Iron-Catalyzed Arene C-H Amidation Using Functionalized Hydroxyl Amines at Room Temperature

Article abstract of DOI:10.1021/acscatal.8b02939

Herein, we report Fe^(III)(TPP)Cl as an effective catalyst for promoting arene C-H amidation through intramolecular cyclization of N-tosyloxyarylcarbamate substrates. The reaction proceeds via nitrene (outer sphere pathway) C(sp²)-H i

Full text of DOI:10.1021/acscatal.8b02939

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Letter
Iron-Catalyzed Arene C—H Amidation using
Functionalized Hydroxyl Amines at Room Temperature
A. V. G. Prasanthi, Samiyara Begum, Hemant Kumar Srivastava, Sandip Kumar Tiwari, and Ritesh Singh
ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.8b02939 • Publication Date (Web): 09 Aug 2018
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ACS Catalysis
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Iron-Catalyzed Arene C—H Amidation using Functionalized Hydroxyl
Amines at Room Temperature
A.V.G. Prasanthi,b Samiyara Begum,c Hemant Kumar Srivastava,c Sandip Kumar Tiwarid and Ritesh Singh,*, a, b
aNational Institute of Pharmaceutical Education and Research (NIPER), Raebareli, Uttar Pradeshꢀ229010, INDIA
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bOrganic Synthesis and Process Chemistry Division, CSIRꢀIndian Institute of Chemical Technology, Hyderabad, Telanganaꢀ500007,
INDIA
cDepartment of Chemistry, Indian Institute of Technology Guwahati, Guwahati, Assamꢀ781039, INDIA
dCentre of BioꢀMedical Research, SGPGI Campus, Raebareli Road, Lucknow, Uttar Pradesh 226014
ABSTRACT: Herein, we report Fe(III)(TPP)Cl as an effective catalyst for promoting arene CꢀH amidation through intramolecular
cyclization of NꢀTosyloxyarylcarbamate substrates. The reaction proceeds via nitrene (outer sphere pathway) C(sp2)—H insertion to yield
benzoxazolones under external oxidant free condition at ambient temperature. The method is operationally simple and scalable with high
functional group tolerance. Preliminary experimental and computational data indicates involvement of electrophilic aromatic substitution
mechanism for this aryl C—H amidation transformation, distinct from operating mechanism reported previously in aryl C—H amination
using azide based substrates.
KEYWORDS: C—H amidation, ironꢀcatalysis, benzoxazolone, chemoselective, nitrene insertion, DFT calculations
Transition metal catalyzed direct C—H bond amination is an
attractive strategy for constructing Nꢀcontaining molecules.1 In this
context, arene C(sp2)—H bond amination constitutes a valuable
transformation for synthesizing benzannulated Nꢀheterocycles.2 In
recent years hydroxyl amine derived aminating agents have gained
significant prominence and several catalytic systems have been
developed for the construction of Nꢀscaffolds using arene C—H
bond functionalization strategy which mostly occur via manifold
outside with that of nitrene intermediate.4 The relatively weak NꢀO
bond of this aminating agent functions as internal oxidant when
cleaved under reaction condition thus obviating the need for
transformations11 but aromatic C—H amidation. To the best of our
knowledge no report exists with Fe(III)(TPP)Cl as an inexpensive,
nonꢀtoxic and biocompatible catalyst for aromatic C—H
amidation.12a, b
Prior reports of aryl C-H amidation using hydroxyl amine based amidating agent via 'nitrene' intermediate
Intermolecular
DG
H
[RhCp*Cl2
Ag(10 mol%), 60 o
]2 (2.5 mol%)
