Silica: An efficient catalyst for one-pot regioselective synthesis of dithioethers

Article abstract of DOI:10.3762/bjoc.10.5

The development of a silica-promoted highly selective synthesis of 1,2 or 1,3-dithioethers via solvent-free one-pot tandem reactions of an allyl bromide with excess thiol at room temperature is described. The choice of silica gel, either pre-calcined or moistened with water, exhibited notable regioselectivity in the formation of dithioethers. Plausible mechanistic routes were explored and postulated.

Full text of DOI:10.3762/bjoc.10.5

Silica: An efficient catalyst for one-pot
regioselective synthesis of dithioethers
Samir Kundu, Babli Roy and Basudeb Basu*
Full Research Paper
Open Access
Address:
Beilstein J. Org. Chem. 2014, 10, 26–33.
Department of Chemistry, North Bengal University, Darjeeling
734013, India, Fax: +91 353 2699001
doi:10.3762/bjoc.10.5
Received: 15 September 2013
Accepted: 11 November 2013
Published: 07 January 2014
Email:
Basudeb Basu* - basu_nbu@hotmail.com
* Corresponding author
Associate Editor: B. Stoltz
Keywords:
© 2014 Kundu et al; licensee Beilstein-Institut.
License and terms: see end of document.
allyl halide; dithioether; silica gel; tandem reactions; thiol
Abstract
The development of a silica-promoted highly selective synthesis of 1,2 or 1,3-dithioethers via solvent-free one-pot tandem reac-
tions of an allyl bromide with excess thiol at room temperature is described. The choice of silica gel, either pre-calcined or moist-
ened with water, exhibited notable regioselectivity in the formation of dithioethers. Plausible mechanistic routes were explored and
postulated.
Introduction
Organosulfur compounds are important building blocks for the metal-catalyzed addition of disulfides to alkenes [19,20] or by
synthesis of various biologically active molecules [1-3]. Versa- the traditional nucleophilic substitution of 1,2-dihalides with
tile applications of organosulfur compounds are known in fields suitable thiols/thiolates [21,22]. They are also prepared by
such as the pharmaceutical, the polymer, the pesticide and the consecutive hydrothiolation of alkynes, both under nucleophilic
food-processing industry [4-8]. For example, organosulfur com- and radical-induced conditions [22,23]. On the other hand, 1,3-
pounds in garlic are often used in food-processing industries as dithioethers can be prepared by the nucleophilic substitution of
flavouring and preservative agents and are also used as herbal compounds bearing suitable leaving groups at 1,3-positions of
medicine [4]. Dithioethers are commonly employed as ligands alkyl chains [21]. Because of their versatile applications, a great
in preparing metal-coordination complexes and also as spacers number of procedures have been developed to synthesize
in metal-organic frameworks [9-14]. For example, vicinal bis(thioethers) with varying degrees of success and a variety of
dithioether-based zirconium and titanium complexes have been limitations [19-31].
used for alkene polymerization and hydroamination [15-18].
Chiral dithioethers have been prepared and their iridium Over the last decade, organic synthesis has taken a major turn
complexes have been employed in asymmetric hydrogenation towards developing reaction conditions that are environmen-
[18]. Vicinal dithioethers are generally synthesised either by the tally friendly and sustainable [32-36]. Mesoporous inorganic
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Beilstein J. Org. Chem. 2014, 10, 26–33.
oxides, which often facilitate various organic reactions, are fide (5–10%) was formed [43,44], which was easily separable
considered suitable to promote eco-friendly chemical processes from the reaction mixture by column chromatography. Since the
[36]. Organic reactions with a high selectivity under eco- choice of silica led to the production of highly regioselective
friendly and sustainable conditions are attractive features in products, we wanted to optimize both conditions to establish
terms of the concepts of Green chemistry. Previously, we have them as general protocols. Table 1 shows the optimization of
developed silica-promoted facile and highly selective methods the reactions of different allylic substrates with benzenethiol.
for N and S-alkylations/acylation from amines or thiols, Silica gel (directly from the container, commercially available)
respectively [37,38]. An equimolar mixture of a benzenethiol was used either pre-activated by heating at 100 °C under
and allyl bromide on treatment with silica afforded vacuum for 1 h and then cooled under vacuum for use under
allyl(phenyl)sulfane in excellent yield. Since alkenes are also conditions A or moist with water (0.1 mL water for 0.5 g of
known to undergo ‘click’ addition with thiols [39,40], excess silica) for use under conditions B. It was observed that allyl bro-
use of thiols could effectively produce dithioethers, and based mide or allyl iodide underwent sequential substitution–addition
on a regioselective addition one could achieve either vicinal or reactions entirely regioselectively with comparable yields
1,3-dithioethers in one-pot consecutive substitution–hydrothio- (Table 1, entries 1–5), whereas allyl chloride showed varying
lation processes (Scheme 1). Although both reactions are well- results under conditions A or B, and allyl acetate did not
known, a search in the literature surprisingly revealed no undergo any desired reaction, but merely produced the disul-
general one-pot protocols for the preparation of dithioethers fide from oxidative dimerization of the thiol (Table 1, entries
from allylic substrates. Recently, Banerjee and co-workers 6–8). Allyl tosylate, however, produced the desired thioethers in
reported on the simple synthesis of thioethers by silica NPs, a regioselective manner, but with relatively low yields (Table 1,
where a single example of a reaction of an allyl bromide and entries 9 and 10). Interestingly, allylphenylsulfane or allyl phe-
excess benzenethiol was studied [41,42]. The reaction was nyl ether entirely followed an anti-Markovnikov addition, under
carried out in the presence of silica NPs and water, and they both conditions, A and B (Table 1, entries 11–14).
isolated 1,3-dithioether by an anti-Markovnikov addition.
However, there is no report on the metal-free hydrothiolation of With the two distinct conditions, we examined the scope of
allylic substrates in a Markovnikov fashion to afford 1,2- these one-pot tandem reactions of allyl bromide with a variety
dithioethers in one-pot reactions. In this paper, we wish to of thiols under both conditions. The results are presented in
report our investigations on the reaction of allyl halides with Table 2. Arylthiols bearing different functional groups like
excess thiols promoted by silica gel, which finally constitutes CH3, OCH3, Cl or F were reacted with allyl bromide in the
distinct protocols for one-pot, solvent-free substitution and presence of pre-calcined and dry silica affording good to excel-
regioselective additions to produce either 1,2 or 1,3- lent yields of the corresponding 1,2-dithioethers (Table 2,
dithioethers.
entries 1, 3, 5, 7, 9, 11 and 17). 2-Naphthylthiol also underwent
a similar regioselective Markovnikov addition, resulting in the
corresponding 1,2-dithioether in 82% yield (Table 2, entry 18).
Results and Discussion
Following our previous experience [37,38], we first attempted Extending the protocol to aliphatic thiols, such as n-pentylthiol
the magnetic stirring of a mixture of allyl bromide and and cyclohexylthiol also afforded regioselective dithioether in
benzenethiol in a 1:2.5 ratio by using pre-calcined silica gel at good yields (Table 2, entries 13 and 15). In all the cases, we
room temperature that indeed led to the formation of 1,2- observed 100% Markovnikov addition products and no anti-
dithioether in 91% yield. On the other hand, if silica gel moist- Markovnikov products were detected. We now turned our atten-
ened with a few drops of water was used for the same reaction, tion to the other conditions B – the use of moist silica gel.
the regioselective anti-Markovnikov addition product, i.e., 1,3- Again, a variety of aromatic thiols, including those that were
dithioether, (1-(3-(phenylthio)propylthio)benzene) was obtained used for the conditions A, were employed to react with allyl
in 83% yield. In both cases, a minimal amount of diphenyldisul- bromide in the presence of silica moist with a few drops of
Scheme 1: Sequential substitution-addition reactions of thiols with allyl halides leading to the formation of 1,2 or 1,3-dithioethers.
27

