Efficient visible light mediated synthesis of quinolin-2(1H)-ones from quinolineN-oxides

Article abstract of DOI:10.1039/d1gc01460a

Quinolin-2(1H)-ones are one of the important classes of compounds due to their prevalence in natural products and in pharmacologically useful compounds. Here we present an unconventional and hitherto unknown photocatalytic approach to their synthesis from easily available quinoline-N-oxides. This reagent free highly atom economical photocatalytic method, with low catalyst loading, high yield and no undesirable by-product, provides an efficient greener alternative to all conventional synthesis reported to date. The robustness of the methodology has been successfully demonstrated with easy scaling up to the gram scale.

Full text of DOI:10.1039/d1gc01460a

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Efficient visible light mediated synthesis of
quinolin-2(1H)-ones from quinoline N-oxides†
Cite this: Green Chem., 2021, 23,
049
5
a
a
b
a
a
Susanta Mandal, Samuzal Bhuyan, Saibal Jana, Jagir Hossain, Karan Chhetri and
Received 26th April 2021,
Accepted 17th June 2021
a
Biswajit Gopal Roy
*
DOI: 10.1039/d1gc01460a
rsc.li/greenchem
Quinolin-2(1H)-ones are one of the important classes of com- and associated expense in obtaining suitably pre-functiona-
pounds due to their prevalence in natural products and in pharma- lized aromatic systems, requirement of other co-precursors,
cologically useful compounds. Here we present an unconventional use of harmful heavy metal complexes or strong acidic or basic
and hitherto unknown photocatalytic approach to their synthesis conditions and lower yields make the first approach funda-
from easily available quinoline-N-oxides. This reagent free highly mentally less attractive in terms of efficiency and cost effective-
atom economical photocatalytic method, with low catalyst ness. The exploitation of easily accessible N-oxides of naturally
loading, high yield and no undesirable by-product, provides an abundant/easily synthesizable quinolines makes the second
efficient greener alternative to all conventional synthesis reported approach (Fig. 2B) comparatively more efficient and cost
to date. The robustness of the methodology has been successfully effective in most cases and therefore more suitable for bulk
demonstrated with easy scaling up to the gram scale.
scale synthesis. These Reissert–Henze type approaches exploit
the high oxide nucleophilicity and weak N–O bonds of quino-
line N-oxides to generate good leaving groups at N-1, which
ultimately makes C-2 highly susceptible to nucleophilic attack.
However, these methodologies require stoichiometric (or
excess) electrophilic reagents for successful functionalization
at C-2. Thus, these methods produce significant amounts of
hazardous polar aqueous soluble waste, which in turn jeopar-
dizes the prospect of their use in bulk/industrial scale syn-
thesis. Moreover, the use of stoichiometric electrophilic
reagents makes these methods unsuitable for quinoline
N-oxide substrates containing free nucleophilic substituents
like hydroxyl or amine groups. Several early endeavors have
used UV light irradiation on easily available quinoline
N-oxides to obtain the desired 2-quinolones under stoichio-
Quinolin-2(1H)-ones and their analogues represent an impor-
tant class of organic compounds due to their high prevalence
1
–4
in natural products
and pharmacologically beneficial
5
–10
compounds
and as useful intermediates in organic syn-
thesis. Numerous compounds having this core skeleton are
found to exhibit intriguing broad range bioactivity as anti-
endothelin receptor antagoists, CDK5
inhibitor, p38aMAP kinase inhibitors, angiotensin II recep-
1
1
1
2–19
20
tumor agents,
5
21
2
2
23–25
tor antagonists, and antibiotics (Fig. 1).
Many quinolin-2
(
1H)-ones are also found to be useful as efficient fluorescent
2
6,27
markers
for amino acids, peptides, amino carbohydrates,
and amino polysaccharides. Their usefulness has attracted
considerable attention from synthetic chemists, which has
consequently resulted in the development of multiple,
efficient synthetic methods. These synthetic approaches can
be divided into two main categories: (i) through the construc-
tion of quinolones/isoquinolone rings starting from a suitably
metric reagent free conditions in a polar protic solvent
48–51
(
Fig. 2C).
However, the high energy UV light generates
multiple unstable undesirable byproducts as the major com-
ponents in the product mixture with a very low yield of
2
8–41
pre-functionalized benzene precursor
(Fig. 2A) and (ii)
52,53
2-quinolones.
Furthermore, the use of high energy UV
through efficient ortho-functionalization of N-oxides of natu-
light makes these methods unsuitable for substrates contain-
ing UV labile substituents (like, nitro, carbonyls, alkenes etc.).
