Metal-Free Three-Component Selenopheno[2,3-b]indole Formation through Double C?H Selenylation with Selenium Powder

Article abstract of DOI:10.1002/adsc.201901023

A facile metal-free entry to novel selenopheno[2,3-b]indole motif is described. The three-component assembly of indoles, aromatic ketones, and selenium powder is enabled by the IBr-promoted highly selective double C–H selenylation/annulation. This protocol provides a novel access to a diverse variety of selenopheno[2,3-b]indoles with good efficacy and broad functional group compatibility. (Figure presented.).

Full text of DOI:10.1002/adsc.201901023

Title: Metal-Free Three-Component Selenopheno[2,3-b]indole
Formation through Double C–H Selenylation with Selenium
Powder
Authors: Penghui Ni, Jing Tan, Wenqi Zhao, Huawen Huang, and
Guojun Deng
This manuscript has been accepted after peer review and appears as an
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To be cited as: Adv. Synth. Catal. 10.1002/adsc.201901023
Link to VoR: http://dx.doi.org/10.1002/adsc.201901023

10.1002/adsc.201901023
Advanced Synthesis & Catalysis
COMMUNICATION
DOI: 10.1002/adsc.201((will be filled in by the editorial staff))
Metal-Free
Three-Component
Selenopheno[2,3-b]indole
Formation through Double C–H Selenylation with Selenium
Powder
Penghui Ni,a Jing Tan,a Wenqi Zhao,a Huawen Huang,a* and Guo-Jun Denga,b*
a
Key Laboratory for Green Organic Synthesis and Application of Hunan Province, Key Laboratory of Environmentally
Friendly Chemistry and Application of Ministry of Education, College of Chemistry, Xiangtan University, Xiangtan
411105, P. R. of China. E-mail: hwhuang@xtu.edu.cn; gjdeng@xtu.edu.cn
b Beijing National Laboratory for Molecular Sciences and CAS Key Laboratory of Molecular Recognition and Function
Institute of Chemistry, Chinese Academy of Sciences (CAS) Beijing 100190, P. R. of China.
Received: ((will be filled in by the editorial staff))
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201######.((Please
delete if not appropriate))
A user-friendly strategy is the direct use of
Abstract.
A
facile metal-free entry to novel
elemental selenium as the selenylation reagent as it is
cheap and readily available and more importantly its
properties of ambient stability make it easy-handling
on bench. While the affinity of selenium may
attenuate the catalytic activity of some metal catalysts,
during the past several years, great progress has been
made on the development of a copper-catalytic
system for C–Se bond formation with selenium
powder.[10] Therein, prefunctionalized substrates such
as aryl halides were generally utilized as coupling
partners. Notably, copper catalysts also proved to be
effective to promote direct C–H oxidative
selenylation.[11] For example, Ji and Wang disclosed
selenium insertion reactions between isocyanides and
amines.[12] Wu and co-workers developed direct C–H
selenylation of alkynes and heteroarenes with
selenium powder.[13]
selenopheno[2,3-b]indole motif is described. The three-
component assembly of indoles, aromatic ketones, and
selenium powder is enabled by the IBr-promoted highly
selective double C–H selenylation/annulation. This
protocol provides a novel access to a diverse variety of
selenopheno[2,3-b]indoles with good efficacy and broad
functional group compatibility.
