Nucleophilic Activation of Hydrosilanes via a Strain-Imposing Strategy Leading to Functional Sila-aromatics

Article abstract of DOI:10.1021/jacs.0c12619

Carefully designed cyclic hydrosilanes enable trans-selective hydrosilylation of unactivated alkynes without transition metal catalysts via silicate formation. Employment of sterically demanding bidentate ligands of silicon increases steric congestion upo

Full text of DOI:10.1021/jacs.0c12619

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Nucleophilic Activation of Hydrosilanes via a Strain-Imposing
Strategy Leading to Functional Sila-aromatics
Toshiya Ikeuchi, Keiichi Hirano,* and Masanobu Uchiyama*
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ABSTRACT: Carefully designed cyclic hydrosilanes enable trans-selective hydrosilylation of unactivated alkynes without transition
metal catalysts via silicate formation. Employment of sterically demanding bidentate ligands of silicon increases steric congestion
upon silicate formation, and this strain-imposing strategy facilitates hydride transfer. This hydrosilylation provides efficient access to
diverse benzosiloles, silaphenalenes, and related silacycles.
catalyst-free trans-selective hydrosilylation reaction leading to
functional sila-aromatics (Scheme 1).
ilicon-centered penta- or hexacoordinate ate complexes, i.e.
Ssilicates, are fascinating reactive species that can mediate
various organic transformations.1 Carbonyl allylation,2 enolate
addition,3 and reduction4 have been especially well studied,
and the increased Lewis acidity of the pentacoordinate silicon
atom upon silicate formation plays a pivotal role in these
reactions.5 On the other hand, ligand transfer from the
pentacoordinate silicate to unpolarized functionalities, such as
carbon−carbon triple bonds, is not promoted by the Lewis
acidity of the silicon center and, thus, is especially challenging.6
Herein, we report a design strategy for highly nucleophilic
pentacoordinate silicates that enable efficient trans-selective
hydrosilylation of unactivated internal alkynes,7,8 leading to
benzosiloles9 and unprecedented sila-aromatics.
Scheme 1. Trans-selective Hydrosilylation of Alkynes for
Silacycle Formation
Hydrofunctionalization reactions of alkynes provide efficient
and straightforward access to functionalized alkenes and have,
thus, been extensively investigated.10 However, trans-selective
transformations remain difficult and further development is
still needed.11,12 Trans-selective hydrosilylation of alkynes is an
attractive approach to construct silacycles. Lewis acid catalyzed
highly trans-selective intermolecular hydrosilylation of unac-
tivated internal alkynes was pioneered by Yamamoto et al.
employing trialkylsilanes, and aluminum(III) proved to be
particularly effective.7a They extended this chemistry to the
synthesis of various aliphatic silacycles including a single
example of benzosilole.7b As for transition-metal-catalyzed
processes, Trost extensively developed the Ru-catalyzed
methodology,7c−f which was utilized for the synthesis of siloles
via sequential trans-selective hydrosilylations of diynes by
Murakami et al.7g As a base-promoted example, Lee reported a
tandem process of alkynylation of carbonyl compounds and
trans-hydrosilylation using alkynylhydrosilanes to form oxasi-
lacyclopentenes involving pentacoordinated silicate.8 We have
recently reported a pseudo-intramolecular strategy for activa-
tion of boron reagents leading to the formation of various
boracycles via trans-selective interelement boration reactions.13
This entropic activation strategy may also be applicable for
activation of the Si−H bond of hydrosilanes especially with an
sp2-carbanion via pentacoordinate silicate formation to achieve
Another key to success is the use of sterically demanding
diols as the bidentate ligand of the silicon reagents. Initial
experiments using commonly employed hydrosilanes such as
HSiEt3 and HSi(OEt)3 were not fruitful due to the poor
silicate-forming ability of the former and undesired ligand
exchange of the latter. For reference, a simple model
calculation for acyclic H2Si(MeO)2 and MeLi showed that
the least basic MeO ligand is far more easily substituted than
Received: December 3, 2020
Published: March 5, 2021
https://dx.doi.org/10.1021/jacs.0c12619
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© 2021 American Chemical Society
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Table 1. Optimization of Reaction Conditions
Entry
Variation from Standard Conditions
Yield (%)
a
1
2
3
4
5
6
7
8
none
1b in lieu of 1a
1c in lieu of 1a
58
a
49
n.d.
