Visible light-induced radical addition/annulation to construct phenylsulfonyl-functionalized dihydrobenzofurans involving an intramolecular 1,5-hydrogen atom transfer process

Article abstract of DOI:10.1021/acs.orglett.0c03038

A visible light-induced radical cascade reaction of 2-alkynylarylethers with sodium sulfinates was established for the synthesis of sulfonyl-functionalized dihydrobenzofurans, and an intramolecular 1,5-hydrogen atom transfer was involved in this transformation. This process provided an efficient and convenient C-C formation protocol for the construction of a dihydrobenzofuran ring. Various substituents on 2-alkynylarylethers and sodium sulfinates were tolerated in the reaction, and the corresponding products were obtained in moderate to good yields.

Full text of DOI:10.1021/acs.orglett.0c03038

pubs.acs.org/OrgLett
Letter
Visible Light-Induced Radical Addition/Annulation to Construct
Phenylsulfonyl-Functionalized Dihydrobenzofurans Involving an
Intramolecular 1,5-Hydrogen Atom Transfer Process
Shentong Xie, Yifan Li, Ping Liu,* and Peipei Sun*
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ABSTRACT: A visible light-induced radical cascade reaction of 2-
alkynylarylethers with sodium sulfinates was established for the
synthesis of sulfonyl-functionalized dihydrobenzofurans, and an
intramolecular 1,5-hydrogen atom transfer was involved in this
transformation. This process provided an efficient and convenient
C−C formation protocol for the construction of a dihydrobenzo-
furan ring. Various substituents on 2-alkynylarylethers and sodium
sulfinates were tolerated in the reaction, and the corresponding
products were obtained in moderate to good yields.
adical addition/cyclization cascade reactions provide an
Rexcellent strategy for the synthesis of various function-
alized heterocycles.1 The radical-mediated intramolecular
hydrogen atom transfer (HAT), as a highly regioselective
and mild approach to the functionalization of remote C(sp3)−
H bonds, offers a special opportunity in the assembly of cyclic
scaffolds.2 Except for the pioneering works of Heiba and
Dessau,3a Curran,3b,c and others, a series of seminal works were
developed in recent years via the 1,5-HAT from C(sp3)− or
C(sp2)−H bonds to vinyl radicals followed by cyclization to
construct five-membered rings.3d−k The cascade can be
initiated from the addition of various radicals to a CC
bond due to the easy accessibility of this group.4
The sulfonyl group plays an important role in drugs and in
organic synthesis.5 Like the available and stable inorganic salts,
sulfinates have been widely used as the sulfonyl sources,
especially in the radical-mediated transformation. The addition
of a sulfonyl radical to alkynes was mainly applied to the 1,2-
difunctionalization of alkynes, and a series of olefins containing
sulfonyl and other active groups such as carbonyl, amino,
phenoxy, etc., were synthesized effectively by employing this
protocol (Scheme 1a).6 Furthermore, by using sodium
sulfinates as the sulfonyl radical sources to add to the CC
bond, some sulfonyl-functionalized heterocycles, including 3-
sulfonylindoles, 2H-azirines, etc., were synthesized via the
radical cascades (Scheme 1b).7
available approaches, for example, the intramolecular coupling
of ortho-halogenated phenylethylalcohols,9 as well as the direct
intramolecular aryl C−H/O−H cross-coupling reactions
(Scheme 2a).10 Using ortho-substituted aryl ethers as the
substrates to carry out intramolecular C−C bond formation is
another strategy for constructing the DHB ring, in which Rh-
catalyzed asymmetric carbene C−H insertion reactions from
diazo group-substituted aryl ethers have been used in the
synthesis of some DHB-containing natural products (Scheme
2b).11 As the highly efficient method for organic synthesis,
photoredox catalysis has been employed in the preparation of
DHBs in recent years. Yoon et al.12 reported a photocatalytic
oxidative [3+2] cycloaddition of phenols and electron-rich
styrenes to produce DHBs (Scheme 2c). Chen, Xiao, and co-
workers13 developed a visible light photoredox-catalyzed
multicomponent reaction of 2-vinylphenols, Umemoto’s
reagent, and sulfur ylides to provide trifluoromethylated
DHBs (Scheme 2d). Polyzos et al.14 synthesized ester-
functionalized DHBs via visible light-photocatalyzed annulative
carbonylation of alkenyl-tethered arenediazonium salts
(Scheme 2e). Regardless, developing novel and efficient
methods to access DHBs is still highly desirable. Inspired by
these seminal reports and our continuing efforts in the field of
radical cascade reactions, we envisioned that the photoredox-
catalyzed addition of sulfonyl radicals to alkynes would initiate
The dihydrobenzofuran (DHB) skeleton is widely found in
natural products and synthetic medicines. Many DHB-
containing natural products have been reported with diverse
biological activities.8 Thus, developing synthetic methods of
dihydrobenzofurans has attracted a great deal of attention
among organic chemists. Transition metal-catalyzed intra-
molecular C−O bond formation provided one of the most
Received: September 11, 2020
https://dx.doi.org/10.1021/acs.orglett.0c03038
© XXXX American Chemical Society
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A

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Scheme 1. Addition of Sulfonyl Radicals to Alkynes with
Scheme 2. Synthesis of DHBs via Different Strategies
Sodium Sulfinates
a 1,5-HAT/cyclization cascade to yield sulfonyl-functionalized
dihydrobenzofurans (Scheme 2f).
