Stereoselective synthesis of alkyl styryl selenides in one-pot: a straightforward approach by in situ dialkyl diselenide formation under transition metal-free conditions

Abstract

A simple procedure for the synthesis of alkyl styryl selenides was developed. This methodology involves the in situ generation of selenolate anions by a one-pot and three-step protocol where a nucleophilic substitution between KSeCN and alkyl halides affords the corresponding alkyl selenocyanate in the first step, giving dialkyl diselenide by a further K₃PO₄-assisted reaction. Subsequent reduction yields the alkyl selenolate anions which react with substituted (E,Z)-β-styryl halides to afford the corresponding vinyl selenides with retention of stereochemistry and from moderate to excellent yields. This procedure does not require a catalyst, proceeds under an air atmosphere and within a short reaction time, and is free from diselenide and malodorous and air-sensitive alkyl selenols as starting materials.

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Introduction

Selenium containing compounds have attracted considerable attention due to their versatility in organic synthetic procedures and their potential biological activities.¹ Among them, vinyl selenides are important synthetic intermediates in organic chemistry.^2,3 In these molecules, it is possible to combine known reactions of organoselenium compounds with the ability to form new C–C bonds involving the double bond of the vinyl selenides, since the selenium atom is able to stabilize both negative and positive charges. Due to the relevance of vinyl selenides, numerous synthetic methods have been reported for their synthesis such as Wittig-type reactions,⁴ addition of nucleophilic and electrophilic selenium reagents to alkynes⁵ and allenes,⁶ use of acetylenic selenides and direct substitution of a halogen atom on the double bond by selenolate anions.^2,3,7 In many cases, the yield and stereoselectivity of the reactions are controlled by the use of transition-metal complexes such as Pd,⁸ Cu,⁹ Rh,¹⁰ La¹¹ and Ni¹² among others.¹³ Nevertheless, the requirement of specific ligands and the air sensitivity of some catalytic systems involve a common limitation to their industrial and large-scale application. Some few examples of the preparation of vinyl selenides from diaryldiselenide without using transition-metal catalyst have also been reported.^5a,14 However, this last methodology is restricted to the commercial and synthetic availability of diaryldiselenide reagent.

Recently we have reported a simple methodology for the stereoselective synthesis of alkyl styryl sulfides by a one-pot base-assisted procedure without transition metals. This reaction proceeds under mild conditions at short times, and is free from malodorous and air-sensitive alkyl thiols.¹⁵

Considering these previous results, we envisage the possibility of extending the methodology above described to the synthesis of alkyl styryl selenides, using potassium selenocyanate (KSeCN, 1) as a selenium source. As in the case of KSCOMe, KSeCN, it is easy to handle, commercially available and widely used as a selenium source for the synthesis of organic seleno- and isoselenocyanates, whose moderated reactivity allows easy interconversion to other interesting functional groups.¹⁶ We now report a simple and convenient synthetic procedure for the preparation of alkyl arylvinyl selenides, by employing KSeCN.

Results and discussion

First, we explored the reactivity of KSeCN (1) towards different organic halides, in DMF as a solvent (Scheme 1). After 5 min at room temperature, salt 1 reacted with benzyl bromide affording exclusively the substitution product benzyl selenocyanate. On the other hand, its reactivity was moderate towards primary alkyl halides, such as n-octyl bromide (2). Thus, there was no reaction at room temperature after 5 min, while the yield of n-octylselenocyanate was quantitative at 100 °C. Finally, (E)-β-bromostyrene (3a) did not react with 1 after 2 h at 100 °C.

Scheme 1 Reactivity of KSeCN (1).

These results suggest the possibility of generating the benzyl or alkylselenolate anion (RSe⁻) from the corresponding organic selenocyanate in the presence of styryl bromide. Furthermore, these selenolate anions could react towards vinyl halides, such as alkane thiolate analogues,¹⁵ giving the corresponding alkyl vinyl selenides in a one-pot procedure. We selected n-octyl bromide (2) as an alkylating reagent, (E)-β-bromostyrene (3a) as a model substrate, and the polar DMF as a solvent to start this study.

The organic selenocyanates (RSeCN) can be reduced to the corresponding RSe⁻ anions, NaBH₄ being one of the reagents more frequently used for this purpose.¹⁷ With the aim of generating selenolate anions, we performed the reaction in the presence of this reduction reagent. Thus, a mixture of 1, 2 and 3a was allowed to react at 100 °C for 10 min in DMF as a solvent, in order to assure that the S_N2 reaction between 1 and 2 was completed. After that time, NaBH₄ was added and the reaction mixture was heated at 100 °C for one hour under nitrogen atmosphere. Fortunately, n-octyl styryl selenide (4a) was detected in 49% yield, together with the symmetric selenoether di-n-octyl selenide (5) (Scheme 2). This by-product probably comes from the nucleophilic attack of n-octyl selenolate anion at carbon 1 of n-octylselenocyanate.¹⁸ To avoid this secondary reaction we increased the amount of 1, 2 and NaBH₄ in relation to 3a, and product 4 was obtained in 57% yield but with a lower amount of compound 5 and a new secondary product 6, indentified as di-n-octyldiselenide (Scheme 2, Table 1, entry 1). Product 6 can proceed from a nucleophilic attack of the in situ generated n-octyl selenolate anion at the selenium electrophilic atom of n-octylselenocyanate.¹⁷

Scheme 2 One-pot synthesis of n-octyl styryl selenide (4a).

Table 1 Solvent screening for the one-pot synthesis of selenide 4a

Entry	Solvent^a	Yield% 4a^b
a The reaction was performed by using: 0.25 mmol of 3a and a ratio of 1 : 1.3 : 1.2 : 2.6 for 3a, 1, 2 and NaBH4 respectively.b Quantified by GC with internal standard. nd: no detected.c Together with ca. 20% of a mixture of 5 and 6 with a ratio 5/6 around 70/30–80/20.
1	DMF	57^c
2	DMSO	56^c
3	MeCN	48^c
4	NMP	59^c
5	Toluene	nd
6	Dioxane	nd
7	THF	nd
8	Ethanol	<5
9	PEG 300	53^c

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After these preliminary results, we performed a screening of solvents whose results are summarized in Table 1. Thus, the best results were obtained in the non protic solvent such as DMF, DMSO, MeCN and NMP, with 50–60% yields of 4a (Table 1, entries 1–4). Conversely, the non polar toluene, dioxane and THF were not effective for this reaction (Table 1, entries 5–7).

