Hantzsch Ester-Mediated Synthesis of Phenanthridines under Visible-Light Irradiation

Article abstract of DOI:10.1002/asia.202000888

An efficient photocatalytic synthesis of phenanthridines mediated by an organo-photoredox initiator Hantzsch ester has been developed via denitrogenative intramolecular annulation of benzotriazolyl chalcones. The highly reducing photoactivated Hantzsch es

Full text of DOI:10.1002/asia.202000888

CHEMISTRY
AN ASIAN JOURNAL
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Accepted Article
Title: Hantzsch Ester-Mediated Synthesis of Phenanthridines under
Visible Light Irradiation
Authors: Savita B. Nagode, Ruchir Kant, and Namrata Rastogi
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10.1002/asia.202000888
Chemistry - An Asian Journal
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Hantzsch Ester-Mediated Synthesis of Phenanthridines under
Visible Light Irradiation
Savita B. Nagode,a,b Ruchir Kantc and Namrata Rastogia,b*
Abstract: An efficient photocatalytic synthesis of phenanthridines
mediated by organo-photoredox initiator Hantzsch ester has been
developed via denitrogenative intramolecular annulation of
benzotriazolyl chalcones. The highly reducing photoactivated
Hantzsch ester facilitates the transformation of benzotriazolyl
chalcones into phenanthridinyl chalcones through photoinduced
However, the literature search disclosed only rare examples of
phenanthridine synthesis via denitrogenative ring opening of
benzotriazoles,14 none under visible light mediated conditions.
For instance, the Flash Vacuum Pyrolysis (FVP) induced
denitrogenation and ring opening of benzotriazol-1-
yl)phenylethanones
led
to
the
formation
of
7H-
electron transfer (PET) and hydrogen atom transfer (HAT) processes. dibenzo[b,d]azepin-7-ones as the major product and
The mild reaction conditions utilizing inexpensive Hantzsch ester as
photosensitizer, wide reaction scope and excellent functional group
tolerance are notable attributes of the methodology.
Introduction
phenanthridines were detected as minor products only when
o
FVP was carried out at 400-500 C temperature.14b In another
example, phenanthridine was isolated as minor product along
with major product indole in the N-,β-unsaturated
alkylidenaminophenyl radical mediated reaction of β-aryl-β-
(benzotriazol-1-yl)--alkyl/aryl-,β-unsaturated ketones with n-
tributyl tin hydride.14c Recently, Ahmad and co-workers
documented an elegant copper-catalyzed synthesis of 6-
The phenanthridine scaffold is one of the most remarkable ones
among the large family of aza-heterocycles owing to its
presence in natural as well as synthetic molecules of medicinal,
agricultural and industrial importance.1 The phenanthridine
containing molecules exhibit diverse biological activities such as
antibacterial, anticancer, antimalarial, antifungal, antiviral,
cytotoxic etc.2 The phenanthridine motif possess unique
photophysical properties and finds use in luminescent materials,
liquid crystals, and nanomaterials etc.3 Therefore, developing
efficient synthetic protocols for phenanthridine motif is much
desired despite already existing plethora of synthetic strategies.
The best known methods for phenanthridine synthesis include
classical name reactions4 such as Diels-Alder reaction, Pictet-
Hubert reaction, and its Morgan-Walls modification in addition to
transition metal-catalyzed couplings,5 radical cyclizations6 and
other miscellaneous7 methods.
aminophenanthridines
through
denitrogenation
and
decarbonylation of ,-diamino carbonyl adduct generated in-
situ from benzotriazole and 2-oxoiminium ions.14d
We envisaged that installing a chalcone moiety in benzotriazole
derivatives would not only enhance the stability of ring-opened
intermediate o-aminated aryl radical species, it would also offer
a template for intramolecular cyclization offering direct access to
the phenanthridinyl chalcones. We hereby report
a
photocatalytic synthesis of phenanthridinyl chalcones from
corresponding benzotriazole derivatives under “organo-
photoredox” mediated conditions (Scheme 1).
