Atom Transfer Oxidative Radical Cascade of Aryl Alkynoates towards 1,1-Dichalcogenide Olefins

Article abstract of DOI:10.1002/asia.201900820

An oxidative trifunctionalization of aryl alkynoates has been devised via the chalcogenide radical triggered intramolecular 1,4-aryl migration/decarboxylation cascade to prepare 1,1-dichalcogenide tetrasubstituted alkenes in high yields (up to 98 %). This operationally simple reaction proceeds under metal-free conditions, can be executed on gram scale, and highlights formal 1,1-difunctionalization of alkynes. Synthetic potential of this protocol was demonstrated through a twofold cascade rearrangement to access highly conjugated tetra-selenylated alkenes along with a cross-dehydrogenative annulation to prepare fluorene derivative.

Full text of DOI:10.1002/asia.201900820

CHEMISTRY
AN ASIAN JOURNAL
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Accepted Article
Title: Atom Transfer Oxidative Radical Cascade of Aryl Alkynoates
towards 1,1-Dichalcogenide Olefins
Authors: Harekrishna Sahoo, Isai Ramakrishna, Anup Mandal, and
Mahiuddin Baidya
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To be cited as: Chem. Asian J. 10.1002/asia.201900820
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Atom
Transfer
Oxidative
Radical
Cascade
of
Aryl
Alkynoates towards 1,1-Dichalcogenide Olefins
Harekrishna Sahoo, Isai Ramakrishna, Anup Mandal, and Mahiuddin Baidya*[a]
Dedicated to Professor K. K. Balasubramanian on the occasion of his 80th birthday
Abstract: Abstract: An oxidative trifunctionalization of aryl
alkynoates has been devised via the chalcogenide radical triggered
intramolecular 1,4-aryl migration/decarboxylation cascade to prepare
1,1-dichalcogenide tetrasubstituted alkenes in high yields (up to
98%). This operationally simple reaction proceeds under metal-free
conditions, can be executed in gram scale, and also highlights
formal 1,1-difunctionalization of alkynes. Synthetic potential of this
protocol was demonstrated through
a
twofold cascade
rearrangement to access highly conjugated tetra-selenylated
alkenes along with a cross-dehydrogenative annulation to prepare
fluorene derivative.
Atom transfer radical cascade strategies have gained significant
attention where multiple bond-forming and bond-breaking take
place in a single operation to build molecular complexity.[1] In
this context, aryl alkynoates encompassing activated alkyne
functionality serve as valuable building blocks.[2,3,4] Among the
various radical cascade reactions established with aryl
alkynoates, syntheses of functionalized alkenes based on 1,4-
aryl migration from oxygen center to carbon center via the spiro-
cyclic intermediate A, which resembles radical based Smiles
rearrangement,[5,6] are intriguing (Scheme 1a). However, most of
these cascade processes holistically account an oxidative
difunctionalization of aryl alkynoates to forge trisubstituted
olefins.[7] Examples of trifunctionalization en route to
tetrasubstituted olefins are scarce. In 2016, Han group[8]
reported an iodination triggered rearrangement of aryl
Scheme 1. Radical-based cascade rearrangement of aryl alkynoates towards
functionalized alkenes.
o
alkynoates at 160 C to access tetrasubstituted olefins (Scheme
With our continuous interest in organochalcogenide[13]
chemistry, recently, we have introduced a selenylative radical
difunctionalization process with aryl alkynoates to prepare highly
functionalized ,-unsaturated acids at room temperature.[13f]
Herein, we report a radical based cascade trifunctioanlization of
aryl alkynoates for the production of tetrasubstituted olefins that
feature 1,1-dichalcogenide framework (Scheme 1c). With a