Finally, only traces of 4a were obtained when the reaction was performed in the polar protic ethanol and its yield increased to 53% in PEG 300 (Table 1, entries 8 and 9). The secondary products 5 and 6 were found in most of the solvents used, as minor by-products. Also, the amount of n-octylselenocyanate, formed in the first step, was detected in traces. It is important to mention that the reduction of one equivalent of 6 by two equivalents of NaBH₄ would generate two equivalents of n-octylselenolate anions. On the other hand, the presence of the secondary product 5 in moderate concentration is a disadvantage since its generation implies a minor availability of selenolate anions to react with styryl bromide 3a.

Then, for a detailed study of the generation of n-octylselenolate anion from the corresponding alkylselenocyanate, diverse bases and reducing agents were screened (Table 2). Using NaH as a reducing species, the yield of 4a dropped to 22% in comparison with that of the reaction performed employing NaBH₄ (Table 2, entries 1 and 2). When t-BuOK was used as a base, instead of 4a, a new product was obtained for the first time, indentified as n-octyl(2-phenylethynyl)selenide (Table 2, entry 3). This product was not detected in the previous reactions and its mechanistic study and synthetic scope are currently being developed. In the presence of other bases such as K₃PO₄, KOH, K₂CO₃ and DABCO, traces of 4a were only detected in better conditions (Table 2, entries 4–7). Although these results did not seem to be encouraging, a detailed analysis of GC profiles showed that 6 was afforded as main product when the reaction was conducted in the presence of K₃PO₄ (Table 2, entry 4). In the presence of K₂CO₃, 6 was also obtained; yet, its yield was notably lower than with K₃PO₄ (Table 2, entry 6). The fact that neither 5 nor 6 were obtained in the presence of DABCO allows excluding the participation of DMF as reducing agent for the formation of selenolate anions from the alkyl selenocyanate. The generation of selenolate anions was ascribed to base-catalysed hydrolysis of n-octyl selenocyanate due to the presence of traces of water in DMF.¹⁹

Table 2 Screening of reducing agents and bases

Entry	Red./base^a	4a^b%	n-Oct-SeCN^b%	(5 + 6)%, ratio 5/6^b
a The reaction was performed by using: 0.25 mmol of 3 and a ratio of 1 : 1.3 : 1.2 : 2.6 for 3a, 1, 2 and reducing agent or base, respectively.b Quantified by GC with internal standard; nd: not detected.c Phenylethynyl n-octyl selenide was also obtained in 33% yield.
1	NaBH4	57	6	17, 70/30
2	NaH	22	nd	52, 100/0
3^c	t-BuOK	<5	<5	65, 32/68
4	K3PO4	<5	<5	92, 0/100
5	KOH	<5	nd	98, 10/90
6	K2CO3	nd	73	26, 0/100
7	DABCO	nd	98	nd

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In addition, the oxidation of selenolate anions to afford 6 can be excluded since these reactions were carried-out under an inert atmosphere and formation of 6 is only attributed to the presence of K₃PO₄ (Table 2, entry 4).

The results summarized in Table 2, evidence that under certain conditions it is possible to obtain only 6 without any trace of 5. As mentioned above, alkyl selenolate anions can be generated by reduction of the corresponding dialkyldiselenide. In consequence, we planned to perform the same reaction in three steps: first, the S_N2 reaction between alkyl halide and KSeCN to afford alkylselenocyanate; then, the K₃PO₄-assisted generation of dialkyldiselenide from the previously obtained alkylselenocyanate; finally, the reduction of dialkyldiselenide to render alkylselenolate anion and the nucleophilic substitution of vinyl halides.

Scheme 3 shows the general strategy for the one-pot synthesis of alkyl vinyl selenide in three steps and Table 3 summarizes the results for the optimization conditions. First, we performed the reaction with a ratio of 1 : 1.3 : 1.2 : 1.3 : 1.3 for 3a, 1, 2, K₃PO₄ and NaBH₄ as a reducing agent, and the reaction time for the three previously mentioned steps was 10 min, 1 h and 1 h, respectively. Under these conditions compound 4a was obtained in 66% yield (Table 3, entry 1). To improve the performance of the reaction, the equivalents of KSeCN and alkyl halide were increased to 1.5 and 1.35 relative to 3a, affording an increase of vinyl selenide 4a to 75% yield. Furthermore, when the reaction was conducted under air, the yield was not modified (76%) (Table 3, entries 2 and 3). The third step was performed for 2 h and the yield of 4a increased to 80% quantified by GC and 71% isolated by column chromatography (Table 3, entry 4).

Scheme 3 One-pot three-step synthesis of alkyl vinyl selenides.

