Remarkable Alkene-to-Alkene and Alkene-to-Alkyne Transfer Reactions of Selenium Dibromide and PhSeBr. Stereoselective Addition of Selenium Dihalides to Cycloalkenes

The original goal of this research was to study stereochemistry of selenium dihalides addition to cycloalkenes and properties of obtained products. Remarkable alkene-to-alkene and alkene-to-alkyne transfer reactions of selenium dibromide and PhSeBr were discovered during this research. The adducts of selenium dibromide with alkenes or cycloalkenes easily exchange SeBr2 with other unsaturated compounds, including acetylenes, at room temperature, in acetonitrile. Similar alkene-to-alkene and alkene-to-alkyne transfer reactions of the PhSeBr adducts with alkenes or cycloalkenes take place. The supposed reaction pathway includes the selenium group transfer from seleniranium species to alkenes or alkynes. It was found that the efficient SeBr2 and PhSeBr transfer reagents are Se(CH2CH2Br)2 and PhSeCH2CH2Br, which liberate ethylene, leading to a shift in equilibrium. The regioselective and stereoselective synthesis of bis(E-2-bromovinyl) selenides and unsymmetrical E-2-bromovinyl selenides was developed based on the SeBr2 and PhSeBr transfer reactions which proceeded with higher selectivity compared to analogous addition reactions of SeBr2 and PhSeBr to alkynes under the same conditions.

In spite of sufficient progress in application of selenium dihalides in synthesis of organoselenium compounds , the stereochemistry of the addition of these novel electrophilic reagents to the double bond was not examined and required careful studying. Besides, the reactions of selenium dichloride and dibromide with cycloalkenes were not described in the literature. However, studying these reactions may make it possible to determine syn or anti process takes place on the addition of selenium dihalides to the double bond. We endeavored to study these reactions and stereochemistry of the addition.

Results and Discussion
We found that reactions of selenium dihalides with cycloalkenes 6a,b proceeded stereoselectively as anti-addition giving hitherto unknown trans,trans-bis (2-halocycloalkyl) selenides 7a,b and 8a,b in quantitative yields (Scheme 2).
Favorable conditions for efficient chemoselective and stereoselective reaction consist in addition of selenium dichloride or dibromide to a solution of cycloalkenes 6a,b at −78 °C in methylene chloride or chloroform. These reactions can be carried out at room temperature; however, the selectivity was decreased in this case, and the formation of some by-products in 2-5% yields was observed.
The reliable evidence of the anti-addition could be obtained by X-ray studying the adducts of selenium dihalides with cycloalkenes. However, the selenides 7a,b and 8a,b are liquid substances. In order to obtain crystals suitable for X-ray analysis, the halogenation of selenides 7a,b and 8a,b, with bromine and sulfuryl chloride, was carried out. We found that the halogenation reaction Stereoselectivity of the addition of selenium halides to unsaturated compounds has been basically studied for acetylenes [45,[50][51][52][53][54][55][56]. The reactions of selenium dichloride and dibromide with acetylene occurred stereoselectively as anti-addition affording (E,E)-bis(2-halovinyl) selenides in high yields [50]. The addition of selenium dihalides to mono-substituted acetylenes, as a rule, proceeded in a regioselective and stereoselective mode, giving anti-Markovnikov products with (E)-stereochemistry [45,51,52].
In spite of sufficient progress in application of selenium dihalides in synthesis of organoselenium compounds , the stereochemistry of the addition of these novel electrophilic reagents to the double bond was not examined and required careful studying. Besides, the reactions of selenium dichloride and dibromide with cycloalkenes were not described in the literature. However, studying these reactions may make it possible to determine syn or anti process takes place on the addition of selenium dihalides to the double bond. We endeavored to study these reactions and stereochemistry of the addition.

Results and Discussion
We found that reactions of selenium dihalides with cycloalkenes 6a,b proceeded stereoselectively as anti-addition giving hitherto unknown trans,trans-bis(2-halocycloalkyl) selenides 7a,b and 8a,b in quantitative yields (Scheme 2).
Favorable conditions for efficient chemoselective and stereoselective reaction consist in addition of selenium dichloride or dibromide to a solution of cycloalkenes 6a,b at −78 • C in methylene chloride or chloroform. These reactions can be carried out at room temperature; however, the selectivity was decreased in this case, and the formation of some by-products in 2-5% yields was observed.
