Cobalt(III)-Catalyzed C?H Activation: Counter Anion Triggered Desilylative Direct ortho-Vinylation of Secondary Benzamides

Article abstract of DOI:10.1002/adsc.201800649

A Co(III)-catalyzed counter anion triggered desilylative direct ortho-vinylation of secondary benzamides is reported. The reaction furnishes the alkenylated product, exclusively, and no formation of the possible cyclic products was observed. Mechanistic studies suggest that the counter anion, [SbF₆]^?, plays a crucial role in the desilylation. The utility of alkynylsilanes instead of terminal alkynes turned out to be a potential and practical approach to obtain the corresponding vinylated products. The developed methodology is compatible with a variety of functional groups. (Figure presented.).

Full text of DOI:10.1002/adsc.201800649

Title: Cobalt(III)–Catalyzed C–H Activation: Counter Anion Triggered
Desilylative Direct ortho-Vinylation of Secondary Benzamides
Authors: Nachimuthu Muniraj and Kandikere Prabhu
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To be cited as: Adv. Synth. Catal. 10.1002/adsc.201800649
Link to VoR: http://dx.doi.org/10.1002/adsc.201800649

10.1002/adsc.201800649
Advanced Synthesis & Catalysis
FULL PAPER
DOI: 10.1002/adsc.201((will be filled in by the editorial staff))
Cobalt(III)–Catalyzed C–H Activation: Counter Anion
Triggered Desilylative Direct ortho-Vinylation of Secondary
Benzamides
Nachimuthu Muniraja and Kandikere Ramaiah Prabhua,*
a Department of Organic chemistry, Indian Institute of Science, Bangalore 560 012, Karnataka, India. E-mail:
prabhu@ iisc.ac.in
Received: ((will be filled in by the editorial staff))
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201######.((Please
delete if not appropriate))
Abstract. A Co(III)-catalyzed counter anion triggered desilylative direct ortho-vinylation of secondary benzamides is
reported. The reaction furnishes the alkenylated product, exclusively, and no formation of the possible cyclic products was
observed. Mechanistic studies suggest that the counter anion, [SbF6]-, plays a crucial role in the desilylation. The utility of
alkynylsilanes instead of terminal alkynes turned out to be a potential and practical approach to obtain the corresponding
vinylated products. The developed methodology is compatible with a variety of functional groups.
Keywords: Cobalt; amides; alkynylsilane; desilylation; alkenylation; C-H activation
Introduction
Scheme 1. Direct vinylation of secondary benzamides
Alkene is an important functional group that can
easily be transformed into a variety of functional
groups. Development of a direct alkenylation method
employing directing group strategy is attractive as
well as atom-economical.[1] Alkenylation is achieved
by employing either alkene[2] or alkyne[3] as coupling
partner using high-valent metal catalysts. In these
alkenylation reactions, use of alkynes as coupling
partners offers a major advantage as alkynes can
undergo a redox neutral process which does not
require external oxidant. There are many reports on
the use of internal alkynes to obtain internal
alkenes,[3] whereas the utility of terminal alkynes for
similar reactions are limited.[4] Generally, terminal
alkynes are less compatible with many of the C-H
activation reactions.[3] Yu and Cheng reported the
ortho-alkenylation of 2-phenyl pyridine derivatives
using terminal alkynes as coupling partners.[4a] The
amide is a weak coordinating group, which has been
well explored as a directing group in C-H activation
reactions such as alkenylation,[2,3] annulations,[5]
etc.[6] Besides, secondary benzamides have a free-NH
group, which are well known to form cyclized
products in the directed alkenylation reactions with
alkynes.[5] However, under similar reaction
conditions, it is challenging to terminate the reaction
without cyclization to yield exclusively the olefin
derivatives. Traditional vinylation methods of
benzamides involve a multistep sequence of Wittig
reaction followed by coupling the acid with an amine
(Scheme 1a). Vinyl acetate, vinyl carboxylic acid or
vinyl stannane have been employed for the direct
vinylation reactions of benzamides in the presence of
various directing groups, which invariably require an
external oxidant or the presence of an oxidizing
directing group.[7] We developed the idea of using
TMS-acetylene as a precursor for the direct
vinylation of secondary benzamides under redox
1
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10.1002/adsc.201800649
Advanced Synthesis & Catalysis
neutral conditions. To our delight, under the reaction
conditions, the reaction of TMS acetylene with
benzamide afforded the corresponding styrene
derivative (Scheme 1b). Interestingly, the -NH group
of the amide remains intact under the reaction
conditions and the possible cyclic products were not
observed in the reaction (Scheme 1c).
