Pd-NHC Catalytic System for the Efficient Atom-Economic Synthesis of Vinyl Sulfides from Tertiary, Secondary, or Primary Thiols

Article abstract of DOI:10.1021/acscatal.5b01815

Vinyl sulfides represent an important class of compounds in organic chemistry and materials science. Atom-economic addition of thiols to the triple bond of alkynes provides an excellent opportunity for environmentally friendly processes. We have found tha

Full text of DOI:10.1021/acscatal.5b01815

Letter
pubs.acs.org/acscatalysis
Pd-NHC Catalytic System for the Efficient Atom-Economic Synthesis
of Vinyl Sulfides from Tertiary, Secondary, or Primary Thiols
Evgeniya S. Degtyareva,† Julia V. Burykina,† Artem N. Fakhrutdinov,† Evgeniy G. Gordeev,†
Victor N. Khrustalev,‡ and Valentine P. Ananikov*,†,§
†Zelinsky Institute of Organic Chemistry, Russian Academy of Sciences, Leninsky Prospect 47, Moscow, 119991, Russia
‡Nesmeyanov Institute of Organoelement Compounds, Russian Academy of Sciences, Vavilov str., 28, Moscow, 119991, Russia
§Department of Chemistry, Saint Petersburg State University, Stary Petergof, 198504, Russia
S
* Supporting Information
ABSTRACT: Vinyl sulfides represent an important class of
compounds in organic chemistry and materials science. Atom-
economic addition of thiols to the triple bond of alkynes provides an
excellent opportunity for environmentally friendly processes. We
have found that well-known and readily available Pd-NHC complex
(IMes)Pd(acac)Cl is an efficient catalyst for alkyne hydrothiolation.
The reported technique provides a general one-pot approach for the
selective preparation of Markovnikov-type vinyl sulfides starting from
tertiary, secondary, or primary aliphatic thiols, as well as benzylic and
aromatic thiols. In all the studied cases, the products were formed in excellent selectivity and good yields.
KEYWORDS: atom economy, hydrothiolation, vinyl sulfides, palladium, N-heterocyclic carbene, catalysis
Scheme 1. Selective Synthesis and Application of the Vinyl
Sulfides
remendous developments in materials science have caused
Tthe emergence of outstanding practical applications of
organic compounds.1−3 Of special importance are vinyl
monomers that contain the Csp2−heteroatom bond. The
monomers can be easily involved in well-known polymerization
routes to produce functionalized polymers.2−5 Heteroatom-
functionalized polymers represent a demanding class of new
materials that are useful in a wide range of applications, such as
high-performance engineering plastics, optics, optoelectronics,
network polymers, fuel cell membranes, proton-conducting
electrolytes, and photochemical materials.2 Some polymers
have applications in biomedicine, especially sulfopolymers that
are used as biomembranes; polysulfates and polysulfonates
exhibit antithrombotic and antiviral activities.2 The incorpo-
ration of a sulfur atom into the polymer structure (Scheme 1)
provides outstanding possibilities for changing its physical and
chemical properties3,4 to increase the refractive index,6 improve
elastic behavior,7 enhance thermal properties,8 and provide
sorption activity for Au, Ag, Pt, Pd, and Hg salts.9
From a practical point of view, an atom-economic addition of
thiols to alkynes opens outstanding possibilities to access vinyl
sulfides. Application of the metal-complex catalysis allows one
to obtain products with high yields and selectivity.10 This
catalytic approach was utilized for the preparation of the
demanding Markovnikov-type vinyl sulfides (Scheme 1) and
has demonstrated excellent tolerance to the nature of the
functional group in the alkyne.
the lanthanide is [Sm], [La], or [Lu]),15 actinide-based (where
the actinide is [U] or [Th]),16 and [In]-based17 catalytic
systems have been developed for the selective synthesis of
Markovnikov-type vinyl sulfides.
However, the addition of aliphatic thiols, especially branched
ones, was much more challenging.18 Indeed, from a practical
Surprisingly, the reaction is strongly sensitive to the nature of
the R substituent in the thiol. Several [Ni]-based,11 [Pd]-
based,12 [Rh]-based,13 [Zr]-based,14 lanthanide-based (where
Received: August 19, 2015
Revised: October 27, 2015
© XXXX American Chemical Society
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a
point of view, the reaction with secondary and tertiary thiols is
of high interest. On the one hand, the steric bulk provides
flexible control over the properties of the resulting polymeric
material. On the other hand, these alkanethiols are naturally
occurring in crude oil and readily available, thus potentially
providing a great opportunity to access inexpensive vinyl sulfide
monomers, especially on the industrial scale.
