Build-up of double carbohelicenes using nitroarenes: Dual role of the nitro functionality as an activating and leaving group

Article abstract of DOI:10.1039/d0sc02058c

The construction of double carbohelicenes is highly fascinating yet challenging work. Disclosed herein is a streamlined and simplified synthetic route to double carbohelicenes starting from nitroarenes through sequential nitro-activated ortho-C-H arylation, denitrative alkenylation and intramolecular cyclodehydrogenation. In this synthetic strategy, the nitro group plays a dual role namely as a leaving group for the denitrative alkenylation and as an activating group for ortho-C-H arylation, which is distinct from those of aryl halides in a conventional coupling reaction. In this work, the palladium-catalyzed Heck-type alkenylation of nitroarenes has been presented, in which the conventionally inert Ar-NO2 bond is cleaved. This work provides a novel synthetic strategy for polycyclic aromatic hydrocarbons (PAHs). This journal is

Full text of DOI:10.1039/d0sc02058c

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Build-Up of Double Carbohelicenes Using Nitroarenes: Dual Role of
the Nitro Functionality as Activation and Leaving Group
Fulin Zhou, Fujian Zhou, Rongchuan Su, Yudong Yang* and Jingsong You*
Received 00th January 20xx,
Accepted 00th January 20xx
The construction of double carbohelicenes is a highly fascinating yet challenging work. Disclosed herein is a streamlined and
simplified synthetic route to double carbohelicenes starting from nitroarenes through sequential nitro-activated ortho-C−H
arylation, denitrative alkenylation and intramolecular cyclodehydrogenation. In this synthetic strategy, the nitro group plays
a dual role including as a leaving group for the denitrative alkenylation and as an activating group for ortho-C−H arylation,
which is distinct from those of aryl halides in conventional coupling reaction. In this work, the palladium-catalyzed Heck-
type alkenylation of nitroarenes has been presented, in which the conventionally inert Ar−NO2 bond is cleaved. This work
provides a novel synthetic strategy for polycyclic aromatic hydrocarbons (PAHs).
DOI: 10.1039/x0xx00000x
and the employment of di- or multiple halogenated aromatic
substrates (Scheme 1b, path I).
Recently, the development of transition metal-catalyzed
Introduction
Nonplanar polycyclic aromatic hydrocarbons (PAHs) have
attracted increasing attention in the past decade because of
reactions using unconventional coupling partners has emerged
as an attractive and challenging topic.8 The reactions involving
the cleavage of inert chemical bonds can not only replenish the
carbon–carbon bond formation toolbox but also improve the
synthetic approaches to complex structures. Nitroarenes, which
could be easily prepared by nitration of aromatics, are common
and cheap chemical feedstocks.9 Because of the strong
electron-withdrawing nature of nitro group, the ortho-position
of nitroarenes easily undergoes the C−H bond arylation with
their
unique
optoelectronic
properties,
fascinating
supramolecular characteristics and conformational dynamics,
distinct from those of planar PAHs.1 Helicenes, helical-shaped
structures with ortho-fused benzene rings, are one of the most
investigated nonplanar PAHs.2,3 In particular, considerable
effort has been devoted to the helicenes with multihelicity,
which can provide more diverse conformations, increased
nonplanarity
and
multidimensional
intermolecular
interactions.4 Among these multi-helicenes, double helicenes,
which usually exhibit effective intermolecular π−π stacking, are
regarded as the promising candidates in organic
semiconductors and chiroptical devices.5 Although a variety of
double helicenes have been reported (Scheme 1a),5-7 the
synthesis of double carbohelicenes typically suffers from
inaccessible precursors and tedious synthetic routes. Therefore,
the development of facile preparation of double carbohelicenes
starting from easily available substrates remains challenging yet
highly demanded.
