Cobalt(III)?Catalyzed C?H Activation: A Secondary Amide Directed Decarboxylative Functionalization of Alkynyl Carboxylic Acids Wherein Amide NH-group Remains Unreactive

Article abstract of DOI:10.1002/adsc.201701406

A Co(III)-catalyzed C?H activation reaction for ortho-alkenylation of benzamides (aryl/heteroaryl) and C2-alkenylation of indole derivatives have been developed using alkynyl carboxylic acid as an alkene source. A high regioselectivity has been achieved i

Full text of DOI:10.1002/adsc.201701406

Title: Cobalt(III)–Catalyzed C–H Activation: A Secondary Amide
Directed Decarboxylative Functionalization of Alkynyl Carboxylic
Acids Wherein Amide NH–group Remains Unreactive
Authors: Nachimuthu Muniraj and Kandikere Prabhu
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Advanced Synthesis & Catalysis
10.1002/adsc.201701406
COMMUNICATION
DOI: 10.1002/adsc.201((will be filled in by the editorial staff))
Cobalt(III)–Catalyzed C–H Activation: A Secondary Amide
Directed Decarboxylative Functionalization of Alkynyl
Carboxylic Acids Wherein Amide NH–group Remains
Unreactive
Nachimuthu Muniraj and Kandikere Ramaiah Prabhu*
a
Department of Organic chemistry, Indian Institute of Science, Bangalore 560 012, Karnataka, India. E-mail:
prabhu@orgchem.iisc.ernet.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######
Abstract. A Co(III)-catalyzed C-H activation reaction
for ortho-alkenylation of benzamides (aryl/heteroaryl)
and C2-alkenylation of indole derivatives have been
developed using alkynyl carboxylic acid as an alkene
source. A high regioselectivity has been achieved in the
formation of disubstituted alkenes, and the possible
cyclic products were not observed. This efficient
alkenylation shows a broad range of substrate scope with
a good functional group tolerance. The application of the
methodology has been showcased by transforming an
benzamide is a reactive directing group, which
participates in the reaction leading to the formation of
a cyclic product. Therefore, obtaining an olefin
derivative, exclusively, in the reaction of a secondary
benzamide with alkyne derivatives without forming
the cyclic product is a challenging task, which offers
the advantage of modifying the olefins into a variety
of functional groups. Nevertheless, coupling the
terminal alkynes is one of the strategies for
synthesizing disubstituted olefin derivatives using
directing group strategy. For example, high-valent
Co-catalysts were used for the alkenylation of indoles,
alkenylated amide to
a
3-hydroxy isoindolinone
derivative.
[13]
pyrroles, 6-aryl purine derivatives, etc.
Whereas
Keywords: Cobalt; amides; alkynyl carboxylic acid;
alkenylation; C-H activation
low-valent Co-catalysts are known to furnish the
[13]
corresponding annulated products. Compared to
Co-Catalyzed C-H activation strategies are emerging
as powerful methods for C-H functionalization,
which are on par with other precious transition metal
catalyzed reactions using Ru-, Rh-, Ir-catalysts. The
ortho-alkenylation of benzamides using directing
group strategy has been achieved using either alkenes
Scheme 1. Comparision with previous work
[1]
(
oxidative process) or alkynes (redox neutral process)
[2-5]
as coupling partners.
In the oxidative process, the
insertion of alkenes followed by β-hydride
elimination leads to the alkenylation of benzamides.
The reaction of benzamides with alkynes involves
either
alkenylation/protonation
or
alkenylation/annulation, which depends on the nature
of the amide (secondary or tertiary) employed in the
reaction. Alkenylation of tertiary benzamides with
alkynes has been accomplished using either Ru- or
Rh-catalysts.[ Due to the unavailability of a free
NH-group, these reactions proceed through an alkyne
insertion followed by the protodemetallation
furnishing the corresponding alkenylated product. A
similar reaction with an amide that has a free NH-
group, most of the time, leads to an annulated product
6,7]
[8]
[9]
[10]
[11a]
[11b]
in the presence of Pd, Ru, Rh, Re,
Ni,
catalysts (Scheme 1). In the reaction of
secondary benzamide with alkynes, the NH-group of
or
Co[
12]
1
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Advanced Synthesis & Catalysis
10.1002/adsc.201701406
the terminal alkynes, it is advantageous to use alkynyl
carboxylic acids as coupling partners as alkynyl
screening of solvents, additives, silver salts, catalyst
loading and temperature did not show any remarkable
improvement in the yield of 3aa (see the SI for
detailed optimization studies).