Keywords: metal-free; selenopheno[2,3-b]indole; C–H
selenylation; selenium powder
Organoselenium compounds are of great importance
in chemistry constituting core structures of a number
of drugs, naturally occurring products, and functional
materials, among others,[1] as well as versatile
building blocks in synthetic chemistry.[2] Accordingly,
studies on the construction of C–Se bond has drawn
considerable interest in synthetic chemistry.[3] With
Indole-involved synthesis is pharmacologically[14]
and strategically important.[15] During the past several
years, few reports on indole selenylation have been
uncovered with different selenating reagents with or
without the assistance of transition-metal catalysts.[16]
In 2016, Wu et al. described copper-catalyzed C–Se
bond formation between indoles and aryl iodides
using selenium powder as the selenating reagent
(Scheme 1, a).[17] Wang and Ji reported a four-
component oxidative selenium insertion using
TEMPO as the catalyst (Scheme 1, b).[18] Given the
usual selectivity mode of selenylation at the C-3
position to give 3-selenyl-indoles, the C-2 position of
indoles is quite less reactive. Silveira reported the first
C-2 selenylation of indoles using selenium
dichlorides as the selenylating reagent.[19] Within our
continuing program on indole functionalization,[20]
respect to selenium sources,[4] active organoselenium
[6]
reagents such as selenol,[5] ArSeSnR3
and
diselenides[7] were frequently used as selenylation
reagents in transition-metal-catalyzed cross coupling
reactions. For example, Gu et al. reported a
palladium-catalyzed selenation using selenoates as
the selenium source via C(O)–Se bond cleavage
process.[8] Recently, direct C–H selenylation with
diselenides or selenium chlorides has emerged as an
alternative approach for C–Se bond construction, in
which the use of stoichiometric amount metal salts as
the oxidant was required in most cases.[9] However,
those selenylation reagents generally bear properties
of unpleasant odors and air-/moisture-instability and
thereby might prevent their practical applications.
1
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10.1002/adsc.201901023
Advanced Synthesis & Catalysis
herein, we describe a general three-component
were used.[20e] This result may indicate nucleophilic
or radical C2 selenylation of indoles.
selection
reaction
for
the
synthesis
of
selenopheno[2,3-b]indoles with elemental selenium
under simple metal-free conditions. To complement
Table 1. Optimization of reaction conditions.[a]
our
previous
selective
benzoselenophene
formation,[20e] this protocol features high levels of
regioselectivity towards C-2 selenylation of indoles
through a tandem indole alkenylation/dual C–H
oxidative selenylation (Scheme 1, c).
Entry
1
Oxidant
I2
Solvent
NMP
NMP
NMP
NMP
NMP
toluene
PhCl
Yield (%)[b]
21
2
NIS
ICl
35
3
47
4
IBr
ICl3
IBr
IBr
IBr
IBr
IBr
IBr
IBr
IBr
IBr
66
5
38
6
n.d.
n.d.
19
7
8
DMF
DMAc
DEF
9
28
10
11
12[c]
13[d]
14[e]
15
36
DMSO
NMP
NMP
NMP
NMP
Trace
65
Scheme 1. Indole C–H selenylation with selenium powder.
59
61
On the basis of our previous indole-template
20e,f
]
synthesis of carbazoles and thieno[2,3-b]indoles,[
n.d.
we reasonably proposed a selenopheno[2,3-b]indole
formation through a similar cascade annulation.
Hence, 1-methylindole (1a), acetophenone (2a) and
selenium powder were initially handled as model
reactants with iodine reagents (Table 1). To our
delight, the desired selenylation/annulation product
3aa was obtained in 21% yield when the reaction was
treated with elemental iodine (1.0 equiv) in N-methyl
pyrrolidone (NMP) at 130 °C for 6 h (entry 1).
Among other iodide-containing oxidants including
NIS, ICl, IBr and ICl3 (entries 2-5), IBr was proved to
be the most effective oxidant to afford the desired
product in an excellent yield (entry 4). Investigations
on reaction media revealed that NMP showed the best
performance, while other organic solvents such as
toluene, chlorobenzene, DMF, DMAc, DEF, and
DMSO decreased the yield (entries 6-11). Increasing
the amounts of IBr or decreasing the reaction
temperature led to a lower yield of product (entries
12-13). The yield slightly declined when the reaction
was performed under Ar atmosphere (entry 14).
Finally, selenylation/annulation product was not
observed in the absence of external oxidant (entry 15).
