Visible-light mediated sulfonylation of thiols via insertion of sulfur dioxide

Article abstract of DOI:10.1039/c9ob01040h

A simple and efficient visible-light mediated synthesis of thiosulfonates via a radical-radical coupling of sulfenyl radicals and arylsulfonyl radicals was developed. The reaction of thiols, aryldiazonium tetrafluoroborates and DABSO proceeded at room temperature using 5 mol% eosin Y. The reaction displayed wide functional group tolerance and delivered the unsymmetrical thiosulfonates in good to excellent yields.

Full text of DOI:10.1039/c9ob01040h

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DOI: 10.1039/C9OB01040H
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Visible-Light Mediated Sulfonylation of Thiols via Insertion of
Sulfur Dioxide
Akshay M. Nair, Shreemoyee Kumar, Indranil Halder and Chandra M. R. Volla*a
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
www.rsc.org/
A
simple and efficient visible-light mediated synthesis of sulfur dioxide.8 They developed an elegant amino sulfonylation
thiosulfonates via a radical-radical coupling sulfenyl radicals and reaction using DABSO under palladium catalysis.8a Subsequently,
arylsulfonyl radicals was developed. The reaction of thiols, Wu pioneered the use of DABSO to generate arylsulfonyl
aryldiazonium tetrafluoroborates and DABSO proceeds at room radicals,9,10 demonstrating an elegant radical amino sulfonylation
temperature using 5 mol% Eosin Y. The reaction displays wide raction (Scheme 1b).10a Since then, arylsulfonyl radical generation
functional group tolerance and delivered the unsymmetrical from DABSO has been extensively studied for the construction of S-
N,10a-e S-C10f-k and S-P bonds.10l However, the formation S-S bonds
thiosulfonates in good to excellent yields.
using DABSO remains elusive until recently. In a recent report Yang
The abilty to adapt variable oxidation states enables sulfur to form
valuable functional groups like mercapto, thioethers, sulfoxides
sulfones and sulfonamides. Consequently, sulfur could be found in
the core of a variety of syntheticallly and comercially important
molecules like active pharmaceutical ingredients, pesticides, and
various advanced materilas.1 As a result, efficient synthesis of
oganosulfur compounds and incorporation of sulfur into organic
molecules has intrigued both synthetic and medicinal chemists.2
The escalating concerns regarding sustainability and energy
efficiency in the recent years has seen the emergence of visible-
light photocatalysis as a powerful tool in synthetic organic
chemistry.3 These reactions permit the generation of organic
radicals under mild catalytic conditions, bypassing the use of
stochiometric radical initiators or oxidants. The seminal work by
Yoon on visible light mediated thiol-ene reactions (Scheme 1a),4
paved way for numerous reports on photocatalytic radical C-S bond
formations using thiols.5 However, the formation of S-heteroatom
bonds using photocatalalysis remains unexplored apart from,
photocatalytic dimerization of thiols reported by Noel and co-
workers6a and visible light mediated synthesis of thiophosphates
from thiols and diaryl phosphine oxides illustrated by Zhang and co-
workers.6b
and co-workers described TFA mediated synthesis of thiosulfonates
using diazonium salts and DABSO.11
Scheme 1: Overview of the work.
On the other hand, the fixation of sulfur dioxide into small
molecules has gained a lot interest in the synthetic community.7
Willis and co-workers introduced DABSO: a charge transfer complex
of SO2 and DABCO, as a bench stable and easy to handle source of
Organic thiosulfonates belong to an important class of
organosulfur compounds as they display an array of biological
activities including antibacterial, antiviral and antifungal.12
Additionally, their synthetic utility as a source of both sulfenyl and
sulfonyl groups has been well documented.13 Consequently, the
synthesis of thiosulfonates has gained widespread attention.
Synthesis of these thiosulfonates from various precursors like
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generation of sulfur centred radicals from thiols.5VieCwarArrtyicinlegOnolinuet
studies in that facet, we were delighteDdOIt:o10s.1e0e39a/C9sOubB0st1a0n4t0iHal
disulfides, sodium sulfinates, and sulfonyl hydrazides have been
achieved.14 Despite the progress in this field, concerns such as poor
yields, sustainability, use of metals, stoichiometric oxidants and amplification in the yield of 3a (78%), when the reaction was carried
high temperatures have limited their widepread utilization. As a
result, it is desirable to develop mild and selective protocols for
accessing thiosulfonates and herein, we report an Eosin Y catalyzed
visible light mediated thiosulfonylation for the synthesis of
thiosulfonates using DABSO as sulfur dioxide source (Scheme 1c).
