Photocatalytic aerobic oxidation/semipinacol rearrangement sequence: A concise route to the core of pseudoindoxyl alkaloids

Article abstract of DOI:10.1016/j.tetlet.2014.06.102

A visible light-induced photocatalytic aerobic oxidation/semipinacol rearrangement has been successfully developed. This methodology allows for efficient conversion of a wide range of indoles into the synthetically significant 2,2-disubstituted indolin-3-ones, the core structure of pseudoindoxyl alkaloids, under mild reaction conditions.

Full text of DOI:10.1016/j.tetlet.2014.06.102

Tetrahedron Letters xxx (2014) xxx–xxx
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Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Photocatalytic aerobic oxidation/semipinacol rearrangement
sequence: a concise route to the core of pseudoindoxyl alkaloids
a,b,
Wei Ding a, Quan-Quan Zhou a, Jun Xuan a, Tian-Ren Li a, Liang-Qiu Lu a, , Wen-Jing Xiao
⇑
⇑
a Key Laboratory of Pesticide & Chemical Biology, Ministry of Education, College of Chemistry, Central China Normal University, 152 Luoyu Road, Wuhan, Hubei 430079, China
b Collaborative Innovation Center of Chemical Science and Engineering, Tianjin 300000, China
a r t i c l e i n f o
a b s t r a c t
Article history:
A visible light-induced photocatalytic aerobic oxidation/semipinacol rearrangement has been success-
fully developed. This methodology allows for efficient conversion of a wide range of indoles into the syn-
thetically significant 2,2-disubstituted indolin-3-ones, the core structure of pseudoindoxyl alkaloids,
under mild reaction conditions.
Received 23 May 2014
Revised 20 June 2014
Accepted 26 June 2014
Available online xxxx
Ó 2014 Published by Elsevier Ltd.
Keywords:
Aerobic oxidation
2,2-Disubstituted indolin-3-one
Semipinacol rearrangement
Photocatalysis
Pseudoindoxyl alkaloids featuring the 2,2-disubstituted indo-
lin-3-one core structure constitute an important family of indole
alkaloids.1 For example, (+)-Aristotelone, Brevianamide A, and
Duocarmycin C1/B1 are some of the representative natural isolates
with diverse biological activities (Fig. 1).1b–d Not surprisingly, a
great deal of efforts has been devoted to the construction of these
core structures.2 In this context, the protocol of oxidation of
indoles to indol-3-ols followed with a semipinacol rearrangement3
represents one of the most powerful routes to 2,2-disubstituted
indolin-3-ones (Scheme 1, A).2b,d,g However, this methodology
suffered from some limitations, including stepwise operation,
involvement of strong or not atom-economic oxidants (i.e. Oxone,
HNO2, H2O2, et al.)2b,g,m for the oxidative step and the requirement
of acidic, basic, or heating conditions2a,c,e,f for the following rear-
rangement process. Thus, the search for more straightforward
methods with the use of ‘green oxidant’ to directly convert 2,3-
disubstituted indoles into the corresponding 2,2-disubstituted
indolin-3-ones still remains highly desirable.
X
O
O
H
O
N
Me
ArOC
O
H
N
N
N
CO2Me
N
N
H
H
N
H
O
OH
Duocarmycin C1, B1
(+)_Aristotelone
Brevianamide A
Figure 1. Representative examples of pseudoindoxyl alkaloids.
reactions, molecular oxygen is used not only as an oxidizing agent
to accomplish the photoredox catalytic cycle but also as a terminal
oxygen source introduced into products.6 For example, in 2003, Zen
and co-workers disclosed a highly selective oxidation reaction of
sulfide to sulfoxide using O2 under the visible light-induced photo-
redox reaction conditions.6a In 2011, the Jiao group has realized an
efficient oxidative transformation of aryl halides to aryl carbonyl
compounds using air as the oxidant, by merging photocatalysis
and organocatalysis.6b Shortly after, Xia and co-workers described
a visible light promoted aerobic oxidative CAC bond cleavage of
aldehydes to prepare a series of ketones with good yields.6e
Recently, our group developed an aerobic oxidation/[3+2] cyclo-
addition/oxidative aromatization sequence via visible light photo-
catalysis.7 As a thematic extension, we proposed that this visible
light induced oxidation could be served as an important platform
for the construction of pseudoindoxyl core via photocatalytic aer-
obic oxidation/semipinacol rearrangement sequence (Scheme 1,
B). Herein, we disclose the successful implementation of this idea,
Over the past decade, molecular oxygen has been widely
utilized as an ideal terminal oxidant for oxidation reactions
because of its environmentally friendly and low-cost advantages.4
Recently, photocatalytic aerobic oxidation under mild reaction
condition has received increasing attention as a result of its
inherent features of green chemistry and sustainability.5 In these
⇑
Corresponding authors. Tel./fax: +86 2767862041.
