Erythrosine B Catalyzed Visible-Light Photoredox Arylation–Cyclization of N-Alkyl-N-aryl-2-(trifluoromethyl)acrylamides to 3-(Trifluoromethyl)indolin-2-one Derivatives

Article abstract of DOI:10.1002/ejoc.201601493

3-Trifluoromethylindoline-2-one derivatives were prepared in a visible-light photocatalytic transformation of acrylamides. The arylation–cyclization sequence was initiated by light-induced aryl radical generation from aryldiazonium salts with the utilization of erythrosine B as an organic photocatalyst.

Full text of DOI:10.1002/ejoc.201601493

A Journal of
Accepted Article
Title: Erythrosine B catalyzed visible-light photoredox arylation-
cyclization of N-alkyl-N-aryl-2-(trifluoromethyl)acrylamides to 3-
(trifluoromethyl)indolin-2-one derivatives
Authors: Zsombor Gonda, Ferenc Béke, Orsolya Tischler, Milán Petró,
Zoltán Novák, and Balázs Tóth
This manuscript has been accepted after peer review and appears as an
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To be cited as: Eur. J. Org. Chem. 10.1002/ejoc.201601493
Link to VoR: http://dx.doi.org/10.1002/ejoc.201601493
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10.1002/ejoc.201601493
European Journal of Organic Chemistry
COMMUNICATION
Erythrosine B catalyzed visible-light photoredox arylation-
cyclization of N-alkyl-N-aryl-2-(trifluoromethyl)acrylamides to 3-
(trifluoromethyl)indolin-2-one derivatives
Zsombor Gonda, Ferenc Béke, Orsolya Tischler, Milán Petró, Zoltán Novák,* Balázs L. Tóth*
Abstract: 3-Trifluoromethyl-indoline-2-one derivatives were prepared
in a visible-light photocatalytic transformation of acrylamides. The
arylation-ring closure was initiated by light induced aryl radical
generation from aryl diazonium salts with the utilization of erythrosine
B as a novel organic photocatalyst.
benzoxazines,[11] 3,4-disubstituted dihydroquinolinones,[12,13] 1-
azaspiro[4.5]decanes[13] and indolin-2-ones[14-20,22-25]
Various
.
radical species can be used for the construction of the target
indolin-2-one derivatives via the cyclization path (Scheme 1). In
the existing methods, radicals for the initial step were originated
from carboxylic anhydrides,[14] carboxylic acids,[15] iodonium
16]
salts,[13b,
arenediazonium salts,[17] solvents such as
dichloromethane[17b]
(phenylsulfonyl)methyl bromide[18] or arylsulfinic acids.[19]
However, the cyclization takes place also by photocatalyzed
acetonitrile,[17c]
or
acetone,[17c]
Introduction
In the last decade the photoredox catalysis evolved rapidly. There
are numerous published transformations which uses the energy
of visible light irradiation to cleave chemical bonds.[1] In a
photocatalytic reaction colored photosensitizer dyes act as
catalysts and after the absorption of appropriate wavelength of a
photon instantaneously an excited state takes place. As the result
of the relaxation charge transfer (CT) occurs which lead to
generate reactive radicals in single electron transfer (SET)
fashion. The library of these photocatalyst dye molecules are wide
and truly diverse.[2] However, it can be concluded, Ir and Ru based
complexes are prevalent, but the use of organic sensitizer
dehalogenation
of
o-iodophenylacrylamides
or
bromodifluoroacetamides.[20]
The fluorinated functional groups have significant impact on
medicinal and agricultural chemistry due to their electronic
properties, metabolic stability and their effect on lipophilicity.[21] In
this field the application of trifluoromethyl group is extensively
investigated.[22]
According to general interest, the presence of fluorine
functionalized groups appeared also in the chemistry of indolin-2-
one derivatives. Based on radical photoredox cyclization
reactions significant synthetic methods were developed to install
fluorine into these cyclic organic compounds, therefore ethyl 2,2-
difluoroacetyl,[23] diethyl (trifluoromethyl)phosphonyl,[24] 2,2,2-
trifluoroethyl[25] or di- and trifluoromethyl group were
introduced.[26]
molecules is
a less developed part of this field. The
photocatalytically generated radicals can recombine resulting a
radical coupling, or attack on a substrate initiating a radical
cascade, which leads to the formation of cyclized product in easily
feasible way.[3]
The indolin-2-one core structure units can be formed by
cyclization reactions of N-aryl acrylamide compounds. These ring
closing reactions were well investigated and several methodology
have already been developed such as transition metal catalyzed
Heck-like coupling,[4] Suzuki-type reactions,[5] oxidative palladium
catalyzed
cyclizations,[6]
oxidative
cyclizations[7]
and
intramolecular radical ring closure initiated electrochemically,[8]
with chemical initiators[9] (e.g. AIBN, TBHP, Bu3SnH, Langlois’
salt) or by UV and visible light.[10] Remarkably, there are
significant examples for Ir and Ru photocatalyzed cyclization
reactions of N-aryl acrylamide derivatives to oxazolines,
Dr. Zs. Gonda, F. Béke, O. Tischler, M. Petró.
