Ynamide Smiles Rearrangement Triggered by Visible-Light-Mediated Regioselective Ketyl-Ynamide Coupling: Rapid Access to Functionalized Indoles and Isoquinolines

Article abstract of DOI:10.1021/jacs.9b13975

In the past decades, significant advances have been made on radical Smiles rearrangement. However, the eventually formed radical intermediates in these reactions are limited to the amidyl radical, except for the few examples initiated by a N-centered radical. Here, a novel and practical radical Smiles rearrangement triggered by photoredox-catalyzed regioselective ketyl-ynamide coupling is reported, which represents the first radical Smiles rearrangement of ynamides. This method enables facile access to a variety of valuable 2-benzhydrylindoles with broad substrate scope in generally good yields under mild reaction conditions. In addition, this chemistry can also be extended to the divergent synthesis of versatile 3-benzhydrylisoquinolines through a similar ketyl-ynamide coupling and radical Smiles rearrangement, followed by dehydrogenative oxidation. Moreover, such an ynamide Smiles rearrangement initiated by intermolecular photoredox catalysis via addition of external radical sources is also achieved. By control experiments, the reaction was shown to proceed via key ketyl radical and α-imino carbon radical intermediates.

Full text of DOI:10.1021/jacs.9b13975

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Article
Ynamide Smiles Rearrangement Triggered by Visible-Light-
Mediated Regioselective Ketyl−Ynamide Coupling: Rapid Access to
Functionalized Indoles and Isoquinolines
Ze-Shu Wang, Yang-Bo Chen, Hao-Wen Zhang, Zhou Sun, Chunyin Zhu, and Long-Wu Ye*
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ABSTRACT: In the past decades, significant advances have been
made on radical Smiles rearrangement. However, the eventually
formed radical intermediates in these reactions are limited to the
amidyl radical, except for the few examples initiated by a N-
centered radical. Here, a novel and practical radical Smiles
rearrangement triggered by photoredox-catalyzed regioselective
ketyl−ynamide coupling is reported, which represents the first
radical Smiles rearrangement of ynamides. This method enables
facile access to a variety of valuable 2-benzhydrylindoles with broad substrate scope in generally good yields under mild reaction
conditions. In addition, this chemistry can also be extended to the divergent synthesis of versatile 3-benzhydrylisoquinolines through
a similar ketyl−ynamide coupling and radical Smiles rearrangement, followed by dehydrogenative oxidation. Moreover, such an
ynamide Smiles rearrangement initiated by intermolecular photoredox catalysis via addition of external radical sources is also
achieved. By control experiments, the reaction was shown to proceed via key ketyl radical and α-imino carbon radical intermediates.
Scheme 1. Radical Smiles Rearrangement
INTRODUCTION
■
The Smiles rearrangement is an X → C aryl migration reaction
and is potentially of great practical interest to synthetic organic
chemists and medicinal chemists.1 However, there remain to
be limited reports detailing radical Smiles rearrangements.2−6
For instance, Nevado et al. disclosed a desulfonylated Smiles
rearrangement in a number of elegant studies by employing
sulfonylamides as precursors.2 In this reaction, α-carbonyl
radical is trapped by aryl to form a spiro intermediate followed
by desulfonylation to generate an amidyl radical, which can
undergo a variety of useful radical reaction cascades (Scheme
1a). Such a sulfonylamide Smiles rearrangement was also
3a
́
nicely exploited by the research groups of Gerard/Sapi and
many others.3b−j Most recently, a major breakthrough was the
combination of a Smiles rearrangement and radical cation
chemistry via photoredox catalysis by Stephenson and co-
workers, leading to the alkene aminoarylation involving the
generation of a β-aminoalkyl radical and amidyl radical.4
Despite these remarkable advances, the eventually formed
radical intermediates via Smiles rearrangement in these
reactions are limited to amidyl radical, except for recent N-
centered radical-initiated Smiles rearrangements, achieved by
Zhang and Belmont by employing N-fluorobenzenesulfoni-
mide (NFSI) and sulfonyl hydrazides as radical initiators,
respectively.5 Thus, it is still highly demanded to explore new
modes of radical Smiles rearrangement, especially with new
types of radical initiators.
