Concise Synthesis of Polysubstituted Carbohelicenes by a C?H Activation/Radical Reaction/C?H Activation Sequence

Article abstract of DOI:10.1002/anie.201811023

Disclosed herein is the merging of C?H activation and radical chemistry, enabling rapid access to a structurally diverse family of fused carbohelicenes through the fusion of α-acetylnaphthalenes with alkynes under oxidative conditions. This cascade process exhibits exquisite chemoselectivity and regioselectivity. The reaction pathway was analyzed by intermediate separations, control experiments, radical trapping, EPR, MALDI-TOF-MS, and ESI-HRMS experiments, and shown to involve a C2?H activation/radical reaction/C8?H activation relay.

Full text of DOI:10.1002/anie.201811023

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Accepted Article
Title: Concise Synthesis of Poly-Substituted Carbohelicenes via a C–H
Activation/Radical Approach/C–H Activation Relay
Authors: Jingsong You and Jiangliang Yin
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COMMUNICATION
Concise Synthesis of Polysubstituted Carbohelicenes by a C–H
Activation/Radical Approach/C–H Activation Sequence
Jiangliang Yin, and Jingsong You*
Abstract: Disclosed herein is the merging of C–H activation and
radical chemistry to enable rapid access to a structurally diverse
family of fused carbohelicenes through the fusion of α-
acetylnaphthalenes with alkynes under oxidation conditions. This
cascade process exhibits exquisite chemoselectivity and
regioselectivity. By intermediate separations, control experiments,
radical trapping, EPR, MAIDL-TOF-MS, and ESI-HRMS experiments,
of radical intermediates has also made significant progress in the
[
7,8]
construction of complex molecules.
merging of ionic-type and radical-type reactions would raise
golden opportunities for discovering new chemical
It is foreseeable that the
transformations. However, few literatures have been focused on
this strategy, rendering it more challenging. Arylketones are
particularly appealing yet challenging research objects in the C–H
activation field due to their weak coordinating ability and keto–
the reaction pathway, involving
a
C2–H activation/radical
[9,6c-f]
approach/C8–H activation relay, has been illustrated.
enol tautomerism,
and have also attracted extensive
attention in the radical chemistry due to the ability of forming α-
[
10]
carbonylmethyl radical species by oxidization.
Herein we
Different from the planar polycyclic aromatic hydrocarbons
describe a selective C2–H activation/annulation, radical approach
and C8–H activation/annulation relay of α-acetylnaphthalenes
with alkynes to assemble fused carbohelicenes (Scheme 1e).
(
PAHs), carbohelicenes feature helically shaped π-skeletons with
ortho-fused aromatic rings, which have been extensively applied
to diverse scientific fields due to their distorted π-surface, unique
dynamic behavior, inherent chirality, characteristic reactivity,
[
1,2]
redox activity and good solubility.
photochemical synthesis, Scholl
For many years,
reaction, Diels-Alder
cycloaddition, benzylic-type coupling, carbonyl or pinacol coupling
and Friedel–Crafts–type reaction remain the main methods to
[
1,2]
access carbohelicenes.
However, these procedures generally
suffer from problems of chemoselectivity, synthetic efficiency and
product diversity. With the development of transition metal-
catalyzed cross-coupling reactions and cycloadditions, the
synthesis of carbohelicenes through catalytic strategy has made
great progress, which may be roughly summarized in the
[
1-3]
following pathways:
(1) intermolecular cross-couplings
between bismetallic (metal = Sn or B) binaphthyls and double
[
1]
halogenated aromatics (Scheme 1a); (2) palladium-catalyzed
intramolecular C–H/C–X (X = Cl, Br or I) couplings (Scheme
[
3b]
1
b);
(3) metal-catalyzed intramolecular [2+2+2] cycloadditions
[
3a,3d]
of triynes (Scheme 1c)
ring closure);
metathesis (Scheme 1d).
and cyclizations of arylated alkynes
and (4) Ru-catalyzed intramolecular olefin
Scheme 1. Transition metal-catalyzed synthesis of carbohelicenes.
[
3e,3f]
(
[
3c]
Despite significant progress, these
existing bottom-up approaches typically require non-easily
accessible synthetic precursors and multi-step operation, leading
to limited types of products. Doubtlessly, the development of
straightforward accesses to the carbohelicenes is an appealing,
yet challenging puzzle in organic synthesis.
The past decades have witnessed the galloping
development of transition metal-catalyzed C–H activation for the
[
4]
discovery of novel functional skeletons. In particular, C–H
activation/annulation of arenes with alkynes has emerged as a
versatile strategy for the construction of polycyclic aromatic
Scheme 2. Discovery of the reaction (a) and proposed reaction pathway (b).
