Visible-Light Induced Radical Silylation for the Synthesis of Dibenzosiloles via Dehydrogenative Cyclization

Article abstract of DOI:10.1002/adsc.201800417

A visible-light induced radical silylation to dibenzosiloles from biarylhydrosilanes is described. The products were obtained in satisfactory yields under mild and water/air compatible conditions, providing an efficient and practical method for the synthesis of difunctionalized siloles by using a cheap organic dye photocatalyst. The method is tolerated by a wide range of functional groups and has a broad substrate scope. Light/dark experiments and quantum yield measurements provided support for a photocatalytic pathway rather than a chain process. (Figure presented.).

Full text of DOI:10.1002/adsc.201800417

Title: Visible-Light Induced Radical Silylation for the Synthesis of
Dibenzosiloles via Dehydrogenative Cyclization
Authors: Chao Yang, Jing Wang, Jianhua Li, Wenchao Ma, Kun An,
Wei He, and Chao Jiang
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To be cited as: Adv. Synth. Catal. 10.1002/adsc.201800417
Link to VoR: http://dx.doi.org/10.1002/adsc.201800417

10.1002/adsc.201800417
Advanced Synthesis & Catalysis
COMMUNICATION
DOI: 10.1002/adsc.201
Visible-Light Induced Radical Silylation for the Synthesis of
Dibenzosiloles via Dehydrogenative Cyclization
Chao Yang,a Jing Wang,b Jianhua Li,a Wenchao Ma,a Kun An,b Wei He,b and Chao
Jiang a*
a
School of Chemical Engineering, Nanjing University of Science and Technology, Nanjing, Jiangsu 210094, China
E-mail: chaojiang@njust.edu.cn
School of Medicine and Tsinghua-Peking Joint Centers for Life Science, Tsinghua University, Beijing 100084, China
b
Received:
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201######.
Abstract. A visible-light induced radical silylation to
dibenzosiloles from biarylhydrosilanes is described. The
products were obtained in satisfactory yields under mild
and water/air compatible conditions, providing an
efficient and practical method for the synthesis of
difunctionalized siloles by using a cheap organic dye
photocatalyst. The method is tolerated by a wide range of
functional groups and has a broad substrate scope.
Light/dark experiments and quantum yield measurements
provided support for a photocatalytic pathway rather than
Scheme 1. Known (formal) dehydrogenative strategy.
a chain process.
Keywords: Visible Light Photoredox Catalysis;
Recently, visible-light photoredox catalysis has
Dibenzosilole; C-Si bond formation; Radical reaction.
emerged as one of the most flourishing and attractive
tools for organic transformations owing to its intrinsic
characteristics of sustainability and mild operating
conditions.[12] Although it has been well demonstrated
Dibenzosilole (9‑silafluorene) derivatives have
attracted much attention due to their applications in
organic electronic and photonic materials, such as light
emitting materials, field effect transistors, photovoltaic
cells, and fluorescent sensors.[1] Conventional methods
to synthesize dibenzosiloles by metal−halogen
exchange involve highly reactive organometallic
reagents.[2] Recently, many transition-metal-catalyzed
couplings or cycloadditions to synthesize diverse
dibenzosiloles were developed employing rhodium,[3]
palladium[4,5,6] and iridium catalysis.[7] However, these
methods require pre-functionalized precursors such as
halides, triflates or boronic acids, or complicated
diynes. In comparison, the construction of C-Si bond
via (formal) dehydrogenative coupling between a C–H
bond and a Si–H bond is conceivably the most efficient
approach (Scheme 1).[8] To this end, three different
approaches have been reported: (a) the rhodium
catalyzed C-H/Si-H coupling reactions reported by the
Takai and He groups (Scheme 1a);[9] (b) the strong
Lewis acid B(C6F5)3 or cationic Ru-S complexes
induced electrophilic aromatic substitution reactions
(SEAr) reported by the Kawashima, Ingleson and
Oestreich groups (Scheme 1b);[10] and (c) the
homolytic aromatic substitution reactions involving
silicon-centered radicals generated under heating
reported by the Studer and Li groups (Scheme 1c).[11]
that visible-light photoredox catalysis was quite
effective for the construction of C–C,[13] C–N,[14] and
C-O[15] bonds under mild conditions,[16] to the best of
our knowledge, construction of C-Si bonds via visible-
light photoredox catalysis still remains scarce.[17] Very
recently, Wu reported a convenient visible-light-
driven metal-free hydrosilylation of alkenes that
proceeds through selective hydrogen atom transfer.[17a]
While decatungstate-photocatalyzed hydrosilylation
of electron-poor alkenes with silane was reported by
Fagnoni group, the reaction was irradiated under UV
