A Bithiophene-Promoted ppm Levels of Palladium-Catalyzed Regioselective Hydrosilylation of Terminal Allenes

Article abstract of DOI:10.1002/adsc.201901552

A bithiophene?alkyne-based compound was synthesized and first utilized as a ligand for the selective hydrosilylation of allenes with primary and secondary phenylsilanes. It shows high selectivity towards the production of branched allylsilanes with a wide range of allenes. It is worth mentioning that the catalytic loading of the palladium can be reduced to 500 ppm. This work opens a new front of using bidentate thiophene ligand as a reaction promoter in transition-metal-catalyzed organic reaction. (Figure presented.).

Full text of DOI:10.1002/adsc.201901552

Title: A bithiophene-promoted ppm level palladium-catalyzed highly
regioselective hydrosilylation of terminal allenes
Authors: Jun-Jia Chen, Jia-Hao Zeng, Ying Yang, Zhi-Kai Liu, YanNan
Jiang, MiaoRan Li, Li Chen, and Zhuang-Ping Zhan
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To be cited as: Adv. Synth. Catal. 10.1002/adsc.201901552
Link to VoR: http://dx.doi.org/10.1002/adsc.201901552

10.1002/adsc.201901552
Advanced Synthesis & Catalysis
COMMUNICATION
DOI: 10.1002/adsc.201((will be filled in by the editorial staff))
A Bithiophene-Promoted ppm Levels of Palladium-Catalyzed
Regioselective Hydrosilylation of Terminal Allenes
Jun-Jia Chen‡, Jia-Hao Zeng‡, Ying Yang, Zhi-Kai Liu, Ya-Nan Jiang, Miao-Ran Li,
Li Chen, and Zhuang-Ping Zhan*
a
Department of Chemistry and Key Laboratory for Chemical Biology of Fujian Province, College of Chemistry and
Chemical Engineering, Xiamen University, Xiamen, Fujian, People’s Republic of China
E-mail: zpzhan@xmu.edu.cn; Tel: 0592-2180318
Received: ((will be filled in by the editorial staff))
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201######.((Please
delete if not appropriate))
Abstract. A bithiophene-alkyne-based compound was
synthesized and first utilized as a ligand for the selective
hydrosilylation of allenes with primary and secondary
phenylsilanes. It shows high selectivity towards the
production of branched allylsilanes with a wide range of
allenes. It is worth mentioning that the catalytic loading of
the palladium can be reduced to 500 ppm. This work opens
a new front of using bidentate thiophene ligand as a
reaction promoter in transition-metal-catalyzed organic
reaction.
Keywords: Bithiophene ligand; Ppm level palladium;
Allenes; Hydrosilylation; Regioselectivity
Introduction
Transition metal complexes with sulfur ligands are
active catalysts in a considerable number of
homogeneous reactions,[1,2] whereas they have been
less developed than other heteroatoms such as
phosphorus and nitrogen ligands because sulfur has a
tendency to poison metal catalysts.[3] As a result,
transition-metal-catalyzed synthetic reactions with
sulfur-containing ligands still await study. Besides,
the sulfur-containing compounds normally occurred
as monodentate, N-S bidentate or P-S bidentate
ligands,[2] while the sulfur bidentate ligands are rarely
investigated. Among those, M. Cristina White has
applied a bis-sulfoxide-promoted system in a variety
of organic transformations.[4] To the best of our
knowledge, there are still no examples of using
bidentate thiophene compounds as ligands in the
organic reactions.
Figure 1. Hydrosilylation of allenes: previous work
and our work.
synthesis of versatile allylsilanes has been paid more
and more attention. The hydrosilylation of allenes is
one of the most straightforward and atom-economical
approaches to prepare such compound, which has
been achieved with various metal catalysts, such as
Mo,[8a] Co,[8b,8c] Ni,[8d,8e] Pd[8d-8h] and Au.[8i] However,
controling the regio- and stereoselectivity is still
challenging in this reaction, as up to six potential
isomeric products can be generated (Figure 1a).
Consequently, developing an efficient method to
regulate the selectivity is quite important.
During the past few years, different ligands for the
regioselective hydrosilylation of allenes have been
developed to afford allylsilanes or vinylsilanes.[8] For
example, in 2016, Takai reported a molybdenum-
catalyzed hydrosilylation at the terminal C=C bonds
Allylsilanes are indispensable synthetic building
blocks in organic synthesis because of their non-
toxicity, high stability and applications in Hiyama
cross-couplings,[5] Sakurai reactions,[6] Tamao
oxidation reactions,[7] and so on. Hence, the
development of new and efficient methods for the
1
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10.1002/adsc.201901552
Advanced Synthesis & Catalysis
of allenes that yielded linear allylic silanes (L-
allylsilane);[8a] meanwhile, Ge and co-workers
demonstrated that high linear selectivity of
allylsilanes could be achieved by cobalt complexes
which generated from Co(acac)2 with binap and
xantphos ligand.[8b] Similarly, Huang and co-workers
developed a cobalt-based catalyst to form linear
allylsilanes.[8c] As for the Pd catalyzed allene
hydrosilylation, several ligands have been applied to
afford vinylsilanes or branched allylsilanes.
Montgomery and co-workers reported that
vinylsilanes could be accessed with Pd(0) NHC
catalysts (Figure 1, L1).[8g] They also established the
classic N-heterocyclic carbene complexes of Pd(0)
catalyst system (Figure 1, L2) for branched
Table 1. Optimization of the Reaction Conditions for
a
Hydrosilylation of undeca-1,2-diene and PhSiH3
entry
1d
Pd-mol %
Pd(PPh3)2Cl2
Ligand- mol %
Yield%b
21
r.r.c
3:1
10
L4
L4
L4
L4
L4
L5
L6
L7
L8
L9
L10
L11
L12
L13
L14
L15
L15
-
10
10
10
1
2
3
Pd(PPh3)2Cl2
Pd(PPh3)2Cl2
Pd(PPh3)2Cl2
Pd(PPh3)2Cl2
Pd(PPh3)2Cl2
Pd(PPh3)2Cl2
Pd(PPh3)2Cl2
Pd(PPh3)2Cl2
Pd(PPh3)2Cl2
Pd(PPh3)2Cl2
Pd(PPh3)2Cl2
Pd(PPh3)2Cl2
Pd(PPh3)2Cl2
Pd(PPh3)2Cl2
Pd(PPh3)2Cl2
Pd(OAc) 2
0.1
82
98
7:1
>20:1
10:1
7:1
0.05
0.05
0.05
0.05
0.05
0.05
0.05
0.05
0.05
0.05
0.05
0.05
0.05
0.05
10
4
68
5
0.1
10
10
10
10
10
10
10
10
10
10
10
10
41
6
40
4:1
allylsilanes[8d]
.
Another method for branched
7
11
10:1
>20:1
>20:1
>20:1
>20:1
>20:1
1:1
allylsilanes was developed by Schmidt et al. by using
their 3-iminophosphine ligands coordinated with
Pd(II) (Figure 1, L3).[8f] It is also worth mentioning
that more recently our group applied a conjugated
microporous polymer as an efficient heterogeneous
catalytic ligand for the Pd-catalyzed regioselective
allene hydrosilylation.[8h] All the approaches implied
that ligands might play a crucial role in controlling
the regioselectivity for the allene hydrosilylation.
Herein we first reported a bithiophene-based ligand
8
95
9
60
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25e
61
75
51
40
17
1:1
63
2:1
as
a homogeneous catalyst for the selective
complex
complex
33
-
hydrosilylation of allenes with primary and secondary
phenylsilanes (Figure 1c). It exhibits high selectivity
towards the production of branched allylsilanes with
a wide range of allenes. It's worth noting that the
catalytic amount of the metal catalyst can be reduced
to 500 ppm. This work opens a new front of using
bidentate thiophene ligands as a reaction promoter in
transition-metal-catalyzed organic transformation.
-
Pd(PPh3)2Cl2
PdCl2
0.05
0.05
0.05
0.05
0.05
0.05
0.05
0.05
4:1
L4
L4
L4
L4
L4
L4
L4
10
10
10
10
10
10
10
complex
complex
complex
30
-
Pd(PhCN)2Cl2
Pd(py)2Cl2
-
-
Pd(dppf)Cl2
Pd(Pcy3)Cl2
Pd(PPh3)4
1:1
40
10:1
4:1
35
Results and Discussion
Pd(PPh3)4
75
12:1
As mentioned above, there are already many
reports about the bipyridine ligands in the literature[9],
whereas the bithiophene as ligands in organic
reactions has not been reported yet. Recently, we
have successfully applied bipyridine and bithiophene
based conjugated microporous polymers as
heterogeneous ligands for highly selective oxidative
Heck reaction.[10] As part of our laboratory’s ongoing
interests in the development of regioselective
catalytic reactions, we found that our bithiophene-
alkyne-based ligand L4 could function as active
ligand for the hydrosilylation of allenes with phenyl-
and diphenylsilane.