Corresponding Authors: Dr. Zoltán Novák, B.
L
Tóth
MTA-ELTE "Lendület" Catalysis and Organic Synthesis Research
Group.
Eötvös
University,
Department
of
Organic
Chemistry
Address: Pázmány Péter stny. 1/A 1117 Budapest, Hungary.
web:
http://zng.elte.hu/
E-mail: novakz@elte.hu
E-mail: tbasileus@caesar.elte.hu
Supporting information for this article is given via a link at the end of
the document
Scheme 1. Photoredox catalyzed cyclization of N-aryl acrylamides to indolin-2-
one structures.
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10.1002/ejoc.201601493
European Journal of Organic Chemistry
COMMUNICATION
In our research, we aimed to prepare trifluoromethylated
oxindole derivatives in the radical cyclization reaction in which the
fluoroalkyl group attaches directly to the heterocyclic system.
irradiation, in absence of catalyst we did not observed the
formation of any product (Table 1. Entry 1). Visible-light irradiation
without any catalyst did not result the formation of the desired
product (Entry 2). Although, it is important to mention, that 10% of
the starting acrylamide was decomposed but no product was
identified. In the presence of 5 mol% tetraacetylriboflavin or
acrydine yellow the heterocycle formation took place in 25% and
18% yield (determined by calibrated GC-FID analysis, Entries 3
and 4) respectively under blue LEDs (Light Emitting Diodes)
irradiation. Eosin B as photocatalyst gave similar results when
blue or green light was used for the activation (Entries 5 and 6).
Ethyl eosin gave slightly higher GC yields both in DMSO and
MeOH for the cyclized product but the efficiency remained
between 27-36% independently from the type of light source
(Entries 7-9). During the optimization studies with other
photocatalyst we recognized that the temperature has significant
influence on the efficiency of the transformation (vide infra). With
the use of LEDs without external cooling the reaction temperature
was 45 °C, due to the heat generation. However, significant
reduction of the temperature to -50 °C had beneficial effect on the
yield and it was revealed, that the ethyl eosin photocatalyzed
reaction in methanol under green light irradiation can convert
quickly the starting N-aryl-2-(trifluoromethyl)acrylamide into
arylated 3-(trifluoromethyl)-indolin-2-one derivative in 77% of GC
yield (Entry 10). Replacement of the light source to blue LEDs
resulted lower 46% yield at -50 °C (Entry 11). In searching the
most efficient sensitizer we found that among the tested
photocatalysts erythrosine B was superior for the transformation.
As a reference for the study of this photocatalyst, the reaction was
carried out in the absence of light and the reaction did not work
(Entry 12). First, the reaction mixture of starting materials and 5
mol% of erythrosine B in DMSO solvent was irradiated by CFL
(Compact Fluorescent Light bulb) and we observed 49% yield at
25 °C (Entry 13). When green LEDs were used for the reaction
the product was formed in 33% GC yield after 2 hours (Entry 14).
Changing the light source from green to blue LEDs resulted higher,
43% GC yield of the oxindole derivative. Considering the
properties of aryldiazonium salts, it is important to exclude or
suppress their thermal decomposition reactions. Therefore, we
carried out the green and blue light irradiated reactions in a wider
temperature range between 45 °C and -50 °C (Entry 15-19). We
found that decreasing the reaction temperature results in higher
yields of the product, and the best result, 89% GC yield of the
cyclic product was obtained at -50 °C under blue light irradiation
after 2 hours reaction time (Entry 19). To compare the light
sources, we carried out the reaction at -50 °C using green LEDs,
but the reaction afforded the desired product only in 31% GC yield,
demonstrating the necessity of blue light (Entry 20). Additionally,
during the optimization studies we demonstrated the importance
of the inert reaction atmosphere. Changing the argon atmosphere
to oxygen for the same reaction gave no product (Entry 21). It is
easy to understand, since the oxygen is triplet in ground state,
which allows to enter into the radical photocatalytic quenching
cycle and thereby inhibit the reaction and the formation of the
desired product. After finding the suitable catalyst and the optimal
reaction temperature we tested further solvents for the
transformation such as DMF, ethanol, hexane, THF but these
solvents were not suitable in this arylation-cyclization reaction
(Entries 21-24).
