Palladium-catalyzed intermolecular C-H silylation initiated by aminopalladation

Article abstract of DOI:10.1039/d0cc00872a

A Pd(ii)-catalyzed intermolecular C-H silylation reaction initiated by aminopalladation has been developed. The C-H bonds were activated by an alkyl Pd(ii) species generated through aminopalladation and then disilylated with hexamethyldisilane to form disilylated indolines as the final products. The reaction provides a new method for the introduction of silyl groups into complex organic molecules.

Full text of DOI:10.1039/d0cc00872a

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DOI: 10.1039/D0CC00872A
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Palladium-Catalyzed Intermolecular C–H Silylation Initiated by
Aminopalladation
eceived 00th January 20xx,
Accepted 00th January 20xx
Xiaoming Ji, Feng Wei, Bing Wan, Cang Cheng, Yanghui Zhang*
DOI: 10.1039/x0xx00000x
www.rsc.org/
A Pd(II)-catalyzed intermolecular C–H silylation reaction initiated
C−H
functionalization
reactions
initiated
by
by aminopalladation has been developed. The C–H bonds were aminopalladation have been developed by the groups of
activated by an alkylPd(II) species generated through Yang,7 Mhaske8 and Liu.9 This type of reaction starts with
aminopalladation and then disilylated with hexamethyldisilane to intramolecular aminopalladation to generate σ-alkylpalladium
form disilylated indolines as the final products. The reaction intermediates. The resulting Pd(II) species then active C−H
provides a new method for the introduction of silyl groups into bonds at appropriate positions to form C,C-palladacycles as the
complex organic molecules.
intermediates (Scheme 1c). In this reaction, C–H bonds that
are distal to the nitrogen-containing group are activated
through cascade reactions. Furthermore, the nitrogen atom is
combined into the final product and the removal or conversion
of directing groups is avoided. The reaction represents an
innovative and atom-economical strategy for C–H activation. In
all the current reactions, the resulted C,C-palladacycles
underwent intramolecular cyclization, and intermolecular
functionalization reactions with external reagents have not
been reported yet.10
Transition metal-catalyzed direct C–H functionalization
represents a powerful method for the construction of carbon–
carbon and carbon–heteroatom bonds.1 One of the major
challenges in C–H functionalization reactions is to activate
target C–H bonds selectively. Currently, the most common
strategy to achieve site-selective C−H functionalization is to
utilize directing groups that usually contain coordinating
heteroatoms (Scheme 1a).2 In the presence of directing
groups, only C−H bonds proximal to the directing groups can
be activated. Although this strategy has achieved great
success, it has drawbacks including preinstallation and removal
of directing groups. Furthermore, the strategy is limited to the
activation of proximal C–H bonds and restricts the
development of C–H functionalization. Notably, a variety of
innovative methods have been developed to activate C–H
bonds remote from directing groups, including U-shaped
a) C–H activation assisted by directing groups
C–H
activation
Pd(II)
DG
H
DG
Pd(II)
DG
H
DG =
Directing Group
Pd(II)
b) C–H activation initiated by carbopalladation
C–H
activation
X
Pd(0)
H
H
Pd(II)
Pd(II)
X = Br, I
template-enabled
meta/para-C–H
functionalization,3
c) C–H activation initiated by aminopalladation
C–H
norbornene-enabled meta-C–H activation,4 and Cu or Ru-
catalyzed meta-C–H acitvation.5 Furthermore, remote C–H
N
Pd(II)
activation
N
NH
H
Pd(II)
H
bonds
can
be
activated
through
intramolecular
Pd(II)
carbopalladation (Scheme 1b).6 However, these reactions still
require the preinstallation of directing groups. It is highly
desirable to develop new C–H activation strategies to expand
the scope of C–H functionalization reactions.
Scheme 1. Selective C−H functionalization.
Organosilicon compounds are widely applied in organic
chemistry,11 materials science12 and medicinal chemistry,13
due to their unique chemical, physical and bioactive
properties. Therefore, the development of new strategies for
the construction of C−Si bonds is of great significance.
Recently, it has been revealed that C,C-palladacycles react
efficiently with hexamethyldisilane and can be disilylated very
efficiently.14 Our group has been interested in the
intermolecular functionalization of C,C-palladacycles obtained
School of Chemical Science and Engineering, Shanghai Key Laboratory of Chemical
Assessment and Sustainability, Tongji University, 1239 Siping Road, Shanghai,
200092, P. R. China. E-mail: zhangyanghui@tongji.edu.cn;
† Footnotes relating to the title and/or authors should appear here.
