Catalysis by ionic liquids: Solvent-free efficient transthioacetalisation of acetals by molten tetrabutylammonium bromide

Article abstract of DOI:10.1039/b204363g

Transthioacetalisation of acetals to dithioacetals by a solvent-free reaction with a thiol or dithiol catalyzed by molten tetrabutylammonium bromide was presented. Acetals were prepared by standard methods and were purified by distillation or chromatography. Tetrabutylammonium bromide in molten state was demonstrated to be an efficient and recyclable catalyst. The methodology offered improvements over other procedures in relation to yield of products, selectivity for acetal of aldehyde over that of ketone, and mildness of reaction conditions.

Full text of DOI:10.1039/b204363g

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Catalysis by ionic liquids: solvent-free efficient
transthioacetalisation of acetals by molten tetrabutylammonium
bromide†
1
Brindaban C. Ranu,* Arijit Das and Sampak Samanta
Department of Organic Chemistry, Indian Association for the Cultivation of Science, Jadavpur,
Calcutta - 700 032, India. E-mail: ocbcr@mahendra.iacs.res.in; Fax: ϩ91 33 4732805
Received (in Cambridge, UK) 7th May 2002, Accepted 29th May 2002
First published as an Advance Article on the web 11th June 2002
Tetrabutylammonium bromide in molten state has been
demonstrated to be a very efficient and recyclable catalyst
for transthioacetalisation of O,O-acetals to S,S-acetals
under solvent-free conditions.
A wide range of structurally varied acetals derived from
aliphatic, aromatic and conjugated aldehydes underwent clean
transthioacetalisations with a variety of aliphatic and aromatic
thiols and dithiols to provide the corresponding thioacetals in
very high yields. The results are summarized in Table 1. The
conversion of an acetal to the corresponding acyclic or
cyclic dithioacetal has been accomplished efficiently with choice
of thiol or dithiol respectively. The presence of electron-
withdrawing or electron-donating substituents on the aromatic
ring of an acetal does not affect the course of reaction.
Although acetals derived from aldehydes undergo transthioace-
talisation by this procedure very easily, the acetals from ketones
remained inert (entry 18). This makes this process selective for
acetals of aldehydes over those of ketones as evident in a couple
of cross experiments.7 A variety of functionalities such as Cl,
NO2, OMe, methylenedioxy, allyloxy also survived the present
reaction conditions.
The toxic and volatile nature of many organic solvents, particu-
larly chlorinated hydrocarbons, that are widely used in organic
synthesis have posed a serious threat to the environment. Thus,
design of solvent-free reactions1 and use of alternative green
solvents like water,2 supercritical fluids,3 and ionic liquids4 have
received a lot of attention in recent times in the area of
green synthesis. Although ionic liquids have been successfully
employed as solvents with catalytic activities for a variety of
important reactions4 their use as real catalyst under solvent-
free conditions has not been explored to any great extent.5 The
high cost of most of the conventional ionic liquids prompted us
to initiate an investigation to explore the catalytic (minimum)
use of less expensive and readily available ionic liquids or
molten ionic salts for useful organic transformations.
The transthioacetalisation of O,O-acetals to S,S-acetals is a
synthetically useful transformation and is usually carried out
under the catalysis of a variety of Lewis acids such as MgBr2,6a
TeCl4,6b WCl6,6c ZrCl4,6d SiO2–SOCl2,6e trichloroisocyanuric
acid6f in methylene chloride and other organic solvents. In view
of the use of toxic reagents and solvents coupled with other
disadvantages like long reaction period in many of these
procedures an alternative improved green procedure for this
useful transformation would be appreciated.
The reactions in molten tetrabutylammonium bromide are, in
general, very fast and clean. The absence of tetrabutylam-
monium bromide makes the conversion sluggish and leads to
considerable polymeric products particularly in reactions with
propane-1,3-dithiol. No conversion has been observed with
solid tetrabutylammonium bromide at room temperature or
under reflux in dichloromethane and dichloroethane. However,
in more polar solvents like dimethyl sulfoxide (DMSO) at
110 ЊC transthioacetalisation proceeds to a certain extent
(40–50%) although the reactions are not always clean being
associated with other undesirable side products. For com-
parison with other salts when the reaction was carried out in the
presence of solid sodium bromide without solvent at 110 ЊC no
appreciable reaction (less than 10%) was apparent. However,
use of DMSO as solvent leads to some conversion (40%).
Presumably, molten tetrabutylammonium bromide acts as a
ready source of bromide ion which by co-ordination with thiol
makes the thiolate anion a stronger nucleophile and leads to
efficient transthioacetalisation, while the partial progress of
reaction with solid tetrabutylammonium bromide or sodium
bromide in highly polar solvent, DMSO is due to the
availability of bromide ion to a certain extent in the reaction
media.
In conclusion, the present solvent-free procedure for
transthioacetalisation of acetals demonstrates the potential of
molten tetrabutylammonium bromide,8 a cheap readily avail-
able ionic liquid as an efficient catalyst and thus broadens the
scope for catalytic uses of molten ionic salts for organic trans-
formations in general. Moreover, this methodology offers sig-
nificant improvements over many existing procedures with
regard to yield of products, mildness of reaction conditions,
simplicity in operation, selectivity for acetal of aldehyde over
that of ketone, cost-efficiency, and above all, green aspects
avoiding toxic catalysts and solvents.
We wish to report here an efficient transthioacetalisation of
acetals to dithioacetals by a simple solvent-free reaction with a
thiol or dithiol catalysed by molten tetrabutylammonium
bromide, a cheap commercially available ionic salt (Scheme 1).
Scheme 1 Reagents and conditions: i, HS(CH2)nSH, TBAB, 110 ЊC.
The experimental procedure is very simple. A mixture of
acetal and thiol (or dithiol) was added to molten tetrabutyl-
ammonium bromide at 110–115 ЊC and was stirred at that tem-
perature for several minutes. The product was isolated either by
direct distillation under reduced pressure or extraction with a
relatively non-hazardous solvent. The tetrabutylammonium
bromide remaining in the reaction flask was washed with hex-
ane, dried under vacuum and was recycled for subsequent uses
without any loss of efficiency. 30 Mol% of tetrabutylammonium
bromide (with respect to acetal) was found to be optimum for
an efficient and fast reaction. Although 10–20 mol% is sufficient
to catalyse the reaction a longer period for full conversion is
necessary. The use of larger quantities of tetrabutylammonium
bromide does not make any significant difference.
Experimental
Acetals were prepared by standard methods and were purified
by distillation or chromatography. Thiols were commercial
† Electronic supplementary information (ESI) avialable: spectral data
of S,S-acetals. See http://www.rsc.org/suppdata/p1/b2/b204363g/
1520
J. Chem. Soc., Perkin Trans. 1, 2002, 1520–1522
This journal is © The Royal Society of Chemistry 2002
DOI: 10.1039/b204363g

