Selective Synthesis of Silacycles by Borane-Catalyzed Domino Hydrosilylation of Proximal Unsaturated Bonds: Tunable Approach to 1,n-Diols

Article abstract of DOI:10.1002/adsc.201700698

The tris(pentafluorophenyl)boron-catalyzed domino hydrosilylation of substrates carrying unsaturated functionalities in a proximal arrangement is presented to produce silacycles. Excellent levels of efficiency and selectivity were achieved in the cyclization by the deliberate choice of the hydrosilane reagents. The key to successful cyclic hydrosilylation is the reactivity enhancement of the second intramolecular hydrosilylation by a proximity effect. Not only dienes but also enones, enynes, ynones and enimines readily afford medium-sized silacycles under convenient and mild conditions. The cyclization proceeds with acceptable diastereoselectivity mainly controlled by the conformational bias towards inducing additional stereogenic centers. The silacycles obtained from this reaction were converted to 1,n-diols or 1,n-amino alcohols upon oxidation, thus rendering the present cyclization a powerful tool for accessing synthetically valuable building blocks. (Figure presented.).

Full text of DOI:10.1002/adsc.201700698

Title: Selective Synthesis of Silacycles by Borane-Catalyzed Domino
Hydrosilylation of Proximal Unsaturated Bonds: Tunable
Approach to 1,n-Diols
Authors: Kwangmin Shin, Seewon Joung, Youyoung Kim, and Sukbok
Chang
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10.1002/adsc.201700698
Advanced Synthesis & Catalysis
FULL PAPER
DOI: 10.1002/adsc.201((will be filled in by the editorial staff))
Selective Synthesis of Silacycles by Borane-Catalyzed Domino
Hydrosilylation of Proximal Unsaturated Bonds: Tunable
Approach to 1,n-Diols
Kwangmin Shin,b,a,c Seewon Joung,b,a,c Youyoung Kim,a,b and Sukbok Chang*,b,a
a
Department of Chemistry, Korea Advanced Institute of Science and Technology (KAIST), Daejeon, 305-701, Republic
of Korea
b
Center for Catalytic Hydrocarbon Functionalization, Institute for Basic Science (IBS), Daejeon, 305-701, Republic of
Korea
c
The authors contributed equally to this work
Received: ((will be filled in by the editorial staff))
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201######.((Please
delete if not appropriate))
Abstract. Tris(pentafluorophenyl)boron-catalyzed domino The cyclization proceeds with acceptable diastereo-
hydrosilylation of substrates carrying unsaturated selectivity mainly controlled by the conformational bias
functionalities in proximal arrangement is presented to towards inducing additional stereogenic centers. The
produce silacycles. Excellent levels of efficiency and silacycles obtained from this reaction were converted to
selectivity were achieved in the cyclization by the deliberate 1,n-diols or 1,n-amino alcohols upon oxidation, thus
choice of the hydrosilane reagents. The key to successful rendering the present cyclization a powerful tool for
cyclic hydrosilylation is the reactivity enhancement of the accessing synthetically valuable building blocks.
second intramolecular hydrosilylation by proximity effect.
Not only dienes but also enones, enynes, ynones and Keywords: silacycles, domino reaction, amino alcohols,
enimines readily afford medium-sized silacycles under B(C6F5)3 catalyst, hydrosilylation
convenient and mild conditions.
