Intramolecular Remote C-H Activation via Sequential 1,4-Palladium Migration to Access Fused Polycycles

Article abstract of DOI:10.1021/acs.orglett.9b02392

An unprecedented intramolecular remote C-H activation via sequential 1,4-palladium migration with an aromatic ring as a conveyor has been described. This reaction provides an efficient route to construct diverse polycyclic frameworks in moderate to good yield via palladium-catalyzed remote C-H activation/alkene insertion, arylation, alkenylation, and the Heck reaction. The preliminary mechanistic studies revealed that the 1,4-palladium migration process was reversible.

Full text of DOI:10.1021/acs.orglett.9b02392

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
Intramolecular Remote C−H Activation via Sequential 1,4-Palladium
Migration To Access Fused Polycycles
Panpan Li, Qiuyu Li, He Weng, Jiaming Diao, Hequan Yao,* and Aijun Lin*
State Key Laboratory of Natural Medicines (SKLNM) and Jiangsu Key Laboratory of Bioactive Natural Product Research,
Department of Medicinal Chemistry, China Pharmaceutical University, Nanjing 210009, P. R. China
S
* Supporting Information
ABSTRACT: An unprecedented intramolecular remote C−H activation via sequential 1,4-palladium migration with an
aromatic ring as a conveyor has been described. This reaction provides an efficient route to construct diverse polycyclic
frameworks in moderate to good yield via palladium-catalyzed remote C−H activation/alkene insertion, arylation, alkenylation,
and the Heck reaction. The preliminary mechanistic studies revealed that the 1,4-palladium migration process was reversible.
n the past two decades, palladium-catalyzed C−H
I
functionalization has provided an efficient method for
constructing carbon−carbon or carbon−heteroatom bonds,1
which has been widely employed to the rapid synthesis of
natural products and biologically active compounds.2 In this
context, σ-chelation-directed ortho-C−H activation of aromatic
compounds via a conformationally rigid five- or six-membered
palladacycle intermediate has been substantially explored;3
however, the selective activation of remote C−H bonds (meta
or para position) of arenes is still a difficult task due to the
distance and geometry within the substrates. With the aid of
the innate steric or electronic bias of substrates, Yu4a and
Hartwig4b independently achieved the palladium-catalyzed
meta-C−H functionalization of aromatic compounds (Figure
1a). In 2012, Yu and coworkers innovatively developed a U-
shaped template that could well overcome the constraint of
spatial geometry and accomplished the meta-C−H function-
alization of arenes (Figure 1b).5a Subsequently, Yu,5b−i Tan,6
Maiti,7 Li,8 Jin,9 and Kong10 further established the versatility
of this U-shaped template in the meta-C−H functionalization
of arenes. In addition, Maiti and coworkers described a
palladium-catalyzed remote para-C−H functionalization of
arenes with a D-shaped biphenyl template-based assembly
(Figure 1b).11 In 2015, Yu and coworkers creatively disclosed a
palladium-catalyzed, ligand-enabled meta-C−H activation
employing norbornene as a transient mediator (Figure 1c).12
Almost simultaneously, Dong and coworkers reported a simple
amine-directed meta-C−H arylation via Pd/norbornene
catalysis with Ph3As as the ligand (Figure 1c).13 The use of
norbornene as a mediator was then extensively applied in the
palladium-catalyzed meta-C−H functionalization of arenes.14
These foregoing and remarkable works have inspired chemists
to unremittingly explore other more strategies or models to
achieve the remote C−H functionalization of aromatic
Figure 1. Palladium-catalyzed remote C−H functionalization of
arenes. DG = directing group. FG = functional group.
compounds. Herein we describe an unprecedented intra-
molecular remote C−H activation via sequential 1,4-palladium
Received: July 11, 2019
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.9b02392
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Letter
a
Table 1. Optimization of the Reaction Conditions
Scheme 1. Substrate Scope of Palladium-Catalyzed Remote
C−H Activation/Alkene Insertion Reaction
a b
,
b
entry
deviation from standard conditions
yield (%)
1
none
84
2
3
4
5
6
7
8
9
10
11
12
13
14
without Pd(OAc)2
without KOPiv
n.d.
n.d.
