Phototandem catalysis: Efficient synthesis of 3-ester-3-hydroxy-2-oxindoles by a visible light-induced cyclization of diazoamides through an aerobic oxidation sequence

Article abstract of DOI:10.1002/asia.201402990

An unprecedented phototandem catalysis based on a single iridium photocatalyst has been successfully developed. This powerful strategy consists of two mechanistically distinct catalytic cycles, namely, photocatalytic energy transfer (ET) and single electron transfer (SET). The novel protocol allows a rapid and efficient construction of biologically and synthetically important 3-ester-3-hydroxy-2-oxindole derivatives from readily available diazoamides through a cyclization/aerobic oxidation sequence under very mild conditions.

Full text of DOI:10.1002/asia.201402990

DOI: 10.1002/asia.201402990
Communication
Photocatalysis
Phototandem Catalysis: Efficient Synthesis of 3-Ester-3-hydroxy-2-
oxindoles by a Visible Light-Induced Cyclization of Diazoamides
through an Aerobic Oxidation Sequence
Xu-Dong Xia,[a] Yan-Liang Ren,[a] Jia-Rong Chen,[a, b] Xing-Long Yu,[a] Liang-Qiu Lu,[a, b] You-
Quan Zou,[a] Jian Wan,*[a] and Wen-Jing Xiao*[a, b]
with respect to different substrates and intermediates are criti-
Abstract: An unprecedented phototandem catalysis
based on a single iridium photocatalyst has been success-
fully developed. This powerful strategy consists of two
mechanistically distinct catalytic cycles, namely, photocata-
lytic energy transfer (ET) and single electron transfer (SET).
The novel protocol allows a rapid and efficient construc-
tion of biologically and synthetically important 3-ester-3-
hydroxy-2-oxindole derivatives from readily available diaz-
oamides through a cyclization/aerobic oxidation sequence
under very mild conditions.
cal to devising new efficient tandem catalytic reactions.
Over the past years, visible-light-induced homogeneous
photocatalysis has been reinvestigated probably owing to its
unique catalytic modes and operational simplicity.[4] In this
context, the photocatalytic process with a single electron-
transfer (SET) mechanism has been well demonstrated in
a wide range of organic transformations.[5] Significantly, syner-
gistic catalysis,[6] merging photocatalysis with other catalytic
processes, has been exploited for the development of uncon-
ventional reactions with high reactivity and selectivity. In 2008,
the MacMillan group pioneered the combination of photoca-
talysis with enamine catalysis to realize a highly enantioselec-
tive a-alkylation of aldehydes under fluorescent light irradia-
tion,[7] wherein the photocatalyst [Ru(bpy)3Cl2] and organocata-
lyst imidazolidinone activated a-bromocarbonyl compounds
and aldehydes simultaneously (Scheme 1a). This strategy has
In analogy to natural multi-enzymatic systems that catalyze se-
quential reactions, tandem reactions have been established as
one of the most efficient synthetic methods for the rapid con-
struction of molecular and skeletal complexity from simple
starting materials in a single flask.[1] In particular, the strategy
of “tandem catalysis” has received increasing attention from
synthetic chemists, wherein a single catalyst system can work
in a successive manner to create new chemical bonds without
further addition of any triggering reagent.[2] Although transi-
tion metal- and organocatalyst-mediated tandem catalyses
have successfully afforded a vast variety of tandem, cascade or
domino chemical reactions, identifying or engineering novel
tandem catalytic transformations still remains a challenging
task.[1,3] In this regard, the judicious selection of suitable cata-
lysts and rational harness of their multiple catalytic activities
[a] X.-D. Xia,+ Y.-L. Ren,+ Prof. Dr. J.-R. Chen, X.-L. Yu, Dr. L.-Q. Lu, Y.-Q. Zou,
Prof. Dr. J. Wan, Prof. Dr. W.-J. Xiao
Key Laboratory of Pesticide & Chemical Biology
Ministry of Education
College of Chemistry, Central China Normal University
152 Luoyu Road, Wuhan, Hubei 430079 (China)
Fax: (+86)27-6786-2041
E-mail: wxiao@mail.ccnu.edu.cn
Homepage: http://chem-xiao.ccnu.edu.cn
[b] Prof. Dr. J.-R. Chen, Dr. L.-Q. Lu, Prof. Dr. W.-J. Xiao
Synergetic Innovation Centre of Chemical Science and Engineering
Tianjin 300072 (China)
[+] These authors contributed equally to this work.
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/asia.201402990.
Scheme 1. The concept of visible-light-induced phototandem catalysis.
