Palladium-Catalyzed (4 + 4) Annulation of Silacyclobutanes and 2-Iodobiarenes to Eight-Membered Silacycles via C-H and C-Si Bond Activation

Article abstract of DOI:10.1021/acscatal.1c00975

Construction of eight-membered silacycles via Pd-catalyzed (4 + 4) annulation of silacyclobutanes and 2-iodobiphenyl derivatives is described. This strategy involves direct C-H and C-Si bond activation followed by a ring annulation and features low catalyst loading, ligand-free conditions, and readily available starting materials. Mechanistic studies revealed the involvement of five-membered palladacycle species in the reaction.

Full text of DOI:10.1021/acscatal.1c00975

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Letter
Palladium-Catalyzed (4 + 4) Annulation of Silacyclobutanes and
2‑Iodobiarenes to Eight-Membered Silacycles via C−H and C−Si
Bond Activation
Ming-Hui Zhu, Xiao-Wen Zhang, Muhammad Usman, Hengjiang Cong, and Wen-Bo Liu*
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ABSTRACT: Construction of eight-membered silacycles via Pd-
catalyzed (4 + 4) annulation of silacyclobutanes and 2-
iodobiphenyl derivatives is described. This strategy involves direct
C−H and C−Si bond activation followed by a ring annulation and
features low catalyst loading, ligand-free conditions, and readily
available starting materials. Mechanistic studies revealed the
involvement of five-membered palladacycle species in the reaction.
KEYWORDS: C−H activation, C−Si cleavage, silylation, (4 + 4) annulation, medium-sized silacycles, palladium catalysis
methods of eight-membered silacycles from readily available
materials is highly appealing. Notably, the Oshima group
utilized the oxygen affinity of silicon to realize palladium-
catalyzed (4 + 4) annulation of silacyclobutanes with enones
leading to eight-membered cyclic silyl enolates (Scheme
1e).14a Formal annulations upon cleavage and exchange of
C−C and C−Si σ-bonds were remarkably demonstrated by
Murakami et al.14b,c In 2018, an inspirational report by Zhang
and co-workers15a detailed a palladium-catalyzed disilylation of
iodobiarenes through a five-membered palladacycle I, which
formed by oxidative addition of aryl halides and subsequent
activation of the neighboring C−H bond (Scheme 1f). On the
basis of this finding and the studies toward the reactivity of
palladacycle I in the literature,15b−d we envisaged that such
palladium species would undergo a formal (4 + 4) annulation
through ring-opening/cross coupling with silacyclobutanes.
Herein, we report a palladium-catalyzed annulation of 2-halo
biarenes and silacyclobutanes to directly assemble eight-
membered silacycles (Scheme 1g). In comparison with the
strained small rings (e.g., cyclobutanones, cyclopropenones)
employed previously, biaryl halides used as an annulation
partner are conveniently accessible.
ilicon is the second most abundant element in the Earth’s
1
Scrust.
Compared with carbon, the silicon atom has a
larger covalent radius, less electronegativity, and extra 3d
orbitals. These distinct properties make organosilicon com-
pounds widely applicable in organic synthesis,2 pharmaceut-
icals,3 agrochemistry,4 and material sciences.5 Notably, the
intriguing strategy of sila-substitution in medicinal chemistry to
replace the parent carbon is continuously developing to seek
less toxic, more stable, and lipophilic drugs.6
Because of the high ring strain and Lewis acidity of the
silicon,7 silacyclobutanes as synthons to access organosilicon
compounds have been extensively studied in the past few
decades.8 The tendency of releasing the strain energy makes
the silacyclobutanes (siletanes) applicable to annulations via
transition-metal catalysis (Scheme 1a). Major efforts have been
spent on the development of (4 + 2) annulation reactions
between alkynes and silacyclobutanes (Scheme 1b),9a−i since
the seminal example reported by Sakurai and Imai in 1975.9a
Recently, an enantioselective annulation with cyclopropenes
was achieved by the Xu group.9j Using cyclopropylideneace-
tates and cyclopropenones as the annulation partners, (4 + 3)
annulations with silacyclobutanes were successfully realized by
the groups of Saito10a and Zhao,10b respectively (Scheme 1c).