E-mail addresses: luliangqiu@mail.ccnu.edu.cn (L.-Q. Lu), wxiao@mail.ccnu.edu.
cn (W.-J. Xiao).
http://dx.doi.org/10.1016/j.tetlet.2014.06.102
0040-4039/Ó 2014 Published by Elsevier Ltd.
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2
W. Ding et al. / Tetrahedron Letters xxx (2014) xxx–xxx
(A). Previsous work: stepwise route
With the optimal reaction conditions in hand, we then investi-
Oxone®, H2O2,
HNO2, et al.
step 1
OH
HCO2H, KOH or
gated the substrate scope of this sequential photocatalytic aerobic
oxidation and semipinacol rearrangement. As illustrated in Table 2,
a variety of structurally diverse 2-phenyl-3-benzyl indoles were
able to undergo this reaction smoothly. As for the effect of elec-
tronic properties of the substituents on the phenyl ring, the elec-
tro-rich substrates (1b and 1c) showed slightly higher reactivity
than electro-deficient one (1d). Importantly, the phenol group
(1e), which can be easily oxidized to quinone, was also well toler-
ated under the present oxidative condition, affording the desired
product in a moderate yield (2e: 45% yield). In addition, the naph-
thyl (1f) and heteroaryl (1g) substituted substrates participated in
the reaction well to give the corresponding products 2f and 2g in
47% and 51% yield, respectively. Notably, several indoles with alkyl
groups at 3-position, such as cyclohexyl (1h), iso-butyl (1i), and
allyl (1j), were also easily transformed into the corresponding
products 2h–2j in moderate to good yields.
R1
R2
heating conditions
step 2
N
R1
O
Semipinacol
R1
R2
R2
Rearrangement
N
H
N
O
H
H
R1
R2
O2
acid/base-free
& room temp.
N
(B) Our work: one-step route
• aerobic oxidation/semipinacol rearrangement sequence
(step-, atom- and redox-economy)
• visible light as driving force (photocatalysis)
• mild reacton conditions and green oxidizing agent (
O
)
2
Scheme 1. Construction of 2,2-disubstituted indolin-3-ones via semipinacol
rearrangement.
In addition to the phenyl group, variation of aryl substituents on
the 2-position of indole substrates were also successfully achieved.
As highlighted in Table 3, all these substrates can be transformed
into the corresponding products (2k–n) with moderate to excellent
yields. For example, highest isolated yield (94%) was obtained in
the case of substrate 1l with a methoxyl group on phenyl ring,
probably due to the higher susceptivity to the single electron oxi-
dation by the excited photocatalyst (please see the mechanism
analysis in Scheme 2). Moreover, we found that indole 1o with a
thienyl at 2-position was well compatible with the reaction condi-
tions and furnished the desired product 2o in 78% yield.
Finally, we turned our attention to the diversity of the benzene
ring of indole. As shown in Table 4, variation of the substation pat-
tern has no deleterious effect on the reaction efficiency (2p, 2q:
76%). Besides, various indoles bearing electron-rich (MeO) and -
deficient (F, Br) substituents at 5-position can react well to furnish
the desired products 2r–2t in moderate to good yields (52–84%).
The structure of representative product 2t was further confirmed
by X-ray crystallographic analysis.10
and document a new step-, atom- and redox-economic8 methodol-
ogy, that can be used for the preparation of a wide range of
2,2-disubstituted indolin-3-ones.