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Table 1 Transthioacetalisation of O,O-acetals catalysed by TBAB
Entry
1
Acetal
Thiol
t/min
Product
Yield(%)a
87
PhCH(OMe)2
HS(CH)2SH
40
2
PhCH(OMe)2
HS(CH)3SH
40
89
3
4
PhCH(OMe)2
PhCH(OMe)2
EtSH
PhSH
60
40
PhCH(SEt)2
PhCH(SPh)2
81
89
5
6
7
8
HS(CH)2SH
HS(CH)2SH
HS(CH)3SH
HS(CH)2SH
45
45
50
60
85
86
89
85
9
PhSH
60
87
10
11
12
HS(CH)2SH
EtSH
70
60
70
83
86
83
HS(CH)2SH
13
14
HS(CH)2SH
HS(CH)2SH
50
50
89
81
15
16
HS(CH)2SH
HS(CH)2SH
80
60
79
85
17
HS(CH)2SH
60
83
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Table 1 (Contd.)
Entry
18
Acetal
Thiol
t/min
Product
Yield(%)a
HS(CH)2SH
120
No reaction
a Yields refer to those of pure isolated products characterised by spectral data (IR, 1H and 13C NMR).
materials and were distilled before use. Tetrabutylammonium
bromide (Lancaster, UK) was also recrystallised before use.
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A representative general procedure
(Entry 1). A mixture of dimethyl acetal of benzaldehyde
(456 mg, 3.0 mmol) and ethane-1,2-dithiol (376 mg, 4.0 mmol,
a little excess was used to compensate any loss during heating)
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liquid (475 mg, 87%) whose spectral data (IR, H NMR, 13C
1
NMR) are in good agreement with those reported.9a
All other reactions listed in Table 1 were carried out follow-
ing this procedure. If any reaction was carried out on a 1 mmol
(or less) scale and the distillation was not very convenient,
the product could be isolated by extraction with diethyl
ether followed by washing with 10% NaOH solution and
brine and purification by column chromatography. All the
products were properly characterised by their IR, H and 13C
1
NMR spectral data, many of which have been reported
earlier.6,9
Acknowledgements
This investigation has enjoyed financial support from CSIR,
New Delhi [Grant No. 01(1739)/02]. A. D. and S. S. are also
thankful to CSIR for their fellowships.
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J. Chem. Soc., Perkin Trans. 1, 2002, 1520–1522