Rhodium(III)-Catalyzed C—H Vinylation of Arenes: Access to Functionalized Styrenes

Article abstract of DOI:10.1002/cjoc.201800354

An effective method has been developed for Rh(III)-catalyzed direct vinylation of arenes to give functionalized styrenes, using vinyltriethoxysilane as a convenient and inexpensive vinyl source. A wide variety of substrates, including 1-aryl-2-pyrrolidinones, anilines, benzamides and ketones, were compatible with this reaction. Moreover, this method can be applied to the two-step synthesis of functionalized indoles. Mechanistic investigation reveals that the reaction probably proceeds through an oxidative Heck/desilylation pathway.

Full text of DOI:10.1002/cjoc.201800354

Rhodium(III)-Catalyzed C–H Vinylation of Arenes: Access to
Functionalized Styrenes
Jun Zhou, a,b Xin Li, a Gang Liao,a and Bing-Feng Shi*,a
ABSTRACT An effective method has been developed for the Rh(III)-catalyzed direct vinylation of arenes to give functionalized styrenes, using
vinyltriethoxysilane as a convenient and inexpensive vinyl source. A wide variety of substrates, including 1-aryl-2-pyrrolidinones, anilines, benzamides and
ketones, were compatible with this reaction. Moreover, this method can be applied to the two-step synthesis of functionalized indoles. Mechanistic
investigation reveals that the reaction probably proceeds through an oxidative Heck/desilylation pathway.
KEYWORDS C-H Activation, Vinylation, Rhodium, Styrenes, Vinyltriethoxysilane
vinylation
Introduction
Functionalized styrenes are important building blocks for
organic synthesis and the polymer industry.[1] Classically, they are
prepared by elimination reactions (mainly via dehydration of
alcohols
or dehydrohalogenation of halides),[2] carbonyl
alkenylation (by using P, Si, or Ti-based reagents),[3] or the
semihydrogenation of terminal alkynes.[4] One of the most useful
methods is the transition metal-catalyzed cross-coupling of aryl
and vinyl units (eg. aryl electrophiles with vinylmetallic reagents,
vinyl electrophiles with arylmetallic reagents, or aryl electrophiles
with ethylene or its synthons).[5] However, extra steps are needed
to prepare the prefunctionliazed coupling partners and undesired
waste salts are generated. Therefore, the direct oxidative
coupling of arenes with ethylene or its synthons is a more
attractive strategy.
Recently, significant efforts have been devoted to the
synthesis of vinylarenes through oxidative coupling of arenes with
alkenes.[6] Since the pioneering work by Fujiwara and coworkers
on the Pd-catalyzed oxidative arylation of ethylene for the
production of styrenes,[7] styrene synthesis via direct coupling
with ethylene has been extensively investigated (Scheme 1a).[8]
Although ethylene is the most atom efficient vinylation reagent,
the use of pressured and flammable ethylene gas is still relatively
inconvenient on laboratory-scale.[7,8] More recently, elegant
studies by the groups of Kakiuchi,[9] Kuang,[10] Ellman,[11] Wei,[12]
and Dong[13] have successfully demonstrated that vinyl acetate
could be used as vinyl source in the directed C-H vinylation of
arenes. Cheng and coworkers reported the Rh(III)-catalyzed
vinylation of benzamides using tributyl(vinyl)sannane as
vinylation reagent.[14] Xu and Fan reported a Rh(I)-catalyzed
decarbonylative C-H vinylation of arenes with acrylic acid as an
efficient ethylene surrogate (Scheme 1b).[15] Although these
elegant examples have significantly broadened the scope of the
vinylation reactions, the survey of more easy to handle and
readily available ethylene surrogates with broad substrate scope
for oxidative vinylation of arenes is still in high demand.
Results and Discussion
We initially applied our previously reported conditions[17] for
the oxidative coupling 1-Phenyl-2-pyrrolidinone (1) with
vinyltriethoxysilane. To our delight, the vinylation product 2 was
obtained in 40% yield (Table 1, entry 1). Considering the
important role of the fluoride source in the activation of
organosilicon reagents, we then investigated various fluoride
reagents (entries 2-6). The yield increases to 63% when NaF was
used instead (entry 6). Solvent effects played a significant role in
the reaction and DCE was found to be the most suitable solvent
(entries 6-11). Further optimization revealed that LiF was the
optimal (entry 12). The yield dramatically increases to 90% when
prolonging the reaction time to 36
commercially available vinylsilanes,
(divinyl-tetramethyldisiloxane, entry 14, 35%) and D4
(1,3,5,7-tetravinyl-1,3,5,7-tetramethylcyclotetrasiloxane,
15, 23%), were ineffective.
h
(entry 13). Other
Herein, we describe a Rh(III)-catalyzed vinylation of aromatic
C-H bonds with vinyltriethoxysilane, which is nontoxic,
commercially available and ease of operation.[16] A wide variety of
substrates, including 1-aryl-2-pyrrolidinones, anilines, benzamides
and ketones, were compatible with this reaction, providing a
general and efficient strategy to access functionalized styrenes
(Scheme 1c).
such as DVDS
v
entry
Scheme
1 Synthesis of styrenes via transition-metal-catalyzed C-H
a Department of Chemistry, Zhejiang University, Hangzhou 310027,China.
b School of Chemistry and Biological Engineering, Changsha University of
Science & Technology, Changsha 410114, China.
