Selenium-catalyzed intramolecular atom- And redox-economical transformation ofo-nitrotoluenes into anthranilic acids

Article abstract of DOI:10.1039/d0gc04407e

Anthranilic acids (AAs) are significant basic chemicals used in pharmaceuticals, agrochemicals, dyes, fragrances,etc. Superfluous steps are always involved in obtaining AAs. Herein, we demonstrate a straightforward strategy to transform abundanto-nitrotoluenes into biologically and pharmaceutically significant AAs without any extra reductants, oxidants and protecting groups. Various sensitive groups, such as halogens, sulfide, aldehyde, pyridines, quinolines,etc., can be tolerated in this transformation. A hundred-gram-scale operation is realized efficiently with almost quantitative selenium recycling. Further mechanistic studies and DFT calculations disclosed the proposed atom-exchange processes and the key roles of the selenium species.

Full text of DOI:10.1039/d0gc04407e

Green Chemistry
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Selenium-catalyzed intramolecular atom-
and redox-economical transformation of
o-nitrotoluenes into anthranilic acids†
Cite this: Green Chem., 2021, 23,
986
2
a
a
b
b
Yiming Li,‡ Yuhong Wang,‡ Tilong Yang,‡ Zhenyang Lin * and
a,c
Xuefeng Jiang
*
Anthranilic acids (AAs) are significant basic chemicals used in pharmaceuticals, agrochemicals, dyes,
fragrances, etc. Superfluous steps are always involved in obtaining AAs. Herein, we demonstrate a straight-
forward strategy to transform abundant o-nitrotoluenes into biologically and pharmaceutically significant
AAs without any extra reductants, oxidants and protecting groups. Various sensitive groups, such as halo-
gens, sulfide, aldehyde, pyridines, quinolines, etc., can be tolerated in this transformation. A hundred-
gram-scale operation is realized efficiently with almost quantitative selenium recycling. Further mechanis-
tic studies and DFT calculations disclosed the proposed atom-exchange processes and the key roles of
the selenium species.
Received 29th December 2020,
Accepted 22nd February 2021
DOI: 10.1039/d0gc04407e
rsc.li/greenchem
the significance of AAs and the abundance of o-nitrotoluene,
we proposed the development of a catalytic strategy (find a
Introduction
Anthranilic acid (AA) is a vital moiety present in more than 70 suitable catalyst) for an intramolecular redox transformation
pharmaceuticals, 50 clinical drugs, 20 agrochemicals, and of o-nitrotoluenes into anthranilic acids, which is highly atom-
5
–7
diverse fine chemicals. Over 400 000 tons of AAs are manufac- economical
tured per year for drug synthesis (e.g., alprazolam, furosemide, ing groups (Fig. 1C).
clonazepam, etc.) and agrochemical production (e.g., benta- Direct transformation of o-nitrotoluenes into anthranilic
zone) (Fig. 1A). It is highly essential to develop a methodology acids is atom-economical and redox-economical, but with
without extra reductants, oxidants and protect-
8
9
–11
12–15
for the synthesis of AAs. Nitroarenes and/or o-methyl nitro- contradictory oxygenation
and hydrogenation
pro-
arenes (e.g., o-nitrotoluene) are common substrates used for cesses. Generally, they are rarely compatible with each other,
the synthesis of AAs. However, the related synthesis procedures due to the quenching effect between related oxidative and
are always very complicated. For example, superfluous steps
such as reduction, protection, oxidation, and deprotection are
always involved/needed to obtain biologically and pharmaceu-
1
–3
tically significant products (Fig. 1B).
In 2010, Yu and co-
workers reported an attractive method for AA synthesis from
anilides via direct C–H activation followed by carbon monox-
4
ide insertion. Nevertheless, strategies without extra protection
and deprotection processes are still urgently needed. In view of
a
Shanghai Key Laboratory of Green Chemistry and Chemical Process, East China
Normal University, 3663 North Zhongshan Road, Shanghai 200062, P. R. China.
