Pd(II) pincer type complex catalyzed tandem C-H and N-H activation of acetanilide in aqueous media: A concise access to functionalized carbazoles in a single step

Article abstract of DOI:10.1039/c5gc02937f

One-pot, tandem C-H and N-H activation of acetanilides with aryl boronic acids to realize functionalized carbazoles was conveniently performed under aerobic conditions using a novel NNO pincer type Pd(ii) complex [Pd(L)Cl] (where L = nicotinic acid (phenyl-pyridin-2-yl-methylene)-hydrazide or furan-2-carboxylic acid (phenyl-pyridin-2-yl-methylene)-hydrazide) as a catalyst in neat water and a very low (0.01 mol%) amount of catalyst. It is worth noting that recyclability up to six consecutive runs and column chromatography free isolation of the title heterocycles in an excellent yield are achieved.

Full text of DOI:10.1039/c5gc02937f

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DOI: 10.1039/C5GC02937F
Journal Name
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Pd(II) pincer type complex catalyzed tandem C-H and N-H
activation of acetanilide in aqueous media: A concise access to
functionalized carbazoles in a single step
Received 00th January 20xx,
Accepted 00th January 20xx
Vignesh Arumugam,a Werner Kaminskyb and Dharmaraj Nallasamya*
DOI: 10.1039/x0xx00000x
www.rsc.org/
One-pot, tandem C–H and N–H activation of acetanilides with aryl boronic acids to realize
functionalized carbazoles were conveniently performed under aerobic conditions using a novel
NNO pincer type Pd(II)complex [Pd(L)Cl] (where L = nicotinic acid (phenyl-pyridin-2-yl-
methylene)-hydrazide or furan-2-carboxylic acid (phenyl-pyridin-2-yl-methylene)-hydrazide) as a
catalyst in neat water and very low (0.01 mol%) amount of catalyst. It is worth to note
recyclability up to six consecutive runs and column chromatography free isolation of the titled
heterocycles in an excellent yield.
Introduction
Huggen Wang’s report7a
Key to pharmaceutical compounds are carbazole cores (Fig.
1).1 They lead to well-known antitumor, antibacterial, anti-
tuberculosis, anti-inflammatory, psychotropic and anti-
histaminic drugs.2 Numerous synthetic methodologies were
reported in literature for the construction of carbazole
architectures.3 However, those strategies involved multiple
steps, such as the preparation of key starting materials
followed by sequential reaction schemes and involved harsh
reaction conditions to produce the target compounds.3
Hence, a facile and sustainable synthetic approach towards
Jeganmohan’s report11
Akhilesh K. Verma’s report7b
the construction of carbazole moieties is desirable.
Transition metal-catalyzed, chelation-assisted activation of
C–H and N–H bonds has emerged as an effective synthetic
method.4 Specifically, utilizing one-pot C–C and C–N bond
forming reactions were key to the synthesis of nitrogen-
containing heterocycles.5 In this context, various functional
groups with coordination ability, such as acetamino (NH–
COR)6a amides,6b imines,6c pyridines,6d,e pyridine-N-oxide,6fo-
phenylcarbamates6g and
This Work
aInorganic & Nanomaterials Research Laboratory, Department of Chemistry,
Bharathiar University, Coimbatore 641 046, India .
E-mail: dharmaraj@buc.edu.in; Fax: +91 4222422387; Tel: +91 4222428316;
Web: http://ndharmaraj.wix.com/inrl
b Department of Chemistry, University of Washington, Seattle, Washington 98195,
USA.
Electronic Supplementary Information (ESI) available: Copies of 1H and 13C NMR
spectra, Single-crystal XRD data of the complexes, experimental procedures See
DOI: 10.1039/x0xx00000x
Scheme 1 Selected reports for the synthesis of carbazoles
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methoxy benzamide6h were employed in the palladium- Based on state-of-the-art techniques, we aVrieew AcruticrlreeOnnltinlye
DOI: 10.1039/C5GC02937F
catalyzed C–C and C–N bond forming reactions. Among interested to develop new pincer type complexes as
those, palladium catalyzed ortho-arylation of acetamino catalysts for activation of C–H and N–H bonds. Owing to high
(NH–COR) coordinated aromatics with several electrophiles stability and reactivity, pincer complexes have been widely
has been extensively studied (Scheme 1).7 A wide range of utilized as catalyst for a variety of organic reactions including
arylating agents like aromatic boranes, stannenes and C–H and N–H bond activation.12 Previously, we examined
silanes were utilized in coupling reactions. Due to the pincer type palladium(II) complexes for C–C bond formation
inherent advantages of boronic acids such as low toxicity, reactions.13 In continuation of our ongoing research,13 we
stability to ambient conditions including humidity and the wish to present herewith two new palladium(II) complexes
facile removal of boron-containing side products, aromatic incorporating NNO pincer type ligand for one-pot, single
boranes are widely chosen in such coupling reactions.8 step synthesis of functionalized carbazoles via C–H and N–H
However, very few reports were published on employing activation of acetanilide with aryl boronic acids. We believe
organoboron reagents as the coupling partner in palladium that the catalytic system presented below deserves merit as
catalyzed C–H and N–H activation reactions.9
it doesn’t require organic solvents, elevated temperatures,
oxidant, additives and phase transfer reagents. To the best
of our insight, this is the first report on the utility of pincer
type palladium(II) complexes for the one-pot construction of
N-acetylcarbazoles with low catalyst loading (0.01 mol%)
under aerobic conditions in aqueous media.
