NaBH 4 / [ bmim ] BF 4 : a New Reducing System to Access Vinyl Selenides and Tellurides

Um método simples e geral foi desenvolvido para a síntese de selenetos e teluretos vinílicos a partir de alquinos terminais e dicalcogenetos de diorganoíla usando NaBH 4 e [bmim]BF 4 como solvente reciclável. Este método eficiente permite a formação preferencial de calcogenetos vinílicos de configuração Z na maioria dos exemplos estudados. Também foi observado que na reação envolvendo fenilacetileno, (E)-bis-fenilcalcogenoestirenos foram obtidos com bons rendimentos e alta seletividade. O líquido iônico foi reutilizado três vezes sem perda da eficiência.


General remarks
Proton nuclear magnetic resonance spectra ( 1 H NMR) were obtained at 200 MHz on a Bruker DPX-200 NMR spectrometer.Spectra were recorded in CDCl 3 solutions.Chemical shifts are reported in ppm, referenced to the solvent peak of CDCl 3 or tetramethylsilane (TMS) as the internal reference.Data are reported as follows: chemical shift (d), multiplicity, coupling constant (J) in Hertz and integrated intensity.Carbon-13 nuclear magnetic resonance spectra ( 13 C NMR) were obtained at 50 MHz on a Bruker DPX-200 NMR spectrometer.Spectra were recorded in CDCl 3 solutions.Chemical shifts are reported in ppm, referenced to the solvent peak of CDCl 3 .Column chromatography was performed using Merck Silica Gel (230-400 mesh) following the standard methods.Thin layer chromatography (TLC) was performed using Merck Silica Gel GF 254 , 0.25 mm thickness.For visualization, TLC plates were either placed under ultraviolet light, or stained with iodine vapor, or acidic vanillin.The reactions were monitored by TLC for disappearance of starting material.Reactions were conducted in flame-dried glassware equipped with tightly fitted rubber septa and under a positive atmosphere of dry nitrogen.Reagents were handled using standard syringe techniques.

General procedure for the synthesis of vinyl chalcogenides
To a mixture of alkyne (2.0 mmol) and diphenyl diselenide (0.156 g; 0.5 mmol) in [bmim]BF 4 (0.5 mL) under N 2 atmosphere, NaBH 4 (0.045 g; 1.2 mmol) was added at room temperature.Then, the temperature was slowly raised to 60 °C.The reaction progress was followed by TLC and after the time described on Table 1 the reaction mixture was extracted using ether (5 × 3 mL).The solvent was evaporated under reduced pressure and the residue was purified by column chromatography over silica gel eluting with hexanes or a mixture hexanes/ethyl acetate.For the synthesis of the vinyl tellurides analogs it was used 1.2 mmol of the alkyne.Spectral data of the products prepared are listed below.

General procedure for the reuse of ionic liquid
The same procedure as for hydrochalcogenation of alkynes described before was followed.After work-up, [bmim]BF 4 was diluted with dichloromethane (3.0 mL) and filtered over Celite.The solution was dried over MgSO 4 and the volatiles were removed under vacuum.The recovered ionic liquid was reused for the next reaction.