The reliable evidence of the anti-addition could be obtained by X-ray studying the adducts of selenium dihalides with cycloalkenes. However, the selenides 7a,b and 8a,b are liquid substances. In order to obtain crystals suitable for X-ray analysis, the halogenation of selenides 7a,b and 8a,b, with bromine and sulfuryl chloride, was carried out. We found that the halogenation reaction occurred efficiently in hexane at 0 • C. Under these conditions, the reaction was accompanied by precipitation of the target products, which can be easily isolated. Using this method, we obtained trans,trans-dihalo[bis (2-halocycloalkyl)]-λ 4 -selanes 9a,b and 10a,b in 96-99% yields (Scheme 2). The materials suitable for single-crystal X-ray diffraction were obtained from selanes 9a and 10a. The X-ray analysis of selanes 9a and 10a exhibited trans,trans-configuration ( Figure 1). The structural assignment of trans,trans-configuration of selenides 7a,b and 8a,b was also proved by NMR spectroscopy, including NOESY experiments (see Supplementary Materials), which indicated trans-disposition of the protons in the group SeCH-CHCl (no cross-peaks between these protons were observed in 1 H 2D NOESY spectra). Each of compounds 7a,b, 8a,b, 9a,b, and 10a,b has trans,trans-configuration and consist of two diastereomers (Scheme 2), which manifest in the NMR spectra. In the 13 C-NMR spectra of these compounds, every carbon atom appears as two close signals which correspond to two diastereomers.
We failed to obtain pure products from the reaction of selenium dihalides and cyclooctene. We supposed that equilibrium may take place between starting compounds and the product in the reaction of selenium dibromide with cyclooctene or other alkenes. The reaction with cyclooctene was carried out in chloroform or methylene chloride, using a 1:2 ratio of the reagents, at room temperature. However, some amount of cyclooctene always remained in the reaction mixture, and the reaction was not accomplished to the end. Adding acetonitrile to the reaction mixture increased conversion and led to halogenation of the double bond, accompanied by the elemental selenium precipitation (Scheme 3). The selenium precipitation led to the shift of the equilibrium, and the formation of 1,2-dibromocyclooctane (11) (~40% yield) was observed. It was noted in [57] that the adduct of PhSeCl with cyclooctene was found to be relatively unstable and suffered decomposition over the course of several hours at room temperature.
The anchimeric assistance effect, also known as neighboring-group participation, is usually considered mainly as a factor accelerating the rate of nucleophilic substitution reaction. In the present study, we are facing the new property of this effect, consisting in discovery of remarkable selenium dibromide and organylselenenylbromide transfer reactions. Using this method, we obtained trans,trans-dihalo[bis(2-halocycloalkyl)]-λ 4 -selanes 9a,b and 10a,b in 96-99% yields (Scheme 2). The materials suitable for single-crystal X-ray diffraction were obtained from selanes 9a and 10a. The X-ray analysis of selanes 9a and 10a exhibited trans,trans-configuration ( Figure 1). The structural assignment of trans,trans-configuration of selenides 7a,b and 8a,b was also proved by NMR spectroscopy, including NOESY experiments (see Supplementary Materials), which indicated trans-disposition of the protons in the group SeCH-CHCl (no cross-peaks between these protons were observed in 1 H 2D NOESY spectra). Each of compounds 7a,b, 8a,b, 9a,b, and 10a,b has trans,trans-configuration and consist of two diastereomers (Scheme 2), which manifest in the NMR spectra. In the 13 C-NMR spectra of these compounds, every carbon atom appears as two close signals which correspond to two diastereomers.
We failed to obtain pure products from the reaction of selenium dihalides and cyclooctene. We supposed that equilibrium may take place between starting compounds and the product in the reaction of selenium dibromide with cyclooctene or other alkenes. The reaction with cyclooctene was carried out in chloroform or methylene chloride, using a 1:2 ratio of the reagents, at room temperature. However, some amount of cyclooctene always remained in the reaction mixture, and the reaction was not accomplished to the end. Adding acetonitrile to the reaction mixture increased conversion and led to halogenation of the double bond, accompanied by the elemental selenium precipitation (Scheme 3). The selenium precipitation led to the shift of the equilibrium, and the formation of 1,2-dibromocyclooctane (11) (~40% yield) was observed. It was noted in [57] that the adduct of PhSeCl with cyclooctene was found to be relatively unstable and suffered decomposition over the course of several hours at room temperature.
The anchimeric assistance effect, also known as neighboring-group participation, is usually considered mainly as a factor accelerating the rate of nucleophilic substitution reaction. In the present study, we are facing the new property of this effect, consisting in discovery of remarkable selenium dibromide and organylselenenylbromide transfer reactions.    Mixing compounds 5a-5c or 8a,b (or their solutions) with alkene or alkyne leads to the formation of new adducts of selenium dibromide. Possible options of the alkene-to-alkene transfer reactions of selenium dibromide are presented in the Scheme 4.
