One pot synthesis of phenanthridines using a palladium-catalyzed cyclization of aromatic ketoximes with aryl iodides: Via Beckmann rearrangement

Article abstract of DOI:10.1039/c6ra07423e

The catalytic reaction of ketoximes with aryl iodides via a Beckmann rearrangement in the presence of a catalytic amount of Pd(OAc)₂, Ag₂O and ZnBr₂ gave substituted phenanthridines in good to excellent yield. In the reaction, aromatic ketoximes converted first to acetanilides in the presence of ZnBr₂/TFA via a Beckmann rearrangement followed by arylation in the presence of palladium complex. Furthermore, ortho-arylated acetanilides were converted to phenanthridine derivatives in the presence of a Hendrickson reagent.

Full text of DOI:10.1039/c6ra07423e

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One pot synthesis of phenanthridines using a palladium- catalyzed
cyclization of aromatic ketoximes with aryl iodides via Beckmann
rearrangement
Received 00th January 20xx,
Accepted 00th January 20xx
Gajula Rajua, Vijayacharan Gugulotha and Battu Satyanarayanaa*
DOI: 10.1039/x0xx00000x
www.rsc.org/
The catalytic reaction of ketoximes with aryl iodides via Beckmann methods, limited substrate scope and in some cases harsh reaction
rearrangement in the presence of catalytic amount of Pd(OAc)2, conditions. Therefore, the development of more milder, general
Ag2O and ZnBr2 gave substituted phenanthridines in good to and convenient methods from easily available starting materials for
excellent yields. In the reaction aromatic ketoximes converted the synthesis of the phenanthridine ring structures. Herein, we wish
first into acetanilides in presence of ZnBr2/TFA via Beckmann to report one pot synthesis of phenanthridines in the presence of
rearrangement then subsequent arylation in presence of palladium catalyst. The catalytic reaction was also compatible with
palladium complex. Later, ortho-arylated acetanilides were various functional groups such as electron-rich, electron-deficient
converting into phenanthridine derivatives in presence of and halogen group substituted aromatic oximes and substituted
Hendrickson reagent.
iodo benzenes. Further, ortho-arylatedanilides were converted into
Phenanthridines and its derivatives are the important core useful heteroaromatics such as phenanthridines in presence of
structures of many hetero cyclic naturally occurring and biologically Ph3PO and Tf2O.15
active molecules. They have many clinical applications including
R2
R4
N
antiprotozoal, antibacterial, anticancer agents, pharmaceutically
and optoelectronic properties.1 Several methods have been
reported for the syntheses of phenanthridine skeleton such as
radical,2 one pot cascade,3 benzyne-mediated,4 photochemical,5
hypervalent iodine-promoted,6 photocyclized,7 microwave assisted8
and transition-metal-catalyzed.9 Palladium-catalyzed C-C bond-
forming reactions involving direct C-H bond activation has a
powerful method for the synthesis of complex polycyclic
heterocyclic structures.10 In 2009, Fagnou group reporteda simple
catalytic method for direct intramolecular arylation reaction for the
synthesis of six- and five-membered ring biaryls.11 Recently, Lautens
described a palladium-catalyzed reaction involving ligand-mediated
C-H activation and cross-coupling for the preperation of substituted
phenanthridine derivatives (Fig 1).12 Fensterbank et al.
demonstrated a palladium catalyzed reaction for the most efficient
synthesis of phenanthridines from benzylamines and aryl iodides
through the oxidative dehydrogenative coupling by palladium and
norbornene mediated domino reaction.13
R2
I
N
Previous work
R3
R3
X
Coupling reaction
R1
R1
ref 12
R2
Two bonds construction
R2
HO
I
N
N
Present work
R3
C-H Bond activation
R3
R1
Three bonds construction
R1
Fig 1. Synthesis of phenanthridines
I
Me
Br
N
Me
Pd(OAc)2 (5 mol %)
Br
OH
N
Ag2O (1.5 eq),
ZnBr2 (50 mol %),
TFA, 110 oC, 16hr.