(a) Selected examples of double carbohelicences
RO
OR
RO
OR
2015, Itami[7a]
2016, V. Alabugin[7c]
2012, Miao[5a]
dibenzo[g,p]chrysene
(b) Synthetic routes of double carbohelicenes
NO2
ortho C-H
arylation
NO2
Ar1
A
retrosynthetic analysis of dibenzo[g,p]chrysene, the
Ar1
H
Ar2
simplest double helicene, indicates that double carbohelicenes
could be constructed by the Heck reaction between 2-
(pseudo)halo-1,1'-biaryls and 2-vinyl-1,1'-biaryls, followed by
intramolecular cyclization of the resultant 1,2-di([1,1'-biaryl]-2-
yl)ethenes. However, both the synthesis of 2-vinyl-1,1'-biaryls
and 2-(pseudo)halo-1,1'-biaryls typically requires multi-steps
denitrative
alkenylation
path II
denitrative
(this work)
alkenylation
Ar2
Ar2
Ar1
Ar1
H
Ar1
Ar1
Ar1
Ar1
Ar2
Ar2
Ar2
path I
multiple steps
alkenylation
Y
selectivity
problem
Ar1
Y
X
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; yangyudong@scu.edu.cn
Ar1
Ar2
X, Y = Cl, Br, I, etc.
† Electronic Supplementary Information (ESI) available: [Experimental procedures,
spectroscopic data of compounds and crystallographic data in CIF or other
electronic formats. CCDC 1975224 (5d) and CCDC 1975225 (5g).]. See
DOI: 10.1039/x0xx00000x
Scheme 1 (a) Selected examples of double carbohelicenes. (b) Synthetic
routes of double carbohelicenes.
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aryl halides via palladium catalysis.10 We envisioned that, if a of 1o and 2m afforded the bis(biaryl) ethylene 5bVʹ iienw 4Ar1tic%le yOinelinlde
denitrative Heck-type reaction could be developed, the double (Scheme 3). The intramolecular oxidatDivOeI: 1c0y.1c0li3z9a/tDio0nSC0in205th8Ce
helicenes would be easily accessed from simple nitroarenes by presence of FeCl3 and DDQ resulted in an isomeric mixture of 5-
ortho-C−H arylation, two consecutive denitrative alkenylation ([1,1'-biphenyl]-2-yl)chrysene
and
9-(2-(naphthalen-2-
and intramolecular cyclodehydrogenation, which avoids the yl)phenyl)phenanthrene, which were subsequently converted
preparation of di- or multiple halogenated arenes (Scheme 1b, into [4,5]helicene 5b in the presence of TfOH/DDQ (two steps,
path II). However, because of the difficulty of oxidative addition 41% yield).
of Ar−NO2 to a metal center, nitroarenes are typically inert in
conventional transition metal-catalyzed coupling reactions.11c
Table 1 The scope of nitroarenes and olefinsa
Recently, pioneered by Nakao, the palladium-catalyzed
Pd(acac)2, X-phos, K3PO4
+
Ar
Ar
R
denitrative amination, arylation, alkylation, alknynylation and
hydrogenation of nitroarenes have been reported.10c,11 Herein
we wish to describe a streamlined and simplified synthetic
route to double carbohelicenes starting from nitroarenes based
on our investigations on the palladium-catalyzed denitrative
alkenylation of nitroarenes. During the preparation of this work,
Yamaguchi and co-workers reported a palladium-catalyzed
Mizoroki-Heck reaction of nitroarenes and styrene
derivatives.12
heptane, 140 oC, 24 h
R
NO2
1
2
3 or 4
Scope of nitroarenes
3e
R = Me, , 69%
(E/Z = 96:4)
R
3b
R = H, ,61%
3f
R = CF3, , 65%
R
(E/Z > 99:1)
(E/Z = 94:6)
R = Ph, 3g, 67%
(E/Z > 99:1)
3c
R = OMe, , 63%
(E/Z > 99:1)
Ph
Ph
Ph
3d
R = Et, , 81%
3a
, 93%