oxidation.[8,9]In fact, procedures for the preparation of
Introduction
silacyclic
compounds
via
metal-catalyzed
Hydroxyl groups are ubiquitous in natural products,
synthetic intermediates, pharmaceuticals, and bio-
materials.[1] In particular, diols are a prominently
featured class of scaffolds in synthesis owing to the
large number of versatile transformations that they
allow.[2] As a result, the development of synthetic
protocols for the synthesis of 1,n-diols, especially
from abundant unsaturated bonds, has been
extensively studied.[3-5] A variety of 1,2- and 1,3-diols
can be synthesized mostly by the 1,2-dihydroxylation
of olefins[3] and the reduction of β-hydroxy carbonyl
compounds,[4] respectively. While approaches to
access 1,4-diols have been rarely explored, recently, a
few elegant examples for the synthesis of 1,4-diols
have been reported.[5] Despite these extensive efforts,
a general and widely applicable synthetic method for
accessing a range of 1,n-diols is elusive thus far. One
traditional strategy to access 1,n-diols is a boracycle
synthesis via tandem hydroboration of dienes and
subsequent oxidation.[6] However, only a single
borylating reagent, the unstable thexylborane, has
been shown to mediate the cyclic hydroboration, thus
limiting the synthetic applicability of this strategy.[7]
These considerations led us to envision another
approach leading to 1,n-diols based on a silacycle
formation via hydrosilylation and subsequent
intramolecular hydrosilylation of olefinic substrates
were reported several decades ago.[10] However, these
procedures suffer from narrow scope in either
substrates and silane reactants. In this context, we
wondered whether the [B(C6F5)3], a Lewis acidic
borane catalyst that has been widely utilized recently
to facilitate hydrosilylation of various unsaturated
systems via an unique 1-activation mode of SiH
[11,12]
bonds in hydrosilane reagents,
can mediate the
desired ‘domino hydrosilylation’ of proximal
unsaturated bonds such as dienes and enones with
dihydrosilanes to form the corrseponding silacycles
(Scheme 1a). It was anticipated that the initial
intermolecular hydrosilylation will occur at the more
reactive unsaturated bonds where more stable
carbocation intermediates[13] can be generated to
afford monohydrosilyl species. This intermediate
may undergo
a
subsequent intramolecular
hydrosilylation at the neighboring unsaturated bonds,
leading to the corresponding silacyclic products. It
was predicted that the proximity effect and the fact
that no translational entropy penalties have to be paid
by turning the intermolecular into an intramolecular
reaction may facilitate this cyclic hydrosilylation
towards the less reactive unsaturated bonds that
1
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10.1002/adsc.201700698
Advanced Synthesis & Catalysis
otherwise do not react under normal borane-catalyzed
hydrosilylation conditions.[14]
B(C6F5)3 catalyst (5 mol %) took place almost
exclusively at the styrenyl position, albeit in poor
efficiency. The use of an analogous monohydrosilane,
Et3SiH (2d, 2.2 equiv.), resulted in a similar
selectivity pattern, but with higher reactivity when
compared to Ph3SiH (Scheme 2a, left). The terminal
olefin of 1a was sluggish toward hydrosilylation even
at high reaction temperature (65 oC).
Described herein is a general methodology to form
medium-sized silacycles (m + n = 5~7) by a borane-
catalyzed domino hydrosilylation of proximal
unsaturated bonds (m = 3~6) with dihydrosilanes (n =
1~4, Scheme 1b). The silylating reagents (n) were
finely tuned to react with proximal unsaturated bonds
while varying the hydrocarbon chain linker (m)
between two unsaturated bonds. As envisioned,
subsequent oxidation of silacycles was facile under
the established oxidative conditions to furnish the
corresponding 1,n-diols.
However, diphenylsilane Ph2SiH2 (2a, 1.2 equiv.)
was reacted smoothly with diene 1a at room
temperature to afford a 6-membered silacycle (4a) in
good yield. Interestingly, diethylsilane Et2SiH2 (2b)
was also facile to form the corresponding silacycle
(4a’) in excellent yield (Scheme 2a, right). These
results clearly support our initial hypothesis that
intrinsically low reacting unsaturated bonds can
greatly enhance their reactivity by altering its reaction
mode from inter- to intramolecular. The efficiency in
the formation of silacycles was changed depending
on the ring size.[10a] For instance, when diene 1a was
treated with a bis-silane 2e, the desired 9-membered
silacycle was not formed (Scheme 2a, bottom). This
tunable reactivity in the silacycle formation was also
seen with other types of dienes. Indeed, 1,3-
conjugated diene 1h was readily reacted with bis-
silane 2e to form a 7-membered disilacycle (4h) in
high yield (Scheme 2b). The resultant connectivity in
this cyclization is notable in that β- and δ-carbon
centers relative to the phenyl group in 1h are
connected to two silicon atoms of 2e. Likewise,
disilane 2f was reacted with trans-1-phenyl-1,3-
butadiene (1h) to furnish a 5-membered ring (4h’)
albeit in moderate yield. On the other hand,
monosilanes 2a or 2b did not undergo the desired
cyclization to generate 4-membered silacycles. Again,
these results address the importance of optimizing
reactivity in the silacycle formation by tuning the
types of silanes.
