Cobalt-catalyzed aryl C-H activation and highly regioselective intermolecular annulation of sulfonamides with allenes

Article abstract of DOI:10.1039/c6cc08622e

Herein we describe a cobalt-catalyzed C-H activation of aryl and heteroaryl sulfonamides and their intermolecular heteroannulation reaction with allenes, providing a convergent strategy for the synthesis of biologically interesting heterocyclic scaffolds. Carbometallation of allenes proceeds selectively through a Co-alkenyl pathway for a wide range of electron-poor and electron-rich allenes.

Full text of DOI:10.1039/c6cc08622e

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Marilyn M. Olmstead, Alan L. Balch, Josep M. Poblet, Luis Echegoyenet al.
Reactivity differences of Sc3N@C2n (2n 68 and 80). Synthesis of the
first methanofullerene derivatives of Sc N@D5h-C80
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Cobalt-Catalyzed Aryl C−H Activation and Highly Regioselective Intermolecular
Annulation of Sulfonamides with Allenes
Neetipalli Thrimurtulu, Rajender Nallagonda and Chandra M. R. Volla*
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
www.rsc.org/
Although allenes have been widely employed in metal-catalyzed
addition reactions,8 insertion reactions with allenes via C-H bond
functionalization were relatively rare.9 Insertion of the
organometallic intermediate at C2 or C1 of the allene leads to
either M-π-allyl M1 or M-alkenyl intermediate M2 respectively
(Scheme 1a).10 The resulting organometallic intermediate can
undergo different types of reactions such as protonation, cyclization
ABSTRACT: Herein we describe a cobalt-catalyzed C−H activation of
aryl and heteroaryl sulfonamides and their intermolecular
heteroannulation reaction with allenes, providing convergent
a
strategy for the synthesis of biologically interesting heterocyclic
scaffolds. Carbometallation of allenes proceeds selectively through a
Co-alkenyl pathway for a wide range of electron-poor and electron-
rich allenes.
or β-elimination. Rh(I)-catalyzed C-H functionalization of ketemines
with terminal olefins was illustrated by Cramer and co-workers
proceeding through M-π-allyl intermediate M1 (Scheme 1a, path
A).11 Ma and co-workers demonstrated Rh(III)-catalyzed
Sulfonamides and their cyclic analogues, sultams, are very
important structural motifs for drug discovery because of their
extensive chemical and biological activities.1 In particular,
benzofused sultams are present in many drugs and pharmaceuticals
with a wide range of medicinal properties as exemplified by the
activities displayed by Ampiroxicam, Brinzolamide and Calpain I
inhibitor (Fig 1).2 Transition-metal catalyzed C−H activation
reactions gained considerable attention in the last few decades for
promoting a diverse range of annulation reactions for synthesizing a
variety of heterocyclic scaffolds.3,4 Although excellent progress was
made in metal-catalyzed directed C-H activation to facilitate a wide
range of directing groups, only a few studies have been reported
wherein easily available and medicinally important sulfonamide
derivatives were used as ortho directing groups for sp2 C−H
activation.5 Cramer in 2012 disclosed an efficient Rh(III)-catalyzed
strategy for the synthesis of benzosultams by the ortho C−H
activation of N-acetylated sulfonamides.6 Among other interesting
C−H functionalization methods for the synthesis of benzosultams
include the Ni(0)-catalyzed denitrogenative annulation reaction of
allenes with 1,2,3,4-benzothiatriazine-1,1(2H)-dioxides developed
by Murakami.7
protonation and β-elimination reactions of M-alkenyl species M2
leading to allylation and allenylation of benzamides (Scheme 1a,
path B).12 Similarly, Krische and co-workers reported cationic Ir-
catalyzed prenylation reactions.13a However, a cyclization pathway
leading to biologically interesting moieties remained unexplored for
the M-alkenyl intermediates M2.
a) Overview of the work :
Ar
NH2
R1
DG
M = Rh(I)
Path A
R
cyclization
Cramer et al.
M-π-allyl
DG
M
R1
R
R2
M-cat.
O
M1
+
OMe
R1
R1
R2
Protonation
Ma et al.
N
H
•
DG
R
R2
M = Rh(III)
Path B
M-alkenyl
O
R1
M
β
-elimination
Ma et al.
