Redox-neutral decarboxylative photocyclization of anthranilic acids

Article abstract of DOI:10.1039/d0gc02789h

A mild metal-, catalyst-, and oxidant-free photoredox neutral system has been found to efficiently enable intramolecular decarboxylative cyclization of anthranilic acids. This facile protocol provides an alternative method for the synthesis of carbazoles. Mechanistic studies reveal a key photoinduced 6π-electrocyclization process and formic acid was released as the sole byproduct.

Full text of DOI:10.1039/d0gc02789h

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DOI: 10.1039/D0GC02789H
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Redox-neutral decarboxylative photocyclization of anthranilic
acids
Received 00th January 20xx,
Accepted 00th January 20xx
Huawen Huang,* Kun Deng and Guo-Jun Deng*
DOI: 10.1039/x0xx00000x
www.rsc.org/
A mild metal-, catalyst-, and oxidant-free photoredox neutral
system has been found to efficiently enable intramolecular
decarboxylative cyclization of anthranilic acids. This facile
protocol provides an alternative method for the synthesis of
carbazoles. Mechanistic studies reveal a key photoinduced 6π-
electrocyclization process and formic acid was released as the
sole byproduct.
(a) transition metal catalysis
[M]
[M]
COO
COOH
[M]
-CO2
base
well-known
[M]
: Pd, Cu, Ag, Rh, Ru, etc.
(b) benzoyl hypohalites
X
COOM
COOH
base
X
COO
X2
Carboxylic acids are cheap, stable, and versatile raw materials
that have been widely employed in organic synthesis and
related studies. Decarboxylative couplings of carboxylic acids
have proved to be a viable strategy for diverse alkylations and
arylations.1 To improve the reactivity of carboxylic acids,
prefunctionalizations are frequently performed to transform
them into ester, thioester, or amide derivatives, which allow a
smooth oxidative addition of a catalytically active low-valent
metal species into the C–X bonds (X = O, S, N). Recently,
alkyl carboxylic N-hydroxyphthalimide esters have been used
in the decarboxylative functionalization under mild metal or
photocatalysis.2 Direct decarboxylative couplings of carboxylic
acids represent a straightforward route in molecular synthesis.
To this end, transition metal catalysis has been intensively
explored to find diverse efficient catalytic systems on the basis
of Pd, Cu, Ag, Rh, and Ru, etc (Fig. 1a).3 Alternatively, the in
situ generation of carboxylate salts was exchanged with
elemental halogen to form benzoyl hypohalites which could
efficiently decompose into aryl halides (Fig. 1b).4
-CO2
M: Ag, K, etc; X = Br or I
(c) photoredox catalysis
≈ 106 s-1
COOH
COO
k
hv, PC
-H, -e-
-CO2
rarely reported
CO2Ar
H-atom abstraction
Ar-H
≈ 108 M-1 s-1
k
≈ 107 M-1 s-1
k
(d) decarboxylation of anthranilic acids
Pd(OAc)2 (10 mol%)
Cu(OAc)2 (3.0 equiv)
DMA, 130 oC, 72 h, N2
previous work
R2 COOH
N
R1
R3
R1
R3
this work
N
H
R2
light-irradiation
room temperature
catalyst-free
oxidant-free
Photoredox catalysis has emerged to be a powerful tool for
organic synthesis.5 Specifically, photoinduced decarboxylation
via aroyloxy radicals provides a promising platform for
couplings,6 which, however, is known to be challenging (k =
1.4 × 106 s-1)7 because of the rapid H-atom abstraction (k = 1.2
Fig. 1 Decarboxylation of benzoic acids
× 107 s-1) or addition to arenes (k = 2.2 × 108 s-1) (Fig. 1c).8 In
2017, Glorius group disclosed visible light-mediated
a
decarboxylation of aryl carboxylic acids via a key benzoyl
hypobromite intermediate that afforded a mild access to
biarenes.9 Recently, Yoshimi and co-workers reported visible-
and UV-light-induced decarboxylative radical reactions of
benzoic acids using organic photoredox catalysts.10 Based on
this concept, divergent arene products by addition, borylation,
and reduction were generated under mild heating conditions.
