Visible light mediated metal-free thiol-yne click reaction

Article abstract of DOI:10.1039/c6sc02132h

The carbon-sulfur bond formation reaction is of paramount importance for functionalized materials design, as well as for biochemical applications. The use of expensive metal-based catalysts and the consequent contamination with trace metal impurities are challenging drawbacks of the existing methodologies. Here, we describe the first environmentally friendly metal-free photoredox pathway to the thiol-yne click reaction. Using Eosin Y as a cheap and readily available catalyst, C-S coupling products were obtained in high yields (up to 91%) and excellent selectivity (up to 60:1). A 3D-printed photoreactor was developed to create arrays of parallel reactions with temperature stabilization to improve the performance of the catalytic system.

Full text of DOI:10.1039/c6sc02132h

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Visible Light Mediated Metal-free Thiol–yne Click Reaction
Sergey S. Zalesskiy,a Nikita S. Shlapakova and Valentine P. Ananikov*a
Received 00th January 20xx,
Accepted 00th January 20xx
ABSTRACT: The carbon-sulfur bond formation reaction is of paramount importance for functionalized materials design, as
well as for biochemical applications. The use of expensive metal-based catalysts and the consequent contamination with
trace metal impurities are challenging drawbacks of the existing methodologies. Here, we describe the first
environmentally friendly metal-free photoredox pathway to the thiol–yne click reaction. Using Eosin Y as a cheap and
readily available catalyst, C-S coupling products were obtained in high yields (up to 91%) and excellent selectivity (up to
60:1). A 3D-printed photoreactor was developed to create arrays of parallel reactions with temperature stabilization to
improve the performance of the catalytic system.
DOI: 10.1039/x0xx00000x
www.rsc.org/
Introduction
We propose a practical approach to access valuable S-
functionalized products using a cheap and readily available
In recent decades, the thiol–yne coupling was established as
powerful and proven tool for carbon–sulfur bond
alkyne made by the condensation of acetylene with acetone
via the Favorskii reaction (Scheme 1). An atom-economic
thiol–yne click reaction yields sulfenylated alkenes, which
can be readily converted to dienes by known dehydration
protocols in one step (Scheme 1). Sulfenylated dienes are
universal building blocks for organic synthesis and polymer
science.9 Their high reactivity in Diels-Alder reactions
facilitates cycloaddition under mild conditions.10,11 The
a
formation.1 Currently, this process is of particular
importance for the preparation of polymers,2 dendrimers,3
smart materials1,4 and functionalized surfaces.1b,5 An
important prerequisite for most of these applications is
flexibility and efficient control with respect to reaction
selectivity. Initially, the addition of thiols to alkynes was
performed via a free-radical pathway.6 However, free-
radical addition suffered from harsh conditions, low
selectivity and extensive formation of by-products, which
diminished the utility of this method in organic synthesis.6c
The application of transitional metal catalysts improved the
selectivity in favor of C–S bond formation;1,7 however,
leaching of metal species8 and unavoidable contamination
with biologically active metal-containing impurities makes
the use of metal catalysts unacceptable for biomedical and
pharma applications.
overall
process
is
totally
atom-economic
and
environmentally benign; the target product is formed from
acetylene, acetone and thiol, releasing only one molecule of
water. The key step of the overall process is the thiol–yne
click reaction, which should be performed with high stereo-
and regioselectivity and without a metal catalyst. It is
important to exclude transition metal catalysts at this step
because their avoidance decreases cost and eliminates the
generation of toxic wastes. Thus, carrying out this
transformation under metal-free, visible light-mediated
conditions would provide an important advance in the
synthetic methodology.
Herein, we present the first example of a metal-free
photoredox thiol–yne click reaction. We have utilized the
outstanding advantages of photocatalytic processes to
render transformations of functionalized organic
molecules.12 A special emphasis was placed on using organic
catalysts13 to avoid metal contamination and to increase
cost efficiency. To the best of our knowledge, no examples
of metal-free photoredox thiol–yne coupling have been
reported to date.14
Scheme 1. The key light-mediated thiol–yne click reaction for the
synthesis of activated dienes from simple precursors
a.
Zelinsky Institute of Organic Chemistry, Russian Academy of Sciences, Leninsky
prospekt, 47, Moscow, 119991, Russia.
Electronic Supplementary Information (ESI) available, DOI: 10.1039/x0xx00000x
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Journal Name
catalyst results in a reaction mixture with optimal optical
density and ensures uniform excitation of the reaction
volume.