light.[17b] The successful employment of visible-light
photoredox catalysis[18] in C-Si bond formation would
provide more efficient and green processes for the
syntheses of silicon containing compounds.[19] Herein,
we report a new, cost-effective, and powerful protocol
for the preparation of dibenzosiloles via visible-light
photoredox catalysis. Notable features of our findings
include (i) C-Si bond formation via visible-light
photoredox catalysis, (ii) cheap organic dye as the
photoredox catalyst, (iii) mild and water/air
compatible conditions, (iv) triflate group compatible
conditions, (v) orthogonally difunctionalized silole
can be obtained directly.
We commenced our study with biarylhydrosilane 1a
as the model substrate and surveyed photoredox
1
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10.1002/adsc.201800417
Advanced Synthesis & Catalysis
Table 1. Optimization of the reaction conditions[a]
using KOH and t-BuOLi as the base, which not only
delivered the desired product 2a in higher yields, but
Table 2. Scope of substituted biarylhydrosilanes (ring A)[a]
[a] The reaction was carried out with 1a (0.2 mmol), TBHP
(3.5 eq.), additive (2 eq.), solvent (3 mL), 36 h, irradiated
with a 25 W CFL (one side; CFL = compact fluorescent
light) under air and at room temperature.
[b] NMR yield.
[c] Isolated yield.
[d]
Conditions: anhydrous DCE (water <30 ppm), 4 Å
molecular sieves, and N2 atmosphere.
[e] Irradiated with 2 x 25 W CFL (both sides). TBHP = tert-
butyl hydroperoxide.
[a] Reaction conditions: 1 (0.2 mmol), rose bengal (0.05 eq.),
TBHP (3.5 eq.), KOH (2 eq.), DCE/H2O = 2:1 (v:v, 3 mL),
irradiated with 2 x 25 W CFL (both sides) for 36 h under air
and room temperature.
catalysts, solvents, and bases (Table 1). Using DMF as
the solvent, no reaction took place with acridine,
fluorescein, and methylene blue (entries 1-3).
Gratifyingly, the desired product 2a was obtained in
the presence of eosin Y-2Na or rose bengal (entries 4-
5), albeit in low yields (12% and 18% repectively).
The target product could also be observed with Ru
photocatalyst, although a large amount of starting
materials remained unreacted (entries 6-7). In order to
improve the conversion and yield, solvents were then
screened (see Supporting Information for the detailed
study). DMSO, DME, CH3OH, or MeCN made no
improvements (entries 8-11). With DCE, the
conversion of 1a was improved dramatically.
However, a significant amount of by-product silanol
was obtained, which might come from trace amount of
water (entries 12).[10d] To suppress this side reaction,
anhydrous DCE (water < 30 ppm), molecular sieves,
and Schlenk techniques were applied, however there
were no noticeable benefit (entry 13). Considering the
effect of base on the deprotonation step, further
optimization was focused on the base additives (entries
14-18). The assumption was proved successful by
also suppressed the side reaction (entries 16-17).
However, no improvements were obtained with
stronger bases, such as t-BuONa, and t-BuOK (not
shown). After extensive experimentation, the optimal
conditions were found by using mixed solvents of
DCE and H2O (entries 19-24). Contrary to our
understanding that the presence of water led to the
formation of byproduct silanol,[20] the addition of H2O
in 1:2 ratio with DCE increased the yield to 70% with
only trace amount of silanol formed (entry 22). The
abundance of light was also important for the reaction,
which further advanced the isolated yield of 2a to 77%
(entry 24).
With the optimized conditions in hand, the visible-
light induced radical silylation was applied to a wide
scope of biarylhydrosilane substrates (Table 2). First,
substituents on aryl ring
A were examined.
Biarylhydrosilanes bearing electron-donating and
2
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10.1002/adsc.201800417
Advanced Synthesis & Catalysis
electron-withdrawing groups at the 4’-position gave
the di-methyl substituted silane delivered the
corresponding product 2o in much lower yield
probably due to their less effective stabilizing effect
the desired dibenzosiloles in moderate to excellent
Table 3. Scope of substituted biarylhydrosilanes (ring B, or
both rings)[a]
Table 4. Control experiments and radical scavenger
experiments[a]
[a]
TEMPO = 2,2,6,6-tetramethylpiperidine-1-oxyl. BHT =
2,6-di-tert-butyl-4-methylphenol.
on the silicon radical than phenyl groups.
The effect of substituents on aryl ring B or both
rings was also investigated (Table 3). Overall, the
position and electronic property of the function groups
on aryl ring B has negligible influence on the reaction
profile. Dibenzosiloles 2p, 2r and 2t were obtained in
good yields, which are similar to the results of 2i, 2d
and 2h in Table 2. For substrates with electron-