The visible light photoredox catalysis has emerged as an
efficient and greener alternative to the classical radical chemistry
in last decade.8 Consequently, several radical reactions for
preparing phenanthridine derivatives under visible light
irradiation have been developed. These photocatalytic strategies
most commonly involve imidoyl radicals derived from biaryl
isonitriles9 or iminyl radicals derived from biaryl vinyl azides10 or
biaryl oxime ethers.11 Additionally, radicals or radical cations
derived from substrates such as imines, cyanides, N-glycine
esters, pyridinium salts etc. too have been exploited for
constructing phenanthridine scaffold under visible light mediated
conditions.12
Recently, Glorius and co-workers revealed that benzotriazoles
can serve as the synthetic equivalents of o-amino arene
diazonium salts and result into the formation of o-aminated aryl
radicals via visible light photoredox catalyzed denitrogenation
process.13a
Subsequently,
several
denitrogenative
functionalizations of benzoyl and sulfonyl benzotriazoles such as
thiolation, borylation, alkylation, phosphorylation as well as
annulation with alkynes to form 2-substituted indoles, were
reported under visible light irradiation.13b-d
Scheme 1. Phenanthridine synthesis via denitrogenative ring opening of
benzotriazole derivatives
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At this juncture, we envisioned that diethyl 2,6-dimethyl-1,4-
dihydropyridine-3,5-dicarboxylate or Hantzsch ester 2a (HE) in
its excited state (E*oxd = -2.28 V vs. SCE) can effect single
electron reduction of 1a without any additional photocatalyst.16
Consequently, we irradiated a solution of 1a with 1 equivalent of
HE 2a in acetonitrile with blue LEDs under nitrogen atmosphere.
Gratifyingly, reaction afforded the desired product 3a in 89%
isolated yield after 5 hours at room temperature (entry 3).
Further, reactions with other HEs 2a-2d also provided product
3a in comparable yields (entries 3-6) whereas reaction
employing N-methyl substituted HE 2e failed to afford the
desired product (entry 7). Finally 2a was selected as the
preferred organo-photoredox initiator for further optimization of
reaction conditions, due to its easy availability. The optimization
of HE’s amount revealed a marked reduction in the yield of 3a
(67%) with 0.5 equivalent of 2a (entry 8) whereas, increasing the
amount of 2a had negligible effect on the reaction yield (entry 9).
After optimizing the substrate-HE ratio to 1:1, several solvents
including DMSO, DMF, DCM and MeOH were screened in the
reaction. While the reaction afforded desired product 3a in high
yields in all the polar aprotic solvents (entries 3, 10-12), only
traces of product was isolated with MeOH as reaction solvent
(entry 13). Further, failure of reactions conducted in the absence
of visible light or HE 2a established the essential role of both
parameters in the reaction (entries 14-15).
Results and Discussion
The irreversible reduction potential of the selected model
substrate 1a (Ered = -1.57 V vs. SCE in MeCN) calculated by
cyclic voltammetry (please see SI) indicated the thermodynamic
feasibility of single electron transfer (SET) from fac-Ir(ppy)3 (Ered
= -1.73 V vs. SCE)15 to 1a. However, no product formation was
observed when a solution of 1a and 2 mol% of fac-Ir(ppy)3 in
MeCN was irradiated with blue LED under nitrogen atmosphere
(Table 1; entry 1). Upon adding 2 equivalents of Hunig’s base
along with 2 mol% of fac-Ir(ppy)3, the expected product 3a was
isolated in mere 38% yield (entry 2).
Table 1. Optimization of reaction conditions
Entry
conditionsa
Yield (%)b
fac-Ir(ppy)3 (2 mol%), CH3CN
fac-Ir(ppy)3 (2 mol%), DIPEA
(2 equiv.), CH3CN
0
38
1
2
After establishing the suitable conditions, several β-benzotriazol-
1-yl chalcones bearing substitutents with varied electronic
characters were employed in the reaction to assess the scope
and limitations of the reaction (Table 2). Evidently, reaction
showed wide substrate scope since electron donating groups
such as Me, Et, OMe in both aryl rings of chalcone moiety
provided the respective products (3a-3g, 3n-3o, 3r) in high
yields (79-91%). Also, substrates featuring electron withdrawing
substitutions on chalcone moiety reacted efficiently and afforded
the corresponding products (3h-3k, 3p-3q, 3s, 3u) in excellent
yields (80-87%). The unsubstituted benzotriazolyl chalcone 1d
and naphthalene bearing benzotriazolyl chalcone 1m provided
corresponding products 3d and 3m in 87% and 82% isolated
yields, respectively. Interestingly, the bromine of 4-bromomethyl
moiety in 1l was substituted with benzotriazole under the
reaction conditions yielding 4-benzotriazolylmethyl bearing
product 3l in 72% yield. The substrate possessing both electron
donating and electron withdrawing groups 1t too underwent this
photocatalytic transformation smoothly, leading to the product 3t
in 81% isolated yield. Additionally, substrates with heteroaryl or
alkyl moiety too could be successfully employed in the reaction
affording corresponding heteroaryl (3v-3y) or alkyl group (3z)
