Auto-tandem PET and EnT photocatalysis by crude chlorophyll under visible light towards the oxidative functionalization of indoles

Article abstract of DOI:10.1039/d1gc00138h

Chlorophyll is the most abundant photocatalytic pigment that enables plants to absorb solar energy and convert it to energy storage molecules. Herein, we report a tandem photocatalytic approach utilizing the natural pigment chlorophyll in crude form to achieve photoinduced electron transfer (PET) and energy transfer (EnT) towards the oxidative functionalization of indoles. Redox potentials, ESR, fluorescence quenching and UV experiments have evidenced the dual catalytic activity of chlorophyll. The highlight of the study is the auto-tandem photocatalytic role of chlorophyll to enable the green oxidation of indoles using molecular oxygen as the oxidant, water as the reaction medium, and photochemical energy from the visible region of the spectrum.

Full text of DOI:10.1039/d1gc00138h

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Auto-tandem PET and EnT photocatalysis by crude
chlorophyll under visible light towards the
oxidative functionalization of indoles†
a,b
Cite this: Green Chem., 2021, 23,
3039
Saira Banu,a,b Shubham Choudhari,a Girija Patela,b and Prem P. Yadav
*
Chlorophyll is the most abundant photocatalytic pigment that enables plants to absorb solar energy and
convert it to energy storage molecules. Herein, we report a tandem photocatalytic approach utilizing the
natural pigment chlorophyll in crude form to achieve photoinduced electron transfer (PET) and energy
transfer (EnT) towards the oxidative functionalization of indoles. Redox potentials, ESR, fluorescence
quenching and UV experiments have evidenced the dual catalytic activity of chlorophyll. The highlight of
the study is the auto-tandem photocatalytic role of chlorophyll to enable the green oxidation of indoles
using molecular oxygen as the oxidant, water as the reaction medium, and photochemical energy from
the visible region of the spectrum.
Received 13th January 2021,
Accepted 17th March 2021
DOI: 10.1039/d1gc00138h
rsc.li/greenchem
encouraging growth in the recent past. The green pigment
“chlorophyll” in the leaves of plants has been the key source of
Introduction
The sustainability aspects of organic syntheses have become inspiration to the community working on photochemical reac-
the prime objective with nature being the foremost inspiration tions,8 and the ultimate aim of mimicking the natural
in this regard. Solar radiations have been the ultimate source phenomenon of photosynthesis has gained much needed
of light and energy on earth.1 Also, they are the most abun- attention to achieve sustainability.
dant, inexpensive and renewable source of green energy.2 The
Chlorophyll is known to be involved in the PSI and PSII pro-
high energy UV region of the light is mostly absorbed by cesses via energy transfer (EnT)9 and photoinduced electron
organic molecules and hence has been most widely used in transfer.10 Surprisingly, in the context of visible light-mediated
organic syntheses;3 however, limited by its harsh nature, safety synthetic reactions, the use of chlorophyll as a photocatalyst
issues and requirement of special photoreactors. Thus, it is remains quite unexplored. The isolation, purification, and
much needed to utilize the wavelength of visible light abun- stability of the natural pigment chlorophyll may have contribu-
dant in the solar spectrum.4 Visible light photocatalytic appli- ted to its scarce exploration as the photocatalyst. In the recent
cations have started to emerge in the recent past and provide a past, some visible light-mediated transformations utilized
greener, cleaner, and more sustainable approach towards mul- chlorophyll as a photocatalyst.11 Some of these reports have
tiplying the organic chemical space.5 The ultimate goal to outlined a detailed mechanistic proposal. Boyer et al. reported
match the ability of nature to harness solar energy by its the chlorophyll a catalyzed electron transfer mechanism to
photosynthetic machinery has led to several discoveries in the enable controlled radical polymerization under visible light;
field of synthetic photochemistry and the quest is still on.6 they have achieved the same feat even with the crude extract of
Mostly, transition metal-based polypyridyl complexes are used chlorophyll a (Scheme 1a).12 Furthermore, the photo-degrad-
for visible light-mediated reactions; simultaneously, organic ability of chlorophyll in the air to polar components shunned
dyes are gaining momentum as the photocatalyst of choice.7 the need for catalyst removal, making the purification process
Organic photoredox catalysts have existed for long; however, easier.12b Very recently, Das et al. reported chlorophyll-cata-
their role as a PET6a catalyst in organic syntheses has seen lyzed singlet oxygen generation via EnT, followed by 1,2-acyl
migration reactions to achieve α-amino carbonyl compounds
from the corresponding hydroperoxides (Scheme 1b).13 In
another report, the EnT-generated singlet oxygen undergoes
aMedicinal and Process Chemistry Division, CSIR-Central Drug Research Institute,
Lucknow-226031, India. E-mail: pp_yadav@cdri.res.in
bAcademy of Scientific & Innovative Research, Ghaziabad-201002, India
SET with the substrate, which ultimately got converted to tetra-
hydroquinolines (Scheme 1b).14 Most of these reactions use
†Electronic supplementary information (ESI) available: Synthetic procedures,
mechanistic investigations, characterization data and 1H, 13C NMR spectra of
synthesized compounds. See DOI: 10.1039/d1gc00138h
either PET or EnT properties of the photocatalyst chlorophyll,
whereas ground state redox potentials were mentioned to sub-
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Scheme 1 Chlorophyll as a mechanistically defined photocatalyst.
