Pd-Catalyzed C(sp3)-C(sp2) cross-coupling of Y(CH2SiMe3)3(THF)2 with vinyl bromides and triflates

Article abstract of DOI:10.1039/c6ob01765g

Pd-Catalyzed C(sp³)-C(sp²) cross-coupling of Y(CH₂SiMe₃)₃(THF)₂ with vinyl bromides and triflates has been developed for efficient synthesis of various allyltrimethylsilanes. The cross-coupling reaction was conducted at room temperature with low catalyst loading of either Pd(PPh₃)₄ or Pd(PPh₃)₂Cl₂, and exhibited high efficiency and a broad substrate scope. In combination with the cross-coupling by the Lewis-acid catalyzed Hosomi-Sakurai reaction, a novel three-component one-pot cascade reaction was then accomplished to deliver homoallylic alcohols and ethers with high regioselectivity and diastereoselectivity. The three-component reaction defined the yttrium complex as a novel one-carbon synthon, which could either trigger bifunctionalization of alkenes or link two electrophiles and would find applications in organic synthesis.

Full text of DOI:10.1039/c6ob01765g

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DOI: 10.1039/C6OB01765G
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Pd-Catalyzed C(sp3)-C(sp2) Cross-Coupling of Y(CH2SiMe3)3(THF)2
with Vinyl Bromides and Triflates
Guilong Cai, Zhibing Zhou, Wenchao Wu, Bo Yao,* Shaowen Zhang, and Xiaofang Li*
Received 00th January 20xx,
Accepted 00th January 20xx
Pd-catalyzed C(sp3)-C(sp2) cross-coupling of Y(CH2SiMe3)3(THF)2 with vinyl bromides and triflates had been developed for
efficient synthesis of various allyltrimethylsilanes. The cross-coupling reaction was conducted under room temperature with
low catalyst loading of either Pd(PPh3)4 or Pd(PPh3)2Cl2, and exhibited high efficiency and broad substrate scope. In
combination of the cross-coupling with Lewis-acid catalyzed Hosomi-Sakurai reaction, a novel three-component one-pot
cascade reaction was then accomplished to deliver homoallylic alcohols and ethers with high regioselectivity and
diastereoselectivity. The three-component reaction defined the yttrium complex as a novel one-carbon synthon, which
could either trigger bifunctionalization of alkenes or link two electrophiles and would find applications in organic synthesis.
DOI: 10.1039/x0xx00000x
www.rsc.org/
we recently reported the first Pd-catalyzed C(sp3)-C(sp2) cross-
coupling of discrete rare-earth metal alkyl complexes with aryl
Introduction
bromides.10 The reaction delivered
a
variety of
Organometallic chemistry of rare-earth metals has been
extensively studied in the past several decades.1 Rare-earth
metal alkyl/aryl complexes were often applied as reactants or
catalysts in organic transformations and polymerizations.2,3,4
Although these compounds were proposed to be key
intermediates in many reactions, examples on organic reactions
using discrete organorare-earth metal complexes as reactants
were rare. In 1984, Fujiwara reported the first cross-coupling
reaction of rare-earth metal aryl complexes (PhYbI) with organic
halides by transition-metal catalysis.5 In 2010, Knochel reported
a novel method for the preparation of triaryl lanthanum (Ar3La)
and demonstrated efficient cross-coupling of these reagents
with aryl iodides in the presence of Pd(PPh3)4.6 Besides these
reports, Molander and his coworkers discovered that
lanthanide salts exhibited positive effects in Pd-catalyzed
C(sp3)-C(sp2) cross-coupling of triethylaluminium with aryl
bromides. They proposed that ethyl lanthanide complexes
might be formed in-situ and act as the actual coupling partner.7
Homoleptic rare-earth metal trialkyl/triaryl complexes are a
type of important compounds with unique structural properties
and reactivity.8 The stability and reactivity of these complexes
is highly dependent on the relative sizes of the ligands to the
central metals. Due to the steric bulk of the trimethylsilylmethyl
ligands, Ln(CH2SiMe3)3(THF)2 exhibited proper stability and
good reactivity at ambient temperature and therefore were
widely used as starting materials in organorare-earth metal
chemistry.8,9 With Ln(CH2SiMe3)3(THF)2 as the coupling partner,
benzyltrimethylsilanes with diverse functional groups efficiently
under mild conditions (Scheme 1). Furthermore, ɑ-
bromostyrene was also found to be coupled with
Ln(CH2SiMe3)3(THF)2
to
furnish
the
corresponding
allyltrimethylsilane, albeit in moderate yield.