71
62
51
74
70
77
33
49
73
75
n.d.
PdCl2 instead of Pd(OAc)2
Pd(dba)2 instead of Pd(OAc)2
Pd(PPh3)4 instead of Pd(OAc)2
PPh3 instead of Davephos
TFP instead of Davephos
PCy3 instead of Davephos
Xantphos instead of Davephos
DPPE instead of Davephos
CsOPiv instead of KOPiv
KOAc instead of KOPiv
reducing the temperature to 80 °C
a
Standard reaction conditions: 1a (0.2 mmol), Pd(OAc)2 (10.0 mol
%), Davephos (10.0 mol %), KOPiv (0.4 mmol), DMF (2.0 mL), 100
b
°C, 24 h, Ar atmosphere, sealed tube. Yield of isolated 2a. TFP:
tri(2-furyl)phosphine.
migration15 with an aromatic ring as a conveyor to construct
fused polycyclic compounds (Figure 1d).
Initially, we commenced our study with 2-iodo-5-((2-
methylallyl)oxy)-1,1′-biphenyl 1a as the model substrate for
the palladium-catalyzed remote C−H activation/alkene
insertion reaction. After an intensive investigation (see the
Supporting Information (SI) for details), we found that the
dihydrophenanthrofuran 2a could be ultimately obtained in
84% yield in the presence of Pd(OAc)2 (10.0 mol %),
Davephos (10.0 mol %), and KOPiv (2.0 equiv) as the base in
DMF at 100 °C under an argon atmosphere (Table 1, entry 1).
Without the palladium catalyst or the base, no desired product
was detected (Table 1, entries 2 and 3). Employing PdCl2,
Pd(dba)2, or Pd(PPh3)4 to replace Pd(OAc)2 resulted in a low
transformation of 1a (Table 1, entries 4−6). A range of
phosphine ligands were also tested, delivering 2a in a
diminished yield (Table 1, entries 7−11). Other bases, such
as CsOPiv and KOAc, slightly impacted the yield of 2a (Table
1, entries 12 and 13). Reducing the reaction temperature to 80
°C, no reaction occurred (Table 1, entry 14).
With the optimized conditions established, we then explored
the substrate scope of this palladium-catalyzed remote C−H
activation/alkene insertion reaction, and the results are shown
in Scheme 1. Decorated 2-iodobiaryls 1 bearing electron-
donating or electron-withdrawing substituents at the para
position of the Ar1 ring offered the corresponding products
2a−f in moderate to good yield. The substrate with a methyl
group at the meta position of the Ar1 ring gave 2g as a single
regioisomer in 45% yield, and the structure was determined by
heteronuclear multiple bond correlation (HMBC) analysis.
(See the SI for details.) The ortho-substituted Ar1 substrates
(1h and 1i) were also compatible candidates, delivering 2h and
2i in good yield. The substrates 1j and 1k with naphthalene
and pyrrole ring occurred smoothly in this sequential 1,4-Pd
migration process. The tethered alkenes with various
substituents were then investigated, and the products 2l−q
a
Reaction conditions: X = I, 1 (0.2 mmol), Pd(OAc)2 (10.0 mol %),
Davephos (10.0 mol %), KOPiv (0.4 mmol), DMF (2.0 mL), 100 °C,
b
c
d
24 h, Ar atmosphere, sealed tube. Isolated yields. 48 h. 1.0 mmol.
e
X = Br, 48 h.