judicious choice of aryl alkynoate building blocks, heretofore
unknown bis-trifunctionalization was also achieved by twofold
radical based Smiles rearrangement and concomitant
decarboxylation/selenylation sequence.
1b). To the best of our knowledge, till now, this is the only report
that deals with cascade reaction of aryl alkynoates towards fully
substituted alkenes. Thus, there is ample scope in radial based
Smiles rearrangement of aryl alkynoates for the synthesis of
tetrasubstituted olefins.
Organochalcogenides are privileged scaffolds found in a
myriad of natural products, agrochemicals and pharma-
ceuticals.[9] They have also received increasing attention due to
their widespread applications in organic transformations,
catalysis, fluorescence probe imaging, and functional
materials.[10] Specifically, poly-selenide alkene derivatives are
known as antinociceptive agents in mice, neuroprotective agents
in rodents, and human erythrocyte aminolevulinate dehydratase
(-ALA-D) inhibitors (Scheme 1d).[11] Recently, they have also
been used as a molecular magnetic superconductor in device.[12]
Therefore, the construction of chalcogenide alkene skeletons in
an efficient manner is of utmost importance.
We commenced our investigation on atom transfer radical
cascade trifunctionalization of aryl alkynoate 1a with readily
available diphenyl diselenide 2a under the influence of different
oxidants. Satisfyingly, when the mixture of 1a and 2a in DCE
o
solvent was exposed to K2S2O8 oxidant at 120 C, the selenium
radical-induced cascade trifunctionalization was realized to
deliver tetrasubstituted gem-diselenoalkene 3a in 27% isolated
yield (Table 1, entry 1). When chlorobenzene was used as
solvent, yield improved to 77% (entry 2). Screening of other
oxidants such as TBHP, H2O2, DCP, and DTBP, gave DTBP as
the most suitable oxidant to furnish product 3a in 97% yield
(entries 3-6). Further, screening of reaction solvents using DTBP
a]
H. Sahoo, A. Mandal, I. Ramakrishna, and Prof. M. Baidya
Department of Chemistry
Indian Institute of Technology Madras
Chennai 600 036, Tamil Nadu, India
E-mail: mbaidya@iitm.ac.in
Supporting information for this article is given via a link at the end of the
document.
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oxidant revealed that the reaction was very effective in toluene
(88%) and acetonitrile (90%); however, yields dropped to 79%
and 81% in DMSO and DMF, respectively (entries 7-10). When
the reaction was performed in the air atmosphere, the yield of
the product was reduced to 54% yield (entry 11). Reduction in
yield was also observed with the decrease of the DTBP loading
and the method completely failed without oxidant, demonstrating
the pivotal role of oxidant in this reaction (entries 12-13).
1x derived from the 2-butynoic acid was an ineffective substrate
for this reaction.
A gram scale synthesis using aryl alkynoate 1o was also
carried out and the efficiency of the small-scale reaction was
comparable upon scale-up, affording 3o in 89% yield.
The efficiency of various diselenides was also evaluated
(Scheme 2). Diaryl diselenides having methoxy (4d) and
different halogen functionalities at para- (4a-c), meta- (4e), and
ortho- (4f) positions gave desired products in synthetically useful
yields (55-89%). The compound 4a was crystallized, and its X-
ray analysis unambiguously confirmed the tetrasubstituted gem-
diselenoalkene framework. 2,2′-Dithienyl diselenide and dimethyl
diselenide were also suitable substrates for this reaction and
Table 1. Optimization of reaction conditions[a]
Entry
1
Oxidant
K2S2O8
K2S2O8
TBHP
H2O2
Solvent
DCE
Yield [%][b]
27
77
72
trace
33
97
88
90
81
79
54
66
0
2
PhCl
3
PhCl
4
PhCl
5
DCP
PhCl
6
DTBP
DTBP
DTBP
DTBP
DTBP
DTBP
DTBP