owing to the fact that this chemistry can achieve a range of
useful carbonyl umpolung reactions and has been widely
applied in organic synthesis. A general approach for the
formation of the ketyl from a carbonyl group is to use
Received: December 31, 2019
Published: January 31, 2020
The generation of ketyl radicals by a single electron transfer
(SET) from a carbonyl group has received intensive attention
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a
stoichiometric SmI2, and various efficient SmI2-mediated
reductive coupling reactions have been developed.7 Recently,
great progress in photocatalytic proton-coupled electron
transfer (PCET)8 has evoked a new round of exploration on
the generation of ketyl radicals.9,10 In 2013, Knowles and co-
workers reported for the first time the photoredox-catalyzed
formation of ketyl radical intermediates by means of a PCET,
which enabled the catalytic ketyl−olefin coupling.9b In
addition, this visible-light-induced intermolecular reductive
ketyl−olefin coupling reaction10 was also elegantly demon-
strated by Chen10a and Ngai,10b successively, where the
formation of ketyl radicals from a single electron reduction was
involved. Although significant advances have been achieved,
these reactions generally rely on the use of the ketyl radical-
generation activators such as Lewis acids, Brønsted acids, in
situ-generated Brønsted acids, and others,9d and examples of
the relevant ketyl−alkyne coupling reaction are quite scarce.11
Ynamides have become prevalent in organic synthesis and
have attracted much attention in the past decades.12
Surprisingly, radical-mediated ynamide transformations have
been rarely explored but are certainly highly desirable.13
Inspired by the above results and our recent study on the
ynamide chemistry in heterocycle synthesis,14 we envisioned
that the intramolecular photoredox-catalyzed ketyl−ynamide
coupling of aryl sulfonyl-substituted ynamides might generate
the vinyl radical species, which would undergo further
desulfonylated Smiles rearrangement, eventually leading to 2-
benzhydrylindoles (Scheme 1b). Herein, we report the
successful implementation of such a photoredox-catalyzed
ketyl−ynamide coupling-triggered desulfonylated Smiles re-
arrangement, and a rare radical attack at the α position of
ynamide is involved.12d,13 Importantly, this protocol represents
the first radical Smiles rearrangement based on ynamides. The
reaction affords a variety of 2-benzhydrylindoles with broad
substrate scope in generally good yields. Moreover, this
chemistry can also be extended to the divergent synthesis of
versatile 3-benzhydrylisoquinolines via similar tandem ketyl−
ynamide coupling/Smiles rearrangement followed by dehydro-
genative oxidation. Notably, 2-benzhydrylindoles and 3-
benzhydrylisoquinolines are important heterocycles found in
various bioactive molecules (Figure 1).15,16 In addition, such
Table 1. Optimization of Reaction Conditions
yield
b
entry
photocatalyst
reaction conditions
(%)
1
2
3
4
5
6
7
8
[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
[Ir(ppy)2(dtbbpy)]PF6
[lr(ppy)2(dtbbpy)]PF6
MeOH, rt, 4 h
i-PrOH, rt, 4 h
1,4-dioxane, rt, 4 h
MeCN, rt, 4 h
DMSO, rt, 4 h
DMA, rt, 2 h
64
53
57
56
63
62
9
72
60
50
57
41
70
42
15
DME, rt, 2 h
DMF, rt, 4 h
9
[Ir(dF(CF)3 ppy)2(dtbbpy)]PF6 DMF, rt, 2 h
10
11
12
13
14
15
fac-Ir(ppy)3
Ru(bpy)3(PF6)2
Ru(bpy)3Cl2
4CzIPN
[Ir(ppy)2(dtbbpy)]PF6
DMF, rt, 2 h
DMF, rt, 4 h
DMF, rt, 4 h
DMF, rt, 4 h
DMF, rt, 4 h
DMF, rt, 4 h
c
a
Reaction conditions: 1a (0.1 mmol), Hantzsch ester (0.4 mmol),
photocatalyst (0.001 mmol), solvent (1 mL), rt, 2−4 h, N2; 30 W blue
LED is used, and the distance between the light source and the
b
Schlenk tubes is about 3 cm. Measured by 1H NMR using Hantzsch
c
ester and Hantzsch pyridine as internal standard. A 3 equiv amount
of Hantzsch ester was used.
