Visible-Light-Enabled Ortho-Selective Aminopyridylation of Alkenes with N-Aminopyridinium Ylides

Article abstract of DOI:10.1021/jacs.0c05025

By utilizing an underexplored reactivity mode of N-aminopyridinium ylides, we developed the visible-light-induced ortho-selective aminopyridylation of alkenes via radical-mediated 1,3-dipolar cycloaddition. The photocatalyzed single-electron oxidation of N-aminopyridinium ylides generates the corresponding radical cations that enable previously inaccessible 1,3-cycloaddition with a broader range of alkene substrates. The resulting cycloaddition adducts rapidly undergo subsequent homolytic cleavage of the N-N bond, conferring a substantial thermodynamic driving force to yield various β-aminoethylpyridines. Remarkably, amino and pyridyl groups can be installed into both activated and unactivated alkenes with modular control of ortho-selectivity and 1,2-syn-diastereoselectivity under metal-free and mild conditions. Combined experimental and computational studies are conducted to clarify the detailed reaction mechanism and the origins of site selectivity and diastereoselectivity.

Full text of DOI:10.1021/jacs.0c05025

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Visible-Light-Enabled Ortho-Selective Aminopyridylation of Alkenes
with N‑Aminopyridinium Ylides
Yonghoon Moon, Wooseok Lee, and Sungwoo Hong*
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ABSTRACT: By utilizing an underexplored reactivity mode of N-
aminopyridinium ylides, we developed the visible-light-induced
ortho-selective aminopyridylation of alkenes via radical-mediated
1,3-dipolar cycloaddition. The photocatalyzed single-electron
oxidation of N-aminopyridinium ylides generates the correspond-
ing radical cations that enable previously inaccessible 1,3-
cycloaddition with a broader range of alkene substrates. The
resulting cycloaddition adducts rapidly undergo subsequent
homolytic cleavage of the N−N bond, conferring a substantial
thermodynamic driving force to yield various β-aminoethylpyr-
idines. Remarkably, amino and pyridyl groups can be installed into
both activated and unactivated alkenes with modular control of ortho-selectivity and 1,2-syn-diastereoselectivity under metal-free and
mild conditions. Combined experimental and computational studies are conducted to clarify the detailed reaction mechanism and
the origins of site selectivity and diastereoselectivity.
controlling regioselectivity and diastereoselectivity.9 Despite
being less reactive nucleophiles, the N-aminopyridinium ylides
INTRODUCTION
■
Alkene carboamination that rapidly constructs vicinal C−N
are capable of undergoing thermally induced 1,3-dipolar
and C−C bonds across an alkene is a highly efficient and
cycloadditions with alkynes.10 Inspired by this strategy, we
powerful approach to direct access to structurally diverse N-
containing molecules in a single operation.1,2 The β-amino-
imagined that the challenging ortho-selective aminopyridyla-
tion of alkenes could be achieved by exploiting the
intermolecular 1,3-dipolar cycloaddition of N-aminopyridi-
nium ylides with alkenes. We speculated that the resulting
cycloaddition adducts could be converted into the desired C2-
pyridyl β-amino compounds by subsequent reductive cleavage
of the N−N bond of the resulting adducts (Scheme 1b).7c In
contrast to cycloaddition reactions with alkynes, however, the
reaction of N-aminopyridinium ylides with olefins has proven
to be challenging in classic two-electron chemistry and, to the
best of our knowledge, is unprecedented. To understand the
curious difference between alkenes and alkynes in behavior
toward 1,3-dipolar cycloaddition with N-aminopyridinium
ylides, we calculated the Gibbs free energies of five-membered
cyclized adducts, as shown in Figure 1. These initial
computational studies clearly revealed that the impediments
to this type of cycloaddition can be attributed to the high-
energy barrier of the resulting adducts. Although the
ethylpyridine motifs consisting of vicinal pyridine and amino
groups are widely employed as versatile building blocks in drug
discovery3 and materials science.4 In this regard, site-selective
and diastereoselective alkene aminopyridylation represents an
efficient and robust protocol for installing two valuable
pyridine and amino groups onto a carbon−carbon double
bond. Despite significant advances, Minisci-type cross-coupling
reactions between alkyl radicals and pyridine scaffolds are
plagued by poor regioselectivity and overalkylation issues,
which precludes the direct use of unblocked pyridine systems.5
The synthetic potential of pyridinium salts has been
demonstrated as versatile pyridine surrogates to address
various chemical transformations.6 In light of these benefits,
our group leveraged the steric and electronic properties of the
N-substituent of pyridinium salts to control the functionaliza-
tion of pyridine at the C4-position over the competing C2-
position (Scheme 1a).7 However, the selective introduction of
alkyl groups at the C2-position of unblocked pyridinium salts
proved uniquely challenging.8 Driven by the need for an
efficient synthetic route to C2-pyridyl β-amino motifs, we
sought to develop a general strategy that enables the
intermolecular ortho-selective aminopyridylation of various
olefins.