Scheme 1. Synthesis of 1,n-diols via [B(C6F5)3]-catalyzed
domino hydrosilylation
Results and Discussion
Domino Hydrosilylation of Dienes. To test our
working hypothesis on the tunable reactivity towards
the silacycle formation, we commenced our studies
on the B(C6F5)3-catalyzed hydrosilylation of 1,4-
diene 1a with various types of silanes (Scheme 2a).
Diene 1a bearing a phenyl group at the 2-position
was deliberately chosen as a model substrate since
two double bonds were predicted to display a distinct
reactivity
towards
the
borane-mediated
hydrosilylation, in which the reaction course was
assumed to be guided by the relative stability of
emerging carbocationic intermediates.[13] We
envisaged that a phenyl-substituted double bond will
react first with the borane-silane adduct[12d] and then
terminal olefin will be engaged by a subsequent
intramolecular hydrosilylation. Indeed, a reaction of
1a with Ph3SiH (2c, 2.2 equiv.) in the presence of
Scheme 2. Borane-catalyzed domino hydrosilylation of
1,3- and 1,4-dienes with hydrosilanes
2
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10.1002/adsc.201700698
Advanced Synthesis & Catalysis
Table 1. Scope of domino hydrosilylation of dienes[a]
[a] Conditions: 1 (0.5 mmol), 2 (0.6 mmol) and [B(C6F5)3] (5 mol %) in CHCl3 (0.5 mL) at room temperature for 24 h under
argon atmosphere. [b] Reaction was conducted for 12 h. [c] Reaction at 65 oC
To test the generality of the present domino
hydrosilylation route to silacycles, the scope of
dienes was first investigated (Table 1). 2-Phenyl-1,4-
pentadiene (1a) smoothly underwent the domino
hydrosilylation with diphenylsilane Ph2SiH2 (2a, 1.2
equiv.) at room temperature in the presence of 5
mol % of B(C6F5)3 catalyst in chloroform solvent (see
Supporting Information for the optimization studies).
A 6-membered silacycle product (4a) was obtained in
good yield. A same molecular skeleton of 1,4-diene
having two substituents at the 2- and 4-position (1b)
was facile for the silacycle formation (4b).
Interestingly, the cyclization was occurred with high
diastereoselectivity favoring a trans-product over cis-
isomer (84:16). The structure of 4b was determined
by X-ray crystallographic analysis (Figure 1, left). A
reaction of diphenylsilane with a 1,4-diene bearing
one endocyclic olefin (1c) gave a bicyclic silacycle
(4c) in 91% yield with high diastereoselectivity
(87:13) to furnish an additional stereocenters at the
newly generated bicyclic junction. A different type of
silabicyclic compound (4d) was obtained at slightly
higher temperature from 1,4-diene bearing one exo-
methylene double bond (1d) that was derived from a
natural product, isopulegol. In this cyclization, a high
level of diastereoselectivity (82:18) was induced. 1,5-
Diene substituted at the 2- and 5-position (1e)
underwent the domino hydrosilylation with moderate
efficiency to afford a 7-membered silacycle (4e). In
contrast to the 6-membered cases (4b-4d), a similar
mixture of two diastereomers (52:48) was observed.
Structure of a trans-isomer of 4e was confirmed by
an X-ray crystallographic analysis (Figure 1, right).
Notably, the diastereoselectivity was significantly
increased when 1,5-diene has one internal double
bond bearing substituents at the 2- and 6-position (1f),
wherein a 6-membered silacycle (4f) was produced
with high diastereoselectivity (90:10). A bicyclic silyl
product 4g was obtained from (+)-limonene (1g), a
1,5-diene bearing one methylene and one cyclic
double bond, in reaction with diphenylsilane (1.2
equiv.) at room temperature with 5 mol % of
B(C6F5)3 catalyst. While three stereogenic centers
were newly generated in this cyclization, a mixture of
two diastereomers were observed.
