Dehydrogenative Silylation of Alkenes for the Synthesis of Substituted Allylsilanes by Photoredox, Hydrogen-Atom Transfer, and Cobalt Catalysis

Article abstract of DOI:10.1002/anie.201904707

A synergistic catalytic method combining photoredox catalysis, hydrogen-atom transfer, and proton-reduction catalysis for the dehydrogenative silylation of alkenes was developed. With this approach, a highly concise route to substituted allylsilanes has been achieved under very mild reaction conditions without using oxidants. This transformation features good to excellent yields, operational simplicity, and high atom economy. Based on control experiments, a possible reaction mechanism is proposed.

Full text of DOI:10.1002/anie.201904707

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Accepted Article
Title: Dehydrogenative Silylation of Alkenes for the Synthesis of
Substituted Allylsilanes via Photoredox, HAT, and Cobalt
Catalysis
Authors: Wan-Lei Yu, Yong-Chun Luo, Lei Yan, Dan Liu, Zhu-Yin
Wang, and Peng-Fei Xu
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201904707
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10.1002/anie.201904707
Angewandte Chemie International Edition
COMMUNICATION
Dehydrogenative Silylation of Alkenes for the Synthesis of Substituted
Allylsilanes via Photoredox, HAT, and Cobalt Catalysis
Wan-Lei Yu, Yong-Chun Luo, Lei Yan, Dan Liu, Zhu-Yin Wang and Peng-Fei Xu*[a]
Abstract: A synergistic catalytic method of combining photoredox
catalysis, hydrogen atom transfer and proton-reduction catalysis for
the dehydrogenative silylation of alkenes was developed. With this
approach, a highly concise route to substituted allylsilanes has been
achieved under very mild reaction conditions without using oxidants.
This transformation features good to excellent yields, operational
simplicity and high atom economy. Based on control experiments, a
possible reaction mechanism was also proposed.
Silicon-containing compounds have played very important
roles in organic chemistry, owing to their unique chemical,
physical, and bioactive properities.[1] Especially, allylsilanes are
regarded as one of vital building blocks in the synthesis of small
molecules and polymers, because of their high stability and low
toxicity.[2] Therefore, the development of a mild and efficient
method for the introduction of silicon groups into organic
Scheme 1. The strategies for the synthesis of substituted allylsilanes.
molecules has attracted lots of interest in recent years. The
traditional synthesis methods usually require highly reactive
reagents such as silylmetal or allylmetal reagents in multiple
step processes and the final products are frequently
scopes were still limited. In contrast to the pathways for cross-
coupling between alkenes and different nucleophiles,[6k-6o] we
envisaged that the dehydrogenative cross C-Si coupling reaction
would be highly beneficial as a reliable synthetic technique.
Considering the similar BDEs of Si-H and C-H bonds, the
hydrogen abstraction process with an electrophilic radical or
radical cation can also enable the activation of a Si-H bond.[7] To
accomplish this goal, the major challenge to address is how to
avoid the hydrosilylation process and vinylsilane byproducts.
Herein, we report a novel tricatalytic activation method, i.e. the
synergistic combination of photoredox,[8] HAT (hydrogen atom
transfer),[9] and cobalt catalysis,[6] which provides an efficient
and general pathway to the selective construction of allylsilanes.
As the most successful surrogates for organotin compounds,
tris(trimethylsilyl)silane (TTMSS) has acted as a stable, green
and commercially available organosilicon compound which has
contaminated with metal reagents.[3] The direct coupling silyl
groups of alkenes to produce allylsilanes offers a step-efficient
and atom-economical synthetic tool to access this valuable
targets. For example, Watson reported Pd-catalyzed silyl-Heck
reaction to prepare allylsilanes utilizing terminal alkenes and
TMSI (Scheme 1a).[4a,4b] High regioselective dehydrogenative
silylation to provide allylsilanes is still challenging. Most of these
reactions need either moisture-sensitive noble metal catalysts or
excess alkenes as sacrificial hydrogen acceptors, and afford
both allyl- and vinylsilane products due to low regioselectivity
(Scheme 1b).[4c-4h] Thus, more efficient and environmentally
benign synthetic methodologies are desirable and urgently
needed.