[
5,6]
compounds.
With few exceptions, these reactions are normally
classified as ionic reactions involving the charged species.
Recently, another promising reaction type proceeding by means
Inspired by our previous work of phenalenyl-fused pyrylium
cation synthesis, we tried to synthesize the C4-substituted α-
acetylnaphthalene derivatives by using nucleophiles such as
phenols, amines, and indoles instead of water for attacking the
C4 position of α-acetylnaphthalene (Scheme 2a). Unexpectedly,
the fused [5]carbohelicene 4aa was obtained in 5% yield by using
α-acetylnaphthalene and diphenylacetylene as the substrates,
[]
J. Yin, Prof. Dr. J. You
[
6g]
Key Laboratory of Green Chemistry and Technology of Ministry of
Education, College of Chemistry, Sichuan University
29 Wangjiang Road, Chengdu 610064, P. R. China
E-mail: jsyou@scu.edu.cn.
[
Cp*RhCl
2
]
2
/AgSbF
6
as the catalyst, and Ag
2
O as the oxidant in
Supporting information for this article is given via a link at the end of
the document.
o
the presence of 2-naphthol and NaSbF
6
at 120 C (Table S1,
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COMMUNICATION
entry 1). Control experiments indicated that 2-naphthol was
yield and the carbohelicene product could not be detected,
probably due to the larger steric hindrance (Scheme 3b). These
results indicated that the fulvene derivative could be one of the
reaction intermediates and the C–H activation of α-
acetylnaphthalene could initially occur at the C2 position rather
than the C8 position. Secondly, in the absence of alkyne, the
furan-fused product 6aa was isolated in 8% yield from the
necessary for this reaction and NaSbF
S1, entries 4 and 7). The yield of 4aa could be slightly improved
to 16% when [RuCl (p-cymene)] /AgOTf, Ag O, 2-naphthol and
,2-dichloroethane (DCE) were used (Tables S2-S4). It is known
that 2-naphthol may form the radical species by the
6
was dispensable (Table
2
2
2
1
[
8a,8c,11]
oxidation.
The rhodium-catalyzed C2–H annulation of alkyl
phenyl ketones with alkynes has also been reported to produce
reaction of 1a with 5aa (Scheme 3c). This transformation also
[
6c,6d]
*
fulvene derivatives.
Thus, we speculated that the resulting
took place without the catalyst [Cp RhCl
2
]
2
. The above results
radical species enabled to abstract the methyl hydrogen of α-
acetylnaphthalene to yield the methyl radical, trapped by the
terminal alkene (detected by high resolution electrospray
ionization mass spectrometry, ESI-HRMS) generated by C2–H
annulation of α-acetylnaphthalene with alkyne (Scheme 2b). To
verify this hypothesis, various radical initiators such as copper
salts, iron salts and potassium persulfate were screened instead
of 2-naphthol (Table 1). As expected, 4aa was obtained in 22%
suggested that this process probably underwent an electrophilic
addition of the methyl radical, generated by the oxidation of α-
acetylnaphthalene, to the terminal alkene. In addition, 5aa could
also react with 1a and 2a to afford 4aa in 50% yield with a
complete consumption of 5aa, and 6aa was not detected
(Scheme 3d). Thirdly, except forming 6aa in 9% yield, the
*
reaction of 1a, 5aa and 2a did not afford 4aa without [Cp RhCl
2
]
2
(Scheme 3e), demonstrating that the last annulation with alkyne
yield when CuBr
Cp*RhCl /AgOTf catalytic system further improved the yield of
aa to 34% yield (Table 1, entry 9). The yield of 4aa could reach
5% by increasing the amount of α-acetylnaphthalene to 5.0
2
was used (Table 1, entry 8). The
might be a C–H activation process rather than a radical approach.
[
4
5
2 2
]
equiv, which is probably attributed to the enhanced concentration
of the yielding α-carbonylmethyl radical, and meanwhile 63% of α-
acetylnaphthalene could be recovered (Table 1, entry 10).
Dropping the catalyst loading from 10 to 5 mol % could deliver
4aa in 59% yield (Table 1, entry 11).
[
a]
Table 1: Optimization of radical initiator.
Scheme 3. Control experiments.
Based on the above experiment results, the outline of the
mechanism could be divided into three sections including C2–H
activation/annulation,
radical
approach
and
C8–H
[
a] Reaction conditions: 1a (0.4 mmol), 2a (0.2 mmol), metal complex (0.01
activation/annulation. To gain more insights into the
transformation relative to the radical, radical trapping and electron
paramagnetic resonance (EPR) experiments were performed.
