An unconventional sulfur-to-selenium-to-carbon radical transfer: Chemo-and regioselective cyclization of yne-ynamides

Article abstract of DOI:10.1039/c9gc03745d

An uncommon sulfur → selenium → carbon radical transfer process is employed to develop an unprecedented selenyl radical-mediated regioselective cyclization of yne-tethered-ynamides. Density functional theory studies and HRMS experiments are used to establish a reactivity scale between thiyl and selenyl radicals. The unique features of this transformation include, (1) the chemoselective reactivity of RSe over RS, (2) regioselective RSe attack on alkyne over ynamide, (3) 5-exo-dig cyclization of yne-ynamide to unusual 4-selenyl-pyrroles, and (4) the green synthetic method. The reaction of methyldiselenide with yne-ynamides to methylselenopyrroles is also described.

Full text of DOI:10.1039/c9gc03745d

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Prabagar, R. Vanjari, V. Gandon and A. K. Sahoo, Green Chem., 2020, DOI: 10.1039/C9GC03745D.
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Green Chemistry
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DOI: 10.1039/C9GC03745D
COMMUNICATION
An Unconventional Sulfur-to-Selenium-to-Carbon Radical
Transfer: Chemo-and Regioselective Cyclization of Yne-Ynamides
Shubham Dutta,† B. Prabagar,†$ Rajeshwer Vanjari,†$ Vincent Gandon‡¥* and Akhila K. Sahoo†*
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
An uncommon sulfur→selenium→carbon radical transfer process is rate constant for the addition to the unsaturated system being
employed to develop an unprecedented selenyl radical-mediated 1050 fold higher than that of PhSe•.5 Surprisingly, a reverse S•
a) Background
regioselective cyclization of yne-tethered-ynamides. Density
Se to S radical transfer
functional theory studies and HRMS experiments are used to
establish a reactivity scale between thiyl and selenyl radicals. The
unique features of this transformation include, 1) the
chemoselective reactivity of RSe• over RS•, 2) regioselective RSe•
attack on alkyne over ynamide, 3) 5-exo-dig cyclization of yne-
ynamide to unusual 4-selenyl-pyrroles, and 4) green synthetic
method. The reaction of methyldiselenide with yne-ynamides to
methylselenopyrroles is also described.
Known
SPh
R2
SPh
R2
R2
R1
R1
PhS
PhSe
R1
Unknown
SePh/SPh
S to Se radical transfer
b) Strategy
R'
SPh R
N
R
Ph
N
R'
Path I
Selenium is a micronutrient essentially found in natural
products, amino acids, and proteins.1 Se-bearing heteroaryls
are invaluable building blocks exhibiting distinct chemical,
physical, and biological properties of pharmaceutical
relevance.2 Therefore, green synthetic methods that allow the
rapid introduction of Se into heteroaromatic scaffolds are highly
desirable. In this regard, binary chalcogens of type (RS)2 and
(RSe)2 exhibit diverse synthetic applications, primarily including
the homo-difunctionalization of unactivated olefins and
alkynes.3 Another notable application is the hetero-
difunctionalization of olefins and alkynes involving the
dominance of two reactive radical species, PhSe• and PhS•,
TS= 15.8 kcal/mol
Ph
R= SO2Ph, R'=
Ph
A
.
catalytic
Initiator
(low concentration of S )
PhS
PhSH
R'
N
R
SePh
R
Ph
N
Path II
PhSe
(PhSe)2
R'
Ph
TS= 4.0 kcal/mol
TS= 8.7 kcal/mol
A
c) This work
Ph
Ph
PhSe
H
Ph
PhSe
(PhSe)2 PhSH
N
H
TsN
Ph
•
•
through the sequential radical transfer: Se•
C , via the
Ts
→
S
→
attack of PhS• to olefin or alkyne (Figure 1a).4 These reactions
are dependent on the preferential reactivity of PhS•, with the
DFT calculations
34 examples High FG tolerance
Figure 1. a) Background: Se to S radical transfer; b) Strategy: S to Se radical transfer; c)
This work: chemo and regioselective selenyl radical attack to alkyne.
