Cp*Co(III)-Catalyzed o-Amidation of Benzaldehydes with Dioxazolones Using Transient Directing Group Strategy

Article abstract of DOI:10.1002/adsc.201901267

Transition metal-catalyzed ortho-selective C(sp²)?H amidation of weakly coordinating aldehydes remains limited to precious metals such as Ir, Rh, Ru, etc. Herein, we put forward a novel report on ortho-amidation of benzaldehydes employing user-

Full text of DOI:10.1002/adsc.201901267

Title: Cp*Co(III)-Catalyzed o-Amidation of Benzaldehydes with
Dioxazolones Using Transient Directing Group Strategy
Authors: Bhuttu Khan, Vikas Dwivedi, and Basker Sundararaju
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10.1002/adsc.201901267
Advanced Synthesis & Catalysis
UPDATE
DOI: 10.1002/adsc.201((will be filled in by the editorial staff))
Cp*Co(III)-Catalyzed o-Amidation of Benzaldehydes with
Dioxazolones Using Transient Directing Group Strategy
Bhuttu Khan,a Vikas Dwivedi,a and Basker Sundararaju,a,*
a
Department of Chemistry, Indian Institute of Technology Kanpur, Kanpur, Uttar Pradesh, India – 208 016.
Email: basker@iitk.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######.
catalysis, which not only take care of the cost but also
Abstract. Transition metal-catalyzed ortho-selective
C(sp2)H amidation of weakly coordinating aldehydes amenable to environmentally benign conditions.
remains limited to precious metals such as Ir, Rh, Ru, etc.
Herein, we put forward a novel report on ortho-amidation
of benzaldehydes employing user-friendly dioxazolones
under cost-effective and air-stable Cp*Co(III) catalysis.
This catalytic transformation involves transient directing
group strategy to enhance the ligating capability of weakly
chelating aldehydes.
Keywords: Aldehydes; amidation; Co-catalysis; transient
directing group, dioxazolones
Scheme 1. Transition metal-catalyzed amidation of
benzaldehydes and its surrogates.
Directing group (DG) assisted CH bond activation
involves a wide range of coordinating structural
In this regard, Cp*Co(III) catalysts[10] have been
elements with strongly or weakly chelating ability
utilized for C-H bond amidation by Chang[11] and
depending upon the ligating atoms.[1] Functional
later by Jiao,[12] Ackermann,[13] Li,[14] Matsunaga,[15]
groups containing oxygen for chelation are
Sundararaju[16] and others.[17] Interestingly, Dong[7c]
considered to be weakly chelating in comparison to
and co-workers reported Cp*Rh(III)-catalyzed
that of nitrogen, sulfur and phosphorus.[2] Aldehyde,
amidation via in situ generation of a traceless
being an easily transformable functional group,
directing group through combination of aldehydes
shows high potential for medicinally important
and dioxazolones. Additionally, Zhang[7d] reported
prototypes as well as synthetic methodologies.[3]
an elegant H2O promoted amidation of benzaldehydes
Despite the fact that formyl group holds a unique
under Cp*Rh catalysis. To the best of our knowledge,
reactivity, its exploration in directed C-H bond
ortho-amidation of aldehydes under first row
activations is limited owing to its weak chelation
transition metal catalysis has not been realized yet.[18]
ability and sensitive nature.[4] In this context, the
Our continuous interest in exploring C-H bond
transient directing group (TDG) strategy[1f,5] has
functionalization using weakly chelating directing
emerged as an excellent alternative for aldehydes
groups under base metal catalysis[19] prompted us to
develop an efficient and sustainable procedure for the
since the pioneering work by Jun and co-workers.[6]
synthesis of 2-aminobenzaldehydes. Towards this
However, transition metal-catalyzed amidation of
goal, herein, we disclose the cobalt-catalyzed ortho-
benzaldehydes is still limited to precious metals such
selective C(sp2)-H amidation of benzaldehydes with
as Rh,[7] Ru,[8] and Ir.[9] For example, Wang,[7e]
dioxazolones as an easily accessible amidating
Kim,[9a] and Cui[9c] reported Cp*Rh(III) or Ir(III)-
reagent[11b,20] using Cp*Co(III) as catalyst.
catalyzed ortho-amidation of aldehydes using
aldimines or nitrones as ‘preinstalled directing
We began our optimization studies by treating
groups’. In addition to that, Shi,[5c] Prabhu,[7a] Jiao,[7b]
benzaldehyde 1a with dioxazolones 2a (1.2 equiv.) in
Rasheed,[8] and He[9b] came up with the ‘transient
the presence of Cp*Co(CO)I2 (10 mol%) along with
directing group strategy (TDG)’ employing Rh, Ir
AgSbF6 (30 mol%) and aniline (20 mol%) under
and Ru(II) complexes for variety of C-H bond
argon in 2,2,2-trifluoroethanol (TFE) (0.2 M) at
functionalizations. From
sustainable point of view, there is great demand for
having an improved procedure under base metal
a
cost-effective and
80 °C for 24 h. The expected ortho-amidated product
3aa was isolated in 45% isolated yield (entry 1, Table
1
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10.1002/adsc.201901267
Advanced Synthesis & Catalysis
1). Next, we screened various amines as the transient
directing group; among them aniline was found to be
as trifluoroacetic acid (TFA) and CH3COOH were
proved to be inefficient (entries 12-13). Increasing
more suitable for the high-product formation (see ESI, the temperature up to 120 °C could offer the amidated
Table-S2). Changing the solvent to 1,2-
dichloroethane (DCE) and elevating the temperature
to 100 °C led to the formation of 3aa in 56% yield
(entries 2-3). Solvents such as acetonitrile, t-AmOH
were also examined but none of them worked better
than DCE (entries 4-5).
product in 61% yield (entry 14). Excellent yield of
3aa was obtained when the loading of aniline
increased to 60 mol% and the reaction time was
reduced to 12 h (entries 15-17, for more details see
ESI). Further decreasing of the loading of the catalyst
as well as silver salt led to reduction in the isolated
yield (entry 18-20, Table 1). No product formation
occurred when aniline was not employed as transient
directing group (entry 21). Further, control
experiments showed that catalyst, AgSbF6 and
additive are important to obtain the expected ortho-
amidated product in high yield (see ESI, Table-S1).
Table 1. Reaction optimization and control experimentsa
Temp.
(°C)
80
Time Yield
Entry Catalyst
Solvent
Additive
(h)
24
24
24
24
(%) b
1
2
3
4
Cp*Co(CO)I2 TFE
Cp*Co(CO)I2 DCE
-
-
-
-
45
80
48
Cp*Co(CO)I2 DCE
100
56
Cp*Co(CO)I2 CH3CN
100
NR
t-
Cp*Co(CO)I2
AmOH
5
100
-
24
26
6
[Cp*CoI2]2
CoCl2.6H2O
Co(acac)2
DCE
TFE
TFE
100
80
-
-
24
24
24
24
24
24
24
NR
NR
NR
59
20
10
25
35
61
69
74
91
38
38
51
NR
7
8
80
-
9
Cp*Co(CO)I2 DCE
Cp*Co(CO)I2 DCE
Cp*Co(CO)I2 DCE
Cp*Co(CO)I2 DCE
Cp*Co(CO)I2 DCE
Cp*Co(CO)I2 DCE
Cp*Co(CO)I2 DCE
Cp*Co(CO)I2 DCE
Cp*Co(CO)I2 DCE
Cp*Co(CO)I2 DCE
Cp*Co(CO)I2 DCE
Cp*Co(CO)I2 DCE
Cp*Co(CO)I2 DCE
100
100
100
100
100
120
120
120
120
120
120
120
100
NaOPiv
PhCOONa
NaOAc
TFA
10
11
12
13
14
15c
16d
17d
18d,e
19d,e
20f
21g
CH3COOH 24
NaOPiv
NaOPiv
NaOPiv
NaOPiv
NaOPiv
NaOPiv
NaOPiv
-
24
24
24
12
12
24
12
24
a
Reaction conditions: benzaldehyde (1a, 0.20 mmol),
aniline (20 mol%), catalyst (10 mol%), AgSbF6 (30 mol%),
3-phenyl-1,4,2-dioxazol-5-one (2a, 1.2 equiv.), additive
(20 mol%), solvent (1.0 mL), heated at given temperature
under Argon atmosphere, hydrolyzed the reaction mixture
with 3 N HCl. bIsolated yield. cAniline (40 mol%).