The utilizing of energy from visible light in synthetic field is
usually attracting because of very mild reaction conditions. A
powerful strategy of combined photocatalysis with other catalytic
methods could realize new transformations which do not
proceed with either catalysis alone. Recently, photoredox-
mediated hydrogen-evolution cross-coupling reactions have
been developed by Wu and Lei groups. The elegant processes
avoided the use of any oxidant and provided an efficient access
to C-C, C-X bond.[5-6] Up to now, many successful methods have
been developed in both heterogeneous[5f-5h] and homogeneous[6]
systems. Despite the advances, reaction patterns and substrate
been widely employed as
a silyl source in free-radical
chemistry.[10] As shown in Table 1, we began our investigation
by screening conditions for the dehydrogenative silylation using
TTMSS [BDE(Si-H) = 82.3 kcal/mol] as the silicon radical
precursor. To our delight, the desirable allylsilane product 1 (in a
yield of 40%) was obtained at room temperature by irradiation
with blue LEDs using Ir[dF(CF3)ppy]2(dtbbpy)PF6 as the
photocatalyst, quinuclidine (A) as the HAT catalyst,
Co(dmgH)2PyCl (II) as the protonreduction catalyst, and N-
methyl-N-phenylmethacrylamide as the silyl radical acceptor
(entry
1).
When
4CzIPN
was
used
instead
of
Ir[dF(CF3)ppy]2(dtbbpy)PF6, the yield of the product 1 was
increased to 53% (entry 2).[11] Meanwhile, the reaction did not
[a]
W-L. Yu, Dr. Y-C. Luo, L. Yan, D. Liu, Prof. Dr. Z.-Y.Wang, Prof.
Dr. P.-F. Xu*
State Key Laboratory of Applied Organic Chemistry, College of
occur when either less oxidizing Ir(ppy)2(dtbbpy)PF6 (
=
+0.66 V) or less reducing Acr+-MesClO4 at the reduced state
( = -0.57 V) (entries 3 and 4) was used as the photocatalyst.
-
Chemistry and Chemical Engineering, Lanzhou University, Lanzhou
730000, P.R. China.
Other cobalt catalysts I and III gave slightly lower yields
E-mail: xupf@lzu.edu.cn
Supporting information for this article is given via a link at the end of
the document.
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10.1002/anie.201904707
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COMMUNICATION
Table 1. Optimization of the reaction conditions.[a]
Table 2. Reaction scope of dehydrogenative silylation of alkenes.[a,b]
Entry
1
Photocatalyst
PC 1
HAT
[Co]
Base
-
Yield (%)[b]
40
A
II
2
3
4CzIPN
PC 2
A
A
A
A
A
B
C
A
A
A
A
II
II
II
I
-
53
trace
0
-
-
4
Acr+-MesClO4
4CzIPN
4CzIPN
4CzIPN
4CzIPN
4CzIPN
4CzIPN
4CzIPN
4CzIPN
-
5
-
36
6
III
II
II
II
II
II
II
-
-
20
7
0
8
-
8
9
DABCO
DMAP
pyridine
2,6-lutidine
trace
trace
83
10
11
12
55
[a] Unless otherwise noted, reaction conditions are as follows: alkene (0.1
mmol), TTMSS (0.2 mmol), photocatalyst (0.002 mmol), Co catalyst (0.005
mmol), HAT (0.02 mmol), base (0.1 mmol), solvent (2 mL), 25 W blue LED
strip, 25 oC for 24 h and under N2 atmosphere. H2 was detected by gas
chromatography. [b] Isolated yield. PC 1 = Ir[dF(CF3)ppy]2(dtbbpy)PF6, PC 2 =
Ir(ppy)2(dtbbpy)PF6, dmg = dimethylglyoximate.
[a] Unless otherwise noted, reaction conditions are as follows: alkenes (0.1
mmol), silanes (0.2 mmol), 4CzIPN (0.002 mmol), quinuclidine (0.02 mmol),
Co(dmgH)2PyCl (0.005 mmol), pyridine (0.1 mmol), MeCN (2 mL), 25 W blue
LED strip, 25 oC and under N2 atmosphere. [b] Isolated yield. [c] quinuclidin-3-
yl acetate (0.02 mmol) instead of quinuclidine, 35 oC.
(entries 5 and 6). When previously reported HAT catalysts such
as thiophenol (B) and triisoproplysilylthiol (C) instead of
quinuclidine (A) were used, either no reaction or a poor yield
was observed (entries 7 and 8). To improve the reaction yield, a
variety of bases were examined (entries 9-12). In the presence
of DABCO and DMAP, only a trace amount of 1 was formed
(entries 9 and 10). Surprisingly, the yield was improved to 83%
when pyridine was employed as the base (entry 11). In addition,
solvent screening showed that MeCN was the optimal medium
(See SI). Furthermore, control experiments revealed that the
photocatalyst, the cobalt catalyst, the HAT catalyst and the
visible-light source were all essential for the reaction, and no
reaction was observed in their absence.
With the reaction conditions optimized, we then explored the
reactivities of various alkenes, and the results were summarized
in Table 2. Employing TTMSS as the silane reagent, a range of
electrophilic alkenes could successfully participate in this silicon
radical addition-dehydrogenation protocol. Notably, a series of
methacrylamide were found to be suitable substrates including
those with alkyl and phenyl groups located at the nitrogen core
and all gave good yields (1-4, 75-87%). The presence of the N-H
bond in 5 appeared to give a slightly lower yield. Substrates with
substituted phenyl groups also led to the products in moderate
yields (6 and 7). We were pleased to find that both of the acyclic
and cyclic unsaturated dialkylamides were well tolerated in this
transformation (8-15). Compared with the five- and six-
membered heterocycles, the seven-membered heterocycle gave
the highest yield (11, 80%). Moreover, other heterocyclic ring
systems which have an active α C-H bond bound to O, S, or N
atoms were also found to be amenable to this reaction (12-15,
52-85%). Other silanes, such as triethylsilane, triisopropylsilane
and tert-butyldimethylsilane, were compatible reaction partners
as well for producing products 16-18, albeit with decreased
yields (29-38%), which was probably owing to the higher Si-H
BDEs, e.g. BDE(Et3Si-H) = 92.8 kcal/mol. To solve this issue,
different HAT catalysts were tested to improve the yield. It was
found that quinuclidin-3-yl acetate had better effect on
o
alkylsilanes at 35 C, which resulted in slightly increased yields
(16-18, 38-47%). We next explored the scope of this
transformation for α,β-unsaturated esters. Benzyl 2-methyl
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Table 3. Dehydrogenative silylation for 5-exo-trig cyclization.[a,b]
[a] Unless otherwise noted, reaction conditions are as follows: alkenes (0.1
mmol), TTMSS (0.2 mmol), 4CzIPN (0.002 mmol), quinuclidine (0.02 mmol),
Co(dmgH)2PyCl (0.005 mmol), pyridine (0.1 mmol), MeCN (2 mL), 25 W blue
LED strip, 25 oC and under N2 atmosphere. [b] Isolated yield.
acrylate was found to be well accommodated (19, 58%). As
expected, tert-butyl 2-methyl acrylate and butyl 2-methyl acrylate
behaved similarly in this reaction (20 and 21). Furthermore, it
was found that the substrates with a bulky group were also well
tolerated (22-26, 40-74%). In an attempt to broaden the
generality of the reaction, we turned our attention to other
alkenes to explore the potential of this transformation. The
results showed that heterocyclic aromatic alkenes were
successfully transformed into the expected products in moderate
yields (27-29). We were pleased to see that 2-propenyl boronic
esters could also react under the optimized reaction conditions
to provide the corresponding product (30, 68%). However, the
scope of alkene sources was limited to 1,1-disubstituted alkenes.
The transformation of monosubstituted alkenes and α,β-
unsaturated ketones failed. To further demonstrate the synthetic
utility of the protocol, we tested several more complex alkene
substrates containing a biologically relevant unit, including the
sugar skeleton of diacetone-D-fructopyranose (31), diacetone-D-
allofuranose (32) and the derivatives of the steroid (33 and 34),
which all were suitable candidates to participate in the reaction
and afforded the desired products in good yields. Finally, a 1
mmol-scale reaction was conducted and pregnenolone
derivative 34 was obtained in 60% yield. These results
demonstrated that the present method would be valuable for the
late-stage functionalization in a synthetic process.
With this tricatalytic system, it was found that the radical 5-
exo-trig cyclization could proceed to form 5-membered α,β-
unsaturated γ-lactam at standard reaction conditions and the
results are summarized in Table 3. First, a series of N-allyl-N-
aryl acrylamides were investigated for extending the substrate
scope, and both electron-withdrawing and electron-donating
substituent groups on the aromatic ring were well tolerated. The
electron-donating groups such as methyl (36, 41), isopropyl (37)
and tertbutyl (38) groups gave better yields (58-66%). However,
nonconjugated substrates such as N-benzyl and N-cyclohexyl
acrylamides were unable to give the desired products, probably
due to the lower 5-exo-trig cyclization rate. Obviously, the use of
cinnamyl instead of allyl group had a better effect on the yield,
giving the corresponding products 42 in 86% yield.