Figure 1. Crystal structure of 4b and 4e
As the next type of diolefinic substrates, we turned
our attention to conjugated dienes to test the
feasibility of the silacycle formation. Domino
3
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10.1002/adsc.201700698
Advanced Synthesis & Catalysis
hydrosilylation of 1,3-dienes was envisioned to be
interesting in that its cyclization regioselectivity may
be changed depending on the type of substituents.
Indeed, 1-phenylbutadiene 1h was reacted with 1,2-
bissilylbenzene 2e or disilane 2f to afford the
corresponding 7- and 5-membered disilacycle 4h and
4h’, respectively. This cyclization outcome to form
CSi bonds at the C-2 and C-4 position can be
understood by assuming that the reaction proceeds
via the most stable carbocationic intermediates
(Scheme 3a).
approach. Moreover, the resulting oxasilinanes
obtainable from enones eventually furnish one-carbon
shortened diols upon oxidation when compared to the
silacycles derived from analogous 1,n-diene
substrates. Since ketones are known to exhibit
generally higher reactivity towards the B(C6F5)3-
catalyzed hydrosilylation than olefinic double
bonds,[11h,k] we hypothesized that the initial
hydrosilylation to enones will occur at the carbonyl
double bond. In addition, the pioneering study of
Dussault and co-workers demonstrated that an
alkoxysilane, a putative intermediate in the present
domino hydrosilylation of enones (see the figure in
Table 2), can undergo the intramolecular
hydrosilylation smoothly to form the corresponding
silacyclic product.[16] Based on these considerations,
we predicted that the desired domino hydrosilylation
of enones would be facile. As a proof of concept, 3-
methyl-1-phenylbut-3-en-1-one (6a) was readily
reacted with diphenylsilane (2a, 1.2 equiv.) at 0 oC by
the action of B(C6F5)3 catalyst (5 mol %) to deliver
3,5-disubstituted 1,2-oxasilinane (Table 2, 7a). More
pleasingly, the reaction was highly diastereoselective
favoring anti-isomer (d.r. >99:1), confirmed by X-ray
crystallographic analysis (Figure 2).
It is noteworthy that
a
reaction of 2-
phenylbutadiene (1i) with 2e gave rise to a disilacycle
product 4i where two CSi bonds formed at the C-1
and C-3 position (Scheme 3b). Interestingly, this
cyclization afforded a single trans-isomeric product,
which was unambiguously determined by NMR
analysis. Preliminary mechanistic studies suggest that
while the hydrosilylation of 1-phenylbutadiene 1h
takes place first at the terminal double bond via an
allylic carbocation intermediate I, reaction of 2-
phenylbutadiene 1i proceeds in a 1,4-hydrosilylation
manner via an allyl benzyl cationic intermediate II
(see Supporting Information for details). Additional
types of conjugated dienes also underwent the
reaction to form the corresponding silacycles. For
instance, a 5-membered silacycle 4j was obtained in a
reaction of 1,1,4,4-tetrasubstituted-1,3-diene 1j with
disilane 2f presumably via an analogous mechanistic
pathway as that of 1h. In addition, 2,3-
dimethylbutadiene 1k was cyclized by domino
hydrosilylation with bissilylbenzene 2e to afford a 7-
membered silacycle 4k, as similar to that of 1i. It
needs to be mentioned that transition metal-catalyzed
diboration, as an alternative route to diols, can afford
only 1,2- or 1,4-diols from 1,3-dienes.[15] Therefore,
the present borane-catalyzed domino hydrosilylation
Figure 2. Crystal structure of 7a
of
conjugated
dienes
offers
a
valuable
complementary approach for the regioselective
synthesis of 1,3-diols upon the subsequent oxidation.