Results and Discussion
Initially, we evaluated the performance of various
photoredox systems for the thiol–alkyne coupling (Table 1).
Carrying out selective transformations is challenging due to
Table 1. Search for the optimal catalyst for thiol–yne photoredox couplinga
a number of parallel processes leading to products
3-6
#
Catalystb
Solvent
LED
3a, %
E:Z
(Scheme 2). The formation of 4a was eliminated by avoiding
an excess of oxygen in the reaction mixture. The addition of
pyridine to the reaction mixture allowed us to increase
selectivity via suppression of the formation of the double
addition products 5a and 6a. The absence of transition
metal complexes excluded the formation of another
1
2
3
4
5
6
7
8
9
[Ru(bpz)3][PF6]2
[Ru(bpy)3][PF6]2
Ir(ppy)3
Fluorescein
Bengal Rose
Eosin Y
MeCN
MeCN
MeCN
MeOH
DMF
MeOH
DMF
MeCN
Et2O
465 nm
465 nm
530 nm
465 nm
530 nm
530 nm
530 nm
530 nm
530 nm
530 nm
34
40
63
60
53
53
15
56
20
63
20:1
30:1
40:1
27:1
25:1
25:1
14:1
27:1
20:1
30:1
regioisomer 7a 1,8
.
Eosin Y
Eosin Y
Eosin Y
Eosin Y
10
DMSO
a
Conditions: 0.3 mol % catalyst loading, stirring under LED light for 4 h; see
Supporting Information for details. b Control experiments in the absence of
photocatalyst were also performed; see Supporting Information for details.
The dissolved photocatalyst slowly degrades during the
course of the reaction, which makes the solution
unsaturated. This naturally leads to the dissolution of
subsequent portions of Eosin Y from the solid phase. This
“saturation feedback” automatically maintains the
concentration of the photocatalyst at the optimal level, thus
compensating for the loss of active species. Under such self-
regulating conditions, yields and E:Z selectivities as high as
85% and 50:1, respectively, were achieved (entry 7, Table 2).
Scheme 2. Plausible products in the thiol–yne coupling process
Known ruthenium (entries 1-2, Table 1) and iridium-based
(entry 3) photoredox systems gave good yields and E:Z
selectivities of the desired product. Three different organic
dyes demonstrated similar yields but afforded lower
selectivity (entries 4-6, Table 1) relative to the Ir catalyst
(entry 3, Table 1). Varying the solvent (entries 6-10, Table 1)
allowed us to achieve 63% yield of 3a and 30:1 selectivity
using Eosin Y as a photosensitizer (entry 10, Table 1).
Although initial screening did not favor organic dyes as
viable catalysts due to lower selectivities (cf. entries 3 and
10, Table 1), we developed this approach further to meet
the requirements of cost-efficiency and avoiding metal
contamination.
Variation of the amount of photocatalyst revealed a non-
uniform trend (Table 2). Decreasing the amount of Eosin Y
initially resulted in an increase in both the yield and
selectivity (entries 1-2, Table 2) but further lowering the
amount of the dye led to a decrease in both parameters
(entries 3-5, Table 2). It is very likely that the combination of
two factors, namely excitation of only the surface layer of
the solution due to increased optical density (because of the
large amount of the dye in solution) and reaction stoppage
due to degradation of Eosin Y (especially for very low dye
loadings), is responsible for the observed behavior. Indeed,
rapid degradation of the photocatalyst was clearly visible at
low catalyst loadings, which resulted in discoloration of the
solution.
Table 2. Optimization of reaction conditions for the studied photoredox thiol–yne
click reactiona
#
1
2
3
4
5
6
7
8
9
Eosin Y, mol%
2
Solvent
DMSO
DMSO
DMSO
DMSO
DMSO
Hexane
Hexane
Hexane
Hexane
Yield, %
15
38
46
39
33
70
85
11
E:Z
30:1
37:1
45:1
38:1
32:1
37:1
50:1
10:1
8:1
0.3 b
0.02
0.005
0.001
0.3
0.3 b
0.02
0.001
9
a The reaction conditions were similar to those in Table 1; b The reaction time
was 5 h (longer reaction times did not further improve the yield).
