Visible-Light-Mediated Metal-Free Hydrosilylation of Alkenes through Selective Hydrogen Atom Transfer for Si?H Activation

Article abstract of DOI:10.1002/anie.201711250

Although there has been significant progress in the development of transition-metal-catalyzed hydrosilylations of alkenes over the past several decades, metal-free hydrosilylation is still rare and highly desirable. Herein, we report a convenient visible-

Full text of DOI:10.1002/anie.201711250

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Accepted Article
Title: Visible-Light-Mediated Metal-Free Hydrosilylation of Alkenes
through Selective Hydrogen Atom Transfer for Si-H Activation
Authors: Rong Zhou, Yi Yiing Goh, Haiwang Liu, Hairong Tao, Lihua Li,
and Jie Wu
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Visible-Light-Mediated Metal-Free Hydrosilylation of Alkenes
through Selective Hydrogen Atom Transfer for SiH Activation
Rong Zhou,[a,b] Yi Yiing Goh, Haiwang Liu, Hairong Tao, Lihua Li, and Jie Wu*
[a]
[a]
[c]
[a]
[a]
Abstract: Although developments on the transition-metal-catalyzed
hydrosilylation of alkenes have achieved great progress over the past
several decades, metal-free hydrosilylation is still rare and highly
desirable. Herein, we report a convenient visible-light-driven metal-
free hydrosilylation of both electron-deficient and electron-rich
alkenes via selective hydrogen atom transfer for SiH activation. By
the synergistic combination of an organo-photocatalyst 4CzIPN and
quinuclidin-3-yl acetate catalyst, the hydrosilylation of electron-
deficient alkenes exclusively occurred with SiH activation.
Additionally, the hydrosilylation of electron-rich alkenes was achieved
by merging photoredox and polarity-reversal catalysis.
to enable selective hydrogen atom abstraction beyond the control
of the relative bond dissociation energies (BDEs).[
9]
Organosilanes are versatile intermediates or products in
medicinal chemistry and material science. The hydrosilylation of
alkenes, a 100% atom-efficient transformation, represents one of
the most important reactions in the silicone industry.[1] The direct
activation of SiH bonds for subsequent hydrosilylation has been
effectively accomplished through the use of Speier’s or Karstedt’s
platinum catalysts in industrial-scale manufacturing.[2] Intensive
efforts in this research field have led to the discovery of earth-
abundant metals, such as iron, nickel, and cobalt, as effective
hydrosilylation catalysts.[3,4] In addition, recent advances have
shown that Lewis acid catalysts, especially electron-deficient
boranes, can also catalyze hydrosilylations in a rare-metal-free
manner (Scheme 1A).[5] Nevertheless, successful examples have
been mostly limited to electron-rich alkenes, as these catalysts
generally show poor reactivity and selectivity toward electron-
deficient alkenes (- vs -adducts, 1,2- vs 1,4-addition).[6] To the
best of our knowledge, a practical methodology for effective
catalytic hydrosilylation of both electron-deficient and electron-
rich alkenes has not been realized, particularly in a metal-free
manner.
Dramatic developments in photocatalysis have occurred over
the past decade, which have enabled previously inaccessible
transformations.[ Other than single-electron transfer (SET) and
energy transfer, hydrogen atom transfer (HAT) is frequently
involved in photocatalysis, offering enormous opportunities for
CH activations.[8] In the context of HAT, polarity plays a vital role
7]
Scheme 1. Hydrosilylation of alkenes.
The Fagnoni and Ravelli group recently disclosed an elegant
hydrosilylation of electron-poor alkenes through the use of
tetrabutylammonium decatungstate as a direct HAT catalyst
under UV light (310 nm) irradiation.[10] Notably, a mixture of SiH
and CH activation products were obtained with trialkylsilanes
[
a]
Dr. R. Zhou, Y. Y. Goh, H. Liu, L. Li, Dr. J. Wu
Department of Chemistry
National University of Singapore
(
Scheme 1B), likely due to the similar BDEs of SiH and CH in
3
Science Drive 3, Republic of Singapore, 117543
[11]
trialkylsilanes. As silicon is more electro-positive than carbon
(electronegativity 1.90 vs 2.55 on the Pauling scale), SiH bonds
are generally more hydridic than CH bonds. We speculated that
by incorporating a highly electrophilic HAT catalyst, such as the
quinuclidinium radical cation applied by MacMillan,[12] the SiH
bond should be selectively activated in the presence of multiple
CH bonds with similar bond strengths, therefore realizing
selective hydrosilylation (Scheme 1C). Herein we reported the
first visible-light-driven hydrosilylation of both electron-deficient
and electron-rich alkenes in a metal-free manner (Scheme 1D).