bearing products in high yields. The substrates 1za and 1zb
possessing substituted benzotriazole moiety afforded products
3za and 3zb in 89% and 92% yields, respectively. Although,
position of substituents in the ring B of chalcone does not have
any significant effect on the reaction outcome, the position of
substituents in the ring A completely changed the end result of
the reaction. For instance, blocking one o-position with Br group
in 1zc afforded a complex reaction mixture and the desired
product 3zc could not be isolated. Further, quite expectedly
2a (1.0 equiv), CH3CN
2b (1.0 equiv), CH3CN
2c (1.0 equiv), CH3CN
89
87
89
3
4
5
84
0
6
7
2d (1.0 equiv), CH3CN
2e (1.0 equiv), CH3CN
2a (0.5 equiv), CH3CN
2a (1.5 equiv), CH3CN
2a (1.0 equiv), DMSO
2a (1.0 equiv), DMF
2a (1.0 equiv), DCM
2a (1.0 equiv), MeOH
2a (1.0 equiv), CH3CN
no PC, CH3CN
67
88
88
84
74
Traces
0
8
9
10
11
12
13
14c
15
0
[a] Unless otherwise mentioned, 0.2 mmol of 1a and specified amount of
photocatalyst/initiator in 2 mL solvent was irradiated with blue LEDs (455 nm)
under N2 atmosphere for 5-8 h. [b] Isolated yields. [c] Without light
[a]
S. B. Nagode, Dr. N. Rastogi
Medicinal & Process Chemistry Division
CSIR-Central Drug Research Institute, Sec. 10, Jankipuram
Extension, Sitapur Road, P.O. Box 173, Lucknow-226031, India
E-mail: namrata.rastogi@cdri.res.in; namrataiit@gmail.com
Academy of Scientific and Innovative Research (AcSIR),
Ghaziabad-201002, India
[b]
[c]
R. Kant
Molecular & Structural Biology Division
CSIR-Central Drug Research Institute, Sec. 10, Jankipuram
Extension, Sitapur Road, P.O. Box 173, Lucknow-226031, India
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substitution at the m-position of ring A led to the formation of an
inseparable mixture of regioisomers in 1:0.25 ratio in case of
3zd. Interestingly, benzotriazolyl chacones derived from drug
molecules such as ibuprofen, flurbiprofen and probenacid could
be successfully subjected to this photocatalytic transformation
affording corresponding products 3ze, 3zf and 3zg, respectively
in high yields.
variable concentrations and temperatures showed only slight
variation in the chemical shift of N-H proton (please see SI for
details), which confirmed the intramolecular H-bonding expected
in the keto-enamine tautomeric structure.
Table 2. Scope and limitations of the reaction.a
Figure 1. Tautomerism in 3 and crystal structure of 3k at 293 K
After establishing the general nature of the reaction, several
control experiments were carried out to gain insight into the
reaction mechanism (Scheme 2).
[a] All reactions were performed with 1 (0.2 mmol) and HE 2a (0.2 mmol)
dissolved in MeCN (2 mL) and irradiated with blue LED (455 nm) under N2
atmosphere. [b] Isolated yields in parentheses.
Notably, the keto-enamine tautomeric structure assigned to the
products initially on the basis of documented literature17 was
confirmed by NMR spectroscopy and single crystal X-ray
diffraction analysis. The existence of keto-imine tautomer was
ruled out promptly due to the missing peaks for the methylene
group in 1H and 13C NMR spectrum. Further, the single crystal X-
ray analysis of a representative compound 3k unambiguously
established the keto-enamine structure of products (Figure 1).18
Moreover, the 1H NMR spectra of compound 3u recorded at
Scheme 2. Control Experiments
The involvement of triplet excited state of HE was suggested
due to reaction failure in presence of oxygen, a strong triplet
quencher (Scheme 2a).19 The interaction between photoexcited
HE 2a and benzotriazolyl chalcone 1a was confirmed by strong
fluorescence quenching (97.18% at 445 nm; please see SI for
details) of HE 2a by 1a (Scheme 2b). However, donor-acceptor
complex formation between HE 2a and substrate 1a was ruled
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out since no appreciable bathochromic shift was observed in the
UV-Vis absorption of 2a upon mixing it with 1a (Scheme 2c;
please see SI for details).20
catalyzed intramolecular sp2 C-H bond amination and isolated
phenanthridinyl-(het)aryl ketones 7a-7c instead of anticipated
quinolino-phenanthridinones.22 Moreover, the phenanthridinyl
chalcone 3s upon treatment with hydroxylamine hydrochloride
provided direct access to novel spiro-isoxazolyl phenanthridine
8.23
The plausible reaction mechanism on the basis of literature
reports and abovementioned control experiments is illustrated in
Scheme 3. The photoinduced electron transfer (PET) from
photoexcited HE* 2a to the substrate 1d leads to the generation
of HE radical cation and benzylic radical A. The intermediate A
upon N2 extrusion forms resonance stabilized radical anion B
which following protonation and intramolecular addition to the
aryl ring, forms radical C. The hydrogen atom elimination from
radical C following hydrogen atom transfer (HAT) from HE
radical cation and subsequent hydrogen elimination affords
product 3d along with dialkylpyridinium dixarboxylate which
upon deprotonation yields dialkylpyridine dixarboxylate
byproduct.