Initially, the feasibility of the proposed reaction was examined
by using 3-benzyl-2-phenyl indole 1a as the model substrate in
the presence of 5 mol % of Ir(ppy)2(dtb-bpy)PF6 under 3W white
LED irradiation. To our delight, the reaction indeed proceeded
smoothly in CHCl3 and afforded the desired product 2a in 64% yield
(Table 1, entry 1). Encouraged by this result, we continued to opti-
mize the condition by varying reaction parameters. It was found
that solvents had a dramatic impact on the reaction, and CHCl3
was identified as the best solvent of choice (entries 1–6). Subse-
quently, a series of iridium, ruthenium, and dye photocatalysts
were screened to further improve the reaction efficiency (entries
7–12). It turned out that the yield could be improved to 69% when
we applied Ru(bpy)3Cl2ꢀ6H2O as a photocatalyst, albeit with a
longer reaction time (entry 9). Notably, when the catalyst loading
was decreased to 3 mol %, the reaction could give the best result
(entry 13, 74% yield). Significantly, the direct application of air as
oxygen source also proved to be suitable for the reaction, while a
slight decrease in yield (entry 14, 56% yield) was observed. Finally,
control experiments confirmed that photocatalyst, visible light,
and oxygen were all critical for this process.9
Importantly, a gram scale reaction with 1a (4 mmol, 1.13 g) has
been carried out to prove the practicability of this sequential pro-
cess, providing the desired product 2a in 71% yield without affect-
ing the efficiency (Eq. (1)). More significantly, this sequential
photocatalytic aerobic oxidation/semipinacol rearrangement
Table 1
Optimization of reaction conditionsa
O
Bn
Photocatalyst (5 mol%)
Solvent, O2, 30 oC, 3W White LED
Bn
Ph
Ph
N
H
N
H
1a
2a
Entry
Photocatalyst
Solvent
Time (h)
Conversionb (%)
Yieldb (%)
1
2
3
4
5
6
7
8
9
10
11
12
13c
14c,e
Ir(ppy)2(dtb-bbpy)PF6
Ir(ppy)2(dtb-bbpy)PF6
Ir(ppy)2(dtb-bbpy)PF6
Ir(ppy)2(dtb-bbpy)PF6
Ir(ppy)2(dtb-bbpy)PF6
Ir(ppy)2(dtb-bbpy)PF6
Ir(ppy)2(bpy)PF6
Ir(ppy)3
CHCl3
CH2Cl2
DCE
CH3CN
DMF
DMSO
CHCl3
CHCl3
CHCl3
CHCl3
CHCl3
CHCl3
CHCl3
CHCl3
48
40
48
25
25
16
48
48
72
96
48
48
72
72
96
95
56
97
90
>99
97
97
90
80
93
99
95
81
64
29
32
20
17
18
60
33
69
Ru(bpy)3Cl2ꢀ6H2O
Ru(bpy)3(PF6)2
56
44
61
Ru(bpm)3(BArF)2
Eosin Y
Ru(bpy)3Cl2ꢀ6H2O
74(76)d
56
Ru(bpy)3Cl2ꢀ6H2O
a
Unless otherwise noted, the reaction was carried out with 1a (0.3 mmol), photocatalyst (5 mol %) in the solvent (3.0 mL) at 30 °C under 3W white LED irradiation.
Determined by GC.
b
c Using 3 mol % of photocatalyst.
d Isolated yield in parentheses.
e Performed under an air atmosphere.
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W. Ding et al. / Tetrahedron Letters xxx (2014) xxx–xxx
3
Table 2
Variation of the substituents on the 3-position of indolesa
R1
O
Ru(bpy)3Cl2.6H2O (3 mol%)
R1
Ph
CHCl3, O2, 30 oC, 3W White LED
Ph
N
N
H
H
1a-j
2a-j
O
O
O
Me
OMe
Ph
Ph
Ph
N
N
N
H
H
H
2a
2b
2c
, 72 h, 76%
, 72 h, 77%
, 72 h, 65%
O
O
O
Br
OH
O
Ph
Ph
N
N
H
2e
Ph
N
H
H
b
2d
, 72 h, 54%
, 96 h, 45%
2f
, 72 h, 50%
O
O
O
S
Ph
Ph
Ph
N
H
N
H
N
H
Ph
, 96 h, 51%
N
H
2g, 72 h, 47%b
2h
2i
, 120 h, 60%, 1.6:1 dr
2j
, 72 h, 25%
a
Unless otherwise noted, the reaction was carried out with 1 (0.3 mmol), Ru(bpy)3-
Cl2ꢀ6H2O (3 mol %) in CHCl3 (3.0 mL) at 30 °C under 3W white LED irradiation; isolated
yield.
b Using 5 mol % of Ir(ppy)2(dtb-bbpy)PF6.
