Modular metal-free catalytic radical annulation of cyclic diaryliodoniums to access π-extended arenes

Article abstract of DOI:10.1039/d0gc04183a

Polycyclic aromatic hydrocarbons (PAHs) are of increasing importance in the advanced material field. Cyclic diaryliodonium salts are non-toxic and environmentally benign arylating reagents. Here, we describe an alkylamine-mediated free radical intramolecular annulation to access PAHs under environmentally friendly conditions. On modulating substituents and their positions in the iodoniums, the free radical reaction controllably underwent three types of cyclization including ring contraction and ring switch to form tricyclic and tetracyclic frameworks of PAHs. Preliminary mechanistic studies implied that alkylamines initiated a radical pathway to complete the cyclization efficiently. Moreover, these acquired products were further converted into diverse complex graphene segments.

Full text of DOI:10.1039/d0gc04183a

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Modular metal-free catalytic radical annulation of
cyclic diaryliodoniums to access π-extended
arenes†
Cite this: Green Chem., 2021, 23,
1972
Received 10th December 2020,
Accepted 10th February 2021
Daqian Zhu, a,b Hui Peng,b Yameng Sun,b Zhouming Wu,b Yun Wang,b
b
Bingling Luo,b Tiantian Yu,b Yumin Hu,b Peng Huang*b and Shijun Wen
*
DOI: 10.1039/d0gc04183a
rsc.li/greenchem
Polycyclic aromatic hydrocarbons (PAHs) are of increasing impor- closing metathesis,6 and metal-catalyzed Suzuki–Miyaura cross-
tance in the advanced material field. Cyclic diaryliodonium salts coupling/intramolecular cyclization.7 These approaches suffer
are non-toxic and environmentally benign arylating reagents. low yields, byproduct formation, limited substrate scope, and
Here, we describe an alkylamine-mediated free radical intra- involvement of sensitive starting materials. Recently, metal-cata-
molecular annulation to access PAHs under environmentally lyzed annulative π-extension arylation of simple aromatic sub-
friendly conditions. On modulating substituents and their positions strates for constructing fused PAHs has been reported by
in the iodoniums, the free radical reaction controllably underwent Itami,8 Glorius,9 Sanford10 and other groups.11 However, this
three types of cyclization including ring contraction and ring arylation strategy still has some limitations such as low reactiv-
switch to form tricyclic and tetracyclic frameworks of PAHs. ity and poor regioselectivity. Effective and convenient accesses
Preliminary mechanistic studies implied that alkylamines initiated a to diverse PAHs are highly appealing.
radical pathway to complete the cyclization efficiently. Moreover,
Over the past few years, diaryliodonium salts have been used
these acquired products were further converted into diverse as mild, nontoxic, and selective arylating agents.12 While linear
complex graphene segments.
diaryliodoniums have generated unwanted aryl iodide waste,
which is against sustainable chemistry in terms of atom
economy, cyclic diaryliodoniums overcome such an issue. More
recently, cyclic diaryliodoniums have shown great promise to
take advantage of their intrinsic double-arylation reactivity to
construct pharmaceutically active compounds,13 fluorescent
chemicals,14 material molecules,15 and axially chiral biarylio-
dides.16 Transition-metal-catalyzed annulation between cyclic
diaryliodoniums and arenes has been reported for the construc-
tion of π-extended arenes under hash temperature conditions
(Scheme 1a).17 Moreover, transition metals have some draw-
backs including high cost, toxicity, and requirement to remove
the metal impurity from the final products. Organocatalysis
offers some advantages over transition-metal catalysts, and
most organocatalysts are stable in air and water, easily handled,
relatively nontoxic, and readily separated from the crude reac-
tion mixture.18 The free radical reaction is widely explored and
studied due to its rapid process and wild reaction characteristics
at the same time.19 The organocatalyst-mediated free radical
reaction through a controllable pathway would provide promis-
ing approaches to build PAHs from cyclic diaryliodoniums
without the use of costly transition metals.
