Ring Expansion to 6-, 7-, and 8-Membered Benzosilacycles through Strain-Release Silicon-Based Cross-Coupling

Article abstract of DOI:10.1002/anie.202001539

The synthesis of silacycles is highly appealing due to their important applications in organic synthesis, medicinal chemistry, and materials chemistry. However, sila-tetralins and sila-benzosuberanes are surprisingly under-represented due to a lack of general methods to access these compounds. We successfully developed a Pd-catalyzed strain-release silicon-based cross-coupling as an unprecedented ring-expansion method, which constitutes a general route for preparing diverse sila-tetralins and sila-benzosuberanes.

Full text of DOI:10.1002/anie.202001539

A Journal of the Gesellschaft Deutscher Chemiker
Angewandte
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Accepted Article
Title: Ring Expansion to 6−, 7− and 8−Membered Benzosilacyles
through Strain−Release Silicon-based Cross-Coupling
Authors: Dongbing Zhao, Ying Qin, Jie-Lian Han, and Cheng-Wei Ju
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.202001539
Angew. Chem. 10.1002/ange.202001539
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Ring Expansion to 6−, 7− and 8−Membered Benzosilacycles
through Strain−Release Silicon-based Cross-Coupling
Ying Qin, Jie-Lian Han, Cheng-Wei Ju, and Dongbing Zhao*[a]
Abstract: Silacycle synthesis is highly appealing due to their
cross-coupling of aryl halides with O or N−nucleophiles during
the reaction[6] by choice of proper nucleophile and catalytic
system. Herein we describe the realization of ring expansion of
4− and 5−membered silacycles in the presence of alcohol by Pd-
catalyzed strain−release silicon-based cross-coupling (Figure
1d). This method provides a modular means of assembling sila-
tetralins & sila-benzosuberanes.
important applications in organic synthesis, medicinal chemistry as
well as materials chemistry. However, sila-tetralins
&
sila-
benzosuberanes are surprisingly under-represented to date due to
the lack of general methodology to approach these compounds.
Herein we successfully develop Pd-catalyzed strain−release silicon-
based cross-coupling as an unprecedented ring expansion method,
which constitutes one of the most general route for preparing
diversified sila-tetralins & sila-benzosuberanes.
“Carbon/Silicon switch” has been regarded as an innovative
strategy for the development of new materials, drugs and
pesticides.[1] The structural units of tetralins & benzosuberanes
at various oxidation levels are prevalent in numerous marketed
drugs, and natural products (Figure 1a).[2] In light of this fact,
investigations of sila-tetralins & sila-benzosuberanes are of great
significance and in great demand. However, sila-tetralins & sila-
benzosuberanes are surprisingly under-represented to date due
to the lack of general methodology to approach these
compounds (Figure 1a). The typical protocols to approach sila-
tetralins & sila-benzosuberanes suffer several disadvantages
such as the lack of flexibility, the need of hash conditions and
multi-step synthesis.[3] Thus, it is highly desired for the
development of the general and efficient strategy to directly
constructing sila-tetralins & sila-benzosuberanes.
The palladium-catalyzed Hiyama-Denmark cross-coupling
has developed over the past 30 years into an efficient and
attractive carbon−carbon bond forming strategy (Figure 1b).[4] In
general, the –SiR3 group would convert to the corresponding
silicon by-products as the wastes in thi cross-coupling. We
wondered whether Hiyama-Denmark cross-coupling could be
high-efficiently utilized as a ring expansion strategy enabling
rapid construction of diverse silacycle skeletons even it has
never been presented. It was revealed that the propensity of the
Figure 1. Research background and motivation for ring expansion of 4- and 5-
membered silacycles via strain−release organosilicon-based cross-coupling.