Inner-sphere pathway
N
O
X
C
R
R
Y= Bz
(1)
(2)
O
O
X
O
CoCp*(CO)I2 (8 mol%)
Ag (16 mol%), 60 o
O
DG
DG
H
N
C
O
X
YO
NH
R
or
Y = Ac
Y = Ns
H
[IrCp*Cl2]2 (2.5 mol%)
exogenous oxidant. Recently, Glorius [with Rh(III) 5a, b
]
and Chang
Ag(10 mol%), rt
DG
H
5c,
d
[with Ir(III) and Co(III)
]
demonstrated the directed (via inner
N
O
Pd(OTs)2(MeCN)2
(10 mol%) 80 o
R
(3)
sphere pathway)5e arene C—H amidation using Nꢀacyloxy and Nꢀ
aryloxy carbamates respectively as putative Nꢀ(nitrene) source (eq 1
and 2, Figure 1, respectively). However, utilization of hydroxyl
amine derivatives as discrete Nꢀ(nitrene) source for arene C—H
C
O
X
Outer-sphere pathway
CCl3
O
O
H
TsO NH
N
O
Z
Z
(4)
O
CCl3
H
[Cu(MeCN)4]PF6 (20 mol%)
amidation are only handful6, which includes Yu et al’s8a (outer
7
2,9 dimethylphenanthroline(20 mol%)
o-, m- & p-
sphere pathway) Pd(II)ꢀcatalyzed and Nicholas et al’s8b (inner sphere
pathway) Cu(I)ꢀcatalyzed intermolecular arene C—H amidation
using NꢀNosyloxy and NꢀTosyloxycarbamate respectively (eq 3 and
4, Figure 1). Where considerable progress has been made in the
direction of intermolecular transformation, no report of
intramolecular arene C—H amidation exists (via outer sphere
140 o
C
non-selective insertion
Intramolecular
Inner-sphere pathway
O
JACS, 2017
Glorius et al5b
OH
O
N
[RhCp*(CH3CN)3] (5 mol%)
O
H
R
R
NaOAc, rt
NH
Bicyclic olefin (20 mol%)
H
pathway) with this system leading to valuable Nꢀheterocycle.5f,
6a
Outer-sphere pathway
O
first
example
N
O
More so, all the reactions mentioned above are associated with one
or more limitations like requirement of either 2nd or 3rd row
transition metal catalysts/additives (or specialized catalyst), harsh
conditions (high temperature), and high catalyst loading for efficient
transformation (Figure 1). Therefore, development of mild reaction
conditions4h using earth abundant 1st row transition metal especially
iron would be advantageous for cost effective and efficient synthesis
of Nꢀheterocycles.9
On the other hand, since the Breslow’s seminal report of
FeIII(TPP)Cl as a catalyst for C(sp3)—H bond amination using
iminoiodinane as N(nitrene)ꢀsource,10 Feꢀporphyrin based catalytic
system has found extensive usage for many different
O
H
N
H
R1
R2
N
H
OTs
F
(this work)
Fe(III)(TPP)Cl (2 mol%)
anti-cancer agent3
O
NH
O
N
R
R
O
O
O
25 oC, DCE
N
H
6
O
O
R2
R
1 H
N
H
N
H
.
Cl
5
iron catalyst/environment benign
chemo/regioselective transformation
room temperature conversion
low catalyst loading/high yield
water soluble side product
Chlorzoxazone
pardoprunox
(anti-parkinson)
(muscle relaxant)
Representative benzoxazolone based drugs
Figure 1. Prior art of arene C-H amidation with
hydroxylamine derived aminating agents via nitrenoid
insertion, this work and benzoxazolone based bio-active
scaffolds
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Page 2 of 6
Therefore, towards our research interest for development
transformation, perhaps the electronics and structure of porphyrin
of new sustainable amidation chemistry, herein, we wish to report
Fe(III)(TPP)Cl catalyzed conversion of NꢀTosyloxyaryl carbamates
to biologically important benzoxazolones3, 13, 14 via intramolecular
aryl C(sp2)—H amidation under extremely mild reaction condition
(at room temperature, external oxidant free, low catalyst loading,
and water soluble side product) (Figure 1).
plays crucial rolein stabilizing the high oxidation state Feꢀ
intermediates during the course of the reaction.
