Cp?Co(III)-catalyzed: Ortho-Amidation of azobenzenes with dioxazolones

Article abstract of DOI:10.1039/c7ob00540g

Cp?Co(iii)-catalyzed C-H amidation of azobenzene with dioxazolones has been developed. The amidation reaction does not require external oxidants and gives carbon dioxide as the only by-product. Both symmetrical and unsymmetrical azobenzenes were found to undergo amidation smoothly with broad functional group tolerance.

Full text of DOI:10.1039/c7ob00540g

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Takeharu Haino et al.
Solvent-induced emission of organogels based on tris(phenylisoxazolyl)
benzene
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Cp*Co(III)‐Catalyzed ortho‐Amidation of Azobenzenes with
Dioxazolones
Gongutri Borah,a,b Preetismita Boraha and Pitambar Patel*,a,b
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
www.rsc.org/
Cp*Co(III)‐catalyzed C–H amidation of azobenzene with on first row transition metal catalyzed C–H functionalization of
dioxazolones has ben developed. The amidation reaction does not azobenzenes are very rare.16b, 22 In recent years, majority of
require external oxidants and gives carbon dioxide as the only by‐ research has been focused on the employment Cp*Co(III)‐
product. Both symmetrical and unsymmetrical azobenzenes were catalyzed C–H activation because of its high natural
found to undergo amidation smoothly with broad functional abundance, and inexpensive nature.23 Following the
group tolerance.
pioneering work from Kanai/Matsunaga,24 on Co(III)‐catalyzed
C–H activation, many other groups including Ackermann,25
Glorius,26 Chang,27 Ellman28 and others29 have also contributed
significantly to further advancing this field on C–C and C–N
bond formation.
Over the past decades, azobenzene derivatives have attracted
a great deal of attention due to their extensive application in
various fields such as photochemical switches,1 molecular
machines,2 protein probes,3 organic dyes,4 nonlinear optical
devices,5 chemosensors,6 and polymers.7 As a consequence
much effort have been put for the efficient synthesis and
functionalization of azobenzene.8 Recently, transition metal
catalyzed ortho‐functionalization of azobenzenes using azo
group as a directing group have found considerable attention
for the synthesis of unsymmetrical azobenzenes.9‐19 In this
context, Fahey et al. reported the Palladium catalyzed first
ortho‐halogenations of azobenzenes.10 Following this,
numerous ortho‐functionalization of azobenzenes including
alkoxylation,11
phosphorylation,15
alkylation/cyclization18 and amidation/cyclization19 have been
widely investigated. Despite of these progresses, the ortho‐
amidation of azobenzenes has not been well explored.
Recently group of Lee, Xu and Jia independently demonstrated
Previous report on Amidation of azobenzene
(a) Rh(III)-catalyzed C H amidation with sulphonyl azide
N
N
cat. Rh(III)
N
N
R
N3
+
NH
R
(b) Rh(III)-catalyzed C H amidation with dioxazolone
O
N
N
cat. Rh(III)
N
N
O
O
R
acylation,12
halogenation,13
nitration,14
+
N
NH
addition,16
alkenylation/cyclization,17
O
R
This work
: Co(III)-catalyzed C H amidation of azobenzene
O
N
N
O
cat. Co(III)
O
R
N
N
+
N
H
NH
the C
−H amidation of azobenzenes with sulphonyl azides
employing Rh(III)‐catalytic system (Scheme 1a).20 Additionally,
Lee and Kim groups independently reported the Rh‐catalyzed
O
R
Scheme 1 Transition metal catalyzed ortho‐amidation of
azobenzene.
C
−H amidation of azobenzenes using dioxazolones as
coupling partner (Scheme 1b).21 However all the above ortho‐
functionalization of azobenzenes had been carried out using
precious transition metals such as Pd, or Rh, where as reports
Despite of these considerable advancement on Co(III)‐
catalyzed C–H activations, to the best of our knowledge Co(III)‐
catalyzed C–H amidation of azobenzene has not been realized
so far. In continuation to our transition metal catalyzed C–H
functionalizations,30 herein we report Cp*Co(III)‐catalyzed C–H
amidation of azobenzene with dioxazolones (Scheme 1c).31
a. Chemical Science & Technology Division, CSIR‐North East Institute of Science and
Technology, Jorhat‐785006, India. E‐mail: patel.pitambar@gmail.com.
b. Academy of Scientific and Innovative Research (AcSIR), CSIR‐North East Institute
of Science and Technology, Jorhat, India‐785006.
.