bc
,
HSiPh2(OtBu) in lieu of 1a
1a: 1.6 equiv in lieu of 1.0 equiv
1a: 2.0 equiv in lieu of 1.0 equiv
Quenching without warming up
37
87
b
b
b
94
<5
b
THF: 0.04 M in lieu of 0.02 M, otherwise under the
conditions of entry 6
TEMPO (3.0 equiv), otherwise under the conditions of
entry 8
98
93
97
b
b
9
10
11
9,10-Dihydroanthracene (3.0 equiv), otherwise under the
conditions of entry 8
iPrMgCl·LiCl (1.0 equiv) in lieu of BuLi, 1-I-2-(phenyl
n.d.
t
ethynyl)benzene in lieu of 2a, otherwise under the
conditions of entry 8
Et2O in lieu of THF
b
12
<5
a
b
Isolated yields. NMR yields determined using mesitylene as an
c
Figure 1. Toward nucleophilic activation of hydrosilanes: (a) our
strategy and (b) selectivity of ligand transfer in nucleophilic activation
of silanes, with ΔG values (kcal/mol) calculated at the level of
B3LYP/6-31+G*.
internal standard. The product has two phenyl groups on silicon as
exocyclic substituents. THF = tetrahydrofuran. n.d. = not detected.
the hydride (ΔG‡TS1 = 1.7 kcal/mol vs ΔGCP2‑CP4 + ΔG‡
=
TS2
17.7 kcal/mol) (Figure 1b). Thus, we adopted a strategy of
tying back two of the alkoxy ligands on the silicon by using
diols. Moreover, envisioning enthalpic activation of hydride,
we adopted a strain-imposing strategy with a multiply
methylated ligand for steric acceleration (Figure 1a-2). To
test this design strategy, we prepared seven- and six-membered
hydrosilanes containing diols equipped with multiple methyl
groups (1a−1c), as shown in Figure 1a-3. Notably, these
silanes, which were obtained in high yields, are stable to air and
moisture, in contrast to frequently employed diols such as
pinacol and 2,2-dimethyl-1,3-propanediol.14
Hydride transfer was very slow at −78 °C and warming to rt
was required for smooth progress (entry 7). The yield reached
the maximum under the conditions of entry 8. Use of 3 equiv
of TEMPO or 9,10-dihydroanthracene under the optimized
reaction conditions had no effect, and 3a was obtained in 93%
and 97% yields, respectively (entries 9 and 10). These results
strongly indicate that the present process does not involve free
radicals as key species promoting the reaction. Use of
magnesiated species only gave diphenylacetylene and un-
changed 1a, with no silicate formation (entry 11). Diethyl
ether basically shut down the reaction, whereas THF was
extremely effective (entry 12). Although tBuLi was found to be
the most suitable metalating agent in this system, we also
Next, we selected 2-phenylethynylphenyllithium, prepared
from bromoarene 2a, as a model substrate (Table 1). Upon
treatment of the aryllithium with 1a (1.0 equiv) hydride
transfer proceeded efficiently and subsequent cyclization gave
the desired benzosilole 3a in 58% yield (entry 1). The regio-
and stereoselectivity of hydrosilylation were perfectly con-
trolled, whereas such control is sometimes problematic in the
case of transition metal or Lewis acid catalysis. Use of 1b
slightly decreased the yield of benzosilole to 49% (entry 2). In
the case of 1c, the product was observed by GCMS analysis of
the crude mixture, but the silylether was cleaved during
chromatographic purification on silica gel (entry 3). The use of
HSiPh2(OtBu), which is involved in Lee’s hydrosilylation,8
afforded the corresponding benzosilole in 37% yield accom-
panied by the formation of the silylated diphenylacetylene in
n
s
evaluated less aggressive BuLi and BuLi. The use of these
alkyllithiums under optimized conditions afforded 3a in good
yield under noncryogenic conditions.
DFT calculations revealed that the activation energy for
hydride transfer from the silicon center to the carbon−carbon
triple bond is 15.0 kcal/mol with a large energy gain of −24.1
kcal/mol resulting from strain release (Scheme 2). The silicate-
forming step and the ring-closing step are highly reversible and
essentially barrierless processes with activation energy values of
1.8 and 2.4 kcal/mol, respectively (for the entire reaction
pathway; see Scheme S1).