4− and 5−membered silacylces toward activation by
a
nucleophile as the pentacoordinate silicate is enhanced because
of their inherent ring-strain-release Lewis acidity.[5] Its origins is
the difference in coordination geometry between four-coordinate
(tetrahedral) and five-coordinate (trigonal bipyramidal) silicon
species (Figure 1c). Thus, the 4− and 5−membered silacylces
appear to be more susceptible to transmetalation in
organosilicon-based coupling reaction. This enhanced reactivity
offers exciting possibilities for ring expansion via intramolecular
Hiyama-Denmark cross-coupling by incorporation of aryl
bromides and 4− or 5−membered silacycles into the same
skeleton at the adjacent position. We envisioned that this
pathway could be initiated if we could inhibit the problematic
Silacyclobutanes (SCBs) have been widely utilized as sila
synthons in transition metal-catalyzed reactions due to its high
ring strain and Lewis acidity.[7] Thus, the silacyclobutane
derivative 1a was subjected to Pd-catalytic conditions to try to
develop an efficient approach to access sila-tetralin in our
preliminary experiments. After extensive survey of the reaction
parameters (more details, seeing SI), the palladium-catalyzed
strain−release organosilicon-based cross-coupling smoothly
occurred to yield sila-tetralin 2a and the optimized conditions
were identified to be: 5 mol % of Pd(PtBu3)2, TMSNEt2 (1.5
t
equiv), and BuOH as the solvent and the nucleophile, wherein
the desired product 2a was afforded in 87% yield at 80 °C for 24
h. Next, we investigated the substrate scope and functional
group tolerance (Figure 2). To our delight, whenever the 3-, 4-,
5- or 6-position of the phenyl ring bears electron-neutral,
electron-rich or electron-withdrawing groups the reaction
proceeded smoothly under the optimized conditions, with good
to excellent yields (2b−i). Furthermore, the 3,6-, 4,5-, 5,6-, 4,6-
and 5,6-disubstituted substrates also underwent the reaction
[a]
Y. Qin, J. Han, C.-W. Ju, Prof. Dr. D. Zhao
State Key Laboratory and Institute of Elemento-Organic Chemistry
College of Chemistry, Nankai University
94 Weijin Road, Tianjin 300071 (China)
E-mail: dongbing.chem@nankai.edu.cn
Supporting information for this article is given via a link at the end of
the document.((Please delete this text if not appropriate))
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smoothly and the yields were comparable to those of the
monosubstituted substrates (2j−q). Heteroaryl bromide bearing
silacyclobutane group at adjacent position also provided the
corresponding ring expansion product (2r). Additionally, we
studied the influence of the nucleophile. We found that it did not
significantly affect the reaction performance to switch of the
tBuOH to different alcohols and phenols as the nucleophile and
the solvent (2s−w). Introducing bromo and silacyclobutane
group to 1-, and 8-position of naphthalenes individually, the
intramolecular strain−release organosilicon-based cross-
coupling reaction also works well, enabling access to the 7-
membered silacycles 2x−y. It is important to stress that this
method was found to be compatible with a variety of important
functional groups, including halogens, trifluoromethyl, cyano,
and alkoxy substituents.
group tolerance of sila-cyclopentanes 4 to approach diverse sila-
benzosuberanes (Figure 3). Various sila-cyclopentanes bearing
substituent at any position of phenyl ring with different electronic
properties proceeded smoothly to afford the desired sila-
benzosuberanes in satisfactory yields (4b−l). Again, our method
could also be applied in ring expansion of multi-substituted
substrates without any drop in yields (4m−q). Furthermore, we
studied the influence of the nucleophile for this reaction. We
found that both cyclohexylmethanol and phenol were competent
nucleophiles, enabling access to silabenzosuberane derivatives
(4r−s). Assembling of bromo- and sila-cyclopentane group to 1-,
and
8-position
of
naphthalenes,
the
intramolecular
strain−release organosilicon-based cross-coupling also works
well to yield the 8-membered silacycles 4t.
Figure 3. Substrate scope to access sila-benzosuberanes. Values represent
isolated yields. [a] DMF/CyCH2OH (1:1; 1 mL) as the solvent. [b] DMF/PhOH
(1:1; 1 mL) as the solvent. [c] DMF/tBuOH (1:1; 1 mL) as the solvent.
To demonstrate the synthetic utility of the silacycles obtained in
our transformation, we first sought to conduct six additional
transformations of the silicon center at our product 4h to
approach structurally diverse important building blocks (Figure 4,
upper panel). Treatment of 4h with 40% HF in THF/H2O at room
temperature for 1 h led to the cleavage of −OCy group at room
temperature to yield product 5a in 85% yield.[8] We prove that
the −OCy group of the sila-benzosuberane 4h can be easily
cleaved and replaced by alkyl group to afford product 5b (79%
yield) by treatment with organolithium reagent.[9] Moreover, 4h
can also be easily hydrolyzed with tBuOOH/SeO2 in DCM to give
the corresponding silanol 5c in 98% yield.[10] The structure of 5c
was clearly confirmed by single-crystal X-ray diffraction.
Additionally, the Si–O bond of the product 4h can be reduced by
Figure 2. Substrate scope for ring expansion of silacyclobutane derivatives to
access sila-tetralins. Values represent isolated yields.
To
access
sila-benzosuberanes,
the
intramolecular
organosilicon-based cross-coupling reaction of aryl bromide
bearing sila-cyclopentane at adjacent position (3a) was
examined under the optimized condition. Unfortunately, only
trace amounts of the desired silabenzosuberane were provided.
After an extensive screening, we found that the reaction
occurred smoothly in 91% yield (4a) by using the mixture of
DMF/CyOH (1:1) as the solvent (CyOH as the nucleophile) at
150 °C for 24 h. We next examined the scope and functional
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LiAlH4 to afford the cyclic hydrosilane 5d (76% yield),[11] which
can be further utilized as the synthetic intermediate in
hydrosilylation.[12] In addition to reduction, we could also submit
compound 4h to the modified Tamao’s oxidation condition to
produce 2-hydroxyarylbutanol 5e in 80% yield.[13] This strategy
took place by employment of silacyclohexane 11 as the
substrate under conditions identical with ring expansion of 4-, or
5-membered silacycles (Eq. 1-2). The results indicate that the
high ring strain of silacycles is indispensable in this
intramolecular Pd-catalyzed organosilicon-based cross-coupling.
