Metal-free, visible-light-promoted oxidative radical cyclization of: N-biarylglycine esters: One-pot construction of phenanthridine-6-carboxylates in water

Article abstract of DOI:10.1039/c9gc01557d

A metal-free, visible-light (blue LED: Hν = 425 ± 15 nm) photoredox-catalyzed intramolecular cyclization reaction of N-biarylglycine esters to phenanthridine-6-carboxylates in water under an open air atmosphere at ambient conditions has been developed. A

Full text of DOI:10.1039/c9gc01557d

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Metal-free, visible-light-promoted oxidative radical cyclization of N-
biarylglycine esters: one-pot construction of phenanthridine-6-carboxylates in
water
Palani Natarajan*, Deachen Chuskit and Priya
Department of Chemistry & Centre for Advanced Studies in Chemistry, Panjab University,
Chandigarh - 160 014, India
pnataraj@pu.ac.in
Abstract
A metal-free, visible-light (blue LED: h = 42515 nm) photoredox-catalyzed intramolecular
cyclization reaction of N-biarylglycine esters to phenanthridine-6-carboxylates in water under
open air atmosphere at ambient condition has been developed. A plausible mechanism is
proposed for the reaction. Using catalytic amount (5 mol%) of rose bengal and a blue LED, the
N-biarylglycine esters have been transformed into radical intermediates, which then underwent
intramolecular cyclization reaction followed by dehydrogenation in one-pot to give desired
products in up to 93% yield. This convention is pertinent for the gram-scale synthesis, eco-
friendly and atom economy as compared to reported methods for the synthesis of
phenanthridines. Moreover, to the best of our knowledge, no instance has hitherto been
accounted for on the visible-light photocatalyzed transformation of inexpensive and
biocompatible N-biarylglycine esters to phenanthridines.
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Introduction
Phenanthridine1 and its derivatives are important structural motifs that are widespread in many
naturally occurring alkaloids, pharmaceuticals, agrochemicals and fluorescence staining agents.2
In pharmaceuticals, phenanthridines are integral to a large number of synthetic drug substances
that exhibit antitumor, antiviral, antifungal, antibacterial, antiseptic, cytotoxic and DNA
intercalation properties.3 Besides, the phenanthridines are also used in functional materials as
hole and electron transporting materials.4 Several synthetic strategies have been accounted for
their preparations.1-4,5 Among, in the course of the most recent decade, the profoundly used
methodology is the visible-light-induced photoredox catalysis including i) a tandem radical
addition-cyclization reaction of 2-isocyanobiphenyls6 and 2-(1-azidovinyl)-1,1'-biaryls7 with
diverse radical precursors (Scheme 1, a and b) and ii) the N-O bond homolysis cyclization
reactions of oxime ethers8 and oxime esters9 (Scheme 1, c and d). Despite the energy efficient,
mild and best usage, these strategies remain associated with specific hindrances, for example,
selection of toxic and explosive reagents, solvents and metal catalysts. As a consequence, the
development of a practical and environmentally benign synthetic protocol to get access to
phenanthridine derivatives is profoundly attractive and proceeding with enthusiasm for the fields
of organic, material and pharmaceutical chemistry.
N-biarylglycine esters10 are biocompatible, can be stored for longer periods of time under
ambient conditions, wide variety of derivatives are commercially available and were extensively
applied for the synthesis of structurally diverse and biologically active functional amines.10,11
However, to the best of our knowledge, no occasion has up to this point been accounted for on
the visible-light photocatalyzed synthesis of phenanthridines using N-biarylglycine esters as a
sole precursor.
In continuation of our studies on the visible-light-induced photoredox catalysis,12 we
demonstrate herein a novel, efficient and eco-friendly protocol for the preparations of
phenanthridine-6-carboxylates from N-biarylglycine esters in the presence of 5 mol% of rose
bengal as a photocatalyst, water as a solvent, molecular oxygen as an oxidant and blue LED (h
= 42515 nm) as irradiation sources at ambient conditions, cf. Scheme 1. A plausible mechanism
is proposed for the reaction. The N-biarylglycine esters have been transformed into radical
intermediates, which at that point experienced intramolecular cyclization reaction followed by
2

