Chlorophyll-catalyzed photochemical regioselective coumarin C-H arylation with diazonium salts

Article abstract of DOI:10.1039/d0nj02012e

This communication describes the development of a mild method for the cross-coupling of the C3-position of coumarin with an array of diazonium salts mediated by chlorophyll as a biocatalyst via visible light catalysis. A natural pigment such as chlorophyll is used as a green photosensitizer and environmentally benign catalyst. This general and easy procedure provides a transition-metal-free alternative for the formation of 3-aryl coumarin derivatives at room temperature with good to excellent yields. This journal is

Full text of DOI:10.1039/d0nj02012e

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DOI: 10.1039/D0NJ02012E
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Chlorophyll-Catalyzed Photochemical Regioselective Coumarin C‒
H Arylation with Diazonium Salts
Received 00th January 20xx,
Accepted 00th January 20xx
Ali Moazzam, Farnaz Jafarpour*
DOI: 10.1039/x0xx00000x
www.rsc.org/
This communication describes the development of a mild method for cross-coupling of C3‒position of coumarin with an
array of diazonium salts mediated by chlorophyll as biocatalyst via visible light catalysis. Natural pigments such as chlorophyll
is used as a green photosensitizer and environmentally benign catalyst. The general and easy procedure provides a
transition-metal-free alternative for the formation of 3‒aryl coumarin derivatives in ambient temperature with good to
excellent yields.
Introduction
In recent years global warming and environmental pollution are
serious threats for today’s world. Dealing with these threats
require using sustainable and environmental friendly
approaches such as using green catalysts and less hazardous
components in chemical processes.1, 2
Visible Light is a low-cost, rich and interminably inexhaustible
wellspring of clean energy source. The visible spectrum of light,
which lies between the UV and IR radiation in terms of energy,
is responsible for many things including driving photosynthesis
in plants.3 Plants involve chlorophyll that can act as a typical
natural photocatalyst in some chemical reactions. Chlorophyll
as a biocatalyst captures the sunlight energy to convert the
water and carbon dioxide into oxygen and glucose. As a green,
environmental friendly and widespread photosensitizer, it has
been seldom used in the photo-driven synthesis.4, 5
Figure 1. 3-Arylcoumarin based bioactive molecules
.
chance to be potent selective monoamine oxidase B inhibitors
(MAO-B inhibitors, used to treat the symptoms of Parkinson's
by preventing the breakdown of the chemical messenger
dopamine in the brain),14 active in neurological disorders15 and
inhibitors of HIV-1 replication16 (Figure 1). Notwithstanding the
significance of 3‐aryl coumarins as biologically active scaffolds,
direct α‐arylation of coumarins has been proved to be
problematic due to high preference of coumarins β‐arylation
observed in Heck‐type reactions.17
Coumarin derivatives are important compounds that are found
widespread in nature and exhibit significant biological activities.
Owing to their abundance in natural products and medicinal
agents, coumarin derivatives have become important synthetic
targets for chemists.6 Consequently, there is consistent
tendency in conducting reactions that allow for a regioselective
coumarin functionalization. In this regard, various
regioselective C-3 functionalization of coumarins including
3-Arylcoumarins can be obtained by several of general
aroylation,7
alkylation,8
aoromination,9
phosphonation,11 and arylation12 have been stated.
amidation,10
methods, for example, utilizing
a
cycloaddition ring
development approach,18-20 by transition metal-catalyzed
coupling reactions,21-23 and metal free reactions.24
3-Arylcoumarins are important scaffolds observed in many
natural products and bioactive compounds.13 In particular, 3-
arylated coumarin derivatives have been indicated with a
The regioselective direct C-H arylation of coumarins at C-3 have
been successfully achieved by using transition-metal catalysed
reactions employing pre-functionalized coumarins at 3-position
such as 3-halocoumarins25 or coumarin-3-carboxylic acids,26
and also via pd-catalyzed C-H functionalization of coumarins
with aryl halids,27 or arenesulfonyl chlorides28 as well as
a. School of Chemistry, College of Science, University of Tehran, 14155-6455 Tehran,
Iran. E-mail: jafarpur@khayam.ut.ac.irElectronic Supplementary Information (ESI)
available: [details of any supplementary information available should be included
here]. See DOI: 10.1039/x0xx00000x
This journal is © The Royal Society of Chemistry 20xx
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a
DOI: 10.1039/D0NJ02012E
Table 1. Optimization of reaction conditions for direct arylation of 1a .
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Yield
Entry Photocatalyst
Condition
Fluorescein (10 mol%) 1a (1 equiv.) Green LED DMSO
PC (10 mol%) 1a (1 equiv.) Blue LED DMSO
Solvent
(%)b
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22
20
33
0
1
2c
3d
4
I
PC II (10 mol%)
Chlorophyll (10 mg)
1a (1 equiv.) Green LED DMSO
1a (1 equiv.) White LED DMSO
1a (4 equiv.)
