S-Acetylation of Thiols Mediated by Triflic Acid: A Novel Route to Thioesters

Article abstract of DOI:10.1021/acs.oprd.7b00378

This paper demonstrates an efficient, mild, and chemoselective synthesis of various thioesters via a HOTf-catalyzed S-acetylation of aromatic and aliphatic thiols using isopropenyl acetate as a cheap and convenient acetylating agent. During our tests, we also investigated the differences between the activities of metal triflates and triflic acid as catalysts in the acetylation of thiols. Finally, the potential of our concept has been increased by the implementation of Nafion as a heterogeneous catalyst for S-acetylation of thiols.

Full text of DOI:10.1021/acs.oprd.7b00378

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S-Acetylation of thiols mediated by triflic acid: a novel route to thioesters
Krzysztof Kuci#ski, and Grzegorz Hreczycho
Org. Process Res. Dev., Just Accepted Manuscript • DOI: 10.1021/acs.oprd.7b00378 • Publication Date (Web): 12 Mar 2018
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SꢀAcetylation of thiols mediated by triflic acid: a novel route to
thioesters
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Krzysztof Kuciński* and Grzegorz Hreczycho*
Faculty of Chemistry, Adam Mickiewicz University in Poznań
ul. Umultowska 89b, 61ꢀ614 Poznań (Poland)
*kucinski.k@amu.edu.pl and g.h@amu.edu.pl; http://ghgroup.home.amu.edu.pl
Table Of Contents Graphic
ABSTRACT: This paper demonstrates an efficient, mild and chemoselective synthesis of various
thioesters via a HOTfꢀcatalyzed Sꢀacetylation of aromatic and aliphatic thiols using isopropenyl acetate as
a cheap and convenient acylating agent. During our tests, we also investigated the differences in the
activity between metal triflates and triflic acid as catalysts in the acetylation of thiols. Finally, the
potential of our concept has been increased by the implementation of Nafion® as heterogeneous catalyst
of Sꢀacetylation of thiols.
Keywords: Sꢀacetylation, Thioesterification, Triflic Acid, Nafion, Thiols, Isopropenyl Acetate
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Thioesters are highly important intermediates in many catalyst in
organic and biosynthetic transformations.1–3 Acetyl this paper, we present a highly chemoselective and
coenzyme A is a most popular example of ꢀacetylated efficient method for the ꢀacetylation of a variety of
S
ꢀacetylation (Brønsted acid catalysis53,54). In
S
S
biomolecule that i.e., plays a vital role in fatty acid thiols with easily available, convenient and inexpensive
metabolism (Figure 1).4
isopropenyl acetate catalyzed by HOTf (Scheme 1).
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Scheme 1. Sꢀacetylation of thiols mediated by HOTf.
We started with the optimization of the process. Initially,
benzenethiol (1.0 eq.) and isopropenyl acetate (2.0 eq.)
were chosen as the coupling partners in the presence of 2
mol% of scandium(III) triflate (Table 1, entry 1). The
conversion was rapid (100% of thiol after 5 minutes) and
selectivity was also very good (95% of Sꢀacetylated
product). However, it was decided to also apply these
reaction conditions for two other thiols (4ꢀ
fluorothiophenol and 4ꢀchlorothiophenol). In their case,
the reaction was again extremely fast (less than 5
minutes), but chemoselectivity was worse than initial
Figure 1. Structure of acetyl coenzyme A.
In this connection, thioester moiety has been extensively
investigated in pharmaceutical chemistry.5,6 Furthermore,
thioesters’ properties make them highly valuable
reagents in organic chemistry.7 It is also true that
Sꢀ
acylation of thiols provides an effective protection of
sulfhydryl groups, which is commonly used in organic8
and polymer chemistry.9 There are several known
methodologies for the synthesis of thioesters.2,10–13
Carboxylic acids,14–16 as well as acid anhydrides17,18 and
chlorides19–22 are traditionally used as acyl sources,
however the direct use of aldehydes has also been widely
investigated recently in these transformations.23–29
Nonetheless, several limitations have been observed in
the aboveꢀmentioned approaches, including the use of
moisture sensitive and highly reactive reagents, long
reaction times, harsh reaction conditions, expensive
catalysts and others. Most recently, isopropenyl acetate,
which is a convenient alternative to other acetyl sources,
testing (Table 1, entries 2 and 3). Medium amounts of Cꢀ
acylated products (35% for 4ꢀfluorothiophenol and 10%
for 4ꢀchlorothiophenol) were first observed. We then
examined a simple triflic acid (5 mol%; Table 1, entries
4 and 5), and found that the chemoselectivity for
thioester formation was excellent (100%).