(E/Z = 95:5)
b,c
3h
R = NO2, , 54%
(E/Z > 99:1)
(E/Z > 99:1)
R
Ph
Ph
3i
R = F, , 63%
(E/Z > 99:1)
Ph
Ph
3j
R = OPh, , 51%
(E/Z > 99:1)
c
c
3l
3m
3n
, 89%
, 66%
, 42%
b,c
3k
R = NO2, , 42%
Results and discussion
(E/Z 93:7)
(E/Z > 99:1)
(E/Z = 97:3)
(E/Z = 97:3)
Initially, 1-nitronaphthalene (1a) and styrene (2a) were selected
as the model substrates for the condition optimization of
catalytic denitrative alkenylation (Table S1). After screening
various reaction parameters, an optimal catalytic system
composed of Pd(acac)2 (10 mol %), X-Phos (20 mol%), K3PO4 (3.0
equiv) and heptane as the solvent was determined to give (E)-
1-styrylnaphthalene (3a) in 93% yield (Table S1, entry 1). No
obvious amount of (Z)-isomeric product was detected by GC-MS
(E/Z > 99:1, Fig. S1). Next, we investigated the substrate scope
of the denitrative Heck-type reaction (Table 1). All the tested
substrates gave the E-products in high steroselectivity (E/Z
ratios range from 93:7 to 99:1). No obvious electronic or steric
Scope of olefins
1-Nap
4b
R = OMe, , 88%
1-Nap
1-Nap
OMe
(E/Z = 94:6)
R = tBu, , 94%
4c
Ph
R
OMe
N
(E/Z = 95:5)
4a
, 52%
(E/Z = 96:4)
4d
R = F, , 54%
4e
, 80%
(E/Z = 96:4)
(E/Z > 99:1)
1-Nap
1-Nap
1-Nap
1-Nap
S
4f
4g
4h
4i
, 65%
(E/Z = 97:3)
, 41%
(E/Z > 99:1)
, 54%
, 82%
(E/Z >99:1)
(E/Z = 97:3)
MeO
1-Nap
1-Nap
TMS
4k
4l
4j
, 62%
(E/Z = 94:6)
, 35%
(E/Z > 99:1)
, 85%
(E,E/other isomers > 99:1)
effect of the substitutions on the E/Z-selectivity was observed. a Reaction conditions: 1 (0.6 mmol), 2 (0.2 mmol), Pd(acac)2 (10 mol%), X-Phos (20
mol%), and K3PO4 (3.0 equiv) in heptane (1 mL) at 140 C under N2 for 24 h. Yield
of isolated products. E/Z ratios were determined by 1H NMR spectroscopy.
BrettPhos (20 mol%) instead of X-Phos. Nitroarene (0.2 mmol) and olefin (0.6
Nitroarenes bearing the substituents with different electronic
nature at the ortho-, meta-, or para-position could react with
styrene to give the desired products in moderate to excellent
yields (3b-3k). Nitro-substituted condensed aromatics such as
b
c
mmol). X-Phos
Brettphos
= 2-(dicyclohexylphosphino)-2',4',6'-tri-i-propyl-1,1'-biphenyl;
2-(dicyclohexylphosphino)-3,6-dimethoxy-2'-4'-6'-tri-i-propyl-1,1'-
=
9-nitrophenanthrene, 3-nitroperylene and 1-nitroperylene also biphenyl sitylene).
worked well (3l-3n). The scope of olefins was also examined. A
variety of substituted phenyl ethylenes could participate into
TMS
R
NO2
1) Pd(acac)2, X-phos
K3PO4, heptane, 140 o
C
ArBr
R
NO2
the denitrative Heck-type reaction to deliver the diaryl
ethylenes (4a-4e). Vinyl naphthalenes and heteroaromatics
such as 2-vinylpyridine and 2-vinylthiophene were
accommodable (4f-4i). Furthermore, terminal conjugated
diene, aliphatic olefin and trimethyl(vinyl)silane were also
applicable in this reaction (4j-4l).
(allyPdCl)2, PCy3 • HBF4
Ar
2) Bu4N+F-, THF, 100 o
C
Ar
2,2'-dimethylbutyric acid
K2CO3, toluene, 130 o
C
2b
Ar = Ph, , 45%
1o
R = H, Ar = Ph, , 74%
1p
R = H, Ar = 4-MeO-Ph, , 70%
R = H, Ar = 2-naphthyl, , 62%
Ar = 2-naphthyl,
2m
, 46%
1q
1r
R = H, Ar = 9-phenanthryl, , 52%
1s
R = 3-Ph, Ar = Ph, , 65%
PhBr
With the denitrative Heck-type reaction in hand, the
construction of double carbohelicences was next investigated.
