Palladium-catalyzed cleavage of the Me-Si bond in ortho-trimethylsilyl aryltriflates: Synthesis of benzosilole derivatives from ortho-trimethylsilyl aryltriflates and alkynes

Article abstract of DOI:10.1039/c3ra42910e

An efficient Pd-catalyzed cleavage of the Me-Si bond in ortho-trimethylsilyl aryltriflates was realized and synthetically applied. Most of the commercially available ortho-trimethylsilyl aryltriflates could undergo the Pd-catalyzed intermolecular coupling with alkynes via cleavage of the Me-Si bond, which represents a new reaction pattern of ortho-trimethylsilyl aryltriflates. Potassium bromide (KBr) was found effective for this process. A variety of benzosilole derivatives were thus generated in high yields. The Royal Society of Chemistry 2013.

Full text of DOI:10.1039/c3ra42910e

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Palladium-catalyzed cleavage of the Me–Si bond in
ortho-trimethylsilyl aryltriflates: synthesis of
benzosilole derivatives from ortho-trimethylsilyl
aryltriflates and alkynes3
Cite this: RSC Advances, 2013, 3, 14273
Received 13th June 2013,
Accepted 25th June 2013
DOI: 10.1039/c3ra42910e
Tianhao Meng,ab Kunbing Ouyangab and Zhenfeng Xi*bc
www.rsc.org/advances
An efficient Pd-catalyzed cleavage of the Me–Si bond in ortho-
trimethylsilyl aryltriflates was realized and synthetically applied.
Most of the commercially available ortho-trimethylsilyl aryltri-
flates could undergo the Pd-catalyzed intermolecular coupling
with alkynes via cleavage of the Me–Si bond, which represents a
new reaction pattern of ortho-trimethylsilyl aryltriflates.
Potassium bromide (KBr) was found effective for this process. A
variety of benzosilole derivatives were thus generated in high
yields.
efficient Pd-catalyzed intermolecular coupling with alkynes via
cleavage of the Me–Si bond to provide a novel synthetic method
for benzosilole derivatives 2 (Scheme 1).
Initially, as given in Table 1, we tested the reaction of o-
(trimethylsilyl)phenyl triflate 1a with diphenylacetylene as a model
example. The expected benzosilole derivative 2a could be
generated under certain reaction conditions. Notably, in addition
to the remarkable effect of different catalysts and ligands, the base
was found to be very sensitive for this transformation.11 When
LiOt-Bu was used as the base, the triflate 1a was partially changed
to its corresponding 2-trimethylsilylphenyl t-butyl ether (entries 1–
3). The base K2CO3 could mediate the reaction to afford the
product 2a in 41% GC yield (entry 6). LiOEt was found to be the
most effective base, while most other bases such as NaOt-Bu,
KOt-Bu, Li2CO3, Na2CO3, Cs2CO3, NaOEt, LiOAc, NaOAc, KOAc
afforded very low yields of products or no product at all.
Meanwhile, the addition of KBr was found to remarkably promote
the catalytic process and increase the yield of the product 2a from
Transition metal-catalyzed selective cleavage of the Me–Si bond in
a SiMe3 group and its synthetic applications have attracted much
recent interest.1–7 However, although remarkable achievements
have been made, this research area is still at a very early stage, with
much limitation in the diversity of suitable SiMe3-substituted
substrates and reaction types. Further development of the
transition-metal-catalyzed coupling accompanied by a selective
cleavage of the C(sp3)–Si bond would lead to useful protocols for
the synthesis of substituted siloles and derivatives, which are very
important organic materials in electronic and optoelectronic
devices.8,9
Commercially available ortho-silyl aryltriflates 1 have been
frequently used as benzyne precursors in organic synthesis.10 As
our continued interest in the search of suitable SiMe3-containing
substrates for the catalytic synthetic application of their Me–Si
bonds,7 we turned our attention to those commercially available
ortho-silyl aryltriflates 1 and envisioned that such compounds
could be good substrates. Thus, in this paper, we report a new
reaction pattern of ortho-silyl aryltriflates 1, which undergo
aBeijing National Laboratory for Molecular Sciences, Institute of Chemistry, Chinese
Academy of Sciences (CAS), Beijing 100190, China
bBeijing National Laboratory for Molecular Sciences, MOE Key Laboratory of
Bioorganic Chemistry and Molecular Engineering, College of Chemistry, Peking
University, Beijing 100871, China. E-mail: zfxi@pku.edu.cn; Fax: +86-10-62751708;
Tel: +86-10-62759728
cState Key Laboratory of Organometallic Chemistry, Shanghai Institute of Organic
Chemistry (SIOC), CAS, Shanghai 200032, China
Scheme 1 Commercially available ortho-silyl aryltriflates 1a–f and transition
metal-catalyzed cleavage of the Me–Si bonds in 1 and subsequent intermolecular
coupling with alkynes for the synthesis of benzosiloles 2.