carboxylic acids are bench stable and easy to
synthesize.[
14a,b]
Besides, most of the aryl acetylene
derivatives are obtained either from the deprotection
of aryl silyl acetylenes or the decarboxylation of the
Table 1. Optimization studies[a]
[14c]
corresponding aryl alkynyl carboxylic acids.
Zhang[
15a]
and Song
[15b]
have reported the sequential
decarboxylative alkynylation followed by annulation
3
2
of sp and sp C-H bonds catalyzed by low-valent
metal catalysts such as Cu(II)- and Co(II)-catalysts
using bidentate directing group (Scheme 1). Recently,
Ellman’s group has used tertiary amide as a directing
group for Co(III)-catalyzed three-component
entry
1a
2a
Additive
NMR yield
[16]
[b]
transformation to obtain alkenyl halides.
This
(equiv)
(equiv)
(20 mol%)
(%)
method relies on thiophene/furan derived amides. In
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1.5
2
2
1.5
1.5
1.5
1.5
1
1
1
1
1
NaOAc
AcOH
PivOH
AdCOOH
AdCOOH
AdCOOH
AdCOOH
KOAc
24
33
24
45
54
[17]
continuation of our efforts,
we have employed
alkynyl carboxylic acid as a source of an alkene, for
the first time, in the presence of high-valent metal
catalytic system. Thus, herein we report a Co(III)-
catalyzed, amide-directed, alkenylation of secondary
benzamides using alkynyl carboxylic acid as an
alkene source, in which the NH-group of the amide
does not facilitate the cyclization.
67
78 (72)[c]
20
43
54
The optimization studies were started by
examining the reaction of N-methyl benzamide 1a
2
CsOAc
NaOPiv
1
0
2
1
(
0.3 mmol) with phenylpropiolic acid 2a (0.45 mmol).
11
12
2
2
1
1
AdCOOH
AdCOOH
AdCOOH
AdCOOH
AdCOOH
none
78[
d]
The reaction of 1a with 2a in the presence of
60[e]
Cp*Co(CO)I
2
(5 mol%) as a catalyst, AgSbF
6
(20
mol%) as an activator and NaOAc (20 mol%) as an
49[f]
1
1
1
1
1
3
4
5
6
7
2
2
2
2
1
1
1
1
o
additive in DCE (2 mL) at 100 C for 1 h furnished
[g]
nd
the corresponding alkenylated product 3aa in 24%
yield (Table 1, entry 1). Changing the additive to
AcOH or PivOH has no significant bearing on the
outcome of the reaction (entries 2 and 3). AdCOOH
as an additive was recently employed in related
hydroarylation reactions.[ Thereby, using AdCOOH
as an additive has resulted in improving the yield of
the product 3aa to 45% (entry 4). Further screening
with 1 equiv of both 1a and 2a furnished the desired
product 3aa in 54% (entry 5). Finally, increasing the
amount of 2a to 1.5 and 2 equiv increased the yield of
[
h]
nd
nd
68[
i]
2
1
AdCOOH
[
a]
Reaction conditions: 1a (XX mmol), 2a (YY mmol),
[Cp*Co(CO)I ] (5 mol%), [AgSbF ] (20 mol%), additive
18]
2
6
o
[b] 1
(20 mol%), DCE (2 mL), at 100 C for 1 h.
yield (using terephthaldehyde as an internal standard).
H NMR
[
c]
[
d]
o
[e]
Isolated yield.
Reaction was performed at 80 C.
[Cp*Co(CH CN) ][SbFb
[Cp*Co(CO)I
AgSbF . nd= not detected. AdCOOH= Adamantane-1-
Reaction was performed at 120 C.
o
[f]
10 mol% of
3
3
6
]
2
was used instead of
[
g]
[h]
3
aa to 67 and 78%, respectively (entries 6 and 7).