Introduction
Polycyclic aromatic hydrocarbons (PAHs) have attracted con-
siderable attention in functional material science due to their
unique physical and chemical properties.1 PAHs exhibit high
electron density, low oxidation–reduction potentials, strong π–π
interactions, and good chemical stability.2 Consequently, these
π-extended arenes are often used as key components in photo-
electronic devices, including organic light-emitting diodes,
organic field-effect transistors and organic photovoltaics. PAH-
derived compounds have also been developed as fluorescent
probes for bioimaging.3 Therefore, it is of high importance to
develop effective synthetic methods to access these functiona-
lized PAH derivatives. Traditional synthetic routes to PAHs have
included Diels–Alder reactions,4 alkyne cyclizations,5 ring-
aSchool of pharmacy, Guangdong Pharmaceutical University, 280 Waihuan East
Road, Guangzhou 510006, China
bState Key Laboratory of Oncology in South China, Collaborative Innovation Center
for Cancer Medicine, Sun Yat-sen University Cancer Center, 651 Dongfeng East Road,
Guangzhou 510060, China. E-mail: wenshj@sysucc.org.cn,
huangpeng@sysucc.org.cn
Considering that amines can be used as single-electron
transfer reagents,20 we envisioned that a combinational use of
amines and cyclic diaryliodoniums could lead to unpre-
cedented reactions. The oxidative state of hypervalent iodine
†Electronic supplementary information (ESI) available. See DOI: 10.1039/
d0gc04183a
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Table 1 Optimization of the reaction conditionsa
Entry
Amine
Base
Solvent
Yieldc (%)
1
2
3
4
5
6
7
8
TEAb
N.A.
N.A.
N.A.
N.A.
N.A.
N.A.
N.A.
N.A.
N.A.
N.A.
N.A.
N.A.
N.A.
N.A.
N.A.
N.A.
Na2CO3
K2CO3
NaOH
Na2CO3
Na2CO3
Na2CO3
Na2CO3
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
H2O
57
52
66
61
65
0
12
45
51
52
84
5
80
81
81
DEAb
b
t-BuNH2 b
n-BuNH2
PDAb
Anilineb
b
t-BuNH2b
t-BuNH2b
t-BuNH2b
t-BuNH2b
t-BuNH2b
t-BuNH2b
t-BuNH2b
t-BuNH2b
t-BuNH2
t-BuNH2
t-BuNH2
t-BuNH2
t-BuNH2
N.A.
EtOH
9
iPrOH
nBuOH
EtOAc
Anisole
Sulfolane
iPrOAc
nBuOAc
EtOAc
EtOAc
EtOAc
EtOAc
EtOAc
10
11
12
13
14
15
16
17
18
19
20
21e
22f
23g
Scheme 1 Approaches for the synthesis of polycyclic aromatic hydro-
carbons from cyclic diaryliodoniums.
14
88 (85d)
83
41
would render cyclic diaryliodoniums vulnerable to accept an
electron from electron-rich tert-butylamine, leading to the
formation of free radical intermediate I-1 or I-2 (Scheme 1b).
There would be three-way intramolecular annulations taking
place in I-1 and I-2. If we modulate substituents and their
positions in cyclic diaryliodoniums, the following annula-
tions could be tuned in the designed directions. As a result,
PAHs A, B and C would be constructed and the accepted elec-
tron from tert-butylamine could be released and re-used for
the next round of free radical generation. Herein, we report a
tert-butylamine-catalyzed free radical reaction for accessing
structurally diverse π-extended arenes via intramolecular
annulation of cyclic diaryliodoniums. Our approach can
realize the substituent-guided ring contraction and ring
switch of cyclic diaryliodoniums with environmentally
friendly ethyl acetate as the solvent. Moreover, the released
iodide was further employed to build more complex graphene
segments.
N.D.
N.D.
N.D.
