Bimetal Cooperatively Catalyzed Arylalkynylation of Alkynylsilanes

Article abstract of DOI:10.1021/acs.orglett.1c02283

An unprecedented Pd/Rh cooperatively catalyzed arylalkynylation of alkynylsilanes was developed to merge an alkynylidene moiety with benzosilacycle. These silaarenes possess a particular aggregation-induced emission behavior. Mechanistic investigations demonstrate that the relay trimetallic transmetalation plays a pivotal role in governing this transformation.

Full text of DOI:10.1021/acs.orglett.1c02283

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Letter
Bimetal Cooperatively Catalyzed Arylalkynylation of Alkynylsilanes
Xing Chen,§ Mengke Li,§ Zhipeng Liu, Can Yang, Haisheng Xie,* Xinwei Hu, Shi-Jian Su,*
Huanfeng Jiang, and Wei Zeng*
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ABSTRACT: An unprecedented Pd/Rh cooperatively catalyzed
arylalkynylation of alkynylsilanes was developed to merge an
alkynylidene moiety with benzosilacycle. These silaarenes possess a
particular aggregation-induced emission behavior. Mechanistic
investigations demonstrate that the relay trimetallic transmetalation
plays a pivotal role in governing this transformation.
Scheme 1. Coupling Strategies of Alkynes with Arylsilanes
and Alkynylsilanes
nyne-condensed arenes are pervasive in numerous natural
1
Eproducts and organic materials. These unsaturated carbon
skeletons represent versatile transformation platforms for
assembling complex molecules through classical coupling
cyclizations.2 Thus, the method development for a concurrent
installation of aryl and alkynyl moieties into arenes, alkenes, and
alkynes remains important in synthetic chemistry. However, the
exploration of the direct arylalkynylation of unsaturated
hydrocarbons was very scarce; the challenges of this method-
ology are mainly derived from its chemo- and regioselectivity of
unsaturated alkenes and alkynes. Until now, although much
progress in the arylalkynylation of alkenes has been made,3 the
arylalkynylation of alkynes has not been well-documented.4
Therefore, it remains valuable to develop efficient methods of
arylalkynylation of alkynes to assemble aryl-enyne skeletons.
A C/Si switch generally endows the corresponding
silahydrocarbons with distinct physical-chemical properties.5
Accordingly, intermolecular arylsilylation strategies of alkynes
with arylsilanes have been pioneeringly explored by Xi,6
Nakamura,7 and Chatani8 to furnish 1-silaindans (Scheme 1a).
However, the development of an arylalkynylation strategy of
alkynylsilanes to merge enyne and the Si element into silacycles
is often difficult to achieve, because alkynylsilanes possess
diversified reaction sites;9 moreover, alkynylsilanes still suffer
from a C−Si bond cleavage transmetalation or oxidative
addition with metal catalysts (Scheme 1b).10
molecules. These enyne-tethered benzosilacycles possibly
possess promising application potential in optoelectronic
materials.11
Our studies began by probing the reaction of ortho-
bromobenzyl alkynylsilane 1a with 1,1,3-triphenylprop-2-yn-1-
ol 2a to screen different catalytic systems in the presence of
Ag2CO3, t-BuOK, and 1,2-bis(diphenylphosphino)ethane
(dppe) in 1.0 mL of 1,4-dioxane under Ar at 100 °C for 12 h
(Table 1, entries 1−7). It was found that the dual catalytical
systems Rh(III)/Pd(OAc)2, Ir(III)/Pd(OAc)2, and Rh(I)/
Pd(OAc)2 could provide the desired arylalkynylation product
In answer to this challenge, we posited that α-alkynylalcohols
(B) possibly possess a higher reactivity than alkynylsilanes (A)
to interact with Rh catalysts, producing alkynylrhodiums (D)
through a β-C elimination. Meanwhile, Pd catalysts could
preferentially attack a phenyl Csp2-Br bond instead of an alkynyl
Csp-Si bond of A via an oxidative addition, affording
arylpalladiums (C), which could further be trapped by D to
enable this arylalkynaylation (Scheme 1c). To verify this
hypothesis, we described herein a bimetal Pd/Rh-catalyzed
arylalkynylation of ortho-bromoaryl-tethered alkynylsilanes with
α-alkynylalcohols. A broad range of substrates efficiently
participate in this reaction, leading to valuable silacyclic
Received: July 9, 2021
https://doi.org/10.1021/acs.orglett.1c02283
© XXXX American Chemical Society
Org. Lett. XXXX, XXX, XXX−XXX
A

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Letter
a
2b could not give 3a at all [see the Supporting Information for
details].