Received: May 7, 2020
The 1,3-dipolar cycloadditions across C−C π-systems are
widely employed in the construction of various heterocycles
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We reasoned that the use of open-shell radical cation species
from either an N-aminopyridinium ylide or alkene would
provide an energetic driving force and enable a stepwise 1,3-
dipolar cycloaddition. To explore such a strategy, we
considered two different approaches to radical-mediated 1,3-
dipolar cycloaddition: (1) single-electron-transfer (SET)
oxidation of alkenes to generate electrophilic alkene radical
cations11,12 that react with nucleophilic N-aminopyridinium
ylides and (2) photocatalytic generation of N-radical
pyridinium salts to enable radical addition to the C−C π-
systems of alkenes. The strategy of the photocatalytic alkene
carboamination through alkene radical cation intermediates
has provided efficient synthetic access to structurally diverse
amine derivatives.12,13 Although remarkable advances have
been realized, this approach restricts the alkene scope because
the oxidation potentials of some common unactivated alkenes
(i.e., terminal aliphatic olefins) prohibitively exceed the redox
potentials of even highly oxidizing excited-state photocatalysts
as illustrated in Figure 2.14 As such, this approach is currently
limited to the carboamination reactions of activated alkenes,
such as styrene analogues and trisubstituted alkyl alkenes.
Scheme 1. Design Plan: Photocatalytic ortho-Selective
Aminopyridylation of Olefins Using N-Aminopyridinium
Ylides
Figure 2. Comparison of redox potentials between highly oxidizing
photocatalysts and typical alkenes (vs SCE).
The Knowles group reported the photocatalytic hydro-
amination reactions of unactivated olefins using nitrogen-
centered radicals generated via single-electron-transfer oxida-
tion.15 With the goal of ortho-selective aminopyridylation of
various olefins, we envisioned that the direct oxidation of N-
aminopyridinium ylides by an excited-state redox photo-
catalyst16 could generate N-radicals that enable previously
inaccessible 1,3-cycloaddition with a broader range of alkene
substrates, including terminal aliphatic alkenes in the reaction
scope. In our working hypothesis, the resulting cycloaddition
adducts would undergo subsequent homolytic cleavage of the
N−N bond, conferring a substantial thermodynamic driving
force to afford the aminopyridylated products, as outlined in
Scheme 1c. Herein, we report the successful realization of the
ortho-selective aminopyridylation of alkenes via previously
underexplored photocatalytic radical-mediated 1,3-dipolar
cycloaddition using a novel reactivity of N-aminopyridinium
ylides as bifunctional reagents.13,17 Remarkably, amino and
pyridyl groups could be installed into both activated and
Figure 1. Free energy comparison between N-aminopyridinium ylide
and their classic [3 + 2] cycloadded product with various alkenes and
an alkyne.
cycloaddition reaction with an alkyne was observed to be a
thermodynamically downhill process (Int-H4, −16.7 kcal/
mol), the corresponding reactions with various alkenes were
considerably endergonic (Int-H1, H2, and H3, from 7.9 to
10.2 kcal/mol). These observations indicate that the formation
of cycloaddition adducts from alkenes is a thermodynamically
uphill process and lacks a driving force, thus favoring a
competitive retro-1,3-dipolar cycloaddition process (Figure 1).
In this regard, an alternative nonclassical strategy is apparently
required to overcome the thermodynamic barrier of classic 1,3-
dipolar cycloaddition.
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unactivated alkenes with excellent diastereocontrol under
metal-free and mild reaction conditions, granting efficient
access to a variety of valuable C2-pyridyl β-amino molecules
with a syn-configuration.