OMe
R2
N
M2
H
•
O
O
R1
TMS
Q
S
Me
O
O
O
O
N
b) Present work :
H
S
Et
O
O
S
H2N
S
S
O
N
N
N
OCH3
N
O
O
O
H
O
O
O
S
R2
O
N
S
Q
Co
O
O
S
H
Q
+
N
N
N
R
H
NHEt
R
•
R
R1
OCO2Et
H
R2
Ph
Ampiroxicam
COX-II inhibitor
Inhibitor of Calpain I
Brinzolamide
carbonic anhydrase inhibitor
R1
R1
R2
Co
Fig. 1: Biologically active sultams
Aryl C−H activation
Co-alkenyl
Excellent regioselectivity
Good functional group tolerance
One-Pot Synthesis
Q =
N
Scheme 1: Overview of the work
The use of earth-abundant, first-row transition metals for
catalyzing various C−H functionalization reactions has emerged as a
promising alternative to noble metals.14 In particular, cobalt-salts
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have proven to be exceptionally versatile for promoting these
transformations because of their unique reactivity.15 In a pioneering
study, Daugulis and co-workers developed an efficient cobalt(II)-
the benzamide C−H activation (20 mol % Co(acac)2, 1.0 equiv.
Mn(OAc)3.2H2O, 2.0 equiv. NaOPiv in TFE at rt). Only traces of the
product were formed under these conditions. However, increasing
o
catalyzed bidentate chelation strategy for the C−H activation of the temperature to 100 C from rt led to the selective formation of
benzamides and phosphinic amides.16
We have recently reported a cobalt-catalyzed heteroannulation elimination from a seven membered cobaltacycle was faster than
benzosultam 3a in 32% yield indicating that the reductive
reaction of arylamides and alkenylamides with allenes under mild
conditions to access various isoquinolinone and pyridone
derivatives.17 In view of the great potential of aryl fused sultam
scaffolds for biological and pharmaceutical studies, we envisaged
developing a direct and straight forward method for synthesizing
these derivatives, and herein, we report our studies on the
regioselective Co-catalyzed intermolecular annulation of
arylsulfonamides and allenes (Scheme 1b). Based on the prior
success of allene interaction with organometallic intermediates,
arylsulfonamide 1 was employed as the substrate. We expected
that the bidentate chelator, 8-aminoquinoline pioneered by
Daugulis would have the prerequisites for stabilizing the
metallacycle intermediate for successful allene insertion.
the protonation or β-elimination. We are also pleased to observe
that the reaction displayed a high regioselectivity for the annulation
proceeding through the Co-alkenyl intermediate. This is in line with
our previous observation that the carbometallation of electron-
poor allenes takes places through
a
M-alkenyl pathway.13a
Encouraged by these initial results, we screened different cobalt
salts to improve the yield. Indeed, the desired reactivity was
achieved when 20 mol % Co(OAc)2 was used along with 2.0 equiv. of
0
Mn(OAc)3.2H2O and 2.0 equiv. NaOPiv in TFE at 100 C and 3a was
isolated in 91% yield (see Supporting Information for more details).
The regioselectivity of 3a was further confirmed by X-ray
crystallography. When 10 mol % of Co(OAc)2 was used instead of 20
mol %, the reaction took longer for completion and the yield
decreased to 45%. Omission of either cobalt salt or oxidant
completely shuts down the reactivity, illustrating the importance of
the combination.
