Selective CH or NH Imidoylative Annulation of 2-(2-Isocyanophenyl)-1H-indoles Leading to Diverse Indole-fused Scaffolds

Article abstract of DOI:10.1002/adsc.201801623

2-(2-Isocyanophenyl)-1H-indoles, a class of functionalized isocyanides containing both aromatic CH and indolo NH functionalities, are first applied in selective imidoylative annulation reactions. Scaffolds of 6-aryl 11H-indolo[3,2-c]quinoline and 6-aryl i

Full text of DOI:10.1002/adsc.201801623

Title: Selective C‒H or N‒H Imidoylative Annulation of 2-(2-
Isocyanophenyl)-1H-indoles Leading to Diverse Indole-fused
Scaffolds
Authors: Fan Teng, Weiming Hu, Huaanzi Hu, Shuang Luo, and Qiang
Zhu
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To be cited as: Adv. Synth. Catal. 10.1002/adsc.201801623
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10.1002/adsc.201801623
Advanced Synthesis & Catalysis
UPDATE
DOI: 10.1002/adsc.201((will be filled in by the editorial sttaff))
Selective C‒H or N‒H Imidoylative Annulation of 2-(2-Isocyanophenyl)-
1H-indoles Leading to Diverse Indole-fused Scaffolds
Fan Teng,ab Weiming Hu,ab Huaanzi Hu,ab Shuang Luo,*ab and Qiang Zhu*ab
a State Key Laboratory of Respiratory Disease, Guangzhou Institutes of Biomedicine and Health, Chinese Academy of
Sciences, 190 Kaiyuan Avenue, Guangzhou 510530 (China), Fax: (+86) 20-3201-5299;
b University of Chinese Academy of Sciences, No. 19(A) Yuquan Road, Shijingshan District, Beijing 100049, China
E-mail: luo_shuang@gibh.ac.cn; zhu_qiang@gibh.ac.cn.
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))
functional groups,[3] such as carbonyl oxygen of
Abstract: 2-(2-Isocyanophenyl)-1H-indoles, a class of
functionalized isocyanides containing both aromatic
C‒H and indolo N‒H functionalities, are first applied in
amides,[3a-b] aromatic C‒H bonds,[3c-h] alkenes,[3i-j] and
alkynes,[3k-l] leading to heterocyclic compounds such
as oxazoles, phenanthridines, cyclic ketoimines, and
indole-fused derivatives. Intramolecular trapping of
the imidoyl radical intermediates is another important
application of functionalized isocyanides, which has
been widely studied in the synthesis of phenanthridine,
quinoxaline, benzoimidazole analogues and so on.[4]
selective imidoylative annulation reactions. Scaffolds of
6-aryl 11H-indolo[3,2-c]quinoline and 6-aryl indolo[1,2-
c]quinazoline, both of which are of biological
importance, are obtained selectively through capturing
the imidoyl radical or imidoyl palladium intermediates
by the C‒H and N‒H bonds, respectively.
However, when
a
functionalized isocyanide
Keywords: Selective cycloimidoylation; Pd-catalyzed;
radical; isocyanide.
contains multiple reactive sites to terminate imidoyl
intermediates, selective annulation is a challenging
task with very limited successful examples.[3d,
3g-h]
Recently, a ligand-controlled site-selective C(sp2)−H
cycloimidoylation process was realized starting from
2-isocyano-2,3-diarylpropanoates to deliver six-
membered 3,4-dihydroisoquinolines and five-
Nonfunctionalized isocyanide (RNC) usually
serves as a C1 building block through α,α-
difunctinalization of the terminal divalent isocyano
carbon to get a C=N or C=O containing product in
which the isocyano carbon turns to the “normal” membered 1,1-disubstituted 1H-isoindoles (a, Scheme
valent state.[1] In this process, the pending group R in
isocyanide does not participate in the reaction by
itself; and even in some cases, it is hydrolyzed
1).[3h] Meanwhile, by discriminating two symmetric
C(sp2)−H bonds, enantioselective construction of
central and planar chiral compounds were
accomplished by Pd-catalyzed cycloimidoylations in
the presence of SPINOL-derived phosphoramidites
(b).[3d, 3g] However, in the aforementioned reactions,
the selectivity was taking place between the same
type of functionality, C(sp2)−H bonds. We envisioned
that 2-(2-isocyanophenyl)-1H-indoles, a class of
simultaneously
or
during
the
following
transformations. In recent years, functionalized
isocyanides, by incorporating functionalities capable
of trapping various isocyano-derived intermediates
intramolecularly, have attracted extensive attention in
the synthesis of a myriad of nitrogen heterocyclic
motifs.[2] By taking this strategy, almost all atoms in
isocyanide are maintained in the final cyclized functionalized isocyanides containing both C−H and
products, which is not only highly atom economic but
also efficient in generating molecular complexity and
structural diversity. For instance, in palladium-
catalyzed imidoylative annulation of functionalized
isocyanides, the key imidoyl Pd(II) intermediates
formed by migratory insertion of isocyanide to a Pd(II)
species could be captured by various intramolecular
N−H bonds capable of imidoylation, would be ideal
substrates to study chemo-selective cycloimidoylation
between different types of functionalities (c).