Hantzsch ester as the reductant in the presence of 1 mol % of
[Ir(ppy)2(dtbbpy)]PF6 as the photocatalyst in MeOH at room
temperature in 4 h afforded the desired 2-benzhydrylindole 2a
in 64% yield (Table 1, entry 1). Further solvent screening
(Table 1, entries 2−8) revealed that the use of DMF as solvent
led to the formation of the desired 2a in 72% yield (Table 1,
entry 8). In addition, other iridium- and ruthenium-based
photocatalysts were also investigated but failed to improve the
reaction (Table 1, entries 9−12). Of note, the reaction could
also furnish the desired 2a in 70% yield by employing 4CzIPN
as photoredox catalyst, which represents a metal-free photo-
redox catalysis (Table 1, entry 13).17 However, low efficiency
was observed (<30%) catalyzed by other organic dyes such as
Eosin Y and Fluorescein. The use of 3 equiv of Hantzsch ester
led to a significantly decreased yield (Table 1, entry 14).
Finally, it was found that 2a could be obtained in 15% yield in
Figure 1. 2-Benzhydrylindoles and 3-benzhydrylisoquinolines in
bioactive molecules.
an ynamide Smiles rearrangement initiated by intermolecular
photoredox catalysis via addition of external radical sources is
also achieved. In this article, we report the results of our
detailed investigations of this radical cascade cyclization,
including the substrate scope, synthetic applications, and
mechanistic studies.
RESULTS AND DISCUSSION
■
Optimization of Reaction Conditions. At the outset,
benzoyl ynamide 1a was used as the model substrate to react
with commercial Hantzsch ester under photocatalysis (Table
1). To our delight, treatment of substrate 1a with 4 equiv of
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the absence of photocatalyst (Table 1, entry 15), while no 2a
formation was detected without light irradiation.
Substrate Scope. With the optimized reaction conditions
in hand, we then evaluated the substrate scope of the reaction.
As depicted in Table 2, a range of substituted benzoyl
on the aromatic ring could be readily converted into the
desired products 2o−2s in good yields. The molecular
structure of 2a was further confirmed by X-ray diffraction
(Figure 2).
a
Table 2. Synthesis of 2-Benzhydrylindoles 2
Figure 2. Structure of 2-benzhydrylindole 2a in its crystal.
Besides benzoyl-substituted ynamides, this tandem reaction
was also viable for other carbonyl-tethered ynamides. As
summarized in Table 3, the reaction proceeded smoothly with
other different benzoyl-substituted ynamides, affording the
desired 2-benzhydrylindoles 2t−2aa in 41−71% yields.
a
Table 3. Synthesis of 2-Benzhydrylindoles 2
a
Reaction conditions: 1 (0.2 mmol), Hantzsch ester (0.8 mmol),
[Ir(ppy)2(dtbbpy)]PF6 (0.002 mmol), DMF (2 mL), rt, 4 h, N2; 30
W blue LED is used, and the distance between the light source and
b
the Schlenk tubes is about 3 cm; isolated yields are reported. A 6
equiv amount of Hantzsch ester is used, 6 h.
ynamides 1 were compatible with this transformation to
furnish the corresponding 2-benzhydrylindoles 2a−2s with
yields ranging from 38% to 88%. Various aryl-substituted
ynamides (R1 = Ar) were first screened, and the reaction
afforded the desired 2-benzhydrylindoles 2a−2h in mostly high
yields. Of note, even cyano and sterically hindered ortho
groups could be tolerated. Interestingly, the reaction was also
applicable to the alkyl-substituted ynamide (R1 = alkyl) to
produce the desired 2i, albeit less efficiently. In addition,
ynamides with different aryl-substituted sulfonyl protecting
groups, even the thienyl sulfonyl, were suitable substrates for
this reaction, delivering the desired 2-benzhydrylindoles 2j−2n
in 60−75% yields. Moreover, ynamides bearing both the
electron-withdrawing and the electron-donating substituents
a
Reaction conditions: 1 (0.2 mmol), Hantzsch ester (0.8 mmol),
[Ir(ppy)2(dtbbpy)]PF6 (0.002 mmol), DMF (2 mL), rt, 4 h, N2; 30
W blue LED is used, and the distance between the light source and
the Schlenk tubes is about 3 cm; isolated yields are reported. Time =
b
10 h.
