Cerium photocatalyzed dehydrogenative lactonization of 2-arylbenzoic acids

Article abstract of DOI:10.1039/c9ob02676b

The first cerium photocatalyzed dehydrogenative lactonization of 2-arylbenzoic acids has been developed. This operationally simple protocol allows rapid access to synthetically useful coumarins on gram scale by employing CeCl₃ as a photocatalyst and O₂ as a terminal oxidant. Overall, this delivers an economical and environmentally amiable entry to diversely substituted coumarins, important structural motifs in bioactive molecules.

Full text of DOI:10.1039/c9ob02676b

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DOI: 10.1039/C9OB02676B
ARTICLE
Cerium photocatalyzed dehydrogenative lactonization of
2‑arylbenzoic acids
Ketan Wadekar, a Suraj Aswale a and Veera Reddy Yatham *a
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
The first cerium photocatalyzed dehydrogenative lactonization of 2-arylbenzoic acids has been developed. This
operationally simple protocol allow rapid access to synthetically useful coumarins on a gram scale by employing CeCl3 as a
photocatalyst and O2 as a terminal oxidant. Overall, this deliver an economical and environmentally amiable entry to
diversely substituted coumarins, an important structural motifs in bioactive molecules.
Derivatives of benzo-3,4-coumarins are widely found in
pharmaceuticals and natural bioactive compounds such as
neo-transshinlactone,6a cell proliferation inhibitors,6b Isodispar
Introduction
The Visible-light photocatalysis has emerged as one of the
fastest growing field in organic synthesis. In such reactions, a
photoactive catalyst absorbs visible light and participates in
either single electron transfer (SET) or energy transfer (ET)
processes with organic substrates to generate various radical
entities under very mild reaction conditions.1 Most photoredox
catalysts at present use are transition metal derived
complexes2 or synthetically elaborate organic dyes,3a-d the cost
and limited availability3e of which can hamper their application
for industrial processes. Recently, visible-light induced Ligand
to Metal Charge Transfer (LMCT) has gained significant
attention for the generation of radical entities by coordination-
LMCT-homolysis process4,5 This strategy substitutes transition
metal (Ir, Ru) based photocatalysts for an abundant and
inexpensive metal catalyst, thus empower a new plot for the
late-stage derivatization of complex molecules.5
B,6c alternariol6d,e and the urolithin family7 (Figure 1). Recently,
in material science these molecules found an interesting
applications.8 Due to the important synthetic applications of
benzo-3,4-coumarins, significant achievements have been
made by using C-H activation strategies for their synthesis9–11
.
Among these synthetic protocols, the intramolecular C−H
lactonization of 2-arylbenzoic acids has emerged as attractive
and efficient method (Figure 2, top). Early methods employing
undesirable stoichiometric toxic reactants12 or UV light13 have
been used for such reactions. Also, transition metal (Pd, Cu)
catalyzed procedures have been developed. Recently, a silver-
catalyzed method has been described to enable this reaction,
Known reports
1. transition metal
(Pd, Cu, Ag)
O
O
O
2. Organo catalyst
(Ar-I, stong oxidant)
OH
O
Me
R
OH
Ph
O
R
O
O
Me
O
3. organo photocatalyst
O
R
+-Mes], (NH4 2S2O8
HO
O
R
[Acr
)
HO
O
O
O
iPr
Me
This work
Isodispar B
Cell proliferation inhibitor
neo-transhinlactone
O
OH
O
(10 mol%)
Me
O
O
= R2 = R3 = OH
= R2 = H; R3 = OH;
= R3 = OH; R2 = H
CeCl3
Urolithin A; R1
Urolithin B; R1
Urolithin B; R1
OH
R
R1
R
OH
O
Blue light, RT
R2
HO
O
O
R
O
alternariol
R
R3
O
R
Figure 1. Natural products and pharmaceuticals containing benzo 3,4-
coumarins.
R
key intermediate
a.Department of Organic Synthesis and Process Chemistry, CSIR-Indian Institute of
Chemical Technology (CSIR-IICT), Hyderabad 500007, India.