Correspondingly, the construction of substituted 1H-
indolo[3,2-c]quinoline and indolo[1,2-c]quinazoline
derivatives will be expected through C−H and N−H
1
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10.1002/adsc.201801623
Advanced Synthesis & Catalysis
bond cycloimidoylation, respectively. Importantly,
both of these motifs are frequently found in natural
products and biologically active molecules.[5]
Yield
Entry
1
Ar source
Reaction conditions
Mn(acac)3, DCE, Ar, 80 oC
4a
54%a
5a
PhB(OH)2
(2a)
PhB(OH)2
(2a)
PhB(OH)2
(2a)
PhI (3a)
0
2
3
4
5
6
Mn(acac)3, MeCN, Ar,
80 oC
62%a
0
Mn(acac)3, MeCN, Ar,
73%a
(82%)b
trace
0
100 oC
Pd(OAc)2, PPh3, Cs2CO3,
PhMe, Ar, 80 oC
56%c
73%c
77%c
PhI (3a)
PhI (3a)
trace
trace
Pd(OAc)2, PPh3, CsOPiv，
PhMe, Ar, 80 oC
Pd(OAc)2, PPh3, Cs2CO3,
PivOH，PhMe, Ar, 80 oC
Reaction conditions: a 1a (0.1 mmol), 2a (0.2 mmol), Mn(acac)3
(0.2 mmol), solvent (2 mL), 80 oC, in Ar, 24 h, sealed tube,
isolated yield. b 2a (0.3 mmol), Mn(acac)3 (0.3 mmol). c 1a (0.1
mmol), 3a (0.12 mmol), Pd(OAc)2 (10 mol %), PPh3 (20 mol %),
base (0.12 mmol), 80 oC, in Ar. A solution of 1a in solvent (1 mL)
was added to the reaction mixture (in 1.0 mL of solvent) via a
syringe pump within 1 h, isolated yield.
Scheme 1. Selective cycloimidoylation of functionalized
isocyanide.
Inspired by recent advances in cascade radical
addition/cyclization of radical intermediates with 2-
isocyano-1,1'-biphenyls,[6] we tested the hypothesis of
chemo-selective cycloimidoylation using 2-(2-
isocyanophenyl)-1H-indole 1a as a representative
model substrate and PhB(OH)2 2a as a phenyl radical
Scheme 2. The scope of ArB(OH)2 in C−H
cycloimidoylation.
a
o
source in the presence of Mn(acac)3 in DCE at 80 C
under Ar. Delightfully, 6-phenyl 11H-indolo[3,2-
c]quinoline
4a,
generated
from
C(sp2)−H
cycloimidoylation, was isolated selectively in 54%
yield (entry 1, Table 1). The other possible cyclization
product 5a was hardly detected. The yield of 4a was
improved to 62% when MeCN was used as a solvent
(entry 2). Elevating the reaction temperature to
100 °C was proven to be beneficial (73%, entry 3).
Finally, 4a could be isolated in 82% yield with 3.0
equivalents of PhB(OH)2 and Mn(acac)3. Then, the
reaction of 1a with PhI 3a under palladium catalysis
was investigated. 6-Phenyl indolo[1,2-c]quinazoline
5a, derived from N−H bond cycloimidoylation, was
formed exclusively under reaction conditions
involving Pd(OAc)2, PPh3 and Cs2CO3 in toluene at
a
Reaction conditions: 1a (0.1 mmol), 2 (0.3 mmol), Mn(acac)3
o
(0.3 mmol), MeCN (2.0 mL), 100 C, in Ar, 24 h, sealed tube,
isolated yield. b 1.0 mmol scale. c N.D.=not detected.
