Photoinduced, Direct C(sp2)?H Bond Azo Coupling of Imidazoheteroarenes and Imidazoanilines with Aryl Diazonium Salts Catalyzed by Eosin Y

Article abstract of DOI:10.1002/chem.201905308

Herein, a greener approach to the eosin Y-Na₂ catalyzed, C(sp²)?H bond azo coupling of imidazoheteroarene with aryl diazonium salts is described, under acid free conditions. This direct photoredox process resulted in the corresponding azo products in good to excellent yields. Besides, this new approach could also be applicable to anilines, which is a poorly reactive substrate by other methods. The main features of this reaction are that it provides high yields and is gram-scalable and applicable to biologically relevant imidazoheteroarenes and -anilines.

Full text of DOI:10.1002/chem.201905308

A Journal of
Accepted Article
Title: Eosin Y-catalyzed photo-induced direct C(sp2)-H bond azo
coupling of imidazo-heteroarenes and anilines with aryl
diazonium salts
Authors: Sumbal Saba, Caio R. Dos Santos, Bruno R. Zavarise, Aline
A. S. Naujorks, Marcelo S. Franco, Alex R. Schneider, Marcos
R. Scheide, Ricardo F. Affeldt, Jamal Rafique, and Antonio
Luiz Braga
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To be cited as: Chem. Eur. J. 10.1002/chem.201905308
Link to VoR: http://dx.doi.org/10.1002/chem.201905308
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Eosin Y-catalyzed photo-induced direct C(sp2)-H bond azo
coupling of imidazo-heteroarenes and anilines with aryl
diazonium salts
Sumbal Saba,[a]† Caio R. Dos Santos,[b] Bruno R. Zavarise,[b] Aline A. S. Naujorks,[c]† Marcelo S.
Franco,[b] Alex R. Schneider,[b] Marcos R. Scheide,[b] Ricardo F. Affeldt,[b] Jamal Rafique,*[c] Antonio L.
Braga*[b]
Abstract: Herein, we describe a greener approach to the eosin-Y
Na2 catalyzed, C(sp2)-H bond azo coupling of imidazo-heteroarene
with aryl diazonium salts, under acid free conditions. This direct
photoredox process resulted in the corresponding azo products in
good to excellent yields. Besides, this new approach could also be
applicable to anilines, which is a poorly reactive substrate by other
methods. The main features of this reaction are that it provides high
yields and is gram-scalable and applicable to biologically relevant
imidazo-heteroarenes and anilines.
conditions which limits their use in complex synthesis, specially,
when the substrate contain an acid-labile group. Only few
studies on the reaction of biologically relevant heteroarenes,
imidazo-heterocycles as well as N,N-disubstituted anilines are
found in the literature, mainly involving mild and clean
transformations or a lack thereof (Scheme 1A-B).[18-21]
Imidazo-heteroarenes are ubiquitous heterocycles that represent
an important “privileged scaffold”.[1,2] Their structure is present in
many pharmaceuticals and also has applications in material
sciences.[3-7] Therefore, it is not surprising that the synthesis and
functionalization of imidazo-heteroarenes have received
considerable attention in different areas of scientific
research.[1,2,8-14] Analogously, aryl-azo compounds are widely
used in several areas, including the chemical industry,
pharmaceuticals, chemo-sensors, electronics and liquid crystals
(1a-d, Figure 1).[15-16] In this way, very recently, Feringa
described the photoisomerization and biological evaluation of a
diazo based privileged series of novel photoswitchable quorum-
sensing agonists and antagonists.[16a]
Figure 1. Examples of important azole-heterocyclic azo derivatives.
The development of new synthetic procedures to obtain
multi-targeted hybrids of these skeletons (aryl-azo imidazo-
heteroarenes) in a single structure would be useful, due to their
diverse applications (1e-f, Figure 1).[17a] Although, there are
several reports on the preparation of aryl-azo compounds,[17b-h]
most of the protocols often suffer from acidic condition, long
reaction time, low temperature, inert atmosphere, low yields and
reduced stability of diazonium salts at room temperature.[17h] The
majority of these methods proceed through strong acidic
Scheme 1. Reaction of biologically relevant heterocyclic with diazonium salts
In recent years, there has been a surge of interest in the
development of new organic transformations via photo-redox
catalysis by the synthetic community, resulting in several
interesting publications.[22-28] In this regard, organic dyes such as
eosin-Y Na2, fluorescein and rose bengal have emerged as
versatile photo-catalysts in the cross-coupling reaction for C-C
and C-X bond formation.[29-34]
As part of our research interest in designing and
developing sustainable processes as well as in the C(sp2)-H
functionalization of biologically relevant heteroarenes,[35-40]
herein, we disclose for the first time a photo-induced eosin-Y
Na2 catalyzed azo coupling of imidazo-heteroarene. Using this
scalable approach, aryl-azo imidazo-heteroarenes were
obtained in good to excellent yields from the corresponding
biologically relevant imidazo-heteroarenes and aryl diazonium
tetrafluoroborate via C(sp2)-H functionalization.
