NBE-Controlled Palladium-Catalyzed Interannular Selective C-H Silylation: Access to Divergent Silicon-Containing 1,1′-Biaryl-2-Acetamides

Article abstract of DOI:10.1021/acs.orglett.9b00690

A novel palladium-catalyzed interannular selective C-H silylation of 1,1′-biaryl-2-acetamides is described. The combination of palladium catalyst with copper oxidant enables meta- or ortho-selective C-H silylation by employing hexamethyldisilane as a trimethylsilyl source, which relies on the control of NBE derivatives as a switch, thus providing straightforward access to divergent silicon-containing 1,1′-biaryl-2-acetamides.

Full text of DOI:10.1021/acs.orglett.9b00690

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
NBE-Controlled Palladium-Catalyzed Interannular Selective C−H
Silylation: Access to Divergent Silicon-Containing 1,1′-Biaryl-2-
Acetamides
Wenguang Li, Wenqi Chen, Bang Zhou, Yankun Xu, Guobo Deng, Yun Liang,* and Yuan Yang*
National & Local Joint Engineering Laboratory for New Petro-chemical Materials and Fine Utilization of Resources, Key Laboratory
of Chemical Biology and Traditional Chinese Medicine Research, Ministry of Education, Key Laboratory of the Assembly and
Application of Organic Functional Molecules of Hunan Province, Hunan Normal University, Changsha, Hunan 410081, China
S
* Supporting Information
ABSTRACT: A novel palladium-catalyzed interannular selective C−H silylation of 1,1′-biaryl-2-acetamides is described. The
combination of palladium catalyst with copper oxidant enables meta- or ortho-selective C−H silylation by employing
hexamethyldisilane as a trimethylsilyl source, which relies on the control of NBE derivatives as a switch, thus providing
straightforward access to divergent silicon-containing 1,1′-biaryl-2-acetamides.
selective C−H functionalization remains underdeveloped,7 yet
he synthesis of organosilicon compounds is interesting to
Tscientific researchers because they possess unique is particularly attractive, since the target C−H bond is more
remote from directed functional groups. Inspired by the
Catellani reaction8,10 that allows sequential ortho-C−H and
ispo-C−X functionalization of aryl halides, which was first
discovered by Catellani9 and further developed by the groups
of Catellani,8a,b Lautens,8c,d Dong,10a−e and others (Scheme 1-
1).10f−l Recently, the elegant strategy for achieving meta-
selective C−H functionalization that combines directed
palladium-catalyzed ortho-C−H activation with norbornene-
mediated Catellani reaction have been developed (Scheme 1-
2).11 The pioneering work was reported by Yu and co-workers
to achieve meta-selective C−H alkylation and arylation of
arene enabled by a highly optimized pyridine ligand.11a
Subsequently, the Yu group further enriched this chemistry
and developed a series of meta-selective C−H functionaliza-
tion,11b−f such as alkylation, arylation, alkynylation, amination,
and chlorination. In 2015, the Dong group also reported a
Pd(II)/NBE-catalyzed meta-C−H arylation.11g Later, the
Shi11h and Zhao11i groups successfully utilized this strategy
to develop meta-selective arylation of biaryl-2-trifluoroaceta-
mides and β-heteroethylamines. However, meta-selective C−H
silylation has never been realized via a similar strategy. Very
recently, the Chen4c,d and Our groups12f simultaneously
demonstrated palladium-catalyzed Catellani-type ortho-silyla-
tion of aryl iodides. On the basis of recent reports and our
ongoing interests in the tandem reactions involving pallada-
chemical, physical and bioactive properties, which could be
widely applied in organic chemistry, medicinal chemistry, and
materials science.1,2 Specifically, the recently emerging studies
in medicinal chemistry have been made toward the
incorporation of silicon groups into drug candidates.2
Consequently, the development of new efficient strategies for
the assembly of organosilicon compounds is of significant
importance. Recently, transition-metal-catalyzed C−H silyla-
tion has emerged as a powerful tool for the preparation of
organosilicon compounds due to atom and step economy.3
Among these established methods, palladium-catalyzed direct
C−H silylation by employing hexamethyldisilane as a
trimethylsilyl source has been attracting increased attention.4,5
However, the vast majority of the reported methods were
limited to C(sp3)−H and aromatic ortho-C(sp2)−H silylation,4
despite the fact that Maiti and Liu have recently made an
impressive breakthrough to achieve regioselective para-C−H
silylation of toluene derivatives via a directed palladium-
catalyzed strategy involving substrate−solvent hydrogen-
bonding interaction.5 So far, meta-C−H silylation of arenes
has not yet been reported. Therefore, the development of new
meta-selective C−H silylation methods is urgently needed.