Results and discussion
Treatment of Pd(II) precursor [PdCl2(PPh3)2] with pincer type
ligands, namely, isonicotinic acid (phenyl-pyridin-2-yl-
methylene)-hydrazide (HL1) or furan-2-carboxylic acid
(phenyl-pyridin-2-yl-methylene)-hydrazide (HL2) yielded
complexes of the type [Pd(L)Cl] as shown in scheme 2
(complex 1 and 2).
Fig. 1 Carbazole based drugs
In 2010, H. Lipshutz’s group demonstrated ortho-arylation of
aryl ureas with aryl boronic acids or aryl iodide catalyzed by
cationic palladium complexes.10a, b However, their approach
required 10 mol % of palladium complex as catalyst to obtain
an ortho-arylated product via C–H activation. Later,
Jeganmohan’s group reported Ru(II) complex catalyzed
ortho-arylation of acetanilide with aryl boronic acids to
ortho-arylated acetanilides, which were subsequently
converted into phenanthridine and carbazole derivatives by
using Ph3PO and Tf2O or palladium or Cu(OTf)2 catalysts and
obtain 90% yield.11a Particularly, M.-J. Wu and co-workers11b
reported one-pot synthesis of 9-(pyridin-2-yl)-9H-carbazoles
through the simultaneous C–H activation and palladium(II)-
catalyzed cross-coupling of N-phenylpyridin-2-amines with
potassium aryltrifluoroborates in presence of silver acetate
as an oxidant.
Where R = 3-pyridyl (complex 1) or 2-furyl (complex 2)
Scheme 2 Synthesis of NNO pincer type ligands and their
Pd(II) complexes 1 and 2.
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Table 1 Optimization of reaction conditions.a
Characterization of the newly synthesized complexes 1 and 2
were carried out by means of elemental analysis, IR, H and
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DOI: 10.1039/C5GC02937F
1
13C NMR spectroscopy (details are given in the Experimental
section). In particular, the exact coordination mode of the
pincer type ligand is fully supported by the single-crystal XRD
data. Infra-red spectra of the ligands were recorded using
KBr pellets. The free pincer type ligands (HL1 and HL2)
showed the characteristic N–H and the C=O bands of the
amide functionality at 3025, 3060 and 1641, 1680cm-1,
respectively. A sharp band was observed at 1571 and 1581
cm-1, respectively due to the presence of C=N stretching of
the free ligands.14 The IR spectra of complexes 1 and 2 didn’t
display any bands assignable to the N–H or the C=O group of
the amide functionality proving that the ligands (HL1 and
HL2) underwent enolisation followed by deprotonation prior
to coordination with the palladium(II) ion.14 Furthermore, a
new band due to the C=N–N=C group was observed at 1478
and 1474 cm-1, respectively.14 Thus, pincer type ligands HL1
and HL2 were coordinated to palladium(II) ion via the pyridyl
Entry
Catalyst
Base
Solvent
Yield
(%)*
1
2
Complex 1
Complex 1
Complex 1
Complex 1
Complex 1
Complex 1
Complex 1
Complex 1
Complex 1
Complex 1
Complex 1
Complex 1
Complex 1
Complex 1
Complex 1
Complex 1
Complex 1
Complex 1
Complex 1
Complex 2
No base
KOH
H2O
H2O
10
72
89
76
70
56
51
38
72
89
74
63
59
72
75
65
53
61
53
93
3
K2CO3
NaOH
Na2CO3
CH3COONa
Et3N
H2O
4
H2O
5
H2O
6
H2O
1
nitrogen, azomethine nitrogen and the imidolate oxygen. H
7
H2O
and 13C NMR spectral data of the complexes 1 and 2,
recorded in CDCl3, are presented in the Experimental
section. The 1H NMR spectra of these complexes didn’t show
any N−H proton signals meaning oxygen is coordinated to
the palladium ion in the imidolate form. Aromatic protons of
the ligands present in complexes 1 and 2 showed resonances
in the region of 7.00−7.81 ppm.15 Downfield resonance in the
13C NMR spectra of the complexes 1 and 2 at 161.6 and
165.6 ppm, respectively, owed their presence to coordinated
carbon of imidolate functionality(N=C–O). Signals featuring
in the region of 165.3–124.7 ppm were assigned to various
aromatic carbons present in complexes 1and 2.15
8
Pyridine
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
H2O
9
THF
10
11
12
13
14
15
16
17
18
19
20
H2O
DMF
CH3CN
DMSO
MeOH
EtOH
toluene
benzene
CHCl3
DCM
H2O
The single-crystal XRD studies of the complexes 1 and 2
showed the pincer type ligands (HL1 and HL2) adopting to
the palladium ion in an uni-negative tridentate manner
constituting pyridyl nitrogen, azomethine nitrogen and the
deprotonated imidol oxygen as donor atoms forming two
different five membered chelate rings with the palladium ion
while the fourth coordination site is occupied by a chloride
ion. The Pd(II) center in complexes 1 and 2 adopted a
distorted square-planar geometry. The bond lengths and