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dehydrogenation in one-pot to give desired products in good to excellent yields. It is worth
nothing that in addition to the use of organic photocatalyst, i.e., rose bengal, this convention
exploits oxygen and water as clean, abundant, and sustainable chemical reagents/solvents.
c)
R
O
O
R
R
a)
b)
NC
O
N
N
R
+
R
R
N
photocatalyst
Ru2+/Ir3+
photocatalyst
Ru2+/Ir3+
R
N3
d)
visible-light
organic solvents
visible-light
organic solvents
+
Scheme 1. Well-known and largely investigated methods for the synthesis of phenanthridines
under visible-light photocatalysis.6-9
COOR
COOR
blue LED (34 W,  = 425 nm)
HN
N
rose bengal (5 mol%)
R'
O2, water, 24 h, rt
(56-93%)
R'
R''
R''
Scheme 2. A metal-free, visible-light photoredox-catalyzed intramolecular cyclization reaction
of N-biarylglycine esters to phenanthridine-6-carboxylates in water at ambient condition reported
in this work.
Results and Discussion
An environmentally friendly and commercially available methyl 2-([1,1'-biphenyl]-2-
ylamino)acetate (1a) and [Ru(bpy)3]2+, respectively, were picked as model substrate and
photocatalyst to optimize the reaction conditions. When acetonitrile solution containing 1a and a
catalytic amount of [Ru(bpy)3]2+ (3.0 mol%) was irradiated by blue LEDs (34 W, h = 42515
nm) in an open vessel under ambient conditions for 36 h, and the desired methyl phenanthridine-
6-carboxylate (2a) was obtained in 47% yield, cf. Table 1 and Entry 1. The product 2a was
characterized by mass and NMR analysis, cf. Electronic Supporting Information (ESI). A few
different photocatalysts including Ir(ppy)3, rose bengal, methylene blue, fluorescein, eosin Y and
rhodamine were then examined, and rose bengal was observed to be the best for this reaction
(Table 1 and Entry 3). Thus, rose bengal was chosen as the photocatalyst for the reaction. Next,-
3

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Table 1. Selected results of screening the optimal conditions for the photocatalytic synthesis of
phenanthridine-6-carboxylates from N-biarylglycine estersa
COOR
COOR
blue LED (34 W, hv = 425 nm)
HN
N
photo-catalyst
solvent, rt
2a
1a
Entry Catalyst (mol%)b
Solventc
CH3CN
CH3CN
CH3CN
Time (h)
Yield (%)
47
1
[Ru(bpy)3]2+ (3)
36
36
36
36
36
36
36
24
22
36
24
24
24
24
24
24
24
24
6
2
Ir(ppy)3 (3)
22
3
rose bengal (3)
84
4
methylene blue (3) CH3CN
13
5
fluorescein (3)
rhodamine (3)
eosin Y (3)
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
hexane
DMSO
CH3OH
DMF
21
6
29
7
16
8
rose bengal (5)
rose bengal (10)
Nil
89
9
91
10
11
12
13
14
15
16
17
18
19
20
21
22
23
NRd
<10
43
rose bengal (5)
rose bengal (5)
rose bengal (5)
rose bengal (5)
rose bengal (5)
rose bengal (5)
rose bengal (5)
rose bengal (5)
rose bengal (5)
rose bengal (5)
rose bengal (5)
rose bengal (5)
rose bengal (5)
33
40
water
93
water-CH3CN
water
95
18e
<5%f
39
water
water
water
12
18
21
30
68
water
83
water
89
water
94
a
Unless stated otherwise all reactions were performed in a vial equipped with 2-([1,1'-
biphenyl]-2-ylamino)acetate (1a, 0.3 mmol) and 5.0 mol% of photocatalyst in solvent were
irradiated using 34 W blue LEDs under an open air atmosphere at room temperature for 24-36 h.
b
c
Commercially available high purity catalyst were purchased and utilized as such. Solvents
d
e
were purified before use. Standard conditions without catalyst. Reaction performed under N2
atmosphere. f Reaction was carried in dark. N.R. no reaction.
4