Results and Discussion
Initially, we chose propargyl alcohol (1a; 2.0 mmol) and diphenyl diselenide (2a; 0.5 mmol) to establish the best conditions for the hydroselenation reaction.We examined the temperature, amounts of IL and NaBH 4 and the use of N 2 atmosphere.It was found that using 1.2 mmol of NaBH 4 and 0.5 mL of IL at room temperature and under N 2 atmosphere, the reaction proceeded slowly furnishing the product in 30% yield after stirring for 26 h.However, when the mixture was heated at 60 °C, the desired products 3a and 4a were obtained in good yields (87%) after 6 h (Table 1, entry 1).When the same protocol was performed without N 2 it was observed, after 10 h, incomplete consume of 2a and the products were isolated in 48% yield.When the reaction was performed in the presence of [bmim]BF 4 alone, without NaBH 4 , no reaction took place in all conditions tested and the starting materials were recovered.Besides, using a smaller amount of NaBH 4 (1.0 mmol), the desired products 3a and 4a were obtained only in 52% yield.The use of a slighter amount of 1a (1.5 mmol) afforded also lower yield of products (68%).
Since the best conditions were established, the protocol was extended to other alkynyl alcohols with diphenyl diselenide (Scheme 1).In all the studied cases, a mixture of (Z)-3 and gem-4 was obtained in good yields (Table 1, entries 1-5) and, except for homopropargyl alcohol 1e, the anti-Markovnikov addut 3 was obtained in higher amount than the Markovnikov one 4 (Table 1, entry 5).For the reaction of phenylselenolate anions with sterically hindered alcohols, it was observed that steric factors are important, because both, the 3:4 ratio and the reaction time, increased with the R group size (Table 1, compare entries 2-4 with 1 and 5).4][15][16][17][18][19][20] When the same protocol was applied to phenyl acetylene 1g, (E)-1,2-bis-phenylseleno styrene 5a was obtained in 85% yield after 3 h at 60 °C (Table 1, entry 7).This result is similar to that obtained under solvent-free conditions. 34iming to extend the scope of this new method, we performed the hydrotelluration of terminal alkynes.Thus, when diphenyl ditelluride 2b (0.5 mmol) reacted with propargyl alcohol 1a (2.0 mmol) in the presence of NaBH 4 (1.2 mmol) and [bmim]BF 4 (0.5 mL), the respective vinyl tellurides 3h and 4h were obtained in 89% yield after stirring at 60 o C for 6.5 h.In contrast to the observed for the hydroselenation reaction, a large excess of alkyne is not necessary.Thus, when 1.2 mmol of propargyl alcohol 1a reacted with 0.5 mmol of C 6 H 5 TeTeC 6 H 5 under the same conditions, a mixture of the vinyl tellurides 3h and 4h was obtained in 88% yield after 7 h (3h:4h ratio = 73:27; Table 1, entry 8).Similarly to the selenium analogs, the reaction works well with other alkynols, giving preferentially the respective anti-Markovnikov adduts 3i-k in good yields (Table 1, entries 9-11).Equally to the observed for diphenyl diselenide, the reaction with phenylacetylene gave the corresponding product 5b in 82% yield, but with a ration (Z)-3m:5b = 5:95 (Table 1, entry 13).When dibutyl ditelluride 2c was used as A plausible mechanism for the reaction of phenyl acetylene with diphenyl dichalcogenides using [bmim]BF 4 as solvent for formation of bis-and mono-phenylchalcogen alkenes respectively, is depicted on Scheme 2. The formation of bis-phenylchalcogen alkenes 5a and 5b can be attributed probably to the low solubility of the starting reagents 2a and 2b in the ionic liquid compared with 2c, which makes the reaction behavior like a solvent-free one, with formation of an intermediate analogous to 6 (Scheme 2). 34A free radical chain addition mechanism could also be involved. 45For the propargylic alcohols, however, the formation of the respective bis-chalcogen alkenes were not observed.This fact suggests that in this case, the intermediates 7 and 8 could be involved in the formation of 3 and 4, respectively. 46o check the efficacy of this method, a reuse study of the ionic liquid medium [bmim]BF 4 was carried out for the reaction showed in Scheme 3.After completion of the hydroselenation of propargyl 1a, the reaction mixture was diluted with ethyl ether and the product was isolated.The ionic liquid was recovered, dried under vacuum and reused for further reactions.The ionic liquid maintained its good level of efficiency even after being reused three times (Table 2).Yields of pure products isolated by column chromatography (hexanes/AcOEt) and identified by 1 H and 13 C NMR. b Determined by GC of the crude reaction mixture and confirmed after isolation of the individual isomers.c Obtained as mixture of (Z)-3g and (E)-5a (ratio 26:74).d Obtained as mixture of (Z)-3m and (E)-5b (ratio = 5:95).

Table 1 .
13drochalcogenation of alkynes using [bmim]BF 4 as recyclable solventYields of pure products isolated by column chromatography (hexanes/AcOEt) and identified by 1 H and13C NMR.b Determined by GC of the crude reaction mixture and confirmed after isolation of the individual isomers.c Obtained as mixture of (Z)-3g and (E)-5a (ratio 26:74).
a d