It is well known, that a metal triflate can serve as a
source of HOTf, which may be generated in situ via
hydrolysis.55,56 Therefore, we performed an experiment
to check which specie – triflate salt or in situ formed
acid, was a true catalyst in this transformation. The
coupling reaction between benzenethiol and isopropenyl
acetate catalyzed by Sc(OTf)3 was carried out with the
addition of a Lewis base ꢀ 2,6ꢀdiꢀtertꢀbutylpyridine
(DTBP),57 which is widely known as a proton scavenger
(Table 1, entry 6). In this experiment, we observed only
a small amount of Cꢀacylated thiol and no traces of
thioester. This confirmed that metal triflate was not an
active catalytic specie. In the absence of a catalyst, using
the same conditions, no reaction was observed (Table 1,
entry 7).
has also been used in
Nꢀ and O
ꢀacetylation.30–32
However, its use in the absence of any catalyst is
extremely rare.32 To the best of our knowledge, the only
one report has been patented,33 which described two
examples of Sꢀacetylation of thiols by using isopropenyl
acetate in the presence of base catalysts.
Our team has recently reported several protocols
describing the use of metal triflates as the catalysts in
Oꢀ
silylation,34–37 ꢀgermylation,38,39 ꢀborylation,40
O
ꢀ and
S
O
S
ꢀbenzylation,41 SꢀS coupling,42 and hydrothiolation
reactions.43,44 Triflate salts are currently widely
investigated in the synthesis of organosulfur
To ensure maximum efficiency and selectivity, we
carried out preliminary tests in a few solvents in order to
choose the most suitable one (Table 1, entries 8ꢀ12).
Collectively, these results indicate that CH2Cl2 is the best
choice (Table 1, entry 4). We also tried to decrease the
amount of the catalyst (for example: Table 1, entry 13),
but the total conversion of thiol was not observed, even
over a longer period of time. With the optimized
conditions achieved, we then proceeded to examine the
generality of the HOTfꢀmediated thioesterification. The
results are summarized in Table 2.
compounds45–47 and their use in
Oꢀ and Sꢀacylation in the
presence of acid anhydrides is well documented.48–52
In view of recent reports and literature, the aim of this
work was to explore the reactivity of various thiols in the
reaction with isopropenyl acetate catalyzed by metal
triflates (Lewis acid catalysis). In our initial tests the
chemoselectivity of this process proved to be
insufficient. However, our further tests indicated that
simple triflic acid is an extremely active and selective
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Table 1. Reaction optimizationa
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Solvent (1
mL)
Sꢀacetylated
productb
Cꢀacetylated
productb
Entry
Thiol
Catalyst (mol%)
Disulfideb
1
2
CH2Cl2
CH2Cl2
Sc(OTf)3 (2)
Sc(OTf)3 (2)
95
55
5
0
5
35
3
4
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
Sc(OTf)3 (2)
HOTf (5)
80
100
100
0
10
0
5
0
0
0
5
HOTf (5)
0
6c
Sc(OTf)3 (2)
15
7
8
CH2Cl2
C2H4Cl2
C6H5CH3
CH3CN
(CH3)2O
CH3NO2
CH2Cl2
ꢀ
0
100
95
60
35
97
80
0
0
2
0
1
0
0
0
1
HOTf (5)
HOTf (5)
HOTf (5)
HOTf (5)
HOTf (5)
HOTf (2)
9
3
10
11
12
13
20
10
3
0
aReaction conditions: room temperature, air atmosphere, thiol/isopropenyl acetate molar ratio = 1.0:2.0, reaction time = 5 min. bThe conversion of
thiols to corresponding products was calculated by GC. c6 mol% of 2,6ꢀdiꢀtertꢀbutylpyridine was added.