Table 1. Optimization of the Reaction Conditions
b
entry
value
Additives
yield of 3(4) [%]
In the current study, we describe the first example of the Pd-
NHC-catalyzed addition for a wide range of thiols and alkynes.
(Here, NHC represents N-heterocyclic carbene.) The prepared
catalyst precursor allowed us to conduct the reaction with
tertiary, secondary, and primary aliphatic thiols, as well as with
aromatic and benzylic thiols. The developed catalytic system
has essential advantages, such as air and moisture stability.
The optimization of the reaction conditions was carried out
using a model reaction involving 1-pentanethiol (1a) and 1-
heptyne (2a). Several Ni, Pd, Pt, Cu, Fe, and Co metal
complexes have been evaluated in the hydrothiolation reaction,
and the Pd complexes exhibited the best performance (see the
Supporting Information). Many different phosphine, nitrogen,
and NHC ligands were evaluated in the reaction involving Pd
complexes (see Tables S1 and S2 in the Supporting
Information) with a superior outcome observed for (NHC)-
Pd(acac)Cl. The application of the NHC = IPr ligand resulted
in the formation of a mixture of Markovnikov and anti-
Markovnikov products (3a:4a = 2:1), while the closely related
IMes-bearing complex allowed for the selective preparation of
the Markovnikov-type product (3a) (Table S2). (Here, IMes
represents N,N′-bis(2,4,6-trimethylphenyl)imidazole-2-ylidene
and IPr represents N,N′-bis(2,6-diisopropylphenyl)imidazole-
2-ylidene.) Therefore, the (IMes)Pd(acac)Cl complex was
employed as the catalyst of choice for further optimization.
In the next step, we optimized the reaction conditions (see
Table 1). The goal of the optimization was to increase the
selectivity and yield toward the formation of product 3a, while
minimizing the formation of byproducts Z-4a and E-4a.
Initially, the reaction was performed under solvent-free
conditions, which furnished 9% yield of 3a and 30% yield of
byproducts 4a (entry 1). The addition of γ-terpinene as a
radical reaction inhibitor prevented the formation of by-
products 4a (entry 2), because of suppression of the radical side
reactions. The addition of triethylamine increased the yield of
the desired product to 54% (entry 3). To determine the
optimal conditions, the triethylamine amount was varied from 1
mol % to 100 mol % (entries 3−6). A catalytic amount of 4
mol % was found to be optimal for the studied reaction (entry
4). The use of pyridine as a base afforded lower yields,
compared to triethylamine (entry 7). In the experiment with
both triethylamine and γ-terpinene additives, product 3a was
selectively obtained in high yield (entry 8). The application of
solvents, such as benzene, tetrahydrofuran, and 1,2-dichloro-
ethane (entries 9−11), did not improve the yield of the product
3a. The best results were obtained under solvent-free
conditions (entry 8).
1
2
3
4
5
6
7
8
none
9(30)
26(<1)
54(22)
87(4)
γ-terpinene
Et3N
1 equiv
1 mol %
Et3N
4 mol %
Et3N
10 mol %
100 mol %
10 mol %
4 mol % + 1 equiv
79(6)
Et3N
77(2)
pyridine
Et3N + γ-terpinene
19(27)
88(<1)
c
Solvents
9
benzene
0.5 mL
0.5 mL
0.5 mL
75(<1)
73(<1)
32(<1)
10
11
tetrahydrofuran, THF
dichloroethane, DCE
c
Temperatures
d
12
13
14
15
16
120 °C
100 °C
80 °C
50 °C
86(4 )
88(<1)
81(<1)
47(<1)
36(<1)
room temperature
a
Reaction conditions: 1a (0.5 mmol) and 2a (0.5 mmol) were added
to (IMes)Pd(acac)Cl (1 mol %). The reaction mixture was stirred for
4 h at 100 °C (entries 1−11). The yields were determined by H
NMR and calculated based on the initial thiol amount. Et3N (4 mol
%), and γ-terpinene (1 equiv) were added. Unidentified mixture of
byproducts.
b
1
c
d
Once the optimal reaction conditions were established, we
explored the scope of alkynes for the synthesis of vinyl sulfides
(Scheme 2). All of the reactions proceeded with good to
excellent yields. The various potentially reactive functional
groups in the alkynes were well tolerated in this reaction. These
groups include alcohols, amines, nitriles, esters, and chlorides
(3b, 3c, 3d, 3e, 3f). To study alkyne steric effects, various R′
substituents were employed and exhibited excellent yields for
3a, 3g, 3h (88%, 99%, and 91%, respectively). Even bulky
product 3i was formed in good yield (76%). Benzylic and
aromatic alkynes also selectively yielded desired Markovnikov
products 3l and 3m.