Likewise, derivatives of β,γ-unsaturated enones
(6b,6c) were highly facile for the cyclic
hydrosilylation with exclusive formation of the
corresponding anti-silacycles. In addition to β,γ-
unsaturated enones, γ,δ-unsaturated enone 6d also
underwent the domino hydrosilylation with
diphenylsilane to afford a 7-membered oxasilepane
product 7d. Interestingly, as observed above in the
cyclization of dienes, diastereoselectivity in the
formation of this 7-membered silacycle (7d) was not
notable (56:44). However, it was greatly increased
(d.r. >99:1) in the formation of a 6-membered
silacycle (7e) from the same type of γ,δ-unsaturated
enone 6e bearing an internal double bond. In addition,
this reaction could be performed on a 5 mmol scale to
afford the desired silacyclic product 7e in 78% (1.21
g) yield, demonstrating the synthetic utility of the
current procedure (see the Supporting Information for
detailed procedure). A dienone (6f) also underwent a
regioselective domino hydrosilylation to afford 7f. As
anticipated, a more distant double bond at the ,-
position relative to the carbonyl group was observed
to be kinetically less favorable than ,-olefinic
double bond. In fact, no 7-membered silacycle was
Scheme 3. Proposed reaction pathways of 1- or 2-
substituted conjugated dienes
Domino Hydrosilylation of Enones. We next turned
our attention to an additional type of substrates:
enones (Table 2). Since the desired 1,2-oxasilinane
products were predicted to be easily oxidized to
afford 1,n-diols,[9] the success in this domino
hydrosilylation of enones could further widen the
substrate scope and synthetic value of the current
4
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10.1002/adsc.201700698
Advanced Synthesis & Catalysis
Table 2. Scope of domino hydrosilylation of enones[a]
o
[a] Conditions: 6 (0.3 mmol), 2a (0.36 mmol) and [B(C6F5)3] (5 mol %) in CHCl3 (0.5 mL) at 0 C for 2 h under argon
atmosphere. [b] Reaction was performed on a 5 mmol scale.
formed in this reaction. In reaction of dieneone 6g,
the γ,-double bond relative to the carbonyl group
reacted selectively with diphenylsilane to furnish an
oxasilinane product 7g via 6-exo cyclization. These
results indicate that the regioselective domino
hydrosilylation can be efficiently achieved by tuning
the ring size of resultant silacyclic products, thus
highlighting the potential utility of the present
method to more complex enone substrates.
the B(C6F5)3 catalyst (5 mol %) at room temperature
to afford a 6-membered cyclic vinylsilane 10 via an
endo-dig cyclization process (Scheme 4a).[10g] When
an ynone (11) was subjected to the present
hydrosilylation protocol, unstable 6-membered
silacyclic product 12 was obtained in moderate yield
(Scheme 4b).[10h] These preliminary results foresee
the potential utility of our strategy for the formation
of sp2 CSi bonds that can readily be transformed by
subsequent cross-coupling reactions, which were
extensively studied by Denmark and Hiyama.[17]
Moreover, we observed that γ,δ-olefinic imine 13
participated in the present hydrosilylation with
diethylsilane. Although we could not isolate the
desired azasilacycle mainly owing to the unstable
nature of NSi bond, we indeed obtained the
corresponding aminosilane product 14 upon in situ
hydrolysis and subsequent N-tosylation for the ease
of isolation (Scheme 4c).
Application: Oxidation of Silacycles. Importantly,
as we anticipated at the beginning of this study,
silacyclic products obtained by the present domino
hydrosilylation procedure were efficiently converted
to the corresponding 1,n-diols and amino alcohols
under the Fleming-Tamao oxidation conditions
(Scheme 5).[9] When silinane (4a) or silepane (4e)
were subjected to the Fleming protocol, the
corresponding 1,5-diol (5a) and 1,6-diol (5e) were
obtained in good yields, respectively (Scheme 5a and
5b). In addition, 1,3-diol (5h) was obtained from the
oxidation of either a disilyl-containing 7-membered
silacycle (4h) or a 5-membered silacycle having a
bis-silyl moiety (4h’) in synthetically acceptable
yields (Scheme 5c). Tamao oxidation of oxasilinane
Scheme 4. Extension of the present procedure: Domino
hydrosilylation of other unsaturated systems
Domino Hydrosilylation of Other Unsaturated
Systems. Besides dienes and enones, we were
pleased to see that the current domino hydrosilylation
strategy could be successfully applied to additional
types of unsaturated systems (Scheme 4). 1,4-Enyne
(9) was smoothly reacted with diphenylsilane using
5
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10.1002/adsc.201700698
Advanced Synthesis & Catalysis
To a solution of B(C6F5)3 (12.8 mg, 0.025 mmol, 5 mol %)
in CHCl3 (0.5 mL) in a screw capped reaction vial was
added dihydrosilane (2, 0.60 mmol, 1.2 equiv.) under
argon atmosphere. Diene (1, 0.50 mmol, 1.0 equiv.) was
then added and the reaction mixture was allowed to heat at
the indicated temperature and time. The reaction mixture
was cooled down to room temperature, concentrated under
reduced pressure and purified by column chromatography
on silica gel with n-hexane/EtOAc to give the desired
silacyclic products 4a-4k. Products 4b, 4c, 4d, 4f and 4g
were further purified by preparative HPLC using Shim-
pack PREP-ODS (H) kit or YMC-pack SIL column to
obtain the major/minor diastereomers.