The selenides 5a-5c and 8a,b are believed to exist in equilibrium with seleniranium cations and the transfer reactions proceed via these intermediate species. The driving force for the generation of seleniranium cations is high anchimeric assistance effect of the selenium atom. It was found that this effect is more than one order of magnitude greater than the effect of the sulfur atom [42]. The anchimeric assistance effects of selenium and sulfur atoms have been quantitatively estimated based on the determination of the absolute and relative rates of nucleophilic substitution of chlorine in 2,6-dichloro-9-selena-and -thiabicyclo[3.3.1]nonanes obtained by the transannular addition of selenium or sulfur dichloride to cis,cis-1,5-cyclooctadiene [42].
We found that a useful reagent to carry out the selenium dibromide transfer reaction is bis(2-bromoethyl) selenide (12). The reactions of selenide 12 with 1-alkenes 1a,c or cyclohexene (a 1:2 molar ratio of selenide 12 and the alkene) were monitored by NMR spectroscopy (Scheme 5). The reagents were mixed in CDCl3 solution and closed NMR tubes were left at room temperature. The 1 H and 13 C-NMR spectra of the samples were recorded at regular intervals. In the case of cyclohexene, a molar ratio of the compounds 12:13:8b was 34:51:15 after 10 days. The appearance of the intensive signal of ethylene (5.36 ppm) is noteworthy. The complete conversion of 12 was observed after ~1 month with the formation of compounds 13 and 8b in approximately equimolar ratio. The reaction of selenide 12 with 1-hexene was faster. The molar ratio of the compounds 12:14a:5a was 18:50:32 after four days, and the complete conversion of 12 was observed after two weeks. Mixing compounds 5a-5c or 8a,b (or their solutions) with alkene or alkyne leads to the formation of new adducts of selenium dibromide. Possible options of the alkene-to-alkene transfer reactions of selenium dibromide are presented in the Scheme 4.
The selenides 5a-5c and 8a,b are believed to exist in equilibrium with seleniranium cations and the transfer reactions proceed via these intermediate species. The driving force for the generation of seleniranium cations is high anchimeric assistance effect of the selenium atom. It was found that this effect is more than one order of magnitude greater than the effect of the sulfur atom [42]. The anchimeric assistance effects of selenium and sulfur atoms have been quantitatively estimated based on the determination of the absolute and relative rates of nucleophilic substitution of chlorine in 2,6-dichloro-9-selena-and -thiabicyclo[3.3.1]nonanes obtained by the transannular addition of selenium or sulfur dichloride to cis,cis-1,5-cyclooctadiene [42].
We found that a useful reagent to carry out the selenium dibromide transfer reaction is bis(2-bromoethyl) selenide (12). The reactions of selenide 12 with 1-alkenes 1a,c or cyclohexene (a 1:2 molar ratio of selenide 12 and the alkene) were monitored by NMR spectroscopy (Scheme 5). The reagents were mixed in CDCl 3 solution and closed NMR tubes were left at room temperature. The 1 H and 13 C-NMR spectra of the samples were recorded at regular intervals. In the case of cyclohexene, a molar ratio of the compounds 12:13:8b was 34:51:15 after 10 days. The appearance of the intensive signal of ethylene (5.36 ppm) is noteworthy. The complete conversion of 12 was observed after 1 month with the formation of compounds 13 and 8b in approximately equimolar ratio. The reaction of selenide 12 with 1-hexene was faster. The molar ratio of the compounds 12:14a:5a was 18:50:32 after four days, and the complete conversion of 12 was observed after two weeks.
Molecules 2020, 25    Alkynes were involved in the selenium dibromide transfer reactions. The reactions of selenide 12 with 1-hexyne and 3-hexyne (a 1:2 molar ratio of selenide 12 and the alkyne) were also monitored by NMR spectroscopy at room temperature, using CDCl3 solutions in closed NMR tubes (Scheme 6). The reactions led to unsymmetrical selenides 15a,b and 16a,b. The complete conversion of selenide 12 was observed after ~15 days in the reaction with 3-hexyne (~90% yield of selenide 16a). For the same period of time, the conversion of compound 12 was 60% (~55% yield of selenide 15a) in the reaction with 1-hexyne.