To achieve the synthesis of the target heterocycle we aimed
to utilize transition-metal-free photoredox catalytic conditions. The
utilization of widely available structurally diverse organic
photocatalysts could make the procedure more economic and
environmentally benign.[27] Beside the utilization of transition
metal based photoredox systems for the ring closure of acrylates,
[14-18,20,23-26]
organic photosensitizer (eosin Y) catalyzed
cyclizations of N-aryl acrylamides were developed by Wang and
coworkers.[19]
Herein we report
photosensitizer catalyzed cyclization reaction of N-aryl-2-
(trifluoromethyl)-acrylamides to access new N-3-
a
novel erythrosine
B
organic
(trifluoromethyl)indolin-2-one derivatives. In our transformation,
aryldiazonium tetrafluoroborate salts serve as radical aryl source.
The reduction potentials of arenediazonium salts are very close
to 0 V, which means the formation of aryl radicals are not energy
demanding procedures, therefore it is easily feasible.[28] This is
one of their advantages, which ease their use in synthetic
applications and in photoredox transformations.
Erythrosine B is a member of the xanthene dye family, its
structure derivable from fluorescein or eosin substances (Scheme
2). It is widely used as food colorant (E127, FD&C Red No. 3) or
painting ink, but the chemical application remained within a
narrow band such as photodegradation,[29] photodynamic therapy
experiments[30]
photoinitiated
radical
polymerization,[31]
photodehydrogenation[32] and used as triplet sensitizer for cis-
trans isomerization of alkenes.[33]
Scheme 2. Members of xanthene dye family used as photosensitizers such as
fluoresceine and its substituted derivatives, eosin Y and erythrosine B.
Results and Discussion
At the beginning of the optimization of the reaction we used
the most frequently applied conditions for our model reaction.[17]
We studied the arylation-cyclization reaction of N-methyl-N-
phenyl-2-(trifluoromethyl)acrylamide
and
4-fluorophenyl-
diazonium tetrafluoroborate, which serves as excellent aryl
radical source under inert (argon atmosphere) photocatalytic
conditions (Table 1). In dimethyl sulfoxide (DMSO) without
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10.1002/ejoc.201601493
European Journal of Organic Chemistry
COMMUNICATION
Table 1. Optimization study for photoredox 4-fluorophenylation-cyclization of N-
methyl-N-phenyl-2-(trifluoromethyl)acrylamide.[a]
As a confirmation for the photoinitiation, the reaction was
followed by in situ time-resolved infrared spectroscopy (IR). The
photoredox
reaction
between
N-methyl-N-phenyl-2-
(trifluoromethyl)acrylamide (1a) and 4- fluorobenzenediazonium
tetrafluoroborate (2d) catalyzed by erythrosine B (5 mol%) was
performed in methanol. Dark and irradiated time periods (by 23 W
CFL bulb) were alternated and the intensity changes of the
carbonyl band of the product (located at 1717 cm-1) was
measured. The product formation was initiated by irradiation and
was paused when the light was switched off. The temperature of
the irradiated periods was between 26-28 °C while in the dark
periods it was 24 °C (Figure 1). The cyclization in methanol was
slower compared to DMSO (see Supporting Information), yet it
reached completion after 6 hours. Therefore, in situ IR-monitoring
demonstrated that the formation of the product occurs only under
light irradiation, while in the absence of light the transformation
stopped.
