Visible-light activated metal catalyst-free vicinal diazidation of olefins with sulfonium iodate(i) species

Article abstract of DOI:10.1039/c9cc00007k

A visible light-induced vicinal diazidation of various alkenes using stable sulfonium bis(acetoxy)iodate(i) and sodium azide as a radical precursor is described. The trimethylsulfonium [bis(azido)iodate(i)] species, intrinsically generated in situ, demonstrated unprecedented reactivity toward CC π bond-promoting selective azidation under photochemical conditions without any photoredox or metal catalyst. The visible-light stimulated diazidation reaction provides a convenient and straightforward approach to highly prevalent vicinal nitrogen scaffolds of myriad therapeutic importance.

Full text of DOI:10.1039/c9cc00007k

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Visible-Light Activated Metal Catalyst-Free Vicinal Diazidation of
Olefins with Sulfonium Iodate (I) Species†
Thurpu Raghavender Reddy,‡a Dodla Sivanageswara Rao,‡a and Sudhir Kashyap*a
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
www.rsc.org/
A visible light-induced vicinal diazidation of various alkenes using the presence of NaN3/TMSN3 or azidoiodine(III) oxidant
stable sulfonium bis(acetoxy)iodate(I) and sodium azide as radical (Zhdankin reagent) as an azide source (Scheme 1). Xu and co-
precursor
is
described.
The
trimethylsulfonium workers developed Fe-catalyzed diazidation utilizing
[bis(azido)iodate(I)] species, intrinsically generated in situ, hypervalent iodine-based oxidants and tridentate ligands.3d
demonstrated unprecedented reactivity toward C=C  bond- Recently, Lin et al. reported a divergent electrochemical
promoting selective azidation under photochemical conditions oxidative diazidation of alkenes with manganese involving
without any photoredox or metal catalyst. The visible light azidyl transfer from active metal-N3 species.3f Studer
stimulated diazidation reaction providing
a convenient and previously demonstrated the intramolecular radical
straightforward approach to highly prevalent vicinal nitrogen azidooxygeneration by employing the Zhdankin reagent as an
scaffolds of myriad therapeutic importance.
N3-radical precursor and in situ trapping of the C-radical with
TEMPONa as the O-nucleophile.2i Advancing the utility of
Zhdankin reagent, Greaney and co-workers investigated the
light-switchable reactivity of copper-based photoredox catalyst
in selective azidation of vinylarenes.2c Visible light activation
offers an attractive and distinctive pathway for a wide range of
green organic transformations and thus has gained significant
momentum in past decades.10
The stereoselective carbon-nitrogen bond formation via
nitrogen transfer continues to gain significant attention owing
to exceptional versatility of vital nitrogen moieties in synthesis
as well in pharmaceutical chemistry.1 Organic azides,2
especially 1,2-diazides,3 are highly versatile synthetic
intermediates for rapidly assembling valuable heterocylic
scaffolds, contributing late-stage utility and alternative route
to underdeveloped transformations. Besides the unique
importance of the azido moiety in chemistry4 and naturally
bioactive structures,5 azido-derivatives have shown promising
biological activities and hence have found intensive
applications in drug discovery6 as well in synthetic
polymer/material science.7 Most importantly, 1,2-diazides has
proven to be ideal precursors for vicinal primary diamines,3d,8 a
privileged functional motif that constitutes an abundant class
of biologically important molecules,8a,9a and an amply
recognized feedstock as chiral catalysts/ligands in
stereoselective syntheses.9
Metal-Catalyzed Diazidation
NH2
N3
[Cu/Fe/Pb/Pd/Mn]
Azide source
R2
R2
R2
R1
R1
R1
Oxidant/Ligand
Electricity/Heat
NH2
N3
Studer and Greaney work
TEMPONa
N
O
R1
O
R1
R2
R3
N3
R3
O
+
R2
I
Photoredox
catalyst
N3
R1
Zhdankin Reagent
Nu
R2
R3
N3
Driven by the exceptional usefulness as powerful probes
for robust chemical transformations, vicinal-diazidation of
olefins has gained considerable attention in recent years.3 In
this context, several researchers actively pursued direct 1,2-
diazidation of alkenes often catalyzed by transition metals in
Visible Light
This work: Visible Light mediated diazidation
N3
Me3SI(OAc)2
NaN3
Vinylarenes
unactivated &
allylic olefins
N3
Visible light
(CFL 27 W)
n
a.Department of Chemistry, Malaviya National Institute of Technology, Jaipur-
302017, INDIA. E-mail: skashyap.chy@mnit.ac.in, skr.kashyap@gmail.com.