•
•
→
†
Se cross-radical transfer to promote the addition of PhSe
radicals to unsaturated bonds remains unexplored (Figure 1).
a.
[ ] School of Chemistry, University of Hyderabad, Hyderabad-500046, India.
E-mail: akhilchemistry12@gmail.com, akssc@uohyd.ac.in
[‡] Vincent Gandon, Institut de Chimie Moléculaire et des Matériaux d’Orsay,
CNRS UMR 8182, Université Paris-Sud, Université Paris-Saclay, Bâtiment 420,
b.
To address this challenge, the density functional theory (DFT)
studies (detailed in Figure 2) were sought; the reaction between
PhS• and alkyne (A) requires 15.8 kcal/mol (Path I, Figure 1b),
whereas the PhS• with (PhSe)2 needs 4.0 kcal/mol (Path II.
Figure 1b). These valuable pieces of information can indeed be
beneficial for circumventing the issue of the PhS• affinity for
91405 Orsay cedex (France)
[¥] Vincent Gandon, Laboratoire de Chimie Moléculaire (LCM), CNRS UMR
c.
9168, Ecole Polytechnique, Institut Polytechnique de Paris, route de Saclay, 91128
Palaiseau cedex (France)
[$] These authors contributed equally.
d.
Electronic Supplementary Information (ESI) available: [details of any supplementary
information available should be included here]. See DOI: 10.1039/x0xx00000x
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Green Chemistry
Page 2 of 7
COMMUNICATION
Journal Name
•
unsaturated bonds. Accordingly, the S• Se radical transfer concentration with (PhSe)2 that requires 4.0 kcal/mol;
→
View Article Online
DOI: 10.1039/C9GC03745D
was anticipated through the interaction of PhS• at a low
Ph
PhS
A
A
Ph
a)
2a)
PhSeSePh (
RN
Ph 2a
PhSe
PhSSePh
+
PhS
RN
Ph
G343 = 4.0 kcal/mol
G343 = -0.4 kcal/mol
IV
G343 = 15.8 kcal/mol
I
III
B'
R= SO2Ph
favoured
disfavoured
TSrad1
Ph
d)
Ph
b)
RN
PhS
RN
PhSe
Ph
Bn
SH
AIBN
PhSeSePh
Ph
Ph
Ph
(20 mol %)
2a
PhS
Path-I
Path-II
R'
H
Neat
N
70 C, 36 h
TSFG
'
TSAF
'
TsN
Ph
Ts
TSBC
'
TSAB
'
3a3e
1a
4a
F' TSAF
Table 1: Screening of thiophenols
TSFG
Products yield
Thiophenols
TSBC
B'
B
TSAB
F
Ph
S
1
2
3
4
R' = H , 3a
p-Me,
3b
p-OMe, 3c
p-NO2, 3d
o-Me, 3e
4a : 5a = 71 : 5
4a : 5b = 70 : 5
4a : 5c = 55 : 25
4a : 5d = 65 : 5
Ph
R'
Bn
N
H
RN
PhSe
Ph
Ph
Ph
Ph
Ts
Ph
PhS
PhSe
5a5e
A (0.0)
4a : 5e
= 72 : 5
5
RN
Ph
PhS
Ph
Ph
5a ,
5b ,
5c ,
R'= H ( ) p-Me ( ) p-OMe ( ) p-NO2
( ) o-Me ( )
5d , 5e
N
Y
R
+
G'
e)
N
R
Ph
Ph
RN
Ph
G
Ph
PhSe
Ph
Y = S or Se
C'
R= SO2Ph
PhSe
N
R
N
C
R
TSrad2
R = p-OMe:
TSAB
(8.7 kcal/mol)
B
(5.5)
TSBC
(12.2)
C
(-20.9)
TSAF
(11.0)
F
(6.5)
TSFG
(15.4)
G
(-16.1)
G343 = 8.1 kcal/mol
G343 = 4.7 kcal/mol
TSAB
'
B'
(7.3)
TSBC
'
C'
(-18.9)
TSAF
'
F'
(11.9)
TSFG
'
G'
(-15.1)
(15.8)
(15.8)
(18.4)
(18.4)
Ph
Ph
Bn
c)
PhSe
PhSe
tautomerization
PhSe
PhSe
Ph
Ph
PhSH
Ph
5-exo-dig
(I)
PhSe
Ph
H
N
N
N
RN
favoured
Ph
Ph
R
R
R
RN
Ph
PhS
E
B
D
C
A
Figure 2. a) Energy comparison between formation of selenyl radical (III) and intermediate formation (B′); b) Energy comparison between various radical attacks and mode of
cyclizations (R = SO2Ph); c) Plausible mechanism; d) Screening of thiophenols; e) Energy calculation for p-OMe thiophenol.