Initially, we investigated our reaction by employing 1-
ethynyl-2-isopropoxybenzene (1a) and sodium benzenesulfi-
nate (2a) as the model substrates to test the possibilities
(Table 1). Without a photocatalyst or in the dark, the reaction
did not take place at all (entry 1 or 2, respectively). In the
presence of Ir[(ppy)2(dtbbpy)]PF6 (2 mol %), the targeted
product 2,2-dimethyl-3-[(phenylsulfonyl)methyl]-2,3-dihydro-
benzofuran (3a) was obtained in 22% yield under the
irradiation of 30 W blue light-emitting diodes (LEDs) in
CH2Cl2 under a N2 atmosphere after 24 h (entry 3).
Encouraged by this result, we then investigated the reaction
conditions. The presence of AcOH could obviously promote
the reaction (entries 4 and 5). When 4 equiv of AcOH was
added, a 52% yield of 3a was obtained. Other photocatalysts
such as Ir[dF(CF3)ppy]2(dtbbpy)PF6, 4CzIPN, and fac-
Ir(ppy)3 led to lower catalytic activity (entries 6−8,
respectively). The solvents were then screened. CH3CN,
acetone, EtOH, and DMF were tested (entries 9−12,
respectively). In CH3CN, almost no product was found
(entry 9). When DMF was used, the yield was improved to
61% (entry 12). Surprisingly, water had a beneficial effect on
the reaction. In the presence of 10 equiv of H2O, product 3a
was obtained in 72% yield (entry 13). Additionally, when the
reaction was performed in open air, the reaction gave a very
poor yield (entry 14).
Cl) substituted at the para position of the alkoxy of 1 had no
obvious effect on the reaction, and the desired products were
obtained in 61−69% yields (3b−3d). The presence of CN and
COOMe caused a slight decrease in the yields (3e and 3f).
When Cl was substituted at the meta position (1g), the
reaction gave a 66% yield, while a lower yield of 49% was
obtained from meta-ester group-substituted reactant 1h. The
reaction was also appropriate for dichloro-substituted substrate
and afforded 3i in 71% yield. A series of R2 and R3 groups were
then investigated. For the same or different R2 and R3 groups,
the reactions did not a show significant difference (3j and 3k).
Interestingly, from the substrates with cycloalkyl ether motifs,
spiro structural products 3l−3r were obtained with good
With the optimized reaction conditions in hand, the scope of
this visible light-induced addition/cyclization was evaluated.