In the preliminary experiment the catalytic amount
of the palladium and the ligand were both set as 10
mol %, and the reaction could afford the product in
moderate yield with low selectivity in 5 hours (Table
1, entry 1). Then we were pleased to find that
reducing the catalytic loading of the palladium to 0.1
mol % could give a greatly increased yield and the r.r.
value was improved to 7:1 (Table 1, entry 2). Further
decreasing the metal catalyst amount could greatly
aReaction conditions: 1a (0.3 mmol), phenylsilane
(0.33 mmol), Pd (0.05 mol %), ligand (10 mol %),
b
o
dioxane (2 mL), 80 C, 12 h, N2 atmosphere. The
yields of 2a were determined by 1H NMR
spectroscopy using 1,3,5-trimethoxybenzene as an
internal standard. cr.r. = 2a/other isomers, determined
by 1H NMR spectroscopy or GC analysis of the crude
d
d
reaction mixture. Reaction time: 5h. Reaction time:
0.5h.
improve the r.r. value to >20:1 with a 98 % yield of
the aimed product 2a (Table 1, entry 3). With these
2
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10.1002/adsc.201901552
Advanced Synthesis & Catalysis
results, we also tried to decrease the amount of the
ligand, but no desired results occurred (Table 1,
entries 4-5). Next, we tested a series of thiophene-
based ligand (L5-L15) in an attempt to investigate
the most effective ligand that favor allylsilanes in the
palladium-catalyzed allene hydrosilylation. A series
of 5,5’-disubstituted 2,2’-bithiophene were tested ,
L5 and L6 gave 2a in poor yields and moderate
regioselectivities (Table 1, entries 6-7), while L7 was
superior in both the yield and the selectivity (Table 1,
entry 8). Although L8-L11 showed excellent
selectities, the yields were relatively low (Table 1,
entries 9-12). Different monodentate thiophene
ligands such as L12, L13 and L14 were also screened
and they all showed a decrease in the r.r. values
(Table 1, entries 13-15). The bidentate sulfoxide
ligand L15 was finally tested but the reaction mixture
was complex (Table 1, entry 16) and it also
performed a complex result with the bis-sulfoxide-
promoted system[4] (Table 1, entry 17). Besides, we
found the transformation became less effective
without the ligand, which demonstrated the
importance of the ligand in this reaction (Table 1,
entry 18). Other palladium sources such as PdCl2,
Pd(PhCN)2Cl2, and Pd(py)2Cl2 were also tried, but
only complicated mixtures were obtained (Table 1,
entries 19-21). Pd(dppf)Cl2, Pd(Pcy3)Cl2 and
Pd(PPh3)4 could give corresponding products but they
were not as effective as Pd(PPh3)2Cl2 (Table 1,
entries 22-24).
summarized in Table 2. Alkyl substituted allenes
including cyclohexylallene reacted to produce the
desired branched allylsilanes (Table 2, 2a-2d) in
moderate to high yields (71–93%) with excellent
regioselectivities. This reaction also showed good
functional group tolerance with a range of reactive
groups, such as chloro (Table 2, 2e), bromo (Table 2,
2f), and imide (Table 2, 2g). Unprotected hydroxyl
allene gave the corresponding hydrosilylation
products (Table 2, 2h) in high yield with consistently
excellent regioselectivity. Several derivatives of 2h
could also be tolerated, and proceeded smoothly to
afford the branched allylsilanes in decent yields with
high selectivities (Table 2, 2i-2l). Aryl substituted
allenes bearing electron-donating groups reacted with
phenylsilane under the standard reaction condition to
give the aimed products (Table 2, 2m and 2n) in
high yields and selectivities. However, electron-
neutral aryl allenes performed good selectivities with
only moderate yields (Table 2, 2o and 2p). Finally,
the 1,1-disubstituted terminal allene could also afford
the hydrosilylation product (Table 2, 2q).
Further exploration of this method involved the
evaluation of terminal allenes (Table 3) with
secondary phenylsilanes. Alkyl substituted allenes
could undergo the hydrosilylation with high
regioselectivity and good to high yields (Table 3, 3a-
3d). Some reactive groups, such as chloro (Table 3,
3e), bromo (Table 3, 3f), and imide (Table 3, 3g),
were also compatible with the reaction conditions to
afford the products in high isolated yields with high
selectivities. The reaction successfully formed the
desired product with excellent regioselectivity in high
Under the identified conditions (Table 1, entry 3),
we explored the substrate scope of different allenes
with primary phenylsilane and the results were
Table 2. Scope of allenes for the Bithiophene-
Table 3. Scope of allenes for the Bithiophene-
a,b,c
a,b,c
Catalyzed Hydrosilylation with PhSiH3
Catalyzed Hydrosilylation with Ph2SiH2
aReaction conditions: allenes (0.3 mmol),
aReaction conditions: allenes (0.3 mmol),
diphenylsilane (0.33 mmol), cat. (0.05 mol %), L4
phenylsilane (0.33 mmol), cat. (0.05 mol %), L4 (10
(10 mol %), dioxane (2 mL), 80 oC, 0.5 h-48 h, N2
mol %), dioxane (2 mL), 80 oC, 0.5-48 h, N2
b
b
atmosphere.
