Visible light initiated hydrothiolation of alkenes and alkynes over ZnIn2S4

Article abstract of DOI:10.1039/c9gc00328b

The construction of C-S bonds is very important. The hydrothiolation of alkenes or alkynes with thiols represents an attractive and atom economical approach for the formation of C-S bonds. In this manuscript, flowerlike microspheres of ZnIn₂S₄ consisting of interweaving nanoflakes were prepared by a solvothermal method and were applied for the first time in the visible light initiated hydrothiolation of alkenes and alkynes. The reactions between a broad range of thiols and alkynes or alkenes over irradiated ZnIn₂S₄ afford the corresponding hydrothiolated products in moderate to excellent yields. The mechanism proposed based on the ESR results suggests that the holes generated over irradiated ZnIn₂S₄ are reductively quenched by the thiols to generate thiyl radicals, which are added to the alkynes/alkenes to generate alkene/alkyl radicals for the propagation of thiol-ene/thiol-yne coupling reactions. The use of solar light and a semiconductor-based photocatalyst to realize the thiol-ene and thiol-yne coupling reactions in a green solvent (methanol), with only stoichiometric thiols required and applicable to a broad substrate scope, makes this reaction protocol a green, sustainable and cost-effective strategy for the synthesis of thiolated products. This study also highlights the great potential of semiconductor-based photocatalysis for advanced organic syntheses.

Full text of DOI:10.1039/c9gc00328b

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Visible Light Initiated Hydrothiolation of Alkenes and Alkynes
DOI: 10.1039/C9GC00328B
over ZnIn2S4
Yuanyuan Li, Jingyu Cai, Mingming Hao, Zhaohui Li
Research Institute of Photocatalysis, State Key Laboratory of Photocatalysis on
Energy and Environment, College of Chemistry, Fuzhou University, Fuzhou 350116,
P. R. China
 Author to whom all correspondences should be addressed. E-mail:
zhaohuili1969@yahoo.com, Tel (Fax): 86-591-22865855
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Abstract
The construction of C-S bonds is very important. The hydrothiolation of alkenes
or alkynes with thiols, represents an attractive and atom economic approach for the
formation of C-S bonds. In this manuscript, flowerlike microspheres of ZnIn2S4
consisting of interweaving nanoflakes were prepared by a solvothermal method and
for the first time applied in visible light initiated hydrothiolation of alkenes and
alkynes. The reactions between a broad range of thiols and alkynes or alkenes over
irradiated ZnIn2S4 afford corresponding hydrothiolated products in moderate to
excellent yields. The mechanism proposed based on the ESR results suggests that the
holes generated over irradiated ZnIn2S4 are reductively quenched by the thiols to
generate the thiyl radicals, which are added to the alkynes/alkenes to generate the
alkene/alkyl radicals for propagation of the thiol-ene/thiol-yne coupling reactions. The
use of solar light and a semiconductor-based photocatalyst to realize the thiol-ene and
thiol-yne coupling reactions in a green solvent (methanol), with only stoichiometric
thiols required and applicable to a broad substrate scope, makes this reaction protocol
to be a green, sustainable and cost-effectively strategy for the synthesis of thiolated
products. This study also highlights the great potential of semiconductor-based
photocatalysis for advanced organic syntheses.
Keywords: hydrothiolation reaction; visible light; ZnIn2S4; alkenes; alkynes.
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Introduction
The construction of C-S bonds is synthetically important because of the large
numbers of sulfur-containing natural products and pharmaceuticals as well as the
increasing importance of the sulfur-containing products in polymer and material
chemistry.1 The most classical and important method for the construction of C-S
bonds is the substitution reaction of alkyl halides and mercaptans under strong
alkaline conditions.2 These reactions have several drawbacks such as low efficiency,
limited substrate scopes and harsh reaction conditions. Although an efficient method
to construct C-S bond has been realized through the transition-metal-catalyzed
cross-coupling reactions between activated Csp2-X (aryl halides/aryl boronic acids)
and thiols,3-6 most of the involved catalysts are based on noble metals like Pd, Rh, Au,
Ag etc.7-8 In particular, this protocol is applied only to prefunctionalized arenes and
harsh reaction conditions are usually required.
