A simple and non-conventional method for the synthesis of selected  -arylalkylchalcogeno substituted alcohols, amines and carboxylic acids

A simple and mild procedure for the reaction of nucleophilic chalcogenium species (Se and Te) with lactones, epoxides or aziridines to prepare chalcogen-containing acids, alcohols and amines in non-conventional media is described. The chalcogenolate nucleophiles were generated in situ from the respective diorganyl dichalcogenide using NaBH 4 /Al 2 O 3 under solvent-free conditions (to prepare the chalcogen-containing acids and alcohols) or a NaBH 4 /PEG-400 system (for the synthesis of chalcogen-containing amines) at 50 °C. The functionalized organochalcogenides were prepared in short reaction times and good yields


Introduction
2][3] Organochalcogen compounds, when associated with other functionalities, are versatile reagents in organic synthesis. 4Additionally, recently they have been applied in materials science, 5 biological and pharmacological studies 6 and organic synthesis.[9][10][11] On the other hand, the ring opening reaction by nucleophilic selenium and tellurium species is a very useful method to incorporate these elements into organic molecules, due to their soft nucleophilicity and low basicity.The use of cleaner procedures for producing nucleophilic species of organochalcogen has been an efficient strategy.Among the alternative methods employed for generation in situ of chalcogenolate anions for ring opening reactions of epoxides, aziridines and lactones are: Zn/HCl/biphasic, 12 Zn/THF/reflux, 13 Zn/HCl/[bmim][BF4], 14 KOH/CuO/[bmim][BF4] 15 and Zn/AlCl3/CH3CN/70 o C. 16 In most cases, these methods employ strong base or acid, high temperatures and volatile organic compounds (VOCs), limiting their use to a few functional groups.
Recently, we developed a new method to the in situ generation of chalcogenolate anions by using the (RY)2/NaBH4/PEG-400 system.This protocol was successfully used to prepare vinyl chalcogenides, 17 bis-chalcogen alkenes 18 and β-chalcogen esters, ketones and carboxylic acids. 19,20y this procedure, the use of odoriferoous, unstable compounds and drastic reaction conditions are avoided, enabling us to explore the soft nucleophilicity of organochalcogen (Se and Te) compounds under mild conditions.
In this work, we describe the ring-opening reaction of lactones, epoxides and non-activated aziridines by chalcogenolate anions using solvent-free NaBH4/Al2O3 or NaBH4/PEG-400 systems at 50 o C to prepare chalcogen-functionalized carboxylic acids, alcohols and amines (Scheme 1).Scheme 1. Synthesis of chalcogen-containing acids, alcohols and amines under mild conditions.

