Hypervalent Iodine-Promoted Aromatization of Exocyclic β-Enaminones for the Synthesis of meta-N,N-Diarylaminophenols

Article abstract of DOI:10.1002/adsc.201700004

A metal- and additive-free milder cascade approach for the synthesis of meta-N,N-diarylaminophenols (DAAP) starting from exocyclic β-enaminones has been developed. The feasibility of the process is rationalized by the suitable molecular geometry of β-enaminones for tandem N-arylative α-iodination and aromatization under milder basic conditions. Furthermore, the developed strategy has been extended to the synthesis of meta-N-benzyl-N-arylaminophenols (BAAP). 4-Ethylpropionyl-2-cyclohexenone has been explored to give 7-diarylaminochroman-2-one (DAAC) by employing a similar one-pot approach. The plausible mechanistic steps were deduced based upon isolation of a stable intermediate and structural identification through X-ray crystallographic analysis. (Figure presented.).

Full text of DOI:10.1002/adsc.201700004

Title: Hypervalent iodine promoted aromatization of exo-cyclic β-
enaminones for the synthesis of meta-N,N-diarylaminophenols
Authors: Dhananjay Bhattacherjee, Vandna Thakur, Arun Shil, and
Pralay Das
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To be cited as: Adv. Synth. Catal. 10.1002/adsc.201700004
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10.1002/adsc.201700004
Advanced Synthesis & Catalysis
COMMUNICATION
DOI: 10.1002/adsc.201((will be filled in by the editorial staff))
Hypervalent iodine promoted aromatization of exocyclic β-
enaminones for the synthesis of meta-N,N-diarylaminophenols†
Dhananjay Bhattacherjee,a,b Vandna Thakur,a,b Arun K. Shil,a,b and Pralay Das*a,b
a
Natural Product Chemistry and Process Development, CSIR-Institute of Himalayan Bioresource Technology,
Palampur-176061, H.P., Fax: +91-1894-230433, E-mail: pdas@ihbt.res.in, pdas_nbu@yahoo.com
Academy of Scientific and Innovative Research, New Delhi, India
b
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######.((Please
delete if not appropriate))
Abstract. A metal and additive free milder cascade
approach for the synthesis of meta-N,N-diarylamino
phenols (DAAP) starting from exocyclic β-enaminones has
been developed. The feasibility of the process is accounted
by the suitable molecular geometry of β-enaminones for
tandem N-arylative α-iodination and aromatization under
milder basic conditions. Furthermore, the developed
strategy has been extended for meta-N-benzyl-N-
arylaminophenols (BAAP) synthesis. 4-Ethylpropionyl-2-
cyclohexenone has been explored to give 7-diarylamino
chroman-2-one (DAAC) employing similar one-pot
approach. The plausible mechanistic steps were drawn
based upon isolation of a stable intermediate and structural
identification through X-ray crystallographic analysis.
Keywords: aromatization; β-enaminones; hypervalent
iodine reagents; triarylamines; metal free reactions
Scheme 1. Synthesis of N,N-disubstituted aminophenol.
Ullmann[9] or Buchwald[10] coupling approaches of N-
arylation of anilines. Traditionally, triaryl amines
have been prepared by ligand-accelerated catalysis in
Ullmann condensation by copper salt,[11] palladium
catalyzed coupling of amine precursors with different
aryl halides[12] or coupling of N,N-bis(4-
Implementation of highly selective, efficient and
environmentally benign chemical processes for
replicating the contemporary synthetic strategies is
the long standing goal of organic chemistry.
Triarylamines are important structural motif with
diverse range of applications in the synthesis of
natural products, dyes and polymers.[1] Particularly
diarylamino phenols and diarylamino coumarins are
potential analogues of triarylamine class of
compounds which have shown prominent
photophysical, electrochemical and pharmacological
properties.[2] Recently, due to their efficient electron
donating and hole transporting abilities, these
compounds have been broadly applied in organic
electroluminescence (EL),[3] organic light emitting
diodes (OLEDs),[4] organic field-effect transistors,[5]
photovoltaic functional materials[6] non-linear
materials7 and xerography.[8]
methoxyphenyl)amine
with
methyl
4-
iodobenzoate,[13] Iron catalyzed aromatic amination
of aryl bromides with Grignard amide as coupling
partner[14]
and amination of chloroarenes with
diarylamines catalyzed by NiCl2(PCy3)2 using
Grignard reagent as base.[15] In all the above
mentioned methods phenolic substrate has been
avoided due to its higher sensitivity with cross
coupling reagents and as a consequence methoxy
derivatives were used as alternatives. Therefore, an
additional demethylation step is required for the
synthesis of anticipated diarylaminophenol (Scheme
1).[4] On the contrary, 7-diarylamino-substituted
coumarin derivatives are generally prepared from the
condensation of corresponding diarylamino phenols
and acetoacetic ester using 70% H2SO4 as catalyst,
but environmental hazards have made the process
limited.[3c] Therefore, development of new efficient
During last few decades,
a
plethora of
methodologies have been developed for the synthesis
of triarylamine derivatives mostly using either
1
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10.1002/adsc.201700004
Advanced Synthesis & Catalysis
method that replaces the tedious approaches to
synthesize diarylaminophenols would be highly
desired from synthetic and mechanistic perspective.