With the optimal set of reaction conditions in hand, the
scope of the benzosiloles was investigated (Table 2). This
reaction can be easily scaled up, and 3a was obtained on a
gram scale in 88% yield. The fluorescence quantum yield of 3a
t
12% yield via undesired substitution of BuO ligand (entry 4).
The poor ligand transfer selectivity observed here can be
attributed to the lack of entropic stabilization of the
pentacoordinate silicate with the monodentate alkoxide ligand
upon activation with the highly basic carbanion. A higher
loading of 1a improved the yield of 3a (entries 5 and 6).
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a
benzosilole 3x in 92% yield. The silole ring was also installed
in a steroid, affording the pentacyclic estrogen analogue 3y in
good yield. In contrast, hydrosilylation of the double bond
using (E)-1-bromo-2-styrylbenzene was not successful under
the current conditions. Compounds of the silole family are
attracting increasing attention, especially in materials science,
owing to potential applications for organic light-emitting
diodes (OLEDs), organic light-emitting field-effect transistors
(OLEFETs), organic solid-state luminescent sensors, photo-
voltaic cells, etc.15 Thus, we believe the present methodology is
of high value in offering access to a range of unprecedented
structures.
Scheme 2. DFT Calculations on Hydride Transfer
a
Calculated at the level of M06-2X/6-31+G* with PCM (THF).
Numerical values are ΔG (kcal/mol). One molecule of THF on a Li
atom is omitted for clarity.
One of the major advantages of this silacycle-forming
reaction is its modularity for modification of the substituent on
the silicon atom owing to the silylether unit acting as a leaving
group (Scheme 3). The reaction with 4-anisyllithium gave the
unsymmetrically substituted benzosilole 4 in 93% yield.
is almost quantitative (ΦFL = 98% in CHCl3) with an emission
wavelength of 410 nm. The electronic character of the para-
substituent on the aryl group at the 2-position was first
examined. The electron-neutral parent phenyl group gave 3a in
93% yield, and the electron-withdrawing Cl-, F-, and CF3-
substituted benzosiloles were obtained in high yields as well
(3b−3d). The substrates with electron-donating substituents
(nBu, OMe) required heating for good conversion (3e, 3f).
Highly electron-deficient 3,5-bis(trifluoromethyl)phenyl and
sterically encumbered mesityl and naphthyl groups were also
tolerated (3g−3i). A heteroaromatic ring was also applicable
(3j). The triple bond proximal to the aryllithium was the
reactive site in the case of enyne and diyne substrates (3k, 3l).
This methodology offers facile access to unique benzosiloles
bearing a heteroelement directly at the 2-position (3m−3o).
Double hydrosilylation of the 1,4-diethynylbenzene-based
substrate afforded 3p in 83% yield. As for annulated aromatic
units, extension of the π-conjugated system (3q, 3r) and
substitutions with heteroelements (3s−3u) proceeded in high
yields. Heteroarene-annulated siloles were also obtained (3v,
3w). This silacycle formation is not limited to benzosiloles, and
1-bromo-2-phenylethynylcyclohexene gave the tetrahydro-
16
Quantitative fluorination with BF3·OEt2 and subsequent
alkynylation proceeded in 68% yield (5). Treatment of the
fluorinated intermediate with silica gel afforded the corre-
sponding silanol 6 quantitatively. Hydrosilane 7, obtained by
reduction of 3a with DIBAL, is an intriguing chemical scaffold.
Anionic activation of 7 was achieved by deprotonation of the
Si−H bond employing TMP-Li, and the generated silyl anion
was captured by methyl iodide. Although the yield of 8 is low
due to competing TMP-silicate formation, this is, to our
knowledge, the first example of deprotonation of an Si−H
bond with lithium amide.17 The relatively high acidity of the
Si−H bond of 7 suggests that there may be some degree of
aromatic stabilization of the benzosilolide (or silolide) anion
by the 10π (or 6π) electron system. Utilization of
unprecedented benzosilolylium species was also demonstrated.
Treatment of hydrosilane 9 with triphenylmethyl cation,
followed by sila-Friedel−Crafts reaction18 with the pendent
biphenyl moiety provided another benzosilole structure 10 in
Table 2. Hydrosilylation Leading to Diverse Benzosiloles
a
b
c
d
e
t
Isolated yields. Run at 70 °C on 3.0 mmol scale. Run at 60 °C. Run at 70 °C. Run with BuLi (4.0 equiv) and 1a (4.0 equiv). Mixture of
diastereomers (3:1).