Sodium benzenesulfinate (2a) was used as the reaction partner,
and the reaction results of various substituted 2-alkynylar-
ylethers (1) are listed in Scheme 3. The electron-donating
group (R1 = CH3) or electron-withdrawing group (R1 = F or
B
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a
a
Table 1. Optimization of the Reaction Conditions
Scheme 3. Reaction Scope of 2-Alkynylarylethers 1
AcOH
yield
b
entry
1
2
3
4
5
6
photocatalyst
(equiv)
solvent
(%)
−
−
−
−
2
4
4
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
0
0
22
45
52
43
c
Ir[(ppy)2(dtbbpy)]PF6
Ir[(ppy)2(dtbbpy)]PF6
Ir[(ppy)2(dtbbpy)]PF6
Ir[(ppy)2(dtbbpy)]PF6
Ir[dF(CF3)ppy]2(dtbbpy)
PF6
7
8
9
10
11
12
13
14
fac-Ir(ppy)3
4CzIPN
4
4
4
4
4
4
4
4
CH2Cl2
CH2Cl2
MeCN
acetone
EtOH
DMF
29
46
trace
53
44
61
Ir[(ppy)2(dtbbpy)]PF6
Ir[(ppy)2(dtbbpy)]PF6
Ir[(ppy)2(dtbbpy)]PF6
Ir[(ppy)2(dtbbpy)]PF6
Ir[(ppy)2(dtbbpy)]PF6
Ir[(ppy)2(dtbbpy)]PF6
d
DMF
DMF
72
<10
e
a
Reaction conditions: 1a (0.2 mmol), 2a (0.4 mmol), solvent (2 mL),
PC (2 mol %) under N2, irradiated with 30 W blue LEDs, 24 h.
b
c
d
Isolated yield based on 1a. In the dark. In the presence of 10 equiv
e
of H2O. In open air.
yields. Notably, this reaction showed excellent compatibility to
the substrates with natural product structural motifs,
exemplified by menthol and epiandrosterone, and products
3q and 3r were obtained in 78% and 63% yields, respectively.
When R2 = H and R3 = benzyl, the reaction gave a 52% yield of
3s. For -CN- or -OMe-substituted benzyls, similar results were
found (3t and 3u). When R2 = H and R3 = ethyl, however, an
only 19% yield was obtained (3v), which might be due to the
less stable nature of the reaction intermediate. Unfortunately,
for 1-ethynyl-2-methoxybenzene (1w) and the reactant with an
internal alkynyl (1x), no desired products were produced. In
addition, in the scale-up reaction using 1 mmol of 1a, product
3a was obtained in 65% yield, indicating that the reaction
efficiency did not substantially decrease.
Subsequently, we employed 1a as the substrate and explored
the reaction with a diverse set of sodium sulfinates (2) under
the standard reaction conditions (Scheme 4). A wide scope of
substituted sodium benzenesulfinates underwent the reaction
smoothly (4a−4i, 64−78% yields). The electronic effect and
the position of the substituents on the phenyl ring of sodium
benzenesulfinates showed no significant influence. Similarly,
the reaction of sodium naphthalene 1-sulfinate gave a 66%
yield (4j). Heterocyclic derivatives such as sodium pyridine 3-
sulfinate and sodium thiophene 2-sulfinate were also applicable
for this transformation to afford products 4k and 4l in 63% and
57% yields, respectively. Upon using sodium ethylsulfinate,
desired product 4m was obtained in 52% yield, but the use of
sodium methylsulfinate did not provide the desired product.
To demonstrate the synthetic application of this reaction, we
explored the further conversion of the product (Scheme 5).
Upon treatment of 3a with 5 equiv of KOSiMe3 in THF under
an argon atmosphere at 70 °C, terminal alkene 5 was obtained
in 84% yield after 12 h.
a
Reaction conditions: 1 (0.2 mmol), 2a (0.4 mmol), AcOH (0.8
mmol), H2O (2 mmol), DMF (2 mL), and Ir[(ppy)2(dtbbpy)]PF6 (2
mol %) were stirred and irradiated with 30 W blue LEDs under N2 at
b
rt for 24 h. All yields are isolated on the basis of 1. 1a (1.0 mmol)
and 2a (2.0 mmol) were used.
To shed light on the possible reaction pathway, several
mechanistic experiments were conducted. The results of the
C
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a
Scheme 4. Reaction Scope of the Sodium Sulfinates
Scheme 6. Mechanistic Studies
a
Reaction conditions: 1a (0.2 mmol), 2 (0.4 mmol), AcOH (0.8
mmol), H2O (2 mmol), DMF (2 mL), and Ir[(ppy)2(dtbbpy)]PF6 (2
mol %) were stirred and irradiated with 30 W blue LEDs under N2 at
rt for 24 h. All yields are isolated on the basis of 1a.