stantiate the mechanistic proposals. However, the excited state PET-mediated reduction of indole to indolyl radical anion
redox potentials of chlorophyll a calculated as per Nicewicz species, followed by reactions with EnT-derived singlet oxygen.
et al.6a and ground state redox potentials15 were found to be The oxidative cleavage of indoles17 is one of the fundamental
Eo*xd ðsinglet stateÞ ¼ ꢀ1:04 V vs. SHE, Eo*xd ðtriplet stateÞ ¼ transformations found to be involved in the peroxidase-cata-
ꢀ0:53 V vs. SHE, and Er*ed ðsinglet stateÞ ¼ 0:73 V vs. SHE, lyzed transformation of L-tryptophan to L-formylkynurenine,
Er*ed ðtriplet stateÞ ¼ 0:22 V vs. SHE (section 3.3A, ESI†). leading to the amino acid kynurenine, and it also happens to
Accordingly chlorophyll a could act as an efficient photoredox be the first key step in the biosynthesis of coenzyme NAD.18
catalyst as Eo*xd ðcat•þ=cat*Þ , 0; Er*ed ðcat*=cat•ꢀÞ > 0.6a
The oxidative cleavage of 2,3-dimethylindoles generates
N-acetylaminoacetophenones (and their downstream products
2′-aminoacetophenones), which are the key starting materials
for the synthesis of different biologically active molecules and
drugs such as linagliptin.19 Witkop oxidation17a,18b is the most
prevalent and general method for the oxidative cleavage of
indoles to N-acetylaminoacetophenones. Besides this method,
ozonolysis20 and a very recent ozone-halide catalysis system
was reported to effect the transformation efficiently.21
Conventional methods usually require organic oxidants or
toxic transition metals, which produce harmful by-products.
Previous work
Henceforth, we lend credence to the fact that with proper
designing, chlorophyll a with appropriate exited state redox
properties and triplet state excitation energy ET = 30.9 kcal
mol−1 (ref. 16) may act as an efficient photocatalyst. Herein, we
report the oxidative functionalization of indoles via the auto-
tandem photocatalysis viz., PET and EnT under the visible-
light excitation of the crude chlorophyll (Scheme 1c), enabling
the 2,3-substitution-driven diverse array of products via the key
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Table 1 Optimization of conditions for the visible light-mediated
photo-oxidative functionalization of 2,3-dimethylindole(s) (1a) to N-(2-
acetylphenyl)acetamide (2a)a
Therefore, it is highly desirable to develop a benign, efficient
reaction protocol using safer oxidants and solvents. In
addition, the synthetic applicability of the downstream pro-
ducts and our interest in the synthetic exploration of substi-
tuted 2′-aminoacetophenones and indoles22 led to the selec-
tion of substituted indoles as substrates to study the Chl-
mediated oxidative functionalization. This natural pigment
chlorophyll (crude extract) catalysis system can offer a general,
sustainable, green oxidation process for indoles, evading the
use of hazardous oxidants, and provide better atom economy
than other reported methods.
PC
Time
(h)b
Yield
(%)c
Entry
Solvent
PC
(Conc.)