E-mail: bfshi@zju.edu.cn
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been through the copyediting, typesetting, pagination and proofreading process, which
may lead to differences between this version and the Version of Record. Please cite this
article as doi: 10.1002/cjoc.201800354
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Table 1 Optimization of reaction conditions a, b
BHT
(2,6-di-tert-butyl-4-methylphenol),
TBC
(4-tert-butylcatechol), and hydroquinone, didn’t give any
improvement.[18]
Table 2 Substrates Scope: 1-Phenyl-2-pyrrolidinonesa, b
a Reactions were carried out with 1 (0.2 mmol), vinyltriethoxysilane (0.6
a Reactions were carried out with 3 (0.2 mmol), vinyltriethoxysilane (0.6
mmol), [Cp*RhCl2]2 (5 mol %), AgSbF6 (20 mo l%), Cu(OAc)2 (0.4 mmol) and
LiF (0.4 mmol) in 2 mL DCE for 36 h. b Isolated Yield.
mmol), [Cp*RhCl2]2 (5 mol%), AgSbF6 (20 mol%), Cu(OAc)2 (0.4 mmol) and
c
fluoride reagent (0.4 mmol) in 2 mL solvent for 24 h. b Isolated Yield.
Starting material 1 was recovered in 50% yield. d Using DVDS instead,
e
v
v
DVDS
=
divinyl-tetramethyldisiloxane.
Using D4 instead, D4
=
Table 3 Substrates Scope: Pivalamides a, b
1,3,5,7-tetravinyl-1,3,5,7-tetramethylcyclotetrasiloxane.
With the optimized conditions in hand, we further
investigated the compatibility of various 1-aryl-2-pyrrolidinones
(Table 2). A variety of functional groups, including methoxy (4d,
4k, 4p and 4m), trifluoromethyl (4e), ester (4f), fluoro (4g and 4l),
chloro (4h), and bromo (4i), were well tolerated, providing great
opportunities for further elaborations. Electronic effect can
greatly affect the reactivity. In general, substrates bearing
electron-donating groups (4a-d, 4j-k) gave higher yields than
those bearing electron-withdrawing groups (4e-g, 4i and 4l). The
reaction is also very sensitive to steric effect, as substitutes in the
ortho-position of the directing group significantly inhibit the
reactivity (4m, 39%; 4n, <5%). Moreover, 4,5-disubstituted
substrates were olefinated effectively to give even higher yields
(4o-p). In addition, 3-phenyl-2-oxazolidinone is also an effective
substrate, affording the desired vinylation product in moderate
yield (4q, 48%).
Arenes and heteroarenes with other kinds of directing groups
(DGs) were tested. Pivalamides were also compatible with this
vinylation protocol (Table 3). Pivalamides bearing both
electron-donating and electron-withdrawing groups on the
aromatic rings reacted smoothly to give the desired vinylation
products in moderate yields (6b-6l). Moreover, disubstituted
substrates were also vinylated effectively to give the
corresponding products (6j-6l). To our delight, the vinylation
happened not only on simply arenes but also on heteroarenes,
including quinolines (6m) and pyridines (6n-o), albeit in relatively
low yields (20%-32%). In addition, 1,2,3,4-tetrahydroquinoline 5p
could also be vinylated using acetamide as DG. It is worth
mentioning that the low yields was largely due to the
decomposition. We have hypothesized that the possible
dimerization and/or polymerization of the vinylation products
might be the problem. However, the addition of various
commonly used radical inhibitors, such as PTZ (phenothiazine),
a Reactions were carried out with 5 (0.4 mmol), vinyltriethoxysilane (1.2
mmol), [Cp*RhCl2]2 (5 mol %), AgSbF6 (20 mol %), Cu(OAc)2 (0.8 mmol) and
LiF (0.8 mmol) in 2 mL DCE. b Isolated Yield.
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The success of the vinylation reaction of anilines led us to
extend the method developed to substrates bearing other DGs.
Benzamides were found to be compatible with this reaction,
affording the vinylation products in moderate yields (Table 4). A
broad range of important functional groups, such as
trifluoromethyl, halogens (F and Cl), and methoxy, were well
tolerated. The yield remains to be moderate regardless of
whether the benzamides bearing electron-withdrawing or
electron-donating groups. To our delight, ketones, generally
considered as weakly coordinating substrates, were also viable
(8j-8k).