Beilstein J. Org. Chem. 2014, 10, 26–33.
Table 1: Optimization of one-pot sequential substitution–hydrothiolation of allylic substrate with excess benzenethiol over silica at room temperature.
Entry
CH2=CH-CH2-X
Conditionsa
Time (h)
Productb/Yieldc (%)
1
X = Br
A
6
1,2-dithioether/77
1,2-dithioether/91
1,3-dithioether/83
1,2-dithioether/89
1,3-dithioether/85
1,2-dithioether/57
diphenyldisulfide/83
diphenyldisulfide/90
1,2-dithioether/75
1,3-dithioether/68
1,3-dithioether/83
1,3-dithioether/80
3-phenoxythioether/89
3-phenoxythioether/82
no dithioether is formed
2
X = Br
A
11
20
12
20
15
30
24
8
3
X = Br
B
4
X = I
A
5
X = I
B
6
X = Cl
A
7
X = Cl
B
8
X = OAc
X = OTs
X = OTs
X = SPh
X = SPh
X = OPh
X = OPh
X = Br
A
9
A
10
11d
12d
13d
14d
15
B
22
5
A
B
12
6
A
B
14
20
Neat mixture
aConditions A: allylic compound and PhSH (1:2.5 mmol) over pre-calcined dry silica gel (0.5 g); conditions B: allylic compound and PhSH
(1:2.5 mmol) over moistened silica gel (0.5 g). bIn each case 5–10% diphenyldisulfide was formed except in entries 6–8. cYield refers to isolated pure
product and no other constitutional isomer was detected. dThiol (1.2 mmol) was used for entries 11–13.
Table 2: Regioselective one-pot synthesis of 1,2 and 1,3-dithioethers using dry (pre-calcined) or moistened silica gel at room temperature.
Entry
1
Thiol
Conditionsa
A
Time (h)
10
Product
Yieldb (%)
91
C6H5-SH
2
3
4
C6H5-SH
B
A
B
22
6
81
87
78
4-(H3C)C6H4-SH
4-(H3C)C6H4-SH
20
5
6
7
4-(H3CO)C6H4-SH
4-(H3CO)C6H4-SH
4-(Cl)C6H4-SH
A
B
A
6.5
18
6
78
76
83
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Beilstein J. Org. Chem. 2014, 10, 26–33.
Table 2: Regioselective one-pot synthesis of 1,2 and 1,3-dithioethers using dry (pre-calcined) or moistened silica gel at room temperature.
(continued)
8
4-(Cl)C6H4-SH
4-(F)C6H4-SH
B
A
15
8
87
80
9
10
11
12
4-(F)C6H4-SH
B
A
B
16
8
84
74
77
2,6-(CH3)2)C6H3-SH
2,6-(CH3)2)C6H3-SH
20
13
n-C5H11-SH
A
9
67
14
15
16
n-C5H11-SH
Cy-SH
B
A
B
16
10
18
71
65
67
Cy-SH
17
18
2-(H3C)C6H4-SH
2-C10H7-SH
A
A
7
9
71
82
19c
C6H5-SH
B
22
29