We envisioned that the arylsulfonyl radical generated from the
aryldiazonium salts and DABSO could be coupled with
photocatalytically generated sulfenyl radicals to access various
unsymmetrical thiosulfonates. To the best of our knowledge,
photocatalytic synthesis of thiosulfonates starting from easily
available precursors has not been reported.
out under irradiation by green LED using 5 mol % of rhodamine B as
photocatalyst (entry 8). Following this, an array of organic
photocatalysts was screened for the reaction (entries 8-12) and
Eosin Y was found to be most effective providing 87 % of 3a
(isolated yield of 84%). Varying the light source from green to either
blue or white LEDs led to slightly lower yields (entries 13 and 14).
The combination of Eosin Y as photocatalyst and green LED as light
source was found to be crucial, as the reaction fared poorly in the
absence of either (entries 15 and 16).
With the optimized conditions in hand, we tested the scope of
various diversely functionalized thiols with 4-methoxy
phenyldiazonium tetrafluoroborate 2a (Table 2). Thiophenols
having substituents like methyl, iso-propyl and methoxy at different
positions of the aromatic ring proceeded well under the standard
conditions giving excellent yields of their corresponding products
(3b-3f, 80-87%). Halogenated thiophenols were also found to be
amenable substrates in the reaction affording the corresponding
functionalized thiosulfonates in good yields (3g-3l). Ortho-
substituted thiophenols also displayed similar reactivity profile as
that of their meta and para counterparts indicating no appreciable
steric effect giving 3c, 3f, 3i and 3k in 77-87% . While 2-napthylthiol
1m gave the corresponding product 3m in 74%, no conversion was
observed when highly electron deficient penta-fluorothiophenol 1n
was employed.
Table 1. Optimization of the reaction conditionsa
Sr.No
Photocatalyst
light
solvent
Yield (%)b
1
2
-
-
MeCN
DMF
DMSO
DCM
THF
16
<5
-
-
3
-
-
12
4
-
-
25
5
-
-
-
15
6c
7d
8
-
DCM
DCM
DCM
DCM
DCM
DCM
DCM
DCM
DCM
DCM
DCM
35
-
-
32
Rhodamine B
Eosin Y
Eosin B
Fluorescein
Rose Bengal
Eosin Y
Eosin Y
Eosin Y
-
Green LED
Green LED
Green LED
Green LED
Green LED
Blue LED
White LED
-
78
87 (84)e
9
Table 2. Substrate scope of the thiols with 2a
10
11
12
13
14
15
16
64
35
64
76
71
26
Green LED
30
aReaction conditions: 1a (0.2 mmol), 2a (1.5 equiv.), DABSO (1 equiv.), photo
catalyst (5 mol %), solvent (2 mL), RT, 8 h, under air. bNMR yield using 1,3,5-
c
trimethoxybenzene as an internal standard. Reaction was performed at 70
oC. dReaction was performed at 90 oC,. eIsolated yields in parenthesis.
To test our hypothesis, thiophenol 1a and 4-
methoxyphenyldiazonium tetrafluoroborate 2a were chosen as the
model substrates and indeed their reaction with 1eq. DABSO in
MeCN at rt, provided the desired thiosulfate 3a albeit in lower yield,
16% after 8h (Table 1, entry 1). In order to improve the reaction
profile, we screened a variety of solvents for the reaction (entries 2-
5) and found DCM to be superior of all, giving a best yield of 25%.
No significant amplification in yield was observed by increasing the
reaction temperature (entries 6-7).15 Since we were sanguine about
the slick formation of arylsulfonyl radical from diazonium salts and
DABSO, we envisaged that facilitating the formation of sulfenyl
radicals could ameliorate the reaction profile. Visible-light
photocatalysis has manifested itself as an revered tool for the
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aReaction conditions: 1 (0.2 mmol), 2a (1.5 equiv.), DABSO (1 equiv.), Eosin Y
(5 mol %), DCM (2 mL), RT, 8 h, under air.