1
2
3
4
5
6
7
8
MeCN
MeCN
MeCN
MeCN
MeCN
MeOH
DMSO
DMF
MeOH : H2O(1 : 10)
H2O/SDS
H2O/SDS
H2O/SDSe
H2O/SDS
H2O/SDS
H2O/SDS
H2O/SDS
H2O/SDS
H2O/SDS
H2O/SDS
MeOH(dry)
c-Chl
Chl a
EY
MB
RB
30 ppm
30 ppm
5 mol%
5 mol%
5 mol%
30 ppm
30 ppm
30 ppm
30 ppm
30 ppm
15 ppm
15 ppm
10 ppm
5 ppm
15 ppm
15 ppm
15 ppm
—
15 ppm
15 ppm
24
16
24
24
24
24
24
24
16
24
18
28
24
24
24
36
48
48
24
48
64
62
17
15
26
67
53
39
Results and discussion
c-Chl
c-Chl
c-Chl
c-Chl
c-Chl
c-Chl
c-Chl
c-Chl
c-Chl
c-Chl
c-Chl
c-Chl
—
First, 2,3-dimethylindole (1a) was selected as the model sub-
strate and crude chlorophyll (c-Chl) derived from spinach was
used as the photocatalyst. The determination of chlorophyll a
concentration in crude extract was done via UV-Vis spec-
troscopy based on the Wellburn equation23 (Fig. FS1, ESI†). As
shown in Table 1, a preliminary screening was conducted with
1a using c-Chl (Chl a, 30 ppm) as the photocatalyst, CH3CN
(5 mL), in an air atmosphere, under the irradiation of 3 W
white LED at room temperature. The desired product N-(2-acet-
ylphenyl)acetamide (2a) was obtained with a satisfactory yield
of 64% (entry 1). The role of Chl a as the catalyst in the crude
form was substantiated by carrying out the reaction with pure
Chl a. A similar result was obtained (entry 2, 62%) as com-
pared to the crude chlorophyll. Thereafter, switching the cata-
lytic system from c-Chl to other commonly used photosensiti-
zers, viz. methylene blue (MB), eosin Y (EY), rose Bengal (RB)
resulted in a reduced yield of 2a (entries 3–5). Next, the optim-
ization of solvents was performed (entries 1, 6–10). The sol-
vents in which substrate 1a and photocatalyst c-Chl were
readily soluble, such as MeCN, MeOH, DMSO, and DMF,
afforded good to moderate yields. Next, to screen water as a
solvent, MeOH–water (1 : 10) was used (1a and c-Chl were in-
soluble in water) and it provided 69% yield of 2a (entry 9). To
9
69
10
11
12
13
14
15f
16g
17h
18
19i
20j
83
90 (79d)
87
54
37
63
67
NR
Trace
85
c-Chl
c-Chl
NR
a Reaction conditions: Air atmosphere and irradiation of visible light
with 3 W white LED, 2,3-dimethyl indole (1a) (0.2 mmol, 1 equiv.),
photocatalyst (PC) solvent (5–10 ml), SDS (1 equiv.), temperature (RT,
approx. 25 °C), time (16–24 h) in a 30 mL glass vial. b Time for the con-
sumption of the substrate as per TLC observation. c Conversion yields
were determined by 1H NMR using dibromomethane as the internal
standard. d Isolated yield. e SDS (0.5 equiv.). f The reaction was per-
formed under irradiation of 3W blue LED. g The reaction was per-
formed under irradiation of 3W red LED. h No light. i Reaction per-
formed under oxygen atmosphere instead of air. j Argon atmosphere
instead of air. NR = No reaction. PC = photocatalyst.
further elaborate water as the solvent system, H2O/SDS was paratively low yields. Chlorophyll a possesses two prominent
also used in the optimization (entries 10–19). Gratifyingly, absorption bands at 430 nm (Soret band, blue region) and at
H2O/SDS provided a better product yield than the other sol- 665 nm (Q-band, red region) of the visible spectrum. However,
vents, although 1a and c-Chl had low solubility in H2O/SDS. the reaction under the irradiation of 3 W blue and red LEDs
Most probably, a low concentration of the substrate in the reac- (entries 15 and 16) further reduced the yield of product 2a.