With the combination of nitro-activated ortho-C−H arylation
and denitrative alkenylation, a library of 2-nitro-1,1'-biaryls and
2-vinyl-1,1'-biaryls, which are the key building blocks of double
carbohelicenes, were easily prepared starting from simple
nitroarenes (Scheme 2). Further denitrative alkenylation
between the two structural units could provide the bis(biaryl)
ethylene precursors. As a representative example, the reaction
(allyPdCl)2, PCy3 • HBF4
2,2'-dimethylbutyric acid
K2CO3, toluene, 130 o
C
NO2
NO2
1t,
71%
1a
Scheme 2 Synthesis of the key building blocks of double carbohelicenes. For
detailed reaction conditions, see the supporting information.
2 | J. Name., 2012, 00, 1-3
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leading to 5g rather than 5gʹ.13 Decreasing the amount of
oxidant or reaction temperature could not inhibit the undesired
View Article Online
DOI: 10.1039/D0SC02058C
Pd(acac)2 (10 mol %)
Ruphos (20 mol %)
FeCl3 (50 mol %)
DDQ (2.0 equiv)
+
DCM (5 mL)
65 °C N2, 1 h
CsF (3.0 equiv), heptane (1 mL)
coupling reaction. In addition, substituted double helicenes
could be easily prepared by using functionalized substrates, but
a simple filtration through a silica gel pad before iron-induced
carbocyclization was required (5e and 5f).
NO2
1o
150 °C N2, 48 h
2m
5b'
, 41%
TfOH (1 mL)
1.431
(c)
1.367
(a)
DDQ (2.0 equiv)
+
1.351
1.411
DCM (10 mL)
0 °C, N2, 0.5 h
B
-10.5
1.427
1 . 4 1 3
1.405
1.414
1.414
-11.8
-12.9
1.368
-12.6
-12.1
unseparated isomeric mixture
5b,
2 steps, 41% yield
f 1.444
1.363
1.447
-5.5
A
1.374
c
Scheme
3
Synthesis
of
[4,5]helicene
5b.
TfOH
=
1.389 e
1.378
1.463
1.407
1.455
1.407
d
b
trifluoromethanesulfonic acid; DDQ = 2,3-dichloro-5,6-dicyano-1,4-
benzoquinone; Ruphos = 2-dicyclohexylphosphino-2',6'-di-i-propoxy-
1,1'-biphenyl.
-5.7
C
1.420
1.408
1.412
1.394
a
1.380
-10.4
1.373
1.452
1.448
1.426
D
1.404
1.425
1.371
Ar2
1.408
Ar2
1.351
1) Pd(acac)2, Ruphos, CsF
heptane, 150 C, 48 h
Ar1
(b)
Ar1
(d)
Ar2
Ar3
-1.55 eV
+
Ar3
Ar1
Ar3
B
2) FeCl3, DDQ, DCM, 65 °C, 1 h
3) TfOH, DDQ, DCM, 0 °C, 0.5 h
Ar4
D
109.24
129.25
NO2
Ar4
Ar4
via
3 . 0 0 2
60.24
3.65 eV
1
2
5

Å
-5.20 eV
C
A
HOMO
LUMO
b
(e)
a
5a
5b
5c
,11%
, 29%
, 21%
c
2.784 Å
2.931 Å
(P,P)
Ph
2.992 Å
2.833 Å
5d,
21%
5e
, 22%
2.931 Å
3.676 Å
3.676 Å
face-to-face
edge-to-face
OMe
2.784 Å
2.833 Å
2.992 Å
(M,M)
5g
5g'
, not detected
, 20%
5f,
15%
(f)
c
b
Scheme 4 One pot synthesis of double carbohelicenes. Reaction conditions:
1) 1 (0.2 mmol), 2 (0.3 mmol), Pd(acac)2 (10 mol%), Ruphos (20 mol%), and
CsF (3.0 equiv) in heptane (1 mL) at 140 °C under N2 for 48 h; 2) FeCl3 (50
mol%), and DDQ (0.4 mmol) in DCM (5.0 mL) at 65 °C under N2 for 1 h; and 3)
DDQ (0.4 mmol), TfOH (1.0 mL) and DCM (5.0 mL) at 0 °C under N2 for 0.5 h.
a The third step was conducted -10 °C.