bite angles are very similar to those observed for other
palladium(II) complexes.16 The perspective ORTEP view of
complexes 1 and 2 are shown in Fig. 2 and 3. Details on the
data collection, structure refinements, bond angles and
bond distances are gathered in ESI (Table S1 &S2)
a
= Reaction conditions: acetanilide (2 mmol), phenylboronic acid (2
mmol) K2CO3 (4, mmol), H2O and catalyst (0.01 mol %) stirred at 60 °C
for 8-10 h. * Isolated yield
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were utilized further on: acetanilide (2 mmol), aryl
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boronic acid (2 mmol) K2CO3 (4 mmoDl)O, IH: 120O.10a39n/dC5cGoCm02p9l3e7xF
2
as catalyst (0.01 mol %), 60 °C. The molecular identity of
the isolated products was confirmed by H, 13C NMR and
mass spectroscopy (see ESI) and compared with
literature. 11
1
The substrate scope of various aryl boronic acids
possessing both activating and deactivating groups was
tested at above described optimized conditions. Excellent
yields with high regioselectivity were observed for most
of these substrates (Table 2 and 3). Aryl boronic acids
featuring strongly activating groups such as −OCH3, −OH
and −N(CH3)2 underwent the reaction smoothly to give
significant yields (Table 2, entries 1e, 1f, 1h and 1m).
Fig. 2 ORTEP diagram of complex 1 with thermal ellipsoids
at the 50 % probability level.
Incorporation of
sterically
demanding
2,
3-
dimethylphenylboronic acid and 4-t-butylphenylboronic
acid did not affect the reaction notably and afforded the
respective products in 75% and 88% yield (Table 2, entries
1d and 1g). On the other hand, substitution of weakly
activating groups like −CH3 at either ortho or para
positions of boronic acid gave 90 and 91 % yield (Table 2,
entries 1b and 1c). Both, naphthyl and biphenyl boronic
acids converted into the desired products (Table 2,
entries 1i and 1j).
Fig. 3 ORTEP diagram of complex 2 with thermal ellipsoids
at the 50 % probability level.
It is gratifying to note that electron-deficient aryl boronic
acids were well tolerated and weakly deactivating
halogens such as −Cl and −Br provided good yields (80 &
85%) of the expected product (Table 1, entries 1k and 1l).
The presence of halogens (chloro/bromo) in the targeted
heterocyclic derivatives (Table 2, entries 1k and 1l) hints
at further functionalization through cross coupling at
respective positions. Moderately deactivating groups like
formyl or acetyl occupying the para position of
phenylboronic acids were well tolerated to afford 87% of
the desired product (Table 1, entry 1p). In addition,
boronic acid bearing a strongly deactivating −CF3 group at
the para position, gave 82% of the desired product and
thus revealed that electronegativity of trifluromethyl as a
substituent was insignificant (Table 1, entry 1q).
We directed our attention on the identification of suitable
reaction conditions for efficient C–H and N–H activation
of acetanilide with phenyl boronic acid. For this purpose,
acetanilide (2 mmol) and phenylboronic acid (2 mmol)
were used as model substrates in presence of complex
1
as catalyst (0.01 mol %). Reaction without any base
realized only 10% of isolated yield. Yields of the
functionalized carbazoles as product were optimized by
screening with complexes
1 and 2 as catalysts and various
bases and solvents. K2CO3 proved to be the most effective
base (Table 1, entry 3). We focused next on finding an
appropriate solvent, where upon H2O emerged as the
optimal choice out of THF, DMF, DMSO, CH3CN, MeOH,
EtOH, toluene, benzene, CHCl3 and DCM (Table 1, entry
10). Complex
enhanced catalytic activity of complex
2
proved to be less effective than
1. The
The same protocol used above was applied to N-
naphthalen-2-yl-acetamide, which possesses a napthyl
unit in place of a phenyl. Aryl boronic acids bearing
2
is correlated to
the presence of a furan moiety (Table 1, entry 23).17 We
also tested the above reaction at a range of temperatures
starting from room temperature. Best yield of the
heterocyclic product (93%) was achieved at 60 °C. The
yield of the product decreased significantly for
temperatures above 60 °C. Thus, the following conditions
electron-
donating/withdrawing
groups
actively
participated and afforded the expected products in good
isolated yields (Table 3 entries 2a-2q). During the course
of this catalytic reaction, we didn’t observe any homo
coupled products of aryl boronic acids.