Scheme 5. Synthetic Utility
Stern−Volmer evperiment showed that the luminescence of
Ir[(ppy)2(dtbbpy)]PF6 could be readily quenched with an
increase in the concentration of 2a (Scheme 6a; see the
Supporting Information for details). When 3 equiv of radical
scavenger 2,2,6,6-tetramethylpiperidinooxy (TEMPO) was
added, the reaction was completely inhibited. Upon addition
of 2 equiv of 1,1-diphenylethylene, no desired product 3a was
detected. Meanwhile, trapping product 6 was observed via LC-
MS analysis of the reaction solution (Scheme 6b). These
results indicated that a radical process might be involved in the
catalytic cycle. When deuterated reactant 1y was used as the
radical acceptor, product 3y with a completely D-substituted
benzyl hydrogen was obtained (Scheme 6c), which indicated
that a 1,5-HAT occurred in this reaction. Using D2O and
AcOD instead of H2O and AcOH, D-containing product 3a-d
was obtained with 87% deuterium incorporation, and the
efficiency of the reaction was not obviously decreased. This
result revealed that the α-hydrogen of the sulfonyl group came
from water or acetic acid (Scheme 6c).
On the basis of these experimental results and previous
reports,3,7 a plausible mechanism for this photoredox radical
cascade process is depicted in Scheme 7. Initial oxidation of
•
red
sodium benzenesulfinate 2a (for PhSO2 /PhSO2Na; E1/2
=
afforded α-oxoalkyl radical species III, which could be
converted to radical IV via a 5-exo-trig radical cyclization.
Reduction of radical IV by the IrII (E1/2red [IrIII/IrII] = −1.58 V
vs SCE in DMF) produced the corresponding anion V,15
followed by the transfer of a proton from H2O or AcOH to
give product 3a.
0.51 V vs SCE in DMF) by the photoexcited Ir-
red
[(ppy)2(dtbbpy)]PF6 (E1/2 [*IrIII/IrII] = 0.68 V vs SCE in
DMF) (see the Supporting Information for details) generated
sulfonyl radical I. The addition of I to the alkynyl of reactant
1a gave vinyl radical II. Then an intramolecular 1,5-HAT of II
D
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Pollution Control, Jiangsu Collaborative Innovation Center of
Biomedical Functional Materials, Nanjing Normal University,
Nanjing 210023, P. R. China
Scheme 7. Proposed Mechanism
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.orglett.0c03038
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by the National Natural Science
Foundation of China (Projects 21672104 and 21502097) and
the Priority Academic Program Development of Jiangsu
Higher Education Institutions.
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In summary, we have established a visible light-induced
radical cascade reaction for the synthesis of sulfonyl-function-
alized dihydrobenzofurans, and an intramolecular 1,5-HAT
was involved in this transformation. This process provided an
efficient and convenient way via C−C bond formation to
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ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.orglett.0c03038.
■
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Experimental procedures, characterization data, and
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Peipei Sun − School of Chemistry and Materials Science, Jiangsu
Provincial Key Laboratory of Material Cycle Processes and
Pollution Control, Jiangsu Collaborative Innovation Center of
Biomedical Functional Materials, Nanjing Normal University,
Nanjing 210023, P. R. China; orcid.org/0000-0002-1716-
2343; Email: sunpeipei@njnu.edu.cn
Ping Liu − School of Chemistry and Materials Science, Jiangsu
Provincial Key Laboratory of Material Cycle Processes and
Pollution Control, Jiangsu Collaborative Innovation Center of
Biomedical Functional Materials, Nanjing Normal University,
Nanjing 210023, P. R. China; Email: pingliu@njnu.edu.cn
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Authors
Shentong Xie − School of Chemistry and Materials Science,
Jiangsu Provincial Key Laboratory of Material Cycle Processes
and Pollution Control, Jiangsu Collaborative Innovation Center
of Biomedical Functional Materials, Nanjing Normal
University, Nanjing 210023, P. R. China
Yifan Li − School of Chemistry and Materials Science, Jiangsu
Provincial Key Laboratory of Material Cycle Processes and
E
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