Table 3 Optimization for the one-pot synthesis of 4a^a

Entry	Ratio^a 3a : 1 : 2 : K3PO4 : NaBH4	Cond.	% Yield^b 4a
a The reaction was performed by using: 0.25 mmol of 3 and a ratio as indicated in the second column, in 2 mL of DMF.b Quantified by GC with the internal standard.c Isolated yield.
1	1.0 : 1.3 : 1.2 : 1.3 : 1.3	1 h, N2	66
2	1.0 : 1.5 : 1.35 : 1.5 : 1.5	1 h, N2	75
3	1.0 : 1.5 : 1.35 : 1.5 : 1.5	1 h, air	76
4	1.0 : 1.5 : 1.35 : 1.5 : 1.5	2 h, air	80 (71)^c

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Next, we explored the scope and limitations of this methodology for the synthesis of alkyl styryl selenides with a variety of substituted styryl and alkyl halides (Table 4). In general, for different primary alkyl halides, including methyl iodide, 3-butenyl bromide, n-octyl bromide and cyclohexylmethyl bromide, the corresponding alkyl styryl selenides were obtained from moderate to excellent (71–95%) yields; n-octyl tosylate afforded 4a in good isolated yield (Table 4, entries 1–5). The secondary cyclohexyl bromide afforded only 21% yield of 4e (Table 4, entry 6) while the tertiary tert-butyl chloride failed to render the alkane selenolate anion. Both results are clear evidences of a competitive E₂ reaction, and this is the main limitation of the present methodology. Benzyl bromide and p-fluorobenzyl chloride afforded the corresponding selenides 4f and 4g in 44% and 48% yield, respectively, while for ortho-substituted benzyl halides the yields of selenides 4h and 4i dropped to 25% and 21% respectively (Table 4, entries 7–10). These results can be ascribed to steric hindrance and secondary reactions involving the benzyl halide and benzyl selenocyanate under basic and reductive conditions (results not shown). Some examples were also performed in PEG 300 as solvent but the yields were not improved (Table 4, entries 2 and 5).

Table 4 One-pot synthesis of alkyl (E) styryl selenides^a

Entry	R–X	Product, yield^b (%)
a Reaction conditions unless otherwise stated: (E)-β-bromostyrene (3a) (0.25 mmol), KSeCN (1) (0.38 mmol), alkyl halide (0.34 mmol), K3PO4 (0.38 mmol) and NaBH4 (0.38 mmol) in 2 mL of DMF.b Isolated yields.c Performed in PEG 300, 51% isolated yield, with minor amount of 5 and 6.d Performed in PEG 300, 70% isolated yield, with minor amount of 5 and 6.
1	n-OctylBr		4a	71
2	n-OctylOTs		4a	83^c
3	MeI		4b	95
4	CH2=CHCH2CH2Br		4c	71
5	Cy-CH2Br		4d	85^d
6	CyBr		4e	21
7	PhCH2Br		4f	44
8			4g	48
9			4h	25
10			4i	21

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Finally, the reactivity of a variety of substituted styryl halides with n-octyl tosylate was also studied (Table 5). Both E-styryl bromide and chloride afforded 4a in good isolated yields and Z-styryl bromide gave the corresponding Z-isomer Z4a in 84% yield (Table 5, entries 1–3). We also tested the effect of the substituents on the styryl moiety. The p-chloro derivative 3c afforded the selenide 4j in comparable yields with the unsubstituted styryl bromide (Table 5, entry 4); however, as the electron donor ability of the substituent increased (Me and MeO), the reactivity of the corresponding vinyl bromide decreased (Table 5, entries 5 and 6). Furthermore the presences of the MeO group at the ortho position decreased the yield to 52% due to steric hindrance, (Table 5, entry 7).

Table 5 One-pot synthesis of arylvinyl n-octyl selenides^a

Entry	3a–e Y, X	Product 4a, j–l, yield^b (%)
a Reaction conditions unless otherwise stated: aryl vinyl halide (3a–e) (0.25 mmol), KSeCN (1) (0.38 mmol), n-octyl tosylate (0.34 mmol), K3PO4 (0.38 mmol) and NaBH4 (0.38 mmol) in 2 mL of DMF.b Isolated yields.c Z-Styryl bromide.d The starting (Z)-vinyl halide contained ca. 10% of (E)-isomer; this led to ca. 10% of the trans-isomer in the product.
1	H, Br 3a		4a	83
2	H, Cl 3b		4a	70
3	(Z)-H, Br^c		(Z)4a	84^d
4	Cl, Br 3c		4j	83
5	Me, Br 3d		4k	80
6	MeO, Br 3e		4l	74
7	o-MeO, Br		4m	52

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The effect of the substituents on the styryl halides, the stereochemical outcome of the reaction (configuration retention) and the occurrence of the reaction without catalysis suggest that the reaction of arylvinyl halides with alkyl selenolate anion follows a classical mechanism of concerted addition–elimination vinylic nucleophilic substitution similar to their analogue alkyl thiolate anion. Thus, the mechanism for the sequential one-pot synthesis of alkyl vinyl selenides follows (Scheme 4). First, alkyl halide reacts with KSeCN by a S_N2 mechanism to afford the corresponding alkyl selenocyanate. In the presence of K₃PO₄, this intermediate (RSeCN) is hydrolyzed by traces of water present in DMF²⁰ to produce alkyl selenolate anion which fast attacks the selenium atom of another molecule of RSeCN affording the corresponding dialkyl diselenide. The dialkyl diselenide was then reduced by NaBH₄ to yield alkane selenolate anion able to react with the vinyl halide by a vinylic nucleophilic substitution (S_NV).²¹

Scheme 4 Proposed mechanism for one-pot formation of alkyl aryl vinyl selenide.

Experimental

Materials and methods

KSeCN, alkyl and benzyl halides were all high-purity commercial samples used without further purification. (E) and (Z)-β-Bromostyrenes were prepared from cinnamic acids following literature procedures.²² Di-octyl selane (5)²³ and 1,2-dioctyl diselane (6)²⁴ were isolated from the reaction mixture by column chromatography and exhibited physical properties identical to those reported in the literature. DMF, acetonitrile and DMSO absolute grade were used without further purification and stored over molecular sieves (4 Å). Toluene, dioxane, THF and pyridine were distilled by standard procedures and stored over molecular sieves (4 Å). PEG 300 and ethanol were used without further purification. All the reaction products were isolated by radial chromatography (silica gel, petroleum/diethyl ether) from the reaction mixture and characterized by ¹H, ¹³C and ⁷⁷Se NMR and mass spectrometry. ¹H, ¹³C and ⁷⁷Se NMR spectra were recorded at 400.16 and 100.62 MHz respectively on a Bruker 400 spectrometer, and all spectra were reported in δ (ppm) relative to Me₄Si, with CDCl₃ as a solvent. The chemical shifts in ⁷⁷Se spectra were given in ppm using diphenyl diselenide (PhSeSePh) diluted in CDCl₃ as an external standard (δ 463 ppm at 25 °C). Gas chromatographic analyses were performed on Agilent 5890 with a flame-ionization detector, on 30 m capillary column of a 0.32 mm × 0.25 μm film thickness, with a 5% phenylpolysiloxane phase. GS-MS analyses were conducted on Agilent 7890 employing a 30 m × 0.25 mm × 0.25 μm with a 5% phenylpolysiloxane phase column. HRMS spectra were recorded on a GCT Premier orthogonal acceleration time-of-flight (oa-TOF) GC mass spectrometer. Ionization was achieved by electronic impact (70 eV) and detection setup positive mode.