Table 3
Variation of the substituents on the 2-position of indolesa
O
Bn
Ru(bpy)3Cl2.6H2O (3 mol%)
CHCl3, O2, 30 oC, 3W White LED
Bn
R2
R2
N
N
H
H
1a, 1k-o
2a, 2k-o
O
O
O
Bn
Bn
Bn
N
H
N
H
N
H
Me
OMe
2a
2k
2n
2l
, 72 h, 76%
, 72 h, 62%
, 72 h, 94%
O
O
O
Bn
Bn
Bn
S
N
H
N
H
N
H
F
Cl
2m
2o
, 72 h, 74%
, 72 h, 55%
, 72 h, 78%
a
Unless otherwise noted, the reaction was carried out with 1 (0.3 mmol), Ru(bpy)3-
Cl2ꢀ6H2O (3 mol %) in CHCl3 (3 mL) at 30 °C under 3W white LED irradiation; isolated
yield.
R1
reaction showed promising synthetic potential in one-step
construction of the spirocyclic indolin-3-one product 2u, the
intrinsic core of (+)-Aristotelone and Brevianamide A, albeit with
a comparatively low yield (Eq. (2)). Further condition optimization
for preparation of spirocyclic indolin-3-ones is still under active
investigation.
To gain some insight into the possible mechanism, several
control experiments were designed and executed. Firstly, the fluo-
rescence quenching experiments with Ru(bpy)3Cl2-6H2O and cyclic
voltammetry (CV) measurements of model substrate 1a were per-
formed. The results indicated that substrate 1a (ꢁ0.04 V (vs SCE))
could be successfully oxidized by the photoexcited state
Ru(bpy)32+⁄ (+0.77 V (vs SCE)).9 Secondly, two 18O-labeling experi-
ments by using 18O2 and H218O were implemented (Eqs. (3) and
(4)), and these results showed that the oxygen atom of
R1
R1
O
O
R2
R2
R2
OH
N
N
N
H
H
H
R1
O
II
1
I
R2
Ru(II)*
Ru(I)
N
IV
O
O
Reductive
Quenching
R1
O2
R1
Visible
Light
R2
R2
Ru(II)
N
H
N
O2
H
III
1
O
R1
HO
3
R1
step-, atom-,
and redox-
economy
Semipinacol
R2
R2
Rearrangement
N
H
N
2
Scheme 2. Proposed mechanism.
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W. Ding et al. / Tetrahedron Letters xxx (2014) xxx–xxx
Table 4
Variation of the substituent on the benzene ring of indole substratesa
O
Bn
Ru(bpy)3Cl2.6H2O (3 mol%)
Bn
Ph
R3
R3
Ph
CHCl3, O2, 30 oC, 3W White LED
N
N
H
H
1a, 1p-t
2a, 2p-t
O
O
O
Bn
Ph
Me
Bn
Bn
N
H
Ph
Ph
N
H
N
H
Me
2a
2p
2s
2q
, 72 h, 76%
, 72 h, 76%
, 72 h, 76%
O
O
O
MeO
F
Br
Bn
Ph
Bn
Ph
Bn
Ph
N
H
N
H
N
H
X-ray structure of 2t
2r
2t
, 72 h, 52%
, 36 h, 84%
, 72 h, 76%
a
Unless otherwise noted, the reaction was carried out with 1 (0.3 mmol), Ru(bpy)3-
Cl2ꢀ6H2O (3 mol %) in CHCl3 (3 mL) at 30 °C under 3W white LED irradiation; isolated
yield.
Gram-scale Reaction:
Bn
O
.
Ru(bpy)3Cl2 6H2O (3 mol%)
Bn
Ph
C1a
C3a
C2a
ð1Þ
ð2Þ
Ph
CHCl3, O2, 30 oC, 3W WhiteLED, 72 h
N
H
N
H
Bn
1a:
2a:
4 mmol, 1.13g
71% yield, 0.85 g
Ph
N
Primary Trial:
H
1a
O
Ir(ppy)2(dtb-bbpy)PF 6 (5 mol%)
OH
Bn
Ph
CHCl3, 2, 30 oC, 3W WhiteLED, 72 h
O
N
H
1u
N
H
one step: the core of
N
3a
(+)-Aristotelone and Austamide
O
2u: 23% yield
Bn
the carbonyl group in the product originated exclusively from
molecular oxygen rather than water.9 Moreover, we have also car-
ried out a series of control experiments to investigate the possible
intermediates. As shown in Eq. (5), the model reaction of 1a could
afford the 3-benzyl-2-phenyl-3H-indolin-3-ol 3a in 17% isolated
yield upon quenching after 6 h. Notably, when the isolated tertiary
alcohol 3a was re-treated under the standard conditions, it was
exclusively converted into product 2a in 90% yield after 24 h.