Discussion
In the initial experiments, we chose diphenyl ditelluride 1a as the chalcogen source and β-methylβ-propiolactone 2a, propylene oxide 2b and 2-methylaziridine 2c as the electrophiles to evaluate the best conditions for the nucleophilic ring-opening reaction (Table 1).In the optimization study, we examined the influence of the amount of electrophile, the temperature and the use of solvent or solid-supported reducing agent.It was observed that the presence of solid support or solvent is essential for the success of the ring-opening of the three electrophiles (Table 1, entries 1, 17 and 22).As can be seen in Table 1, the use of 50.0 mg of solid support or 50L of solvent and an excess of electrophile (1.5 mmol) provided the best results (entries 5, 13 and 20).The reactions were monitored by thin layer chromatography (TLC) and gas chromatography (GC).
Among the conditions that were tested for the ring-opening of lactone 2a to obtain the tellurium-containing acid 3a, the most effective approach was that using NaBH4/Al2O3 at 50 o C, which afforded the product in 76% yield (Table 1, entry 3).A decrease in the yield of 3a was observed when the reaction was performed either at room temperature or at 80 o C (Table 1, entries 2 and 4), while 83% of the product was obtained using an excess of 2a (1.5 equiv) with respect to the ditelluride 1a (Table 1, entry 5).We also tested alternative solvents in the reaction, such as PEG-400, glycerol and ethanol.A satisfactory yield of 3a was obtained only when NaBH4/PEG-400 was used (Table 1, entries 9-10).Ethanol delivered 3a in only 34% yield, while using glycerol caused the formation of a solid in the reaction vessel, thus preventing mixing of reagents (Table 1, entries 11-12).When propylene oxide 2b was used as the electrophile, the profile of the reaction remained the same, with the NaBH4/Al2O3 system, affording the desired tellurium-containing alcohol 4a in 88% yield after 2 h at 50 o C (Table 1, entry 13).The NaBH4/PEG-400 system afforded the alcohol 4a in 80% yield (Table 1, entry 15).We observed that SiO2 is not as good as Al2O3 as the solid support and the ring-opening was less efficient for both lactone 2a and epoxide 2b (Table 1, entries 8 and 14).Ethanol was not a good solvent to prepare 4a, which was obtained in only 41% yield after 2 h (Table 1, entry 16).A longer reaction time did not improve the yield.
In striking contrast to these results, NaBH4/PEG-400 system gave the best result in the ringopening of 2-methylaziridine 2c, affording the tellurium-containing amine 5a in 76% yield (Table 1, entry 20).Clearly, to produce chalcogen amines the presence of a hydrogen source is essential for success.These results are corroborated by those using conventional methodologies. 21,22owever, when the reaction was carried out in ethanol, the yield of 5a decreased to 57% (Table 1, entry 21).The use of additional ethanol (2.0, 5.0 and 10.0 mL) did not change the outcome.Neither increasing the temperature (80 and 100 o C) nor using a larger excess of aziridine 2c (2.0, 3.0 and 5.0 equiv) improved the yield of 5a using the NaBH4/PEG-400 system.----2c (1.5) 50 18 a Reaction performed in the presence of 0.5 mmol of diphenyl ditelluride 1a and 1 mmol of NaBH4 for 2 h.b Yields are given for isolated products 3a, 4a or 5a.
With the standard reaction conditions defined, we next investigated the scope of our methodology by employing a variety of chalcogenolate anions and electrophiles.To establish the generality for the lactone 2a ring-opening reaction, various diorganyl dichalcogenides 1 were used in the presence of NaBH4/Al2O3 system at 50 o C. The reactions proceeded with good yields employing diaryl and dialkyl dichalcogenides and they are not sensitive to electronic effects in the aromatic ring of the diaryl ditellurides (Scheme 2).This approach was successfully extended to diphenyl diselenide 1e and the respective β-phenylselanyl carboxylic acid 3e was obtained in 85% yield (Scheme 2).A good result was obtained when -butyrolactone 2d was reacted with diphenyl diselenide 1e in the presence of NaBH4/Al2O3, yielding the -phenylselanyl acid 3f in 79% yield (Scheme 3).Excellent results were obtained in the reaction of the chalcogenolate anions generated in situ with epoxides (Scheme 4).As can be seen in Scheme 4, β-chalcogen alcohols 4a-k were obtained in good to excellent yields from different epoxides and a variety of ditellurides and diselenides.For instance, 2-benzyloxirane 2e reacted with diphenyl ditelluride 1a and diphenyl diselenide 1e in the presence of NaBH4/Al2O3 to afford, after 2 h, the respective phenyltelluro alcohol 4j and Next, we explored our protocol using NaBH4/PEG-400 in the ring-opening of 2methylaziridine 2c, aiming to prepare β-chalcogen-containing amines 5a-h (Scheme 5).We found that the ring-opening of aziridine 2c is more efficient when nucleophilic selenium is used, producing the respective selenium-containing amines in better yields that the telluro-products.As in the ring-opening of lactone and epoxides (Schemes 2 and 4), the presence of electronwithdrawing or electron-releasing groups in the aromatic ring of the ditelluride and diselenide did not influence the yields of products in a predictable way.The yields of tellurium-containing aziridines ranged from 58 to 76%, while the selenium-containing aziridines were obtained with yields from 69 to 85% (Scheme 5).Because of the basicity of the chalcogen-containing amines, in these reactions aqueous NaCl was used instead NH4Cl in the work up, to avoid product loss.Regarding the stability of the obtained chalcogen compounds, we have observed the following order: chalcogen-containing amines > chalcogen-containing alcohols > chalcogen-containing acids.In the presence of solvent, light or at high temperatures, the chalcogen-containing acids are decomposed.Thus, work-up and purification steps must be performed rapidly.The same care should be taken when working with the chalcogen-containing alcohols, but the degradation rate is lower.In contrast, the chalcogen-containing amines are very stable and do not require the same attention.
Scheme 5. Synthesis of chalcogen-containing amines 5a-h by ring opening reactions of 2methylaziridine.

