Iron-Catalyzed Silylation of (Hetero)aryl Chlorides with Et3SiBpin

Article abstract of DOI:10.1021/acs.orglett.0c00809

To date, the iron-catalyzed construction of C-heteroatom bonds has been less developed due to the difficulty of transmetalation with heteroatom anions and the sluggish reductive elimination. Herein we report an iron-catalyzed method for the silylation of

Full text of DOI:10.1021/acs.orglett.0c00809

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Iron-Catalyzed Silylation of (Hetero)aryl Chlorides with Et3SiBpin
Jia Jia,∥ Xiaoqin Zeng,∥ Zhengli Liu, Liang Zhao, Chun-Yang He,* Xiao-Fei Li,* and Zhang Feng*
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ABSTRACT: To date, the iron-catalyzed construction of C−
heteroatom bonds has been less developed due to the difficulty of
transmetalation with heteroatom anions and the sluggish reductive
elimination. Herein we report an iron-catalyzed method for the
silylation of (hetero)aromatic chlorides. It features high efficiency,
a broad substrate scope, and excellent functional group
compatibility. Moreover, this protocol enables the late-stage
silylation of some pharmaceuticals, thus providing an excellent
method to access valuable intermediates in medicinal chemistry.
reductive elimination from the iron center (Scheme 1). For
he development of efficient methods for the construction
Tof carbon−heteroatom bonds is an essential objective in instance, to facilitate the transmetalation process, reactive
organic synthesis.1 Tremendous achievements have been made
magnesium phenylamides were employed in the iron-catalyzed
C−N bond formation cross-coupling reaction.6 Recently, the
in the transition-metal-catalyzed construction of C(aryl)−
heteroatom bonds in the past several decades.2 Compared with
Nakamura group disclosed an important work on the iron-
catalyzed borylation of aryl chlorides,7 but the substrate scope
noble metals, iron catalysis has become more and more
attractive due to its nontoxicity and cheapness.3 To date, iron
was restricted to the reactive aryl chlorides.
Organosilanes are very important synthons in organic
catalysis has been widely employed in cross-coupling reactions
synthesis due to their wide applications in medicinal chemistry
for the construction of C−C bonds using organometallics as
and material science.8 To our knowledge, the iron-catalyzed
reaction partners (Scheme 1).4 However, advances in the
silylation of electrophilic reagents, such as aryl halides, has
development of iron-catalyzed C−heteroatom bonds forma-
scarcely been reported,9 even though great progress has been
tion have been rather limited,5 especially through cross-
achieved in the iron-catalyzed hydrosilylation of alkenes and
coupling reactions, which might be due to the difficulty of
transmetalation with heteroatom anions as well as the sluggish
alkynes.10 Aryl chlorides are cheap and commercially available
and have been extensively used as coupling partners in
transition-metal-catalyzed reactions.11 However, the transition-
Scheme 1. Iron-Catalyzed Formation of C−Heteroatom
metal-catalyzed silylation of aryl chlorides has been less
Bonds
documented. This is because of the relatively strong
dissociation energy of the C−Cl bond, which requires the
use of electron-rich ligands to facilitate the oxidative addition
process (Scheme 2).12 For instance, the Buchwald group
reported the palladium-catalyzed silylation of aryl chlorides
using a disilane reagent with the aid of electron-rich phosphine
ligands.12a The reaction of hydrosilane starting materials with
aryl chlorides has also been explored with noble metals,
palladium, and rhodium, but this transformation has proceeded
Received: March 3, 2020
https://dx.doi.org/10.1021/acs.orglett.0c00809
© XXXX American Chemical Society
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A