Because N-aminopyridinium salts with different N-protecting
groups could exhibit distinctive chemical and physical
properties, our initial investigations focused on extensive
screening of a variety of N-aminopyridinium ylides to
determine the effect of the N-substituent unit on the oxidizable
ability of each pyridinium ylide (see the Supporting
Information for further details). To our delight, we found
that the desired aminopyridylated product 3 could be formed
in an ortho-selective manner when the sulfonamide group was
used as an N-substituent (entries 2 and 3). With this promising
result in hand, we investigated a series of amide-substituted
pyridinium ylides by varying the electron density (entries 4−
7). Whereas electron-deficient benzoyl groups showed poor
yields (entries 4 and 5), the electron-rich para-methoxybenzoyl
group 1g was found to be a superior N-substituent to promote
aminopyridylation, consistent with its higher oxidizable ability
(entry 7). Thus, ylide 1g was selected for further study. The
employment of C as a photocatalyst was found to be more
productive (entry 9). The evaluation of the solvent revealed
that this transformation proceeds best in dichloromethane and
produces the product 3a in 78% yield (entries 10 and 11). We
conducted control experiments to confirm the essential role of
light and photocatalyst in the success of this transformation
(entries 12 and 13). As expected, the desired product was not
observed with the recovery of the starting materials when the
reaction was conducted with 2,2,6,6-tetramethylpiperidine-1-
oxyl (TEMPO) (entry 14).
RESULTS AND DISCUSSION
■
To evaluate the feasibility of using N-aminopyridinium ylides
for the prospective radical-mediated 1,3-cycloaddition, the
model reactions of N-aminopyridinium ylide 1 and styrene
(2a) were conducted in the presence of a photocatalyst under
irradiation with blue LEDs (Table 1). Attempts to use N-
aminopyridinium ylide 1a failed to produce any product (entry
1). Thus, the success of the proposed strategy requires a
suitable N-aminopyridinium ylide that can readily undergo
single-electron oxidation under mild reaction conditions.
a
Table 1. Optimization of the Reaction Conditions
With the optimized conditions in hand, we set out to
examine the substrate scope to verify the generality of the
current aminopyridylation reaction, as shown in Table 2.
Regarding the alkene scope, this method was applicable to a
variety of styrene derivatives, unactivated alkenes, and
electron-rich alkenes and allowed excellent functional group
tolerance. Styrenes bearing various C4 substituents, including
halides (3b−3d), bulky tert-butyl (3e), methoxy (3f),
trifluoromethyl (3g), and trimethylsilyl (3h) groups reacted
well to form β-aminopyridine derivatives with excellent ortho-
regiocontrol on the pyridyl scaffold. The meta- or ortho-
substituted (3j and 3k) styrenes showed similar reactivity with
para-substituted styrene (3i). Mesityl styrene and naphthyl-
styrene were also suitable substrates (3l and 3m). Moreover,
various β-aminoethylpyridines containing quaternary carbon
centers were readily prepared (3n−3r). Typically, the alkene
carboamination of unactivated alkenes has proven difficult to
achieve through the single-electron oxidation of alkenes.
Remarkably, this strategy was not limited to styrene analogues
but could also be successfully extended to more challenging
substrates, such as terminal aliphatic alkenes (3s−3y),
highlighting the advantage of the current method involving
N-centered radical-mediated alkene functionalization. In
addition, the scope could be expanded to the electron-rich
enol ether and vinyl lactam to afford the desired product 3z
and 3aa, respectively.
b
entry
ylide
photocatalyst (5 mol %)
solvent
yield (%)
c
1
2
3
4
5
6
7
8
1a
A
A
A
A
A
A
A
B
C
C
C
1,2-DCE
1,2-DCE
1,2-DCE
1,2-DCE
1,2-DCE
1,2-DCE
1,2-DCE
1,2-DCE
1,2-DCE
DCM
n.d.
13
13
n.d.
19
37
59
69
71
78
1b
1c
1d
1e
1f
1g
1g
1g
1g
1g
1g
1g
1g
9
10
11
12
13
14
MeCN
DCM
DCM
DCM
51
n.d.
n.d.
n.d.
Following the investigations of the alkene scope, we
subsequently examined the utility of our method by exploring
pyridinium ylide scope. A wide range of pyridinium ylides
bearing various functional groups, such as methyl (4a), phenyl
(4b), ketone (4f), and substituted aryl groups (methoxy,
bromo, and trifluoromethyl) at the C2-position, effectively
underwent aminopyridylation to afford the desired products.