which to our knowledge is revealed for the first time. Moreover, but as explained above, it is challenging. The envisaged
organic solvents and toxic transition-metals are being largely synthetic plan to reach 4-selenylated pyrroles is outlined in
•
used in the synthetic transformations, which ultimately poses Figure 2b. It relies on the following hypotheses: i) S• Se cross-
→
serious concern to the environment as well as the human body; radical transfer; ii) chemoselective trapping of RSe• over RS•, iii)
thus, the development of organic reactions in the absence of regioselective RSe• attack to alkyne over ynamide, iv) 5-exo-dig
these reagents have aroused intensive interest.6 As the current cyclization, v) trapping of the cyclic vinyl radical with H-atom of
transformation does not primarily require transition metal PhSH.
catalyst and solvent; thus, synthesis of 4-selenylated pyrroles DFT computations were carried out to validate the possibility of
through selenyl radical mediated cyclization of yne-ynamide is PhSe• formation by sulfur-to-selenium radical transfer from
•
truly green. Thus, a reverse S• Se cross-radical transfer and PhS• and PhSeSePh (see the Supporting Information for details).
→
chemo- and regioselective cyclization of yne-tethered The transition state (TSrad1) for the addition of PhS• (III) to
ynamides,
cyclization/cycloisomerization processes to construct complex step is exergonic by 0.4 kcal/mol (Figure 2a). On the other hand,
N-heterocycles,7-9 is envisioned. the reaction of PhS• to yne-ynamide A requires 15.8 kcal/mol of
which
have
been
used
for
diverse PhSeSePh (2a) lies 4.0 kcal/mol above the reactants and the
As part of our earlier investigations, we reported the ArS• free energy of activation. Thus, attack of PhS• to PhSeSePh
radical-triggered cyclization of yne-ynamides to 4-sulfinylated seems feasible, even in the presence of the yne-ynamide.
pyrroles.[10] The development of a novel synthetic method for We next studied the reactivity and cyclization diversity of PhS•
the construction of unusual 4-selenylated pyrroles is important, and/or PhSe• with the yne-ynamide. The addition of PhS• or
2 | J. Name., 2012, 00, 1-3
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Green Chemistry
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COMMUNICATION
PhSe• to one or the other alkyne moiety of A would lead to vinyl
radicals B (PhSe• attack to alkyne), B (PhS• attack to alkyne)
(Path-I, Figure 3b), or F (PhSe• attack to ynamide), F (PhS• attack
to ynamide) (Path-II, Figure 2b).11 Next, intramolecular 5-exo-
dig radical cyclization of B/B and F/F independently forms C/C
and G/G, respectively. The formation of B/B (Y = Se/S) is
endergonic by 5.5/7.3 kcal/mol, while formation of C/C is
exergonic by 20.9/18.9 kcal/mol. The transition states TSAB and
TSBC involved in PhSe• attack have free energies of 8.7 and 12.2
kcal/mol, respectively, whereas the identical PhS• attack
requires free energies of 15.8 kcal/mol for both TSAB and TSBC.