Scheme 2. Reaction Scope.a
a
f
dAniline (60 mol%). eCatalyst (5 mol%). AgSbF6 (20
Reaction conditions: benzaldehyde (1a-1s, 0.20 mmol),
mol%). gWithout aniline.
dioxazolone (2, 1.2 equiv), aniline (60 mol%),
Cp*Co(CO)I2 (10 mol%), AgSbF6 (30 mol%) and NaOPiv
(20 mol%) in DCE at 120 °C for 12 h under argon
b
c
atmosphere. Isolated yield. 3-(trifluoromethyl)aniline (60
mol%).
Other cobalt precursors did not afford the amidated
product, however no product formation observed with
Cp*CoI2 dimer; this is unexpected and unexplainable
at this stage (entries 6-8). In order to improve the
efficiency of the product formation, a series of
carboxylate additives such as NaOPiv, PhCO2Na,
NaOAc were tested and among them NaOPiv offered
satisfactory yield (entries 9-11). Acid additives such
With the optimized reaction conditions in hand,
ortho-amidation was applied to various aldehydes
(Scheme 2). In general,
a
wide range of
benzaldehydes was employed under optimized
reaction conditions. Delightedly, various electron-
withdrawing groups and electron-donating groups on
2
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10.1002/adsc.201901267
Advanced Synthesis & Catalysis
benzaldehydes afforded desired amidated products in
good to excellent yields. To begin with, halogens at
ortho- and para-positions worked well and expected
products were isolated in excellent yields (Scheme 2a,
3ba-3fa). Next, with trifluoromethyl group at meta
position, C-H activation occurred selectively at less
hindered side but a very low yield of 23% was
observed, possibly due to the electron deficient ortho-
position, but when 3-(trifluoromethyl)aniline was
used as TDG in the place of aniline then improved
yield of 42% (3ga) was obtained. It is noteworthy
that nitro group at ortho-position, which appears to be
problematic in C-H activations offered excellent yield
(Scheme 1, 3ha). Under the standard conditions,
alkyl groups at ortho-, meta- and para-positions
afforded good yields (Scheme 1, 3ia-3la). Methoxy
group at ortho- and para-positions gave lower yields
but improved yields were obtained when 3-
(trifluoromethyl)aniline was replaced by aniline
(3ma-3na). Delightedly, sensitive yet useful group
such as allylphenyl ethers afforded 3oa in 66% yield.
Similarly, 2-phenyl, 1-naphthyl and 2-naphthyl
benzaldehydes worked well under the optimized
conditions (3pa-3ra). Finally, we also tested 3,4-
dichlorobenzaldehyde 1s, which afforded the
amidated product in satisfactory yield of 68% (3sa).
We next investigated the generality of the
dioxazolones using benzaldehyde 1a as coupling
partner (Scheme 2b). Aryl dioxazolones with alkyl
and methoxy substitution (2b-2d) worked well to
afford the amidated products in good yields (3ab-
Scheme 3. Synthetic applications and transformations.
Keeping in mind the importance of olefins in organic
synthesis, aldehydic group was transformed to the
alkene with methyltriphenylphosphonium iodide,
affording
5
in 91% yield.[7e] An efficient
intermolecular cyclization of 3aa with aq. NH3 in
isopropanol solvent at 90 °C afforded 2-
phenylquinazoline
6
in 79% yield.[21] Next,
CoII/TBHP system converted 3aa to benzoxazin-4-
one derivative 7 in 80% yield via intermolecular
cyclization.[22] Finally, quinolone derivative 8 was
synthesized using dimethyl acetylenedicarboxylate as
a coupling partner.[23]
3ad).
Furthermore,
3-(naphthalen-1-yl)-1,4,2-
dioxazol-5-one afforded lower yield, but with 3-
(trifluoromethyl)aniline on the place of aniline gave
good yield of 74% (3ae). It is noteworthy that the
benzyl, pentyl and cyclohexyl dioxazolones
performed well to afford desired products in excellent
yields (3af-3ah). Finally, we also tested the
heteroaromatic thiophene derived dioxazolone 3-
(thiophen-2-yl)-1,4,2-dioxazol-5-one, which also
worked well and afforded 3ai in 61% yield.
To elucidate the reaction mechanism, we performed a
series of control experiments (See ESI for more
details). The kinetic isotope effect (KIE) value was
found to be 1.7, which indicates the possible
involvement of C-H activation in rate-limiting step of
the catalytic cycle (ESI, Va). Additionally, a
competitive reaction of electron-releasing 4-
methylbenzaldehyde (1k) and electron-withdrawing
4-chlorobenzaldehyde (1e) with phenyl-1,4,2-
dioxazol-5-one (2a) furnished the amidated products
in a ratio of 2:1, suggesting that reaction might
proceed through electrophilic C-H bond activation.
(ESI, Vb). We further synthesized the aldimine (4) of
benzaldehyde and aniline, which worked well under
standard conditions confirming that the aniline acts as
transient directing group (ESI, Vc).
Scheme 4. Plausible mechanism.
Based on literature reports[7a,c-d,10-11,13,15-16] and our
preliminary control experiments, we proposed a
plausible reaction mechanism (Scheme 4). Initially,
the cationic CoIII complex A can undergo initial
ligand exchange with in situ generated aldimines
from benzaldehyde 1a and aniline to give complex B.
Next, carboxylate-assisted concerted metalation–
deprotonation afforded cobaltacycle C. Then, the
coordination of dioxazolones to the cobalt center of
the intermediate C, followed by CO2 extrusion would
To demonstrate the synthetic utility of ortho-
amidated benzaldehydes, 3aa was transformed to
valuable products (5-8) as described in Scheme 3.
3
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10.1002/adsc.201901267
Advanced Synthesis & Catalysis
lead to the Co-nitrenoid species E. The formation of a
new C-N bond by migratory insertion of nitrenoid
species between Co-C bond would lead to six
membered cobaltacycle F. Finally, protodemetalation
will regenerate the active catalyst and acidic
hydrolysis of intermediate G will deliver the desired
product 3aa.
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In conclusion, Cp*Co(III)-catalyzed ortho-C(sp2)-H
amidation of benzaldehydes has been achieved using
dioxazolones as user friendly amidating reagent
under transient directing group strategy. A key
feature of this protocol involves readily available
cobalt catalyst, ambient conditions, broad substrate
scope as well as useful synthetic transformations of
amidated benzaldehydes. Further applications and
other mechanistic studies are currently under
investigation in our laboratory.
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Experimental Section
General Procedure for the o-Amidation of Aryl
Aldehyde: An oven-dried Schlenk tube was charged with
a Teflon coated magnetic stir bar; under argon atmosphere
was added aryl aldehyde (0.20 mmol, 1.0 equiv.), Aniline
(0.12 mmol, 60 mol%), Cp*Co(CO)I2 (9.5 mg, 0.02 mmol,
10 mol%), silver hexafluoroantimonate (AgSbF6) (20 mg,
0.06 mmol, 30 mol%), dioxazolone derivative (0.24 mmol,
1.2 equiv.) and sodium pivalate (5 mg, 0.04 mmol, 20
mol%) followed by 1,2-dichloroethane (DCE) (1 mL).The
closed Schlenk tube was placed in preheated oil bath at
120 °C for 12 hours. After 12 h the reaction mixture was
allowed to cool to room temperature. The reaction mixture
was filtered through a small silica pad, worked up with 3 N
HCl, and extracted with ethyl acetate (2 x 10 mL). The
organic layer was washed with brine solution and dried
over anhydrous Na2SO4. Removal of organic solvent
followed by column chromatography (EtOAc/Hexane) on
silica gel afforded the amidated product.
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Acknowledgements
Financial support to carry out this work by SERB
(EMR/2019/000136) is gratefully acknowledged. BK and VD
thanks to IITK for institute postdoctoral fellowship and BS
express thanks to IITK for PK Kelkar young faculty award.
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5
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10.1002/adsc.201901267
Advanced Synthesis & Catalysis
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