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exhibited poor efficiency in the previous report.7 After testing
various ligands, we found that the phosphine ligands and
nitrogen ligands could both promote this transformation using
1a as a substrate (Table 1, entries 1−5). The dinitrogen ligand,
3,4,7,8-tetra-Me-phen, stood out, providing the desired
product in 65% yield (Table 1, entry 5). Additionally, other
iron sources were evaluated as well, and FeI2 gave the best
result (Table 1, entries 5−7; for details, see the Supporting
Information). Interestingly, when the highly pure iron catalyst
was employed, a higher yield was obtained (Table 1, entry 8,
75% yield upon isolation). Control experiments revealed the
necessity for a iron catalyst and ligand. No silylated product
was observed in the absence of the iron catalyst, and only a
21% yield of 1 was afforded without the use of a ligand (Table
1, entries 9 and 10). To evaluate the trace-metal effect in this
transformation, some transition metals were tested. Copper,
palladium, and nickel sources could promote this reaction as
well, but lower yields were presented (Table 1, entries 11−13).
These results suggest that this reaction was catalyzed by the
iron catalyst not other transition metals.
With the optimized reaction conditions established, various
aryl chlorides were examined. As shown in Table 2, the
monoaryl chlorides and biphenyl chlorides could both undergo
this transformation smoothly, providing the corresponding
products in moderate to good yield. This reaction exhibited
good functional group tolerance. Functional groups, such as
BnO, hydroxyl, morpholinyl, amine, Boc, CF3, Bpin, and
alkenyl, could be well-tolerated (5, 8−14). Electron-rich
substrates reacted well, affording silylated products in
moderate to good yield (4−7, 11; 5−83%). Electron-deficient
aryl chloride 12a underwent this transformation smoothly, and
a moderate yield was obtained. The alkenyl group could not
always be tolerated in hydrosilylation reactions, but substrate
14a could react well and generated the desired product 14 in
76% yield. Importantly, substrate 24a with a bulky steric
hindrance could also undergo this transformation, affording the
silylated product 24 in 70% yield. The heteroaromatic
chlorides were also suitable for this catalytic system. The
Scheme 2. Iron-Catalyzed Silylation of Aryl Chlorides
with low efficiency, even in the presence of electron-rich
ligands.13 In 2015, He and coworkers reported the palladium-
catalyzed silylation of aryl halides, in which only one example
of aryl chloride, 4-chloroanisole, was studied using a reactive
silylborane reagent.14 Therefore, the development of an
efficient and economical method for the synthesis of aryl
silane is highly appealing. With our continuing interest in
transition-metal catalysis,15 we herein report the first example
of the iron-catalyzed silylation of aryl chlorides using a nitrogen
ligand.
With these considerations in mind, we began our initial
studies on the silylation of the relatively reactive 4-biphenyl
chloride 15a with silylborane reagent 2b.9a To our delight, the
corresponding silylated product 15 could be obtained in
moderate to excellent yield (90% yield upon isolation) when
this reaction proceeded in the presence of FeI2 and nitrogen
ligands or phosphine ligands. (For details, see the Supporting
Information.) Encouraged by these results, we started to
investigate the silylation of the more inert substrate, which
a
Table 1. Representative Results for the Optimization of the Iron-Catalyzed Silylation of Aryl Chloride 1a
b
entry
[Fe]
ligand
yield (%)
1
2
3
4
5
6
7
FeI2
FeI2
FeI2
FeI2
XantPhos (10 mol %)
XPhos (10 mol %)
PCy3 (20 mol %)
2,9-di-Me-phen (10 mol %)
3,4,7,8-tetra-Me-phen (10 mol %)
3,4,7,8-tetra-Me-phen (10 mol %)
3,4,7,8-tetra-Me-phen (10 mol %)
3,4,7,8-tetra-Me-phen (10 mol %)
20
28
38
35
65
FeI2
FeBr2
Fe(OAc)2
FeI2
43
33
c
8
c
77 (75)
21
9
FeI2
10
11
12
13
3,4,7,8-tetra-Me-phen (10 mol %)
3,4,7,8-tetra-Me-phen (10 mol %)
3,4,7,8-tetra-Me-phen (10 mol %)
3,4,7,8-tetra-Me-phen (10 mol %)
0
28
24
44
CuI (5 mol %)
PdI2 (0.1 mol %)
Ni(OAc)2·4H2O (0.1 mol %)
a
Reaction conditions (unless otherwise specified): 1a (0.2 mmol, 1.0 equiv), silylborane 2b (0.7 mmol, 3.5 equiv), [Fe] (0.02 mmol, 0.1 equiv),
b
Ligand (0.1 to 0.2 equiv), t-BuONa (0.5 mmol, 2.5 equiv), THF (1.5 mL), 120 °C, 12 h. Determined by 1H NMR using mesitylene as an internal
standard. The isolated yield is shown in parentheses. FeI2(99.99%) was used.
c
B
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a
Table 2. Scope of the Iron-Catalyzed Silylation of (Hetero)aryl Chlorides
a
Reaction conditions (unless otherwise specified): aryl chlorides (0.2 mmol, 1.0 equiv), silylborane 2b (0.7 mmol, 3.5 equiv), FeI2 (0.02 mmol, 0.1
b
equiv), 3,4,7,8-tetra-Me-phen (0.02 mmol, 0.1 equiv), t-BuONa (0.5 mmol, 2.5 equiv), THF (1.5 mL), 120 °C, 12 h. Aryl chlorides (0.2 mmol,
1.0 equiv), silylborane 2b (0.74 mmol, 3.7 equiv), FeI2 (0.02 mmol, 0.1 equiv), 3,4,7,8-tetra-Me-phen (0.02 mmol, 0.1 equiv), t-BuONa (0.5 mmol,
2.5 equiv), THF (1.5 mL), 135 °C, 12 h. FeI2(99.99%) was used.
c
chloropyridines and chloroindoles could react well, furnishing
the corresponding products in moderate yield (25, 26, 28, and
29). However, to our surprise, the chloroquinolone could not
undergo this transformation (27, 0%). It is worth mentioning
that the substrates containing an unprotected hydroxyl or
amine group could undergo this silylation smoothly, albeit in