(7b) and aminosilane (14) was also facile to afford
the corresponding 1,4-diol (8b) and 1,4-amino
alcohol (15), respectively (Scheme 5d and 5e).
General Procedure for B(C6F5)3-Catalyzed Domino
Hydrosilylation of Enones
To a solution of B(C6F5)3 (7.7 mg, 0.015 mmol, 5 mol %)
in CHCl3 (0.4 mL) in a screw capped reaction vial was
added diphenylsilane (2a, 66.8 μL, 0.36 mmol, 1.2 equiv.)
under argon atmosphere. The reaction mixture was allowed
o
to cool down to 0 C. Ketone substrate (6, 0.30 mmol, 1.0
equiv.) was subsequently added under argon atmosphere
o
and allowed to stir for 2 h at 0 C. After 2 h, all volatiles
were removed under reduced pressure and the crude
product was purified by flash column chromatography with
n-hexane/EtOAc to give the desired silacyclic products.
Products 7d, 7f and 7g were further purified by
preparative HPLC using Shim-pack PREP-ODS (H) kit or
YMC-pack SIL column to obtain the major/minor
diastereomers.
Scheme 5. Conversion of silacycles to the corresponding
diols via oxidation
Acknowledgements
This research was supported by the Institute for Basic Science
(IBS-R010-D1) in Korea.
Conclusion
In summary, we have shown that a borane-catalyzed
domino hydrosilylation of readily available
compounds having proximal unsaturated bonds with
various dihydrosilanes furnishs silacycles. Substrate
scope was broad to include remote dienes, conjugated
dienes, enones, enimines, enynes, and ynones. While
the selectivity of the initial intermolecular
hydrosilylation is governed by the relative stability of
in situ generated carbocation intermediates, the
subsequent intramolecular hydrosilylation takes place
at the remaining less reactive unsaturated bonds that
are otherwise unreactive under the borane cataysis.
Another attractive feature of this approach lies in the
availability of a broad range of dihydrosilanes
altering their reactivity and ring size leading to
silacycles of variable sizes. Thus, the reaction
efficiency in the domino hydrosilylation can be tuned
effectively by the choice of flexible dihydrosilanes as
well as by the design of proximal unsaturated bonds
of substrates. The obtained silacycles were readily
oxidized to the corresponding 1,n-diols or 1,n-amino
alcohols as versatile synthons. Investigations on
further synthetic utility of silacycles that become
readily accessible through this study are now in
progress to include a direct cross-coupling of CSi
bonds with organic electrophiles.
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General Procedure for B(C6F5)3-Catalyzed Domino
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6
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Advanced Synthesis & Catalysis
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Supporting Information. CCDC 1552310 (4b), CCDC
7
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10.1002/adsc.201700698
Advanced Synthesis & Catalysis
1469939 (4e) and CCDC 1468851 (7a) contain the
crystallographic data for this paper. These data can be
obtained free of charge from The Cambridge
Crystallographic
Data
Centre
via
www.ccdc.com.ac.uk/data_request/cif.
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10.1002/adsc.201700698
Advanced Synthesis & Catalysis
FULL PAPER
Selective Synthesis of Silacycles by Borane-
Catalyzed Domino Hydrosilylation of Proximal
Unsaturated Bonds: Tunable Approach to 1,n-Diols
Adv. Synth. Catal. Year, Volume, Page – Page
Kwangmin Shin, Seewon Joung, Youyoung Kim,
and Sukbok Chang*
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