DDQ-mediated synthesis of functionalized unsymmetrical disulfanes

Article abstract of DOI:10.1039/c5ra04173b

We developed a simple and efficient method for the synthesis of functionalized unsymmetrical disulfanes under mild conditions in good yields. The designed method is based on the reaction of bis(5,5-dimethyl-2-thioxo-1,3,2-dioxaphosphorinan-2-yl)disulfane with thiols in the presence of 2,3-dichloro-5,6-dicyanobenzoquinone (DDQ). The developed method allows the preparation of unsymmetrical disulfanes bearing additional hydroxy, carboxy, or amino functionalities.

Full text of DOI:10.1039/c5ra04173b

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DDQ-mediated synthesis of functionalized
unsymmetrical disulfanes†
Cite this: RSC Adv., 2015, 5, 31347
*
Mateusz Musiejuk, Tomasz Klucznik, Janusz Rachon and Dariusz Witt
Received 9th March 2015
Accepted 25th March 2015
DOI: 10.1039/c5ra04173b
www.rsc.org/advances
We developed a simple and efficient method for the synthesis of and thiolsulfonates,28–31 sulfanylsulnamidines,32 thionitrites,33
functionalized unsymmetrical disulfanes under mild conditions in sulfenylthiocarbonates,34 thioimides,35–37 and thiophosphonium
good yields. The designed method is based on the reaction of bis(5,5- salts.38 Other practical procedures involve the reaction of a thiol
dimethyl-2-thioxo-1,3,2-dioxaphosphorinan-2-yl)disulfane with thiols with a sulnylbenzimidazole,39 the rhodium-catalyzed disulde
in the presence of 2,3-dichloro-5,6-dicyanobenzoquinone (DDQ). The exchange,40,41 an electrochemical method,42 ring opening of azir-
developed method allows the preparation of unsymmetrical disulfanes idines using tetrathiomolybdate in the presence of symmetrical
bearing additional hydroxy, carboxy, or amino functionalities.
disulfanes,43,44 and the use of diethyl azodicarboxylate (DEAD)45 or
a solid support46 in a sequential coupling of two different thiol
groups. Recently, the oxidation of a mixture of two different thiols
by 2,3-dichloro-5,6-dicyanobenzoquinone (DDQ) to produce an
unsymmetrical disulfane has also been reported.47,48
Disulfanes have been used for the preparation of self-
assembled monolayers (SAMs)49,50 and monolayer-protected
clusters (MPCs) with a number of versatile properties.51,52
Compounds containing the disulde linkage have also been
used for the preparation of dynamic combinatorial libraries,53
catenanes,54,55 macrocycles,5,56 carceplexes,57 dendrimers,58
rotaxanes, micelles,59,60 and a wide range of chemosensors and
pro-drugs.61 These species illustrate the wide applications of
disulfanes and show that the synthesis of the disulde bond is a
critical transformation in organic chemistry.6–9
Compounds with the structure R–S–S–R, where the R group can
be alkyl, vinyl or aryl are termed symmetrical disuldes when
the R groups are the same. The wide range of unsymmetrical
disuldes in which the R groups are different is also well
known. These compounds are oen termed organic disuldes
in the literature; however, the IUPAC recommended nomen-
clature is disulfanes.1 The name disulde should only be
applied to ionic compounds, such as sodium disulde (Na2S2).
Moreover, the term disulfane is more widely applicable than
disulde because it facilitates naming, even when the R groups
are acyl and/or phosphoryl.