DMSO
5
1a (4 equiv.), White LED DMSO
<5
25
20
15
0
<5
12
<5
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46
50
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62
6
7
8
9
Chlorophyll (10 mg)
Chlorophyll (10 mg)
Chlorophyll (10 mg)
Chlorophyll (10 mg)
Chlorophyll (10 mg)
Chlorophyll (10 mg)
Chlorophyll (10 mg)
Chlorophyll (20 mg)
Chlorophyll (30 mg)
Chlorophyll (40 mg)
Chlorophyll (45 mg)
Chlorophyll (60 mg)
Chlorophyll (45 mg)
DMF
ACN
MeOH
H2O
THF
Acetone
DMSO/H2O
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
1a (1 equiv.), White LED
1a (1 equiv.), White LED
1a (1 equiv.), White LED
1a (1 equiv.), White LED
1a (1 equiv.), White LED
1a (1 equiv.), White LED
1a (1 equiv.), White LED
1a (1 equiv.), White LED
1a (1 equiv.), White LED
1a (1 equiv.), White LED
1a (1 equiv.), White LED
1a (1 equiv.), White LED
1a (2 equiv.), White LED
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Scheme 1. Metal-catalysed and Metal-free approaches to 3-arlylcoumarins.
dehydrogenative cross‐coupling of coumarins and arenes in a
CDC reaction (Scheme 1).29
Limits to using metals in C3-arylation of coumarins are the
requisite for prefunctionalizations, use of transition-metals,
Chlorophyll (45 mg) 1a (4 equiv.), White LED DMSO
ligands, high temperatures, low yields, the inevitability of 20
transition metals with high cost, toxicity and long reaction
times. It is highly significant and desirable to develop an
efficient transition metal-free methodology towards
regioselectivity at 3-position of coumarins.
Radical mediated C‐H arylation methods have proved as an
appropriate method due to the inherent reactivity of
heterocycles towards aryl radical species.30 They have been
accomplished through the use of individual arylating reagents
to produce the aryl radical in the reaction course, for example
hypervalent iodine,31 arylboronic acids,32 phenyl hydrazine,33
and diazonium salts.34 Resonance stabilized benzylic radical and
allylic radical intermediates affect the α‐regioselectivity in case
of coumarins in C-H arylation reactions.35, 36
Diazonium salts are well-known highly reactive species in
organic syntheses.37 Despite their important application in the
dye industry, diazonium salts function as ideal sources of
organic radicals that engage in several important
transformations such as the Meerwein arylation.38
Since the advent and renaissance of photoredox catalysis,39
many photocatalysts have been shown to catalyse the single
electron transfer reduction of diazonium salts to organic
radicals. In particular, ruthenium(II),40 gold41 and iridium
complexes,42 as well as eosin Y,43 have been successfully applied
in many reactions that exploit their remarkable reactivity in
either reductive or oxidative quenching pathways upon visible
light excitation.
aReaction conditions: Coumarin 1a (0.4 mmol), diazonium tetrafluoroborate (0.1
mmol), solvent (0.4 mL), rt, 14 h; bIsolated yields; c2,4,6-triphenylthiopyrylium
perchlorate (I); d2,4,6-triphenylpyrylium perchlorate (II)
coumarins with aryl diazonium salts using Chlorophyll as green
catalyst and 5 W LED lamp as the light source (Scheme 1).
Results & discussion
Initially, the reaction conditions for the direct arylation of
coumarin 1a with diazonium salt 2a employing various
photocatalysts such as chlorophyll, Fluorescein, 2,4,6-
triphenylthiopyrylium
perchlorate
(
I
)
and
2,4,6-
triphenylpyrylium perchlorate (II) under different reaction
conditions were investigated. Among different photocatalysts
used, chlorophyll resulted the best result under white LED lamp
light source (Table 1, entries 1-4). Next, control experiments
were performed to establish that both chlorophyll and visible
light are necessary for the arylation reaction. The reaction
conducted without chlorophyll and visible light did not proceed
at all (entry 5). The protocol was also ineffective for arylation of
coumarin when no chlorophyll was used in the photocatalytic
According to our interest in regioselective arylation of
coumarins,44-45 herein we report a new photocatalyzed single-
electron transfer (SET)-mediated direct C‒H bond arylation of
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Table 2. Scope of regioselective photochemical arylation of coumarin a ,b
.
system where, only trace amount of product was detected by
1H NMR (entry 6). The results indicated that both visible light
and chlorophyll are necessary for the model reaction.
View Article Online
DOI: 10.1039/D0NJ02012E
Next, various solvents, different amounts of the catalyst and
also coumarin 1a were examined. A solvent screen improved
DMSO to be the best choice for the photoreaction among other
solvents such as DMF, MeCN, THF, MeOH, H2O, acetone and
DMSO/H2O (entries 7‒13). Fortunately, increasing the amount
of chlorophyll from 10 mg to 45 mg, increased the yield
remarkably (entries 14‒17). When the amount of chlorophyll
was further increased to 60 mg, the yield of 3a was almost
remained unchanged (entry 18). The yield was further improved
using an extra amount of 4 equivs of 1a (entry 20). Therefore,
DMSO, 45 mg of catalyst dosage and 4 equivs of 1a were chosen
as the suitable conditions for the model reaction.
