Visible-Light-Driven Photocatalytic Activation of Inert Sulfur Ylides for 3-Acyl Oxindole Synthesis

Article abstract of DOI:10.1002/chem.201600871

Bicarbonyl-substituted sulfur ylide is a useful, but inert reagent in organic synthesis. Usually, harsh reaction conditions are required for its transformation. For the first time, it was demonstrated that a new, visible-light photoredox catalytic annulation of sulfur ylides under extremely mild conditions, permits the synthesis of oxindole derivatives in high selectivities and efficiencies. The key to its success is the photocatalytic single-electron-transfer (SET) oxidation of the inert amide and acyl-stabilized sulfur ylides to reactive radical cations, which easily proceeds with intramolecular C?H functionalization to give the final products.

Full text of DOI:10.1002/chem.201600871

DOI: 10.1002/chem.201600871
Communication
&
CÀH Functionalization
Visible-Light-Driven Photocatalytic Activation of Inert Sulfur
Ylides for 3-Acyl Oxindole Synthesis
Xu-Dong Xia+,[a] Liang-Qiu Lu+,[a] Wen-Qiang Liu,[b] Dong-Zhen Chen,[a] Yu-Han Zheng,[a]
Li-Zhu Wu,*[b] and Wen-Jing Xiao*[a]
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Abstract: Bicarbonyl-substituted sulfur ylide is a useful,
but inert reagent in organic synthesis. Usually, harsh reac-
tion conditions are required for its transformation. For the
first time, it was demonstrated that a new, visible-light
photoredox catalytic annulation of sulfur ylides under ex-
tremely mild conditions, permits the synthesis of oxindole
derivatives in high selectivities and efficiencies. The key to
its success is the photocatalytic single-electron-transfer
(SET) oxidation of the inert amide and acyl-stabilized
sulfur ylides to reactive radical cations, which easily pro-
ceeds with intramolecular CÀH functionalization to give
the final products.
Since the pioneering work in 1960s,[1] sulfur ylides have been
a kind of privileged reagents in the field of carbocycle and het-
erocycle synthesis.[2] The unique reactivity of this broadly ap-
plied reagent is based on its carbanion structure, which is sta-
bilized by an adjacent sulfonium ion. The additional delocaliza-
tion of the carbanion by electron-withdrawing groups such as
keto, ester, and amide groups makes this type of reagent more
bench-stable, practical, and functionalized (Scheme 1). Howev-
er, with these benefits, the sulfur ylides with double-electron-
deficient functional groups usually exhibit low reactivity. Thus,
notably few successful cyclizations of such sulfur ylides have
been disclosed and harsh conditions (e.g., heating and Lewis
acid catalysis) and highly electron-deficient alkenes are usually
required.[3] In 2012, Maulide and co-workers reported an im-
pressive annulation reaction of dicarbonyl-stabilized sulfur
ylides and alkynes under mild conditions by subtly applying
the strategy of gold catalysis to activate the sulfur ylide accept-
ors (Scheme 1a).[4a,5] Importantly, this strategy is general and
can be widely applied to many other transformations of al-
kenes and allenes with stable sulfur ylides.[4b–e] In contrast to
this strategy, we recently questioned whether such sulfur
ylides themselves could be activated by a single-electron-trans-
fer (SET) oxidation using visible-light photoredox catalysis,
forming radical-cation-type electrophiles for annulation reac-
tions.
Scheme 1. Reactivity of dicarbonyl-stabilized sulfur ylides.
ylides could be photolysed at 254 nm to generate active car-
bene intermediates (Scheme 1b).[6e] These active species easily
react with excess cyclohexene to produce cyclopropanation or
allylation products, with moderate chemoselectivities. Recently,
visible-light-induced photocatalysis has been widely applied in
organic synthesis to construct many kinds of heterocyclic com-
pounds.[7] For example, Yu’s group reported a very nice work
on the synthesis of oxindoles by visible-light photoredox catal-
ysis with the bromosubstituted analogue acting as the
quenching reagent.[8] As part of our research program on het-
erocycle syntheses[9] using sulfur ylides and visible-light photo-
catalysis, we developed a new annulation reaction of amide
and acyl-stabilized sulfur ylides for the first time (Scheme 1c).
This process provides an alternative route to biologically signif-
icant 3-acyl oxindole products[10] under extremely mild reaction
conditions. Notably, the construction of oxindoles has been an
important platform to develop new reaction methodologies,
including visible-light photocatalysis strategy with diazo,[9g]
and N-arylacrylamide compounds[11] as starting materials.