Allylsilanes are a type of bench-stable nucleophilic allylation
reagents in organic synthesis. They have broad applications in
the synthesis of homoallylic alcohols11 and amines,12 ethers,13
a.Key Laboratory of Cluster Science of Ministry of Education, School of Chemistry,
Beijing Institute of Technology, 5 South Zhongguancun Street, Haidian District,
Beijing 100081, China. E-mail: yaobo@bit.edu.cn, xfli@bit.edu.cn.
†Electronic Supplementary Information (ESI) available: Experimental details,
compound characterization, copies of 1H NMR and 13C NMR spectra. See
DOI: 10.1039/x0xx00000x
Scheme 1. Application of Ln(CH2SiMe3)3(THF)n as a one-carbon synthon
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unsaturated ketones14 and so on by Lewis acid-catalyzed activity. In both of the two reactions, 3a was isolaVtieewdAinrticalelmOnolinset
Hosomi-Sakurai reactions with eletrophiles.15 Although many quantitative yield (96% and 97%). After catalyst screening, the
DOI: 10.1039/C6OB01765G
synthetic methods for the preparation of allylsilanes existed,16 substrate scope for vinyl bromides was next examined under
development of new methods by transition metal catalysis17 the conditions as below (Table 1): Y(CH2SiMe3)3(THF)2 (0.33
remains significant. In connection with our ongoing project in equiv., 0.1 mmol), vinyl bromide (1 equiv., 0.3 mmol),
application of organorare-earth metal complexes to organic Pd(PPh3)2Cl2 (1 mol%), toluene (4 mL), room temperature, 1 h.
synthesis, we developed Pd-catalyzed cross-coupling of Several ɑ-bromostyrene derivatives bearing methyl, ethyl and
Y(CH2SiMe3)3(THF)2 with vinyl bromides and triflates for the methoxy groups were found to be coupled with the yttrium
construction of allyltrimethylsilanes. Since the obtained complex
allyltrimethylsilanes could undergo the Hosomi-Sakurai allyltrimethylsilanes (3b
reaction with carbon electrophiles, we further developed a bromostyrenes prepared from cinnamic acid and its derivatives
three-component reaction18 by a one-pot cascade method to were also coupled with
to deliver the coupling products (3e
1
nicely to afford the corresponding
-
3d) in very excellent yield. And β-
1
-
give homoallylic alcohols and ethers efficiently (Scheme 1). This 3h) in almost quantitative yield. It is worthwhile to mention that
three-component reaction applied Y(CH2SiMe3)3(THF)2 as a functional groups such as chlorine and bromine on the phenyl
novel nucleophilic one-carbon synthon19 to trigger rings were well tolerated. The functional groups in the products
bifunctionalization of alkenes (Type
A
)
or to link two provided a handle for further transformations (Scheme 3).
electrophilic moieties (Type B).
Cross-Coupling of Y(CH2SiMe3)3(THF)2 with Vinyl Triflates
To further enlarge the substrate scope of the cross-coupling
reaction for the synthesis of allyltrimethylsilanes, we continued
Results and discussion
to test vinyl triflates
could be easily accessible from a large number of commercial
ketones and aldehydes. Initial study on the model reaction of 1-
phenylvinyl triflate 4a with Y(CH2SiMe3)3(THF)2
presence of various palladium catalysts revealed that Pd(PPh3)4
4 as the coupling partners because they
Cross-Coupling of Y(CH2SiMe3)3(THF)2 with Vinyl Bromides
We commenced our study with the investigation of the cross-
coupling of Y(CH2SiMe3)3(THF)2
1 with vinyl bromides. When ɑ-
1 in the
bromostyrene 2a was chosen as the test substrate, several Pd
catalyst/ligand systems were screened. Under the
Pd2(dba)3/XPhos conditions, which were optimal for the cross-
Table 2.