1
2
3
4
5
6
7
8
Notably, this is the first report of Feꢀporphyrin catalyzed
decomposition of Nꢀhydroxy substituted carbamates as nitrene
source. NꢀNosyloxy (3) and NꢀMesyloxy (4) based substrates
provided lower yield of the desired product 6 (entries 10 and 11,
Table 1). Conducting the reaction in open atmosphere provided
slightly diminished but practical yield of 6a, thus making this
protocol operationally convenient (entry 14, Table 1). Control
reactions demonstrated the requirement of both base as well as
catalyst for the success of this amidation reaction (entries 12 and 13,
Table 1). Reducing the base (1 equiv) took longer time for full
conversion, providing diminished yield of 6a (entry 19, Table 1).
However, comparable yields were obtained from 2 and 1 mmol
catalyst, latter taking about 18 h for full conversion (entries 17 and
18, Table 1). Dichloroethane (DCE) turned out to be the best
reaction medium among solvents screened (see ESI for details).
Scale up of this reaction led to no significant change in isolated
yield (entry 20, Table 1).
Our studies began with catalyst screening for
decomposition of Oꢀphenyl azidoformate (1) an ideal substrate, at
room temperature for benzoxazolone synthesis, unfortunately, none
of the catalysts screened including Rh2(OAc)4 were able to
decompose this substrate at rt (entries 1ꢀ3; Table 1). This prompted
us to introduce other leaving groups like an ester or a sulfonate in
place of –N2 as potential Nꢀsource. Although, Nꢀacetoxy based
carbamate (2) remained inert with all the catalyst screened, we were
pleased to observe decomposition of NꢀTosyloxy based carbamate
(5) with Fe(III)(TPP)Cl using K2CO3 as base under ambient condition
providing benzoxazolone (6a) in excellent yield of 94% (entry 8,
Table 1). K2CO3 proved as optimal base for current transformation
(see ESI for details). Rh2(OAc)4 on the other hand could only
provide modest (35%) yield upon full conversion (entry 7, Table 1).
Among other catalytic systems screened, Cu(II) failed to provide any
desired product (entry 9, Table 1) whereas nonꢀheme iron based
catalyst (FeCl2 and FeCl3) tested under otherwise similar reaction
condition failed to initiate the reaction at rt (entries 15 and 16, Table
1) or gave. Catalytic activity of Fe(III) with other ligands (salen and
box) gave inferior results (entry 10 and 11, Table 1, respectively).12c
9
10
11
12
13
14
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Table 2. Scope of Fe(III)(TPP)Cl catalyzed C(sp2)—H amidation
on N-Tosyloxy arylcarbamatesa
Br
Table 1. Optimization of arene C—H amidation with different
carbonyl based nitrene precursorsa
O
1
O
6b, R = OMe; 96%
6c, R = t-butyl; 95%
7
4
O
O
O
6
O
O
N
2
R1
O
6d, R = Me;
6e, R = F;
6f, R = I;
