Electro- and acid-catalysis in tetrafluoroborate-based ionic liquid: new alternative routes for the oxidation of sulfides with UHP

Article abstract of DOI:10.1016/j.tetlet.2008.07.051

By means of anodic activation, BF₄-based ionic liquid was found to play the triple role of electrolysis/reaction medium, supporting electrolyte and pre-catalyst versus the oxidation of sulfides with UHP. Galvanostatic electrolysis in the presence of substrate and UHP molecules allowed the fast, efficient and selective achievement of sulfoxides. Comparable results have been attained by acid catalysis based on R-CSA.

Full text of DOI:10.1016/j.tetlet.2008.07.051

Tetrahedron Letters 49 (2008) 5611–5613
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Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Electro- and acid-catalysis in tetrafluoroborate-based ionic liquid:
new alternative routes for the oxidation of sulfides with UHP
a,*
a
a
b
a
Laura Palombi , Carmen Bocchino , Tonino Caruso , Rosaria Villano , Arrigo Scettri
a
Dipartimento di Chimica, Università di Salerno, Via Ponte don Melillo 84084 Fisciano (Salerno), Italy
Istituto di Chimica Biomolecolare-CNR, trav. La Crucca 3, Reg. Baldinca, 07040 Li Punti (Sassari), Italy
b
a r t i c l e i n f o
a b s t r a c t
Article history:
By means of anodic activation, BF -based ionic liquid was found to play the triple role of electrolysis/reac-
4
Received 20 May 2008
Revised 4 July 2008
Accepted 7 July 2008
Available online 11 July 2008
tion medium, supporting electrolyte and pre-catalyst versus the oxidation of sulfides with UHP. Galvano-
static electrolysis in the presence of substrate and UHP molecules allowed the fast, efficient and selective
achievement of sulfoxides. Comparable results have been attained by acid catalysis based on R-CSA.
Ó 2008 Elsevier Ltd. All rights reserved.
Keywords:
Sulfoxides
Electrochemical oxidation
Ionic liquid
1
a: R= p-MeC H
6 4
As evidenced by a number of literature reports, room tempera-
ture ionic liquids (RTILs) are recently finding large application in
organic synthesis both as reaction media and as catalysts or co-
catalysts. The versatile use of this class of compounds arises from
their unique and tunable properties such as high thermal stability,
negligible vapour pressure and high polarity.
1b: R= p-MeOC
6
H
4
1
1
c: R= o-MeOC H
O
S
6 4
d: R= p-BrC H
6
4
electrolysis
S
1e: R= o-BrC H
1
6
4
R
Me
UHP /IL
R
Me
1f: R= p-NO C H
2 6 4
1g: R= 2-naphthyl
1h: R= n-C
1
2
H
8 17
i: R= C H CH
6 5 2
IL(A): EMImEtSO4
IL(B): EMImBF4
IL(C): BMImBF4
1
On the contrary, despite their recognized potential as conduct-
ing media, which avoid the employment of supporting electrolytes,
N
2
the use of RTILs in electrosynthesis remains rare. Furthermore, the
1
j: R=
S
few examples concerning electrosynthetic procedures in RTIL
mainly refer to cathodic processes, and very little references have
addressed attention to the anodic ones.
Scheme 1.
Here we describe the first electro-activation of urea hydrogen
peroxide (UHP) adduct in tetrafluoroborate-based ionic liquid at
a platinum anode, usefully exploited for the in situ oxidation of
sulfides (Scheme 1). In parallel with the electrochemical procedure,
we also report a purely chemical method, based on the (1R)-(À)-
As summarized in Table 1, the preliminary investigations
pertained to the choice of the suitable ionic liquid and the optimal
setting of the electrochemical conditions, focusing on the oxidation
of sulfide 1a as a model substrate. Interestingly, all the RTILs
explored ensured a good transport of current through the cell.