We found that the transfer reactions are considerably accelerated by using acetonitrile as a solvent. Acetonitrile is a polar aprotic solvent with a high dielectric constant (~38), exhibiting the ability to accelerate ionic reactions. The reactions of selenide 12 with terminal (1-hexyne and 1-heptyne) and internal (3-hexyne and 4-octyne) alkynes were carried out in acetonitrile in closed flasks, using a 1:3 molar ratio of selenide 12 and the alkyne (Scheme 6). The complete conversion of selenide 12 and the formation of products 16a,b in 91-93% yield were observed in the reactions with internal alkynes (3-hexyne or 4-octyne) after overnight stirring (14 h) at room temperature. In the case of terminal alkynes (1-hexyne and 1-heptyne), the reactions gave compound 15a,b in 90-92% yield after 30 h of stirring at room temperature.
With the goal to purify the product, selenide 16b was subjected to column chromatography (Al2O3, hexane → hexane/chloroform 4:1). However, instead of compound 16b, hydroxyl derivative 17 was isolated in 70% yield (Scheme 7). Obviously, the bromine atom in compound 16b was substituted by the hydroxyl group due to traces of moisture on the alumina. It is worth noting that the bromine atom in the 2-bromoethylselenide group is very reactive with respect to nucleophilic substitution due to high anchimeric assistance effect of the selenium atom [42]. Alkynes were involved in the selenium dibromide transfer reactions. The reactions of selenide 12 with 1-hexyne and 3-hexyne (a 1:2 molar ratio of selenide 12 and the alkyne) were also monitored by NMR spectroscopy at room temperature, using CDCl 3 solutions in closed NMR tubes (Scheme 6). The reactions led to unsymmetrical selenides 15a,b and 16a,b. The complete conversion of selenide 12 was observed after~15 days in the reaction with 3-hexyne (~90% yield of selenide 16a). For the same period of time, the conversion of compound 12 was 60% (~55% yield of selenide 15a) in the reaction with 1-hexyne.
We found that the transfer reactions are considerably accelerated by using acetonitrile as a solvent. Acetonitrile is a polar aprotic solvent with a high dielectric constant (~38), exhibiting the ability to accelerate ionic reactions. The reactions of selenide 12 with terminal (1-hexyne and 1-heptyne) and internal (3-hexyne and 4-octyne) alkynes were carried out in acetonitrile in closed flasks, using a 1:3 molar ratio of selenide 12 and the alkyne (Scheme 6). The complete conversion of selenide 12 and the formation of products 16a,b in 91-93% yield were observed in the reactions with internal alkynes (3-hexyne or 4-octyne) after overnight stirring (14 h) at room temperature. In the case of terminal alkynes (1-hexyne and 1-heptyne), the reactions gave compound 15a,b in 90-92% yield after 30 h of stirring at room temperature.
With the goal to purify the product, selenide 16b was subjected to column chromatography (Al 2 O 3 , hexane → hexane/chloroform 4:1). However, instead of compound 16b, hydroxyl derivative 17 was isolated in 70% yield (Scheme 7). Obviously, the bromine atom in compound 16b was substituted by the hydroxyl group due to traces of moisture on the alumina. It is worth noting that the bromine atom in the 2-bromoethylselenide group is very reactive with respect to nucleophilic substitution due to high anchimeric assistance effect of the selenium atom [42].
Taking into account that selenides containing 2-bromoethyl moiety may decompose on purification by column chromatography, the bromine atom was substituted by methoxy group in compound 15a by reaction with methanol in the presence of NaHCO 3 at room temperature (Scheme 7). The target product, (18), was isolated in 71% yield by column chromatography (Al 2 O 3 , hexane → hexane/chloroform 9:1). The reactions (Schemes 6 and 7) demonstrate the possibility for selective preparation of unsymmetrical selenides of the type 15a,b-18 based on selenium dibromide transfer reactions.
The reaction of selenide 12 with 4-octyne (a 1:2 molar ratio) was also monitored by NMR spectroscopy at room temperature, using CD 3  Taking into account that selenides containing 2-bromoethyl moiety may decompose on purification by column chromatography, the bromine atom was substituted by methoxy group in compound 15a by reaction with methanol in the presence of NaHCO3 at room temperature (Scheme 7). The target product, (1E)-1-bromo-2-[(2-methoxyethyl)selanyl]hex-1-ene (18), was isolated in 71% yield by column chromatography (Al2O3, hexane → hexane/chloroform 9:1). The reactions (Schemes 6 and 7) demonstrate the possibility for selective preparation of unsymmetrical selenides of the type 15a,b-18 based on selenium dibromide transfer reactions. The reaction of selenide 12 with 4-octyne (a 1:2 molar ratio) was also monitored by NMR spectroscopy at room temperature, using CD3CN solution in a closed NMR tube. The decrease of the content of starting selenide 12 with proportional increasing the contents of compound 16b and bis[(E)-2-bromo-1-propyl-1-pentenyl] selenide (19b) was observed. The formation of symmetrical selenide 19b occurred by the reaction of compound 16b with 4-octyne. After 145 h, the molar ratio of the compounds 12:16b:19b was 34:51:15 (see Supplementary Materials).