2
]. Absence of Co-catalyst. Absence of
Switching the acid additive to KOAc, CsOAc, and
6
[
i]
NaOPiv did not help in improving the yield of 3aa
carboxylic acid. Phenylacetylene instead of 2a.
(
entries 8, 9, and 10). There was no considerable
improvement in the yield when the reaction was
With the optimal conditions (entry 7, Table 1), the
scope of the alkenylation reaction was evaluated
(Scheme 2). The reaction of N-methylbenzamide, N,
o
o
performed at 120 C or 80 C (entries 11 and 12).
Using cationic cobalt complex,
Cp*Co(CH CN) ][SbF (10 mol%), furnished the
product 3aa in 49% yield (entry 13). Reactions in the
absence of [Cp*Co(CO)I ] or AgSbF or AdCOOH
[
3
3
6
]
2
4-dimethylbenzamide,
and
4-methoxy-N-
methylbenzamide with phenylpropiolic acid 2a under
the optimal reaction conditions furnished the
corresponding alkenylated products 3aa, 3ba, and
3ca in good yields (72, 67 and 69%, respectively).
Halogen substituted benzamides such as 4-F, 4-Br,
and 4-I substituted N-methyl benzamide derivatives
afforded the corresponding products 3da, 3ea, and
3fa in good yields (66, 72, and 77%, respectively).
2
6
did not provide the product 3aa (entries 14-16).
Performing the reaction with phenyl acetylene instead
of phenylpropiolic acid (2a) afforded the product 3aa
in 68% yield (entry 17). As aryl acetylene derivatives
are obtained either from the deprotection of aryl silyl
acetylenes or from the decarboxylation of aryl
alkynyl carboxylic acids,[
14]
and the reaction of
Electron-withdrawing groups such as 4-CF
NO -substituted benzamide derivatives
and 4-
also
3
phenylpropiolic acid yielded a better result than
phenyl acetylene, the scope of the reaction has been
explored using arylalkynyl carboxylic acids. Further
2
underwent smooth reaction leading to the
corresponding alkenylated products 3ga and 3ha in
2
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Advanced Synthesis & Catalysis
10.1002/adsc.201701406
6
4 and 65% yields, respectively. 3,4,5-Trimethoxy-
performing the reaction of several derivatives of
propiolic acids such as 3-(4-cyanophenyl)propiolic
acid, 3-(4-(tert-butyl)phenyl)propiolic acid, 3-(4-
and 4-vinyl-substituted N-methyl benzamides
furnished the products 3ia and 3ja in moderate to
bromophenyl)propiolic
acid,
3-(4-
Scheme 2. Scope for benzamide derivatives[a,b]
(ethoxycarbonyl)phenyl)propiolic acid, and 3-(4-
acetylphenyl)propiolic acid with 1a afforded the
corresponding alkenylated products 4ag–4ak in
moderate to good yields. Additionally, substituted
benzamides such as N,4-dimethylbenzamide, and 4-
iodo-N-methylbenzamide reacted well with 3-(4-
bromophenyl)propiolic acid and 3-(p-tolyl)propiolic
acid, respectively, furnishing the products 4al and
4am in 81% and 68%, respectively.
However, 4-
methoxyphenylpropiolic acid under the optimal
reaction conditions failed to afford the corresponding
alkenylated product but the corresponding cyclic
product 4ag was obtained in 19% yield. Alkyl
substituted alkynyl carboxylic acids were also found
to be effective substrates. Thus, the reaction of but-2-
ynoic acid and hex-2-ynoic acid with 1a furnished
their corresponding alkenylated products 4ah and 4ai
in good yields (76 and 66%, respectively).
Scheme 3. Scope for alkynylcarboxylic acids[a,b]
[
a]
Reaction conditions: 1 (0.6 mmol), 2a (0.3 mmol),
2
Cp*Co(CO)I ] (5 mol%), [AgSbF ] (20 mol%), AdCO H
[
2
6
o
[b]
(
20 mol%), DCE (2 mL), at 100 C for 1 h. Measured by
H NMR with terephthalaldehyde as the internal standard.