Electronic Supplementary Information (ESI) available: [details of any
supplementary information available should be included here]. See
DOI: 10.1039/x0xx00000x
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 1
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by C–H activation.15 Since C–H activation via aminopalladation proceed in toluene or DMF, and only a trace amount of 3a was
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also form C,C-palladacycles as the intermediates, we obtained in 1,4-dioxane (entries 2-4).DOSIu: b10s.e10q3u9e/Dn0tlCyC, 0o08th72eAr
envisioned that the C,C-palladacycles could also be disilylated oxidants were investigated, such as AgOAc, AgTFA and O2
with hexamethyldisilane. However, in all the current (entries 5-7). The results revealed that AgTFA was the most
disilylation reactions, aryl halides were used as the substrates. efficient and 3a was not observed in the absence of a silver
Mechanistically, the reactions were initiated by the oxidative salt. Next, we examined the impact of bases on the reaction,
addition of aryl halides to Pd(0) and involved Pd(0)-Pd(0) including NaHCO3, KOAc and KHCO3 (entries 8-10). We found
catalytic cycle. For the aminopalladation-initiated C–H that replacing K2CO3 with KHCO3 resulted in a slight increase in
activation reactions, the catalytic cycle starts with Pd(II) and yield (entry 9). The yield was improved to 40% by using 40 mol
ends up with Pd(0). Therefore, a new catalyst system should % of pyridine as the ligand (entry 11) and further enhanced to
be developed and extensive studies should be conducted to 50% by adding 8 equiv of H2O (entry 12). By increasing the
find suitable reaction conditions. Herein, we report a new amount of hexamethyldisilane 2 and KHCO3, the yield was
Pd(II)-catalyzed intermolecular C−H silylation reaction initiated improved to 62% and 65%, respectively (entries 13 and 14),
aminopalladation. The reaction represents an innovative and the addition of 20 mol % of 2,5-DMBQ afforded product
method for the introduction of silyl groups into complex 3a in 72% yield (entry 15). Finally, the reaction was more
organic molecules.
effective in 2 mL of DMSO and a yield of 81% was obtained
We commenced our study by investigating the model (entry 16). Under N2 atmosphere, the yield decreased to 25%
reaction of 2-phenylacrylamide 1a with hexamethyldisilane (2) (entry 17).
(Table 1). To our delight, the desired disilylation product 3a
Having developed an efficient protocol for Pd(II)-catalyzed
was obtained in 7% yield in the presence of Pd(OAc)2 (10 oxidative double cyclization/disilylation reaction, we next
o
mol%), Ag2CO3 (2 equiv), K2CO3 (2 equiv) in DMSO at 90 C investigated the substrate scope of this transformation (Table
under an air atmosphere (entry 1). The reaction did not
2). First, we examined the performance of acrylamides bearing
different substituents on the benzene rings of the anilines. The
substrates bearing a 4-methyl or methoxy group were suitable
(3b and 3c), and the fluoro and chloro groups were tolerated
under the standard conditions (3d and 3e). In these reactions,
the corresponding disilylated products were formed in
moderate or high yields. 5-Substituted substrates by methyl,
fluoro or chloro were also reactive (3f-3h), and the reactions
were slightly less effective than those of the corresponding 4-
substituted substrates. The structure of 3h was unambiguously
confirmed by single-crystal X-ray structural analysis.16 The
methyl group is cis to the (trimethylsilyl)methyl group. Next,
the compatibility of functionalities on the other phenyl group
was examined. The substrates containing a methyl or phenyl
group at para-positions underwent the domino reaction
smoothly (3i and 3j). The phenoxy group and a range of alkoxy
groups were compatible, and moderate yields were obtained
(3k-3n). The presence of electron-withdrawing trifluoromethyl
group led to a low yield (3o). Fluoro and chloro groups were
well tolerated, and the reactions gave the desired products in
moderate yield (3p-3r). The reaction selectively took place at
the less hindered position for meta-substituted 1r. The
substrates bearing substituents on both of the benzene rings
could also be disilylated with hexamethyldisilane (3s and 3t).
The impact of substituents on the allyl groups was also
examined. Ethyl-substituted alkene 1u was disilylated in 68%
yield (3u). The reaction did not occur in the presence of a
terminal phenyl group (3v). It should be mentioned that
unsubstituted alkene (1w-1y) also underwent the disilylation
reaction (3w-3y), and the potential side products from β-
hydride elimination were not observed. The NOESY spectrum
of 3w indicates that the α-proton of the amide group is cis to
the (trimethylsilyl)methyl group. An alkylamine was not
compatible and failed to give the desired product (3z). The
substrate containing a pyrrole ring was not reactive and the
starting material was recovered (3aa).