The resulting 2-phenylpyridine moiety (4b−4e) could be
widely employed as versatile ligands for designing transition-
metal catalysts. For the C2/C3 disubstituted pyridine core, the
d
C
C
e
a
Reaction conditions: Ylides (0.1 mmol), 2a (0.1 mmol), and
photocatalyst (5.0 mol %) in solvent (1.0 mL) under irradiation using
blue LEDs at rt for 22 h under N2. Yields were determined by H
b
1
c
NMR spectroscopy. 1a was generated from N-aminopyridinium
d
e
iodide and K2CO3. Reaction was performed in the dark. TEMPO
(1.0 equiv) was added. 1,2-DCE = 1,2-dichloroethane. DCM =
dichloromethane, n.d. = not detected.
C
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a
Table 2. Substrate Scope of ortho-Selective Aminopyridylation
a
Reaction conditions: 1 (0.1 mmol), 2 (0.1 mmol), and 9-Mes-2,7-di-Me-10-Ph-Acr+BF4− (5.0 mol %) in DCM (1.0 mL) under irradiation using
b
blue LEDs at rt for 22 h under N2. Isolated yields. Regioisomeric ratios were determined by 1H NMR spectroscopy. 5.0 equiv of alkene was used.
c
d
3.0 equiv of alkene was used. Ir[(dFCF3ppy)2(5,5′-dCF3bpy)]PF6 (2.5 mol %) was used as the catalyst.
reactivity was consistent to deliver aminopyridylated products
4g and 4h. With respect to the C3-substituted pyridine
substrates, radical addition occurred preferably at the sterically
more congested C2-position of the pyridine core over the
competing C6-position to forge a challenging C−C bond at a
sterically more hindered site (4i−4p). The structure of 4i was
unambiguously confirmed by X-ray analysis of a single crystal
(see the Supporting Information for further details). These
results could be explained by considering that the initial
formation of free radicals on tertiary carbons is likely to be
more favorable than the free radicals on secondary carbons as
the radical addition converts the sp2 carbons of the pyridine
segment into sp3 carbons.
D
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Next, C4-substituted pyridinium ylides bearing methyl,
phenyl, cyano, ester, and ether were tested with styrene, all
showing excellent reactivity (4q−4u). In addition, amino-
pyridylation of the unactivated terminal alkene with C4-ester-
substituted pyridinium ylide was tolerated under the optimized
conditions (4v). Further exploration demonstrated that the N-
isoquinolinium ylide was a suitable substrate and provided the
desired product 4w.
various pyridinium ylides with internal olefins to demonstrate
the generality of the diastereoselectivity (6j−6q). As such, the
current method provides an opportunity for accessing 1,2-syn-
aminopyridylation products.
To further demonstrate the synthetic utility of the current
transformation, we explored the late-stage modifications of
more structurally complex biorelevant molecules, as high-
lighted in Table 4. The isoeugenol derivative and β-pinene
We then attempted to evaluate the scope of internal alkenes,
as shown in Table 3. We were pleased to find that this protocol
Table 4. Late-Stage Site-Selective Functionalization of
Biorelevant Molecules
a
a
Table 3. Substrate Scope of Internal Alkenes
a
Reaction conditions: 1 (0.1 mmol), alkene (0.1 mmol), and 9-Mes-
−
2,7-di-Me-10-Ph-Acr+BF4 (5.0 mol %) in DCM (1.0 mL) under
irradiation using blue LEDs at rt for 22 h under N2. Isolated yields.
1
Regioisomeric ratios were were determined by H NMR spectrosco-
b
c
py. 2 equiv of 1g was used. Ir[(dFCF3 ppy)2(5,5′-dCF3 bpy)]PF6
(2.5 mol %), 3 equiv of 2a was used.
a
Reaction conditions: 1 (0.1 mmol), 5 (0.1 mmol), and 9-Mes-2,7-di-
Me-10-Ph-Acr+BF4− (5.0 mol %) in DCM (1.0 mL) under irradiation
using blue LEDs at rt for 22 h under N2. Isolated yields. 3.0 equiv of
were subjected to the standard reaction conditions, leading to
the respective products 7a and 7b. Likewise, nateglinide,
fenofibric acid, and nalidixic acid derivatives were well-
tolerated, affording aminoethylpyridine products 7c, 7d, and
7e. Moreover, substrates derived from pyridine-based drugs
with various functional groups, such as pyriproxyfen, bisacodyl,
and vismodegib, could be applied to successfully deliver
products 7f, 7g, and 7h, respectively. Taken together, these
results highlight the synthetic utility of this transformation for
rapidly accessing a wide range of valuable C2-aminoalkyl-
functionalized pyridine motifs with excellent functional group
tolerance, such as ester, ketone, amide, sulfone, ether, and
pyridyl groups.