Given these intermolecular findings by photoinduced
Key Laboratory for Green Organic Synthesis and Application of Hunan Province,
Key Laboratory of Environmentally Friendly Chemistry and Application of Ministry
of Education, College of Chemistry, Xiangtan University, Xiangtan 411105, China.
Fax:
(+86)0731-5829-2251;
Tel:
(+86)0731-5829-8601;
E-mail:
hwhuang@xtu.edu.cn; gjdeng@xtu.edu.cn
Electronic 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
J. Name., 2013, 00, 1-3 | 1
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decarboxylation of benzoic acids, we speculate the decarboxylation in the present carbazole formation. The result
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intramolecular variant would proceed with much higher from inert atmosphere further confirmedDtOhiI:s1s0u.1p0p39o/sDe0(GeCn0t2ry7859H).
reactivities. Hence, we designed N-arylanthranilic acids as the The use of 1.5 equiv of pyridine slightly enhanced the
model substrate to explore the ability of decarboxylative intramolecular decarboxylative coupling that afforded 1a in
cyclization under photocatalysis. Notably, this reaction has moderate yield (50% yield, entry 6). Inorganic bases such as
been previously realized by a palladium-based catalytic system KHCO3 and NaHCO3 were inferior to pyridine (entries 7 and 8).
involving stoichiometric copper(II) salt as the oxidant.11 Herein, These results above indicated that the present photoinduced
within our continuous program on sustainable photochemical decarboxylative coupling of anthranilic acid probably
synthesis,12 we report the metal-, oxidant- and catalyst-free proceeded through 6π-electrocyclization. Hence, the UV light
photocyclization of N-arylanthranilic acids, which led to facile (365 nm) was used in the absence of photocatalyst. To our
formation of structurally useful carbazoles13 (Fig. 1d).
delight, this catalyst-free treatment led to a good reactivity of
To commence our studies, the DMSO solution of N-methyl- carbazole formation (68% yield, entry 9). Then, the test of
N-arylanthranilic acid (1a) was treated with Ir(dF- reaction media (entries 10-15) revealed that while protic
CF3ppy)2(dtbbpy)PF6 ([Ir]PF6) as the photocatalyst (PC) and solvents such as water and methanol quenched the desired
pyridine as base under blue light irradiation and air atmosphere. transformation, others including actone, acetonitrile (MeCN),
This reaction afforded N-methylcarbazole (2a) in 46% yield 1,4-dioxane, and ethyl acetate all enabled this reaction. We then
(Table 1, entry 1). This result encouraged us to screen different screened a series of bases which were all found inferior to
photocatalysts. Unfortunately, among others both 4CzIPN and pyridine (entries 16-21). Control experiments (entries 22-25)
Rose Bengal did not enable the decarboxylative annulations suggested, again, that oxygen was not necessary in the UV light
(entries 2 and 3). Unexpectedly, the reaction performed under system. And light irradiation found to be indispensable. Finally,
an oxygen atmosphere completely prohibited the desired the yield of 2a was dramatically decreased when using 0.5
transformation (entry 4), suggesting
a
non-oxidative equiv. of pyridine (entry 26).