Beilstein J. Org. Chem. 2014, 10, 26–33.
Table 2: Regioselective one-pot synthesis of 1,2 and 1,3-dithioethers using dry (pre-calcined) or moistened silica gel at room temperature.
(continued)
20
C6H5-SH
Ad
15
82
aConditions A: allylic compound and PhSH (1:2.5 mmol) over pre-calcined dry silica gel (0.5 g); conditions B: allylic compound and PhSH
(1:2.5 mmol) over moist silica gel (0.5 g). bYield refers to isolated pure product; in each case 5–10% diphenyldisulfide was formed and isolated. cD2O
(0.5 mL for 0.5 g silica gel) was used instead of H2O. dMixture of silica and sodium silicate (1:1 w/w; 0.5 g for 1 mmol of allyl bromide) was used after
drying under vacuum.
water, and we isolated entirely regioselective 1,3-dithioethers the subsequent addition takes place in an anti-Markovnikov ap-
(Table 2, entries 2, 4. 6, 8, 10 and 12). The same selectivity was proach (Scheme 2, conditions B).
observed in the reaction of aliphatic thiols (acyclic or alicyclic),
viz. n-pentane-1-thiol and cyclohexanethiol, with allyl bromide In order to find evidence for the role of silica adsorbed water,
to afford the corresponding 1,3-dithioethers in 71% and 67% we conducted the following experiments: (i) the reaction was
yield, respectively (Table 2, entries 14 and 16). In these cases, carried out under conditions A in the presence of an exogenous
we did not detect any Markovnikov addition products. Thus, base (sodium silicate; see Experimental), which leads to the for-
moistened silica gel turns out to be effective for sequential mation of anti-Markovnikov product only (1,3-dithioether)
substitution reactions, and entirely anti-Markovnikov addition, (Table 2, entry 20); (ii) reactions under conditions B with
while pre-calcined dry silica gel could efficiently give rise to varying quantities of H2O (0.5 mL and 1.0 mL) did not exhibit
only Markovnikov addition products. The reactions over dry any significant changes producing only 1,3-dithioethers in
silica gel appear to be faster than the procedure using moist quantities; (iii) dry HCl gas was passed through pre-calcined
silica. Moreover, the 1,2-dithioethers are formed in slightly silica and was used for the hydrothiolation of allylphenylsul-
better yields than the corresponding 1,3-analogues. We also fane, exclusively yielding an Markovnikov addition product
experienced that aromatic thiols, under both conditions A and (1,2-dithioether); (iv) an experiment was carried out with silica
B, give better yields than aliphatic thiols.
moistened with D2O (Table 2, entry 19), which afforded a mix-
ture of 1,3-bis(phenylthio)propane and [2-D]1,3-dithioether as
We assume that the nature of the silica surface and its possible seen from the 1H NMR spectrum of the mixture and calculated
interactions with thiols is responsible for the notable regioselec- to be in the ratio of 1:3.9. In the 13C NMR spectrum, the deuter-
tivity in the hydrothiolation of allylsulfane. It is known that ated carbon appeared as a triplet at δ 27.96, J = 20 Hz (see
amorphous or mesoporous silica consists of silanol groups and Supporting Information File 1). This observation supports that
siloxane bridges that determine its surface properties, and the conditions B might occur through an initial thiol proton
concentration of these OH groups depends mostly on the actual exchange with D2O (PhS–H → PhS–D).
process of calcinations [45-47]. Based on Zhuravlev’s physico-
chemical model of silica surface [45], it may be presumed that Conclusion
the moistened silica surface is covered with a single layer or We have demonstrated that the choice of silica gel, either dry or
multilayer of adsorbed water, which might disappear during the moistened, could lead to highly selective pathways for the
calcination process. Since allylphenylsulfane on hydrothiola- preparation of different dithioethers. The sequential reactions in
tion affords the anti-Markovnikov product under both condi- one-pot protocols are robust, neutral, metal-free and notably
tions A and B (Table 1, entries 11 and 12), we presume that selective with a broad range of substrates. The diverse reactiv-
there might be an influence of the generated acid in the first step ity of silica gel in the formation of vicinal or 1,3-dithioethers
under dry conditions A. In the absence of water, the generated might not only spur the adaptation of existing procedures for
HBr in the first step might activate the double bond and subse- selective dithioether preparation but also attract novel applica-
quent assistance by the neighbouring sulfur atom coupled with tions. Further utilizations of this diverse reactivity are currently
the stability of the secondary carbocation lead to the being explored in our laboratory.
Markovnikov addition resulting in the exclusive formation of
1,2-dithioether (Scheme 2, conditions A). On the other hand, Experimental
the moist silica consisting of a single layer or multilayered All chemicals were purchased from commercial suppliers and
adsorbed water promotes thiols to bind with allylsulfane, and used without further purification. IR spectra were recorded on
30