Generation of a complex mixture of products, with a low yield
of the desired quinolone, harsh reaction conditions and
limited substrate scope, makes this method synthetically unat-
tractive. Therefore, there remains a genuine need for the devel-
opment of an easy, mild, environment friendly general method
for the synthesis of quinolin-2(1H)-ones with a wide substrate
scope.
4
2–47
rally abundant quinolines (Fig. 2B).
The inherent difficulty
a
th
Department of Chemistry, Sikkim University, 6 Mile, Tadong, Gangtok,
Sikkim – 737102, India. E-mail: bgroy@cus.ac.in
b
Department of Chemistry, University of Liverpool, Crown Street, Liverpool, L69 7ZD,
UK
†
Electronic supplementary information (ESI) available: General information,
1
13
characterization data, and copies of H and C NMR spectra of products. See
DOI: 10.1039/d1gc01460a
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The application of visible light photoredox catalysis has
revolutionized the field of organic synthesis by providing an
easier, cleaner and greener alternative to numerous strategi-
cally important conventional reactions. Since its inception in
organic synthesis using expensive rare metal complexes as cat-
alysts, to date with much greener and inexpensive organo-
catalysts, it has travelled a highly successful path overcoming
numerous practical and environmental challenges posed by
various conventional methods. Surprisingly, not a single
report to date has addressed the above-mentioned problems
associated with conventional synthetic methods of quinolin-2
(
1H)-ones through the application of visible light photoredox
catalysis. Herein, we report a highly efficient synthesis of qui-
nolin-2(1H)-ones using a visible light photoredox catalyst start-
ing from easily obtainable quinoline N-oxides under ambient
conditions. This metal free highly atom economical method
with very low loading of an easily synthesizable inexpensive
organic photocatalyst, thus, provides a much greener and cost-
effective alternative to all reported synthesis so far.
Fig. 1 Natural products and bioactive drugs containing a quinolin-2
(
1H)-one moiety.
Fig. 2 Strategies to access various acridine derivatives. (A) Typical existing approaches for the synthesis of quinolone through ring construction. (B)
Typical existing approaches for the synthesis of quinolone from quinolone-N-oxide through activation of the 2-position. (C) Early endeavor towards
UV light mediated synthesis of 2-quinolones. (D) The present work: reagent free visible light mediated photochemical synthesis of quinolin-2(1H)-
ones.
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Table 1 Optimization of the reaction conditions
Entry
Catalyst (mol%)
Solvent
CH CN
Time
Yield (%)
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
PC 1 (1 mol%)
PC 1 (1 mol%)
PC 5 (1 mol%)
PC 5 (1 mol%)
PC 5 (1 mol%)
PC 5 (1 mol%)
PC 5 (1 mol%)
PC 5 (1 mol%)
PC 5 (0.5 mol%)
PC 5 (0.5 mol%)
PC 5 (0.5 mol%)
PC 5 (0.2 mol%)a
PC 5 (0.5 mol%)b
PC 5 (0.5 mol%)
3
8 h
8 h
8 h
24 h
24 h
8 h
8 h
8 h
8 h
5 h
10 h
8 h
8 h
8 h
24 h
24 h
8 h
8 h
8 h
73
82
81
ND
Trace
77
50
90
89
65
89
50
86
89
ND
ND
71
DMSO
CH CN
3
H
2
O
MeOH
Toluene
DCM
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
1
1
1
1
1
1
1
1
1
1
Fig. 3 Photocatalysts used for the optimization of the reaction.
c
PC 5 (1 mol%)
—
ness (entry 9, Table 1) compared to 1 mol% of iridium
entries 2 and 17, Table 1) and ruthenium based expensive
PC 2 (1 mol%)
PC 3 (1 mol%)
PC 4 (1 mol%)
(
51
54
heavy metal complexes (entry 18, Table 1) or the Fukuzumi
catalyst (entry 19, Table 1). Considering the high efficiency,
the low catalyst loading requirement and the inexpensive and
easy synthetic procedure, we chose PC5 as the optimized
photo-catalyst for the current reaction. With the optimized
solvent, conditions and catalyst in hand, we became interested
a
b
c
In the presence of O
2
.
In the presence of argon. In the absence of
light; ND means not detected.