Since TSAB and TSBC were located at much higher free energies
than TSAB and TSBC, formation of C (Y = Se) appears facile (Path
I).11 Indeed, TSAF (11.0 kcal/mol) and TSFG (15.4 kcal/mol)
involved in the PhSe• attack to ynamide lie 2.3 and 3.2 kcal/mol
higher on the free energy surface respective to TSAB and TSBC.
Moreover, G (Y = Se) is located at 16.1 kcal/mol and is less
stable than C (Y = Se) by 3.8 kcal/mol. Likewise, the free energy
of activation for the attack of PhS• to ynamide through TSAF
(18.4 kcal/mol) or TSFG (18.4 kcal/mol) is higher. From these
pieces of information, Path-I is found kinetically and
thermodynamically favored over Path-II, and the formation of
C seems viable (Figure 2b).11 Finally, quenching of C by PhSH
provides D and subsequent tautomerization of D affords the
desired 4-selenylated pyrrole E (Figure 2c). These steps, as well
as other cyclization modes, were not computed. Considering
our previous DFT analysis on the PhS• triggered cyclization of
yne-ynamides,10 other cyclization modes are actually very
unlikely.
Ph
PhSeSePh
SePh
AIBN
(20 mol %)
SH
S View Article Online
2a
DOI: 10.1039/C9GC03745D
Me
Me
[Eq 1]
a)
TsN
Neat
70 C, 45 min
Ph
6
3e
1a
Observed : m/z = 302.3027
Calculated : m/z = 302.9723
[M+Na+]
AIBN
Me
Me Me
Me Me
Me
Me
(20 mol %)
TEMPO Me
(3.0 equiv)
N
O
N
O
b)
[Eq 2]
1a
3e
2a
Se
S
Neat
Me
70 C, 45 min
7
8
Not observed
Observed : m/z = 336.1569
Calculated : m/z = 336.0843
[M+Na+]
Ar
AIBN
(20 mol %)
c)
d)
S
TEMPO (3.0 equiv)
1a
2a
[Eq 3]
Ar
1a
3e
S
9
-
2a
Neat
70 C, 36 h
Ar = o MeC6H4
AIBN
SH
(20 mol %)
Me
S
Me
TEMPO (3.0 equiv)
[Eq 4]
[Eq 5]
S
Neat
Me
70 C, 36 h
9
PhSe
Ph
AIBN
(20 mol %)
1a
2a
e)
Ph
Neat
N
70 C, 36 h
Ts
4a, trace
Scheme 1. a) & b) HRMS experiments, c) & d) TEMPO experiments e) Blank reaction
(without 3e).
These results led us to examine the reaction between 1a and
PhSeSePh (2a) in the presence of phenylthiol (3a) and radical
initiator (see Table S3 for the detailed optimization). Upon
several trials, the expected 4-selenylated pyrrole 4a12 was
obtained in 71% yield along with 4-thiyl pyrrole (5a; 5%) (entry
1, Figure 2d), when the reaction conducted with 1a (1.0 equiv),
2a (2.5 equiv), and 3a (2.0 equiv) in the presence of 20 mol%
responsible to this poor product selectivity. A high energy
barrier TSrad2 (8.1 kcal/mol) is essential for the addition of p-
MeOC6H4S• to 2a (a consequence to the formation of electron-
rich p-MeOC6H4S•), and the reaction is endergonic by 4.7
kcal/mol (Figure 2e). This large energy barrier necessary for the
radical exchange of 3c to PhSeSePh (8.1 kcal/mol) in comparison
to 3a (4.0 kcal/mol) validates the observation of entry 3, Figure
1d. The identical transformation in presence of 3e afforded 4a
in 72% yield (entry 5, Figure 2d).
