Electrochemical Chalcogenation of β,γ-Unsaturated Amides and Oximes to Corresponding Oxazolines and Isoxazolines

Article abstract of DOI:10.1002/adsc.201901262

The current report represents a transition-metal-free synthesis of oxazoline and isoxazoline derivatives by a tandem electro-oxidative chalcogenation-cyclization process. Both C?Se and C?S bond-forming protocols were developed without using any external oxidant and the reaction was performed at room temperature, open to the air. Using this methodology, 29 substituted oxazoline and 16 substituted isoxazoline derivatives were synthesized with up to 91% isolated yield. (Figure presented.).

Full text of DOI:10.1002/adsc.201901262

Title: Electrochemical Chalcogenation of <i>β,γ</i>-Unsaturated
Amides and Oximes to Corresponding Oxazolines and
Isoxazolines
Authors: Samrat Mallick, Mrinmay Baidya, Kingshuk Mahanty,
Debabrata Maiti, and Suman De Sarkar
This manuscript has been accepted after peer review and appears as an
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published online in Early View as soon as possible and may be different
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To be cited as: Adv. Synth. Catal. 10.1002/adsc.201901262
Link to VoR: http://dx.doi.org/10.1002/adsc.201901262

10.1002/adsc.201901262
Advanced Synthesis & Catalysis
COMMUNICATION
DOI: 10.1002/adsc.201((will be filled in by the editorial staff))
Electrochemical Chalcogenation of β,γ-Unsaturated Amides and
Oximes to Corresponding Oxazolines and Isoxazolines
Samrat Mallick,a Mrinmay Baidya,a Kingshuk Mahanty,a Debabrata Maiti,a and Suman
De Sarkara*
a
Department of Chemical Sciences, Indian Institute of Science Education and Research Kolkata, Mohanpur-741246,
West Bengal, India
E-mail: sds@iiserkol.ac.in
Received: ((will be filled in by the editorial staff))
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201######.((Please
delete if not appropriate))
In recent years tandem oxidative cyclization reactions
Abstract. The current report represents a transition-metal-
free synthesis of oxazoline and isoxazoline derivatives by have been proven as an effective synthetic means for
a tandem electro-oxidative chalcogenation-cyclization
these core heterocycle skeletons.[5] N-Allyl amides and
process. Both C–Se and C–S bond-forming protocols were
developed without using any external oxidant and the
reaction was performed at room temperature, open to the
air. Using this methodology, 29 substituted oxazoline and
16 substituted isoxazoline derivatives were synthesized
with up to 91% isolated yield.
β,γ-unsaturated oximes are recognized as versatile
precursors for oxidative cyclization to produce
diversely functionalized oxazoline[6] and isoxazoline[7]
frameworks. Regardless of the accomplished
advancements, scopes of the above reactions have
several shortcomings due to the use of harsh reaction
conditions, stoichiometric amount of oxidants, the
involvement of transition metals and prolonged
reaction times. Therefore, to develop environmentally
benign sustainable efficient synthetic methods are still
in high demand.
Keywords: Electrochemistry; Chalcogenation;
Oxazoline; Isoxazoline; Oxidation
Oxazolines[1] and isoxazolines[2] represent two
branches of privileged heterocycle motifs numerously
present in several biologically active natural products
(Figure 1). These are the key scaffolds in many
pharmaceuticals
and
agronomical
industry.
Furthermore, oxazoline core has widespread
utilization as protecting group, chiral ligand and
auxiliary in asymmetric synthesis and material
science.[3] Therefore, elegant methodologies for the
construction of these venerable heterocycles allure
many synthetic and medicinal chemists. In last few
decades, an extensive endeavor has been made to
develop proficient methods for the synthesis of these
five-membered heterocycles.[4]
Scheme 1. Electrochemical synthesis of chalcogenated
oxazoline and isoxazoline scaffolds.
Organochalcogen moieties are the essential structural
core found in many bioactive molecules and natural
products.[8] Thus, C–S and C–Se bond forming
reactions are in high demand. Furthermore, these
chalcogen functionalities have potential applications
in a wide number of organic transformations.[9] As
Figure 1. Biologically active a) oxazolines, isoxazolines
and b) organochalcogens.
1
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10.1002/adsc.201901262
Advanced Synthesis & Catalysis
sulfur and selenium groups can be modified easily via
oxidation or reduction, so incorporation of such atoms
can lead to many useful conversions.[10] Due to the
great potential of these ubiquitous structural motifs,
tireless endeavors have been made towards the
synthesis of organochalcogen compounds.[11] In the
last decade various reports have been published to
following screening of the solvents, such as MeOH,
MeCN/H2O mixtures were investigated and found that
MeOH is also equally effective while the mixture of
MeCN/H2O leads to a decrease in yield. Among the
various electrolytes used, tetrabutylammonium
tetrafluoroborate ends up with comparable yield to
87% with that of lithium perchlorate (Table 1, entry 7).
