Carbazole based Electron Donor Acceptor (EDA) catalysis for the synthesis of biaryl and aryl-heteroaryl compounds

Article abstract of DOI:10.1039/d0ob00282h

A highly regioselective, carbazole based Electron Donor Acceptor (EDA) catalyzed synthesis of biaryl and aryl-heteroaryl compounds is described. Various indole and carbazole derivatives were screened for the Homolytic Aromatic Substitution (HAS) reaction. Tetrahydrocarbazole (THC) was very efficient for the HAS transformation and proceeded via a complex formation between diazonium salt and electron rich tetrahydrocarbazole. The UV-Vis spectroscopy technique has been used to confirm the complex formation. The in situ generated EDA complex even in a catalytic amount is found to be efficient for the Single Electron Transfer (SET) process without any photoactivation. Biaryl compounds, 2-phenylfuran, 2-phenylthiophene, and 2-phenylpyrrole and bioactive compounds such as dantrolene and canagliflozin have been synthesized in moderate to excellent yields.

Full text of DOI:10.1039/d0ob00282h

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21 December 2017
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DOI: 10.1039/D0OB00282H
Paper
Carbazole based Electron Donor Acceptor (EDA) Catalysis for the
Synthesis of biaryls and aryl-heteroaryl compounds.
Rajendhiran Saritha,a Sesuraj Babiola Annes,a Saravanan Subramanian,b Subburethinam Ramesh*a
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
Scheme 1. Carbazole catalyzed single electron transfer process for C-C
bond formations.
A highly regioselective, carbazole based electron donor acceptor
(EDA) catalyzed synthesis of biaryl and aryl-heteroaryl compounds
is described. Various indole and carbazole derivatives were
screened for the homolytic aromatic substitution (HAS) reaction.
The tetrahydrocarbazole was very efficient for the HAS
transformation and proceed via a complex formation between
diazonium salt and electron rich tetrahydrocarbazole. The UV-Vis
spectroscopy technique has been used to confirm the complex
formation. The in-situ generated EDA complex even in catalytic
amount is found to be efficient for the single electron transfer
(SET) process without any photoactivation. Biaryl, 2-phenylfuran,
2-phenylthiophene, 2-phenylpyrrole and bioative compounds such
as dantrolene and canagliflozin have been synthesized in
moderate to excellent yields.
The biaryl and aryl-heteroaryl compounds are important
structural motif and have been extensively used in various
fields such as agrochemical, fragrance, pharmaceuticals and
dyes.1 The C-H bond functionalization in cross coupling
strategy using transition metal is the general method for the
synthesis of biaryls. Because of the variable oxidation states,
the transition metals are involved in redox switching process
for the cross coupling reactions. However, these strategies
involve pre-functionalized starting materials such as arylhalide,
silane, arylboronic acid, aryltriflate, mesylate etc. These
methods work only in the presence of expensive metal catalyst
and importantly, with stoichiometric co-oxidant in-order to
recycle the active catalyst. In addition, stoichiometric amount
of base is used to increase the electrophilicity of the coupling
partner. Furthermore, the heavy metal traces in active
pharmaceutical ingredient (API) as well as in medicinal
excipients, shall be detrimental to human health and more
cost involved in the purification process. The Gomberg
a.Department of Chemistry, School of Chemical and Biotechnology, SASTRA
Deemed University, Thanjavur 613401, Tamil Nadu, India.
b.Discipline of Inorganic Materials and Catalysis, Central Salt and Marine Chemicals
Research Institute (CSMCRI), Council of Scientific and Industrial Research (CSIR),
G.B. Marg, Bhavnagar - 364 002, Gujarat, India.
† Footnotes relating to the title and/or authors should appear here.