Table 1 Optimization of reaction conditions.a
COOH
conditions
N
N
H
1a
2a
Entry
1
2
PC
Light source
Blue LEDs
Blue LEDs
Blue LEDs
Blue LEDs
Blue LEDs
Blue LEDs
Blue LEDs
Blue LEDs
UV light
UV light
UV light
UV light
UV light
UV light
UV light
UV light
UV light
UV light
UV light
UV light
UV light
UV light
UV light
UV light
In dark
Base
Solvent
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
acetone
H2O
MeCN
1,4-dioxane
MeOH
EtOAc
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
Yield (%)b
46
trace
ND
trace
40
50 (46)
46
28
68 (65)
45
ND
51
[Ir]PF6
4CzIPN
Rose Bengal
[Ir]PF6
[Ir]PF6
[Ir]PF6
[Ir]PF6
[Ir]PF6
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
pyridine
pyridine
pyridine
pyridine
pyridine
pyridine
KHCO3
NaHCO3
pyridine
pyridine
pyridine
pyridine
pyridine
pyridine
pyridine
DBU
3
4c
5d
6e
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23c
24d
25 f
26g
51
ND
60 (58)
39
37
48
34
37
45
30
35
65
ND
55
Et3N
2,6-lutidine
DIPEA
NaHCO3
Cs2CO3
pyridine
pyridine
pyridine
pyridine
UV light
a Reaction conditions: 1a (0.2 mmol), photocatalyst (PC, 2.0 mol%) base (1.0 equiv), DMSO (2 mL), 36 W blue LED (405 nm) or
b
36 W UV light (365 nm), air, 48 h. [Ir]PF6 = Ir(dF-CF3ppy)2(dtbbpy)PF6. Yields were determined by GC analysis using
2 | J. Name., 2012, 00, 1-3
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c
e
dodecane as the internal standard. Isolated yields were given in parentheses. Under O2 atmosphere. d Under N2 atmosphere.
View Article Online
Pyridine (1.5 equiv). f No light. g Pyridine (0.5 equiv).
DOI: 10.1039/D0GC02789H
With the established UV light-induced reactivity of N- generated in generally moderate yields. Unfortunately, meta-
methyl-N-arylanthranilic acid in the metal- and catalyst-free substituted anilines exhibited poor regioselectivities. For
decarboxylative cyclization, we subsequently probed the example, the meta-methyl substrate afforded a mixture of 4,9-
generality and substrate scope of this novel photochemistry (Fig. dimethyl-9H-carbazole and 2,9-dimethyl-9H-carbazole in a
2).
total 80% yield with a 2.3:1 ratio of regioselectivity (2f), while
the meta-fluoro reactant gave a 1:1 mixture of two isomers (2g).
The electron properties of the functionalities attached at this
aryl ring affected dramatically on the reactivity; therein,
strongly electron-donating para-methoxy group was not
R2 COOH
N
R1
pyridine (1.0 equiv)
R3
R1
N
DMSO (0.1 M), 48 h, air
H
hv (365 nm)
R3
R2
1
2
accommodated. Subsequently, we screened
a series of
Et
substrates with substituents at the benzoyl acid moiety. Fluoro
anthranilic acids afforded 2j and 2k in 22% and 41% yield,
respectively. Comparably, chloro reactants enhanced the yield
to 46% and 70% yield, respectively. While the anthranilic acid
with methoxy group delivered 2n in 38% yield, some multi-
functional carbazoles were furnished in good yields (2o-2q).
Regarding N-substituents, other alkyls including ethyl (2r) and
butyl (2s) featured good reactivities. Allyl (2t) and
benzylcarbazoles (2u) could also be produced in modest yields.
Four reacting sites exist when N,N-diarylanthranilic acids were
used, in which this kind of reactants exhibited good efficiency,
affording N-arylcarbazoles 2v-2x in satisfactory yields, with
poor selectivity in the case of unsymmetric triaryl amine (2x).
Notably, the effective gram-scale reaction of 1a under redox-
neutral conditions mirrored the robust nature of our
photochemical decarboxylative couplings (56% yield for 10.0
mmol scale).
N
N
N
a
2b
, 57%
2c
, 50%
2a
, 65%, 56%
t-Bu
i-Pr
N
N
80% (rr = 2.3:1)
N
2e,
45%
2f,
F3CO
2d
, 50%
F
F
N
N
N
2g
, 63% (rr = 1:1)
2h
, 62%
2i
, 67%
F
Cl
F
N
N
N
2k
, 41%
During our studies on the substrate scope, some unexpected
results including dehalogenation and demethylation were
obtained (Fig. 3). The N-methyl-N-(2-fluorophenyl)anthranilic
acid afforded exclusively N-methylcarbazole 2a with the flouro
group completely eliminated (Fig. 3a). Dehalogenation of the
para-halogen such as chloro and bromo was also observed,
leading to the formation of mixtures of halocarbazoles (2z and
2aa) and 2a (Fig. 3b,c). This result may be attributed to
decomposition in C-X bond of the excited intermediate with
high-energy under UV light. The anthranilic acids bearing o-
toluidine moiety proceeded through the decarboxylative
cyclization with methyl group partially eliminated. While 2-
methyl and 2,4-dimethyl substrates underwent decarboxylative
cyclization with minor demethylation products (6:1 and 3:1,
respectively, Fig. 3d,e), the 2,3-dimethylphenyl anthranilic acid
afforded carbazole product via demethylation process (1:25, Fig.