94%
81%
84%
H
N
H
N
H
3 O
5
N 3
R
6h, R1 = NO2; 0%
6cz, R1 = CF3;0%
H
6i; 75%
6g; 92%
O
Me
O
O
O
HN
O
O
O
O
O
N
H
O
N
+
H
N
Time Yieldb
c6m; 87%
c6j; 86%
N
H
Entr X (substrate) Catalyst
Base
(equiv)
none
c6k; 85%(1:1)
H
6-Me:4-Me(1:2)
6-ethyl:4-ethyl(1:1)
y
6q; 82%
(mol%)
(h)
24
24
24
(%)
NR
NR
NR
NR
NR
NR
35
Large
steric hindrance
1
2
3
4
5
6
7
8
9
ꢀN3 (1)
Rh2(OAc)4(5)
Fe(TPP)Cl(5)
Cu(OTf)2(5)
Rh2(OAc)4(5)
Fe(TPP)Cl(5)
Cu(OTf)2(5)
Rh2(OAc)4(5)
Fe(TPP)Cl(5)
Cu(OTf)2(5)
O
Cl
O
O
O
O
O
O
O
O
N
H
ꢀN3 (1)
none
O
N
H
N
N
H
N
H
6r; 93%
H
ꢀN3 (1)
none
6n; 77%
6p; 80%
6o; 84%
6l; 77%
O
O
ꢀNHOAc(2)
ꢀNHOAc(2)
ꢀNHOAc(2)
ꢀNHOTs(5a)
ꢀNHOTs(5a)
ꢀNHOTs(5a)
K2CO3(2) 24
K2CO3(2) 24
K2CO3(2) 24
O
O
O
N
H
Cl
O
H
H
O
O
H
H
N
NH
6dz; 75%
O
H
muscle-relaxant
+
O
N
H
O
H
H
O
O
6s; 89%
NH
O
(minor)
6t; 92%
(major)
K2CO3(2)
K2CO3(2)
K2CO3(2)
2
EtO
N
H
O
c6u :6u'(1.2:1); 79%
2
94
6ez; 54%
2
NR
42
O
Fe(III)ꢀ(salen)Cl(5) K2CO3(2)
Fe(III)ꢀ(box)Br(5) K2CO3(2)
2
O
O
O
10 ꢀNHOTs(5a)
11 ꢀNHOTs(5a)
10 ꢀNHONs(3)
11 ꢀNHOMs(4)
12 ꢀNHOTs(5a)
13 ꢀNHOTs(5a)
14c ꢀNHOTs(5a)
15 ꢀNHOTs(5a)
16 ꢀNHOTs(5a)
O
N
H
O
N
2
34
H
N
6v; 83%
H
6w; 79%
do not survive
Fe(TPP)Cl(5)
Fe(TPP)Cl(5)
Fe(TPP)Cl(5)
none
K2CO3(2)
K2CO3(2)
none
2
52
6x; 75%
under strong oxidative
condition
2
64
O
O
O
O
24
NR
NR
82
O
O
N
O
O
H
N
N
H
K2CO3(2) 24
K2CO3(2)
N
H
H
6y; 80%
6z; 82%
6bz; 90%
6az; 72%
Fe(TPP)Cl(5)
FeCl2(5)
2
aReactions were conducted in a round bottom flask under argon at 0.31 mmol scale in
DCE (2 mL) with FeIII(TPP)Cl(2 mol%), K2CO3 (2.0 equiv), 4 Å MS (100 wt %), 6ꢀ12 h;
bIsolated yields. cRegioselectivity determined from 1H NMR.
K2CO3(2) 24
K2CO3(2) 24
NR
NR
93
FeCl3(5)
Subsequently, substrate scope was investigated with unoptimized
reaction time. As observed from results shown in Table 2, various
substituents like halogens, alkyl, aryl and alkoxy groups were well
tolerated under the reaction condition. Influence of electronic effect
was clearly observed on 4ꢀsubstituted carbamates, where substrates
with electron donating substitutents (alkoxy, Me, tꢀbutyl) cyclized
efficiently to provide desired products in good to excellent yields
(entries 6b-6g), while electron withdrawing substituted (F and I)
substrates provided slightly diminished yields of their respective
benzoxazolones (entries 6e and 6f).13a, b Strong electron withdrawing
17 ꢀNHOTs (5a) Fe(TPP)Cl(2)
18 ꢀNHOTs (5a) Fe(TPP)Cl(1)
K2CO3(2)
6
K2CO3(2) 18
K2CO3(1) 24
84
19 ꢀNHOTs(5a)
Fe(TPP)Cl(5)
51
20d ꢀNHOTs (5a) Fe(TPP)Cl(2)
K2CO3(2)
2
90
aReactions were conducted in a round bottom flask under argon at 25 oC with substrate
(0.31 mmol), cayalyst (1ꢀ5 mol%), 4 Å MS (100 wt %), base (1ꢀ2 equiv), in anhyd DCE
(2.0 mL) (see ESI for details). bIsolated yields. cConducted in open atmosphere.