Altering the output current to 10 mA or 20 mA proved
to be less efficient (Table 1, entries 8 and 9). Different
proportions of diphenyl diselenide (2a) were tested
and a substoichiometric amount (0.8 equiv) was found
to be the most effective. The desired oxazoline did not
form when no electricity was run (entry 12).
construct
C–S
and
C–Se
bonds
using
photocatalysis,[12] transition metal catalysis,[13] or by
activating with different Lewis acids and Brønsted
acids.[14] Despite the major progress, these
methodologies have been largely restricted from some
drawbacks like toxic and pungent reagents, harsh
reaction conditions, excess use of oxidants, transition
metals, and low scalability.[15] The majority of these
reactions require super-stoichiometric amount of
chalcogen source, resulting in low atom-economy and
often causes undesirable byproducts. Therefore
sustainable, oxidant and metal-free environment-
friendly approach to synthesize organochalcogen
moieties, especially attached with heteroaryl groups
are still under-explored from the green chemistry
perspective.
Although electro-organic synthesis was first
established over 200 years ago,[16] in recent years it
emerged as an attractive protocol for organoredox
transformations due to its environmentally benign and
economically viable features.[17] This electro-synthetic
strategy is globally accepted as a smart alternative for
stoichiometric redox reagents and successfully
excluding chemical wastes and toxic byproducts.[18]
However, fruitful electrochemical approach to
synthesize C–chalcogen bond is underexplored.[19] In
2017, Aiwen Lei disclosed electrochemical thiolation
of indole derivatives starting from thiol,[19e] but
thiolation using disulfide is yet to be investigated. To
the best of our knowledge oxidant free chalcogenation
of β,γ-unsaturated oxime to form isoxazoline
derivative under electrochemical conditions has not
been reported so far. Given the importance of
oxazoline and isoxazoline moieties, herein we report a
sustainable method for the electro-oxidative
chalcogenation of different amides[20] and oximes to
synthesize chalcogen-functionalized heterocycles, in a
mediator free undivided electrochemical cell (Scheme
1).
To ensure the synthetic utility of our
electrochemical methodology, we commenced our
study by treating N-allyl benzamide (1a) with diphenyl
diselenide (2a) in an undivided cell equipped with
graphite as electrodes. To our delight, 79% isolated
yield of selenylated oxazoline 3aa was achieved when
electrolyzed in 0.1 M LiClO4 containing acetonitrile
solution (Table 1, entry 2). We further investigated the
effect of different electrode materials. Replacing
cathode as platinum plate instead of graphite increased
the efficacy of the reaction to 91% isolated yield
(Table 1, entry 1). The yield of the desired product
was diminished slightly when platinum was employed
both as anode and cathode (Table 1, entry 3).
Subsequently, Ni foam cathode was found not very
efficient in product formation (Table 1, entry 4). The
Table 1. Optimization of the reaction conditions[a]
Entry Deviation from Standard
Conditions[a]
Yield
[%][b]
91
79
84
1
2
3
4
5
None
Graphite both as anode and cathode
Pt both as anode and cathode
Ni foam as cathode
77
86
MeOH instead of MeCN
6
MeCN/H2O Instead of MeCN
70
7
Bu4NBF4 instead of LiClO4
10 mA for 3 hr
20 mA for 1.5 hr
87
83
82
90
72
NR
8
9
10
11
12
0.25 mmol of 2a was used
0.13 mmol of 2a was used
Without current
[a] Standard conditions: 1a (0.25 mmol), 2a (0.20 mmol),
LiClO4 (0.1 M), MeCN (5 mL), C anode, Pt cathode,
undivided cell, constant current = 15 mA, at r.t. under air for
2 h. [b] Isolated yield.
After establishing the optimized conditions, we
next subjected a range of N-allyl amides 1 and
diselenide 2 into electro-oxidative cyclization to
acquire various oxazoline derivatives 3 (Table 2). We
first examined the electronic and steric effects of a
series of substrates and gratifyingly found that the
protocol is widely applicable. Electron rich arenes
containing –Me, –tBu, –OMe groups at different
positions of the aryl ring reacted cleanly to furnish the
corresponding selenylated oxazoline in excellent
yields. Amide 1k comprising free hydroxy group was
also delivered desired oxazoline 3ka in good yield
without forming any noticeable byproduct. A series of
halogen-containing arenes smoothly delivered very
good isolated yield ranging from 81-90%. Electron
withdrawing –CF3, –CN groups at the para-position of
arenes were also well tolerable. It is worth noting that
heterocyclic group (pyridine, thiophene) bearing
amides were tolerated well to delivered corresponding
2
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10.1002/adsc.201901262
Advanced Synthesis & Catalysis
selenylated products in satisfactory yields (3oa and
3pa). The reaction scope was further reviewed with a
series of ortho-, meta- and para-substituted diaryl
diselenide derivatives. In all cases, excellent yields of
the oxazolines were observed (3ab-3ag). The reaction
was also found to be general with respect to the
electronic influence, both electron-rich and electron
poor diselenides displayed good to excellent
efficiency.