Electronic Supplementary Information (ESI) available: [details of any
supplementary information available should be included here]. See
DOI: 10.1039/x0xx00000x
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Beckmann reaction, the homolytic aromatic substitution the arylation reaction and found that the light hVaieswnAorticrleoOlenlinine
DOI: 10.1039/D0OB00282H
strategy avoids transition metal catalyst and involves the transformation. When we tried the reaction in the absence
diazonium salt as starting precursor. However, the reactions of light, the product 12a was observed in slightly lesser yield
undergo harsh conditions and give the many side products (table 1, entry 3) than that of in the presence of light (table 1,
such as triazenes (ArN=NNHAr), arenes (ArH) and diazenes entry 2). Prolonging the reaction
(ArN = NAr’).2 Recent past, metal-free radical arylation process
for the synthesis of biaryl compounds3 have been studied
Table 1. Optimization of the carbazole catalyzed Gomberg−Bachmann reaction.
involving organocatalyst,4 organophotocatalyst,5 electrolytic
system6 and electron donor acceptor (EDA) complex.7 Photo
physical studies of the EDA complexes have been focussed
since last 50 decades. However, the application of EDA
complex for chemical transformation is less explored.8a The
EDA complex is formed between an electron rich organic
reductant and an electron deficient oxidant. The single
electron transfer (SET) mechanism of a radical process is
concorded with the charge transfer band in the coloured EDA
complex formed between an electron donor and an
acceptor.8b-d The carbazole, an important heterocyclic
compound has broad spectrum of active profile in
pharmaceutical, natural product, and other application.9 In
the very recent time, the carbazole derivatives, electron rich
organic reductant are involved in single electron transfer
radical process either through photocatalytic mechanism or via
electron donor of an EDA complex.10a-d
Zhang, et al. reported photoredox/metal based dual
catalytic system using carbazole in the presence of Ni metal for
the C-C bond formation between carboxylic acid and
alkylhalide (scheme 1, a).10a Photoactivation of chiral carbazole
S. No
1
2
3
4
5
6
7
8
Catalyst
Solvent Yieldb
Catalyst 19 in white LED
Catalyst 18 in white LED
Catalyst 18, without light
Cat. 20
Cat. 21
Cat. 22
Cat. 23
Cat. 24
Catalyst 18, 48h
Cat. 18, 5 mmol of 10a
Cat. 18
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMF
trace
86
84
29
64
36
31
75
80
42
71
11
10
63
83
amine
7 for the enantiopure conjugate addition reactions was
reported by Melchiorre, et. al. (scheme 1, b).10b Carbazole
amine (11) based photocatalytic generation of arylradical from
alkylhalides for the biaryl synthesis was reported by Matsubara
and co-workers (scheme 1, c).10c Yu, et al. reported
photocatalyzed radical translocation process for the remote
heteroarylation of amide using an EDA complex generated
from carbazole based iminium ion (scheme 1, d).10d
Additionally, some of organic transformation using these
carbazole derivatives and other catalytic system for similar
type of C-C bond forming reactions have been reported.11-19 In
this context, we report here the synthesis of aryl-aryl and aryl-
biaryl compound using an EDA complex formed between
tetrahydrocarbazole (THC) and diazonium salt, an organic
oxidant. Note to mention that EDA complex formed in catalytic
amount was found to be active and facile the single electron
transfer process without any photo activation under milder
condition.
9
10
11
12
13
14
15
Cat. 18
Cat. 18
Cat. 18 (5 mol %)
Cat. 18 (20 mol %)
CH3CN
THF
DMSO
DMSO
Unless otherwise noted: aReaction conditions: 20 min. addition of furan (10
mmol), 1.0 mmol of diazonium salt, 10 mol % of catalyst, in dark, under N2, room
temperature, 14 h. bIsolated yield
time did not improve further (table 1, entry 9), but it showed
the product stability in the reaction conditions. Lowering the
equivalent of heteroarene
conversion and 12a was obtained in lower yield (table 1, entry
10). Following catalyst screening, tetrahydrocarbazole (THC,
18) was found to be superior than other structurally related
catalysts such as indole (20), 2,3-dimethylindole (21), 2-
(10a) reduced the product
Thus, we hypothesized that aryldiazonium salts (17), have
high reduction potential might produce aryl radical in the
presence of simple carbazole (Cat. 19). We initiated our
optimization for the reaction condition using p-
bromophenyldiazonium salt (17a) and the coupling partner,
furan 10a with catalytic amount of 19 in the presence of white
LED light (table 1, entry 1), but only trace of product was
methylindole
(22), 2-phenylindole (Cat. 23
)
and N-
alkyltetrahydrocarbazole (Cat. 24). Among various solvents
screened, DMSO was most efficient (table 1, entry 11-13). We
then tested the catalytic loading and 10 mol % of the catalyst
18 was the optimal level. 5 mol % of THC, 18 lowered the yield
and 20 mol % of it did not improved the product yield
significantly.
