Selective Electrochemical Hydrolysis of Hydrosilanes to Silanols via Anodically Generated Silyl Cations

Article abstract of DOI:10.1002/anie.202010437

The first electrochemical hydrolysis of hydrosilanes to silanols under mild and neutral reaction conditions is reported. The practical protocol employs commercially available and cheap NHPI as a hydrogen-atom transfer (HAT) mediator and operates at room temperature with high selectivity, leading to various valuable silanols in moderate to good yields. Notably, this electrochemical method exhibits a broad substrate scope and high functional-group compatibility, and it is applicable to late-stage functionalization of complex molecules. Preliminary mechanistic studies suggest that the reaction appears to proceed through a nucleophilic substitution reaction of an electrogenerated silyl cation with H₂O.

Full text of DOI:10.1002/anie.202010437

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Accepted Article
Title: Selective Electrochemical Hydrolysis of Hydrosilanes to Silanols
via Anodically Generated Silyl Cations
Authors: Hao Liang, Lu-Jun Wang, Yun-Xing Ji, Han Wang, and Bo
Zhang
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10.1002/anie.202010437
Angewandte Chemie International Edition
RESEARCH ARTICLE
Selective Electrochemical Hydrolysis of Hydrosilanes to Silanols
via Anodically Generated Silyl Cations
Hao Liang+, Lu-Jun Wang+, Yun-Xing Ji, Han Wang, and Bo Zhang*
Abstract: The first electrochemical hydrolysis of hydrosilanes to
silanols under mild and neutral reaction conditions is reported. The
practical protocol employs commercially available and cheap NHPI
as a hydrogen-atom transfer (HAT) mediator and operates at room
temperature with high selectivity, leading to various valuable silanols
in moderate to good yields. Notably, this electrochemical method
exhibits
a broad substrate scope and high functional-group
compatibility, and it is applicable to late-stage functionalization of
complex molecules. Preliminary mechanistic studies suggest that the
reaction appears to proceed through a nucleophilic substitution
reaction of an electrogenerated silyl cation with H2O.
Introduction
Silanols are highly important and valuable compounds, which
are frequently employed as building blocks for silicon-based
polymeric materials,[1] nucleophilic partners in metal-catalyzed
cross-coupling reactions,[2] directing groups for C-H
functionalization,[3] organocatalysts for activating carbonyl
compounds,[4] and isosteres of bioactive molecules.[5] Wide
applications trigger continued interest in the synthetic chemistry
of silanols.[1c,e] The hydrolysis of chlorosilanes is a conventional
method for the preparation of less bulky silanols.[6] However, this
approach requires strictly controlled reaction conditions and it is
difficult to prepare sterically hindered silanols that easily
condense to produce disiloxanes. The direct transformation of
hydrosilanes into silanols by oxidation of Si-H bonds with strong
stoichiometric oxidants, such as silver salts,[7a] ozone,[7b]
peracids,[7c] dioxiranes,[7d] oxaziridines,[7e] permanganate,[7f] and
osmium tetroxide,[7g] has also been developed. Nevertheless,
selectivity and compatibility issues remain impediments of these
methods. For example, these oxidation reactions often lead to
the unwanted formation of disiloxanes by condensation of silanol
products or byproducts resulting from silanol decomposition,[7]
thus deterring broad utilization. Transition-metal catalysis has
provided an attractive alternative to access silanols. Several
metal catalysts, such as Ti,[8a] Re,[8b] W,[8c] Te,[8d] Co,[8e] and
Mn,[8f] have been exploited to convert hydrosilanes to silanols
with H2O2 or O2 as an oxidant (Scheme 1a, top). Water is a more
Scheme 1. Preparation of silanols through Si-H bond activation.
appealing oxygen donor because of its safety (non-toxic, non-
explosive, and non-flammable) and its abundant and
inexpensive character. Based on this concept, a series of
catalytic systems based on noble metal catalysts, such as Pd,[9a]
Cu,[9b] Ru,[9c] Ir,[9d] Re,[9e] Ag,[9f,j] Pt,[9g] Au,[9h,i] and Rh,[9k] have
been successfully established for the hydrolysis of hydrosilanes
to silanols (Scheme 1a, bottom). Despite these advances, most
of these metal-catalyzed methods still resulted in the formation
of disiloxane byproducts. Moreover, most of catalysts used in
these procedures are expensive and commercially unavailable.