On the other hand, the site-selective C−H functionalization
of arenes is a significant challenge. In past decades, directed
transition-metal-catalyzed C−H functionalization has displayed
superior potential, in terms of site selectivity.6,7 Nevertheless,
considerable efforts have been devoted to the ortho-selective
C−H functionalization of arenes.6 By contrast, directed meta-
Received: February 23, 2019
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.9b00690
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Letter
a
Scheme 1. Site-Selective C−H Functionalization in
Palladium/NBE Chemistry
Table 1. Optimization of the Reaction Conditions
b
entry
variation from the standard conditions
yield (%)
c
1
none
81 (74 )
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
under oxygen atmosphere
under nitrogen atmosphere
without Pd(OAc)2 or Cu(OAc)2
PdCl2(PPh3)2 instead of Pd(OAc)2
Pd(dba)2 instead of Pd(OAc)2
PdCl2 instead of Pd(OAc)2
Pd(PPh3)4 instead of Pd(OAc)2
CuCl2 instead of Cu(OAc)2
AgOAc instead of Cu(OAc)2
K3PO4 instead of K2CO3
Cs2CO3 instead of K2CO3
DMF instead of CH3CN
toluene or dioxane instead of CH3CN
without DABCO
21
64
0
35
12
65
69
11
10
44
55
47
0
41
76
70
at 120 °C
at 140 °C
cycles,12 herein we successfully achieved palladium-catalyzed
interannular meta-C−H silylation/ortho-C−H alkylation for
building silicon-containing 1,1′-biaryl-2-acetamides from 1,1′-
biaryl-2-acetamides with hexamethyldisilane in the presence of
NBE (Scheme 1-3). Interestingly, the strategy could be
expanded to interannular meta-C−H silylation/ortho-C−H
alkenylation of 1,1′-biaryl-2-acetamides by the use of ONBD,
instead of NBE and interannular ortho-C−H silylation of 1,1′-
biaryl-2-acetamides, in the absence of NBE.
a
Reaction conditions: 1a (0.2 mmol), NBE (3 equiv), TMS-TMS (5
equiv), Pd(OAc)2 (10 mol %), Cu(OAc)2 (3 equiv), K2CO3 (2
equiv), DABCO (1 equiv), and MeCN (2 mL) at 130 °C under air
atmosphere for 24 h. Isolated yield. 1a (1 mmol) and CH3CN (8
b
c
mL) were used.
entry 15). Finally, investigation of the reaction temperature
showed that a lower or higher temperature had a negative
effect on the reaction (Table 1, entries 16 and 17).
Gratifyingly, performing the reaction, scaled up to 1 mmol of
1a, could also afford the product 2a in a 74% yield (Table 1,
entry 1).
We commenced our studies with the interannular meta-C−
H silylation/ortho-C−H alkylation reaction of N-([1,1′-
biphenyl]-2-yl)acetamide (1a) with norbornene (NBE), and
hexamethyldisilane for optimization of the reaction conditions
(Table 1). After extensive screening the reaction parameters,
we found that the optimal conditions were the use of 1a (0.2
mmol), NBE (3 equiv), TMS-TMS (5 equiv), Pd(OAc)2 (10
mol %), Cu(OAc)2 (3 equiv), K2CO3 (2 equiv), DABCO (1
equiv), and MeCN (2 mL) at 130 °C under air atmosphere for
24 h, and the desired meta-C−H silylation product 2a was
obtained in 81% yield (Table 1, entry 1). When the reaction
was conducted under an oxygen or nitrogen atmosphere, the
desired product was provided in 21% and 64% yields,
respectively (Table 1, entries 2 and 3). Notably, control
experiments demonstrated that palladium catalysts and copper
oxidants were crucial for this interannular meta-C−H silylation
reaction, since the target product was not detected when any
one of them was omitted (Table 1, entry 4). Other alternative
palladium catalysts, such as PdCl2(PPh3)2, Pd(dba)2, PdCl2,
and Pd(PPh3)4, displayed lower catalytic efficiency (Table 1,
entries 5−8). Furthermore, the yields were dramatically
reduced by replacing Cu(OAc)2 with CuCl2 or AgOAc as
oxidant (Table 1, entries 9 and 10). Further screening of other
bases, including K3PO4 and Cs2CO3, indicated that both of
them were inferior to K2CO3 (Table 1, entries 11 and 12). The
reaction proved sensitive to the effect of solvent: using DMF as
solvent delivered 47% of the target product, even the reaction
could not occur when CH3CN was replaced by toluene or
dioxane as medium (Table 1, entries 13 and 14). The reaction
exhibited lower efficiency in the absence of DABCO (Table 1,
With the optimal reaction conditions in hand, we set out to
examine the substrate scope of this palladium-catalyzed
interannular meta-C−H silylation/ortho-C−H alkylation pro-
tocol (Scheme 2). Initially, the compatibility of various
functional groups on the benzene ring Ar1 of 1,1′-biaryl-2-
acetamides was investigated (1b−1m). To our delight, a range
of substituents, including electron-donating groups (Me and
OMe) and electron-withdrawing groups (F, Cl, CF3, CO2Me,
CN, and NO2), at the 4- or 5-position on the benzene ring Ar1
were well tolerated, and the corresponding products 2b−2k
were obtained in 73%−97% yields. Their electronic effect was
not obvious. Importantly, the disubstituted 1,1′-biaryl-2-
acetamides could successfully be converted into the target
products (2l and 2m), albeit with a diminished yield.