GC
Entry
Photocatalyst
Light Source
Solvent
T/°C
yield %d
1
2
-
-
dark
DMSO
DMSO
25
25
0
0
23W CFL[b]
Tetraacetyl-
riboflavin
3
4
blue LEDs[c]
blue LEDs[c]
MeOH
MeOH
45
45
25
18
Acrydine
yellow
5
Eosin B
blue LEDs[c]
green LEDs[c]
blue LEDs[c]
green LEDs[c]
green LEDs[c]
green LEDs[c]
blue LEDs[c]
dark
MeOH
MeOH
MeOH
MeOH
DMSO
MeOH
MeOH
DMSO
DMSO
DMSO
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
DMF
45
45
4
6
Eosin B
23
30
36
27
77
46
0
7
Ethyl eosin
45
8
Ethyl eosin
45
9
Ethyl eosin
45
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
Ethyl eosin
-50
-50
25
Ethyl eosin
Erythrosine B
Erythrosine B
Erythrosine B
Erythrosine B
Erythrosine B
Erythrosine B
Erythrosine B
Erythrosine B
Erythrosine B
Erythrosine B
Erythrosine B
Erythrosine B
Erythrosine B
23W CFL[b]
green LEDs[c]
blue LEDs[c]
blue LEDs[c]
blue LEDs[c]
blue LEDs[c]
green LEDs[c]
blue LEDs[c]
blue LEDs[c]
blue LEDs[c]
blue LEDs[c]
blue LEDs[c]
25
49
33
43
44
53
89
31
0[e]
33
19
3
Figure 1. Evolution of peak 1717 cm-1 referring to the characteristic carbonyl
band of 3-(4-fluorobenzyl)-1-methyl-3-(trifluoromethyl)indolin-2-one in a MeOH
solution recorded by ReactIR.
45
45
25
As the result of our optimization studies we determined the
most efficient condition for the synthesis of the 3-trifluoromethyl-
2-oxindole derivatives through the photoredox catalyzed ring
closure. Thus, we explored the substrate scope of the
methodology in MeOH at -50 °C under argon atmosphere in the
presence of 5 mol% erythrosine B photocatalyst under blue light
irradiation by LEDs. The trifluoromethylated acrylamide 1a was
reacted with various diazonium salts under the optimized
conditions (Table 1. Entry 18).
-20
-50
-50
-50
-50
-50
-50
-50
EtOH
The reaction with phenyldiazonium salt provided the oxindole
product 3a in 41% yield. Reactions with aryldiazonium salts
substituted with electron donating groups, such as methyl and
methoxy, on the aromatic ring in para position afforded the
desired products 3b and 3c in 83% and 38% yield, respectively.
Halogenated aryldiazonium salts were also suitable reactants for
the arylation-cyclization reaction. The presence of bromo, chloro
and fluoro functions on the aromatic ring in para positions
tolerated well the reaction conditions and the desired
heterocycles (3d-3f) were obtained in good yields (70-80%).
Hexane
THF
0
[a] General conditions: 0.005 mmol photocatalyst, 0.1 mmol N-methyl-N-
phenyl-2-(trifluoromethyl)acrylamide, 0.25 mmol 4-fluorophenydiazonium
tetrafluoroborate, 1.2 mL solvent, argon atmosphere. [b] CFL = Compact
Fluorescent Light Bulb, [c] Green (520-530 nm) LEDs or blue (460-470 nm)
LEDs. [d] Calibrated GC yield after 2 h. [e] O2 atmosphere.
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10.1002/ejoc.201601493
European Journal of Organic Chemistry
COMMUNICATION
When the chlorine atom was in meta and ortho position the
appropriate products 3g and 3h was obtained in relatively lower
yields (34% and 35% respectively). The reaction carried out with
4-nitrophenyldiazonium salt provided the oxindole derivative 3i in
51% yield, demonstrating the applicability of electron deficient
reaction partners. In contrast, electron withdrawing nitrile group in
ortho position had deleterious effect on the transformation and the
desired product 3j was not formed. Further transformations were
performed on the N-ethyl derivative of the acrylamide (1b) and
similar reactivity pattern was observed when different diazonium
salts were applied. Both the para and ortho fluorophenyl-
diazonium salt provided the appropriate product 3k and 3l in 67%
and 26% yield, respectively. The dimethylphenyl derivative was
isolated in 50% yield, while the utilization of electron deficient
bis(trifluoromethyl)phenyldiazonium salt in the transformation
enabled the access of the appropriate oxindole 3n in 47% yield.
The ring closure was successfully achieved in the reaction of
chloro- substituted aryl acrylamide and methoxy-, fluoro-, chloro-
and bromophenyldiazonium salts, and the halide substituted
heterocycles (3o-r) were obtained in 9-57% yields. The reaction
was repeated under the developed photocatalytic conditions with
electron rich 4-methoxyphenylacrylamide. The reaction of this
substrate both with para fluorophenyl- and para tolyldiazonium
salt provided the appropriate oxindole derivatives 3s and 3t in
42% and 67% yields, respectively. Phenylacrilamides with methyl
and chloro substituents in ortho position to the amide function on
the phenyl ring were also subjected to the transformation and the
expected indolinones 3u and 3v were obtained in 46% and 25%
yield. Reactions between the meta-fluoro substituted acrylamide
and various aryldiazonium salts afforded two regioisomers.