Besides transition-metal catalysis, organocatalytic[10] and
enzymatic[11] approaches have been explored for selective
oxidation of hydrosilanes to silanols in the presence of H2O2 or
O2, but the former method needs to be conducted in a strictly
controlled buffer solution and the latter method suffers from
limited substrate scope (Scheme 1b). Consequently, it would be
much more fascinating and yet challenging to develop a general
and practical protocol that allows for highly selective conversion
of hydrosilanes to silanols without the use of a noble metal
catalyst under mild and neutral reaction conditions.
In recent years, synthetic organic electrochemistry has
emerged as a powerful technique for realizing numerous
challenging reactions in a sustainable manner.[12] In particular,
electrochemical oxidation and functionalization of Csp3-H bonds
represents an attractive and atom-economical strategy to
decorate alkanes, which has been studied intensively.[13] In
sharp contrast, electrochemical oxidation of Si-H bonds to Si-OH
bonds remains thus far unexplored. Considering the importance
of silanols, we were intrigued by the possibility of developing a
practical and complementary electrosynthetic route to silanols.
We envisioned that upon electrochemical oxidation,
organosilanes could be oxidized to the corresponding silyl cation
intermediates and subsequently trapped with H2O, thereby
delivering silanol products. However, we recognized the
[*] H. Liang, L.-J. Wang, Y.-X. Ji, H. Wang, Prof. Dr. B. Zhang
State Key Laboratory of Natural Medicines
China Pharmaceutical University
Nanjing 210009 (China)
E-mail: zb3981444@cpu.edu.cn
[+] These authors contributed equally to this work.
Supporting information for this article is given via a link at the end of the
document.
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10.1002/anie.202010437
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RESEARCH ARTICLE
underlying difficulties to achieving high selectivity for silanol
synthesis under electrochemical conditions. First, organosilanes
have high redox potentials[14] and their selective oxidation in the
presence of other functionalities that have low redox potentials is
challenging. Second, several undesired reactions, such as
reduction of the formed silyl cation intermediates at the cathode
and condensation of two molecules of silanol products, are likely
to occur to form disilanes and disiloxanes,[15] which would
impede selective synthesis of silanols. Herein, we report our
success in the development of
a general and practical
electrochemical method for the hydrolysis of organosilanes to
silanols at room temperature (Scheme 1c). Our electrochemical
approach presented here exhibits several advantages: 1) high
selective preparation of silanols without the formation of
undesired disilane or disiloxane. 2) the use of commercially
available and cheap NHPI as the electrochemical mediator and
inexpensive materials, reticulated vitreous carbon (RVC) and
nickel, as the electrodes. 3) the utilization of simple water as the
oxygen donor. 4) a wide scope of substrate silanes, high
functional-group tolerance, neutral reaction conditions, and
amenability to late-stage modification of complex molecules.
Results and Discussion
We initiated our study by identifying the optimal reaction
conditions for the electrochemical hydrolysis of triphenylsilane 1
to triphenylsilanol 2 (Table 1). The electrolysis was first
conducted for 8 h under a constant current of 12 mA in an
undivided cell equipped with an RVC anode and a nickel foam
cathode using nBu4NPF6 as the supporting electrolyte in the
mixed solvent of CH3CN/H2O (7:0.4) at room temperature.
Table 1. Optimization of reaction conditions.[a]
Entry Variation from the standard reaction conditions
Yield [%][b]
1
none
85
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
no NHPI
n.r.
82
49
67
79
75
65
trace
trace
74
69
60
71
60
78
n.r.
n.r.
quinuclidine instead of NHPI
TEMPO instead of NHPI
DABCO instead of NHPI
NHSI instead of NHPI
Cl4NHPI instead of NHPI
acetone instead of CH3CN
CH2Cl2 instead of CH3CN
DMF instead of CH3CN
nBu4NClO4 instead of nBu4NPF6
nBu4NBF4 instead of nBu4NPF6
graphite as anode
6 mA instead of 12 mA
NHPI (0.5 equiv)
NHPI (1.5 equiv)
no electric current
no nBu4NPF6
Scheme 2. Substrate scope of electrochemical hydrolysis of hydrosilanes.