When the reactions were carried out in open-air flasks, there was a better possibility for ethylene evolution compared to closed NMR tubes, and the reactions proceeded faster in the open-air flasks. The use of excess alkyne with respect to bis(2-bromoethyl) selenide was also useful for shifting the equilibrium. When the reaction of selenide 12 with 4-octyne (a 1:3.3 molar ratio) was carried out in a closed flask in acetonitrile at room temperature, pure selenide 19b was obtained in quantitative yield in 90 h (Figure 2). Taking into account that selenides containing 2-bromoethyl moiety may decompose on purification by column chromatography, the bromine atom was substituted by methoxy group in compound 15a by reaction with methanol in the presence of NaHCO3 at room temperature (Scheme 7). The target product, (1E)-1-bromo-2-[(2-methoxyethyl)selanyl]hex-1-ene (18), was isolated in 71% yield by column chromatography (Al2O3, hexane → hexane/chloroform 9:1). The reactions (Schemes 6 and 7) demonstrate the possibility for selective preparation of unsymmetrical selenides of the type 15a,b-18 based on selenium dibromide transfer reactions. R = C 4 H 9 (a), C 5  The reaction of selenide 12 with 4-octyne (a 1:2 molar ratio) was also monitored by NMR spectroscopy at room temperature, using CD3CN solution in a closed NMR tube. When the flasks were equipped with a tube containing drying agent (CaCl2) in order to allow ethylene releasing without moisture access in the flask, the complete conversion of 12 in the reaction with internal alkynes was observed after overnight stirring, and pure divinyl selenides 19a,b were obtained in quantitative yields (Scheme 8).   When the flasks were equipped with a tube containing drying agent (CaCl2) in order to allow ethylene releasing without moisture access in the flask, the complete conversion of 12 in the reaction with internal alkynes was observed after overnight stirring, and pure divinyl selenides 19a,b were obtained in quantitative yields (Scheme 8). In the case of 1-hexyne, 40 h stirring at room temperature was required in order to complete the reaction and to produce selenide 20 in a quantitative yield (Scheme 8). The reaction occurred in a regioselective and stereoselective mode affording anti-Markovnikov product of (E,E)-stereochemistry.
The fastest version of these reactions consisted in carrying out the process with inert gas bubbling (argon or nitrogen) into the mixture, in order to remove ethylene. The reactions of selenide 12 with excess internal alkynes (3-hexyne and 4-octyne) in acetonitrile were accomplished in 1 h, and the reaction with 1-hexyne was completed in 2 h at room temperature. However, the yields (90-96%) and purity (90-95%) of the crude products 19a,b and 20 were slightly lower compared to those achieved by reactions proceeding in flasks (Scheme 8).
We also studied the direct reactions of selenium dibromide with 3-hexyne and 4-octyne under the same conditions as in Scheme 8. These reactions proceeded in acetonitrile faster than the Scheme 8. The alkene-to-alkyne transfer reactions of selenium dibromide.
In the case of 1-hexyne, 40 h stirring at room temperature was required in order to complete the reaction and to produce selenide 20 in a quantitative yield (Scheme 8). The reaction occurred in a regioselective and stereoselective mode affording anti-Markovnikov product of (E,E)-stereochemistry.
The fastest version of these reactions consisted in carrying out the process with inert gas bubbling (argon or nitrogen) into the mixture, in order to remove ethylene. The reactions of selenide 12 with excess internal alkynes (3-hexyne and 4-octyne) in acetonitrile were accomplished in 1 h, and the reaction with 1-hexyne was completed in 2 h at room temperature. However, the yields (90-96%) and purity (90-95%) of the crude products 19a,b and 20 were slightly lower compared to those achieved by reactions proceeding in flasks (Scheme 8).
We also studied the direct reactions of selenium dibromide with 3-hexyne and 4-octyne under the same conditions as in Scheme 8. These reactions proceeded in acetonitrile faster than the reactions of compound 12 with 3-hexyne and 4-octyne but less selectively and the formation of some by-products was observed. Changing acetonitrile for methylene chloride as a solvent and decreasing temperature of the reaction to 0 • C allowed to increase the selectivity of the reactions of selenium dibromide with 3-hexyne and 4-octyne and to obtain pure products 19a,b in near quantitative yields.