Electronic Supplementary Information (ESI) available: [Full experimental details and
NMR spectra of the synthesized products]. 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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We instigated our investigation of Co(III)‐catalyzed ortho‐
Journal Name
With the optimized conditions in hand, we next explored
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DOI: 10.1039/C7OB00540G
amidation of azobenzenes using (E)‐1,2‐diphenyldiazene 1a as the scope of azobenzenes in reaction with 3‐phenyl‐1,4,2‐
a model substrate and 3‐phenyl‐1,4,2‐dioxazol‐5‐one 2a as dioxazol‐5‐one 2a (Table 2). We were delighted to observe
amide source (Table 1). Reaction of azobenzene 1a with 2a in that the amidation took place efficiently irrespective of the
presence of Cp*Co(CO)I2 (5 mol %) and AgSbF6 (10 mol %) in position and electronic nature of substituents on azobenzene
2,2,2‐trifluoroethanol (TFE) at 110 °C did not afford the desired and furnished the corresponding amidated products in good to
amidated product 3a (entry 1, Table 1). Gratifyingly, addition high yields. In fact, symmetrical azobenzenes bearing alkyl (3b
of acetic acid as additive (1.0 equiv), lead to the formation of and 3c), alkoxy (3d), bromo (3e), chloro (3f) fluoro (3g),
mono amidated product 3a in excellent yield (entry 2). trifluoromethyl (3h) and ester (3i) groups at the para‐position
However, formation diamidated product
4 (5%) was also were found to be facile for the present amidation reactions.
observed in very little amount (see the Electronic Supporting Similarly, ortho‐substituted azobenzene underwent amidation
Information for details). Amongst different acid additives to furnish the desired amidated product (3j) in moderate yield.
screened, acetic acid was found to be most effective (entries However, amidation of azobenzene bearing a methyl group at
3
−
4). Reaction efficiency decreased on reducing the amount meta‐position gave a mixture of regioisomers 3k‐i and 3k‐ii in
of additive (entry 5). Screening of various silver salts revealed 78% overall yield. Nevertheless, formation of diamidated
AgSbF6 as the best choice (entry 6 8). As expected, both product was observed in trace amount in most of the cases.
Cp*Co(CO)I2 and AgSbF6 were found to be essential for the
reaction (entries 9
10). Decreasing the reaction temperature Table 2 Scope of symmetrical azobenzenea
−
−
also resulted in a reduced yield of the product 3a (entry 11).
Role of solvent was found to be crucial for the reaction, as use
of other solvents result in decreased product yields (entries
12
−14). Replacing the cobalt catalyst with pre‐generated
cationic Cp*Co(III) complex resulted in slight decrease in the
product yield (entry 15).
Table 1 Optimization studiesa
entry [Ag] salt
additive
‐‐
solvent
TFE
TFE
TFE
TFE
TFE
TFE
TFE
TFE
TFE
TFE
TFE
Toluene
EtOH
DCE
TFE
yield (%)b
<5
AgSbF6
1
2
3
AgSbF6
AgSbF6
AgSbF6
AgSbF6
AgOAc
AgBF4
AgOTf
‐‐
AcOH
PivOH
PhCO2H
AcOH
AcOH
AcOH
AcOH
AcOH
AcOH
AcOH
AcOH
AcOH
AcOH
AcOH
85 (82)c
80
4
65
60
<10
65
20
<5
<5
25
<5
5d
6
7
8
9
aReaction conditions:
1 (0.1 mmol), 2a (1.1 equiv.), Cp*Co(CO)I2 (5.0
mol%), AgSbF6 (10.0 mol%), and AcOH (1.0 equiv) in TFE (1 mL) at 110
°C for 6 h. Isolated yields are given.
10e
11f
12
13
14
15g
AgSbF6
AgSbF6
AgSbF6
AgSbF6
AgSbF6
‐‐
Subsequently, amidation of symmetrical azobenzene was
next investigated under optimized cobalt catalytic conditions
(Table 3). Amidation of unsymmetrical azobenzenes bearing
both electron‐donating (3l) as well as electron‐withdrawing
groups (3m) were smoothly amidated to give a mixture of
<5
15
70
aReaction conditions: 1a (0.10 mmol, 1.0 equiv), 2a (0.11 mmol, 1.1 isomeric product. It was observed that the amidation reaction
equiv), Cp*Co(CO)I2 (5.0 mol %), Ag‐salt (10.0 mol %) and additive (1.0 was comparatively more favorable on the electron‐rich
equiv) in solvent (1 mL) at 110 °C for 6 h. b1H NMR yield (CH2Br2 as aromatic ring.
c
d
internal standard). Isolated yield in parentheses. 0.5 Equiv of AcOH
After successful exploration of substrate scope for
was used. eReaction without Co‐catalyst. fReaction at 80 °C. azobenzenes, we next examine the scope of various
g[Cp*Co(CH3CN)3](SbF6)2 was used as catalyst.
dioxazolones with azobenzene 1a under standard reaction
conditions (Table 4). Dioxazolones bearing both electron‐
donating as well as electron‐withdrawing aryl substituents
2 | J. Name., 2012, 00, 1‐3
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were found to couple efficiently with 1a to give the aReaction conditions: 1a (0.1 mmol), 2 (1.1 equiv.), Cp*Co(CO)I2 (5.0
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DOI: 10.1039/C7OB00540G
corresponding amidated product 5a–5e in high yields. mol%), AgSbF6 (10.0 mol%), and AcOH (1.0 equiv) in TFE (1 mL) at 110
Similarly, polycyclic dioxazolone 3‐(naphthalen‐1‐yl)‐1,4,2‐ °C for 6 h. Isolated yields are given.
