Arylsilylation of Electron-Deficient Alkenes via Cooperative Photoredox and Nickel Catalysis

Article abstract of DOI:10.1021/acscatal.9b04916

Carbosilylation of alkenes can be an efficient approach to the synthesis of organosilicon compounds. However, few general methods of carbosilylation are known. Here, we introduce a strategy for arylsilylation of electron-deficient terminal alkenes by combining photoredox-catalyzed silyl radical generation, innate reactivity of silyl radical with alkene, and Ni-catalyzed aryl-Alkyl cross-coupling. This cooperative photoredox and nickel catalysis operates under mild conditions. It employs readily available alkenes, aryl bromides, and silane as reagents, and it produces useful synthetic building blocks in a modular manner.

Full text of DOI:10.1021/acscatal.9b04916

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Letter
Arylsilylation of Electron-Deficient Alkenes via
Cooperative Photoredox and Nickel Catalysis
Zhikun Zhang, and Xile Hu
ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.9b04916 • Publication Date (Web): 13 Dec 2019
Downloaded from pubs.acs.org on December 13, 2019
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Arylsilylation of Electron-Deficient Alkenes via
Cooperative Photoredox and Nickel Catalysis
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Zhikun Zhang and Xile Hu*
Laboratory of Inorganic Synthesis and Catalysis, Institute of Chemical Sciences and Engineering,
École Polytechnique Fédérale de Lausanne (EPFL), ISIC-LSCI, Lausanne 1015, Switzerland
ABSTRACT: Carbosilylation of alkenes can be an efficient approach to the synthesis of
organosilicon compounds. However, few general methods of carbosilylation are known. Here we
introduce a strategy for arylsilylation of electron-deficient terminal alkenes by combining
photoredox-catalyzed silyl radical generation, innate reactivity of silyl radical with alkene, and Ni-
catalyzed aryl-alkyl cross-coupling. This cooperative photoredox and nickel catalysis operates
under mild conditions. It employs readily available alkenes, aryl bromides, and silane as reagents,
and produces useful synthetic building blocks in a modular manner.
KEYWORDS: nickel catalysis • arylsilylation • photoredox catalysis • cooperative catalysis •
organosilicon
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INTRODUCTION
Silicon-containing compounds have very wide applications in organic synthesis, medicinal
chemistry and material science.1 Hydrosilylation is a widely used method to synthesize
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organosilicon compounds from readily available alkenes (Scheme 1a). However, hydrosilylation
introduces only one functional (silyl) group. Carbosilylation of alkenes, on the other hand, can
introduce simultaneously two functional groups, providing rapid access to functionalized
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organosilicon compounds. Despite progress in difunctionlization of alkenes, carbosilylation
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4b-
remains under-developed. In earlier examples, only dienes and allenes were suitable substrates.
d, 4f
Kambe and co-workers then reported Cp
2
TiCl -catalyzed carbosilylation of alkenes through
2
4
e
silylation of an in-situ generated alkyl Grignard reagent (Scheme1b) . The latter intermediate,
however, limited the functional group compatibility. Engle and co-workers recently reported Pd-
4
h
catalyzed, 8-aminoquinoline (AQ) directed aryl/alkenylsilylation of alkenes (Scheme1b) . The
reaction occurred via quenching of an organopalladium intermediate with PhMe Si-Bpin, which
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nonetheless generated some 1, 2-carboborylation byproduct. Additionally, the AQ directing group
limited the scope of alkenes. In the above two examples, silylation was achieved by reaction of a
silicon reagent on an organometallic intermediate. We wondered whether carbosilylation via silyl
radicals would become a complementary approach to overcome some of the limitations of previous
methods.
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Silyl radicals are well known intermediates in organic synthesis. Traditionally, silyl radicals are
generated by oxidation of silanes with radical initiators (e.g., azobisisobutyronitrile (AIBN),
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peroxides.) under thermal conditions. Recently, photoredox catalysis was developed to generate
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silyl radicals under much milder conditions. We thought that if a silyl radical could be generated
by photoredox-oxidation of a silane, it could be trapped by an alkene to give a β-silyl alkyl radical
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(
Scheme 1c). This radical could be further trapped by a Ni(II) aryl Br intermediate generated from
oxidation addition of an aryl bromide to a Ni(0) species (Scheme 1c).8-9 Reductive elimination of
the resulting Ni(III) alkyl aryl intermediate would yield the arylsilylation product together with a
Ni(I) species which could be transformed into the original Ni(0) catalyst by the reduced
photoredox catalyst. Although transformations of β-silyl alkyl radical intermediates were
previously applied for carbosilylation of alkenes, those transformations were based on further
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oxidation followed by reactions with nucleophiles (Scheme 1b), further reduction followed by
reactions with electrophiles, or intramolecular addition to arenes4i, 4j, 4m, 4n. A general, cross-
coupling type carbosilylation remained elusive. In view of this state of the art, our strategy
appeared attractive.
4l
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Scheme 1. (a) A general scheme for hydrosilylation of alkenes; (b) Selected prior examples of
carbosilylation of alkenes; (c) Our design strategy for arylsilylation of alkenes via cooperative
photoredox and nickel catalysis.
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RESULTS AND DISCUSSION
Following this design, we explored conditions for arylsilylation of methyl acrylate (1a), which
SiH (2a) as the silane due to its weak Si-H bond,6d, 6e
is a good radical acceptor. We chose TMS
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prone to formation of a silyl radical. We chose bromobenzene (3a) as an archetypical aryl halide.
7b
.DME as the Ni
We first applied [Ir(dF(CF
3
)ppy)
2
(dtbbpy)]PF
6
(PC1) as the photocatalyst, NiBr
2
source, and 4,4′-di-tert-butyl-2,2′-dipyridyl as ligand (for Ni). An encouraging yield of 15% of the
desired product 4a was obtained upon illumination with blue LED in DME at room temperature
after 12 h, when 2.0 equiv of Na
2
CO was used as base (Table 1, entry 1). Other ligands and bases
3
were used, but the yields were lower (Tables S1 and S2, SI). The solvents had a strong influence
on the yields of the reaction (Table S3, SI). 1,4-dioxane was the best solvent, giving a yield of 54%
for 4a (Table 1, entry 2). A slight modification on the substrate ratio improved the yield to 60%
(
Table 1, entry 3). Other Ni salts were screened as the Ni sources (Table S4, SI), and Ni(COD)
was found to be the best, probably by facilitating the oxidative addition of phenyl bromide (Table
1, entry 4). We then screened other photocatalysts. [Ir(dF(Me)ppy) (dtbbpy)]PF (PC2) improved
the yield to 76% (Table 1, entry 5), while [Ir(ppy) (dtbbpy)]PF (PC3) and 4CzIPN (PC4) were
2
2
6
2
6
not efficient at all (Table 1, entry 6,7). The performance of the photocatalysts PC1-3 correlated
with their excited-state oxidation potentials (see below for details). A further tuning of substrate
ratio (Table S5, SI) led to the optimized conditions, under which 4a could be obtained in 88%
yield. (Table 1, entry 8). Control experiments showed that the nickel catalyst, photocatalyst, and
light are all necessary for the reaction (Table S6, SI). Chlorobenzene and iodobenzene were not
suitable substrates (Table S6, SI). Without base, the yield decreased sharply (Table S6, SI),
probably due to the decomposition of catalysts under acidic conditions.
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Table 1. Summary of key reaction parameters.a
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Yields (%)[b]
Entry
1a : 2a : 3a
1.5 : 1.0 : 1.5
1.5 : 1.0 : 1.5
1.5 : 1.5 : 1.0
1.5 : 1.5 : 1.0
1.5 : 1.5 : 1.0
1.5 : 1.5 : 1.0
1.5 : 1.5 : 1.0
1.0 : 1.5 : 2.0
Ni salt
PC
Solvent
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NiBr
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2
2
.DME
PC1
PC1
PC1
PC1
PC2
PC3
PC4
PC2
DME
15
54
NiBr
NiBr
.DME
.DME
1,4-Dioxane
1,4-Dioxane
1,4-Dioxane
1,4-Dioxane
1,4-Dioxane
1,4-Dioxane
1,4-Dioxane
60
Ni(COD)
Ni(COD)
Ni(COD)
Ni(COD)
Ni(COD)
2
2
2
2
2
65
76
<5%
<5%
88 (80)
[a] The reaction scale was 0.2 mmol. Loadings and substrate ratios were shown in the Table.
Volume of solvent was 2.0 mL. Reactions were conducted under nitrogen at room temperature for
1
2 hours. Additional details can be found in the SI. [b] Yields were determined by GC-FID analysis
with dodecane as internal standard; the yield in parentheses for entry 8 is isolated yield. TMS =
trimethylsilyl, DME = dimethoxyethane, COD = 1, 5-cyclooctadiene, dtbbpy = 4, 4′-di-tert-butyl-
2
, 2′-dipyridyl, LED = light emitting diode, PC = photoredox catalyst.