Cross-coupling of 1 with vinyl bromides 4a
coupling of Y(CH2SiMe3)3(THF)2
bromostyrene 2a was coupled with
1
with aryl bromides,10 ɑ-
smoothly to deliver
1
allyltrimethylsilane 3a in moderate yield (76%). Further
screening other palladium catalysts showed that both of the
less bulky ligand coordinated Pd(0) complex Pd(PPh3)4 and the
air-stable Pd(II) complex Pd(PPh3)2Cl2 exhibited better catalytic
Table 1.
Cross-coupling of 1 with vinyl bromides 2a
aConditions: Vinyl triflate
Y(CH2SiMe3)3(THF)2 (0.33 equiv., 0.1 mmol), dry toluene (4 mL) at room
temperature, N2, 45 min.
4 (1.0 equiv., 0.3 mmol), Pd(PPh3)4 (1 mol%),
aConditions: Vinyl bromide 2 (1.0 equiv., 0.3 mmol)
, Pd(PPh3)2Cl2 (1 mol%),
Y(CH2SiMe3)3(THF)2 (0.33 equiv., 0.1 mmol), dry toluene (4 mL) at room
temperature, N2, 1 h.
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was the best catalyst and the cross-coupling product 5a was catalysts in many organic transformations,4c,20 we wondered if
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isolated in 96% yield under the new optimum conditions. Then the Y(III) salts could trigger the subseqDuOenI:c1t0.1H0o39so/Cm6Oi-BS0a1k7u6r5aGi
a series of acyclic 1-aryl-vinyl triflates were synthesized from reaction of the in-situ generated allyltrimethylsilanes with
various methyl ketones and the cross-coupling of them carbon electrophiles. To test the feasibility of this idea, we
furnished the allyltrimethylsilanes (5b
the reactions of vinyl bromides. And a lot of cyclic vinyl triflates coupling of Y(CH2SiMe3)3(THF)2
prepared from cyclic ketones were also found to be coupled the cross-coupling conditions for 2 h followed by the reaction
with the yttrium complex very well, delivering the desired with 1.1 equiv. of benzaldehyde 6a at 100 °C for 3 h afforded
products (5f 5k) in very excellent yield. Both electron-donating the trimethylsilyl-protected homoallylic alcohol in74% yield.
-
5e) in the yield as high as conducted the experiment by a one-pot cascade way. The cross-
1
with ɑ-bromostyrene 2a under
1
-
7
methyl (5b and 5d), tert-butyl (5c), and methoxy (5h) groups, However, another parallel reaction with acidic workup
and electron-withdrawing chlorine (5i) and bromine (5j) in delivered the homoallylic alcohol 8a instead in similar yield
these substrates were well tolerated. Particularly, the aromatic (Scheme 2A). Encouraged by this result, we then investigated
C-Cl and C-Br bonds in the products could be further various carbon electrophiles for the three-component one-pot
transformed into various C-C and C-heteroatom bonds by many cascade reaction and found that only aromatic aldehydes and
synthetic methods. The cross-coupling of 2,2-diphenylvinyl acetals were well tolerant to the reaction conditions above. A
triflate 4l gave the corresponding product (5l) in much lower variety of aromatic aldehydes bearing either electron-
yield than other reactions.