The thiol-ene and thiol-yne reactions, the hydrothiolation reaction of alkenes or
alkynes with thiols, represent an attractive and atom economic approach for the
formation of C-S bond.9-10 Initially, the addition of thiols to alkenes or alkynes is
achieved via a radical pathway initiated by thermal or UV activation of a radical
initiator or by direct irradiation with UV light.11-12 However, this kind of free-radical
addition suffers from harsh reaction conditions and low selectivity with the possible
formation of by-products, which makes its application to only simple systems.13 The
development of hydrothiolations of alkynes/alkenes with thiols based on metal
catalysts represents an important progress in this field since it offers great potential
for hydrothiolation with high degree of region- and stereoselectivity.14-16 Recently,
with the aim of developing green and sustainable process for organic syntheses, the
use of light to drive chemical transformations has attracted extensive current research
interest.17-21 The development of the photoredox catalysis has provided an attractive
alternative to traditional methods to generate synthetically useful radical intermediates
in complex systems.22-24 Up to date, the hydrothiolation of alkenes and alkynes have
been realized over homogeneous photocatalysts like Ru(II) complexes25-26 , dyes
(Eosin Y)27 as well as heterogeneous semiconducting TiO2.28 Using the photocatalyst
to mediate the electron transfer not only enables these hydrothiolation reactions to be
more selective with expanded substrate scopes, but also can be realized under mild
conditions without the requirement of special UV equipments.
The first demonstration of the photoredox hydrothiolation reaction was reported
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by Yoon and co-workers, in which the reactions between alkenes and thiols were
realized under visible light over [Ru(bpz)3](PF6)2 (bpz = 2,2’-bipyrazine).25 It was
proposed that when irradiated, [Ru(bpz)3](PF6)2 is excited to its excited state, which
via a reductive quenching by a thiol generates [Ru(bpz)3]+ accompanied with a thiyl
radical cation. Deprotonation of the thiyl radical cation generates a thiyl radical,
which abstracts hydrogen from an un-reacted thiol to produce the hydrothiolated
product and another equivalent of thiyl radical to propagate the radical reaction, while
the as-formed [Ru(bpz)3]+ can be regenerated by reacting with oxygen. To realize
such a hydrothiolation system, a photoredox cycle for initiation of the thiyl radical
and the radical propagation cycle for the generation of the thiolated products are both
required and operate synergistically. However, due to the consumption of the protons
by oxygen in the system, excess thiols (4 equivalents) are required to achieve a high
yield to the thiolation products, which not only is not economic, but also produces
unnecessary by-products. Later, Ru(bpy)3Cl2-based photocatalytic radical initiation
system for thiol-ene reactions was also developed.26 Unlike that in [Ru(bpz)3](PF6)2
system, via a single electron oxidation of the sacrificial electron donor in the system,
the photoexcited [Ru(bpy)3Cl2]* is transform to a [Ru] species capable of reduction of
bromotrichloromethane to form trichloromethyl radicals. The further reaction between
the thiols and the trichloromethyl radicals yield the active thiyl radicals for the radical
thiol-ene coupling. Similar hydrothiolation reactions between thiols and alkynes were
also realized over Eosin Y,27 a metal-free dye photocatalyst, via a similar strategy as
that over [Ru(bpz)3](PF6)2. The use of a cheap dye to replace expensive Ru complexes
as photocatalyst in light initiated hydrothiolation reactions represents a significant
advance in the synthetic methodology since it can significantly reduce the cost for the
reaction. However, the organic dyes usually lack a long-term stability due to their
degradations upon irradiations.
Recently, semiconducting TiO2 was also reported for the hydrothiolation
reactions between alkenes and thiols.28 The light initiated thiol-ene reactions over
TiO2 also involves the thiyl radicals, which are generated by the deprotonation of the
thiyl radical cations via the reaction between the thiol and the photogenerated holes.