Reaction conditions: see entry 1, Table 1. Yields of isolated products are given.
[a] CH2Cl2 (1.0 mL) was added to increase solubleness of hydrosilanes.
Regrettably, under direct electrolysis, the transformation did not
take place, and starting material was completely recovered
(entry 2). We hypothesized that by incorporating a hydrogen-
atom transfer (HAT) mediator that is usually utilized to facilitate
the electrochemical oxidation of C-H bonds,[13a,c,d] the Si-H bond
might be efficiently activated. With this hypothesis in mind, we
next examined the electrochemical hydrolysis of triphenylsilane
[a] Reaction conditions: in an ElectraSyn 2.0 cell, 1 (0.6 mmol) and NHPI (0.6
mmol) in CH3CN/H2O (7.0/0.4 mL) with 0.12 M nBu4NPF6 as supporting
electrolyte were electrolyzed under a constant current of 12 mA at room
temperature under N2 for
8
h. [b] Isolated yields. NHPI
=
N-
hydroxyphthalimide; TEMPO = 2,2,6,6-tetramethyl-1-piperidinyloxy; DABCO =
1,4-diazabicyclo[2.2.2]octane; NHSI
reaction.
= N-hydroxysuccinimide; n.r. =
no
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RESEARCH ARTICLE
1
in the presence of
a
HAT mediator. After extensive
many drugs, organocatalysts, and silicon-based materials.[1,4,5]
We also showed that highly selective synthesis of important
geminal silanediols is possible using this novel method. As a
representative example, dihydro diphenylsilane could be
successfully converted to silanediol 32 in 62% yield. To
demonstrate the amenability of the electrochemical hydrolysis
reaction to late-stage modification of more complex small
molecules, a series of natural product and drug derivatives were
examined. It was found that substrates derived from naturally
occurring alcohols, such as (-)-borneol, (-)-nopol, L-menthol, (+)-
fenchol, and cholestanol, delivered the corresponding silanols
33-37 in 46-78% yields. In addition, several drug derived
substrates, including ibuprofen, febuxostat, and gemfibrozil,
could engage in this transformation, giving rise to silanols 38-40
in moderate to good yields. We expected that these silanol
products with more elaborate molecular architectures could have
potential applications in medicinal chemistry. Notably, in all
cases, formation of disilane or disiloxane was not detected, and
the incomplete conversions of the starting hydrosilanes
accounted for the missing yield. It was reported that phthalimido-
N-oxyl (PINO) electrogenerated from NHPI can undergo
degradation under electrolysis conditions through both
bimolecular and unimolecular pathways.[16] We discovered that
in our electrochemical conditions PINO can get decomposed to
produce some decomposition products (e.g., phthalimide). The
inevitable degradation of PINO during the electrolysis process
could prevent full conversion.
investigation, we were delighted to find that readily available
NHPI can serve as a highly efficient HAT mediator, which
afforded 2 in 85% yield (entry 1). Intriguingly, formation of
disilane or disiloxane byproduct was not observed. Running the
reaction with quinuclidine as the HAT mediator furnished 2 in
82% yield (entry 3). The use of TEMPO and DABCO resulted in
inferior efficiency (entries 4 and 5). Testing of other imidoxyl
mediators, such as NHSI and Cl4NHPI, did not improve the yield
(entries 6 and 7). Changing the solvent to acetone, CH2Cl2, and
DMF has a negative influence on the yield (entries 8-10).
Replacing nBu4NPF6 with other supporting electrolytes, such as
nBu4NClO4 and nBu4NBF4, was found to be detrimental to this
electrochemical oxidation process (entries 11 and 12). When
graphite was used instead of RVC as the anode, the yield of 2
dropped to 60% probably due to the small surface area of the
graphite anode for substrate contact (entry 13). An attempt to
reduce the electric current to 6 mA led to a decrease in yield
(entry 14). Reducing or increasing the NHPI loading resulted in
poor results (entries 15 and 16). Notably, no reaction occurred
without electric current (entry 17). Moreover, this
electrochemical progression was completely suppressed when
the reaction was carried out in the absence of nBu4NPF6 (entry
18). These results revealed that both electric current and
nBu4NPF6 are indispensable for the success of the reaction.