The synthesis of unsymmetrical disulfanes is an important
transformation in organic synthesis and medicinal chemistry.2–5
Recent developments in disulde bond formation have been
reviewed.6–9 Although many different methods exist for the prep-
aration of unsymmetrical disulfanes, the most prevalent approach
involves substitution of a sulfenyl derivative with a thiol or its
derivative. To date, the most commonly utilized sulfenyl deriva-
tives are the following: sulfenyl chlorides,10–12 S-alkyl thiosulfates
and S-aryl thiosulfates (Bunte salts),13,14 S-alkylsulfanyl-iso-
thioureas,15 benzothiazol-2-yl disulfanes,16,17 benzotriazolyl-sul-
We have previously demonstrated the preparation of func-
tionalized unsymmetrical molecules, such as dia-
lkyldisulfanes,62
alkyl–aryl
disulfanes,63
‘bioresistant’
disulfanes,64 the unsymmetrical disulfanes of L-cysteine and
L-cystine,65 and diaryldisulfanes,66 based on the readily available
5,5-dimethyl-2-thioxo-1,3,2-dioxaphosphorinane-2-disulfanyl
derivatives 1. These disulfanyl derivatives 1 of phosphor-
odithioic acid were also convenient for the preparation of
a-sulfenylated carbonyl compounds,67 functionalized phos-
phorothioates,68 as well as symmetrical69,70 and unsymmet-
rical71,72 trisulfanes (Fig. 1).
fanes,18
dithioperoxyesters,19
(alkylsulfanyl)dialkylsulfonium
salts,20 2-pyridyl disulfanes and derivatives,21,22 N-alkylte-
trazolyldisulfanes,23 sulfenamides,24 sulfenyldimesylamines,25 sul-
fenylthiocyanates,26 4-nitroarenesulfenanilides,27 thiolsulnates
As part of our continued interest in the preparation of
functionalized unsymmetrical disulfanes, in this study, we
report an efficient and convenient synthesis of unsymmetrical
disulfanes 1 directly from phosphorodithioic acid disulfane 2a
and functionalized thiols 3.
Department of Organic Chemistry, Chemical Faculty, Gdansk University of Technology,
Narutowicza 11/12, 80-233 Gdansk, Poland. E-mail: chemwitt@pg.gda.pl
† Electronic supplementary information (ESI) available: Experimental details and
spectroscopic data for all new compounds 1. See DOI: 10.1039/c5ra04173b
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dimethyl-2-thioxo-1,3,2-dioxaphosphorinan-2-yl) disulfane 2a
(variable amount) and dodecane-1-thiol 3a (1 equivalent) in the
presence of DDQ (0.5 equivalent) (Table 1).
As the data in Table 1 demonstrate, the excess symmetrical
disulfane 2a improved the yield of unsymmetrical product 1a.
The reactions using 1 equivalent of disulfane 2a in CH2Cl2 and
CH3CN produced unsymmetrical product 1a in a 99% and 96%
yield, respectively (entry 4). Further studies were performed
with 1 equivalent of disulfane 2a because the yield of product 1a
was high when the excess amount of 2a was acceptable.
Under the optimized conditions, the scope and limitations
of this new unsymmetrical disulfane formation reaction were
investigated, and the results are summarized in Table 2.
A broad range of thiols reacted smoothly under the opti-
mized reaction conditions. The reaction was tolerant of various
functional groups, including the hydroxy, carboxy, azide,
ferrocene, active ester and carbon–carbon double bond groups.
Aromatic thiols (3g–i) underwent disulde bond formation to
furnish the desired products (1g–i) with excellent yields (95–
99%, entries 7–9, Table 2). Steric hindrance does not appear to
affect the progress of the reaction (entry 9, Table 2). The
aliphatic thiols also provided unsymmetrical disulfanes 1 in
high yields (77–99%, entries 1–6, Table 2). Note that the
aliphatic thiol 3j underwent the reaction in the presence of a
carbon–carbon double bond (entry 10, Table 2), and the hydroxy
and carboxy groups did not require protection (entries 2–3,
Table 2). In the case of N-acetyl-L-cysteine 3k, the reaction did
not occur in CH2Cl2 due to the low solubility of the starting
material in the solvent. However, using acetonitrile as the
solvent allowed the preparation of the corresponding unsym-
metrical disulfane 1k in high yield (89%, entry 11, Table 2).