4a
2a
1a
2. Hendrickson reagent
Scheme 1.Phenanthridine synthesis of 4a
More recently, Bin Li described a direct and efficient method for
the synthesis of phenanthridines using a palladium-catalyzed C-H
activation and C-C bond forming intramolecular reaction of N-(2-
haloaryl)-imines.14 Although a number of useful synthetic methods
are available for the synthesis of phenanthridine core structures,
there are remain many limitations such as multi step synthetic
The oxidative cyclization of 3-bromo acetophenone oxime 1a with
iodobenzene 2a in the presence of Pd(OAc)2 (5 mol %), Ag2O (1.5
equiv), and ZnBr2 (0.5 equiv) in TFA at 110 °C for 16 h gave 3-
bromo-6-methylphenanthridine 4a in 87% isolated yield (scheme 1).
In the reaction, 3-bromo acetophenone oxime 1a converted first
into acetanilide via Beckmann rearrangement in presence of ZnBr2
and TFA then subsequent ortho-arylation of acetanilide in presence
of Pd(OAc)2 and Ag2O. Further, ortho-arylatedacetanilides 3a were
a.Department of Chemistry, University College of Science, Osmania University,
Hyderabad, India 500 007. E-mail: satyambchem@yahoo.co.in; Tel: +91-
9440065576; Fax: +91-40-27090020.
Electronic Supplementary Information (ESI) available: [This material is available
free of charge via the Internet]. 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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converted in to 3-bromo-6-methylphenanthridine 4a 87% yield in The catalytic reaction was also tested with various oxidants such as
presence of Hendrickson reagent.
Ag2O, AgOTf, AgOCOCF3, AgF, and Ag2CO3 (Table 1, entry 4-8). The
ortho-arylated anilide 3a was observed in the presence of Ag2O in
65% yield (entry 4), AgOTf, AgOCOCF3, AgF, and Ag2CO3 were less
effective for giving 3a in 30%, 10%, 5%, 3% yields respectively (entry
5-8). The reaction was tested without palladium catalyst and just
only in the presence of Ag2O and TFA. In meantime the catalytic
reaction was also tested without Ag2O only with palladium and TFA.
In both the reactions no ortho-arylated anilide product 3a was
observed (entry 9 and 10). The catalytic reaction was also studied
with various acids such as TfOH, ZnBr2, ZnCl2 and p-TsCl (entry 11-
14). Among them ZnBr2 is the best for the Beckmann
rearrangemant for providing the ortho-arylated anilide 3a in
excellent 89% yield (entry 12), TfOH, ZnCl2, p-TsCl ortho-arylated
anilide 3a was observed in 30%, 43% and 65% yields (entry 11, 13
and 14). In meantime the catalytic reaction was also studied with
Lewis acids such as AlCl3 and PCl5 under the same optimized
conditions in the reaction no arylation compound 3a was observed,
only minor amount of 3-bromo acetanilide 7a was obtained (entry
15 and 16). The yield of product 3a was determined by the 1H NMR
integration methods using mesitylene as an internal standard.
These results clearly revealed that both palladium and oxidant such
as Ag2O are crucial for the ortho-arylation reaction. In meanwhile,
Intra-molecular cyclization reaction for the synthesis of
phenanthridine was also tested with 3-bromo acetophenone oxime
1a under the optimized reaction conditions and TFAA, in the
reaction cyclization compound 4a was observed only 35% yield.
When the cyclization reaction was performed in the presence of
Hendrickson reagent instead of TFAA in the reaction 3-bromo-6-
methylphenanthridine 4a was observed in 83% yield.
O
Me
NH
I
Me
Br
Pd(OAc)2 (5 mol %)
Br
OH
N
Ag2O (1.5 eq),
ZnBr2 (50 mol %),
TFA, 110 oC, 16 hr.
2a
1a
3a, 89%
In the beginning of the project, our aim was to synthesize
phenanthridines from cyclization reaction of ketoximes with aryl
iodides. For this strategy we have started our optimization studies.
The cyclization reaction of 3-bromo acetophenone oxime 1a,
iodobenzene 2a was examined in the presence of Pd(OAc)2 (5 mol
%) and AgOAc (1.5 eq) in AcOH at 110 °C for 16 h. However, in the
reaction, no cyclization product 4a was observed only a minor
amount of ortho-arylatedanilide 3a was observed. The catalytic
reaction was also tested with various solvents such as TFA and TfOH
provided ortho-arylatedanilide 3a in 50%, 28% yields, surprisingly.
In presence of acidic solvents ketoxime was converted in to
acetanilide via Beckmann rearrangement followed by ortho-
arylation in presence of palladium complex.
Table 1. Optimization studiesa
Entry Pd cat.