Electronic Supplementary Information (ESI) available: [General procedures,
characterization data, spectral charts are provided]. See DOI: 10.1039/x0xx00000x
‡Thurpu Raghavender Reddy and Dodla Sivanageswara Rao contributed equally.
n
Scheme 1. Representative azidation of alkenes and our work using sulfonium
bis(acetoxy)iodate(I) salt.
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Table 1. Optimization of selective azidation using sulfonium iodate reagenta
Inspired by these precedents, we speculated that selective
azidation of a C=C bond with Me3SI(N3)2 might be possible by
taking advantage of intriguing photoinduced electron-transfer
under visible light to generate requisite radicals. To the best of
our knowledge, the diazidation of alkenes under visible light
irradiation without any heavy metal-salt/ligands or
photocatalyst/sensitizer have been scarcely investigated. In
this context and with our continuing interest in vicinal
bisfunctionalization of C-C multiple bond,11 herein we report a
novel metal-free visible light accelerated diazidation of olefins
with Me3SI(N3)2 electrophilic species11a (Scheme 1).
Reagent system
Yields %b
Entry
Solvent
2
3
1
Me3SI(OAc)2/TEMPO/TMSN3
Me3SI(OAc)2/TEMPO/NaN3
Me3SI(OAc)2/TEMPO/NaN3
Me3SI(OAc)2/TEMPO/NaN3
Me3SI(OAc)2/TEMPO/NaN3
Me3SI(OAc)2/TEMPO/NaN3
Me3SI(OAc)2/TEMPO/NaN3
Me3SI(OAc)2/TEMPO/NaN3
Me3SI(OAc)2/NaN3
CH3CN
CH3CN
THF
56
0
0
0
0
0
0
0
0
2
62
3
48
We conceptually envisioned that electrophilic azidyl radical
instigated
[bis(azido)iodate(I)] species facilitated via visible-light adds on
C=C
bond2i of vinylarene to give an initial benzylic radical
(Scheme 2). Subsequently, radical comprising a terminal
azidyl group could be trapped by a radical scavenger, for
from
homolysis
of
trimethylsulfonium
4
H2O
42
5
MeOH
AcOH
DMF
0

A
6c
7
trace
A
92
instance
2,2,6,6-tetramethylpiperidin-1-yl)oxyl
(TEMPO),
8d
9
DMF
48 (55)
unambiguously generating oxyazidation product
B. However,
lack of an efficient competing radical quenching in the absence
of TEMPO would prevent this radical transformation from to
Thus, another azido radical from trimethylsulfonium
iodoazide species could presumably add to the incipient
benzylic radical and eventually undergo an onward reaction
to deliver the expected diazide in a two step process.
DMF
-
-
-
-
-
-
95 (84)
68
A
10
11e
12e
13e
14e
Me3SI(OAc)2/NaN3
DMSO
DMF
B.
I2/PhI(OAc)2/NaN3
trace
trace
0
A
Me3SI/K2S2O8/NaN3
DMF
C
Me3SI/TBHP/NaN3
DMF
Me3SI/H2O2/NaN3
DMF
0
aReaction conditions: 1a (104 mg, 1.0 mmol, 1.0 equiv), Me3SI(OAc)2 (1.1
equiv), TMSN3 or NaN3 (2.5 equiv), TEMPO (1.1 equiv), solvent (2 mL),
stirred at 35 °C for 12 h, irradiated with a 27 W CFL unless otherwise noted.
c
bThe isolated yields after column chromatography. Mixture of iodoacetoxy
compound. dThe reactions was conducted in normal daylight. eOther
oxidant combinations, I2 or K2S2O8 (1.1 equiv), 70% aq. TBHP (1.2 equiv),
50% aq H2O2 (1.2 equiv) were used. Parenthesis refer to isolated yields with
7 W blue LEDs.
Scheme 2. Rationalization for visible-light-driven stereodivergent azidation with
trimethylsulfonium [bis(azido)iodate(I)] species.