Direct aromatic C−H activation/silylation of silacyclobutanes
catalyzed by rhodium for the synthesis of siloles was also
established by the He group (Scheme 1d).11
Initially, 4,4′-difluoro-2-iodo-1,1′-biphenyl 1a and 1,1-
diphenylsiletane 2a were selected as model substrates for
condition optimization (see Tables S1−S5 in Supporting
Information for details). The reaction in DMF with Pd(OAc)2
Despite the growing utility of silacyclobutanes as highly
enabling reagents to access silacycles, their application in the
construction of eight-membered silacycles is underdeveloped
due to the kinetic and thermodynamic penalties during ring
formation processes.12 Additionally, dimerization of silacyclo-
butanes under transition-metal catalysis would also be
problematic.13 Hence, the development of efficient synthetic
Received: March 2, 2021
Revised: April 14, 2021
Published: April 26, 2021
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Scheme 1. (a−g) Works on Annulation of Silacyclobutanes
Table 1. Optimization of Reaction Conditions
a
Reactions conducted with 0.1 mmol of 1a, 0.12−0.15 mmol of 2a in
b
1 mL of solvent for 12 h. Determined by 19F NMR of the crude
product using 2-fluoropyridine as an internal standard and isolated
c
yield included in the parentheses. With CpPd(π-allyl) as the catalyst.
d
With 2.5 mol % Pd(OAc)2. N.D. = not detected. Blue text indicates
optimal conditions.
ing five-membered palladacycle species is conformationally
unfavorable. Next, the silacyclobutanes with different sub-
stituents were elucidated. The biaryl (2b, 2c)-substituted
silacyclobutanes were found to be appropriate substrates and
delivered the corresponding products (3i, 3j) in 87% and 81%
yields, respectively. The reactions of the dialkyl (2d, 2e)-
substituted silacyclobutanes gave products 3k and 3l in
moderate yields. In comparison, the reactions with 1,1-
dialkylsilacyclobutane showed lower yields than that with
1,1-diarylsilacyclobutane, and deiodination of the biarene
substrate was observed. The annulation of 4,4′-dicarboxylate-
substituted 2-iodobiphenyl 1i and silacyclobutane 2c resulted
in the formation of product (3m) in 83% yield. Silacyclobutane
2f bearing two different exocyclic substituents was also feasible
to the reaction, albeit poor diastereoselectivity was observed
(3n, 78% yield, 1.2:1 dr). It should be mentioned that the
annulation of 1a with 1,1-dimethylsilolane under the standard
conditions failed to deliver the desired nine-membered
silacycle. Instead, dimerization and deiodination of 1a were
obtained (see Scheme S1 in Supporting Information for
details). Notably, the reaction on a 3.5 mmol scale provided
1.09 g of the desired product 3a in 76% yield, which
highlighted the robustness of this chemistry.
(5 mol %) and K2CO3 (2 equiv) at 100 °C provided the
desired product 3a in 34% yield (entry 1, Table 1). The use of
aprotic polar solvents (entries 1−2, Table 1) is crucial for the
reaction, and poor reactivity was observed in nonpolar solvents
such as toluene and THF (entries 3−4). The screening of
bases identified that KOAc significantly improved the yield
(entries 5−7), possibly because of its vital role in the process
of concerted metalation deprotonation (CMD).16 When we
modified the molar ratio of 1a versus 2a as 1:1.2, the yield was
slightly increased to 74% (entry 9). Raising the temperature to
120 °C also improved the yield (entry 10). Replacement of
Pd(OAc)2 with other palladium sources resulted in lower
reactivity (entry 11 and Table S5). To our delight, the yield
(78%) was maintained by lowering Pd(OAc)2 loading from 5
mol % to 2.5 mol % (entries 10 vs 12). Therefore, the optimal
conditions were identified: using 2.5 mol % of Pd(OAc)2 as the
catalyst, KOAc as the base, and DMF as the solvent to perform
the reaction at 120 °C.
