Metal-free visible-light-mediated aerobic oxidation of silanes to silanols

Article abstract of DOI:10.1007/s11426-018-9289-9

Oxidation of silanes into silanols using water/air has attracted considerable attention. The known methods with no exception required a metal catalyst. Herein we report the first metal-free method: 2 mol% Rose Bengal as the catalyst, air (O₂) as the oxidant, water as the additive and under visible light irradiation. While this method produces various silanols in a simple, cost-effective, efficient (92%–99% yields) and scalable fashion, its reaction mechanism is very different than the reported ones associated with metal catalysis.

Full text of DOI:10.1007/s11426-018-9289-9

SCIENCE CHINA
Chemistry
•ARTICLES• . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . https://doi.org/10.1007/s11426-018-9289-9
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Metal-free visible-light-mediated aerobic oxidation of silanes to
silanols
Jing Wang, Bin Li, Li-Chuan Liu, Chenran Jiang, Tao He & Wei He*
School of Pharmaceutical Sciences, Tsinghua University, Beijing 100084, China
Received May 3, 2018; accepted May 21, 2018; published online August 2, 2018
Oxidation of silanes into silanols using water/air has attracted considerable attention. The known methods with no exception
required a metal catalyst. Herein we report the first metal-free method: 2 mol% Rose Bengal as the catalyst, air (O2) as the
oxidant, water as the additive and under visible light irradiation. While this method produces various silanols in a simple, cost-
effective, efficient (92%–99% yields) and scalable fashion, its reaction mechanism is very different than the reported ones
associated with metal catalysis.
metal-free, photoredox, aerobic oxidation, silanes
Citation:
Wang J, Li B, Liu LC, Jiang C, He T, He W. Metal-free visible-light-mediated aerobic oxidation of silanes to silanols. Sci China Chem, 2018, 61,
https://doi.org/10.1007/s11426-018-9289-9
1 Introduction
been ingeniously exploited as enzyme inhibitors [7] and
isosteres of pheromones [8]. The classic syntheses of silanols
involve hydrolysis of chlorosilanes [9] and less commonly
nucleophilic substitution of siloxanes [10]. The direct oxi-
dation of hydrosilanes into silanols represents a more de-
sirable approach since it produces less byproducts [11].
Visible light photoredox catalysis has received rapidly
growing attention because it may constitute an ideal platform
for sustainable chemistry. It has many conceived advantages
due to the characteristics of light (abundant, non-toxic and
generating no waste) as well as the related process (oper-
ationally simple and scalable) [1]. The past decade has wit-
nessed the development of various powerful photoredox
catalyst systems [2]. Among them, transition metal dyes [2a–
2d] are extensively studied and most successful. In com-
parison, the application of organic dyes [2e–2j] is still limited
but warrant further investigations since they are more cost-
effective and environmental benign.
Conventional methods entail
stoichiometric oxidants
[11a,11b]. The focus has now shifted to the usage of water
and air as the green oxidants [11c–11f]. Our longstanding
interest in silicon chemistry has also led to the single-site
Au(I) catalyzed water oxidation of silanes [11f]. While these
methods provide greener access to silanols, the universal
employment of a transition metal catalyst [11g] causes bur-
dens in product purification and waste treatment.
Silanols are ubiquitous building blocks in silicone industry
[3a,3b]. They are also versatile synthons in organic synthesis
[3c,3d], serving as nucleophilic partners in cross-coupling
reactions [4], organocatalysts for activating carbonyl com-
pounds [5] and directing groups for C−H bond activation
reactions [6]. In medicinal chemistry, the Si−OH moiety has
We thus became curious if it is possible to develop a metal-
free aerobic oxidation of slianes using photoredox catalysis.
In this context, Fu’s group [12] has reported the first pho-
toredox oxidation of silanes with water using a Rh(III) por-
phyrins photocatalyst. Herein, we reported the first examples
of metal-free visible-light-mediated oxidation of silanes to
silanols (Scheme 1). Thus, with low loading (2 mol%) of
Rose Bengal [13] as the photocatalyst under white LED ir-
*Corresponding author (email: whe@tsinghua.edu.cn)
© Science China Press and Springer-Verlag GmbH Germany, part of Springer Nature 2018 . . . . . . . . . . . . . . . . . . . . . chem.scichina.com link.springer.com

