Synthesis of Hexylselenol and Hexylselenides from Hexylthiol Involving ­Hexylthiolanium Salts

Abstract

Selenium nucleophiles react regioselectively at the endo­cyclic or exocyclic carbon of hexylthiolanium salts depending upon the nature of the salt.

Key words

halides - S-alkylations - selenium - nucleophiles

References and Notes

• 1a Krief A. Dumont W. Robert M. Synlett  2006,  484 
• 1b Krief A. Dumont W. Robert M. Chem. Commun.  2005,  2167 
• 2a Anklam E. Synthesis  1987,  841 
• 2b

  It is necessary to employ 1.5 equivalents of 1,4-dibromo-butane to avoid competing formation of 1,4-di(hexylthio)butane; the latter is produced in up to 16% yield along with 3b (75%) if a stoichiometric amount of the substrate is used.

• 2c

  1-Bromo-4-chloro-butane behaves similarly to 1,4-dibromo-butane.

• 3 Eliel E. Hutchins RO. Mebane R. Willer RL. J. Org. Chem.  1976,  41:  1052 
• 4a Badet B. Jacob L. Julia M. Tetrahedron  1981,  37:  887 
• 4b Badet B. Julia M. Ramirez-Mundoz M. Sarrazin CA. Tetrahedron  1983,  39:  3111 
• 7a

  S -1-Hexylthiolanium Bromide: A mixture of hexane-thiol (590 mg, 5.0 mmol), 1,4-dibromobutane (1.6 g, 1.5 equiv, 7.5 mmol), and K₂CO₃ (690 mg, 1.5 equiv, 5.0 mmol) in EtOH (10 mL) was stirred at r.t. for 5 h under argon. EtOH was then removed, the residue was suspended in CHCl₃, and KBr was removed by filtration. CHCl₃ was removed under vacuum, the resulting oil was first triturated with anhyd Et₂O and then dried under vacuum to produce 1.1 g of the oily S-1-hexylthiolanium bromide (86% yield of crude product).

• 7b

  1-Hexylselenocyanate: A solution of S-1-hexylthiolanium bromide (254 mg, 1 mmol) in DMF (1 mL) was added at r.t. to a stirred solution of KSeCN (143 mg, 1 mmol) in DMF (1 mL). The resulting solution was stirred at 80 °C for 24 h, cooled to r.t., then washed with H₂O (5 mL) and extracted with Et₂O (3 × 5 mL). The organic layer was washed with H₂O (2 × 5 mL) and dried over MgSO₄. After filtration, the solvent was evaporated under vacuum (10 mm Hg). Purification of the resulting liquid by preparative TLC (silica gel; pentane-Et₂O, 9:1) leads to 1-hexylselenocyanate (6; 130 mg, 68% yield).

• 7c

  Hexylselenol: NaBH₄ (190 mg, 5 mmol) was added at r.t., in small portions, to a stirred solution of hexylseleno-cyanate (950 mg, 5 mmol) in EtOH (5 mL). After 0.2 h, aq HCl (10%) was added. The resulting solution was extracted with Et₂O (3 × 20 mL), the organic layer was washed with H₂O (2 × 5 mL) and then dried over MgSO₄. After filtration and evaporation of the solvent the residue was distilled to give hexylselenol (82 °C, 20 mm Hg, 638 mg, 75% yield).

• 8 Krief A. Delmotte C. Dumont W. Tetrahedron  1997,  53:  12147 
• 9a For a synthesis of selenides from aryl iodides and stannylselenides using a palladium catalyst: Beletskaya IP. Sigeev AS. Peregudov AS. Petrovskii PV. J. Organomet. Chem.  2000,  605:  96 
• 9b

  For a related reaction recently performed on thiolates see ref. 1a.

• 10a Carey FA. Sundberg RJ. In Advanced Organic Chemistry, Part A: Structure and Mechanisms   Kluwer Academic/Plenum Publishers; New York: 2000. 
• 10b Carey FA. Sundberg RJ. In Advanced Organic Chemistry, Part B: Reactions and Synthesis   Kluwer Academic/Plenum Publishers; New York: 2000. 

5

We have also performed this reaction successfully in acetone. On mixing a solution of KSeCN (1 mmol) in acetone (3 mL) at r.t. with a solution of the tetrafluoroborate 3c (1 mmol) in the same solvent (1 mL), a red-brown solid precipitates almost instantaneously. This solid mainly contains potassium tetrafluoroborate and some KSeCN, which co-precipitates. The resulting solution [IR (neat): 2060 cm^-1 (s), attributed to the selenocyanate anion] slowly delivers 1-hexyl 1-(4-selenocyanato)butyl sulfide (8; 20 °C, 72 h, 3c/8: 39:61). Evaporation of acetone and distillation of the residue (short-path distillation, 150 °C, 200 mmHg) leads to 6 in 68% yield; IR(neat): 2150 cm^-1 (s), attributed to the selenocyanato group attached to an alkyl chain.

6

Prepared from potassium selenocyanate and 1,4-dibromo-butane. IR and NMR spectra support this assignment rather than that of its cyclic isomer.