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paved the way for the development of new reactions, which
are otherwise not easily accessible by either catalyst independ-
ently.[8] In addition, the Sanford group nicely demonstrated
a high-yielding aromatic CÀH arylation reaction which involves
a Pd-catalyzed CÀH activation and a visible-light-induced pho-
toredox catalysis (Scheme 1b).[9] Shortly thereafter, the Rovis
group disclosed an efficient catalytic asymmetric a-acylation of
tetrahydroisoquinolines with aldehydes, in which the photoca-
talyst [Ru(bpy)3Cl2] and chiral N-heterocyclic carbene catalyst
have been elegantly employed (Scheme 1c).[10] Furthermore,
MacMillan and co-workers have developed a photocatalytic a-
amino CÀH arylation (Scheme 1d),[11] in which the single pho-
tocatalyst Ir(ppy)3 works in a bifunctional catalytic mode;
therefore no extra co-catalyst, reductive or oxidative reagent is
required. Quite recently, by combining two SET processes, the
Rueping group successfully developed a photocatalytic Mi-
chael addition/aromatization/CÀC bond cleavage cascade reac-
tion, which enabled the formation of indole-3-carboxaldehyde
derivatives in high yields.[5h] Shortly after, the Cho group re-
ported a hydrotrifluoromethylation of alkynes through a photo-
catalytic iodotrifluoromethylation and deiodination sequen-
ce.[5i]
In addition to the SET process, the Yoon group[12c] and our
laboratory[12d] have independently exploited an energy transfer
(ET) process[12] for visible-light-photocatalytic [2+2] cycloaddi-
tions. As part of our ongoing projects on photocatalysis[13] and
tandem catalysis,[14] we wondered whether we could develop
a phototandem catalysis that combines two mechanistically
distinct processes (e.g. ET and SET). It is well known that some
azides can undergo a synthetic transformation through an ET
pathway in the presence of photosensitizers under irradiation
of visible light.[12e,13f] We assumed that structurally similar diaz-
oamides might be activated through an ET process as well.
Based on this hypothesis, we have recently developed a visi-
ble-light-induced phototandem catalysis that relies on a single
iridium photocatalyst. This unprecedented process allows an
efficient transformation of diazoamides[15] into biologically and
synthetically important 3-ester-3-hydroxy-2-oxindole deriva-
Table 1. Optimization of reaction conditions for the first cyclization
step.[a]
Entry
Solvent
Catalyst
t [h] Yield [%][b]
1
2
3
4
5
6
7
8
CH2Cl2
EtOH
MeCN
DMF
CHCl3
DCE
(CF3)2CHOH
CF3CH2OH
CF3CH2OH
CF3CH2OH
CF3CH2OH
CF3CH2OH
CF3CH2OH
CF3CH2OH
CF3CH2OH
CF3CH2OH
[Ir(ppy)2(dtbbpy)]PF6
[Ir(ppy)2(dtbbpy)]PF6
[Ir(ppy)2(dtbbpy)]PF6
[Ir(ppy)2(dtbbpy)]PF6
[Ir(ppy)2(dtbbpy)]PF6
[Ir(ppy)2(dtbbpy)]PF6
[Ir(ppy)2(dtbbpy)]PF6
[Ir(ppy)2(dtbbpy)]PF6
–
48
48
48
48
48
48
48
48
48
48
48
48
6
72
<5
<5
<5
73
71
84
90
20
<5
16
33
94
43
57
27
9
10[c]
11[d]
12[e]
13
14
15
16
–
[Ir(ppy)2(dtbbpy)]PF6
[Ir(ppy)2(dtbbpy)]PF6
[Ir(dF(CF3)ppy)2(dtbbpy)]PF6
.
Ru(bpy)3Cl2 6H2O
48
48
48
Eosin Y
Ir(ppy)3
[a] Unless otherwise noted, the reaction was carried out with 1a
(0.2 mmol), photocatalyst (2 mol%), solvent (4.0 mL), 18W CFL, at room
temperature. [b] Determined by 1H NMR analysis using 1,3,5-trimethoxy-
benzene as an internal standard. [c] With 3W red LED strip as light
source. [d] Without a light source. [e] Without degasings. ppy=2-phenyl-
pyridine; dtbbpy=4,4’-di-tert-butyl-2,2’-bipyridine; dF(CF3)ppy=2-(2,4-di-
fluorophenyl)-5-(trifluoromethyl)pyridine; bpy=2,2’-bipyridine; DMF=
N,N-dimethylformamide; DCE=1,2-dichloroethane. CFL=compact fluo-
rescent lamp. LED=light-emitting diode.