1
0-Camphor sulfonic acid (R-CSA) catalysis in 1-ethyl-3-methyl-
Nevertheless, the presence of the BF4
À
counteranion was strictly
imidazolium tetrafluoroborate (EMImBF
4
).
required to enable the efficient conversion of the starting material
The electrochemical experiments were carried out under galva-
nostatic conditions, using a U-two compartment cell equipped
with a Pt anode and cathode and a G4-glass diaphragm as separa-
tor septum.
1a.
Concerning the set-up of the optimal operational conditions, we
can highlight the following features of the electrochemical
procedure:
In a typical experiment, the anolyte was made up of a solution
of sulfide 1 and UHP in ionic liquid, while the catholyte consisted
in pure ionic liquid. The cell was put in a water bath to avoid the
overheating during the electrolysis.
– the reaction requires sub-stoichiometric amount of electricity to
proceed (Q = 0.3 F/mol);
– the application of a higher current density and quantity (from
2
2
5
0 mA/cm to 150 mA/cm ; from 0.1 F/mol to 0.3 F/mol) mark-
edly reduces the reaction time without affecting the chemose-
lectivity of the process;
*
Corresponding author.
E-mail address: lpalombi@unisa.it (L. Palombi).
0
040-4039/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2008.07.051

5
612
L. Palombi et al. / Tetrahedron Letters 49 (2008) 5611–5613
Table 1
Electrochemically promoted oxidation of 1a with UHP in ionic liquids
IL(A): 81% yield
IL/BF Et O
3
2
1
a
2a
Reaction 2a Yieldb
0˚C, 10 min
Entry Electrolysis Ionic
Current density Current
IL(C): > 99% yield
methoda
liquid (mA/cm )
2
quantity time (h)
F/mol)
(%)
(
Scheme 2.
1
2
3
4
5
1
1
1
2
2
IL(A)
IL(B)
IL(C)
IL(C)
IL(C)
50
50
50
150
150
0.1
0.1
0.1
0.1
0.3
18
3
2
Trace
58
65
67
>99
shown in Scheme 2, an efficient conversion of the starting material
1a, although with different yields, occurred both in IL(A) (which
was ineffective towards the electrochemical activation) and in
2
n.d.c
a
IL(C), providing support to the hypothesized role of BF
3
.
Method 1: catholyte: pure ionic liquid 1; anolyte: UHP/IL. At the end of the
electrolysis, substrate 1 was added to the anode compartment and the reaction
prolonged for the time reported in the table. Method 2: catholyte: pure ionic liquid;
anolyte: 1a/UHP/IL (1 mmol/1.06 mmol/0.6 ml).
It must be pointed out, however, that the direct use of BF
3
ÁEt
2
O
required the reaction to be performed at 0 °C, with careful slow
addition of the catalyst to moderate the generation of HF gas.
Conversely, the electrochemical induction facilitated the set-up
and allowed the reaction to be performed at room temperature.
Despite the great number of procedures available, the prepara-
tion of sulfoxides from sulfides still represents a topical issue for
b
The yields refer to isolated, chromatographically pure products.
TLC analysis at the end of the electrolysis (time: 7 min) showed a complete
c
consumption of the starting material 1a.
5
–
the electrolysis may be successfully performed in the presence of
the substrate 1a, showing the best result in term of efficiency,
selectivity and reaction time.
organic chemists. In view of environmental reasons, particularly
attractive are new, selective methodologies based on the use of
6
benign oxidants, catalysts and solvents. On the other hand, the
use of RTILs or metal-free catalysts, in combination with hydrogen
7
Under a similar electrochemical set-up (Table 1, entry 5), we
peroxide or derivatives, is yet little explored.
tested the general applicability of the one-pot procedure by per-
forming the electrolysis of the UHP/BMImBF in the presence of a
With this in mind, we decided to explore the UHP/ EMImBF4
system to establish an alternative sulfoxidation methodology,
resorting to the ‘purely chemical’ acid catalysis.