[a]
[b]
Asst. Prof. Dr. S. Saba
Centro de Ciências Naturais e Humanas-CCNH,
Universidade Federal do ABC,
Santo André, 09210-580, SP, Brazil.
C. R. Dos Santos, Bruno R. Zavarise, M. S. Franco, A. R.
Schneider, M. R. Scheide, Dr. R. A. Affeldt, Prof. Dr. A. L. Braga
Departamento de Química,
Universidade Federal de Santa Catarina - UFSC
Florianópolis, 88040-900, SC - Brazil. http://labselen.ufsc.com/
E-mail: braga.antonio@ufsc.br
[c]
A. A. S. Naujorks, Assoc. Prof. Dr. J. Rafique
Instituto de Química,
Universidade Federal do Mato Grosso do Sul - UFMS
Campo Grande, 79074-460, MS - Brazil. https://sintmol.ufms.br
E-mail: jamal.rafique@gmail.com , jamal.rafique@ufms.br
In this study, we firstly attempted the C-C coupling of
activated heteroarenes and aryl diazonium salt, based on the
Supporting information for this article is given via a link at the end of
the document
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report by König (Scheme 1E).[34] However, in the case of
imidazo-heteroarenes, we observed the formation of C-N in the
azo-compound, exclusively (Scheme 1D).
For the optimization of this azo-coupling via C(sp2)-H bond
functionalization, we used 7-methyl-2-phenylimidazo[1,2-
in hand (Table 1, entry 4), the generality and scope of the
C(sp2)-H bond azo coupling of other imidazo[1,2-a]pyridine (IPs)
2
with various aryldiazonium tetrafluoroborates
3
were
investigated (Scheme 2).
The reaction worked effectively for structurally diverse
aryldiazonium tetrafluoroborates 3. A variety of diazonium salts
having both electron withdrawing groups (-Br, -NO2, -COMe) and
electron donating groups (-OMe, -OCH2O-) as well as
unsubstituted phenyl ring were tested, affording the
corresponding azo coupled 4a-g product in good to excellent
yields. There was a slight negative effect on the yield when 4-
nitrobenzenediazonium tetrafluoroborate was used as the
substrate. The system also tolerated bulky group, i.e., 2-
naphthyl, resulting in 4h in yield of 91%. Similarly to the previous
report,[21] only E isomers were formed in this transformation.
To check the synthetic versatility of this protocol, the reaction
scope was also tested using different IP cores 2 with diazonium
salt 3a (Scheme 2), under the optimized conditions. The reaction
afforded the corresponding azo coupled products 4a,i-q in 76%
to 99% yields. In general, the electronic effect of the substituents
on the phenyl group, attached at the C-2 position of IP 2, had no
major effect on the yields of the azo products 4a,i-m. Similarly, a
bulky substrate (R = 2-naphthyl) resulted in the desired product
4q in 86% yield.
a]pyridine
2a
and
4-methoxybenzene
diazonium
tetrafluoroborate 3a as model substrates (Table 1). Initial
screening was performed for the catalyst, in the presence of
blue LEDs, using CH3CN as the solvent (entries 1-6). The
desired azo product 4a was obtained in very low yield when the
reaction was performed in the absence of a photo-catalyst (entry
1). Subsequently, different photo-catalysts (5 mol%) were tested
for this coupling reaction and eosin-Y Na2 was found to be the
most efficient, affording the C(sp2)-H bond azo product 4a in
96% yield (entry 4 vs 2,3,5,6).
In the next step, the loading of the catalyst was screened
for this transformation (entries 7-8) and 5 mol% was found to be
ideal. With regard to the influence of the solvent on this coupling
reaction (entries 9-15), CH3CN provided an excellent result in
terms of promoting the formation of 4a (entry 4 vs 9-15).
Subsequently, the influence of the reaction time was
monitored for this transformation (entries 4,16,17) and the
optimized reaction time was found to be 2 h (entry 4). Lastly, the
effect of light on this transformation was examined (entries 18-
20). The coupled azo product 4a was obtained in low yield when
the reaction was performed in the dark. In the presence of white
LEDs, 4a was obtained in 77% yield (entry 19). In the case of
green LEDs, 4a was obtained quantitatively, showing the
superiority of green LEDs for this transformation.