E-mail: xfjiang@chem.ecnu.edu.cn
b
Department of Chemistry, The Hong Kong University of Science and Technology,
Clear Water Bay, Kowloon, Hong Kong, P. R. China. E-mail: chzlin@ust.hk
c
State Key Laboratory of Organometallic Chemistry, Shanghai Institute of Organic
Chemistry, Chinese Academy of Sciences, Shanghai 200032, P. R. China
†
Electronic supplementary information (ESI) available. CCDC 1948537 and
991082. For ESI and crystallographic data in CIF or other electronic format see
1
Fig. 1 Significance and synthesis of anthranilic acids (AAs). (A)
Remarkable anthranilic acids. (B) Traditional procedures for AA synthesis.
(C) Our strategy.
DOI: 10.1039/d0gc04407e
These authors contributed equally to this work.
‡
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a
reductive intermediates. Therefore, a rare reaction realizes the Table 1 Optimization of conditions
transfer of both oxygen and hydrogen atoms in one reaction.
To solve this problem, finding a specific and powerful catalyst
becomes extremely important to achieve this challenging and
meaningful transformation. In this paper, we report a sel-
1
6–19
enium-catalyzed
intramolecular redox transformation of
b
Chalcogen
Entry (x equiv.)
Solvent
(z mL)
Yield of 2
(%)
o-nitrotoluenes to anthranilic acids (Fig. 1C), an atomic and
Base (y equiv.)
electronic redox strategy capable of achieving hydrogen and
2
0,21
c
oxygen metathesis
between the methyl carbon and nitro
1
2
3
4
5
6
7
8
9
1
1
1
S (1)
S (1)
Inorganic bases (2) EtOH/H
2
2
2
2
2
2
O = 1/2 3–8
d
Organic bases (2)
KOH (2)
NaOH (2)
EtOH/H
EtOH/H
EtOH/H
EtOH/H
EtOH/H
MeOH (2)
MeOH (2)
MeOH (2)
MeOH (2)
MeOH (2)
MeOH (2)
MeOH (2)
MeOH (2)
O = 1/2 ND
O = 1/2 27
O = 1/2 50
O = 1/2 18
O = 1/2 ND
nitrogen centers for the collective synthesis of an AA library. A
hundred-gram-scale operation is also realized efficiently with
almost quantitative selenium recycling.
Se (1)
Se (1)
Se (1)
LiOH (2)
Te/Br
Se (1)
Se (1)
2 2
/I (1) NaOH (2)
e
NaOH (2)
NaOH (2)
NaOH (2)
NaOH (2)
NaOH (2)
NaOH (2)
NaOH (2)
—
61
65
22
65
66
75
ND
ND
e, f
e, f
e, f
e, f
e, f,g
e, f,g
Se (0.1)
Se (0.3)
Se (0.5)
Se (0.3)
—
Results and discussion
0
1
2
As mentioned in the Introduction, the intramolecular redox
transformation of o-nitrotoluenes into anthranilic acids 13
14
Se (0.3)
involves both the oxidation of the methyl group and the
reduction of the nitro group. On the basis of our experiences
a
Standard conditions: chalcogen was added into a nitrogen-filled
tube. After dehydration and degassing, o-nitrotoluene, base, and
solvent were successively added and stirred at 90 °C. LC yields.
2
2–29
on chalcogen chemistry,
chalcogens, sulfur and selenium,
b
are proposed to act as potential catalysts for the transform-
c
t
t
Inorganic bases include NaO Bu, LiO Bu, CsOH, KOH, NaOH, LiOH,
ation in view of their redox properties, i.e., capable of oxidizing K CO , NaHCO , etc. Organic bases include DBU, DABCO, Et N,
d
2
3
3
3
e
DIPEA, etc. MeOH (1 mL) and DMSO (0.2 mL) were added to o-nitro-
toluene and the catalyst, and then NaOH (2 equiv.) in MeOH (1 mL)
electron-rich C–H bonds and reducing the electron-poor nitro
group. Indeed, our experimental attempts gave encouraging
f
was added dropwise over 5 hours with a syringe pump at 90 °C.
results that elemental sulfur delivered the desired product 2 in (2.0 equiv.) was added. PhNO (20 mol%) as an additive.