Thus, a typical photoredox system with Eosin Y suffers from
gradual photocatalyst degradation during the course of the
reaction when all of the catalyst is dissolved in the reaction
mixture (Figure S1, Supporting Information). This
degradation can halt the progress of the reaction at the
middle or even initial stages of the reaction (entries 10-11,
Table 2). In the developed approach, the low solubility of
the photocatalyst increases the efficiency of light utilization
by maximizing reactive excitations. The inevitable
degradation of the photocatalyst is immediately
compensated by the undissolved portion (Figure S1). It
should be noted that when identical catalyst loadings were
We found that the dye degradation issues can be overcome
by switching the reaction solvent to hexane (Table 2, entries
6-7). Eosin Y exhibits very low solubility in hexane, and at
the beginning of the reaction, the majority of the Eosin Y
remains undissolved. A low concentration of the dissolved
2 | J. Name., 2012, 00, 1-3
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used in different solvents, namely DMSO and hexane (i.e.,
2.1 mg of Eosin Y for 0.3 mol%), the reaction was governed
by the solubility of the photocatalyst. Notably, the reaction
in hexane proceeded with higher selectivity and yield than
the reaction conducted in DMSO (Table 2).
After establishing the optimal reaction conditions, efforts
were focused toward performing thiol–yne click reactions
with different substrates. The developed synthetic approach
provides excellent opportunities for incorporating various
sulfur substituents (R) by using readily available thiols
(Scheme 1). However, investigation of the substrate scope
has shown that the process suffered from low
reproducibility due to poor temperature control and
variations in light intensity from one LED set up to another.
depending on the particular setup. Commonly used LED
strips wrapped around the photocatalytic system
substantially heat the reaction vessel. The inner
temperature of such an assembly can reach up to 60 °C,
with large temperature variations of ±10 °C (Figure 1A).
Another standard setup involves placement of the reaction
vessel on top of a single LED or LED matrix (Figure 1B), which
may increase the temperature up to 45 °C with temperature
variations of ±5 °C. The optimal solution would provide
irradiation of the reaction mixture from above, with the LED
source incorporated into the vessel’s cap (Figure 1C). In such
a setup, temperature stabilization and stirring can be easily
performed using a standard oil or water bath equipped with
a magnetic stirrer.
Because LED sources incorporated into reaction vessel caps
are not readily available, we designed a custom cap using 3D
computational modeling. The outstanding advantage of
computer-aided modeling is the ability to virtually assemble
all components of the setup together and to check for
possible clearance issues and design pitfalls prior to
manufacturing. We have developed a concept optimized for
photoredox applications that is designed for compatibility
with standard glass reaction vessels (Figure 2A). The
designed photoreactor was fabricated using a commonly
available FDM 3D printer (Figure 2B).
Figure 1. Comparison of the possible designs of the photoreactor:
irradiation (LED strip); B – bottom irradiation (LED matrix); C – developed design
with temperature stabilization.
A – side
A closer examination revealed that experimental conditions
(e.g., temperature and light intensity) can significantly vary
Scheme 3. The scope of the developed photocatalytic thiol–yne click reaction.a (a Isolated yields are given in parentheses, reaction time – 3 h. b Reaction time - 10 h.
c Reaction time - 20 h)
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aliphatic thiols in this reaction (R1=Alk); however, significantly
lower selectivity was observed (<10:1). An important factor is
the structure of the alkyne, where steric effect played a key
role in obtaining high selectivity (see Supporting Information).
Amazingly, high selectivity was observed for the alkynes
bearing α-substituted carbon atom, which is required to
implement the synthetic methodology proposed in the present
study (Scheme 1). For the studied class of alkynes the
corresponding products 3a-3m were synthesized in good to
high yields and excellent selectivity (Scheme 3) including
sterically hindered alkynes, where the products 3n-3p were
formed in good yields (89-96%) and high selectivity (35:1–
40:1).
A plausible mechanism for the photoredox thiol–yne click
reaction is presented in Scheme 4A.13,15 A photoexcited
molecule of Eosin Y (E*) undergoes oxidative quenching,
furnishing a radical cation of arylthiol. Pyridine abstracts a
proton from this species, yielding thiyl radicals and preventing
side reactions of ArSH·+. In the next step, ArS· is involved in
the radical addition to the alkyne, giving the desired product 3
and regenerating ArS· radicals. Within the mechanism the E-
species of Eosin Y are regenerated upon interaction with
molecular oxygen from ambient air. According to
thermodynamic properties single electron oxidation of
benzenethiol by excited state of Eosin Y is a feasible process:
the potential EE*/E- = 1.18 V vs. SCE is much higher as
compared to EPhSH/PhSH+ ≈ 0.95 V vs. SCE.16
Figure 2. Computer model of designed photoreactor (A) and ready to use assembled
photoreactor (B).