tion solution may be favorable for the control of reaction rate, The broad spectrum of wavelength (400–700 nm) in the case of
thereby leading to a relatively clean reaction.
white light11c,25 comprising both the significant regions of Chl
Furthermore, higher concentrations of chlorophyll a are absorption bands, viz., 350–450 nm and 650–700 nm, may be
presumed to undergo self-quenching; therefore, the analysis of required for efficient PET and EnT processes. There was no sig-
the effects of the Chl a concentration was carried out (entries nificant conversion in the absence of light (entry 17); however,
10–11, 13–14). The Chl a concentration of 15 ppm provided the without photocatalyst c-Chl, a trace amount of the product was
highest yield within reduced time as compared to that of obtained (entry 18). Furthermore, the reaction under oxygen
30 ppm. As per literature reports,12a,24 fluorescence intensity atmosphere resulted in an almost similar yield as that under
decreases at higher concentrations of Chl a due to the fast air atmosphere (entry 19); however, no product was observed
transfer of excitation energy to the statistical pairs of Chl a under an argon atmosphere (entry 20). Furthermore, as per
present in close vicinity with each other in the solution. our observations and literature reports, Chl is degraded to
However, further lowering the Chl a concentration to 10 ppm unreactive polar species,26 simplifying the final removal of
(entry 13) and 5 ppm (entry 14), afforded product 2a in com- photocatalyst from the reaction mixture.12b
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With the determination of the optimized condition for excess of c-Chl a (Chl a = 10 ppm) was required. It could be
visible light-mediated aerobic oxidation conditions, the sub- envisaged that indole N–Me protection rendered it less prone
strate scope of indoles was assessed. It was delightful to to the oxidative quenching of excited Chl at the initial step
witness the reaction method’s versatility with indole moieties, (see plausible mechanism; Scheme 2), thereby decreasing its
which were either commercially available or synthesized fol- reactivity. So far, there are several reports for the synthesis of
lowing literature methods,27 affording mechanistically diver- C-2 quaternary indolinones,31,33 the majority of them utilized
gent products based on the substitutions of indole on 2,3-posi- a specialized metal catalyst, high temperature, external oxidant
tions (Table 2). Indole-bearing alkyl substitution at C-2,3 (1a– viz., peroxides (TBHP). The present visible light-mediated
m) and C-3 (1n) smoothly undergo photo-oxidative cleavage c-Chl catalyzed system could provide a benign, useful strategy
rendering N-(2-acetylphenyl)acetamides (2a–m), N-(2-formyl- for the synthesis of 2-(indol-3-yl) indolin-3-one and 2,2-bis
phenyl)acetamide (2n) in good to moderate yields. On the (indol-3-yl)indolin-3-ones from their corresponding indoles.
other hand, 3-acetylindole did not react under optimized con- Overall, the findings suggest that indoles’ photooxidative
ditions. 2,3-Dimethylindole-bearing electron-donating groups functionalization is highly substrate selective with respect to
(–OMe, –Me, –iPr etc.) and electron-withdrawing groups (–Br, the C2–C3 substitution patterns, leading to diverse products.
–Cl, –F etc.) on the phenyl ring exhibited adequate reactivity The protocol developed herein afforded better chemo and
and afforded the corresponding oxidized products (2b–e, 2k) regioselectivity, alleviating the need of indole N–H protection,
and (2f–j, 2l) in good to moderate yields. The reaction time which seems to be prerequisite in earlier methods.17b,c
was mainly dependent on the comparative ease of substrate-
A gram scale reaction was performed with 2,3-dimethyl-
solubility in the water–SDS system. Subsequently, the reactivity indole (1a) (1.5 g) under optimized reaction conditions to
of differently substituted tetrahydrocarbazoles and hexahydro- demonstrate the present visible light-mediated method’s syn-
cyclohepta[b]indole (1B) was investigated, and they were found thetic workability. It provided the desired product N-(2-acetyl-
to undergo photooxidative functionalization to fused quino- phenyl)acetamide (2a) with 69% yield (1.27 g).