sence of photocatalyst. It was found that no cyclized product
2a was formed even after 48 h (Table 1, entry 10). In addition,
the reaction did not occur in the presence of photocatalyst
without irradiation of visible light (Table 1, entry 11). These re-
sults showed that both visible light and photocatalyst were
critical to the cyclization. Degassing was necessary, as exposure
of the reaction to the air only gave rise to 2a in 33% yield
(Table 1, entry 12). Catalyst screening disclosed that [Ir(dF-
(CF3)ppy)2(dtbbpy)]PF6 was optimal in terms of the reaction ef-
ficiency (Table 1, entry 13 vs entries 8, 14–16).[17]
tives through
(Scheme 1e).[16]
a
cyclization/aerobic oxidation cascade
We initially chose diazoamides 1a as the model substrate to
examine the feasibility of the first cyclization step. To our de-
light, irradiation of 1a with 18W white LED afforded 3-ester-2-
oxindole (2a) in 72% yield in the presence of [Ir(ppy)2-
(dtbbpy)]PF6 in dichloromethane at room temperature. A brief
screen of reaction media identified CF3CH2OH to be the best
solvent of choice, and the cyclization product 2a was obtained
in 90% yield (Table 1, entry 8). Interestingly, a 20% yield (as de-
termined by NMR spectroscopy) of 2a was observed in the ab-
sence of photocatalyst after 48 h, when a solution of 1a was
subjected to 18W compact fluorescent light source (Table 1,
entry 9). UV/Vis spectra of 1a indicated that 1a could absorb
UV light (lmax =242 and 266 nm). As we noted some adsorp-
tion in the UV range under illumination with 18W compact flu-
orescent light, the reaction might proceed through a traditional
UV light-mediated cyclization. Thus, we have performed the
control experiment with a 3W red LED (l=617 nm) in the ab-
With the optimal reaction conditions in hand for the first
cyclization, we turned our attention to optimize the reaction
conditions for the tandem cyclization/oxidation sequence with
oxygen as a terminal oxidant in a one-pot fashion (Table 2). Ini-
tially, upon complete consumption of 1a, the reaction mixture
was exposed to air to initiate further oxidation (Table 2,
entry 1). Unfortunately, no hydroxylated product was detected
after 12 h. Inspired by the recent base-mediated a-hydroxyl-
ation of oxindoles,[18] we briefly examined several inorganic
and organic bases and it was found that the addition of base
was indeed essential to the photocatalytic oxidation step
(Table 2, entries 2–6). NaHCO3 was determined to be the best
choice, affording the desired product 3a in 68% overall yield
(Table 2, entry 2).[19] Notably, further optimization revealed that
this tandem reaction with only 1 mol% catalyst loading and
0.25 equivalents of NaHCO3 could proceed smoothly to deliver
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Table 2. Optimization of reaction conditions of the second step.[a]
Table 3. Substrate scope.[a]
Entry
Base
Equiv.
t [h]
Yield [%][b]
1
2
3
4
5
6
7[c]
8[c]
9[c]
10[d]
11[e]
–
–
6+12
6+4
6+4
6+4
6+4
6+4
6+4
6+4
6+4
6+4
4
0
68
66
57
48
40
70
70
NaHCO3
KOH
K2CO3
DABCO
DBU
NaHCO3
NaHCO3
NaHCO3
NaHCO3
NaHCO3
2.0
2.0
2.0
2.0
2.0
2.0
1.0
0.25
2.0
2.0
69
trace
trace
[a] Unless otherwise noted, the reaction conditions are as follows: 1a
(0.2 mmol), [Ir] (2 mol%), CF3CH2OH (4.0 mL), 18W CFL, at room tempera-
ture. After the consumption of substrate 1a, base was added and the re-
action was exposed to the air, and stirred under 18W CFL. [b] Determined
by 1H NMR analysis using 1,3,5-trimethoxybenzene as an internal stan-
dard. [c] 1 mol% of photocatalyst was used. [d] No visible light source
was used for the second step. [e] Using 2a (0.2 mmol) directly for the
second step in the absence of photocatalyst. [Ir]=[Ir(dF(CF3)ppy)2-
(dtbbpy)]PF6. CFL=compact fluorescent lamp. DABCO=1,4-diazabicyclo-
[2.2.2]octane.