To complete a photocatalytic cycle over TiO2, oxygen as a sacrificial electron
acceptor should also be used in the reaction system. As compared with homogeneous
Ru complexes and the dyes, the use of TiO2 as photocatalyst for hydrothiolation
reactions obviously has advantages since TiO2 is cheap, nontoxic and readily
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accessible. In addition, as a heterogeneous semiconductor photocatalyst, TiO2 can be
easily removed from the reaction system by filtering for recycling, which not only can
avoid the possible contamination of the products by the catalyst, but also reduce the
cost of the reaction. However, bare TiO2 only absorb weakly in the visible light due to
its relatively large band gap. The visible light initiated hydrothiolation reactions over
TiO2 are only applicable to those substrates that adsorbed and bound to its surface
since the formation of the surface complexes between the substrates and the surface of
TiO2 is responsible for the absorption of TiO2 in visible light region.29-30 In addition,
like that in [Ru(bpz)3](PF6)2 system, oxygen introduce into the reaction system
consume the protons and therefore an excess of the thiol are required.
Hexagonal ZnIn2S4, a ternary chalcogenide semiconductor with a band gap well
responsive in visible light region, has been extensively studied as a photocatalyst for
hydrogen evolution and advanced organics synthesis.31-38 In particular, our recent
studies revealed that PtS/ZnIn2S4 nanocomposites can split thiols to produce
disulfides under visible light, via thiyl radicals (RS∙) generated between the reaction
of thiols and the photogenerated holes over ZnIn2S4.39 Stimulated by this previous
observation, in this manuscript, we investigated the visible light initiated
hydrothiolation reactions over ZnIn2S4. Both aromatic and aliphatic thiols as well as
thiophenols were found to react with a variety of alkynes and styrene to afford the
corresponding thiolated products in moderate to excellent yields. This study provides
a green, sustainable and cost-effective strategy for realization of hydrothiolation
reactions and highlights the great potential of semiconductor-based photocatalysis in
light initiated organics synthesis.
Experimental
Preparations
All the reagents were commercial available and used without further
purifications. ZnIn2S4 was synthesized according to our previously reported method.40
Characterizations
Powder XRD data were collected by using a Bruker D8 Advance X-ray
diffractometer (CuKa irradiations). The morphology of the samples was characterized
by field-emission SEM (JSM-6700F). UV-visible diffuse reflectance spectra
(UV-DRS) of the powders were obtained for the dry-pressed disk samples using a
Cary 500 Scan Spectrophotometer (Varian, USA), with BaSO4 used as a reflectance
standard. The ESR spectra were recorded on a Bruker A300 ESR spectrometer.
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Catalytic reactions
The visible light initiated hydrothiolation reactions were carried out in a sealed
reaction tube with a LED lamp (Beijing Perfect Light, PCX50B). The catalyst (10
mg), thiols (0.1 mmol) and alkynes/styrene (0.12 mmol) were suspended in a schlenk
tube containing solvent (4 ml). Before the reaction, the suspension was degassed and
saturated with N2 to remove any dissolved O2. The reaction was performed under
irradiation with a LED lamp (65 mW/cm2) in room temperature. After the reaction,
the products were analyzed by GC (Shimadzu GC-2014) equipped with an HP-5
capillary column and GC-MS.
Results and discussion
ZnIn2S4 was prepared by a solvothermal method from ZnCl2, InCl3·4H2O and
thioacetamide (TAA) in a mixture of ethylene glycol (EG) and N,N-dimethyl
formamide (DMF). The XRD patterns of the product was indexed to be hexagonal
ZnIn2S4 (JCPDS 65-2023), in agreement with our previous report (Fig. 1a).41 The
SEM image shows that the product is composed of flowerlike microspheres with a
dimension of ca. 4 μm, with the microspheres made up of numerous interweaving
nanoflakes (Fig. 1b). The UV/Vis DRS of the product shows that the absorption edge
extends to ca. 490 nm, in consistent with its reported band gap of ca. 2.4 eV (Fig. 1c).