1
Isolated yields are in parentheses.
good yields (53 and 65%, respectively). N,2-
Dimethylbenzamide afforded the corresponding
product 3ka in a trace amount, which may be
attributed to the steric factors. The reaction of N,3-
dimethylbenzamide, which has a sterically free ortho-
position, afforded the corresponding alkenylated
product 3la in 69% yield. N-Methyl-2-napthamide
and N-isopropyl benzamides furnished the desired
products 3ma and 3na in good yields (75 and 70%,
respectively). The reaction of benzofuran-derived
amide furnished the corresponding alkenylated
product (3oa) in low yield, whereas thiophene-
derived amides furnished their corresponding
alkenylated products 3pa and 3qa in 69 and 79%
yields, respectively. A scale up experiment of
phenylpropiolic acid 2a (3.42 mmol, 500 mg) with 1a
under optimal reaction conditions afforded the
corresponding product 3aa in 64% yield.
[a]
Reaction conditions: 1a (0.6 mmol), 2 (0.3 mmol),
[
2 6 2
Cp*Co(CO)I ] (5 mol%), [AgSbF ] (20 mol%), AdCO H
o
[b]
(
20 mol%), DCE (2 mL), at 100 C for 1 h. Measured by
1
H NMR with terephthalaldehyde as the internal standard.
Isolated yields are in parentheses.
After successfully demonstrating the scope of the
alkenylation reaction of benzamide derivatives with
alkynyl carboxylic acid derivatives, we turned our
attention to the scope of the reaction with indole
systems (Scheme 4). In the initial experiments, the
reaction of N,N-dimethyl-1H-indole-1-carboxamide
with phenylpropiolic acid (2a) under the optimal
reaction conditions found to be less effective (17%,
see the SI). As it is well-known that 2-pyrimidinyl
After examining the scope of the reaction with a
variety
of
benzamide
derivatives
with
phenylpropiolic acid, an investigation was undertaken
to explore the scope of the alkenylation reaction with
a few alkynyl carboxylic acid derivatives (Scheme 3).
Thus, 1-naphthyl, 4-methyl, 2-methyl, 4-fluoro and 3-
chloro substituted phenylpropiolic acid derivatives
underwent a facile reaction with N-methyl benzamide
[1],[19]
group is compatible with Co-catalyzed reactions,
we employed N-pyrimidyl indole as a substrate. The
reaction of N-pyrimidyl indole with phenylpropiolic
acid 2a under slightly modified conditions (NaOAc
was used instead of AdCOOH see SI) afforded the
corresponding 2-alkenylated indole derivative in
good yields. Thus, the reaction of N-pyrimidyl indole
(
1a) furnishing the corresponding alkenylated
products 4ab, 4ac, 4ad, 4ae, and 4af in good yields
70, 72, 69, 69, and 74%, respectively). Further,
(
3
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Advanced Synthesis & Catalysis
10.1002/adsc.201701406
and its derivatives such as 5-methoxy indole, 5-
bromo indole, and methyl indole-6-carboxylate with
performing
the
same
experiment
with
phenylacetylene instead of 2a, 15% of deuterium
incorporation was observed in the one of the olefinic
carbon (Scheme 6e). These reactions suggest the
involvement of the acid additive in the
protodemetallation step.
phenylpropiolic
acid
(2a)
furnished
their
corresponding alkenylated products 6aa, 6ba, 6ca,
and 6da in excellent yields (98, 92, 96, and 80%,
respectively, Scheme 4).
Scheme 4. Substrate scope for indole derivatives[a,b]
Scheme 6. Mechanistic investigation
[
a]
Reaction conditions: 5 (0.2 mmol), 2a (0.3 mmol),
Cp*Co(CO)I ] (5 mol%), [AgSbF ] (20 mol%), NaOAc
20 mol%), DCE (2 mL), at 100 C for 1 h.
[
(
2
6
o
[b]
Isolated
yields.