2
H O
g
2
trace amounts with the aid of inorganic bases (3–8%).
2
However, sodium sulfide (Na S), phenyl disulfide (PhSSPh),
sodium sulfite (Na SO ), and sodium benzenesulfinate
2
3
(PhSO
2
Na) could not catalyze this reaction (Table 1, entry 1), even with easily oxidized methylene groups (e.g., BnO) (7) can
and organic bases did not help (Table 1, entry 2) in increasing be transformed into the corresponding anthranilic acids
the yield of the reaction. To our delight, we found that sel- smoothly in 59–70% yields. Electron-neutral groups with an
enium powder afforded a great yield, probably related to the extended conjugated effect were well compatible (8 and 9).
fact that selenium is softer than sulfur (Table 1, entries Diverse electron-withdrawing functional groups such as tri-
3
0–34
3
–5).
We also tried tellurium, bromide, and iodine, but fluoromethyl (10), nitrile (11), and sulfone (12) were tolerated
they were incapable of achieving this transformation (Table 1, as well. Halogens, such as fluoro (13 and 14), chloro (15–17),
entry 6). Methanol with an additive of dimethyl sulfoxide and bromo (18 and 19) groups remain intact during transform-
increases the efficiency to 61% (Table 1, entry 7). Additional ation. Even the iodo group (20), which is sensitive to common
Lewis acids and phase transfer catalysts were also tested, in reductive conditions, afforded 4-iodo-anthranilic acid.
the hope of accelerating the oxygen transfer process, but Functional groups sensitive to traditionally oxidative con-
without improvement. Further addition of two equivalents of ditions, such as sulfide (13) and aldehyde (21), were success-
water slightly increased the yield to 65% (Table 1, entry 8). fully survived under these conditions. Molecules with miscella-
Reducing the amount of selenium to 30 mmol% maintains the neous active hydrogens, such as amide (22), sulfonamide (23),
efficiency (Table 1, entries 9–11). Trace amounts of side pro- and acids (24 and 25), were converted to the corresponding
ducts, o-aminobenzaldehyde and o-aminotoluene were also products in reasonable yields (44–72%). Generally, hetero-
detected after the reaction was complete. Hence, multifarious cycles and condensed arenes are not tolerated under oxidative
oxidants were investigated to see if they are capable of acceler- conditions. However, compounds 26–31 were produced well in
ating the oxygenation process. We found that additional this system.
PhNO
2
(20 mol%) instead of conventional oxidants success-
Considering the significant value of AAs, the scalability of
fully enhances the yield to 75% (Table 1, entry 12). In the this protocol was evaluated. A hundred-gram-scale operation
absence of selenium or base, the intramolecular redox trans- was conducted to afford the product at 1.2 mol with 60% yield
formation does not occur (Table 1, entries 13 and 14).
Subsequently, the compatibility of the transformation was quantitatively with the assistance of sodium bisulfite
comprehensively investigated (Fig. 2) under the optimized (NaHSO ) and air (ESI, section VI†), which was further con-
(Fig. 3A). Remarkably, red selenium could be readily recycled
3
reaction conditions discussed above (Table 1, entry 12). firmed by PXRD and the undecayed reactivity (68% yield)
o-Methyl nitroarenes with electronic donating groups (3–7), (Fig. 3B). The minute fluctuations of yields, originating from
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distinct allotropy of selenium (red vs. black selenium), were
attributed to the fact that the thousand-membered ring (black
selenium) possesses stronger electron-receiving ability and
lower open-loop fracture energy, compared to the eight-mem-
bered crown-shaped ring (red selenium).
Mechanistic studies
After achieving the unusual transformation, we carried out
several experiments in the hope of gaining insights into the
reaction mechanism, which is expected to be very complex in
view of the fact that the intramolecular redox reaction involves
the transfer of 6 electrons from the carbon center to the nitro-
gen center. The experiments were designed to (1) learn more
about the active selenium species; (2) trace the migration of
1
8
the O atom (via O labeling) and (3) explore the substituent
effects.