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DOI: 10.1039/C9OB01040H
Captivatingly, aliphatic thiols were found to be effective
furnishing the corresponding thiosulfonates in excellent yields (3o-
3q). Notably, when tetra-acetylated glucose thiol 1r was employed,
82% of glycosylated thiosulfonate 3r was obtained. It’s worth a
mention that these glycosylated thiosulfonates have been studied
as precursors in glycoprotein synthesis.16
Subsequently, we expanded the scope of the developed
protocol with various diazonium salts
2 using either p-
Scheme 2: Synthesis of dihydrobenzofuran-3-yl thiosulfonates.
chlorothiophenol 1j or cyclohexane thiol 1p (Table 3). Gratifyingly,
diazonium salts bearing both electron withdrawing as well as
donating substituents were found to be suitable substrates for the
transformation. While, electron rich diazonium salts displayed good
reaction profile, only moderate yields were obtained in case of
electron deficient nitro substituent (3v, 56%). Unlike the case of
thiophenols, steric factors seemed to have a marked effect in the
case of diazonium salts, where lower yields were observed when
ortho substituents were present 3y (67%) and 3ac (54%).
The unique reactivity and low toxicity of diaryliodonium salts
has led to their application as alternative arylating agents.3c Inspired
by their utility in visible-light photoredox catalysis, thiol 1e was
reacted with the diphenyliodonium salt 6a under the standard
conditions and we observed the formation of the thiosulfonate 3ad
in a moderate yield of 47% (Scheme 3). It is worth mentioning that
along with 3ad, symmetrical thiosulfonate 3e was isolated in 34%.
These symmetrical thiosulfonate might be formed by the oxidation
of the in situ generated disulphide. The absence of similar
symmetrical thiosulfonates in the case of aryl diazonium salts
implies their superior reactivity in compared to diaryliodonium salts
in the visible light mediated thiosulfonylation.
Table 3: Scope of aryldiazonium salts.
Scheme 3: Three-component coupling of thiols, iodonium salts and SO2.
To study the scalability of this protocol 1.0 g of 1a was reacted with
of 2a under standard conditions to furnish 2.05 g (81 %) of 3a
(scheme 4a). We found that 3a could be utilized for selective
sulfonylation of morpholine using NBS as an oxidant under basic
conditions to obtain sulfonamide 7 in 86% yield.12a Base mediated
benzylation of 3a using Cs2CO3 led to the formation of sulfone 8 in
82% (scheme 4b).12a Furthermore, 3a was demonstrated as an
efficient source of sulfenyl group towards the formation of
thiocarbamate 9 from tert-butylisonitrile in 87% (scheme 4c).12e
aReaction conditions: 1 (0.2 mmol), 2 (1.5 equiv.), DABSO (1 equiv.), Eosin Y
(5 mol %), DCM (2 mL), RT, 8 h, under air.
In order to improve the scope of the transformation, allyl tethered
diazonium salt 4 was reacted with thiophenol 1a under the
standard condition and we were pleased to observe the formation
the dihydrobenzofuran derived thiosulfonate 5a in 79% (Scheme 2).
While the formation of 5a imparted significant synthetic
importance to the developed protocol, it also provided basic
insights into the reaction mechanism (see supporting information).
The process involves an intramolecular 5-exo-cyclization of aryl
radical followed by insertion of sulfur dioxide (vide infra).
Consequently we tested various thiols for the aforementioned
transformation and found that the reaction proceeds with good
yields regardless of the electronic and steric aspects of the thiol.
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arylsulfonyl radical formed from DABSO and aryldiazonium
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DOI: 10.1039/C9OB01040H
salts or diaryliodonium salts. The protocol displayed good
functional group tolerance and furnished good to excellent
yields of the product. Furthermore,
cyclization/radical-radical coupling
a
of
cascade radical
(2-allyloxy)-
phenyldiazonium salts and thiols was demonstrated.
ACKNOWLEDGMENT
The activity was generously supported by Department of Science
and Technology, Government of India (DST/TMD/SERI/S111(G)). S.K
would like to thank UGC, India and I. H would like to thank DST-
Inspire for the fellowship.
References
Scheme 4: Gram scale synthesis and synthetic utility.
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transfer would result in the formation of the aryl radical B, SO2 and
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2
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*
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Scheme 5: Proposed mechanism.
Conclusions
In summary, we have developed a visible-light mediated
synthesis of thiosulfonates via SO2 insertion. The reaction
proceeds via
a
radical-radial cross coupling of
photoctalytically-generated sulfenyl radicals and the
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15 At this juncture, to improve the yields Yang and co-workers chose to
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employed photocatalysis at room temperature. This also highlights
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