Synthetic procedures

General experimental procedures for the reactions of 1 and benzyl, alkyl and vinyl bromides (Scheme 1). The reactions were carried out in a 10 mL three-necked Schlenk tube, equipped with a magnetic stirrer. The tube was charged with DMF (2.0 mL). Potassium selenocyanate (0.25 mmol) and benzyl, n-octyl or (E)-β-styryl bromides (0.25 mmol) were added and stirred at the corresponding temperature for 5 min (2 h for (E)-β-bromostyrene). The reaction mixture was cooled to room temperature. Diethyl ether (15 mL) and water (15 mL) were added and the mixture was stirred. The organic layer was separated and the aqueous layer was extracted with diethyl ether (2 × 15 mL). The combined organic extract was dried over anhydrous Na₂SO₄, and the product was isolated by column chromatography (silica gel, eluent: pentane). The identity of all the products was confirmed by ¹H, ¹³C and ⁷⁷Se NMR and EI MS.
1-Selenocyanatooctane²⁵. Following the general procedure, using potassium selenocyanate (36.0 mg, 0.25 mmol) and n-octyl bromide (48.3 mg, 0.25 mmol) for 5 min at 100 °C. Purification was performed by column chromatography (pentane) providing the title compound as a pale yellow oil (53.5 mg, 95% yield). ¹H NMR (400 MHz, CDCl₃): δ = 3.06 (t, J = 7.4 Hz, 2H), 1.90 (quint, J = 7.3 Hz, 2H), 1.48–1.39 (m, 2H), 1.34–1.24 (m, 8H), 0.89 (t, J = 6.9 Hz, 3H). ¹³C NMR (100 MHz, CDCl₃): δ = 101.5, 31.7, 30.8, 29.7, 29.1, 29.0, 28.8, 22.6, 14.0. ⁷⁷Se NMR (76 MHz, CDCl₃): δ = 207.6. GC-MS (EI) m/z 219 (72) [M]⁺, 137 (100), 101 (82), 75 (52), 50 (40).
(Selenocyanatomethyl)benzene²⁶. Following the general procedure, using potassium selenocyanate (36.0 mg, 0.25 mmol) and benzyl bromide (42.7 mg, 0.25 mmol) for 5 min at room temperature. Purification was performed by column chromatography (pentane) providing the title compound as a white solid (53.5 mg, 98% yield). ¹H NMR (400 MHz, CDCl₃): δ = 7.38–7.32 (m, 5H), 4.31 (s, 2H). ¹³C NMR (100 MHz, CDCl₃): δ = 135.4, 129.2, 129.0, 128.7, 101.8, 32.8. ⁷⁷Se NMR (76 MHz, CDCl₃): δ = 281.9. GC-MS (EI) m/z 91 (100) [M − SeCN]⁺, 65 (18).