Scheme 2. Gram-scale reaction and mechanistic investigations.
To confirm the practicability of this process, a gram-scale
experiment was performed, which afforded the corresponding
allylsilane 1 in good yield (Scheme 2a). Several mechanistic
experiments were also carried out to gain insights into the
dehydrogenative silylation pathway. In the presence of 2
equivalents of the radical scavenger TEMPO, the formation of 1
was completely inhibited, and H2 was not detected by gas
chromatography, which implied that the reaction was a radical
process (Scheme 2b). When N-methyl-N-phenylacrylamide was
used, no corresponding vinylsilane product 43 was detected
(Scheme 2c), showing that the reaction had high regioselectivity.
Moreover,
N-methyl-N-phenylethylacrylamide
was
also
compatible with this reaction, which resulted in only Z isomeric
product (Scheme 2d). A kinetic isotope effect (KIE) value of 2.8
was determined by the intermolecular competitive isotope effect
experiment, which indicated that the methyl C-H bond rupture
could be a kinetically relevant step in this reaction (Scheme 2e).
Subsequently, luminescence quenching studies were performed.
As shown in Stern-Volmer plot (Scheme 2f), Co(dmgH)2PyCl
had much larger quenching extent and revealed a kinetic
preference for the single-electron oxidation of excited state
4CzIPN*. To reveal more details of the radical mechanism, the
electron paramagnetic resonance (EPR) experiments were
designed and then performed to observe the capture of the
radical intermediate by DMPO as the spin trapping reagent (see
SI for details). The EPR signal clearly indicated the formation of
spin-adduct products, where g = 2.00738, aN = 14.57 G and aH =
20.47 G. We could conclude that a carbon-centered radical[12]
was involved in the reaction, resulting from the addition of the
silyl radical to N-methyl-N-phenylmethacrylamide.
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10.1002/anie.201904707
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Based on the above observations as well as other reports,[13-
a plausible reaction mechanism was proposed and illustrated
Keywords: allylsilanes, photochemistry, hydrogen atom transfer,
cobalt catalysis, radicals
14]
in Scheme 3. Irradiation of the ground-state photocatalyst
4CzIPN with visible light produces the excited-state
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*
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B
III
II
( [Co /Co ] = -0.68 V vs. SCE). Then the generated oxidizing
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abstracts the hydrogen atom from the hydridic Si-H bond of
TTMSS [BDE(Si-H)
=
82.3 kcal/mol] to produce the
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Scheme 3. A proposed mechanism
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In summary, we developed a synergistic combination of
photoredox catalysis, hydrogen atom transfer and cobalt
catalysis to synthesize allylsilanes from various of alkenes with
H2 evolution. The dehydrogenative silylation reaction features
high regioselectivity, excellent tolerance of functional groups,
wide scopes of substrates and mild reaction conditions.
Moreover, this oxidant-free system offers a cleaner and more
efficient method beyond traditional catalysis using stoichiometric
or excess amounts of oxidants. Notably, this reaction model has
been successfully used in the introduction of a silyl group into
many biologically active molecule skeletons.
Acknowledgements
We thank the NSFC (21632003, 21871116 and 21572087), the
key program of Gansu province (17ZD2GC011) and the ‘‘111’’
program from the MOE of P. R. China for financial support.
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10.1002/anie.201904707
Angewandte Chemie International Edition
COMMUNICATION
COMMUNICATION
W.-L.Yu, Y.-C. L, L. Yan, D.Liu, Z.-
Y. Wang P.-F.Xu*
Page No. – Page No.
Dehydrogenative Silylation of Alkenes
for the Synthesis of Substituted
Allylsilanes via Photoredox, HAT, and
Cobalt Catalysis
Text for Table of Contents
A synergistic catalytic method of combining photoredox catalysis, hydrogen atom transfer and proton-reduction catalysis for the
dehydrogenative silylation of alkenes was developed. The reaction features high regioselectivity, excellent tolerance of functional
groups, wide scopes of substrates and mild reaction conditions. Moreover, this oxidant-free system offers a cleaner and more
efficient method beyond traditional catalysis using stoichiometric or excess amounts of oxidants.
This article is protected by copyright. All rights reserved.