[a] Unless otherwise noted, the reaction conditions are as follows:
1 (0.3 mmol), [Ir] (1 mol%), CF3CH2OH (6.0 mL), 18W CFL, at room temper-
ature. After the consumption of 1a, NaHCO3 (0.075 mmol) was added, and
the mixture was exposed to the air, and stirred under 18W CFL. [b] Yield
of isolated product. [c] 9.0 mL of CF3CH2OH was used. [Ir]=Ir(dF(CF3)ppy)2-
(dtbbpy)PF6, PMB=p-methoxybenzyl.
the corresponding product in 69% yield (Table 2, entry 9). Note
that both photocatalyst and visible light were essential to the
aerobic oxidation step, because very low yields of the product
were obtained in the absence of either light or the photocata-
lyst (Table 2, entries 10 and 11).
To verify the effectiveness of the newly developed visible-
light-induced tandem catalytic system, we evaluated a variety
of N-protected diazoamides 1 under the optimized conditions,
and representative results are highlighted in Table 3. Replace-
ment of the methyl group on nitrogen with PMB
instability of the initially formed hydroxylation product
[Eq. (1)], and the benzenesulfonyl group acted as the leaving
group in the presence of KOH.
and Bn did not change the reaction efficiency and in
both cases, the corresponding prouducts 3b and 3c
were isolated in 62% and 72% overall yields, respec-
tively. In the case of diazoamide 1d with a phenyl
group on the nitrogen, the reaction only afforded
the product 3d in 38% yield. Importantly, various
electron-donating (e.g. MeO, Me) and electron-with-
drawing (e.g. F, Br, CN, CO2Et, CF3) groups could be well toler-
ated in the aromatic ring of N-benzyl diazoamides, and prod-
ucts 3e–3l were obtained in a range of 48–86% yields. Nota-
bly, the reaction with N-naphthyl diazoamide also participated
in the reaction to give rise to product 3m in 30% yield. More-
over, the variation of the ester moiety in diazoamide could be
successfully accomplished. For example, switching ethyl group
to benzyl, tert-butyl, and allyl substituents did not obviously
affect the tandem process, and the reaction provided the de-
sired products 3n–3p in 58–61% yields.
To gain some insights into the reaction mechanism, we have
measured the cyclic voltammetry (CV) data of substrate 1a in
CH3CN (À1.51 V vs SCE).[20] The results showed that diazoamide
1a was not sufficiently electron-deficient to be readily reduced
by iridium photocatalyst, as the reduction potentials of [Ir]*
and 1a are À0.89 and À1.51 V vs. SCE, respectively.[20] More-
over, theoretical calculations were performed and the results
indicated that the energy of the excited triplet state of [Ir]*
(ET =61 kcalmolÀ1) was sufficient for sensitizing 1a to its first
electronically excited triplet state (1a, ET =34 kcalmolÀ1).[21]
These facts are well consistent with the hypothesis that the
first step of this tandem process proceeds through a photocata-
lytic energy transfer pathway.
Interestingly, when the ester moiety in diazoamide was re-
placed with benzenesulfonyl group (1r), the reaction delivered
N-benzyl substituted isatin (3r) in 38% yield probably due to
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overall yield [Eq. (4)]. Subse-
quently, we performed a fluores-
cence quenching experiment
with 2a, and the results indicat-
ed that 2a could quench the
excited [Ir]* catalyst.[19] More-
over, the CV data of 2a was de-
termined to be +0.12 V, which
suggested that it could be
easily oxidized by the excited
[Ir]* (+1.21 V) through a reduc-
tive quenching pathway.[19] All
this evidence showed that the
second step of the phototan-
dem catalysis went predomi-
nantly through a SET mecha-
nism.
Taking all the above experi-
Scheme 2. Control experiments.
