Switchable cascade C-H annulation to polycyclic pyryliums and pyridiniums: Discovering mitochondria-Targeting fluorescent probes

Article abstract of DOI:10.1039/d0cc06997c

Disclosed herein is a counterion additive-switched rhodium-catalyzed cascade triple C-H annulation of 4-hydroxy-1-naphthaldehydes with alkynes, in which six chemical bonds are formed in one-pot. This reaction enables the rapid assembly of diverse polycycl

Full text of DOI:10.1039/d0cc06997c

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Switchable cascade C–H annulation to polycyclic
pyryliums and pyridiniums: discovering
Cite this:DOI: 10.1039/d0cc06997c
mitochondria-targeting fluorescent probes†
Received 21st October 2020,
Accepted 9th November 2020
Xingyu Chen, Lipeng Yan, Yanhong Liu, Yudong Yang * and Jingsong You
*
DOI: 10.1039/d0cc06997c
rsc.li/chemcomm
Disclosed herein is a counterion additive-switched rhodium-catalyzed of the most concise and efficient approaches for the construc-
cascade triple C–H annulation of 4-hydroxy-1-naphthaldehydes with tion of heteroaromatics.7 In terms of step- and atom-economy,
alkynes, in which six chemical bonds are formed in one-pot. This the cascade oxidative C–H annulation of simple aromatics and
reaction enables the rapid assembly of diverse polycyclic pyrylium and alkynes, which allows the straightforward formation of multiple
pyridinium fluorophores, which leads to two specific mitochondria- chemical bonds in one pot, would provide an ideal synthetic route
labeling reagents with low cytotoxicity and superior photostability.
to structurally complex PHAs and their cationic analogues.8 In
2018, we disclosed a rhodium-catalyzed annulation cascade of
Fluorescent probes, which enable the molecular or cellular level arylnitriles with alkynes to obtain stable delocalized PAH carbo-
activities to be visualized, are essential chemical tools for cations, which exhibited specific lysosome-targeting ability in
bioimaging.1 In order to detect the malfunction and abnormality HepG2 cells.8d Recently, polymethine-bridged pyrylium (flavylium
of organelles, an ideal organelle specific targeted fluorescent type) fluorophores have been reported as potential candidates for
probe should feature superior cell permeability, good quantum near- and shortwave infrared imaging (Scheme 1a).9 Herein,
yield, high photostability and low toxicity.2 In this regard, we describe a counterion additive-switched rhodium-catalyzed
fluorescent oxonium or azanium scaffolds are considered as cascade C–H annulation of 4-hydroxy-1-naphthaldehydes with
priviledged candidates for bioimaging agents that possess the alkynes to rapidly assemble a library of polycyclic pyrylium and
above properties.3 Therefore, the pursuit of new methods for the pyridinium cations, which enables the discovery of two mitocho-
rapid assembly of fluorescent probes with such structures repre- ndria trackers in living HepG2 cells (Scheme 1b).
sents a highly fascinating and significant work.
We started our investigations by using 4-hydroxy-1-
On the other hand, polycyclic heteroaromatics (PHAs) and naphthaldehyde 1a and diphenylacetylene 2a as the model
their stable cations have broad applications in the fields of
fluorescent labeling agents, photocatalysts and optoelectronic
materials.4 The biological and photophysical properties (emis-
sion wavelengths and quantum yields) of (cationic) PHAs are
highly dependent on the structures of conjugated skeletons and
could be manipulated by regulating the doped heteroatoms
as well as the p-extension degree.5 However, conventional
approaches to polycyclic oxonium and azanium scaffolds often
suffer from tedious synthetic routes, harsh reaction conditions
and inaccessible starting materials, which restrict the diversity
of molecular skeletons.6 In the past two decades, transition
metal-catalyzed oxidative annulation of aromatics and alkynes
through chelation-assisted C–H activation has emerged as one
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: yangyudong@scu.edu.cn, jsyou@scu.edu.cn
† Electronic supplementary information (ESI) available. CCDC 2027219 (5a),
Scheme 1 Design and synthesis of polycyclic pyrylium and pyridinium
2027222 (4a) and 2027223 (3a). For ESI and crystallographic data in CIF or other
cation type fluorescent probes for bioimaging.
electronic format see DOI: 10.1039/d0cc06997c
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Table 1 Optimization of the cascade C–H annulation reactiona
Table 2 Scope of cascade C–H annulation for the synthesis of cationic
PHAs 3 or 4ab
Entry
Oxidant
Additive
Solvent
Yield (%)b
1
2
3
4
5
6
7
8
Ag2CO3
AgOAc
Ag2O
CuO
Cu(NO3)2
Cu(acac)2
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
NaBF4
NaBF4
NaBF4
NaBF4
NaBF4
NaBF4
NaBF4
NaBF4
NaBF4
NaBF4
NaBF4
NaBF4
—
THF
THF
THF
THF
THF
THF
THF
1,4-Dioxane
DCE
Toluene
t-AmylOH
HFIP
THF
Trace
52
Trace
38
34
Trace
65
34
9
n.d.
n.d.