b
5i was used.
was amenable to a range of internal olefins, leading to the
formation of 1,2-syn-isomeric products with excellent diaster-
eoselectivity (>20:1 d.r.) in all of these examples. The syn-
configuration between the pyridine and amide groups was
unambiguously confirmed by X-ray analysis of a single crystal
(6a, see the Supporting Information for further details). The
applicability of the protocol of indene and cinnamyl acetate
was also examined, which led to the formation of desired
products 6c and 6d. For internal styrenes containing a Michael
acceptor or terminal olefin, high chemoselectivity for the styryl
double bond was observed (6e and 6f). Substrates bearing
cyclohexene were also tolerable to form syn-product 6g and 6h.
Aliphatic trisubstituted alkene underwent aminopyridylation to
give 6i in good yield and regioselectivity. Next, we examined
To gain deeper insights into the reaction mechanism and the
diastereo-determining step, we carried out computational
studies based on density functional theory (DFT). The
computed reaction energy profiles are depicted in Figure 3
(see the Supporting Information for details). The reaction is
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Figure 3. Proposed reaction pathway and related free energy profile. Geometry, vibrational frequency, and solvation energy were calculated using
the B3LYP-D3 and LACVP**, and B3LYP-D3 and cc-pVTZ(-f) were employed to determine single-point energy.
initiated by the photoexcitation of PC to the excited state PC*,
which oxidizes 1g to 1g* via a SET event. The resulting
pyridinium radical cation 1g* reacts with alkene 5a via 1g*-TS
with a barrier of 4.0 kcal/mol, generating alkyl radical
intermediate I. Once radical I is formed, it is poised for
intramolecular radical addition to the C2-position of the
pyridinium segment. Our calculations indicate that this radical
addition step via TS-I determines the diastereoselectivity,
leading to the formation of two possible cycloaddition adducts
II and II′. The transition-state TS-I that leads to the syn-
isomer 3n is 2.7 kcal/mol lower than TS-I′. This finding is in
good agreement with the experimentally observed excellent
diastereoselectivity. Next, the resulting intermediate II under-
goes deprotonation to form intermediate III, and N−N
cleavage of intermediate III through TS-III is fast and
energetically downhill (18.4 kcal/mol).
We conducted a kinetic isotope effect (KIE) experiment
with 1g and d5-1g as substrates, in which the KH/KD ratio was
found to be 1.01 (see Scheme 2a), indicating that the
deprotonation step is not involved in a rate-determining step in
a significant way. Next, N-centered radical IV is reduced by the
PC radical anion, which completes the photocatalytic cycle.
Subsequent protonation generates the desired product 6a.
Once the radical intermediate 1g* is generated, the reaction is
highly exergonic, making the overall reaction exothermic by
84.1 kcal/mol, and the intramolecular radical addition is
assigned as a diastereo-determining step based on the reaction
energy profile. The energy calculation of 6a and 6a′ indicated
that the syn-isomer 6a is a kinetically and thermodynamically
more favorable isomer. The reaction quantum yield was found
to be Φ = 0.141 (see the Supporting Information for further
details), which indicates that a radical chain pathway is not the
major pathway in this transformation.
Next, Stern−Volmer quenching studies were conducted,
which revealed that the excited state of photocatalyst C can be
quenched and linearly correlated with the concentration by
both pyridinium ylide 1g and styrene 2a (Figure 4). However,
the Stern−Volmer quenching constant, Ksv, of 1g is
significantly higher (>19 times) than that of styrene 2a.
Moreover, a series of Stern−Volmer experiments showed that
the luminescence of the photoexcited C is barely quenched by
unactivated alkenes 2u and 5g that were successfully employed
in this transformation. In addition, the measured reduction
potential of pyridinium ylide 1g was 1.58 V (peak potential vs
SCE, see Figure S7), which is much lower than the reduction
potential of styrene (1.97 V vs SCE) and the excited-state
catalyst C (2.09 V vs SCE). These observations suggested that
pyridinium ylides are more favorable for oxidation in the
reductive quenching photocatalytic cycle, and therefore the
photocatalytic cycle could be reasonably postulated to
commence with single-electron oxidation of N-aminopyridi-
nium ylides.