Unfortunately, an unprotected amino group was not toler-
ated under the developed conditions. Although thiol 3l was
consumed during the reaction, the unsymmetrical disulfane
was not produced (entry 12, Table 2). DDQ most likely reacted
with an amino group to produce a complex reaction mixture
Fig. 1 Synthetic applications of unsymmetrical disulfanes 1.
Wang and co-workers have developed a new method for the
synthesis of unsymmetrical disulfanes from simple aliphatic
and aromatic thiols using DDQ as the oxidant.47 This method is
particularly interesting due to its apparent selectivity for the
exclusive formation of unsymmetrical disulfanes, despite the
presence of two different thiols in the reaction mixture in a 1 : 1
ratio before the addition of DDQ. We expected that this
method might also be applicable to the synthesis of unsym-
metrical disulfanes 1. As a test reaction, 5,5-dimethyl-2-sulfanyl-
2-thioxo-1,3,2-dioxaphosphorinane and dodecane-1-thiol 3a
(1 : 1 ratio) were mixed and then treated with DDQ (0.5 equiv-
alent) following Wang's procedure.47 Unfortunately, the yield of
unsymmetrical disulfane 1a aer separation was moderate, and
the observed ratio of products (1a : 2a : 4) was typical for the
oxidation of most thiol mixtures8 (Fig. 2).
Although Wang and co-workers did not discuss the mecha-
nism of the formation of the unsymmetrical disulfanes, it can
be speculated that DDQ converts the more easily oxidized thiol
to the symmetrical disulfane and the second thiol to the
alkylthio radical (thiyl radical RSc) which then reacts with
symmetrical disulfane to produce the unsymmetrical product. Table 1 Reaction of disulfane 2a with thiol 3a and DDQa
To verify this hypothesis, we performed a reaction of bis-(5,5-
Yieldb (%)
Entry
2a (equiv.)
CH2Cl2
CH3CN
1
2
3
4
5
6
6c
2
1.5
1
0.6
0.5
100
100
100
99
81
72
100
99
98
96
78
69
a
Conditions: disulfane 2a (0.5–6.0 mmol), thiol 3a (1 mmol), DDQ (0.5
mmol), 4.0 mL of solvent. b Isolated yields. c 10 mL of solvent was used.
Fig. 2 Oxidation of phosphorodithioic acid and thiol 3a mixture by
DDQ.
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Table 2 Synthesis of unsymmetrical disulfanes 1 from functionalized
thiols 3a
the preparation of unsymmetrical disulfanes 1m and 1n under
the developed reaction conditions.
We were curious to continue our investigation with other
symmetrical disulfanes, such as 2,20-dipyridyl disulfane 2b and
diphenyldisulfane 2c (Table 4).
As demonstrated, the formation of unsymmetrical disul-
fanes 1 strongly depends on the structure of the starting
material 2. In the case of disulfane 2a and 2b, the unsymmet-
rical product 1 was obtained in 99% and 23% yield, respectively.
However, the reaction of diphenyldisulfane 2c with dodecane-1-
thiol 3a in the presence of DDQ did not produce an unsym-
metrical product, and exclusive formation of didodecyldi-
sulfane 4 was observed. The determination of the scope of
disulfanes 2 capable of producing unsymmetrical disulfanes
under the developed conditions is under investigation.
The suggested mechanism to explain this transformation
involves the initial formation of the thiyl radical (RSc) through
the oxidation of thiol RSH 3 by DDQ. The resulting thiyl radical
reacts with the symmetrical disulfane 2a to yield the unsym-
metrical product 1 and the phosphorodithioic acid radical,
which undergoes recombination to produce symmetrical
disulfane 2a (Fig. 3).
The phosphorodithioic acid radical cannot extract hydrogen
from the starting thiol because in this case, the catalytic amount of
DDQ would be sufficient to produce unsymmetrical disulfane 1 in
high yield. When a smaller amount of DDQ was used, a lower yield
of product was obtained. In addition, we did not observe the
formation of phosphorothioic acid in the reaction mixture.