The developed catalytic system was tolerant to a wide range
of thiols. First, the most challenging tertiary and secondary
thiols were studied, and the products were obtained in very
good yields of 74%−83% (3o, 3p, 3q). The developed catalytic
system exhibited very good performance for various primary
thiols (3r−3x) with product yields of 67%−99%.
Aromatic and benzylic thiols provided high yields (73%−
99%) of the desired products (3y, 3z, 3ab−3am). A series of
experiments with aromatic thiols, containing electron-with-
drawing or electron-donating groups, were examined, and very
good functional group tolerance was observed (3ab−3am).
Phenylselenol has also reacted with a high yield of 99% (3aa).
The developed catalytic system has shown excellent regiose-
lectivity. A minor amount of byproducts was observed in one
case only (3m:4m = 5:1). In all of the other cases, compound 3
was obtained as a single product (controlled by 1H NMR), and
amount of the byproducts was below the trace level. The
catalyst operation in a wide temperature range is a key factor
for optimizing the outcome of the addition process. Temper-
Variations in the temperature revealed an important
advantage of the developed catalytic system. High selectivity
toward the formation of desired product 3a was retained over a
wide temperature range from room temperature (rt) to 100 °C
(entries 13−16). Reaction at higher temperature of 120 °C also
gave a good product yield; however, some amount of
byproducts was found (entry 12). For a particular combination
of reagents (1a and 2a), a temperature of 100 °C was the
optimal choice (entry 13).
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a b c
, ,
Scheme 2. Catalytic Regioselective Addition of Thiols to Alkynes
a
b
Reaction conditions: 1 (0.5 mmol), 2 (0.5 mmol), Et3N (4 mol %), (IMes)Pd(acac)Cl (1 mol %), γ-terpinene 1 equiv at 50 °C 18 h. Yields by 1H
c
d
NMR and isolated yields shown in parentheses. For additional substrate scope see also Scheme S1 in the Supporting Information. Conditions: 100
°C, 8 h. Conditions: 100 °C, 4 h. Conditions: 50 °C, 8 h. Conditions: 35 °C, 24 h. 3m:4m ratio = 5:1. Two equivalents (2 equiv) of alkyne 2
were used. PhSeH was used instead of thiol.
e
f
g
h
i
j
atures ranging from 35 °C to 100 °C were utilized to render the
selective transformation, depending on the reactivity of thiol
and alkyne (Scheme 2). The ability to vary the temperatures is
important for reagents that have a low boiling point or thiols
that are prone to generate disulfides at high temperatures.
To investigate the reaction mechanism, we performed a
(I), 1 equiv of thiophenol (1b), and Et3N was carried out in
benzene-d6. The appearance of a set of new signals was
observed: half of the initial (IMes)Pd(acac)Cl was converted to
a new NHC-containing complex, with the liberation of
acetylacetone. The addition of a second equivalent of PhSH
(1b) led to complete conversion of the initial precatalyst (I)
into [(IMes)Pd(SPh)2]n, indicating the substitution of both the
acac and Cl ligands in I. The crystals of the [(IMes)Pd(SPh)2]2
1
sequence of stoichiometric reactions (see Scheme 3). The H
NMR monitoring of the reaction between the catalyst precursor
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bond lengths may be due to the relative effects of the trans
influence, as well as steric effects.
Scheme 3. Stoichiometric Reaction of Pd-Mediated PhSH
(1b) Addition to Alkyne (2b)
It is interesting to note that, in contrast to complex IIa, the
two Pd2S2 four-membered rings in the related trinuclear
complex, (IPr)(SPh)Pd−(μ-SPh)2-Pd-(μ-SPh)2−Pd(SPh)
(IPr) has a butterfly conformation, and the square planar
geometry of the terminal Pd atoms was distorted to a greater
extent.19 Regardless of these facts, the Pd−C and correspond-
ing Pd−S distances in both complexes are very similar.
Complex IIa can also be compared with related structures
containing PPh3
triisopropyl phosphite has a trans configuration and the
distance between Pd and S2a atom is shorter than Pd−S2, in
contrast to IIa.
dimer (IIa) were grown, and its molecular structure was
determined by X-ray analysis (see Figure 1). In the last step of
19b
or P(O-iPr)19c ligands. Complex with
The catalytic activity of the isolated dimeric complex was also
examined. The addition of alkyne (2b) and PhSH to the
catalytic amount of complex IIa, followed by heating at 80 °C
for 4 h, resulted in formation of the product 3y in 90% NMR
yield. Isolated complex IIa was found less reactive and the
catalytic addition reaction was rather slow at rt or at 50 °C.
Hydrothiolation of the alkyne was initiated upon heating of the
reaction mixture up to 80 °C.