Majority of the investigated thiols were fully converted with carboxylic groups, we observed only traces of
to corresponding thioesters with very good isolated thioesters.
yields (80ꢀ93%). In each case, the reaction was
extremely fast (approx. 5 min.). Disappointedly, thiols Finally, the reaction of benzenethiol with isopropenyl
bearing
aminobenzenethiol
heterocyclic
the
amine
(2ꢀaminobenzenethiol,
4ꢀaminobenzenethiol), phenyl thioacetate at
(2ꢀthiazolineꢀ2ꢀthiol, 2ꢀ Experimental Section). On the basis of our results and
plausible mechanism was
ꢀacetylation with isopropenyl acetate can be
3ꢀ acetate also proceeds well on a gram scale, providing
Sꢀ
and
a
91% yield level see
(
mercaptobenzothiazole and 2ꢀmercaptoꢀ5ꢀmethylꢀ1,3,4ꢀ the known literature,
thiadiazole) and carboxylic (3ꢀmercaptopropionic acid, proposed.
a
S
2ꢀmercaptopropionic acid and thioglycolic acid) groups explained as simple Brønsted acid catalysis. It assumes
are not suitable to our catalytic system. Heterocyclicꢀ the activation of a carbonyl group in acidic conditions.
and amineꢀsubstituted thiols immediately react with the This is followed by the attack of thiol to form an
catalyst, which was predictable. In the case of substrates intermediate, which undergoes a rapid conversion to
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thioester and corresponding enol. At room temperature
an equilibrium between keto and enol form is highly
shifted to ketone. Because of this, the acetone
molecule is observed as the only byꢀproduct.
6
7
8
Table 2. Results for the Sꢀacetylation of various thiols with isopropenyl acetate leading to thioestersa
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S
O
1 (89%)
6 (90%)
11 (89%)
16 (88%)
2 (85%)
3 (90%)
8 (92%)
13 (80%)
4 (93%)
5 (89%)
S
S
S
O
O
O
Cl
OMe
7 (89%)
12 (92%)
17 (90%)
9 (91%)
10 (90%)
14 (89%)
15 (85%)
aReaction conditions: rt, 1 mL of CH2Cl2, air atmosphere, thiol (1.00 mmol)/isopropenyl acetate (2.00 mmol) molar ratio = 1.00:2.00, reaction time
= 5 min.
The aboveꢀmentioned concept of the Brønsted acid (4.5 mmol of benzenethiol and 9.0 mmol of isopropenyl
catalysis encourage us to check a heterogeneous system acetate at rt). As the result, the recovered Nafion® NR50
based on Nafion®. This perfluorinated copolymer was successfully reused without a significant loss of
containing sulfonic acid groups has found use especially performance over 10 cycles (each time, 85ꢀ91% isolated
in alkylation58–61 and acylation62–64 reaction. Initially, yield with good selectivity (around 88ꢀ90% of Sꢀphenyl
benzenethiol (4.5 mmol) and isopropenyl acetate (9.0 thioacetate)).
mmol) were chosen as the coupling partners in the
presence of 2.2 g of pellets (approx. 50) of Nafion® Lastly, we performed an experiment with a column filled
NR50 (Scheme 2).
with Celite/Nafion® powder (ratio 3:1). Both reactants
(0.05 g of benzenethiol and 0.091 g of isopropenyl
acetate) were pumped through this column with 1.5 mL
of CH2Cl2. The procedure had to be repeated 10 times to
afford a high conversion. Pleasingly, under these
Scheme 2
.
S
ꢀacetylation of benzenethiol mediated by conditions, thiophenol was selectively converted to its
acylated derivative ( ) in 90% isolated yield. In case of
Nafion® powder the selectivity for ꢀacetylation (vs. Cꢀ
Sꢀ
Nafion® NR50.
1
S
To our delight, the conversion was very good (100% of acetylation) was higher (ratio 97:3). These results can
thiol after 1 hour) and selectivity was also good (90% of contribute to further use of our catalytic system in flow
Sꢀphenyl thioacetate). Next, the reusability of Nafion® chemistry65 by using efficient flow devices.
pellets was also examined (Figure 2). For the reꢀcycling
test, 50 pellets were used in the same reaction conditions
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80
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0
Representative procedure for the synthesis of thioesters
(Table 2, entries 1ꢀ17):
Thiol (1.00 mmol, 1.0 eq.), isopropenyl acetate (2.00 mmol,
2.0 eq.), CH2Cl2 (1 mL) and triflic acid (0.05 mmol, 0.05 eq.)
were added to a 10 mL oneꢀnecked roundꢀbottom flask closed
with a stopper under air atmosphere. The reaction mixture
was stirred at room temperature for 5 minutes. Reaction
progress was monitored by GC analyses. After the reaction
was completed, in order to neutralize triflic acid the
potassium carbonate (0.15 mmol, 0.15 eq.) was added. Next,
the mixture was filtered over a plug of celite under suction. In
next step, all volatiles (an excess amount of isopropenyl
acetate, acetone (byꢀproduct) and solvent) were removed
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Run
Figure 2. Recycling of Nafion® NR50 in
of benzenethiol.