were achieved in 51−80% yield. Substrates 1r−w with the
nitrogen atom as a linker delivered tetrahydronaphthoindole 2r
in 86% yield and dihydronaphthoindolones 2s−w in 72−84%
yield. The 2-brombiaryl 1r′ provided 2r in 45% yield, even
when the reaction time was extended to 48 h. Two additional
isomers of 1s, such as N-(2-iodo-[1,1′-biphenyl]-3-yl)-N-
methylmethacrylamide and N-(2′-iodo-[1,1′-biphenyl]-3-yl)-
N-methylmethacrylamide, were also synthesized to explore the
reactivity, and the product 2s was achieved in 8 and 81% yield
under our standard reaction conditions, respectively. (See the
SI for details.) It should be noted that when N-ally and N-
B
DOI: 10.1021/acs.orglett.9b02392
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Table 2. Palladium-Catalyzed Remote C−H Activation/
Scheme 2. Mechanistic Experiments
a b
,
Arylation, Alkenylation, and Heck Reaction
Scheme 3. Proposed Mechanism
a
Reaction conditions: 3 (0.2 mmol), Pd(OAc)2 (10.0 mol %),
Davephos (10.0 mol %), KOPiv (0.4 mmol), DMF (2.0 mL), 100 °C,
b
24 h, Ar atmosphere, sealed tube. Isolated yields.
methacrylamide existed simultaneously, product 2u was
preferentially generated with the allyl group untouched.
After exploring the palladium-catalyzed remote C−H
activation/alkene insertion reaction, we then turned our
attention to expand this strategy in the remote C−H
activation/arylation process. The products 1-phenyldibenzo-
[b,d]furan 4a, 4-phenyl-9H-carbazole 4b, and fluoranthene 4c
could be obtained in good yield (Table 2, entries 1−3).
C
DOI: 10.1021/acs.orglett.9b02392
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Substrates 3d and 3e delivered the products 4d and 4e in
moderate yield (Table 2, entries 4 and 5). The potential C−N
coupling side reaction to offer phenoxazine and phenazine did
not occur. A remote C−H activation/alkenylation was also
explored, and the dihydrocarbazolone 4f was isolated in 44%
yield (see the SI for the detailed reaction pathway) (Table 2,
entry 6). The palladium-catalyzed remote C−H activation/
Heck reaction provided indole derivative 4g in moderate yield
(see the SI for the detailed reaction pathway) (Table 2, entry
7). Product 4h was also acquired in an acceptable yield via a
complicated palladium-catalyzed remote C−H activation/
redox-relay Heck cascade process (see the SI for the detailed
reaction pathway) (Table 2, entry 8).16
To elucidate some mechanistic insight into this reaction, the
H/D exchange and kinetic isotope experiments were
conducted. The reversibility of the C−H cleavage was
determined by running the reaction in the presence of D2O
(Scheme 2a). 1H NMR analysis of the product [D4]-5
indicated that the deuteration occurred at four different
positions. The analysis of the product [D2]-2a revealed that
the deuteration occurred at both the C1 and C10 positions.
These results demonstrated that the 1,4-palladium migration
process was reversible. The KH/KD value was determined to be
3.0 by the intermolecular kinetic isotope effect (KIE)
competition experiment (Scheme 2b), whereas a KH/KD
value of 0.8 was observed when two parallel reactions were
conducted (Scheme 2c). These two inconsistent KIE results
revealed that the C−H activation or the cleavage of the C−H
bond might occur after the rate-limiting step, and the oxidative
addition might be involved in the rate-limiting step.17
Experimental procedures, characterization data for all
new compounds, and copies of 1H and 13C NMR
spectra (PDF)
AUTHOR INFORMATION
Corresponding Authors
■
*E-mail: hyao@cpu.edu.cn, cpuhyao@126.com (H.Y.).
*E-mail: ajlin@cpu.edu.cn (A.L.).
ORCID
Aijun Lin: 0000-0001-5786-4537
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We thank the National Natural Science Foundation of China
(NSFC21572272) and the Innovation Team of “the Double-
First Class” Disciplines (CPU2018GY04 and CPU2018GY35)
for the financial support.
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* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.9b02392.
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