d7 mmol scale. NR = No Reaction
These results clearly demonstrate the superiority of tetradentate
(N,N,N,N) (porphyrin) ligand around the metal centre for this
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group (NO2 and CF3) substituted carbamates failed to undergo
radical) (eq 1, Scheme 1)under standard reaction condition did not
hamper the reaction and cyclized product 6a, was obtained in good
yield, indicating no involvement of free radical in the
transformation.15c In addition, no product formation from Nꢀmethyl
Oꢀtosylated carbamate, 7 under similar reaction conditions (eq 2,
Scheme 1), indicated putative involvement of Feꢀnitrenoid species
incurrent aromatic C—H bond amidation, as 7 is incapable of
generating nitrene intermediate. Experiments were then conducted
to gather evidence for involvement of Feꢀnitrenoid species
cyclization (entries 6h and 6cz). While substrate 5h remained inert
under reaction condition, 5cz decomposed to give corresponding
uncyclized carbamate (4ꢀCF3C6H5OCONH2). However, ester
substituted analog was readily synthesized in modest yield (entry
6ez). These results are consistent with involvement of putative
electrophilic aromatic substitution (EAS) pathway.6 It is worth
mentioning here that under present external oxidant free reaction
condition even oꢀhalo (entry 6i) substituted derivative underwent
smooth cyclization to give product in high yield. Further,
Chlorzoaxazone, a muscle relaxant was efficiently accessed using
this protocol in good yield (entry 6dz). Halogen substituted
benzoxazolones thus provides opportunity for further synthetic
manipulations thereof. Regioselective product formation was
observed on oꢀPhenyl substituted and αꢀnaphthol derived
carbamates in good yield (entries 6q and 6t).
1
2
3
4
5
6
7
8
undergoing stepwise, (either electrophilic or
H
atom
16, 17
abstraction/radical rebound (HAA/RB))11g,
or concerted
9
asynchronous pathway. Consequently, two set of one pot
competition reactions were conducted under standard reaction
condition. Substrate with electron donating group (5b) and electron
neutral substrate (5a) provided product in the ratio of 2.5:1(eq 3,
Scheme 1, see ESI for details), whereas, substrate with electron
withdrawing group (5e) provided product ratio of 1:1.9 with respect
to 5a (eq 4, Scheme 1, see ESI for details).This indicated the
beneficial effect of electron donating groups.
10
11
12
13
14
15
16
17
18
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41
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43
44
45
46
47
48
49
50
51
52
53
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57
58
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Excellent yield with no side product formation was
obtained under present protocol on dimethyl substituted substrates
owing to nonꢀoxidative and mild reaction condition (entries 6r and
6s). Likewise, sterically hindered oꢀalkyl and diꢀalkyl substituted
substrates (entries 6p and 6l) furnished product in very good yields.
Further, to observe the steric effect on the C—H bond amidation
mediated cyclization process, mꢀsubstituted Nꢀtosyloxycarbamates
were tested. mꢀmethyl substituted carbamate gave moderate
regioselectivity with major C—H amidation product obtained from
sterically more encumbered C—H site in good yields (entry 6j). mꢀ
ethyl and βꢀnaphthol derived carbamates furnished the regioisomeric
products 6k and 6m respectively in 1:1 ratio. Although, mꢀtꢀbutyl
substituted substrate provided single regioisomer owing to its sterics
(entry 6o), unexpectedly, single isomer (from sterically less
hindered CꢀH site) was isolated from the reaction of mꢀchloro
carbamate (entry 6n). Exact reason for observation of single isomer
is unclear this stage.