of 1a with various silyl acetylenes such as
(triisopropylsilyl)acetylene
butyldimethylsilyl)acetylene
(2b),
(tert-
(2c),
(triethylsilyl)acetylene (2d), and bis-(trimethylsilyl)
acetylene (2e) was examined. The reaction of 1a with
2b did not furnish the product 3 (entry 2, Table 1).
The reaction of 1a with 2c furnished the product 3 in
55% yield along with divinylated product 3’ in 8%
(entry 3). The reaction of 1a with 2d and 2e furnished
the product 3 in 20 and 40% yields, respectively
(entries 4 and 5). Replacing the methyl group of
amide 1a with isopropyl or phenyl group did not
bring any significant change in the yield of the
Further, we examined the scope of the reaction
employing (arylalkynyl)silanes, which are found to
be better precursors than the terminal alkynes for
obtaining the corresponding disubstituted alkenes.
Moreover, desilylation of (arylalkynyl)silanes is one
of the easiest ways to obtain terminal alkynes.[8] Thus, reaction (entries 6 and 7). Performing the reaction 1a
in continuation of our efforts,[9] herein we report an
air-stable, abundant, and cost effective[10] Co(III)-
catalyst for the desilylative direct ortho-vinylation of
secondary benzamides, in which the counter anion,
SbF6- plays a crucial role in the desilylation.
with 2c at 60 C furnished the product 3 in a trace
amount, and the corresponding mono- and di-
activated vinylsilanes 4ac and 4ac’ were obtained in
54, and 8% yields, respectively (entry 8 and see SI,
Table S-5). Efforts to improve the yields of vinylated
product 3 or/and vinylsilane 4 by varying reaction
conditions were unsuccessful (see the SI for more
details).
o
Results and Discussion
Optimization of the reaction started by
examining the reaction of N-methyl benzamide 1a
(0.3 mmol) with TMS acetylene 2a (0.36 mmol). The
reaction in the presence of Cp*Co(CO)I2 (5 mol%) as
a catalyst, AgSbF6 (20 mol%) as an activator and
AdCO2H (1 equiv) as an additive in DCE (2 mL) at
120 oC for 3 h furnished the corresponding
alkenylated product 3 in 48% yield along with the
Using the optimal conditions (entry 3, Table 1), the
scope of the reaction with benzamide derivatives has
been examined (Scheme 2). Thus, 4-Me, 3-Me, and
4-OMe substituted N-methyl benzamides afforded the
corresponding vinylated products 3ba, 3ca, and 3da
in 54, 30 and 58% yields, respectively. In the case of
3-methyl-substituted benzamide, we observed the
vinylation at the sterically free ortho position, and the
dialkenylated product 3’ in 5% yield (entry 1, Table 1, other regioisomer was not observed. It is worth
and also see SI, Table SI-1). Further investigation of
various solvents, activators, and additives (see SI,
Table S-1, S-2, and S-3) revealed that the reaction
proceeds well in DCE (2 mL) as a solvent in the
presence of AgSbF6 (20 mol%) and AdCO2H (1
equiv). During the optimization studies, the reaction
noting that the other uncommon regioisomer was
observed by Hamilton and Whiteoak in their reaction
of 3-fluorobenzamide with methyl vinyl ketone.[11]
Halogenated N-methylbenzamides underwent smooth
Scheme 2. Substrate scope for vinylation of amides[a]
Table 1. Optimization studies[a]
NMR yield (%) [b]
entry
1
2
3
4
R1
Me
Me
Me
Me
Me
iPr
Ph
2
3
3'
5
4
4'
nd
nd
nd
nd
nd
nd
nd
8
2a
2b
2c
2d
2e
2c
2c
50
nd
55
20
40
22
30
nd
nd
nd
nd
nd
nd
nd
54
nd
8
traces
4
5
6
traces
traces
nd
7
8[c]
Me
2c traces
[a]
Reaction conditions: 1 (0.3 mmol), 2 (0.36 mmol),
[Cp*Co(CO)I2] (5 mol%), AgSbF6 (20 mol%), AdCO2H (1
o
[b]
equiv), DCE (2 mL), at 120 C for 3 h. Measured by 1H
[a] Under standard conditions. [b] 2 equiv of 1 and 1 equiv of
2 was used.