E-mail: reddy.iisc@gmail.com, yatham.342@csiriict.in
†Electronic Supplementary Information (ESI) available:
See DOI: 10.1039/x0xx00000x
Figure 2. Dehydrogenative lactonization of 2-arylbenzoic acids
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Journal Name
most likely via SET oxidation-radical cyclization.14 In addition,
Martin group reported the first organocatalyzed
dehydrogenative lactonization process that avoids the use of
expensive transition metal and toxic catalysts15 However, a
common feature associated with all these methods are
employing stoichiometric amounts of oxidants [e.g.,
PhCO2OtBu, (PhCO2)2, AcOOH, K2S2O8, (NH4)2S2O8] that
leads to waste generation.Photo catalysis has also provided an
alternative strategy for this dehydrogenative lactonization
reaction. In 2013, Wei et al. developed the NIS-mediated
lactonization of 2-arylbenzoic acids for the synthesis of benzo-
3,4-coumarins.16 In 2015, Gonzalez-Gomez et al. also reported
a visible-light photocatalytic intramolecular dehydrogenative
lactonization of 2-arylbenzoic acids using (NH4)2S2O8 as the
stoichiometric oxidant.17a In 2019 Song Ye et al. also reported
photocatalytic oxidative lactonization of 2-methyl-1,1′-biaryls
using oxygen as the final oxidant.17b More recently, three
groups described a dual cobalt-photoredox system for the
dehydrogenative lactonization of 2-arylbenzoic acids18
View Article Online
(10 mol%)
(2.0 eq.)
O
O
CeCl3
NaHCO3
DOI: 10.1039/C9OB02676B
O
OH
CHCl
3, 25°C,
Blue-LEDs, rt, air
2a
1a
(%)[a]
2a
Deviation from standard conditions
Entry
1
[b]
95(90)
none
instead of
·7H
CeCl3
2O
CeCl
85
60
80
66
25
92
90
85
62
56
6
2
instead of C3eCl
(NH4)2Ce(NO3)6
3
3
tBu
2CeCl
)
6 instead of CeCl3
(
4
4
instead of
K
NaHCO3
instead of
2CO3
Cs2CO3
5
NaHCO3
6
EtOAc instead of CHCl3
Acetone instead of CHCl3
CH3CN instead of CHCl3
THF instead of CHCl3
DMF instead of CHCl3
7
8
9
10
11
12
13
14
15
16
instead of air
with 2.0 eq. of(NH4 2S2O8
)
ballon
78
6
with O2
Under nitrogen
without CeCl3
without light
0
0
Table 1. Optimization of the reaction conditions. 1a (0.2 mmol), CeCl3 (10
mol%), CHCl3 (0.1 M) at 25°C, 455 nm blue LED for 20 h. [a]NMR yields using
trimethoxy benzene as internal standard. [b]Isolated yield.
Results and Discussions
Motivated by the recent work of Zuo5a and König5e et al., who
developed the cerium photocatalyzed 1,5-HAT functionali-
zation of alcohols and the cerium photocatalyzed
decarboxylative hydrazination of alkyl carboxylic acids. We
wondered how 2-arylbenzoic acids would behave under
cerium photocatalysis. Given that decarboxylation of aromatic
carboxyl radicals is slower than for their aliphatic
homologues,19 we thus reasoned that homolysis of 2-aryl
benzoicacids under cerium photocatalysis would provide
benzoyloxy radicals (key intermediate, Figure 2) that could be
trapped by the aryl substituent in an intramolecular fashion,
further oxidation would generate the benzo-3,4-coumarins
(Scheme 2). Herein, we report the first cerium photocatalyzed
dehydrogenative lactonization of 2-arylbenzoic acids in the
presence of visible light at room temperature, which could
provide an alternative robust method for the synthesis of
benzo-3,4-coumarins (Figure 2, bottom).
oxygen atmosphere afforded, 2a in 78% yield (Table 1, entry
13), while employing (NH4)2S2O8 instead of air diminished in
the yield (Table 1, entry 12). Additionally, control experiments
indicated that a light irradiation, catalytic amount of cerium
salt and an air atmosphere were required for the reaction to
occur. Not even traces of 2a were detected in the absence of
cerium catalyst or the light (Table 1, entries 15 and 16).