52
Trace
n.d.
49
10
11
12
13
14c
NaBF4
THF
a
Reaction conditions: 1a (0.1 mmol), 2a (0.4 mmol), [Cp*RhCl2]2
(5.0 mol%), oxidant (0.4 mmol, 4.0 equiv.), NaBF4 (0.5 mmol,
b
5.0 equiv.), and solvent (1.0 mL) at 150 1C for 24 h under N2. Isolated
c
yield. The reaction was carried out at 120 1C. DCE = 1,2-dichloroethane;
t-AmylOH = tert-amyl alcohol; HFIP = 1,1,1,3,3,3-hexafluoro-2-propanol;
THF = tetrahydrofuran; n.d. = not detected.
substrates (Table 1). In the presence of [RhCp*Cl2]2 (5.0 mol%),
Ag2CO3 (4.0 equiv.), NaBF4 (5.0 equiv.) as the additive and THF
as the solvent, only trace amounts of the desired products were
detected, whereas mono-annulated product naphtha[1,8-bc]pyran
aldehyde 5a (see eqn (S1), Scheme 2) was formed in a high yield
a
Reaction conditions: 1 (0.1 mmol, 1.0 equiv.), 2 (0.4 mmol, 4.0 equiv.),
[Cp*RhCl2]2 (5.0 mol%), Cu(OAc)2 (0.4 mmol, 4.0 equiv.), NaBF4
(0.5 mmol, 5.0 equiv.) or NH4BF4 (0.3 mmol, 3.0 equiv.) and THF
(1.0 mL) at 150 1C for 24 h under N2. Isolated yield.
b
(entry 1). To our delight, when the oxidant was changed to AgOAc, chloro, bromo and alkyl could participate in the cascade C–H
the desired polycyclic oxonium cation 3a was afforded in 52% annulation to provide polycyclic pyrylium cations in moderate
yield via cascade triple C–H annulation (entry 2). Further optimi- to high yields (3a–3h). The ortho-substituted diaryl alkynes,
which have higher steric hindrance, exhibited less efficiency
than their para-substituted analogues (3b vs. 3c). The substi-
tuted 4-hydroxy-1-naphthaldehydes could also work in the
present rhodium catalytic system, affording the corresponding
products in moderate yields (3i and 3j).
zation of the oxidants indicated that Cu(OAc)2 was the most
suitable oxidant in this transformation (entries 3–7). Attempts
with other solvents such as 1,4-dioxane, DCE, toluene, t-AmylOH
and HFIP did not deliver a higher yield (entries 8–12). Control
experiments indicated that a counterion additive was essential
(entry 13). Lowering the reaction temperature resulted in a
diminished yield (entry 14).
Interestingly, when NH4BF4 was used as the additive, p-fused
pyridinium 4a was obtained in 85% yield (Table 2). Following the
With the optimized reaction conditions in hand, the sub- rhodium-catalyzed cascade C–H O- and N-annulation protocol,
a set of polycyclic pyridinium cations 4 was constructed with
moderate to excellent yields (4b–4d). The structures of 3a and 4a
were demonstrated by 1H NMR, 13C NMR, high-resolution mass
spectroscopy and single-crystal X-ray analysis.
strate scope of this reaction was next explored. As shown in
Table 2, diaryl alkynes with functional groups such as methoxy,
To gain more insight into this cascade process, preliminary
investigation on the mechanism was conducted (Scheme 2).
When the reaction was performed in the presence of only
1.0 equiv. of diaryl alkynes without NaBF4, the mono-annulated
products, naphtha[1,8-bc]pyran aldehyde 5, could be afforded in
excellent yields (eqn (S1), ESI†). Further reaction of the resulting
product 5b with 1,2-bis(4-methoxyphenyl)ethyne 2b under the
standard conditions could deliver the desired pyrylium cation
3b in 44% yield (eqn (S2), ESI†). These results imply that the
present cascade reaction involves a pyran ring formation/pyrylium
formation sequence.
Scheme 2 Control experiments.
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Scheme 3 Proposed mechanism.