Indeed, employing a more polar solvent such as DMSO
accomplished the desired transformation with a moderate
yield (entry 10). Further iteration using different oxidizing
reagents such as molecular iodine, K2S2O8, TBHP or H2O2
proved to be unsuccessful (entries 11-14).
Having an optimized protocol in hand, a wide range of
electronically and sterically diverse vinylarenes and
unactivated alkenes were subjected to access the
corresponding diazides (Table 2). The reaction employing
styrene with electron-donating functional groups including
methyl, tert-butyl, methoxy and ethoxy proceeded smoothly,
providing the corresponding products 4-7 in 78-97% isolated
yields. The substrate with a methyl ester and an electron-rich
4-vinylphenyl group performed well under direct diazidation to
To test this hypothesis, we initially investigated the direct
azidooxygenation using styrene 1a as the model substrate with
11a
Me3SI(OAc)2 and TMSN3 in the presence of TEMPO under
mild photochemical conditions (Table 1). The visible-light
activation of 1a in acetonitrile using compact fluorescent light
(household CFL bulb, 27 W) furnished azidooxygenated
product
2 in 56% yield (entry 1). Subsequently, switching the
azide source to NaN3 facilitated the regioseletive azidation
process with an improved yield; 62% (entry 2). Unappreciated
results were obtained when the reaction was examined in the
presence of THF and water, whereas running the reaction in
the presence of MeOH as the solvent proved unsuccessful
(entry 3-5). Reaction using acetic acid found to be too sluggish,
producing
2 in traces alongside a substantial amount of
competitive by-product (entry 6).11b Admirably, a considerable
improvement was observed in DMF as the solvent, affording
produce the corresponding 1,2-diazides
8 (75%) and 9 (92%),
respectively. A variety of synthetic useful yet challenging
functional groups such as cyano, halogens, N-tosyl and N-Boc
groups, are compatible with this protocol, affording the
desired products 10-14 in good yields (77-93%). Importantly,
sterically congested 2,5-di- and 2,4,6-trimethyl styrenes
participated cleanly in the diazide forming reaction to afford
the desired products 15-16 in 80% and 94%, respectively.
the desired product
2
in 92% yield (entry 7).12 Subsequent
attempts under normal daylight (entry 8) or other visible-light
sources (7 W blue/green LEDs) led to no improvement. To
probe the elusive diazidation, the reaction was conducted
successfully in the absence of TEMPO to afford the
corresponding vicinal diazide product 3 in 95% yield (entry 9).
2 | J. Name., 2012, 00, 1-3
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The diazidation of potentially sensitive 2-vinyl pyridine and 2- Indeed, an unactivated alkene, including a linear aliphatic
vinyl naphthalene were also achieved, providing the desired olefin, uniformly reacted under operationally facile diazidation
diazides 17-18 in good yields (75-78%). To demonstrate the conditions, affording the desired product 31 in 74% isolated
generality further, we next examined the effect of substituents yield. Notably, diazidation of electron-rich enol ether (vinyl
at the olefin and the feasibility of using non-styrenyl alkenes. acetate) and enamine (N-vinyl carbazole) proceeded smoothly
We observed that
be efficiently converted into their corresponding diazides 19- products 32-33 in 85% and 88% yields, respectively.
20 in 84% and 94% yields, respectively. Meanwhile, -methyl To gain further insights into radical intermediate in
-methyl styrene and diphenyl ethylene can under the applied conditions, affording the corresponding

styrene and various protected cinnamyl alcohols (OAc, OMe, intramolecular azidation, an experiment was performed using
OTr) were evaluated under standard conditions, affording the trans-stilbene in the presence of TEMPO under visible light
desired products 21-25 in 80-89% yields albeit with moderate irradiation (Scheme 3). The reaction resulted a mixture of
selectivity (dr up to 1:1).12 In contrast, indene and 1,2- stereoconvergent product 34 in 95% yields with a dr of 1.3:1.