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the impact of amount of rose bengal on the product yield was likewise explored. An increase in
the catalyst amount from 3.0 mol% to 5.0-10.0 mol%, the reaction time was clearly abbreviated
with 89-91% yield of 2a (Table 1 and Entries 8-9). In any case, no product was seen without a
photocatalyst (Table 1 and Entry 10). Afterwards, the influence of solvents on the formation of
2a was explored (Table 1 and Entries 11-16). The examination uncovered that acetonitrile, water
and their combinations were the most effective medium to promote the reaction with 93-95%
yield (Table 1 and Entries 15-16). Water was picked as a solvent for this transformation (Scheme
2). Later, we explored the influence of atmosphere on the reaction. Switching the reaction
atmosphere from open air/O2 (balloon) to N2, inferior yields of 2a were obtained (Table 1 and
Entry 17). Finally, the control experiments proposed that photocatalyst, light, and oxygen were
essential for this reaction to proceed (Table 1 and Entries 10, 17-18). Thus, the optimized
conditions for the synthesis of phenanthridine-6-carboxylates from N-biarylglycine esters were
5.0 mol% of rose bengal, blue LED lights, water and molecular oxygen at ambient condition for
about 24 h. Under the optimized reaction conditions, we also performed a model reaction at
different time intervals (6 h, 12 h, 18 h, 21 h and 30 h) to check the reaction progress. As
expected, product yield is directly proportional to the reaction time (Table 1 and Entries 19-21)
until 24 h.
To explore the substrate scope of this methodology (Scheme 2), the optimized reaction
conditions (Table 1) were applied to a series of N-biarylglycine esters. First, we studied the
effect of the substituents in the aromatic ring with glycine ester group and the outcomes are
summarized in Table 2. Notice that the reaction was slightly affected by the electronic effects of
substituents in the aromatic ring with glycine ester group. The substrates bearing electron-
donating groups (CH3 and OCH3) provided expected phenanthridine-6-carboxylates (2b, 2c, 2d
and 2e) in higher yields than those bearing electron-deficient groups (F, Cl, NO2, and COOCH3):
probably due to the lower oxidation potentials of latter precursors.13 Afterwards, the effects of
substituents at the 4-position of the aromatic ring without the glycine ester group were examined.
Obviously, both electron-rich and electron-deficient substrates underwent cyclization readily
producing the desired products 2j-2p in moderate to good yields (Table 2). Therefore, different
ester groups (ethyl, n-propyl and C6H5) were explored, and corresponding products 2q, 2r and 2s
were obtained in good yield (Table 2). Interestingly, no desired products were detected for the
5

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reactions with 1t and 1u as substrates, most likely due to poor stabilization of radical
intermediates as well as over oxidation.6,10 In this way, a wide range of electronically and
structurally diverse N-biarylglycine esters can proficiently be transformed to phenanthridines
with moderate to good yields, cf. Table 2. Notably, various functional groups such as methoxy,
chloro, fluoro, trifluoromethyl, and nitro groups were tolerated well under the present
experimental conditions, which will be suitable for potential further functionalization.1-3
Table 2. Substrate scope for the transformation of biarylglycine esters into phenanthridine-6-
carboxylatesa
COOR
COOR
blue LED (34 W,  = 425 nm)
HN
N
rose bengal (5 mol%)
R'
O2, water, 24 h, rt
R'
R''
R''
2
1
Cl
Cl
Cl
Cl
COONa
I
I
NaO
O
O
I
I
Rose Bengal (RB)
6

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H3C
O
O
N
O
N
H3C
N
O
O
O
2a
, 93%
2b
2e
2c
, 88%
, 92%
H3CO
O
O
O
N
H3CO
N
O2N
N
O
O
O
2d
2g
2f
, 94%
, 90%
, 56%
Cl
F
O
O
O
N
N
F
N
O
O
O
, 78%
2h
, 69%
2i
, 77%
CH3
O
OCH3
O
O
N
N
N
O
O
O
O
2j
, 91%
2k
2l
, 88%
, 90%
CF3
O
F
O
N
N
N
O
O
O
O
2m
, 92%
2n
2o
, 84%
, 89%
Cl
O
H3C
H3C
O
O
N
N
N
O
O
, 93%
2q
2r
, 92%
2p
2s
, 87%
H3C
H3C
H3C
O
N
C6H5
N
C6H5
N
CH3
O
2t
2u
, < 5%
, trace
, 83%
a Unless stated otherwise all reactions were performed in crimp vials equipped with 1 (0.3 mmol)
and rose bengal (5.0 mol%) in water (3-5 mL) was irradiated using a 34 W blue LEDs under an
open air/O2 atmosphere at room temperature for 24 h.
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To further illustrate the preparative utility of this methodology, a scale-up synthesis of 2b was
conducted (Scheme 3). A gram-scale reaction of 1b (10.0 mmol, Scheme 3) was done under the
optimized reaction conditions (Table 1), and product 2b was obtained in 92% isolated yield,
which demonstrated the efficacy of this protocol.
COOCH3
COOCH3
blue LED
rose bengal (5 mol%)
HN
N
O2, water, 24 h, rt
(92%)
H3C
H3C
1b
2b
Scheme 3. A scale-up synthesis of 2b from 1b using optimized reaction conditions.
Based on the control experiments and our experience with visible light photocatalysis,12
a
conceivable mechanism for the formation of phenanthridine-6-carboxylates from N-biarylglycine
esters is proposed in Scheme 4. Upon irradiation by blue LEDs, rose bengal (RB) got excited,
i.e., RB* [Ered(RB*/RB•) = +0.99 V vs SCE].14 RB* is single electron reduced by glycine ester
1a [Eox(1a/1a•+) = +0.86 V vs SCE]13 to form RB• and amine radical cation of 1a (1-I, Scheme
4). Afterward, RB• is single electron oxidized by molecular oxygen and leading to the ground
•
state RB and superoxide anion radical (O2 ). A deprotonation reaction between 1-I and O2• was
afforded biarylglycine ester radical 1-II and hydroperoxyl radical HOO•. At that point, radical 1-
II experienced intramolecular CH aromatic coupling to form a new radical intermediate 1-III.
Hydrogen radical transfer between 1-III and HOO• gives methyl-5,6-dihydrophenanthridine-6-
carboxylate (1-IV) and H2O2 (Scheme 4). A second photoredox catalytic cycle between excited-
state rose bengal and 1-IV, and subsequent loss of proton (H+) and H• loss managed the desired
product 2a, cf. Scheme 4.
8