Table 1. Optimization of the Reaction Parameters
Next, we evaluated the coupling cyclization of different 3-
arylprop-2-yn-1-ols 2 with alkynylsilane 1a, as shown in Scheme
2. A comparison with electroneutral 3-phenylprop-2-yn-1-ol 2b,
a b
,
Scheme 2. Arylalkyne Scope
b
entry
Rh/Pd cat
[Rh(cod)Cl]2
alkyne
ligand
yield (%)
c
1
2a
2a
2a
2a
2a
2a
2a
2b
2c
2d
2e
2b
2b
2b
2b
2b
2b
2b
dppe
0
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
[Cp*RhCl2]2
[Cp*IrCl2]2
dppe
dppe
dppe
dppe
dppe
dppe
dppe
dppe
dppe
dppe
0
0
[Cp*RhCl2]2/Pd(OAc)2
[Cp*IrCl2]2/Pd(OAc)2
RhCl3·3H2O/Pd(OAc)2
[Rh(cod)Cl]2/Pd(OAc)2
[Rh(cod)Cl]2/Pd(OAc)2
[Rh(cod)Cl]2/Pd(OAc)2
[Rh(cod)Cl]2/Pd(OAc)2
[Rh(cod)Cl]2/Pd(OAc)2
[Rh(cod)Cl]2/Pd(OAc)2
[Rh(cod)Cl]2/Pd(OAc)2
[Rh(cod)Cl]2/Pd(OAc)2
[Rh(cod)Cl]2/Pd(OAc)2
[Rh(cod)Cl]2/Pd(OAc)2
[Rh(cod)Cl]2/Pd(OAc)2
Pd(OAc)2
16
21
22
23
23
<5
<5
<5
15
31
49
a
The mixture of 1a (0.20 mmol), alcohols 2 (0.10 mmol), Ag2CO3
(1.0 equiv), PPh3 (40 mol %), and t-BuOK (1.0 equiv) with
[Rh(cod)Cl]2 (10 mol %) and Pd(OAc)2 (20 mol %) in dioxane (1.0
mL) was stirred at 100 °C for 24 h under Ar in a sealed tube, followed
d
dppm
dppp
e
PPh3
PPh3
PPh3
PPh3
PPh3
b
f
by flash chromatography on SiO2. Isolated yields were based on the
alkynylating reagent 2. The catalytical reaction was performed on a
1.0 mmol scale.
60
c
f g
75 ,
f gh
,
88 ,
g hi
, ,
18
a
arylalkynyl alcohols that contain alkyl-, methoxyl-, and fluoro-
substituted phenyl groups, showed that 2b could efficiently react
with alkynylsilane 1a to furnish the desired products (3b−3f, 3g,
3i, and 3j) in 53−82% yields. Among them, meta- or ortho-
substituted congeners resulted in slightly sluggish conversions
(53−61%), possibly due to the steric hindrance (3c, 3d, and 3j).
Meanwhile, 4-acetamido-, 4-chloro-,13 and 4-CF3-phenyl-
substituted alkynylalcohols, naphthylalkynyl alcohol, 3-pyridy-
lalkynyl alcohol, and even 2-thiothylalkynyl alcohol also
furnished the corresponding 2-silaindan-fused enynes (3h,
3k−3o) in acceptable yields (32−54%). Unfortunately, when
n-heptynyl alcohol 2t was subjected to the standard reaction
system, no desired product 3p was obtained.