As quoted by us as long ago as 1996, Brønsted acid CSA usefully
promotes the chemoselective conversion of sulfides to sulfoxides
4
series of sulfides (1b–j). At the end of the electrolysis, the reaction
was monitored by TLC analysis and was prolonged until complete
consumption of the starting material 1 (Table 2). After extraction
with ethyl acetate, the crude mixtures were purified by filtration
over silica gel providing pure sulfoxides 2 with fair to very good
yields in almost all the cases.
8
by t-butyl hydroperoxide in CHCl . On the other hand, recently,
3
9
Firouzabadi et al. have reported the highly selective oxidation of
sulfides by H O in aqueous medium promoted by the acid surfac-
2 2
As shown, the reaction was found to proceed with satisfactory
efficiency and excellent chemoselectivity both with dialkyl (Table
tant dodecyl hydrogen sulfate. According to what was proposed for
the surfactant catalyst, we envisaged that R-CSA could activate the
oxidant molecule by hydrogen bonding, favouring the oxygen
transfer to the sulfide molecule.
2, entries 7 and 8) and with aryl–alkyl sulfides (Table 2, entries
–6) bearing electron-donating and electro-withdrawing substitu-
1
ents in ortho- or para- position. Conversely, the ambident substrate
j showed a lower reactivity under a variety of experimental con-
So, we decided to investigate the catalytic ability of R-CSA ver-
sus the oxidation process of sulfides with UHP/EMImBF4.
The data collected in Table 3 show a good reactivity of this sys-
tem towards almost all the sulfides investigated, with reaction
time in the order of about few hours and an excellent chemoselec-
tivity towards sulfoxides. On the other hand, the system was not
found to be effective from a stereochemical point of view, leading
to sulfoxides with little enantioselectivity (3-4% ees).
1
ditions. However, the reaction proceeded in a highly regio- and
chemoselective way, with exclusively the exocyclic sulfur atom
prone to oxidation. Finally, the use of stoichiometric amount of
electricity as well as more prolonged reaction time or higher tem-
peratures compromised the chemoselectivity affording a mixture
of 2j and the corresponding methyl-sulfone.
Examination of a recent study on the electrochemistry of
As already noted for the electrochemical procedure, attempts to
improve the chemical yield for the substrate 1j, by increasing the
temperature, the catalyst loading and the reaction time, gave
unsatisfactory results. Finally, it is important to note that the back-
ground reactions (in the absence of electrolysis or R-CSA) were
BMImBF
ment of the electrogenerated Lewis acid BF
4
reported by Xiao and Johnson3 suggested the involve-
3
as possible catalyst
of the oxidation process. Thus, two experiments, without electrol-
4
ysis, were carried out in the presence of 0.3 equiv of BF
3
ÁOEt
2
. As
Table 3
Table 2
R-CSA-promoted oxidation of 1 with UHP in IL(B)
Electrochemically promoted oxidation of 1b–j
R-CSA(0.1 eq)/UHP (1 eq.)
Entry
1
2
Reaction time (h)
Yield (%)a
1
2
b
EMIMBF (0.3 ml/mmol), r.t.
4
1
2
3
4
5
6
7
8
1b
1c
1d
1e
1f
1g
1h
1i
2b
2c
2d
2e
2f
2g
2h
2i
n.d.
>99
75
78
87
92 [8]
90
70
b
n.d.
Yielda (%)
Entry
1
2
Reaction time (h)
2
2
n.d.
1
2
3
4
5
6
7
8
9
1a
1b
1c
1d
1e
1f
2a
2b
2c
2d
2e
2f
4
2
6
3
24
2
>99
96
>99
92
>99
81
94
b
3
6
3
4
24
75
9
1j
1j
2j
2j
43 (57)
55 (16) [24]
c
1
0
1g
2g
2h
2j
6
5
24
1h
1j
74
a
Yields refer to isolated, chromatographically pure compounds. Yields in round
36 (64)b
parentheses refer to recovered starting material 1. Yields in square brackets refer to
a
isolated sulfones.
Yields refer to isolated, chromatographically pure compounds. Yields in
parentheses refer to recovered starting material 1.
b
See footnote c, Table 1.