The photochemical reactions of sulfur ylides remain under-
developed because of their poor selectivity and because they
typically require high-energy UV irradiation.[6] For example,
Jenks’ group in 2007 reported that diester-stabilized sulfur
We first examined this visible-light-driven photochemical re-
action with amide- and acetyl-stabilized sulfur ylide 1a as the
model substrate. The reaction was performed in THF at room
temperature with [Ir(dF(CF3)ppy)2(dtbbpy)]PF6 (abbreviation:
[Ir]; E/1/2 = +1.21 versus saturated calomel electrode (SCE) in
CH3CN; dF(CF3)ppy=2-(2,4-difluorophenyl)-5-trifluoromethyl-
pyridine; dtbbpy=4,4’-di-tert-butyl-2,2’-bipyridine)[7f] as the
photocatalyst (PC). The starting material 1a was completely
consumed after 7 h, and the enol-formed 3-acetyl oxindole
product 2a was obtained. Although the product contained
some unconfirmed impurities, this problem was well resolved
by adding 2.0 equivalents of KH2PO4,[12] and the pure product
2a was obtained in 82% yield under a prolonged reaction
time (Table 1, entry 1). The solvent effect was further investi-
gated, but no better result was obtained in other reaction
media. For example, ether solvents such as dioxane and diethyl
[a] X.-D. Xia,+ Dr. L.-Q. Lu,+ D.-Z. Chen, Y.-H. Zheng, Prof. Dr. W.-J. Xiao
Key Laboratory of Pesticide & Chemical Biology
College of Chemistry, Central China Normal University (CCNU)
152 Luoyu Road, Wuhan, Hubei 430079 (P. R. China)
E-mail: wxiao@mail.ccnu.edu.cn
[b] W.-Q. Liu, Prof. Dr. L.-Z. Wu
Key Laboratory of Photochemical Conversion and Optoelectronic Materials
Technical Institute of Physics and Chemistry & University of Chinese
Academy of Sciences, Chinese Academy of Sciences
Beijing 100190 (P. R. China)
E-mail: lzwu@mail.ipc.ac.cn
[+] X.-D. Xia and L.-Q. Lu contributed equally to this work.
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201600871.
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Table 1. Optimization of the photochemical annulation of 1a.[a]
Table 2. Photochemical annulation: Sulfur ylide scope.[a]
ox
Entry
Photocatalyst (E1/2
)
Solvent
Result [%][b]
1
2
3
4
5
6
7
8
[Ir] (+1.21 V)
[Ir] (+1.21 V)
[Ir] (+1.21 V)
[Ir] (+1.21 V)
[Ir] (+1.21 V)
[Ir] (+1.21 V)
[Ir] (+1.21 V)
[Ir] (+1.21 V)
Mes-Acr (+2.06 V)
Eosin Y (+0.83 V)
THF
dioxane
Et2O
CH2Cl2
DMF
CH3CN
iPrOH
EtOH
THF
THF
THF
THF
THF
82
62
44
40
20
24
15
trace
17
NR
NR
NR
NR
NR
0
9
10
11
12
13[c]
14
15[d]
[Ru(bpy)3(PF6)2] (+0.77 V)
fac-[Ir(ppy)3] (+0.31 V)
[Ir] (+1.21 V)
–
THF
THF
[Ir] (+1.21 V)
[a] Conditions: sulfur ylide 1a (0.3 mmol), KH2PO4 (2.0 equiv), PC
(2 mol%), solvent (6.0 mL), Ar atmosphere, 3 W blue-LEDs irradiation, rt,
16 h. [b] Isolated yields. [c] Without visible light. [d] Air atmosphere. NR =
No reaction. DMF = N,N-Dimethylformamide. bpy = 2,2’-Bipyridine. ppy
= 2-phenylpyridine.
[a] Conditions: sulfur ylide 1 (0.3 mmol), KH2PO4 (2.0 equiv), [Ir] (2 mol%),
THF (6.0 mL), Ar atmosphere, 3 W blue-LEDs irradiation, rt, indicated time;
isolated yields. NR = No reaction. PMB = p-Methoxylbenzyl.
ether provided the product in 62 and 44% yield, respectively,
at the level of full conversion (entries 2–3). Solvents such as
CH2Cl2, DMF, and MeCN did not efficiently furnish the transfor-
mation and delivered the desired product 2a in modest to
moderate yields (entries 4–6). Alcohol solvents such as isopro-
panol and ethanol were also studied; however, only 15% yield
or a trace amount of 2a was detected, with most of the start-
ing material intact (entries 7 and 8). We next evaluated numer-
ous other photocatalysts for this transformation. For example,
Mes-Acr, which has a high oxidative potential in its excited
state[13] to catalyse the oxidative transformation of alkenes, sul-
finates, 2H-azirines, and carboxylic acids, only accomplished
this annulation reaction in a low yield (entry 9, 17% yield).