Moreover, the suggested mechanism explains why the excess of
disulfane 2 improved the yield of the reaction. The initially formed
thiyl radical (RSc) can undergo either recombination or reaction
with product 1 to produce symmetrical disulfane RSSR. These side
reactions can be avoided when an excess of disulfane 2 is present
in the reaction mixture. The reactivity of symmetrical disulfane 2
with the thiyl radical appears to play a vital role in the formation of
unsymmetrical disulfane 1. Most likely, 2,20-dipyridyl disulfane 2b
and diphenyldisulfane 2c are less reactive toward thiyl radicals
Yieldb (%)
Entry
RSH
Product
CH2Cl2
CH3CN
1
2
3
4
5
6
7
8
HS–(CH2)11CH3 3a
HS–(CH2)11OH 3b
HS–(CH2)10CO2H 3c
HS–(CH2)11N3 3d
HS–(CH2)10CONHS 3e
HS–(CH2)10CO–ferrocene 3f
HS–Ph 3g
HS–C6H4–4-CH3 3h
HS–C6H4–2-CO2H 3i
HS–(CH2)9–CH]CH2 3j
Ac–Cys–OH 3k
1a
1b
1c
1d
1e
1f
1g
1h
1i
99
88
77
93
88
82
99
98
96
87
—
0
96
89
82
96
89
85
98
97
95
92
89
0
9
10
11
12
1j
1k
1l
HS–(CH2)11NH2$HCl 3l
a
Conditions: disulfane 2a (1 mmol), thiol 3 (1 mmol), DDQ (0.5 mmol),
4.0 mL of solvent. b Isolated yields.
without the formation of unsymmetrical disulfane 1l. Similar
unsuccessful results have also been observed by Wang and co-
workers in the case of amine-substituted thiols.47 To overcome
that limitation, we decided to use protected amino-thiols under
the developed conditions (Table 3).
Our data show that the protection of the amino group plays a
vital role in the yield of the unsymmetrical disulfanes 1m and
1n. When the amino group is protected by a single Boc group,
the corresponding unsymmetrical disulfane 1m is produced in
moderate yield (53–56%, entry 2, Table 3). However, using thiol
3n with a double-protected amino group provided unsymmet-
rical product 1n in high yields (94–96%, entry 3, Table 2). The
appropriate protection of the amino group appears to facilitate
Table 4 Reaction of symmetrical disulfane 2 with thiol 3a and DDQa
Table 3 Reaction of disulfane 2a with amino-thiol and DDQa
Yieldb (%)
Entry
1
R
1
4
99
0
Yieldb (%)
Entry
RSH
Product
CH2Cl2
CH3CN
2
23
0
70
99
1
2
3
HS–(CH2)11NH2$HCl 3l
HS–(CH2)11NHBoc 3m
HS–(CH2)11NBoc2 3n
1l
1m
1n
0
53
94
0
56
96
3
Ph-2c
a
a
Conditions: disulfane 2a (1 mmol), thiol 3 (1 mmol), DDQ (0.5 mmol),
Conditions: disulfane 2 (1 mmol), thiol 3a (1 mmol), DDQ (0.5 mmol),
4.0 mL of solvent. b Isolated yields.
4.0 mL of CH2Cl2. b Isolated yields.
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Acknowledgements
We gratefully acknowledge the National Science Centre (NCN)
for nancial support (grant no. 2013/09/B/ST5/01261).
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In summary, we developed an efficient and convenient
method for the preparation of unsymmetrical disulfanes 1
directly from the disulfane of phosphorodithioic acid 2a and
functionalized thiols 3 in the presence of DDQ. A wide range
of functional groups is tolerated, including the hydroxy,
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the preparation of functionalized unsymmetrical disulfanes,
especially derivatives of phosphorodithioic acid with versa-
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General procedure for the preparation
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