The selenium powder, which was used as a (pre)catalyst
2
−
can be dismutated to selenite (SeO ) and polyselenides
3
2
−
35
(
Se
Na
entry 1). The selenide Na Se (30 mol%) was also catalytically
n
) in the presence of hydroxide (Fig. 4A). The selenite
2
SeO (30 mol%) could not catalyze this reaction (Fig. 4A,
3
2
3
6
inactive (Fig. 4A, entry 2). The polyselenides such as Na
Na Se , Na Se , and Na Se
2 4 2 6 2 8
2
Se ,
2
3
7
achieved comparative efficiency
a
Fig. 2 Compatibility. Standard conditions: black selenium (30 mmol%)
was added into a nitrogen-filled tube. After dehydration and degassing,
o-nitrotoluene (1.2 mmol), PhNO
1 mL), and DMSO (0.2 mL) were successively added and stirred at 90 °C
with the addition of NaOH (2 equiv.) in MeOH (1 mL) dropwise over
2
(20 mol%), water (2 equiv.), MeOH
(
b
t
c
5
hours with a syringe pump. LiO Bu (2 equiv.). PhNO
2
(10 mol%) and
2 3
Cs CO (2 equiv.). LiOMe (2 equiv.).
d
e
CoCl
f
2
·6H
2
O
(0.1 mol%).
Substrate in MeOH (3 mL) followed by dropwise addition of NaOH (4
equiv.) in MeOH (1 mL) over 5 hours.
Fig. 4 Experiments designed to learn more about the active selenium
Fig. 3 Hundred-gram-scale reaction. (A) Hundred-gram-scale syn-
thesis. (B) The efficiency and PXRD analyses of selenium. LC yields.
species. (A) Studies with inorganic selenides. (B) Studies with selenium-
containing compounds 32 and 33. LC yields.
a
a
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to selenium powder (Fig. 4A, entries 3–6), which indicated that
polyselenides should be the active catalysts in the systems.
More control experiments were carried out to determine the
active selenium species (ESI, Fig. S3†). We also made a lot of
effort to see if we could separate possible intermediates but
failed, which might be due to the high activity and low stability
of the intermediates. Then, relatively stable selenium-contain-
ing compounds 32 and 33 were prepared. To our surprise, 32
and 33 afforded the desired product 2 in reasonable yields in
the presence of sodium hydroxide (Fig. 4B). Furthermore, the
HRMS trapping studies found the characteristic signals of sel-
enium-containing intermediates with different substrates (ESI,
section 3.1†). The above results further indicated the impor-
tance of possible selenium-containing catalytic species.
Then, the migration of oxygen from the nitro to carboxylic
1
8
group was proved via the O-labelling experiment (Fig. 5, eqn
(
H
1)). To further understand the decrease of the labelling ratio, Fig. 6 Hammett plot experiments.
O instead of H O was introduced into the standard
2 2
1
8
16
system (Fig. 5, eqn (2)), which afforded the labelling product
as well. The above results and control experiment with anthra-
1
8
nilic acid and H
2
O under standard conditions (Fig. 5, eqn
(3)) indicated the exchange between the water or hydroxy anion
and the intermediate(s) (e.g. aldehyde/acetal), instead of the
final product (ESI, Fig. S20†). Beyond those results shown
above, the substituent effect was studied with Hammett plot
experiments, indicating that electron-withdrawing groups
accelerate the progress of this system (ρ = 1.302) (Fig. 6).
With the limited experimental mechanistic studies
described above, we decided to carry out DFT calculations at
the M062X/6-311++G** level of theory (see the ESI† for compu-
tational details) in the hope of gaining more insights into this
transformation. We do not expect to derive an accurate/exact
reaction mechanism. However, a working hypothesis is extre-
mely important for gaining insight into the reaction.
Fig. 7 Proposed mechanism. The detailed energy profiles for the trans-
Fig. 7 gives a feasible/plausible mechanism supported by formation from o-nitrotoluene 1 to o-aminobenzoic acid 2 catalyzed by
2
−
our DFT calculations (ESI, Fig. S13† for the detailed energy the model catalyst Se
profiles). The mechanistic cycle consists of four major stages.
