Neutral-Eosin-Y-Photocatalyzed Silane Chlorination Using Dichloromethane

Article abstract of DOI:10.1002/anie.201908556

Chlorosilanes are versatile reagents in organic synthesis and material science. A mild pathway is now reported for the quantitative conversion of hydrosilanes to silyl chlorides under visible-light irradiation using neutral eosin Y as a hydrogen-atom-transfer photocatalyst and dichloromethane as a chlorinating agent. Stepwise chlorination of di- and trihydrosilanes was achieved in a highly selective fashion assisted by continuous-flow micro-tubing reactors. The ability to access silyl radicals using photocatalytic Si?H activation promoted by eosin Y offers new perspectives for the synthesis of valuable silicon reagents in a convenient and green manner.

Full text of DOI:10.1002/anie.201908556

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Accepted Article
Title: Neutral-Eosin-Y-Photocatalyzed Silane Chlorination Using
Dichloromethane
Authors: Xuanzi Fan, Pin Xiao, Zeqing Jiao, Tingting Yang, Xiaojuan
Dai, Wengang Xu, Jin Da Tan, Ganglong Cui, Hongmei Su,
Weihai Fang, and Jie Wu
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201908556
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10.1002/anie.201908556
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Neutral-Eosin-Y-Photocatalyzed Silane Chlorination Using
Dichloromethane
Xuanzi Fan, Pin Xiao, Zeqing Jiao, Tingting Yang, Xiaojuan Dai, Wengang Xu, Jin Da Tan, Ganglong
Cui*, Hongmei Su, Weihai Fang, and Jie Wu*
Abstract: Chlorosilanes are versatile reagents in organic synthesis
and material science. We herein report a mild pathway for the
quantitative conversion of hydrosilanes to silyl chlorides under visible-
light irradiation using neutral eosin Y as a hydrogen atom transfer
photocatalyst and dichloromethane as a chlorinating agent. Stepwise
chlorination of di- and trihydrosilanes was achieved in a highly
selective fashion assisted by continuous-flow micro-tubing reactors.
The ability to access silyl radicals using photocatalytic SiH activation
promoted by eosin Y offers new perspectives for the synthesis of
valuable silicon reagents in a convenient and green manner.
accommodates an extremely broad substrate scope due to no
limitation on redox potentials.[5] In view of the electrophilic nature
of the oxygen radical in the photo-excited eosin Y triplet state, it
was anticipated that this catalytic HAT protocol might be extended
to abstraction of protons from hydridic SiH bonds, to access
transient and reactive silyl radicals for the synthesis of value-
added silicon reagents.[6]
Photo-induced catalytic hydrogen atom transfer (HAT) can
activate substrates without the limitation of redox potentials, and
has provided enormous opportunities for CH activations.[1]
A
photoredox catalyst and a HAT mediator are normally involved in
these reactions, where a reactive species is generated by photo-
promoted single-electron oxidation of the mediator for the
subsequent hydrogen atom abstraction. A more reagent- and
redox-economical strategy is by employing the direct photo HAT
catalyst, which can directly abstract the hydrogen atom after
assimilating a photon, omitting the redox activation step (Scheme
1a).[1] Photo-induced direct HAT catalysis has endowed with a
wide range of reaction patterns to allow CH functionalization.[2]
However, SiH activation has rarely been investigated as a
means of synthesizing value-added silicon compounds through
the use of modern photocatalysis strategy.[3] As silicon is more
electropositive than carbon (electronegativity of 1.90 vs. 2.55 on
the Pauling scale), SiH bonds are generally more hydridic than
CH bonds. In this context, we have recently achieved a visible-
light mediated hydrosilylation of both electron-deficient and
electron-rich alkenes through the synergistic merger of a
photoredox catalysts and a HAT mediator.[4] Additionally, we
recently disclosed the unique property of neutral eosin Y, which
can function as an ideal direct HAT photocatalyst due to its visible-
light-absorption, metal-free, readily available, and low-costing
nature.[5] It has been applied to the CH alkylation which
Scheme 1. Chlorination of hydrosilanes through photo-induced HAT.