Our journey started with initial success in generation of in exploring the substrate scope and generality of the reaction.
quinolin-2(1H)-one 2a on exposure of acetonitrile solution of As shown in Fig. 4 the reaction occurred successfully irrespec-
our model compound, quinoline N-oxide 1a, to a blue LED tive of the electronic nature and the position of various substi-
in presence of the commercially available photocatalyst PC1 tuents on quinoline N-oxides. From the trend it has been
(entry 1, Table 1). The reaction successfully yielded the observed that the steric effect of the substituents seems to
desired product 2a in 73% yield in 8 h. Unfortunately, our play a more prominent role compared to the electronic effect
simultaneous effort to carry out the reaction in an aqueous on the reaction yield or reaction time. For example, both elec-
solvent failed to initiate the desired change in the starting tron withdrawing bromine and electron donating methyl sub-
N-oxides even after 24 h. Environmentally hazardous nonpolar stituents at C-3 resulted in similar yields of quinolone (3 and
or mildly polar organic solvents like THF, chloroform or 4, Fig. 4). All the C-4 substituted quinoline N-oxides (5–12)
dichloromethane were not an option as highly polar quino- were found to give better yields compared to the C-3 substi-
lone N-oxide substrates are not soluble in these solvents. tuted system irrespective of the nature of the substituents.
Therefore, we wanted to explore the possibility of using Again, quinoline N-oxides with substituents at C-6 (13–19)
DMSO, a greener and versatile solvent, for the above-men- were found to give comparatively higher yields compared to
tioned transformation. We were pleasantly surprised to find the C-3 and C-4. The best yield is obtained in the case of sub-
that the reaction was even more efficient in DMSO compared stituents at C-8 (20–24), probably the close proximity of peri
to acetonitrile (entry 2 vs. entry 1, Table 1). Therefore, consid- substituents assists in the formation of oxaziridine by
ering the broad range solubility, environment friendly nature pushing N-oxide towards C-2. However, the bulkiness of the
and favorable solvent effect on the reaction we selected DMSO substituent is found to play a significant role. For example,
as the optimized solvent for this photocatalytic reaction. After quinoline N-oxides with a bulky toluyl substituent at C-8 were
choosing the best solvent for the reaction, we turned to pick found to give a higher yield of 24 than in the case of the less
the best photocatalyst in DMSO through an optimization bulky fluorine substituent at the same position (20). Our
study using a pool of commercially available expensive efforts to extend the methodology to the isoquinoline N-oxide
iridium (PC1, PC2) and ruthenium metal complexes PC3, system have successfully generated differently substituted
Fukuzumi catalyst (PC4), and a new acridinium based photo- 1-isoquinolones 25–28 with almost equal efficiency. The possi-
5
4
catalyst PC5 developed by our group (Fig. 3). We were bility of scaling up the reaction was explored by successfully
pleased to find that the inexpensive and easily synthesizable carrying out the reaction with 1 g of 1a in blue light to obtain
PC5 with 0.5 mol% catalyst loading displayed more effective- 2a in 0.78 g yield in 10 h.
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oxygen of quinolone is not coming from the external sources
but the oxygen of the N-oxide is being transferred either intra-
molecularly or intermolecularly. To determine the intra or
intermolecular nature of the oxygen transfer mechanism, we
carried out the photo reaction in argon saturated acetonitrile
with three different concentrations of quinoline N-oxide and
1
observed their progress (Fig. 5) by recording the H NMR at
certain time intervals (ESI Fig. S1A–C†). In the case of inter-
molecular oxygen transfer, the NMR tube with a higher quino-
line N-oxide concentration was expected to exhibit a higher
rate of reaction. However, in our case, the NMR tube with the
lowest N-oxide concentration was found to display the highest
reaction rate (Fig. 5). Quinoline N-oxide (2) has negligible
absorbance in the blue LED region (400–450 nm) and the
photocatalyst is present at a much lower concentration
(
0.5 mol%) in the reaction mixture; these observations
indirectly indicate the possibility of intramolecular oxygen
transfer from quinoline N-oxide to the vicinal carbon at C-2 to
form 2-quinolone.
As the reaction occurs only in the presence of light and
photocatalyst and in the absence of any reagent or stoichio-
metric oxygen source, a photocatalytic intramolecular oxygen
transfer of N-oxide from N-1 to C-2 through a three membered
oxaziridine ring formation is presumed. Photocatalytic organic
transformation may occur through a single electron transfer
(
SET) or through the energy transfer (EnT) mechanism. To
understand the nature of the photocatalysis of the present
transformation, we have calculated the redox potential of the
excited photocatalyst PC5 and the ground state redox potential
of quinoline N-oxide 2 and calculated their triplet energies. We
found that the excited state reduction potentials of PC5 were
higher, 1.97 V vs. standard calomel electrode (SCE), than the
ground state oxidation potential of 2 (1.24 V vs. SCE).