Ph
N
H
2a
Time (h)
Figure 2. Reaction profile of photocatalytic aerobic oxidation/semipinacol rear-
rangement sequence analyzed by 1H NMR.
Bn
18O
Ru(bpy)3Cl2.6H2O (3 mol%)
Bn
Ph
Ph
Ph
Ph
18O
ð3Þ
CHCl3,
2, 30 oC, 3W White LED
present sequential reaction most probably proceeded through the
formation of key tertiary alcohol 3a, an important precursor for
the product 2a via semipinacol rearrangement.
N
N
H
H
72 h
1a
18O-2a
: 73% yield
According to these experimental results, a plausible mechanism
was proposed. As illustrated in Scheme 2, indole substrate 1 was
oxidized to cation radical I by the photoexcited Ru(II)⁄ through a
reductive quenching process. The resultant Ru(I) was oxidized by
the oxygen to regenerate Ru(II) with the formation of superoxide
radical anion, which could react with I to give the intermediate II
or III.6f,h The following proton transfer and OAO cleavage processes
would afford the tertiary alcohol 3.6h,11 Finally, the formation of
product 2 occurs via the chemospecific semipinacol rearrangement
of the key intermediate 3 (R1 migration).2e,3a Note that, although
the possibility of singlet oxygen-mediated process cannot be
completely excluded at the current stage,12 the results of the
singlet oxygen quenching experiment indicated that it was not
the predominating pathway for product formation.9
Bn
16O
Ru(bpy)3Cl2.6H2O (3 mol%)
CHCl3, O2, H218O (5.0 equiv), 30 o
Bn
C
ð4Þ
ð5Þ
ð6Þ
Ph
N
H
1a
N
H
3W White LED, 72 h
2a: 78% yield
OH
Bn
Bn
Ru(bpy)3Cl2.6H2O (3 mol%)
CHCl3, O2, 30 oC, 3W White LED, 6 h
Ph
N
H
1a
N
3a, 17% yield
OH
O
Ru(bpy)3Cl2.6H2O (3 mol%)
Bn
Ph
Bn
CHCl3, 2, 30 oC, 3W White LED, 24 h
O
Ph
N
N
H
Preliminary studies have also revealed that the use of chiral
phosphoric acid 4 as organocatalyst can induce the oxidation
rearrangement reaction with promising levels of enantioselectivity
(Eq. (7), 83% yield, 60% ee).13 Further to improve the enantioselec-
tivitiy of this process is still ongoing in our laboratory.
3a
2a, 90% yield
More remarkably, the variation process of starting material 1a,
transient species 3a, and final product 2a can be recorded through
1H NMR analysis (Fig. 2). All these results suggested that the
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W. Ding et al. / Tetrahedron Letters xxx (2014) xxx–xxx
5
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Ru(bpy)3Cl2.6H2O
(3 mol%)
CHCl3, O2, 30 oC,
3W White LED
4 (10 mol%)
60 h
O
Bn
O
O
O
Bn
Ph
P
Ph
OH
N
N
H
H
1a
2a
83% yield
60% ee
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In conclusion, a highly efficient sequential photocatalytic aero-
bic oxidation/semipinacol rearrangement reaction has been
accomplished in our laboratory. This reaction can transform a wide
range of readily available indoles into synthetically significant 2,2-
disubstituted indol-3-ones in generally good yields under mild
reaction conditions. Moreover, a plausible mechanism for this
reaction was proposed based on 18O labeling experiments, lumi-
nescence quenching experiments, and the detection of reaction
intermediates. Further application of this new sequential reaction
is currently underway in our laboratory.
Acknowledgments
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We are grateful to the National Natural Science Foundation of
China (Nos. 21272087, 21202053 and 21232003) and the National
Basic Research Program of China (2011CB808603) for support of
this research.
Supplementary data
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.tetlet.2014.06.
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References and notes
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Please cite this article in press as: Ding, W.; et al. Tetrahedron Lett. (2014), http://dx.doi.org/10.1016/j.tetlet.2014.06.102