We then explored the scope of this arylsilylation using the optimized conditions (Table 2). A
large number of aryl bromides (3a-3aa) could be coupled. Reactions of 4-substituted aryl bromides
typically gave high yields of arylsilylation (4a-4d, 4f, 4g-4o). The substituents could be electron
rich (4b, 4c, 4f), neutral (4a), or poor (4g-4o). 3-Substituted aryl bromides were coupled in
reasonable yields as well (4q-4s). A crystal structure of 4p was determined (for details, see section
6
in SI). The reactions tolerated potentially coordinating functional groups such as ester, amide,
aldehyde, and nitrile. Moreover, groups potentially reactive in cross-coupling such as boryl (4t)
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and aryl-bromide (4w) were also tolerated. But styrene (4u) and alkynyl (4v) groups posed a
problem and the yields were low. While 2-bromonaphthalene was easily coupled (4e), 1-
bromonaphthalene (3aj) was not a viable substrate, likely due to steric hindrance. Likewise, 2-
methyl phenyl bromide (3ak) could not be coupled. These results might be rationalized by the
difficulty in trapping of an alkyl group with a large tri(trimethylsilyl)silyl group at the α-position
by a nickel intermediate with a hindered aryl group. Unhindered, disubstituted aryl bromides could
be coupled to give good yields (4w-4y). Coupling of aryl bromides conjugated to a heteroaryl
group was achieved as well (4z and 4aa), despite potential addition of the silyl radical to the
heterocycle via Minisci-type reaction.10
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The arylsilylation also worked with heteroaryl bromides such as 3- and 4-bromo pyridines (4ab-
4
ai), although the yields were more modest compared to those of coupling with aryl bromides. A
gram-scale reaction was conducted for the synthesis of 4l, and the product was obtained in 73%
yield, demonstrating the robustness of the reaction for scale-up.
We then probed the scope of alkenes using methyl 4-bromobenzoate (1k) as the arylation
reagent. Different alkyl acrylates including phenol, benzyl alcohol, 3-oxobutanol, N-boc-4-
hydroxypiperidine and ethyl 6-hydroxyhexanoate derived acrylates were viable alkene partners
(4aj-4an). Reaction with acrylonitrile also gave a high yield (4ao). Vinyl acetate could be used,
but the yield was low (4ap). Arylsilylation of styrenes (1i), alkyl alkenes (1j), and multi-
substituted methyl acrylates (1k), however, were unsuccessful.
The synthetic utility of arylsilylation products was demonstrated by several further
transformations of compound 4a (Scheme 2). The ester group in 4a could be reduced to an alcohol
group by LiAlH (Scheme 2a). The tri(trimethylsilyl)silyl group in the resulting compound 6 could
4
then be oxidized to a second alcohol group, yielding 2-phenyl-1,3- propanediol (7). The
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tri(trimethylsilyl)silyl group in 4a could also be converted to a disilane group (as in 8). Oxidation
of the disilane group gave a siloxane 9. Both propanediol and siloxane are useful building blocks
in organic and polymer synthesis.
Table 2. Scope of arylsilylation of alkenes. [a, b]
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In the test arylsilylation reaction (Table 1), the major side product (ca. 10%) was methyl β-
TMS Si-propionate (5a), originated from hydrosilylation of methyl acrylate (1a) with silane (2a).
3
When 5a was treated with bromobenzene (3a) under conditions of arylsilylation, no reaction
occurred (Fig. 1a). This result excluded the possibility that arylsilylation proceeded first by
hydrosilylation followed by α-arylation. When the reaction was conducted in the presence of
radical scavengers such as (2, 2, 6, 6-tetramethylpiperidin-1-yl)oxyl (TEMPO) and 1,1-
diphenylethane, the yields decreased (Fig. 1b). However, a significant amount of TEMPO (1.5
equiv) was needed to completely quench the reaction. The silylated TEMPO was not observed in
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GC-MS, probably due to its instability, but 1, 1-diphenyl-2-(TMS) Si-ethane was observed. Light–
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dark interval experiments showed that the product was formed only under light (Fig. S1, SI). The
quantum yield was 0.93 (for details, see section 5 in SI). These results suggest that arylsilylation
is mostly a non-chain radical process. Fluorescence quenching experiments were conducted on
PC2 using all possible quenchers in the reaction mixture, including 1a, 2a, 3a and
[
(dtbbpy)Ni(Ph)(Br)] (10), in-situ prepared from reaction of Ni(COD) , dtbbpy, and PhBr (for
2
details, see section 5 in SI). No significant quenching was observed with 1a, 2a, or 3a (Fig. S2,
SI). However, 10 efficiently quenched the excited state of PC2 (Fig. 1c), following a linear Stern-
n