Unless otherwise noted, all the reactions were performed using
alkynylsilane 1a (0.10 mmol), alkynyl alcohols 2 (0.10 mmol),
Ag2CO3 (1.0 equiv), ligand (40 mol %), and t-BuOK (1.0 equiv) with
Rh catalysts (5 mol %) and Pd(OAc)2 (PdL2, 10 mol %) in 1,4-
dioxane (1.0 mL) at 100 °C for 12 h under Ar in a sealed tube,
followed by flash chromatography on SiO2. Isolated yields were
based on the alkynylating reagent 2. dppe refers to 1,2-
bis(diphenylphosphino)ethane. dppm refers to 1,2-bis-
(diphenylphosphino)methane. dppp refers to 1,2-bis-
(diphenylphosphino)propane. 10 mol % of [Rh(cod)Cl]2 and 20
b
c
d
e
f
g
mol % of Pd(OAc)2 were employed. 2.0 equiv of 1a was used.
h
i
Reaction time was extended to 24 h. Only 20 mol % of Pd(OAc)2
was employed.
Subsequently, the scope of the present procedure with regard
to different ortho-bromophenyl-tethered alkynylsilanes was also
tested systematically (Scheme 3). Generally, this transformation
is sensitive to the electronic effect of substituents in silylalkynyl
and silylalkyl phenyl rings. Electron-donating (Me-, MeO-) and
electron-withdrawing (F-) groups on the silylalkylnyl and
silylalkyl benzene ring of silanes 1 could efficiently react with
α-phenylethynyl isopropyl alcohol 2b to furnish good yields
(61−77%) of the products (4a−4c, 4g, and 4h). On the
contrary, electron-withdrawing CF3- or Cl-substituted phenyl
ring-containing silanes 1 gave the corresponding silacycles 4d
(46%),13 4e (51%), and 4i (50%)13 in moderate yields. We
stress that this transformation was also compatible with
alkylalkynyl-substituted phenylmethylsilane, producing the
desired 2-silaindan 4f in a 42% yield.
More importantly, the scope of ortho-bromophenylmethyl
alkynylsilanes could be extended to ortho-bromophenylethyl
alkynylsilanes, in which 2-bromophenethyl/phenylethynyl
silanes smoothyl reacted with various arylethynyl isopropyl
alcohols 2 via a coupling cyclization to assemble six-membered
2-silatetralins 4j−4t in 46−73% yields. Among them, the
branched β-methylphenethyl/phenylethynyl silane still showed
a high reactivity toward arylethynyl isopropyl alcohols regardless
3a in 16−23% yields (entries 4−7). By contrast, the monometal
catalysts [Rh(cod)Cl]2, [Cp*RhCl2]2, and [Cp*IrCl2]2 did not
enable this transformation at all (entries 1−3). Then, we
employed [Rh(cod)Cl]2 (5 mol %)/Pd(OAc)2 (10 mol %) as a
bimetal cocatalyst to evaluate the effect of alkynyl alcohols (2b,
2c, 2d) and even alkynyl bromide (2e) on this transformation
and found that tertiary alcohols 2a and 2b were the most
effective alkynylating reagents (entries 9−11 vs 7 and 8).