Notably, with indole 1a we did not observe the
formation of benzo[b]selenophene, which was
generated as the major product when 2-arylindoles
[a]
Conditions: 1a (0.3 mmol), 2a (0.2 mmol), Se (0.3
mmol), oxidant (0.12 mmol), solvent (0.6 mL), 130 °C, 6 h,
under air atmosphere. DMF = N,N-dimethylformamide,
DMAc
=
N,N-dimethylacetamid. DEF
=
N,N-
diethylformamide.
[b] GC yield based on 2a.
[c] IBr (0.2 mmol).
[d] 120 oC.
[e] Under Ar.
With the optimized reaction conditions in hand, the
substrate scope of the tandem indole C-3
alkenylation/dual C–H oxidative selenylation was
probed. First,
a
broad range of substituted
acetophenones were subjected to the IBr-based
system (Table 2). Among them, a series of para-
substituted acetophenones bearing electron-donating
groups (methyl, methoxyl) were smoothly converted
into the corresponding products in moderate yields
(3ab,
3ae).
Acetophenones
bearing
halo
functionalities (F, Cl, Br, and I) were compatible to
give the corresponding products in good yields (3ag-
3aj, 3ap-3ar). The structure of 3aj was confirmed by
X-ray crystallography. Strong electron-withdrawing
substituents such as CO2CH3, CN, NO2 and SO2CH3
were also compatible with the reaction conditions
2
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10.1002/adsc.201901023
Advanced Synthesis & Catalysis
(3ad, 3ak-3am and 3at). When substituents were
presented at the ortho position, lower yields were
observed (3au-3aw). Acetophenones 2x and 2y
bearing two substituents reacted well to yield the
desired products 3ax and 3ay, respectively. The
products (3az-3aa’, 3ae’) were given in modest
yields when bulky aromatic ketones such as 1-
(naphthalen-1-yl)ethanone (2z), 1-(naphthalen-2-
yl)ethanone (2a’), and 1-(phenanthren-3-yl)ethanone
(2e’) were used. Notably, heteroaromatic ketones
(2b’-2d’) also reacted efficiently to afford the
corresponding products (3ab’-3ad’) in good yield.
Unfortunately, aliphatic ketones such as 1-
cyclohexylethanone and 3-methylbutan-2-one were
not suitable substrates for this cascade transformation.
We also tried non-methyl ketones such as
transformation, in which only a trace amount of
product was observed by GC-MS. Moderate yields
were obtained when substituents such as methyl and
methoxy were presented at the indole C-5 position
(3ba and 3ca). Halogen functionalities such as chloro
and bromo were well tolerated, giving the halo-
substituted products in moderate yields (3da and 3fa).
While free NH-indole (1h) afforded the
corresponding 3ha in modest yield, other N-
alkylindoles were also suitable to couple with ketone
and selenium powder to give the target
selenopheno[2,3-b]indoles in good yields (3ia-3ja).
Table 3. Substrate scope with respect to indoles.[a]
propiophenone
and
1,2-diphenylethan-1-one.
Unfortunately, we did not observe the target products.
Hence, non-methyl ketones featured no reactivity in
the present system. 2-Bromo-1-phenylethan-1-one
instead of acetophenone was also productive toward
3aa, albeit in a low yield (34%). The robust nature of
this metal-free system was reflected by the effective
gram-scale preparation (3aa, 3ai).
Table 2. Substrate scope with respect to ketones.[a]
[a] Conditions:
mmol), IBr (0.12 mmol), NMP (0.6 mL), 130 C, 6 h,
under air atmosphere, isolated yield based on 2a
1 (0.3 mmol), 2a (0.2 mmol), Se (0.3
o
.
The selenophenoindoles product 3ai was obtained in
58% yield in a gram-scale reaction (Scheme 2).