With optimized experimental conditions in hand, we set out to
evaluate the scope of the electrochemical hydrolysis of
hydrosilanes. As summarized in Scheme 2, a wealth of
triphenylsilane derivatives carrying different electronically and
sterically varied phenyl substituents reacted well, affording the
corresponding silanols 3-14 in 70-80% yields. A variety of
substituents, including alkyl (3, 4, 12, 14), methoxyl (5), silyl (6),
fluoro (13), chloro (7), trifluoromethyl (8), phenyl (9), ester (10),
and cyano (11) groups, were well-tolerated under the current
reaction conditions. Remarkably, benzylic C-H bonds otherwise
susceptible to oxidation under previously reported NHPI-
mediated electrochemical conditions remained intact (e.g., 3, 12
and 14),[13d] thus showing high chemoselectivity of our method.
Apart from phenyl substituents, naphthyl-substituted substrates
also participated in the desired reaction to deliver silanols in
good yields (15). Substrates bearing heteroaromatic substituents,
such as thiophene, dibenzothiophene, and dibenzofuran, proved
to be compatible with this electrochemical hydrolysis process,
leading to the targeted products 16-18 in high yields. Both
methyldiphenylsilane and butyldiphenylsilane were competent
substrates, furnishing silanols 19 and 20 in 78% and 65% yields,
respectively. Dimethyl(phenyl)silanes containing different
functional groups were efficiently transformed into silanols 21-26
in good yields. We found that the substrate scope of the reaction
could be expanded to alkyl-substituted hydrosilanes, which
reacted selectively to afford the expected products in
synthetically useful yields (27 and 28). It is worth noting that
alkenyl- and alkynyl-substituted hydrosilanes underwent this
transformation smoothly to afford silanols 29 and 30. As for 29,
epoxidation of the C=C bond was not observed.[8f] Interestingly,
a substrate having two Si-H bonds could be completely
transformed into the corresponding silanediol 31 in a reasonable
yield. Geminal silanediols are key structural motifs found in
To further illustrate the practicality of our methodology, the
electrochemical hydrolysis of organosilanes has been conducted
on a preparative scale. As shown in Scheme 3, silanol 2 and
silanediol 32 were obtained in 81% (1.62 g) and 55% yields
(0.86 g), respectively. Moreover, we performed cycling
experiments and found that after the first round hydrolysis of
triphenylsilane 1 (0.60 mmol), the electrodes, the RVC anode
and the nickel cathode, could be reused at least three times
without any reduction in yield or selectivity (the first run: 85%;
the second run: 86%; the third run: 84%; the fourth run: 85%).
Scheme 3. Larger-scale synthesis.
To gain insight into the reaction mechanism, we performed
some mechanistic experiments (Scheme 4). When this reaction
was carried out in the presence of H218O, an 18O-labeled silanol
[18O]-2 was isolated in 80% yield, thereby confirming that H2O is
the source of the oxygen atom of the silanol product (Scheme
4a). Kinetic isotope effect (KIE) experiment using substrates 1
and [D]-1 was conducted, and the value of kH/kD = 2.9 from two
parallel reactions suggests that Si-H bond cleavage might be
involved in the rate-determining step of the reaction (Scheme
4b). When 5.0 equivalents of 2,2,6,6-tetramethyl-1-piperidinyloxy
(TEMPO) were employed, the TEMPO-trapped product 43 was
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10.1002/anie.202010437
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RESEARCH ARTICLE
identified by high-resolution mass spectrometry (HRMS)
(Scheme 4c). When an optically active hydrosilane, (+)-α-
naphthylphenylmethylsilane, was subjected to our standard
reaction conditions, the racemic product was obtained in 79%
yield. These results indicate that a silyl radical might be involved.