Reaction with 2-fluorophenyl diazonium salt afforded
regioisomers 3w and 3w’ in 1:1 ratio, which were separated and
obtained in 17% and 18 % yields. The appropriate regioisomers
were isolated in higher yields when 4-methylphenyldiazonium salt
was used for the same substrate. In this case the isomers were
formed also in 1:1 ratio but each product was obtained in higher
yield (41% and 47% for 3x and 3x’). Unfortunately, the
appropriate heterocyclic products of the reaction of 4-
fluorophenyldiazonium salt and the meta substituted
fluorophenyl-acrylamide were isolated as 1:1 mixture of the
regioisomers in 66% yield.
Conclusions
In conclusion, we have developed a novel method for visible-light
photoredox
arylation-cyclization
of
N-alkyl-N-aryl-2-
(trifluoromethyl)acrylamides resulting 3-(trifluoromethyl)indolin-2-
one derivatives. The aryl radicals were generated from
aryldiazonium tetrafluoroborate salts. In this reaction erythrosine
B was used as novel organic photocatalyst. The photocatalytic
transformation was carried out at -50 °C, irradiation by blue light
LEDs, and the reaction was followed by in situ time-resolved
infrared spectroscopy at room temperature. With the utilization of
the optimized reaction conditions, wide variety of 3-
(trifluoromethyl)indolin-2-one compounds were synthesized.
Scheme 3. Scope and limitations of the cyclization. Reaction conditions:
erythrosine
B (0.025 mmol, 5 mol%). N-alkyl-N-aryl-2-(trifluoromethyl)-
acrylamide (0.5 mmol), aryldiazonium tetrafluoroborate (1.25 mmol, 2.5 equiv),
6 mL degassed methanol, 3x3 W blue (460-470 nm) Epistar power LEDs, -50 °C,
4-6 hours. Isolated yields. [a] Started from 1 mmol acrylamide.
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European Journal of Organic Chemistry
COMMUNICATION
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A 20 mL septum-screw-cap vial was charged with erythrosine B (0.025
mmol, 5 mol%). N-alkyl-N-aryl-2-(trifluoromethyl)acrylamide (0.5 mmol),
aryldiazonium tetrafluoroborate (1.25 mmol, 2.5 equiv) and equipped with
stirring bar. The atmosphere was evacuated and backfilled with argon
(repeated 3 times). 6 mL degassed methanol was added by syringe
through the septa. 3x3 W blue (460-470 nm) or green (520-530 nm) Epistar
Power LEDs and wires were insulated and the light sources were wrapped
around the vessel. The reaction vessel and LEDs were placed into -50 °C
cryostated isopropyl alcohol cooling bath. The reaction mixture was
homogenized for 5 minutes then the lights were switched on and the
mixture was stirred for 4-6 hours. The mixture was quenched with water,
ad extracted with diethyl ether. The organic phase was collected and
extracted 2 times with saturated NaHCO3 solution, followed by brine then
dried over MgSO4 and evaporated in rotary evaporator. The crude product
was purified by flash column chromatography (hexane/ethyl acetate).
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Acknowledgements
This work was supported by a “Lendület” Research Scholarship
of the Hungarian Academy of Sciences (LP2012-48/2012) and
NKFIH NN118172. The Authors thank the help in structure
determination of the oxindoles to Orsolya Egyed, in HRMS
measurements to Ágnes Gömöry at the Hungarian Academy of
Sciences.
Keywords: erythrosine B • photoredox catalysis • arylation-
cyclization • arenediazonium salts • trifluoromethyl group
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10.1002/ejoc.201601493
European Journal of Organic Chemistry
COMMUNICATION
COMMUNICATION
3-Trifluoromethyl-indoline-2-one
derivatives were prepared in a visible
light photocatalytic transformation of
Organic Dye
Zsombor Gonda, Ferenc Béke, Orsolya
Tischler, Milán Petró, Zoltán Novák*
Balázs L. Tóth*
acrylamides.
The
arylation-ring
closure was initiated by light induced
aryl radical generation from aryl
diazonium salts with the utilization of
Page No. – Page No.
Erythrosine B catalyzed visible-light
photoredox arylation-cyclization of
N-alkyl-N-aryl-2-
erythrosine
photocatalyst.
B as novel organic
(trifluoromethyl)acrylamides to 3-
(trifluoromethyl)indolin-2-one
derivatives
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