To study the performance of ZnIn2S4 for light initiated hydrothiolation reactions,
the reaction between phenylacetylene (1a) and benzyl mercaptan (2a) was selected as
the model reaction and the reaction was initially carried out in CH3CN under visible
light. It was found that after irradiated for 15h, 80% of benzyl mercaptane was
converted, with a yield of 75% to the desirable product benzyl styryl sulfide (3a)
(Table 1, entry 1). An almost undetectable trace amount of H2 was also produced. On
the contrary, no product was detected when the reaction was carried out in absence of
either light or ZnIn2S4 (Table 1, entries 2-3), indicating that the hydrothiolation
between phenylacetylene and benzyl mercaptan to produce benzyl styryl sulfide is
actually induced by irradiated ZnIn2S4. To investigate the effect of solvent on this
reaction, the reactions carried out in toluene, ethylacetate, 2-propanol and methanol
were also studied. Among all the solvents investigated (Table 1, entries 4-7),
methanol showed the highest conversion of benzyl mercaptan (94%) and the best
yield to benzyl styryl sulfide (90%) in 15 h (Table 1, entry 7). E-benzyl styryl sulfide
was found to be the predominant product and the ratio of E/Z was determined to be
72:28 based on the NMR result (Supporting Fig. S1). Since methanol was found to be
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the best solvent in the hydrothiolation reaction, it was chosen as the reactionDmOIe: 1d0i.1u0m39/C9GC00328B
for the following investigations.
The visible light initiated thiol-yne reaction over ZnIn2S4 was applicable to a
wide range of substrate scopes. Aryl alkynes bearing both electron-donating and
electron-withdrawing substituents reacted effectively with benzyl mercaptane to
produce vinyl sulfides in good to excellent yields (73%-92%, Table 2, entries 1-3). As
compared with alkynes containing electron-donating substituent like CH3- in their aryl
rings, those with electron-withdrawing substituents like Cl- and Br-showed lower
reactivity to form vinyl sulfides, indicating the existence of electronic effect in this
reaction. Alkyl alkynes like 3-methyl butynol also reacted with benzyl mercaptane to
form 2-methyl-4-[(phenylmethyl)thio]-3-buten-2-ol with a moderate yield (58%,
Table 2, entry 4). Although varied in the activity, a broad range of thiols, including
both aromatic and aliphatic thiols as well as thiophenols also reacted with
phenylacetylene, with moderate to excellent yield to their corresponding products
(51%-95%). Aromatic thiols like 4-chlorophenethyl mercaptan and 2-phenylethyl
mercaptan show high activity, giving a good to excellent yield (82%-93%) to the
hydrothiolated products (Table 2, entries 5-6). On the contrary, the reaction on
tert-butyl mercaptan was lower, with a yield of 61% to the thiolated product (Table 2,
entry 7), probably due to the existence of a steric effect. Aliphatic thiols such as
cyclohexyl mercaptan also reacted effectively with phenylacetylene to generate the
hydrothiolated product in excellent yield (95%, Table 2, entry 8). Similar to that
observed on tert-butyl mercaptan, bulky aliphatic thiols such as 1-dodecanethiol
showed moderate yield (56%, Table 2, entry 9) to the hydrothiolated product.
Thiophenols such as p-toluenethiol and 2-naphthyl mercaptans also participate
smoothly in this process, with moderate to high yields (51%-70%) to the
hydrothiolated products (Table 2, entries 10-11).
In addition to alkynes, alkenes like styrene also reacted efficiently with the both
aromatic and aliphatic thiols as well as thiophenols to produce the corresponding alkyl
sulfides over ZnIn2S4 under visible light (Table 3). After irradiated for 15 h, the
thiol-ene coupling reactions between styrene and aromatic thiols such as
2-phenylethyl mercaptan, 4-chlorophenethyl mercaptan and tert-butyl mercaptan gave
the hydrothiolated products with moderate yields (49%-58%, Table 3, entries 1-3).