To scrutinize the proposed mechanism, several control
experiments were conducted. First, the KIE was not observed,
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sensitivity of this method was investigated to determine the
influence of the variation of the standard conditions (Scheme
2e). Specifically, the transformation was shown to be
insensitive toward moisture. In contrast, the reaction was
markedly inhibited under a high-oxygen atmosphere.
Scheme 2. Control Experiments
CONCLUSIONS
■
In summary, we have developed a general photocatalytic
platform that uses N-aminopyridinium ylides as bifunctional
reagents to enable ortho-selective aminopyridylation of alkenes
via radical-mediated 1,3-dipolar cycloaddition with high levels
of atom and step economy. Intriguingly, the unprecedented
reactivity of the N-aminopyridinium radical cations in situ
generated from N-aminopyridinium ylides via SET has been
demonstrated by enabling previously underexplored 1,3-
cycloaddition with both activated and unactivated alkenes.
The synthetic advances of this transformation allow the
functionalization of the pyridine core exclusively at the C2-
position. Moreover, the reactions with internal olefins lead to
the formation of single diastereomers with a syn-configuration.
This straightforward and environmentally friendly method
exhibited a broad substrate scope at room temperature, and the
synthetic value was further demonstrated by late-stage
modifications of complex biorelevant molecules. We anticipate
that the current method will provide an efficient synthetic
toolbox for the modular assembly of valuable β-amino-
ethylpyridines by addressing key challenges of alkene amino-
pyridylation, including ortho-selectivity and diastereoselectivity
issues.
ASSOCIATED CONTENT
■
sı
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/jacs.0c05025.
Experimental procedure and characterization of new
compounds (1H and 13C NMR spectra) (PDF)
X-ray crystallographic data (6a) (CIF)
X-ray crystallographic data (4i (major)) (CIF)
X-ray crystallographic data (4i′ (minor)) (CIF)
AUTHOR INFORMATION
■
Corresponding Author
Sungwoo Hong − Department of Chemistry, Korea Advanced
Institute of Science and Technology (KAIST), Daejeon 34141,
Korea; Center for Catalytic Hydrocarbon Functionalizations,
Institute for Basic Science (IBS), Daejeon 34141, Korea;
orcid.org/0000-0001-9371-1730; Email: hongorg@
kaist.ac.kr
Figure 4. Stern−Volmer quenching plots of photocatalyst C with 1g,
2a, 2u, and 5g.
which indicates that the deprotonation step and intramolecular
radical addition step are unlikely to be rate-determining
(Scheme 2a,b). To determine whether the pyridine in situ
generated from pyridinium ylide via single-electron reduction
could act as an alkyl radical acceptor, we subjected mixtures of
1g and PMP-substituted pyridine to the standard reaction
conditions (Scheme 2c), which showed that the functionaliza-
tion took place only at the pyridinium ylide 1g. Notably, the
reactions with both Z- and E-methylstyrenes provided the
same syn-isomeric product 6a, suggesting that the bond
rotation of intermediate I is operative in the proposed stepwise
cycloaddition process (Scheme 2d and Figure 3). Next, the
Authors
Yonghoon Moon − Department of Chemistry, Korea Advanced
Institute of Science and Technology (KAIST), Daejeon 34141,
Korea; Center for Catalytic Hydrocarbon Functionalizations,
Institute for Basic Science (IBS), Daejeon 34141, Korea
Wooseok Lee − Department of Chemistry, Korea Advanced
Institute of Science and Technology (KAIST), Daejeon 34141,
Korea; Center for Catalytic Hydrocarbon Functionalizations,
Institute for Basic Science (IBS), Daejeon 34141, Korea
Complete contact information is available at:
https://pubs.acs.org/10.1021/jacs.0c05025
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a Trimeric Circular Helicate. J. Am. Chem. Soc. 2018, 140, 4982−
4985.
Notes
The authors declare no competing financial interest.
(5) For selected examples of acid-promoted Minisci reaction, see:
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arenes: Scope, Tunability, and Predictability. J. Am. Chem. Soc. 2013,
135, 12122−12134. (b) Jin, J.; MacMillan, D. W. C. Direct α-
Arylation of Ethers through the Combination of Photoredox-
Mediated C-H Functionalization and the Minisci Reaction. Angew.
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ACKNOWLEDGMENTS
■
This research was supported financially by the Institute for
Basic Science (IBS-R010-A2). We thank Dr. Dongwook Kim
(IBS) for XRD analysis.
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