Eosin y photoredox catalyzed net redox neutral reaction for regiospecific annulation to 3-sulfonylindoles: Via anion oxidation of sodium sulfinate salts

Article abstract of DOI:10.1039/c7ob02977b

An eosin Y photoredox catalyzed net redox neutral process for 3-sulfonylindoles via the anionic oxidation of sodium sulfinate salts and its radical cascade cyclization with 2-alkynyl-azidoarenes was developed with visible light as a mediator. The reaction offers metal and oxidant/reductant free, visible light mediated vicinal sulfonamination of alkynes to 2-aryl/alkyl-3-sulfonylindoles and proceeds via the generation of a sulfur-centered radical through direct oxidation of the sulfinate anion by an excited photocatalyst with a reductive quenching cycle. The mild conditions, use of an organic dye as photo-catalyst, bench stability and easily accessible starting materials make the present approach green and attractive.

Full text of DOI:10.1039/c7ob02977b

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Eosin Y Photoredox Catalyzed Net Redox Neutral Reaction for
Regiospecific Annulation to 3‐Sulphonylindoles via Anion
Oxidation of Sodium Sulfinate Salts
Received 00th January 20xx,
Accepted 00th January 20xx
Rajendra S. Rohokalea,b Shrikant D. Tambeb and Umesh A. Kshirsagar*b
DOI: 10.1039/x0xx00000x
www.rsc.org/
Eosin Y photoredox catalyzed net redox neutral process for 3‐ of an electron from substrate or reductant.1 Nicewicz1d further
sulfonylindoles via the anionic oxidation of sodium sulfinate salts classified the general redox outcomes of these photo‐
and its radical cascade cyclization with 2‐alkynyl‐azidoarenes was catalyzed reactions as (i) net oxidative reactions, which require
developed with visible light as a mediator. The reaction offers an external electron acceptor (oxidant), (ii) net reductive
metal and oxidant/reductant free, visible light mediated vicinal reactions, which require an external electron donor
sulfonamination of alkynes to 2‐aryl/alkyl‐3‐sulfonylindoles and (reductant), and (iii) net redox‐neutral reactions.
proceeds via generation of sulfur‐centered radical through direct
A single‐electron oxidation of anions is one of the potential
oxidation of sulfinate anion by excited photocatalyst with concepts to generate reactive heteroatom‐centered radicals,
reductive quenching cycle. Mild conditions, organic dye as photo‐ due to the bench stability, easy availability and handling of
catalyst, bench stable and easily accessible starting materials inorganic anion salts and their further applications in organic
make the present approach green and attractive.
synthesis such as bond‐forming and oxidation reactions.2,5
Despite this, the reports on visible light‐induced anion
oxidation generating reactive radical intermediates or their
utilization for further synthesis are rather scarce.2 Recently, Gu
et al., reported the visible light initiated Eosin Y photo‐
catalyzed net reduction reaction for the synthesis of
unsymmetrical 2,3‐diarylsubstituted indoles from 2‐alkynyl‐
azidoarenes and sulfonyl chlorides in the presence of base and
1,4‐cyclohexadiene (1,4‐CHD) as a hydrogen radical donor,
which involved an oxidative quenching cycle (Scheme 1).6 This
reaction proceeds by generation of an aryl radical via
desulfynative pathway of sulfonyl chloride through photo‐
catalysis and subsequent annulation. Herein, we report our
outcomes on visible light photoredox catalysis anion oxidation
of sodium sulfinate salts in the presence of 2‐alkynyl‐
azidoarenes for vicinal sulfonamination of alkynes to construct
2‐aryl/alkyl‐3‐sulfonylindole derivatives via net redox neutral
Visible light‐mediated photo‐redox catalysis has emerged as an
alternative, green, economical and practical approach for
generating reactive radicals/intermediates in
a catalytic
manner over the past decade.1 Chemical and electrochemical
oxidation methods have some limitations such as formation of
by‐products, need of a specially designed set‐up and suffers
from cost‐effectiveness.2 Consequently, visible light photo‐
redox catalysis has re‐invoked great attention from the
synthetic community, not only as an alternative for
photochemical and electro‐chemical reactions but also for
classical synthetic methods.1 Metal‐based3 photo‐redox
catalysts and organic dye‐based4 photo‐redox catalysts have
proven immensely useful for various bond‐forming reactions.
In particular, non‐toxic, cost‐effective organic dyes are of
growing interest, and offer advantages in comparison to
metal‐based photo‐catalysts.
6
Pr evious W or k: Gu e t al.