Table 2. Substrate Scope for the Synthesis of Selenylated
Oxazolines [a], [b]
Scheme 2. Scope of Amides and Thiobenzamide.
Amides with varying chain length and unsaturation
were also compatible in the oxidative cyclization
process (Scheme 2). N-Propargyl amide 4a delivered
the oxazoline 5a with an exocyclic double bond as a
single diastereomer. Pleasingly, the six-membered
heterocycle 5b was formed efficiently with N-
homoallyl amide 4b. Finally, the thiazoline derivative
5c was synthesized from corresponding N-allyl
thiobenzamide 4c following the standard reaction
conditions.
Table 3. Substrate Scope for the Synthesis of Selenylated
Isoxazoline [a], [b]
[a]
[a]
Reaction conditions: 1 (0.25 mmol), 2 (0.20 mmol),
Reaction conditions: 6 (0.25 mmol), 2a (0.20 mmol),
LiClO4 (0.1 M), MeCN (5 mL), C anode, Pt cathode,
undivided cell, constant current = 15 mA, at r.t. under air for
2 h. [b] Isolated yield.
LiClO4 (0.1 M), MeCN (5 mL), C anode, Pt cathode,
undivided cell, constant current = 10 mA, at r.t. under air for
2 h. [b] Isolated yield.
3
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10.1002/adsc.201901262
Advanced Synthesis & Catalysis
Next, we investigated the synthesis of selenylated
lower efficiency was observed and thiolated
isoxazoline moieties 7 by electrooxidation of oximes 6
and diphenyl diselenide (2a) with minor modification
of the standard conditions (Table 3). para-Substituted
electron-rich aryl oximes underwent smooth
conversion to the isoxazoline in high yields (7aa-da).
Oxime 6e containing free hydroxyl group was found
as a competent substrate under electrochemical
conditions. A series of halo-substituted oximes were
tested, in all cases, the smooth formation of
selenylated isoxazolines were observed (7fa-ha).
meta-Substituted aryl oximes were also yielded the
corresponding isoxazolines efficiently (7ia and 7ja).
Finally, good to excellent yields were obtained with
thiophenyl and naphthyl oximes (6ka and 6la).
isoxazoline 10 were formed in decent yields.
Synthetic practicability of the developed method
was proved in a gram-scale synthesis of 3aa, by
maintaining the short reaction time and production
efficiency (Scheme 3a). Derivatization of 3aa was
performed by oxidation to form selenoxide 11. In
another reaction, ring-opening hydrolysis of 3aa
delivered selenylated alcohol 12 (Scheme 3b).
Table 4. Substrate Scope for the Synthesis of Thio
Oxazolines and Isoxazolines [a], [b]
Scheme 3. Gram-scale synthesis and derivatizations.
To get the mechanistic insight, the reaction was
performed in the presence of the radical quencher
TEMPO. The amide 1a underwent product formation
with diminished yield (Scheme 4a), whereas shutdown
in the selenylation reaction was observed for the oxime
6a (Scheme 4b). Moreover, the adduct 13 which
corresponds to the trapping of the radical intermediate
by TEMPO was isolated in high yield. These
observations indicate the possibility of both radical
and ionic reaction pathways for the overall annulation
and chalcogenation process.
[a] Reaction conditions: 1 or 6 (0.25 mmol), 8a (0.20 mmol),
LiClO4 (0.1 M), MeCN (5 mL), C anode, Pt cathode,
undivided cell, constant current = 10-15 mA, at r.t. under air
for 2 h. [b] Isolated yield.
To extend the generality of the electro-synthetic
protocol we explored the reaction of disulfide 8a with
various amides 1 and oximes 6 (Table 4). A series of
electronically and sterically diverse amides underwent
smooth cyclization to form the thiolated oxazoline 9 in
good yields. In the case of oximes, comparatively
Scheme 4. Mechanistic studies.
4
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10.1002/adsc.201901262
Advanced Synthesis & Catalysis
Based on the evidence gathered from the mechanistic
studies, cyclic voltammetric studies (see SI for
details), and literature reports, the probable
mechanistic paths are depicted in Scheme 5. Oxidative
activation of the diphenyl diselenide 2a (oxidation
peak at 1.66 V vs. Ag/AgCl) generates phenyl
selenium cation A and phenyl selenium radical B.