Various types of phosphine ligands were then evaluated for
further improving the reaction conversions of 1a with 2b; PPh3
was the best ligand, which could give 49% yield of 3a (entries 8,
12, and 13 vs 14). Increasing the catalyst loading (Rh salts: 10
mol %; Pd salts: 20 mol %) dramatically increased the reaction
yield to 60% (entry 14 vs 15). Meanwhile, switching the ratio of
1a/2a from 1/1 to 2/1 could further enhance this trans-
formation for yielding 75% of 3a, possibly due to 1a being easily
suffered from an intramolecular coupling cyclization and
decomposition12 (entry 15 vs 16). Extending the reaction time
to 24 h was also beneficial for achieving a higher yield (entry 17,
88%). By the way, a very poor yield of 3a (18%) was obtained in
the absence of [Rh(cod)Cl]2 (entry 18), and the monometal
Rh(I)-, Pd(II)-, and Ag(I)-catalyzed arylalkynaylation of 1a with
B
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a b
,
Scheme 3. Alkynylsilane Scope
Scheme 4. Synthetic Applications
opening/coupling cyclization of 3a with Na2S could also afford
2,3,5-triarylthiophene 3−2a (49%) via a C−Si bond cleavage/
C−S bond formation. On the other hand, the 1,3-enyne moiety
of the benzosilacycle 3a would regioselectively react with Et3SiH
employing H2PtCl6 (1 mol %) as catalysts to produce 1,3-bis-
silyl-1,3-diene-containing silacycle 3−3a in 75% yield. Mean-
while, the 1,3-enyne group of 3a still underwent Pd-catalyzed
hydrostannation with (n-Bu)3SnH, delivering vinyltin 3−4a in
88% yield.
a
The mixture of silanes 1 (0.20 mmol), alcohols 2 (0.10 mmol),
Ag2CO3 (1.0 equiv), PPh3 (40 mol %), and t-BuOK (1.0 equiv) with
[Rh(cod)Cl]2 (10 mol %) and Pd(OAc)2 (20 mol %) in dioxane (1.0
mL) was stirred at 100 °C for 24 h under Ar in a sealed tube.
To get a better insight into the mechanism, we ran control
experiments to evaluate the behavior of α-alkynylalcohols,
alkynylsilanes, arylbromides, and bimetal catalysts (Scheme 5).
b
c
Isolated yields were based on the alkynylating reagent 2. Recovered
Scheme 5. Preliminary Mechanism Studies
yield.
of the steric hindrance from methyl-substitutent effect,
producing 4s and 4t in good yields of 64−67%. However,
when a phenylpropylsilane was subjected to the standard
reaction system, only the coupling product 4u′ (see the
Supporting Information for details) was obtained in 4% yield;
no desired benzo-fused seven-membered silacyclic compound
4u was observed.
To evaluate the luminescent properties of these enyne-fused
benzosilacycles, the photophysical properties of different sizes of
silacycles 3a, 3m, 4j, and 4r were investigated by UV−vis
absorption and photoluminescence (PL) spectra in a diluted
tetrahydrofuran (THF) solution. As illustrated in the absorption
spectra (Figure S1a in the Supporting Information), the
naphthalene-containing benzosilacycles 3m and 4r show
bathochromic absorption bands relative to that of the phenyl-
substituted silacycles 3a and 4j. Also, the hypsochromic shifted
absorption bands were noticed more for 2-silatetralins 4j and 4r
than those of 2-silaindans 3a and 3m, respectively. Similar red-
shifted emission profiles were further observed in their PL
spectra from phenyl-substituted structures to naphthalene-
substituted structures, also from 2-silatetralin derivatives to 2-
silaindan derivatives (Figure S1b), indicating that these enyne-
fused benzosilacyclical skeletons could be a promising building
block for deep blue/violet luminescent materials. More
interesting, a striking fluorescence enhancement phenomenon
was observed for all the compounds 3a, 3m, 4j, and 4r in a mixed
THF/water solvent, indicating that all these silacyclic skeletons
exhibit an aggregation-induced emission (AIE) characteristic14
(Figure S1c), possibly due to the incorporation of multiple
rotational phenyl or naphthyl groups. A flexible enyne group and
a twisted multimembered silacycle endow the isolated molecule
adequately with active intramolecular motions to dissipate an
exciton energy in the isolated state.15
It is well-known that terminal alkynes would produce
alkynylmetal species under base conditions. However, the
arylalkynylation of 1a with 4-methoxyphenyl ethyne 2u under
the standard conditions only provided phenylethynyl-contain-
ing 2-silaindan 3a (9%) instead of 4-methoxylphenylethynyl-
substituted congener 3g (Equation (Eq) a), implying that 2-
alkynylpropan-2-ols 2 more easily underwent metalation than
terminal alkynes and alkynylsilanes; accordingly, alkynylsilanes
possessed a higher reactivity than terminal alkynes to form
The postsynthetic transformations (Scheme 4) demonstrated
that the silacycle of 3a could undergo a ring-opened reaction to
produce α-silylalcohol 3−1a (49%) by employing a PtCl2/CO
catalytical system. On the one hand, of course, the tandem ring-
C
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alkynyl metallates. On the one hand, a performance of the
arylalkynylation of 1a with 2u in the absence of Rh(I) catalysts
only gave the 1,3-diynes 5 (7%) and 6 (30%) and could not
furnish the products 3a and 3g (Eq b). On the other hand, a
switch of the alkynylsilylmethyl phenylbromide 1a to
alkynylethyl phenylbromide 1n only afforded the low yield of
the arylalkynylation product 4v (30%), confirming that Si
element possibly played an important silyl hyperconjugation
effect11,16 in this coupling cyclization (Eq c).