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a
Scheme 3. Transformation of Benzosilole
Table 3. Hydrosilylation for Other Silacycles
a
b
Isolated yields. Run with 1a (2.0 equiv). Optimized Cartesian
coordinates obtained at the level of M06-2X/6-311++G** with PCM
c
(THF) were used. ΔG values at the level of M06-2X/6-31+G* with
PCM (THF).
a
Isolated yields. R: 1,1,4-trimethyl-4-hydroxypenthyl. (a) 4-MeO-
C6H4Li (2.2 equiv), THF, 80 °C, 72 h. (b) BF3·EtO2 (2.0 equiv),
THF, rt, 20 h, then phenylethynyllithium (5.0 equiv), THF, 60 °C, 12
h. (c) BF3·EtO2 (2.0 equiv), THF, rt, 20 h, then SiO2. (d) DIBAL
(4.0 equiv), THF, 80 °C, 20 h. (e) TMPLi (2.0 equiv), THF, −78 °C,
15 min, then CH3I (10 equiv), rt, 15 min. (f) Ph3CB(C6F5)4 (2.0
equiv), 2,6-lutidine (2.0 equiv), CH2Cl2, rt, 16 h.
and 13). The silacycle fused with benzothiophene 14 was
obtained in 58% yield. Ynamide was also a competent substrate
and was smoothly hydrosilylated to give an unprecedented 1H-
[1,4]azasilino[3,2,1-hi]indole framework (15). There are a few
early reports on silaphenalene dealing with the physical and
chemical properties of its cationic, anionic, and radical forms.21
However, the reported preparation required a severe pyrolytic
condition (685 °C) and afforded the silaphenalene in only 10%
yield.21a Our methodology is incomparably milder and is
compatible with peripherally decorated silaphenalenes.
In conclusion, we report an efficient hydrosilylation reaction
of unactivated carbon−carbon triple bonds via silicate
formation. The keys to success were the use of unprecedented
seven- and six-membered dialkoxyhydrosilanes designed for
entropic stabilization in the silicate state, and the strain-
imposing strategy to enhance the nucleophilicity of hydride.
The diol ligand is also beneficial as a leaving group for
subsequent chemical transformations of the obtained benzo-
siloles at the silicon center. The present methodology provides
a powerful tool for flexible design of functional silacycles.
Detailed analyses of the optical and physical properties of the
novel silacycles and applications of the cyclic hydrosilanes for
construction of other ring systems are in progress.
77% yield. Compound 10 is weakly fluorescent in solution
(ΦFL = 5% in CHCl3), but is strongly fluorescent in the solid
state. This compound might thus find an application as a dye
with AIEE character.
Finally, this hydrosilylation reaction was extended to the
synthesis of six-membered sila-aromatics (Table 3a). Naph-
thyllithium prepared from 1-bromo-8-phenylethynyl-
naphthalene was converted to the silaphenalene 11 (ΦFL
=
57% in CHCl3) in 76% yield by employing 1b. The structure
was unambiguously determined by single-crystal X-ray
diffraction analysis of compound 11′ obtained by reduction
of 11 with DIBAL. Contrary to the case of benzosilole
formation, use of 1a significantly decreased the yield of the
silaphenalene to 14% (Table 1, entry 1 vs entry 2).
In order to shed light on the reason for this drastic
difference, the %V buried19 values of the ligands of 1 were
calculated using SambVca, developed by Cavallo.20 The results
suggest that 1a is sterically more congested than 1b in the
coordination sphere of the silicon center (Table 3b). This is
consistent with the difference in the stability of silicate formed
from the naphthyllithium and 1 (Table 3c). More facile silicate
formation with 1b due to the smaller repulsive interaction with
the sterically demanding peri-substituted naphthyllithium, as
compared with 1a, should be one of the reasons for the much
better conversion with 1b. As for silaphenalenes, a large
substituent such as a mesityl group at the 2-position and
methoxy groups on the naphthyl moiety were tolerated (12
ASSOCIATED CONTENT
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Experimental procedures, characterization data, compu-
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1988, 110, 4599−4602. (c) Kobayashi, S.; Nishio, K. Facile and
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Tetracoordinated Allylic Silanes into Pentacoodinated Allylic Silicates.