Electronic supplementary information (ESI) available: Materials including experi-
3
mental procedures, NMR spectra of all new products. See DOI: 10.1039/c3ra42910e
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Table 1 Optimization of reaction conditions for the reaction of 1a with
diphenylacetylene (see ESI3 for details)
Table 2 Formation of benzosiloles 2 from 1 and aromatic alkynes
2b: Ar = Ph, 86%
Entry [Pd]
Ligand Base
Additive Yield of 2a (%)a
2c: Ar = 4-Tol, 64%
2d: Ar = 1-Naph, 67%
2e: Ar = 4-FC6H4, 59%
2f: Ar = 2-Thenyl, 73%
1
2
3
[Pd(p-allyl)Cl]2 Pt-Bu3 LiOt-Bu
[Pd(p-allyl)Cl]2 Pt-Bu3 LiOt-Bu KI
[Pd(p-allyl)Cl]2 Pt-Bu3 LiOt-Bu KBr
—
15
trace
32
4
5
6
7
8
9
10
11
12
13
14
15
[Pd(p-allyl)Cl]2 Pt-Bu3 Li2CO3
[Pd(p-allyl)Cl]2 Pt-Bu3 Na2CO3 KBr
[Pd(p-allyl)Cl]2 Pt-Bu3 K2CO3 KBr
[Pd(p-allyl)Cl]2 Pt-Bu3 Cs2CO3 KBr
KBr
NR
10
41
12
16
2g: Ar = Ph, 65%
2h: Ar = 4-Tol, 53%
2i: Ar = 1-Naph, 56%
2j: Ar = 2-Thenyl, 51%
[Pd(p-allyl)Cl]2 Pt-Bu3 LiOEt
[Pd(p-allyl)Cl]2 Pt-Bu3 LiOEt
[Pd(p-allyl)Cl]2 Pt-Bu3 NaOEt
[Pd(p-allyl)Cl]2 Pt-Bu3 LiOAc
[Pd(p-allyl)Cl]2 Pt-Bu3 NaOAc
[Pd(p-allyl)Cl]2 Pt-Bu3 KOAc
—
KBr
KBr
KBr
KBr
KBr
84 (80)
b
—
2k: R = Me, 56%
2l: R = OMe, 63%
NR
22
18
b
[Pd(p-allyl)Cl]2 Pt-Bu3 NaOt-Bu KBr
[Pd(p-allyl)Cl]2 Pt-Bu3 KOt-Bu KBr
—
—
b
a
b
GC yields. Isolated yields are given in parenthesis. The substrate
decomposed.
15% (entry 1) to 32% (entry 3). When LiOEt was used as the base,
the yield of product 2a was increased from 16% (entry 8) to 84% in
the presence of two equivalents of KBr (entry 9). Detailed
investigation into the effect of additives was summarized and
mol%), PPh3 (10 mol%), KBr (2 equiv), LiOEt (3 equiv), in toluene,
120 uC, 24 h (Scheme 2). The 2,3-bisalkyl-substituted benzosiloles
3a–c could be obtained in moderate to good isolated yields.