Scheme 2. Scope of aryl diazonium 3 and IP 2.[a]
Table 1. Optimization of reaction conditions.[a]
Entry catalyst [mol %] Solvent
LEDs t [h] Yield [%][b]
1
2
3
4
5
6
7
8
--
CH3CN
Blue
Blue
Blue
Blue
Blue
Blue
Blue
Blue
Blue
Blue
Blue
Blue
Blue
Blue
Blue
Blue
Blue
Dark
White
Green
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
1
3
2
2
2
31
50
66
96
69
78
97
81
39
60
75
60
50
19
27
75
Rose Bengal (5) CH3CN
Fluorescein (5) CH3CN
Eosin-Y Na2(5) CH3CN
Alizarin (5)
CH3CN
p-Terphenyl (5) CH3CN
Eosin-Y Na2 (6) CH3CN
Eosin-Y Na2 (4) CH3CN
Eosin-Y Na2 (5) 1,4-Dioxane
Eosin-Y Na2 (5) DCM
Eosin-Y Na2 (5) DMF
Eosin-Y Na2 (5) DMC
Eosin-Y Na2 (5) MeOH
Eosin-Y Na2 (5) H2O
9
10
11
12
13
14
15
16
17
18c
19d
20e
Eosin-Y Na2 (5) THF
Eosin-Y Na2 (5) CH3CN
Eosin-Y Na2 (5) CH3CN
Eosin-Y Na2 (5) CH3CN
Eosin-Y Na2 (5) CH3CN
Eosin-Y Na2 (5) CH3CN
97
30
77
Quantitative
[a] Reaction conditions: 2a (0.25 mmol), 3 (0.26 mmol), catalyst (5 mol%),
solvent (2 mL). [b] Isolated yields.
LCMS analysis of 2a (ESI S2S19) demonstrated the
presence of a single product. With the best reaction parameters
[a] Isolated yields.
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Furthermore, there was a mild effect on the yields due to the
position of the substituent attached to the pyridine moiety of the
IP 2, affording 4a,n,o in 99%, 86% and 76% yields, respectively.
Lastly, when C-2(H) IP 2 was used as a substrate, no reaction
was observed.
Following the success obtained in the eosin-Y Na2-catalyzed
C(sp2)–H bond azo coupling of IP nucleus 2, the reaction scope
was further extended to structurally different imidazo[2,1-
b]thiazole (IT) cores 5 and diazonium salts 3a and 3b as the
coupling partner, under the optimized reaction conditions
(Scheme 3). To our delight, the reaction worked effectively with
the IT 5 as substrates. In comparison with IP 2, there was no
effect of the substituent and its position on the thiazole ring,
resulting in 6a-b in excellent yields. Furthermore, the electronic
effect of the substituents on the phenyl group at the C-6 position,
with electron withdrawing groups (-F, -Cl, Br) as well as an
electron donating group (-OMe), had no major influence on the
yields of 6c-f. Similarly, a bulky substrate (R = 2-naphthyl)
resulted in the desired product 6g in 85% yield. We were also
encouraged to find that 5-chlorothiophen-2-yl substituted IT 5h
afforded the desired product 6h quantitatively. Lastly, the
diazonium salt with a 4-methoxy group 3a resulted in the
coupled products (6i,j) in slightly lower yields compared to those
with the 4-bromo group 3b (6a vs 6i). There was no product
formation when other N-heterocyclic arenes (benzothiazole,
indole, 1,3,4-oxadiaozle) were tested.
Scheme 4. Reaction of biologically relevant azole-heterocyclics with
diazonium salts.
Furthermore, another main advantage of our method is that it is
applicable to substituted-anilines. The use of aryl diazonium
tetrafluoroborate in such type of coupling are rare and only
limited number of literate is available.[17g-h] To our delight, when
the optimized reaction conditions were tested for N,N-
disubstituted anilines 7, the resulted diazo products 8 were
obtained in good yields (Scheme 5).
Scheme 5. Scope of aryl diazonium 3 and N,N-disubstituted anilines 7.[a]
Scheme 3. Scope of aryl diazonium 3 and IT 5.[a]
[a] Isolated yields.
To further establish the synthetic utility of this protocol,
scale-up reactions were carried out at the gram scale (Scheme
6). IP 2a, IT 5a and diazonium salt 3b were selected as the
reagents under the optimized conditions, affording 4b and 6a,
respectively, with no major decrease in the yields. Thus, this
protocol could be used as a robust methodology for the gram
scale synthesis of azo-coupled IPs and ITs, which are
precursors for important bioactive molecules.[1-7]
[a] Isolated yields.
The preparation of diazo product of IP and IT with alkyl
moiety at C-2 and C-6 position, respectively, is difficult.[21] To our
delight, using this photoinduced diazotization reaction allowed
us to access this kind of compounds in good yields (Scheme 4).
Scheme 6. Gram-scale reactions.