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Accession Codes
CCDC 2021313 and 2046045 contain 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
Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
AUTHOR INFORMATION
Corresponding Authors
■
Keiichi Hirano − Graduate School of Pharmaceutical Sciences,
The University of Tokyo, Tokyo 113-0033, Japan;
orcid.org/0000-0002-2702-9183; Email: k1hirano@
mol.f.u-tokyo.ac.jp
(3) For comprehensive studies on aldol reaction with a combination
of trichlorosilylenolates and phosphoramidite by Denmark, see:
(a) Denmark, S. E.; Winter, S. B. D.; Su, X.; Wong, K.-T. Chemistry
of Trichlorosilyl Enolates. 1. New Reagents for Catalytic, Asymmetric
Aldol Additions. J. Am. Chem. Soc. 1996, 118, 7404−7405.
(b) Denmark, S. E.; Wong, K.-T.; Stavenger, R. A. Chemistry of
Trichlorosilyl Enolates. 2. Highly-selective Asymmetric Aldol
Additions of Ketone Enolates. J. Am. Chem. Soc. 1997, 119, 2333−
2334. (c) Denmark, S. E.; Ghosh, S. K. The First Catalytic,
Diastereoselective, and Enantioselective Crossed-aldol Reactions of
Aldehydes. Angew. Chem., Int. Ed. 2001, 40, 4759−4762. (d) Den-
mark, S. E.; Stavenger, R. A. Asymmetric Catalysis of Aldol Reactions
with Chiral Lewis Bases. Acc. Chem. Res. 2000, 33, 432−440. For
recent contributions on enantioselective aldol reaction of unprotected
carboxylic acid, see: (e) Kotani, S.; Yoshiwara, Y.; Ogasawara, M.;
Sugiura, M.; Nakajima, M. Catalytic Enantioselective Aldol Reactions
of Unprotected Carboxylic Acids under Phosphine Oxide Catalysis.
Angew. Chem., Int. Ed. 2018, 57, 15877−15881.
Masanobu Uchiyama − Graduate School of Pharmaceutical
Sciences, The University of Tokyo, Tokyo 113-0033, Japan;
Cluster of Pioneering Research (CPR), Advanced Elements
Chemistry Laboratory, RIKEN, Saitama 351-0198, Japan;
Research Initiative for Supra-Materials (RISM), Shinshu
University, Nagano 386-8567, Japan; orcid.org/0000-
0001-6385-5944; Email: uchiyama@mol.f.u-tokyo.ac.jp
Author
Toshiya Ikeuchi − Graduate School of Pharmaceutical
Sciences, The University of Tokyo, Tokyo 113-0033, Japan
Complete contact information is available at:
https://pubs.acs.org/10.1021/jacs.0c12619
Notes
The authors declare no competing financial interest.
(4) (a) Boyer, J.; Corriu, R. J. P.; Perz, R.; Reye, C. Reduction
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(d) Kohra, S.; Hayashida, H.; Tominaga, Y.; Hosomi, A. Pentaco-
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(5) For design and utilization of highly Lewis acidic silicon reagents
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L. Strained Silacycles in Organic Synthesis: The Tandem Aldol−
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ACKNOWLEDGMENTS
■
This work was partly supported by JSPS Grants-in-Aid for
Scientific Research on Innovative Areas (No. 17H05430), JSPS
KAKENHI (S) (No. 17H06173), and JST CREST (No.
JPMJCR19R2), as well as grants from NAGASE Science
Technology Foundation and Sumitomo Foundation (to M.U.),
a JSPS Grant-in-Aid for Challenging Research (Exploratory)
(No. 18K19390), and a grant from the Naito Foundation (to
K.H.). RIKEN Integrated Cluster of Clusters (RICC),
HOKUSAI-GreatWave, and HOKUSAI-BigWaterfall provided
the computer resources for the DFT calculations. Helpful
advice on the synthesis of hydrosilanes from Prof. Dr. Seiji
Shirakawa (Nagasaki University), insightful discussion with
Prof. Dr. Kazunori Miyamoto (The University of Tokyo), and
assistance in obtaining data on the photophysical properties of
the compounds by Prof. Dr. Atsuya Muranaka (RIKEN) are
gratefully acknowledged.
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