General experimental procedures for one-pot synthesis of alkyl styryl selenides. The reactions were carried out in a 10 mL three-necked Schlenk tube, equipped with a magnetic stirrer. The tube was charged with DMF (2.0 mL). Potassium selenocyanate (0.375 mmol), alkyl halide (0.335 mmol), β-halostyrene (0.25 mmol) were added and stirred for 10 min at 100 °C. K₃PO₄ (0.375 mmol) was then added and the mixture was stirred for 1 h. Finally, NaBH₄ (0.375 mmol) was also added and the mixture was stirred for 2 h. The reaction mixture was cooled to room temperature. Diethyl ether (15 mL) and water (15 mL) were added and the mixture was stirred. The organic layer was separated and the aqueous layer was extracted with diethyl ether (2 × 15 mL). The combined organic extract was dried over anhydrous Na₂SO₄ and the products were isolated by radial chromatography from the crude reaction mixture. The identity of all the products was confirmed by ¹H, ¹³C, ⁷⁷Se NMR and EI MS.
(E)-Octyl(styryl)selane (4a). Following the general procedure, using potassium selenocyanate (54.0 mg, 0.375 mmol), n-octyl tosylate (95.7 mg, 0.337 mmol), (E)-β-bromostyrene (45.5 mg, 0.25 mmol) for 10 min at 100 °C, potassium phosphate (79.9 mg, 0.375 mmol) for 1 h at 100 °C and sodium borohydride (14.2 mg, 0.375 mmol) for 2 h at 100 °C. Purification was performed by radial chromatography (pentane) providing 4a as a pale yellow oil (61.4 mg, 83% yield). ¹H NMR (400 MHz, CDCl₃): δ = 7.33–7.24 (m, 4H), 7.24–7.15 (m, 1H), 7.04 (d, J = 15.9 Hz, 1H), 6.73 (d, J = 15.9 Hz, 1H), 2.81 (t, J = 7.3 Hz, 2H), 1.76 (q, J = 7.3 Hz, 2H), 1.46–1.37 (m, 2H), 1.34–1.23 (m, 8H), 0.88 (t, J = 6.9 Hz, 3H). ¹³C NMR (100 MHz, CDCl₃): δ = 137.5, 132.1, 128.6, 127.1, 125.7, 119.5, 31.8, 30.4, 29.9, 29.2, 29.1, 26.3, 22.7, 14.1. ⁷⁷Se NMR (76 MHz, CDCl₃): δ = 267.6. GC-MS (EI) m/z 296 (69) [M]⁺, 184 (100), 102 (39), 91 (39), 77 (21), 71 (39), 57 (75), 55 (33), 43 (82), 41 (70). HRMS (ESI-TOF) m/z calcd for C₁₆H₂₄Se [M]⁺ 296.1038, found 296.1040.
(E)-Methyl(styryl)selane (4b). Following the general procedure, using potassium selenocyanate (54.0 mg, 0.375 mmol), methyl iodide (21.0 μL, 0.337 mmol), (E)-β-bromostyrene (45.5 mg, 0.25 mmol) for 10 min at 100 °C, potassium phosphate (79.9 mg, 0.375 mmol) for 1 h at 100 °C and sodium borohydride (14.2 mg, 0.375 mmol) for 2 h at 100 °C. Purification was performed by radial chromatography (pentane) providing 4b as a pale yellow oil (47.0 mg, 95% yield). ¹H NMR (400 MHz, CDCl₃): δ = 7.27–7.31 (m, 4H), 7.18–7.23 (m, 1H), 7.07 (d, J = 15.8 Hz, 1H), 6.61 (d, J = 15.8 Hz, 1H), 2.26 (s, 3H). ¹³C NMR (100 MHz, CDCl₃): δ = 137.4, 130.4, 128.6, 127.0, 125.6, 120.0, 5.5. ⁷⁷Se NMR (76 MHz, CDCl₃): δ = 183.8. GC-MS (EI) m/z 198 (89) [M]⁺, 183 (100), 102 (99), 77 (56), 63 (23), 51 (57). HRMS (ESI-TOF) m/z calcd for C₉H₁₀Se [M]⁺ 197.9942, found: 197.9943.
(E)-But-3-en-1-yl(styryl)selane (4c). Following the general procedure, using potassium selenocyanate (54.0 mg, 0.375 mmol), 4-bromobut-1-ene (34.2 μL, 0.337 mmol), (E)-β-bromostyrene (45.5 mg, 0.25 mmol) for 10 min at 100 °C, potassium phosphate (79.9 mg, 0.375 mmol) for 1 h at 100 °C and sodium borohydride (14.2 mg, 0.375 mmol) for 2 h at 100 °C. Purification was performed by radial chromatography (pentane) providing 4c as a pale yellow oil (72.2 mg, 71% yield). ¹H NMR (400 MHz, CDCl₃): δ = 7.32–7.28 (m, 4H), 7.24–7.18 (m, 1H), 7.04 (d, J = 15.9 Hz, 1H), 6.75 (d, J = 15.9 Hz, 1H), 5.86 (ddt, J = 16.9, 10.2, 6.6 Hz, 1H), 5.11 (ddd, J = 16.9, 2.8, 1.9 Hz, 1H), 5.07 (ddd, J = 10.2, 2.8, 1.3 Hz, 1H), 2.87 (t, J = 7.5 Hz, 2H), 2.57–2.49 (m, 2H). ¹³C NMR (100 MHz, CDCl₃): δ = 137.4, 137.0, 132.8, 128.6, 127.2, 125.7, 119.0, 116.1, 34.5, 25.0. ⁷⁷Se NMR (76 MHz, CDCl₃): δ = 270.2. GC-MS (EI) m/z 238 (45) [M]⁺, 183 (54), 182 (40), 157 (19), 129 (58), 116 (87), 102 (77), 91 (65), 77 (53), 55 (100), 51 (37). HRMS (APCI) m/z calcd for C₁₂H₁₅Se [M + H]⁺ 239.03338, found: 239.03357.