[c]
NMR with terephthalaldehyde as an internal standard.
At 60 oC for 12 h. nd=not detected
2
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10.1002/adsc.201800649
Advanced Synthesis & Catalysis
[a] Under standard conditions. [b] 3 mmol scale.
reaction furnishing the products 3ea, 3fa, and 3ga in
moderate yields (56, 52, and 50% yields,
respectively). The reaction of N-methyl-4-vinyl
benzamide with 2a furnished the product 3ha in 53%
yield. Substituting methyl group at the ortho-position
of 1a furnished the product 3ia in a trace amount,
which might be due to the steric factor. Thiophene
derived amide, a heterocyclic amide, afforded the
product 3ja in 36% yield. Additionally, we have
examined the reaction of terminal alkynes such as
phenylacetylene, 1-pentyne, 1-heptyne, 1-octyne, and
5-cyano-1-pentyne with 1a under a slightly modified
Benzamides substituted with vinyl, and an electron-
donating methoxy group at the para-position also
reacted with 6a affording the products 7ia-7ka in
moderate yields. 2-Napthamide and isopropyl
protected benzamide furnished the products 7la and
7ma in 87 and 83% yields, respectively. Additionally,
heterocyclic amides such as thiophene and
benzofuran derived amides displayed
a
good
reactivity with the acetylene 6a to furnish the
products 7na-7pa in 60, 88, and 18% yields,
respectively.
4-Acetamido-N-methylbenzamide
reaction
conditions,
which
delivered
the
reacted with 6a, selectively at the ortho-position of
the amide and furnished the corresponding product
7qa in 34% yield. Interestingly, subjecting the
tertiary benzamide such as N,N-dimethybenzamide
under the optimal conditions, furnished the exo-
alkenylated product 7ra in 46% yield which may be
due to the steric crowding on nitrogen. A scale-up
experiment has been performed in 3 mmol scale to
showcase the efficacy of the reaction, which afforded
the product 7aa in 73% yield.
corresponding alkenylated products 5aa, 5ab, 5ac,
5ad, and 5ae in 68, 66, 60, 63, and 15% yields,
respectively (Scheme 2). However, in all the cases,
the unreacted N-methyl benzamide was recovered.
As most of the phenylacetylene derivatives are
obtained
from
the
desilylation
of
(arylalkynyl)silanes,[8] we have turned our attention
to explore the scope of the reaction of amide 1a with
(phenylalkynyl)silane 6a (Scheme 3). Thus, the
reaction of 1-phenyl-2-trimethylsilylacetylene 6a
with the amide 1a under optimal conditions (Table 1,
entry 3) furnished the corresponding alkenylated
product 7aa in 76% isolated yield along with the
diactivated product 7aa’ in 13% yield (see SI, Table
S-6). The reaction of 6a with benzamide having
methyl and halogen substitution at meta- and para-
position provided the corresponding alkenylated
products 7aa-7fa in good to excellent yields, whereas
a methyl substitution at the ortho-position of 1a
under optimal conditions afforded the product 7da in
a trace amount. Benzamide derivatives having
substitution with electron-withdrawing groups such
as NO2 and CF3 at the para-position underwent
smooth reactions and gave the desired products 7ga
and 7ha in good yields.