dioxazol‐5‐one was found to smoothly react with 1a to furnish
the amidated product 5f in good yield. The dioxazolone
Next, we were inquisitive to see the selectivity of
derived from heterocycles was also well compatible to give the diamidation reaction of azobenzenes on the two aryl rings
desired amidated product 5g in 80 % yield. Furthermore, after the first amidation. When 1a was subjected for amidation
amidation with aliphatic dioxazolones was also found to be with 3 equivalent of 2a under the standard reaction
facile to give the corresponding amidated product 5i in 67 % conditions, gave the diamidated product
yield. However, amidation with 3‐benzyl‐1,4,2‐dioxazol‐5‐one mono amidated product 3a (Scheme 2a). It is important to
4 (51%) along with
was found to be sluggish.
note that the second amidation occur exclusively on the other
phenyl ring, which is in contrast to the Rh‐catalytic system
where both amidation occurred on the same phenyl ring.21
Similarly, when 3a was treated with 2a under optimum
conditions, the amidation reaction proceed smoothly,
Table 3 Amidation of unsymmetrical azobenzenea
providing the corresponding diamidated azobenzenes
yield (Scheme 2).
4 in 63%
aReaction conditions:
1 (0.1 mmol), 2a (1.1 equiv), Cp*Co(CO)I2 (5.0
mol%), AgSbF6 (10.0 mol%), and AcOH (1.0 equiv) in TFE (1 mL) at 110
°C for 6 h. Isolated yields are given.
Table 4 Scope of dioxazolonea
Scheme 2 Diamidation of azobenzene
In order to shed light on the plausible mechanism of
amidation reaction, several controlled experiments were
carried out (Scheme 3). Deuterium exchange experiments of
1b using CD3OD as co‐solvent under the standard cobalt
catalytic system in absence of 2a did not show a significant
amount of H/D scrambling at the ortho‐positions, suggesting
an irreversible C
−H activation process (Scheme 3a). A
relatively low KIE value was observed both in parallel reaction
(kH/kD = 1.3, Scheme 3b) as well as in intramolecular
competitive reaction (kH/kD = 1.4, Scheme 3c), indicating that
the C−H bond cleavage is not involved in the rate determining
step. Finally, the C–H amidation of 1a performed in presence
of radical scavenger gave the desired amidated product 3a
with only a minor loss in reaction yield (Scheme 3d). This result
ruled out the possible involvement of a radical‐based
mechanism.
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Cp*Co(CO)I2 to AgSbF6 and AcOH provides the active cationic
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I, which coordinates to azobenzene 1a
DOI: 10.1039/C7OB005a40nGd
Co(III) species
further undergoes C–H activation to afford a five‐membered
cobalt metallacycle II Subsequent coordination of
a) Deuterium scrambling
.
Me
D/H
Me
H/D
standard conditions
MeOD (100 mL), 2h
dioxazolones 2a with II generates intermediate III, which upon
decarboxylation followed by migratory insertion delivers the
six‐membered amido species IV. Finally, proto‐decobaltation
of IV furnish the desired product 3a along with regeneration of
the active Co(III) species.
N
N
N
N
H/D
Me
Me
H/D
1b-dn
(<10% D)
1b
b) Intermolecular parallel reactions
H5/D5
N
H5/D5
N
N
N
2a (1.0 equiv)
Conclusions
D4/H4
standard conditions
D5/H5
NH
In summary, we have successfully disclosed an efficient Co(III)‐
catalyzed C–H amidation of azobenzenes with 1,4,2‐dioxazol‐5‐
ones. The reaction does not require any external oxidant, and
generates gaseous carbondioxide as the only side product.
More significantly, this reaction represents the first example of
cobalt‐catalyzed C–H amidation of azobenzenes.
2 h
O
Ph
KIE = 1.3
1a or 1a-d10
3a or 3a-d9
c) Intramolecular competitive reaction
D5/H5
N
N
2a (1.0 equiv)
standard conditions
N
D4/H4
N
D5
NH
2 h
KIE = 1.4
O
Ph
1a-d5
3a-d5 or 3a-d4
Acknowledgements
d) Amidation in presence of redical scavenger
We thank DST‐INSPIRE Faculty program for fellowship and funding
(GAP‐0738). We also thank Dr. D. Ramaiah, Director, CSIR‐NEIST,
Jorhat, for his support and providing all the facility. We also thank
Dr. Arup Roy, scientist, CSTD, CSIR‐NEIST, for his helpful discussion.
N
N
N
N
standard conditions
TEMPO (1.0 equiv)
2a
+
NH
1a-d5
O
Ph
3a
(55%)
Notes and references
Scheme 3 Preliminary mechanistic investigation
1
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Cp*Co(CO)I2 + AgSbF6
AcOH
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Me
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Me
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Scheme 4 Plausible catalytic cycle
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