Table 1. Optimization of reaction conditions.
Pd(OAc)2 (10 mol %)
Ligand (40 mol %)
TMS
H
Oxidant, Base, Additive
O
TMS-TMS
+
N
DMSO (1 mL), 90 o
air, 24 h
C
N
2 (5 equiv)
O
TMS
H
1a (0.1 mmol)
3a
Entry
Ligand
Oxidant
(equiv)
Base
Solvent
Yield
(%)a
(equiv)
1
/
/
/
/
/
/
/
/
/
/
Ag2CO3 (2)
Ag2CO3 (2)
Ag2CO3 (2)
Ag2CO3 (2)
AgOAc (2)
AgTFA (2)
O2
K2CO3 (2)
DMSO
Toluene
Dioxane
DMF
7
2
K2CO3 (2)
K2CO3 (2)
K2CO3 (2)
K2CO3 (2)
K2CO3 (2)
K2CO3 (2)
KOAc (2)
KHCO3 (2)
0
3
trace
0
4
5
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
6
6
20
0
7
8
AgTFA (2)
AgTFA (2)
6
9
25
7
10
11
AgTFA (2) NaHCO3 (2)
Pyridine AgTFA (2)
KHCO3 (2)
40
12b
13b,c
14b,c
15b,c,d
16b,c,d,e
Pyridine AgTFA (2)
Pyridine AgTFA (2)
Pyridine AgTFA (2)
Pyridine AgTFA (2)
Pyridine AgTFA (2)
KHCO3 (2)
KHCO3 (3)
KHCO3 (4)
KHCO3 (4)
KHCO3 (4)
KHCO3 (4)
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
50
62
65
72
81(79)f
17b,c,d,e,g Pyridine AgTFA (2)
25
aThe yields were determined by 1H NMR analysis of the crude reaction
mixture using CHCl2CHCl2 as the internal standard. bH2O (8 equiv) was
added. cTMS-TMS (7 equiv). d20 mol % of 2, 5-DMBQ was added. eDMSO (2
f
mL). Isolated yield. gUnder N2 atmosphere. 2,5-DMBQ = 2,5-dimethyl-p-
benzoquinone, TMS-TMS = 1,1,1,2,2,2-hexamethyldisilane.
2 | J. Name., 2012, 00, 1-3
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cyclic product 5a. The reaction provides an efficient method
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DOI: 10.1039/D0CC00872A
for the construction of medium-sized cyclic compounds
containing silicons.
Table 2. Substrate scope of the disilylation reaction.a,b
Pd(OAc)2 (10 mol %)
R2
Pd(OAc)2 (10 mol %)
Pyridine (40 mol %)
AgTFA (2 equiv)
pyridine
R2
TMS
H
R3
AgTFA
2,5-DMBQ
R3
O
R1
H
2,5-DMBQ (20 mol %)
Si
Si
TMS-TMS
N
+
O
Si
3
R1
N
+
KHCO3,
N
KHCO3 (4 equiv)
H2O (8 equiv)
DMSO (2 mL)
90 oC, air, 24 h
H
2 (7 equiv)
O
H2O, DMSO
90 oC, air, 24 h
N
TMS
O
Si
H
1a-aa
3a-aa
4
(7 equiv)
5a
, 50%
1a
(0.1 mmol)
TMS
TMS
TMS
Scheme 2. Disilylation Reaction with a Cyclic Disilane.
MeO
N
N
N
H1 (75% D)
(75% D)
TMS
O
O
O
TMS
TMS
TMS
H2
3a, 79%
3b,78%
3c, 80%
H1
H2
H
MeO
MeO
TMS
TMS
TMS
standard conditions
MeO
O
N
F
Cl
syn-amidopalladation
N
N
N
N
H
MeO
O
TMS
3ab-D
, 38%
O
O
O
TMS
TMS
TMS
3f, 72%
TMS
1ab-D
(0.1 mmol)
3d, 65%
3e, 56%
TMS
TMS
Scheme 3. Deuterium-Labeling Experiment on the
Determination of Stereochemistry in the Aminopalladation
Step.
N
N
N
F
Cl
O
O
O
TMS
TMS
3h, 45%
TMS
TMS
3g, 60%
3i, 61%
Aminopalladtion may proceed via a syn- or anti-pathway.17
To determine the stereochemical course of the
aminopalladation step in the disilylation reaction, we prepared
substrate 3ab-D with 75% deuterium incorporated into the H1.