The scope of the reaction was further extended
by reacting (arylalkynyl)silane derivatives with N-
methyl benzamide 1a (Scheme 4). Thus, the reaction
of 1a with (arylalkynyl)silanes possessing
alkyl/halogen substitution on the aromatic ring
proceeded well under the optimal conditions
furnishing the products 8ab-8af in good to excellent
yields.
(Arylalkynyl)silanes
that
contained
synthetically useful functional groups such as
aldehyde, ketone, ester, and cyano groups are also
well tolerated and afforded the products 8ag-8aj in
good to moderate yields. However, the thiophene-
derived alkynylsilane furnished the corresponding
cyclic product 8ak in 40% yield which might be due
to the presence of an electron-rich heteroaryl ring that
can lead to an acid promoted cyclization. Reaction of
(arylalkynyl)silane (6l) with 4-methyl substitution on
Scheme 3. Scope of alkenylation of secondary benzamide
derivatives[a]
Scheme 4. Scope of alkynylsilane derivatives[a]
3
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10.1002/adsc.201800649
Advanced Synthesis & Catalysis
[a] Under standard conditions. [b] 2 equiv of 1a, 1 equiv of 6
[c]
[d]
and 20 mol% of AdCO2H was used.
NMR yield.
Inseparable mixture along with 1a.
the aromatic ring with 1a furnished the corresponding
alkenylated product 8al along with the cyclic product
8al’ in 33 and 40% yield, respectively. However, the
yield of 8al was improved to 70% by using 2 equiv of
1a and 20 mol% of AdCO2H (Scheme 4). This may
be due to the reduced amount of acid additive present
in the reaction medium. The reaction of an alkyl
group substituted alkynylsilane also furnished the
corresponding alkenylated product 8am in 38%.
Scheme 5. Control experiments
(arylalkynyl)silane
that
contained
electron-
withdrawing group such as a nitro group on aryl ring
furnished the product 8an in 32% yield (1H NMR
yield) as an inseparable mixture along with 1a.
To probe the reaction mechanism, a few control
experiments were carried out. Variation of the
temperature indicated that the desilylation requires a
o
high temperature (Scheme 5a). The reaction at 80 C
furnished 20% of 3aa and 30% of 4ac indicating that
4ac can be the intermediate in the reaction (Scheme
5a). Further, when the reaction was performed for
only 15 min under optimal conditions, the products
3aa and 4ac were obtained in 5 and 20% yields,
respectively (Scheme 5b). This reaction emphasizes
that 4ac is the probable intermediate. To find the
reagent that is responsible for the desilylation, a
reaction of 4ac with AgSbF6 and AdCO2H was
performed, which furnished 3aa in 98% yield.
Surprisingly, the same reaction furnished the product
3aa even in the absence of AdCO2H without affecting
the yield of 3aa (Scheme 5c). This experiment
clarifies that there is no role of the acid in the
desilylation. Further screening of different silver salts
has been carried out (Scheme 5d). Thus, the reaction
of 4ac with AgNO3 or AgOAc did not furnish the
corresponding desilylated product 3aa, whereas the
similar reaction of 4ac with AgBF4 furnished the
corresponding desilylated product 3aa in 98% yield.
These experiments revealed that fluoride source is
necessary for the desilylation. This observation was
further confirmed by the reaction of 4ac with
Cp*Co(CO)I2, which did not furnish the product 3aa,
involvement of AdCO2H in the protodemetallation.