After the seminal work by Varvoglis on
iodophenylation of 2-amino-1,4-benzoquinones, Du
and Zhao reported the α-iodination and concurrent N-
phenylation of β-enaminones in high yields.[16d, 22c]
Recently, the same author has also demonstrated
dehalogenative aromatization of N,N-diaryl α-iodo
enaminone in a single step for -NO2 substituted
compounds.[16b] Also, the reported procedures[16]
revealed that aromatization of exocyclic β-
enaminones still remained a big challenge for
synthetic organic chemists especially under one-pot
metal free conditions. In recent years, hypervalent
iodine reagents[17,18,19] have been widely used as
oxidants for the construction of C-N and C-C bonds,
but there is no single precedent found in literature
where PhI(OAc)2 was used for one-pot aromatization
of exocyclic enaminones.
approach under metal and additive free milder basic
conditions. Furthermore, such a transformation would
complement the recent methods of transition metal
catalysed triarylamine synthesis and strengthening the
potential efficacy of the reaction platform.
In this study, the role of PhI(OAc)2 for N-arylation
and aromatization of exocyclic β-enaminones to N,N-
diarylaminophenol synthesis under metal free milder
basic conditions were critically investigated in one-
pot fashion. To get highest yield of the product,
optimization studies were performed to fix the
reaction co-ordinates such as base, solvent,
temperature and time (Table 1, entries 1-19).
Exhaustive examinations revealed that 4-(3-
oxocyclohex-1-enylamino)benzonitrile (1a) (1 equiv),
PhI(OAc)2 (1.2 equiv), K2CO3 (1.2 equiv) in
Table 2. Metal free dehydrohalogenative aromatization of
exocyclic β-enaminones.
Table 1 Optimization of N-phenylation and aromatization
of exo-cyclic β-enaminones[a]
[a]Reaction conditions: 1a (0.38 mmol), PhI(OAc)2 (0.45
mmol) and K2CO3 (0.45 mmol) in solvent (2 mL) at
given temperature for selected time;
[b]Isolated yields of the product; nd = not detected;
[c]1.2 equiv of PhI(OAc)2 was used.
[d]1 equiv of PhI(OAc)2 was used;
[e] 2 equiv of PhI(OAc)2 was used.
[a] Isolated yields of the products; Reaction conditions: β-
enaminone (100 mg), PhI(OAc)2 (1.2 equiv) and K2CO3
(1.2 equiv) in CH3CN at 100 ºC;
Herein, PhI(OAc)2 was explored for the synthesis
of triarylaminophenols (DAAP, BAAP and DAAC)
from exocyclic β-enaminones following one-pot α-
iodination, N-arylation and aromatization in a cascade
[b] reaction time 8 h. [c] reaction time 7 h.
[d] reaction time 4 h.
2
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10.1002/adsc.201700004
Advanced Synthesis & Catalysis
o
acetonitrile (2 ml) at 100 C for 6 h were found to
be suitable condition to give the meta-(N-(4-
cyanophenyl)-N-phenyl)aminophenol (2a) in 89%
yield (Table 1, entry 9).
Table 3. Dehydrohalogenative aromatization of aryl and
alkyl substituted exocyclic-β-enaminones[a]
With the optimised reaction conditions in hand, we
further proceeded to explore the substrates scope with
different arylamine substituted β-enaminones (both
electron rich and deficient). Under the standard
conditions, electron deficient functional groups such
as p-CO2CH3, p-COCH3 and p-CF3 on the arylamine
substituted β-enaminones 1b-d afforded the
corresponding DAAPs 2b-d in 88-95% yield (Table
2). Notably, electron withdrawing p-NO2-aryl
substituted β-enaminone 1e excellently gave in 95%
yield of DAAP 2e, contrary to the previously reported
reaction.[16] Gratifyingly, electron rich arylamine
substituted β-enaminones 1f and 1g also gave the
respective DAAPs 2f and 2g in 78 and 82% yields
respectively, but the reaction took modest longer time
for its completion (Table 2). Moreover,
haloarylamine substituted β-enaminones 1h and 1i
were found to be highly reactive and gave 2h and 2i
in considerably good yields (89-97%) (Table 2).