PhCl
toluene
MeCN
DMSO
DMF
7
8
9
10
11[c]
12[d]
13
PhCl
PhCl
PhCl
[a] Reaction conditions: 1a (0.2 mmol), 2a (0.2 mmol), oxidant (0.4 mmol),
solvent (1.5 mL), 120 oC, 24 h. [b] Isolated yields. [c] Reaction was performed
under air instead of N2 atmosphere. [d] Reaction was performed using 0.2
mmol of DTBP. TBHP: tert-butyl hydrogen peroxide. DCP: dicumyl peroxide.
DTBP: di-tert-butyl peroxide.
After identifying the optimal reaction conditions, we next
explored the scope of this cascade trifunctionalization and the
results are summarized in Scheme 2. The reaction is quite
general. The aryl alkynoates bearing electron-releasing groups,
for examples methyl (3b,j), alkoxy (3c,m,p) and electron-
withdrawing substituents, for instances, chloro (3d,n), bromo
(3e,o), iodo (3f), ester (3g), fluoro (3k) and trifluoromethyl (3l) at
para- and meta-positions of the O-aryl ring delivered expected
gem-diselenoalkenes in uniformly high yields (78-98%).
Sensitive functionalities such as ketone and aldehyde were well
tolerated to provide 3h and 3i in 82% and 67% yields,
respectively. The presence of bulky naphthyl and biphenyl
groups did not impede the cascade reaction, delivering 3q-r in
high yields. Changes in the alkyne aryl moiety were also
examined. The para- (3s-u), meta- (3v), and ortho- (3w)
functionalized aryl alkynoates offered expected products in high
to excellent yields (66-97%). However, the alkynoate compound
Scheme 2. Substrate scope for trifunctionalization of aryl alkynoates with
dichalcogenides.
desired products 4g-h were obtained in moderate yields. The
protocol was not limited to diselenide only, it worked equally well
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with diphenyl disulfide 5 to give the gem-dithioalkenes 7a-b in
very high yields. However, the diphenyl ditelluride 6 was turned
out ineffective substrate in this radical cascade reaction and the
origin for this shortcoming is not apparent at this juncture
(Scheme 2).
not detected. However, a diselenylated compound 13 was
obtained in 27% yield (Scheme 5c). All these findings are in line
with a spiro-cyclic intermediate for this cascade reaction.
The versatility of this protocol was further extended for a
twofold radical based Smiles rearrangement employing suitably
substituted aryl alkynoates 9 (Scheme 3). Pleasingly, when
sulfinylbis (4,1-phenylene) bis(3-phenylpropiolate) 9a and thio-
bis(4,1-phenylene) bis(3-phenylpropiolate) (9b) were reacted
under the optimized conditions with diphenyl diselenide 2a,
desired radical cascade proceeded smoothly and the
polyselenylated olefins 10a,b were isolated in 69% and 73%
yields, respectively. The diselenide with halogen substituents
also gave products with a thioether link (10c-e) in moderate
yields (44-59%). These compounds would be very useful in
material chemistry.
Scheme 5. Control experiments.
Considering these observations and preceding literature
findings, a plausible reaction mechanism is depicted in Scheme
6. The aryl selenium radical generated from diselenide upon
heating with DTBP adds to alkynyl bond of 1 and gives the vinyl
radical species A, which consequently undergoes an
intramolecular radical ipso-cyclization to produce B. The
intermediate B undergo a 1,4-aryl migration and decarboxylation
to provide the vinyl radical D. The vinyl radical D reacts with
another molecule of diaryl diselenide (or aryl selenium radical) to
give the desired product 3.
Scheme 3. Substrate scope for twofold smiles rearrangement of aryl
alkynoates.
To showcase further synthetic utility, a palladium catalyzed
cross-dehydrogenative annulation reaction was accomplished to
obtain the fluorene derivative 11 in 51% yield (Scheme 4).[14]
Also, exposure of product 3d to m-CPBA produced selenoxide
12 in 62% yield.
Scheme 6. Plausible reaction mechanism.
In summary, we have disclosed an atom transfer radical-
based Smiles rearrangement cascade for trifunctionalization of
aryl alkynoates to furnish diverse tetrasubstituted alkenes
comprising 1,1-dichalcogenide framework in good to excellent
yields. The protocol features operational simplicity, scalability,
and displayed ample substrate scope. A twofold cascade
sequence was also accomplished to prepare selenium-enriched
alkenes. These products are also good substrates for cross-
dehydrogenative annulation reaction to access fluorene
derivatives. The reaction involves a radical mechanism, which
was supported through various control experiments. Further
Scheme 4. Post-functionalizations.
To probe the reaction mechanism, few control experiments
were executed (Scheme 5). The reaction yield drastically fall
down in the presence of TEMPO, butylated hydroxytoluene
(BHT), and 1,1-diphenylethylene as radical scavengers,
suggesting the participation of radical species in the cascade
process (Scheme 5a). The product 3y was isolated in 73% yield
from the substrate 1y bearing ortho-occupied O-aryl ring
(Scheme 5b). When methyl alkynoate 1z was treated under the
standard reaction conditions, the rearrangement product 3z was
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We gratefully acknowledge DST-SERB India for financial
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funding support through the Institute Research Development
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Keywords: Cascade Reaction • Selenium Radical •
Dichalcogenides • Aryl alkynoates • gem-Diselenoalkenes
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Entry for the Table of Contents (Please choose one layout)
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Harekrishna Sahoo, Isai Ramakrishna,
Anup Mandal, and Mahiuddin Baidya*
Page No. – Page No.
Atom Transfer Oxidative Radical
Cascade of Aryl Alkynoates towards
1,1-Dichalcogenide Olefins
A chalcogenide radical triggered cascade rearrangement is reported for oxidative
trifunctionalization of aryl alkynoates to prepare 1,1-dichalcogenide tetrasubstituted
alkenes in high yields (up to 98%).
For internal use, please do not delete. Submitted_Manuscript
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