On the basis of previous reports and our experimental results, including absorptions, emissions, and quantum yields were investi-
a mechanism is proposed (Scheme 3).7c,8b,c Firstly, a phenolic gated (see Table S2 in the ESI†). These cationic PHAs typically
OH-directed C–H activation process takes place to afford a five- exhibited yellow, orange or red luminance in CH2Cl2 under UV
membered rhodacycle A (detected by MALDI-TOF-MS, calcd: irradiation (365 nm). Most of these cations exhibited good quan-
408.06, found: 408.05), which then undergoes alkyne insertion tum yields up to 69% with maximum emissions ranging from 550
and reductive elimination to deliver the mono-annulated product to 623 nm. To our delight, Pyrylium-Fluors and Pyridium-Fluors 3a,
5a. Next, a sequential carbonyl-directed C–H activation and alkyne 3b, 3d, 3e, 3h and 4a exhibited good cell penetration ability and
insertion forms an eight-membered rhodacycle D (detected by labelled the HepG2 cells (Fig. 1 and Fig. S7–S12, ESI†). Co-stained
MALDI-TOF-MS, calcd: 763.21, found: 763.21) via intermediate C experiments of these cationic PHAs with commercially available
(detected by MALDI-TOF-MS, calcd: 585.13, found: 585.12). Mito Tracker Red FM (MTR) or Mito Tracker Green FM (MTG)
Following intramolecular cyclization/oxidation/C–H activation rho- indicated that they could specifically label the mitochondria of
dacycle F is obtained, which further undergoes the third alkyne living cells, with Pearson’s coefficient values of 0.89, 0.86, 0.75, 0.94,
insertion and C–O reductive elimination to afford oxonium-doped 0.86 and 0.89, respectively (Table S6, ESI†).13 In comparison, the
PHA 3a. The released [RhI] species would be oxidized by Ag2O to neutral pyran derivative 5a exhibited blue emission in HepG2 cells
regenerate [RhIII] to complete the catalytic cycle (Path I). Alterna- (Fig. S6 and Table S6, ESI†) with a specific labeling ability at the
tively, in the presence of NH4BF4, rhodacycle I (detected by MALDI- endoplasmic reticulum (Pearson’s coefficient = 0.92).14
TOF-MS, calcd: 583.13, found: 583.14) is generated from the in situ
The photostabilities of the cationic PHAs 3a, 3b, 3d, 3h and 4a
formed aldimine derivative H.10 Following alkyne insertion and were investigated by using a confocal microscope together with
reductive elimination N,O-doped PAH K was obtained ([M + H]+ MTR and MTG (Fig. 2a and Fig. S13, ESI†), indicating that 3a, 3b
detected by MALDI-TOF-MS, calcd: 524.20, found: 524.26). Then a and 4a showed better photostability than MTR and MTG after the
pyridine-directed oxidative C–H annulation with alkyne delivers the same irradiation time. Cytotoxicity tests showed that Pyrylium-
azanium-doped PHA 4a (Path II).11
Fluor 3a exhibited almost no cytotoxicity and Pyridium-Fluor 4a
Mitochondria are regarded as the energy factory of cells. exhibited hypotoxicity to the HepG2 cells in the concentration
Because the abnormal function of mitochondria is closely related range of 0.125–4.0 mM (Fig. 2b and Fig. S14, ESI†).
to many diseases, the development of mitochondria-targeting
In conclusion, we have developed a counterion additive-
fluorescent probes is of high importance in chemical biology controlled rhodium-catalyzed cascade C–H annulation of
and clinical diagnosis.3,12 With a library of polycyclic pyrylium and 4-hydroxy-1-naphthaldehydes with alkynes. Six chemical bonds
pyridinium cations 3 and 4 in hand, named Pyrylium-Fluors and three aromatic rings are constructed in this one-pot triple
and Pyridium-Fluors, respectively, their photophysical properties annulation relay. Following this protocol, a library of polycyclic
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This work was supported by the National NSF of China
(21772128). We thank the Comprehensive Training Platform
of Specialized Laboratory, College of chemistry, Sichuan
University.
Conflicts of interest
There are no conflicts to declare.
Fig. 1 Co-staining of HepG2 cells with 3a, 4a and a commercially available
mitochondria-specific tracker MTR: (a) fluorescent image of HepG2 cells stained
with MTR (1.0 mM, lex = 543 nm, lem = 600–700 nm), (b) fluorescent image of
HepG2 cells stained with 3a (1.0 mM, lex = 488 nm, lem = 500–600 nm),
(c) merged image of (a and b), (d) intensity profile of the regions of interest across
HepG2 cells, (e) fluorescent image of HepG2 cells stained with MTR (1.0 mM,
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