Table 1: Scope of sulfonamides with allenylphosphonatea
Co(OAc)2 (20 mol% )
Mn(OAc)3.2H2O (2 equiv )
O
O
O
O
S
Q
S
Q
N
H
N
O
•
O
Ph
NaOPiv (2 equiv )
TFE(2 mL), 1000 C, air
R
R
Table 2: Synthesis of benzosultams from different allenesa
+
P
P
Ph
Ph
Ph
2a
3
1, 0.4 mmol
Co(OAc)2 (20 mol% )
Mn(OAc)3.2H2O (2 equiv )
O
O
O
O
R2
O
O
S
Q
S
Q
O
O
N
N
S
Q
R
NaOPiv (2 equiv )
TFE(2 mL), 1000 C, air
•
R
H
+
S
Q
R1
N
O
N
O
R1
P
2
P
Ph
1, 0.4 mmol
R2
Q
3
Me
Ph
Ph
Ph
O
O
O
O
O
3b, 90%
O
3a, 91%
CCDC 1504459
S
Q
S
Q
S
N
O
N
O
N
O
O
O
O
O
O O
P
Me
Me
Ph
Me
OEt
OBn
S
Q
S
Q
S
Q
Ph
N
O
N
O
N
O
P
Me
P
Me
P
Me
Me
3a, 91%
3q, 60%
3r, 76%
Me
Ph
Ph
Ph
Ph
Ph
Ph
Me
Me
O
S
O
3c, 82%
3d, 84%
3e, 86%
Q
O
O
S
N
O
Me
Me
Q
Me
N
O
OBn
F
O
O
O
S
O
O
O
MeO
OBn
S
Q
Q
S
Q
N
O
N
O
3t, 77%
3s, 80%
N
O
P
P
MeO
Ph
F
Ph
P
Cl
Ph
Ph
Ph
Ph
O
Ph
O
O
O
S
O
O
S
Q
3g, 60%
S
Q
Q
3f, 78%
3h, 85%
N
O
N
O
N
O
Me
Me
Me
O
O
MeO
OBn
Me
3u, 48%b
OEt
OBn
O
O
S
Q
N
O
S
Q
Me
Me
N
O
Me
3v, 41%c
Me
P
F3C
P
3w, 48%c
Ph
Br
Ph
Ph
O
O
3i, 68%
3j, 73%
O
O
S
Q
CCDC 1504464
N
S
Q
N
O
MeO
Me
O
O
O
O
O
Me
3y, 35%d
Me
O
O
S
S
Q
S
Q
N
O
N
O
Q
3x, 38%d
CCDC 1504460
N
O
P
P
Ph
F3CO
Ph
P
O
O
S
Ph
Ph
O
S
O
O
S
O
AcO
Ph
Ph
Q
Q
Q
3m, 40%
3k, 72%
3l, 62%
N
N
N
Me
Me
Ph
Me
Me
Me
3ab, trace
O
O
O
O
O
O
3z, 24%e (70%)f
S
Q
3aa, 22 % (64%)f
S
Q
S
N
O
S
Q
N
O
P
N
O
P
P
Ph
Ph
aReaction conditions: 1 (0.4 mmol), allene 2 (0.8 mmol), Co(OAc)2.4H2O (20
mol %), Mn(OAc)3.2H2O (0.8 mmol), NaOPiv.H2O (0.8 mmol), TFE (4 mL), 36-
48 h, 100 0C, isolated yield, b2:1 regioisomers, c4:1 regioisomers, d5:1
Ph
Ph
Ph
Ph
3n, 59%
3o, 76%b
3p, 80%
aReaction conditions: 1 (0.4 mmol), allene 2a (0.48 mmol), Co(OAc)2.4H2O
(20 mol %), Mn(OAc)3.2H2O (0.8 mmol), NaOPiv.H2O (0.8 mmol), TFE (4 mL),
36-48 h, 100 0C, yield of the isolated product, b5:1 regioisomers
regioisomers, e10:1 regioisomers, brsm yield.
f
With these optimized conditions in hand, we probed the
substrate scope of the arylsulfonamides 1 with allenylphosphonate
2a. As shown in Table 1, different functional groups on the
arylsulfonamide were tolerated under the standard conditions.
For the initial investigation, we chose tosylsulfonamide 1a and
allenylphosphonate 2a as the coupling partners, and the reaction
was tested under the conditions which we developed previously for
2 | J. Name., 2012, 00, 1-3
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Several valuable functional groups such as methoxy (3f), fluoro (3g),
The efficiency of this protocol was further tested by performing
chloro (3h), bromo (3i), trifluoro (3j), trifluoromethoxy (3k), and a gram-scale reaction to obtain 3i in 61% yield (Scheme 2). The
acetyl (3l) are compatible at different position of 1. In addition, this structure and the regioselectivity of bromoaryl sultam 3i were
strategy was further extended to 4-biphenyl and 1-naphthyl confirmed unambiguously by the X-ray crystallography.
sulfonamides to obtain the corresponding products 3m and 3n in
good yields (40-59%). When 2-naphthylsulfonamide was used, a
mixture of 5:1 regioisomers was isolated in 76% yield. In addition,
heterocyclic derivatives such as thiophenesulfonamide (3p) was
also found to be compatible. Unfortunately, 1,1-disubstituted
allenylphosphonates (see Supporting Information) failed to undergo
the reaction probably because of the increased steric hindrance.