Radical scavengers, such as 2,2,6,6-tetramethylpiperidine
(TEMPO) and 2,6-di-tert-butyl-4-methylphenol (BHT), significantly
inhibited the reaction, suggesting the possible radical pathway
mmol), AgOTf (0.04 mmol), Ag
at 120 C under N
2
[
OTf = trifluormethanesulfonate). Cp* = 1,2,3,4,5-pentamethylcyclopentadiene.
n.d. = not detected.
2
O (0.3 mmol), radical initiator and DCE (1.0 mL)
for 12 h. [b] Isolated yield. [c] The reaction was carried out in
.0 mL of DCE at 150 C. [d] 1.0 mmol of 1a was used. [e] 0.005 mmol of
o
2
o
Cp*RhCl and 0.32 mmol of Ag O were used. (p-cymene = p-isopropyltoluene;
]
2 2
2
(
Table S6). In addition, we detected an adduct of α-
acetylnaphthalene with TEMPO by ESI-HRMS, indicating the
presence of the reactive α-acetylnaphthalene radical (Scheme 4a
and Figure S1), but the fulvene and its radical adduct were not
observed, which is probably because that the radical scavengers
could inhibit the C2–H activation/annulation of acetylnaphthalene
with alkyne to form fulvene intermediate. Subsequently, an EPR
To further understand this cascade process, a series of control
experiments were conducted (Scheme 3 and SI, Part VII). Firstly,
besides the carbohelicene 4fa, the reaction of 1f with 2a gave the
fulvene derivative 5fa in 7% yield (Scheme 3a). When 2l was
used instead of 2a, the fulvene derivative 5al was obtained in 40%
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COMMUNICATION
analysis indicated that the reaction of 1a and 5aa was EPR active, aromatic ring and sequential oxidation form I, followed by
giving the g value of 2.0029 (Scheme 4c), close to the typical g
factor of 2.0023 for organic radicals, while no obvious EPR signal
was observed in the absence of both 1a and 5aa (Figure S3). The
furan-fused product 6aa was also EPR negative under standard
reaction conditions (Figure S4). These results demonstrated that
the radical processes could be involved in this cascade reaction.
deprotonation and rhodium-catalyzed C8–H activation to yield
cyclometalated intermediate J. The carbohelicene is formed by
sequential alkyne insertion, electrophilic cyclization and
dehydration along with the release of the active Rh(III) species. In
addition, although path 1 is more reasonable, the intramolecular
electrophilic addition of F to the aromatic ring to form radical M
(
path 2) could not be excluded in the current stage.
3
320 3340 3360 3380
field (G)
Scheme 4. Radical detection experiments.
Scheme 6. Substrate scope. Standard conditions: 1 (1.0 mmol), 2 (0.2 mmol),
Cp*RhCl (0.005 mmol), AgOTf (0.04 mmol), Ag O (0.32 mmol), CuBr (0.2
mmol) and DCE (2.0 mL) at 150 C under N
cymene)]
[
]
2 2
2
2
o
o
2
for 12 h. [a] 120 C. [b] [RuCl
2
(p-
2
was used as the catalyst. [c] 24 h. [d] The ratio of lk with 2a was 2:1.
[
13]
Next, investigation of substrate scope was conducted.
As
shown in Scheme 6, the cascade reaction of α-acetylnaphthalene
allows a variety of alkynes, affording a family of aryl-substituted
[
5]carbohelicenes. The symmetrical diaryl alkynes with either an
electron-donating or electron-withdrawing group could deliver the
desired products (4ab-4aj). It is noteworthy that unsymmetrical
alkyl aryl alkyne gave a good regioselectivity (4ak). To obtain
diversely substituted and larger π-conjugated carbohelicenes, a
variety of α-acetylnaphthalene derivatives were screened. First,
the α-acetylnaphthalenes with a 4-substituent group (e.g, chloro,
phenyl, 2-thiophenyl, 2-naphthyl, and triphenylamine) could
produce the carbohelicenes (4ea-4ia). Although the α-
acetylnaphthalenes substituted by 4-Me, 4-OMe and 4-Br did not
give the desired products under the standard conditions, the
Scheme 5. Proposed mechanism. MALDI-TOF-MS: matrix-assisted laser
desorption ionization time-of-flight mass spectroscopy.
Based on the experimental results and the reported literatures,
[
7b,6c-f,10]
a pathway is proposed in Scheme 5.