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Table 2 Coupling of acetanilide and aryl boronic acids to functionalized carbazoles (1a–1q) via C-H and N-H activation
under optimized reaction conditions.a
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Table 3 Coupling of N-naphthalen-2-yl-acetamide and aryl boronic acids to functionalized benzo[b]carbazoles (2a–2q)
via C-H and N-H activation under optimized reaction conditions.a
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1
The gram scale synthesis was examined using 14.7 mmol identified by melting point data, Rf value of TLC and H
of acetanilide with 14.7 mmole of phenylboronic acid as NMR spectra. (Detailed procedure of recovery and
representative examples. The reaction readily occurred to reusability is given in the experimental section).
afford the target product 1a in 85% yield (Scheme 3).
Scheme 3 Gram scale synthesis of 1a
.
Recently, M.-J. Wu et al11b documented a Pd(II) catalyzed
synthesis of N-pryidylcarbazoles via tandem C–H
activataion of N-phenylpyridin-2-amines with potassium
aryltrifluoroborates. However, their strategy suffered
from the formation of a side product, use of an oxidant,
higher temperature and longer duration (48 h). Hence, we
are of the opinion that our present methodology is
superior to that of them.
Fig. 4 Reusability of catalyst
2
Based on the reported metal-catalyzed C–H and N–H
bond activation reactions, we propose a plausible
mechanism to understand the formation of carbazole
We tried above reaction with low catalyst loading,
ranging from 0.01–0.0001 mol % under optimized
conditions, but only low yields were obtained, as was
reported earlier.19 However, use of 0.0001 mol% catalyst
resulted in high turnover numbers (Table 4).
derivatives in Scheme 4 6, 9, 10, 18
active species to undergo an oxidative addition of
acetanilide to form intermediate , a six membered
palladacycle. Transmetallation of phenylboronic acid into
.
The first step involves the
1
2
Table 4 Catalyst loading test
intermediate
intermediate
2
3
in the presence of K2CO3 generates
which, on reductive elimination, yielded
Entry
Mol %
Isolated yield
TON
%
the ortho-arylated intermediate
of Pd(0) active species The catalytic cycle proceeds
with subsequent deprotonation of the N-H group in to
form a palladium complex . Through palladium as C-H
activator, intermediate undergoes a C-H activation at
the appropriate carbon resulting in a six membered
palladacycle, . Reductive elimination of lets access N-
acetylcarbazole with regeneration of active Pd(0) species
for the next catalytic cycle.
4 with the regeneration
1
2
3
0.01
0.001
0.0001
93
9,300
85,000
760,000
1.
85
76
4
5
5
The lifetime of a catalytic system and its level of
reusability are vital factors in homogeneous catalysis.20 In
this regard, we found that the present catalytic system
remains active after recycling for up to 6 consecutive runs
(see Fig. 4), with a gradual decrease of the activity after
each cycle.21 The stability of recovered catalyst was
6
6
1
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Notes and references
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DOI: 10.1039/C5GC02937F
1
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Scheme 4 Proposed mechanism for the catalytic reaction
3
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of acetanilide with phenylboronic acid.
Conclusions
In summary, we presented the design and synthesis of a
couple of new, water-soluble pincer type Pd(II) complexes
[Pd(L)Cl]as catalysts for the C-H and N-H activation of
acetanilide/N-naphthalen-2-yl-acetamide with diverse aryl
boronic acids in aqueous medium to achieve N-acetyl
carbazoles. The chosen catalytic reaction proceeded
efficiently with low catalyst loading (0.01 mol %) under
aerobic conditions, displaying a broad substrate scope and
functional compatibility. The catalytic system can be
effectively recycled up to six consecutive runs. We feel that
the present palladium catalyzed methodology offers a
simple and green route to synthesize a series of biologically
important functionalized carbazoles in one-pot synthesis
with excellent yields. The scientific merit of this interesting
study is highlighted by the fact that this is a simple but
valuable approach towards a series of highly functionalized
carbazoles by excluding the need of product isolation using
column chromatography.
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Acknowledgements
Mr. Vignesh Arumugam thanks the University Grants
Commission (UGC), New Delhi, India. for the award of Senior
Research Fellowship (SRF) under UGC-BSR RFSMS.
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DOI: 10.1039/C5GC02937F