Additionally, an important feature of this new methodology is
the access to C-3(NH2) IP from the synthesized diazo-
compounds 4. For example, the diazo IP compound 4a on
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reduction with elemental zinc in acetic acid leads to the amino
derivative 9, in 96% isolated yield (Scheme 7), which is an
important building block for other complex structures and
biologically active compounds.[41-42]
is inert under these conditions being the first responsible for
initiating the photocatalytic cycle in the reaction.
Based on the control experiments reported in Scheme 8, ESI
S20-21 (Figure SI-II, Table SI), and Table 1 (entry 18), as well
as previous reports in the literature,[29-34,43] two possible
mechanisms, via ionic and radical pathways, could be
simultaneously operating in this transformation, as proposed in
Scheme 9.
When the reaction follows the ionic pathway (Scheme 9A),
the imidazole-heteroarene 2 reacts with the diazonium salt 3 to
generate the azo species 9a. This azo species 9a undergoes
deprotonation to furnish the desired azo product 4.
In the radical pathway (Scheme 9B), the photo-catalyst
(EY) would be excited by the light to give EY*, which generates
EY+ through a single electron transfer (SET) to 3, generating
9b●.[43] Radical species 9b● on reaction with IP 2 generates 9c●.
These species suffer SET to EY+ results 9c+, while regenerating
the catalyst (EY). Lastly, the azo IP cation 9d+ on deprotonation
would result in the desired product 4.
Scheme 7. Access of amine containing IP from 4a.
In order to gain an insight into the mechanism associated
with this eosin-Y Na2-catalyzed photo-induced azo coupling of
azoles via C(sp2)−H functionalization, control experiments were
performed (Scheme 8). With the use of a 2 molar equiv. of
radical inhibitors (TEMPO, BHT, hydroquinone), the reaction
was not completely inhibited, resulting in the desired product 4a
in lower yields (Scheme 8A). This result may indicate that the
transformation proceeds by both radical and ionic pathways. The
standard reaction under an oxygen atmosphere and an inert
atmosphere afforded the azo coupled products 4a in 99% and
96% yields, respectively (Scheme 8B-C), indicating that the
oxygen has no active role in this transformation.
Scheme 9. Plausible Mechanisms.
Scheme 8. Control Experiments.
In conclusion, we have developed an acid free, eosin-Y Na2-
catalyzed procedure for the direct C(sp2)-H bond azo coupling of
imidazo-heteroarenes with aryl diazonium salts, at room
temperature. Under the optimized reaction conditions, this
alternative photo-redox approach worked efficiently to form the
azo products in good to excellent yields from the corresponding
imidazo-heteroarenes and anilines. This gram-scalable protocol
utilizes economical catalyst and is applicable to biologically
relevant imidazo-heteroarene and anilines. This is an important
contribution considering the mildness of the reaction conditions
as well as the potential therapeutic application of these
compounds.
To support which reagent is the photoactive specie towards
excitation of eosin-Y Na2, a fluorescence emission intensity
experiment was performed using different concentrations of
imidazopyridine 2a and aryl diazonium salt 3a. (See Supporting
Information section, ESI E20-21). The slope of the graph refers
to the Stern-Volmer constant (KSV), which shows a value of
6.025 103 M-1 for aryl diazonium salt 3a only, which is clearly
ascribed to interaction with eosin-Y Na2 in the excited state
leading to fluorescence quenching by electron transfer. Since
absorption and emission bands for both compounds are in
different areas of the spectra, this quenching suggests that the
4-methoxybenzenediazonium salt 3a is activated by the
photocatalyst, while 7-methyl-2-phenylimidazo[1,2-a]pyridine 2a
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Conflicts of interest
The authors declare no conflict of interest
Acknowledgements
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We gratefully acknowledge CAPES (001), CNPq, CERSusChem
GSK/FAPESP (grant 2014/50249-8), INCT-Catálise and
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Keywords: Imidazo-heteroarenes • eosin-Y Na2 • diazo • aniline
• sustainable chemistry • aryl diazonium salts
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10.1002/chem.201905308
Chemistry - A European Journal
COMMUNICATION
COMMUNICATION
Asst. Prof. Dr. Sumbal Saba, Caio R.
Dos Santos, Bruno R. Zavarise, Aline A.
S. Naujorks, Marcelo S. Franco, Alex R.
Schneider, Marcos R. Scheide, Dr.
Ricardo F. Affeldt, Assoc. Prof. Dr.
Jamal Rafique,* Prof. Dr. Antonio L.
Braga*
Page No. – Page No.
Highly efficient, clean and an acid free, eosin-Y Na2-catalyzed method for the azo
coupling of imidazo-heteroarenes/anilines were achieved through direct C(sp2)-H
functionalization using aryl diazonium via photo-redox approach.
Eosin Y-catalyzed photo-induced
direct c(sp2)-H bond azo coupling of
imidazo-heteroarenes and anilines
with aryl diazonium salts
This article is protected by copyright. All rights reserved.