Having optimized the reaction conditions, we examined the
scope of the reaction of a series of coumarin derivatives with a
range of substituted aryl diazonium salts (Table 2). It can be
seen that aryl diazonium salts with either electron-donating
methyl, methoxy and hydroxyl groups or electron-withdrawing
fluoro, bromo, and carboxyl groups reacted well with coumarin
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to give the corresponding products 3a‒3t in good to high yields,
supporting the captodative nature of radicals.46 Introducing
fluoro, bromo, and carboxcylic groups on diazonium salts,
provide versatile synthetic intermediates suitable for further
functionalization reactions.
Next the substrate scope with respect to coumarins was
investigated where coumarins with various alkyl, alkoxy,
hydroxy and halo substituents participated well in this arylation
approach to afford the desired products in 38-89% yields.
Gratifyingly, the hydroxy-substituted coumarin also afforded 3-
arylcoumarin 3m in 82% yield, eliminating a prerequisite for a
protection/deprotection process. Furthermore, 4-methyl
substituted coumarin also tolerated well the arylation reaction
at C-3 with a promising 89% outcome (3q). Unfortunately, aThe reaction was performed with 1 (4 equiv.), 2 (0.1 mmol), and Chlorophyll (45
employing coumarin with electron-withdrawing nitro
mg) in 0.4 mL of DMSO. bIsolated yields.
substituent no desired product 3r was detected. We also
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a plausible mechanism for this photoreaction is proposed
investigated this protocol on some heterocyclic diazonium salts
(Scheme 3). Initially, aryl radical (Ι) is formed by SET from the
and fortunately obtained the desired 3-heteroarylcoumarins
albeit in low yields (3s and 3t). Furthermore, the photochemical
arylation of coumarin 1a was successfully scaled up to gram
scale and the desired 3-aryl coumarin 3a was obtained in 40%
isolated yield after 24 hrs of irradiation (see SI for details).
To gain insight into the reaction mechanism, we performed a
control experiment for radical trapping. The reaction of
coumarin and phenyldiazonium salt in presence of (2,2,6,6-
tetramethylpiperidin-1-yl)oxidanyl (TEMPO) (2 equiv), was fully
suppressed and the desired product 3a was not detected,
indicating the involvement of a radical mechanism (Scheme 2).
On the basis of the above observations and literature reports,47-
Scheme 3. Proposed mechanism for C‒3 regioselective arylation of coumarins.
Scheme 2. Control experiment.
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Olmedo, R. Sancho, L. M. Bedoya, J. L. Lopez-Perez, E.
excited state of chlorophyll to aryl diazonium salt 2a. Aryl radical
(Ι) can be introduced on two C-3 as well as C-4 positions of
coumarin where, the attachment of aryl radical to C-3 may
result in a more stable benzyl radical intermediate (II). Next the
Del Olmo, E. Muñoz, J. Alcamí, M. P. GuVpietwa Aartnicdle OAn.linSe.
DOI: 10.1039/D0NJ02012E
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radical intermediate
(ΙΙ) is further transformed to the
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carbocation intermediate (IV) by two possible pathways: (a)
oxidation of the radical intermediate by the chlorophyll radical
cation; (b) the oxidation of (ΙΙ) by aryl diazonium salt 2a in a
radical chain transfer mechanism. Finally, intermediate (IV) is
deprotonated, regenerating the aromatic system and leading to
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the desired 3-arylated product 3a
.
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Conclusions
In summary, we have reported
a metal-free, direct
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and B. K. Singh, Eur. J. Org. Chem., 2018, 896.
intermolecular C−H arylation of coumarin by chlorophyll as a
photoredox biocatalyst with white LED light. The reaction
proceeds fluently at room temperature, which does not require
transition-metal catalysts or bases. The reaction involves the
generation of a phenyl radical via photoinduced electron
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transfer
from
a
chlorophyll
in
its
excited
state to a diazonium salt followed by addition to coumarin. The
formed radical is oxidized by the chlorophyll cation radical thus
closes the catalytic cycle. The protocol is green because it
utilizes visible light and natural pigment chlorophyll as the
photosensitizer to deliver the product at room temperature in
a simple procedure. This novel process provides an example for
exploring an environmentally friendly, simple, and convenient
synthetic route utilizing chlorophyll and light energy in organic
chemistry.
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Conflicts of interest
There are no conflicts to declare.
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Acknowledgements
We acknowledge the financial support from University of
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New Journal of Chemistry
Page 6 of 6
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DOI: 10.1039/D0NJ02012E
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Graphical Abstract:
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Chlorophyll-Catalyzed
Coumarin C‒H Arylation with Diazonium Salts
Photochemical
Regioselective
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Ali Moazzam, and Farnaz Jafarpour*
A metal-free, direct C−H arylation of coumarins with aryl diazonium salts at room
temperature using chlorophyll as a green photosensitizer is devised.