Under the standard conditions, product 3ab-D was obtained
with 75% deuterium incorporated into the H1 positions, which
indicates a syn-amidopalladation pathway (Scheme 3).
TMS
TMS
Ph
OMe
OiPr
N
N
N
O
O
O
TMS
3j, 75%
TMS
TMS
3k, 58%
TMS
TMS
3l, 73%
TMS
OBn
OPh
CF3
N
N
N
O
O
N
O
TMS
TMS
TMS
3m, 67%
3n, 73%
3o, 30%
TMS
TMS
TMS
O
Ag0
F
Cl
PdII
amidopalladation
N
N
N
AgI
H
Cl
1a
Pd0
O
O
O
TMS
TMS
TMS
3a
3p, 58%
3q, 66%
3r, 63%
reductive
elimination
PdII
N
Et
TMS
TMS
TMS
H
TMS
PdII
O
OMe
Ph
TMS
N
N
O
N
A
F
N
O
O
N
TMS
TMS
TMS
or
reductive
elimination
PdII
B
O
3s, 70%
3t, 53%
3u, 68%
O
G'
G
TMS
TMS
Deprotonation
TMS
H
H
TMS
TMS
Ph
O
BH
MeO
N
N
N
O
N
PdII
TMS
3w, 52%
PdIV
TMS
O
O
N
O
TMS
TMS
E
TMS
3x, 55% (d.r. 5:1)c
3v, 0%
N
or
N
TMS
TMS
PdII
O
TMS TMS
PdII
B
migratory
insertion
H
H
TMS
TMS
TMS
F'
O
F
OMe
N
N
N
N
oxidative
addition
metathesis
N
2
O
TMS
O
O
TMS
TMS
PdII
O
3y, 50% (d.r. 5:1)c
3aa, 0%
3z, 0%
C
C-H
activation
2
N
aReaction conditions: 2-phenylacrylamide 1 (0.1 mmol), TMS-TMS 2 (7
equiv), Pd(OAc)2 (10 mol %), pyridine (40 mol %), AgTFA (2 equiv), KHCO3
(4 equiv), 2,5-DMBQ (20 mol %), H2O (8 equiv) in DMSO (2 mL) at 90 oC for
24 h. bYields of the isolated products. CRatio determined by 1H NMR
analysis; the major diastereomer is depicted.
PdII
O
D
Scheme 4. Proposed Mechanism for the Disilylation Reaction.
Notably, a cyclic disilane was also an effective disilylating
reagent. As shown in Scheme 2, 1a could react with 1,1,2,2-
tetramethyl-1,2-disilacyclohexane (4) to form ten-membered
Based on the above experimental results and previous
reports,9,13 we proposed a tentative mechanism for the Pd(II)-
catalyzed oxidative double cyclization/disilylation reaction. As
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5
6
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shown in Scheme 4, substrate 1a undergoes intramolecular
aminopalladation to form σ-alkylpalladium intermediate A, in
which the double bond might coordinate to the alkyl-Pd(II)
View Article Online
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a base generates
intermediate B. The subsequent intramolecular olefin insertion
yields a second alkyl-Pd(II) intermediate C. The resulting Pd(II)
cleaves an aryl C–H bond to give palladacycle D. D can react
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with
hexamethyldisilane
via
either
an
oxidative
addition/reductive elimination or σ-bond metathesis
pathway.18 In these steps, an alkyl-Pd(II) or aryl-Pd(II)
intermediate may be formed. Finally, the reductive elimination
of G or G’ generates product 3a and releases Pd(0) species.
The catalytic cycle is completed by the regeneration of Pd(II)
from Pd(0) with Ag(I) as the oxidant.
In conclusion, we have developed a new Pd(II)-catalyzed
intermolecular C–H silylation reaction initiated by
aminopalladation. The reaction formed disilylated indolines as
the final products in moderate to good yields, providing a
novel and straightforward method for the introduction of silyl
groups into complex organic molecules.
7
8
9
10 Although σ-alkylpalladium generated by aminopalladation
could activate C–H bonds of simple arenes intermolecularly,
these have been found to preferentially cleave the C–H
bonds of an external arene substrate, rather than a C–H
bond tethered to the nitrogen atom of the substrate. C. F.
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Conflicts of interest
There are no conflicts to declare.
Notes and references
‡ The work was supported by the National Natural Science
Foundation of China (No.216721626) and Shanghai Science and
Technology Commission (14DZ2261100).
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