Subsequently, an array of 1,1′-biaryl-2-acetamides with
substituents on the benzene ring Ar2 or both of benzene
rings Ar1 and Ar2 were examined, which underwent this
reaction with NBE and hexamethyldisilane to afford the target
products 2n−2x in moderate to good yields. The results
indicated that the substituents on the benzene ring Ar2 have a
detrimental effect, in terms of yields. It was noteworthy that N-
(2-(1H-pyrrol-1-yl)phenyl)acetamide 1y was also a suitable
substrate, albeit giving decreased yield in comparison with N-
([1,1′-biphenyl]-2-yl)acetamide 1a.
Next, the reaction of norbornadiene with N-([1,1′-
biphenyl]-2-yl)acetamide (1a) and hexamethyldisilane was
B
DOI: 10.1021/acs.orglett.9b00690
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
of substrates bearing substituents (Me, OMe, F, and Cl) could
uneventfully be converted to the corresponding products with
a Z/E ratio value in 47%−61% overall yield (3b, 3d, 3e, 3p,
and 3u).
Scheme 2. Substrate Scope for Interannular meta-C−H
silylation/ortho-C−H Alkylation from 1,1′-Biaryl-2-
a
acetamides (1) with NBE and TMS-TMS
Further survey showed that the C−H silylation at the 2′-
position of N-([1,1′-biphenyl]-2-yl)acetamide could be
achieved in the absence of NBE. After examining various
reaction parameters, we were pleased to found that treatment
of N-([1,1′-biphenyl]-2-yl)acetamide 1a with hexamethyldisi-
lane, Pd(dba)2 (10 mol %), Cu(OAc)2 (3 equiv), K2CO3 (2
equiv), AgBF4 (20 mol %), and DMF (2 mL) at 120 °C under
air atmosphere for 24 h could deliver the desired product 4a in
75% yield (see Table S1 in the Supporting Information (SI)).
Inspired by these results, a variety of the silylation products
were synthesized in moderate to good yields by checking the
compatibility of functional groups (Scheme 4). For example,
Scheme 4. Substrate Scope for Interannular ortho-C−H
Silylation from 1,1′-Biaryl-2-acetamides (1) with TMS-
a
TMS
a
Reaction conditions: 1 (0.2 mmol), NBE (3 equiv), TMS-TMS (5
equiv), Pd(OAc)2 (10 mol %), Cu(OAc)2 (3 equiv), K2CO3 (2
equiv), DABCO (1 equiv), and MeCN (2 mL) at 130 °C under air
atmosphere for 24 h.
explored under the above optimal reaction conditions.
According to the previous reports, we speculated that N-(3′-
(trimethylsilyl)-2′-(2-(trimethylsilyl)vinyl)-[1,1′-biphenyl]-2-
yl)acetamide 3a could be afforded via a retro-Diels−Alder
reaction of the NBE-containing meta-C−H silylation product.