Volmer plot (Fig. 1d). Additionally, strong quenching was observed using Bu
4
NBr, a soluble
-
source of Br , as the quencher (Fig. S3, SI). These results suggest quenching of excited PC2 by
-
-
10 leads to oxidation of its Br ligand to a Br. Similar photoredox generation of Br∙ from Br was
7b
n
previously reported. Addition of extra Bu
4
NBr during the reaction slightly decreased the yield
due to an increase of side product 5a (Table S6, SI). This mode of initiation is consistent with the
푟푒푑
*
performance of different catalysts. Both PC1 (퐸1/2 [Ir(III) /Ir(II)] = +1.21 V vs saturated calomel
푟푒푑
*
electrode (SCE) in CH
3
CN)11 and PC2 (퐸1/2 [Ir(III) /Ir(II)] = 0.97 V vs saturated calomel
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CN) could oxidize Br ((퐸
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/2
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/2
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퐸1 [Ir(III) /Ir(II)] = 0.66 V vs saturated calomel electrode (SCE) in CH CN ) , however, is not
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oxidizing enough for oxidation of Br , which explains why PC3 is not a viable photocatalyst. For
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PC4, the excited-state oxidation potential is sufficient to oxidize Br anion, but the catalyst might
be too unstable for this reaction. The higher catalytic efficiency of PC2 compared to PC1 might
also be related to their excited state oxidation potentials. The higher oxidation power for PC1*
induced more side reactions (according to GC-MS analysis). The involvement of Br∙ also explains
-
-
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the inactivity of PhCl and PhI as substrates. Cl is more difficult than Br to oxidize. Although I is
expected to be oxidized to I∙ under the photocatalytic conditions, the latter has a too low oxidation
power to abstract an H atom from a silane.
Scheme 2. Further transformations of an arylsilylation product 4a. (a) Synthesis of 2-phenyl-1, 3-
propanediol (7); (b) Synthesis of a disilane (8) and a siloxane (9).
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Figure 1. Mechanistic investigations. (a) Reaction of 5a with PhBr under standard arylsilylation
conditions. (b) Arylsilylation in the presence of radical scavengers. For experimental details, see
*
the Supporting Information. (c) Quenching of excited Ir (III) photocatalyst PC2 with 10 (10 =
[
(dtbbpy) Ni(Ph)(Br)]); (d) Stern-Vomer linear fitting of the quenching in (c).
7b
Based on the above results and literature, we proposed a mechanism for the arylsilylation (Fig.
2
). Oxidative addition bromobenzene (3a) to a Ni (0) catalyst (A) gives a Ni(II)(Ph)(Br)
III
intermediate (B). Meanwhile, the Ir photocatalyst is excited by blue LED light. The excited-state
II
is then reductively quenched by Br to give Ir and a Br∙. The latter abstracts a hydrogen atom from
silane (2a) to give a silyl radical (E). Addition of E to methyl acrylate (1a) generates a β-silyl alkyl
radical (F), which is trapped by nickel species B to give a Ni(III) intermediate (C). C-C reductive
elimination from C yields the arylsilylation 4a and a Ni(I) species (D), which is reduced by Ir(II)
to regenerate both the Ir(III) photocatalyst and the Ni(0) catalyst.
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Figure 2. A proposed mechanism for arylsilylation of alkenes via cooperative photoredox and
nickel catalysis.
CONCLUSIONS
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In summary, by combing photoredox and nickel catalysis, we achieved arylsilylation of electron-
deficient terminal alkenes using aryl bromides and TMS SiH as reaction partners. The silylation
3
involved a photo-generated silyl radical intermediate, whereas the arylation proceeded via Ni-
catalyzed cross coupling. The reactions could be conducted under mild conditions, exhibiting
broad scope and high functional group tolerance. The products are useful synthetic intermediates.
ASSOCIATED CONTENT
AUTHOR INFORMATION
Corresponding Author
*
E-mail: xile.hu@epfl.ch
ORCID
Zhikun Zhang: 0000-0002-0591-3680
Xile Hu: 0000-0001-8335-1196
Author Contributions
The authors declare no competing financial interests.
Funding Sources
This work is supported by the Swiss National Science Foundation.
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Supporting Information.
The Supporting Information is available free of charge on the ACS Publications website.
Experimental procedures, catalyst and compounds characterization (PDF)
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ACKNOWLEDGMENT
We thank José Luis Bila of the Severin's Group at EPFL for the help in fluorescence quenching
experiments and Dr. Farzaneh Fadaei Tirani (EPFL) for X-ray crystallographic analysis.
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