In this entry the electrolysis was performed using 1 F/mol of current.
c
b 1
H NMR yields.

L. Palombi et al. / Tetrahedron Letters 49 (2008) 5611–5613
5613
generally negligible within the reaction time. Nevertheless, for oxi-
dations requiring more than 6 h a significant contribution by the
background reaction cannot be excluded.10
In summary, we have shown here the first electrochemical ano-
dic activation of the UHP adduct in tetrafluoroborate-based ionic
liquid for the chemoselective oxidation of sulfides. The proposed
B.; Keyrouz, R.; Gmouh, S.; Vaultier, M.; Jouikov, V. Chem. Commun. 2004, 674–
6
1
75; (c) Tang, M. C.-Y.; Wong, K.-Y.; Chan, T. H. Chem. Commun. 2005, 1345–
347.
3
.
Xiao, L.; Johnson, K. E. J. Electrochem. Soc. 2003, 150, E307–E311.
4. The use of BF to mediate the oxidation of sulfides with KO in CH
recently reported: Chen, Y.-J.; Shen, J.-Y. Tetrahedron Lett. 2005, 46, 4205–
208.
3
2
3
CN has been
4
5
.
For recent reviews see: (a) Kowalski, P.; Mitka, K.; Ossowska, K.; Kolarska, Z.
Tetrahedron 2005, 61, 1933–1953; (b) Kaczorowska, K.; Kolarska, Z.; Mitka, K.;
Kowalski, P. Tetrahedron 2005, 61, 8315–8327.
(a) Varma, R. S.; Naicker, K. P. Org. Lett. 1999, 1, 189–191; (b) Bäckvall, J.-E. In
Modern Oxidation Methods; Bäckvall, J.-E., Ed.; Wiley-VCH: Weinheim,
Germany, 2004; pp 193–222; (c) Legros, J.; Bolm, C. Chem. Eur. J. 2005, 11,
reaction pathway (i.e., via electrogenerated Lewis acid BF
3
from
the pre-catalyst BMImBF ) discloses a new approach to oxidation
4
6
.
processes, combining the advantages of electrocatalysis and use
of the benign oxidant and solvent. As a comparison, a purely chem-
ical method, based on the use of the Brønsted acid R-CSA in an
ionic liquid, has also been demonstrated.
1
086–1092 and references cited therein.
Lindén, A. A.; Johansson, M.; Hermanns, N.; Bäckvall, J.-E. J. Org. Chem. 2006,
1, 3849–3853.
Bonadies, F.; De Angelis, F.; Locati, L.; Scettri, A. Tetrahedron Lett. 1996, 37,
129–7130.
Firouzabadi, H.; Iranpoor, N.; Jafari, A. A.; Riazymontazer, E. Adv. Synth. Catal.
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7
8
9
.
.
.
7
7
References and notes
2
1
.
.
For recent reviews see: (a) Sheldon, R. Chem. Commun. 2001, 2399–2407; (b)
Welton, T. Coord. Chem. Rev. 2004, 248, 2459–2477; (c) Muzart, J. Adv. Synth.
Catal. 2006, 348, 275–295; (d) Chowdhury, S.; Mohan, R. S.; Scot, J. L.
Tetrahedron 2007, 63, 2363–2389; (e) Pârvulescu, V. I. Chem. Rev. 2007, 107,
1
0. In actual fact, the ionic liquids can interact with peroxy species and increase
the electrophilicity of the oxygen by hydrogen bonding, which involve the
+
[
Im] cation moiety, see: Cimpeanu, V.; Parvulescu, V. I.; Amorós, P.; Beltrán,
D.; Thompson, J. M.; Hardacre, C. Chem. Eur. J. 2004, 10, 4640–4646.
2
615–2665.
2
Selected articles on electrochemically-promoted oxidation reactions in ionic
liquid: (a) Gaillon, L.; Bedioui, F. Chem. Commun. 2001, 1458–1459; (b) Martiz,