4
are given in Fig. S13 of the ESI.†
Stage 1 (1 → C) is related to deprotonation and Se atom
abstraction. Sodium hydroxide first deprotonates one benzylic
hydrogen of o-nitrotoluene (1) to form a dearomatized anionic
species (A). A is then able to abstract a selenium atom from
2
−
2−
Se
n
to form Sen−1 and B, from which a second deprotona-
tion occurs to give C. The DFT calculations indicate that the
second deprotonation (B → C) is the rate-determining step in
the whole catalytic cycle, with an overall activation free-energy
barrier of 27.8 kcal mol− . This finding gains support from the
Hammett plot showing that electron-withdrawing groups on
the aryl ring promote/accelerate the reactions.
1
In stage 2, C undergoes a series of re-arrangements to give
F, formally completing an exchange of O for Se to achieve the
mutual redox process: first re-aromatization via formation of a
5-membered-ring species (D), followed by a ring N–O bond
cleavage leading to E having a nitroso substituent and then
2
−
transfer of Se to the nitroso N to form F having an aldehyde
substituent. In this process, two pairs of electrons were for-
mally transferred from the carbon centre to the nitrogen
1
8
Fig. 5 The O labeling experiments to trace the migration of the O
atom.
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centre. In the O-for-Se exchange process, the re-aromatization
Author contributions
C → D step shows a moderately high barrier (22.9 kcal mol−
1
)
likely due to the need for rotating the exo CvC double, but the Xuefeng Jiang conceived the project, Yiming Li and Yuhong
other two steps are very facile with the barriers being less than Wang performed the experiments and analyzed the data,
0 kcal mol−
1
.
Zhenyang Lin and Tilong Yang performed the DFT calcu-
1
Stage 3 (F → J) corresponds to the second O-transfer: for- lations. Xuefeng Jiang, Zhenyang Lin, Yiming Li, and Tilong
mation of a new 5-membered-ring species (G) through an Yang prepared the manuscript.
intramolecular O-nucleophilic attack on the aldehyde carbon
in F, followed by a ring N–O bond cleavage leading to H and
then a hydride transfer to give I and then J (after proton
migration), completing the second O-transfer and the formal
Conflicts of interest
transfer of the third pair of electrons. The ring N–O bond clea- There are no conflicts to declare.
vage together with the hydride transfer requires a barrier of
1
7.5 kcal mol− , while all the other steps are very facile with
1
−
1
barriers being less than 10 kcal mol . The selenium-contain-
Acknowledgements
ing species trapped by HRMS (ESI, section 3.1†) are proposed
to be the dehydrated product of J. During this process, the O We are grateful for financial support provided by NSFC
exchange between H and water (or hydroxide) was proposed to (21971065), S&TCSM of Shanghai (20XD1421500, 20JC1416800
1
8
occur (ESI, Fig. S20†), which was coincident with the O label- and 18JC1415600), the Innovative Research Team of High-
ling experiments in Fig. 5.
In the final stage, abstraction of a Se atom from J by Sen−1
regenerates the catalyst Se
Level Local Universities in Shanghai (SSMU-ZLCX20180501),
Professor of Special Appointment (Eastern Scholar) at
and gives K, the hydrolysis of Shanghai Institutions of Higher Learning, and Research
2
−
2
−
n
which produces the final product anthranilic acid (2). The Grants Council of Hong Kong (HKUST 16305119). Y. L. thanks
−
1
barrier for the Se atom abstraction is 18.7 kcal mol
.
China Postdoctoral Science Foundation (2018M632059). We
Here, one may ask whether intermolecular redox reactions are also grateful to Ms. Yanxia Zhang for electron paramag-
between different intermediates are possible. However, the netic resonance studies and advices.
thermodynamic and kinetic disadvantages (related to the
entropy effect) for intermolecular reactions between different
intermediates (when compared with intramolecular reactions
described in the mechanistic proposal in Fig. 7) exclude the
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