The reaction between aliphatic thiols such as cyclohexyl mercaptan and
1-dodecanethiol and styrene also generate 2-(cyclohexylthio)ethyl benzene and
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dodecyl (2-phenylethyl) sulfane in moderate to good yields (50%-63%, Table 3,
entries 4-5). Thiophenols such as p-toluenethiol was also tolerated in this reaction
protocol, with a yield of 61% to 2-phenylethyl 4-methylphenyl sulfide in 15 h (Table
3, entry 6). Aryl alkenes bearing both electron-donating and electron-withdrawing
substituents as well as alkyl alkenes like cyclohexene also reacted with benzyl
mercaptane to produce corresponding alkyl sulfides, although with lower yields
(30%-59%, Table 3, entries 7-9).
Previous studies on light induced hydrothiolation reactions revealed that the
formation of thiyl radical is involved.27-28 Our previous studies on the photocatalytic
splitting of thiols over PtS/ZnIn2S4 nanocomposite also suggest the involvement of
the photogenerated thiyl radicals. To elucidate the mechanism, the ESR of the
reaction system was studied. The ESR of the irradiated reaction system containing
ZnIn2S4, benzyl mercaptan and CH3OH in the presence of 5,5-dimethyl-1-pyrroline
N-oxide (DMPO) as a trapping agent shows four typical signals for the DMPO-thiyl
adducts, a confirmation of the formation of thiyl radicals in the reaction system
(Figure 2).42 The addition of 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO), a radical
scavenger, into the reaction system, led to a significantly decreased yield (3%) to the
benzyl styryl sulfide, suggesting that the thiyl radicals are indeed involved in the
visible light initiated hydrothiolation reactions over ZnIn2S4.
Based on the above observations, a possible reaction mechanism for visible light
initiated hydrothiolation reactions over ZnIn2S4 was proposed (Scheme 1). When
irradiated with visible light, electrons and holes are generated over ZnIn2S4. The
photogenerated holes are reductively quenched by the thiols, generating thiyl radical
cations (step 1), which followed by deprotonation to give thiyl radicals (step 2). The
as-formed thiyl radicals initiate the thiol-ene/thiol-yne coupling by addition to the
alkynes/alkenes to generate alkene/alkyl radicals (step 3), which abstract protons from
the un-reactive thiols to form the hydrothiolated products and produces thiyl radicals
to propagate the radical reaction (step 4). A controlled experiment by carrying out the
reaction in deuterated methanol under otherwise similar conditions gave the same
benzyl styryl sulfide instead of the deuterated product, an evidence to prove that the
origin of the proton is indeed from the thiol instead of methanol. As shown in the
Scheme 1, although the light initiated hydrothiolation reactions over ZnIn2S4 also
contains a photoredox cycle for initiation of the thiyl radicals and the radical
propagation cycle to generate the thiolated products, the generation of the thiyl
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radicals is only required in the beginning of the reaction as evidenced from the almost
undetectable trace amount of hydrogen evolved in the system. Therefore, unlike that
over TiO2-based system, only stoichiometric thiols are required in the thiol-ene and
thiol-yne coupling reactions over ZnIn2S4, which not only reduce the cost, but also
save the time required for the separation of the products. In addition, since bare TiO2
only absorb weakly in the visible light, the visible light initiated hydrothiolation
reactions over TiO2 are only applicable to those substrates which can form surface
complexes with TiO2. On the contrary, ZnIn2S4 is well responsive in the visible light
region. Therefore, the hydrothiolation reactions realized over ZnIn2S4 can be applied
to a wider range of substrate scopes.
To study the stability of ZnIn2S4 for visible light initiated hydrothiolation
reactions, the photocatalyst was centrifuged and filtered for a repeated use. It was
found that ZnIn2S4 was highly stable in the reaction between phenylacetylene and
benzyl mercaptane. No obvious loss of its activity was observed after three cycling
tests and the photocatalyst after three runs showed similar XRD patterns (Fig. 3 and
Fig. 4).
Conclusion
In summary, a general and efficient hydrothiolation of alkenes and alkynes can
be realized over ZnIn2S4 under visible light irradiation. The use of solar light and a
semiconductor-based photocatalyst to realize the thiol-ene and thiol-yne coupling
reactions in a green solvent (methanol), with only stoichiometric thiols required and
applicable to a broad substrate scope, makes this reaction protocol to be a green,
sustainable and cost-effectively strategy for the synthesis of thiolated products. This
study also highlights the great potential of semiconductor-based photocatalysis for
advanced organic syntheses.