Chlorosilanes are widely used reagents in organic chemistry
and material science. They are extensively utilized in multistep
organic synthesis for the installation of silyl protecting groups on
alcohols, amines, acids, and alkynes.[7] They are also applied in
the preparation of silyl metallic reagents or silyl enol ethers to
access “hard” or “soft” nucleophiles.[8] Chlorosilanes are
extensively involved in the production of silicon-based polymers,[9]
and coating for silicon and glass surfaces.[10] However, the
synthesis of chlorosilanes by chlorination of SiH bonds normally
requires stoichiometric amounts of metal salts in conjunction with
hazardous chlorine reagents, such as tin chloride, lead chloride,
Cl2, and CCl4, or the use of expensive transition-metal catalysts
(Scheme 1b).[11] Notably, Dobrovetsky et al. recently reported a
B(C6F5)3-catalyzed selective chlorination of hydrosilanes using
HCl as the chlorinating agent.[12] However, a large excess
gaseous HCl was employed in this method in an uncontrollable
way. Consequently, the development of an operationally simpler
and green catalytic protocol for chlorinating of hydrosilanes is
appealing and highly desirable. Herein, we describe a green and
selective method to generate chlorosilanes through neutral eosin
Y-catalyzed HAT with hydrosilanes under visible-light irradiation
[*]
X.-Z. Fan, T.-T. Yang, W.-G. Xu, J. D. Tan, Dr. J. Wu
Department of Chemistry,
National University of Singapore
3 Science Drive 3, 117543 (Republic of Singapore)
E-mail: chmjie@nus.edu.sg
P. Xiao, Z.-Q. Jiao, X.-J. Dai, Prof. Dr. G.-L. Cui, Prof. Dr. H.-M. Su,
Prof. Dr. W.-H. Fang
College of Chemistry, Beijing Normal University
Beijing, 100875 (China)
E-mail: ganglong.cui@bnu.edu.cn
T.-T. Yang,
College of Chemistry and Materials Science,
Anhui Normal University
Wuhu, 241002 (China)
Supporting information and the ORCID identification number(s) for
the author(s) of this article can be found under: https://doi.org/
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10.1002/anie.201908556
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using CH2Br2 as an internal standard. [c] Irradiated using an 18 W white LED
strip.
using dichloromethane (DCM) as both the chlorinating agent and
solvent (Scheme 1c). Different from the industrial Müeller-
Rochow process to catalytically prepare chlorosilanes using alkyl
halides and elemental silicon at high temperature (e.g. 300 oC),[13]
our method provides a convenient and complement way to access
chlorosilanes from hydrosilanes at a laboratory scale.
The generality of silane chlorination was evaluated employing
the optimal reaction conditions (Table 1, entry 1). As depicted in
Scheme 2, aryl-substituted silanes including tri-, di-, and mono-
aryl silanes with different steric properties were well-tolerated and
afforded the corresponding chlorosilanes (2-8) quantitatively.
Trialkylsilanes with different sized alkyl groups performed
effectively to give the corresponding products (9-15) in excellent
yields. Disilanes such as para-phenylenebis(dimethylsilane) were
also effective substrates, smoothly delivering the dichlorinated
product (16). The dichlorosilanes (17-20) could be quantitatively
generated by chlorination of either R1R2SiH2 or the mono-
Our initial investigation showed that treatment of
diphenylmethyl silane (1) in dichloromethane in the presence of 1
mol% neutral eosin Y under blue LED irradiation (λmax = 470 nm,
18 W), produced quantitative yield of diphenylmethylchlorosilane
(2) (Table 1, entry 1). Dichloromethane is widely utilized as
chlorinated solvent in pharmaceutical chemistry, which has been
proposed as a replacement for other chlorinated solvents in cases
when a chlorine-free solvent is not applicable.[14] It acted as both
a chlorine source and solvent media in our protocol, avoiding the
usage of hazardous chlorinated reagents. The reaction became
less efficient when dichloromethane was replaced by chloroform
or dichloroethane (DCE) (entries 2 and 3). A lower efficiency was
also detected in more concentrated dichloromethane solution
(entry 4). Notably, employment of Ru(bpy)3(PF3)2 as the
photocatalyst, which possesses redox properties similar to those
of eosin Y, failed to produce any of the desired product (entry 5).