With the optimized reaction conditions in hand, we next
investigated the substrate scope (Scheme 1). First, the
electronic variation in the para position of the Ar2 ring was
studied. The results indicates that ethyl (1b), tert-butyl (1c),
phenyl (1d) halogens such as fluoro (1e), chloro (1f), bromo
(1g), trifluoromethyl (1h), trifluoromethoxy (1i), groups were
all well tolerated, giving benzo-3,4-benzocoumarins in 67−91%
yields. Next, the electronic variation in the meta- substitution
of the Ar2 ring was investigated. Electron donating (Me, OMe;
1j and 1k) groups provide the corresponding benzo-3,4-
coumarins in mixture of regio isomers. While electron-
We were pleased to find that the dehydrogenative
lactonization of biphenyl-2-carboxylic acid (1a) took place
efficiently at room temperature upon irradiation with blue
LEDs (455 nm) under cerium photocatalysis. Specifically, using
CeCl3 (10 mol%) as the photocatalyst and O2 (air) as the
terminal oxidant in the presence of NaHCO3 (2.0 eq) in CHCl3
gave compound 2a in 90% isolated yield after 20 h (Table 1,
entry 1). The reaction using CeCl3.7H2O as a photocatalyst
works with similar efficiency to give 2a in 85% yield (Table 1,
entry 2), while the yield of 2a slightly dropped upon use of
other cerium salts (Table 1, entry 3 and 4). When NaHCO3 was
replaced with K2CO3, 2a was afforded in 66% yield (Table 1,
entry 5), where as employing Cs2CO3 instead of NaHCO3
caused a drastic reduction in the yield (Table 1, entry 6). The
reaction worked with similar efficiency in EtOAc, Acetone and
CH3CN, while other solvents (e.g., DMF or THF) were less
effective (Table 1, entries 10-11). The substitution of air for an
withdrawing (F, Cl; 1l and 1m
) groups furnished the
corresponding benzo-3,4-coumarins as a single regio isomer in
moderate to excellent yields (Scheme 1). Notably, this reaction
could tolerate functional groups such as ketone (1n), amide
(1o), nitro (1p) and benzyloxy (1x) give the corresponding
benzo-3,4-coumarins as a single regio isomer in moderate to
good yields (Scheme 1). Meanwhile, the corresponding benzo-
3,4-benzocoumarins (Scheme 1, 1q-t) were obtained in good
regioselectivities with good yields.20 However, when the ortho
position of Ar2 was substituted, the reaction was significantly
inhibited.20 A naphthalene (1u
, 1v), dibenzothiophene (1aa),
dibenzofuran (1ab) ring could also be tolerated in this reaction
(Scheme 1). Arylcinnamic acids 1w-1y was converted to the
corresponding coumarin 2w-2y in good yield, which shows
2 | J. Name., 2012, 00, 1-3
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O
O
O
O
(10 mol%)
(2 eq.)
View Article Online
CeCl3
NaHCO3
OH
O
DOI: 10.1039O/C9OB02676B
OH
Ar1
Ar1
base
Ar2
Ar2
CHCl
RT, Blue-LEDs
3, 25°C
IVLn
Ce
1
2
1a
2a
2a; R = H, 90%, 70%a
2b; R = Et, 67%
2c; R = tBu, 73%
2d; R = Ph, 81%
2e; R = F, 87%
2j; R = Me, 81% (1:1)
2k; R = OMe, 76% (1:0.15)
base.H
base.H
O2
O
O
O
2l; R = F, 56%
2m; R = Cl, 84%
CeIVLn
O
O
*
base
O
O
, 63%
2n; R = -COCH3
O2
, 60%c
2o; R = -NHCOCH3
2f; R = Cl, 89%
R
O
IIILn
, 30%c
A
2g; R = Br, 91%, 75%b
Ce
R
2p; R = -NO2
, 85%
2h; R = CF3
, 83%
D
2i; R = OCF3
R
O
*
Me
O
O
R'
O
O
O2
O
O
Me
O
O
2s; R = Cl, R' = F, 66%
2t; R = OEt, R' = OMe, 57%
1:0.6
2r; 84%
O2
O
R
2q; 80%
B
O
C
O
O
O
Scheme 2. Proposed mechanism for the dehydrogenative lactonization
O
O
R = H, 2w, 61%
R = Cl, 2x, 73%
2u, 80%
2v, 78%
R
prompted us to conduct some preliminary mechanistic
studies.20 As anticipated, ON/OFF experiments revealed that
our reaction required a continuous visible light irradiation to
proceed.20 The inhibition of catalysis upon addition of TEMPO
further indicate that the reaction proceeds via radical
intermediates.