lones, and 1,2,3,4-tetrahydroacridin-9(10H)-ones28 (3), respect-
ively, in 10–15% isolated yields. Such type of an indole-quino-
lone rearrangement was reported by Winterfeldt,18b,29 who
found out different conditions that could accomplish the one-
Mechanistic investigations
pot Witkop oxidation, followed by Camps cyclization to Next, to delve into the mechanism, some control experiments
provide quinolones, which widely exist in numerous marketed were performed using 2,3-dimethylindole (1a) as the model
drugs and bioactive molecules.30 After going through screen- substrate. When 2,2,6,6-tetramethylpiperidinooxy (TEMPO) or
ing of different bases and acids (Table TS1, ESI†), it was 2,6-di-tert-butyl-4-methylphenol (BHT) was added to the reac-
observed that 1 equiv. of K3PO4 in addition to the optimized tion under optimized conditions, the yield of 2a decreased sig-
condition for 2,3-dimethyl indoles afforded the Witkop– nificantly (Fig. 1a). It indicated that the reaction might involve
Winterfeldt products (3a–g, Table 2) in moderate yields. On radical generation (Fig. FS2a and b, ESI†). On the other hand,
the other hand, the indole-bearing substitutions at C-2 only the use of 1,4-diazabicyclo [2,2,2]octane (DABCO) and sodium
(1C) underwent oxidative dearomatization instead of photooxi- azide, both singlet oxygen quenchers (Fig. 1b), made the reac-
dative cleavage, attaining 2-indole-substituted 3-oxindoles (4) tion sluggish, with the reduction in yield of 2a (Fig. FS2c and
with 43% yield. Herein, also the addition of 0.5–1.0 equiv. of d, ESI†). The addition of both radical quenchers (TEMPO and
base (K3PO4 or Cs2CO3) resulted in the enhancement of yield BHT) and singlet oxygen quenchers (DABCO and NaN3)
(60–65%) of the desired products (4a–e). Mass spectrometry of resulted in a significant decrease in the yield of 2a; this led to
the crude mixture indicated the presence of 2-substituted hypothesize that the reaction follows both PET and EnT pro-
indolin-3-one (section 2.2A, ESI†), suggesting the involvement cesses. The formation of electron donor–acceptor (EDA) com-
of in situ generated iminium species as an intermediate.31 plexes between chlorophyll (PC) and indoles was ruled out via
Similarly, C-2,3 unsubstituted 1H-indoles and 1-methyl-1H- UV-Vis spectroscopy of 1a and PC in DMSO (FS11, ESI†). Next,
indole, under the same reaction condition, yielded corres- Stern–Volmer fluorescence quenching experiments were
ponding 2,2-bis(indol-3-yl)indolin-3-ones (5a–d) in a regio- carried out, (Fig. FS3, ESI†). When the chlorophyll–DMSO solu-
selective manner. Indole-bearing electron-donating and mod- tion was excited at 433 nm, a 671 nm fluorescence was
erately electron-withdrawing functionalities on the phenyl ring observed, and bubbling oxygen through the photocatalyst-
afforded the products in good to excellent yields, while the DMSO solution did not have any significant effect on the fluo-
strong electron-withdrawing group (–CN, –NO2, –COOH)-con- rescence intensity (Fig. FS6, ESI†). However, on adding 2,3di-
taining indoles remained unreacted (5e–g, Table 2). methylindole, the photocatalyst fluorescence intensity
Furthermore, the absence of the product 3,3-bis(indol-3-yl) decreased significantly (Fig. 1c and d and Fig. FS4 and 5,
indolin-2-one implied that isatin, proposed as an intermediate ESI†). These results suggest that the excited Chl initiates
by Thakur et al.,32 was not involved in the present study. The single-electron transfer with the quencher. The observation
observation also substantiated the proposed mechanism was further supported by redox potential values of the catalyst
indirectly (Scheme 2). In the case of 1-methyl-1H-indole, the and substrate, as E1/2 red of 2,3-dimethylindoles were found to
reaction needs to be continued for more than 48 h and an be +0.087 V vs. SHE (see Fig. FS7, ESI†), which is higher than
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Table 2 Substrate scope for the Visible light-mediated photo-oxidative functionalization of 2,3-dimethylindoles and related N-heterocyclesa,b
a Reaction conditions: Air atmosphere and irradiation of visible light with 3 W white LED, indole (1A, 1B, 1C, 1D) (0.2 mmol, 1 equiv.), photo-
catalyst crude chlorophyll extract (Chl a = 15 ppm), solvent (water, 10 ml), SDS (0.2 mmol, 1 equiv.) temperature (RT, approx. 25 °C), time
(10–48 h) in a 30 mL glass vial. K3PO4 (0.1–0.2 mmol, 1 equiv.) was added in case of 1B, 1C, and 1D. b Reaction performed with CH3CN (5 mL),
Cs2CO3 (0.2 mmol, 1 equiv.).