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Importantly, various functional groups such as F, Cl, CF3, and
CO2Et were compatible with this tandem cyclization. Addi-
tionally, ynamides bearing different ketone groups were also
suitable substrates to produce the desired products 2y (54%)
and 2z (42%). This chemistry could also be extended to the
synthesis of 3-unsubstituted 2-benzhydrylindole 2aa in a
serviceable yield starting from the corresponding formyl−
ynamide. Of note, attempts to extend the reaction to styryl-
substituted ynamide 1ab, terminal ynamide 1ac, and aliphatic
alkyl-linked ynamide 1ad only gave a complex mixture of
products.18
with various R2 substitution patterns were also good substrates
for this tandem process, with the desired products 4r−4t
afforded in moderate yields. The molecular structure of 4i was
confirmed by X-ray diffraction (Figure 3). Thus, this one-pot
This photoredox-catalyzed ketyl−ynamide coupling could
also be extended to benzoyl ynamides 3, and the correspond-
ing 3-benzhydrylisoquinolines 4 were obtained in 23−83%
yields. Of note, significantly improved yields could be achieved
under air atmosphere, which should accelerate the dehydro-
genative oxidation.19 We then investigated this reaction scope
under the optimized reaction conditions, as shown in Table 4.
a
Table 4. Synthesis of 3-Benzhydrylisoquinolines 4
Figure 3. Structure of 3-benzhydrylisoquinoline 4i in its crystal.
strategy provides a convenient and practical route for
preparation of the synthetically useful 3-benzhydrylisoquino-
lines. Our attempts to extend the reaction to one more carbon
homologue of ynamide 3u only led to the formation of
complicated mixtures.18
Besides intramolecular ketyl radical-initiated ynamide Smiles
rearrangement, this strategy was also applicable to intermo-
lecular photoredox catalysis by addition of external radical
sources. In particular, most of the radical precursor reagents
would introduce fluorine to products, thus suggesting potential
applications in medicinal chemistry.20 For example, Togni
reagent was chosen as a trifluoromethyl reagent to react with
styryl-tethered ynamides 5,21a and the desired trifluoromethyl
tetracyclic N-heterocycles 6a−6c were furnished in 45−72%
yields (eq 1). Of note, improved yields could be achieved in
a
Reaction conditions: 3 (0.2 mmol), Hantzsch ester (0.8 mmol),
[Ir(ppy)2(dtbbpy)]PF6 (0.002 mmol), DCE (2 mL), rt, 2 h, air; 30 W
blue LED is used, and the distance between the light source and the
b
vial is about 3 cm; isolated yields are reported. A 6 equiv amount of
Hantzsch ester is used, 3 h.
the latter two cases by addition of 3 Å molecular sieves. The
molecular structure of 6a was also confirmed by X-ray
diffraction.19 Here, it should be mentioned that the reaction
of ynamide 1a or 3a with Togni reagent resulted in the
formation of a complex product mixture. In addition, CF2H-
substituted N-heterocycle 6d was also obtained in 42% yield by
employing N-tosyl-S-difluoromethyl-S-phenylsulfoximine as
CF2H reagent (eq 2).21b Finally, the use of other types of
radical reagents such as BrCF2CO2Et and BrCH(CO2Et)2 also
The reaction occurred smoothly with ynamides bearing
different R1 groups and Ar-substituted sulfonyl protecting
groups, affording the desired 3-benzhydrylisoquinolines 4a−4j
in generally moderate to good yields. In addition, ynamides
with various R groups, even the methyl group, were suitable
substrates for this coupling to deliver the corresponding
products 4k−4q in generally good yields. Finally, ynamides
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benzhydrylindole 2ah was observed. Instead, 8a was formed in
37% yield via a radical cyclopropane ring-opening process (eq
4), indicating that the ketyl radical is likely involved as the
delivered the expected products 6e and 6f in 56% and 58%
yields, respectively (eq 3). Once again, regioselective radical
reactive species in this process. In addition, cascade cyclization
of ynamide 1ai under the same standard conditions led to the
formation of the corresponding indole-fused tetracyclic
compound 8b and indole-fused pentacyclic compound 8c in
30% and 42% yields, respectively (eq 5). This result also
addition on the α position of ynamides was achieved in all of
these cases. Of note, the use of Hantzsch ester (2 or 4 equiv)
as additive in these cases only led to the formation of
complicated mixtures. The reaction is most likely to involve a
radical addition/Smiles rearrangement/SET cascade.19
Synthetic Applications and Preliminary Biological
Tests. To substantiate the synthetic utility of this method-
ology, we also carried out several chemical transformations
(Scheme 2). First, it was found that the tandem reaction of
suggests that the ketyl radical is presumably generated and
initiates the subsequent tandem radical cyclization.19 Of note,
the relevant Smiles rearrangement did not occur in this case,
and the whole tosyl group was eventually removed. Instead, the
formed vinyl radical species was trapped by the styryl moiety.