o
AIBN under neat condition at 70 C for 36 h. To increase the
reaction selectivity and productivity, various substituted aryl
thiols [p-MeC6H4SH (3b), p-MeOC6H4SH (3c), p-O2NC6H4SH (3d),
o-MeC6H4SH (3e) were screened (Figure 2d). Compound 4a was
isolated as major product in most cases, (entries 2, 4, and 5,
Figure 1d). In contrast, the reaction in presence of 3c delivered
substantial amount of 5c (4-thio-pyrrole, 25%) and 4a (55%)
(entry 3, Figure 2d). We believe that the radical transfer
between p-MeOC6H4S• and PhSeSePh is inefficient and
To further investigate the affinity of o-MeC6H4S• for 2a to
produce PhSe•, a few HRMS experiments were conducted. A
mass peak at m/z 302.3027 [corresponding to
(phenylselenyl)(o-tolyl)sulfane (6)] was detected from the
o
reaction of 1a, 2a, and 3e in presence of AIBN at 70 C for 45
min [Eq 1, Scheme 1]; thus, affinity of o-MeC6H4S• for 2a is
higher than 1a. Next, radical quenching experiment of 1a, 2a,
3e, and TEMPO in presence of AIBN at 70 oC for 45 min
produced TEMPO-SePh adduct 7 (m/z 336.1569) and not
TEMPO-S-o-tolyl 8; the Se• thus seems to be active (Eq 2,
Scheme 1), whereas the same reaction after 36 h did not
provide 7/4a but rather the disulfide adduct 9 (Eq 3, Scheme 1).
Moreover, exposing 3e with AIBN in presence of TEMPO
exclusively provided the disulfide adduct 9 (Eq 4, Scheme 1).The
reaction in the absence of 3e did not deliver 4a (Eq 5, Scheme
1), thus o-MeC6H4S• is essential. However, we could not detect
the radical through EPR, when the reaction
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J. Name., 2013, 00, 1-3 | 3
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Green Chemistry
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COMMUNICATION
Journal Name
Ar
PhSeSePh
Ar
PhSeSePh
Ar
PhSe
AIBN
View Article Online
Ar
PhSe
AIBN
(20 mol %)
SH
SH
2a
2a
(20 mol %)DOI: 10.1039/C9GC03745D
Me
(2.5 equiv)
Ar'/Alk
Me
3e
Ar'
R
(2.5 equiv)
Neat
Me
Neat
H
Me
N
N
70 C, 36 h
Ts
70 C, 36 h
TsN
Ar'/Alk
N
O2S
Ar'
3e
(2.0 equiv)
SO2
Alk/Ar''
1
10a l
1

(1.0 equiv)
(2.0 equiv)
4am
Ar''/Alk
(1.0 equiv)
NC
COMe
R
R
PhSe
Me
PhSe
Me
PhSe
PhSe
H
PhSe
Me
N
Ph
Ph
Ts
H
N
N
N
N
4a
Ts
10a
10b
R = H,
m-Me,
, 68%
, 67%
p-F, 10c, 46%
Ts
Ts
4a
Ts
4c
R = p-COMe , 63%
10d
R = H , 72%
, 65%
10e
, 51%
[a]
4d
m-NO2 , 42%
4b
m-Me , 59%
F3C
PhSe
Me
R
PhSe
Me
H2N
PhSe
F
PhSe
PhSe
PhSe
Me
S
N
Ph
Ph
Ph
N
Ts
Ph
H
H
Ts
H
N
N
N
N
Ph
10h
R = p-Me, , 75%
Ts
Ts
Ts
Ts
10g,
70%
Me
m-F, 10i, 42%
[a]
4g
, 73%
10f, 76%
4f
, 57%
Ar
4e
, 57%
Me
OBn
Cl
Ph
PhSe
H
PhSe
H
MeO
PhSe
Ph
PhSe
H
PhSe
Me
S
PhSe
Me
N
N
N
Me
Ts
Ts
4i
N
N
Ts
Ph
N
Ph
Ph
Ts
Ts
[b]
4j, 32%
, 29%
Ar = 3,4-diMeC6H4
Ts
4h
, 68%
10j
, 58%
10l
, 67%
10k
, 80%
PhSe
H
Ar
Ph
PhSe
H
PhSe
Ph
Scheme 3. Scope of tetra-substituted 4-selenylated pyrroles.
