F.L. Coelho et al.
CatalysisCommunications121(2019)19–26
2.3. General procedure for chalcogenoacetylenes
minutes. When the reaction was conducted for a prolonged time, sub-
product 6 was formed by a second step of arylselanyl coupling.
As expected, the model reaction did not occur in the absence of
copper and presented only 14% yield when the reaction was performed
without ligand (Table 1, entry 6). Reducing the temperature to 25 °C
decreased the yield to 30%, and only the selenoacetylene 5a was se-
lectively generated. Raising the temperature to 80 °C increased the yield
moderately to 67%, and at 115 °C an inversion of selectivity occurred.
The catalyst load (2 and 10 mol%) was also investigated (entries 10
and 11, Table 1); it negatively affected the yield at 2 mol% and se-
lectivity at 10 mol%. Among the solvents (entries 12–15, Table 1) and
bases (entries 16–18, Table 1) tested, Cs2CO3 and DMSO achieved the
best results. Note that the use of potassium hydroxide as a base resulted
in 55% of solely the selenoacetylene 5a, while use of DMF favoured the
formation of 6. For the reactions with dichloromethane and toluene as
solvent, no product was obtained.
Structural variations of the arylselanylpyrazole ligands reveal that
the ligand environment, i.e., the substituents on the arylselanyl and
pyrazolyl moieties, influences the catalytic performance of the reaction.
The presence of methyl groups at the 3- and 5- positions of the pyrazolyl
ring in 2a–c generates a more active catalytic system (entries 8, 19 and
be associated with the greater electron-donating ability of the pyrazolyl
unit in 2a–c, which promotes better stabilization of the catalytically
active species, and thus improves the catalytic performance of the re-
action.
On the other hand, the presence of a methoxy group at the para-
position of the arylselanyl moiety in ligand 2b leads to a small increase
in selectivity, while the presence of the electron-withdrawing group
chlorine in the arylselanyl moiety of 2c decreases the formation of 5a.
Despite the small increase in yield obtained with ligand 2b, we kept
using ligand 2a because it was easier to prepare.
Looking at entries 7 and 16 (Table 1) that showed excellent se-
lectivity, two additional time investigations were done with the re-
spective conditions. While the reaction with potassium hydroxide at
80 °C showed formation of product 6 after 15 min of reaction, the re-
action with caesium carbonate at room temperature (entry 22, Table 1)
achieved 84% yield in 25 min without the formation of subproduct 6.
Thereby, the optimal reaction conditions were as follows: phenylace-
tylene (0.5 mmol), diphenyl diselenide (0.5 mmol), CuI (5 mol%), li-
gand 3 (5 mol%) and Cs2CO3 (0.5 mmol) stirred in DMSO for 25 min at
25 °C.
The versatility of this method was established by carrying out sev-
eral reactions under the optimized reaction conditions with a series of
aromatic and aliphatic diselenides (3a–i) and terminal alkynes (4a–g)
(Table 2). Either diphenyl diselenide (3a) or phenylacetylene (4a) can
be replaced successfully with other derivatives highlighting ar-
ylacetylenes containing different substituents such as nitro (4n), amino
(4o) and aliphatic (4j) and also aliphatic diselenides (4 g), besides
sulfur and tellurium examples of chalcogenoacetylenes (4 h and 4i)
affording the desired products 5a–o.
In all cases, the reaction is chemically efficient (only formation of
5), albeit there are differences in yields observed between the different
products (from 14% to 88% yield) arising from the difficulty in solu-
bilizing diselenides. The lower yield of the series for 5f (14%) is at-
tributed to poor solubility of the respective diselenide in the reaction
media. Reaction conditions were not changed to evaluate method ef-
ficiency according to substituent variation. Furthermore, an increase in
temperature, which would circumvent the solubility limitations, pro-
motes formation of subproduct 6, as was observed in the optimization
reactions (Table 1). Despite this limited drawback, the process allows
the rapid preparation of a wide array of structurally interesting chal-
cogenoacetylene compounds.
To a round-bottom flask was added the following: DMSO (2 mL),
phenylacetylene (120 mg, 1.0 mmol) the respective diorganoyl dichal-
cogenide (0.5 mmol), Cs2CO3 (650 mg, 1 mmol), selanylpyrazole 2a
(13 mg, 0.05 mmol), CuI (10 mg, 0.05 mmol). The reaction mixture was
stirred for 25 min at room temperature. The crude product was diluted
with water, extracted with ethyl acetate (3 × 25 mL), dried and pur-
ified by column chromatography (hexane:ethyl acetate, 1:99).
2.4. General procedure for sulfides
To a round-bottom flask was added the following: DMSO (2 mL),
aryl iodide (1.0 mmol) the respective thiophenol (1.0 mmol), Cs2CO3
(650 mg, 1 mmol), selanylpyrazole 2a (13 mg, 0.05 mmol), CuI (10 mg,
0.05 mmol). The reaction mixture was stirred for 6 h at 110 °C. The
crude product was diluted with water, extracted with ethyl acetate
(3 × 25 mL), dried and purified by column chromatography (hex-
ane:ethyl acetate, 1:99).
3. Results and discussion
3.1. Synthesis of chalcogenoacetylenes
Thinking of chalcogenoacetylenes as building blocks for more
complex structures, the present work arose from the need to access
these compounds by a method combining easy preparation, chemos-
electivity, and high yields. In this context, we expected that employ-
ment of arylselanylpyrazoles as auxiliary ligands could avoid the for-
mation of byproducts, mainly vinylic ones, leading to satisfactory yields
associated with short reaction time, low temperature, and aerobic
conditions.
The arylselanylpyrazole analogues were readily prepared according
to the reference work [23a]. Briefly, starting from pyrazole or 3,5-di-
methylpyrazole (1a and 1b), the reaction with paraformaldehyde forms
the N-substituted aminoalcohol product that is converted to its chlori-
nated analogue with thionyl chloride. Lastly, the arylselanyl moiety is
linked by a nucleophilic reaction between a phenylseleno(trihydro)
borate complex, generated in situ from diaryl diselenides and NaBH4,
[23b] and a chlorinated alkylpyrazole, leading to the arylselanylpyr-
Reactants: (i) paraformaldehyde (ii) SOCl2, CHCl3 (iii) diphenyl
diselenide, NaBH4.
For initial verification of the activity of arylselanylpyrazole‑copper
complexes in selenoacetylene synthesis, diphenyl diselenide (3a) and
phenylacetylene (4a) were reacted in the presence of a catalyst under
aerobic conditions, achieving 60% yield of phenyl(phenylethynyl)se-
lane (5a) in a homogeneous catalytic system. The results and reaction
parameters are summarized in Table 1. In the initial experiment, the
reaction was monitored for 60 min (Table 1, entries 1–4), and it could
be observed that product 5a was formed at a fast rate in the initial
Scheme 2. Arylselanylpyrazole ligands of copper complexes.
21