E-mail: chmjie@nus.edu.sg
Dr. R. Zhou
College of Chemistry and Chemical Engineering
Taiyuan University of Technology
Taiyuan, China, 030024
Dr. H. Tao
College of Chemistry
Beijing Normal University
Beijing, China, 100875
[
[
b]
c]
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.2017XXXXX
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COMMUNICATION
without any CH activation byproducts detected, highlighting the
effectiveness of the electrophilic quinuclidinium radical cation
HAT catalyst for the selective SiH activation. Changing the
photocatalyst to Ir[dF(CF
redox potential as that of 4CzIPN, delivered a comparable result.
Other photocatalysts including Ir(ppy) (dtbpy) PF Ir(ppy)
Ru(bpy) Cl , and eosin Y, failed to afford the desired product.
3 2 6
)ppy] (dtbpy)PF , which has a similar
2
3
6
,
3
,
3
2
Among the range of solvents examined, MeCN resulted in the
best reactivity. Finally, no hydrosilylation product was detected in
the absence of either photocatalyst 1, quinuclidinium catalyst 2,
or light, demonstrating the need for all these components.
Adopting the optimal metal-free conditions, we sought to
investigate the generality of this hydrosilylation. Our recently
developed “stop-flow” micro-tubing (SFMT) reactor was applied in
reactions where conversions were low in batch reactors due to its
enhanced light penetration efficiency.[ Employing Et
14]
SiH and
3
2
Me PhSiH as two representative silane reagents, a diverse range
of electron-deficient alkenes were examined. As depicted in
Scheme 2a, acrylates with different steric and electronic
properties were well tolerated (4-12), with functionalities including
silane (10), ether (11), and epoxide (12). Dimethyl maleates and
lactones were also viable substrates, readily affording the
corresponding hydrosilylation products in 84% and 37% yields,
respectively (13 and 14). Cyclic enones with different ring sizes
(
(
15-17) and acyclic enones with either alkyl or aryl substituents
18-20) were all accommodated. Furthermore, the hydrosilylation
was successfully applied to other types of electron-deficient
alkenes such as acryamides (21 and 22), vinyl sulfone (23),
acrylonitrile (24), and methylene-malonates (25-27). Notably,
activated CH bonds adjacent to heteroatoms, such as -oxy (11
and 12), -nitro (21 and 22), and -silyl (10) CH bonds were well
tolerated in this selective SiH activation protocol. Experiments
probing silane scope using benzyl acrylate (Scheme 2b)
illustrated that a series of trialkylsilanes with different sizes
uneventfully underwent the hydrosilylation to deliver products 28-
34 in moderate to excellent yields. Notably, only mono-
hydrosilylation product 35 was obtained with p-
phenylenebis(dimethylsilane). The incorporation of heteroaryl (36)
or phenyl (37 and 38) substituents in the silane were all feasible.
Tris(trimethylsilyl)silane gave product 39 in excellent yield, likely
due to the enhanced stability of the super-silyl radical intermediate.
As shown in Scheme 3, a plausible mechanism for the
hydrosilylation of electron-deficient alkenes was proposed based
on several control experiments. No reaction was detected when
adding the radical scavenger TEMPO or hydroquinone,
[
a] For typical conditions, see the Supplementary Materials. Isolated yields
obtained from batch reactors. [b] Results obtained from SFMT reactors.
Scheme 2 Scope of metal-free hydrosilylation of electron-deficient alkenes.[a]
radical-based process.[15] The Stern-Volmer
suggesting
quenching studies supported SET from 3-acetoxyquinuclidine (E
a
p
[12b]
=
+1.22 V vs SCE in MeCN)
to the light-activated photocatalyst
Our study was commenced by evaluating the photo-promoted
hydrosilylation of benzyl acrylate with triethylsilane under blue
LED irradiation (λmax = 470 nm). After careful investigation of
various photocatalysts and solvents (Table S1, Supplementary
Materials), the combination of a catalytic amount of organo
photocatalyst 1,2,3,5-tetrakis(carbazol-9-yl)-4,6-dicyanobenzene
-
[13]
1* [E1/2 (*P/P ) = +1.35 V vs SCE in MeCN] to afford the reduced
photocatalyst 1’ and amine radical cation I. No obvious quenching
of 1* was observed by the silane or benzyl acrylate.[15] Due to its
high electrophilicity, radical cation I would selectively abstract a
hydrogen atom from the more hydridic SiH bond to deliver silyl
radical III, along with the generation of quinuclidinium cation II.