(E)-(Cyclohexylmethyl) (styryl)selane (4d). Following the general procedure, using potassium selenocyanate (54.0 mg, 0.375 mmol), bromomethyl cyclohexane (46.9 μL, 0.337 mmol), (E)-β-bromostyrene (45.5 mg, 0.25 mmol) for 10 min at 100 °C, potassium phosphate (79.9 mg, 0.375 mmol) for 1 h at 100 °C and sodium borohydride (14.2 mg, 0.375 mmol) for 2 h at 100 °C. Purification was performed by radial chromatography (pentane) providing 4d as a pale yellow oil (59.5 mg, 85% yield). ¹H NMR (400 MHz, CDCl₃): δ = 7.31–7.28 (m, 4H), 7.24–7.17 (m, 1H), 7.03 (d, J = 15.9 Hz, 1H), 6.72 (d, J = 15.9 Hz, 1H), 2.73 (d, J = 6.8 Hz, 2H), 1.93–1.85 (m, 2H), 1.79–1.69 (m, 2H), 1.69–1.63 (m, 1H), 1.63–1.55 (m, 1H), 1.32–1.20 (m, 2H), 1.19–1.09 (m, 1H), 1.06–0.94 (m, 2H). ¹³C NMR (100 MHz, CDCl₃): δ = 137.6, 131.7, 128.6, 127.0, 125.6, 120.3, 38.6, 34.4, 33.5, 26.3, 26.1. ⁷⁷Se NMR (76 MHz, CDCl₃): δ = 239.4. GC-MS (EI) m/z 280 (12) [M]⁺, 184 (32), 97 (28), 91 (12), 55 (100), 41 (22). HRMS (ESI-TOF) m/z calcd for C₁₅H₂₀Se [M]⁺ 280.0725, found: 280.0734.
(E)-Cyclohexyl(styryl)selane (4e). Following the general procedure, using potassium selenocyanate (54.0 mg, 0.375 mmol), bromocyclohexane (40.9 μL, 0.337 mmol), (E)-β-bromostyrene (45.5 mg, 0.25 mmol) for 10 min at 100 °C, potassium phosphate (79.9 mg, 0.375 mmol) for 1 h at 100 °C and sodium borohydride (14.2 mg, 0.375 mmol) for 2 h at 100 °C. Purification was performed by radial chromatography (pentane) providing 4e as a pale yellow oil (14.0 mg, 21% yield). ¹H NMR (400 MHz, CDCl₃): δ = 7.35–7.29 (m, 4H), 7.22 (td, J = 5.8, 2.6 Hz, 1H), 7.11 (d, J = 15.9 Hz, 1H), 6.83 (d, J = 15.9 Hz, 1H), 3.27–3.16 (m, 1H), 2.12 (d, J = 10.1 Hz, 2H), 1.78 (dd, J = 9.0, 4.3 Hz, 2H), 1.63 (dd, J = 10.3, 3.7 Hz, 2H), 1.43–1.26 (m, 4H). ¹³C NMR (100 MHz, CDCl₃): δ = 137.6, 134.1, 128.6, 127.2, 125.8, 118.3, 41.6, 34.4, 26.8, 25.7. ⁷⁷Se NMR (76 MHz, CDCl₃): δ = 338.4. GC-MS (EI) m/z 266 (32) [M]⁺, 184 (96), 102 (32), 91 (22), 77 (16), 55 (100), 51 (12), 41 (36). HRMS (APCI) m/z calcd for C₁₄H₁₉Se [M + H]⁺ 267.06469, found: 267.06476.
(E)-Benzyl(styryl)selane (4f). Following the general procedure, using potassium selenocyanate (54.0 mg, 0.375 mmol), benzyl bromide (64.1 mg, 0.337 mmol), (E)-β-bromostyrene (45.5 mg, 0.25 mmol) for 10 min at 100 °C, potassium phosphate (79.9 mg, 0.375 mmol) for 1 h at 100 °C and sodium borohydride (14.2 mg, 0.375 mmol) for 2 h at 100 °C. Purification was performed by radial chromatography (pentane) providing 4f as a light yellow solid (30.1 mg, 44% yield). ¹H NMR (400 MHz, CDCl₃): δ = 7.38–7.26 (m, 8H), 7.25–7.19 (m, 2H), 7.03 (d, J = 15.9 Hz, 1H), 6.78 (d, J = 15.9 Hz, 1H), 4.04 (s, 2H). ¹³C NMR (100 MHz, CDCl₃): δ = 138.3, 137.3, 133.6, 128.9, 128.6, 128.6, 127.3, 127.1, 125.8, 118.9, 29.9. ⁷⁷Se NMR (76 MHz, CDCl₃): δ = 340.9. GC-MS (EI) m/z 274 (10) [M]⁺, 183 (5), 91 (100), 65 (13). HRMS (ESI-TOF) m/z calcd for C₁₅H₁₄Se [M]⁺ 274.0256, found 274.0260.
(E)-(4-Fluorobenzyl)(styryl)selane (4g). Following the general procedure, using potassium selenocyanate (54.0 mg, 0.375 mmol), 4-fluorobenzyl chloride (48.7 mg, 0.337 mmol), (E)-β-bromostyrene (45.5 mg, 0.25 mmol) for 10 min at 100 °C, potassium phosphate (79.9 mg, 0.375 mmol) for 1 h at 100 °C and sodium borohydride (14.2 mg, 0.375 mmol) for 2 h at 100 °C. Purification was performed by radial chromatography (pentane) providing 4g as a yellow solid (35.0 mg, 48% yield). ¹H NMR (400 MHz, CDCl₃): δ = 7.33–7.27 (m, 6H), 7.23 (dt, J = 9.2, 4.2 Hz, 1H), 7.02–6.97 (m, 3H), 6.77 (d, J = 15.8 Hz, 1H), 4.01 (s, 2H). ¹³C NMR (100 MHz, CDCl₃): δ = 161.8 (d, ¹J_C–F = 244.5 Hz), 137.2, 134.1, 134.1 (d, ⁴J_C–F = 4.3 Hz), 130.4 (d, ³J_C–F = 8.1 Hz), 128.7, 127.5, 125.8, 118.5, 115.5 (d, ²J_C–F = 21.4 Hz), 29.1. ⁷⁷Se NMR (76 MHz, CDCl₃): δ = 343.1. GC-MS (EI) m/z 292 (6) [M]⁺, 183 (7), 109 (100), 102 (8), 77 (3), 51 (4). HRMS (APCI) m/z calcd for C₁₅H₁₂FSe [M − H]⁺ 291.00832, found 291.00867.²⁷ HRMS (APPI) m/z calcd for C₁₅H₁₃FSe [M]⁺ 292.01669, found 292.01742.