Cl
Cl
O
O
O
X
O
O
O
O
X = S, 2aa, 66%
X = O, 2ab; 60%
2z; 59%
2y, 64%
F
methyl protected Urolithin family
(
Application of our methodology
of natural products):
Based on the investigations of Zuo5a and König5d,e et al., we
propose that the dehydrogenative lactonization of 2-
arylbenzoic acids to benzo-3,4-coumarins occurs via
benzoyloxy radical generation upon ligand-to-metal-charge-
transfer (LMCT). The detailed mechanistic proposal is shown in
Scheme 2. By oxidation of CeCl3 with O2, a cerium(IV) species
is generated. Next the co-ordination of aryl carboxylic acid
OMe
OMe
MeO
O
O
O
O
O
O
Urolithin-B
Urolithin-C
2ad, 54%d
Urolithin-A
2ac, 68%
2ae, 65%d
OMe
OMe
OMe
Scheme 1. Dehydrogenative lactonization of 2-aryl benzoic acids. with CeIV forms complex
A, which undergoes the photoinduced
Reaction conditions as given in Table 1 (entry 1). Isolated yields,
average of at least two independent runs. Reaction performed at 5.0
mmol. Reaction performed at 3.6 mmol scale. Mixture of DMSO and
CHCl3 (1:1) was used as a solvent. dAcetone was used as a solvent.
*other regio isomer.
Ce-O(CO) homolytic cleavage and regenerates the catalytically
competent CeIII species and carboxyl radical B. Followed by an
intramolecular attack by the carboxyl radical22 to the aryl
a
b
c
substituent generate the cyclized intermediate
further undergoes a single electron transfer (SET) process with
O2, would be converted to cation intermediate . Finally, a
deprotonation process, from would deliver the desired
product
C. Which
C
D
further synthetic application of our lactonization. To show
potential application of our methodology,
D
a gram-scale
2
.
synthesis of this lactonization was performed. A 5 mmol and
3.6 mmol portion of 1a and 1g could be cyclized to 2a and 2g
in 70% and 75% yield. This result indicated that the cerium
photocatalyzed dehydrogentaive lactonization had great
potential in a practical organic synthesis.
Conclusions
In conclusions, we have developed a general procedure for the
catalytic dehydrogenative lactonization of 2-aryl carboxylic
acids using an inexpensive cerium photocatalyst under
transition metal free conditions. This operationally simple
protocol allows for the efficient synthesis of benzo-3,4-
coumarins derivatives and has been applied to the family of
natural products. We further demonstrated the application of
our methodology by a gram scale reaction, which is an
important industrial application.
To further demonstrate the synthetic utility of our
methodology, we targeted the synthesis of the methyl
protected urolithin family of natural products (Scheme 1).
Starting from commercially available starting materials, we
synthesized substituted 2-aryl benzoic acids, then applied our
dehydrogenative lactonization to convert compounds (1ac-
1ae) to cyclized products (2ac-2ae) in good yields.20,21 The
efficiency of our dehydrogenative lactonization reaction
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 3
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Soc. 2015, 137, 9234; (j) Y. Qiao, Eric J. Schelter, Acc. Chem.
Conflicts of interest
There are no conflicts to declare
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Acknowledgements
V.R.Y. acknowledges SERB, New Delhi, for the financial support
of Ramanujan Fellowship (SB/S2/RJN-138/2018). V.R.Y. thanks
to Dr. Ch. Raji Reddy for his kind support and helpful
discussions. V.R.Y. thanks to Prof. Burkhard König for providing
the blue LED set up and proof reading of the manuscript. V.R.Y
thank to Kasam Vishali Reddy for nmr support. V.R.Y thank to
Dr. Prathama S. Mainkar and Dr. S. Chandrasekhar (Director,
CSIR-IICT) for their kind support and encouragement. CSIR-IICT
7
Communication
No.
for
this
manuscript
is
IICT/Pubs./2019/398.
8
9
For selected examples of the synthesis of benzo-3,4-
coumarins, see: a) W. Zhang, B. I. Wilke, J. Zhan, K.
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