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Scheme 2 Plausible mechanism.
3
1
the E1/2 oxd of triplet excited chlorophyll a, i.e., −0.53 V vs. SHE which implies that the triplet photosensitization of O2 to O2
3
(−1.04 V vs. SHE for the singlet excited state of chlorophyll [E(1Δ − 3Σ) = 22.5 kcal mol−1],17c by Chl* is feasible. Further,
1
a).6a,15,16 However, for molecular oxygen to get reduced to to find out the photosensitized generation of O2 under opti-
superoxide ion O2•−, the potential was reported to be −1.23 V mized reaction conditions, 1,3-diphenylisobenzofuran (DPBF)
vs. SCE.17c Hence, there would not be any admissible single (6) was used to trap the in situ generated singlet oxygen. The
electron transfer between the triplet excited state of Chl a and reaction led to the isolation of the diketone product 1,2-pheny-
molecular oxygen. Furthermore, the Stern Volmer fluorescence lenebis(phenylmethanone) (7) with 69% yield, confirming the
1
quenching experiment performed with molecular oxygen generation of O2 in the operative EnT mechanism (see Fig. 2a
(Fig. FS6; ESI†) is in line with the aforementioned redox poten- and ESI†).
tials. On the other hand triplet state energy (theoretical value)
It was further substantiated by UV-Vis spectroscopy using
of chlorophyll a was reported to be 30.9 kcal mol−1 (1.34 eV),16 DPBF (with a characteristic peak at 412 nm),34 for detecting
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Fig. 2 (a) Diketone product 1,2-phenylenebis (phenylmethanone) (7)
isolated from the reaction of DPBF (6) under optimised reaction con-
dition. (b) UV-Visible experiment to investigate singlet oxygen gene-
ration; DPBF, PC, 3 W White LED. (c) (A) ESR spectrum of mixture of
crude Chlorophyll a (PC), DMPO in DMSO under irradiation of 3 W white
LED for 10 min. (B) ESR spectrum of the mixture of PC, 1a, DMPO in
DMSO under dark conditions for 10 min. (C) ESR spectrum of mixture of
PC, 1a, DMPO in DMSO under irradiation of 3 W white LED for 10 min.
Fig. 1 (a) Control experiments performed with free radical scavengers
TEMPO and BHT, respectively. (b) Control experiments performed with
singlet oxygen quenchers DABCO and NaN3, respectively. (c)
Fluorescence emission spectra of c-Chl (Chla = 1 μM) with different
concentrations of 1a excited at 433 nm. (d) Linear fit plot of fluor-
singlet oxygen generation in the reaction medium. In the pres-
ence of light and photocatalyst, the intensity of DPBF gradually
decreased with time (Fig. 2b and Fig. FS12, ESI†). The ESR
study using 5,5-dimethyl-1-pyrroline N-oxide (DMPO) as the
escence quenching. The decrease in the emission intensity is correlated spin-trapping agent indicated the type of radical species or
with the Stern–Volmer equation, I0/I = 1 + kqτ0[Q], where I0 and I are
the emission intensity in the absence and presence of a quencher, kq is
the quenching rate constant, τ0 is the excited lifetime, [Q] is the
quencher concentration.
ROS involved in the reaction system (Fig. 2c).
A mixture of Chl a crude extract (containing Chl a concen-
tration = 15 ppm), DMPO (0.7 mM) in DMSO was irradiated
with 3 W white LED for 10 min under air atmosphere, and a
small amount of the solution was transferred to a capillary.