N.D.
t-BuNH2
t-BuNH2
t-BuNH2
EtOAc
EtOAc
EtOAc
a Reaction conditions: cyclic diphenyliodonium 1a (0.2 mmol), amine
(0.1 equiv.), base (2 equiv.), solvent (1 mL), 100 °C, 24 h, argon, sealed
tube. b 2 equiv. c NMR yield. d Isolated yield. e 2 equiv. TEMPO added.
f 2 equiv. BHT added. g 2 equiv. 1,1-diphenylethylene added. N.A.: not
added. N.D.: not detected. TEA: triethylamine; DEA: diethylamine;
PDA: 1,3-propyldiamine.
best choices with tert-butylamine as the organocatalyst (entries
7–15 and Table S2†). The reaction efficiency was obviously
affected by various combinations of tert-butyl amine and bases
(entries 16–19 and Table S3†). In the presence of Na2CO3, tert-
butylamine could be decreased to a catalytic amount (entries
16 and 17), indicating the importance of a base. K2CO3 was
also effective; however, the strong base NaOH gave a low yield
(entries 18 and 19 and Table S3†). When tert-butylamine was
removed, no 2a was observed, indicating the indispensable
role of tert-butylamine (entry 20). When the typical radical sca-
vengers tetramethylpiperidinooxy (TEMPO), butylated hydroxy-
Results and discussion
We started our investigation by performing reactions of cyclic toluene (BHT), and 1,1-diphenylethylene were subjected to the
diaryliodonium 1a in the absence of a metal catalyst to gene- standard reaction conditions, the formation of 2a was substan-
rate a fluorene motif that is present in simple PAHs (Table 1). tially inhibited (entries 21–23). It is worth noting that radical
To our delight, iodofluorene 2a could be obtained in 57% yield trapping intermediates of 1a with either BHT or 1,1-diphenyl-
when 1a was treated with triethylamine (entry
Table S1†). The following screening showed that other alkyl
1
and ethylene were detected by EI-MS (Fig. S1†).
With the optimized reaction conditions in hand, the scope
amines also facilitated the reactions to give the desired of cyclic diaryliodoniums was investigated (Scheme 2).
product, but aniline failed to undergo the conversions Iodoniums bearing one or two methyl groups gave the desired
(entries 2–6 and Table S1†). In order to select more sustainable iodofluorenes in good yields (2a, 2b). Simple cyclic diaryliodo-
solvents, some solvents recommended by the “CHEM21 selection nium 1c with no substituent also underwent the transform-
guide” were tested.21 EtOAc was demonstrated as one of the ation smoothly (2c). Electron-withdrawing groups, including
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to form another five-membered ring. The outcome might be
the formation of a new ring switching from the iodonium ring.
Thus, a series of five-member iodoniums 3 were designed and
synthesized (Scheme 3). When subjected to the above standard
conditions, the expected fluorenes were successfully con-
structed indeed (4a–4c). Heterocyclic iodonium was also suit-
able for the ring switch (4d).
Since an aryl tethered at the ortho position to the hyperva-
lent iodine of five-membered iodoniums 3 was able to accept
the free radical attack, we wondered whether an aryl installed
on the counterclockwise meta position would realize a similar
annulation. A series of five-membered iodoniums 5 were
designed and synthesized. When subjected to the standard
reaction conditions, phenyl directly linked to the aryl side of
the iodonium indeed enabled the formation of a new six-mem-
bered ring through
a ring switch pathway (Scheme 4).
Regardless of the presence and electron properties of substitu-
ents on the iodoniums, the ring expansion proceeded
smoothly, resulting in the formation of triphenylenes (6a–6h).
Furthermore, thiophene sided iodonium also gave the
expected product in a moderate yield (6i). Moreover, while the
phenyl was installed via methylene to the iodonium side, the
seven-membered ring was able to be formed successfully
(6j–6l), indicating the generality of the ring switch approach.
Based on our results, a plausible radical pathway for the
ring contractive annulation of cyclic diaryliodoniums 1 to
produce 2 was proposed (Scheme 5). Hypervalent iodonium 1
grabbed an electron donated by t-BuNH2 to generate radical
1-i, and subsequent rapid intramolecular annulation yielded the
new radical 1-ii. The following electron transfer to the positive
tert-butylamine radical species generated the positive arene
Scheme 2 Substrate scope of six-membered and large ring size diary-
liodoniums 1 for intramolecular ring contractive annulation. Reaction
conditions: cyclic diaryliodoniums 1 (0.2 mmol), Na2CO3 (2.0 equiv.),
tert-butylamine (0.1 equiv.), EtOAc (1.0 mL), 100 °C, 24 h, argon, sealed
tube.