3f). Mechanistically, demethylation as well as defluorination in
ortho positions may occur in the decarboxylative elimination
process.
2j
, 22%
2l
, 46%
OMe
OMe
Cl
N
N
N
2n
, 38%
2m
, 70%
2o
, 60%
F3CO
OMe
F
OMe
N
N
N
Et
2q
, 69%
2p
2s
2v
2r
, 72%
, 63%
N
N
N
Bn
n-Bu
2u
, 68%
, 68%
2t
, 43%
N
N
N
Ph
Ph
Ph
2w
, 65%
, 72%
2x
, 59% (rr = 1:1)
a
Fig. 2 Redox-neutral decarboxylative photocyclization. yield
of 10.0 mmol scale reaction.
With respect to the N-aryl groups (marked by red color), para-
substituted aniline moieties bearing alkyl (2a-2e), fluoro (2h),
and trifluoromethoxy (2i) functionalities were compatible with
this system. The corresponding decorated carbazoles were
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dehalogenation:
we studied the steric hindrance effect of the cyclization process
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DOI: 10.1039/D0GC02789H
F
COOH
by using meta-substituted anthranilic acids (Fig. 4d). While
N
methyl substrate afforded C2-cyclized product 2f as the major
product, the formation o C6-participated annulation product
was enhanced when using isopropyl anthranilic acid (1ae).
Furthermore, C6-annulation product was exclusively generated
when tert-butyl substrate was subjected to the system (1af).
These results are in line with the proposed 6π photocyclization
process. To determine the role of pyridine additive, we
measured UV-Vis absorption of anthranilic acid 1a and its
mixture with pyridine. The results reveal that the addition of
pyridine reduced the absorption density of 1a in the UV light
range (Fig. 4e). Hence, the pyridine additive may only play a
positive role in the process of formic acid elimination. Finally,
the result of light-on/off experiments rules out a radical chain
reaction pathway (Fig. 4f).
(a)
(b)
(c)
+
standard
conditions
N
N
F
2y
2a
62% (<1:25)
COOH
Cl
N
N
+
standard
conditions
N
N
Cl
2z
2a
58% (4:1)
+
COOH
Br
standard
conditions
Br
N
N
2aa
2a
62% (3.3:1)
demethylation:
COOH
N
(d)
(e)
+
(a) radical scavenger
standard
conditions
N
N
H
COOH
standard conditions
2a
2ab
N
TEMPO
BHT
DPE
: 47% yield
: 54% yield
: 47% yield
32% (6:1)
+
additive
(1.5 equiv)
N
COOH
N
2a
1a
standard
N
N
(b) formation of silver mirror
conditions
(c) kinetic isotope effect
standard
conditions
2b
H
COOH
2ac
42% (3:1)
N
N
kH/D = 1:1
COOH
H(D)
D
N
(f)
2a + D1-2a
standard
conditions
D1-1a
N
N
2ad
2f
51% (1:25)
(d) steric hindrance effect
COOH
R
standard
conditions
Fig. 3 Photoinduced decarboxylative couplings along with
dehalogenation and demethylation.
R
N
+
R
N
N
2
1
2'
Decarboxylative couplings in many cases proceed under
oxidative conditions via radical pathway. The control
experiments under inner atmosphere (Table 1) reveal that our
UV-light-induced decarboxylative cyclization of anthranilic
acids is achieved in a redox-neutral reaction system. Moreover,
in our photo system, the pyridine additive and DMSO solvent
were dispensable, which rules out intermolecular electron
transfer and hydrogen-atom transfer. Instead, the reaction
initially proceeds through direct light-irradiation of substrate
followed by intramolecular cycloisomerization. The addition of
a radical scavenger such as TEMPO, butylated hydroxytoluene
(BHT), and 1,1-diphenylethylene (DPE) had no significant
influence on the reaction efficiency (Fig. 4a), which could also
probably suggest a non-radical mechanism of this reaction. On
the basis of some literature, we speculate a redox-neutral 6π
photocyclization mechanism14 that ultimately releases formic
acid as the side product (Fig. 5). Thus, the reaction mixture
after competition was guardedly added dropwise to a silver
ammonia solution. To our delight, a silver mirror formed on the
solution surface (Fig. 4b), which suggests that formic acid was
probably generated. The intramolecular kinetic isotope effect
was determined to be kH/D = 1:1 (Fig. 4c), indicating a
kinetically irrelevant C-H bond cleavage. Hence, the
photocyclization is probably the rate determining step. Then,
1f
1ae
1af
2f 2f'
/ = 2.3 : 1
, R = i-Pr:
, R = t-Bu:
, R = Me:
2ae 2ae'
/ = 1.4 : 1
2af 2af'
/ = <1 : 20
(f) light on/off experiment
(e) UV-Vis absorbtion
.10
.08
-5
1a 10 mol/L
-5
1a + pyridine 10 mol/L
.06
.04
.02
0.00
280
300
320
340
360
380
400
Wavelength (nm)
Fig. 4 Mechanistic studies.
*
H
COOH
H
COOH
H
COOH
hv
N
N
[6]
Photocyclization
N
A
1a
B
COO
H
COO
H
base
HCOO-
N
N
H
py
py-H+
N
2a
D
C
Fig. 5 Possible reaction mechanism.
4 | J. Name., 2012, 00, 1-3
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Aggarwal, Science, 2017, 357, 283-286; (g) T. QiVnie,wJ.ArCticolernOenllilnae,
Notably, N-methyldiphenylamine did not work in our redox-
neutral photochemical system and majority of the starting
reactant was recovered. We rationally speculate that the
electron-withdrawing carboxylic acid group could probably
stabilize the charge polarized intermediate (B) that thereby
enables the electrocyclization process.
C. Li, L. R. Malins, J. T. Edwards, S. Kawamura, B. D. Maxwell,
DOI: 10.1039/D0GC02789H
M. D. Eastgate and P. S. Baran, Science, 2016, 352, 801-805.
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P. Perry, D. M. Cannas and I. Larrosa, Chem. Sci., 2018, 9,
3860-3865.
Conclusions
In summary we have developed a catalyst- and oxidant-free
decarboxylative photocyclization reaction of anthranilic acids
under ambient temperature and air conditions. This redox-
neutral protocol provides a mild alternative approach for the
construction of structurally significant carbazole compounds.
Generally moderate yields were obtained with tolerance of
some useful functional groups. Dehalogenation and
demethylation of some special substrates were unexpectedly
observed, which may inspire other designation of synthesis
through inner bond cleavage. Experimental results of some
mechanistic studies reveal a reaction cascade involving photo-
mediated 6π-electrocyclization and subsequent pyridine-
promoted formic acid elimination.
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Conflicts of interest
9. L. Candish, M. Freitag, T. Gensch and F. Glorius, Chem. Sci.,
2017, 8, 3618-3622.
There are no conflicts of interest to declare.
10. S. Kubosaki, H. Takeuchi, Y. Iwata, Y. Tanaka, K. Osaka, M.
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Acknowledgements
Support by the National Natural Science Foundation of China
(22071211), the Science and Technology Planning Project of Hunan
Province (2019RS2039), Hunan Provincial Natural Science
Foundation of China (2020JJ3032), and the Collaborative Innovation
Center of New Chemical Technologies for Environmental Benignity
and Efficient Resource Utilization is gratefully acknowledged.
12. For recent examples of sustainable photosynthesis, see: (a) Z.
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10, 154-159; (b) Z. Wang, X. Ji, T. Han, G. J. Deng and H.
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Jiang, Green Chem., 2020, 22, DOI: 10.1039/D0GC02091E; (g)
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