Oxidant
AgOAc
AgOAc
AgOAc
Ag2O
Acid
Solvent
AcOH
TfOH
TFA
TFA
TFA
TFA
TFA
TFA
TFA
TFA
TFA
TFA
TFA
TFA
TFA
TFA
Yield (%)b
1
2
Pd
Pd
Pd
Pd
Pd
Pd
Pd
Pd
Pd
-
-
nr
28
50
65
30
10
5
-
3
-
4
-
5
AgOTf
AgOCOCF3
AgF
-
6
-
-
7
8
Ag2CO3
-
-
3
9
-
nr
nr
30
89
43
65
nr
nr
10
11
12
13
14
15
16
Ag2O
-
Pd
Pd
Pd
Pd
Pd
Pd
Ag2O
TfOH
ZnBr2
ZnCl2
p-TsCl
AlCl3
PCl5
Ag2O
Ag2O
Scheme 2. Scope of the aromatic oximes
Ag2O
Ag2O
Under the similar reaction conditions, the catalytic reaction was
Ag2O
examined with various substituted ketoximes 1a-e. Thus, 3-bromo
aAll reactions were carried out using 1a (1.0 mmol), iodo benzene 2a 1a, 3-methoxy 1b, 2-chloro 1c, 1-naptho 1d, 3,4-dimethoxy 1e
(3.0mmol), oxidant (1.5mmol), acid (0.5 mmol) and Pd(OAc)2 (5 mol %) in
substituted ketoximes with aryl iodides 2a-e underwent cyclization
solvent (2.0 mL) at 110 °C for 16h. bYields were determined by the 1H NMR
reaction selectively at the less hindered ortho C-H bond yielding the
integration method, using mesitylene as an internal standard.
corresponding 6-methyl phenanthridines 4b-i in good to excellent
In order to increase the yield of 3a, initially reaction was studied
yields (scheme 2). The cyclization reaction of 3-bromo
with different type of solvents such as AcOH, TfOH and TFA with
acetophenone oxime 1a with 4-methyl 2b, 4-methoxy 2c, 4-bromo
AgOAc (Table 1, entry 1-3). Among these TFA solvent is providing
2d, 4-CO2Me 2e gave the corresponding 6-methyl phenanthridines
ortho-arylated anilide compound 3a in 50% yield (entry 3) and TfOH
4b-e in excellent 80%, 73%, 78% and 68% yields respectively. Next
solvent is less effective and it is giving 28% yield (entry 2). AcOH
the cyclization reaction of 3-methoxy acetophenone oxime 1b with
solvent is the totally inactive for the arylation reaction (entry 1).
iodo benzene 2a, 2-chloro acetophenone oxime 1c with 4-methyl
2 | J. Name., 2012, 00, 1-3
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iodo benzene
COMMUNICATION
bromo acetophenone oxime 1a, in this reaction, there are two
ortho C-H bonds for cyclization. In both the reactions, two types of
3-bromo phenenthridines cyclization products were observed 4qa,
4qb in 61%, 15% and 4ra, 4rb in 60%, 13% yields (scheme 4).
2b
affording
the
corresponding
6-
methylphenanthridines 4f and 4g in 79% and 65%, yields
respectively. 1-naptho 1d, 3,4-dimethoxy 1e acetophenone oximes
with iodo benzene 2a also proceeded smoothly under similar
reaction conditions providing the 6-methylphenanthridines 4h and
4i in 70% and 79% yields, respectively.