However, the product 3a was unexpectedly not detected in our
attempts. Fortunately, after extensive screening, we found that
dimethyl7-oxabicyclo[2.2.1]hepta-2,5-diene-2,3-dicarboxylate
(ONBD) could react with N-([1,1′-biphenyl]-2-yl)acetamide
(1a) and hexamethyldisilane to produce a mixture of Z/E
isomers 3a with a ratio of 5:1 in 50% overall yield. Encouraged
by this results, the substrate scope of the reaction with regard
to 1,1′-biaryl-2-acetamides was explored (Scheme 3). A range
a
Reaction conditions: 1a (0.2 mmol), TMS-TMS (5 equiv), Pd(dba)2
(10 mol %), Cu(OAc)2 (3 equiv), K2CO3 (2 equiv), AgBF4 (20
mol %), and DMF (2 mL) at 120 °C under an air atmosphere for 24
h.
substrates with substituents, such as Me, OMe, F, Cl, CF3,
CO2Me, CN, and NO2, on the benzene ring Ar1 underwent the
C−H silylation reaction to obtain the target products 4b-k in
53−75% yields. The reaction of 1,1′-biaryl-2-acetamides
bearing substituents on the benzene ring Ar2 or both of
benzene rings Ar1 and Ar2 with hexamethyldisilane proceeded
smoothly, leading to the silylation products in 35%−53% yields
(4o, 4t, 4z, and 4aa).
Scheme 3. Substrate Scope for Interannular meta-C−H
Silylation/ortho-C−H Alkenylation from 1,1′-Biaryl-2-
a
acetamides (1) with ONBD and TMS-TMS
On the basis of the present results (see the SI) and previous
reports, a possible mechanism for the palladium-catalyzed
interannular selective C−H silylation from 1,1′-biaryl-2-
acetamides was proposed (Scheme 5).4−8 Initially, with the
aid of the acetamide directing group, the reaction of Pd(OAc)2
with 1a generates six-membered palladacycle by a interannular
ortho-C−H activation. Seminal works by Yu and Dong et al.
have shown the six-membered palladacycle undergoes an
analogous Catellani reaction pathway to afford the aryl
norbornyl palladacycle (ANP) in the presence of NBE or
ONBD. Subsequently, reaction of the ANP with hexamethyl-
disilane forms the intermediate D via two paths (path a:
oxidative addition of the ANP to hexamethyldisilane, followed
by reductive elimination; path b: the σ-bond metathesis
pathway involving intermediate C). Finally, reductive elimi-
nation of the intermediate D occur to provide the desired
a
Reaction conditions: see Table 1 and Scheme 1.
C
DOI: 10.1021/acs.orglett.9b00690
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
Scheme 5. Possible Reaction Mechanism
Yun Liang: 0000-0002-2550-9220
Yuan Yang: 0000-0002-9061-1758
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by the National Natural Science
Foundation of China (Nos. 21572051 and 21602057), the
Opening Fund of Key Laboratory of Chemical Biology and
Traditional Chinese Medicine Research (Ministry of Educa-
tion of China), Hunan Normal University (Nos.
KLCBTCMR201707 and KLCBTCMR201708), and Science
and Technology Planning Project of Hunan Province (No.
2018TP1017).
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In summary, we have developed a novel palladium-catalyzed
interannular meta-C−H silylation for the synthesis of divergent
silicon-containing 1,1′-biaryl-2-acetamides from 1,1′-biaryl-2-
acetamides with hexamethyldisilane in the presence of NBE
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Notably, the method exhibits high regioselectivity and broad
substrate scope. Further applications of the method are still in
progress in our laboratory.
ASSOCIATED CONTENT
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Accession Codes
CCDC 1892356 and 1892357 contain the supplementary
crystallographic data for this paper. These data can be obtained
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emailing data_request@ccdc.cam.ac.uk, or by contacting The
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AUTHOR INFORMATION
Corresponding Authors
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Maiti, D. Angew. Chem., Int. Ed. 2017, 56, 14903.
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see: (a) Chen, Z.; Wang, B.; Zhang, J.; Yu, W.; Liu, Z.; Zhang, Y. Org.
Chem. Front. 2015, 2, 1107. (b) Zhang, M.; Zhang, Y.; Jie, X.; Zhao,
H.; Li, G.; Su, W. Org. Chem. Front. 2014, 1, 843. (c) Rouquet, G.;
*E-mail: yliang@hunnu.edu.cn (Y. Liang).
*E-mail: yuanyang@hunnu.edu.cn (Y. Yang).
ORCID
Guobo Deng: 0000-0002-5470-5706
D
DOI: 10.1021/acs.orglett.9b00690
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DOI: 10.1021/acs.orglett.9b00690
Org. Lett. XXXX, XXX, XXX−XXX