Ar
Ar
Photoredox catalyzed reactions are generally categorized in
two major mechanistic paths in which quenching of excited
photo catalysts is facilitated by either (a) ‘oxidative quenching
cycle’ with the transfer of an electron to the substrate or the
oxidant, or (b) ‘reductive quenching cycle’ with the acceptance
Eoosin Y
1,4-CCDH, CH CN
Ar
ArSO2Cl
3
N
Naa HPO , rt
2
4
N3
H
BBlue Light
(31-84%)
• Net reduction reaction
• Ox idative quenching cycle
P resent W ork:
O
O
Ar
S
Ar/R
EEosin Y
Ar/R
ArSO2Na
DMMF:H O, rt
2
GGreen Lig ht
N
H
N3
• Net redox neutral reaction
• Oxidant/reductan free
• Reductive quenching cycle
(38-84%)
Scheme 1 Photoredox catalyzed reactions off 2‐alkynyl‐azidoarenes
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Figure 1: Biologically active sulfonylindole derivatives.
reaction, which involved the reductive quenching cycle with
the preserved sulfonyl unit.
Sulfonylated molecules7 in general and 3‐sulfonyl‐indole8
derivatives in particular have been a focus of attention due to
their crucial role in the medicinal and pharmaceutical fields
owing to their remarkable biological activities9 such as
norepinephrine reuptake inhibitor (I), HIV‐1 non‐nucleoside
reverse transcriptase inhibitor (II), 5‐HT6 receptor (III) and
orexin receptor antagonist (IV) (Figure 1). Hence sulfonylation
reactions and the synthesis of sulfonylated heterocycles has
received great attention.7,8 Recently, tert‐Butylhydroperoxide
(TBHP) mediated vicinal sulfonamination of alkynes was
reported for 3‐sulfonylindoles at elevated temperature.8a
For optimization of the reactions in the present work,
initially,
the
reaction
between
1‐azido‐2‐
(phenylethynyl)benzene 1a10 and phenyl sodium sulfinate salt
2a was conducted in dichloromethane (DCM) at room
temperature in the presence of 5 mol % Eosin Y as
photocatalysts (entry 1, Table 1). Formation of product 3a was
observed in low yield (28%). This positive result encouraged us
to carry out the further optimization of reaction conditions.
The reaction was examined by changing the solvents (entries
1‐7) in which a combination of DMF/H2O (9:1) was found to be
the best solvents to afford the highest yield (81% entry 7).11
The screening by different photocatalysts, for example, Na2‐
Eosin Y, Rose Bengal, Ru(bpy)3Cl2 and Ir(ppy)3 (entries 8‐11) did
not improve the yield of 3a. It has been realized that both
Scheme 2 Scope of 2‐Alkynyl Arylazides.a,b aReaction condition: 0.14 mmol 1a, 0.41
mmol 2, 5 mol % Eosin Y, DMF/H2O (9:1, 1.5 mL), rt, 48 h; bIsolated yields are given.
cReaction was performed using THF/H2O (9:1) as solvent.
photoredox catalysts (entries 12) and visible light (entries 13)
are necessary for the reaction to occur. With the optimized
reaction conditions, the substrate scope and limitations of 2‐
alkynyl‐azidoarenes were examined (Scheme 2).11 Various 2‐
alkynyl‐azidoarenes were prepared and reacted with phenyl
sodium sulfinate salt (2a). The reaction was successfully
amenable to a wide range of 2‐phenylethynyl aryl azides (1a‐
Table 1 Optimization Studiesa,b
e
), affording the desired 3‐sulfonylindoles 3a‐e in 69‐81%
yields. When 1‐azido‐4‐nitro‐2‐(phenylethynyl)benzene (1f
)
reacted under the same condition, the corresponding product
3f was not formed. Next, azido benzenes with both aryl alkynyl
and alkyl alkynyl were further investigated. A variety of
azidobenzenes with arylethynyl i.e., p‐Me, p‐Et, p‐nBu, p‐Br, p‐
OEt proceeded well in the reaction, giving rise to the
corresponding products 3g‐k in 73‐84% yields. The
azidobenzene with 2‐pyridinyl ethynyl delivered the respective
product 3i (65% yield). When azidobenzenes with different
entry
1
2
3
4
5
6
7
8
photo‐catalyst
Eosin Y
solvents
DCM
PhCF3
CH3OH
THF
yield (%)b
28
Eosin Y
Eosin Y
Eosin Y
Eosin Y
Eosin Y
Eosin Y
Na2‐Eosin Y
Rose Bengal
Ru(bpy)3Cl2
Ir(ppy)3
NR
NR
44
20
25
81
60
trace
trace
trace
NR
HFIP
DMSO
DMF:H2O
DMF:H2O
DMF:H2O
DMF:H2O
DMF:H2O
DMF:H2O
DMF:H2O
aliphatic
alkynyls
such
as
4‐methylpent‐1‐yne,
9
ethynylcyclopropane as well as but‐3‐yn‐1‐ylbenzene were
treated with sodium phenyl sulfinate, the corresponding
products 3m‐o were formed in moderate yields (46, 38 and
55%, respectively). Further to explore the scope of sodium
sulfinate salts, a variety of sodium sulfinate salts were applied
under the optimised reaction condition with 1‐azido‐2‐
(phenylethynyl)benzene 1a (Scheme 3). Aryl sodium sulfinate
salts such as p‐Me, p‐F, p‐Br, p‐Cl and naphthalene‐2‐sulfinate
10
11
12c
13d
none
Eosin Y
NR
aReaction conditions: 1a (0.04 mmol), 2a (0.12 mmol), photo‐catalyst (5 mol %),
solvent (1 mL) and stirring for 48 h under nitrogen atmosphere. bIsolated yield.