Next, the scope of the cyclization reaction of substituted
symmetrical acetophenone oximes was examined under the
optimized reaction conditions (scheme 3). Initially, the reaction was
tested with 4-chloro acetophenone oxime 1f with iodo benzene 2a
in the reaction two types of cyclization products were observed
mono arylated 4ja and di arylated 4jb in 64% and 13% yields. The
cyclization reaction was tested with O-methyl 4-chloro ketoxime 5a
and ido benzene 2a under similar reaction conditions forming mono
arylated 4ja and di arylated 4jb in 47% and 39% yields. The catalytic
reaction was also tested with O-acyl 4-chloro ketoxime 6a,
cyclization with iodo benzene 2a, under the optimezed reaction
conditions providing mono arylated 4ja and di arylated 4jb in 40%
and 18% yields. The cyclization of 4-methoxy ketoxime 1g, with 4-
methoxy iodo benzene 2c, giving 4ka and 4kb, in 47% and 39%
Scheme 4. Scope of the unsymmetrical iodo benzenes
The Beckmann rearrangement of 3-bromo acetophenone oxime 1a
in presence of ZnBr2/TFA providing 3-bromo acetanilide 7a in 95%
yield. In meantime the catalytic reaction was also studied with 3-
bromo acetanilide 7a with iodo benzene 2a in presence of
palladium catalyst and Ag2O gave ortho-arylated 3-bromo
yields, 4-methoxy ketoxime 1g, with 4-methyl iodo benzene 2b, acetanilide 3a in 86% yield. Further ortho-arylated 3-bromo
acetanilide 3a was converted in to phenenthridine derivatives by
giving 4la and 4lb, in 60% and 23% yields. The cyclization of 4-
methyl ketoxime 1h, with iodo benzene 2a, was also providing 4ma
and 4mb, in 55% and 21% yields. The cyclization reaction of
acetophenone oxime 1i with iodo benzene 2a in the raection 6-
methylphenanthridine 4na and 4nb was observed in 49% and 32%
yields. The reaction was tested with propiophenone oxime 1j, with
4-methyl iodo benzene 2b, 6-ethylphenanthridines 4oa and 4ob
were observed in 57% and 31% yields. The reaction of Isobutero
acetophenone oxime 1k, with iodo benzene 2a, was also giving 6-
isopropyl phenanthridines 4pa and 4pb in 53% and 22% yields.
using Hendrickson reagent in 83% yield (scheme 5).
Scheme 5. Phenanthridine synthesis
Me
OH
N
1
ZnBr2/ TFA
H
N
Me
O
H
N
N
Me
Me
7
Pd(OAc)2
O
Pd
4
OAc
8
Tf2O/ PPh3PO
H
Ph
I
2
H
Ag2O
N
Me
N
Me
O
O
Ph
Pd
Ph
I
Scheme 3. Scope of the symmetrical oximes
3
OAc
9
The catalytic reaction was also tested with unsymmetrical iodo
Scheme 6. Proposed mechanism
benzenes such as 3-methyl 2f, 3-fluoro 2g, iodo benzenes with 3-
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A possible reaction mechanism was explained for the catalytic
reaction in Scheme 6.9,16 The catalytic reaction first ketoxime 1
4
1
converting in to
acetanilide 7 in presence of ZnBr2/TFA via
oxygen chelating with
Beckman rearrangement. Acetanilide
7
palladium complex followed by ortho metallation of aromatics
provided intermediate 8. Trans metallation of palladium in to the
aryl iodide 2 bond in presence of Ag2O provided intermediate 9.
Next reductive elimination of intermediate 9 in presence of AcOH or
TFA giving ortho-arylated acetanilide 3 and regenerate the active
5
6
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catalyst for next catalytic
reaction. Later,
ortho-arylated
7
8
acetanilides 3 were coverted into phenanthridine derivatives 4 in
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Conclusions
In conclusion, we have developed a palladium-catalyzed
synthesis of phenanthridine derivatives from the catalytic
reaction of substituted aromatic ketoximes with aryl iodides.
The present catalytic reaction is highly regioselective in case of
unsymmetrical oximes, yielding substituted phenanthridines in
good to excellent yields. In the reaction, meta- substituted
ketoximes cyclization takes place very selectively at the less
hindered aromatic C-H bond and also in the reaction oxime
moiety is converting into the anilide moiety. Further,
substituted arylated anilides were converted into
phenanthridine derivatives in the presence of Hendrickson
reagent.
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Acknowledgements
Gajula Raju thankful to the Head, Department of Chemistry,
Osmania Univeristy, Hyderabad, India for providing research
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4 | J. Name., 2012, 00, 1-3
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Graphical Abstract
One pot synthesis of phenanthridines using a palladium - catalyzed
cyclization of aromatic ketoximes with aryl iodides via Beckmann
rearrangement
Gajula Rajua, Vijayacharan Gugulotha and Battu Satyanarayanaa*
Abstract:
The catalytic reaction of ketoximes with aryl iodides via Beckmann rearrangement in the presence of catalytic amount of Pd(OAc)2,
Ag2O and ZnBr2 gave substituted phenanthidines in good to excellent yields. In the reaction aromatic ketoximes converted first into
acetanilides in presence of ZnBr2/TFA via Beckmann rearrangement then subsequent arylation in presence of palladium complex. Later,
ortho-arylated acetanilides were converting into phenanthridine derivatives in presence of Hendrickson reagent.