S
ꢀacetylation under reduced pressure and the crude product was purified by
“bulbꢀtoꢀbulb” distillation to give the corresponding
thioesters* (Table 2, entries 1ꢀ17).
CONCLUSIONS
*All of the synthesized thioesters were previously reported in
literature and their physical and spectral data were
consistent with the ones reported in literature.
In summary, we have shown that inexpensive and readily
available isopropenyl acetate can be used as a highly
efficient source of acetyl group in the Sꢀacetylation of
various aliphatic and aromatic thiols. This reaction is
mediated by simple triflic acid, which is less expensive
than its metal salts. What is more, our investigations
revealed that, in the case of metal triflates, they undergo
hydrolysis and generated hidden Brønsted acid, which is
A gramꢀscale synthesis of Sꢀphenyl thioacetate:
To a 50 mL oneꢀnecked roundꢀbottom flask were added
benzenethiol (2.20 g, 20.0 mmol, 1.0 eq.), isopropenyl acetate
(4.00 g, 40.0 mmol, 2.0 eq.), triflic acid (0.15 g, ~88 µL, 1.0
mmol, 0.05 eq.) and dichloromethane (25 mL) under air
atmosphere and the flask was closed with a stopper. The
reaction mixture was stirred at room temperature until
monitoring of the reaction progress by GCꢀFID proved full
consumption of benzenethiol (within 5 min.). After the
reaction was completed, in order to neutralize triflic acid the
potassium carbonate (3.0 mmol, 0.15 eq.) was added. Next,
indeed a true catalytic specie in
Sꢀacetylation. The
favourable features of this solution are: mild conditions
(rt, air atmosphere), nonꢀtoxic and nonꢀmoisture
sensitive acetyl source, very good isolated yields (80ꢀ
93%), high chemoselectivity (exclusively thioesters),
satisfactory atom economy (acetone as the only one and
recyclable byꢀproduct), and short reaction time (less than the mixture was filtered over a plug of celite under suction. In
next step, all volatiles (an excess amount of isopropenyl
acetate, acetone (byꢀproduct) and solvent) were removed
under reduced pressure and the crude product was purified by
“bulbꢀtoꢀbulb” distillation to give Sꢀphenyl thioacetate in
91% yield (2.77 g, 18.2 mmol).
5 min.), as well as the simplicity of the experimental
techniques. Finally, the potential of our concept has been
increased by the implementation of Nafion® as
heterogeneous catalyst, that may also contribute to
further use of our catalytic system in flow chemistry.
Representative procedure for the Sꢀacetylation of
benzenethiol in the presence of Nafion® NR50:
EXPERIMENTAL SECTION
General Information
Benzenethiol (4.5 mmol, 1.0 eq.), isopropenyl acetate (9.0
mmol, 2.0 eq.), CH2Cl2 (10 mL) and Nafion® NR50 (2.2 g,
approx. 50 pellets) were added to a 50 mL oneꢀnecked roundꢀ
bottom flask closed with a stopper under air atmosphere. The
reaction mixture was stirred at room temperature for 1 hour.
Reaction progress was monitored by GC analyses. After the
reaction was completed, the solution was separated from the
catalyst. Next, all volatiles (an excess amount of isopropenyl
acetate, acetone (byꢀproduct) and solvent) were removed
under reduced pressure to give the mixture of Sꢀ and Cꢀ
acetylated products in 90% yield (Selectivity 90:10).
Thiols and isopropenyl acetate used for experiments were
purchased from SigmaꢀAldrich Co. and used without further
purification. Triflic acid was purchased from ABCR.