Second, we submitted silacyclobutane 1x to the standard
condition in the absence of Pd0 catalyst (Eq. 3). No reaction took
place. This result indicates that our starting material doesn’t form
provides
a
general method for the preparation of 2-
hydroxyarylbutanol skeletons, which have been widely utilized
as the synthetic intermediates to approach numerous bioactive
molecules.[14] Bisiodination via ring opening reaction of
compound 4h by treatment of NIS/AgF also smoothly proceeded
to give 1-iodo-2-(4-iodobutyl)benzene 5f in 56% yield,[15] which
was synthesized in six steps.[16]
t
free pentacoordinate-silicon intermediate in BuOH. Concerted
formation of pentacoordinate-silicon species and transmetalation
might be more preferred. Third, 100% of the starting material
has
been
recovered
if
we
exposed
1-methyl-1-
(naphthalenyl)siletane 13 to the standard condition in the
presence of stoichiometric Pd(PtBu3)2 (Eq. 4). The result is
consistent with a scenario in which silacyclobutane is inactive
until it undergoes intramolecular transmetalation. Furthermore,
the competition experiment between silacyclobutane 13 and aryl
bromide 14 also elucidates that oxidative addition of C−Br bond
is much faster than insertion of C−Si bond and the reaction
started from the oxidative addition of C−Br bond because the
dimerization of 14 smoothly happened to yield compound 15 in
the presence of stoichiometric Pd(PtBu3)2 (Eq. 5).
Figure 4. Synthetic applications of sila-benzosuberane 4h.
Furthermore, we wondered if the carbocycle of our products
could also been further functionalized resulting structurally
diverse sila-tetralins or sila-benzosuberanes at various oxidation
levels. Herein, we for the first time show that bromination at the
5-position of sila-benzosuberane 5b occurred smoothly to yield
the product 6 in 70% yield,[17] which could serve as a useful
building block for further transformations (Figure 4, lower panel).
Compound 6 could smoothly undergo oxidation in the presence
of DMSO to yield brominated sila-benzosuberanol 7 with 58%
yield.[18] Furthermore, the bromo group of 6 can be easily
eliminated in the presence of DBU to form the unsaturated sila-
benzosuberene 8. The sila-benzosuberene 8 could be further
oxidized either by PCC or DDQ to produce the corresponding
sila-benzosuberone 9 or the conjugated system 10.
On the basis of literatures and our control experiments,[19] we
propose a catalytic cycle that initiates with oxidation addition of
Pd0 to C–Br bond of 1 or 3 to form PdII intermediate A, which is
followed by intramolecular transmetalation, thus affording the
intermediate B (Figure 5, Path A). Complex C further undergoes
an intermolecular etherification in the presence of ROH and
base to deliver the intermediate C bearing a Si–OR bond,
followed by reductive elimination to form the product 2 or 4 and
regenerate the Pd0 catalyst. Notably, an alternative pathway
involving a Br/OR exchange of the PdII intermediate A in the
presence of base and HOR to afford the Ar−PdII−OR species B'
and subsequently concerted σ-bond metathesis between
PdII−OR bond and C(sp3)–Si bond to reach intermediate C was
also possible (Figure 5, Path B).
We then carried out a series of control experiments to
elucidate the mechanism. First, we found that no ring expansion
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[3]
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Figure 5. Proposed catalytic cycle.
In summary, we have described a mechanistically unique
ring expansion process of 4− and 5−membered silacylces. With
the aid of an alcohol, diversified 6−, 7− and 8−membered
silacycles, along with wide range of functional group tolerance
were synthesized. In fact, our study constitutes one of the most
efficient and general route for preparing sila-tetralins & sila-
(benzo)suberanes, which would trigger the development of new
drug-like candidates due to the importance of the structural units
of tetralins & benzosuberanes in medicinal chemistry and natural
products.
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Acknowledgements
We are grateful for the financial support from the National
Natural Science Foundation of China (21871146, 21602115),
1000-Talent Youth Program (020/BF180181), the Natural
Science Foundation of Tianjin (18JCYBJC20400), the
Fundamental Research Funds for the Central Universities and
Nankai University.
[7]
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Keywords:
Organosilicon compounds • Silacycles • Ring expansion •
Hiyama coupling • Strained ring
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Entry for the Table of Contents (Please choose one layout)
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Ying Qin, Jie-Lian Han, Cheng-Wei Ju,
and Dongbing Zhao*
Page No. – Page No.
Ring Expansion to 6−, 7− and
8−Membered Benzosilacycles
through Strain−Release Silicon-based
Cross-Coupling
Ring Expansion via Hiyama-Denmark Coupling: Herein, the first intramolecular
strain−release organosilicon-based cross-coupling proceeds smoothly by treatment
of palladium catalyst, which constitutes the most efficient and general route for
preparing diverse sila-tetralins & sila-(benzo)suberanes.
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