Further synthetic modification of the bromo group of
product 3ai was investigated. Pd-catalyzed
Suzuki−Miyaura cross coupling (Scheme 2, a) and
Buchwald−Hartwig amination (Scheme 2, b) of 3ai
proceeded smoothly to afford products 4a and 4b in
90% and 71% yields, respectively.[21] When 3aa was
treated with I2 in DMF under argon atmosphere, the
iodization product 4c was obtained in 72% yield
(Scheme 2, c).[22] The Suzuki–Miyaura cross-
coupling reaction of 4c with p-tolylboronic acid
proceeded well to give 4d in 95% yield (Scheme 2, d).
Finally, the 4d was transformed to the 6,10-dimethyl-
10H phenanthro[9',10':4,5]selenopheno[2,3-b]indole
4e through dehydrogenative annulations with
FeCl3.[23] (Scheme 2, e).
[a] Conditions: 1a (0.3 mmol),
mmol), IBr (0.12 mmol), NMP (0.6 mL), 130 C, 6 h,
2 (0.2 mmol), Se (0.3
o
[b]
under air atmosphere, isolated yield based on
2.
[c]
Yield of 6 mmol scale reaction.
2-Bromo-1-
phenylethan-1-one instead of acetophenone.
Thereafter, the substrate scope with respect to
functionalized indoles was explored (Table 3).
Unexpectedly, a methyl group at the C-4 position of
indole
component
almost
prevented
the
3
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10.1002/adsc.201901023
Advanced Synthesis & Catalysis
treated indole 1a and ketone 2a in the absence of
selenium powder, which afforded 3-vinylindole 3a
within 0.5 h (Scheme 3, a). As expected, 3a could
smoothly convert into 3aa with high efficacy by the
treatment of elemental selenium under the standard
reaction conditions (Scheme 3, b). Furthermore, we
did not observe any product with C–Se bond
formation in the absence of ketone or indole
component (Scheme 3, c and d), in which indole and
ketone were mainly recovered, respectively. This
result also indicated that higher reactive 3-vinylindole
3a was initially generated and then coupled with
selenium. Finally, while the addition of scavenger
BHT (butylated hydroxytoluene) or ethene-1,1-
diyldibenzene did not affect the efficiency of the
desired
transformation,
TEMPO
(2,2,6,6-
tetramethylpiperidine-1-oxyl) and DDQ (2,3-
dicyano-5,6-dichlorobenzoquinone) as the additive
completely prevented the reaction (Scheme 3, e).
The experimental results above indicated 3-
vinylindole 3a to be the key intermediate for the
three-component selenylation/annulation. On the
basis of this and previous reports,[24] a possible
reaction mechanism was proposed (Scheme 4).
Starting from the 3-vinylindole 3a, the process of
dual C–H oxidative selenylation probably proceeded
through two different pathways: path a) nucleophilic
Scheme 2. Application of the present work. (a) 3ai (0.2
mmol), phenylboronic acid (1.5 equiv), Pd(PPh3)4 (3
mol%), Na2CO3 (8.0 equiv), toluene-H2O-EtOH (0.7, 0.7,
0.2 mL, respectively), 100 oC, 12 h, Ar; (b) 3ai (0.2 mmol),
carbazole (1.5 equiv), Pd(OAc)2 (10 mol%), K2CO3 (3.0
.
equiv), P(t-Bu)3 HBF4 (20 mol%), o-xylene (0.8 mL), 130
oC, 16 h, Ar; (c) 3aa (0.2 mmol), I2 (1.5 equiv.), KOH (3
o
equiv ), DMF (0.6 mL), 40 C, 6 h, Ar; (d) 4c (0.2 mmol),
p-tolylboronic acid (1.5 equiv), Pd(PPh3)4 (3 mol%),
Na2CO3 (8.0 equiv), toluene-H2O-EtOH (0.7, 0.7, 0.2 mL,
respectively), 100 oC, 12 h, Ar; (e) 4d (0.1 mmol), FeCl3 (6
equiv.), CH2Cl2 (0.8 mL), 45 oC, 12 h, Ar.
attack-involved
cascade
annulation
and
dehydrogenative aromatization;[10h,11a] and path b)
SET (single electron transfer)-initiated radical
annulation.[11b,12b]
Scheme 4. Possible reaction mechanism.