Fe(III)(TPP)Cl
H
O
O
N
OTs
O
(eq 1)
O
K2CO3
DCE, 2 h
TEMPO (1 equiv)
N
H
5a
6a, 88%
Fe(III)(TPP)Cl
O
N
O
N
OTs
OTs
(eq 2)
O
O
O
K2CO3
DCE, 24 h
7
(>95% starting recovered)
H
O
N
O
O
Fe(III)(TPP)Cl
O
(eq 3)
(eq 4)
O
+
O
5b
N
N
O
H
H
6b
2.5
6a
H
N
O
:
1
OTs
K2CO3
DCE, 6 h
O
O
O
O
5a
O
O
+
H
N
N
F
N
H
H
Chemoselective transformation took place when we
OTs
OTs
6a
1.9
6e
O
:
subjected various oꢀalkyl substituted carbamates with relatively
weaker (1o, 2o, and 3o) C—H bonds under cyclization condition.
Only oꢀaromatic C—H amidation products were obtained in
excellent yields (entries 6v-6z). Bicyclic substrate too provided
chemoselective product in exellent yield (entry 6bz). On contrary,
under Fe(III)(TPP)Cl catalytic condition it’s sister analog carbene
undergoes C(sp3)—H insertion (major product) with alkyl
benzenesin the absence of directing group.11a Substrates with
sustituents like orthoꢀalkene and orthoꢀalkyne were well tolerated
under present reaction condition and gave only chemoselective five
membered products (entry 6z and 6az). The unsaturated moieties
present in the molecule thus allow for generation of new scaffolds in
chemical space. Noteworthily, alkyne moieties are often prone to
undergo side reactions under oxidative reaction conditions,15a
therefore, preferential aromatic C(sp2)—H bond nitrenoid insertion
over more labile oꢀsubstitutents (C(sp3)—H bond, alkene and
alkyne) at room temperature demonstrates the uniqueness of the
current system under Feꢀporphyrin catalysis. Finally, we
investigated the application of this C(sp2)—H amidation reaction to
demonstrate late stage functionalization of a natural scaffold.
Consequently, estrone derivative 5u, was subjected under standard
reaction condition which gave the regioisomeric products in 1.2:1
ratio (79% yield) (entry 6u and 6u', Table 2).
1
F
5e
D
H
H
O
O
H
Fe(III)(TPP)Cl
(eq 5)
O
O
D
N
+
O
N
H
6a-d1
N
K2CO3
DCE, 6 h
H
O
6a
5a-d1
KIE (PH/PD) = 0.23±0.02
i) Fe(III)(TPP)Cl
K2CO3, DCE, 6h
O
N
(no o-aryl C-H/C-D scrambling)
95% starting material recovered
OTs
(eq 6)
O
7
ii) D2O, 1 h
Scheme 1.Mechanistic investigation experiments
Due to virtue of present reaction conditions being
extremely mild, no side products with substrates capable of forming
benzoxazinone as well as aziridine derivatives (entries 6v-6z)were
observed. Such results also points towards nonꢀinvolvement of free
nitrene intremediate in this process as free nitrene generated under
thermolytic conditions often lead to nonꢀselective product
formation.15b Thereafter, a set of experiments were conducted to
underpin the rationale mechanism prevalent in this Fe(III)ꢀporphyrin
catalyzed transformation. Subjecting the substrate 5a, with free
radical scavenger TEMPO (2,2,6,6ꢀtetramethylpiperidineꢀ1ꢀoxy
Figure 2. DFT calculated energy profiles of C(sp2)—H
bond amidation (red and black colour) and C(sp3)—H
bond amidation (blue colour) reaction on substrate 5w.