Several experiments were carried out to investigate the reaction
mechanism. (1) There are several examples of palladium-catalyzed
conversion of aryl and vinyl triflates to their corresponding
bromides in the presence of KBr or LiBr.12 However, as given in
Scheme 3, under our palladium-catalyzed condition, no formation
of its corresponding bromide 4 was observed.
(2) When the 2-silylarylbromide 4 was subjected to the optimal
reaction condition for 1, the product 2a was obtained in 60% yield,
along with 30% of the six-membered silacycle 5 (Scheme 4).13
These results indicate that the 2-silylarylbromide 4 may not be
directly involved in this current reaction.
provided in the ESI. An optimal reaction condition was realized as
3
follows: [PdCl(p-allyl)]2 (2.5 mol%), Pt-Bu3 (10 mol%), LiOEt (3.0
equiv), KBr (2.0 equiv), in toluene, 120 uC, 24 h.
Under the optimized reaction condition, the benzosilole
derivatives 2b–n were obtained in 51% to 86% isolated yields
(Table 2). Aromatic alkynes substituted with electron-withdrawing
groups generally afforded moderate to good yields of their
corresponding products. Those alkynes with electron-donating
groups gave the products in slightly lower yields. Unsymmetrically
substituted alkynes with a TMS group and an aromatic group
afforded their products in moderate yields, regioselectively with
the TMS group at the 2-position of the benzosiloles (2g–j). When
1b (R9 = Me) and 1c (R9 = OMe) were treated with diphenylace-
tylene, their corresponding products 2k and 2l were also obtained.
Similarly, as given in Table 2, the naphthyl derivatives 1e and 1f
could afford their corresponding benzosiloles 2m and 2n in 58%
and 84% isolated yields, respectively.
(3) The gas composition of the reaction was analyzed by using
GC-MS analysis. Experimental results showed that when KBr was
When an aliphatic alkyne, such as 3-hexyne or 4-octyne, was
subjected to the optimal reaction condition for the reaction of
aromatic alkynes, the expected products 3a and 3b were formed in
very low yields (Scheme 2). Most of the starting materials 1
decomposed. Optimization of reaction conditions for the reaction
of 1a with 4-octyne was provided in the ESI. Again, the addition of
3
KBr was proved to be necessary and LiOEt was found to be the
most effective base. The optimal reaction condition for the
reaction of 1 with aliphatic alkynes is as follows: Pd(PPh3)2Cl2 (5
Scheme 2 Formation of benzosiloles 3 from 1a and aliphatic alkynes.
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potassium bromide (KBr) was found to be very effective for this
reaction.
This work was supported by the Natural Science Foundation of
China, and the Major State Basic Research Development Program
(2012CB821600).
Scheme 3 Control experiment: palladium-catalyzed conversion of aryl triflates to
bromides.
Notes and references
1 For a recent review on the formation of silacycles via metal-
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Scheme 4 Comparison of reactivity between 2-silylaryl bromide
(trimethylsilyl)phenyl triflate 1a.
4 with o-
introduced, the formation of MeBr was obviously observed. On the
contrary, no MeBr was detected in the absence of KBr, and most of
the starting compound 1a was decomposed.
Based on all the above preliminary experimental results, a
plausible mechanism for this process is proposed in Scheme 5.
The intermediate 6 would be first generated via oxidative addition
of 1 to Pd(0). When KBr was added, a nucleophilic substitution
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Reductive elimination of the MePdBr would then release MeBr
and regenerate the active Pd(0) species for the catalytic cycle.
Alternatively, the nucleophilic substitution reaction with KBr may
take place at the species 79.
In summary, an efficient Pd-catalyzed cleavage of the Me–Si
bond in ortho-trimethylsilyl aryltriflates was realized and synthe-
tically applied. Most commercially available ortho-trimethylsilyl
aryltriflates could undergo the Pd-catalyzed intermolecular cou-
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variety of benzosilole derivatives. A new reaction pattern of ortho-
trimethylsilyl aryltriflates was thus developed. The addition of
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