Although the reaction condition screening in Table 1 (entry
18) suggested that the arylalkynylation between alkynylsilane 1a
and α-phenylethynyl isopropyl alcohol 2b still occurred with a
poor yield in the absence of Rh(I) catalysts, the alkynyl source
was not clear. We therefore performed the coupling cyclization
of alkynylsilane 1a with 4-methoxyphenylethynyl alcohol 2k in
the absence of Rh catalysts and obtained 3a (11%) and 3g
(24%), in which both alkynylsilane 1a and alkynylalcohol 2k
acted as an alkynyl source to enable the arylalkynaylation (Eq d).
More importantly, in comparison with the 60% yield of 3g in
Scheme 2, this result clearly demonstrated that Rh catalysts
significantly increased the reactivity and chemical selectivity of
alkynyl alcohols.
intramolecular migratory insertion to furnish vinyl palladiums B.
On the other hand, 3-arylprop-2-yn-1-ols 2 are converted to
alkynylrhodium species C via a β-alkynyl elimination,20 and the
subsequent transmetalation with Ag salts gives alkynylsilvers
D.21 Finally, the transmetalation between alkynylsilver D and
the vinyl palladium species B occurs again and delivers the
alkynyl/alkenyl palladiums E, which further afford the
arylalkynylation products 3 via a reductive elimination with
the regenernation of Pd catalysts.
In summary, we have presented an unprecedented bimetal
Pd/Rh cooperatively catalyzed arylalkynylation cyclization of
alkynylsilanes with alkynyl alcohols through a sequential
arylmetalation, alkynylmetalation, and transmetalation process.
Intriguing mechanistic details triggering these perfect processes
were elucidated. This newly developed methodology enables a
direct access to distinctive alkynylidene-fused benzosilacycles
that possess an unusual AIE feature.
ASSOCIATED CONTENT
■
sı
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.orglett.1c02283.
To establish the role of Rh(I) catalysts and Pd(II) catalysts
during this transformation, we conducted the cross-coupling of
alkynyl alcohol 2a with phenyl bromide 7 in the absence of
Pd(OAc)2 (Eq e). It was found that Rh(I) catalysts could
efficiently interact with 2a to produce alkynyl rhodium species
and ketone 9 (95%) via a β-alkynyl elimination,17 and Pd(II)
catalysts mainly participated in trapping aryl bromides through
an oxidative addition and subsequent reductive elimination.
Given that Rh(I) salts could be oxidized to Rh(III) species by
Ag2CO3,18 a Rh(III) (10 mol %)/Pd(II) (20 mol %) catalyzed
coupling cyclization of alkynylsilane 1a with 2k was performed
in the absence of Ag2CO3, and both silacycles 3a and 3g were not
detected (Eq f), implying that stoichiometric Ag salts19 possibly
played an important role of transmetalation instead of acting as
an oxidant. Finally, the intramolecular cyclization of 1a in the
presence of CH3OD gave 8% yield of 1-benzylidene-2-silaindan
d1-10, in which the incorporation of deuterium into the vinyl
Csp2-H bond (13% D) was observed. On the contrary, this
transformation did not happen at all in the absence of Pd
catalysts (Eq g), indicating a vinyl Pd(II) species was possibly
involved in this reaction.