Interestingly, m-CF3, p-CH3, H- and m-Cl-benzyl
amine substituted β-enaminones 1j-m underwent
aromatization delivering the corresponding BAAPs
2j-m in good yields (Table 2). Eencouraged by these
results, we next tested different electron rich and
electron deficient diacetoxyiodobenzene derivatives
and checked the versatility and regiospecificity of the
present
method.
Electron
rich
p-
[a] Yield refers to isolated products; Reaction conditions: β-
enaminone (100 mg), PhI(OAc)2 (1.2 equiv) and K2CO3
(1.2 equiv) in CH3CN at 100 ºC.
[b]reaction time 11 h.
methyldiacetoxyiodobenzene smoothly reacted with
β-enaminone 1n to afford 2n in good yield.
Gratifyingly, electron deficient diacetoxyiodobenzene
also gave excellent result when it reacted with 1o to
give 2o in 84% yield. Electron withdrawing β-
enaminone 1a satisfactorily gave 2p in 81% yield
when it reacted with p-bromo substituted PhI(OAc)2
derivative. Interestingly, both electron withdrawing
substrate 1b and iodine(III) reagent reacted well
under optimized condition gave corresponding 2q in
89% yields in 3 h. In all the reactions with substituted
diacetoxyiodobenzene regiospecific N-arylation was
observed. Moreover, the N-alkyl amine substituted β-
enaminones were attempted under the same
conditions unfortunately no products 2r-s were
observed (Table 2). To see the substrate versatility
and effect of functional groups methyl, 5-Ph and 4-
C2H4CO2C2H5 substituted β-enaminones 1r-w
(supporting information) were selected for this study.
5-Ph substituted β-enaminone 1r was treated under
same conditions no significant effect of phenyl group
was observed for its aromatization and corresponding
phenyl-substituted DAAP 3a was obtained in 76%
yield (Table 3). Further, 4-alkylester substituted β-
enaminones (1s-w, supporting information) as novel
molecule was synthesized following our earlier
reports[20] and subjected to similar reaction. No major
role of alkyl group was observed on their reactivity
and subsequently alkyl ester-substituted DAAPs 3b-e
were produced in considerably good yields (Table 3).
In continuation of our study, methyl substituted
[c] reaction time 6h.
[d] reaction time 12 h.
electron rich and deficient β-enaminones (1x-za,
supporting information) were also tested with various
substituted and unsubstituted PhI(OAc)2 derivatives.
5-methyl substituted electron deficient substrates (1x-
y) were subjected to aromatization under optimized
conditions gave excellent yields of the desired
products 3f-g. However, electron rich PhI(OAc)2 took
longer time to furnish desired product in satisfactory
yield. Halogen substituted PhI(OAc)2 derivatives
smoothly reacted with substrate 1z-za afforded 3h-i
in 71-74% yields respectively.
Previous
reports
indicated
that
7-N,N-
diarylsubstituted coumarins are well known for their
huge applications in organic light emitting diodes
(OLEDs).[4] Interestingly, when the same reactions in
Table 3 (1s-w) were subjected to prolonged heating
(36 h) under same condition, the DAACs 4a-d were
identified as major products in reasonably good
yields (Table 4) and could be useful intermediate for
7-N,N-diarylsubstituted coumarin synthesis.
3
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10.1002/adsc.201700004
Advanced Synthesis & Catalysis
Table 4. One-pot approach for dehydrohalogenative
enaminone might stabilize the molecule and hinders
the aromatization to arylamine synthesis. However, in
the present study, when the β-enaminone gets N-
arylated under the optimized conditions, the planarity
as well as long conjugation of the molecule gets
distorted resulting into a facile structural orientation
for rapid C=C bond generation and simultaneous
removal of C2 iodine atom to give DAAP products.
aromatization and cyclization[a]
[a]
Isolated yields; Reaction conditions: ethyl 3-(4-N-
arylamino-2-oxocyclohex-3-enyl)propanoate (100 mg),
PhI(OAc)2 (1.2 equiv) and K2CO3 (1.2 equiv) in CH3CN
at 100 oC for 36 h.
Scheme 2. Aromatization of 2-iododiaryl substituted β-
enaminone to DAAP synthesis.
Further, the addition of TEMPO in the reaction
mixture did not affect the formation of desired
product indicated the non-radical pathway of the
reaction (supporting information). Moreover, two
additional experiments were also conducted to
understand selectivity and mechanistic pathway of
the reaction. When 2-iodo-arylamino-substituted
enaminone[21b] (7a) subjected to aromatization under
conditions A and B, no product was formed (Scheme.