We supposed that similar transfer reactions may also occur with addition products of organylselenenyl bromides to alkenes. We obtained 2-bromoethyl, 2-bromocyclopentyl and 2-bromocyclohexyl phenyl selenides 21-23 by addition of phenylselenenyl bromide to ethylene, cyclopentene and cyclohexene (Scheme 9) and studied the PhSeBr transfer reactions of these reagents with some alkenes, cycloalkenes, and alkynes. Indeed, compounds 21-23 participated in the phenylselenenyl bromide transfer reactions, which proceeded smoothly at room temperature in acetonitrile. Preliminary results demonstrate that the PhSeBr transfer reactions occur faster compared to the SeBr 2 transfer reactions with the same alkenes or alkynes under the same conditions. reactions of compound 12 with 3-hexyne and 4-octyne but less selectively and the formation of some by-products was observed. Changing acetonitrile for methylene chloride as a solvent and decreasing temperature of the reaction to 0 °C allowed to increase the selectivity of the reactions of selenium dibromide with 3-hexyne and 4-octyne and to obtain pure products 19a,b in near quantitative yields. We supposed that similar transfer reactions may also occur with addition products of organylselenenyl bromides to alkenes. We obtained 2-bromoethyl, 2-bromocyclopentyl and 2-bromocyclohexyl phenyl selenides 21-23 by addition of phenylselenenyl bromide to ethylene, cyclopentene and cyclohexene (Scheme 9) and studied the PhSeBr transfer reactions of these reagents with some alkenes, cycloalkenes, and alkynes. Indeed, compounds 21-23 participated in the phenylselenenyl bromide transfer reactions, which proceeded smoothly at room temperature in acetonitrile. Preliminary results demonstrate that the PhSeBr transfer reactions occur faster compared to the SeBr2 transfer reactions with the same alkenes or alkynes under the same conditions.

PhSe Br
PhSeBr c-C 5  The reactions of selenide 21 with excess 1-hexyne, 3-hexyne, and 4-octyne were carried out by stirring the reagents in acetonitrile overnight at room temperature in the flasks equipped with a tube containing a drying agent (CaCl2). The reactions proceeded in a stereoselective mode, as anti-addition giving products with (E)-stereochemistry 24 and 25a,b in quantitative yields (Scheme 10). The regioselective formation of anti-Markovnikov adduct 24 was observed in the reaction with 1-hexyne. The addition of PhSeBr to 1-hexyne under the same conditions in acetonitrile was less selective compared to the PhSeBr transfer reaction giving some by-products (5-7%) along with the The reactions of selenide 21 with excess 1-hexyne, 3-hexyne, and 4-octyne were carried out by stirring the reagents in acetonitrile overnight at room temperature in the flasks equipped with a tube containing a drying agent (CaCl 2 ). The reactions proceeded in a stereoselective mode, as anti-addition giving products with (E)-stereochemistry 24 and 25a,b in quantitative yields (Scheme 10). The regioselective formation of anti-Markovnikov adduct 24 was observed in the reaction with 1-hexyne. The addition of PhSeBr to 1-hexyne under the same conditions in acetonitrile was less selective compared to the PhSeBr transfer reaction giving some by-products (5-7%) along with the target compound 24. The reactions of selenide 21 with excess 1-hexyne, 3-hexyne, and 4-octyne were carried out by stirring the reagents in acetonitrile overnight at room temperature in the flasks equipped with a tube containing a drying agent (CaCl2). The reactions proceeded in a stereoselective mode, as anti-addition giving products with (E)-stereochemistry 24 and 25a,b in quantitative yields (Scheme 10). The regioselective formation of anti-Markovnikov adduct 24 was observed in the reaction with 1-hexyne. The addition of PhSeBr to 1-hexyne under the same conditions in acetonitrile was less selective compared to the PhSeBr transfer reaction giving some by-products (5-7%) along with the target compound 24. Surprisingly, adduct 22 can also serve as the efficient PhSeBr transfer reagent liberating cyclopentene (bp 44 • C), which is evaporated during 40 h stirring (Scheme 10).

PhSe
The alkene-to-alkene PhSeBr transfer reactions also occurred easily at room temperature in acetonitrile. Examples of the alkene-to-alkene PhSeBr transfer (the reactions of selenide 21 with cycloalkenes and 1-hexene) are presented in Scheme 11. The reactions (Schemes 10 and 11) proceeded very fast (~1 h) when inert gas (argon or nitrogen) was slowly bubbled into the mixture in order to remove ethylene.