N
H
sulfonyl protected yne-ynamides 1l and 1m having different aryl
groups at the alkyne terminus (C6H5, p-FC6H4) and at the
ynamide terminus (o-PhC6H4 or C6H5,) were subjected to the
standard conditions to yield the desired highly substituted 4-
selenyl-N-sulfonyl pyrrole derivatives N-Ms (4l), N-SO2Ph (4m),
as shown in Scheme 2.
N
N
Ph
Ph
Ts
SO2
SO2
[a]
Me
4m
, 78%
4k
, 49%
4l
, 75%
Ar = 4-FC6H4
Scheme 2. Scope of yne-ynamides. [a] 50 C, [b] 60 C.
conducted at room temperature.13
We then examined the scope of Se• triggered cyclization of yne-
ynamides under the established conditions (entry 5, Figure 1d)
for the construction of unusual peripheral substituted 4-
selenylated pyrroles (Schemes 24). To start with, the yne-
ynamides having aryl substitutions at the propargyl terminus
[electron-neutral phenyl, electron-rich (m-Me), and electron-
poor (p-COMe, m-NO2, m-F)] provided the desired products
4a4e (4272%; Scheme 2). An aniline substituted product 4f
was obtained from 1f; the free NH2 group did not actually
obstruct the reaction. Likewise, thienyl-substituted pyrrole 4g
was synthesized in 73% yield. The electron distribution via
inherent N-lone pair polarization of the ynamide could possibly
affect the reaction outcome. Thus, the Se• triggered cyclization
of yne-ynamides having electronic variation of substituents at
the ynamide terminus was next investigated (Scheme 2). The 4-
selenylated pyrrole 4h was fabricated from 1h having aryl
substitutions (p-Cl). The cyclization of 1i with a O-benzyl
protected flexible alkyl group at the ynamide terminus afforded
4i, albeit in only 29% yield. A noteworthy observation is the
synthesis of N-Ts-protected pyrrole 4j (32%) with a cyclopropyl
motif on the skeleton; cyclopropyl rings are prone to radical
clock reactions when located adjacent to a radical (Scheme
2).The ortho-biphenyl substituted yne-ynamide 1k provided 4k
in moderate yield. To evaluate the electronic properties of N-
protecting groups in this cyclization, various N-
SH
Ar
(R'Se)2
2bf
(2.5 equiv)
Ph
R'Se
AIBN
(20 mol %)
Ar'
R
R
Neat
70 C, 36 h
N
N
Ar'
NO2
3d
(2.0 equiv)
SO2
O2S
Ar''
1 (1.0 equiv)
Ar''
11ai
Ph
Se
Ph
Se
R'
Me
Ph
Ph
H
N
H
N
11b
11c
R' = p-Br,
p-Cl,
, 45%
, 31%
Ts
Ts
11a
, 51%
Cl
Ph
Ph
MeSe
Ph
Se
MeSe
H
Ph
F
Ph
Cl
H
H
N
N
N
Ts
11e
Ts
Ts
, 64%
11f
11d
, 44%
COMe
, 39%
MeSe
H
MeSe
Me
MeSe
Ph
Ph
H
N
N
N
Ph
Ts
Ts
SO2
Ph
11h
11g
, 60%
11i
, 73%
, 49%
Scheme 4. Scope of diselenides.