With the optimized conditions in hand, we next investigated
the scope of substrates (Table 2). The reactions with chloro-
and trifluoromethyl substituted 2-iodobiaryl substrates deliv-
ered the corresponding products 3b and 3c in excellent yields.
Meanwhile, biaryl substrates with electron-neutral and
electron-rich substituents gave the corresponding silacycles
(3d−3f) in 71−94% yields. In addition, the annulation of 3′,5-
dimethyl-substituted 2-iodobiphenyl 1g delivered product 3g
in 90% yield. However, 2′,6-dimethyl-substituted substrate 1h
failed to give any desired product under the standard
conditions, probably because the formation of the correspond-
Next, the regioselectivity of the reaction using unsymmetric
substrates was explored (Table 3). Interestingly, substrate 1j,
with an electron-withdrawing (3′-CO2Me) substituent pro-
vided silacycle 3o (58%) in high regioselectivity together with
3p (3%) as a minor product. An electron-donating group, 3′-
OMe-substituted substrate (1k) was converted into the
annulation products with silacycle 3r as the major one (3q/
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Letter
a
a
Table 2. Substrate Scope
Table 3. Selectivity of Unsymmetric Substrates
a
Reactions conducted with 0.2 mmol of 1, 0.24 mmol of 2a, 2.5 mol
% of Pd(OAc)2, and 0.4 mmol of KOAc in 2 mL of DMF for 12 h at
b
1
120 °C. Determined by H NMR analysis of the crude reaction
mixture.
According to the above results, it is concluded that the
annulation process tends to form silacycles with silicon
attached on the electron-rich aromatics. This regioselective
trend is in consistent with previous report.10,14b,c
A series of experiments were carried out to get insights into
the mechanism of this unprecedented reaction. Subjecting 4-
ester-substituted biphenyl iodide 1o to the reaction conditions
offered similar results with 1j (Table 3) in terms of yields and
regioselectivities (Scheme 2a), which suggests the involvement
of a common five-membered palladacycle I. To further confirm
this, a bipyridine-coordinated palladacycle 5 was prepared
according to the literature.17 A stoichiometric reaction of
complex 5 and silacyclobutane 2a delivered the corresponding
product 3e in 86% yield (Scheme 2b). With the palladacycle 5
as a catalyst (2.5 mol %), the reaction of 2-iodobiphenyl 1e
and silacyclobutane 2a generated eight-membered silacycles 3e
in 71% yield (Scheme 2c). These results provided convincing
evidence of the palladacycle intermediacy. Previously mecha-
nistic studies toward a related chemistry, namely, palladium-
catalyzed annulation of 2-iodobiphenyls and CH2Br2 for the
a
Reactions conducted with 0.1 mmol of 1, 0.12 mmol of 2, 2.5 mol %
of Pd(OAc)2 and 0.2 mmol of KOAc in 1 mL of DMF for 12 h at 120
b
c
°C. 0.2 mmol scale. With 45% of starting material 1h and 45% of
deiodination product isolated (see Scheme S1 in Supporting
Information).
3r = 1:4.7). Moreover, the reaction with 1l bearing a 3′-Me
substituent gave a mixture of products 3s and 3t in 86% total
yield with the formation of 3t slightly favored (3s/3t = 1:1.3).
Finally, the compatibility of the reaction to heteroaryl
substrates was evaluated. Both pyrrole- and thiophene-
substituted aryl iodides (1m and 1n) were feasible substrates
delivering the products in good yields, and a C2′- and C4′-
silylation mixture were obtained for the reaction with 1n.
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Scheme 2. (a−d) Mechanistic Investigations
Scheme 3. Proposed Mechanism
straightforward approach to medium-sized silacycles from
readily available starting materials.
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acscatal.1c00975.
synthesis of fluorenes, by Zhang et al.15d revealed that the rate-
determining step is the oxidative addition of 2-iodobiphenyl
with Pd(0) to generate Pd(II) species. They also found that
the second slowest step is the concerted metalation
deprotonation (CMD) to form the five-membered palladacycle
I as evidenced by an intramolecular competition experiment.