Two possible pathways of the SeBr 2 or PhSeBr transfer reactions can be discussed. If the selenides containing the 2-bromoethyl moiety exist in some equilibrium with the starting compounds, generated SeBr 2 or PhSeBr may add to other unsaturated compound (pathway A is depicted in Scheme 12 on the example of compounds 12, 21, and 1-hexyne). Another reaction pathway is based on the assumption that the intermediates involved in the transfer reactions are seleniranium species (pathway B, Scheme 12). The alkene-to-alkene PhSeBr transfer reactions also occurred easily at room temperature in acetonitrile. Examples of the alkene-to-alkene PhSeBr transfer (the reactions of selenide 21 with cycloalkenes and 1-hexene) are presented in Scheme 11. The reactions (Schemes 10 and 11) proceeded very fast (~1 h) when inert gas (argon or nitrogen) was slowly bubbled into the mixture in order to remove ethylene.

General Information
X-ray diffraction experiments were carried out on a Bruker D8 Venture Photon 100 CMOS diffractometer with Mo-K α radiation (λ = 0.71073 Å). X-ray crystallographic data for compounds 9a (CCDC 1965943) and 10a (CCDC 1502244) are shown in Supplementary Materials. These data can be obtained free of charge via http://www.ccdc.cam.ac.uk/conts/retrieving.html. We recorded 1 H (400.1 MHz) and 13 C (100.6 MHz) NMR spectra on a Bruker DPX-400 spectrometer, in 5-10% solution in CDCl 3 . Of note, 1 H and 13 C chemical shifts (δ) are reported in parts per million (ppm), relative to the residual solvent peak of CDCl 3 (δ = 7.27 and 77.00 ppm in 1 H-and 13 C-NMR, respectively). Mass spectra were recorded on a Shimadzu GCMS-QP5050A, with electron impact (EI) ionization, at 70 eV. Elemental analysis was performed on a Thermo Flash EA 1112 Elemental Analyzer (USA). 7a,b-10a,b trans,trans-Bis(2-chlorocyclopentyl) selenide (7a). Typical Procedure. A solution of selenium dichloride (2.5 mmol) in CH 2 Cl 2 (20 mL) was added dropwise to a cooled (−78 • C) solution of cyclopentene (0.347 g, 5.1 mmol) in CH 2 Cl 2 (30 mL). The mixture was stirred at −78 • C for 4 h and 1 h at room temperature. The solvent was removed on a rotary evaporator. The residue was dried in vacuum, giving the product as a yellowish oil. Yield: 0.715 g (quantitative). 1  trans,trans-Bis(2-bromocyclohexyl) selenide (8b) was obtained as a light yellow oil (1.008 g) in quantitative yield from selenium dibromide and cyclohexene under the same conditions as compound 7a. 1  trans,trans-Dichloro[bis(2-chlorocyclopentyl)]-λ 4 -selane (9a). A solution of sulfuryl chloride (0.27 g, 2 mmol) in hexane (10 mL) was added to a cooled to −0 • C solution of selenide 7a (0.572 g, 2 mmol) in hexane (15 mL) and the mixture was stirred at −0 • C for 8 h and allowed to warm to room temperature. The precipitate was filtered off and dried in vacuum to give the product as a white powder, mp = 114-115 • C. Yield: 0.685 g (96%). The crystals suitable for single-crystal X-ray diffraction were obtained by recrystallization from chloroform. 1  trans,trans-Dibromo[bis(2-bromocyclopentyl)]-λ 4 -selane (10a). A solution of bromine (0.32 g, 2 mmol) in hexane (10 mL) was added to a cooled to 0 • C solution of selenide 10a (0.75 g, 2 mmol) in hexane (15 mL), and the mixture was stirred at 0 • C for 4 h and allowed to warm to room temperature. The precipitate was filtered off and dried in vacuum, to give the product as a yellow powder, mp = 101-102 • C. Yield: 1.06 g (99%). The crystals suitable for single-crystal X-ray diffraction were obtained by recrystallization from chloroform. 1

Synthesis of Compounds 17-26