Performing the same experiment in the presence of
D2O (1 equiv) under the optimal conditions furnished
the product with 23% deuteration at the terminal
carbon, which further confirmed the involvement of
moisture during desilyation step (Scheme 5h).
while
the
reaction
of
4ac
with
[Cp*Co(CH3CN)3][SbF6]2 furnished the product 3aa
in 98% yield (Scheme 5e). From these experiments,
we conclude that the counter anion plays a crucial
role in the desilylation of 4ac. The desilylation was
also observed even in the absence of AdCO2H
(Scheme 5c), indicating that the protodesilylation is
occurring with the help of moisture present in the
medium. To ensure this observation, 4ac was reacted
with AgSbF6 in the presence of D2O (1 drop), which
led to the deuteration at the terminal carbon (60%)
confirming our assumption (Scheme 5f). Treating 1a
with 2c under the optimal conditions with CD3COOD,
instead of AdCO2H, resulted in the formation of the
product with 33% of deuteration at the α-position of
the olefinic carbon and, 7 and 10% of terminal carbon
of the olefin (Scheme 5g). This reaction suggests the
Based on the control experiments, previous
studies,[4f] and literature precedence,[9,11] a plausible
mechanism has been proposed (Scheme 6). The in
situ generated catalytically active species A reacts
with 1a leading to the cobaltacycle B. Further
insertion of alkyne 2c to B forms the intermediate C.
Subsequently,
AdCO2H
promotes
the
protodemetallation of C forming the corresponding
vinylsilane 4ac and regenerating the active catalyst A.
Finally, 4ac undergoes desilylation in the presence of
-
the counter anion SbF6 , which is present in the
medium, furnishing the product 3aa.
4
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10.1002/adsc.201800649
Advanced Synthesis & Catalysis
In
a
8-mL screw-cap reaction vial, N-methyl
benzamide
derivative
(0.3
mmol),
(tert-
butyldimethylsilyl)acetylene
or
1-phenyl-2-
trimethylsilylacetylene derivatives (0.36 mmol), cobalt
catalyst (Cp*CoCOI2, 7.1 mg, 5 mol%), AdCOOH (54
mg, 1 equiv), AgSbF6 (20.6 mg, 20 mol%) in DCE (2
mL) were taken. The vial was sealed with a screw cap
and placed in a pre-heated metal block at 120 °C, and
the reaction mixture was stirred at the same
temperature for 3 h. After completion of the reaction
(monitored by TLC), the reaction mixture was cooled
to room temperature and concentrated under vacuum.
The crude products were purified on a silica gel
column using EtOAc/petroleum ether mixture.
Scheme 6. Plausible Mechanism
Note: Compounds 7ma and 8ai were purified by using
EtOAc/DCM mixture instead of EtOAc/petroleum
ether mixture.
Experimental
procedure
for
synthesizing vinylsilane (4ac)
In
a 8-mL screw-cap reaction vial, N-methyl
benzamide derivative (40.5 mg, 0.3 mmol), (tert-
butyldimethylsilyl)acetylene (50.4 mg, 0.36 mmol),
cobalt catalyst (Cp*CoCOI2, 7.1 mg, 5 mol%),
AdCOOH (54 mg, 1 equiv), AgSbF6 (20.6 mg, 20
mol%) in DCE (2 mL) were taken. The vial was sealed
with a screw cap and placed in a pre-heated metal
block at 60 °C, and the reaction mixture was stirred at
the same temperature for 12 h. After completion of the
reaction (monitored by TLC), the reaction mixture was
cooled to room temperature and concentrated under
vacuum. The crude products were purified on a silica
gel column using EtOAc/petroleum ether mixture.
Conclusion
In conclusion, we have developed a Co(III)-
catalyzed desilylative direct ortho-vinylation of N-
methyl benzamide derivatives using alkynyllsilane as
a coupling partner. The salient feature of the reaction
is the exclusive formation of the alkenylated product
rather than the cyclic products. The alkynylsilane was
found to be an excellent alternative for terminal
alkynes. The mechanistic studies revealed that the
-
counter anion, SbF6 , is playing a crucial role in
desilylation, which is a novel and unprecedented
observation.
Acknowledgments
This work was supported by, SERB (EMR/2016/006358),
New-Delhi, CSIR (No. 02(0226)15/EMR-II), Indian Institute
of Science, and R L Fine Chem, Bangalore. N. M.. thanks, UGC,
New-Delhi for a fellowship.
Experimental Section
General information
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10.1002/adsc.201800649
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