c
a
In order to avoid troublesome isolation, a three-step, one pot
procedure was developed. This process allowed the rapid
construction of structurally diverse double carbohelicenes,
including [4,4]helicene (5a), [4,5]helicene (5b), [4,6]helicene
(5c), and [5,5]helicene (5d) (Scheme 4).13 Notably, the double
carbohelicene 5d was previously synthesized via sequential ICl-
mediated aromatization and Pd-catalyzed intramolecular
Fig. 1 (a, e) Crystal structure of (M,M)-5d. (b) ORTEP drawing of (M,M)-5d
with 50% probability. (c) Calculated ACID (only contributions from π-electrons
of 5d are considered) plots and NICS (0) values of 5d at the B3LYP/6-31G(d)
level. (d) Molecular orbitals of 5d. (f) Crystal packings of 5d. (P,P)-isomer,
yellow; (M,M)-isomer, blue.
The crystals of 5d were grown by slowly volatilizing the
saturated dichloromethane solution and the helical structure
was clearly confirmed by X-ray crystallographic analysis (Fig.
1a).13 The geometry of helicene 5d slightly deviates from the C2
symmetry, which is reflected by the unequal torsion angles of
two [5]helicene substructures. The dihedral angles between the
terminal rings A and D, and rings B and C, are 60.2 and 60.5,
respectively (Fig. 1b), which are larger than those of recently
coupling
starting
from
1,2-bis(2-(naphthalen-2-
yl)phenyl)ethyne, which was typically prepared by 1,2-
dihalobenzene.14 Our protocol enables the synthesis of double
carbohelicenes from simple nitroarenes rather than di- or
multiple halogenated arenes. In the reaction of 9-(2-
nitrophenyl)phenanthrene and 2-vinyl-1,1'-biphenyl, further
coupling at the 1-position of phenanthrene was observed,
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 3
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presented double carbo[5]helicene (47.8)7b and OBO-fused thermodynamic stability, but are more stable than the (P,M)-
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double [5]helicene (48).5d Additionally, because of the enantiomer. This result coincides with the fact that only (P,P)-
DOI: 10.1039/D0SC02058C
repulsion of terminal benzene rings, the central naphthalene and (M,M)-enantiomers were observed in the single crystals of
ring is distorted with the dihedral angles of 32.0 for a-b-e-d and 5d, whereas no (P,M)-enantiomer was observed. The (P,P)-to-
31.8 for c-b-e-f, which are among the largest twisting (M,M) interconversion proceeds through the transition state
deformations of naphthalene ring in nonplanar PAH.15 The TS-1 and TS-2, in which the terminal benzene rings stretch
embedded naphthalene unit (C−C bond lengths: 1.39−1.46 Å) outward and bend inward. The calculated activation free energy
possesses less aromaticity than the pristine naphthalene (approximately 33.0 kcalmol-1) was much higher than that of
(1.36−1.42 Å) presumably owing to the twisted structure. This carbo[5]helicene (22.9 kcalmol-1),16 and slightly higher than
observation is consistent with the computational results of the that of the [5,5]helicenes reported by Miao (approximately 28.6
nucleus-independent chemical shift (NICS) and anisotropy of kcalmol-1)5a and Kamikawa (31.8 kcalmol-1).7b Because the
the induced current density (ACID) (Fig. 1c). As shown in Fig. 1d, calculated activation free energy approximates to that of
the HOMO and LUMO energies of 5d were estimated by carbo[6]helicene (35.0 kcalmol-1),16 the racemization process
molecular orbital (MO) calculation to be -5.20 and -1.55 eV, of 5d might be slow.
respectively, corresponding to a HOMO-LUMO gap of 3.65 eV,
which is larger than that of the carbo[5]helicene (3.41 eV) and
double carbo[5]helicenes reported by Miao (2.73 eV)5a and
Kamikawa (2.92 eV).7b This result might be attributed to the
decreased π-conjugation induced by the significant geometric
distortion of the central naphthalene ring. In the crystal packing
of 5d, one unit cell contains four pairs of (P,P)- and (M,M)-
enantiomers (Fig. 1e). The homochiral isomer layers lying on the
bc plane stack alternatively along the a-axis (Fig. 1f), with a
slipped face-to-face - distance of 3.67 Å to the adjacent
heterochiral layer (Fig. 1e). In addition, every molecule interacts
with the neighboring homochiral molecules through edge-to-
face contact within the range of 2.78 to 2.99 Å (Fig. 1e).