The overall catalytic transformation starts with the activation
of the precatalyst I by replacement of the Cl and acac ligands
with SR ligands (see Scheme 4). The resulting Pd complexes
may exist in a monomeric (II) or dimeric (IIa) forms. The
ligand replacement reactions were confirmed by a step-by-step
stoichiometric sequence (see Scheme 3), and structure IIa was
established by X-ray analysis (see Figure 1).
To define the nature of the catalytic species, kinetic studies
and theoretical calculations were performed (see the Support-
Scheme 4. Plausible Mechanism of Alkyne Hydrothiolation
Figure 1. X-ray crystal structure of the catalyst resting state (IIa).
Selected bond distances: Pd1−C2, 2.0212(17) Å; Pd1−S1, 2.3190(5)
Å; Pd1−S2, 2.3431(4) Å; Pd1−S2a, 2.3648(4) Å; N1−C2, 1.362(2)
Å; C2−N3, 1.360(2) Å. Selected bond angles: C2−Pd1−S1,
91.22(5)°; C2−Pd1−S2, 95.93(5)°; C2−Pd1−S2a, 174.92(5)°; S1−
Pd1−S2, 170.822(18)°; S1−Pd1−S2a, 88.665(17)°; S2−Pd1−S2a,
83.699(16)°; Pd1−S2−Pd1a, 96.303(16)°; and N1−C2−N3,
103.73(15)°.
stoichiometric transformations, product 3y was formed in high
yield after heating with PhSH (Scheme 3). In the reaction of
alkyne 2b and isolated complex IIa, product formation was
detected only in the presence of PhSH.
Compound IIa (Figure 1) is a centrosymmetric binuclear
complex formed by two bridging phenylthiolato ligands.
Because of the intrinsic inversion center, the central Pd2S2
four-membered ring is strictly planar. The Pd···Pd separation in
IIa was 3.5070(3) Å, which indicates the absence of a bonding
interaction. Each Pd(II) atom adopts an approximately square
planar geometry that is coordinated by three S atoms from the
PhS ligands and one C atom from the NHC ligand. The
distortion of the square planar configuration is manifested in
the slight deviation of the Pd atoms from the mean planes that
pass through the four coordinated atoms by 0.1032(5) Å. Both
the bridging and terminal thiolate ligands are arranged anti,
relative to the central four-membered ring, and the carbene
ligands occupy the mutually trans positions. The phenyl rings of
the terminal and bridging PhS ligands are nearly parallel with a
dihedral angle of 11.22(14)°. The bridging Pd−S bond lengths
were similar to each other, and the differences between these
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ing Information). Under catalytic conditions, the reaction was
approximately first order, with respect to (IMes)Pd(acac)Cl.
Therefore, we assume that a monomeric complex (II) is
involved in the catalytic cycle as an intermediate, and complex
IIa is a resting state (see Scheme 4).
stable in air, and this approach avoids the use of complicated
experimental techniques. In summary, a simple and efficient
catalytic system has been developed for the atom-economic
preparation of vinyl sulfides with excellent selectivity and very
good yields.
Theoretical modeling of the alkyne insertion into Pd−S bond
was carried out using DFT calculations at the PBE1PBE/6-
311G(d) and SDD levels (see the Supporting Information).
The monomeric complex is three-coordinated and provides the
necessary option for binding of the alkyne and formation of π-
complex III with further transition into complex IV after alkyne
insertion (see Scheme 4). A relatively small activation barrier of
ΔG⧧ = 12.9 kcal/mol and favorable thermodynamic driving
force with ΔG = −10.7 kcal/mol were found in the calculations
(Scheme 4A). Pd atoms in the dimeric complex are four-
coordinated and do not have coordination vacancy for strong
binding of alkyne and activation of the triple bond. Rather high
activation barriers were calculated for alkyne insertion into the
terminal ΔG⧧ = 42.9 kcal/mol (Scheme 4B) and bridging ΔG⧧
= 40.4 kcal/mol (Scheme 4C) Pd−S bonds. In total agreement
with the experimental findings monomeric complex is more
reactive toward alkynes, compared to the dimeric palladium
complex.
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acscatal.5b01815.
■
S
Mechanistic study, kinetic measurements, theoretical
calculations, experimental procedures, spectral data,
details of the X-ray analysis (PDF)
Crystallographic data for C66H68N4Pd2S4 (CIF)
AUTHOR INFORMATION
Corresponding Author
*E-mail: val@ioc.ac.ru.
Notes
■
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
This work was supported by Russian Science Foundation (RSF
Grant No. 14-50-00126).
■
The nature of the insertion step and observed selectivity of
the reaction were confirmed by performing deuterium labeling
experiments (see Scheme 5 and the Supporting Information).
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