(4E)-4-Bromo-5-[(2-hydroxyethyl)selanyl]oct-4-ene (17). A solution of 4-octyne (0.11 g, 1 mmol) in MeCN (1 mL) was added to a solution of selenide 12 (0.1 g, 0.34 mmol) in MeCN (1.5 mL), and the mixture was stirred in a 200 mL round-bottomed closed flask overnight (14 h) at room temperature. The solvent was removed by a rotary evaporator. The residue contained compounds 16b (0.117 g, 91% yield) and 19b in a 20:1 molar ratio (the NMR data). The residue was subjected to column chromatography (Al 2 O 3 , hexane → hexane/chloroform 4:1). Instead of compound 16b, hydroxyl derivative 17 (0.068 g, 70% yield based on compound 16b) was isolated as a colorless oil. 1 (18). A solution of 1-hexyne (0.082 g, 1 mmol) in MeCN (1 mL) was added to a solution of selenide 12 (0.1 g, 0.34 mmol) in MeCN (1.5 mL), and the mixture was stirred in a 200 mL round-bottomed closed flask for 30 h at room temperature. The solvent was removed by a rotary evaporator. The residue contained compounds 15a (0.107 g, 90% yield) and unconverted selenide 12 in a 9:1 molar ratio (the NMR data). The residue was dissolved in chloroform (1 mL) and methanol (0.3 mL) and NaHCO 3 (0.084 g, 1 mmol) were added. The mixture was stirred for 24 h at room temperature and then filtered; solvents were removed by a rotary evaporator. The residue was subjected to column chromatography (Al 2 O 3 , hexane → hexane/chloroform 9:1), giving compound 18 (0.065 g, 71% yield based on compound 15a) as a colorless oil. 1  Bis(E-2-bromo-1-ethyl-1-butenyl) selenide (19a). A solution of 3-hexyne (0.14 g, 1.7 mmol) in MeCN (1 mL) was added to a solution of selenide 12 (0.1 g, 0.34 mmol) in MeCN (1 mL). The mixture was stirred in a 100 mL round-bottomed flask equipped with a tube containing a drying agent (CaCl 2 ) overnight (18 h) at room temperature. The solvent was removed by a rotary evaporator, and the residue was dried in vacuum, giving compound 19a (0.137 g) as a light-yellow oil in quantitative yield. 1  Bis(E-2-bromo-1-propyl-1-pentenyl) selenide (19b) was obtained in quantitative yield as a light-yellow oil (0.156 g) from 4-octyne and selenide 12 in acetonitrile under the same conditions as compound 19a. 1  Bis[(1E)-1-bromohex-1-en-2-yl] selenide (20). A solution of 1-hexyne (0.137 g, 1.67 mmol) in MeCN (1 mL) was added to a solution of selenide 12 (0.065 g, 0.22 mmol) in MeCN (1 mL). The mixture was stirred in a 100 mL round-bottomed flask equipped with a tube containing a drying agent (CaCl 2 ) for 40 h at room temperature. The solvent was removed by a rotary evaporator, and the residue was dried in vacuum, giving compound 20 (0.089 g) as a light-yellow oil in quantitative yield. 1 (21). Dry ethylene was bubbled to a flask containing CH 2 Cl 2 (10 mL) with stirring for 10 min at room temperature. A solution of PhSeBr [(4 mmol), prepared from Ph 2 Se 2 (0.624 g, 2 mmol) and bromine (0.320 g, 2 mmol) in CH 2 Cl 2 (15 mL)] was added dropwise to the flask for 30 min with stirring. The ethylene bubbling (~30 mL/min) was continued during the PhSeBr addition and 40 min after the addition. The mixture was stirred additionally for 1 h at room temperature and filtered. The solvent was removed in vacuum, giving selenide 21 (1.046 g, 99% yield) as a light-yellow oil. 1 (22). A solution of PhSeBr [(2 mmol), prepared from Ph 2 Se 2 (0.312 g, 1 mmol) and bromine (0.16 g, 1 mmol) in CH 2 Cl 2 (10 mL)] was added dropwise to a solution of cyclopentene (0.15 g, 2.2 mmol) in CH 2 Cl 2 (10 mL), at such a rate that discoloration of the reaction mixture occurred after each drop. The mixture was stirred for 1 h at room temperature, and the solvent was removed on a rotary evaporator. The residue was dried in vacuum, giving the product as a light-yellow oil. Yield: 0.608 g (quantitative). 1   quantitative yields was developed based on the SeBr 2 and PhSeBr transfer reactions, which proceeded with higher selectivity compared to analogous addition reactions of SeBr 2 and PhSeBr to alkynes under the same conditions. High selectivity, quantitative yields, mild-reaction conditions, and very simple work-up procedures are important features of this approach.
Supplementary Materials: The following are available online, synthesis of compounds 12-16a,b and monitoring data, Figure S1, examples of 1 H-and 13 C-NMR spectra of the obtained compounds, X-ray crystallographic data of compounds 9a and 10a.