The ESR spectrum displayed no signal (Fig. 2cA). Similarly, the
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ESR spectrum for the mixture of Chl a crude extract (Chl a = oxygen provides intermediate IX and O2•−. After that, the inter-
15 ppm), 1a (0.7 mM), DMPO (0.7 mM) in DMSO without mediate IX undergoes oxidation providing another iminium
irradiation of visible light showed no signal (Fig. 2cB). After intermediate X, which upon further attack by the indole
irradiation with 3 W white LED for 10 min, the ESR spectrum moiety afford the product 2,2-bis(indol-3-yl)indolin-3-ones (5).
for the mixture of Chl a crude extract (Chl a = 15 ppm), 1a
(0.7 mM), DMPO (0.7 mM) in DMSO, exhibited a new broad
signal (Fig. 2cC) without any detectable hyperfine splitting over
a wide range of temperature (g = 2.0024, with epr line width =
Conclusion
We demonstrated the auto-tandem photocatalysis by crude
chlorophyll and its mechanistic understanding. The photo-
catalytic process served as an efficient, green method towards
the C-2,3 substitution-dependent oxidative transformation of
9 Gauss). The signal may correspond to the pi-radical cation of
Chl a, formed upon the reduction of the substrate indole (1a).
Borg et al. and other groups have shown that the pi radical
cation of chlorophyll a generates an epr signal with g = 2.0025
indoles to afford oxidative cleavage products such as 2′-N-acety-
0.0001 and epr line width = 7–13 Gauss, with Gaussian line
shape, and absence of hyperfine splitting.35 On the other
laminoacetophenones, tetrahydroacridin-9-ones, and dearoma-
tized products such as 2-indolyl-3-oxindoles and 2,2-bis(indol-3-
hand, the indolyl radical-DMPO adducts could not be detected
in the ESR spectrum (Fig. FS13, ESI†).
A plausible mechanism for the visible light mediated
aerobic oxidation of indoles is proposed in Scheme 2. At first,
yl)indolin-3-ones. The chlorophyll in the crude form is an envir-
onmentally benign, cost-effective, and ubiquitous photocatalyst
and probably bears the potential to achieve sustainability needs.
Further exploration of the photocatalytic transformations using
the photocatalyst (Chl) transforms into its excited state Chl*
crude chlorophyll as the photocatalyst is currently underway.
under the irradiation of visible light; next, a photoinduced
electron transfer (PET) from Chl* to 1 lead to a radical anion I
and oxidized Chl•+ (confirmed by fluorescence quenching, ESR
studies). Unlike transition metal polypyridyl complexes, the
Conflicts of interest
electron originates from the aromatic pi-electron system of the
The authors declare no competing financial interest.
porphyrinic chromophore of the Chl molecule and not from
the (closed shell element) Mg2+ ion.12 However, Mg2+ ion is
required for the structural and functional integrity of Chls,36
Acknowledgements
and it is also found to be necessary for effective functioning of
Chls in the present method (de-metalation of Chls was per-
formed and subsequent reaction with crude pheophytin
thankful to CSIR, New Delhi, India for financial assistance.
resulted in 15% yield of 2a, see FS15, ESI†). Although there are
S. B. and S. C. are thankful to UGC, New Delhi, and G. P. is
The authors are thankful to SAIF-CDRI, Lucknow, India for
providing spectral and analytical data. This work was sup-
ported in part by CSIR project MLP0123. The authors are grate-
ful to Instrumentation Facility, UGC-DRS II, DST-FIST &
DST-PURSE Assisted, Department of Chemistry, Aligarh
Muslim University (AMU), Aligarh, India for providing ESR and
very few reports on the single electron reduction of the indole
moiety,37 mechanistic investigation (viz. fluorescence quench-
ing, ESR spectroscopy, CV data) strongly indicated the gene-
ration of an indole radical anion (I) via PET. The nucleophili-
city of the indole ring is enhanced on being converted to the
indole radical anion, which easily reacts with the singlet oxygen
CV Data. This is CSIR-CDRI communication no. 10212.
giving rise to the aminium radical ion (II). The aminium
ion intermediate in turn donates an electron to Chl•+, affording
a zwitterionic species (III) with the regeneration of the photo-
catalyst (Chl). Moreover, a radical chain propagation reaction
Notes and references
via SET may also lead to species (III). Next, the zwitterionic
species (III) forms dioxetane intermediate (IV), which
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upon subsequent oxidative cleavage affords the product
N-acetylacetaminophenone (2). Similarly, in the case of sub-
strate 1B, the dioxetane intermediate (IV) undergoes cleavage to
keto amide (V), which upon Camps cyclization under basic con-
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