In summary, we have developed a three-
component selenylation/annulation to give facile
access to novel selenopheno[2,3-b]indole motif
through a dual C–H oxidative selenylation under
metal-free conditions. In this protocol, simple and
readily available indoles, aromatic ketones, and
selenium powder were used as the starting materials
and a variety of functional groups attached to indoles
and aromatic ketones were well tolerated. The C–Se
bond formation unexpectedly occurred at the C-2
position of indole moiety with high levels of
selectivity, which may inspire other cases of selenium
powder-involved design for regioselective C–H
selenylation.
Scheme 3. Control experiments.
Based
on
our
previous
indole-template
synthesis,[20b,c] we suspected the coupling of indole
and ketone would be the initial step. Hence, we
4
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10.1002/adsc.201901023
Advanced Synthesis & Catalysis
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Experimental Section
General procedure for the Synthesis of 3aa
IBr (25.0 mg, 0.12 mmol) and selenium powder (24.0 mg,
0.3 mmol) were added to an oven-dried reaction vessel (20
mL). The reaction vessel was sealed, 1-methyl-1H-indole
(1a, 37.0 μL, 0.3 mmol), acetophenone (2a, 24.0 μL, 0.2
mmol) and N-methyl pyrrolidone (0.6 mL) were added by
o
syringe. The reaction vessel was stirred at 130 C for 6 h
under air atmosphere. After cooling to room temperature,
the reaction was diluted with ethyl acetate (5 mL) and
washed with saturated salt water. The organic layer was
separated, and the aqueous layer was extracted with ethyl
acetate for three times. The combined organic layer was
dried over sodium sulfate and the volatiles were removed
under reduced pressure. The residue was purified by
column chromatography on silica gel (petroleum
ether/EtOAc = 300:1) to yield the desired product 3aa as
colourless oily liquid (43.4 mg, 70 % yield). Rf = 0.65
1
(100:1 petroleum ether/EtOAc). H NMR (400 MHz,
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CDCl3, ppm) δ 7.75-7.68 (m, 3H), 7.49 (t, J = 7.4 Hz, 2H),
7.43-7.37 (m, 1H), 7.36-7.32 (m, 1H), 7.27-7.20 (m, 2H),
7.07 (t, J = 7.5 Hz, 1H), 3.83 (s, 3H); 13C NMR (100 MHz,
ppm) δ 144.9, 142.2, 138.5, 137.9, 128.5, 128.3, 127.5,
123.3, 122.1, 121.5, 119.2, 119.0, 116.1, 108.8, 33.2;
HRMS (ESI) m/z calcd. for C17H14NSe+ (M+H)+ 312.0286,
found 312.0283.
CCDC-1828980
contains
the
supplementary
crystallographic data for this paper. These data can be
obtained free of charge from The Cambridge
Crystallographic
Data
Centre
via
www.ccdc.cam.ac.uk/data_request/cif.
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Acknowledgements
This work was supported by the National Natural Science
Foundation of China (21871226, 21602187), the Collaborative
Innovation Center of New Chemical Technologies for
Environmental Benignity and Efficient Resource Utilization, the
Hunan Provincial Innovative Foundation for Postgraduate
(CX2018B362, CX2018B046), Hunan Provincial Natural Science
Foundation of China (16JJ3112, 17JJ3299).
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10.1002/adsc.201901023
Advanced Synthesis & Catalysis
COMMUNICATION
Metal-Free Three-Component Selenopheno[2,3-
b]indole Formation through Double C–H
Selenylation with Selenium Powder
Adv. Synth. Catal. Year, Volume, Page – Page
Penghui Ni,a Jing Tan,a Wenqi Zhao,a Huawen
Huang,a* and Guo-Jun Denga,b*
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