Further, to demonstrate the usefulness of the
alkenylated product, an ozonolysis experiment was
performed on 3aa, which furnished the 3-hydroxy
isoindolinone derivative 7 in 98% yield (Scheme 4).
Next, the pyrimidinyl group was successfully
deprotected by treating 6aa with NaOEt/DMSO to
obtain 8 in 60% yield (Scheme 5).[20]
Scheme 5. Synthetic utility and removal of directing
group
we presumed that phenylpropiolic acid (2a) could
form silver phenylacetylide 9, in the presence of
AgSbF
6
and can lead to the corresponding
To probe the reaction mechanism, a few control
experiments were performed (Scheme 6). In the
alkenylated product. Therefore, we performed the
reaction of 1a with 9, which failed to give the product
reaction of 1a with D O, under optimal reaction
2
3
aa, indicating that the silver phenylacetylide 9 may
conditions, the ortho-hydrogens of benzamide were
not deuterated (Scheme 5a). The reaction of deuterio-
not be an intermediate in this reaction (Scheme 6f).
Further to check whether the reaction is proceeding
through a radical pathway, we have performed a
reaction of benzamide (1a) with phenyl propionic
acid (2a) in the presence of BHT and TEMPO under
the optimal reaction conditions. These reactions
furnished the product 3aa in 70 and 65% yields,
respectively, which rules out the radical pathway
1
a with 2a under the optimal reaction conditions did
not furnish the alkenylated product (Scheme 6b).
Further, experiments with deuterio-1a (2 equiv) with
2a (1 equiv) under the optimal reaction conditions,
the product deuterio-3aa was isolated in 65% yield
Scheme 6c). No deuterium/proton exchange was
(
observed either in the product deuterio-3aa or in the
recovered starting material. The lack of scrambling is
not common in Cp*Co(III) catalytic system, which
has been also observed by Ellman.[16] Further, the
(
Scheme 6g). A competitive experiment using
phenylacetylene and 4-methylphenyl propiolic acid
with benzamide (1a) provided the corresponding
alkenylated products in a ratio of 1:1 (Scheme 6h).
reaction of 1a with 2a in the presence of CD COOD,
3
Based on the control experiments, and the
under the optimal reaction conditions, deuterium
incorporation was observed on the olefinic carbon of
[6,7,13,14]
literature precedence,
a plausible mechanism
has been proposed in Scheme 7. The reaction of
3aa (10 and 20% D, Scheme 6d). Additionally,
Cp*Co(CO)I
2
with AdCOOH and AgSbF
6
forms the
4
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Advanced Synthesis & Catalysis
10.1002/adsc.201701406
catalytically active species A, which reacts with 1a
forming a cobaltocycle B and AdCOOH. An alkyne
insertion to the species B leads to the formation of 7-
membered intermediate C, which undergoes
decarboxylation forming the species D. Subsequently,
AdCOOH promotes the protodemetallation thereby
furnishing the product 3aa along with active catalyst
A. Alternatively, phenyl acetylene, which is
generated from 2a, can react with the species B
furnishing the product 3aa. Further work is underway
to understand the mechanism of this reaction.
In a 8-mL screw cap reaction vial, N-Pyrimidinyl
indole derivatives (0.2 mmol), alkynyl carboxylic acid
(0.24 mmol), cobalt catalyst (4.76 mg, 5 mol%),
6
NaOAc (3.28 mg, 20 mol%), AgSbF (13.7 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 100 °C and the reaction mixture was stirred at
the same temperature for 1h. 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 7. Proposed mechanism
Acknowledgements
This work was supported by, SERB (NO.SB/S1/OC-56/2013),
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.
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10.1002/adsc.201701406
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Advanced Synthesis & Catalysis
10.1002/adsc.201701406
COMMUNICATION
Cobalt(III)–Catalyzed C–H Activation:
A
Secondary Amide Directed Decarboxylative
Functionalization of Alkynyl Carboxylic Acids
Wherein Amide NH–group Remains Unreactive
Adv. Synth. Catal. Year, Volume, Page – Page
Nachimuthu Muniraj and Kandikere Ramaiah
Prabhu*
7
This article is protected by copyright. All rights reserved.