donating methoxyl and electron-withdrawing chloro
substituents, 2q and 2s were formed in comparable
yields. Inspired by Oestreich's work, di-substituted
precursors were also applied to our conditions, di-tert-
butyl substituted dibenzosilole 2u and dichloro-
substituted dibenzosiloles (2v, 2w) could be obtained
in moderate to good yields. Moreover, orthogonally
difunctionalized silole monomer 2x, which can
undergo site-selective cross-coupling reactions, could
be efficiently obtained in 79% yield in one step under
the standard conditions. The crystal structure of 2x was
shown in the table.[21] The results in Table 2 and 3
demonstrated the excellent functional group
compatibility of this mild visible-light photoredox-
catalyzed radical silylation reaction to access a variety
of benzannulated silole building blocks.
Aseries of experiments were then carried out to gain
insights into the mechanism. When 1a was irradiated
under the standard conditions in the absence of TBHP
or KOH, no desired product was observed (Table 4a).
It is worth noting that without KOH, the reaction gave
silanol in 92% yield. The reaction stopped completely
without light irradiation, however, trace amount of 2a
could be observed in the absence of the photocatalyst.
In addition, when different amounts of TEMPO were
added to the reaction conditions, the reaction was
suppressed gradually (Table 4b). The reaction was
inhibited completely with butylated hydroxytoluene
(BHT). The light/dark experiment was then conducted,
where the reaction didn’t take place in the dark and can
be switched on by turning on the light (see the
Supporting Information, Figure S1). Moreover, with
blue light (450 nm) irradiation, the quantum yield of
[a] Reaction conditions: 1 (0.2 mmol), rose bengal (0.05 eq.),
TBHP (3.5 eq.), KOH (2 eq.), DCE/H2O = 2:1 (v:v, 3 mL),
irradiated with 2 x 25 W CFL (both sides) for 36 h under air
and room temperature.
yields (2b–2i, 2m). Overall, the 2-biarylhydrosilanes
with electron-withdrawing trifluoromethyl (2d),
halogen (2h, 2i), triflate (2m) groups usually gave
better yields than those with electron-donating groups
such as methyl (2b), methoxyl (2f), and t-butyl (2g).
The yields of the target products were also influenced
by the position of functional groups. The 2’-fluoro
substituted biarylhydrosilanes gave the desired
product 2k in 65% yield, while 3’ and 4’-fluoro
substituted dibenzosiloles could be obtained in 72%
and 81% yield, respectively. For meta-fluoro system
and naphthalene ring systems, similar regioselectivity
was observed as in Studer’s studies,[11a] but higher
yields could be achieved under the visible-light
photoredox conditions (2j, 2l, 2n). To our delight,
precursors with oxygen donor, such as silicon-
protected hydroxy group, could deliver the
corresponding product 2e in 81% yield. It is worth
emphasizing that substrate with a triflate group, which
could directly undergo a series of cross-coupling
reactions, showed excellent compatibility with the
visible-light photoredox conditions and gave 2m in
77% yield comparing to the low conversion under
Oestreich's conditions. Furthermore, other substituents
on the silicon center were also tested. Not surprisingly,
3
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10.1002/adsc.201800417
Advanced Synthesis & Catalysis
this reaction was determined to be 0.28% (see the
Supporting Information). These results ruled out the
possibility of a chain reaction process but supported
the photocatalytic pathway where oxidation of
mmol) in the mixture solvent of DCE/H2O = 3 mL (2:1).
After sealing the tube, the mixture was stirred at room
temperature and irradiated with 2 x 25 W CFL (both sides)
for 36 h under air and room temperature. After the reaction
finished, the reaction mixture was poured into 10 mL water
and extracted with DCM (3 × 20 mL), the combined organic
phase was dried over Na2SO4. The reaction mixture was
concentrated under vacuum and purified by column
chromatography to afford the desired product.
Acknowledgements
This work was supported by the National Natural Science
Foundation of China (NSFC) (Grant No. 21402092, 21772092),
Natural Science Foundation of Jiangsu Province – China (Grant
No. BK20140775), the Priority Academic Program development of
Jiangsu Higher Education Institutions – China (PAPD), the
Fundamental Research Funds for the Central Universities” (No.
30917012202). We thank Dr. Jianwei Bian and Dr. Xiangbing Qi
for helpful discussions.
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Scheme 2. Proposed catalytic cycle of C-H silylation via
visible-light photoredox catalysis. FG = functional group.
PC = photocatalyst, rose bengal.
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