mental observations into ac-
count, we postulated a possible
pathway for the phototandem
Furthermore, some control experiments were also carried
out with 2a as the model substrate to investigate the possible
mechanism for the aerobic oxidation step (Scheme 2). When
a singlet oxygen-generating system TPP/CH2Cl2 was used, the
reaction proceeded very slowly and only furnished the oxida-
tion product 3a in 35% yield [Eq. (2)].[22] In sharp contrast, the
standard reaction with iridium as photocatalyst resulted in the
formation of 3a in 85% yield [Eq. (3)]. It is well known that
DABCO is usually used to quench the singlet oxygen.[23] The
tandem reaction, however, worked very well even with the ad-
dition of two equivalents of DABCO and affords 3a in 67%
catalytic reaction of diazoamides to 3-ester-3-hydroxy-2-oxin-
doles (Scheme 3). Take 1a as the example, visible light firstly
transferred its energy to the photocatalyst [Ir], and then 1a
obtained energy from the excited [Ir]* to form excited 1a*. A
release of N2 afforded the intermediate A, which underwent an
intramolecular cyclization and H-transfer to generate inter-
mediate 3-ester-2-oxindole 2a. NMR analysis showed that
there was an equilibration between the ketone (2a) and relat-
ed enol (2a’). Upon sensitization of [Ir] to the excited [Ir]*,
a SET from excited state [Ir]* to 2a’ gave rise to the key radical
intermediate C, while the photocatalyst [Ir]* was reduced to
[Ir]À. Subsequently, the [Ir]À was reoxidized to [Ir] by air, and
this resulted in a highly reactive oxygen radical anion coupled
with radical C to afford the hydroperoxide intermediate E
through intermediate D.[18] Reduction of E by another enolate
2a’ then furnished the corresponding product 3a.[25]
In conclusion, we have successfully developed a visible-light-
induced phototandem catalysis strategy that is based on
a single iridium photocatalyst. A wide range of diazoamides
were efficiently transformed into the corresponding biological-
ly and synthetically important 3-ester-3-hydroxy-2-oxindole de-
rivatives through a cyclization/aerobic oxidation cascade under
mild conditions. The combination of two mechanistically dis-
tinct transformations into an operationally simple process
makes this phototandem catalysis particularly useful. Current
research efforts are directed towards further application of this
phototandem catalysis to other new reaction development.
Experimental Section
General Procedure
1 (0.3 mmol, 1.0 equiv), [Ir(dF(CF3)ppy)2(dtbbpy)]PF6 (1 mol%), and
CF3CH2OH (for 1a–1e 6.0 mL; for 1e–1p 9.0 mL) were added to
a 25 mL Schlenk tube equipped with a magnetic stirrer bar. The re-
sulting mixture was degassed through a “freeze-pump-thaw” pro-
Scheme 3. Proposed mechanism for the phototandem catalysis.
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[10] D. A. DiRocco, T. Rovis, J. Am. Chem. Soc. 2012, 134, 8094–8097.
cedure three times, and then the solution was stirred at room tem-
perature under irradiation by 18W CFL at a distance of approxi-
mately 5 cm. Upon the completion of the cyclization reaction, as
monitored by TLC, NaHCO3 (0.075 mmol, 0.25 equiv) was added to
the reaction mixture, and the system was exposed to the air under
irradiation of 18 W white LEDs at a distance of approximately 5 cm.
After the reaction was completed, the product was subjected to
flash chromatography on silica gel (silica: 200–300; eluent: petrole-
um ether/ethyl acetate=5:1) to provide 3-ester-3-hydroxy-2-oxin-
dole 3.
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Acknowledgements
We are grateful to the National Science Foundation of China
(NO. 21232003, 21202053, and 21272087) and the National
Basic Research Program of China (2011CB808603) for support
of this research.
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Keywords: cycloaddition
photocatalysis · visible light
·
diazoamides
·
iridium
·
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[19] Please see the Supporting Information for more details.
[20] CV measurements of 1a, 2a, and 2a’ were carried out in the presence
of 2.0 equiv of NaHCO3 in 0.1m of Bu4NPF6/MeCN at a scan rate of
100 mVsÀ1. The working electrode is a glassy carbon, the counter elec-
trode is a Pt wire, and the reference electrode is SCE.
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Received: August 25, 2014
Published online on && &&, 0000
Chem. Asian J. 2014, 9, 1 – 6
www.chemasianj.org
5
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
&
&
These are not the final page numbers! ÞÞ

Communication
COMMUNICATION
&
Photocatalysis
X.-D. Xia, Y.-L. Ren, J.-R. Chen, X.-L. Yu,
L.-Q. Lu, Y.-Q. Zou, J. Wan,* W.-J. Xiao*
&& – &&
Phototandem Catalysis: Efficient
Synthesis of 3-Ester-3-hydroxy-2-
oxindoles by a Visible Light-Induced
Cyclization of Diazoamides through an
Aerobic Oxidation Sequence
ET, SET, match: A phototandem cataly-
sis based on a single iridium photocata-
lyst has been successfully developed.
This powerful strategy consists of two
mechanistically distinct catalytic cycles,
namely, photocatalytic energy transfer
(ET) and single electron transfer (SET).
The protocol allows a rapid and efficient
construction of biologically and synthet-
ically important 3-ester-3-hydroxy-2-ox-
indole derivatives under very mild con-
ditions.
&
&
Chem. Asian J. 2014, 9, 1 – 6
www.chemasianj.org
6
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ÝÝ These are not the final page numbers!