Relative energies (kcal/mol) are in parentheses using
various methods (see Table S2 in ESI for details)
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Next, Kinetic Isotopic Effect (KIE) study was conducted
benzoxazolone 5a was readily cleaved under standard reaction
condition to provide oꢀhydroxyl aniline, thus providing
on a deuterated substrate (5a-d1).2e Observation of inverse secondary
KIE (PH/PD = 0.23±0.02 at 25 oC), under standard reaction condition
indicated that cleavage of C—H bond is not involved in rate
determining step, and suggested involvement of stepwise
mechanism favoring EAS pathway in Fe(III)ꢀporphyrin catalyzed
cyclization (eq 5, Scheme 1, see ESI for details).
a
1
2
3
4
5
6
7
8
complementary approach to access substituted oꢀhydroxyl
anilines.5b (see ESI; Scheme S2).
In conclusion, we have developed an iron (heme) based
catalytic system for efficient aryl C(sp2)—H bond amidation on
aryl Nꢀtosyloxycarbamates to access privileged benzoxazolones.
Experimental findings were validated by DFT calculations.
Fe(III)(TPP)Cl as earth abundant and innocuous catalyst
decomposes Nꢀtosyloxycarbamates under external oxidant free
reaction condition at room temperature. Low catalyst loading (2
mol%) along with generation of water soluble side product
underscores the importance of current transformation. Moreover,
this transformation adds to the repertoire of Feꢀcatalyzed reactions
in an effort towards development of sustainable amidation
chemistry. Further exploration and deep mechanistic insight of
this relatively new transformation is currently underway in our
laboratory.
Theoretical studies were then conducted to obtain
further mechanistic insight, as well as for rationalization of
selective C(sp2)—H amidation over more labile and weaker
C(sp3)—H bond. Therefore, DFT calculations were performed
with substrate 5w and FeIII(por)Cl as simplified catalyst model,
which in accordance with the earlier report,10 indicated slight
preference for C(sp3)—H amidation over C(sp2)—H in gas phase.
However, C(sp2)—H amidation reaction path was found to be
energetically more favourable when calculations were performed
in solution phase using DCE as solvent (Figure 2). Probably
solvent plays a major role and controls the path of this reaction.
Interestingly, the energy barrier height is the lowest for the
formation of CꢀN bond (via intermediate 9) followed by Hꢀshift in
gas phase as well as in solvent. Thus, product 6w follows the EAS
path depicted with black line in Figure 2. Calculations were
performed at a few different levels and methods in DCM solvent
which clearly show that the level of the calculation does not affect
the overall trend of the results. M06 level DFT calculations
indicate clearer picture compared to B3LYP level; however both
the levels show the same trend.
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ASSOCIATED CONTENT
Supporting Information
General and experimental information on preparation of
carbamates, optimization studies, procedure for C—H amidation,
details for mechanism studies and characterization data (1H NMR,
13C NMR, HRMS) for all new compounds. This material is
available free of charge on the ACS Publications website.
Supporting Information is available free of charge via the Internet
at http://pubs.acs.org.
Based on the experimental observation and theoretical
calculations in this report and previous literature reports,18a, b, 5f, 6
tentative mechanism has been proposed as depicted in Scheme 2.
a
AUTHOR INFORMATION
Corresponding Author
riteshsingh@niperraebareli.edu.in;ritesh.cdri@gmail.com
Notes
The authors declare no competing financial interests.
ACKNOWLEDGMENT
RS and Prasanthi thanks DST, New Delhi for INSPIRE faculty
award (IF15ꢀCHꢀ179) and doctoral fellowship, respectively. HKS
and SB thanks DSTꢀSERB for the financial assistance through
SB/S2/RJNꢀ004/2015
respectively. Director NIPERꢀRaebareli (Dr. S.J.S. Flora) is
highly acknowledged for support and encouragement.
and
PDF/2016/002053
projects
Scheme 2.Tentative mechanism for aryl C(sp2)—H
amidation
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A
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