No transformation was observed using the commercial dianionic
form of eosin Y as the photocatalyst (entry 6), and this is
consistent with our previous recognition of neutral eosin Y as the
active HAT photocatalyst.[5] The yield of chlorosilane decreased
significantly in the presence of air (entry 7), which led to
production of siloxane by-products. The reaction was suppressed
in the presence of 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO),
chlorosilane,
R1R2SiClH.
Dichloromethylsilane
delivered
methyltrichlorosilane (21) smoothly. Phenylsilane was a good
candidate in this protocol and produced phenyltrichlorosilane (22).
In contrast to tetrabutylammonium decatungstate-catalyzed
hydrosilylation under UV light irradiation, in which a mixture of the
SiH and CH activation products were obtained,[6] the eosin Y-
catalyzed protocol demonstrated an excellent selectivity of SiH
activation in the presence of activated CH bonds such as
benzylic and -silyl CH bonds.
suggesting
a
radical-based process (entry 8). Control
experiments demonstrated that light and the photocatalyst were
both essential for efficient chlorination (entry 9).
Table 1 Investigation of reaction parameters for silane chlorination[a]
entry
deviation from standard conditions[a]
none
yield(%)[b]
1
2
3
4
5
6
7
8
9
99
85
67
71
0
CHCl3 instead of CH2Cl2
ClCH2CH2Cl instead of CH2Cl2
CH2Cl2 (0.5 M)
Ru(bpy)3(PF3)2 instead of eosin Y
ionic eosin Y instead of neutral eosin Y[c]
with air balloon
0
31
0
adding TEMPO (2.0 equiv)
no photocatalyst or no light
0
[a] Standard conditions: 1 (1 mmol), neutral eosin Y (1 mol%) and CH2Cl2 (5
mL) under N2 in a 10 mL seal vial, irradiated under 18 W blue LED for 4 h at
ambient temperature. [b] Yields determined by analysis of the crude 1H NMR
Scheme 2. Scope of silane chlorination.
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DFT calculations (see the Supporting Information for
details)[15] of transition states for hydrosilane HAT with light-
activated eosin Y at various sites were conducted to probe the
kinetic origins of selectivity. As illustrated in Scheme 3, the
calculated activation energies revealed an appreciable kinetic
preference for abstraction of SiH rather than the benzylic and -
silyl CH bonds by the photo-activated eosin Y.
studies (Figure S8) indicated that the SiH activation was not
induced by either an electron- or an energy-transfer process. The
HAT process was further supported by laser flash photolysis
experiments. Upon laser excitation at 532 nm, a new absorption
peak emerged after 30 μs at 360 nm after addition of silanes in
eosin Y CH2Cl2 and MeCN solution, which was not observed in
the absence of silanes (Figures S10 and S11). It was
corresponding to the radical intermediate of eosin YH generated
from HAT between *eosin Y and the silane.[5] Both light on/off
experiments (Figure S12) and the calculated quantum yield ( =
0.35) suggested a nonchain pathway.[18]
Scheme 3. Calculated relative activation energies (G‡) for HAT with photo-
activated eosin Y.
Compared to stirred reactor vessels, continuous-flow micro-
tubing reactors have significant processing advantages including
improved mass/heat transfer and mixing efficiency, and precise
control of residence time.[16] They provide an excellent platform for
photochemical reactions due to the enhanced and uniform light
irradiation and ease of scaling up.[17] As demonstrated in Scheme
4, the silane chlorination assisted by an operationally simple
continuous-flow micro-tubing reactor was amenable to gram-
scale production (2,3,11). More intriguingly, stepwise chlorination
of di- and trihydrosilanes (R1R2SiH2 and RSiH3) was successfully
achieved in a highly selective manner (23 vs 17; 24 vs 25 vs 22).