Beilstein J. Org. Chem. 2014, 10, 26–33.
Scheme 2: Plausible mechanisms for the regioselective formation of vicinal and 1,3-dithioethers by using dry or moistened silica gel.
an FTIR spectrophotometer (8300 Shimadzu) using Nujol General procedure for Table 2 (route A or B)
mulling for liquid compounds and KBr pellets for solid com- Route A: A mixture of allyl bromide (1 mmol) and thiol
pounds. NMR spectra were recorded on a Varian AV-300 spec- (2.5 mmol) was mixed with pre-calcined dry silica gel (Table 2,
trometer with CDCl3 as a solvent. Chemical shifts (δ) are for entries 1, 3, 5, 7, 9, 11, 13, 15, 17 and 18)
reported in ppm and referenced to TMS for 1H NMR spectra
and residual solvent signals for 13C NMR spectra as internal Route B: A mixture of allyl bromide (1 mmol) and thiol
standards. Coupling constants (J) are reported in Hertz (Hz). (2.5 mmol) was mixed with silica gel (0.5 g), moistened with
Standard abbreviations indicating multiplicity were used as two drops of water, (Table 2, for entries 2, 4, 6, 8, 10, 12, 14
follows: s = singlet, d = doublet, t = triplet, q = quartet, qnt = and 16), and stirred magnetically by using a spin bar for the
quintet, m = multiplet. Melting points were determined by respective times listed in Table 2. The reaction was monitored
heating in an open capillary tube. High resolution mass spectra by TLC. After completion the product was purified by column
(HRMS) were performed in a Micromass Q-TOF Spectrometer chromatography over silica gel. Elution with light petroleum or
under ESI (positive mode) by the services at the Indian Associ- mixtures of ethyl acetate/light petroleum (see Supporting Infor-
ation for the Cultivation of Science, Kolkata.
mation File 1) furnished the desired dithioether. All products
were characterized by IR, 1H NMR, 13C NMR and HRMS data.
Calcination: Commercially available silica gel (Merck, India;
Grade: TLC; HF254) was heated under vacuum at 100 °C for
1 h, cooled, and then be used for the reaction or stored in a
glass-stoppered flask for at least two weeks.
Procedure for the reaction using a mixture of
silica and sodium silicate under conditions A
(Table 2, entry 20)
Equal quantities of silica gel and sodium silicate (1 g each)
were mixed, dried under vacuum at 100 °C for 1 h, cooled, and
used for the reaction. The mixture (500 mg) was stirred in water
(10 mL) and its pH was measured to be 12.7. A mixture of allyl
bromide (1 mmol) and benzene thiol (2.5 mmol) was thor-
oughly mixed with the mixture of dry silica gel and sodium sili-
Moistened silica: Commercially available silica gel (Merck,
India; Grade: TLC; HF254) was mixed with water and used for
the reactions. For column chromatography: silica (60–120 µm)
(Thomas Baker, India), and for TLC, Merck plates coated with
silica gel 60, F254 were used.
31

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