(E)-(2-Iodobenzyl)(styryl)selane (4h). Following the general procedure, using potassium selenocyanate (54.0 mg, 0.375 mmol), 2-iodobenzyl chloride (84.6 mg, 0.337 mmol), (E)-β-bromostyrene (45.5 mg, 0.25 mmol) for 10 min at 100 °C, potassium phosphate (79.9 mg, 0.375 mmol) for 1 h at 100 °C and sodium borohydride (14.2 mg, 0.375 mmol) for 2 h at 100 °C. Purification was performed by radial chromatography (pentane) providing 4h as a yellow solid (25.0 mg, 25% yield). ¹H NMR (400 MHz, CDCl₃): δ = 7.84 (d, J = 7.3 Hz, 1H), 7.41–7.17 (m, 7H), 7.08 (d, J = 15.8 Hz, 1H), 6.91 (td, J = 7.7, 1.6 Hz, 1H), 6.79 (d, J = 15.8 Hz, 1H), 4.13 (s, 2H). ¹³C NMR (100 MHz, CDCl₃): δ = 141.3, 139.9, 137.2, 134.8, 129.9, 128.7, 128.6, 128.5, 127.5, 125.9, 118.3, 100.6, 35.6. ⁷⁷Se NMR (76 MHz, CDCl₃): δ = 338.4. GC-MS (EI) m/z 400 (21) [M]⁺, 217 (100), 183 (23), 102 (28), 91 (65), 90 (65), 89 (35), 77 (11), 63 (21), 51 (13). HRMS (ESI-TOF) m/z calcd for C₁₅H₁₃INaSe [M + Na]⁺ 422.9120, found: 422.9113.
(E)-5-Bromo-6-((styrylselanyl)methyl)benzo[d][1,3]dioxole (4i). Following the general procedure, using potassium selenocyanate (54.0 mg, 0.375 mmol), 5-bromo-6-(bromomethyl)benzo[d][1,3]dioxole (99.0 mg, 0.337 mmol), (E)-β-bromostyrene (45.5 mg, 0.25 mmol) for 10 min at 100 °C, potassium phosphate (79.9 mg, 0.375 mmol) for 1 h at 100 °C and sodium borohydride (14.2 mg, 0.375 mmol) for 2 h at 100 °C. Purification was performed by radial chromatography (pentane) providing 4i as a yellow solid (20.8 mg, 21% yield). ¹H NMR (400 MHz, CDCl₃): δ = 7.35–7.27 (m, 4H), 7.24–7.19 (m, 1H), 7.10 (d, J = 15.8 Hz, 1H), 7.00 (s, 1H), 6.85 (s, 1H), 6.79 (d, J = 15.8 Hz, 1H), 5.96 (s, 2H), 4.07 (s, 2H). ¹³C NMR (100 MHz, CDCl₃): δ = 147.6, 147.5, 137.2, 134.4, 131.2, 128.6, 127.4, 125.9, 118.4, 114.8, 112.9, 110.2, 101.8, 30.4. ⁷⁷Se NMR (76 MHz, CDCl₃): δ = 342.3. GC-MS (EI) m/z 396 (2) [M]⁺, 215 (100), 213 (93), 104 (17), 102 (22), 78 (37), 77 (25), 76 (32), 75 (29), 51 (24), 50 (22). HRMS (ESI-TOF) m/z calcd for C₁₆H₁₃BrNaO₂Se [M + Na]⁺ 418.9154, found 418.9145.
(Z)-Octyl(styryl)selane (Z4a). Following the general procedure, using potassium selenocyanate (54.0 mg, 0.375 mmol), n-octyl tosylate (95.7 mg, 0.337 mmol), (Z)-β-bromostyrene (45.5 mg, 0.25 mmol) for 10 min at 100 °C, potassium phosphate (79.9 mg, 0.375 mmol) for 1 h at 100 °C and sodium borohydride (14.2 mg, 0.375 mmol) for 2 h at 100 °C. Purification was performed by radial chromatography (pentane) providing Z4a as a pale yellow oil (62.2 mg, 84% yield). The starting (Z)-vinyl halide contained ca. 10% of (E)-isomer; this led to ca. 10% of the trans-isomer in the product. ¹H NMR (400 MHz, CDCl₃): δ = 7.46–7.15 (m, 5H), 6.87 (d, J = 10.6 Hz, 1H), 6.60 (d, J = 10.6 Hz, 1H), 2.78 (t, J = 7.3 Hz, 2H), 1.75 (q, J = 7.3 Hz, 2H), 1.44–1.35 (m, 2H), 1.29 (m, 8H), 0.88 (t, J = 6.9 Hz, 3H). ¹³C NMR (100 MHz, CDCl₃): δ = 137.7, 129.3, 128.3, 126.8, 123.2, 31.8, 30.7, 29.7, 29.2, 29.1, 22.6, 14.1. ⁷⁷Se NMR (76 MHz, CDCl₃): δ = 263.1. GC-MS (EI) m/z 296 (58) [M]⁺, 184 (97), 102 (59), 91 (30), 71 (36), 43 (100), 41 (55). HRMS (ESI-TOF) m/z calcd for C₁₆H₂₄Se [M]⁺ 296.1038, found 296.1041.
(E)-(4-Chlorostyryl)(octyl)selane (4j). Following the general procedure, using potassium selenocyanate (54.0 mg, 0.375 mmol), n-octyl tosylate (95.7 mg, 0.337 mmol), (E)-1-(2-bromovinyl)-4-chlorobenzene (54.4 mg, 0.25 mmol) for 10 min at 100 °C, potassium phosphate (79.9 mg, 0.375 mmol) for 1 h at 100 °C and sodium borohydride (14.2 mg, 0.375 mmol) for 2 h at 100 °C. Purification was performed by radial chromatography (pentane) providing 4j as a pale yellow oil (68.5 mg, 83% yield). ¹H NMR (400 MHz, CDCl₃): δ = 7.28–7.19 (m, 4H), 7.04 (d, J = 15.9 Hz, 1H), 6.66 (d, J = 15.9 Hz, 1H), 2.82 (t, J = 6.8 Hz, 2H), 1.76 (q, J = 6.8 Hz, 2H), 1.47–1.37 (m, 2H), 1.30 (m, 8H), 0.88 (t, J = 6.8 Hz, 3H). ¹³C NMR (100 MHz, CDCl₃): δ = 136.0, 132.6, 130.5, 128.8, 126.8, 120.6, 31.8, 30.3, 29.8, 29.2, 29.1, 26.3, 22.6, 14.1. ⁷⁷Se NMR (76 MHz, CDCl₃): δ = 271.6. GC-MS (EI) m/z 330 (40) [M]⁺, 218 (28), 183 (40), 102 (20), 71 (51), 57 (78), 43 (100), 41 (71). HRMS (APCI) m/z calcd for C₁₆H₂₄ClSe [M + H]⁺ 331.07241, found: 331.07245.