Other photosensitizers such as organic dye Eosin Y[13] and
metal complexes fac-[Ir(ppy)3] and [Ru(bpy)3(PF6)2][7f] did not re-
spond to the optimized reaction conditions. Finally, the control
experiments revealed that visible light and the photocatalyst
were indispensable elements for the present annulation reac-
tion of sulfur ylides (entries 13 and 14). When the photochemi-
cal annulation of 1a was performed under an air atmosphere,
this reaction system was complex and no desired product was
observed (entry 15).
ble-light-induced photocatalytic annulation reaction. As out-
lined in Table 2, the tolerance of substituent groups on the ni-
trogen atom was first investigated. The results show that these
mild reaction conditions tolerate a series of substituents, such
as methyl, phenyl, benzyl, and p-methoxylbenzyl, and produce
the corresponding oxindole products with large variations on
the nitrogen atom (2a–d: 62–64%). The Boc-protected (Boc=
tert-butoxycarbonyl) substrate (1e) did not react under opti-
mized conditions. We then selected N-methyl amide-stabilized
sulfur ylides to further study the electronic and steric effect on
the benzene ring of aniline. Consequently, a wide range of
substituted patterns on the aromatic ring was found to be
compatible with these photoredox catalysis conditions. For ex-
ample, all sulfur ylides with halogen atoms (e.g., Cl and F) and
electron-donating groups (e.g., MeO and Me) on the benzene
ring can be converted into the corresponding products in
moderate to good yields (2 f–i: 52–80% yields). When a methyl
group was substituted on the ortho-position of the nitrogen
atom, the 7-methyl oxindole derivative 2j was isolated in 77%
yield; when two substituents were introduced (e.g., Me and
CF3) at the meta-positions, ketone-formed oxindole derivatives
2k and 2l were obtained in 92 and 46% yield, respectively.
We further evaluated the diversity of the acyl group on
sulfur ylides (R3). For example, sulfur ylides with aryl- and het-
After having determined the optimal conditions, we evaluat-
ed the scope of sulfur ylides that could participate in this visi-
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eroaryl-substituted acyls were also useful and were trans-
formed into oxindole products in acceptable yields (2m: R3 =
phenyl, 57% yield; 2n: R3 =2-furanyl, 56% yield). In the case
of aliphatic substituents, this transformation can also convert
the sulfur ylides into the desired products in satisfactory yields
(e.g., 2o: R3 =benzyl, 72% yield; 2p: R3 =cyclopropyl, 67%
yield). In addition to the acyl moieties on sulfur ylides, other
electron-withdrawing groups were demonstrated to be suc-
cessful for this visible-light-induced photochemical annulation.
For example, when the sulfur ylides with an ester group (1q)
or a benzenesulfonyl group (1r) were subjected to the stan-
dard conditions, 3-ester oxindole 2q and 3-sulfonyl oxindole
2r were produced in 70 and 48% yield, respectively [Eqs. (1)
and (2)].
After the generation of intermediate B, two possible pathways
are proposed. In path a, an SET reduction of B by the strong
reducing reagent [Ir]À delivers a transient radical intermediate
C and simultaneously closes the photoredox catalytic cycle.[15]
Intermediate C is then converted into ketone-formed 3-acetyl
oxindole 2a’ through a 1,2-hydrogen shift/aromatization pro-
cess. Finally 2a’ spontaneously transforms into the more
stable, enol-formed product 2a through a tautomerization pro-
cess. In path b, KH2PO4 promotes the elimination process to
furnish the intermediate D with its resonance form E. They
could then be reduced by the photocatalyst and reprotonated
to give the final compound 2a.