Having established the optimal condition, we began to
explore the scope of the reaction in-order to understand the
electronic effect of the substitutions on diazonium compound
observed. Surprisingly, upon addition of 10 mol
% of
tetrahydrocarbazole (18) in the presence of light the desired
product 12a was observed in 86% yield (table 1, entry 2). Next,
we sought to determine the exact role of each component for
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(scheme 2). Halogen substitution in a molecule is synthetically 51% (12l) were obtained. On the other hand, metVaieswuAbrtsictlietOuntleinde
DOI: 10.1039/D0OB00282H
more useful for further chemical transformation. Hence, first, phenyldiazonium salt also tolerated the reaction conditions
halogen substituted diazonium derivatives were converted and produced the compound 12m and 12n in good yields. The
into the desired products 12a, 12c and 12d in good yields (65 - success of the arylation of furan 10a prompted us to explore
83%). Electron withdrawing groups such as –NO2, -CN, and -CF3 the synthesis biaryl compound and other aryl-heteroaryl
were similarly tolerated the reaction to produce the (thiophene and pyrrole) compounds using the EDA-complex
compounds 12e-g (72 - 77%). Phenyldiazonium salt 17b was
catalyzed HAT reactions (scheme 3). As expected, electronic
property on the phenyl ring of the phenyhydrazine has
predominantly influenced the arylation reaction on benzene
Scheme 2. Substrate scope of the reactiona using various phenyldiazonium salts
(
17a-n)
Scheme 3. Substrate scope of the reactiona using benzene (10b), thiophene (10c
and pyrrole (10d
)
)
aReaction conditions:. 0.37 mmol of diazonium salt (1 equiv.), 1 mL of benzene , 10
mol % of THC, 18 in dark, under N2 at room temperature for 14 h. bReaction
conditions:. 0.37 mmol of diazonium salt (1 equiv.), 20 equiv. of heteroarene (10), 10
mol % of catalyst in dark, under N2 at room temperature for 14 h
aReaction conditions:. 0.37 mmol of diazonium salt (1 equiv.), 3.37 mmol of
heteroarene (10a), 10 mol % of THC, using DMSO as solvent in dark, under N2 at
room temperature for 14 h.
10b. The electron withdrawing groups including –NO2, -Cl, -Br,
-F were more favourable (12p-s, 52-60%), while electron
donating group –OMe completely declined the arylation
reaction (12t, 5%). On the other hand, heteroarylation using
thiophene 10c and pyrrole 10d were tolerated the condition in
the presence of diazonium salt and catalyst EDA complex and
also used to obtain the heteroaryl compound 12b in 61% yield.
On the other hand, electron donating groups such as –OMe
and –Me were also participated the reaction and gave the
corresponding products 12h and 12i in modest yields of 42%
and 35% respectively. The yield comparison among the
substitution at para position of diazoniun salt clearly suggested
us that the electrondonating groups are stabilizing the possible
radical species and subsequently reduces the yield of the
provided the heteroarylated compounds 12u-y and 12z, 12aa
in moderated yields. To showcase the synthetic utility of this
EDA complex, the arylation reactions were performed to
provide the desire compounds 5-(4-nitrophenyl)furan-2-
carbaldehyde (12ab) and 2-(4-fluorophenyl)thiophene (12ac),
which were used for synthesis of important drug molecules
product. Phenyldiazonium salts with ortho substitution (17j-l
)
declined the conversion due to steric factor against the
arylation, albeit noticeable yield of 64% (12j), 41% (12k) and
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such as dantrolene (26) and canagliflozin (27) respectively TEMPO (28) in the standard reaction conditions which results
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(scheme 4). We next sought to determine the mechanism in the TEMPO-Ar(NO2), 29 and TEMDPOOI-:A10r.(1B0r3)9,/D300OBa0d02d8u2cHt
for the homolytic aromatic substitution reaction (HAS). During formation. The above radical inhibition experiments showed
our initial experiments, when we mix the colourless solutions explicitly the aryl radical formation in the reaction.
of diazoniumsalt in DMSO and THC in DMSO, we observed the
Scheme 5. Control experiments.
formation of coloured solution which probably due to
Scheme 4. Formal synthesis of bioactive drug molecules Dantrolene (26) and
Canagliflozin (27).
Scheme 6. Possible mechanism for EDA complex catalyzed HAS for biaryl/aryl-
heteroaryl synthesis.
Figure 1. a) UV-Vis Spectrum of 17a, THC (18) and mixture of 17a + THC (18).
b) Visual appearance of the coloured EDA complex. C) Job’s plot between molar
fraction of 1a and absorbance.
formation of EDA complex between THC and 17a (figure 1, b).
We next wanted to confirm if the EDA complex is indeed
formed using UV-vis spectroscopy technique (scheme 5). We
prepared a 0.1 mM solution of compound 17a and THC in
DMSO with 1:1 ratio. When we record a UV for the mixture,
we clearly observed a red shift (370 nm) compared to the
starting precursor’s λmax (THC (284 nm) and 17a (285 nm))
(figure 1, a). From Job’s plot between molar fraction of 17a
with corresponding molar extinction coefficient (figure 1, c),
we inferred that the formation of EDA complex with 1:1 ratio
of THC and 17a (scheme 5). In the optimization study, 10 mol%
of THC was found to be the optimal catalytic loading for this
reaction. Note to mention here that 10 mol % of complex
formation is efficient for the arylation reaction. We also
performed a radical inhibition study with radical scavenger,
4 | J. Name., 2012, 00, 1-3
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8330; (e) S. B. Annes, S. Ramesh, Asian J. Org. VCiehweAmrti.c,le2O0n1lin9e,
8, 1398.
Based on the spectroscopic studies, control experiments
and yield variations during scope of the reactions, we
proposed a plausible mechanism for the arylation reaction
5
(a) D. P. Hari, P. Schroll, B. Konig, J. DAOmI:.10C.h10e3m9/.DS0oOcB.,002208122H,
134, 2958; (b) J. Lee, B. Hong, A. Lee, J. Org. Chem., 2019,
84, 9297.
(scheme 6). The reaction begins with EDA complex (
formation between THC, 18 and 17. The formed EDA complex
) undergoes a facile single electron transfer without any
A)
6
7
8
D. Hata, M. Tobisu, T. Amaya, Bull. Chem. Soc. Jpn. 2018, 59,
1749.
A. de. A. Bartolomeu,R. C. Silva, T. J. Brocksom,T. Noe, K. T.
de-Oliveira, J. Org. Chem., 2019, 84, 10459.
(a) C. G. S. Lima, T. deM. Lima, M. Duarte, I. D. Jurberg and
M. W. Paixao, ACS Catal., 2016, 6, 1389; (b) M. J. James, F. S.
Kalthoff, F. Sandfort, F. J. R. Klauck, F. Wagener, F. Glorius,
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(d) H. Y. Tu, S. Zhu, F. L. Qing, L. Chu,
Chem. Commun., 2018, 54, 12710.
A. W. Schmidt, K. R. Reddy, H. J. Knölker, Chem. Rev., 2012,
112, 3193.
(A
photoexcitation. The THC, 18 a nitrogen containing organic
reductant and one of the components of EDA, is probably
donating an electron to the diazonium salt 17 of the EDA
component. The then formed adduct of
radical anion species is probably decomposed into radical
cation , arylradical and nitrogen gas. The radcial cationic
species could be easily stabilized due to the presence of
B, a radical cation and
C
D
C
resonance structures which possess the stable tertiary free
radical. The arylradical would then undergo HAS reaction with
9
furan 10 and provide the radical species
intermediate can abstract a single electron from the radical
arylated species and could be recycled back into its original
catalytic form, the THC, 18 compound. Deprotonation of
cationic species by the borontetrafluoride anion would then
E. At this stage, the
C
10 (a) J. Luo, J. Zhang, ACS Catal., 2016,
6
, 873; (b) Z. Y. Cao, T.
, 1; (c) R.
E
Ghosh, P. Melchiorre, Nat. Commun., 2018,
9
Matsubara, T. Yabuta, U. MdIdros, M. Hayashi, F. Ema, Y.
Kobori, K. Sakata, J. Org. Chem., 2018, 83, 9381; (d) H. Chen,
W. Fan, X. A. Yuan, S. Yu, Nat. Commun., 2019, 10, 1.
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10, 467.
12 T. Yi Shang, L. Hui Lu, Z. Cao, Y. Liu, W. He, B. Yu, Chem.
Commun., 55, 5408.
F
provide the arylated compound 12 and HBF4 as byproduct.
Conclusions
In conclusion, we, for the first time have developed a metal-
free carbazole based EDA catalyzed synthesis of biaryl and
aryl-heteroaryl compounds in moderate to excellent yields.
The method utilizing an EDA complex formed between
diazonium salt and carbazole. The strategy reaction requires
inexpensive and easily available starting precursor. The scope
of the reaction, synthetic application and possible reaction
pathway of the reaction has been demonstrated.
13 Z. Wang, Q. Liu, X. Ji, G. Deng, H. Huang, ACS Catal., 2020, 10
,
154.
14 Z. Wang, X. Ji, J. Zhao, H. Huang, Green Chem., 2019, 21
5512.
15 L. Wang, G. Liu, Cat. Commun., 2019, 131, 105785.
,
16 C. Huang, J. Wang, J. Qiao, X. Fan, B. Chen, C. Tung, L. Wu J.
Org. Chem., 2019, 84, 12904.
17 M. Giedyk, R. Narobe, S. Wei, D. Touraud, W. Kunz, B.
König, Nat.Catal., 2020, 3, 40.
18 D. C. Cameron, H. Garret, M. Miyake, ACS Catal., 2020, 10
,
2609.
ACKNOWLEDGMENT
19 X. Liang, L. Niu, S. Wang, J. Liu, A. Le, Org. Lett., 2019, 21
,
S.R. sincerely thanks DST-SERB, Government of India, New
Delhi for financial support under DST INSPIRE Faculty Program
(Grant No. DST/INSPIRE/04-I/2017/000002). R.S. thanks
SASTRA Deemed University, Thanjavur, India for the research
fellowship.
2441.
Notes and references
1
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Slanga, G.W.Gokel, J. Org. Chem., 1984, 49, 1594; (b)
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3
4
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This journal is © The Royal Society of Chemistry 20xx
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DOI: 10.1039/D0OB00282H
Carbazole based Electron Donor Acceptor (EDA) Catalysis for the Synthesis
of biaryls and aryl-heteroaryl compounds.
Rajendhiran Saritha,a Sesuraj Babiola Annes,a Saravanan Subramanian,b Subburethinam Ramesh*a
N2BF4
Biaryl/
aryl-heteroaryl
Synthesis
X
diazoniumsalt
or
Biaryl/
aryl-heteroaryl
X
or
N
H
X = O, N, S
aryl/heteroaryl
Tetrahydrocarbazole
N2BF4
N2BF4
SET
N
H
N
H
EDA complex
Radical cation/
radical anion