4 | J. Name., 2012, 00, 1-3
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Green Chemistry
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The PhSe• mediated intramolecular 5-exo-trig cyclization of
yne-ynamides having a methyl substitution at the propargyl
position could furnish fully substituted pyrroles (Scheme 3).
Hence, the -methyl substituted propargyl tethered yne-
ynamides 1n1y directly led to peripherally substituted
selenylated pyrroles 10al (4280%). The steric and electronic
effects of the aryl substituents both at the propargyl and
ynamide terminus did not prevent the reaction outcome
(Scheme 3).
The effect of other diaryl diselenides 2b2e was next
scrutinized (Scheme-4). The reaction of 1a and 2b (p-
MeC6H4Se)2 in presence of 3e led to 11a (27%); however, the
identical transformation with p-O2NC6H4SH (3d) proved more
efficient, delivering 11a in 51% yield. Likewise, the reaction of
several diaryl diselenides {[(p-BrC6H4Se)2 (2c)], [(p-ClC6H4Se)2
(2d)], [(m-ClC6H4Se)2 (2e)]} with 1a in the presence of 3d yielded
the respective products 11bd. Interestingly, the cyclization
thank SERB-NPDF, India, for fellowship. VG thankVsieCwNArRticSle, OUnPlinSe,
and Ecole Polytechnique for financial support. We thank Prof.
DOI: 10.1039/C9GC03745D
D. Basavaiah, UoH for his valuable suggestions. This paper is
dedicated to Dr. A. V. RamaRao for his 85th Birthday and his
invaluable contribution to synthetic organic chemistry.
Notes and references
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was
compatible
with
(MeSe)2
to
provide
4-
methylselenopyrroles 11ei, and is remarkable (Scheme 4); as
the identical reaction with aliphatic thiyl radicals was not viable.
The MeSe• mediated intramolecular 5-exo-trig cyclization of
yne-ynamides having various substitutions both at the
propargyl and ynamide terminus furnished the respective 4-
methylselenopyrroles 11eg in moderate to good yield. Other
than N-Ts-protected ynamides, the N-benzene sulfonyl
protected 4-methylselenopyrrole 11h was constructed in good
yield. The fully peripheral substituted selenopyrrole 11i was
also synthesized (Scheme 4).
4
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CONCLUSIONS
In summary, a chemo and regioselective RSe• radical mediated
cyclization of yne-tethered ynamides to fully substituted 4-
selenylated pyrroles has been developed. This reaction involves
an uncommon S• Se radical transfer and is contrary to the
•
→
well-explored radical dominance and reactivity of S• over Se•.
This green synthetic method does not require transition metal
catalyst and solvent and is general exhibiting a broad substrate
scope and high functional group tolerance (the free NH2 and the
common radical trap electron-withdrawing groups did not
affect the reaction outcome). Computational studies provided
mechanistic insights for this transformation. Various control
HRMS experiments validated the role of the arylthiol initiator
for the in situ formation PhSe•. We believe these findings will
open new avenues in trapping selenium radical for the
construction of novel selenylated scaffolds.
6
7
8
Conflicts of interest
There are no conflicts to declare
Acknowledgements
This research was supported by the CEFIPRA (grant no.: 5505-
2). We thank University of Hyderabad (UoH; UPE-CAS and
PURSE-FIST) for overall facility. SD, PB, thank CSIR, India and RV
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COMMUNICATION
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DOI: 10.1039/C9GC03745D
9
10 S. Dutta, R. K. Mallick, R. Prasad, V. Gandon and A. K. Sahoo,
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11 See the Supporting Information.
12 CCDC 1945002 contains the supplementary crystallographic
data for compound 4a.
13 Detection of selenyl radical cannot be completely ruled out at
70 C. Due to inaccessibility of the EPR-probe at high
temperature, such experiment was not possible.
6 | J. Name., 2012, 00, 1-3
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DOI: 10.1039/C9GC03745D