To support this proposed redox-neutral photocatalytic pro-
cess, we first characterized substrate 1a in CH3CN using CV,
which revealed the oxidation potential of 1a to be +0.99 V
versus SCE.[16] Hence, the SET oxidation of sulfur ylide 1a by
the excited state of [Ir]* (+1.21 V vs. SCE) should be thermody-
namically favourable. We then carried out the fluorescence
quenching experiment with 1a, and substrate 1a can unam-
biguously quench the excited [Ir]*.[16] Moreover, the quantum
yields (F) of the process were measured at four stages of con-
version,[17] which were 2, 4, 6, and 8 min, respectively and the
corresponding quantum yields were 28, 29, 28, and 30%.[16]
Quantum yields of these magnitudes (F<1) support the feasi-
bility of the proposed mechanism, although it cannot com-
We then selected 3-acetyl oxindole 2a as a versatile
platform for further synthetic manipulations. For in-
stance, as highlighted in Scheme 2, 20 mol% of chiral
amine catalyst 3, 40 mol% of trifluoroacetic acid, and
2.0 equivalents of benzalacetone 4 were added to
the reaction mixture after full conversion of the start-
ing material 1a and replacement of the solvents. As
a result, this two-step, one-pot operation rapidly pro-
duced the chiral spirocyclic oxindole 5 in good yield
with excellent enantio- and diastereocontrol (63%
yield, 94% ee, >20:1 d.r.).[14] This sequence of photo-
Scheme 2. Synthetic manipulation: asymmetric synthesis of chiral spirocyclic oxindole 5.
catalytic annulation and organocatalytic asymmetric
Michael addition/aldol condensation undoubtedly
demonstrates the synthetic utility of the present visi-
ble-light-driven photochemical reaction of sulfur
ylides.
To better understand this photochemical annula-
tion of sulfur ylides, we propose a possible reaction
mechanism as illustrated in Scheme 3. The photoca-
talyst [Ir] is well known to readily absorb a photon
under blue-LEDs irradiation to generate the excited
photocatalyst [Ir]*, which has a strong oxidation po-
tential.[7f] Sulfur ylide 1a, which is a special carbanion,
is assumed to be oxidized by the excited state of the
photocatalyst [Ir]* to form the sulfonium-ion-connect-
ed radical species A (SET oxidation), which can be sta-
bilized by the adjacent amide and acetyl groups. Pre-
sumably, species A is highly electrophonic and readily
undergoes an intramolecular radical addition to the
benzene ring to forge a new chemical bond, which
concomitantly generates the radical intermediate B. Scheme 3. Possible reaction mechanism.
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pletely rule out the chain propagation process. In addition, the
reaction was completely shut down by the addition of 2,2,6,6-
tetramethylpiperidine N-oxide (TEMPO), which suggests that
radicals were involved in this photochemical annulation reac-
tion of sulfur ylides. On the other hand, in order to rule out
the generation of the carbene intermediate through an
energy-transfer pathway,[9g] we added 2.0 equivalents of 1,4-
diazobicyclo[2.2.2]octane (DABCO), an electron-donating
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intact in the alcohol solvents (Table 1, entries 7–8). Therefore,
we believed that the carbene intermediate was less likely in-
volved in this reaction based on the current experimental evi-
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In conclusion, we have developed a new, visible-light photo-
redox catalytic annulation reaction of sulfur ylides. This photo-
chemical process enables the synthesis of oxindole derivatives
in high selectivity and efficiency under extremely mild reaction
conditions. Its success relies on the activation of inert dicar-
bonyl-stabilized sulfur ylides by photocatalytic SET oxidation,
which generates reactive radical species that easily induce the
intramolecular CÀH functionalizations. We believe that the
strategy of visible-light photoredox catalysis will pave the way
to exploit a new annulation chemistry of sulfur ylides.
Experimental Section
General
To a 10 mL Schlenk tube equipped with a magnetic stirrer bar was
added 1 (0.3 mmol, 1.0 equiv), KH2PO4 (0.6 mmol, 2.0 equiv), [Ir]
(2 mol%), and THF (6 mL). The resulting mixture was degassed
through a ‘freeze-pump-thaw’ procedure three times, and then the
solution was stirred at room temperature under irradiation of 3 W
blue LEDs. Upon the completion of the cycloaddition, as moni-
tored by TLC analysis, the isolated yield was determined by flash
chromatography on silica gel (petroleum ether/EtOAc=4:1).
Acknowledgements
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[12] Many different carbonates, phosphates, and organic acids were
screened, and finally KH2PO4 was identified as the best choice. Maybe,
it could better convert ketone-formed or other enol-formed oxindoles
into 2a; see references in ref. [9].
We are grateful to the National Science Foundation of China
(no. 21232003, 21202053, 21272087 and 21372266), FANEDD
(no. 201422) and Central China Normal University
(CCNU15A02007) for supporting this research.
Keywords: CÀH functionalization · oxindoles · photocatalysis ·
sulfur ylides · visible light
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Received: February 24, 2016
Published online on April 26, 2016
[16] Please see the Supporting Information for more details.
Chem. Eur. J. 2016, 22, 8432 – 8437
www.chemeurj.org
8437
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim