Elemental Chalcogen (Se, S) in PEG-400 to the Synthesis of Seleno- and Thioflavones from 2-Chlorophenyl Ethynyl Ketone and Nucleophilic Species of Chalcogen

An alternative green method was developed for the synthesis of thioand selenoflavones by the ring closure of 2-chlorophenyl ethynyl ketone with NaHY (Y = S, Se). These nucleophilic chalcogen species were generated in situ using NaBH4 to reduce the elemental chalcogen in the presence of polyethylene glycol-400 (PEG-400). The efficiency of this reaction is strongly dependent on the PEG-400 solvent, acting like a crown ether, complexing with the sodium atom of NaHY species, making the chalcogen nucleophile more active. The synthetic protocol proceeded efficiently at 100 °C under argon, using a range of 2-chlorophenyl ethynyl ketone containing alkyl, aryl, or vinyl groups and the sulfur and selenium chalcogen. By this efficient and simple approach, 18 chalcogenoflavones were obtained in good to excellent yields after 2 h.


Introduction
Interest in organochalcogen compounds has been continuously increasing due to their reputable synthetic and biologic applications. 1 In this field, the modification of heterocyclic compounds with sulfur, selenium, and tellurium atom is a good strategy to obtain more active species and increase the biological applicability. 2 Among the several heterocyclic cores, flavones and chromone derivatives constitute part of a large family of natural products with pharmaceutical applications. The flavone system (2-phenyl-4H-chromen-4-one, shown in Figure 1) is a key structural which play important roles in numerous biological processes. 3 Replacement of the oxygen atom in the pyran ring with selenium and sulfur results in selenoflavone and thioflavone, respectively ( Figure 1). The chalcogenoflavones have received intensive attention from medicinal chemists due to their unique structure-activity relationship showing antioxidant, anti-inflammatory, anti-cancer, antimicrobial, and antifungal activities, among others. 4 Regarding the synthesis of these bioactive compounds, they can be prepared by two main procedures: intramolecular cyclization of 2-alkylchalcogenolated alkynones; 5 and ring closure of 2-halophenyl ethynyl ketones with -SeH or -SH moiety. 6 Also, the thioflavone was prepared using non-conventional methodologies, such as multicomponent reactions, 7 intramolecular cyclization 8 and rearrangement, 9 decarbonylative cycloaddition, 10 condensation, 11 among others. 12 However, greener synthetic methods for the synthesis of both selenoflavones and thioflavones are still limited. In the context of green chemistry, several synthetic approaches have been progressed, producing a wide variety of compounds without or less environmental hazard. 13 A pivotal parameter to improve the sustainability of a reaction is the use of a suitable medium for environmentally friendly and safe chemical reactions. 14 Thus, polyethylene glycol-400 (PEG-400) might be a solvent with higher preference because of its versatile blessings as a green reaction media. PEG solvent is perceived to be cheap, easily accessible, non-volatile, biodegradable, biocompatible, with low flammability, recyclable, and thermally stable. 14,15 Noticeable organic transformations involving PEG as a green reaction media were reported. 16 Specifically, this solvent has been described as the reaction medium for procedures involving organochalcogen compounds. 17 Our research group has recently described the in situ generation of chalcogenolate anions using the system RYYR/NaBH 4 / PEG-400 (Y = S, Se, and Te) and their use in selective reactions. 18 So far, only two procedures were described using NaBH 4 /PEG-400 system to reduce elemental chalcogen and their application to prepare bis(2-pyridyl) diselenide derivatives and divinyl selenides/tellurides. 19 We hypothesized that the nucleophilic chalcogen species generated in situ could be applied in other transformations. To test this conjecture and develop our continued interest in green chemistry, we herein disclose our latest work on the synthesis of selenoflavones 3 (Y = Se) and thioflavones 4 (Y = S) by ring closure of 2-chlorophenyl ethynyl ketone 2 under mild conditions (Scheme 1).

General information
All reagents and solvents used were purchased from commercial sources (Sigma-Aldrich ® , São Paulo, Brazil). The reactions were monitored by thin-layer chromatography (TLC) carried out on Merk silica gel (60 F 254 ) by using UV light as visualization agent and the mixture between 5% of vanillin in 10% of H 2 SO 4 under heating conditions as developing agents. Merck silica gel (particle size 0.040-0.063 mm) was used to flash chromatography. Hydrogen nuclear magnetic resonance spectra ( 1 H NMR) were obtained on Bruker Avance III HD 400 MHz (Uster, Switzerland) employing a direct broadband probe at 400 MHz. The spectra were recorded in CDCl 3 solutions. The chemical shifts are reported in ppm, referenced to tetramethylsilane (TMS) as the internal reference. Coupling constants (J) are reported in hertz. Abbreviations to denote the multiplicity of a particular signal are s (singlet), d (doublet), dd (doublet of doublet), dt (doublet of triplet), t (triplet), q (quartet), quint (quintet) and m (multiplet). Carbon-13 nuclear magnetic resonance spectra ( 13 C NMR) were obtained on Bruker Avance III HD 400 MHz (Uster, Switzerland) employing a direct broadband probe at 100 MHz. The chemical shifts are reported in ppm, referenced to the solvent peak of CDCl 3 (d 77.0 ppm). Selenium-77 nuclear magnetic resonance spectra ( 77 Se NMR) were obtained on Bruker Avance III HD 400 MHz (Uster, Switzerland) employing a direct broadband probe at 76 MHz, measured without 1 H decoupling. The chemical shifts are reported in ppm, using as solvent the CDCl 3 and as an internal standard the diphenyl diselenide (d 463.0 ppm). The high-resolution mass spectrometry (HRMS) quadrupole time-of-flight (QTOF) analyses were performed on a Bruker (Billerica, MA, USA) Daltonics micrOTOF-Q II instrument in positive mode. The samples were solubilized in high performance liquid chromatography (HPLC)-grade acetonitrile and injected into the atmospheric pressure chemical ionization (APCI) source by means of a syringe pump at a flow rate of 5.0 µL min -1 . The follow instrument parameters were applied: capillary and cone voltages were set to +3500 and -500 V, respectively, with a desolvation temperature of 180 ºC. For data acquisition and processing, Compass 1.3 for micrOTOF-Q II software (Bruker Daltonics, USA) was used. The data were collected in the m/z range of 50-1200 at the speed of two scans per s. Low-resolution mass spectra were obtained with a Shimadzu (Kyoto, Japan) GC-MS-QP2010P mass spectrometer. Melting point (mp) values were measured in a Marte (São Paulo, Brazil) PFD III instrument with a 0.1 ºC precision. dry tetrahydrofuran (THF) (25 mL) was added n BuLi (2.5 mol L -1 in n hexane, 2.2 mL, 5.5 mmol, 1.1 equiv) dropwise at -78 ºC under nitrogen atmosphere. After the reaction mixture had been stirred at -78 °C for 1 h, 2-chlorobenzaldehyde (5.0 mmol, 1.0 equiv) was added dropwise at -78 °C. Upon stirring at same temperature for 1 h, the reaction solution was quenched with saturated solution of NH 4 Cl and extracted with ethyl acetate (3 × 20 mL). The combined organic layer was dried over anhydrous MgSO 4 , filtered, and concentrated under reduced pressure to give the crude alcohol product. After, dichloromethane (15 mL) and MnO 2 (10 equiv) were added and the solution was stirred at room temperature till alcohol disappeared by TLC analysis. The reaction solution was filtered over celite, solvents were removed, and the crude product was purified by column chromatography on silica gel with hexane/EtOAc and concentrated to afford the corresponding 2-chlorophenyl ethynyl ketone 2.
General procedure for synthesis of chalcogenoflavones 3 and 4 In a 10.0 mL reaction vial containing Se or S (0.3 mmol) in PEG-400 (2.0 mL) under argon atmosphere was added NaBH 4 (0.7 mmol), and the mixture was slowly heated to 50 °C being stirred for 30 min. Then, compound 2 (0.25 mmol) was added, and the temperature was raised to 100 °C. The reaction mixture remained under magnetic stirring for 2 h. Then, water was added (25.0 mL) and the reaction was extracted with ethyl acetate (3 × 10.0 mL). The organic phase was separated, dried over MgSO 4 , and the solvent was evaporated under reduced pressure. The product 3 or 4 was isolated by column chromatography using hexane/ethyl acetate (95/5) as eluent.     Procedure for identify the nucleophilic species formed through 77 Se NMR spectroscopy In a 10.0 mL reaction vial containing Se (0.3 mmol) in PEG-400 (2.0 mL) under argon atmosphere was added NaBH 4 (0.7 mmol), and the mixture was slowly heated to 50 °C being stirred for 30 min. Then, 500 µL of this solution was transferred to a 5 mm NMR tube, purged with argon gas and a capillary tube with a solution of the diphenyl diselenide in CDCl 3 was used as the chemical shift reference (d 463 ppm), after the NMR tube was capped with a rubber septum and analyzed by 77 Se NMR. The spectra were recorded at 298 K and with 845 scans.

Results and Discussion
Based on the results of our recent studies using elemental chalcogens, 19 we chose selenium powder 1a, NaBH 4 and 1-(2-chlorophenyl)-3-phenylprop-2-yn-1-one 2a as model substrates to identify the optimum conditions under argon (Ar) atmosphere and PEG-400 as the solvent (Table 1). For this test, a mixture of selenium (0.25 mmol) and NaBH 4 (2.3 equiv) in PEG-400 (2.0 mL) was stirred at 50 °C, for the in situ generation of the nucleophilic selenium species. This formation was monitored by the color change of the reaction medium, changing from grey to colorless after 0.5 h of stirring. After that, the 2-chlorophenyl ethynyl ketone 2a (0.25 mmol) was added to the reaction vessel and the temperature was raised to 100 °C. In these conditions, the corresponding 2-phenyl-4H-selenochromen-4-one 3a was obtained with 56% gas chromatography (GC) yield in a reaction time of 2 h (Table 1, entry 1). To our delight, increasing the amount of selenium powder to 0.3 mmol provides an improvement in the reaction performance, and the target compound 3a could be achieved with 99% GC yield and an isolated yield of 98% (Table 1, entry 2). Reducing the reaction time to 1 h showed a negative effect to produce the desired Se-flavone 3a, and only 69% GC yield was observed (Table 1, entry 3). We also investigated the effect of the temperature on the reaction yield. However, decreasing the temperature results in lower yields ( Table 1, entries 4-6). Subsequently, other solvents were investigated under similar reaction conditions. Polar solvents such as EtOH, glycerol and water provided product 3a, but in lower yields compared to the use of PEG-400, furthermore, several by-products were observed in gas chromatography mass spectrometry (GC-MS, Table 1, entries 7-9 vs. entry 2). Additionally, aprotic solvents were inefficient in our study and the use of dimethyl sulfoxide (DMSO) or MeCN failed to afford the desired product (Table 1, entries 10-11). Thus, we established the best reaction conditions as being a two steps onepot procedure, which starts with the reaction of selenium 1a (0.3 mmol) and NaBH 4 (0.7 mmol) in PEG-400 Reactions were performed using Se 0 1a, NaBH 4 (2.33 equiv in relation to selenium powder) in 2.0 mL of the solvent at 50 °C under argon for 0.5 h. Then, 0.25 mmol of 2a was added. b Yield determined by GC using n dodecane as an internal standard. c Yield for the isolated product. PEG-400: polyethylene glycol-400; DMSO: dimethyl sulfoxide; NR: no reaction.
(2.0 mL) at 50 °C under Ar for 0.5 h. Then, compound 2a (0.25 mmol) is added into the reaction medium, and the resulting mixture is stirred for an additional 2 h at 100 °C (Table 1, entry 2). With the optimal reaction conditions identified, various 2-chloroaryl ethynyl ketone 2 for this ring closure reaction were investigated and the results are summarized in Scheme 2. In the beginning, the electronic effects on the substituent R 1 of the compound 2 were investigated. Briefly, this method proved to be more efficient for electron-donating substituents when compared with electron-withdrawing substituents in the aromatic ring. When using electron-donating groups in the aromatic ring (2-CH 3 and 4-CH 3 ), the corresponding Se-flavones 3b and 3c were obtained in high yields. Since by using electronwithdrawing atoms in the aromatic ring (2-Cl and 4-Cl), the Se-flavones 3d and 3e were obtained 61 and 76%, respectively. To our delight, the substitution of the aryl group by naphthyl group did not affect the efficiency of this transformation, and the Se-flavone 3f was obtained in an excellent yield of 98%. In the same manner, substrate possessing styryl moiety can also be effectively converted into Se-flavone functionalized with styryl derivative 3g in 74% yield under well-established conditions. Despite employing a reduction system (NaBH 4 /PEG-400), this method proved to be chemoselective, since no reaction in the carbon-carbon double bond of the styryl group was observed. We can also prepare an analogous compound with an aliphatic chain, through the reaction compound 2h, leading to the formation of the product 3h in an excellent yield of 97%. We also explored the R position of the 2-chloro ethynyl ketone 2, substrate 2i (R = Cl) reacted under well-established conditions, affording the desired product 3i in 72% yield.
In addition, we explored the change of the chlorine atom by bromine atom in the substrate 2 under optimized conditions. After 2 h of reaction time, the Se-flavone 3a was also obtained in a good yield, however lower than the chlorine one. This result is consistent because it clearly supports the point about halogen's atom role, in which electron-withdrawing groups improve the reactivity for the aromatic nucleophilic substitution reaction. Considering the straightforward method herein developed, it was successfully applied in the gram-scale synthesis of Se-flavone 3a (4 mmol), leading to 81% yield of the corresponding product.
Encouraged by the Se-flavones results, we next studied the general applicability of the method for the synthesis of thioflavones 4 by the reaction of sulfur powder 1b with a variety of 2-chloroaryl ethynyl ketones 2, as shown in the Scheme 3. Thus, when using the compound 2a, with a neutral group, the 2-phenyl-4H-thiochromen-4-one 4a was afforded in 99% yield. Next, we explore the influence of the substituents on the 2-chlorophenyl ethynyl ketone 2 in the R 1 position, firstly, containing electron-donating groups (2-CH 3 and 4-CH 3 ), and subsequently with electronwithdrawing atoms  in the aromatic ring. In both cases, substituents at the ortho position in the aromatic ring showed better results, when compared with the para position, affording the desired products 4c and 4e in 81 and 83% yield, respectively. According to the groups in the para position in the aromatic ring, the S-flavones 4b and 4d were obtained in 75 and 62% yield, respectively. Similar to the selenium reactivity, 2-naphthyl substituent did not influence the reaction performance, leading to product 4f in an excellent yield of 94%. To our delight, when using the compound 2g, possessing a styryl moiety, the respective S-flavone was obtained in 61% yield and no parallel reaction in the C-C double bond of the styrene group being observed. In addition, changing the substituent R 1 to an aliphatic chain was possible, and the desired product 4h was obtained in 91% yield. We next evaluated the reaction of 2,4-dichlorophenyl ethynyl ketone 2i containing a chlorine atom in the R position, under optimized conditions, and the S-flavone 4i was obtained in a good yield. In counterpart, our attempts to synthesize the Te-flavones derivatives through the reaction between tellurium powder and compound 2a under the optimized conditions have been proved fruitless. In all cases, only reduction by-products derived from substrate 2a were observed by GC-MS analysis. 22 In order to clarify the possible mechanism of the ring closure reaction, a set of control experiments were carried out (Scheme 4). Polyethylene glycols, openchain analogs of crown ethers, are well documented to have the tendency to bind with alkali cations as crown ethers. 23 Our hypothesis is that the PEG-400 could be forming a crown-like structure in this reaction. For this purpose, the 2-chlorophenyl ethynyl ketone 2a was reacted with selenium powder 1a, NaBH 4 using ethanol as solvent (since among the tested solvents, it was able to form Se-flavone in a low yield; Table 1, entry 7) and 1.0 equiv of the 15-crown-5 was added in the reaction. In this condition, the desired product was obtained in 78% yield (Scheme 4, i). This result demonstrates that the presence of a crown-like species favors the formation of Se-flavone, once, in the absence of the crown ether, only 32% yield of the product was obtained (Table 1, entry 7). Similarly, compound 2a was reacted with selenium powder 1a, NaBH 4 using ethanol as solvent and 1.0 equiv of the PEG-400 was added in the reaction. Gratifyingly, the desired product was obtained in 71% yield. This result indicates a possible coordination of the PEG-400, such crown-like, to sodium atom and this complex cause the selenium anion to be more activated, favoring the formation of the Se-flavone 3a (Scheme 4, ii). Scheme 3. Substrate scope to the synthesis of S-flavones 4a-4i. Reactions was performed using S 0 1b (0.3 mmol), NaBH 4 (0.7 mmol, 2.33 equiv in relation to sulfur powder) in 2.0 mL of the solvent at 50 °C under argon for 0.5 h. Then, 0.25 mmol of 2 was added. Yields for isolated products.
Our next endeavor was to identify the nucleophilic species formed by the reaction of the selenium powder with NaBH 4 in PEG-400 solvent, through 77 Se NMR spectroscopy. In 2016, Oliveira et al. 24 reported the identification of the nucleophilic selenium species from the reaction between Se 0 and NaBH 4 in ethanol solvent through a proton-coupled 77 Se NMR experiment. Thus, a similar procedure was performed, and a mixture of selenium powder 1a (0.3 mmol) and NaBH 4 (0.7 mmol) was stirred at 50 °C in PEG-400 for 0.5 h and then a 77 Se NMR spectrum was immediately recorded. It was observed in the selenium spectrum a doublet resonance at -458.1 ppm with a coupling constant of 16.6 Hz, confirming the Se-H bonding ( Figure 2). Oliveira et al. 24 reported a similar result (d = -496 ppm), corroborating the characterization of this nucleophilic selenium species. It is noteworthy that as 15 N and 17 O NMR profile, 25,26 the deshielding of the 77 Se chemical shift in PEG-400 solvent indicates a solvent effect, producing a less effective solvation and, consequently, obtaining a more nucleophilic selenium species. 27 On the basis of the above results and reported literature, 6 a plausible mechanism is proposed (Scheme 5). Firstly, the chalcogen 1 in its elemental form is reduced by NaBH 4 , forming the corresponding sodium hydrochalcogenide (NaHSe or NaHS). In the presence of PEG-400, which behaves as a crown ether, the in situ formation of sodium complex is favored, making the chalcogen nucleophilic species species more active. Subsequently, the nucleophilic  species is added to the carbon-carbon triple bond of the 2-chlorophenyl ethynyl ketone 2 in a Michaeltype addition, giving the intermediate A, that after an intramolecular cyclization result in the chalcogenoflavones 3 or 4 (Scheme 5, path I). The nucleophilic species can also attack the carbon atom attached to the chlorine atom of the compound 2 in an aromatic nucleophilic substitution reaction giving the intermediate B, that after the intramolecular cyclization results in the chalcogenoflavones 3 or 4 (Scheme 5, path II).

Conclusions
In summary, we have developed an efficient method for the synthesis of chalcogenoflavones by ring closure reaction of 2-chlorophenyl ethynyl ketones with nucleophilic chalcogen species. The nucleophiles were generated in situ using PEG-400 as solvent and NaBH 4 as reduction agent. This green and environmentally friendly strategy demonstrates great compatibility with selenium and sulfur derivatives, affording a series of chalcogenoflavones in good to excellent yields. Moreover, PEG-400 proved to be crucial in this method, acting as a green solvent and mainly forming crown-like complexes. In addition, the nucleophilic species was identified by proton-coupled 77 Se NMR experiments, contributing to the understanding about the reactivity of this nucleophilic chalcogen species in organic synthesis.

Supplementary Information
Supplementary information is available free of charge at http://jbcs.sbq.org.br as PDF file.