Nucleophilic addition of silane radical III to an electron-deficient
alkene formed a transient radical adduct IV. Single-electron
(
4CzIPN) 1[13] and quinuclidin-3-yl acetate 2 in acetonitrile (MeCN)
was found to provide the best result, isolating hydrosilylation
product 4 in 81% yield. Remarkably, compared to Fagnoni and
Ravelli’s study,[10] only the hydrosilylation product was obtained
-
reduction of radical adduct IV by 1’ [E1/2 (P/P ) = -1.21 V vs SCE
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COMMUNICATION
in MeCN][13] afforded the hydrosilylation product after
protonation,[14b] while regenerating photocatalyst 1. The
deuterium-labeling experiments indicated that anion V would
most likely abstract a free proton from solvent.[15] The BDE
difference between the NH bond of the quinuclidinium cation
We next attempted to extend this visible-light-promoted metal-
free hydrosilylation protocol to electron-rich alkenes. However, no
product was formed with 1-octene under the optimal conditions
for electron-deficient alkenes (Table S2, entry 1, Supplementary
Materials). Compared to reactions with electron-deficient alkenes,
the reason of failure may be that the radical adduct generated
from addition of silyl radical III to an electron-rich alkene was not
oxidative enough to oxidize 1’ to turn over the catalytic cycle. We
envisioned that merging the photoredox catalyst with a polarity-
reversal catalyst for hydrogen atom abstraction may solve this
+
[12b]
(
N H BDE = 100 kcal/mol)
and the CH bond  to the
electron-withdrawing group of the hydrosilylation product (CH
BDE = 85.2 kcal/mol for 4)[11] indicated that a radical chain
process between radical IV and cation II was unlikely.
Furthermore, both the light on/off experiment and the calculated
quantum yield ( = 0.048) further excluded the possibility of a
chain mechanism.[15,16]
problem.[9b] Employing dimethylphenylsilane and 1-octene as the
[17]
model substrates, a series of thiol catalysts,
additives, and
solvents were examined under the photoredox conditions (Table
S2). It was found that the combination of PC 1 (3 mol%), N,N-
diisopropylethylamine
(DIPEA,
5
mol%),
and
triisopropylsilanethiol (5 mol%) in dioxane under blue LED
irradiation delivered the hydrosilylation product 40 in excellent
yield. Control experiments unveiled that the reaction proceeded
smoothly even without DIPEA, albeit with a lower efficiency (60%
yield). Changing DIPEA to a catalytic amount of K
resulted in an efficient transformation (85% yield).
2 3
CO also
With the optimal conditions, the scope of electron-rich alkenes
was investigated (Scheme 4a). A broad range of aliphatic terminal
alkenes were shown to be good substrates for this protocol,
delivering the corresponding silane products in moderate to
excellent yields (40-49). Functional groups such as ester (44 and
Scheme 3. Plausible mechanism for hydrosilylation of electron-poor alkenes.
49), ketone (45), chloride (46), epoxide (47), and amide (48) were
well tolerated. Both vinyl silanes and allyl silanes could be applied
to introduce disilane products (50 and 51). Different silane
reagents were examined as well (Scheme 4b). Trialkylsilanes
such as triethylsilane, tripropylsilane, triisopropylsilane,
tributylsilane,
and
benzyldimethylsilane
all
afforded
hydrosilylation products in good yields (52-56). Moreover, the
hydrosilylations proceeded smoothly with aryl substituted silanes
including diphenylmethylsilane and triphenylsilane (57 and 58).
Tris(trimethylsilyl)silane resulted in hydrosilylation product 59 in
excellent yield. Notably, this protocol could be expanded to
secondary silane to give product 60 bearing one SiH bond.