3, (a)). Similarly, treatment of N-phenylated β-
enaminone 1zb under the reaction condition A, did
not led to the formation of desired product (Scheme.
3 (b)). Therefore, these experiments indicated that the
exact structural pattern or active sites present in β-
enaminones (1a-zb) are essential for its reactivity
with PhI(OAc)2 to produce DAAP, BAAP and
DAAC products.
To understand the possible mechanistic pathway,
series of reactions have been performed to identify
intermediates. In our screening, when we treated 1a
under the same conditions at room temperature we
observed a new spot on TLC in major quantity. This
compound was further isolated and identified by
NMR and ESI-MS spectroscopic studies and detected
as 2-iodo-diarylamino-substituted enaminones (5a) in
65% yield. Finally, the structure of 5a was confirmed
by the single crystal X-ray crystallographic analysis
(Figure 1, 5a).
Figure 1. Molecular structures of 2a and 5a determined by
X-ray crystallography.
Next to prove the involvement of 5a in the reaction
mechanism, when we treated it with K2CO3 in
o
Scheme
3.
(a)
aromatization
of
2-iodo-3-
CH3CN solvent at 100 C the corresponding DAAP
(phenylamino)cyclohex-2-enone; (b) aromatization of 3-
(diphenylamino)cyclohex-2-enone.
product 6a was formed in 96% yield (Scheme 2).
Similar method was applied for the synthesis of 6b-c
from intermediates 5b-c (supporting information) in
93-97% yields respectively (Scheme 2). The presence
of long conjugation of “N-C=C-C=O” in β-
4
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10.1002/adsc.201700004
Advanced Synthesis & Catalysis
On the basis of the precedent literature[21a, 22] and
β-enaminone
1a
(100
mg,
0.4717
mmol),
diacetoxyiodobenzene (0.5660 mmol) and K2CO3 (0.5660
mmol) were taken in an oven dried reaction vial (40 mL)
and acetonitrile (2 ml) was added to it. The reaction
mixture was heated to 100 ºC for 6 h under magnetic
stirring. The progress of reaction was monitored by TLC
(EtOAc and n-hexane = 5: 95 as the mobile phase). After
the completion, the reaction mixture was extracted with
ethyl acetate (3x2 ml) by adding 1 mL water and dried on
anhydrous Na2SO4. Evaporation of the combined organic
layer under reduced pressure followed by column
chromatography (hexane: ethyl acetate = 95:5) over silica
gel (mesh 60-120) afforded 89% yield of the 3-N,N-
disubstituted aminophenol (1) as white solid. mp: 158-160
C.
intermediate detection, a plausible mechanism for the
aromatization process has been drawn in Scheme 4.
The first step involves the ligand exchange reaction
between 1a and PhI(OAc)2 which leads to the
formation of stable intermediate 5a through iodine-
to-nitrogen 1,3 phenyl migration mechanism.
Recently, Shafir and co-workers described the N-
arylation, α-iodination sequence of imidazoles that
proceeds via iodine-to-nitrogen 1,3 phenyl migration
pathway.[21a] Moreover, the ligand exchange reaction
further supported by the regiospecific formation of
only one aromatised product (2n-q and 3f-i) with
substituted diacetoxyiodobenzenes (Table 2 and 3).
Intermediate 5a might exist in equilibrium with its
enol form 5aʹ under basic condition. Due to the α-
iodinated N-phenylation of β-enaminone, the
conjugation of N-C=C-C=O moiety gets destroyed
hence facilitating the keto-enol tautomerization
̊
“CCDC for compound 2a: 1049274 and for compound 5a:
1049341 contain the supplementary crystallographic data
for this paper. These data can be obtained free of charge
from the Cambridge Crystallographic Data Centre via.
www.ccdc.cam.ac.uk/data_request/cif.”
Acknowledgements
We are grateful to Director CSIR-IHBT for providing necessary
facilities during the course of work. The authors thank CSIR, New
Delhi for financial support as part of XIIth Five Year Plan
programme under title ORIGIN (CSC-0108). We also thank SAIF,
IIT Bombay for Single Crystal X-Ray Diffraction analysis. DB,
VT and AKS thank UGC and CSIR, New Delhi for awarding
fellowships.
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10.1002/adsc.201700004
Advanced Synthesis & Catalysis
COMMUNICATION
Hypervalent iodine promoted aromatization of
exocyclic β-enaminones for the synthesis of meta-
N,N-diarylaminophenols
Adv. Synth. Catal. Year, Volume, Page – Page
Dhananjay Bhattacherjee, Vandna Thakur, Arun
K. Shil, and Pralay Das*
7
This article is protected by copyright. All rights reserved.