To investigate mechanism of the rhodium(III)-catalyzed C-H
vinylation reaction, a series of experiments were conducted.
Intermolecular competition experiments between 3d and 3g
showed that electron-rich N-aryl-pyrrolidinone 3d was
transformed preferentially (Eq 1). To determine the
rate-determining step of the catalytic cycle, the deuterium kinetic
isotope effect was measured (eq 2). Therefore, [d5]-1 was
prepared and submitted to the standard conditions together with
1. A kH/kD value of 1.9 was observed, indicating that the cleavage
of the C-H bond might be involved as the rate-determining step.
Subsequently, to check the reversibility of the C-H activation step,
reactions of 1 with vinyltriethoxysilane were carried out in the
presence of MeOD as co-solvent, 75% deuterium incorporation
in [D]-1 and 50% deuterium incorporation in [D]-2a was observed,
suggesting that the C-H bond activation step is reversible (eq 3).
We also observed that partial deuterium incorporation was
detected on ethylene moiety of product [D]-2. The vinylation
product 2 was then subjected to the standard conditions with
MeOD as co-solvent, deuterium incorporation into the ethylene
moiety was also detected (eq 4).
Table 4 Substrates Scope: Benzamides and Aromatic Ketones a, b
a Reactions were carried out with 7 (0.4 mmol), vinyltriethoxysilane (1.2
mmol), [Cp*RhCl2]2 (5 mol %), AgSbF6 (20 mol %), Cu(OAc)2 (0.8 mmol) and
LiF (0.8 mmol) in 2 mL DCE. b Isolated Yield.
Furthermore, we found that N-acylindoles could be
synthesized through
Rh-catalyzed C-H vinylation of acetanilides
a
two-step sequence involving the
with
9
vinyltriethoxysilane followed by Pd-catalyzed cyclization (Table
5).[19]
Table 5 Synthesis of Indoles a, b
We further tried to monitor the reaction by GC-MS to identify
possible intermediates. A peak with mass value of 349 was
detected when the reaction proceeded for 1 hour, which was
identical to the Heck-type product 2b and further confirmed by 1H
and 13C NMR analysis (eq 5). The Heck-type product 2b could be
readily transferred to the vinylation product 2 under the standard
reaction conditions. These results indicated that 2b might be a
possible intermediate and an oxidative Heck coupling/desilylation
sequence might be involved.
a
Reaction Conditions. Step 1: 9 (0.4 mmol), vinyltriethoxysilane (1.2
mmol), [Cp*RhCl2]2 (5 mmol %), AgSbF6 (20 mmol %), Cu(OAc)2 (0.8 mmol)
and LiF (0.8 mmol) in 2 mL DCE. Step 2: PdCl2 (0.04 mmol), CuCl (0.4
mmol) and 1,3-propdiol (0.4 mmol) in 2 mL of DME. b Isolated Yield in two
steps.
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Acknowledgement
Financial support from the NSFC (21572201, 21772170), the
National Basic Research Program of China (2015CB856600), and
the Fundamental Research Funds for the Central Universities
(2018XZZX001-02) is gratefully acknowledged.
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access functionalized styrenes through direct Rh(III)-catalyzed
C−H vinylation. Vinyltriethoxysilane serves as an inexpensive and
convenient vinyl source for a range of substrates. Moreover, this
method can be applied to the two-step synthesis of functionalized
indoles. Mechanistic investigation reveals that the reaction
probably proceeds through an oxidative Heck/desilylation
pathway.
Experimental
To 20 mL Schlenck tube was added 1-phenyl-2-pyrrolidinone 1
(0.2 mmol, 1.0 equive), vinyltriethoxysilane (0.6 mmol, 3.0
equive), [Cp*RhCl2]2 (5 mmol %), AgSbF6 (20 mmol %), Cu(OAc)2
(0.4 mmol), LiF (0.4 mmol) and DCE (2 mL), the solution was
stirred at 120 °C for 36 h. After cooling to room temperature, H2O
(2 mL) was added and stirred for 5 min. The resulting mixture was
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chromatography to give the vinylation product 2.
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(The following will be filled in by the editorial staff)
Manuscript received: XXXX, 2018
Revised manuscript received: XXXX, 2018
Accepted manuscript online: XXXX, 2018
Version of record online: XXXX, 2018
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Entry for the Table of Contents
Page No.
Rhodium(III)-Catalyzed C–H Vinylation of
Arenes: Access to Functionalized Styrenes
An effective Rh(III)-catalyzed direct vinylation of arenes has been developed for the synthesinons
of functionalized styrenes, using vinyltriethoxysilane as a convenient and inexpensive vinyl
source. The reaction is compatible with a wide range of directing groups and various functional
groups.
J. Zhou, X. Li, G. Liao, B.-F. Shi*
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