The results from competition experiments highlight electron-rich
silanes to be preferentially converted (Scheme 4d). Additionally,
we investigated the reaction using alcohols as the nucleophiles
to trap the in situ generated silyl cation intermediates (Scheme
4e). As anticipated, the electrochemical alcoholysis of
triphenylsilane 1 with MeOH as co-solvent occurred, and the
corresponding silyl ether 44 was obtained in 51% yield. Further
optimization revealed that alcohol can be used as a limiting
reagent when the electrolysis was conducted for 3 h under a
constant current of 12 mA in an undivided cell equipped with an
RVC anode and a nickel foam cathode using nBu4NBF4 as the
supporting electrolyte and quinuclidine as the HAT mediator.
Under these conditions, MeOH, 1-adamantanol, L-menthol, and
(-)-nopol, provided the silyl ethers 44-47 in 51-89% yields. These
results further verify the intermediacy of a silyl cation and show
the potential of the current electrochemical strategy to prepare
silyl ethers under very mild reaction conditions.[17] To validate
that the reactive intermediates are formed at the anode, we
performed a reaction using H-type divided cell separated by an
anion exchange membrane (AMI-7001 membrane). Both the
divided anodic chamber and cathodic chamber containing the
same silane substrate, mediator, supporting electrolyte, and
solvent were electrolyzed under the standard conditions. As
expected, in the case of 1, only anodic chamber could furnish
the desired silanol product 2 in 60% yield. Finally, we conducted
electricity on/off experiments and confirmed that the formation of
silanol products requires continuous electricity (see the
Supporting Information (SI) for details).
To elucidate the role of NHPI as a HAT mediator of the
reaction, a series of cyclic voltammetry (CV) experiments were
conducted.[18] The oxidation potential of NHPI in CH3CN/H2O
was observed at about 0.98 V (Ep/2). Meanwhile, the oxidation
potential of triphenylsilane 1 was observed at about 1.52 V (Ep/2).
These results show that NHPI should be first oxidized under
electrolytic conditions. Moreover, the oxidation potential of NHPI
was also measured in the presence of tetrabutylammonium
hydroxide (TBAOH). A significantly low oxidation potential was
observed at approximately 0.41 V (Ep/2), implying that the
oxidation of NHPI to PINO is a hydroxide-mediated proton-
coupled process. In the CV of the mixture of NHPI and 1, a
catalytic current was observed, thus indicating that a HAT event
between PINO and 1 occurs.
Based on these experimental observations described above, a
plausible reaction mechanism for the electrochemical hydrolysis
of hydrosilanes is depicted in Scheme 5. First, NHPI undergoes
a proton-coupled oxidation process facilitated by hydroxide
generated from H2O reduction at the cathode to afford
PINO.[13a,d,19] Subsequently, the resulting PINO abstracts a
hydrogen atom from the hydridic Si-H bond to generate a silyl
radical A, which is further anodically oxidized to produce a silyl
cation B. Finally, the silyl cation B gets trapped by H2Oto furnish
the desired silanol product C while liberating a proton that is
then reduced at the cathode to produce H2. Because of the low
concentration of hydroxide compared to H2O, quenching the silyl
cation B by hydroxide is likely to represent only a minor reaction
pathway.
Scheme 5. Proposed reaction mechanism.
Conclusion
In summary, we have described the first effective protocol of
electrochemical hydrolysis of hydrosilanes, providing a general
and highly selective method for preparing a wide array of
synthetically valuable silanols. The practical protocol uses
Scheme 4. Mechanistic studies.
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RESEARCH ARTICLE
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methodology are currently underway in our laboratory.
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RESEARCH ARTICLE
H. Liang, L.-J. Wang, Y.-X. Ji, H. Wang,
B. Zhang*
Page No. – Page No.
Selective Electrochemical Hydrolysis
of Hydrosilanes to Silanols via
Anodically Generated Silyl Cations
A practical electrochemical method for selective hydrolysis of hydrosilanes to
furnish diverse silanols has been achieved. These reactions occur with high
chemoselectivity and functional-group tolerance under mild and neutral reaction
conditions. The addition of NHPI as a HAT mediator is crucial for promoting the
generation of silyl cation species.
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