2 . . . . . . . . . . . . . . . . . . . . . . . . . . . .Wang et al. Sci China Chem . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2
Scheme 1 Synthetic methods for silanols (color online).
radiation at room temperature, an array of silanes was con-
verted into the corresponding silanols in exceptionally high
yields (92%–99%) within 12 h. Moreover, our studies un-
covered a reaction mechanism that is very different than the
ones reported in metal catalysis including Fu’s work.
were tested but none of them is comparable with THF (Table
1, entries 11–17). By note, the reaction in methanol only
produced the alcoholysis product Ph3SiOMe but not silanol
2a (Table 1, entry 15).
A number of controlled experiments were carried out to
dissect the roles of oxidants. Under nitrogen atmosphere,
water alone could not oxidize the silane (Table 1, entry 18).
Consistent with this observation, when an oxygen balloon
was used instead of open air, 90% conversion was achieved
in dry THF (i.e., without appreciable amount of water) after
72 h (Table 1, entry 19) and complete conversion could be
achieved within 12 h when water is added which gave silanol
2a (Table 1, entry 20). These experiments collectively sug-
gested that molecular oxygen but not water was likely the
oxidant. This is remarkably different from metal catalysis
[11c] as well as Rh(III) mediated photochemistry [12], where
water itself was a competent oxidant. Rose Bengal, visible
light and oxygen were all necessary for the success of this
transformation. The optimal conditions were thus identified
as listed in entry 2 (Table 1). The reaction time could be
shorten to 3 h when heating at 60 °C, which might be more
desirable for industrial application.
2 Experimental
To a solution of Rose Bengal (0.008 mmol, 2 mol%) in THF
2 mL was added silane 1 (0.4 mmol, 1 equiv.) and H2O (50
μL). The reaction mixture was open to the air (with a syringe
needle inserted into the rubber cover to intoduce the air into
the system and avoid the solvent evaporation heavily, the
same below), and stirred under the irradiation of a household
200 W white LED at room temperature for 12 h. After
completion of the reaction (monitored by GC-MS or TLC),
the solvent was removed under reduced pressure, the crude
product was purified by column chromatography with pet-
roleum ether/ethyl acetate (10:1) as eluent to give the desired
silanol 2.
With the optimal conditions in hand, we successfully ex-
tended the reaction to a scope of silane substrates. In all the
reactions, silanols were obtained as the only isolable product
and the formation of troubling siloxanes was not observed. In
the case of the hindered silane 1b, the reaction was more
sluggish: the desired silanol 2b was obtained in 78% yield
after 12 h irradiation (Table 2). However, extending the re-
action time to 24 h could achieve complete conversion and
an excellent yield (95%). Replacing one of the phenyl group
with either bulky tert-butyl group (1c) or smaller methyl
group (1d) did not affect the reaction, giving the product in
97% and 99% yield, respectively. Further introducing of a
second alkyl group was also well tolerated as shown in the
case of 1e (97% yield). Trialkylsilanes are also amenable to
this reaction despite of the alkyl group sizes, giving silanols
2f, 2g and 2h in excellent yields. Unsaturated carbon-carbon
bonds were compatible with this method: alkynyl silane 1i
and alkenyl silane 1j furnished the desired products in almost
quantitative yields, albeit the former required longer reaction
time. It is worthy to point out that dihydrosilane 1k, as well
as silane bearing an electron withdrawing group 1m, reacted
smoothly under the current reaction conditions. Importantly,
without tuning of reaction conditions except for reaction
3 Results and discussion
We started our investigation using the less reactive triphe-
nylsilane 1a as the model substrate. In the presence of
2 mol% of Rose Bengal in tetrahydrofuran (THF) at room
temperature in open air, triphenylsilanol 2a was isolated as
the single product in 67% yield (70% conversion as mon-
itored by crude 1 H NMR) after 12 h irradiation of white LED
(Table 1, entry 1). Gratifyingly, complete conversion of 1a
was obtained when 7 equiv. of water was added to the re-
action (Table 1, entry 2). Three other photocatalysts were
also examined but none of them appeared to be effective
(Table 1, entries 3–5). Importantly, very minimal conversion
was observed when the reaction was carried out in the ab-
sence of a photocatalyst (Table 1, entry 6) or without light
(Table 1, entry 7). The catalyst loading of the Rose Bengal
was examined, showing that increased loading from 2 mol%
to 5 mol% to 10 mol% slightly diminished the yields (cf.
Table 1, entry 2, 8, 9). When 1 equiv. of Rose Bengal was
added, the yield drastically dropped to 43% (Table 1, entry
10), probably due to the shield of light resulting from the
incomplete resolution of catalysts. A number of solvents

3. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Wang et al. Sci China Chem
Table 1 Optimization of reaction conditionsa)
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
Entry
1
Catalyst
Solvent
THF
Oxidants
Air
Conversion/Yield (%)b)
70/67
Rose Bengal
Rose Bengal
Eosin B
2
THF
Air/H2O
Air/H2O
Air/H2O
Air/H2O
Air/H2O
Air/H2O
Air/H2O
Air/H2O
Air/H2O
Air/H2O
Air/H2O
Air/H2O
Air/H2O
Air/H2O
Air/H2O
Air/H2O
N2/H2O
O2
>99/98
<5/–
3
THF
4
Eosin Y
THF
<5/–
5
Fluorescein
–
THF
<5/–
6
THF
<5/–
7c)
8d)
9e)
10f)
11
12
13
14
15g)
16
17
18
19
20
Rose Bengal
Rose Bengal
Rose Bengal
Rose Bengal
Rose Bengal
Rose Bengal
Rose Bengal
Rose Bengal
Rose Bengal
Rose Bengal
Rose Bengal
Rose Bengal
Rose Bengal
Rose Bengal
THF
<5/–
THF
>99/97
>99/95
47/43
THF
THF
DME
DMF
90/86
50/47
DMA
MeCN
MeOH
Acetone
DMSO
THF
13/10
<5/–
60/53
77/72
55/51
<5/–
90/86h)
Dry THF
Dry THF
O2/H2O
>99/97
a) Reaction conditions unless otherwise stated: triphenysilane 1a (0.4 mmol), H2O (50 μL), photocatalyst (2 mol%), otherwise noted, solvent (2 mL)
without anhydrous treatment, using a 200 W white LED as the visible light source, 12 h. b) Yields after chromatography. c) Reaction performed in dark. d) 5
mol%, e) 10 mol% and f) 100 mol% catalyst was used, respectively. g) Alcoholysis product was isolated. h) The reaction time was 72 h.
time, this operationally simple method gave universally ex-
cellent yields to all types of silanes. We also demonstrated its
applicability to gram scale reaction of silane 1a as example
(1.04 g, 4 mmol), which gave silanol 2a in 99% yield.
A series of controlled experiments were carried out to
better understand the reaction mechanism. H218O was added
to a reaction performed under 16O2 atmosphere and the re-
action products were monitored by gas chromatography-
mass spectrometer (GC-MS) (Table 3). It was observed that
in the initial stage of the reaction (entry 1), mainly the Ph3
Si16OH was produced. However, as the reaction proceeded
Ph3Si18OH started to dominate (Table 3, entries 2–4). Im-
portantly, our control experiments showed that there was no
oxygen exchange between H2O and the silanol under reac-
1
tion conditions (see Supporting Information online). Alto-
Scheme 2 Controlled Experiments to Verify O2 (color online).
gether these indicated that in the initial stage the oxygen in
the silanol was mainly originated from the oxygen molecule
(16O2) and then gradually from the water (H218O). The deu-
PO), a radical scavenger, completely inhibited the production
terium kinetic isotope effect (KIE) was measured to be a
small KIE (kH/kD is about 1.21), suggesting that Si–H bond
cleavage was not the rate-limiting step.
of the silanol (Scheme 2, Eq. (1)), suggesting a radical
pathway might be operative in this reaction. It is well re-
cognized that Rose Bengal could react with molecular oxy-
gen [2g,13e–13f,14] under visible light irradiation to
Addition of 2,2,6,6-tetramethyl-1-piperdinyloxy (TEM-

4 . . . . . . . . . . . . . . . . . . . . . . . . . . . .Wang et al. Sci China Chem . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
Table 2 Substrate scope of the organocatalytic oxidation of silane a,b)
2a
2b
2c
2d
98% (12 h)
78% (12 h)
95% (24 h)
97% (12 h)
99% (12 h)
99% (gram scale)
2e
2f
2g
2h
97% (12 h)
92% (12 h)
97% (12 h)
99% (12 h)
2i
2j
2k
2m
81% (12 h)
97% (24 h)
98% (12 h)
98% (12 h)
97% (12 h)
a) Standard conditions unless otherwise stated: silane (0.4 mmol), Rose Bengal (2.0 mol%), H2O (50 μL),THF (2 mL), 200 W white LED, air, r.t., 12 h or
24 h. b) Isolated yields.
generate singlet oxygen.
To verify the presence of 1O2 in our reaction, we carried out
the trapping and quenching experiments. The trapping ex-
periment [15] with 9,10-dimethylanthracene 3 under the
standard conditions furnished the endoperoxide product 4
(Scheme 2, Eq. (2)), while such an adduct product 4 was not
observed in the reactions performed in the dark or without
Rose Bengal. The reaction was completely quenched in the
presence of DABCO, which is known as a strong physical
quencher of singlet oxygen (Scheme 2, Eq. (3)). These ex-
1
Figure 1 Proposed reaction mechanism (color online).
periments indicated that O2 was produced in the reaction
system. The reaction didnot take place in the dark and can be
switched on by turning on the light, suggesting that a pho-
tocatalytic pathway but not the chain propagation is likely
the main mechanism. Such a notion was further supported by
fact the quantum yield of the oxidation of 1a was measure to
be around 0.017 (see Supporting Information online). In
addition, fluorescence quenching experiments showed that
the emission intensity of excited Rose Bengal (RB*) was not
affected by the silane concentration, which suggested that
there was no energy transfer between silane 1a and Rose
Bengal.
state species RB*, which interacted with 3O2 to generate 1O2
via the energy transfer (ET). In this process, the excited state
RB* returned to its ground state RB. Subsequently, the
generated 1O2 abstracted an electron from silane 1 to give the
•−
radical cation A along with superoxide anion O2 . The ab-
straction of a proton from the radical cation A by the su-
peroxide radical anion produced a hydroperoxy radical
HOO• and a silyl radical B. The oxidation of Si–H bond by
O2•− is a good analogue to the proposed oxidation of S–H
bond under similar conditions, reported independently by
Wang’s group [16a] and the Barman’s group [16b] in their
inspiring work. The silyl radical B and the HOO• radical
formed a Si–O bond in the form of silylperoxide C. The
On the basis of the above experimental observations as
well as literature examples [16], a plausible reaction me-
chanism is proposed (Figure 1). First, Rose Bengal (RB) was
excited under visible light irradiation to produce its excited

5. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Wang et al. Sci China Chem
Table 3 Controlled Experiments of Reaction with H218O and 16O2a)
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
Conversion
Ph3Si18OH
Ph3Si16OH
Entry
Time (h)
(%)
in 2a (%)b)
in 2a (%)b)
1
2
3
4
0.3
1
2.7
33
66
65
67
67
34
35
33
5.8
6
71.5
12
100
a) Reaction conditions: triphenysilane 1a(0.4 mmol), H218O (50 μL), Rose Bengal (2 mol%), dry THF (2 mL), 200 W white LED, reacting in a freshly
dried Schlenk tube under the 16O2 atmosphere. b) The relative amount of Ph3Si18OH and Ph3Si16OH in 2a was analyzed by GC-MS.
direct excursion of an oxygen atom from silylperoxide C to
form the silanol product 2 has not been reported, which is
also very unlikely. Instead, we propose that water acted as a
nucleophile to attack the silicon, forming a pentavalent ate
complex D. Such types of pentavalent Si complex has been
documented in many previous studies [17]. This would ex-
plain the rate difference of the reaction with and without
external water (cf. entries 19 and 20, Table 1). The decom-
position of intermediate D into the silanol product 2 should
involve several different σ metatheses pathways, where ei-
ther of the two Si–O bonds could be cleaved. It is possible
that the O–O bond can also be cleaved, which accounts for
the 2:1 statistical ratio of Ph3Si16OH/ Ph3Si18OH in the initial
stage of the reaction (cf. entry 1, Table 3).
The production of silanol 2 is accompanied with the pro-
duction of equal molar of hydrogen peroxide E. As the re-
action proceeded, accumulated hydrogen peroxide E started
to react with silyl radical B, which also produced inter-
mediate C. In other words, the OOH group in silylperoxide C
can come from either dioxygen (pathway a) or HOOH
(pathway b). In the early stage, as there was little HOOH,
pathway a was dominant, while in later stage pathway b
became significant. This could explain the increasing per-
centages of the Ph3Si18OH as the reaction proceeded. Even
though we know that the Si–H cleavage is not the rate de-
termining step, it is hard to deduce the rate limiting step.
Much more in-depth kinetic studies are required to uncover
the mechanistic aspects of this interesting reaction. None-
theless, the known aspects of the reaction mechanism is very
different than the reported ones [11,12].
reaction operates under simple, mild conditions yet produces
a wide scope of silanols in extremely high efficiency. Given
these advantages and the conceived advantage of visible
light photoredox catalysis in mass scale production, we be-
lieved this method might serve as the first step to produce
silanols by photocatalysis at industrial scale.
Acknowledgements This work was supported by the National Key R&D
Program of China (2017YFA0505200) and the National Natural Science
Foundation of China (21625104, 21521091).
Conflict of interest The authors declare that they have no conflict of
interest.
Supporting information The supporting information is available online at
http://chem.scichina.com and http://link.springer.com/journal/11426. The
supporting materials are published as submitted, without typesetting or
editing. The responsibility for scientific accuracy and content remains en-
tirely with the authors.
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4 Conclusions
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low cost, readily available organic dyes, Rose Bengal, as the
photocatalyst and oxygen in air as the green oxidant. The
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