0 equiv)
in dry Et2O (0.2 M) was stirred at 30 °C in an oil bath for 12 h under
b
c
an argon atmosphere. Isolated yield. 16 h.
Email: xyh0709@ustc.edu.cn
Scheme 5. Possible Mechanistic Pathways
Authors
Meng Zhao − Hefei National Laboratory for Physical Sciences
at the Microscale, and Department of Chemistry, University
of Science and Technology of China, Hefei, Anhui 230026, P.
R. China
Ying Wang − Hefei National Laboratory for Physical Sciences
at the Microscale, and Department of Chemistry, University
of Science and Technology of China, Hefei, Anhui 230026, P.
R. China
Zi-Lu Wang − Hefei National Laboratory for Physical Sciences
at the Microscale, and Department of Chemistry, University
of Science and Technology of China, Hefei, Anhui 230026, P.
R. China
Jian-Lin Xu − Hefei National Laboratory for Physical Sciences
at the Microscale, and Department of Chemistry, University
of Science and Technology of China, Hefei, Anhui 230026, P.
R. China
Kai-Yang Dai − Hefei National Laboratory for Physical
Sciences at the Microscale, and Department of Chemistry,
University of Science and Technology of China, Hefei, Anhui
230026, P. R. China
lization would undergo two different pathways: (1) Direct
cyclization/protonation reactions to give the cyclobuta[l]-
phenanthrene 3a would happen.16 (2) The exocyclic diene
product 4a would be furnished via a protonation process. The
LCu-OR species released as catalyst V would be involved in
another catalytic cycle to activate silylboronate 2 again. Then,
the regenerated LCu-SiMe2Ph species I would further react
with the exocyclic diene product 4a. Finally, the bis((silyl)-
methyl)phenanthrene product 5a would also be generated.
In conclusion, we have developed general and straightfor-
ward catalytic approaches to access three silyl-substituted
polycyclic phenanthrene derivatives, respectively. Starting from
the readily available 2,2′-diethynylbiaryl derivatives, these
copper-catalyzed silylative cyclization domino reactions can
be applied to construct complex molecules with high efficiency
under mild conditions. The transformations of the cyclobuta-
[l]phenanthrene and the exocyclic diene products indicate
their potential applications in organic synthesis.17 Further
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.orglett.1c00968
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We gratefully acknowledge research support of this work by
the funding of the National Natural Science Foundation of
China (21871240), the State Key Laboratory of Elemento-
organic Chemistry Nankai University (202001), and the
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Fundamental Research Funds for the Central Universities
(WK2060000017).
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