withdrawing (Cl, Br, NO2) or electron-donating (Me) groups
were applied as the electrophiles for the three-component
reaction with Y(CH2SiMe3)3(THF)2 1 and ɑ-bromostyrene 2a. All
these reactions went smoothly to deliver the terminal products
Three-Component Reactions of Y(CH2SiMe3)3(THF)2, Alkenes with
Aldehydes (or Acetals)
In the cross-coupling reactions above, only 0.33 equiv. of in good yields (8b
Y(CH2SiMe3)3(THF)2 was needed for the full conversion of the needed. Besides, the reaction of
-
8e) without any other catalysts or reagents
and 2a with benzaldehyde
1
vinyl bromides or triflates. Therefore, all the three alkyl groups dimethyl acetal 6f gave the methyl homoallyl ether 8f in 80%
of the yttrium complex were transferred into the products, yield.
leading to the formation of 0.33 equiv of YBr3 or Y(OTf)3 as the
byproduct. As Ln(III) salts had been widely used as Lewis-acid
Although the three component reaction with ɑ-
bromostyrene 2a had been successfully developed, the reaction
for vinyl triflate
4 or β-bromostyrene 2e only gave poor yield
probably due to the lower Lewis acidity of Y(OTf)3 from vinyl
triflates or the lower reactivity of the allyltrimethylsilane 3e
from β-bromostyrene. To improve the reaction efficiency, the
conditions were modified for the second step by using TiCl4 as
the catalyst and CH2Cl2 as the solvent. Under the new conditions,
the reaction of some representative alkenes was carefully
investigated and the regioselectivity and diastereoselectivity in
the reaction was also discussed. First, the reaction of β-
bromostyrene 2e with
1 and 6f furnished the γ-substituted
product 9a in very excellent yield and high diastereoselectivity
(92% yield and 3:1 dr) without α-substituted product 9b
observed by NMR (Scheme 2B). Second, the cyclic vinyl triflate
with a six-membered ring 4g reacted with
1 and 6f to give the
α-substituted product 10b in 93% yield without the γ-
substituted product 10a detected by NMR (Scheme 2C). The
regioselectivity might be controlled by the steric hindrance of
α- and γ-positions of the allylsilane intermediate. Therefore, the
reaction of the cyclic vinyl triflate with a seven-membered ring
4k delivered both of the α- and γ-substituted products with the
α-product 11b as the major (Scheme 2D). The yttrium complex
1
had been utilized as a nucleophilic one-carbon synthon, which
reacted with two types of electrophiles. The first reaction
formally applied this reagent to trigger the bifunctionalization
of alkenes, while the other two reactions utilized it as a CH2
linker.
Application in the Multi-Functionalization of Styrene
To demonstrate the applications of the new methodologies
above in organic synthesis, we further performed a three-step
reaction of vinyl bromide 2g with
1
and 6f. After the first cross-
Scheme 2. The three-component reaction
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Sakurai reaction with 6f in the presence of TiCl4, and the second
cross-coupling with in the presence of Pd2(dba)3/XPhos, the
vinyl bromide 2g was finally transformed into the product 12 in
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In summary, we had developed the Pd-catalyzed cross-coupling
of Y(CH2SiMe3)3(THF)2
1 with various vinyl bromides 2 and
triflates for the construction of allytrimethylsilanes. The easy
4
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,
,
access to various vinyl bromides and triflates from commercial
alkynes, ketones and aldehydes, the mild conditions for the
cross-coupling, high efficiency and good functionality tolerance
made this new method a powerful tool for the synthesis
of allyltrimethylsilanes. In combination of the cross-coupling
with Lewis-acid catalyzed Hosomi-Sakurai reaction, a novel
three-component one-pot cascade reaction were realized for
the efficient synthesis of homoallylic alcohols and ethers with
high regioselectivity and good diastereoselectivity. This three-
component reaction applied Y(CH2SiMe3)3(THF)2
one-carbon synthon, which either triggered
bifunctionalization of alkenes or was connected with two
electrophiles. And the application of these methods in the
synthesis of complex molecules such as natural products or
unnatural organic functional materials is under way.
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as a novel
the
Acknowledgements
All authors have given approval for the final version of the
manuscript. The authors declare no competing financial interest.
We thank the National Natural Science Foundation of China
(21274012, 21322401 and 21490570) and Beijing Institute of
Technology (the 111 Project B07012) for financial support.
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