Yields of isolated products. cThe
atmosphere.
Yields of isolated products. cThe
selectivity for product (r.r. = 3/other isomers) was ≥
selectivity for product (r.r. = 2/other isomers) was ≥
1
20:1 unless otherwise noted, determined by H NMR
1
20:1 unless otherwise noted, determined by H NMR
spectroscopy. dr.r. = 10:1
spectroscopy. dr.r. = 10:1 spectroscopy.
3
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10.1002/adsc.201901552
Advanced Synthesis & Catalysis
yield when using unprotected hydroxyl allene (Table
3, 3h). Besides, several derivatives of 3h were also
tolerated, and the results showed excellent
selectivities to afford the branched allylsilanes in
decent yields (Table 3, 3i-3l). In addition, when aryl
substituted allenes bearing electron-donating groups
(Table 3, 3m and 3n) were tested, good selectivities
were also observed. However, electron-neutral aryl
allenes led to good selectivities and relatively lower
yields (Table 3, 3o and 3p). A 1,1-disubstituted allene
(Table 3, 3q) was finally examed to function well
with this reaction condition. Furthermore, we also
tried this hydrosilylation of allenes with tertiary
hydrosilanes such as Ph3SiH and Et3SiH，but the
reaction did not occur.
Conclusion
In conclusion, we first synthesized a bithiophene-
alkyne-based ligand and utilized it to catalyse the
hydrosilylation reactions of allenes with primary and
secondary phenylsilanes. It shows high selectivity
and catalytic efficiency towards a wide range of
allenes to produce the branched allyllsilanes, and the
catalytic loading of the palladium can be reduced to
500 ppm. This work opens a new front of using
bidentate thiophene ligand as a reaction promoter in
transition-metal-catalyzed organic transformation.
Experimental Section
We proposed a possible mechanism as shown in
Scheme 1. Firstly, Pd(PPh3)2Cl2 was reduced to form
the Pd(0) species, and oxidative addition of
hydrosilane to Pd(0) with the formation of complex
(I) was achieved.[8d] Next, hydrometalation of the
palladium allene complex intermediate (II) formed an
allylpalladium intermediate (III). Finally, C−Si
reductive elimination of (III) would afford allylsilane
product 2 or 3. As a support for this proposed
mechanism, a double-labelled crossover experiment
was done.[8f] We used Ph2SiD2 and PhMeSiH2 (0.5
equiv. each) in this catalytic hydrosilylation with
cyclohexylallene. The reaction finished over twelve
hours and 1H NMR spectroscopy afforded the
Typical Experimental Procedure for the Synthesis of
2a : In a nitrogen filled Schlenk tube, Pd(PPh3)2Cl2 (0.05
mol %), L4 (10 mol %) and dioxane (2 mL) were added,
then 1a (0.3 mmol) and phenylsilane (0.33 mmol) were
added under N2. The reaction mixture was stirred at 80oC
for 12h. Upon completion, the solvent was removed by
vacuum and the crude residue was purified by silica gel
column chromatography to afford the corresponding
products 2a.
Author Contributions
‡These authors contributed equally.
Acknowledgements
production of
a
nearly 1:1 mixture of
Ph2SiDCH(Cy)CD=CH2 and PhMeSiHCH(Cy)CH=
CH2 while Ph2SiDCH(Cy)CH=CH2 and PhMeSiHC-
H(Cy)CD=CH2 were not detected (See SI for workup
procedures and experimental data ). Thus, there was
no H/D scrambling at the vinylic position or Si-H/Si-
D crossover, illustrating that the R2Si and H units in
the product were derived from a single molecule of
the hydrosilane. The result of this experiment
discounts the machinism related to [Pd-H][8f] active
species and supports the possibility of [Si-Pd-H][8d]
intermediate I formed by silane oxidative addition to
Pd(0) species, which is in consistent with our
proposed mechanism.
We are grateful to the National Natural Science Foundation of
China (No. 21772166 and 91845101), the Chinese Universities
Scientific Fund (No. 20720190132) and NFFTBS (No. J1310024).
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5
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10.1002/adsc.201901552
Advanced Synthesis & Catalysis
COMMUNICATION
A bithiophene-promoted ppm level palladium-
catalyzed highly regioselective hydrosilylation of
terminal allenes
Adv. Synth. Catal. Year, Volume, Page – Page
Jun-Jia Chen‡, Jia-Hao Zeng‡, Ying Yang, Zhi-Kai
Liu, Ya-Nan Jiang, Miao-Ran Li, Li Chen, and
Zhuang-Ping Zhan*
6
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