Firstly, the highly
III
electrophilic [Rh Cp*] A is formed by the reaction of [Cp*RhCl ]
2 2
with AgOTf, followed by carbonyl-assisted C2–H activation of α-
acetylnaphthalene to form B. Next, B undergoes alkyne insertion,
electrophilic cyclization and dehydration to produce the fulvene
derivative E. The terminal alkene of E is attacked by the methyl
radical generated by the oxidation of α-acetylnaphthalene to yield
reaction could proceed smoothly by using [RuCl
2 2
(p-cymene)]
instead of [Cp*RhCl (4ba-4da). In other cases, the ruthenium
2 2
]
catalysis could deliver the products but with lower yields. The 7-
phenyl substituted acetylnaphthalene afforded the carbohelicene
with atropisomer and the two isomers (4ja-1 and 4ja-2) could be
separated via column chromatography in 25% and 17% yields,
respectively. Notably, 9-acetylphenanthrene could afford the
[
10]
F.
The intramolecular electrophilic addition of F to the C=O
double bond forms radical G, followed by C–C bond cleavage and
[12]
rearrangement to radical H. In the absence of alkyne, the furan
product 6aa is formed by the oxidation and deprotonation of H. In
the presence of alkyne, intramolecular radical addition to the
[
6]carbohelicene (4ka). 1-Acetylpyrene underwent the cascade
reaction to provide the larger π-conjugated carbohelicene 4la.
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Considering the chiroptical properties of carbohelicenes, chiral
separation was conducted. The two enantiomers of 4ac were
separated by a chiral high performance liquid chromatography
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in toluene (Figure 1a-b). The first portion eluted by chiral HPLC
exhibited a positive Cotton effect at 357 nm in CD spectrum. The
second portion had an opposite sign as the mirror image. On the
basis of the comparison of CD spectra with those of known
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14]
[
(
5]helicenes, the absolute configuration of 4ac is determined as
P)-(+)-4ac and (M)-(−)-4ac. The two isomers exhibited almost
the same absorption and fluorescence spectra in CH Cl (Figure
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2
1
two isomers showed two reversible oxidation waves around 0.59
V and 1.07 V for (P)-4ac and 0.61 V and 1.11 V for (M)-4ac
(
Figure 1d), indicating that the radicals could be probably formed
upon oxidation.
(4)
(
P)-4ac
(
P)-4ac
(M)-4ac
8787.
(
M)-4ac
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0
2
4
6
8
300 350 400 450 500 550
Wavelength (nm)
Retention time/min
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Abs of (P)-4ac
(P)-4ac
(M)-4ac
1
0
0
0
0
0
.0
Abs of (M)-4ac
Em of (P)-4ac
Em of (M)-4ac
.8
.6
.4
.2
.0
3
00
400
500
600
-2
-1
0
1
Wavelength (nm)
Volts vs. ferrocene/ferrocecenium (V)
Figure 1. (a) Chiral HPLC analysis of 4ac, eluted by hexane/dichloromethane =
:1 using CHIRALPAK IF. (b) Circular dichroism spectra in dry toluene. (c)
Absorption and fluorescence spectra in CH Cl (d) Cyclic voltammogram
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In conclusion, we have described the merging of C–H
activation and radical chemistry to pave an unparalleled road to
poly-substituted [5]-, and [6]carbohelicenes by using easily
available α-acetylnaphthalenes and alkynes as starting materials.
The rapid route to carbohelicenes developed herein has
exemplified the high compatibility of ionic-type and radical-type
transformations.
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This work was supported by the National NSF of China (No.
16928.
2
1772128 and 21432005) and the Fundamental Research Funds
(13) For the discussion of single crystal X-ray structures of 4aa, 4ak and 4ca,
see Part XI in the supporting information. CCDC 1862779 (4aa),
for the Central Universities (2012017yjsy108).
1862780 (4ak), 1862781 (4ca), 1862782 (5al), and 1862783 (6aa)
contain the supplementary crystallographic data for this paper. These
data can be obtained free of charge from The Cambridge
Crystallographic Data Centre.
Keywords: carbohelicene • C–H activation • radical approach •
rhodium-catalysis • annulation
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COMMUNICATION
COMMUNICATION
Jiangliang Yin, and Jingsong You*
Page No. – Page No.
Concise Synthesis of Polysubstituted
Carbohelicenes by a C–H
Activation/Radical Approach/C–H
Activation Sequence
An unparalleled ease of access to the carbohelicenes through
a C–H
activation/radical approach/C–H activation relay of α-acetylnaphthalenes with
alkynes has been achieved. The rapid route to carbohelicenes developed herein
has exemplified the high compatibility of ionic-type and radical-type transformations.
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