dihydronaphthalene were reacted with a complementary The stereo-chemical outcome indicates the involvement of a
regioselectivityyielding syn-vicinal diazides 26 (72%) and 27 putative benzylic radical in a step-wise radical process. Indeed,
(77%) in high dr; 96:4. Subsequently, phenylpropene and the non-stereospecific oxyazidation of cis-stilbene, presumably
Eugenyl derivatives, containing allylic double bonds, were going through an identical carbon radical species, ensures a
conveniently diazidated into their corresponding 1,2-diazides consistently low dr, albeit with a different yield (84%). These
28-30 in good yields.
observations and the stereochemical infidelity unambiguously
established the role carbon-radicals in the diastereoselectivity-
determining step.13 Subsequently, the second azidation of
internal benzylic radical generated from trans- or cis-stilbene
in the penultimate step enabled the desired vicinal diazide 35
in 92% isolated yield with dr = 2.6:1. Furthermore, the reaction
Table 2. Substrate scope for the visible-light promoted diazidationa,b
of 1,6-heptadiene producing the uncyclized diazide 36
,
suggesting that the second azide addition is fast, a general
feature of a stepwise radical process.3e In addition, the control
experiment with a stoichiometric amount of radical scavengers
such as 2,6-di-tert-butyl-4-methylphenol (BHT) as an additive
showed complete inhibition of this photoinduced azidation.
Scheme 3. Control experiments for mechanistic investigation of the photoinduced
azidyl and carbon radical generation.
Next,
a late-stage synthetic application was amply
demonstrated in the rapid transformation of valuable diazide
and oxyazide products prepared using the present protocol
(Scheme 4). Sequential transformation of
3 employing a
Staudinger reduction following N-protection conveniently
produced the functionalized vicinal diamine 37 in 85% yield. In
addition, the alkoxyamine functional group in
readily cleaved under mild conditions to obtain the highly
diverse and versatile 1,2-azido alcohol14 38
which was
2 could be
aReaction conditions: Olefins (1.0 equiv), Me3SI(OAc)2 (1.1 equiv), NaN3 (2.5
equiv), DMF (2 mL), stirred at 35 °C for 12 h, irradiated with a 27 W CFL. bThe
isolated and unoptimized yields after chromatography.
,
subsequently subjected to a CuAAC click reaction furnishing
drug-analogs 39-40 of medicinal value.15
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a stereodivergent
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under mild visible-light conditions. The photoinduced
homolysis of C=C bond originated from trimethylsulfonium
[bis(azido)iodate(I)] conceptually applied to intramolecular
7
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a
operationally simple, utilizing the readily accessible and
relatively less-toxic oxidant/azides, and tolerates a wide range
of synthetically useful functional groups. The ameliorated
strategy facilitating the direct bisfunctionalization under
transition-metal-free conditions offers further perspective for
the stereodivegent induction of heteroatoms based on a
visible-light approach.
SK gratefully acknowledge the Department of Science and
Technology, India for the Early Career Research Award and
Research Grant for Young Scientists (SERB-ECR/2017/001477).
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Visible-Light Activated Metal Catalyst-Free ViciDnOaI: 1l0.1039/C9CC00007K
Diazidation of Olefins with Sulfonium Iodate (I) Species
Thurpu Raghavender Reddy,+ Dodla Sivanageswara Rao,+ and
Sudhir Kashyap*
aDepartment of Chemistry, Malaviya National Institute of Technology (MNIT),
Jaipur-302 017, INDIA.
 Corresponding author. Tel.: +91-9549657150;
Email: skashyap.chy@mnit.ac.in, skr.kashyap@gmail.com
Graphical Abstract for Table of Contents
N3
n
R
n
R
Visible
light
R
R'
R'
R'
N3
N3
Me3SI(OAc)2
NaN3
n
possibly via 2-steps
Abundant chemical
feedstocks
Household
CFL bulb (27 W)
Transition metal-free >35 valuable products
Ligand free
yield up to 97%
An unprecedented visible-light inspired selective radical azidation of unactivated and diverse
substituted vinylarenes with sulfonium iodate reagent has been realized. The intrinsic radical process
triggered by light, tolerated several synthetic useful functionalities enabling two new carbon-hetero
bonds which display distinctive late-stage applications to biologically relevant scaffolds.