Visible light catalyzed methylsulfoxidation of (het)aryl diazonium salts using DMSO

Article abstract of DOI:10.1039/c6cc04142f

The visible light catalyzed methylsulfoxidation of (het)aryl diazonium salts using DMSO is illustrated. This is the first example of DMSO being used as the source of the methylsulfinyl group. The procedure tolerates a wide range of functional groups on (het)aryl diazonium salts and provides aryl methyl sulfoxides in excellent yields under mild reaction conditions.

Full text of DOI:10.1039/c6cc04142f

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Visible light catalyzed methylsulfoxidation
of (het)aryl diazonium salts using DMSO†
Cite this:DOI: 10.1039/c6cc04142f
Mukund M. D. Pramanikab and Namrata Rastogi*ab
Received 18th May 2016,
Accepted 9th June 2016
DOI: 10.1039/c6cc04142f
www.rsc.org/chemcomm
The visible light catalyzed methylsulfoxidation of (het)aryl diazonium
salts using DMSO is illustrated. This is the first example of DMSO
being used as the source of the methylsulfinyl group. The procedure
tolerates a wide range of functional groups on (het)aryl diazonium
salts and provides aryl methyl sulfoxides in excellent yields under mild
reaction conditions.
Organic sulfoxides occur extensively in natural1/synthetic bio-
active molecules and drugs (Fig. 1).2 Sulfoxides are useful as
intermediates in material chemistry,3 directing/activating groups
or starting substrates for several organic transformations4 and
used as chiral auxiliaries or ligands in asymmetric reactions.5
The conventional methods for the synthesis of sulfoxides
include: (i) sulphide oxidation with peracids and common
inorganic oxidants,6 organometallic catalytic complexes,7 metal
catalyzed oxidations utilizing molecular oxygen as an oxidant8 or
visible light promoted oxidations;9 (ii) nucleophilic substitution
of electrophilic sulfoxide derivatives;10 and (iii) metal catalyzed
arylation of sulfenate anions.11 Despite their widespread use most
of these approaches suffer from one or more limitations such
as the production of large amount of toxic wastes, the use of
considerable amount of supplementary oxidants or expensive
catalysts, limited functional group tolerance, overoxidation to
sulfones and operational difficulties. Evidently all the methods
for the synthesis of sulfoxides require pre-existing sulfur function-
Fig. 1 Sulfoxide group bearing natural and synthetic bioactive molecules/
drugs.
Scheme 1 DMSO as a source of S-containing functional groups.
Recently, the use of visible light to develop greener synthetic
ality in the molecule. Among the established sulfur-introducing protocols has attracted the attention of the chemist community.16
reagents12 in organic molecules DMSO is arguably the safest, While there are numerous reports on the visible light catalyzed C–C
cheapest, and innocuous reagent.13 DMSO has been utilized as and C–X (X = N, O, P, F, Cl) bond formation,17 examples of C–S
a source of thiomethyl14 as well as the methylsulfonyl15 group; bond formation under visible light catalysis are limited.18 We
however to the best of our knowledge, use of DMSO as a source hereby report the synthesis of aryl methyl sulfoxides from the
of the methylsulfinyl group is unprecedented (Scheme 1).
corresponding aryl diazonium salts under visible light catalyzed
conditions using DMSO as the source of the methylsulfinyl group.
We started our investigation with 4-methoxybenzenedia-
zonium tetrafluoroborate 1a and 1 mol% of the photocatalyst
[Ru(bpy)3Cl2] in DMSO (Table 1). Upon irradiation of the
reaction mixture with blue light for 4 h at room temperature,
the 1-methoxy-4-(methylsulfinyl)benzene 3a was isolated in 52%
yield (entry 1). While changing the photocatalyst from [Ru(bpy)3Cl2]
a Medicinal & Process Chemistry Division, CSIR-Central Drug Research Institute,
Sector 10, Jankipuram Extension, Sitapur Road, P.O. Box 173, Lucknow 226031,
India. E-mail: namrataiit@gmail.com, namrata.rastogi@cdri.res.in
b Academy of Scientific and Innovative Research, New Delhi 110001, India