Accordingly, an intramolecular competition reaction with a 2′-
deuterated substrate (1e-d) was carried out (Scheme 2d), and
a similar kinetic isotope effect was observed.
■
sı
Experimental details, characterization, and spectra of
new compounds (PDF)
Crystallographic data of 3o (CCDC 2018282) (CIF)
AUTHOR INFORMATION
Corresponding Author
■
On the basis of above investigation and previous reports,15,18
a plausible mechanism is depicted in Scheme 3. Oxidative
addition of 2-iodobiphenyl 1 and Pd(0) generates Pd(II) species
II. Subsequently, five-membered palladacycle I is formed via a
concerted metalation deprotonation (CMD) mechanism.17b
Then, oxidative insertion of palladacycle I into silacyclobutane
2 delivers spiro Pd(IV) species III. Upon the reductive
elimination sequence of the Pd−Si bond and Pd−C bond,
two possible nine-membered palladacycles IV and IV′ are
formed correspondingly. Because of the formation of the C−Si
bond can significantly relieve the steric congestion of the
palladium center, the corresponding reductive elimination to
IV is likely favorable.18c Finally, the eight-membered silacycle 3
is produced through reductive elimination of IV or IV′ with
the release of Pd(0) to continue the catalytic cycle. However, a
direct σ-bond metathesis between silacyclobutane 2 and
palladacycle I cannot be excluded at this stage (path 2).15b
In conclusion, we have developed an efficient approach for
the synthesis of eight-membered silacycles from 2-iodobiaryls
and silacyclobutanes. With 2.5 mol % of Pd(OAc)2 as the
catalyst, the reaction performed smoothly in good yields.
Mechanistic investigations indicate the involvement of a five-
membered palladacycle species. This methodology offers new
insights into silacyclobutanes transformation and provides a
Wen-Bo Liu − Engineering Research Center of Organosilicon
Compounds & Materials (Ministry of Education), Sauvage
Center for Molecular Sciences, College of Chemistry and
Molecular Sciences, Wuhan University, Wuhan, Hubei
430072, China; State Key Laboratory of Organometallic
Chemistry, Shanghai Institute of Organic Chemistry, Chinese
Academy of Sciences, Shanghai 200032, China; orcid.org/
0000-0003-2687-557X; Email: wenboliu@whu.edu.cn
Authors
Ming-Hui Zhu − Engineering Research Center of
Organosilicon Compounds & Materials (Ministry of
Education), Sauvage Center for Molecular Sciences, College of
Chemistry and Molecular Sciences, Wuhan University,
Wuhan, Hubei 430072, China
Xiao-Wen Zhang − Engineering Research Center of
Organosilicon Compounds & Materials (Ministry of
Education), Sauvage Center for Molecular Sciences, College of
Chemistry and Molecular Sciences, Wuhan University,
Wuhan, Hubei 430072, China
Muhammad Usman − Engineering Research Center of
Organosilicon Compounds & Materials (Ministry of
Education), Sauvage Center for Molecular Sciences, College of
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Chemistry and Molecular Sciences, Wuhan University,
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Letter
Synthesis of Silacyclobutanes and Their Catalytic Transformations
Enabled by Transition-Metal Complexes. Coord. Chem. Rev. 2018,
374, 93.
Wuhan, Hubei 430072, China
Hengjiang Cong − Engineering Research Center of
Organosilicon Compounds & Materials (Ministry of
Education), Sauvage Center for Molecular Sciences, College of
Chemistry and Molecular Sciences, Wuhan University,
Wuhan, Hubei 430072, China; orcid.org/0000-0002-
9225-0095
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Complete contact information is available at:
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Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We gratefully acknowledge the NSFC (21772148 and
21971198), the Natural Science Foundation of Hubei Province
(Grant No. 2020CFA036), and Wuhan University (WHU) for
financial support. Core Research Facilities of CCMS (WHU)
are acknowledged for access to analytic equipment.
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