Furthermore, it was found that the reaction of Ms-protected
ynamide 1aj could afford the corresponding Ms-substituted
eneindolin-3-ol 8d in 87% yield with a 3/1 E/Z ratio (eq 6).
Scheme 2. Synthetic Applications
Thus, generation of vinyl radical species from the ketyl radical
via regioselective ketyl−ynamide coupling is most likely
involved, but the Smiles rearrangement is interrupted in this
case. The molecular structures of 8b, 8c, and 8d were
confirmed by X-ray diffraction.19 Light on/off experiments
were also conducted19 and indicated that continuous
irradiation is essential for this transformation. Finally, the
luminescence quenching experiments indicated that the
Hantzsch ester is more likely to quench the excited state of
[Ir(ppy)2(dtbbpy)]PF6 instead of the ynamide (Figure 4),19
which is in accordance with Chen’s protocol.10a
On the basis of the above experimental observations, a
plausible mechanism to rationalize the formation of 2-
benzhydrylindole 2j and 3-benzhydrylisoquinoline 4h is
depicted in Scheme 3. First, irradiated by a light source, IrIII
is activated to IrIII* (excited state) which undergoes SET with
Hantzsch ester (HE) to produce HE•+ and IrII. Then ynamide
1j or 3h reacts with HE•+ via SET with IrII to generate the
ketyl radical intermediate A.23 Subsequently, this ketyl radical
undergoes regioselective ketyl−ynamide coupling followed by
Smiles rearrangement via radical intermediate B, affording N-
centered radical species C under extrusion of SO2. This imidyl
radical exists at its resonance structure α-imino carbon radical
D, which could be stabilized by two aromatic rings and a C
N double bond.5a Next, the intermediate D may abstract
hydrogen from HE• via hydrogen atom transfer (HAT, path a)
or undergoes SET followed by protonation (path b) to
generate the intermediate F, which can be isolated in the case
where benzoyl ynamide 3h is employed.19 Notably, prelimi-
nary density functional theory (DFT) computations indicate
ynamide 1ae under standard conditions led to the formation of
the desired 2-benzhydrylindole 2ae, which could be further
converted into the corresponding indole-fused tetracyclic
compound 7a via copper-mediated C−N coupling. The
molecular structure of 7a was further confirmed by X-ray
diffraction.19 Of note, this structural motif frequently occurs in
natural products and bioactive molecules.22 Moreover, the
synthesis of 2-benzhydrylindoles 7b with antitumor activity15a
and 7c with antibacterial activity15b was also achieved using
this protocol by starting from the corresponding ynamides 1af
and 1ag.
In addition, preliminary biological tests revealed that 2-
benzhydrylindoles 2i and 2ae and 3-benzhydrylisoquinolines
4f and 4l displayed their bioactivity as antitumor agents against
melanoma cells A375,19 suggesting a potential application of
these heterocyclic compounds in medicinal chemistry.
Mechanistic Study. To understand the reaction mecha-
nism, several control experiments were conducted. First, when
a standard radical clock cyclopropane substrate 1ae was
subjected to the above optimal conditions, no expected 2-
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CONCLUSION
■
In summary, we developed a photoredox-catalyzed regiose-
lective ketyl−ynamide coupling-triggered cascade cyclization.
Importantly, an unprecedented desulfonylation Smiles rear-
rangement was discovered, which represents the first radical
Smiles rearrangement based on ynamides. The reaction
proceeds under mild reaction conditions and has excellent
scope for the synthesis of various valuable 2-benzhydrylindoles
in generally good yields. Moreover, this chemistry can also be
extended to the divergent synthesis of versatile 3-benzhy-
drylisoquinolines via a similar ketyl−ynamide coupling, Smiles
rearrangement, and dehydrogenative oxidation. In addition,
such an ynamide Smiles rearrangement initiated by inter-
molecular photoredox catalysis via addition of external radical
sources is also achieved. Mechanistic studies revealed that the
reaction presumably proceeds via key ketyl radical and α-imino
carbon radical intermediates. The development of other novel
ynamide Smiles rearrangements and mechanistic investigations
are the subjects of ongoing research in our laboratory.