† Electronic supplementary information (ESI) available: Experimental procedures
and analytical data for all products and new starting materials. See DOI: 10.1039/
c6cc04142f
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Table 1 Optimization of reaction conditions
Table 2 Scope of diazonium salts
Entry Conditions
Yielda (%)
1
2
3
4
5
6
7
8
9
[Ru(bpy)3Cl2] (1 mol%), DMSO (4 mL), blue light 52
Eosin Y (1 mol%), DMSO (4 mL), green light
[Ir(ppy)3] (1 mol%), DMSO (4 mL), blue light
20
53
[Ru(bpy)3Cl2] (2 mol%), DMSO (4 mL), blue light 64
Eosin Y (2 mol%), DMSO (4 mL), green light
[Ir(ppy)3] (2 mol%), DMSO (4 mL), blue light
14
59
[Ru(bpy)3Cl2] (3 mol%), DMSO (4 mL), blue light 62
[Ru(bpy)3Cl2] (2 mol%), DMSO (2 mL), blue light 66
[Ru(bpy)3Cl2] (2 mol%), DMSO (1 mL), blue light 68
a
Isolated yields.
to eosin Y resulted in a considerable reduction in the yield (entry 2),
[Ir(ppy)3] provided the product in comparable yields (entry 3). The
same pattern in the yields was observed with an increased loading
of the photocatalysts (entries 4–6) with [Ru(bpy)3Cl2] giving the
best yield. Further optimization revealed that 2 mol% of the
[Ru(bpy)3Cl2] was the optimum amount considering the reaction
economy since only a marginal increase in the product yield was
noticed with 3 mol% of the photocatalyst (entry 7). An amount of
DMSO that is only sufficient to dissolve the diazonium salt (1 mL)
is required in the reaction (entries 4, 8–9).
After finding the suitable conditions for methylsulfoxidation
of 4-methoxybenzenediazonium tetrafluoroborate 1a, we set out
to examine the scope of diazonium salts in the reaction. Aryl
diazonium salts bearing neutral, electron-donating, and electron-
withdrawing substituents at the o, m, p-positions reacted smoothly
under the optimized reaction conditions affording the corres-
ponding products in high yields in most of the cases (Table 2).
The reaction also tolerated several functional groups such as
nitrile 3u, thiocyanate 3v, ketone 3w, ester 3x–3z etc. very well
on aryl as well as heteroaryl diazonium salts. The reaction also
works well with deuterated DMSO since methylsulfoxidation of
substrates 1a, 1l and 1v using deuterated DMSO was carried out
smoothly yielding corresponding deuterated methylsulfoxides
3a0, 3l0 and 3v0 in high yields.
Scheme 2 Methylsulfoxidation of drug molecules.
The mechanism proposed on the basis of control experiments
and related literature reports16 is illustrated in Scheme 4.
2+
Initially, the photocatalyst in its excited state Ru(bpy)3
*
reduces the diazonium salt 1a by single-electron transfer resulting
in the formation of aryl radical A and generation of the oxidized
photocatalyst.20 The aryl radical A then traps a DMSO molecule
yielding radical B which is oxidized to the sulfoxide cation C.
We could also successfully utilize the diazonium salt of
two drug molecules, benzocaine and dapsone in the reaction
(Scheme 2). While benzocaine diazonium salt 1za provided the
corresponding methylsulfoxide 3za in an excellent yield, in
the case of dapsone only one amino group was replaced by the
methylsulfinyl group whereas another one simply underwent the
deamination reaction19 resulting in the formation of 3zb.
Additionally, key control experiments were conducted to
substantiate the fact that both visible light and the photocatalyst
were necessary for the reaction since no product formation
was noticed in the absence of the photocatalyst or visible light.
Further, a test reaction in the presence of (2,2,6,6-tetramethyl-
piperidin-1-yl)oxyl (TEMPO) did not furnish the methyl sulfoxide
product 3a (see the ESI† for data), suggesting the involvement of a
radical mechanism (Scheme 3).
Scheme 3 Control experiments.
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Scheme 4 Proposed mechanism.
The cation C transfers one methyl group to the weakly nucleo-
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In conclusion, we have demonstrated that DMSO can be
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