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COOCH3
HN
R
R
RB
*
COOCH3
R
HN
COOCH3
R
HN
SET
R
1-II
photoredox
cycle - 1
R
1-I
HOO
HOO
O2
RB
RB
COOCH3
R
COOCH3
HOOH
HN
HN
R
R
SET
R
R
1-III
O2
O2
1-IV
COOCH3
R
RB
COOCH3
*
HN
visible
light
N
+O2
-H2O2
SET
R
photoredox
cycle - 2
R
1-V
RB
RB
SET
O2
O2
Scheme 4. A plausible mechanism for the formation of phenanthridine-6-carboxylates from N-
biarylglycine esters under visible-light photocatalysis. SET, single electron transfer. RB, rose
bengal.
A proposed reaction mechanism (Scheme 4) was confirmed by conducting an additional
experiment as shown in Scheme 5. When methyl 5,6-dihydrophenanthridine-6-carboxylate (1-
IV) was irradiated in the presence of rose bengal (5.0 mol%) under the optimized reaction
conditions, as expected; a methyl phenanthridine-6-carboxylate (2a) was obtained in a
quantitative yield, cf. Scheme 5.
COOCH3
COOCH3
blue LED
rose bengal (5 mol%)
HN
N
O2, water, rt
(98%)
1-IV
2a
Scheme 5. Conversion of a compound 1-IV into 2a under optimized reaction conditions at room
temperature.
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As mentioned at the beginning, in recent time, synthesis of phenanthridines is to a great extent
being led by either tandem radical addition-cyclization of 2-isocyanobiphenyls and 2-(1-
azidovinyl)-1,1'-biaryls with diverse radical precursors or the N-O bond homolysis cyclization
reactions of oxime ethers and oxime esters. Nevertheless, method reported here is based on the
use of N-biarylglycine esters as a sole starting material. Moreover, this convention meets a
considerable lot of the necessities of green chemistry as: i) the reaction utilizes O2 from air as the
oxidant and water as a solvent; ii) it is atom-economy and iii) organic dye is used as catalyst in
the reaction. Thus, we trust that the present protocol may locate a bright future particularly in the
pharmaceuticals industries where the necessity for final products to be totally free of traces
metals and halogenated solvents.
Conclusion
In summary, we have built up a novel, an atom-economical and ecologically amiable method for
the conversion of promptly accessible N-biarylglycine esters into the corresponding
phenanthridine-6-carboxylates. The transformation proceeds in water employing rose bengal as
an organic photoredox catalyst. The desired products are obtained in pure form by simple
filtration. Further usage of the strategy to create other heterocyclic compounds is at present under
investigation and will be revealed at the appropriate time.
Acknowledgments
The authors are pleased to acknowledge the financial support from the Department of Science &
Technology (DST-SERB), New Delhi, India, through start-up research grant for young scientists,
vide project no. SB/FT/CS-117/2013. Also, we are indebted to anonymous reviewers for their
insightful comments and suggestions.
Supplementary data
Supplementary data (general aspects, experimental characterization data, references and copies
of 1H, 13C and 19F NMR spectra) associated with this article can be found, in the online version,
at ..
10

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Page 13 of 14
Green Chemistry
View Article Online
DOI: 10.1039/C9GC01557D
For table of contents only
COOR
COOR
blue LED (34 W,  = 425 nm)
HN
N
rose bengal (5 mol%)
R'
O2, water, 24 h, rt
(> 90%)
R'
R''
R''
13

Green Chemistry
Page 14 of 14
View Article Online
DOI: 10.1039/C9GC01557D
124x27mm (300 x 300 DPI)