Figure 4. Stern−Volmer quenching studies.
Scheme 3. Plausible Reaction Mechanism
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/jacs.9b13975.
■
sı
Experimental procedures, characterization and analytical
data of products, and NMR spectra (PDF)
Crystallographic structure of 2a (CIF)
Crystallographic structure of 4i (CIF)
Crystallographic structure of 6a (CIF)
Crystallographic structure of 7a (CIF)
Crystallographic structure of 8b (CIF)
Crystallographic structure of 8c (CIF)
Crystallographic structure of 8d (CIF)
AUTHOR INFORMATION
Corresponding Author
■
Long-Wu Ye − iChEM, State Key Laboratory of Physical
Chemistry of Solid Surfaces, Key Laboratory of Chemical
Biology of Fujian Province, and College of Chemistry and
Chemical Engineering, Xiamen University, Xiamen 361005,
China; State Key Laboratory of Organometallic Chemistry,
Shanghai Institute of Organic Chemistry, Chinese Academy of
Sciences, Shanghai 200032, China; orcid.org/0000-0003-
3108-2611; Email: longwuye@xmu.edu.cn
Authors
Ze-Shu Wang − iChEM, State Key Laboratory of Physical
Chemistry of Solid Surfaces, Key Laboratory of Chemical
Biology of Fujian Province, and College of Chemistry and
Chemical Engineering, Xiamen University, Xiamen 361005,
China
Yang-Bo Chen − iChEM, State Key Laboratory of Physical
Chemistry of Solid Surfaces, Key Laboratory of Chemical
Biology of Fujian Province, and College of Chemistry and
Chemical Engineering, Xiamen University, Xiamen 361005,
China
Hao-Wen Zhang − iChEM, State Key Laboratory of Physical
Chemistry of Solid Surfaces, Key Laboratory of Chemical
Biology of Fujian Province, and College of Chemistry and
Chemical Engineering, Xiamen University, Xiamen 361005,
China
that path a is more favorable than path b.19 The next steps
presumably involve the combination of second photoredox
catalysis with spin-center shift (SCS)24 and subsequent SET,
generating the intermediate I. In the case of ynamide 1j as
substrate (n = 0), the final protonation delivers the desired
indole 2j. When ynamide 3h is employed (n = 1), protonation
followed by dehydrogenative oxidation,25 which is probably
assisted by photoredox catalysis,26 affords the corresponding
isoquinoline 4h.
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Zhou Sun − iChEM, State Key Laboratory of Physical Chemistry
of Solid Surfaces, Key Laboratory of Chemical Biology of Fujian
Province, and College of Chemistry and Chemical Engineering,
Xiamen University, Xiamen 361005, China
Chunyin Zhu − School of Chemistry and Chemical Engineering,
Jiangsu University, Zhenjiang 212013, China
Complete contact information is available at:
https://pubs.acs.org/10.1021/jacs.9b13975
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS
■
We are grateful for financial support from the National Natural
Science Foundation of China (21622204 and 21772161), the
Natural Science Foundation of Fujian Province of China
(2019J02001), the President Research Funds from Xiamen
University (20720180036), NFFTBS (No. J1310024),
PCSIRT, and Science & Technology Cooperation Program
of Xiamen (3502Z20183015). We thank Professor Xianming
Deng from Xiamen University (School of Life Sciences) for
assistance with biological tests and Mr. Zanbin Wei from
Xiamen University (College of Chemistry and Chemical
Engineering) for assistance with X-ray crystallographic analysis.
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Cascade Reaction of Alkynes with N-Fluoroarylsulfonimides and
Alcohols. Nat. Commun. 2015, 6, 7011. (b) Brachet, E.; Marzo, L.;
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(25) Control experiments revealed that much higher efficiency was
obtained when the reaction of 3a ran in the presence of air
atmosphere. Please see the SI for details.
(26) For selected examples, see: (a) Hossain, A.; Pagire, S. K.;
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(17) For selected examples on the use of Hantzsch ester as reductant
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Angew. Chem., Int. Ed. 2019, 58, 5697−5701. (b) Zhang, J.; Li, Y.; Xu,
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̈
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(18) For the structures of ynamides 1ab−1ad and ynamide 3u see
below:
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J. Am. Chem. Soc. 2020, 142, 3636−3644