cNo photo catalyst. dNo light
2 | J. Name., 2012, 00, 1‐3
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Scheme 5 A plausible mechanism pathway
was carried out with allyl benzene instead of sodium sulfinate
salt (eq 7). No reaction was observed and 1a was recovered
which rule out the possibility of formation of nitrene.
Scheme 3 Scope of sodium sulfinate salts.a aReaction conditions: 0.14 mmol 1a, 0.41
mmol 2, 5 mol % Eosin Y, DMF/H2O (9:1, 1.5 mL), rt, 48 h.
The present reaction, the above control experiments, and
the literature1d,2,5 suggest that the reaction follows a reductive
quenching cycle in net redox neutral pathway. A plausible
radical cascade reaction pathway has thus been proposed
(Scheme 5). Visible light induced photo‐excitation of Eosin Y
proceeded smoothly to deliver the products 3p‐t in 43‐73%
yields. Interestingly, the methyl sulfinate salts afforded the
targeted products 3u in 48% yield.
To have mechanistic insight accompanying the presented
reaction, some control experiments were carried out. When
the reaction was performed under the standard condition in
the presence of classical radical inhibitors such as BHT and
TEMPO, the formation of product was not observed (Scheme
4, eq 1). The absence of water led to the product with its yield
<10% (eq 2). To analyze the role of anion of sulfinate, the
reaction was carried out using the phenylsulfinic acid instead
of sulfinate salt; the product 3a was not obtained (eq 3).
Surprisingly, in the presence of Na2CO3, the desired product
was obtained with <10% yield (eq 4). When the reaction was
performed in the presence of an external oxidant such as
oxygen or nitrobenzene (eq 5), and the DIPEA as an external
reductant (eq 6) reaction failed to give 3a. To check the
formation of nitrene from azide by photocatalysis,12 reaction
(
EY) generates excited state of Eosin Y (EY*). Direct one
electron oxidation of the sulfinate anion by this ambivalent
reactive excited state EY* provides S‐centered sulfonyl radical
(
A2) with reduction of EY* to EY•‐ 13
.
Regioselective addition of
the sulfonyl radical on the triple bond gives olefinic radical
intermediate . Intramolecular radical cyclization of with the
azide nitrogen provides N‐centered radical intermediate
upon N2 release. Oxidation of EY•‐ to the ground state EY by
electron transfer to radical intermediate may provide
intermediate , which on proton abstraction from the reaction
mixture led to 3a
B
B
C
C
D
.
In conclusion, we disclosed visible light induced, Eosin Y
photo‐redox catalyzed net redox neutral reaction by direct
oxidation of sulfinate anion through a reductive quenching
cycle for a radical cascade cyclization of 2‐alkynyl‐azidoarenes
to produce 2‐aryl/alkyl‐3‐sulfonyl‐indoles. To our knowledge,
this is the first visible light induced Eosin Y photo‐redox
catalyzed vicinal sulfonamination of alkynes. Mild conditions
such as room temperature, visible light as a traceless energy
source, metal free, oxidant/reductant free, stable and easily
accessible starting materials are further highlights. The
potential of anion oxidation for the construction of a variety
heterocyclic compounds and natural products of biological
importance and details on the mechanistic aspects are in
progress in our laboratory.
Conflicts of interest
There are no conflicts to declare.
Acknowledgments
Scheme 4 Control experiment
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Table of Contents:
• Net redox neutral Reaction
• Anion oxidation
• Reductive quenching cycle
• Oxidant/reductant free
• Photo-redox catalysis (organic dye)
• Mild reaction condition