Scheme 5. Synthetic transformations of phenanthridinyl chalcones
Conclusions
In conclusion, the β-benzotriazolyl chalcones were smoothly
converted into corresponding phenanthridinyl chalcones under
visible light organophotoredox mediated conditions. The reaction
exploits high reducing capacity of photoactivated Hantzsch ester,
which acts as photosensitizer as well as photoreductant under
visible light irradiation. This mild photocatalytic transformation
could be achieved effectively with selected drug molecules-
derived substrates as well. Further, the phenanthridinyl chalcone
products were manipulated into several other valuable scaffolds.
Scheme 3. Plausible mechanism
The formation of radical anion B was confirmed by subjecting o-
disubstituted substrate 1zh to the standard reaction conditions in
the presence of excess tributyl allyl tin. The presence of
substitutions at both o-positions prevented intramolecular
annulation and radical anion B could be trapped with tributyl allyl
tin affording adduct 4 (Scheme 4).
Experimental Section
General procedure for photoinitiated annulation of β-
benzotriazolylchalcones 1 into phenanthridinyl chalcones 3
In an oven dried 5 mL snap vial equipped with a magnetic
stirring bar, the β-benzotriazolyl chalcone 1 (0.2 mmol) and
Hantzsch ester 2a (51 mg, 0.2 mmol) were dissolved in
anhydrous CH3CN (2.0 mL, 0.1 M). The resulting reaction
mixture was degassed by three “pump-freeze-thaw” cycles via a
syringe needle. The vial was irradiated using 455 nm blue LEDs
o
Scheme 4. Radical trapping experiment
with a cooling device maintaining the temperature around 25 C.
After 5-8 h of irradiation (TLC monitoring), the reaction mixture
was concentrated under reduced pressure. The residue was
dissolved in ethyl acetate (10 mL) and washed with water (20
mL x 3). The organic layer was dried over Na2SO4 and
concentrated under reduced pressure. The crude product was
purified by column chromatography on silica gel (100-200 mesh)
using hexane/ethyl acetate as eluent to afford the pure product 3.
Further, the practical utility of phenanthridinyl chalcone products
was demonstrated by manoeuvring them into several other
valuable scaffolds, as shown in Scheme 5. As a representative
example, compound 3d was used as ligand for BF2 complex 5
formation with boron trifluoride etherate, confirming the preferred
keto-enamine tautomeric form and thus Z-configuration.17
Another elegant transformation of phenanthridinyl chalcones into
phenanthridinyl triazoles 6a-6c was achieved via Regitz diazo
transfer reaction upon treating them with tosyl azide.21 Further,
we subjected selected products 3d, 3i and 3x to copper
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Acknowledgements
S.B.N. thanks UGC, New Delhi for the Ph.D fellowship. We
thank SAIF division of CSIR-CDRI for the analytical support. We
thank Dr. Tejender S. Thakur of Molecular and Structural
Biology Division, CSIR-Central Drug Research Institute for
supervising the X-ray data collection and structure determination
of 3k. We achknowledge Dr. Iti Gupta, Department of Chemistry,
Indian Institute of Technology, Gandhi Nagar for recording CV
data of 1a. We are grateful to Dr. Vijay Luxami, School of
Chemistry & Biochemistry, Thapar Institute of Engineering &
Technology, Patiala for fluoresence quenching experiments. We
sincerely acknowledge Department of Science & Technology
(DST), New Delhi for the financial support (Project ref. No.:
EMR/2016/006975). CDRI Communication No: (will be inserted
if accepted for publication).
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Keywords: visible-light • benzotriazolyl chalcones •
phenanthridines • Hantzsch ester • denitrogenation
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For internal use, please do not delete. Submitted_Manuscript
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