We further tested the scope of different allenes for the cobalt-
catalyzed heteroannulation of arylsulfonamides (Table 2). Under
the optimized conditions, ethyl and benzyl 2,3-butadienoates
served to provide the expected arylsultams (3q-3t) in good to
excellent yields as a single regioisomeric product. The reaction
occurs exclusively at the less substituted double bond of the allene.
To our delight, 1,3-disubstituted allenoate provided the
corresponding products 3u-3w in 41-48% (mixture of regioisomers
Scheme 3: Intermolecular competitive experiments
arising from 1,3-hydrogen shift, vide infra Scheme 4). Intriguingly,
changing the allene partner from electron poor to electron rich
allene such as methoxyallene also led to the selective formation of
benzosultams 3x and 3y in 38% and 35% respectively. The structure
and regioselectivity of the 3x was confirmed by X-ray
crystallography. Reaction of sulfonamide 1a with phenylallene
afforded 3z with the same regioselectivity in 24% yield. Pleasingly,
1,1-dimethylallene under the same reaction conditions furnished
the heteroannulated sultam 3aa without 1,3-hydrogen shift in 22%
yield. Surprisingly, no cyclic π-allyl product was observed in these
cases.
The mechanism of the reaction was studied utilizing
intermolecular competition experiments. When a mixture of 1:1 4-
methoxy and 4-trifluoromethyl benzene sulfonamides 1f and 1j was
treated with allenylphosphonate 2a under the standard conditions,
a 1.2:1 ratio of the products was obtained suggesting that
electrophilic cobaltation is unlikely (Scheme 3a). To acquire further
understanding, another intermolecular competitive experiment was
carried out using a 1:1 mixture of allenylphosphonate 2a and
methoxyallene under standard conditions with p-tert-
butylbenzenesulfonamide derivative 1e. A ratio of 4.4:1 was
observed, indicating that the migratory insertion of the arylcobalt
species proceeds faster with electron-poor allenes (Scheme 3b).
Table 3: One-pot sequential protocola
aYield of the isolated product after column chromatography.
To highlight the synthetic applicability of this method further,
we set out to develop a one-pot protocol for the sequential
amidation of arylsulfonyl chlorides followed by cobalt-catalyzed
C−H activation and heteroannulation with allenes. Thus,
arylsulfonyl chlorides 4 were treated with 8-aminoquinoline 5
utilizing Et3N in CH2Cl2 and upon completion of the reaction, the
mixture was concentrated to dryness. Subjection of the crude
sulfonamide 1 to optimized conditions furnished the corresponding
sultams 3 in comparable yields (Table 3).
Scheme 4: Proposed reaction mechanism
Based on the above experiments and previous reports on Co-
catalyzed C−H bond activation,
a plausible mechanism was
proposed (Scheme 4). The catalytic cycle begins with the in situ
generation of Co(III)-species by the oxidation of Co(II) precatalyst.
Coordination of 8-aminoquinoline derived sulfonamide 1 to Co(III)
forms the intermediate A,13a which upon base assisted C−H
activation leads to the key metallacycle Co(III) intermediate B. The
formation of the intermediate B can be proposed based on ESI-MS
(See Supporting Information). The key Co(III)-metalacycle
Scheme 2: Gram scale synthesis of sultam
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8. M. Jeganmohan and C.-H. Cheng, Chem. Commun. 2008, 3101-
3117.
intermediate was observed by treating stoichiometric amounts of
Co(OAc)2 with arylsulfonamide 1e. Coordinative insertion of C-Co
bond of intermediate C with the less-substituted double bond of
allene affords seven membered cobaltacycle intermediate D.
Reductive elimination from D gives the cyclic product E and
regenerates the Co(III) species after oxidation of Co(I) with oxidant.
Intermediate E undergoes 1,3-hydrogen shift to provide the
corresponding final product 3.
In summary, we developed a novel cobalt(II)-catalyzed chelation
assisted ortho C−H activation of sulfonamides to furnish the
biologically relevant aryl and heteroaryl fused sultams in a highly
regioselective manner. The reaction proceeds in air under
operationally simple conditions with inexpensive cobalt-salts.
Notable features of this transformation include high regioselectivity
for the intermolecular annulation proceeding through Co-alkenyl
pathway and excellent functional group compatibility. The reaction
conditions are amenable to scale up and further development of a
one-pot three-component protocol illustrates the practicality of the
method.
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Acknowledgement
This activity is supported by DST (EMR/2015/002047, funding to
CMRV). Financial supports were received from DST Fast Track
Scheme (N.T.)
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