Conclusions
In conclusion, we have shown that the use of NaBH4/PEG-400 and NaBH4/Al2O3 as reducing systems to prepare chalcogenolate anions can be successfully applied in the synthesis of telluriumand selenium-functionalized acids, alcohols and amines.This atom-economic strategy involves the ring-opening of lactones, epoxides and aziridines and is general for dialkyl and diaryl ditellurides and diselenides.Moreover, this simple procedure does not involve harsh reaction conditions and is not time consuming, with good-to-excellent yields of products being obtained in only a twohour reaction.

Experimental Section
Analytical thin-layer chromatography (TLC) was performed by using aluminum-backed silica plates coated with a 0.25 mm thickness of silica gel 60 F254 (Merck), visualized with an ultraviolet light (λ = 254 nm).Either 300 MHz or 500 MHz acquired the NMR spectra.The 1 H NMR chemical shifts are reported in parts per million (ppm) relative to tetramethylsilane (TMS) peak ( 0.0 ppm).The data are reported in chemical shift (), multiplicity, coupling constant (J) in Hertz and integrated intensity.The 13 C NMR chemical shifts were reported at either 75 or 125 MHz in ppm relative to CDCl3 signal (77.0ppm).The 77 Se NMR chemical shifts are reported in ppm relative to internal standard C6H5SeSeC6H5 (467 ppm).The 125 Te NMR chemical shifts are reported in ppm relative to internal standard C6H5TeTeC6H5 (422 ppm).High-resolution mass spectra (HRMS) were acquired using a Bruker Daltonics MicroTOF instrument, operating electrospray ionization (ESI) mode with ion mass/charge (m/z) ratios as values in atomic mass units.General procedure: To a 5 mL vial equipped with magnetic stirrer and a rubber septum under nitrogen, was added dialkyl or diaryl dichalcogenide (0.5 mmol) and the electrophile (1.5 mmol) followed by the catalyst system.To synthesize the chalcogen-containing acids and alcohols a NaBH4/Al2O3 (1 mmol/50 mg) system was employed and to prepare chalcogen-containing amines a NaBH4/PEG-400 (1 mmol/50 L) system was used.The mixture was then stirred for 120 min at 50 o C. The reaction progress was monitored by thin layer chromatography (TLC) and gas chromatography (GC).After 1 h at rt the reaction medium was diluted with AcOEt (20 mL) and washed with saturated aq solution of NH4Cl (15 mL) for acids and alcohols and NaCl (15 mL) for amines.The phases were separated and the aq phase was extracted with AcOEt (2 × 20 mL).The organic phase was dried over MgSO4 and the solvents were evaporated under reduced pressure.The product was purified by flash column chromatography eluting first with hexane to remove alkyl or aryl chalcogen byproducts and then with hexane/AcOEt (8:2) to remove the acids or alcohols and AcOEt only to remove the amine.

Table 1 .
Optimization of the ring-opening reactions of lactone, epoxide and aziridine by phenyltellurolate anion a