With the optimal reaction conditions for selective
formation of 4a established, the scope of arylboronic
acids 2 in this Mn-mediated radical cyclization was
first explored (Scheme 2). 6-Aryl 11H-indolo[3,2-
c]quinolines bearing electron-donating (4b-4d, 4h-4j)
or electron-withdrawing groups (4e-4g) on 6-aryl
o
80 C under Ar (56%, entry 4). When Cs2CO3 was
replaced by CsOPiv, the yield of 5a was increased
remarkably to 73%. To avoid handling hygroscopic
CsOPiv, in-situ formation of CsOPiv by mixing
Cs2CO3 and PivOH was preferred, which gave a
slightly improved yield of 5a (77%, entry 6). Notably, could be formed in moderate to good yields (41-87%)
with the corresponding aryl boronic acids as the aryl
source. Functionalities including Cl, Br, and acetyl
tolerated well under the reaction conditions. Notably,
amino substituted derivative 4j which showed good
under all of the above conditions for Pd catalysis,
cycloimidolylation at the C-3 position of indole was
suppressed, and 4a was only detected in trace amount.
This result might be assigned to the coordination
ability of the NH free nitrogen of indole.[7]
activity
against
Mycobacterium
tuberculosis
according to our previous report (MIC = 2.5
μg/mL),[5f] could be obtained in 42% yield. In
addition, the bulky 1-naphthyl group could also be
Table 1. Optimization of the reaction conditions.
2
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10.1002/adsc.201801623
Advanced Synthesis & Catalysis
installed at the C6 position of 11H-indolo[3,2-
c]quinoline scaffold in 64% yield (4k). Secondary
alkylboronic acids could also provide the
corresponding 6-alkyl substituted products in
moderate yields (4l, 4m). However, the reaction
with 1-butanylboronic acid as a radical source
could not deliver the corresponding product.
Then, studies by varying substitutents on 2-(2-
isocyanophenyl)-1H-indoles 1 were carried out
(Scheme 3). Preliminary results showed that electron-
withdrawing groups (F, Cl, Br) on the indole moiety
gave slightly better results than electron-donating
methyl (4o-4q vs. 4n and 4r). When 2-(2-
isocyanophenyl)-1-methyl-1H-indole was subjected
to the reaction conditions, the annulation product 4s
was isolated in 65% yield.
a
Reaction conditions: 1a (0.1 mmol), 3 (0.12 mmol), Pd(OAc)2
(10 mol %), PPh3 (20 mol %), Cs2CO3 (0.12 mmol), 80 oC, in Ar.
A solution of 1a in toluene (1 mL) was added to the reaction
mixture (in 1.0 mL of toluene) via a syringe pump within 1 h,
isolated yield. b 1.0 mmol scale.
Scheme 3. The scope of isocyanide in C−H
Finally, the tolerance of functionalized isocyanides
was tested in reactions with 3a, and the results were
summarized in Scheme 5. 2-(2-Isocyanophenyl)-1H-
indoles containing different substituents on the indole
moiety were investigated, and the desired N−H
cycloimidoylation products 5k-5p were generated in
51-76% yields. When 2-(2-isocyanophenyl)-7-
methyl-1H-indole bearing a methyl group adjacent to
the indolo N−H was used, the reaction proceeded
equally well to produce 5p in 68% yield.
cycloimidoylation.a
Scheme 5. The scope of isocyanide in N−H
cycloimidoylation.a
Reaction conditions: a 1 (0.1 mmol), 2a (0.3 mmol), Mn(acac)3
(0.3 mmol), MeCN (2.0 mL), 100 oC, in Ar, 24 h, sealed tube,
isolated yield.
Next, the feasibility for indolo[1,2-c]quinazoline
formation under Pd catalysis was investigated
(Scheme 4). In general, electron-deficient ArI gave
better yields of 5 than those of electron-rich ones. For
example, 4-iodobenzonitrile and 4-iodoanisole
delivered the corresponding products in 80% (5f) and
56% (5e) yields, respectively. However, when 2-
iodobenzonitrile was applied, the annulated product
5c was generated in only 47% yield, indicating the
negative effect of steric hindrance of iodide.