Further, one-electron oxidation of B forms another
cation A. Addition of A to the double bond can
generate intermediate C which upon nucleophilic
cyclization by the amide oxygen produce oxazoline
3aa (pathway I). Alternatively, reduction of 2a is also
feasible (reduction peak at -1.44 V vs. Ag/AgCl) at the
reaction conditions to deliver radical B and phenyl
selenium anion D. One electron oxidation of D forms
another radical B. On the other hand, oxidation of the
amide 1a (oxidation peak at 1.32 V vs. Ag/AgCl) can
produce delocalized radical E, follow up radical
cyclization and coupling with B generate 3aa
(pathway II).
applying 0.5 F/mol electricity at different potentials
(Scheme 4c). At 1.0 V, no redox process is operating,
thus product formation was not observed. Next, the
potential was increased to 1.55 V, at this point only the
reduction of 2a and oxidation of 1a are feasible, which
implies that radical cyclization is the only viable
pathway. Indeed, 27% of 3aa was formed which is in
support of the radical annulation. The potential was
further increased to 1.77 V which allows both the
reduction and oxidation of 2a, thus opens up the
possibility of both pathways I and II. Under this
conditions, the observed 3aa production was 39%,
which is higher than the maximum possible yield (33%
using 0.5 F/mol electricity) considering only pathway
II. Therefore, the ionic pathway I must also be
operational in the product formation. A similar
mechanistic hypothesis is also applicable for the
oxidative selenylation of allyl oximes to produce
isoxazolines. However, the formation of TEMPO
adduct 13 and comparatively low potential of 6a
(oxidation peak at 1.13 V vs. Ag/AgCl) indicate that
the radical pathway may have a major contribution in
isoxazoline formation.
Herein, we report the sustainable synthesis of
chalcogenated oxazoline and isoxazoline derivatives
under electrochemical conditions. Only sub-
stoichiometric amount of chalcogen source was used
and the reaction was performed at room temperature,
open to air under neutral reaction conditions which
results in the compatibility of a wide range of
functional groups. Mechanistic studies indicate both
radical and ionic reaction pathways are operative for
the tandem C–O and C–chalcogen bond formations.
This external oxidant and metal-free green protocol
reveals synthetic benefits in terms of easily accessible
amides and oximes as starting materials, short reaction
time and also claims potential applications in large
scale synthesis.
Experimental Section
General Procedure for Electrochemical Synthesis of
Selenylated Oxazolines 3:
In an oven-dried undivided reaction flask (10 mL) equipped
with a stir bar were added N-allylbenzamide derivatives (1,
0.25 mmol), diselenide (2, 0.2 mmol), LiClO4 (0.1 M) and
MeCN (5 mL). The flask was equipped with a platinum
sheet (8 mm×30 mm×0.25 mm) as the cathode and a
graphite plate (5.2 cm×0.8 cm×0.2 cm) as the anode.
Thereafter the solution was electrolyzed at a constant
current of 15 mA at room temperature open to the air for 2
h. After completion of the reaction, the solvent was
evaporated in vacuum and purified by silica gel column
chromatography to deliver oxazolines 3.
Scheme 5. Proposed mechanism.
The electricity demand of the above-mentioned
processes is different. The ionic pathway I requires
1.0 F electricity for per mole of 3aa production,
whereas the radical pathway II needs 1.5 F per mole
(see SI for details). To validate the possibility of these
two pathways, 1a and 2a were allowed to react by
General Procedure for Electrochemical Synthesis of
Selenylated Isoxazolines 7:
In an oven-dried undivided reaction flask (10 mL) equipped
with a stir bar were added allyl-oxime derivatives (6, 0.25
5
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10.1002/adsc.201901262
Advanced Synthesis & Catalysis
mmol), diphenyl diselenide (2a, 0.2 mmol, 62 mg), LiClO4
(0.1 M) and MeCN (5 mL). The flask was equipped with a
platinum sheet (8 mm×30 mm×0.25 mm) as the cathode and
a graphite plate (5.2 cm×0.8 cm×0.2 cm) as the anode.
Thereafter the reaction mixture was stirred and electrolyzed
at a constant current of 10 mA under room temperature for
2 hrs. After the completion of the reaction the solvent was
evaporated in vacuum and purified by silica gel column
chromatography to deliver isoxazolines 7.
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Acknowledgements
[9]
Generous support by IISER Kolkata [Fellowship to SM and
infrastructure], CSIR [Fellowship to MB and DM], UGC
[Fellowship to KM] and SERB, DST, India [ECR/2016/000225 to
SDS] is gratefully acknowledged.
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Advanced Synthesis & Catalysis
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10.1002/adsc.201901262
Advanced Synthesis & Catalysis
COMMUNICATION
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8
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