Conclusions
In conclusion, we have presented the promising potential of
nitroarenes in the construction of polycyclic aromatic
hydrocarbons (PAHs) based on our investigations on the
palladium-catalyzed denitrative alkenylation reaction. The nitro
group can act as a leaving group for the denitrative alkenylation
and an activating group for ortho-C−H arylation, which provides
a rapid access to double carbohelicenes through sequential
nitro-activated ortho-C−H arylation, two consecutive
denitrative
alkenylation
and
intramolecular
cyclodehydrogenation. Further applications of this strategy in
the construction of other twisted and π-extended aromatic
systems are undergoing in our laboratory.
The UV-vis absorption and fluorescence emission in
dichloromethane of 5a-5g were investigated (Fig. S3). All the
double carbohelicenes exhibit multiple absorption and blue
fluorescence emission with the peaks in the range of 324–374
nm and 394-455 nm, respectively. A clear redshift of emission
was observed with the extension of -system (Table S2, 5a-5c).
The incorporation of functional groups such as phenyl and
methoxy on the terminal rings of helicene could improve the
fluorescence quantum yield (Table S2, 5a, 5b, 5e and 5f), which
would be valuable for the future application of double
carbohelicenes in optoelectronics and photonics.
Conflicts of interest
There are no conflicts to declare.
Acknowledgements
This work was supported by grants from the National Natural
Science Foundation of China (No. 21772128 and 21502123). We
appreciate the support from the Comprehensive Training
Platform of Specialized Laboratory, College of Chemistry,
Sichuan University.
TS-1
TS-2
33.1
33.0
Notes and references
1
(a) Y. Segawa, H. Ito and K. Itami, Nat. Rev. Mater., 2016, 1,
15002; (b) M. Rickhaus, M. Mayor and M. Juríĕk, Chem. Soc.
Rev., 2017, 46, 1643; (c) S. H. Pun and Q. Miao, Acc. Chem.
Res., 2018, 51, 1630; (d) M. A. Majewski and M. Stępień,
Angew. Chem., Int. Ed., 2019, 58, 86.
(P,M)
6.1
(M,M)
(P,P)
2
3
4
(a) Y. Shen and C.-F. Chen, Chem. Rev., 2012, 112, 1463; (b) M.
Gingras, Chem. Soc. Rev., 2013, 42, 968; (c) M. Gingras, G. Félix
and R. Peresutti, Chem. Soc. Rev., 2013, 42, 1007.
(a) M. Gingras, Chem. Soc. Rev., 2013, 42, 1051; (b) W.-L.
Zhao, M. Li, H.-Y. Lu and C.-F. Chen, Chem. Commun., 2019,
55, 13793.
For reviews, see: (a) M. Ball, Y. Zhong, Y. Wu, C. Schenck, F.
Ng, M. Steigerwald, S. Xiao and C. Nuckolls, Acc. Chem. Res.,
2015, 48, 267; (b) C. Li, Y. Yang and Q. Miao, Chem. Asian J.,
0.0
0.0
Gibbs free energy (kcal•mol-1
)
Fig. 2 Isomerization process from (P,P)-5d to (M,M)-5d.
The thermal stability and isomerization process of 5d were
evaluated by DFT calculations at the B3LYP/6-31G(d) level (Fig.
2). The (P,P)- and (M,M)-enantiomers possess the same
4 | J. Name., 2012, 00, 1-3
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DOI: 10.1039/D0SC02058C
Table of Contents
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Build-Up of Double Carbohelicenes Using Nitroarenes: Dual Role of the Nitro
Functionality as Activation and Leaving Group
Fulin Zhou, Fujian Zhou, Rongchuan Su, Yudong Yang* and Jingsong You*
Ar2
H
Ar2Br
Ar2
Ar1
Ar1
NO2-activated
ortho C-H arylation
Ar1
NO2
1) denitrative
alkenylation
NO2
Ar3
2) intramolecular
dehydrogenative
cyclization
O2
N
Ar4
Ar3
TMS
Ar3
denitrative
alkenylation
Ar4
Ar4
Dual role of -NO2: activating group and leaving group
Disclosed herein is a concise route to double carbohelicenes starting from nitroarenes, in which
the nitro group serves as an activating group for ortho-C−H arylation and a leaving group for the
denitrative alkenylation.