fluoro, chloro, trifluoromethyl, cyano, and CO2Me, were com- species 1-iii. Meanwhile, tert-butylamine was regenerated to
patible to afford the desired products (2d–2h). In addition, the initiate the next cycle of radical transformation. The presence
iodonium functionalized with a combination of an electron- of a base facilitated the proton removal from 1-iii to generate
donating group (MeO) and an electron-withdrawing group product 2. However, a similar ring contraction with five-mem-
(CO2Me) also successfully generated a more complex product bered iodoniums 3 and 5 would be energy-unfavourable to
(2i). Moreover, an iodonium with a benzene ring positioning form a four-membered ring. They instead underwent a ring
on the methylene underwent the reaction smoothly to provide switch to generate products 4 and 6 (Scheme S1†).
the fluorene derivative (2j). Furthermore, heterocyclic iodo-
nium was also applicable to give the expected heterocyclic skel-
eton (2k). Since this intramolecular ring contractive annulation
of six-membered cyclic diaryliodoniums was realized, seven-
membered cyclic diaryliodonium 1l has also been explored.
When subjected to the reaction conditions, 9,10-dihydrophe-
nanthrene iodide was obtained in a moderate yield (2l).
However, a larger size cyclic diaryliodonium failed to form the
expected product (2m).
Compared with six-membered and other large ring size dia-
ryliodoniums, five-membered diaryliodoniums are more fre-
quently employed in hypervalent iodine chemistry. However,
simple five-membered diphenyliodonium failed to undergo a
ring contraction to form a four-membered product, likely due
to the high energy barrier for the opposite aryl to attack the
initiated aryl free radical (Scheme S1†). We envisioned that
Scheme 3 Substrate scope of five-membered diaryliodoniums 3 for
intramolecular ring switch. Reaction conditions: 3 (0.2 mmol), Na2CO3
installation of a tethered aryl ortho to the hypervalent iodine
(2.0 equiv.), tert-butylamine (0.1 equiv.), EtOAc (1.0 mL), 100 °C, 24 h,
could be possible to accept in situ generated free radical attack argon, sealed tube.
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Scheme 6 Construction of graphene segments using 6a. Reaction
conditions: (i) Suzuki–Miyaura cross-coupling: 6a (0.1 μmol), arylboronic
acid (1.2 equiv.), K3PO4 (2.5 equiv.), DMF (1 mL), 90 °C, 12 h, argon.
(ii) Scholl reaction: FeCl3, dichloromethane, 0 °C.
Scheme 4 Substrate scope of five-membered diaryliodoniums 5 for
intramolecular ring switch and ring expansion. Reaction conditions: 5
(0.2 mmol), Na2CO3 (2.0 equiv.), tert-butylamine (0.1 equiv.), EtOAc
(1.0 mL), 100 °C, 24 h, argon, sealed tube.
scale provided 6a, which was further used for Suzuki–Miyaura
cross-coupling with three different boronic acids to generate
products 7a–7c. Following FeCl3 oxidation Scholl reactions,23
small graphene nanoribbon segments were prepared (8a–8c).
Conclusions
In summary, a sustainable protocol to access polycyclic
π-extension aromatic hydrocarbons was established via a
tBuNH2-catalyzed intramolecular free radical annulation of
nontoxic and environmentally benign cyclic diaryliodoniums.
This unprecedented method enables the preparation of PAHs
with a high tolerance of functional groups in the absence of
transition-metal catalysts, while ethyl acetate was used as a
safe and clean solvent. By modulating substituents and their
positions on the iodoniums, various polycyclic π-extension aro-
matic hydrocarbon structures were successfully obtained via
ring contraction or ring switch. The resulting PAHs bear an
intact iodide functional group, serving as a versatile platform
for further transformation to construct graphene nanoribbons.
Scheme 5 The proposed mechanism exemplified by the transform-
ation from 1 to 2.
The electronic properties of PAHs depend on the degree of
π-extension, the fusion topology, and the periphery.22 The
unique feature of our synthetic PAHs was the presence of
iodide, providing an opportunity for further structure derivati-
zation to prepare more complex PAHs. As shown in Scheme 6,
Conflicts of interest
the scale-up reaction using iodonium 5a over the one gram There are no conflicts to declare.
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