Remarkably, 3-iodopyridine and 2-iodothiophene
could also afford the heteroaromatic substituted
products 5i and 5j in moderate yields. In addition,
both of the reactions for selective annulation could be
scaled up using 1 mmol of 1a in almost identical
yields of 4a and 5a, verifying the practicality of the
methods.
a
Reaction conditions: 1 (0.1 mmol), 3a (0.12 mmol), Pd(OAc)2
(10 mol %), PPh3 (20 mol %), Cs2CO3 (0.12 mmol), 80 oC, in Ar.
A solution of 1a in toluene (1 mL) was added to the reaction
mixture (in 1.0 mL of toluene) via a syringe pump within 1 h,
isolated yield.
This kind of chemo-selective C‒H or N‒H
imidoylative annulation was also applicable to other
structurally similar isocyanide. When 2-(2-
isocyanophenyl)-1H-pyrrole 6 was subjected to both
aforementioned reactions, 4-phenyl-11H-pyrrolo[3,2-
c]quinoline 7 and 5-phenylpyrrolo[1,2-c]quinazoline
8 could be obtained in moderate yields and good
selectivity following the pathways of C−H and N−H
cycloimidoylation, respectively (1, Scheme 6). In
addition, C−H cycloimidoylation of 1a with other
Scheme 4. The scope of ArI in N−H cycloimidoylation.a
3
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10.1002/adsc.201801623
Advanced Synthesis & Catalysis
radical species generated from 1,4-dioxane,[7g] was concentrated in vacuum, and the product was
Togni’s reagent[7a] and diphenylphosphine oxide[7q, 8] purified by flash column chromatography on silica gel
took place smoothly to give C6 diversified 11H- with petroleum ether-EtOAc as the eluent to give the
indolo[3,2-c]quinolines 9-11 in yields ranging from desired product.
58% to 64% (2-4).
Scheme 6. Other examples leading to annulation productsa
Annulation toward 6-arylindolo[1,2-c]quinazoline:
An oven-dried 25 mL Schlenk tube charged with
Pd(OAc)2 (0.01 mmol, 2.2 mg), PPh3 (0.02 mmol, 5.2
mg) and Cs2CO3 (0.12 mmol, 39.0 mg) was
vacuumed and refilled with Ar for 3 times. Then, a
solution of ArI (0.12 mmol, 1.2 equiv), PivOH (0.12
mmol, 14.0 µL, 1.2 equiv) in 1.0 mL of toluene was
added by syringe and the tube was placed in an 80 oC
oil-bath. A solution of isocyanide 1 or 6 (0.1 mmol,
1.0 equiv) in 1.0 mL of toluene was added dropwise
with a syringe pump to the reaction mixture. The
addition was finished within 1 h. The crude reaction
mixture was extracted with EA (2 mL × 3) and
washed with brine (5 mL). The organic phase was
concentrated in vacuo and the residue was purified by
silica gel flash column chromatography to afford the
corresponding products.
b
a Performed under corresponding conditions. For details, see SI.
BPO (dibenzoyl peroxide) used as an oxidant.
(tetrabutylammonium iodide) used as a catalyst.
Acknowledgements
c
TBAI
We are grateful to the National Natural Science Foundation of
China (NNSFC) (21532009, 21772198, and 21871268) for
financial support.
In conclusion, we reported a chemo-selective
cycloimidoylation of 2-(2-isocyanophenyl)-1H-
indoles, class of functionalized isocyanides
a
containing both aromatic C‒H and indolo N‒H
functionalities capable of trapping imidoyl
intermediates. Under radical conditions, homolytic
aromatic substitution (HAS) of the imidoyl radical
intermediate takes place preferentially regardless of
radical sources, generating C6 diversified 11H-
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4
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10.1002/adsc.201801623
Advanced Synthesis & Catalysis
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5
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10.1002/adsc.201801623
Advanced Synthesis & Catalysis
UPDATE
Selective C‒H or N‒H Imidoylative Annulation of
2-(2-Isocyanophenyl)-1H-indoles
Leading
to
Diverse Indole-fused Scaffolds
Adv. Synth. Catal. 2018, XX, Page – Page
Fan Teng, Weiming Hu, Huaanzi Hu, Shuang
Luo,* and Qiang Zhu*
6
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