Acknowledgements
This work was supported by NSFC (21872031, U1705251). Z. Li thanks the
Award Program for Minjiang Scholar Professorship for financial support.
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[42] S. Pou and G. M. Rosen, J. Chem. Soc., Perkin Trans. 2, 1998, 0, 1507-1512.
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DOI: 10.1039/C9GC00328B
Captions for Figures
Fig. 1 (a) XRD diffraction pattern, (b) SEM image, (c) UV-visible absorption
spectrum of ZnIn2S4.
Fig. 2 DMPO spin-trapping ESR spectra of the reaction system containing ZnIn2S4,
benzyl mercaptan and CH3OH (a) in the dark; (b) under visible light irradiation.
Fig. 3 Cycling of the hydrothiolation reaction between phenylacetylene and benzyl
mercaptane over ZnIn2S4 under visible light irradiation.
Fig. 4 XRD diffraction patterns of (a) fresh ZnIn2S4; (b) used ZnIn2S4 (after three
cycling reactions).
Caption for Scheme
Scheme 1 Proposed mechanism for visible-light-initiated hydrothiolation of alkenes
and alkynes reaction over ZnIn2S4.
Captions for Tables
Table 1 Light initiated reaction between phenylacetylene and benzyl mercaptane over
ZnIn2S4 under different conditions.
Table 2 Investigation of the substrate scope of visible-light-initiated hydrothiolation
of alkynes over ZnIn2S4.
Table 3 Investigation of the substrate scope of visible-light-initiated hydrothiolation
of styrene over ZnIn2S4.
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Page 13 of 20
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Fig. 1
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Green Chemistry
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DOI: 10.1039/C9GC00328B
Fig. 2
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Fig. 3
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Green Chemistry
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DOI: 10.1039/C9GC00328B
Fig. 4
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Page 17 of 20
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DOI: 10.1039/C9GC00328B
Scheme 1
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Green Chemistry
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DOI: 10.1039/C9GC00328B
Table 1
Yield (%)b
Entry
Solvent
Conv (%)a
3a
1
2 c
3 d
4
CH3CN
CH3CN
CH3CN
Toluene
Et2O
80
0
75
0
trace
67
trace
62
5
73
69
6
i-PrOH
CH3OH
81
78
7
94
90
Conditions: 1a (0.12 mmol), 2a (0.1 mmol), ZnIn2S4 (10 mg)，solvent (4mL)，
a
b
visible light irradiation for 15h; conv was calculated based on 2a; The
c
d
amount of the products was calculated based on GC analyses, No catalyst,
Without light.
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Page 19 of 20
Green Chemistry
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DOI: 10.1039/C9GC00328B
Table 2
Entry
Alkenes
Thiols
Vinyl sulfides
Yield (%)a, b
1
2a
2a
2a
2a
92
81
73
58
93
2
3
4
5
1a
1a
6
7
82
61
1a
8
9
1a
1a
1a
95
56
70
CH3(CH2)11SH
10
11
1a
51
Conditions: alkynes (0.12 mmol), thiols (0.1 mmol), ZnIn2S4 (10 mg), CH3OH
a
b
(4mL), visible light irradiation for 15h; conv was calculated based on thiols;
The amount of the products was calculated based on GC and GC-MS.
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Green Chemistry
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DOI: 10.1039/C9GC00328B
Table 3
Entry
Alkenes
Thiols
Alkyl sulfides
Yield (%)a, b
1
58
49
2
3
4
55
63
5
6
7
CH3(CH2)11SH
50
61
59
2a
8
9
2a
2a
34
30
Conditions: styrene (0.12 mmol), thiols (0.1 mmol), ZnIn2S4 (10 mg), CH3OH
a
b
(4mL), visible light irradiation for 15h; conv was calculated based on thiols;
The amount of the products was calculated based on GC and GC-MS.
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