Nafion® NR50 (pellets) and Nafion® perfluorinated resin
powder were purchased from SigmaꢀAldrich Co. Reactions
were carried out under air atmosphere. 1H NMR and 13C
NMR were recorded on a Bruker Avance III HD NanoBay
(400 MHz and 600 MHz) spectrometer using C6D6 as
solvent. GC analyses were carried out on a Bruker Scion 436ꢀ
GC (column: DBꢀ5 30 m I.D. 0.53 mm) equipped with TCD.
The GC yields of the crude products were calculated using
decane as an internal standard. Mass spectra of few products
were determined by GCꢀMS analysis on a Varian Saturn
2100T, equipped with a BDꢀ5 capillary column (30 m) and
Finnigan Mat 800 ion trap detector.
Recycling test in the presence of Nafion® NR50:
Benzenethiol (4.5 mmol, 1.0 eq.), isopropenyl acetate (9.0
mmol, 2.0 eq.), CH2Cl2 (10 mL) and Nafion® NR50 (2.2 g,
approx. 50 pellets) were added to a 50 mL oneꢀnecked roundꢀ
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bottom flask closed with a stopper under air atmosphere. The (3)
Hirschbeck, V.; Gehrtz, P. H.; Fleischer, I. Chem.
Eur. J. 2017, DOI: 10.1002/chem.201705025.
reaction mixture was stirred at room temperature for 1 hour.
Reaction progress was monitored by GC analyses. After the
reaction was completed, the solution was separated from the
catalyst. Next, all volatiles (an excess amount of isopropenyl
acetate, acetone (byꢀproduct) and solvent) were removed
under reduced pressure to give Sꢀphenyl thioacetate.
Recovered Nafion® NR50 was reused in the next
experiment. The catalytic performance of Nafion® NR50
was well maintained over the 10 recycling steps with yields
of 85–91%.
(4)
AbuꢀElheiga, L.; Matzuk, M. M.; AboꢀHashema, K.
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4701.
The experiment with a column filled with Celite/Nafion®
powder:
The mixture of benzenethiol (0.45 mmol, 1.0 eq.),
isopropenyl acetate (0.9 mmol, 2.0 eq.) and CH2Cl2 (1.5 mL)
were pumped through the column filled with Celite (0.564
g)/Nafion® perfluorinated resin powder (0.188 g) (ratio 3:1).
The procedure had to be repeated 10 times to afford very
good conversion. Next, all volatiles (an excess amount of
isopropenyl acetate, acetone (byꢀproduct) and solvent) were
removed under reduced pressure to give Sꢀphenyl thioacetate
(1) in 90% isolated yield.
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ASSOCIATED CONTENT
Supporting Information.
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Feng, J.; Lu, G.ꢀP.; Cai, C. RSC Adv. 2014, 4 (97),
54409–54415.
The Supporting Information is available free of charge on the
ACS Publications website at DOI:
Yan, K.; Yang, D.; Wei, W.; Zhao, J.; Shuai, Y.;
Tian, L.; Wang, H. Org. Biomol. Chem. 2015, 13
(26), 7323–7330.
Characterization and NMR spectra of compounds synthesized
in this paper (PDF). The following files are available free of
charge.
(13)
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Feng, J.; Lv, M.ꢀF.; Lu, G.ꢀP.; Cai, C. Org. Biomol.
Chem. 2015, 13 (3), 677–681.
Funding Sources
This work was supported by the National Science Centre
Grants: UMOꢀ2015/19/B/ST5/00240 (OPUS, G.H) and
UMOꢀ2017/24/T/ST5/00130 (ETIUDA, K.K).
Neises, B.; Steglich, W. Angew. Chem. Int. Ed. 1978,
17 (7), 522–524.
Notes
Iranpoor, N.; Firouzabadi, H.; Khalili, D.; Motevalli,
S. J. Org. Chem. 2008, 73 (13), 4882–4887.
The authors declare no competing financial interest.
ACKNOWLEDGMENT
Chou, Y.ꢀL.; Jhong, Y.; Swain, S. P.; Hou, D.ꢀR. J.
Org. Chem. 2017, 82 (19), 10201–10208.
This work was supported by the National Science Centre
Grants: UMOꢀ2015/19/B/ST5/00240 (OPUS, G.H) and
UMOꢀ2017/24/T/ST5/00130 (ETIUDA, K.K). Krzysztof
Kuciński acknowledges Adam Mickiewicz University
Foundation scholarship in 2017/2018 academic year. We
would like to thank the Reviewers for their valuable and very
important suggestions.
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