low yield (8, 34%; 28, 44%). We found that this trans-
formation is very sensitive to substrates, and the side reactions,
such as protonation and dimerization, were always observed.
The more reactive substrates, 1-bromo-4-tert-butylbenzene and
4-tert-butyliodobenzene, prefer to generate the protonated
byproducts, not the silylated products. Moreover, other
silylation reagents were also evaluated, such as PhMe2Si-Bpin
and Ph3Si-SiPh3, but no desired products were found.
Scheme 3. Late-Stage Functionalization of Pharmaceuticals
The iron-catalyzed silylation reaction was further demon-
strated by its applicability in the late-stage functionalization of
pharmaceuticals (Scheme 3). Cloperastine, an antitussive and
antihistamine, could undergo this silylation with high
efficiency, affording the corresponding product 31 in 88%
yield. In addition, this reaction could be conducted on a 1 g
scale, and a 72% yield was provided. Buclizine 32a is
considered as an antiemetic and could also be silylated under
standard conditions. Clomipramine 33a, which is used for the
treatment of obsessive-compulsive disorder and for decreasing
the risk of suicide, could react well, providing the silylated
product in 54% yield. Moreover, sibutramine 34a, an appetite
suppressant that is widely employed as an adjunct in the
treatment of obesity along with diet and exercise, furnished the
silylated product in 60% yield. Thus this protocol offers an
excellent synthetic route for the diversification of some
pharmaceuticals.
To gain insight into the mechanism of this silylation
reaction, radical inhibition experiments were carried out.
Drastically decreased yields of 1 were observed when a radical
scavenger TEMPO (100 mol %) or a radical inhibitor BHT
(100 mol %) was added as an additive under the standard
C
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reaction conditions (Scheme 4A). Furthermore, a radical clock
experiment was conducted, but no standard radical ring-
Guizhou Province, School of Pharmacy, Zunyi Medical
University, Zunyi, Guizhou 563003, P. R. China; orcid.org/
0000-0002-0599-7335; Email: hechy2002@163.com
Scheme 4. Mechanistic Studies
Authors
Jia Jia − Key Laboratory of Biocatalysis & Chiral Drug Synthesis
of Guizhou Province, Generic Drug Research Center of Guizhou
Province, School of Pharmacy, Zunyi Medical University, Zunyi,
Guizhou 563003, P. R. China
Xiaoqin Zeng − Chongqing Key Laboratory of Natural Product
Synthesis and Drug Research, School of Pharmaceutical Sciences,
Chongqing University, Chongqing 401331, P. R. China
Zhengli Liu − Chongqing Key Laboratory of Natural Product
Synthesis and Drug Research, School of Pharmaceutical Sciences,
Chongqing University, Chongqing 401331, P. R. China
Liang Zhao − Key Laboratory of Biocatalysis & Chiral Drug
Synthesis of Guizhou Province, Generic Drug Research Center of
Guizhou Province, School of Pharmacy, Zunyi Medical
University, Zunyi, Guizhou 563003, P. R. China
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.orglett.0c00809
opening product 36 was observed, and a trace amount of 1 was
obtained (Scheme 4B). These results indicated that the silane
radical might not be involved in this transformation. On the
basis of these results and the radical nature of iron-catalyzed
cross-coupling reactions,7,16 we decided that the radical
pathway could not be ruled out in this catalytic system.
In conclusion, we have developed an efficient iron-catalyzed
method for the silylation of (hetero)aryl chlorides. This
reaction features a broad substrate scope and good functional
group tolerance. Moreover, this transformation has exhibited
the possibility of the late-stage functionalization of some
pharmaceuticals, thus providing excellent opportunities for
applications in drug discovery and development. Further
mechanistic studies to explain this silylation reaction are
ongoing in our lab.
Author Contributions
∥J.J. and X.Z. contributed equally.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We are grateful for the financial support from the National
Natural Science Foundation of China (nos. 21801029,
81760624, and 21702241), the Graduate Scientific Research
and Innovation Foundation of Chongqing (no. CYS18046),
the 100 Talent Plan from Chongqing University
(0247001104405), the Natural Science Foundation of
Chongqing (no. cstc2019jcyjmsxmX0048), Sichuan Key
Laboratory of Medical Imaging (North Sichuan Medical
College, no. SKLMI201901), and Programs of Guizhou
Province (nos. 2017-1225, 2018-1427).
ASSOCIATED CONTENT
* Supporting Information
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REFERENCES
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.orglett.0c00809.
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AUTHOR INFORMATION
Corresponding Authors
■
Zhang Feng − Chongqing Key Laboratory of Natural Product
Synthesis and Drug Research, School of Pharmaceutical Sciences,
Chongqing University, Chongqing 401331, P. R. China;
Sichuan Key Laboratory of Medical Imaging & Department of
Chemistry, School of Preclinical Medicine, North Sichuan
Medical College, Nanchong, Sichuan 637000, P. R. China;
orcid.org/0000-0001-7776-8200; Email: fengzh@
cqu.edu.cn
Xiao-Fei Li − Key Laboratory of Biocatalysis & Chiral Drug
Synthesis of Guizhou Province, Generic Drug Research Center of
Guizhou Province, School of Pharmacy, Zunyi Medical
University, Zunyi, Guizhou 563003, P. R. China;
Email: lixiaofei@zmu.edu.cn
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