Therefore, excited PC5 has the power to grab a single electron
from 2 to generate a highly unstable quinolinium-oxo radical,
Fig. 4 Substrate scope of the methodology.
To explore the mechanism of the reaction, we carried out
several controlled reactions. The reaction does not take place
under dark conditions in the presence of a photocatalyst
(entry 16, Table 1) or in a blue LED but in the absence of a
photocatalyst (entry 16, Table 1). This proves that both blue
LED exposure and presence of a photocatalyst are necessary
conditions for the reaction. To understand if the carbonyl
oxygen is coming from an external source like air, water, etc.,
we carried out the reaction in a dry acetonitrile solvent at three
different partial pressures of oxygen. In three different vials,
an equal amount of quinoline N-oxide was dissolved in an
equal amount of dehydrated acetonitrile. The solvent in these
vials was then completely saturated with (i) dry argon, (ii) dry
oxygen and (iii) air with 2 mol% water respectively and the pro-
gress of the reaction was monitored at a fixed time interval
using NMR spectroscopy. The reaction in argon saturated dry
acetonitrile was found to display a higher reaction rate com-
pared to that in air or oxygen saturated dry acetonitrile. From
the above experiment it can be easily concluded that the C-2 Fig. 5 Concentration dependent changes in the rate of the reaction.
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Fig. 6 A. Computed Gibbs free energy profile for the photocatalytic synthesis of quinolin-2(1H)-one. B. Proposed mechanism for visible light
photoredox catalytic synthesis of quinolin-2(1H)-ones.
which may undergo fast intramolecular oxygen transfer to
generate quinolin-2(1H)-one 2a. To explore the possibility of
Conclusions
the Dexter energy transfer mechanism, we compared the An efficient, straightforward method for the synthesis of an
triplet energies of the light absorbing donor PC5 (194.9 kJ array of quinolones and isoquinolones using visible light cata-
−
1
−1
mol ) and acceptor 2 (185.25 kJ mol ). Photocatalyst PC5 lysis has been successfully demonstrated. This highly atom
with higher triplet energy can transfer energy to the lower economical environment-friendly approach does not require
triplet energy acceptor quinoline N-oxide. Therefore, there is a any stoichiometric reagents or generate any undesirable by-
possibility of simultaneous occurrence of the EnT mechanism. product. Being the only photocatalytic method with a low cata-
However, for successful Dexter energy transfer a good overlap lyst loading, high yield and hassle-free clean methodology, it
between the emission spectra of the donor and the absorption provides a much greener alternative to all reagent based tra-
spectra of the acceptor is required, which is not present in the ditional alternatives reported so far. Based on the available
donor and acceptor system we are working with. Therefore, a experimental evidence and theoretical calculations of energy
single electron transfer (SET) has been considered as a more profiles, a mechanism of the reaction has been proposed.
feasible mechanism for this reaction. Our efforts to trap any Further experimental, spectroscopic and theoretical studies
resulting radical during the course of the reaction through the towards a complete and in-depth understanding of mechanis-
addition of TEMPO remained unsuccessful. However, the tic details are being actively pursued.
addition of 1 equiv. of TEMPO was found to competitively
inhibit the reaction which ultimately resulted in 43% of 2a
after 10 h and the rest of the substrate remained unreacted. Conflicts of interest
This fact asserts that TEMPO interferes with the N-oxide photo-
redox process by competitively giving its electrons to the singly
occupied HOMO of the excited photocatalyst. Based on the
A patent application has been filed for the synthesis of the
catalyst used in this reaction.
above-mentioned experimental evidence and TD-DFT Gibb’s
free energy calculations using Gaussian 16 at the M06-2X/6-
Acknowledgements
3
11G(d,p) level of theory (Fig. 6A), a SET mechanism has been
proposed as shown in Fig. 6B. Single electron donation of
N-oxide 1a to the HOMO of the photocatalyst from the oxide
resulted in a highly unstable nucleophilic quinolinium N-oxo
radical, which ultimately participates in attacking the vicinal
electrophilic C-2 centre to form the kinetically favourable three
membered oxaziridine radical cation ring III. The formed
radical cation then acts as an acceptor of electrons from the
singly occupied LUMO of the photocatalyst to form neutral but
relatively unstable oxaziridine IV. The weak N–O bond of oxa-
ziridine IV finally isomerizes to form stable carbonyls through
simultaneous hydride migration to generate the corresponding
quinolin-2(1H)-ones 2a as the final product.
We thank SERB, DST, India (EMR/2016/000175), and DBT,
India (BT/PR24668/NER/95/1339/2017), for project funding.
SM and SB thank DBT & DST respectively for their fellowship.
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