Detailed experimental procedures, characterization data,
crystallographic data for 3a, 4d, and 4o (CIF), copies of
1H NMR and 13C NMR spectra for all isolated
compounds (PDF)
Accession Codes
CCDC 2074954, 2074953, and 2074955 contain the supple-
mentary crystallographic data for this paper. These data can be
obtained free of charge via www.ccdc.cam.ac.uk/data_request/
cif, or by emailing data_request@ccdc.cam.ac.uk, or by
contacting The Cambridge Crystallographic Data Centre, 12
Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
AUTHOR INFORMATION
■
Corresponding Authors
Haisheng Xie − Key Laboratory of Functional Molecular
Engineering of Guangdong Province, School of Chemistry and
Chemical Engineering, South China University of Technology,
Guangzhou 510641, China; Email: xiehs@scut.edu.cn
Shi-Jian Su − State Key Laboratory of Luminescent Materials
and Devices, Institute of Polymer Optoelectronic Materials and
Devices, South China University of Technology, Guangzhou
510641, China; orcid.org/0000-0002-6545-9002;
Email: mssjsu@scut.edu.cn
Wei Zeng − Key Laboratory of Functional Molecular
Engineering of Guangdong Province, School of Chemistry and
Chemical Engineering, South China University of Technology,
Guangzhou 510641, China; orcid.org/0000-0002-6113-
2459; Email: zengwei@scut.edu.cn
On the basis of the above experimental observations, a
plausible mechanism is proposed in Scheme 6. On the one hand,
ortho-bromobenzyl alkynylsilanes 1 preferentially undergo an
oxidative addition with Pd(0) catalysts in the presence of
phosphine ligands to produce arylpalladiums A, followed by an
Scheme 6. Proposed Mechanism
Authors
Xing Chen − Key Laboratory of Functional Molecular
Engineering of Guangdong Province, School of Chemistry and
Chemical Engineering, South China University of Technology,
Guangzhou 510641, China
Mengke Li − State Key Laboratory of Luminescent Materials
and Devices, Institute of Polymer Optoelectronic Materials and
Devices, South China University of Technology, Guangzhou
510641, China
D
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(6) Liang, Y.; Geng, W.; Wei, J.; Xi, Z. Palladium-Catalyzed
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Synthesis of Benzosiloles and Heteroarene-Fused Siloles by Catalytic
Cleavage of the C(sp3)-Si Bond. Angew. Chem., Int. Ed. 2012, 51, 1934−
1937.
Zhipeng Liu − Key Laboratory of Functional Molecular
Engineering of Guangdong Province, School of Chemistry and
Chemical Engineering, South China University of Technology,
Guangzhou 510641, China
Can Yang − Key Laboratory of Functional Molecular
Engineering of Guangdong Province, School of Chemistry and
Chemical Engineering, South China University of Technology,
Guangzhou 510641, China
Xinwei Hu − Key Laboratory of Functional Molecular
Engineering of Guangdong Province, School of Chemistry and
Chemical Engineering, South China University of Technology,
Guangzhou 510641, China
Huanfeng Jiang − Key Laboratory of Functional Molecular
Engineering of Guangdong Province, School of Chemistry and
Chemical Engineering, South China University of Technology,
Guangzhou 510641, China; orcid.org/0000-0002-4355-
0294
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Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.orglett.1c02283
Author Contributions
§(X.C. and M.L.) These two authors contributed equally to this
work.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
The authors thank the KARDPGP (No. 2020B010188001),
NSFC (Nos. 21871097 and 51625301), NSFGD
(2019A1515011790), and STPG (No. 201904010113) for
financial support.
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