Compared to the dramatically reduced selectivity obtained in
batch stirred reactors (Scheme S4), the key to success relied on
the excellent mixing efficiency and precise control of reaction
temperature and residence time associated with continuous-flow
micro-tubing reactors.[16]
To gain further insight into the nature of the chlorination,
several control experiments were performed. Reaction monitoring
by UV/Vis spectroscopy (Figure S7) proved that the neutral eosin
Y species was the active photocatalyst during the reaction
process.[5] Hydrosilylation product 27 was generated in good
yields with the same photocatalytic system by addition of alkene
26 in either acetone or DCE, while no reaction was observed in
the absence of silanes (Scheme 5a), which supported the
generation of silyl radicals in different solvents. This eosin Y-
based photocatalytic protocol therefore represents one of the
most mild and convenient methods to introduce silyl radicals from
hydrosilanes. When the silane chlorination was performed in
deuterated chloroform, mono-deuterated DCM (CDHCl2) was
clearly detected in the 1H NMR spectra of the crude reaction
mixture (Scheme 5b, Figure S13). Luminescence quenching
Scheme 4. Gram-scale and stepwise chlorination in continuous-flow micro-
tubing reactors.
In light of all the experimental data, a plausible mechanism
was proposed as illustrated in Scheme 5c. The silyl radical III is
generated smoothly by the HAT between light-activated *eosin Y
(I) and hydrosilane. The formation of an intermediate eosin Y-H
(II) was confirmed by laser flash photolysis experiments. The
derived silyl radical subsequently abstracts the chloride from
DCM to deliver the silyl chloride product and the chloromethyl
radical species (IV). A reverse hydrogen atom transfer (RHAT)
between radical IV and eosin Y-H (II) regenerates ground state
eosin Y and produces chloromethane, which was detected by 1H
NMR analysis of the crude reaction mixture. Based on DFT
calculations, the RHAT process has an energy barrier of 11.8
kcal/mol, which is significantly lower than the 37.4 kcal/mol
associated with a single electron transfer pathway between II and
IV (Scheme S1, Figures S4 and S5).
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In conclusion, we have developed a visible-light-mediated
silane chlorination catalyzed by neutral eosin Y, using DCM as
both
a chlorinating agent and the solvent media. This
transformation accommodates a wide range of silanes, achieving
products with exclusive SiH activation in the presence of
activated CH bonds. This protocol is also distinguished by its
operational simplicity, metal-free character, use of ambient
conditions, green reagents, and delivery of quantitative yields in
most cases, and can be directly streamlined with a sequential
transformation. A highly selective stepwise chlorination of di- and
trihydrosilanes was achieved assisted by the unique advantages
of continuous-flow micro-tubing reactors. More importantly, the
neutral eosin Y-based HAT provides a very convenient and green
approach for the generation of silicon radicals under visible-light
irradiation, which promise to find wide applications in the
synthesis of valuable silicon compounds in both academic and
industrial settings.
Acknowledgements
We are grateful for the financial support provided by the Ministry
of Education (MOE) of Singapore (MOE2017-T2-2-081), GSK-
EDB (R-143-000-687-592), National Natural Science Foundation
of China (Grant No. 21702142, 21871205, 21425313) and the
National University of Singapore (Suzhou) Research Institute.
Scheme 5. Proposed mechanism and supporting evidence.
The synthetic value of this straightforward photo-mediated
silane chlorination was further demonstrated by one-pot synthesis
of various types of valuable compounds. As illustrated in Scheme
6, the crude DCM solution of silane chlorination could be directly
utilized in subsequent transformations for silanol synthesis (path
a), alcohol protection (path b), silyl enol ether formation (path c),
silylation of nitroalkanes (path d), alkyne protection (path e), and
phenyl silylation (path f), achieving products in good to excellent
yields in all cases.
Keywords: hydrogen atom transfer • SiH activation • neutral
eosin Y • silyl chloride • dichloromethane
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COMMUNICATION
Xuanzi Fan, Pin Xiao, Zeqing Jiao,
Tingting Yang, Xiaojuan Dai, Wengang
Xu, Jin Da Tan, Ganglong Cui*,
Hongmei Su, Weihai Fang, and Jie Wu*
Page No. – Page No.
Neutral-Eosin-Y-Photocatalyzed
Silane Chlorination Using
Dichloromethane
A mild and convenient silane chlorination has been achieved using neutral eosin Y as a direct hydrogen
atom transfer photocatalyst and dichloromethane as both solvent and the chlorinating reagent. Selective
stepwise chlorination of di- and trihydrosilanes was achieved in a controllable manner using
continuous-flow micro-tubing reactors.
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