(E)-(4-Methylstyryl)(octyl)selane (4k). Following the general procedure, using potassium selenocyanate (54.0 mg, 0.375 mmol), n-octyl tosylate (95.7 mg, 0.337 mmol), (E)-1-(2-bromovinyl)-4-methylbenzene (49.2 mg, 0.25 mmol) for 10 min at 100 °C, potassium phosphate (79.9 mg, 0.375 mmol) for 1 h at 100 °C and sodium borohydride (14.2 mg, 0.375 mmol) for 2 h at 100 °C. Purification was performed by radial chromatography (pentane) providing 4k as pale yellow oil (62.0 mg, 80% yield). ¹H NMR (400 MHz, CDCl₃): δ = 7.20 (d, J = 8.1 Hz, 2H), 7.10 (d, J = 8.1 Hz, 2H), 6.97 (d, J = 15.9 Hz, 1H), 6.71 (d, J = 15.9 Hz, 1H), 2.80 (t, J = 7.3 Hz, 2H), 2.31 (s, 3H), 1.75 (q, J = 7.3 Hz, 2H), 1.46–1.37 (m, 2H), 1.35–1.22 (m, 8H), 0.88 (t, J = 6.9 Hz, 3H). ¹³C NMR (100 MHz, CDCl₃): δ = 136.9, 134.8, 132.5, 129.3, 125.6, 118.0, 31.8, 30.4, 29.9, 29.9, 29.1, 26.3, 22.6, 21.2, 14.1. ⁷⁷Se NMR (76 MHz, CDCl₃): δ = 264.3. GC-MS (EI) m/z 310 (82) [M]⁺, 198 (61), 197 (29), 183 (65), 115 (63), 105 (19), 91 (20), 71 (36), 57 (64), 43 (100), 41 (86). HRMS (ESI-TOF) m/z calcd for C₁₇H₂₆Se [M]⁺ 310.1195, found: 310.1169.
(E)-(4-Methoxystyryl)(octyl)selane (4l). Following the general procedure, using potassium selenocyanate (54.0 mg, 0.375 mmol), n-octyl tosylate (95.7 mg, 0.337 mmol), (E)-1-(2-bromovinyl)-4-methoxybenzene (53.2 mg, 0.25 mmol) for 10 min at 100 °C, potassium phosphate (79.9 mg, 0.375 mmol) for 1 h at 100 °C and sodium borohydride (14.2 mg, 0.375 mmol) for 2 h at 100 °C. Purification was performed by radial chromatography (pentane) providing 4l as a white solid (60.3 mg, 74% yield). ¹H NMR (400 MHz, CDCl₃): δ = 7.26–7.21 (m, 2H), 6.87 (d, J = 15.8 Hz, 1H), 6.84–6.82 (m, 2H), 6.71 (d, J = 15.8 Hz, 1H), 3.80 (s, 3H), 2.79 (t, J = 6.8 Hz, 2H), 1.75 (q, J = 6.8 Hz, 2H), 1.46–1.37 (m, 2H), 1.34–1.22 (m, 8H), 0.88 (t, J = 6.9 Hz, 3H). ¹³C NMR (100 MHz, CDCl₃): δ = 158.9, 132.5, 130.5, 126.9, 116.3, 114.1, 55.3, 31.8, 30.4, 29.8, 29.2, 29.1, 26.4, 22.6, 14.1. ⁷⁷Se NMR (76 MHz, CDCl₃): δ = 262.1. GC-MS (EI) m/z 326 (25) [M]⁺, 214 (16), 198 (11), 134 (100), 57 (12), 43 (26), 41 (23). HRMS (ESI-TOF) m/z calcd for C₁₇H₂₆OSe [M + Na]⁺ 349.1042, found 349.1050.
(E)-(2-Methoxystyryl)(octyl)selane (4m). Following the general procedure, using potassium selenocyanate (54.0 mg, 0.375 mmol), n-octyl tosylate (95.7 mg, 0.337 mmol), (E)-1-(2-bromovinyl)-2-methoxybenzene (53.2 mg, 0.25 mmol) for 10 min at 100 °C, potassium phosphate (79.9 mg, 0.375 mmol) for 1 h at 100 °C and sodium borohydride (14.2 mg, 0.375 mmol) for 2 h at 100 °C. Purification was performed by radial chromatography (pentane) providing 4m as pale yellow oil (42.4 mg, 52% yield). ¹H NMR (500 MHz, CDCl₃): δ = 7.34 (dd, J = 7.6, 1.7 Hz, 1H), 7.19 (ddd, J = 8.2, 7.5, 1.7 Hz, 1H), 7.09 (d, J = 16.0 Hz, 1H), 7.03 (d, J = 16.0 Hz, 1H), 6.91 (td, J = 7.5, 1.0 Hz, 1H), 6.85 (dd, J = 8.2, 0.8 Hz, 1H), 3.84 (s, 3H), 2.85–2.78 (m, 2H), 1.76 (q, J = 7.4 Hz, 2H), 1.47–1.38 (m, 2H), 1.35–1.23 (m, 8H), 0.88 (t, J = 7.0 Hz, 3H). ¹³C NMR (126 MHz, CDCl₃): δ = 156.0, 128.1, 127.2, 126.6, 126.5, 120.7, 120.1, 110.9, 55.4, 31.8, 30.4, 29.9, 29.2, 29.1, 26.2, 22.7, 14.1. ⁷⁷Se NMR (76 MHz, CDCl₃): δ = 269.6. GC-MS (EI) m/z 326 (57) [M]⁺, 214 (29), 198 (27), 183 (23), 134 (59), 119 (43), 105 (43), 91 (38), 77 (21), 57 (38), 41 (100). HRMS (APCI) m/z calcd for C₁₇H₂₇OSe [M + H]⁺ 327.12222, found 327.12075.

Conclusions

In summary, we have developed a new one-pot methodology for the stereoselective synthesis of alkyl arylvinyl selenides in moderate to excellent yields, free of any transition metal/ligand systems, from malodorous and air-sensitive selenolate anions and diselenide as starting reagents. This procedure employs the commercially available potassium selenocyanate, DMF as a solvent, short reaction time and air atmosphere.

Acknowledgements

This work was supported partly by Consejo Nacional de Investigaciones Científicas y Técnicas (CONICET) and Fondo de Ciencia y Tecnología (FONCYT), Argentina. A. A. H. gratefully acknowledges the receipt of a fellowship from CONICET.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Spectra (¹H, ¹³C and ⁷⁷Se NMR) for all the products (4a–m and Z4a). See DOI: 10.1039/c5ra20883a

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