Mechanistically, as shown in Scheme 5, the hydrosilylation of
electron-rich alkenes was initiated with the formation of thiyl
radical VI via SET between excited photocatalyst 1* and
triisopropylsilanethiol 3 (E1/2 = +0.28 V vs SCE in MeCN)[15]
followed by deprotonation. The Stern-Volmer quenching studies
illustrated that 3 could quench the excited catalyst 1*, but both the
silane and electron-rich alkene cannot. The presence of a
ox
2 3
catalytic amount of base (DIPEA or K CO ) presumably promoted
the deprotonation first to facilitate the formation of thiyl radical
VI.[ The electrophilic thiyl radical VI then abstracted a hydrogen
atom from the hydridic SiH bond to deliver silyl radical III, along
with the regeneration of thiol 3. Regioselective addition of the silyl
radical III at the less sterically hindered site of electron-rich
alkenes afforded radical adduct VII. The nucleophilic radical VII
would undergo another polarity matched HAT process to
generate the hydrosilylation product, accompanying with the
formation of thiyl radical VI. The BDE difference between the SH
bond of 3 (SH BDE = 88.2 kcal/mol) and the CH bond  to the
electron-donating group of the hydrosilylation product (CH BDE
15]
[
a] For typical conditions, see the Supplementary Materials. Isolated yields were
obtained from batch reactors. [b] Results were obtained from SFMT reactors.
Scheme 4. Scope of metal-free hydrosilylation of electron-rich alkenes.[a]
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=
93.2 kcal/mol for 40) indicated the HAT was feasible.[11] Thus
We are grateful for the financial support provided by the National
University of Singapore, the Ministry of Education (MOE) of
Singapore (R-143-000-645-112, R-143-000-665-114, and R-143-
000-696-114), GSK-EDB (R-143-000-687-592), and National
Natural Science Foundation of China (Grant No. 21502135 and
thiol 3 behaved as a polarity-reversal catalyst to enable the
efficient transformation. However, both the light on/off study and
quantum yield measurements ( = 0.02)
[15]
did not support a
chain process.[17] The ineffectiveness of several photoredox
catalysts with stronger oxidative potentials also indicated that
catalyst 1 played roles more than a simple radical initiator.[15]
Therefore, other than abstracting a hydrogen atom from silane,
the thiyl radical VI (E1/2red = -0.82 V vs SCE in MeCN) may
21702142).
Keywords: metal-free • photocatalysis • hydrosilylation •
hydrogen atom transfer • polarity-reversal catalysis
[15]
-
preferentially oxidize 1’ [E1/2 (P/P ) = -1.21 V vs SCE in MeCN] to
regenerate photocatalyst 1 to finish the catalytic cycle. However,
at the current stage, we were unable to exclude the possibility of
a chain process with quick termination triggered by HAT between
radical VI and the silane.[16]
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Scheme 5. Plausible mechanism for hydrosilylation of electron-rich alkenes.
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metal-free hydrosilylation of both electron-deficient and electron-
rich alkenes through the synergistic combination of photoredox
and HAT catalysis. By merging the organo-photocatalyst 4CzIPN
and HAT catalyst quinuclidin-3-yl acetate, SiH activations
exclusively occurred over CH activations to achieve the
hydrosilylation with electron-deficient alkenes. Additionally, the
hydrosilylation of electron-rich alkenes was realized by dual
organo photoredox and polarity-reversal catalysis. Both reactions
featured merits such as atom- and redox-economy, mild
conditions, working efficiently with an extremely broad substrate
scope, and easy scaled up by continuous-flow technology. Our
study also provides convenient approaches to introduce silicon
radicals under visible-light conditions, which will likely find further
application for the synthesis of functionalized silicon-containing
molecules.
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Acknowledgements
[
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COMMUNICATION
COMMUNICATION
We accomplished a metal-free visible
light-driven hydrosilylation of electron-
deficient alkenes through the
Rong Zhou, Yi Yiing Goh, Haiwang Liu,
Hairong Tao, Lihua Li, Jie Wu*
synergistic combination of an organo-
photoredox catalyst and a highly
electrophilic hydrogen atom transfer
catalyst, while the hydrosilylation of
electron-rich alkenes was achieved by
merging the photocatalyst and a
polarity-reversal catalyst.
Page No. – Page No.
Visible-Light-Mediated Metal-Free
Hydrosilylation of Alkenes through
Selective Hydrogen Atom Transfer for
SiH Activation
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