The reactor includes an efficient twist-lock LED mount that
allows the LED light source to be changed easily. The bottom
part can house a range of common glass vials available in the
lab, and the presence of an O-ring ensures a tight fit with the
reaction vessel. A detailed description of the reactor and the
3D model are provided in the Supporting Information. The
developed model can be directly printed on a regular 3D
printer in approximately 30 minutes and requires only 6 grams
of the plastic source material (< $ 1 cost). The developed
photoreactor was totally compatible with common magnetic
stirrers and allowed for very good temperature stabilization at
25±0.5 °C (Figure 1C). The ability to maintain stable
experimental conditions (e.g., temperature and light intensity)
drastically improves the selectivity and reproducibility of
photochemical reactions. An array of the custom-built
photoreactors allowed us to execute a study of the substrate
scope involving various substituents in a parallel fashion
(Scheme 3). Four important advantages deserve particular
attention: i) the catalytic system showed excellent
performance for various aryl thiols;9 ii) excellent
stereoselectivity in the range of 60:1–25:1 was
achieved; iii) complete regioselectivity was maintained and
the formation of another regioisomer (7a) was not observed;
and iv) the hydroxyl group remained intact during the reaction,
which is a necessary prerequisite to access the diene backbone
after dehydration (Scheme 1). The developed synthetic
approach was tolerant toward electronic and steric effects of
the substituents in the thiol moiety (Scheme 3). Even for thiols
containing strongly electron-withdrawing substituents, such as
CF3 or COOMe, the desired products were formed in very high
Scheme 4. The proposed mechanism (A) for the Eosin
Y mediated thiol–yne
photoredox coupling (Eosin is denoted with E), plausible origins of reaction
Y
regioselectivity (B) and stereoselectivity (C).
Rather different stability of the primary and secondary
intermediate radical species governs high regioselectivity in
yield and high selectivity (i.e., 3e, 3g, 3h). We also tested
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the studied reaction (Scheme 4B). According to DFT the rapid assessment of optimal reaction coVienwdAitrtiioclne sOnlitnoe
calculations secondary radical is more stable by > 8 kcal/mol improve the performance of the catalyticDsOyIs:t1e0m.10.39/C6SC02132H
compared to primary radical (see Supporting Information). In An efficient photocatalytic system was created using a
total agreement with relative stability of the radical species combination of Eosin Y and hexane as solvent. Utilization of
linear (anti-Markovnikov type) products
reactions, whereas formation of branched (Markovnikov type) maintain the optimal concentration of the catalyst may
products was not observed. substantially facilitate development of a variety of other
3 were formed in the the “saturation feedback” approach described here to
Steric effect of the substituent R provides the necessary photocatalytic transformations.
control for stereoselectivity in the studied transformation
(Scheme 4C). Decreasing steric bulk resulted in lower reaction
selectivity or non-selective transformation (see Supporting
Experimental details
Information). The presence OH group and two other
substituents at the α-carbon atom in the developed concept
(Scheme 1) perfectly matches the structure of the alkyne
towards getting a high stereoselectivity.
Radical nature of the studied process was confirmed by
performing a control experiment using a radical trap. Indeed,
addition of γ-terpinene suppressed the reaction between the
thiol and alkyne. Several other control experiments were also
carried out to confirm the proposed mechanism and to
demonstrate regeneration of the dye in the presence of
oxygen (see Supporting Information).
General procedures
Photochemical reactions were carried in 2 mL glass vials
covered with aluminium tape. Magnetic stirrer bars were
cleaned with boiling solution of alkali followed by boiling
solution of aqua regia and further rinsing with distilled water
to ensure that all traces of absorbed catalyst were removed.
All reagents and catalysts were purchased from commercial
sources and used without futher purification. Solvents were
purificated by standard procedures. Flash chromatography was
performed using Merck 60 μm silica. LC-MS-grade solvents
(MeCN and DCM) for ESI-MS experiments were ordered from
Merck and used as received. All samples (solutions in MeCN or
DCM) for the ESI-MS experiments were prepared in 1.5 mL
Eppendorf tubes. All plastic disposables (Eppendorf tubes and
tips) used for sample preparation were washed with MeCN or
1
DCM prior to use. H and 13C NMR data are referenced to TMS
and residual solvent signal, respectively.
Formation of double addition products was observed in a few
studied cases at
performed using a regular flash chromatography as described
below.
a trace level. Purification was easily
Scheme 5. The synthesis of diene 8a using the developed
approach
High-resolution mass spectra were recorded on a Bruker maXis
Q-TOF instrument (Bruker Daltonik GmbH, Bremen, Germany)
equipped with electrospray ionization (ESI) ion source. The
measurements were performed in a positive (+MS; +MS/MS)
ion mode (HV capillary: 4500 V; HV End Plate offset: -500 V)
with a scan range m/z: 50 - 3000. External calibration of the
mass spectrometer was performed using an electrospray
calibrant solution (Fluka). A direct syringe injection was used
for all of the analyzed solutions in MeCN or DCM (flow rate: 3
μL/min). Nitrogen was used as the nebulizer gas (0.4 bar), dry
gas (4.0 L/min) and collision gas for all of the MS/MS analyses
and experiments; the dry temperature was set at 180 ºC. All of
the recorded spectra were processed using the Bruker
DataAnalysis 4.0 software package.
Finally, as a proof of concept, we have prepared the target
diene 8a using the industrially available starting materials
acetylene, acetone and benzenethiol. In the first step, alkyne
1a was prepared according to the published procedure.17
Alkyne 1a was converted to vinyl sulfide 3a using the
developed photochemical protocol. Dehydration of 3a under
mild conditions furnished diene 8a in 94% isolated yield. Thus,
a demanding S-functionalized 1,3-diene was prepared starting
from simple precursors through a sequence of atom-economic
addition reactions with the only release of water.
Conclusions
Synthesis of 3a-p
To summarize, the use of cheap and commercially available
Eosin Y as a photocatalyst provided an attractive and
environmentally friendly protocol for thiol–yne coupling,
furnishing valuable sulfur-functionalized products in good
yields. The developed catalytic system demonstrated
unprecedented selectivities (up to 60:1) considering the broad
scope of substituted aryl thiols that could be used. The use of a
custom-designed 3D-printed chemical photoreactor facilitated
Alkyne (1 mmol), thiol (1.1 mmol), Eosin Y (0.003 mmol),
pyridine (0.25 mmol) and 250 μl of hexane were placed into a
2 mL glass vial (covered with aluminium tape) equipped with a
stirrer bar. The mixture was suspended with ultrasound. The
vial was placed on a magnetic stirrer and fitted with the
developed photoreactor reactor equipped with green LED
(λmax = 533 nm). After completing the reaction, the solvent
was removed under reduced pressure. The remaining viscous
5
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(a) Kuniyasu, H.; Ogawa, A.; Sato, K.; Ryu, I.V;ieKwaAmrticbleeO, nNlin.e;
bright pink oil was purified with flash chromatograhy
Sonoda, N. J. Am. Chem. Soc. 1992, 114, 5902; (b) Ananikov,
DOI: 10.1039/C6SC02132H
(petroleum ether/ethyl acetate/triethylamine
elution) to obtain a pure product.
= 4:1:0.075
V. P.; Orlov, N. V.; Beletskaya, I. P.; Khrustalev, V. N.; Antipin,
M. Y.; Timofeeva, T. V. J. Am. Chem. Soc. 2007, 129, 7252; (c)
Ananikov, V. P.; Orlov, N. V.; Zalesskiy, S. S.; Beletskaya, I. P.;
Khrustalev, V. N.; Morokuma, K.; Musaev, D. G. J. Am. Chem.
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Synthesis of 8a
Vinylsulfide 3a (10.6 mg, 5.45•10-2 mmol) was dissolved in 2
mL of 1,4-dioxane, followed by addition of HCl (1 µL of 38%
aqueos solution). The reaction mixture was stirred at room
temperature for 4 hours. The reaction mixture was diluted
with 10 mL of dioxane and the solvent was removed on a
rotary evaporator. The water bath temperature should not
exceed 30 °C during evaporation to avoid product
decomposition. Product 8a was isolated as a slightly yellow oil
and identified according to the published NMR data.18 Product
yield – 9.3 mg (94%).
8
9
Trzeciak, A. M.; Ziółkowski, J. J.; Tylus, W. Top. Catal. 2006
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,
Aryl derivatives (R
= Ar) of sulfenylated dienes are
particularly valuable due to their stronger S-Сsp2 bond and
have more reliable applications in the synthesis of sulfur-
functionalized compounds. Alkyl derivatives (R = Alk) are less
stable due to a relatively weaker S-Сsp3 bond. For the
difference in bond energy see: Ananikov, V.P.; Gayduk, K.A.;
Beletskaya, I.P.; Khrustalev, V.N.; Antipin, M. Yu. Chem.-Eur.
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Acknowledgements
This work was supported by the Russian Science Foundation
(RSF grant 14-50-00126). The authors thank Dr. Evgeny
Gordeev for helpful discussion.
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